JP2009195937A - Method for producing continuously cast slab, and continuous casting machine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a continuously cast slab and a continuous casting machine with which, even in the case a wide difference is caused in the solidification completion positions in a slab width direction, the improvement of central segregation uniform in the slab width direction is made possible. <P>SOLUTION: In the method for producing a slab and a continuous casting machine, when, using a continuous casting machine 1 provided with a light rolling reduction zone 4 composed of two or more pairs of rolling reduction rolls, the rolling reduction of a slab 9 is started from when the solid phase ratio in the thickness central part of the slab 9 is ≤0.4 at the light rolling reduction zone, and, while applying rolling reduction force to the slab, the solidification is completed within the range of the light rolling reduction zone so as to produce a continuously cast slab, a solidification completion position 12 in the slab width direction is detected during the casting, and, on the basis of information on the detected solidification completion position, a static magnetic field in the slab thickness direction is applied to a part in which the solidification completion position is elongated to the casting direction from a static magnetic field application apparatus 14 installed in the light rolling reduction zone. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a continuous casting slab manufacturing method and a continuous casting machine capable of manufacturing a continuous casting slab of light steel with a central segregation.

  In continuous casting of steel, since there is not a sufficient amount of molten steel in the unsolidified phase at the end of solidification at the center of the slab, if a void is formed in the center of the slab or negative pressure is generated, the final It is known that the concentrated molten steel between dendritic trees, which are solidified parts, flows and accumulates in the center of the slab and solidifies to form so-called “center segregation”.

  This central segregation degrades the quality of the steel product. For example, in line pipe materials for oil transportation and natural gas transportation, hydrogen-induced cracking (also referred to as “HIC”) occurs from the center segregation due to the action of sour gas, and is also used in beverage can products. In deep drawn materials, anisotropy appears in workability due to segregation of components.

  Therefore, many measures for reducing the center segregation of the slab have been proposed from the casting process to the rolling process. Among them, as a means to reduce the center segregation of the slab at low cost and effectively, a method of gradually reducing the slab at the end of solidification with a reduction amount corresponding to the solidification shrinkage amount of the slab in a continuous casting machine ( Hereinafter, it is called “under light pressure”) (see, for example, Patent Document 1).

  This light reduction technology gradually reduces the volume of the unsolidified phase by reducing the slab by a reduction amount commensurate with the amount of solidification shrinkage, and prevents central segregation by preventing the flow of concentrated molten steel between dendrites. It is a technology to prevent. Therefore, it is necessary to control the solidification completion position of the slab (also referred to as “crater end position”) within the range of the light reduction belt composed of a roll group for carrying out the light reduction.

  By the way, in a slab slab (hereinafter, also simply referred to as “slab”), since the cross section is flat, the solidification completion position is not necessarily uniform in the slab width direction. In addition, it is known that the shape of the mold fluctuates with time due to changes in the flow of molten steel in the mold and changes in the clogging state of the secondary cooling spray nozzle.

  If the solidification completion position is different in the width direction of the slab, the amount of reduction in the light reduction zone will be different in each position in the width direction of the slab.Therefore, the position where the amount of reduction is difficult, that is, the solidification completion position is in the casting direction. At the extended position, a sufficient center segregation improvement effect cannot be obtained, and there are cases where center segregation cannot be suppressed even if light reduction is performed. In the width direction of the slab, for example, when solidification is completed early in the width center, even if it is lightly reduced, the deformation resistance of the fully solidified width center is large, so the reduction force is applied to the parts other than the width center. Does not work, and the desired amount of reduction cannot be applied. This is because light reduction devices generally do not have a load resistance enough to reduce a completely solidified slab because of limitations on equipment space and equipment cost. This is because, when the part is completely solidified and the rolling load exceeds the specified load, the rolling roll is released by a disc spring or hydraulic setting to protect the equipment.

  Thus, light reduction is not performed on a portion that is slowly solidified in the width direction of the slab, that is, a portion where the final solidification position is extended, and as a result, the center segregation of that portion is deteriorated.

Therefore, Patent Document 2 includes a transverse wave ultrasonic sensor and a longitudinal wave ultrasonic sensor for the purpose of extinguishing the portion where the final solidification position is extended and applying a light reduction force in the slab width direction. The solidification completion position detection device is used to detect the solidification completion position in the slab width direction, and the difference in the casting direction between the least expanded portion and the most extended portion of the detected solidification completion position is the reference range. Adjust the molten steel flow in the mold or adjust the width of the secondary cooling zone so that it is inside, or at least one of them, while rolling down the slab in the light reduction zone It has been proposed.
JP 54-107831 A JP 2006-198664 A

  However, Patent Document 2 has the following problems.

  That is, in Patent Document 2, if the difference in the solidification completion position in the slab width direction is within 2 m, the light reduction force works almost evenly in the slab width direction, but the difference in the solidification completion position is about 2 m. In other words, the center segregation of the portion where the solidification completion position is extended is worse than that of the other portions. In Patent Document 2, the difference in solidification completion position in the slab width direction is set to be within 2 m by adjusting the flow of molten steel in the mold or by adjusting the width of the secondary cooling zone. If the casting conditions are in a steady state, the difference in solidification completion position can be sufficiently adjusted within 2 m. In the non-steady state, for example, one of the discharge holes of the immersion nozzle is clogged, resulting in a difference in solidification completion position. If there is a difference in the solidification completion position due to clogging of the secondary cooling spray nozzle or the like, the difference in the solidification completion position becomes too large and the molten steel flow in the mold according to Patent Document 2 is adjusted. In addition, it is difficult to adjust the difference in solidification completion position within a predetermined value only by the method of adjusting the width of the secondary cooling zone. Therefore, in such a case, the center segregation of the portion where the solidification completion position is extended is worsened.

  The present invention has been made in view of the above circumstances, and the object thereof is to improve uniform center segregation in the slab width direction even when a large difference occurs in the solidification completion position in the slab width direction. It is providing the manufacturing method and continuous casting machine of continuous casting slab which become.

  The present inventors have intensively studied and studied to solve the above problems.

  As a result, depending on the shape of the solidification completion position detected by the solidification completion position detection device in the width direction of the slab, the secondary cooling water amount is not only cut in the secondary cooling zone but also in the entire width direction of the slab. It was found from the heat transfer calculation that the shape of the solidification completion position in the slab width direction can be made uniform if changed. However, in order to carry out this method, the amount of secondary cooling water from a large number of spray nozzles installed in the casting direction and the slab width direction must be controlled independently by each spray nozzle. Therefore, it was concluded that not only the cost of remodeling the equipment was great, but also the control of each spray nozzle was extremely complicated, so that practical application was difficult.

  Therefore, when solidifying the slab while lightly reducing, the solidification completion position in the slab width direction is not uniform, and even if part of the solidification is completed in the slab width direction, the solidification completion position is in the casting direction. A measure to reduce the center segregation of the stretched part was studied. As a result, when the slab is solidified while being lightly reduced, a light reduction force is applied to the slab, and at the same time, the shape in the width direction of the solidification completion position is grasped online, and the solidification completion position is extended to the casting direction. By applying a static magnetic field that can suppress the flow of molten steel, the center segregation of the portion where the solidification completion position extends in the casting direction is improved, and a small slab with center segregation over the entire width of the slab can be obtained. The knowledge of was obtained.

  The present invention has been made on the basis of the above knowledge, and the method for producing a continuous cast slab according to the first invention uses a continuous casting machine provided with a light reduction belt comprising a plurality of pairs of reduction rolls, Starting the rolling of the slab in the light rolling zone from the time when the solid phase ratio at the thickness center of the piece is 0.4 or less, solidification is completed within the range of the light rolling zone while applying a rolling force to the slab. When the continuous cast slab is manufactured, the solidification completion position in the width direction of the slab is detected during casting. Based on the information on the detected solidification completion position, the position where the solidification completion position extends in the casting direction is detected. Then, a static magnetic field in the slab thickness direction is applied in the light pressure lower belt.

  Moreover, the continuous casting machine which concerns on 2nd invention is for detecting the solidification completion position of the light reduction belt | band | zone for giving a reduction force to a slab which consists of several pairs of reduction rolls, and the width direction of a slab. Solidification completion position detection means, solidification completion position shape calculation means for obtaining a portion where the solidification completion position extends in the casting direction based on the detection result by the solidification completion position detection means, and the light pressure lower belt In addition, a static magnetic field application device for applying a static magnetic field in the thickness direction of the slab, and the static magnetic field application device with a solidification completion position in the casting direction based on a signal input from the solidification completion position shape calculation means. And a static magnetic field application position control means for installing in the extended part.

  According to the present invention, when solidification is completed while applying a light reduction force to the slab, a static magnetic field is applied to a portion where the solidification completion position extends in the casting direction to suppress the flow of molten steel in the portion. Therefore, the center segregation of the portion where the solidification completion position extends in the casting direction is suppressed, and as a result, even if a large difference in the casting direction occurs in the solidification completion position in the slab width direction, It is possible to obtain a slab with a slight center segregation.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a view showing an embodiment of the present invention, and is a schematic side view of a slab continuous casting machine according to the present invention.

  In FIG. 1, 1 is a slab continuous casting machine, 2 is a mold for solidifying molten steel, 3 is a slab support roll for supporting a cast slab, and is injected into the mold 2 of the slab continuous casting machine 1 The molten steel is cooled by the mold 2 to form a solid phase portion 10 at a portion in contact with the mold 2, the periphery is a solid phase portion 10, and the inside is a slab 9 having an unsolidified liquid phase portion 11. Is pulled out below the mold 2 while being supported by a plurality of pairs of slab support rolls 3 arranged opposite to each other below the mold 2. A spray nozzle (not shown) such as an air mist spray nozzle or a water spray nozzle that blows cooling water toward the surface of the slab 9 is disposed in the gap between the slab support rolls 3 adjacent to each other in the casting direction. A secondary cooling zone is provided below the mold, and the slab 9 is cooled in the secondary cooling zone while being drawn out downstream in the casting direction, and completely solidified to the center. The position where the solidification has been completely achieved is the solidification completion position 12. The slab 9 that has been solidified is cut into a predetermined length by a slab cutting machine 7 installed on the downstream side of the slab support roll 3, and is carried out as a slab 9A by a transport roll 8.

  A light pressure lower zone 4 is disposed downstream of the secondary cooling zone. The light pressure lower belt 4 is set so that the interval between the opposed rolls of the slab support roll 3 (referred to as “roll interval”) is gradually narrowed toward the downstream in the casting direction. This is a site where there is a group of slab support rolls 3 that can apply a rolling force.

  In the present invention, the time when the light reduction is started is the time when the solid phase ratio in the central portion of the thickness of the slab 9 is 0.4 or less, and the solidification needs to be completed within the range of the light reduction zone 4. Therefore, in order to satisfy these conditions, the slab drawing speed or the amount of secondary cooling water is adjusted based on heat transfer calculation or the like. This is because, even if light reduction starts after the solid phase ratio at the center of the slab thickness exceeds 0.4, the flow of concentrated molten steel may occur before that, which causes center segregation. Therefore, when the effect of light reduction cannot be fully exhibited, and when the solidification completion position 12 extends downstream beyond the light pressure reduction band 4, the reduction force does not work and the center segregation is improved. This is because the effect cannot be obtained.

  By using the lightly lower belt 4, the slab 9 at the end of solidification is crushed, the flow of the concentrated molten steel based on the solidification shrinkage is suppressed, and the center segregation of the slab 9 is improved. What is necessary is just to set the gradient of the roll space | interval in this light reduction belt 4 to such an extent that a reduction speed becomes the range of 0.6-1.5 mm / min. When the rolling speed is less than 0.6 mm / min, the effect of reducing the center segregation is small. On the other hand, when the rolling speed exceeds 1.5 mm / min, the concentrated molten steel is squeezed out in the direction opposite to the casting direction. This is because negative segregation may be generated at the center of the piece. Further, it is sufficient that the total reduction amount is about 2 to 6 mm.

  In this light pressure lower belt 4, one static magnetic field applying device 14 is installed in the gap between adjacent slab support rolls 3, one on each side in the width direction of the slab 9. In FIG. 2, the installation situation of the static magnetic field application apparatus 14 is shown. The static magnetic field application device 14 magnetizes the pair of magnetic poles 14c that are opposed to each other with the slab 9 interposed therebetween, the return yoke 14b that connects the N pole and the S pole of the magnetic pole 14c, and the magnetic pole 14c that goes around the return yoke 14b. And a DC power supply device (not shown) connected to the excitation coil 14a. The DC power supply supplied from the DC power supply device penetrates the slab 9 between the opposing magnetic poles 14c. A static magnetic field is applied. Note that the static magnetic field application device 14 is arranged at intervals of two slab support rolls 3 in FIG. 1, and may be arranged at intervals of several slab support rolls 3 in this way. However, in order to reduce the center segregation, it is desirable to arrange in the gaps of all the slab support rolls 3 of the light pressure lower belt 4. In this case, since the effect is small if the static magnetic field applying device 14 is only at one place, it is necessary to install at least two places in the casting direction. Moreover, when applying a static magnetic field, you may utilize a permanent magnet other than said electromagnet.

  Each of the static magnetic field application devices 14 is connected to each of the position adjusting actuators 16 and can be moved independently in the slab width direction by each of the position adjusting actuators 16. The position adjusting actuator 16 is connected to the static magnetic field application position control device 15 and is operated by a signal input from the static magnetic field application position control device 15. On the other hand, the static magnetic field application position control device 15 is connected to a static magnetic field application position determination device 6 described later, and sends a signal to the position adjustment actuator 16 based on a signal input from the static magnetic field application position determination device 6. Send. The static magnetic field application position control device 15 and the position adjusting actuator 16 have a role as static magnetic field application position control means for determining the installation position of the static magnetic field application device 14. Also, part of the static magnetic field application position determination device 6 functions as a static magnetic field application position control means. The shape of the magnetic pole 14c is such that the length corresponding to the casting direction of the slab is 40 mm and the length corresponding to the width direction of the slab is about 100 to 300 mm, which is sufficient for the gap between the adjacent slab support rolls 3. It is a dimension that fits. Thus, by arranging the magnetic pole 14c in the gap between the adjacent slab support rolls 3, a static magnetic field can be applied efficiently, but it is not essential to place it in the gap between the slab support rolls 3, and the slab support is supported. It may be arranged above the roll 3.

  Furthermore, in the slab continuous casting machine 1 according to the present invention, as shown in FIG. 1, a solidification completion position detection device 5 is installed as solidification completion position detection means for detecting the solidification completion position 12 of the slab 9. Has been. Further, based on the detection signal of the solidification completion position detection device 5, the distribution of the solidification completion position 12 in the slab width direction is obtained, and the portion where the solidification completion position 12 extends in the casting direction is obtained from the obtained distribution. Furthermore, a static magnetic field application position determination device 6 for determining a position where a static magnetic field needs to be applied is installed. The static magnetic field application position determination device 6 also has a function as solidification completion position shape calculation means.

  The solidification completion position detection device 5 used in the present invention measures the thickness of the solid phase portion 10 of the slab 9 from the propagation time of the longitudinal wave ultrasonic waves in the slab 9, and thereby detects the solidification completion position 12. However, the propagation speed of the longitudinal ultrasonic wave is greatly influenced by the steel type and the thickness of the slab, and the thickness of the solid phase portion 10 cannot be measured accurately unless these are calibrated. Therefore, in the coagulation completion position detection device 5 used in the present invention, the longitudinal wave ultrasonic wave is detected using the position of the coagulation completion position 12 measured based on the fact that the transverse wave ultrasonic wave does not pass through the liquid phase part 11. Calibrate the measurement results.

  That is, when the transmission signal from the transverse wave ultrasonic sensor appears from the disappearance state or disappears from the detection state, the solidification completion position 12 and the placement position of the transverse wave ultrasonic sensor are determined regardless of the steel type and casting conditions. Since the absolute value can be measured to coincide with each other, the calculation formula for calculating the coagulation completion position 12 from the propagation time of the longitudinal wave ultrasonic wave is calibrated under the condition that the coagulation completion position 12 is the arrangement position of the transverse wave ultrasonic sensor. The estimation method of the coagulation completion position 12 using the propagation time of longitudinal ultrasonic waves, which is excellent in relative measurement accuracy, can be used as a detection means excellent in absolute accuracy.

  The solidification completion position detection device 5 is disposed so as to face the transverse wave ultrasonic sensor including the transverse wave ultrasonic transmitter 17 and the transverse wave ultrasonic receiver 18 that are opposed to each other with the slab 9 interposed therebetween. An electrical signal is given to the longitudinal wave ultrasonic sensor including the longitudinal wave ultrasonic transmitter 19 and the longitudinal wave ultrasonic receiver 20, and the transverse wave ultrasonic transmitter 17 and the longitudinal wave ultrasonic transmitter 19 to ultrasonically transmit the slab 9. An ultrasonic transmission unit 21 that is an electric circuit for transmitting a signal, a transverse wave transmission intensity detection unit 22 and a coagulation completion position arrival detection unit 23 for processing a reception signal received by the transverse wave ultrasonic receiver 18, A longitudinal wave propagation time detection unit 24 and a coagulation completion position calculation unit 25 for processing a reception signal received by the wave ultrasonic receiver 20 are provided. The ultrasonic waves transmitted by the transverse wave ultrasonic transmitter 17 and the longitudinal wave ultrasonic transmitter 19 are transmitted through the slab 9 and received by the transverse wave ultrasonic receiver 18 and the longitudinal wave ultrasonic receiver 20, respectively, and are supplied as electrical signals. Is converted to In FIG. 1, the transverse wave ultrasonic sensor and the longitudinal wave ultrasonic sensor are installed in the light pressure lower belt 4, but this installation position is not limited to the light pressure lower belt 4, and as long as it is a secondary cooling belt, It does not matter.

  The transverse wave ultrasonic sensor and the longitudinal wave ultrasonic sensor are attached to, for example, a frame that can move in the width direction of the slab 9, and the width of the slab 9 is obtained by moving the transmitter and the receiver in synchronization. The solidification completion position 12 at each position in the direction can be detected. In this case, the transverse wave ultrasonic sensor and the longitudinal wave ultrasonic sensor are configured to move synchronously.

  The transverse wave transmission intensity detection unit 22 is a device that detects the intensity of the transverse wave ultrasonic signal received by the transverse wave ultrasonic receiver 18, and the coagulation completion position arrival detection unit 23 is detected by the transverse wave transmission intensity detection unit 22. The apparatus determines whether the coagulation completion position 12 is upstream or downstream in the casting direction with respect to the arrangement position of the transverse wave ultrasonic wave transmitter 17 and the transverse wave ultrasonic wave receiver 18 from the change of the transmitted signal of the transverse wave ultrasonic wave. is there. The longitudinal wave propagation time detection unit 24 is a device that detects the propagation time of longitudinal wave ultrasonic waves that pass through the slab 9 from the reception signal received by the longitudinal wave ultrasonic receiver 20. Reference numeral 25 denotes an apparatus for calculating and determining the coagulation completion position 12 from the propagation time of the longitudinal wave ultrasonic wave detected by the longitudinal wave propagation time detection unit 24. Here, the transverse wave transmission intensity detection unit 22, the coagulation completion position arrival detection unit 23, the longitudinal wave propagation time detection unit 24, and the coagulation completion position calculation unit 25 are calculated by a computer.

  Note that an ultrasonic signal amplifier and an A / D converter for taking a waveform into the computer are required between the transverse wave ultrasonic receiver 18 and the longitudinal wave ultrasonic receiver 20 and the computer. It is omitted inside. Further, in the coagulation completion position detection device 5 shown in FIG. 1, the transverse wave ultrasonic transmitter 17 and the longitudinal wave ultrasonic transmitter 19 are integrally configured, and similarly, the transverse wave ultrasonic receiver 18 and the longitudinal wave ultrasonic wave transmitter 19 are combined. The sound wave receiver 20 is integrally formed.

  Here, the operation of the solidification completion position detection device 5 will be described. First, the operation of the transverse wave transmission intensity detector 22 will be described with reference to FIG.

  FIG. 3 is a diagram showing the operation of the transverse wave transmission intensity detection unit 22 and shows a reception signal waveform corresponding to one transmission signal. The first wave in FIG. 3 is a signal in which the transmission signal is electrically leaked into the transverse wave ultrasonic receiver 18, and the second wave is a transmission signal of the transverse wave ultrasonic wave.

  The time position at which the transmission signal of the transverse wave ultrasonic wave appears is roughly known from the thickness of the slab 9, the approximate temperature of the slab 9, and the propagation speed of the transverse wave ultrasonic wave. And the maximum value of the signal in the gate is obtained. This processing can be easily realized by calculation processing by taking the waveform of the received signal into the computer by A / D conversion. As a method of taking the maximum value of the signal, either an absolute value based on 0 V or a peak-to-peak value may be used. Actually, since the transmission signal is repeated with a period of several tens of Hz to several hundreds of Hz, the transmission intensity of the transverse ultrasonic wave is obtained after averaging each waveform, or the transmission intensity of each waveform. It is effective to reduce the influence of fluctuation caused by noise.

  Next, the operation of the solidification completion position arrival detection unit 23 will be described with reference to FIG. FIG. 4 is a diagram showing an example of the operation of the solidification completion position arrival detection unit 23. The transverse wave ultrasonic wave sent from the transverse wave transmission intensity detection unit 22 while changing the casting conditions over several tens of minutes of continuous casting operation. It is the chart which detected the intensity | strength of the transmitted signal of.

  As shown in FIG. 4, the intensity of the transmitted signal of the transverse ultrasonic wave changes according to the change in the casting conditions of the continuous casting operation. In the range of A and B in FIG. 4, the intensity of the transmission signal is very small, and the coagulation completion position 12 is downstream in the casting direction from the arrangement position of the transverse wave ultrasonic transmitter 17 and the transverse wave ultrasonic receiver 18. It represents the state that exists on the side.

  The coagulation completion position arrival detection unit 23 determines that the coagulation completion position 12 has passed the arrangement position of the transverse wave ultrasonic sensor when the intensity of the transmission signal crosses a predetermined determination threshold value. This determination threshold value may be either a predetermined fixed value or a fluctuation threshold value obtained by obtaining a noise level from a signal level in a time domain in which a transverse ultrasonic transmission signal does not appear. . When the coagulation completion position arrival detection unit 23 determines that the coagulation completion position 12 has passed the arrangement position of the transverse wave ultrasonic sensor in this way, the coagulation completion position arrival detection unit 23 sends a timing signal to the coagulation completion position calculation unit 25.

  Next, the operation of the longitudinal wave propagation time detector 24 will be described with reference to FIG. FIG. 5 is a diagram illustrating the operation of the longitudinal wave propagation time detection unit 24, and is a diagram illustrating a waveform of a reception signal corresponding to one transmission signal. The first wave in FIG. 5 is a transmission signal that has leaked into the longitudinal wave ultrasonic wave receiver 20 electrically, and the second wave is a transmission signal of longitudinal wave ultrasound.

  The longitudinal wave propagation time detection unit 24 detects the time from the transmission timing of the transmission signal to the appearance timing of the transmission signal of the longitudinal wave ultrasonic wave. As a method for detecting a transmission signal of longitudinal ultrasonic waves, as shown in FIG. 5, either the time when the threshold value is exceeded or the time when the maximum value in the gate is reached may be used. Similar to the transverse wave transmission intensity detector 22, this processing can be easily realized by calculation processing by capturing the waveform of the received signal into the computer by A / D conversion. In practice, since the transmission signal is repeated at a period of several tens Hz to several hundreds Hz, the propagation time of longitudinal ultrasonic waves is obtained after averaging each waveform or the propagation time of each waveform. It is effective to reduce the influence of fluctuation caused by noise.

  Finally, the operation of the coagulation completion position calculation unit 25 will be described with reference to FIG. FIG. 6 is a diagram illustrating the operation of the coagulation completion position calculation unit 25, and illustrates an approximate expression for calculating the coagulation completion position 12 from the propagation time of longitudinal wave ultrasonic waves. As the solidification completion position 12 is located downstream of the arrangement position of the longitudinal wave ultrasonic transmitter 19 and the longitudinal wave ultrasonic receiver 20 in the casting direction, the thickness of the liquid phase portion 11 increases, and the propagation time becomes longer. Therefore, the propagation time and the distance from the molten steel surface 13 in the mold 2 to the solidification completion position 12 are approximately proportional to each other, and the relationship shown in FIG. 6 is shown. Therefore, in order to obtain the solidification completion position 12 from the propagation time, an approximate expression of a polynomial, for example, a linear expression shown in the following expression (1) may be used.

CE = a1 × Δt + a0 (1)
In the equation (1), CE is the distance from the molten steel surface 13 in the mold to the solidification completion position 12, Δt is the propagation time of longitudinal ultrasonic waves, and a1 and a0 are polynomial coefficients.

  In FIG. 6, the line indicated by A represents an approximate expression before calibration of the distance from the molten steel surface 13 in the mold to the solidification completion position 12 based on longitudinal wave ultrasonic waves.

  However, since the propagation speed of longitudinal ultrasonic waves differs depending on the steel type, the coefficient a0 needs to be corrected depending on the steel type. Therefore, the correction method will be described below.

  When the timing signal for determining the passage of the coagulation completion position 12 is sent from the coagulation completion position arrival detection unit 23 to the coagulation completion position calculation unit 25, the coagulation completion position calculation unit 25 transmits the propagation time of longitudinal ultrasonic waves at that time ( Δt1) is obtained, and further, the following equation (2) is used so that the distance (CE) from the molten steel surface 13 in the mold to the solidification completion position 12 matches the arrangement position of the transverse ultrasonic sensor ( 1) Modify the coefficient (a0) in the equation.

a0 = CE1-a1 * [Delta] t1 (2)
However, in the formula (2), CE1 is the distance from the molten steel surface 13 in the mold to the arrangement position of the transverse wave ultrasonic sensor, and Δt1 is the time when it is determined that the solidification completion position 12 has passed the arrangement position of the transverse wave ultrasonic sensor. Is the propagation time of longitudinal ultrasonic waves.

  Thus, the approximate expression for obtaining the solidification completion position 12 is calibrated, for example, after calibration indicated by B in FIG. After the calibration, the solidification completion position 12 can be detected on-line during casting with high accuracy based on the propagation time of the longitudinal wave ultrasonic wave using the approximate expression after calibration indicated by B.

  The calibration is performed only once every time a new steel type is cast, every time the solidification completion position 12 crosses the position of the transverse ultrasonic sensor during continuous casting, or at the discretion of the operator. Any suitable time may be used.

  Next, the operation of the static magnetic field application position determination device 6 will be described. The static magnetic field application position determination device 6 is a solidification completion position shape calculation means that calculates the shape of the solidification completion position 12 in the slab width direction based on the calculation result of the solidification completion position calculation unit 25 of the solidification completion position detection device 5. Based on the solidification completion position shape calculation unit 26 and the shape of the solidification completion position 12 calculated by the solidification completion position shape calculation unit 26, a range in which a static magnetic field needs to be applied in the width direction and the casting direction of the slab 9, That is, the solidification completion position 12 extends in the casting direction, the liquid phase portion 11 remains in the center portion of the slab, and the range in which the flow of the remaining liquid phase portion 11 needs to be suppressed by applying a static magnetic field is set. And a static magnetic field application region determination unit 27 as a static magnetic field application position control means to be determined.

  Among these, the solidification completion position shape calculation unit 26 includes information on the solidification completion position 12 at each position in the slab width direction calculated by the solidification completion position calculation unit 25, and information on the slab width direction position at which it is measured. Based on the above, the shape of the solidification completion position 12 over the entire width of the slab 9 is calculated.

  The static magnetic field application region determination unit 27 determines a region where static magnetic field application is required as follows. That is, in the case of the shape of the solidification completion position 12 as shown in FIG. 7, in the region in the width direction other than the short side of the slab, from the position where even a part is completely solidified, the downstream side in the casting direction, A region where the liquid phase portion 11 that is an unsolidified phase remains (a hatched range in FIG. 7) is determined as a static magnetic field application region. The static magnetic field application region determination unit 27 outputs the static magnetic field application position control device 15 so that the static magnetic field is applied to the region determined as the static magnetic field application region. In this case, the output is output to the static magnetic field application position control device 15 so that the center position of the extended solidification completion position 12 and the center position of the static magnetic field application device 14 coincide. Further, when there are three or more extended portions of the solidification completion position 12 in the slab width direction, the static magnetic field application position control device 15 is arranged so that the static magnetic field application devices 14 are arranged in descending order of the degree of expansion. Output.

  In the slab continuous casting machine 1 configured as described above, the slab 9A is manufactured as follows.

  Molten steel is cast on the mold 2 through an immersion nozzle (not shown). The molten steel cast in the mold 2 is cooled by the mold 2 to form a solid phase portion 10, and continuously drawn as a slab 9 having a liquid phase portion 11 therein while being supported by the slab support roll 3. It is. While the slab 9 passes through the slab support roll 3, it is cooled in the secondary cooling zone, increases the thickness of the solid phase portion 10, and reaches the light pressure lower zone 4. And it is reduced by the appropriate amount of reduction in the light reduction zone 4, and solidification to a center part is completed eventually.

  At that time, the solidification completion position detection device 5 and the static magnetic field application position determination device 6 detect the position of the solidification completion position 12 in the slab width direction. Then, the static magnetic field application position determination device 6 determines the static magnetic field application area according to the detected shape of the solidification completion position 12 in the slab width direction, and the static magnetic field application is performed by the position adjustment actuator 16 based on the signal. The apparatus 14 is moved, and a static magnetic field is applied to the slab 9 from the static magnetic field application apparatus 14 arranged at a predetermined position.

  It should be noted that the solidification completion position 12 is within the range of the light pressure lower belt 4 and the reduction by the light pressure lower belt 4 is started even in the extended portion of the solidification completion position 12 based on heat transfer calculation in advance. When the casting conditions are set so that the solid phase ratio at the central portion of the thickness of the slab 9 is 0.4 or less, but the detection result by the solidification completion position detector 5 is different from this In order to satisfy the above conditions, the slab drawing speed and the amount of secondary cooling water are adjusted. The slab 9 that has been solidified is cut into a predetermined length by the slab cutting machine 7, and is carried out to the next process by the transport roll 8 as a slab 9A.

  In general, the shape in the width direction of the solidification completion position 12 of the slab 9 tends to be the shape as shown in FIG. 7 due to the influence of “width cutting” in the secondary cooling zone. Since a static magnetic field is applied according to the width direction shape of the completion position 12, the center segregation can be reduced even if the solidification completion position 12 is a portion extending in the casting direction. The “width cutting” in the secondary cooling is a method of cooling without spraying cooling water intentionally in the vicinity of the corner of the long side surface of the slab slab in the secondary cooling zone of the continuous casting machine.

  The reason why the center segregation of the slab 9 is reduced by the application of the static magnetic field is as follows.

  When a liquid with good electrical conductivity flows perpendicular to the magnetic flux of a static magnetic field in an environment where a static magnetic field is applied, an induced current is generated in the liquid, and this induced current and the applied static magnetic field It is known that an electromagnetic force in a direction opposite to the flow direction acts on the liquid due to the interaction with the liquid and suppresses the flow. That is, the magnetic phase 14c opposed to each other with the slab 9 interposed therebetween is disposed, and the liquid phase portion 11 inside the slab is cast in a state in which a static magnetic field having a magnetic field strength B is applied from the magnetic pole 14c in the thickness direction of the slab 9. When moving in the direction and the width direction at a speed V, an electromotive force E based on the following equation (3) is generated.

E = V × B (3)
Due to this electromotive force E, an eddy current I (I = σ × E) is generated in the liquid phase portion 11, and the flow direction of the liquid phase portion 11 is reversed by the interaction between the eddy current I and the magnetic field strength B. The body force F shown by the following formula (4) works in the direction. Where σ is the specific electrical resistance (m / Ω).

F = −I × B = −σ × V × B ² (4)
As apparent from the equation (4), the body force F is proportional to the liquid phase portion 11, that is, the flow velocity of the molten steel and the strength of the static magnetic field. This bulk force F suppresses the flow of molten steel and reduces central segregation. In the final solidified portion, the liquid phase portion 11 is thin. Therefore, the moving direction of the liquid phase portion 11 in the final solidified portion is a direction parallel to the slab surface, that is, a direction perpendicular to the magnetic flux of the static magnetic field. .

  As described above, according to the present invention, the solidification completion position 12 is always detected during casting while the slab 9 is lightly pressed, and the solidification completion position 12 extends in the casting direction based on the detection result. Since a static magnetic field is applied to the light pressure lower zone 4, the flow of unsolidified molten steel is suppressed even at the solidification completion position 12 extending in the casting direction, and the center segregation is uniformly reduced in the slab width direction. The high-quality cast slab 9A can be manufactured stably.

  In the solidification completion position detection device 5 shown in FIG. 1, the transverse wave ultrasonic sensor and the longitudinal wave ultrasonic sensor are arranged at the same position, but need not be arranged at the same position, and the same in the slab width direction. As long as it is installed at a position, it may be a position away from the casting direction.

  Here, the same position in the slab width direction means a range in which it can be considered that there is almost no change in the casting direction of the solidification completion position 12. In the slab continuous casting machine 1, the solidification completion position 12 may be different in the width direction of the slab 9, so that the solidification completion position 12 detected by the transverse wave ultrasonic sensor and the longitudinal wave ultrasonic sensor is the same, or Even if a change in the casting direction occurs at the solidification completion position 12, it is necessary to arrange the transverse wave ultrasonic sensor and the longitudinal wave ultrasonic sensor within a range in the width direction where it can be considered that there is almost no difference in change. Specifically, when the shape of the solidification completion position 12 in the slab width direction can be regarded as flat, it may be several hundred mm away, and conversely, the shape of the solidification completion position 12 in the slab width direction changes greatly. If it is, it must be within several tens of mm. This is because the wavelength of the ultrasonic wave used for this purpose is several tens of millimeters and the size of the sensor is about several tens of millimeters. Therefore, if the influence of diffraction is taken into consideration, it is regarded as the same position within several tens of millimeters. Because you can.

  In addition, when the transverse ultrasonic sensor and the longitudinal ultrasonic sensor are arranged at the same position, it is not always necessary to use an electromagnetic ultrasonic sensor that generates and detects longitudinal ultrasonic waves and transverse ultrasonic waves at the same position. As long as the arrangement interval between the transverse wave ultrasonic sensor and the longitudinal wave ultrasonic sensor is within several tens of millimeters, the transverse wave ultrasonic sensor and the longitudinal wave ultrasonic sensor may be arranged separately.

  Examples of the present invention will be described below. Chemical component composition is C: 0.15 mass%, Si: 0.15 mass%, Mn: 1.0 mass%, P: 0.015 mass%, S: 0.005 mass%, Ti: 0.01 1% by mass, sol.Al: 0.03% by mass of medium carbon steel was melted, and using the slab continuous casting machine shown in FIG. 1, the slab drawing speed was 1.5 m / min, width: 2100 mm, Thickness: Cast into a 250 mm slab slab.

  According to the slab width direction shape of the solidification completion position detected by the solidification completion position detection device in the light pressure lower band, while rolling down the slab by setting the narrowing gradient of the roll interval in the light pressure lower band to 0.53 mm per mm, A static magnetic field was applied to the portion where the solidification completion position extended in the casting direction. The static magnetic field application device is arranged at about 2 m intervals between the rolls in the range of 26 to 32 m from the lower end of the mold so that the center position of the static magnetic field application device coincides with the portion where the solidification completion position extends most in the casting direction. A static magnetic field was applied at a magnetic field intensity of 5000 gauss.

  For comparison, the steel type was cast under the same casting conditions without applying a static magnetic field and setting the narrowing gradient of the roll interval in the light pressure zone to 0.53 mm per mm.

A slab full width sample was taken from the cast slab and the center segregation of the slab was investigated. The center segregation was investigated by collecting chips with a 5 mm diameter drill at 100 pitches in the width direction of the slab from the center of the slab, determining the carbon concentration (C) with a combustion carbon analyzer, and 1/4 of the slab. The degree of segregation of carbon (C / C ₀ ), which is a ratio to the carbon analysis value (C ₀ ) of the chips collected from the thickness position, was investigated.

FIG. 8 shows the slab width direction shape of the solidification completion position detected by the solidification completion position detection device at the time of the slab casting, together with the slab width direction distribution of the carbon segregation degree (C / C ₀ ) investigated. (Crater end shape) is shown. As is apparent from FIG. 8, the carbon segregation degree (C / C ₀ ) is made uniform in the width direction in the present invention example even though the solidification completion position is non-uniform in the slab width direction. I understand that. On the other hand, in the comparative example, the segregation degree (C / C ₀ ) of carbon deteriorated in the portion where the solidification completion position extended in the casting direction.

It is a side schematic diagram of the slab continuous casting machine concerning the present invention. It is a figure which shows the installation condition of the static magnetic field application apparatus shown in FIG. It is a figure which shows operation | movement of a transverse wave transmission intensity detection part. It is a figure which shows an example of operation | movement of the coagulation completion position arrival detection part. It is a figure which shows operation | movement of a longitudinal wave propagation time detection part. It is a figure which shows operation | movement of the coagulation completion position calculating part. It is a figure which shows one example of the width direction shape of a coagulation completion position. In conjunction with the slab width direction distribution of segregation ratio (C / C ₀₎ of the carbon is a diagram showing a slab width direction shape of the solidification completion position.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Mold 3 Cast slab support roll 4 Light pressure lower belt 5 Solidification completion position detection device 6 Static magnetic field application position determination device 7 Cast slab cutting machine 8 Roll for conveyance 9 Cast slab 10 Solid phase part 11 Liquid phase part 12 Solidification completion position 13 Molten steel surface 14 Static magnetic field application device 14a Excitation coil 14b Return yoke 14c Magnetic pole 15 Static magnetic field application position control device 16 Actuator for position adjustment 17 Transverse wave ultrasonic transmitter 18 Transverse wave ultrasonic receiver 19 Longitudinal wave ultrasonic transmission 20 Longitudinal wave ultrasonic wave receiver 21 Ultrasonic wave transmission unit 22 Transverse wave transmission intensity detection unit 23 Coagulation completion position arrival detection unit 24 Longitudinal wave propagation time detection unit 25 Coagulation completion position calculation unit 26 Coagulation completion position shape calculation unit 27 Static magnetic field application Area determination unit

Claims (2)

  Using a continuous casting machine equipped with a light reduction belt consisting of a plurality of pairs of reduction rolls, the reduction of the slab is started in the light reduction zone when the solid phase ratio at the thickness center of the slab is 0.4 or less. When producing a continuous cast slab by completing solidification within the range of the light rolling zone while applying a rolling force to the slab, the solidification completion position in the slab width direction is detected during casting, and the detected solidification completion Based on the position information, a static magnetic field in the thickness direction of the slab is applied to the portion where the solidification completion position extends in the casting direction, using the light pressure lower zone, and the continuous cast slab is manufactured. Method. A light reduction belt for applying a reduction force to the slab, comprising a plurality of pairs of reduction rolls;
A solidification completion position detection means for detecting a solidification completion position in the width direction of the slab;
A solidification completion position shape calculating means for obtaining a portion where the solidification completion position extends in the casting direction based on a detection result by the solidification completion position detection means;
A static magnetic field application device for applying a static magnetic field in the thickness direction of the slab, disposed in the light pressure lower belt,
A static magnetic field application position control means for installing the static magnetic field application device in a portion where the solidification completion position extends in the casting direction based on a signal input from the solidification completion position shape calculation means;
A continuous casting machine comprising:

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