JP2008238257A - Method for producing continuously cast slab and continuous caster - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a method for producing a continuously cast slab and a continuous caster with which a crater-end shape can precisely be controlled and the improvement of uniform center segregation in the width direction under light pressure can be obtained. <P>SOLUTION: In the method for producing the continuously cast slab by using the continuous caster having the light pressing zone for light-pressing the cast slab: a solidification-completing positional information is obtained by using a solidification-completing position detecting instrument, with which the solidification-completing position of the cast slab can be detected in on-line; and based on the obtained solidification-completing positional information, the light pressing is applied in the light pressing zone while adjusting non-solidified thickness in the cast slab by widening an interval between supporting rolls set at the upstream on the light pressing zone so that the difference between the shortest solidification-completing position (CE<SB>u</SB>) and the longest solidification-completing position (CE<SB>d</SB>) comes in the preset standard range. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a continuous casting slab manufacturing method and a continuous casting machine, and more specifically, detects the shape of the solidification completion position of the slab during casting so that the detected solidification completion position has a predetermined shape. The present invention relates to a method for producing a cast slab having a small central segregation in the entire cross section by performing light reduction under control, and a continuous casting machine.

In continuous casting of steel, there is not a sufficient amount of molten steel in the unsolidified phase at the end of solidification of the slab, so the concentrated molten steel between the dendritic trees, which is the final solidification zone, flows, which is the center of the slab. It is known that so-called “central segregation” is generated by accumulating in a part and solidifying.
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 it is also used for canned products for drinking water. In the deep drawn material, anisotropy appears in workability due to segregation of components.

Therefore, many countermeasures for reducing the center segregation have been proposed from the casting process to the rolling process.
Among them, as a means of reducing the center segregation of the slab at a low cost and effectively, a method of gradually reducing the unsolidified slab at a reduction amount corresponding to the solidification shrinkage of the slab at the end of solidification (hereinafter referred to as “ Has been proposed (for example, see Patent Document 1).

Under this light pressure, the slab is gradually reduced with a light reduction amount commensurate with the amount of solidification shrinkage to reduce the volume of the unsolidified phase, and central segregation is performed without causing the flow of concentrated molten steel between dendrites. It is preventing.
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 group of rolls for light reduction.

By the way, in the slab slab (hereinafter referred to as “slab”), since the cross section is flat, the solidification completion position is not necessarily uniform in the width direction.
Moreover, it is known that the shape of the molten steel changes due to changes in the flow of molten steel due to nozzle clogging and time.
If the solidification completion position is different in the slab width direction, the light reduction amount in the light reduction zone will be different in each position in the slab width direction, so the position where the light reduction amount is small, that is, the position where the solidification completion position is extended is sufficient. There are cases where the center segregation improvement effect cannot be obtained and the center segregation cannot be suppressed even if light reduction is performed.
Therefore, controlling the solidification completion position in the slab width direction to be the same is extremely important for improving center segregation by light reduction.
In order to perform such control, a method of accurately measuring the solidification state in the slab width direction, that is, the width direction distribution of the crater end, and correcting it when it is not uniform can be considered.

The following have been proposed as adopting such a method.
A transverse wave ultrasonic sensor that transmits a transverse wave ultrasonic wave to the continuous cast slab and receives the transmitted transverse wave ultrasonic wave, and a casting position separated from the position of the transverse wave ultrasonic sensor at the same position of the continuous casting machine or in the casting direction. A longitudinal wave ultrasonic sensor that transmits longitudinal wave ultrasonic waves to a continuous cast slab and that receives the transmitted longitudinal wave ultrasonic waves, and is received by the longitudinal wave ultrasonic sensor. A solidification completion position calculation unit for obtaining a solidification completion position of the slab using a calculation formula based on the received signal, and by changing the intensity of the reception signal of the transverse wave ultrasonic sensor, the arrangement position of the transverse wave ultrasonic sensor and the casting When it is confirmed that the solidification completion position of the piece matches, the calculation formula is calibrated so that the solidification completion position calculated by the calculation formula matches the position of the transverse ultrasonic sensor. The detection range is used to detect the solidification completion position in the slab width direction of the continuous cast slab, and the difference between the detected shortest solidification completion position and the detected longest solidification completion position is set in advance. A method for producing a continuous cast slab, characterized by adjusting the flow of molten steel in a mold or adjusting the width of secondary cooling so as to be within (Patent Document 2).
JP 54-107831 A JP 2006-198664 A

The method disclosed in Patent Document 2 is intended to control the shape of the crater end in the slab width direction in the lightly squeezed belt based on the study of the relationship between the width of the secondary cooling and the crater end position. is there.
However, it is not sufficient to control the precise crater end shape only by adjusting the width of the secondary cooling.

  The present invention has been made to solve the above-described problems, and can provide a method for producing a continuous cast slab capable of precise control of a crater end shape and improvement of uniform center segregation in the width direction due to light reduction. The aim is to obtain a continuous casting machine.

As the one for controlling the crater end shape based on the solidification completion position information of the solidification completion position detection device, only the one described in Patent Document 2 is known.
However, the method of adjusting the width of secondary cooling shown in Patent Document 2 is insufficient for precise control of the crater end shape as described above.

Therefore, in order to precisely control the shape of the crater end, the inventor controls the unsolidified thickness inside the slab on the upstream side by adjusting the interval between the support rolls installed upstream of the light pressure lower belt, thereby creating a crater. We considered controlling the end shape.
However, in order to control the crater end shape appearing on the downstream side upstream of the light pressure lower belt, it is not easy to specifically adjust the support roll interval. This is even more difficult because the crater end shape changes over time.

Therefore, in order to precisely control the crater end shape by adjusting the support roll interval, it is necessary to grasp the crater end shape online and control it by feedback while constantly observing it.
The inventor pays attention to the solidification completion position detection method of the continuous cast slab that can accurately measure the crater end shape, and by using this method and controlling the support roll interval based on the information that can be obtained by this method, We obtained knowledge that the end shape can be precisely controlled.
The inventor has also found that there is an optimum range for the total expansion amount of the support roll interval according to the detected “shortest solidification completion position” and “longest solidification completion position” of the crater end.
The present invention has been made based on these findings, and specifically has the following configuration.

(1) A method for producing a continuous cast slab according to the present invention is a method for producing a continuous cast slab using a continuous casting machine having a light reduction belt for lightly reducing the slab,
Obtain solidification completion position information using a solidification completion position detector that can detect the solidification completion position of the slab online, and based on the obtained solidification completion position information, the shortest solidification completion position (CE _u ) and the longest solidification completion While adjusting the unsolidified thickness in the slab by widening the interval between the support rolls installed upstream of the light pressure lower belt so that the difference from the position (CE _d ) falls within the preset reference range A light reduction is applied in the light reduction zone.

  In this specification, the solidification completion position closest to the molten steel surface in the mold is referred to as the “shortest solidification completion position”, and conversely, the solidification completion position farthest from the molten steel surface in the mold is referred to as the “longest solidification completion position”. "

(2) Further, in the above described (1), the total expansion amount d _IB (mm) of the interval between the support rolls installed upstream of the light pressure lowering zone is within the following range. .
0.15d _L ≦ d _IB ≦ d _L (1)
d _L = D _u (1-√ (CE _u / CE _d )) (2)
However,
D _u : slab thickness (mm) at the detected shortest solidification completion position
CE _u : Distance from the molten steel surface in the mold at the shortest solidification completion position detected (m)
CE _d : Distance from the molten steel surface in the mold at the longest solidification completion position detected (m)

(3) Moreover, in the above-described (1) or (2), the solidification completion position detecting device transmits a transverse wave ultrasonic wave to the continuous cast slab and receives the transmitted transverse wave ultrasonic wave. Longitudinal wave ultrasonic waves were transmitted to and transmitted to a continuous cast slab installed at the same position in the slab width direction as the position where the ultrasonic wave sensor and the transverse wave ultrasonic sensor are disposed. A longitudinal wave ultrasonic sensor that receives longitudinal wave ultrasonic waves, and a solidification completion position calculation unit that obtains a solidification completion position of the slab using a calculation formula based on a reception signal received by the longitudinal wave ultrasonic sensor, The solidification completion position calculation unit is calculated by the above calculation formula when it is confirmed that the arrangement position of the transverse wave ultrasonic sensor matches the solidification completion position of the slab by the intensity change of the received signal of the transverse wave ultrasonic sensor. Coagulation Ryo such position matches the position of the transverse ultrasonic wave sensor, and is characterized in that it has a function of calibrating the calculation formula.

In the present invention, 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 at the solidification completion position. In slab continuous casting machines, the solidification completion position may differ in the width direction of the slab, so the solidification completion position detected by the transverse wave ultrasonic sensor and the longitudinal wave ultrasonic sensor is the same, or the solidification completion position Even if there is a change in the casting direction, 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 in the slab width direction at the solidification completion position can be regarded as flat, it may be several hundred mm away, and conversely, the shape in the slab width direction at the solidification completion position is greatly changed. In some cases, it is necessary to make it 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. Moreover, the same position of the continuous casting machine means that not only the slab width direction is the same position but also the same position in the casting direction. The same position in the casting direction means that the positions of the slab support roll gaps where the sensors are arranged are the same.

(4) A continuous casting machine according to the present invention detects on-line the shortest solidification completion position and the longest solidification completion position in the width direction of the slab in a continuous casting machine having a light reduction belt for lightly reducing the slab. Based on the position information of the solidification completion position detection device and the shortest solidification completion position and the longest solidification completion position, it is determined whether these differences are within a predetermined reference range. Calculation means for calculating the change amount of the support roll interval installed upstream of the light pressure lower belt so that the difference falls within the reference range, and installed upstream of the light pressure lower belt based on the calculation result of the calculation means And a support roll interval control device that adjusts the supported support interval.

(5) In addition, in the above described (4), the change amount of the support roll interval calculated by the calculation means is the total expansion amount d _IB (mm) of the support roll interval installed upstream of the light pressure lower belt. It is calculated on the premise that it is in the following range.
0.15d _L ≦ d _IB ≦ d _L (1)
d _L = D _u (1-√ (CE _u / CE _d )) (2)
However,
D _u : slab thickness (mm) at the detected shortest solidification completion position
CE _u : Distance from the molten steel surface in the mold at the shortest solidification completion position detected (m)
CE _d : Distance from the molten steel surface in the mold at the longest solidification completion position detected (m)

(6) Moreover, the solidification completion position detection device in the above (4) or (5) includes a transverse wave ultrasonic sensor that transmits a transverse wave ultrasonic wave to the continuous cast slab and receives the transmitted transverse wave ultrasonic wave, The longitudinal wave ultrasonic wave transmitted to the continuous cast slab installed at the same position in the width direction of the slab separated from the casting direction or the same position as the position of the transverse wave ultrasonic sensor and the transmitted longitudinal wave ultrasonic wave A longitudinal wave ultrasonic sensor for receiving, and a solidification completion position calculation unit for obtaining a solidification completion position of a slab using a calculation formula based on a reception signal received by the longitudinal wave ultrasonic sensor, the solidification completion position calculation unit Is the solidification completion position calculated by the calculation formula when it is confirmed that the arrangement position of the transverse wave ultrasonic sensor matches the solidification completion position of the slab by the intensity change of the received signal of the transverse wave ultrasonic sensor. Is Yokonami To match the position of the acoustic sensors, and is characterized in that it has a function of calibrating the calculation formula.

According to the present invention, even if the crater end shape fluctuates during casting, it can be corrected, and uniform center segregation in the width direction can always be improved by light reduction, which is extremely industrially useful for improving productivity. It has a beneficial effect.
Further, according to the solidification completion position detecting device according to the present invention, the solidification completion position of the slab is obtained from the propagation time of the longitudinal ultrasonic wave measured by the longitudinal wave ultrasonic sensor when the transverse wave ultrasonic sensor detects the solidification completion position. Since the solidification completion position is calibrated, the solidification completion position of the slab can be accurately determined only from the measured values of the transverse wave ultrasonic sensor and longitudinal wave ultrasonic sensor, without performing laborious calibration work such as hammering into the slab. It is possible to detect well. This makes it possible to accurately grasp the solidification completion position during casting under various casting conditions for all steel types.

  FIG. 1 is an explanatory view for explaining an outline of a continuous casting machine according to an embodiment of the present invention. The continuous casting machine according to the present embodiment includes a mold 1 and a plurality of pairs of slab support rolls 3 arranged to face the lower side of the mold 1. The gap between adjacent slab support rolls 3 in the casting direction includes an air mist spray nozzle or a water spray nozzle that blows cooling water toward the surface of the slab passing through the opposing slab support roll 3. A secondary cooling zone (not shown) is installed.

Further, in a part of the slab support roll 3 installed below the mold 1, the roll interval of the slab support roll 3 is arbitrarily expanded and gradually widened toward the downstream side in the casting direction of the slab 1. A roll group (referred to as a bulging band 4) that can be set as described above is installed.
The roll interval of the roll group in the bulging band 4 is adjusted based on a control signal of a crater end shape controller 15 described later by the roll interval controller 9.

A light pressure lower zone 11 is installed downstream of the secondary cooling zone. The light pressure lower belt 11 is set so that the interval between the opposing rolls of the slab support roll 3 (hereinafter simply referred to as “roll interval”) becomes gradually narrower toward the downstream side in the casting direction. Thus, there is a group of support rolls 102 that can apply a rolling force.
The reason why the lightly lower belt 11 is provided is to improve the center segregation by reducing the flow of the concentrated molten steel based on the solidification shrinkage by reducing the slab at the end of solidification. What is necessary is just to set the gradient of the roll space | interval in this light reduction belt 11 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 segregation is small. On the other hand, when the rolling speed exceeds 1.5 mm / min, the concentrated molten steel is squeezed in the direction opposite to the casting direction, and the slab This is because negative segregation may be generated at the center. Further, the total reduction amount is about 2 to 6 mm.

FIG. 2 is a diagram showing the relationship between the roll interval and the distance from the molten steel surface in the present embodiment configured as described above.
As shown in FIG. 2, in the present embodiment, with respect to the slab thickness at the lower end of the mold, the interval between the support rolls is expanded stepwise from the upstream side from the molten steel surface in the mold to the light pressure zone. (Hereinafter referred to as “intentional bulging”), and after that, the roll interval can be reduced stepwise in the light pressure zone thereafter.

The light pressure lower belt 11 is provided with a solidification completion position detection device 13 for detecting a solidification completion position of a slab passing through the opposed slab support roll 3, and based on a detection signal of the solidification completion position detection device 13. A crater end shape control device 15 for controlling the crater end shape is installed.
Further, a slab cutting machine 17 for cutting the slab that has been solidified into a predetermined length is disposed on the downstream side of the light pressure lower belt 11, and the downstream side thereof is used for transporting the slab that has been cut. A roll 19 is installed.

  In the continuous casting machine configured as described above, the molten steel injected into the mold 1 is cooled from the surface of the mold 1 to form a solid phase portion 21 and cast as a slab 25 having a liquid phase portion 23 therein. It is cooled in the secondary cooling zone while being drawn downstream in the direction, and completely solidifies to the center in the light pressure lower zone 11. The solidified position 27 is a position where the solidified portion is completely solidified. The slab 25 that has been solidified is cut into a predetermined length by the slab cutting machine 17, and is carried out by the transport roll 19 as a slab 25A.

Here, the solidification completion position detection device 13 and the crater end shape control device 15 will be described in detail.
<Coagulation completion position detection device>
The solidification completion position detection device 13 includes a transverse wave ultrasonic sensor composed of a transverse wave ultrasonic transmitter 31 and a transverse wave ultrasonic receiver 33 arranged opposite to each other with a slab interposed therebetween, and a longitudinal wave ultrasonic wave arranged oppositely across the slab. An electrical signal is sent to the slab 25 by giving an electrical signal to the longitudinal wave ultrasonic sensor including the acoustic wave transmitter 35 and the longitudinal wave ultrasonic receiver 37, the transverse wave ultrasonic transmitter 31, and the longitudinal wave ultrasonic transmitter 35. An ultrasonic transmission unit 39 that is an electric circuit for the above, a transverse wave transmission intensity detection unit 41 for processing reception signals received by the transverse wave ultrasonic receiver 33 and the longitudinal wave ultrasonic receiver 37, and a transverse wave transmission intensity detection unit 41, a coagulation completion position arrival detection unit 43 for detecting whether the coagulation completion position has reached the installation position of the transverse wave ultrasonic sensor based on the detection signal 41, and a longitudinal wave propagation time detection unit for detecting the propagation time of the longitudinal wave 45, Longitudinal wave propagation time Coagulation completion position calculating unit 47 for calculating a coagulation completion position based on the detection time of the detection section 45, and a.

The ultrasonic waves transmitted by the transverse wave ultrasonic transmitter 31 and the longitudinal wave ultrasonic transmitter 35 pass through the slab 25, and are received by the transverse wave ultrasonic receiver 33 and the longitudinal wave ultrasonic receiver 37, respectively, to generate an electrical signal. Is converted to
The transverse wave ultrasonic sensor and the longitudinal wave ultrasonic sensor are attached to, for example, a frame movable in the width direction of the slab 25, and the width of the slab 25 is obtained by the transmitter and the receiver moving in synchronization. The solidification completion position 27 at each position in the direction can be detected. In this case, the transverse wave ultrasonic sensor and the longitudinal wave ultrasonic sensor are also configured to move synchronously.

The transverse wave transmission intensity detection unit 41 detects the intensity of the transverse wave ultrasonic signal received by the transverse wave ultrasonic receiver 33. Further, the coagulation completion position arrival detection unit 43 determines that the coagulation completion position 27 of the transverse wave ultrasonic transmitter 31 and the transverse wave ultrasonic receiver 33 is based on a change in the transmission signal of the transverse wave ultrasonic wave detected by the transverse wave transmission intensity detection unit 41. It is determined whether it is upstream or downstream in the casting direction from the arrangement position.
Further, the longitudinal wave propagation time detection unit 45 detects the propagation time of the longitudinal wave ultrasonic wave transmitted through the slab 25 from the reception signal received by the longitudinal wave ultrasonic receiver 37. Further, the coagulation completion position calculation unit 47 calculates and determines the coagulation completion position 27 from the propagation time of the longitudinal wave ultrasonic wave detected by the longitudinal wave propagation time detection unit 45.

The transverse wave transmission intensity detection unit 41, the coagulation completion position arrival detection unit 43, the longitudinal wave propagation time detection unit 45, and the coagulation completion position calculation unit 47 are realized by the CPU executing a program.
Note that an ultrasonic signal amplifier and an A / D converter for taking a waveform into the CPU are required between the transverse wave ultrasonic receiver 33 and the longitudinal wave ultrasonic receiver 37 and the CPU. Is omitted. Further, in the coagulation completion position detection device 13 according to the embodiment of the present invention shown in FIG. 1, the transverse wave ultrasonic transmitter 31 and the longitudinal wave ultrasonic transmitter 35 are configured integrally, and similarly, the transverse wave An ultrasonic receiver 33 and a longitudinal wave ultrasonic receiver 37 are integrally formed.

Here, the operation of the solidification completion position detection device 13 will be described.
First, the operation of the transverse wave transmission intensity detector 41 will be described with reference to FIG. FIG. 3 is a diagram showing the operation of the transverse wave transmission intensity detection unit 41, and shows a received 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 33, and the second wave is a transmission signal of the transverse wave ultrasonic wave.

Since the time position at which the transmission signal of the transverse wave ultrasonic wave appears is roughly known from the thickness of the slab 25, the approximate temperature of the slab 25, and the propagation speed of the transverse wave ultrasonic wave, a gate for extracting only the signal at that position. 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 coagulation completion position arrival detection unit 43 will be described with reference to FIG. FIG. 4 is a diagram illustrating an example of the operation of the solidification completion position arrival detection unit 43. The transverse wave ultrasonic wave transmitted from the transverse wave transmission intensity detection unit 41 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 transverse wave ultrasonic wave does not pass through the liquid phase. Therefore, the coagulation completion position 27 is at the transverse wave ultrasonic transmitter 31 and the transverse wave ultrasonic wave reception. The state which exists in the downstream of a casting direction rather than the arrangement position of the container 33 is represented.
The coagulation completion position arrival detection unit 43 determines that the coagulation completion position 27 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 where a transmission signal of a transverse wave ultrasonic wave does not appear. . When the coagulation completion position arrival detection unit 43 determines that the coagulation completion position 27 has passed the arrangement position of the transverse wave ultrasonic sensor in this way, the coagulation completion position arrival detection unit 43 sends a timing signal to the coagulation completion position calculation unit 47.

  Next, the operation of the longitudinal wave propagation time detection unit 45 will be described with reference to FIG. FIG. 5 is a diagram illustrating the operation of the longitudinal wave propagation time detection unit 45, 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 receiver 37 electrically, and the second wave is a transmission signal of the longitudinal wave ultrasonic wave.

The longitudinal wave propagation time detection unit 45 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 detection unit 41, this processing can be easily realized by calculation processing by taking the waveform of the received signal into the computer by A / D conversion.
In practice, the transmission signal is repeated with a period of several tens of Hz to several hundreds of Hz, so that each waveform is averaged and then the propagation time of longitudinal ultrasonic waves is obtained, and the propagation of each waveform is performed. It is effective to reduce the influence of fluctuation due to noise by averaging time.

  Finally, the operation of the coagulation completion position calculation unit 47 will be described with reference to FIG. FIG. 6 is a diagram illustrating the operation of the coagulation completion position calculation unit 47 in the first embodiment, and illustrates an approximate expression for calculating the coagulation completion position 27 from the propagation time of longitudinal wave ultrasonic waves. Since the thickness of the liquid phase portion 23 at the arrangement position increases as the solidification completion position 27 becomes more downstream in the casting direction than the arrangement position of the longitudinal wave ultrasonic transmitter 35 and the longitudinal wave ultrasonic receiver 37, The propagation time at becomes longer.

Therefore, the propagation time and the distance from the molten steel surface 49 (see FIG. 1) in the mold to the solidification completion position 27 are approximately proportional to each other, and the relationship shown in FIG. 6 is shown. Therefore, in order to obtain the solidification completion position 27 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)
CE: distance from molten steel surface 49 in mold to solidification completion position 27
Δt: Propagation time of longitudinal ultrasonic wave
a1, a0: Polynomial coefficients

In FIG. 6, the line indicated by A represents an approximate expression before distance calibration from the molten steel surface 49 in the mold to the solidification completion position 27 based on longitudinal wave ultrasonic waves.
However, since the propagation speed of longitudinal ultrasonic waves varies 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 a timing signal for determining the passage of the coagulation completion position 27 is sent from the coagulation completion position arrival detection unit 43 to the coagulation completion position calculation unit 47, the coagulation completion position calculation unit 47 transmits the longitudinal wave propagation time ( Δt1) is obtained, and further, the following equation (2) is used so that the distance (CE) from the molten steel surface 49 in the mold to the solidification completion position 27 matches the arrangement position of the transverse ultrasonic sensor ( 1) Modify the coefficient (a0) in the equation.
a0 = CE1-a1 · Δt1 (2)
However, CE1: Distance from the molten steel surface 49 in the mold to the position where the transverse wave ultrasonic sensor is placed
Δt1: It is determined that the coagulation completion position 27 has passed the arrangement position of the transverse wave ultrasonic sensor
Propagation time of longitudinal ultrasonic wave at the time

As a result, the approximate expression for obtaining the solidification completion position 27 is calibrated, for example, after the calibration indicated by B in FIG. After the calibration, the solidification completion position 27 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 27 crosses the position of the transverse ultrasonic sensor during continuous casting operation, or at the discretion of the operator. Any suitable time may be used.

Next, the crater end shape control device 15 will be described.
<Crater end shape control device>
The crater end shape control device 15 includes a crater end shape calculation unit 51 that calculates a crater end shape based on the calculation result of the solidification completion position calculation unit 47 in the solidification completion position detection device 13, and the calculated crater end shape is preset. A shape determination unit 53 that determines whether the shape is within the range of the shape, and a roll interval calculation unit 55 that calculates the roll interval in the bulging band 4 based on the determination result of the shape determination unit 53.
Based on the solidification completion position information at each position in the slab width direction calculated by the solidification completion position calculation unit 47 and the slab width direction position information from which the crater end shape calculation unit 51 is calculated, The shape of the crater end over the entire width is calculated.

The shape determination unit 53 determines whether the crater end shape peak-and-valley calculated by the crater end shape calculation unit 51 is within a preset reference range.
The threshold for determining whether or not the crater end shape peak-to-valley difference is within a preset reference range is basically determined by accumulating results as shown below.
FIG. 7 is a graph showing the relationship between the crater end-shaped ridge-and-valley difference and the carbon segregation degree (C / C0) at the longest solidification completion position investigated by the present inventors using the actual machine. (M) is shown, and the vertical axis shows the degree of segregation (C / C0). As shown in FIG. 7, the degree of segregation (C / C0) worsens as the crater end shape peak-to-valley difference increases. Therefore, the threshold for determining how much the difference between the peaks and valleys of the crater end shape should be reduced, that is, whether the crater end shape is flat, may be determined according to the required level for the center segregation of the product.

The roll interval calculation unit 55 includes a roll interval in the bulging band 4 in order to allow the crater end shape peak-to-valley difference to enter the reference range when the crater end shape peak-to-valley difference exceeds a preset reference range by the shape determination unit 53. And the calculated result is output to the roll interval control device 9.
The roll interval may be set in advance based on experience.

Here, the reason why the crater end-shaped peak-valley difference can be eliminated by the incremental bulging before light pressure reduction will be described.
Usually, when the slab 1 is cooled in a state where the spray nozzle is not clogged, the shortest solidification completion position exists at the center of the slab width, and the longest solidification completion position has a slab thickness of 220 to 250 mm. In the case of a grade, it exists in the position about 200-300 mm away from the slab short side surface. In that case, the thickness of the unsolidified molten steel in the cross section before the completion of solidification has a distribution as shown in FIG. 8A, that is, a thin state at the center of the width and a thick state at a position 200 to 300 mm away from the short side. By applying intentional bulging, as shown in FIG. 8 (b), the center of the width is intentionally bulged, the thickness of the unsolidified molten steel at the center of the width is increased, and the thickness of the unsolidified molten steel in the width direction is made uniform. By doing so, the width direction of the crater end shape is made uniform.

In addition, when the roll interval total expansion amount from the slab thickness at the lower end of the mold in the bulging band 4 to a constant thickness is d _IB , the roll interval total expansion amount d _IB differs in the width direction in the light pressure lower belt 11. An amount that compensates for the difference (d _L ) in the thickness of the unsolidified molten steel is preferable.
In this regard, in the present embodiment, the solidification completion position detection device 13 is used to detect the solidification completion position of the continuous cast slab in the width direction, and the longest solidification completion position with the slowest solidification progress and the fastest solidification progress. Since the shortest solidification completion position can be detected, the difference (d _L ) in unsolidified molten steel thickness can be predicted as follows.

In general, it is known that the solidified shell thickness S grows as the following equation.
S = k√t (5)
However, S: Shell thickness (mm)
k: Coagulation constant (mm / min ^0.5 )
t: Solidification time (min)

The distance from the molten steel surface in the mold at the shortest solidification completion position detected is CE _u (m), and the distance from the molten steel surface in the mold at the longest solidification completion position detected is CE _d (m). The solidification constants k _u and k _d at the position and the longest solidification position are as follows.
k _u = D _u / 2 √ (Vc / CE _u ) (6)
k _d = D _d / 2 √ (Vc / CE _d ) (7)
Where D _u is the slab thickness at the shortest solidification completion position (mm)
D _d: Slab thickness at the longest solidification completion position (mm)
Vc: Casting speed (m / min)

Unsolidified thickness at distance CE _u from the molten steel in the mold melt surface is the shortest clotting completion position in the slab width direction position longest solidification completion position is present (d _L) is as follows.
d _L = Du-2 × k _d √ (CEu / Vc) = D _u −2 × D _d / 2 √ (Vc / CE _d ) √ (CE _u / Vc)
= D _u -D _d √ (CE _u / CE _d ) (8)
D _u and D _d are slightly different under light pressure conditions, but can be regarded as almost the same.
d _L = D _u (1-√ (CE _u / CE _d )) (2)

In this manner, the maximum difference in the thickness of the unsolidified molten steel between the shortest solidification completion position and the longest solidification completion width position can be estimated.
Therefore, in this embodiment, in order to compensate for the difference in unsolidified thickness,
0.15d _L ≦ d _IB ≦ d _L (1)
Intensive bulging is applied within the range of.

The reason why 0.15d _L ≦ d _IB was set is that, when 0.15d _L > d _IB , the effect of uniformizing the crater end shape was hardly recognized.
Also, d _IB ≦ d _L is set because the bulging amount becomes excessive under the condition of d _IB > d _L , an internal crack occurs in the slab, and the CE at the center of the thickness shifts to the downstream side. This is because the segregation at the center of the width rather deteriorates.

<Description of method for producing continuous cast slab>
Next, the manufacturing method of the continuous cast slab by the continuous casting machine comprised as mentioned above is demonstrated.
Molten steel is poured into the mold 1 through an immersion nozzle (not shown). The molten steel injected into the mold 1 is cooled by the mold 1 to form a solid phase part 21 and continuously drawn downward as a slab 25 having a liquid phase part 23 inside while being supported by the slab support roll 3. It is. The slab 25 is cooled in the secondary cooling zone while an appropriate amount of light reduction is added by the light reduction zone 11, the thickness of the solid phase portion 21 is increased, and solidification is eventually completed to the center portion.

In the light pressure lower belt 11, the position of the coagulation completion position 27 is detected by the coagulation completion position detection device 13 by the above-described operation. Based on this detection information, the casting speed and the amount of secondary cooling water are adjusted so that both the shortest solidification completion position and the longest solidification completion position are within the range of the light pressure lower belt 11.
Further, the roll interval of the bulging band 4 is adjusted so that the crater end shape becomes flat, that is, the difference between peaks and valleys of the crater end shape falls within a predetermined reference value range.

When the slab 25 is cooled in a state where the spray nozzle is not clogged, the shortest solidification completion position exists at the center of the slab width, and the longest solidification completion position is, for example, when the slab thickness is about 250 mm Is present at a position about 200 to 300 mm away from the short side surface of the slab. For this reason, the crater end shape is often W-shaped as shown in FIG. The specific shape of the W shape is detected and the water amount control in the width direction of the secondary cooling is performed to correct the shape to the U shape as shown in FIG. 9B.
The operation for flattening the crater end shape is specifically as follows.

Based on the information of the solidification completion position 27 in the slab width direction detected by the solidification completion position detection device 13, the crater end shape calculation unit 51 of the crater end shape control device 15 calculates the crater end shape over the entire width of the slab. To do. Then, the shape determination unit 53 determines whether or not the crater end shape peak and valley difference calculated by the crater end shape calculation unit 51 is within a preset reference range.
If it is determined by this determination that the crater end shape peak-to-valley difference is within the reference range, the operation is continued under the same operation conditions.
On the other hand, when the shape determination unit 53 determines that the crater end shape peak-to-valley difference exceeds a preset reference range, the roll interval calculation unit 55 places the crater end shape peak-to-valley difference in the reference range. The roll interval expansion amount in the bulging band 4 is calculated, and the calculation result is output to the roll interval control device 9. At this time, the roll interval total extension amount d _IB is set to 0.15d _L ≦ d _IB ≦ d _L as described above.

The roll interval control device 9 receives the calculation result from the roll interval calculation unit 55 and adjusts the roll interval of the bulging band 4 based on the input value.
The roll interval of the bulging band 4 is changed as described above, and then, after a predetermined time has passed, the same operation is performed to determine whether or not the crater end shape peak-to-valley difference is within the reference range, and the crater end shape is still flat. If it is determined that it is not, the same operation is repeated until it is determined that the crater end shape has become flat.
By repeating such feedback control, the crater end shape becomes a flat shape as shown in FIG. By rolling down the slab having a flat crater end shape with the light reduction belt 11, the center segregation can be reduced uniformly in the width direction, and a high-quality continuous cast slab can be manufactured.

  As described above, according to the present invention, the shape of the solidification completion position of the cast slab 25 to be cast is accurately measured, and based on this accurate measurement value, the difference in the crater end shape is set to the reference range. Since shape control is performed and light reduction is performed on the shape control, central segregation can be reduced uniformly in the width direction, and the quality of the steel material can be maintained at a high level.

Since the effect of the present invention has been confirmed as follows, it will be described.
Composition: C: 0.15 mass% (hereinafter simply referred to as “%”), Si: 0.15%, Mn: 1.0%, P: 0.015%, S: 0.005%, Ti: 0 .01%, sol.Al: 0.03% of medium carbon steel was melted and a cross-sectional shape: width 1950 mm, thickness 220 mm was used, mold powder was added into the mold, Casting was performed at a narrowing gradient of 0.53 mm per meter and a casting speed of 1.5 m / min.

A specimen full width sample for inspection was collected from the cast slab and used for segregation investigation. From the slab, the slab was sampled with a 5 mm diameter drill at a pitch of 100 mm from the half thickness position of the slab, and the carbon was analyzed by a combustion type carbon analyzer, and the segregation degree of carbon (C / C0 )investigated.
In FIG. 10, the width direction crater end distribution in the said test and the width direction distribution of segregation degree (C / C0) are shown collectively. As can be seen from FIG. 10, the solidification completion position is made uniform, and the segregation degree (C / C0) is also made uniform in the width direction.

It is explanatory drawing explaining the outline | summary of the continuous casting machine which concerns on one embodiment of this invention. It is explanatory drawing of the intentional bulging which concerns on one embodiment of this invention. It is explanatory drawing explaining operation | movement of the transverse wave transmission intensity detection part 41 in the coagulation completion position detection apparatus which concerns on one embodiment of this invention. It is explanatory drawing explaining operation | movement of the coagulation completion position arrival detection part 43 in the coagulation completion position detection apparatus which concerns on one embodiment of this invention. It is explanatory drawing explaining operation | movement of the longitudinal wave propagation time detection part 45 in the coagulation completion position detection apparatus which concerns on one embodiment of this invention. It is explanatory drawing explaining operation | movement of the coagulation completion position calculating part 47 in the coagulation completion position detection apparatus which concerns on one embodiment of this invention. It is a figure which shows the relationship between the ridge-and-valley difference of a crater end shape, and the segregation degree (C / C0) of carbon in the longest solidification completion position. It is explanatory drawing of the manufacturing method of the continuous cast slab which concerns on one embodiment of this invention. It is explanatory drawing of the manufacturing method of the continuous cast slab which concerns on one embodiment of this invention. It is explanatory drawing of the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Mold 3 Slab support roll 9 Roll space | interval control apparatus 11 Light pressure lower belt 13 Solidification completion position detection apparatus 15 Crater end shape control apparatus 21 Solid phase part 23 Liquid phase part 25 Cast piece 27 Solidification completion position 31 Transverse wave ultrasonic transmitter 33 Transverse wave ultrasonic wave receiver 35 Longitudinal wave ultrasonic wave transmitter 37 Longitudinal wave ultrasonic wave receiver 39 Ultrasonic wave transmission unit 41 Transverse wave transmission intensity detection unit 43 Coagulation completion position arrival detection unit 45 Longitudinal wave propagation time detection unit 47 Coagulation completion position calculation unit 51 Crater end shape calculation unit 53 Shape determination unit 55 Roll interval calculation unit

Claims (6)

A method for producing a continuous cast slab using a continuous casting machine having a light reduction belt for lightly reducing the slab,
Obtain solidification completion position information using a solidification completion position detector that can detect the solidification completion position of the slab online, and based on the obtained solidification completion position information, the shortest solidification completion position (CE _u ) and the longest solidification completion While adjusting the unsolidified thickness in the slab by widening the interval between the support rolls installed upstream of the light pressure lower belt so that the difference from the position (CE _d ) falls within the preset reference range A method for producing a continuous cast slab characterized by applying light reduction in a light reduction zone. 2. The method for producing a continuous cast slab according to claim 1, wherein a total expansion amount d _IB (mm) of a space between support rolls installed upstream of the light pressure lower zone is in the following range.
0.15d _L ≦ d _IB ≦ d _L (1)
d _L = D _u (1-√ (CE _u / CE _d )) (2)
However,
D _u : slab thickness (mm) at the detected shortest solidification completion position
CE _u : Distance from the molten steel surface in the mold at the shortest solidification completion position detected (m)
CE _d : Distance from the molten steel surface in the mold at the longest solidification completion position detected (m) The solidification completion position detecting device includes a transverse wave ultrasonic sensor that transmits a transverse wave ultrasonic wave to the continuous cast slab and receives the transmitted transverse wave ultrasonic wave, and the same position as the arrangement position of the transverse wave ultrasonic sensor or in the casting direction. A longitudinal wave ultrasonic sensor installed at the same position in the width direction of the separated slab and transmitting longitudinal wave ultrasonic waves to the continuous cast slab and receiving the transmitted longitudinal wave ultrasonic waves, and the longitudinal wave ultrasonic waves A solidification completion position calculation unit that obtains the solidification completion position of the slab using a calculation formula based on a reception signal received by the sensor, the solidification completion position calculation unit according to a change in intensity of the reception signal of the transverse wave ultrasonic sensor When it is confirmed that the arrangement position of the transverse wave ultrasonic sensor coincides with the solidification completion position of the slab, the solidification completion position calculated by the above calculation formula matches the arrangement position of the transverse wave ultrasonic sensor. Method for producing a continuous casting slab according to claim 1 or 2, characterized in that it has a function of calibrating the calculation formula. In a continuous casting machine having a light reduction belt for lightly reducing a slab, a solidification completion position detecting device for detecting the shortest solidification completion position and the longest solidification completion position in the width direction of the slab online, and the shortest solidification completion position And whether or not these differences are within a predetermined reference range based on each position information of the longest solidification completion position, and if not, the light pressure is reduced so that the difference is within the reference range. A calculation means for calculating a change amount of the support roll interval installed upstream of the belt, and a support roll interval for adjusting the support roll interval installed upstream of the light pressure lowering belt based on the calculation result of the calculation means And a continuous casting machine. The change amount of the support roll interval calculated by the calculation means is calculated on the assumption that the total extension amount d _IB (mm) of the support roll interval installed upstream of the light pressure lowering zone is in the following range. The continuous casting machine according to claim 4.
0.15d _L ≦ d _IB ≦ d _L (1)
d _L = D _u (1-√ (CE _u / CE _d )) (2)
However,
D _u : slab thickness (mm) at the detected shortest solidification completion position
CE _u : Distance from the molten steel surface in the mold at the shortest solidification completion position detected (m)
CE _d : Distance from the molten steel surface in the mold at the longest solidification completion position detected (m) The solidification completion position detecting device includes a transverse wave ultrasonic sensor that transmits a transverse wave ultrasonic wave to the continuous cast slab and receives the transmitted transverse wave ultrasonic wave, and the same position as the arrangement position of the transverse wave ultrasonic sensor or in the casting direction. A longitudinal wave ultrasonic sensor installed at the same position in the width direction of the separated slab and transmitting longitudinal wave ultrasonic waves to the continuous cast slab and receiving the transmitted longitudinal wave ultrasonic waves, and the longitudinal wave ultrasonic waves A solidification completion position calculation unit that obtains the solidification completion position of the slab using a calculation formula based on a reception signal received by the sensor, the solidification completion position calculation unit according to a change in intensity of the reception signal of the transverse wave ultrasonic sensor When it is confirmed that the arrangement position of the transverse wave ultrasonic sensor coincides with the solidification completion position of the slab, the solidification completion position calculated by the above calculation formula matches the arrangement position of the transverse wave ultrasonic sensor. Continuous casting machine according to claim 4 or 5, characterized in that it has a function of calibrating the calculation formula.

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