WO2011065023A1

WO2011065023A1 - Low-carbon steel sheet and process for producing same

Info

Publication number: WO2011065023A1
Application number: PCT/JP2010/006958
Authority: WO
Inventors: 多津彦平谷; 善彦尾田; 峰男村木
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2009-11-30
Filing date: 2010-11-29
Publication date: 2011-06-03
Also published as: CN102639745B; JP5655295B2; JP2011119298A; TW201134953A; CN102639745A; KR20120112435A; TWI461544B; KR101399995B1

Abstract

Disclosed is a low-carbon steel sheet which is a clad steel sheet comprising: a center layer, in terms of sheet thickness, that has a ferrite mixture structure containing at most 1.0 mass% Si and including any one or more of a pearlite phase, a bainite phase, and a martensite phase; and surface layers each comprising a ferrite phase, as the only phase, that contains 3-5 mass% Si. The surface layers have an in-plane tensile stress of 70-160 MPa imposed thereon as an internal stress, and the low-carbon steel sheet therefore has excellent high-frequency characteristics and is inhibited from suffering an increase in iron loss caused by an external stress.

Description

Low carbon steel sheet and method for producing the same

The present invention relates to a low-carbon steel sheet suitable as a high-frequency transformer, a reactor, and a motor core for power electronics, and particularly aims to improve high-frequency characteristics and reduce iron loss deterioration due to external stress.

The iron loss of an electrical steel sheet is composed of a hysteresis loss that strongly depends on precipitates in the steel, crystal grain size, texture, and the like, and an eddy current loss that strongly depends on the plate thickness, specific resistance, magnetic domain structure, and the like.
A general electromagnetic steel sheet increases crystal grain growth by reducing impurities in the steel as much as possible, thereby reducing hysteresis loss. Moreover, eddy current loss is reduced by adding 0.5 to 3.5% by mass of Si to increase the specific resistance or to reduce the plate thickness.

Here, since the hysteresis loss is proportional to the frequency and the eddy current loss is proportional to the square of the frequency, the ratio of the hysteresis loss to the iron loss of the electrical steel sheet is large at the commercial frequency (50/60 Hz). When the frequency becomes several kHz or higher, the ratio of eddy current loss increases.
In recent years, high frequency switching elements have been developed in the field of power electronics, and it has been strongly desired to reduce high-frequency iron loss even for electrical steel sheets used as iron core materials for transformers, reactors, motors, etc. It was.

In response to this demand, eddy current loss can be reduced by reducing the thickness of the electromagnetic steel sheet to 0.2 mm or less or increasing the Si content to nearly 4% by mass. However, in the future, driving at a high frequency exceeding 10 kHz is also expected, and there is a need for innovative material development that is not an extension of the conventional development.

Conventionally, materials such as soft ferrite, metal powder compact, and amorphous have been used under such excitation conditions in the high frequency region. However, since ferrite has a low magnetic flux density, the iron core becomes large. Amorphous has low iron loss. On the other hand, the building factor is significantly inferior to that of electrical steel sheets. Furthermore, metal dust such as Sendust alloy powder has magnetostriction and iron. The loss is low, but the cost is high, and the saturation magnetic flux density is lower than that of the electromagnetic steel sheet. For this reason, various studies have recently been conducted on the reduction of high-frequency iron loss in electrical steel sheets.

As means for reducing high-frequency iron loss of an electromagnetic steel sheet, Patent Document 1 describes a Si enrichment method for a steel sheet surface by a siliconization method.
This Si enrichment technology, for example, as described in Patent Document 2, reacts a 3 mass% Si steel sheet with a thickness of 0.1 to 0.35 mm with silicon tetrachloride gas at a high temperature to reduce the Si concentration in the steel. It is a process of enhancing.
In addition, as is known for a long time, a 6.5 mass% Si steel sheet has a specific resistance approximately twice that of a 3 mass% Si steel sheet, and can effectively reduce eddy current loss. At the same time, since the magnetostriction is substantially zero, it exhibits an excellent effect in reducing the noise of the iron core.

Patent Document 2 further shows that, in the silicidation process, sufficient magnetic properties can be obtained by adjusting the surface Si concentration without making the Si concentration uniform in the thickness direction from the viewpoint of shortening the diffusion time. ing.

In Patent Document 3, regarding a silicon steel plate having a Si concentration gradient in the plate thickness direction, in order to reduce high-frequency iron loss, the Si concentration difference (maximum-minimum) in the plate thickness direction, the surface Si concentration, and the Si on the front and back surfaces of the steel plate It defines the difference in concentration. In particular, there is a description that the lowest iron loss is obtained when the surface Si concentration is 6.5 mass%.

Furthermore, Patent Document 4 discloses an electromagnetic steel sheet having excellent high-frequency characteristics by subjecting the electromagnetic steel sheet to a silicidation treatment with a ferrite phase and forming an electromagnetic steel sheet having a high Si concentration in the surface layer and a low Si concentration in the central portion of the plate thickness. Are listed.

Patent Document 5 discloses that a low carbon steel is subjected to a siliconization treatment in a temperature range of 900 to 1000 ° C. where the austenite phase is small to obtain a steel sheet having a high Si concentration in the surface layer, thereby improving workability and excellent high frequency characteristics. In addition, a silicon steel sheet for motors is described.

Japanese Patent Publication No. 6-45881 Japanese Patent Publication No. 5-49744 JP 2005-240185 A JP 2009-263778 A JP 2000-328226 A

As described above, the iron loss is represented by the sum of hysteresis loss and eddy current loss. It is known that the higher the excitation frequency, the higher the ratio of eddy current loss to the total iron loss. Since the eddy current is more difficult to flow as the specific resistance of the material is larger, a material having a higher specific resistance is used for the high-frequency magnetic core.
In general, in a magnetic steel sheet, the specific resistance increases as the Si concentration increases. Therefore, a high frequency magnetic core material containing 3% by mass or more of Si is suitable.

On the other hand, as the Si concentration increases, the steel tends to become harder and more brittle, making cold rolling difficult. In addition, when the Si concentration increases, the transformation from austenite to ferrite during slab cooling (hereinafter referred to as γ / α transformation) does not occur, and ferrite forms a coarse structure as it is, so slab cracks and surface defects are likely to occur. Become. Therefore, in the actual electrical steel sheet manufacturing process, the upper limit of the Si concentration in the steel is 4% by mass.

According to Patent Document 2, the 6.5% by mass Si steel sheet has the highest magnetic permeability and the lowest magnetostriction among the electromagnetic steel sheets. However, in order to use this as an iron core, it is necessary to perform secondary processing such as slitting, punching, and bending. However, since 6.5 mass% Si steel sheet is brittle and easily cracked as compared with a normal electromagnetic steel sheet, advanced processing technology is required in the secondary processing as described above. Further, the Vickers hardness Hv of the 6.5 mass% Si steel sheet is about 390, which is very hard as compared with the conventional magnetic steel sheet Hv: about 200. For this reason, there also existed a fault that a press metal mold | die was easy to shorten a lifetime. Especially for high frequency applications above 10kHz, the ratio of eddy current loss is higher than hysteresis loss, so even if the hysteresis loss is bad, another material such as an inexpensive dust core with low eddy current loss (high specific resistance) can be used. Widely used.

According to Patent Document 2, a 6.5% by mass Si steel plate can be manufactured by a siliconization process in which a silicon tetrachloride is sprayed at a high temperature after rolling a 3% by mass Si steel plate to a final thickness.
However, in order to actually use it as a magnetic core, it is necessary to slit, press or bend the 6.5% by mass Si steel sheet after siliconization, which often causes cracking and chipping. It was.
Further, in Patent Document 2, as shown in FIG. 13, when the Si concentration is too nonuniform, the iron loss is greatly increased. On the other hand, when the Si concentration nonuniformity is suppressed to a certain level, the Si concentration is uniform. An example in which a low iron loss comparable to a simple steel plate is obtained is described. However, no case is shown in which when the Si concentration is not uniform, the iron loss is even lower than that of a steel plate with a uniform Si concentration. Moreover, no specific numerical value is described about the high frequency iron loss.

Patent Document 3 describes a steel plate that leaves a Si concentration gradient in the thickness direction as a material that is further superior in high-frequency characteristics than a 6.5 mass% Si steel plate. Even if the surface layer has a high Si concentration, since the Si concentration in the central portion in the thickness direction of the plate is about 3% by mass, the average of the entire steel material is a material having a lower Si than the 6.5% by mass Si steel plate described above. it is conceivable that.
However, in this case, the γ / α transformation does not occur because the lower limit of the Si concentration in the steel is about 3% by mass. For this reason, the steel sheet structure when cooled to room temperature is composed of coarse ferrite grains that longitudinally cut the plate thickness, and the problem remains that cracks and chips are likely to occur when slitting or pressing.

According to Patent Document 4, a grain oriented electrical steel sheet having a large magnetic domain width and a large abnormal eddy current loss due to a coarse secondary recrystallized structure is inherently tensile on the surface layer of the steel sheet and compressed internally. By applying each stress distribution, it is possible to effectively reduce eddy current loss. However, since a grain-oriented electrical steel sheet is used as the material, the cost is higher than other high-frequency iron core materials. In addition, siliconization is used as a means for imparting stress distribution. However, in grain oriented electrical steel sheets, in order to sufficiently reduce eddy current loss, silicon is immersed from the surface layer until the average Si concentration reaches 4% by mass or more. It was necessary to increase the surface layer Si concentration to 5% by mass or more.

In Patent Document 5, low carbon steel having an austenite phase is subjected to siliconizing treatment. However, if siliconizing treatment is performed in a temperature range exceeding 1000 ° C., cracking occurs at the interface of γ / α transformation. Silica treatment is performed at a relatively low temperature range of ~ 1000 ° C.
However, in order to reduce eddy current loss, it is preferable to perform a silicidation treatment at 1000 ° C. or higher, and in the above-described interface, manufacturing of a steel plate material and a steel plate that does not cause cracking even when a silicon carbide treatment at 1000 ° C. or higher is performed. A method was desired. In addition, since the steel material described in Patent Document 5 has a high surface layer Si concentration of 5 to 6.5% by mass and a coarse secondary recrystallized structure, it is still cracked or chipped when slitting or pressing. The problem that often occurs was left.

Other high frequency magnetic core materials include a dust core formed by compacting iron powder, a ferrite core of iron oxide powder, and an Fe-based amorphous alloy. These are characterized by low eddy current loss because of their higher specific resistance than 6.5 mass% Si steel sheet.
However, since the ferrite core has a low saturation magnetic flux density, the ferrite core is normally used only for high-frequency applications with a low output and several hundred kHz or more. On the other hand, the dust core and the Fe-based amorphous alloy have a slightly lower saturation magnetic flux density than the magnetic steel sheet, but have a low eddy current loss.

However, any of the above-described materials has a problem that iron loss increases remarkably when an external stress such as compression is applied.
As described above, many magnetic materials having excellent high-frequency characteristics are inferior in workability, and react sensitively to external stress, and particularly when compressive stress is applied, iron loss increases remarkably. .

On the other hand, examples of the material excellent in workability include low-carbon steel plates that are widely used as structural materials or exterior materials. Further, the magnetism of low carbon steel is not as sensitive to external stress as a general magnetic material, and iron loss does not increase remarkably even when compressive stress is applied.
However, since the structure of a general low carbon steel sheet is composed of a fine ferrite mixed structure including a pearlite phase, a bainite phase, and a martensite phase, the DC magnetic characteristics are extremely poor. Therefore, a low-carbon steel plate was rarely used for a commercial frequency magnetic core mainly composed of hysteresis loss.

However, if the low-carbon steel sheet can reduce the eddy current loss and take advantage of the low iron loss rise against compressive stress, the high-frequency iron loss is low and the iron loss is low against external stress. An excellent magnetic core material can be obtained.

The present invention has been developed in view of the above-mentioned present situation, and an object thereof is to provide a low carbon steel sheet having excellent high frequency characteristics and less iron loss deterioration due to external stress, together with its manufacturing method.

According to Patent Document 3, an Si concentration gradient is formed in the plate thickness direction in the siliconization process, and the Si concentration difference between the front and back surfaces of the steel plate is controlled to reduce eddy current loss.
This technique is considered to be applied to low carbon steel in the same manner to reduce eddy current loss.

However, in the case of low carbon steel, the γ / α transformation occurs. Therefore, when the siliconizing treatment is performed in the high-temperature austenite phase region, a phenomenon occurs in which the surface layer where the Si concentration increases is transformed into the ferrite phase. At this time, a Si concentration gap exists between the low Si concentration austenite phase and the high Si concentration ferrite phase, and therefore, the Si concentration gradient at the heterophase interface becomes discontinuous. When cooling with such a Si concentration gradient remaining, the high Si concentration ferrite phase in the surface layer does not transform, as shown in FIG. 1, whereas the low Si concentration austenite phase in the center layer of the plate thickness is the pearlite phase. And a fine ferrite mixed structure containing a bainite phase and a martensite phase.
That is, the low carbon steel that has been subjected to a siliconization treatment to give a Si concentration gradient is greatly different from the technique described in Patent Document 3 in which no γ / α transformation occurs.

Hereinafter, the experiment that led to the completion of the present invention will be described. Table 1 shows the compositions of the A to D4 low carbon steels used in this experiment.
In the table, the steel indicated by the symbol (C)
Annealed at 1200 ° C in nitrogen gas (no siliconization) ... Steel treatment I
Thoroughly diffused Si after siliconizing at 1200 ° C (siliconized + long-time diffusion) ... Steel treatment II
3 minutes of siliconization + Si diffusion at 1200 ° C (siliconization + short-time diffusion) ... Steel treatment III
The sample used for this experiment was produced by performing these three types of treatments.

Here, the silicon treatment of steel treatments II and III was adjusted such that the sample average Si concentration after treatment was 3 mass% Si. These samples are magnetized by direct current and alternating current using a 30 × 100mm single plate measurement frame, and the iron loss under the condition of magnetic flux density 0.05T and frequency 20kHz is separated into hysteresis loss and eddy current loss. And compared the results.
The result of sample No. (C) is shown in FIG. In the figure, the iron loss values of electromagnetic steel sheets (3 mass% Si steel sheet and 6.5 mass% Si steel sheet) having the same thickness are also shown.

Compared with the steel treatment I sample, the steel treatment II sample has a lower eddy current loss due to an increase in specific resistance due to an increase in Si, and in the process of homogenizing Si, a γ / α transformation is performed over the entire plate thickness, resulting in coarseness. Due to the formation of a proper ferrite structure, the hysteresis loss is also reduced.
In addition, the iron loss of the steel treatment II sample (representing hysteresis loss + eddy current loss, hereinafter the same for the present invention) is a large value compared to the iron loss of the electrical steel sheet (3 mass% Si) having the same Si concentration. It was. The reason for this is that even in the same coarse ferrite structure of 3 mass% Si, the amount of C is less than 50 ppm in the case of electrical steel sheets, whereas the amount of C is included in the case of low carbon steel is 500 ppm or more. Therefore, it is considered that the history loss increases.

On the other hand, the iron loss of the steel treatment III sample is surprisingly lower than that of the 3 mass% Si electrical steel sheet as well as the 6.5 mass% Si electrical steel sheet, and in particular, reduced eddy current loss. Admitted.
It was expected that by adding a Si concentration gradient in the plate thickness direction, the magnetic flux could be concentrated on the surface layer, and eddy current loss could be reduced. It was only estimated that the eddy current loss could be reduced by 20 to 30% compared to the sample of which the Si concentration was made uniform. That is, the result of this experiment was an effect of reducing eddy current loss exceeding 50%, which was far beyond expectations.

Next, in order to investigate this phenomenon in more detail, the four types of materials shown in Table 1 were used, and siliconization was performed under the conditions of the steel treatment III described above to prepare a sample. The structure of these samples was observed in a longitudinal section, and the iron loss was measured in the same manner as in the above experiment. The results are shown in FIGS.

FIGS. 3A to 3D show photographs of the cross-sectional structures of the samples after the steel treatment III was applied to the sample Nos. (A) to (D), respectively.
In FIG. 3A, although a boundary is observed between the surface layer and the plate thickness center layer, each has a coarse grain structure of a ferrite single phase. On the other hand, FIGS. 3B and 3C include a bainite structure, a pearlite structure, and a martensite structure that are found when air cooling is performed after annealing a low carbon steel at a temperature at which an austenite phase is generated in the center thickness layer. A ferrite mixed structure is observed, which is clearly different from the ferrite single-phase structure of the surface layer. FIG. 3D shows a martensite structure including a small amount of ferrite structure in the center layer of the plate thickness.

From FIG. 4, it was found that not all steel materials subjected to the steel treatment III were able to obtain a low iron loss that surpassed the 6.5% by mass Si magnetic steel sheet.
Therefore, when the components of each sample were confirmed, when the amount of C was 200 ppm or more and when Mn was contained by 0.3 mass% or more, the effect of reducing eddy current loss was noticeable, It was found that low iron loss was obtained that surpassed 6.5% by mass Si magnetic steel sheet.
Moreover, although the sample D shows a lower iron loss than a normal electromagnetic steel sheet (3 mass% Si), its superiority tends to be lower than those of the samples B and C.

Next, the plate warpage was observed when the sample was removed from one side of the surface of the sample to the center of the plate thickness by chemical polishing. As a result, the warp was convex on the thickness center side. As a result, it was found that a tensile stress was generated in the surface layer and a compressive stress was generated in the center before the removal by polishing.

Here, in the present invention, as shown in FIG. 5, the internal stress means that the original plate thickness is d (mm) and the curvature radius at the time of the above-described plate warp is r (mm). (Internal tensile stress) = Compressive stress acting on the thickness center
In-plane tensile stress = E x d / (2r) [MPa] (E represents Young's modulus of the steel sheet)
Is defined.
Further, samples having the compositions to be Sample A and Sample C in Table 1 were used, and the samples were prepared by variously changing the Si diffusion time under the above-described steel treatment III conditions. About these samples, while measuring internal stress by said method, the eddy current loss was measured. The result is shown in FIG.
From FIG. 6, in the case of Sample C containing 200 ppm or more of C and 0.3% by mass or more of Mn, the internal stress after the above-described siliconization treatment tended to be large. In addition, the reduction of eddy current loss was remarkable when the internal stress (in-plane tensile stress) was in the range of 70 to 160 MPa.

The cause of the above-mentioned tendency is not clear at present, but the inventors speculate as follows.
In Fe-Si alloys, when the amount of C in steel increases at the low carbon steel level, the γ / α boundary line on the Fe-Si phase diagram is shifted to the high Si side, and siliconization is applied. In this case, the Si concentration gap between the portion transformed into the ferrite phase and the portion remaining in the austenite phase increases. When the Si concentration gap increases at high temperatures, it is considered that an internal stress is generated between the central layer that is about to expand due to a γ / α phase transformation during cooling and the surface ferrite phase that no longer undergoes transformation.

Also, the effect of Mn addition is that Mn is an element that stabilizes the austenite phase. Therefore, when the amount of Mn increases, the phase transformation point of γ / α shifts to the low temperature side. Therefore, it is considered that the internal stress generated during cooling further increases.

Furthermore, the plate thickness center layer has a fine mixed structure and is in a state where compressive stress is applied, so it is difficult to magnetize, whereas the surface layer is coarse ferrite crystal grains and tensile stress is applied. Since it is in a state, it tends to be easily magnetized.
Therefore, when such a steel plate is magnetized in the in-plane direction of the plate, the magnetic flux concentrates on the surface layer, and as a result, it is considered that the eddy current loss of the steel plate is reduced.

It has also been found that the iron loss value does not increase even when an external stress is applied if the sample has a large internal stress as in the sample described above.
In other words, even when the external stress is zero, if the internal stress of about 70 to 160 MPa is generated like the sample, the tensile state of the surface layer is maintained even if a compressive stress of about several tens of MPa is applied from the outside. Is done. On the other hand, the compressive stress further increases in the center portion of the plate thickness, but it is a portion that is originally difficult to be magnetized, and its influence is negligible.
As a result, there is no change in the situation where the magnetic flux tends to concentrate on the surface layer, and it is considered that the effect of reducing the eddy current loss of the sample is not lost.

In addition, as described above, when annealing is performed for a long time at a high temperature, the Si concentration distribution of the steel sheet is relaxed, or the internal stress is relaxed, the above-described effect of reducing the eddy current loss and the deterioration of the iron loss with respect to the external compressive stress. The prevention effect is reduced, and the superiority over electrical steel sheets having the same Si concentration is lost.
Accordingly, it has been found that it is preferable to take into consideration the time of heat treatment performed before the completion of the magnetic core including the diffusion time from the siliconization treatment.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. Si: 1.0% by mass or less, C: 0.02 to 0.16% by mass, Mn: 0.3 to 2.0% by mass, P: 0.03% by mass or less and S: 0.01% by mass or less, the balance is Fe and inevitable impurities. A plate thickness central layer which is a ferrite mixed structure containing one or more of pearlite phase, bainite phase and martensite phase, Si: 3 to 5 mass%, C: 0.02 to 0.16 mass%, Mn: It is a clad-type low-carbon steel plate comprising 0.3 to 2.0% by mass, P: 0.03% by mass or less and S: 0.01% by mass or less, the composition of the balance Fe and inevitable impurities, and a surface layer that is a ferrite single phase. A low carbon steel sheet, wherein the surface layer has an in-plane tensile stress of 70 to 160 MPa as an internal stress.

2. 2. The low-carbon steel sheet according to 1 above, wherein the total thickness of the surface layer is 30 to 60% of the total thickness.

3. 3. The low carbon steel plate according to 1 or 2 above, wherein the low carbon steel plate has a thickness of 0.05 to 0.35 mm.

4. The sheet thickness center layer and the surface layer of the low-carbon steel plate are further Al: 0.002-0.6 mass%, Cr: 0.01-1.5 mass%, V: 0.0005-0.1 mass%, Ti: 0.0005-0.1 mass%, Nb: 0.0005. -1 mass%, Zr: 0.0005-0.1 mass%, B: 0.0005-0.01 mass%, and N: 0.002-0.01 mass%, one or more elements selected from the above, 4. The low carbon steel plate according to any one of 3 to 3.

5. Heating a steel plate containing Si: 1.0% by mass or less, C: 0.02 to 0.16% by mass, Mn: 0.3 to 2.0% by mass, P: 0.03% by mass or less and S: 0.01% by mass or less, the balance being Fe and inevitable impurities Then, in the austenite region of 1050 to 1250 ° C., by reacting with a Si-based gas, a ferrite phase having a Si content of 3 to 5% by mass is formed on the surface layer of the steel sheet, and then Si in the steel is homogenized. A method for producing a low-carbon steel sheet, characterized by cooling before.

6. 6. The production of the low-carbon steel sheet as described in 5 above, wherein the Si-based gas is one or more gases selected from silicon tetrachloride, trichlorosilane, dichlorosilane, monosilane, and disilane. Method.

7. The steel sheet is further made of Al: 0.002-0.6% by mass, Cr: 0.01-1.5% by mass, V: 0.0005-0.1% by mass, Ti: 0.0005-0.1% by mass, Nb: 0.0005-0.1% by mass, Zr: 0.0005- 7. The low carbon steel sheet according to 5 or 6 above, which contains one or more elements selected from 0.1% by mass, B: 0.0005 to 0.01% by mass and N: 0.002 to 0.01% by mass. Manufacturing method.

According to the present invention, a low-carbon steel sheet having excellent high-frequency characteristics and less iron loss deterioration due to external stress can be obtained together with its manufacturing method, so that an iron core material excellent in workability can be provided.

It is a schematic diagram which shows the structure | tissue of the low carbon steel of this invention. It is a graph which compares and shows the iron loss value according to the siliconization treatment method of low carbon steel. It is a structure | tissue photograph in the steel plate cross section after the siliconization process according to steel plate component composition. It is a graph which compares and shows the iron loss value after the siliconization process according to steel plate component composition. It is a figure which shows the measuring point of the internal stress (in-plane tensile stress) in this invention. It is a figure which shows the relationship between internal stress (in-plane tensile stress) and eddy current loss. It is a schematic diagram of a siliconization furnace suitable for use in the present invention.

Hereinafter, the present invention will be specifically described.
First, the reasons for limiting the structure and composition of the steel sheet will be described. In addition, unless otherwise indicated, the% display in a steel plate component composition represents the mass%.

In the present invention, as described above, it is important to further apply tensile stress to the surface layer with increased specific resistance.
For this purpose, a steel sheet that is in an austenite phase (hereinafter referred to as γ phase) at high temperature is subjected to a siliconization treatment to increase the specific resistance by increasing the amount of Si in the surface layer. , It is necessary to cool before Si in steel becomes uniform.
As a result of the cooling described above, as shown in FIG. 1, a plate thickness center layer composed of a ferrite mixed structure containing at least one of a pearlite phase, a bainite phase and a martensite phase, and a ferrite having a high Si concentration A steel sheet having a so-called “clad type” three-layer structure having two surface layers on the front and back surfaces of a steel sheet having a single-phase structure is obtained. Since there is a difference in Si concentration between each surface layer and the plate thickness center layer, as described above, internal stress is generated due to the Si concentration gap, and tensile stress is applied to the surface layer.
In addition, about a plate | board thickness center layer, since sufficient internal stress is not obtained in the case of a ferrite single phase, a pearlite phase, a bainite phase, and a martensite phase are included in total 30% (area%) or more, and the remainder is It is preferable that it is substantially a ferrite phase.

Here, when the amount of Si in the surface layer is 3% or more, the magnetostriction takes a large positive value. Therefore, when the tensile stress as described above is applied, it becomes easy to be magnetized by the magnetoelastic effect. As a result, when the steel plate is magnetized, the concentration of magnetic flux on the surface layer is promoted, and the effect of reducing eddy current is increased.
However, when the surface layer Si content exceeds 5%, the magnetostriction is conversely reduced, the magnetoelastic effect due to the tensile stress is reduced, and the surface becomes hard, resulting in a decrease in workability. Therefore, the Si content in the surface layer is set to 3 to 5%.

On the other hand, if the average value of the Si amount in the thickness center layer exceeds 1.0%, the difference in Si concentration from the surface layer described above decreases, and the internal stress of the steel sheet decreases, so the effect of reducing eddy current loss is diminished. End up. Therefore, the Si concentration of the plate thickness center layer is set to 1.0% or less. The lower limit of the Si concentration is not particularly limited, but is preferably about 0.1% in order to remove oxygen in the steel during steelmaking.

In the present invention, before the Si in the steel is made uniform, the surface layer Si increased by the silicon immersion penetrates to the inside by sufficient diffusion, and before the Si concentration in the surface layer and the central layer becomes uniform. Meaning.
Accordingly, there is a Si concentration gradient in the surface depth portion (center) direction in both the surface layer portion and the plate thickness central layer portion, but this gradient is extremely small and can be almost ignored. Accordingly, the surface Si concentration (amount) in the present invention means the average Si concentration (amount) of the surface layer portion. Moreover, although the surface layer may have a dot-like or linear carbide, there is no particular problem in this case, and the ferrite single phase may be substantially formed.

The tensile stress of the surface layer described above must be in-plane tensile stress of 70 to 160 MPa. This is because if the tensile stress of the surface layer is less than 70 MPa, there is a problem that the effect of reducing eddy current loss is weakened. On the other hand, if it exceeds 160 MPa, the hysteresis loss increases too much and the effect of reducing eddy current loss is reduced. The problem of canceling out occurs. Therefore, in the present invention, the tensile stress of the surface layer is limited to 70 to 160 MPa.

The thickness of the surface layer described above is preferably about 30 to 60% of the total thickness of the two steel layers. This is because the hysteresis loss increases if the total thickness of the steel sheet is less than 30%. On the other hand, if it exceeds 60%, the effect of reducing eddy current loss is reduced, resulting in an increase in iron loss.
The surface layer described above is not necessarily the same for each of the upper and lower surfaces, such as the thickness and the component composition, but it is desirable that the surface layer be the same level.

Furthermore, the thickness of the steel sheet used in the present invention is desirably about 0.05 to 0.35 mm. This is because if the thickness of the steel sheet is less than 0.05 mm, the production efficiency is lowered and the manufacturing cost is increased. On the other hand, if it exceeds 0.35 mm, the eddy current loss increases and it is not suitable as a magnetic core material for high frequency. However, even if the thickness is not satisfied, the iron loss reduction effect in the present invention is not lost.

Hereinafter, the reasons for limiting the components of the surface layer and the plate thickness center layer of the steel plate will be described. In addition, the remainder of the steel plate component shown below is Fe and inevitable impurities.
Among the components, for Si, as described above, the surface layer needs to be 3 to 5%, and the plate thickness center layer needs to be 1.0% or less, but the other components are common to both the surface layer and the plate thickness center layer. .
C: 0.02 to 0.16%,
C is an element necessary for increasing the internal stress of the steel material and obtaining a sufficient eddy current loss reduction effect, and needs to contain at least 0.02%. On the other hand, if it exceeds 0.16%, cracking is likely to occur at the interface between the surface layer and the thickness center layer. Therefore, C is limited to the range of 0.02 to 0.16%.
More preferably, C is set in the range of 0.03 to 0.10% from the viewpoint of obtaining a lower iron loss than the 6.5% Si electromagnetic steel sheet even at high frequencies.

Mn: 0.3-2.0%
Mn is an element necessary for obtaining a sufficient eddy current loss reduction effect, and needs to be contained at least 0.3%. On the other hand, if it exceeds 2.0%, the γ phase tends to remain in the thickness center layer of the steel sheet even after cooling to room temperature, and the internal stress with the surface layer of the steel sheet is reduced. Therefore, Mn is limited to a range of 0.3 to 2.0%.

P: 0.03% or less P is an embrittlement element, and cracking is likely to occur at the interface between the surface layer of the steel sheet and the thickness center layer. Therefore, it is desirable to reduce as much as possible, but 0.03% is acceptable.

S: 0.01% or less S is an element that causes hot brittleness. Since productivity decreases as the concentration increases, it is desirable to reduce it as much as possible, but it is acceptable up to 0.01%.

As mentioned above, although the basic component of the steel plate has been described, in the present invention, in addition, one or more selected from the elements described below are contained in common in both the surface layer and the center layer of the plate thickness. can do.
Al: 0.002 to 0.6%
The addition of Al increases the specific resistance and is therefore an effective element for reducing eddy current loss. If it is less than the lower limit, the effect of addition is poor. On the other hand, if it exceeds the upper limit, the α phase is present at a high temperature before silicidation, making it impossible to produce the clad steel sheet proposed by the present invention.

Cr: 0.01-1.5%
The addition of Cr increases the specific resistance and is therefore an effective element for reducing eddy current loss. If the amount is less than the lower limit, the effect of addition is poor. On the other hand, if the amount exceeds the upper limit, carbides precipitated in the grains and at the grain boundaries are the starting points, and brittle fracture is likely to occur.

V: 0.0005 to 0.1%, Ti: 0.0005 to 0.1%, Nb: 0.0005 to 0.1%, Zr: 0.0005 to 0.1%
The addition of V, Ti, Nb, and Zr is effective in reducing eddy current loss because it lowers the magnetic permeability by forming carbide and nitride at the center of the plate thickness and enhances the effect of concentrating the magnetic flux on the surface layer. When the content is less than the lower limit, the effect of addition is poor. On the other hand, when the content exceeds the upper limit, carbide and nitride precipitated in the grains and at the grain boundaries are the starting points, and brittle fracture is likely to occur.

B: 0.0005 to 0.01%, N: 0.002 to 0.01%
The addition of B and N increases the hardenability of the central thickness layer in the cooling process after the siliconization treatment, so that the permeability of that portion is reduced and the effect of concentrating the magnetic flux on the surface layer is increased, thereby reducing eddy current loss. Each is effective. If each is less than the lower limit, the effect of addition is poor, whereas if the upper limit is exceeded, embrittlement tends to occur.

Next, the suitable manufacturing method of the low carbon steel plate of this invention is demonstrated.
There is no restriction | limiting in particular about the manufacturing method of the low carbon steel plate before performing a siliconization process, Any conventionally well-known method can be used conveniently. For example, the slab having the composition of the plate thickness center layer of the steel plate is heated and then subjected to hot rolling, and cold rolling or cold rolling with one or more intermediate annealings is repeated to obtain a predetermined slab. A thick steel plate may be used. Moreover, you may give finish annealing as needed.

The steel plate obtained as described above is subjected to a siliconization treatment to increase the Si concentration of the surface layer. After forming a ferrite phase having a Si content of 3 to 5% on the surface layer of the steel plate, the Si in the steel is reduced. The low carbon steel sheet of the present invention can be produced by cooling before homogenization.
Here, as a method of infiltrating (siliciding) Si, any conventionally known method can be applied. For example, a vapor phase siliconization method, a liquid phase siliconization method, a solid phase siliconization method, or the like can be used. Can be mentioned. In addition, the Si-based gas used at that time is not particularly limited, but is a silane gas such as one or more gases selected from silicon tetrachloride, trichlorosilane, dichlorosilane, monosilane, and disilane. It is desirable to be.
Hereinafter, a method of infiltrating Si by vapor phase siliconization will be described.

In the case of vapor phase siliconization, if the Si-based reaction gas is sufficiently supplied, the temperature history (temperature of each zone in the furnace and the residence time of the steel plate) from the start to the end of the siliconization and further cooling is determined. Corresponding to the plate thickness and the amount of Si added (silicon immersion amount), the Si concentration distribution in the plate thickness direction is determined almost uniquely.

As the blast furnace used in the present invention, any conventionally known blast furnace can be suitably used. For example, there is an equipment having a structure as shown in FIG.

In the present invention, when siliconizing is performed on a low carbon steel sheet having a Si concentration of 1% or less, performing the siliconizing process under a condition satisfying the following formula 1 particularly reduces the high-frequency iron loss significantly. It is preferable for obtaining a Si concentration distribution.
Formula 1: 1.3 × 10 ⁻⁴ ≦ (Σt _k × exp (−25000 / T _k )) / (d ² × [mass% Si] _add ) ≦ 2.2 × 10 ⁻⁴

Here, T _k is the temperature of each zone in the furnace through which the steel plate passes after the start of the siliconization treatment, t _k is the residence time of the steel plate in each zone, d is the plate thickness (mm), [mass% Si] _add Represents the amount of Si added to the steel sheet during the siliconization treatment (increase in the average Si concentration in the thickness direction).
In the present invention, when the furnace temperature changes, it can be considered that the heat treatment is performed at a constant temperature and a constant time so that the value of Σt _k × exp (−25000 / T _k ) is the same. For example, when cooling from 1200 ° C. to 700 ° C. in 5 minutes, Σt _k × exp (-25000 / T _k ) ≈1.9 × 10 ⁻⁶ , and when 1200 ° C. is constant, the value of t _k is 45 Second. Therefore, the cooling can be regarded as the same as that subjected to a heat treatment at 1200 ° C. for 45 seconds.

Even when the lower limit of the value of the above formula is smaller than 1.3 × 10 ⁻⁴ , it is possible to make the Si concentration distribution appropriate by performing a post-process such as strain relief annealing at a relatively high temperature. . However, if it is smaller than 1.3 × 10 ⁻⁴ , the Si concentration in the surface layer is actually too high, and as a result, the steel plate is deformed during the siliconizing treatment, or the sheared portion is cracked or chipped during the subsequent processing. It is desirable to satisfy the above value because it is likely to occur.

On the other hand, when the upper limit value of the above formula is larger than 2.2 × 10 ⁻⁴ , the internal stress is relaxed and the effect of reducing eddy current is reduced. Therefore, it is desirable that the above value is also satisfied.

In addition, when performing the siliconization treatment in a continuous line, when the temperature is 700 ° C. or lower, the Si concentration of the steel sheet does not change in a realistic time, so the calculation of Equation 1 may be up to 700 ° C.

The silicidation temperature in the present invention is 1050 to 1250 ° C. This is because if the temperature is less than 1050 ° C., internal stress may not be sufficiently generated when it is cooled, while if it exceeds 1250 ° C., the surface layer with high Si concentration becomes a semi-molten state during the siliconization treatment, and the steel plate It is because there is a possibility of breaking.

The low carbon steel sheet that has been subjected to the siliconization treatment is subjected to a drying / baking process after the insulating coating is applied.
When passing through the above-described steps, if the heat treatment is performed at a temperature lower than 600 ° C., stress relaxation of the steel sheet does not occur, and high-frequency iron loss does not increase. However, when the heat treatment is performed at 600 ° C. or higher, the internal stress is relaxed with time, so that the high-frequency iron loss is increased.

Therefore, we investigated the optimum thermal history for heat treatment in the range of 600-800 ° C. As a result, it was confirmed that the iron loss was lower than that of a uniform material having the same plate thickness and the same Si concentration as long as the following expression 2 was satisfied. Therefore, when the heat treatment is performed in the range of 600 to 800 ° C., it is preferable that the heat history satisfying the following formula 2 is set.
Formula 2: (Σt ′ _k × exp (−25000 / T ′ _k )) / (d ² × [mass% Si] _add ) ≦ 0.2 × 10 ⁻⁴

Here, _T'k is the temperature of the heat treatment process steel sheet is passed after the siliconizing treatment, _t'k residence time of the steel sheet in each heat treatment step, d is the thickness (mm), [mass% Si ] add Represents the amount of Si added to the steel sheet during the siliconization treatment (increase in the average Si concentration in the thickness direction).
As in the case of Equation 1, when the furnace temperature changes, heat treatment is performed at a constant temperature and a constant time so that the value of Σt ′ _k × exp (−25000 / T ′ _k ) is the same. It can be regarded as a thing.

The low-carbon steel sheet that has been subjected to the siliconization treatment is assembled as an iron core through various processing steps such as slitting, shearing, pressing, and the like, and may be subjected to strain relief annealing. Also in this case, since the internal stress is relieved by annealing at 600 ° C. or higher, it is preferable to set the strain relief annealing temperature and time so as to satisfy the above-mentioned formula 2.
In addition, when the insulating coating is dried and baked at 400 ° C. or more and subjected to strain relief annealing after processing, the heat treatment step and strain relief annealing step of the coating are totaled, and the temperature and It is preferable to set the time.
From the above, it is possible to set the manufacturing conditions in consideration of the time for the heat treatment performed until the completion of the magnetic core.

<Example 1>
A sample having the component composition shown in Table 2 was rolled to a sheet thickness of 0.2 mm, heated to 1200 ° C., and subjected to siliconizing treatment equivalent to 3% Si and Si diffusion treatment in a SiCl ₄ + N ₂ atmosphere. After 3 minutes, it was cooled to room temperature at 10 ° C./min.
The high frequency iron loss of these samples was measured by the Epstein test method (JIS C 2550). The results are shown in Table 3 together with the Si concentrations of the surface layer and the plate thickness center layer.

As shown in the table, it can be seen that all of the inventive examples (Nos. 3 to 5 and 7) obtained according to the present invention have lower iron loss than the 3% Si electrical steel sheet.

<Example 2>
The samples shown as Nos. 2 to 5 in Table 2 were subjected to a change in iron loss by applying a compressive stress of ± 50 MPa parallel to the magnetization direction. These high-frequency iron losses were measured by the Epstein test method (JIS C 2550).
Table 4 shows the obtained results.

As shown in the table, the conventional 3% Si electrical steel sheet showed a significant increase in iron loss by more than twice due to external compressive stress, whereas the steel sheets according to the present invention (sample Nos. 3 to 5) However, it has stopped at a slight rise (up to 14W / kg iron loss). It can also be seen that the steel sheet according to the present invention has a sufficiently low iron loss even when subjected to an external tensile stress, and remains at a maximum of 12 W / kg.

In the present invention, it is possible to obtain a low-carbon steel sheet having excellent high-frequency characteristics and less iron loss deterioration due to external stress. As a result, it is possible to obtain a high-frequency iron core with low iron loss, and thus it is possible to manufacture a highly energy efficient transformer and other electric devices.

Claims

Si: 1.0% by mass or less, C: 0.02 to 0.16% by mass, Mn: 0.3 to 2.0% by mass, P: 0.03% by mass or less and S: 0.01% by mass or less, and the balance is Fe and inevitable impurities. A plate thickness central layer which is a ferrite mixed structure containing one or more of pearlite phase, bainite phase and martensite phase, Si: 3 to 5 mass%, C: 0.02 to 0.16 mass%, Mn: It is a clad-type low-carbon steel plate comprising 0.3 to 2.0% by mass, P: 0.03% by mass or less and S: 0.01% by mass or less, the composition of the balance Fe and inevitable impurities, and a surface layer that is a ferrite single phase. A low carbon steel sheet, wherein the surface layer has an in-plane tensile stress of 70 to 160 MPa as an internal stress.
The low-carbon steel sheet according to claim 1, wherein the total thickness of the surface layer is 30 to 60% of the total sheet thickness.
The low-carbon steel sheet according to claim 1 or 2, wherein a thickness of the low-carbon steel sheet is 0.05 to 0.35 mm.
The sheet thickness center layer and the surface layer of the low-carbon steel plate are further Al: 0.002-0.6 mass%, Cr: 0.01-1.5 mass%, V: 0.0005-0.1 mass%, Ti: 0.0005-0.1 mass%, Nb: 0.0005- 2. One or more elements selected from 0.1% by mass, Zr: 0.0005 to 0.1% by mass, B: 0.0005 to 0.01% by mass and N: 0.002 to 0.01% by mass are included. 4. The low carbon steel plate according to any one of 3 to 3.
Heating a steel plate containing Si: 1.0% by mass or less, C: 0.02 to 0.16% by mass, Mn: 0.3 to 2.0% by mass, P: 0.03% by mass or less and S: 0.01% by mass or less, the balance being Fe and inevitable impurities Then, in the austenite region of 1050 to 1250 ° C., by reacting with a Si-based gas, a ferrite phase having a Si content of 3 to 5% by mass is formed on the surface layer of the steel sheet, and then Si in the steel is homogenized. A method for producing a low-carbon steel sheet, characterized by cooling before.
6. The low carbon steel sheet according to claim 5, wherein the Si-based gas is one or more gases selected from silicon tetrachloride, trichlorosilane, dichlorosilane, monosilane, and disilane. Production method.
The steel sheet is further made of Al: 0.002-0.6% by mass, Cr: 0.01-1.5% by mass, V: 0.0005-0.1% by mass, Ti: 0.0005-0.1% by mass, Nb: 0.0005-0.1% by mass, Zr: 0.0005- The low carbon according to claim 5 or 6, which contains one or more elements selected from 0.1% by mass, B: 0.0005 to 0.01% by mass and N: 0.002 to 0.01% by mass. A method of manufacturing a steel sheet.