【０００１】
【発明の属する技術分野】
本発明は、平均粒径が３μｍ以下の微細フェライト組織を有し、強度・靭性に優れた熱延鋼帯の製造方法に関するものである。
【０００２】
【従来の技術】
タンデム圧延中に逆変態を生じさせる圧延方法が従来から多数提案されている。
【０００３】
たとえば、特開平１１−１５２５２２号には、連続熱延の仕上げ圧延における加工発熱、及び、又は、鋼板の復熱による温度上昇を利用して、フェライト（α）をオーステナイト（γ）に逆変態させることを特徴とする深絞り性に優れた薄鋼板の製造方法が示されている。
【０００４】
また、特公平７−９０３３号には、連続鋳造鋳片又はインゴットを熱片又は熱塊状態から冷却し、（ａ）Ａｒ３点以下の温度域で合計圧下率３０％以上の圧延を施す、（ｂ）続いてＡｃ３点〜〔Ａｃ３点＋１００℃〕の温度域に１０℃／秒以上の加熱速度で昇温し、フェライトからオーステナイトへ逆変態を生じさせる、（ｃ）そして、該オーステナイト相温度域で合計圧下率１０％以上の圧延を施す、 なる工程で順次加工熱処理し冷却することを特徴とする、超微細組織鋼板の製造方法が示されている。 さらに、連続鋳造鋳片又はインゴットを熱片又は熱塊状態のまま乃至は加熱炉に装入してから再結晶温度域で合計圧下率３０％以上の圧延を行った後、これを冷却し、（ａ）Ａｒ３以下の温度域で合計圧下率３０％以上の圧延を施す、 （ｂ）続いてＡｃ３点〜〔Ａｃ３点＋１００℃〕の温度域に１０℃／秒以上の加熱速度で昇温し、フェライトからオーステナイトへ逆変態を生じさせる、（ｃ）そして、該オーステナイト相温度域で合計圧下率１０％以上の圧延を施す、なる工程で順次加工熱処理し冷却することを特徴とする、超微細組織鋼板の製造方法が示されている。
【０００５】
また、特開平１０−８１４２号には、重量比で Ｃ： ０．０００５〜０．０１５０％ Ｓｉ：０．２％以下 Ｍｎ：０．０５〜０．６％ Ｐ： ０．０２％以下 Ｓ： ０．０２％以下 Ａｌ：０．１５％以下 Ｎ： ０．０２％以下を含み、さらに必要に応じて Ｎｂ：０．００３〜０．０２０％ Ｔｉ：０．００３〜０．０２０％ Ｂ： ０．０００２〜０．００２０％ Ｃｕ：０．５％以下 Ｎｉ：０．５％以下 Ｃｒ：０．５％以下 Ｍｏ：０．２％以下の１種又は２種以上を含み、残部はＦｅおよび可避不純物よりなる組成の鋼スラブに、 熱間粗圧延を全圧下量８０％以上、そのうち、最終パスを２０％以上とする条件下で行い、 仕上熱間圧延を、被圧延材の温度が仕上圧延機列のいずれかの圧延スタンド通過の際、圧延加工に伴う発熱により逆変態させ、仕上圧延温度がＡｒ３−５０℃以上となるように終了し、 ５５０〜７５０℃の温度で巻取って熱間圧延鋼帯を得、 該熱間圧延鋼帯に対してスケール除去、冷間圧延、再結晶焼鈍および３０％以下の調質圧延を行うことを特徴とする加工性が良好でかつ肌荒れのない製缶用鋼板の製造方法が示されている。
【０００６】
【発明が解決しようとする課題】
上記の従来技術には、いずれもタンデム圧延中に逆変態を生じさせる圧延方法が示されているが、圧下率、温度範囲を規定しているものの、被圧延材の全長に亘って均一な材質の鋼板を製造する具体的な方法が示されていない。
【０００７】
一般に被圧延材の仕上圧延機入側における温度は、放射、対流による材料の温度降下により、先端で高く後端に向かうほど低くなる。これを補償するためにタンデム圧延では圧延速度を後端になるほど加速する加速圧延が行われている。
【０００８】
逆変態をタンデム仕上圧延機の特定のスタンドで被圧延材の先端から後端まで安定して生じさせるためには、当該スタンドにおける被圧延材の温度と加工発熱量を長手方向にほぼ一定に保つ必要がある。ところが、通常のタンデム圧延では被圧延材の温度は最終スタンドで一定であるものの、それ以外のスタンドでは圧延位置により異なり、前段スタンドほど圧延中の材料温度低下が著しい。また、加速圧延を行えば圧延加工のひずみ速度が徐々に速くなるため、材料の変形抵抗が上昇し、加工発熱量が増加することとなる。
【０００９】
これらの現象をすべて考慮に入れ、特定の同一の圧延スタンドで圧延材の先端から後端まで安定して逆変態を生じるよう、圧延条件を設定することは極めて困難であり、従来技術では圧延材の全長に亘って逆変態による微細組織を有する鋼板を製造することはできなかった。
【００１０】
本発明は、上記のような従来技術の問題点を解決し、圧延材の全長に亘って逆変態による微細組織を有する熱延鋼帯を安定して製造することのできる熱延鋼帯の製造方法を提供することを目的とする。
【００１１】
【課題を解決するための手段】
すなわち、本発明の熱延鋼帯の製造方法は以下のような特徴を有する。
【００１２】
（１）連続タンデム仕上圧延機により仕上圧延して熱延鋼帯を製造するに際し、仕上圧延機入側に被圧延材を加熱するための加熱装置を設置して、被圧延材の仕上圧延機入側温度を長手方向にＡｒ３以下の略一定温度に制御するとともに、被圧延材先端から後端までの圧延速度を一定とすることにより、被圧延材の長手方向に略一定の圧延加工に伴う加工発熱を生じさせ、被圧延材の先端から後端まで同一の圧延スタンドでフェライトからオーステナイトに逆変態させることを特徴とする熱延鋼帯の製造方法。
【００１３】
（２）仕上圧延の最終圧延パス終了直後の急速冷却を、５０℃／秒以上の冷却速度で行うことを特徴とする上記（１）に記載の熱延鋼帯の製造方法。
【００１４】
【発明の実施の形態】
図１は、本発明の実施に供される熱延鋼帯の製造設備列の一実施形態を示す説明図である。
【００１５】
図１に示す熱延鋼帯の製造ラインは、所定温度に加熱されたスラブを所定の粗さの粗バ−に圧延するための粗圧延機１と、粗圧延機出側直近に設置された中間冷却装置２と、粗バ−を加熱するための加熱装置３と、この加熱装置３により長手方向に温度を一定に制御された粗バ−を所定の厚さの熱延鋼帯に圧延するための仕上圧延機４と、仕上圧延機出側直近に位置し、熱延鋼帯を急速冷却するための急速冷却装置５と、巻取り温度を調整するための冷却装置６と、この熱延鋼帯を巻取るための巻取り機７とを備えている。
【００１６】
前記中間冷却装置２は、粗圧延最終圧延スタンドの出側直近に設置し、粗圧延後の粒成長を防止するとともに、被圧延材の仕上圧延機入側温度を長手方向にＡｒ３以下の略一定温度とする前の温度調整冷却を行う冷却装置である。
【００１７】
粗圧延での圧下にて生成された再結晶粒オ−ステナイト粒は、通常は粗圧延と仕上圧延の間の数十秒の間に大きな粒成長挙動を示す。オーステナイトから変態して生じるフェライト組織の粒径はオーステナイト粒径が小さいほど小さくなるため、またＡｒ３以下の温度で残留するオーステナイト粒径をできるだけ微細に保つため、粗圧延後は直ちに冷却することが望ましい。
【００１８】
前記加熱装置３は、中間冷却装置２で温度調整冷却された被圧延材を加熱して、被圧延材の仕上圧延機入側温度を長手方向にＡｒ３以下の略一定温度に制御するための加熱装置である。このような加熱装置として、誘導加熱装置を使用すれば、制御応答性がよく、仕上圧延機４の入側における粗バーの温度をその長さ方向全長に亘って一定温度に加熱することができる。
【００１９】
急速冷却装置５は、圧延直後の急速冷却により粒成長を抑制するとともに、フェライトへの変態核成長速度を速め、より微細な組織を得るための冷却装置である。この仕上圧延の最終圧延パス終了直後の急速冷却を５０℃／秒以上の冷却速度で行うことが好ましい。
【００２０】
圧延機直後での急速冷却を可能とするため、極力圧延機出側直近に配置することが望ましく、図１の実施形態では仕上圧延機出側直近に急速冷却装置を配置している。
【００２１】
また、材質調整の観点からは、巻取り機７に巻取られた後の温度も重要であり、図１の実施形態では、巻取り機７の直前に巻取り温度調整用の冷却装置６を設置している。
【００２２】
以下、上記装置構成を用いた本発明法の一実施形態を説明する。また、図２には、上記装置構成における被圧延材の長手方向中央部の断面平均温度の変化を示す。
【００２３】
図１に示す熱延鋼帯の製造ラインにおいて、所定温度に加熱されたスラブを粗圧延機１により所定の厚さの粗バ−に減厚して圧延する。次に圧延された粗バーを、粗圧延機出側直近に設置した中間冷却装置２により仕上圧延で目標とする温度付近への温度調整冷却を行う。温度調整冷却を行った粗バ−を、仕上圧延機入側温度が粗バー全長に亘ってＡｒ３以下の略一定温度となるように加熱装置３で加熱する。次に加熱装置３で加熱された粗バーを仕上圧延機４により所定の厚さの熱延鋼帯に減厚して圧延する。このとき被圧延材先端から後端までの圧延速度を一定とすることにより、被圧延材の長手方向に略一定の圧延加工に伴う加工発熱を生じさせ、被圧延材の先端から後端まで同一の圧延スタンドでフェライトからオーステナイトに逆変態させている。
【００２４】
上記の被圧延材先端から後端までの圧延速度を一定とするとは、仕上圧延機４の全圧延スタンドについての圧延速度をそれぞれ略一定とすることである。
【００２５】
ここで、加工発熱での温度上昇は３０〜４０℃が限界で、かろうじて逆変態を生じさせられる程度である。一方、組織微細化の観点からは、逆変態前の温度をできるだけ低くし、フェライト相の比率を多くしておいた後に逆変態させることが望ましい、つまり、粗圧延後の中間冷却ではより低温にする方が微細化鋼の製造に有利となる。ところが、中間冷却での温度を低くすると逆変態温度までの昇温量が大きくなり、加工発熱だけでは不足する。そこで図１に示すように加熱装置によりある程度の加熱を行い、仕上圧延機のいずれかの圧延スタンド通過の際に、加工発熱で逆変態が生じる温度に仕上圧延機入側温度を制御すればよい。
【００２６】
具体的には、例えば、第５スタンドで逆変態を生じさせる場合、仕上圧延機入側から第５スタンド圧延終了までに被圧延材が受ける冷却などによる温度降下ΔＴｃ、加工発熱および摩擦発熱による温度上昇ΔＴｈは計算による予測が可能で、被圧延材の逆変態温度をＡｃ３、加熱装置の入側温度をＴとすると、加熱装置での目標昇温温度ΔＴは下記の（１）式で表される。
【００２７】
ΔＴ＝Ａｃ３−（ΔＴｈ−ΔＴｃ）−Ｔ …（１）
さらに、上記の仕上圧延においては、逆変態を生じさせるスタンドにおける被圧延材の温度と加工発熱量を長手方向にほぼ一定に保つ必要があるため、加速圧延は行なわないで被圧延材先端から後端までの圧延速度を一定として圧延する。
【００２８】
その後、熱延鋼帯を急速冷却するための急速冷却装置５により仕上圧延の最終圧延パス終了直後に５０℃／秒以上の冷却速度で急速冷却し、巻取り温度を調整するための冷却装置６で温度調整後、巻取り機７で鋼帯を巻取る。
【００２９】
図２に示す被圧延材の長手方向中央部の断面平均温度の変化において、粗バーが中間冷却装置を出て、加熱装置に入る間に若干温度が下がる区間を「搬送」と表示する。この他の「粗圧延機」、「冷却装置」、「急速加熱」、「仕上圧延機」、「急速冷却」の表示は、それぞれ図１に示す熱延鋼帯の製造ラインにおいて粗圧延機１、中間冷却装置２、加熱装置３、仕上圧延機４、急速冷却装置５でのプロセスに対応している。
【００３０】
【実施例】
図１に示す熱延鋼帯の製造ラインを用いて、板厚２５０ｍｍの低炭素鋼スラブを加熱炉にて約１１００℃に加熱後、該スラブを粗圧延機での５パスの圧延にて５０ｍｍまで減厚し、粗バーとした。この粗バーを中間冷却装置により８３０℃程度まで冷却して、Ａｒ３（８６０℃）以下の温度とした後、仕上圧延機の入側温度が時間の経過とともに低下する温度降下を誘導加熱装置により補償し、仕上圧延機入側の被圧延材温度を全長に亘ってほぼ一定の８５０℃に制御して、被圧延材の先端から後端にかけて全圧延スタンドでの圧延速度をそれぞれ一定速度にて７パスの仕上圧延を行った。このときの圧延条件は、仕上板厚４ｍｍ、仕上圧延速度１０００ｍｐｍとした。
【００３１】
また、この鋼種の逆変態温度が８７５℃であったため、逆変態の生じるスタンドを５スタンド目とし、このスタンドでの圧延後の温度が８８０℃となるよう、３スタンド目から５スタンド目にかけての被圧延材の圧下率を調整した。仕上圧延速度を一定にして圧延を行ったところ、各スタンドの温度は以下に示す値で被圧延材の全長に亘りほぼ一定であった。
【００３２】
これに対し、従来の方法による仕上圧延４ｍｍの熱延鋼帯の仕上圧延として、仕上圧延機入側での加熱を行わず、被圧延材先端から後端へ圧延が進行するにつれて最終スタンドの圧延速度を９００ｍｐｍから１１００ｍｐｍまで加速する条件での圧延を行った。被圧延材の先端では５スタンド目の圧延温度が８８０℃となっていたが、後端になるにしたがって温度低下が生じ、被圧延材全長のほぼ半分まで圧延したときに圧延温度が８７５℃まで低下した。さらに後端付近では５スタンド目における被圧延材の温度は８７０℃まで低下した。
【００３３】
以上の条件で製造した本発明と比較例の熱延鋼帯から、先端、中央、後端の３箇所でサンプルを切り出し、それぞれのフェライト粒径を調査した。その結果、本発明の熱延鋼帯では圧延方向のフェライト粒径がほぼ２μｍで一定であったのに対して、比較例の熱延鋼帯ではフェライト粒径が圧延方向で２〜９μｍまで変化しており、先端で微細粒、後端では５μｍ以上のフェライト粒径の組織となっていた。つまり比較例の熱延鋼帯では先端以外は逆変態してなかったと考えられる。
【００３４】
【発明の効果】
以上説明したように、本発明によれば、合金元素を添加することなく、粒径３μｍ以下の極微細なフェライト組織を有する熱延鋼板が製造でき、高強度・高靭性を有する鋼板の製造が可能となる。
【図面の簡単な説明】
【図１】本発明の実施に供される熱延鋼帯の製造設備列の一実施形態を示す説明図
【図２】図１に示す装置構成における被圧延材の長手方向中央部の断面平均温度の変化を示すグラフ
【符号の説明】
１ 粗圧延機
２ 中間冷却装置
３ 加熱装置
４ 仕上圧延機
５ 急速冷却装置
６ 冷却装置
７ 巻取り機[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a hot-rolled steel strip having a fine ferrite structure having an average particle diameter of 3 μm or less and having excellent strength and toughness.
[0002]
[Prior art]
Many rolling methods for causing reverse transformation during tandem rolling have been conventionally proposed.
[0003]
For example, Japanese Patent Application Laid-Open No. H11-152522 discloses that a ferrite (α) is inversely transformed into austenite (γ) by utilizing the heat generated during processing in the finish rolling of continuous hot rolling and / or the temperature rise due to reheating of a steel sheet. A method for producing a thin steel sheet excellent in deep drawability, characterized by the above, is disclosed.
[0004]
Japanese Patent Publication No. Hei 7-9033 discloses that a continuous cast slab or ingot is cooled from a hot slab or a hot lump state, and (a) is subjected to rolling at a total reduction rate of 30% or more in a temperature range of three points or less of Ar. (B) Subsequently, the temperature is raised to a temperature range of from Ac 3 points to [Ac 3 points + 100 ° C.] at a heating rate of 10 ° C./sec or more to cause reverse transformation from ferrite to austenite. (C) A method for producing an ultrafine-structured steel sheet is characterized in that rolling is performed in a phase temperature region at a total reduction rate of 10% or more. Furthermore, after rolling the continuous cast slab or ingot in a hot slab or hot lump state or in a heating furnace and then performing rolling at a total reduction rate of 30% or more in a recrystallization temperature range, this is cooled, (A) Rolling is performed with a total draft of 30% or more in a temperature range of Ar 3 or less. (B) Subsequently, a heating rate of 10 ° C./sec or more is applied to a temperature range of Ac 3 points to [Ac 3 points + 100 ° C.]. (C) rolling at a total reduction of 10% or more in the austenite phase temperature range; and sequentially working and cooling. , A method for producing an ultrafine-structured steel sheet.
[0005]
Japanese Patent Application Laid-Open No. 10-8142 discloses that, by weight, C: 0.0005 to 0.0150% Si: 0.2% or less Mn: 0.05 to 0.6% P: 0.02% or less S: 0.02% or less Al: 0.15% or less N: 0.02% or less, and if necessary, Nb: 0.003 to 0.020% Ti: 0.003 to 0.020% B: 0 0.0002% to 0.0020% Cu: 0.5% or less Ni: 0.5% or less Cr: 0.5% or less Mo: One or more of 0.2% or less, the balance being Fe and Hot rough rolling is performed on a steel slab with a composition consisting of impurities to avoid total rolling reduction of 80% or more, of which the final pass is 20% or more. When passing through any of the rolling stands in the rolling mill row, reverse transformation occurs due to the heat generated by rolling. Finish rolling temperature is completed so that the Ar 3 -50 ° C. or more, to obtain a hot rolled steel strip by winding at a temperature of 550 to 750 ° C., descaling against heat-rolled steel strip, cold rolled A method for producing a steel sheet for cans with good workability and without roughening, characterized by performing recrystallization annealing and temper rolling at 30% or less.
[0006]
[Problems to be solved by the invention]
In the above prior art, any rolling method that causes reverse transformation during tandem rolling is shown. However, although the rolling reduction and the temperature range are specified, uniform rolling is performed over the entire length of the material to be rolled. No specific method for producing the steel sheet is disclosed.
[0007]
Generally, the temperature of the material to be rolled at the entrance to the finishing mill is higher at the leading end and lower toward the rear end due to the temperature drop of the material due to radiation and convection. In order to compensate for this, in tandem rolling, accelerated rolling in which the rolling speed is accelerated toward the rear end is performed.
[0008]
In order to generate the reverse transformation stably from the front end to the rear end of the material to be rolled at a specific stand of the tandem finish rolling mill, the temperature of the material to be rolled and the calorific value of the work at the stand are kept substantially constant in the longitudinal direction. There is a need. However, in ordinary tandem rolling, although the temperature of the material to be rolled is constant at the final stand, it varies depending on the rolling position in other stands, and the temperature of the material during rolling is more remarkable in the former stand. In addition, when the accelerated rolling is performed, the strain rate of the rolling process is gradually increased, so that the deformation resistance of the material is increased, and the calorific value of the process is increased.
[0009]
Taking all these phenomena into account, it is extremely difficult to set the rolling conditions so that the reverse transformation occurs stably from the leading end to the trailing end of the rolled material at the same specific rolling stand. A steel sheet having a microstructure by reverse transformation over the entire length of the steel sheet could not be manufactured.
[0010]
The present invention solves the above-mentioned problems of the prior art, and manufactures a hot-rolled steel strip capable of stably manufacturing a hot-rolled steel strip having a microstructure by reverse transformation over the entire length of a rolled material. The aim is to provide a method.
[0011]
[Means for Solving the Problems]
That is, the method for producing a hot-rolled steel strip of the present invention has the following features.
[0012]
(1) When a hot-rolled steel strip is manufactured by finish rolling by a continuous tandem finishing mill, a heating device for heating the material to be rolled is installed on the entrance side of the finishing mill, and the finishing mill for the material to be rolled is installed. By controlling the entry temperature to a substantially constant temperature of Ar 3 or less in the longitudinal direction and keeping the rolling speed from the leading end to the trailing end of the material to be rolled, a substantially constant rolling process in the longitudinal direction of the material to be rolled is achieved. A method for producing a hot-rolled steel strip, which generates accompanying heat generated by processing and reversely transforms ferrite to austenite in the same rolling stand from the front end to the rear end of the material to be rolled.
[0013]
(2) The method for producing a hot-rolled steel strip according to the above (1), wherein the rapid cooling immediately after the final rolling pass of the finish rolling is performed at a cooling rate of 50 ° C./sec or more.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Drawing 1 is an explanatory view showing one embodiment of a manufacturing equipment row of a hot-rolled steel strip used for carrying out the present invention.
[0015]
The hot-rolled steel strip production line shown in FIG. 1 is installed near a rough rolling mill 1 for rolling a slab heated to a predetermined temperature into a rough bar having a predetermined roughness, and in the vicinity of a rough rolling mill outlet side. An intermediate cooling device 2, a heating device 3 for heating the rough bar, and a rough bar whose temperature is controlled to be constant in the longitudinal direction by the heating device 3 are rolled into a hot-rolled steel strip having a predetermined thickness. Mill 4 for cooling the hot-rolled steel strip, a cooling device 6 for adjusting the winding temperature, and a cooling device 6 for cooling the hot-rolled steel strip, which is located immediately adjacent to the exit side of the finishing mill. And a winding machine 7 for winding a steel strip.
[0016]
The intermediate cooling device 2 is installed in the vicinity of the exit side of the rough rolling final rolling stand to prevent grain growth after the rough rolling, and to reduce the temperature of the material to be rolled into the finishing mill at the entry side of Ar 3 or less in the longitudinal direction. This is a cooling device that performs temperature-adjusted cooling before a constant temperature is reached.
[0017]
The recrystallized austenite grains produced under the rough rolling reduction usually show large grain growth behavior during several tens of seconds between the rough rolling and the finish rolling. Since the grain size of the ferrite structure formed by transformation from austenite becomes smaller as the austenite grain size becomes smaller, and in order to keep the austenite grain size remaining at a temperature of Ar 3 or less as small as possible, it is necessary to immediately cool the steel after rough rolling. desirable.
[0018]
The heating device 3 heats the material to be rolled, the temperature of which is controlled and cooled by the intermediate cooling device 2, and controls the temperature of the material to be rolled into the finishing mill at the longitudinal direction to a substantially constant temperature of Ar 3 or less in the longitudinal direction. It is a heating device. If an induction heating device is used as such a heating device, the control responsiveness is good, and the temperature of the coarse bar on the entry side of the finishing mill 4 can be heated to a constant temperature over its entire length in the longitudinal direction. .
[0019]
The rapid cooling device 5 is a cooling device for suppressing grain growth by rapid cooling immediately after rolling, increasing the rate of growth of transformation nuclei into ferrite, and obtaining a finer structure. It is preferable to perform the rapid cooling immediately after the final rolling pass of the finish rolling at a cooling rate of 50 ° C./sec or more.
[0020]
In order to enable rapid cooling immediately after the rolling mill, it is desirable to arrange as close as possible to the exit side of the rolling mill. In the embodiment of FIG. 1, the rapid cooling device is arranged immediately near the exit side of the finishing mill.
[0021]
In addition, from the viewpoint of material adjustment, the temperature after winding by the winder 7 is also important. In the embodiment of FIG. 1, the cooling device 6 for adjusting the winding temperature is provided immediately before the winder 7. Has been installed.
[0022]
Hereinafter, an embodiment of the method of the present invention using the above-described apparatus configuration will be described. FIG. 2 shows the change in the cross-sectional average temperature at the center in the longitudinal direction of the material to be rolled in the above-described apparatus configuration.
[0023]
In the production line of a hot-rolled steel strip shown in FIG. 1, a slab heated to a predetermined temperature is reduced to a rough bar having a predetermined thickness by a rough rolling mill 1 and rolled. Next, the rolled rough bar is subjected to temperature adjustment and cooling to near the target temperature in finish rolling by the intermediate cooling device 2 installed in the immediate vicinity of the rough rolling mill outlet side. The rough bar which has been subjected to the temperature control cooling is heated by the heating device 3 so that the temperature on the finishing mill entry side is substantially constant at not more than Ar 3 over the entire length of the rough bar. Next, the rough bar heated by the heating device 3 is reduced in thickness to a hot-rolled steel strip having a predetermined thickness by the finish rolling mill 4 and rolled. At this time, by making the rolling speed from the front end to the rear end of the material to be rolled constant, processing heat is generated due to substantially constant rolling in the longitudinal direction of the material to be rolled, and the same heat is generated from the front end to the rear end of the material to be rolled. Reverse transformation from ferrite to austenite at a rolling stand.
[0024]
To make the rolling speed from the front end to the rear end of the material to be rolled constant means to make the rolling speeds of all the rolling stands of the finishing mill 4 substantially constant.
[0025]
Here, the temperature rise due to the heat generated during processing is limited to 30 to 40 ° C., which is a degree at which reverse transformation can be barely caused. On the other hand, from the viewpoint of refining the structure, it is desirable to lower the temperature before the reverse transformation as much as possible and to reverse the transformation after increasing the ratio of the ferrite phase, that is, to lower the temperature in the intermediate cooling after the rough rolling. This is advantageous for the production of micronized steel. However, if the temperature in the intermediate cooling is lowered, the amount of temperature rise up to the reverse transformation temperature becomes large, so that the heat generated by processing alone is insufficient. Therefore, as shown in FIG. 1, a certain degree of heating is performed by a heating device, and when passing through any one of the rolling stands of the finishing rolling mill, the temperature on the finishing rolling mill entrance side may be controlled to a temperature at which reverse transformation occurs due to processing heat. .
[0026]
Specifically, for example, when the reverse transformation is caused in the fifth stand, the temperature drop ΔTc due to cooling or the like to the material to be rolled from the entrance of the finishing mill to the end of the fifth stand rolling, the temperature due to processing heat and frictional heat The rise ΔTh can be predicted by calculation. Assuming that the reverse transformation temperature of the material to be rolled is Ac 3 and the inlet side temperature of the heating device is T, the target heating temperature ΔT in the heating device is expressed by the following equation (1). Is done.
[0027]
ΔT = Ac 3 − (ΔTh−ΔTc) −T (1)
Furthermore, in the above-mentioned finish rolling, since it is necessary to keep the temperature of the material to be rolled and the calorific value of the processing at the stand that causes the reverse transformation almost constant in the longitudinal direction, the rolling from the leading end of the material to be rolled is performed without performing the accelerated rolling. Rolling is performed at a constant rolling speed to the end.
[0028]
Thereafter, the cooling device 5 for rapidly cooling the hot-rolled steel strip at a cooling rate of 50 ° C./sec or more immediately after the end of the final rolling pass of the finish rolling by the rapid cooling device 5 for adjusting the winding temperature. Then, the steel strip is wound by the winder 7.
[0029]
In the change in the cross-sectional average temperature at the center in the longitudinal direction of the material to be rolled shown in FIG. 2, a section where the temperature of the coarse bar slightly decreases while leaving the intermediate cooling device and entering the heating device is indicated as “transport”. The other indications of “rough rolling mill”, “cooling device”, “rapid heating”, “finishing rolling mill” and “rapid cooling” are respectively used in the hot rolling steel strip production line shown in FIG. , An intermediate cooling device 2, a heating device 3, a finishing mill 4, and a rapid cooling device 5.
[0030]
【Example】
Using a hot-rolled steel strip manufacturing line shown in FIG. 1, a low-carbon steel slab having a thickness of 250 mm was heated to about 1100 ° C. in a heating furnace, and then the slab was rolled into a 50 mm rough-rolling machine with 5 passes. To a coarse bar. The coarse bar is cooled to about 830 ° C. by an intermediate cooling device to reach a temperature of Ar 3 (860 ° C.) or less, and then the temperature drop at which the inlet side temperature of the finishing mill decreases over time is reduced by an induction heating device. Compensate and control the temperature of the material to be rolled on the entrance side of the finishing mill to 850 ° C., which is almost constant over the entire length, and set the rolling speed at all rolling stands from the front end to the rear end of the material to be rolled at a constant speed. 7 passes of finish rolling were performed. The rolling conditions at this time were a finished plate thickness of 4 mm and a finish rolling speed of 1000 mpm.
[0031]
Further, since the reverse transformation temperature of this steel type was 875 ° C., the stand where the reverse transformation occurred was set to the fifth stand, and the temperature after the rolling at this stand was set to 880 ° C., so that the stand from the third stand to the fifth stand was set. The rolling reduction of the material to be rolled was adjusted. When rolling was performed at a constant finish rolling speed, the temperature of each stand was substantially constant over the entire length of the material to be rolled at the following values.
[0032]
On the other hand, as the finish rolling of the hot-rolled steel strip of 4 mm in the finish rolling according to the conventional method, the heating at the entrance side of the finishing rolling mill is not performed, and the rolling of the final stand is performed as the rolling progresses from the leading end to the trailing end of the material to be rolled. Rolling was performed under the condition of increasing the speed from 900 mpm to 1100 mpm. The rolling temperature at the 5th stand at the leading end of the material to be rolled was 880 ° C., but the temperature dropped toward the rear end, and the rolling temperature reached 875 ° C. when rolling was performed to almost half of the total length of the material to be rolled. Dropped. Further, near the rear end, the temperature of the material to be rolled at the fifth stand dropped to 870 ° C.
[0033]
From the hot-rolled steel strips of the present invention and the comparative example manufactured under the above conditions, samples were cut out at three points of the front end, the center, and the rear end, and the ferrite grain size of each was investigated. As a result, the ferrite grain size in the rolling direction was constant at approximately 2 μm in the hot-rolled steel strip of the present invention, whereas the ferrite grain size changed from 2 to 9 μm in the rolling direction in the hot-rolled steel strip of the comparative example. The structure had a fine grain at the front end and a ferrite grain size of 5 μm or more at the rear end. That is, it is considered that the hot rolled steel strip of the comparative example did not undergo reverse transformation except at the tip.
[0034]
【The invention's effect】
As described above, according to the present invention, a hot-rolled steel sheet having an ultrafine ferrite structure with a grain size of 3 μm or less can be manufactured without adding an alloying element, and a steel sheet having high strength and high toughness can be manufactured. It becomes possible.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing one embodiment of a hot-rolled steel strip manufacturing facility line used for carrying out the present invention; FIG. Graph showing temperature change [Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Rough rolling mill 2 Intermediate cooling device 3 Heating device 4 Finish rolling mill 5 Rapid cooling device 6 Cooling device 7 Winding machine
