JPH0551623A - Method for adding ca in refining of stainless steel - Google Patents

Abstract

PURPOSE:To eliminate the loss in the addition of Ca and to improve its yield in the final stage of the refining process of stainless steel by adding Al to stainless molten steel, removing silicic acid up to a prescribed concn. and thereafter adding Ca. CONSTITUTION:In the final stage of the refining process of stainless steel, the molten steel in a ladle is mixed with Al to regulate the concn. of SiO2 to <=2%. Next, Ca is added thereto to increase the basicity of slag and to improve the capacity of desulfurization. Then, the loss in the addition of Ca is eliminated to improve the yield of Ca.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of adding Ca at a high yield for desulfurization and the like at the end of a refining process of stainless steel.

[0002]

2. Description of the Related Art As is generally known, in particular α
System and duplex stainless steels, Ca alloys such as CaSi and CaC ₂ are added at the end of the refining process in order to remove S and O that hinder hot workability and promote removal of oxide inclusions. A defluent such as a flux containing CaO as a main component may be added in some cases. A specific example of the former case of removing S and O is addition of Ca to pipe steel which requires hydrogen-induced cracking resistance. This is for the purpose of controlling the inclusion morphology of residual sulfur after flux desulfurization and subsequent pickup sulfur, and is performed by blowing CaSi alloy or the like. As a specific example of the latter removal of oxide inclusions, addition of Ca-based flux to steel for electric resistance welded pipes and the like can be mentioned. In this method, Al ₂ O ₃ inclusions in molten steel are floated by coalescence with flux particles blown in. As the addition method, there are various addition methods such as addition by an injection pipe, addition by chute, and addition by blowing into the molten steel together with gas.

[0003]

However, the Ca-based additive added is SiO ₂ , MnO, Fe in the molten steel.
Since it reacts with O and Cr ₂ O ₃ , the Ca addition loss is large and the yield is very poor. For this reason,
The unit consumption of expensive Ca has increased, which has been a cause of cost increase. In addition, the added Ca content may adhere to the nozzle of the blowing lance to cause blockage, or the excessive addition of Ca may cause product defects.

Therefore, the main object of the present invention relates to the refining of stainless steel, and aims to improve the yield by eliminating the addition loss of Ca added at the final stage of the refining process.

[0005]

In the final stage of the refining process of stainless steel, Al is added to the molten stainless steel in the ladle to reduce the SiO ₂ concentration to 2% or less, and then Ca is added. It can be solved by featuring that.

Further, at the final stage of the refining process of stainless steel, Ca may be added to the molten stainless steel in the ladle after removing slag.

[0007]

For example, the slag on the molten stainless steel reduced and refined by AOD refining contains about 10 to 20% of SiO _2.
It is contained around%. According to the present invention, when Al is added, 3SiO ₂ + 4Al → 2Al ₂ O ₃ + 3Si-(1) By the reaction of the above formula (1), Al is reduced and S in the slag is reduced.
The iO ₂ concentration is significantly reduced.

In the reaction of the above formula (1), the SiO ₂ concentration is 2
Perform until it becomes less than or equal to%. As the SiO ₂ concentration decreases, the yield of Ca to be added next increases, and the slag basicity (CaO / SiO ₂ ) also increases, improving desulfurization ability. In the case where SiO ₂ concentration exceeds 2%, SiO ₂ in the slag is not desirable cause Ca yield deterioration is reduced to Ca.

On the other hand, FeO, MnO, and C added with Al
r ₂ O ₃ concentration is [% FeO] ≦ 1.0%,
[% MnO] ≦ 1.0%, [% Cr ₂ O ₃ ] ≦ 3.0%
Is desirable.

On the other hand, the addition amount of Al can be obtained as a cumulative value of the equations (1) to (4) by the reaction with each oxide.

3MnO + 2Al → Al ₂ O ₃ + 3Mn −−− (2) 3FeO + 2Al → Al ₂ O ₃ + 3Fe −−− (3) Cr ₂ O ₃ + 2Al → Al ₂ O ₃ + 2Cr −− (4) (1) to (3) From the formula 4), the added Al amount (V ₁ ) = SiO ₂ amount × 4/3 × [Al] / [SiO ₂ ] − 〃 (V ₂ ) = MnO amount × 2/3 × [Mn] / [SiO ₂ ] - 〃 (V ₃₎ = FeO amount × 2/3 × [Fe] / [SiO 2] - 〃 _{(V 4) = Cr 2 O} 3 × 2 × [Cr] / [SiO 2] --- Therefore, the Al addition amount is obtained as a cumulative value of V _{1 to} V ₄ . Actually, Al ₂ O ₃ before Al addition
Since the amount varies depending on the amount and the amount of O ₂ in the molten steel, the amount to be added is determined in consideration of the amount.

[0012]

EXAMPLES The present invention will be described in detail below based on examples. The stainless steel having the components shown in Table 1 was refined under the AOD operation.

[0013]

[Table 1]

In the case of AOD refining, as compared with other stainless refining methods such as VOD and RH, in the final step, slag and
Slag control such as slag can be controlled easily, and high desulfurization ability can be obtained due to its large stirring ability and high refining temperature. Further, strong stirring with the slag brings advantages such as promotion of floating separation of oxide inclusions.

The operating conditions in the AOD refining are shown in Table 2.

[0016]

[Table 2]

As shown in the flow chart of FIG. 1 and FIG. 2, the method of the present invention is applied when Ca is added after the deoxidation reaction by Si addition in the AOD refining. A
11% of SiO ₂ is contained in the slag on the molten stainless steel after Si reduction refining by OD refining. As a pretreatment for Ca addition, 0.7 kg / t of Al was added to the molten stainless steel along with Ar gas blowing, and Ar stirring was performed for 5 minutes until the SiO ₂ concentration in the slag became 2%. When the SiO ₂ concentration in the slag became 2%, 2.6 kg of CaSi was added by the injection method.

FIG. 2 shows the transition of the amount of each component of the steel slag from the Si reduction refining to the reduction treatment by adding Al to CaSi injection. The comparison is shown in FIG.

C with and without reduction pretreatment by addition of Al
The a yield was stable in the range of 12 to 15%, and a high yield could be obtained, whereas the Ca yield without pretreatment was 0.5 to 5%. C
It is found that the yield is significantly improved.

FIG. 4 shows the redox balance in the pretreatment by adding Al.
90% of the lower oxide containing O ₂ is reduced by Al, Ca
It was found that the reduction loss of the lower oxides of was able to be limited to 10%.

The case of performing Al pre-reduction treatment by adding Al before Ca addition has been described in detail above.
The amount of SiO ₂ in the ladle can also be reduced by removing the slag. However, in this case, there is a cost disadvantage due to the loss of metal outflow due to slag removal,
It is necessary to consider it in consideration of profit and loss.

[0022]

As described above in detail, according to the present invention,
With respect to the refining of stainless steel, it is possible to improve the Ca yield by eliminating the addition loss of Ca added at the end of the refining process.

[Brief description of drawings]

FIG. 1 is a refining flow chart according to the method of the present invention.

FIG. 2 is a diagram showing a transition of the amount of each steel slag component in slag.

FIG. 3 is a comparison diagram of Ca yields with and without reduction pretreatment by adding Al.

FIG. 4 is a diagram showing an oxidation / reduction balance.

Claims (2)

[Claims] 1. At the end of the stainless steel refining process, Al is added to the molten stainless steel in the ladle to reduce the SiO ₂ concentration to 2% or less, and then Ca is added. Method of adding Ca in refining. 2. A method for adding Ca in the refining of stainless steel, characterized in that, at the final stage of the refining process of stainless steel, Ca is added to the molten stainless steel in the ladle after removing slag.