JPH0692614B2 - Dechromization method of molten steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for effectively removing chromium (Cr) from molten steel after converter blowing.

<Prior art and its problems> In general, it is known that an increase in Cr content in steel causes deterioration of processing characteristics such as cold workability and deep drawability. When melting carbon steel with good workability such as, the Cr content in the molten steel should be 0.03
It is strictly required to adjust to ~ 0.05% or less.
However, recently, it has been gradually becoming difficult to completely prevent a sudden increase in the Cr content in the carbon steel melting operation by the converter.

This is because, in the melting operation of carbon steel in a converter, scrap is usually mixed into the hot metal, but in recent years, the growth of demand for stainless steel has been reflected in the scrap used as a raw material for converter conversion. This is because the chances of mixing steel scraps were increasing.

Of course, although the management of scrap has been further strengthened, the actual situation was that the problem of out-of-specification of the Cr content in the carbon steel after converter blowing could not be eradicated.

Therefore, as a countermeasure against the above-mentioned out-of-specification of the Cr content, a method of blowing oxygen in the converter to further extend the blowing to remove Cr is also taken. However, in this case, the Cr dissolved in the hot metal
There is a limit to the reduction of Cr because a part of it has already been oxidized during the decarburization refining and moved into the slag as Cr oxide,
In addition, there is a problem that the melting loss of the converter refractory during the treatment becomes severe.

Therefore, under the present circumstances, it is inevitable to "change steel grade" for molten steel having a large degree of Cr content deviation from the standard.

On the other hand, scrap is showing signs of increasing due to the recent trend of stabilizing steel demand.If more scrap can be obtained at a lower cost in the future, the molten pig iron ratio will be lowered when carbon steel is melted. It is visible that the situation will raise. Therefore, in such a situation, it is considered that the probability that the Cr content of the steel material will deviate from the standard due to the inclusion of stainless steel scrap will increase further, and the deviance range will further increase, and it will not be possible to deal with it only by "steel type change". it is obvious.

<Means for Solving the Problems> As described above, when the present inventors anticipate that the mixing of Cr into molten steel through scraps and the like will increase more and more in the future, “oxygen blowing in the converter will be performed. In extension of down blowing
From the viewpoint that it is not possible to cope with the Cr content out of specification and the melting of low Cr steel (carbon steel for cold-rolled steel sheets for automobiles)
A study was conducted on the possibility of simple and effective molten steel Cr removal by smelting outside the furnace, and it was shown that "undeoxidized and unvacuum-treated molten steel after converter blowing was stored in a container such as a ladle. Highly oxidative
If the flux is a low basicity flux and the oxidizing property, that is, the (T.Fe) value is adjusted according to the basicity, the Cr removal of molten steel will proceed effectively, and Even if there is Cr mixed in, it is possible to stably manufacture low Cr steel that sufficiently satisfies the strict Cr regulations required for cold-rolled steel sheets for automobiles, etc. " Of.

The present invention has been made based on the above findings, "a method for dechromizing molten steel in which an oxidizing low basicity flux in which no carbonate component is added is brought into contact with molten steel after being blown into a container and stored in a container Therefore, the value of (T.Fe) in the slag is adjusted according to the basicity of the flux. Adjusting as described above makes it easier and more stable to remove molten steel.
It has a feature in "the point of performing Cr".

Here, the container for containing the molten steel is not particularly limited, but it is generally said that a ladle is the most simple and preferable. Further, the degree of oxidizing power and basicity of the flux are appropriately adjusted within the range described below depending on the application of the molten steel, the Cr content, and the like. The flux composition is mainly composed of iron oxide (FeO content of iron ore, dust, scale, etc.) and SiO ₂ , and Al ₂ O ₃ (an acidic oxide that is effective for Cr removal) In addition, the refractory of molten steel container such as ladle is Al ₂ O.
_In case of ₃ , it is preferable to prevent melting of refractory materials), CaO
Add one or more of (effective for adjusting basicity and preventing melting of refractories), MgO (effective for preventing melting of refractories) and CaF ₂ (having an effect of improving the slag slag slag). It is preferable that the composition is mixed or synthesized and the Cr-free composition of the present invention is used.
Suitable for processing.

The components, basicity and degree of oxidizing power of this flux will be described in more detail below.

In order to effectively carry out the Cr removal treatment according to the present invention, it is important to note the following two points when selecting the flux components.

The first point is to select the components so that the slag composition has a lower basicity when the "oxidizing power of the slag" is similar. This is because Cr ₂ O ₃ generated in the slag by the deCr treatment is considered to be a weak but basic oxide, and the slag is low in order to reduce the activity of Cr ₂ O ₃ in the slag so as not to reconstitute Cr. This is because basicity is more effective.

Figure 1 shows the basicity of slag at various iron oxide activities (α _Feo ) {(CaO + MgO + MnO) / (SiO ₂ + Al ₂ O ₃ )} and Cr.
Is a graph showing the relationship with the distribution ratio of {(Cr) / [Cr]}. Here, α _Feo is the FeO concentration in the slag, that is, (Fe
O) and the basicity of the slag, "Turkdogan and Pearso
α _Feo isoactivity diagram by n (“Journal of the Iron and
It is an "index showing the oxidizing power of slag" as estimated from Steel Institute "1953 MARCH, pages 217-223)".
From FIG. 1 also, even if the slag has the same α _Feo , the lower the basicity of the slag, the more the distribution ratio of chromium {(Cr) /
It can be seen that the value of [Cr]} increases.

Generally, it is desirable that the basicity of the slag is 1 or less, but if it is 2 or less, a good effect is obtained, and if it is 3 or less, the progress of Cr removal is ensured by other conditions. be able to.

The second point is "to select the flux component so that the slag has a strong oxidizing power". This is because the removal of Cr in molten steel by oxidation with slag is performed by a phenomenon in which Cr is removed while being accompanied by decarburization due to the strong oxidizing power of slag.

This point is also clear from FIG. 1 shown above. That is,
Examination of Fig. 1 shows that α _Feo is high for slag of the same basicity.
It should be understood that the slag with has a higher (Cr) / [Cr] value.

When the basicity of the slag is 1.5 to 3, the (FeO) concentration is preferably 20 to 30% by weight or more, and in other regions, it is preferably higher than the above value.

Furthermore, similar results can be obtained by studying this point by using (T.Fe), which is used as a standard for estimating the oxidizing power of general slag.

That is, Fig. 2 shows the effect of (T.Fe) on the Cr removal rate for three types of relatively low flux basicity {(CaO + MgO / SiO ₂ + Al ₂ O ₃ )}. However, from FIG. 2 as well, for example, in order to obtain a Cr-free flux with a Cr-free rate exceeding 20%, slag (T.Fe) It turns out that it is necessary to adjust to a higher value.

Now, it is desirable to remove the converter slag when performing the Cr removal treatment according to the present invention, but when recondensation does not pose a problem, the above-mentioned condition is left with a small amount of the converter slag. The added flux component may be controlled so that a slag component having a property is obtained.

Further, as a method for adding the flux, the injection method from the upper portion of the molten steel is sufficient, but the method of injecting into the molten steel is more effective.

The amount of flux added is determined by the target amount of Cr removal and the amount of decarburization that occurs at the same time, but usually 10 to 50 kg / t ・ st
eel is good. If it is more than this, the slag overflows due to the forming phenomenon and the temperature drop of the molten steel becomes large, which is not suitable. However, 50-100 kg / t · steel may be used if the ladle can be raised and the temperature can be raised after adding flux.

By the way, it goes without saying that stirring of molten steel and slag is important in the treatment according to the present invention, but as the stirring method, bubbling stirring with a general argon gas is of course added to the molten steel by adding a flux at the time of tapping the converter. It is also effective to use stirring by gravity. This is because the treatment according to the present invention has a property of rapidly advancing as long as the oxidizing power of the flux is sufficient.

It is desirable to remove slag after Cr removal treatment to prevent reoxidation Cr in the next step of deoxidation and RH treatment, but even if you adopt the method of slag chilling by solidifying slag with lumped quick lime etc. Good.

Next, the effect of the present invention will be described with reference to Examples in comparison with Comparative Examples.

<Example> First, using a Tammann furnace, 2 kg of carbon steel was melted in the atmosphere in an MgO crucible to reproduce molten steel in the same state as that after converter blowing, and this was kept at 1600 ° C. Then, this molten steel is first
100 g of the flux under the conditions shown in the table was added, and bubbling and stirring were performed by blowing in an argon gas to perform the Cr removal treatment.

The results of this Cr removal treatment are also shown in Table 1.

As is clear from the results shown in Table 1, in the present invention examples (Test Nos. 1 to 6) in which the flux having high oxidizability and low basicity was used, decarburization was caused by contact with the flux. It can be seen that the Cr removal of molten steel proceeded well.

On the other hand, the result of the test number 7 (comparative example) shows that even if a low basicity flux is used, when the T.Fe value in the slag is small and the oxidizing power is insufficient, almost no Cr removal occurs. It shows that it does not progress.

Further, Test No. 8 (Comparative Example) is an example using a flux having a high basicity. From the results, it can be seen that even if the flux has a strong oxidizing power, it is sufficient if the basicity is high. It can be seen that Cr removal cannot be performed.

In the above explanation, the Cr removal from the molten steel melted in the converter was
However, it is needless to say that the method of the present invention can be applied to the case where carbon steel is melted only from scrap using an electric furnace.

<Summary of Effects> As described above, according to the present invention, it is possible to easily and effectively carry out the Cr removal treatment of molten steel, to relieve the Cr out of specification of molten steel, and to stably produce high-quality carbon steel. It is extremely easy to do without being influenced by the blended raw materials, and industrially useful effects are brought about.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the basicity of slag having various oxidizing powers and the Cr distribution ratio. FIG. 2 is a graph showing the relationship between (T.Fe) and the Cr removal rate of slag under various basicity of flux composition.

Claims (1)

[Claims] 1. A method for dechromizing molten steel, in which an oxidizing low basicity flux without addition of a carbonate component is brought into contact with molten steel after it has been blown in a converter stored in a container, which is adjusted to the basicity of the flux. The value of (T.Fe) in the slag A method for dechromizing molten steel, which is characterized in that it is adjusted as follows.