KR20120112435A - Low-carbon steel sheet and process for producing same - Google Patents

Abstract

The low carbon steel sheet according to any one of claims 1 to 3, wherein the low carbon steel sheet comprises a plate thickness central layer of a ferrite mixed structure containing at least one of a pearlite phase, a bainite phase and a martensite phase having a Si content of 1.0 mass% % By weight, and an in-plane tensile stress of 70 to 160 MPa is applied to the surface layer as an internal stress, whereby a low-carbon steel sheet having excellent high-frequency characteristics and low iron loss- .

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a low-

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

The iron loss of the electromagnetic steel sheet is affected by the hysteresis loss strongly dependent on the precipitates in the steel, the crystal grain size, the aggregate structure and the like, as well as the plate thickness, the specific resistance, It is composed of strongly dependent eddy-current loss.

The general electric steel sheet increases the growth of the crystal grains by reducing impurities in the steel as much as possible, thereby reducing the hysteresis loss. In addition, by adding 0.5 to 3.5 mass% of Si to increase the specific resistance, or to reduce the thickness of the plate, the eddy current loss is reduced.

Here, since the hysteresis loss is proportional to the frequency and the eddy current is proportional to the square of the frequency, the hysteresis loss occupies a large proportion in the iron loss of the electromagnetic steel sheet at the commercial frequency (50/60 Hz) Or more, the ratio of the eddy current hand increases.

BACKGROUND ART [0002] In recent years, in the field of power electronics, high frequency switching of switching elements is progressing, and therefore, reduction of high frequency iron loss is strongly desired also for an electromagnetic steel sheet used as an iron core material such as a transformer, a reactor and a motor.

With respect to this demand, the eddy current loss can be reduced by setting the plate thickness of the electromagnetic steel sheet to 0.2 mm or less or increasing the Si to about 4 mass%. However, in the future, driving at a high frequency exceeding 10 kHz is also expected, and development of an innovative material that is not in line with the conventional development is required.

Conventionally, materials such as soft ferrite, metal compact, and amorphous have been used under such exciting conditions in the high frequency range. However, since ferrite has a low magnetic flux density, the iron core is enlarged. The amorphous iron has a low iron loss, but the building factor is significantly lower than that of an electromagnetic steel sheet. In addition, The powders are low in both magnetostriction and iron loss, but are expensive, and have a single-stage end, such as a low saturation magnetic flux density as compared with an electromagnetic steel sheet. For this reason, various studies have been made recently on reduction of high-frequency iron loss of an electromagnetic steel sheet.

As a means for reducing the high-frequency iron loss of an electromagnetic steel sheet, Patent Document 1 discloses a method of enriching the surface of a steel sheet by a siliconizing method.

This Si embedding technique is a process for increasing the Si concentration in steel by reacting a 3 mass% Si steel sheet having a plate thickness of 0.1 to 0.35 mm with silicon tetrachloride gas at a high temperature, as described in Patent Document 2 .

Further, as known from the past, the 6.5 mass% Si steel sheet has an intrinsic resistance twice as high as that of the 3 mass% Si steel sheet and can effectively reduce the eddy current loss. Therefore, the 6.5 mass% Si steel sheet is advantageous as a high- Since it is substantially zero, it exerts an excellent effect in reducing the noise of the iron core.

Patent Document 2 shows that sufficient magnetic properties can be obtained by adjusting the surface layer Si concentration even if the Si concentration is not made uniform in the thickness direction in view of shortening the diffusion time in the penetration process.

Patent Document 3 discloses a silicon steel sheet having a Si concentration gradient in the sheet thickness direction, in order to reduce high-frequency iron loss, the Si concentration difference (maximum-minimum) in the sheet thickness direction and the Si concentration in the surface layer and the Si concentration The difference is stipulated. In particular, there is a description that the lowest iron loss is obtained when the surface layer Si concentration is 6.5 mass%.

Patent Document 4 discloses that an electromagnetic steel sheet having a high Si content and a low Si concentration at the central portion of the sheet thickness can be obtained by sputtering an electromagnetic steel sheet in a ferrite phase, The steel sheet is described.

Patent Document 5 discloses a silicon steel sheet for motor having good workability and high frequency characteristics by forming a steel sheet having a high Si concentration in the surface layer by treating the low carbon steel in a temperature range of 900 to 1000 占 폚 with a small austenite phase have.

Japanese Patent Publication No. 6-45881 Japanese Patent Publication No. 5-49744 Japanese Patent Application Laid-Open No. 2005-240185 Japanese Patent Application Laid-Open No. 2009-263782 Japanese Patent Application Laid-Open No. 2000-328226

As described above, the iron loss is represented by the sum of the hysteresis loss and the eddy current loss, and it is known that the higher the excitation frequency is, the more the ratio of the eddy current to the total iron loss increases. As the resistivity of the material increases, the eddy current becomes difficult to flow. Therefore, a material having a high resistivity is used for a magnetic core for a high frequency.

Generally, in an electromagnetic steel sheet, since the specific resistance increases with an increase in the Si concentration, it is preferable that the high-frequency magnetic core material contains 3 mass% or more of Si.

On the other hand, as the Si concentration becomes higher, the steel tends to be hard and tends to break, making cold rolling difficult. Further, when the Si concentration is increased, transformation from austenite to ferrite (hereinafter referred to as? /? Transformation) does not occur at the time of cooling the slab, and ferrite intactly forms a coarse structure, And surface defects are likely to occur. Therefore, in an actual steel sheet manufacturing process, the Si concentration in the steel is set to an upper limit of 4 mass%.

According to Patent Document 2, the 6.5% by mass Si steel sheet has the highest magnetic permeability and small magnetostriction among the electromagnetic steel sheets. However, in order to use it as an iron core, it is necessary to perform secondary processing such as slit processing, punching processing, and bending processing. However, the 6.5 mass% Si steel sheet is more fragile as compared with the ordinary electric steel sheet, and therefore, in the secondary processing as described above, a high processing technique is required. Further, the Vickers hardness Hv of the 6.5 mass% Si steel sheet is about 390, which is very hard as compared with about Hv: 200 of the conventional electromagnetic steel sheet. For this reason, there is also a drawback that the press die is liable to become short-lived. Particularly, in the high-frequency application of 10 kHz or more, since the ratio of the eddy current loss increases rather than the hysteresis loss, other materials such as a compressed powder magnetic core having a low eddy current loss (high resistivity) .

According to Patent Document 2, a 6.5 mass% Si steel sheet can be produced by a sprinkling process in which a 3 mass% Si steel sheet is rolled to a final sheet thickness and then attached with silicon tetrachloride at a high temperature.

However, in order to actually use it as a core, a 6.5 mass% Si steel sheet after the penetration has to be slit, press or bend, and cracks and bending often occur.

In Patent Document 2, in FIG. 13, when the Si concentration is excessively uneven, the iron loss is greatly increased. On the other hand, when the Si concentration unevenness is suppressed to a certain degree or less, An example in which a low iron loss is obtained is described. However, in the case where the Si concentration is nonuniform, there is no case in which the Si concentration becomes lower than the uniform steel sheet. Further, specific numerical values are not described for the high-frequency iron loss.

Patent Document 3 describes a steel sheet having a Si concentration gradient in the sheet thickness direction as a material superior in high frequency characteristics to a 6.5 mass% Si steel sheet. Even if the Si concentration in the surface layer is high, since the Si concentration at the central portion in the thickness direction of the steel sheet is about 3 mass%, the steel sheet can be considered as an average of all the steel materials as a material of lower Si than the above 6.5 mass% Si steel sheet.

However, in this case, since the lower limit of the Si concentration in the steel is about 3% by mass, the? /? Transformation does not occur. Therefore, the steel sheet structure at the time of cooling to room temperature is composed of coarse ferrite particles that terminate the plate thickness, and there is a problem that cracks and fatigue easily occur when the slits or presses are performed.

According to Patent Document 4, because of the inherently coarse secondary recrystallized structure, even in the case of a grain-oriented electrical steel sheet having a large magnetic domain width and an excessive eddy current loss, stress distribution is imparted to the surface layer of the steel sheet, It is possible to effectively reduce hands. However, since a grain-oriented electromagnetic steel sheet is used for the material, the cost is relatively high compared with other high-frequency iron core materials. In order to reduce sufficient eddy currents, the grain-oriented electrical steel sheet is sprinkled from the surface layer so that the average Si concentration is increased to 4 mass% or more, and the surface Si concentration It was necessary to increase it to 5% by mass or more.

In Patent Document 5, a low-carbon steel having an austenite phase is subjected to a sprinkling treatment. However, when the sprinkling treatment is performed in a temperature range exceeding 1000 캜, cracks are generated at the interface of? /? And the penetration process is performed in a relatively low temperature range.

However, in order to reduce eddy currents, it is preferable to carry out a steeping treatment at 1000 DEG C or higher, and a steel sheet material and a method for producing a steel sheet which do not cause cracking even when a steeping treatment of 1000 DEG C or higher is performed at the above- . In addition, the steel material described in Patent Document 5 has a high surface layer Si concentration of 5 to 6.5 mass% and has a coarse secondary recrystallized structure, so that cracks and fatigue occur when slits or presses are produced Leaving many problems.

As the high-frequency magnetic core material, there are dust core, ferrite core of iron oxide powder and Fe-based amorphous alloy which are powder compacted with iron powder. They are characterized by low eddy current loss because of their large resistivity compared to 6.5 mass% Si steel sheets.

However, since the ferrite core has a low saturation magnetic flux density, the ferrite core is generally used for high frequency applications of several hundred kHz or more at low output. On the other hand, the dust cores and Fe-based amorphous alloys have a slightly lower saturation magnetic flux density as compared with the electromagnetic steel sheet, but have low eddy current loss, so they may be used in the same way as the electromagnetic steel sheet for high output high frequency applications.

However, all of the above-mentioned materials have a problem that the iron loss remarkably increases when external stress such as compression is applied.

As described above, magnetic materials having excellent high-frequency characteristics are often inferior in workability and are sensitive to external stresses. In particular, when compressive stress is applied, many iron losses are remarkably increased.

On the other hand, as a material having excellent workability, a low-carbon steel sheet widely used as a structural material or an exterior material can be mentioned. Further, the magnetic properties of the low carbon steel are not sensitive to external stress as much as a general magnetic material, and the iron loss is not remarkably increased even when the 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, its DC magnetic property is very bad. Therefore, the low-carbon steel sheet is rarely used for the magnetic core of the commercial frequency where the hysteresis loss is the main subject.

However, it is possible to obtain a magnetic core material excellent in low-frequency iron loss and low in iron loss, even with respect to external stress, if it can reduce the eddy current loss of the low carbon steel sheet and can reduce the increase in iron loss with respect to the compressive stress.

The present invention has been developed in view of the above phenomenon and aims to provide a low carbon steel sheet excellent in high frequency characteristics and having little deterioration in iron loss due to external stress as well as a method for producing the same.

According to Patent Document 3, the Si concentration gradient is formed in the sheet thickness direction in the penetration process, and the difference in Si concentration on the front and back surfaces of the steel sheet is controlled, thereby reducing eddy current hands.

This technique can also be applied to low-carbon steels in the same way, so that eddy currents can be reduced.

However, in the case of a low-carbon steel, a γ / α transformation occurs, so that when the steel is subjected to a steeping treatment in a high-temperature austenite phase region, transformation from a surface layer with an increased Si concentration to a ferrite phase occurs. At this time, since the Si concentration gap exists between the austenite phase of the low Si concentration and the ferrite phase of the high Si concentration, the Si concentration gradient at the abnormal phase interface becomes discontinuous. 1, the ferrite phase of the high Si concentration in the surface layer is not transformed, whereas the low-Si concentration austenite phase of the plate thickness center layer is in the pearlite phase, the bainite phase And a martensite phase.

That is, the Si concentration gradient is imparted to the low-carbon steel by the steeping treatment is significantly different from the technique described in Patent Document 3 in which the γ / α transformation does not occur.

Hereinafter, an experiment leading to completion of the present invention will be described. Table 1 shows the composition of four kinds of low carbon steels A to D used in this experiment.

In the same table, in the steel material indicated by the symbol (C)

Nitrogen gas annealed at 1200 ° C (no sparking) ... Steel treatment I

Si uniformly diffused after ignition at 1200 ° C (spurious + prolonged diffusion) ... Steel Treatment II

Incineration at 1200 ° C plus Si diffusion for 3 minutes (spurious + short-time diffusion) ... Steel Treatment III

To prepare samples to be used in this experiment.

Here, the steeping treatment of the steel treatments II and III was adjusted so that the sample mean Si concentration after treatment was 3 mass% Si. These samples were subjected to magnetization measurement by direct current and alternating current using a single plate measuring frame of 30 x 100 mm, and the iron loss under the conditions of the magnetic flux density of 0.05 T and the frequency of 20 kHz was measured by separating by the hysteresis loss and the eddy current , And compared the results.

The results of the sample No. (C) are shown in Fig. In the drawings, the iron loss values of the electromagnetic steel sheets (3 mass% Si steel sheet and 6.5 mass% Si steel sheet) having the same sheet thickness are also shown.

With respect to the sample of the steel treatment I, the sample of the steel treatment II has an eddy current loss due to an increase in resistivity due to the Si increase, and in the Si uniformization process, the γ / Since the ferrite structure is formed, the hysteresis loss is also reduced.

Also, the iron loss (hysteresis loss + eddy current loss) of the sample of the steel treatment II was large as compared with the iron loss of the electromagnetic steel sheet (3 mass% Si) having the same Si concentration. This is because even in the case of the same 3 mass% Si coarsely ferritic coarse texture, the C content is less than 50 ppm, whereas in the case of the low carbon steel, the C content is 500 ppm or more, I can think.

On the other hand, the iron loss of the sample of the steel treatment III surprisingly showed a lower iron loss than that of the electronic steel sheet of 3 mass% Si but also 6.5 mass% of Si, and particularly the eddy current loss was recognized to be reduced.

It was expected that the magnetic flux would be concentrated on the surface layer and the eddy current loss could be reduced by allocating the Si concentration gradient in the plate thickness direction. The effect of this case is that the Si concentration is made uniform It was estimated that the reduction of the eddy current by 2 to 3% could be achieved. In other words, the result of this experiment was an effect of reducing the eddy current loss of more than 50%, far exceeding the expectations.

Next, in order to investigate this phenomenon in more detail, four kinds of materials shown in Table 1 were used to make a specimen by sprinkling or the like under the conditions of the above-mentioned steel treatment III. These samples were examined for the structure on the longitudinal section, and iron loss was measured in the same manner as in the above experiment. The results are shown in Figs. 3 and 4. Fig.

Figs. 3 (a) to 3 (d) show cross-sectional photographs of samples after the steel treatments III were performed on the specimens No. (A) to (D), respectively.

In Fig. 3 (a), although the boundary between the surface layer and the plate thickness central layer is recognized, all of the ferrite single phase has a coarse texture. On the other hand, FIGS. 3 (b) and 3 (c) show a bainite structure, a pearlite structure, and a martensite structure which are visible when the low carbon steel is annealed at a temperature at which an austenite phase occurs, And a structure which is clearly different from the ferrite single phase structure of the surface layer is obtained. 3 (d) shows a martensite structure including a small amount of ferrite structure in the plate thickness central layer.

From Fig. 4, it was found that low iron loss exceeding that of the 6.5% by mass Si electronic steel sheet was not necessarily obtained in all of the steel materials subjected to the steel treatment III.

When the content of C is 200 ppm or more, when the content of Mn is 0.3 mass% or more, the effect of reducing the eddy current is remarkably exhibited, and at the same time, the electric steel sheet of 6.5 mass% Lt; RTI ID = 0.0 > of < / RTI >

In addition, although Sample D shows lower iron loss than a conventional electromagnetic steel sheet (3 mass% Si), its superiority tends to be lowered compared with Samples B and C.

Subsequently, plate warpage was observed by chemical polishing from the side of the surface of the sample to the center of the plate thickness. As a result, the warp was convex on the plate thickness center side. As a result, it can be seen that tensile and compressive stresses are generated in the surface layer before the removal by polishing.

Herein, in the present invention, the internal stress refers to the tensile stress acting on the surface (surface tension) when the original plate thickness is d (mm) and the curvature radius at the plate bending is r (mm) Internal tensile stress) = the compressive stress acting on the central portion of the plate thickness,

Plane tensile stress = E x d / (2r) [MPa] (E represents the Young's modulus of the steel sheet)

.

In addition, samples were prepared by changing the Si diffusion time in various ways under the conditions of the above-mentioned steel treatment III by using the materials of the compositions to be the samples A and C of Table 1. [ For these samples, the internal stress was measured by the above-described method, and eddy current hands were measured. The results are shown in Fig.

From Fig. 6, in the case of Sample C containing C of not less than 200 ppm and Mn of not less than 0.3% by mass, the internal stress after the penetration treatment tends to be large. In addition, in the range of the internal stress (in-plane tensile stress) of 70 to 160 MPa, the reduction of the eddy current hands is remarkable.

The cause of the above tendency is not clear at present, but the inventors assume as follows.

In the Fe-Si alloy, when the C amount in the steel is increased at the low carbon steel level, the γ / α boundary line on the Fe-Si phase diagram shifts to the high Si side, The Si concentration gap in the portion of the austenite phase increases. When the Si concentration gap is increased at high temperature, it is conceivable that an internal stress is generated between the central layer in which a phase transformation of? /? Occurs during cooling and which tends to expand and the ferrite phase of the surface layer which is not transformed already .

Further, since Mn is an element stabilizing the austenite phase, the effect of Mn addition is such that when the amount of Mn is increased, the phase transformation point of? /? Shifts to the low temperature side. Therefore, it can be considered that the internal stress generated at the time of cooling further increases.

Though the plate thickness center layer is a fine mixed structure and is in a state in which compressive stress is applied, the surface layer is coarse ferrite crystal grain and hardly magnetized, and since the surface layer is in a state of being subjected to tensile stress, have.

Therefore, when such a steel sheet is magnetized toward the in-plane direction of the plate, its magnetic flux concentrates on the surface layer, and consequently, the eddy current of the steel sheet is consequently considered to be lowered.

It was also found that, even if external stress is applied, the iron loss value does not rise if the sample has a large internal stress as in the above-described samples.

That is, even in the state of the external stressor, if an internal stress of about 70 to 160 MPa is generated as in the sample, even if compressive stress of about tens of MPa is applied from the outside, the tensile state of the surface layer is maintained. On the other hand, at the central portion of the plate thickness, the compressive stress is further increased, but the portion is hardly magnetized and the effect is very slight.

As a result, there is no change in the situation in which the magnetic flux is concentrated on the surface layer, and the effect of reducing the eddy current hand of the sample is not lost.

In addition, as described above, when annealing is performed at a high temperature for a long period of time to alleviate the Si concentration distribution of the steel sheet or to mitigate the internal stress, the effect of reducing the eddy current hand and the effect of preventing iron loss deterioration to external compressive stress are reduced , The superiority to an electromagnetic steel sheet having the same Si concentration is lost.

Accordingly, it was found that it is preferable to consider the time of the heat treatment including the diffusion time from the penetration treatment to the completion of the core.

The present invention is based on the above recognition.

That is, the structure of the present invention is as follows.

1. A ferritic stainless steel comprising: 1.0% by mass or less of Si; 0.02 to 0.16% by mass of C; 0.3 to 2.0% by mass of Mn; 0.03% , A ferrite-mixed center-layer which is a ferrite mixed structure containing at least one of a pearlite phase, a bainite phase and a martensite phase, and at least one of a Si: 3 to 5 mass%, a C: 0.02 to 0.16 mass% Carbon steel sheet having a surface layer of ferrite single phase and containing 0.3 to 2.0 mass%, P: 0.03 mass% or less and S: 0.01 mass% or less, and the balance of Fe and inevitable impurities, And the surface layer has an in-plane tensile stress of 70 to 160 MPa as an internal stress.

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 plate thickness.

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

4. The plate thickness of the low carbon steel sheet The steel sheet according to any one of claims 1 to 3, wherein the central layer and the surface layer further contain 0.002 to 0.6 mass% of Al, 0.01 to 1.5 mass% of Cr, 0.0005 to 0.1 mass% of V, 0.0005 to 0.1 mass% of Ti, : 0.0005 to 0.1 mass%, Zr: 0.0005 to 0.1 mass%, B: 0.0005 to 0.01 mass% and N: 0.002 to 0.01 mass%. Carbon steel sheet according to any one of < 1 >

5. A ferritic stainless steel comprising: 1.0% by mass or less of Si, 0.02 to 0.16% by mass of C, 0.3 to 2.0% by mass of Mn, 0.03% or less by mass of P and 0.01% or less of S by mass, wherein the balance Fe and inevitable impurities A steel sheet is heated to react with a Si-based gas in an austenite zone at 1050 to 1250 ° C to form a ferrite phase having an Si content of 3 to 5 mass% in the surface layer of the steel sheet, Wherein the Si is cooled before being homogenized.

6. The method for producing a low-carbon steel plate according to 5 above, wherein the Si-based gas is one or more gases selected from silicon tetrachloride, trichlorosilane, dichlorosilane, monosilane and disilane.

7. The steel sheet according to any one of claims 1 to 6, further comprising 0.002 to 0.6 mass% of Al, 0.01 to 1.5 mass% of Cr, 0.0005 to 0.1 mass% of V, 0.0005 to 0.1 mass% of Ti, 0.0005 to 0.1 mass% Carbon steel sheet according to the fifth or sixth aspect, wherein the steel sheet contains one or more elements selected from the group consisting of 0.0005 to 0.1 mass%, B: 0.0005 to 0.01 mass% and N: 0.002 to 0.01 mass% .

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

1 is a schematic view showing the structure of a low carbon steel of the present invention.
FIG. 2 is a graph showing iron loss values of low-carbon steels according to a method for treating irrigation.
Fig. 3 is a photograph of the structure at the cross section of the steel sheet after the steeping treatment for each steel sheet component composition.
4 is a graph showing the iron loss values after the impregnation treatment for each steel sheet component composition.
Fig. 5 is a view showing the measuring method of the internal stress (in-plane tensile stress) in the present invention. Fig.
6 is a diagram showing the relationship between internal stress (in-plane tensile stress) and eddy current hands.
Fig. 7 is a schematic view of a burning furnace suitable for use in the present invention. Fig.

(Mode for carrying out the invention)

Hereinafter, the present invention will be described in detail.

First, the reasons for limiting the structure, composition, and the like of the steel sheet will be described. The percentages in the composition of the steel sheet composition represent mass% unless otherwise specified.

In the present invention, as described above, it is important to further apply tensile stress to the surface layer with the increased resistivity.

Therefore, the steel sheet subjected to the austenite phase (hereinafter referred to as? Phase) at high temperature is subjected to a sprinkling treatment to increase the specific surface area of Si by increasing the specific resistance, and only the surface layer is referred to as a ferrite phase ), It is necessary to cool the steel in the steel before it is uniformized.

By the said cooling, the plate thickness center layer which consists of a ferrite mixed structure containing any 1 type, or 2 or more types of a pearlite phase, bainite phase, and martensite phase as shown in FIG. 1, and a ferrite single phase with high Si concentration are shown. It becomes the so-called "clad-type" three-layered steel plate which has a two-layer surface layer of the steel plate front and back surface which consists of structures. Since there is a difference in the Si concentration between each surface layer and the plate thickness central layer, internal stress due to the Si concentration gap is generated as described above, and tensile stress is added to the surface layer.

In the case of the ferrite single phase, the sheet thickness central layer contains a total of 30% (area%) or more of pearlite phase, bainite phase and martensite phase in view of not obtaining sufficient internal stress, It is preferable that it is substantially ferrite phase.

If the amount of Si in the surface layer is 3% or more, the magnetostriction takes a large positive value, so that when the tensile stress acts as described above, magnetization tends to occur due to the self-elastic effect. As a result, when the steel sheet is magnetized, the magnetic flux concentration to the surface layer is promoted, and the eddy current hand reduction effect becomes greater.

However, when the amount of surface layer Si exceeds 5%, on the contrary, the magnetostriction becomes small, the self-elastic effect by the tensile stress becomes small, and the surface becomes hard and the workability is lowered. Therefore, the amount of Si in the surface layer was 3 to 5%.

On the other hand, if the average value of the amount of Si in the plate thickness central layer exceeds 1.0%, the Si concentration difference with the above-mentioned surface layer becomes small and the internal stress of the steel sheet decreases, and the effect of reducing eddy currents becomes dim. Therefore, the Si concentration of the plate thickness central layer was 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 steel during steelmaking.

In the present invention, before the Si in the steel is uniformed, it means that Si of the surface layer increased due to penetration penetrates to the inside by sufficient diffusion, and before the Si concentration of the surface layer and the center layer becomes uniform.

Therefore, there is a Si concentration gradient in the surface layer portion toward the plate thickness depth (center) direction also in the plate thickness central layer portion, but this gradient is very small and can be almost neglected. Therefore, the surface layer Si concentration (amount) in the present invention means the average Si concentration (amount) in the surface layer portion. In addition, there may be a point-like or line-shaped carbide present on the surface layer. However, in this case, there is no particular problem, and the ferrite single phase can be substantially used.

The above-mentioned tensile stress of the surface layer needs to be an in-plane tensile stress of 70 to 160 MPa. That is, when the tensile stress of the surface layer is less than 70 MPa, there is a problem that the effect of reducing the eddy current hand becomes faint. On the other hand, when the tensile stress is more than 160 MPa, the hysteresis loss excessively increases, A problem arises. 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 is preferably about 30 to 60% of the total thickness of the steel sheet as a total of two layers. That is, when the total thickness of the steel sheet is less than 30%, the hysteresis loss becomes large. On the other hand, when it exceeds 60%, the effect of reducing the eddy current loss is faint and consequently the iron loss increases.

In addition, the surface layer is not necessarily the same in each of the two layers of the upper and lower layers, such as the thickness, the component composition, and the like, but is preferably the same.

The thickness of the steel sheet used in the present invention is preferably about 0.05 to 0.35 mm. That is, 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 is more than 0.35 mm, the eddy current loss increases and it is not suitable as a high-frequency magnetic core material. However, even if the thickness is not satisfied, the iron loss reduction effect of the present invention is not lost.

Hereinafter, the reasons for limiting the constituents of the surface layer and the plate thickness central layer of the steel sheet will be described. In addition, the remainder of the steel sheet component shown below is Fe and inevitable impurities.

As for the Si content, it is necessary to set the surface layer to 3 to 5% and the plate thickness central layer to 1.0% or less as described above, and for the other components, both the surface layer and the plate thickness center layer Common.

C: 0.02 to 0.16%

C is required to contain at least 0.02% as an element necessary for increasing the internal stress of the steel and obtaining sufficient eddy current reduction effect. On the other hand, if it exceeds 0.16%, cracks tend to occur at the interface between the surface layer and the plate thickness central layer. Therefore, C is limited to a 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 an iron loss lower than that of the 6.5% Si electronic steel sheet even at high frequencies.

Mn: 0.3 to 2.0%

Mn is an element necessary for obtaining a sufficient eddy current reducing effect, and it is required to contain Mn of at least 0.3%. On the other hand, if it exceeds 2.0%, the? -Phase easily remains in the center thickness of the steel sheet after cooling to room temperature, and the internal stress with the surface layer of the steel sheet is lowered. Therefore, Mn is limited to the range of 0.3 to 2.0%.

P: not more than 0.03%

P is an element to be embrittlement, since cracks tend to occur at the interface between the surface layer of the steel sheet and the center thickness of the plate layer, so it is preferable to reduce the maximum to 0.03%.

S: not more than 0.01%

S is an element causing hot brittleness. Since the productivity is lowered when the concentration is increased, it is preferable to reduce it as much as possible, but up to 0.01% is acceptable.

Although the basic components of the steel sheet have been described above, the present invention may also include one or more species selected from the following elements, which are common to both the surface layer and the plate thickness central layer .

Al: 0.002 to 0.6%

The addition of Al is an element effective for reducing the eddy current hand because it increases the intrinsic resistance. If the lower limit is less than the upper limit, the addition effect is insufficient. On the other hand, if the upper limit is exceeded, since the α phase is present at a high temperature before the precipitation, the clad steel sheet proposed by the present invention can not be produced.

Cr: 0.01 to 1.5%

The addition of Cr is an element effective for reducing the eddy current loss because it increases the intrinsic resistance. When the amount is less than the lower limit, the effect of addition is insufficient. On the other hand, when the upper limit is exceeded, carbides precipitated in the grain and grain boundaries become the starting point and brittle fracture tends 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 for reducing eddy currents, because the magnetic permeability is lowered by forming carbides and nitrides in the central portion of the plate thickness, thereby enhancing the magnetic flux concentration effect on the surface layer. If the lower limit is less than the lower limit, the effect of addition is insufficient. If the upper limit is exceeded, carbides and nitrides precipitated in the grain and in the grain boundaries become the starting point, and brittle fracture tends to occur.

B: 0.0005 to 0.01%, N: 0.002 to 0.01%

The addition of B and N enhances the quenching of the plate thickness central layer during the cooling process after the impregnation treatment, so that the magnetic permeability of the portion is lowered and the magnetic flux concentration effect on the surface layer is enhanced. If it is less than the lower limit, the effect of addition is insufficient, while if it exceeds the upper limit, it becomes easy to brittle.

Next, a suitable manufacturing method of the low carbon steel sheet of the present invention will be described.

The production method of the low carbon steel sheet before the impregnation treatment is not particularly limited, and all conventionally known methods can be suitably used. For example, the slab serving as the composition of the plate thickness central layer of the above-mentioned steel sheet is subjected to hot rolling after the heating, and cold rolling in which cold rolling or intermittent annealing is performed once or twice is repeated, It may be a steel plate having a plate thickness. Further, finish annealing may be performed as necessary.

The steel sheet thus obtained is subjected to a sprinkling treatment to increase the Si concentration in the surface layer. However, after forming a ferrite phase having an Si content of 3 to 5% on the surface layer of the steel sheet and cooling the steel in the steel before the Si is homogenized, The low carbon steel sheet of the present invention can be produced.

As a method for infiltrating (impinging) Si, all conventionally known methods can be applied. For example, vapor-phase immersion method, liquid phase immersion method, solid immersion method and the like can be given. The Si-based gas to be used at this time is not particularly limited, but may be a silane gas such as one or more gases selected from among silicon tetrachloride, trichlorosilane, dichlorosilane, monosilane and disilane .

Hereinafter, a method of infiltrating Si by the vapor phase silencing method will be described.

In the case of the vapor phase deposition method, the temperature history (the temperature of each zone in the furnace and the residence time of the steel sheet) from when the Si-based reaction gas is sufficiently supplied to the end of the burning, When determined, the Si concentration distribution in the plate thickness direction is determined almost uniquely corresponding to the plate thickness and the Si addition amount (needle acceptance amount).

As the needle bed used in the present invention, all conventionally known ones can be suitably used, and for example, facilities having the structure shown in Fig. 7 can be mentioned.

In the present invention, when the low carbon steel sheet having the Si concentration of 1% or less is subjected to the impregnation treatment, it is preferable that the impregnation treatment is carried out under the condition satisfying the following formula 1, particularly, the Si concentration distribution in which the high- .

Equation ^{1: 1.3 × 10 -4 ≤ (} Σt k × exp (-25000 / T k)) / (d 2 × [mass% Si] add) ≤2.2 × 10 -4

Here, T _k is, after the start of chimgyu treated with the temperature of each zone of the steel sheet is passed, t _k is the residence time of the steel plate at each zone, d is the thickness (㎜), [mass% Si ] add the (Amount of increase in Si average concentration in the sheet thickness direction) added to the steel sheet during the steeping treatment.

Further, in the present invention, when the furnace inner temperature changes, it can be regarded as a heat treatment at a constant temperature and a constant time such that the value of Σt _k × exp (-25000 / T _k ) becomes equal. For the value of the example, when the at 1200 ℃ to 700 ℃ cooling for 5 _{minutes, Σt k × exp (-25000 /} T k) ≒ 1.9 × 10 -6 , and is, if t _k a constant 1200 ℃ 45 seconds. Therefore, it can be considered that the cooling is the same as the heat treatment at 1200 ° C for 45 seconds.

Even when the lower limit of the value of the above expression is smaller than 1.3 x 10 < ^-4 >, it is possible to make the Si concentration distribution appropriate by performing the subsequent step such as the deformation-correcting annealing at a relatively high temperature. However, if it is smaller than 1.3 × 10 ^-4 , the Si concentration in the surface layer becomes too high in practice, and as a result, deformation of the steel sheet occurs during the steeping treatment, or cracks or bending It is preferable that the above value is satisfied.

On the other hand, when the upper limit value of the above formula is larger than 2.2 x 10 < ^{-4 &} gt ;, the internal stress is relaxed and the eddy current hand reducing effect is lowered.

Further, in the case where the continuous line is subjected to the penetration treatment, the Si concentration of the steel sheet does not change at a realistic time when the temperature is 700 캜 or lower, so that the calculation of the formula 1 may be up to 700 캜.

The intoxication treatment temperature in the present invention is 1050 to 1250 캜. That is, if the temperature is lower than 1050 占 폚, there is a possibility that internal stress is not sufficiently generated at the time of cooling. If the temperature is higher than 1250 占 폚, the surface layer having a high Si concentration becomes semi-molten during the steeping treatment, There is.

The low carbon steel sheet subjected to the penetration treatment is subjected to a drying and burn in process after the insulating film is applied.

When passing through the above-described process, if the heat treatment is performed at a temperature lower than 600 캜, the stress relaxation of the steel sheet does not occur and the high-frequency iron loss does not rise. However, when heat treatment is performed at 600 占 폚 or higher, the internal stress is relaxed with time, so that the high-frequency iron loss thereof is increased.

Therefore, the optimum thermal history was examined in the case of performing the heat treatment in the range of 600 to 800 deg. As a result, it was confirmed that the iron loss was lower than that of the uniform material having the same Si concentration under the condition that the following equation 2 was satisfied. Therefore, when the heat treatment is performed in the range of 600 to 800 deg. C, it is preferable to use the thermal history satisfying the following expression (2).

Equation _{2: (Σt 'k × exp} (-25000 / T' k)) / (d 2 × [mass% Si] add) ≤0.2 × 10 -4

Here, T _'k is the temperature of each heat treatment step of the steel sheet passes after chimgyu processing, t' _k is the residence time of the steel strip in each annealing process, d is the thickness (㎜), [mass% Si ] add the (Amount of increase in Si average concentration in the sheet thickness direction) added to the steel sheet during the steeping treatment.

As in the case of Equation 1, when the in-furnace temperature changes, heat treatment is performed at a constant temperature and for a predetermined time such that the value of Σt ' _k × exp (-25000 / T' _k ) Can be considered.

The low carbon steel sheet subjected to the irregularity treatment is assembled as an iron core through various processing steps such as slit, shearing, press, and the like, but at that time, the deformed calibration annealing may be performed. Also in this case, since the internal stress is relaxed by annealing at 600 占 폚 or more, it is preferable to set the deformation-calibrated annealing temperature and time so as to satisfy the above-described formula (2).

When the drying and burning of the insulating film is performed at 400 ° C or higher and deformation calibration annealing is performed after the fabrication, the temperature and the time are set so as to satisfy the above formula 2 in total by the heat treatment process of the film and the deformation calibration annealing process .

From the above, it is possible to set the manufacturing conditions in consideration of the time of the heat treatment performed until completion of the magnetic core.

(Example)

≪ Example 1 >

The sample having the composition shown in Table 2 was rolled to a thickness of 0.2 mm and then heated to 1200 캜 and subjected to a 3% Si impregnation treatment and a Si diffusion treatment for 3 minutes in a SiCl ₄ + N ₂ atmosphere And then 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 concentration of the surface layer and the plate thickness center layer.

As shown in the table, all of the inventive examples (Nos. 3 to 5 and 7) obtained according to the present invention are found to 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 compressive stress of ± 50 MPa in parallel with the magnetization direction to investigate changes in iron loss. These high frequency iron loss was measured by the Epstein test method (JIS C 2550).

The obtained results are shown in Table 4.

As shown in the table, the conventional 3% Si electrical steel sheet shows a remarkable increase in the iron loss by more than two times due to the external compressive stress, whereas the steel sheet (Sample Nos. 3 to 5) according to the present invention exhibits a slight rise (Iron loss of 14 W / kg at the maximum). The steel sheet according to the present invention is sufficiently low in iron loss even when subjected to an external tensile stress, and it can be seen that the steel sheet is still at a maximum of 12 W / kg.

In the present invention, it is possible to obtain a low-carbon steel sheet excellent in high-frequency characteristics and having little deterioration in iron loss due to external stress. As a result, it is possible to obtain a high-frequency iron core with a small iron loss, thereby making it possible to manufacture a transformer and other devices having high energy efficiency.

Claims (7)

And a balance of Fe and inevitable impurities, wherein the composition contains at most 1.0 mass% of Si, 0.02 to 0.16 mass% of C, 0.3 to 2.0 mass% of Mn, 0.03 mass% or less of P and 0.01 mass% or less of S, A plate thickness central layer which is a ferrite mixed structure containing at least one of a pearlite phase, a bainite phase and a martensite phase, and a laminated body composed of 3 to 5 mass% of Si, 0.02 to 0.16 mass% of C, : 0.3 to 2.0 mass%, P: 0.03 mass% or less, and S: 0.01 mass% or less, and the balance Fe and inevitable impurities, wherein the low carbon steel sheet is a clad- And the surface layer has an in-plane tensile stress of 70 to 160 MPa as an internal stress. The method of claim 1,
And the total thickness of the surface layer is 30 to 60% of the total plate thickness. The method according to claim 1 or 2,
Wherein the low-carbon steel sheet has a thickness of 0.05 to 0.35 mm. 4. The method according to any one of claims 1 to 3,
The steel sheet according to any one of claims 1 to 3, wherein the low-carbon steel sheet has a plate-thickness central layer and a surface layer of at least one selected from the group consisting of Al: 0.002 to 0.6 mass%, Cr: 0.01 to 1.5 mass%, V: 0.0005 to 0.1 mass%, Ti: 0.0005 to 0.1 mass% By mass of Ti, 0.0005 to 0.1% by mass of Zr, 0.0005 to 0.01% by mass of B, and 0.002 to 0.01% by mass of B, based on the total mass of the low carbon steel sheet. A steel sheet comprising Si: not more than 1.0 mass%, C: 0.02 to 0.16 mass%, Mn: 0.3 to 2.0 mass%, P: not more than 0.03 mass%, and S: not more than 0.01 mass%, and the balance Fe and inevitable impurities The steel sheet is heated and reacted with an Si-based gas at a temperature of 1050 to 1250 占 폚 to form a ferrite phase having an Si content of 3 to 5 mass% in the surface layer of the steel sheet, And cooling the steel sheet. The method of claim 5,
Wherein the Si-based gas is one or more gases selected from silicon tetrachloride, trichlorosilane, dichlorosilane, monosilane, and disilane. The method according to claim 5 or 6,
Wherein the steel sheet further contains 0.002 to 0.6 mass% of Al, 0.01 to 1.5 mass% of Cr, 0.0005 to 0.1 mass% of V, 0.0005 to 0.1 mass% of Ti, 0.0005 to 0.1 mass% of Zr, 0.0005 to 0.1 mass% To 0.1 mass% of B, 0.0005 to 0.01 mass% of B, and 0.002 to 0.01 mass% of N, based on the total mass of the low-carbon steel sheet.

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