Hot-rolled steel plate Magnetic flux leakage testing

Magnetic flux leakage flaw detection method and method for manufacturing hot-rolled steel plate using the magnetic flux leakage flaw detection method
Subject
The present invention relates to a magnetic flux leakage flaw detection method for detecting internal inclusions by applying a magnetic field to a strong magnet and detecting the magnetic flux leakage of the strong magnet, and a method for producing hot-rolled steel plates and descaled steel plates using the magnetic flux leakage flaw detection method.
Background art
As a method for detecting defects such as inclusions in a strong magnet in a steel plate, the magnetic flux leakage flaw detection method is widely used.
As an example, FIG. 1 shows the structure of a magnetic flaw detection device using magnetic attraction assembled on a steel plate inspection line. A magnetic flaw detection device 4 is set along the transfer path of the steel plate 1 (strong magnet) being transferred on the product inspection line at a substantially fixed speed V using transfer rollers 2 and 3. This magnetic flaw detection device 4 is composed of a magnetizer 5 that magnetizes the steel plate 1 in motion, a magnetic clamp 6 that clamps the steel plate in the middle and is relatively and equally set on the magnetizer 5, and a signal processing device 7 that processes the output signal of the magnetic clamp 6.
When the steel plate 1 is magnetized by the magnetizer 5, there is a defect 8 inside the steel plate 1. Due to the magnetic flux effect caused by the defect 8 passing through the steel plate 1, a part of it leaks to the outside of the steel plate 1. The magnetic shovel 6 detects this leakage magnetic flux, and the signal processing device 7 processes the output signal to detect the defect 8. Since the intensity of the leakage magnetic flux is related to the size of the defect, the size of the defect 8 can be judged according to the output signal level of the magnetic probe 6.
On the other hand, the leakage magnetic flux detected by the magnetic probe includes the leakage magnetic flux caused by the local bias unevenness in the steel plate (uneven thickness of the oxide scale and the unevenness of the oxide scale/sintered steel plate surface) and the interaction of the leakage magnetic flux caused by the surface roughness in addition to the leakage magnetic flux caused by the defect. From the perspective of defect detection, this interaction of magnetic flux is unnecessary magnetic flux, which is obviously noise.
In order to eliminate the influence of this noise, the following method is sometimes used to utilize the different frequency characteristics of the output signal generated by the defect (hereinafter referred to as the defect signal) and the noise signal (hereinafter referred to as the noise signal).
Figure 2 shows an example of measuring the frequency characteristics of the defect signal and the noise signal when the steel plate is moved at a certain speed. Generally, as shown in the figure, the defect signal has a higher frequency distribution than the noise signal, so the high-pass filter with a frequency f in the setting of the signal processing device can be used to relatively enhance the extraction of the defect signal. In order to improve the resolution of defect detection by the leakage magnetic flaw detection method, a method of using an appropriate Parker filter is proposed in the Utility Model Publication No. 61-119760.
As shown in Figure 2, since the frequency characteristics of the defect signal and the noise signal overlap, when the defect is small or the noise is large, for example, even if a high-pass filter is set to distinguish the frequency of the defect signal and eliminate the influence of the noise, it is difficult to accurately detect the defect.
Contents of the invention
The purpose of the present invention is to provide a magnetic flux leakage testing method that can highly accurately detect defects even in situations where defects are small or unnecessary magnetic flux is present, as well as in situations where clutter is high, and to provide a method for manufacturing hot-rolled steel plates using such a method.
To achieve this goal, magnetic flux leakage testing is used to sequentially magnetize the same position of the strong magnet at two different magnetization strengths; Using magnetic detection to detect the leakage of magnetic flux at the same position of the strong magnet magnetized at the two different intensities mentioned above; And the handicrafts produced by multiplying the output signal of the magnetic detector corresponding to the smaller magnetization intensity from the output signal of the magnetic detector corresponding to the larger magnetization intensity among the two mentioned above. In addition, the magnetic leakage testing method used may also include a program of magnetizing the strong magnet with one or more magnetizers; Move the above-mentioned strong magnet along the edges of two magnetic probes set at positions with different magnetization strengths, and use the two magnetic probes set at positions with different magnetization strengths to sequentially detect the leakage magnetic flux from the same position of the magnetized strong magnet; The large city where the output signals of the magnetic detectors corresponding to the small magnetization intensity are concentrated from the output signals of the magnetic detectors corresponding to the large magnetization intensity at the same position of the magnetized strong magnet mentioned above.
Provide a production method for hot-rolled steel plates, which utilizes the leakage magnetic flux testing method of the present invention to detect defects and determine defect information indicators. This method is used to manufacture hot-rolled steel plates with accurate pre known defect locations and surface density information covered with oxide scales, as well as hot-rolled steel plates that have undergone descaling treatment.