JPH0741822A - Deoxidizing method for molten steel - Google Patents

Abstract

PURPOSE:To exceedingly decrease the alumina inclusions and total oxygen quantity in steel by simultaneously adding CaO and oxides of specific metals having the oxygen affinity weaker than the oxygen affinity of Al together with metallic Al and flocculating and floating the alumina nonmetallic inclusions at the time of deoxidizing the Al. CONSTITUTION:SiO2, FeO, MnO, TiO2 and V2O3, etc., are applicable as the metal oxides to be simultaneously mixed with Al and CaO and more particularly the SiO2 and FeO advantageous. The weight ratio at which the CaO and SiO2 are added to the molten steel are defined as X, Y respectively and these metals are added to the molten steel in a range satisfying a relation 0.6X<=Y<=2.4X in the case of using the SiO2. The fine alumina inclusions of the products formed by deoxidation are rapidly flocculated and floated by the deoxidation method, by which the total oxygen quantity in the steel after the deoxidation is lowered down to 10ppm level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a deoxidizing method for molten steel, and particularly to rapid agglomeration / decomposition of fine Al ₂ O ₃ which is a decarburization product.
It is intended for floating separation.

[0002]

2. Description of the Related Art In recent years, particularly as represented by thin steel, the demand for high quality has been particularly increased, and along with this, the development and improvement of corresponding refining technology is also required. For the purpose of improving and stabilizing the quality of such steel materials, molten steel is subjected to deoxidation treatment. Among them, the deoxidation treatment method using metal Al is used for so-called Alkymi killed steel melting and It is carried out by directly adding metallic Al to later undeoxidized molten steel or weak deoxidized molten steel obtained by reducing the residual slag.

However, in the case of melting aluminum-killed steel as described above, metal Al reacts with oxygen in the steel, and a large amount of fine Al ₂ O ₃ (solid inclusions) is produced. Such Al ₂ O ₃ particles gradually aggregate and coalesce in the secondary refining process to form clusters, which eventually float up and are separated from the steel ingot, or are not fully floated and are trapped in the steel ingot. Will result in product defects.

Therefore, since the generated Al ₂ O ₃ particles need to be removed from the molten steel as much as possible, many methods for promoting the floating separation have been proposed so far. For example, in the Ar gas bubbling method (Arima et al., Iron and Steel, 60 (1974) 11, S397), Ar gas was blown into molten steel for 10 minutes to produce Al ₂
O ₃ particles are floated and separated, and the oxygen concentration in molten steel ([TO])
Achieves deoxidation up to 20-40ppm.

However, it was practically impossible to deoxidize to a higher level by the above method. The reason for this is that among the Al ₂ O ₃ particles produced, fine particles of 10 μm or less do not float up and separate once suspended in molten steel, even if Ar gas stirring (same for other molten steel stirring) is performed. It is thought to be because of this.

Therefore, fine Al ₂ O ₃ having a particle size of 10 μm or less is used.
Also for particles, a method was devised in which the particles were coarsened immediately after formation and floated and separated. For example, in Japanese Unexamined Patent Publication (Kokai) No. 3-47910, at the time of deoxidation, at the same time as Al, a compound such as CaO that easily reacts with the deoxidation product Al ₂ O ₃ is added to produce Al ₂ O ₃ fine particles at an early stage. I have proposed a method to coarsen.

However, the above method has the following problems. That is, Al ₂ O ₃ has a high melting point of around 2200 ° C., is a solid phase in molten steel, and CaO to be added is also a solid phase particle, and an oxide reduced by Al exists near CaO. From not doing, Al ₂ O ₃
The probability of collision / coalescence between CaO and CaO is extremely low, resulting in α
The formation rate of CaO.βAl ₂ O ₃ (lime aluminate) is extremely slow. Therefore, as a secondary reaction,
The trapping speed of Al ₂ O ₃ particles by αCaO / βAl ₂ O ₃ is also slow, and therefore, it takes a long time to float and separate.

[0008]

The present invention advantageously solves the above problems and is a deoxidation product when deoxidizing and refining undeoxidized or weakly deoxidized molten steel with metallic Al. By condensing and floating fine alumina inclusions in a short time, the concentration of alumina inclusions in the steel after refining is reduced, and as a result, the total oxygen content (TO) in the steel is 10 ppm.
The purpose is to propose a new deoxidation refining method for molten steel that can be reduced to the following levels.

[0009]

[Means for Solving the Problems] That is, the present invention, in the deoxidation of molten steel with Al, together with metal Al, CaO and at least one selected from the group of metal oxides having a weaker affinity for oxygen than Al. A method for deoxidizing molten steel is characterized in that it is added to molten steel at the same time.

In the present invention, the oxides of metals having a weaker affinity for oxygen than Al include SiO ₂ , FeO, MnO and Ti.
Although 0 ₂ and V ₂ O ₅ can be applied, SiO ₂ and FeO are particularly advantageous.

In the present invention, a particularly preferred embodiment uses SiO ₂ as an oxide of a metal having a weaker affinity for oxygen than Al, and the addition weight ratios of CaO and SiO _{2 to} molten steel are X and Y, respectively. Then, CaO and SiO ₂ are added in such a range that X and Y satisfy the relationship of the following formula 0.6X ≦ Y ≦ 2.4X.

The method of adding the deoxidizing agent is not particularly limited, and any method may be used as long as the above-mentioned components can be added at the same time, but a mixture obtained by mixing all of them as powders. , Or CaO powder and SiO ₂ powder, etc.
The method of adding as a binder filled in an Al pipe is advantageous.

[0013]

[Function] Now, the inventors have found that the oxygen concentration in steel during deoxidation of Al [T.
O]] over time, and as a result of earnest research on the remaining amount, morphology, particle size distribution, etc. of the alumina-based inclusions remaining in the molten steel after deoxidation, the findings described below were obtained, and the present invention was completed. Came to. Hereinafter, the clarification process of the present invention will be described together with the deoxidizing mechanism of the present invention.

Considering the Al deoxidizing step as a basic step, it can be divided into the following three steps. (1) Melting and mixing of metallic Al. (2) Deoxidation reaction (oxidation reaction of metallic Al, generation of deoxidation product nuclei, nuclear growth). (3) Aggregation and floating separation of the deoxidation product Al ₂ O ₃ . To melt low [TO] aluminum killed steel,
The step of flocculation and flotation of the deoxidation product Al ₂ O ₃ of (3) above is particularly important. That is, the key point of this technique is how quickly the fine Al ₂ O ₃ particles generated by the deoxidation reaction are coarsened and floated and separated.

Therefore, as a result of intensive studies on this point, the following findings were obtained. That is, the metals Al and Ca
When O is simultaneously added to the molten steel, Al dissolved in the molten steel reacts with residual oxygen in the molten steel to form fine Al ₂ O ₃ particles. At this time, in parallel with the reaction of producing Al ₂ O ₃ particles, when dissolved Al remaining in the molten steel reduces oxides of metals such as SiO ₂ and FeO, which have a weaker affinity for oxygen than Al, The Al ₂ O ₃ thus produced immediately reacts with CaO added at the same time to form αCaO · βAl ₂ O ₃ (lime aluminate). This αCaO / βAl ₂ O
_{Since 3} is a low-melting point oxide, which is usually a liquid in molten steel, it can easily capture and agglomerate the fine Al ₂ O ₃ particles that are the above-mentioned deoxidation product to promote coarsening, and cause floating separation. ,
Inclusions in steel are reduced.

That is, the present invention is based on the above-mentioned Japanese Patent Laid-Open No.
-47910 publication, that is, only CaO
Al ₂ O ₃ and Ca, which are the products after deoxidation reaction,
Rather than reacting O, SiO ₂ and FeO added at the same time by Al in molten steel are reduced, and αCaO · βAl ₂ O ₃ in a liquid state is formed by reaction with CaO.
β Al ₂ O ₃ captures and coarsens alumina-based inclusions finely dispersed in molten steel in a short period of time, thereby achieving rapid levitation separation.

As described above, according to the present invention, Japanese Patent Laid-Open No.
-47910 can form a low melting point compound, αCaO.βAl ₂ O ₃ (lime aluminate), earlier than the method disclosed in Japanese Patent Publication No. 47910/1990, and thus is formed immediately after addition of a deoxidizing agent. The fine Al ₂ O ₃ particles can be collected and agglomerated at an early stage, and then can be floated and separated, and thus advantageous reduction of inclusions in steel can be realized.

Table 1 shows the addition of a deoxidizer in the method of the present invention (when SiO ₂ is added as an oxide of a metal having a weaker affinity for oxygen than Al) and the conventional method (JP-A-3-47910). The deoxidation process of is shown by a chemical formula in time series. The reaction of the conventional method was estimated from the description in the publication.

[0019]

[Table 1]

As is clear from the table, the method of the present invention is
Al (Al dissolved in molten steel) nCaO + mSiO ₂ or nCaO
・ By reducing mSiO ₂ , nCaO ・ (p / 2 +
q) forming a Al ₂ O ₃ based low-melting oxide, so do this by collecting and agglomeration of Al ₂ O ₃ particles produced as a solid phase, as compared to the conventional method, capturing of Al ₂ O ₃ particles Not only the collection and aggregation, but also the floating separation efficiency can be remarkably enhanced.

The present invention enhances the efficiency of trapping / aggregating / floating separation of fine Al ₂ O ₃ particles by forming αCaO / βAl ₂ O ₃ (lime aluminate) -based low-melting oxide at an early stage. Therefore, the oxides to be added at the same time as Al are nCaO + mSiO ₂ and nCaO · mSiO ₂ -based materials, as well as nCaO + mFeO and nCaO · mFe.
The same effect can be obtained with any oxide of a metal such as an O-based oxide that has a weaker affinity for oxygen than Al.

Further, in the present invention, when SiO ₂ is used as an oxide of a metal having a weaker affinity for oxygen than Al, in order to more suitably form a low melting point oxide, the addition ratio of SiO ₂ is set. It is desirable to adjust. Now, SiO ₂ and Al ,
As elementary reaction of _{CaO, SiO 2 +4/3 Al = 2} /3 Al 2 O 3 + Si ----- (6) 2/3 Al 2 O 3 + 2/3 CaO = 2/3 CaO · Al 2 Consider the reaction O ₃ ----- (7). Furthermore, in αCaO / βAl ₂ O ₃ (lime aluminate), the ratio of CaO to Al ₂ O ₃ (β / α) is
Since the melting point is lowered at about 0.4 to 1.5, if the weight% of added CaO to the weight of molten steel is X and the weight% of added SiO ₂ is Y, the optimum addition rate of SiO ₂ is given by the following formula (9). To be

[Equation 1]

Regarding the method of adding the deoxidizing agent according to the present invention, (1) a method of adding the deoxidizing agent when tilting the converter after tapping the converter and tapping the ladle, 2) A method in which a deoxidizing agent is added by blowing while stirring molten steel with an inert gas after tapping, and a method in which the deoxidizing agent is added by blowing together with an inert gas in a vacuum degassing tank. However, it has been confirmed that the same good deoxidation efficiency can be obtained by any method.

[0024]

【Example】

Example 1 2 g of electrolytic iron (TO = 200 ppm) was filled with various deoxidizing agents shown in Table 2, and each sample was dissolved in a vacuum by electron beam irradiation to carry out deoxidizing. The results of examining the particle size distribution of inclusions under each experimental condition are shown in FIG.

[0025]

[Table 2]

As is apparent from the results of FIG. 1, compared with the conventional example (No. 1) in which only Al is embedded and the conventional example (No. 2) in which Al and CaO are embedded, an invention example in which Al + CaO + SiO ₂ is embedded In (No.3), coarsening of inclusions is greatly promoted.

Example 2 Molten steel melted in a 230 ton converter was divided into three equal parts at the time of tapping, and (1) only metal Al (0.20%) and (2) metal Al (0.20
%) + CaO (0.10%) and (3) Metallic Al (0.20%) + CaO
(0.10%) + SiO ₂ (0.08%) was added for deoxidation, followed by RH treatment and continuous casting under the same conditions. The results of evaluating the amount of large non-metallic inclusions in the obtained slab by the slime extraction method are shown in FIG.

As is clear from the figure, according to the method of the present invention, Al ₂ O ₃ is coarsened at an early stage and is then floated and separated, and as a result, the amount of large inclusions in the slab after continuous casting is remarkably increased. is decreasing. The oxygen concentration of the slab obtained according to the present invention was 7.0 ppm, and the target value of 10 ppm or less was achieved.

[0029]

As described above, according to the present invention, the fine Al ₂ O ₃ particles formed after deoxidation can be coarsened at an early stage and can be floated and separated, so that the amount of non-metallic inclusions in the steel after refining and, by extension, in the steel. The total oxygen content can be significantly reduced. As a result, not only the surface defects of the product can be reduced, but also cracks due to non-metallic inclusions during processing can be reduced, so that the product quality of the steel sheet or the like is significantly improved.

[Brief description of drawings]

FIG. 1 is a graph showing a particle size distribution of non-metallic inclusions in Example 1.

FIG. 2 is a graph showing the amount of nonmetallic inclusions in Example 2.

Claims (3)

[Claims] 1. At the time of deoxidizing Al of molten steel, Ca together with metallic Al
A method for deoxidizing molten steel, characterized in that O and at least one selected from the group of metal oxides having a weaker affinity for oxygen than Al are simultaneously added to the molten steel. 2. The oxide of a metal according to claim 1, which has a weaker affinity for oxygen than Al, is SiO ₂ and / or
Method for deoxidizing molten steel that is FeO. 3. The oxide according to claim 1, wherein the selected oxide of a metal having a weaker affinity for oxygen than Al is SiO ₂ , and the addition weight ratios of CaO and SiO _{2 to} the molten steel are X, respectively.
A method for deoxidizing molten steel, in which CaO and SiO ₂ are added in a range where X and Y satisfy the following formula: 0.6X ≦ Y ≦ 2.4X, where Y is Y.