JP4012370B2

JP4012370B2 - Method for producing slab for thin steel sheet without inclusion physical defect

Info

Publication number: JP4012370B2
Application number: JP2001103652A
Authority: JP
Inventors: 昌光若生; 敏彦小関; 輝樹林田
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2001-04-02
Filing date: 2001-04-02
Publication date: 2007-11-21
Anticipated expiration: 2021-04-02
Also published as: JP2002294326A

Description

【０００１】
【発明の属する技術分野】
本発明は、薄鋼板向け炭素鋼の連続鋳造鋳片とその製造方法に関し、特に介在物性欠陥の少ない鋳片およびその製造方法に係わるものである。
【０００２】
【従来の技術】
近年、連続鋳造法で製造した鋳片における介在物性の欠陥は非常に少なくなってきている。これは、溶鋼段階での脱酸法の技術改善や、連続鋳造における種々の介在物対策が効を奏した結果である。（第１２６・１２７回西山記念技術講座「高清浄鋼」社団法人日本鉄鋼協会，１９８８）
しかしながら、薄板向け鋳片、特に飲料缶素材用鋳片においては、一層の介在物低減が要求されており、介在物個数の低減とともにそのサイズを小さくすることが求められている。鋳片内の介在物個数を低減する技術としては、例えば、特開平０７−３００６１２号公報、特開平０５−３３１５２２号公報等がある。また、微細介在物をつくる技術としては、例えば、特開昭５８−２０４１１７号公報、特開平０３−２６７３１１号公報等がある。
【０００３】
飲料缶用鋳片内の介在物個数を低減する技術として、上記特開平０７−３００６１２号公報には、二次精錬において、溶鋼中にガス吹き込みランスからフラックスを吹き込んで、該フラックスを介在物と凝集合体させ、浮上させることが記載されているが、吹き込んだフラックスが溶鋼中に残留して介在物となる恐れがあった。
【０００４】
また、上記特開平０５−３３１５２２号公報では、転炉内へＭａＯを投入してスラグを固化させた後、取鍋内に出鋼し、その後取鍋上のスラグにＡｌを添加して、スラグ中ＦｅＯ濃度を２％以下にすることを記載しているが、スラグ中ＦｅＯ濃度を安定的に２％以下にするには、多量のＡｌ投入が必要となり、コスト的に高くなる。また、スラグ中ＦｅＯ濃度を２％以下にしても、Ａｌ脱酸を行なう限り、脱酸生成物であるアルミナが生成してクラスタ状になる。これは比重が大きいため、溶鋼表面への浮上によるアルミナクラスタ個数の大幅減少は、期待出来ない。
【０００５】
介在物のサイズを小さくする技術としては、特開昭５８−２０４１１７号公報では、Ｍｎ，ＳｉとＴｉまたはＡｌ、或いは更にＲＥＭまたはＣａを脱酸力の弱い順で加える技術が示されているが、Ｍｎが０．８質量％以上と規定されており、Ｍｎの低い薄板向けでは適用できない。また、特開平０３−２６７３１１号公報では、ＴｉとＣａを用いた脱酸法が開示されているが、０．００５質量％以上のＺｒが必須となっているため、コスト的に高くなる。
【０００６】
そこで、本発明者らは先に、特開２０００−１２９３３２号公報、特開２０００−１２９３３３号公報、特開２０００−１４４２３０号公報、特開２０００−１４４３３０号公報を提示した。これらの発明は、脱酸の方法を規定して、鋳片内の５３μｍ以上の介在物の個数を低減させるための技術で、大きな効果が得られた。しかしながら、近年、ユーザーの要求は益々厳格になり、特に用途によっては２０μｍ程度の小さな介在物の個数までも大幅に低減させることが要求されるようになったため、これらの技術のみでは完全な対応が出来くなくなった。
【０００７】
このようなことから、前記各号公報の技術では、薄板向鋼板用鋳片の介在物個数の低減と介在物サイズの微細化、特に２０μｍレベルの介在物個数を安定して低減させることは困難であった。
【０００８】
【発明が解決しようとする課題】
本発明の目的は、鋳片の介在物個数の低減と介在物サイズの微細化を安定して達成することによって、介在物性欠陥の少ない薄鋼板用鋳片とその製造方法を提供することである。すなわち、本発明は、薄板製品で介在物性欠陥が発生しないための鋳片内介在物条件を達成できる鋳片とその鋳片の製造方法である。特に、介在物制約が厳しい用途で用いられる薄鋼板用鋳片での、２０μｍ以上の介在物個数を大幅に低減し、介在物性欠陥のない鋳片とその製造方法を提供することを課題とする。
【０００９】
【課題を解決するための手段】
本発明は、溶鋼に脱酸材を添加する前に、減圧雰囲気でのＣ脱酸や微量Ａｌによって溶鋼中の酸素濃度を所定の値以下に制御し、その後、脱酸材としてＴｉ、Ａｌ、Ｃａの順で金属または合金として添加することにより、製品加工において介在物欠陥の生じにくい鋳片、特に２０μｍ以上の介在物（以下、これを有害介在物と称す）の個数の少ない鋳片とその製造方法を提供するものである。
【００１０】
上記目的を達成するために、本発明は以下の構成を特徴とする。
（１）減圧雰囲気の二次精錬工程で、溶鋼中の酸素濃度が１００ｐｐｍ超の場合に、ＡｌまたはＡｌ合金を添加して該溶鋼中の酸素濃度を１００ｐｐｍ以下とし、酸素濃度が１００ｐｐｍ以下の溶鋼にＴｉ，Ａｌ，Ｃａのそれぞれを含む金属または合金をＴｉ，Ａｌ，Ｃａの順で添加して、質量％で、Ｃ：０．００１〜０．２％，Ｓｉ：０．００１〜２．５％，Ｍｎ：０．０１〜２．０％，Ｐ：０．００１〜０．２％，Ｓ：０．０００５〜０．０５％，Ａｌ：０．００５〜０．０３％，Ｔｉ：０．００５〜０．０６％，Ｃａ：０．０００５〜０．００５％，Ｎ：０．０００５〜０．００７％，Ｏ：０．０００５〜０．００５０％を含み、残部鉄および不可避的不純物からなる溶鋼とし、この溶鋼を連続鋳造工程で鋳造することにより、得られた鋳片の酸化物系介在物のうち、粒子直径が２０μｍ以上のものが２０００個／kg以下となるようにすることを特徴とする介在物欠陥のない薄鋼板用鋳片の製造方法である。
（２）前記二次精錬工程における溶鋼中の酸素濃度が１００ｐｐｍ超の場合に、前記金属または合金の添加に先立ち減圧雰囲気でＡｌまたはＡｌ合金の代わりにＣを添加してＣ脱酸により該溶鋼中の酸素濃度を１００ｐｐｍ以下とすることを特徴とする前記（１）記載の介在物欠陥のない薄鋼板用鋳片の製造方法である。
（３）前記溶鋼が、更に、質量％で、Ｎｂ：０．００１〜０．０５％，Ｖ：０．００５〜０．０５％，Ｃｒ：０．０１〜０．５０％，Ｍｏ：０．０１〜０．５０％，Ｃｕ：０．０１〜０．５０％，Ｎｉ：０．０１〜０．５０％，Ｂ：０．０００２〜０．００２０％，Ｍｇ：０．０００１〜０．００５０％以下の一種または二種以上を含有することを特徴とする前記（１）または（２）記載の介在物欠陥のない薄鋼板用鋳片の製造方法である。
【００１１】
【発明の実施の形態】
本発明者らは、まず、製品にとって介在物性欠陥の発生しにくい鋳片の介在物条件について検討した。ここで、介在物とは、製品欠陥に悪影響を与えやすい酸化物系のものを示す。鋳片内の介在物個数が多くなると、製品での介在物性欠陥が発生しやすくなる。そこで、鋳片内の介在物の大きさや個数と製品欠陥発生との関係を調査した結果、特に介在物欠陥が発生しやすいシャドウマスクについては、鋳片内の介在物のうち直径２０μｍ以上の大きさのものが、鋳片１ｋｇあたり２０００個以上あった場合に製品欠陥発生率が高い、すなわち製品欠陥が発生しやすい傾向にあることが判明した。
【００１２】
以下に本発明の鋳片について詳細に説明するために、発明の条件を規定した理由を述べる。
まず、鋳片中の酸化物系介在物のうち、２０μｍ以上の介在物の個数を２０００個／ｋｇ以下としたのは、前述したように、一番厳しい製品での欠陥発生率が小さくなる条件から決定したものである。
【００１３】
次に、このような鋳片内の介在物条件を満たすための製造方法について検討した。本発明者らは、まず脱酸元素について着目した。溶鋼の脱酸元素としては、一般にＡｌが広く用いられている。しかしながら、Ａｌ脱酸後の生成物であるアルミナは、一つ一つの粒子は小さいが、生成後、直ぐに粒子同士が凝集し、クラスタ状となってサイズが大きくなる。また、このクラスタは、構成粒子同士の間に鉄を含むので、比重が大きく浮上しにくい。従って、Ａｌ脱酸で生成したアルミナ介在物を浮上・除去するためには、静置時間を非常に長くとる、Ａｒガスを多量に溶鋼中へ吹き込んで、ガスと介在物を合体させて浮上を促進する等の対策が必要であった。
【００１４】
そこで、本発明者らはＡｌを単独で脱酸材として用いず、複数の脱酸材で脱酸することを検討した結果、Ｔｉ、Ａｌ、Ｃａの脱酸元素を、Ｔｉ、Ａｌ、Ｃａの順で添加し脱酸した場合に、鋳片内の介在物個数のうち、粒子直径２０μｍ以上の介在物が非常に少なくなることを見いだした。Ｔｉ、Ａｌ、Ｃａの脱酸元素を、Ｔｉ、Ａｌ、Ｃａの順で添加し脱酸した場合に、鋳片内の０．００５〜２．０μｍの介在物個数が非常に多くなることは、先に本発明者らが知見し、その技術を発明として提示しているが、この脱酸法により、粒子直径２０μｍ以上の介在物が非常に少なくなることは、全く新しい発見である。
【００１５】
次に、Ｔｉを添加する前の溶鋼酸素濃度について検討した。表１に示す溶鋼で、表２の条件を用いて脱酸前の溶鋼酸素濃度を変化させ、上述のＴｉ、Ａｌ、Ｃａの脱酸元素を、Ｔｉ、Ａｌ、Ｃａの順で添加する実機試験を行なった。その結果、図１に示すように、Ｔｉ脱酸前の溶鋼酸素濃度が１００ｐｐｍより高い場合には、良い結果が出ないことが判った。すなわち、鋳片内の介在物個数のうち、粒子直径２０μｍ以上の介在物が非常に少なくなる効果は出せなかった。従って、Ｔｉ脱酸前の溶鋼酸素濃度が１００ｐｐｍ以下にする必要がある。
【００１６】
Ｔｉ脱酸前の溶鋼酸素濃度を１００ｐｐｍ以下にする手段としては、熱力学的に検討すると、Ｍｎ−Ｓｉ複合脱酸が挙げられるが、本発明が対象とする薄鋼板用鋳片では、材質上ＭｎやＳｉ濃度を低く制約される場合がある。従って、ＭｎやＳｉ濃度に依存しない脱酸法を考える必要があった。そこでＣに着目し、減圧下でＣ脱酸を行なうことにより、溶鋼酸素濃度を１００ｐｐｍ以下にすることを考えた。Ｃ脱酸平衡から検討すると、例えばＣ濃度０．０４質量％の場合、溶鋼温度１６００℃で雰囲気中のＣＯ分圧が約０．２であれば、平衡する溶鋼酸素濃度は約１００ｐｐｍとなり、本発明で要求される条件を満足する事が出来る。Ｃ脱酸は、脱酸生成物がＣＯガスであるため、溶鋼中に残留して介在物とならないことも大きな特徴である。
【００１７】
また、微量Ａｌを添加しても、溶鋼酸素濃度を１００ｐｐｍ以下にすることが可能である。ここで微量Ａｌとは、添加した後、溶鋼中の濃度で０．００５質量％以下の場合であり、介在物生成に大きな悪影響を与えないので、Ｔｉ添加前にＡｌを添加して脱酸しても構わない。
【００１８】
次に、成分について、規定した理由を述べる。
Ｃは鋼の強度を持たす為に用いられる元素であるが、薄板向けでは深絞り用鋼板等でＣを極力低減させたほうが望ましい場合もある。しかしながら、Ｃが０．００１質量％以下では本発明におけるＣ脱酸が非常に困難になるので、下限を０．００１質量％とし、上限は薄板材で用いられる最大炭素量として０．２質量％とした。
【００１９】
また、Ｍｎも強度を得るためやＳによる脆化を抑制するために必要であり、上限はハイテン材等で使用される場合の最大値２．０質量％とした。また、下限は不可避的に混入するために０．０１質量％とした。
Ｓｉも強度を得るためや高温特性を改善するために用いられる元素であり、上限は薄板の特殊なハイテンや電磁鋼で用いられる２．５質量％とした。また、不可避的に混入するためその下限を０．００１質量％とした。
【００２０】
Ｐは鋼に有害な元素であるため、極力少ないほうが望ましいが、不可避的に混入するため下限値０．００１質量％が現実的である。しかしながら、鋼の強度や耐食性向上の観点から多量のＰ添加を求められる場合があるので、その上限を０．２質量％とした。これ以上では、Ｐによる脆化の影響が強くなる。
Ｓも同様に製品特性に害をなす場合が多く、極力低位とすることが望ましいが、不可避的に混入するため下限値０．０００５質量％が現実的である。また上限は連続鋳造時の割れを防ぐために０．０５質量％とした。
【００２１】
Ａｌは脱酸元素として一般的に使用されているが、Ａｌで脱酸すると、生成したアルミナ同士が凝集して大きな介在物となり易いので、本発明では多くを添加しないことが基本思想である。
しかしながら、本発明の対象となる薄鋼板用鋳片は、一般的にＣが低いので、脱酸前の溶鋼酸素濃度は高くなりやすい。溶鋼酸素が高い状態で脱酸すると、前述したように介在物が多量に生成し、かつ介在物のサイズも大きくなるので、脱酸前の溶鋼酸素濃度を下げる必要がある。また、前述したように、Ｔｉ、Ａｌ，Ｃａの順序で脱酸する時にＡｌが必要なので、上限を０．０３質量％とした。また、下限値は不可避的に混入するため、０．００１質量％とした。
【００２２】
ＴｉおよびＣａは本発明の重要な元素である。鋳片中の酸化物系介在物のうち、２０μｍ以上の介在物の個数を２０００個／ｋｇ以下にするためには、前述したようにＴｉやＡｌ、Ｃａを用いる必要があることを、本発明者らは知見した。Ｔｉの下限値は、脱酸効果を得るために０．００５質量％とし、上限については、多量に添加するとＣａ脱酸の効果を阻害するので、０．０６質量％と規定した。
【００２３】
Ｃａについても、十分な脱酸効果を得るために、下限値は０．０００５質量％とした。上限値は、過剰に入れても効果が飽和するレベルとして０．００５質量％とした。
Ｎは、Ａｌと化合してＡｌＮをつくり、結晶粒の成長を抑えることに利用される。この観点から用いられている添加量の上限値として、０．００７質量％とした。また、不可避的に混入される分を考慮して、下限値として０．０００５質量％とした。
【００２４】
鋳片中の酸素量は、そのほとんどが鋳片内の酸化物系介在物として含まれる分である。製品で有害となる２０μｍ以上の介在物については、極力少ないほうが望ましいが、大きな介在物が少なくなれば、必ず酸素量が低くなるという訳ではない。すなわち、製品に無害な微細介在物が多数あっても、酸素量は高くなる。従って、酸素量があるレベル以下では、必ずしも酸素量は介在物個数の指標とは成り得ないが、酸素値が非常に高い場合には、大きな介在物個数が多くなる傾向が見られるので、上限を０．００５０質量％とした。また、下限については、不可避的に混入する分を考慮して、０．０００５質量％とした。
【００２５】
以上が、本発明が対象とする鋼の基本成分であるが、強度や耐食性、焼き入れ性を初めとする材料の諸特性を向上させるために、鋼の用途に応じてＮｂ，Ｖ，Ｃｒ，Ｍｏ，Ｃｕ，Ｎｉ，Ｂ，ＲＥＭの一種または二種以上を添加しても、本発明の効果は何ら損なわれるものではない。すなわち、その添加量の範囲は、質量％で、それぞれＮｂ：０．００１〜０．０５％、Ｖ：０．００５〜０．０５％、Ｃｒ：０．０１〜０．５０％、Ｍｏ：０．０１〜０．５０％、Ｃｕ：０．０１〜０．５０％、Ｎｉ：０．０１〜０．５０％、Ｂ：０．０００２〜０．００２０％、Ｍｇ：０．０００１〜０．００５０％以下とする。
【００２６】
この他の元素として、ＲＥＭの元素が溶鋼中に含まれる場合もあるが、当該１元素につき１０ｐｐｍまでなら、含まれても本発明の効果に影響を与えることはない。
なお、実際の製造プロセスでは、添加した元素が１００％溶鋼中に含まれることになるわけではないので、歩留を考慮して余分に添加する必要がある。また、添加方法については、特に規定はしない。上記条件を満足するように鋼中に含有できる方法であれば、どのような方法でも構わない。
【００２７】
【表１】

【００２８】
【表２】

【００２９】
【実施例】
表３に示す成分の炭素鋼を表４に示す製造条件で製造し、得られた鋳片の介在物個数と、鋳片を圧延して得られた鋼板および、それを素材として加工した場合の結果について調査した。調査方法としては、表５に示した方法で行なった。なお、水準Ａ−１、Ｂ−１では、Ｃ脱酸前に溶鋼３４０ｔにつき、ＣａＯを５００ｋｇ、Ａｌを１００ｋｇ、取鍋内のスラグ上に添加した。
【００３０】
結果を表６に示す。表より、本発明の場合の条件を満たす場合には、鋳片内の介在物個数が少なく、表面疵や内部欠陥による不合格が発生せず、更に加工時の欠陥も発生しないという良好な結果が得られた。
一方、本発明を満たさない比較材については、次の通り問題のある結果となった。すなわち、比較材Ａ−２では、脱酸元素添加前の溶鋼酸素濃度が高いこと、および脱酸元素が異なるために鋳片内介在物が多くなった。Ｂ−２、Ｃ−２、Ｅ−２、Ｆ−２、Ｇ−２では、脱酸元素添加前の溶鋼酸素濃度は低いが、脱酸元素やその添加順が異なるために鋳片内介在物が多くなった。Ｄ−２では、脱酸元素や添加順は同じでも、溶鋼酸素濃度が高いために鋳片内介在物が多くなった。ＨおよびＩは、Ｔｉ−Ｃａ−Ａｌの順序で添加を行なっても成分範囲が満たさないために、すなわちＨではＡｌ量が多いため、またＩではＴｉ量が多いために、鋳片内介在物個数が多くなっている。
【００３１】
この結果、本発明の条件を満たさない場合には、鋳片内介在物の個数が多く、圧延後のコイル欠陥や製品加工時の欠陥も発生している。
【００３２】
【表３】

【００３３】
【表４】

【００３４】
【表５】

【００３５】
【表６】

【００３６】
【発明の効果】
以上説明したように、本発明により有害な介在物の個数が大幅に減少した薄鋼板用鋳片が得られ、圧延後のコイル欠陥や製品加工時の欠陥が非常に少ないものが得られ、加工の厳しい材料においても、介在物性欠陥のない薄鋼板用鋳片の製造が可能となる。
【図面の簡単な説明】
【図１】Ｔｉ添加前の溶鋼酸素量と介在物サイズとの関係を示した図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a continuous cast slab of carbon steel for a thin steel plate and a method for manufacturing the same, and particularly to a slab having few inclusion physical property defects and a method for manufacturing the same.
[0002]
[Prior art]
In recent years, defects of inclusion physical properties in slabs produced by a continuous casting method have been extremely reduced. This is a result of the technical improvement of the deoxidation method in the molten steel stage and various measures for inclusions in continuous casting. (126th and 127th Nishiyama Memorial Technology Course “High Clean Steel” Japan Iron and Steel Institute, 1988)
However, in the slab for thin plates, particularly the slab for beverage can material, further reduction of inclusions is required, and it is required to reduce the size as well as the number of inclusions. As a technique for reducing the number of inclusions in a slab, there are, for example, JP-A-07-300612 and JP-A-05-331522. Moreover, as a technique for producing fine inclusions, there are, for example, Japanese Patent Application Laid-Open Nos. 58-204117 and 03-267711.
[0003]
As a technique for reducing the number of inclusions in a slab for a beverage can, JP-A-07-300612 discloses that in secondary refining, a flux is blown into a molten steel from a gas blowing lance, and the flux is used as an inclusion. Although it is described that the particles are aggregated and coalesced and floated, the blown flux may remain in the molten steel and become inclusions.
[0004]
Moreover, in the said Unexamined-Japanese-Patent No. 05-331522, after putting MaO in a converter and solidifying slag, it is steel-rolled in a ladle, and after that, Al is added to the slag on a ladle, slag Although it is described that the intermediate FeO concentration is 2% or less, in order to stably reduce the FeO concentration in the slag to 2% or less, a large amount of Al is required, which increases the cost. Further, even if the FeO concentration in the slag is 2% or less, as long as Al deoxidation is performed, alumina, which is a deoxidation product, is generated and becomes clustered. Since this has a large specific gravity, it cannot be expected that the number of alumina clusters will greatly decrease due to floating on the molten steel surface.
[0005]
As a technique for reducing the size of inclusions, Japanese Patent Application Laid-Open No. 58-204117 discloses a technique of adding Mn, Si and Ti or Al, or REM or Ca in order of decreasing deoxidizing power. , Mn is defined as 0.8 % by mass or more, and cannot be applied to a thin plate having a low Mn. Japanese Patent Application Laid-Open No. 03-267311 discloses a deoxidation method using Ti and Ca. However, since Zr of 0.005 % by mass or more is essential, the cost becomes high.
[0006]
In view of this, the present inventors previously presented Japanese Patent Application Laid-Open Nos. 2000-129332, 2000-129333, 2000-144230, and 2000-144330. These inventions are a technique for defining the deoxidation method and reducing the number of inclusions in the slab of 53 μm or more, and a great effect has been obtained. However, in recent years, the demands of users have become more and more strict, and it has become necessary to significantly reduce the number of inclusions as small as about 20 μm depending on the application. I can't do it.
[0007]
For these reasons, it is difficult to reduce the number of inclusions in the slabs for thin steel sheets and the refinement of the inclusion size, in particular, to stably reduce the number of inclusions at the 20 μm level with the techniques of the above-mentioned publications. Met.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to provide a slab for a thin steel sheet with few inclusion physical property defects and a method for manufacturing the slab by stably reducing the number of inclusions in the slab and miniaturizing the inclusion size. . That is, the present invention is a slab capable of achieving the inclusion conditions in the slab so that no inclusion physical defect occurs in the thin plate product, and a method for manufacturing the slab. In particular, an object of the present invention is to provide a slab having no inclusion physical property defect and a method for producing the slab, by significantly reducing the number of inclusions of 20 μm or more in a thin steel plate slab used for applications in which inclusion restrictions are severe. .
[0009]
[Means for Solving the Problems]
In the present invention, before adding a deoxidizer to molten steel, the oxygen concentration in the molten steel is controlled to a predetermined value or less by C deoxidation or a small amount of Al in a reduced pressure atmosphere, and thereafter, Ti, Al, By adding in the order of Ca as a metal or an alloy, a slab which is less likely to cause inclusion defects in product processing, particularly a slab having a small number of inclusions of 20 μm or more (hereinafter referred to as harmful inclusions) and its A manufacturing method is provided.
[0010]
In order to achieve the above object, the present invention is characterized by the following configurations.
(1) In the secondary refining process in a reduced pressure atmosphere, when the oxygen concentration in the molten steel exceeds 100 ppm, Al or Al alloy is added to make the oxygen concentration in the molten steel 100 ppm or less, and the molten steel having an oxygen concentration of 100 ppm or less A metal or an alloy containing each of Ti, Al, and Ca is added to Ti, Al, and Ca in this order, and in terms of mass%, C: 0.001 to 0.2%, Si: 0.001 to 2.5 %, Mn: 0.01-2.0%, P: 0.001-0.2%, S: 0.0005-0.05%, Al: 0.005-0.03%, Ti: 0.00. 005-0.06%, Ca: 0.0005-0.005%, N: 0.0005-0.007%, O: 0.0005-0.0050%, consisting of the balance iron and inevitable impurities and the molten steel, by casting the molten steel in a continuous casting process, to obtain Among oxide type inclusions of the slab, is a manufacturing method of the inclusions without defects thin steel sheet for insert piece, characterized in that to ensure that the particle diameter of more than 20μm is less than 2000 / kg.
( 2 ) When the oxygen concentration in the molten steel in the secondary refining process exceeds 100 ppm , C is added instead of Al or Al alloy in a reduced-pressure atmosphere prior to the addition of the metal or alloy, and the molten steel is subjected to C deoxidation. The method for producing a slab for a thin steel sheet having no inclusion defect according to the above (1), wherein the oxygen concentration in the steel is 100 ppm or less.
( 3 ) The molten steel is further in mass%, Nb: 0.001 to 0.05%, V: 0.005 to 0.05%, Cr: 0.01 to 0.50%, Mo: 0.00. 01 to 0.50%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, B: 0.0002 to 0.0020%, Mg: 0.0001 to 0.0050% It is the manufacturing method of the slab for thin steel plates without the inclusion defect of said (1) or (2) characterized by containing the following 1 type, or 2 or more types.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The inventors of the present invention first examined the inclusion conditions of a slab in which inclusion physical property defects are unlikely to occur in a product. Here, the inclusion refers to an oxide-based material that easily affects product defects. Increasing the number of inclusions in the slab tends to cause inclusion physical defects in the product. Therefore, as a result of investigating the relationship between the size and number of inclusions in the slab and the occurrence of product defects, particularly for shadow masks where inclusion defects are likely to occur, among the inclusions in the slab, the diameter is 20 μm or more. It has been found that when there are 2000 or more per kg of slab, the product defect occurrence rate is high, that is, product defects tend to occur.
[0012]
In order to describe the slab of the present invention in detail, the reason for defining the conditions of the invention will be described below.
First, among the oxide-based inclusions in the slab, the number of inclusions of 20 μm or more was set to 2000 pieces / kg or less, as described above, under the condition that the defect occurrence rate in the most severe product is reduced. It was decided from.
[0013]
Next, a manufacturing method for satisfying the inclusion in the slab was examined. The inventors first focused on deoxidizing elements. Generally, Al is widely used as a deoxidizing element for molten steel. However, alumina, which is a product after Al deoxidation, is small in each particle, but immediately after production, the particles are aggregated to form clusters and increase in size. Moreover, since this cluster contains iron between constituent particles, specific gravity is large and it is hard to float. Therefore, in order to float and remove the alumina inclusions generated by Al deoxidation, the standing time is very long, Ar gas is blown into the molten steel in large quantities, and the gas and inclusions are combined to float. Measures such as promotion were necessary.
[0014]
Therefore, as a result of studying deoxidization with a plurality of deoxidation materials, the present inventors did not use Al alone as a deoxidation material. As a result, the deoxidation elements of Ti, Al, and Ca were changed to Ti, Al, and Ca. It was found that inclusions with a particle diameter of 20 μm or more were very small in the number of inclusions in the slab when added in order and deoxidized. When the deoxidizing elements of Ti, Al, and Ca are added in the order of Ti, Al, and Ca and deoxidized, the number of inclusions of 0.005 to 2.0 μm in the slab becomes very large. The present inventors have previously discovered and presented the technique as an invention, but it is a completely new discovery that inclusions having a particle diameter of 20 μm or more are extremely reduced by this deoxidation method.
[0015]
Next, the molten steel oxygen concentration before adding Ti was examined. In the molten steel shown in Table 1, using the conditions of Table 2, the molten steel oxygen concentration before deoxidation was changed, and the above-described deoxidation elements of Ti, Al, and Ca were added in the order of Ti, Al, and Ca in this order. Was done. As a result, as shown in FIG. 1, it was found that when the molten steel oxygen concentration before Ti deoxidation is higher than 100 ppm, good results are not obtained. That is, the effect that the number of inclusions having a particle diameter of 20 μm or more out of the number of inclusions in the slab was very small could not be obtained. Therefore, the molten steel oxygen concentration before Ti deoxidation needs to be 100 ppm or less.
[0016]
As a means for reducing the oxygen concentration of molten steel before Ti deoxidation to 100 ppm or less, Mn-Si composite deoxidation can be cited as a thermodynamic study. There are cases where the Mn and Si concentrations are restricted to be low. Therefore, it was necessary to consider a deoxidation method that does not depend on the Mn or Si concentration. Accordingly, attention was focused on C, and it was considered to reduce the molten steel oxygen concentration to 100 ppm or less by performing C deoxidation under reduced pressure. Examining from the C deoxidation equilibrium, for example, when the C concentration is 0.04 mass% , if the CO partial pressure in the atmosphere is about 0.2 at a molten steel temperature of 1600 ° C., the equilibrium molten steel oxygen concentration is about 100 ppm. The conditions required by the invention can be satisfied. A major feature of C deoxidation is that the deoxidation product is CO gas, so that it remains in the molten steel and does not become inclusions.
[0017]
Further, even if a small amount of Al is added, the molten steel oxygen concentration can be made 100 ppm or less. Here, the trace amount Al is a case where the concentration in the molten steel is 0.005 % by mass or less after addition and does not have a great adverse effect on the formation of inclusions. Therefore, Al is added and deoxidized before Ti addition. It doesn't matter.
[0018]
Next, the reasons for defining the ingredients will be described.
C is an element used to give steel strength. However, for thin plates, it may be desirable to reduce C as much as possible with a deep drawing steel plate or the like. However, when C is 0.001% by mass or less, C deoxidation in the present invention becomes very difficult. Therefore, the lower limit is set to 0.001% by mass, and the upper limit is 0.2% by mass as the maximum carbon amount used in the thin plate material. It was.
[0019]
Further, Mn is also necessary for obtaining strength and suppressing embrittlement due to S, and the upper limit is set to 2.0% by mass when it is used as a high tensile material or the like. Also, the lower limit is set to 0.01% by mass in order to inevitably mix.
Si is also an element used for obtaining strength and improving high-temperature characteristics, and the upper limit is set to 2.5% by mass used for special high tensile steel and electromagnetic steel. Moreover, in order to mix inevitably, the minimum was made into 0.001 mass%.
[0020]
Since P is an element harmful to steel, it is desirable that P be as small as possible. However, since it is inevitably mixed, a lower limit of 0.001% by mass is realistic. However, since there is a case where a large amount of P addition is required from the viewpoint of improving the strength and corrosion resistance of steel, the upper limit is set to 0.2% by mass. Above this, the influence of embrittlement by P becomes stronger.
Similarly, S often harms the product characteristics and is desirably as low as possible. However, since it is inevitably mixed, the lower limit of 0.0005% by mass is practical. The upper limit was set to 0.05% by mass to prevent cracking during continuous casting.
[0021]
Al is generally used as a deoxidizing element. However, when deoxidizing with Al, the produced alumina tends to aggregate and form large inclusions, so the basic idea is not to add much in the present invention.
However, since the slab for thin steel sheet which is the object of the present invention generally has a low C, the molten steel oxygen concentration before deoxidation tends to be high. If deoxidation is performed in a state where the molten steel oxygen is high, a large amount of inclusions are generated as described above, and the size of the inclusions also increases, so it is necessary to reduce the molten steel oxygen concentration before deoxidation. Further, as described above, since Al is required when deoxidizing in the order of Ti, Al, and Ca, the upper limit was set to 0.03% by mass. Moreover, since the lower limit is inevitably mixed, it is set to 0.001% by mass.
[0022]
Ti and Ca are important elements of the present invention. Of the oxide inclusions in the slab, in order to reduce the number of inclusions of 20 μm or more to 2000 pieces / kg or less, it is necessary to use Ti, Al, and Ca as described above. They found out. The lower limit of Ti is set to 0.005% by mass in order to obtain a deoxidation effect, and the upper limit is defined as 0.06% by mass because the effect of Ca deoxidation is inhibited when added in a large amount.
[0023]
Also for Ca, in order to obtain a sufficient deoxidizing effect, the lower limit was set to 0.0005 mass%. The upper limit value was set to 0.005% by mass as a level at which the effect is saturated even if it is excessively added.
N combines with Al to produce AlN and is used to suppress the growth of crystal grains. The upper limit of the amount used added from this viewpoint is 0.007% by mass. Moreover, considering the amount inevitably mixed, the lower limit is set to 0.0005% by mass.
[0024]
Most of the oxygen content in the slab is contained as oxide inclusions in the slab. For inclusions of 20 μm or more that are harmful to the product, it is desirable to reduce the amount of inclusions as much as possible. However, if the amount of large inclusions decreases, the amount of oxygen does not necessarily decrease. That is, even if there are many harmless fine inclusions in the product, the amount of oxygen becomes high. Therefore, if the oxygen amount is below a certain level, the oxygen amount is not necessarily an indicator of the number of inclusions, but if the oxygen value is very high, the number of large inclusions tends to increase, so the upper limit Was 0.0050 mass% . The lower limit is set to 0.0005 % by mass in consideration of the inevitable mixing.
[0025]
The above are the basic components of steel targeted by the present invention. In order to improve various properties of materials including strength, corrosion resistance, and hardenability, Nb, V, Cr, Addition of one or more of Mo, Cu, Ni, B, and REM does not impair the effects of the present invention. That is, the range of the addition amount is mass%, Nb: 0.001 to 0.05%, V: 0.005 to 0.05%, Cr: 0.01 to 0.50%, Mo: 0, respectively. 0.01 to 0.50%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, B: 0.0002 to 0.0020%, Mg: 0.0001 to 0.0050 % Or less.
[0026]
As other elements, an element of REM may be included in the molten steel, but if it is up to 10 ppm per one element, the effect of the present invention is not affected.
In the actual manufacturing process, the added element is not necessarily contained in the 100% molten steel, so it is necessary to add it in consideration of the yield. Further, the addition method is not particularly specified. Any method may be used as long as it can be contained in steel so as to satisfy the above conditions.
[0027]
[Table 1]

[0028]
[Table 2]

[0029]
【Example】
When carbon steel having the components shown in Table 3 is manufactured under the manufacturing conditions shown in Table 4, the number of inclusions in the obtained slab, the steel plate obtained by rolling the slab, and the case where it is processed as a raw material The results were investigated. As the investigation method, the method shown in Table 5 was used. At levels A-1 and B-1, 500 kg of CaO and 100 kg of Al were added on slag in the ladle for 340 t of molten steel before C deoxidation.
[0030]
The results are shown in Table 6. From the table, if the conditions of the present invention are satisfied, the number of inclusions in the slab is small, no defects due to surface flaws or internal defects occur, and no defects during processing occur. was gotten.
On the other hand, about the comparative material which does not satisfy | fill this invention, it brought a problematic result as follows. That is, in comparative material A-2, the inclusion in the slab increased because the molten steel oxygen concentration before the deoxidation element addition was high and the deoxidation elements were different. In B-2, C-2, E-2, F-2, and G-2, the molten steel oxygen concentration before the deoxidation element addition is low, but the inclusions in the slab are different because the deoxidation elements and their order of addition are different. Increased. In D-2, although the deoxidizing elements and the order of addition were the same, inclusions in the slab increased because the molten steel oxygen concentration was high. Since H and I do not satisfy the component range even if they are added in the order of Ti-Ca-Al, that is, the amount of Al is large in H and the amount of Ti is large in I, inclusions in the slab The number is increasing.
[0031]
As a result, when the conditions of the present invention are not satisfied, the number of inclusions in the slab is large, and coil defects after rolling and defects during product processing also occur.
[0032]
[Table 3]

[0033]
[Table 4]

[0034]
[Table 5]

[0035]
[Table 6]

[0036]
【The invention's effect】
As described above, the present invention provides a slab for thin steel sheet in which the number of harmful inclusions is greatly reduced, and a product with very few coil defects after rolling and defects during product processing. Even in a severe material, it is possible to manufacture a slab for a thin steel plate free from inclusion physical property defects.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the amount of molten steel oxygen before inclusion of Ti and the size of inclusions.

Claims

In the secondary refining process in a reduced-pressure atmosphere, when the oxygen concentration in the molten steel exceeds 100 ppm, Al or an Al alloy is added to make the oxygen concentration in the molten steel 100 ppm or less, and the molten steel with an oxygen concentration of 100 ppm or less is Ti, A metal or alloy containing each of Al and Ca is added in the order of Ti, Al, and Ca. By mass%, C: 0.001 to 0.2%, Si: 0.001 to 2.5%, Mn : 0.01-2.0%, P: 0.001-0.2%, S: 0.0005-0.05%, Al: 0.005-0.03%, Ti: 0.005-0 0.06%, Ca: 0.0005 to 0.005%, N: 0.0005 to 0.007%, O: 0.0005 to 0.0050%, and a molten steel composed of the remaining iron and inevitable impurities, by casting the molten steel in a continuous casting process, the resulting slab Among oxide type inclusions, the manufacturing method of the inclusions defect-free thin steel sheet for insert piece, characterized in that to ensure that the particle diameter of more than 20μm is less than 2000 / kg.

When the oxygen concentration in the molten steel in the secondary refining process exceeds 100 ppm, oxygen is added to the molten steel by C deoxidation by adding C instead of Al or Al alloy in a reduced pressure atmosphere prior to the addition of the metal or alloy. The method for producing a slab for a thin steel plate free from inclusion defects according to claim 1, wherein the concentration is 100 ppm or less.

The molten steel is further in mass%, Nb: 0.001 to 0.05%, V: 0.005 to 0.05%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0 .50%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, B: 0.0002 to 0.0020%, Mg: 0.0001 to 0.0050% or less Or the 2 or more types are contained, The manufacturing method of the slab for thin steel plates without the inclusion defect of Claim 1 or 2 characterized by the above-mentioned.