JP4133419B2 - Hot-rolled steel sheet material prediction method, pass schedule correction or setting method, and hot-rolled steel sheet manufacturing method - Google Patents

Description

【００１７】
上記（２）式は、金属材料に内在するエネルギーの減少速度を表しており、金属材料に内在するエネルギーＺを、時間ｔで微分することによって求められる。ここで、時間ｔとは、Ｆｅ原子が金属材料内部を自己拡散する時間（ｓｅｃ）であり、熱間圧延を想定した場合、パス間時間（ｓｅｃ）を代入すればよい。パス間時間とは、当該パスにおいてＺ_inが導入される時刻と次パスにおいてＺ_inが導入される時刻との間の時間である。但し、当該パスが最終パスのときは、最終パスにおいてＺ_inが導入される時刻と計算打ち切り温度に達する時刻との間の時間である。なお、Ｚ_inについては後述する。
【００１８】
式中、Ｔ（絶対温度、単位：Ｋ）は、当該パスの平均圧延温度であり、当該パスにおいてＺ_inが導入される時の圧延温度と次パスにおいてＺ_inが導入される時の圧延温度との平均値である。但し、当該パスが最終パスのときは、最終パスにおいてＺ_inが導入される時刻での圧延温度と計算打ち切り温度を平均して求めた平均圧延温度である。
【００１９】
ここで、計算打ち切り温度とは、熱間圧延終了後、相変態が開始する温度であり、別途変態点測定実験で求めれば良い。但し、一般的な鋼材の相変態は約750℃で起こるので、変態点測定実験を省くときは計算打ち切り温度を750℃とすれば良い。後述する実施例でも、計算打ち切り温度を750℃としている。
【００２０】
式中、Ｑは、Ｆｅ原子が金属材料内部を自己拡散する際に要する活性化エネルギー（291000Ｊ／ｍｏｌ）であり、Ｒは、気体定数（8.3145Ｊ／ｍｏｌ・Ｋ）である。また、ｋは実験によって決まる定数を示しており、定数ｋの求め方は後述する。
【００２１】
前記（２）式の微分方程式を解くと共に、Ｑ＝291000Ｊ／ｍｏｌ、Ｒ＝8.3145Ｊ／ｍｏｌ・Ｋを夫々代入すると、下記（３）式が得られる。
Ｚ＝Ｚ_e×ｅｘｐ［−ｋ×ｔ×ｅｘｐ（35000／Ｔ）］ ・・・（３）
【００２２】
上記（３）式中、Ｚ_eは金属材料に内在するエネルギーであり、エネルギーが開放される前の値である。従って、熱間圧延において当該パスがｎ回目のパスであるときＺ_eは、当該パスまでに金属材料内に残留しているエネルギー（Ｚ_n-1）と、当該パスにおいて導入されるエネルギー（Ｚ_n・_in）の和（Ｚ_n-1＋Ｚ_n・_in）で表すことができる。よって、当該パスにおけるパス間時間をｔ_n、平均圧延温度をＴ_nとすると、前記（３）式は下記（１）式で表される。なお、当該パスが最初パス（第一回目のパス）であれば、当該パスまでに導入された総エネルギー（Ｚ₀）は0となるので、初期値は0である。
Ｚ_n＝（Ｚ_n-1＋Ｚ_n・_in）×ｅｘｐ［−ｋ×ｔ_n×ｅｘｐ（35000／Ｔ_n）］
・・・（１）
【００２３】
本発明では、当該パスにおいて新たに導入されるエネルギーは単位を限定するものではないが、例えば圧延によって導入される相当歪とすれば良い。相当歪は工業的に算出が容易だからである。この場合、当該パスをｎ回目のパスとすると、圧延条件に基づいて例えば下記（４）式で求めることができる。
Ｚ_n・_in＝0.03913−7.43612×10⁵×ｈ_n・_in＋0.00256×ｈ_n・_in×ｒ_n＋0.82293×ｒ_n＋0.94855×（ｒ_n）² ・・・（４）
【００２４】
上記（４）式中、ｒ_n＝（ｈ_n・_in−ｈ_n・_out）／ｈ_n・_inであり、ｈ_n・_inは当該圧延パスにおける入側の板厚（ｍｍ）、ｈ_n・_outは当該圧延パスにおける出側の板厚（ｍｍ）である。
【００２５】
従って、当該パスがｎ回目であるとき、次パスはｎ＋１回目となるので、次パスにおけるＺ値を算出する際には、当該パスにおいて算出したＺ_n値と予め設定されている次パス以降の圧延条件を前記（１）式に代入すると、次パスにおけるエネルギーＺ_n+1値を求めることができる。
【００２６】
＜当該パス＞
Ｚ_n＝（Ｚ_n-1＋Ｚ_n・_in）×ｅｘｐ［−ｋ×ｔ_n×ｅｘｐ（35000／Ｔ_n）］
＜次パス＞
Ｚ_n+1＝（Ｚ_n＋Ｚ_n+1・_in）×ｅｘｐ［−ｋ×ｔ_n+1×ｅｘｐ（35000／Ｔ_n+1）］
【００２７】
この様に計算を繰り返すと、最終パスにおけるＺ_z値を算出することができ、最終パスにおけるＺ_z値は、未知数を一つ（つまり未知数は定数ｋのみ）含む式として算出できる。そして、この未知数ｋに任意の数値を代入し、算出されたＺ_z値と熱延鋼板の材質との関係について検討したところ、良好な相関関係が認められ、このＺ_z値から熱間鋼板の材質を予測できることを本発明者らは見出した。
【００２８】
すなわち、鋼片をＡ_r3点以上の温度域で数段階に分けて熱間圧延することによって製造される熱延鋼板の材質を予測するには、熱間圧延の最終パスを経て得られた熱延鋼板の材質を調べると共に、このときの各パス毎の圧延条件から前記最終パスを経て得られた熱延鋼板に導入されたエネルギーＺ_z値を算出し、前記Ｚ_z値と前記材質の相関関係を予め求めておく。次に、当該相関関係を調べた鋼材と同一鋼種の鋼片を、予め設定されたパススケジュールで製造する際に、最初パスから任意の時点におけるパスが終了するまでのパススケジュールを実際パススケジュールとして記録しておき、前記実際パススケジュールおよび次パス以降の予め設定された予定パススケジュールから当該鋼片の最終パス終了後の熱延鋼板に導入されるエネルギーＺ_z'値を求める。そして、求められたＺ_z'値と、前記Ｚ_z値と前記材質の相関関係を用いてやれば、熱延鋼板の材質を予測できるのである。以下、Ｚ_z値を用いて熱延鋼板の材質を予測する方法について説明する。
【００２９】
圧延条件から熱延鋼板の材質を予測するためには、Ｚ_z値と材質とのデータベースを予め構築する必要がある。このデータベースの構築方法について説明する。
【００３０】
鋼片を任意に定められたパススケジュールで熱間圧延して鋼板を製造する。このとき、最終パス終了までの圧延条件を記録しておき、記録した圧延条件を前記（１）式に代入して最終パスにおけるＺ_z値を算出する。これを同一の鋼種について少なくとも2回以上繰り返し、夫々の場合についてＺ_z値を算出する。但し、算出されたＺ_z値は、何れも定数ｋを未知数として含む関数で表される。なお、鋼板を製造してＺ_z値を算出する回数は、少なくとも2回以上としたけれども、材質予測の精度を高めるためには多い方が良く、好ましくは5回以上、より好ましくは10回以上とするのが望ましい。
【００３１】
一方、得られた熱延鋼板を引張試験して鋼板の材質を測定する。鋼板の材質とは、降伏点（ＹＰ）や引張り強さ（ＴＳ）、あるいは降伏点と引張り強さから求められる降伏比（ＹＲ）などを指す。なお、以下では説明の便のため、鋼板の材質として降伏点（ＹＰ）を取り挙げて説明するが、降伏点の代わりに引張り強さや降伏比などを採用すると、これらの材質を予測できる。
【００３２】
次に、上記算出されたＺ_z値を示す関数中の定数ｋに任意の数値を代入して夫々のＺ_z値を算出し、得られたＺ_z値と降伏点との関係をプロットし、相関係数を夫々算出する。そして、この操作（すなわち定数ｋに任意の数値を代入してＺ_z値を算出すると共に、前記材質との相関係数を算出する操作）を繰り返して、前記相関係数が最も高くなるときの定数ｋを選び出す。前記相関係数を算出する操作は少なくとも2回以上とし、相関係数の算出回数は多い方が信頼性は高くなる。定数ｋには、任意の数値を代入すれば良いが、例えば最終圧延パスにおける圧延温度が750〜1000℃のときに例示したＺ_n・_inの式を用いた場合は、ｋに10⁵〜10²⁰の範囲で数値を代入すれば良い。
【００３３】
決定された定数ｋを前記（１）式に代入すると共に、記録しておいた最終パス終了までの圧延条件を代入してＺ_z値を算出し、Ｚ_z値と前記降伏点との関係をデータベースとする。以下、定数ｋを代入したときのＺ_z値を、説明の便宜上「Ｚ_z・_k」で示す。
【００３４】
次に、前記データベースを構築した鋼材と同一鋼種の鋼片を、予め設定されたパススケジュールで製造する際に、製造される熱延鋼板の材質を予測する方法について説明する。
【００３５】
本発明の材質予測方法で採用することのできる鋼種は、前記データベースを構築したときの鋼材と成分組成は大きく異なってはならない。すなわち、Ｚ_z・_k値は金属材料内部のエネルギーとして算出されるが、このエネルギーは回復や再結晶、粒成長といった冶金現象に伴って金属材料内部から開放されると考えられる。従って、これらの冶金現象は、鋼種が等しければ個々の元素の含有量が若干異なっていても、Ｚ_z・_k値は殆ど変動しないけれども、例えば、Ｎｂ等のマイクロアロイと称される元素が添加されると、微細析出物により回復や再結晶、粒成長が遅延される等、組織内での現象が大きく異なるためエネルギー開放速度も遅くなり、Ｚ_z・_k値は変動する。よって、鋼材に含まれる合金元素種が変わるときは、予備実験でｋ値を改めて設定し、データベースを再構築する必要がある。
【００３６】
一方、熱延鋼板を製造する際のパススケジュールは、前記データベースを構築したときのパススケジュールと同一である必要はない。すなわち、前記データベースは、最終パスにおける金属材料内部のエネルギー（Ｚ_z・_k値）と最終鋼板の材質との関係を示しているので、データベースを構築したときのパススケジュールと当該操業時のパススケジュールが相違していたとしても、金属材料が最終パスにおいて有しているエネルギー（Ｚ_z'・_k）を当該操業時のパススケジュールから算出できれば、このＺ_z'・_k値と前記データベースから材質を予測できるからである。
【００３７】
つまり、予め設定されたパススケジュールの中から任意に選択されたパスまで実際に熱間圧延が行われているときは、最初パスから任意の時点における当該パスまでのパススケジュールを実際パススケジュールとして記録しておき、実際パススケジュールおよび次パス以降の予め設定された予定パススケジュールを前記（１）式に代入すると共に、データベースを構築したときに算出した定数ｋを代入すると、最終パスにおける予測Ｚ_z値を算出することができる。なお、予測して得られたＺ値を、以下、説明の便宜上「Ｚ_z'・_k値」で示す。
【００３８】
そして、このＺ_z'・_k値と前記データベースを用いると、次パス以降を予定パススケジュールで操業したときに製造される熱延鋼板の材質を予測することができる。
【００３９】
ところで、圧延条件が一定であれば、前記（１）式で算出されるＺ_n値も一定となり、このとき得られる材質も等しくなる。しかし、目標材質を得るための圧延条件は一つには決まらず、鋼材の成分組成や圧延温度，圧下率，パス間時間など種々の条件の組み合わせが考えられる。従って、熱間圧延によって得られる材質を安定化させるには、圧延条件を固定すれば良いが、条件を固定すると製造条件に融通が利かなくなり、生産性が低下する原因となる。
【００４０】
そこで、本発明では、任意の時点のパスにおいて前記（１）式を用いてＺ_z'・_k値を算出し、得られる鋼板の材質を前記Ｚ_z'・_k値から予測し、予測した材質と目標材質とを比較して許容範囲外の差があれば、前記データベースを用いて算出される目標材質に対応するＺ_z値（以下、便宜のため「Ｚ_z''・_k値」とする）に近づく様にパススケジュールを修正する。すなわち、予測した材質と目標材質との間に格段の差があれば、夫々の材質に対応するＺ_z値にも差が生じるので、次パス以降のパススケジュールを修正して、予測したＺ_z'・_k値がＺ_z''・_k値に近づく様にパススケジュールを変更すれば良いのである。
【００４１】
予測材質と目標材質との差がどの程度許容できるかは、鋼板の用途によっても変わってくるので一概に定めることはできないが、例えば、建築用鋼板として用いるときは、予測した降伏点と目標とする降伏点の差が30ＭＰａ程度以内であれば許容範囲内と考えられる。
【００４２】
パススケジュールの修正は、各種圧延条件（例えば、圧延温度やパス間時間，パス出側の板厚，圧延荷重，圧延速度など）を制御することで実現できるが、好ましいのはパス間時間を制御する方法である。その理由は、圧延条件の中でも制御しやすいからであり、予測材質に対応するＺ_z'・_k値が、目標材質に対応するＺ_z''・_k値よりも小さい場合は、パス間時間を短くすれば良い。一方、予測材質に対応するＺ_z'・_k値が、目標材質に対応するＺ_z''・_k値よりも大きい場合は、パス間時間を長くすれば良い。なお、パス間時間を長くするには、当該パスにおける圧延が終了した後、次パスにおける圧延を開始するまでの時間を長くすれば良く、パス間時間を短くするには、当該パスにおける圧延が終了した後、次パスにおける圧延を開始するまでの時間を短くすれば良い。また、パス間時間は、圧延速度を変更することによっても制御できるが、パス毎に圧延速度を変更することは実操業では難しい。
【００４３】
Ｚ値を算出して熱延鋼板の材質を予測する時期は特に限定されず、最終パスまでに少なくとも一度行えば良く、各パスにおいてＺ値を算出して得られる材質を適宜予測しても良い。但し、圧延初期（例えば、圧延を8回行うときの1回目や2回目のパス）のみで材質を予測しても、後パス条件によっては目標材質から外れる場合があるので、最終パスの直前パスや二つ前のパスで材質評価方法を適用するのが好まれる。
【００４４】
また、Ｚ値を算出して熱延鋼板の材質を圧延前に予測してやれば、圧延のパススケジュールを新規に設定することもできる。
【００４５】
予め設定されたパススケジュールに沿って鋼片をＡ_r3点以上の温度域で数段階に分けて熱間圧延して鋼板を製造するにあたっては、最初パスにおける熱間圧延条件から任意の時点におけるパスが終了するまでパススケジュールを実際パススケジュールとして記録しておくと共に、次パス以降を、前記予め設定された予定パススケジュールに沿って熱間圧延するときに、前記材質予測方法を適用して、前記実際パススケジュールおよび次パス以降の予定パススケジュールから熱延鋼板の材質を予測し、予測された材質が目標材質の許容範囲内であれば、前記予め設定された予定パススケジュールで引き続き熱間圧延を行い、予測された材質が目標材質の許容範囲外であれば、目標材質に対応するＺ_z値に近づく様に次パス以降のパススケジュールを変更すれば、目標材質を有する熱間圧延を製造することができる。このとき、前記パススケジュールの変更は、パス間時間の制御によって行うのが好ましい理由は、上述した通りである。
【００４６】
本発明における熱間圧延鋼板の製造方法では、種々の鋼種を製造することができるが、上述した様に、鋼材の成分組成が大きく変われば予備実験によって前記（１）式中の定数ｋを算出し直す必要がある。
【００４７】
【実施例】
以下、本発明を実施例によって更に詳細に説明するが、下記実施例は本発明を限定する性質のものではなく、前・後記の趣旨に適合し得る範囲で適当に変更して実施することも可能であり、それらはいずれも本発明の技術的範囲に含まれる。
【００４８】
［データベースの構築］
成分組成が0.16％Ｃ−0.35％Ｓｉ−1.3％Ｍｎ−0.03％Ｖ−残部Ｆｅおよび不可避的不純物（鋼種ＪＩＳ−ＳＮ４９０）の鋼片（初期板厚：250ｍｍ）を、800〜1000℃の温度範囲で熱間圧延した後、大気中で冷却して鋼板（板厚：36ｍｍ）を20枚作製した。なお、前記「％」は、「質量％」の意味であり（以下、同じ）、この成分組成を有する鋼片のＡ_r3点は約750℃である。
【００４９】
熱間圧延時のパス数は5〜10回であり、最初パスから最終パスまでの圧延条件を記録しておくと共に、該圧延条件を前記（１）式に代入してＺ_z値を算出する。
【００５０】
一方、得られた夫々の鋼板について引張試験を行い、降伏点（ＹＰ）を測定した。
【００５１】
前記（４）式の定数ｋに種々の数値を代入してＺ_z値を計算し、得られたＺ_z値と測定した降伏点（ＹＰ）の関係をプロットしたときに相関関係が最も良くなったときのｋ値を定数ｋとする。この実施例では、ｋ＝1.0×10¹⁰であった。得られた定数ｋ（1.0×10¹⁰）を前記（１）式に代入すると共に、最終パスまでの熱延条件を代入するとＺ_z・_k値が算出される。
【００５２】
算出されたＺ_z・_k値と降伏点の間には、下記関係が成り立つ。これをデータベースとする。なお、この式から算出される降伏点を「予測降伏点」と称する。
（予測）降伏点＝49.14×Ｚ_z・_k＋308.5
【００５３】
［材質評価］
成分組成が0.16％Ｃ−0.35％Ｓｉ−1.3％Ｍｎ−0.03％Ｖ−残部Ｆｅおよび不可避的不純物（鋼種ＪＩＳ−ＳＮ４９０）の鋼片（初期板厚：250ｍｍ）を、800〜1000℃の温度範囲で熱間圧延し、圧延後冷却して鋼板（板厚：36ｍｍ）を製造する（圧延No.１〜8）。
【００５４】
実際に鋼板を製造した時の各パスにおける圧延温度Ｔ（℃）、当該パスで導入される相当歪Ｚ_n・_in、パス間時間ｔ_n（ｓｅｃ）を夫々表１に示す。また、上記データベースを構築したときに定めた定数ｋを、前記（１）式に代入して算出したＺ_z・_k値を下記表１または表２に合わせて示す。
【００５５】
次に、算出したＺ_z・_k値と前記データベースを用いて得られる鋼板の材質を予測する。材質として降伏点を予測し、予測した降伏点を「予測ＹＰ」として下記表１または表２に併記する。
【００５６】
一方、実際に製造された鋼材を引張試験して降伏点（ＹＰ）と降伏比（ＹＲ）を測定した。引張試験から得られた降伏点を「実測ＹＰ」、降伏比を「実測ＹＲ」として下記表１または表２に結果を併記する。また、予測ＹＰと実測ＹＰの相関関係をプロットしたものを図１に示す。
【００５７】
【表１】

【００５８】
【表２】

【００５９】
図１から明らかな様に、本発明の材質予測方法を適用すると、実際に製造される鋼板の材質（降伏点）を予測できることが分かる。
【００６０】
【発明の効果】
本発明によれば、変数は定数ｋのみであるので、熱延鋼板を製造するに際し、少ない予備実験で且つ簡単な方法で熱延鋼板の材質を予測することができる。また本発明によれば、熱延時のパススケジュールを適量修正することにより目標材質を有する熱延鋼板を製造できる。さらに、これら材質予測方法および前記パススケジュール修正方法を適用することにより、目標通りの材質を有する熱延鋼板を効率良く確実に製造することができる。
【図面の簡単な説明】
【図１】 予測ＹＰと実測ＹＰの相関関係をプロットしたグラフである。[0001]
BACKGROUND OF THE INVENTION
In the present invention, the steel bill is A_r3More specifically, the present invention relates to a technology for producing a hot-rolled steel sheet by rolling it in several stages in a temperature range above the point, and more specifically, a material prediction method capable of easily predicting the material of the steel sheet obtained by hot rolling, and this material The present invention relates to a pass schedule correction method or setting method for manufacturing a hot-rolled steel sheet having a target material by applying a prediction method, and a method for manufacturing a hot-rolled steel sheet by applying these methods.
[0002]
[Prior art]
The production line for hot-rolled steel sheets includes a heating facility for heating a steel slab, a rolling facility for hot rolling the heated steel slab, and a cooling facility for cooling the steel plate after hot rolling. Generally, the rolling equipment is provided with a rough rolling mill and a finish rolling mill._r3Hot rolling is performed by repeatedly passing through these rolling mills at a point or more (that is, the γ single phase region). Passing through a rolling mill is generally called a “pass”, and the steel slab is rolled to a predetermined steel plate size by a series of passes. A set of rolling conditions (for example, rolling temperature, time between passes, plate thickness on the pass exit side, rolling load, rolling speed, etc.) in a series of passes is referred to as a “pass schedule”.
[0003]
The hot-rolled steel sheet obtained by the said production line is used for various uses, for example, is used as a steel sheet for construction. When used as a steel sheet for construction, characteristics such as a low yield point, strength, and a low yield ratio are required. The hot-rolled steel sheet having such characteristics is desired to have a large absorbed energy from the viewpoint of earthquake resistance. Hereinafter, the yield point may be represented by “YP”, the strength may be represented by “tensile strength” or “TS”, and the yield ratio may be represented by “YR”. The yield ratio is the ratio of the yield point to the tensile strength [YP / TS × 100].
[0004]
As a method for producing a high-strength steel sheet having a low yield ratio, a method has been proposed in which the steel sheet structure is a composite structure such as ferrite + martensite or ferrite + bainite (see, for example, Patent Documents 1 to 3). Moreover, the technique which accelerates cooling to the cooling stop temperature after completion | finish of hot rolling as another method is proposed previously (for example, refer patent documents 4-5). However, since the material of the hot-rolled steel sheet is greatly affected by fluctuations in rolling conditions, the rolling conditions must be strictly controlled. Therefore, a method and an apparatus for predicting the material of the hot-rolled steel sheet in an intermediate path during hot rolling have been proposed. For example, Patent Documents 6 and 7 listed below propose techniques for predicting the material of a hot-rolled steel sheet using an initial state model, a hot working model, a precipitation model, a transformation model, and a structure-material model.
[0005]
In addition, as a material prediction control method for a steel sheet using this material prediction method, the following Patent Document 8 compares the prediction result with the actual production results to obtain the target material, and the conditions of each process are related to the material. A method for predicting and controlling the material is disclosed. However, in these techniques, it is necessary to construct a database in order to predict the material of the hot-rolled steel sheet. However, since there are many variables, preliminary experiments for constructing the database become extremely complicated.
[0006]
[Patent Document 1]
JP 58-93814 ([Claims] etc.)
[Patent Document 2]
JP-A 64-47815 ([Claims] etc.)
[Patent Document 3]
JP-A-5-2171761 ([Claims] etc.)
[Patent Document 4]
JP-A-2-197521 ([Claims] etc.)
[Patent Document 5]
JP-A-5-339631 ([Claims] etc.)
[Patent Document 6]
Japanese Patent No. 2563844 ([Claims] etc.)
[Patent Document 7]
Japanese Patent Publication No. 7-102378 ([Claims] etc.)
[Patent Document 8]
Japanese Patent No. 2509481 ([Claims] etc.)
[0007]
[Problems to be solved by the invention]
The present invention has been made in view of such circumstances, and a first object is to accurately predict the material of a hot-rolled steel sheet by a simple method with few preliminary experiments when manufacturing the hot-rolled steel sheet. Is to provide. A second object is to provide a pass schedule correction method and a setting method for manufacturing a hot-rolled steel sheet having a target material by applying this material prediction method. A third object is to provide a method capable of reliably obtaining a hot-rolled steel sheet having a target material by applying the material prediction method, the path schedule correction method, and the setting method.
[0008]
[Means for Solving the Problems]
The method for predicting the material quality of a hot-rolled steel sheet according to the present invention, which has achieved the first object, refers to a steel slab of A_r3A method for predicting the quality of a hot-rolled steel sheet manufactured by hot rolling in several stages in a temperature range above the point, and examining the material of the hot-rolled steel sheet obtained through the final pass of hot rolling The energy Z introduced into the hot-rolled steel sheet obtained through the final pass from the rolling conditions for each pass at this time_zValue is calculated and Z_zThe correlation between the value and the material is obtained in advance, and when a billet of the same steel type as the steel material for which the correlation has been examined is manufactured according to a preset path schedule, the path at an arbitrary time from the first path is completed. Energy Z to be introduced into the hot-rolled steel sheet after completion of the final pass of the steel slab from the actual pass schedule and the preset scheduled pass schedule after the next pass._{z '}The value of Z_zUsing the correlation between the value and the material, the Z_{z '}The point is that the material of the hot-rolled steel sheet is predicted based on the value.
[0009]
In addition, the first purpose is to change the steel piece to A._r3A method for predicting the quality of a hot-rolled steel sheet manufactured by hot rolling in several stages in a temperature range above the point, and examining the material of the hot-rolled steel sheet obtained through the final pass of hot rolling The energy Z introduced into the hot-rolled steel sheet obtained through the final pass by substituting the rolling conditions for each pass at this time into the following equation (1)_zValue is calculated and Z_zA constant k having the highest correlation between the value and the material is determined, and the determined constant k is substituted into the following equation (1) and the rolling conditions are substituted into Z._z・_kCalculate the value and get Z_z・_kThe relationship between the value and the material is prepared in advance as a database, and when a billet of the same steel type as the steel material for which the relationship has been examined is manufactured according to a preset pass schedule, a pass at an arbitrary time from the first pass Is recorded as an actual path schedule, and the actual path schedule, a preset scheduled path schedule after the next path, and the determined constant k are substituted into the following equation (1). Energy Z introduced into the hot rolled steel sheet after the final pass of the slab_{z '}・_kFind the value and use the database to_{z '}・_kIt can also be achieved by predicting the material of the hot-rolled steel sheet based on the value.
Z_n= (Z_n-1+ Z_n・_in) × exp [−k × t_nXexp (35000 / T_n]]
... (1)
[0010]
In the formula, Z indicates the energy in the hot-rolled steel sheet, Z_n-1Is the energy introduced in the pass immediately before the n pass, and the initial value of Z (Z₀) Is zero. Z_n・_inIs energy introduced into the hot-rolled steel sheet in n passes, and is calculated by the following equation.
Z_n・_in= 0.03913-7.43612 × 10^Five× h_n・_in+ 0.00256 × h_n・_inXr_n+ 0.82293 × r_n+ 0.94855 × (r_n)²
[0011]
In the above formula,
r_n= (H_n・_in-H_n・_out) / H_n・_in
And h_n・_in: Thickness (mm) of entry side in n passes, h_n・_out: Thickness (mm) of exit side in n pass, k: constant, t_n: N-pass time (sec), T_n: Average rolling temperature (K) in n pass is shown respectively. Here, n represents the number of paths, n = 1, 2,..., Z, and z represents the final path.
[0012]
The path schedule correction or setting method according to the present invention that has achieved the second object described above uses the database when the material predicted by the material prediction method is outside the allowable range of the target material. Z corresponding to the target material_{z ''}・_kCalculate the value_{z ''}・_kThe point is that the path schedule is corrected or newly set to approach the value. The correction or new setting of the path schedule is preferably performed by controlling the time between paths.
[0013]
The method for producing a hot-rolled steel sheet according to the present invention, which has achieved the third object, refers to a method of manufacturing a steel slab according to a preset pass schedule._r3It is a method of manufacturing a hot-rolled steel sheet that is manufactured by hot rolling in several stages in a temperature range above the point, and the pass schedule from the first pass to the end of the pass at an arbitrary time is used as the actual pass schedule In addition to recording, when the next pass and after are hot-rolled according to the preset scheduled pass schedule, the material predicting method is applied, and the actual pass schedule and the scheduled pass schedule after the next pass are applied. If the material of the hot-rolled steel sheet is predicted from the above, and the predicted material is within the allowable range of the target material, the hot rolling is continued according to the preset scheduled pass schedule, and the predicted material is the allowable value of the target material. If out of range, Z corresponding to the target material_{z '''}・_kThe point is that hot rolling is performed by correcting the pass schedule after the next pass so as to approach the value. The path schedule is preferably corrected by controlling the time between paths.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
A piece of steel_r3As described above, the material of the hot-rolled steel sheet obtained by hot rolling in several stages in the temperature range above the point varies greatly depending on the rolling conditions. The reason for this is considered that energy is introduced into the metal material as a grain boundary or dislocation in the rolling process. Since the introduced energy is released to the outside of the metal material by a so-called metallurgy phenomenon called so-called recovery or recrystallization, the material changes with time after hot rolling. At this time, the energy reduction (opening) rate is considered to be proportional to the amount of energy inherent in the metal material. The greater the amount of energy, the greater the rate of energy decrease with the passage of time. It is considered that the rate of energy decrease with the course of the time becomes smaller.
[0015]
Therefore, as a result of intensive studies focusing on the relationship between the energy inherent in the metal material and the material of the hot-rolled steel sheet, the inventors have determined that the degree of energy decrease is due to the self-diffusion phenomenon of Fe atoms in the metal material. It was clarified that the rate of energy reduction in the metal material can be expressed by the following equation (2).
[0016]
[Expression 1]

[0017]
The above equation (2) represents the rate of decrease of energy inherent in the metal material, and is obtained by differentiating the energy Z inherent in the metal material with time t. Here, the time t is the time (sec) for Fe atoms to self-diffusion inside the metal material. When hot rolling is assumed, the time between passes (sec) may be substituted. The time between passes is the Z_inIs introduced at the time of the next pass and Z_inIt is the time between the time when is introduced. However, if the path is the final path, Z_inIs the time between the time when is introduced and the time it reaches the calculated censoring temperature. Z_inWill be described later.
[0018]
In the formula, T (absolute temperature, unit: K) is an average rolling temperature of the pass, and Z in the pass_inRolling temperature when Z is introduced and Z in the next pass_inIs the average value with the rolling temperature when. However, if the path is the final path, Z_inIs the average rolling temperature obtained by averaging the rolling temperature at the time when is introduced and the calculated cutoff temperature.
[0019]
Here, the calculated cutoff temperature is a temperature at which the phase transformation starts after the end of hot rolling, and may be obtained separately by a transformation point measurement experiment. However, since the phase transformation of a general steel material occurs at about 750 ° C., when the transformation point measurement experiment is omitted, the calculated cutoff temperature may be set to 750 ° C. In the examples described later, the calculation cutoff temperature is 750 ° C.
[0020]
In the formula, Q is an activation energy (291000 J / mol) required for Fe atoms to self-diffusion inside the metal material, and R is a gas constant (8.3145 J / mol · K). Further, k represents a constant determined by experiments, and a method for obtaining the constant k will be described later.
[0021]
When the differential equation (2) is solved and Q = 291000 J / mol and R = 8.3145 J / mol · K are substituted, the following equation (3) is obtained.
Z = Z_e× exp [−k × t × exp (35000 / T)] (3)
[0022]
In the above formula (3), Z_eIs energy inherent in the metal material, and is a value before the energy is released. Therefore, when the pass is the n-th pass in hot rolling, Z_eIs the energy remaining in the metal material (Z_n-1) And energy introduced in the path (Z_n・_in) Sum (Z_n-1+ Z_n・_in). Therefore, the time between paths in the path is t_nThe average rolling temperature is T_nThen, the formula (3) is expressed by the following formula (1). If the path is the first path (first pass), the total energy introduced up to the path (Z₀) Is 0, so the initial value is 0.
Z_n= (Z_n-1+ Z_n・_in) × exp [−k × t_nXexp (35000 / T_n]]
... (1)
[0023]
In the present invention, the energy newly introduced in the path is not limited in unit, but may be equivalent strain introduced by rolling, for example. This is because the equivalent strain is industrially easy to calculate. In this case, if the pass is the n-th pass, it can be obtained by, for example, the following equation (4) based on the rolling conditions.
Z_n・_in= 0.03913−7.43612 × 10^Five× h_n・_in+ 0.00256 × h_n・_inXr_n+ 0.82293 × r_n+0.94855 x (r_n)²      ... (4)
[0024]
In the above formula (4), r_n= (H_n・_in-H_n・_out) / H_n・_inAnd h_n・_inIs the thickness (mm) of the entry side in the rolling pass, h_n・_outIs the thickness (mm) of the delivery side in the rolling pass.
[0025]
Therefore, when the path is the nth time, the next path is the (n + 1) th time. Therefore, when calculating the Z value in the next path, the Z value calculated in the path is calculated._nBy substituting the value and rolling conditions after the preset next pass into the formula (1), energy Z in the next pass_{n + 1}The value can be determined.
[0026]
<Path concerned>
Z_n= (Z_n-1+ Z_n・_in) × exp [−k × t_nXexp (35000 / T_n]]
<Next pass>
Z_{n + 1}= (Z_n+ Z_{n + 1}・_in) × exp [−k × t_{n + 1}Xexp (35000 / T_{n + 1}]]
[0027]
If the calculation is repeated in this way, Z in the final pass_zValue can be calculated and Z in the final pass_zThe value can be calculated as an expression including one unknown (that is, the unknown is only a constant k). Then, an arbitrary numerical value is substituted for the unknown k, and the calculated Z_zWhen the relationship between the value and the material of the hot-rolled steel sheet was examined, a good correlation was observed._zThe present inventors have found that the material of the hot steel sheet can be predicted from the value.
[0028]
That is, the steel piece is A_r3In order to predict the material of the hot-rolled steel sheet produced by hot rolling in several stages in the temperature range above the point, the material of the hot-rolled steel sheet obtained through the final hot rolling pass is examined. The energy Z introduced into the hot-rolled steel sheet obtained through the final pass from the rolling conditions for each pass at this time_zValue is calculated and Z_zThe correlation between the value and the material is obtained in advance. Next, when manufacturing a steel slab of the same steel type as the steel material for which the correlation has been investigated with a preset pass schedule, the pass schedule from the first pass to the end of the pass at an arbitrary time is taken as the actual pass schedule. The energy Z to be recorded and introduced into the hot-rolled steel sheet after the final pass of the steel slab from the actual pass schedule and the preset scheduled pass schedule after the next pass_{z '}Find the value. And the requested Z_{z '}Value and Z_zIf the correlation between the value and the material is used, the material of the hot-rolled steel sheet can be predicted. Hereinafter, Z_zA method for predicting the material of the hot-rolled steel sheet using the values will be described.
[0029]
In order to predict the material of the hot-rolled steel sheet from the rolling conditions, Z_zIt is necessary to build a database of values and materials in advance. A method for constructing this database will be described.
[0030]
A steel plate is manufactured by hot rolling a billet according to a pass schedule determined arbitrarily. At this time, the rolling conditions up to the end of the final pass are recorded, and the recorded rolling conditions are substituted into the equation (1) to obtain Z in the final pass._zCalculate the value. This is repeated at least twice for the same steel type, and in each case Z_zCalculate the value. However, the calculated Z_zEach value is represented by a function including the constant k as an unknown. In addition, Z_zAlthough the number of times of calculating the value is at least 2 or more, it is better to increase the accuracy of material prediction, preferably 5 or more, more preferably 10 or more.
[0031]
On the other hand, the obtained hot-rolled steel sheet is subjected to a tensile test to measure the material of the steel sheet. The material of the steel sheet refers to the yield point (YP), the tensile strength (TS), or the yield ratio (YR) obtained from the yield point and the tensile strength. In the following, for convenience of explanation, the yield point (YP) is taken as an example of the material of the steel sheet, but these materials can be predicted by adopting tensile strength, yield ratio, or the like instead of the yield point.
[0032]
Next, the above calculated Z_zSubstituting an arbitrary numerical value for the constant k in the function indicating the value, each Z_zCalculate the value and get Z_zThe relationship between the value and the yield point is plotted, and the correlation coefficient is calculated respectively. And this operation (that is, assigning an arbitrary numerical value to the constant k, Z_zThe calculation of the value and the operation of calculating the correlation coefficient with the material are repeated to select the constant k when the correlation coefficient is the highest. The operation for calculating the correlation coefficient is performed at least twice, and the more the correlation coefficient is calculated, the higher the reliability. An arbitrary numerical value may be substituted for the constant k. For example, Z exemplified when the rolling temperature in the final rolling pass is 750 to 1000 ° C._n・_inWhen using this formula, k is 10^Five~Ten²⁰Substituting numerical values within the range of.
[0033]
Substituting the determined constant k into the equation (1), and substituting the recorded rolling conditions up to the end of the final pass, Z_zCalculate the value Z_zThe relationship between the value and the yield point is used as a database. Hereafter, Z when the constant k is substituted_zFor convenience of explanation, set the value to “Z_z・_k".
[0034]
Next, a method for predicting the material of the hot-rolled steel sheet to be manufactured when manufacturing a steel slab of the same steel type as the steel material for which the database has been constructed according to a preset pass schedule will be described.
[0035]
The steel types that can be employed in the material prediction method of the present invention should not differ greatly from the steel material when the database is constructed. That is, Z_z・_kAlthough the value is calculated as the energy inside the metal material, this energy is considered to be released from the inside of the metal material due to metallurgy such as recovery, recrystallization, and grain growth. Therefore, these metallurgical phenomena can be achieved even if the content of each element is slightly different if the steel types are equal._z・_kAlthough the value hardly fluctuates, for example, when an element called microalloy such as Nb is added, the phenomenon in the structure is greatly different, such as recovery, recrystallization, and grain growth delayed by fine precipitates. Therefore, the energy release speed is also slowed down._z・_kThe value varies. Therefore, when the alloying element type contained in the steel material changes, it is necessary to set a new k value in a preliminary experiment and rebuild the database.
[0036]
On the other hand, the pass schedule when manufacturing the hot-rolled steel sheet does not have to be the same as the pass schedule when the database is constructed. That is, the database stores the energy inside the metal material (Z_z・_kValue) and the material of the final steel plate, even if the pass schedule when the database is constructed differs from the pass schedule during the operation, the energy that the metal material has in the final pass (Z_{z '}・_k) Can be calculated from the pass schedule at the time of operation, this Z_{z '}・_kThis is because the material can be predicted from the value and the database.
[0037]
In other words, when hot rolling is actually performed from a preset pass schedule to a pass arbitrarily selected, the pass schedule from the first pass to the relevant pass at any time is recorded as the actual pass schedule. When the actual path schedule and the preset scheduled path schedule after the next path are substituted into the equation (1) and the constant k calculated when the database is constructed is substituted, the predicted Z in the final path is calculated._zA value can be calculated. The Z value obtained by the prediction is hereinafter referred to as “Z” for convenience of explanation._{z '}・_kIndicated by value.
[0038]
And this Z_{z '}・_kBy using the value and the database, it is possible to predict the material of the hot-rolled steel sheet manufactured when the subsequent pass and subsequent operations are performed according to the scheduled pass schedule.
[0039]
By the way, if the rolling conditions are constant, Z calculated by the above equation (1)_nThe value is also constant, and the materials obtained at this time are also equal. However, there is no single rolling condition for obtaining the target material, and combinations of various conditions such as the composition of steel materials, rolling temperature, rolling reduction, and time between passes are conceivable. Therefore, in order to stabilize the material obtained by hot rolling, the rolling conditions may be fixed. However, if the conditions are fixed, the manufacturing conditions are not flexible and the productivity is lowered.
[0040]
Therefore, in the present invention, Z is obtained by using the equation (1) in a path at an arbitrary time._{z '}・_kValue is calculated, and the material of the obtained steel sheet is Z_{z '}・_kIf the predicted material and the target material are compared with each other and there is a difference outside the allowable range, the Z corresponding to the target material calculated using the database is predicted._zValue ("Z" for convenience)_{z ''}・_kCorrect the pass schedule so that it approaches "value". That is, if there is a significant difference between the predicted material and the target material, the Z corresponding to each material_zBecause there is a difference in the value, the predicted Z_{z '}・_kThe value is Z_{z ''}・_kYou can change the pass schedule to get closer to the value.
[0041]
How much difference between the predicted material and the target material can be tolerated depends on the use of the steel sheet, so it cannot be determined unconditionally.For example, when used as a steel sheet for construction, the predicted yield point and target If the difference in yield point is within 30 MPa, it is considered to be within the allowable range.
[0042]
The correction of the pass schedule can be realized by controlling various rolling conditions (for example, rolling temperature, pass-to-pass time, pass exit plate thickness, rolling load, rolling speed, etc.), but it is preferable to control the pass-to-pass time. It is a method to do. The reason is that it is easy to control even in rolling conditions, and Z corresponding to the predicted material._{z '}・_kThe value Z corresponding to the target material_{z ''}・_kIf the value is smaller than the value, the time between passes may be shortened. On the other hand, Z corresponding to the predicted material_{z '}・_kThe value Z corresponding to the target material_{z ''}・_kIf it is greater than the value, the time between passes may be lengthened. In order to lengthen the time between passes, it suffices to lengthen the time from the end of rolling in the pass to the start of rolling in the next pass. To shorten the time between passes, rolling in the pass is performed. What is necessary is just to shorten time until it starts rolling in the next pass after it complete | finishes. The time between passes can also be controlled by changing the rolling speed, but changing the rolling speed for each pass is difficult in actual operation.
[0043]
The time for calculating the Z value and predicting the material of the hot-rolled steel sheet is not particularly limited, and may be performed at least once before the final pass, and the material obtained by calculating the Z value in each pass may be appropriately predicted. . However, even if the material is predicted only at the beginning of rolling (for example, the first and second passes when rolling is performed 8 times), it may be out of the target material depending on the post-pass conditions, so the pass immediately before the final pass. It is preferred to apply the material evaluation method in the previous two passes.
[0044]
  Further, if the Z value is calculated and the material of the hot-rolled steel sheet is predicted before rolling, a rolling pass schedule can be newly set.
[0045]
A slab in accordance with a preset pass schedule_r3When manufacturing steel sheets by hot rolling in several stages in the temperature range above the point, record the pass schedule as an actual pass schedule from the hot rolling conditions in the first pass until the pass at any point in time is completed. In addition, when the subsequent pass and subsequent passes are hot-rolled according to the preset scheduled pass schedule, the material prediction method is applied to perform hot rolling from the actual pass schedule and the scheduled pass schedule after the next pass. If the steel plate material is predicted and the predicted material is within the allowable range of the target material, hot rolling is continued with the preset scheduled pass schedule, and the predicted material is outside the allowable range of the target material. Z corresponding to the target material, if any_zIf the pass schedule after the next pass is changed so as to approach the value, hot rolling having the target material can be manufactured. At this time, the reason why the path schedule is preferably changed by controlling the time between paths is as described above.
[0046]
In the method for producing a hot-rolled steel sheet according to the present invention, various steel types can be produced. As described above, if the component composition of the steel material changes greatly, the constant k in the equation (1) is calculated by a preliminary experiment. It is necessary to redo.
[0047]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.
[0048]
[Database construction]
Steel composition (initial plate thickness: 250 mm) with an ingredient composition of 0.16% C-0.35% Si-1.3% Mn-0.03% V-balance Fe and inevitable impurities (steel grade JIS-SN490) in a temperature range of 800-1000 ° C. After being hot-rolled at 20, it was cooled in the air to produce 20 steel plates (plate thickness: 36 mm). The “%” means “mass%” (hereinafter the same), and A of a steel slab having this component composition_r3The point is about 750 ° C.
[0049]
The number of passes during hot rolling is 5 to 10 times, and the rolling conditions from the first pass to the final pass are recorded, and the rolling conditions are substituted into the equation (1) to obtain Z_zCalculate the value.
[0050]
On the other hand, each obtained steel plate was subjected to a tensile test, and the yield point (YP) was measured.
[0051]
By substituting various numerical values for the constant k in the equation (4), Z_zCalculate the value and get Z_zThe k value when the correlation becomes the best when the relationship between the measured value and the measured yield point (YP) is plotted is the constant k. In this embodiment, k = 1.0 × 10^TenMet. The obtained constant k (1.0 × 10^Ten) In the formula (1) and the hot rolling conditions up to the final pass are substituted._z・_kA value is calculated.
[0052]
Calculated Z_z・_kThe following relationship holds between the value and the yield point. This is a database. The yield point calculated from this equation is referred to as “predicted yield point”.
(Forecast) Yield point = 49.14 x Z_z・_k+308.5
[0053]
[Material evaluation]
Steel composition (initial plate thickness: 250 mm) with an ingredient composition of 0.16% C-0.35% Si-1.3% Mn-0.03% V-balance Fe and inevitable impurities (steel grade JIS-SN490) in a temperature range of 800-1000 ° C. To produce a steel plate (plate thickness: 36 mm) (rolling No. 1 to 8).
[0054]
Rolling temperature T (° C.) in each pass when actually manufacturing the steel sheet, equivalent strain Z introduced in the pass_n・_in, Time between passes t_nTable 1 shows (sec). Further, Z calculated by substituting the constant k determined when the database is constructed into the equation (1)._z・_kValues are shown in Table 1 or Table 2 below.
[0055]
Next, the calculated Z_z・_kThe steel plate material obtained using the value and the database is predicted. The yield point is predicted as the material, and the predicted yield point is also shown in Table 1 or Table 2 below as “Predicted YP”.
[0056]
On the other hand, the steel material actually manufactured was subjected to a tensile test to measure the yield point (YP) and the yield ratio (YR). The results are shown in Table 1 or 2 below, with the yield point obtained from the tensile test as “actually measured YP” and the yield ratio as “actually measured YR”. Moreover, what plotted the correlation of prediction YP and measurement YP is shown in FIG.
[0057]
[Table 1]

[0058]
[Table 2]

[0059]
As is apparent from FIG. 1, it can be seen that the material (yield point) of the steel sheet actually produced can be predicted by applying the material prediction method of the present invention.
[0060]
【The invention's effect】
According to the present invention, since the variable is only the constant k, the material of the hot-rolled steel sheet can be predicted with a few preliminary experiments and a simple method when manufacturing the hot-rolled steel sheet. Moreover, according to this invention, the hot-rolled steel plate which has a target material can be manufactured by correcting the pass schedule at the time of hot-rolling by an appropriate amount. Furthermore, by applying the material prediction method and the pass schedule correction method, a hot-rolled steel sheet having a target material can be efficiently and reliably manufactured.
[Brief description of the drawings]
FIG. 1 is a graph plotting the correlation between predicted YP and measured YP.

Claims (5)

A method for predicting the quality of a hot-rolled steel sheet produced by hot-rolling a steel slab in several stages at a temperature range of _{Ar 3} or higher,
While examining the material of the hot rolled steel sheet obtained through the final pass of hot rolling,
By substituting the rolling conditions for each pass at this time into the following formula (1), the energy Z _z value introduced into the hot-rolled steel sheet obtained through the final pass is calculated,
A constant k that maximizes the correlation between the Z _z value and the material is determined;
Substituting the determined constant k into the following equation (1) and substituting the rolling conditions to calculate the Z _{z · k} value,
The relationship between the obtained Z _{z · k} value and the material is prepared in advance as a database,
When producing a billet of the same steel type as the steel material for which the relationship has been investigated, with a preset pass schedule,
Record the pass schedule from the first pass to the end of the pass at any point in time as the actual pass schedule,
Energy that is introduced into the hot-rolled steel sheet after completion of the final pass of the steel slab by substituting the actual pass schedule and the preset scheduled pass schedule after the next pass and the determined constant k into the following equation (1) Find Z _{z '· k} value,
A material prediction method for a hot-rolled steel sheet, wherein the material of the hot-rolled steel sheet is predicted based on the Z _{z ′ · k} value using the database.
_{Z n = (Z n-1} + Z n · in) × exp [-k × t n × exp (35000 / T n)]
... (1)
Wherein, Z is shows the energy in the hot-rolled steel sheet, Z _n-1 is introduced energy in the immediately preceding pass of n paths, the initial value of Z (Z ₀₎ is zero. Z _{n · in} is energy introduced into the hot-rolled steel sheet in n passes, and is calculated by the following equation.
Z _{n · in} = 0.03913-7.43612 × 10 ⁵ × h _{n · in} + 0.00256 × h _{n · in} × r _n + 0.82293 × r _n + 0.94855 × (r _n ) ²
_{r n = (h n · in} -h n · out) / h n · in
h _{n · in} : Thickness (mm) on the entry side _in n passes
h _n.out : Thickness (mm) on the exit side in n passes
k: constant t _n : time between paths in n paths (sec)
T _n : Average rolling temperature (K) in n passes
Here, n represents the number of paths, n = 1, 2,..., Z, and z represents the final path. ] When the material predicted by the material prediction method according to claim 1 is outside the allowable range of the target material,
Calculate Z _{z ″ · k} value corresponding to the target material using the database,
A path schedule correction or setting method, wherein a path schedule is corrected or newly set so as to approach the Z _{z ″ · k} value. The path schedule correction or setting method according to claim 2 , wherein the path schedule is corrected or newly set by controlling a time between paths. A method of manufacturing a hot-rolled steel sheet manufactured by hot-rolling a steel slab in several stages at a temperature range of _{Ar 3} points or more according to a preset pass schedule,
Record the pass schedule from the first pass until the end of the pass at an arbitrary time as an actual pass schedule,
After the next pass, when hot rolling along the preset scheduled pass schedule,
Applying the material prediction method according to claim 1 , predicting the material of the hot-rolled steel sheet from the actual pass schedule and the scheduled pass schedule after the next pass,
If the predicted material is within the allowable range of the target material, the hot rolling is continued with the preset scheduled pass schedule,
If the predicted material is outside the allowable range of the target material, hot rolling is performed by correcting the pass schedule after the next pass so as to approach the Z _{z ′ ″ · k} value corresponding to the target material. A method for manufacturing a hot-rolled steel sheet. The manufacturing method according to claim 4 , wherein the correction of the pass schedule is performed by controlling the time between passes.

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