JP2011202968A - Method and device for measurement of oxide film thickness on surface of steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To propose a method and device for measurement of an oxide film thickness on the surface of a steel plate, capable of performing an accurate on-line and continuous measurement of the film thickness of an iron-based oxide formed on the surface of the steel plate, from outside a heating furnace being far from the steel plate.SOLUTION: The surface of the steel plate is irradiated intermittently with infrared light. When irradiated with the infrared light, the energy being the total of spontaneous light radiation energy radiated from the surface of the steel plate and reflected light energy of the radiated infrared light from the surface of the steel plate is detected in two different infrared wavelength bands. When irradiation with the infrared light is intercepted, only the spontaneous radiation energy from the steel plate is detected likewise. Out of detection values detected by the intermittent irradiation with the infrared light, the detection value detected upon interception of the infrared light is deducted from that detected upon irradiation, so as to calculate the reflected light energy. As to the calculated reflected light energy, the ratio between the energies in the two different infrared wavelength bands is computed on the entrance and exit sides of the heating furnace, and the ratio between them computed on these sides is computed.

Description

  The present invention relates to a method and an apparatus for measuring the thickness of an iron-based oxide film on the surface of an iron-based oxide continuously formed on the surface of a steel sheet, and in particular, an annealing process of a hot-dip galvanized steel sheet manufacturing process. The present invention relates to a method and apparatus for measuring an oxide film thickness on the surface of a steel sheet, which is suitable for application to the direct-fired heating furnace exit side.

  The hot dip galvanized steel sheet has excellent properties such as corrosion resistance, workability, and surface aesthetics, and is used in large quantities, for example, as a steel sheet for automobiles. For hot dip galvanized steel sheets for automobiles used for automobile outer plates, inner plates, etc., high strength is required even with the same plate thickness for the purpose of improving collision safety characteristics and reducing weight. In order to satisfy these requirements, in recent years, high-tensile hot-dip galvanized steel sheets (hereinafter referred to as high-ten GA steel sheets) that use high-strength steel sheets as original sheets have been used in some automotive parts, and the usage ratio Is growing.

  Further, various high-tensile GA steel plates, that is, steel plates having different strength values are selected depending on the members used. In the production of high-tensile GA steel plates, oxidizable elements such as Si and Mn are added to the steel plates in order to improve the strength. These easily oxidizable elements are selectively oxidized on the surface of the steel sheet during annealing, which is an intermediate process of the hot-dip galvanized steel sheet manufacturing process, and when plating is performed on the surface of the steel sheet in the plating process, which is a post-annealing process, It is known to adversely affect That is, components such as Si and Mn are oxidized, and iron-based oxides generated on the surface of the steel sheet may cause non-plating.

  On the other hand, the direct-fired heating furnace that performs annealing is superior in the compactness of equipment, the improvement of sheet passing, thermal response characteristics, etc., has great economic merit, and can secure good plating properties. There is also an advantage that the addition limit can be increased. For this reason, the direct-fired heating furnace installed in the hot dip galvanized steel sheet production line has the feature of expanding the degree of freedom in designing the components of the steel sheet and enabling the production of hot dip galvanized steel sheets with better material properties. . The direct-fired heating furnace is divided into a plurality of zones, and a combustion pattern according to the load can be set. In the divided zones, oxidation promotion heating and reduction heating are performed continuously, but the combustion time is controlled in units of zones according to the load, and heating up to several hundred degrees Celsius is achieved as the steel plate temperature on the heating furnace exit side. The The steel sheet heated rapidly in a short time is annealed in a radiant tube heating furnace in the next process, and then guided to the plating process.

  As mentioned above, in the production of high-tensile GA steel sheets, one of the important points is how to prevent oxidation of easily oxidizable elements, but in high-tensile GA materials of 590 MPa class and higher, the amount of Si and Mn added is also large. Therefore, prevention of oxidation of easily oxidizable elements is essential for improving plating properties. In order to cope with this problem, it has been found that direct fire heating is an effective means. In other words, in the radiant heating furnace zone after the direct-fired heating furnace, by changing the iron-based oxide produced on the steel sheet surface to a pure iron layer, the surface oxidation of Si and Mn can be prevented and the plating characteristics can be improved. It can be done.

  Therefore, in order to improve the plating characteristics of the final high-tensile GA material, it is important to understand the characteristics of the oxide layer generated after the direct-fired heating furnace. Moreover, since the amount of the easily oxidizable element added to the high-tensile GA material varies depending on the material characteristics such as strength, the thickness of the oxide layer to be generated is different for each high-ten GA material.

  As a conventional analysis method, the relationship between the plating characteristics and the direct flame heating conditions is investigated in detail for each addition amount of Si and Mn, and the air ratio, burner flame strength, and combustion, which are the changes in the heating conditions The gas composition was optimized. However, with this method, the actual variation in the amount of process in the actual hot dip galvanized steel sheet manufacturing process, the occurrence of burner clogging and flame abnormalities, and the effects of gas composition fluctuations are actually optimal on the outlet side of a direct-fired heating furnace. It was difficult to confirm that an oxide layer was formed properly.

  For this reason, it is useful to directly measure the thickness of the oxide layer, and many methods for measuring the thickness of the oxide have been proposed.

  Usually, the method of measuring the oxide film thickness on the steel sheet surface, which continuously measures the film thickness of the iron-based oxide generated on the steel sheet surface, includes methods using X-ray fluorescence, ellipsometry (ellipsometry), Reflection and absorption methods are applied. However, when these methods are applied to the outlet side of a direct-fired heating furnace, the steel sheet is heated to several hundred degrees Celsius or higher, so that there is thermal radiation from the steel sheet itself, which causes an error factor. Furthermore, there is a problem that it is difficult to apply from an economical point of view, such as complicated heat countermeasures for installation and high installation costs.

  Therefore, a method using a color sensor (Patent Document 1) is disclosed as a new method. In the method of measuring the oxide film thickness on the steel sheet surface using the color sensor of Patent Document 1, since the oxide film thickness value and the brightness and hue value of the steel sheet are related, the relationship is obtained in advance. Thickness is estimated.

  As another method, a relational expression between a relationship between two spectral radiances measured under different measurement conditions and a change in emissivity of a measurement target is obtained in advance, and after estimating the emissivity, the oxide film thickness (Patent Document 2) is disclosed. In this case, the different measurement conditions include a case where the measurement is performed at two wavelengths, a case where the measurement is performed at two different measurement angles, and a case where the measurement is performed with two polarization components. This is an application of the Trace thermometer that measures the temperature.

  Another method is to irradiate the surface of the steel sheet with infrared light intermittently, and receive infrared light signals of multiple wavelengths received when the infrared light is irradiated and when the infrared light is blocked. Discloses a method of measuring the oxide film thickness (Patent Document 3).

  Furthermore, as another method, although it is an apparatus for measuring the oxide film thickness of a hot-rolled steel sheet, similarly to Patent Document 3, the infrared light is intermittently irradiated on the surface of the steel sheet, and reflection at a specific wavelength of 8 μm or more. An apparatus (Patent Document 4) that measures light at a solid angle of 15 degrees or more and measures the oxide film thickness without being affected by the surface roughness of the steel sheet is disclosed.

The scattering of electromagnetic waves from rough surfaces, Beckmann, Pergamon Press, 1963

JP-A-4-43905 Japanese Patent Laid-Open No. 7-18341 JP 2007-10464 A JP-A-10-206125

  However, the method for measuring the oxide film thickness on the steel sheet surface using the color sensor of Patent Document 1 and the method for measuring the oxide film thickness on the steel sheet surface using the emissivity correction thermometer described in Patent Document 2 When applied to the measurement of the oxide film thickness on the steel sheet surface, there is a problem that the oxide film thickness cannot be measured accurately because part of the iron-based oxide is reduced and the reduced Fe is scattered in the surface layer portion. I found out. That is, as described above, in the latter zone of the direct-fired heating furnace, heating is performed in a reducing atmosphere, so that a part of the iron-based oxide generated in the former zone is reduced, and reduced Fe is scattered on the surface layer. It will remain.

  In addition, in the oxide film measuring apparatus of Patent Document 3, the problem of reduced Fe has been solved, but since the water-cooled light-shielding tube is installed in the furnace and used close to the steel sheet surface, engineering is difficult, There was also a problem that when the measurement was performed from the outside of the furnace away from the steel plate surface, the measurement accuracy was lowered.

  Furthermore, in the oxide film thickness measuring apparatus of Patent Document 4, an optical system has been proposed in which the effect of the steel sheet surface roughness is reduced by taking a large solid angle for receiving reflected light. There was a problem that it had to be installed close to.

  The present invention is applied to a direct-fired heating furnace, which is an annealing process of a steel sheet manufacturing process, and the film thickness of the iron-based oxide generated on the steel sheet surface is continuously and continuously online from outside the heating furnace far from the steel sheet. It is an object of the present invention to propose a method and an apparatus for measuring an oxide film thickness on a steel sheet surface that can be measured with high accuracy.

  The above problem can be solved by the following means.

[1] A method for measuring an oxide film thickness on a steel sheet surface, in which a film thickness of an oxide generated on the surface of the steel sheet is measured in a continuous annealing step provided with a heating furnace,
Energy obtained by intermittently irradiating the surface of the steel plate with infrared light, and the sum of the self-luminous radiant energy radiated from the surface of the steel plate and the reflected light energy of the irradiated infrared light from the surface of the steel plate when irradiated with the infrared light. Detecting only the self-luminous radiant energy from the steel sheet in two different infrared wavelength bands when the irradiation of infrared light is interrupted,
The reflected light energy is calculated by subtracting the detected value when shut off from the detected value at the time of infrared light irradiation among the detected values detected by the intermittent irradiation of infrared light, and the calculated reflected light energy A method for measuring an oxide film thickness on a surface of a steel sheet, wherein a ratio between two different infrared wavelength bands is calculated on an inlet side and an outlet side of the heating furnace, and a film thickness is obtained by calculating a ratio between them. .

[2] A device for measuring an oxide film thickness on a steel sheet surface installed in a continuous annealing line equipped with a heating furnace,
An infrared light source for irradiating the surface of the steel plate with infrared light;
A light blocking device for intermittently irradiating infrared light from the infrared light source;
When the infrared light is irradiated on the steel sheet surface when the infrared light is irradiated, the total energy of the self-luminous radiant energy emitted from the steel sheet surface and the reflected light energy from the steel sheet surface of the irradiated infrared light, A photodetection device that detects only self-luminous radiant energy emitted from the steel sheet surface in two different infrared wavelength bands when the infrared light irradiation is interrupted;
For each of the two different infrared wavelength bands, the reflected energy is calculated by subtracting the detected value when the infrared light is blocked from the detected value when the infrared light is irradiated, and further, the two different wavelengths are calculated. An arithmetic processing unit that calculates a ratio between the bands,
An apparatus for measuring an oxide film thickness on the surface of a steel sheet, which is installed on both the entry side and the exit side of the heating furnace and obtains the film thickness from a ratio of outputs from the entry side and exit side arithmetic units.

  According to the present invention, it is generated on the surface of the steel sheet without being affected by the surface roughness of the steel sheet, even from the outside of the heating furnace far from the steel sheet, on the outlet side of the direct heating furnace that is an annealing process of the steel sheet manufacturing process. In addition, the thickness of the iron-based oxide can be continuously and accurately measured online. In addition, by using the present invention instead of the conventional control and control of direct fire heating conditions, the radiation of the steel sheet is radiated based on the information on the thickness of the iron-based oxide generated on the steel sheet surface on the exit side of the direct fire heating furnace. By estimating the rate, the temperature of the steel plate on the outlet side of the direct-fired heating furnace can be accurately estimated at the same time, so that more accurate direct-fire combustion control can be realized.

  As a result, by applying it to the continuous hot-dip galvanized steel sheet manufacturing process, accurate direct-fire combustion control can be realized regardless of the components of the hot-dip galvanized steel sheet and the speed of the steel sheet running on the continuous hot-dip galvanizing line. The number of cases to be suppressed is reduced, and it becomes possible to achieve an improvement in productivity and an improvement in plating characteristics.

It is a figure which shows the example of installation of the oxide film thickness measuring apparatus of the steel plate surface concerning this invention. It is a figure showing the reflection spectrum characteristic in the steel plate from which the film thickness of the oxide film produced | generated on the steel plate surface differs. It is a figure which shows the apparatus structural example of a reflected and emitted light intensity detection apparatus. It is a figure which shows the structural example of an oxide film thickness measuring apparatus and a measurement window part. It is a characteristic view which shows the relationship between an oxide film thickness and a measured value at the time of installing an oxide film thickness measuring apparatus close to a steel plate. It is a characteristic view which shows the relationship between an oxide film thickness and a measured value at the time of installing an oxide film thickness measuring apparatus far away from a steel plate. It is a characteristic view which shows the relationship between an oxide film thickness and a measured value in the case of this invention. It is a figure which shows the relationship between g parameter and reflection. It is a figure which shows the condition which installs the oxide film thickness measuring apparatus disclosed by patent document 3 in the position far away from the steel plate, and measures the film thickness of the iron-type oxide produced | generated on the steel plate surface.

  Before explaining the present invention below, the oxide film thickness measuring device disclosed in Patent Document 3 is not equipped with a water-cooled light-shielding tube. An embodiment for measuring the film thickness will be described.

  FIG. 9 is a diagram illustrating a situation in which the oxide film thickness measuring device disclosed in Patent Document 3 is installed at a position far away from the steel sheet and the film thickness of the iron-based oxide generated on the steel sheet surface is measured. .

  In the figure, 1 is a traveling steel plate, 2 is a direct-fired heating furnace, 4 is an oxide film thickness measuring device, 5 is a roll, and 6 is a measurement window.

  The interior of the direct-fired heating furnace 2 is covered with a heat insulating material such as a refractory, and continuously heats the steel plate 1 traveling by a direct-fire burner arranged in a plurality of zones. A plurality of zones with different air ratios, gas compositions, and burner flame conditions are arranged in series along the line. The traveling steel plate 1 is heated according to a predetermined combustion pattern by such a direct fire heating furnace 2. The oxide film thickness measuring device 4 measures the oxide film thickness on the surface of the traveling steel plate 1 through the measurement window 6.

  Further, the measurement position is set near the in-furnace roll 5 in order to avoid the influence of the pass line fluctuation. If there is no existing roll, a support roll may be newly provided. In FIG. 9, the measurement is performed using a path in which the steel plate is vertical, but a horizontal path may be used.

FIG. 4 is a diagram illustrating a configuration example of the oxide film thickness measuring device and the measurement window. In the figure, 1 is a traveling steel plate, 2 is a direct-fired heating furnace, 3 is a reflected / radiant light intensity detector, 4 is an oxide film thickness measuring device, 6 is a measurement window, 33 is an amplification processing device, and 41 is an infrared light source. , 42 is a rotating chopper, 43 is a half mirror, 45 is a rotation timing detecting device, 61 is an N ₂ purge blowing unit, 62 is an infrared transmission window glass, and 63 is a heat insulating material.

The oxide film thickness measuring device 4 is disposed outside the direct-fired heating furnace 2 and performs measurement through a measurement window 6 installed in the furnace body so as to penetrate the heat insulating material 63 of the direct-fired heating furnace 2. Measuring window section 6 is, for example, sealed by infrared-transparent window glass 62 that transmits infrared light, such as BaF _2, so as not dirty window glass is performed and N ₂ purged from the N ₂ purge blower unit 61. The straight line connecting the center of the light receiving window of the reflected / radiant light intensity detection device 3 and the center of the measurement window 6 is installed so as to be orthogonal to the surface of the traveling steel plate 1.

  The rotating chopper 42 intermittently irradiates the surface of the traveling steel plate 1 with the infrared light from the infrared light source 41 via the half mirror 43 that branches into two paths. For this reason, the rotation timing detection device 45 detects the rotation timing of the rotation chopper 42.

  FIG. 3 is a diagram illustrating a device configuration example of the reflected / radiated light intensity detection device. In the figure, 3 is a reflected / radiant light intensity detecting device, 21 is a condensing lens system, 22-24 are half mirrors, 25-28 are spectroscopic elements, 29-32 are light detecting elements, 33 is an amplification processing device, 34 is An arithmetic processing unit 35 and an output unit are shown.

  The reflected / radiant light intensity detection device 3 has a condensing lens system 21, and is irradiated from the infrared light source 41 and the self-luminous radiant energy radiated from the steel plate surface of the traveling steel plate 1 when irradiated with infrared light. The total energy of the reflected light energy from the steel sheet surface of infrared light is measured by the reflected / radiated light intensity detection device 3 (also referred to as a light detection device) including the spectroscopic elements 25 to 28 and the light detection elements 29 to 32. Is configured to do. On the other hand, when the infrared light irradiation is interrupted, only the self-luminous radiant energy radiated from the steel plate surface of the traveling steel plate 1 is measured by the light detection device including the spectroscopic elements 25 to 28 and the light detection elements 29 to 32. It is configured.

  The amplification processing device 33 electrically amplifies the signal from the reflected / radiated light intensity detection device 3 including the spectroscopic elements 25 to 28 and the light detection elements 29 to 32. Then, the signal amplified by the amplification processing device 33 is sent to the arithmetic processing device 34. The arithmetic processing unit 34 is configured to obtain a film thickness formed on the surface of the traveling steel plate 1 and a temperature of the traveling steel plate 1 by performing predetermined arithmetic processing described later, and to output it to the output device 35.

  Next, a method for continuously measuring the film thickness of the iron-based oxide generated on the surface of the traveling steel sheet by the oxide film thickness measuring apparatus equipped with the reflected / radiant light intensity detection apparatus will be described including the background. FIG. 2 is a diagram showing the reflection spectrum characteristics of steel plates having different thicknesses of oxide films generated on the steel plate surface.

  A steel plate sample was prepared by changing the heating conditions (for example, air ratio, combustion temperature, time, etc.) of the direct-fired heating furnace 2 and changing the film thickness of the iron-based oxide generated on the steel plate surface in five stages. The infrared reflectance spectrum of the steel plate sample from which thickness differs is shown as a result of having measured the relative reflectance with respect to the base steel plate in which the iron-type oxide is not formed with the Fourier spectrophotometer (FTIR).

To measure the oxide film thickness on the steel sheet, create a calibration curve based on the comparison between the chemical analysis results of the standard sample and the analysis results of the glow discharge analyzer (GDS). The result of fracture measurement by GDS analysis was used. This steel sheet has a Si content of 0.25%. The unit of film thickness is g / m ² obtained by chemical analysis, and is the total amount (weight conversion) of iron-based oxides converted per unit area, such as Fe ₂ O ₃ , Fe ₃ O ₄ , and FeO. is there. The film thickness of the oxide measured by analysis with the glow discharge analyzer (GDS) is (1) in Fig. 2 with the largest film thickness of iron-based oxide, followed by (2), (3 ) And (4) become thinner in order, and (5) has the thinnest thickness of the iron-based oxide.

  From the result shown in FIG. 2, an interference phenomenon occurs as the oxide film thickness increases, and the wavelength at which the reflectance becomes maximum and minimum tends to shift to the longer wavelength side. From this result, when a specific infrared wavelength was selected, a certain relationship was obtained between the oxide film thickness and the reflectance. Therefore, it can be seen that the oxide film thickness on the steel sheet may be estimated by measuring the reflectance. A method of using such reflectance measurement for film thickness measurement is common.

  A method that uses the reflection ratio (reflected light intensity ratio) at two specific wavelengths in order to eliminate the effects of vibration and flickering of the measurement object during online measurement and reduce the effects of fluctuations in the irradiation light source. Are also commonly used. By considering the advantage of this measure, it is possible to use the two-wavelength reflection ratio for the film thickness measurement even for the oxide film thickness measurement on the steel sheet surface. In the present invention, this method is basically applied.

That is, the oxide film thickness [d] on the steel sheet is estimated from the measured two-wavelength reflection ratio R (λ ₂ ) / R (λ ₁ ). Here, λ ₂ <λ ₁ .

  Here, the influence when the surface roughness of the original steel sheet is different will be considered. The measured average arithmetic roughness Ra is 0.27 μm (root mean square roughness Rq is 0.35 μm), 0.51 (0.64), and 0.88 (1.08). FIG. 6 shows the result of measurement performed in the same arrangement as that in FIG. FIG. 5 shows the result when the oxide film thickness measuring device is measured sufficiently close to about 200 mm from the steel plate. Thus, in the case of FIG. 6 measured from outside the heating furnace far from the steel plate, the value of the reflectance ratio itself is small, and the variation in the measured value due to the difference in surface roughness is relatively large. I understand. Here, the cause of such a result was examined.

  FIG. 8 is a diagram illustrating the relationship between the g parameter and reflection. According to Non-Patent Document 1 described above, as shown in FIG. 8, the reflection pattern of the reflected light is determined by the g parameter having arguments of the incident angle, the surface roughness, and the wavelength. Then, for example, the reflection pattern for the classification of whether the value of g is 0, sufficiently small with respect to 1, 1 or more, or sufficiently larger than 1 is specified.

When the g parameter was calculated for the three levels of samples with different roughnesses, the results are shown in the table of FIG. For reference, FIG. 8 also shows the calculation results for λ ₃ of 3.5 μm and λ ₄ of 2.5 μm, which will be described later. Corresponding to FIG. 4, the incident angle θ = 0.

It was found that for any surface roughness, the g parameter was larger in λ ₂ than in λ ₁ , and the diffuse reflection component of the reflection pattern was increased, so that the regular reflection component was reduced. Further, the above-described division of g with respect to λ ₁ and λ ₂ is different, and further, the division with respect to λ ₁ and λ ₂ varies depending on the surface roughness. Therefore, in addition to the decrease in the reflectance due to the oxide film thickness, It turned out that the influence of the fall of the reflectance has gone up.

  This effect is small if the measurement distance is small and the reflected solid angle of reflected light is large, including diffusely reflected light, but this effect is small, but the measured solid angle is small enough to receive only specularly reflected light. In the case of the arrangement of, it was found that it would have a big influence.

FIG. 1 is a diagram showing an installation example of an apparatus for measuring an oxide film thickness on a steel sheet surface according to the present invention. In order to correct the influence described above, in the present invention, as shown in FIG. 1, the steel plate before the oxide film is generated is also measured with the same apparatus, and R (λ ₂ ) after the oxide film is generated. / R (lambda ₁₎ is divided by the previous generation _{R (λ 2) / R (} λ 1), according to g parameter (surface roughness, due to wavelength) R a _{(λ 2) / R (λ} 1) Variations can be corrected. FIG. 7 shows the result of measurement performed in the same arrangement as in FIG. Thus, not only the comparison with FIG. 6 measured from the outside of the heating furnace far away from the steel plate, but also the variation due to the surface roughness is suppressed to a small extent compared with FIG. 5 measured close to about 200 mm. I understand that.

In addition, as described in Patent Document 3, if the steel type is different, the film properties (refractive index, etc.) are different, so that the infrared reflection spectrum may be different, and in that case, it is used for film thickness estimation. It is necessary to select another wavelength as the wavelength. For example, the film thickness [d] of the oxide on the steel sheet can be estimated from the two-wavelength reflection ratio R (λ ₃ ) / R (λ ₂ ) using another wavelength λ ₃ . Naturally, even if the steel type is different, there is no great difference in the film properties, and the difference in the infrared reflection spectrum may be negligible. In that case, it is only necessary to use two wavelengths λ ₁ and λ ₂ .

Here, λ ₃ <λ ₂ <λ ₁ including the two wavelengths. For measuring the reflectance, it is necessary to irradiate the surface of the steel sheet with infrared light having sufficient radiation intensity in the infrared wavelength region including the _three wavelengths λ ₁ , λ ₂ , and λ ₃ . In order to use such a method for measuring the oxide film thickness generated on the surface of the steel sheet on the outlet side of the direct heating furnace, the measures described below are required. That is, the traveling steel plate 1 on the direct-fired heating furnace exit side emits heat in the infrared wavelength region from the steel plate surface because the steel plate itself is heated to several hundred degrees Celsius or higher. Therefore, when the steel plate surface is irradiated with infrared light having sufficient radiant intensity from the infrared light source, the self-luminous radiant energy emitted from the steel plate surface and the reflected light energy of the irradiated infrared light from the steel plate surface Thus, the oxide film thickness measuring device 3 detects the total energy.

  Therefore, in the present invention, the surface of the traveling steel sheet 1 is intermittently irradiated with infrared light on the exit side of the direct-fired heating furnace, and the self-luminous radiant energy radiated from the steel sheet surface when irradiated with infrared light and the irradiated red The total energy of the reflected light energy from the surface of the steel sheet of outside light, when the irradiation of infrared light is interrupted, only the self-emitting radiant energy from the steel sheet, respectively, in a plurality of different wavelength bands in the infrared wavelength range. It was made to detect at the same time.

The oxide film thickness measuring method according to the present invention will be described below using equations.
However, the symbols are as follows.
Ire (λ, T): measured intensity (temperature; T) at a wavelength λ when the object is irradiated with a light source,
Ira (λ, T): self-luminous intensity at temperature λ (temperature; T),
R (λ): reflectance at wavelength λ,
ε (λ): Emissivity at wavelength λ,
I ₀ (λ): Light source irradiation intensity at wavelength λ

First, the following formulas (1) to (4) are obtained from the following Planck's law.
Lb (λ, T) = (2c ₁ / λ ⁵ ) {1 / (exp (c ₂ / λT) −1)}
Here, Lb (λ, T) is the spectral radiance of the black body, and c ₁ and c ₂ are physical constants.
(i) Measurement when infrared light is irradiated on the steel sheet surface from an infrared light source Ire (λ ₁ , T) = ε (λ ₁ ) · Lb (λ ₁ , T) + I 0 (λ ₁ ) · R (λ ₁ (1)
Ire (λ ₂ , T) = ε (λ ₂ ) · Lb (λ ₂ , T) + I 0 (λ ₂ ) · R (λ ₂ ) (2)
(ii) Measurement when infrared light applied to the steel sheet surface from an infrared light source is cut off (Self-luminance measurement)
Ira (λ ₁ , T) = ε (λ ₁ ) · Lb (λ ₁ , T) (3)
Ira (λ ₂ , T) = ε (λ ₂ ) · Lb (λ ₂ , T) (4)

Formula (5) is obtained by using Formulas (1) to (4).
R (λ ₂ ) / R (λ ₁ ) = {Ire (λ ₂ , T) −Ira (λ ₂ , T)} / {Ire (λ ₁ , T) −Ira (λ ₁ , T)} I 0 ( λ ₁ ) / I 0 (λ ₂ ) (5)

As described above, the relationship between the reflectance ratio and the oxide film thickness [d] is classified into a certain grade of steel according to the amount of Si content. That is, if the reflectance ratio is determined, a relationship that can be expressed by a function whose film thickness is uniquely determined is obtained. Therefore, if a relational expression for a certain steel type is a function of f _1, it can be expressed as Expression (6).
d = f ₁ {R (λ ₂ ) / R (λ ₁ )} (6)
If the function representing the relationship in the same manner to another steel grade and f _2, can be expressed as Equation (7).
d = f ₂ {R (λ ₃ ) / R (λ ₂ )} (7)

Further, when the expressions (6) and (7) are generalized, they can be expressed as the following expression (8).
d = A ₁ × f ₁ {R (λ ₂ ) / R (λ ₁ )} + A ₂ × f ₂ {R (λ ₃ ) / R (λ ₂ )} (8)

There are several types of high-strength steel sheets depending on the content of components such as Si and Mn, but by appropriately setting the coefficients in formula (8); A ₁ and A ₂ , the reflectivity ratio and oxidation It is possible to derive a relational expression that links the film thickness [d] of the object. Here, as the function expressions f ₁ and f ₂ , for example, a quadratic function is used.

  The above-described method can be used with sufficient accuracy when there is little reduced Fe produced by reducing some of the iron-based oxides produced in the former zone of the direct-fired heating furnace in the latter zone. It has been confirmed that the film thickness of the system oxide can be estimated. However, in actual hot-dip galvanized steel sheets, some of the iron-based oxides produced in the front zone of the direct-fired heating furnace are heat-treated in a reducing atmosphere in the zone after the direct-fired heating furnace. In many cases, reduced Fe is scattered on the outermost surface of the iron-based oxide produced in the preceding zone.

  Therefore, the film thickness of the iron-based oxide generated on the steel sheet surface is selected by selecting the three wavelengths described so far and applying it to the direct heating furnace exit side, which is an intermediate process of the hot dip galvanized steel sheet manufacturing process. When measuring on-line continuously, large errors may occur. That is, a part of the iron-based oxide generated in the preceding zone is reduced, and reduced Fe is scattered in the surface layer portion, so that the outermost surface layer is not covered. However, when a part of the iron-based oxide is reduced and reduced Fe is scattered on the surface layer, not only the reflection characteristics on the steel sheet surface are different, but also the light to the oxide layer Therefore, the light absorption characteristics in the oxide layer are different from those in the case where the reduced Fe is not scattered in the surface layer portion.

Therefore, an attempt was made to fuse reflection information at the _fourth wavelength λ ₄ where the sensitivity at the outermost part of the iron-based oxide layer formed on the steel plate surface was relatively excellent. That is, whether the reflection state on the surface of the iron-based oxide generated on the steel sheet surface by irradiating infrared light of the fourth wavelength λ ₄ from the infrared light source can be indirectly measured and utilized for correction, As a result of investigating whether or not, by adding the reflection information of the fourth wavelength λ ₄ as a correction term to the relational expression (8), the film thickness of the iron-based oxide generated on the surface of the steel plate is continuously online. It was confirmed that the measurement can be performed with high accuracy. Equation (9) including information on the reflection ratio of the fourth wavelength λ ₄ as the correction term obtained is shown below.

d = A ₁ × f ₁ {R (λ ₂ ) / R (λ ₁ )} + A ₂ × f ₂ {R (λ ₃ ) / R (λ ₂ )}
+ A ₃ × f ₃ {R (λ ₄ ) / R (λ ₃ )} (9)
Here, λ ₄ <λ ₃ <λ ₂ <λ ₁ including the aforementioned three wavelengths.

As described above, by combining a reflectance ratio measured by combining four wavelengths and a relational expression obtained in advance, even when reduced Fe is scattered on the outermost surface, It is possible to accurately measure the film thickness d of the iron-based oxide generated on the steel plate surface. Further, the case where the reduction in the surface layer portion Fe is not scattered, without λ _4, λ _1, λ ₂ or, it is a film thickness only information plus lambda ₃ in some cases can be measured accurately.

Further, since the emissivity and the film thickness of the iron-based oxide generated on the steel sheet surface have a substantially constant relationship for each steel type, for example, by making f ₄ a function representing the relationship, emissivity of λ ₄ , for example ε (λ ₄ ) can be obtained from the following equation (10).
ε (λ ₄ ) = f ₄ (d) (10)
In this way, the emissivity is identified, and the temperature T is obtained from Ira (λ ₄ , T) measured in the same manner as in the equation (3) and the identified ε (λ ₄ ).

  Therefore, by using the present invention instead of the conventional direct fire heating condition management and control, the film thickness of the iron-based oxide generated on the steel plate surface of the traveling steel plate 1 on the direct fire heating furnace exit side is accurately measured. can do. In addition, according to the present invention, it becomes possible to estimate the emissivity of the steel sheet based on the information on the film thickness of the iron-based oxide generated on the steel sheet surface. Temperature can be measured, and the combustion control of the direct-fired heating furnace can be performed more strictly.

  In that case, as shown in FIGS. 3 and 4, a rotating chopper 42 having a window is used, the surface of the traveling steel plate is intermittently irradiated with infrared light, and the optical path is branched by a plurality of half mirrors 22 to 24. It is preferable to use a photodetection device that simultaneously detects energy in four wavelength ranges.

  The reason for this is that even when the longitudinal variation of the thickness of the oxide film on the surface of the traveling steel plate 1 is abrupt, it is possible to intermittently irradiate the surface of the traveling steel plate with infrared light at short time intervals, Moreover, since energy can be detected simultaneously in four wavelength regions, the film thickness can be quickly obtained by calculation using eight different detection values detected by intermittent irradiation of infrared light. Therefore, it can be set as the oxide film thickness measurement apparatus of the steel plate surface excellent in responsiveness.

  However, when the longitudinal fluctuation of the thickness of the oxide film on the surface of the traveling steel plate 1 is moderate, the following measurement method using a single optical path can be used. For example, using a FTIR spectrometer to remotely measure the spectrum to extract the necessary wavelength information, rotating the continuous spectral filter to measure the spectrum sequentially, and mounting multiple transmission interference filters for the detection wavelength band For example, a method of sequentially performing measurement in each detection wavelength band by rotating the rotating plate.

  The configuration of a suitable apparatus for measuring the oxide film thickness on the surface of a steel sheet installed in a production line for a continuous hot-dip galvanized steel sheet equipped with a direct-fired heating furnace 2 will be described with reference to FIG. In FIG. 3, reference numeral 21 denotes a condensing lens system of the reflected / radiated light intensity detecting device 3 which is a light detecting device including the spectroscopic elements 25 to 28 and the light detecting elements 29 to 32. The photodetector of the preferred embodiment installed on the exit side of the direct-fired heating furnace 2 has a first wavelength of 12 μm, a second wavelength of 7 μm, a third wavelength of 3.5 μm, Using 2.5 μm as the fourth wavelength, the reflected light intensity and radiance for each wavelength are simultaneously measured by the spectroscopic elements 25 to 28 and the light detecting elements 29 to 32.

  The light that has passed through the condenser lens system 21 is divided into two paths by the first half mirror 22, a part of the light is guided to the second half mirror 23, and the remaining light is sent to the third half mirror. Led to 24. The light is further branched into two paths by the second half mirror 23, and the light transmitted through the second half mirror 23 is transmitted through the spectroscopic element 25 such as an interference filter and is a 12 μm detection element. The reflected light intensity and radiance are measured by the element 29.

  The light reflected by the second half mirror 23 passes through the spectroscopic element 26, and the reflected light intensity and radiance are measured by the light detection element 30 which is a 7 μm detection element. The light transmitted through the first half mirror 22 is branched into two paths by the third half mirror 24, and the light reflected by the third half mirror 24 passes through the spectroscopic element 27 and is a detection element for 3.5 μm. The detection element 31 measures reflected light intensity and radiance. The light transmitted through the third half mirror 24 passes through the spectroscopic element 28, and the reflected light intensity and radiance are measured by the light detection element 32 which is a detection element for 2.5 μm. The four sets of spectroscopic elements 25 to 28 and the light detection elements 29 to 32 measure the reflected light intensity and radiance for each wavelength at the same time, and the measured light intensity signals are sent to the amplification processing device 33.

It mentioned example here using four wavelengths of lambda ₁ to [lambda] _4, in some cases suffice lambda 3 wavelengths of ₁ to [lambda] ₃ or lambda ₁ and lambda ₂ of 2 wavelengths, as described above. In this embodiment, an optical system is used that detects the light transmitted through the condensing lens system by dividing it with a half mirror. However, after the light is divided by the half mirror, a plurality of detections having a condensing lens system for each wavelength are used. It is also conceivable to detect with an apparatus.

Thereafter, the amplified signal is sent to the arithmetic processing unit 34. The signal before the oxide film generation is already input to the arithmetic processing unit 34, and the position of the steel sheet is tracked to obtain the ratio of the corresponding data. Thus, the influence of the surface roughness is corrected. Furthermore, the relational expressions f ₁ , f ₂ , f ₃ , the coefficients A ₁ , A ₂ , A ₃ , and the relational information of f ₄ described above are stored for each steel type to be measured in advance. The arithmetic processing is performed, the thickness of the oxide film generated on the surface of the traveling steel plate 1 and the temperature value of the traveling steel plate 1 are obtained, and the results are output to the output device 35. At that time, the surface of the traveling steel plate 1 is intermittently irradiated with infrared light from the infrared light source 41.

  This will be described with reference to FIG. Infrared light from the infrared light source 41 is divided by the half mirror 43 through the window of the rotating chopper 42, and a part of the infrared light is vertically irradiated on the surface of the traveling steel plate 1. A window is formed in the rotating chopper, and when the portion without the window rotates and the infrared light from the infrared light source 41 is blocked, the surface of the steel plate is not irradiated with light. In the rotating chopper 42, a plurality of windows may be opened at a certain angle. If a window is formed at a certain angle and a window is formed, infrared light is intermittently applied to the surface of the steel sheet every certain time. If the rotation timing device 45 detects the passage of the rotation chopper 42 through the window, the irradiation timing is detected.

  Infrared light reflected on the surface of the steel sheet returns vertically, passes through the half mirror 43 again, and the reflected light intensity and radiance for each wavelength are simultaneously measured by the spectroscopic elements 25 to 28 and the light detecting elements 29 to 32. The The detection signal from the rotation timing detection device 45 is sent to the amplification processing device 33 shown in FIG. 3 and used for measurement timing control with four detection elements. In the above description, an example of vertical irradiation is shown. However, infrared light irradiation and infrared light reflected on the surface of the steel sheet may be received with a certain angle.

  As a relational expression for calculating the thickness of the oxide film, three functions of the reflectance ratio at two wavelengths are obtained and weighted by multiplying them by a certain coefficient. It is also possible to substitute one function formula that calculates a multiple regression formula from the value of the rate ratio.

DESCRIPTION OF SYMBOLS 1 Traveling steel plate 2 Direct-fired heating furnace 3 Reflection and synchrotron radiation intensity detection apparatus 4 Oxide film thickness measurement apparatus 5 Roll 6 Measurement window part 7 Thermocouple 8 Arithmetic apparatus 9 Storage apparatus 10 Output apparatus 11 Postnatal apparatus for process management 21 Condensing lens System 22-24 Half mirror 25-28 Spectroscopic element 29-32 Photodetection element 33 Amplification processing device 34 Arithmetic processing device 35 Output device 41 Infrared light source 42 Rotation chopper 43 Half mirror 45 Rotation timing detection device 61 N ₂ purge blowing part 62 Infrared transmitting window glass 63 Insulation

Claims (2)

In the continuous annealing step equipped with a heating furnace, a method for measuring the thickness of an oxide film on the surface of a steel sheet, measuring the thickness of the oxide generated on the surface of the steel sheet,
Energy obtained by intermittently irradiating the surface of the steel plate with infrared light, and the sum of the self-luminous radiant energy radiated from the surface of the steel plate and the reflected light energy of the irradiated infrared light from the surface of the steel plate when irradiated with the infrared light. Detecting only the self-luminous radiant energy from the steel sheet in two different infrared wavelength bands when the irradiation of infrared light is interrupted,
The reflected light energy is calculated by subtracting the detected value when shut off from the detected value at the time of infrared light irradiation among the detected values detected by the intermittent irradiation of infrared light, and the calculated reflected light energy A method for measuring an oxide film thickness on a surface of a steel sheet, wherein a ratio between two different infrared wavelength bands is calculated on an inlet side and an outlet side of the heating furnace, and a film thickness is obtained by calculating a ratio between them. . An apparatus for measuring the oxide film thickness on the surface of a steel plate installed in a continuous annealing line equipped with a heating furnace,
An infrared light source for irradiating the surface of the steel plate with infrared light;
A light blocking device for intermittently irradiating infrared light from the infrared light source;
When the infrared light is irradiated on the steel sheet surface when the infrared light is irradiated, the total energy of the self-luminous radiant energy emitted from the steel sheet surface and the reflected light energy from the steel sheet surface of the irradiated infrared light, A photodetection device that detects only self-luminous radiant energy emitted from the steel sheet surface in two different infrared wavelength bands when the infrared light irradiation is interrupted;
For each of the two different infrared wavelength bands, the reflected energy is calculated by subtracting the detected value when the infrared light is blocked from the detected value when the infrared light is irradiated, and further, the two different wavelengths are calculated. An arithmetic processing unit that calculates a ratio between the bands,
An apparatus for measuring an oxide film thickness on the surface of a steel sheet, which is installed on both the entry side and the exit side of the heating furnace and obtains the film thickness from a ratio of outputs from the entry side and exit side arithmetic units.

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