JP2008183607A - Continuous casting method of steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method of a steel where, when, after the drawing of a slab is temporarily stopped for exchange of a tundish or immersion nozzle and again, the drawing is started, and consecutive continuous casting is continued, the surface temperature of the slab in the part corresponding to the secondary cooling zone whose drawing is stopped in the machine is controlled to the prescribed one. <P>SOLUTION: The temperature of a slab 12 occurred by the stop in a continuous casting machine 1 is measured on and after the outlet side of a secondary cooling zone 6, the portion having a temperature lower than the previously set objective temperature is specified, the cooling zone 6a to 6i in the secondary cooling zone as the cause of the occurrence of the low temperature portion is specified, and the cooling water quantity in the specified cooling zone is set in such a manner that the difference between the surface temperature of the slab upon the stop in the machine on and after the following time and the previously set objective temperature reaches ≤50°C. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel continuous casting method, and more specifically, when slab drawing is temporarily stopped during casting, and then continuous casting is resumed after tempering or dip nozzle replacement. The present invention relates to a method for performing continuous casting by optimizing the secondary cooling pattern.

  In continuous casting of steel, the molten steel in the ladle is once poured into the tundish, and with a predetermined amount of molten steel staying in the tundish, the molten steel in the tundish is passed through an immersion nozzle installed at the bottom of the tundish. Are injected into each mold. The molten steel injected into the mold is cooled to form a solidified shell on the contact surface with the mold, and this slab with the solidified shell as the outer shell and the unsolidified molten steel inside is a secondary provided below the mold. While being cooled by cooling water (referred to as “secondary cooling water”) in the cooling zone, it is continuously pulled out below the mold, and eventually solidification to the center is completed. The slab that has been solidified to the center is cut into a predetermined length to produce a slab that is a rolling material.

  The cooling of the slab in the secondary cooling zone is one of the factors that greatly affects the quality of the slab. For example, if the cooling strength is too strong, cracks such as vertical cracks and transverse cracks occur on the slab surface, while if the cooling strength is too low, the slab surface temperature becomes too high and the solidified shell The strength decreases, and the bulging of the solidified shell due to the static pressure of the molten steel increases (the slab expands), resulting in internal cracking or deterioration of center segregation due to this bulging.

Therefore, in order to accurately control the cooling of the slab, it is common to divide the secondary cooling zone that extends in the casting direction into a plurality of parts in the casting direction and determine the amount of cooling water for each divided cooling zone. is there. Then, the flow rate (Q) of the cooling water in each cooling zone is adjusted using a function such as Q = aV ² + bV + c (a, b, and c are constants) with the slab drawing speed (V) as a parameter. It is common (see, for example, Patent Document 1). Here, when the drawing speed of the slab is zero, that is, the reason for spraying the cooling water on the slab when the continuous casting machine is stopped in the machine is as follows. That is, in a slab having an unsolidified layer inside, the solidified shell always receives heat from the unsolidified layer. When the cooling water spray is stopped, the temperature of the solidified shell rises and the bulging of the solidified shell increases. This is because operational troubles such as difficulty in redrawing or breakage of the solidified shell due to bulging and breakout occur.

  By the way, normally, in the continuous casting of steel, the slab is continuously drawn, and the case where the slab is stopped in the machine is generally the case of the next operation.

  That is, in continuous casting of steel, in order to increase the productivity of a continuous casting machine, not only continuous casting of several heats (referred to as “continuous casting”) is performed, but the tundish is replaced after continuous casting of several heats. Alternatively, it is common practice to continuously cast several heats by replacing the immersion nozzle disposed on the bottom of the tundish. During the tundish exchange and the immersion nozzle exchange, the molten steel injection into the mold is interrupted and the slab drawing is temporarily stopped. Then, wait for a new tundish or new immersion nozzle to be installed with the slab stopped in the machine, and as soon as a new tundish or new immersion nozzle is installed, more molten steel is poured into the mold, and accordingly The slab drawing is resumed and the speed is increased to the steady drawing speed. In this case, the slab remains stopped in the machine for about 1 minute to several minutes.

Even when the slab stops, the secondary cooling water is sprayed on the slab as described above. However, in a narrow slab, the short side of the slab cools the long side of the slab. Therefore, the corner portion of the slab is often supercooled because it is cooled by the secondary cooling water. In the continuous casting operation of steel, it is common to correct the slab bending and unbending at a higher temperature side than the brittle region of the steel. Enters the brittle region, and when the slab is bent or unbent, it is a type of transverse crack at the slab corner, which is a corner crack from the long side to the short side of the slab. Say) occurs. When a corner crack occurs, the surface of the slab needs to be cleaned, and various problems such as inability to perform hot strip direct feed rolling, a decrease in yield, and an extension of the manufacturing process occur.
Japanese Patent Laid-Open No. 61-238453

  The present invention has been made in view of the above circumstances. The purpose of the present invention is to temporarily stop drawing of a slab and then start drawing again, such as when changing a tundish or changing an immersion nozzle. Provided is a steel continuous casting method capable of controlling the surface temperature of a slab at a portion corresponding to a secondary cooling zone, in which drawing is stopped in the machine when continuous casting is continued, to a predetermined temperature. That is.

  In the continuous casting method of steel according to the first invention for solving the above-mentioned problem, the temperature of the slab generated by the in-machine stop in the continuous casting machine is measured after the outlet side of the secondary cooling zone and set in advance. The temperature zone lower than the target temperature is identified, and the cooling zone of the secondary cooling zone that caused the low temperature zone is identified from the slab drawing speed and elapsed time after the machine stops, and the identified cooling zone The amount of cooling water in is set so that the difference between the surface temperature of the slab at the next and subsequent in-machine stops and a preset target temperature is 50 ° C. or less.

  In the continuous casting method of steel according to the second invention, in the first invention, the amount of cooling water in the cooling zone is individually adjusted by adjusting the amount of cooling water from a spray nozzle arranged in the long side direction of the slab. It is characterized by setting.

  According to a third aspect of the present invention, there is provided the continuous casting method of steel according to the first or second aspect, wherein at least a corner of the slab out of spray nozzles in which a plurality of cooling water amounts in the cooling zone are arranged in the long side direction of the slab. It sets by reducing the amount of cooling water from the spray nozzle arrange | positioned in the part.

  According to the present invention, every time the slab stops in the continuous casting machine due to tundish replacement or the like, the surface temperature of the slab is measured, and when the measured surface temperature is lower than a preset target temperature. Identifies the secondary cooling zone corresponding to the low part and resets the amount of cooling water in the identified cooling zone so that the difference from the preset target temperature is 50 ° C or less. The slab surface temperature at the time can be close to the target temperature. And, since the surface temperature of the slab including the corner part becomes equal to the target temperature, the slab corner part is out of the brittle region, and the slab corner part has a corrective stress at the bent part or the bent back part of the slab. Even if it acts, it is possible to prevent the occurrence of corner cracks, and industrially beneficial effects such as improvement in yield and direct hot rolling of the slab can be achieved.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic side view of a vertical bending type continuous casting machine to which the present invention is applied.

  As shown in FIG. 1, the vertical bending type continuous casting machine 1 includes a mold 4, a tundish 2 disposed above the mold 4, a roller apron 5 disposed immediately below the mold 4, and a roller apron 5. A plurality of pairs of pinch rolls 7 installed below, a bending roll 8 for bending the cast slab 12 into an arc shape, and a straightener for bending back the arc-shaped slab 12 into a flat plate shape 9. An immersion nozzle 3 is installed at the bottom of the tundish 2.

  A plurality of pairs of guide rolls (not shown) are arranged on the roller apron 5, and the region of the roller apron 5 is a secondary cooling zone 6. The secondary cooling zone 6 includes a first zone 6a, a second zone 6b, a third zone 6c, a fourth zone 6d, a fifth zone 6e, a sixth zone 6f, and a seventh zone 6g in order from the lower side of the mold 4. The cooling zone is divided into nine cooling zones, an eighth zone 6h and a ninth zone 6i. In each of these cooling zones, a spray nozzle 13 comprising a water spray nozzle or an air mist spray nozzle as shown in FIG. It is installed in a gap between guide rolls adjacent in the casting direction. 2 is a schematic view showing the arrangement of the spray nozzle 13, FIG. 2 (A) is an arrangement view seen from a direction parallel to the casting direction, and FIG. 2 (B) is seen from a direction perpendicular to the casting direction. FIG.

  In this case, the cooling water supply pipe 14 to the spray nozzle 13 is divided into three systems of routes 1, 2, and 3, and the central portion is cooled with respect to the slab of the maximum width that can be cast by the route 1. Route 3 is configured to cool its end and Route 2 is configured to cool between Route 1 and Route 3. In each of the cooling zones from the first zone 6a to the ninth zone 6i, the supply amount of the secondary cooling water can be adjusted independently, as well as the routes 1, 2, and 3 of each cooling zone. Even in the two systems, the amount of secondary cooling water supplied can be adjusted independently. The spray nozzles 13 are arranged in a staggered manner in the casting direction, and the slab 12 is cooled by the secondary cooling water sprayed from the spray nozzles 13.

  The molten steel 11 was poured into the tundish 2 from a ladle (not shown), and the tundish 2 was accommodated in the tundish 2 via the immersion nozzle 3 while maintaining a state where a predetermined amount of molten steel 11 was accommodated. Molten steel 11 is poured into the mold 4. Then, the slab 12 having a solidified shell (not shown) formed by contact of the molten steel 11 with the mold 4 as an outer shell and an inside as an unsolidified layer (not shown) is continuously casted by a pinch roll 7. Pull out 4 below. The slab 12 having an unsolidified layer inside drawn out from the mold 4 is cooled by the secondary cooling water in the secondary cooling zone 6 provided in the region of the roller apron 5 while being drawn downward, and the pinch roll 7 Solidification to the center of the slab is completed in the region of the roller apron 5 before reaching. That is, solidification is completed at the vertical portion of the vertical bending type continuous casting machine 1.

  The slab 12 that has been solidified and has passed through the pinch roll 7 is bent into an arc shape by the bending roll 8, and then the slab 12 bent into the arc shape is bent back into a flat plate shape by the straightener 9. The slab 12 bent back into a flat plate shape is cut into a predetermined length by a torch type cutting machine (not shown) disposed on the downstream side of the straightener 9. A temperature measurement sensor 10 is installed between the bending roll 8 and the straightener 9 (at a position 32.6 m from the molten steel surface in the mold) to measure the temperature of the corner of the slab 12 where the temperature tends to decrease. Has been. A measurement value obtained by the temperature measurement sensor 10 is input to a calculator (not shown), and the temperature measurement value is displayed. The installation position of the temperature measurement sensor 10 does not have to be the position shown in FIG. 1, and may be anywhere after passing through the secondary cooling zone 6.

  In such a vertical bending type continuous casting machine 1, the position where the tensile stress acts on the surface of the slab 12 is mainly the lower surface side of the slab when the slab 12 is bent into an arc shape by the bending roll 8, And it is the slab upper surface side when the arc-shaped slab 12 is bent back into a flat plate shape by the straightener 9, and when the corner surface temperature of the slab 12 enters the brittle region of steel when the tensile stress acts, A corner crack occurs in the slab 12.

  In the vertical bending type continuous casting machine 1 configured as described above, the slab 12 is temporarily stopped in the machine to replace the tundish 2, the tundish 2 is replaced, and then the molten steel 11 is injected into the mold 4. FIG. 3 shows an example of the transition of the surface temperature of the slab lower surface side corner when the slab 12 is resumed after being resumed. The slab surface temperature is measured by the temperature measurement sensor 10.

  FIG. 3 shows the slab corner temperature by comparing the case where the cooling of the slab 12 is normal and the case where the cooling is abnormal. The horizontal axis in FIG. 3 is the elapsed time after resuming the drawing of the slab. In the example of the cooling abnormality, the slab corner temperature becomes less than 550 ° C. from about 25 minutes after the resumption of drawing. The part at the time when 25 minutes have elapsed indicates that it corresponds to the eighth zone 6 h of the secondary cooling zone 6. In the case of the cooling abnormality shown in FIG. 3, it can be seen that only the eighth zone 6h is not supercooled, but the cooling is too strong in the range from the first zone 6a to the eighth zone 6h. On the other hand, when the cooling is normal, the slab corner temperature of the portion stopped in the range of the secondary cooling zone 6 is almost 650 ° C., and it is less than 600 ° C. It can be seen that this is only a part of one zone 6a and there is no part below 550 ° C.

  In addition, in FIG. 3, the position at the time of the stop of the slab 12 is displayed with the dashed-dotted line. For example, when the drawing of the slab 12 is resumed and 25 minutes have passed, the portion that has passed the position of the temperature measuring sensor 10 is about 17 m below the casting direction from the mold surface in the mold at the time of stoppage. ing.

  FIG. 4 shows that the target temperature of the slab corner temperature at the installation position of the temperature measurement sensor 10 is set to 600 ° C., and when the slab corner temperature is lower than the target value of 600 ° C., the difference from the target temperature, It is a figure which shows the relationship with a slab corner crack. As apparent from FIG. 4, when the slab corner temperature measured by the temperature measuring sensor 10 falls below 50 ° C. from the target temperature of 600 ° C., slab corner cracking occurs, and the temperature difference increases. It can be seen that the slab corner cracks become severe.

  From these results, it can be seen that the slab corner crack does not occur at the “normal time” shown in FIG. In addition, since the corner temperature of the slab 12 decreases due to heat radiation as it becomes downstream in the casting direction, the absolute value of the target temperature should be set in consideration of this temperature drop according to the installation position of the temperature measurement sensor 10. Is preferred.

  Therefore, in the present invention, when the slab 12 is stopped in the machine for the tundish replacement, after the slab 12 is resumed withdrawn after the tundish replacement, the slab corner portion at the outlet of the secondary cooling zone 6 is resumed. The temperature is measured, the measured corner temperature is compared with a preset target temperature, and the cooling zone of the secondary cooling zone 6 at the time of in-machine stop corresponding to the slab corner portion of the temperature portion lower than the target temperature is determined. It is determined from the slab drawing speed and the elapsed time after stopping in the machine. Then, the cooling water amount in the specified cooling zone is reset so that the difference between the surface temperature of the slab when the machine stops and the target temperature set in advance is 50 ° C or less, and the next tundish change is performed. carry out. By resetting the cooling water amount each time the tundish is replaced, a more accurate secondary cooling pattern can be constructed.

  The present invention aims to prevent corner cracking caused by overcooling of the corner portion of the slab 12, and since the cooling strength of the corner portion of the slab 12 can be adjusted accurately, the amount of cooling water in the cooling zone 2 is preferably adjusted using a device capable of controlling the flow rate of the secondary cooling water in the width direction of the slab 12 as shown in FIG. Specifically, it is preferable to reduce the cooling strength on the slab corner side by decreasing or stopping the supply flow rate from the spray nozzle 13 at the position corresponding to the slab corner.

  By performing continuous casting operation of steel in this way, even if the surface temperature of the slab corner is lower than the target temperature, each time, specify the cooling zone corresponding to the low temperature part, Since the cooling water amount in the specified cooling zone is reset, the slab surface temperature at the next and subsequent in-machine stop can be brought close to the target temperature. As a result, the occurrence of slab corner cracks is greatly reduced, and the manufacturing cost can be greatly reduced.

  In the above description, the case where the slab 12 is stopped in the machine for the tundish replacement has been described. However, the present invention can be applied to a case where only the immersion nozzle 3 is replaced instead of the tundish replacement. The present invention can also be applied to cases where the slab 12 has to be stopped once in the machine for some reason.

  Further, the type of the continuous casting machine is not the vertical bending type continuous casting machine 1 shown in FIG. 1, but is a continuous casting machine in which a bending part and a bending correction part are installed after the secondary cooling zone 6. The present invention can be applied. However, since the object of the present invention is to prevent slab corner cracking during straightening due to overcooling, therefore, in a continuous casting machine in which a straightening section is installed within the secondary cooling zone 6, the above method is used. It cannot be applied as it is. This is because in a continuous casting machine in which a correction part is installed within the range of the secondary cooling zone 6, it is necessary to arrange a place for measuring the corner temperature of the slab 12 upstream of the correction part, or a correction part. This is because the above-described method alone may not be suitable, for example, because it is necessary to consider the cooling strength of secondary cooling.

  An example in which the present invention is applied to the vertical bending type continuous casting machine shown in FIG. 1 will be described.

  Tundish exchange was carried out when continuously casting slabs of medium carbon steel having a thickness of 310 mm and a width of 2450 mm. The slab was stopped for about 1 minute in the machine, and simultaneously with the start of injection of molten steel from the newly installed tundish into the mold, the slab that had been stopped in the machine was started to be drawn. And the corner part of this slab was measured with the temperature measurement sensor.

  Table 1 shows the differences between the secondary cooling water amounts in the second to ninth zones of the secondary cooling zone and the target temperature (600 ° C.) of the corner temperature when the slab is stopped in the machine. As shown in Table 1, the corner temperature of the slab corresponding to the position of the sixth zone was 63 ° C. lower than the target temperature. In other cooling zones, it was within 50 ° C. with respect to the target temperature.

  Based on the results in Table 1, the amount of secondary cooling water in the sixth zone was reduced and reset from 226 L / min to 199 L / min. In this case, the amount of secondary cooling water decreased the amount of water in route 3 shown in FIG. And the slab corner temperature of the site | part which stopped in the machine at the time of the next tundish exchange was measured with the temperature measurement sensor.

  Table 2 shows the difference between the amount of secondary cooling water in each of the second to ninth zones of the secondary cooling zone and the corner temperature target temperature (600 ° C.) when the slab is stopped in the machine. As shown in Table 2, the corner temperature of the slab corresponding to the position of the sixth zone rises, the difference from the target temperature is 37 ° C, and within 50 ° C with respect to the target temperature in all the cooling zones. It was.

  After casting, the slab was cooled and the occurrence of corner cracks in the slab was investigated using the penetrant flaw detection method. FIG. 5 shows the occurrence rate of slab corner cracks when the present invention is applied (invention example) and when the present invention is not applied and cast with the secondary cooling water amount shown in Table 1 (conventional example). It is the result of comparative investigation. As is apparent from FIG. 5, the corner cracking of the slab was greatly reduced by the present invention.

1 is a schematic side view of a vertical bending type continuous casting machine to which the present invention is applied. It is the schematic which shows arrangement | positioning of a spray nozzle. It is a figure which shows the example of transition of the surface temperature of a slab lower surface side corner part when pulling out of a slab is restarted after tundish exchange. It is a figure which shows the relationship between the difference with target temperature, and a slab corner crack. It is the result of having investigated the incidence rate of a corner crack in the example of the present invention and the conventional example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Vertical bending type continuous casting machine 2 Tundish 3 Immersion nozzle 4 Mold 5 Roller apron 6 Secondary cooling zone 7 Pinch roll 8 Bending roll 9 Straightener 10 Temperature measuring sensor 11 Molten steel 12 Cast slab 13 Spray nozzle 14 Cooling water supply piping

Claims (3)

  The temperature of the slab generated by the in-machine stop in the continuous casting machine is measured after the exit side of the secondary cooling zone, the temperature part lower than the preset target temperature is specified, the slab drawing speed after the in-machine stop and From the elapsed time, specify the cooling zone of the secondary cooling zone that caused the low temperature part, and set the cooling water amount in the specified cooling zone in advance with the surface temperature of the slab at the next in-machine stoppage A continuous casting method for steel, characterized in that the difference between the target temperature and the target temperature is set to 50 ° C. or less.   2. The continuous casting method for steel according to claim 1, wherein the cooling water amount in the cooling zone is set by individually adjusting the cooling water amount from a plurality of spray nozzles arranged in the long side direction of the slab. .   The amount of cooling water in the cooling zone is set by reducing the amount of cooling water from at least the spray nozzles arranged at the corners of the slab out of a plurality of spray nozzles arranged in the long side direction of the slab. The continuous casting method of steel according to claim 1 or claim 2.

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