JP3542913B2

JP3542913B2 - Slab for thin steel sheet with less inclusion defect and method for producing the same

Info

Publication number: JP3542913B2
Application number: JP32438898A
Authority: JP
Inventors: 昌光若生; 秀里間渕; 弘昭飯星
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1998-10-30
Filing date: 1998-10-30
Publication date: 2004-07-14
Anticipated expiration: 2018-10-30
Also published as: JP2000144230A

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）。
【０００３】
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、益々の介在物低減が要求されており、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３−２６７３１１号公報がある。
【０００４】
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。
【０００５】
また、上記特開平０５−３３１５２２号公報では、転炉内へＣａＯ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。
【０００６】
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またあは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重量％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３−２６７３１１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重量％以上のＺｒが必須となっているため、コスト的に高くなる。また、ＴｉやＣａ添加前の溶鋼酸素濃度が高い場合には、ＴｉやＣａを添加して脱酸を行なっても、介在物の微細化効果が十分に発揮されないことから、生成した介在物は大きなものとなってしまう。
【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を安定して達成す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満足する鋳片とその鋳片の製造方法であり、特に、薄鋼板用鋳片で制約を受ける、ＭｎやＳｉそしてＡｌ含有量に依存しない、介在物性欠陥の少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Ｃ脱酸を行なって溶鋼中の酸素濃度を低減し、その後、脱酸材としてＴｉ、Ｃａの順で金属または合金として添加して脱酸し、その後Ａｌを添加することにより、５３μｍ以上の酸化物系介在物の個数が２００個／ｋｇ以下で、かつ、その内、アルミナクラスタ介在物の個数が２０個／ｋｇ以下の鋳片にするものであり、その手段１は、Ｃ：０．００１〜０．２重量％、Ｍｎ：０．０１〜０．５重量％、Ｓｉ：０．００１〜０．５重量％、Ｐ：０．００１〜０．３重量％、Ｓ：０．０００５〜０．０５重量％、Ａｌ：０．００６超〜０．１重量％、Ｔｉ：０．００５〜０．０６重量％、Ｃａ：０．０００５〜０．０１重量％、Ｎ：０．０００５〜０．０１重量％、酸素：０．０００５〜０．００５０重量％を含み、残部鉄および不可避的不純物からなる炭素鋼で、鋳片中の酸化物系介在物のうち、５３μｍ以上の介在物の個数が２００個／ｋｇ以下で、かつ、その内、アルミナクラスタ介在物の個数が２０個／ｋｇ以下である介在物性欠陥の少ない薄鋼板用鋳片である。
【００１０】
更に手段２は、前記手段１に、Ｎｂ：０．００１〜０．１０重量％、Ｖ：０．００５〜０．２０重量％、Ｃｒ：０．０１〜０．５０重量％、Ｍｏ：０．０１〜０．５０重量％、Ｃｕ：０．０１〜０．５０重量％、Ｎｉ：０．０１〜０．５０重量％、Ｂ：０．０００２〜０．００２０重量％の一種または二種以上を含有せしめるものである。
【００１１】
手段３は、Ｃ：０．００１〜０．２重量％、Ｍｎ：０．０１〜０．５重量％、Ｓｉ：０．００１〜０．５重量％、Ｐ：０．００１〜０．３重量％、Ｓ：０．０００５〜０．０５重量％、Ａｌ：０．００６超〜０．１重量％、Ｔｉ：０．００５〜０．０６重量％、Ｃａ：０．０００５〜０．０１重量％、Ｎ：０．０００５〜０．０１重量％、酸素：０．０００５〜０．００５０重量％を含み、残部鉄および不可避的不純物からなる炭素鋼溶鋼を連続鋳造設備で鋳造して鋳片を製造する際に、脱炭を行なった溶鋼を、減圧雰囲気でＣ脱酸を行なって該溶鋼中の酸素濃度を３００ｐｐｍ以下とし、その後、Ｔｉ、Ｃａの順で金属または合金として添加して脱酸し、その後Ａｌを添加する介在物性欠陥の少ない薄鋼板用鋳片の製造方法である。
【００１２】
手段４は、前記手段３に、Ｎｂ：０．００１〜０．１０重量％、Ｖ：０．００５〜０．２０重量％、Ｃｒ：０．０１〜０．５０重量％、Ｍｏ：０．０１〜０．５０重量％、Ｃｕ：０．０１〜０．５０重量％、Ｎｉ：０．０１〜０．５０重量％、Ｂ：０．０００２〜０．００２０重量％の一種または二種以上を含有せしめるものである。
【００１３】
手段５は、Ｃ：０．００１〜０．２重量％、Ｍｎ：０．０１〜０．５重量％、Ｓｉ：０．００１〜０．５重量％、Ｐ：０．００１〜０．３重量％、Ｓ：０．０００５〜０．０５重量％、Ａｌ：０．００６超〜０．１重量％、Ｔｉ：０．００５〜０．０６重量％、Ｃａ：０．０００５〜０．０１重量％、Ｎ：０．０００５〜０．０１重量％、酸素：０．０００５〜０．００５０重量％を含み、残部鉄および不可避的不純物からなる炭素鋼溶鋼を連続鋳造設備で鋳造して鋳片を製造する際に、脱炭を行なった溶鋼を、減圧雰囲気でＣ脱酸を行なって該溶鋼中の酸素濃度を３００ｐｐｍ以下とし、その後、ＭｎまたはＭｎ，ＳｉまたはＭｎ，Ｓｉそして溶鋼中のAl濃度が０．０１重量％以下となるように微量Alを金属または合金として添加して脱酸を行ない、次にＴｉを金属または合金として添加して脱酸し、更にＣａ金属または合金として添加して脱酸し、その後残りのＡｌを添加する介在物性欠陥の少ない薄鋼板用鋳片の製造方法である。
【００１４】
手段６は、前記手段５に、Ｎｂ：０．００１〜０．１０重量％、Ｖ：０．００５〜０．２０重量％、Ｃｒ：０．０１〜０．５０重量％、Ｍｏ：０．０１〜０．５０重量％、Ｃｕ：０．０１〜０．５０重量％、Ｎｉ：０．０１〜０．５０重量％、Ｂ：０．０００２〜０．００２０重量％の一種または二種以上を含有せしめるものである。
【００１５】
【発明の実施の形態】
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、図１に示すように、鋳片内の介在物のうち、５３μｍ以上の大きさのものが、鋳片１ｋｇあたり２００個以下で、しかも、５３μｍ以上のアルミナクラスタが鋳片１ｋｇ当たり２０個以下であると、製品欠陥発生率が極めて低く良好なものである。
【００１６】
これに対し、これ以外の場合（５３μｍ以上の介在物が２００個超、アルミナクラスタが２０個超）には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。
【００１７】
ここで、アルミナクラスタとは、複数の粒子が凝集したもので、この集合体を１個と数える。一般的に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、生成後すぐに粒子どうしが凝集し、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、構成粒子どうしの間に鉄を含むので、比重が大きく浮上しにくい。また、アルミナクラスタのほうが他の介在物よりも、製品欠陥に与える影響が大きい。なお、５３μｍという数字は、介在物分析法におけるフィルターの編み目のサイズである。
【００１８】
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。
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重量％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重量％とし、上限は板材で用いられる最大炭素量として０．２重量％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０．５重量％とした。また、下限は不可避的に混入するために０．０１重量％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０．５重量％とした。また、不可避的に混入するためその下限を０．００１重量％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重量％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０．３重量％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重量％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重量％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、鋳片中の酸化物系介在物のうち、５３μｍ以上の介在物の個数が２００個／ｋｇ以下であり、かつその内、アルミナ粒子が２個以上合体したアルミナクラスタ介在物の個数が２０個／ｋｇ以下であることを満たすためには、本発明では極力Ａｌを脱酸元素として用いないことが、基本思想である。
【００２２】
しかしながら、発明の対象となる薄鋼板用鋳片においては、材質上Ａｌが必要とされる。すなわち、Ａｌは鋼中でＡｌＮとなって鋼の結晶粒の成長を抑える働きがあり、この観点から、必用成分として規格化されている。そこで、下限を０．００６重量％超とした。また、上限は本発明で用いるＴｉおよびＣａの効果を阻害しないために、０．１重量％とした。
【００２３】
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、５３μｍ以上の介在物の個数が２００個／ｋｇ以下であり、かつその内、アルミナ粒子が２個以上合体したアルミナクラスタ介在物の個数が２０個／ｋｇ以下であることを満たすためには、Ａｌを脱酸材として用いるのではなく、後述するようにＴｉやＣａを用いる必要があることを、発明者らは知見した。
【００２４】
Ｔｉの下限値は、脱酸効果を得るために０．００５重量％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重量％と規定した。
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重量％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０．０１重量％とした。
【００２５】
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、０．０１重量％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重量％とした。
【００２６】
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５３μｍ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重量％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重量％とした。
【００２７】
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、Ｎｂ：０．００１〜０．１０重量％、Ｖ：０．００５〜０．２０重量％、Ｃｒ：０．０１〜０．５０重量％、Ｍｏ：０．０１〜０．５０重量％、Ｃｕ：０．０１〜０．５０重量％、Ｎｉ：０．０１〜０．５０重量％、Ｂ：０．０００２〜０．００２０重量％とする。
【００２８】
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。
【００２９】
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。
また、鋳片中の酸化物系介在物のうち、５３μｍ以上の介在物の個数を２００個／ｋｇ以下とし、かつその内のアルミナクラスタの個数を２０個／ｋｇ以下としたのは、図１に示したように、製品欠陥の発生率が小さくなる条件から決定したものである。
【００３０】
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。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、生成後すぐに粒子どうしが凝集し、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、構成粒子どうし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。
【００３１】
そこで、発明者らはＡｌを脱酸材として用いないことを考え、Ａｌに代わる脱酸元素として、Ｃａに着目した。Ｃａで脱酸すると、脱酸生成物であるＣａＯが生成するが、そのサイズが他の脱酸元素に較べて小さい特徴がある。しかしながら、このＣａＯのサイズは、Ｃａ添加前の溶鋼酸素濃度に大きく依存する。
【００３２】
発明者らは、ＣａＯ介在物のサイズが小さくなるＣａ添加前の溶鋼酸素濃度について、ラボ実験により求めた。鋼の成分は０．０４％Ｃ−０．００１０％ＮでＴｉ，Ｃａ，酸素量を変化させた。なお、他の成分は含まれていない。図２には、Ｃａ脱酸直後のＣａＯ介在物平均粒径と、Ｃａ脱酸前の溶鋼酸素濃度の関係を示すが、溶鋼酸素濃度が５０ｐｐｍ以下の場合に、生成したＣａＯ介在物の平均サイズが１０μｍ以下と非常に小さくなることが判った。
【００３３】
次に、Ｃａ添加前の溶鋼酸素濃度を５０ｐｐｍ以下に制御する手段について検討した。熱力学的に検討すると、溶鋼酸素濃度を５０ｐｐｍ以下にするためには、Ｓｉよりも酸素親和力の強い脱酸元素を選択するのが良い。これは、Ｓｉを０．５重量％と比較的多量に入れて脱酸した時に、溶鋼温度１６００℃で熱力学的に平衡する溶鋼酸素濃度が約７０ｐｐｍであることから推測出来る。
【００３４】
これに当てはまる脱酸元素としては、Ｔｉ、Ａｌ、Ｍｇ、Ｃａが挙げられるが、Ｃａはその後の脱酸で用いるので、除外される。また、ＭｇはＣａに近い強脱酸元素なので、除かれる。また、Ａｌについては、脱酸元素として用いないことが本発明の基本思想であるから除いた。
以上の考察から、Ｃａ添加前の溶鋼酸素濃度を５０ｐｐｍ以下に制御する手段として、Ｔｉを用いることにした。Ｔｉ脱酸は、脱酸するために必要な濃度が数百ｐｐｍと、ＭｎやＳｉの場合に較べて非常に少ないことも特徴である。
【００３５】
しかしながら、Ｔｉ脱酸においても、Ｃａ脱酸と同様に、Ｔｉ添加前の溶鋼酸素濃度が、生成するＴｉ酸化物のサイズに大きく影響する。すなわち、溶鋼酸素濃度が高い場合には、生成するＴｉ酸化物が大きいものになり、本発明の意図と矛盾することになる。
そこで、発明者らは、Ｔｉ酸化物のサイズが小さくなるＴｉ添加前の溶鋼酸素濃度について、ラボ実験により求めた。図３には、Ｔｉ脱酸直後のＴｉ酸化物の平均粒径と、Ｔｉ脱酸前の溶鋼酸素濃度の関係を示すが、溶鋼酸素濃度が３００ｐｐｍ以上では、生成したＴｉ酸化物のサイズが急激に大きくなることが判った。従って、Ｔｉ添加前の溶鋼酸素濃度を３００ｐｐｍ以下とする必要があることが判明した。
【００３６】
次に、Ｔｉ添加前の溶鋼酸素濃度を３００ｐｐｍ以下に制御する手段について検討した。熱力学的に検討すると、溶鋼酸素濃度を３００ｐｐｍ以下にするためには、Ｍｎ脱酸やＳｉ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用があった。
【００３７】
発明者らは、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３００ｐｐｍ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重量％の場合、溶鋼温度１６００℃で雰囲気中のＣＯ分圧が約０．４であれば、平衡する溶鋼酸素濃度は約３００ｐｐｍ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。
【００３８】
次に、Ａｌ添加について検討した。本発明では、Ａｌを脱酸元素として使用しないことが基本的思想であるが、発明の対象となる薄鋼板用鋳片では、材質上
Ａｌが必要とされる。
すなわち、Ａｌは鋼中でＡｌＮとなって鋼の結晶粒の成長を抑える働きがあり、この観点から、必用成分として規格化されている。しかしながら、Ａｌは酸素との親和力が非常に大きいので、溶鋼酸素が高いうちにＡｌを添加すると、多量のアルミナ介在物が生成し、本発明の意図が満たされなくなる。
【００３９】
そこで、本発明では、Ａｌを添加する時期として、Ｃａ添加の後と規定した。Ａｌに較べて、Ｃａのほうが酸素親和力が大きいので、Ｃａ添加により溶鋼酸素濃度は非常に低下している。そこにＡｌを添加しても、Ａｌは酸素と結合することがほとんどなく、溶鋼中に溶解する。すなわち、この場合Ａｌは脱酸元素としては働かない。従って、Ｃａ脱酸後にＡｌを添加することが重要である。
なお、Ｃ脱酸を行なった後でＭｎやＭｎ，Ｓｉおよび微量Ａｌを添加してからＴｉ添加をしても構わない。ここで、微量あｌとは、添加した後、溶鋼中の濃度で０．０１重量％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。そして、Ｃａ添加の後で残りのＡｌを添加すればよい。
【００４０】
また、脱酸を行なう前に、取鍋内溶鋼上のスラグにＣａＯやＡｌを添加して、スラグ中の酸素ポテンシャルを低下させる、いわゆるスラグ改質を行なうことは、本発明の効果にとっても有利な方法であり、スラグ改質を行なうほうが、更なる介在物個数の低減と介在物の微細化が期待できる。
【００４１】
【実施例】
表１に示す成分の炭素鋼を表３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４に示した方法で行なった。なお、水準Ａ−１、Ｃ−１、Ｄ−１はスラグ改質として、Ｃ脱酸前に、取鍋内のスラグ上に、溶鋼３００ｔにつきＣａＯを１．５ｔ、Ａｌを５００ｋｇ添加した。また、Ｅ−１ではＭｎ，Ｓｉ，微量をＡｌをC脱酸後Ｔｉ添加前に添加し、Ｇ−１，Ｈ−１ではＭｎ，ＳｉをＣ脱酸後Ｔｉ添加前に添加した。
【００４２】
【表１】

【００４３】
【表２】

【００４４】
【表３】

【００４５】
【表４】

【００４６】
結果を表２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。
【００４７】
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Ｂ−２、Ｅ−２、Ｆ−２では、脱酸用合金元素であるＴｉ，Ｃａの添加前にＣ脱酸を行なっていないので、結果的に脱酸用合金元素添加前の溶鋼酸素濃度が３００ｐｐｍより高くなり、Ｔｉ−Ｃａ−Ａｌの順序で添加を行なっても、鋳片内介在物個数が多くなっている。
【００４８】
比較材Ａ−２、Ｄ−２では、脱酸用合金元素添加前にＣ脱酸を行なったにもかかわらず、脱酸用合金元素添加前の溶鋼酸素濃度が３００ｐｐｍ以下を満たしていないので、鋳片内介在物個数が多くなっている。比較材Ｃ−２、Ｇ−２、Ｈ−２では、脱酸元素の添加順序が本発明を満たしていなかったので製品加工時に欠陥が発生した。
【００４９】
また、比較材Ｉ−１ではＴｉ濃度が低く本発明を満たさないため、またＪ−１ではＡｌが高く、本発明を満たさないため、Ｋ−１ではＴｉ、Ｃａ、酸素の濃度が高く本発明を満たしていないために、鋳片内介在物個数が多くなっている。特にＪ−１の場合には、５３μｍ以上の介在物個数は、条件を満たしているが、アルミナクラスタ個数が本発明範囲よりも多くなっている。
【００５０】
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。ここで、表３中の加工欠陥の欄で、−印となっているものは、コイル段階で不合格になったために、製品にはならず、加工に至らなかったものである。
【００５１】
【発明の効果】
以上の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た。従って、本発明により、介在物性欠陥の少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】鋳片内介在物個数と製品欠陥の発生率の関係を示した図。
【図２】Ｍｇ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。
【図３】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]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a continuous cast slab of carbon steel for thin steel sheets and a method for producing the same, and more particularly, to a slab having few inclusion defects and a method for producing the same.
[0002]
[Prior art]
In recent years, defects in inclusions in slabs manufactured by the continuous casting method have been extremely reduced. This is the result of the technical improvement of the deoxidation method at the molten steel stage and the measures taken against various inclusions in continuous casting. (The 126th and 127th Nishiyama Memorial Technical Lecture “High Purity Steel” Nippon Steel Corp. Association, 1988).
[0003]
However, in cast pieces for thin plates, particularly cast pieces for beverage can materials, more and more inclusions are required to be reduced, and it is required to reduce the size as well as to reduce the number of inclusions. As a technique for reducing the number of inclusions in a slab, there are, for example, JP-A-07-200612 and JP-A-05-331522.
Techniques for forming fine inclusions include, for example, JP-A-58-204117 and JP-A-3-267311.
[0004]
As a technique for reducing the number of inclusions in a slab for a beverage can, Japanese Patent Application Laid-Open No. H07-300012 discloses a technique in which, during secondary refining, a flux is blown into a molten steel from a gas injection lance, and the flux is mixed with the inclusions. Although it is described that the flux is caused to agglomerate and coalesce, it is feared that the blown flux remains in the molten steel and becomes an inclusion.
[0005]
Further, in the above-mentioned Japanese Patent Application Laid-Open No. 05-331522, after CaO is charged into a converter to solidify slag, steel is tapped into a ladle, and then Al is added to the slag on the ladle, and slag is added. It is described that the concentration of FeO in the medium is 2% or less. However, in order to stably reduce the concentration of FeO in the slag to 2% or less, a large amount of Al needs to be introduced, which is costly. 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 formed to form a cluster.
Since the specific gravity is large, it is not expected that the number of alumina clusters greatly decreases due to floating on the molten steel surface.
[0006]
As a technique for reducing the size of inclusions, Japanese Patent Application Laid-Open No. 58-204117 discloses a technique in which Mn, Si and Ti or Al, or REM or Ca is added in order of decreasing deoxidizing power. However, since Mn is specified to be 0.8% by weight or more, it cannot be applied to a thin plate having a low Mn.
Japanese Patent Application Laid-Open No. 3-267311 discloses a deoxidation method using Ti and Ca. However, since Zr of 0.005% by weight or more is essential, the cost is high. Further, when the oxygen concentration of molten steel before the addition of Ti or Ca is high, even if deoxidation is performed by adding Ti or Ca, the effect of miniaturization of inclusions is not sufficiently exhibited. It will be big.
[0007]
For these reasons, it has been difficult to stably achieve the reduction in the number of inclusions and the miniaturization of the size of the inclusions in the slabs for thin steel plates for steel sheets with the techniques of the above-mentioned publications.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to provide a slab for a thin steel sheet having few inclusion defect and a method for producing the same by stably achieving the reduction of the number of inclusions and the miniaturization of the inclusion size of the slab.
That is, the present invention is a method for producing a slab and a slab that satisfies the inclusion condition in the slab for the inclusion property defect not to occur in the thin plate product, and is particularly limited by the slab for the thin steel plate. It is an object of the present invention to provide a slab having few inclusion defect and independent of Mn, Si and Al contents and a method for producing the same.
[0009]
[Means for Solving the Problems]
The present invention reduces the oxygen concentration in the molten steel by performing C deoxidation in a reduced-pressure atmosphere before adding the deoxidizing material to the molten steel, and then adding Ti and Ca as the metal or alloy in the order of deoxidizing material. Then, by adding Al, the number of oxide-based inclusions of 53 μm or more is 200 / kg or less, and among them, the number of alumina cluster inclusions is 20 / kg or less. Means 1 is as follows: C: 0.001 to 0.2% by weight, Mn: 0.01 to 0.5% by weight, Si: 0.001 to 0.5% by weight, P : 0.001 to 0.3% by weight, S: 0.0005 to 0.05% by weight, Al: more than 0.006 to 0.1% by weight, Ti: 0.005 to 0.06% by weight, Ca: 0.0005 to 0.01% by weight, N: 0.0005 to 0.01% by weight, oxygen: 0.0005 to 0.01% by weight Carbon steel containing 0.0050% by weight, the balance being iron and unavoidable impurities, and among the oxide-based inclusions in the slab, the number of inclusions with a size of 53 μm or more is 200 / kg or less, and And a cast piece for a thin steel sheet having a small number of inclusion defect, in which the number of alumina cluster inclusions is 20 / kg or less.
[0010]
Further, the means 2 is different from the means 1 in that Nb: 0.001 to 0.10% by weight, V: 0.005 to 0.20% by weight, Cr: 0.01 to 0.50% by weight, Mo: 0. 01 to 0.50% by weight, Cu: 0.01 to 0.50% by weight, Ni: 0.01 to 0.50% by weight, B: 0.0002 to 0.0020% by weight. It is to be included.
[0011]
Means 3 is as follows: C: 0.001 to 0.2% by weight, Mn: 0.01 to 0.5% by weight, Si: 0.001 to 0.5% by weight, P: 0.001 to 0.3% by weight %, S: 0.0005 to 0.05 wt%, Al: more than 0.006 to 0.1 wt%, Ti: 0.005 to 0.06 wt%, Ca: 0.0005 to 0.01 wt% , N: 0.0005 to 0.01% by weight, Oxygen: 0.0005 to 0.0050% by weight, and cast carbon steel molten steel consisting of iron and unavoidable impurities by a continuous casting facility to produce a slab. When performing decarbonization, the decarbonized molten steel is subjected to C deoxidation in a reduced-pressure atmosphere to reduce the oxygen concentration in the molten steel to 300 ppm or less, and then added as a metal or an alloy in the order of Ti and Ca to be deoxidized. And a method of manufacturing a cast piece for a thin steel sheet having a small number of inclusion defects, to which Al is added thereafter.
[0012]
The means 4 is different from the means 3 in that Nb: 0.001 to 0.10% by weight, V: 0.005 to 0.20% by weight, Cr: 0.01 to 0.50% by weight, Mo: 0.01. 0.50% by weight, Cu: 0.01 to 0.50% by weight, Ni: 0.01 to 0.50% by weight, B: 0.0002 to 0.0020% by weight. It is a hurry.
[0013]
Means 5 is as follows: C: 0.001 to 0.2% by weight, Mn: 0.01 to 0.5% by weight, Si: 0.001 to 0.5% by weight, P: 0.001 to 0.3% by weight. %, S: 0.0005 to 0.05 wt%, Al: more than 0.006 to 0.1 wt%, Ti: 0.005 to 0.06 wt%, Ca: 0.0005 to 0.01 wt% , N: 0.0005 to 0.01% by weight, Oxygen: 0.0005 to 0.0050% by weight, and cast carbon steel molten steel consisting of iron and unavoidable impurities by a continuous casting facility to produce a slab. The decarbonized molten steel is subjected to C deoxidation in a reduced pressure atmosphere to reduce the oxygen concentration in the molten steel to 300 ppm or less, and then the Mn or Mn, Si or Mn, Si and the Al concentration in the molten steel are reduced. Add a trace amount of Al as a metal or alloy so as to be 0.01% by weight or less to deoxidize And then deoxidizing by adding Ti as a metal or alloy, further deoxidizing by adding Ca as a metal or alloy, and then adding the remaining Al. It is.
[0014]
The means 6 is different from the means 5 in that Nb: 0.001 to 0.10% by weight, V: 0.005 to 0.20% by weight, Cr: 0.01 to 0.50% by weight, Mo: 0.01. 0.50% by weight, Cu: 0.01 to 0.50% by weight, Ni: 0.01 to 0.50% by weight, B: 0.0002 to 0.0020% by weight. It is a hurry.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
The inventors first studied the inclusion conditions of a cast slab in which inclusion defects are unlikely to occur in a product. Here, the inclusion means an oxide-based material that easily affects a product defect. Increasing the number of inclusions in the slab tends to cause inclusion defect 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, as shown in FIG. 1, among the inclusions in the slab, those having a size of 53 μm or more were found to be cast. When the number of alumina clusters is 200 or less per 1 kg of a piece and the number of alumina clusters of 53 μm or more is 20 or less per 1 kg of a slab, the product defect occurrence rate is extremely low and good.
[0016]
On the other hand, in other cases (exceeding 200 inclusions of 53 μm or more and exceeding 20 alumina clusters), it was found that the product defect occurrence rate was high, that is, product defects tended to occur.
[0017]
Here, the alumina cluster is an aggregate of a plurality of particles, and this aggregate is counted as one. In general, alumina, which is a product after Al deoxidation, has small individual particles, but immediately after generation, the particles aggregate to form a cluster and increase in size. In addition, since this cluster contains iron between constituent particles, it has a large specific gravity and is difficult to float. In addition, alumina clusters have a greater effect on product defects than other inclusions. The number 53 μm is the size of the stitch of the filter in the inclusion analysis method.
[0018]
Hereinafter, in order to explain the cast slab of the present invention in detail, the reason for defining the conditions of the present invention will be described.
C is an element used for imparting the strength of steel, but for thin sheets, it may be desirable to reduce C as much as possible by using a steel sheet for deep drawing. However, when C is 0.001% by weight or less, C deoxidation in the present invention becomes very difficult. Therefore, the lower limit is set to 0.001% by weight, and the upper limit is set to 0.2% by weight as the maximum carbon amount used in the sheet material. did.
[0019]
Also, Mn is necessary for obtaining strength and suppressing embrittlement due to S, and the upper limit is set to 0.5% by weight, which is the maximum value when used as a high-tensile material or the like. In addition, the lower limit is set to 0.01% by weight to be inevitably mixed.
Si is also an element used for obtaining strength and improving high-temperature characteristics, and the upper limit is set to 0.5% by weight. In addition, the lower limit was set to 0.001% by weight because of inevitable mixing.
[0020]
Since P is a harmful element to steel, it is desirable that P be as small as possible. However, since P is inevitably mixed, the lower limit of 0.001% by weight is practical. However, from the viewpoint of improving the strength and corrosion resistance of steel, a large amount of P may be required in some cases, so the upper limit is set to 0.3% by weight. Above this, the effect of embrittlement by P becomes stronger.
Similarly, S also often impairs the product characteristics, and is desirably as low as possible. However, the lower limit is 0.0005% by weight because it is inevitably mixed. The upper limit is set to 0.05% by weight in order to prevent cracking during continuous casting.
[0021]
Al is generally used as a deoxidizing element, but among the oxide-based inclusions in the slab, the number of inclusions of 53 μm or more is 200 / kg or less, and among them, alumina particles In order to satisfy that the number of the alumina cluster inclusions of two or more coalesced is 20 or less, it is a basic idea that in the present invention, Al is not used as a deoxidizing element as much as possible.
[0022]
However, in the slab for a thin steel sheet, which is an object of the present invention, Al is required due to its material. That is, Al acts as AlN in the steel to suppress the growth of crystal grains of the steel. From this viewpoint, Al is standardized as a necessary component. Therefore, the lower limit is set to more than 0.006% by weight. The upper limit is set to 0.1% by weight so as not to impair the effects of Ti and Ca used in the present invention.
[0023]
Ti and Ca are important elements of the present invention. Among the oxide-based inclusions in the slab, the number of inclusions of 53 μm or more is 200 / kg or less, and among them, the number of alumina cluster inclusions in which two or more alumina particles are combined is 20 / kg. The inventors have found that in order to satisfy the requirement of not more than kg, it is necessary to use Ti or Ca as described later instead of using Al as a deoxidizing material.
[0024]
The lower limit of Ti is set to 0.005% by weight in order to obtain a deoxidizing effect, and the upper limit is set to 0.06% by weight, because adding a large amount impairs the effect of Ca deoxidizing.
The lower limit of Ca was set to 0.0005% by weight in order to obtain a sufficient deoxidizing effect. The upper limit was set to 0.01% by weight as a level at which the effect would be saturated even if added excessively.
[0025]
N combines with Al to form AlN and is used to suppress the growth of crystal grains. From this viewpoint, the upper limit of the addition amount is set to 0.01% by weight. The lower limit was set to 0.0005% by weight in consideration of the inevitable mixing.
[0026]
Most of the oxygen amount in the slab is included as oxide-based inclusions in the slab. It is desirable to minimize the inclusions of 53 μm or more that are harmful to the product, but the smaller the number of large inclusions, the lower the oxygen content.
That is, even if there are many harmless fine inclusions in the product, the amount of oxygen is high. Therefore, when the oxygen amount is below a certain level, the oxygen amount cannot always be an index of the number of inclusions, but when the oxygen value is very high, the tendency is that the number of large inclusions tends to increase. Was set to 0.0050% by weight. The lower limit was set to 0.0005% by weight in consideration of the inevitable mixing.
[0027]
The above are the basic components of the steel targeted by the present invention. In order to improve various properties of the material such as strength, corrosion resistance, and hardenability, Nb, V, Cr, Even if one or more of Mo, Cu, Ni, and B are added, the effects of the present invention are not impaired at all.
That is, the range of the addition amount is as follows: Nb: 0.001 to 0.10% by weight, V: 0.005 to 0.20% by weight, Cr: 0.01 to 0.50% by weight, Mo: 0.01. To 0.50% by weight, Cu: 0.01 to 0.50% by weight, Ni: 0.01 to 0.50% by weight, and B: 0.0002 to 0.0020% by weight.
[0028]
As other elements, REM elements may be included in the molten steel, but if the content of each element is up to 10 ppm, the effect of the present invention is not affected even if it is included.
[0029]
In an actual manufacturing process, the added elements are not necessarily included in 100% molten steel. Therefore, it is necessary to add extra elements in consideration of the yield. There is no particular limitation on the method of addition. Any method may be used as long as it can be contained in steel so as to satisfy the above conditions.
Further, among the oxide-based inclusions in the slab, the number of inclusions having a size of 53 μm or more was set to 200 / kg or less, and the number of alumina clusters was set to 20 / kg or less in FIG. As shown in FIG. 7, the conditions are determined from the condition that the incidence of product defects is reduced.
[0030]
Next, a manufacturing method for satisfying such inclusion conditions in the slab was examined. The inventors first paid attention to deoxidizing elements. Generally, Al is widely used as a deoxidizing element of molten steel. However, alumina, which is a product after Al deoxidation, has small individual particles, but immediately after the generation, the particles aggregate to form clusters and increase in size.
In addition, since this cluster contains iron between constituent particles, it has a large specific gravity and is difficult to float. Therefore, in order to float and remove the alumina inclusions generated by Al deoxidation, a very long standing time is required, a large amount of Ar gas is blown into the molten steel, and the gas and the inclusions are united to float. Measures such as promotion were necessary.
[0031]
Then, the inventors considered not using Al as a deoxidizing material, and focused on Ca as a deoxidizing element instead of Al. Deoxidation with Ca produces CaO, which is a deoxidation product, but its size is smaller than other deoxidizing elements. However, the size of this CaO largely depends on the oxygen concentration of molten steel before adding Ca.
[0032]
The inventors determined by laboratory experiments the oxygen concentration of molten steel before the addition of Ca at which the size of CaO inclusions was reduced. The composition of steel was 0.04% C-0.0010% N, and the amounts of Ti, Ca and oxygen were changed. In addition, other components are not included. FIG. 2 shows the relationship between the average particle size of CaO inclusions immediately after Ca deoxidation and the oxygen concentration of molten steel before Ca deoxidation. When the oxygen concentration of molten steel is 50 ppm or less, the average size of CaO inclusions generated Was found to be very small at 10 μm or less.
[0033]
Next, means for controlling the oxygen concentration of molten steel before adding Ca to 50 ppm or less was examined. Considering thermodynamically, in order to reduce the oxygen concentration of molten steel to 50 ppm or less, it is preferable to select a deoxidizing element having a higher oxygen affinity than Si. This can be inferred from the fact that when oxygen is added in a relatively large amount of 0.5% by weight and deoxidized, the oxygen concentration of molten steel which equilibrates thermodynamically at a molten steel temperature of 1600 ° C. is about 70 ppm.
[0034]
Examples of the deoxidizing element applicable to this include Ti, Al, Mg, and Ca. However, Ca is excluded because it is used in the subsequent deoxidizing. Also, Mg is a strong deoxidizing element close to Ca, and is therefore excluded. In addition, Al was omitted because it is a basic idea of the present invention that it is not used as a deoxidizing element.
From the above considerations, Ti was used as a means for controlling the oxygen concentration of molten steel before adding Ca to 50 ppm or less. Ti deoxidation is also characterized in that the concentration required for deoxidation is several hundred ppm, which is very small as compared with the case of Mn or Si.
[0035]
However, in the case of Ti deoxidation, the oxygen concentration of molten steel before the addition of Ti greatly affects the size of the generated Ti oxide as in the case of Ca deoxidation. That is, when the molten steel oxygen concentration is high, the generated Ti oxide is large, which contradicts the intention of the present invention.
Then, the inventors obtained through a laboratory experiment the oxygen concentration of molten steel before the addition of Ti at which the size of the Ti oxide was reduced. FIG. 3 shows the relationship between the average particle diameter of the Ti oxide immediately after Ti deoxidation and the oxygen concentration of molten steel before Ti deoxidation. When the oxygen concentration of molten steel is 300 ppm or more, the size of the generated Ti oxide sharply increases. It turned out to be bigger. Therefore, it became clear that the oxygen concentration of molten steel before adding Ti needs to be 300 ppm or less.
[0036]
Next, means for controlling the oxygen concentration of molten steel before adding Ti to 300 ppm or less was studied. According to thermodynamic studies, Mn deoxidation and Si deoxidation are mentioned in order to reduce the molten steel oxygen concentration to 300 ppm or less. May be constrained lower. Therefore, it was necessary to consider a deoxidation method independent of Mn and Si concentrations.
[0037]
The present inventors have focused on C and considered that the oxygen concentration of molten steel is reduced to 300 ppm or less by performing C deoxidation under reduced pressure. Considering the C deoxidation equilibrium, for example, when the C concentration is 0.04% by weight, if the molten steel temperature is 1600 ° C. and the CO partial pressure in the atmosphere is about 0.4, the equilibrium molten steel oxygen concentration is about 300 ppm. The requirements required by the invention can be satisfied. C deoxidation is also characterized in that since the deoxidation product is CO gas, it remains in molten steel and does not become inclusions.
[0038]
Next, the addition of Al was studied. In the present invention, the basic idea is not to use Al as a deoxidizing element, but the cast piece for a thin steel sheet, which is the subject of the present invention, requires Al because of its material.
That is, Al acts as AlN in the steel to suppress the growth of crystal grains of the steel. From this viewpoint, Al is standardized as a necessary component. However, since Al has a very high affinity for oxygen, if Al is added while molten steel oxygen is high, a large amount of alumina inclusions will be generated, and the intention of the present invention will not be satisfied.
[0039]
Therefore, in the present invention, the timing of adding Al is defined as after the addition of Ca. Since Ca has a higher oxygen affinity than Al, the oxygen concentration of molten steel is significantly reduced by the addition of Ca. Even if Al is added thereto, Al hardly combines with oxygen and dissolves in molten steel. That is, in this case, Al does not work as a deoxidizing element. Therefore, it is important to add Al after Ca deoxidation.
After C deoxidation, Mn, Mn, Si and a small amount of Al may be added, and then Ti may be added. Here, the trace amount refers to a case in which the concentration in the molten steel is 0.01% by weight or less after the addition, and does not significantly affect the formation of inclusions. It may be acid. Then, the remaining Al may be added after the addition of Ca.
[0040]
Before performing deoxidation, performing so-called slag reforming, in which CaO or Al is added to the slag on the molten steel in the ladle to reduce the oxygen potential in the slag, is also advantageous to the effects of the present invention. The slag reforming can be expected to further reduce the number of inclusions and refine the inclusions.
[0041]
【Example】
When carbon steel having the components shown in Table 1 was manufactured under the manufacturing conditions shown in Table 3, the number of inclusions in the obtained slab, the steel plate obtained by rolling the slab, and the case where the steel plate was processed as a raw material The results were investigated. The survey method was as shown in Table 4. For the slag reforming of the levels A-1, C-1, and D-1, 1.5 tons of CaO and 500 kg of Al were added per 300 tons of molten steel to the slag in the ladle before C deoxidation. For E-1, Mn, Si, and trace amounts of Al were added after deoxidizing C and before adding Ti. For G-1 and H-1, Mn and Si were added after deoxidizing C and before adding Ti.
[0042]
[Table 1]

[0043]
[Table 2]

[0044]
[Table 3]

[0045]
[Table 4]

[0046]
Table 2 shows the results. From the table, when the condition of the present invention is satisfied, the number of inclusions in the slab is small, good results that no rejection due to surface flaws or internal defects does not occur, and further no defects during processing occur was gotten.
[0047]
On the other hand, the comparative materials that do not satisfy the present invention have the following problematic results.
That is, in the comparative materials B-2, E-2, and F-2, C deoxidation was not performed before the addition of the deoxidizing alloy elements Ti and Ca. And the molten steel oxygen concentration becomes higher than 300 ppm, and the number of inclusions in the slab is increased even if the addition is performed in the order of Ti-Ca-Al.
[0048]
In the comparative materials A-2 and D-2, the oxygen concentration of molten steel before the addition of the deoxidizing alloy element did not satisfy 300 ppm or less, although the C deoxidation was performed before the addition of the deoxidizing alloy element. The number of inclusions in the slab is increasing. In the comparative materials C-2, G-2, and H-2, the order of adding the deoxidizing element did not satisfy the present invention, and thus defects occurred during product processing.
[0049]
Further, since the comparative material I-1 has a low Ti concentration and does not satisfy the present invention, and the J-1 has a high Al and does not satisfy the present invention, the K-1 has a high concentration of Ti, Ca and oxygen, and thus the present invention has a high concentration. Is not satisfied, the number of inclusions in the slab increases. In particular, in the case of J-1, the number of inclusions of 53 μm or more satisfies the condition, but the number of alumina clusters is larger than the range of the present invention.
[0050]
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. Here, in the column of the machining defect in Table 3, those marked with a minus sign indicate that the product was not rejected at the coil stage and thus did not become a product and did not undergo machining.
[0051]
【The invention's effect】
As described above, according to the present invention, a slab for a thin steel sheet in which the number of harmful inclusions has been significantly reduced was obtained, and a coil defect after rolling and a defect during processing of a product were extremely small. Therefore, according to the present invention, it is possible to manufacture a cast piece for a thin steel sheet having few inclusion defect.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the number of inclusions in a slab and the incidence of product defects.
FIG. 2 is a diagram showing the relationship between molten steel oxygen content and inclusion size before Mg addition.
FIG. 3 is a graph showing the relationship between the oxygen content of molten steel and the size of inclusions before the addition of Ti.

Claims

C: 0.001 to 0.2% by weight, Mn: 0.01 to 0.5% by weight, Si: 0.001 to 0.5% by weight, P: 0.001 to 0.3% by weight, S: 0.0005 to 0.05% by weight, Al: more than 0.006 to 0.1% by weight, Ti: 0.005 to 0.06% by weight, Ca: 0.0005 to 0.01% by weight, N: 0 0.0005 to 0.01% by weight, oxygen: 0.0005 to 0.0050% by weight, the balance being iron and unavoidable impurities. A slab for a thin steel sheet having few inclusion defects, wherein the number of inclusions is 200 / kg or less, and among them, the number of alumina cluster inclusions is 20 / kg or less.

Nb: 0.001 to 0.10% by weight, V: 0.005 to 0.20% by weight, Cr: 0.01 to 0.50% by weight, Mo: 0.01 to 0.50% by weight, Cu: 2. The composition according to claim 1, wherein one or more of 0.01 to 0.50% by weight, Ni: 0.01 to 0.50% by weight, and B: 0.0002 to 0.0020% by weight are contained. A slab for a thin steel sheet having few described inclusion defects.

C: 0.001 to 0.2% by weight, Mn: 0.01 to 0.5% by weight, Si: 0.001 to 0.5% by weight, P: 0.001 to 0.3% by weight, S: 0.0005 to 0.05% by weight, Al: more than 0.006 to 0.1% by weight, Ti: 0.005 to 0.06% by weight, Ca: 0.0005 to 0.01% by weight, N: 0 0.0005 to 0.01% by weight, oxygen: 0.0005 to 0.0050% by weight, and when casting carbon steel molten steel consisting of iron and unavoidable impurities with a continuous casting facility to produce a slab, The decarbonized molten steel is subjected to C deoxidation in a reduced-pressure atmosphere to reduce the oxygen concentration in the molten steel to 300 ppm or less, and then added as a metal or an alloy in the order of Ti and Ca to be deoxidized. A method for producing a slab for a thin steel sheet having less inclusion defect, characterized by adding.

Nb: 0.001 to 0.10% by weight, V: 0.005 to 0.20% by weight, Cr: 0.01 to 0.50% by weight, Mo: 0.01 to 0.50% by weight, Cu: 4. The composition according to claim 3, wherein one or more of 0.01 to 0.50% by weight, Ni: 0.01 to 0.50% by weight, and B: 0.0002 to 0.0020% by weight are contained. A method for producing a slab for a thin steel sheet having few inclusion defect as described above.

C: 0.001 to 0.2% by weight, Mn: 0.01 to 0.5% by weight, Si: 0.001 to 0.5% by weight, P: 0.001 to 0.3% by weight, S: 0.0005 to 0.05% by weight, Al: more than 0.006 to 0.1% by weight, Ti: 0.005 to 0.06% by weight, Ca: 0.0005 to 0.01% by weight, N: 0 0.0005 to 0.01% by weight, oxygen: 0.0005 to 0.0050% by weight, and when casting carbon steel molten steel consisting of iron and unavoidable impurities with a continuous casting facility to produce a slab, The decarbonized molten steel is subjected to C deoxidation in a reduced pressure atmosphere to reduce the oxygen concentration in the molten steel to 300 ppm or less, and then the Mn or Mn, Si or Mn, Si and the Al concentration in the molten steel are reduced to 0.01 wt. % Or less as a metal or alloy to perform deoxidation. Characterized in that Ti is added as a metal or an alloy to deoxidize, and further added as a Ca metal or an alloy to be deoxidized, and then the remaining Al is added, and the slab for a thin steel sheet with few inclusion defects is characterized in that Manufacturing method.

Nb: 0.001 to 0.10% by weight, V: 0.005 to 0.20% by weight, Cr: 0.01 to 0.50% by weight, Mo: 0.01 to 0.50% by weight, Cu: 6. One or two or more of 0.01 to 0.50% by weight, Ni: 0.01 to 0.50% by weight, and B: 0.0002 to 0.0020% by weight. A method for producing a slab for a thin steel sheet having few inclusion defect as described above.