JP2004238707A - Calcium treatment method of low-aluminum molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for stably controlling Ca yield when subjecting low-Al molten steel to Ca treatment. <P>SOLUTION: In the Ca treatment method for the low-Al molten steel, the molten steel comprising, by mass, ≤0.5% C, 0.1-1.0% Si, 0.1-1.3% Mn and <0.008% sol. Al and the balance being Fe and impurities (wherein impurities include ≤0.004% S) is subjected to Ca treatment. Here, Ca is added to the molten steel so that the mass concentration ratio R äCaO (mass%)/Al<SB>2</SB>O<SB>3</SB>(mass%)} of CaO to Al<SB>2</SB>O<SB>3</SB>in ladle slag and the amount A of Ca added satisfy the relation: A≥-0.11×R+0.24, provided that 0.75<R<1.5. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００１５】
結果を図１に示す。前記成分範囲の鋼では、Ａｌ以外の成分がＣａ歩留まりに与える影響は顕著ではなかった。
【００１６】
この図から明らかなように、Ａｌ濃度が０．００８％以上では、Ｃａ歩留まりはほぼ一定しており、Ｃａ歩留まりに与えるＡｌの影響は大きくないが、Ａｌ濃度が０．００８％未満ではＣａ歩留まりが低下するとともに、ばらつきが大きくなる。そこで、Ａｌ濃度が０．００８％未満の溶鋼を対象として、以下の検討を行った。
【００１７】
前記のように、Ａｌ濃度が低下するとＣａ歩留まりが低下するのは、次に述べる理由によるものと推察される。すなわち、Ａｌ濃度が低くなると、溶鋼の溶存酸素濃度が上昇して酸素活量が増加し、添加されたＣａはこの酸素と下記（３）式に従い反応する。
Ｃａ＋Ｏ→ＣａＯ ・・・（３）
この反応の平衡状態におけるＣａの活量を平衡Ｃａ活量［Ｃａ］_ｅとすると、［Ｃａ］_ｅは、平衡定数Ｋ、酸素活量［Ｏ］を用いて、下記（４）式で表すことができる。
［Ｃａ］_ｅ＝（１／Ｋ）・（ＣａＯ／［Ｏ］） ・・・（４）
この（４）式において、Ａｌ濃度の低下に伴い酸素活量［Ｏ］が増加すると、ＣａＯの活量は一定とみなせるから、平衡Ｃａ活量［Ｃａ］_ｅが低下し、Ｃａの蒸発速度が速くなるためＣａ歩留まりが低下することになる。
【００１８】
したがって、低Ａｌ溶鋼でＣａ歩留まりを制御するには、酸素濃度（活量）とＣａ濃度（活量）の関係を把握することが重要である。しかし、低Ａｌ溶鋼で酸素濃度を制御するのは容易ではない。そこで、溶鋼にフラックスを添加してスラグを形成させ、「スラグ−メタル間反応」で酸素濃度を制御する方法について検討することとし、スラグ組成とＣａ歩留まりの関係を調査した。スラグ−メタル間反応で酸素濃度を推算することは熱力学的に可能であるが、このスラグ組成とＣａ歩留まりの関係を熱力学的推算により求めることは難しいからである。
【００１９】
スラグ組成とＣａ歩留まりの関係の調査では、先ず、マグネシア坩堝内で、Ｓｉ：０．１〜０．７％、Ｍｎ：０．５〜１．３％、Ｓ：０．０００４〜０．００２５％およびＣ：０．００２％未満で、残部がＦｅである鋼１５ｋｇをＡｒ雰囲気下で溶解し、溶鋼温度を１６００℃に調整した。次いで、溶鋼のＡｌ濃度を０．００２〜０．００７％の範囲に調整した後、この溶鋼にＣａＯとＡｌ_２Ｏ_３の質量比（ＣａＯ／Ａｌ_２Ｏ_３）が０．７５〜１．５で、ＳｉＯ_２濃度が２８％以下のフラックスを３００ｇ添加した。その後、Ｃａを０．０５〜０．３ｋｇ／ｔの範囲で添加した。
【００２０】
Ｃａ添加後、溶鋼サンプルとスラグサンプルをそれぞれ採取し、スラグ組成と溶鋼のＣａ濃度を定量し、前記（２）式により「Ｃａ歩留まり」を求めた。
【００２１】
この調査で得られた、スラグ中のＣａＯ／Ａｌ_２Ｏ_３質量比とＣａ歩留まりの関係を図２に示す。なお、スラグのＳｉＯ_２濃度が２８％以下の範囲では、前記ＣａＯ／Ａｌ_２Ｏ_３質量比とＣａ歩留まりの関係に及ぼすＳｉＯ_２濃度の影響は小さかった。
【００２２】
図２に示した結果から、ＣａＯ／Ａｌ_２Ｏ_３質量比の低下に伴いＣａ歩留まりが低下することが解る。ＣａＯ／Ａｌ_２Ｏ_３質量比が低下すると、アルミナの活量が増加するため、酸素濃度が上昇（酸素活量が増加）し、その結果、前記（４）式で示したように、平衡Ｃａ活量［Ｃａ］_ｅが低下し、Ｃａ歩留まりが低下する、と考えられ、先の推察と一致する。
【００２３】
また、図２には、枠外にＣａ添加量をパラメータとして示したが、Ｃａ添加量が０．１７ｋｇ／ｔ以上（図中の□印、△印および○印）であれば、Ｃａ歩留まりのＣａＯ／Ａｌ_２Ｏ_３質量比に対する依存性は同一で、Ｃａ歩留まりとＣａＯ／Ａｌ_２Ｏ_３質量比とが一定の比率（勾配）で変化していることが解る。すなわち、Ｃａ添加量が０．１７ｋｇ／ｔ以上であれば、ＣａＯ／Ａｌ_２Ｏ_３質量比に対して、一義的にＣａ歩留まりが求まる。
【００２４】
しかし、Ｃａ添加量が０．１７ｋｇ／ｔ未満では、Ｃａ歩留まりのＣａＯ／Ａｌ_２Ｏ_３質量比に対する依存性がＣａ添加量によって変化し、例えば、Ｃａ添加量が０．１４ｋｇ／ｔでは、ＣａＯ／Ａｌ_２Ｏ_３質量比＞１．０の範囲ではＣａ添加量が０．３ｋｇ／ｔ以上の場合と同一の依存性を示すが、ＣａＯ／Ａｌ_２Ｏ_３質量比が１．０より小さくなるとＣａ歩留まりが急速に低下する。さらに、Ｃａ添加量が０．１ｋｇ／ｔでは、ＣａＯ／Ａｌ_２Ｏ_３質量比＞１．３の範囲でのみ同一の依存性を示す。これは、スラグによって溶鋼の酸素濃度を低下させても、Ｃａ添加量が少なすぎると、Ｃａ脱酸が進行しないためと考えられる。すなわち、ＣａＯ／Ａｌ_２Ｏ_３質量比が高ければ、溶鋼の酸素濃度（活量）が低いためＣａ添加量が０．１ｋｇ／ｔでもＣａ脱酸が進行するが、ＣａＯ／Ａｌ_２Ｏ_３質量比が１．０より小さくなると、Ｃａ添加量が０．１４ｋｇ／ｔでも不足することになる。
【００２５】
このことから、あるＣａＯ／Ａｌ_２Ｏ_３質量比で安定したＣａ歩留まりを得るには、ある最低限のＣａ添加量が存在するということができ、その量は図２から容易に求めることができる。例えば、ＣａＯを大量に添加し、ＣａＯ／Ａｌ_２Ｏ_３質量比＝１．５でＣａ添加を行う場合、Ｃａ添加量が０．１ｋｇ／ｔ以上であれば安定したＣａ歩留まりが得られる。一方、スラグ量を低減するために、ＣａＯ／Ａｌ_２Ｏ_３質量比＝１．０で操業する場合は、Ｃａ添加量は０．１４ｋｇ／ｔ以上必要であり、それ未満ではＣａ歩留まりは不安定になる。
【００２６】
以上の結果をまとめると、Ａｌ濃度が０．００８％未満の溶鋼では、下記▲１▼および▲２▼のとおりである。
▲１▼Ｃａ歩留まりはＣａＯ／Ａｌ_２Ｏ_３質量比に依存し、ＣａＯ／Ａｌ_２Ｏ_３質量比の低下に伴いＣａ歩留まりが低下する（つまり、不安定になる）。
▲２▼Ｃａ添加量が、ＣａＯ／Ａｌ_２Ｏ_３質量比によって決まるある一定値よりも低くなると、Ｃａ歩留まりが急激に低下する（つまり、不安定になる）。
【００２７】
そこで、前記のＣａＯ／Ａｌ_２Ｏ_３質量比によって決まるＣａ添加量の“ある一定値”をより正確に把握するために、前述のスラグ組成とＣａ歩留まりの関係の調査をさらに継続して実施した。
【００２８】
この調査で得られた、スラグ中のＣａＯ／Ａｌ_２Ｏ_３質量比とＣａ添加量の関係を図３に示す。図３における○印は、Ｃａ歩留まりとＣａＯ／Ａｌ_２Ｏ_３質量比とが、Ｃａ添加量によらず、一定の比率（勾配）で変化した場合（前記図２に示した□印、△印および○印の場合）を、●印は、前記一定の比率（勾配）から外れてＣａ歩留まりが低下（不安定化）した場合を示す。同図中に示した境界線Ｂが、それぞれのＣａＯ／Ａｌ_２Ｏ_３質量比における安定したＣａ歩留まりが得られる最低のＣａ添加量を示し、この境界線Ｂより下のＣａ添加量が少ない領域ではＣａ歩留まりが不安定になる。この境界線Ｂは下記（５）式で表される。
【００２９】
ｙ＝−０．１１ｘ＋０．２４ ・・・（５）
（５）式において、縦軸のＣａ添加量をＡ、「ＣａＯ／Ａｌ_２Ｏ_３質量比」と、「ＣａＯとＡｌ_２Ｏ_３の質量濃度比〔ＣａＯ（質量％）／Ａｌ_２Ｏ_３（質量％）〕」とは同じであるから、横軸のＣａＯ／Ａｌ_２Ｏ_３質量比を〔ＣａＯ（質量％）／Ａｌ_２Ｏ_３（質量％）〕に置き換え、Ｒとすると、前記（５）式は下記（６）式に書き換えることができる。
Ａ＝−０．１１×Ｒ＋０．２４ ・・・（６）
なお、（６）式において、Ｒの範囲は、０．７５＜Ｒ＜１．５である。Ｒが０．７５以下の場合は、Ａｌ_２Ｏ_３飽和なので、（６）式にＲ＝０．７５を代入して求められるＣａ添加量Ａを、安定したＣａ歩留まりが得られる最低のＣａ添加量として適用すればよい。また、Ｒが１．５以上の場合は、ＣａＯ飽和なので、（６）式にＲ＝１．５を代入して求められるＣａ添加量Ａを、同様に適用すればよい。
【００３０】
本発明は、以上述べた検討結果に基づきなされたもので、その要旨は、下記の低Ａｌ溶鋼のＣａ処理方法にある。
【００３１】
『Ｃ：０．５％以下、Ｓｉ：０．１〜１．０％、Ｍｎ：０．１〜１．３％およびｓｏｌ．Ａｌ：０．００８％未満を含有し、残部がＦｅおよび不純物からなり、不純物中のＳが０．００４％以下である溶鋼をＣａ処理するに際し、取鍋スラグ中のＣａＯとＡｌ_２Ｏ_３の質量濃度比〔ＣａＯ（質量％）／Ａｌ_２Ｏ_３（質量％）〕ＲとＣａ添加量Ａが下記（１）式
Ａ≧−０．１１×Ｒ＋０．２４ ・・・（１）
ただし、０．７５＜Ｒ＜１．５
を満足するように、前記溶鋼にＣａを添加する低Ａｌ溶鋼のＣａ処理方法。』
ところで、スラグのＣａＯ／Ａｌ_２Ｏ_３質量比とＣａ添加量とによっては、スラグから溶鋼にＡｌが供給（Ａｌ ｐｉｃｋ ｕｐ）され、溶鋼中Ａｌ濃度が上昇する。Ａｌ濃度を厳格に管理する必要がない場合は、Ｃａ添加量Ａは、前記（１）式のみを満足する量であればよい。しかし、溶鋼中Ａｌ濃度を厳格に管理する場合は、Ａｌ濃度の上限を抑えることが望ましい。
【００３２】
図４は、前述した一連の調査に関連して得られた結果で、Ｃａ添加量と溶鋼中Ａｌ濃度の上昇量の関係を示す図である。なお、同図の枠外に示した「Ｃ／Ａ」は、スラグのＣａＯ／Ａｌ_２Ｏ_３質量比を表す。このような調査結果から、溶鋼中Ａｌ濃度の上昇が認められない条件を求め、整理した結果が、前記図３に示した境界線Ａ（Ａ＝−０．１１×Ｒ＋０．３９）である。この境界線Ａよりも上、すなわちＣａ添加量が多い場合は、溶鋼中Ａｌ濃度が上昇するので、Ａｌ濃度の上昇を抑制する必要がある場合は、下記（７）式を満たすようにＣａを添加すればよい。
Ａ＜−０．１１×Ｒ＋０．３９ ・・・（７）
【００３３】
【発明の実施の形態】
以下、本発明の低Ａｌ溶鋼のＣａ処理方法について、詳細に説明する。
【００３４】
はじめに、本発明のＣａ処理方法において、溶鋼の化学組成を前記のように定めた理由を説明する。
【００３５】
Ｃ：０．５％以下
ＣはＣａの活量を低下させるので、Ｃ濃度が高いと、Ｃａと介在物、ＣａとＯ（酸素）、ＣａとＳなどとの間の反応の速度が変化する。また、一方で、ＣはＳの活量を高くするので、Ｃ濃度が著しく高い場合は、ＣａＳの生成が容易になる。したがって、本発明の方法で処理の対象とする溶鋼のＣ濃度は、０．５％以下とする。
【００３６】
Ｓｉ：０．１〜１．０％
Ｓｉは脱酸力を有する元素である。そのため、Ｓｉ濃度が０．１％未満になると、溶鋼のＯ（酸素）濃度が１００ｐｐｍと高くなり、製品清浄度が悪化する。一方、Ｓｉ濃度が１．０％を超えて高くなると、Ｓの活量を高め、Ｃａ−Ｓ間の反応に影響する。したがって、溶鋼のＳｉ濃度は、０．１〜１．０％とする。
【００３７】
Ｍｎ：０．１〜１．３％
Ｍｎは弱脱酸元素であるが、その濃度が０．１％未満になると、溶鋼の脱酸が著しく不足する。一方、Ｍｎ濃度が０．１％以上であると、脱酸性に大きな影響はないが、Ｍｎ濃度が高いとコスト高になる。したがって、上限を１．３％とし、溶鋼のＭｎ濃度は、０．１〜１．３％とする。
【００３８】
ｓｏｌ．Ａｌ：０．００８％未満
先に述べたように、Ａｌ濃度が０．００８％以上では、Ｃａ歩留まりはほぼ一定しており、Ｃａ歩留まりに与えるＡｌの影響は大きくはない（前記の図１参照）。したがって、溶鋼のＡｌ濃度（ｓｏｌ．Ａｌとして定量される）は、０．００８％未満とする。
【００３９】
本発明の方法で処理の対象とする溶鋼は、前記の成分以外、残部がＦｅおよび不純物からなる溶鋼である。不純物としては、Ｓの上限を抑えることが必要である。
【００４０】
Ｓ：０．００４％以下
ＳはＣａと親和性が強く、ＣａＳ化合物等を生成させる。Ｓ濃度が高いと、生成するＣａＳにより鋼の清浄度が著しく悪化するので、溶鋼のＳ濃度は、０．００４％以下とする。
【００４１】
本発明のＣａ処理方法は、前記化学組成の低Ａｌ溶鋼に対して、前記の（１）式を満足するように、Ｃａを添加する方法である。この（１）式と前記の（６）式とを比較すると明らかなように、（１）式が等号をとる場合、（１）式は前記図３の境界線Ｂを表しており、したがって、「（１）式を満足する」ということは、図３における境界線Ｂを含めてそれより上であること、すなわち、Ｃａ添加量を境界線Ｂが示す値以上とすることを意味している。
【００４２】
（１）式において、Ｒの範囲が定められているが、これは、（１）式が前記Ｒの範囲（０．７５＜Ｒ＜１．５）で成り立つことを示すものである。先に述べたように、Ｒが０．７５以下の場合、またはＲが１．５以上の場合は、ＣａＯとＡｌ_２Ｏ_３の質量濃度比〔ＣａＯ（質量％）／Ａｌ_２Ｏ_３（質量％）〕（以下、単に「ＣａＯ／Ａｌ_２Ｏ_３質量濃度比」と記す）によりＣａ添加量を変化させる必要はないので、（１）式において、Ｒ＝０．７５、またはＲ＝１．５を代入して求められるＣａ添加量Ａを、それぞれ、Ｒが０．７５以下、またはＲが１．５以上の範囲において、安定したＣａ歩留まりが得られる最低のＣａ添加量として適用すればよい。
【００４３】
Ｃａの添加は溶鋼が取鍋内に収容されている間に行うので、（１）式からＣａ添加量を求めるに際し、（１）式のＲには、取鍋内のスラグについて求めたＣａＯ／Ａｌ_２Ｏ_３質量濃度比Ｒを適用する。
【００４４】
このＣａ処理方法によれば、Ｃａ歩留まりが低く、安定したＣａ処理の実施が困難な低Ａｌ溶鋼に対し、高いＣａ歩留まりで安定した処理を行うことができる。
【００４５】
次に、本発明のＣａ処理方法を、転炉を用い、循環脱ガス（ＲＨ）法およびガス吹き込み精錬を行って低Ａｌ溶鋼を溶製する際に実施する場合を例にとって説明する。
【００４６】
溶鋼を、転炉脱炭処理した後、取鍋に出鋼する。続いて、溶鋼を収容した取鍋をＲＨ装置内に移動してＲＨ処理を行い、溶鋼の成分および温度を調整する。
【００４７】
ＲＨ処理の前または後にガス吹き込み精錬を実施してもよい。また、溶鋼の成分および温度の調整は、ガス吹き込み精錬で行ってもよいし、ＲＨ処理およびガス吹き込み精錬の両方で分担して実施してもよい。いずれにせよ、Ｃａ添加を行う前までに実施して、溶鋼の成分を前記規定の範囲内に入るように制御する。
【００４８】
スラグ組成の制御は、転炉からの出鋼時、ＲＨ処理中、またはガス吹き込み精錬時に、フラックスを添加することにより行う。
【００４９】
ＣａＯ／Ａｌ_２Ｏ_３質量濃度比の制御は、Ａｌ脱酸による生成アルミナ量、転炉から取鍋への流出スラグ量、および溶鋼の温度上昇処理によるアルミナ生成量を勘案し、それに応じてＣａＯまたはＣａＯ含有物質の添加量を調整することにより制御する。スラグ組成の制御は、Ｃａ添加前に完了していることが望ましい。Ｃａ添加後にＣａＯ／Ａｌ_２Ｏ_３質量濃度比を大きく変化させると、再びＣａ蒸発速度が変化するからである。なお、スラグを均一組成とし、かつ、スラグ−メタル間反応を十分に促進させておくために、Ｃａ添加前にガス吹き込み精錬を行うことが望ましい。
【００５０】
スラグ中の低級酸化物（Ｔ．Ｆｅ＋ＭｎＯ）の濃度は１０％以下であることが望ましい。スラグ中の低級酸化物濃度が１０％を超えて高いと、Ｃａと低級酸化物が反応してしまい、Ｃａ歩留まりが悪化する。
【００５１】
スラグ量は１０ｋｇ／ｔ以上であることが望ましい。１０ｋｇ／ｔに満たないと、スラグ量が少なく、溶鋼表面の被覆が不十分となって、スラグ−メタル間反応が停滞する場合がある。
【００５２】
溶鋼成分の調整、およびスラグの組成や量の制御が完了した後、溶鋼に前記の（１）式を満足するようにＣａを添加する。溶鋼のＡｌ濃度の上昇を回避したい場合には、前記の（７）も同時に満足させることが望ましい。
【００５３】
前記添加するＣａとしては、金属Ｃａの他、ＣａＳｉ、ＦｅＣａなどのＣａ合金を使用してもよい。また、Ｃａの添加方法は、これらＣａまたは前記Ｃａ合金からなるＣａ系粉末をキャリアガスとともに吹き込む「Ｃａ吹き込み」、Ｃａ系ワイヤを溶鋼中に送り込む「ワイヤ添加」など、従来使用されている如何なる方法でもよい。また、Ｃａ添加中に不活性ガス吹き込みを同時に行うと、スラグ−メタル間反応が促進されるので、Ｃａ歩留まりがより安定する。
【００５４】
このように、溶鋼の温度、成分調整、スラグの組成や量の調整およびＣａ添加を行って得られた低Ａｌ溶鋼は、連続鋳造他に供することができる。
【００５５】
【実施例】
（実施例１）
Ｃ：０．０４５〜０．０６％、Ｓｉ：０．３〜０．５％、Ｍｎ：０．７％およびＳ：０．００２５％を含有する溶鋼１．５ｔを、１．３３３×１０^４Ｐａ（１００Ｔｏｒｒ）のＡｒ雰囲気中で溶製し、Ａｌ濃度を０．００１〜０．００４％に調整した。続いて、雰囲気をＡｒ１．０１３×１０^５Ｐａ（７６０Ｔｏｒｒ）、溶鋼温度を１６００℃とし、５分間保持した。その後、溶鋼にＣａＳｉ（Ｃａ純分：３５％）を添加した。なお、ＣａＳｉの添加は、ホッパーから一括添加することにより行った。
【００５６】
続いて、溶鋼およびスラグのサンプルを採取し、溶鋼組成およびスラグ組成を分析した。スラグ中のＣａＯ／Ａｌ_２Ｏ_３質量濃度比の制御は、ＣａＯ−Ａｌ_２Ｏ_３フラックスにおけるＣａＯとＡｌ_２Ｏ_３の配合割合を変更ことにより行った。
【００５７】
表１に、溶鋼中のＡｌ濃度、スラグ組成（ＣａＯ／Ａｌ_２Ｏ_３質量濃度比およびＳｉＯ_２濃度）ならびにＣａの添加量と歩留まりを示す。表１において（表２においても同じ）、「スラグ中ＣａＯ／Ａｌ_２Ｏ_３比」はＣａＯ／Ａｌ_２Ｏ_３質量濃度比を表す。また、「（１）式右辺から求められるＣａ添加量」とは、（１）式が等号をとる場合で、安定したＣａ歩留まりが得られる最低のＣａ添加量を示し、「（７）式右辺から求められるＣａ添加量」とは、溶鋼のＡｌ濃度の上昇が認められない最高のＣａ添加量を示す。なお、Ｃａ歩留まりは、前記の（２）式から計算により求めた。
【００５８】
【表１】

【００５９】
表１に示した結果から、Ｃａ添加量が（１）式を満足する試験Ｎｏ．１〜１２（実施例）では、安定したＣａ歩留まりが得られていることが解る。また、そのうちの試験Ｎｏ．１〜１０は（１）式および（７）式を同時に満足する場合であるが、この場合は、溶鋼のＡｌ濃度の上昇も抑制された。
【００６０】
一方、Ｃａ添加量が（１）式から外れる試験Ｎｏ．１３〜２０（比較例）ではＣａ歩留まりは０．７〜１．３％と低く、しかも、ばらつきが大きかった。
【００６１】
以上の結果から、（１）式を満足するようにＣａを添加すれば、スラグ中のＣａＯ／Ａｌ_２Ｏ_３質量濃度比の値如何にかかわらず、安定したＣａ歩留まりを確保することが可能であり、また、同時に（７）式をも満たすものであれば、溶鋼のＡｌ濃度の上昇も抑制することができる。
【００６２】
（実施例２）
転炉で精錬した溶鋼２５０ｔを取鍋へ出鋼し、この取鍋をＲＨ装置内へ移動した。ＲＨ処理を行い、溶鋼の成分を、Ｃ：０．０４〜０．０６３％、Ｓｉ：０．２〜０．４％、Ｍｎ：０．５〜０．８％およびＳ：０．００１８〜０．００２２％に、また、溶鋼温度を、１６００〜１６１０℃に調整した後、取鍋をＲＨ装置外へ移動し、ＣａＳｉをワイヤ添加した。ＣａＳｉのＣａ純分は３０％である。また、ＣａＳｉの添加速度は、Ｃａ純分に換算して０．０４２ｋｇ／（ｔ・ｍｉｎ）であった。スラグ中のＣａＯ／Ａｌ_２Ｏ_３質量濃度比の制御は、転炉出鋼時にＣａＯ、Ａｌ_２Ｏ_３フラックスを添加することにより行った。なお、スラグ量は１３〜２１ｋｇ／ｔであった。
【００６３】
表２に、溶鋼中のＡｌ濃度、スラグ組成（ＣａＯ／Ａｌ_２Ｏ_３質量濃度比、ＳｉＯ_２濃度および「Ｔ．Ｆｅ＋ＭｎＯ」濃度）ならびにＣａの添加量と歩留まりを示す。なお、Ｃａ歩留まりは、前記の（２）式から計算により求めた。
【００６４】
【表２】

【００６５】
本実施例２では、溶鋼２５０ｔで、反応系が大きく、実施例１の場合よりも見かけの反応速度がやや低下するため、Ｃａ歩留まりが表２に示すように高くなった。
【００６６】
しかし、表２に示した結果から、Ｃａ添加量が（１）式を満足する試験Ｎｏ．２１〜２７（実施例）では、（１）式から外れる試験Ｎｏ．２８〜３４（比較例）に比べて、安定して高いＣａ歩留まりが得られることが解る。試験Ｎｏ．２１〜２６は（１）式および（７）式を同時に満足する場合であるが、この場合は、溶鋼のＡｌ濃度の上昇も抑制された。
【００６７】
また、スラグ中低級酸化物（Ｔ．Ｆｅ＋ＭｎＯ）濃度が低い方が高いＣａ歩留まりが得られたが、Ｃａ添加量が（１）式を満足する場合は、Ｔ．Ｆｅ＋ＭｎＯ濃度が高くても、高いＣａ歩留まりが得られた。
【００６８】
【発明の効果】
本発明の低Ａｌ溶鋼のＣａ処理方法によれば、Ｃａ歩留まりが低く、Ｃａ処理を安定して行うことが困難な低Ａｌ溶鋼をＣａ処理するに際し、Ｃａ歩留まりを安定して制御することができる。
【図面の簡単な説明】
【図１】溶鋼中Ａｌ濃度とＣａ歩留まりの関係を示す図である。
【図２】スラグ中のＣａＯ／Ａｌ_２Ｏ_３質量比とＣａ歩留まりの関係を示す図である。
【図３】スラグ中のＣａＯ／Ａｌ_２Ｏ_３質量比とＣａ添加量の関係を示す図である。
【図４】Ｃａ添加量と溶鋼中Ａｌ濃度の上昇量の関係を示す図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a Ca treatment method for low Al molten steel, which has a low Ca yield and makes it difficult to perform Ca treatment stably.
[0002]
[Prior art]
The Ca treatment of molten steel is one of the typical methods of treating molten steel, and its purpose is to reduce hydrogen-induced cracking and MnS inclusions or Al that reduce weldability. ₂ O ₃ It is very diverse, such as controlling the form of harmful inclusions such as inclusions, preventing nozzle blockage during continuous casting, and improving free-cutting.
[0003]
Thus, although the purpose of the Ca treatment is various, the most important thing in the Ca treatment is to stabilize the Ca yield. The boiling point of Ca is 1440C, which is lower than the general molten steel processing temperature of 1580 to 1620C. For this reason, Ca easily evaporates from the molten steel during the Ca treatment, and it is difficult to control its concentration, that is, the Ca yield. If the Ca yield is difficult to control, the Ca concentration in the molten steel will vary, making it impossible to obtain steel having the desired properties.
[0004]
Many techniques have been proposed to stabilize the Ca yield. For example, in Patent Document 1, when adding Ca to molten steel stored in a ladle, CaO / SiO in slag is added. ₂ (Content ratio) is set to be larger than 2.5, and T.C. After reducing the total content of Fe and MnO to less than 2.0%, Ca is added to prevent the added Ca from reacting with the slag to prevent Al from being consumed. ₂ O ₃ A method of adding Ca to molten steel that achieves reproducible morphological control (low melting point) of system inclusions is disclosed.
[0005]
In Patent Document 2, in continuous casting of a bloom billet, Ca is added to molten steel containing Al: 0.010 to 0.050% and S: 0.005 to 0.050% to obtain [% Ca] / [ % Al] ratio within the range of 0.06 to 0.20, and the composition of inclusions in the steel is changed to low melting point 12CaO-7Al. ₂ O ₃ Patent Document 3 discloses a method for preventing the formation of large inclusions by promoting coagulation flotation by bringing the composition close to the composition of the system oxide, and in Patent Document 3, Al (0.100% or less), S (0.150%). %) Is added to molten steel containing Ca, by controlling the rate of addition of Ca in accordance with the C concentration, thereby suppressing the generation of CaS to a low level and increasing the Al content. ₂ O ₃ To low melting point 12CaO · 7Al ₂ O ₃ A Ca treatment method has been proposed in which a small-section slab having a small cross-section and having few inclusion defects is obtained without causing nozzle clogging.
[0006]
Patent Document 4 discloses that, after controlling the S content to 20 ppm or less and the Al content to 0.001 to 0.020%, by adding 70 ppm or less of Ca, CaS cluster inclusions are reduced and the cleanliness is excellent. A method for producing a steel material for a line pipe is disclosed.
[0007]
Further, in Patent Document 5, Al is put into molten steel in a ladle, deoxidized, stirred to reduce FeO + MnO in the slag to 5% or less, and then Ca is added, so that a small amount of Ca is added. A method is disclosed that can reliably prevent nozzle clogging of a ladle.
[0008]
As described above, a large number of techniques relating to Ca treatment have been proposed for Al deoxidized molten steel. Improvement of the yield of Ca to molten steel, improvement of operations by controlling the form of inclusions based on the improvement, and quality of steel have been proposed. Has been improved. This is because the unstable Ca yield is not only due to the easiness of evaporation of Ca, but the biggest reason is that the strong affinity between Ca and O (oxygen) causes the deoxidation state of molten steel (the oxygen potential of molten steel). ) Strongly influences the reaction of Ca in the molten steel. Therefore, most of the prior art is directed to Al deoxidized steel to which Al is added to stabilize the oxygen potential of molten steel, and the content of Al is higher than 0.01%.
[0009]
On the other hand, in recent years, for the purpose of improving weldability and reducing costs, a steel in which the concentration of Al in the steel has been reduced has been desired. Al molten steel ”). Along with this, it became necessary to perform Ca treatment on the low Al molten steel, but the conventional Ca treatment technology requires that the Al concentration be equal to or higher than a certain value, and it is difficult to stably perform the Ca treatment on the low Al molten steel. It is. Further, it was known that the Ca yield decreased due to the high oxygen potential in low Al molten steel, but it was not quantitatively grasped and the cause of the decrease was not clear, so a technical solution for it was found. I couldn't do that.
[0010]
[Patent Document 1]
JP-A-63-7318
[Patent Document 2]
JP-A-1-299742
[Patent Document 3]
JP-A-3-183721
[Patent Document 4]
JP-A-57-9822
[Patent Document 5]
JP-A-64-75621
[0011]
[Problems to be solved by the invention]
The present invention has been made in view of the above situation, and an object of the present invention is to provide a method for stably controlling a Ca yield when performing Ca treatment on low Al molten steel.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, the present inventors have investigated the relationship between the Al concentration in molten steel and the Ca yield, which has not been quantified conventionally. In the following, “%” of the concentration (content) of the chemical components of molten steel and slag means “% by mass”.
[0013]
First, in a magnesia (MgO) crucible, Si: 0.1 to 0.7%, Mn: 0.5 to 1.3%, S: 0.0004 to 0.0025%, and C: less than 0.002% Then, 15 kg of steel whose balance is Fe is melted in an Ar atmosphere, and the molten steel temperature is set to 1600 ° C., and then metal Al is added to the molten steel, and the Al concentration in the molten steel ranges from 0.0001 to 0.08%. Was adjusted to Subsequently, metallic Ca was added to the molten steel, and the Ca concentration in the molten steel was measured to determine the Ca yield. The “Ca yield” was defined by the following equation (2).
[0014]
(Equation 1)

[0015]
The results are shown in FIG. In the steels in the above component ranges, components other than Al did not significantly affect the Ca yield.
[0016]
As is clear from this figure, when the Al concentration is 0.008% or more, the Ca yield is almost constant, and the effect of Al on the Ca yield is not significant. However, when the Al concentration is less than 0.008%, the Ca yield is low. And the variation increases. Therefore, the following study was conducted on molten steel having an Al concentration of less than 0.008%.
[0017]
As described above, the reason why the Ca yield decreases when the Al concentration decreases is presumed to be as follows. That is, when the Al concentration decreases, the dissolved oxygen concentration of the molten steel increases, the oxygen activity increases, and the added Ca reacts with this oxygen according to the following equation (3).
Ca + O → CaO (3)
The activity of Ca in the equilibrium state of this reaction is represented by the equilibrium Ca activity [Ca]. _e Then, [Ca] _e Can be expressed by the following equation (4) using the equilibrium constant K and the oxygen activity [O].
[Ca] _e = (1 / K) · (CaO / [O]) (4)
In the equation (4), when the oxygen activity [O] increases with a decrease in the Al concentration, the activity of CaO can be considered to be constant, so the equilibrium Ca activity [Ca] _e Is reduced, and the Ca evaporation rate is increased, so that the Ca yield is reduced.
[0018]
Therefore, in order to control the Ca yield with low Al molten steel, it is important to understand the relationship between the oxygen concentration (activity) and the Ca concentration (activity). However, it is not easy to control the oxygen concentration with low Al molten steel. Therefore, a method of controlling the oxygen concentration by "slag-metal reaction" by adding a flux to molten steel to form slag was examined, and the relationship between slag composition and Ca yield was investigated. This is because it is thermodynamically possible to estimate the oxygen concentration by the slag-metal reaction, but it is difficult to obtain the relationship between the slag composition and the Ca yield by thermodynamic estimation.
[0019]
In the investigation of the relationship between slag composition and Ca yield, first, in a magnesia crucible, Si: 0.1 to 0.7%, Mn: 0.5 to 1.3%, S: 0.0004 to 0.0025% And C: 15 kg of steel having less than 0.002% and the balance being Fe was melted in an Ar atmosphere, and the temperature of the molten steel was adjusted to 1600 ° C. Next, after adjusting the Al concentration of the molten steel to a range of 0.002 to 0.007%, CaO and Al were added to the molten steel. ₂ O ₃ Mass ratio (CaO / Al ₂ O ₃ ) Is 0.75 to 1.5 and SiO ₂ 300 g of a flux having a concentration of 28% or less was added. Thereafter, Ca was added in the range of 0.05 to 0.3 kg / t.
[0020]
After the addition of Ca, a molten steel sample and a slag sample were respectively collected, the slag composition and the Ca concentration of the molten steel were quantified, and the “Ca yield” was determined by the above equation (2).
[0021]
CaO / Al in slag obtained by this survey ₂ O ₃ FIG. 2 shows the relationship between the mass ratio and the Ca yield. The slag SiO ₂ When the concentration is 28% or less, the CaO / Al ₂ O ₃ Effect of SiO on the relationship between mass ratio and Ca yield ₂ The effect of concentration was small.
[0022]
From the results shown in FIG. ₂ O ₃ It can be seen that the Ca yield decreases as the mass ratio decreases. CaO / Al ₂ O ₃ When the mass ratio decreases, the activity of alumina increases, so that the oxygen concentration increases (the oxygen activity increases). As a result, as shown in the above equation (4), the equilibrium Ca activity [Ca] _e Is considered to decrease, and the Ca yield decreases, which is consistent with the above assumption.
[0023]
Further, FIG. 2 shows the amount of added Ca as a parameter outside the frame, but if the amount of added Ca is 0.17 kg / t or more (indicated by □, Δ and ○ in FIG. 2), the CaO yield of CaO / Al ₂ O ₃ Dependence on the mass ratio is the same, Ca yield and CaO / Al ₂ O ₃ It can be seen that the mass ratio changes at a constant ratio (gradient). That is, if the Ca addition amount is 0.17 kg / t or more, CaO / Al ₂ O ₃ The Ca yield is uniquely determined with respect to the mass ratio.
[0024]
However, if the amount of Ca added is less than 0.17 kg / t, the CaO / Al ₂ O ₃ The dependency on the mass ratio changes depending on the amount of Ca added. For example, when the amount of Ca added is 0.14 kg / t, CaO / Al ₂ O ₃ When the mass ratio is in the range of> 1.0, the same dependence is exhibited as in the case where the amount of Ca added is 0.3 kg / t or more, but CaO / Al ₂ O ₃ When the mass ratio is less than 1.0, the Ca yield decreases rapidly. Furthermore, when the amount of Ca added is 0.1 kg / t, CaO / Al ₂ O ₃ The same dependence is shown only in the range of mass ratio> 1.3. This is presumably because even if the oxygen concentration of the molten steel is reduced by the slag, if the amount of Ca added is too small, Ca deoxidation does not proceed. That is, CaO / Al ₂ O ₃ If the mass ratio is high, the oxygen concentration (activity) of the molten steel is low, so even if the Ca addition amount is 0.1 kg / t, Ca deoxidation proceeds, but CaO / Al ₂ O ₃ If the mass ratio is less than 1.0, the Ca addition amount will be insufficient even at 0.14 kg / t.
[0025]
From this, a certain CaO / Al ₂ O ₃ In order to obtain a stable Ca yield by mass ratio, it can be said that a certain minimum amount of Ca is present, and the amount can be easily obtained from FIG. For example, a large amount of CaO is added and CaO / Al ₂ O ₃ When Ca is added at a mass ratio of 1.5, a stable Ca yield can be obtained if the Ca addition amount is 0.1 kg / t or more. On the other hand, in order to reduce the amount of slag, CaO / Al ₂ O ₃ When operating at a mass ratio of 1.0, the amount of Ca added must be 0.14 kg / t or more, and if it is less than that, the Ca yield becomes unstable.
[0026]
The above results are summarized as follows in (1) and (2) in molten steel having an Al concentration of less than 0.008%.
(1) Ca yield is CaO / Al ₂ O ₃ Depending on the mass ratio, CaO / Al ₂ O ₃ As the mass ratio decreases, the Ca yield decreases (that is, becomes unstable).
(2) Ca added amount is CaO / Al ₂ O ₃ If it becomes lower than a certain value determined by the mass ratio, the Ca yield sharply decreases (that is, becomes unstable).
[0027]
Therefore, the above CaO / Al ₂ O ₃ In order to more accurately grasp the “certain value” of the Ca addition amount determined by the mass ratio, the above-described investigation of the relationship between the slag composition and the Ca yield was further continued.
[0028]
CaO / Al in slag obtained by this survey ₂ O ₃ FIG. 3 shows the relationship between the mass ratio and the amount of Ca added. The circles in FIG. 3 indicate the Ca yield and CaO / Al ₂ O ₃ When the mass ratio changes at a constant ratio (gradient) irrespective of the amount of Ca added (in the case of □, Δ and ○ shown in FIG. 2), the mark ● indicates the constant ratio. (Gradient) and the case where the Ca yield is lowered (stabilized). The boundary line B shown in FIG. ₂ O ₃ This indicates the lowest Ca addition amount at which a stable Ca yield at a mass ratio can be obtained. In a region below the boundary line B where the Ca addition amount is small, the Ca yield becomes unstable. This boundary line B is expressed by the following equation (5).
[0029]
y = −0.11x + 0.24 (5)
In the formula (5), the Ca addition amount on the vertical axis is A, and “CaO / Al ₂ O ₃ Mass ratio "and" CaO and Al ₂ O ₃ Mass concentration ratio [CaO (mass%) / Al ₂ O ₃ (% By mass)]], the CaO / Al on the horizontal axis ₂ O ₃ The mass ratio is [CaO (mass%) / Al ₂ O ₃ (% By mass)] and R as the above, the above equation (5) can be rewritten as the following equation (6).
A = −0.11 × R + 0.24 (6)
In the equation (6), the range of R is 0.75 <R <1.5. When R is 0.75 or less, Al ₂ O ₃ Since it is saturated, the Ca addition amount A obtained by substituting R = 0.75 into the equation (6) may be applied as the minimum Ca addition amount at which a stable Ca yield can be obtained. When R is 1.5 or more, CaO is saturated. Therefore, the Ca addition amount A obtained by substituting R = 1.5 into the equation (6) may be similarly applied.
[0030]
The present invention has been made based on the above-described examination results, and the gist lies in the following Ca treatment method for low Al molten steel.
[0031]
"C: 0.5% or less, Si: 0.1 to 1.0%, Mn: 0.1 to 1.3%, and sol. Al: CaO and Al in ladle slag when performing Ca treatment on molten steel containing less than 0.008%, the balance being Fe and impurities, and S in the impurities being 0.004% or less. ₂ O ₃ Mass concentration ratio [CaO (mass%) / Al ₂ O ₃ (% By mass)] When R and the amount A of Ca added are the following formula (1)
A ≧ −0.11 × R + 0.24 (1)
However, 0.75 <R <1.5
A Ca treatment method for low Al molten steel, wherein Ca is added to the molten steel so as to satisfy the following. 』
By the way, the slag CaO / Al ₂ O ₃ Depending on the mass ratio and the amount of Ca added, Al is supplied from the slag to the molten steel (Al pick up), and the Al concentration in the molten steel increases. When it is not necessary to strictly control the Al concentration, the Ca addition amount A may be an amount that satisfies only the expression (1). However, when strictly controlling the Al concentration in the molten steel, it is desirable to suppress the upper limit of the Al concentration.
[0032]
FIG. 4 is a graph showing the relationship between the amount of Ca added and the amount of increase in the Al concentration in molten steel, as a result obtained in connection with the above-described series of investigations. Note that “C / A” shown outside the frame of FIG. ₂ O ₃ Represents the mass ratio. From the results of such investigations, conditions under which an increase in the Al concentration in the molten steel was not found were determined, and the result of arrangement was the boundary line A (A = −0.11 × R + 0.39) shown in FIG. Above this boundary line A, that is, when the amount of Ca added is large, the Al concentration in the molten steel increases. Therefore, when it is necessary to suppress the increase in the Al concentration, Ca is added so as to satisfy the following equation (7). What is necessary is just to add.
A <−0.11 × R + 0.39 (7)
[0033]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the Ca treatment method of the low Al molten steel of the present invention will be described in detail.
[0034]
First, the reason why the chemical composition of molten steel is determined as described above in the Ca treatment method of the present invention will be described.
[0035]
C: 0.5% or less
Since C reduces the activity of Ca, when the C concentration is high, the rate of the reaction between Ca and inclusions, Ca and O (oxygen), Ca and S, and the like changes. On the other hand, C increases the activity of S, and therefore, when the C concentration is extremely high, generation of CaS becomes easy. Therefore, the C concentration of the molten steel to be treated by the method of the present invention is set to 0.5% or less.
[0036]
Si: 0.1 to 1.0%
Si is an element having deoxidizing power. Therefore, when the Si concentration is less than 0.1%, the O (oxygen) concentration of the molten steel becomes as high as 100 ppm, and the product cleanliness deteriorates. On the other hand, when the Si concentration is higher than 1.0%, the activity of S is increased, which affects the reaction between Ca and S. Therefore, the Si concentration of the molten steel is set to 0.1 to 1.0%.
[0037]
Mn: 0.1 to 1.3%
Mn is a weakly deoxidizing element, but if its concentration is less than 0.1%, the deoxidation of molten steel becomes significantly insufficient. On the other hand, when the Mn concentration is 0.1% or more, there is no significant effect on deacidification, but when the Mn concentration is high, the cost increases. Therefore, the upper limit is set to 1.3%, and the Mn concentration of the molten steel is set to 0.1 to 1.3%.
[0038]
sol. Al: less than 0.008%
As described above, when the Al concentration is 0.008% or more, the Ca yield is almost constant, and the effect of Al on the Ca yield is not significant (see FIG. 1 described above). Therefore, the Al concentration of the molten steel (quantified as sol. Al) is set to less than 0.008%.
[0039]
The molten steel to be treated by the method of the present invention is a molten steel consisting of Fe and impurities with the balance other than the above components. It is necessary to suppress the upper limit of S as an impurity.
[0040]
S: 0.004% or less
S has a strong affinity for Ca and generates CaS compounds and the like. If the S concentration is high, the cleanliness of the steel is significantly deteriorated by the CaS generated, so the S concentration of the molten steel is set to 0.004% or less.
[0041]
The Ca treatment method of the present invention is a method in which Ca is added to a low Al molten steel having the above chemical composition so as to satisfy the above-mentioned formula (1). As is apparent from a comparison between the expression (1) and the expression (6), when the expression (1) takes an equal sign, the expression (1) represents the boundary line B in FIG. The expression "satisfies the expression (1)" means that the value is higher than that including the boundary line B in FIG. I have.
[0042]
In the equation (1), the range of R is determined, which indicates that the equation (1) is satisfied in the range of R (0.75 <R <1.5). As described above, when R is 0.75 or less, or when R is 1.5 or more, CaO and Al ₂ O ₃ Mass concentration ratio [CaO (mass%) / Al ₂ O ₃ (Mass%)] (hereinafter simply referred to as “CaO / Al ₂ O ₃ Mass concentration ratio), it is not necessary to change the amount of Ca to be added. , R is 0.75 or less, or R is 1.5 or more, it is only necessary to apply the minimum amount of Ca to obtain a stable Ca yield.
[0043]
Since the addition of Ca is performed while the molten steel is accommodated in the ladle, when calculating the Ca addition amount from the equation (1), R in equation (1) includes CaO / calculated for the slag in the ladle. Al ₂ O ₃ The mass concentration ratio R is applied.
[0044]
According to this Ca treatment method, stable treatment can be performed at a high Ca yield on low Al molten steel having a low Ca yield and difficult to perform stable Ca treatment.
[0045]
Next, the case of carrying out the Ca treatment method of the present invention at the time of smelting low Al molten steel by performing a circulation degassing (RH) method and gas blowing refining using a converter will be described as an example.
[0046]
After the converter is decarburized, the molten steel is output to a ladle. Subsequently, the ladle containing the molten steel is moved into the RH device to perform the RH treatment, and the components and the temperature of the molten steel are adjusted.
[0047]
Gas blowing refining may be performed before or after the RH treatment. The adjustment of the components and the temperature of the molten steel may be performed by gas blowing refining, or may be performed by sharing both of the RH treatment and the gas blowing refining. In any case, the control is performed before the addition of Ca to control the composition of the molten steel so as to fall within the above-mentioned specified range.
[0048]
The slag composition is controlled by adding a flux when tapping from the converter, during RH treatment, or during gas blowing refining.
[0049]
CaO / Al ₂ O ₃ The mass concentration ratio is controlled by taking into account the amount of alumina produced by Al deoxidation, the amount of slag flowing out of the converter to the ladle, and the amount of alumina produced by increasing the temperature of the molten steel, and adding CaO or CaO-containing substances accordingly. Control by adjusting the amount. It is desirable that the control of the slag composition is completed before the addition of Ca. CaO / Al after Ca addition ₂ O ₃ This is because if the mass concentration ratio is largely changed, the Ca evaporation rate changes again. In order to make the slag a uniform composition and sufficiently promote the reaction between the slag and the metal, it is desirable to perform gas blowing refining before adding Ca.
[0050]
It is desirable that the concentration of the lower oxide (T.Fe + MnO) in the slag is 10% or less. If the concentration of the lower oxide in the slag is higher than 10%, Ca and the lower oxide react, and the Ca yield deteriorates.
[0051]
It is desirable that the amount of slag is 10 kg / t or more. If it is less than 10 kg / t, the amount of slag is small, the coating on the molten steel surface becomes insufficient, and the reaction between slag and metal may be stagnated.
[0052]
After the adjustment of the molten steel component and the control of the composition and amount of the slag are completed, Ca is added to the molten steel so as to satisfy the above equation (1). When it is desired to avoid an increase in the Al concentration of the molten steel, it is desirable that the above (7) is also satisfied.
[0053]
As the Ca to be added, a Ca alloy such as CaSi and FeCa may be used in addition to metal Ca. The method of adding Ca may be any conventionally used method such as “Ca blowing” in which a Ca-based powder composed of the Ca or the Ca alloy is blown together with a carrier gas, and “wire addition” in which a Ca-based wire is fed into molten steel. May be. Further, when the inert gas is blown simultaneously during the addition of Ca, the reaction between the slag and the metal is promoted, so that the Ca yield is further stabilized.
[0054]
Thus, the low Al molten steel obtained by adjusting the temperature and composition of the molten steel, adjusting the composition and amount of the slag, and adding Ca can be used for continuous casting and the like.
[0055]
【Example】
(Example 1)
1.5 tons of molten steel containing 0.045 to 0.06% of C, 0.3 to 0.5% of Si, 0.7% of Mn and 0.0025% of S was 1.333 × 10 ⁴ It was melted in an Ar atmosphere of Pa (100 Torr), and the Al concentration was adjusted to 0.001 to 0.004%. Subsequently, the atmosphere was changed to Ar 1.013 × 10 ⁵ Pa (760 Torr), the molten steel temperature was set to 1600 ° C., and the temperature was maintained for 5 minutes. Thereafter, CaSi (Ca content: 35%) was added to the molten steel. The addition of CaSi was performed by batch addition from a hopper.
[0056]
Subsequently, samples of molten steel and slag were collected and analyzed for molten steel composition and slag composition. CaO / Al in slag ₂ O ₃ The mass concentration ratio is controlled by CaO-Al ₂ O ₃ CaO and Al in flux ₂ O ₃ Was carried out by changing the compounding ratio.
[0057]
Table 1 shows the Al concentration in the molten steel and the slag composition (CaO / Al ₂ O ₃ Mass concentration ratio and SiO ₂ Concentration), the amount of Ca added, and the yield. In Table 1 (same in Table 2), “CaO / Al in slag ₂ O ₃ Ratio ”is CaO / Al ₂ O ₃ Represents the mass concentration ratio. In addition, “the amount of Ca added from the right side of equation (1)” indicates the minimum Ca addition amount at which a stable Ca yield can be obtained when equation (1) takes the equal sign, and the equation “(7) The “Ca addition amount determined from the right side” indicates the highest Ca addition amount at which no increase in the Al concentration of the molten steel is observed. Note that the Ca yield was calculated from the above equation (2).
[0058]
[Table 1]

[0059]
From the results shown in Table 1, it was found that Test No. 3 in which the amount of Ca added satisfied the expression (1). In Examples 1 to 12 (Examples), it can be seen that a stable Ca yield was obtained. In addition, among the test Nos. 1 to 10 are cases where the expressions (1) and (7) are simultaneously satisfied. In this case, the increase in the Al concentration of the molten steel was also suppressed.
[0060]
On the other hand, Test No. 1 in which the amount of Ca added deviated from the expression (1). In 13 to 20 (Comparative Example), the Ca yield was as low as 0.7 to 1.3%, and the variation was large.
[0061]
From the above results, if Ca is added so as to satisfy the expression (1), CaO / Al in the slag is obtained. ₂ O ₃ Irrespective of the value of the mass concentration ratio, a stable Ca yield can be ensured, and at the same time, if the expression (7) is satisfied, an increase in the Al concentration of the molten steel can be suppressed. .
[0062]
(Example 2)
250 t of molten steel refined in the converter was tapped into a ladle, and the ladle was moved into an RH device. The RH treatment is performed, and the components of the molten steel are as follows: C: 0.04 to 0.063%, Si: 0.2 to 0.4%, Mn: 0.5 to 0.8%, and S: 0.0018 to 0. After adjusting the molten steel temperature to 1600 to 1610 ° C., the ladle was moved out of the RH apparatus, and CaSi was added to the wire. The Ca pure content of CaSi is 30%. The addition rate of CaSi was 0.042 kg / (t · min) in terms of pure Ca. CaO / Al in slag ₂ O ₃ The control of the mass concentration ratio is controlled by CaO, Al ₂ O ₃ This was done by adding flux. The slag amount was 13 to 21 kg / t.
[0063]
Table 2 shows the Al concentration in the molten steel and the slag composition (CaO / Al ₂ O ₃ Mass concentration ratio, SiO ₂ Concentration and “T.Fe + MnO” concentration), the amount of Ca added and the yield. Note that the Ca yield was calculated from the above equation (2).
[0064]
[Table 2]

[0065]
In Example 2, the reaction system was large with 250 t of molten steel, and the apparent reaction rate was slightly lower than that in Example 1, so that the Ca yield was increased as shown in Table 2.
[0066]
However, from the results shown in Table 2, it can be seen from Test No. that the amount of Ca added satisfies the expression (1). In Test Examples 21 to 27 (Examples), test Nos. It can be seen that a higher Ca yield can be stably obtained as compared with 28 to 34 (Comparative Example). Test No. 21 to 26 are the cases where the expressions (1) and (7) are simultaneously satisfied. In this case, the increase in the Al concentration of the molten steel was also suppressed.
[0067]
In addition, a higher Ca yield was obtained when the lower oxide (T.Fe + MnO) concentration in the slag was lower, but when the Ca addition amount satisfied the formula (1), the T.C. Even with a high Fe + MnO concentration, a high Ca yield was obtained.
[0068]
【The invention's effect】
According to the Ca treatment method for low Al molten steel of the present invention, when Ca treatment is performed on low Al molten steel having a low Ca yield and it is difficult to perform Ca treatment stably, the Ca yield can be controlled stably. .
[Brief description of the drawings]
FIG. 1 is a diagram showing a relationship between Al concentration in molten steel and Ca yield.
FIG. 2 CaO / Al in slag ₂ O ₃ It is a figure which shows the relationship between a mass ratio and Ca yield.
FIG. 3 CaO / Al in slag ₂ O ₃ It is a figure which shows the relationship between a mass ratio and Ca addition amount.
FIG. 4 is a graph showing the relationship between the amount of Ca added and the amount of increase in the Al concentration in molten steel.

Claims (1)

C: 0.5% or less, Si: 0.1 to 1.0%, Mn: 0.1 to 1.3%, and sol. Al: Less than 0.008%, the balance consists of Fe and impurities, and when Ca in molten steel containing 0.004% or less of S in impurities, CaO and Al ₂ O ₃ in ladle slag are removed. A mass concentration ratio [CaO (mass%) / Al ₂ O ₃ (mass%)] wherein Ca is added to the molten steel so that R and Ca addition amount A satisfy the following formula (1). Ca treatment method for Al molten steel.
A ≧ −0.11 × R + 0.24 (1)
However, 0.75 <R <1.5

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