JP2020175420A - Metal mold for punching, method for manufacture thereof, punching method and processed product - Google Patents

Abstract

To punch a magnetic steel sheet while suppressing degradation due to iron loss to minimum.SOLUTION: When punching a magnetic steel sheet, a tool tip is previously sharpened before punching with respect to a punch (male mold) and a die (female mold) as a tool to be fitted to a metal mold. The sharpened tool tip of both or one of the punch and the die is used for punching after ensuring a tool radius or a tool missing width to be 1.5 μm or less. Punching for minimizing a range of magnetic domain change is realized, whereby a punched product in which iron loss is suppressed to minimum, can be processed.SELECTED DRAWING: Figure 1

Description

The present invention relates to a die for punching, a manufacturing method thereof, a punching method, and a work piece (product), and more particularly to a processing technique suitable for punching an electromagnetic steel sheet used for an iron core of a motor or the like.

In recent years, there has been an urgent need to realize a society that suppresses global warming by controlling carbon dioxide emissions. In automobiles, mainly hybrid automobiles and electric automobiles, the adoption of motors and generators, weight reduction, and high performance are required.
An iron core for a motor (hereinafter referred to as a motor core) is manufactured by punching an electromagnetic steel sheet with a mold, laminating a plurality of sheets, and then caulking, or integrating by bonding or welding.
It is a well-known fact that in addition to the loss between plates during integration, plastic strain and elastic strain during punching in the punching process of electrical steel sheets cause iron loss.

As an effort to reduce iron loss in electrical steel sheet punching, first, as the cause is identified, as shown in the investigation shown in Non-Patent Document 1, the magnetic domain of the non-oriented electrical steel sheet changes after punching, and this magnetic domain change changes. Deterioration of the electrical steel sheet structure due to punching, such as causing hysteresis loss (history loss) and eddy current loss, leading to iron loss.
Further, as an effort to reduce iron loss in the punching process of electrical steel sheets, as shown in Patent Document 1, as a result of investigating the optimum value of the clearance, which is the gap between the punch and the die, with the thickness ratio of the electromagnetic steel sheet, the plate thickness ratio is 7. It is recommended that the clearance be less than%.

As a measure for reducing plastic strain in punching of an electromagnetic steel plate, as shown in Patent Document 2, a method of relaxing the concentration of shear stress by adding a shear angle to the punch, or as shown in Patent Document 3, punching is performed. After that, a method of cutting off only the processing affected portion by cutting processing (shaving processing) is disclosed.

Iron loss and change in magnetic domain structure of non-oriented electrical steel sheets due to shearing / Kunihiro Senda, Masayoshi Ishida, Yoichi Nakasu, Masaaki Yagi / Institute of Electrical Engineers of Japan A 125, No. 3, 2005/241-246 JP-A-2016-129902 JP-A-2014-207783 JP 2000-175414

FIG. 1 is a diagram showing a mold structure that is the basis of shearing.
Punching is a destructive method in which the work piece is sandwiched between a male punch and a female die, and the displacement and load of the press are transmitted to the tool to shear and break the work piece.
In order to keep the work material flat during punching, it is necessary to apply a plate pressing force from the stripper plate to the work material.
The stripper plate also has a role of peeling the punch from the work material after the punching process.

As shown in FIG. 2, a drool, a sheared surface, a fracture surface, and a burr are formed on the punched material to be processed. These are collectively called the cut surface.
The length of the cut surface changes depending on the clearance that is the gap between the punch and the die.
In general, the clearance is actually machined by selecting an appropriate value within the range where the wear of the tool and the length of the drool and sheared surface can meet the required quality.

Shear stress is applied to the tool being punched, and the shear stress is transmitted to the work material to promote deformation.
FIG. 3 is a diagram schematically showing the addition range of shear stress with a non-oriented electrical steel sheet having a plate thickness of 0.5 mm when the tool tip R of the punch is 10 μm, 3.0 μm, and 1.0 μm.
FIG. 3 qualitatively shows that the stress state applied to the work material changes only when the shape of the tool tip changes in units of micrometers.
It can be seen that under the condition that the tool tip R is sharpened to 1.0 μm, the shear stress is concentrated on the tool tip, and the range in which the stress is applied is concentrated in a narrower range than the other two conditions.

If the punching process is continuously performed in the actual processing, the tool is worn due to the reaction force and friction received from the work material and the adhesion of the work material during the work.
Tool wear changes the cut surface when the work piece is punched. When the burr becomes large and the required quality is no longer met, the tool reaches the end of its useful life and begins maintenance and replacement work.

Generally, in the punching process of a metal material, a large equivalent plastic strain is applied to the crystal grains located at the punched end portion of the material to be processed, and the processed affected portion generated by the distortion deformation or transformation of the crystal lattice remains.
In the punching process of electrical steel sheets, elastic strain and plastic strain are applied to the crystal grains due to the application of processing stress when the punch punches the electrical steel sheets.

When these two types of strains are applied to the crystal grains of the electrical steel sheet being punched, the magnetic domain of the electrical steel sheet changes (Non-Patent Document 1).
It is known that when the magnetic domain pattern width is expanded by adding strain, the moving speed of the domain wall increases and the eddy current loss increases. It is also well known that when dislocations occur inside the crystal grains, domain wall movement is hindered and hysteresis loss increases.
Therefore, in the shearing of electrical steel sheets, in addition to general clearance selection and control of cut surface length, the development of shearing conditions that reduce iron loss has been promoted.

So far, Non-Patent Document 1 has reported that elastic strain and plastic strain affect the change in magnetic domain in shearing of non-oriented electrical steel sheets.
Further, regarding the clearance selection in the punching process of the electromagnetic steel sheet, Patent Document 1 has shown that the clearance is 7% or less.
Patent Document 2 has shown a method of controlling the magnetic flux density by giving a punch an angle called a shear angle.
Further, Patent Document 3 discloses a method of removing the plastic strain influence range shown in Non-Patent Document 1 by a shaving method by providing a separate step.

Patent Document 1 pays attention to the clearance and the plate pressing force, and shows the processing conditions capable of reducing the iron loss.
However, although it is reported that the magnetic domain change range is affected by shear stress, the effect of the tool radius and tool missing width on the tool tip on the magnetic domain is not reported.
Therefore, it is considered that the optimum values of clearance and plate retainer may change depending on the shape of the tool tip.
So far, no investigation has been conducted on the influence of the shape of the tool tip in punching of electrical steel sheets on the magnetic domain change range and iron loss.

In Patent Document 2, since the shear angle is given in the tool manufacturing, the tool manufacturing process is increased. Also in Patent Document 3, the shearing process is increased to two processes.
For this reason, in Patent Document 2 and Patent Document 3, since machining is carried out in a complicated punch shape and a plurality of processes, a simple tool manufacturing method, a tool manufacturing method that can minimize iron loss in a single punching process, and a punching method. Development is shown as a new challenge.
However, the reality is that no measures have been taken to prevent iron damage with simple tools and construction methods.

As described above, in the current electrical steel sheet punching process, it can be said that the process conditions and construction methods for reducing iron loss have been developed from the optimization of the punching conditions, and have been put into practical use.
However, there are still issues to be solved for reducing iron loss, and there is still room for the development of simpler tools and machining conditions.

In view of the above situation, in view of the above situation, in order to limit the strain influence range at the time of punching of an electromagnetic steel sheet to a small size, a die for punching that can minimize iron loss of the punched product, a manufacturing method thereof, and punching. It is intended to provide a method.

In order to solve the above-mentioned problems and achieve the object, in the punching tool according to the present invention, the tool tips of one or both of the punch and the die are sharpened to 1.5 μm or less with the tip R or the tool missing width. It is characterized by securing.
Further, in the punching method according to the present invention, a tool having a tool tip R or a tool missing width of 1.5 μm or less is used, and one or a plurality of electromagnetic steel sheets are laminated and then punched. It is characterized by performing.
Further, by such a processing method, the workpiece according to the present invention has a vertical dimension corresponding to the plate thickness in the cross section of the punched hole and a horizontal dimension corresponding to the plate thickness toward the inside of the workpiece. In the substantially square region, the portion where the shape and dimensions of the magnetic domain changed before and after punching (the range affected by the magnetic domain) is less than 25%.

According to the present invention, in the punching process of one or a plurality of laminated electromagnetic steel sheets, the punching process that minimizes the magnetic domain change range is realized, so that the punching product can be processed with the minimum iron loss. It will be possible.

It is a structural drawing for demonstrating the structure and role of the punching die which concerns on one Embodiment of this invention. It is an electron micrograph which shows the cut end surface of a punched hole. It is a schematic diagram showing how the addition range of the shear stress applied to the work material changes according to the tool tip R at the time of punching of an electromagnetic steel sheet. It is a figure which shows the measurement position of the tool tip R when the shear angle is given to the punch. It is a figure which defines the tool tip missing width when the punch tip edge portion is not R shape. It is a figure which defines the tool tip missing width when the die tip edge portion is not R shape. It is an external photograph of the small servo press used in an Example. It is a chart which showed the diameter measurement result and clearance of a punch and a die used in an Example. It is an electron micrograph and the shape measurement result figure of the tip shape of a drooping punch used in an Example. It is an electron micrograph and the shape measurement result figure of the tip shape of a normal grinding punch used in an Example. It is an electron micrograph and the shape measurement result figure of the ion polishing punch tip shape used in an Example. It is a figure which shows the result of having observed the reflected electron image with an electron microscope, using the sample which was intermediate-stopped at the initial stage of processing as a cross-sectional sample in the plate thickness direction in an Example. In the example, it is the figure which superposed the analysis result in the EBSD IPF (Inverse pole figure) ND direction on the reflected electron image of FIG. In the example, it is the figure which superposed the analysis result of EBSD KAM (Kernel average misorientation) on the reflected electron image of FIG. It is a schematic diagram which shows the magnetic domain structure of the non-oriented electrical steel sheet before processing. In the example, it is a magnetic domain structure image which shows the magnetic domain structure of the drooling large punch intermediate stop sample of FIG. 12A. It is a schematic diagram which shows the magnetic domain structure of the intermediate stop material by a big punch. In the example, it is a magnetic domain structure image which shows the magnetic domain structure of the normal grinding punch intermediate stop sample of FIG. 12 (b). It is a schematic diagram which shows the magnetic domain structure of the intermediate stop material by a normal grinding punch. In the example, it is a magnetic domain structure image which shows the magnetic domain structure of the ion polishing punch intermediate stop sample of FIG. 12C. It is a schematic diagram which shows the magnetic domain structure of the intermediate stop material by an ion polishing punch. In the Example, it is a magnetic domain structure image which showed the magnetic domain structure of the cross section of a sample punched by using a large punch. In the Example, it is a magnetic domain structure image which showed the magnetic domain structure of the sample cross section punched by using a normal grinding punch. In the Example, it is the magnetic domain structure image which showed the magnetic domain structure of the sample cross section punched by using an ion polishing punch. It is a schematic diagram which shows the magnetic domain structure of the punched product by a large punch and a normal grinding punch. It is a schematic diagram which shows the magnetic domain structure of the punched product by an ion polishing punch. It is a schematic diagram for defining a comparison target area.

Hereinafter, a method of punching an electromagnetic steel sheet by a punch and a die in which the tool tip R or the tool missing width is sharpened to 1.5 μm or less, which is an embodiment of the present invention, will be described in detail with reference to FIGS. 1 to 26.
The present invention is not limited to this embodiment.

[Principle of invention]
The present invention keeps the tool tip shape of the tool for punching electrical steel sheets below a certain size, and when punching one to two or more pieces of electrical steel sheets, the strain influence range at the time of punching is limited to a small magnetic domain. It exerts the effect of limiting the range of influence and reduces the iron loss of the motor core.

[Construction of die for punching]
FIG. 1 shows a die 10 for punching according to the present invention, in which a punch 12, a die 14, a stripper plate 16, a punch plate 18, a back plate 20, a spring 22, and a die plate 24 are shown. And the guide post 26.

The lower end side of the guide post 26 is fixed to the die plate 24, and the upper end side thereof is inserted into the through holes formed in the stripper plate 16 and the punch plate 18.
Further, the lower end side of the punch 12 is inserted into the through hole of the stripper plate 16, and the upper end side thereof penetrates the punch plate 18 and is fixed to the back plate 20.
An electromagnetic steel plate as a work material 28 is arranged between the stripper plate 16 and the die plate 24.

Here, when a pressing force from a press machine (not shown) is applied to the punch plate 18, the punch 12 descends and punches the work piece 28 with the die 14.
At this time, the punch 12 can be peeled off from the work material 28 while the work material 28 is kept flat by the plate pressing force applied to the stripper plate 16.

The materials used for the punch 12 and the die 14 are not particularly limited, but the tool tip shapes of the punch tip edge portion and the die tip edge portion are ensured to be sharpened to 1.5 μm or less at the tool tip R, respectively.
That is, as shown in FIG. 4A, when the punch 12 is given a shear angle in advance, as shown in FIG. 4B, the R portion at the position where the shear angle at the bottom of the punch and the side surface of the punch intersect. Let be "tool tip R".

When the tool tip shape of the punch 12 is not R-shaped, as shown in FIG. 5, the width A in which the edge is missing on the punch bottom surface side and the width B in which the edge portion is missing on the punch side surface side are referred to as “tool tip”. Treat as "missing width".
In this case, the width A and the width B are ensured to be within 1.5 μm, respectively.

When the tool tip shape of the die 14 is not R-shaped, as shown in FIG. 6, the width C in which the edge is missing on the upper surface side of the die and the width D in which the edge portion is missing on the side surface side of the die are referred to as "tool tip missing width". Treat as.
In this case, the width C and the width D are ensured to be within 1.5 μm, respectively.

[Punching material and punching conditions]
The thickness and number of laminated electromagnetic steel sheets to be processed are not limited.
In addition, punching is performed without limiting the clearance, plate pressing force, and punching speed as punching conditions.

In order to demonstrate that the shape of the tool tip in punching of electrical steel sheets limits the range of influence of strain and the range of influence of magnetic domains, the effect was confirmed from the following experiments.
As the press machine, as shown in FIG. 7, a small servo press of 10 kN was used. A nanometer-precision digital clearance adjustment mold to which Japanese Patent No. 6240864 was applied was used as the mold.
This mold incorporates a piezo-type digital control stage directly under the die, and the clearance bias can be digitally adjusted with a resolution of 0.04 μm.
Therefore, punching can be realized while maintaining the clearance in nanometer units without bias.
Therefore, it is possible to reproduce the processing affected portion in which the crystal grains existing on the left and right of the punched hole are affected by the strain without bias.

Ultra-fine particle cemented carbide was used as the punch used in the experiment. Ultrafine particle cemented carbide was used as the die.
The punch diameter was set to φ0.6 mm, the clearance was set to 1.8%, and the die diameter was set to φ0.618 mm.

Three types of punch tip shapes were prepared.
The first is a "large punch" in which the tip of a normal normal grinding punch is ground with a diamond grindstone and the tip R is increased to about 8 μm.
The second is a "normal grinding punch" with a general grinding finish.
The third is an "ion polishing punch" in which a normal grinding punch is polished with argon ions.
The die has a normal grinding finish.

The diameters of the manufactured punch and die and the calculated clearance are shown in FIG.
The outer and inner diameters of the punch and die were measured with an ultra-high precision coordinate measuring device (UA3P-400L) manufactured by Panasonic (registered trademark).

The electron microscope results of the punch tip shape and the shape measurement results of the tip R are shown in FIGS. 9 to 11. The die shape measurement result is shown in FIG.
The tip shape of the punch and die was measured with an all-around three-dimensional measuring device (MLP-3) manufactured by Mitaka Kohki (registered trademark).

The punching experiment was carried out as a non-oriented electrical steel sheet with a thickness of 0.5 mm.
The above three types of punches were punched in order with one die.
The punching process was performed in the order of ion polishing punch, normal grinding punch, and large punch.
For each punch, a sample was prepared for 3 shots in which the work material first came into contact with the punch and then intermediately stopped at a position of about 0.1 mm.
After this, 10 shots were punched out.
The punching speed is 4.5 mm / s as measured.

The intermediate stopper and the sample punched out of the hole were mechanically polished to the center position of each hole to prepare a sample for cross-section observation.
After mechanical polishing, the sample surface was polished with argon ions to remove strain by mechanical polishing.

A reflected electron image was obtained by an electron microscope using the prepared cross-sectional observation sample, and then crystal orientation analysis was performed by electron backscatter diffraction (EBSD).

Subsequently, a magnetic field of ± 1000 O e was applied to the sample subjected to EBSD analysis by a Kerr effect microscope, and the magnetic domain image was acquired by dividing it into 23 steps.
The conditions for applying the magnetic field were that -1000Oe was first added, then the magnetic domain was inverted to + 1000Oe, and then the magnetic domain was inverted again to -1000Oe.

As for the magnetic domain contrast, the data of the two places where the contrast was the strongest were acquired.
Specifically, it is -233Oe data in the first half route with -1000Oe to + 1000Oe added, and + 233Oe in the second half route with + 1000Oe to -1000Oe added.

Since the acquired magnetic domain contrast is too thin to distinguish the magnetic domain structure, image processing was added to improve the distinguishability of the magnetic domain structure.
That is, photo editing software manufactured by Adobe (registered trademark) so that two images of -233Oe and + 233Oe are superimposed and the part where the magnetic domain is inverted is displayed in white and the part where the magnetic domain is not inverted as a domain wall is displayed in black. The difference image was acquired using Photoshop (registered trademark) Elements 2019.
This image difference was subjected to black-and-white shading enhancement processing at a ratio of 8/255 to obtain a magnetic domain structure image.

FIG. 12 is an SEM-reflected electron image of a sample whose punching process is intermediately stopped at a position 0.1 mm from the punch contact position.
The crystal grains of the non-directional electromagnetic steel plate processed by (a) large punch, (b) normal grinding punch, and (c) ion polishing punch are strained to the left and right of the line connecting the punch and die. It can be seen that there are crystal grains whose grain boundaries are difficult to see, and the processing-affected parts are generated in a band shape.

FIG. 13 is a diagram in which the results of EBSD analysis performed on the reflected electron image of FIG. 12 centered on the line connecting the punch tip and the die tip are superimposed.
EBSD analysis measured a range of 180 μm in the horizontal direction and 550 μm in the vertical direction at a pitch of 0.3 μm.
In the figure, the measurement results in the IPF ND direction are superimposed on the EBSD analysis results. The IPF diagram can display the crystal orientation and grain boundaries.

From the analysis results of EBSD IPF ND displayed in FIG. 13, the crystal orientation inside the crystal grains in the processing-affected part is determined by each of (a) large punch, (b) normal grinding punch, and (c) ion polishing punch. You can see that it is changing. Black grain boundaries extend linearly from the tip of the punch toward the inside of the work piece. This linear portion is a portion where dislocations have occurred and is a portion where considerable plastic strain has occurred.

FIG. 14 is a diagram in which the EBSD IPF ND direction analysis result of FIG. 13 is replaced with the KAM analysis result of EBSD.
KAM is a diagram showing the crystal orientation difference between the measurement points at six adjacent measurement points as an average value of angles.
In general, it is known that the measurement result of KAM corresponds to the equivalent plastic strain. From this figure, the difference was shown in each punch in the part shown in gray in the figure in which the crystal orientation difference was about 2 °.

That is, in the two punches (a) large punch and (b) normal grinding punch, the spread of the gray part is limited at the bottom of the punch tip, but in (c) the ion polishing punch, the area directly below the punch tip is black. It can be seen that in the portion where the crystal orientation difference of 5 ° occurs, the portion where the gray crystal orientation difference of about 2 ° extends below this portion extends vertically and horizontally in the figure.
The tip R of the normal punch shown in FIG. 10 was 1.68 μm on average on the left and right, and 1.18 μm in the ion polishing punch of FIG.
Although the R difference between the tips of the two punches is 0.5 μm, it was shown that a crystal orientation difference of about 2 ° occurs during punching, and the processing influence range to which a considerable plastic strain is applied is different.

Next, the change in magnetic domain due to punching will be considered.
FIG. 15 is a schematic view showing the magnetic domain structure of the non-oriented electrical steel sheet before processing, and it can be seen that the magnetic domains shown in white and the magnetic domains shown in gray are scattered in any direction in the figure. ..

FIG. 16 is a magnetic domain structure image showing the magnetic domain structure of the large punched product of FIG. 12 (a). The magnetization direction is the left-right direction in the figure.
From the comparison between FIGS. 12 (a) and 16 (a), magnetic domains were formed heterogeneously in the crystal grains of the portion (the right portion of FIG. 16) left after punching in which strain could not be confirmed in FIG. 12 (a). You can see that there is.
A complex magnetic domain structure is also observed on the punched material side on the left side of FIG.
The portion indicated by the white arrow in the figure is a portion where the magnetic domains are aligned in the vertical (vertical) direction in the figure, which is rotated by 90 ° with respect to the magnetization direction.
FIG. 17 is a schematic view showing the magnetic domain structure of the intermediate stop material by the large punch, and shows how the magnetic domain is generated in the vertical direction of the figure due to elastic strain.

In a large punch, the shear stress is considered to be widely dispersed rather than concentrated at the tip of the punch. It is reported in Non-Patent Document 1 that the magnetic domain of a non-oriented electrical steel sheet changes due to elastic strain.
In the intermediate stop sample of FIG. 16, shear stress is accumulated as elastic strain inside the sample, and the vertical streak-shaped magnetic domain indicated by the arrow in the figure may be caused by the elastic strain.
Since the vertical streak-shaped magnetic domain is rotated by 90 ° with respect to the magnetization direction, this portion becomes an obstacle to the movement of the domain wall, and it is considered that the iron loss increases.

FIG. 18 is a magnetic domain structure image showing the magnetic domain structure of the normal grinding punch intermediate stop processed product of FIG. 12 (b).
From the comparison between FIGS. 12 (b) and 18 (b), magnetic domains were formed heterogeneously in the crystal grains of the portion (the right portion of FIG. 18) left after punching in which strain could not be confirmed in FIG. 12 (b). You can see that there is.
A complex magnetic domain structure is also observed on the punched material side on the left side of FIG.
The portion indicated by the white arrow in the figure is a portion where the magnetic domains are aligned in the vertical direction in the figure, which is rotated by 90 ° with respect to the magnetization direction.

The vertical streaky magnetic domains of the large punched product in FIG. 16 also spread to the part left after punching on the right side of the figure, but in FIG. 18, the white arrows are concentrated on the material part to be punched on the left side in the figure. doing.
In FIG. 18, there is a region in which the magnetic domain indicated by the black-and-white striped arrow is enlarged in gray. This portion corresponds to the portion where the crystal orientation difference is 5 ° in FIG. 13 (b), and it is considered that the magnetic domain structure has changed due to the addition of equivalent plastic strain.
It is known that when a gray magnetic domain of the same color spreads, it becomes difficult to move the domain wall when the magnetic domain is inverted, and the eddy current loss increases.

FIG. 19 is a schematic view showing the magnetic domain structure of the intermediate stop material by the normal grinding punch, and shows that the magnetic domain generated in the vertical direction exists on the material side to be punched.
In addition, a magnetic domain whose magnetic domain range has been expanded due to the influence of plastic strain exists immediately below the punch tip.

FIG. 20 is a magnetic domain structure image showing the magnetic domain structure of the ion polishing punch intermediate stop processed product of FIG. 12 (c).
From the comparison between FIG. 12 (c) and FIG. 20, it can be seen that the magnetic domain structure is unevenly distributed in FIG. 20 as in the case of other punched products.
In FIG. 20, the vertical streak-shaped magnetic domains indicated by the white arrows remain at three locations, and it can be said that the range of influence is smaller than that of other punched products.
The extent of the gray magnetic domain indicated by the black-and-white striped arrow is wider than that of the normal grinding punched product of FIG. The spread of this gray magnetic domain corresponds to the portion where the crystal orientation difference in FIG. 14 (c) is widened to about 2 °, and it is considered that the magnetic domain structure changed in the portion where a considerable plastic strain was applied.

FIG. 21 is a schematic view showing the magnetic domain structure of the intermediate stop material by the ion polishing punch. The magnetic domain in the vertical direction due to the influence of elastic strain is smaller than that of the normal grinding punch product, and the magnetic domain range is expanded due to the influence of plastic strain. Has a wider range than the normal grinding punch.

From FIGS. 20 and 14 (c), in the ion polishing punch, the corresponding plastic strain is concentrated in the linear shape connecting the punch tip and the die tip, and the vertical streaks caused by the elastic strain indicated by the white arrow in FIG. 20. The influence of magnetic domains is also reduced.
This has the effect that the elastic strain and the equivalent plastic strain are concentrated in a line connecting the punch tip and the die tip due to the R average value of 1.18 μm at the tip of the ion polishing punch, which has the effect of minimizing the magnetic domain structural change. It can be said that there is.

FIG. 22 is a magnetic domain structure image of a sample that has been punched using a large punch.
The arrow portion shown by the black-and-white striped pattern in the figure is a crystal grain whose magnetic domain width is widened and is affected by processing.
The vertical streaks appearing in the intermediate-stopped sample of FIG. 16 have disappeared. It is considered that this is because in the observation range this time, the influence of the elastic strain was replaced by the change in the magnetic domain structure of the arrow part shown by the black and white striped pattern by the end of punching.
In FIG. 22, the magnetic domain structure changed significantly from the initial stage of shearing in most of the observation range, and showed the same tendency as the previously reported Non-Patent Document 1.

FIG. 23 is a magnetic domain structure image of a sample that has been punched using a normal grinding punch.
In the normal grinding punch punched product of FIG. 23, the magnetic domain structure inside the punched material is significantly changed from the magnetic domain structure at the time of the intermediate stop processing of FIG. 18, similar to the drool large punch punched product of FIG.
The magnetic domain structure of the normal grinding punch punched product, which is a normal construction method, showed the same tendency as that of the previously reported Non-Patent Document 1.

FIG. 24 is a magnetic domain structure image of a sample that has been punched using an ion polishing punch.
The magnetic domain structure of the ion polishing punch of FIG. 24 has not changed significantly from the magnetic domain structure of the intermediate stop processed product of FIG. 20.
In particular, the magnetic domain width expansion locations indicated by the black-and-white striped arrows are limited to two locations.
It can be said that it was demonstrated that the ion polishing punch of FIG. 24 does not change the magnetic domain structure as the punching process progresses with respect to the other two punches shown in FIGS. 22 and 23.

FIG. 25 is a schematic view showing the magnetic domain structure of a punched product by a large punch and a normal grinding punch, and the range shown in black and white in the figure shows the range in which the magnetic domain is changed by shearing.
As shown in the figure, if the magnetic domain length before shearing is L0 and the magnetic domain length of the portion where the magnetic domain is changed by shearing is L1, the relationship of L1> L0 is established.
In addition, the magnetic domain changed by shearing extends not only around the hole but also on the upper surface side (punch side) and the lower surface side (die side) of the hole in the range where L1> L0.

On the other hand, FIG. 26 is a schematic view showing the magnetic domain structure during shearing with an ion polishing punch. As shown in the drawing, in the embodiment in which ion polishing was performed and the punch tip R was 1.18 μm, the magnetic domain was sheared. There was almost no change before and after, and only a part of the magnetic domain width expanded due to plastic strain.

FIG. 27 is a schematic view showing a cross section of the work piece 30 after punching, and has a vertical dimension t1 corresponding to the plate thickness of the inner surface of the hole and a horizontal dimension t2 corresponding to the plate thickness toward the inside of the work piece. It defines a substantially square-shaped comparison target area S.

Here, from FIGS. 22, 23, and 25, the magnetic domains of the large-drip punch punched product and the normal grinding punch punched product have a portion (magnetic domain influence range) in which the length and width of the magnetic domain are extended and the shape is changed before and after punching. It can be seen that the comparison target area S extends to about 70%.
On the other hand, in the ion-polished punch punched products shown in FIGS. 24 and 26, the magnetic domain influence range is only about 5% in the comparison target region S.
That is, it can be seen that the ion-polished punch punched product can suppress the magnetic domain influence range to 1/10 or less of that of the punched product of a large punch or a normal grinding punch.

Although the present invention was developed for the purpose of reducing characteristic deterioration that occurs during punching of electrical steel sheets, the effect of reducing the range of influence due to strain during punching is widely applied to metal materials other than electrical steel sheets. Needless to say, it is possible, and therefore, the object to be processed is not limited to the electrical steel sheet, and it can be widely applied to the punching process of thin metal sheets.

10 Die for punching
12 punch
14 die
16 stripper plate
18 punch plate
20 back plate
22 spring
24 die plate
26 Guide post
28 Work material (electrical steel sheet)
30 Processed product S Comparison target area

Claims (5)

A die used for punching a thin metal plate, wherein at least one tool tip radius of a punch and a die or a tool missing width is set to 1.5 μm or less. A method for manufacturing a punching die, which comprises setting the tool tip radius or the tool missing width to 1.5 μm or less by performing ion polishing on the tips of punches and dies. A punching process method according to claim 1, wherein the punching process is performed after stacking one or a plurality of thin metal plates using the punching die. The punching method according to claim 3, wherein the thin metal plate is made of an electromagnetic steel plate. A work piece processed by the punching method according to claim 3 or 4, which corresponds to a vertical dimension corresponding to a plate thickness in a cross section of a punched hole and a plate thickness toward the inside of the work piece. A work piece characterized in that, in a substantially square region having horizontal dimensions, the portion where the shape and dimensions of magnetic domains change before and after punching is 25% or less.

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