JPS63188463A - Method for detecting molten steel surface condition in mold for continuous casting - Google Patents

Abstract

PURPOSE:To quicken detection and to improve detection accuracy by constituting a member of detector by non-magnetic body having the specific magnetic permeability, exciting a detecting coil by current having the specific frequency and further arranging a storage device of magnetic flux unbalanced value. CONSTITUTION:The member and parts containing the detector 1 composing of a magnetic sensor, cable 14, etc., and making to support and work the detector 1 are all constituted by using the non-magnetic body having 1.003 the max. magnetic permeability. The frequency of the power source exciting the coil 1c of detector 1 is set to the high frequency avoiding a higher harmonics zone by the frequency for each current except the detector 1. Further, a RAM15 storing environmented effective value to the detector 1 is arranged in the control circuit. By this mechanism, as the magnetic flux balance type magnetic sensor detector 1 detects the molten steel surface condition under completely cutting off the effect of environmented magnetic field formed by the electromagnetic function, the accuracy of detection is improved and the detection is quickened.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to the surface level of molten steel in a continuous casting mold (hereinafter referred to as continuous casting mold) and the behavior of the molten steel surface at each level ( This invention relates to a method for accurately detecting lip ring (hereinafter referred to as lip ring).

[Conventional technology]

The applicant has developed a device for detecting the surface level of molten steel in a continuous casting mold and the lip ring condition at each level, and has already filed an application for this (Japanese Patent Laid-Open No. 55-14
No. 9017: Special Publication No. 60-8900). When this device is used to detect the surface level and lip ring of molten steel in a continuous casting mold using a magnetic flux balance type magnetic sensor, the magnetic flux balance condition is disturbed due to the influence of long or short pieces of the continuous casting mold, resulting in false detection. It causes

Therefore, in order to eliminate this drawback, the present applicant has previously investigated and memorized the magnetic flux imbalance situation due to magnetic materials including long pieces and short pieces of continuous casting molds that exist in the operating environment of the magnetic sensor. (Refer to Japanese Patent Laid-Open No. 161828/1982) . With this method, the influence of the magnetic material present in the operating environment described above on the magnetic sensor is substantially rendered harmless.

However, in order to meet the needs of steel materials for steel structures in the extremely low temperature range, which has expanded in recent years, and thin steel sheets, which continue to strongly demand stable and economically superior workability, In order to improve the cleanliness of molten steel during continuous casting by reviewing steel materials from the molten steel stage, it has been proposed to incorporate electromagnetic stirring functions, electromagnetic braking functions, etc. (hereinafter referred to as electromagnetic functions) into continuous casting molds. There is. An example of this is JP-A-56-160862, and another example is JP-A-57-160862.
-17356 publication.

FIG. 3 shows an example of a continuous casting apparatus equipped with this electromagnetic function. That is, a rectangular casting space is defined by a long mold piece a and a short mold piece forming a continuous casting mold, and molten steel is poured into the rectangular casting space. This continuous casting mold is made of water-cooled 1Ti
It is cooled by Ic and is equipped with an electromagnetic coil d. The molten steel injected into the casting space is sent out from the continuous casting mold while being cooled and solidified, and is carried out as a slab f by guide rollers e.

[Problem that the invention seeks to solve]

In such a continuous casting mold, conventionally, the surface level and/or lip ring of molten steel in the casting space is detected using the above-mentioned flux-balanced magnetic sensor. However, since the continuous casting mold is equipped with the electromagnetic coil d, a new environmental magnetic field is introduced into the operating environment of the flux-balanced magnetic sensor. As a result, flux-balanced magnetic sensors, which had been used smoothly and stably up until then, began to malfunction, cause erroneous detections, and output erroneous signals, making them no longer practical.

An object of the present invention is to restore the practicality of this flux-balanced magnetic sensor and to accurately detect the surface condition of molten steel in a continuous casting mold.

[Means for solving problems]

In order to solve the above-mentioned problems, the method for detecting the surface condition of molten steel in a mold for continuous casting of the present invention is provided by arranging a detector consisting of a magnetic flux-balanced magnetic sensor to face the surface of molten steel in a mold for continuous casting. and the molten steel surface, and driving the detector in a direction toward and away from the molten steel surface so as to eliminate magnetic flux imbalance that occurs when the molten steel surface is displaced; In the method of detecting the position of the molten steel surface based on the advancing and retreating positions of the detector that has achieved magnetic flux balance, the housing of the detector and a mount supporting or holding the housing include: 1. An excitation current for an electromagnetic function or a transformer for the excitation current, in which members or parts such as cables are made of a non-magnetic material with a maximum magnetic permeability of 1.003 or less, and the detector is installed in the continuous casting mold in which the detector is used. The excitation coil of the detector is excited with an excitation current of a frequency outside the harmonic range of each frequency of the power supply to the The means is to store the magnetic flux unbalance value in advance in a storage device, and to compensate the output of the detector based on the magnetic flux unbalance value during actual measurement.

By separating the compensated output signal of the detector into a molten steel surface level signal in a continuous casting mold and a surface behavior signal at each molten steel surface level for control and/or display, the lip ring can be controlled and/or displayed. Automatic control or monitoring becomes possible.

[Effect]

The present inventors investigated the reason why the flux-balanced magnetic sensor lost its practicality.

First, the detection coil (FIG. 1) of a magnetic flux-balanced magnetic sensor (for example, the detector indicated by reference numeral 1 in FIGS. 1 and 2).

In the circuit (indicated by the reference number IC in FIG. 2), the molten steel surface level signal in the continuous casting mold when the electromagnetic function installed in the continuous casting mold is activated and when it is not activated (the amplifier 8 in FIGS.
), the level signal, and a superimposed signal of the rippling signal (each output of the comparator 5 in FIG. 1 and the D/A converter 24 in FIG. 2, that is, the output signal of the detection coil IC) We investigated changes in the signal obtained by correcting the influence value due to the magnetic material and further subtracting the reference distance signal. The result is the fifth
As shown in the figure.

As a result, it was found that the deviation signal contained harmonic components, the level signal showed a higher level than the actual one, and the ripple was displayed more intensely than the actual one.

Next, following this factor, in the excitation coil (indicated by reference numerals 1a + lb in Figs. 1 and 2) circuit of the magnetic flux balanced type magnetic sensor, when the electromagnetic function installed in the continuous casting mold is activated, the excitation is changed when it is not activated. A frequency analysis of the current was performed.

The results are as shown in Figure 4. There are many harmonic components near the excitation frequency of 1.6 kHz in the excitation coils la and lb, and there are also many and strong harmonic components above and behind 5. It turns out that it exists.

In other words, in order to detect the surface condition of molten steel inside the continuous casting mold using a flux-balanced magnetic sensor and to understand the level and lip ring, it is necessary to detect the various harmonics formed by the newly introduced continuous casting mold with electromagnetic function. make it disappear,
It has become clear that unless this problem is avoided, practicality in new fields will not be established.

Based on these findings, the present inventors have investigated the extinction of various harmonics, the confinement and cutoff of harmonics, and the avoidance of harmonics.Currently, the latter method is industrially economical and technically reliable. I learned that it is advantageous in terms of. Therefore, we analyzed the causes of the generation of the two types of harmonic components. As a result, the component near 1.6 is a harmonic component of the frequency of the excitation current for operating the electromagnetic function with the continuous casting mold, and the component around 5 KHz is the excitation current for operating the electromagnetic function with the continuous casting mold. It turned out to be a harmonic component of the power supply frequency supplied to the frequency converter used to obtain current.

Therefore, the present inventors determined that the frequency of the excitation current of the excitation coils la and lb is the harmonic frequency range of the excitation current of the electromagnetic function installed in the continuous casting mold, and the frequency of the power supply that is the source of the excitation current. A confirmation test was conducted by exciting the excitation coil at 8KHz, avoiding harmonics.

The results are shown in FIG. 6, and it was found that both the comparator 5 and the amplifier 8 produced abnormal outputs, although the amplitude variation was different from the data shown in FIG. 5.

Since this cause remains after removing the influence of the environmental magnetic field, the present inventors have investigated the excitation coil and detection coil of the detector 1 in the magnetically balanced magnetic sensor shown in FIGS. 1 and 2. Considering the influence of the mutual inductance M, the parts constituting the housing that supports or holds these, the cable 10 that operates the detector l shown in FIGS. 1 and 2, and the like. Components such as a pedestal (not shown) that supports the

When we investigated the materials of all the parts and parts, we found that the same stainless steel cable as Cable IO has a permeability of about 1.03, which is generally considered the maximum permeability of non-magnetic materials (Institute of Electrical Engineers of Japan,
Published May 15, 1971 [Electrical Engineering Handbook] 4
SOS 301 (page 04) emits false signals due to the magnetic influence of the environmental magnetic field, but once annealed, the maximum permeability is 1.
．． It was found that the magnetic field was about 0.003, which means that the magnetic field was substantially unaffected by the environmental magnetic field.

Furthermore, even if all the members and parts are made of non-magnetic materials, if the surface treatment of the small screws etc. that attach each part to the members is plated with a magnetic material, such as nickel, as shown in Figure 6. It was found that this resulted in

Based on these findings, the inventors of the present invention repeatedly conducted experiments and studies, including the surface treatment of metals for screws, and found that the maximum magnetic permeability was 1.
．． It has been found that if the device is constructed from a non-magnetic material of 003 or less, the state shown in FIG. 6 becomes the state shown in FIG. 7, and the problem is solved.

This demonstrated that the surface level and surface behavior of the molten steel in the continuous casting mold could be detected completely reliably and accurately, whether or not the electromagnetic function was activated.

In order to verify the above-mentioned measures, the present inventors returned the frequency of the excitation current of the excitation coil of the detector (magnetic sensor) 1 from 1.6 to 7 using the device that obtained the results shown in Fig. 7. We conducted an experiment. The results are shown in FIG. Looking at Figure 8, there is a difference in level as a level signal! However, it has been found that the rip ring signal, which causes powder entrainment and the like and therefore requires accurate detection and appropriate countermeasures, exhibits an abnormality that is greater than the actual situation.

From the above experiments conducted by the present inventors, the magnetic material interposed in the level and rip ring detection side devices increases the strength or magnitude of the detection signal under the influence of the environmental magnetic field, falsely reporting the level, It was discovered that the harmonics generated during the formation locally caused the amplitude of the detection signal to be excessive, causing false alarms about the rip ring situation. It was confirmed that the measures proposed by the invention were also appropriate.

The present invention was made based on these findings. That is, the present invention detects and measures the level and lip ring of the surface of molten steel in a continuous casting mold using a magnetic flux balanced magnetic sensor in continuous casting. and a cable, and a cable actuation mechanism), so that the influence of environmental magnetic materials on the magnetic field formed by the sensor, that is, the change in mutual inductance M between the excitation coil and the detection coil, is substantially rendered harmless. At the same time, it eliminates the influence of the magnetic field existing in the environment and the magnetic material of the sensor on the magnetic field formed by the sensor, that is, the change in the mutual inductance M, and eliminates the harmonics generated due to the formation of the environmental magnetic field. The influence, ie the superimposition with the excitation frequency of the excitation coil, can be completely avoided. This makes it possible to restore the practicality of the flux-balanced magnetic sensor.

〔Example〕

Next, the present invention will be described in detail with reference to the drawings.

FIGS. 1 and 2 show examples of the configuration of the apparatus used in the first and second embodiments of the present invention, respectively. In either example, the detector (magnetic sensor) is excited by the excitation current of the electromagnetic function installed in the mold and the frequencies of 3 to 57 and 50 to 60 Hz of the power supply used to generate the excitation current. An 8 KHz power source was used, avoiding frequencies below 6 KHz where harmonics generated from (3-phase) etc. exist. Moreover, the detector 1. The nine components of the frame 9 that support and operate the cables 14 and the like are all made of non-magnetic materials, including plated metal, with a maximum magnetic permeability μ of 1.003 or less.

Next, the embodiment shown in FIG. 1 will be explained in detail.

In this example, a detector 1 to which a magnetic flux balanced type magnetic sensor is applied is used. This detector 1 was developed in Japanese Unexamined Patent Publication No. 58
It has a structure similar to that disclosed in Japanese Patent No.-161828.

When the molten steel 13 in the mold 12 moves up and down, the detector 1 detects the change in the magnetic field as a voltage with the detection coil IC placed between the excitation coils la and lb, and the distance detection circuit 3 amplifies the signal and sends it to the comparator. Enter 5. Since the comparator 5 receives the set voltage manually from the reference level setter 4, the signals are compared here and the deviation value is input to the motor driver 6, which drives the servo motor 7 and sends it through the winding drum 11. The drive cable 10 is driven and the detector l is controlled via the conductive foil 14 so that the detected value of the magnetic field is constant. In this case, the value may be inductance or impedance.

On the other hand, the potentiometer 2 is also driven by the servo motor. The output of the potentiometer 2 is input to an amplifier 8, and the level is displayed on a level display 9. Here, solid lines indicate signals, and dashed lines indicate mechanical connections. In this way, when detecting changes in the magnetic field and keeping the value constant, or when keeping the inductance value of the electromagnetic coil always at a constant value, it is not always possible to make contact with the detector 1 due to the influence of the mold 12. The distance r between 113 and the hot water level is not constant.

Therefore, measured values that vary depending on factors other than the distance between the metal surface and the detector, that is, environmental impact values, are stored in advance in a storage device.
During actual measurement, it is read out from the storage device and added to the measurement control system from the detector to the motor driver 6 as a correction value.

Simply put, the value obtained by removing the environmental influence value from the detected value is used as the detector-metal surface distance signal.

Static RAM (Random Access Memory H5
The read/write address of is determined by the output data of the A/D converter 16. The A/D converter 16 has a selector switch 1.
7, set the environmental impact value in RAM15 (memory)
When performing measurement, the output of the amplifier 8, that is, the detector position signal is applied, and during actual measurement, the addition output of the adder 18 is applied. During actual measurement, the vibrator 19 applies vertical vibration with a constant period and constant amplitude to the mold 12, and the relative position between the detector 1 and the mold 12 changes periodically. Therefore, the displacement detector 20 applies the vibration displacement amount based on the bottom dead center of the vibration to the adder 18 and adds it to the position amount of the detector 1, that is, the output of the amplifier 8. The relative position of is given to the A/D converter 16. RAM15, A/D converter 2
1 gives write data. A distance detection signal is applied to the A/D converter 21 from the circuit 3 via the open/close switch 22 when setting the environmental influence value in the RAM 15.

When the R/W (read/write) instruction switch 23 is closed, the RAM 15 transfers stored data (l! environment influence value) at an address determined by the output data of the A/D converter 16 to the D/A converter 24. The D/A converter 24 converts the read data into analog and applies it to the adder 25.

The adder 25 adds the reference value (distance instruction value between the metal surface and the detector 1) from the setter 4 to the read value and provides the result to the comparator 5. Environmental impact values (hereinafter referred to as environmental data) are stored in RAM.
15, a selector switch 26 is inserted between the comparator 5 and the motor driver 60, and one of the selection contacts of the switch 26 is connected to the motor forward/reverse direction instruction switch 2.
Connect 7.

In FIG. 1, 28 is a switch for storing environmental data, 29 is a write instruction switch, and 30 generates a write instruction pulse (plus H) of a predetermined time width when there is a change in the output code of the A/D converter 16. This is a code change detector. The code change detector 30 constantly compares the human code with the gamma code, and if the input code differs from the latch code (the initial state is a code indicating "0"), the code change detector 30 updates the input code, latches it, and outputs a pulse with a predetermined time width. do.

Next, the operation of the environmental data memory will be explained. First, the operator closes the switch 28. In conjunction with this, the changeover switch 26 connects the switch 27 to the motor driver 6, the open/close switch 22 closes, and the changeover switch 17 connects the output line of the amplifier 8 to the A/D converter 16. Next, the operator places the mold 12 at the bottom dead center of vibration, and switches the switch 27
is closed to the +VCC side or -VCC side and the servo motor 7 is driven in forward or reverse rotation to move the detector 1 to the lowest or highest point of the vertical movement range. For convenience of the following explanation, it is assumed that the servo motor 7 is driven in normal rotation (+Vcc is applied) and positioned at the lowest point. Next, the operator closes switch 29 and switches 27 to 1. Connect to C side. By closing the switch 27 to the -vce side, the servo motor 7 is reversely energized at a constant speed, and the detector 1 is raised at a constant speed. With this rise, the output voltage (detector position signal) of the amplifier 8 changes linearly, and the output code of the A/D converter 16 changes at a predetermined period.

At the time of this change, the detector 30 emits a pulse, which closes the switch 23 via the switch 29 and stores the information in the RAM 15.
Instruct "memory". As a result, every time the detector 1 increases by an amount equivalent to the resolution of the A/D converter 16, that is, the A/D
Every time the output code of the converter 16 changes, the detected output data, that is, the environmental data at that time, is stored in the address determined by the output code. When detector l reaches the uppermost point, the operator returns switch 27 to the neutral position to stop motor 7 and opens switch 29.

Next, the operation during actual measurement will be explained. Before pouring the molten steel 13 into the mold 12, the operator switch 28 is opened. When the switch 28 opens, the switch 26 connects the comparator 5 to the motor driver 6, the switch 22 opens, and the switch 17
connects the adder 18 to the A/D converter 16. As a result, the adder 18 provides the A/D converter 16 with a relative position signal obtained by adding the correction for the displacement due to vibration of the mold 12 to the position of the detector 1 (amplifier 8 output).
The access address 5 is determined by the relative position data between the vibrating mold 12 and the detector 1, and the environmental data at that address is converted into an analog value by the D/A converter 24 and given to the adder 25, where the predetermined value is It is added to the value indicating the distance l and applied to the comparator 5. The comparator 5 compares the output of the distance detection circuit 3 and the output of the adder 25 and determines the difference between the two, that is, the accurate distance measurement value that is the value obtained by subtracting the environmental impact value from the detection output, and the set value, that is, the target value. A signal (plus, minus) indicating the difference with the distance is given to the motor driver 6 via the switch 26. As a result, follow-up control is performed to maintain the distance between the metal surface and the detector 1 at the target value even if the metal surface moves up and down, as in the conventional case.

In the past, as mentioned above, since the detector is inaccurate, the distance between the metal surface and the detector 1 deviates greatly from the target value.
Therefore, the indicated value of the level indicator 9 had a large error, but in this embodiment, the environmental influence value that causes such an error is canceled out by the comparator 5, so that the detector can be accurately placed above and below the metal surface. 1 follows, the distance between the metal surface and the detection is maintained at a constant value, and therefore the indicated value of the level indicator 9 is accurate.

The memorization operation described above was performed without molten steel in the mold, but in order to further reduce the error, a metal plate, such as a lead plate or an aluminum plate, was fixed below the detector for a certain period of time. However, it is preferable to use a method in which the detector is moved up and down within the mold in this state and the values of the detector corresponding to each level position are stored.

FIG. 2 shows another device configuration. For this purpose, a micro combinator system 31 is used, and a key switch 32 is used.
According to the operation, environmental data memory and detector tracing control during actual measurement are performed. This microcomputer system 31 includes a central processing unit CPU31.
a, read-only memory ROM31 b, input/output circuit l1031C, A/D:I inverter 31d and RA
Equipped with M2S. This system 31 controls the motor drive to position the detector 1 at the highest point when the key switch 32 instructing the memory of environmental data is closed, and then sets the motor driver 6 to low-speed descending drive. Performs memory of environmental data. In this memory operation, the central processing unit CPU31a instructs the A/D converter 31d to A/D convert the input channel 3 at regular intervals, reads the conversion data, and determines whether the conversion data is the previously captured conversion data, If it is different, the converted data imported this time is
A Set the write address of M2S and converter 31
Instruct A/D conversion of input channel 2 to d, set the conversion data to the memory data line of RAM M2S, and
Instruct AM15 to write. While repeating this, when the conversion data of input channel 3 reaches a value indicating the lowest point of detector l, the motor driver 6 is instructed to stop and drive upward, and the conversion data of input channel 3 reaches a value indicating the highest point. When this happens, the motor driver 6 is instructed to stop the motor.

When the key switch 32 that instructs actual measurement is closed, the system 31 sequentially instructs the A/D converter 31d to perform manual channel 1.2 and A/D conversion to obtain vibration displacement data and detection distance. Read the data and detector position data, and store the vibration displacement amount + detector position in RAM15.
is accessed to read the environmental data, and the balance obtained by subtracting the environmental data value from the detection distance data value, that is, the correct measured value after compensation, is compared with the preset target distance data, and the former is compared to the latter. An operation of energizing and setting the motor in the matching direction is performed at a predetermined cycle.

The results of measurement using the above-mentioned apparatus are as shown in FIG. 7, and normal level signals and ripple signals were obtained without being affected by the environmental magnetic material or the environmental magnetic field.

To separate the rippling signal from this mixed signal, as shown in FIGS. 1 and 2, the superimposed signal of both signals is passed through the rectifier circuit 40 to obtain a DC conversion output, and this is set to the rippling permissible limit, that is, the alarm. The transmission reference value R0 is output to the set comparator 41 to obtain a rippling alarm signal. Said direct/
Recording the current conversion output provides a special record of the rip ring level, and feeding the outputs of the rectifier circuit 40 and comparator 41 to a rip ring controller (not shown) enables automatic rip ring control.

In this embodiment, the elevation of the detector 1 is controlled by the wire cable 1.
The drum 11 wrapped with 0 is driven by the servo motor 7, but instead of this, a convex cable is connected to the piston rod of a hydraulic cylinder, and the pressure of the piston rod is adjusted by a servo valve instead of the servo motor 7. The detector 1 may be raised and lowered by being driven by oil.

This is preferable because the detector 1 can be lowered more quickly and smoothly.

〔Effect of the invention〕

As explained above, the present invention completely blocks the influence of the environmental magnetic field formed by the electromagnetic function added to the continuous casting mold, and makes it possible to detect the surface condition of molten steel inside the mold using a magnetic flux balanced type magnetic sensor. There is. This makes it possible to perform continuous casting while accurately and quickly performing level control and rip ring control using a mold with an electromagnetic function, which is most effective when manufacturing steel materials that meet the improvement of material quality due to expanding demand for applications. This has great effects on the field of art, such as making it possible to economically and dramatically improve the quality of steel materials.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the configuration of a device used in an embodiment of the present invention;
Fig. 2 is an explanatory diagram of the configuration of the device used in another example, Fig. 3 is an explanatory diagram of the installation situation of the mold with electromagnetic function, and Fig. 4 is the detector when the electromagnetic function installed in the mold is activated. Figure 5 shows the frequency distribution of the current flowing through the circuit of the excitation coil. Figure 5 shows the level and deviation of the electromagnetic function when it is turned on and off when a conventional mold level detection device is used in a mold with an electromagnetic function. A diagram showing the output signal, Figure 6, shows a conventional mold hot water bath in which the excitation coil of the detector is excited with an excitation current that avoids the noise frequency range that has harmonics of each frequency of the excitation current of the electromagnetic function and the power source of the excitation current. The output signals of the level and deviation when the electromagnetic function is turned on and off are shown when using the device in the surface detection method. FIG. 7 shows the level and deviation output signals in a test similar to FIG. 6 by an apparatus implementing the method of the invention. Further, FIG. 8 shows the detector 1. Cable 10 and members 9 such as a stand supporting the cables 10 and the like
FIG. 7 is a diagram showing output signals similar to FIGS. 6 and 7 when a device in which all parts are made of non-magnetic material is excited with an excitation current without conventional frequency countermeasures. Patent applicant New Japan! l! m stock company (and 1 other person) agent Masu Kobori (
and 2 others) No. 3vl Fig. 4 (Excitation coil la, lti + n! η magnetic circumference) No. 5
Mouth time Figure 7 Satoru 6 r'i Time No. 8171

Claims (1)

[Claims] 1. A detector consisting of a magnetic flux balanced type magnetic sensor is placed opposite to the surface of the molten steel in the mold for continuous casting, and the magnetic flux equilibrium state between the detector and the surface of the molten steel can be detected; The detector is driven in a direction approaching and away from the molten steel surface so as to eliminate the magnetic flux unbalance that occurs when the surface is displaced, and the position of the molten steel surface is determined by the advancing and retreating position of the detector that achieves magnetic flux balance. In the method of detecting, a housing of the detector and a pedestal supporting or holding them;
An excitation current for an electromagnetic function or a transformer for the excitation current, in which members or parts such as cables are made of a non-magnetic material with a maximum magnetic permeability of 1.003 or less, and the detector is installed in the continuous casting mold in which the detector is used. The excitation coil of the detector is excited with an excitation current of a frequency outside the harmonic range of each frequency of the power supply to the 1. A method for detecting a surface condition of molten steel in a mold for continuous casting, characterized in that the output of the detector is compensated based on the magnetic flux unbalance value stored in a storage device in advance during actual measurement. 2. The output signal of the detector to which the compensation has been added is separated into a molten steel surface level signal in a mold for continuous casting and a surface behavior signal at each molten steel surface level and used for control and/or display. A method for detecting the surface condition of molten steel in a continuous casting mold according to claim 1.