JPH03197614A - Method for melting extremely low carbon steel with rh degassing treatment - Google Patents

Abstract

PURPOSE:To smoothly progress a decarbonizing reaction in the latter half period of producing an extremely low carbon steel by specifying the circulating gas flow rate in the first half period of reaction until the carbon content in the steel is decarbonized to <=20ppm and in the latter half period after that. CONSTITUTION:In the melting method for the extremely low carbon steel varying the molten steel circulating flow rate during RH degassing treatment based on C content [C] in the steel, in the first half period of decarbonizing reaction when [C] is still >20ppm, the circulating gas flow rate is made to 33-42Nl/min, cm. Then, in the latter half period of decarbonizing reaction of [C]<=20ppm, it is lowered to 8-25Nl/min,cm. By reducing the circulating gas flow rate in the latter half period in such a way as to be contrary to the conventional technique, the decarbonizing reaction is accelerated without stagnating even in the latter half period of decarbonization and the extremely low carbon steel having [C]<=10ppm can be produced.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a method for producing ultra-low carbon steel by RH degassing treatment, and in particular, a method for producing ultra-low carbon steel that promotes decarburization in the low carbon range of C≦20 ppm. Regarding the method.

[Conventional technology]

Generally, in the melting process of ultra-low carbon steel, oxygen is first injected under atmospheric pressure to reduce iron oxidation loss by C: 0.03 to 0.05.
%, and exposing the molten steel under reduced pressure to reduce the partial pressure of 500 to 700%.
It is roughly divided into a process of promoting Co gasification between dissolved oxygen adjusted to ppm and [C] in the steel, and decarburizing to C; 10 to 20 PP.

The present invention relates to the latter process.

Conventional techniques in the above-mentioned fields include a method of blowing in large amounts of inert gas, a method of blowing gaseous oxygen under reduced pressure, and a method of blowing iron oxide powder into molten steel under reduced pressure. There are the following problems.

In other words, regarding the injection of large amounts of inert gas, Matsunaga et al.
There are several documents such as Tetsu to Hagane, Vol. 63, No. 13, 1977) and Japanese Patent Application No. 63-093467, which discuss improvements in the molten steel circulation speed during degassing treatment or an increase in splash. The aim is to improve the decarburization rate by increasing the gas-liquid interface. However, although these methods improve the decarburization rate, they also increase the amount of metal adhesion in the vacuum degassing tank, which not only destabilizes the ferroalloy yield for adjusting the molten steel composition, but also reduces the iron content yield. Many operational problems have arisen, such as reduction in fuel consumption, downtime to remove metal deposits in the vacuum degassing tank, and reduced lifespan of refractories in the RH degassing recirculation pipe.

In addition, the method of blowing gaseous oxygen into the molten steel bath surface or below the bath surface under reduced pressure increases the amount of dissolved oxygen and can improve the reaction rate, but in the case of molten steel bath surface spraying, Similar to large amounts of inert gas injection, there is a problem with an increase in splash, and the method of blowing below the bath surface not only causes a significant increase in splash, but also reduces the vacuum degassing tank due to wear and tear on the refractories around the tuyeres. It has the disadvantage of shortening its lifespan. Furthermore, since both methods increase the amount of dissolved oxygen in the entire molten steel, a large amount of deoxidation products are generated after deoxidation treatment, which worsens the cleanliness of molten steel and increases the occurrence of defects in steel products due to this. There was a lack of consideration in this aspect.

Furthermore, Japanese Patent Application Laid-open No. 181217/1983, Collection of Lectures by the Iron and Steel Institute of Japan, CAMP-IS I J Vol. 1 (1
988, P, 1185), etc., or the method of top-blowing oxide powder onto the surface of molten steel under reduced pressure, or the method of adding oxygen potential to molten steel under reduced pressure, as disclosed in JP-A-63-169321. In the method of directly injecting high-oxygen powder into molten steel through tuyeres installed below the bath surface, areas with high oxygen potential are generated near the powder that has entered and diffused into the steel bath, causing As a result of accelerated decarburization, it is possible to improve the decarburization limit and increase the decarburization speed in the extremely low carbon range without controlling the deterioration of the cleanliness of the entire molten steel. The problem has not been resolved, and the fact that inert gas must be continuously blown to prevent the tuyeres from clogging, even when ultra-low carbon treatment is not performed, contributes to the wear and tear of the refractories around the tuyeres. It must be said that this technology is not only wasteful in terms of cost, but also wasteful in terms of cost.

Furthermore, in order to solve the problem that the decarburization rate drops rapidly in the ultra-low carbon range of C≦20ppm in the melting operation of ultra-low carbon steel in RH degassing treatment, JP-A-60-6
3311 and JP-A-63-213617,
A method of increasing the amount of recirculated molten steel by increasing the amount of reflux inert gas such as Ar is disclosed. That is, in JP-A-60-63311, the average speed of decompression is 120
C
o In addition to promoting the generation of bubbles and increasing the reaction interface area,
The lower limit of the reflux inert gas injection amount Q (Nm3/5in) is set as follows.

However, D: Immersion pipe inner diameter (m) Furthermore, in JP-A-63-213617, a method is adopted in which the flow rate of molten steel during RH degassing treatment is gradually increased based on the C content [C] in the steel as follows. There is.

That is, (a) 15 from the start of processing until [C]≦1100pp1
Less than 0 t/in (B) [C] from less than 1100 ppm to 30 ppm 1
50 t/win or more and less than 200 t/akin (c) If [C] is less than 30 pP hazy, 200 t/win
Limited to win or above.

However, the inventor of the present invention
Results of an experiment using a H degassing tank and extending the processing time C
It was confirmed that decarburization stagnates in the low carbon range of ≦20 ppm.

[Problem that the invention seeks to solve]

The purpose of the present invention is to solve the above-mentioned problems of the prior art in a method for producing ultra-low carbon steel by RH degassing treatment, and [C]
The object of the present invention is to provide an effective melting method that can smoothly promote the decarburization reaction even in the extremely low carbon range of ≦20 ppm.

[Means for solving problems]

The gist of the present invention is as follows.

That is, in a method for producing ultra-low carbon steel in which the flow rate of molten steel during RH degassing treatment is changed based on the C content [C] in the steel, decarburization is performed until the above [C] is [C]>20 ppm. In the first half of the reaction, the reflux gas amount was set at 33 to 42 NQ/
min, tx, and in the latter half of the decarburization reaction when [C]≦20 ppm, the reflux gas amount is 8 to 25 N Q /min, aa
This is a method for producing ultra-low carbon steel by RH theory gas treatment, which is characterized by reducing the amount of carbon. Here, the amount of reflux gas indicates the amount per minute when the inner diameter of the immersion tube is 1aIl.

The present invention will be explained based on experiments conducted by the inventor. Roughly decarburized in a converter [C]' = approximately 160 of 300 ppm
The results of RH treatment on molten steel at 0°C are shown in Figure 1. Figure 1 also shows the results obtained by the conventional method, which are shaded.

In other words, the conventional method can reduce the RH without changing the amount of recirculated gas.
When treated, [C] stagnates at approximately 20 ppm and further decarburization is impossible; however, in the present invention, from the start of RH treatment to [C] 420 ppm, 42 N
Q/win, ca (recirculation gas flow rate per minute per inner diameter of the immersion tube), and after [C] valve 20 ppm, the recirculation gas flow rate was reduced to 17NQ/min, cm, and the decarburization reaction continued after that. Also promoted, 20 minutes after the start of treatment
After a few minutes passed, I was able to achieve [C]≦l0PP garden.

In addition, in this case, the average oxygen O in the molten steel in the latter half of RH is 2
0 ppm, N is 20 ppm on average, final [C] = 8 p
It was p@.

[For production]

In the case of the conventional method, in the ultra-low carbon range of [C]≦20ppm, many fine bubbles 4 are generated in the reflux molten steel 2 as shown in Fig. 2, and when the amount of reflux molten steel is large, that is, the reflux velocity is low. When the bubbles are large, the fine bubbles 4 also flow back into the ladle 6, so decarburization is considered to be stagnant.

In addition, circulating molten steel amount: W (t/w+in) Inert gas injection depth: H (m) Inert gas injection amount: QNrn'/Ugly in immersion pipe inner diameter: D
(m) Since the amount of circulating molten steel is proportional to the amount of inert gas blown, in the present invention, the amount of inert gas blown is
C] ≦20pP- by decreasing in the extremely low carbon range,
The reflux rate decreases, and the residence time of fine CO bubbles in the molten steel in the vacuum chamber is extended, and the decarburization reaction is promoted even in the extremely low carbon range, and the decarburization reaction is promoted even in the extremely low carbon range. It is thought that this made it possible to melt carbon steel.

〔Effect of the invention〕

When melting ultra-low carbon steel by RH decarburization gas treatment, it is better to flow a large amount of recirculation gas and increase the recirculation molten steel cycle, especially in order to promote decarburization in the ultra-low carbon range of [C]≦20ppm. Contrary to the conventional technology which is said to be effective, the present invention sets the reflux gas amount in the first half of decarburization to [C] 20 ppm to 33 to 42 N Q /min, all [C]
] ≦20ppm In the latter half of the decarburization reaction, the amount of reflux gas is reduced to 8~
25 N Q /n+in.

■By reducing the temperature, the decarburization reaction is promoted without stagnation even in the latter half of decarburization, and [C]≦l0PPI
We were able to achieve the effect of making ultra-low carbon steel (I) possible.

[Brief explanation of drawings]

Figure 1 is a diagram comparing the relationship between the RH treatment time and the C concentration of molten steel in an example of ultra-low carbon steel melting by RH degassing treatment according to the present invention with a conventional example, and Figure 2 is a diagram comparing [C]≦ 20p
It is a schematic cross-sectional view explaining the stagnation of the decarburization reaction in the conventional method ↓ in the extremely low carbon range of pm. ４・・・・・・・・・Fine bubbles ６・・・・・・Ladle

Claims (1)

[Claims] (1) In an ultra-low carbon steel melting method in which the molten steel circulation flow rate during RH degassing treatment is changed based on the C content [C] in the steel, the above [C] is degassed until [C] > 20 ppm. In the first half of the carbon reaction, the reflux gas amount was set at 33 to 42 Nl/min, c
A method for producing ultra-low carbon steel by RH degassing treatment, characterized in that the amount of recirculated gas is reduced to 8 to 25 Nl/min, cm in the latter half of the decarburization reaction when [C]≦20 ppm.