JPH08176644A - Manufacturing method of low-sulfur steel with reduced killing time - Google Patents

Abstract

(57) [Summary] [Purpose] To develop a method for producing low-sulfur steel with S ≦ 0.002% or less in a killing time of 60 minutes or less. [Constitution] After a basic solvent of 8 to 40 was added at the time of tapping the converter, CaO was used as a desulfurizing agent as a base, and Al ₂ O ₃ , CaF ₂ , S
A low melting point flux containing at least one of iO ₂ and a slag modifier based on Al and CaCO ₃ are put into molten steel, and an inert gas is blown during ladle refining.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing low-sulfur steel capable of shortening the killing time.

[0002]

2. Description of the Related Art Today, the specifications for steel materials are becoming stricter, and inexpensive and higher quality steel materials are required. In particular, regarding the S content, there is a demand for an inexpensive manufacturing method of low-sulfur steel in which desulfurization treatment is strengthened, for example, S ≦ 0.002%.

Conventionally, various methods for producing low-sulfur steel have been proposed. For example, according to JP-A-63-169320, a desulfurizing agent containing lime as a main component when tapping a steel from a converter to a ladle. At the time of charging, the amount of charging was determined from the [O] concentration in the converter blown steel, and the slag composition after charging was CaO: 60-75%, Al ₂ O ₃ : 15-
25%, SiO ₂ : 5 to 15%, and then desulfurization treatment is performed while blowing a desulfurizing agent containing lime as a main component into molten steel, while the slag composition is CaO: 60 to 70%, Al ₂ O ₃ : 25-35%, SiO
₂ : A manufacturing method of low-sulfur steel is disclosed, which is maintained at 10% or less.

However, blowing a desulfurizing agent containing lime as a main component into molten steel as in this method lowers the temperature of the molten steel, and raises the temperature of the molten steel in the out-furnace refining after tapping and before casting. The need arises, leading to an extension of the killing time (time from the steel output from the converter to the casting).

Further, for example, means for introducing Al is adopted to raise the temperature of the molten steel, and if the temperature of the molten steel is raised before tapping in such a manner, the melting loss of the refractory in the furnace increases. ,
Both of them will result in increased manufacturing costs.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing low-sulfur steel capable of shortening the killing time without lowering the temperature of molten steel even by desulfurization treatment. is there.

[0007]

Means for Solving the Problems Accordingly, the present inventors have:
Various studies have been repeated to solve such problems, and the following findings have been obtained.

That is, in the conventional method, first, hot metal desulfurization is performed, then steelmaking is performed in a converter, and a desulfurizing agent is added at the time of tapping to perform slag reforming, while adjusting the slag composition, and further in molten steel. Although the desulfurizing agent is blown in, the temperature of the molten steel is lowered due to the blowing of the desulfurizing agent. Therefore, the temperature is raised by introducing so-called Al prior to continuous casting.

Since the temperature drop of the molten steel at the time of blowing the desulfurizing agent is caused by charging the desulfurizing agent during ladle refining, the desulfurizing agent is charged at the time of tapping, and then Al + CaCO ₃ is the main component. By adding a slag modifier as described below and only blowing Ar gas during ladle refining, for example, effective desulfurization can be performed while preventing the temperature drop of the molten steel, so the killing time is almost 60 minutes. The inventors have completed the present invention, knowing that it is significantly shortened.

According to the present invention, Al ₂ O ₃ is used as a desulfurizing agent based on CaO at the time of tapping the converter after introducing a solvent medium having a basicity of 8 to 40 in the initial stage or during the blowing of the converter. , CaF ₂ , Si
A low melting point flux containing at least one of O ₂ is charged, an slag modifier based on Al and CaCO ₃ is charged into the molten metal, and then an inert gas, preferably molten steel 250, is added during ladle refining. It is a method for producing low-sulfur steel with shortened killing time, which is characterized by blowing 10 to 60 Nm ³ per ton.

[0011]

As described above, as compared with the conventional method, in the present invention, the desulfurizing agent is not blown in the secondary refining in the ladle after tapping the converter, and the desulfurization is promoted only by bubbling the inert gas. Of.

In the present invention, the desulfurization can be promoted only by such bubbling of an inert gas. The desulfurization can be promoted by introducing a high basicity medium solvent having a basicity of 8 to 40 at the time of blowing the converter. is because it is, also when the converter tapping CaO-Al ₂ O ₃ based low-melting premelt flux and Al
This is because the slag modifier based on + CaCO ₃ is added.

Further, since it is not necessary to add a desulfurizing agent at the time of secondary refining in the ladle, the temperature of the molten steel is not substantially decreased, and since Al is added as a slag modifier, the temperature of the molten steel is decreased. Can be made as small as possible, and since they are added at the time of tapping, no increase in inclusions in the steel is observed. Here, the reason for defining the processing conditions and the processing material as described above in the present invention will be described in detail.

Highly basic solvent medium : In order to promote slag slag formation and viscosity reduction at the same time, basicity:
CaO, SiO ₂ , and CaF ₂ are added so that CaO / SiO ₂ = 8-40. The basicity is limited to 8 to 40 because the desulfurization ability of the slag increases by increasing the basicity, but if it is too large, the melting point of the slag increases and the reactivity with molten steel decreases. Preferably, the basicity is 8-20.

Flux (desulfurizing agent): for changing the slag in the ladle to a slag composition having a low melting point and a low viscosity, and for exerting a desulfurizing action. Al ₂ O ₃ based on CaO,
It contains at least one of CaF ₂ and SiO ₂ .

That is, the CaO--Al ₂ O ₃ (Ca
F ₂ , SiO ₂ ) -based low melting point premelt flux contains at least one of Al ₂ O ₃ , CaF ₂ , and SiO ₂ based on CaO, and its composition is not particularly specified, but preferably CaO 40 % By weight or more, and Al ₂ O ₃ : 20 to 50% by weight,
It has a composition of at least one of CaF ₂ : 20 to 40% by weight and SiO ₂ : 5 to 40% by weight.

Slag modifier : The slag modifier is added prior to the secondary refining in order to accelerate the desulfurization in order to reduce lower oxides in the slag, and is based on Al and CaCO ₃ . As a result, the slag composition has a high basicity to increase the fluidity and ensure desulfurization ability. In addition, Al ₂ O ₃ may be blended to replace Al.

In the present invention, when secondary refining in a ladle is performed, Al + CaCO ₃ is added to molten steel before the treatment to reduce lower oxides. Al + CaCO, not Al
_The reason for adding ₃ is to increase the stirring power.

Inert gas : In the present invention, by bubbling with an inert gas, inclusions are floated to promote desulfurization between slag and metal. In this case, the inert gas is Ar, Ne, He or the like, but Ar is preferable from the economical viewpoint.

That is, in the present invention, in order to accelerate the desulfurization reaction at the slag / molten steel interface, the molten steel is stirred with an inert gas. Bubbling is over blowing with a lance,
Alternatively, the bottom is blown from the plug at the bottom of the ladle, or both are used in combination, and the blowing amount is not particularly limited in the present invention. Although the desulfurization capacity increases when the amount of blown in is large, the flow rate is 5 to 100 Nm ³ with respect to 250 tons of molten steel, preferably 1060 because the possibility of entraining slag increases.
Nm ³ will do. 5 to 100 Nm ³ for 250 tons of molten steel
The flow rate of is equivalent to 0.02-0.4Nm ³ / Ton.

Here, it has been considered that the killing time, that is, the time from the steel output from the converter to the casting is generally required to be about 75 to 80 minutes in the conventional example in which desulfurization is performed to S ≦ 0.002% or less. In the present invention, since the temperature drop of molten steel is not seen,
Since it can be sent to the continuous casting process immediately after the secondary refining of the ladle, such a killing time is desulfurized to S ≦ 0.002% or less, but it is approximately 60 minutes or less, which is approximately 20 to 25% compared to the conventional method. Can also be shortened.

[0022]

[Examples] In the examples of the present invention, in the hot metal desulfurization, [S] = 16
After thorough desulfurization to ppm, the slag in the hot metal ladle was removed to prevent re-sulfurization, and it was put in a 250 ton converter. After blowing the high basicity medium solvent shown in Table 1 at the time of blowing of the converter, 1.0 ton of flux shown in Table 1 was put into the tapping, and 300 kg of the slag modifier shown in Table 1 after tapping was completed. I put it in.

Subsequently, the injection lance is immersed in the ladle molten steel so that the lance discharge port comes close to the bottom of the ladle,
Ar gas was blown in the amount shown in Table 1 to continuously cast molten steel to obtain a slab.

As a result, steels having the compositions shown in Table 2 were obtained. Examples Nos. 1 to 14 are all examples of the present invention, and S ≦ 0.
It can be seen that the killing time is within 60 minutes while effectively desulfurizing to less than 002%.

On the other hand, Examples Nos. 15 to 20 are conventional examples and comparative examples, and in any of them, desulfurization up to S≤0.002% cannot be performed, or even if desulfurization up to S≤0.002% is possible, the killing time is reduced. You can see that it is over 60 minutes.

That is, Example No. 15 is a conventional example,
In a ladle refining equipment that can inject powder under reduced pressure after adding the high basicity solvent shown in Table 1 at the time of blowing 250 tons converter.
1 ton of CaO and 0.25 ton of CaF ₂ were blown into the molten steel for desulfurization and continuous casting to obtain a slab. As a result, Table 2
Although low-sulfur steel was obtained as shown in, the killing time was extended.

Example No. 16 is also a conventional example. After the high-tonicity medium solvent shown in Table 1 was charged at the time of blowing 250 tons of converter, slag reforming flux was 1.0 tons during tapping, and after tapping was completed. 300
I put in kg. Next, 50Nm ³ of oxygen gas was blown in with an injection lance, and the desulfurizing agent (CaO powder) was also injected with 50Nm ³ of Ar gas by an injection lance.
After blowing 750 kg, molten steel was continuously cast to obtain a slab. As a result, low-sulfur steel was obtained as shown in Table 2,
Killing time has been extended.

Example No. 17 is a comparative example, in which the slag modifier of Table 1 was blown into molten steel, and the killing time was extended as in Example No. 16. In Examples No.15, 16 and 17, the extended killing time was caused by the temperature decrease of the molten steel due to the blowing of powder, so it was necessary to raise the temperature of the molten steel in the outside furnace refining after tapping and before casting. It is due to the fact.

[0029] Examples No.18, 19 is a comparative example, was the amount blowing Ar gas and 4 Nm ^3, 110 nm ^3. As a result, desulfurization in 4 Nm ³ is not sufficiently, entrainment of the slab was confirmed although desulfurization capacity in 110 nm ³ was ensured.

Example No. 20 is a comparative example, in which 400 kg of the slag modifier was introduced after tapping without introducing the desulfurizing agent. afterwards,
Immerse the injection lance in the molten steel and remove Ar gas.
Blowing only 40 Nm ³ and continuously casting molten steel to obtain a slab. As a result, the desulfurization ability was insufficient.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

EFFECTS OF THE INVENTION According to the present invention, the temperature of molten steel is lowered by introducing a low melting point solvent, a desulfurizing agent and a slag modifier at the time of tapping a converter, and then performing an interfacial desulfurization reaction by only blowing an inert gas. Can be as small as possible,
The killing time can be kept within 60 minutes to efficiently produce low-sulfur steel.

Claims (2)

[Claims] 1. A basicity of 8 during or during the initial blowing of a converter.
As a desulfurizing agent when tapping the converter, after adding ~ 40 medium solvent
Based on CaO, a low melting point flux containing at least one of Al ₂ O ₃ , CaF ₂ and SiO ₂ is added to the Al and CaCO ₃
A method for producing low-sulfur steel with reduced killing time, which comprises introducing a slag modifier based on slag into a molten metal and then injecting an inert gas during ladle refining. 2. The method according to claim 1, wherein 5 to 100 Nm ^{3 of} 250 ton of molten steel is blown with an inert gas during ladle refining.