JP4706507B2 - Continuous casting method - Google Patents

Description

  In the present invention, when a slab including an unsolidified portion is reduced using a reduction roll pair, the lower roll of the reduction roll pair protrudes from the lower pass line of the slab in the continuous casting machine or at the machine end. The present invention relates to a continuous casting method for reducing the center segregation of a slab by rolling down, and more particularly, to a continuous casting method characterized by an operation at the time of drawing the first slab.

  In general, in rolling a steel sheet, the reduction amount by the upper roll and the reduction amount by the lower roll are reduced so as to be substantially equal.

  Recently, even in continuous casting, a slab including an unsolidified portion has been reduced in a curved or vertical bending type continuous casting machine. When rolling down a slab including an unsolidified part, a dummy bar is generally used at the start of casting.

  FIG. 1 is a diagram schematically showing a situation at the start of casting in a vertical bending die continuous casting machine. Continuous casting is started by inserting the head 9a of the dummy bar 9 into the lower portion of the mold 3 having no bottom portion to form a temporary bottom portion, and pouring the molten steel 4 from the immersion nozzle 1 therein. After the molten steel surface 2 in the mold 3 reaches a preset level and a solidified shell having a predetermined thickness is formed, the dummy bar 9 is started to be pulled out, and the pulling speed is increased to shift to a steady state. The dummy bar 9 is taken obliquely upward after the pinch roll (not shown).

  Normally, when rolling down a slab containing an unsolidified part using a dummy bar, it is necessary to pass the pair of rolling rolls through the dummy bar without hindrance, so the lower roll of the rolling roll pair has a rolling point below the slab. The height position is fixed so that it is at the same level as the pass line, and only the upper roll of the reduction roll pair is lowered and reduced. For this reason, there has been no proposal to reduce the slab by causing the lower roll of the reduction roll pair to protrude in the continuous casting machine.

  The present inventors, unlike rolling of a steel sheet, in a continuous casting method in which only the upper roll is lowered to reduce the unsolidified slab, the reduction force is consumed in the bending deformation of the slab and the slab is efficiently reduced. In Patent Document 1, a method was proposed in which the lower roll of the reduction roll pair protrudes upward from the lower pass line to reduce the slab. Specifically, in the continuous casting method proposed in Patent Document 1, the slab including the unsolidified portion is bulged, and then the lower roll of the reduction roll pair is placed above the lower pass line of the slab in the continuous casting machine. It is a continuous casting method of steel that is projected and reduced.

  In this way, by allowing the lower roll to protrude upward from the lower pass line of the slab, the slab including the unsolidified portion can be efficiently reduced without loss of the reduction force. It is possible to reduce the segregation that occurs in the central part in the thickness direction of the slab, compared to the reduction in which only the upper roll is lowered. Furthermore, by rolling down the slab containing the unsolidified portion and then rolling it down, the removal of the unsolidified molten steel to the upstream side of the slab is further promoted, and segregation is further reduced.

  However, in the continuous casting method proposed in Patent Document 1, it is assumed that the dummy bar for pulling out the first slab of casting cannot be pulled out smoothly during actual operation.

  FIG. 2 is a diagram showing a passing state of the dummy bar when the lower roll of the conventional rolling roll pair is not protruded. When the lower roll is not protruded, the dummy bar 9 moves in parallel with the lower pass line of the slab and does not interfere with the upper guide roll 6, and therefore can pass through the reduction roll pair 7 without any trouble. In this way, if the passage of the dummy bar 9 proceeds smoothly and the joint portion (not shown) between the slab and the dummy bar head passes the position of the reduction roll pair 7, the slab including the unsolidified portion is immediately reduced. While the rolling using the roll pair 7 is started, the dummy bar 9 is pulled out at the machine end.

  On the other hand, before the dummy bar passes the rolling roll pair, the lower roll protrudes above the lower pass line of the slab, passes the dummy bar and the slab, and then the slab is rolled down. In the method, it becomes difficult for the dummy bar to pass through the pair of rolling rolls without hindrance.

  FIG. 3 is a diagram showing a passing state of the dummy bar when the lower bar is protruded before the dummy bar passes through the rolling roll pair. In the case where the lower end of the lower roll 7 is projected and the leading portion of the slab is allowed to pass, the dummy bar 9 rides on the protruding portion of the lower roll 7, and the maximum height portion 10 of the unit block of the dummy bar 9 is the upper guide roll 6a. The movement of the dummy bar 9 stops in the state shown in FIG. In such a case, it becomes difficult to pull out the dummy bar 9, and the casting operation cannot be continued.

  As a method of avoiding the state in which it is difficult to pull out the dummy bar as described above, at the start of casting, the lower roll is not projected as shown in FIG. A method of projecting the lower roll after the slab has reached the reduction position after the joint portion with the reduction roll pair has passed can be considered.

  However, a very large force is required to raise the lower roll from the state where the slab is loaded on the lower roll. Therefore, when the slab reaches the reduction position and the lower roll of the pair of reduction rolls is projected, a large-scale reduction facility is required, and such a large-scale reduction facility is installed in the continuous casting machine. Is not realistic.

  Patent Document 2 discloses a method of stabilizing the operation by preventing a dummy bar head from being detached. Specifically, this is a continuous casting method in which the distance between the guide rolls in the bulging zone is increased after the dummy bar passes through the uppermost guide roll in the bulging zone.

  However, the continuous casting method disclosed in Patent Document 2 is a method in which only the upper roll of the rolling roll pair is lowered and rolled down, and the purpose is to prevent the dummy bar head from detaching from the slab. It is not a reference for enabling easy withdrawal.

JP 2004-001079 A Japanese Patent No. 3111953

  As described above, the continuous casting method proposed in Patent Document 1 is an excellent continuous casting method that can reduce the center segregation of the slab. However, when the first slab is drawn, It is assumed that it is difficult to pull out and casting cannot be continued. In order to solve this problem, a large-scale reduction facility is required to allow the lower bar to be raised after the dummy bar passes through the reduction roller pair.

  The present invention has been made in view of the above problems, and when a slab including an unsolidified portion is pressed down by projecting the lower roll of a pair of reduction rolls above the lower pass line of the slab, An object of the present invention is to provide a continuous casting method that does not require a large-scale reduction equipment and can perform the extraction of the dummy bar without any trouble.

The inventors of the present invention have studied a continuous casting method capable of pulling out the dummy bar without any trouble even when the lower roll of the pair of reduction rolls is projected to perform reduction, and the following (a) to (c) The new knowledge shown in was obtained.
(A) When the dummy bar passes over the lower roll protruding from the pair of rolling rolls and passes through the upper guide roll on the machine end side from the position of the rolling roll pair, the maximum height of the unit block of the dummy bar that rides on the lower roll The dummy bar can be pulled out smoothly without interfering with the upper guide roll. Therefore, it is possible to uniformly reduce from the upper and lower surfaces of the slab by a relatively simple reduction equipment, and it is possible to efficiently perform unsolidified reduction.
(B) In (a), the upper guide roll in the range from the position of the reduction roll pair to at least the horizontal position where the unit block of the dummy bar riding on the lower roll reaches the maximum height is further smoothened. The dummy bar can be pulled out.
(C) In the continuous casting method in which the slab is bulged and then squeezed using a pair of squeezing rolls, by applying the methods (a) and (b), a higher quality slab can be manufactured at low cost. it can.

This invention is completed based on said knowledge, and makes the summary the following continuous casting method of (1)-(3).
(1) In a continuous casting machine or at the machine end, when a slab including an unsolidified portion is squeezed using a squeezing roll pair having an upper roll and a lower roll, the lower roll that can be moved up and down is reduced. In a continuous casting method in which a slab protrudes upward from a lower pass line of the slab and is rolled down, a dummy bar for pulling out the first slab of casting is structured such that a plurality of unit blocks are connected to bendable, and the dummy bar Before passing the roll pair, the lower roll is protruded, and when the dummy bar passes over the protruded lower roll, the upper guide roll on the machine end side is moved from the position of the reduction roll pair to the lower part. It is raised from the maximum height of the unit block of the dummy bar that rides on the roll, and the gap between the upper guide roll and the lower guide roll is widened for casting. Continuous casting method to be.
(2) In the continuous casting method described in (1) above, the upper guide roll in a range from the position of the reduction roll pair to at least the horizontal position where the unit block of the dummy bar riding on the lower roll has the maximum height. It is desirable to raise.
(3) In the continuous casting method described in the above (1) or (2), it is desirable to squeeze the slab including the unsolidified portion and then reduce the slab.

  According to the continuous casting method of the present invention, even if the lower bar protrudes before the dummy bar passes the reduction roll pair, it interferes with the guide roll located on the machine end side (downstream side) of the reduction roll pair. Since it is possible to pass through the dummy bar, it is possible to quickly shift to the reduction of the unsolidified slab by the subsequent reduction roll pair.

  This eliminates the need to project the lower roll of the reduction roll pair after the dummy bar and the first cast slab have passed through the position of the reduction roll pair. The center segregation of the piece can be reduced.

  Furthermore, if the slab containing the unsolidified portion is bulged and then reduced using a pair of reduction rolls, the center segregation can be further reduced.

  As described above, in the continuous casting method of the present invention, when a slab including an unsolidified portion is squeezed using a squeezing roll pair having upper and lower rolls, a lower roll that can be moved up and down is reduced. In the continuous casting method, the dummy bar has a structure in which a plurality of unit blocks are connected so as to be able to bend, and the lower roll protrudes before the dummy bar passes the reduction roll pair. When the dummy bar passes over the projected lower roll, the upper guide roll on the machine end side is raised from the position of the reduction roll pair above the maximum height of the unit block of the dummy bar riding on the lower roll, and the upper side The casting is characterized in that the gap between the guide roll and the lower guide roll is increased.

  Below, the continuous casting method of this invention is demonstrated concretely based on drawing.

  FIG. 4 is a view showing a passing state of the dummy bar when the lower roll protrudes before the dummy bar passes through the rolling roll pair after the start of casting using the continuous casting method of the present invention. When the lower roll 7 is protruded before the dummy bar 9 passes through the rolling roll pair 7, as shown in the figure, the unit block of the dummy bar 9 rides on the protruding portion, and the unit block of the dummy bar 9 is the maximum. Reach up to height position 10.

  For this reason, in the continuous casting method of the present invention, the upper guide roll 6 a located on the machine end side (downstream side) of the reduction roll pair 7 is raised above the maximum height position 10 of the block constituting the dummy bar 9. As a result, the dummy bar 9 does not interfere with the upper guide roll 6a and can pass through the reduction roll pair 7 without any trouble. Thus, if the passage of the dummy bar 9 proceeds smoothly, the dummy bar 9 is pulled out at the machine end.

  In the continuous casting method of the present invention, as shown in the above (1) and (2), the range of the casting direction in which the upper guide roll should be raised is from the position of the pair of reduction rolls to the machine end side, and at least to the lower roll. This is the range up to the horizontal position where the unit block of the mounted dummy bar has the maximum height. By defining the range in the casting direction in this way, smooth passage of the dummy bar can be ensured. The horizontal maximum allowable range in which the upper guide roll can be raised is not particularly limited as long as the driving force for drawing the slab can be secured by the guide roll that does not widen the upper and lower roll intervals.

  Further, in the continuous casting method of the present invention, when the slab including the unsolidified portion is squeezed, the lower roll is caused to protrude upward from the lower pass line of the slab, thereby causing a reduction in the reduction force. Can be efficiently reduced. For this reason, it is possible to reduce the segregation that occurs in the central portion in the thickness direction of the slab, compared to the conventional reduction in which only the upper roll is lowered, but in order to obtain the effect more prominently, It is desirable to squeeze the slab using a pair of reduction rolls after bulging the slab including the solidified portion. When bulging is performed, the amount of reduction is preferably the bulging equivalent, but may exceed the bulging equivalent. The reduction may be performed according to the configuration of the apparatus, and may be performed in a continuous casting machine or at the end of the continuous casting machine.

  Moreover, since the improvement effect is acquired even if it makes the protrusion amount of the lower roll of a reduction roll pair project slightly, the range is not specifically limited.

  In the continuous casting method of the present invention, a parallel roll may be used as a reduction roll, and a reduction force can be effectively applied over the entire width of the slab. Moreover, since the protrusion amount of the lower roll of the reduction roll pair is fixed to a constant value, flexibility in operation may be lacking, so that it can be raised and lowered by hydraulic pressure or the like according to operation conditions. Thereby, the method of the present invention can be implemented flexibly.

  In the continuous casting method of the present invention, the dummy bar is not particularly limited in size and shape, and a commonly used one can be applied. However, the entire dummy bar can be smoothly curved along the shape of a continuous casting device such as a curved type or a vertical bending type, and can pass over the lower roll of a pair of rolling rolls protruded by a dummy bar block. Thus, a structure in which a plurality of unit blocks are connected so as to be bendable is essential. The connection method of the unit blocks is not particularly specified, but connection using a connection pin can be applied.

  The dummy bar charging method may be either a top charging method in which the dummy bar is charged from the upper part of the mold or a bottom charging method in which the dummy bar is charged after the pinch roll.

  Furthermore, when carrying out the continuous casting method of the present invention, an electromagnetic stirring device may be installed on the upstream side of the reduction roll pair. Segregation can be dispersed by applying a flow to the molten steel in the unsolidified part, which is effective.

In order to confirm the effect of the present invention, the following tests were conducted and the results were evaluated.
(Preliminary test)
In continuous casting, the upper guide on the machine end side (downstream side) of the reduction roll pair with the lower roll protruding 12 mm so that the reduction roll can be efficiently reduced without loss of the reduction force. A test was carried out to confirm the relationship between the amount of roll rise and the passage of dummy bars.

  In the preliminary test, the vertical bending type continuous casting machine shown in FIG. 1 was used, the dummy bar had a total length of about 12 m, a distance between the connecting pins of about 480 mm, and had a structure that was connected so as to be able to be bent. Was 190 mm, and a dummy bar head set in a mold for casting a 235 mm thick slab was used by a top charging method in which the dummy bar head was charged from the upper part of the mold.

  As described above, this preliminary test is a test for confirming the relationship between the amount of rise of the upper guide roll on the machine end side (downstream side) with respect to the pair of reduction rolls and the passing state of the dummy bar. There wasn't.

First, when the upper guide roll is not raised from the normal standby position and the distance from the lower guide roll is set to 280 mm, the upper end of the dummy bar on the pulling-side end rides on the protruding part of the lower roll of the reduction roll pair. Interference with the guide roll. As a result of continuously increasing the amount of upward movement of the upper guide roll, it was confirmed that the dummy bar and the upper guide roll would not interfere with each other if the distance from the lower guide roll was 330 mm or more.
(Casting test)
In the present invention, the following test was conducted in order to confirm the smooth pulling effect of the dummy bar for pulling out the first cast piece.

  FIG. 5 is a diagram schematically showing a continuous casting apparatus provided with a vertical bending type bulging mechanism used for testing the continuous casting method of the present invention. A slab having a thickness of 235 mm and a width of 2300 mm was cast by a method using bulging described in Patent Document 1. Similar to the preliminary test, the dummy bar has a total length of about 12 m, a distance between the connecting pins of about 480 mm, and has a structure that can be bent, and has a thickness of 190 mm and is cast in a mold for casting a 235 mm thick slab. A dummy bar head to be set was used.

  Before the dummy bar passes the rolling roll pair 7, the lower roll protrudes 12 mm, and the upper guide roll rises to 330 mm based on the result of the preliminary test, which is closer to the machine end side (downstream side) than the rolling roll pair 7. Four upper guide rolls were raised. At this time, it was confirmed that the pulling force could be secured by the upper and lower guide rolls other than the four upper guide rolls.

  The steel types used in the casting test were C: 0.02 to 0.20%, Si: 0.04 to 0.60%, Mn: 0.50 to 2.00%, P: 0.020% in mass%. Hereinafter, the chemical composition was S: 0.006% or less. The casting speed was 1.03 m / min, and the amount of secondary cooling water was 1.31-1.35 L / kg-steel. Casting was performed with the bulging amount of the slab set to 20 mm. Although the continuous casting machine used for the test is a vertical bending type continuous casting machine, the bending type continuous casting machine can be used without being limited thereto.

  Regarding the adjustment of the central solid phase ratio during rolling, the heat transfer in the slab is calculated under various conditions mainly by giving the casting speed and the thickness of the central part of the slab, and the predetermined solidity in this casting test is calculated. The conditions for achieving the phase ratio were determined. Moreover, the molten steel temperature in the tundish was made substantially constant between ΔT (degree of superheat) = 40 ° C. to 50 ° C. ΔT is the difference between the molten steel temperature and the liquidus temperature.

  In continuous casting, a dummy bar head (not shown) is inserted into the bottom of a bottomless mold 3 by a top charging method to form a temporary bottom, and molten steel 4 is injected from the immersion nozzle 1 therein. Started by. After the molten steel surface 2 in the mold 3 reached a preset level and a solidified shell having a predetermined thickness was formed, the dummy bar was started to be extracted, and the extraction speed was increased to shift to a predetermined casting speed. .

  The molten steel 4 injected into the mold 3 from the tundish (not shown) through the immersion nozzle 1 is cooled by spray water sprayed from the mold 3 and a group of secondary cooling spray nozzles (not shown) below it, A solidified shell 5 was formed to form a slab 8.

The guide rolls 6 group are arranged so that the interval in the thickness direction of the slab 8 can be controlled to a constant value, and bulging is performed when an unsolidified portion exists in the slab 8 as shown in FIG. I let you. The slab 8 was squeezed by the squeezing roll pair 7 through the group of guide rolls 6 while holding the unsolidified portion therein. A pair of reduction rolls 7 was installed at a position 21.5 m below the molten steel surface (meniscus) 2 formed inside the mold 3. The diameter of the rolling rolls constituting the rolling roll pair 7 was 450 mm, and the maximum rolling force was 5.88 × 10 ³ kN (600 ton).

After starting casting and shifting to a steady state, the dummy bar was taken out upward after a pinch roll (not shown).
(Casting test results)
FIG. 6 is a view showing a passing state of the dummy bar head portion when the lower roll is protruded before the dummy bar passes the reduction roll pair after the casting is started by using the continuous casting method of the present invention. As shown in the drawing, after the drawing by the dummy bar 9 is started and the predetermined casting speed is reached and the position of the reduction roll pair 7 is reached, the dummy bar 9 causes the protruding lower roll 7 to interfere with the upper guide roll 6a. I was able to pass without.

  Subsequently, after the dummy bar head 9a passes, when the slab steady portion 8 having the unsolidified portion with the target central solid phase ratio reaches the position of the reduction roll pair 7, the upper roll of the reduction roll pair 7 is reduced. (Descent) was started, and the upper roll reduction amount was 21 mm, that is, the total reduction amount including the reduction amount by the lower roll was 33 mm.

  As a result, the central segregation ratio C / Co, which is the ratio of the Mn content C (mass%) in the center of the slab in the thickness direction and the Mn average content Co (mass%) of the slab, over the entire width, Since 0.7 ≦ C / Co ≦ 1.2, it was possible to reduce segregation that occurred at the center of the slab in the thickness direction.

  According to the continuous casting method of the present invention, even if the lower bar protrudes before the dummy bar passes the reduction roll pair, it interferes with the guide roll located on the machine end side (downstream side) of the reduction roll pair. Since it is possible to pass through the dummy bar, it is possible to quickly shift to the reduction of the unsolidified slab by the subsequent reduction roll pair.

  This eliminates the need to project the lower roll of the reduction roll pair after the dummy bar and the first cast slab have passed through the position of the reduction roll pair. The center segregation of the piece can be reduced.

  Furthermore, if the slab containing the unsolidified portion is bulged and then reduced using a pair of reduction rolls, the center segregation can be further reduced.

  The continuous casting method of the present invention can be carried out in any of the continuous casting apparatus of the curved type and the vertical bending type, and the shape of the slab can be applied not only to the slab but also to the bloom. It can be widely applied as a method for casting a low-cost and high-quality slab.

It is a figure which shows typically the condition at the time of the casting start of a vertical bending type | mold continuous casting machine. It is a figure which shows the passage condition of the dummy bar when not making the lower roll of the conventional reduction roll pair project. It is a figure which shows the passage condition of the dummy bar in the case of making the lower roll project before the dummy bar passes the reduction roll pair. It is a figure which shows the passage condition of the dummy bar in case the lower bar rolls out before the dummy bar passes a rolling-down roll pair after casting starts using the continuous casting method of this invention. It is the figure which showed typically the continuous casting apparatus provided with the vertical bending type bulging mechanism used in order to test the continuous casting method of this invention. It is a figure which shows the passage condition of the dummy bar head part in the case of making the lower roll project before the dummy bar passes the reduction roll pair after casting using the continuous casting method of the present invention.

Explanation of symbols

1. Immersion nozzle Molten steel surface (meniscus)
3. Mold 4. Molten steel 5. Solidified shell 6, 6a. Guide roll 7. Rolling roll 8. Cast slabs 9. Dummy bar 9a. Dummy bar head 10. Maximum height of the unit block of the dummy bar, maximum height position

Claims (3)

In the continuous casting machine or at the machine end, when the slab including the unsolidified portion is squeezed by using a squeezing roll pair having an upper roll and a lower roll, the lower roll that can be moved up and down is reduced. In the continuous casting method of projecting upward and lowering from the lower pass line,
The dummy bar for pulling out the first slab of casting has a structure in which a plurality of unit blocks are connected so as to be bendable. The dummy bar protrudes before the dummy bar passes through the pair of rolling rolls, and the dummy bar protrudes. When passing over the lower roll, the upper guide roll on the machine end side from the position of the pair of reduction rolls is raised from the maximum height of the unit block of the dummy bar riding on the lower roll, and the upper guide roll and lower A continuous casting method, wherein casting is performed with a gap between the guide roll and the casting roll increased.   2. The continuous guide roll according to claim 1, wherein the upper guide roll is raised in a range from the position of the reduction roll pair to a horizontal position where at least a unit block of the dummy bar riding on the lower roll has a maximum height. Casting method. The continuous casting method according to claim 1 or 2, wherein after bulging the slab including an unsolidified portion, the slab is reduced using a reduction roll pair.

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