JPH0841530A - Production of low aluminum and low sulfur stainless steel - Google Patents

Abstract

PURPOSE:To provide a method for producing a high quality stainless steel by enabling an effective desulfurizing operation. CONSTITUTION:At the time of producing the low aluminum and low sulfur stainless steel, Al for deoxidizing and CaO for desulfurizing are added to molten steel and slag in a ladle after finish decarburizing, and vacuum reduction of the molten steel and slag is executed. After this vacuum reduction, the slag in the ladle is removed and also, the excess Al contained in the molten steel is removed by blowing oxygen, and the lowering in S content S is achieved. While lowering oxidizing degree of the slag and molten steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of stainless steel, and particularly to the composition ratio of aluminum and sulfur [Al].
Low Al with ≦ 0.015% and [S] ≦ 0.005%
/ A method for producing low S stainless steel using a pan degassing facility such as VOD.

[0002]

2. Description of the Related Art In the production of stainless steel, a molten steel in a ladle is made of Ca alloy or simultaneously with or before Al ₂
It has been widely known that O ₃ is added to desulfurize.

For example, Japanese Patent Laid-Open No. 61-27964 discloses a ladle slag with a basicity = (CaO + MgO) / (S
iO ₂ + Al ₂ O ₃ ) ≧ 1 and 0.3 to 200 Tor
There is described a method of desulfurizing by spraying a desulfurizing agent containing CaO as a main component on the surface of molten steel while bottom-blown and stirring with an inert gas of 0.5 to 10 Nl / t under a vacuum of r.

Further, Japanese Patent Laid-Open No. 1-75622 discloses a desulfurization method in which a Ca alloy such as CaSi is added to molten steel in a ladle and Ca having a strong affinity with [S] is used. At the same time as or before the addition of Al ₂ O _3, there is described a production method in which the morphology of sulfide can be controlled as a CaO—Al ₂ O ₃ —S composite inclusion.

[0005]

Generally, in the desulfurization reaction, the reaction of CaO + S = CaS + O proceeds, so that it is essential to reduce the oxidation degree of slag and molten steel in order to reduce S.

However, in the manufacturing method disclosed in Japanese Patent Laid-Open No. 61-27964, CaO under basic vacuum ≧ 1 under strong vacuum stirring.
Since the desulfurization material is sprayed, the formation of Cr ₂ O ₃ is unavoidable when the stainless steel is subjected to deoxidation with carboxylic acid. Therefore, in the production under such conditions, the desulfurization efficiency is suppressed to be low, and it is difficult to produce low S stainless steel.

In the manufacturing method disclosed in Japanese Patent Laid-Open No. 1-75662, desulfurization and inclusion morphology control are performed by adding Al ₂ O ₃ at the same time as the CaSi alloy. There is formation of Cr ₂ O ₃ as in the previous example. Therefore, not only the desulfurization efficiency is low, but also Ca
Since the raw Si unit price is low and the CaO—Al ₂ O ₃ —CaS compound generated as a reaction product of the added CaSi is water-soluble, the corrosion resistance of the stainless steel is deteriorated.

The problem to be solved in the present invention is to provide a method for producing high quality stainless steel by enabling an efficient desulfurization operation.

[0009]

[Means for Solving the Problems] The method for producing low aluminum and low sulfur stainless steel according to the present invention comprises adding deoxidizing Al and desulfurizing CaO to molten steel and slag in a ladle after finish decarburization. The molten steel and the slag are vacuum-reduced, and after the vacuum reduction, the slag in the ladle is discharged and removed, and excess Al contained in the molten steel is removed by propellant.

[0010]

[Operation] In the production of Cr-containing steel such as stainless steel in ladle refining such as VOD, vacuum blow acid decarbonization is carried out to suppress Cr oxidation loss, but the content of Cr ₂ O ₃ after blow acid is 10% or more. It becomes a high concentration. Desulfurization reaction is CaO + S = C
This is a reduction reaction of aS + O, and it is essential to reduce the oxidation degree of slag and molten steel in order to reduce S.

Therefore, in the present invention, first, Al for deoxidation is added to the slag and molten steel so that [Al]> 0.030% is obtained after deoxidation of C by blowing acid, and CaO for S removal is further added to 30
1 to 9.5 FNl / min at high vacuum of 0 Torr or more
Stirring is performed using an inert gas of / t. Here, 0.030% or more of deoxidizing [Al] is added in order to deoxidize molten steel and reduce oxides (lower oxides such as Cr ₂ O ₃ ) in slag in molten steel [Al]. is there. Also, 300T
Efficient slag reforming is carried out by stirring at or or more and 1 to 9.5 Nl / min / t.

If the gas agitation is less than 1 Nl / min / t, efficient slag reduction cannot be achieved. Further, if the high vacuum degree is 9.5 Nl / min / t, the molten steel rise will be high, and an operation hindrance will occur from the viewpoint of freeboarding.

In such an operation, when the molten steel [Al] becomes more than the target range after the strong stirring de-S treatment, the slag thickness in the ladle is once removed to 30 to 150 mm, and the propelling acid for Al removal is carried out again. To do. At this time, the recovery [S] after slag removal is suppressed to 0.002% or less.

Similarly, after the strong stirring de-S treatment, the molten steel [A
When l] is above the target range, Al-propelling acid for Al removal is performed after removing the slag in the immersion pipe by bottom blowing bubbling using an immersion pipe such as CAS. Even in this case, the rocking of the molten steel outside the dip pipe is suppressed, and at the same time, the returning S from the pan de-S slag is also suppressed, and the difference Δ between the molten steel [S] and the target [S] is reduced.
[S] is operated at 0.002% or less.

[0015]

[Example] Before manufacturing a low Al, low S stainless steel using VOD equipment, before VOD treatment [S] = 0.01
After 5% stainless steel molten steel was usually subjected to VOD blowing acid treatment, a production test was conducted in which the molten steel [Al] was strongly reduced to obtain an extremely low S.

FIG. 1 is a diagram showing the relationship between molten steel [Al] and reduced [S] obtained by this test, and the effects of the degree of vacuum and the flow rate of stirring gas are as shown in Table 1.

[0017]

[Table 1] From these diagrams and tables, it was confirmed that [S] <0.003% is stably achieved when [Al] is 0.030% or more.

Here, S removal of molten steel in the ladle is [Al] ≧
0-300 Torr at 0.030% and bottom blowing gas 1Nl
/ Sm / min / t or more, and removes S mainly from CaO from above
The material was added. After that, molten steel [Al] and target [A
l) as a de-Al treatment of a difference (ΔAl), a propellant acid lance is used for propelling acid, but two methods shown in FIGS. 2 and 3 are used to prevent recovery [S] from de-S slag. went.

FIG. 2 shows an equipment in which a blowing acid lance 2 is provided in the upper portion of the ladle 1 and argon (Ar) can be blown from the bottom portion.

In this equipment, the (S) -containing slag after S removal is removed, and 1.0 to 1.2 times as much propellant acid as the stoichiometric amount of ΔAl is added by means of propellant lance 2. Done,
Furthermore, it is 300 Torr or more and the bottom blowing Ar is 1 Nl / m.
Stirring was performed while supplying more than in / t.

At this time, for the slag removal, an operation of removing the reducing slag was carried out using the existing slag equipment, and the temperature drop in this operation was 10 to 25 ° C. Moreover, when the thickness of the reduced slag and the S-return behavior are confirmed, the width of de-Al is 0.
By recovering the slag to a thickness of 30 to 150 mm at an operation of 020%, the S-reversion amount was reliably suppressed to 0.002% or less.

Here, the de-Al brominated acid is 1.0 to the theoretical value.
The reason for increasing 1.2 times is that, since Al removal depends on the oxygen efficiency for the Al oxidation reaction, if excessive blowing acid causes the deterioration of the yield due to the oxidation of molten steel and the production of lower oxides, the quality will deteriorate. Is. It was also confirmed that the vacuum agitation treatment after blowing acid promotes the slag reduction due to the molten steel residue [Al], which stabilizes the quality.

On the other hand, FIG. 3 shows a facility for removing Al by removing the (S) -containing slag from the inside of the dip pipe 3 using the dip pipe 3 after the de-S, and directly spraying the molten steel with the propionic acid lance 2. Is an example of.

By eliminating the (S) -containing slag, the recovery of S that accompanies the oxidation reaction by propellant can be suppressed.
As with (1), the amount of acid recovery is 1.0 to 1.2 of the theoretical value of ΔAl.
Molten steel oxidation is also suppressed by carrying out double blowing acid with stirring at a bottom blowing gas of 1 Nl / min / t or more.

[0025]

INDUSTRIAL APPLICABILITY According to the present invention, low A, which is rich in low temperature impact resistance, is used.
l, low-S stainless steel can be manufactured favorably,
Easy to use low Al and low S without using expensive low S alloy
Stainless steel can be manufactured. Further, by performing the reduction treatment of the slag in the pot, it is possible to maintain the stability of the continuous casting operation and to manufacture low Al / low S stainless steel with stable quality.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between molten steel [Al] and reduced [S].

FIG. 2 is a schematic view showing blowing acid and bottom blowing into a ladle.

FIG. 3 is a schematic view showing a ladle to which a dipping tube is applied.

[Explanation of symbols]

 1: Ladle 2: Lactic acid lance 3: Immersion tube

Claims (1)

[Claims] 1. The molten steel and slag in the ladle after the final decarburization are added with Al for deoxidation and CaO for desulfurization to vacuum-reduce the molten steel and slag, and after this vacuum reduction, the slag in the ladle. Is removed and excess Al contained in the molten steel is removed.
Method for producing low-aluminum, low-sulfur stainless steel by removing broccid acid.