JP2002038212A - Slag modifier and method for producing high purity ultra-low carbon steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a slag modifier in which a carbon pick-up amount to a molten steel in a slag modification for performing after decarburization can be suppressed to less than 5 ppm in the production of an ultra-low carbon steel using a vacuum degassing apparatus, and also provide a method for producing the ultra-low carbon steel using the same. SOLUTION: After a non-deoxidized molten steel from a steelmaking furnace is decarburized to an ultra-low carbon region by using a vacuum degassing apparatus and successively deoxidized, this slag modifier is used in a slag modification process. The total carbon concentration of the slag modifier is less than 5 mass%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a slag reforming treatment performed after a vacuum decarburization treatment, in which the increase in carbon concentration is 0.0%.
The present invention relates to a slag modifier that can be suppressed to not more than 005 mass% (hereinafter, simply expressed as mass%), that is, 5 ppm or less, and a method for producing a high-purity ultra-low carbon steel using the same.

[0002]

2. Description of the Related Art Ultra-low carbon steels are often used as exterior materials for automobiles, and these exterior materials are required to have a steel sheet with few surface defects and excellent formability. In order to meet these requirements, in the steelmaking process, it is necessary to obtain high cleanliness and to refine with high accuracy up to an extremely low carbon concentration region.

[0003] Usually, in order to produce ultra-low carbon steel, a steelmaking furnace such as a converter is tapped as undeoxidized molten steel having a C content of 0.03 to 0.06%. Then, as secondary refining, a reaction between dissolved oxygen [O] and carbon [C] in the molten steel using a vacuum degassing device: [C] + [O] → CO is used to reduce the carbon concentration to 0.0001. From 0.005% to the target composition of 0.005%. After the decarburization treatment by the vacuum refining, the secondary refining process is further completed through Al deoxidation and component adjustment, and the steel is subjected to continuous casting.

The vacuum degassing process for producing ultra low carbon steel is generally performed by a molten steel recirculation type vacuum degassing apparatus (hereinafter referred to as RH), in which molten steel is sucked into a vacuum chamber and refluxed by the principle of a gas lift pump. Is abbreviated). This R
H consists of two dip tubes and a vacuum tank, but the ladle slag is present on the molten steel between the dip tube and the ladle, and the slag does not generate large flow or stirring.

[0005] In order to obtain a sufficient decarburization rate by this vacuum degassing treatment, the dissolved oxygen concentration is required to be 0.04% or more before the decarburization treatment. In order to obtain by the lower oxide concentration in the slag (generally (FeO)
And (MnO), hereinafter referred to as (Fe) in slag.
O + MnO) concentration) as high as 15 to 20%.

As described above, (FeO + MnO) in slag
When vacuum decarburization is performed at RH in a state where the concentration is high, the state where the (FeO + MnO) concentration in the slag is high is maintained even after vacuum decarburization. After that, even if Al is added to deoxidize the molten steel, the slag has low fluidity and poor reactivity,
The reduction reaction rate between lower oxide and Al in slag is slow,
The (FeO + MnO) concentration in the slag is hard to decrease.

Therefore, the slag containing a large amount of the remaining lower oxide continuously supplies oxygen to the molten steel from the deoxidizing treatment following the vacuum degassing treatment to the casting.
Due to the supply of oxygen, the molten steel undergoes reoxidation, and a large amount of Al ₂ O ₃ -based inclusions are generated in the molten steel, thereby deteriorating the cleanliness of the ultra-low carbon steel.

[0008] That is, Al ₂ O ₃ -based inclusions in molten steel that are initially generated by Al deoxidation are removed by reflux treatment with RH, but ladle after RH treatment is re-oxidized from slag after RH. Inclusions in the molten steel that occur at, through the tundish and mold without being sufficiently removed,
It will remain as Al ₂ O ₃ -based inclusions in the cast slab.

The Al ₂ O ₃ inclusions directly cause various surface defects near the slab surface. Also,
In the operation of continuous casting, there is a problem that Al ₂ O ₃ -based inclusions are a main cause of clogging of the immersion nozzle, so that multiple casting cannot be performed and the operation is hindered.

In order to suppress the nozzle blockage,
Ar gas is blown in from the upper part of the immersion nozzle, but if the degree of cleanliness of molten steel is insufficient, a large amount of Ar gas is blown. If this remains as bubbles in the vicinity of the slab surface, it also causes surface defects.

In order to cope with such surface defects, care may be performed at the slab or hot-rolled coil stage.
It requires enormous costs and requires some countermeasures.

[0012]

As a countermeasure for this, for example, Japanese Patent Laid-Open No. 6-256837 discloses a method in which a slag deoxidizing agent is added to a ladle slag at the time of undeoxidized tapping from a converter. Total Fe concentration in slag (hereinafter, (TF
e) also referred to as “concentration”) of 5% or less, and then, at the time of decarburization by a vacuum degassing apparatus, oxygen blowing is performed to reduce the [C] concentration in the molten steel to less than 0.006%. Add slag modifier for slag reforming,
A method for subsequent deacidification is disclosed. However, this method does not show any solution to the problem of C pickup of molten steel that may be caused by slag reforming after decarburization, and mentions the effect of lowering the (T.Fe) concentration in slag. However, it cannot be used in a method for producing an ultra-low carbon steel that requires high-precision carbon concentration control.

As a method of melting ultra-low carbon and ultra-low sulfur steel, Japanese Patent Application Laid-Open No. Hei 7-316637 discloses that after decarburizing by RH, oxygen is blown during deoxidation to raise the temperature of molten steel. Then, Al slag and CaO are added to the slag to modify the ladle slag, the basicity (CaO / SiO ₂ mass ratio) is 3 to 6, and ((T.Fe) + (MnO)) ≦ 1.
As No. 5, a method for performing desulfurization by flux injection is disclosed.

In this method, the total carbon concentration (hereinafter also referred to as TC concentration) as a part of the slag modifier is 0.4 to 0.4%.
Although 0.8% of Al slag is used, the content of C in CaO used at the same time is not described, and T.C. It is not clear about the C concentration.

[0015] Further, in the examples, the C concentration was 7 pp after desulfurization by flux injection after RH decarburization.
m, and it is difficult to employ the method disclosed in the same gazette for the production of ultra-low carbon steel, which requires precise control of the C concentration at a level of 5 ppm or less.

According to the present invention, there is provided a slag modifier capable of suppressing the amount of C pickup to molten steel at the time of slag reforming performed after decarburization to 5 ppm or less in the production of ultra-low carbon steel using a vacuum degassing apparatus. An object of the present invention is to provide a method for producing ultra-low carbon steel.

[0017]

Means for Solving the Problems The present inventors have studied the slag modifier capable of suppressing the amount of C pickup to 5 ppm or less, and have obtained the following findings.

(A) The slag modifier is (F) in the slag.
Generally, it refers to a reducing agent containing a metal having a strong affinity for oxygen, such as Al, which can reduce eO) and (MnO).

Also, an oxide of the metal, for example, Al ₂ O
_{When 3} coexists with a melting promoter such as CaO or SiO _2, it can be easily dissolved to promote the reduction reaction. Therefore, the slag modifier does not simply refer to the reducing agent itself containing a metal having a strong affinity for oxygen, but includes a melting promoter.

(B) The slag modifier includes a binder for forming the slag modifier into a predetermined shape so that it can be easily handled. (C) The carbon content in the slag modifier is derived from the raw materials of these reducing agents, melting promoters and binders, and their existence forms are classified as follows.

One of them is aluminum carbide (for example, Al ₄ C ₃ ) in Al ash as a reducing agent. The two are carbonates such as CaCO ₃ in the melt accelerator.

In the case of using three organic binders, volatile oil is mainly used. (D) As described above, since various carbon components are contained in the slag modifying agent, the slag modifying agent is considered as a raw material for the slag, and the slag modifying agent is not reduced in its performance as much as possible. With a small amount, C pickup of molten steel during slag reforming can be prevented.

(E) Before the slag modifier is added to the slag, the slag is exposed to a high-temperature atmosphere on the slag, so that a large amount of the volatile C contained at this point volatilizes,
If only the remaining C component is used as an index, the amount of C pickup can be more precisely suppressed.

The present invention has been made based on the above findings, and the gist is as follows. (1) The steel is removed from the steelmaking furnace without deoxidation, and decarburized to an extremely low carbon region using a vacuum degasser, and then used for slag reforming after deoxidation. A slag modifier, wherein the total carbon concentration of the slag modifier is 5% by mass or less.

(2) When removing steel from a steelmaking furnace without deoxidization, decarburizing to a very low carbon region using a vacuum degassing apparatus, and subsequently performing deoxidation treatment, and then performing slag reforming. A slag modifier used in a slag modifier, wherein the total non-volatile carbon concentration is 2% by mass or less.

(3) The steel is removed from the steelmaking furnace without deoxidation, decarburized to an extremely low carbon region using a vacuum degassing device, and subsequently deoxidized, and then slag is added while bubbling molten steel. A method for producing ultra-low carbon steel in which slag is modified by adding a modifier, wherein the total amount of carbon contained in the slag modifier is 50 g or less per 1 ton of molten steel. Manufacturing method of low carbon steel.

(4) The steel is removed from the steelmaking furnace without deoxidation, decarburized to an extremely low carbon region using a vacuum degassing device, and subsequently deoxidized, and then slag is added while bubbling molten steel. A method for producing ultra-low carbon steel in which slag is modified by adding a modifier, wherein the total amount of non-volatile carbon contained in the slag modifier is 20 g or less per 1 ton of molten steel. Production method of ultra-clean ultra low carbon steel.

(5) The method for producing a high-purity ultra-low carbon steel according to (3), wherein the slag modifier according to (1) is used. (6) The method for producing a high-purity ultra-low carbon steel according to (4), wherein the slag modifier according to (2) is used.

(7) The method for producing a high-purity ultra-low carbon steel according to any one of the above (3) to (6), wherein the bubbling of the molten steel is performed at 100 W or less with stirring power per 1 ton of molten steel. .

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS With regard to steel sheets for automobiles and the like to which ultra-low carbon steel is applied, customers have recently demanded a further improvement in both deep drawability and high strength.

That is, deep drawability requires further reduction of the carbon concentration in the extremely low carbon concentration region, and control of the carbon concentration in the extremely low carbon concentration region is required for high strength. You. In order to increase the strength, it is necessary to leave a carbon concentration of 0.0010% or more with an accuracy of 0.0002 to 0.0005% in this extremely low carbon concentration region according to the required strength of each steel type. You. RH to control carbon concentration in a trace area
It is performed by adjusting the decarburization processing time in consideration of the ultimate vacuum degree in the above.

Particularly, in the case of a BH steel sheet (bake hardening type steel sheet), the steel sheet is strengthened by the residual carbon content at the stage of baking coating, so that, for example, the carbon concentration in the steel is 0.001.
Control at 5 to 0.0030% is required. Decarburization and refining control in the extremely low carbon region must be performed within the target value of ± 5 ppm, and pickup exceeding 5 ppm after decarburization and refining cannot be tolerated at all.

In order to control the carbon concentration to such an extremely low level, a method of performing excessive decarburization in anticipation of the C pickup amount and then adding and adjusting the C content is performed by removing the carbon in the extremely low carbon region. Charcoal control requires a considerable amount of time, which leads to an increase in the production cost, and it is difficult to implement the method from the viewpoint of the production cost and its controllability.

On the other hand, it is premised that the slag modifier of the present invention is applied to slag reforming after the decarburization period by a vacuum degassing apparatus, which is an ultra-low carbon steel production process. Therefore, the slag modifier in the present invention means a slag modifier added to a ladle slag for slag reforming after the vacuum decarburization period.

In order to reduce lower oxides (transition metal oxides, such as FeO and MnO, which are easily reduced) in the slag, the slag modifier contains a metal Al having a strong affinity for oxygen as a reducing agent. An oxide such as CaO or SiO ₂ is contained as a melting accelerator so that such a reduction reaction is easy, and the particle size, shape and strength are so set as to further facilitate the use of a slag modifier. A conditioning binder is included.

As the reducing agent, when applied to Al-killed steel, Al is the most suitable.
Al ash (Al slag) which is a by-product of the re-dissolution step is often used.

[0037] Al ash, metal Al, made of SiO ₂ or the like as Al ₂ O ₃ and the balance, Al ₂ around the metal Al
Since O ₃ is formed, it is presumed that it also has a supplementary function of preventing the dissolution of metallic Al in molten steel and promoting the reaction with slag.

As the melting accelerator, the main constituent is an oxide (CaO, SiO _2, CaF _{2 and the} like). These oxide concentrations are selected in view of the slag composition in the applied steelmaking process.

The binder is necessary in order to form the above-mentioned reducing agent and the melting accelerator as particles and obtain the strength required for use as a slag modifier. As the binder, the same types as those used in refractory manufacturing are widely used, and specifically, resins, oils, starchy materials, water glass, and the like are used according to the granulation method.

The C content contained in the slag modifier is as follows. Al ash as a reducing agent: a by-product generated in the Al re-dissolution step using combustion of heavy oil as a heat source, and contains C component. Examples of the C content include Al ₄ C ₃ and free C. To obtain a particularly low C content of Al ash,
There is a problem that sorting and the like are required, yield is deteriorated, and the price is increased.

The melting promoting agent CaCO as _3: When using ₃ minutes CaCO, is C content in the slag modifier is contained mainly as CaO source. The C component as a binder is mainly organic.

The composition of the reducing agent is, for example, that the Al content in the normally available Al ash is 30 to 70% and the balance is Al
₂ O ₃ : 20 to 60% and SiO ₂ : 0.5 to 5%. Further, the melting accelerator contains 30% to 60% of CaO and 100% of SiO ₂ with respect to 100% of Al ₂ O _{3 in} Al ash.
May be blended in an amount of 1 to 20%. With this mixing ratio,
It reacts with the Al ₂ O ₃ film formed on the surface of the Al metal particles and can lower the melting point.

In addition, about CaF ₂ ,
About 0 to 50% may be added to 100% of the O content.
Further, the amount of the binder varies depending on the binder used, the molding method, the particle size of Al ash and other melting promoters, and the strength required after molding, but is about 1 to 5% of the slag modifier.

From the above, the composition of the slag modifier is Al: 2
_{0~65%, Al 2 O 3:} 18~40%, CaO: 5~
_{25%, SiO 2: 1~16%} , CaF 2: 0~12%
And a binder: 1 to 5% and unavoidable impurities.

The T.C. The C concentration is 5% or less, but preferably 3% or less. Al ash, which is a main component of the slag modifier, is a by-product generated in the Al re-dissolving step as described above, and has a metal Al content of about 15% to 75% and its oxide Al ₂ O ₃ , It is made of aluminum carbide (for example, Al ₄ C ₃ ), aluminum nitride (for example, AlN), and SiO ₂ unavoidably contained.

The metal Al content in the Al ash varies depending on the timing and method of collecting the Al ash, but the price of the Al ash is determined substantially by the content of the metal Al and the degree of slag reforming. Metal Al to be contained
Since the amount is determined, it should be determined by cost-effectiveness such as the allowable amount of C pickup and the degree of slag reforming.

The content of the aluminum carbide in the Al ash varies depending on the Al raw material at the time of re-dissolving Al, the fuel serving as a heat source at the time of melting, and the combustion mode. Conventionally, the content of such aluminum carbide has not been taken into consideration, or conversely, it has been required to be more strictly regulated than necessary. However, it may be determined according to the allowable amount of C pickup.

As for the binder, T.D.
It plays an important role in controlling the C concentration. Conventionally, the C content in these binders has not been considered so much. For example, when it is desired to mix a large amount of Al ash which is inexpensive but contains a large amount of C, granulation is carried out. By using a binder, T.V. In the case where the C concentration is reduced or Al ash having a small amount of C is used as a main raw material, a resin-based binder containing a resin-based C component that is more easily granulated may be used.

The following test was conducted to confirm the effect of suppressing the C pickup according to the present invention. That is, about 1
Using a high-frequency induction furnace capable of holding 5 kg of extremely low carbon molten steel at 1600 ° C., various modifiers were added to the slag on the molten steel, and after a predetermined time, the molten steel was sampled, and a test for examining C pickup was performed.

The slag used is a mixture of CaO, Al ₂ O ₃ , SiO ₂ , FeO and MnO simulating the slag after vacuum degassing, and the amount of slag is 1 k of molten steel.
The slag amount in actual operation was set to 30 g per g, and the slag amount was simulated.

The modifier used was obtained by granulating Al ash and quick lime using a binder (2 to 20 mm in diameter).
Metal Al content is 45 to 50%, balance Al ₂ O ₃ , Si
It consists of O ₂ and CaO.

Further, Al ash is used as a T.A. Using a 1.0 to 2.8% C concentration, the C content in the binder was adjusted and the T.C. C concentration was controlled. FIG. 1 shows the T.D. in the slag modifier. Change of C concentration and [C] concentration in molten steel:
It is a graph which shows the relationship with (C).

The amount of change in the [C] concentration in the molten steel is hereinafter also referred to as the C pickup amount. As shown in the figure, in order to suppress the amount of C pickup to molten steel to 5 ppm or less, the T.P. C concentration needs to be 5% or less.

Further, it is more preferable to set the C pickup amount to 2
In order to make it below ppm, it is necessary to make it below 3%. Next, the T.I. Factors for suppressing the C pickup amount more accurately than the C concentration were examined.

Since the slag modifier is temporarily exposed to the high-temperature atmosphere on the slag before reacting with the slag at the stage of being introduced into the slag on the ladle, the C contained at this time is Most of the C is volatilized, and if only the remaining C is considered, the amount of C pickup can be more quantitatively predicted.

That is, the nonvolatile C concentration was defined as follows. The raw material is a slag modifier composed of Al ash, a melting accelerator, and a binder, which is once formed into a shape to be used in the steelmaking process, and then coarsely pulverized. The particle size is 100-325.
After the mesh (0.044 to 0.149 mm),
r or 600 ° C. in a stream of an inert gas such as nitrogen.
Hold for 600 seconds. T. of the slag modifier after this heat treatment.
C concentration analysis was performed and T.C. The value obtained for the C concentration is converted to the non-volatile C
Defined as concentration.

In order to confirm the relationship between the nonvolatile C concentration and the C pickup amount in the present invention, about 15 kg
Using a high-frequency induction furnace capable of holding the ultra-low carbon molten steel at 1600 ° C., various modifiers were added to the slag on the molten steel, the molten steel was sampled after a predetermined time, and a test for examining the amount of C pickup was performed.

The slag used is a mixture of CaO, Al ₂ O ₃ , SiO ₂ , FeO and MnO which simulates the slag after the vacuum degassing treatment.
The amount was 30 g per g, which was a ratio corresponding to the amount of slag in the actual operation.

The modifier used was obtained by granulating Al ash and quick lime using a binder (2 to 20 mm in diameter).
Metal Al content is 45 to 50%, balance Al ₂ O ₃ , Si
It consists of O ₂ and CaO.

Further, Al ash is used as a T.A. Using a 1.0 to 2.8% C concentration, the C content in the binder was adjusted and the T.C. C concentration was controlled. FIG. 2 shows the non-volatile T.C. in the slag modifier. It is a graph which shows the relationship between C concentration and the variation of [C] concentration in molten steel: △ [C].

As shown in the figure, in order to suppress the amount of C pickup to molten steel to 5 ppm or less, the non-volatile T.P. C content needs to be 2% or less. In order to reduce the C pickup amount to 2 ppm or less, which is more desirable, 0.4
% Or less.

Next, molten steel 1 brought in from the slag modifier
T. per ton of mass The relationship between the amount of C (g) and the amount of change in the [C] concentration in the molten steel: Δ [C] will be described below. In the converter, after decarbonization is performed to a carbon concentration of 0.03 to 0.06%, the steel is removed from the converter at 1650 to 1680 ° C. The slag flowing out of the converter will reduce the total amount of the modifier later,
Reduce the amount as much as possible by the existing slag control method. At this stage, the (FeO + MnO) concentration in the ladle slag is in a high state, and decarburization and deoxidation are directly performed using RH or the like.

After the vacuum decarburizing treatment is completed and the deoxidizing treatment is completed, a slag modifier in an amount capable of reducing the (FeO + MnO) concentration in the ladle slag is sprayed on the upper surface of the slag. Then, slag modification by molten steel bubbling ((FeO + Mn
O) concentration reduction processing).

When a very low carbon molten steel having a [C] concentration of 15 to 40 ppm in the molten steel is manufactured by the test in such a process, the variation of the [C] concentration in the molten steel: Δ [C] and the slag modifier T. per 1 ton of molten steel brought in C amount (g)
The relationship with was investigated.

FIG. 3 is a graph showing T.T./m. Ton of molten steel introduced from the slag modifier. It is a graph which shows the relationship between the amount of C and the variation of [C] concentration in molten steel: △ [C]. As shown in FIG. C amount is 50g
In the following, Δ [C], that is, the C pickup amount is 5 ppm
Below 30g, and especially below 30g can be suppressed below 2ppm.

On the other hand, T. When the C amount exceeds 50 g, the C pickup amount increases, and the T.C. When the amount of C is 80 to 90 g, 6 to
At 11 ppm, an unacceptable C pickup was found for ultra-low carbon steel.

Next, molten steel 1 introduced from the slag modifier
Non-volatile T.V. The relationship between the amount of C (g) and the amount of change in the [C] concentration in the molten steel: Δ [C] will be described below.

In the converter, the carbon concentration is 0.03 to 0.06.
%, And then the steel is removed from the converter at 1650-1680 ° C. The slag flowing out of the converter is reduced as much as possible by the existing slag suppressing method in order to suppress the total amount of the modifier later. (FeO +) in the ladle slag at this stage
MnO) concentration is in a high state, and decarburization and deoxidation are directly performed using RH or the like.

After the vacuum decarburizing treatment is completed and the deoxidizing treatment is completed, a slag modifier in an amount capable of reducing the (FeO + MnO) concentration in the ladle slag is sprayed on the upper surface of the slag. Then, slag modification by molten steel bubbling ((FeO + Mn
O) concentration reduction processing).

When a very low carbon molten steel having a [C] concentration of 15 to 40 ppm in the molten steel is manufactured by the test in such a process, the amount of change in the [C] concentration in the molten steel: Δ [C] and the slag modifier Non-volatile T.O./m.t.
The relationship with the C amount (g) was investigated.

FIG. 4 shows non-volatile T.P./m.t. Of molten steel brought from the slag modifier. It is a graph which shows the relationship between the amount of C and the variation of [C] concentration in molten steel: △ [C]. As shown in FIG.
When the C amount is 20 g or less, Δ [C], that is, the C pickup amount can be suppressed to 5 ppm or less.
It can be suppressed to below ppm.

On the other hand, when the amount of non-volatile C exceeds 20 g, the amount of C pickup increases, and when the amount of non-volatile C is about 26 g, the amount of C is 6 to 11 ppm, which is unacceptable as an extremely low carbon steel.
Pick-up was allowed.

In order to suppress the variation of the [C] concentration in the molten steel to 5 ppm or less, as described above, the T.C. It is appropriate to use a modifier with a C concentration of at least 5%, but the amount of the modifier added is (FeO + Mn) in the ladle slag.
O) It is essential to contain an amount of metallic Al capable of suppressing the concentration to a desired concentration.

Further, it is desirable to contain a small amount of CaO and SiO ₂ so that the reaction with the slag is quick.
Therefore, while ensuring the amount of Al in the modifier, T.P. It is necessary to suppress the amount of C.

FIG. 5 shows T.V. in the modifier. The required amount of Al in the slag modifier per 1 ton of molten steel and the T.C. It is a graph which shows the relationship with C amount. Incidentally, as a conventional example, T.I. There is a modifier having a composition having a C concentration of 6.7% and an Al concentration of 36 to 40%. There are modifiers with compositions of C concentration: 3.8% and 2.5% and Al concentration: 40 to 50%.

As shown in FIG. 5, in the conventional modifier,
While securing the required amount of Al, T.T. The region where the amount of C can be reduced to 50 g or less is extremely small. However, when the modifier of the present invention is used, the T.I. The area where the amount of C can be reduced to 50 g or less becomes large.

The change in the [C] concentration in the molten steel is 5 ppm
As described above, the non-volatile T.D. It is appropriate to use a modifier having a C concentration of 2% or less, but the amount of the modifier added depends on the amount of (FeO) in the ladle slag.
+ MnO) metal Al capable of suppressing the concentration to a desired concentration
It is essential to include the amount.

Further, it is desirable to contain a small amount of components such as CaO and SiO ₂ so that the reaction with the slag is accelerated. FIG. 6 shows the non-volatile T.C. C. The required amount of Al in the slag modifier per 1 ton of molten steel and the non-volatile T.C. It is a graph which shows the relationship with C amount.

Conventionally, a non-volatile T.I.
C concentration: 2.8% and Al concentration: 36 to 40%. C concentration: 1.8% and 0.38% and Al concentration: 40 to 50
There are %%.

As shown in FIG. 6, with the conventional modifier, the required amount of Al was ensured and the T.I. C
Although the region where the amount can be reduced to 20 g or less is extremely small, when the modifier of the present invention is used, while securing the necessary Al amount,
T. per 1 ton of molten steel The area where the amount of C can be reduced to 20 g or less becomes large.

Finally, the main purpose of the bubbling stirring in the slag reforming is to stir the modifier and the slag, and the disturbance of the interface between the molten steel and the slag promotes the reoxidation reaction of the molten steel to form inclusions, Or slag entrainment.
Therefore, it is necessary to reform the slag with the smallest possible amount of bubbling.

Similarly, disturbance of the interface between molten steel and slag increases the chance of the slag modifier contacting the molten steel without reacting with the slag. Therefore, the C pickup amount is large and may vary.

Then, the relationship with the C pickup amount was arranged using the stirring power ε per 1 ton of molten steel expressed by the following equation (1) as an index of the stirring amount. ε = 371 · Q _c · T _L / W _L × {ln (1 + 9.8 · ρ _L · H / P) + (1-T _G / T _L )｝ (1) where ε: stirring power (W / t) Q: Inert gas flow rate for stirring (m ³ (standard state) / s) WL: Weight of molten steel (t) ρ _L : Density of molten steel (about 7000 kg / m ³ ) _TL : Temperature of molten steel (K) _TG : Gas temperature before blowing (K) H: Blowing lance depth (m) R: Bath radius (m) P: Atmospheric pressure (1.013 × 10 ⁵ N / m ³ ) In this test, molten steel amount: 270 mass tons, Bath depth: 3.
The bubbling gas flow rate was changed under the ladle conditions of 2 m, radius: 1.95 m, lance immersion depth: 2.4 m, and the stirring power of molten steel was calculated according to the equation (1).

A slag modifier containing 50% metallic Al and having a TC concentration of 2.5% (A) and a non-volatile C
The test was performed using two types (B) having a concentration of 0.38% and a constant bubbling time of 60 seconds. The amount of the modifier added was 1 kg per ton of molten steel, and the amount of T.I. The C amount is 25 g, and the non-volatile C amount is 3.8 g.

FIG. 7 shows the stirring power when two types of slag modifiers (A) and (B) were used and the [C] in the molten steel.
6 is a graph showing a relationship with a change in density: △ [C].
As shown in the figure, if the stirring power per 1 ton of molten steel is 100 W or less, Δ [C], that is, the C pickup amount can be reduced to 5 ppm or less.
If it exceeds 00W, the variation in the amount of C pickup increases, and the T.C. Even if the amount of C or the amount of non-volatile C is appropriate, it is difficult to reduce the amount of C pickup to 5 ppm or less.

On the other hand, when the stirring power per 1 ton of molten steel is 50 W or less, the variation of the C pickup amount is further reduced, which is more desirable for controlling the C concentration with high accuracy.

The gas flow rate Q under which the stirring power per 1 ton of molten steel was 100 W and 50 W under the test conditions.
Was about 0.020 m ³ (standard state) / s and about 0.011 m ³ (standard state) / s, respectively.

The bubbling after the addition of the modifier is suitably performed by stirring with an inert gas from a lance or a bolus plug at the bottom of the pot. When a lance is used, the shape and flow rate of the lance are not particularly limited. However, from the viewpoint of stirring the slag, it is preferable that the lance is a three- or four-hole lance and the slag is stirred in the entire region.

[0089]

EXAMPLE An example of applying the present invention based on an example in which a 270 mass ton ultra-low carbon steel is melted by an RH degassing apparatus and a slab is manufactured by a continuous casting machine will be described below.

In the converter, [C]: 0.03-0.06%,
[Mn]: 0.01-0.2%, [Si]: 0.01-
After refining to 0.03%, temperature: 1650 ° C to 1680 ° C, tapping is performed on a ladle by applying a conventional converter slag outflow prevention method. The amount of outflow slag is 8-12 per 1 ton of molten steel.
kg.

Next, the ladle was transported to an RH vacuum degassing device, and [C] ≦ 0.005 at a high vacuum according to a conventional method.
Decarburization was performed to the desired carbon concentration of 5%. Thereafter, Al was added from the vacuum chamber to perform deoxidation, and [Al]: 0.02%
-0.06%. Reflux for a predetermined time after adding Al
Processing terminated. After the treatment, the slag modifier was sprayed on the upper surface of the slag, and the ladle was transported to a bubbling treatment device.

The slag modifier contains 50% metal Al, 3% CaO, 7% SiO ₂ , and the balance Al ₂ O.
Modifier particles consisting of No. 3 and a binder (particle size: 2 to 20 mm)
Was used.

Table 1 shows the component composition of the slag modifier used in the tests of the examples.

[0094]

[Table 1] Table 2 shows the amount of the slag modifier used in each of the examples and the T.E. per 1 ton of molten steel in the slag modifier. The C amount and the nonvolatile C amount are shown.

[0095]

[Table 2] Table 3 shows the bubbling conditions for slag reforming when the tests of each example were performed.

[0096]

[Table 3] Note that Ar gas was used as the bubbling gas, and the stirring power was calculated using the above equation (1) with the molten steel temperature set at 1873K and the gas temperature set at 298K.

The bubbling time was 60 seconds. In each test, a three-hole lance was used as the lance, and the immersion depth was 2400 mm. In order to evaluate the C pickup during slag reforming, before the RH decarburization treatment is completed and before adding the slag modifying agent (hereinafter simply before slag reforming) and after adding the slag modifying agent and bubbling A sample (hereinafter simply after slag reforming) was collected, and the difference in carbon concentration in this sample was defined as the C pickup amount (ppm).

FIG. 8 is a graph showing the relationship between each example and the variation of the [C] concentration in the molten steel: Δ [C]. In each of Inventive Examples 1 and 2, the C pickup amount, which is essential for controlling the trace carbon concentration of the ultra-low carbon steel, satisfied 5 ppm or less. In particular, in Example 2 of the present invention, it was as low as 2 ppm or less.

On the other hand, in the comparative example, T.D. The C concentration and the nonvolatile C concentration were high, and as a result, the C pickup amount was 5 ppm or more.

[0100]

According to the present invention, it has become possible to reduce the amount of C pickup to molten steel after decarburization and deoxidation during the production of ultra-low carbon steel to 5 ppm or less.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. C concentration and [C] in molten steel
6 is a graph showing a relationship with a change in density: △ [C].

FIG. 2: Non-volatile T.C. in slag modifier. It is a graph which shows the relationship between C concentration and the variation of [C] concentration in molten steel: △ [C].

FIG. 3 shows T.T./ton of molten steel brought from a slag modifier. Change of C amount and [C] concentration in molten steel: △
It is a graph which shows the relationship with [C].

FIG. 4 shows a non-volatile T.V./ton of molten steel brought from a slag modifier. It is a graph which shows the relationship between the amount of C and the variation of [C] concentration in molten steel: △ [C].

FIG. 5: T.A. The required amount of Al in the slag modifier per 1 ton of molten steel and the T.C. It is a graph which shows the relationship with C amount.

FIG. 6: Non-volatile T.C. Required Al in slag modifier per 1 ton of molten steel with C concentration as a parameter
Amount and non-volatile T. in the modifier. It is a graph which shows the relationship with C amount.

FIG. 7 is a graph showing the relationship between stirring power and the amount of change in [C] concentration in molten steel: Δ [C] when two types of slag modifiers (A) and (B) are used. .

FIG. 8 shows the amount of change in [C] concentration in molten steel in each of the examples: Δ
It is a graph which shows the relationship with [C].

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kenji Saka 3 Oaza Hikari, Kashima City, Ibaraki Prefecture Sumitomo Metal Industries Kashima Works F-term (reference) 4K013 AA07 BA02 BA08 CE07 CF01 EA30

Claims (7)

[Claims] Claims: 1. A steelmaking furnace that has not yet been deoxidized, and
A slag modifier used when performing slag reforming after decarburizing to an extremely low carbon region using a vacuum degassing device and subsequently performing deoxidation treatment, and the total carbon of the slag modifier is A slag modifier having a concentration of 5% by mass or less. 2. A steelmaking furnace that has not yet been deoxidized, and
After decarburizing to an extremely low carbon region using a vacuum degassing device and then performing deoxidation, it is a slag modifier used when performing slag reforming. 2
A slag modifier characterized by being not more than mass%. 3. A steelmaking furnace that has not yet been deoxidized, and
Production of ultra-low carbon steel that uses a vacuum degasser to decarburize to an ultra-low carbon region, then performs deoxidation, and then adds slag modifier while bubbling molten steel to reform slag A method for producing a highly clean ultra-low carbon steel, wherein the total amount of carbon contained in the slag modifier is not more than 50 g per 1 ton of molten steel. 4. A method for producing steel from a steelmaking furnace without deoxidation,
Production of ultra-low carbon steel that uses a vacuum degasser to decarburize to an ultra-low carbon region, then deoxidizes, and then adds slag modifier while bubbling molten steel to reform slag The method for producing a high-purity ultra-low carbon steel according to the method, wherein the total amount of non-volatile carbon contained in the slag modifier is 20 g or less per 1 ton of molten steel. 5. The method for producing a highly clean ultra-low carbon steel according to claim 3, wherein the slag modifier according to claim 1 is used. 6. The method for producing a highly clean ultra-low carbon steel according to claim 4, wherein the slag modifier according to claim 2 is used. 7. The method for producing a high-purity ultra-low carbon steel according to claim 3, wherein the bubbling of the molten steel is performed with a stirring power per 1 ton of molten steel at 100 W or less.