JP2006231397A - Continuous casting method for aluminum-killed steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method for aluminum-killed steel under the high speed casting condition where the amount of the molten steel to be poured per minute is ≥4 ton, by which the clogging of an immersion nozzle caused by Al<SB>2</SB>O<SB>3</SB>in the molten steel is prevented, and consecutive continuous casting is performed. <P>SOLUTION: At the time when aluminum-killed molten steel 17 in a tundish is poured into a die 2 using a molten steel pouring means installed in the bottom part of a tundish 1, and composed of an upper nozzle 3 with a gas blowing part 3, a sliding nozzle 4 connected to the upper nozzle, and an immersion nozzle 9 connected to the sliding nozzle, the average pore size of porous brick composing the gas blowing part is controlled to 30 to 50 μm, and further, the aluminum-killed molten steel of ≥4.0 ton per minute is poured into the die while gaseous Ar is blown therein from the gas blowing part in such a manner that the amount of the gaseous Ar to be blown reaches ≥2.0 NL per ton of the amount of the molten steel to be poured. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a continuous casting method of aluminum killed steel, and more particularly to a continuous casting method capable of preventing the immersion nozzle from being blocked by Al ₂ O _{3 in} molten steel.

Aluminum killed steel is manufactured by deoxidizing molten steel that has been oxidatively decarburized and refined with Al, and removing oxygen in the molten steel that has been increased by oxidative decarburization and refining. Al ₂ O ₃ particles (alumina particles) generated in this deoxidation process are removed from the molten steel by utilizing the density difference between the molten steel and Al ₂ O ₃ , but a minute Al ₂ O _{3 of} several tens μm or less. Since the floating speed of the particles is extremely slow and it takes a long time for the floating separation, it is extremely difficult to completely float and separate such fine Al ₂ O ₃ particles in the actual process. Inside, fine Al ₂ O ₃ particles remain in a suspended state.

In continuous casting of steel, molten steel is poured from a tundish into a mold using a refractory immersion nozzle. The properties required for the immersion nozzle is that excellent melting loss resistance to thermal shock resistance and mold powder and the molten steel, therefore, is excellent in these properties Al ₂ O ₃ - immersion of graphite or Al ₂ O ₃ Quality Nozzles are widely used. However, Al ₂ O ₃ - when casting the aluminum-killed steel using immersion nozzle of graphite or Al ₂ O ₃ quality, Al ₂ O ₃ particles suspended in molten steel adheres and accumulates on the immersion nozzle inner wall surface The problem that the submerged nozzle is blocked occurs.

When the immersion nozzle is blocked, various problems occur in the casting operation and the slab quality. For example, the slab drawing speed has to be reduced, and not only the productivity is lowered, but in a severe case, the casting operation itself must be stopped. In addition, if the Al ₂ O ₃ particles that have accumulated on the inner wall surface of the immersion nozzle suddenly peel off and are ejected into the mold and trapped by the solidified shell in the mold, they become product defects, resulting in a decrease in product yield. Leads to.

For these reasons, conventionally, in continuous casting of aluminum killed steel, means for preventing adhesion of Al ₂ O ₃ to the surface of the inner wall of the immersion nozzle have been studied and many proposals have been made. As one of the means, an inert gas such as Ar gas is blown into the inner wall of the immersion nozzle to forcibly clean Al ₂ O ₃ adhering to the inner wall of the immersion nozzle by the inert gas, or the inner wall of the immersion nozzle A technique has been proposed in which a gas film is formed between steel and molten steel so that Al ₂ O ₃ does not contact the wall.

For example, Patent Document 1 discloses a molten steel injection means that includes an upper nozzle having an Ar gas blowing portion, a sliding nozzle connected to the upper nozzle, and an immersion nozzle connected to the sliding nozzle, installed at the bottom of the tundish. When pouring the aluminum killed steel into the mold using Ar, and blowing the Ar gas with the brick pore diameter of the gas blowing part being 30 μm to 50 μm, the molten steel is 1.0 to 2.0 tons per minute. A continuous casting method for casting is disclosed.
JP-A-3-297545

However, Patent Document 1 has the following problems. That is, in order to prevent the adhesion of Al ₂ O ₃ by utilizing the cleaning effect of Ar gas, it is necessary to set the amount of Ar gas blown with respect to the amount of molten steel to be cast. Absent. Moreover, although the casting amount per minute in Patent Document 1, that is, the molten steel injection amount into the mold is 1.0 to 2.0 tons, the slab drawing speed is increased with the productivity improvement technology of the continuous casting machine. In recent years, the amount of molten steel injected per minute has reached 4 tons or more, and it is not clear how to blow in such high-speed casting conditions.

The present invention has been made in view of such circumstances, and the object of the present invention is to continuously cast aluminum killed steel under high-speed casting conditions in which the molten steel injection amount per minute is 4 tons or more. It is an object of the present invention to provide a continuous casting method of aluminum killed steel that can prevent the immersion nozzle from being blocked by Al ₂ O ₃ .

  An aluminum killed steel continuous casting method according to the present invention for solving the above-described problems is an upper nozzle having a gas blowing portion installed at the bottom of a tundish, a sliding nozzle connected to the upper nozzle, and a connection to the sliding nozzle. When injecting aluminum killed molten steel in the tundish into the mold using molten steel injecting means composed of an immersion nozzle, the average pore diameter of the porous brick constituting the gas blowing portion is set to 30 μm to 50 μm In addition, while blowing Ar gas from the gas blowing portion so that the Ar gas blowing amount is 2.0 NL or more per ton of molten steel, 4.0 tons or more of molten aluminum killed steel is injected into the mold per minute. It is what.

According to the present invention, since the average pore diameter of the porous brick constituting the gas blowing section is 30 μm to 50 μm, 2.0 NL or more of Ar gas per ton of molten steel can be stably blown from the upper nozzle for 1 minute. Even with continuous casting of aluminum killed molten steel with a molten steel injection amount of 4.0 tons or more, Al ₂ O ₃ adhesion to the inner wall surface of the immersion nozzle can be suppressed, and the immersion nozzle is blocked by Al ₂ O _3. Is achieved. As a result, the casting time can be dramatically extended, and at the same time, due to defects in large inclusion physical properties of the slab caused by coarse Al ₂ O ₃ peeling from the inner wall of the immersion nozzle, and due to the closure of the immersion nozzle Mold powder defects caused by the drift of molten steel in the mold can be greatly reduced, and an industrially beneficial effect is brought about.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic view of a mold part of a slab continuous casting machine used in carrying out a continuous casting method according to the present invention.

  In FIG. 1, the outer shell is covered with an iron shell 15 at a predetermined position above the mold 2 constituted by the opposed mold long side 13 and the opposed mold short side 14 provided inside the mold long side 13. The tundish 1 with the refractory 16 applied inside is arranged. At the bottom of the tundish 1, an upper nozzle 3 that fits the refractory 16 is installed, and in contact with the lower surface of the upper nozzle 3, the upper fixing plate 5, the sliding plate 6, the lower fixing plate 7, and the rectification A sliding nozzle 4 comprising a nozzle 8 is disposed, and further, an immersion nozzle 9 having a pair of discharge holes 10 in contact with the lower surface of the sliding nozzle 4 is disposed below the sliding nozzle 4, and the molten steel outflow hole from the tundish 1 to the mold 2 11 is formed. That is, an upper nozzle 3, a sliding nozzle 4 and an immersion nozzle 9 are installed as means for pouring molten steel from the tundish 1 into the mold 2.

  The immersion nozzle 9 is used with its tip immersed so that the discharge hole 10 installed in the lower part is buried in the molten steel 17 in the mold. The sliding plate 6 is connected to the reciprocating actuator 12, and the reciprocating actuator 12 is operated to move between the upper fixing plate 5 and the lower fixing plate 7 while being in contact with these fixing plates. The amount of molten steel passing through the molten steel outflow hole 11 is controlled by adjusting the opening area formed by the plate 6, the upper fixing plate 5 and the lower fixing plate 7.

  An enlarged view of the upper nozzle 3 is shown in FIG. As shown in FIG. 2, the upper nozzle 3 is composed of three parts: an upper blowing part 3a, a lower blowing part 3b, and a main body part 3e located between the upper blowing part 3a and the lower blowing part 3b. An iron skin 3f is arranged on the outer periphery. The upper blowing portion 3a and the lower blowing portion 3b are gas blowing portions, and are formed of alumina porous brick. The main body 3e is made of a relatively dense alumina material. In FIG. 2, the upper blowing portion 3a and the lower blowing portion 3b are displayed so as to be clearly distinguishable from the main body portion 3e, but there is actually no clear boundary, and the alumina brick forming the main body portion 3e, The alumina porous brick forming the upper blowing portion 3a and the lower blowing portion 3b is formed so as to gradually change the blending ratio. That is, they are integrally formed.

  Two gas introduction pipes 3c and 3d are disposed through the iron skin 3f, the gas introduction pipe 3c opens to the upper blowing part 3a, and the gas introduction pipe 3d opens to the lower blowing part 3b. That is, Ar gas supplied from the gas introduction pipe 3c is blown into the molten steel outflow hole 11 through the upper blowing part 3a, while Ar gas supplied from the gas introduction pipe 3d is passed through the lower blowing part 3b. It is configured to be blown into the molten steel outflow hole 11. The iron skin 3 f disposed on the outer periphery of the upper nozzle 3 also has the purpose of ensuring the strength of the upper nozzle 3, but prevents Ar gas from flowing out from the outer peripheral surface of the upper nozzle 3. Therefore, the Ar gas supplied from the gas introduction pipes 3 c and 3 d is surely blown into the molten steel outflow hole 11. The gas introduction pipes 3c and 3d are connected to independent Ar gas supply devices, respectively, and the Ar gas supply amount is controlled independently.

  In this invention, the average pore diameter of the porous brick of the upper blowing part 3a which is a gas blowing part, and the lower blowing part 3b shall be 30 micrometers-50 micrometers. In other words, an upper nozzle in which the upper blowing portion 3 a and the lower blowing portion 3 b are made of porous bricks having an average pore diameter of 30 μm to 50 μm is used as the upper nozzle 3. As will be described later, in the present invention, when molten steel 17 of 4.0 tons or more per minute is injected into the mold 2, Ar gas of 2.0 NL or more is injected from the upper nozzle 3 per ton of molten steel injection. When the average porosity of the porous brick is less than 30 μm, Ar gas of 2.0 NL or more per 1 ton of molten steel injection cannot be stably blown. On the other hand, if the average pore diameter of the porous brick exceeds 50 μm, the Ar gas bubbles become too large, the cleaning effect is lowered, and the fluctuation of the molten metal surface on the molten steel surface 9 is increased.

  FIG. 3 is a result of investigating the bubble diameter of Ar gas in water using a porous brick having an average pore diameter of 40 μm and a porous brick having an average pore diameter of 20 μm. As the average pore diameter of the porous brick increases, the average bubble diameter of Ar gas increases under the same Ar gas flow rate. Therefore, the average pore diameter of the porous brick can be grasped by measuring the Ar gas bubbles in the water. In order to maintain and produce an average pore diameter of porous brick in the range of 30 μm to 50 μm, the particle size of the refractory powder raw material is adjusted, and the compression molding force when compressing the porous brick from the powder raw material is adjusted. It can obtain by specifying raw material conditions and manufacturing conditions.

  In addition, as a method for directly measuring the average pore diameter of the porous brick, there are the following methods. That is, when the porous brick is buried in mercury and a pressure corresponding to a specific pore diameter is applied, the amount of mercury sucked into the porous brick is obtained, and the ratio of the corresponding pore diameter is obtained from this mercury amount. This measurement is performed under various pressures to obtain the distribution of the entire pore diameter and then determine the average diameter. If the distribution map of the average bubble diameter of Ar gas as shown in FIG. 3 is determined using the porous brick whose average pore diameter is measured in this way, then it is not necessary to directly measure the pore diameter, The average pore diameter can be grasped by measuring the Ar gas bubble diameter in water.

  The continuous casting method of this invention is implemented as follows using the slab continuous casting machine comprised in this way.

  A molten steel 17 of aluminum killed steel melted in a primary refining furnace such as a converter or electric furnace or a secondary refining furnace such as an RH vacuum degassing apparatus is poured into a tundish 1 from a ladle (not shown). When the amount of molten steel in the dish reaches a predetermined amount, the sliding plate 6 is opened, and the molten steel 17 is injected into the mold 2 through the molten steel outflow hole 11. The molten steel 17 is injected into the mold as a discharge flow 18 from the discharge hole 10 toward the mold short side 14. The molten steel 17 injected into the mold is cooled by the mold 2 to form a solidified shell 21. Then, if a predetermined amount of molten steel 17 is injected into the mold, a pinch roll (not shown) installed below the mold 2 is driven in a state where the discharge hole 10 is immersed in the molten steel 17 in the mold, The outer shell is the solidified shell 21 and the drawing of the slab having the unsolidified molten steel 17 inside is started. After the start of drawing, the slab drawing speed is increased so that molten steel 17 of 4.0 tons or more per minute is injected into the mold while controlling the position of the molten steel surface 19 to a substantially constant position in the mold. And maintain that speed. Mold powder 20 is added on the molten steel surface 19 in the mold. The mold powder 20 melts to prevent oxidation of the molten steel 17 and flows between the solidified shell 21 and the mold 2 to exert an effect as a lubricant.

  During the casting, Ar gas is introduced into the molten steel outflow hole 11 while adjusting the flow rate of gas blown in accordance with the passing mass of the molten steel 17 flowing down the molten steel outflow hole 11 from the upper blowing portion 3a and the lower blowing portion 3b of the upper nozzle 3. Infuse.

  Specifically, the total amount of Ar gas blown from the upper blower 3a and the lower blower 3b when pouring 4.0 tons or more of aluminum killed molten steel per minute into the mold is per ton of molten steel. In contrast, Ar gas is blown so as to be 2.0 NL (standard state conversion) or more. In this case, the ratio of the blowing amount from the upper blowing portion 3a and the blowing amount from the lower blowing portion 3b is not particularly specified and may be about 1: 1, but the upper blowing portion 3a is blown. Since the area is large, in order to blow in uniformly, it is preferable to be about 10: 4 to 10: 7.

If the Ar gas blowing amount is less than 2.0 NL per ton of molten steel, the adhesion of Al ₂ O ₃ to the immersion nozzle 9 becomes intense, so the Ar gas blowing amount must be 2.0 NL or more per ton of molten steel. There is. However, if the Ar gas blowing amount becomes too large, the molten metal surface 19 may fluctuate due to the Ar gas bubbles floating on the molten steel 17 in the mold, and the mold powder 20 may be entrained. It is preferably 3.6 NL or less per ton. In addition, when the injected amount of molten steel 17 per minute is less than 4.0 tons, the flow rate of the discharge flow 18 is slow and the flow rate of the molten steel surface 19 is also slowed. Although it is necessary to change, since this invention intends high-speed casting, it shall not mention especially here.

Thus, the average pore diameter of the porous brick constituting the gas blowing section is set to 30 μm to 50 μm, and 2.0 NL or more of Ar gas is blown from the upper nozzle 3 per ton of molten steel, so that 4.0 tons or more per minute Even in the continuous casting of molten aluminum killed steel, Al ₂ O ₃ adhesion to the inner wall surface of the immersion nozzle can be suppressed, and the immersion nozzle can be prevented from being blocked by Al ₂ O ₃ .

  In the above description, the example of two gas blowing portions has been described. However, the present invention can be applied to the above-described case even when the number of gas blowing portions is one or three or more. In the above description, the example of the sliding nozzle 4 having the three-plate configuration is given, but the present invention can also be applied to the sliding nozzle having the two-plate configuration.

  Using a slab continuous casting machine shown in FIG. 1 in which an upper nozzle having a porous brick having an average pore diameter of 40 μm as an upper blowing portion and a lower blowing portion was arranged, a test was performed by changing the Ar gas blowing amount.

The capacity of the tundish was 50 tons, and a slab slab having a thickness of 250 mm and a width of 950 mm was cast at a drawing speed of 2.5 m / min. In this case, the molten steel injection amount per minute is 4.66 tons, and the Ar gas blowing amount according to the molten steel pouring amount is Ar gas blowing amount (NL / min) / molten steel pouring amount (ton / min). The ratio was changed in the range of 1.3 to 4.2 NL / ton. The ratio of the Ar gas blowing amount from the upper blowing portion and the Ar gas blowing amount from the lower blowing portion was 10: 6 in all tests. Under these conditions, 6-charge continuous continuous casting (“continuous casting”) was performed, and after casting, the amount of Al ₂ O ₃ deposited on the inner wall of the immersion nozzle was measured. Further, the cast slab was hot-rolled and further cold-rolled to produce a thin steel plate, and the surface defects of the inclusion physical properties in this thin steel plate were investigated.

FIG. 4 shows the result of measuring the Al ₂ O ₃ adhesion amount on the inner wall of the immersion nozzle. As shown in FIG. 4, when the ratio of Ar gas injection amount / molten steel injection amount is 2.0 NL / ton or more, the Al ₂ O ₃ adhesion amount is small, the 6-charge continuous casting has no problem, and the continuous casting is continued. It was confirmed that it was possible. On the other hand, when the ratio of Ar gas injection amount / molten steel injection amount is less than 2.0 NL / ton, Al ₂ O ₃ adhesion on the inner wall of the immersion nozzle is severe, and it is impossible to continue casting over 6 charges. It was confirmed.

  In FIG. 5, the result of having investigated the inclusion physical property surface defect in a thin steel plate is shown. In this case, when the ratio of Ar gas blowing amount / molten steel injection amount was in the range of 1.3 to 4.2 NL / ton, the occurrence rate of surface defects was low, and it was confirmed that there was no problem. However, when the ratio of the Ar gas injection amount / molten steel injection amount exceeds 3.6 NL / ton, the incidence of surface defects tends to be slightly higher. Therefore, from the viewpoint of suppressing the inclusion surface defects in the thin steel sheet, It was confirmed that the ratio of Ar gas blowing amount / molten steel injection amount is preferably 3.6 NL / ton or less.

It is the schematic of the casting_mold | template part of the slab continuous casting machine used when implementing the continuous casting method by this invention. It is an enlarged view of the upper nozzle shown in FIG. It is a figure which shows the result of having investigated the bubble diameter of Ar gas generated from a porous brick in water. In Example 1, a diagram illustrating the results of measurement of the Al ₂ O ₃ deposition amount of the immersion nozzle inner wall. In Example 1, it is a figure which shows the result of having investigated the inclusion physical property surface defect in a thin steel plate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Tundish 2 Mold 3 Upper nozzle 3a Upper blowing part 3b Lower blowing part 3c Gas introduction pipe 3d Gas introduction pipe 3e Main part 3f Iron skin 4 Sliding nozzle 5 Upper fixed plate 6 Sliding plate 7 Lower fixed plate 8 Rectification nozzle 9 Immersion Nozzle 10 Discharge hole 11 Molten steel outflow hole 12 Reciprocating actuator 13 Mold long side 14 Mold short side 15 Iron skin 16 Refractory 17 Molten steel 18 Discharge flow 19 Molten steel surface 20 Mold powder 21 Solidified shell

Claims (1)

  The tundish is installed in the tundish using a molten steel injection means comprising an upper nozzle having a gas blowing portion, a sliding nozzle connected to the upper nozzle, and an immersion nozzle connected to the sliding nozzle. When pouring aluminum killed molten steel into the mold, the average pore diameter of the porous brick constituting the gas blowing portion is set to 30 μm to 50 μm, and the Ar gas blowing amount is set to 2.0 NL or more per ton of molten steel. A continuous casting method of aluminum killed steel, wherein 4.0 tons or more of molten aluminum killed steel is injected into the mold while Ar gas is being blown into the gas blower.

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