JPH11158537A - Production of extra-low carbon steel excellent in cleanliness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently improve the cleanliness of an extra-low carbon steel by executing decarburization treatment with a vacuum degassing apparatus after adjusting FeO+MnO concn. in slag of undeoxidizing low carbon steel tapped into a ladle and successively, adding powdery Al-Fe alloy onto the slag to execute deoxidization treatment. SOLUTION: The underoxidizing low carbon steel having about 0.02-0.07 wt.% C produced in a refining furnace for steel, is tapped into the ladle 2 and a reducing agent of Al, etc., is added onto the slag 3 of this molten steel 1 and FeO+MnO in the slag 3 is made to 4-8%. Successively, the decarburization treatment of the molten steel 1 is executed with the vacuum degassing apparatus provided with a vacuum vessel 5. In this way, C concn. in the molten steel 1 is made to be 0.001-0.006%. Thereafter, a deoxidizer of metallic Al, etc., is added into the vacuum vessel 5 from a chute 6 to execute the deoxidization treatment. At this time, the powdery Al-Fe alloy 7 having 87-95% Al content and 0.01-5 mm grain diameter, is added onto the ladle slag 3 through an adding device 8, and FeO+MnO in the ladle slag 3 is reduced to <=2% to restrain the reaction with Al in the molten steel 1.

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 ultra-low carbon steel having excellent cleanliness.

[0002]

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

[0003] In the smelting of ultra low carbon steel, the concentration of [C] in molten steel discharged from a steelmaking furnace is 0.02 to 0.07% (hereinafter, all concentrations are indicated by weight%). The molten steel is subjected to vacuum degassing, and the carbon concentration is reduced to 0.006% or less using [C] + [O] → CO ↑ which is a reaction between dissolved carbon [C] and dissolved oxygen [O] in the molten steel. It is generally reduced to
The concentration of dissolved oxygen in molten steel of 0.04 to 0.05% is sufficient for promoting this decarburization reaction.
It is present in excess of 06% or more.

[0004] When the dissolved oxygen in the molten steel is present at a high level, the (FeO + MnO) concentration in the coexisting slag is as high as about 15%. (FeO + Mn in slag
O) The high concentration state continues even after the completion of the vacuum decarburization treatment,
In this state, the Al residue added in the deoxidizing treatment reacts with FeO and MnO in the slag to produce Al ₂ O ₃ , and this Al ₂ O ₃ remains in the molten steel and significantly lowers the cleanliness of the steel sheet. This causes a problem.

As a method for solving the above problem, the following proposals (1) and (2) have been made. (1) JP-A-60-152611 discloses that when tapping steel,
When adding the slag reducing agent (Al-containing material), a gas generating substance such as CaCO ₃ is added together with the slag reducing agent in order to mix the slag reducing agent and the slag, and FeO in the slag is added. Efficient reduction of MnO and Al ₂ O
_A method for suppressing formation of _three- system inclusions is disclosed.

(2) JP-A-6-256837 discloses that a slag reducing agent is added to slag in a ladle during or immediately after tapping from a converter to a ladle, and the slag reducing agent is further evacuated to a vacuum. A method of adding the slag in the ladle after the decarburization treatment by the degassing treatment device is disclosed.

[0007]

However, the above conventional method has the following problems (1) and (2). (1) In the method disclosed in Japanese Patent Application Laid-Open No. 60-152611, the (FeO + MnO) concentration in the slag is reduced to about 4% or less at a time by a reduction treatment at the tapping stage. There is an advantage that the reaction between FeO and MnO therein and Al dissolved in the molten steel hardly occurs. Therefore, it is effective in suppressing the generation of Al ₂ O ₃ -based inclusions. However, if the oxygen concentration in the coexisting molten steel is too low, the decarburization reaction will not proceed in this state, and it will be difficult to produce ultra-low carbon steel.

A method of separately supplementing the oxygen deficiency by blowing it from the tuyere is also conceivable. However, at the same time, the slag is oxidized and the (FeO + MnO) concentration in the slag increases, so that the intended purpose cannot be achieved. Becomes

(2) In the method disclosed in JP-A-6-256837, the slag reducing agent is added in two stages, at the time of tapping and after the completion of the decarburization treatment. The problem of the lack of dissolved oxygen in the molten steel for decarburization, which is caused by the reduction of carbon dioxide, is solved. But on the other hand,
After the decarburization treatment, the (FeO + MnO) concentration in the slag is 2 to
In order to reduce the slag to 3% or less, it is necessary to mechanically agitate the slag or perform forced agitation such as agitation by gas bubbling. It is difficult to perform such forced stirring during the RH processing, and it is necessary to add another processing step after the RH processing. For this reason, there are problems such as an increase in processing time, a decrease in molten steel temperature, and an increase in production cost, and this is not an effective method. In addition, this method has a risk of increasing inclusions caused by entrainment of generated Al ₂ O ₃ and slag due to forced agitation, thereby further deteriorating cleanliness.

It is an object of the present invention to improve the above-mentioned problems of the prior art and to appropriately secure dissolved oxygen necessary for decarburization in a decarburization treatment and deoxidation treatment operation of molten steel using a vacuum degassing apparatus. At the same time, it is an object of the present invention to provide a method for melting ultra-low carbon steel having excellent cleanliness and suppressing new generation of Al ₂ O ₃ -based inclusions.

[0011]

As a result of various experiments and studies, the present inventors have obtained the following findings (A) to (D).

(A) In the decarburization treatment by the vacuum degassing apparatus, it is important to control the (FeO + MnO) concentration in the ladle slag in order to appropriately maintain the dissolved oxygen concentration as a decarburization reaction source. (B) (FeO + Mn) in the ladle slag managed above
In the case of the O) concentration, Al ₂ O ₃ inclusions are generated after the deoxidizing treatment step, so that a countermeasure must be taken.

(C) In order to suppress the formation of Al ₂ O ₃ inclusions, (FeO +) in the ladle slag after vacuum degassing and decarburizing treatment is used.
It is necessary to reduce the MnO) concentration below a certain concentration. (D) In order to reduce the (FeO + MnO) concentration in the ladle slag to a certain concentration or less, at the time of the deoxidation treatment after the decarburization treatment in the vacuum degassing apparatus, the density of the ladle slag is almost equal to the density of the ladle slag. It is effective to disperse a low melting point reducing agent having a particle size in the slag.

The present invention has been made based on the above findings, and the gist thereof is as follows. In a steel refining furnace, low carbon undeoxidized steel is melted, and tapping is performed in a ladle in an undeoxidized state, and a reducing agent is added to make the sum of the FeO concentration and the MnO concentration in the slag 4-8%. Then, after performing decarburization treatment with a vacuum degassing apparatus to adjust the C concentration to 0.001 to 0.006%, a ladle slag is added when a deoxidizing agent is added to the vacuum tank to perform the deoxidizing treatment. On top, the Al content is 87-95% and the particle size is 0.
A method for producing an ultra-low carbon steel having excellent cleanliness, characterized by adding a powdery Al-Fe alloy of 1 to 5 mm to reduce the sum of the FeO concentration and the MnO concentration in a ladle slag to 2% or less. .

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION In the smelting of ultra-low carbon steel, vacuum degassing is performed on a low-carbon undeoxidized steel having a C concentration of 0.02 to 0.07%, which has been discharged from a steelmaking furnace. Decarbonization is performed by reacting dissolved oxygen and dissolved carbon therein.

The decarburization of the molten steel in the vacuum degassing apparatus is as follows:
Reaction between carbon and oxygen dissolved in molten steel, [C] + [O]
→ Proceed by CO ↑. Normally, the dissolved oxygen after tapping has an unnecessarily high concentration and must be reduced.

FIG. 1 shows (F) in the ladle slag before the decarburization treatment.
4 is a graph showing the relationship between (eO + MnO) concentration and the dissolved oxygen concentration in molten steel. As shown in FIG. 1, the dissolved oxygen concentration has a positive correlation with the (FeO + MnO) concentration in the ladle slag. Therefore, utilizing this relationship, the dissolved oxygen concentration required for decarburization is reduced to such an extent that the dissolved oxygen is not excessively present. It is necessary to adjust the (FeO + MnO) concentration in the slag by adding a slag reducing agent.

If the dissolved oxygen is in the range of 0.04 to 0.05%, ultra low carbon steel (C concentration 0.001 to 0.006)
%) Is possible. (FeO + Mn in slag
O) By controlling the concentration to 4% to 8%, the dissolved oxygen in the molten steel can be made appropriate as such.

The slag reducing agent preferably contains metal Al or an Al alloy. After that, decarburization and deoxidation are performed by a vacuum degasser, and the [C] concentration in the molten steel is 0.00.
1 to 0.006%. At this time, as a deoxidizing agent,
In the ultra-low carbon steel used for the steel sheet of the present invention, metal Al or Al
An alloy is used. Although the dissolved oxygen concentration is reduced by the deoxidation treatment, the (FeO + MnO) concentration in the slag remains almost unchanged, that is, 4% to 8%.

When continuous casting is performed in this state, (FeO + MnO) in the slag sequentially reacts with Al in the molten steel, and Al
_It generates ₂ O ₃ inclusions and causes inclusion defect of the slab. For this reason, it is necessary to suppress the reaction between FeO and MnO in the slag and Al in the molten steel.

According to the present invention, a powdery Al-Fe alloy having an Al content of 87 to 95% and a particle size of 0.1 to 5 mm is added to the ladle slag at the time of the deoxidation treatment, thereby improving the efficiency. Can be reduced, the (FeO + MnO) concentration in the slag can be reduced to 2% or less, and Al ₂ O ₃ due to the reaction with residual Al in the molten steel can be reduced.
Can be suppressed.

The present invention relates to the above prior art (Japanese Patent Laid-Open No. 6-2).
56837), there is no need to add a special treatment for stirring the slag-metal interface after the vacuum degassing treatment, and it can be applied in parallel with the ordinary deoxidation treatment step.
There is an advantage that problems such as an increase in processing time, a decrease in molten steel temperature, and an increase in production cost do not occur.

The method of carrying out the present invention will be described below by taking as an example a case where the method is carried out in an RH vacuum degassing apparatus. FIG. 2 is a schematic view of a longitudinal section of an RH vacuum degassing apparatus for carrying out the method of the present invention.

After the decarburization by the RH vacuum degassing apparatus, Al necessary for deoxidation is added from the shooter 6 to the molten steel in the vacuum chamber, and the molten steel is deoxidized. The Al content is 87-95% and the particle size is 0.1-5 on the ladle slag from the powdery Al-Fe alloy adding device 8.
1 mm of powdery Al—Fe alloy 7 is supplied.

The powder of the Al—Fe alloy is dispersed in the molten slag to be in a molten state, and Al in the alloy is changed to Fe in the slag.
O and MnO are reduced quickly, and the FeO concentration and MnO
The sum of the concentrations can be efficiently reduced to 2% or less.
Deoxidation products generated during molten steel deoxidation treatment (mainly Al
A reflux treatment is performed for a predetermined time for the purpose of uniting and floating of ₂ O ₃ ). This reflux treatment is usually performed for 5 to 10 minutes.

The reason for limiting the Al content in the added Al-Fe alloy to 87 to 95% is that the density of the added alloy is made almost the same as the density of the ladle slag so that the Al-Fe
This is because the alloy powder can be dispersed in the molten slag, and since it has a low melting point, can easily be formed into droplets, and the reduction of FeO and MnO in the slag can be rapidly advanced.

The density of the ladle slag is about 2.8 g / cm.
³ , powder Al-Fe having an Al content of 87 to 95%
The density of the alloy is 2.6~3.0g / cm ^3. The above Al
When the content is out of the range, the ratio of the amount of Al consumed in the reduction of FeO and MnO in the slag to the amount of Al in the added Al-Fe alloy (hereinafter, referred to as effective Al ratio) is significantly reduced.

When the Al content exceeds 95%, the added Al-Fe alloy has a lower density than the ladle slag, so that it floats on the slag and is oxidized by air to lower the effective Al ratio. If the Al content is less than 87%, the added Al-F
The e-alloy sediments in the ladle slag due to its high density and is absorbed by the molten steel, and the effective Al ratio similarly decreases.

If the particle size of the Al-Fe alloy is less than 0.1 mm, the added alloy powder is scattered, and if the particle size exceeds 5 mm, the reaction speed decreases due to a decrease in the reaction interface area between the alloy and the slag. The effective Al ratio decreases.

FIG. 3 is a graph summarizing the relationship between the effective Al ratio and the Al content of the added Al-Fe alloy in terms of the particle diameter as a parameter. As shown in FIG. 3, the effect of the Al content and the particle size of the Al—Fe alloy on the effective Al rate is remarkable.
It is necessary that the l content is 87 to 95% and the particle size is 0.1 to 5 mm.

Al content on the ladle slag is 87-95%
In order to add the powdery Al-Fe alloy, a device capable of supplying the powdery alloy at a constant flow rate while moving the addition position is used. An example of such an apparatus is a powder adding apparatus of a continuous casting facility. In addition, it is desirable to provide a plurality of supply devices for efficient addition.

The temperature of the ladle slag surface layer after the RH vacuum degassing treatment is lower than the molten steel temperature (about 1600 ° C.) and may be in a solid or semi-molten state. By adding a low melting point flux before adding the Fe-Al alloy and melting the slag surface layer portion, more effective slag reduction can be easily performed. Examples of the flux to be added include CaO—SiO ₂ —CaF ₂ and C
aO-Al ₂ O ₃ -CaF ₂ based flux is considered.

Next, FeO in the slag after the RH deoxidation treatment
The reason for reducing the sum of the concentration and the concentration of MnO to 2% or less will be described below. As shown in FIG. 4, T in molten steel
In order to obtain a stable and clean molten steel with [O] of 20 ppm or less, (FeO + MnO) in the ladle slag after the RH treatment is required.
It is necessary to reduce the concentration to less than 2%.

In summary of the present invention, FeO and M in the slag are concurrently deoxidized by the vacuum degassing apparatus.
The reduction of nO becomes possible, and it becomes possible to produce ultra-low carbon steel excellent in cleanliness without increasing the number of processing steps such as stirring. In the present invention, since stirring is not required, there is no problem of increase of inclusions due to entrainment from generated Al ₂ O ₃ and slag.

Also, as in the method disclosed in JP-A-60-152611, C and C are used together with a reducing agent such as Al.
A method of adding a gas generating substance such as aCO ₃ or a carbonate or carbon and dispersing and mixing the reducing agent in the ladle slag while stirring the slag with the generated gas is considered, but it is included in the gas generating substance. [C] concentration in molten steel is increased by carbon, and there is a problem that the regulation of [C] concentration is not more than 20 to 30 ppm and cannot be used for processing of extremely low carbon steel. Then there is no such problem.

[0036]

[Example] (Example of the present invention)
250 tons of 05% undeoxidized molten steel was poured into a ladle. At this time, by using a slag stopper, the outflow slag amount is suppressed to about 10 kg per 1 ton of molten steel (hereinafter, abbreviated as t), and the target of the (FeO + MnO) concentration in the slag is set to 6%, so that the slag flow is changed to Slag reducing agent (Al content 4
0%, CaCO ₃ content 60%)
Was added.

The obtained molten steel was subjected to vacuum decarburization treatment with an RH vacuum degassing apparatus, and decarbonized to a carbon concentration of 0.0025%. At this time, the (FeO + MnO) concentration in the slag was about 6%.

Next, a screen for a deoxidizing agent installed in a vacuum chamber
Metal Al was charged from the tar to deoxidize the molten steel, and the Al concentration in the molten steel was set to about 0.03%. Simultaneously with the addition of the metal Al, 0.14 kg / t of a powdery Al-Fe alloy having a particle size of 1 to 3 mm and an Al content of 90% was slag in a ladle using two addition devices capable of moving the addition position. Added above.

Thereafter, RH reflux treatment was performed for 5 minutes, and the generated Al ₂ O ₃ was floated. At this point, the sum of the concentrations of FeO and MnO in the ladle slag was reduced to 1.6%. The obtained molten steel was subjected to continuous casting under the same conditions as in a comparative example described later, and a sample was collected from a steady portion of the obtained slab, and nonmetallic inclusions of 30 μm or more were counted by a microscopic method.

Comparative Example 1 Powdery Al-F after Deoxidation Treatment
The dealloying treatment, deoxidizing treatment, and reflux treatment of the molten steel were performed under the same conditions as in the present invention, except that no e-alloy was added. The obtained molten steel was subjected to continuous casting under the same conditions as those of the present invention, and a sample was collected from a steady portion of the obtained slab.
Non-metallic inclusions greater than 0 μm were counted by microscopy.

(Comparative Example 2) Except that the (FeO + MnO) concentration in the slag after the vacuum decarburization treatment was about 9%, the decarburization treatment, deoxidation treatment, and A reflux treatment was performed. The obtained molten steel was subjected to continuous casting under the same conditions as those of the present invention, and a sample was taken from a steady portion of the obtained slab, and nonmetallic inclusions of 30 μm or more were counted by a microscopic method.

(Comparative Example 3) 250 tons of undeoxidized molten steel having a carbon concentration of 0.05% produced in a converter was tapped into a ladle. At this time, by using a slag stopper, the amount of outflow slag is suppressed to about 10 kg / t per 1 t of molten steel, and the target of (FeO + MnO) concentration in slag is set to 2%, and the slag reducing agent (Al-containing Rate of 40% and CaCO ₃ content of 60%) were added at 2 kg / t.

The obtained molten steel was subjected to a vacuum decarburization treatment using an RH vacuum degassing apparatus. Due to lack of dissolved oxygen in the molten steel,
100 g / t-oxygen from the immersion tuyere installed in the vacuum chamber
The gas was blown at a speed of 3 minutes for 3 minutes to decarbonize to a carbon concentration of 0.0025%.

Next, a screen for a deoxidizing agent installed in a vacuum chamber
Metal Al was charged from the tar to deoxidize the molten steel, and the Al concentration in the molten steel was set to about 0.03%. At the same time as the metal Al is charged, powder A having a particle size of 1 to 3 mm and an Al content of 90%
0.14 kg / t of the l-Fe alloy was added to the slag in the ladle using two addition devices capable of moving the addition position.

Thereafter, an RH reflux treatment was performed for 5 minutes, and a floating treatment of the generated Al ₂ O ₃ was performed. The obtained molten steel was subjected to continuous casting under the same conditions as in the present invention, and a sample was collected from a steady portion of the obtained slab, and inclusions of 30 μm or more were counted by a microscopic method.

In each of the present invention and Comparative Examples 1, 2, and 3,
Each charge was processed for 10 charges. FIG.
Next, in the ladle slag from the time of tapping the converter to the end of the RH degassing process (FeO + M) for the inventive examples and Comparative Examples 1, 2, and 3
nO) shows the transition of the concentration. In the example of the present invention, the (FeO + MnO) concentration in the ladle slag is reduced to about 6% by the slag reduction treatment at the time of tapping from the converter, and is maintained at this value until the end of the RH decarburization treatment. Thereafter, by adding a powdery Al-Fe alloy at the time of deoxidation treatment, after the RH treatment, (FeO
+ MnO) concentration could be reduced to 1.6%.

The case of Comparative Example 1 was the same as that of the present invention until the end of the RH decarburization treatment, but the (FeO + MnO) concentration in the ladle slag after the RH treatment was constant at 6%. In the case of Comparative Example 2, the (FeO + MnO) concentration in the ladle slag is reduced to about 9% by the slag reduction treatment at the time of converter tapping, and is maintained at this value until the end of the RH decarburization treatment. After that, the concentration of (FeO + MnO) in the ladle slag was reduced after the RH treatment by the addition of the powdery Al-Fe alloy at the time of the deoxidation treatment, but was reduced only to 4.5%.

In the case of Comparative Example 3, (Fe) in the ladle slag
O + MnO) concentration is about 2 due to slag reduction during tapping of converter.
%, But the oxidation of slag progressed due to oxygen blowing during the decarburization treatment with RH, and increased to about 8% at the end of the decarburization treatment. Then, powdery Al-Fe at the time of deoxidation treatment
The addition of the alloy reduced the (FeO + MnO) concentration in the ladle slag after the RH reflux treatment, but only to 3.6%.

FIG. 6 shows T [O] in molten steel in a tundish.
(Total oxygen amount) is shown by comparing the present invention example with Comparative Examples 1, 2, and 3. As shown in FIG. 6, T [O] in the present invention example
Average value of 15 ppm, compared with 35 to 45 ppm of the comparative example,
It was low enough.

FIG. 7 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 of the slab counted by the microscopic method of Comparative Example 1 as 1.0. The slab defect index in the present invention example was about 0.2, indicating that the cleanliness was the best.

[0051]

According to the method of the present invention, in the decarburizing and deoxidizing operations of molten steel using a vacuum degassing apparatus,
At the same time properly to ensure a dissolved oxygen necessary for decarburization Al ₂
New generation of O ₃ -based inclusions can be efficiently suppressed, and extremely low carbon steel with excellent cleanliness can be produced.

[Brief description of the drawings]

FIG. 1: (FeO + Mn) in ladle slag before decarburization treatment
3 is a graph showing the relationship between O) concentration and oxygen concentration in steel.

FIG. 2 is a schematic view of an RH vacuum degassing apparatus as an example of the present invention.

FIG. 3 Effective Al of Al content and particle size of Al—Fe alloy
It is a graph which shows the influence which affects a rate.

FIG. 4 Ladle slag after RH treatment (FeO + MnO)
Concentration and T [O] (total oxygen content) in molten steel in a tundish
6 is a graph showing a relationship with the graph.

FIG. 5 shows the method of the present invention and the conventional method,
It is the graph which compared with the transition of the (FeO + MnO) density | concentration in the ladle slag after H processing.

FIG. 6 is a graph comparing the method of the present invention and the conventional method with T [O] (total oxygen content) in molten steel in a tundish.

FIG. 7 is a graph comparing the method of the present invention and the conventional method with a slab defect index.

[Explanation of symbols]

 1: molten steel 2: ladle 3: slag 4a: riser 4b: descender 5: vacuum tank 6: shooter 7: powdery Al-Fe alloy 8: powdery Al-Fe alloy addition device

Claims (1)

[Claims] Claims 1. A low-carbon undeoxidized steel is melted in a steel refining furnace.
Tapping the ladle in a non-deoxidized state, adding a reducing agent to make the sum of the FeO concentration and the MnO concentration in the slag 4 to 8%, and then performing a decarburization treatment with a vacuum degassing device to perform C concentration. 0.0
After setting the content to 0.01 to 0.006%, when a deoxidizing agent is added in the vacuum chamber to perform a deoxidizing treatment, the Al content on the ladle slag is 8%.
A cleaner characterized by adding a powdery Al-Fe alloy having a particle size of 0.1 to 5 mm at 7 to 95% to reduce the sum of FeO concentration and MnO concentration in a ladle slag to 2% or less. Production method of ultra low carbon steel with excellent degree.