JPH02182823A - Method for vacuum degassing and decarburization of molten steel - Google Patents

Abstract

PURPOSE:To quicken decarburization in an extremely low carbon range and to quickly obtain extremely low carbon steel without degrading workability by blowing steam and/or water-containing gas in the prescribed timing at the time of top-blowing O2-series gas into a treating vessel to execute the subject treatment. CONSTITUTION:A circulating flow tube 4 at lower end of the vacuum vessel 3 is submerged into upper part of molten steel 2 in a ladle 1 and Ar gas for circulating flow is blown from an introducing pipe 5 to circulate the molten steel 2 into the vacuum vessel 3. At first, in the high carbon range, O2 gas is blown to the upper part of the molten steel surface in the vacuum vessel 3 with a top-blowing lance 6 to progress the decarburizing reaction in the molten steel 2 and also CO gas generated during degassing treatment is burnt. In this method, the carbon content in the molten steel is lowered and at the time of getting behind with the decarburizing reaction, the steam or water-containing gas is blown from the lance 6. By this method, the decarburization in the extremely low carbon range is quickly executed and the extremely low carbon steel having <=10ppm carbon content can be stably obtd.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention provides a vacuum degassing process for molten steel that enables the rapid production of ultra-low carbon steel without impairing the operability of the equipment. The present invention relates to a method for producing ultra-low oxygen steel through degassing treatment.

<Conventional technology> In recent years, continuous annealing equipment has been increasingly adopted in order to make the annealing process of cold-rolled steel sheets continuous and improve efficiency. Carbon steel is now in demand. Traditionally, low carbon steel has been used in converters at C70.02 to 0.05.
Molten steel that has been decarburized to % by weight (hereinafter referred to as %) has been decarburized under reduced pressure using a vacuum degassing device such as the RH method.

A considerable amount of research has been conducted on the decarburization process under reduced pressure, and the main theory is iron and steel, vo jj
69 (1983) A 37, the decarburization rate is expressed by equation (1).

d (C) d t = (C) * exp(-K
c t L---41) Also, the rate constant Kc is (
2) It is shown by the formula.

Kc= (Q' /V) (ak/ (Q' + a k)
1・-・・・・・・(2) Here, (C). : (C11 degrees, Kc: apparent rate constant (eiin-') at the start of treatment.

ak: Mass transfer capacity coefficient (nf/s) in the vacuum chamber.

Q': Circulation flow rate of molten steel (Po/S).

V: Molten steel amount (po).

Therefore, when trying to improve decarburization efficiency, Q',
It is known that measures should be taken to increase ak.

In order to increase the circulating flow flQ' of molten steel, there are two methods: enlarging the diameter of the reflux pipe and increasing the amount of gas blown into the reflux. The problem is that the tube life is short, and the latter problem is that it impairs the high degree of vacuum required in the ultra-low carbon range.

On the other hand, in order to increase the mass transfer capacity coefficient ak,
In the extremely low carbon region where C≦soppm, the mass transfer process of C to the reaction site is the rate-determining reaction rate, so it is essential to increase the reaction interfacial area a (rrr).The zero reaction site is molten steel. The internal gas bubbles/molten steel interface, the surface of the steel bath in the vacuum chamber, and the splash that accompanies gas bubbles when they leave the steel bath are all assumed, but their respective contributions are not necessarily clear. Increasing the amount
From the viewpoint that it would be effective for the above three points, 5000 N
(Currently, Technique 4+7, which involves sucking in a large amount of static gas as much as 1/min, is being adopted.

By the way, when a large amount of static gas is injected in this way, it is difficult to maintain a high vacuum due to the exhaust characteristics of the vacuum evacuation equipment under high vacuum, and the decarburization rate in the extremely low carbon range is actually reduced and a large amount of gas is generated. There is no means to deal with the adhesion of splash to the inner surface of the vacuum chamber, which causes problems such as impairing operability, and is still technically insufficient from the viewpoint of rapid decarburization technology with a carbon content of 10 ppm or less. I am in a situation.

Furthermore, in order to produce ultra-low carbon steel, attempts have been made to enrich the dissolved oxygen in the molten steel by adding gases such as oxygen and carbon dioxide, or powders such as iron oxide, and to accelerate the decarburization reaction (
JP-A-49-34414, JP-A-51-1512
11, JP-A-51-151212, etc.), but in the extremely low carbon region where c<5opp-, the mass transfer process of C to the reaction site is the rate-determining reaction rate. These methods are not effective in that the deoxidation reaction after decarburization generates a large amount of deoxidation products, resulting in an increase in nonmetallic inclusions, which is undesirable in terms of producing a steel sheet with good workability.

In addition, as a method to advance the decarburization reaction, when molten steel is subjected to vacuum decarburization treatment, hydrogen gas is blown into the molten steel through an immersion tube to increase the hydrogen concentration in the molten steel, and then dehydrogenation is performed to promote the decarburization reaction. is disclosed in Japanese Patent Application Laid-Open No. 194206/1983. However, this method has a problem in that the reducing hydrogen gas reduces oxides in the refractory, causing the refractory to be eroded. Hydrogen is also difficult to use economically due to its high gas cost.

<Problems to be Solved by the Invention> In view of the above-mentioned current situation, the present invention is designed to quickly decarburize in an extremely low carbon range of 1 oppm or less, and to prevent splash metal from adhering to the inside of a vacuum chamber. This was done in order to provide an effective and simple method that can be used. In other words, in order to speed up decarburization in the extremely low carbon range, Ar
The present invention provides a method for increasing the reaction interface area a (rrf) while suppressing the decrease in the degree of vacuum due to an increase in the amount of gas, and furthermore, provides a method for increasing the reaction interface area a (rrf), which is one method for increasing the amount of gas. This provides a simple method that can prevent gold adhesion.

Means for Solving the Problems> The present invention provides: (1) Vacuum degassing treatment of undeoxidized molten steel or weakly deoxidized molten steel produced in a steelmaking furnace by applying the RH method, D11 method, VOD method, etc. In a method in which oxygen gas or oxygen-containing gas is blown from above the bath surface of molten steel in a vacuum degassing treatment tank to advance the decarburization reaction of molten steel and to burn CO gas generated during the degassing treatment, This is a method for vacuum degassing and decarburization of molten steel, which is characterized by blowing steam and/or water-containing gas through a top blowing lance when the decarburization reaction is stagnant. Or, the time when water-containing gas is blown is at the end of the decarburization reaction, and the carbon concentration in the steel is 5.
The method for vacuum degassing and decarburization of molten steel as described in the preceding paragraph (■) in which the concentration is 0 ppm or less, and the method for vacuum degassing and decarburization of molten steel as described in the preceding paragraph (■) or the preceding paragraph (■) in which the water-containing gas is an inert gas. It is.

<Process and effects that led to the invention> The present inventors have found that the decarburization reaction is particularly stagnant [C] at 50pp.
We conducted a study to improve the decarburization reaction in m+ and obtained the following findings. In other words, the reasons for the decrease in the decarburization reaction when (C)
It has been suggested that the decarburization reaction is rate-limited by the gas-liquid interfacial reaction, but the reason is not clear.However, in any case, increasing the gas-liquid reaction interface improves the decarburization reaction. is valid. However, if the amount of blown gas is simply increased, the diameter of the gas bubbles when they separate from the molten steel will monotonically increase, and the majority of the molten steel droplets that accompany the bubbles will also be of large diameter. The inventors have found that the amount of splash increases and the amount of base metal adhering to the vacuum chamber increases.

If the WL droplet has a large diameter, in the extremely low carbon N range, the reaction rate-limiting process is C diffusion in the molten steel, and it cannot fully contribute to the decarburization reaction. In order to increase the stirring at
Alternatively, they focused on using a gas containing water. In other words, by blowing steam and/or water-containing gas onto the surface of a steel bath in a vacuum chamber, small-diameter droplets can be easily obtained on the surface of the steel bath, and furthermore, agitation under the gas-liquid interface can be increased. In addition, they discovered that by lowering the CO gas partial pressure at the gas-liquid interface and reducing the thickness of the film on the gas side, it is possible to improve the decarburization rate in the low carbon region. When a gas containing water vapor and/or water is blown, the blown surface has a
□ Since 0 decomposes and 112 is generated, a large number of small-diameter droplets are generated on the droplet surface, and the generation of llx increases the agitation at the gas-liquid interface, and the generated H2 causes This is because decarburization can be advantageously led by lowering the CO gas partial pressure and reducing the thickness of the gas-side film.

If this method is used, it is not necessary to increase the amount of Ar gas in the steel bath, so it is possible to maintain a high vacuum and perform highly efficient decarburization.

Japanese Patent Publication No. 49-12810 proposes a method of blowing oxygen gas into a vacuum chamber;
However, in the extremely low carbon region where O is present in excess in the steel bath and decarburization is rate-limited by C diffusion, it is meaningless to supply 08. It was found that it is ineffective in increasing the reaction area of the gas-liquid interface because it is rapidly absorbed by the gas-liquid interface.

In addition, a method of blowing ^r gas from a top-blowing lance with the aim of increasing the reaction area at the gas-liquid interface is also proposed in Kobe Steel Technical Report 36 (
1986), p. 40, ``The present inventors also confirmed that simply blowing gas does not lead to an increase in the reaction interfacial area.The reason for this is that the gas leaving the steel bath 11□ on the gas bubble surface covering the bubble surface
When 0 is sprayed, a decomposition reaction occurs immediately on the surface, resulting in an increase in the gas-liquid reaction interface area, an increase in stirring power, and a decrease in the 00 partial pressure due to the generated h.
This is because when a gas that does not involve a reaction, such as a static gas, is blown, such an effect cannot be expected and it is not possible to reduce the diameter of the bubbles or increase the number of bubbles.

The method for generating a large amount of splash in the extremely low carbon range is as follows:
Although it is effective in improving the decarburization speed, it is accompanied by operational troubles due to metal attachment. As a countermeasure to this problem, the present invention adopts a method in which the base metal is dissolved and removed using the heat of burning CO generated during decarburization to 008 in a tank. The purpose of 02 spraying, which aims to burn the generated CO gas necessary to overheat large objects in the tank to prevent metal melting or metal adhesion, is preferably in a high carbon concentration range that does not inhibit the decarburization reaction. The present inventors found through a series of experiments that the C concentration should be limited to 1100 pp or more.

In the present invention, when performing decarburization to an extremely low carbon range using a vacuum degassing device, a top-blowing lance is provided to supply gas onto the surface of the steel bath in the vacuum chamber, and when the steel bath carbon concentration is high, Preferably, oxygen gas is supplied from the top-blown lance until the C concentration reaches 100 pps+, and when the steel bath carbon concentration has decreased, preferably 50 pp- or less, water vapor or water-containing gas is supplied from the top-blown lance. This is a method that achieves rapid decarburization to an extremely low carbon range by supplying a small amount of inert gas (usually a small amount of inert gas) without causing problems with metal ingots.

Furthermore, according to the present invention, even when steel is tapped in a converter, it is possible to stop blowing (C).
) can be raised and steel can be tapped while (0) remains lower than in the conventional method. This is because the top-blown lance has 0 in the high carbon range! This is because it supplies water vapor that decomposes and generates O2 even in a low carbon region. As a result, low-temperature steel tapping in the converter becomes possible, extending the life of the furnace body, and further improving quality by preventing overoxidation of the slag.

<Example> An example using a RH vacuum chamber of scale 230 as shown in FIG. 1 will be described below.

The reflux pipe 4 at the lower end of the vacuum tank 3 is immersed in the upper part of the molten steel 2 of 23OL in El l, and meter gas is blown through the reflux Ar gas introduction pipe 5 to circulate the molten steel into the vacuum tank by the lift pump effect. The method is the well-known RH method.

In the present invention, first, in a high carbon region, oxygen gas is blown from above the bath surface in a degassing treatment vacuum chamber to advance the decarburization reaction of molten steel, and at the same time, the CO gas generated during the degassing treatment is combusted. This allows the decarburization reaction of the molten steel to proceed and prevents metal from adhering within the vacuum chamber.

Furthermore, this oxygen top blowing makes it possible to raise the molten steel temperature and the dissolved oxygen, so decarburization in the converter can be omitted and (C
) is higher than conventional 200-300pp steel, making it 400-5
Steel is tapped at 00 pρ. The result (0) is 500-60
It is now possible to reduce the iron oxide concentration (T・Fe) from 0 ppm to 200-300 ppm, and accordingly
), it is possible to lower the molten steel temperature and extend the life of the converter bricks.

Now, in the present invention, from the top of the vacuum chamber, the top blow lance 6
As mentioned above, oxygen is blown into this lance to decarburize and raise the temperature.In addition to the oxygen gas pipe 7, this top-blowing lance has a steam pipe 8.
and Ar gas piping 9 were connected, and water vapor was blown in when the low carbon region was reached.

In the comparative example, decarburization was performed without using top-blown gas. In addition, in the converter, the temperature is 1620"C1 (C) -0.04~0
．． 05%, (0) -300 to 350 ppm.

Table 1 shows the embodiments of the above Examples and Comparative Examples and the results obtained after 15 minutes of decarburization treatment.

The degree of vacuum achieved when oxygen gas was blown from the top blowing lance was 8 to 12 torr. In addition, the distance between the top blowing lance and the molten steel in the vacuum chamber is 2 to 2.5 m when blowing oxygen gas, and the distance when blowing steam is 1 =
1.3m + closed.

The decarburization rate constant Kc shown in Table 1 is defined by formula (I).As is clear from Table 0, the rate constant in the examples is significantly large in the low carbon region, and due to the top blowing of steam, It can be seen that the decarburization process in the low carbon region has been improved. As a result, (C) was stable at 10 after a short treatment of 15 minutes.
ppm or less, and while conventional processing can only produce ingots of about 20 ppm, cold-rolled plate material is advantageously suitable for high-quality continuous annealing. 8 can be made.

In addition, in the case of continuous treatment, metal buildup in the tank due to the active generation of splash is a big problem, but in the first half of the decarburization reaction of the present invention, in order to combust a part of the generated CO gas into CO2, The 9 methods have also been recognized as effective in preventing metal adhesion due to the effect of elevating the temperature of the refractories in the vacuum chamber by spraying gas from an acid lance.The method involves introducing metering gas from below the surface of the steel bath for stirring the steel bath. Vacuum precision method, i.e. DH method, l'OD
The same applies to law.

<Effects of the Invention> According to the method of the present invention, decarburization in the ultra-low carbon range can be performed quickly, and ultra-low carbon steel with a carbon content of 1Qpps+ or less can be stably obtained. It is possible to prevent a large amount of splash from adhering to the inside of the vacuum chamber.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing one aspect of equipment for implementing the present invention. ...Ladle, ...Vacuum chamber, ...Recirculation needle gas introduction pipe, ...Oxygen gas piping, ...＾rGas piping. 2... Molten steel, 4... Circulation pipe, 6... Top blowing lance, 8... Steam piping,

Claims (1)

[Claims] 1. Vacuum degassing treatment in vacuum degassing treatment of undeoxidized molten steel or weakly deoxidized molten steel melted in a steelmaking furnace by applying the RH method, DH method, VOD method, etc. A method in which oxygen gas or oxygen-containing gas is blown into the molten steel from above the bath surface in the tank to advance the decarburization reaction of the molten steel and to burn the CO gas generated during the degassing process. A method for vacuum degassing and decarburization of molten steel, characterized by blowing in a gas containing steam and/or water during a period of stagnation. 2. The method for vacuum degassing and decarburization of molten steel according to claim 1, wherein the gas containing steam and/or water is blown from the top blowing lance at the end of the decarburization reaction, when the carbon concentration in the steel is 50 ppm or less. 3. The method for vacuum degassing and decarburization of molten steel according to claim 1 or 2, wherein the water-containing gas is an inert gas.