GB2142729A - Method and apparatus for non-contact measurement of solidified shell of a metal casting having unsolidifed inner part - Google Patents

Abstract

The thickness of a solidified shell (1c) of a metal casting (1) under solidification having an unsolidified part (1b) therein is measured in a non-contact manner by means of the electromagnetic induction effect. An alternating magnetic field is applied to the casting by means of exciter (3) and primary coil (22) and the magnitude of eddy currents produced within the casting is detected by a non-contact detecting coil (23) on the basis of the electromagnetic induction effect. The thickness of the solidified shell is measured from the magnitude of the eddy currents in accordance with the difference in electrical resistivity between the unsolidified part and the solidified part of the casting. A dummy coil may be connected in series opposition with coil (23) for temperature compensation and the coils wound on an H shaped core. A plurality of detecting coils may be arranged in line and the outputs multiplexed. A plurality of frequencies may be applied to the primary coil and in order to compensate for lift-off the exciting current may include a low frequency component and a high frequency component the latter being indicative of the variation in lift off and being used to compensate the low frequency component. <IMAGE>

Description

SPECIFICATION Method and apparatus for non-contact measurement of solidified shell of a metal casting having unsolidified inner part The present invention relates to a measuring method of the eddy current detection type and measuring apparatus used therefor for measuring in a non-contact manner the thickness of a solidified shell of a metal casting having the unsolidified inner part, e.g., a cast slab under solidification emerging from the exit of a mold of a continuous casting machine or a metal block under solidification within a mold by the batch casting.

In the casting of metal particularly in the continuous casting of steel the thickness of a solidified shell of a cast slab at the exit of amold of a continuous casting machine must always be maintained constant. The shell thickness relates directly to the rate of withdrawal of the cast slab from the mold and it also relates to the cooling conditions of the mold. Variations in the thickness of the solidified shell just emerging from the exit of the mold give rise to difficult problems such as causing the slab to bulge at a position downstream of the mold exit or causing defects in the interior of the slab.

In a known shell-thickness measuring method of the non-contact type, a strong pulse magnetic field is applied to one surface of a cast slab just emerging from the exit of a mold of a continuous casting machine so that the electromagnetic induction effect due to the oscillatory magnetic field produces ultrasonic waves due to the Lorentz force in the case slab and the ultrasonic waves propagating within the cast slab are detected by an ultrasonic sensor employing an eddy-current detecting coil at the other surface of the cast slab thereby measuring only the whole thickness of the case slab by a Magne-scale or the like and also measuring the propagation time of the ultrasonic waves propagating within the cast slab to measure the solidified shell thickness of the cast slab from the propagation time in accordance with the difference between the average velocity of sound on the solid phase of the metal and the average velocity of sound in the liquid phase of the metal.

The disadvantages of this type of known method may be summarized as follows. (1) Since the total thickness of the solidified layers on the sides of a cast slab in the thickness direction are measured, it is impossible to make a distinction between a case where the solidified layer on one side is greater in thickness than the solidified layer on the other side and a case where the two solidified layers are equal in thickness. (2) Since the generation of ultrasonic waves is effected by supplying a high-voltage pulse current to an excitation coil, it is necessary to use a source of high voltage (15 to 30 kV) and this requires a very difficult insulation problem from the practical point of view. (3) Due to the use of ultrasonic waves, the detection sensitivity is low and therefore it is difficult to increase the amount of lift-off of the detecting head from the case slab.Incidentally, the amounts of lift-off in the systems now in practical applications are on the order of several mm at the maximum and particularly the cast slab and the detecting head inevitably contact or strike against each other if the irregularities in the surface of the cast slab are large.

It is the primary object of the present invention to provide a method and apparatus for measuring in a non-contact manner the thickness of a solidified shell of a cast slab at rest or in movement.

It is another object of the invention to provide such method and apparatus capable of making the desired measurement even if the detection head is at a distance for example of more than ten-odd mm from the cast slab surface.

It is still another object of the invention to provide such method and apparatus capable of making the desired measurement without using a high-voltage power source.

It is still another object of the invention to provide such method and apparatus capable of taking stable measurements against variations of the temperature conditions.

It is still another object of the invention to provide such method and apparatus further capable of measuring for example the distribution of solidified shell thicknesses in the width direction of a cast siab.

It is still another object of the invention to provide such method and apparatus capable of selecting a desired thickness measuring span or simultaneously making measurements with respect to a plurality of measuring spans which are different from one another.

It is still another object of the invention to provide such method and apparatus capable of preventing any measuring error due to variations in the separating distance between the detecting head and the cast slab caused by the irregularities in the slab surface, the vibrations of the slab or the like particularly in the case of continuous measurement of the solidified shell thickness of the moving cast slab emerging from the mold of a continuous casting machine.

In accordance with one embodiment of the invention, the measuring method basically comprises the steps of: applying an alternating magnetic field to a metal casting having an unsolidified interior; detecting in a non-contact manner the magnitude of eddy currents produced in a casting by a detecting coil; and measuring the thickness of the solidified shell of the casting from the magnitude of the eddy currents in accordance with the difference in electric resistance between the unsolidified part and the solidified part of the casting.

The basic concept of the present invention resides in the fact that when a certain metal in the liquid state changes to a solid state upon cooling, that is, when the metal is solidified, the electric resistivity of the metal discontinuously changes rapidly and the rate of change of the resistivity with temperature changes during cooling of the metal from the solidification temperature to about 1 000,C is small.

Then, if Vp represents the voltage value of the ac voltage applied to the excitation coil which generates the alternating magnetic field applied to the cast slab, the voltage Vs given by the following equation is induced in the detection coil for detecting eddy currents within the cast slab Vs = K.Vp where K is a coupling coefficient and its value is determined by the sizes of the detecting coil and its magnetic core, the relative distance of the detecting coil and the cast slab, the frequency of the ac voltage applied to the excitation coil, the resistivity of the liquid phase of the cast slab, the resistivity of the solid phase, the thickness of the solidified shell, etc.

When the thickness of the solidified shell is increased in a predetermined thickness region from the surface of the cast slab, the proportion of the solid part in this region is increased so that the electric resistivity of the region on the whole is decreased considerably as compared with that before the shell thickness was increased and this in turn results in an increase in the eddy currents flowing into the region. This means a change of the coupling coefficient in the previously mentioned equation so that by measuring the magnitude V5 of the ac voltage induced in the detecting coil, it is possible to measure the thikness of the solidified shell of the cast slab facing the detecting coil.

With the measuring method in accordance with another, embodiment of the invention, the step of noncontact measurement of the magnitude of the eddy currents within the casting by the detecting coil employs a dummy coil having the equivalent temperature characteristic to the detecting coil and a relatively weak magnetic coupling with the casting in differential combination with the detecting coil thereby providing a temperature compensation for the detection signal from the detecting coil.

With the measuring method according to still another embodiment of the invention, a plurality of detecting coils are arranged in a line and a further step is added which measures the distribution of solidified shell thicknesses along the direction of the coil arrangement in accordance with the detection signals from the plurality of detecting coils. In this case, the method of sequentially selecting the detection signals from the plurality of detecting coils by a multiplexer and thereby repeatedly performing the scanning in the direction of the coil arrangement also falls within the scope of the present invention.

In accordance with the present invention, more practically a low frequency alternating magnetic field is applied to one surface of the casting and the frequency of the low frequency alternating magnetic field is predetermined in such a manner that the penetration depth of the eddy currents induced within the casting is limited midway within the casting thereby arbitrarily determining the measuring span through adjustment of the setting of the frequency.

The low frequency alternating magnetic field needs not have a single low frequency component and it may include a plurality of frequency components which are different from one another.

In this case, induced in the detecting coil is a detection signal including a plurality of low frequency signal components each thereof corresponding to one of the plurality of frequency components and the low frequency signal components are separately taken out thereby simultaneously making the measurement for each of a plurality of measuring spans which are different from or overlaped with each other.

With the measuring method in accordance with still another embodiment of the invention, a high frequency alternating magnetic field is superposed on the low frequency alternating magnetic field and applied to the casting. This high frequency alternating magnetic field is a field of such a high frequency that ensures the generation of high frequency eddy currents concentrically flowing into the surface portion of the casting or produces the skin effect of the eddy currents. Then, the low frequency signal components and the high frequency signal component are separately taken from the detection signal from the detecting coil and the high frequency signal component compensates the low frequency signal components for any measuring error due to the irregularities in the surface of the casting and variation in the separating distance between the detecting coil and the surface of the casting. This method is particularly well suited as a high-accuracy continuous measuring method in cases where the cast slab is moving continuously.

Where the present invention is applied to the ordinary continuously cast slab of steel, the frequency of the low frequency alternating magnetic field is selected 1000 Hz or less, preferably 800 Hz or less and the frequency of the compensating high frequency alternating magnetic field is selected 1000 Hz or over, preferably 1 600 Hz or over.

The present invention also relates to a measuring apparatus for performing the abovementioned measuring method and in accordance with its basic embodiment the solidified shell thickness measuring apparatus comprises: detecting head means including a primary coil and a secondary coil and positioned at a predetermined distance from the surface of a metal casting having an unsolidified inner part to face the later such that the two coils and the casting are magnetically coupled to each other; exciting means for supplying an ac current to the primary coil so that the alternating flux produced from the primary coil penetrates into the casting; and detecting circuit means for detecting the amplitude value of an ac voltage induced in the secondary coil magnetically coupled to the casting and generating a measured output corresponding to the thickness of the solidified shell of the casting under solidification.

With the apparatus according to another embodiment of the invention, in addition to the above-mentioned basic construction, the secondary coil includes a detecting coil and a dummy coil and the dummy coil has the same temperature characteristic as the detecting coil and positioned within the detecting head so that the magnetic coupling between it and the casting is weak as compared with that between the detecting coil and the casting. The detecting coil and the dummy coils are both magnetically coupled with the primary coil and they are differentially connected to each other such that their induced voltages are combined with a phase difference of qt rad.In the case of this specific embodiment, the detecting head means includes an H-shaped magnetic core and the H-shaped magnetic core includes a pair of parallel legs and a single central connecting portion connecting the two legs substantially at the center of their lengths. The primary coil is wound on the central connecting portion and the detecting coil and the dummy coil are respectively wound on the sides of the central portion of one of the legs.

With the apparatus according to still another embodiment of the invention, the detecting head means includes a plurality of secondary coils arranged in a line and it is connected to a multiplexer which successively selects and generates the detection signals of the secondary coils thereby allowing the detecting head means to effect a self-scanning in the direction of arrangement of the secondary coils.

The exciting means basically supplies a single low frequency ac current to the primary coil.

With the apparatus according to still another embodiment of the invention, the exciting means generates an ac current including a plurality of frequency components having predetermined low frequencies which are different from one another and the detecting circuit means includes frequency selective means for separating a plurality of low frequency signal components each corresponding to one of the frequency components from an ac voltage signal generated from the secondary coil thereby generating a shell thickness measurement output from the amplitude value of the signal from each of the frequency selective means.

With the apparatus according to still another embodiment of the invention, the exciting means generates a low frequency ac current and a high frequency ac current, and the high frequency ac current is supplied to the primary coil so that a high frequency alternating magnetic field is generated from the primary coil and the high frequency alternating magnetic field produces high frequency eddy currents which concentrically flow into the surface portion of the casting.Also, the detecting circuit means includes means for extracting a high frequency signal component corresponding to the high frequency eddy currents from the detection signal generated by the secondary coil and means responsive to the high frequency signal component to compensate the low frequency signal components of the detection signal for a measuring error due to the irregularities in the casting surface and variation in the amount of lift-off of the detecting head from the casting surface.

In accordance with the invention, due to the measurment of a solidified shell thickness by means of the electromagnetic induction effect, there are advantages that the thickness of a solidified shell on one side of a cast slab at rest or in movement can be measured in a non-contact manner, that the separating distance (the amount of lift-off) between the detecting head and the cast slab surface can be selected as large as 20 to 30mm and that there is no need to use a high voltage source.

Also, by virtue of the fact that a plurality of low frequency ac fields of different frequencies are applied and the results of measurements according to the respective frequency components are combined, the number of measuring spans is increased without any deterioration of the linearity. Further, the use of the high frequency alternating magnetic field in superposition is advantageous in that the high frequency signal component of the detection signal provides information on the variations in the surface conditions of the cast slab and the amount of lift-off and therefore the high frequency signal component is utilized to provide a signal compensation so as to cancel any measuring error due to these variations thereby making it possible to obtain satisfactory results even in the continuous measurement of a cast slab in movement.Still further, if a plurality of detecting heads are arranged in a line and selected simultaneously or selected successively through a multiplexer to make measurements, it is possible to measure a distribution pattern of shell thicknesses in the direction of head arrangement. Still further, by arranging the plurality of detecting heads circumferentially round the cast slab, it is possible to measure shell thicknesses and a distribution pattern thereof as well as a crosssectional shape of the cast slab.

Other objects, constructions and features of the present invention will become more apparent from the following description of its preferred embodiments taken in conjunction with the accompanying drawings.

Brief Description of the Drawings Fig. 1 is a graph showing the electric resistivities of iron in liquid and solid states, respectively, with the abscissa showing the temperature T ("C) and the ordinate the resistivity p X 10-6 (s2-cm).

Fig. 2 is a graph showing in elarged form the portion of Fig. 1 around the melting point (1 539"C).

Fig. 3 is a block diagram showing the construction of a basic apparatus according to the invention.

Fig. 4a is a circuit diagram showing another example of the coil connection of the detecting head.

Fig. 4b is a diagram showing the arrangement of the magnetic core and the coils of the detecting head shown in Fig. 4a.

Fig. 5 is a graph showing a solidified shell thickness versus output characteristic of the measuring apparatus according to the invention with the abscissa showing the actual shell thickness d (mm) and the ordinate showing the output voltage e0 (V) of the detecting circuit.

Fig. 6 is a block diagram showing another embodiment of the invention.

Fig. 7 is a block diagram showing still another embodiment of the invention.

Fig. 8 is a block diagram showing still another embodiment of the invention.

Description of the preferred Embodiments Figs. 1 and 2 are graphs showing by way of an example variations in the electric resistivity of iron in its liquid and solid states. The electric resistivity of the iron differs discontinuously between the liquid state and the solid state with the melting point (M.P.) as a borderline and moreover the variation of the electric resistivity is small in the range from the melting point to about 1 00O"C. This characteristic is the same practically for all kinds of steels mainly consisting of iron except that the electric resistivity in the liquid state differs more or less depending on the carbon content.

Fig. 3 shows a basic embodiment of the invention. In the Figure, a cast slab 1 is continuously withdrawn by rolls 6 from the exit of a mold 5 of a continuous casting machine. Arranged at a position just below the mold 5 is a detecting head 2 which faces the cast slab 1 at a predetermined distance from the surface of the cast slab 1 on one side thereof. The detecting head 2 includes a core 21 of magnetic material and a primary coil 22 and a secondary coil 23 which are wound on the core 21. An ac voltage of a predetermined frequency is applied from an exiter 3 to the primary coil 22 and the secondary coil 23 is connected to the input terminal of an amplifier 41 of a detecting circuit 4.

The output of the amplifier 41 is detected by a detector 42. Note that the cast slab 1 includes a head-side solidified shell 1 a, an opposite-side solidified shell 1c and an unsolidified part 1 c forming the inner part between the former.

When the ac voltage is applied to the primary coil 22 from the exiter 3, alternating flux flows to a closed magnetic path which passes through the core 21 and the cast slab 1 via a gap. Of course, the primary and secondary coils 22 and 23 are included in the closed magnetic path. Eddy currents are produced in the cast slab 1 by the alternating flux passing therethrough and the eddy currents have a magnitude corresponding to the amplitude value of an ac voltage signal induced in the secondary coil 23. The ac voltage signal induced in the secondary coil 23 is amplified by the amplifier 41 of the detecting circuit 4 and then the signal is detected by the detector 42 thereby generating a measured output e0 corresponding to the amplitude value of the signal.

Fig. 5 shows an examplary output characteristic of the measuring apparatus according to the invention. This characteristic graph is one obtained by using the actual molten steel and actually measuring the relation between the solidified shell thickness and the output voltage e0 of the detecting circuit 4. In this case, the actual measurement of the shell thickness d (mm) was made on the basis of the shell formation (solidification) temperatures measured by thermocouples and the values of the output voltage e0 (V) corresponding to the solidification temperatures of the cast slab for the respective thicknesses were recorded. The frequency used is 400 Hz and the lift-off is 20mm. As will be seen from Fig.

5, there is a proportional relation between the solidified shell thickness and the output voltage eO.

Fig. 4a is a coil connection diagram showing another example of the detecting head 2 and in this case the head 2 includes a primary winding 22 and a secondary winding 23 including a pair of coils 23a and 23b having the same temperature characteristic. One coil 23a forming a part of the secondary coil 23 is a detecting coil which is arranged at a position nearer to the cast slab 1 and the other coil 23b is a dummy coil arranged remote from the cast slab 1. In other words, the detecting coil 23a is higher in the amount of magnetic coupling to the cast slab 1 than the dummy coil 23b.The detecting coil 23a and the dummy coil 23b are both wound on a single magnetic core 21 and also the detecting coil 23a and the dummy coil 23b are differentially connected to each other such that ac voltage signals induced in the two coils are combined with a mutual phase difference of 7r (rad). More specifically, this can be accomplished by for example winding the detecting coil 23a and the dummy coil 23b in the opposite directions and connecting in series with each other.

Fig. 4b shows a good example of the connection for causing a difference in the amount of magnetic coupling with the cast slab 1 between the detecting coil 23a and the dummy coil 23b. More specifically, an Hshaped core 21 B is used as the magnetic core of the detecting head 2 and the H-shaped core 21 B includes a pair of parallel legs 25 and 26 and a connecting portion 24 for connecting the legs 25 and 26 at their central positions. The primary coil 22 is wound on the connecting portion 24 and the detecting coil 23a is wound on a portion 25a of one leg 25 which is on the side of the cast slab 1.

The dummy coil 23b is wound on a portion 25b of the leg 25 which is remote from the cast slab 1. In this example, the detecting coil 23a is included in a closed magnetic path through which the alternating flux generated from the primary coil 22 flows round the cast slab 1 and the dummy coil 23b is positioned outside the closed magnetic path. Thus, only a signal component corresponding only to the magnitude of the eddy currents within the cast slab 1 appears in the induced voltage of the secondary coil 23 on the whole and the dummy coil 23b serves the function of eliminating not only the variations due to temperature changes but also any undesired signal components from the induced voltage of the secondary coil 23.

Fig. 6 is a block diagram of a measuring apparatus for performing a method according to another embodiment of the invention. In this embodiment, a detecting head 2 is of the same type as shown in Fig. 3.

In connection with the previously mentioned embodiment, this embodiment features that ac voltages of different frequencies (e.g., 400, 800 and 1 600 Hz) are applied simultaneously to a single primary coil. An exciter 3A includes three oscillators 33A, 33B and 33C, a mixer 31A for mixing the outputs of the three oscillators and a power amplifier 32A for amplifying and supplying the output of the mixer 31A to a primary coil 22 of the head 2.

A detecting circuit 4A includes an amplifier 41A for amplifying the detection signal from the head 2, three band-pass filters 43A, 43B and 43C respectively corresponding to the oscillation frequencies f1, f2 and f3 of the three oscillators, three detectors 42A, 42B and 42C for respectively detecting the signals separated by the respective band-pass filters and two compensating differential amplifiers 44A and 44B.

The oscillators 33A, 33B and 33C respectively supply ac voltages (of1, Vf2 and V,3) of the frequencies f1, f2 and f3 to the mixer 31A so that the resulting mixed ac voltage is power amplified by the power amplifier 32A and then supplied to the primary coil 22 of the detecting head 2. In the like manner as the previously mentioned embodiment, the secondary coil 22 generates a detection signal corresponding to the eddy currents so that after the signal has been amplified to a given level by the amplifier 41A the signal is separated into the frequencies f1, f2 and f3 and detected by the detectors 42A, 42B and 42C thereby generating measured outputs e, to e3.

Assuming that f1 = 400 Hz, f2 t 800 Hz and f3 = 1 600 Hz, the eddy currents corresponding to each of these frequencies are produced within the cast slab 1 and the penetration depth of the eddy currents corresponds to the frequency. As a result, the eddy currents of f1 = 400 Hz reach the relatively deep portion of the cast slab 1 and the eddy currents of f2 = 800 Hz flow into the less deep portion of the cast slab 1. On the other hand, dueto the high frequency, the eddy currents of f3 = 1 600 Hz concentrate in the surface portion of the cast slab 1 due to the skin effect.

Noting the frequencies f1 and f2, the measurements by these low frequency magnetic fields differ in measuring range or measuring span from each other and an output characteristic having an excellent linearity is obtained within the measuring span corresponding to each of these frequencies. Therefore, if measurements are made by simultaneously using the two frequencies, it is possible to make shell thickness measurements over the wide measuring spans while ensuring a satisfactory linearity for the output characteristics. In the case of the measurement by the high frequency f3, the measurement results in an output voltage e3 corresponding to the surface contour of the cast slab 1 or its relative distance with the head 2 rather than the thickness of the solidified shell.Thus, the outputs e, and e2 are respectively applied to the positive input terminal of the liftoff compensating differential amplifiers 44A and 44B and the output e3 is applied as a compensation signal to the negative input terminal of each of these differential amplifiers, thus differentially processing the signals and thereby measuring the thickness of the solidified shell for each of the measuring spans with a high degree of accuracy. It is to be noted that the differential amplifiers 44A and 44B may each be replaced with a multiplier.

Fig. 7 is a block diagram showing still another embodiment of the invention. With the illustrated embodiment, it is to be noted that a linear sensor is formed by a plurality of detecting heads arranged in a line and having the same construction as described with reference to Figs. 4a and 4b.

An exciter 3B includes a single low frequency oscillator 33 and a power amplifier 32B. The low frequency ac voltage from the power amplifier 32B is supplied in common to the primary conl7Wplurality of detecting heads 2A, 2B, 2C ...., 2N. As shown in Fig.

4a, the secondary coil of each detecting head generates an output representing the difference between the induced voltage Ea of the detecting coil and the induced voltage Eb of the dummy coil or Eo = Ea; - Eb and the outputs of the detecting heads are successively sent to a signal amplifier 41 through a multiplexer 7. The timing of this operation is controlled by the timing pulses generated from a gate signal generator 8. The output of the signal amplifier 41 is detected by a detector 42 and generated as an output signal egO.

Then, the current output signal e10 corresponds to an output produced by scanning the outputs of the detecting heads 2A, 2B, ....

2N in time sequence. In this way, the distribution of shell thicknesses in the direction of arrangement of the detecting heads can be grapsed.

Fig 8 is a block diagram of a measuring apparatus for performing a method according to still another embodiment of the invention.

In connection with the embodiment of Fig.

7, the present embodiment features that ac voltages of different frequencies (e.g., 400, 800 and 1 600 Hz) are simultaneously applied to the primary coil of each of detecting heads.

In other words, the embodiment of Fig. 8 applies the multifrequency measuring method of Fig. 6 to the embodiment of the linear sensor type shown in Fig.7 and this embodiment differs from the embodiment of Fig. 7 in that an exciter 3A is of the same type as used in the embodiment of Fig. 6 and a detecting circuit 4A is also of the same type as used in Fig. 6.

The oscillators 33A, 33B and 33C respectively supply ac voltages Vf1, Vf2 and Vf3 of frequencies fl, f2 and f3 to the mixer 31A so that the resulting combined voltage is power amplified by the power amplifier 32A and then supplied in common to the primary coil of the detecting heads 2A, 2B, ..., 2N, re spectively. In the like manner as the embodi ment of Fig. 7, the difference output (Eo = Ea - Eb) is generated from the secondary coil of each of the heads and the outputs from the heads are successively sent to the signal amplifier 41A through the multiplexer 7.Then, after the signal has been amplified to a given level by the signal amplifier 41A, the signal is separated into the frequencies f1, f2 and f3 by the band-pass filters 43A, 43B and 43C and detected by the detectors 42A, 42B and 42C, respectively.

As a result, the detector 42A successively generates a measured output e" corresponding to the frequency f1 for each of the detect ing heads 2A, 28 ...., 2N in time sequence and the detector 42B similarly generates successively a measured output e,2 corresponding to the frequency f2 for each of the detecting heads 2A, 28 ..., 2N in time sequence.

Similarly, the detector 42C successively generates a measured output e,3 corresponding to the frequency f3 for each of the detecting heads 2A, 28 ..., 2N in time sequence. The outputs e", e,2 and e,3 are synchronized with one another.

The differential amplifiers 44A and 448 are of the same lift-off compensating type as in the embodiment of Fig. 6 and their resulting outputs e,4 and e,5 respectively correspond to one obtained by compensating the output e" by the output e,3 and one obtained by compensating the output e,2 by the output e,3.

Claims (14)

CLAIMS;

1. A solidified shell thickness measuring method comprising the steps of: applying an alternating magnetic field to a metal casting having an unsolidified inner part; detecting the magnitude of eddy currents produced within said casting in a non-contact manner by detecting coil means; and measuring the thickness of a solidified shell of said casting from the magnitude of said eddy currents in accordance with the difference in electric resistivity between an unsolidified molten part and a solidified part of said casting.

2. A measuring method according to claim 1, wherein dummy coil means having an equivalent temperature characteristic as said detecting coil means and a relatively weak magnetic coupling with said casting is differentially combined with said detecting coil means thereby temperature compensating a detection signal from said detecting coil means.

3. A measuring method according to claim 1, wherein said detecting coil means includes a plurality of detecting coils arranged in a line whereby a distribution of solidified shell thicknesses along the direction of arrangement of said detecting coils is measured in accordance with a plurality of detection signals from said plurality of detecting coils.

4. A measuring method according to claim 3, wherein the detection signals from said plurality of detecting coils are sequentially selected by a multiplexer thereby repeatedly performing a scanning with respect to said direction of coil arrangement.

5. A measuring method according to claim 1, wherein a low frequency alternating magnetic field is applied to one surface of said casting, and wherein the frequency of said low frequency alternating magnetic field is predetermined such that the penetration depth of eddy currents produced within said casting is midway within said casting whereby a measuring span is determined by said frequency.

6. A measuring method according to claim 1, wherein a low frequency alternating magnetic field comprising a plurality of different frequency components is applied to one surface of said casting whereby a plurality of low frequency signal components each corresponding to one of said frequency components are separated from a detection signal from said detecting coil means thereby simultaneously making a measurement for each of a plurality of different measuring spans.

7. A measuring method according to claim 5 or 6, wherein the frequency or frequencies of said low frequency alternating magnetic field are 1000 Hz or less, preferably 800 Hz or less.

8. A measuring method according to claim 5 or 6, wherein a high frequency alternating magnetic field for producing high frequency eddy currents flowing only into a surface portion of said casting is superposed on said low frequency alternating magnetic field and applied to said casting whereby said low frequency signal component or components and a high frequency signal component are separated from a detection signal from said detecting coil means and each said low frequency signal component is compensated by said high frequency signal component for a measuring error due to irregularities in the surface of said casting and variation in the amount of lift-off of said detecting coil means from said casting surface.

9. A measuring method according to claim 8, wherein the frequency of said high frequency alternating magnetic field is 1000 Hz or over, preferably 1600 Hz or over.

10. A measuring method according to any one of claims 1 to 9, wherein said casting is a continuously cast slab under solidification emerging from an exit of a mold of a continuous casting machine.

11. An apparatus for measuring the thickness of a solidified shell of a casting comprising: detecting head means including a primary coil and at least one secondary coil and positioned to face said metal casting having an unsolidified inner part at a predetermined distance apart from a surface thereof and to magnetically couple said coils and said casting; exciting means for supplying an alternating current to said primary coil such that alternating flux generated from said primary coil passes through said casting; and detecting circuit means for detecting the amplitude value of an alternating voltage induced in said secondary coil magnetically coupled to said casting to generate a measured output corresponding to the thickness of a solidified shell of said casting under solidification.

1 2. A measuring apparatus according to claim 11, wherein said secondary coil includes a detecting coil and a dummy coil, wherein said dummy coil has the same temperature characteristic as said detecting coil and disposed within said detecting head means in such a manner that said dummy coil has a smaller amount of magnetic coupling to said casting than said detecting coil, wherein each of said detecting coil and said dummy coil is magnetically coupled to said primary coil, and wherein said detecting coil and said dummy coil are differentially connected to each other such that induced voltages thereof are combined with a phase difference of sr rad.

1 3. A measuring apparatus according to claim 12, wherein said detecting head means includes an H-shaped magnetic core, wherein said H-shaped magnetic core includes a pair of parallel legs and a central connecting portion connecting said legs at substantially central portions of lengths thereof, wherein said primary coil is wound on said central connecting portion, and wherein said detecting coil is wound on one side of the central portion of one of said legs and said dummy coil is wound on the other side of the central portion of said one leg.

14. A measuring apparatus according to claim 11, wherein said detecting head means includes a plurality of secondary coils arranged in a line and connected to a multiplexer for sequentially selecting and delivering detection signals from said secondary coils whereby said detecting head means is selfscanned in the direction of arrangment of said secondary coils.

1 5. A measuring apparatus according to claim 11, wherein said exciting means generates an alternating current of a single predetermined low frequency.

1 6. A measuring apparatus according to claim 11, wherein said exciting means generates an alternating current including a plurality of frequency components each having a predetermined different low frequency, and wherein said detecting circuit means includes a plurality of frequency selective means each adapted to separate from an alternating voltage signal from said secondary coil one of a plurality of low frequency signal components corresponding to one of said frequency components whereby a shell thickness measurement output is generated from the amplitude value of a signal from each of said frequency selective means.

1 7. A measuring apparatus according to claim 1 5 or 16, wherein said exciting means generates said low frequency alternating current and a high frequency alternating current, wherein said high frequency alternating current is supplied to said primary coil such that a high frequency alternating magnetic field is generated from said primary coil thereby producing high frequency eddy currents concentrically flowing into a surface portion of said casting, and wherein said detecting circuit means further includes means for extracting a high frequency signal component corresponding to said high frequency eddy currents from a detection signal from said secondary coil and means for compensating each said low frequency signal component of said detection signal by said high frequency signal component for a measuring error due to irregularities in the surface of said casting and variation in the amount of lift-off of said detecting head means from said casting surface.

1 8. a method according to claim 1 and substantially as described herein with reference to the accompanying drawings.

1 9. apparatus for measuring the thickness of a solidified shell of a casting, substantially as described herein with reference to the accompanying drawings.