JPH0211865B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetizer for magnetic flaw detection of strips.

As is well known, magnetic flaw detection is a method for detecting defects by magnetizing a material to be inspected using direct current or alternating current, and detecting magnetic flux leaking from a defective portion of the material to be inspected. In the magnetic flaw detection, the quality of the magnetization means of the material to be inspected is so important that it determines the flaw detection accuracy, and the conventional magnetic flaw detection is as follows.

That is, as shown in FIG. 1a, the test material 2 is placed between electrodes 1 arranged opposite each other, and the test material 2 is directly magnetized for flaw detection, or as shown in b,
A method of flaw detection by passing the test material 2 through the space of the cylindrical coil 3 is adopted, and flaw detection is performed on steel bars and pipes.

In addition, as shown in FIG. 2a, a magnetizing device in which a coil 5 is provided on a U-shaped core 4, or a magnetic device in which a coil 5 is provided in a leg portion of a U-shaped core 4 as shown in FIG. Flaw detection is performed on the welded parts 6 of steel and the corners of steel pieces.

By the way, recently, surface-treated steel sheets such as tinplate, stain-free steel sheets, and cold-rolled steel sheets are used for various purposes and are subjected to severe processing, so it is difficult to detect even minute defects. , it is necessary to ensure quality and improve quality. In order to detect minute defects in the strip, it is essential to create a uniform magnetic field with high magnetic flux density within the strip, which is parallel to the plate surface.
It is necessary to develop a magnetizer for magnetic flaw detection that forms such a magnetic field. In conventional magnetization devices for magnetic flaw detection, the magnetic flux is higher directly under or near the core than elsewhere, and lower elsewhere, so it is not suitable for flaw detection of thin steel plates such as strips.

Some studies have been made regarding strip magnetic flaw detection, and a strip magnetic flaw detection apparatus has been proposed, for example, in Japanese Patent Laid-Open No. 55271/1983. This is a magnetizing device in which a ferromagnetic yoke is provided with a center leg and an outer leg, a conductor array is provided between the center leg and the outer leg, and
The magnetizing devices are placed opposite each other with a strip in between. In this case, since the center leg and the outer leg of the ferromagnetic yoke are opposed to each other, the magnetic flux density between the legs is greater than elsewhere, which deteriorates the uniformity of the magnetic field in the horizontal direction of the strip surface. For this reason, the area of the portion where a uniform magnetic field is formed becomes narrow, and a large flaw detection area per scan cannot be obtained.

In addition, as a strip magnetic flaw detection device, the third
As shown in the figure, there is one in which an air-core magnetized coil 7 is provided inside a hollow roll (not shown) of non-magnetic material.
Flaws are detected by wrapping a strip around a hollow roll. The strength distribution of the magnetic field of this magnetic flaw detection device is strong almost at the center in the length direction of the air-core magnetized coil 7;
It becomes weaker as it approaches both ends.

By the way, in this Figure 3, 8-1, 8-
Reference numeral 2 denotes a lead-out portion of the coil, and points 9 and 10 correspond to both ends of a line parallel to the cylindrical centerline drawn on the outside of the circumferential surface of the coil. Reference numeral 11 indicates the direction of a magnetic field when a voltage is applied to the coil lead-out portions 8-1 and 8-2 and a direct current flows. This follows Fleming's right-hand rule. Now, Figure 3b shows the strength of the magnetic field in the axial direction on the coil center axis at this time,
This figure shows the strength of the magnetic field in the axial direction on the axis connecting both ends 9 and 10 on the outside of the coil circumferential surface. 13 is the strength of the magnetic field on the central axis of the coil, and 14 is the strength of the magnetic field on the axis connecting both ends 9 and 10 on the outside of the circumferential surface of the coil. The strength 13 of the magnetic field on the coil center axis is always in the positive direction, is strong at the midpoint between points 9 and 10, and becomes weaker as it approaches both ends. On the other hand, the strength 14 of the magnetic field on the axis connecting both ends 9 and 10 on the outside of the coil circumferential surface takes a negative value between points 9 and 10,
In the vicinity of points 9 and 10, it takes a somewhat large negative value, and as it moves further outward, it becomes a positive value, increasing once and then decreasing.
Therefore, in this case, a uniform magnetic field can only be obtained in a narrow range on the outer surface of the air-core magnetized coil 7. It is also weak in strength, and requires an extremely large amount of current to flow through the coil.

In view of these circumstances, the present inventors have developed a strip with a uniform magnetic field strength distribution over a wide range, a high magnetic field strength, which can detect even minute defects in the strip with high accuracy, and a high flaw detection efficiency. A number of studies were conducted with the aim of developing a magnetization device for magnetic flaw detection. As a result, a magnetizer is constructed in which the coil is wound around a cylindrical iron core, and D/L between the diameter D of the cylindrical iron core and the length L of the cylindrical iron core around which the coil is wound is 0.06 or more and 0.6 or less, By providing a magnetizing device for magnetic flaw detection of strips, which is characterized in that the magnetizer is installed in a hollow roll on which the strip is wound and passed, the strength is uniform on the outer surface of the magnetizer in the cylindrical direction. It has been found that a magnetic field is generated and the magnetic field strength is large, making it suitable for detecting defects in strips.

The present invention has been made based on this knowledge, and will be described in detail below with reference to the drawings.

FIG. 4 shows a magnetization device for magnetic flaw detection according to one embodiment of the present invention. In the drawing, 15 is a hollow roll, and inside this, a cylindrical iron core 16,
A magnetizer 18 formed by winding a coil 17 around the outer periphery of the cylindrical iron core 16 is inserted. Reference numeral 19 denotes a rotating shaft, and a power supply cable (not shown) runs through the center of the shaft 19, and is adapted to supply power to the coil 17 via a slip ring. S
is a strip of the material to be tested, and the hollow roll 1
5 and threaded. 20 is strip S
This is a detector for detecting magnetic leakage due to internal inclusions or surface flaws, and has a built-in searching coil or magnetically sensitive semiconductor.
It is designed to detect the entire width of the area. 2
1 is a guide roller.

Now, the magnetizer 18 will be described in detail with reference to FIG. 5. In order to use a magnetizer 18 that can perform magnetic flaw detection on the strip S with high precision and ensure a wide flaw detection area, it is necessary to make the strength distribution of the magnetic field uniform in the cylindrical direction on the outer surface of the magnetizer 18. It is desirable that the strength of the magnetic field be large. For this reason, the present inventors have created various magnetizers in which the diameter of the cylindrical iron core and the length of the cylindrical iron core around which the coil is wound are changed, and the strength distribution of the magnetic field in the cylindrical direction on the outer surface of the magnetizer 18 has been determined. investigated. Figure 6 shows the results, where the horizontal axis is the ratio D/L of the diameter D of the cylindrical iron core 16 and the length L of the coil wound around the cylindrical iron core 16, and the vertical axis is the ratio of the magnetic field. It is the distortion rate.

Note that the definition of the strain rate of the magnetic field will be described with reference to FIG. 22 shows the distribution of magnetic field strength,
23 is the length obtained by cutting 5% of each end of the cylindrical iron core length, and the maximum value of the magnetic field within that range is 24,
When the minimum value is 25, the distortion rate of the magnetic field is defined as the following equation.

Distortion rate of magnetic field = (maximum value - minimum value) / maximum value Now, in magnetic flaw detection of a strip, the strength distribution of the magnetic field in the horizontal direction where the magnetizer 18 faces the strip via the hollow roll 15 is uniform. You need to be like that. For this reason, since the purpose is met if the strain rate of the magnetic field is within 5%, the ratio D/ L was found to be between 0.06 and 0.6.

When the D/L is less than 0.06, when a current is passed through the coil 17 and a magnetic field is generated in the magnetizer 18, the center of the magnetizer 18 is magnetically saturated and the magnetic field at the center does not become strong. The magnetic field strength distribution at this time is 8th
As shown by [A] in the figure, the magnetic field distribution becomes non-uniform in the axial direction (horizontal direction) of the magnetizer 18.

On the other hand, when D/L exceeds 0.6, a strong magnetic field can be generated even at the center of the magnetizer 18, but on the other hand, the demagnetizing field becomes strong and the magnetic field strength at the ends decreases, as shown in [B] in FIG. The magnetic field distribution in the axial direction of the magnetizer 18 also becomes non-uniform. [C] in Figure 8 shows that D/L is
The magnetic field distribution in the axial direction of the magnetizer 18 is shown between 0.06 and 0.6.

The magnetic field distribution in Figure 8 is based on the fact that a strong magnetic field is required to detect defects in strips.
This is the distribution when the magnetic field strength at 20 mm above the surface of the magnetizer 18 (corresponding to the strip contact position) is targeted to be 400 Oe (Oersted).

Therefore, in the magnetizer 18 of the present invention, the ratio D/L between the diameter D of the cylindrical iron core 16 and the length L of the coil winding is set to be 0.06 or more and 0.6 or less. This magnetizer 18 is attached to the hollow roll 1 as shown in FIG.
When the strip S is inserted into the tube 5 and tested for flaws, the strip S is magnetized uniformly over its entire width, and the entire width is detected with high accuracy.

Next, examples will be shown.

A magnetizer was manufactured by winding a coil around a cylindrical iron core with a diameter D of 276 mmφ and a body length of 1100 mm. The length L of the cylindrical iron core around which this coil was wound is 1060 mm.
and D/L was set to 0.260. The magnetizer is placed in a hollow stainless steel roll with a diameter of 315 mmφ and a length of 1340 mm, and a strip is wound around the hollow roll as shown in Figure 4, and while passing through the coil, an exciting current is applied to the coil. It was magnetized by flowing 20A. At this time, the magnetic velocity density of the magnetic field in the width direction (horizontal magnetic field) of the strip in contact with the hollow roll was measured, and the results are shown in FIG. As shown in the figure, the magnetic field was uniform in the width direction of the strip, and its value was high, proving that strip flaw detection could be performed with high accuracy over the entire width.

Although the vertical magnetic field distribution is also shown, the magnetic field strength in the thickness direction of the strip does not affect the strip defect detection and can be ignored.

[Brief explanation of drawings]

Fig. 1 and Fig. 2 are diagrams showing conventional magnetic flaw detection, Fig. 3 is a diagram showing another conventional magnetic flaw detection, Fig. 4 is a cross-sectional view of an embodiment of the present invention, and Fig. 5 is a diagram showing the present invention. FIG. 6 is a side view of a magnetic flaw detection coil in one embodiment, and FIG. FIG. 7 is a diagram for explaining the definition of the strain rate of the magnetic field. FIG. 8 is a diagram for explaining the influence of the ratio D/L of the diameter D of the cylindrical iron core and the length L of the coil wound on the magnetic field strength distribution in the length direction of the magnetizer body.
FIG. 9 is a diagram showing the measurement results of the magnetic flux density of the magnetic field in the width direction of the strip in contact with the hollow roll in one embodiment. S... Strip, 15... Hollow roll, 16
... Cylindrical iron core, 17 ... Coil, 18 ... Magnetic flaw detection coil, 19 ... Rotating shaft, 20 ... Detector, 21
...Guidance roller.

Claims (1)

[Claims] 1. Constructing a magnetizer in which a coil is wound around a cylindrical iron core, and the ratio D/L of the diameter D of the cylindrical iron core and the length L of the cylindrical iron core around which the coil is wound is 0.06 or more and 0.6 or less,
A magnetizing device for magnetic flaw detection of a strip, characterized in that the magnetizer is provided in a hollow roll on which the strip is wound and passed.