JP2003155516A - Method for desulfurizing molten steel with ladle- refining - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for desulfurizing molten steel with a ladle- refining at a low cost, with which the desulfurizing efficiency can be kept similar to the conventional method without using fluorite. SOLUTION: When the desulfurizing treatment is applied by tapping off the molten steel into the ladle after refining in a converter and stirring the molten steel held in this ladle, slag produced in the desulfurize-refining is made to flow out into the ladle together with the molten steel at the above tapping-off time. Further, after charging a flux which has the components and the amount simultaneously satisfying the following ranges in the finish composition of the slag applying the desulfurizing treatment into this ladle, and beforehand mixes lime-base material and Al2 O3 source respectively having <=10 mm grain diameter, the molten steel and slag are stirred. CaO+MgO=55-65 mass%, Al2 O3 + SiO2 =30-45 mass%. Wherein, MgO=8-15 mass%, Al2 O3 =20-35 mass% and SiO2 =0-15 mass%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a desulfurization method of molten steel by ladle refining, and in particular, does not use fluorite as a flux to be added and effectively utilizes slag generated in prior converter refining. The technology for desulfurization.

[0002]

2. Description of the Related Art In recent years, consumer demands for quality of steel materials have become strict, and it has become necessary to further refine molten steel obtained by converter refining outside the furnace after tapping to remove impurity elements. Therefore, the so-called "ladle refining" called the ASEA-SKF method, VAD method, LF method, etc. has come to be performed. The basic of these ladle refining is to take molten steel 1 from a converter into a ladle 2 and, as shown in FIG.
(Hereinafter, referred to as flux 3) is added, stirring is performed, and a so-called "slag-metal reaction" is actively performed between the formed slag 4 and the molten steel 1 to remove a desired impurity element in the molten steel. It is a thing. In this case, before the addition of the flux, all the slag that is generated in the converter refining and flows out into the ladle 2 along with the molten steel 1 is removed for the reason of phosphorus reconstitution. Further, the stirring of the molten steel 1 in the ladle 2 may be electromagnetic stirring (ASEA-SKF method),
As shown in FIG. 3, it is carried out by injecting an inert gas 6 (VDA method, LF method) through a porous plug provided at the bottom of the ladle 2 or the tuyere 5, and further adding the added flux 3 To melt, the electrode 7 is immersed in the molten steel 1 for heating as necessary.

By the way, sulfur contained in steel materials (symbol S
Represents the steel to be melted, because it impairs the toughness of the steel.
For some species, reduce this sulfur content as much as possible
Is desirable. Therefore, the molten steel that is the material of the relevant steel material
After the converter refining, further desulfurization by the ladle refining
Be done. For example, Japanese Examined Patent Publication (Kokoku) No. 58-2575 discloses a conventional method.
Powder mixed type or molten type Ca used in pot refining
O-Al₂O₃-CaF ₂Solving the problems of system flux
Therefore, a new sintering type (a raw material temperature
Indirect heating with gas etc.) Use of flux is proposed.
This sinter-type flux melts raw materials once by arc heating.
Compared to the melting type flux that melts, it uses less electric power.
The cost is very low.

However, the production of the sintered flux requires many steps such as pulverization of raw materials-homogeneous mixing-calcination-re-milling-classification, and simply powdery raw materials (for example, calcined lime, silica stone, etc.). The cost is still higher than the method of using the flux for mixing and adding). In addition, fluorite (CaF ₂ ) is added to accelerate the melting of raw materials that are difficult to melt, which has a great influence on the melting damage of the refractory lining the ladle. Furthermore, recently, a demand for controlling the amount of fluorine outflowing into the environment has been strongly exclaimed, and in order to meet the demand, addition of fluorite is desired. In addition, as described above, the removal of the outflow slag from the converter before the start of ladle refining not only lowers the temperature of the molten steel, but also removes the molten steel together with the slag, thereby increasing the molten steel yield. Lowers and causes economic disadvantage.

[0005]

SUMMARY OF THE INVENTION In view of such circumstances, the present invention provides a desulfurization method by ladle refining of molten steel which is inexpensive and can maintain desulfurization efficiency similar to the conventional one without using fluorite. Is intended.

[0006]

In order to achieve the above-mentioned object, the inventor diligently studied the flux used in desulfurizing molten steel in ladle refining such as the LF method, and the results of the present invention Embodied.

That is, according to the present invention, when steel is tapped into a ladle after converter refining and the molten steel held in the ladle is agitated for desulfurization, the refining is performed together with the molten steel during tapping. The resulting slag is drained into a ladle, and inside the ladle,
The final composition after desulfurization treatment of the slag has a component and an amount that simultaneously satisfy the following ranges, and has a particle size of 10 mm or less, and after introducing a flux in which a Al ₂ O ₃ source is premixed, , A method for desulfurizing molten steel by ladle refining, which comprises stirring molten steel and slag. CaO + MgO = 55 to 65 wt%, Al ₂ O ₃ + SiO
₂ = 30 to 45 wt%, however, MgO = 8 to 15 wt%, Al ₂ O ₃ = 20 to 35 wt% and SiO ₂ = 0 to 1
5 mass% Moreover, the present invention is a desulfurization method by ladle refining of molten steel, characterized in that the mixing ratio of the lime-based substance and the Al ₂ O ₃ source is set to 1.5 to 4.0.

Further, the present invention is produced by the above converter refining,
A desulfurization method by ladle refining of molten steel, characterized in that a deoxidizing agent is added to the slag discharged into the ladle to reduce the slag, and the (FeO + MnO) content of the slag is set to 1% by mass or less. . And in this case, the flux is
It is preferable to add the slag that is flowing to the ladle.

In addition, the present invention is a desulfurization method by ladle refining of molten steel, characterized in that the stirring is performed with an inert gas blown into the molten steel from the bottom of the ladle.

According to the present invention described above, fluorite (CaF ₂ )
Even without the addition of
It becomes possible to stably produce molten steel having a sulfur concentration of 10 ppm or less. In addition to being a measure against the release of fluorine to the environment, it also reduces the cost of flux,
Moreover, since the melting loss of the ladle refractory is reduced, the refining cost is significantly reduced as compared with the conventional one. Furthermore, there is a secondary effect that it is possible to avoid stockpiling of converter slag in the steelworks for which a clear reuse method has not been established.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below together with the background of the invention.

First, the inventor has noticed that in the conventional ladle refining, the slag formed in the previous converter refining is completely removed before the desulfurization treatment. In other words, the slag is still very high temperature, and after its removal, it is put into a ladle and effectively used to promote the slag formation of the flux that adjusts the composition of the slag formed during the desulfurization treatment so as to have a high desulfurization ability. I thought that I could do it. Therefore, based on this idea, many slags formed by converter refining were not removed from the ladle, and many test operations were conducted to examine the flux to be added to the slag. As a result, it was found that the slag formation can be carried out quickly by simply mixing the lime-based material in the form of particles and the Al ₂ O ₃ source without fluorite, and this was made one of the requirements of the present invention.

Specifically, calcined lime and lightly calcined dolomite are used as the lime-based substance, Al dross is used as the Al ₂ O ₃ source,
The use of banded shale is preferred. If these are mixed in advance, the CaO and Al ₂ O ₃ contained therein easily form a low melting point compound. The particle size of each is preferably 10 mm or less (desirably 3 mm or less).
This is because if it exceeds 10 mm, the slag formation will be delayed and the contact area between the two will be increased. Therefore, in the present invention,
We decided to add this particle size range to one of the requirements.

Further, in order to increase the desulfurization ability of the slag generated during the desulfurization treatment, it is necessary to make the composition of the slag as desired. Therefore, in the present invention, as the third requirement, the slag composition at the end of the desulfurization treatment is determined within the following range with reference to the conventional slag composition.

CaO + MgO = 55-65%, Al ₂ O ₃
+ SiO ₂ = 30 to 45%, where MgO = 8 to 15
%, Al ₂ O ₃ = 20 to 35%, SiO ₂ = 0 to 15%,
CaF ₂ = 0 to 10% Specifically, based on the composition and weight of the slag that flowed out into the ladle at the time of tapping, the amount and composition of the flux necessary to obtain the composition within the above range at the end of desulfurization were calculated. After that, the flux is added.

Then, the inventor has completed the present invention by combining the three main matters described above. As a result, it is not necessary to use the flux of the melting type or the sintering type as in the past, and the flux cost can be reduced.

Next, the inventor also examined another aspect of the above invention and made various inventions described below.

The first one is the above-mentioned lime-based material and Al ₂ O ₃
The mixing ratio with the source (lime-based substance / Al ₂ O ₃ source) is in the range of 1.5 to 4.0 in terms of mass ratio. If it is less than 1.5, the desulfurization ability of the formed slag is low, and if it exceeds 4.0, the amount of Al ₂ O ₃ is small and the slag formation is delayed without fluorite.

Secondly, a deoxidizing agent is added to the slag flowing out of the converter along with molten steel at the time of tapping, and the slag is reduced by the deoxidizing agent (Al, aluminum ash, etc.) and modified. The content of (FeO + MnO) in the slag is set to 1% by mass or less. As a result, the slag removal process in the ladle after tapping can be omitted, the temperature drop of the molten steel can be further suppressed, and it is very effective in promoting the slag formation of the flax that is subsequently charged. In this case, if the flux is added to the slag that is flowing into the ladle, the flux can be easily slagged by the stirring energy of the tapped steel flow and the heat of the molten steel. Therefore, the addition of the flux to the slag flowing into the ladle is the third mode. In any of the present inventions, it is free to energize by using an electrode to heat molten steel, slag and flux to promote the slag formation of the flux, if necessary.

[0020]

[Example] (Example 1) Hot metal was desiliconized as a preliminary treatment,
The dephosphorized and desulfurized hot metal is subjected to decarburization refining by adding slag material in a converter and blowing oxygen gas, and then the obtained molten steel is ladle refined (LF method in this case) and RH vacuum degassed. Sequentially performed, extremely low S, (Al + Si) killed steel was melted.

At that time, the above-described method for desulfurizing molten steel according to the present invention was applied to ladle refining. Molten steel after decarburization and refining in the converter is finished together with slag in a ladle (processing capacity: 250
Steel, and then, the flux and slag material having the components and amounts determined by the calculation were put into the ladle and stirred.
As flux, quick lime 60 mass% and band shale 4
A mixture with 0 mass% (particle size: 3 mm under) was used, and the addition amount was 7.4 kg / t. In addition to this flux, 7.8 kg / t of calcined lime was added. In the conventional ladle refining, as the flux and the slag material, 5.9 kg / t of burnt lime, 7.4 kg / t of light burned dolomite, 1.9 kg / t of band shale and 1. 9 kg / t had been input.

The desulfurization treatment in this ladle refining furnace was carried out from the start to the end of stirring, and the stirring was carried out by bottom blowing of argon gas. The flow rate of the argon gas used is 0.0052 Nm ³ /min/(t.stee1).

Regarding the operational results, first, the improvement of the desulfurization rate is
Admitted. In other words, 83% of conventional ladle refining
The sulfurization rate was 87%, which was an improvement of 4%. And in molten steel
Concentration of sulfur (symbol [S]) is 30ppm by the conventional method
Was 5 ppm. Also, the processing requirements
As shown in Figure 1, the time was 45 minutes in the past.
It was 36 minutes and could be reduced by 9 minutes. This is a visual observation
However, in the practice of the present invention, the melting time of the flux is
Significantly contributed to shortening from 12 minutes to 5 minutes
It is thought that he was doing. Furthermore, to the target slag composition
As shown in Fig. 2 (a), the arrival time of
(See Fig. 2 (b)). In addition, these
In FIG. 2, the composition of the slag is SiO₂-(CaO + Mg
O) -Al₂O ₃Before the desulfurization process is started
Flux and slag material in the slag (after flowing out of the converter)
Change by the end of treatment (corresponding to the target composition)
The process is indicated by arrows. (Example 2) The same steel type as in Example 1 was melted and used for ladle refining.
Light burned dolomite is used instead of burnt lime as the flux.
It was In other words, 70 pieces of lightly baked dolomite as flux
% And band shale 30% by mass (particle size: 3 mm under)
The mixture is used as the slag material, and the flux is 10.
It was 4 kg / t and 5.9 kg / t of burnt lime. Stirring, etc.
The other conditions are the same as in Example 1.

As a result, almost the same operation results as in Example 1 were obtained.

[0025]

As described above, according to the present invention, even if fluorite (CaF ₂ ) is not added, rapid slag formation of flux progresses, and molten steel having a sulfur concentration of 10 ppm or less is stabilized. You will be able to melt. Further, not only is it a countermeasure against the release of fluorine to the environment, but also the cost of flux is reduced and the melting loss of the refractory of the ladle is reduced, so that the refining cost is greatly reduced from the conventional one. In addition, a side effect of avoiding the stockpiling of converter slag in steelworks, for which a clear reuse method has not been established, can be expected.

[Brief description of drawings]

FIG. 1 is a diagram showing changes over time in the concentration of sulfur in molten steel during desulfurization treatment of molten steel by ladle refining.

FIG. 2 is a diagram showing changes in the composition of slag in the desulfurization treatment of molten steel by ladle refining, where (a) is the present invention and (b) is the case where the conventional method is carried out.

FIG. 3 is a diagram showing a desulfurization treatment state in ladle refining by a general LF method.

[Explanation of symbols]

1 Molten steel 2 ladle 3 flux 4 slag 5 tuyere (porous plug) 6 Inert gas 7 electrodes

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Momoki Kamo             1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama             Shi) Kawasaki Steel Co., Ltd. Mizushima Steel Works F-term (reference) 4K013 BA05 CA01 CB09 CC02 CD02                       DA05 DA10 EA01 EA03 EA05                       FA05

Claims (5)

[Claims] 1. When the molten steel held in the ladle after being smelted by the converter is subjected to desulfurization by stirring the molten steel held in the ladle, at the time of tapping, the slag produced by the converter smelting together with the molten steel is removed. The lime-based substance and Al ₂ O having a component and an amount such that the final composition of the slag after desulfurization treatment simultaneously satisfies the following ranges while being discharged into a ladle and having a particle diameter of 10 mm or less and Al ₂ O A desulfurization method by ladle refining of molten steel, which comprises mixing molten steel and slag after introducing a flux in which the _three sources are premixed. CaO + MgO = 55 to 65 wt%, Al ₂ O ₃ + SiO
₂ = 30 to 45 wt%, however, MgO = 8 to 15 wt%, Al ₂ O ₃ = 20 to 35 wt% and SiO ₂ = 0 to 1
5 mass% 2. The mixing ratio of the lime-based substance and the Al ₂ O ₃ source is set to 1.5 to 4.0.
A method for desulfurizing molten steel by ladle refining as described. 3. The slag generated in the converter refining and flowed into the ladle is reduced by adding a deoxidizer to the slag (F
The desulfurization method by ladle refining of molten steel according to claim 1 or 2, wherein the eO + MnO) content is set to 1% by mass or less. 4. The method for desulfurizing molten steel by ladle refining according to any one of claims 1 to 3, wherein the flux is added to the slag flowing out to the ladle. 5. The desulfurization method by ladle refining of molten steel according to claim 1, wherein the stirring is performed with an inert gas blown into the molten steel from the bottom of the ladle.