JP2005342788A - Method and apparatus for diagnosing flash-butt weld zone of steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus for diagnosing a flash-butt weld zone, which can easily and precisely diagnose a state of a weld zone, when a preceding steel sheet and a succeeding steel sheet are joined by flash-butt welding and then continuously fed into a continuous cold rolling mill for example, employing a flash-butt welder for an uncoiled steel sheet of about 1.0-10 mm in thickness and about 400-2500 mm in width. <P>SOLUTION: In the flash-butt welder for joining the preceding steel sheet and the succeeding steel sheet in a continuous cold processing line, the quality of the weld zone is diagnosed based on a temperature distribution obtained by measuring temperature on the weld zone in the designated width from the weld line as a center, in the state before or after trimming of a bead within 10 seconds after the completion of upsetting. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to, for example, a diagnostic method and a diagnostic device for a welded portion in a case where a preceding steel plate and a subsequent steel plate, which are continuously supplied in a continuous cold rolling line or a continuous pickling line, are joined by flash butt welding. is there.

For example, in a cold continuous rolling line for steel plates, in order to improve productivity, the ends of the front and rear steel plates are joined by flash butt welding and continuously supplied to a continuous rolling mill for continuous rolling. As an example of the flash butt welding equipment used here, as shown in FIG. 7 (a), the rear end of the preceding steel plate Sa being rolled with a loop is formed by clamps (electrodes) 3a and 3b of the fixing base 1 as shown in FIG. Clamp and trim the rear end with the shear 4, and clamp the tip of the subsequent steel plate Sb introduced into the moving table 2 with the clamps (electrodes) 3 a and 3 b of the moving table 2, and the tip with the shear 4. The pedestal is moved, the moving table 2 is gradually approached to the fixed table 1 side, a voltage is applied between the electrodes 3a and 3b on the fixed table 1 side and the electrodes 3a and 3b on the moving table 2 side, and the preceding steel plate Sa and By bringing the succeeding steel plate Sb close to each other, the flash is blown between the rear end surface of the preceding steel plate Sa and the front end surface of the succeeding steel plate Sb, and is rapidly pressed and joined at a predetermined upset cost.
When the up-setting is completed, a burr (bead) bb as shown in FIG. 7B is generated along the weld line. This burr is raised by the trimmer 5 which is arranged so as to be movable forward and backward with respect to the weld line. Trimming can be performed without moving the welded part after completion of setting, and since the welded part is immediately after upsetting and the temperature is high and the cutting resistance by the trimmer 5 is small, the burr bb is easily trimmed to improve the quality of the welded part. It is possible to ensure the efficiency of the welding operation including trimming by shortening the time required for moving the welded portion to the trimmer 5.
However, when flash butt welding is used, it is inevitable that flash irregularities will occur on the joining end faces of the preceding and succeeding steel plates to be joined, as well as minute deviations in the thickness direction of the steel plates. As a result, uniform bonding cannot be achieved, and a desired bonding strength (here, a bonding strength that does not break in the continuous cold rolling process or the winding / rewinding process may be sufficient) cannot be secured stably. If the desired joint strength cannot be obtained, the joint will be destroyed during the continuous cold rolling process, and the continuous rolling operation becomes impossible, and a semi-finished product treatment is required, resulting in a decrease in yield as well as a reduction in work efficiency. become.

Therefore, conventionally, several methods have been proposed as a method for diagnosing a welded portion by flash butt welding in addition to a method by visual inspection. For example, a method for determining the presence or absence of an abnormality in a welded part from changes in device operation, voltage, or current value, a method for inspecting a weld internal defect using ultrasonic waves, and the like have been proposed.
Also, as a welding abnormality monitoring method for the welding machine, a welding abnormality monitoring method for the welding machine that compares the amount of power input to the welding machine in the pre-flush, late, and upset periods with the reference power amount, before and after the flash, and the upset period. In addition, a welding abnormality monitoring method for a welder that compares the heat input of the welder with a reference heat input has been proposed.
However, in the case of visual inspection, there is a problem that it is difficult to ensure stable inspection accuracy in addition to a large human load. In addition, in the case of equipment operation, voltage, current value, electric energy, heat input, etc., even if these indicate a normal state, a joint failure may occur depending on the conditions of the material to be welded. In some cases, even if it seems to be joined at first glance with ultrasonic waves, welding with insufficient joining strength, for example, welding defects such as cold welding may not be detected, so it is more reliable. Development of a diagnostic (inspection) method for flash butt welds has been requested. In recent years when the production volume of difficult-to-weld materials such as high-strength steel sheets has increased, this demand has been increasing.
(1) Patent Document 1 discloses a welding abnormality monitoring method for a welding machine that compares the heat input amount of the welder with a reference heat input amount in the pre-flush, late period, and upset period.
(2) Patent Document 2 discloses a welding abnormality monitoring method for a welding machine that compares the amount of power input to the welding machine before, late, and upset with a reference power amount.
JP-A-3-32473 JP-A-3-32474

  The present invention is a flash butt welding machine for a steel sheet having a thickness of about 1.0 to 10 mm and a width of about 400 to 2500 mm that has been rewound from a coil. A diagnostic method and diagnostic device for a flash butt weld, which can easily and accurately diagnose the state of a weld when continuously supplying to a continuous cold rolling mill, and which can advantageously solve the problems in the conventional welding diagnostic method. It is to provide.

The present invention has been made in order to advantageously solve the above-mentioned problems, and has the following (1) to (5).
(1). In flash butt welding that joins the preceding steel plate and the following steel plate, the temperature of the welded portion having a predetermined width centered on the weld line is within 10 seconds after completion of the upset, before or after trimming the burr (bead). And diagnosing the quality of the welded portion from the temperature distribution thereof.
(2). In (1), the portion where the temperature in the plate width direction at the welded portion is distributed on the high temperature side and the low temperature side outside the range of the set ratio with respect to the preset reference average temperature is determined as the abnormal welding portion. The length of the abnormal welded portion in the plate width direction is summed, and the case where the sum of the lengths is equal to or greater than the set ratio with respect to the plate width is determined to be a welding failure. Diagnosis method.
(3). In (1), the temperature width in the plate longitudinal direction at the welded portion is measured to obtain the temperature width distribution in the plate width direction, and the wide portion and the narrow portion outside the range of the reference temperature width where the temperature width is set in advance are determined. It is judged as an abnormal welded part, the length in the plate width direction of this abnormal welded part is summed, and the case where the sum of the length is a set ratio or more with respect to the plate width is judged as a welding defect. A diagnostic method for flash butt welds on steel plates.
(4). From the upset stage, two-dimensional images with a predetermined width and length centered on the weld line are taken continuously in time, and the image immediately after the average luminance value of the image exceeds a preset threshold is selected. Then, the brightness of the image is converted into temperature, a temperature distribution is calculated, and the quality of the weld is diagnosed from the temperature distribution, according to any one of (1) to (3), A diagnostic method for flash butt welds on steel plates.
(5). In a diagnostic apparatus for diagnosing the quality of flash butt welding for joining a preceding steel plate and a succeeding steel plate, a material information input unit for inputting information on a material to be welded, and an imaging condition output unit for outputting an imaging condition from the information on the welding material And a two-dimensional camera that continuously captures a two-dimensional image having a predetermined width and a predetermined length of the welded portion according to the imaging conditions, and a luminance average value of the two-dimensional image is preset from the continuous two-dimensional image. An image selection unit that selects the first image that captures the welded portion after exceeding the threshold, and a temperature conversion that calculates the temperature distribution in the vicinity of the predetermined welded portion by converting the brightness of the selected image of the selected image portion into a temperature And a pass / fail determination unit for determining whether or not the joining state is acceptable based on the temperature distribution.

  In the present invention, whether the flash butt weld is good or bad is diagnosed based on the temperature distribution of the weld immediately after completion of the upset, for example, whether it is a joined state that does not cause breakage during continuous processing, and Judgment with high accuracy, and when the joint is prone to breakage, it is possible to select a process such as joint removal / rejoining promptly and perform a smooth continuous processing operation. In addition, it is possible to avoid the occurrence of inconveniences such as semi-finished product processing and to realize a smoother continuous rolling operation.

The present invention is applied to, for example, in a cold continuous rolling line for steel sheets, in order to increase productivity, the ends of front and rear steel sheets are joined by flash butt welding and continuously supplied to a continuous rolling mill for continuous rolling. This is a method for diagnosing a welded portion. Basically, a welded portion having a predetermined width centered on the weld line in a state where the temperature of the welded portion is high (for example, about 600 ° C. to 1000 ° C.) within 10 seconds after the completion of upset The temperature is measured and the quality of the weld is diagnosed from the temperature distribution.
Regarding the temperature measurement of the welded part, the abnormal welded part is often at a lower temperature than the normal welded part in the high-temperature state immediately after the completion of the upset. Although it is advantageous to do so within 10 seconds after completion of upset, more preferably within 5 seconds, it is possible to accurately determine the weld defect. If this condition is satisfied, this temperature measurement may be performed before trimming of burrs (beads) or after trimming.

In addition, when the temperature is measured after 10 seconds have elapsed after the completion of the upset, the temperature of the weld decreases to 500 ° C. or less, and there is no significant temperature difference between the abnormal weld and the normal weld. This makes it impossible to accurately determine the part.
This temperature measurement is performed either on the upper surface side and the lower surface side of the steel plate or on the upper surface side and the lower surface side of the steel plate. As thermometers used for temperature measurement, various radiation thermometers such as image measurement type radiation thermometers and two-dimensional radiation thermometers that can easily measure the temperature distribution in a desired region and can be arbitrarily analyzed and processed by image processing, A non-contact type thermometer such as an infrared camera thermometer is suitable, but a contact-type thermometer such as a thermocouple can also be used to measure the temperature of the weld after trimming the burr. (Equivalent to the embodiment of claim 1)

The present invention will be described below with reference to FIGS.
In the flash butt welding, the present inventors have recognized that if the flash can be blown uniformly, a normal weld will be formed and the temperature distribution of the weld will be uniform, and the temperature of the weld will be Focusing on the relationship between distribution and weld defects, the following knowledge was obtained through experiments.
(1) In the case of flash butt welding, as shown in FIG. 1, the temperature of the welded portion is the highest at about 1400 ° C. when the upset is completed, and drops to about 600 ° C. after 10 seconds. When the temperature of the welded portion is measured in this temperature region, as shown in FIG. 2, there is a clear temperature distribution on the weld line and its periphery.
(2) When the current is increased and a lot of flash is generated, the melted portion of the material to be welded is locally scattered excessively and missing, and when it is upset as it is, that portion becomes lower than the temperature of other portions. On the other hand, when the current is lowered, the occurrence of flash is reduced and the upsetting is performed without sufficiently forming the melted portion, so that the portion with the lower temperature becomes dominant.

(3) Also, in the case of step welding with a step in the thickness direction, as shown in FIG. 3, since the joint surface is inclined, the weld line is shifted between the upper surface side and the lower surface side. When the temperature distribution in the longitudinal direction of the plate is measured with reference to the weld line, the temperature width is biased.
(4) Even if the temperature is high, a sufficient melted portion cannot be obtained if a local excessive melted portion caused by non-uniformity of flash scattering is lost. In this case, for example, as shown in FIG. 4A, in the temperature distribution in the plate longitudinal direction of the welded portion, A having a temperature range wider than the temperature range of the normal welded portion (equal temperature region) within a certain temperature range. Produce. Alternatively, for example, as shown in FIG. 4B, in the temperature distribution in the plate longitudinal direction of the welded portion, B having a temperature width narrower than the temperature width of a normal welded portion (equal temperature region) within a certain temperature range. Arise. The former is presumed to be the result of the flash portion remaining at the time of upset and most of the melted part is pushed out, and the latter is presumed to be a result of the molten part largely scattered by flashing.
From the above findings, the inventors have concluded that the state of the weld can be accurately diagnosed by the temperature distribution of the weld in the high-temperature region immediately after completion of the upset.

In the present invention, welding conditions (voltage, current, flash cost, upset cost, etc.) that can ensure stable normal welds according to the steel plate conditions (material, size) to be joined, normal welds and abnormal welds Welding conditions (basically, normal welded parts) that can ensure discrimination criteria and desired joining strength (here, joining strength that does not break during continuous cold rolling and winding / rewinding processes) And the ratio of abnormal welds). In addition, for example, for abnormal welds as shown in FIGS. 4 and 5, the quality of the welds is diagnosed as follows.
The temperature measurement is performed within 10 seconds (more preferably within 5 seconds) after completion of the upset, and before or after trimming the burr (bead), but the conditions are different before and after trimming the burr (bead). The conditions for the above diagnosis are set separately.

The weld pass / fail judgment in the welded portion diagnosis method of the present invention is performed, for example, as follows.
(1) The temperature in the plate width direction at the welded portion immediately after the upset is measured, and the temperature in the plate width direction is high outside the preset ratio e with respect to the preset reference temperature as shown in FIG. Similar to the distributed part A distributed on the side, the part B distributed on the low temperature side outside the range of the set ratio e ′ is determined as an abnormal weld, and the length in the plate width direction of this abnormal weld is determined. x is summed (summed when there are a plurality of parts A and B), and welding is performed when the sum xt of the length x is equal to or greater than a set ratio (xt / w is, for example, 15% or more) with respect to the plate width w. The weld is judged to be defective, the weld is retracted, cut and removed, and then welded again. In addition, fine adjustment of welding conditions and tension change during rolling are taken into consideration as necessary. (Equivalent to the embodiment of claim 2)

(2) The temperature width that is equal to or higher than the reference temperature in the plate longitudinal direction at the welded portion immediately after the upset is measured to obtain the temperature width distribution in the plate width direction, as shown in FIGS. 4 (a) and 4 (b). In addition, a part A having a temperature width wider than a preset reference temperature width a and a part B narrower than the reference temperature width b are determined as abnormal parts, and the length x of the abnormal part in the plate width direction is summed up. When the sum xt is equal to or greater than the set ratio with respect to the plate width w (xt / w is, for example, 15% or more), it is determined that the welded portion is defective. Weld. In addition, fine adjustment of welding conditions and tension change during rolling are taken into consideration as necessary. (Equivalent to the embodiment of claim 3)

The flash butt welding equipment for carrying out the present invention is basically the same as that shown in FIG. 7 (a), but as shown in FIGS. 6 (a) and 6 (b), in the vicinity of the upset position. For example, a radiation thermometer 7 is disposed on the support 6 of the trimmer 5 that can be moved back and forth in the vertical and horizontal directions, and when the burr bb of the welded portion is trimmed with the trimmer 5 immediately after the upset, the preceding (before trimming) In this case, a structure for measuring the temperature of the welded portion without moving the welded portion is added. In FIG. 6B, the radiation thermometer 7 is arranged on the upper surface side of the steel plate Sa (Sb) so that the burr bb measures the temperature of the welded part before trimming. The radiation thermometer 7 is arranged on one or both of the upper surface side and the lower surface side of the steel plate.
When the temperature measurement information from the radiation thermometer 7 is input to a pass / fail determination device (not shown) incorporating the weld pass / fail diagnosis system of (1) or (2) above to determine pass / fail, and when it is determined that the weld is defective. For example, it is possible to automatically shift to a reprocessing operation such as retreating the welded portion, cutting and removing it, and welding again, or to issue a reprocessing command by a display device or an alarm device. For this purpose, we have added the calculation control function for fixed base clamp (electrode), shear, trimmer, movable base clamp (electrode), shear, welding condition setting change and rolling schedule change to optimize welding conditions during re-welding. It is also possible to automatically control the prevention of welding breakage during rolling.

As described above, it is advantageous to obtain the temperature distribution output from the radiation thermometer 7 in a state where the temperature is high, and within 10 seconds after completion of the upset, more preferably within 5 seconds. Therefore, it is possible to accurately determine whether or not the welded portion is acceptable. Therefore, an embodiment of a diagnostic method and apparatus for measuring the temperature distribution of the welded portion within 5 seconds after completion of the upset and then determining whether the joining state is acceptable will be described with reference to FIGS. (Corresponding to the embodiments of claims 4 and 5)
FIG. 8 is a diagram showing an embodiment of the diagnostic apparatus according to the present invention, and a portion surrounded by a one-dot chain line is the range. Other than that is a flash butt welding facility for carrying out the present invention (hereinafter referred to as “welding machine main body”), which is basically the same as that shown in FIG. Here, the radiation thermometer 7 includes a two-dimensional camera 13, an image selection unit 14, and a temperature conversion unit 15. As shown in FIG. 8 (a), a two-dimensional camera 13 is disposed on the support base 6 of the trimmer 5 that can be moved back and forth in the vertical and horizontal directions in the vicinity of the upset position. Prior to trimming with the trimmer 5, a two-dimensional image of the weld is taken.

A two-dimensional camera 13 having a high resolution (for example, 2000 × 2000 pixels) for obtaining a detailed weld temperature distribution and having a partial readout function for continuous imaging at a short interval is suitable. That is, as shown in FIG. 8B, when the entire welded part is imaged with a single two-dimensional camera 13 for a material having a short plate width, the welded part indicated by the burr bb enters as shown in FIG. The camera field of view is set at a position (a range surrounded by a one-dot chain line in the figure), and an image of only a readout portion (a range surrounded by a broken line in the figure) including the welded portion and its periphery is captured. For example, when imaging a welded portion of a steel plate having a width of 400 mm with a two-dimensional camera of W × W = 2000 × 2000 pixels, taking a walk of the steel plate into consideration and taking a field of view of 500 × 500 mm, the resolution is 500/500. 2000 = 0.25 mm (1)
It becomes.
Since the width of the burr (bead) bb is 3 to 5 mm in the longitudinal direction, it is sufficient to take a visual field of about 25 mm. Therefore, the required number of pixels L is L = 25 / 0.25 = 100 pixels (2)
It becomes. Therefore, the readout pixel is 2000 × 100, which is 1/20 of the entire pixel. By performing partial reading in this way, the amount of image data captured by the two-dimensional camera 13 can be reduced, and even a high-resolution image can be continuously captured at a short pitch.

  Further, as shown in FIG. 10A, when a wide steel plate is imaged by a plurality of (three in this figure) two-dimensional cameras 13-1, 13-2, and 13-3, FIG. As described above, the fields of view 17-1, 17-2, and 17-3 of the respective two-dimensional cameras may be extracted as read portions 18-1, 18-2, and 18-3. In FIG. 9, the reading width W is set slightly larger than the plate width in consideration of the walk of the steel plate, and the reading length L is determined from the range affected by the welding heat. Further, the position of the weld line in the length direction of the steel plate is substantially constant depending on the material to be welded, and when the two-dimensional camera 13 is installed, it is determined as the pixel value Xc. Therefore, the readout start point (X, Y) shown in FIG. Becomes (Xc-L / 2, 0). The values X, Y, L, and W necessary for determining the reading portion are determined in this way and set in the two-dimensional camera 13 in advance.

Hereinafter, with respect to one flash butt welding, the operation of each part of the diagnostic apparatus from before the start until the judgment of the joining state is made will be described with reference to FIG. The material information input unit 11 inputs material information from the welding machine main body before starting welding. Material information includes (1) steel plate conditions (material, size) for joining control, (2) welding conditions (voltage, current, flash charge, upset charge, etc.), (3) pixel value (Xc) of weld point position, etc. Is included.
The material condition output unit 12 uses a table to show the relationship between the material information and the optimum camera exposure time based on the results of a large number of tests performed in advance, and uses the table to determine the exposure time that matches the received material information. 13 and the temperature conversion unit 15. The two-dimensional camera 13 with the exposure time set in this way is in a state of waiting for the start of imaging, and starts continuous imaging when a trigger signal is received. At the upset stage, the trigger signal is transmitted from the welding machine body to the two-dimensional camera via the material information input unit 11 and the material condition output unit 12.

In the image selection unit 14, partial readout images of W × L pixels are sequentially sent from the two-dimensional camera 13 at a short time pitch, but the welding machine body is shielded so that spatter does not fly outside during welding. Thus, although the two-dimensional camera 13 is not damaged by sputtering during welding, a welded portion is not seen immediately after the start of imaging, so a dark image is formed. Entered.
After that, when the sputter shield is opened, an image including a high-luminance weld is formed. Therefore, in order to acquire an image at the earliest possible stage after upset, the image selection unit 14 calculates the average brightness value of the input images that are sequentially sent, and if the value exceeds a preset threshold value, the sputter shield It is determined that the image has been opened, and the next image sent is sent to the temperature converter 15. Here, the reason why the image when the threshold value is exceeded is not sent is because there is a high possibility that the image is an incomplete welded part while the sputter shield is being opened. As described above, the input pitch of the partial readout image is about 0.1 second, and the time from the upset to the opening of the sputter shield is 3 to 4 seconds at the longest. Images can be selected.

In the temperature conversion unit 15, a black body furnace whose uniform and accurate temperature is known is imaged with the two-dimensional camera 13 in advance, and the obtained image luminance value and the black body furnace temperature are used as the emissivity of the joining target. It is stored in correspondence with the corrected temperature value. That is, the calibration data indicates the relationship between the gradation indicating the image brightness of the two-dimensional camera 13 and the temperature for each exposure time as shown in FIG. Using the exposure time and the calibration data, the image data sent to the temperature conversion unit 15 is converted from a luminance value to a temperature and sent to the pass / fail judgment unit 16 as temperature distribution data.
The pass / fail determination unit 16 performs processing corresponding to the above embodiments (2) and (3) based on the input temperature distribution data, and outputs a pass / fail determination result.

In the above embodiment, the case where one two-dimensional camera 13 is installed is described as an example. However, even when a plurality of two-dimensional cameras are installed as shown in FIG. 9, each two-dimensional camera 13-1, 13- 2 and 13-3, the read images 18-1, 18-2, 18-3 obtained from the image selection unit 14 may be combined into one image and sent from the temperature conversion unit 15.
The present invention is not limited to the above example. For example, in the above example, a steel plate continuously supplied to a continuous cold rolling mill is a welding target, but a steel plate continuously supplied to a continuous pickling facility is also applicable as a welding target.

The conceptual explanatory view which shows the example of a time-dependent change of the temperature of a flash butt welding part. Side surface explanatory drawing which shows the weld line and temperature distribution example of a flash butt weld part. Side surface explanatory drawing which shows the example of a weld line and temperature distribution when there exists a position shift in the thickness direction in a flash butt weld part. The plane conceptual explanatory drawing which shows the abnormal weld part example of a flash butt weld part with the temperature width distribution of a plate length direction. The conceptual explanatory drawing which shows the abnormal weld part example of a flash butt weld part with the temperature distribution of a plate width direction. (A) The figure is a side conceptual view showing an example of the structure of the flash butt welding equipment used in the present invention, (b) The figure is a front concept partially showing an arrangement example of the radiation thermometer in (a). Illustration. (A) A side conceptual explanatory drawing which shows the structural example of the conventional flash butt welding equipment, (b) A side conceptual explanatory drawing which shows the burr | flash example which generate | occur | produces in a flash butt welding part. (A) is explanatory drawing which concerns on one Example of this invention, (b) is a front conceptual explanatory drawing which shows partially the example of arrangement | positioning of the two-dimensional camera in (a). The conceptual explanatory drawing which shows a camera visual field and an image read-out part. 8A is a front conceptual explanatory view partially showing an example in which three cameras are arranged in the steel plate width direction in FIG. 8A, and FIG. 8B is a camera view corresponding to FIG. FIG. Explanatory drawing which shows the outline | summary of calibration data.

Explanation of symbols

Sa Preceding steel plate sb Subsequent steel plate 1 Fixed base 2 Moving base 3a, 3b Electrode (clamp)
DESCRIPTION OF SYMBOLS 4 Shear 5 Trimmer 6 Support stand 7 Radiation thermometer 10 Diagnosis apparatus of flash butt welding part 11 Material information input part 12 Imaging condition output part 13 Two-dimensional camera 13-1, 13-2, 13-3 Two-dimensional camera 14 Image selection Section 15 Temperature conversion section 16 Pass / fail judgment section 17-1, 17-2, 17-3 Camera field of view 18-1, 18-2, 18-3 Reading part

Claims (5)

  In flash butt welding, which joins the preceding steel plate and the following steel plate, the temperature of the welded part with a predetermined width centered on the weld line is measured within 10 seconds after completion of the upset, with the bead being trimmed before or after trimming. A method for diagnosing a flash butt weld of a steel sheet, wherein the quality of the weld is diagnosed from the temperature distribution.   The part where the temperature in the plate width direction at the welded part is distributed on the high temperature side and the low temperature side outside the set ratio range with respect to the preset reference average temperature is determined as an abnormal welding part, and this abnormal welding part 2. The flash butt weld of a steel sheet according to claim 1, wherein a total of the lengths in the plate width direction is determined, and a case where the sum of the lengths is equal to or greater than a set ratio with respect to the plate width is determined to be poor welding. Diagnosis method.   The temperature width in the plate longitudinal direction at the welded part is measured to obtain the temperature distribution in the plate width direction, and the wide and narrow parts outside the range of the reference temperature range where the temperature width is set in advance are determined as abnormal welded parts. The sum of the lengths in the plate width direction of the abnormal weld sites is determined as a welding failure when the sum of the lengths is equal to or greater than a set ratio with respect to the plate width. A diagnostic method for flash butt welds on steel plates.   From the upset stage, two-dimensional images with a predetermined width and length centered on the weld line are taken continuously in time, and the image immediately after the average luminance value of the image exceeds a preset threshold is selected. The steel sheet according to any one of claims 1 to 3, wherein the brightness of the image is converted into temperature to calculate a temperature distribution, and the quality of the weld is diagnosed from the temperature distribution. Diagnostic method for flash butt welds.   In a diagnostic apparatus for diagnosing the quality of flash butt welding for joining a preceding steel plate and a succeeding steel plate, a material information input unit for inputting information on a material to be welded, and an imaging condition output unit for outputting an imaging condition from the information on the welding material And a two-dimensional camera that continuously captures a two-dimensional image having a predetermined width and a predetermined length of the welded portion according to the imaging conditions, and a luminance average value of the two-dimensional image is preset from the continuous two-dimensional image. An image selection unit that selects the first image that captures the welded portion after exceeding the threshold, and a temperature conversion that calculates the temperature distribution in the vicinity of the predetermined welded portion by converting the brightness of the selected image of the selected image portion into a temperature And a pass / fail determination unit for determining whether or not the joining state is acceptable based on the temperature distribution.

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