JP3390478B2 - Melting method of high cleanliness steel - Google Patents

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 an extremely low carbon steel excellent in cleanliness, and particularly to a vacuum decarburizing treatment by adding a slag deoxidizing agent in two stages in vacuum degassing treatment. We propose a method for producing a steel with high cleanliness by promoting deoxidation and suppressing the residual of deoxidation products in molten steel as much as possible.

[0002]

2. Description of the Related Art A general ultra-low carbon steel is produced by vacuum degassing undeoxidized steel with a C content of 0.01 to 0.06 wt% produced in a refining furnace to cause a C + O → CO ↑ reaction. It is manufactured by decarburization, and the content of C obtained is 0.006
wt% or less.

In this known decarburization method, for example,
C: To decarburize 400 ppm molten steel to 30 ppm, 493 pp
m oxygen is required. By the way, the oxygen required for such vacuum decarburization is covered by the oxygen supplied to the molten steel from the dissolved oxygen (free oxygen) contained in the undeoxidized steel and the iron oxide that is a lower oxide in the slag. Has been.
It is known that the undeoxidized steel has a free oxygen concentration in the steel and a lower oxide concentration in the slag in equilibrium with each other. Therefore, in order to efficiently carry out the decarburization reaction to the extremely low carbon region by vacuum decarburization, it is effective to control the lower oxide concentration in the slag to be high. . However, lower oxides (FeO, MnO, etc.) in these slags are
After decarburization of molten steel, it is not sufficiently reduced, and there is a high possibility that it will be present in the slag in a high concentration as it is even after secondary refining. Then, these residual lower oxides react with deoxidizing agents such as [Al] and [Ti] in the steel after deoxidation of the molten steel to form fine inclusions, thereby lowering the cleanliness of the molten steel.

In order to overcome such problems, conventionally,
Japanese Patent Application Laid-Open No. 2-30711 discloses, "Low carbon undeoxidized steel is melted in a steel refining furnace, tapped in a ladle, and then a deoxidizer is put on the slag in the ladle, and T. Fe concentration of 5% or less, decarburization treatment while continuing to blow oxygen into the tank with a vacuum degassing device, and C content of 0.006% or less, which is an extremely clean electrode. "Producing method of low carbon steel" is proposed.

According to this method, although (T.Fe) and (MnO) are lowered by the vacuum degassing treatment, oxygen is blown into the vacuum chamber, so that oxygen in steel necessary for decarburization is insufficient. There is no such thing.

Further, as another slag reforming method, Japanese Patent Publication Sho
JP-A-62-39205 proposes a method of deoxidizing molten steel at the time of tapping from a steelmaking furnace, adding a flux that promotes coalescence of the deoxidized product, and further adding a slag reducing agent together.

Further, in Japanese Patent Publication No. 2-19168, as a method for promoting the slag reforming reaction, a gas generating substance such as CaCO ₃ (a premixed slag reducing agent is also possible) is added together with the slag reducing agent. Proposing a method.

[0008]

Of the above-mentioned conventional techniques,
The method disclosed in the JP-A 2-3071 1 discloses, in order to oxygen-flow in the tank during the vacuum degassing treatment, at the time of tapping (T.Fe) <5
% Even after slag reforming up to 100%, at the end of acid transfer
As shown in, (T.Fe) increases by 2 to 4%.
Therefore, there is a problem that slag reforming becomes insufficient and sufficient molten steel cleanliness cannot be obtained. Also, the Japanese Patent Publication Sho 62
The method disclosed in Japanese Patent Publication No. 39205 is a method of deoxidizing molten steel at the time of tapping, and thus is not suitable for melting ultra-low carbon steel. Further, the method disclosed in Japanese Patent Publication No. 2-19168 requires not only a gas generating substance separately but also a smoke generating treatment facility. In addition, these Japanese Patent Publications Sho 62-3920
Even if the slag reforming method described in Japanese Patent Publication No. 5 or Japanese Patent Publication No. 2-19168 is carried out at the time of tapping a steelmaking furnace, an increase in (T.Fe) during vacuum degassing treatment cannot be avoided. was there.

An object of the present invention is to establish a technique for producing an ultra-low carbon steel of high cleanliness without inhibiting the decarburization reaction during vacuum degassing.

[0010]

DISCLOSURE OF THE INVENTION As a result of earnest research aimed at overcoming the above-mentioned problems of the above-mentioned respective prior arts, the inventors of the present invention have found that the above-mentioned vacuum degassing treatment (T.
In order to cope with the increase in Fe), not only slag reforming at the time of tapping but also two-stage treatment of performing slag reforming after vacuum decarburization treatment, so-called ultra-clean carbon The present invention was conceived by finding that steel can be easily manufactured.
That is, the gist of the present invention is as follows. That is, low-carbon undeoxidized steel is smelted in a steel refining furnace,
Then, in the method for producing highly clean steel by performing decarburization while feeding acid into a vacuum tank with a vacuum degassing device, the addition of the slag deoxidizer is first performed in steel refining. During tapping from the furnace to the ladle, or to the slag in the ladle immediately after tapping, add only a part of the amount required for slag deoxidation,
A slag deoxidizing agent for the remaining amount is added to the slag in the ladle after the decarburizing treatment by the vacuum degassing apparatus, which is a method for melting highly clean steel. Further, in the above melting method, the slag deoxidizer to be added during tapping or immediately after tapping is 5 wt% in the slag in the ladle (T.Fe).
Add enough to make it less than.

[0011]

FIG. 1 shows the relationship between the slag (T.Fe) before the start of vacuum degassing and after the vacuum degassing. According to this figure, vacuum degassing (acid transfer in the tank) It can be seen that the low-grade oxide (T.Fe) surely rises. Especially in the slag before the start of vacuum degassing (T.Fe)
The lower the heat is, the greater the degree of increase in (T.Fe) due to acid transfer. Even if the reforming process is performed once, (T.Fe) increases due to the vacuum decarburization process, which is satisfactory in terms of molten steel cleanliness. Not in a state.

Therefore, in the present invention, a slag modifier is added immediately after the completion of tapping from the refining furnace so that the slag (T.Fe) content is less than 5%, and then the vacuum degassing is performed. Even after carrying out the charcoal treatment, in order to modify the (T.Fe) that has risen due to the vacuum decarburization treatment, by adding the remaining amount of the total added slag modifier, the molten steel cleanliness is improved. It is something to try.

On the other hand, a method is conceivable in which the slag modification is not carried out at the time of tapping, and the slag modification is carried out only after the vacuum decarburizing process in the secondary refining. This method has a large decarburization rate due to the large amount of oxygen supplied to the molten steel during vacuum decarburization and shortens the decarburization treatment time, but has the problem of an excessive reforming amount. That is, the deoxidation product generated by the slag modification penetrates into the molten steel from the slag-metal interface, and then,
It will rise again and be absorbed in the slag. Therefore, when the time from the end of the reforming process to the start of casting is short, or when the molten steel is not sufficiently stirred in the secondary refining process and the deoxidized products are not sufficiently aggregated and coalesced, By carrying out the slag reforming, the casting will start with the molten steel contaminated.

In order to prevent such a problem, it can be said that it is desirable to minimize the amount of reforming after the vacuum decarburization treatment. That is, in order to suppress the slag reforming after the completion of vacuum decarburization to a minimum amount, first add a predetermined amount of slag deoxidizer at the time of tapping the refining furnace, and set the slag (T.Fe) <5% as a guide. It has been found that a two-stage slag reforming treatment is effective, in which preliminary slag reforming is performed, and then only the amount increased by the acid feeding during vacuum decarburizing is vacuum decarburized and then slag reforming is performed again.

In order to confirm this, the inventors conducted the following experiment. This experiment, as a conventional method,
After smelting furnace tapping on the total amount of Al ash required amount of composition shown in Table 1 was added to the ladle as a slag reductant, decarburization while oxygen-flow into the vacuum chamber at a vacuum degassing facility, as shown in FIG. 2 Treatment was performed to melt ultra low carbon steel. In this method, the slag that has been once modified (reduced) by the acid transfer in the vacuum tank is again oxidized. Therefore, not only the efficiency of Al ash is low, but also the slag reforming at the time of casting is insufficient and the level is not satisfactory. During the slag (T.
The transition of Fe) is shown as c method in FIG.

In order to solve the above-mentioned problems, the same amount as in the above-mentioned method c is applied after the acid feeding by vacuum degassing, that is, after the degassing treatment is completed.
Al ash was added and the slag was stirred for modification (a in FIG. 4).
Law). In the method a (batch reforming after RH), since the (T.Fe) before vacuum decarburization was high, the phenomenon that the slag was further oxidized during the acid feeding in the tank was not observed. As a result, the above c
(T.Fe) before casting was lower than that of the method, but because the degree of modification (ΔT.Fe) was large and slag stirring was necessary, non-metallic inclusions were sufficiently floated. Therefore, it has been found that it is necessary to take a sufficient standing time after the modification treatment.

Next, as the method of the present invention, after tapping the refining furnace,
Al ash having the composition shown in Table 1 was added to the ladle as a slag reducing agent only in a necessary amount, and decarburization was performed while feeding acid into the vacuum tank with a vacuum degassing equipment to melt the ultra-low carbon steel. After the production, the remaining Al ash was added again after the degassing treatment was completed, and the slag was stirred to perform reforming again (method b in FIG. 4).

According to the method of the present invention, even if slag reoxidation occurs during vacuum decarburization, it can be reduced by reforming after the decarburization treatment, and the decarburization rate during vacuum decarburization can be reduced. It does not reduce the slag enough to cause a drop. Further, since the degree of modification after the decarburization treatment is smaller than that of the method a, it was found that the standing time after the modification treatment can be shorter than that of the method a. Moreover, the slag before casting (T.Fe) was the lowest among the three methods, and good results were obtained. Although the ratio of Al ash added separately before and after the vacuum decarburization treatment was halved in this example, 5 wt% of slag (T.Fe) before the vacuum decarburization treatment was used.
It has been found that even better results can be obtained by controlling the slag oxidation degree at the time of tapping the converter, and if necessary controlling the standing time after the treatment so that it is less than%.

[0019]

[Example] For slag reforming performed immediately after tapping the smelting furnace,
Al ash having the chemical composition shown in Table 1 was added as a slag reducing agent under the conditions shown in Table 2, and then from the lance 2 into the RH degassing tank 1 of the vacuum degassing equipment as shown in FIG. O
_The molten steel 4 in the ladle 3 was subjected to vacuum decarburization while blowing ₂ .
Furthermore, for some of the heat, Al ash of Table 1 was added on the slag 5 in the ladle 3 after the vacuum decarburization treatment, as shown in FIG.
The slag 5 and Al ash were reacted using Ar gas by the method as shown in FIG. Table 2 shows an example of the operation results at this time.

As is clear from the results shown in Table 2,
In the case of the comparative method as shown in Examples 4 to 6 (without slag re-reforming treatment after vacuum decarburizing treatment), (T.Fe) in the slag remarkably rises due to O ₂ blowing during vacuum decarburizing treatment. However, in the heat of Examples 1 to 3 according to the method of the present invention, in which the slag reforming was performed again after the so-called vacuum decarburization treatment, (TF
e) is lower than (T.Fe) of the heat that has not been modified.

[0021]

[Table 1]

[0022]

[Table 2]

FIG. 5 shows 1.0 based on the cleanliness level in molten steel. The method c is a comparative example in which modification is performed only during tapping, the method b is modified only during tapping, and the other is T.I. F
e) When <5%, it is a case where reforming is performed again after vacuum decarburization. However, the above method a is a case where reforming is performed after vacuum decarburization without reforming at the time of tapping. From the above results,
The case of method b is the most T.I. It was found that Fe can be reduced, and that this is a suitable melting method for obtaining high-cleanliness steel.

[0024]

As described above, according to the present invention,
By simply dividing the timing of the single slag reforming into two stages, it is possible to reliably manufacture a steel having a high cleanliness and an extremely low carbon.

[Brief description of drawings]

FIG. 1 is a graph showing changes in (T.Fe) before and after vacuum degassing treatment.

FIG. 2 is a sectional view showing an example of vacuum degassing equipment.

FIG. 3 is a schematic diagram showing an example of an Ar bubbling method.

FIG. 4 is a schematic diagram showing how the (T.Fe) changes under various reforming methods.

5 is a schematic diagram showing how (TO) fluctuations under various reforming methods are shown.

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Ryuichi Asaho 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Chiba Works (72) Inventor Masaaki Kuga 1 Kawasaki-cho, Chuo-ku, Chiba Chiba Steel Manufacturing Co., Ltd. Chiba Steel Works (56) References JP-A-2-30711 (JP, A) JP-A-6-256836 (JP, A) JP-A-61-246310 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21C 7/10 C21C 7/00 C21C 7/068

Claims (3)

(57) [Claims] 1. A low carbon undeoxidized steel is melted in a steel refining furnace,
Add a slag deoxidizer and use a vacuum degassing device to vacuum the chamber.
In the method for producing highly clean steel by performing decarburization while feeding acid into the steel, the addition of the slag deoxidizer is first performed during or after tapping from the steel refining furnace to the ladle. To the slag in the ladle, add only a part of the amount necessary for deoxidizing slag, and add the remaining amount of the slag deoxidizer in the ladle after decarburizing treatment by the vacuum degassing device. A method for smelting highly clean steel, which is characterized by adding to slag. 2. The melting method according to claim 1, wherein the slag deoxidizing agent to be added during tapping or immediately after tapping is less than 5 wt% in the slag in the ladle (T.Fe). A method for producing a highly clean steel, which comprises adding a sufficient amount of water. 3. The smelting method according to claim 1, wherein after the addition of the total amount of the slag deoxidizer is completed, the slag in the ladle is stirred mechanically or by blowing gas. Method for producing high-purity steel.