JPH0949011A - Method for deoxidizing molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a low carbon steel, etc., excellent in the cleanliness by specifying the ratio of CaO, etc., to Al quantity in a deoxidizer, at the time of executing deoxidizing treatment by adding the deoxidizer composed of metallic Al and CaO, etc., to non-deoxidized molten steel. SOLUTION: The non-deoxidized molten steel melted in a converter, etc., is tapped into a ladle, and slag reforming agent is added at need to execute the reforming of the slag. The deoxidizer composed of metallic Al or Al- containing alloy and CaO or Cao-containing flux is added into this molten steel in the ladle by using a bubbling device, electromagnetic stirring device, etc., to execute the deoxidizing treatment. At this time, in the case of using Y wt% for the pure CaO content in the deoxidizer and X wt% for the pure Al content, the ratio of Y/X is regulated to be 1-2. By this method, the low carbon steel or the extra-low carbon steel remarkably improved in the cleanliness is obtd.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a deoxidizing method for carbon steel having excellent cleanliness.

[0002]

2. Description of the Related Art As a deoxidizing method for obtaining aluminum-killed steel, a method of adding metal Al to undeoxidized molten steel at the time of tapping the converter or during RH treatment is general. But,
This method produces a large amount of Al ₂ O ₃ as a deoxidation product,
The fine Al ₂ O ₃ inclusions are suspended and dispersed in the molten steel, making separation and removal at the molten steel stage extremely difficult. After adding deoxidizing Al
As a ladle refining method for separating Al ₂ O ₃ inclusions, there are a bubbling method, an RH reflux method, an electromagnetic stirring method, and the like.

The bubbling method is a method in which molten steel after deoxidation of Al is stirred by an inert gas such as Ar, and Al ₂ O ₃ is agglomerated and coalesced to promote the floating.

[0004] RH perfusion method is to introduce discharged molten steel was Al deoxidized in a vacuum chamber having a dip tube of the two legs to reflux, Al ₂ O
_This is a method of aggregating and coalescing ₃ to promote floating.

In the electromagnetic stirring method, a moving magnetic field is applied from the outside of a ladle containing Al-deoxidized molten steel to stir the molten steel in the ladle by electromagnetic force to cause Al ₂ O ₃ to aggregate and coalesce to promote levitation. It is a method to let.

In the method for producing a highly clean ultra-low carbon steel disclosed in Japanese Patent Laid-Open No. 3-158412, FeO in the slag reacts with [Al] in the molten steel to form an alumina-based inclusion to form a steel. The following methods have been proposed as measures against the deterioration of cleanliness.

That is, in melting the ultra low carbon steel by vacuum decarburization, the ladle during vacuum degassing is used in the initial stage of the killing process by adding Al following the rimd process of molten steel in the decarburizing process. In the slag floating above the molten steel inside, the molar ratio of CaO to the predicted Al ₂ O ₃ generated in the molten steel
The slag is modified by adding CaO so that CaO / Al ₂ O ₃ is in the range of 0.5 to 2.0. According to this method, the absorption capacity of alumina in molten steel can be improved by this slag modification.

In the deoxidizing method for molten steel disclosed in Japanese Patent Laid-Open No. 3-47910, defects caused by Al ₂ O ₃ are generated at a certain ratio even when the conventional countermeasures for levitation separation of Al ₂ O ₃ are implemented. However, in order to solve the problem that defects can not be eradicated, in the undeoxidized molten steel metal aluminum, CaO or metal aluminum, CaO
, A CaF ₂ binder or a mixture thereof is added, and the resulting inclusions have a CaO—Al ₂ O ₃ system composition. In this method, suspended Al ₂ O ₃ is simultaneously deoxidized and CaO-Al
By it allowed to the ₂ O ₃ based inclusions, and be able to control the form harmless form different from the deleterious Al ₂ O ₃ inclusions that are detected surface flaws portion and internal defect.

[0009]

However, the above conventional method has the following problems.

The inventors of the present invention have conducted tests and examinations on the contents of Japanese Patent Laid-Open No. 3-158412 and found that the cleanliness of molten steel is not improved. The reason may be that CaO is added after Al is added in the RH vacuum degassing apparatus, so that there is a time lag in the addition timing of Al and CaO. That is,
Since CaO is added after the addition of Al, the generated Al ₂ O ₃ is dispersed in the molten steel in the ladle when CaO is added. Al ₂ O ₃ is dispersed, CaO concentration is added in a low state of Al ₂ O ₃ inclusions is merely only some contribute to lower the melting point of the Al ₂ O ₃ inclusions, large Part of CaO floats in the molten steel without capturing Al ₂ O ₃ dispersed in the molten steel and is captured by slag. After that, it only captures Al ₂ O ₃ that has reached the slag-molten steel interface and cannot contribute to the modification of Al ₂ O ₃ that does not reach this interface.

In the method disclosed in Japanese Patent Laid-Open No. 3-47910,
Although the occurrence rate of defects due to Al ₂ O ₃ is slightly lower than in the past, it was found that problems such as the inability to significantly reduce it and the inability to eradicate product defects occur.

The reason is that CaO (Y) and metal to be blended
It is considered that the ratio Y / X with Al (X) is not appropriate. For example, in the examples of the invention described above, Y / X is as low as 0.22 and the blending ratio of CaO is small, so that the melting point of Al ₂ O ₃ inclusions is not sufficiently lowered.

As described above, if Al and CaO are added at different times or at different places, the cleanliness of molten steel cannot be ensured. Further, the cleanliness of molten steel cannot be sufficiently improved only by mixing Al and CaO as a deoxidizing agent and adding them.

An object of the present invention is to provide a deoxidizing method in which Al and CaO are simultaneously added to the same place, by optimizing the composition ratio of the deoxidizing method, thereby improving the cleanliness of molten steel. Especially.

[0015]

The gist of the present invention resides in the following method for deoxidizing molten steel.

Undeoxidized molten steel with metallic Al or Al-containing alloy
A method of adding a deoxidizing agent consisting of CaO or a CaO-containing flux and then performing a deoxidizing treatment.
A deoxidizing method for molten steel, characterized in that a ratio Y / X of CaO pure content Y (Wt%) and Al pure content X (Wt%) is set to 1-2.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention will be described specifically.

First, undeoxidized or weakly deoxidized molten steel produced in a converter or the like is tapped into a ladle. At this time, in-furnace slag inevitably flows into the ladle. This outflow slag contains lower oxides (FeO, MnO), and after deoxidation, it reacts with deoxidizing elements to form inclusions, which may deteriorate the cleanliness of molten steel. Therefore, if the concentration of lower oxides in the outflowing slag is extremely high, it is necessary to reduce the concentration of the lower oxides in the slag in advance and reduce the concentration of the lower oxides in the tapping flow or slag in the ladle during tapping. A slag modifier containing metallic Al or Al alloy may be added.

The slag modifier may have a composition such as Al: 40Wt% + CaCO ₃ : 60Wt% or Al: 50Wt% + Al ₂ O ₃ : 40Wt% + CaO: 10Wt%. Al in the presence is metallic Al or
An Al alloy may be used. Metal Al in slag modifier or
The Al alloy is contained as a reducing agent for lower oxides in the slag. Metal Al or Al alloy has a strong reducing power and is effective in reducing lower oxides in slag.

In order to improve the cleanliness of steel, the method of the present invention uses a bubbling device, an electromagnetic stirring device, and a refining device capable of stirring molten steel such as LF, RH, and DH to recirculate the molten steel in the ladle. The deoxidizing treatment is performed by adding a predetermined deoxidizing agent.

Depending on the steel type, vacuum decarburization treatment may be carried out before adding the deoxidizing agent.

In this case, as a matter of course, the refining device to be used is to put the entire ladle in a vacuum container and inject a inert gas from the porous plug at the bottom of the ladle, or a tank degassing device, or dip two legs in molten steel in the ladle. RH in which the pipe is immersed, the vacuum tank is evacuated, and an inert gas for reflux is blown from one of the immersion pipes.
It must be a device that can decarburize molten steel, such as a degasser.

In the case of steel types requiring vacuum decarburization, if the dissolved oxygen concentration in the molten steel is low and interferes with the decarburizing reaction of C + O = CO, the molten steel in the ladle or vacuum tank is blown upward. Alternatively, the oxidizing gas may be supplied by injection. The ultimate carbon concentration at this time depends on the required component specifications of the steel type. For example, [C] <30 ppm is usually required for ultra-low carbon steel.

For steel types that do not require vacuum decarburizing treatment, after arriving at the refining equipment, for steel types that require vacuum decarburizing treatment, after performing predetermined vacuum decarburizing treatment, metal Al or Al-containing alloy and CaO or CaO-containing Deoxidizing treatment is performed by adding a deoxidizing agent composed of flux and high-cleanliness steel.

As CaO-containing flux, CaO and Ca
Use is made of one or more of F ₂ , MgO and BaO. The desirable content of CaO in the CaO-containing flux is 50
Wt% or more. When CaF ₂ is used, the content of CaF ₂ is preferably 10 Wt% or less in order to suppress melting loss of the refractory. About 5 Wt% is desirable.

One of the contents of the deoxidizer is metallic Al or Al.
The reason for limiting the content of the alloy is that it has a strong reducing power and the dissolved oxygen in the molten steel can be reduced. The reason for limiting to CaO or CaO-containing flux as other contents of the deoxidizer is as follows.
This is because this reacts with the Al ₂ O ₃ inclusions generated by the addition of Al, and has the effect of reforming to a CaO—Al ₂ O ₃ -based inclusion having a low melting point.

The low melting point CaO—Al ₂ O ₃ inclusions have a melting point of 20.
Unlike Al ₂ O ₃ inclusions at 00 ° C or higher, since they have a liquid phase composition, the inclusion and aggregation efficiency of inclusions can be increased.

Therefore, the number of fine Al ₂ O ₃ inclusions decreases, and the large CaO—Al ₂ O ₃ inclusions that have aggregated and coalesced increase. As can be seen from Stokes' law, the floating speed of the inclusions increases in proportion to the square of the particle size, so that the inclusions enlarged due to aggregation / coalescence are quickly floated and separated.

In the method of the present invention, the ratio of CaO pure content Y (Wt%) to Al pure content X (Wt%) in the deoxidizer (hereinafter referred to as weight ratio) Y / X is further applied in the above series of steps. Is 1 to 2. The reason for limiting this condition will be described with reference to FIG.

The influence of the above weight ratio Y / X in the deoxidizer on the cleanliness of molten steel was examined by the following method.

Molten steel discharged from the converter was put in a ladle and subjected to RH treatment without being deoxidized. 2 minutes after RH treatment starts
H The deoxidizer was added to the molten steel in the vacuum tank from the bunker provided at the top of the tank and refluxed for 10 minutes, and the weight ratio Y /
X and RH T. in molten steel after reflux The relationship with [O] was investigated. The result is shown in FIG.

FIG. 1 shows the weight ratio Y / X in the deoxidizer and the T.V. in molten steel in the tundish after RH reflux. It is a figure which shows the relationship with [O]. As shown in the figure, when Y / X is less than 1, T. It can be seen that [O] exceeds 20 ppm and deteriorates rapidly.

When Y / X is less than 1, the CaO—Al ₂ O ₃ type inclusions generated immediately after the addition of the deoxidizing agent become Al ₂ O ₃ excess, so that the melting point of the inclusions is sufficiently lowered. This is because the inclusion of the inclusions is not sufficiently absorbed.
As a result, it becomes difficult to promote the floating separation of inclusions, and the T. The [O] did not decrease.

On the other hand, when Y / X exceeds 2, T.
[O] exceeds 20ppm and deteriorates rapidly. The reason for this will be described below.

If Y / X exceeds 2, CaO-Al ₂ O ₃ -based inclusions formed immediately after the addition of the deoxidizing agent are CaO-excessive. Therefore, the melting point of the inclusions is not sufficiently lowered, and the combined absorption of the inclusions is not sufficiently generated. As a result, it becomes difficult to promote the floating separation of inclusions, and the T. [O] does not decrease.

Therefore, the weight ratio Y / X needs to be in the range of 1-2.

In the method of the present invention, it is desirable that the following conditions are further satisfied.

The preferred amount of slag in the ladle is 15 kg / t
Hereafter, more preferable is 10 kg / t or less. If the amount of slag is large, the absolute amount of low-grade oxide will increase, and
This is because the reoxidation of [l] cannot be suppressed. On the other hand, the lower limit is 1 kg / t. Below this, inclusions (A
This is because the absorption capacity of both l ₂ O ₃ and CaO-Al ₂ O ₃ ) is saturated.

Before adding the deoxidizing agent, the slag in the ladle (Wt% FeO)
+ (Wt% MnO) is preferably 0.1 to 10, and more preferably 0.1 to 5.

Metal Al or Al-containing alloy and CaO or CaO
A desirable form of the deoxidizing agent composed of the contained flux is a combination or mixture of these. Al and CaO
When added separately in the state where and are separated, low melting point CaO-Al
₂ O ₃ -based inclusions cannot be effectively generated. The method for producing the mixture deoxidizing agent may be, for example, mixing and mixing metal Al powder and CaO powder, and encapsulating them in an iron capsule.
The powder may be enclosed inside an Al ball or the like. In addition, Al
Fill the pipe made of flux powder containing CaO with both ends
It may be closed by Al or Fe. In the case of the binder deoxidizing agent, there is a method of briquetting metal Al powder and CaO 2 -containing flux powder, for example.

The addition of the deoxidizer is carried out from a bunker at the top of the vacuum chamber, top blowing provided on the inner wall of the vacuum chamber or spraying or blowing from a dipping tuyere, top blowing from an elevating lance provided inside the vacuum chamber, etc. The molten steel in the vacuum chamber is subjected to the above method.

Desirable molten steel reflux conditions after the addition of the deoxidizer are as follows:
The reflux time is 5 to 15 minutes, and the Ar flow rate for the reflux is 0.6 to 20 Nl / min.

By the method of the present invention, it is possible to lower the melting point of the inclusions formed immediately after the addition of the deoxidizer, but when the concentration of the lower oxide (FeO, MnO, etc.) in the slag in the ladle is high. It was found that after addition of the deoxidizer, [Al] in the molten steel reacts with the lower oxide in the slag in the ladle. As a result, for example, when an RH vacuum degasser is used, Al ₂ O ₃ -based inclusions are generated by the above reaction during the RH reflux after the addition of the deoxidizer or during the RH treatment and before the continuous casting. As a result, they may be suspended in the molten steel and deteriorate the cleanliness of the molten steel.

Therefore, the effect of the amount of lower oxides in the slag before the addition of the deoxidizer on the cleanliness of molten steel was examined.
In the same treatment as in the case of FIG. 1 described above, the weight ratio Y / X in the deoxidizing agent added by RH was controlled to 1 to 2, and after the deoxidizing agent was added, a reflux treatment was performed for 10 minutes at RH. It was performed and cast into a slab with a continuous casting device. T. in samples taken from molten steel in the tundish. The relationship between [O] and (Wt% FeO) + (Wt% MnO) in the slag in the ladle before the addition of the deoxidizer was investigated. The result is shown in FIG.

FIG. 2 shows T.D. in molten steel in a tundish.
(Wt% FeO) + (Wt in the slag in the ladle before adding [O] and the deoxidizer
It is a figure which shows the relationship with% MnO). As shown in the figure, as the content of (Wt% FeO) + (Wt% MnO) in the slag in the ladle before the addition of the deoxidizer increased, the T.O. It can be seen that [O] increases. When (Wt% FeO) + (Wt% MnO) exceeds 5, T.
[O] exceeds 10 ppm, and when it exceeds 10, it also exceeds 20 ppm and deteriorates rapidly.

Therefore, (Wt% FeO) + in the slag in the ladle
It can be seen that (Wt% MnO) needs to be 10 or less, preferably 5 or less.

On the other hand, in the slag in the ladle before adding the deoxidizer (Wt%
The desirable lower limit of FeO) + (Wt% MnO) is 0.1. If it is made less than this, not only does Al in the slag modifier act to reduce the lower oxides in the slag, but also the dissolved oxygen in the molten steel is excessively lowered. That is, Al ₂ O ₃ inclusions are generated when the slag modifier is added.

For this reason, (Wt% FeO) + (Wt% MnO) in the slag in the ladle before the addition of the deoxidizer is 0.1 to 10, more preferably 0.1 to 5
Is desirable.

[0049]

【Example】

(Basic condition) Molten steel that was roughly decarburized in the converter until [C] reached 0.02 to 0.3 Wt% (converter end point temperature 1630 to 1670 ° C) was tapped into a 250 ton ladle (pot temperature after tapping 1610). ~ 1650 ℃). After tapping, slag Aratameshitsuzai slag top _{(Al: 40Wt%, CaCO 3} : 60Wt
%) 0.2 to 2 kg / t was sprinkled and added to perform slag modification. After that, deoxidation treatment was performed with RH.

The immersion tube diameter of RH was 660 mm, the flow rate of Ar gas for recirculation was 2000 Nl / min, and the vacuum degree during processing was 1 to 5 Torr. The pure Al content X in the deoxidizer was 0.8 to 1.2 kg / t.

The deoxidizing agent is a metal in a plurality of iron capsules.
Al powder (particle size 0.2-1mm) and 95Wt% CaO-5Wt% CaF
The mixed flux of ₂ (particle size 0.2 to ₂ mm) was mixed and used. The outer shape of this capsule is oval, and the internal volume of each capsule is about 20 cm ³ .

Example 1 Processing was carried out under the above basic conditions. However, [C] after rough decarburization in the converter is 0.04 to 0.06 Wt
% The amount of slag that unavoidably flowed out of the converter into the ladle at the time of tapping was 8 to 10 kg / t. Then, 0.6 kg / t of the above slag modifier was uniformly added to carry out slag modification. As a result, the concentration of lower oxides (Wt% Fe
O + Wt% MnO) was 5.5-7.9.

Then, after refluxing for 2 minutes in the RH vacuum degassing apparatus while not deoxidizing, a deoxidizing agent having various weight ratios Y / X as shown in Table 1 was added to the RH vacuum tank. And refluxed for 10 minutes. At this time, the dissolved oxygen concentration in the molten steel before adding the deoxidizer was 300 to 600 ppm.

Next, the T.
[O] was investigated. Furthermore, slabs were made by continuous casting and product coils were made by rolling, and product flaws were examined by ultrasonic flaw detection.

The survey results are also shown in Table 1. The standard for the product defect index was the case of heat No.1.

[0056]

[Table 1]

From Table 1, T.
It can be seen that the weight ratio Y / X needs to be in the range of 1 to 2 in order to make [O] 20 ppm or less and the product defect index 0.1 or less. If Y / X is less than 1 or more than 2, T. [O] exceeded 20 ppm, and the product defect index deteriorated to 0.7 or more.

(Example 2) Crude decarburization in a converter is an end point [C].
= 0.02-0.08Wt%, weight ratio Y / X in the deoxidizer is 1.2-1.5
And (Wt% FeO) + (Wt% M in the slag in the ladle before adding the deoxidizer.
A test was conducted to confirm the effect of nO). The basic conditions are the same as in Example 1.

When steel was tapped from the converter, the amount of slag unavoidably flowed out of the converter into the ladle was 6 to 8 kg / t. In order to reduce and modify the lower oxides in this outflowing slag, a modifier of the above composition 0.1 to 5.0 kg / t is sprinkled on the upper surface of the slag after completion of tapping, and (Wt% FeO in the slag in the ladle is added. The range of) + (Wt% MnO) is 0.05 to 20 as shown in Table 2.

[0060]

[Table 2]

After that, vacuum decarburization treatment is carried out, and [C] becomes 0.
It was reduced to below 003Wt%.

At that time, in the initial stage of vacuum decarburization, oxygen gas of 50 to 300 N
m ³ was sprayed from the upper tuyere provided on the side wall of the vacuum chamber. After decarburization, add a deoxidizer by the capsule addition method,
As it was, reflux treatment was carried out at RH for 10 minutes. The dissolved oxygen concentration in the molten steel before the addition of the acid agent was 200 to 500 ppm.

Then, in the same manner as in Example 1, the T. [O] and coil product defects were investigated. The survey results are also shown in Table 2. The product defect index was based on the results of heat No.1.

From Table 2, it can be seen that the cleanliness deteriorates with the increase of (Wt% FeO) + (Wt% MnO) in the slag in the ladle before the addition of the deoxidizer. Molten Steel in Tundish Medium T.I. [O] 20
In order to keep the ppm or less and the product defect index to 0.1 or less,
Before adding the deoxidizer, set the concentration of lower oxides in the slag in the ladle to 10 Wt.
It is clear that it is desirable to set it to be less than or equal to%.

[0065]

By using the molten steel deoxidizing method of the present invention, not only low carbon steel but also extremely low carbon steel can be remarkably improved in cleanability.

[Brief description of drawings]

FIG. 1 T. T. in molten steel in the tundish after Y / X and RH reflux. It is a figure which shows the relationship with [O].

FIG. 2 T. in molten steel in tundish It is a figure which shows the relationship between [O] and (Wt% FeO) + (Wt% MnO) in slag in a ladle before adding a deoxidizer.

Claims (1)

[Claims] 1. A non-deoxidized molten steel and a metal Al or an Al-containing alloy
A method of adding a deoxidizing agent consisting of CaO or a CaO-containing flux and then performing a deoxidizing treatment.
A deoxidizing method for molten steel, characterized in that a ratio Y / X of CaO pure content Y (Wt%) and Al pure content X (Wt%) is set to 1-2.