JP3716638B2 - Method for producing high-tensile hot-rolled steel strip having ferrite + bainite structure - Google Patents
Method for producing high-tensile hot-rolled steel strip having ferrite + bainite structure Download PDFInfo
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Description
【０００１】
【発明の属する技術分野】
本発明は、伸びフランジ性に優れた高張力熱延鋼帯、特に、フェライト＋ベイナイト組織を有し、強度が４００〜５９０ＭＰａの高張力熱延鋼帯の製造方法に関する。
【０００２】
【従来の技術】
近年、自動車用の構造部材には、省エネルギーのための軽量化や安全性向上のための高強度化が要請されており、高張力鋼板の適用される機会が増加している。なかでも、ロアアーム、メンバー類、ホイール類などの自動車足廻り部品には、伸びフランジ成形を主体とする過酷な成形を受けることから、伸びフランジ性に優れたフェライト＋ベイナイト組織を有する熱延鋼板が使用される場合が多い。
【０００３】
このフェライト＋ベイナイト組織を有する熱延鋼板の製造方法としては、熱延後の鋼帯を連続焼鈍設備などを用いて熱処理する方法もあるが、コスト的に有利な熱延ままで製造する方が望ましく、特公昭６２−３７０８９号公報、特公平７−７４３７８号公報、特公平８−２６４０７号公報などには、そのための方法が開示されている。
【０００４】
こうした方法により熱延ままでフェライト＋ベイナイト組織を有する熱延鋼帯を製造する場合、鋼帯内における特性の均一化を図るために仕上温度やその後の冷却条件を厳密にコントロールする必要がある。
【０００５】
【発明が解決しようとする課題】
しかしながら、仕上温度と冷却条件を共に厳密にコントロールすることは難しく、圧延速度を一定にして冷却条件を厳密にコントロールしようとすると仕上温度に、また加速圧延を行って仕上温度を厳密にコントロールしようとすると冷却条件に、大きな変動が生じて鋼帯内における特性の均一化を十分に図れないのが実情である。
【０００６】
本発明は、このような課題を解決するためになされたもので、鋼帯内において均一な特性の得られるフェライト＋ベイナイト組織を有する高張力熱延鋼帯の製造方法を提供することを目的とする。
【０００７】
【課題を解決するための手段】
上記課題は、ｗｔ％で、Ｃ：０．００３〜０．１５％、Ｓｉ：１．５％以下、Ｍｎ：０．５〜２．５％、Ｐ：０．１％以下、Ｓ：０．０１％以下、ｓｏｌ．Ａｌ：０．００５〜０．０８％、Ｎ：０．００８％以下、あるいはさらにＣｕ：０．６ｗｔ％以下、Ｎｉ：０．６ｗｔ％以下、Ｍｏ：０．７ｗｔ％以下、Ｓｎ：０．１ｗｔ％以下、Ｎｂ：０．０６ｗｔ％以下、Ｔｉ：０．１５ｗｔ％以下、Ｖ：０．1ｗｔ％以下、Ｚｒ：０．１ｗｔ％以下、Ｂ：０．０1ｗｔ％以下、Ｃｒ：０．８ｗｔ％以下、Ｗ：０．５ｗｔ％以下、Ｃａ：０．００６ｗｔ％以下、ＲＥＭ：０．１ｗｔ％以下のうちから選ばれた少なくとも１種の元素を含み、残部Ｆｅおよび不可避的不純物からなる鋼の粗バーまたは薄スラブを製造する工程と、前記粗バーまたは薄スラブを加熱することにより、１００〜８５０ｍｐｍの速度範囲の一定圧延速度で、鋼帯内の仕上温度をＡｒ3変態点〜（Ａｒ3変態点＋５０℃）の温度範囲に納めて仕上圧延する工程と、前記仕上圧延後の鋼帯を５℃／ｓ以上の冷却速度で５５０〜７５０℃の温度範囲に冷却し、前記温度範囲で１秒以上中間保持する工程と、前記中間保持後の鋼帯を５℃／ｓ以上７４℃／ｓ以下の冷却速度で冷却し、３００〜５５０℃の温度範囲に巻取る工程と、を有してなるフェライト＋ベイナイト組織を有する高張力熱延鋼帯の製造方法により解決される。
【０００８】
以下に、成分および製造条件の限定理由について説明する。
Ｃ：０．１５ｗｔ％を超えると硬質・低延性となり、０．００３ｗｔ％未満では所定の強度を得るためには多量の合金元素の添加が必要になりコスト高となる。耐食性が所望される場合には、Cの添加量を０．００３ｗｔ％以上で比較的少量に制限し、Ｐ、Ｃｕなどで補うと効果的である。
【０００９】
Ｓｉ：目標強度レベルに応じて適宜添加する必要があるが、１．５ｗｔ％を超えると溶接性が劣化する。
【００１０】
Ｍｎ：ベイナイト組織生成に必要であり、目標強度レベルに応じて適量添加する必要がある。０．５ｗｔ％未満では所望の組織または強度が得られず、２．５ｗｔ％を超えると溶接性が劣化する。
【００１１】
Ｐ：耐食性が所望される場合は添加が有効であるが、０．１ｗｔ％を超えると低延性・低靭性となる。耐食性が所望されない場合は、０．０１５ｗｔ％以下が好ましい。
【００１２】
Ｓ：０．０１ｗｔ％を超えるとＭｎＳを形成して伸びフランジ性を大きく低下させる。好ましい範囲は０．００５ｗｔ％以下である。
【００１３】
ｓｏｌ．Ａｌ：鋼の脱酸を安定して行うために０．００５ｗｔ％以上必要であるが、０．０８ｗｔ％を超えるとその効果は飽和しコスト高になる。好ましい範囲は０．０１〜０．０５ｗｔ％である。
【００１４】
Ｎ：０．００８ｗｔ％を超えると低延性・低靭性となる。好ましい範囲は０．００５ｗｔ％以下である。
【００１５】
これらの元素の他、耐食性の向上を目的としてＣｕを０．６ｗｔ％以下、Ｎｉを０．６％以下、Ｍｏを０．７ｗｔ％以下、Ｓｎを０．１ｗｔ％以下、析出強化による溶接時のＨＡＺ軟化防止を目的としてＮｂを０．０６ｗｔ％以下、Ｔｉを０．１５ｗｔ％以下、Ｖを０．１ｗｔ％以下、Ｚｒを０．１ｗｔ％以下、焼入れ性改善を目的としてＢを０．０１ｗｔ％以下、Ｃｒを０．８ｗｔ％以下、Ｗを０．５ｗｔ％以下の範囲で添加しても、本発明の効果が損なわれることはない。
【００１６】
また、Ｃａを０．００６ｗｔ％以下、ＲＥＭを０．１ｗｔ％以下の範囲で添加すると伸びフランジ性がさらに改善される。
【００１７】
こうした成分を含有する鋼を仕上圧延するに際しては、鋼を溶製後粗バーあるいは粗バー相当の厚みの薄スラブを製造する必要がある。その製法は特に限定しないが、通常は、鋼を溶製後、連続鋳造あるいは造塊ー分解圧延によりスラブとなし、そのまま直接あるいは加熱炉で再加熱して粗圧延することにより粗バーが、また、溶製後、連続鋳造により粗バー相当の厚みを有する薄スラブが製造される。
【００１８】
スラブの再加熱を行う場合は、スケール欠陥の発生防止や仕上圧延前のオーステナイト粒の微細化を図る上で、１２５０℃以下の加熱が好ましい。
【００１９】
前述したように、鋼帯内において特性の均一なフェライト＋ベイナイト組織を有する高張力熱延鋼帯を製造するには、仕上温度と冷却条件を共に厳密にコントロールする必要がある。
【００２０】
本発明者等がそのための条件を検討したところ、１００〜８５０ｍｐｍの速度範囲の一定圧延速度で、鋼帯内の仕上温度をＡｒ3変態点〜（Ａｒ3変態点＋５０℃）の範囲内に納めて仕上圧延を行うことが必要であり、その手段として粗バーや薄スラブを仕上圧延前に加熱することが有効であることが明らかになった。
【００２１】
圧延速度が１００ｍｐｍ未満では生産性が著しく低下し、８５０ｍｐｍを超えると圧延後の冷却条件の厳密なコントロールが困難になるとともに、圧延中の温度や板厚のコントロールも困難になる。なお、上記一定圧延速度においては、±５０ｍｐｍ程度の速度変動があっても本発明の効果が損なわれることはない。
【００２２】
仕上温度がＡｒ3変態点未満では、加工性が著しく劣化するとともに、鋼帯内において均一な特性が得られない。また、（Ａｒ3変態点＋５０℃）を超えると、加工ひずみが解放されるとともにオーステナイト粒が大きくなりフェライトの核生成サイトが少なくなることから、鋼帯内において均一な特性が得られ難くなるとともに、延性も低下する。
【００２３】
仕上温度をこのような温度範囲に納めるために粗バーや薄スラブを加熱するとき、その温度は粗バーや薄スラブの加熱前の温度や鋼の変態点、圧延速度などに応じて適宜決められる。粗バーや薄スラブには、通常、位置による温度分布が生じているので、その温度分布に応じて粗バーや薄スラブの加熱条件を変えるのが好ましい。
【００２４】
仕上圧延後は、微細なフェライト粒を強度に応じて適正量析出させるために、５℃／ｓ以上の冷却速度で５５０〜７５０℃の温度範囲に冷却し、この温度範囲で１秒以上中間保持する必要がある。
【００２５】
中間保持後は、未変態のオーステナイト相を安定してベイナイト相に変態させるために５℃／ｓ以上の冷却速度で冷却し、３００〜５５０℃の温度範囲で巻取る必要がある。
【００２６】
鋼帯内の仕上温度をＡｒ3変態点〜（Ａｒ3変態点＋３０℃）の温度範囲に納めて仕上圧延すれば、鋼帯内においてより均一な特性が得られる。
【００２７】
３００〜５００ｍｐｍの速度範囲の一定圧延速度で仕上圧延を行えば、より厳密に冷却条件をコントロールできるので、より均一な特性が得られる。
【００２８】
３５０〜５００℃の温度範囲に巻取れば、組織の均一化が進み、安定して所望の特性が得られる。
【００２９】
粗バーまたは薄スラブの加熱は、コイルボックスなどを用いて行ってもよいが、粗バーまたは薄スラブを搬送しながらその幅方向全体を加熱できる粗バー加熱装置により行えば、生産性を損なうことなくより均一な加熱を行える。
【００３０】
なお、コイルボックスで巻かれた粗バーまたは薄スラブを巻き戻して仕上圧延する場合は、巻き戻された後に上記の粗バー加熱装置で加熱するのが効果的である。
【００３１】
粗バーまたは薄スラブの加熱を誘導加熱コイルを用いて行えば、加熱を迅速に行えるので、より生産性をアップできる。
【００３２】
【実施例】
表１に示す成分を有する鋼Ａ〜Ｇを溶製し、連続鋳造により厚さ約２５０ｍｍのスラブを製造し、１２１０℃に加熱後，粗圧延機により厚さ約３０ｍｍの粗バーに圧延した。そして、表２〜４に示す条件で、この粗バーを加熱し、仕上圧延ー冷却ー巻取りを順次行い、板幅８００ｍｍ、板厚１．４〜８．０ｍｍ、強度レベル４００〜５９０ＭＰａのフェライト＋ベイナイト組織を有する熱延鋼帯１〜２２を作製した。なお、粗バー加熱は、誘導コイルタイプの加熱手段を備え、このコイル中に粗バーを通過させながら加熱する粗バー加熱装置を用いて行った。
【００３３】
そして、作製した鋼帯１〜２２の長手方向始端部（Ｔ）、中央部（Ｍ）、終端部（Ｂ）の幅中央部より圧延直角方向に採取した２本のＪＩＳ５号試験片により引張試験を行い、平均値で引張特性を評価した。特に、鋼帯内における特性の均一性は、強度ＴＳのバラツキΔＴＳ、すなわちＴ、Ｍ、Ｂにおける強度の最大値と最小値の差で評価した。同様に、仕上温度のバラツキΔＦＴを、Ｔ、Ｍ、Ｂにおける仕上温度の最大値と最小値の差から求めた。
【００３４】
伸びフランジ性の評価については、圧延鋼帯の幅中央部より採取した１５０×１５０ｍｍの試験片中央にｄ0＝１０ｍｍφの穴を打抜き、これを頂角６０°の円錐ポンチにてバリをポンチ側として押し広げ、穴縁に板厚を貫通して亀裂が入った時点での穴径（ｄf）を測定し、次式により穴拡げ率（λ）を求めた。
穴拡げ率：λ＝（ｄf−ｄ0）／ｄ0×１００ （％）
【００３５】
結果を表２〜４に示す。
【００３６】
本発明例である鋼帯Ｎｏ．１、２、５〜１３、１５〜２０は、いずれもΔＴＳが５０ＭＰａ以下で、長手方向にわたって均一な特性が得られる。
【００３７】
特に、仕上温度がＡｒ3変態点〜（Ａｒ3変態点＋３０℃）の温度範囲にある鋼帯Ｎｏ．１１、１２、１５、２０や圧延速度が３００〜５００ｍｐｍの速度範囲にある鋼帯Ｎｏ．５、６、８、９、１６では、ΔＴＳが３０ＭＰａ以下となり、より均一な特性が得られる。
【００３８】
また、仕上温度がＡｒ3変態点〜（Ａｒ3変態点＋３０℃）の温度範囲にあり、しかも圧延速度が３００〜５００ｍｐｍの速度範囲にある鋼帯Ｎｏ．２、７、１３、１５、１７では、均一性に著しく優れた特性が得られる。
【００３９】
一方、比較例である鋼帯Ｎｏ．３、２１では、仕上温度が圧延終端部で低下するためΔＴＳが大きくなり、また、鋼帯Ｎｏ．４、２２では、空冷時間が圧延終端部で短くなるため降伏比が大きくなり、いずれも長手方向にわたる特性の均一性に劣る。
【００４０】
なお、本発明のいずれの鋼帯もフェライト＋ベイナイト組織を有しており、ベイナイト分率が高いほど高強度になっている。
【００４１】
【表１】
【表２】
【表３】
【表４】
【発明の効果】
本発明は以上説明したように構成されているので、、鋼帯内において均一な特性の得られるフェライト＋ベイナイト組織を有する高張力熱延鋼帯の製造方法を提供できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a high-tensile hot-rolled steel strip excellent in stretch flangeability, particularly a high-tensile hot-rolled steel strip having a ferrite + bainite structure and having a strength of 400 to 590 MPa.
[0002]
[Prior art]
2. Description of the Related Art In recent years, structural members for automobiles are required to be lightened for energy saving and increased in strength to improve safety, and opportunities for applying high-tensile steel sheets are increasing. In particular, automotive undercarriage parts such as lower arms, members, and wheels are subjected to severe forming mainly of stretch flange molding, so hot rolled steel sheets having a ferrite + bainite structure with excellent stretch flangeability are used. Often used.
[0003]
As a method for producing a hot-rolled steel sheet having this ferrite + bainite structure, there is a method of heat-treating a steel strip after hot-rolling using a continuous annealing facility or the like. Desirably, Japanese Patent Publication No. 62-37089, Japanese Patent Publication No. 7-74378, Japanese Patent Publication No. 8-26407, etc. disclose methods for that purpose.
[0004]
When a hot-rolled steel strip having a ferrite + bainite structure is produced while being hot-rolled by such a method, it is necessary to strictly control the finishing temperature and the subsequent cooling conditions in order to achieve uniform characteristics in the steel strip.
[0005]
[Problems to be solved by the invention]
However, it is difficult to strictly control both the finishing temperature and the cooling conditions. If the rolling speed is kept constant and the cooling conditions are strictly controlled, the finishing temperature and the accelerated rolling are used to strictly control the finishing temperature. Then, the actual condition is that the cooling conditions fluctuate and the characteristics in the steel strip cannot be sufficiently uniformed.
[0006]
The present invention has been made to solve such a problem, and an object of the present invention is to provide a method for producing a high-tensile hot-rolled steel strip having a ferrite + bainite structure in which uniform characteristics are obtained in the steel strip. To do.
[0007]
[Means for Solving the Problems]
The above problems are in wt%, C: 0.003 to 0.15%, Si: 1.5% or less, Mn: 0.5 to 2.5%, P: 0.1% or less, S: 0.00. 01% or less, sol. Al: 0.005 to 0.08%, N: 0.008% or less, or Cu: 0.6 wt% or less, Ni: 0.6 wt% or less, Mo: 0.7 wt% or less, Sn: 0.1 wt % Or less, Nb: 0.06 wt% or less, Ti: 0.15 wt% or less, V: 0.1 wt% or less, Zr: 0.1 wt% or less, B: 0.01 wt% or less, Cr: 0.8 wt% or less , W : 0.5 wt% or less, Ca: 0.006 wt% or less, REM: A steel coarse bar comprising at least one element selected from 0.1 wt% or less , the balance being Fe and inevitable impurities Alternatively, the step of producing a thin slab and heating the rough bar or thin slab allows the finishing temperature in the steel strip to be changed from the Ar 3 transformation point to the (Ar 3 transformation point) at a constant rolling speed in the speed range of 100 to 850 mpm. + 50 ° C) Finishing rolling, cooling the steel strip after the finish rolling to a temperature range of 550 to 750 ° C. at a cooling rate of 5 ° C./s or more, and maintaining the intermediate temperature for 1 second or more in the temperature range; The steel strip after intermediate holding is cooled at a cooling rate of 5 ° C./s or more and 74 ° C./s or less and wound up in a temperature range of 300 to 550 ° C., and has a high tensile strength having a ferrite + bainite structure It is solved by a method for manufacturing a hot-rolled steel strip.
[0008]
Below, the reason for limitation of a component and manufacturing conditions is demonstrated.
C: If it exceeds 0.15 wt%, it becomes hard and low ductility, and if it is less than 0.003 wt%, it is necessary to add a large amount of alloy elements in order to obtain a predetermined strength, resulting in an increase in cost. When corrosion resistance is desired, it is effective to limit the addition amount of C to a relatively small amount of 0.003 wt% or more and supplement with P, Cu or the like.
[0009]
Si: It is necessary to add appropriately according to the target strength level, but if it exceeds 1.5 wt%, the weldability deteriorates.
[0010]
Mn: Necessary for formation of a bainite structure, and an appropriate amount needs to be added according to the target strength level. If it is less than 0.5 wt%, a desired structure or strength cannot be obtained, and if it exceeds 2.5 wt%, weldability deteriorates.
[0011]
P: Addition is effective when corrosion resistance is desired, but when it exceeds 0.1 wt%, low ductility and low toughness are obtained. When corrosion resistance is not desired, 0.015 wt% or less is preferable.
[0012]
S: If it exceeds 0.01 wt%, MnS is formed and stretch flangeability is greatly reduced. A preferred range is 0.005 wt% or less.
[0013]
sol. Al: 0.005 wt% or more is necessary to stably perform deoxidation of steel, but if it exceeds 0.08 wt%, the effect is saturated and the cost is increased. A preferred range is 0.01 to 0.05 wt%.
[0014]
N: When it exceeds 0.008 wt%, low ductility and low toughness are obtained. A preferred range is 0.005 wt% or less.
[0015]
In addition to these elements, in order to improve corrosion resistance, Cu is 0.6 wt% or less, Ni is 0.6 wt% or less, Mo is 0.7 wt% or less, Sn is 0.1 wt% or less, and during welding by precipitation strengthening. For the purpose of preventing HAZ softening, Nb is 0.06 wt% or less, Ti is 0.15 wt% or less, V is 0.1 wt% or less, Zr is 0.1 wt% or less, and B is 0.01 wt% for the purpose of improving hardenability. Hereinafter, even if Cr is added in a range of 0.8 wt% or less and W in a range of 0.5 wt% or less, the effect of the present invention is not impaired.
[0016]
Further, when Ca is added in the range of 0.006 wt% or less and REM in the range of 0.1 wt% or less, the stretch flangeability is further improved.
[0017]
When the steel containing these components is finish-rolled, it is necessary to produce a rough bar or a thin slab having a thickness corresponding to the coarse bar after the steel is melted. The production method is not particularly limited. Usually, after melting the steel, it is formed into a slab by continuous casting or ingot-making / decomposition rolling, and the coarse bar is obtained by directly rolling directly or by reheating in a heating furnace. After melting, a thin slab having a thickness corresponding to a coarse bar is produced by continuous casting.
[0018]
When reheating the slab, heating at 1250 ° C. or lower is preferable in order to prevent the occurrence of scale defects and to refine the austenite grains before finish rolling.
[0019]
As described above, in order to produce a high-tensile hot-rolled steel strip having a ferrite + bainite structure with uniform characteristics in the steel strip, it is necessary to strictly control both the finishing temperature and the cooling conditions.
[0020]
When the present inventors examined the conditions for this, the finishing temperature in the steel strip was kept within the range of Ar 3 transformation point to (Ar 3 transformation point + 50 ° C.) at a constant rolling speed in the speed range of 100 to 850 mpm. It has become clear that it is effective to heat the coarse bars and thin slabs before the finish rolling.
[0021]
If the rolling speed is less than 100 mpm, the productivity is remarkably reduced. If it exceeds 850 mpm, it becomes difficult to strictly control the cooling conditions after rolling, and it becomes difficult to control the temperature and thickness during rolling. At the above-mentioned constant rolling speed, the effect of the present invention is not impaired even if there is a speed fluctuation of about ± 50 mpm.
[0022]
If the finishing temperature is less than the Ar 3 transformation point, the workability is remarkably deteriorated and uniform characteristics cannot be obtained in the steel strip. Further, if it exceeds (Ar 3 transformation point + 50 ° C.), the processing strain is released and the austenite grains are enlarged and the nucleation sites of ferrite are reduced, so that it is difficult to obtain uniform characteristics in the steel strip. , Ductility also decreases.
[0023]
When heating a rough bar or thin slab in order to keep the finishing temperature in such a temperature range, the temperature is appropriately determined according to the temperature before heating the rough bar or thin slab, the transformation point of the steel, the rolling speed, etc. . The coarse bar or thin slab usually has a temperature distribution depending on the position. Therefore, it is preferable to change the heating conditions of the coarse bar or thin slab according to the temperature distribution.
[0024]
After finish rolling, in order to precipitate an appropriate amount of fine ferrite grains according to the strength, the steel is cooled to a temperature range of 550 to 750 ° C. at a cooling rate of 5 ° C./s or more, and held at this temperature range for 1 second or longer. There is a need to.
[0025]
After the intermediate holding, in order to stably transform the untransformed austenite phase into a bainite phase, it is necessary to cool at a cooling rate of 5 ° C./s or more and to wind in a temperature range of 300 to 550 ° C.
[0026]
If the finish temperature in the steel strip falls within the temperature range of Ar 3 transformation point to (Ar 3 transformation point + 30 ° C.) and finish rolling is performed, more uniform characteristics can be obtained in the steel strip.
[0027]
If finish rolling is performed at a constant rolling speed in a speed range of 300 to 500 mpm, cooling conditions can be controlled more strictly, and thus more uniform characteristics can be obtained.
[0028]
If it winds in the temperature range of 350-500 degreeC, the structure | tissue homogenization will advance and a desired characteristic will be acquired stably.
[0029]
The coarse bar or thin slab may be heated using a coil box, etc., but if the coarse bar or thin slab is heated by a coarse bar heating device that can heat the entire width direction of the coarse bar or thin slab, productivity will be impaired. More uniform heating.
[0030]
In addition, when rewinding and finishing rolling the rough bar or thin slab wound by the coil box, it is effective to heat with the above-mentioned rough bar heating device after being unwound.
[0031]
If the coarse bar or the thin slab is heated by using the induction heating coil, the heating can be performed quickly, so that the productivity can be improved.
[0032]
【Example】
Steels A to G having the components shown in Table 1 were melted, a slab having a thickness of about 250 mm was produced by continuous casting, heated to 1210 ° C., and then rolled into a rough bar having a thickness of about 30 mm by a roughing mill. Then, this rough bar is heated under the conditions shown in Tables 2 to 4, and finish rolling, cooling, and winding are sequentially performed, and a ferrite having a plate width of 800 mm, a plate thickness of 1.4 to 8.0 mm, and a strength level of 400 to 590 MPa. + Hot-rolled steel strips 1 to 22 having a bainite structure were produced. Note that the coarse bar heating was performed using a coarse bar heating device provided with an induction coil type heating means and heating while passing the coarse bar through the coil.
[0033]
And the tensile test by the two JIS5 test pieces extract | collected from the width direction center part of the longitudinal direction start end part (T), center part (M), and termination | terminus part (B) of the produced steel strips 1-22 in the perpendicular direction of rolling. The tensile properties were evaluated with average values. In particular, the uniformity of characteristics in the steel strip was evaluated by the difference ΔTS in strength TS, that is, the difference between the maximum value and the minimum value of strength in T, M, and B. Similarly, the finishing temperature variation ΔFT was determined from the difference between the maximum and minimum finishing temperatures at T, M, and B.
[0034]
For the evaluation of stretch flangeability, a hole of d 0 = 10 mmφ was punched in the center of a 150 × 150 mm specimen taken from the center of the width of the rolled steel strip, and this was punched on the punch side with a conical punch with an apex angle of 60 ° Then, the hole diameter (d f ) was measured when the crack penetrated through the plate thickness at the hole edge, and the hole expansion ratio (λ) was obtained by the following equation.
Hole expansion rate: λ = (d f −d 0 ) / d 0 × 100 (%)
[0035]
The results are shown in Tables 2-4.
[0036]
Steel strip No. which is an example of the present invention. As for 1, 2, 5-13, 15-20, (DELTA) TS is 50 Mpa or less, and a uniform characteristic is acquired over a longitudinal direction.
[0037]
In particular, the steel strip No. having a finishing temperature in the temperature range of Ar 3 transformation point to (Ar 3 transformation point + 30 ° C.). No. 11, 12, 15, 20 and steel strip No. in which the rolling speed is in the speed range of 300 to 500 mpm. In 5, 6, 8, 9, and 16, ΔTS is 30 MPa or less, and more uniform characteristics are obtained.
[0038]
Further, the steel strip No. having a finishing temperature in the temperature range of Ar 3 transformation point to (Ar 3 transformation point + 30 ° C.) and a rolling speed in the speed range of 300 to 500 mpm. In 2, 7, 13, 15, and 17, characteristics with extremely excellent uniformity can be obtained.
[0039]
On the other hand, steel strip No. which is a comparative example. In Nos. 3 and 21, the finishing temperature decreases at the end of rolling, so ΔTS increases. In Nos. 4 and 22, the air cooling time is shortened at the rolling end portion, so that the yield ratio becomes large, and both are inferior in the uniformity of characteristics in the longitudinal direction.
[0040]
Note that any steel strip of the present invention has a ferrite + bainite structure, and the higher the bainite fraction, the higher the strength.
[0041]
[Table 1]
[Table 2]
[Table 3]
[Table 4]
【The invention's effect】
Since the present invention is configured as described above, it is possible to provide a method for producing a high-tensile hot-rolled steel strip having a ferrite + bainite structure that can obtain uniform characteristics in the steel strip.
Claims (6)
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP25701998A JP3716638B2 (en) | 1998-09-10 | 1998-09-10 | Method for producing high-tensile hot-rolled steel strip having ferrite + bainite structure |
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| Application Number | Priority Date | Filing Date | Title |
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| JP25701998A JP3716638B2 (en) | 1998-09-10 | 1998-09-10 | Method for producing high-tensile hot-rolled steel strip having ferrite + bainite structure |
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| JP3716638B2 true JP3716638B2 (en) | 2005-11-16 |
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| KR101066691B1 (en) | 2008-10-28 | 2011-09-21 | 현대제철 주식회사 | Hot-rolled steel sheet having ultra-high strength and high burring workability, and method for producing the same |
| KR101510272B1 (en) * | 2012-12-27 | 2015-04-08 | 주식회사 포스코 | Method for manufacturing hot rolled steel plate and hot rolled steel sheet |
| KR101767839B1 (en) * | 2016-06-23 | 2017-08-14 | 주식회사 포스코 | Precipitation-hardening hot-rolled steel sheet having excellent uniformity and hole expansion and method for manufacturing the same |
| CN112522574A (en) * | 2019-09-19 | 2021-03-19 | 宝山钢铁股份有限公司 | Thin-strip continuous casting high-reaming steel and production method thereof |
| CN114635079A (en) * | 2022-01-29 | 2022-06-17 | 安阳钢铁股份有限公司 | 650MPa lightweight high-strength steel for rim and production method thereof |
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