JP2009063441A - Device and method for measuring internal structure of magnetic body - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device capable of measuring an welded condition of a magnetic body such as steel, and an internal structure thereof, such as an internal defect, with a nondestructive means, without being influenced by a surface condition, and to provide a method therefor. <P>SOLUTION: A fluctuating external magnetic field is applied to a spot welding place 92 of steel sheets 90, 91 of magnetic bodies to be measured. However, the external magnetic field is applied to the extent that magnetization of steel sheets 90, 91 do not become stabilized. This reflects the internal structure of the welding location to be measured that is sensitive to the extent of progress of magnetization. The return value is detected by measuring the return quantity of the magnetization, after current cutoff. The difference between such a return values is determined, which is measured by using two types of exciting patterns, having different developing magnetizabilities. This method enables proper measurement of the internal structure of the welding location, without being influenced by the surface conditions. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an apparatus and a method for measuring an internal structure such as a weld state and internal defects of a magnetic material such as a steel plate. More specifically, the present invention relates to an apparatus and method that can measure the internal structure of a magnetic material that is an object in a non-destructive manner and with as little influence as possible from the surface state.

  Conventionally, magnetic materials such as steel plates have been widely used as structural materials for various industrial products including automobiles. Here, a steel plate or the like, which is a structural material, may be used by spot welding because of the necessity as a product. A spot welded portion of a steel plate usually has a portion called a nugget portion. This is a place where welding was performed by melting once and then solidifying during welding. The diameter of the nugget portion, that is, the nugget diameter is an important index indicating the quality of spot welding. Further, in a spot welded portion, the surface portion is usually slightly recessed compared to the surroundings. This is because of the pressurization received during welding.

As a conventional technique for nondestructively measuring the internal structure of a magnetic material such as a spot welded part, one described in Patent Document 1 can be cited. In the technique described in Patent Document 1, basically, information on the internal structure of the object to be measured is acquired by applying a static magnetic field to the magnetic body that is the object to be measured. The reason why information on the internal structure can be obtained by such a method is that the magnetic characteristics are greatly different between a part that experienced a remarkable thermal history such as a nugget part and a part that does not. For this reason, even when a static magnetic field of the same condition is applied, the state of magnetization of the object to be measured is affected by the internal structure.
Japanese Patent No. 3098193

  However, the conventional techniques described above have the following problems. In other words, the measurement results are affected not only by the internal structure but also by the surface state. For this reason, it is difficult to remove the part caused by the surface state from the measurement result and to take out the part purely caused by the internal structure. In Patent Document 1, it is stated that the surface shape and the nugget diameter can be separated and measured separately. However, even if the nugget diameter is measured there, it is difficult to eliminate the influence of the surface shape itself.

  The present invention has been made to solve the above-described problems of the prior art. In other words, the problem is to provide an apparatus and method that can measure the internal structure of a magnetic material such as a steel plate, such as the welded state and internal defects, nondestructively and without being affected by the surface state as much as possible. There is to do.

  An apparatus for measuring the internal structure of a magnetic body according to the present invention, which has been made for the purpose of solving this problem, is applied to a magnetic body to be measured by an excitation coil that generates a magnetic field to be applied to the magnetic body to be measured, and the excitation coil An apparatus having a magnetic detection sensor arranged in a magnetic field path, a current application unit for applying a current to an excitation coil, and a sensor output acquisition unit for acquiring an output of the magnetic detection sensor. A current that changes with time according to the first excitation pattern is applied to the coil, and then the current is cut off. After the applied current according to the first excitation pattern is cut off to the excitation coil, the first excitation pattern A current that changes with time is applied in accordance with a second excitation pattern different from that of the first excitation pattern, and then the current is interrupted. The output of the magnetic detection sensor when the applied current due to the current is interrupted is acquired, and the output of the magnetic detection sensor when the applied current due to the second excitation pattern is interrupted is acquired. At least one of the two excitation patterns is a pattern that does not stabilize the degree of magnetization of the magnetic substance to be measured even when the current is interrupted. The internal structure includes not only the mechanical structure but also the magnetic properties, chemical composition, and crystal structure location.

  In addition, the method for measuring the internal structure of a magnetic material according to the present invention is arranged in an excitation coil that generates a magnetic field to be applied to the magnetic material to be measured, and a path of the magnetic field applied to the magnetic material to be measured by the excitation coil. This is performed using a magnetic detection sensor. That is, a current that changes with time according to the first excitation pattern is applied to the excitation coil, and then the current is interrupted, and the output of the magnetic detection sensor when the current is interrupted is obtained. After the applied current due to the first excitation pattern is cut off, a current that changes with time is applied according to a second excitation pattern different from the first excitation pattern, and then the current is cut off and the current is cut off. The output of the magnetic detection sensor is acquired. Here, as at least one of the first and second excitation patterns, a pattern that does not stabilize the degree of magnetization of the magnetic substance to be measured even when the current is interrupted is used.

  In the present invention, a varying magnetic field is applied to the magnetic material to be measured by the first or second excitation pattern. Thereby, the magnetization of the magnetic substance to be measured is advanced. The degree of progress of magnetization at this time depends on the internal structure of the object to be measured. That is, if the structure contains a lot of easily magnetized structures, the degree of magnetization progress is large. On the other hand, if the structure contains a lot of hard-to-magnetize structures, the degree of magnetization progress is small. Moreover, this difference appears quite sensitive. This is because at least one of the first and second excitation patterns is a pattern that does not stabilize the degree of magnetization of the magnetic substance to be measured even when the current is interrupted.

  Of course, it also depends on the ability of the first and second excitation patterns to magnetize the object. The difference due to the excitation pattern is particularly prominent when the object to be measured has a structure containing many tissues that are difficult to be magnetized. On the other hand, in the case of a structure containing a lot of easily magnetized structures, the difference due to the excitation pattern does not appear so much. For this reason, it can be said that the difference in the degree of progress of magnetization due to the first and second excitation patterns represents the ease of magnetization of the measurement object. Therefore, the output of the magnetic detection sensor when the current due to both excitation patterns is interrupted is acquired. Thereby, the return amount of magnetization can be measured. Therefore, by comparing the return amounts after both excitation patterns, it is possible to measure whether the measurement object contains a lot of hard-to-magnetize tissues or little.

  In the present invention, the object of measurement is mainly the spot welded portion of the steel sheet. In this case, the spot welding location is determined by the difference between the output of the magnetic detection sensor when the first excitation pattern is interrupted after application of current and the output of the magnetic detection sensor when the second excitation pattern is interrupted after application of current. The nugget formation status can be measured.

  Here, a preferable example of the first excitation pattern is a pattern in which the current value is continuously increased from the initial pattern to the final pattern. A preferable example of the second excitation pattern in that case is a pattern in which a pulse is added to the latter half of the first excitation pattern.

  According to the present invention, there is provided an apparatus and a method capable of measuring an internal structure of a magnetic material such as a steel plate, such as a welded state and internal defects, in a nondestructive manner and without being affected by the surface state as much as possible. It is in.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best mode for embodying the present invention will be described in detail with reference to the accompanying drawings. The present embodiment embodies the present invention as a measuring device for measuring the internal structure of a joint portion obtained by superposing two thin steel plates and joining them by spot welding and a measuring method using the measuring device.

  The internal structure measuring apparatus according to this embodiment is configured as shown in FIG. The internal structure measuring apparatus of FIG. 1 has an iron core 21, an excitation coil 22 wound around the iron core 21, and a magnetic sensor array 3 arranged on the surface of one magnetic pole 11 of the iron core 21. The magnetic sensor array 3 has a large number of coils arranged two-dimensionally. Each coil outputs a current that is proportional to the rate of change of the magnetic field strength at that location. That is, the magnetic sensor array 3 has position resolution. The internal structure measuring apparatus of this embodiment further includes a current applying unit 4 for applying current to the exciting coil 22, a pattern storing unit 5 for storing the current application pattern, and a data acquiring unit 6 for acquiring the output of the magnetic sensor array 3. Have.

  The magnetic body to be measured in this embodiment is a thin steel plate. More specifically, a spot welded portion 92 in which two thin steel plates 90 and 91 are overlapped and joined by spot welding is a measurement target portion of this embodiment. A nugget portion 100 exists inside the spot welded portion 92. In addition, the thin steel plates 90 and 91 in the spot welded portion 92 are slightly indented on the surfaces 93 and 94, although not apparent in FIG.

  In the nugget portion 100, the crystal structure is significantly different from the original crystal structure of the thin steel plates 90 and 91 due to the thermal history. Specifically, the crystal grains are fine and the orientation is random. For this reason, in the nugget portion 100, compared to portions other than the nugget portion 100, magnetization is less likely to proceed when a magnetic field is applied. Also, the magnetization is difficult to return even when the application of the magnetic field is interrupted. For this reason, the nugget part 100 has a lower magnetic permeability and a higher residual magnetization than parts other than the nugget part 100.

  In this embodiment, as shown in FIG. 1, a magnetic field H generated from the iron core 21 by energizing the excitation coil 22 by the current application unit 4 is applied to a spot weld 92 that is a measurement target location. Then, the magnetic sensor array 3 monitors the state of the magnetic field between the magnetic pole 11 and the thin steel plate 90 after the energization of the exciting coil 22 is cut off. Specifically, as shown in FIG. 2, the current applied to the exciting coil 22 (hereinafter referred to as coil current) is increased from zero to the peak current I. Then, the coil current is rapidly reduced.

  Here, the time from the start of application of the coil current to the peak current I (excitation time, time t0 to time t1) is about 0.3 to 4.0 milliseconds (preferably 0.3 to 1.5 milliseconds). Seconds). The peak current I is a current that does not reach the saturation magnetization of the thin steel plates 90 and 91 to be measured. Although it depends on factors such as the number of turns of the exciting coil 22, the value here is, for example, about 0.5 to 1.5 amperes. The time until the coil current returns from the peak current I to zero (breaking time, time t1 to time t2) is much shorter than the excitation time, and is about 0.5 to 1.5 microseconds. That is, the horizontal axis in the graph of FIG. 2 is greatly enlarged on the right side from the time t1.

  As described above, in this embodiment, first, a varying magnetic field is applied to the spot welded portion 92 during the excitation time from time t0 to time t1. The strength of the applied magnetic field at the end (time t1) is such a strength that the thin steel plates 90 and 91 do not reach saturation magnetization. For this reason, the excitation state of the thin steel plates 90 and 91 at time t1 has not been stabilized. The inside of the thin steel plates 90 and 91 at the time t1 is still in the process of progressing magnetization due to the influence of the applied magnetic field.

  Therefore, the actual degree of magnetization of the thin steel plates 90 and 91 at time t1 is mainly governed by the following two factors. One is the ease of magnetization in the thin steel plates 90 and 91. Here, the influence of the internal structure of the spot welded portion 92 appears. The other is the current waveform at the excitation time in the graph of FIG. This is because the degree of actual magnetization varies depending on the current waveform. On the other hand, it is not greatly affected by the state of the surfaces 93 and 94 of the thin steel plates 90 and 91.

  Therefore, in this embodiment, two types of excitation patterns are used as the coil current pattern in the excitation time. FIG. 2 shows an example of the first excitation pattern. There are various second excitation patterns, but examples include the one shown in FIG. In the excitation pattern of FIG. 3, the value of the peak current I and the length of the excitation time are the same as those in FIG. However, it differs from the excitation pattern of FIG. 2 in that spike-like pulses P exist during the excitation time. In the excitation pattern of FIG. 3, the coil current at the time of the pulse P exceeds the peak current I. This is not a problem unless the excited state of the thin steel plates 90 and 91 is stabilized at the time t1. The excitation pattern of the coil current is stored in the pattern storage unit 5.

  In this embodiment, the output of the magnetic sensor array 3 is acquired by the data acquisition unit 6 and integrated over the interruption time (time t1 to time t2) after the coil current is interrupted at time t1. The coil current decrease pattern and required time during the cut-off time are constant regardless of the excitation pattern during the excitation time. Thereby, the difference in magnetization of the thin steel plates 90 and 91 at time t1 and time t2 is detected. Of course, the magnetization of the thin steel plates 90 and 91 is on the way to decay. That is, the amount of return of magnetization developed by the magnetic field of the coil current is detected. This is performed after excitation by the first excitation pattern and after excitation by the second excitation pattern.

  Here, since the current pattern at the time of interruption after both excitation patterns is the same, the part resulting from the decrease in the external magnetic field H emitted from the iron core 21 is the same among the outputs of the magnetic sensor array 3 here. Therefore, the difference in the output of the magnetic sensor array 3 between the two excitation patterns is due to the difference in the amount of decrease in magnetization of the thin steel plates 90 and 91.

  In the method of this embodiment, at the time t1, the magnetization state of the measurement object has not yet been stabilized for both excitation patterns. For this reason, the influence of the difference in the excitation pattern up to that point appears in the amount of progress of magnetization at this point. And it is taken as it is as a return amount of magnetization after interruption. Thus, in the method of this embodiment, the difference in internal structure appears more sensitively in the measurement result. For this reason, even if there is a dent on the surface of the object to be measured, the effect is relatively small compared to the case of the conventional method. In this way, it is possible to obtain mainly internal structure information rather than surface state information.

  In the present embodiment, if the magnetized state after excitation is not stable with at least one of the excitation patterns, there is an advantage over the conventional method. However, it is even better if both excitation patterns are the same.

  Here, there are various types of spot welds 92 of the thin steel plates 90 and 91 that are mainly measured in this embodiment, due to the following factors. For example, steel sheet thickness and combination, steel sheet type and combination, steel sheet surface treatment type and combination, angle formed by the rolling direction of the steel sheet, surrounding processing conditions, electrode size and current during spot welding Quantity, etc. Furthermore, the internal structure measuring apparatus of the present embodiment is not limited to the spot welded portion 92 of the thin steel plates 90 and 91, and any other object can be used as a measurement target as long as it is a magnetic material.

  However, the combination of the excitation patterns illustrated in FIGS. 2 and 3 is not necessarily effective for any of the various types of measurement objects as described above. Depending on the type of measurement object, a good measurement result may not be obtained with the combination of the excitation patterns shown in FIGS. Even in such a case, a good measurement result may be obtained by using another excitation pattern. In other words, a combination of excitation patterns should be selected according to the type of measurement target.

  The relationship between the type of measurement object and the preferred combination of excitation patterns is not theoretically elucidated, but a preferred combination can be selected by trial and error. That is, for the same type of measurement object, various samples having different actual nugget diameters are prepared. A combination having a good correlation between the actual nugget diameter of these samples and the measurement result is a preferable combination for the type of measurement object.

  As described above in detail, in the present embodiment, a varying external magnetic field is applied to a spot welded portion of a thin steel plate that is a magnetic body to be measured. However, the magnetization of the thin steel plate should not be stabilized. As a result, the internal structure of the object to be measured is sensitively reflected in the degree of magnetization. This is detected by measuring the return of magnetization after current interruption. Furthermore, this is done with two types of excitation patterns that have a difference in the ability to develop magnetization, and the difference is taken so that the internal structure of the magnetic material can be measured appropriately without being affected by the surface condition. .

  Note that this embodiment is merely an example, and does not limit the present invention. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof. For example, the shape of the iron core 1 is not limited to that illustrated. If the shape of the measurement object permits, it is possible to sandwich the measurement object from the front and back with two magnetic poles. The order of application of the first and second excitation patterns may be reversed.

It is a block diagram which shows the internal structure measuring apparatus which concerns on embodiment. It is a graph which shows one example of the excitation pattern in embodiment. It is a graph which shows another example of the excitation pattern in embodiment.

Explanation of symbols

21 Iron core 22 Excitation coil 3 Magnetic sensor array 4 Current application unit 5 Pattern storage unit 6 Data acquisition unit

Claims (4)

An exciting coil that generates a magnetic field to be applied to the magnetic material to be measured;
A magnetic detection sensor disposed in a path of a magnetic field applied to a magnetic material to be measured by the excitation coil;
A current application unit for applying a current to the excitation coil;
In the internal structure measuring device for a magnetic body having a sensor output acquisition unit for acquiring an output of the magnetic detection sensor,
The current application unit is:
Applying a current that changes over time according to the first excitation pattern to the excitation coil, and then interrupting the current,
After the applied current by the first excitation pattern is cut off to the excitation coil, a current that changes with time is applied according to a second excitation pattern different from the first excitation pattern, and then the current is cut off To do,
The sensor output acquisition unit
Obtaining an output of the magnetic detection sensor when an applied current by the first excitation pattern is interrupted;
Obtaining an output of the magnetic detection sensor when an applied current due to the second excitation pattern is interrupted;
At least one of the first and second excitation patterns is a pattern that does not stabilize the degree of magnetization of the magnetic substance to be measured even when the current is cut off. . An exciting coil that generates a magnetic field to be applied to the magnetic material to be measured;
In the method for measuring the internal structure of a magnetic material, which is performed using a magnetic detection sensor disposed in a magnetic field path applied to the magnetic material to be measured by the excitation coil,
Applying a current that changes with time according to a first excitation pattern to the excitation coil, and then interrupting the current and obtaining an output of the magnetic detection sensor when the current is interrupted,
After the applied current by the first excitation pattern is cut off to the excitation coil, a current that changes with time is applied according to a second excitation pattern different from the first excitation pattern, and then the current is cut off And obtaining the output of the magnetic detection sensor when the current is interrupted,
A method for measuring the internal structure of a magnetic material, wherein at least one of the first and second excitation patterns uses a pattern that does not stabilize the degree of magnetization of the magnetic material to be measured even when the current is interrupted. . The method for measuring the internal structure of a magnetic body according to claim 2,
The spot welded part of the steel plate is the object of measurement,
The spot welding location is determined by the difference between the output of the magnetic detection sensor when the first excitation pattern is interrupted after application of current and the output of the magnetic detection sensor when the second excitation pattern is interrupted after application of current. A method for measuring the internal structure of a magnetic material, comprising measuring a nugget formation state of the magnetic material. In the internal structure measuring method of the magnetic body according to claim 2 or claim 3,
The first excitation pattern is a pattern that continuously increases the current value from the initial pattern to the final pattern,
The method for measuring the internal structure of a magnetic material, wherein the second excitation pattern is a pattern obtained by adding a pulse to the second half of the first excitation pattern.

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