JP4227478B2 - Low carbon steel slab manufacturing method - 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、表面疵も発生し難い低炭素鋼鋳片の製造方法と溶鋼中介在物の改質剤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数１００μｍ以上の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く、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、従来は特許文献１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、或いは特許文献２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、特許文献３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や、特許文献４のようにＡｌを添加せずにＴｉで脱酸した薄鋼板も開示されている。
【０００４】
【特許文献１】
特開平５−１０４２１９号公報
【特許文献２】
特開昭６３−１４９０５７号公報
【特許文献３】
特開平５−３０２１１２号公報
【特許文献４】
特開平８−２３９７３１号公報
【０００５】
【発明が解決しようとする課題】
しかしながら、上述の特許文献１〜特許文献２に開示されている様な、アルミナ系介在物を除去する方法では、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、特許文献３に開示されている様な、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、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。
さらに、特許文献４に開示されている様な、Ｔｉで脱酸する場合には、溶鋼中に生成した固相状態のＴｉ酸化物が鍋ノズルやタンディッシュノズルの内面に地金と共に付着堆積し、通常のＡｌ脱酸よりもかえってノズル詰まりを助長するといった問題があった。
これらの問題を鑑み、本発明は溶鋼中介在物の凝集合体とノズルへの付着を防止し、鋼板中に介在物を微細分散させることにより、確実に表面疵を防止できる、溶鋼中介在物の改質剤および低炭素鋼鋳片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。即ち、
（１）溶鋼の炭素濃度を０．０１質量％以下まで脱炭した後、該溶鋼にＴｉを添加して脱酸し、その後、金属Ｓｉが５〜２５質量％、少なくともＬａ、Ｃｅ、Ｎｄが４０〜９５質量％、Ｆｅが０〜５５質量％で、残部が不可避的不純物元素からなる組成である溶鋼中介在物の改質剤を添加した溶鋼を鋳造することを特徴とする低炭素鋼鋳片の製造方法。
（２）溶鋼の炭素濃度を０．０１質量％以下まで脱炭した後、該溶鋼にＡｌを添加して予備脱酸処理を行い、溶鋼中の溶存酸素濃度を０．０１質量％以上０．０４質量％以下とし、次いでＴｉを添加して脱酸し、その後、金属Ｓｉが５〜２５質量％、少なくともＬａ、Ｃｅ、Ｎｄが４０〜９５質量％、Ｆｅが０〜５５質量％で、残部が不可避的不純物元素からなる組成である溶鋼中介在物の改質剤を添加した溶鋼を鋳造することを特徴とする低炭素鋼鋳片の製造方法。
（３）溶鋼の炭素濃度を０．０１質量％以下まで脱炭した後、該溶鋼にＡｌを添加し３分以上攪拌して予備脱酸処理を行い、溶鋼中の溶存酸素濃度を０．０１質量％以上０．０４質量％以下とし、次いでＴｉを添加して溶鋼中のＴｉ濃度を０．００３質量％以上０．４質量％以下とし、その後、金属Ｓｉが５〜２５質量％、少なくともＬａ、Ｃｅ、Ｎｄが４０〜９５質量％、Ｆｅが０〜５５質量％で、残部が不可避的不純物元素からなる組成である溶鋼中介在物の改質剤を添加し、少なくともＬａ、Ｃｅ、Ｎｄの溶鋼中濃度を０．００１質量％以上０．０３質量％以下とした溶鋼を鋳造することを特徴とする低炭素鋼鋳片の製造方法。
（４）溶鋼の炭素濃度を０．０１質量％以下まで脱炭するに際し、真空脱ガス装置を用いることを特徴とする（１）〜（３）のいずれかに記載の低炭素鋼鋳片の製造方法。
（５）溶鋼を鋳造するに際し、電磁攪拌機能を有する鋳型で鋳造することを特徴とする（１）〜（４）のいずれかに記載の低炭素鋼鋳片の製造方法。
（６）溶鋼を鋳造するに際し、１３００℃における粘性が４ｐｏｉｓｅ以上のモールドフラックスを用いて鋳造することを特徴とする（１）〜（５）のいずれかに記載の低炭素鋼鋳片の製造方法。
（７）溶鋼を鋳造するに際し、連続鋳造することを特徴とする（１）〜（６）のいずれかに記載の低炭素鋼鋳片の製造方法。
【０００７】
【発明の実施の形態】
以下に本発明を詳細に説明する。
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（下記（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）
これらの介在物は脱酸直後からお互いに凝集合体し、数１００μｍ以上の粗大なアルミナクラスターとなり、鋼板製造時に表面欠陥の原因となる。
そこで、アルミナクラスターを生成させないために、脱炭処理後の溶存酸素をＡｌ以外の脱酸剤と改質剤で脱酸することに着目した。
【０００８】
本発明方法として、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０．０１質量％以下とした溶鋼にＴｉを添加して脱酸し、その後金属Ｓｉが５〜２５質量％、少なくともＬａ、Ｃｅ、Ｎｄが４０〜９５質量％、Ｆｅが０〜５５質量％で、残部が不可避的不純物元素からなる改質剤を添加した溶鋼を鋳造する方法を考案した。
ここで、少なくともＬａ、Ｃｅ、Ｎｄというのは、Ｌａ、Ｃｅ、Ｎｄの１種以上という意味である。以降も同様の意味で用いている。
【０００９】
本発明者らは、溶鋼へ添加する脱酸剤として、ＡｌまたはＴｉや、これに少なくともＬａ、Ｃｅ、Ｎｄを添加したものを適宜組み合わせて、これらの介在物の凝集挙動を実験的に評価したところ、アルミナ系介在物、Ｔｉ酸化物系介在物、アルミナ系介在物とＴｉ酸化物系介在物からなる複合介在物、或いはアルミナと少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物（例えば、アルミナ−Ｌａ酸化物系複合介在物、アルミナ−Ｃｅ酸化物−Ｎｄ酸化物系複合介在物、アルミナ−Ｌａ酸化物−Ｃｅ酸化物−Ｎｄ酸化物系複合介在物等）は比較的容易に凝集合体し、るつぼ壁にも付着し易いのに対し、Ｔｉ酸化物と少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物（例えば、Ｔｉ酸化物−Ｌａ酸化物系複合介在物、Ｔｉ酸化物−Ｃｅ酸化物−Ｎｄ酸化物系複合介在物、Ｔｉ酸化物−Ｌａ酸化物−Ｃｅ酸化物−Ｎｄ酸化物系複合介在物等）は凝集合体し難く、且つるつぼ壁にも付着せず、溶鋼中に微細分散することを見いだした。この理由は、アルミナ系介在物、Ｔｉ酸化物系介在物、アルミナ系介在物とＴｉ酸化物系介在物からなる複合介在物、或いはアルミナと少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物に比べて、Ｔｉ酸化物と少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物の方が、介在物と溶鋼間の界面エネルギーの低下が大きく、介在物同士の凝集合体と耐火物への付着を抑制したためである。これらの知見を基に、溶存酸素をＴｉで脱酸し、さらに少なくともＬａ、Ｃｅ、Ｎｄを添加することによりＴｉ酸化物系介在物をＴｉ酸化物と少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物に改質し、溶鋼中に介在物を微細に分散させることに成功した。
【００１０】
しかし、実機設備を用いて少なくともＬａ、Ｃｅ、Ｎｄからなる合金を溶鋼中に添加する試験を実施したところ、合金をベルトコンベヤーでホッパーまで輸送する際、合金をホッパーに挿入する際、或いは合金を溶鋼に添加する際に、合金同士が衝突することによる発火現象が観察された。そこで、少なくともＬａ、Ｃｅ、Ｎｄからなる合金に他の金属を含有させた介在物改質剤を種々作製し、ＪＩＳ−Ｇ−０５６６の火花試験法を用いて発火を抑制する成分を検討した。
本発明者らは、少なくともＬａ、Ｃｅ、Ｎｄからなる合金に金属Ｓｉを５質量％以上含有させると火花の発生が急激に減少すること、また金属Ｓｉをさらに増加させ２５質量％まで含有させると火花の発生は全くなくなり、これを超えて添加しても火花抑制効果はほとんど変わらないことを見いだした。よって、介在物改質剤中の金属Ｓｉ含有率は５〜２５質量％である。
但し、溶鋼中のＳｉ濃度が増加すると、得られた鋼板の加工性が低下するため、介在物改質剤中の金属Ｓｉ含有率に応じて、介在物改質剤の添加量を適宜考慮することが重要である。
【００１１】
また、介在物改質剤中の、少なくともＬａ、Ｃｅ、Ｎｄの含有率は４０〜９５質量％である。これは、少なくともＬａ、Ｃｅ、Ｎｄの含有率が９５質量％超では相対的に金属Ｓｉの含有率が５質量％未満となり火花発生を抑制できないため、４０質量％未満では介在物改質剤の改質効果が大きく低下するためである。
さらに、介在物改質剤中のＦｅ含有率は０〜５５質量％とする。介在物改質剤中のＦｅはＬａ、Ｃｅ、Ｎｄの含有率を相対的に減少させることにより、火花発生を抑制する効果がある。上記の通り、介在物改質剤中の金属Ｓｉ含有率の下限が５質量％であり、また、少なくともＬａ、Ｃｅ、Ｎｄの含有率の下限が４０質量％であるため、介在物改質剤中のＦｅ含有率の上限は５５質量％となる。一方、金属Ｓｉの火花発生抑制効果が得られている条件では、Ｆｅ含有率を０質量％としても良い。
但し、Ｆｅ含有率が高くなり過ぎると、Ｆｅが溶解するために溶鋼から熱を奪うことにより、改質剤が冷却剤として作用し、溶鋼の温度を低下させるといった問題が生じる。従って、介在物改質剤中のＦｅ含有率に応じて、介在物改質剤の添加量を適宜考慮することが重要である。
【００１２】
上記成分の介在物改質剤は、発火現象を抑制して、または全く発火することなく、溶鋼中に添加することが可能である。合わせて、少なくともＬａ、Ｃｅ、Ｎｄを、そのまま溶鋼に添加した場合と比べて、介在物改質効果が損なわれることがないため、溶存酸素をＴｉで脱酸し、その後上記改質剤を添加することにより、製品鋼材の材質低下なく、溶鋼中に介在物を微細に分散させ、ノズル付着をも解消できる結果、確実に表面疵を防止できる。
【００１３】
本発明方法の別の形態として、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して、炭素濃度を０．０１質量％以下とした溶鋼にＡｌを添加して予備脱酸処理を行い、溶鋼中の溶存酸素濃度を０．０１質量％以上０．０４質量％以下とし、次いでＴｉを添加して脱酸し、それから金属Ｓｉが５〜２５質量％、少なくともＬａ、Ｃｅ、Ｎｄが４０〜９５質量％、Ｆｅが０〜５５質量％で、残部が不可避的不純物元素からなる改質剤を添加した溶鋼を鋳造する方法を考案した。
この方法は、製造コストの面からより実用的なプロセスを考え、脱炭処理後の溶存酸素を全部Ａｌで脱酸するのではなく、溶存酸素を残すようにＡｌを添加して予備脱酸を行い、害にならない程度までアルミナ系介在物量を短時間で浮上除去し、その後改めてＡｌ以外の元素を用いて脱酸することを考案し、品質向上と製造コスト低減を両立させるものである。
【００１４】
上述したように、本発明者らは、Ｔｉ酸化物と少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物は凝集合体し難く、るつぼ壁にも付着せず、溶鋼中に微細分散することを明らかにした。これらの知見を基に、脱炭処理後の溶存酸素をＴｉだけで脱酸するのではなく、溶存酸素の一部をまずＡｌで予備脱酸し、害にならない程度までアルミナ系介在物を短時間で攪拌等により浮上除去した後、改めて残った溶存酸素をＴｉで脱酸し、さらに前述した介在物改質剤を添加することにより、アルミナ系介在物を含まないＴｉ酸化物と少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物を生成させ、溶鋼中に介在物を微細分散させることに成功した。このことで、溶鋼中介在物の凝集合体を防止し、鋼板中に介在物を微細分散させることにより、確実に表面疵を防止できる。ここで、上記記載のＡｌ予備脱酸後の害にならない程度のアルミナ系介在物濃度は、鋼板の表面疵を防止できれば特に規定するものではないが、通常は例えば全Ａｌ濃度で高々５０ｐｐｍ程度以下である。
【００１５】
Ｌａ、ＣｅとＮｄはＴｉに比べて非常に脱酸能が高いため、Ｔｉ添加後に生成したＴｉ酸化物系介在物を少量のＣｅ、ＬａもしくはＮｄで還元し、Ｔｉ酸化物と少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物に改質することは容易である。しかし、Ａｌ予備脱酸後の溶存酸素が０．０４質量％を超えると、Ｔｉ添加後に多量のＴｉ酸化物系介在物が生成するため、少なくともＬａ、Ｃｅ、Ｎｄを添加しても一部未改質のＴｉ酸化物系介在物が残留し、粗大なチタニアクラスターとなりやすい。一方、Ａｌ添加量を増大させ予備脱酸後の溶存酸素濃度を低下させると、多量のアルミナ系介在物を生成するため、これを分離除去することが困難になる。そこで、粗大化し易いアルミナ系介在物をできるだけ低減する条件から、Ａｌ脱酸後の溶存酸素濃度は０．０１質量％以上にすることが好ましい。したがって、本発明では、Ａｌ予備脱酸後の溶存酸素濃度を０．０１質量％以上０．０４質量％以下の範囲に制御することが好ましい。
【００１６】
さらに、本発明方法の詳細な形態として、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して、炭素濃度を０．０１質量％以下とした溶鋼にＡｌを添加し３分以上攪拌して予備脱酸処理を行い、溶鋼中の溶存酸素濃度を０．０１質量％以上０．０４質量％以下とし、次いでＴｉを添加して溶鋼中のＴｉ濃度を０．００３質量％以上０．４質量％以下とし、それから金属Ｓｉが５〜２５質量％、少なくともＬａ、Ｃｅ、Ｎｄが４０〜９５質量％、Ｆｅが０〜５５質量％で、残部が不可避的不純物元素からなる改質剤を添加した溶鋼を鋳造する方法を考案した。
実験的な検討では、予備脱酸におけるＡｌ添加後の溶存酸素濃度を０．０１質量％以上とし、且つＡｌ添加後の攪拌時間を３分以上確保することで、殆どのアルミナ系介在物を浮上除去できることを明らかにした。特に、真空脱ガス装置を用いた場合は、Ａｌ添加後の攪拌方法として還流することが一般的である。
【００１７】
予備脱酸後に、少量のＴｉを添加して脱酸すると、ＴｉはＡｌ等に比べて脱酸力が弱いため、一部溶存酸素が溶鋼中に残存する。Ｃ濃度が０．０１質量％以下の薄鋼板用の溶鋼では、溶存酸素濃度が０．０２質量％を超えると鋳造時にＣＯ気泡が発生することから、溶鋼中のＴｉ濃度は溶存酸素濃度が０．０２質量％以下になるように添加する必要があり、平衡計算からＴｉ濃度を算出すると０．００３質量％以上となる。一方、Ｔｉは脱酸力が比較的弱い方であるが、それでも溶鋼中に多量に添加すると、溶鋼中の溶存酸素濃度が大きく低下するため、その後に介在物改質剤を添加してもＴｉ酸化物と少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物に改質することが難しくなり、本発明の介在物微細化効果が損なわれる。このため、Ｔｉ濃度は数ｐｐｍ程度の溶存酸素を残せるように、０．４質量％以下にする必要がある。以上から、Ｔｉ濃度は０．００３質量％以上０．４質量％以下にすることが望ましい。
【００１８】
介在物改質剤を添加することは、介在物の微細化に効果的であるが、改質剤中のＬａ、Ｃｅ、Ｎｄは非常に強い脱酸材であるため、耐火物やモールドフラックスと反応して、溶鋼を汚染させると共に、耐火物やモールドフラックスを劣化させる。このため、改質剤の添加量は、生成したＴｉ酸化物系介在物を改質するに必要な量以上であって、且つＬａ、Ｃｅ、Ｎｄが耐火物やモールドフラックスと反応して溶鋼を汚染させない量以下である。実験的検討では、少なくともＬａ、Ｃｅ、Ｎｄの溶鋼中濃度の適正範囲は、０．００１質量％以上０．０３質量％以下である。また、介在物改質剤の添加は、必ずしも真空脱ガス装置内で添加する必要はなく、Ｔｉ添加後から鋳型内に流入するまでの間で添加すれば良く、例えばタンディッシュ内で添加することも可能である。
【００１９】
最近では、連続鋳造機に鋳型内電磁攪拌装置が装備されるようになっており、鋳造時に溶鋼を電磁攪拌すれば、鋳片表層の凝固シェルへの介在物捕捉を抑制できる。本発明では、溶鋼中の介在物は微細に分散しており、鋳片に捕捉されても表面欠陥にはならないが、例えばモールドフラックス、耐火物、スラグ等の外来系介在物が混入した場合には、電磁攪拌を行うことに、これらの外来系介在物の捕捉を確実に防止できる。このため、本発明の効果を安定的に得られると言う観点で、電磁攪拌を実施することが好ましい。
【００２０】
さらに、本発明の溶鋼を鋳造する場合、鋳造時間の経過と共にＴｉ酸化物と少なくともＬａ酸化物、Ｃｅ酸化物、Ｎｄ酸化物からなる複合介在物がモールドフラックス中に吸収され、それと共に、モールドフラックスの粘性が低下する可能性がある。モールドフラックスの粘性低下は、フラックス巻き込みを助長し、モールドフラックス起因の欠陥を引き起こす原因となる。このため、本発明の溶鋼を鋳造する場合、介在物吸収による粘性低下を考慮して、モールドフラックスの粘性を予め高めに設計しておくことが有効である。実験によれば、１３００℃におけるモールドフラックスの粘性を４poise以上にしておけば、モールドフラックス起因の欠陥は発生しなかった。モールドフラックスはモールドと鋳型間の潤滑機能を有しており、その機能が損なわれない程度であれば、特に粘性の上限値を規定するものではない。
【００２１】
本発明は、インゴット鋳造および連続鋳造でも可能であり、連続鋳造であれば通常の２５０mm厚み程度のスラブ連続鋳造に適用されるだけでなく、連続鋳造機の鋳型厚みがそれより薄い、例えば１５０mm以下の薄スラブ連続鋳造に対しても十分な効果が発現し、極めて表面疵の少ない鋳片を得ることができる。
本発明での低炭素というのは、炭素濃度の上限は特に規定するものではなく、他の鋼種と比較して相対的に炭素濃度が低いという意味であるが、特に、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から、Ｃ濃度を０．０５質量％以下、好ましくは０．０１質量％以下にするのが良い。Ｃ濃度の下限値は特に規定するものではない。
【００２２】
【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炭素濃度を０．００３質量％とした３００ｔの取鍋内溶鋼をＴｉで脱酸し、その後金属Ｓｉが１０質量％、Ｌａ、Ｃｅ、Ｎｄの合計が８０質量％、Ｆｅが１０質量％からなる改質剤（残部に微量の不可避的不純物元素を含む）を添加して、Ｔｉ濃度を０．０５質量％、Ｃｅ、Ｌａ、Ｎｄの合計濃度を０．００８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。鋳造の際に使用したモールドフラックスの粘性は６poiseであった。タンディッシュノズルや鍋ノズルの閉塞はなく、安定鋳造であった。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、表面欠陥は発生しなかった。
【００２３】
実施例２：転炉での精錬と真空脱ガス装置での処理により炭素濃度を０．００３質量％とした３００ｔの取鍋内溶鋼に予備脱酸Ａｌを１００kg添加して３分間環流させ、溶存酸素濃度０．０２質量％の溶鋼とした。さらに、この溶鋼にＴｉを添加して３分間環流し、その後金属Ｓｉが１５質量％、Ｌａ、Ｃｅ、Ｎｄの合計が７０質量％、Ｆｅが１５質量％からなる改質剤（残部に微量の不可避的不純物元素を含む）を添加し、Ｔｉ濃度を０．０３質量％、Ｃｅ、Ｌａ、Ｎｄの合計濃度を０．００７質量％にした溶鋼を溶製した。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。鋳造の際に使用したモールドフラックスの粘性は６poiseであった。タンディッシュノズルや鍋ノズルの閉塞はなく、安定鋳造であった。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。鋼板品質については、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。
【００２４】
実施例３：転炉での精錬と真空脱ガス装置での処理により炭素濃度を０．００５質量％とした３００ｔの取鍋内溶鋼に予備脱酸Ａｌを１５０kg添加して５分間環流させ、溶存酸素濃度０．０１２質量％の溶鋼とした。さらに、この溶鋼にＴｉを添加して４分間環流し、その後金属Ｓｉが１２質量％、Ｌａ、Ｃｅ、Ｎｄの合計が８８質量％からなる改質剤（残部に微量の不可避的不純物元素を含む）を添加し、Ｔｉ濃度を０．０４５質量％で、Ｃｅ、Ｌａ、Ｎｄの合計濃度を０．０１質量％にした溶鋼を溶製した。この溶鋼を電磁攪拌機能を有する鋳型を用いて、連続鋳造法で厚み７０mm、幅１８００mmの薄スラブに鋳造した。鋳造の際に使用したモールドフラックスの粘性は１５poiseであった。タンディッシュノズルや鍋ノズルの閉塞はなく、安定鋳造であった。このようにして得られた薄スラブは、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。鋼板品質については、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。
【００２５】
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０．００３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。鋳造後半で、タンディッシュノズルが閉塞傾向となり、湯面変動が発生した。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。
【００２６】
比較例２：転炉での精錬と真空脱ガス装置での処理により炭素濃度を０．００３質量％とした取鍋内溶鋼をＴｉで脱酸し、Ｔｉ濃度０．０４質量％、溶存酸素濃度０．００４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。鋳造後半で、鍋ノズルが閉塞し、５０ｔ程度の溶鋼を鍋ごと転炉に返送した。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７個／コイルの表面欠陥が発生した。
【００２７】
【発明の効果】
以上に説明したように、本発明によると、溶鋼中の介在物を微細分散させ、ノズル閉塞をも抑制することができるため、確実に表面疵を防止できる加工性、成形性に優れた低炭素鋼鋳片を製造することが可能となる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a low carbon steel slab that is excellent in workability and formability and hardly generates surface flaws, and a modifier for inclusions in molten steel.
[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 of several hundreds of μm or more. 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, 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, a very high rate of surface defects, and a reduction in alumina inclusions. Countermeasures are a major issue.
[0003]
On the other hand, conventionally, the inclusion adsorption flux of Patent Document 1 is added to the molten steel surface to remove alumina inclusions, or the CaO flux is added to the molten steel using the injection flow of Patent Document 2. Thus, a method for adsorbing and removing alumina inclusions has been proposed and implemented. On the other hand, as a method that does not remove the alumina inclusions but does not generate them, as disclosed in Patent Document 3, molten steel is deoxidized with Mg, and Al is hardly deoxidized with Al. A thin steel sheet deoxidized with Ti without addition of Al as in Reference 4 is also disclosed.
[0004]
[Patent Document 1]
Japanese Patent Laid-Open No. 5-104219 [Patent Document 2]
JP 63-149057 A [Patent Document 3]
Japanese Patent Laid-Open No. 5-302112 [Patent Document 4]
Japanese Patent Laid-Open No. 8-239731
[Problems to be solved by the invention]
However, in the method for removing alumina inclusions as disclosed in the above-mentioned Patent Documents 1 to 2, the alumina inclusions produced in large amounts in the low carbon molten steel are not affected by surface flaws. It is very difficult to reduce.
In addition, Mg deoxidation which does not produce any alumina inclusions as disclosed in Patent Document 3 has a high vapor pressure of Mg and a very low yield to molten steel, so it dissolves like low carbon steel. In order to deoxidize molten steel having a high oxygen concentration with only Mg, a large amount of Mg is required, which is not a practical process in view of manufacturing costs.
Furthermore, when deoxidizing with Ti as disclosed in Patent Document 4, Ti oxide in a solid state generated in molten steel adheres and accumulates on the inner surface of the pan nozzle or tundish nozzle together with the metal. There is a problem that nozzle clogging is promoted rather than normal Al deoxidation.
In view of these problems, the present invention prevents agglomeration and inclusion of inclusions in the molten steel and adhesion to the nozzle, and finely disperses the inclusions in the steel sheet, thereby reliably preventing surface flaws. It aims at providing the manufacturing method of a modifier and a low carbon steel slab.
[0006]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention has the following configuration. That is,
(1) After decarburizing the carbon concentration of the molten steel to 0.01% by mass or less, Ti is added to the molten steel for deoxidation, and thereafter 5 to 25% by mass of metal Si, at least La, Ce, and Nd are contained. Low carbon steel casting characterized by casting molten steel to which a modifier for inclusions in molten steel having a composition of 40 to 95% by mass, Fe of 0 to 55% by mass and the balance consisting of inevitable impurity elements is added. A manufacturing method of a piece.
(2) After decarburizing the molten steel to a carbon concentration of 0.01% by mass or less, Al is added to the molten steel to perform a preliminary deoxidation treatment, so that the dissolved oxygen concentration in the molten steel is 0.01% by mass or more and 0.0. 04 mass% or less, then Ti is added to deoxidize, then metal Si is 5 to 25 mass%, at least La, Ce, Nd is 40 to 95 mass%, Fe is 0 to 55 mass%, the balance A method for producing a low-carbon steel slab characterized by casting molten steel to which a modifier for inclusions in molten steel having a composition comprising inevitable impurity elements is added.
(3) After decarburizing the carbon concentration of the molten steel to 0.01% by mass or less, Al is added to the molten steel and stirred for 3 minutes or longer to perform a preliminary deoxidation treatment, so that the dissolved oxygen concentration in the molten steel is 0.01 Mass% or more and 0.04 mass% or less, then Ti is added to make the Ti concentration in the molten steel 0.003 mass% or more and 0.4 mass% or less, and thereafter, metal Si is 5 to 25 mass%, at least La , Ce, Nd is 40 to 95% by mass, Fe is 0 to 55% by mass, the remainder of the inclusion in the molten steel is a composition consisting of inevitable impurity elements , at least La, Ce, Nd A method for producing a low-carbon steel slab characterized by casting molten steel having a concentration in molten steel of 0.001 to 0.03% by mass.
(4) When decarburizing the molten steel to a carbon concentration of 0.01% by mass or less, a vacuum degassing device is used. The low carbon steel slab according to any one of (1) to (3) , Production method.
(5) The method for producing a low-carbon steel slab according to any one of (1) to (4) , wherein the molten steel is cast with a mold having an electromagnetic stirring function.
(6) The method for producing a low-carbon steel slab according to any one of (1) to (5) , wherein the molten steel is cast using a mold flux having a viscosity at 1300 ° C. of 4 poise or more. .
(7) The method for producing a low carbon steel slab according to any one of (1) to (6) , wherein the molten steel is continuously cast when cast.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
The molten steel decarburized in the 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) below). Therefore, a large amount of alumina inclusions are generated.
2Al + 3O = Al 2 O 3 (1)
These inclusions aggregate and coalesce with each other immediately after deoxidation to form coarse alumina clusters of several hundred μm or more, which cause surface defects during the production of the steel sheet.
Therefore, in order not to generate alumina clusters, attention was focused on deoxidizing dissolved oxygen after decarburization with a deoxidizer and a modifier other than Al.
[0008]
As a method of the present invention, refining is performed in a steelmaking furnace such as a converter or an electric furnace, or further vacuum degassing is performed, and Ti is added to molten steel having a carbon concentration of 0.01% by mass or less to deoxidize. Then, a molten steel to which 5 to 25% by mass of metal Si, at least La, Ce, and Nd are 40 to 95% by mass, Fe is 0 to 55% by mass, and the balance is an inevitable impurity element is added. A method of casting was devised.
Here, at least La, Ce, and Nd mean one or more of La, Ce, and Nd. Hereinafter, the same meaning is used.
[0009]
The present inventors experimentally evaluated the agglomeration behavior of these inclusions by appropriately combining Al or Ti with at least La, Ce, or Nd added thereto as a deoxidizer to be added to molten steel. However, alumina inclusions, Ti oxide inclusions, composite inclusions comprising alumina inclusions and Ti oxide inclusions, or composite inclusions comprising alumina and at least La oxide, Ce oxide, and Nd oxide. (For example, alumina-La oxide composite inclusions, alumina-Ce oxide-Nd oxide composite inclusions, alumina-La oxide-Ce oxide-Nd oxide composite inclusions, etc.) are relatively It easily aggregates and coalesces and easily adheres to the crucible wall, while it contains composite inclusions composed of Ti oxide and at least La oxide, Ce oxide and Nd oxide (for example, Ti oxide-La oxide based composites). Inclusions, Ti oxide-Ce oxide-Nd oxide composite inclusions, Ti oxide-La oxide-Ce oxide-Nd oxide composite inclusions, etc.) are difficult to agglomerate and coalesce on the crucible wall. It was found that it was not adhered and was finely dispersed in the molten steel. This is because alumina inclusions, Ti oxide inclusions, composite inclusions consisting of alumina inclusions and Ti oxide inclusions, or alumina and at least La oxide, Ce oxide, Nd oxide. Compared to composite inclusions, composite inclusions composed of Ti oxide and at least La oxide, Ce oxide, and Nd oxide have a greater decrease in interfacial energy between inclusions and molten steel, and the inclusions are aggregated. This is because adhesion to coalescence and refractory was suppressed. Based on these findings, the dissolved oxygen is deoxidized with Ti, and at least La, Ce, and Nd are added to make the Ti oxide inclusions into Ti oxide and at least La oxide, Ce oxide, and Nd oxidation. The composite inclusions were improved, and the inclusions were successfully finely dispersed in the molten steel.
[0010]
However, when a test was performed to add an alloy consisting of at least La, Ce, and Nd into the molten steel using actual equipment, the alloy was transported to the hopper by a belt conveyor, the alloy was inserted into the hopper, or the alloy was When added to molten steel, an ignition phenomenon due to collision of the alloys was observed. Therefore, various inclusion modifiers containing at least other metals in an alloy composed of La, Ce, and Nd were prepared, and components for suppressing ignition were examined using the spark test method of JIS-G-0567.
When the present inventors contain metal Si in an alloy composed of at least La, Ce, and Nd in an amount of 5% by mass or more, the generation of sparks is drastically reduced. Further, when the metal Si is further increased to 25% by mass. It was found that the generation of sparks was completely eliminated, and that the spark suppression effect was hardly changed even if it was added beyond this. Therefore, the metal Si content in the inclusion modifier is 5 to 25% by mass.
However, when the Si concentration in the molten steel increases, the workability of the obtained steel sheet decreases, so the amount of inclusion modifier added is appropriately considered according to the metal Si content in the inclusion modifier. This is very important.
[0011]
Further, the content of at least La, Ce, and Nd in the inclusion modifier is 40 to 95% by mass. This is because at least the content of La, Ce, and Nd is less than 95% by mass, and the content of metal Si is relatively less than 5% by mass, so that the generation of sparks cannot be suppressed. This is because the reforming effect is greatly reduced.
Furthermore, the Fe content in the inclusion modifier is 0 to 55% by mass. Fe in the inclusion modifier has the effect of suppressing the occurrence of sparks by relatively reducing the content of La, Ce, and Nd. As described above, the lower limit of the metal Si content in the inclusion modifier is 5% by mass, and the lower limit of at least La, Ce, and Nd is 40% by mass. The upper limit of the Fe content is 55% by mass. On the other hand, the Fe content may be 0% by mass on the condition that the effect of suppressing the occurrence of sparks of metal Si is obtained.
However, if the Fe content becomes too high, Fe dissolves and heat is taken away from the molten steel, causing a problem that the modifier acts as a coolant and lowers the temperature of the molten steel. Therefore, it is important to appropriately consider the amount of inclusion modifier added depending on the Fe content in the inclusion modifier.
[0012]
The inclusion modifier of the above components can be added to molten steel while suppressing the ignition phenomenon or without igniting at all. In addition, compared with the case where at least La, Ce, and Nd are added to the molten steel as they are, the inclusion modification effect is not impaired, so the dissolved oxygen is deoxidized with Ti, and then the above modifier is added. By doing so, the inclusions can be finely dispersed in the molten steel without deteriorating the quality of the product steel, and the nozzle adhesion can be eliminated. As a result, surface flaws can be reliably prevented.
[0013]
As another form of the method of the present invention, Al is added to molten steel having a carbon concentration of 0.01% by mass or less by refining in a steelmaking furnace such as a converter or an electric furnace, or further by vacuum degassing. The pre-deoxidation treatment is performed, the dissolved oxygen concentration in the molten steel is set to 0.01% by mass or more and 0.04% by mass or less, then Ti is added for deoxidation, and then the metal Si is 5 to 25% by mass, at least A method has been devised for casting molten steel to which La, Ce, and Nd are 40 to 95% by mass, Fe is 0 to 55% by mass, and the balance is added with a modifier composed of inevitable impurity elements.
This method considers a more practical process from the viewpoint of manufacturing cost, and does not deoxidize all dissolved oxygen after decarburization with Al, but pre-deoxidizes by adding Al so that dissolved oxygen remains. It is devised to float and remove the amount of alumina inclusions in a short time to such an extent that it does not cause harm, and then deoxidize using an element other than Al again, thereby achieving both improvement in quality and reduction in manufacturing cost.
[0014]
As described above, the present inventors have found that composite inclusions composed of Ti oxide and at least La oxide, Ce oxide, and Nd oxide are difficult to agglomerate and do not adhere to the crucible wall, and are fine in the molten steel. It was revealed that it was dispersed. Based on these findings, rather than deoxidizing the dissolved oxygen after decarburization with Ti alone, a part of the dissolved oxygen is first pre-deoxidized with Al, and the alumina inclusions are shortened to the point where they do not harm. After floating and removing by stirring, etc. over time, the remaining dissolved oxygen is again deoxidized with Ti, and by adding the inclusion modifier described above, Ti oxide not containing alumina inclusions and at least La oxidation Composite inclusions made of copper, Ce oxide, and Nd oxide were produced, and the inclusions were successfully finely dispersed in the molten steel. Thus, surface flaws can be reliably prevented by preventing aggregation and inclusion of inclusions in the molten steel and finely dispersing the inclusions in the steel sheet. Here, the concentration of alumina inclusions that does not cause harm after the Al preliminary deoxidation described above is not particularly specified as long as the surface flaws of the steel sheet can be prevented, but usually, for example, at most about 50 ppm or less at the total Al concentration. It is.
[0015]
Since La, Ce and Nd have a very high deoxidation capacity compared to Ti, Ti oxide inclusions generated after addition of Ti are reduced with a small amount of Ce, La or Nd. Ti oxide and at least La oxide It is easy to modify the composite inclusions composed of Ce oxide and Nd oxide. However, if the dissolved oxygen after Al pre-deoxidation exceeds 0.04% by mass, a large amount of Ti oxide inclusions are generated after Ti addition. Modified Ti oxide inclusions remain and tend to be coarse titania clusters. On the other hand, if the amount of Al added is increased and the dissolved oxygen concentration after the preliminary deoxidation is lowered, a large amount of alumina inclusions are produced, which makes it difficult to separate and remove them. Therefore, it is preferable to set the dissolved oxygen concentration after Al deoxidation to 0.01% by mass or more from the condition of reducing alumina inclusions that are likely to be coarsened as much as possible. Therefore, in this invention, it is preferable to control the dissolved oxygen concentration after Al preliminary deoxidation to the range of 0.01 mass% or more and 0.04 mass% or less.
[0016]
Furthermore, as a detailed form of the method of the present invention, Al is added to molten steel having a carbon concentration of 0.01% by mass or less by refining in a steelmaking furnace such as a converter or an electric furnace, or further by vacuum degassing. Add and stir for 3 minutes or longer to perform a pre-deoxidation treatment, so that the dissolved oxygen concentration in the molten steel is 0.01 mass% or more and 0.04 mass% or less, and then Ti is added to bring the Ti concentration in the molten steel to 0.0. 003 mass% to 0.4 mass%, and then metal Si is 5 to 25 mass%, at least La, Ce and Nd are 40 to 95 mass%, Fe is 0 to 55 mass%, and the balance is inevitable impurity element A method for casting molten steel with a modifier consisting of
In experimental studies, the dissolved oxygen concentration after Al addition in the preliminary deoxidation is 0.01% by mass or more, and the stirring time after Al addition is secured for 3 minutes or more, so that most alumina inclusions emerge. Clarified that it can be removed. In particular, when a vacuum degassing apparatus is used, it is common to reflux as a stirring method after the addition of Al.
[0017]
When a small amount of Ti is added and deoxidized after preliminary deoxidation, Ti has a weaker deoxidizing power than Al or the like, and thus some dissolved oxygen remains in the molten steel. In molten steel for thin steel sheets having a C concentration of 0.01% by mass or less, CO bubbles are generated during casting when the dissolved oxygen concentration exceeds 0.02% by mass, so the Ti concentration in the molten steel has a dissolved oxygen concentration of 0. It is necessary to add it to 0.02 mass% or less, and when the Ti concentration is calculated from the equilibrium calculation, it becomes 0.003 mass% or more. On the other hand, Ti has a relatively weak deoxidizing power, but if it is still added in a large amount in molten steel, the dissolved oxygen concentration in the molten steel is greatly reduced. It becomes difficult to modify the composite inclusions composed of the oxide and at least La oxide, Ce oxide, and Nd oxide, and the inclusion refinement effect of the present invention is impaired. For this reason, the Ti concentration needs to be 0.4% by mass or less so as to leave about several ppm of dissolved oxygen. From the above, it is desirable that the Ti concentration be 0.003 mass% or more and 0.4 mass% or less.
[0018]
Adding inclusion modifiers is effective for making inclusions finer, but La, Ce, and Nd in the modifiers are very strong deoxidizers, so that refractories and mold flux It reacts to contaminate the molten steel and degrade the refractory and mold flux. For this reason, the addition amount of the modifier is more than the amount necessary for modifying the generated Ti oxide inclusions, and La, Ce, and Nd react with the refractory and the mold flux to give molten steel. Less than the amount that does not contaminate. In the experimental study, the appropriate range of at least the concentrations of La, Ce, and Nd in the molten steel is 0.001% by mass or more and 0.03% by mass or less. In addition, the inclusion modifier need not be added in the vacuum degassing device, but may be added after the Ti addition until it flows into the mold, for example, in a tundish. Is also possible.
[0019]
Recently, a continuous casting machine is equipped with an in-mold electromagnetic stirrer. If molten steel is electromagnetically stirred during casting, inclusion trapping in the solidified shell of the slab surface layer can be suppressed. In the present invention, the inclusions in the molten steel are finely dispersed and do not cause surface defects even when trapped by the slab, but when foreign inclusions such as mold flux, refractory, slag, etc. are mixed. Can reliably prevent these foreign inclusions from being trapped by performing electromagnetic stirring. For this reason, it is preferable to implement electromagnetic stirring from the viewpoint that the effects of the present invention can be stably obtained.
[0020]
Further, when the molten steel of the present invention is cast, a composite inclusion composed of Ti oxide and at least La oxide, Ce oxide, and Nd oxide is absorbed in the mold flux as the casting time elapses, and the mold flux The viscosity of can be reduced. The decrease in the viscosity of the mold flux promotes flux entrainment and causes defects due to the mold flux. For this reason, when casting the molten steel according to the present invention, it is effective to design the viscosity of the mold flux to be high in advance in consideration of viscosity reduction due to inclusion absorption. According to the experiment, when the viscosity of the mold flux at 1300 ° C. was set to 4 poise or more, defects due to the mold flux did not occur. The mold flux has a lubrication function between the mold and the mold, and the upper limit of the viscosity is not particularly defined as long as the function is not impaired.
[0021]
The present invention is also applicable to ingot casting and continuous casting. If continuous casting is used, the present invention is not only applied to a normal slab continuous casting of about 250 mm thickness, but the mold thickness of the continuous casting machine is thinner, for example, 150 mm or less. A sufficient effect is exhibited even for continuous casting of a thin slab, and it is possible to obtain a slab with extremely little surface flaws.
The low carbon in the present invention does not particularly define the upper limit of the carbon concentration, and means that the carbon concentration is relatively low compared to other steel types. Since it is used for applications in which processing of the outer plate or the like is severe, it is necessary to add workability, so the C concentration is 0.05% by mass or less, preferably 0.01% by mass or less. The lower limit value of the C concentration is not particularly specified.
[0022]
【Example】
Hereinafter, the present invention will be described with reference to examples and comparative examples.
Example 1: 300 t of molten steel in a ladle having a carbon concentration of 0.003 mass% was deoxidized with Ti by refining in a converter and treatment in a reflux vacuum degassing apparatus, and then metal Si was 10 mass%. , La, Ce and Nd are added in a total amount of 80% by mass and Fe is added by 10% by mass (the balance contains a trace amount of inevitable impurity elements), and the Ti concentration is 0.05% by mass, Ce. , La and Nd total concentration was 0.008 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 viscosity of the mold flux used for casting was 6 poise. There was no blockage of the tundish nozzle and pan nozzle, and the casting was stable. 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, no surface defects occurred.
[0023]
Example 2: 100 kg of pre-deoxidized Al was added to 300 t of molten steel in a ladle with a carbon concentration of 0.003 mass% by refining in a converter and treatment in a vacuum degassing apparatus, and refluxed for 3 minutes. It was set as molten steel having an oxygen concentration of 0.02% by mass. Further, Ti was added to the molten steel and refluxed for 3 minutes, and thereafter a modifier comprising 15% by mass of metal Si, 70% by mass of La, Ce, and Nd and 15% by mass of Fe (with a trace amount in the balance). Including inevitable impurity elements) was added, and molten steel with a Ti concentration of 0.03% by mass and a total concentration of Ce, La, and Nd of 0.007% by mass was melted. 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 viscosity of the mold flux used for casting was 6 poise. There was no blockage of the tundish nozzle and pan nozzle, and the casting was stable. 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 steel sheet 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.
[0024]
Example 3: 150 kg of pre-deoxidized Al was added to 300 t of molten steel in a ladle with a carbon concentration of 0.005 mass% by refining in a converter and processing in a vacuum degassing apparatus, and refluxed for 5 minutes. A molten steel having an oxygen concentration of 0.012% by mass was obtained. Further, Ti is added to the molten steel and refluxed for 4 minutes, and then a modifier comprising 12% by mass of metal Si and 88% by mass of La, Ce, and Nd (the balance contains a trace amount of inevitable impurity elements). ) Was added, and a molten steel having a Ti concentration of 0.045 mass% and a total concentration of Ce, La, and Nd of 0.01 mass% was melted. This molten steel was cast into a thin slab having a thickness of 70 mm and a width of 1800 mm by a continuous casting method using a mold having an electromagnetic stirring function. The mold flux used for casting had a viscosity of 15 poise. There was no blockage of the tundish nozzle and pan nozzle, and the casting was stable. The thin 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 steel sheet 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.
[0025]
Comparative Example 1: Molten steel in a ladle having a carbon concentration of 0.003% by mass by refining in a converter and treatment in a reflux vacuum degassing device was deoxidized with Al, and the Al concentration was 0.04% by mass. The oxygen concentration was 0.0002% by 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. In the second half of casting, the tundish nozzle tended to close and the molten metal surface level changed. 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.
[0026]
Comparative Example 2: Molten steel in a ladle with a carbon concentration of 0.003% by mass by refining in a converter and treatment in a vacuum degassing device was deoxidized with Ti, and a Ti concentration of 0.04% by mass and dissolved oxygen concentration The amount was 0.004% by 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. In the latter half of casting, the pan nozzle was closed, and about 50t of molten steel was returned to the converter together with the pan. 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 7 pieces / coil were generated on an average slab.
[0027]
【The invention's effect】
As described above, according to the present invention, inclusions in molten steel can be finely dispersed and nozzle clogging can be suppressed, so that low carbon with excellent workability and formability that can reliably prevent surface flaws. Steel slabs can be manufactured.
Claims (7)
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