CN106289037B - A kind of continuous casting steel billet shell thickness eddy current detection method - Google Patents

Abstract

The invention discloses an eddy current detection method for the shell thickness of a continuous casting billet, belonging to the technical field of continuous casting billet detection. The purpose is to solve the deficiency of the prior art in measuring the thickness of the continuous casting solidified slab shell, and provide a method for measuring the thickness of the continuous casting slab shell by using the multi-frequency eddy current measurement method to layer the inside of the slab according to the electrical conductivity corresponding to the temperature . The method is specifically as follows: place an eddy current sensor on the surface of the slab at the outlet of the continuous casting mold. The sensor is a mutual induction differential type. The adder performs frequency mixing, the measurement signal is amplified and filtered by the signal conditioning circuit, and the cross-correlation operation is performed with the reference signals of different frequencies in the correlator, the signal is separated and the amplitude and phase separation of the eddy current impedance signal is performed, and the separated signal After collection and conversion, it is transmitted to the computer for data processing to obtain the thickness of the continuous casting billet shell.

Description

An eddy current detection method for continuous casting billet shell thickness

technical field

The invention specifically relates to an eddy current detection method for the shell thickness of a continuous casting billet, belonging to the technical field of continuous casting billet detection.

Background technique

In the process of continuous casting of iron and steel, through the online non-destructive detection measurement and control of the thickness of the solidified billet shell at the outlet of the crystallizer, timely and efficient detection of breakouts and the uniformity of billet shell thickness in the continuous casting production process is carried out to improve The production quality of continuous casting slabs and the improvement of continuous casting automation level are embodied in the following two aspects: real-time detection of the thickness of the billet shell at the exit of the crystallizer to withstand the static pressure of molten steel, and prevention of steel breakout production accidents in the continuous casting process; online non-destructive Detect the change trend of the thickness of the solidified billet shell, adjust the cooling intensity of the secondary cooling water online, and control the cooling water volume of the secondary cooling water in a closed loop.

Due to the harsh on-site environment of continuous casting production, the influence of high-temperature heat radiation, cooling water spray, dust, etc., there are solid-phase, two-phase and liquid-phase regions in the billet shell at the outlet of the crystallizer. These factors determine that the on-line non-destructive testing of continuous casting slab shell thickness requires long-term operation in high temperature and high water mist environments.

At present, there are three main methods for detecting the thickness of the solidified slab shell in the continuous casting process: experimental measurement method, numerical model method, and strain sensor detection method. In the experimental measurement method, the nail shooting method is mainly used. The steel nail containing sulfur tracer is injected into the continuous casting slab in the secondary cooling zone, and the sulfur print analysis and low magnification are used to detect the concentration of the nail in the solid phase and liquid phase area of the casting slab. Determining the thickness of the solidified shell and the position of the liquid phase hole by using the topography is a mature experimental method with high precision and intuitive and reliable results, but it belongs to off-line destructive testing, and the measurement workload is heavy. Piercing slab shell measurement method, pierces the solidified slab shell in the secondary cooling zone of continuous casting, so that the unsolidified liquid steel flows out, and then conducts online detection of the thickness of the solidified slab shell. This detection method is a destructive test, and the on-site operation is very difficult . The bulge method is to determine the position of the liquid core by detecting the position of the bulge of the continuous casting slab. This measurement method is intuitive and the device is simple, but it can only roughly estimate the position of the end of the liquid core. The radiation method is mainly based on the use of drilling ray sources, and the thickness of the solidified slab shell is detected by Geiger counters to detect the changes in the ray intensity after the ray source passes through the continuous casting slab. cause a certain impact. The numerical model method is based on one-dimensional or two-dimensional unsteady-state heat transfer control equations, using the finite element difference calculation method to calculate the internal temperature field of the continuous casting slab, and then determine the internal shape of the continuous casting slab. This method is low in cost, but the model solution Accuracy depends on accurate boundary conditions, but unsteady continuous casting production determines that it is difficult to determine accurate boundary conditions. At the same time, the model calculation takes a long time to converge, and it is difficult to predict the change of solidified slab shell thickness online in real time. The strain test method determines the position of the solidification end of the continuous casting slab by installing foil resistance strain gauges on the relevant sectors of the continuous casting slab and monitoring the mutation point of the strain change online. This method has poor reliability and sensitivity. Due to the influence of radiation, water mist in the secondary cooling zone, complex forces on the fan section or work rolls, etc., the repeatability of the test results is poor, and it cannot be popularized and applied in industrial production sites.

Based on the importance of the solidified slab shell at the exit of the mold, accurate characterization of its thickness and uniformity is essential for optimizing the continuous casting mold process and equipment, improving the quality of the continuous casting slab and continuous casting production efficiency. At present, the detection methods for the thickness of the solidified billet shell at home and abroad have not fundamentally solved the problem of field application in industrial production.

Contents of the invention

Therefore, aiming at the above-mentioned shortcomings of measuring the thickness of the continuous casting solidified slab shell in the prior art, the present invention aims to provide an electric eddy current detection method for the thickness of the continuous casting slab shell. The invention can effectively overcome the influence of harsh environments such as surface high temperature radiation and water mist environment in the continuous casting production site, and is respectively suitable for measuring thin-slab continuous casting requirements of different surface temperatures of casting slabs and different pouring steel types.

The principle of this method is based on the fact that the surface temperature of the continuous casting billet is above the Curie point, and the resistivity of the billet material changes with the temperature, and when the steel is melted into a liquid state, its resistivity changes abruptly, and its resistivity is 2- 3 times, so the steel billet can be divided into three layers according to the resistivity, the solidified layer, the solid-liquid layer and the liquid layer, and then use the eddy current technology to measure the thickness of the solidified layer.

The purpose of this program is achieved through the following programs:

An eddy current sensor is placed on the surface of the slab at the outlet of the continuous casting crystallizer, and the sensor is a mutual induction differential type. The eddy current sensor is excited by mixing frequency through a digital frequency synthesizer, and the excited three different frequency signals are mixed by an adder, and the measurement signal is amplified and filtered by a signal conditioning circuit, and is respectively different from three different The frequency reference signal is subjected to cross-correlation calculation, the signal is separated and the amplitude and phase of the eddy current impedance signal are separated, and the separated signal is collected and converted, and then transmitted to the computer for data processing. The process of data processing by the computer is as follows: When calibrating, firstly, simulate calibration by using the temperature field and electromagnetic field coupling measurement model pre-established by the finite element simulation software, and obtain the relationship curve between the measurement data and the layer thickness. Then use a hot steel plate with variable thickness to replace the solidified layer, and another hot steel plate to replace the solid-liquid layer. The molten steel is a liquid layer. The measured data is calculated by the least square algorithm to complete the measurement data of the solidified layer thickness and the solid-liquid layer. The correction of the fit is obtained by measuring the inversion of the mathematical model. During the measurement, the measurement data is brought into the measurement inversion mathematical model to obtain the thickness of the continuous casting billet shell.

Further, in the method, the eddy current sensor is placed at a position 2mm away from the surface of the slab at the outlet of the continuous casting mold.

Further, the digital frequency synthesizer in the method is an AD9959 digital frequency synthesizer.

Further, in the method, in the parameter setting of the eddy current coil of the eddy current sensor, the excitation is 700HZ to 12KHZ, the inner radius is 12mm, the outer radius is 17mm to 19mm, the height is 4mm, and the number of primary and secondary turns is 60 .

Further, the establishment method of the measurement model is to use the finite element simulation software ANSYS to simulate and calculate the temperature field of the slab at the exit of the mold according to formula 1 and the boundary conditions. The calculation can obtain the cloud map of the temperature gradient distribution inside the slab. The calculation formula 2 of the chemical composition and solid-liquidus temperature of the steel type can obtain the solidus temperature value and liquidus temperature value of the steel type, and the billet parameters during calculation are:

Mold wall thickness: 20mm, effective length: 1000mm, billet casting speed: 2-5.5m/min, pouring temperature: 1547°C, billet thickness: 50mm.

In the formula, λ—heat conductivity coefficient of material (w/m.℃), C—specific heat (J/Kg.℃), ρ—density (Kg/m ³ ), τ—slab solidification time (s), L _f - latent heat of solidification of molten steel (J/kg).

^T _l ＝T _f -∑ΔT·i％ (Formula 2)

In the formula: T _l is the solid-liquidus temperature, T _f is the melting point of the steel, and ΔT is the lowering value of the melting point without adding 1% element i to the iron.

According to the simulation calculation, determine the required measurement range. The object to be tested is selected as a thin slab, the thickness of the slab is 20-100 mm, and the shell thickness is 5-20 mm. According to the change of the electromagnetic properties of the metal with temperature, the billet surface temperature at the outlet below the continuous casting crystallizer is higher than the Curie temperature, and when the steel melts into a liquid state, the resistivity changes abruptly, and the billet is divided according to the resistivity and material temperature. Correspondingly, the slab at the outlet of the continuous casting mold is stratified.

The process of carrying out simulation calibration to the measurement model in the method specifically includes:

Step 1: Use phase rotation and subtraction algorithm in MATLAB to process data of three groups of different impedance values;

Step 2: Change the thickness of the solidified slab shell layer, and the thickness of the other two layers will change accordingly. Repeat the steps one or more times to obtain the calibration data of the multi-frequency multi-layer board; obtain the measurement curves and mathematical models corresponding to different frequencies by the least square method . Mathematical model such as formula three

Where u _fi is the voltage value corresponding to the phase of the measured signal at different frequencies. α _i , β _i , χ _i are obtained through multiple sets of data and the least square method, and d _i is the layer thickness. Then solve the equations to get the thickness inversion model as Equation 4

Step 3: Replace the measured object with two layers of hot steel plates and molten steel to measure the thickness of the billet shell, only change the thickness of the two layers of steel plates, and keep the thickness of the molten steel unchanged, and use the measurement system described in the plan to obtain the sensor measurement signal The magnitude and phase corresponding to three different frequencies are used to correct the measurement model.

The beneficial effect of the present invention is that: the present invention applies the multi-frequency eddy current technology to the continuous casting slab thickness measurement, and its advantage is that it can measure the thickness of the continuous casting slab on-line without contact in the continuous casting production environment of high temperature and water mist. Solidification thickness, specifically reflected in the following points:

1. High temperature resistant flat excitation coil is adopted. The coil is in the form of mutual inductance and differential. The coil material is metal tungsten. The insulation is made of ceramic paint while winding and painting. The coil is added with a water cooling jacket to effectively overcome the high temperature radiation on the surface of the continuous casting production site. , water mist environment and other harsh environments, and at the same time ensure that the eddy current coil can work reliably on high-temperature metal surfaces above 1000 °C.

2. For the first time, the interior of the billet at the exit of the mold is stratified according to the material resistivity corresponding to the temperature, and the thickness of the billet shell is obtained by using the eddy current multi-frequency measurement method, which makes the interior of the billet more intuitive and clear, and can measure the solidified billet shell Thick is more precise.

3. Use finite element simulation software to analyze the temperature field inside the continuous casting slab, and obtain the thickness measurement range of the slab shell under the crystallizer. According to the measurement range requirements, the size, structure and excitation frequency range of the probe coil are designed, which is more targeted, and it makes it possible to measure the thickness of the slab in the high temperature environment of continuous casting production.

4. For the first time, the multi-frequency multi-layer conductive structure thickness measurement is used to measure the thickness of the continuous casting slab shell, so that the application of the multi-frequency eddy current detection theory is verified in practice in high temperature environments.

Description of drawings

Fig.1 Contour diagram of cross-section temperature of slab at casting speed V=4.5m/min

Fig. 2 is a schematic diagram of the stratification inside the slab at the outlet of the continuous casting crystallizer of the present invention.

Fig. 3 is a schematic diagram of measurement by the eddy current sensor of the continuous casting slab multi-layer plate of the present invention.

Fig. 4 is a schematic diagram of the installation of the eddy current measuring device for the thickness of the solidified shell of the present invention.

Fig. 5 is a schematic diagram of the penetration depth of eddy current under the excitation frequency of 4kHZ in the present invention.

Fig. 6 is an outline drawing of the water cooling jacket of the high temperature probe of the present invention.

Fig. 7 is a calculation schematic diagram of simulation calibration performed by the finite element simulation software of the present invention.

Fig. 8 is a graph showing the relationship between thickness and voltage variation at different frequencies in the present invention.

Fig. 9 is an overall principle block diagram of the measurement system of the method of the present invention.

Detailed ways

The specific embodiment of the present invention is described below in conjunction with accompanying drawing:

The invention measures the thickness of the solidified slab shell at the exit of the continuous casting mold online. The eddy current coil of the invention measures the thickness of the solidified slab at the exit of the continuous casting mold. Due to the particularity of the object, the eddy current coil is independently designed. When the slab just leaves the continuous casting mold, the interior of the slab is a solid region, a solid-liquid two-phase region, and a liquid phase region, and the thickness of these regions is difficult to measure. First of all, it is necessary to know the thickness range of the billet shell, and design the probe and measurement system according to the measurement range.

Through the finite element simulation software ANSYS, the temperature field of the slab at the outlet of the mold is simulated and calculated according to formula 1 and a series of boundary conditions. The calculation can get the cloud diagram of the temperature gradient distribution inside the slab as shown in Figure 1, combined with the steel type. The calculation formula 2 of chemical composition and solid-liquidus temperature can get the solidus temperature value and liquidus temperature value of the steel grade. The billet parameters during calculation are shown in Table 1.

Table 1

Crystallizer wall thickness Effective length Throwing speed pouring temperature Slab Thickness 20mm 1000mm 2-5.5m/min 1547°C 50mm

In the formula, λ—heat conductivity coefficient of material (w/m.℃), C—specific heat (J/Kg.℃), ρ—density (Kg/m ³ ), τ—slab solidification time (s), L _f - latent heat of solidification of molten steel (J/kg).

^T _l ＝T _f -∑ΔT·i％ Formula 2

In the formula: T _l is the solid-liquidus temperature, T _f is the melting point of the steel, and ΔT is the lowering value of the melting point without adding 1% element i to the iron.

According to the simulation calculation, determine the required measurement range. The object to be tested is selected as a thin slab, the thickness of the slab is 20-100 mm, and the shell thickness is 5-20 mm.

According to the change of the electromagnetic properties of the metal with the temperature, the surface temperature of the billet at the outlet below the continuous casting crystallizer is higher than the Curie temperature, and when the steel melts into a liquid state, the resistivity changes abruptly. Correspondingly, the cast slab at the outlet of the continuous casting crystallizer can be stratified, and the built model is shown in Figure 2, in which 4 is the solid zone, 5 is the solid-liquid two-phase zone, and 6 is the liquid phase zone.

The sensor model is established on the detection object model, as shown in Figure 3. Among them, R ₁ is the inner radius of the coil, R ₂ is the outer radius of the coil, D is the height of the eddy current coil, H is the lift-off distance, t ₁ is the thickness of the solid-state layer removed, t ₂ is the thickness of the solid-liquid two-phase region, t ₃ is the thickness of the liquid phase region. Sensor optimization calculations are performed on this model.

The calculation results support the placement of the eddy current sensor as shown in Figure 4. In Figure 4, 1 is the tundish, 2 is the crystallizer, and 3 is the eddy current sensor. Through the finite element simulation software ANSYS, it is concluded that when the lift-off distance is 2mm, the eddy current density can meet the measurement requirements. Therefore, the eddy current sensor 3 is placed at a position 2 mm below the continuous casting crystallizer 2 and away from the surface of the slab.

According to the particularity of the measured target and the relevant theory of eddy current detection, the eddy current coil is simulated by using finite element simulation software and impedance analysis method, so that the eddy current coil can detect the thickness of the solidified shell on-line and the efficiency of the eddy current coil can reach the best excellent.

Where f is the main excitation frequency of the eddy current coil, ρ is the magnetic permeability, μ is the relative magnetic permeability, d is the skin depth, and R is the inner radius of the coil.

Based on Formula 5 and Formula 6, the main excitation frequency of the eddy current coil and the inner radius of the coil can be obtained. Based on the above two factors, the finite element simulation calculation is performed on the outer radius, height and number of turns of the eddy current coil respectively. The calculated The result data is imported into MATLAB, analyzed and processed by MATLAB, and finally the excitation of the eddy current coil is 700HZ to 12KHZ, the inner radius is 12mm, the outer radius is 17mm to 19mm, the height is 4mm, and the number of primary and secondary turns is 60 turns can obtain the maximum efficiency, and the finite element simulation skin depth is shown in Figure 5.

In order to be suitable for the environment of high temperature and water mist in the continuous casting site, the excitation coil winding is formed by spiral winding of high temperature resistant tungsten wire and ceramic paint, the probe shell of the sensor adopts a metal shell with water cooling effect, and the probe shell The water cooling design is shown in Figure 6.

Firstly, finite element simulation software is used for simulation calibration, and then the measurement model is corrected by using hot steel plate and liquid steel.

In multi-frequency eddy current detection, the eddy current sensor is excited by the signal synthesized by three frequency waveforms. Since the superposition principle is applied to the coil linear system, the impedance change of the probe under the mixed frequency excitation is the linear synthesis of the impedance change under the separate excitation of each frequency. . The effect is the same as three independent eddy current instruments controlling the same probe, and then summing the outputs of the three instruments, so in the finite element simulation, three separate frequencies are used to excite the coil.

Step 1: Different excitation frequencies will produce different impedance values, and three different excitation frequencies will produce three different impedance values. Use the phase rotation subtraction algorithm in MATLAB to process the data of the three different impedance values;

Step 2: Change the thickness of the solidified slab shell layer, and the thickness of the other two layers will change accordingly. Repeat the steps one or more times to obtain the calibration data of the multi-frequency multi-layer board; obtain the measurement curves and mathematical models corresponding to different frequencies by the least square method . Mathematical model such as formula three

Where u _fi is the voltage value corresponding to the phase of the measured signal at different frequencies. α _i , β _i , χ _i are obtained through multiple sets of data and the least square method, and d _i is the layer thickness. Just get the thickness of the top two layers. Then solve the equations to get the thickness inversion model as Equation 4

The calculation principle of simulation calibration using finite element simulation software is shown in Figure 7. In the figure, since the surface temperature of the slab is higher than the Curie point, the relative magnetic permeability μ _i is 1, the electrical conductivity corresponds to the temperature, and the coil is the mutual inductance difference dynamic. Probe coil parameters: r ₁ , r ₂ - inner diameter and outer diameter of probe coil, (1 ₂ -l ₁ ,) - probe length; l ₁ - lift-off; material conductivity: σ ₁ , σ ₂ ,…,σ _m , σ _m+1 ,σ _m+2 ; material relative permeability: μ ₁ , μ ₂ ,…, μ _m , μ _m+1 , μ _m+2 at the junction of each layer in the z direction: z ₁ , z ₂ , …, z _m , z _1+m , z _i =-(d _m +d _m-1 ,…d _i+2 +d _i+1 ), i=1,2,3…m-1,d _m + d _m-1 ,...d _i+2 +d _i+1 - the thickness of the individual layers.

Step 3: Replace the measured object with two layers of hot steel plates and molten steel to measure the thickness of the billet shell, only change the thickness of the two layers of steel plates, and keep the thickness of the molten steel unchanged, and use the measurement system described in the plan to obtain the sensor measurement signal The amplitude and phase corresponding to the three different frequencies are corrected to the measurement model, and finally the relationship curve between the thickness and the voltage change value at different frequencies is shown in Figure 8.

The schematic diagram of the online measurement of the thickness of the solidified shell by the eddy current coil of the present invention is shown in FIG. 9 . It consists of multi-frequency signal excitation source AD9959, eddy current probe coil, signal processing circuit, correlator, signal acquisition and conversion and computer.

The eddy current sensor requires three different excitation frequencies to simultaneously excite the eddy current detection coil. Therefore, the excitation signals of three different frequencies generated by the AD9959 must be mixed by an adder, so as to satisfy the multi-frequency excitation of the eddy current detection coil at the same time. mixing.

It is impossible to realize single-frequency signal excitation for shell thickness measurement, and the traditional impedance analysis method can only suppress one interference factor. Therefore, in order to obtain more information about the inspected workpiece, it is necessary to excite the eddy current sensor with multiple frequencies. For the measurement of the thickness of the three-layer conductive structure, three sinusoidal signals with different frequencies are required to be mixed and excited, and it must be ensured that each excitation signal can pass through the shell thickness. The information contained in the response signal is separated to obtain a DC signal under single-frequency excitation.

The online measurement of the thickness of the solidified slab shell of the present invention is carried out in such a way: the above-mentioned eddy current sensor is placed at the outlet of the continuous casting mold at a distance of 2mm from the surface of the slab, and the eddy current sensor is excited by AD9959. Three different The frequency is mixed by the adder, the signal at the measuring end is amplified and filtered by the signal conditioning circuit, and the cross-correlation operation in the correlator realizes the separation of different frequency signals and the amplitude and phase separation of the eddy current impedance signal. The separated signal After collection and conversion, it is finally transmitted to a computer for data processing.

The measured object is replaced by two layers of hot steel plates and molten steel with known thickness to measure the thickness of the billet shell, and the sensor and measurement system shown in Figure 9 are used to obtain data. The data processed by the computer is used to correct the measurement model calibrated by simulation in advance to obtain a practical measurement model.

When measuring, the measured values obtained by the above-mentioned sensors and measurement system are brought into the measurement model to calculate the thickness of the solidified shell. When the steel type is changed, it needs to be re-calibrated.

In order to improve the measurement accuracy, the stratification in the billet can be further subdivided, but more frequencies need to be added to the measurement frequency, and the measurement speed will be reduced. This problem needs to be considered as a compromise when calibrating.

The above description is a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications can also be made. It should be regarded as the protection scope of the present invention.

Claims (7)

1. a kind of continuous casting steel billet shell thickness eddy current detection method, which is characterized in that the method is：Go out in continuous cast mold Casting billet surface at mouthful places current vortex sensor, and sensor is mutual inductance differential type, and current vortex sensor is closed by numerical frequency It grows up to be a useful person and carries out mixing excitation, three different frequency signals of excitation are mixed by adder, and measuring signal passes through signal Amplification, the filtering of modulate circuit, computing cross-correlation is carried out in three correlators with the reference signal of three different frequencies respectively, By Signal separator and the width phase separation for being vortexed impedance signal is carried out, the signal separated is acquired and converts, and is transferred to calculating Machine carries out the processing of data, and the data procedures of computer disposal are as follows：When calibration, first by using finite element emulation software The temperature field pre-established and electromagnetic field couples measurement model carry out analogue simulation calibration, obtain measurement data and layer thickness relationship Curve, then replace solidification layer, another hot steel plate to replace solid-liquid layer with the hot steel plate of Varying-thickness, molten steel is layer liquid, measured Data by least-squares algorithm, complete the amendment of the fitting to solidified layer thickness and solid-liquid layer measurement data, measured Inverting mathematical model, when measurement, measurement data, which is brought into, measures the thickness that inverting mathematical model can be obtained continuous casting steel billet green shell.

2. continuous casting steel billet shell thickness eddy current detection method as described in claim 1, which is characterized in that electric in the method The positions casting billet surface 2mm of eddy current sensor being placed on apart from continuous cast mold exit.

3. continuous casting steel billet shell thickness eddy current detection method as described in claim 1, which is characterized in that number in the method Word frequency synthesizer is AD9959 digital frequency synthesizers.

4. continuous casting steel billet shell thickness eddy current detection method as described in claim 1, which is characterized in that in the method The parameter of the eddy-current coils of current vortex sensor is set as, and is actuated to 700HZ to 12KHZ, inside radius 12mm, and outer radius is 17mm to 19mm is highly 4mm, and former pair side the number of turns is 60 circles.

5. continuous casting steel billet shell thickness eddy current detection method as described in claim 1, which is characterized in that surveyed in the method The method for building up for measuring model is to carry out temperature field according to public affairs to the strand in crystallizer exit by finite element emulation software ANSYS Formula one and boundary condition carry out simulation calculating, and calculating can obtain strand internal temperature gradient cloud charts, in conjunction with the change of steel grade It studies point and the calculation formula two of solid-liquid phase line temperature, it can be deduced that the solidus temperature values and liquidus of steel grade, meter Strand parameter is when calculation：

Crystallizer wall thickness：20mm, effective length：1000mm, casting speed：2-5.5m/min, pouring temperature：1547 DEG C, strand Thickness：50mm,

The coefficient of heat conduction of λ-material, unit in formula：DEG C, w/m. C-compares heat unit：DEG C, J/Kg. ρ-density unit：Kg/m³, τ-casting blank solidification chronomere：s,L_fMolten steel solidification latent heat unit：J/kg,

T_l=T_f∑ Δ Ti% formula two

In formula：T_lFor solid-liquid phase line temperature, T_fIt is that 1% element i is not added in iron to make fusing point decreasing value for steel grade fusing point, Δ T,

According to simulation calculation, required measurement range is determined, select measurand for thin plate strand, slab thickness ranging from 20- 100 millimeters, ranging from 5-20 millimeters of range of shell thickness is varied with temperature according to the electromagnetic property of metal, under continuous cast mold Square exit base surface temperature is higher than Curie temperature, and when steel is molten into liquid, the characteristic of transition occurs for resistivity, by steel billet It is corresponding with material temperature according to resistivity, the strand in continuous cast mold exit is layered.

6. continuous casting steel billet shell thickness eddy current detection method as claimed in claim 5, which is characterized in that right in the method The process that measurement model carries out emulation calibration specifically includes：

Step 1：It is mutually cut algorithm using phase place in MATLAB and three groups of different impedance values of corresponding three frequencies is carried out Data processing,

Step 2：Change casting speed, the thickness of solidified shell changes correspondingly, remaining two layers thickness consequently also becomes, and repeats to walk Rapid one, it obtains multifrequency and surveys multiple-plate calibration curve.

7. continuous casting steel billet shell thickness eddy current detection method as claimed in claim 6, which is characterized in that used in the method The hot steel plate of known thickness and molten steel composition shell thickness measure and substitute measurand, only change the thickness of steel plate, the thickness of molten steel It remains unchanged, measurement model is modified.