JP6451380B2 - Steel continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting method of steel in which molten steel is supplied into a mold to produce a slab.

  During continuous casting of steel, bubbles in the molten steel rise near the molten steel surface (hereinafter referred to as meniscus) in the mold, so that the raised bubbles are particularly easily captured by the solidified shell at the meniscus position.

  Since the trapped air bubbles may become surface defects during rolling, in order to improve the surface properties of the slab in continuous casting of steel, an electromagnetic stirrer installed on the outer periphery near the meniscus position of the mold is used. In general, the molten steel in the mold is stirred and fluidized.

  It is possible to suppress trapping of bubbles in the molten steel by the solidified shell due to the molten steel flow in front of the solidified shell generated by the electromagnetic stirring of the molten steel in the mold.

  However, since the distance between the immersion nozzle and the long side wall surface becomes narrow at the central part of the long side of the mold, the molten steel flow in the horizontal plane at the meniscus position of the molten steel in the mold generated by electromagnetic stirring stagnates.

  Also, in the short side portion of the mold at the meniscus position, the discharge flow toward the short side collides with the short side wall surface and rises along the short side wall surface, and at the meniscus position, it becomes a flow toward the long side center of the mold. The molten steel flow stagnates due to interference with the molten steel flow generated by the electromagnetic stirring. In the stagnation portion of the molten steel flow, bubbles are easily trapped by the solidified shell.

  Therefore, a stirring flow having a uniform flow velocity has been demanded on the front surface of the solidified shell at the center of the long side of the mold at the meniscus position.

  For example, in Patent Document 1, left and right discharge holes facing the short side wall of the mold are arranged symmetrically with respect to the side wall, and one end of the bottom wall reaches the right discharge hole and the other end reaches the left discharge hole. A continuous casting method of steel using an immersion nozzle provided with a slit is disclosed.

  However, when the inventor investigated the continuous casting method disclosed in Patent Document 1, the solidified shell in which the discharge flow that collided with the long side wall surface of the mold was formed along the long side wall surface of the mold when the slit width was increased. It has been found that slab surface defects accompanying remelting and the worst case breakout have an adverse effect on operations. It was also found that the floating amount of bubbles and the like increased at the center of the long side of the mold where the molten steel flow stagnated, and defects increased at the center of the long side of the mold.

  Patent Document 2 discloses continuous casting of steel using a mold that is convexly curved toward the electromagnetic stirrer side.

  However, in the case of continuous casting using a mold that is convexly convex toward the electromagnetic stirrer side, the distance between the molten steel and the electromagnetic stirrer increases at the long side end of the mold, and the molten steel flow decreases. In addition, as a result of investigation by the inventors, the molten steel flow at the end of the long side of the reduced mold and the reverse flow of the discharge from the short side wall surface of the mold interfere with each other, and the molten steel flow stagnates or flows due to electromagnetic stirring. It was found that the flow was in the opposite direction. Therefore, there is a restriction on the distance between the mold and the immersion nozzle or the size of the convex portion of the mold.

JP 2000-343188 A JP2011-224635A

  The problem to be solved by the present invention is that in the prior art, it is difficult to obtain a stirring flow having a uniform flow velocity in front of the solidified shell at the center of the long side of the mold at the meniscus position.

The present invention
In order to obtain a stirring flow having a uniform flow velocity in front of the solidified shell at the center of the long side of the mold at the meniscus position,
A method of continuously casting steel while electromagnetically stirring molten steel near a meniscus in a mold,
At the time of continuous casting, using a submerged nozzle formed with slit-like discharge holes connected to two discharge holes provided on the side wall portion facing the short side wall surface of the mold below the meniscus in the mold,
The central part of each long side of the upper part of the mold has a curved part that is convexly curved toward the outside of the mold, and the curvature of the curved part becomes smaller toward the lower part of the mold, and hot water is supplied to the mold without the curved part at the lower part of the mold. When
The relationship between the width a in the thickness direction of the slit-shaped discharge hole formed in the bottom wall of the immersion nozzle and the shortest horizontal distance b between the curved portion of the mold and the lower end of the immersion nozzle is the shortest horizontal distance b. The main feature is continuous casting so that a / b ≦ 0.5 at 50 mm or more .

  In the present invention, the width a in the thickness direction of the slit-like discharge hole formed on the bottom wall of the immersion nozzle, and the shortest horizontal distance b between the curved portion formed at the center of each long side of the mold and the lower end of the immersion nozzle By defining the relationship within the optimum range, it is possible to obtain a molten steel flow having a uniform flow velocity on the front surface of the solidified shell at the center of the long side of the mold at the meniscus position.

  In the present invention, a molten steel flow having a uniform flow velocity in the width direction can be obtained in front of the solidified shell in the horizontal plane at the meniscus position, so that the molten steel flow stagnates at the long side center and long side end of the mold. Therefore, the solidified shell at the center of the long side of the mold does not redissolve. Accordingly, it is possible to prevent trapping of bubbles in the solidified shell at the long side center portion and the long side end portion of the mold at the meniscus position, and it is possible to cast a slab having good surface quality without any trouble.

It is a figure explaining the mold shape used for the continuous casting method of steel of this invention, the shape of an immersion nozzle, and both relative position relationship, (a) is the figure seen from the plane direction, (b) is A of (a). It is the figure seen from -A direction. It is a figure explaining the sample position of the long side center part at the time of calculating the bubble capture density in the long side center part of slab. It is a figure explaining the sample position of the long side edge part at the time of calculating the bubble capture density in the long side edge part of a slab. The width a in the thickness direction of the slit-shaped discharge hole formed in the bottom wall of the immersion nozzle and the central part of the long side of the mold when the electromagnetic brake is not used together with the case created based on Tables 1 and 2 It is the figure which showed ratio a / b of the shortest horizontal distance b of the formed curved part and the lower end of an immersion nozzle, and the relationship of a casting condition. The width a in the thickness direction of the slit-shaped discharge hole formed in the bottom wall of the immersion nozzle and the case where the electromagnetic brake is not used together with the case where the electromagnetic brake is used together based on Tables 1 and 2, and the long side end of the mold It is the figure which showed the relationship of the bubble trap index. The shortest horizontal distance b ′ or b between the long side center part of the mold and the lower end of the submerged nozzle and the long side center part of the mold when the electromagnetic brake is not used together and when it is used based on Tables 1 and 2 It is the figure which showed the relationship of the bubble trapping index. It is the figure which created based on Table 1 and showed the relationship between a / b 'or a / b and the bubble capture index | exponent of the long side edge part of a casting_mold | template when not using an electromagnetic brake together.

  The purpose of the present invention is to obtain a stirring flow having a uniform flow velocity on the front surface of the solidified shell at the center of the long side of the mold at the meniscus position. This is realized by defining the relationship between a and the shortest horizontal distance b between the curved portion formed at the center of each long side of the mold and the lower end of the immersion nozzle within the optimum range.

First, the new idea about the continuous casting method of steel of the present invention will be described.
When a moving magnetic field is applied in the long side direction of the mold by an electromagnetic stirrer, the molten steel flow in the long side direction of the mold on the front surface of the solidified shell on the long side wall surface of the mold is stagnated at the central part of the long side and at the end of the long side. It's easy to do.

  Therefore, the inventor prevents the stagnation of the molten steel flow at the central part of the long side of the mold by curving the central part of the long side of the mold so as to protrude outward from the mold provided with the electromagnetic stirring device. By providing a slit-like discharge hole (hereinafter referred to as a slit) in the bottom wall of the immersion nozzle to weaken the discharge flow toward the short side of the mold, the stagnation of the molten steel flow at the long side end of the mold can be prevented. Thought.

  However, as a result of investigation by the inventor, in the case of an immersion nozzle provided with a slit in the bottom wall, the downward discharge flow collides with a solidified shell formed along the long side wall surface of the mold, particularly in high-throughput casting. It has been found that there is a case where the solidified shell is remelted and the molten steel leaks.

  In addition, a curve formed at the center of the long side of the mold simply by combining a mold with the center of the long side convexly protruding outward from the mold and an immersion nozzle provided with a slit in the bottom wall of the immersion nozzle. It was found that the flow stagnation and remelting of the solidified shell at the center of the long side of the mold could not be prevented without adjusting the shortest horizontal distance between the bottom part and the lower end of the immersion nozzle and the width of the slit of the bottom wall of the immersion nozzle. .

  The present invention has been made through experiments described below based on the above-described new idea.

[Experiment A]
In the mold having a rectangular cross section with both long side walls straight, an immersion nozzle provided with a slit in the bottom wall, and the shortest horizontal distance b ′ between the lower end of the immersion nozzle and each long side wall surface of the mold is 37.5 mm. The steel was continuously cast while applying a moving magnetic field to the molten steel in the mold by an electromagnetic stirrer while injecting 4.0 tons of molten steel per minute.

  Next, the central part of each long side of the upper part of the mold has a curved part that is convexly curved toward the electromagnetic stirrer, and the curvature of the curved part becomes smaller toward the lower part of the mold, and there is no curved part at the lower part of the mold. Casting was performed under the same conditions as described above by changing the shortest horizontal distance b between the curved portion of the mold and the lower end of the immersion nozzle by changing the convex curve amount of the curved portion using a mold.

The casting results are shown in Table 1 below.
Table 1 below shows the width a of the slit provided on the bottom wall of the immersion nozzle used for continuous casting, the shortest horizontal distance b ′ or b, the relationship a / b ′ or a / b, and the breakout. And the occurrence density of bubbles, the trap density index of bubbles at the center and the end of the long side of the mold, and whether or not claim 1 is satisfied.

  Further, the width a of the slit 2b provided in the bottom wall of the immersion nozzle 2 connected to the positions of the long side central portion 1a and the long side end portion 1b of the mold 1 and the two discharge holes 2a of the immersion nozzle 2 in FIG. And the shortest horizontal distance b of the curved part 1c provided in the long side center part 1a of the casting_mold | template 1 at the time of casting, and the lower end of the immersion nozzle 2 is shown.

  In the breakout column in Table 1 below, the case where breakout did not occur was marked as ◯, and the case where breakout occurred was marked as x. Moreover, although the breakout did not occur, the temperature of the thermocouple installed in the mold at a position 100 mm away from the lower end of the immersion nozzle at the center of the long side of the mold in the slab drawing direction was the same height position. A case where the temperature rises 1.2 times or more than the average value of the temperature of other thermocouples installed at positions near the short side of the long side is indicated by Δ.

  The bubble trapping density index is an index of the bubble trapping density trapped in the slab, and an immersion nozzle having two slits of 70 mm in length and 55 mm in width without slits on the bottom wall is 2 Bubbles in the case of using a mold having two discharge holes facing both short sides of the mold, the shortest horizontal distance b 'being 37.5 mm, and both long side walls of the mold having a straight transverse section The trap density was 1.

  That is, when the bubble trapping density is larger than 1, the trapped amount of bubbles is increased as compared with the slab cast under the casting condition, and when the bubble trapping density is smaller than 1, the slab cast under the casting condition. This means that the amount of trapped air bubbles has decreased.

  As shown in FIG. 2, the bubble trapping density at the center of the long side of the mold is 100 mm each from the center of the long side of the cast slab 3 to both ends of the long side (total 200 mm), and short from both ends of the short side. A sample 4 having 10 mm in the center direction of the side and 10 mm in the casting direction was prepared, and then the number of bubbles was measured by transmitting X-rays through the sample 4, and the number of bubbles measured by the volume of the sample 4 was divided and calculated. .

  On the other hand, as shown in FIG. 3, the bubble trapping density at the long side end portion of the mold is 100 mm from the both ends of the long side of the cast slab 3 toward the center of the long side from the both ends of the long side, and from both ends of the short side. A sample 4 having a length of 10 mm in the central direction of the short side and 10 mm in the casting direction was prepared, and the number of bubbles was measured by transmitting X-rays through the sample, and the number of bubbles measured by the volume of the sample was divided.

  In addition, about what a breakout generate | occur | produced, the bubble capture density was investigated from the part cast before breakout generate | occur | produced.

Further, in the column of claim 1, “ Yes” is satisfied when each claim is satisfied, and “-” is not satisfied.

  According to Table 1 and FIG. 4 created based on Table 1, a / b ′ or a / b exceeded 0.6 in execution numbers 11, 14, 16, and 17 in which breakout occurred in Experiment A. Moreover, although breakout did not generate | occur | produce in the implementation numbers 8 and 12 with a / b 'or a / b exceeding 0.5 and 0.6 or less, from the lower end of the immersion nozzle in the long side center part of a casting_mold | template. The temperature of the thermocouple installed in the mold at a position 100 mm apart in the slab drawing direction is 1 than the average value of the temperatures of other thermocouples installed near the short side of the long side of the mold at the same height. Increased more than 2 times.

  This suggests that breakout may occur under conditions exceeding a / b ′ (= 0.53) of the execution number 8. Actually, for example, in No. 12, the breakout did not occur and the temperature at the central part of the long side of the mold only increased. However, the width a of the slit, that is, a / b, was made larger than that of No. 12. In run number 17, a breakout occurred. Therefore, it has been found that it is desirable to set a / b to a value smaller than 0.5 in order to surely prevent breakout.

  Moreover, from FIG. 5 created based on Table 1 and Table 1, in the implementation numbers 5 to 7 where the width a of the slit provided on the bottom wall of the immersion nozzle is 10 mm, the bubble trapping index at the long side edge of the mold is 1 Although not sufficiently reduced to 2 or 0.8, in the case of run numbers 8 to 18 where the width a of the slit was 20 mm or more, the bubble trapping index at the long side edge of the mold was 0.2 or 0.1. , Markedly decreased.

  Moreover, from FIG. 6 created based on Table 1 and Table 1, the shortest horizontal distance b provided the curved part in each long-side center part of a casting_mold | template upper part, and the implementation numbers 2-4, 6, 7, In 9, 10, 12 to 15, 17, and 18, the bubble trapping index at the center of the long side of the mold was 0.5 or less, which was significantly reduced.

  The above experiment A was based on the case of using an immersion nozzle having two outlet holes of 70 mm in length and 55 mm in width, which has no slit in the bottom wall and is installed in the direction of both short sides. 1) Needless to say, when the size of the ejection hole is changed, a suitable slit width a can be obtained by the same method as described above, assuming that the bubble trapping density at the changed size is 1.

  In order to prevent remelting of the solidified shell caused by the discharge flow from the slit connected to the two discharge holes formed in the bottom wall of the submerged nozzle colliding with the solidified shell at the center of the long side of the mold by the above experiment A It was found that the relationship a / b between the slit width a and the shortest horizontal distance b should be 0.6 or less.

  On the other hand, even when the a / b is 0.6 or less, the thermocouple installed in the mold at a position 100 mm away from the lower end of the immersion nozzle at the center of the long side of the mold in the slab drawing direction. Since the temperature is rising, in order to prevent local remelting of the solidified shell at the center of the long side of the mold and completely prevent the occurrence of breakout, a / b should be 0.5 or less. Is desirable.

  Further, from FIG. 7 created based on Table 1 and Table 1, when a / b is smaller than 0.3, when the width a of the slit is 10 mm, the bubble trapping index at the end of the long side of the mold increases. Yes. Accordingly, in order to effectively reduce the bubble trapping index at the long side end of the mold, it is desirable that a / b is 0.3 or more.

[Experiment B]
Under the same conditions as in Experiment A, the mold has a rectangular shape with both long side walls straight, and a curved part in which the central part of each long side of the upper part of the mold is curved outwardly from the mold, The curve of the curved part becomes smaller as it goes to the bottom, and a mold without the curved part is used at the lower part of the mold, and the electromagnetic brake is operated together with the electromagnetic stirrer so that the discharge hole part of the immersion nozzle is uniform in the short side direction of the mold. Table 2 below shows the casting results when a DC magnetic field having a magnetic flux density of 0.3 T was applied. The indices in Table 2 below are the same as in Table 1 above.

  As shown in FIGS. 4 to 6 created based on Table 2 and Table 2, when the electromagnetic brake is used in combination with the electromagnetic stirring, the solidification at the center of the long side is compared with the case where the electromagnetic brake is not operated only by the electromagnetic stirring. The same or equivalent effect was seen in the local remelting of the shell and the prevention of trapping of bubbles in the slab.

  In addition, when an electromagnetic brake is used in combination with electromagnetic stirring, when the width a of the slit formed on the bottom wall of the immersion nozzle is 20 mm or more under the same conditions as when the electromagnetic brake is not operated only by electromagnetic stirring. In addition, it was possible to remarkably reduce bubble trapping at the long side edge of the mold.

The present invention has been made based on the results of the experiments A and B by the inventors.
That is, the present invention is a method of continuously casting steel while electromagnetically stirring molten steel near the meniscus in a mold,
At the time of continuous casting, using a submerged nozzle formed on the bottom wall with a slit connected to two discharge holes provided in the side wall portion opposite to the short side wall surface of the mold below the meniscus in the mold,
The central part of each long side of the upper part of the mold has a curved part that is convexly curved toward the outside of the mold, and the curvature of the curved part becomes smaller toward the lower part of the mold, and hot water is supplied to the mold without the curved part at the lower part of the mold. When
The relationship between the width a in the thickness direction of the slit formed in the bottom wall of the immersion nozzle and the shortest horizontal distance b between the curved portion of the mold and the lower end of the immersion nozzle is a / b ≦ 0. This is a continuous casting method of steel that is continuously cast so as to be 5 .

The Keep present invention, when the shortest horizontal distance b between the lower end of the cast type immersion nozzle and the curved portion of the above 50mm, the bubbles trapped in the long side central portion and the long side edge portion of the mold more effectively It can be carried out.

  At that time, even if an electromagnetic brake is used in combination, the same or equivalent effect can be obtained with respect to remelting of the long side of the mold and prevention of trapping of bubbles in the slab.

  The present invention is not limited to the above example, and it goes without saying that the embodiments may be changed as appropriate within the scope of the technical idea described in each claim.

  For example, the above description does not describe the conditions of electromagnetic stirring performed in the present invention, but the inventors have confirmed through experiments that the effects of the present invention can be achieved as long as the conditions are in a range that is normally performed during continuous casting. ing.

  In addition, the inventors have confirmed through experiments that the electromagnetic brake conditions are not limited to those performed in Experiment B, and that the effects of the present invention can be achieved as long as the conditions are in a range that is normally performed during continuous casting.

DESCRIPTION OF SYMBOLS 1 Mold 1a Long side center part 1c Curved part 2 Immersion nozzle 2a Discharge hole 2b Slit

Claims (1)

A method of continuously casting steel while electromagnetically stirring the molten steel near the molten steel surface in the mold,
At the time of continuous casting, using a submerged nozzle in which slit-like discharge holes connected to two discharge holes provided on the side wall portion facing the short side wall surface of the mold below the molten steel surface in the mold are formed on the bottom wall,
The central part of each long side of the upper part of the mold has a curved part that is convexly curved toward the outside of the mold, and the curvature of the curved part becomes smaller toward the lower part of the mold, and hot water is supplied to the mold without the curved part at the lower part of the mold. When
The relationship between the width a in the thickness direction of the slit-shaped discharge hole formed in the bottom wall of the immersion nozzle and the shortest horizontal distance b between the curved portion of the mold and the lower end of the immersion nozzle is the shortest horizontal distance b. A continuous casting method of steel, wherein continuous casting is performed so that a / b ≦ 0.5 at 50 mm or more .

Images