JPH08304008A - Method and apparatus for measurement of solidification thickness of cast piece - Google Patents

Abstract

PURPOSE: To obtain a method and an apparatus which reduce a drop in measuring accuracy due to an increase in a solidification thickness by applying magnetic fields having at least two mudually different frequencies to a continuous- casting cast piece and detecting a voltage which is due to an eddy current generated in an unsolidified part inside the cast piece. CONSTITUTION: When the solidification thickness (t) of a cast piece 1 is measured, frequencies from an AC power supply 9 are changed alternately from f1 to f2 , and an alternating current is applied to an exciting coil 6 at a low- velocity sensor 4. At this time, the component of an exciting magnetic flux is removed by a phase regulating circuit 10 from sensor output voltages V1 , V2 according to frequencies obtained from a search coil 7. In addition, by using respective gap detection voltages Vg1 , Vg2 obtained from a coil 8 for gap detection, gap change positions in the sensor output voltages V1 , V2 are corrected by a correction circuit 12 so as to be removed. By using the ratio V2 /V1 of the sensor output voltages V1 , V2 of which inessential portions have been removed in this manner, a computing and processing operation is performed by a signal processing circuit 13, and the solidification thickness (t) is found.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for measuring the solidification thickness of continuously cast slabs, which is devised so as to reduce the decrease in measurement accuracy due to an increase in the solidification thickness.

[0002]

2. Description of the Related Art As a conventional technique for measuring the solidified thickness of a continuously cast slab, a method disclosed in Japanese Patent Laid-Open No. 59-151003 is known.

In this type of prior art, as shown in FIG. 7, a sensor 30 in which an exciting coil 31 is sandwiched between secondary coils 32A and 32B from the front and back is arranged close to the surface 1A of the slab 1, and FIG. As shown in FIG.
The solidification thickness t is detected by passing a current through the amplifier 1 and amplifying the difference between the electromotive forces generated in the secondary coils 32A and 32B by the amplifier 34. In FIG. 7, reference numerals 2 are solidified shell portions, 3
Indicates an unsolidified portion (molten metal such as molten steel).

The operating principle is that, when an alternating magnetic field generated by the exciting coil 31 causes an eddy current to flow through the solidification shell portion 2, the penetration depth of the eddy current (skin depth: skin depth) changes depending on the solidification thickness t. The secondary coil 32
This means that the difference in electromotive force generated between 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 be lowered as the solidification thickness t increases. The reason for this will be described below for steel.

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

[0007]

[Equation 1]

In the case of steel, since μ = 1 at 700 ° C. or higher, the penetration depth δ is determined from the formula (1) if the frequency f is determined.
It will change depending on the electrical resistivity ρ.

Considering the change in the electrical resistivity of the slab 1 due to the solidification thickness t, the average values of the solidified shell portion 2 and the unsolidified portion (molten steel) 3 are obtained, and the amount of change is small. If the solidification thickness t is further increased, the proportion of the molten steel 3 having a high electric resistivity is reduced, and the average variation is further reduced, which may cause a decrease in measurement accuracy.

It is an object of the present invention to provide a measuring method and a measuring device in which the measurement accuracy is less likely to decrease as the solidified thickness increases.

[0011]

A method for measuring the solidification thickness of a cast product according to the present invention which solves the above-mentioned problems is such that at least two different continuous cast products are provided in a direction perpendicular to the direction of travel of the cast product.
It is characterized in that a magnetic field of two frequencies is applied, a voltage based on an eddy current generated in an unsolidified portion inside a cast piece by the magnetic field is detected, and a solidification thickness is obtained from a ratio of the same voltage.

In addition, the solidification thickness measuring device for cast slabs according to the present invention which solves the above-mentioned problems, has a C-shaped core, an exciting coil wound around the core, and a search wound around both NS magnetic poles at the tip of the core. A flow velocity sensor having a coil and having an NS pole oriented in a direction perpendicular to the direction of travel of the slab, a power supply for applying alternating current of different frequency to the exciting coil, and a different frequency from the search coil. And a means for calculating the solidification thickness from the output,
Furthermore, a gap detection coil wound around the core, 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 cast piece, by means of an output from the gap detection coil, Or a means for stirring the unsolidified portion inside the slab.

[0013]

Function A flow exists in the unsolidified portion of the continuously cast slab due to the discharge of the molten metal from the nozzle. Therefore, as shown in FIG. 4, assuming that the traveling direction of the slab 1 is the vertical direction orthogonal to the paper surface and that the molten metal flow has a component v _{H in the} horizontal direction 17 parallel to the paper surface, the exciting coil 6 is attached to the core 5. Winding and N
The sensor 4 formed by winding the search coil 7 so as to surround the S magnetic poles is arranged in a direction 16 perpendicular to the direction of travel of the slab,
Consider a case where a magnetic field is applied at a right angle 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 in the solidified shell portion 2 and reaches the unsolidified portion 3,
An eddy current 1 is generated in the unsolidified portion 3 at the interface with the solidified shell portion 2 in proportion to the horizontal molten metal flow rate v _H and the conductor length (core size) L.
8 flows, and this eddy current 18 creates a magnetic flux φ _v (see FIG. 5). Further, this magnetic flux φ _v is attenuated in the solidification shell portion 2 and interlinks with the search coil 7, an electromotive force is generated in the search coil 7, and the output voltage V thereof 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 solidification thickness measurement of the slab, both the molten metal flow rate v _H and the solidification thickness t are unknown quantities, and therefore the solidification thickness t cannot be obtained as it is.

Therefore, the attenuation rate α is the penetration rate (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]

[Equation 3] V ₁ ∝ B · v _H · L · exp (-2α ₁ t) …… Equation (3)

[0019]

[Equation 4] V ₂ ∝ B · v _H · L · exp (-2α ₂ t) …… Equation (4)

When the ratio of the two sensor output voltages, for example, V ₂ / V ₁ is taken, the following equation (5) is established from the equations (4) / (3), and the damping rates α ₁ , α ₂ and coagulation It is an exponential function of the thickness t. Further, if the equation (5) is logarithmized, the following equation (6) is obtained.

[0021]

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

[0022]

(Equation 6)

Now consider equation (6). If f ₁ <f ₂ is set, α ₁ <α ₂ is established, and therefore V ₁ > V ₂ from the equations (3) and (4). Therefore, the numerator 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, the solidification thickness t increases as the voltage ratio V ₂ / V ₁ decreases.

Then, by performing the calculation of equation (6),
The solidification thickness t can be obtained directly from the voltage ratio V ₂ / V _1, and the decrease in measurement accuracy due to the increase in the solidification thickness as in the conventional case 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 Expression (6).

[0026]

(Equation 7)

By the way, as shown in FIG. 6, the slab 1 moves in the slab advancing direction 15, so that an eddy current 19 flows in the unsolidified portion 3 even at the slab moving speed v _{V. In} this case, respectively. Since the currents of 1 to 5 cancel each other, no electromotive force based on the slab moving speed v _V is generated in the search coil 7.

Further, the gap g between the core 5 and the slab 1
Changes, the sensor output voltage changes and the measurement accuracy decreases. 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. As a result, it is possible to prevent a decrease in measurement accuracy due to a gap change.

Further, as described above, since the flow velocity due to the discharge of the molten metal from the nozzle is used, the measurement accuracy is lowered when the flow velocity decreases at the end of solidification. In such a case, a linear motor type electromagnetic stirrer is used. The flow velocity is increased by agitating the inside of the uncoagulated portion. This makes it 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 solidification thickness measuring device for a cast product according to an embodiment of the present invention, and FIG. 2 is a diagram showing a positional relationship between the cast product and a flow velocity sensor as seen from the front of the cast product. FIG. 3 is a side view of FIG. 2 showing the cast piece in a cutaway manner. However, in FIG.
The slab is shown in a state of being horizontally broken along the line II in FIG. FIG. 4 is a diagram showing the measurement principle of the present invention, FIG. 5 is a diagram showing eddy currents generated by the flow of molten metal, and FIG. 6 is a diagram showing eddy currents generated by the movement of the slab.

In this embodiment, as shown in FIGS.
It is assumed that the traveling direction 15 of the continuously cast slab 1 is the vertical direction. Reference numeral 16 indicates the horizontal direction. Further, the slab 1 has a rectangular cross section, and one of the four surfaces 1A to 1D is
In the description, A is the sensor side, 1B is the non-sensor side, and 1C and 1D are the side surfaces. Further, the unsolidified portion 3 of the cast slab 1 is assumed to be molten steel.

As shown in FIG. 1, the apparatus for measuring the solidification thickness of a cast product according to this embodiment has a flow velocity sensor 4, an AC power supply 9, a phase rectifying circuit 10, a signal amplifier 11, and a correcting circuit 12.
, A signal processing circuit 13, and a linear motor type electromagnetic stirring device 14.

The flow velocity sensor 4 has a C shape which can be said to be U-shaped.
The mold core 5, the exciting coil 6, the search coil 7, and the gap detecting core 8 are wound around the core 5 so that the exciting coil 6 and the gap detecting coil 8 are adjacent to each other. The search coil 7 is wound so as to surround it.

The AC power source 9 is different from each other in the exciting coil 6.
Frequency f₁, F₂The alternating current is applied. Implementation
In the example, f₁<F₂And the frequency of the AC power supply 9 is f₁Or
F ₂It is assumed that measurement is performed by alternately changing to
It

As shown in FIGS. 1 to 3, the flow velocity sensor 4 is moved in the direction 16 in the horizontal plane perpendicular to the direction 15 of movement of the slab.
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 slab surface 1A with a gap g.
As a result, an AC magnetic field in a direction 16 perpendicular to the slab traveling direction is applied to the slab surface 1A from the flow velocity sensor 4 (FIG. 4).
reference).

Therefore, when the exciting coil 6 is excited by the AC power source 9, a magnetic flux φ _S is generated as shown in FIG. 4, and this magnetic flux φ _S and the molten metal flow 17 in the unsolidified portion 3 cause the magnetic flux to flow as shown in FIG. An eddy current 18 is generated in the unsolidified portion 3, and the eddy current 18 generates a magnetic flux φ _{V as} shown in FIGS. 4 and 5.

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, V ₁ is the sensor output voltage when the frequency is f ₁ , and V ₂ is the sensor output voltage when the frequency is f ₂ .

On the other hand, the gap detecting coil 8 detects the gap g between the slab surface 1A and the core 5, and since the magnetic flux φ _S changes depending on the gap g, the magnetic flux φ _S
Is detected, a voltage V _g corresponding to the gap _g is output.
Here, V _g1 is the gap detection voltage when the frequency is f ₁ and V _g2 is the gap detection voltage when the frequency is f ₂ .

In the phase rectifier circuit 10, since the magnetic flux φ _S due to the excitation and the magnetic flux φ _V due to the flow velocity are linked to the search coil 7 at the same time, the component of the magnetic flux φ _S is removed from the sensor output voltages V ₁ and V _2. And these magnetic fluxes φ _S , φ _V
The fact that there is a 90 ° phase difference between the AC power supply 9
By phase rectifying the sensor outputs V ₁ and V ₂ with the frequencies f ₁ and f ₂ of, the signals corresponding to the magnetic flux φ _V due to the flow velocity are separated and output.

The signal amplifier 11 is a gap detecting coil 8
Of the output voltage V _g1 and V _g2 .

The correction circuit 12 uses the sensor output voltages V ₁ , V ₂
Is to remove the influence of the fluctuation of the gap g from the sensor output voltages V ₁ and V ₂ to the gap detection voltages V _g1 and V _g2.
A signal that is not affected by the gap variation is obtained by correcting with.

The signal processing circuit 13 outputs the sensor output voltage V ₁ ,
The solidification thickness t is obtained by performing a predetermined calculation using V ₂ and the damping ratios α ₁ and α ₂ , and in this embodiment, the voltage ratio V ₂ / V ₁ is used to calculate the above equation ( 6) That is, t =-
_{(L n (V 2 / V} 1)) / performs arithmetic processing of ₂ (α 2 -α _1). Where α ₁ is the attenuation rate when the frequency is f ₁ ,
α ₂ is the attenuation rate when the frequency is f ₂ .

The electromagnetic stirrer 14 stirs the uncoagulated portion 3 electromagnetically in a non-contact manner according to the principle of a linear motor, and the stirring increases the flow velocity in the uncoagulated portion 3.

Next, the overall operation of the above-mentioned apparatus
Will be explained. When measuring the solidification thickness t of the slab 1, use an alternating current
Frequency f from source 9₁To f₂Flow 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 reach coil 7
₁, V₂From the phase rectifier circuit 10_Sof
Remove the ingredients. Also, obtain from the gap detection coil 8.
Gap detection voltage V_g1, V_g2Using the correction circuit
12 the sensor output voltage V₁, V₂Gap fluctuation
Is corrected and removed. In this way, the unnecessary components are removed.
Output voltage V ₁, V₂Ratio V of₂/ V₁Using the signal
The processing circuit 13 performs the arithmetic processing of the above equation (6) to coagulate
Calculate the thickness t. Also, when the flow velocity decreases, such as at the end of coagulation
The electromagnetic stirrer 14 automatically or manually.
To increase the flow rate and prevent a decrease in measurement accuracy.

As a concrete 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, the output of about 0.2 mV was obtained from the search coil 7.

Here, when the gap g is not changed or is negligibly small, the gap detection coil 8 and the correction circuit 12 are not necessarily required. The electromagnetic stirrer 14 is not always necessary if the flow velocity is sufficient even at the final stage of coagulation, and conversely, the electromagnetic stirrer may be constantly operated at times other than the final stage of coagulation if the flow velocity is insufficient. Further, most of the magnetic flux φ _S due to the excitation interlinking with the search coil 7 is canceled on the N-pole side and the S-pole side. Therefore, when the remaining amount is negligibly small, the phase rectifying circuit 10 is not always necessary. . Further, the core 5 may be substantially U-shaped or C-shaped, including the case where a part of the H-shaped core is used.

Further, the frequency of the alternating current applied to the exciting coil 6 is alternately switched from f ₁ to f _2, and 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. In this case, a signal corresponding to f ₁ and a signal corresponding to f ₂ may be obtained separately from each output voltage of the search coil 7 and the gap detection coil 8 by using a filter or the like.

[0048]

According to the present invention, the solidification thickness is directly detected by utilizing the flow velocity at the interface between the solidification shell portion and the non-solidification portion. Less loss of accuracy.

Further, according to the present invention, when the output of the search coil is corrected by using the output of the coil for detecting the gap, it is possible to eliminate the influence of the gap variation between the core and the surface of the slab. Further, when the uncoagulated portion is agitated, the flow velocity can be increased even at the final stage of coagulation or the like, so that the output of the search coil is increased even if the coagulated thickness is increased, and the measurement accuracy can be prevented from lowering. That is, it is possible to measure even if the solidified thickness becomes thick.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a solidification thickness measuring device for cast slabs according to an embodiment of the present invention.

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

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

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

FIG. 5 is a diagram showing eddy currents generated by the flow of molten metal.

FIG. 6 is a diagram showing an eddy current generated by the 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 a relationship between electric resistivity and temperature in steel.

[Explanation 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 detection 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 Direction of slab movement 16 Direction perpendicular to direction of slab movement 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 by flow velocity is generated Magnetic flux

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Hiroshi Nakajima 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture Mitsubishi Heavy Industries Ltd. Hiroshima Research Institute (72) Inventor Motoki Nakajima 4 Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture 6-22 No. 6 Hiroshima Research Laboratory, Mitsubishi Heavy Industries, Ltd.

Claims (4)

[Claims] 1. A voltage based on an eddy current generated in an unsolidified portion inside a cast by applying a magnetic field of at least two frequencies different from each other to a continuously cast cast in a direction perpendicular to a cast advancing direction. Is detected and the solidification thickness is determined 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 magnetic poles at the tip of the core, and the NS pole is directed in a direction perpendicular to the slab traveling direction. A flow velocity sensor provided, a power supply for applying alternating currents of different frequencies to the exciting coil, and means for calculating solidification thickness from outputs of different frequencies from the search coil. Instrument for measuring solidification thickness of cast slab. 3. The gap detection coil wound around the core according to claim 2, and a change in the output of the search coil caused by a change in the gap between the core and the slab,
An apparatus for measuring the solidification thickness of a slab, comprising: a means for correcting the output from the gap detecting coil. 4. The solidified thickness measuring device for a cast product according to claim 2, further comprising means for stirring an unsolidified portion inside the cast product.