JP3760144B2 - Ultra-low carbon steel sheet, ultra-low carbon steel slab and method for producing the same - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、加工性、成形性に優れた極低炭素鋳片、極低炭素鋼板およびその製造方法に関するものである。
【０００２】
【従来の技術】
転炉や真空処理容器で精錬された溶鋼中には、多量の溶存酸素が含まれており、この過剰酸素は酸素との親和力が強い強脱酸元素であるＡｌにより脱酸されるのが一般的である。しかし、Ａｌは脱酸によりアルミナ系介在物を生成し、これが凝集合体して粗大なアルミナクラスターとなる。このアルミナクラスターは鋼板製造時に表面疵発生の原因となり、薄鋼板の品質を大きく劣化させる。特に、炭素濃度が低く、精錬後の溶存酸素濃度が高い薄鋼板用素材である極低炭素溶鋼では、アルミナクラスターの量が非常に多く、表面疵の発生率が極めて高いため、アルミナ系介在物の低減対策は大きな課題となっている。
【０００３】
これに対して、従来は特開平５−１０４２１９号公報の介在物吸着用フラックスを溶鋼表面に添加してアルミナ系介在物を除去する方法、或いは特開昭６３−１４９０５７号公報の注入流を利用してＣａＯフラックスを溶鋼中に添加し、これによりアルミナ系介在物を吸着除去する方法が提案、実施されてきた。一方、アルミナ系介在物を除去するのではなく、生成させない方法として、特開平５−３０２１１２号公報にあるように溶鋼をＭｇで脱酸し、Ａｌでは殆ど脱酸しない薄鋼板用溶鋼の溶製方法も開示されている。
【０００４】
【発明が解決しようとする課題】
しかしながら、上述したアルミナ系介在物を除去する方法では、極低炭素溶鋼中に多量に生成したアルミナ系介在物を表面疵が発生しない程度まで低減することは非常に難しい。また、アルミナ系介在物を全く生成しないＭｇ脱酸では、Ｍｇの蒸気圧が高く、溶鋼への歩留まりが非常に低いため、極低炭素鋼のように溶存酸素濃度が高い溶鋼をＭｇだけで脱酸するには多量のＭｇを必要とし、製造コストを考えると実用的なプロセスとは言えない。
【０００５】
これらの問題に鑑み、本発明は溶鋼中で殆ど介在物を生成させることなく、凝固時に酸化物を微細に析出させることにより、確実に表面疵を防止できる極低炭素鋼板、極低炭素鋳片とその製造方法を提供することを目的とする。
【０００６】
【課題を解決するための手段】
上記課題を解決するために、本発明は以下の構成を要旨とする。
（１）溶鋼の炭素濃度を０．００２質量％以下まで脱炭した後、該溶鋼にＮｂを添加して、溶鋼中のＮｂ濃度を０．００５質量％以上、０．０５質量％以下にし、さらにＡｌあるいはＴｉで脱酸を行って溶鋼中の溶存酸素濃度を０．０２質量％以上、０．０６質量％以下に調整した溶鋼、またはＮｂを添加した際の溶鋼中の溶存酸素濃度が０．０２質量％以上、０．０６質量％以下である溶鋼、を鋳造することを特徴とする極低炭素鋼鋳片の製造方法。
（２）真空脱ガス処理により炭素濃度を０．００２質量％以下まで脱炭した後、該溶鋼にＮｂを添加して、溶鋼中のＮｂ濃度を０．００５質量％以上、０．０５質量％以下にし、さらにＡｌあるいはＴｉで脱酸を行って溶鋼中の溶存酸素濃度を０．０２質量％以上、０．０６質量％以下に調整した溶鋼、またはＮｂを添加した際の溶鋼中の溶存酸素濃度が０．０２質量％以上、０．０６質量％以下である溶鋼、を鋳造することを特徴とする極低炭素鋼鋳片の製造方法。
（３）溶鋼を鋳造するに際し、電磁攪拌、或いは電磁場の印加を行いながら鋳造することを特徴とする（１）または（２）に記載の極低炭素鋼鋳片の製造方法。
（４）溶鋼を鋳造するに際し、電磁攪拌を行って、メニスカス位置における溶鋼を４０ｃｍ／ｓ以上、１００ｃｍ／ｓ以下の平均流速で旋回させながら鋳造することを特徴とする（１）または（２）に記載の極低炭素鋼鋳片の製造方法。
（５）溶鋼を鋳造するに際し、電磁場の印加を行い、メニスカス位置における溶鋼を０．１Ｈｚ以上、１００Ｈｚ以下で水平方向に振動させながら鋳造することを特徴とする（１）または（２）に記載の極低炭素鋼鋳片の製造方法。
（６）（１）〜（５）のいずれかに記載の方法により製造された鋳片を加工して得られる極低炭素鋼板であって、直径０．５μｍから３０μｍの微細酸化物が１０００個／ｃｍ ２ 以上、１００００００個／ｃｍ ２ 未満分散し、且つその酸化物に少なくともＳｉ、Ｍｎ、Ｆｅを含んでいることを特徴とする極低炭素鋼板。
（７）（１）〜（５）のいずれかに記載の方法により製造された鋳片を加工して得られる極低炭素鋼板であって、鋼板中に存在する酸化物の個数割合で４０％以上が少なくともＳｉ、Ｍｎ、Ｆｅを含んでいることを特徴とする極低炭素鋼板。
（８）（１）〜（５）のいずれかに記載の方法により製造された鋳片を加工して得られる極低炭素鋼板であって、鋼板中に存在する酸化物の個数割合で４０％以上が少なくともＳｉ、Ｍｎ、Ｆｅを含んだ球状酸化物であることを特徴とする極低炭素鋼板。
（９）（１）〜（５）のいずれかに記載の方法により製造された鋳片を加工して得られる極低炭素鋼板であって、鋼板中に存在する酸化物の個数割合で４０％以上が少なくともＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物の含有率で、２０質量％以上であることを特徴とする極低炭素鋼板。
（１０）（１）〜（５）のいずれかに記載の方法により製造された鋳片を加工して得られる極低炭素鋼板であって、鋼板中に存在する酸化物の個数割合で４０％以上が少なくともＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物の含有率で、２０質量％以上の球状酸化物であることを特徴とする極低炭素鋼板。
（１１）（１）〜（５）のいずれかに記載の方法により製造された鋳片を加工して得られる極低炭素鋼板であって、直径０．５μｍから３０μｍの微細酸化物が１０００個／ｃｍ ２ 以上、１００００００個／ｃｍ ２ 未満分散し、且つその酸化物の個数割合で４０％以上が少なくともＳｉ、Ｍｎ、Ｆｅを含んでいることを特徴とする極低炭素鋼板。
（１２）（１）〜（５）のいずれかに記載の方法により製造された鋳片を加工して得られる極低炭素鋼板であって、直径０．５μｍから３０μｍの微細酸化物が１０００個／ｃｍ ２ 以上、１００００００個／ｃｍ ２ 未満分散し、且つその酸化物の個数割合で４０％以上が少なくともＳｉ、Ｍｎ、Ｆｅを含んだ球状酸化物であることを特徴とする極低炭素鋼板。
（１３）（１）〜（５）のいずれかに記載の方法により製造された鋳片を加工して得られる極低炭素鋼板であって、直径０．５μｍから３０μｍの微細酸化物が１０００個／ｃｍ２以上、１００００００個／ｃｍ２未満分散し、且つその酸化物の個数割合で４０％以上が少なくともＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物の含有率で、２０質量％以上であることを特徴とする極低炭素鋼板。
（１４）（１）〜（５）のいずれかに記載の方法により製造された鋳片を加工して得られる極低炭素鋼板であって、直径０．５μｍから３０μｍの微細酸化物が１０００個／ｃｍ２以上、１００００００個／ｃｍ２未満分散し、且つその酸化物の個数割合で４０％以上が少なくともＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物の含有率で、２０質量％以上の球状酸化物であることを特徴とする極低炭素鋼板。
（１５）（１）〜（５）のいずれかに記載の製造方法により得られる極低炭素鋼鋳片であって、直径０．５μｍから３０μｍの微細酸化物が鋳片表層から２０ｍｍの範囲内に１０００個／ｃｍ２以上、１００００００個／ｃｍ２未満分散し、且つその酸化物の一部または全部に少なくともＳｉ、Ｍｎ、Ｆｅを含んでいることを特徴とする極低炭素鋼鋳片。
（１６）（１）〜（５）のいずれかに記載の製造方法により得られる極低炭素鋼鋳片であって、鋳片表層から２０ｍｍの範囲内に存在する酸化物の個数割合で４０％以上が少なくともＳｉ、Ｍｎ、Ｆｅを含んでいることを特徴とする極低炭素鋼鋳片。
（１７）（１）〜（５）のいずれかに記載の製造方法により得られる極低炭素鋼鋳片であって、鋳片表層から２０ｍｍの範囲内に存在する酸化物の個数割合で４０％以上が少なくともＳｉ、Ｍｎ、Ｆｅを含んだ球状酸化物であることを特徴とする極低炭素鋼鋳片。
（１８）（１）〜（５）のいずれかに記載の製造方法により得られる極低炭素鋼鋳片であって、鋳片表層から２０ｍｍの範囲内に存在する酸化物の個数割合で４０％以上が少なくともＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物の含有率で、２０質量％以上であることを特徴とする極低炭素鋼鋳片。
（１９）（１）〜（５）のいずれかに記載の製造方法により得られる極低炭素鋼鋳片であって、鋳片表層から２０ｍｍの範囲内に存在する酸化物の個数割合で４０％以上が少なくともＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物の含有率で、２０質量％以上の球状酸化物であることを特徴とする極低炭素鋼鋳片。
（２０）（１）〜（５）のいずれかに記載の製造方法により得られる極低炭素鋼鋳片であって、直径０．５μｍから３０μｍの微細酸化物が鋳片表層から２０ｍｍの範囲内に１０００個／ｃｍ ２ 以上、１００００００個／ｃｍ ２ 未満分散し、且つその酸化物の個数割合で４０％以上が少なくともＳｉ、Ｍｎ、Ｆｅを含んでいることを特徴とする極低炭素鋼鋳片。
（２１）（１）〜（５）のいずれかに記載の製造方法により得られる極低炭素鋼鋳片であって、直径０．５μｍから３０μｍの微細酸化物が鋳片表層から２０ｍｍの範囲内に１０００個／ｃｍ ２ 以上、１００００００個／ｃｍ ２ 未満分散し、且つその酸化物の個数割合で４０％以上が少なくともＳｉ、Ｍｎ、Ｆｅを含んだ球状酸化物であることを特徴とする極低炭素鋼鋳片。
（２２）（１）〜（５）のいずれかに記載の製造方法により得られる極低炭素鋼鋳片であって、直径０．５μｍから３０μｍの微細酸化物が鋳片表層から２０ｍｍの範囲内に１０００個／ｃｍ２以上、１００００００個／ｃｍ２未満分散し、且つその酸化物の個数割合で４０％以上が少なくともＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物の含有率で、２０質量％以上であることを特徴とする極低炭素鋼鋳片。
（２３）（１）〜（５）のいずれかに記載の製造方法により得られる極低炭素鋼鋳片であって、直径０．５μｍから３０μｍの微細酸化物が鋳片表層から２０ｍｍの範囲内に１０００個／ｃｍ２以上、１００００００個／ｃｍ２未満分散し、且つその酸化物の個数割合で４０％以上が少なくともＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物の含有率で、２０質量％以上の球状酸化物であることを特徴とする極低炭素鋼鋳片。
【０００７】
【発明の実施の形態】
以下に本発明を詳細に説明する。
本発明の製造法では、転炉や電気炉等の製鋼炉で精錬して、或いはさらに真空脱ガス処理等を行って、炭素濃度を０．００２質量％以下とした溶鋼にＮｂを添加し、且つ溶存酸素濃度を０．０２〜０．０６質量％になるように調整する。
この溶製法の基本思想は、鋳造時に酸素と反応してＣＯガスを発生させない程度まで炭素濃度を低減し、且つＡｌを殆ど添加せず、溶存酸素を多量に残すことにより、溶鋼中に殆ど介在物を生成させず、且つ脱酸力の極めて弱いＮｂを添加してＣやＮを固定することで、薄板用鋼板としての材質をも確保することにある。
【０００８】
転炉や真空処理容器で脱炭処理された溶鋼中には、多量の溶存酸素が含まれており、この溶存酸素は通常Ａｌの添加により殆ど脱酸される（（１）式の反応）ため、多量のアルミナ系介在物を生成する。
２Ａｌ＋３Ｏ＝Ａｌ2Ｏ3 （１）
このアルミナ系介在物は脱酸直後からお互いに凝集合体し、粗大なアルミナ系介在物となり、鋼板製造時に表面欠陥発生の原因となる。しかし、脱炭処理後の溶鋼中にＡｌを全く添加しないか、或いは添加する場合でも少量を添加し、殆ど脱酸しなければ、多量の溶存酸素が溶鋼中に含まれているが、介在物は殆ど生成せず、非常に清浄性の高い溶鋼が得られる。通常、このような溶存酸素の高い溶鋼を鋳造すると、凝固時にＣＯガスが発生し、激しい突沸現象が生じると共に、鋳片内に多量の気泡が捕捉されるため、鋳造性が悪化するだけでなく、鋳片品質も大きく低下する。
【０００９】
そこで、本発明では、Ａｌを全く添加しない、あるいは殆ど添加せずに溶存酸素を残す代わりに、Ｃ濃度を極力低下させることにより、凝固時のＣＯガス発生を抑制することに着目した。その結果、実験的検討からＣ濃度を０．００２質量％以下にすれば、凝固時のＣＯガス発生速度は極めて低下することが判明した。また、特に薄板用鋼板等においては加工性を高めるために、Ｃ濃度を極力低下させるとともに、鋼中に固溶したＣとＮを他元素の添加により固定することが重要である。通常、ＡｌやＴｉ等が鋼中のＣとＮを固定する元素として使用されるが、これらの元素をＣやＮを固定するに十分な量を添加すると溶鋼を強く脱酸してしまう。そこで、本発明ではＮやＣを十分に固定できる程度の量を添加しても、殆ど溶鋼を脱酸しないような、脱酸力が極めて弱い元素としてＮｂを添加することを見出した。
【００１０】
上記の様にＣ濃度を０．００２質量％以下まで脱炭しても、溶鋼中の溶存酸素濃度が高過ぎると、凝固時のＣＯガス発生を抑制することはできないため、この場合溶存酸素濃度もある程度低くする必要がある。これら過剰な溶存酸素分だけであれば、ＡｌやＴｉ等で脱酸することは可能であるが、実験的な検討から溶存酸素濃度で０．０２質量％よりも低下させると、アルミナやチタニア等の介在物が多くなり過ぎ、浮上除去されずに溶鋼中に残留してしまう。また、Ｎｂを添加した際に、溶存酸素濃度が本発明の範囲であれば、ＡｌやＴｉ等を全く添加しなくても良い。反対に、溶存酸素濃度が０．０６質量％を超えると、Ｃ濃度を０．００２質量％以下に下げても鋳片内にＣＯ気泡が捕捉されてしまうため、圧延後に気泡系の欠陥が発生する。よって、溶鋼中の溶存酸素濃度は０．０２質量％以上、０．０６質量％以下にする必要がある。なお、溶鋼中の溶存酸素濃度は固定電解質を用いた酸素センサーにより、Ｃ濃度については溶鋼サンプリング法により分析することができる。
【００１１】
次に、溶鋼に添加されたＮｂの好ましい溶鋼中の濃度について説明する。溶鋼中のＮｂ濃度が０．００５質量％未満ではＣ、Ｎを十分固定しにくくなり、０．０５質量％超では加工性が低下し易くなることから、Ｎｂの添加量は溶鋼中のＮｂ濃度が０．００５質量％以上、０．０５質量％以下になる様にすることが好ましい。また、この範囲のＮｂ添加量であれば、Ｎｂと平衡する酸素濃度は０．０２質量％以上であり、Ｎｂを添加しても溶存酸素を０．０２質量％以上確保できる。
【００１２】
また上記の様に、溶鋼のＣ濃度を０．００２質量％以下まで脱炭する方法としては、通常は真空脱ガス装置を用いることで達成できる。
さらに、最近では、連続鋳造機内に鋳型内電磁攪拌装置、あるいは電磁コイルが装備されるようになっており、これらを用いることで、ＣＯ気泡を鋳片に捕捉させることなく、鋳造できることを知見した。
【００１３】
本発明者らは凝固時に電磁攪拌を行う際の、鋳型内メニスカスにおける溶鋼流速を４０〜１００cm／s程度確保すれば、溶存酸素濃度を０．０６質量％程度にしても、ＣＯ気泡を鋳片に捕捉させることなく鋳造できるため好ましいことを知見している。なお、電磁攪拌による溶鋼の旋回流速が４０cm／s未満では十分なＣＯ気泡の洗浄効果が得られにくく、旋回流速が１００cm／s超ではＣＯ気泡は洗浄されるが、溶鋼表面にあるモールドパウダーを巻き込み、表面欠陥が発生し易くなる。
【００１４】
また、鋳片へのＣＯ気泡の捕捉防止に対しては、鋳型内に装備された電磁コイルにより鋳型内の溶鋼を０．１から１００Hzの周波数で振動させることも有効であることを見いだしている。この場合、周波数１００Hz超では振動方向の変化に溶鋼流が追従しにくくなるため、０．１Hz未満では反対に振動方向の変化速度が遅いため、何れも振動による凝固界面の気泡洗浄効果は十分に得られにくい。
【００１５】
次に、本発明の鋳片について説明する。
溶鋼中のＣ濃度を非常に低くすると、溶存酸素は鋳造中にＦｅ酸化物系介在物として析出する。このＦｅ酸化物系介在物は溶鋼中で生成するのではなく、凝固時に析出するため、凝集合体することなく、鋳片内に微細に分散する。なお、Ｆｅ酸化物系介在物とは純粋なＦｅ酸化物だけでなく、Ｓｉ酸化物やＭｎ酸化物等と複合化した酸化物も含む。従って、本発明の様な極低炭素鋼においては、少なくとも酸化物としてＳｉ、Ｍｎ、Ｆｅが含まれている。言い換えれば、Ｓｉ、Ｍｎ、Ｆｅの各酸化物の１種以上が含まれている。
【００１６】
また、本発明の鋳片の表層から２０mmの範囲内にある介在物分散状態を評価したところ、直径０．５μｍから３０μｍの微細酸化物が鋳片内に１０００個／cm2以上１００００００個／cm2未満分散しており、この様に介在物が微細に分散していることで、表面欠陥の防止を達成できる。
尚、上記微細酸化物の直径を０．５μｍから３０μｍとしたのは、本発明の鋳片と鋼板における介在物の大きさがほぼ０．５μｍから３０μｍの範囲にほぼ収まっているためである。
また、介在物分散状態として１０００個／cm2以上１００００００個／cm2未満としたのは、本発明における鋳片と鋼板の介在物がこの個数密度にある場合、表面欠陥が発生しなかったためである。
ここで、介在物の分散状態は、鋳片の研磨面を１００倍と１０００倍の光学顕微鏡で観察し、単位面積内の介在物粒径分布を評価した。この介在物の粒径、すなわち直径とは長径と短径を測定し、（長径×短径）0.5とした。
【００１７】
なお、鋳片において表層から２０mmの範囲内における介在物分布に注目したのは、この範囲の介在物が圧延後に表層に露出して、表面欠陥になる可能性が高いためである。
以降も同様に、鋳片においては、表層から２０mmの範囲内における介在物分布に注目した。
【００１８】
また、鋳片の表層から２０mmの範囲内に存在する酸化物の個数割合で４０％以上が少なくともＳｉ、Ｍｎ、Ｆｅを含んでいれば、殆どの介在物が凝固時に生成し、凝集合体する時間が短いので、微細に分散でき、表面欠陥が発生しにくいためである。
さらに、通常このような介在物は球状酸化物である。
【００１９】
また、鋳片の表層から２０mmの範囲内に存在する酸化物の個数割合で４０％以上が少なくともＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物の含有率で２０質量％以上、より好ましくは５０質量％以上であれば、酸化物は殆ど凝固完了に近い時期に生成し、凝集合体する時間が非常に短いので、介在物が微細分散し、表面欠陥が発生し難いためである。
【００２０】
また、本発明の鋳片の表層から２０mmの範囲内にある介在物分散状態として、直径０．５μｍから３０μｍの微細酸化物が鋳片内に１０００個／cm2以上１００００００個／cm2未満分散していて、且つ上記記載の酸化物の個数割合の両方を満足していると、さらに好ましいことは言うまでもない。
【００２１】
また、上記の酸化物分散状態、組成および形状を有した鋳片を熱間圧延して得られる熱延鋼板、さらに冷間圧延して得られる冷延鋼板等の、鋳片を加工して得られる鋼板を、本発明では鋼板と定義する。
鋳片と同様に、鋼板の介在物分散状態についても評価したところ、鋳片表層２０mm内の酸化物分散状態とほぼ同じであった。このような酸化物分散状態、組成および形状を有する鋳片を加工して得られる鋼板では、表面欠陥は発生しない。以上の結果から、本発明により溶鋼中で殆ど介在物を生成させることなく、凝固時にＦｅＯ系の酸化物を析出させ微細に分散させることができるため、鋼板製造時に介在物は表面疵発生の原因とならず、薄板用鋼板の品質は大きく向上できる。
【００２２】
薄板用鋼板は、自動車用外板等の加工が厳しい用途に用いられるため、加工性を付加する必要がある。薄板用鋼板の加工性を高めるためには、Ｃ濃度を極力低下させ、その上で鋼中に固溶したＣとＮを他元素の添加により固定することが重要である。Ｃ濃度に関しては、加工性の観点から０．０１質量％以下、好ましくは０．００５質量％以下にするのが良い。しかし、凝固時のＣＯ気泡発生防止の条件はＣ濃度０．００２質量％以下であるので、本発明では加工性の条件から決まるＣ濃度は十分に満足されている。なお、Ｃ濃度の下限値は特に規定するものではない。
【００２３】
また、鋼板中の成分の作用について言及する。
鋼板中のＳｉ濃度は、０．００５質量％以上、０．０３質量％以下であることが好ましい。Ｓｉ濃度は０．００５質量％未満では板の強度が不足するため、またＳｉ濃度が０．０２質量％以上では板の加工性が低下するためである。また、Ｓｉ濃度が０．０３質量％以下であれば平衡酸素濃度も０．０２質量％超となり、溶存酸素濃度を０．０２質量％以上確保することは可能である。
【００２４】
鋼板中のＭｎ濃度が０．０８質量％未満になると熱間圧延時にへげ疵が発生し易くなり、またＭｎ濃度は０．３質量％を超えると板の加工性が低下する。このため、鋼板中のＭｎ濃度は０．０８質量％以上、０．３質量％以下であることが好ましい。また、ＭｎはＳｉに比べても非常に脱酸力が弱いため、Ｍｎ濃度を０．３質量％にしても平衡酸素濃度は０．１質量％超であり、溶鋼中に０．０２質量％から０．０６質量％の溶存酸素を確保できる。
【００２５】
本発明では、凝集合体し易いアルミナ系介在物を生成させないように、溶鋼中にＡｌを全く添加しない必要があるが、耐火物等から不可避的に侵入するアルミナ系介在物については問題とならない。これは、少量のアルミナ系介在物であれば、溶鋼中の溶存酸素が高いため、溶鋼とアルミナ系介在物の界面エネルギーは低下しており、凝集合体が殆ど生じないためである。また、鋼中のＴｉはＣとＮをＴｉＮやＴｉＣとして固定するため、加工性を向上させる上で有効であるが、Ｔｉの添加量も多くなると、例えばＴｉ濃度が０．００３質量％以上になると平衡酸素濃度が０．０２質量％未満になるため、十分な溶存酸素濃度を確保できない。よって、加工性をさらに高める必要からＴｉを添加する場合には、０．００３質量％以下の範囲で添加しても良い。
【００２６】
【実施例】
以下に、実施例及び比較例を挙げて、本発明について説明する。
［実施例１］
転炉での精錬と環流式真空脱ガス装置での処理により、Ｃ濃度を０．００１８質量％とした溶鋼３００ｔを溶製した。この溶鋼に合金を添加し、０．０１質量％Ｓｉ、０．１５質量％Ｍｎ、０．０１５質量％Ｎｂ、０．０４５質量％溶存酸素とした。この溶鋼を連続鋳造法で厚み２５０mm、幅１８００mmのスラブに鋳造した。鋳造した鋳片は８５００mm長さに切断し、１コイル単位とした。
【００２７】
鋳片表層２０mmの範囲における介在物を調査したところ、直径０．５μｍから３０μｍの微細酸化物が鋳片内に３００００個／cm2分散しており、その７０％はＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物を合計で６０質量％以上含有する球状酸化物であった。このようにして得られたスラブは、常法により熱間圧延、冷間圧延し、最終的には０．７mm厚みで幅１８００mmコイルの冷延鋼板とした。品質については、冷間圧延後の検査ラインで目視観察を行い、１コイル当たりに発生する表面欠陥の発生個数を評価した。その結果、表面欠陥は発生しなかった。また、鋳片と同様に、冷延鋼板内の介在物を調査したところ、直径０．５μｍから３０μｍの微細酸化物が鋳片内に３３０００個／cm2分散しており、その７０％はＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物を合計で６０質量％以上含有する球状酸化物であった。
【００２８】
［実施例２］
転炉での精錬と環流式真空脱ガス装置での処理によりＣ濃度を０．００１５質量％とした溶鋼３００ｔを溶製した。この溶鋼に合金を添加し、０．０１質量％Ｓｉ、０．１５質量％Ｍｎ、０．０１５質量％Ｎｂ、０．００１質量％Ｔｉ、０．０４質量％溶存酸素とした。この溶鋼を鋳型内電磁攪拌装置を有する連続鋳造機を用いて、メニスカスにおける溶鋼を平均流速４５cm／sで電磁攪拌しながら、厚み２５０mm、幅１８００mmのスラブに鋳造した。鋳造した鋳片は８５００mm長さに切断し、１コイル単位とした。
【００２９】
鋳片表層２０mmの範囲における介在物を調査したところ、直径０．５μｍから３０μｍの微細酸化物が鋳片内に２７０００個／cm2分散しており、その７０％はＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物を合計で６０質量％以上含有する球状酸化物であった。このようにして得られたスラブは、常法により熱間圧延、冷間圧延し、最終的には０．７mm厚みで幅１８００mmコイルの冷延鋼板とした。鋳片品質については、冷間圧延後の検査ラインで目視観察を行い、１コイル当たりに発生する表面欠陥の発生個数を評価した。その結果、表面欠陥は発生しなかった。また、鋳片と同様に、冷延鋼板内の介在物を調査したところ、直径０．５μｍから３０μｍの微細酸化物が鋳片内に２９０００個／cm2分散しており、その７０％はＳｉ酸化物、Ｍｎ酸化物、Ｆｅ酸化物を合計で６０質量％以上含有する球状酸化物であった。
【００３０】
［比較例１］
転炉での精錬と環流式真空脱ガス装置での処理により炭素濃度を０．００１５質量％とした取鍋内溶鋼をＡｌで脱酸し、Ａｌ濃度０．０４質量％、溶存酸素濃度０．０００２質量％とした。この溶鋼を連続鋳造法で厚み２５０mm、幅１８００mmのスラブに鋳造した。鋳造した鋳片は８５００mm長さに切断し、１コイル単位とした。このようにして得られたスラブは、常法により熱間圧延、冷間圧延し、最終的には０．７mm厚みで幅１８００mmコイルの冷延鋼板とした。鋳片品質については、冷間圧延後の検査ラインで目視観察を行い、１コイル当たりに発生する表面欠陥の発生個数を評価した。その結果、スラブ平均で５個／コイルの表面欠陥が発生した。
【００３１】
【発明の効果】
以上に説明したように、本発明によると、溶鋼中に殆ど介在物を生成させることなく、凝固時に酸化物を微細に析出させることができるため、確実に表面疵を防止できる加工性、成形性に優れた薄鋼板用の極低炭素溶鋼を製造することが可能となる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an ultra-low carbon slab excellent in workability and formability, an ultra-low carbon steel sheet, and a method for producing the same.
[0002]
[Prior art]
The molten steel refined in a converter or vacuum processing vessel contains a large amount of dissolved oxygen, and this excess oxygen is generally deoxidized by Al, a strong deoxidizing element with a strong affinity for oxygen. Is. However, Al produces alumina inclusions by deoxidation, which aggregate and coalesce into coarse alumina clusters. This alumina cluster causes surface flaws during the production of the steel sheet and greatly deteriorates the quality of the thin steel sheet. In particular, ultra-low carbon molten steel, which is a material for thin steel sheets with a low carbon concentration and a high dissolved oxygen concentration after refining, has a very high amount of alumina clusters and a very high rate of surface flaws. Measures to reduce this are a major issue.
[0003]
On the other hand, conventionally, the inclusion adsorption flux described in JP-A-5-104219 is added to the molten steel surface to remove alumina inclusions, or the injection flow disclosed in JP-A-63-149057 is used. Thus, a method has been proposed and implemented in which CaO flux is added to molten steel, and thereby alumina inclusions are adsorbed and removed. On the other hand, as a method that does not remove the alumina inclusions but does not generate them, the molten steel is deoxidized with Mg as disclosed in JP-A-5-302112, and the molten steel for thin steel sheet is hardly deoxidized with Al. A method is also disclosed.
[0004]
[Problems to be solved by the invention]
However, in the method of removing the alumina inclusions described above, it is very difficult to reduce the alumina inclusions produced in a large amount in the ultra low carbon molten steel to the extent that no surface defects are generated. In addition, Mg deoxidation, which does not generate any alumina inclusions, has a high vapor pressure of Mg and a very low yield to molten steel. Therefore, demolition of molten steel with a high dissolved oxygen concentration such as ultra-low carbon steel with only Mg. A large amount of Mg is required for the acid, and it cannot be said that it is a practical process in view of manufacturing costs.
[0005]
In view of these problems, the present invention provides an ultra-low carbon steel sheet and an ultra-low carbon slab that can reliably prevent surface flaws by causing fine oxides to precipitate during solidification without generating inclusions in the molten steel. And its manufacturing method.
[0006]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention has the following configuration.
(1) After decarburizing the carbon concentration of the molten steel to 0.002% by mass or less, Nb is added to the molten steel so that the Nb concentration in the molten steel is 0.005% by mass to 0.05% by mass, Further, deoxidation with Al or Ti is performed to adjust the dissolved oxygen concentration in the molten steel to 0.02 mass% or more and 0.06 mass% or less, or the dissolved oxygen concentration in the molten steel when Nb is added is 0. A method for producing an ultra-low carbon steel slab characterized by casting molten steel that is 0.02 mass% or more and 0.06 mass% or less.
(2) After decarburizing the carbon concentration to 0.002% by mass or less by vacuum degassing treatment , Nb is added to the molten steel, and the Nb concentration in the molten steel is 0.005% by mass or more and 0.05% by mass. In the following, the dissolved oxygen concentration in the molten steel is adjusted to 0.02 mass% or more and 0.06 mass% or less by deoxidizing with Al or Ti, or dissolved oxygen in the molten steel when Nb is added A method for producing an ultra-low carbon steel slab characterized by casting molten steel having a concentration of 0.02 mass% or more and 0.06 mass% or less .
(3) The method for producing an extremely low carbon steel slab according to (1) or (2), wherein the molten steel is cast while electromagnetic stirring or application of an electromagnetic field is performed .
(4) When casting molten steel, electromagnetic stirring is performed and the molten steel at the meniscus position is cast while turning at an average flow velocity of 40 cm / s or more and 100 cm / s or less (1) or (2) The manufacturing method of the ultra-low-carbon steel slab described in 1.
(5) When casting molten steel, an electromagnetic field is applied, and the molten steel at the meniscus position is cast while being oscillated in a horizontal direction at 0.1 Hz or more and 100 Hz or less. (1) or (2) Method for producing ultra-low carbon steel slabs.
(6) An ultra-low carbon steel plate obtained by processing a slab manufactured by the method according to any one of (1) to (5), wherein 1000 fine oxides having a diameter of 0.5 μm to 30 μm / Cm 2 or more and less than 1,000,000 pieces / cm 2 and the oxide contains at least Si, Mn and Fe.
(7) An ultra-low carbon steel sheet obtained by processing a slab manufactured by the method according to any one of (1) to (5), wherein the percentage of the number of oxides present in the steel sheet is 40%. An ultra-low carbon steel sheet characterized by containing at least Si, Mn, and Fe.
(8) An ultra-low carbon steel sheet obtained by processing a slab produced by the method according to any one of (1) to (5), wherein the percentage of the number of oxides present in the steel sheet is 40%. The above is a spherical oxide containing at least Si, Mn, and Fe .
(9) An ultra-low carbon steel plate obtained by processing a slab manufactured by the method according to any one of (1) to (5), wherein the number ratio of oxides present in the steel plate is 40%. The above is the content of at least Si oxide, Mn oxide, and Fe oxide, and is an ultra-low carbon steel sheet characterized by being 20% by mass or more .
(10) An ultra-low carbon steel sheet obtained by processing a slab manufactured by the method according to any one of (1) to (5), wherein the number ratio of oxides present in the steel sheet is 40%. The ultra-low carbon steel sheet is characterized in that the content of at least Si oxide, Mn oxide and Fe oxide is 20% by mass or more .
(11) An ultra-low carbon steel plate obtained by processing a slab produced by the method according to any one of (1) to (5), wherein 1000 fine oxides having a diameter of 0.5 μm to 30 μm / cm 2 or more, 1,000,000 / cm 2 less than dispersed, and ultra-low carbon steel sheet 40% or more by the number ratio of the oxide is characterized in that it comprises at least Si, Mn, and Fe.
(12) An ultra-low carbon steel plate obtained by processing a slab manufactured by the method according to any one of (1) to (5), wherein 1000 fine oxides having a diameter of 0.5 μm to 30 μm An ultra-low carbon steel sheet characterized in that it is a spherical oxide containing at least Si, Mn, and Fe at a rate of 40% or more in terms of the number ratio of oxides dispersed at / cm 2 or more and less than 1 million pieces / cm 2 .
(13) An ultra-low carbon steel plate obtained by processing a slab manufactured by the method according to any one of (1) to (5), wherein 1000 fine oxides having a diameter of 0.5 μm to 30 μm / Cm 2 or more and less than 1,000,000 pieces / cm 2 , and 40% or more of the number ratio of the oxides is at least 20% by mass or more in terms of the content of Si oxide, Mn oxide, and Fe oxide. An ultra-low carbon steel sheet characterized by
(14) An ultra-low carbon steel plate obtained by processing a slab manufactured by the method according to any one of (1) to (5), wherein 1000 fine oxides having a diameter of 0.5 μm to 30 μm / Cm 2 or more, less than 1 million pieces / cm 2 , and 40% or more of the number of oxides is at least Si oxide, Mn oxide, Fe oxide, and 20% by mass or more of spherical oxidation An ultra-low carbon steel sheet characterized by being a product.
(15) An ultra-low carbon steel slab obtained by the production method according to any one of (1) to (5), wherein a fine oxide having a diameter of 0.5 μm to 30 μm is within a range of 20 mm from the surface of the slab 1000 / cm 2 or more, dispersed than 1,000,000 / cm 2, and at least Si, Mn, ultra low carbon steel cast piece, characterized in that it contains Fe in a part or all of the oxides.
(16) An ultra-low carbon steel slab obtained by the production method according to any one of (1) to (5), wherein the percentage of the number of oxides present within a range of 20 mm from the slab surface layer is 40%. An ultra-low carbon steel slab characterized in that the above contains at least Si, Mn, and Fe .
(17) An ultra-low carbon steel slab obtained by the production method according to any one of (1) to (5), wherein the percentage of the number of oxides present within a range of 20 mm from the slab surface layer is 40%. or at least Si, Mn, ultra low carbon steel cast piece, characterized in that the spherical oxide that contains Fe.
(18) An ultra-low carbon steel slab obtained by the production method according to any one of (1) to (5), wherein the ratio of the number of oxides present within a range of 20 mm from the slab surface layer is 40%. The above is the content of at least Si oxide, Mn oxide, and Fe oxide, and is an ultra-low carbon steel slab characterized by being 20% by mass or more .
(19) An ultra-low carbon steel slab obtained by the production method according to any one of (1) to (5), wherein the percentage of the number of oxides present within a range of 20 mm from the slab surface layer is 40%. The above is the content of at least Si oxide, Mn oxide, and Fe oxide, and is a spherical oxide of 20% by mass or more .
(20) An ultra-low carbon steel slab obtained by the production method according to any one of (1) to (5), wherein a fine oxide having a diameter of 0.5 μm to 30 μm is within a range of 20 mm from the slab surface layer. 1000 / cm 2 or more, 1,000,000 / cm 2 less than dispersed, and at least Si is 40% or more in the number of oxides, Mn, ultra low carbon steel cast piece, characterized in that it contains Fe .
(21) An ultra-low carbon steel slab obtained by the production method according to any one of (1) to (5), wherein a fine oxide having a diameter of 0.5 μm to 30 μm is within a range of 20 mm from the surface of the slab In addition, 1000 % / cm 2 or more and less than 1000000 / cm 2 are dispersed, and 40% or more of the oxides are in a spherical oxide containing at least Si, Mn, and Fe. Carbon steel slab.
(22) An ultra-low carbon steel slab obtained by the production method according to any one of (1) to (5), wherein a fine oxide having a diameter of 0.5 μm to 30 μm is within a range of 20 mm from the surface of the slab 1000 / cm 2 or more, 1,000,000 / cm 2 less than dispersed, and at least Si oxide 40% or more by the number ratio of the oxide, Mn oxide, at a content of Fe oxide, or 20 wt% ultra low carbon steel cast piece, characterized in that it.
(23) An ultra-low carbon steel slab obtained by the production method according to any one of (1) to (5), wherein a fine oxide having a diameter of 0.5 μm to 30 μm is within a range of 20 mm from the surface of the slab 1000 / cm 2 or more, 1,000,000 / cm 2 less than dispersed, and at least Si oxide 40% or more by the number ratio of the oxide, Mn oxide, at a content of Fe oxide, or 20 wt% An ultra-low carbon steel slab characterized by being a spherical oxide.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
In the production method of the present invention, Nb is added to molten steel having a carbon concentration of 0.002% by mass or less by refining in a steelmaking furnace such as a converter or an electric furnace, or by performing vacuum degassing treatment or the like. The dissolved oxygen concentration is adjusted to 0.02 to 0.06% by mass.
The basic idea of this melting method is to reduce the carbon concentration to such an extent that it does not generate CO gas by reacting with oxygen during casting. Almost no intercalation is present in molten steel by adding little Al and leaving a large amount of dissolved oxygen. It is to secure a material as a steel plate for a thin plate by adding Nb having a very weak deoxidizing power without generating a product and fixing C and N.
[0008]
The molten steel decarburized in a converter or vacuum processing vessel contains a large amount of dissolved oxygen, and this dissolved oxygen is usually almost deoxidized by the addition of Al (reaction (1)). A large amount of alumina inclusions are produced.
2Al + 3O = Al 2 O 3 (1)
The alumina inclusions aggregate and coalesce with each other immediately after deoxidation to form coarse alumina inclusions, which cause surface defects during steel plate production. However, if Al is not added at all in the molten steel after decarburization treatment or if a small amount is added and almost no deoxidization is performed, a large amount of dissolved oxygen is contained in the molten steel. Is hardly produced, and a very clean steel can be obtained. Normally, when casting such molten steel with high dissolved oxygen, CO gas is generated during solidification, and a severe bumping phenomenon occurs, and a large amount of bubbles are trapped in the slab, which not only deteriorates the castability. The slab quality is also greatly reduced.
[0009]
Therefore, in the present invention, attention was focused on suppressing CO gas generation during solidification by reducing the C concentration as much as possible instead of leaving Al with little or no addition of Al. As a result, it was found from experimental examination that the CO gas generation rate during solidification is extremely reduced when the C concentration is 0.002% by mass or less. In particular, in the case of a steel sheet for thin plates, it is important to reduce the C concentration as much as possible and to fix C and N dissolved in the steel by addition of other elements in order to improve workability. Usually, Al, Ti or the like is used as an element for fixing C and N in the steel, but if these elements are added in an amount sufficient to fix C or N, the molten steel is strongly deoxidized. Therefore, in the present invention, it has been found that Nb is added as an element having a very weak deoxidizing power so as to hardly deoxidize molten steel even if an amount sufficient to fix N or C is added.
[0010]
Even if the C concentration is decarburized to 0.002% by mass or less as described above, if the dissolved oxygen concentration in the molten steel is too high, CO gas generation during solidification cannot be suppressed. Needs to be lowered to some extent. If it is only these excessive dissolved oxygen content, it is possible to deoxidize with Al, Ti, etc., but if the dissolved oxygen concentration is reduced below 0.02% by mass from experimental studies, alumina, titania, etc. The amount of inclusions increases and remains in the molten steel without being lifted and removed. Further, when Nb is added, if the dissolved oxygen concentration is within the range of the present invention, it is not necessary to add Al or Ti at all. On the other hand, if the dissolved oxygen concentration exceeds 0.06% by mass, CO bubbles are trapped in the slab even if the C concentration is lowered to 0.002% by mass or less, resulting in bubble-type defects after rolling. To do. Therefore, the dissolved oxygen concentration in molten steel needs to be 0.02 mass% or more and 0.06 mass% or less. The dissolved oxygen concentration in the molten steel can be analyzed by an oxygen sensor using a fixed electrolyte, and the C concentration can be analyzed by a molten steel sampling method.
[0011]
Next, the preferable concentration of Nb added to the molten steel in the molten steel will be described. If the Nb concentration in the molten steel is less than 0.005% by mass, it becomes difficult to sufficiently fix C and N, and if it exceeds 0.05% by mass, the workability tends to be reduced. Therefore, the amount of Nb added is the Nb concentration in the molten steel. Is preferably 0.005 mass% or more and 0.05 mass% or less. Further, if the amount of Nb added is within this range, the oxygen concentration in equilibrium with Nb is 0.02% by mass or more, and even if Nb is added, 0.02% by mass or more of dissolved oxygen can be secured.
[0012]
Further, as described above, the method for decarburizing the C concentration of molten steel to 0.002% by mass or less can be usually achieved by using a vacuum degassing apparatus.
Furthermore, recently, in-mold electromagnetic stirrers or electromagnetic coils have been installed in continuous casting machines, and it has been found that by using these, casting can be performed without trapping CO bubbles in the slab. .
[0013]
If the present inventors secure a flow rate of molten steel at the meniscus in the mold of about 40 to 100 cm / s when performing electromagnetic stirring during solidification, the CO bubbles can be cast into a slab even if the dissolved oxygen concentration is about 0.06% by mass. It is known that it is preferable because it can be cast without being trapped. In addition, if the swirling flow velocity of molten steel by electromagnetic stirring is less than 40 cm / s, it is difficult to obtain a sufficient effect of cleaning CO bubbles. If the swirling flow velocity exceeds 100 cm / s, CO bubbles are washed, but mold powder on the molten steel surface is removed. Entrainment and surface defects are likely to occur.
[0014]
In addition, it has been found that it is effective to vibrate molten steel in a mold at a frequency of 0.1 to 100 Hz by an electromagnetic coil provided in the mold for preventing the capture of CO bubbles in the slab. . In this case, if the frequency exceeds 100 Hz, it becomes difficult for the molten steel flow to follow the change in the vibration direction. If the frequency is less than 0.1 Hz, the change speed in the vibration direction is slow. It is difficult to obtain.
[0015]
Next, the slab of the present invention will be described.
When the C concentration in the molten steel is very low, dissolved oxygen precipitates as Fe oxide inclusions during casting. The Fe oxide inclusions are not formed in the molten steel, but are precipitated during solidification, so that they are finely dispersed in the slab without agglomeration and coalescence. The Fe oxide inclusions include not only pure Fe oxides but also oxides complexed with Si oxides, Mn oxides, and the like. Accordingly, the ultra-low carbon steel as in the present invention contains at least Si, Mn, and Fe as oxides. In other words, one or more oxides of Si, Mn, and Fe are included.
[0016]
Further, when the inclusion dispersion state in the range of 20 mm from the surface layer of the slab of the present invention was evaluated, fine oxides having a diameter of 0.5 to 30 μm were 1000 / cm 2 or more and 1000000 / cm 2 in the slab. The dispersion of less than 2 and the inclusions are finely dispersed in this way can prevent surface defects.
The reason why the diameter of the fine oxide is set to 0.5 μm to 30 μm is that the size of inclusions in the slab and the steel plate of the present invention is almost within the range of 0.5 μm to 30 μm.
Further, the inclusion dispersion state was set to 1000 / cm 2 or more and less than 1000000 / cm 2 because surface defects did not occur when the inclusions of the slab and the steel plate in the present invention had this number density. is there.
Here, regarding the dispersion state of inclusions, the polished surface of the slab was observed with an optical microscope of 100 times and 1000 times, and the inclusion particle size distribution within a unit area was evaluated. The particle size of the inclusion, that is, the diameter was measured by measuring the major axis and the minor axis, and was set to (major axis × minor axis) 0.5 .
[0017]
The reason why the inclusion distribution in the range of 20 mm from the surface layer in the slab was noted is because inclusions in this range are likely to be exposed to the surface layer after rolling and become surface defects.
Similarly, in the slab, attention was paid to the inclusion distribution in the range of 20 mm from the surface layer.
[0018]
In addition, when 40% or more of the number of oxides existing within a range of 20 mm from the surface layer of the slab contains at least Si, Mn, and Fe, most of the inclusions are generated during solidification and agglomerate and coalesce. This is because it can be dispersed finely and surface defects are less likely to occur.
Furthermore, such inclusions are usually spherical oxides.
[0019]
Further, 40% or more in terms of the number of oxides present within a range of 20 mm from the surface layer of the slab is 20% by mass or more, more preferably 50% by mass in terms of the content of at least Si oxide, Mn oxide and Fe oxide. If it is at least%, the oxide is formed almost at the completion of solidification and the time for agglomeration and coalescence is very short, so that inclusions are finely dispersed and surface defects are unlikely to occur.
[0020]
In addition, as an inclusion dispersion state within a range of 20 mm from the surface layer of the slab of the present invention, fine oxides having a diameter of 0.5 to 30 μm are dispersed in the slab at 1000 / cm 2 or more and less than 1000000 / cm 2. Needless to say, it is more preferable that both of the above-described oxide number ratios are satisfied.
[0021]
Also obtained by processing a slab such as a hot-rolled steel sheet obtained by hot rolling a slab having the above oxide dispersion state, composition and shape, and a cold-rolled steel sheet obtained by cold rolling. The steel plate to be used is defined as a steel plate in the present invention.
Similar to the slab, the inclusion dispersion state of the steel sheet was also evaluated, and was almost the same as the oxide dispersion state in the slab surface layer of 20 mm. In a steel sheet obtained by processing a slab having such an oxide dispersion state, composition and shape, no surface defects occur. From the above results, according to the present invention, it is possible to precipitate and finely disperse FeO-based oxides during solidification without generating inclusions in the molten steel. However, the quality of the steel sheet for thin plate can be greatly improved.
[0022]
Since the steel sheet for thin plates is used for applications in which processing of an outer plate for automobiles and the like is severe, it is necessary to add workability. In order to improve the workability of the steel sheet for thin plates, it is important to reduce the C concentration as much as possible and fix C and N dissolved in the steel by adding other elements. The C concentration is 0.01% by mass or less, preferably 0.005% by mass or less from the viewpoint of workability. However, since the condition for preventing the generation of CO bubbles during solidification is 0.002% by mass or less, the present invention sufficiently satisfies the C concentration determined from the workability conditions. The lower limit value of the C concentration is not particularly specified.
[0023]
Reference is also made to the action of the components in the steel sheet.
The Si concentration in the steel sheet is preferably 0.005% by mass or more and 0.03% by mass or less. If the Si concentration is less than 0.005% by mass, the strength of the plate is insufficient, and if the Si concentration is 0.02% by mass or more, the workability of the plate is deteriorated. Further, if the Si concentration is 0.03% by mass or less, the equilibrium oxygen concentration also exceeds 0.02% by mass, and it is possible to ensure the dissolved oxygen concentration to be 0.02% by mass or more.
[0024]
If the Mn concentration in the steel sheet is less than 0.08% by mass, cracks are likely to occur during hot rolling, and if the Mn concentration exceeds 0.3% by mass, the workability of the plate is lowered. For this reason, it is preferable that Mn density | concentration in a steel plate is 0.08 mass% or more and 0.3 mass% or less. Further, since Mn has a very weak deoxidizing power compared with Si, even if the Mn concentration is 0.3% by mass, the equilibrium oxygen concentration is over 0.1% by mass, and 0.02% by mass in the molten steel. From 0.06 mass%, dissolved oxygen can be secured.
[0025]
In the present invention, it is necessary to add no Al to the molten steel so as not to generate alumina inclusions that easily aggregate and coalesce. However, there is no problem with alumina inclusions that inevitably infiltrate from refractories and the like. This is because, if a small amount of alumina inclusions are present, the dissolved oxygen in the molten steel is high, so that the interfacial energy between the molten steel and the alumina inclusions is lowered, and almost no agglomeration occurs. Moreover, Ti in steel fixes C and N as TiN and TiC, which is effective in improving workability. However, when the amount of Ti added is increased, for example, the Ti concentration becomes 0.003 mass% or more. Then, since the equilibrium oxygen concentration is less than 0.02% by mass, a sufficient dissolved oxygen concentration cannot be ensured. Therefore, when adding Ti to further improve the workability, it may be added in a range of 0.003% by mass or less.
[0026]
【Example】
Hereinafter, the present invention will be described with reference to examples and comparative examples.
[Example 1]
300 t of molten steel with a C concentration of 0.0018% by mass was produced by refining in a converter and treatment in a reflux-type vacuum degassing apparatus. An alloy was added to the molten steel to obtain 0.01 mass% Si, 0.15 mass% Mn, 0.015 mass% Nb, and 0.045 mass% dissolved oxygen. This molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm by a continuous casting method. The cast slab was cut to a length of 8500 mm to make one coil unit.
[0027]
When the inclusions in the range of 20 mm of the slab surface layer were investigated, 30,000 / cm 2 of fine oxide having a diameter of 0.5 to 30 μm was dispersed in the slab, 70% of which was Si oxide, Mn oxide And a spherical oxide containing 60% by mass or more of Fe oxide in total. The slab thus obtained was hot-rolled and cold-rolled by a conventional method, and finally formed into a cold-rolled steel sheet having a thickness of 0.7 mm and a coil width of 1800 mm. Regarding quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, no surface defects occurred. Further, as in the case of the slab, when the inclusions in the cold-rolled steel sheet were examined, fine oxides having a diameter of 0.5 to 30 μm were dispersed in the slab at 33,000 / cm 2 , 70% of which was Si It was a spherical oxide containing 60% by mass or more of oxide, Mn oxide, and Fe oxide in total.
[0028]
[Example 2]
300 t of molten steel having a C concentration of 0.0015% by mass was produced by refining in a converter and treatment in a reflux vacuum degassing apparatus. An alloy was added to this molten steel to obtain 0.01 mass% Si, 0.15 mass% Mn, 0.015 mass% Nb, 0.001 mass% Ti, and 0.04 mass% dissolved oxygen. The molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm while electromagnetically stirring the molten steel at the meniscus at an average flow rate of 45 cm / s using a continuous casting machine having an in-mold electromagnetic stirring device. The cast slab was cut to a length of 8500 mm to make one coil unit.
[0029]
When the inclusions in the slab surface layer of 20 mm were investigated, 27,000 / cm 2 of fine oxide having a diameter of 0.5 to 30 μm was dispersed in the slab, 70% of which was Si oxide and Mn oxide. And a spherical oxide containing 60% by mass or more of Fe oxide in total. The slab thus obtained was hot-rolled and cold-rolled by a conventional method, and finally formed into a cold-rolled steel sheet having a thickness of 0.7 mm and a coil width of 1800 mm. Regarding the slab quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, no surface defects occurred. Further, as in the case of the slab, when the inclusions in the cold-rolled steel sheet were investigated, 29000 pieces / cm 2 of fine oxide having a diameter of 0.5 μm to 30 μm was dispersed in the slab, 70% of which was Si It was a spherical oxide containing 60% by mass or more of oxide, Mn oxide, and Fe oxide in total.
[0030]
[Comparative Example 1]
The molten steel in the ladle having a carbon concentration of 0.0015% by mass by refining in a converter and treatment in a reflux-type vacuum degassing apparatus is deoxidized with Al, and the Al concentration is 0.04% by mass and the dissolved oxygen concentration is 0.00. It was 0002 mass%. This molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm by a continuous casting method. The cast slab was cut to a length of 8500 mm to make one coil unit. The slab thus obtained was hot-rolled and cold-rolled by a conventional method, and finally formed into a cold-rolled steel sheet having a thickness of 0.7 mm and a coil width of 1800 mm. Regarding the slab quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, surface defects of 5 pieces / coil were generated on average on the slab.
[0031]
【The invention's effect】
As described above, according to the present invention, the oxide can be finely precipitated at the time of solidification without generating any inclusions in the molten steel, so that the workability and formability can be reliably prevented. It is possible to produce an ultra-low carbon molten steel for thin steel sheets that is excellent in the quality.
Claims (23)
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| JP4486518B2 (en) * | 2005-01-27 | 2010-06-23 | 新日本製鐵株式会社 | Alloyed hot-dip galvanized steel sheet with excellent press formability and coating adhesion during press forming |
| JP4873921B2 (en) * | 2005-02-18 | 2012-02-08 | 新日本製鐵株式会社 | Method for producing ultra-low carbon steel sheet and ultra-low carbon cast slab excellent in surface properties, workability and formability |
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| JPH03104844A (en) * | 1989-09-18 | 1991-05-01 | Nippon Steel Corp | Nonoriented silicon steel sheet excellent in magnetic characteristics and its manufacture |
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