JP3575400B2

JP3575400B2 - Direct-feed rolling method of continuous cast slab

Info

Publication number: JP3575400B2
Application number: JP2000177688A
Authority: JP
Inventors: 幹雄鈴木; 浩淡路谷; 康一堤; 正之中田
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2000-06-14
Filing date: 2000-06-14
Publication date: 2004-10-13
Anticipated expiration: 2020-06-14
Also published as: JP2001353563A

Description

【０００１】
【発明の属する技術分野】
本発明は、連続鋳造機により鋳造された高温鋳片を直接、あるいは表面温度を中心温度と同じにする程度の保温・加熱を行った後に熱間圧延するか、又は、連続鋳造機により鋳造された高温鋳片を加熱炉に装入して加熱した後に熱間圧延するか、何れかの方法（本発明ではこれらをまとめて「直送圧延」と定義する）で鋼板を製造する直送圧延方法に関し、詳しくは、表面性状に優れた鋼板を製造することができる直送圧延方法に関するものである。
【０００２】
【従来の技術】
連続鋳造機で鋳造された鋳片を直接、あるいは表面温度を中心温度と同じにする程度の保温・加熱を行った後に熱間圧延する、所謂直接圧延（ＨＤＲとも云う）や、連続鋳造機で鋳造された高温の鋳片を加熱炉に装入して加熱後に熱間圧延する、所謂ホットチャージ圧延（ＨＣＲとも云う）は、工程の大幅な合理化や省エネルギ−及び歩留りの向上が期待でき、今後更に開発が進められ発展が予想される。
【０００３】
ところで、連続鋳造鋳片の表層部には、縦割れ、横割れ、ノロカミ、ブローホール等の表面欠陥があり、例えば特開昭６１−２６４１３５号公報や特開昭６２−３４６０２号公報に開示されるように、連続鋳造鋳片を直送圧延して鋼板を製造する場合、鋳片にはこれらの表面欠陥が無いことが必要であり、そのため、表面欠陥を回避する様々な手段を用いて直送圧延が実施されている。
【０００４】
具体的な対策例として、縦割れは鋳型内の冷却速度を低下すると減少することが知られており、鋳型内での抜熱を少なくするモールドパウダーが開発され、縦割れ防止に効果をあげている。横割れは鋳片表面温度を鋼の脆化温度範囲外の８５０℃以上に保持して連続鋳造機の矯正帯を通過させることで防止可能であり、二次冷却強度及び鋳片引き抜き速度の制御により８５０℃以上の表面温度を確保して横割れが防止されている。ノロカミの低減にはモールドパウダーの性状を最適化すると共に鋳型内の溶鋼流動を制御することが必要であり、鋳造条件毎にその条件に合致するモールドパウダーが開発され、又、電磁力を用いた鋳型内溶鋼の流動制御が行われ、ノロカミ低減に効果をあげている。ブローホールは浸漬ノズル内に吹き込まれるＡｒに起因して発生するため、Ａｒの吹き込み流量を必要最低限に制御してその発生を抑えている。このような対策を積み上げ実施することで直送圧延が実用化している。
【０００５】
【発明が解決しようとする課題】
しかしながら、上記対策を実施してもＨＤＲプロセスやＨＣＲプロセスにより製造された鋼板には表面疵の発生が多く、表面品質は再加熱材に比べて劣ると云う問題がある。ここで再加熱材とは、常温まで冷却した鋳片を無手入れのまま加熱炉に装入して圧延温度まで加熱し、次いで、圧延して製造した鋼板である。
【０００６】
ＨＤＲプロセスやＨＣＲプロセスでは、凝固・冷却後の鋳片をできるだけ高温のまま熱間圧延機に搬送、若しくは加熱炉に装入することにより、鋳片を加熱する燃料を節約するのが狙いである。このようなプロセスにおいては、炭素濃度が０．１質量％以下の炭素鋼を凝固させると、δ−フェライトが初晶として晶出し、更に冷却していくと、δ−フェライト相はオーステナイト（以下「γ」と記す）相に変態する。γ相中の硫黄（Ｓ）の溶解度はδ−フェライト相中のＳの溶解度よりも小さいため、γ相中ではＳはγ粒界に偏析する。そのため、γ粒界ではＳとＭｎが反応して硫化物（ＭｎＳ）が析出する。粒界でのＳの偏析は著しく大きいため、ＭｎＳの析出量は粒内に比べて粒界の方が圧倒的に多くなる。又、Ｍｎの拡散はＳの拡散に比べて１オーダー小さいので、ＭｎＳが生成すると、その周囲にＭｎの欠乏層が生成する。尚、Ｓは拡散が早く、欠乏層は生成しない。
【０００７】
Ｍｎ欠乏層の生成等によりＭｎ濃度が低くなると、硫化物としてＦｅリッチな［Ｆｅ、Ｍｎ］Ｓが形成・析出してくる。この硫化物組成では、硫化物自体の融点が非常に低く、１１００℃〜１２００℃程度まで低下する。そのため、Ｆｅリッチな［Ｆｅ、Ｍｎ］Ｓが析出した鋼の熱間延性は非常に低く、熱間圧延すると割れが発生し易くなる。
【０００８】
特に、このような硫化物の析出と低融点化は、鋼成分組成のＭｎ含有量とＳ含有量との比（Ｍｎ／Ｓ）が５０以下で顕著になるため、Ｍｎ／Ｓ比が５０以下の鋼をＨＤＲプロセス若しくはＨＣＲプロセスで製造すると、鋼板表面の疵発生頻度が高くなる。Ｍｎ／Ｓ比が５０以下の鋼では、Ｍｎ含有量がＳ含有量に較べて相対的に少なく、上記のＭｎ欠乏層の生成に相まって、Ｆｅリッチの複合硫化物が生成し易くなるためである。
【０００９】
本発明は上記事情に鑑みなされたもので、その目的とするところは、炭素濃度が０．１質量％以下の低炭素Ａｌキルド鋼鋼板をＨＤＲプロセスやＨＣＲプロセスにより製造する際に、表面疵を防止して表面性状に優れた鋼板の製造を可能とする連続鋳造鋳片の直送圧延方法を提供することである。
【００１０】
【課題を解決するための手段】
第１の発明による連続鋳造鋳片の直送圧延方法は、質量％で、Ｃ：０．１％以下、Ｐ：０．０２％以下、Ｓ：０．０３％以下、ｓｏｌＡｌ：０．０１〜０．０５％を含有するＡｌキルド鋼鋳片の直送圧延方法であって、垂直曲げ型連鋳機を用いて２ｍ／ｍｉｎ以上の引き抜き速度で鋳造し、鋳片が上部矯正帯を通過後に、凝固率が１０〜５０％の鋳片を５〜１５℃／ｓｅｃの冷却速度で冷却して鋳片表面温度をＡr1点以下まで低下させ、次いで、復熱させてＡc3点以上の鋳片表面温度で下部矯正帯を通過させ、その後、鋳片表面温度をＡc3点以上に保持したまま、切断すると共に熱間圧延機まで搬送して、熱間圧延することを特徴とするものである。
【００１１】
第２の発明による連続鋳造鋳片の直送圧延方法は、質量％で、Ｃ：０．１％以下、Ｐ：０．０２％以下、Ｓ：０．０３％以下、ｓｏｌＡｌ：０．０１〜０．０５％を含有するＡｌキルド鋼鋳片の直送圧延方法であって、垂直曲げ型連鋳機を用いて２ｍ／ｍｉｎ以上の引き抜き速度で鋳造し、鋳片が上部矯正帯を通過後に、凝固率が１０〜５０％の鋳片を５〜１５℃／ｓｅｃの冷却速度で冷却して鋳片表面温度をＡr1点以下まで低下させ、次いで、復熱させてＡc3点以上の鋳片表面温度で下部矯正帯を通過させ、その後、鋳片表面温度をＡc3点以上に保持したまま、切断すると共に加熱炉まで搬送し、加熱炉で加熱した後に熱間圧延することを特徴とするものである。
【００１２】
第３の発明による連続鋳造鋳片の直送圧延方法は、第１の発明又は第２の発明において、Ａｌキルド鋼鋳片のＭｎ含有量とＳ含有量との比が５０以下であることを特徴とするものである。
【００１３】
第４の発明による連続鋳造鋳片の直送圧延方法は、第１の発明乃至第３の発明の何れかにおいて、鋳片が上部矯正帯を通過後、少なくとも２０秒経過してから鋳片表面温度をＡｒ１点以下に冷却することを特徴とするものである。
【００１４】
本発明では、Ｃ：０．１質量％（以下「％」と記す）以下、Ｐ：０．０２％以下、Ｓ：０．０３％以下、ｓｏｌＡｌ：０．０１〜０．０５％を含有するＡｌキルド鋼を対象とする。直送圧延には強度が低くて加工性が良い鋼が適しており、上記成分組成のＡｌキルド鋼はこの条件を満たすためである。そして、特に、Ｍｎ含有量とＳ含有量との比（Ｍｎ／Ｓ）が５０以下のＡｌキルド鋼鋳片を対象とすることが好ましい。前述したように、Ｍｎ／Ｓが５０以下の鋼では、析出する硫化物がＦｅリッチの複合硫化物になり、熱間圧延中に割れ易く、製品欠陥が発生して歩留まりが低下するという問題が大きく、本発明の効果を十分に発揮させることができるからである。
【００１５】
本発明では一旦鋳片の表面温度をＡｒ１点以下まで下げ、鋳片表層部をパーライト変態させ、次いで、鋳片表面をＡｃ３点以上の温度まで復熱させ、再度γ相に変態させる。この２回の相変態により、凝固・冷却段階で最初に形成したγ粒の粒界に沿って析出していた硫化物と新しく生成したγ粒界とが分離される。本発明では、このようにして硫化物析出位置とγ粒界とを分離させるので、鋼の高温脆化を著しく改善でき、熱間圧延時の表面疵を低減させることができる。
【００１６】
本発明では、２ｍ／ｍｉｎ以上の引き抜き速度で鋳造する必要がある。その理由は、ＨＤＲプロセスにおける熱間圧延機の生産量を確保するためには、熱間圧延機の生産量に見合った引き抜き速度で鋳造する必要があるからである。又、圧延前の鋳片温度は、圧延性や省エネルギーの観点から高い方が良く、２ｍ／ｍｉｎ以上の引き抜きで鋳造することにより鋳片温度を高めることができるからである。
【００１７】
本発明では垂直曲げ型連続鋳造機を用いる。本発明が対象とする鋼種はＡｌキルド鋼であり、高清浄性が要望される鋼である。Ａｌキルド鋼では溶鋼中に脱酸生成物であるアルミナ性介在物が多数、懸濁しており、これを除去するために様々な対策が採られている。そのひとつに、垂直曲げ型連鋳機を用いて鋳型を含む垂直部でアルミナ性介在物を浮上させ、モールドパウダーに吸収させて除去する方法があり、この方法によるアルミナ性介在物の除去効果は特に優れている。これが本発明で垂直曲げ型連続鋳造機を用いる理由である。
【００１８】
垂直曲げ型連続鋳造機では鋳片は２回矯正される。即ち、垂直方向に鋳造された鋳片は上部矯正帯（１点で矯正される場合には上部矯正点と云う）で平板状から円弧状に矯正され、下部矯正帯（１点で矯正される場合には下部矯正点と云う）で円弧状から平板状に矯正されて水平方向に搬出される。
【００１９】
鋳片が上部矯正帯を通過する際の鋳片表面温度は高い方が良い。鋳片温度が高くなると、鋳片の強度が低下して小さな矯正力で矯正可能になり、且つ、矯正時の応力緩和が起り易くなるためである。鋳片表面温度が低過ぎると、鋳片強度が高過ぎて矯正できないという問題が起こる。そのため、本発明では鋳片が上部矯正帯を通過してから鋳片表面温度をＡｒ１点以下に冷却することとした。更に、矯正後直後は鋳片に矯正応力が残留するので、矯正終了後少なくとも２０秒間は応力緩和させて残留応力を消失させ、その後鋳片表面温度をＡｒ１点以下に冷却することが好ましい。
【００２０】
又、下部矯正帯での矯正時に矯正応力により、鋳片に横割れが発生することがある。本発明で対象とする鋼は本来矯正応力による横割れの発生が少ないが、本発明では鋼の脆性域より高温側のＡｃ３点以上の鋳片表面温度に復熱させてから矯正するので、横割れを完全に防止することができる。
【００２１】
そして、鋳片表面温度をＡｃ３点以上に保持したまま、切断すると共に熱間圧延機又は加熱炉まで搬送する。これは、ＨＤＲプロセスの場合には圧延開始前の鋳片温度はＡｃ３点以上必要であり、又、ＨＣＲプロセスの場合も鋳片温度が高い程省エネルギー効果が高くなるからである。
【００２２】
【発明の実施の形態】
以下、添付図面を参照して本発明を説明する。図１は本発明による直送圧延方法のうちＨＤＲプロセスにおける鋳片表面の温度履歴を示す概要図、図２は本発明による直送圧延方法のうちＨＣＲプロセスにおける鋳片表面の温度履歴を示す概要図、図３は本発明の実施の形態の例を示す図であって、垂直曲げ型のスラブ連続鋳造機の側面概要図である。尚、図１及び図２のＴ_Ｌは液相線温度、Ｔ_Ｓは固相線温度である。
【００２３】
Ｃ：０．１％以下、Ｐ：０．０２％以下、Ｓ：０．０３％以下、ｓｏｌＡｌ：０．０１〜０．０５％を含有する溶鋼を転炉や二次精錬炉等により溶製し、この溶鋼を図３に示すように連続鋳造機にて鋳造する。前述したように本発明は、これら成分に加えて、更に、Ｍｎ含有量とＳ含有量との比（Ｍｎ／Ｓ）が５０以下の鋼に適用することが好ましい。その他の成分は特に限定する必要はないが、通常、直送圧延される薄鋼板用のＡｌキルド鋼では、およそＳｉ：０．１％以下、Ｍｎ：０．４％以下である。直送圧延には強度が低くて加工性が良い鋼が適しており、このような薄鋼板用のＡｌキルド鋼は、合金成分が少なく、この条件を満たすため、直送圧延には最適である。
【００２４】
浸漬ノズル５を介して鋳型６内に鋳造された溶鋼は、鋳型６内で冷却されて凝固殻３を形成し、内部に未凝固相２を有する鋳片１として、鋳型６の下方に設けたサポートロール７、ガイドロール８、及びピンチロール９に支持されつつ、ピンチロール９の駆動力により鋳型６の下方に、２．０ｍ／ｍｉｎ以上の引き抜き速度で連続的に引き抜かれる。この間鋳片１は複数のガイドロール８からなる上部矯正帯１０で平板状から円弧状に曲げられ、又、複数のガイドロール８からなる下部矯正帯１１で円弧状から平板状に曲げ戻される。鋳片１は、これらのロール間に設けられた水スプレー又はエアーミストスプレーから構成される二次冷却帯（図示せず）で冷却され、下方に引き抜かれつつ凝固殻３の厚みを増大する。尚、上部矯正帯１０では鋳片表面温度を７００℃以上に保持して矯正するのが一般的であるが、フェライト相とγ相が共存すると、脆弱なフェライト相が優先的に矯正変形を起こし、フェライト部分に割れが発生する可能性があるので、γ単相の温度域で矯正することが望ましい。
【００２５】
上部矯正帯１０と下部矯正帯１１との間には、鋳片表層部を強冷却するための強冷却装置４が設置されている。強冷却装置４は、通常の二次冷却帯に較べて鋳片表面を急速に冷却することが可能な装置であり、３〜１５℃／ｓｅｃ、好ましくは５℃／ｓｅｃ以上の冷却速度で冷却可能な装置である。そのためには、例えば水プレーノズルで冷却する場合は、鋳片表面１ｍ^２当たりの１分間の冷却水量を２００〜２０００ｌとし、噴射圧力を２〜１００ｋｇ／ｃｍ^２とすれば良い。但し、強冷却装置４は水スプレーノズルに限るものではなく、例えば鋳片表面に層流の冷却水を流すような冷却方法としても良い。
【００２６】
又、強冷却装置４の設置位置は、鋳片１が上部矯正帯１０を通過して少なくとも２０秒経過した時点から強冷却されるように、上部矯正帯１０との設置間隔を設定することが好ましい。上部矯正帯１０から強冷却装置４までの鋳片１の通過時間は鋳片引き抜き速度により決定されるので、採用する引き抜き速度の最速値において通過時間が２０秒以上となるように強冷却装置４を配置すれば良い。
【００２７】
この強冷却装置４を用いて鋳片表面を急速に冷却し、鋳片表面温度がＡｒ１点以下となるまで冷却する。Ａｒ１点以下となる保持時間は１０秒間以上であれば上限値は特に限定されないが、冷却し過ぎると鋳片１の温度が下がり、直送圧延の省エネルギー効果が低下するので６０秒程度で十分である。このように急速冷却する理由は、上部矯正帯１０の通過後から下部矯正帯１１に入るまでに、鋳片表面温度を一旦Ａｒ１点以下に下げ、少なくとも１０秒以上Ａｒ１点以下に保持した後、復熱させて鋳片表面温度をＡｃ３点以上に保持する必要があるからである。１０秒間以上保持する理由は、パーライト変態を十分に起こさせるためである。短すぎると変態が完全に終了せずに復熱してしまい、γ粒界と硫化物析出位置との分離が完全には実現しない可能性があるからである。急速冷却しても鋼の熱拡散が小さいために凝固殻３の内部は温度低下せず、表面から深さ方向１５ｍｍ程度にしか熱拡散が及ばない。この結果、鋳片１の有する顕熱はほとんど失われることがない。ゆっくり冷却すると、表層がＡｒ１点以下になるまでに凝固殻３の内部も冷却され、結果的に顕熱が失われてしまう。急速に冷却することで顕熱を失わず、効果的に復熱させることが可能となる。
【００２８】
Ａｒ１点は冷却速度により影響を受ける。即ち、パーライト変態を開始する際には過冷却が必要となるので、Ａｒ１点は冷却速度の影響を受けて低下する。冷却速度とＡｒ１点との関係を調べた結果、上記成分範囲の鋼種のＡｒ１点は、冷却速度１０℃／ｍｉｎでは約５５０℃となるので、この温度を目安として冷却するのが良い。
【００２９】
又、強冷却中の鋳片１の凝固率を１０〜５０％の範囲に制御することが好ましい。内部に未凝固相２を有し、その凝固率が１０〜５０％である鋳片１を強冷却すると、未凝固相２の潜熱及び顕熱により復熱時の温度上昇量を大きくさせること、換言すれば、高温の鋳片１を得ることができるからである。この条件を満足させるために、鋳片引き抜き速度及び鋳型６直下から強冷却装置４入り口までの二次冷却強度を制御し、凝固殻３の厚みを調整することが必要である。尚、凝固率とは鋳片１の厚みの半分に対する凝固殻３の厚みの比率を百分率で表示したものである。
【００３０】
鋳片１が強冷却装置４を通過したならば、二次冷却強度を調整して鋳片表面を復熱させる。鋳片１は、多量の未凝固相２を内部に有しているので、二次冷却強度を調整することで、未凝固相２の凝固潜熱及び顕熱により容易に復熱する。そして、連続鋳造機の湾曲部で鋳片表面温度がＡｃ３点以上となるまで復熱させ、次いで、下部矯正帯１１で鋳片１を矯正する。
【００３１】
上部矯正帯１０の出口から下部矯正帯１１の入り口までの距離は、例えば半径１０ｍの円弧状に鋳片１を矯正する場合には１５．７ｍとなる。図３に示すように、通常の連続鋳造機では、複数本のガイドロール８による逐次矯正が採用されているので、実質的には約１２〜１３ｍの長さの間で上述した冷却・復熱を実施する必要がある。引抜き速度は少なくとも２ｍ／ｍｉｎ以上であるので、この湾曲部を通過する時間は最大６．５分となる。復熱速度は鋼の熱拡散率によって支配され、連鋳機内にある鋳片１では最大で約２０℃／ｓｅｃであるので、強冷却装置４の設置位置及び冷却速度を考慮することが重要である。
【００３２】
そして、鋳片表面温度をＡｃ３点以上に保持したまま、鋳片中心部まで完全に凝固させ、凝固した鋳片１を連続鋳造機出側に設置した切断機（図示せず）で所定長さに切断し、Ａｃ３点以上の高温の鋳片１を搬送台車等により熱間圧延機に搬送して熱間圧延するか、若しくは加熱炉で加熱後に熱間圧延する。
【００３３】
鋳片温度を高めるためには、連続鋳造機の出口（機端）付近まで未凝固相２を残し、連続鋳造機の出口直前で鋳片１の凝固を完了させることが効果的である。溶鋼が凝固する際には潜熱を放出するため、この潜熱により凝固殻３を復熱させることができる。尚、ＨＤＲプロセスの場合、鋳片１を熱間圧延機で圧延する前に必要に応じて表面温度を中心温度と同じにする程度の保温・加熱を行っても、又、同調カッターの切断時に形成されるバリを除去しても良い。
【００３４】
このようにして直送圧延を行うことで、表面疵が極めて少なく、表面性状に優れた鋼板を直送圧延により製造することが可能となる。又、一旦鋳片表面をＡｒ１点以下まで低下させるが、その後の復熱により鋳片は容易にＡｃ３点以上となり、鋳造後の鋳片加熱を必要としない程度まで鋳片温度を高めることができる。
【００３５】
【実施例】
［実施例１］
図３に示す垂直曲げ型スラブ連続鋳造機を用いて、引き抜き速度及び冷却強度を変更したＨＤＲ試験を実施し、鋳片表面温度と薄鋼板での疵発生率との関係を調査した。用いた連続鋳造機は、鋳型長さが０．９ｍ、鋳型上端から上部矯正帯出口までの長さが３．５ｍ、湾曲半径が１０ｍ、鋳型上端から下部矯正帯入り口までの長さが１７．５ｍ、鋳型上端から下部矯正帯出口までの長さが１９．５ｍ、連続鋳造機の機長（鋳型上端から機端まで）が４５ｍであり、急速冷却装置は鋳型上端から４．５〜６．５ｍの範囲に設置した。鋳片厚みが２２０ｍ、鋳片幅が１０００ｍｍであり、Ｃ：０．１％以下、Ｓｉ：０．１％以下、Ｍｎ：０．４％以下、Ｐ：０．０２％以下、Ｓ：０．０３％以下、ｓｏｌＡｌ：０．０１〜０．０５％を含有し、Ｍｎ／Ｓが３５〜３８のＡｌキルド鋼の鋳片を１．８〜３．２ｍ／ｍｉｎの引き抜き速度で鋳造した。
【００３６】
鋳型上端から６．６ｍ、１７．４ｍ、機端、及び熱間圧延機直前の合計４箇所の位置に２色温度計設置し、鋳片幅方向中央の表面温度を測定した。上部矯正帯を通過時の鋳片表面温度は９００℃以上になるように冷却強度を制御し、上部矯正帯通過後急速に冷却した。その後、二次冷却強度を調整して１０００℃を目標に復熱させ、下部矯正帯を通過させた。鋳造した鋳片は直ちに所定長さ（約９ｍ）にガスカッターで切断し、鋳片コーナー部はガスバーナー式のヒーターで１０００℃以上になるように加熱した後、ローラーテーブルにより熱間圧延機まで高速搬送した。ＨＤＲが可能な温度は経験的に熱間圧延前の鋳片表面温度が９３０℃以上であるので、９３０℃以上を確保できるかどうかをＨＤＲ可否の判断基準とした。表１に試験条件及び試験結果を示す。これら鋼種のＡｒ１点は約５５０℃、Ａｃ３点は９００℃である。
【００３７】
【表１】

【００３８】
引き抜き速度が１．８ｍ／ｍｉｎの試験Ｎｏ．１では、熱間圧延前の鋳片表面温度が８８０℃となり、ＨＤＲ可否の判断基準である９３０℃を確保できなかった。一方、引き抜き速度が２．０ｍ／ｍｉｎ以上の試験では、熱間圧延前の鋳片表面温度は、９３０℃以上であった。これらの結果から、ＨＤＲを行うためには引き抜き速度を２．０ｍ／ｍｉｎ以上とする必要があることが分かった。
【００３９】
鋳片表面温度がＡｒ１点以下にならないように冷却した試験Ｎｏ．９では、ＨＤＲは問題なく実施できたが、熱間圧延後の薄鋼板に表面疵が多数発生した。又、強冷却装置でＡｒ１点以下に冷却した後、下部矯正帯をＡｃ３点以下で通過させた試験Ｎｏ．１０では、連鋳機出口での鋳片温度が低くなり、そのため、熱間圧延機直前での鋳片温度も低く、圧延荷重が高まると同時に圧延後の疵の発生も多かった。
【００４０】
引き抜き速度が３．２ｍ／ｍｉｎの試験Ｎｏ．８では、鋳片下面コーナー部にコーナー割れが発生し、薄鋼板の疵発生率が若干高くなった。急速冷却すると鋳片表面には引張り応力が作用する。従って、上部矯正から急速冷却開始までの時間が短かいと、急速冷却による引張り応力に加えて矯正時の残留応力が作用し、鋳片下面コーナー部に引き抜き方向に直交する割れが発生する。これが試験Ｎｏ．８における鋳片下面コーナー部のコーナー割れの発生原因である。一方、上部矯正から強冷却開始までの時間が２０秒以上の他の全ての試験ではコーナー割れは発生しなかった。これらの結果から、上部矯正から強冷却開始までの時間を２０秒以上にすれば、コーナー割れを防止できることが分かった。
【００４１】
上部矯正から強冷却開始までの時間を２０秒以上とし、更に、引き抜き速度及び鋳片表面温度を本発明の範囲内に制御した試験Ｎｏ．２〜７では薄鋼板の表面疵が少なく、且つ、安定してＨＤＲを実施することができた。
【００４２】
［実施例２］
実施例１と同一の連続鋳造機を用い、実施例１と同一のＡｌキルド鋼を鋳造してＨＣＲを実施した。連続鋳造機の鋳造量と熱間圧延機の生産量とを一致させるために、連続鋳造機の引き抜き速度を２．４ｍ／ｍｉｎとして厚みが２２０ｍｍ、幅が１０００ｍｍの鋳片を鋳造した。
【００４３】
強冷却装置による急速冷却をした後の鋳片表面温度は、４５０〜５２０℃の範囲であった。その後、復熱させて下部矯正帯を通過時の鋳片表面温度を９５０〜１０２０℃の範囲に制御した。連鋳機出口の鋳片表面温度は９８０〜１０２０℃であり、ガスカッターで切断後に鋳片コーナーを加熱せずに加熱炉に装入した。加熱炉温度は１１５０℃に設定した。加熱炉からの抽出時間を３０分、６０分、９０分、１２０分、１５０分、１８０分に設定して、この時間で抽出し、熱間圧延を実施して薄鋼板の表面疵の発生率を調査した。表２に試験条件及び試験結果を示す。これら鋼種のＡｒ１点は約５５０℃、Ａｃ３点は９００℃である。表２に示す加熱燃料原単位指数は、加熱時間が３時間以上の燃料原単位の平均値を基準（１．０）としたものである。
【００４４】
【表２】

【００４５】
加熱時間が３０分の試験Ｎｏ．１１では、鋳片コーナー部分の温度が上がりきらず、熱間圧延後の薄鋼板に表面疵が若干発生したが、６０分以上加熱すると鋳片温度が均一化して表面疵の発生率は少なくなった。一方、加熱時間が長くなるほど加熱燃料の原単位は増加した。省エネルギーと表面疵防止の観点からは加熱時間は６０〜１２０分とすることが望ましい。
【００４６】
【発明の効果】
本発明では、Ｍｎ／Ｓが５０以下のＡｌキルド鋼鋳片の直送圧延において、硫化物の析出形態を制御しながら、且つ、直送圧延が可能な温度まで連鋳機内で復熱させるので、鋳片の凝固組織に起因する熱延コイルの表面疵を完全に防止することが可能となると共に、省エネルギー効果を増大させることができ、工業上有益な効果がもたらされる。
【図面の簡単な説明】
【図１】本発明によるＨＤＲプロセスにおける鋳片表面の温度履歴を示す図である。
【図２】本発明によるＨＣＲプロセスにおける鋳片表面の温度履歴を示す図である。
【図３】本発明の実施の形態の例を示す図であって、垂直曲げ型のスラブ連続鋳造機の側面概要図である。
【符号の説明】
１鋳片
２未凝固相
３凝固殻
４強冷却装置
５浸漬ノズル
６鋳型
７サポートロール
８ガイドロール
９ピンチロール
１０上部矯正帯
１１下部矯正帯[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention is a hot cast slab directly cast by a continuous casting machine, or hot-rolled after performing heat retention and heating to the extent that the surface temperature is the same as the center temperature, or cast by a continuous casting machine The hot rolling slab is heated in a heating furnace and then hot rolled, or a direct rolling method for producing a steel sheet by any method (these are collectively defined as "direct rolling" in the present invention). More specifically, the present invention relates to a direct rolling method capable of producing a steel sheet having excellent surface properties.
[0002]
[Prior art]
So-called direct rolling (also referred to as HDR), in which a slab cast by a continuous casting machine is directly or hot-rolled after performing heat retention and heating so that the surface temperature is the same as the center temperature, or a continuous casting machine. The so-called hot charge rolling (HCR), in which a cast high-temperature slab is charged into a heating furnace and heated and then hot-rolled, can be expected to significantly reduce the process, save energy, and improve the yield. Further development is expected in the future.
[0003]
By the way, the surface layer portion of the continuous cast slab has surface defects such as vertical cracks, horizontal cracks, norokami and blowholes, and is disclosed in, for example, JP-A-61-264135 and JP-A-62-34602. As described above, when a steel plate is manufactured by directly rolling a continuous cast slab, it is necessary that the slab does not have these surface defects, and therefore, the direct rolling is performed using various means for avoiding the surface defects. Has been implemented.
[0004]
As a specific countermeasure example, it is known that vertical cracks decrease when the cooling rate in the mold is reduced, and a mold powder that reduces heat removal in the mold has been developed, which is effective in preventing vertical cracks. I have. Lateral cracking can be prevented by maintaining the slab surface temperature at 850 ° C or higher, which is outside the range of brittleness of steel, and passing it through the straightening zone of a continuous casting machine, and controlling the secondary cooling strength and slab drawing speed. As a result, a surface temperature of 850 ° C. or more is secured to prevent lateral cracking. It is necessary to optimize the properties of mold powder and control the flow of molten steel in the mold in order to reduce slag, and mold powder that meets the conditions has been developed for each casting condition. The flow of molten steel in the mold is controlled, which is effective in reducing norokami. Since blow holes are generated due to Ar blown into the immersion nozzle, the flow rate of Ar blow is controlled to a necessary minimum to suppress the occurrence. By directly stacking and implementing such measures, direct-feed rolling has been put to practical use.
[0005]
[Problems to be solved by the invention]
However, even if the above measures are taken, there is a problem that the steel plate manufactured by the HDR process or the HCR process has many surface flaws, and the surface quality is inferior to that of the reheated material. Here, the reheated material is a steel sheet manufactured by charging a cast slab cooled to normal temperature into a heating furnace without care, heating the slab to a rolling temperature, and then rolling.
[0006]
In the HDR process and the HCR process, the aim is to save the fuel for heating the slab by transferring the slab after solidification and cooling to a hot rolling mill at as high a temperature as possible or loading it into a heating furnace. . In such a process, when carbon steel having a carbon concentration of 0.1% by mass or less is solidified, δ-ferrite is crystallized as primary crystals, and when further cooled, the δ-ferrite phase becomes austenite (hereinafter referred to as “ γ ”) phase. Since the solubility of sulfur (S) in the γ phase is lower than the solubility of S in the δ-ferrite phase, S segregates at the γ grain boundary in the γ phase. Therefore, at the γ grain boundary, S and Mn react to precipitate sulfide (MnS). Since the segregation of S at the grain boundaries is remarkably large, the precipitation amount of MnS is overwhelmingly greater at the grain boundaries than within the grains. Further, since the diffusion of Mn is one order of magnitude smaller than the diffusion of S, when MnS is generated, a Mn deficient layer is generated around it. Note that S diffuses quickly and does not generate a depletion layer.
[0007]
When the Mn concentration decreases due to the formation of a Mn-deficient layer, Fe-rich [Fe, Mn] S is formed and precipitated as sulfide. In this sulfide composition, the melting point of the sulfide itself is very low, and drops to about 1100 ° C to 1200 ° C. Therefore, the hot ductility of steel in which Fe-rich [Fe, Mn] S is precipitated is extremely low, and cracks are easily generated when hot rolling is performed.
[0008]
In particular, such precipitation of sulfides and lowering of the melting point become remarkable when the ratio of the Mn content to the S content (Mn / S) of the steel component composition is 50 or less, so that the Mn / S ratio is 50 or less. Is produced by the HDR process or the HCR process, the frequency of occurrence of flaws on the surface of the steel sheet increases. This is because in a steel having an Mn / S ratio of 50 or less, the Mn content is relatively small as compared with the S content, and the Fe-rich composite sulfide is easily generated in combination with the formation of the Mn-deficient layer. .
[0009]
The present invention has been made in view of the above circumstances, and an object of the present invention is to produce a low-carbon Al-killed steel sheet having a carbon concentration of 0.1% by mass or less by an HDR process or an HCR process. It is an object of the present invention to provide a method of direct rolling of a continuous cast slab which can prevent the production of a steel sheet having excellent surface properties.
[0010]
[Means for Solving the Problems]
The method for directly rolling a continuous cast slab according to the first invention is as follows: C: 0.1% or less, P: 0.02% or less, S: 0.03% or less, solAl: 0.01 to 0% by mass. A method for directly rolling an Al-killed steel slab containing 0.05% by weight using a vertical bending type continuous casting machine at a drawing speed of 2 m / min or more, and solidifying after the slab passes through the upper straightening zone. the rate is reduced to below Ar1 point the billet surface temperature is cooled at a cooling rate of 5 to 15 ° C. / sec from 10 to 50% of the slab, then, at the slab surface temperature above Ac3 point by recuperator After passing through the lower straightening zone, the slab is cut and transported to a hot rolling mill while maintaining the slab surface temperature at the Ac3 point or higher, where hot rolling is performed.
[0011]
In the method for directly rolling a continuous cast slab according to the second invention, C: 0.1% or less, P: 0.02% or less, S: 0.03% or less, solAl: 0.01 to 0% by mass. A method for directly rolling an Al-killed steel slab containing 0.05% by weight using a vertical bending type continuous casting machine at a drawing speed of 2 m / min or more, and solidifying after the slab passes through the upper straightening zone. the rate is reduced to below Ar1 point the billet surface temperature is cooled at a cooling rate of 5 to 15 ° C. / sec from 10 to 50% of the slab, then, at the slab surface temperature above Ac3 point by recuperator After passing through the lower straightening zone, the slab is cut and transported to a heating furnace while maintaining the slab surface temperature at the Ac3 point or higher, and then hot-rolled after heating in the heating furnace.
[0012]
According to a third aspect of the present invention, there is provided a method for directly rolling a continuous cast slab, wherein the ratio between the Mn content and the S content of the Al-killed steel slab is 50 or less. It is assumed that.
[0013]
According to a fourth aspect of the present invention, there is provided a method for directly feeding and rolling a continuous cast slab according to any one of the first to third aspects, wherein the slab surface temperature is at least 20 seconds after the slab has passed the upper straightening zone. Is cooled to Ar1 point or less.
[0014]
In the present invention, C: 0.1% by mass (hereinafter referred to as "%") or less, P: 0.02% or less, S: 0.03% or less, and solAl: 0.01 to 0.05%. For Al-killed steel. A steel having low strength and good workability is suitable for direct rolling, and an Al-killed steel having the above-mentioned composition is to satisfy this condition. In particular, it is preferable to target an Al-killed steel slab having a ratio of Mn content to S content (Mn / S) of 50 or less. As described above, in a steel having an Mn / S of 50 or less, the sulfide that precipitates becomes a Fe-rich composite sulfide, which is liable to crack during hot rolling, causing a product defect to lower the yield. This is because the effect of the present invention can be sufficiently exhibited.
[0015]
In the present invention, the surface temperature of the slab is once lowered to the Ar1 point or lower, the slab surface layer is transformed into pearlite, and then the slab surface is reheated to the temperature of the Ac3 point or higher and transformed again into the γ phase. By the two phase transformations, sulfides precipitated along the grain boundaries of the γ grains formed first in the solidification and cooling stage are separated from newly formed γ grain boundaries. In the present invention, since the sulfide precipitation position and the γ grain boundary are separated in this manner, high-temperature embrittlement of steel can be significantly improved, and surface defects during hot rolling can be reduced.
[0016]
In the present invention, it is necessary to cast at a drawing speed of 2 m / min or more. The reason is that in order to secure the production volume of the hot rolling mill in the HDR process, it is necessary to perform casting at a drawing speed commensurate with the production volume of the hot rolling mill. In addition, the slab temperature before rolling is preferably higher from the viewpoint of rollability and energy saving, and the slab temperature can be increased by casting by drawing at 2 m / min or more.
[0017]
In the present invention, a vertical bending type continuous casting machine is used. The steel type targeted by the present invention is Al-killed steel, which is a steel required to have high cleanliness. In Al-killed steel, a large number of alumina-based inclusions, which are deoxidation products, are suspended in molten steel, and various measures have been taken to remove them. One of the methods is to lift the alumina inclusions in the vertical part including the mold using a vertical bending type continuous casting machine, absorb them into mold powder and remove them, and the effect of removing alumina inclusions by this method is Especially excellent. This is the reason for using the vertical bending type continuous casting machine in the present invention.
[0018]
In the vertical bending type continuous casting machine, the slab is straightened twice. That is, a slab cast in the vertical direction is straightened from a flat plate to an arc by an upper straightening zone (or an upper straightening point when straightened at one point) and a lower straightening zone (one straightened). In this case, it is corrected from an arc shape to a flat shape at a lower correction point, and is conveyed horizontally.
[0019]
The higher the surface temperature of the slab when the slab passes through the upper straightening zone, the better. If the slab temperature is high, the strength of the slab is reduced, so that the slab can be corrected with a small correction force, and stress relaxation at the time of correction is more likely to occur. If the slab surface temperature is too low, there is a problem that the slab strength is too high to correct. Therefore, in the present invention, the slab surface temperature is cooled to the Ar1 point or less after the slab has passed through the upper straightening zone. Further, since the straightening stress remains in the slab immediately after the straightening, it is preferable to relax the stress for at least 20 seconds after the completion of the straightening to eliminate the residual stress, and then cool the slab surface temperature to the Ar 1 point or lower.
[0020]
In addition, a horizontal crack may be generated in the cast slab due to the correction stress during the correction in the lower correction band. The steel targeted in the present invention originally has few lateral cracks due to the straightening stress. However, in the present invention, the steel is corrected to a slab surface temperature of three or more points higher than the brittle zone of the steel, and then straightened. Cracks can be completely prevented.
[0021]
Then, the slab is cut and transported to a hot rolling mill or a heating furnace while maintaining the slab surface temperature at the Ac3 point or higher. This is because in the case of the HDR process, the slab temperature before the start of rolling must be equal to or higher than Ac, and also in the case of the HCR process, the higher the slab temperature, the higher the energy saving effect.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic diagram showing the temperature history of the slab surface in the HDR process in the direct rolling method according to the present invention. FIG. 2 is a schematic diagram showing the temperature history of the slab surface in the HCR process in the direct rolling method according to the present invention. FIG. 3: is a figure which shows the example of embodiment of this invention, and is a side schematic drawing of the slab continuous casting machine of a vertical bending type | mold. In addition, _{TL in} FIGS. 1 and 2 is a liquidus temperature, and T _S is a solidus temperature.
[0023]
Molten steel containing C: 0.1% or less, P: 0.02% or less, S: 0.03% or less, and solAl: 0.01 to 0.05% is produced by a converter or a secondary refining furnace. Then, the molten steel is cast by a continuous casting machine as shown in FIG. As described above, in addition to these components, the present invention is preferably applied to steel having a ratio of Mn content to S content (Mn / S) of 50 or less. The other components need not be particularly limited, but are usually about 0.1% or less for Si and 0.4% or less for Mn in an Al-killed steel for a thin steel sheet which is directly rolled. Steel having low strength and good workability is suitable for direct rolling, and such an Al-killed steel for a thin steel sheet has a small alloy component and satisfies these conditions, and is therefore most suitable for direct rolling.
[0024]
The molten steel cast in the mold 6 through the immersion nozzle 5 is cooled in the mold 6 to form a solidified shell 3 and is provided below the mold 6 as a slab 1 having an unsolidified phase 2 therein. While being supported by the support roll 7, the guide roll 8, and the pinch roll 9, the sheet is continuously pulled out below the mold 6 by a driving force of the pinch roll 9 at a drawing speed of 2.0 m / min or more. The cast slab 1 is bent from a flat plate shape into an arc shape by an upper straightening band 10 composed of a plurality of guide rolls 8, and is bent back from an arc shape to a flat shape by a lower straightening band 11 composed of a plurality of guide rolls 8. The slab 1 is cooled by a secondary cooling zone (not shown) composed of a water spray or an air mist spray provided between these rolls, and is drawn downward to increase the thickness of the solidified shell 3. In the upper straightening zone 10, it is general to straighten the slab while maintaining the slab surface temperature at 700 ° C. or higher. However, when the ferrite phase and the γ phase coexist, the brittle ferrite phase preferentially causes deformation. Since cracks may occur in the ferrite portion, it is desirable to correct the temperature within the temperature range of a single gamma phase.
[0025]
Between the upper straightening belt 10 and the lower straightening belt 11, a strong cooling device 4 for strongly cooling the slab surface layer is installed. The strong cooling device 4 is a device capable of rapidly cooling the slab surface as compared with a normal secondary cooling zone, and cools at a cooling rate of 3 to 15 ° C./sec, preferably 5 ° C./sec or more. It is a possible device. For that purpose, for example, when cooling with a water spray nozzle, the amount of cooling water per 1 m ² of the slab surface per minute may be 200 to 2000 l and the injection pressure may be ² to 100 kg / cm ² . However, the strong cooling device 4 is not limited to the water spray nozzle, and may be a cooling method in which, for example, laminar cooling water flows on the slab surface.
[0026]
The installation position of the strong cooling device 4 may be set to an interval between the slab 1 and the upper straightening band 10 so that the slab 1 is strongly cooled at least 20 seconds after passing through the upper straightening band 10. preferable. Since the passage time of the slab 1 from the upper straightening zone 10 to the strong cooling device 4 is determined by the slab drawing speed, the strong cooling device 4 is set so that the passing time is 20 seconds or more at the maximum value of the adopted drawing speed. Should be arranged.
[0027]
The slab surface is rapidly cooled using the strong cooling device 4 until the slab surface temperature becomes equal to or lower than the Ar1 point. The upper limit is not particularly limited as long as the holding time at which the Ar 1 point or less is 10 seconds or more. However, if the temperature is too low, the temperature of the slab 1 decreases, and the energy saving effect of direct-feed rolling decreases, so that about 60 seconds is sufficient. . The reason for such rapid cooling is that, after passing through the upper straightening zone 10 and before entering the lower straightening zone 11, the slab surface temperature is once reduced to an Ar1 point or less, and maintained at least 10 seconds or more and an Ar1 point or less. This is because it is necessary to recover the heat to maintain the slab surface temperature at the Ac 3 point or more. The reason for holding for 10 seconds or more is to sufficiently cause the pearlite transformation. If the length is too short, the transformation is not completed and the reheating occurs, and the separation between the γ grain boundary and the sulfide precipitation position may not be completely realized. Even if it is rapidly cooled, the temperature inside the solidified shell 3 does not decrease because the heat diffusion of the steel is small, and the heat diffusion reaches only about 15 mm in the depth direction from the surface. As a result, the sensible heat of the slab 1 is hardly lost. When the cooling is performed slowly, the inside of the solidified shell 3 is also cooled until the surface layer becomes lower than the Ar1 point, and as a result, sensible heat is lost. By cooling rapidly, it is possible to effectively recover the heat without losing the sensible heat.
[0028]
The Ar1 point is affected by the cooling rate. That is, since supercooling is required at the time of starting the pearlite transformation, the Ar1 point decreases under the influence of the cooling rate. As a result of examining the relationship between the cooling rate and the Ar1 point, the Ar1 point of the steel type in the above component range is about 550 ° C. at a cooling rate of 10 ° C./min.
[0029]
Further, it is preferable to control the solidification rate of the slab 1 during strong cooling to be in the range of 10 to 50%. When the slab 1 having the unsolidified phase 2 therein and having a solidification rate of 10 to 50% is strongly cooled, the amount of temperature rise at the time of reheating by the latent heat and sensible heat of the unsolidified phase 2 is increased; In other words, it is possible to obtain a high-temperature slab 1. In order to satisfy this condition, it is necessary to control the thickness of the solidified shell 3 by controlling the slab withdrawal speed and the secondary cooling strength from immediately below the mold 6 to the entrance of the strong cooling device 4. In addition, the solidification rate is a percentage of the thickness of the solidified shell 3 to half the thickness of the slab 1.
[0030]
When the slab 1 has passed through the strong cooling device 4, the secondary cooling strength is adjusted to reheat the slab surface. Since the cast slab 1 has a large amount of the unsolidified phase 2 therein, by adjusting the secondary cooling strength, the slab 1 easily recovers heat by solidifying latent heat and sensible heat of the unsolidified phase 2. Then, the slab 1 is reheated at the curved portion of the continuous casting machine until the slab surface temperature becomes equal to or higher than the Ac3 point, and then the slab 1 is corrected by the lower correction band 11.
[0031]
The distance from the exit of the upper straightening zone 10 to the entrance of the lower straightening zone 11 is 15.7 m when the slab 1 is straightened into an arc having a radius of 10 m, for example. As shown in FIG. 3, in the ordinary continuous casting machine, since the successive straightening by the plurality of guide rolls 8 is adopted, the cooling and reheating described above is substantially performed for a length of about 12 to 13 m. Need to be implemented. Since the drawing speed is at least 2 m / min or more, the time required to pass through this curved portion is at most 6.5 minutes. Since the recuperation rate is governed by the thermal diffusivity of the steel, and is a maximum of about 20 ° C./sec in the slab 1 in the continuous casting machine, it is important to consider the installation position of the strong cooling device 4 and the cooling rate. is there.
[0032]
Then, while keeping the slab surface temperature at the Ac3 point or more, the slab is completely solidified to the center of the slab, and the solidified slab 1 is cut to a predetermined length by a cutting machine (not shown) installed on the continuous casting machine exit side. The cast slab 1 having a high temperature of 3 or more Ac is conveyed to a hot rolling mill by a carrier or the like and hot-rolled, or hot-rolled after heating in a heating furnace.
[0033]
In order to increase the slab temperature, it is effective to leave the unsolidified phase 2 near the exit (machine end) of the continuous casting machine and complete the solidification of the slab 1 immediately before the exit of the continuous casting machine. Since the latent heat is released when the molten steel is solidified, the solidified shell 3 can be recovered by the latent heat. In the case of the HDR process, if necessary, before the slab 1 is rolled by the hot rolling mill, the surface temperature may be maintained and heated to the extent that the surface temperature is the same as the center temperature. The burrs formed may be removed.
[0034]
By performing direct-feed rolling in this way, it is possible to produce a steel sheet having very few surface flaws and excellent in surface properties by direct-feed rolling. Further, the slab surface is once lowered to the Ar1 point or less, but the slab easily becomes the Ac3 point or more due to reheating, and the slab temperature can be increased to such an extent that slab heating after casting is not required. .
[0035]
【Example】
[Example 1]
Using the vertical bending type slab continuous casting machine shown in FIG. 3, an HDR test was performed in which the drawing speed and the cooling strength were changed, and the relationship between the slab surface temperature and the flaw occurrence rate in the thin steel plate was investigated. The continuous casting machine used had a mold length of 0.9 m, a length from the upper end of the mold to the exit of the upper straightening zone of 3.5 m, a radius of curvature of 10 m, and a length from the upper end of the mold to the entrance of the lower straightening zone of 17.1 m. 5 m, the length from the upper end of the mold to the outlet of the lower straightening zone is 19.5 m, the machine length of the continuous casting machine (from the upper end of the mold to the machine end) is 45 m, and the rapid cooling device is 4.5 to 6.5 m from the upper end of the mold. It was installed in the range. The slab thickness is 220 m, the slab width is 1000 mm, C: 0.1% or less, Si: 0.1% or less, Mn: 0.4% or less, P: 0.02% or less, S: 0. A cast piece of Al-killed steel containing not more than 03% and solAl: 0.01 to 0.05% and Mn / S of 35 to 38 was cast at a drawing speed of 1.8 to 3.2 m / min.
[0036]
A two-color thermometer was installed at a total of four positions 6.6 m and 17.4 m from the upper end of the mold, at the mill end, and immediately before the hot rolling mill, and the surface temperature at the center in the slab width direction was measured. The cooling strength was controlled so that the slab surface temperature at the time of passing through the upper straightening zone was 900 ° C. or higher, and was rapidly cooled after passing through the upper straightening zone. Thereafter, the secondary cooling strength was adjusted to reheat the target to 1000 ° C. and passed through the lower straightening zone. The cast slab is immediately cut to a predetermined length (approximately 9 m) by a gas cutter, and the slab corner is heated to a temperature of 1000 ° C. or higher by a gas burner type heater, and then heated to a hot rolling mill by a roller table. Conveyed at high speed. Since the slab surface temperature before hot rolling is 930 ° C. or higher empirically as a temperature at which HDR is possible, whether or not 930 ° C. or higher can be secured was used as a criterion for judging the possibility of HDR. Table 1 shows test conditions and test results. The Ar1 point of these steel types is about 550 ° C., and the Ac point is 900 ° C.
[0037]
[Table 1]

[0038]
Test No. with a drawing speed of 1.8 m / min. In No. 1, the slab surface temperature before hot rolling was 880 ° C., and 930 ° C., which was a criterion for judging the possibility of HDR, could not be secured. On the other hand, in the test in which the drawing speed was 2.0 m / min or more, the slab surface temperature before hot rolling was 930 ° C. or more. From these results, it was found that the drawing speed had to be 2.0 m / min or more in order to perform HDR.
[0039]
Test No. 1 was cooled so that the slab surface temperature did not fall below the Ar1 point. In No. 9, although HDR could be performed without any problem, a number of surface flaws occurred in the thin steel sheet after hot rolling. In addition, after cooling to a point of Ar1 or less by a strong cooling device, the test No. 3 was passed through the lower straightening zone at a point of Ac3 or less. In No. 10, the slab temperature at the outlet of the continuous caster was low, so that the slab temperature immediately before the hot rolling mill was low, the rolling load was increased, and at the same time, the number of flaws after rolling was increased.
[0040]
Test No. with a drawing speed of 3.2 m / min. In No. 8, corner cracks occurred at the corners of the lower surface of the slab, and the flaw occurrence rate of the thin steel plate was slightly increased. Upon rapid cooling, a tensile stress acts on the slab surface. Therefore, if the time from the straightening of the upper part to the start of the rapid cooling is short, the residual stress at the time of the straightening acts in addition to the tensile stress caused by the rapid cooling, so that a crack perpendicular to the drawing direction occurs at the corner of the lower surface of the slab. This is Test No. 8 is the cause of the occurrence of corner cracks at the corners of the slab lower surface. On the other hand, corner cracking did not occur in all other tests in which the time from the upper straightening to the start of the strong cooling was 20 seconds or more. From these results, it was found that corner cracking can be prevented if the time from the top correction to the start of strong cooling is set to 20 seconds or more.
[0041]
Test No. 2 in which the time from the upper straightening to the start of the strong cooling was 20 seconds or more, and the drawing speed and the slab surface temperature were controlled within the range of the present invention. In Nos. 2 to 7, the surface flaw of the thin steel plate was small and HDR could be stably performed.
[0042]
[Example 2]
Using the same continuous caster as in Example 1, the same Al-killed steel as in Example 1 was cast and HCR was performed. In order to make the casting amount of the continuous casting machine equal to the production amount of the hot rolling mill, a slab having a thickness of 220 mm and a width of 1000 mm was cast with the drawing speed of the continuous casting machine being 2.4 m / min.
[0043]
The slab surface temperature after rapid cooling by the strong cooling device was in the range of 450 to 520 ° C. Then, it was reheated and the slab surface temperature when passing through the lower straightening zone was controlled in the range of 950 to 1020 ° C. The slab surface temperature at the outlet of the continuous casting machine was 980 to 1020 ° C., and after cutting with a gas cutter, the slab corner was charged into a heating furnace without heating. The heating furnace temperature was set at 1150 ° C. The extraction time from the heating furnace was set to 30 minutes, 60 minutes, 90 minutes, 120 minutes, 150 minutes, and 180 minutes, and extraction was performed at this time, hot rolling was performed, and the occurrence rate of surface defects on the thin steel sheet investigated. Table 2 shows the test conditions and test results. The Ar1 point of these steel types is about 550 ° C., and the Ac point is 900 ° C. The heating fuel consumption rate index shown in Table 2 is based on the average (1.0) of the fuel consumption rate for which the heating time is 3 hours or more.
[0044]
[Table 2]

[0045]
Test No. with a heating time of 30 minutes. In No. 11, the temperature of the corner of the slab did not rise completely, and some surface flaws occurred in the thin steel sheet after hot rolling. . On the other hand, the unit of heating fuel increased as the heating time became longer. From the viewpoint of energy saving and prevention of surface flaws, the heating time is desirably 60 to 120 minutes.
[0046]
【The invention's effect】
In the present invention, in the direct rolling of Al-killed steel slabs having an Mn / S of 50 or less, while controlling the precipitation form of sulfide, and reheating in a continuous casting machine to a temperature at which the direct rolling can be performed, casting is performed. The surface flaws of the hot-rolled coil caused by the solidified structure of the piece can be completely prevented, and the energy-saving effect can be increased, resulting in an industrially beneficial effect.
[Brief description of the drawings]
FIG. 1 is a diagram showing a temperature history of a slab surface in an HDR process according to the present invention.
FIG. 2 is a diagram showing a temperature history of a slab surface in the HCR process according to the present invention.
FIG. 3 is a view showing an example of an embodiment of the present invention and is a schematic side view of a vertical bending type slab continuous casting machine.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Cast piece 2 Unsolidified phase 3 Solidified shell 4 Strong cooling device 5 Immersion nozzle 6 Mold 7 Support roll 8 Guide roll 9 Pinch roll 10 Upper straightening zone 11 Lower straightening zone

Claims

Direct feeding rolling method of Al-killed steel slab containing, by mass%, C: 0.1% or less, P: 0.02% or less, S: 0.03% or less, and solAl: 0.01 to 0.05% It is cast at a drawing speed of 2 m / min or more using a vertical bending type continuous casting machine, and after the slab passes through the upper straightening zone, a slab having a solidification rate of 10 to 50% is cast at 5 to 15 ° C. / The slab surface temperature is lowered to Ar1 point or lower by cooling at a cooling rate of sec, and then reheated to pass through the lower straightening zone at the slab surface temperature of Ac3 point or higher, and then the slab surface temperature is reduced to Ac3. A method for direct rolling of continuous cast slabs, wherein cutting is carried out while being held at a temperature of not less than a point, and the cut is conveyed to a hot rolling mill and hot-rolled.

Direct feeding rolling method of Al-killed steel slab containing, by mass%, C: 0.1% or less, P: 0.02% or less, S: 0.03% or less, and solAl: 0.01 to 0.05% It is cast at a drawing speed of 2 m / min or more using a vertical bending type continuous casting machine, and after the slab passes through the upper straightening zone, a slab having a solidification rate of 10 to 50% is cast at 5 to 15 ° C. / The slab surface temperature is lowered to Ar1 point or lower by cooling at a cooling rate of sec, and then reheated to pass through the lower straightening zone at the slab surface temperature of Ac3 point or higher, and then the slab surface temperature is reduced to Ac3. A method of direct rolling of continuous cast slabs, comprising cutting, transporting to a heating furnace, heating in a heating furnace and then hot rolling while maintaining the temperature at or above the point.

3. The method according to claim 1, wherein the ratio between the Mn content and the S content of the Al-killed steel slab is 50 or less.

The continuous casting according to any one of claims 1 to 3, wherein the slab surface temperature is cooled to the Ar1 point or less at least 20 seconds after the slab has passed through the upper straightening zone. Direct slab rolling method.