JP3716639B2 - Manufacturing method of bainite-based high-tensile hot-rolled steel strip - Google Patents
Manufacturing method of bainite-based high-tensile hot-rolled steel strip 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、強度が５９０ＭＰａ以上の高張力熱延鋼帯、特に、ベイナイト、アシキュラーフェライト、ベイニティックフェライトの単相組織あるいはそれらの混相組織からなるベイナイト系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、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使用される熱延鋼板は、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ベイナイト、アシキュラーフェライト、ベイニティックフェライトの単相組織あるいはそれらの混相組織からなるベイナイト系熱延鋼板が使用される場合が多い。
【０００３】
このベイナイト系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、特開平３−１４６６４０号公報、特開平４−３２５６５７号公報、特開平８−１５７９５７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ベイナイト系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ベイナイト系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：０．０２〜０．１５％、Ｓｉ：０．３〜２．５％、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、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工程と、前記加熱された粗バーまたは薄スラブを１００〜８５０ｍｐｍの速度範囲の一定圧延速度によりＡｒ3変態点以上の仕上温度で仕上圧延する工程と、前記仕上圧延後の鋼帯を１０℃／ｓ以上の冷却速度で７００℃以下に冷却後、３００〜５５０℃の温度範囲に巻取る工程と、を有してなり、しかも前記粗バーまたは薄スラブの加熱により鋼帯内の仕上温度の変動を５０℃以下に納める引張強度が５９０ＭＰａ以上であるベイナイト系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、０．０２ｗｔ％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０．０２ｗｔ％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。０．３ｗｔ％未満では所望の強度が得られず、２．５ｗｔ％を超えると溶接性が劣化する。
【００１０】
Ｍｎ：ベイナイト相生成に必要であり、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する必要があるが、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、中央偏析を避けるため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ベイナイト系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、仕上温度をＡｒ3変態点以上で仕上圧延し、しかも鋼帯内の仕上温度の変動を５０℃以下に納める必要があり、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、フェライト相の晶出によりベイナイト系組織が得られなくなる。仕上温度がＡｒ3変態点以上であれば、その後の冷却速度のコントロールによりベイナイト系組織が得られるため、その上限については特に限定しない。しかし、あまり高いとエネルギーコストが大きくなり、また、冷却速度が十分でなかった場合にフェライト相が晶出するため、Ａｒ3変態点より数十℃上が好ましい。
【００２３】
鋼帯内の仕上温度の変動が５０℃を超えると、再結晶によるオーステナイト粒径の変動が鋼帯圧延方向で大きく異なり、均一な特性が得られない。
【００２４】
鋼帯内の仕上温度の変動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。
【００２５】
仕上圧延後は、オーステナイトからのフェライトの晶出を避けつつ均一な形態のベイナイトに変態させるために、１０℃／ｓ以上の冷却速度で７００℃以下に冷却し、３００〜５５０℃の温度範囲に巻取る必要がある。この時、巻取温度を本発明の成分系のＭｓ点より高くしているためベイナイト変態しか起こらないので、７００℃以下に冷却後巻取りまでの冷却を特にコントロールする必要はないが、ベイナイト系組織をより均一化するために一定速度で冷却することが望ましい。
【００２６】
鋼帯内の仕上温度の変動を３０℃以下に納めて仕上圧延すれば、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】
仕上圧延後の鋼帯を４０℃／ｓ以上の冷却速度で７００℃以下に冷却すれば、均一なベイナイト系組織をより安定して形成できる。
【００２９】
３５０〜５００℃の温度範囲に巻取れば、ベイナイト系組織をより安定して形成できる。
【００３０】
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。
【００３１】
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。
【００３２】
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。
【００３３】
【実施例】
表１に示す成分を有する鋼Ａ〜Ｄ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、表３に示す条件で、この粗バーを加熱し、仕上圧延ー冷却−巻取りを順次行い、板幅８００ｍｍ、板厚１．４〜８．０ｍｍ、強度レベル５９０〜７８０ＭＰａのベイナイト系熱延鋼帯１〜１３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。
【００３４】
そして、作製した鋼帯１〜１３の長手方向始端部（Ｔ）、中央部（Ｍ）、終端部（Ｂ）の幅中央部より採取した２本のＪＩＳ５号試験片（圧延直角方向）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、鋼帯内の特性の均一性は、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。
【００３５】
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 （％）
【００３６】
結果を表２、表３に示す。
【００３７】
本発明例である鋼帯Ｎｏ．１〜３、６〜１１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。
【００３８】
特に、仕上温度の変動が３０℃以下である鋼帯Ｎｏ．２、８や圧延速度が３００〜５００ｍｐｍの速度範囲にある鋼帯Ｎｏ．６、７、９ではであり、ΔＴＳが３０ＭＰａ以下となり、より均一な特性が得られる。
【００３９】
また、仕上温度の変動が３０℃以下であり、しかも圧延速度が３００〜５００ｍｐｍの速度範囲にある鋼帯Ｎｏ．３、１０では、均一性に著しく優れた特性が得られる。
【００４０】
一方、比較例である鋼帯Ｎｏ．４、１２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．５、１３では、加速圧延により冷却条件のコントロールが困難になるためΔＴＳ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。
【００４１】
なお、本発明例のいずれの鋼帯においてもベイナイト系組織を有していることを確認している。
【００４２】
【表１】
【表２】
【表３】
【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ベイナイト系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 is a high-tensile hot-rolled steel strip excellent in stretch flangeability and having a strength of 590 MPa or more, in particular, a bainite-based high-tensile heat composed of a single-phase structure of bainite, acicular ferrite, bainitic ferrite or a mixed phase structure thereof. The present invention relates to a method of manufacturing a steel strip.
[0002]
[Prior art]
In recent years, structural members for automobiles are required to be lightened for energy saving and increased in strength to improve safety, and the opportunity to apply high-tensile steel sheets is increasing. Among them, hot-rolled steel sheets used for automobile undercarriage parts such as lower arms, members, and wheels are subjected to severe forming mainly of stretch flange forming, so bainite and acicular with excellent stretch flangeability. Bainitic hot-rolled steel sheets composed of a single phase structure of ferrite or bainitic ferrite or a mixed phase structure thereof are often used.
[0003]
As a method for producing the bainite-based hot-rolled steel sheet, there is a method of heat-treating the steel strip after hot-rolling using a continuous annealing facility or the like. Japanese Patent Laid-Open No. 3-146640, Japanese Patent Laid-Open No. 4-325657, Japanese Patent Laid-Open No. 8-157957, and the like disclose methods for that purpose.
[0004]
When producing a bainite-based hot-rolled steel strip as it is by such a method, it is necessary to strictly control the finishing temperature and the subsequent cooling conditions in order to achieve uniform properties 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 bainite-based high-tensile hot-rolled steel strip capable of obtaining uniform characteristics in the steel strip.
[0007]
[Means for Solving the Problems]
The above-mentioned problems are wt%, C: 0.02-0.15%, Si: 0.3-2.5%, Mn: 0.5-2.5%, P: 0.1% or less, S : 0.01% or less, sol. A step of producing a rough bar or thin slab of steel comprising Al: 0.005 to 0.08%, N: 0.008% or less, the balance Fe and inevitable impurities, and a step of heating the coarse bar or thin slab When a step of finishing rolling in Ar 3 transformation point or more of the finishing temperature of a constant rolling speed of the speed range of 100~850mpm the heated crude bar or thin slab, the steel strip after the finish rolling 10 ° C. / s After cooling to 700 ° C. or less at the above cooling rate, and winding it in a temperature range of 300 to 550 ° C., and further, by changing the finishing temperature in the steel strip by heating the rough bar or thin slab This can be solved by a method for producing a bainite-based high-strength hot-rolled steel strip having a tensile strength of 590 MPa or more that is kept at 50 ° C. or lower.
[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 0.02 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 0.02 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. If it is less than 0.3 wt%, the desired strength cannot be obtained, and if it exceeds 2.5 wt%, the weldability deteriorates.
[0010]
Mn: Necessary for bainite phase generation, 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: When corrosion resistance is desired, it is necessary to add, 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 in order to avoid central segregation.
[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 manufacture a bainite-based high-tensile hot-rolled steel strip having 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 that purpose, a constant rolling speed in a speed range of 100 to 850 mpm and finish temperature were finish-rolled at an Ar 3 transformation point or more, and the fluctuation of the finish temperature in the steel strip was 50 ° C. or less. It has become clear that it is effective to heat a rough bar or thin slab before finish rolling as a means for this.
[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, a bainite structure cannot be obtained due to crystallization of the ferrite phase. If the finishing temperature is equal to or higher than the Ar 3 transformation point, the upper limit is not particularly limited because a bainite structure can be obtained by controlling the subsequent cooling rate. However, if it is too high, the energy cost is increased, and if the cooling rate is not sufficient, the ferrite phase is crystallized, and therefore, it is preferably several tens of degrees Celsius above the Ar 3 transformation point.
[0023]
If the variation in the finishing temperature in the steel strip exceeds 50 ° C., the variation in the austenite grain size due to recrystallization varies greatly in the steel strip rolling direction, and uniform characteristics cannot be obtained.
[0024]
When heating a rough bar or thin slab to keep the finishing temperature fluctuation in the steel strip within this temperature range, the temperature is the temperature before heating the coarse bar or thin slab, the transformation point of the steel, the rolling speed, etc. It is determined appropriately according to 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.
[0025]
After finish rolling, in order to transform into a uniform form of bainite while avoiding crystallization of ferrite from austenite, it is cooled to 700 ° C. or less at a cooling rate of 10 ° C./s or more, and is in a temperature range of 300 to 550 ° C. It is necessary to wind up. At this time, since the coiling temperature is higher than the Ms point of the component system of the present invention, only bainite transformation occurs. Therefore, it is not necessary to particularly control the cooling after cooling to 700 ° C. or less. It is desirable to cool at a constant rate to make the tissue more uniform.
[0026]
More uniform characteristics can be obtained by finishing and rolling the variation in the finishing temperature in the steel strip to 30 ° C. or less.
[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 the steel strip after finish rolling is cooled to 700 ° C. or less at a cooling rate of 40 ° C./s or more, a uniform bainite structure can be formed more stably.
[0029]
If it winds up in the temperature range of 350-500 degreeC, a bainite type structure | tissue can be formed more stably.
[0030]
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.
[0031]
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.
[0032]
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.
[0033]
【Example】
Steels A to D 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, under the conditions shown in Table 2 and Table 3, this rough bar is heated, and finish rolling-cooling-winding is sequentially performed, with a plate width of 800 mm, a plate thickness of 1.4 to 8.0 mm, and a strength level of 590 to 780 MPa. Bainitic hot-rolled steel strips 1 to 13 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.
[0034]
And it pulled by two JIS No. 5 test pieces (in the direction perpendicular to the rolling direction) taken from the width direction center part of the longitudinal direction start end part (T), center part (M), and end part (B) of the produced steel strips 1-13. Tests were conducted and 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.
[0035]
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 (%)
[0036]
The results are shown in Tables 2 and 3.
[0037]
Steel strip No. which is an example of the present invention. 1 to 3 and 6 to 11 all have ΔTS of 50 MPa or less, and uniform characteristics can be obtained in the longitudinal direction.
[0038]
In particular, the steel strip No. having a finishing temperature variation of 30 ° C. or less. Nos. 2 and 8 and steel strip Nos. With rolling speed in the speed range of 300 to 500 mpm 6, 7 and 9, and ΔTS is 30 MPa or less, and more uniform characteristics can be obtained.
[0039]
Further, the steel strip No. having a finishing temperature variation of 30 ° C. or less and a rolling speed in the speed range of 300 to 500 mpm. In 3 and 10, the characteristic which was remarkably excellent in uniformity is obtained.
[0040]
On the other hand, steel strip No. which is a comparative example. In Nos. 4 and 12, since the finishing temperature decreases at the rolling end portion, ΔTS increases. In 5 and 13, since it becomes difficult to control the cooling conditions by accelerated rolling, ΔTS increases, and both are inferior in the uniformity of characteristics in the longitudinal direction.
[0041]
In addition, it has confirmed that all the steel strips of this invention example have a bainite type | system | group structure.
[0042]
[Table 1]
[Table 2]
[Table 3]
【The invention's effect】
Since this invention is comprised as demonstrated above, the manufacturing method of the bainite type high-tensile hot-rolled steel strip which can obtain a uniform characteristic within a steel strip can be provided.
Claims (8)
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP25702098A JP3716639B2 (en) | 1998-09-10 | 1998-09-10 | Manufacturing method of bainite-based high-tensile hot-rolled steel strip |
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| JP25702098A JP3716639B2 (en) | 1998-09-10 | 1998-09-10 | Manufacturing method of bainite-based high-tensile hot-rolled steel strip |
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| JP3716639B2 true JP3716639B2 (en) | 2005-11-16 |
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| JP3915460B2 (en) * | 2001-09-26 | 2007-05-16 | Ｊｆｅスチール株式会社 | High strength hot rolled steel sheet and method for producing the same |
| JP3954411B2 (en) * | 2002-02-28 | 2007-08-08 | 日新製鋼株式会社 | Manufacturing method of high-strength hot-rolled steel sheet with excellent material uniformity and hole expandability |
| KR100942088B1 (en) | 2005-03-28 | 2010-02-12 | 가부시키가이샤 고베 세이코쇼 | High strength hot rolled steel sheet excellent in bore expanding workability and method for production thereof |
| JP4515370B2 (en) * | 2005-10-06 | 2010-07-28 | 株式会社神戸製鋼所 | High strength steel plate and manufacturing method thereof |
| JP5821929B2 (en) * | 2013-10-29 | 2015-11-24 | Ｊｆｅスチール株式会社 | High-strength hot-rolled steel sheet with excellent material stability and weldability and method for producing the same |
| US20230272499A1 (en) * | 2021-11-24 | 2023-08-31 | United States Steel Corporation | Process for manufacturing high strength steel |
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