JPH06306444A - Method for melting extra low carbon and extra low nitrogen steel in vacuum degassing apparatus - Google Patents

Abstract

PURPOSE:To prevent the invasion of the air from an atmosphere exposing part which is not immersed into molten steel in an immersed tubes arranged at the lower part of a degassing vessel, and to efficiently melt an extra low carbon and extra low nitrogen steel. CONSTITUTION:The outer periphery of refractory 5 in each of the immersed tubes 1 is surrounded with an enclosure 2, and into the enclosure 2, hydrogen or mixed gas of the hydrogen and inert gas is supplied as the seal gas from a gas introducing pipe 4 to shut off the invasion of the air from the refractory 5. By shutting off the air, the absorption of nitrogen into the molten steel is prevented and also, hydrogen atom H generated by decomposing the gaseous hydrogen (H2), becomes nuclei for generating CO bubble to promote the decarburization of the molten steel.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to gas components in molten steel,
In particular, the present invention relates to a method for melting ultra-low carbon and ultra-low nitrogen steel by a vacuum degassing device for reducing nitrogen and carbon in the molten steel.

[0002]

2. Description of the Related Art The vacuum degassing method was developed for the purpose of removing gases such as hydrogen in steel by degassing treatment, but recently, deoxidation of molten steel, decarburization, composition adjustment, etc. Applications are expanding rapidly. Some molten steels processed by vacuum degassing are extremely reluctant to mix nitrogen depending on the steel type. This is because nitrogen reduces the toughness of the steel, and recently, the reduction thereof has been required as a factor that reduces the workability together with the carbon of the ultra-low carbon steel. Most of this nitrogen contamination comes from nitrogen contained in the air. When degassing steel with a RH degassing tank, the pair of ascending and descending dipping pipes at the bottom of the degassing tank are immersed in molten steel, and the degassing treatment is performed while the molten steel is shielded from the outside air. To do.

The vacuum degassing apparatus is hermetically sealed during the degassing process, and gas is blown into the molten steel from the dipping pipe portion in order to cause reflux of the molten steel. In this case, the gas is argon gas. It does not mix nitrogen. Therefore, the inventors of the present invention have investigated the path through which air, which causes nitrogen contamination, enters, and found that it is from the outer circumference of the immersion pipe, which is the only place where the refractory is exposed during the degassing process. .

The dip tube is generally constructed by covering the inner and outer circumferences of a cylindrical cored bar and its lower end with a refractory material. It has been conventionally thought that air does not enter from the outer peripheral portion of the dip tube due to the refractory material being covered and the presence of the cylindrical core metal. However, the vacuum degasser differs from the ordinary industrial kiln in that the atmospheric pressure in the degassing tank is 0.2 to 200 mmH during the degassing process.
Under special conditions, the pressure is reduced to about g. For this reason, air is sucked into the outer periphery of the immersion pipe, passes through cracks in the refractory structure, minute pores, gaps between the refractory and the core bar, and even if the core bar is buried, it bypasses the lower end. It seems to be mixed in molten steel.

As a conventional technique for preventing nitrogen pickup in the degassing process, there is an invention described in JP-A-60-174815. According to this, since the degassing process is performed while injecting an inert gas into the vicinity of the tip of the dip tube, the atmosphere is prevented from entering the system through the dip tube.
However, as the melting loss of the tip of the dip tube progresses, the inert gas passage provided inside the dip tube is damaged, and it is not possible to prevent nitrogen pickup from the beginning to the end of use.

Further, JP-A-61-295315 or JP-A-2-228416 discloses a method of sealing the outer circumference of the immersion tube with argon gas or an inert gas. With this method, it is possible to prevent air from entering from the outer circumference regardless of the wear of the dip tube, but a large amount of expensive argon gas for sealing is required, resulting in an increase in cost.

[0007]

In view of the above points, the problem to be solved by the present invention is to prevent absorption of nitrogen in the atmosphere without using expensive inert gas such as argon gas or by reducing it as much as possible. In addition to the above, it is an object of the present invention to provide a method for melting ultra-low carbon and ultra-low nitrogen steel by a vacuum degassing apparatus capable of more efficiently performing degassing.

[0008]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides a vacuum provided with an ascending-side dipping pipe and a descending-side dipping pipe in which a cylindrical cored bar is covered with a refractory material on its inner and outer circumferences. In the melting process of ultra-low carbon and ultra-low nitrogen steel with a degasser, at least the refractory porous body of the ascending-side dipping pipe and the descending-side dipping pipe is not exposed to the atmosphere during vacuum degassing. To enclose the exposed portion of the refractory porous body with a gas by supplying hydrogen or a mixed gas of hydrogen and an inert gas into the enclosure to block the contact with the atmosphere and seal the gas. It is a method of melting ultra-low carbon and ultra-low nitrogen steel using a characteristic vacuum degassing device.

In the present invention, hydrogen or a mixed gas of hydrogen and an inert gas is positively mixed into the molten steel through the porous refractory provided at the lower end of the immersion pipe refractory. Is preferred.

[0010]

As shown in FIG. 1, the degassing tank 9 is used during the degassing process.
The outer circumference of the dip pipe 1 connected to the lower part of the via a joint flange 8 is covered with an enclosure 2, and a seal gas 3 containing hydrogen or a mixture of hydrogen and an inert gas is fired using a gas introduction pipe 4 When it is sent around the object 5 and gas sealed,
Since air 6 cannot enter the inside of the enclosure 2, it is a ladle.
It is possible to prevent nitrogen from being mixed into the molten steel 7 in 14. The reason why the seal gas containing hydrogen is used is that it has no reactivity with molten steel and does not deteriorate the quality of steel. It's cheap.

Further, the volume change due to heating of the hydrogen gas is in the same order as that of an inert gas such as Ar, and in addition, there is an effect of promoting the degassing treatment of molten steel as described below by mixing hydrogen. Is the main reason. That is, the hydrogen molecules taken into the steel decompose into hydrogen atoms (H ₂ → 2
H). It was conventionally thought that hydrogen should be removed rather by degassing, but it can be removed relatively easily, and the decarburization reaction occurs only near the surface of the molten steel under reduced pressure. In the extremely low carbon region of C ≦ 50ppm, hydrogen atom H becomes CO
It becomes a bubble generating nucleus and helps the decarburization reaction. Therefore, hydrogen may be positively mixed into the molten steel. Further, when it is desired to avoid mixing of hydrogen during the dehydrogenation process as much as possible, a mixed gas of hydrogen and an inert gas is used. The inert gas mixed at this time is preferably an argon gas, helium or the like that does not deteriorate the quality of steel.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1, two immersion pipes 1 are joined flanges 8
Is detachably attached to the lower end of the degassing tank 9. 13 indicates an O-ring. In this dip tube 1, a cylindrical cored bar 10 as a skeleton has an outer periphery, an inner periphery and a lower end covered with a dip pipe refractory material 5. The immersion pipe refractory 5 may be either an irregular refractory or a regular refractory, but generally, the outer periphery and the inner periphery are either irregular refractory, or the outer periphery is an irregular refractory and the inner periphery is a regular refractory. To be done.

An Ar gas introducing pipe 12 is embedded in the ascending side immersion pipe 1, and Ar gas introduced from the outside of the degassing tank 9 through the introducing pipe 12 is blown into the inside of the ascending side immersion pipe 1 and taken in. Pot 14
The molten steel 7 therein is introduced into a degassing tank 9, where the molten steel 7 is degassed, and thereafter, the molten steel 7 is refluxed to the ladle 14 through the dipping pipe 1 on the descending side. An enclosure 2 is provided on the outer circumference of the immersion tube 1 constructed as described above. The enclosure 2 may be made of any material as long as it is heat-resistant, but is preferably made of a metal covered with a refractory material in terms of durability and economy. When there are two dip tubes 1 as in the RH type vacuum degassing device, both may be enclosed by one enclosure, but it is easy to attach or detach the dip tube 1 or repair it, so that each dip It is preferable to provide a separate enclosure 2 for the tube 1. It is preferable that the enclosure 2 is also attached to the joint flange 8 so as not to interfere with the attachment / detachment or repair of the dip tube 1. The sealing gas introduction pipe 4 is connected to the enclosure 2 so that hydrogen or a sealing gas 3 composed of hydrogen and an inert gas can be introduced into the interior from the enclosure 2 and the outer periphery of the immersion pipe refractory 5.

When hydrogen gas is introduced into the outer circumference of the enclosure 2 and the immersion pipe refractory 5, first, an inert gas such as Ar is supplied from the gas introduction pipe 4 so that the enclosure 2 and the immersion pipe refractory 5 are separated from each other. The air existing between them is replaced with an inert gas, oxygen gas is removed, and then hydrogen gas is supplied to prevent explosion of hydrogen gas. At the end of the degassing, conversely, the hydrogen existing between the enclosure 2 and the immersion pipe refractory 5 is replaced with an inert gas, and then the immersion pipe 1 is raised to complete the degassing to prevent the hydrogen gas explosion. To do.

In order to prevent hydrogen gas (H ₂ ) from leaking from the contact portion between the immersion pipe 1 and the enclosure 2, an irregular refractory material is press-fitted and sealed. Even if some hydrogen gas leaks, it just burns and there is no danger of hydrogen explosion. In FIG. 2, the horizontal axis represents the nitrogen content in the molten steel before the degassing treatment, and the vertical axis represents the nitrogen content in the molten steel after the degassing treatment. The seals of the method of the present invention were compared with the conventional method. It is a thing. In the figure, the black circles indicate the case where the sealing method of the present invention is used and hydrogen gas is fed at a rate of 500 Nl / min per dipping tube for sealing.

Molten steel heat size is 275 to 295 t
The molten steel reflux rate in the RH degassing tank is 180 t / min.
As is clear from FIG. 2, the use of the method of the present invention makes it possible to prevent absorption of nitrogen in the air through the dip tube refractory, and to a level of 10 ppm or less, which was difficult to achieve in the conventional molten steel. It was possible to reduce nitrogen. In addition, since cheap hydrogen was used, the cost of the seal gas could be kept extremely low. Furthermore, the hydrogen and oxygen levels after the killing treatment were at the same level as in the conventional case.

FIG. 3 shows the results of investigating changes in the decarburization rate by allowing hydrogen to be more positively absorbed in the molten steel. When hydrogen is absorbed in the molten steel as shown in the figure, the decarburization rate can be further improved as compared with the conventional example. In order to positively absorb hydrogen in the molten steel, for example, as shown in FIG. 4, the lower end portion of the immersion pipe refractory 5 is provided with a porous refractory 5a having high porosity and the gas introduction pipe 4 is provided. The amount of hydrogen gas supplied from 1000 Nl / min is not only introduced into the immersion pipe refractory 5 but hydrogen gas may be mixed into the molten steel 7 through the porous refractory 5a. By doing so, an improvement in the decarburization rate and a decrease in the reached C in the extremely low coal area as shown in FIG. 3 are observed.

It should be noted that hydrogen H, which has been positively mixed with the molten steel 7 and once dissolved in the molten steel, is not released into the atmosphere unless the pressure is reduced. Since it has been proved that there is no danger of explosion even if hydrogen gas is blown directly into the molten steel, there is no concern about explosion due to hydrogen gas being mixed into the molten steel directly.

[0019]

As described above, according to the present invention, since inexpensive hydrogen is used to seal the periphery of the refractory for the immersion pipe to prevent nitrogen from being mixed in the air, it is possible to pick up nitrogen in an extremely low carbon region. It came to be able to prevent it. Further, preferably, by utilizing the characteristics of hydrogen and by degassing while more positively mixing hydrogen into the molten steel through the immersion pipe refractory, a very pure ultra low carbon steel can be produced.

[Brief description of drawings]

FIG. 1 is a sectional view showing a vacuum degassing apparatus according to the method of the present invention.

FIG. 2 is a graph showing the relationship between N (ppm) in molten steel before degassing and N (ppm) in molten steel after degassing, comparing the method of the present invention with the conventional method.

FIG. 3 is a graph showing changes in the C concentration in molten steel, comparing the method of the present invention with the conventional method.

FIG. 4 is a sectional view showing a vacuum degassing apparatus according to a conventional method.

[Explanation of symbols]

 1 Immersion pipe 2 Enclosure 3 Seal gas 4 Gas introduction pipe 5 Immersion pipe Refractory 6 Air 7 Molten steel 8 Joint flange 9 Degassing tank 10 Core 11 Slag 12 Ar gas introduction pipe 13 O-ring 14 Ladle

Claims (2)

[Claims] 1. A method for melting ultra-low carbon and ultra-low nitrogen steel by a vacuum degassing apparatus equipped with an ascending-side dipping pipe and a descending-side dipping pipe in which the inner circumference and the outer circumference of a cylindrical cored bar are covered with a refractory material. In the above, in the vacuum degassing process, at least the refractory of the ascending side immersion pipe and the descending side immersion pipe is surrounded by an atmosphere which is not immersed in the molten metal and is surrounded by an enclosure. Of the refractory is covered with a gas to supply a mixed gas of the gas, the contact with the atmosphere is cut off to seal the gas by vacuum degassing equipment, . 2. The ultra-low carbon according to claim 1, wherein hydrogen or a mixed gas of hydrogen and an inert gas is positively mixed into the molten steel through a porous refractory provided at the lower end of the immersion pipe refractory. , Melting method of ultra low nitrogen steel.