JP4384451B2 - Oriented electrical steel sheet with excellent magnetic properties and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention provides a grain-oriented electrical steel sheet that can be used for a wound iron core while maintaining excellent magnetic properties even after strain relief annealing by forming grooves and a melt-resolidified layer by laser processing on the surface of the grain-oriented electrical steel sheet. It relates to the manufacturing method.
[0002]
[Prior art]
The grain oriented electrical steel sheet is required to reduce iron loss from the viewpoint of energy saving. As a method for this, a method of subdividing magnetic domains by laser irradiation has already been disclosed in Japanese Patent Publication No. 58-26405. In this method, the stress loss is introduced into the grain-oriented electrical steel sheet by the reaction force of the thermal shock wave generated by irradiating the laser beam, and the iron loss is reduced by subdividing the magnetic domain. However, this method has a problem that the strain introduced by laser irradiation disappears during annealing, and the magnetic domain refinement effect is lost. Therefore, the grain-oriented electrical steel sheet manufactured by this method can be used for a laminated iron core transformer that does not require strain relief annealing, but cannot be used for a wound iron core transformer that requires strain relief annealing.
[0003]
Therefore, as a method for improving the iron loss of grain-oriented electrical steel sheets that has the effect of reducing the iron loss value even after strain relief annealing, grooves are formed on the steel sheet surface, and magnetic domains are subdivided using the permeability change in the air gaps. Various methods have been proposed. For example, Patent Document 1 discloses a method of pressing a steel sheet with a tooth profile roll to form grooves on the surface of the steel sheet, Patent Document 2 discloses a method of forming grooves by chemical etching, and Patent Document 3 discloses A method of forming a point-row groove on a steel sheet surface with a Q-switched CO ₂ laser is disclosed. Patent Documents 4 and 5 describe a method in which a melt resolidified layer is formed on the surface of a steel sheet by a laser instead of a groove. Patent Document 4 forms a thin melt resolidified layer in a groove and a groove lower layer by a laser. And the like.
[0004]
[Patent Document 1]
Japanese Patent Publication No. 63-44804 [Patent Document 2]
US Pat. No. 4,750,949 [Patent Document 3]
JP-A-7-220913 [Patent Document 4]
Japanese Patent Application No. 10-284034 [Patent Document 5]
Japanese Patent Application Laid-Open No. 6-212275
[Problems to be solved by the invention]
Among the prior arts described above, the mechanical method using a tooth profile roll has a problem that the tooth profile is worn in a short period of time due to the high hardness of the magnetic steel sheet, and as a result, the maintenance frequency is high. The method by chemical etching does not have a problem that the tooth profile is worn, but requires a masking, etching process, and mask removing process, and there is a problem that the process becomes complicated as compared with the mechanical method. The method of forming a point-sequence groove in a steel plate with a Q-switched CO ₂ laser does not have a problem that the tooth profile is worn out and the process becomes complicated because a recess is formed in a non-contact manner. There is a problem that a Q switch device needs to be added separately. In addition, the simple groove formation method reduces the magnetic permeability in the gap and causes a decrease in magnetic flux density and a decrease in the space factor (the ratio of steel sheets in the laminated iron cores), thereby improving the transformer performance. There is a problem to lower. The method for forming the melt-resolidified layer eliminates the decrease in the space factor because there is no volume reduction due to the grooves, but the iron loss improvement effect tends to be inferior to the groove forming method. In addition, the method of forming a thin melt-resolidified layer in the groove and the lower layer of the groove with a laser can be expected to have a synergistic effect between the groove and the melt-resolidified layer. In the same manner, there is a problem that the magnetic flux density is deteriorated and the space factor is lowered.
[0006]
Therefore, the problem to be solved by the present invention is to provide a grain-oriented electrical steel sheet and a manufacturing method that have iron loss improvement equal to or greater than that of groove formation and minimize deterioration of magnetic flux density and space factor. There is to do.
[0007]
[Means for Solving the Problems]
The present invention provides a grain-oriented electrical steel sheet having improved iron loss characteristics by forming a linear groove and a convex melt-resolidified layer at the center of the groove at a constant period on the surface of the steel sheet. In the grain-oriented electrical steel sheet, the groove depth is d ₁ and the height of the cross section of the melt-resolidified layer at the center of the groove is d ₂ .
10 ≦ d ₁ ≦ 40 μm
0.5 ≦ d ₂ / d ₁ ≦ 1.0
The present invention provides a method of manufacturing a grain-oriented electrical steel sheet, forming grooves with molten resolidified layers of the laser beam by scanning irradiation, and its irradiation conditions, the irradiation power of the continuous wave laser beam P (W), when the diameter of the focused beam in the beam scanning direction is dc (mm), the focused beam area is S (mm ² ), and the beam scanning linear velocity is Vc (mm / sec), the beam passage time Tc (sec) ) Is defined as Tc = dc / Vc, and the irradiation energy density Up (mJ / mm ² ) is defined as Up = (P / S) × Tc, where Tc and Up are within the following ranges (however, the laser beam is irradiated) A method of manufacturing a grain-oriented electrical steel sheet, except that an assist gas is supplied to a machining point to be processed) .
0.2 ≦ Up ≦ 10 J / mm ²
1 ≦ Tc ≦ 400μsec
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The inventors of the present invention have studied in detail the laser irradiation conditions, the cross-sectional shape of the groove and the melt-resolidified layer, and the magnetic characteristics in the technology for improving the iron loss by forming the groove and the molten layer by laser beam scanning irradiation processing. Went. As a result, it has been found that, in a specific cross-sectional shape, the iron loss improvement rate is maximized, and the magnetic flux density deterioration and the space factor deterioration are minimized. In order to realize this, a new processing method using the melting and re-solidification phenomenon by laser processing was devised. The present invention will be described below with reference to examples.
[0009]
First, the shapes and effects of the groove and the melt-resolidified layer of the present invention will be described. FIG. 1 is a schematic cross-sectional view of a groove and a melt-resolidified layer according to the present invention. 2, 3 and 4 are cross-sectional schematic views of grooves or melt-resolidified layers in the prior art. The shape of only the groove in FIG. 2 is formed by etching or a tooth profile roll. The shape of FIG. 3 and FIG. 4 in which the molten layer is present in the lower layer and the molten layer without the groove are, for example, a laser beam. Formed by scanning irradiation. The feature of the cross-sectional shape in the present invention is that the convex portion of the melted and resolidified layer is present at substantially the center of the groove. The shape is the depth d _{1 of the} groove, the height d ₂ of the convex portion, and the total width H of the groove. Characterized. On the other hand, the shapes of FIGS. 2 to 4 which are conventional techniques are also characterized by d ₁ , H, or the thickness d ₃ of the molten layer.
[0010]
The present inventors examined in detail the relationship between the above parameters, the iron loss improvement rate η, the magnetic flux density deterioration amount ΔB8, and the space factor γ. Here, η is an improvement rate of iron loss W _17/50 (W / kg), and is defined by the following equation.
[0011]
η (%) = (groove, iron loss before forming molten layer−groove, iron loss after forming molten layer) / groove, iron loss groove before forming molten layer, iron loss after forming molten layer is 4 at 800 ° C. It is a measured value after performing time-relief annealing. W _17/50 is the iron loss when the frequency is 50 Hz and the maximum magnetic flux density is 1.7 T.
[0012]
ΔB8 is the amount of decrease in the magnetic flux density before and after the formation of the groove and molten layer, and is defined by the following equation.
[0013]
ΔB8 (Gauss) = groove, B8 before formation of molten layer-groove, B8 after formation of molten layer
Here, B8 is defined as the magnetic flux density that can be generated at a magnetic field of 800 A / m. The higher this value, the higher the crystal orientation of the grain-oriented electrical steel sheet, and the higher magnetic flux can be easily obtained. .
[0014]
The space factor γ is the ratio of the steel sheet in the laminated iron cores. The mass m (g) of the iron core and the density D ( g / cm ³ ), length L (cm), width b (cm), and iron core thickness h (cm).
[0015]
γ (%) = m / (b × L × h × D)
FIG. 5 shows the experimental results obtained by investigating the relationship between the depth d ₁ and d ₃ of the groove and the molten layer, or d, which is the total value thereof, and the iron loss improvement rate η in the prior art and the present invention. Here, d in the case where only the groove is formed by etching or the like in the product of the present invention (FIG. 1) and the conventional product (FIG. 2) corresponds to the groove depth d ₁ . In the case where a groove and a molten layer are formed with a conventional product (FIG. 3), d is the sum of d ₁ and d ₃ . Further, when only the molten layer is formed (FIG. 4), d corresponds to d ₃ . The groove or the molten layer is formed on both surfaces of the steel plate in parallel to the direction substantially perpendicular to the rolling direction, and the interval is 3 mm. The depth d is the depth per side. The height d ₂ of the convex portion at the center of the groove, which is a feature of the product of the present invention, was fixed at d ₂ / d ₁ = 0.5 by controlling the laser irradiation conditions described later. Groove depth d ₁ is present in two places, but the deeper is defined as d _1.
[0016]
From the results of FIG. 5, the dependence of η on d is almost the same in any case, and a high improvement rate is obtained at 10 ≦ d ≦ 40 μm regardless of the method. It is considered that the mechanism of iron loss improvement by the air gap is that magnetic poles are generated on the groove wall surface due to the difference in permeability between air and the steel sheet, and the magnetic domains are subdivided. Therefore, it is presumed that the depth d, which is a component perpendicular to the rolling direction, has a great influence on the magnetic pole generation amount, and the results shown in FIG. 5 support this.
[0017]
Here, when d is small, it is considered that sufficient iron loss improvement effect cannot be obtained from the above consideration. On the other hand, if d becomes too large, the cross-sectional area of the steel plate through which the magnetic flux passes decreases. As a result, the effective magnetic flux density per unit area in the groove increases. W17 / 50 is the value obtained by dividing the maximum magnetic flux generated by the average cross-sectional area of the steel plate, that is, the iron loss when the magnetic flux density is 1.7T, but the magnetic flux density is small because the cross-sectional area is small in the part with the groove. Is estimated to be higher than 1.7T. Generally, the higher the generated magnetic flux density, the higher the iron loss. Therefore, the iron loss near the groove portion increases relatively. As a result, if the groove becomes too deep, the effect of the local iron loss increase due to the excessive decrease in the cross-sectional area of the steel sheet becomes larger than the effect of iron loss improvement by magnetic domain refinement, and the region where d in FIG. It is thought that this led to a decrease in the iron loss improvement rate. As described above, the basic knowledge has been obtained that there is an optimum range for d and the range is 10 ≦ d ≦ 40 μm.
[0018]
When paying attention to the absolute value of the improvement rate, in the case of the cross-sectional shape of the present invention, a slightly higher iron loss improvement rate was seen at the same depth d than in the case of the groove formed by etching or the like. This is presumed to cause some magnetic domain control effect because residual stress is generated when the melted portion at the center of the groove resolidifies. Moreover, a higher iron loss improvement rate was obtained compared to the case of only the molten layer. The iron loss improvement effect by the molten layer is considered to be the root of the magnetic domain subdivision due to the residual stress and the change in magnetic permeability of the melted part. It is thought that there is.
[0019]
Next, the magnetic flux density deterioration amount ΔB8 was examined. B8 is an index of how much magnetic flux can be generated with a magnetic field of 800 A / m. Here, when the cross-sectional area of the steel sheet decreases, it is considered that the magnetic flux generation resistance penetrating therethrough increases. That is, as the groove depth d increases, B8 decreases. Furthermore, since the magnetic flux generation direction coincides with the rolling direction, it is presumed that the magnetic flux generation resistance further increases when the groove width H increases and the portion with a small cross-sectional area becomes longer in the rolling direction. Therefore, the dependence of ΔB8 on d in various methods was examined for ΔB8. The height d ₂ of the convex portion at the center of the groove, which is a feature of the product of the present invention, was fixed at d ₂ / d ₁ = 0.5 by controlling the laser irradiation conditions described later.
[0020]
FIG. 6 shows the result of examining the relationship between the depth d and ΔB8 with the width H fixed at 0.2 mm. From this, it can be seen that in the conventional grooved product, ΔB8 greatly increases as d increases. In the method using a molten layer, ΔB8 is hardly seen because basically no groove is formed. On the other hand, in the product of the present invention, ΔB8 tends to increase in the groove depth as in the conventional product, but the effective width H is considered to be narrow because the convex portion of the molten layer exists in the center of the groove. ΔB8 is smaller than the conventional groove product.
[0021]
Next, the present invention and the conventional product were compared with respect to the space factor γ. Here, the space factor was examined with all products fixed at H = 200 μm and d = 20 μm. The height d ₂ at the center of the groove of the present invention was fixed at d ₂ / d ₁ = 0.5. FIG. 7 shows the comparison results of the space factor. In the case of only the molten layer, the highest space factor can be obtained because there is no volume reduction of the steel sheet. The conventional groove product has a reduced space factor due to the effect of volume removal. In the method of the present invention, the effective removal volume is small due to the effect of the convex portion, and as a result, it has been found that the space factor decrease is relatively smaller than that of the conventional groove product.
[0022]
Next, the optimum range of the center melt resolidified layer height d ₂ in the groove shape of the present invention will be described using the space factor γ. In the product of the present invention, the proportion of the convex portion at the center of the groove in the groove depth is significant. So we evaluated the relationship between d ₂ and d ₁ ratio d ₂ / d ₁ and the space factor gamma. As shown in FIG. 8, in the range where d ₂ / d ₁ exceeds 0.5, γ shows a relatively high value, but when it exceeds 1.0, γ greatly decreases. This is a case where the melting degree of condensation is intense and the surface is convex beyond the groove depth. In this case, since space is generated between the laminated steel plates, the space factor is greatly reduced. From these results, the optimum range of d ₂ / d ₁ is _{0.5 <d 2 / d 1 <} 1.0.
[0023]
Next, a method for producing the product of the present invention will be described. A feature of the product of the present invention is that it has a convex portion at the center of the groove. With regard to the method for forming this convex portion, the present inventors pay attention to the convex portion formation that occurs during the re-solidification process of the melted portion in the high-speed beam scanning processing method of continuous wave laser light, and the desired method by optimizing the laser irradiation conditions. It has been found that the groove depth and the convex portion height can be obtained. The optimum laser conditions for the present invention will be described below with reference to FIG. 9 and formulas (1) to (3). FIG. 9 is a schematic view of laser beam irradiation.
[0024]
Ip = P / S = P / (π / 4 × dl × dc) Formula (1)
Tc = dc / Vc (2)
Up = Ip × Tc (3)
It is considered that the groove formation mechanism is that the steel sheet temperature exceeds the melting point and further exceeds the boiling point and evaporates and scatters. Therefore, this is a processing phenomenon that requires a very high power density locally. Therefore, it is considered that the groove formation depends on the spatial power density Ip (W / mm ² ) obtained by dividing the laser power by the irradiation beam area. However, when the beam moves at a high speed, it is necessary to consider the influence of the processing time due to the movement, that is, the beam passage time. Therefore, the local energy density Up (mJ / mm ² ) is defined as a laser parameter, which is a parameter that affects the groove depth. Up is defined as the total energy density irradiated by the beam having the power density Ip during the transit time Tc (s) determined by the scanning direction beam diameter dc (mm) and the scanning speed Vc (mm / s). Here, dl is a focused beam diameter in a direction orthogonal to the beam scanning direction.
[0025]
Next, the convex portion at the center of the groove is generated in the condensation process when the metal melted by the laser irradiation is cooled after passing through the beam. Therefore, it is considered that the shape of the convex portion is determined by the melting and condensation. That is, Tc has the greatest influence on the convex portion. As a tendency, a groove cannot be formed by rapid heating / cooling, or solidification occurs in the groove faster than the thermal relaxation rate to the peripheral portion, so that the unevenness of the protrusion is intense and a partially high protrusion is generated. When the heating / cooling rate is low, relatively slow solidification occurs due to thermal relaxation to the peripheral portion, and as a result, there is a tendency that a relatively flat molten portion is formed.
[0026]
Next, specific optimum ranges of Up and Tc obtained from the experiment are shown. First, when Up is 0.2 J / mm ² or less, that is, when Ip and / or Tc are very small, the groove loss is not formed at all, and the iron loss improvement rate is not excellent. On the other hand, when Up exceeds 10 J / mm ² , that is, when Ip and / or Tc are very large, the groove becomes deep, and in particular, ΔB8 and the decrease in the space factor are large. Therefore, the range of Up is optimally 0.2 to 10 J / mm ² .
[0027]
The optimal range of Up is determined mainly from the viewpoint of the groove depth, but the range of Tc is determined in order to further optimize the height of the convex portion. 10A, 10B, and 10C are schematic views of cross-sectional shapes depending on Tc. In the region where Tc is 1 μs or less, the solidification rate is very fast, so there is no thermal relaxation to the peripheral part, and the rapid solidification of the convex part occurs inside the groove, and the convex part is formed beyond the groove depth d ₁ and beyond the steel plate surface. Is done. As a result, d ₂ / d ₁ > 1 and the space factor is greatly reduced. On the other hand, in the range of Tc> 400 μs, the solidification process is very gradual, and thermal relaxation with the peripheral part occurs, so that, for example, a flat melted part as shown in FIG.
[0028]
From the above embodiments, the optimum method for forming the groove / molten layer of the present invention is to collect the continuous wave laser beam in a circle or ellipse, as shown in FIG. 9, equations (1) to (3). Irradiation parameters Up and Tc are in the following ranges.
0.2 ≦ Up ≦ 10J / mm ²
1 ≦ Tc ≦ 400μs
[0029]
【The invention's effect】
As explained above, a high iron loss improvement rate is obtained by the cross-sectional shape of the groove and the convex portion of the molten re-solidified metal at the center of the groove in the present invention, and the deterioration of the magnetic flux density and the space factor is minimized. There is an advantage that it can be suppressed. In the present invention, the formation of the groove and the convex portion of the melt-resolidified steel sheet in the center of the groove was effective on one or both surfaces of the steel sheet surface. In the product manufacturing method of the present invention, an excellent grain-oriented electrical steel sheet can be manufactured by performing high-speed scanning processing of a laser beam in the condition range of the present invention.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of a groove and a melt-resolidified layer of a grain-oriented electrical steel sheet according to the present invention.
FIG. 2 is a schematic cross-sectional view of only a groove according to the prior art.
FIG. 3 is a schematic cross-sectional view of a groove and a melt-resolidified layer according to the prior art.
FIG. 4 is a schematic cross-sectional view of only a melt-resolidified layer, which is a conventional technique.
FIG. 5 is a relationship diagram of a groove or melt layer depth d and an iron loss improvement rate η.
FIG. 6 is a relationship diagram between a groove or melt layer depth d and a magnetic flux density deterioration amount ΔB8 when the width H is fixed.
FIG. 7 is a comparison diagram of the space factor between the present invention and a conventional example.
FIG. 8 is a relationship diagram between d ₂ / d ₁ and the space factor in the present invention.
FIG. 9 is an explanatory diagram of laser beam irradiation parameters of the present invention.
FIGS. 10A, 10B, and 10C are correlation schematic views of a laser beam passage time Tc and a groove / molten layer cross section formed in the present invention.
[Explanation of symbols]
d ₁ ... groove depth d ₂ ... convex height d ₃ ... melted portion depth d ... total groove depth and / or melted portion depth H ... width in the rolling direction of the groove or melted portion dl ... irradiation beam Diameter dc in rolling direction ... Diameter P in irradiation plate width (beam scanning direction) P ... Laser power S ... Irradiation beam area Ip ... Irradiation power density Up ... Irradiation energy density Tc ... Beam passage time η ... Iron loss improvement rate ΔB8 ... Magnetic flux density decrease γ ... Space factor

Claims (2)

In a grain-oriented electrical steel sheet having improved iron loss characteristics by forming a linear groove and a convex melt-resolidified layer at the center inside the groove at a constant period on the surface of the steel sheet, A grain-oriented electrical steel sheet characterized by satisfying all of the following conditions, where the depth is d ₁ and the height of the cross-section of the melt-resolidified layer at the center of the groove is d ₂ .
10 ≦ d1 ≦ 40 μm
0.5 ≦ d2 / d1 ≦ 1.0 The method of manufacturing a grain-oriented electrical steel sheet according to claim 1, in a condition that the laser beam is scanned with a groove and the molten resolidified layer, the irradiation power of the continuous wave laser beam P (W), the condenser When the beam scanning direction diameter of the beam is dc (mm), the focused beam area is S (mm ² ), and the beam scanning linear velocity is Vc (mm / sec), the beam passing time Tc (sec) is Tc = dc / Vc, irradiation energy density Up (mJ / mm ² ) is defined as Up = (P / S) × Tc, and Tc and Up are in the following ranges, respectively (however, assist gas is applied to the processing point where the laser beam is irradiated) In a grain-oriented electrical steel sheet.
0.2 ≦ Up ≦ 10 J / mm ²
1 ≦ Tc ≦ 400μsec

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