JPH10298629A - Method for melting extra-low carbon steel having high cleanliness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a melting method for a steel having high cleanliness and extra-low carbon concn. which suitably secures the necessary dissolved oxygen to decarburization and at the same time, restrains the formation of Al2 O3 in the decarburizing treatment of molten steel and in succession to this, deoxidizing treatment by using a vacuum degassing apparatus. SOLUTION: Firstly, a part of slag reforming agent is added into the tapping steel stream at the time of tapping from a converter to a ladle or the slag in the ladle and low grade oxide concn. in the slag is lowered and the oxygen concn. in the molten steel coexisting with the above oxide is adjusted to the necessary concn. to the decarburizing reaction. Thereafter, the decarburizing treatment and Al deoxidizing treatment are executed in order, and further, at least in one side of before and after the Al deoxidizing treatment, the difficult-to-collapse slag reforming agent having the difficult-to-collapse to the molten steel and easy-to-react to the slag, is added into a vacuum vessel and uniformly dispersed and arrived to the slag on the ladle accompanied with the molten steel stream.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a smelting method for steel having a high cleanliness and an extremely low carbon concentration, and more particularly to a smelting method capable of efficiently suppressing the formation of inclusions.

[0002]

2. Description of the Related Art As an exterior material for an automobile, a steel sheet having few surface defects and good formability (particularly, deep drawability) is required. In order to meet these demands, the steelmaking process requires high-purity steel and ultra-low carbon.

[0003] Usually, in the decarburization method for extremely low carbon,
A vacuum degassing process is applied to undeoxidized molten steel having a carbon concentration of about 0.03% by weight (hereinafter, “% by weight” indicating the composition is simply expressed as “%”) from a steelmaking furnace. Generally, the carbon concentration is reduced to 0.001 to 0.006% by utilizing C + O → CO ↑, which is a reaction between dissolved oxygen ( O 2 ) and carbon ( C ). The dissolved oxygen concentration in molten steel for promoting this reaction is stoichiometrically about 0.04%, which is sufficient, but is actually about 0.06% in excess.

[0004] When the dissolved oxygen in the molten steel is present at a high level, the lower oxide (FeO + MnO) concentration in the coexisting slag is as high as about 10%. Such a state in which the concentration of the lower oxides in the slag is high continues even after the carbon in the molten steel after the completion of the vacuum decarburization treatment is reduced, and the aluminum added by the deoxidation treatment (hereinafter, Al)
) Reacts with the lower oxides in the slag to form A
l ₂ O ₃ is generated, and this Al ₂ O ₃ remains to cause a problem of significantly lowering the cleanliness of the steel sheet.

[0005] As a method for solving this problem, several techniques have been proposed so far. For example, Japanese Patent Application Laid-Open No. 60-152611 discloses that, when a slag modifier is added at the time of tapping, CaCO ₃ is added together with the slag modifier in order to uniformly disperse and mix the slag modifier in the slag. A technique has been proposed in which a gas generating substance such as is added in combination to efficiently reduce lower oxides in the slag and suppress the generation of Al ₂ O ₃ -based inclusions. Also, JP-A-6-256837
In the official gazette, only part of the amount required for deoxidation of slag is added to the slag in the ladle during or immediately after tapping from the converter to the ladle, and the remaining slag is removed. A two-stage addition technique has been proposed in which an acid agent is added to slag in a ladle after decarburization treatment by the vacuum degassing device.

[0006]

However, the above prior art has the following problems.

In the method disclosed in Japanese Patent Application Laid-Open No. 60-152611, the concentration of lower oxides in the slag is reduced to a target concentration at a time by reduction in the tapping stage. When the lower oxide concentration is set to 2 to 3% in the tapping stage, it is effective in suppressing the formation of Al ₂ O ₃ , but on the other hand, the oxygen concentration in the coexisting molten steel is too low, resulting in insufficient decarburization. It becomes difficult to melt ultra-low carbon steel.

That is, a substantially positive correlation shown in FIG. 1 is established between the dissolved oxygen concentration in the molten steel in the ladle before the decarburization treatment and the lower oxide concentration in the slag. From this correlation,
The oxygen concentration in molten steel corresponding to the lower oxide concentration of 2 to 3% is about 0.03%, and the carbon concentration during tapping is usually 0.03%.
%, The achievable carbon concentration in molten steel is 0.01%
%, The target carbon concentration of the method of the present invention of 0.0
It is difficult to melt 01-0.006% ultra low carbon steel.
During the decarburization reaction, oxygen is not replenished from the lower oxides in the slag. Therefore, the relationship shown in FIG. 1 does not hold, and the concentration of the lower oxides in the slag changes before the decarburization treatment. A method of separately supplementing the oxygen deficiency with a top blowing lance or the like is also conceivable, but at this time, the slag is simultaneously oxidized and the concentration of lower oxides in the slag rises, so that the intended purpose cannot be achieved.

According to the method disclosed in Japanese Patent Application Laid-Open No. 6-256687, the slag modifier is added in two stages, at the time of tapping and after the completion of the decarburization treatment. The problem of insufficient oxygen dissolved in molten steel for decarburization caused by reducing slag is solved. But,
On the other hand, a large amount of slag modifier is required to reduce the total amount of lower oxides in the slag to a concentration of 2 to 3% or less after the decarburization treatment, and the slag is mechanically stirred because the fluidity of the slag is low. Or forced agitation such as stirring with gas ring fluidity, which causes problems such as longer processing time, lower molten steel temperature, and higher production cost, and cannot be said to be a viable method. .

An object of the present invention is to solve the above-mentioned problems of the prior art, and to appropriately remove dissolved oxygen necessary for decarburization in a decarburization treatment of molten steel using a vacuum degassing device and a subsequent deoxidation treatment. It is another object of the present invention to provide a method of melting steel having high cleanliness and an extremely low carbon concentration while ensuring the formation of Al ₂ O ₃ while securing the same.

[0011]

SUMMARY OF THE INVENTION The gist of the present invention is that a slag modifier is added to a tapping flow when tapping from a converter to a ladle or slag on low-carbon, non-deoxidized molten steel in the ladle. After adding and adjusting the concentration of the lower oxide in the slag, and then decarburizing using a molten steel reflux type vacuum degassing device, at least one of before and after Al deoxidation treatment, the molten steel in the vacuum chamber In addition, there is provided a method for producing ultra-low carbon steel having high cleanliness, characterized by adding a slag modifier having a property of being hardly collapsible to molten steel and having a high reactivity to slag. .

First, a slag modifier is added to a tapping flow at the time of tapping from a converter to a ladle or slag in a ladle to determine the concentration of lower oxides in the slag and the molten steel coexisting therewith. The concentration of oxygen is adjusted to the minimum necessary concentration so that the oxygen concentration therein becomes the concentration required for the decarburization reaction.

After that, Al deoxidation treatment is performed after decarburization treatment. Before completion of Al deoxidation after completion of the decarburization treatment,
At the time when the Al deoxidation treatment is completed, or at both times, a slag modifier that is hardly collapsible for molten steel and easily reactive for slag is added to molten steel in a vacuum chamber. It is added and is accompanied by the molten steel stream to uniformly disperse and reach the slag on the ladle outside the vacuum chamber. By this treatment, the lower oxide concentration in the slag on the lower surface of the slag in contact with the molten steel can be reduced, and as a result, the generation of inclusions can be suppressed.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The essential points in melting steel having an extremely low carbon concentration and a small amount of Al ₂ O ₃ -based deoxidation products are as follows.

The difference between the carbon concentration in the molten steel and the target carbon concentration after the decarburization treatment is defined as the decarburization amount. Oxygen dissolved in molten steel is used as oxygen required for decarburization, but usually the dissolved oxygen after tapping has an excessively high concentration and needs to be reduced. Since the dissolved oxygen concentration is positively correlated with the lower oxide in the slag as shown in FIG. 1, by utilizing this relationship, the concentration of the lower oxide in the slag can be adjusted to a predetermined concentration by adding a slag modifier. To reduce the dissolved oxygen in the molten steel to the minimum concentration required for the decarburization reaction.

However, although the dissolved oxygen is reduced by the decarburization reaction, the lower oxides in the slag remain at the same concentration after the decarburization treatment and react with the residual Al after the deoxidation treatment.
Produces Al ₂ O ₃ . For this reason, it is necessary to further reduce the concentration of the lower oxide in the slag.

Therefore, after the decarburizing treatment, at least one of before and after the subsequent deoxidizing treatment, a slag modifier is further added to the molten steel in the vacuum chamber in order to further lower the concentration of lower oxides in the slag. The vacuum processing apparatus using the immersion tube cannot sufficiently agitate the slag, but can sufficiently agitate the molten steel. Therefore, if the slag modifier is transported along with the molten steel stream, the slag modifier floats above the ladle outside the vacuum tank,
The slag modifier can be uniformly dispersed on the surface where the slag and the molten steel come into contact.

It is not necessary that the slag be entirely reformed, and the slag may be reformed only in such a portion so that the portion in contact with the molten steel does not contaminate the molten steel. As a result, the slag modifier can be saved. As described in the prior art (the invention of Japanese Patent Application Laid-Open No. 6-256837), when attempting to reform the entire amount of slag, a large amount of slag modifier is required, and a special slag is required for stirring the slag. This is not advisable because cost factors such as the necessity of a stirrer increase.

The hard-to-disintegrate slag modifier has a hard-to-disintegrate property in molten steel and needs to have a chemical composition and a shape that is easily reactive with slag.

The time of addition of the hard-to-disintegrate slag modifier is
1 At least one of before and after the deoxidation treatment is good.

The case where the method of the present invention is performed in an RH vacuum degassing apparatus will be described as an example.

FIG. 2 is a schematic view of a longitudinal section of an RH device for carrying out the method of the present invention.

First, undeoxidized molten steel 1 produced in a converter or the like
Tapping into ladle 2 At this time, the slag 3 which has inevitably flowed into the ladle contains a low concentration of a low oxide and has a high dissolved oxygen concentration in the molten steel. Therefore, a slag modifier containing metal Al or an Al alloy is added to the tapping flow during tapping or the slag in the ladle to reduce the concentration of lower oxides in the slag. At this time, the dissolved oxygen concentration in the molten steel is adjusted to a dissolved oxygen amount corresponding to a target decarburization amount.

Next, a decarburization treatment is performed. The two immersion tubes, the riser tube 4a and the downcomer tube 4b, are immersed in the molten steel 1 in the ladle 2, the vacuum tank 5 is evacuated and the molten steel is pulled up into the vacuum tank while reducing the pressure, and Ar gas is supplied from the riser tube 4a side. The molten steel 1 is recirculated by blowing, and the decarburization is performed by promoting the C + O → CO ↑ reaction.

After the decarburization process is completed, for example, Al necessary for deoxidation is added to the molten steel in the vacuum chamber from the Al shooter 6 to deoxidize the molten steel. Subsequently, a recirculation treatment is performed for a predetermined time necessary for uniting and floating the deoxidation products (mainly Al ₂ O ₃ ) generated during the deoxidation treatment.

Then, while continuing the molten steel recirculation, the slag modifier in the vacuum tank 1 is supplied from the shooter 7 for the hardly collapsible slag modifier to the molten steel in the vacuum chamber 1 while maintaining the hardly collapsible property in the molten steel and easily reacting with the slag. Add the filler 8. The slag modifier having the property of being hardly collapsible in the molten steel is discharged from the downcomer pipe 4b to the molten steel in the ladle along with the flow of the molten steel flowing between the ladle and the vacuum tank. The discharged slag modifier is dispersed in the molten steel and then gradually floats to finally reach the slag in the ladle. Here, the Al and CaO components in the modifier exhibit the effect of reducing the lower oxide concentration in the lower surface of the slag in the ladle, that is, the portion of the slag in contact with the molten steel, so that the slag is reformed. This avoids the reaction between the Al in the molten steel remaining during the deoxidizing treatment and the lower oxide in the slag in the ladle, so that the generation of Al ₂ O ₃ can be suppressed.

The hard-to-disintegrate slag modifier means one which does not easily react with molten steel in molten steel and does not break its shape.

The basic components are metal Al for reducing the lower oxides of the slag in the ladle and CaO for preventing erosion by molten steel. However, in addition, CaF ₂ , Al ₂ O ₃ and SiO ₂ may be contained as melting point adjusting components of the slag in the ladle, and Al ash (A
l Refining by-product) can also be used.

The basic structure is C for preventing erosion by molten steel.
aO becomes the outer shell, and the center becomes metal Al. Although the shape is not particularly limited, a pellet or briquette shape is desirable from the viewpoint of mass production. FIG. 3 shows an example of a cross section of the hard-to-disintegrate slag modifier. (A) is a briquette formed by mixing and compressing metal Al powder and CaO powder, (b) is a pellet containing (a) in CaO, and (c) is a pellet containing metal Al particles in CaO. (A) is the most desirable in terms of ease of fabrication. However, (b) and (c)
There is no particular problem because the outer shell is CaO, but in the case of (a), since some Al is exposed in the surface layer, this elutes into the molten steel between immersion in the molten steel and reaching the slag. Could collapse. However, the Al content is 80%
Since it does not collapse before reaching the slag, the Al content is preferably set to 80% or less.

The strength only needs to be strong enough to withstand handling. In terms of crushing strength, one standard is 20 kg or more. Like the briquette type shown in FIG.
If the raw material is molded with a compressive force, a crushing strength of 20 kg can be exhibited even when the raw material is not fired. However, in the case of the pellet type shown in FIGS. 3B and 3C, the bonding strength of the raw materials adhering to the surface of the central nucleus depends only on the cohesive strength of the binder added together, so that the bonding strength is high. Is weak and the crushing strength is less than 20 kg. Therefore, in the case of a pellet-type molding in which the fine powder material is coated on the surface of the core, it is preferable to preliminarily bake using a binder such as a resin to achieve the strength.

The size is desirably 10 mm or less in outer shape.
That is, it is desirable that the briquette has a long side length and the pellet has a diameter of 10 mm or less. If it exceeds 10 mm, the floating speed of the modifier in the molten steel is high, it is discharged from the downcomer and rises immediately to reach the slag around the downcomer, and evenly distributed over the entire slag in the ladle. Because it does not.

The addition of the hard-to-disintegrate slag modifier may be at least one of before and after the decarburizing treatment of the molten steel, but more preferably after the deoxidizing treatment. The reason is that the dissolved oxygen in the molten steel is higher after decarburization than after deoxidation,
If a portion of Al is exposed as in the briquette-type hard-to-disintegrate slag modifier shown above, Al dissolves in the molten steel and reacts with dissolved oxygen to reach the slag in the ladle. This is because Al is consumed before, and as a result, the reforming efficiency of the slag modifier is reduced. Therefore, the timing of adding the hard-to-disintegrate slag modifier is desirably after the deoxidation treatment with lower dissolved oxygen in the molten steel.

FIG. 4 shows the relationship between the timing of adding the hard-to-disintegrate slag modifier and the slag reforming efficiency. FIG. 4 shows the average value as well as the data variation.

Here, the slag reforming efficiency means the reaction efficiency of Al in the modifying agent added as a reducing agent represented by the following formula.

(Slurry reforming efficiency) = (weight of Al reduced from lower oxide in slurry) / (weight of Al in modifier)

[0036]

【Example】

(Example of the present invention) 250t of undeoxidized molten steel having a carbon concentration of 0.03% produced in a converter was tapped into a ladle. At this time, the amount of slag flowing out is suppressed to about 10 kg per ton of molten steel by using a slag stopper, and a low-grade oxide (FeO
+ MnO) concentration was set at 6%, and a slag modifier (composition: Al 40%, CaCO ₃ 60%) was added to the tapping flow at a rate of 1.5 kg / t. At this time, the concentration of dissolved oxygen in the molten steel was 0.045%. The slag modifier used here has a normal shape, and is not particularly subjected to pretreatment such as molding.

The obtained molten steel is subjected to vacuum decarburization treatment with an RH vacuum degassing apparatus to decarbonize the carbon steel to a carbon concentration of 0.0025%, and then to deoxidization of the molten steel by adding metal Al to obtain dissolved oxygen in the molten steel. The concentration was 0.001%. At this point, the concentration of lower oxides in the slag is still at a high level of 6%,
It is difficult to suppress the production of Al ₂ O ₃ due to the reaction with Al in the molten steel.

Then, in order to continuously reduce the concentration of lower oxides in the slag to 2%, a brittle slag modifier (composition: CaO 50% , Al 50%)
Was added into a vacuum tank of 0.25 kg / molten steel t, and then RH reflux treatment was performed for about 10 minutes to achieve uniform dispersion of the slag modifier to the slag in the ladle.

In the above-described series of processes, before starting the converter,
Before and after the RH treatment, the molten steel and the slag were sampled, and changes in the oxygen concentration in the molten steel, the carbon concentration in the molten steel, and the lower oxide concentration in the slag were investigated.

The obtained molten steel was continuously cast, a sample was taken from a steady portion of the slab, and inclusions having a size of 5 μm or more were counted and examined by a microscopic method.

(Comparative Example 1) A decarburizing treatment and a deoxidizing treatment of molten steel were performed under the same conditions as those of the present invention except that the hardly decayable slag modifier after the deoxidizing treatment was not added.

(Comparative Example 2) Molten steel having a carbon concentration of 0.03% produced in a converter was completely deoxidized, and was discharged to a ladle. At this time, the amount of outflow slag is suppressed to about 10 kg per ton of molten steel by using a slag stopper, and a slag modifier (composition: Al 40%, CaCO ₃ 60%) is applied to the tapping flow.
Is added at 2 kg / t of molten steel, and a low-grade oxide (FeO
+ MnO) was set to a target value of 2%. At this time, the dissolved oxygen concentration in the molten steel was 0.003%.

The molten steel obtained by the RH vacuum degassing apparatus was subjected to vacuum decarburization treatment. Due to lack of dissolved oxygen in the molten steel,
Oxygen top blowing was performed at a supply rate of 0.01 Nm ³ / (t molten steel · minute) for 3 minutes from an upper blowing lance installed in the upper portion of the vacuum tank, and the carbon concentration in the molten steel was set to 0.0025%.

Subsequently, the molten steel was deoxidized by adding metal Al, and the dissolved oxygen concentration in the molten steel was set to 0.001%.

Investigation of the oxygen concentration in the molten steel, the carbon concentration in the molten steel, the lower oxide concentration in the slag, and the number of inclusions in the slab were carried out.
It carried out according to the present invention example.

In each of the examples of the present invention and Comparative Examples 1 and 2, processing was performed for 10 charges for each condition, and the average value and the data variation were also shown. The results are summarized below. FIG. 5 shows the transition of the concentration of the lower oxides in the slag from the time of tapping the converter to the end of the RH vacuum degassing process for the present invention example and Comparative Examples 1 and 2.

In the present invention, the lower oxide concentration in the slag is reduced sequentially in accordance with the addition of an appropriate amount of the slag modifier, and it is possible to finally achieve the target lower oxide concentration in the slag of 2%. You can see that there is.

On the other hand, the lower oxide concentration in the slag of Comparative Example 1 is adjusted to the target of about 6% before the RH treatment, but cannot be reduced thereafter. In Comparative Example 2, the slag lower oxide concentration was once at 2% of the final target.
Although it is reduced to a degree, oxidation of the slag proceeds during the oxygen overblowing by the lance during the vacuum decarburization treatment, and eventually increases to a value close to Comparative Example 1, and the target cannot be achieved.

FIG. 6 shows the state of inclusions in the slab after continuous casting. Here, the slab defect index is expressed as an index with the number of inclusions (sizes of 5 μm or more) of the slab of Comparative Example 1 as 1.0. The slab defect index in the present invention example was about 0.4, indicating that the cleanliness was the best.

[0050]

According to the method of the present invention, it is possible to reduce the concentration of low-grade oxides in slag to about 2%. As a result, it is possible to obtain ultra-low carbon steel of high cleanliness which cannot be obtained by the conventional method. It can be produced reliably and at low cost.

[Brief description of the drawings]

FIG. 1 is a view showing a relationship between a lower oxide in slag and an oxygen concentration in molten steel before RH vacuum degassing treatment.

FIG. 2 is a schematic sectional view of an RH vacuum degassing apparatus for carrying out the present invention.

FIG. 3 is a view showing an example of a cross section of a hardly collapsible slag modifier.

FIG. 4 is a diagram comparing slag reforming efficiencies when a hardly disintegratable slag modifier is added after decarburization treatment and when it is added after Al deoxidation treatment.

FIG. 5 is a graph comparing the present invention example and the comparative example with respect to the transition of the concentration of the lower oxides in the slag from before the converter tapping to after the RH vacuum degassing treatment.

FIG. 6 is a graph comparing the present invention example and a comparative example with respect to a slab defect index.

[Brief description of reference numerals]

 1: molten steel 2: ladle 3: slag 4a: riser 4b: downcomer 5: vacuum tank 6: shooter for Al 7: shooter for hard-to-collapse slag modifier 8: hard-to-collapse slag modifier

Claims (1)

[Claims] A slag modifier is added to a tapping flow at the time of tapping from a converter to a ladle or slag in a ladle to adjust the concentration of lower oxides in the slag. After decarburizing treatment using a vacuum degassing treatment device, at least one of before and after aluminum deoxidation treatment, in molten steel in a vacuum chamber, it is hardly collapsible with molten steel and slag with respect to slag A method for melting ultra-low carbon steel with high cleanliness, characterized by adding a slag modifier having easy reactivity.