JP2001264298A - Online measurement method of steel sheet softening degree - Google Patents

Abstract

(57) [Summary] [Problem] To provide an online measuring method of steel sheet softening degree which can more accurately determine whether a steel sheet exiting a heating zone of a continuous annealing line is in a partially recrystallized state or a recrystallized state. . SOLUTION: An eddy current is generated by applying an alternating magnetic field to a steel sheet 10 that has exited a heating zone of a continuous annealing line, a magnetic field generated by the eddy current is measured, and the softening degree of the steel sheet is determined based on the measurement result. In the on-line measuring method for the degree of softening of a steel sheet to be measured, the frequency of application of the AC magnetic field is set to 100 kHz or more, and the measurement area of the magnetic field created by the eddy current is set to 25 mm ² or less. Preferably, a fluctuation range of at least one, more preferably both, of the magnetic field strength and phase is derived from the measurement result, and the softening degree of the steel sheet is determined using the result.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for online measurement of the degree of softening of a steel sheet.

[0002]

2. Description of the Related Art In the course of the production of ordinary cold-rolled steel sheets, ultra-thin steel sheets for cans, and certain types of continuous hot-dip galvanized steel sheets, the processing strain due to cold rolling is removed during the production to obtain the desired material (strength). , Ductility). The annealing method uses coils (steel strip coils) in terms of quality characteristics and productivity.
Is a continuous annealing method in which the steel sheet is continuously passed through a high-temperature furnace while being continuously unwound.

[0003] Annealing conditions are set based on the softening characteristics of the steel sheet. The softening characteristics of the steel sheet include, for example, a temperature vs. hardness curve A and a temperature vs. elongation (ductility) curve B as schematically shown in FIG.
It is represented by The process of softening, as the temperature rises,
The process sequentially proceeds to each stage of recovery in which dislocation density decreases, partial recrystallization in which locally recrystallized grains are formed, and recrystallization in which recrystallized grains are entirely formed. The material change (hardness decrease, elongation increase) due to temperature rise is small in the recovery stage, but increases in the partial recrystallization stage and decreases again in the recrystallization stage. Note that grain growth occurs on the high temperature side of the recrystallization stage. In the recrystallization stage, the material is stable, but if grain growth occurs, it causes problems such as roughening during molding,
In many cases, the annealing temperature in the production range of the actual machine is set to a temperature that does not cause grain growth in the recrystallization stage, that is, a narrow range near the lower limit within the recrystallization temperature range.

[0004] The softening characteristics of a steel sheet vary depending on the type of steel (composition) and the conditions of hot rolling and cold rolling. For example, FIG. 4 is a graph showing the effect of the amount of Nb on the softening characteristics of the ultra-low carbon steel sheet for cans. As shown in the figure, the lower limit of the recrystallization temperature range greatly changes due to the addition of a small amount of Nb. Further, as shown in FIG. 5, for example, the hardness changes depending on the annealing time, and the softening progresses as the time is extended. HR30T is an index indicating hardness, and the larger the value, the harder it is.

Therefore, in order to stably soften a steel sheet to a predetermined level, operating conditions (temperature (furnace temperature) and time (line speed)) of continuous annealing are appropriately set according to the steel composition and other factors. There is a need. However, as described above, the softening characteristics of steel fluctuate considerably due to minute fluctuations of these factors. Therefore, the fluctuations of these factors are detected online during continuous annealing, and the operating conditions are promptly determined in accordance with the results. It is extremely difficult to control to a proper value.

It is considered that this difficulty is overcome by a technique in which the degree of softening (the degree of progress of recovery and recrystallization) of a steel sheet during continuous annealing is directly measured in real time, and operating conditions are controlled based on the result. Japanese Patent Application Laid-Open No. H11-153581 discloses an annealing method characterized by measuring magnetic properties of a steel sheet in at least one place after a heating zone of an annealing furnace in a continuous annealing step of the steel sheet. An online measuring method of the progress of recovery and recrystallization of a steel sheet in the inside has been proposed. More specifically, an AC magnetic field is applied to a steel sheet to generate an eddy current in the steel sheet, and the strength of the magnetic field based on the eddy current is measured. According to this, it is expected that the release state of the strain of the steel sheet during annealing can be quickly measured on-line, and continuous annealing operation under optimum conditions can be performed based on the obtained measurement results.

[0007]

However, the above-mentioned Japanese Patent Application Laid-Open No.
According to the method of JP-153581-A, particularly in continuous annealing of ultra-low carbon steel for cans, a relatively high rate of material defects due to overheating occurred. Note that the raw burning refers to a phenomenon in which the furnace is released at the stage of partial recrystallization, resulting in excess strength and insufficient ductility.
That is, in the method disclosed in the publication, the partial recrystallization state may be erroneously determined to be the recrystallization state, and there is still room for improvement.

In view of the problems of the prior art, the present invention provides a softening degree of a steel sheet which can more accurately determine whether a steel sheet exiting a heating zone of a continuous annealing line is in a partially recrystallized state or a recrystallized state. It aims to provide an online measurement method.

[0009]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, have limited the frequency of applying an alternating magnetic field to a steel sheet to a specific value or more, and have reduced the measurement area of the magnetic field due to eddy current. When it is limited to a specific value or less, the strength and phase of the magnetic field created by the eddy current will show a characteristic change according to the degree of softening of the steel sheet, so that the partially recrystallized state and the recrystallized state can be more clearly distinguished. And made the present invention.

That is, the present invention provides a method for adding a continuous annealing line.
An eddy current is generated by applying an AC magnetic field to a steel plate that has left the tropics.
And measure the magnetic field created by the eddy current, and based on the measurement results
Measuring the degree of softening of the steel sheet online
In the measurement method, the applied frequency of the AC magnetic field is set to 100 k.
Hz or more, the measurement area of the magnetic field created by the eddy current is 25 mm
^TwoOnline of steel sheet softening degree characterized by the following
It is a measuring method.

In the present invention, it is preferable to derive at least one variation range of the strength and phase of the magnetic field from the measurement result, and to determine the degree of softening of the steel sheet using the result. In the present invention, it is most preferable to derive the fluctuation ranges of both the strength and the phase of the magnetic field from the measurement results and determine the softening degree of the steel sheet using the results.

The above-mentioned "phase" means a phase difference from an applied AC magnetic field (the same applies hereinafter).

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, as shown in FIG. 1, for example, at least one location on the heating zone side of a continuous annealing line is formed so as to face a steel sheet 10 coming out of the heating zone.
An eddy current sensor 6 composed of a secondary coil 1 and a secondary coil 2 is provided, and an AC current is input to the primary coil 1 from an AC power supply 3,
The output of the secondary coil 2 is processed by the signal processing means 4, and the degree of softening of the steel sheet is determined based on the result.

The signal processing means 4 includes an analog signal processing and display device such as a pen recorder and an oscillograph, a digital computer having a function of binarizing an input analog signal, storing and calculating the result, and displaying a result. An appropriate one can be used. In this example, a nested eddy current sensor in which a primary coil and a secondary coil are nested is used, but the present invention is not limited to this.

A magnetic field that penetrates the steel plate 10 is applied to the steel plate 10 by the alternating current input to the eddy current sensor 6,
An eddy current flows through 10 so as to repel the applied magnetic field, and the eddy current creates a magnetic field (referred to as an eddy current magnetic field) that repels the applied magnetic field. The eddy current magnetic field varies depending on the electrical conductivity, magnetic permeability, shape, size, distance between the steel sheet and the coil, and input frequency of the alternating current of the steel sheet. On the other hand, the electrical conductivity and the magnetic permeability differ depending on the dislocation density, the precipitate, the crystal grain size, etc., which govern the degree of softening of the steel sheet. Therefore, when the shape and dimensions of the steel sheet, the distance between the steel sheet and the coil, and the input frequency of the alternating current are constant, the eddy current magnetic field is uniquely determined by the degree of softening of the steel sheet 1, and the state is determined by the output signal of the eddy current sensor 6. (The output voltage), the softening degree of the steel sheet can be measured based on the output signal (ie, the measurement result of the eddy current magnetic field).

Conventionally, the effective value of the output voltage of an eddy current sensor is measured, and the degree of softening is determined from the difference between the result (actually measured value) and the result of similar measurement (reference value) for a standard sample (recrystallized sample). The judgment method was taken. Further, there is no limitation on the input frequency to the eddy current sensor, and the measurement area of the eddy current magnetic field is relatively large. Here, the measurement area refers to the measurement surface area 5, that is, the area of the end face of the secondary coil 2 facing the steel plate (including both the winding portion and the hollow portion).

However, in such a conventional method, the annealing temperature is reduced.
Must be set in a narrow range near the lower limit within the recrystallization temperature range
If not, the partial recrystallization state is erroneously determined as the recrystallization state.
May cause considerable defects due to raw burning.
Was. The present inventors believe that the cause of this erroneous determination is that the measurement range
The volume fraction of the recovered part (unrecrystallized part) that is too large
Is so small that it cannot affect the eddy current
Assuming that there is, an experiment to verify this point
As a result, the input frequency of the AC current (that is, the
(Wave number) 100kHz or more to enhance the skin effect and increase the thickness of the steel sheet
Direction measurement range is limited to the steel sheet surface side, and eddy current magnetic field
Measurement area of 25mm ^TwoThe measurement range within the steel plate surface is controlled as follows.
Limits the number of erroneous determinations and reduces the
Even if the efficacy value is used to judge raw burning,
It was found that the incidence was reduced to about 70% of the conventional rate. Yo
Therefore, in the present invention, the AC magnetic field application frequency is 100 kHz or more,
Measurement area of current magnetic field is 25mm^TwoLimited to the following.

Further, for a steel sheet sample (0.2 mm thick × 150 mm width × 200 mm length) adjusted to various softening degrees by laboratory heat treatment, an eddy current is generated by the method of FIG. And the output voltage is displayed on the amplitude vs phase display screen owned by the measuring means 4.
As shown in the schematic diagram of FIG. 2, the amplitude and phase of the output voltage from the steel sheet sample in each state of unannealed, recovered, partially recrystallized, and recrystallized vary within regions I to IV in FIG. I understand.

That is, a raw burn state (partially recrystallized state)
The characteristic of the steel sheet is that the amplitude and phase of the output voltage continuously measured in the passing direction fluctuate more greatly than the steel sheet in the recrystallized state. Such a variation could not be detected in the prior art where the measurement range was large. Therefore, a threshold value is set for at least one of the fluctuation amounts of the amplitude and the phase derived from the output voltage of the eddy current sensor, and the burning is determined in accordance with the threshold value of the fluctuation amount, that is, the strength of the magnetic field is determined from the measurement result of the magnetic field generated by the eddy current. It is preferable to derive at least one of the fluctuation ranges of the phase and the phase and use the result to determine the degree of softening of the steel sheet in order to further reduce the number of erroneous determinations.

However, if the degree of softening is determined based on only the variation of either the amplitude or the phase, even if the steel sheet is in a partially recrystallized state, it may be rarely found that the steel sheet has somewhere in the length of the steel sheet. It is conceivable that the fluctuation range of the amplitude and the phase is biased toward the upper left portion of the region III in FIG. 2, and in such a case, the fluctuation amount becomes smaller than the threshold value, which may cause erroneous determination. Therefore, from the viewpoint of making the erroneous determination less likely to occur, the area IV in FIG. 2 is formulated by approximating the area IV with, for example, an ellipse or the like, and the determination of the raw burn is made when the fluctuation range of the amplitude and the phase is located outside the area IV. That is, it is most preferable to derive the fluctuation range of both the strength and the phase of the magnetic field from the measurement result of the magnetic field generated by the eddy current, and use the result to determine the degree of softening of the steel sheet.

[0021]

[Example] The typical composition is 0.0011% C-0.16% Mn-0.045%
The present invention was carried out in a continuous annealing process of an ultra-low carbon cold rolled steel sheet for a can (sheet thickness 0.15 to 0.60 mm) which became Al-0.017% Nb-0.0019% N. The annealing operation conditions for this steel sheet are as follows:
0 to 800 ° C, annealing time 60 to 100 seconds (line speed 600
10001000 m / min).

In the practice, the eddy current sensor shown in FIG. 1 was arranged and used on the heating orifice side of the continuous annealing line. The measurement area of the eddy current sensor was 20 mm ^2, and the input frequency of the alternating current was 150 kHz. Regarding the determination of raw burning, when the data of the amplitude and phase is derived every 2 m of the steel plate length section from the output voltage of the eddy current sensor, and all the data of the continuous 10 sections are out of the predetermined recrystallization correspondence range. It was decided that it was a raw burn, and an alarm was issued.

Before the embodiment of the present invention, the measurement area of the eddy current sensor was 100 mm ² , the input frequency was 80 kHz, and the burning was caused by the difference between the effective value of the output voltage and the reference value (the value measured similarly for the standard sample). Had been determined. By implementing the present invention, a favorable result was obtained in which the rate of occurrence of defects due to unburnt burning, which had been about 0.3% until then, was greatly reduced to about 0.1%.

[0024]

According to the present invention, it is possible to more accurately determine whether a steel sheet that has exited the heating zone of a continuous annealing line is in a partially recrystallized state or a recrystallized state. This has the effect of greatly reducing the rate.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of an eddy current sensor used in the present invention.

FIG. 2 is a schematic diagram showing a correspondence relationship between each state of unannealing, recovery, partial recrystallization, and recrystallization, and a fluctuation range of an amplitude and a phase of a secondary coil output voltage.

FIG. 3 is a schematic diagram illustrating an example of expressing softening characteristics of a steel sheet.

FIG. 4 is a graph showing an example of the effect of the amount of Nb on the softening characteristics of an ultra-low carbon steel sheet for cans.

FIG. 5 is a graph showing an example of the effect of annealing temperature and annealing time on the hardness of a steel sheet.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Primary coil 2 Secondary coil 3 AC power supply 4 Signal processing means 5 Measurement surface area 6 Eddy current sensor 10 Steel plate

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hideo Kukuminato 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Chiba Works of Kawasaki Steel Corporation (72) Inventor Makoto Araya 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki (72) Inventor Koji Ito 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term (reference) 2G053 AA17 AB21 BA15 BB03 BC02 BC07 BC14 CA03 CB24 CB25 DA01

Claims (3)

[Claims] 1. An eddy current is generated by applying an AC magnetic field to a steel sheet that has exited a heating zone of a continuous annealing line, a magnetic field generated by the eddy current is measured, and a degree of softening of the steel sheet is determined based on the measurement result. An on-line measuring method for a steel sheet softening degree, wherein an applied frequency of the AC magnetic field is 100 kHz or more, and a measurement area of a magnetic field generated by the eddy current is 25 mm ² or less. 2. The method according to claim 1, wherein a variation range of at least one of a magnetic field intensity and a phase is derived from the measurement result, and the degree of softening of the steel sheet is determined using the result. 3. The method according to claim 2, wherein a variation range of both the strength and the phase of the magnetic field is derived from the measurement result, and the degree of softening of the steel sheet is determined using the results.