NON-ORIENTED ELECTRICAL STEEL SHEET AND METHOD OF MANUFACTURING THE SAME

A non-oriented electrical steel sheet includes chemical compositions including, in terms of mass %: C: 0.0001% to 0.01%; Si: 0.05% to 7.0%; Mn: 0.01% to 3.0%; Al: 0.0020% to 3.0%; S: 0.0001% to 0.1%; P: 0.0010% to 0.15%; N: 0.0010% to 0.01%; Cu: 0.01% to 5.0%; and a remainder including Fe and impurities, in which I2θ=46.4 which is a diffraction intensity of Cu sulfide having a hexagonal structure shown at 2θ=46.4° and I2θ=32.3 which is a diffraction intensity of Cu sulfide having a cubic structure shown at 2θ=32.3°, which are obtained through a X-ray diffraction of an electrolytic extraction residue, satisfy I2θ=46.4/I2θ=32.3≦0.5.

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Description

This application is a national stage application of International Application No. PCT/JP2014/060164, filed on Apr. 8, 2014, which claims priority to Japanese Patent Application No. 2013-081078, filed on Apr. 9, 2013, each of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a non-oriented electrical steel sheet which is used as a core material of an electrical device and a method of manufacturing the same, and more particularly, to a non-oriented electrical steel sheet having excellent core loss and a method of manufacturing the same.

RELATED ART

A non-oriented electrical steel sheet is used as a core material of various types of motors for heavy electrical apparatuses, home appliances, and the like. The non-oriented electrical steel sheet is commercially graded according to core loss, and is classified according to the design features of motors or transformers. Recently, from the viewpoint of energy saving, a further reduction in core loss and an increase in magnetic flux density have been strongly demanded of the non-oriented electrical steel sheet.

In general, when fine precipitates are present in a steel sheet, grain growth during annealing is retarded, and core loss is deteriorated. Particularly, Cu which is unavoidably incorporated into the steel sheet generates Cu sulfide, and the fine Cu sulfide inhibits the grain growth of the non-oriented electrical steel sheet. As a result, core loss is deteriorated. In addition, the fine Cu sulfide which is present in the steel sheet causes a deterioration in hysteresis loss. The deterioration in hysteresis loss also causes the deterioration in core loss.

Here, in the related art, for the purpose of improving the core loss of a non-oriented electrical steel sheet, methods such as precipitation control of sulfide during hot rolling, a method of reducing the amount of sulfide through desulfurization, and suppression of precipitation of Cu sulfide through rapid cooling after final annealing have been proposed.

For example, in Patent Document 1, a method of controlling the dispersion state of Cu sulfide to a preferable state for magnetic properties of a non-oriented electrical steel sheet, that is, core loss and magnetic flux density by holding a slab containing 0.2% or less of Cu in a range of 900° C. to 1100° C. for 30 minutes or longer, thereafter holding the slab at a higher temperature of 1150° C. and subsequently starting rolling, and limiting a cooling rate during finish hot rolling to be 50° C./sec or lower is disclosed. However, in this method, there are problems in productivity, such as an increase in rolling load due to a reduction in slab heating temperature and a difficulty in strict control of the cooling rate.

In Patent Document 2, a method of avoiding the generation of fine precipitates by adding CaSi to molten steel until the completion of casting to control the S content to be 0.005% or less, heating a slab to a temperature of 1000° C. or higher and then hot-rolling the slab, and coiling a coil at a specific temperature range is disclosed. In this method, high purity steel is essential. However, the formation of fine Cu sulfide due to Cu which is incorporated at an unavoidable level cannot be avoided. Therefore, there is a problem in that magnetic properties are rather deteriorated by the incorporation of Cu.

In addition, in Patent Document 3, a technique of suppressing the precipitation of Cu sulfide by performing rapid cooling from a temperature range of 500° C. to 600° C. to 300° C. at a cooling rate of 10° C./sec to 50° C./sec after final annealing is disclosed. However, the fact that Cu sulfide is precipitated even during cooling at a cooling rate of 50° C./sec or higher is known in Non-Patent Documents 1 and 2, and the like. That is, in the technique of Patent Document 3 in which cooling is performed at a cooling rate of 10° C./sec to 50° C./sec, it is difficult to completely eliminate the precipitation of Cu sulfide.

In Patent Documents 4 to 6, a technique in which an enhancement in magnetic properties is expected by suppressing the cooling rate after final annealing is disclosed. However, in this method, it may not be possible to make Cu sulfide harmless.

PRIOR ART DOCUMENT Patent Document

  • [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2010-174376
  • [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H10-183244
  • [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. H09-302414
  • [Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2011-006721
  • [Patent Document 5] Japanese Unexamined Patent Application, First Publication No. 2006-144036
  • [Patent Document 6] Japanese Unexamined Patent Application, First Publication No. 2003-113451

Non-Patent Document

  • [Non-Patent Document 1] CAMP-ISIJ Vol. 25 (2012), p 1080
  • [Non-Patent Document 2] CAMP-ISIJ Vol. 22 (2009), p 1284
  • [Non-Patent Document 3] J. Flux Growth Vol. 5 (2010), p 48
  • [Non-Patent Document 4] Materials Transactions Vol. 53 (2012), P 645
  • [Non-Patent Document 5] Tetsu-to-Hagane Vol. 83 (1997), p 479
  • [Non-Patent Document 6] Tetsu-to-Hagane Vol. 92 (2006), p 619

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made taking the foregoing circumstances into consideration, and an object thereof is to provide a non-oriented electrical steel sheet having excellent core loss and a method of manufacturing the same without causing an increase in cost or a reduction in productivity by making Cu sulfide harmless and increasing the grain size.

Means for Solving the Problem

In order to solve the above-described problems, in the present invention, an effect of the chemical components and manufacturing conditions of a steel sheet on the relationship between the state of sulfide and core loss was repeatedly examined. As a result, it was found that there was a case where non-oriented electrical steel sheets obtained under different manufacturing conditions had significantly different core losses even though the sizes or the number densities of sulfide, which are hitherto known to affect core loss, were at the same level. Here, the inventors conducted more detailed examination on the morphology or structure of sulfide, and found a possibility that a difference in core loss might be caused by a difference in atomic structure of Cu sulfide, and specifically, a possibility that the consistency between the crystal lattice of Fe as a primary phase and Cu sulfide might affect magnetic domain wall motion. The present invention has been made based on the above-described findings, and is summarized in the following (1) to (8).

(1) That is, a non-oriented electrical steel sheet according to an aspect of the present invention includes chemical compositions including, in terms of mass %: C: 0.0001% to 0.01%; Si: 0.05% to 7.0%; Mn: 0.01% to 3.0%; Al: 0.0020% to 3.0%; S: 0.0001% to 0.1%; P: 0.0010% to 0.15%; N: 0.0010% to 0.01%; Cu: 0.01% to 5.0%; and a remainder including Fe and impurities, in which I2θ=46.4 which is a diffraction intensity of Cu sulfide having a hexagonal structure shown at 2θ=46.4° and I2θ=32.3 which is a diffraction intensity of Cu sulfide having a cubic structure shown at 2θ=32.3°, which are obtained through a X-ray diffraction of an electrolytic extraction residue, satisfy the following Expression 1.


I2θ=46.4/I2θ=32.3≦0.5  Expression 1

(2) In the non-oriented electrical steel sheet described in (1), when a Cu content, in terms of mass %, is denoted as [% Cu] and an S content, in terms of mass %, is denoted as [% S], the [% Cu] and the [% S] may satisfy [% Cu]/[% S]≧2.5.

(3) In the non-oriented electrical steel sheet described in (1) or (2), 0.5 pieces/μm3 to 50 pieces/μm3 of sulfide containing Cu and having a diameter of 5 nm to 500 nm may be contained.

(4) A method of manufacturing a non-oriented electrical steel sheet according to another aspect of the present invention, is a method of manufacturing the non-oriented electrical steel sheet described in any one of (1) to (3), and includes: performing a hot rolling on a slab to obtain a hot-rolled steel sheet; annealing the hot-rolled steel sheet; pickling the hot-rolled steel sheet; performing a cold rolling on the hot-rolled steel sheet to obtain a cold-rolled steel sheet; and annealing the cold-rolled steel sheet, in which, in the annealing of the cold-rolled steel sheet, after the cold-rolled steel sheet is held at T1° C., which is represented in the following Expression 2, to 1530° C. for 30 seconds to 3600 seconds, when an average cooling rate from the T1° C. to T2° C., which is shown in Expression 3, is denoted as CR1 in the unit of ° C./sec and an average cooling rate from the T2° C. to T3° C., which is shown in Expression 4, is denoted as CR2 in the unit of ° C./sec, the cold-rolled steel sheet is cooled to a temperature range of the T3° C. or lower so that the CR1 and the CR2 satisfy Expressions 5, 6 and 7:


T1=17000/(14−log10([% Cu]2×[% S]))−273  Expression 2


T2=17000/(14−log10([% Cu]2×[% S]))−323  Expression 3


T3=17000/(14−log10([% Cu]2×[% S]))−473  Expression 4


CR1>CR2  Expression 5


5≦CR1≦500  Expression 6


0.5≦CR2≦50  Expression 7

where [% Cu] is a Cu content in terms of mass % and [% S] is an S content in terms of mass %.

(5) In the method of manufacturing a non-oriented electrical steel sheet described in (4), the CR1 may further satisfy the following Expression 8.


CR1>20  Expression 8

(6) In the method of manufacturing a non-oriented electrical steel sheet according to claim 4) or (5), the CR2 may further satisfy the following Expression 9.


CR2≦20  Expression 9

(7) the method of manufacturing a non-oriented electrical steel sheet described in any one of (4) to (6), may further include: subsequent to the annealing of the cold-rolled steel sheet, holding the cold-rolled steel sheet in a temperature range of the T2° C. or lower to the T3° C. or higher for 30 seconds or longer as an additional annealing.

(8) In the method of manufacturing a non-oriented electrical steel sheet described in any one of (4) to (7), in the annealing of the hot-rolled steel sheet, the hot-rolled steel sheet may be cooled so that CR3 which is an average cooling rate from the T1° C. to room temperature is 15° C./sec or higher.

Effects of the Invention

According to the above aspects of the present invention, even when high purification, a reduction in slab heating temperature, optimization of hot rolling conditions, and the like are not performed on the non-oriented electrical steel sheet, it is possible to make fine Cu sulfide harmless. Accordingly, a non-oriented electrical steel sheet having excellent core loss can be provided.

In addition, according to the above aspects of the present invention, properties (magnetic flux density, workability, and the like) other than core loss, required of a grain-oriented electrical steel sheet, can be ensured to be the same or a higher level than a material in the related art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between I2θ=46.4/I2θ=32.3 and core loss.

FIG. 2 is a flowchart showing an example of a process of manufacturing a non-oriented electrical steel sheet according to an embodiment.

 EMBODIMENTS OF THE INVENTION

Hereinafter, a non-oriented electrical steel sheet according to an embodiment of the present invention (may also be referred to as a non-oriented electrical steel sheet according to this embodiment) and a method of manufacturing the same will be described in detail. All of % of contents are mass %.

C: 0.0001% to 0.01%

C causes significant deterioration in core loss through magnetic aging. Therefore, the upper limit of the C content is 0.01%. From the viewpoint of the improvement in core loss, the C content is preferably 0.0020% or less. On the other hand, when the C content is less than 0.0001%, the magnetic flux density is deteriorated. Therefore, in order to ensure a sufficient magnetic flux density, the lower limit of the C content is 0.0001%. The C content is preferably 0.0005 to 0.0015%, and more preferably 0.0007 to 0.0010%.

Si: 0.05% to 7.0%

The Si content is 0.05% to 7.0% for a balance between ensuring core loss and a sheet travelling property. When the Si content is less than 0.05%, good core loss is not obtained. On the other hand, when the Si content is more than 7.0%, the steel sheet becomes embrittled, and the sheet travelling property during a manufacturing process is significantly deteriorated. The Si content is preferably 2.3% to 3.5%, more preferably 2.9% to 3.3%, and still more preferably 3.0% to 3.2%.

Mn: 0.01% to 3.0%

Mn reacts with S and forms sulfide, and is thus an important element in the present invention. In a case where Mn is present in steel, MnS is precipitated and thus the transition temperature of the crystal structure of Cu sulfide is reduced. In this case, Cu sulfide having a cubic structure is less likely to be generated. Therefore, the upper limit of the Mn content is 3.0%. On the other hand, when the Mn content is less than 0.01%, the steel sheet becomes embrittled during hot rolling. Therefore, the lower limit of the Mn content is 0.01%. The Mn content is preferably 0.05% to 2.0%, and more preferably 0.1% to 1.0%.

Al: 0.0020% to 3.0%

Al is solutionized in steel, resulting in the electric resistance of steel is increased and core loss is reduced. Therefore, in order to improve core loss (reduce core loss), increasing the Al content in steel is advantageous. However, molten steel having a high Al content causes deterioration in operability during casting and thus causes embrittlement of the steel sheet. Therefore, the upper limit of the Al content is 3.0%. On the other hand, when the Al content is low, AlN which accelerates grain growth in the steel sheet is not sufficiently generated, and fine TiN that impedes the grain growth is generated instead of AlN, resulting in significant deterioration in the magnetic flux density. Therefore, the lower limit of the Al content is 0.0020%. The Al content is preferably 0.1% to 2.0%, and more preferably 1.0% to 1.5%.

S: 0.0001% to 0.1%

The S content is directly associated with the amount of sulfide. When the S content is excessive, S is present in steel in a solid solution state, and steel becomes embrittled during hot rolling. Therefore, the upper limit of the S content is 0.1%. On the other hand, when the S content is less than 0.0001%, the precipitation temperature range (a temperature range of T2° C. to T3° C., which will be described later) of Cu sulfide (cubic) is more significantly reduced than the grain growth temperature of the steel sheet, and thus the effect of improving core loss is not obtained. Therefore, the lower limit of the S content is 0.0001%. The S content is preferably 0.01% to 0.05%, and more preferably 0.02% to 0.03%.

P: 0.0010% to 0.15%

P has an effect of increasing the hardness of the steel sheet and enhancing a blanking property. In addition, a small amount of P has an effect of improving the magnetic flux density. In order to obtain this effect, the lower limit of the P content is 0.0010%. Here, when the P content is excessive, the magnetic flux density is deteriorated, and thus the upper limit of the P content is 0.15%. The P content is preferably 0.005% to 0.1%, and more preferably 0.01% to 0.07%.

N: 0.0010% to 0.01%

N is an element which forms nitride with Ti and the like. When the N content is excessive, the precipitation amount of nitride such as TiN is increased, and this nitride impedes the grain growth. Therefore, the upper limit of the N content is 0.01%. Here, a small amount of N contained suppresses precipitation of fine TiC, and thus an effect of accelerating the grain growth of the steel sheet is obtained. Therefore, for the purpose of ensuring a sufficient magnetic flux density, the lower limit of the N content is 0.0010%. The N content is preferably 0.0030% to 0.0080%, more preferably 0.0040% to 0.0080%, and still more preferably 0.0050% to 0.0070%.

Cu: 0.01% to 5.0%

Cu is an element which forms sulfide like Mn, and is a particularly important element. When the Cu content is too high, Cu is solutionized in the steel sheet, and the solid solution Cu causes embrittlement of the steel sheet during hot rolling. Therefore, the upper limit of the Cu content is 5.0%. On the other hand, in order to allow Cu sulfide to be precipitated prior to MnS during hot rolling, the generation temperature of Cu sulfide needs to be a high temperature, and the lower limit of the Cu content needs to be 0.01%. The Cu content is preferably 0.1% to 1.5%, and more preferably 0.8% to 1.2%.

The non-oriented electrical steel sheet according to this embodiment basically contains the above-described chemical components, and the remainder including Fe and impurities. However, for the purpose of further enhancement in magnetic properties, the enhancement in properties such as strength, corrosion resistance, and fatigue properties required of a structural member, the enhancement in casting properties or sheet travelling properties, and the enhancement in productivity by using scrap or the like, a small amount of elements such as Mo, W, In, Sn, Bi, Sb, Ag, Te, Ce, V, Cr, Co, Ni, Se, Re, Os, Nb, Zr, Hf, and Ta may be contained in a range of 0.5% or less in total. In addition, when such elements are incorporated within a range of 0.5% or less in total, the effect of this embodiment is not damaged. Elements which generate sulfides such as Mg, Ca, Zn, and Ti affect the solid solution temperature of Cu sulfide, and thus the sum of the amounts thereof is preferably 0.2% or less.

Next, the state of Cu sulfide which is an important control factor in the non-oriented electrical steel sheet according to this embodiment will be described.

The inventors found that there are at least two types of structures as the structure of Cu sulfide contained in the steel sheet. One is a cubic structure, and the other is a hexagonal structure (hexagonal close-packed structure). The cubic structure has a stable phase, and the hexagonal structure has a metastable phase.

It is difficult to completely remove the presence of sulfide in the steel sheet. Therefore, in the non-oriented electrical steel sheet according to this embodiment, S is allowed to be actively precipitated as Cu sulfide and the precipitated Cu sulfide is controlled to mainly contain sulfide having the cubic structure, thereby avoiding the deterioration in core loss. Therefore, controlling the crystal structure of Cu sulfide is very important.

In this embodiment, for example, when X-ray diffraction (XRD) is performed on the electrolytic extraction residue of the steel sheet, I2θ=46.4 which is the diffraction intensity of Cu sulfide (hexagonal) at 2θ=46.4±2° and I2θ=32.3 which is the diffraction intensity of Cu sulfide (cubic) at 2θ=32.3±2° are controlled to satisfy the following Expression 1.


I2θ=46.4/I2θ=32.3≦0.5  Expression 1

As shown in FIG. 1, as I2θ=46.4/I2θ=32.3 is reduced, core loss is improved.

The lower limit of I2θ=46.4/I2θ=32.3 does not need to be particularly limited. However, in a case where Cu sulfide having the hexagonal structure is absent, I2θ=46.4/I2θ=32.3 becomes zero, and this value may be the lower limit.

In addition, in this embodiment, Cu sulfide (hexagonal) indicates Cu sulfide having the hexagonal structure, and Cu sulfide (cubic) indicates Cu sulfide having the cubic structure. In addition, the identification of diffraction peaks may be collated by using JCPDS-CARD which is a database of crystal lattices. For example, Cu sulfide (hexagonal) can be identified by using JCPDS-CARD: 00-023-0958 or the like, and Cu sulfide (cubic) can be identified by using JCPDS-CARD: 00-024-0061 or the like. In addition, in Cu sulfide in iron, the chemical bond ratio of S to Cu is changed in a range of 1:1 to 2:1 due to the solid solution of Fe or Mn atoms, and the like. Therefore, 2θ has a margin of error of ±2°. In general, an XRD diffraction intensity is a height from the background to the peak of a spectrum. The XRD diffraction intensity (peak intensity) in this embodiment is also obtained by removing the background using the software described in Non-Patent Documents 3 and 4.

There is concern that fine FeS or fine MnS which is sulfide other than Cu sulfide may cause the deterioration in core loss. Therefore, it is preferable that Cu sulfide is allowed to be actively precipitated by sufficiently increasing the Cu content with respect to the S content. Specifically, when the Cu content is denoted as [% Cu] and the S content is denoted as [% S] in terms of mass %, it is preferable that the Cu content and the Mn content are controlled to satisfy [% Cu]/[% S]≧2.5. 120≧[% Cu]/[% S]>40 is more preferable, and 70>[% Cu]/[% S]>50 is still more preferable.

Furthermore, when the Mn content is denoted as [% Mn] in terms of mass %, a case where ([% Cu]×[% Mn])/[% S]≧2 is satisfied is still more preferable from the viewpoint of the improvement in core loss. The reason why core loss is improved by satisfying ([% Cu]×[% Mn])/[% S]≧2 is not clear. However, the inventors think the reason is that the generation of Cu sulfide (cubic) tends to be accelerated by the effect of Mn. ([% Cu]×[% Mn])/[% S]≧15 is more preferable.

In addition, in the non-oriented electrical steel sheet according to this embodiment, in order to further improve core loss, it is preferable that sulfide which contains Cu and has a diameter of 5 nm to 500 nm is present in the steel sheet at a number density per unit area of 0.5 pieces/μm3 to 50 pieces/μm3. When the number density of sulfide is less than 0.5 pieces/μm3, the effect cannot be sufficiently obtained. Therefore, the number density of sulfide is preferably 0.5 pieces/μm3 or higher. On the other hand, when the number density is higher than 50 pieces/μm3, grain growth properties are deteriorated, and thus there is concern of the deterioration in magnetic flux density. Therefore, the upper limit of the number density is preferably 50 pieces/μm3. In order to reliably improve core loss, the number density of sulfide is preferably in a range of 0.5 pieces/μm3 to 1.0 pieces/μm3, and is more preferably in a rage of 0.5 pieces/μm3 to 0.7 pieces/μm3. The observation of precipitates containing sulfide described above may be performed on the steel sheet having a corroded surface with an SEM (scanning electron microscope) or a TEM (transmission electron microscope) according to an extraction replica method or a thin film method. In general, Cu sulfide is extremely fine (for example, smaller than 5 nm). However, in the non-oriented electrical steel sheet according to this embodiment, the crystal structure of Cu sulfide is mainly cubic, and thus sulfide becomes coarse. Accordingly, the diameter of Cu sulfide can be controlled to be in a range of 5 nm to 500 nm. Regarding core loss, a preferable Cu sulfide diameter is 50 nm to 300 nm, and a more preferable Cu sulfide diameter is 100 nm to 200 nm.

Cu sulfide needs to mainly contain sulfide having the cubic structure as its crystal structure, and thus the X-ray diffraction intensity obtained by XRD may satisfy I2θ=46.4/I2θ=32.3≦0.5 as described above. On the other hand, in a case where Cu sulfide is directly observed by a microscope, it is preferable that most of the observed Cu sulfide has the cubic structure, that is, the volume fraction of Cu sulfide having the cubic structure is 50% or more of the total of Cu sulfide. The volume fraction of Cu sulfide having the cubic structure is more preferably 66.7%, and still more preferably 80%. Here, Cu sulfide includes not only the precipitates of Cu sulfide alone, but also the precipitates which are compositely precipitated with other sulfides, oxides or carbides such as MnS and TiS. Furthermore, precipitates in which metal atoms such as Mn or Fe are solutionized in Cu sulfide, such as Cu(Mn)S or Cu(Fe)S, are also included.

In a case where Cu sulfide precipitates with MnS as composite precipitates, according to X-ray diffraction (XRD) performed by using the electrolytic extraction residue, it is preferable that I2θ=34.3 which is the diffraction intensity of Mn sulfide (cubic) at 2θ=34.3° and I2θ=32.3 which is the diffraction intensity of Cu sulfide (cubic) at 2θ=32.3° satisfy the conditions of the following Expression 1-2.


0.001<I2θ=32.3/I2θ=34.3<10  Expression 1-2

It is more preferable to satisfy 0.02<I2θ=32.3/I2θ=34.3<5, and it is still more preferable to satisfy 0.05<I2θ=32.3/I2θ=34.3<1.5.

A preferable method of manufacturing the non-oriented electrical steel sheet according to this embodiment will be described.

The non-oriented electrical steel sheet according to this embodiment can be manufactured by performing hot rolling, hot-rolled sheet annealing, cold rolling, final annealing, and the like on a slab which is melted in a converter and is subjected to continuous casting, in the same manner as a typical electrical steel sheet.

In the hot rolling, regardless of a hot rolling method such as hot direct rolling or continuous hot rolling and a slab heating temperature, the effect of improving core loss can be obtained. In the cold rolling, regardless of a cold rolling method such as cold rolling performed two or more times or warm rolling and a cold rolling reduction, the effect of improving core loss can be obtained. In addition to these processes, a process of forming an insulating film, a decarburization process, and the like may also be performed. In addition, there is no problem even when the non-oriented electrical steel sheet is manufactured by not a typical process but processes using a thin slab without hot rolling, a process using a thin strip according to a rapid cooling solidification method or, a continuous casting method.

However, in a case of obtaining the non-oriented electrical steel sheet according to this embodiment, in a final annealing process, it is important to undergo a thermal history described as follows. That is, it is important (A) to allow the total amount of Cu sulfide to be solutionized during the final annealing, and (B) to reduce the amount of time for staying in a temperature range in which sulfides other than Cu sulfide having the cubic structure are precipitated and increase the amount of time for staying in a temperature range in which Cu sulfide having the cubic structure [Cu sulfide (cubic)] is precipitated.

In this embodiment, three temperatures T1° C., T2° C., and T3° C. which are described below have important meanings. T1° C. is a solid solution temperature of Cu sulfide obtained by calculation, T2° C. is a precipitation start temperature of Cu sulfide having the cubic structure obtained by calculation, and T3° C. is a lower limit temperature, at which Cu sulfide having the cubic structure is precipitated, obtained by calculation.


T1=17000/(14−log10([% Cu]2×[% S]))−273  Expression 2


T2=17000/(14−log10([% Cu]2×[% S]))−323  Expression 3


T3=17000/(14−log10([% Cu]2×[% S]))−473  Expression 4

Where [% Cu] is the Cu content in terms of mass %, and [% S] is the S content in terms of mass %.

Hereinafter, a method of controlling sulfide on the basis of such temperatures will be described.

First, (A): to allow the total amount of Cu sulfide to be solutionized during the final annealing will be described.

In the non-oriented electrical steel sheet according to this embodiment, the total amount of Cu sulfide can be solutionized by holding the steel sheet at T1° C. or higher which is the calculated solid solution temperature of Cu sulfide for 30 seconds or longer. When the holding temperature is lower than T1° C., Cu sulfide cannot be sufficiently solutionized, Cu sulfide having the hexagonal structure or having a crystal lattice damaged by cold rolling remains and has an adverse influence on core loss, which is not preferable. Here, there may be a case where sulfides such as TiS are solutionized and are finely precipitated during cooling such that grain growth in the steel sheet is suppressed and the magnetic flux density and core loss thereof are deteriorated. Therefore, the holding temperature at which Cu sulfide is reliably solutionized and solutionizing of other sulfides is avoided as much as possible is preferably T1+30° C. or higher and T1+200° C. or lower, and more preferably T1+50° C. or higher and T1+100° C. or lower. Here, when the temperature of the steel sheet becomes higher than its melting point, the steel sheet cannot travel, and thus the upper limit of T1 is 1530° C.

In addition, when the holding time is less than 30 seconds, solutionizing does not sufficiently progress. In order to allow Cu sulfide to be more reliably solutionized, it is preferable that the holding time is 35 seconds or longer. On the other hand, when heating is performed for a long period of time, there is a possibility that other sulfides such as TiS having a low precipitation rate may be generated and the amount of generated Cu sulfide (cubic) which is effective in improving core loss may be reduced. Therefore, the holding temperature (a stay time at T1° C. or higher) is preferably 3600 seconds or shorter, and more preferably 300 seconds or shorter.

Next, (B): to reduce the amount of time for staying in a temperature range in which sulfides other than Cu sulfide having the cubic structure are precipitated and increase the amount of time for staying in a temperature range in which Cu sulfide having the cubic structure is precipitated will be described.

In the non-oriented electrical steel sheet according to this embodiment, the effect of improving core loss is obtained by allowing a large amount of Cu sulfide to have the cubic structure which is a stable structure at a low temperature and thus increasing the ratio of Cu sulfide having the cubic structure [Cu sulfide (cubic)] to the total amount of sulfide.

In order to increase the ratio of Cu sulfide (cubic), solid solution S needs to be precipitated as Cu sulfide (cubic) as much as possible. For this, it is important to avoid precipitation of sulfides other than Cu sulfide during cooling as much as possible by rapidly cooling the temperature range of from the solid solution temperature T1° C. of Cu sulfide to the precipitation start temperature T2° C. of Cu sulfide (cubic) and to allow Cu sulfide (cubic) to be sufficiently precipitated by holding the steel sheet in the precipitation temperature range of Cu sulfide (cubic) between T2° C. and T3° C. for a certain amount of time.

Specifically, when the average cooling rate from the solid solution temperature T1° C. of Cu sulfide to the precipitation start temperature T2° C. of Cu sulfide (cubic) is denoted as CR1 (° C./sec) and the average cooling rate from T1° C. to the precipitation temperature range of Cu sulfide (cubic) between T2° C. and T3° C. is denoted as CR2 (° C./sec), the steel sheet is cooled to a temperature of T3° C. or lower to satisfy the following Expressions 5 to 7.


CR1>CR2  Expression 5


5≦CR1≦500  Expression 6


0.5≦CR2≦50  Expression 7

The cause of deterioration in core loss is the precipitation of fine FeS, fine MnS, and fine Cu sulfide having the hexagonal structure [fine Cu sulfide (hexagonal)]. These precipitates are precipitated in a temperature range between the solid solution temperature T1° C. and the precipitation start temperature T2° C. of Cu sulfide (cubic). Therefore, the average cooling rate CR1 from T1° C. to T2° C. is set to 5° C./sec or more. When CR1 is lower than 5° C./sec, precipitation of fine FeS, fine MnS, and fine Cu sulfide (hexagonal) cannot be sufficiently avoided. In order to further increase core loss, CR1 is preferably higher than 20° C./sec, more preferably higher than 50° C./sec, and still more preferably higher than 100° C./sec.

On the other hand, it is difficult to set CR1 to be higher than 500° C./sec due to the facilities, and thus the upper limit thereof may be 500° C./sec. A preferable upper limit of CR1 is 300° C./sec.

In addition, a large amount of Cu sulfide (cubic) is allowed to be precipitated by holding the steel sheet in the above-described precipitation temperature range of Cu sulfide (cubic) between T2° C. and T3° C. for a predetermined period of time or longer. Thereby, even when fine FeS, fine MnS, and fine Cu sulfide (hexagonal) are present, the adverse influence thereof can be cancelled out. Since a certain amount of time is needed for the precipitation of Cu sulfide (cubic), it is important to set the average cooling rate CR2 from T2° C. to T3° C. to be 50° C./sec or lower. When CR2 is higher than 50° C./sec, the amount of time for staying in the precipitation temperature range is not sufficient, and the amount of precipitated Cu sulfide (cubic) is not sufficient. In order to ensure a sufficient precipitation amount, CR2 is preferably 20° C./sec or lower, more preferably 10° C./sec or lower, and still more preferably 5° C./sec or less.

On the other hand, when CR2 is lower than 0.5° C./sec, productivity is degraded, which is not preferable. Therefore, the lower limit of CR2 is 0.5° C./sec. The lower limit of CR2 is preferably 1° C./sec.

In addition, when CR1 is lower than CR2, the precipitates mainly contain fine Cu sulfide (hexagonal), fine FeS, and fine MnS having an adverse influence on core loss, which is not preferable.

In the method of manufacturing the non-oriented electrical steel sheet according to this embodiment, from the viewpoint of the holding in the precipitation temperature range of Cu sulfide (cubic), final annealing may be performed two times or more. For example, as described above, the steel sheet may also be subjected to first final annealing at T1° C. or higher, be temporarily cooled to T3° C. or lower, and thereafter be held in the temperature range between T2° C. to T3° C. for 30 seconds as second final annealing (additional annealing). By performing the additional annealing, the amount of time that the steel sheet stays at T2° C. or lower to T3° C. or higher can be increased, and thus good core loss can be obtained. A more preferable temperature range of the additional annealing is T2−30° C. to T3+30° C., and an still more preferable temperature range is T2−50° C. to T3+50° C.

The soaking time (holding time) in the temperature range of T2° C. or lower and T3° C. or higher is preferably 35 seconds or longer and 3600 seconds or shorter, and is preferably 35 seconds or longer and 300° C. or lower.

In addition, in the non-oriented electrical steel sheet according to this embodiment, as described above, it is effective to temporarily dissolve the total amount of Cu precipitates during the final annealing process. In consideration of the state of Cu sulfide before the final annealing, a large amount of Cu sulfide is precipitated during cooling in the hot rolling process. When Cu sulfide is fine Cu sulfide (hexagonal) in a metastable phase, the total amount of Cu sulfide is rapidly solutionized during the final annealing, which is preferable. In order to allow Cu sulfide before the final annealing to be fine Cu sulfide (hexagonal) in a metastable phase, it is preferable that the total amount of Cu sulfide is solutionized by heating the steel sheet to T1° C. or higher in the hot-rolled sheet annealing process after the hot rolling process and the steel sheet is cooled at a CR3 of 15° C./sec or higher when the cooling rate from T1° C. to room temperature is denoted as CR3. CR3 is more preferably 30° C./sec or higher, and still more preferably 60° C./sec or higher.

Furthermore, the steel sheet is subjected to slow heating at an average temperature rising rate of 100° C./sec or lower during the final annealing, and thus Cu sulfide is more easily solutionized, which is preferable.

Here, room temperature indicates 23±5° C. specified in JIS C 2556.

FIG. 2 is a flowchart showing an example of a process of manufacturing the non-oriented electrical steel sheet according to this embodiment.

In general, as precipitates become coarse, the resistance of magnetic domain wall movement due to the precipitates is reduced and core loss is improved. In addition, as the lattice mismatch between the precipitates and the interface of steel is suppressed, the magnetic domain wall movement becomes smooth and core loss is improved. In the non-oriented electrical steel sheet according to this embodiment, Cu sulfide is transformed so as to have the cubic structure which is a stable crystal system by holding the steel sheet in a predetermined temperature range between T2° C. to T3° C. described above. The Cu sulfide (cubic) has good consistency with the interface of steel, has a high growth rate, and is thus easily coarsened. As a result, in the non-oriented electrical steel sheet according to this embodiment, it is thought that the magnetic domain wall movement becomes easy and good core loss is exhibited.

EXAMPLES Example 1

An ingot having the components shown in Table 1 was melted in vacuum, and the ingot was heated to 1150° C. and was hot rolled at a hot rolling finish temperature of 875° C. and a coiling temperature of 630° C., thereby producing a hot-rolled steel sheet having a sheet thickness of 2.0 mm. The hot-rolled steel sheet was subjected to hot-rolled sheet annealing, was subjected to pickling, and was cold-rolled at a rolling reduction of 75%, thereby producing a cold-rolled steel sheet having a sheet thickness of 0.50 mm. Heat treatments performed on the test materials and the precipitation states of observed precipitates are shown in Table 2, and the magnetic properties (magnetic flux density and core loss) of each of the obtained steel sheets are shown in Table 3. Evaluation results of core loss evaluated as VG for very good, G for good, F for effective, and B for level in the related art are also shown in Table 3.

In addition, the evaluation of magnetic properties was performed on the basis of JIS C 2550:2000. Regarding core loss, W15/50 (W/kg) was evaluated. W15/50 is a core loss at a frequency of 50 Hz and at a maximum magnetic flux density of 1.5 T. In addition, the magnetic flux density was evaluated by using B50. B50 indicates a magnetic flux density at a magnetic field strength of 5000 A/m. In addition, the minimum target value of B50 was set to 1.65 T as in the related art.

The core loss evaluation criteria of the samples are as follows.

VG (Very Good): W15/50 (W/kg)<2.28

G (Good): 2.28≦W15/50 (W/kg)<2.36

F (Fair): 2.36≦W15/50 (W/kg)<2.50

B (Bad): 2.50≦W15/50 (W/kg)

Samples of which the magnetic properties could not be measured due to hot rolling breaking or cold rolling breaking were also evaluated as B (Bad).

In addition, for X-ray diffraction, only inclusions which were collected with a filter by a general extraction residue method described in Non-Patent Documents 4 and 5 were used as analysis samples. XRD measurement was performed by wide angle X-ray diffraction using Cu-Kα rays described in Non-Patent Documents 4 and 6 as a probe.

In addition, for precipitate observation, a surface perpendicular to the rolling direction of the steel sheet was etched and measured through SEM observation. At this time, after ten visual fields of 100 μm2 were observed, the surface was polished by about 20 μm, and then ten visual fields of 100 μm2 were observed. This was repeated five times.

TABLE 1 STEEL (mass %) TYPE C Si Mn P S Al Cu N T1(° C.) T2(° C.) T3(° C.) A 0.0024 3.11 0.03 0.016 0.017 0.51 1.131 0.0026 812.4 762.4 612.4 B 0.0024 3.11 0.03 0.016 0.020 0.51 0.045 0.0026 651.3 601.3 451.3 C 0.0024 3.11 1.15 0.016 0.021 0.51 0.095 0.0026 686.2 636.2 486.2 D 0.0024 3.11 0.03 0.016 0.003 0.51 0.008 0.0026 547.6 497.6 347.6 E 0.0030 2.53 0.33 0.0011 0.009 1.01 0.65  0.0033 762.3 712.3 562.3 F 0.0019 1.51 0.05 0.015 0.015 0.67 0.78  0.0070 786.9 736.9 586.9 G 0.0012 0.52 0.85 0.002 0.008 0.006 0.56  0.0035 751.1 701.1 551.1 COMPONENTS WHICH ARE OUTSIDE OF SPECIFIED RANGES ARE UNDERLINED. REMAINDER INCLUDES Fe AND IMPURITIES.

TABLE 2 FIRST FINAL AVERAGE COOLING ANNEALING RATE AFTER FINAL HOLDING HOLDING ANNEALING MANUFACTURE TEMPERATURE TIME CR1 CR2 NO. STEEL TYPE [% Cu]/[% S] (° C.) (SEC) (° C./SEC) (° C./SEC) CR1 > CR2  1 A 66.5 1000 30 24 5 OK  2 B 2.3 1000 30 24 5 OK  3 A 66.5 1000 30 10 5 OK  4 C 4.5 1000 30 10 5 OK  5 A 66.5 1000 30 24 22 OK  6 A 66.5 1000 30 24 22 OK  7 A 66.5 1000 30 52 5 OK  8 A 66.5 1000 30 105 5 OK  9 A 66.5 1000 30 10 25 NG 10 A 66.5 1000 30 10 25 NG 11 A 66.5 1000 30 10 25 NG 12 D 2.7 1000 30 24 5 OK 13 E 72.2 950 30 24 5 OK 14 E 72.2 950 30 10 25 NG 15 F 52.0 850 30 24 5 OK 16 F 52.0 850 30 10 25 NG 17 G 70.0 820 30 24 5 OK 18 G 70.0 820 30 10 25 NG HOT-ROLLED COOLING RATE SECOND FINAL SHEET AFTER HOT- ANNEALING ANNEALING ROLLEDSHEET NUMBER HOLDING HOLDING HOLDING HOLDING ANNEALING DENSITY MANUFACTURE TEMPERATURE TIME TEMPERATURE TIME CR3 I2θ = 46.4/ (PIECES/ NO. (° C.) (SEC) (° C.) (SEC) (° C./SEC) I2θ = 32.3 μm3)  1 — — 1050 30 10 0.25 1.13  2 — — 1050 30 10 0.41 0.75  3 — — 1050 30 10 0.20 1.03  4 — — 1050 30 10 0.24 1.11  5 — — 1050 30 10 0.25 0.88  6 700 60 1050 30 20 0.20 1.12  7 700 60 1050 30 18 0.17 0.95  8 700 60 1050 30 23 0.15 0.72  9 — — 1050 30 10 0.87 0.33 10 700 60 1050 30 10 0.67 0.30 11 — — 1050 30 20 0.58 0.44 12 400 120  1050 30 20 1.33 0.24 13 — — 1000 30 10 0.23 1.02 14 — — 1000 30 10 0.80 0.48 15 — — 950 30 10 0.23 1.16 16 — — 950 30 10 0.73 0.33 17 — — — — — 0.26 1.19 18 — — — — — 0.97 0.33 ITEMS WHICH ARE OUTSIDE OF SPECIFIED RANGES ARE UNDERLINED

TABLE 3 MAGNETIC CORE MANU- FLUX LOSS FACTURE DENSITY W15/50 EVALU- NO. B50(T) (W/kg) ATION NOTE 1 1.70 2.25 VG INVENTION STEEL 2 1.67 2.38 F INVENTION STEEL 3 1.69 2.29 G INVENTION STEEL 4 1.71 2.24 VG INVENTION STEEL 5 1.68 2.31 G INVENTION STEEL 6 1.71 2.26 VG INVENTION STEEL 7 1.70 2.22 VG INVENTION STEEL 8 1.70 2.20 VG INVENTION STEEL 9 1.63 2.67 B COMPARATIVE STEEL 10 1.65 2.67 B COMPARATIVE STEEL 11 1.64 2.67 B COMPARATIVE STEEL 12 1.61 2.99 B COMPARATIVE STEEL 13 1.71 2.34 VG INVENTION STEEL 14 1.69 2.92 B COMPARATIVE STEEL 15 1.73 2.47 F INVENTION STEEL 16 1.70 3.18 B COMPARATIVE STEEL 17 1.79 2.49 F INVENTION STEEL 18 1.77 4.83 B COMPARATIVE STEEL

Example 2

An ingot having the chemical components shown in Table 4 was melted in vacuum, and the ingot was heated to 1150° C. and was hot rolled at a hot rolling finish temperature of 850° C., thereby producing a hot-rolled steel sheet having a sheet thickness of 2.3 mm. The hot-rolled steel sheet was subjected to hot-rolled sheet annealing, was subjected to pickling, and was cold-rolled at a rolling reduction of 85%, thereby producing a cold-rolled steel sheet having a sheet thickness of 0.5 mm. Thereafter, final annealing was performed at a holding temperature of T1+50° C. for a holding time of 45 seconds. Thereafter, furnace cooling was performed so that the average cooling rates between T1° C. and T2° C. and between T2° C. and T3° C. were respectively 35° C./sec and 15° C./sec. X-ray diffraction results, the precipitation states of precipitates, magnetic properties (magnetic flux density and core loss), brittleness, and overall evaluation results are shown in Table 5.

Regarding X-ray diffraction, measurement of the magnetic properties, and measurement of the precipitates, the same evaluations as in Example 1 were performed. Furthermore, in this example, a repeat bending test was performed on the basis of JIS C 2550:2000 to evaluate workability. In a case where breaking occurs with one time of bending, working properties were insufficient and were evaluated as fail, and a level at which breaking had not occurred after two times of bending was evaluated as pass (PASS).

In addition, in a case where a sample was broken during the repeat bending test, the sample was evaluated as B regardless of core loss, and the evaluation of core loss was performed only on samples which passed the repeat bending test. In addition, regarding the samples which could not be subjected to the repeat bending test due to breaking during rolling or the like, the test results thereof are indicated by “-”.

TABLE 4 STEEL (mass %) (° C.) TYPE C Si Mn P S Al Cu N T1 T2 T3 H1 0.0002 2.89 0.03 0.003 0.004 0.049 0.09 0.0034 647 597 447 H2 0.0002 2.88 0.04 0.002 0.061 0.047 0.12 0.0033 724 674 524 H3 0.009  2.88 0.02 0.004 0.022 0.043 0.07 0.0082 674 624 474 H4 0.0008 1.32 0.04 0.063 0.007 0.06  0.08 0.0059 654 604 454 H5 0.0017 6.98 0.04  0.0034  0.0091 0.082 0.03 0.0023 620 570 420 H6 0.0018 2.86 0.01 0.116  0.0075 0.076 0.08 0.0026 657 607 457 H7 0.0018 3.32 2.99 0.123  0.0062 0.053 0.08 0.0074 649 599 449 H8 0.0016 3.31 0.01 0.113  0.0063 0.051 0.08 0.0023 650 600 450 H9 0.0017 3.29 2.97 0.12   0.0059 0.055 0.07 0.0022 645 595 445 H10 0.0017 2.85 0.05  0.0012  0.0072 0.034 0.04 0.0037 627 577 427 H11 0.0016 3.31 0.09 0.148 0.009 1.24  0.05 0.0026 642 592 442 H12 0.0018 2.87 0.02 0.003  0.0002 0.053 0.06 0.0026 568 518 368 H13 0.0004 2.85 0.02 0.134 0.098 0.041 1.33 0.0082 879 829 679 H14 0.0018 2.98 0.04 0.129 0.041  0.0021 1.11 0.0028 638 788 638 H15 0.0016 3.32 0.55  0.0029  0.0074 2.99  0.06 0.0019 642 592 442 H16 0.0004 3.33 0.03 0.074  0.0021 0.034 0.01 0.0021 549 499 349 H17 0.0016 2.89 0.02 0.121  0.0081 0.068 4.91 0.0025 883 833 683 H18 0.0014 2.88 0.02 0.121 0.091 0.068 4.9  0.0027 971 921 771 H19 0.0016 2.89 0.02 0.116  0.0003 0.071 0.01 0.0023 517 467 317 H20 0.0017 3.11 0.02  0.0042  0.0079 1.73  0.03 0.0011 615 585 415 H21 0.0018 2.87 0.02 0.126  0.0081 0.049 0.06 0.0098 644 594 444 H22 0.0006 3.22 1.24 0.08  0.018 0.78  0.73 0.0044 788 738 588 H23 0.0009 3.12 0.35 0.03  0.021 1.13  1.18 0.0062 821 771 621 h1 <0.0001  3.01 0.05 0.12  0.005 0.045 0.12 0.0055 664 614 464 h2 0.022  3.16 0.03 0.005 0.017  0.0027 0.17 0.0073 709 659 509 h3 0.0012  0.047 0.03 0.002 0.019 0.077 0.23 0.0012 727 677 527 h4 0.0016 7.11 0.03 0.123 0.012 0.021 0.06 0.0021 653 603 453 h5 0.0017 3.32  0.007 0.11  0.018 0.034 0.12 0.0026 694 644 494 h6 0.0013 3.21 3.03 0.12  0.017 0.056 0.16 0.0034 706 656 508 h7 0.0014 3.44 0.03  0.0008 0.019 0.075 2.12 0.0026 855 805 655 h8 0.0014 3.18 0.03 0.151 0.016 0.045 1.97 0.0022 845 795 645 h9 0.0016 3.09 0.04 0.118 <0.0001 0.098 0.84 0.0045 864 614 464 h10 0.0013 3.11 0.03 0.127 0.123 0.043 2.56 0.0026 933 863 733 h11 0.0016 3.27 0.06 0.106 0.012  0.0019 2.12 0.0019 840 790 640 h12 0.0016 3.23 0.02 0.129 0.011 3.15  1.15 0.0026 800 750 600 h13 0.0013 3.21 0.04 0.104 0.002 0.021  0.007 0.0077 536 486 336 h14 0.0016 2.91 0.05 0.098 0.012 0.022 5.11 0.0029 899 849 699 h15 0.001  3.01 0.03 0.111 0.014 0.056 0.55 0.0006 765 715 565 h16 0.0015 3.12 0.04 0.138 0.012 0.017 0.78 0.011  781 731 581 REMAINDER INCLUDES Fe AND IMPURITIES. COMPONENTS WHICH ARE OUTSIDE OF SPECIFIED RANGES ARE UNDERLINED.

TABLE 5 NUMBER CORE LOSS MANUFACTURE STEEL I2θ = 46.4/ DENSITY I2θ = 32.3/ W15/50 NO. TYPE I2θ = 32.3 [% Cu]/[% S] (PIECES/μm3) I2θ = 34.3 (W/kg) 201 H1 0.44 23 1.87 7.21 2.40 202 H2 0.48 2 1.56 6.84 2.49 203 H3 0.36 3 2.34 5.64 2.32 204 H4 0.09 12 0.52 8.11 2.07 205 H5 0.45 4 3.18 2.89 2.40 206 H6 0.47 11 1.12 8.10 2.33 207 H7 0.31 12 1.20 0.01 2.17 208 H8 0.33 12 1.18 12.34 2.41 209 H9 0.33 12 1.21 <0.001 2.42 210 H10 0.34 6 1.55 7.21 2.32 211 H11 0.29 6 1.88 0.01 2.25 212 H12 0.18 280 0.88 8.34 2.28 213 H13 0.38 14 9.48 6.11 2.36 214 H14 0.09 27 0.67 7.12 2.11 215 H15 0.17 8 0.92 0.01 2.17 216 H16 0.39 5 3.27 8.95 2.37 217 H17 0.43 608 41.30 5.79 2.36 218 H18 0.43 608 51.70 7.99 2.46 219 H19 0.43 608 0.41 6.45 2.47 220 H20 0.28 4 1.10 6.33 2.25 221 H21 0.43 8 1.21 7.11 2.40 222 H22 0.19 41 0.86 7.81 1.99 223 H23 0.05 56 0.69 0.07 1.92 224 h1 0.43 24 1.10 8.11 2.68 225 h2 0.33 10 2.13 9.45 2.76 226 h3 0.39 12 1.06 8.31 3.11 227 h4 — 5 — — — 228 h5 — 7 — — — 229 h6 2.10 9 14.40 <0.001 2.65 230 h7 0.33 112 0.91 8.01 2.77 231 h8 0.17 123 1.23 7.78 2.73 232 h9 1.80 >5200 0.57 6.54 2.69 233 h10 — 21 — — — 234 h11 0.32 177 1.20 8.12 2.71 235 h12 0.46 105 0.77 8.91 2.41 236 h13 12.20  4 0.31 5.42 2.67 237 h14 0.40 426 17.30 5.63 2.46 238 h15 0.25 39 1.10 7.47 2.65 239 h16 0.35 65 0.78 8.34 2.72 MAGNETIC WORKABILITY MANUFACTURE FLUX DENSITY REPEAT NO. B50(T) BENDING TEST EVALUATION NOTE 201 1.69 PASS F INVENTION 202 1.69 PASS F STEEL 203 1.68 PASS G 204 1.72 PASS VG 205 1.65 PASS F 206 1.67 PASS G 207 1.68 PASS VG 208 1.69 PASS F 209 1.70 PASS F 210 1.88 PASS G 211 1.88 PASS VG 212 1.89 PASS G 213 1.70 PASS F 214 1.71 PASS VG 215 1.68 PASS VG 216 1.68 PASS F 217 1.67 PASS F 218 1.67 PASS F 219 1.71 PASS F 220 1.66 PASS VG 221 1.68 PASS F 222 1.70 PASS VG 223 1.70 PASS VG 224 1.54 PASS B COMPARATIVE 225 1.68 PASS B STEEL 226 1.73 PASS B 227 — — B 228 — — B 229 1.68 PASS B 230 1.50 PASS B 231 1.55 PASS B 232 1.65 PASS B 233 — — B 234 1.52 PASS B 235 1.69 FRACTURE B AT ONE TIME 236 1.64 PASS B 237 1.67 FRACTURE B AT ONE TIME 238 1.48 PASS B 239 1.51 PASS B

Example 3

An ingot having the same components as Steel type No. H23 shown in Table 4 was heated to 1100° C. and was hot-rolled at a finish temperature of 850° C. and a coiling temperature of 630° C., thereby producing a hot-rolled sheet having a sheet thickness of 2.0 mm. The hot-rolled sheet was subjected to final annealing under the conditions shown in Table 5, and was subjected to hot-rolled sheet annealing at 1000° C. for 120 seconds in some examples. Other manufacturing conditions, X-ray diffraction results, the precipitation states of precipitates, and the evaluation results of magnetic properties (magnetic flux density and core loss) are shown in Table 6. Regarding X-ray diffraction, measurement of the magnetic properties, and measurement of the precipitates, the same evaluations as in Example 1 were performed.

TABLE 6 FIRST FINAL SECOND FINAL ANNEALING AVERAGE COOLING ANNEALING CALCURATED HOLDING RATE AFTER FINAL HOLDING TEMPERATURE TEMPER- HOLDING ANNEALING TEMPER- MANUFACTURE STEEL T1 T2 T3 ATURE TIME CR1 CR2 ATURE NO. TYPE (° C.) (° C.) (° C.) (° C.) (SEC) (° C./SEC) (° C./SEC) CR1 > CR2 (° C.) 23A H23 821 771 521 822 33 34 17 OK — 23B H23 821 771 521 843 31 35 15 OK — 23C H23 821 771 521 845 34 21 17 OK — 23B H23 821 771 521 841 32 42 19 OK — 23E H23 821 771 521 839 33 38 16 OK — 23F H23 821 771 521 841 32 45 7 OK 622 23G H23 821 771 521 858 34 41 12 OK 769 23H H23 821 771 521 832 33 39 2 OK 753 23I H23 821 771 521 912 32 34 16 OK — 23J H23 821 771 521 843 33 29 14 OK 730 23K H23 821 771 521 835 3490 40 15 OK 834 23L H23 821 771 521 832 38 39 17 OK — 23M H23 821 771 521 844 34 93 15 OK 896 23N H23 821 771 521 846 34 298 17 OK — 23O H23 821 771 521 1052 406 52 8 OK 869 23P H23 821 771 521 997 118 112 3 OK 711 23a H23 821 771 521 819 120 185 4 OK 887 23b H23 821 771 521 885 29 302 2 OK 701 23c H23 821 771 521 876 120 17 33 NG 717 23d H23 821 771 521 855 120 3 2 OK 750 23e H23 821 771 521 881 120 98 53 OK 750 23f H23 821 771 521 855 120 2 31 NG 720 23g H23 821 771 521 861 120 16 52 NG 750 COOLING RATE MAGNETIC SECOND FINAL AFTER HOT- PROPERTY ANNEALING ROLLEDSHEET NUMBER MAGNETIC HOLIDNG ANNEALING DENSITY CORE LOSS FLUX MANUFACTURE TIME CR3 I2θ = 46.4/ (PIECES/ W15/50 DENSITY NO. (SEC) (° C./SEC) I2θ = 32.3 μm3) (W/kg) B50(T) EVALUATION NOTE 23A — — 0.48 2.12 2.38 1.66 F INVENTION 23B — — 0.49 1.91 2.39 1.69 F STEEL 23C — — 0.45 1.88 2.37 1.67 F 23D — — 0.47 1.72 2.38 1.68 F 23E — — 0.38 1.66 2.36 1.69 F 23F 33 — 0.35 1.34 2.35 1.88 G 23G 34 43 0.15 0.80 2.25 1.70 VG 23H 31 — 0.31 1.12 2.31 1.68 G 23I — 16 0.33 1.55 2.29 1.67 G 23J 32 16 0.33 1.23 2.35 1.69 G 23K 310  21 0.05 0.66 2.24 1.69 VG 23L — — 0.30 1.32 2.29 1.67 G 23M 38 — 0.19 0.83 2.12 1.70 VG 23N — 61 0.14 0.85 2.22 1.69 VG 23O 3540  32 0.13 0.88 2.04 1.71 VG 23P 129  71 0.07 0.67 1.93 1.69 VG 23a 45 54 0.89 1.86 2.58 1.68 B COMPARATIVE 23b 45 57 1.21 2.14 2.57 1.89 B STEEL 23c 45 58 0.75 2.20 2.53 1.67 B 23d 40 63 0.77 2.17 2.51 1.68 B 23e 40 53 0.69 1.54 2.52 1.89 B 23f 40 52 0.71 1.61 2.55 1.68 B 23g 40 64 0.68 1.93 2.56 1.69 B

According to Examples 1 to 3 described above, in a case where the chemical components and the manufacturing method were preferable as in Manufacture Nos. 1 to 8, 12, 114, 16, 18, 201 to 223, and 23A to 23P, it was seen that the ratio of Cu sulfide (cubic) satisfied the present invention and thus a non-oriented electrical steel sheet having excellent core loss could be obtained. On the other hand, in a case where any of the chemical components and the manufacturing method were outside of the range of the present invention, sufficient core loss could not be obtained, and thus basic properties required of the non-oriented electrical steel sheet could not be obtained. Otherwise, as in Manufacture Nos. 227, 228, and 233, breaking had occurred during rolling, and magnetic properties, XRD (X-ray diffraction), and number densities could not be evaluated.

INDUSTRIAL APPLICABILITY

According to the present invention, even when high purification, a reduction in slab heating temperature, optimization of hot rolling conditions, and the like are not performed on the non-oriented electrical steel sheet, it is possible to make fine Cu sulfide harmless. Accordingly, a non-oriented electrical steel sheet having excellent core loss can be provided.

Claims

1-8. (canceled)

9. A non-oriented electrical steel sheet comprising chemical compositions including, in terms of mass %:

C: 0.0001% to 0.01%;
Si: 0.05% to 7.0%;
Mn: 0.01% to 3.0%;
Al: 0.0020% to 3.0%;
S: 0.0001% to 0.1%;
P: 0.0010% to 0.15%;
N: 0.0010% to 0.01%;
Cu: 0.01% to 5.0%; and
a remainder including Fe and impurities,
wherein I2θ=46.4 which is a diffraction intensity of Cu sulfide having a hexagonal structure shown at 2θ=46.4° and I2θ=32.3 which is a diffraction intensity of Cu sulfide having a cubic structure shown at 2θ=32.3°, which are obtained through a X-ray diffraction of an electrolytic extraction residue, satisfy the following Expression 1. I2θ=46.4/I2θ=32.3≦0.5  Expression 1

10. The non-oriented electrical steel sheet according to claim 9,

wherein, when a Cu content, in terms of mass %, is denoted as [% Cu] and an S content, in terms of mass %, is denoted as [% S], the [% Cu] and the [% S] satisfy [% Cu]/[% S]≧2.5.

11. The non-oriented electrical steel sheet according to claim 9,

wherein 0.5 pieces/μm3 to 50 pieces/μm3 of sulfide containing Cu and having a diameter of 5 nm to 500 nm are contained.

12. The non-oriented electrical steel sheet according to claim 10,

wherein 0.5 pieces/μm3 to 50 pieces/μm3 of sulfide containing Cu and having a diameter of 5 nm to 500 nm are contained.

13. A method of manufacturing the non-oriented electrical steel sheet according to any one of claims 9 to 12, the method comprising:

performing a hot rolling on a slab to obtain a hot-rolled steel sheet;
annealing the hot-rolled steel sheet;
pickling the hot-rolled steel sheet;
performing a cold rolling on the hot-rolled steel sheet to obtain a cold-rolled steel sheet; and
annealing the cold-rolled steel sheet,
wherein, in the annealing of the cold-rolled steel sheet, after the cold-rolled steel sheet is held at T1° C., which is represented in the following Expression 2, to 1530° C. for 30 seconds to 3600 seconds, when an average cooling rate from the T1° C. to T2° C., which is shown in Expression 3, is denoted as CR1 in the unit of ° C./sec and an average cooling rate from the T2° C. to T3° C., which is shown in Expression 4, is denoted as CR2 in the unit of ° C./sec, the cold-rolled steel sheet is cooled to a temperature range of the T3° C. or lower so that the CR1 and the CR2 satisfy Expressions 5, 6 and 7: T1=17000/(14−log10([% Cu]2×[% S]))−273  Expression 2 T2=17000/(14−log10([% Cu]2×[% S]))−323  Expression 3 T3=17000/(14−log10([% Cu]2×[% S]))−473  Expression 4 CR1>CR2  Expression 5 5≦CR1≦500  Expression 6 0.5≦CR2≦50  Expression 7
where [% Cu] is a Cu content in terms of mass % and [% S] is an S content in terms of mass %.

14. The method of manufacturing a non-oriented electrical steel sheet according to claim 13,

wherein the CR1 further satisfies the following Expression 8. CR1>20  Expression 8

15. The method of manufacturing a non-oriented electrical steel sheet according to claim 13,

wherein the CR2 further satisfies the following Expression 9. CR2≦20  Expression 9

16. The method of manufacturing a non-oriented electrical steel sheet according to claim 14,

wherein the CR2 further satisfies the following Expression 9. CR2≦20  Expression 9

17. The method of manufacturing a non-oriented electrical steel sheet according to claim 13, further comprising:

subsequent to the annealing of the cold-rolled steel sheet, holding the cold-rolled steel sheet in a temperature range of the T2° C. or lower to the T3° C. or higher for 30 seconds or longer as an additional annealing.

18. The method of manufacturing a non-oriented electrical steel sheet according to claim 14, further comprising:

subsequent to the annealing of the cold-rolled steel sheet, holding the cold-rolled steel sheet in a temperature range of the T2° C. or lower to the T3° C. or higher for 30 seconds or longer as an additional annealing.

19. The method of manufacturing a non-oriented electrical steel sheet according to claim 15, further comprising:

subsequent to the annealing of the cold-rolled steel sheet, holding the cold-rolled steel sheet in a temperature range of the T2° C. or lower to the T3° C. or higher for 30 seconds or longer as an additional annealing.

20. The method of manufacturing a non-oriented electrical steel sheet according to claim 16, further comprising:

subsequent to the annealing of the cold-rolled steel sheet, holding the cold-rolled steel sheet in a temperature range of the T2° C. or lower to the T3° C. or higher for 30 seconds or longer as an additional annealing.

21. The method of manufacturing a non-oriented electrical steel sheet according to claim 13,

wherein, in the annealing of the hot-rolled steel sheet, the hot-rolled steel sheet is cooled so that CR3 which is an average cooling rate from the T1° C. to a room temperature is 15° C./sec or higher.

22. The method of manufacturing a non-oriented electrical steel sheet according to claim 14,

wherein, in the annealing of the hot-rolled steel sheet, the hot-rolled steel sheet is cooled so that CR3 which is an average cooling rate from the T1° C. to a room temperature is 15° C./sec or higher.

23. The method of manufacturing a non-oriented electrical steel sheet according to claim 15,

wherein, in the annealing of the hot-rolled steel sheet, the hot-rolled steel sheet is cooled so that CR3 which is an average cooling rate from the T1° C. to a room temperature is 15° C./sec or higher.

24. The method of manufacturing a non-oriented electrical steel sheet according to claim 16,

wherein, in the annealing of the hot-rolled steel sheet, the hot-rolled steel sheet is cooled so that CR3 which is an average cooling rate from the T1° C. to a room temperature is 15° C./sec or higher.

25. The method of manufacturing a non-oriented electrical steel sheet according to claim 17,

wherein, in the annealing of the hot-rolled steel sheet, the hot-rolled steel sheet is cooled so that CR3 which is an average cooling rate from the T1° C. to a room temperature is 15° C./sec or higher.

26. The method of manufacturing a non-oriented electrical steel sheet according to claim 18,

wherein, in the annealing of the hot-rolled steel sheet, the hot-rolled steel sheet is cooled so that CR3 which is an average cooling rate from the T1° C. to a room temperature is 15° C./sec or higher.

27. The method of manufacturing a non-oriented electrical steel sheet according to claim 19,

wherein, in the annealing of the hot-rolled steel sheet, the hot-rolled steel sheet is cooled so that CR3 which is an average cooling rate from the T1° C. to a room temperature is 15° C./sec or higher.

28. The method of manufacturing a non-oriented electrical steel sheet according to claim 20,

wherein, in the annealing of the hot-rolled steel sheet, the hot-rolled steel sheet is cooled so that CR3 which is an average cooling rate from the T1° C. to a room temperature is 15° C./sec or higher.
Patent History
Publication number: 20160273064
Type: Application
Filed: Apr 8, 2014
Publication Date: Sep 22, 2016
Inventors: Takashi KATAOKA (Kitakyushu-shi), Yoshihiro ARITA (Funabashi-shi), Fumiaki TAKAHASHI (Kisarazu-shi), Yousuke KUROSAKI (Tokyo)
Application Number: 14/778,459
Classifications
International Classification: C21D 8/12 (20060101); C21D 6/00 (20060101); C22C 38/00 (20060101); C22C 38/06 (20060101); C22C 38/02 (20060101); C22C 38/04 (20060101); C21D 8/02 (20060101); C22C 38/16 (20060101);