JP3994641B2 - Manufacturing method of high clean ultra low carbon steel - Google Patents

Description

表２に各実施例におけるスラグ改質剤の使用量と、スラグ改質剤中の溶鋼１質量トン当たりのＴ．Ｃ量および不揮発性のＣ量を示す。
【００９５】
【表２】

表３に各実施例の試験を行った際のスラグ改質のためのバブリング条件を示す。
【００９６】
【表３】

なお、バブリングガスにはＡｒガスを用い、攪拌動力の計算は、溶鋼温度１８７３Ｋ、ガス温度２９８Ｋ一定として前記（１）式を用いた。
【００９７】
バブリング時間は６０秒間実施した。いずれの試験でもランスには３孔ランスを用い、浸漬深さは２４００mm とした。
スラグ改質時のＣピックアップを評価するために、ＲＨ脱炭処理が終了しスラグ改質剤を添加する前（以下、簡単にスラグ改質前）とスラグ改質剤を添加してバブリングした後（以下、簡単にスラグ改質後）のサンプルを採取し、このサンプル中の炭素濃度の差をＣピックアップ量（ppm ）とした。
【００９８】
図８は、各実施例と溶鋼中の［Ｃ］濃度の変化量：△［Ｃ］との関係を示すグラフである。
本発明例１および２は、いずれも極低炭素鋼の微量炭素濃度制御に必須であるＣピックアップ量が５ppm 以下を満足した。特に本発明例２では、２ppm 以下と非常に低かった。
【００９９】
一方、比較例では、改質剤中のＴ．Ｃ濃度および不揮発Ｃ濃度が高く、その結果Ｃピックアップ量は５ppm 以上となった。
【０１００】
【発明の効果】
本発明により、極低炭素鋼溶製時の脱炭・脱酸処理以降の溶鋼へのＣピックアップ量を５ppm 以下に低減することが可能となった。
【図面の簡単な説明】
【図１】スラグ改質剤中のＴ．Ｃ濃度と溶鋼中の［Ｃ］濃度の変化量：△［Ｃ］との関係を示すグラフである。
【図２】スラグ改質剤中の不揮発性Ｔ．Ｃ濃度と溶鋼中の［Ｃ］濃度の変化量：△［Ｃ］との関係を示すグラフである。
【図３】スラグ改質剤から持ち込まれる溶鋼１質量トン当たりのＴ．Ｃ量と溶鋼中の［Ｃ］濃度の変化量：△［Ｃ］との関係を示すグラフである。
【図４】スラグ改質剤から持ち込まれる溶鋼１質量トン当たりの不揮発性のＴ．Ｃ量と溶鋼中の［Ｃ］濃度の変化量：△［Ｃ］との関係を示すグラフである。
【図５】改質剤中のＴ．Ｃ濃度をパラメータとした溶鋼１質量トン当たりのスラグ改質剤中の必要Ａｌ量と改質剤中のＴ．Ｃ量との関係を示すグラフである。
【図６】改質剤中の不揮発性Ｔ．Ｃ濃度をパラメータとした溶鋼１質量トン当たりのスラグ改質剤中の必要Ａｌ量と改質剤中の不揮発性Ｔ．Ｃ量との関係を示すグラフである。
【図７】（Ａ）および（Ｂ）の２種類のスラグ改質剤を使用したときの攪拌動力と溶鋼中の［Ｃ］濃度の変化量：△［Ｃ］との関係を示すグラフである。
【図８】各実施例と溶鋼中の［Ｃ］濃度の変化量：△［Ｃ］との関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
  In the present invention, the increase in carbon concentration during the slag reforming treatment performed after the vacuum decarburization treatment can be suppressed to 0.0005% by mass (hereinafter simply referred to as% by mass) or less, that is, 5 ppm or less.Using slag modifierThe present invention relates to a method for producing a highly clean ultra low carbon steel.
[0002]
[Prior art]
Extremely low carbon steel is often used as an automotive exterior material, and these exterior materials are required to have a steel sheet with few surface defects and excellent formability. In order to meet these demands, in the steelmaking process, it is necessary to obtain a high cleanliness and to refine with a high accuracy up to an extremely low carbon concentration range.
[0003]
Usually, in order to melt extremely low carbon steel, steel is produced as undeoxidized molten steel having a C content of 0.03 to 0.06% from a steelmaking furnace such as a converter.
After that, as secondary refining, the carbon concentration is set to 0.0001 using the reaction of dissolved oxygen [O] and carbon [C] in the molten steel: [C] + [O] → CO using a vacuum degassing apparatus. Is accurately reduced to a target composition of 0.005%. After the decarburization process by vacuum refining, the secondary refining process is completed through further Al deoxidation and component adjustment, and then subjected to continuous casting.
[0004]
In the vacuum degassing process for melting this ultra-low carbon steel, generally, a molten steel circulating type vacuum degassing apparatus (hereinafter abbreviated as RH) that sucks and circulates the molten steel into the vacuum tank by the principle of a gas lift pump. ) Is used. This RH is composed of two dip tubes and a vacuum tank, but the ladle slag exists on the molten steel between the dip tube and the ladle, and the slag does not flow or stir.
[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. To obtain such a state in a steelmaking furnace such as a converter. Is a lower oxide concentration in slag (generally expressed as the sum of (FeO) and (MnO), hereinafter also referred to as (FeO + MnO) concentration) in the slag is as high as 15 to 20%.
[0006]
When vacuum decarburization is performed in RH with a high (FeO + MnO) concentration in the slag as described above, a high (FeO + MnO) concentration in the slag is maintained even after vacuum decarburization. Even when Al is added to deoxidize the molten steel, the slag is low in fluidity and poor in reactivity, so the reduction reaction rate between the lower oxide in the slag and Al is slow, and (FeO + MnO) in the slag Concentration is difficult to decrease.
[0007]
Therefore, the slag containing a large amount of the remaining lower oxide continues to supply oxygen to the molten steel from the deoxidation process following the vacuum degassing process to the time of casting. By supplying such oxygen, the molten steel undergoes reoxidation and a large amount of Al.₂O_ThreeSystem inclusions are generated in the molten steel, and the cleanliness of the ultra-low carbon steel deteriorates.
[0008]
That is, Al in molten steel that is initially generated by Al deoxidation₂O_ThreeThe system inclusions are removed by the recirculation treatment with RH, but the inclusions in the molten steel generated in the ladle after the RH treatment due to reoxidation from the slag after the RH pass through the tundish and mold without being sufficiently removed, Al in the cast slab₂O_ThreeIt will remain as a system inclusion.
[0009]
This Al₂O_ThreeSystem inclusions directly cause various surface defects in the vicinity of the slab surface.
In continuous casting operations, Al₂O_ThreeThere is a problem in that the inclusions in the system are the main cause of the clogging of the immersion nozzle, making it impossible to perform multiple casting and hindering operation.
[0010]
Moreover, in order to suppress this nozzle obstruction | occlusion, although Ar gas blowing is implemented from the immersion nozzle upper part, if the molten steel cleanliness is inadequate, Ar gas blown in will be needed in large quantities. If this stays in the vicinity of the surface of the slab as air bubbles, it similarly causes surface defects.
[0011]
In order to deal with such surface defects, care may be performed at the slab or hot-rolled coil stage, but a great deal of cost is required and some countermeasure is required.
[0012]
[Problems to be solved by the invention]
As this countermeasure, for example, in JP-A-6-256837, by adding a slag deoxidizer to the ladle slag at the time of undeoxidized steel from the converter, the total Fe concentration in the slag (hereinafter, (Also referred to as T.Fe) concentration) is reduced to 5% or less, and then [C] concentration in the molten steel is reduced to less than 0.006% by performing oxygen blowing during decarburization by a vacuum degassing apparatus. Then, a method of adding a slag modifier for further slag reforming and subsequently deoxidizing is disclosed. However, this method does not show any solution for the C pick-up problem of molten steel that may occur due to slag reforming after decarburization, and mentions the effect of reducing 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 control of the carbon concentration.
[0013]
In addition, as a method of melting ultra-low carbon and ultra-low sulfur steel, Japanese Patent Application Laid-Open No. 7-316636 discloses that after decarburizing with RH, oxygen is blown during deoxidation to raise the molten steel temperature, and then Al To improve the ladle slag by adding cocoon and CaO to the slag, the basicity (CaO / SiO₂A method of performing desulfurization by flux injection with a mass ratio of 3 to 6 and ((T.Fe) + (MnO)) ≦ 1.5 is disclosed.
[0014]
In this method, Al soot having a total carbon concentration (hereinafter also referred to as TC concentration) of 0.4 to 0.8% is used as a part of the slag modifier. The C content of T. is not described, and is included in all of the slag modifiers. The C concentration is not clear.
[0015]
Furthermore, in the examples, the C concentration increased by 7 ppm after desulfurization by flux injection after RH decarburization, and it is necessary to manufacture ultra-low carbon steel that requires precise control of the C concentration at a level of 5 ppm or less. It is difficult to adopt the method disclosed in the publication.
[0016]
  The present invention relates to a slag modifier that can suppress the amount of C pickup to molten steel at 5 ppm or less during slag reforming after decarburization in the production of ultra-low carbon steel using a vacuum degassing apparatus.UsedAn object of the present invention is to provide a method for producing ultra-low carbon steel.
[0017]
[Means for Solving the Problems]
As a result of studying a slag modifier capable of suppressing the C pickup amount to 5 ppm or less, the present inventor has obtained the following knowledge.
[0018]
(A) The slag modifier generally refers to a reducing agent containing a metal having a strong affinity for oxygen, such as Al, which can reduce (FeO) and (MnO) in the slag.
[0019]
Also, metal oxides such as Al₂O_ThreeIs CaO or SiO₂When it coexists with such a melting accelerator, it dissolves easily and can promote the reduction reaction.
Therefore, the slag modifier does not simply refer to a reducing agent containing a metal having a strong affinity for oxygen but includes a melting accelerator.
[0020]
(B) The slag modifier includes a binder for forming the slag modifier into a certain shape so that the slag modifier can be easily handled.
(C) The carbon content in the slag modifier is derived from the raw materials of these reducing agent, melting accelerator and binder, and the existence forms are classified as follows.
[0021]
One of them is aluminum carbide in Al ash as reducing agent (eg Al_FourC_Three).
Two of them are CaCO in the melting accelerator._ThreeSuch as carbonates.
[0022]
The three are mainly volatile oils when organic binders are used.
(D) As described above, since various slag components are contained in the slag modifier, considering the slag modifier raw material, this is as much as possible without reducing the performance as a slag modifier. If the amount is too small, C pick-up of the molten steel during slag reforming can be prevented.
[0023]
(E) Further, since the slag modifier is exposed to a high temperature atmosphere on the slag before the slag modifier is added to the slag, most of the volatile C content contained at this time is volatilized and remains C. If only the minute is used as an index, the amount of C pickup can be suppressed more precisely.
[0026]
  The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) Unsteeled steel from the steelmaking furnace, decarburized to a very low carbon range using a vacuum degassing device, followed by deoxidation treatment, and then added slag modifier while performing molten steel bubbling In the method for producing ultra-low carbon steel that performs slag reforming, the slag modifier is% By massAl: 20 to 65%, Al₂O₃: 18-40%, CaO: 5-25%, SiO₂: 1-16%, CaF₂: 0 to 12%, binder: 1 to 5%, and an inevitable impurity composition, and the total amount of carbon contained in the slag modifier is 50 g or less per 1 ton of molten steelIn addition, the increase in the carbon concentration of the molten steel in the slag reforming is suppressed to 5 ppm or less.A method for producing a highly clean ultra-low carbon steel.
[0027]
(2) Unsteeled steel from the steelmaking furnace, decarburized to a very low carbon range using a vacuum degassing device, followed by deoxidation treatment, and then added slag modifier while performing molten steel bubbling In the method for producing ultra-low carbon steel that performs slag reforming, the slag modifier is% By massAl: 20 to 65%, Al₂O₃: 18-40%, CaO: 5-25%, SiO₂: 1-16%, CaF₂: 0 to 12%, binder: 1 to 5% and unavoidable impurities, and the total amount of non-volatile carbon contained in the slag modifier is 20 g or less per 1 ton of molten steel is thereIn addition, the increase in the carbon concentration of the molten steel in the slag reforming is suppressed to 5 ppm or less.A method for producing a highly clean ultra-low carbon steel.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
In automotive steel sheets and the like to which ultra-low carbon steel is applied, there has recently been a demand from customers for both deep drawability and high strength.
[0031]
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 this extremely low carbon concentration region is necessary for increasing the strength.
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 range according to the required strength for each steel type. The In order to control the carbon concentration in a minute amount range, it is performed by adjusting the decarburization processing time while considering the ultimate vacuum at RH.
[0032]
In particular, in a BH steel sheet (baking hardening type steel sheet), the steel sheet is strengthened by the residual carbon content at the stage of baking coating. For example, the carbon concentration in the steel is required to be controlled at 0.0015 to 0.0030%. The Decarburization refining control in the extremely low carbon region must be performed within the target value ± 5ppm, and pickup exceeding 5ppm after decarburization refining is far from acceptable.
[0033]
In addition, in order to control such an extremely low carbon concentration, after performing excessive decarburization in anticipation of the amount of the C pickup, the method of adding and adjusting the C component is useful for decarburization control in the extremely low carbon region. Takes a considerable amount of time and leads to an increase in manufacturing cost, so that it is difficult to implement from the viewpoint of manufacturing cost and controllability.
[0034]
On the other hand, the slag modifier of the present invention is premised on application to slag reforming after the decarburization period by a vacuum degassing apparatus, which is an extremely low carbon steel manufacturing process.
Therefore, the slag modifier in the present invention means a slag modifier added to the ladle slag for slag modification after the vacuum decarburization period.
[0035]
The slag modifier contains a metal Al or the like having a strong affinity for oxygen as a reducing agent in order to reduce lower oxides (transition metal oxides such as FeO and MnO that are easily reduced) in the slag. CaO and SiO so that the reduction reaction becomes easier₂And the like, and a binder that adjusts the particle size, shape, and strength so as to facilitate the use of the slag modifier.
[0036]
As the reducing agent, Al is most suitable when applied to Al killed steel, and Al ash (Al 滓), which is a byproduct of the Al remelting step, is often used as the Al source.
[0037]
Al ash is metal Al, Al₂O_ThreeAnd the balance is SiO₂Etc., Al around the metal Al₂O_ThreeTherefore, it is presumed that it also has a supplementary function that prevents the dissolution of metal Al in molten steel and promotes reaction with slag.
[0038]
As a melting accelerator, main components are oxides (CaO, SiO₂And CaF₂Etc.). These oxide concentrations are selected in consideration of the slag composition in the applied steelmaking process.
[0039]
The binder is necessary for forming the above reducing agent and melting accelerator as particles and obtaining the strength necessary for use as a slag modifier. As the binder, the same kind as that used for refractory production is widely used, and specifically, resin, oil, starch, water glass and the like are used according to the granulating method.
[0040]
The C content contained in the slag modifier is as follows.
Al ash as a reducing agent: a by-product generated in the Al redissolution process using combustion of heavy oil as a heat source, and contains C. As a form of C component, Al_FourC_ThreeAnd free C. In order to obtain a particularly low Al ash content for C, there is a problem that sorting and the like are required, yield is deteriorated, and price is increased.
[0041]
CaCO as a melting accelerator_Three: CaCO mainly as a source of CaO_ThreeWhen the minute is used, the C component is contained in the slag modifier.
The C component as the binder is mainly organic.
[0042]
The composition of the reducing agent is, for example, that Al content in usually available Al ash is 30 to 70%, and the balance is Al.₂O_Three: 20-60%, SiO₂: It may be 0.5 to 5%.
The melting accelerator is Al in Al ash.₂O_Three30% to 60% of CaO, SiO₂1 to 20% may be blended. If this blending ratio, Al formed on the surface of the Al metal particles₂O_ThreeCan react with the film to lower the melting point.
[0043]
CaF₂About, you may add about 0 to 50% with respect to 100% of CaO content to mix | blend.
Furthermore, the compounding amount of the binder varies depending on the binder to be used, the molding method, the particle size of Al ash and other melting accelerators, and the strength required after molding, but is about 1 to 5% of the slag modifier.
[0044]
From the above, the composition of the slag modifier is Al: 20 to 65%, Al₂O_Three: 18-40%, CaO: 5-25%, SiO₂: 1-16%, CaF₂: 0-12% and binder: 1-5% and unavoidable impurities.
[0045]
The T.I. The C concentration is 5% or less, preferably 3% or less. Al ash, which is the main component of the slag modifier, is a by-product generated in the Al remelting process as described above, and is about 15% to 75% metal Al and its oxide Al.₂O_Three, Aluminum carbide (eg Al_FourC_Three), Aluminum nitride (eg AlN) and inevitably contained SiO₂Etc.
[0046]
The metal Al content in the Al ash varies depending on the time and method of collection of the Al ash, but the Al ash is substantially determined by the content of the metal Al content and is included depending on the degree of slag modification. Since the metal Al content to be determined is determined, it should be determined cost-effectively such as the allowable amount of C pickup and the degree of slag modification.
[0047]
The content of aluminum carbide in the Al ash varies depending on the Al raw material when Al is remelted, the fuel that is the heat source during melting, and the combustion mode. Conventionally, the content of such aluminum carbide has not been taken into account, or conversely, it has been necessary to be strictly regulated more than necessary, but it may be determined by the allowable amount of C pickup.
[0048]
As for the binder, the T.I. It plays an important role in controlling the C concentration. Conventionally, the C content in these binders has not been considered much. For example, when it is desired to granulate a large amount of Al ash that is inexpensive but contains a large amount of C, a water glass system that does not contain the C content. By using a binder, the T.I. When the C concentration is reduced or, on the contrary, when the main ash is Al ash with a small C content, a resin-based binder containing a C-based resin can be used.
[0049]
In order to confirm the effect of suppressing the C pickup according to the present invention, the following test was conducted.
That is, using a high-frequency induction furnace capable of holding about 15 kg of ultra-low carbon molten steel at 1600 ° C., various modifiers were added to the slag on the molten steel, and the molten steel was collected after a predetermined time, and a test for examining the C pickup was performed. .
[0050]
The slag used is CaO, Al simulating slag after vacuum degassing treatment₂O_Three, SiO₂, FeO and MnO, and the amount of slag was 30 g per 1 kg of molten steel, which was a ratio that could simulate the amount of slag in actual operation.
[0051]
The used modifier is granulated (2 to 20 mm in diameter) of Al ash and quicklime using a binder, the metal Al content is 45 to 50%, and the balance is Al.₂O_Three, SiO₂And CaO.
[0052]
In addition, Al ash is the T.V. A C concentration of 1.0 to 2.8% was used, the C content in the binder was adjusted, and the modifier T.V. C concentration was controlled.
FIG. 1 shows the T. slag modifier. It is a graph which shows the relationship between C density | concentration and the variation | change_quantity of a [C] density | concentration in molten steel: (DELTA) [C].
[0053]
The amount of change in the [C] concentration in the molten steel is hereinafter also referred to as C pickup amount.
As shown in the figure, in order to suppress the amount of C pickup to the molten steel to 5 ppm or less, the T.I. The C concentration needs to be 5% or less.
[0054]
Further, to make the C pickup amount more desirable 2 ppm or less, it is necessary to make it 3% or less.
Next, the T.I. Factors that suppress the amount of C pickup more accurately than the C concentration were examined.
[0055]
The slag modifier is temporarily exposed to a high temperature atmosphere on the slag before it reacts with the slag at the stage when it is added to the slag on the ladle. The amount of C pick-up can be predicted more quantitatively by considering only the remaining C component as a problem.
[0056]
That is, the non-volatile C concentration was defined as follows.
A slag modifier comprising raw materials such as Al ash, a melting accelerator, and a binder is once molded into a shape used in the steel making process and then roughly pulverized. After the particle size is 100 to 325 mesh (0.044 to 0.149 mm), it is kept at 600 ° C. for 3600 seconds in an inert gas stream such as Ar or nitrogen. The slag modifier T.I. C concentration analysis was performed and T.P. The value obtained for the C concentration is defined as a non-volatile C concentration.
[0057]
Therefore, in order to confirm the relationship between the non-volatile C concentration and the amount of C pickup in the present invention, a high-frequency induction furnace capable of holding about 15 kg of ultra-low carbon molten steel at 1600 ° C. is used, and various modifiers are added to the slag on the molten steel. After a predetermined time, the molten steel was collected and a test for examining the amount of C pickup was performed.
[0058]
The slag used is CaO, Al simulating slag after vacuum degassing treatment₂O_Three, SiO₂, FeO and MnO, and the amount of slag was 30 g per 1 kg of molten steel, and the ratio was equivalent to the amount of slag in actual operation.
[0059]
The used modifier is granulated (2 to 20 mm in diameter) of Al ash and quicklime using a binder, the metal Al content is 45 to 50%, and the balance is Al.₂O_Three, SiO₂And CaO.
[0060]
In addition, Al ash is the T.V. A C concentration of 1.0 to 2.8% was used, the C content in the binder was adjusted, and the modifier T.V. C concentration was controlled.
FIG. 2 shows the non-volatile T.I. It is a graph which shows the relationship between C density | concentration and the variation | change_quantity of a [C] density | concentration in molten steel: (DELTA) [C].
[0061]
As shown in the figure, in order to suppress the amount of C pickup to the molten steel to 5 ppm or less, the nonvolatile T.V. C amount needs to be 2% or less.
Further, in order to make the C pickup amount more preferably 2 ppm or less, it is necessary to make it 0.4% or less.
[0062]
Next, the T.E. per 1 ton of molten steel brought in from the slag modifier. The relationship between the amount of C (g) and the amount of change in [C] concentration in molten steel: Δ [C] will be described below.
In the converter, after decarburization to a carbon concentration of 0.03 to 0.06%, steel is output from the converter at 1650 to 1680 ° C. In order to reduce the total amount of the modifier later, the outflow slag from the converter is made as small as possible by the existing outflow slag control method. At this stage, the concentration of (FeO + MnO) in the ladle slag is high, and decarburization and deoxidation are performed with RH or the like as it is.
[0063]
After the vacuum decarburization process is completed and the deoxidation process is completed, an amount of slag modifier that can reduce the (FeO + MnO) concentration in the ladle slag is sprayed on the slag upper surface.
After that, slag reforming by molten steel bubbling ((FeO + MnO) concentration reduction treatment) is performed.
[0064]
When an ultra-low carbon molten steel having a [C] concentration of 15 to 40 ppm in the molten steel is experimentally produced by such a process, the amount of change in the [C] concentration in the molten steel: Δ [C] and the molten steel brought in from the slag modifier T.D. per ton of mass. The relationship with C amount (g) was investigated.
[0065]
FIG. 3 shows the T.V. per ton of molten steel brought in from the slag modifier. It is a graph which shows the relationship between C amount and the variation | change_quantity of [C] density | concentration in molten steel: (DELTA) [C].
As shown in the figure, the T.O. When the C amount is 50 g or less, Δ [C], that is, the C pickup amount can be suppressed to 5 ppm or less, and particularly 30 g or less, it can be suppressed to 2 ppm or less.
[0066]
T. on the other hand. When the C amount exceeds 50 g, the C pickup amount increases. When the amount of C was 80 to 90 g, 6 to 11 ppm, which was an unacceptable C pickup as an extremely low carbon steel, was recognized.
[0067]
Next, non-volatile T.C. per 1 ton of molten steel brought in from the slag modifier. The relationship between the amount of C (g) and the amount of change in [C] concentration in molten steel: Δ [C] will be described below.
[0068]
In the converter, after decarburization until the carbon concentration is 0.03 to 0.06%, the steel is removed from the converter at 1650 to 1680 ° C. In order to reduce the total amount of the modifier later, the outflow slag from the converter is made as small as possible by the existing outflow slag control method. At this stage, the concentration of (FeO + MnO) in the ladle slag is high, and decarburization and deoxidation are performed with RH or the like as it is.
[0069]
After the vacuum decarburization process is completed and the deoxidation process is completed, an amount of slag modifier that can reduce the (FeO + MnO) concentration in the ladle slag is sprayed on the slag upper surface.
After that, slag reforming by molten steel bubbling ((FeO + MnO) concentration reduction treatment) is performed.
[0070]
When an ultra-low carbon molten steel having a [C] concentration of 15 to 40 ppm in the molten steel is experimentally produced by such a process, the amount of change in the [C] concentration in the molten steel: Δ [C] and the molten steel brought in from the slag modifier Non-volatile T.C. The relationship with C amount (g) was investigated.
[0071]
FIG. 4 shows the non-volatile T.V. per ton of molten steel brought in from the slag modifier. It is a graph which shows the relationship between C amount and the variation | change_quantity of [C] density | concentration in molten steel: (DELTA) [C]. As shown in the figure, the T.O. 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, and particularly 10 g or less, it can be suppressed to 2 ppm or less.
[0072]
On the other hand, when the amount of non-volatile C exceeded 20 g, the amount of C pick-up increased, and when the amount of non-volatile C was about 26 g, 6 to 11 ppm, C pick-up unacceptable as an extremely low carbon steel was recognized.
[0073]
In order to suppress the change amount of the [C] concentration in the molten steel to 5 ppm or less, as described above, the T.V. Although it is appropriate to use a modifier having a C concentration of at least 5%, the amount of modifier added is the amount of metal Al that can suppress the (FeO + MnO) concentration in the ladle slag to a desired concentration. It is essential to contain it.
[0074]
In addition, CaO, SiO so that the reaction with slag becomes rapid₂It is desirable to contain a small amount. Therefore, while securing the amount of Al in the modifier, T.I. It is necessary to suppress the amount of C.
[0075]
FIG. 5 shows T.I. The required amount of Al in the slag modifier per 1 ton of molten steel with the C concentration as a parameter and the T.O. It is a graph which shows the relationship with C amount. As a conventional example, T.W. There is a modifier having a composition of C concentration: 6.7% and Al concentration: 36-40%. There are modifiers with compositions of C concentration: 3.8% and 2.5% and Al concentration: 40-50%.
[0076]
As shown in FIG. 5, in the conventional modifier, the T.O. The region where the amount of C can be reduced to 50 g or less is extremely small. However, when the modifier of the example of the present invention is used, the T.O. The region where the amount of C can be reduced to 50 g or less becomes large.
[0077]
In order to suppress the change amount of the [C] concentration in the molten steel to 5 ppm or less, as described above, the nonvolatile T.V. Although it is appropriate to use a modifier having a C concentration of 2% or less, the amount of modifier added is the amount of metal Al that can suppress the (FeO + MnO) concentration in the ladle slag to a desired concentration. It is essential to contain it.
[0078]
Also, CaO and SiO so that the reaction with the slag becomes rapid.₂It is desirable to contain a small amount of such components.
FIG. 6 shows the non-volatile T.V. The required amount of Al in the slag modifier per 1 ton of molten steel with the C concentration as a parameter and the non-volatile T.V. in the modifier. It is a graph which shows the relationship with C amount.
[0079]
Conventional modifiers include non-volatile T.I. There are compositions having a C concentration of 2.8% and an Al concentration of 36 to 40%. There are C concentration: 1.8% and 0.38% and Al concentration: 40-50%.
[0080]
As shown in FIG. 6, with the conventional modifier, the T.O. The region where the amount of C can be made 20 g or less is extremely small. However, when the modifier of the present invention is used, the T.O. The region where the amount of C can be made 20 g or less becomes large.
[0081]
Finally, the main purpose of bubbling agitation in slag reforming is the agitation of the modifier and slag. Disturbance at the interface between the molten steel and slag promotes the reoxidation reaction of the molten steel to generate inclusions and slag entrainment. To wake you up. Therefore, it is necessary to reform slag with as little bubbling amount as possible.
[0082]
Similarly, disturbance of the molten steel / slag interface increases the chance that the slag modifier will contact the molten steel unreacted with the slag. Therefore, the amount of C pickup is large and may vary.
[0083]
Therefore, the relationship with the C pickup amount was arranged by using the stirring power ε per 1 ton of molten steel expressed by the following formula (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^Three(Standard condition) / s)
WL: Molten steel weight (t)
ρ_L: Molten steel density (approx. 7000kg / m^Three)
T_L: Molten steel temperature (K)
T_G: Gas temperature before blowing (K)
H: Blowing lance depth (m)
R: Bath radius (m)
P: Atmospheric pressure (1.013 × 10^FiveN / m^Three)
In this test, the bubbling gas flow rate was changed under the ladle conditions of molten steel amount: 270 mass tons, bath depth: 3.2 m, radius: 1.95 m, lance immersion depth: 2.4 m, and the equation (1) Therefore, the molten steel stirring power was calculated.
[0084]
As the slag modifier, two types of TC concentration containing 50% metal Al (A) having a TC concentration of 2.5% and non-volatile C concentration being 0.38% (B) were used. The test was performed with a bubbling time constant at 60 seconds. The amount of the modifier added is 1 kg per ton of molten steel, and the T.I. The amount of C is 25 g, and the amount of non-volatile C is 3.8 g.
[0085]
FIG. 7 is a graph showing the relationship between the stirring power and the amount of change in [C] concentration in molten steel: Δ [C] when using two types of slag modifiers (A) and (B). .
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 amount of C pickup can be reduced to 5 ppm or less, but if the stirring power of molten steel exceeds 100 W, the C pickup The variation in the amount of T.I. Even if the amount of C or the amount of non-volatile C is made appropriate, it is difficult to reduce the amount of C pickup to 5 ppm or less.
[0086]
On the other hand, when the stirring power per 1 ton of molten steel is 50 W or less, the variation in the amount of C pickup is further reduced, which is more desirable for highly accurate control of the C concentration.
[0087]
In this test condition, the gas flow rate Q at which the stirring power per 1 ton of molten steel is 100 W and 50 W is about 0.020 m, respectively.^Three(Standard state) / s and about 0.011 m^Three(Standard state) / s.
[0088]
In addition, the bubbling after the addition of the modifier is appropriately performed by stirring with an inert gas from a lance or a pan bottom bolus plug.
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 desirable that the lance is a 3 to 4 hole lance and the slag is stirred throughout.
[0089]
【Example】
An embodiment to which the present invention is applied based on an example in which 270 mass tons of 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.
[0090]
[C]: 0.03-0.06%, [Mn]: 0.01-0.2%, [Si]: 0.01-0.03%, temperature: 1650 ° C.-1680 ° C. After refining, steel is put into a ladle using the conventional converter slag outflow prevention method. The outflow slag amount was 8 to 12 kg per 1 ton of molten steel.
[0091]
Next, this ladle was conveyed to the RH vacuum degassing apparatus, and decarburized to a desired carbon concentration of [C] ≦ 0.005% at a high vacuum degree according to a conventional method. Then, Al was added from the vacuum tank and deoxidation was performed to make [Al]: 0.02% to 0.06%. The RH treatment was completed by refluxing for a predetermined time after the addition of Al. After the treatment, a slag modifier was sprayed on the upper surface of the slag, and the ladle was conveyed to a bubbling treatment apparatus.
[0092]
The slag modifier contains 50% metallic Al, CaO: 3%, SiO₂: 7% and balance Al₂The modifier grains (particle size 2-20 mm) consisting of O3 and binder were used.
[0093]
Table 1 shows the component composition of the slag modifier used when the test of each example was conducted.
[0094]
[Table 1]

Table 2 shows the amount of slag modifier used in each example and the T.O per mass ton of molten steel in the slag modifier. C amount and non-volatile C amount are shown.
[0095]
[Table 2]

Table 3 shows the bubbling conditions for slag reforming when the tests of the respective examples were conducted.
[0096]
[Table 3]

In addition, Ar gas was used for the bubbling gas, and the calculation of the stirring power was performed using the above equation (1) assuming that the molten steel temperature was 1873 K and the gas temperature was 298 K constant.
[0097]
The bubbling time was 60 seconds. In any test, a 3-hole lance was used as the lance, and the immersion depth was 2400 mm.
In order to evaluate the C pickup at the time of slag reforming, before the RH decarburization process is completed and before the slag modifier is added (hereinafter simply referred to as slag reformer) and after the slag modifier is added and bubbled A sample (hereinafter simply after slag reforming) was taken, and the difference in carbon concentration in this sample was defined as the amount of C pickup (ppm).
[0098]
FIG. 8 is a graph showing the relationship between each example and the amount of change in [C] concentration in molten steel: Δ [C].
In each of Invention Examples 1 and 2, the amount of C pickup essential for controlling the trace carbon concentration of the ultra-low carbon steel satisfied 5 ppm or less. In particular, in Invention Example 2, it was very low at 2 ppm or less.
[0099]
On the other hand, in the comparative example, the T.I. C concentration and non-volatile C concentration were high, and as a result, the amount of C pickup was 5 ppm or more.
[0100]
【The invention's effect】
By this invention, it became possible to reduce the amount of C pick-up to the molten steel after the decarburization and deoxidation process at the time of melting ultra-low carbon steel to 5 ppm or less.
[Brief description of the drawings]
FIG. 1 shows T. slag modifier. It is a graph which shows the relationship between C density | concentration and the variation | change_quantity of a [C] density | concentration in molten steel: (DELTA) [C].
FIG. 2 shows non-volatile T.V. in slag modifier. It is a graph which shows the relationship between C density | concentration and the variation | change_quantity of a [C] density | concentration in molten steel: (DELTA) [C].
FIG. 3 is a graph showing T.E. per ton of molten steel brought from a slag modifier. It is a graph which shows the relationship between C amount and the variation | change_quantity of [C] density | concentration in molten steel: (DELTA) [C].
FIG. 4 shows non-volatile T.C. per ton of molten steel brought in from a slag modifier. It is a graph which shows the relationship between C amount and the variation | change_quantity of [C] density | concentration in molten steel: (DELTA) [C].
FIG. 5 shows T.V. The required amount of Al in the slag modifier per 1 ton of molten steel with the C concentration as a parameter and the T.O. It is a graph which shows the relationship with C amount.
FIG. 6 shows nonvolatile T.V. The required amount of Al in the slag modifier per 1 ton of molten steel with the C concentration as a parameter and the non-volatile T.V. in the modifier. It is a graph which shows the relationship with C amount.
FIG. 7 is a graph showing the relationship between the 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 is a graph showing the relationship between each example and the amount of change in [C] concentration in molten steel: Δ [C].

Claims (2)

Steel is removed from the steelmaking furnace without deoxidation, decarburized to a very low carbon range using a vacuum degasser, and subsequently deoxidized, and then slag modifier is added while bubbling molten steel. In the method for producing ultra-low carbon steel that performs slag reforming, the slag modifier is mass%, Al: 20 to 65%, Al ₂ O ₃ : 18-40%, CaO: 5-25%, SiO ₂ : 1-16%, CaF ₂ : 0-12%, binder: 1-5% and a composition consisting of unavoidable impurities, and the total amount of carbon contained in the slag modifier is 50 g or less per 1 ton of molten steel, A method for producing a highly clean ultra-low carbon steel characterized by suppressing an increase in carbon concentration of molten steel in the slag reforming to 5 ppm or less. Steel is removed from the steelmaking furnace without deoxidation, decarburized to a very low carbon range using a vacuum degasser, and subsequently deoxidized, and then slag modifier is added while bubbling molten steel. In the method for producing ultra-low carbon steel that performs slag reforming, the slag modifier is mass%, Al: 20 to 65%, Al ₂ O ₃ : 18-40%, CaO: 5-25%, SiO ₂ : 1-16%, CaF ₂ : 0 to 12%, binder: 1 to 5% and unavoidable impurities, and the total amount of non-volatile carbon contained in the slag modifier is 20 g or less per 1 ton of molten steel In addition, a method for producing a highly clean ultra-low carbon steel characterized by suppressing an increase in carbon concentration of molten steel in the slag reforming to 5 ppm or less.
  Here, the “non-volatile total carbon concentration” means that the slag modifier is once formed into a shape used in a steelmaking process, and then roughly pulverized, and the particle size is 100 to 325 mesh (0.044). To 0.149 mm), held at 600 ° C. for 3600 seconds in an inert gas stream of Ar or nitrogen, and a value obtained by performing a total carbon concentration analysis of the slag modifier after this heat treatment. .

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