JPH08178902A - Inspection method for abnormal microstructure defects on the surface of ultra low carbon steel sheet - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a method for inspecting abnormal structure defects on the surface layer of ultra-low carbon steel sheet. [Structure] C: During the steel plate manufacturing process of a cold rolled steel plate or a galvanized steel plate manufactured using an ultra low carbon steel of 0.01% or less, a part of the surface of the steel plate is carburized and the steel plate has a fine crystal structure. In a method of nondestructive inspection of a steel sheet having an abnormal abnormal structure defect portion, a magnetic saturation type eddy current sensor placed close to the steel sheet surface is scanned in the width direction of the steel sheet, and flaw detection is performed at a flaw detection frequency of 30 to 150 kHz. This makes it possible to detect the vertical streak defects of the steel sheet online.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for inspecting an abnormal structure defect portion of the surface layer of an ultra low carbon steel sheet, and more particularly to a method for inspecting the quality of a cold rolled steel sheet or a galvanized steel sheet produced from an ultra low carbon steel. .

[0002]

2. Description of the Related Art C: When cold-rolled steel sheet or galvanized steel sheet of 0.01% or less is pressed, the width of the steel sheet is from 3 mm
Irregularity defects of about 10 μm at 40 mm may occur continuously or intermittently along the rolling direction. This uneven streak pattern is a so-called vertical streak defect (or a striped defect). When such a vertical stripe defect occurs, the uneven portion can be visually identified as a vertical stripe shape, and therefore cannot be used for a steel sheet product that is aesthetically problematic.

Further, vertical stripe defects tend to occur continuously or intermittently by 10% or more in the longitudinal direction of the coil of the steel sheet. The cause of the vertical stripe defect is that somewhere in the course of the steel sheet manufacturing process, carbon is traced to an extremely small amount (about 10 to 20 ppm) on the surface of the steel sheet.
) It is presumed that due to the difference in the work deformability between the normal part and the abnormal part during press working, it becomes uneven due to carburization and generation of a fine crystal structure with a finer crystal structure than the surrounding normal part. .

Since the vertical streak defects cannot be identified at all by just looking at the steel sheet before press working and cannot be identified because the cause in the steel sheet manufacturing process is an extremely small amount of carbon source, measures for quality assurance of the steel sheet are taken. There was no method, and there was a longing for establishment of a new inspection method. The methods used so far are:
(Coil width) x (50 mm ~ 100 mm) strip-shaped test pieces were sampled from both ends of the coil, and after applying elongation strain of several% in the coil width direction, the front and back surfaces of the test pieces were ground with a grindstone.
There is a destructive test method of inspecting the presence or absence of uneven portions, that is, the occurrence of vertical stripe defects.

As other nondestructive methods, ultrasonic flaw detection and magnetic flaw detection have been proposed as ideas, but there is no method that has reached a level that can be detected in a laboratory.

[0006]

By the way, in the case of the above-mentioned destructive test method of sampling from both ends of the coil, pulling, and grinding and polishing, it is effective only when the abnormal portion exists at the coil end. is there. However, this abnormal portion may exist only inside the coil or may be discontinuous even at the coil end, and the end sampling method alone is not sufficient.

Therefore, a method of inspecting by cutting the coil every several meters can be considered, but it is effective in the case of a sheet-sized steel plate, but even if it takes a lot of time and effort, extremely limited portions at both ends of the sheet are considered. Can only be inspected, not the entire sheet. In addition, of course, there is no means and method for inspecting the inside of the coil in the steel sheet, and even if an abnormal portion is detected, the only method is to make the entire coil defective, and the yield will be poor. There's a problem.

The present invention solves the above-mentioned problems of the prior art, and provides a technique for detecting an abnormal portion where vertical stripe defects occur in a steel sheet over the entire length and width of the coil by using a nondestructive method online. With the goal.

[0009]

DISCLOSURE OF THE INVENTION According to the present invention, a part of the surface of a steel sheet is carburized during a steel sheet manufacturing process of a cold-rolled steel sheet or a galvanized steel sheet produced by using C: 0.01% or less of ultra-low carbon steel. Then, in the method of non-destructive inspection of a steel sheet having an abnormal structure defect portion where the steel sheet crystal structure of that portion is refined, a magnetic saturation type eddy current sensor arranged close to the steel sheet surface is scanned in the width direction of the steel sheet. A method for inspecting an abnormal structure defect portion of a surface layer of an ultra-low carbon steel sheet, which comprises detecting an abnormal structure part by detecting a defect at a detection frequency of 30 to 150 kHz.

[0010]

In order to solve the above-mentioned problems, the present invention is intended to detect a vertical streak defect abnormal portion of a steel sheet online by a nondestructive method over the entire length and width of the coil by using the eddy current flaw detection technique. is there. In other words, the vertical streak defect part has extremely slight carburization and refinement of crystal structure only in the pole surface layer within about 200 μm from the surface of the steel sheet, and it is highly sensitive and has high resolution to detect it. A detection method is needed. An attempt was made to measure the crystal grain size by ultrasonic flaw detection, which is often used as a non-destructive method, but the S / N for evaluating the detection sensitivity was extremely poor. Also, in the test using the leakage magnetic flaw detection method, a large number of noise peaks of unknown cause appear in the normal part, and it appears stronger than the vertical streak defect signal.
It turned out to be unsuitable for detection.

As a result of detailed examination of the detection method by the eddy current flaw detection method, it was found that the defect output becomes remarkably strong when the eddy current sensor is scanned in the width direction of the steel sheet. Sufficient sensitivity was not obtained by fixing the eddy current sensor at a fixed position or by scanning in the longitudinal direction of the coil. It was also found that applying a DC magnetic field to the steel sheet in the sensor detection portion to magnetically saturate it is effective as a means for removing the base noise in the normal portion of the steel sheet. Furthermore, it was found that the flaw detection frequency in the range of 30 to 150 kHz was good for both output and S / N, and detection performance was insufficient at higher or lower frequencies.

Therefore, according to the present invention, a magnetic saturation type eddy current sensor is arranged close to the surface of a steel sheet on a steel sheet production line, and the sensor is scanned in the width direction of the steel sheet at a flaw detection frequency of 30 to 150 kHz. By doing so, it is possible to completely detect the vertical stripe defect portion of the steel sheet.

[0013]

Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 schematically shows the configuration of a vertical stripe defect detection device according to the present invention, in which (a) is a side view,
(b) is an AA arrow line view. Further, FIG. 2 is a (a) front view and a (b) side view showing an outline of the flaw detection head.

In the figure, reference numeral 1 is a flaw detection head, which is shown in FIG.
, (B), the eddy current sensor 11 and this eddy current sensor 11
And a magnetic saturation yoke 12 surrounding the eddy current sensor 11
Is output from the eddy current output terminal 14 via the line 13, and the exciting voltage is applied to the magnetic saturation yoke 12 from the exciting terminal 15 via the line 16. 2 is the flaw detection head 1 is a steel strip S
The scanning device is, for example, an LM guide that scans in the width direction. A rail member 3 movably supports the scanning device 2 and is supported by columns 4a and 4b. An eddy current flaw detection device 5 supplies an excitation voltage to the excitation terminal 15 of the flaw detection head 1 and inputs an output signal from the eddy current output terminal 14. 6
Is a rubber-lined bridle roll that winds and passes the steel strip S, and its shafts 6a and 6b have bearing bases 7a and 7b.
Is pivoted by.

The results of the strip inspection of the coil, which is concerned about the occurrence of the vertical stripe defect, by using the vertical stripe defect detection apparatus thus constructed will be described below. First, the steel coil used for the flaw detection test was melted by a converter → RH degassing → continuous casting method. The main components are C = 0.0020%, Si = 0.01
%, Mn = 0.15%, P = 0.010%, S = 0.008%. Then, a steel strip S was manufactured by hot-rolling->cold-rolling-> hot dip galvanizing process, and galvanizing the coil having a size of 0.8 mm x 1600 mm width with zinc alloy.

Then, an exciting voltage with a flaw detection frequency of 32 kHz was applied to the steel strip S, and lift-off was 3 mm and phase angle was 40.
°, magnetizing current; 2 A, scanning speed; 0.8 m / sec, plate passing speed;
The flaw was detected under the flaw detection condition of 30 m / min. The results are shown in FIG. 3 (a) for the normal part and FIG. 3 (b) for the defective part. It can be seen that the output at the normal portion is below the defect discrimination level of 2V, whereas the output at the defective portion is as high as around 8V.

Next, as a result of sampling a test piece from the abnormal portion of the steel strip S detected in this inspection, performing elongation strain processing and grinding with a grindstone, it was possible to detect the expected vertical stripe defect. A sketch of the polished state of the abnormal portion of the test piece is shown in FIG. 4, and a micrograph of the metal structure in the cross section of the vertical line defect is shown in FIG. The abnormal tissue portion was ground to 200 μm from the surface, and the result of analysis of the shavings was C = 0.0039%, which was almost twice as much as the component at the time of melting.

FIGS. 6 (a) and 6 (b) show the results of comparison of the detection sensitivities when the eddy current sensor 11 is scanned in the width direction and the longitudinal direction of the steel strip. It can be seen that the detection performance of the widthwise scanning is outstanding. In addition, the flaw detection condition at this time is flaw detection frequency; 64 kHz,
Lift-off: 3 mm, phase angle: 40 °, magnetizing current: 2 A, scanning speed: 0.5 m / sec. 7 (a) and 7 (b) are the results of comparison of detection sensitivities with and without magnetic saturation. Fig. 7 (a) shows flaw detection frequency: 64kHz, lift-off: 3mm, phase angle: 40
7 °, magnetizing current: 2 A, scanning speed: 0.3 m / sec, and Fig. 7 (b) shows flaw detection frequency: 64 kHz, lift-off: 3 mm, phase angle: 40 °, magnetizing current: 0 A, scanning speed:
This is the case when the condition is 0.3 m / sec. If not magnetically saturated,
Defects cannot be detected because base noise such as distortion from the normal part of the steel plate appears stronger than defects.

FIG. 8 shows the results of examining the flaw detection frequency and the vertical line defect detection ability. Longitudinal stripe defect is S / at frequency of 30-150kHz
There is a feature that N is improved. Below 30kHz, the penetration depth of the steel plate surface into the vortex is too deep, and the sensitivity to detect a small amount of carburization is insufficient. It is presumed that it is easily affected by.

[0020]

As described above, according to the present invention,
By applying the eddy current flaw detection technology, which is a non-destructive method, it is possible to inspect longitudinal stripe defects on the entire length and width of the steel strip online, so that the abnormal parts corresponding to the flaw detection results in the steel strip are cut and removed. Therefore, it contributes to the improvement of product quality.

[Brief description of drawings]

FIG. 1 is a drawing schematically showing a configuration of a vertical stripe defect detecting device according to the present invention, in which (a) is a side view and (b) is an AA arrow view.

FIG. 2A is a front view and FIG. 2B is a side view showing an outline of a flaw detection head.

FIG. 3 is a characteristic diagram of (a) a normal part and (b) a defective part showing an example of detecting a vertical line defect according to the present invention.

FIG. 4 is a sketch diagram of a vertical stripe defect.

FIG. 5 is a micrograph of a metal structure in a cross section of a vertical line defect portion.

FIG. 6 is a characteristic diagram showing the detection sensitivity when the eddy current sensor is scanned in (a) the coil width direction and (b) the longitudinal direction.

FIG. 7 is a characteristic diagram showing detection sensitivity with (a) magnetic saturation and (b) without magnetic saturation.

FIG. 8 is a characteristic diagram showing a relationship between a flaw detection frequency and detection sensitivity.

[Explanation of symbols]

 1 flaw detection head 2 scanning device 3 rail member 4 support 5 eddy current flaw detection device 6 bridle roll 7 bearing base 11 eddy current sensor 12 magnetic saturation yoke S steel strip

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akira Ashidate 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Inside Kawasaki Steel Co., Ltd. Chiba Works (72) Inventor Takashi Ono 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Chiba Steel Works (72) Inventor Hiroyuki Yokota 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Kawasaki Steel Co., Ltd. Chiba Steel Works

Claims (1)

[Claims] 1. C: A part of the surface of a steel sheet is carburized during the steel sheet manufacturing process of a cold-rolled steel sheet or a galvanized steel sheet produced by using an ultra-low carbon steel of 0.01% or less, and a steel sheet crystal structure of that portion In a non-destructive inspection method for steel sheets with abnormal microstructure defects, the magnetic saturation type eddy current sensor placed close to the steel sheet surface is scanned in the width direction of the steel sheet and the flaw detection frequency is 30 to 150 kHz. A method for inspecting an abnormal structure defect portion of a surface layer of an ultra-low carbon steel sheet, which is characterized by detecting an abnormal structure portion.