JP2021041410A - Method for preventing contamination of molten steel flow in continuous casting tundish - Google Patents

Abstract

To provide a method for preventing the contamination of an injection flow from a ladle in continuous casting of steel.SOLUTION: A tundish cover 5 is air-tightly provided, and comprises a part through which an injection flow falls from a ladle 1, and which is formed with an opening 6 fitted and suspended with a large-diameter projective pipe 7 that is not contacted with an injection flow, and a lower edge of that is immersed into a molten steel 8. An upper edge is positioned close to a ladle nozzle 3, provided with a gap, placed thereon with an annular gas curtain nozzle 11 blowing an inert gas to a bottom face of the ladle nozzle so as to make an inside of the pipe into a nonoxidizing atmosphere to prevent oxidation of molten steel. In a non-immersed part of the protective pipe, at least an inner wall is made of graphite to prevent the sticking-deposition of scattered molten steel. The gap is provided with an optical sensor 12, and monitored.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for preventing contamination of molten steel injected from a ladle into a tundish in continuous steel casting.

In continuous steel casting, the molten steel in the ladle is continuously cast into the mold via a tundish, which is an intermediate container for controlling the casting flow rate and casting temperature. In high-end models, excellent functions are added to the tundish. The first is a tundish heater, which is a device for reheating molten steel in a timely manner. Precise temperature control is performed in combination with continuous temperature measurement, which not only stabilizes operations (prevents accidents due to low temperatures) and quality (reduces segregation and non-metal inclusions), but also reduces the heat load in the upstream melting process. A solid effect can be obtained.

The second is prevention of contamination of molten steel. The molten steel in the ladle is exposed to air three times between the ladle and the tandish, inside the tandish, and between the tundish mold, and reoxidation proceeds. Contamination is followed by an increase in dissolved oxygen, which is direct pollution due to reoxidation, suspension of suspended slag and fixed slag mainly composed of FeO caused by falling flow in molten steel, and generation of macro inclusions. To do.
In response to this problem, low-grade steel for reinforcing bars is left unattended, but pollution control is indispensable for high-grade steel. Is provided with a refractory protective tube called a dipping nozzle to block air exposure (FIG. 5 of the present application, FIG. 1 of Patent No. 5170353).

Third is the induction of refining effect and reduction of non-metallic inclusions. As a means, in addition to the above-mentioned protection of molten steel, the capacity, structure, flow (stirring and standing), adsorbent, etc. of the tundish are devised to promote the floating separation of deoxidized products and mixed slag particles.

Consider upper and lower protective pipes to prevent contamination of molten steel. When applying the protective tube, the immersion nozzle, which is fixedly connected under the tundish nozzle between the tundish and the mold, is particularly affected by quality problems (air suction), workability (cracking, number of attachments / detachments, mass, etc.), cost, etc. There is no problem, but there are many problems with the long nozzle installed between the ladle and tundish.
First, it requires high-grade materials, is large and heavy in size, has problems with durability, and is costly. Durability is due to oxygen fusing treatment of the fixed bullion and cracking due to thermal shock that occur every time the ladle is replaced.
Second, the work is complicated. The ladle nozzle is a slide gate type and often slides horizontally to adjust the flow rate. The long nozzle must follow its movement. Every time the ladle is replaced, it is attached and detached and adjusted to follow, which is complicated and troublesome, and the work is dangerous and difficult in the part where the molten steel flow drops violently. An expensive robot arm (FIG. 4 of the present application, FIG. 3 of JP-A-2005-254257) is indispensable.

Thirdly, it is difficult to make the connection portion airtight like the immersion nozzle. There are cases where the long nozzle is fixedly connected to the ladle, but it is usually impossible to fix it. Since the connection is odd, air is always drawn in by a violent falling flow. The prevention of reoxidation of molten steel is incomplete.
Fourth, when the injection from the ladle is completed, the outflow of slag is confirmed and the gate is closed immediately, but since the confirmation is visual, the long nozzle must be removed during that time.
Fifth, preheating work is naturally required for each replacement.

Consider an example of improving Tandish related to long nozzles.
Patent Document 1 discloses the latter support holder so as to ensure airtightness by pressing the lower end of the slide gate nozzle attached to the ladle and the upper end of the long nozzle, and to follow the opening / closing operation of the slide gate nozzle. There is.
According to this, a spherical bearing is provided between the long nozzle and the support holder for suspending the long nozzle, and the long nozzle is automatically tilted and followed with the spherical bearing as a fulcrum.
The problem is that the robot arm is indispensable and a complicated holder must be provided at the tip of the arm. It is complicated only in the part where there is a risk of being exposed to molten steel.
Air suction is unavoidable at the degree of pressing between the ladle nozzle and the long nozzle, and it may be insufficient to prevent the reoxidation of molten steel.

In Patent Document 2, the bottom of the radle nozzle is made into a female type and the upper part of the long nozzle is made into a male type to fit, and the latter is provided with a vent to blow an inert gas into the fitting portion to prevent the inflow of air. It is disclosed to do. Although the above-mentioned problems are solved by this method, work problems such as high cost, necessity of a robot arm, nozzle removal for end point observation, and nozzle attachment / detachment for each ladle replacement remain.

Published Patent Publication 2011-083775 Published Patent Gazette Hei 05-023808 Patent No. 5690015

In continuous steel casting, the long nozzle, which is a refractory protective tube for downward immersion that is provided between the ladle and the tundish to protect the injection flow from air oxidation and prevent entrainment due to the injection flow of floating slag, improves the quality of steel. Is an indispensable element, but has the following problems.
1) It is a consumable item and has a large cost burden.
2) An expensive robot arm is indispensable for attaching / detaching and following the ladle nozzle.
3) Before exchanging the ladle, the nozzle must be removed to observe the end point. In the meantime it gets polluted.
4) Every time the ladle is replaced, there is a complicated attachment / detachment work.
An object to be solved by the present invention is to provide a method for protecting molten steel at low cost and easy to work, for the purpose of preventing the molten steel contamination.

The first invention is a method for preventing contamination of molten steel due to a ladle injection flow in continuous steel casting.
1) Suspend a large-diameter protective tube that surrounds the injection flow and does not come into direct contact with the injection flow on the tundish cover.
2) The lower end of the protective tube is immersed in molten steel, and the upper end is concentrically faced with the lower end surface of the ladle nozzle with a gap.
3) The material of the protective tube is refractory for the immersed part and graphite for the inner wall at least for the non-immersed part.
4) It is characterized in that an inert gas is blown to the lower end surface of the radle nozzle by an upward annular gas curtain nozzle surrounding the injection flow provided on the upper end surface of the protective tube to make the atmosphere inside the protective tube non-oxidizing. This is a method for preventing pollution of molten steel.

The second invention has the following three conditions,
1) The inner diameter of the large-diameter protective tube is 2 times or more and 3 times or less the diameter of the ladle nozzle at the upper end, and 4 times or more and 10 times or less at the lower end.
2) Cover the tundish with an airtight cover, blow an inert gas into the molten steel inside the tundish to make the atmosphere inside the tundish non-oxidizing, and provide a graphite resistance heating element in the space.
3) An optical sensor for monitoring the injection flow is provided at the upper end of the large-diameter protective tube.
The method for preventing contamination of molten steel according to the first invention, which comprises incorporating any one or more of them.

As a definition of a predicate
"Concentric" means that the position of the ladle nozzle slides in the front-rear direction to adjust the flow rate, so that the position is concentric with respect to the average position.
The "airtightness" is such that the atmosphere is maintained by injecting the inert gas, and the gas easily flows out.
An "optical sensor" is a measuring instrument called a fiberscope that uses light as a medium, such as a small camera and a radiation thermometer.

The first effect of the present invention is quality improvement. Similar to conventional long nozzles, the large diameter protective tube prevents air reoxidation of the injection flow from the ladle to the tundish. Since reoxidation is prevented, FeO-based slag that grows near the injection flow drop point is not generated, and therefore is not mixed. The generation of macro inclusions is suppressed.

Secondly, it is durable. Normally, in the vicinity of the drop point of the injection flow, a bullion is generated in the vicinity due to the jumping up. The same applies to a large-diameter protective tube. A large-diameter protective tube was tried, but it was not successful because of the adhesion of the bullion, the stop of casting due to the snowball-type bullion, the deterioration of quality, and the complexity of post-treatment. In the present invention, since the inner wall of the protective tube is made of graphite, there is no wettability of the refractory, and the adhered metal does not stick and easily peels off and falls. Continuous casting is maintained stably.

Third is the improvement of workability. The protective tube of the present invention is fixed, and 1) the protective tube itself does not need to be preheated (accompanied by the tundish preheating), and 2) it is necessary to follow the attachment / detachment, position adjustment, and sliding of the ladle nozzle every time the ladle is replaced. All the complicated work is omitted, and 3) the robot arm is unnecessary. FIG. 4 shows a commonly used robot arm (FIG. 3 of JP-A-2005-254257).

Fourthly, it is excellent in durability, which greatly affects the cost. Today, continuous / continuous casting that connects ladle number charges is progressing for about 10 hours, and work improvement is progressing to the extent that it exceeds 10 charges. The durability of the long nozzle cannot keep up with it and requires frequent repairs and replacements, but in the present invention, the durability is high due to the thick graphite in a non-oxidizing atmosphere. Even if the unit price of the protective tube is the same or higher, the service life will be several times longer, which will contribute to cost reduction.

Finally, the present invention is very familiar with the method for heating the molten steel in Tandish disclosed in Cited Document 3. 1) Stability of operation by heating the tundish produces durability of the protective tube. 2) The essential condition that the disclosed tundish is divided into a steel receiving chamber and a casting chamber for controlling the atmosphere inside the tundish is automatically provided by the present invention. And it's easy. They complement each other in terms of effectiveness, profitability, and workability, and are conveniently combined.

It is the outline of the structure of the tundish which carries out the pollution prevention method of molten steel of this invention. The structure of a large-diameter protective tube, which is a main part of the present invention, is shown. A is a vertical cross-sectional view, and B is a view of the ladle nozzle from below. The structure of another example of the large-diameter protective tube which is the main part of the present invention is shown. An example of a robot arm used for adjusting the attachment / detachment of a normal long nozzle is shown. The pollution prevention method using a normal long nozzle is shown.

Hereinafter, embodiments will be described with reference to the drawings.
In FIG. 1, the molten steel 2 in the ladle 1 is dropped and injected from the ladle nozzle 3 into the tundish 4. The tandish 4 is provided with an airtight cover 5. A cylindrical opening 6 is provided at a portion of the airtight cover 5 through which the injection flow 10 penetrates, and a large-diameter protective tube 7 that surrounds and protects the injection flow 10 is fitted into the opening 6 and suspended in the tundish 4. The lower end is immersed in the molten steel 8 in the tundish, and the upper end faces the lower end of the ladle nozzle 3 with a gap.
An annular gas curtain nozzle 11 surrounds the injection flow 10 and is concentrically provided on the upper end surface of the large-diameter protective tube 7, and the inert gas is sprayed upward on the lower end surface of the radle nozzle 3. The intrusion of air into the large-diameter protective tube 7 is prevented, the atmosphere inside the tube becomes non-oxidizing, and air oxidation of the injection flow 10 is prevented.

The molten steel 8 in the tundish 4 is appropriately radiated and heated by a graphite resistance heating element 9 arranged in the space above the molten steel 8. The large-diameter protective tube is also heated to maintain a high temperature. For the durability of the resistance heating element 9 and the soaking of the heat of the molten steel, an inert gas is blown from the breathable refractory plug 13 provided at the bottom, and the atmosphere becomes non-oxidizing. No metal is attached to the inner and outer surfaces of the protective tube due to heating.

An optical sensor 12 for monitoring the injection flow 10 is provided by utilizing the gap between the lower end surface of the radle nozzle 3 and the gas curtain nozzle 11, and a fiberscope is used to confirm the slag outflow at the end of casting and to perform radial temperature measurement. ..

FIG. 2 shows the structure of the large-diameter protective tube 7. The large-diameter protective pipe 7 mainly consists of three parts, an upper pipe 21 made of graphite, a lower pipe 22 made of a refractory material immersed in molten steel, and an annular gas curtain nozzle 23 provided on the upper surface. The gas curtain nozzle is damaged and should be easy to replace.
The upper pipe 21 receives the scattered liquid due to the falling flow and returns to the molten steel without being fixed. Graphite has no wettability with molten steel and does not adhere to refractories.
The gas curtain nozzle 2 forms a non-oxidizing atmosphere in the pipe.
The lower pipe 22 closes the drop area to prevent the slag 24 from entering.

Injection without a protective tube induces quality and work problems near the drop point of the injection stream 10. Normally, slag coating is not applied to keep warm or prevent oxidation, and the injection is maintained in the state of bare water.
Air oxidation of molten steel occurs due to falling energy, and malicious slag mainly composed of FeO is generated and increased. In addition, molten steel and floating slag are scattered around and adhere to the back of the cover and the inner wall of the tundish. Adhesion, oxidation and remelting are repeated and laminated to promote reoxidation of the molten steel, and FeO-based slag is suspended in the molten steel to increase macro inclusions.
The treatment of the adhered bullion not only makes the work complicated, but also adversely affects the durability of the Tandish body and the cover, and the repair cost increases.

The large-diameter protective tube of the present invention solves the above two problems, but the atmosphere control and the material of the protective tube are not sufficient, and the shape is also related.
First, the injection stream 10 does not come into direct contact with the inner wall. When in direct contact, graphite is easily eroded by molten steel, causing durability problems. The inner diameter of the large-diameter protective tube shall be 4 times or more and 10 times or less the inner diameter of the ladle nozzle in order to stop only scattering. Beyond this, cost increases become a problem.
The second is the inner diameter of the upper end. For the gas curtain to function normally, the smaller the inner diameter of the upper end surface or the inner diameter of the annular nozzle, the better. On the other hand, the outer diameter of the ladle nozzle 3 is not so large. Moreover, the ladle nozzle slides during casting to control the flow rate. It is appropriate that the inner diameter of the upper end of the large-diameter protective tube is 2 times or more and 3 times or less the inner diameter of the ladle nozzle. If the outer diameter of the ladle nozzle is insufficient, a flange 25 is provided on the lower surface of the nozzle.

If the tandish is not provided with an airtight cover, the atmosphere cannot be controlled, so that the outer wall of the large-diameter protective tube made of graphite is oxidatively consumed, causing a problem in terms of durability.
FIG. 3 shows a large-diameter protective tube that can cope with such adverse conditions. The upper pipe 21 is doubly configured, the inner pipe 21'is made of graphite, and the outer pipe 21'is made of refractory.

When preheating Tandish, a graphite heating element is used as a heat source, a temporary lid is attached to the upper surface of the large-diameter protective tube, and an inert gas is blown into the tube to make the atmosphere inside the tube non-oxidizing. Prevents oxidative consumption. Even when the burner is used as a heat source, the protective tube is protected from oxidation to some extent by the lid.

The dimensional relationship when carrying out the present invention in continuous casting with a casting efficiency of 100 t / h is shown.
1) Ladle nozzle diameter; 50 mmφ
2) Protective tube inner diameter; 400 mmφ
3) Protective tube outer diameter; 600 mmφ
4) Protective tube upper diameter; 120 mmφ
5) Circular gas curtain nozzle diameter; 150 mm
6) Upper gap; 20 mm
7) Length of upper part of protective tube; 500 mm (depending on the distance between the existing ladle and tandish)
8) Protective tube lower length; 350 mm

The molten steel contamination method according to the present invention can be easily applied to existing continuous casting and contributes to quality and productivity.

1: Radle 2; Molten steel 3; Radle nozzle 4; Tandish 5; Tandish cover 6; Opening 7; Large diameter protective pipe 8; Molten steel 9; Resistive heating element 10; Injection flow 11; Gas curtain nozzle 12; Optical sensor 13 Breathable fireproof plug 21; Upper pipe 21'; Inner pipe 21 "; Outer pipe 22; Lower pipe 23; Gas curtain nozzle 24; Slug 25; Flange 41; Robot arm 51; Long nozzle

Claims (2)

It is a method to prevent the contamination of molten steel caused by the radle injection flow in continuous steel casting. 1) A large-diameter protective pipe that surrounds the injection flow and is not in direct contact with the injection flow is suspended on the tundish cover, and 2) ) The lower end of the protective tube is immersed in molten steel, and the upper end is concentrically facing the lower end surface of the radle nozzle with a gap. 4) It is characterized in that an inert gas is blown to the lower end surface of the radle nozzle by an upward annular gas curtain nozzle surrounding the injection flow provided on the upper end surface of the protective tube to make the atmosphere inside the protective tube non-oxidizing. How to prevent contamination of molten steel. The following 3 conditions,
1) The inner diameter of the large-diameter protective tube should be 2 to 3 times the diameter of the ladle nozzle at the upper end and 4 times to 10 times the diameter at the lower end.
2) Cover the tundish with an airtight cover, blow an inert gas into the molten steel inside the tundish to make the atmosphere inside the tundish non-oxidizing, and provide a graphite resistance heating element in the space.
3) An optical sensor for monitoring the injection flow is provided at the upper end of the large-diameter protective tube.
The method for preventing contamination of molten steel according to claim 1, wherein any one or more of them is incorporated.

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