JP2011224583A - Method for determining centerline segregation of continuously cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for determining the centerline segregation of a continuously cast slab, by which when a steel slab at a terminal period of solidification is cast while being subjected to soft reduction, the degree of the centerline segregation of the slab is correctly determined on line by obtaining a rolling reduction actually applied to the slab even when a casting condition changes.SOLUTION: In the method for determining centerline segregation, when continuous casting is performed while the slab is subjected to reduction in such a manner that a solid phase rate at least in the center part of the thickness of the slab reaches from the point of ≤0.4 to the point of ≥0.7 by using a continuous casting machine provided with a soft reduction zone and composed of a plurality of rolling reduction rolls, required time (Te) for which the solid phase rate in the center part of the thickness of the slab reaches from the preset fsl to fsh is obtained by calculation, further a rolling reduction (De) at which the solid phase in the center part of the thickness of the slab reaches from the fsl to the fsh is actually measured, the effective rolling reduction rate (Re) shown by the following formula (1) is obtained from the obtained required time (Te) and the rolling reduction (De) per cross section of the slab, and degree of the centerline segregation is determined online based on the effective rolling reduction rate (Re): Re=De/Te...(1).

Description

  The present invention occurs at the center of a cast slab when a steel slab at the end of solidification having an unsolidified layer is gradually reduced and cast by a slab support roll at a reduction rate of about the solidification shrinkage. The present invention relates to a method for determining online the degree of central segregation during casting.

  In the solidification process of steel, solute elements such as carbon, phosphorus and sulfur are concentrated on the unsolidified liquid phase side by redistribution during solidification. This is the microsegregation formed between dendrite trees. If a void is formed in the center of the slab or negative pressure occurs due to solidification shrinkage of the slab being cast by the continuous casting machine or bulging of the solidified shell generated between the rolls of the continuous casting machine, this part Although a sufficient amount of molten steel does not exist in the unsolidified layer at the end of solidification, the molten steel concentrated by the microsegregation flows and accumulates in the center of the slab and solidifies. In the segregation spot formed in this way, the concentration of the solute element is much higher than the initial concentration of the molten steel. This is generally called macrosegregation, and is called central segregation because of its existence site.

  Central segregation degrades the quality of steel products. 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. Further, in deep drawn materials used for beverage can products, anisotropy may appear in workability due to segregation of components. Therefore, many measures for reducing the center segregation of the slab have been proposed from the continuous casting process to the rolling process.

  Among them, as a means for effectively and inexpensively reducing the center segregation of the slab, in the continuous casting machine, the slab at the end of solidification having an unsolidified layer is reduced by a slab support roll at a reduction speed of about the solidification shrinkage. A method of casting while gradually reducing (hereinafter referred to as “light reduction”) has been proposed (see, for example, Patent Document 1).

  This light reduction technology uses multiple pairs of rolls aligned in the casting direction and gradually reduces the volume of the unsolidified layer by gradually reducing the slab at a reduction speed commensurate with the amount of solidification shrinkage. Or it is the technique of preventing the center segregation of a slab by preventing the flow of the concentrated molten steel formed between dendrite trees simultaneously by preventing formation of a negative pressure part. Therefore, in the light reduction technology, generally, a specific range of the solid phase ratio at the center of the slab is controlled within the range of the light reduction zone. Here, the light reduction belt is a group of a plurality of pairs of rolls that imparts a reduction in an amount corresponding to the amount of solidification shrinkage to the slab.

  By the way, the solidification completion position of the slab slab, that is, the transition of the solid fraction in the casting direction at the center of the slab, changes every moment depending on the casting conditions. It is known that not only the casting speed and secondary cooling conditions, but also changes in the flow of molten steel due to the clogging of the immersion nozzle, the clogging state of the cooling spray, and changes depending on the time.

  When the solidification completion position changes, the light reduction amount in the light reduction zone may change. For example, solidification was completed over the entire thickness at the center of the slab width due to a sudden decrease in casting speed or fluctuations in the temperature of the secondary cooling water, or the solidified shell thickness on the slab short piece side increased. In some cases, since the deformation resistance against the reduction of these parts increases, it may not be possible to apply a required reduction force to other parts having an unsolidified layer. This is because, generally, the light reduction device of a continuous casting machine does not have a load resistance enough to roll a completely solidified slab due to the limitation of equipment space and equipment cost, and the width other than the short side of the slab. This is because, when a part of the direction is completely solidified or the like, and the load due to the reduction becomes more than the withstand load, the structure is such that the reduction roll is released by a disc spring or hydraulic setting for protection of equipment.

  In such a case, substantially no light reduction is performed, and the center segregation of the slab deteriorates. Further, the center segregation of the cast slab is not necessarily constant in the casting direction even when light reduction is performed, and it may occur that the target level is deviated from the site where the casting conditions are greatly changed.

  Conventionally, in order to determine the degree of center segregation of a slab, a sample for inspection is taken from the slab or rolled steel, and subjected to a macro structure test, a component analysis test, a Charpy test, etc. I was inspecting and judging the degree. These test methods are off-line inspection methods and have the drawback of being delayed in feedback, and there has been a demand for a method for determining the degree of center segregation of a slab on-line even in light pressure casting.

  In Patent Document 2, in the continuous casting method of molten metal that is drawn out while reducing the slab with one or more pairs of rolls, whether the slab speed is a segregation worsening slab caused by a reduction in the casting speed or not. Although a continuous casting method is disclosed in which a slab heating condition corresponding to the segregation level is selected by determining the solidification time of the slab while decelerating, Patent Document 2 determines center segregation only by the casting speed. Although it is an on-line determination, the accuracy is low, and eventually it is necessary to confirm the degree of central segregation in a slab macro structure test, component analysis test, and the like.

JP 49-121738 A JP-A-5-220556

  The present invention has been made in view of the above circumstances, and the object of the present invention is to provide a reduction that is actually applied to a slab when casting a steel slab at the end of solidification having an unsolidified layer while lightly reducing it. To provide a method for determining the center segregation of a continuous cast slab, which can accurately determine the degree of center segregation of a slab on-line even if the casting conditions change by grasping the amount. It is.

The center segregation determination method for a continuous cast slab according to the first invention for solving the above-described problem is a method of determining a light reduction belt comprising a plurality of reduction rolls capable of applying a reduction force to a slab during casting. Using the continuous casting machine provided, at least from the time when the solid phase ratio of the thickness center portion of the slab becomes 0.4 or more to the time when it becomes 0.7 or more, the steel while rolling down the slab at the end of solidification with the reduction roll In continuous casting of the slab, the required time (Te) in the light pressure zone from the solid state ratio of the center of the slab to the preset fsl to fsh is calculated, and the center of the slab thickness is calculated. Measure the reduction amount (De) in the light reduction zone from the solid phase ratio of fsl to fsh, and calculate the following (from the required time (Te) obtained by calculation and the reduction amount (De) obtained by measurement: 1) The effective reduction speed (Re) shown in the equation is the cross section of the slab. To seek, and judging on-line during casting the degree of center segregation of the slab on the basis of the effective pressure rate (Re) determined.
Re = De / Te (1)
However, in the formula (1), Re is an effective reduction speed (mm / min), De is a light reduction zone where the solid phase ratio at the center of the thickness of the slab obtained from measurement is fsl to fsh. The amount of reduction (mm) and Te in are the required time (min) in the light reduction zone until the solid phase ratio at the center of the thickness of the slab reaches from fsl to fsh. Further, fsl is the solid phase ratio at the center portion of the slab thickness at the upstream position between two points in the casting direction in the light reduction zone for obtaining the effective reduction speed, and fsh is in the light reduction zone for obtaining the effective reduction speed. It is a solid phase rate at the center portion of the slab thickness at a downstream position between two points in the casting direction.

  In the center segregation determination method of the continuous cast slab according to the second invention, in the first invention, the fsl exceeds 0 and is 0.4 or less, and the fsh is 0.7 or more and less than 1.0. It is characterized by that.

  The center segregation determination method of the continuous cast slab according to the third invention is the information on the solidification completion position using the solidification completion position detection device capable of detecting the solidification completion position of the slab online in the first or second invention. And calibrate the calculation formula to obtain the required time (Te) until the solid phase ratio at the thickness center of the slab reaches fsh to fsh so that it matches the acquired solidification completion position information It is characterized by.

  According to the present invention, when the continuous cast slab is manufactured, the center segregation degree of the slab is determined online based on the actual slab reduction condition due to light reduction. Even if the rolling condition is changed and the center segregation of the slab is deteriorated, it is possible to reliably and accurately determine the center segregation without missing a portion where the center segregation is deteriorated. In addition, it is possible to specify the inspection position in the slab and the steel material rolled from the slab based on the determination result, the number of inspections can be greatly reduced, and the inspection cost is greatly reduced. Supply of high quality steel products is achieved, which has an industrially beneficial effect.

It is a figure which shows the relationship between the effective reduction speed (Re) and the segregation degree of the center segregation of a slab. It is a side surface schematic diagram of the vertical bending type slab continuous casting machine which implemented the present invention. It is a figure which shows the fluctuation | variation of the slab casting length direction of the effective reduction speed (Re) calculated | required online.

  Hereinafter, the present invention will be specifically described. First, the background to the present invention will be described.

  In order to investigate the relationship between the light reduction condition and the degree of center segregation, the present inventors used a vertical bending slab continuous casting machine having a light reduction zone with a casting direction length of 14 m and cast in the light reduction zone. Tests were performed with various changes in the rolling conditions of the slabs, and samples were taken from the resulting slabs to investigate the degree of center segregation of the slabs. In addition, the light reduction belt means that the interval between the slab support rolls facing each other across the slab (referred to as “roll interval”) is gradually narrowed toward the downstream in the casting direction in order to apply a reduction force to the slab. The slab support roll group set as described above, and the state of the roll interval set so as to become narrower sequentially toward the downstream in the casting direction is referred to as “roll gradient” or “rolling gradient”. The roll gradient is usually displayed as a roll interval narrowing amount (mm / m) per 1 m.

  As a result, the center segregation is generated by the discharge of concentrated segregated molten steel between dendritic trees to the solidification front due to the flow of unsolidified molten steel accompanying solidification shrinkage in the unsolidified region at the end of solidification. In the range of fsl to fsh, where the solidification state of the slab center in a range that can affect the center segregation of the slab, that is, the solid phase ratio of the center of the slab thickness can affect the center segregation, It was found that the center segregation can be evaluated by obtaining the rolling speed at the light rolling zone actually applied to the slab.

  Therefore, when continuously casting a slab slab having a thickness of 250 mm and a width of 1950 mm at a casting speed of 1.4 m / min while adding mold powder into the mold, the slab slab is within a range of 15 to 29 m from the molten steel surface in the mold. Adjust the roll arrangement of the light reduction belt for each experiment so that the reduction speed of the slab is 0.7 to 1.4 mm / min in the light reduction belt consisting of multiple segments installed at a distant position. A slab was cast.

  The specific water amount of the secondary cooling water is 1.48 L / kg, the superheat degree of the molten steel in the tundish is adjusted to 37 to 39 ° C., and the chemical component is C: 0.05 mass% (hereinafter referred to as “%”). ), Si: 0.3%, Mn: 1.3%, P: 0.005%, S: 0.005%, Ti: 0.01%, sol. Al: 0.04%, Nb: 0.00. A molten steel of 04%, Cu; 0.15% was cast. The specific water amount is a numerical value representing the cooling water amount (liter) per 1 kg of cast slab.

  At that time, in order to measure the actual amount of light reduction in the segments constituting the light pressure lower belt, differential transformers are installed at the upper and lower corners of the segment on the upper surface side of each segment. The displacement of the segment under light pressure was measured with a differential transformer (differential transformers arranged at four corners are called “roll interval measuring device”). As described above, this is because the surface temperature of the entire slab decreases compared to the steady casting region at the initial or final stage of the unsteady casting region or when the casting speed is suddenly reduced. By increasing the thickness of the solidified shell on the short side, the deformation resistance of the entire slab increases, a load that exceeds the load resistance of the segment is applied, and the segment itself moves open to make the roll interval larger than the set value, This is because the displacement of the segment itself is observed. In addition, the segment of the tested light pressure lower belt is configured such that the lower surface side segment is fixed and the upper surface side segment is opened by a load.

The sampling position of the center segregation investigation sample from the slab is the fluctuation of the casting speed, the fluctuation of the secondary cooling water, the fluctuation of the slab surface temperature, the fluctuation of the molten metal surface in the mold until the portion injected into the mold is solidified. The site where no change in casting conditions such as the above was observed, and the site where the set amount of reduction was applied from the segment displacement measurement results were carefully selected to eliminate as much as possible the disturbance factors affecting the center segregation. The segregation degree of the center segregation of the slab is defined as 1 in the width direction of the slab, with the carbon analysis value at the position of the slab thickness direction 1/4 (= slab thickness / 4) as the reference value (C _O ) without segregation. / 2 (= slab width / 2) position and 1/4 (= slab width / 4) position, cut out a longitudinal section sample in the casting direction, slice from this section in the slab thickness direction by 1 mm and analyze sample The ratio (C _i / C _O ) between the highest carbon analysis value (C _i ) and the reference value (C _O ) in this analytical sample was evaluated as the degree of segregation. The closer the segregation degree is to 1.0, the smaller the segregation is. The segregation degree is larger than 1.0 and the positive segregation is less than 1.0.

  In consideration of the segment displacement during light reduction, the actual reduction speed of the slab (referred to as “effective reduction speed”) was determined as follows.

  First, in the light reduction zone, the solid fraction of the center of the slab thickness at the upstream position between the two casting directions in the light reduction zone for obtaining the effective reduction speed is fsl, and the thickness center of the slab at the downstream position. The solid phase ratio of the part is fsh, and the distance from the molten metal surface in the mold at the position where the solid phase ratio at the center of the slab thickness is arbitrary fsl and the distance from the molten metal surface in the mold at the position where arbitrary fsh is transmitted. Obtained by thermal solidification calculation. The position at which the central solid fraction is fsl and the position at which fsh are determined are the temperature of the slab center determined by heat transfer solidification calculation considering the casting conditions such as casting speed and secondary cooling conditions, and the liquid phase due to the molten steel components. It can be determined by comparing the line and the solid phase line. Here, the most downstream position corresponds to the solidification start position at the center of the slab at the position where the solid phase ratio at the center of the slab becomes zero, and the solid ratio at the center of the thickness of the slab is 1. The position on the most upstream side corresponding to the position corresponding to 0 corresponds to the solidification completion position. Therefore, although the central solid phase ratios fsl and fsh are arbitrary, it is inevitably necessary to satisfy the condition of 0 <fsl <fsh <1.0. By determining the position where the solid phase ratio at the center of the slab becomes fsl and the position where it becomes fsh, the distance in the casting direction of both is determined, and the time required to move this distance is determined from the casting speed. In the present invention, the time required to move this distance is defined as the required time (Te). The unit of the required time (Te) is “min”.

  Next, the actual roll interval narrowing amount of the reduction roll in the light reduction zone from the position where the solid phase ratio at the thickness center of the slab becomes fsl to the position where it becomes fsh is obtained by the roll interval measuring device. That is, the roll interval of the reduction roll at the position where the solid phase ratio at the thickness center portion of the slab is fsl and the roll interval of the reduction roll at the position where the solid phase ratio of the thickness center portion of the slab is fsh. The difference is the amount of narrowing the roll interval between these two points. This difference is the actual reduction amount from the position where the solid phase ratio at the thickness center of the slab becomes fsl to the position where the solid phase ratio becomes fsh. In the present invention, the actually measured reduction amount is defined as a reduction amount (De). The unit of the reduction amount (De) is mm. In this case, it is assumed that the segment on the upper surface side itself is not deformed, and the roll interval is obtained geometrically from the measured value by the roll interval measuring device.

And the value shown to the following (1) formula which divided | segmented the amount of reduction (De) calculated | required by measurement by the required time (Te) calculated | required was defined as the effective reduction speed (Re). The unit of the effective rolling speed (Re) is “mm / min”.
Re = De / Te (1)
The effective reduction speed (Re) corresponds to the reduction amount actually applied to the slab.

FIG. 1 shows the relationship between the effective reduction speed (Re) and the segregation degree of the slab center segregation when the solid phase ratio is fsl = 0.3 and fsh = 0.7. As shown in FIG. 1, there is a good correlation between the effective rolling speed (Re) derived above and the segregation degree. When the effective rolling speed (Re) is about 0.9 mm / min or more, the segregation degree ( C _i / C _O ) is in the range of 0.9 to 1.2, and it has been found that center segregation can be effectively reduced.

  In order to efficiently reduce the center segregation by lightly reducing the slab, the reduction roll is at least from the time when the solid phase ratio in the central part of the thickness of the slab is 0.4 or less to 0.7 or more. It is necessary to reduce the slab at the end of solidification. 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, that is, center segregation may occur. On the other hand, the flow of the concentrated molten steel may occur until the solid phase ratio reaches about 0.7, and the light reduction is stopped earlier than that. If this occurs, the flow of the concentrated molten steel occurs, which causes central segregation, and the effect of light reduction cannot be fully exhibited. In other words, the center segregation of the slab can be efficiently reduced by applying light reduction until the solid phase ratio at the center of the slab thickness is 0.4 or less to 0.7 or more.

  In accordance with this, the solid phase rate fsl is preferably greater than 0 and not greater than 0.4, and the solid phase rate fsh is preferably not less than 0.7 and less than 1.0. By doing so, the reduction speed in the range of the solid phase ratio that requires light reduction to reduce the center segregation is grasped, and the center segregation can be reliably evaluated. Of course, the solid phase ratio fsl may exceed 0.4 and the solid phase ratio fsh may be less than 0.7.

  The present invention has been made on the basis of the above knowledge, and the effective time shown in the above formula (1) from the required time (Te) obtained by calculation and the reduction amount (De) obtained by actual measurement during casting. The reduction speed (Re) is obtained for each cross section of the slab, and the degree of center segregation of the slab is determined online during casting based on the obtained effective reduction speed (Re). If the effective reduction speed (Re) is 0.9 mm / min or more, it is determined that the center segregation is slight.

  The upper limit value of the effective reduction speed (Re) may be 1.5 mm / min. When the rolling speed exceeds 1.5 mm / min, the concentrated molten steel is squeezed in the direction opposite to the casting direction, and negative segregation is generated at the center of the slab, which is not preferable. Since the effective reduction speed (Re) is smaller than the reduction speed determined by the roll slope (mm / m) of the light reduction belt, the upper limit value of the effective reduction speed (Re) is controlled by adjusting the roll slope of the light reduction belt. be able to. The reduction speed (mm / min) can be determined by the product of the roll gradient (mm / m) and the casting speed (m / min) in the light reduction zone.

  Next, a specific implementation method of the present invention will be described with reference to the drawings. FIG. 2 is a schematic side view of a vertical bending slab continuous casting machine embodying the present invention.

  As shown in FIG. 2, the slab continuous casting machine 1 is provided with a mold 5 for injecting and solidifying molten steel to form the outer shell shape of the slab 10, and at a predetermined position above the mold 5. A tundish 2 for relaying and supplying molten steel 9 supplied from a ladle (not shown) to the mold 5 is installed. On the other hand, a plurality of pairs of slab support rolls 6 including a support roll, a guide roll, and a pinch roll are arranged below the mold 5. In the range in which the slab support roll 6 is disposed, a secondary cooling zone in which a spray nozzle such as a water spray nozzle or an air mist spray nozzle is disposed in the gap between the slab support rolls 6 adjacent in the casting direction is configured. The slab 10 is cooled while being drawn out by the secondary cooling water sprayed from the spray nozzle in the secondary cooling zone.

  A sliding nozzle 3 for adjusting the flow rate of the molten steel 9 is installed at the bottom of the tundish 2, and an immersion nozzle 4 is installed on the lower surface of the sliding nozzle 3. A plurality of transport rolls 7 for transporting the cast slab 10 are installed on the downstream side of the slab support roll 6. Above the transport roll 7, the cast slab 10 to be cast is provided. A slab cutting machine 8 for cutting a slab 10a having a predetermined length is disposed.

  On the upstream side and the downstream side in the casting direction across the solidification completion position 13 of the slab 10, the interval between the slab support rolls facing each other across the slab 10, that is, the roll interval is gradually narrowed toward the downstream in the casting direction. A light pressure belt 14 composed of a plurality of pairs of slab support rolls set to be installed is provided. In the light reduction belt 14, it is possible to perform light reduction on the slab 10 in the entire region or a partially selected region. A spray nozzle for cooling the slab 10 is also disposed between the slab support rolls of the light pressure lower belt 14. The slab support roll 6 of the light reduction belt 14 is also referred to as a “reduction roll” because it is a roll for light reduction.

  The light rolling belt 14 shown in FIG. 2 is configured by three rolling roll groups having a segment structure in which three pairs of rolling rolls are set as one set. The upper surface side segment of each segment has an upstream side. Roll interval measuring devices 15 for measuring the roll interval of the rolling roll during casting are installed at a total of four locations on both sides of the end portion and both ends of the downstream end portion. The roll interval measuring device 15 directly measures the position of the end of each segment with a differential transformer, and the measurement value is input to the roll interval measurement calculation unit 20 and the roll interval measurement calculation unit 20 is input. The roll interval of each rolling roll is calculated from the measured value. In FIG. 2, transmission from the roll interval measurement device 15 to the roll interval measurement calculation unit 20 is partially omitted, but all the roll interval measurement devices 15 are connected to the roll interval measurement calculation unit 20.

  In this slab continuous casting machine 1, the lower surface side segment of each segment constituting the light pressure lower belt 14 is fixed to the foundation (frame or foundation) of the continuous casting machine, and the upper surface when the load exceeds the load resistance. The side segment is structured to move, and the roll interval is measured only by measuring the displacement of the upper surface side segment. And since the position of the edge part of each segment is measured, the roll space | interval of each reduction roll of each segment can be known. Although the roll interval measuring device 15 is in contact with the segment for measurement, it is not necessary to perform the measurement in contact with the embodiment of the present invention, and it can be measured in a non-contact manner using a laser beam or the like. I do not care.

  An ultrasonic sensor 16 for detecting a coagulation completion position is installed in the gap between the segments constituting the light pressure lower belt 14, and measurement data from the ultrasonic sensor 16 is input to the coagulation completion position measurement calculation unit 18. Has been. That is, the ultrasonic sensor 16 and the solidification completion position measurement calculation unit 18 constitute a solidification completion position detection device for detecting the solidification completion position 13 of the slab 10. The ultrasonic sensor 16 is a device for transmitting longitudinal wave ultrasonic waves or transverse wave ultrasonic waves and receiving the transmitted longitudinal wave ultrasonic waves or transverse wave ultrasonic waves. A transverse wave ultrasonic signal (pulse signal) is transmitted to the ultrasonic sensor 16, and reception data at the ultrasonic sensor 16 input from the ultrasonic sensor 16 is processed to cast a longitudinal wave or a transverse wave ultrasonic wave. In this device, the solidification completion position 13 is calculated from the transmission time of the piece 10.

  This solidification completion position detection apparatus transmits longitudinal wave ultrasonic waves and / or transverse wave ultrasonic waves to the slab 10 via a pair of opposed ultrasonic sensors 16, and the transmission speed of the longitudinal wave ultrasonic waves and the transverse wave ultrasonic waves is increased. Using the dependence on the temperature of the slab 10, the temperature at the center of the slab is obtained from the transmission time, and the solidification completion position 13 is detected from the obtained temperature at the center of the slab using heat transfer solidification calculation. It is a device to do. Note that the transverse wave ultrasonic wave does not pass through the slab 10 when the unsolidified layer 12 is present inside the slab 10, and therefore solidifies only when the solidification completion position 13 exists upstream of the ultrasonic sensor 16. The completion position 13 can be detected.

  As the ultrasonic sensor 16, a sensor in which a longitudinal wave ultrasonic sensor and a transverse wave ultrasonic sensor are integrally combined is used, and the solidification completion position 13 is obtained from the transmission time of the longitudinal wave ultrasonic wave through the slab 10. It is preferable to use a coagulation completion position detection device (see JP 2005-177860 A) that calibrates the calculation formula based on the detection result of the coagulation completion position 13 by the transverse wave ultrasonic sensor.

  The measurement result of the solidification completion position 13 by the solidification completion position measurement calculation unit 18 is input to the slab center solid phase ratio calculation unit 19, and the slab center part solid phase ratio calculation unit 19 controls the continuous casting machine. Casting conditions (slab thickness, casting speed, amount and temperature of secondary cooling water, superheat of molten steel temperature in the tundish, liquidus temperature, solidus temperature, steel grade, etc. input from the computer (process computer) 17 ) To calculate the solidification state of the slab 10 by heat transfer solidification calculation, so that the calculation result matches the data input from the solidification completion position measurement calculation unit 18. The calculation formula for calculating the solid phase ratio in the direction is calibrated, and the solid phase ratio at the center of the slab thickness is calculated using the corrected calculation formula. The slab center portion solid phase rate calculation unit 19 can calculate the solid phase rate at the center portion of the slab thickness only from the casting conditions input from the control computer 17 for the continuous casting machine. By adding the information on the solidification completion position 13 from the portion 18, it is possible to accurately calculate the solid phase ratio at the center portion of the slab thickness.

  The slab center portion solid phase rate calculation unit 19 transmits the calculated data (referred to as solidification data) to the effective reduction speed calculation unit 21. In addition, the effective reduction speed calculation unit 21 also receives the roll interval (referred to as roll interval data) of each reduction roll obtained by the roll interval measurement calculation unit 20. Based on the input solidification data, the effective reduction speed calculation unit 21 calculates a required time (Te) in the light reduction zone 14 until the solid phase ratio at the center of the thickness of the slab 10 reaches a preset fsl to fsh. And a reduction amount (De) in the light reduction zone 14 until the solid phase ratio at the thickness center portion of the slab 10 reaches from fsl to fsh based on the input roll interval data. Then, from the required time (Te) and the amount of reduction (De), the effective reduction speed (Re) is obtained by the above equation (1). The effective reduction speed calculation unit 21 transmits the obtained effective reduction speed (Re) to the pass / fail determination unit 22. The acceptance / rejection determination unit 22 determines the degree of central segregation for each cross section of the slab from the effective rolling speed (Re) with reference to the grade of steel grade, application, product sheet thickness, and the like. For example, in the case of a high-grade grade, it is determined that the effective segregation speed (Re) is 0.9 mm / min or more and the center segregation is good, and less than 0.9 mm / min.

  Using the slab continuous casting machine 1 configured as described above, the molten steel 9 is continuously cast as follows.

  The molten steel 9 is poured from the ladle into the tundish 2 to retain a predetermined amount of molten steel 9 in the tundish 2, and the molten steel 9 retained in the tundish 2 is poured into the mold 5 through the immersion nozzle 4. The molten steel 9 injected into the mold 5 is cooled by the mold 5 to form a solidified shell 11, the outer shell is the solidified shell 11, and the slab 10 having an unsolidified layer 12 is formed on the slab support roll 6. While being supported, it is continuously pulled out below the mold 5 by a pinch roll. The slab 10 is cooled by the secondary cooling water in the secondary cooling zone while passing through the slab support roll 6, thereby increasing the thickness of the solidified shell 11, and solidification is completed while being lightly reduced in the light pressure lower zone 14. At position 13, solidification to the inside is completed. The slab 10 after solidification is cut by the slab cutting machine 8 to produce a slab 10a. In this case, the heat transfer solidification calculation is performed so that at least the range of the solid phase ratio at the center of the slab from a certain value of 0.4 or less to a certain value of 0.7 or more falls within the installation range of the light pressure lower belt 14. The casting speed is set using a method such as

  During casting, the slab center portion solid phase rate calculation unit 19 calculates the solid phase rate in the casting direction of the slab thickness center portion every few seconds to every tens of seconds, and the calculated data is used as the effective reduction speed calculation unit 21. Send to. Similarly, the roll interval measurement calculation unit 20 transmits the roll interval of each reduction roll to the effective reduction speed calculation unit 21 every few seconds to every several tens of seconds. The effective reduction speed calculation unit 21 calculates the effective reduction speed (Re) by the equation (1) based on data input from the slab center part solid phase ratio calculation unit 19 and the roll interval measurement calculation unit 20. Then, the effective reduction speed calculation unit 21 transmits the obtained effective reduction speed (Re) to the pass / fail determination unit 22.

  The acceptance / rejection determination unit 22 determines acceptance / rejection based on the input effective reduction speed (Re) by comparing with a preset reference, and rejected slabs are changed in destination, center segregation investigation, scrapping treatment Implement measures such as

  As described above, according to the present invention, when producing a continuous cast slab, the degree of center segregation of the slab is determined online based on the actual slab reduction situation due to light reduction. Accordingly, even if the center segregation of the slab changes during casting, it is possible to reliably and accurately determine the center segregation without missing a portion where the center segregation has deteriorated.

  Using a vertical bending type slab continuous casting machine having a light pressure lower belt having a length of 14 m installed in a range of 15 to 29 m from the molten steel surface in the mold, the reduction speed in the light pressure lower belt is 1.2 mm / min. The slab slab was cast by adjusting the roll gradient so that

  Chemical components are C: 0.05%, Si: 0.3%, Mn: 1.3%, P: 0.005%, S: 0.005%, Ti: 0.01%, sol.Al: Molten steel of 0.04%, Nb: 0.04%, Cu; 0.15% was cast into a slab having a width of 1950 mm and a thickness of 250 mm at a casting speed of 1.4 m / min. The degree of superheated molten steel in the tundish was 35 to 48 ° C., and the amount of secondary cooling water was 1.48 L / kg in terms of specific water. In addition, 250 tons of molten steel was continuously cast for 2 charges.

  FIG. 3 shows fluctuations in the cast casting length direction of the effective reduction speed (Re) obtained on-line when the solid fraction fsl is 0.3 and fsh is 0.7. As is clear from FIG. 3, a clear change in effective rolling speed (Re) was observed during casting. The area of A at the beginning of casting is a part caused by the fact that the supercooled slab at the time of casting disclosure passed through the light pressure zone, the load on the segment was overloaded and the roll interval was larger than the set value. there were. In addition, the effective reduction speed (Re) suddenly decreased in the area B near the casting length of 100 m, when the second charge casting was finished, the casting speed was reduced and top treatment was performed (injecting metal into the molten steel surface in the mold). Occasionally, it was present in the light pressure zone, so that the central solid fraction of the cross section increased from 0.3 to 0.7, and no substantial reduction was applied.

  As a result of collecting slab samples from the A region and the B region and investigating the center segregation, the result was consistent with the relationship between the effective rolling speed and the segregation degree shown in FIG.

  As described above, according to the present invention, it is possible to determine the center segregation online without involving an offline segregation investigation, and to greatly reduce the inspection cost, and at the same time, supply a stable quality steel product. Is achieved with industrially beneficial effects.

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Casting piece support roll 7 Conveying roll 8 Cast piece cutting machine 9 Molten steel 10 Cast piece 11 Solidified shell 12 Unsolidified layer 13 Solidification completion position 14 Light pressure lower belt 15 Roll interval measuring device 16 Ultrasonic sensor 17 Control computer for continuous casting machine 18 Solidification completion position measurement calculation unit 19 Slab center solid phase ratio calculation unit 20 Roll interval measurement calculation unit 21 Effective rolling speed calculation unit 22 Pass / fail judgment unit

Claims (3)

Using a continuous casting machine equipped with a light reduction belt composed of a plurality of reduction rolls capable of applying a reduction force to a slab during casting, a solid phase ratio of at least the thickness center of the slab is 0.4 or less From the time fsl to the time when the steel slab is continuously cast while the slab at the end of solidification is being squeezed with the above-described rolling roll from the point of time to 0.7 or more. The required time (Te) in the light pressure zone until fsh is obtained by calculation, and the amount of reduction (De) in the light pressure zone until the solid phase ratio at the center of the slab thickness reaches fsh to fsh is measured. The effective reduction speed (Re) shown in the following equation (1) is obtained for each cross section of the slab from the required time (Te) obtained by calculation and the reduction amount (De) obtained by actual measurement, and the obtained effective reduction is obtained. Casting the degree of center segregation of the slab based on the speed (Re) And judging online, center segregation judgment method of continuous casting slab.
Re = De / Te (1)
However, in the formula (1), Re is an effective reduction speed (mm / min), De is a light reduction zone where the solid phase ratio at the center of the thickness of the slab obtained from measurement is fsl to fsh. The amount of reduction (mm) and Te in are the required time (min) in the light reduction zone until the solid phase ratio at the center of the thickness of the slab reaches from fsl to fsh.   2. The center segregation determination method for a continuous cast slab according to claim 1, wherein the fsl is greater than 0 and less than or equal to 0.4 and the fsh is greater than or equal to 0.7 and less than 1.0.   Obtain solidification completion position information using a solidification completion position detection device that can detect the solidification completion position of the slab online, so that the solid phase at the center of the slab thickness matches the acquired solidification completion position information. The center segregation determination method for a continuous cast slab according to claim 1 or 2, wherein a calculation formula for obtaining a required time (Te) from the rate fsl to fsh is calibrated.

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