JP2005262283A - Method for detecting solidifying condition in mold during continuous casting - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for accurately measuring the thickness of a solidified shell in a mold and the inflow thickness of molding powder on real time by utilizing ultrasonic waves in continuous casting. <P>SOLUTION: An ultrasonic transmitter 7 is placed in the continuous casting mold 1. An ultrasonic wave is transmitted from the ultrasonic transmitter, and respective propagation times are measured with regard to reflected waves from the boundary between the mold and a molding powder layer 5, the boundary between the molding powder layer and the solidified shell 3, and the boundary between the solidified shell and molten steel 2. The inflow thickness of the molding powder and the thickness of the solidified shell are found on the basis of the measured propagation times. In that event the reflected wave from the boundary between the molding powder layer and the solidified shell is distinguished from the reflected wave from the boundary between the solidified shell and the molten steel because their phases are different. Accurate measurement is possible by using ultrasonic waves having a wavelength well smaller than the inflow thickness of the molding powder. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for detecting a solidification state in a mold in continuous casting. The present invention relates to a state detection method.

  In the continuous casting process of steel, it is important to control the cooling state in the mold, that is, to monitor the thickness of the solidified shell, in order to ensure the quality of the slab and the stability of the operation. In particular, when the thickness of the initial solidified shell thickness is large, problems such as breakout of the slab during casting and vertical cracking of the slab occur. The cause of such inhomogeneity in the solidified shell is mainly due to instability of the cooling state. The instability is caused by fluctuations in the amount of molten steel supplied into the mold and cooling of the mold. Unevenness, fluctuations in the molten steel surface of the molten steel in the mold, etc. can be considered. In addition, if mold powder is used to keep the molten steel in the mold warm and prevent oxidation, absorb inclusions floating from the molten steel, and lubricate the solidified shell and mold, The inflow thickness of the mold powder that flows in between is also related to the cooling state. That is, it is possible to grasp the cooling state in the mold by monitoring the initial solidified shell thickness and the inflow thickness of the mold powder.

The following methods have been proposed as conventional methods for such measurement. For example, in Patent Document 1, an ultrasonic probe is provided in the wall of a continuous casting mold, ultrasonic waves are transmitted toward the inside of the mold, and the thickness of the solidified shell in the mold is obtained based on the reflected wave. A method has been proposed. Patent Document 2 measures the temperature distribution in the casting direction of a continuous casting mold, solves the heat conduction equation using the heat flux value predicted from the measured value as a boundary condition, and determines the thickness of the solidified shell and the inflow thickness of the mold powder. A method for obtaining the value has been proposed. Patent Document 3 discloses a method of measuring a temperature distribution in a casting direction of a continuous casting mold, calculating a heat flux in the casting direction in the mold from the measured value, and sequentially calculating a solidified shell thickness based on the obtained heat flux. Has been proposed.
JP 59-156558 A JP 2000-317594 A Japanese Patent Laid-Open No. 10-277716

  However, the above prior art has some problems, and the main ones are as follows. That is, in Patent Document 1, only the thickness of the solidified shell is measured, and the inflow thickness of the mold powder is not measured. This is probably because the inflow thickness of the mold powder was smaller than the thickness of the solidified shell and could not be measured accurately. In addition, regarding the measurement of the solidified shell thickness, there is a lot of noise at a portion where the thickness of the solidified shell near the molten steel surface in the mold is thin, and the method disclosed in Patent Document 1 cannot be measured with high accuracy.

  On the other hand, in Patent Document 2 and Patent Document 3, since the thickness of the solidified shell or the inflow thickness of the mold powder is obtained based on the temperature measurement value of the mold, when the solidified state in the mold is changed due to the molten metal surface fluctuation, The time of heat conduction from the solidified shell to the mold is always a time delay in the measured value and cannot be measured in real time. In order to prevent this, even if a measure of prediction is taken by referring to / learning a past profile, it cannot be surely a value such as thickness at the time of measurement.

  The present invention has been made in view of the above circumstances, and an object thereof is to exceed the thickness of the solidified shell in the mold and the inflow thickness of the mold powder flowing between the mold and the solidified shell in continuous casting. It is to provide a method for detecting a solidification state in a mold, which can be measured online in real time and with high accuracy by using sound waves.

  The solidification state detection method in the casting mold in the continuous casting according to the first invention for solving the above-mentioned problem is that an ultrasonic transmitter is installed in the casting mold for continuous casting, and the ultrasonic wave transmitted from the ultrasonic transmitter is transmitted. The propagation time of each of the reflected wave from the boundary between the mold and the mold powder layer, the reflected wave from the boundary between the mold powder layer and the solidified shell, and the reflected wave from the boundary between the solidified shell and the molten steel was measured and measured. The inflow thickness of the mold powder and the thickness of the solidified shell are obtained based on the propagation time.

  According to a second aspect of the present invention, there is provided a method for detecting a solidification state in a mold in continuous casting. In the first invention, a reflected wave from a boundary between a mold powder layer and a solidified shell and a reflected wave from a boundary between the solidified shell and molten steel. In this case, each reflected wave is identified by utilizing the fact that the phase of the reflected wave is different.

  The solidification state detection method in the mold in the continuous casting according to the third invention is generated in the mold powder layer using the ultrasonic wave having a wavelength sufficiently smaller than the inflow thickness of the mold powder in the first or second invention. The inflow thickness of the mold powder is obtained by analyzing the frequency of the multiple reflected wave signal to be detected, and the reflected wave from the boundary between the solidified shell and the molten steel is identified by removing the multiple reflected wave component from the measurement signal. It is characterized by.

  According to the present invention having the above-described configuration, in continuous casting, the thickness of the solidified shell in the mold and the inflow thickness of the mold powder flowing between the mold and the solidified shell can be measured in real time with high accuracy. It is possible to grasp the solidification / cooling state in the mold in real time in real time, and it is possible to prevent operational abnormalities and quality abnormalities such as breakouts and vertical cracks of the slab, so that the casting speed can be increased. An improvement in slab quality is achieved and an industrially beneficial effect is achieved.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a diagram showing an example of an embodiment of the present invention, and is a schematic cross-sectional view of a continuous casting mold part embodying the present invention.

  In FIG. 1, molten steel 2 is injected from an immersion nozzle (not shown) into a space surrounded by a water-cooled copper mold 1, and the injected molten steel 2 is cooled by the mold 1 and comes into contact with the mold 1. A solidified shell 3 is produced on the side. The slab with the solidified shell 3 as an outer shell and the inside of the unsolidified molten steel 2 is continuously drawn below the mold 1 by a pinch roll (not shown) installed below the mold 1. During this drawing, the position of the molten steel surface 6 in the mold 1 is adjusted to a substantially constant position. Molded powder 4 is added on the molten steel surface 6 for the purpose of keeping the molten steel 2 warm and preventing oxidation, and lubricating the solidified shell 3 and the mold 1. The mold powder 4 is heated and melted by the heat of the molten steel 2, and a molten powder layer 4 a is formed in a range in contact with the molten steel 2. The molten powder layer 4 a flows into the gap between the mold 1 and the solidified shell 3 to form a mold powder layer 5. The mold powder layer 5 in the vicinity of the molten steel surface 6 and the portion of the mold powder layer 5 in contact with the solidified shell 3 are in a molten state, and the mold powder layer 5 in the molten state is mainly formed between the mold 1 and the solidified shell 3. Functions as a lubricant.

  If the thickness of the solidified shell 3 becomes non-uniform in the width direction, vertical cracks occur in the slab, and the thickness of the mold powder layer 5, that is, the inflow thickness of the mold powder 4 increases, so that heat does not easily escape to the mold 1. If the thickness of the solidified shell 3 is insufficient due to factors such as Therefore, it is necessary to monitor not only the thickness of the solidified shell 3 but also the thickness of the mold powder layer 5.

  An installation hole 8 for embedding the ultrasonic probe 7 is provided in the vicinity of the molten steel surface 6 of the mold 1 to transmit ultrasonic waves toward the solidified shell 3 and receive reflected waves. An ultrasonic probe 7 is installed in the installation hole 8. The point to be noted when embedding the ultrasonic probe 7 is that the transmitted ultrasonic wave propagates at a predetermined angle with respect to the inner surface of the mold 1, and the reflected wave returns to the position of the ultrasonic probe 7. For this purpose, an oblique ultrasonic probe may be used, or a reflector may be provided. In addition, in order to bring the ultrasonic probe 7 into close contact with the mold 1, threading or the like may be performed to fix the ultrasonic probe 7 to the mold 1.

  FIG. 1 shows an example in which an ultrasonic probe 7 is embedded from the back surface of a mold 1. An electromagnet or the like for controlling the flow of molten steel in the mold is installed on the back surface of the mold 1. When it is difficult to embed the ultrasonic probe 7, the ultrasonic probe 7 may be embedded from the upper surface of the mold 1, for example, as shown in FIG. 2 in which the ultrasonic probe 7 is embedded from the upper surface of the mold 1. . Alternatively, multiple channels may be installed in the lateral direction and longitudinal direction of the mold 1 so as to obtain a solidified state distribution. FIG. 3 is a diagram showing an example in which multiple channels are arranged in the lateral direction of the mold 1, and is a view of the mold 1 as viewed from the top. In FIG. 3, 1 a is a mold long side and 1 b is a mold short side.

  The size of the ultrasonic probe 7 varies depending on the frequency used. When the ultrasonic wave to be used is a low frequency, the ultrasonic probe 7 becomes large, and it is difficult to install the ultrasonic probe 7 in the mold 1, so the outer side of the mold 1 (opposite to the surface on which the molten steel is in contact) Side). In this case, in order to promote the propagation of the ultrasonic wave, the gap between the installed ultrasonic probe and the mold 1 is made as small as possible, and at the same time, a filler with a small change in sound speed due to temperature is used as the ultrasonic probe and the mold 1. It is preferable to fill the gap between When the ultrasonic wave used has a high frequency, the ultrasonic probe 7 becomes small and can be embedded in the mold 1.

  The ultrasonic probe 7 is controlled by an ultrasonic transmission / reception processor 9 to transmit / receive ultrasonic waves. FIG. 4 is a schematic view showing how the ultrasonic wave oscillated from the ultrasonic probe 7 propagates. FIG. 4 is an enlarged view of a portion A shown in FIG.

As shown in FIG. 4, the ultrasonic wave 10 oscillated from the ultrasonic probe 7 propagates inside the mold 1, and the reflected wave 11 and the transmitted wave 12 are transmitted at the interface 17 between the mold 1 and the mold powder layer 5. become. When the acoustic impedance of the mold 1 is Z ₁ and the acoustic impedance of the melted mold powder layer 5 is Z ₂ , the reflectance r ₁₂ and the transmittance t _{12 at} the boundary surface 17 are expressed by the following equations (1) and (2), respectively. It is expressed by a formula.

The transmitted wave 12 transmitted through the boundary surface 17 propagates through the melted mold powder layer 5, and becomes a reflected wave 13 and a transmitted wave 14 at the boundary surface 18 between the mold powder layer 5 and the solidified shell 3. When a high frequency ultrasonic wave having a wavelength shorter than the thickness of the mold powder layer 5 is used, the reflected wave 13 may be detected by the ultrasonic probe 7 as multiple reflections. Assuming that the acoustic impedance of the solidified shell 3 is Z ₃ , the reflectance r ₂₃ and the transmittance t _{23 at} the interface 18 are expressed by the following equations (3) and (4), respectively. Z ₂ in the equations (3) and (4) is the acoustic impedance of the molten mold powder layer 5.

The transmitted wave 14 transmitted through the boundary surface 18 propagates through the solidified shell 3, becomes a reflected wave 15 and a transmitted wave 16 at the boundary surface 19 between the solidified shell 3 and the molten steel 2, and the reflected wave 15 is converted into the ultrasonic probe 7. Is detected. When the acoustic impedance of the molten steel 2 is Z ₄ , the reflectance r ₃₄ and the transmittance t _{34 at} the boundary surface 19 are expressed by the following formulas (5) and (6), respectively. Z ₃ in the equations (5) and (6) is the acoustic impedance of the solidified shell 3.

  Since the reflected wave 13 (or multiple reflected wave) and the reflected wave 15 are detected after detecting the reflected wave 11, the mold powder layer 5 and the solidified shell 3 are detected from their propagation time and the sound velocity in each layer. Thickness is detected in near real time. For example, if the propagation time in the solidified shell 3 is Δt, the speed of sound in the solidified shell 3 is Vs, and the thickness of the solidified shell 3 is Ds, the thickness of the solidified shell 3 can be obtained by the following equation (7). In this case, only the reflected wave 13 is detected in a situation where the mold powder layer 5 is not present, for example, when the mold 1 and the solidified shell 3 are in direct contact.

Typical numerical values of acoustic impedance of each layer (Z ₁ = 4x10 ⁷ kg / m ² · s, Z ₂ = 7.42 x 10 ⁶ kg / m ² · s, Z ₃ = 3.56 x 10 ⁷ kg / m ² · s, Z ₄ = 2.84 × 10 ⁷ kg / m ² · s), the reflectivity r ₂₃ has a positive sign, but the reflectivity r ₃₄ has a negative sign, and the phase is inverted. That is, even when the thickness of each layer is small and multiple reflection cannot be clearly removed, the reflected wave 13 and the reflected wave 15 can be distinguished by examining the phase of the reflected wave.

  In consideration of measuring the thin mold powder layer 5, a high frequency wave having a wavelength sufficiently smaller than the thickness of the mold powder layer 5 is used, and signal processing such as frequency analysis is added to the obtained signal. The thickness of the mold powder layer 5 and the thickness of the solidified shell 3 can also be obtained. In the present invention, since the propagation time is measured and the thickness of each layer is obtained from the measured propagation time and the sound speed of each layer, it is necessary to investigate and grasp the sound speed of each layer in advance offline. Therefore, when changing the type of mold powder 4 to be used, it is necessary to check the sound speed in advance.

  Thus, according to the present invention, in continuous casting, the thickness of the solidified shell 3 in the mold 1 and the inflow thickness of the mold powder 4 flowing between the mold 1 and the solidified shell 3 are real-time and accurate. It is possible to measure well, it is possible to grasp in real time the solidification and cooling state in the mold during continuous casting, and it is possible to prevent operational abnormalities and quality abnormalities such as breakout and vertical cracking of the slab. Become.

  In addition, by continuously feeding the measured thickness of the mold powder layer 5 and the thickness of the solidified shell 3 to the operating conditions, continuous casting can always be performed under optimum conditions. For example, the data of the thickness of the mold powder layer 5 and the thickness of the solidified shell 3 obtained by the ultrasonic transmission / reception processing device 9 are input to a casting control device (not shown), and the obtained data is compared with a preset threshold value. By determining the predetermined action and inputting the determined action into the slab drawing speed control device, the secondary cooling control device, the mold vibration control device, the mold cooling water control device, etc., and changing the casting conditions, It is possible to maintain optimum casting conditions. Table 1 shows examples of specific actions. For example, when the threshold value of the thickness of the mold powder layer 5 is 0.1 mm and the thickness of the mold powder layer 5 is 0.1 mm or less, the action shown in Table 1 may be taken.

  In the above description, the method is such that one ultrasonic probe 7 performs transmission and reception. However, for the purpose of preventing multiple reflections from other than the target, the transmission-only ultrasonic probe and the reception-dedicated probe are used. A two-sonic probe method may be used in which an ultrasonic probe is provided and these are set apart vertically or horizontally. Further, the transmission waveform may be transmitted in a certain pattern, and the correlation between the reception waveform and the pattern waveform may be taken to make it less susceptible to noise. Further, a method of dynamically performing focusing and measuring each boundary surface while changing the focus depth is also applicable. Furthermore, the present invention can also be applied to continuous casting of other metals other than continuous casting of molten steel.

  The present invention was implemented when continuously casting low carbon aluminum killed steel at a drawing speed of 1.5 m / min using the continuous casting mold shown in FIG. An ultrasonic probe 7 having a diameter of 6 mm and a band of 5 MHz was embedded in the mold at a position where the distance to the inner surface of the mold was 15 mm. The position in the mold height direction at the installation position is set to be in the vicinity of the molten steel surface 6 (target is 20 mm directly below the molten steel surface), and in this embodiment, the position is 100 mm from the upper surface of the mold. Then, ultrasonic waves adjusted to a sine wave (Gaussian distribution) as shown in FIG. 5 were transmitted from the ultrasonic probe 7 by the ultrasonic transmission / reception processing device 9.

  A waveform observed as a reflected wave is shown in FIG. As shown in FIG. 6, the reflected wave 20 from the boundary surface 17 between the mold and the mold powder layer was first observed with the transmission time of the ultrasonic wave set to 0 second. Since the distance from the ultrasonic probe to the boundary surface 17 was 15 mm, the propagation time was 6.7 microseconds (μs) (sound speed on the mold copper plate: 4500 m / second). Next, what was observed was a reflected wave 21 from the boundary surface 18 between the mold powder layer and the solidified shell, which was 360 nanoseconds (ns) behind the reflected wave 20, so that the thickness of the mold powder layer was It was found to be about 0.5 mm (sound velocity in the mold powder layer: 2750 m / sec).

  The reflected waves 22, 23, and 24 are multiple reflected waves of the reflected wave 21, and the reflected wave 25 from the boundary surface 19 between the solidified shell and the molten steel was confirmed immediately before the reflected wave 24. This delay was 840 nanoseconds behind the reflected wave 21, and the thickness of the solidified shell was found to be 2 mm (sound velocity in the solidified shell: 4750 m / second). The sound speed used is the average value of each layer.

  Depending on the thickness relationship between the mold powder layer 5 and the solidified shell 3, the reflected wave 25 is hidden by the multiple reflection of the reflected wave 21 and a dead zone of the solidified shell is generated. However, the reflected waves 23 and 24 are caused by the multiple reflection. The reflected waves 25, 23, and 24 are easily discriminated from the reflected waves 25 using the fact that the amplitude of the reflected wave 25 is greatly attenuated and the reflected wave 25 is inverted in phase from the reflected wave 21. I was able to. Further, since multiple reflected waves are generated repeatedly, the attenuation amount and observation position of the multiple reflected waves can be predicted from the calculation based on the reflected wave 20, so that the reflected wave is obtained by signal processing such as removal with reference to the calculated values. It is also possible to observe 25.

  When the attenuation in each layer was large, it was forced to lower the frequency. For example, when an ultrasonic probe having a bandwidth of 2.5 MHz is used, the waveform of the reflected wave as shown in FIG. 6 cannot be obtained, and the waveform as shown in FIG. In the method of the present invention, even in such a case, the thickness can be measured by frequency analysis.

  The reflected wave 26 to reflected wave 30 in FIG. 7 are multiple reflections in the mold powder layer. However, the respective positions cannot be determined, and it is difficult to calculate the thickness based on the reflection time. However, by analyzing the frequency of this signal, a frequency distribution as shown in FIG. 8 was obtained, and it was found that there was a peak at about 2.8 MHz. Therefore, the thickness of the mold powder layer was calculated to be 0.5 mm from the sound speed and the peak frequency. Further, when the 2.8 MHz wave is selectively removed, the reflected wave 31 is emphasized, and the position can be specified as the reflected wave 32 as shown in FIG. 9, and the thickness of the solidified shell can be obtained. It was.

It is a figure which shows the example of embodiment of this invention, Comprising: It is a schematic sectional drawing of the casting_mold | template part for continuous casting which implemented this invention. It is a figure which shows the example of embodiment of this invention, Comprising: It is a figure which shows the example which embedded the ultrasonic probe from the upper surface of the casting_mold | template. It is a figure which shows the example of embodiment of this invention, Comprising: It is a figure which shows the example which has arrange | positioned the multichannel ultrasonic probe in the horizontal direction of a casting_mold | template. It is a figure which shows the example of embodiment of this invention, Comprising: It is the schematic which shows the mode of the propagation of the ultrasonic wave oscillated from the ultrasonic probe. It is a figure which shows the sine wave used as a transmission wave in Example 1. FIG. It is a figure which shows the waveform observed as a reflected wave in Example 1. FIG. 6 is a diagram illustrating another example of a waveform observed as a reflected wave in Example 1. FIG. It is a figure which shows the frequency distribution obtained by frequency-analyzing the signal of FIG. It is a figure which shows the example of the waveform obtained when the wave of 2.8 MHz is selectively removed from the signal of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Mold 2 Molten steel 3 Solidified shell 4 Mold powder 5 Mold powder layer 6 Molten steel surface 7 Ultrasonic probe 8 Installation hole 9 Ultrasonic transmission / reception processing apparatus 10 Ultrasonic 11 Reflected wave 12 Transmitted wave 13 Reflected wave 14 Transmitted wave 15 Reflected Wave 16 Transmitted wave 17 Boundary surface 18 Boundary surface 19 Boundary surface

Claims (3)

  An ultrasonic transmitter is installed in a continuous casting mold, and the ultrasonic wave transmitted from the ultrasonic transmitter is reflected from the boundary between the mold and the mold powder layer, from the boundary between the mold powder layer and the solidified shell. Continuous casting characterized by measuring the propagation time of each reflected wave and the reflected wave from the boundary between the solidified shell and molten steel, and determining the inflow thickness of the mold powder and the thickness of the solidified shell based on the measured propagation time Method for detecting solidification state in mold in   The reflected wave from the boundary between the mold powder layer and the solidified shell and the reflected wave from the boundary between the solidified shell and the molten steel are identified by utilizing the difference in the phase of the reflected wave. The solidification state detection method in the casting_mold | template in the continuous casting of Claim 1.   Using ultrasonic waves with a wavelength sufficiently smaller than the inflow thickness of the mold powder, the inflow thickness of the mold powder is obtained by frequency analysis of the multiple reflected wave signal generated in the mold powder layer, and the multiple reflection from the measurement signal The method for detecting a solidification state in a mold in continuous casting according to claim 1 or 2, wherein a reflected wave from a boundary between the solidified shell and the molten steel is specified by removing a wave component.

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