JPS5836302B2 - Method for detecting surface flaws on hot steel pieces - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method for detecting surface flaws on hot steel pieces.

In recent years, direct rolling has attracted attention as a rolling method for steel manufacturing.

This direct rolling method omits the surface flaw inspection and flaw cleaning of the cold billet, which were previously included in the blooming and hot rolling process. This is the rolling method used to send the product to the process.

Therefore, according to this direct rolling method, the step of cooling the steel billet and the step of reheating it are not necessary, so it is extremely advantageous for saving energy and labor.

However, in this case, in exchange for omitting cold inspection and maintenance, it is necessary to devise measures to prevent surface flaws from occurring on hot steel pieces, or to detect and remove surface flaws on hot steel pieces. However, it is generally difficult to directly obtain hot steel pieces without surface flaws.

For this reason, recently, methods and devices have been developed for detecting surface flaws on hot steel pieces using infrared detectors or industrial televisions (hereinafter abbreviated as ITV).

In other words, the hot flaw detection method takes advantage of the fact that the surface flawed area of a hot steel piece is either hotter or colder than the normal area. Alternatively, an infrared detector detects the flaw as an abnormal value of the input signal.

However, with these methods, it is impossible to detect all flaws that need to be removed for the following reasons.

In other words, ``Cracks'' such as vertical cracks and chevron-shaped cracks are difficult to cause temperature differences, so they are easy to overlook.
Since it is a high-temperature body, scale is easily generated, and a portion of this scale is released from the steel piece.

The part where the scale is free becomes much cooler than the part where no scale is formed.

For this reason, there is a strong possibility that the scale will be sensitively detected by the ITV or infrared detector, and will be mistaken for a flaw.

Furthermore, various measures have been proposed to avoid the above-mentioned problems.

For example, (1) the hot steel piece is forcibly cooled before flaw detection in order to increase the temperature difference between the flawed part and the normal part.

(11) In order to prevent temperature unevenness from occurring in areas other than the flaw, the hot steel piece is lightly rolled down before flaw detection.

and so on.

Although the above measures certainly improve the detection rate of surface flaws on hot steel pieces, they have drawbacks such as complicating the equipment and limiting the time for detection.

As mentioned above, when detecting surface flaws on hot steel pieces with an infrared detector or ITV, it is extremely difficult to distinguish between scale and flaws.

For this reason, it has conventionally been necessary to detect flaws in a scale-free state.

On the other hand, in order to first increase the temperature difference between the flawed part and the normal part, and to detect the flaw in a scale-free state, the applicant hot-cut the surface of the hot steel piece in a manner similar to that shown below. We have researched and developed a method for detecting surface flaws on hot steel pieces that detects surface flaws within 1 to 30 seconds (Japanese Patent Application No. 34995/1982).

However, although this invention certainly improves the flaw detection rate,
Since the entire surface of the test object (hot steel billet) is melt-cut, the yield of blooming tends to decrease.

Therefore, the present inventors observed the surface condition of a hot steel billet after blooming and before hot cutting, with the aim of developing a method for detecting defects that does not require a full-surface solvent.

As a result, the following was revealed.

(1) The amount of scale generated varies depending on the steel type and billet temperature.

(2) It is difficult to distinguish scale from flaws in steel pieces where scale is not easily generated and scale is sparsely generated on the surface.

(3) Similar to (2) above, even in steel pieces with a large amount of scale, the scale is easily mistaken for a flaw signal.

In this case, scale is not always uniformly generated over the entire surface, and there are some areas where it is not oxidized.

(4) There is no clear relationship between the phenomenon described in (3) above and the steel type and billet temperature.

(5) When a steel piece on which scale has not uniformly developed is visually inspected after cooling, "cracks" are often observed.

It is difficult to detect these flaws in the hot state using either an infrared detector or a TV.

Furthermore, the present inventors have determined that the hot surface condition of a steel billet that is not uniformly oxidized and the cold surface condition of this steel billet are 357
Detailed recordings were made using an nrIL camera and a comparative study was conducted.

An example of this is the two photographs in Figure 1. FIG. 1a shows the surface condition in hot condition, and FIG. 1b shows the surface condition in cold condition.

As is clear from FIG. 1, it was found that the areas where scale did not occur accurately corresponded to the areas where there were flaws.

Incidentally, since scale is sensitively detected by the infrared detector and ITV as described above, it is extremely easy to detect places where scale is not uniformly generated using these detectors.

The present invention has been made in view of the above-mentioned points, and provides a hot steel billet that can easily detect "cracks," which were conventionally considered difficult to detect, without having to melt the entire surface of the hot steel billet. The present invention aims to provide a method for detecting surface flaws.

The present invention is characterized in that, in detecting surface flaws on a hot steel piece, the surface flaws are detected after blowing a gas containing oxygen onto the surface of the hot steel piece to generate scale.

The present invention will be described in detail below with reference to the drawings.

FIG. 2 is a configuration diagram showing an example of an apparatus for carrying out the method for detecting surface flaws on a heated steel billet according to the present invention.

The hot steel billet 1 after blooming and rolling moves at a constant speed in the direction of the arrow.

In this example, the hot steel piece 1 is perpendicular to the moving direction of the hot steel piece 1.
A tube 2 is arranged so as to surround the hot steel slab 1, and a gas containing oxygen (for example, air) is uniformly sprayed from the tube 2 onto the surface of the hot steel piece 1 to oxidize it and generate scale.

Furthermore, an infrared detector or an ITV flaw detector 3 for detecting surface flaws on the oxidized hot steel piece 1 is arranged in front of the tube 2 when viewed in the moving direction of the hot steel piece 1.

FIG. 3 is a diagram showing how surface flaws are detected before and after oxidation of the hot steel piece 1 shown in FIG. 2.

Now, it is assumed that there is a chevron-shaped crack 4 in the heated steel piece 1, as shown in FIGS. 2 and 3a.

However, as shown in FIG. 3b, it is difficult to detect this chevron-shaped crack 4 using an ITV image before oxidation for the reasons mentioned above.

Furthermore, for the same reason, as shown in Figure 3C, even if the temperature distribution of the A-A line (see Figure 3a) including the chevron-shaped crack 4 is taken out with an infrared detector before oxidation, an abnormal waveform is not detected. It is difficult to detect.

When a gas containing oxygen is uniformly blown from the tube 2 to the hot steel billet 1 containing the above-mentioned chevron-shaped cracks 4, as shown in FIG. 2, the hot steel billet 1 is It is uniformly oxidized except for the portion including the chevron-shaped cracks 4, and scale 5 is generated.

Therefore, when the flaw detector 3 shown in FIG. 2 is an ITV, the distribution state of the scale 5 as shown in FIG. 3e is projected on the IT■ image.

Therefore, the location of the chevron-shaped crack 4 can be easily detected.

In addition, if the flaw detector 3 is an infrared detector, the flaw shown in FIG.
As shown in , the temperature distribution display (input waveform) shows a continuous high temperature over a certain range.

Therefore, the chevron-shaped crack 4 can be detected.

As mentioned above, in the present invention, we focused on the experimental fact that scale is less likely to form on the flawed part of a hot steel billet compared to a normal flat part, and, before detecting hot flaws, we created a uniform scale on the surface of the hot steel billet. Since scale is generated by blowing oxygen-containing gas onto the surface, cracks such as chevron cracks and vertical cracks can be easily detected using a conventional hot flaw detector.

Furthermore, in carrying out the present invention, it is sufficient to install a device that uniformly sprays oxygen-containing gas between the blooming mill and the flaw detector, so the equipment cost is low.
Also, maintenance of the equipment is easy.

According to the present invention, it is not possible to detect the exact shape of the flaw, but this does not necessarily need to be known for the following reason.

In other words, a flaw removing device that removes flaws based on flaw signals sent from a flaw detector generally includes a hot cutting device that has a plurality of independently operable fire units in the width direction of the steel billet. used.

Therefore, as information on the location of flaws, it is sufficient to divide the steel piece into squares for each fire unit and to know in which square the flaw is located.

Furthermore, according to the present invention, after oxidation, surface flaws on the steel piece can be identified with the naked eye (see FIG. 1b), so it is also possible to manually clean up the flaws after marking.

[Brief explanation of drawings]

Fig. 1 is a diagram showing a comparison between the hot and cold surface conditions of a steel billet, and Fig. 2 is an example of an apparatus for carrying out the method for detecting surface flaws on a hot steel billet according to the present invention. The configuration diagram and FIG. 3 are diagrams showing how surface flaws are detected before and after oxidation of a hot steel piece. 1... Hot steel billet, 2... Tube, 3...
...Flaw detector, 4...Flaw, 5...Scale.

Claims (1)

[Claims] 1. A method for detecting surface flaws on a hot steel billet, which comprises: detecting surface flaws on a hot steel billet by blowing oxygen-containing gas onto the surface of the hot steel billet to generate scale; .