JP2004025272A

JP2004025272A - Continuously cast slab and method of producing steel sheet using the same

Info

Publication number: JP2004025272A
Application number: JP2002188129A
Authority: JP
Inventors: Kingo Sasame; 笹目　欽吾
Original assignee: Sumitomo Metal Industries Ltd
Current assignee: Nippon Steel Corp
Priority date: 2002-06-27
Filing date: 2002-06-27
Publication date: 2004-01-29

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of producing a steel sheet having reduced surface defects by which the problem of surface flaws generated in the vicinity of the end part in the width direction of a hot rolled steel sheet is solved without providing special equipment nor increasing special stages. <P>SOLUTION: As for the shape of the short edge in the cross-sectional face of a slab, when the minimum value (α) of the crossed angle between the long edge and a straight line obtained by connecting a slab edge part and a point on the short edge is ≤80°, the distance in a width direction from the slab edge part to the center of the short edge (the amount ofbulging in the short side: d) is controlled to -3 to 5 mm, and in the case of α>80°, (d) is controlled to -3 to 10 mm. Further, α is monitored on continuous casting, and the continuous casting conditions are controlled so that (d) lies within the above range in accordance with α. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、熱間圧延時に発生する表面疵が少ない鋼板の製造方法、すなわち、熱間圧延鋼板の幅方向端部近傍に発生する表面疵を、スラブの短辺形状を特定することにより減少させる方法に関する。
【０００２】
【従来の技術】
一般に矩形断面を有するスラブを熱間圧延して製造された鋼板では、幅方向端部近傍にシーム疵（圧延方向に発生する線状の連続欠陥）などの表面疵が発生しやすいことが知られている。この原因として、矩形断面スラブの隅部は他の部位に比較して、加熱後の温度低下が著しいことや、熱間圧延時の材料流動がスラブ中央部と端部で異なることなどが原因と考えられている。
【０００３】
前者の対策としては、加熱炉抽出温度を高く設定する方法や、スラブエッジ部を圧延前に加熱する方法などがあるが、これらの方法はエネルギコストの増大を招くため、製造コスト低減の観点から、必ずしも良好な方法とはいえない。
【０００４】
特開２００１−１８０４０　号公報には、エネルギコストの増大を抑制して熱間圧延時の庇を防止する方法として、連続鋳造設備の鋳型の出側でスラブ隅部に面取り形状の連続ロール加工を施し、スラブ隅部の加熱炉抽出後の大幅な温度低下を抑制し幅方向端部近傍の庇を防止する方法が開示されている。
【０００５】
後者の対策としては、スラブ形状を工夫して材料流動を制御し、これにより表面疵を減少させる方法が検討されている。例えば特開平６−２６９８０４号公報には、熱間圧延に供するスラブの形状を、最エッジ部のスラブ厚みを中央部のスラブ厚みより厚くして、熱間圧延時の表面疵を少なくさせる製造方法が提案されている。また、上記スラブを得る方法としては、所望の形状に鋳造部を加工した鋳型を使用して連続鋳造する方法や、所定の断面形状が得られるように加工したロールを用いて鋼塊を分塊圧延する方法、スラブに研削加工を施す方法などが開示されている。
【０００６】
【発明が解決しようとする課題】
しかしながら特開２００１−１８０４０　号公報に記載の方法は、連続鋳造設備の大幅な改造を必要とするため、容易に実施できる方法とは言えない。また、特開平６−２６９８０４号公報で開示された方法は、これを特定の断面形状をした鋳型を用いておこなう場合には、連続鋳造時に鋳片の幅変更を行う操業方法への適用は困難である。また、異形ロールによる圧延やスラブ研削により所定のスラブを得る方法は工程負荷増や歩留低下などの問題がある。
【０００７】
本発明の目的は、特段の設備対応や工程負荷増なく、熱間圧延鋼板の幅方向端部近傍に発生する表面庇を無害化できる、表面欠陥の少ない鋼板の製造方法およびそれに利用するスラブを提供することにある。
【０００８】
【課題を解決するための手段】
熱間圧延鋼板製造時には、熱間圧延後の鋼板の幅を所定の寸法に仕上げるために余幅を除去する作業（トリミング作業）がおこなわれる。また、軽度の表面疵は、グラインダ研削などの方法で手入れ除去することも一般的におこなわれている。
【０００９】
熱間圧延時に生じる表面疵の強度（疵の深さ）を通常の手入れ工程で除去できる範囲に軽減するか、その発生位置をトリミング作業で除去される範囲（トリミング代）に制限することができれば、これらの表面疵が発生しても、特別な工程や設備を設けることなく良好な製品を得ることができる。
【００１０】
本発明者は、矩形断面を有するスラブの熱間圧延に際し、幅方向端部近傍に生じる表面疵と、スラブの断面形状、特に短辺近傍における断面形状との関係を種々調査した。その結果、幅方向端部近傍に生じる表面庇の強度および発生位置は、矩形断面スラブの短辺形状により変動し、形断面スラブの短辺形状を特定範囲に制御することにより、幅方向端部近傍に生じる表面疵を容易に無害化できることを知った。
【００１１】
熱間圧延鋼板の素材となる連続鋳造スラブは、溶鋼を鋳型に注入し、鋳型内で凝固シェルを形成させた後鋳型下方から引き抜き、鋳片の上下面をガイドロールなどで拘束しつつ凝固を完了させて製造される。鋳片の断面形状は鋳型直下では完全な矩形である。その短辺形状は、短辺がロールなどにより拘束されている間は直線状に維持されているが、ロールなどによる短辺のサポートがなくなると、内部の溶鋼静圧によりスラブが膨張変形する。このため、通常、圧延素材としてのスラブの短辺は厳密には直線状ではなく、短辺中央部は凸状のいわゆるバルジング形状を示す。
【００１２】
図１は、矩形断面を有する連続鋳造スラブの横断面における断面形状を１／２　幅について示す概念図である。図１で符号１は長辺、符号２は短辺、符号３は長辺上の幅方向端部（以下、スラブエッジ部と記す）、符号４はスラブ幅が最小となる位置、一点鎖線はスラブ幅方向中央を意味する。
【００１３】
図１で、ｄはスラブエッジ部と短辺中央との間の幅方向距離を表す（以下、短辺バルジング量と記す）。ｄは、図１に示すように、短辺中央がスラブの外側に凸である場合を正とする。
【００１４】
図１に示すように、αは、長辺１と、スラブエッジ部３とスラブエッジ部３から短辺中央までの間の短辺上の任意の点とを結ぶ直線とがスラブエッジ部３で交わってなす角度のうちの最小値である（以下、スラブエッジ部角度と記す）。
【００１５】
短辺の両端部（長辺近傍部）には、長辺側のバルジングにより生じる曲げモ−メントの影響により、凹部が形成される場合が多い。αは、短辺上のスラブ各エッジ部より１／２　厚み位置までの間で、最も凹んだ位置４　（スラブ幅が最小となる位置）とスラブエッジ部とを結んだ線と長辺とのなす角度でもある。本発明による調査結果によれば、αは通常７５〜９０°の範囲で変動する。
【００１６】
スラブエッジ部角度αは、幅方向端部近傍に発生する表面庇の強度（本発明においては鋼板表面からの深さを意味する）に影響する。すなわち、αが小さくなるほど水平ロールで圧延して得られた熱間圧延鋼板の幅方向端部近傍には、深さが大きい表面疵が発生し、αが８０°以下である場合には、表面疵の深さは鋼板厚さ方向で０．３ｍｍ　以上になる。疵の深さが０．３ｍｍ　以下であれば、通常の板厚下限公差内の手入れで除去できるため、素材スラブのαが８０°以下である場合には、仮に表面疵が発生したとしても通常の手入れで無害化することができる。
【００１７】
図２は、鋼板端部近傍の疵発生位置を説明するための熱間圧延鋼板の平面図であり、図２に示すように、本発明では、鋼板端部から疵発生位置までの幅方向距離を「奥行き量」と表示する。
【００１８】
図３は、本発明者の調査結果による、Ｃ　：０．１５〜０．１６％クラスの中炭素鋼を、厚さ：２５０　ｍｍ、幅：２２６０ｍｍの鋳型を用いて、鋳込速度１．０　〜１．３　ｍ／分の条件で連続鋳造したスラブを熱間圧延して得た、厚さ：７　〜３０ｍｍ、幅：１０５０〜３３７０ｍｍの厚鋼板について、スラブの短辺バルジング量ｄと、厚鋼板の幅方向端部に発生した表面疵の発生位置（奥行き量で表示）との関係を示すグラフである。
【００１９】
図３からわかるように、スラブの短辺バルジング量が増すにつれて鋼板の表面疵発生位置は鋼板中心部方向にも生じるようになる。図３からわかるように、ｄが５　ｍｍを超えると幅方向端部から２５ｍｍ以上の位置にも表面疵が発生する。
【００２０】
熱間圧延鋼板のトリミング作業におけるトリミング代（除去量）は、一般的な圧延条件である幅出し比（＝圧延後幅／スラブ幅）が１．５　以下の範囲での圧延を施した場合、鋼板端部から片側あたりでおよそ２５ｍｍ以内である。従って奥行き量が２５ｍｍ以下であれば、仮に表面疵が発生したとしてもトリミング作業で除去することができる。
【００２１】
また、本発明者の研究結果によれば、短辺バルジング量ｄが−３ｍｍを下回る場合にはスラブコ−ナ部に割れが認められ、これが原因となって圧延後の鋼板にはエッジ部にへげ状の表面欠陥が生じる場合がある。さらに、ｄが１０ｍｍを超える場合には、スラブ短辺近傍の表皮下で内部割れが生じ、圧延後の鋼板にはへげ疵が生じる懸念がある。これらの欠陥が生じるといずれの場合とも大幅な歩留まり低下が生じる。
【００２２】
本発明はこれらの知見を基にして完成されたものであり、その要旨は下記（１）　、（２）　に記載の連続鋳造スラブおよび（３）　、（４）　に記載の表面欠陥の少ない鋼板の製造方法にある。
【００２３】
（１）　連続鋳造によって製造されたスラブであって、その横断面における短辺形状が、長辺と、スラブエッジ部から短辺中央までの間の短辺上の任意の点とを結ぶ直線とがスラブエッジ部で交わってなす角度のうちの最小値であるスラブエッジ部角度をαとし、スラブ中央部からのエッジ部での幅方向距離とスラブ中央部からの最大幅方向距離との差である短辺バルジング量をｄとするとき、αが８０°以下でかつｄが−３ｍｍ以上、５ｍｍ以下であることを特徴とする連続鋳造スラブ。
【００２４】
（２）　連続鋳造によって製造されたスラブであって、その横断面における短辺形状が、長辺と、スラブエッジ部から短辺中央までの間の短辺上の任意の点とを結ぶ直線とがスラブエッジ部で交わってなす角度のうちの最小値であるスラブエッジ部角度をαとし、スラブ中央部からのエッジ部での幅方向距離とスラブ中央部からの最大幅方向距離との差である短辺バルジング量をｄとするとき、αが８０°を超え、かつ、ｄが−３ｍｍ以上、１０ｍｍ以下であることを特徴とする連続鋳造スラブ。
【００２５】
（３）　矩形断面を有する連続鋳造スラブを熱間圧延して鋼板を製造するに際し、スラブ横断面における短辺形状を、長辺と、スラブエッジ部から短辺中央までの間の短辺上の任意の点とを結ぶ直線とがスラブエッジ部で交わってなす角度のうちの最小値であるスラブエッジ部角度をαとし、スラブ中央部からのエッジ部での幅方向距離とスラブ中央部からの最大幅方向距離との差である短辺バルジング量をｄとするとき、αが８０°以下である場合はｄが−３ｍｍ以上、５ｍｍ以下の範囲にあり、αが８０°を超える場合にはｄが−３ｍｍ以上、１０ｍｍ以下の範囲にあるように定めることを特徴とする鋼板の製造方法。
【００２６】
（４）　矩形断面を有する連続鋳造スラブを熱間圧延して鋼板を製造する方法であって、スラブ横断面における、長辺と、該長辺上の幅方向端部であるスラブエッジ部とスラブエッジ部から短辺中央までの間の短辺上の任意の点とを結ぶ直線がスラブエッジ部で交わってなす角度のうちの最小値であるスラブエッジ部角度をαとし、スラブ中央部からのエッジ部での幅方向距離とスラブ中央部からの最大幅方向距離との差である短辺バルジング量をｄとするとき、連続鋳造に際してαを監視し、αが８０°以下である場合はｄが−３ｍｍ以上、５ｍｍ以下の範囲となり、αが８０°を超える場合にはｄが−３ｍｍ以上、１０ｍｍ以下の範囲になるように連続鋳造条件を調整することを特徴とする鋼板の製造方法。
【００２７】
【発明の実施の形態】
本発明の実施の形態を詳細に述べる。
矩形断面を有するスラブの幅方向断面において、ｄが−３ｍｍを超えて小さくなると、これを熱間圧延した際にスラブ端部に割れが発生し、圧延後の鋼板でへげ疵などの欠陥が生じて歩留まりが低下するおそれがある。これを避けるために、ｄは−３ｍｍ以上とする必要がある。望ましくは０　ｍｍ以上である。また、ｄが１０ｍｍを超えるとスラブ短辺近傍の表皮下で内部割れを生じ、圧延後の鋼板の歩留まりが低下するおそれがある。従ってｄは１０ｍｍ以下とする。ｄが５ｍｍ以下の範囲であれば、鋼板端部にシーム疵のような表面疵が生じたとしても、その発生位置が通常のトリミング代で除去可能な範囲に収まるので容易に除去できる。このため、望ましくはｄは５ｍｍ以下である。
【００２８】
スラブエッジ部角度αが８０°以下となる場合は、強度の表面疵が発生するおそれがある。このような場合は、表面疵発生部位を通常のトリミング作業で除去できる範囲とするために、短辺バルジング量ｄは−３ｍｍ以上、かつ、５ｍｍ以下の範囲にあるようにする。αが８０°を超える場合には、幅方向端部近傍に表面疵が生じるとしても比較的軽度のものであるので、通常の手入れ作業で容易に除去できる。スラブ短辺近傍の表皮下での内部割れや、その結果として生じる鋼板のへげ状欠陥を防止するために、ｄは−３ｍｍ以上、１０ｍｍ以下の範囲にあるようにする必要がある。
【００２９】
スラブエッジ部角度αおよび短辺バルジング量ｄは、スラブ端面の形状をレーザ距離計などにより計測することで容易に確認できる。
連続鋳造において、スラブエッジ部角度αに対しては、鋳型直下の冷却水量や鋳込み速度の実績と、鋳型の厚さ、幅との関係を調査することにより整理することが可能である。従ってこれらの関係を用いれば、連続鋳造条件からαを予測することができる。また、鋳型から引き抜いた鋳片について、テレビカメラなどによる熱間観察や、冷却したサンプル材の観察により、αを測定することも可能である。
【００３０】
短辺バルジング量ｄに対しては、鋳型直下における短辺サポート条件（ロールなどで短辺を拘束する範囲、幅方向の絞り角度を意味するサポ−ト角度、幅方向間隔などの取り合いなど）や、短辺スプレーの水量が影響する。ｄは、例えば、ロールなどで短辺を拘束する範囲を長くすると小さくなり、サポ−ト角度を大きくすれば大きくなる。また、幅方向間隔を大きくすればｄは大きくなり、短辺スプレーの水量を増せばｄは小さくなる。
【００３１】
本発明の製造方法は、連続鋳造時に予測するか、測定するなどの手段を備えることでスラブエッジ部角度αを監視し、そのαに応じてｄが本発明が規定する範囲になるように、冷却条件などの連続鋳造条件を調整しておこなうのが好適である。
【００３２】
本発明の製造方法は、特段の設備や特有の工程を必要としないで、熱間圧延鋼板の幅方向端部近傍に発生しがちな表面疵を、容易に実質上無害化できるので、表面欠陥の少ない鋼板の製造方法として極めて優れた方法である。中でも、寸法公差などの関係から表面疵除去作業が容易に行える厚鋼板の表面疵の減少に特に有効である。
【００３３】
【実施例】
表１に示す化学組成を有する溶鋼を転炉で溶製し、これを連続鋳造して、幅：２２６０ｍｍ、厚さ：２３５ｍｍ　のスラブを作成した。
【００３４】
【表１】

【００３５】
連続鋳造に際しては、切断後のスラブを工業用テレビカメラを用いて熱間観察することによりαを実測し、鋳造速度と鋳型直下の短辺冷却水量を種々変更することによりスラブの短辺形状を種々変更した。得られたスラブは、そのまま加熱炉に装入して１１３０℃に加熱し、圧延終了温度を　７５０℃とする熱間圧延を施し、厚さ：２０〜２５ｍｍ、幅：２５１９〜３１１１ｍｍの熱間圧延鋼板を得た。幅出し比は１．１１〜１．３８の範囲であった。こられの鋼板は常温に冷却した後、幅方向端部近傍に発生した表面疵（シーム疵）の発生位置と強度を調査した。
【００３６】
表２に、上記プロセスの諸条件、スラブ形状および疵発生状況をまとめて示す。
【００３７】
【表２】

【００３８】
表２に示すように、本発明が規定する条件範囲で製造された試験番号１の鋼板では深い表面疵が生じたが、その発生位置は通常のトリミング作業で除去可能な範囲であった。試験番号２の鋼板では幅方向内部に疵が生じたが、その強度は通常のグラインダ手入れで除去できるものであった。これらの鋼板からはいずれも良好な製品が得られる。これに対し試験番号３の鋼板では、通常のグラインダ手入れで除去できない深さの疵が通常のトリミング作業で除去できない幅方向内部に発生した。試験番号４の鋼板では幅方向端部近傍に大きい割れ部があり、試験番号５の鋼板では幅方向端部近傍に内部割れがあった。こられの鋼板からは、良好な製品が得られなかった。
【００３９】
【発明の効果】
本発明の製造方法によれば、特段の設備対応や工程負荷増加を必要としないで、熱間圧延鋼板の幅方向端部近傍に発生する表面庇を効率よく無害化し、製品の歩留を著しく向上させることができる。
【図面の簡単な説明】
【図１】図１（ａ）　は、矩形断面を有する連続鋳造スラブの断面形状を１／２　幅について示す概念図、図１（ｂ）　はその部分の拡大図である。
【図２】鋼板端部近傍の疵発生位置を説明するための熱間圧延鋼板の平面図である。
【図３】スラブ短辺バルジング量ｄと庇発生位置との関係を示すグラフである。
【符号の説明】
１　　スラブ横断面の長辺、
２　　スラブ横断面の短辺、
３　　スラブエッジ部、
４　　端辺上のスラブ幅が最小となる位置。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention is a method for producing a steel sheet having a small number of surface flaws generated during hot rolling, that is, a surface flaw generated in the vicinity of a width direction end of a hot-rolled steel sheet is reduced by specifying a short side shape of a slab. About the method.
[0002]
[Prior art]
In general, it is known that a steel sheet manufactured by hot rolling a slab having a rectangular cross section tends to have surface flaws such as seam flaws (linear continuous flaws generated in the rolling direction) near a width direction end. ing. This is due to the fact that the corners of the rectangular section slab have a remarkable drop in temperature after heating compared to other parts, and that the material flow during hot rolling differs between the center and the end of the slab. It is considered.
[0003]
As the former countermeasures, there are a method of setting a high heating furnace extraction temperature and a method of heating a slab edge portion before rolling, but these methods cause an increase in energy costs, and from the viewpoint of reducing manufacturing costs. This is not always a good method.
[0004]
Japanese Patent Application Laid-Open No. 2001-18040 discloses, as a method of suppressing an increase in energy cost and preventing eaves during hot rolling, performing continuous roll processing of a chamfered shape at a corner of a slab on the exit side of a mold of a continuous casting facility. A method is disclosed in which a large temperature drop after heating furnace extraction at a corner of a slab is suppressed to prevent eaves near a widthwise end.
[0005]
As the latter countermeasure, a method of controlling the material flow by devising the slab shape and thereby reducing surface flaws has been studied. For example, JP-A-6-269804 discloses a manufacturing method in which the shape of a slab to be subjected to hot rolling is made thicker at a slab thickness at an outermost edge portion than at a central portion to reduce surface defects during hot rolling. Has been proposed. Further, as a method of obtaining the slab, a method of continuously casting using a mold having a cast portion processed into a desired shape, or a method of dividing a steel ingot using a roll processed so as to obtain a predetermined cross-sectional shape. A method of rolling, a method of grinding a slab, and the like are disclosed.
[0006]
[Problems to be solved by the invention]
However, the method described in Japanese Patent Application Laid-Open No. 2001-18040 cannot be said to be a method that can be easily implemented because it requires a significant remodeling of a continuous casting facility. Further, the method disclosed in JP-A-6-269804 is difficult to apply to an operation method in which the width of a slab is changed during continuous casting when this method is performed using a mold having a specific cross-sectional shape. It is. Further, the method of obtaining a predetermined slab by rolling with a deformed roll or slab grinding has problems such as an increase in process load and a decrease in yield.
[0007]
An object of the present invention is to provide a method for manufacturing a steel sheet having a small number of surface defects and a slab used for the same, which can detoxify a surface eaves generated in the vicinity of a width direction end of a hot-rolled steel sheet without increasing equipment load or process load. To provide.
[0008]
[Means for Solving the Problems]
When manufacturing a hot-rolled steel sheet, an operation (trimming operation) of removing an excess width is performed to finish the width of the hot-rolled steel sheet to a predetermined size. Further, minor surface flaws are also generally removed by a method such as grinder grinding.
[0009]
If the strength of surface flaws generated during hot rolling (depth of flaws) can be reduced to a range that can be removed by a normal care process, or if the occurrence position can be limited to a range that can be removed by trimming work (trimming allowance) Even if these surface flaws occur, a good product can be obtained without providing any special process or equipment.
[0010]
The present inventor has conducted various investigations on the relationship between the surface flaw generated near the width direction end and the cross-sectional shape of the slab, particularly near the short side, during hot rolling of a slab having a rectangular cross section. As a result, the strength and location of the surface eaves generated near the width direction end vary depending on the short side shape of the rectangular cross section slab, and by controlling the short side shape of the shaped cross section slab to a specific range, the width direction end portion is controlled. It has been found that surface flaws generated in the vicinity can be easily rendered harmless.
[0011]
The continuous cast slab, which is the material for hot-rolled steel sheets, is prepared by injecting molten steel into a mold, forming a solidified shell in the mold, pulling it out from below the mold, and consolidating while restraining the upper and lower surfaces of the slab with guide rolls. Manufactured to complete. The cross-sectional shape of the slab is a perfect rectangle immediately below the mold. The short side shape is maintained linearly while the short side is constrained by a roll or the like, but when the short side is no longer supported by the roll or the like, the slab expands and deforms due to the internal molten steel static pressure. For this reason, usually, the short side of the slab as the rolling material is not strictly linear, and the central part of the short side shows a convex so-called bulging shape.
[0012]
FIG. 1 is a conceptual diagram showing a cross-sectional shape of a continuous cast slab having a rectangular cross section in a 断面 width. In FIG. 1, reference numeral 1 denotes a long side, reference numeral 2 denotes a short side, reference numeral 3 denotes a width direction end on the long side (hereinafter, referred to as a slab edge), reference numeral 4 denotes a position where the slab width is minimum, and It means the center in the slab width direction.
[0013]
In FIG. 1, d represents the width direction distance between the slab edge portion and the center of the short side (hereinafter, referred to as short side bulging amount). As shown in FIG. 1, d is positive when the center of the short side is convex outside the slab.
[0014]
As shown in FIG. 1, α is a slab edge 3 that is a line connecting the long side 1 and an arbitrary point on the short side between the slab edge 3 and the center of the short side. This is the minimum value of the angles formed by the intersection (hereinafter, referred to as the slab edge angle).
[0015]
A concave portion is often formed at both ends of the short side (near the long side) due to the bending moment caused by bulging on the long side. α is the distance between the longest side and the line connecting the most depressed position 4 (the position where the slab width is the smallest) and the slab edge between the slab edges on the short side and the １／ thickness position. It is also an angle to make. According to the findings of the present invention, α usually varies in the range of 75-90 °.
[0016]
The slab edge angle α affects the strength of the surface eaves generated near the width direction end (in the present invention, it means the depth from the steel sheet surface). That is, as the α becomes smaller, near the width direction end of the hot-rolled steel sheet obtained by rolling with a horizontal roll, a surface flaw having a large depth occurs, and when α is 80 ° or less, The flaw depth becomes 0.3 mm or more in the thickness direction of the steel sheet. If the depth of the flaw is 0.3 mm or less, it can be removed by care within the normal lower limit of the thickness of the sheet, and when α of the material slab is 80 ° or less, even if surface flaws are generated, Can be detoxified by care.
[0017]
FIG. 2 is a plan view of a hot-rolled steel sheet for explaining a position where a flaw occurs near an end of the steel sheet. As shown in FIG. Is displayed as “depth amount”.
[0018]
FIG. 3 shows that the medium carbon steel of C: 0.15 to 0.16% class was cast at a casting speed of 1.0 mm using a mold having a thickness of 250 mm and a width of 2260 mm according to the results of the present inventors' investigation. For a thick steel plate having a thickness of 7 to 30 mm and a width of 1050 to 3370 mm obtained by hot rolling a slab continuously cast under a condition of ~ 1.3 m / min, the short side bulging amount d and the thickness It is a graph which shows the relationship with the generation | occurrence | production position (indicated by depth amount) of the surface flaw which generate | occur | produced in the width direction edge part of the steel plate.
[0019]
As can be seen from FIG. 3, as the short side bulging amount of the slab increases, the position at which the surface flaw of the steel sheet occurs also occurs toward the center of the steel sheet. As can be seen from FIG. 3, when d exceeds 5 mm, surface flaws also occur at a position 25 mm or more from the end in the width direction.
[0020]
The trimming allowance (removal amount) in the trimming operation of a hot-rolled steel sheet is as follows: When rolling is performed in a range where the tentering ratio (= width after rolling / slab width) which is a general rolling condition is 1.5 or less, It is within about 25 mm on one side from the end of the steel plate. Therefore, if the depth amount is 25 mm or less, even if a surface flaw is generated, it can be removed by a trimming operation.
[0021]
Further, according to the research results of the present inventor, when the short side bulging amount d is less than -3 mm, cracks are recognized in the slab corner portion, and due to this, the rolled steel sheet has no edge portion. Spiny surface defects may occur. Further, when d exceeds 10 mm, internal cracks occur under the skin near the short side of the slab, and there is a concern that the steel sheet after rolling may have scoring. When these defects occur, the yield is greatly reduced in any case.
[0022]
The present invention has been completed on the basis of these findings, and the gist of the present invention is to provide a continuous cast slab described in the following (1) and (2) and a steel sheet having few surface defects described in the following (3) and (4). Manufacturing method.
[0023]
(1) A slab manufactured by continuous casting, wherein the short side shape in the cross section is a straight line connecting the long side and an arbitrary point on the short side between the slab edge portion and the center of the short side. Is the slab edge angle which is the minimum value of the angles formed at the slab edge, and α is the difference between the width direction distance at the edge from the slab center and the maximum width direction distance from the slab center. A continuous cast slab characterized by α being 80 ° or less and d being -3 mm or more and 5 mm or less, where d is a certain short side bulging amount.
[0024]
(2) A slab manufactured by continuous casting, wherein the short side shape in the cross section is a straight line connecting the long side and any point on the short side between the slab edge and the center of the short side. Is the slab edge angle which is the minimum value of the angles formed at the slab edge, and α is the difference between the width direction distance at the edge from the slab center and the maximum width direction distance from the slab center. A continuous cast slab wherein α is more than 80 ° and d is -3 mm or more and 10 mm or less when a certain short side bulging amount is d.
[0025]
(3) In producing a steel sheet by hot rolling a continuous cast slab having a rectangular cross section, the shape of the short side in the slab cross section is determined on the long side and the short side between the slab edge and the center of the short side. The slab edge angle which is the minimum value of the angle formed by the straight line connecting the arbitrary points and the slab edge at the slab edge is α, and the width direction distance at the edge from the slab center and the distance from the slab center are When the short side bulging amount that is the difference from the maximum width direction distance is d, when α is 80 ° or less, d is in the range of −3 mm or more and 5 mm or less, and when α exceeds 80 °, A method for producing a steel sheet, wherein d is set to be in a range of not less than -3 mm and not more than 10 mm.
[0026]
(4) A method for hot-rolling a continuous cast slab having a rectangular cross section to produce a steel sheet, comprising: a long side in a slab cross section; and a slab edge, which is a widthwise end on the long side. Α is the slab edge angle which is the minimum value of the angle formed by the straight line connecting the arbitrary point on the short side between the edge and the center of the short side at the slab edge, and α from the slab center. When the short side bulging amount, which is the difference between the width direction distance at the edge portion and the maximum width direction distance from the center of the slab, is d, α is monitored during continuous casting. If α is 80 ° or less, d is monitored. Is in the range of -3 mm or more and 5 mm or less, and when α exceeds 80 °, the continuous casting conditions are adjusted so that d is in the range of -3 mm or more and 10 mm or less.
[0027]
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiments of the present invention will be described in detail.
In the width direction cross section of a slab having a rectangular cross section, when d becomes smaller than -3 mm, cracks occur at the end of the slab when hot-rolled, and defects such as scuffs on the rolled steel sheet. This may cause the yield to decrease. In order to avoid this, d needs to be -3 mm or more. Desirably, it is 0 mm or more. On the other hand, if d exceeds 10 mm, internal cracks may occur under the skin near the short side of the slab, and the yield of the steel sheet after rolling may be reduced. Therefore, d is set to 10 mm or less. If d is within a range of 5 mm or less, even if surface flaws such as seam flaws are generated at the end of the steel sheet, the generation positions are within a range that can be removed by a normal trimming allowance, and thus can be easily removed. Therefore, desirably, d is 5 mm or less.
[0028]
When the slab edge angle α is 80 ° or less, a strong surface flaw may be generated. In such a case, the short side bulging amount d is set to be in a range of not less than -3 mm and not more than 5 mm in order to make the area where the surface flaw occurs can be removed by a normal trimming operation. When α exceeds 80 °, even if surface flaws are generated near the width direction end, they are relatively light, and can be easily removed by ordinary care. In order to prevent internal cracks under the epidermis near the short side of the slab and consequent baldness of the steel sheet, d must be in the range of -3 mm or more and 10 mm or less.
[0029]
The slab edge angle α and the short-side bulging amount d can be easily confirmed by measuring the shape of the slab end face with a laser distance meter or the like.
In the continuous casting, the slab edge angle α can be arranged by examining the relationship between the actual amount of cooling water immediately under the mold and the casting speed and the thickness and width of the mold. Therefore, if these relationships are used, α can be predicted from the continuous casting conditions. Further, it is also possible to measure α by hot observation with a television camera or the like or observation of a cooled sample material with respect to the cast slab extracted from the mold.
[0030]
For the short side bulging amount d, the short side support condition immediately below the mold (a range in which the short side is constrained by a roll, a support angle indicating a narrowing angle in the width direction, a width direction interval, and the like) and , The amount of water in the short side spray affects. For example, d becomes smaller when the range in which the short side is constrained by a roll or the like is increased, and becomes larger when the support angle is increased. Also, d increases as the widthwise interval increases, and d decreases as the amount of short-side spray water increases.
[0031]
The production method of the present invention predicts at the time of continuous casting, or monitors the slab edge portion angle α by providing means such as measuring, so that d falls within the range defined by the present invention according to the α, It is preferable to adjust the continuous casting conditions such as cooling conditions.
[0032]
The production method of the present invention does not require any special equipment or a special process, and can easily and substantially detoxify surface flaws that are likely to occur near the widthwise end of a hot-rolled steel sheet. This is an extremely excellent method as a method for producing a steel sheet having a small amount of steel. Above all, it is particularly effective in reducing the surface flaws of a thick steel plate in which the work of removing surface flaws can be easily performed due to dimensional tolerances and the like.
[0033]
【Example】
Molten steel having the chemical composition shown in Table 1 was smelted in a converter and continuously cast to form a slab having a width of 2260 mm and a thickness of 235 mm.
[0034]
[Table 1]

[0035]
In continuous casting, the slab after cutting is hot-observed using an industrial television camera to measure α, and the short-side shape of the slab is changed by variously changing the casting speed and the amount of short-side cooling water just below the mold. Various changes. The obtained slab is directly charged into a heating furnace and heated to 1130 ° C., subjected to hot rolling at a rolling end temperature of 750 ° C., and hot rolled to a thickness of 20 to 25 mm and a width of 2519 to 3111 mm. A steel plate was obtained. The tentering ratio was in the range of 1.11 to 1.38. After cooling these steel sheets to room temperature, the occurrence position and strength of surface flaws (seam flaws) generated near the width direction end were investigated.
[0036]
Table 2 summarizes various conditions, slab shapes and flaw occurrences of the above process.
[0037]
[Table 2]

[0038]
As shown in Table 2, a deep surface flaw occurred in the steel sheet of Test No. 1 manufactured under the condition range specified by the present invention, but the occurrence position was within a range that can be removed by a normal trimming operation. In the steel sheet of Test No. 2, flaws occurred in the width direction inside, but the strength could be removed by ordinary grinder care. Good products can be obtained from these steel plates. On the other hand, in the steel sheet of Test No. 3, flaws having a depth that could not be removed by ordinary grinder care occurred in the width direction inside that could not be removed by ordinary trimming work. The steel sheet of Test No. 4 had a large crack near the width direction end, and the steel sheet of Test No. 5 had an internal crack near the width direction end. Good products could not be obtained from these steel sheets.
[0039]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to the manufacturing method of this invention, the surface eaves which generate | occur | produce in the vicinity of the width direction edge part of a hot-rolled steel sheet are made harmless efficiently, without requiring special equipment correspondence and an increase in process load, and the product yield is remarkably reduced. Can be improved.
[Brief description of the drawings]
FIG. 1A is a conceptual diagram showing a cross-sectional shape of a continuous cast slab having a rectangular cross section with respect to a half width, and FIG. 1B is an enlarged view of the portion.
FIG. 2 is a plan view of a hot-rolled steel sheet for explaining a position where a flaw occurs near an end of the steel sheet.
FIG. 3 is a graph showing a relationship between a slab short side bulging amount d and an eaves occurrence position.
[Explanation of symbols]
1 Long side of slab cross section,
2 Short side of slab cross section,
3 Slab edge,
4 Position where the slab width on the edge is minimum.

Claims

A slab manufactured by continuous casting, wherein the short side shape in the cross section is a straight line connecting the long side and any point on the short side between the slab edge and the center of the short side. Α is the slab edge angle which is the minimum value of the angles formed by the intersections, and the short side is the difference between the width direction distance at the edge from the slab center and the maximum width direction distance from the slab center. A continuous cast slab characterized by α being 80 ° or less and d being -3 mm or more and 5 mm or less, where bulging amount is d.

A slab manufactured by continuous casting, wherein the short side shape in the cross section is a straight line connecting the long side and any point on the short side between the slab edge and the center of the short side. Α is the slab edge angle which is the minimum value of the angles formed by the intersections, and the short side is the difference between the width direction distance at the edge from the slab center and the maximum width direction distance from the slab center. A continuous cast slab wherein α is more than 80 ° and d is -3 mm or more and 10 mm or less when a bulging amount is d.

When hot rolling a continuous cast slab having a rectangular cross section to produce a steel sheet, the short side shape in the slab cross section is changed to the long side and any point on the short side between the slab edge and the center of the short side. The angle of the slab edge, which is the minimum value of the angles formed by the straight lines connecting the slab edges at the slab edge, is α, the width direction distance at the edge from the slab center and the maximum width direction from the slab center. When the short side bulging amount, which is the difference from the distance, is d, when α is 80 ° or less, d is in the range of -3 mm or more and 5 mm or less, and when α exceeds 80 °, d is − A method for manufacturing a steel sheet, wherein the thickness is set to be in a range of 3 mm or more and 10 mm or less.

A method for producing a steel sheet by hot rolling a continuous cast slab having a rectangular cross section, the long side, in the slab cross section, from the slab edge portion and the slab edge portion that is the width direction end on the long side Α is the slab edge angle which is the minimum value of the angle formed by the straight line connecting any point on the short side up to the center of the short side at the slab edge, and the edge from the slab center When the short side bulging amount, which is the difference between the width direction distance of the slab and the maximum width direction distance from the center of the slab, is d, α is monitored during continuous casting. If α is 80 ° or less, d is -3 mm. The method for producing a steel sheet according to claim 1, wherein the continuous casting conditions are adjusted so that d is in the range of -3 mm to 10 mm when α exceeds 80 °.