JP3834094B2

JP3834094B2 - Manufacturing method of hot-rolled steel sheet for processing with excellent formability and good toughness using hot-rolling continuous process

Info

Publication number: JP3834094B2
Application number: JP06030096A
Authority: JP
Inventors: 淳一脇田; 史郎米園
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1996-02-23
Filing date: 1996-02-23
Publication date: 2006-10-18
Anticipated expiration: 2016-02-23
Also published as: JPH09227948A

Description

【０００１】
【発明の属する技術分野】
本発明は、熱延連続化法による熱延鋼板の製造方法に係り、特に自動車や産業機械及びパイプ素材等に用いられる成形性に優れ靱性の良好な加工用熱延鋼板を連続的に熱間圧延して製造する方法に関するものである。
【０００２】
【従来の技術】
従来、自動車等の加工用鋼板の技術分野では、加工性の良い冷延鋼板が使用されていたが、素材のコストダウンのため最近は冷延鋼板に代わる素材として比較的安価な加工用熱延鋼板が使用されるようになってきている。
【０００３】
自動車、産業機械やパイプ素材等に用いられる加工用熱延鋼板の製造方法は、連続鋳造した鋼スラブを加熱炉で約１２００℃に加熱し、次いで熱間圧延機で粗圧延し、仕上圧延をした後に冷却水により冷却してコイルに捲取るのが一般的である。
【０００４】
この様な従来の熱延鋼板の製造方法では、鋼スラブ毎に熱間圧延して、仕上鋼板をランナウトテーブルに設けた冷却装置で冷却水によりラミナー冷却して捲取温度となった熱延鋼板をコイルに捲取っている。ところが、熱間仕上鋼板を冷却する際に、鋼板の先端から冷却しようとしても、鋼板の先端が冷却水の水柱と衝突し、水柱の影響力によって、鋼板が変形失速して通板上のトラブルが発生するので冷却処理をすることができなかった。この現象は、板厚が薄くなればなるほど顕著である。
【０００５】
そのため、従来は、熱延鋼板を捲取温度まで冷却する際は、通板上のトラブルを避けるために熱延鋼板の先端部を冷却することなしに、熱延鋼板の先端をピンチロールに噛み込ませて、その後に冷却を行っていた。このような従来の熱延鋼板の冷却方法では、鋼板の先端部は冷却処理されていないから、その部分は材質不良となり、製品として出荷する際に鋼板の冷却処理されていない先端部を切り捨てることが行われていて、製品歩留りが悪いという問題があった。
【０００６】
次いで、析出処理について説明すると、熱延鋼板に加工性を持たせるためには加工性に有害なＳやＮを除くために、ＭｎＳやＡｌＮとして析出処理する必要があり、曲げ歪を鋼板に付与してＭｎＳやＡｌＮを析出させるようにした方法が知られている。例えば、特公平７−７４３７６号公報には、粗圧延後の被圧延材に１１００℃以下Ａｒ₃点以上の温度域で曲げ加工を施し、かつ上記温度域に１０秒以上保持する方法が開示されている。
【０００７】
本発明者は、曲げ歪による析出処理について更に研究した結果、先行技術に開示されているように単に曲げ歪を付与するだけでなく、特定の曲げ歪と、曲げ歪速度との両方の条件を鋼板に付与すると、ＭｎＳやＡｌＮの析出が促進され、鋼板の伸びが向上することを知見した。
【０００８】
また、加工用熱延鋼板の材質については、加工性と靱性とを兼ね備える必要がある。そこで、本発明者は、加工性を向上させると共に靱性をも向上させた加工用熱延鋼板の材質改善方法について研究し、仕上圧延条件を熱間仕上温度をＡｒ₃＋５０℃以下の範囲で、且つ熱間仕上圧延機入側の温度と熱間仕上圧延機出側の温度との差を１００℃以下となる低等温圧延条件で、そして鋼板の全長に亘って連続的に熱間仕上圧延を行うことによって、γ粒の微細化を生じさせると加工性と共に靱性をも向上させることができることを知見して本発明を完成した。
【０００９】
そして、熱延鋼板の仕上温度分布について検討すると、加工性を劣化させないで、熱間圧延をするためには、圧延される鋼板の温度を少なくともＡｒ₃変態点以上の温度とする必要がある。図１は従来の熱延鋼板の仕上げ温度分布を示す図である。図１に示すように、熱間圧延される鋼板の先端部の仕上温度が一番低く、後端部になるに従い仕上温度が高くなる。後端部の仕上温度が高くなる理由は、加工発熱によるものと考えられる。
【００１０】
なお、鋼板の仕上温度は鋼板の全長に亘って均一でなくばらついているため、従来の熱延方法では上記したようなγ粒の微細化に必要な狭い熱間仕上温度範囲に制御できず問題がある。
【００１１】
従来の加熱炉での加熱は、熱間加工される鋼板の最低仕上温度、即ち、鋼板の先端部の仕上げ温度がＡｒ₃変態点以上の温度となるように加熱温度を選定しＡｒ₃変態点よりもかなり高温の約１２００℃の温度に加熱することが行われていた。
【００１２】
この加熱を省エネルギー上のコストバランスから見ると、鋼板の先端部以外では、過剰加熱が行われていることとなっていて、コストバランスが悪いという問題がある。
【００１３】
鋼片を１１５０℃以下の低温で加熱し、熱間圧延をして加工性、耐２次加工脆性に優れた熱延鋼板とする方法が特開平１−１４９９２２号公報に提案されている。この方法は、低温加熱によって結晶粒の粗大化を防止して、圧延後の再結晶による結晶の細粒化を狙っているが、この方法も前述した熱延鋼板の先端部の温度低下の問題を解決するに至っておらず、鋼板全体に亘って均質な鋼板を得ることは技術的に困難であり、先端部の材質不良による歩留りを向上させることはできない。
【００１４】
【発明が解決しようとする課題】
そこで、本発明は、ＭｎＳやＡｌＮの析出物を微細化すると共に結晶粒を細粒化することにより、熱延鋼板の材質を全長に亘って向上させ、かつ均一にし、また先端部の材質不良による歩留りを向上させること、及び加熱炉における省エネルギーによるコストメリットを向上させ、且つ、生産性を向上させた成形性に優れ靭性の良好な加工用熱延鋼板を連続的に製造する方法を提供することを課題とするものである。
【００１５】
【課題を解決するための手段】
（１）炭素含有量０．５質量％以下の鋼スラブを加熱炉で加熱し、熱間粗圧延機で粗圧延して鋼板となし、次いで熱間仕上圧延機で仕上圧延した後に冷却してコイルに捲取ることを特徴とする成形性に優れた加工用熱延鋼板の製造方法において、粗圧延鋼板をＡｒ_３変態点以上の温度で曲げ歪０．５％以上、曲げ歪速度０．０５Ｓ^−１以上で捲取り、次いで、３秒以上１℃／Ｓ以下の冷却速度で保持し、その後捲戻し、そして、該鋼板の先端を、その前に粗圧延され圧延ラインを先行する鋼板の後端に接合して、熱間仕上温度をＡｒ _３〜Ａｒ_３＋５０℃以下の範囲で、かつ、熱間仕上圧延機入側の温度と熱間仕上圧延機出側の温度との差を１００℃以下となる条件で連続的に熱間仕上圧延を行うことを特徴とする成形性に優れ靭性の良好な加工用熱延鋼板の製造方法。
【００１６】
（２）炭素含有量０．５質量％以下の鋼スラブを加熱炉で加熱し、熱間粗圧延機で粗圧延して鋼板となし、次いで熱間仕上圧延機で仕上圧延した後に冷却してコイルに捲取ることを特徴とする成形性に優れた加工用熱延鋼板の製造方法において、前記加熱炉での加熱を１１５０℃以下の低温加熱とし、そして、粗圧延鋼板をＡｒ_３変態点以上の温度で曲げ歪０．５％以上、曲げ歪速度０．０５Ｓ^−１以上で捲取り、次いで、３秒以上１℃／Ｓ以下の冷却速度で保持し、その後捲戻し、そして、該鋼板の先端を、その前に粗圧延され圧延ラインを先行する鋼板の後端に接合して、熱間仕上温度をＡｒ _３〜Ａｒ_３＋５０℃以下の範囲で、かつ、熱間仕上圧延機入側の温度と熱間仕上圧延機出側の温度との差を１００℃以下となる条件で連続的に熱間仕上圧延を行うことを特徴とする成形性に優れ靭性の良好な加工用熱延鋼板の製造方法。
【００１７】
以下本発明を詳細に説明する。なお、鋼成分の％は質量％を意味する。
【００１８】
本発明で製造する成形性に優れ靱性の良好なた加工用熱延鋼板は、自動車や産業機械及びパイプ素材等に用いられる引張り強さ（ＴＳ）が４００〜５００ＭＰａのＡｌキルド鋼、Ａｌ−Ｓｉキルド鋼、或いは、引張り強さ（ＴＳ）が５００ＭＰａ以上のハイテンやパイプ素材を対象としており、これら鋼板の成分及び成分範囲は以下の如くなっている。
【００１９】
Ｃ：０．５％以下、Ｍｎ：１．６％以下、Ｓｉ：０．８％以下、Ｐ：０．０２５％以下、Ｓ：０．０２５％以下を含有し、及びＴｉ：０．１００％以下、Ｎｂ：０．０６０％以下、Ｖ：０．０８０％以下、Ｃａ：０．００６０％以下、Ｎｉ：０．４０％以下の内から選択された一種以上を含有し、残部実質的にＦｅから成る成形性に優れた加工用熱延鋼板。
【００２０】
成形性に優れた加工用熱延鋼板中に含有されるＣは、硬化元素でありＣ含有量が多くなると硬質となり成形性が悪くなるので、成形性を向上させるにはＣ含有量は少ない方が好ましい。Ｃは最大０．５％迄含有させることができる。
【００２１】
Ｍｎは、靱性を付与するために必要な元素であるが、１．６％を超えると加工性を劣化させる。Ｓｉは、脱酸剤として添加するが多くなると硬化するので０．８％以下とした。Ｐ，Ｓは、不可避的に含有されるが、それぞれ０．０２５％を超えると加工性に悪影響がでる。Ｔｉ、Ｎｂ、Ｖ、Ｃａ、Ｎｉは靱性を向上させるが、多くなると加工性を劣化させるので、Ｔｉ：０．１００％以下、Ｎｂ：０．０６０％以下、Ｖ：０．０８０％以下、Ｃａ：０．００６０％以下、Ｎｉ：０．４０％以下とした。
【００２２】
この様な理由で、上記に示す様な成分、成分範囲に調整されている。
【００２３】
次いで、析出処理について説明する。
【００２４】
ＭｎＳやＡｌＮの析出物をオーステナイト域で出来るだけ析出させることが高延性化につながるものである。そのため粗圧延鋼板にＡｒ₃点以上で曲げ歪０．５％以上、曲げ歪速度０．０５Ｓ^-1以上で捲取り、次いで、３秒以上１℃／Ｓ以下の冷却速度で保持し、その後捲戻すことによりＭｎＳやＡｌＮの析出物を析出させ、その後の仕上圧延でこれが核となり析出を著しく促進するものである。曲げ歪は、大きいほうが析出物の生成に効果があり、曲げ歪０．５％未満ではその効果が期待できないので０．５％以上とした。また、曲げ歪は、高温で付与しているため時間をかけて曲げ歪（転移）を付与しても、その歪みは消滅してしまい曲げ歪の効果が得られないので、短時間の曲げ歪速度で形成させることが大切である。実験の結果によれば、曲げ歪速度０．０５Ｓ^-1以上でなければ延性を向上させる効果は得られないから、曲げ歪速度０．０５Ｓ^-1以上とした。
【００２５】
また、析出物の生成のためには、３秒以上１℃／Ｓ以下の冷却速度に保持する必要があり、これ以上では析出物の生成に効果がない。
【００２６】
これらの要件が欠けると、ＭｎＳやＡｌＮの析出が効果的に行われない。
【００２７】
本発明の析出処理には、鋼板の巻取りを行うコイルボックス法（ＩｒｏｎａｎｄＳｔｅｅｌＥｎｇｉｎｅｅｒ，１９８１，Ｎｏ．１１，Ｐ．４５２）が使用できる。この方法は、鋼板を曲げると同時にコイル状に巻き取るため、保温効果を有していて、３秒以上１℃／Ｓ以下の冷却速度に保持するのに特別の加熱装置なしで行うことができる。
【００２８】
そして、後で述べる低温加熱によるＭｎＳやＡｌＮの１次析出を行わせて核を作っておくと、上記曲げ歪及び曲げ歪速度等で規制する析出処理による析出が著しく促進することが判明した。従って、低温加熱による析出処理と上記析出処理との両方の析出処理を組み合わせて析出処理を行うことが好適である。
【００２９】
引き続き結晶粒の微細化処理について説明する。
【００３０】
本発明者は、加工性を向上させると共に靱性をも向上させる加工用熱延鋼板の材質改善法について研究し、熱間仕上温度をＡｒ₃＋５０℃以下の範囲で、且つ熱間仕上圧延機入側の温度と熱間仕上圧延機出側の温度との差を１００℃以下となる低等温圧延条件で、そして連続的に熱間仕上圧延を行うことによってγ粒の細粒化が生じて、鋼板の全長に亘って加工性と共に靱性が向上することを知見した。図２により説明する。図２は伸び（Ｅｌ）と引張り強さ（ＴＳ）の積と、及び入側温度（ＦＴ0 ）と出側温度（ＦＴ）との温度差（ΔＴ）との関係を示す図である。図に示すようにΔＴが小さくなるに従ってＥｌとＴＳとの積（Ｅｌ×ＴＳ）が高くなることが分かる。また、ΔＴが１００℃以下の場合にＥｌ×ＴＳが急激に高くなり細粒化の効果が著しいので、本発明では１００℃以下とした。また、このような効果が生じるのは、仕上温度がＡｒ₃＋５０℃以下の範囲であって、これ以上の温度ではγ粒が粗大となり、再結晶による細粒化の効果が得られない。従来法では、鋼板の中間部において細粒化が可能としても、鋼板の先端部では、後で述べる図３に示す様に温度低下が大きくて細粒化可能の温度範囲を外れてしまうので、全長に亘って細粒化処理することは技術的に困難である。
【００３１】
ところが、本発明では仕上圧延を連続化することにより、鋼板の先端部が実質上なくなり、鋼板の全長に亘って低等温圧延をすることができるので、この熱延連続化と細粒化処理とを組み合わせることにより、初めて鋼板の全長に亘って細粒化を実現できた。
【００３２】
その結果、鋼板の全長に亘りて成形性に優れ靱性の良好な加工用熱延鋼板とすることができ、先端部の材質不良による製品歩留りを向上することができた。
【００３３】
更に、熱延鋼板の温度分布について説明する。
【００３４】
図３は、加熱炉温度と熱延鋼板の仕上温度との関係を模式的に示す図である。図３に示すように、従来の熱間圧延方法では、通常、加熱炉で約１２００℃に加熱した状態の鋼スラブを熱間圧延しているが、熱延仕上鋼板の中間部の温度は約９００℃、熱延仕上鋼板の先端部はＡｒ₃変態点近傍の温度にそれぞれ低下していた。この様に熱延仕上鋼板の先端部の温度低下が著しいものであった。
【００３５】
ところが、本発明では、粗圧延された熱延鋼板の先端を、その前に粗圧延され熱延ラインを先行する熱延鋼板の後端に接合してあるので、連続的に熱間圧延をすることが可能となり、しかも、その熱間圧延は等速圧延とすることができるので、鋼板の全長に亘って圧延条件が同じとなり、従来のバッチ型の熱間圧延の加速圧延とは異なって、熱延仕上鋼板の温度低下のバラツキが生じない。即ち、本発明の熱延連続化法によれば、鋼板の先端部が存在しないので、熱延条件が従来の熱延仕上鋼板の中間部に相当するだけの圧延となるので、熱延仕上鋼板の温度低下は一定となり、図３の●印に示すようにその温度低下も少ないし、鋼板の全長に亘って均質な材質にすることができる。
【００３６】
このような理由により、本発明では、加熱炉での温度を従来の温度よりも低く設定でき、実験によれば、熱延仕上鋼板の温度をＡｒ₃変態点以上にするためには１１５０℃以下の低温加熱であれば充分であることが分かった。また、従来のように１２００℃の加熱温度では、鋼中にＭｎＳやＡｌＮの析出が充分でないが、１１５０℃以下の低温加熱を行えば、鋼中にＭｎＳやＡｌＮの析出が生じ、このまま共に再結晶による結晶の細粒化が生じて、鋼板の加工性、特に伸びと靱性を向上する効果が生じ、鋼板の全長に亘ってその材質が改善されたものとなるのである。
【００３７】
即ち、１１５０℃を越える加熱温度は過剰加熱となり省エネルギー上のコストメリットが得られないし、鋼中にＭｎＳやＡｌＮの析出が生じず、また、結晶粒が粗大化して材質改善が行われなわれず好ましくないので、本発明での加熱温度は高温加熱が必須となる１１５０℃以下の低温加熱とした。また、９５０℃以下では熱間仕上温度がＡｒ₃変態点以下となって加工性を劣化させるから好ましくない。
【００３８】
なお、ＮｂやＴｉ等を添加しその析出強化を利用する鋼は、高温加熱により、ＮｂやＴｉを一度溶体化処理する必要がある。このような鋼については低温加熱はできないが、仕上圧延時の細粒化処理をとることにより加工性と靭性の向上が期待できる。
【００３９】
また、本発明では、粗圧延された熱延鋼板の先端と、先行する熱延鋼板の後端とを溶接によって接合する。接合された鋼板は一体となるから、連続的に熱間仕上圧延をすることができ、熱延仕上鋼板の最初の先端部を捲取機のピンチロールに噛み込ませれば、それ以降は連続して冷却装置によって冷却水による冷却が可能となるものであり、捲取温度に冷却された熱延仕上鋼板は、捲取機で捲取る。なを、熱延仕上鋼板は、所定の長さで切断機によって切断され捲取機で捲取られるが、切断部位は接合部であることが好ましい。
【００４０】
本発明によれば、最初の熱延鋼板の先端部は従来と同様に冷却されていないので材質不良となるものの、それ以降に連続的に熱延された鋼板は、鋼板の先端部が存在しないので全て冷却することが可能となり、材質不良部分が存在しないこととなり、先端部の材質不良による製品歩留りが向上できる。
【００４１】
更に、請求項２の発明の様に、１１５０℃以下の低温加熱の析出処理と、曲げ歪０．５％以上、曲げ歪速度０．０５Ｓ^-1以上で３秒以上１℃／Ｓ以下の冷却速度で保持する析出処理と熱間仕上温度を狭い範囲に制御する結晶の細粒化処理とを組み合わせると、加工用熱延鋼板の延性と靱性とが向上できる。
【００４２】
【発明の実施の形態】
本発明を図に基づいて説明する。
【００４３】
図４は、熱延連続化法における成形性に優れた加工用熱延鋼板の製造方法の概要を示す図である。
【００４４】
図４に示すように、加熱炉１で例えば１１５０℃以下に加熱された炭素含有量０．５％以下の鋼スラブは、粗圧延機２で熱間圧延され、これをＡｒ₃変態点以上の温度で曲げ歪０．５％以上、曲げ歪速度０．０５Ｓ^-1以上で巻取って粗圧延コイル３とし、次いで３秒以上１℃／Ｓ以下の冷却速度で保持し、その後捲戻す。捲戻された粗圧延コイル３の先端は、溶接用切断機４でもって切断され溶接に適する先端開先が形成される。圧延ラインを先行する粗圧延鋼板が仕上圧延機に搬送され仕上圧延されるが、その後端は同じく溶接用切断機４でもって切断され溶接に適する後端開先が形成される。先行する粗圧延鋼板の後端と後行の粗圧延鋼板の先端とは、溶接装置５により溶接して接合される。
【００４５】
溶接装置５は、移動台車からなっており粗圧延鋼板の後端の移動速度と同期して移動することができるように制御されていて、移動台車を移動させながら先行する粗圧延鋼板の後端と後行の粗圧延鋼板の先端とを溶接する。溶接法は、レーザービーム溶接法が適するが、他の公知の溶接法も適用できる。
【００４６】
溶接装置５によって一体に接合され長尺となった粗圧延鋼板は、仕上圧延機６で熱間仕上温度をＡｒ₃＋５０℃以下の範囲で、且つ熱間仕上圧延機入側の温度と熱間仕上圧延機出側の温度との差を１００℃以下となる条件で連続的に仕上圧延され、次いで、ランナウトテーブルに設置された冷却装置７により捲取温度に水冷却された後に、コイルとして捲取機９で捲取られる。仕上鋼板は所定の長さを捲取られると、切断機８で切断され別のコイルとして捲取機９で捲取られる。なお、切断機８による切断部位は、溶接装置５で接合した部位を切断することが好ましい。
【００４７】
本発明では、粗圧延鋼板の先端を圧延ラインを先行する粗圧延鋼板の後端と接合して長尺の鋼板とするので、連続して熱間仕上圧延をすることができる。そのため、熱延鋼板は、最初の先端部以外に先端部が存在しなく、鋼板は捲取機のピンチロールで常に支持されることとなるので、熱間仕上鋼板の冷却は、連続的に冷却装置で水冷却することが可能となる。したがって、従来のバッチ型熱延方法のように冷却されずに材質不良となる鋼板先端部分は最初を除いて無くなる。
【００４８】
また、本発明では、上記に述べた様に曲げ歪及び曲げ歪速度等を規制した析出処理、および仕上温度範囲を規制した細粒化処理と、必要に応じて請求項２のように低温加熱を組み合わせて析出処理と細粒化処理を施すみものであるため、仕上圧延中にＭｎＳやＡｌＮの析出物を核として微細析出物および結晶粒の細粒化の促進がはかられ、高延性で靱性に優れた加工用熱延鋼板が得られる。そして、連続的に熱間仕上圧延を行うものであるため、熱間仕上圧延中の鋼板全体に温度低下のバラツキがなく、材質も鋼板全長に亘って均質となり、鋼スラブを加熱する加熱温度も、従来の熱延方法の加熱温度１２００℃よりも低温の１１５０℃以下の低温加熱温度に設定できる。
【００４９】
【実施例】
以下、本発明の実施例と比較例とについて述べる。
【００５０】
表１に示す成分の鋼材を用いて、表２に示す処理条件で加工用熱延鋼板を製造した。
【００５１】
表２に示すようＮｏ．１〜３の本発明の実施例はＮｏ．４〜１１の比較例よりもＡ鋼材及びＢ鋼材についてみると、それぞれＴＳ×Ｅｌと脆性遷移温度が格段に優れていて、材質評価は良好であった。
【００５２】
なお、Ｎｏ．２のＡ鋼材を用いて１１００℃の低温加熱を施した実施例は、Ｎｏ．１のＡ鋼材を用いて１２００℃に加熱した実施例よりもＴＳ×Ｅｌが高く、低温加熱した方が材質が優れたものとなっていた。
【００５３】
【表１】

【００５４】
【表２】

【００５５】
【発明の効果】
本発明の熱延連続化法による成形性に優れた加工用熱延鋼板の製造方法によれば、析出処理および結晶の細粒化処理によって高延性で靱性に優れた加工用熱延鋼板を得ることができ、鋼板の全長に亘って均質なものになり、また、低温加熱により加熱炉原単位の低下がはかれ、更に、熱延鋼板の先端部の材質不良による製品歩留りを向上させることがでる。その上、高い生産性を達成することができる。
【図面の簡単な説明】
【図１】熱延鋼板の仕上温度分布を示す図である。
【図２】伸び（Ｅｌ）と引張り強さ（ＴＳ）の積と、及び入側温度（ＦＴ0 ）と出側温度（ＦＴ）との温度差（ΔＴ）との関係を示す図である。
【図３】加熱温度と熱延鋼板の仕上温度との関係を模式的に示す図である。
【図４】本発明の熱延連続化法による成形性に優れた加工用熱延鋼板の製造方法の概要を示す図である。
【符号の説明】
１加熱炉
２粗圧延機
３粗圧延コイル
４溶接用切断機
５溶接装置
６仕上圧延機
７冷却装置
８切断機
９捲取機[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a hot-rolled steel sheet by a hot-rolling continuation method, and in particular, hot-worked steel sheets for processing that have excellent formability and good toughness used for automobiles, industrial machines, pipe materials, etc. are continuously hot. The present invention relates to a method of rolling and manufacturing.
[0002]
[Prior art]
Conventionally, cold-rolled steel sheets with good workability have been used in the technical field of steel sheets for processing such as automobiles, but recently, hot-rolling steel for processing is relatively inexpensive as an alternative to cold-rolled steel sheets in order to reduce material costs. Steel plates are being used.
[0003]
The manufacturing method of hot-rolled steel sheets for processing used in automobiles, industrial machines, pipe materials, etc., heats a continuously cast steel slab to about 1200 ° C in a heating furnace, then roughly rolls it with a hot rolling mill, and finish-rolls. After that, the coil is generally cooled with cooling water and wound into a coil.
[0004]
In such a conventional hot-rolled steel sheet manufacturing method, hot-rolled steel sheets that are hot rolled for each steel slab and laminar-cooled with a cooling water by a cooling device provided on the run-out table are used as finished steel sheets to have a cutting temperature. Is coiled into a coil. However, when the hot-finished steel sheet is cooled, even if it tries to cool from the front end of the steel sheet, the front end of the steel sheet collides with the water column of the cooling water, and the steel plate deforms and stalls due to the influence of the water column, causing trouble on the plate. As a result, the cooling process could not be performed. This phenomenon becomes more prominent as the plate thickness becomes thinner.
[0005]
Therefore, conventionally, when cooling the hot-rolled steel sheet to the scraping temperature, the tip of the hot-rolled steel sheet is bitten by the pinch roll without cooling the front-end part of the hot-rolled steel sheet in order to avoid trouble on the plate. And then cooled. In such a conventional method for cooling a hot-rolled steel sheet, the tip portion of the steel plate is not cooled, so the portion becomes a poor material, and when the product is shipped as a product, the tip portion that is not cooled is cut off. There was a problem that the product yield was bad.
[0006]
Next, the precipitation treatment will be described. In order to give hot-rolled steel sheet workability, it is necessary to perform precipitation treatment as MnS or AlN in order to remove S and N harmful to workability, and impart bending strain to the steel sheet. Thus, a method in which MnS and AlN are deposited is known. For example, Japanese Examined Patent Publication No. 7-74376 discloses a method in which a material to be rolled after rough rolling is bent in a temperature range of 1100 ° C. or lower and an Ar ₃ point or higher and held in the temperature range for 10 seconds or longer. ing.
[0007]
As a result of further research on the precipitation treatment by bending strain, the present inventor has not only applied bending strain as disclosed in the prior art but also the conditions of both specific bending strain and bending strain rate. It has been found that when applied to a steel sheet, precipitation of MnS and AlN is promoted and the elongation of the steel sheet is improved.
[0008]
Moreover, about the material of the hot-rolled steel plate for a process, it is necessary to have workability and toughness. Therefore, the present inventor has studied the material improvement method of the hot-rolled steel sheet for work that has improved workability and improved toughness, and finish rolling conditions in the range of hot finishing temperature Ar ₃ + 50 ° C. or less, In addition, the hot finish rolling is continuously performed over the entire length of the steel sheet under low isothermal rolling conditions where the difference between the temperature at the inlet side of the hot finish mill and the temperature at the exit side of the hot finish mill is 100 ° C. or less. As a result, the inventors have found that if the γ grains are refined, both workability and toughness can be improved, and the present invention has been completed.
[0009]
Then, considering the finishing temperature distribution of the hot-rolled steel sheet, in order to perform hot rolling without degrading workability, it is necessary to set the temperature of the steel sheet to be rolled to at least the Ar ₃ transformation point or higher. FIG. 1 is a diagram showing a finishing temperature distribution of a conventional hot-rolled steel sheet. As shown in FIG. 1, the finishing temperature at the front end of the steel sheet to be hot-rolled is the lowest, and the finishing temperature increases as the rear end is reached. The reason why the finishing temperature at the rear end is increased is considered to be due to processing heat generation.
[0010]
In addition, since the finishing temperature of the steel plate is not uniform and varies over the entire length of the steel plate, the conventional hot rolling method cannot be controlled to the narrow hot finishing temperature range necessary for the refinement of γ grains as described above. There is.
[0011]
The heating of a conventional heating furnace, the lowest finishing temperature of the steel sheet to be hot-worked, i.e., selects the heating temperature so that the finishing temperature of the tip portion of the steel sheet becomes the Ar ₃ transformation point or more of the temperature Ar ₃ transformation point Heating to a temperature of about 1200 ° C., which is considerably higher than that.
[0012]
From the viewpoint of energy saving cost balance, overheating is performed at portions other than the tip of the steel plate, and there is a problem that the cost balance is poor.
[0013]
Japanese Laid-Open Patent Publication No. 1-149922 proposes a method in which a steel slab is heated at a low temperature of 1150 ° C. or lower and hot-rolled to form a hot-rolled steel sheet having excellent workability and secondary work brittleness resistance. This method is intended to prevent crystal grain coarsening by low-temperature heating and to refine the crystal by recrystallization after rolling. However, this method also has the problem of temperature drop at the tip of the hot-rolled steel sheet described above. However, it is technically difficult to obtain a uniform steel sheet over the entire steel sheet, and it is impossible to improve the yield due to defective material at the tip.
[0014]
[Problems to be solved by the invention]
Therefore, the present invention refines the precipitates of MnS and AlN and refines the crystal grains, thereby improving and uniforming the material of the hot-rolled steel sheet over the entire length, and the poor quality of the tip part. A method for continuously producing hot-rolled steel sheets for processing with improved formability and improved toughness with improved productivity and improved cost merit due to energy saving in a heating furnace is provided. This is a problem.
[0015]
[Means for Solving the Problems]
(1) A steel slab having a carbon content of 0.5% by mass or less is heated in a heating furnace, roughly rolled with a hot roughing mill to form a steel plate, and then cooled after finishing with a hot finishing mill. the method of manufacturing a superior processing hot rolled steel sheet in formability, characterized in that take-wound into a coil, the rough rolling steel plate Ar ₃ in the above transformation point temperature bending strain of 0.5% or more, the bending strain rate 0.05S ^-1 or higher, and then held at a cooling rate of 3 ° C or higher and 1 ° C / S or lower, and then rewinded, and the steel sheet tip is roughly rolled before and after the steel sheet preceding the rolling line. Joined to the end, the hot finishing temperature is in the range of Ar ₃ to Ar ₃ + 50 ° C. or lower, and the difference between the temperature on the hot finishing mill entry side and the temperature on the hot finishing mill exit side is 100 ° C. Excellent formability and good toughness, characterized by continuous hot finish rolling under the following conditions Method for producing a Do processing hot rolled steel sheet.
[0016]
(2) A steel slab having a carbon content of 0.5% by mass or less is heated in a heating furnace, roughly rolled with a hot roughing mill to form a steel plate, and then cooled after finishing with a hot finishing mill. In the method for producing a hot-rolled steel sheet for processing excellent in formability characterized by winding into a coil, heating in the heating furnace is performed at a low temperature of 1150 ° C. or lower, and the rough-rolled steel sheet is heated to an Ar ₃ transformation point or higher. At a temperature of 5% or more at a bending strain of 0.05 S ⁻¹ or higher, and then held at a cooling rate of 1 ° C./S for 3 seconds or more, and then tempered. The front end is joined to the rear end of the steel plate that is roughly rolled before and precedes the rolling line, and the hot finishing temperature is in the range of Ar ₃ to Ar ₃ + 50 ° C. or lower, and the hot finishing rolling mill is on the inlet side. The difference between the temperature and the temperature at the exit side of the hot finishing mill is continuously 100 ° C. or less. Hot finishing method for producing a good processability for hot rolled steel sheets of excellent toughness moldability and performs rolling.
[0017]
The present invention will be described in detail below. In addition,% of a steel component means the mass%.
[0018]
The hot-rolled steel sheet having excellent formability and good toughness produced in the present invention is an Al killed steel having a tensile strength (TS) of 400 to 500 MPa and Al-Si used for automobiles, industrial machines, pipe materials and the like. It is intended for killed steel, or high tensile steel or pipe material having a tensile strength (TS) of 500 MPa or more. The components and component ranges of these steel plates are as follows.
[0019]
C: 0.5% or less, Mn: 1.6% or less, Si: 0.8% or less, P: 0.025% or less, S: 0.025% or less, and Ti: 0.100% Hereinafter, one or more selected from Nb: 0.060% or less, V: 0.080% or less, Ca: 0.0060% or less, Ni: 0.40% or less, with the balance being substantially Fe. Hot-rolled steel sheet for processing with excellent formability.
[0020]
C contained in the hot-rolled steel sheet for processing excellent in formability is a hardening element, and as the C content increases, it becomes hard and the formability deteriorates. Therefore, in order to improve the formability, the one with less C content Is preferred. C can be contained up to a maximum of 0.5%.
[0021]
Mn is an element necessary for imparting toughness, but if it exceeds 1.6%, workability is deteriorated. Si is added as a deoxidizer, but hardens as it increases, so it was made 0.8% or less. P and S are inevitably contained, but if each exceeds 0.025%, workability is adversely affected. Ti, Nb, V, Ca, and Ni improve toughness, but when they increase, workability deteriorates, so Ti: 0.100% or less, Nb: 0.060% or less, V: 0.080% or less, Ca : 0.0060% or less, Ni: 0.40% or less.
[0022]
For this reason, the components and component ranges as described above are adjusted.
[0023]
Next, the precipitation process will be described.
[0024]
Precipitation of MnS and AlN precipitates in the austenite region as much as possible leads to high ductility. For this reason, the rough rolled steel sheet is scraped at an Ar ₃ point or higher at a bending strain of 0.5% or higher and a bending strain rate of 0.05 S ⁻¹ or higher, and then held at a cooling rate of 3 ° C. or higher and 1 ° C./S or lower. By returning, precipitates of MnS and AlN are precipitated, and this becomes a nucleus in the subsequent finish rolling to significantly accelerate the precipitation. A larger bending strain has an effect on the formation of precipitates, and if the bending strain is less than 0.5%, the effect cannot be expected. In addition, since bending strain is applied at a high temperature, even if bending strain (transition) is applied over time, the strain disappears and the effect of bending strain cannot be obtained. It is important to form at speed. According to the results of experiments, the effect of improving the ductility unless bending strain rate 0.05 S ^-1 or higher can not be obtained, and a bending strain rate 0.05 S ^-1 or more.
[0025]
In addition, in order to generate precipitates, it is necessary to maintain the cooling rate at 3 ° C. or higher and 1 ° C./S or lower.
[0026]
If these requirements are lacking, precipitation of MnS and AlN will not be performed effectively.
[0027]
For the precipitation treatment of the present invention, a coil box method (Iron and Steel Engineer, 1981, No. 11, P. 452) for winding a steel plate can be used. This method has a heat retention effect because it is wound in a coil shape at the same time as bending the steel plate, and can be performed without a special heating device to maintain a cooling rate of 3 ° C. or more and 1 ° C./S or less. .
[0028]
Then, it was found that if the nucleus is formed by performing primary precipitation of MnS or AlN by low-temperature heating, which will be described later, precipitation by the precipitation treatment regulated by the bending strain and the bending strain rate is remarkably accelerated. Therefore, it is preferable to perform the precipitation treatment by combining both the precipitation treatment by low-temperature heating and the above-described precipitation treatment.
[0029]
Next, crystal grain refinement processing will be described.
[0030]
The present inventor has studied a material improvement method for hot-rolled steel sheets for working that improves workability and toughness, and enters a hot finish rolling mill with a hot finish temperature in the range of Ar ₃ + 50 ° C. or less. The difference between the temperature on the side and the temperature on the exit side of the hot finish rolling mill is 100 ° C. or lower under low isothermal rolling conditions, and by continuously performing hot finish rolling, γ grains are refined, It has been found that toughness improves with workability over the entire length of the steel sheet. This will be described with reference to FIG. FIG. 2 is a graph showing the relationship between the product of elongation (El) and tensile strength (TS) and the temperature difference (ΔT) between the inlet side temperature (FT 0) and the outlet side temperature (FT). As shown in the figure, it can be seen that the product of El and TS (El × TS) increases as ΔT decreases. In addition, when ΔT is 100 ° C. or lower, El × TS is rapidly increased and the effect of refining is remarkable, so in the present invention, it is set to 100 ° C. or lower. Further, such an effect occurs when the finishing temperature is in the range of Ar ₃ + 50 ° C. or lower, and the γ grains become coarse at a temperature higher than this, and the effect of refining by recrystallization cannot be obtained. In the conventional method, even if the fine graining is possible in the middle part of the steel sheet, the tip of the steel sheet has a large temperature drop as shown in FIG. It is technically difficult to make a fine grain over the entire length.
[0031]
However, in the present invention, by continuously performing finish rolling, the tip of the steel sheet is substantially eliminated, and low isothermal rolling can be performed over the entire length of the steel sheet. For the first time, it was possible to achieve fine graining over the entire length of the steel sheet.
[0032]
As a result, it was possible to obtain a hot-rolled steel sheet having excellent formability and good toughness over the entire length of the steel sheet, and it was possible to improve product yield due to poor material quality at the tip.
[0033]
Furthermore, the temperature distribution of the hot rolled steel sheet will be described.
[0034]
FIG. 3 is a diagram schematically showing the relationship between the heating furnace temperature and the finishing temperature of the hot-rolled steel sheet. As shown in FIG. 3, in the conventional hot rolling method, the steel slab in a state heated to about 1200 ° C. in a heating furnace is usually hot-rolled, but the temperature of the intermediate part of the hot-rolled steel sheet is about The tip of the hot rolled steel sheet at 900 ° C. was lowered to a temperature near the Ar ₃ transformation point. In this way, the temperature drop at the tip of the hot rolled steel sheet was significant.
[0035]
However, in the present invention, the hot-rolled steel plate that has been roughly rolled is joined to the rear end of the hot-rolled steel plate that has been roughly rolled before and preceded by the hot-rolled steel plate, so that hot rolling is continuously performed. In addition, since the hot rolling can be a constant speed rolling, the rolling conditions are the same over the entire length of the steel sheet, and unlike the conventional batch type hot rolling accelerated rolling, There is no variation in the temperature drop of the hot rolled steel sheet. That is, according to the hot rolling continuous method of the present invention, since there is no tip portion of the steel sheet, the hot rolling condition is rolling corresponding to the intermediate part of the conventional hot rolled steel sheet. The temperature drop is constant, as shown by the ● marks in FIG. 3, the temperature drop is small, and a uniform material can be formed over the entire length of the steel sheet.
[0036]
For this reason, in the present invention, the temperature in the heating furnace can be set lower than the conventional temperature. According to experiments, in order to set the temperature of the hot rolled steel sheet to the Ar ₃ transformation point or higher, it is 1150 ° C. or lower. It has been found that low temperature heating of is sufficient. In addition, MnS and AlN are not sufficiently precipitated in the steel at a heating temperature of 1200 ° C. as in the past, but if low-temperature heating at 1150 ° C. or lower is performed, MnS and AlN are precipitated in the steel, and both of them are restarted as they are. Crystal refinement due to crystals occurs, and the effect of improving the workability of the steel sheet, particularly the elongation and toughness, is produced, and the material is improved over the entire length of the steel sheet.
[0037]
In other words, heating temperatures exceeding 1150 ° C. result in excessive heating, resulting in no energy saving cost benefits, no precipitation of MnS or AlN in the steel, and no coarsening of crystal grains to improve the material. Since it is not preferable, the heating temperature in the present invention is a low-temperature heating of 1150 ° C. or lower, which requires high-temperature heating. On the other hand, when the temperature is 950 ° C. or lower, the hot finishing temperature is lower than the Ar ₃ transformation point, which is not preferable.
[0038]
In addition, steel which adds Nb, Ti, etc. and uses the precipitation strengthening needs to solution-treat Nb and Ti once by high temperature heating. Although such steel cannot be heated at low temperature, improvement of workability and toughness can be expected by taking a fine graining process during finish rolling.
[0039]
Moreover, in this invention, the front-end | tip of the hot-rolled steel plate rough-rolled and the rear end of a preceding hot-rolled steel plate are joined by welding. Since the joined steel plates are united, hot finish rolling can be performed continuously, and if the first tip of the hot rolled finish steel plate is bitten into the pinch roll of the scraper, the subsequent steps are continuous. The cooling device enables cooling with cooling water, and the hot-rolled finished steel sheet cooled to the cutting temperature is scraped by a scraper. The hot-rolled finished steel sheet is cut by a cutter with a predetermined length and scraped by a scraper, but the cutting site is preferably a joint.
[0040]
According to the present invention, the tip portion of the first hot-rolled steel sheet is not cooled in the same manner as in the prior art, resulting in a defective material, but the steel plate continuously hot-rolled thereafter does not have the tip part of the steel plate. Therefore, it is possible to cool all, and there is no defective material portion, and the product yield due to the defective material at the tip can be improved.
[0041]
Furthermore, as in the second aspect of the invention, a low temperature heating precipitation treatment at 1150 ° C. or lower and a cooling at a bending strain of 0.5% or higher and a bending strain rate of 0.05 S ⁻¹ or higher for 3 seconds to 1 ° C./S or lower. Combining the precipitation treatment maintained at a speed and the crystal grain refining treatment for controlling the hot finishing temperature within a narrow range can improve the ductility and toughness of the hot-rolled steel sheet for processing.
[0042]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described with reference to the drawings.
[0043]
FIG. 4 is a diagram showing an outline of a method for producing a hot-rolled steel sheet for processing excellent in formability in the hot-rolling continuous method.
[0044]
As shown in FIG. 4, the steel slab having a carbon content of 0.5% or less heated to 1150 ° C. or less in the heating furnace 1 is hot-rolled by the roughing mill 2, and the steel slab is heated above the Ar ₃ transformation point. The coil is wound up at a bending strain of 0.5% or more at a temperature and a bending strain rate of 0.05 S ⁻¹ or more to form a rough rolled coil 3, and then held at a cooling rate of 3 ° C. or more and 1 ° C./S or less, and then rewinded. The tip of the rough rolled coil 3 that has been rolled back is cut by a welding cutter 4 to form a tip groove suitable for welding. The rough rolled steel sheet preceding the rolling line is transported to the finishing mill and finish-rolled, but the rear end is similarly cut by the welding cutter 4 to form a rear end groove suitable for welding. The rear end of the preceding rough rolled steel sheet and the front end of the subsequent rough rolled steel sheet are welded and joined by the welding device 5.
[0045]
The welding device 5 is made of a moving carriage and is controlled so as to be able to move in synchronization with the moving speed of the trailing edge of the rough rolled steel sheet. The trailing edge of the preceding rough rolled steel sheet while moving the moving carriage And the tip of the subsequent rough rolled steel sheet. Laser beam welding is suitable for the welding method, but other known welding methods can also be applied.
[0046]
The coarsely rolled steel sheet that is integrally joined by the welding device 5 and has a long length is hot rolled by the finishing mill 6 at a hot finishing temperature of Ar ₃ + 50 ° C. or less, and at the hot finishing mill entry side temperature and hot. The steel sheet is continuously finish-rolled under a condition that the difference from the finish rolling mill outlet side temperature is 100 ° C. or lower, and then water-cooled to the cutting temperature by the cooling device 7 installed on the run-out table. It is picked up by the take-up machine 9. When the finished steel plate is scraped to a predetermined length, it is cut by the cutting machine 8 and cut by the scraping machine 9 as another coil. In addition, it is preferable to cut | disconnect the site | part joined by the welding apparatus 5 as the cutting site | part by the cutting machine 8. FIG.
[0047]
In this invention, since the front-end | tip of a rough-rolled steel plate is joined to the rear end of the rough-rolled steel plate which precedes a rolling line to make a long steel plate, hot finish rolling can be performed continuously. For this reason, the hot-rolled steel sheet has no tip other than the first tip, and the steel sheet is always supported by the pinch roll of the scraper. Therefore, the hot-finished steel sheet is cooled continuously. It becomes possible to perform water cooling with the apparatus. Therefore, the tip portion of the steel sheet that becomes a material defect without being cooled unlike the conventional batch type hot rolling method is eliminated except at the beginning.
[0048]
Further, in the present invention, as described above, the precipitation treatment in which the bending strain and the bending strain rate are regulated, the fine graining treatment in which the finishing temperature range is regulated, and, if necessary, the low temperature heating as in claim 2 Because of the combination of precipitation treatment and refinement treatment in combination, fine precipitation and grain refinement are promoted with MnS and AlN precipitates as the core during finish rolling, resulting in high ductility. Thus, a hot-rolled steel sheet for processing excellent in toughness is obtained. And since the hot finish rolling is performed continuously, there is no variation in temperature drop across the steel plate during hot finish rolling, the material is uniform over the entire length of the steel plate, and the heating temperature for heating the steel slab is also high. The heating temperature of the conventional hot rolling method can be set to a low temperature of 1150 ° C. or lower, which is lower than 1200 ° C.
[0049]
【Example】
Examples of the present invention and comparative examples will be described below.
[0050]
Using the steel materials having the components shown in Table 1, hot-rolled steel sheets for processing were produced under the processing conditions shown in Table 2.
[0051]
As shown in Table 2, no. 1 to 3 of the present invention are No. When A steel material and B steel material were seen rather than the comparative examples of 4-11, TSxEl and the brittle transition temperature were remarkably excellent, respectively, and material evaluation was favorable.
[0052]
In addition, No. No. 2 was used as an example in which low temperature heating at 1100 ° C. was performed using the A steel material. TS × El was higher than that of Example 1 heated to 1200 ° C. using the A steel material, and the material was superior when heated at low temperature.
[0053]
[Table 1]

[0054]
[Table 2]

[0055]
【The invention's effect】
According to the manufacturing method of a hot-rolled steel sheet for processing excellent in formability by the hot-rolling continuous method of the present invention, a hot-rolled steel sheet for processing having high ductility and excellent toughness is obtained by precipitation treatment and crystal grain refinement processing It can be uniform over the entire length of the steel sheet, the heating furnace unit can be reduced by low-temperature heating, and the product yield can be improved due to poor material quality at the tip of the hot-rolled steel sheet. Out. In addition, high productivity can be achieved.
[Brief description of the drawings]
FIG. 1 is a diagram showing a finishing temperature distribution of a hot-rolled steel sheet.
FIG. 2 is a diagram showing the relationship between the product of elongation (El) and tensile strength (TS) and the temperature difference (ΔT) between the inlet side temperature (FT 0) and the outlet side temperature (FT).
FIG. 3 is a diagram schematically showing a relationship between a heating temperature and a finishing temperature of a hot-rolled steel sheet.
FIG. 4 is a diagram showing an outline of a method for producing a hot-rolled steel sheet for processing excellent in formability by the hot-rolling continuous method of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Coarse rolling mill 3 Coarse rolling coil 4 Welding cutting machine 5 Welding device 6 Finishing mill 7 Cooling device 8 Cutting machine 9 Trimmer

Claims

A steel slab having a carbon content of 0.5% by mass or less is heated in a heating furnace, roughly rolled with a hot roughing mill to form a steel plate, then finish-rolled with a hot finishing mill, cooled, and then rolled into a coil. the method of manufacturing a superior processing hot rolled steel sheet in formability, characterized in that taking the rough rolling steel plate Ar ₃ bending lower than the transformation point of the temperature strain of 0.5% or more, the bending strain rate 0.05 S ^-1 or And then held at a cooling rate of 3 ° C. or more and 1 ° C./S or less, and then rewinded, and the front end of the steel plate is joined to the rear end of the steel plate that has been roughly rolled before and preceded by the rolling line. Then, the hot finishing temperature is in the range of Ar ₃ to Ar ₃ + 50 ° C. or less, and the difference between the temperature on the hot finishing mill entry side and the temperature on the hot finishing mill exit side is 100 ° C. or less. Excellent formability and good toughness, characterized by continuous hot finish rolling under certain conditions For producing hot-rolled steel sheets for use.

A steel slab having a carbon content of 0.5% by mass or less is heated in a heating furnace, roughly rolled with a hot roughing mill to form a steel plate, then finish-rolled with a hot finishing mill, cooled, and then rolled into a coil. In the method for producing a hot-rolled steel sheet for processing excellent in formability, the heating in the heating furnace is performed at a low temperature of 1150 ° C. or lower, and the rough-rolled steel sheet is heated at a temperature equal to or higher than the Ar ₃ transformation point. Taken at a bending strain of 0.5% or more and a bending strain rate of 0.05 S ⁻¹ or more, then held at a cooling rate of 3 ° C. or more and 1 ° C./S or less, then rewound, and the tip of the steel sheet was Before that, it is roughly rolled and the rolling line is joined to the rear end of the preceding steel plate, the hot finishing temperature is in the range of Ar ₃ to Ar ₃ + 50 ° C. or lower, and the temperature and heat on the hot finishing rolling mill inlet side. Continuously hot on the condition that the difference from the temperature on the exit side of the finishing mill is 100 ° C or less A method for producing a hot-rolled steel sheet for processing having excellent formability and good toughness, characterized by performing finish rolling.