JP3082828B2 - Method and apparatus for measuring cast solidification thickness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring the solidification thickness of a continuously cast slab, and is designed to reduce the decrease in measurement accuracy accompanying an increase in the solidification thickness.

[0002]

2. Description of the Related Art As a conventional technique for measuring the solidification thickness of a continuous cast slab, a method disclosed in Japanese Patent Application Laid-Open No. 59-150003 is known.

In this type of prior art, as shown in FIG. 7, a sensor 30 having an exciting coil 31 sandwiched between secondary coils 32A and 32B from the front and rear is arranged close to the surface 1A of the slab 1 as shown in FIG. As shown in FIG.
1, the difference between the electromotive forces generated in the secondary coils 32A and 32B is amplified by the amplifier 34, and the solidification thickness t is detected. 7, reference numeral 2 denotes a solidified shell portion;
Indicates an unsolidified portion (a molten metal such as molten steel).

The principle of operation is that when an AC magnetic field generated by the exciting coil 31 causes an eddy current to flow through the solidified shell portion 2, the penetration depth (skin depth: skin depth) of the eddy current changes depending on the solidified thickness t. By doing so, the secondary coil 32
That is, the difference between the electromotive forces generated in A and 32B corresponds to the solidification thickness t.

[0005]

However, in the above-mentioned prior art, there is a concern that the measurement accuracy may decrease as the solidification thickness t increases. The reason for this will be described below for steel.

The electrical resistance of steel depends on temperature as shown in FIG. 9, and changes gradually from 1000 ° C. to the freezing point, but rises sharply by about 7% in the molten steel portion. The magnetic field penetration depth δ is represented by the following equation (1). Here, ρ is the electrical resistivity, f is the frequency, μ is the magnetic permeability, and π is the circular constant.

[0007]

(Equation 1)

In the case of steel, μ = 1 at 700 ° C. or higher. Therefore, if the frequency f is determined from the equation (1),
It will vary with the electrical resistivity ρ.

Considering the change in the electrical resistivity of the slab 1 depending on the solidification thickness t, the change is small since the average value of the solidified shell portion 2 and the unsolidified portion (molten steel) 3 is obtained. When the solidification thickness t is further increased, the ratio of the molten steel 3 having a high electric resistivity is reduced, and the average change amount is further reduced, so that the measurement accuracy may be reduced.

[0010] It is an object of the present invention to provide a measuring method and a measuring apparatus in which a decrease in measurement accuracy due to an increase in solidified thickness is small.

[0011]

According to the present invention, there is provided a method for measuring the solidification thickness of a slab, comprising the steps of: forming a continuous cast slab having at least two different slabs in a direction perpendicular to the slab advancing direction;
A magnetic field of two frequencies is applied, a voltage based on an eddy current generated in an unsolidified portion inside the slab by the magnetic field is detected, and a solidification thickness is obtained from a ratio of the voltages.

In addition, a solidified thickness measuring apparatus for a cast slab according to the present invention that solves the above-mentioned problems is a C-shaped core, an exciting coil wound around the core, and a search coil wound around both NS magnetic poles at the tip of the core. A flow sensor having a coil, NS poles oriented in a direction perpendicular to the slab traveling direction, a power supply for applying an alternating current of a different frequency to the exciting coil, and a different frequency from the search coil. Means for calculating the solidification thickness from the output, comprising:
Further, a gap detection coil wound around the core, and a means for correcting a change in the output of the search coil caused by a change in the gap between the core and the slab by an output from the gap detection coil, Or a means for agitating an unsolidified portion inside the slab.

[0013]

In the unsolidified portion of the continuous cast slab, a flow exists due to discharge of the molten metal from the nozzle. Therefore, as shown in FIG. 4, assuming that the direction of travel of the slab 1 is a vertical direction perpendicular to the plane of the paper and that the molten metal flow has a component v _{H in the} horizontal direction 17 parallel to the plane of the paper, the excitation coil 6 And N
The sensor 4 formed by winding the search coil 7 so as to surround the S magnetic poles is disposed in a direction 16 perpendicular to the slab advancing direction,
Consider a case where a magnetic field is applied at right angles to the surface of the slab by applying an alternating current to the exciting coil 6.

In this case, the magnetic flux φ _S generated by the exciting coil 6 is attenuated by the solidified shell portion 2 and reaches the unsolidified portion 3.
The eddy current 1 in the unsolidified portion 3 at the interface with the solidified shell portion 2 in proportion to the horizontal melt flow velocity v _H and the conductor length (core dimension) L
8 flows, and this eddy current 18 creates a magnetic flux φ _v (see FIG. 5). Further, the magnetic flux φ _v is attenuated in the solidified shell portion 2 and interlinks with the search coil 7 to generate an electromotive force in the search coil 7, and the output voltage V is expressed by the following equation (2). However,
The gap g between the slab 1 and the core 5 is constant.

[0015]

(Equation 2)

That is, the sensor 4 is an electromagnetic non-contact type flow velocity sensor, and detects the flow velocity v _H at the interface between the solidified shell portion 2 and the non-solidified portion 3. The output voltage V changes depending on the solidification thickness t and the attenuation rate α. However, in the measurement of the solidified thickness of the slab, since the molten metal flow velocity v _H and the solidified thickness t are both unknown quantities, the solidified thickness t cannot be obtained as it is.

Therefore, the attenuation rate α is determined by the degree of penetration (skin depth).
Paying attention to the fact that it is the reciprocal of δ and depends on the frequency f, if at least two frequencies f ₁ and f ₂ are used, the frequency f
Sensor output voltages V ₁ at ₁ is expressed by the following equation (3), the sensor voltage output V ₂ at frequency f ₂ is expressed by the following equation (4). Where α ₁ is the attenuation rate at frequency f ₁ , α ₂ is the attenuation rate at frequency f ₂ , and μ =
Since 1 and ρ are almost constant because they are the electrical resistivity of the solidified shell portion 2, α ₁ and α ₂ can be obtained in advance.

[0018]

V ₁ ∝B · v _H · L · exp (−2α ₁ t) (3)

[0019]

V ₂ ∝B · v _H · L · exp (−2α ₂ t) Equation (4)

Then, when the ratio of the two sensor output voltages, for example, V ₂ / V ₁ is taken, the following expression (5) is established from the expressions (4) / (3), and the attenuation rates α ₁ , α ₂ and the coagulation It becomes an exponential function of the thickness t. Further, when Expression (5) is logarithmized, the following Expression (6) is obtained.

[0021]

V ₂ / V ₁ = exp (−2 (α ₂ −α ₁ ) t) Equation (5)

[0022]

(Equation 6)

Here, equation (6) will be considered. If f ₁ <f ₂ is set, α ₁ <α ₂ , so that V ₁ > V ₂ from the equations (3) and (4). Therefore, the molecule l _n (V
₂ / V ₁ ) is negative and the voltage ratio V ₂ / V ₁
The smaller is the larger. Denominator -2 (α ₂ −α ₁ )
Is negative. Therefore, when the voltage ratio V ₂ / V ₁ decreases, the solidification thickness t increases.

Then, by performing the operation of equation (6),
The solidification thickness t can be obtained directly from the voltage ratio V ₂ / V _1, and a decrease in measurement accuracy due to an increase in the solidification thickness as in the related art is reduced.

When the voltage ratio V ₁ / V ₂ is used, the following equation (7) is obtained. If f ₁ <f ₂ , the solidification thickness t increases as the voltage ratio V ₁ / V ₂ increases. This is equivalent to the case of equation (6).

[0026]

(Equation 7)

Since the slab 1 moves in the slab advancing direction 15 as shown in FIG. 6, an eddy current 19 flows in the unsolidified portion 3 depending on the slab moving speed v _V. , The electromotive force based on the slab moving speed v _V is not generated in the search coil 7.

The gap g between the core 5 and the slab 1
Changes the sensor output voltage, so that the measurement accuracy is reduced. In such a case, if the coil 8 is wound around the core 5, the output voltage changes according to the gap g. The output voltage of the search coil 7 is corrected by the voltage. Thereby, it is possible to prevent a decrease in measurement accuracy due to a change in the gap.

Further, as described above, since the flow rate due to the discharge of the molten metal from the nozzle is used, if the flow rate is reduced at the end of solidification, the measurement accuracy will be reduced. In such a case, a linear motor type electromagnetic stirrer or the like is required. The flow rate is increased by agitating the inside of the unsolidified portion by using. Thereby, it is possible to prevent a decrease in measurement accuracy.

[0030]

Embodiments of the present invention will be described below with reference to the drawings. In the drawings, FIG. 1 is a diagram showing a configuration of a device for measuring a solidified thickness of a slab according to one embodiment of the present invention, and FIG. 2 is a diagram showing a positional relationship between the slab and a flow rate sensor when the slab is viewed from the front. FIG. 3 is a side view of FIG. However, in FIG.
The slab is shown in a state of being broken horizontally along line II in FIG. 4 is a diagram illustrating the measurement principle of the present invention, FIG. 5 is a diagram illustrating an eddy current generated by the flow of molten metal, and FIG. 6 is a diagram illustrating an eddy current generated by movement of the slab.

In this embodiment, as shown in FIGS. 2 and 3,
A description will be given on the assumption that the traveling direction 15 of the slab 1 of continuous casting is a vertical direction. Reference numeral 16 indicates the horizontal direction. The slab 1 has a rectangular cross section, and one of the four surfaces 1A to 1D has
A description is given on the assumption that A is the sensor side, 1B is the opposite sensor side, and 1C and 1D are the side faces. Further, it is assumed that the unsolidified portion 3 of the slab 1 is molten steel.

As shown in FIG. 1, the apparatus for measuring the solidified thickness of a slab according to the present embodiment comprises a flow velocity sensor 4, an AC power supply 9, a phase rectifier circuit 10, a signal amplifier 11, and a correction circuit 12.
, A signal processing circuit 13 and a linear motor type electromagnetic stirring device 14.

The flow rate sensor 4 is C-shaped.
A mold core 5, an exciting coil 6, a search coil 7, and a gap detecting core 8 are provided. The exciting coil 6 and the gap detecting coil 8 are wound around the core 5 next to each other. The search coil 7 is wound so as to surround it.

The AC power supply 9 is different from the excitation coil 6
Frequency f₁, F_TwoIs applied. This implementation
In the example, f₁<F_TwoAnd the frequency of the AC power supply 9 is f₁Or
F _TwoIt is assumed that the measurement is performed by alternately changing
You.

As shown in FIGS. 1 to 3, the flow velocity sensor 4 has N in a direction 16 in a horizontal plane perpendicular to the slab traveling direction 15.
It is arranged close to the surface 1A of the slab 1 with the S magnetic pole facing. In other words, the NS magnetic poles are on the same horizontal plane and face the cast slab surface 1A with a gap g.
Thus, an alternating magnetic field in a direction 16 perpendicular to the slab traveling direction is applied from the flow velocity sensor 4 to the slab surface 1A (FIG. 4).
reference).

Therefore, when the exciting coil 6 is excited by the AC power supply 9, a magnetic flux φ _S is generated as shown in FIG. 4, and the magnetic flux φ _S and the molten metal flow 17 in the unsolidified portion 3 cause the magnetic flux φ _S as shown in FIG. An eddy current 18 is generated in the unsolidified portion 3, and a magnetic flux φ _V is generated by the eddy current 18 as shown in FIGS.

The search coil 7 is for detecting the magnetic flux phi _V, resulting in electromotive force search coil 7 corresponding to the flux phi _V, which is the output voltage of the flow sensor 4 (flow rate signal). Here, frequency V ₁ is the sensor output voltage when the f _1, the frequency of V ₂ is the sensor output voltage when the f _2.

On the other hand, the gap detecting coil 8 is for detecting the gap g between the slab surface 1A and the core 5. Since the magnetic flux φ _S changes with the gap g, the magnetic flux φ _S
Detected by outputting a voltage V _g in accordance with the gap g.
Here, frequency V _g1 gap detection voltage for f _1, the frequency of V _g2 is a gap detection voltage in the case of f _2.

The phase rectifier circuit 10 also removes the component of the magnetic flux φ _S from the sensor output voltages V ₁ and V ₂ because the magnetic flux φ _S generated by excitation and the magnetic flux φ _V generated by the flow velocity simultaneously link the search coil 7. And these magnetic fluxes φ _S , φ _V
Utilizing the fact that there is a 90 ° phase difference between
By phase rectifying the sensor outputs V ₁ , V ₂ with the frequencies f ₁ , f ₂ , a signal corresponding to the magnetic flux φ _V due to the flow velocity is separated and output.

The signal amplifier 11 has a gap detecting coil 8.
To amplify the output voltages V _g1 and V _g2 of FIG.

The correction circuit 12 detects the sensor output voltages V ₁ , V ₂
To remove the influence of the change of the gap g from the sensor output voltages V ₁ , V ₂ and the gap detection voltages V _g1 , V _g2.
, A signal which is not affected by gap fluctuation is obtained.

The signal processing circuit 13 outputs the sensor output voltage V ₁ ,
V ₂ and the attenuation factor alpha _1, by using the alpha ₂ is intended to obtain the solidification thickness t by performing a prescribed operation, wherein in this embodiment supra using the voltage ratio V ₂ / V ₁ ( 6) That is, t = −
_{(L n (V 2 / V} 1)) / performs arithmetic processing of ₂ (α 2 -α _1). Here, α ₁ is an attenuation rate when the frequency is f ₁ ,
α ₂ is an attenuation rate when the frequency is f ₂ .

The electromagnetic stirring device 14 stirs the unsolidified portion 3 electromagnetically in a non-contact manner based on the principle of a linear motor, and increases the flow rate in the unsolidified portion 3 by the stirring.

Next, the overall operation of the above-described device
Will be described. When measuring the solidification thickness t of the slab 1,
F from source 9₁To f_TwoFlow rate
An alternating current is applied to the exciting coil 6 of the sensor 4. At this time,
Sensor output voltage V for each frequency obtained from the
₁, V_TwoFrom the excitation flux φ by the phase rectifier circuit 10._Sof
Remove components. In addition, it is obtained from the gap detecting coil 8.
Gap detection voltage V_g1, V_g2Using the correction circuit
12, the sensor output voltage V₁, V_TwoGap variation
Is corrected and removed. In this way, unnecessary components are removed.
Sensor output voltage V ₁, V_TwoRatio V_Two/ V₁Using the signal
The arithmetic processing of the above equation (6) is performed by the processing circuit 13 and
Find the thickness t. Also, when the flow velocity decreases, such as at the end of solidification
The electromagnetic stirrer 14 automatically or manually.
To increase the flow rate and prevent a decrease in measurement accuracy.

In a specific measurement example, the surface magnetic flux of the exciting coil 6 is 30 Gauss, and the diameter of the search coil 7 is 55φ.
(3000 turns), frequency 100Hz, solidification thickness 2
In the case of 0 mm, an output of about 0.2 mV was obtained from the search coil 7.

Here, when there is no change in the gap g or when it is negligibly small, the gap detecting coil 8 and the correction circuit 12 are not necessarily required. Also, if there is a sufficient flow rate even at the end of solidification, the electromagnetic stirrer 14 is not necessarily required. Conversely, if the flow rate is not sufficient even at times other than the end of solidification, the electromagnetic stirrer may be operated at all times. Further, since most of the magnetic flux φ _S due to the excitation linked to the search coil 7 is canceled on the N-pole side and the S-pole side, the phase rectifier circuit 10 is not necessarily required when the remainder is negligibly small. . Further, the core 5 may be of any shape as long as it is substantially U-shaped or C-shaped, including the case where a part of the H-shaped core is used.

Further, in addition to alternately switching the frequency of the alternating current applied to the exciting coil 6 from f ₁ to f ₂ , the alternating current of the frequency f _{1 and} the alternating current of the frequency f ₂ may be superimposed and applied to the exciting coil 6. , from the output voltage of this search coil 7 and the gap detection coil 8 may be Ere separates a signal corresponding to the signal and f ₂ corresponding to f ₁ by such a filter.

[0048]

According to the present invention, the solidification thickness is directly detected by using the flow velocity at the interface between the solidified shell portion and the non-solidified portion. Little decrease in accuracy.

Further, according to the present invention, when the output of the search coil is corrected using the output of the gap detecting coil, the influence of the gap variation between the core and the slab surface can be eliminated. Further, when stirring the unsolidified portion, the flow velocity can be increased even in the final stage of solidification or the like, so that even if the solidified thickness is increased, the output of the search coil is increased and a decrease in measurement accuracy can be prevented. That is, the measurement can be performed even when the solidification thickness increases.

[Brief description of the drawings]

FIG. 1 is a view showing a configuration of an apparatus for measuring a solidified thickness of a slab according to one embodiment of the present invention.

FIG. 2 is a view showing a positional relationship between a slab and a flow sensor as viewed from the front of the slab.

FIG. 3 is a side view of FIG. 2 showing a slab broken.

FIG. 4 is a diagram showing a measurement principle of the present invention.

FIG. 5 is a diagram showing an eddy current generated by a flow of a molten metal.

FIG. 6 is a diagram showing an eddy current generated by movement of a slab.

FIG. 7 is a diagram showing a coil configuration of a conventional example.

FIG. 8 is a diagram showing a circuit configuration of a conventional example.

FIG. 9 is a diagram showing the relationship between electrical resistivity and temperature in steel.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Cast piece 1A Cast piece surface 2 Solidified shell part 3 Unsolidified part 4 Flow velocity sensor 5 Core 6 Exciting coil 7 Search coil 8 Gap detecting coil 9 AC power supply 10 Phase rectifier circuit 11 Signal amplifier 12 Correction circuit 13 Signal processing circuit 14 Linear Motor type electromagnetic stirrer 15 Slab traveling direction 16 Direction perpendicular to the slab traveling direction 17 Molten metal flow 18 Eddy current t Solidification thickness v _H flow velocity φ _S Magnetic flux generated by exciting coil φ _V Eddy current generated at flow velocity Magnetic flux

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hiroshi Nakajima 4-2-2 Kannon Shinmachi, Nishi-ku, Hiroshima-shi, Hiroshima Mitsubishi Heavy Industries, Ltd. Hiroshima Research Laboratory (72) Inventor Motoki Nakajima Kannon-Shimmachi 4 in Nishi-ku, Hiroshima-shi, Hiroshima No. 6-22, Mitsubishi Heavy Industries, Ltd. Hiroshima Laboratory (56) References JP-A-59-151003 (JP, A) JP-A-57-139603 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01B ^7/ 00-7/34 102 B22D 11/00-11/22

Claims (4)

(57) [Claims] 1. A magnetic field of at least two different frequencies is applied to a continuous cast slab in a direction perpendicular to a slab advancing direction, and a voltage based on an eddy current generated in an unsolidified portion inside the slab by the magnetic field. And measuring the solidification thickness from the ratio of the same voltage. 2. A C-shaped core, an exciting coil wound around the core, and a search coil wound so as to surround both NS poles at the tip of the core, wherein the NS pole is directed in a direction perpendicular to the direction of slab travel. And a power supply for applying an alternating current of a different frequency to the excitation coil, and a means for calculating a solidification thickness from an output of a different frequency from the search coil. A device for measuring the solidification thickness of cast slabs. 3. The method according to claim 2, wherein a change in output of the search coil caused by a change in the gap detection coil wound around the core and a change in the gap between the core and the slab is obtained.
A means for correcting with an output from the gap detecting coil. 4. The apparatus for measuring the solidified thickness of a slab according to claim 2, further comprising means for stirring an unsolidified portion inside the slab.