JP2011194420A - Method of producing high cleanliness steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture high cleanliness steel by efficiently preventing slag from flowing out from a ladle to a tundish when pouring molten steel from the ladle to the tundish.SOLUTION: In the method of producing high cleanliness steel, porous bricks 6 are arranged in a manner surrounding a molten steel outflow hole 8 in the bottom of the ladle 1 and, while an inert gas is blown from the porous bricks into the molten steel 9 in the ladle, the molten steel in the ladle is poured into the tundish through the molten steel outflow hole. In this case, it is desirable to adjust the flow rate of blowing the inert gas in accordance with the quantity of the molten steel in the ladle, so that a relation between the inert gas flow rate G (Nm/sec) blown from the porous bricks into the molten steel and the depth H (m) of the molten steel inside the ladle is within the range of an expression (1): G×H<0.010.

Description

  The present invention relates to a method for producing high cleanliness steel by preventing the outflow of slag from a ladle to a tundish when pouring molten steel from a ladle into a tundish.

  Due to the increasing demand for higher functionality and higher quality of steel materials, it is desired to reduce the impurity components (P, S, etc.) and impurities (nonmetallic inclusions, etc.) in steel to the utmost limit. In the stage, there is a need for techniques for increasing the purity and cleanliness of steel.

At present, general steel is produced by decarburizing and refining hot metal in a converter, and then performing secondary refining, such as vacuum degassing, on the molten steel discharged from the converter to the ladle. Thereafter, the molten steel in the ladle is manufactured by a process of pouring the molten steel outflow hole provided in the bottom of the ladle into the tundish and casting from the tundish into a continuous casting mold to obtain a slab. In this manufacturing process, when the molten steel in the ladle is poured into the tundish, a vortex is generated above the molten steel outflow hole of the ladle as the amount of molten steel remaining in the ladle decreases. Slag existing on the molten steel is entrained, and a phenomenon occurs in which the slag flows out to the tundish together with the molten steel. When the slag flowing out to the tundish itself or Al ₂ O ₃ inclusions generated by the reaction of the lower oxides in the slag with Al in the molten steel remains in the molten steel, The cleanliness of the steel deteriorates and causes defects in the steel sheet after rolling.

  Various methods have been proposed to prevent the slag from flowing out of the ladle into the tundish. For example, in Patent Document 1, at the end of the pouring of molten steel from the ladle to the tundish, the molten steel outflow hole of the ladle is blocked at the stage of the remaining steel height in the ladle that can prevent the vortex flow in the ladle. A method of doing this has been proposed. According to Patent Document 2, when the level of molten steel in the ladle is lowered and vortex is generated, a refractory floating valve is placed directly above the molten steel outflow hole of the ladle to prevent the generation of vortex. A method has been proposed.

  In Patent Document 3, a static magnetic field is applied to the molten steel injection flow from the ladle to the tundish before the vortex flow is generated above the molten steel outflow hole of the ladle. A method of applying a rotating magnetic field in the opposite direction has been proposed. Patent Document 4 discloses a method for preventing slag from being mixed in the steel flow from the converter to the ladle, not the molten steel injection flow from the ladle. A method has been proposed in which gas is blown from a plurality of pipe-shaped nozzles provided around the slag to prevent generation of vortex flow and to eliminate slag directly above the steel outlet.

JP 2003-230947 A JP-A-62-203667 JP-A-2-54711 JP-A-62-230930

  However, the above prior art has the following problems.

  That is, in the method described in Patent Document 1, the amount of molten steel remaining in the ladle increases, such as treatment of residual molten steel, reduction in molten steel yield, increased load of ladle maintenance due to adhesion of metal in the ladle, Various adverse effects other than the quality of steel occur, and application to actual operation is not realistic.

  In the method described in Patent Document 2, a device for raising and lowering the floating valve and a device for crushing the slag surface are also required when the slag surface in the ladle is solidified before dropping the floating valve. Also, the floating valve itself, which is a consumable item, is expensive and expensive.

  The method described in Patent Document 3 requires not only a magnetic field generator for applying a static magnetic field or a rotating magnetic field, but also a device for detecting the time when eddy currents are generated, which increases equipment costs. In addition, a magnetic field is applied to the molten steel injection flow flowing down the molten steel outflow hole, and the flowing molten steel injection flow certainly rotates, but the molten steel injection flow immediately flows down. It is extremely questionable whether the effect of the rotating magnetic field works.

  In the method described in Patent Document 4, since the gas is blown through the pipe-shaped nozzle, if the steel outlet is in contact with the molten steel only at the time of steel output like a converter, the gas is blown only at that time. However, if the molten steel outflow hole is always in contact with the molten steel from the time of receiving the steel from the converter to the end of pouring of the molten steel into the tundish, such as a ladle, gas will always be blown in during that period. Otherwise, the pipe-shaped nozzle that is a gas blowing pipe will be blocked, and a necessary amount of gas cannot be blown when molten steel is poured from the ladle into the tundish. That is, the method described in Patent Document 4 cannot be applied to a ladle.

  The present invention has been made in view of the above circumstances, and its purpose is to efficiently prevent the outflow of slag from the ladle to the tundish at the time of pouring molten steel from the ladle to the tundish. It is to provide a method for producing steel.

  The method for producing a high cleanliness steel according to the first aspect of the present invention for solving the above-mentioned problem is that porous brick is disposed so as to surround the molten steel outflow hole at the bottom of the ladle, and the molten steel in the ladle is not disposed from the porous brick. The molten steel in the ladle is poured into the tundish through the molten steel outflow hole while blowing the active gas.

The manufacturing method of the high cleanliness steel according to the second invention is the first invention, wherein the relationship between the flow rate of inert gas blown into the molten steel from the porous brick and the molten steel depth in the ladle is the following (1) The flow rate of the inert gas blown from the porous brick is adjusted according to the amount of molten steel in the ladle so as to be within the range of the formula.
G × H ² <0.010 (1)
However, in the formula (1), G is the flow rate of the inert gas blown from the porous brick (Nm ³ / sec), and H is the depth of molten steel (m) in the ladle.

  The method for producing a high cleanliness steel according to the third invention is the method according to the first or second invention, wherein the upper nozzle constituting the molten steel outflow hole is made of porous brick, and an inert gas is blown from the porous brick. It is characterized by.

  According to a fourth aspect of the present invention, there is provided a method for producing a high cleanliness steel according to the first or second aspect, wherein a porous brick is arranged around an upper nozzle constituting the molten steel outflow hole, and an inert gas is supplied from the porous brick. It is characterized by blowing.

  According to the present invention, since the molten steel in the ladle is injected into the tundish while blowing the inert gas from the porous brick arranged around the molten steel outflow hole, conventionally, it occurs when the amount of molten steel in the ladle decreases. The generation of eddy currents in the ladle that has been used can be suppressed, and this prevents the slag in the ladle from flowing into the tundish, that is, the cause of defects in steel products. Therefore, it is possible to suppress the outflow of the ladle slag to the tundish, so that steel with high cleanliness can be obtained, and industrially beneficial effects such as improvement of the quality and yield of the steel product are brought about.

It is the schematic of the continuous casting installation which implemented this invention. It is the schematic when the ladle shown in FIG. 1 is seen from upper direction. It is a figure which shows the result of having investigated the grade of the air oxidation of molten steel by changing variously the molten steel depth in the ladle at the time of inert gas blowing flow and inert gas blowing. It is a figure which shows the component analysis result of the slag in a tundish in contrast with the example of this invention, and a comparative example.

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

  When fluid is suddenly collected at a particular location, such as an outlet, if each portion of fluid has a momentum in a direction horizontal to that location, it will be centered on that location. While rotating, it gradually moves toward the center, and as it approaches the center, the rotational speed increases according to the law of conservation of angular momentum. Actually, since each portion has a momentum in different directions, they gradually collide with each other and eventually rotate in a certain direction, resulting in a vortex.

  Therefore, in the ladle containing the molten steel, we studied to suppress the formation of vortex flow by dispersing the horizontal momentum of the molten steel toward the molten steel outflow hole, which causes vortex generation.

  As a result, in the ladle, the molten steel outflow hole is located at the bottom of the ladle, and the horizontal momentum of the molten steel increases as it approaches the bottom of the ladle. It was found that blowing gas into the molten steel was effective. That is, the momentum of the molten steel toward the molten steel outflow hole is dispersed upward by the injected gas, and the generation of vortex is suppressed. In this case, the gas was blown from the bottom of the ladle so as to surround the molten steel outflow hole.

  The gas was blown from porous brick. This is because when a gas is blown by a nozzle, there is a region where no gas exists between adjacent nozzles, and the molten steel flows preferentially there, thereby increasing the momentum of the molten steel and promoting the generation of eddy currents. In addition, in a ladle, gas must always be blown to prevent clogging of the nozzle due to insertion of molten steel. On the other hand, in the case of porous brick, it is possible to blow gas uniformly around the molten steel outflow hole, and even if the molten steel is accommodated in the ladle, the pore diameter is small. It is possible to prevent molten steel from being inserted without blowing, and even if the time from receiving the steel to pouring into the tundish becomes longer, it is possible to start gas blowing at the stage of pouring into the tundish. is there.

  The present invention was made on the basis of the above examination results, placing porous brick so as to surround the molten steel outflow hole at the bottom of the ladle, while blowing inert gas from the porous brick into the molten steel in the ladle, The molten steel in the ladle is poured into the tundish through the molten steel outflow hole.

  In the present invention, the gas blowing direction is set to be upward in the vertical direction, that is, the direction toward the molten steel surface in the ladle. This is because when the gas is blown into the molten steel outflow hole, most of the blown gas flows downward along with the injection flow flowing down the molten steel outflow hole, and there is a concern that it is not used for suppressing the generation of vortex flow.

  As the method of blowing gas upward in the vertical direction, the refractory at the upper end of the refractory group constituting the molten steel outflow hole is made into porous brick with the upper exposed, and the molten steel outflow hole is configured. There are two methods of arranging a porous brick so as to surround the refractory at the upper end of the refractory group, but the same effect can be obtained by using either method. In addition, the molten steel outflow hole of the ladle is generally configured by combining an upper nozzle, a sliding nozzle, a rectifying nozzle, and a long nozzle from the upper side, and the above-mentioned refractory group constituting the molten steel outflow hole. "Upper nozzle" means "upper nozzle".

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic view of a continuous casting facility embodying the present invention, and FIG. 2 is a schematic view when the ladle shown in FIG. 1 is viewed from above.

  As shown in FIG. 1, a ladle 1 containing molten steel 9 is arranged at a predetermined position above the tundish 2. In the ladle, slag 10 that has flowed out of the molten steel 9 when the steel is discharged from the converter (not shown) to the ladle 1 stays on the molten steel 9. At the bottom of the ladle 1, an upper nozzle 3 that fits with the bottom refractory of the ladle 1 is disposed, and a sliding nozzle 4 is disposed in connection with the lower portion of the upper nozzle 3. A long nozzle 5 is connected to the lower part, and a molten steel outflow hole 8 from the ladle 1 to the tundish 2 is formed by the upper nozzle 3, the sliding nozzle 4 and the long nozzle 5. The sliding nozzle 4 is a device that controls the flow rate of the molten steel injection flow that flows down the molten steel outflow hole 8 and is composed of three plates, an upper fixed plate, a sliding plate, and a lower fixed plate. Is configured to be injected into the tundish 2 through the inner holes of the upper nozzle 3, the sliding nozzle 4 and the long nozzle 5 while the flow rate is controlled by the sliding nozzle 4. In general, a rectifying nozzle is disposed between the sliding nozzle 4 and the long nozzle 5, but is not disposed in FIG.

  As shown in FIG. 2, a porous brick 6 connected to the gas supply pipe 7 is disposed around the upper nozzle 3 at the bottom of the ladle 1 so as to surround the upper nozzle 3. The porous brick 6 is exposed at the bottom of the ladle so that the surface thereof is in direct contact with the molten steel 9. That is, an inert gas such as Ar gas supplied from the gas supply pipe 7 is blown from the porous brick 6 into the ladle. 1 and 2, the porous brick 6 is disposed around the upper nozzle 3, but the upper end of the upper nozzle 3 is composed of porous brick with the upper surface exposed, and the upper end of the upper nozzle 3. An inert gas may be blown from.

  As shown in FIG. 1, the ladle 1 containing the molten steel 9 that has been discharged from the converter to the ladle 1 and further subjected to secondary refining such as vacuum degassing refining is placed at a predetermined position above the tundish 2. Installed, the sliding nozzle 4 is opened, and the molten steel 9 in the ladle is poured into the tundish 2. When a predetermined amount of molten steel 9 has accumulated in the tundish, casting from the tundish 2 into a mold (not shown) is started. During casting, a predetermined amount of molten steel 9 stays in the tundish 2 and the lower part of the long nozzle 5 is maintained in a state immersed in the molten steel 9 in the tundish.

  If casting progresses and the amount of molten steel in the ladle is reduced, blowing of inert gas from the porous brick 6 is started before the vortex is generated in the molten steel 9 at the latest. If it is difficult to observe the eddy current, it is desirable to start the gas blowing at an early stage. Naturally, the injection of the inert gas from the porous brick 6 may be started simultaneously with the start of the injection of the molten steel 9 into the tundish 2.

  However, if the flow rate of the inert gas blown from the porous brick 6 is too large, the slag 10 existing on the molten steel is discharged by the rising inert gas bubbles, and the molten steel surface is exposed. When the molten steel surface is exposed, there is a concern about oxidation of the molten steel 9 in the ladle by air. Therefore, it is preferable to control the flow rate of the inert gas so that the molten steel surface is not exposed.

  In this regard, the present inventors investigated the influence of oxidation of the molten steel 9 by air by variously changing the flow rate of the inert gas blown from the porous brick 6 and the depth of the molten steel in the ladle when the inert gas was blown. . In addition, the grade of the air oxidation of the molten steel 9 was evaluated by the reduction | decrease amount of dissolved Al in molten steel. Al has a strong affinity for oxygen, and when air oxidation occurs, it can be regarded as a decrease in dissolved Al in molten steel. When there is little air oxidation, the amount of reduction of dissolved Al in molten steel is small.

The survey results are shown in FIG. As shown in FIG. 3, when the inert gas blowing flow rate from the porous brick 6 is G (Nm ³ / sec) and the molten steel depth in the ladle when the inert gas is blown is H (m), the inert gas Depending on the molten steel depth (H) in the ladle, that is, the amount of molten steel, so that the blowing flow rate (G) and the molten steel depth (H) in the ladle are within the range of the following formula (1): It was confirmed that air oxidation of the molten steel 9 can be prevented by adjusting the flow rate (G) of the inert gas blown from the porous brick 6.
G × H ² <0.010 (1)
Therefore, in the present invention, it is preferable to adjust the flow rate of the inert gas blown from the porous brick 6 in accordance with the amount of molten steel in the ladle within the range satisfying the expression (1). Here, the molten steel depth (H) in the ladle is the distance (m) from the molten steel surface in the ladle to the bottom of the ladle, as shown in FIG.

  In order to satisfy the above equation (1), it is necessary to grasp the molten steel depth (H) in the ladle. The molten steel depth (H) in the ladle may be calculated from the molten steel mass in the ladle measured with a load cell based on the design dimensions of the ladle 1, or from within the ladle from above with an appropriate distance meter. The distance to the surface of the molten steel may be measured, and the difference from the thickness of the bottom of the ladle 1 measured in advance may be obtained.

  In order to satisfy the formula (1), it is necessary to reduce the flow rate (G) of the inert gas blown at the beginning of the injection. In addition, the flow rate of the gas blowing is not particularly required as long as the expression (1) is satisfied. However, from the viewpoint of preventing the slag 10 from flowing out to the tundish 2, the vicinity of the upper limit satisfying the expression (1) is satisfied. It is preferable to use a value. In particular, when the injection flow rate of the molten steel 9 into the tundish 2 increases, eddy currents are likely to be generated. Therefore, when the injection flow rate is high, it is preferable to set the upper limit value that satisfies the expression (1).

  When the gas is blown into the molten steel, the apparent specific gravity of the molten steel 9 is reduced, and the injection flow rate of the molten steel 9 from the ladle 1 to the tundish 2 may be reduced as compared with the case where the gas blowing is not performed. In that case, it can be coped with by increasing the inner diameter of the molten steel outflow hole 8. Further, if the inner diameter of the molten steel outflow hole 8 is increased, the vortex flow becomes more difficult to be generated (see 1992 STEELMAKING CONFERENCE PROCEEDINGS, p.665), and further effects can be expected.

  As described above, according to the present invention, the molten steel 9 accommodated in the ladle 1 is injected into the tundish 2 while blowing an inert gas from the porous brick 6 arranged around the molten steel outflow hole 8. The generation of eddy currents in the ladle can be suppressed, and thereby, the slag 10 in the ladle is suppressed from flowing out to the tundish 2, that is, it causes a defect in the steel product. Since the outflow of the ladle slag 10 to the tundish 2 can be suppressed, steel with high cleanliness can be obtained.

  About 250 tons of molten steel was produced by decarburizing and refining the hot metal in a converter, and after removing the molten steel in a ladle, degassing and refining were performed using an RH vacuum degassing apparatus. During this degassing refining, yttrium oxide was added to the slag on the molten steel in the ladle so as to have a concentration of about 5% by mass in order to observe the outflow situation of the slag in the ladle to the tundish.

Thereafter, the ladle was conveyed above the tundish installed above the mold of the continuous casting machine, and a long nozzle was connected to the lower part of the sliding nozzle. Next, the sliding nozzle was opened and injection of molten steel into the tundish was started. When the amount of molten steel in the tundish reaches a predetermined amount, supply of molten steel from the tundish to the two strands of the continuous casting machine is started, and then the molten steel supply amount to each strand becomes 5 tons / min. Was controlled as follows. Further, when the amount of molten steel in the tundish reached a predetermined amount, a flux containing CaO, SiO ₂ and MgO was added onto the molten steel in the tundish. The flux melts into slag and functions as a heat insulator and an air antioxidant for the molten steel in the tundish.

Then, from the point when the molten steel depth in the ladle reaches 1.0 m, 100 NL / min (0.00167 Nm ³ / sec) of Ar gas from the porous brick arranged around the upper nozzle constituting the molten steel outflow hole. Blowing started.

  When the pouring of molten steel from the ladle to the tundish was completed, the ladle was replaced with the next processing order, and the molten steel was subsequently poured from the ladle into the tundish, and casting was continued continuously.

  During the treatment, slag on the molten steel in the tundish was collected, and component analysis of the collected slag was performed. In addition, as a comparative example, the same process as described above was performed using a ladle not provided with porous brick, and the component analysis of the slag in the tundish was performed.

  The component analysis result of the slag in the tundish is shown in FIG. As shown in FIG. 4, in the example of the present invention in which a porous brick was arranged so as to surround the molten steel outflow hole, and gas was blown from the porous brick and injected, yttrium oxide was slightly observed in the tundish slag when the ladle was replaced. However, in the comparative example, the yttrium oxide concentration in the tundish slag tends to increase with the passage of time, and this tendency continued until immediately after the start of the injection of the molten steel in the next processing order.

  That is, by applying the present invention, it was confirmed that outflow of ladle slag was minimized.

DESCRIPTION OF SYMBOLS 1 Ladle 2 Tundish 3 Upper nozzle 4 Sliding nozzle 5 Long nozzle 6 Porous brick 7 Gas supply pipe 8 Molten steel outflow hole 9 Molten steel 10 Slag

Claims (4)

  The porous brick is arranged so as to surround the molten steel outflow hole at the bottom of the ladle, and while the inert gas is blown from the porous brick into the molten steel in the ladle, the molten steel in the ladle is introduced into the tundish through the molten steel outflow hole. A method for producing high cleanliness steel, characterized by being injected into the steel. From the porous brick according to the amount of molten steel in the ladle so that the relationship between the flow rate of inert gas blown into the molten steel from the porous brick and the molten steel depth in the ladle is within the range of the following formula (1): The method for producing a high cleanliness steel according to claim 1, wherein the flow rate of the inert gas is adjusted.
G × H ² <0.010 (1)
However, in the formula (1), G is the flow rate of the inert gas blown from the porous brick (Nm ³ / sec), and H is the depth of molten steel (m) in the ladle.   The method for producing a high cleanliness steel according to claim 1 or 2, wherein an upper portion of the upper nozzle constituting the molten steel outflow hole is made of porous brick, and inert gas is blown from the porous brick.   The method for producing a high cleanliness steel according to claim 1 or 2, wherein porous brick is disposed around an upper nozzle constituting the molten steel outflow hole, and inert gas is blown from the porous brick. .

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