JP2004176133A - Method for producing stainless steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a stainless steel with an AOD method in which erosion of a refractory in an AOD vessel is restrained and also, a dumping cost is reduced and the environmental problem is reduced by utilizing waste electroslag remelting (ESR) slag caused by the production of a high cleanliness steel with ESR method. <P>SOLUTION: Al<SB>2</SB>O<SB>3</SB>together with CaF<SB>2</SB>are charged at reducing period into molten steel in the AOD vessel and the slag having the composition composed of CaO-SiO<SB>2</SB>-CaF<SB>2</SB>-Al<SB>2</SB>O<SB>3</SB>-MgO which contains Al<SB>2</SB>O<SB>3</SB>by 3-30 wt%, desirably 5-20 wt%, is formed on the molten steel and refining is performed while restraining the fusion of MgO from the refractory for furnace wall. As the supplying source of CaF<SB>2</SB>and Al<SB>2</SB>O<SB>3</SB>, the waste ESR slag is utilized. <P>COPYRIGHT: (C)2004,JPO

Description

スラグは精錬作用をもつが、使用の前後において、組成は実質上変化しない。使用後のスラグは、ほとんど利用価値のないものとして廃棄されているが、廃棄それ自体がコストを要するものであり、かつ、環境への負担を考えるとなおさら、有効利用の途が求められている。
【０００６】
発明者らは、ＭｇＯを含む耐火物と溶融スラグとが接しているとき、スラグ中のＡｌ_２Ｏ_３含有量が少ない領域から多い領域に向かってＭｇＯのスラグ中飽和濃度がいったん低下し、さらに増大すると再び上昇することを知り、適量のＡｌ_２Ｏ_３を還元期のスラグ中に存在させることによってＭｇＯの溶出を抑制することを考え、一方、このＡｌ_２Ｏ_３含有量をＥＳＲのスラグ廃棄物から供給することを着想した。
【０００７】
そこで、ＡＯＤ法の実施に当たって還元期のスラグ形成のために投入する材料の一部を、ＥＳＲのスラグ廃棄物で置き換えることを実験した。この実験は成功し、期待どおり、ＭｇＯの溶出を抑制することができた。図１は、還元期スラグ中に存在するＡｌ_２Ｏ_３の量と、耐火物中のＭｇＯが溶出する割合との関係をプロットしたグラフであって、このグラフから、スラグ中３〜５％のＡｌ_２Ｏ_３の存在がＭｇＯの溶出をよく押さえていることがわかる。
【０００８】
【発明が解決しようとする課題】
本発明の第一の目的は、発明者らが得た上述の知見を活用し、ＡＯＤ法によるステンレス鋼の製造方法を改良して、ＡＯＤベッセルの耐火物の溶損を抑制した製造方法を提供することにある。本発明の第二の目的は、ＡＯＤ法の還元期にスラグ形成材料の一部にＥＳＲスラグ廃棄物を利用することにより、その再利用をはかり、廃棄コストを削減ないしゼロにし、かつ、環境問題を軽減することにある。
【０００９】
【課題を解決するための手段】
上記の目的を達成する本発明のステンレス鋼の製造方法は、ＡＯＤ法によるステンレス鋼の製造方法であって、ＡＯＤベッセル内の溶鋼に対して、脱炭期にはＣａＯおよびＭｇＯを投入し、還元期にはフェロシリコンを添加するともにＣａＦ_２を投入してスラグを形成し、精錬を行なう製造方法において、還元期にＣａＦ_２とともにＡｌ_２Ｏ_３を投入して、その３〜３０重量％をＡｌ_２Ｏ_３が占める、ＣａＯ−ＳｉＯ_２−ＣａＦ_２−Ａｌ_２Ｏ_３−ＭｇＯからなるスラグを溶鋼上に形成し、炉壁の耐火物中からのＭｇＯ溶出を抑制しつつ精錬を行なうことを特徴とする。
【００１０】
【発明の実施形態】
還元期のスラグ中に存在させるべきＡｌ_２Ｏ_３の量は、上記の下限であるＡｌ_２Ｏ_３３重量％でもその効果が認められるが、５重量％以上で顕著になる。多量に存在させてもその効果が飽和するので、上記のように３０％の上限を設けた。一般に、２０重量％までを占めるようにすることが好ましい。
【００１１】
還元期のスラグ組成として適切な範囲は、このようなわけで、ＣａＯ：３０〜７０％（好ましくは４５〜５５％）、ＳｉＯ_２：１５〜２５％（好ましくは１８〜２２％）、ＣａＦ_２：１〜２５％（好ましくは５〜１５％）、Ａｌ_２Ｏ_３：３〜３０％（好ましくは５〜２０％）、ＭｇＯ：５〜１５％（好ましくは７〜１２％）となる。
【００１２】
投入するＡｌ_２Ｏ_３の源は、任意に選択することができる。バイヤー法アルミナや、その前駆体である水酸化アルミニウムにように純粋なものには問題ないことはもちろんであり、アルミドロスのような原料も、ＡＯＤ法の実施にとって支障がないかぎり、使用可能である。
【００１３】
Ａｌ_２Ｏ_３の源としてＥＳＲ法のスラグ廃棄物を利用する場合、当然にＣａＦ_２もそこから供給される。この場合に、還元期スラグを形成するためにベッセルに投入する材料のうち、どのくらいの割合でＥＳＲスラグ廃棄物を使用すればよいかは、ＥＳＲスラグ廃棄物の組成にもよるが、還元期スラグの５〜４０％、通常は２０〜３５％を占めるように添加することが適切である。それにより、上記の適切なスラグ組成が実現できるであろう。
【００１４】
耐火物の溶損はまた、スラグの塩基度の低下によっても生じる。塩基度の低下は、還元工程におけるＳｉＯ_２の生成に起因するものである。塩基度を高めることにより、スラグの液相中に飽和するＭｇＯの量を低減させることができるから、Ａｌ_２Ｏ_３のスラグへの添加は、この面からも有用である。
【００１５】
還元期スラグが示す塩基度と、耐火物中のＭｇＯが溶出する割合との関係をプロットして、図２のグラフを得た。この場合の塩基度は、（ＣａＯ＋ＭｇＯ）／（ＳｉＯ_２＋Ａｌ_２Ｏ_３）として定義される。塩基度の増大がＭｇＯ溶出の防止に有効であることが、このグラフから明らかである。
【００１６】
【実施例】
容量２０トンのＡＯＤベッセルを使用し、ＳＵＳ３０４の溶鋼２０トンを製造した。脱炭期のはじめにＣａＯ８００ｋｇおよびＭｇＯ２００ｋｇを投入し、還元期に、つぎの配合でスラグ形成材料を投入した。

使用したＥＳＲスラグ廃棄物の組成は、７０％ＣａＦ_２＋３０％Ａｌ_２Ｏ_３である。本発明によりベッセルの耐火物の寿命が、従来の８５チャージから１００チャージまで延ばすことができた。
【００１７】
【発明の効果】
本発明の製造方法は、今日のステンレス製造技術の主流を占めるＡＯＤ法を実施するに当り、還元期スラグに適切な量のＡｌ_２Ｏ_３を存在させるという簡単な手段により、ＡＯＤベッセルの耐火物からＭｇＯがスラグ中に溶出することを抑制３し、寿命を延ばすことに成功した。とくに、スラグのＡｌ_２Ｏ_３およびＣａＦ_２の源としてＥＳＲスラグ廃棄物を利用する態様によれば、上記の効果を得た上で、スラグ廃棄物の処理の問題を解消することができ、ＥＳＲのコスト低減およびスラグ廃棄が引き起こす環境問題の解決という、多くの利益が一挙に得られる。
【図面の簡単な説明】
【図１】ＡＯＤ法ステンレス製造の還元期スラグ中に存在するＡｌ_２Ｏ_３の量と、耐火物中のＭｇＯが溶出する割合との関係をプロットしたグラフ。
【図２】ＡＯＤ法ステンレス製造の還元期スラグが示す塩基度と、耐火物中のＭｇＯが溶出する割合との関係をプロットしたグラフ。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an improvement in a method for producing stainless steel by an AOD method, and provides a method for producing a refractory lining a furnace wall of an AOD furnace with less erosion.
[0002]
[Prior art]
As is well known, in the production of stainless steel by the AOD method, in a AOD furnace, a mixed gas of oxygen and argon is blown into molten steel containing chromium to perform decarburization (decarburization stage), and then ferromagnetic steel is used. It consists of adding silicon or aluminum to reduce oxidized chromium and recovering it in molten steel (reduction phase).
[0003]
Slag forming material introduced in these processes, quick lime CaO and magnesia MgO decarburization phase, a ferrosilicon Fe-Si and fluorite CaF ₂ in the reduction stage. C / S = CaO / SiO ₂ = 1.8 to 2.2 is usually selected as the basicity, which is the ratio between CaO and SiO ₂ generated by the Cr reduction reaction of Si in ferrosilicon. Fluorite is used to reduce the melting point of slag and reduce the viscosity so that desulfurization refining by slag and reduction of chromium are performed quickly. One example of the amount of the slag forming material used is: CaO + MgO: 1200 kg, 75% ferrosilicon (75% Si content): 200 to 250 kg, and CaF ₂ : 200 to 300 kg. CaF ₂ is not limited to 100% fluorite, but also has a composition of 80% CaF ₂ -20% SiO ₂ .
[0004]
The operation of the AOD furnace increases in temperature (the maximum temperature during decarburization reaches as high as 1750 ° C), so the refractory (dolomite brick) that constitutes the furnace wall is eroded, and the elution of MgO into slag in particular proceeds. Elution of the portion in contact with the slag, so-called slag line elution occurs. This tendency is remarkable when Al is used as a reducing agent with the intention of producing stainless steel having a low Si content. This is considered to be due to the fact that Al ₂ O ₃ generated by the reduction is present in a large amount in the slag.
[0005]
On the other hand, as one of means for producing extremely clean steel, there is an electroslag remelting method ESR. The slag used for the ESR method is composed of the following components (% by weight),

Although slag has a refining action, its composition does not substantially change before and after use. Slag after use is discarded as having little value, but disposal itself is costly, and considering the burden on the environment, there is a need for more effective use. .
[0006]
The present inventors have found that when the refractory containing MgO and the molten slag are in contact with each other, the saturation concentration of MgO in the slag once decreases from a region where the content of Al ₂ O ₃ in the slag is small to a region where the content is large, and furthermore. knows that rises when increasing again considered to suppress the elution of MgO by the presence of a suitable amount of Al ₂ O ₃ in the slag in the reduction stage, while the slag disposal of ESR the content of Al ₂ O ₃ I conceived to supply from things.
[0007]
Therefore, in conducting the AOD method, an experiment was conducted to replace a part of the material input for slag formation in the reduction period with slag waste of ESR. This experiment was successful and the elution of MgO could be suppressed as expected. FIG. 1 is a graph plotting the relationship between the amount of Al ₂ O ₃ present in the slag in the reduction phase and the rate of elution of MgO in the refractory. It can be seen that the presence of Al ₂ O ₃ well suppresses the elution of MgO.
[0008]
[Problems to be solved by the invention]
A first object of the present invention is to provide a manufacturing method in which the above-mentioned knowledge obtained by the inventors is improved and the method of manufacturing stainless steel by the AOD method is improved to suppress the erosion of the refractory of the AOD vessel. Is to do. A second object of the present invention is to use ESR slag waste as a part of the slag forming material in the reduction period of the AOD method, thereby reusing the waste, reducing or eliminating disposal costs, and reducing environmental problems. Is to reduce.
[0009]
[Means for Solving the Problems]
The method for producing a stainless steel of the present invention for achieving the above object is a method for producing a stainless steel by an AOD method, wherein CaO and MgO are introduced into a molten steel in an AOD vessel during a decarburization period to reduce the steel. the period by both put CaF ₂ adding ferrosilicon to form a slag, in the manufacturing method of performing refining, with CaF ₂ was charged with _Al 2 _{O 3} the reduction period, the 3 to 30 wt% Al _A slag composed of CaO—SiO ₂ —CaF ₂ —Al ₂ O ₃ —MgO occupied by ₂ O ₃ is formed on molten steel, and refining is performed while suppressing MgO elution from the refractory of the furnace wall. And
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The effect of the amount of Al ₂ O ₃ to be present in the slag in the reduction period can be recognized at the above-mentioned lower limit of 3% by weight of Al ₂ O ₃ , but becomes remarkable at 5% by weight or more. Since the effect is saturated even if a large amount is present, the upper limit of 30% is set as described above. Generally, it is preferred to account for up to 20% by weight.
[0011]
Suitable ranges as slag composition of the reducing period, This is why, CaO: 30 to 70% (preferably _{45~55%), SiO 2: 15~25} % ( preferably 18 to 22%), CaF ₂ : 1 to 25% (preferably _{5~15%), Al 2 O 3} : 3~30% ( preferably 5 to 20%), MgO: a 5-15% (preferably 7 to 12%).
[0012]
The source of Al ₂ O ₃ to be charged can be arbitrarily selected. Of course, there is no problem with pure substances such as the Bayer method alumina and its precursor aluminum hydroxide, and raw materials such as aluminum dross can be used as long as the AOD method is not hindered. is there.
[0013]
When utilizing slag waste of the ESR method as a source of Al ₂ O ₃ , CaF ₂ is naturally also supplied therefrom. In this case, depending on the composition of the ESR slag waste, the proportion of the ESR slag waste to be used among the materials to be charged into the vessel to form the reduction slag depends on the composition of the reduction slag. Is suitably added so as to account for 5 to 40%, usually 20 to 35%. Thereby, the above-mentioned appropriate slag composition may be realized.
[0014]
Refractory erosion is also caused by a decrease in the basicity of the slag. The decrease in basicity is due to the formation of SiO _{2 in} the reduction step. By increasing the basicity, the amount of MgO saturated in the liquid phase of the slag can be reduced. Therefore, the addition of Al ₂ O _{3 to} the slag is also useful from this aspect.
[0015]
The graph of FIG. 2 was obtained by plotting the relationship between the basicity of the slag during the reduction period and the rate of elution of MgO in the refractory. The basicity in this case is defined as (CaO + MgO) / (SiO ₂ + Al ₂ O ₃ ). It is clear from this graph that an increase in basicity is effective in preventing MgO elution.
[0016]
【Example】
Using an AOD vessel having a capacity of 20 tons, 20 tons of SUS304 molten steel was manufactured. At the beginning of the decarburization period, 800 kg of CaO and 200 kg of MgO were introduced, and during the reduction period, a slag-forming material having the following composition was introduced.

The composition of the ESR slag waste used is 70% CaF ₂ + 30% Al ₂ O ₃ . According to the present invention, the life of the refractory of the vessel can be extended from the conventional 85 charge to 100 charge.
[0017]
【The invention's effect】
In the production method of the present invention, in carrying out the AOD method, which is the mainstream of today's stainless steel production technology, the refractory of the AOD vessel can be obtained by a simple means of allowing an appropriate amount of Al ₂ O ₃ to be present in the reducing slag. , MgO was suppressed from being eluted into the slag, and the life was extended. In particular, according to the aspect in which the ESR slag waste is used as a source of Al ₂ O ₃ and CaF ₂ of the slag, the above-described effects can be obtained, and the problem of the treatment of the slag waste can be solved. Many benefits are obtained at once, including lower costs and solving environmental problems caused by slag disposal.
[Brief description of the drawings]
FIG. 1 is a graph plotting the relationship between the amount of Al ₂ O ₃ present in the slag during the reduction period in the production of AOD stainless steel and the rate of elution of MgO in the refractory.
FIG. 2 is a graph plotting the relationship between the basicity represented by the slag during the reduction period in the production of AOD stainless steel and the rate of elution of MgO in the refractory.

Claims (6)

A method of manufacturing a stainless steel by AOD method for the molten steel in the AOD vessel, the decarburization period was charged with CaO and MgO, the reduction period to both put CaF ₂ addition of ferrosilicon slag In the manufacturing method of forming and refining, CaO—SiO ₂ —CaF ₂ —Al ₂ , in which Al ₂ O ₃ is introduced together with CaF ₂ during the reduction period, and Al ₂ O ₃ occupies ₃ to 30% by weight. A method for producing stainless steel, comprising forming a slag made of O ₃ -MgO on molten steel and performing refining while suppressing elution of MgO from a refractory of a furnace wall. The process of claim 1 _{in which Al} 2 _{O 3} in the reduction period slag carried to occupy 5 to 20 wt%. The slag composition of the reducing phase, in _{weight%, CaO: 30~70%, SiO} 2: 15~25%, CaF 2: 1~25%, Al 2 O 3: 3~30%, MgO: 5~15% 2. The method according to claim 1, wherein the method is selected from the following ranges. The method according to claim 1, wherein the source of Al ₂ O ₃ to be charged is selected from Bayer method alumina, aluminum dross or aluminum hydroxide. The method according to claim 1, wherein CaF ₂ and Al ₂ O ₃ are supplied from slag waste of an electroslag remelting method. 2. The production method according to claim 1, wherein the slag waste of the electroslag re-dissolution method is added so as to occupy 5 to 40% of the slag in the reduction period.

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