JP6624103B2 - High strength hot rolled steel sheet and method for producing the same - Google Patents
High strength hot rolled steel sheet and method for producing the same Download PDFInfo
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Description
本発明は、表面品質、延性亀裂伝播停止特性および低温靱性に優れた高強度熱延鋼板およびその製造方法に関し、特にラインパイプ向け高強度電縫鋼管および高強度スパイラル鋼管の用途に供して好適な高強度鋼板に関する。 The present invention relates to a high-strength hot-rolled steel sheet excellent in surface quality, ductile crack propagation arrestability and low-temperature toughness and a method for producing the same, and particularly suitable for use in high-strength ERW steel pipes and high-strength spiral steel pipes for line pipes. Related to high strength steel sheet.
近年、石油危機以来の原油の高騰や、エネルギー供給源の多様化の要求などから、北海、カナダ、アラスカ等のような極寒地での石油、天然ガスの採掘およびパイプラインの敷設が活発に行われるようになっている。さらに、パイプラインにおいては、天然ガスやオイルの輸送効率向上のため、大径で高圧操業を行う傾向となっている。パイプラインの高圧操業に耐えるため、輸送管（ラインパイプ）は厚肉の鋼管とする必要があり、厚鋼板を素材とするＵＯＥ鋼管が使用されるようになってきている。しかし、最近では、パイプラインの施工コストの更なる低減という強い要望にしたがい、鋼管の材料コスト低減の要求も強く、輸送管として、厚鋼板を素材とするＵＯＥ鋼管に代わり、生産性が高くより安価な、コイル形状の熱延鋼板（熱延鋼帯）を素材とした高強度電縫鋼管、あるいは高強度スパイラル鋼管が用いられるようになってきた。 In recent years, oil prices and natural gas mining and laying of pipelines in arctic regions such as the North Sea, Canada and Alaska have been actively promoted due to the rising prices of crude oil since the oil crisis and the demand for diversification of energy supply sources. It has become to be. Furthermore, in pipelines, there is a tendency to operate large-diameter high-pressure operations in order to improve the transport efficiency of natural gas and oil. In order to withstand high-pressure operation of pipelines, transport pipes (line pipes) need to be made of thick steel pipes, and UOE steel pipes made of thick steel plates have been used. However, recently, in response to the strong demand for further reducing the construction cost of pipelines, there is also a strong demand for reducing the material cost of steel pipes, and instead of UOE steel pipes made of thick steel plates as transport pipes, higher productivity has been achieved. High-strength ERW steel pipes or high-strength spiral steel pipes made of inexpensive coil-shaped hot-rolled steel sheets (hot-rolled steel strips) have come to be used.
これら高強度鋼管には、ラインパイプの破壊を防止する観点から、同時に優れた低温靭性を保持することが要求されている。このような強度と靭性とを兼備した鋼管を製造するために、鋼管素材である鋼板では、熱間圧延（熱延）後の加速冷却を利用した変態強化や、Ｎｂ、Ｖ、Ｔｉ等の合金元素の析出物を利用した析出強化等による高強度化と、制御圧延等を利用した組織の微細化等による高靭性化が図られてきた。また、さらに最近は、極寒地用の鋼管に対しては、パイプラインのバースト破壊を防止する観点から、破壊靭性、特に優れたＣＴＯＤ特性（延性亀裂伝播停止特性）および優れたＤＷＴＴ特性（低温靱性）を具備することが要求される場合がある。 These high-strength steel pipes are required to maintain excellent low-temperature toughness at the same time from the viewpoint of preventing breakage of the line pipe. In order to manufacture a steel pipe having both such strength and toughness, in a steel sheet as a steel pipe material, transformation strengthening using accelerated cooling after hot rolling (hot rolling), and alloys such as Nb, V, and Ti are used. Attempts have been made to increase the strength by precipitation strengthening or the like using elemental precipitates and to increase the toughness by refining the structure using controlled rolling or the like. Further, more recently, for steel pipes for extremely cold regions, from the viewpoint of preventing burst fracture of the pipeline, fracture toughness, particularly excellent CTOD characteristics (ductile crack propagation arresting characteristics) and excellent DWTT characteristics (low temperature toughness). ) May be required.
このような要求に対し、例えば特許文献１には、重量％で、Ｃ：０．０５〜０．１２％、Ｃａ：０．００２０〜０．００６０％を含み、Ｓｉ、Ｍｎ、Ａｌ、Ｐ、Ｓを適正量調整して含む連鋳製スラブに、９５０℃以下で１０〜５０％の圧下を行い、引続き表面の冷却速度が２℃／ｓ以上で表面温度がＡｒ３以下の温度になるまで冷却し、２５０ｓ未満の復熱後、未再結晶領域にて５０％以上の圧延を行い、７２０〜８２０℃の範囲で圧延を終了し、引続いて平均冷却速度５〜３０℃／ｓで冷却した後、４００〜６００℃の範囲で巻取る高靭性耐サワー鋼管用ホットコイルの製造方法が開示されている。特許文献１に記載された技術によれば、耐ＨＩＣ特性と低温靭性の両特性に優れたホットコイルが製造でき、寒冷地でのラインパイプの製造が可能となるとしている。 In response to such a requirement, for example, Patent Document 1 contains C: 0.05 to 0.12% and Ca: 0.0020 to 0.0060% by weight, and contains Si, Mn, Al, P, The continuous cast slab containing S adjusted in an appropriate amount is subjected to a reduction of 10 to 50% at 950 ° C. or less, and then cooled until the surface cooling rate becomes 2 ° C./s or more and the surface temperature becomes Ar 3 or less. Then, after reheating less than 250 s, rolling of 50% or more was performed in the non-recrystallized region, rolling was completed in the range of 720 to 820 ° C, and subsequently cooled at an average cooling rate of 5 to 30 ° C / s. Then, a method for manufacturing a hot coil for a high toughness sour resistant steel pipe wound at a temperature of 400 to 600 ° C. is disclosed. According to the technique described in Patent Document 1, a hot coil excellent in both HIC resistance and low-temperature toughness can be manufactured, and a line pipe can be manufactured in a cold region.
また、特許文献２には、重量％で、Ｃ：０．０１〜０．２０％を含み、Ｓｉ、Ｍｎ、Ａｌ、Ｎを適正量含有する鋼片を、Ａｃ３変態点以上１２５０℃以下に加熱し、９００℃以上の温度での累積圧下率が１０〜８０％の粗圧延を行ったのち、２〜４０℃／ｓの加速冷却を、該冷却速度におけるＡｒ３変態点＋５０℃〜Ａｒ３変態点−５０℃まで行って、加速冷却後、累積圧下率３０〜９０％の仕上げ圧延を６５０℃以上で終了し、さらに仕上げ圧延終了後、５〜４０℃／ｓの冷却速度で２０〜４５０℃まで再び加速冷却する低温靭性に優れた低降伏比高張力鋼材の製造方法が記載されている。特許文献２に記載された技術によれば、複雑な熱処理工程を必要とすることなく、低降伏比と、優れた低温靭性とを両立させた熱延鋼板を製造することができるとしている。 Further, in Patent Document 2, a steel slab containing C: 0.01 to 0.20% by weight and containing appropriate amounts of Si, Mn, Al, and N is heated to a temperature from the Ac3 transformation point to 1250 ° C. or less. Then, after performing rough rolling at a temperature of 900 ° C. or more with a cumulative draft of 10 to 80%, accelerated cooling at 2 to 40 ° C./s is performed at the cooling rate at the Ar3 transformation point + 50 ° C. to Ar3 transformation point− Performed to 50 ° C., accelerated cooling, finished finish rolling at a cumulative rolling reduction of 30 to 90% at 650 ° C. or higher, and after finishing rolling, again at a cooling rate of 5 to 40 ° C./s to 20 to 450 ° C. A method for producing a high yield strength low tensile strength steel material excellent in low temperature toughness by accelerated cooling is described. According to the technique described in Patent Literature 2, a hot-rolled steel sheet having both a low yield ratio and excellent low-temperature toughness can be manufactured without requiring a complicated heat treatment step.
また、特許文献３には、質量百分率でＣ：０．０１〜０．１０％、Ｎｂ：０．０１〜０．１％を含み、Ｓｉ、Ｍｎ、Ｐ、Ｓ、Ｎを適正量含み、かつＭｎ／Ｓｉ：５〜８を満足するように調整した鋼片に、１１００℃以上で行う最初の圧下率：１５〜３０％、１０００℃以上での合計圧下率：６０％以上、最終圧延の圧下率：１５〜３０％の条件下で粗圧延を行い、５℃／ｓ以上の冷却速度で鋼板表層部をＡｒ３点以下まで冷却し、復熱または強制加熱により、表層部の温度が（Ａｃ３−４０℃）〜（Ａｃ３＋４０℃）となった時点で仕上圧延を開始し、９５０℃以下の合計圧下率：６０％以上の条件で仕上圧延を終了し、ついで２ｓ以内に冷却を開始し、１０℃／ｓ以上の速度で６００℃以下まで冷却し、６００〜３５０℃の範囲で巻き取る低温靭性及び溶接性に優れた高強度電縫鋼管用熱延鋼板の製造方法が開示されている。特許文献３に記載された技術によれば、高価な合金元素を添加することなく、また鋼管全体を熱処理する必要なしに、低温靭性および溶接性に優れた高強度電縫鋼管を製造することができるとしている。 Patent Literature 3 contains C: 0.01 to 0.10% and Nb: 0.01 to 0.1% by mass percentage, contains Si, Mn, P, S, and N in appropriate amounts, and Mn / Si: Initial reduction at 1100 ° C. or higher: 15 to 30% for steel slabs adjusted to satisfy 5 to 8; total reduction at 1000 ° C. or higher: 60% or higher; final rolling reduction Rate: 15 to 30%, rough rolling is performed, the surface layer of the steel sheet is cooled to an Ar3 point or less at a cooling rate of 5 ° C./s or more, and the temperature of the surface layer is reduced to (Ac3- (40 ° C.) to (Ac3 + 40 ° C.), the finish rolling is started, the finish rolling is completed under the condition of a total draft of 950 ° C. or less: 60% or more, and then cooling is started within 2 s, and 10 ° C. / S is cooled to 600 ° C or lower at a speed of at least 200 ° C / s and wound up in the range of 600 to 350 ° C. Method of producing a high strength electric resistance welded steel pipe for hot rolled steel sheet excellent in temperature toughness and weldability are disclosed. According to the technique described in Patent Document 3, it is possible to manufacture a high-strength ERW steel pipe excellent in low-temperature toughness and weldability without adding an expensive alloy element and without heat treatment of the entire steel pipe. I can do it.
また、特許文献４には、質量％で、Ｃ：０．０２〜０．０８％、Ｓｉ：０．５％以下、Ｍｎ：０．８〜１．８％、Ｐ：０．０２５％以下、Ｓ：０．００５％以下、Ａｌ：０．００５〜０．１０％、Ｎ：０．００５％以下、Ｎｂ：０．０３〜０．１０％、Ｔｉ：０．００５〜０．０５％を含み、残部Ｆｅおよび不可避的不純物からなる組成を有する鋼素材に、粗圧延を施しシートバーとする粗圧延工程と、該シートバーに仕上圧延を施し熱延板とする仕上圧延工程と、該熱延板を巻き取る巻取工程とを順次施す熱延鋼板の製造方法において、前記粗圧延工程後で、前記仕上圧延工程前に、前記シートバーに、表層部を５０℃／ｓ以上の冷却速度でＡｒ３変態点以下の温度に達するまで急冷する加速冷却を施したのち、該加速冷却を停止し、しかる後に、前記仕上圧延工程における仕上圧延を、１パス当たりの圧下率が（１．１×一様伸び）％以下（ここで、一様伸び：９５０℃まで加熱したのちＡｒ３変態点以下まで冷却し、ついで９５０℃まで再加熱して高温引張試験を行ったときに、得られた応力−歪曲線における一様伸び値（％））である圧延とすることを特徴とする、引張強さＴＳが４９０ＭＰａ以上で、深さ１００μｍ以上の表面割れがない表面品質に優れ、ＣＴＯＤ試験で−１０℃での限界開口変位量δｃが０．２５ｍｍ以上である延性亀裂伝播停止特性に優れる熱延鋼板が製造できるとしている。 Further, in Patent Document 4, in mass%, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.005% or less, Nb: 0.03 to 0.10%, Ti: 0.005 to 0.05% A rough rolling step of subjecting a steel material having a composition comprising the balance of Fe and inevitable impurities to rough rolling to form a sheet bar; a finish rolling step of subjecting the sheet bar to finish rolling to form a hot rolled sheet; In the method for producing a hot-rolled steel sheet, in which a winding step of winding a sheet is sequentially performed, after the rough rolling step and before the finish rolling step, the surface portion of the sheet bar is cooled at a cooling rate of 50 ° C./s or more. After performing accelerated cooling in which rapid cooling is performed until the temperature reaches the Ar3 transformation point or lower, the accelerated cooling is stopped. Thereafter, in the finish rolling in the finish rolling step, the rolling reduction per pass is (1.1 × uniform elongation)% or less (here, uniform elongation: after heating to 950 ° C., cooling to the Ar3 transformation point or less). Then, when subjected to a high-temperature tensile test by reheating to 950 ° C., when the tensile strength TS is determined to be rolling which is a uniform elongation value (%) in the obtained stress-strain curve, Manufacture of hot-rolled steel sheet with excellent surface quality at 490 MPa or more and no surface cracks with a depth of 100 μm or more, and with a critical opening displacement δc at −10 ° C. of 0.25 mm or more in CTOD test with excellent ductile crack propagation arrestability I can do it.
しかし、特許文献１に記載された技術で製造された熱延鋼板では、耐ＨＩＣ特性の向上は顕著であるが、ＤＷＴＴ特性やＣＴＯＤ特性の向上は顕著ではなく、さらに表面割れが発生する場合があるという問題があった。また、特許文献１に記載された技術では、未再結晶領域における圧下率を８５％以上と極めて大きくする必要があり、圧延機に過大な負荷がかかるうえ、厚肉製品の製造が難しいなどの問題もあった。また、特許文献２および特許文献３に記載された技術で製造された熱延鋼板では、表面割れが多発する場合があるという問題があった。また、特許文献４に記載された技術で製造された熱延鋼板では、低温靱性の向上が不十分であり、引張強さＴＳも低く、ＴＳ６４０ＭＰａ以上の高強度熱延鋼板にしようとすると低温靱性が不十分であるという問題があった。 However, in the hot-rolled steel sheet manufactured by the technique described in Patent Document 1, although the improvement in the HIC resistance is remarkable, the improvement in the DWTT property and the CTOD property is not remarkable. There was a problem. Further, in the technique described in Patent Document 1, the rolling reduction in the non-recrystallized region needs to be extremely large as 85% or more, and an excessive load is applied to the rolling mill, and it is difficult to produce a thick product. There were also problems. Further, the hot-rolled steel sheets manufactured by the techniques described in Patent Literature 2 and Patent Literature 3 have a problem that surface cracks may frequently occur. Further, in the hot-rolled steel sheet manufactured by the technique described in Patent Document 4, the low-temperature toughness is insufficiently improved, the tensile strength TS is low, and the high-strength hot-rolled steel sheet having a tensile strength of TS 640 MPa or more has a low temperature toughness. There was a problem that was insufficient.
本発明はかかる事情に鑑みてなされたものであって、表面品質、延性亀裂伝播停止特性および低温靭性に優れた高強度熱延鋼板およびその製造方法を提供することを目的とする。 The present invention has been made in view of such circumstances, and an object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in surface quality, ductile crack propagation stopping characteristics, and low-temperature toughness, and a method for manufacturing the same.
本発明者らは、上記した課題を達成するために、表面品質、延性亀裂伝播停止特性および低温靭性に及ぼす各種要因について鋭意研究を重ねた。その結果、表面品質、延性亀裂伝播停止特性および低温靭性に優れた高強度熱延鋼板は、所定の成分組成を有するとともに、鋼組織が、平均粒径５．０μｍ以下のベイニティックフェライト単相からなり、引張強さＴＳが６４０ＭＰａ以上であることを知見した。 The present inventors have intensively studied various factors affecting the surface quality, ductile crack propagation arrestability, and low-temperature toughness in order to achieve the above object. As a result, a high-strength hot-rolled steel sheet excellent in surface quality, ductile crack propagation arrestability, and low-temperature toughness has a predetermined component composition, and has a single-phase bainitic ferrite having an average grain size of 5.0 μm or less. And found that the tensile strength TS was 640 MPa or more.
また、本発明者らは、表面割れ等の表面欠陥は、優れた延性亀裂伝播停止特性および低温靭性（低温での高靭性）を確保するために低温圧延を指向したことによって生じた、表層部の過冷却による延性の低下や、表層部への過大な圧下による粒界フェライトの割れに、その主因があることを突き止めた。そして、優れた延性亀裂伝播停止特性および低温靭性を有する高強度熱延鋼板を得るためには、被圧延材の温度を高靭化に有効な温度域に冷却した後、所定範囲の圧下を施す粗圧延および仕上圧延を行うことが肝要であり、そのために、粗圧延と仕上圧延とを有する熱間圧延において、粗圧延の途中で所定の加速冷却（急冷）を施すことが有効であることを知見した。 In addition, the present inventors have found that surface defects such as surface cracks are caused by directing low-temperature rolling in order to secure excellent ductile crack propagation stopping characteristics and low-temperature toughness (high toughness at low temperatures). The main causes were found to be the decrease in ductility due to excessive cooling of the steel and the cracking of grain boundary ferrite due to excessive reduction in the surface layer. Then, in order to obtain a high-strength hot-rolled steel sheet having excellent ductile crack propagation stopping characteristics and low-temperature toughness, after the temperature of the material to be rolled is cooled to a temperature range effective for toughening, a predetermined range of reduction is performed. It is important to perform rough rolling and finish rolling, and therefore, in hot rolling having rough rolling and finish rolling, it is effective to perform predetermined accelerated cooling (rapid cooling) during rough rolling. I learned.
本発明は、上記した知見に基づき、さらに詳細な検討を加えて完成されたものである。すなわち、本発明の要旨は次の通りである。 The present invention has been completed based on the above findings and further detailed studies. That is, the gist of the present invention is as follows.
［１］ 質量％で、
Ｃ：０．０２〜０．０８％、 Ｓｉ：０．５％以下、
Ｍｎ：０．８〜２．０％、 Ｐ：０．０２５％以下、
Ｓ：０．００５％以下、 Ａｌ：０．００５〜０．１０％、
Ｎ：０．００５％以下、 Ｎｂ：０．０３〜０．１０％、
Ｔｉ：０．００５〜０．０５％、 Ｃａ：０．０００５〜０．００５％、
Ｏ：０．００５％以下
を含有し、残部Ｆｅおよび不可避的不純物からなる成分組成を有し、
鋼組織が、平均粒径５．０μｍ以下のベイニティックフェライト単相からなり、
引張強さＴＳが６４０ＭＰａ以上であることを特徴とする高強度熱延鋼板。
[1] In mass%,
C: 0.02-0.08%, Si: 0.5% or less,
Mn: 0.8 to 2.0%, P: 0.025% or less,
S: 0.005% or less, Al: 0.005 to 0.10%,
N: 0.005% or less, Nb: 0.03 to 0.10%,
Ti: 0.005 to 0.05%, Ca: 0.0005 to 0.005%,
O: contains 0.005% or less, has a component composition consisting of the balance Fe and inevitable impurities,
The steel structure is composed of a bainitic ferrite single phase having an average grain size of 5.0 μm or less,
A high-strength hot-rolled steel sheet having a tensile strength TS of 640 MPa or more.
［２］ 前記成分組成に加えてさらに、質量％で、Ｃｕ：０．００５〜０．５％、Ｎｉ：０．００５〜０．５％、Ｃｒ：０．００５〜０．５％、Ｍｏ：０．００５〜０．５％およびＶ：０．００５〜０．３％から選択される１種または２種以上を含有することを特徴とする前記［１］に記載の高強度熱延鋼板。 [2] In addition to the above component composition, in mass%, Cu: 0.005 to 0.5%, Ni: 0.005 to 0.5%, Cr: 0.005 to 0.5%, Mo: The high-strength hot-rolled steel sheet according to the above [1], comprising one or more selected from 0.005 to 0.5% and V: 0.005 to 0.3%.
［３］ 鋼素材に、粗圧延を施しシートバーとする粗圧延工程と、該シートバーに仕上圧延を施し熱延板とする仕上圧延工程と、該熱延板を冷却する仕上圧延後冷却工程と、仕上圧延後冷却工程の後に巻き取る巻取工程とをこの順に施して熱延鋼板とするにあたり、
前記鋼素材を、前記［１］または［２］に記載の成分組成を有する鋼素材とし、
前記粗圧延工程が、鋼素材を加熱した後、粗圧延を施す第一粗圧延工程と、第一粗圧延工程後に、表面温度で平均冷却速度：５０℃／ｓ以上となる冷却速度でＡｒ３変態点以下の温度に達するまで急冷した後、該急冷を停止する粗圧延冷却工程と、粗圧延冷却工程後に、９３０℃以下の未再結晶オーステナイト域で粗圧延を１パス以上５パス以下施しシートバーとする第二粗圧延工程とを有し、
前記第二粗圧延工程および前記仕上圧延工程における９３０℃以下の未再結晶オーステナイト域での全圧下率が６５％以上であり、
前記仕上圧延後冷却工程が、前記熱延板を板厚中心で平均冷却速度：２０℃／ｓ以上となる冷却速度で冷却することを特徴とする高強度熱延鋼板の製造方法。
[3] A rough rolling step of subjecting a steel material to rough rolling to form a sheet bar, a finish rolling step of subjecting the sheet bar to finish rolling to form a hot rolled sheet, and a cooling step after finish rolling for cooling the hot rolled sheet And, in performing the winding process after the finish rolling after the cooling step and in this order to obtain a hot-rolled steel sheet,
The steel material is a steel material having a component composition according to the above [1] or [2],
In the rough rolling step, a steel material is heated and then rough rolling is performed, and after the first rough rolling step, an Ar3 transformation is performed at a cooling rate at which the average cooling rate at the surface temperature is 50 ° C./s or more. After the quenching is performed until the temperature reaches the temperature equal to or lower than the point, the quenching is stopped, and after the rough rolling cooling step, the rough rolling is performed in the unrecrystallized austenite region at 930 ° C. or less by 1 pass or more and 5 passes or less. And a second rough rolling step,
The total rolling reduction in the unrecrystallized austenite region of 930 ° C. or less in the second rough rolling step and the finish rolling step is 65% or more;
The method for producing a high-strength hot-rolled steel sheet, wherein the cooling step after finish rolling cools the hot-rolled sheet at a cooling rate of 20 ° C./s or more at an average cooling rate at a thickness center.
［４］ 前記巻取工程が、巻取り温度：３５０〜７００℃で巻取り、巻取り完了後、コイル中央部で平均冷却速度が５〜２０℃／ｈとなる冷却速度で冷却することを特徴とする前記［３］に記載の高強度熱延鋼板の製造方法。 [4] The winding step is characterized in that winding is performed at a winding temperature of 350 to 700 ° C., and after completion of the winding, cooling is performed at a cooling rate at an average cooling rate of 5 to 20 ° C./h at the center of the coil. The method for producing a high-strength hot-rolled steel sheet according to the above [3].
本発明によれば、表面割れ等の表面欠陥の発生がなく表面品質に優れ、しかも延性亀裂伝播停止特性および低温靭性に優れた高強度熱延鋼板、具体的には、引張強さＴＳが６４０ＭＰａ以上で、深さ１００μｍ以上の表面割れがなく、ＣＴＯＤ試験で−１０℃での限界開口変位量δｃが０．２５ｍｍ以上であり、ＤＷＴＴ８５％ＳＡＴＴが−２５℃以下である熱延鋼板を提供することができる。このような高強度熱延鋼板は、本発明の高強度鋼板の製造方法によって、容易にかつ生産性高く製造でき、さらには、表面欠陥の生じやすい低温大気圧下の圧延を施して製造されても、表面割れ等の表面欠陥の発生がなく表面品質に優れる。したがって、産業上格段の効果を奏する。また、鋼管の材料コストを低減できるため、パイプラインの施工コストの更なる低減が可能となる。 According to the present invention, a high-strength hot-rolled steel sheet excellent in surface quality without generation of surface defects such as surface cracks and excellent in ductile crack propagation stopping characteristics and low-temperature toughness, specifically, having a tensile strength TS of 640 MPa As described above, there is provided a hot-rolled steel sheet having no surface cracks having a depth of 100 μm or more, a critical opening displacement δc at −10 ° C. in a CTOD test of 0.25 mm or more, and a DWTT 85% SATT of −25 ° C. or less. be able to. Such a high-strength hot-rolled steel sheet can be easily and highly productively manufactured by the method for manufacturing a high-strength steel sheet of the present invention, and further, is manufactured by performing rolling under low temperature and atmospheric pressure where surface defects are likely to occur. Also, no surface defects such as surface cracks are generated and the surface quality is excellent. Therefore, a remarkable industrial effect is achieved. Further, since the material cost of the steel pipe can be reduced, the construction cost of the pipeline can be further reduced.
本発明の熱延鋼板は、質量％で、Ｃ：０．０２〜０．０８％、Ｓｉ：０．５％以下、Ｍｎ：０．８〜２．０％、Ｐ：０．０２５％以下、Ｓ：０．００５％以下、Ａｌ：０．００５〜０．１０％、Ｎ：０．００５％以下、Ｎｂ：０．０３〜０．１０％、Ｔｉ：０．００５〜０．０５％、Ｃａ：０．０００５〜０．００５％、Ｏ：０．００５％以下を含有し、残部Ｆｅおよび不可避的不純物からなる成分組成を有し、鋼組織が、平均粒径５．０μｍ以下のベイニティックフェライト単相からなり、引張強さＴＳが６４０ＭＰａ以上である高強度熱延鋼板（以下単に「本発明の熱延鋼板」とも記載する）である。なお、本明細書において、「延性亀裂伝播停止特性に優れた」とは、ＢＳ７４４８：Ｐａｒｔ１ １９９１の規定に準拠して、試験温度：−１０℃で行ったＣＴＯＤ試験における限界開口変位量δｃ（ｍｍ）が０．２５ｍｍ以上である場合をいう。また、「低温靱性に優れた」とは、ＤＷＴＴ試験における８５％ＳＡＴＴ（Ｓｈｅａｒ ａｒｅａ ｔｒａｎｓｉｔｉｏｎ ｔｅｍｐｅｒａｔｕｒｅ）が−２５℃以下である場合をいう。また、「表面品質に優れた」とは、深さ１００μｍ以上の表面割れがない場合をいう。また、「高強度」とは、引張強さＴＳが６４０ＭＰａ以上である場合をいう。 The hot-rolled steel sheet according to the present invention is, by mass%, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.005% or less, Nb: 0.03 to 0.10%, Ti: 0.005 to 0.05%, Ca : 0.0005% to 0.005%, O: 0.005% or less, having a component composition comprising the balance of Fe and unavoidable impurities, and having a steel structure having a mean grain size of 5.0 μm or less. It is a high-strength hot-rolled steel sheet (hereinafter, also simply referred to as “hot-rolled steel sheet of the present invention”) made of a ferrite single phase and having a tensile strength TS of 640 MPa or more. In the present specification, “excellent in ductile crack propagation arrestability” means that the critical opening displacement δc (mm) in a CTOD test performed at a test temperature of −10 ° C. in accordance with the provisions of BS7448: Part1 1991. ) Is 0.25 mm or more. Further, “excellent in low-temperature toughness” refers to a case where 85% SATT (Shear area transition temperature) in a DWTT test is −25 ° C. or less. Further, “excellent in surface quality” means that there is no surface crack having a depth of 100 μm or more. The term “high strength” refers to a case where the tensile strength TS is 640 MPa or more.
まず、本発明の熱延鋼板の成分組成について説明する。なお、特に断らないかぎり質量％は単に％と記す。 First, the component composition of the hot-rolled steel sheet of the present invention will be described. Unless otherwise specified, the mass% is simply described as%.
Ｃ：０．０２〜０．０８％、
Ｃは、鋼の強度を上昇させる作用を有する元素であり、本発明では所望の高強度を確保するために、０．０２％以上の含有を必要とする。一方、０．０８％を超える過剰な含有は、パーライト等の第二相の組織分率を増大させ、母材靭性および溶接熱影響部靭性を低下させる。このため、Ｃは０．０２〜０．０８％の範囲に限定した。なお、好ましくは０．０２〜０．０５％である。
C: 0.02 to 0.08%,
C is an element having an effect of increasing the strength of steel, and in the present invention, it is necessary to contain 0.02% or more in order to secure a desired high strength. On the other hand, an excessive content exceeding 0.08% increases the structural fraction of the second phase such as pearlite, and reduces the base metal toughness and the weld heat affected zone toughness. For this reason, C was limited to the range of 0.02 to 0.08%. In addition, it is preferably 0.02 to 0.05%.
Ｓｉ：０．５％以下
Ｓｉは、固溶強化、焼入れ性の向上を介して、鋼の強度を増加させるが、同時に靭性を低下させる作用を有し、また、Ｓｉは電縫溶接時にＳｉの酸化物を形成し、電縫溶接部の靭性を低下させる。このため、本発明では、Ｓｉはできるだけ低減することが望ましいが、０．５％までは許容できることから、Ｓｉは０．５％以下に限定した。なお、好ましくは０．３％以下である。
Si: 0.5% or less Si increases the strength of steel through solid solution strengthening and improvement of hardenability, but at the same time, has the effect of lowering toughness. Form oxides and reduce toughness of ERW welds. For this reason, in the present invention, it is desirable to reduce Si as much as possible, but since it is acceptable up to 0.5%, Si is limited to 0.5% or less. In addition, it is preferably 0.3% or less.
Ｍｎ：０．８〜２．０％
Ｍｎは、焼入性を向上させる作用を有し、焼入性向上を介し鋼板の強度を増加させる。また、Ｍｎは、ＭｎＳを形成しＳを固定することにより、Ｓの粒界偏析を防止してスラブ（鋼素材）割れを抑制する。このような効果を得るためには、０．８％以上の含有を必要とする。一方、２．０％を超える含有は、偏析を助長する。この偏析を消失させるには、１３００℃を超える温度に加熱する必要があり、このような熱処理を工業的規模で実施することは現実的でない。このため、Ｍｎは０．８〜２．０％の範囲に限定した。なお、好ましくは０．９〜１．７％である。
Mn: 0.8-2.0%
Mn has an effect of improving hardenability and increases the strength of a steel sheet through improvement of hardenability. Further, Mn forms MnS and fixes S, thereby preventing grain boundary segregation of S and suppressing slab (steel material) cracking. In order to obtain such an effect, the content needs to be 0.8% or more. On the other hand, a content exceeding 2.0% promotes segregation. In order to eliminate this segregation, it is necessary to heat to a temperature exceeding 1300 ° C., and it is not practical to perform such a heat treatment on an industrial scale. For this reason, Mn was limited to the range of 0.8 to 2.0%. In addition, it is preferably 0.9 to 1.7%.
Ｐ：０．０２５％以下
Ｐは、鋼中に不純物として不可避的に含まれるが、鋼の強度を上昇させる作用を有する。しかし、０．０２５％を超えて過剰に含有すると溶接性が低下する。このため、Ｐは０．０２５％以下に限定した。なお、好ましくは０．０１５％以下である。
P: 0.025% or less P is inevitably contained as an impurity in steel, but has an effect of increasing the strength of steel. However, if the content exceeds 0.025%, the weldability decreases. Therefore, P is limited to 0.025% or less. In addition, it is preferably 0.015% or less.
Ｓ：０．００５％以下
Ｓは、Ｐと同様に鋼中に不純物として不可避的に含まれるが、０．００５％を超えて過剰に含有すると、スラブ割れを生起させるとともに、熱延鋼板においては粗大なＭｎＳを形成し、延性の低下を生じさせる。このため、Ｓは０．００５％以下に限定した。なお、好ましくは０．００３％以下である。
S: 0.005% or less S is inevitably contained as an impurity in steel like P, but if it is contained in excess of 0.005%, it causes slab cracking, and in hot-rolled steel sheets, It forms coarse MnS and causes a decrease in ductility. Therefore, S is limited to 0.005% or less. In addition, it is preferably 0.003% or less.
Ａｌ：０．００５〜０．１０％
Ａｌは、脱酸剤として作用する元素であり、このような効果を得るためには、０．００５％以上含有する。一方、０．１０％を超える含有は、電縫溶接時の、溶接部の清浄性を著しく損なう。このようなことから、Ａｌは０．００５〜０．１０％に限定した。なお、好ましくは０．０８％以下である。
Al: 0.005 to 0.10%
Al is an element that acts as a deoxidizing agent. To obtain such an effect, Al is contained in an amount of 0.005% or more. On the other hand, if the content exceeds 0.10%, the cleanliness of the welded portion during electric resistance welding is significantly impaired. For these reasons, Al is limited to 0.005 to 0.10%. In addition, it is preferably 0.08% or less.
Ｎ：０．００５％以下
Ｎは、鋼中に不可避的に含まれる元素であるが、過剰な含有はスラブ鋳造時の割れを多発させる。このため、Ｎは０．００５％以下に限定した。なお、好ましくは０．００３％以下である。
N: 0.005% or less N is an element inevitably contained in steel, but excessive content causes many cracks during slab casting. For this reason, N was limited to 0.005% or less. In addition, it is preferably 0.003% or less.
Ｎｂ：０．０３〜０．１０％
Ｎｂは、オーステナイト粒の粗大化、再結晶を抑制する作用を有する元素であり、仕上圧延におけるオーステナイト未再結晶温度域圧延を可能にするとともに、炭窒化物として微細析出することにより、溶接性を損なうことなく、少ない含有量で熱延鋼板を高強度化する作用を有する。このような効果を得るためには、０．０３％以上の含有を必要とする。一方、０．１０％を超える過剰な含有は、熱間仕上圧延中の圧延荷重の増大をもたらし、熱間圧延が困難となる場合がある。このため、Ｎｂは０．０３〜０．１０％の範囲に限定した。なお、好ましくは０．０３〜０．０７％である。
Nb: 0.03 to 0.10%
Nb is an element having an effect of suppressing coarsening and recrystallization of austenite grains, and enables austenite non-recrystallization temperature range rolling in finish rolling, and also improves weldability by precipitating finely as a carbonitride. It has the effect of increasing the strength of a hot-rolled steel sheet with a small content without loss. In order to obtain such an effect, a content of 0.03% or more is required. On the other hand, an excessive content exceeding 0.10% causes an increase in the rolling load during hot finish rolling, which may make hot rolling difficult. For this reason, Nb was limited to the range of 0.03 to 0.10%. In addition, it is preferably 0.03 to 0.07%.
Ｔｉ：０．００５〜０．０５％
Ｔｉは、窒化物を形成しＮを固定しスラブ（鋼素材）割れを防止する効果を有するとともに、炭化物として微細析出することにより、鋼板を高強度化させる。このような効果は、０．００５％以上の含有で顕著となる。一方、０．０５％を超える含有は析出強化により降伏点が著しく上昇する。このため、Ｔｉは０．００５〜０．０５％に限定した。なお、好ましくは０．００５〜０．０３％である。
Ti: 0.005 to 0.05%
Ti forms nitrides, fixes N, and has the effect of preventing slab (steel material) cracks, and enhances the strength of steel sheets by precipitating finely as carbides. Such effects become remarkable when the content is 0.005% or more. On the other hand, when the content exceeds 0.05%, the yield point is significantly increased by precipitation strengthening. For this reason, Ti was limited to 0.005 to 0.05%. In addition, it is preferably 0.005 to 0.03%.
Ｃａ：０．０００５〜０．００５％
Ｃａは、粗大な硫化物を球状の硫化物とする硫化物の形態制御に寄与する元素である。このような効果を得るためには、Ｃａは０．０００５％以上含有させる。一方、Ｃａ：０．００５％を超える含有は、鋼板の清浄度を低下させる。また、Ｃａの含有量が０．０００５〜０．００５％の範囲外の場合は、ＭｎＳの粗大化やＣａＯの凝集・粗大化により、表面品質、延性亀裂伝播停止特性や、低温靱性が悪くなる。このため、Ｃａ：０．０００５〜０．００５％の範囲に限定する。
Ca: 0.0005 to 0.005%
Ca is an element that contributes to morphological control of sulfides, which turn coarse sulfides into spherical sulfides. In order to obtain such an effect, Ca is contained at 0.0005% or more. On the other hand, if the content of Ca exceeds 0.005%, the cleanliness of the steel sheet is reduced. When the content of Ca is out of the range of 0.0005 to 0.005%, the surface quality, ductile crack propagation stopping characteristics, and low-temperature toughness deteriorate due to coarsening of MnS and aggregation and coarsening of CaO. . For this reason, Ca is limited to the range of 0.0005 to 0.005%.
Ｏ：０．００５％以下
本発明でＯは不可避的不純物であり含有量の上限を規定する。Ｏは、粗大で靱性に悪影響を及ぼす介在物生成を抑制するため、０．００５％以下とする。
O: 0.005% or less O is an unavoidable impurity in the present invention and defines the upper limit of the content. O is set to 0.005% or less to suppress the formation of inclusions that are coarse and adversely affect toughness.
上記した成分が基本の成分組成であるが、この基本の成分組成に加えてさらに、Ｃｕ：０．００５〜０．５％、Ｎｉ：０．００５〜０．５％、Ｃｒ：０．００５〜０．５％、Ｍｏ：０．００５〜０．５％およびＶ：０．００５〜０．３％から選択される１種または２種以上を含有する組成としてもよい。 The above-mentioned components are the basic component compositions. In addition to the basic component compositions, Cu: 0.005 to 0.5%, Ni: 0.005 to 0.5%, Cr: 0.005 to The composition may include one or more selected from 0.5%, Mo: 0.005 to 0.5%, and V: 0.005 to 0.3%.
Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ、Ｖはいずれも、焼入れ性を向上させ、鋼板の強度を増加させる元素であり、必要に応じて１種または２種以上を選択して含有できる。 Cu, Ni, Cr, Mo, and V are all elements that improve the hardenability and increase the strength of the steel sheet, and one or more of them can be selected and contained as needed.
Ｃｕは、焼入れ性を向上させるとともに、固溶強化あるいは析出強化により鋼板の強度を増加させる作用を有する元素である。このような効果を得るためには、０．００５％以上含有することが望ましい。一方、０．５％を超える含有は熱間加工性を低下させる。このため、Ｃｕは０．００５〜０．５％に限定することが好ましい。 Cu is an element having the function of improving the hardenability and increasing the strength of the steel sheet by solid solution strengthening or precipitation strengthening. In order to obtain such an effect, it is desirable to contain 0.005% or more. On the other hand, a content exceeding 0.5% lowers hot workability. For this reason, Cu is preferably limited to 0.005 to 0.5%.
Ｎｉは、焼入れ性を向上させ、鋼板の強度を増加させるとともに、靭性を向上させる作用を有する元素である。このような効果を得るためには、０．００５％以上含有することが望ましい。一方、０．５％を超えて含有しても効果が飽和し含有量に見合う効果が期待できなくなり、経済的に不利となる。このため、Ｎｉは０．００５〜０．５％に限定することが好ましい。 Ni is an element having an effect of improving the hardenability, increasing the strength of the steel sheet, and improving the toughness. In order to obtain such an effect, it is desirable to contain 0.005% or more. On the other hand, if the content exceeds 0.5%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, Ni is preferably limited to 0.005 to 0.5%.
Ｃｒは、焼入性を向上させ、鋼板強度を増加させる作用を有する元素である。このような効果は、０．００５％以上の含有で顕著となる。一方、０．５％を超える過剰の含有は、電縫溶接時に溶接欠陥を多発させる傾向となる。このため、Ｃｒは０．００５％以上０．５％以下に限定することが好ましい。なお、より好ましくは０．１５〜０．３％である。 Cr is an element having an effect of improving hardenability and increasing the strength of a steel sheet. Such effects become remarkable when the content is 0.005% or more. On the other hand, an excessive content exceeding 0.5% tends to cause a large number of welding defects during electric resistance welding. Therefore, the content of Cr is preferably limited to 0.005% or more and 0.5% or less. In addition, it is more preferably 0.15 to 0.3%.
Ｍｏは、焼入性を向上させるとともに、炭化物を形成して鋼板を高強度化する作用を有する元素であり、このような効果は０．００５％以上の含有で顕著となる。一方、０．５％を超える多量の含有は、溶接性を低下させる。このため、Ｍｏは０．００５〜０．５％に限定することが好ましい。なお、より好ましくは０．１〜０．３％である。 Mo is an element that has the effect of improving the hardenability and forming carbides to increase the strength of the steel sheet. Such an effect is remarkable when the content is 0.005% or more. On the other hand, a large content exceeding 0.5% lowers the weldability. For this reason, Mo is preferably limited to 0.005 to 0.5%. In addition, more preferably, it is 0.1 to 0.3%.
Ｖは、焼入性を向上させるとともに、炭窒化物を形成して鋼板を高強度化する作用を有する元素であり、このような効果は０．００５％以上の含有で顕著となる。一方、０．３％を超える過剰の含有は、溶接性を劣化させる。このため、Ｖは０．００５〜０．３％とすることが好ましい。なお、より好ましくは０．００５〜０．１５％である。 V is an element having the effect of improving hardenability and forming carbonitride to increase the strength of the steel sheet, and such an effect becomes remarkable when the content is 0.005% or more. On the other hand, an excessive content exceeding 0.3% deteriorates weldability. For this reason, V is preferably set to 0.005 to 0.3%. In addition, more preferably, it is 0.005 to 0.15%.
上記した成分以外の残部は、Ｆｅおよび不可避的不純物からなる。 The balance other than the components described above consists of Fe and inevitable impurities.
そして、本発明の熱延鋼板の鋼組織は、平均粒径５．０μｍ以下のベイニティックフェライト単相である。鋼組織を構成するベイニティックフェライトの平均粒径が５．０μｍを超えると、表面品質、延性亀裂伝播停止特性や、低温靱性が悪くなるため、本発明においては、鋼組織が、平均粒径５．０μｍ以下のベイニティックフェライト単相である必要がある。なお、「ベイニティックフェライト単相」とは、ベイニティックフェライトの面積分率が９５％以上である場合をいい、「平均粒径」は、熱延鋼板の鋼組織全体のベイニティックフェライトの平均粒径である。 The steel structure of the hot-rolled steel sheet of the present invention is a single phase of bainitic ferrite having an average grain size of 5.0 μm or less. If the average grain size of the bainitic ferrite constituting the steel structure exceeds 5.0 μm, the surface quality, ductile crack propagation arresting property, and low-temperature toughness are deteriorated. It is necessary that the bainitic ferrite has a single phase of 5.0 μm or less. In addition, "bainitic ferrite single phase" means that the area fraction of bainitic ferrite is 95% or more, and "average grain size" means bainitic ferrite of the entire steel structure of a hot-rolled steel sheet. Is the average particle size.
また、本発明の熱延鋼板の引張強さは、６４０ＭＰａ以上であり、好ましくは６７０ＭＰａ以上であり、さらには７５０ＭＰａ以上とすることもできる。 Further, the tensile strength of the hot-rolled steel sheet of the present invention is 640 MPa or more, preferably 670 MPa or more, and can be 750 MPa or more.
また、本発明の熱延鋼板は、深さ（鋼板表面から板厚方向の大きさ）１００μｍ以上の表面割れがなく表面品質に優れ、ＣＴＯＤ試験で−１０℃での限界開口変位量δｃが０．２５ｍｍ以上であり延性亀裂伝播停止特性に優れ、ＤＷＴＴ８５％ＳＡＴＴが−２５℃以下であり低温靱性に優れる。そして、本発明の熱延鋼板は、試験温度−８０℃でのシャルピー衝撃試験の吸収エネルギーｖＥ−８０が１６０Ｊ以上や、破面遷移温度ｖＴｒｓが−９０℃以下となる低温靭性を具備することもできる。 In addition, the hot-rolled steel sheet of the present invention has excellent surface quality without surface cracks having a depth (size in the thickness direction from the steel sheet surface) of 100 μm or more, and has a critical opening displacement δc at −10 ° C. of 0 in a CTOD test. 0.25 mm or more, excellent in ductile crack propagation arrestability, DWTT 85% SATT is -25 ° C or less, and excellent in low-temperature toughness. And the hot-rolled steel sheet of the present invention may have low-temperature toughness in which the absorbed energy vE-80 in the Charpy impact test at the test temperature of −80 ° C. is 160 J or more and the fracture surface transition temperature vTrs is −90 ° C. or less. it can.
本発明の熱延鋼板の板厚は特に限定されず、例えば板厚（ＷＴ）は１２〜２６ｍｍであるが、厚肉、すなわち板厚が１７．５ｍｍ以上、好ましくは１９．１ｍｍ以上であることが好ましい。表面割れは表面と板厚中心との温度差が大きい厚肉鋼板で特に生じやすいので、本発明の効果は厚肉鋼板ほど顕著となるためである。本発明の熱延鋼板は、ＡＰＩ規格にしてＸ７０以上の高強度厚肉のものに適用することができる。特にラインパイプ向け高強度電縫鋼管および高強度スパイラル鋼管の用途に供して好適である。 The thickness of the hot-rolled steel sheet of the present invention is not particularly limited, and for example, the sheet thickness (WT) is 12 to 26 mm, but the thickness, that is, the sheet thickness is 17.5 mm or more, preferably 19.1 mm or more. Is preferred. This is because surface cracks are particularly likely to occur in a thick steel plate having a large temperature difference between the surface and the center of the thickness, and the effect of the present invention is more remarkable in a thick steel plate. The hot-rolled steel sheet of the present invention can be applied to a high-strength steel sheet having an API standard of X70 or more. It is particularly suitable for use in high strength electric resistance welded steel pipes for line pipes and high strength spiral steel pipes.
次に、上記本発明の熱延鋼板の製造方法について説明する。 Next, a method for manufacturing the hot-rolled steel sheet of the present invention will be described.
本発明の高強度熱延鋼板の製造方法は、鋼素材に、粗圧延を施しシートバーとする粗圧延工程と、該シートバーに仕上圧延を施し熱延板とする仕上圧延工程と、該熱延板を冷却する仕上圧延後冷却工程と、仕上圧延後冷却工程の後に巻き取る巻取工程とをこの順に施して熱延鋼板とするにあたり、鋼素材を、上記した成分組成を有する鋼素材とし、粗圧延工程が、鋼素材を加熱した後、粗圧延を施す第一粗圧延工程と、第一粗圧延工程後に、表面温度で平均冷却速度：５０℃／ｓ以上となる冷却速度でＡｒ３変態点以下の温度に達するまで急冷した後、該急冷を停止する粗圧延冷却工程と、粗圧延冷却工程後に、９３０℃以下の未再結晶オーステナイト域で粗圧延を１パス以上５パス以下施しシートバーとする第二粗圧延工程とを有し、第二粗圧延工程および仕上圧延工程における９３０℃以下の未再結晶オーステナイト域での全圧下率（ＣＲ率）が６５％以上であり、仕上圧延後冷却工程が、熱延板を板厚中心で平均冷却速度：２０℃／ｓ以上となる冷却速度で冷却することを特徴とする。以下、各工程について、詳細に説明する。以下の製造方法の説明において、温度は特に断らない限り鋼素材、シートバー、熱延板や鋼板等の表面温度とする。該表面温度は、放射温度計等で測定することができる。また、平均冷却速度は特に断らない限り（（冷却前の温度−冷却後の温度）／冷却時間）とする。 The method for producing a high-strength hot-rolled steel sheet of the present invention includes a rough rolling step of subjecting a steel material to rough rolling to form a sheet bar; a finish rolling step of subjecting the sheet bar to finish rolling to form a hot-rolled sheet; After the finish rolling cooling step of cooling the rolled sheet and the winding step of winding up after the finishing rolling cooling step, in order to perform a hot rolling steel sheet in this order, the steel material is a steel material having the above component composition. In the rough rolling step, the steel material is heated and then rough rolling is performed, and after the first rough rolling step, the Ar3 transformation is performed at a cooling rate of 50 ° C./s or more at an average cooling rate at the surface temperature after the first rough rolling step. After the quenching is performed until the temperature reaches the temperature equal to or lower than the point, the quenching is stopped, and after the rough rolling cooling step, the rough rolling is performed in the unrecrystallized austenite region at 930 ° C. or less by 1 pass or more and 5 passes or less. And a second rough rolling step The total reduction ratio (CR ratio) in the unrecrystallized austenite region of 930 ° C. or lower in the rolling step and the finish rolling step is 65% or more, and the cooling step after the finish rolling is performed by cooling the hot rolled sheet at an average cooling rate at the center of the sheet thickness. : Characterized by cooling at a cooling rate of 20 ° C./s or more. Hereinafter, each step will be described in detail. In the following description of the production method, the temperature is the surface temperature of a steel material, a sheet bar, a hot rolled sheet, a steel sheet, or the like unless otherwise specified. The surface temperature can be measured with a radiation thermometer or the like. Unless otherwise specified, the average cooling rate is ((temperature before cooling−temperature after cooling) / cooling time).
上記した成分組成を有する鋼素材の製造方法は特に限定されず、上記した成分組成の溶鋼を通常の溶製法で溶製し、連続鋳造法、造塊−分塊法等の通常の鋳造方法で鋼素材とすることができる。 The method for producing the steel material having the above-described composition is not particularly limited, and the molten steel having the above-described composition is smelted by a normal smelting method, and is then cast by a normal casting method such as a continuous casting method or an ingot-bulking method. It can be a steel material.
まず、上記した成分組成の鋼素材に、粗圧延工程および仕上圧延工程を有する熱間圧延（熱延）を施す。そして、本発明においては、粗圧延工程は、第一粗圧延工程と、第一粗圧延工程後に所定の冷却を施す粗圧延冷却工程と、粗圧延冷却工程後に、所定の粗圧延を施す第二粗圧延工程とを有する。すなわち、本発明においては、粗圧延の途中であって粗圧延終了５パス前以降に所定の冷却をし、該冷却の後に粗圧延を再び行なう。なお、粗圧延は、粗圧延機として単独の圧延機を用いこれに複数回通過させることにより複数パスの圧延を行なうようにしてもよく、粗圧延機として複数の圧延機を直列に並べた圧延機を用いこれに連続的に通過させることにより複数パスの圧延を行なうようにしてもよく、また、これらを組み合わせた粗圧延機を用いてもよい。 First, a steel material having the above-described composition is subjected to hot rolling (hot rolling) having a rough rolling step and a finish rolling step. In the present invention, the rough rolling step includes a first rough rolling step, a rough rolling cooling step of performing predetermined cooling after the first rough rolling step, and a second rough rolling step of performing predetermined rough rolling after the rough rolling cooling step. And a rough rolling step. That is, in the present invention, predetermined cooling is performed in the course of rough rolling and five passes before the end of rough rolling, and rough rolling is performed again after the cooling. The rough rolling may be performed in a plurality of passes by using a single rolling mill as a rough rolling mill and passing through the rolling multiple times, and a rolling in which a plurality of rolling mills are arranged in series as a rough rolling mill. Rolling may be performed in a plurality of passes by continuously passing through a mill, or a rough rolling mill in which these are combined may be used.
第一粗圧延工程では、鋼素材を加熱した後、粗圧延を施す。鋼素材の加熱温度は、特に限定されないが、１０００〜１２５０℃の範囲の温度とすることが好ましい。加熱温度が１０００℃未満では、変形抵抗が高く、圧延機への負荷が過大となりすぎる。一方、１２５０℃を超えて高温とすると、結晶粒が粗大化しすぎて熱延板の靭性が低下する。また、スケールロスが多くなり、歩留が低下する。なお、第一粗圧延工程では、圧下率や粗圧延終了温度等の圧延条件は特に限定されない。 In the first rough rolling step, rough rolling is performed after heating the steel material. The heating temperature of the steel material is not particularly limited, but is preferably in the range of 1000 to 1250 ° C. If the heating temperature is lower than 1000 ° C., the deformation resistance is high, and the load on the rolling mill becomes too large. On the other hand, if the temperature is higher than 1250 ° C., the crystal grains become too coarse, and the toughness of the hot-rolled sheet is reduced. Also, the scale loss increases and the yield decreases. In the first rough rolling step, rolling conditions such as a draft and a rough rolling end temperature are not particularly limited.
第一粗圧延工程後に行う粗圧延冷却工程では、表面温度で平均冷却速度：５０℃／ｓ以上となる冷却速度でＡｒ３変態点以下の温度に達するまで急冷した後、該急冷を停止する。表面温度で平均冷却速度：５０℃／ｓ以上となる冷却速度でＡｒ３変態点以下の温度に達するまで急冷することにより、延性亀裂伝播停止特性および低温靭性の向上に有効な温度域に被圧延材を冷却して、その後の第二粗圧延工程および仕上圧延工程により、延性亀裂伝播停止特性および低温靭性を有効に向上させることができる。該急冷をすることにより、延性亀裂伝播停止特性および低温靭性を向上させるために有効な温度域に冷却された板厚方向の領域が拡大でき、９３０℃以下の未再結晶オーステナイト域温度での全圧下率である有効圧延率を増加させて６５％以上とし、延性亀裂伝播停止特性および低温靭性向上の程度を大きくすることができる。粗圧延冷却工程の平均冷却速度が５０℃／ｓ未満では、延性亀裂伝播停止特性および低温靭性の向上に有効な温度域への冷却時間が多大となり、生産性が低下する。粗圧延冷却工程の冷却停止温度がＡｒ３変態点超えの場合には、延性亀裂伝播停止特性および低温靭性を向上させるために有効な温度域に冷却される板厚方向の範囲が狭く、延性亀裂伝播停止特性および低温靭性の向上代が少ない。また、粗圧延冷却工程の冷却停止温度は、表面温度でＡｒ３変態点以下（Ａｒ３変態点−１００℃）以上とすることが好ましい。粗圧延冷却工程の冷却停止温度がＡｒ３変態点−１００℃未満では、延性亀裂伝播停止特性および低温靱性の向上が望めないうえ、Ａｃ３変態点以上の温度に復熱させることが困難となる場合があるためである。 In the rough rolling cooling step performed after the first rough rolling step, the quenching is stopped after quenching until the temperature reaches a temperature equal to or lower than the Ar3 transformation point at a cooling rate of 50 ° C./s or more at the surface temperature. Average cooling rate at surface temperature: Rapid cooling until the temperature reaches the Ar3 transformation point or lower at a cooling rate of 50 ° C./s or more, so that the material to be rolled is brought into a temperature range effective for improving ductile crack propagation arrestability and low-temperature toughness. , And the subsequent second rough rolling step and the finish rolling step can effectively improve the ductile crack propagation stopping characteristics and the low-temperature toughness. By performing the quenching, the region in the sheet thickness direction cooled to a temperature range effective for improving the ductile crack propagation arrest property and the low temperature toughness can be expanded, and the total temperature at an unrecrystallized austenite region temperature of 930 ° C. or less is reduced. By increasing the effective rolling reduction, which is the rolling reduction, to 65% or more, it is possible to increase the degree of improvement in ductile crack propagation stopping characteristics and low-temperature toughness. If the average cooling rate in the rough rolling cooling step is less than 50 ° C./s, the cooling time to a temperature range effective for improving the ductile crack propagation stopping characteristics and low-temperature toughness becomes large, and the productivity is reduced. If the cooling stop temperature in the rough rolling cooling step is higher than the Ar3 transformation point, the range in the sheet thickness direction to be cooled to a temperature range effective for improving the ductile crack propagation stop characteristic and low temperature toughness is narrow, and the ductile crack propagation Less improvement in stopping characteristics and low-temperature toughness. Further, the cooling stop temperature in the rough rolling cooling step is preferably not more than the Ar3 transformation point (Ar3 transformation point −100 ° C.) at the surface temperature. If the cooling stop temperature of the rough rolling cooling step is lower than the Ar3 transformation point of −100 ° C., improvement in ductile crack propagation stop characteristics and low-temperature toughness cannot be expected, and it may be difficult to recover the temperature to a temperature higher than the Ac3 transformation point. Because there is.
粗圧延冷却工程において、急冷を停止すると、内部からの熱によって表面温度が上昇して自然に所定の粗圧延温度まで復熱する。なお、誘導加熱等の手法により強制的に復熱させても何ら問題ない。 In the rough rolling cooling step, when the rapid cooling is stopped, the surface temperature rises due to the heat from the inside, and the material naturally recovers to a predetermined rough rolling temperature. It should be noted that there is no problem even if the temperature is forcibly restored by a method such as induction heating.
このような粗圧延冷却工程の後の第二粗圧延工程では、９３０℃以下の未再結晶オーステナイト域で粗圧延を１パス以上５パス以下施しシートバーとする。本発明においては、第二粗圧延工程の前に所定の急冷を行なう粗圧延冷却工程を行なっているため、第二粗圧延工程では、９３０℃以下の未再結晶オーステナイト域で粗圧延を行なうことができる。第二粗圧延工程で、６パス以上粗圧延を行なうと温度低下量が大きく、鋼板の反りが発生し、通板性が悪くなる。また、第二粗圧延工程を行なわない、すなわち、従来技術のように粗圧延と仕上圧延との間で冷却工程を行なうと仕上げ圧延時のスケール噛み込みにより、表面欠陥発生の恐れがある。 In the second rough rolling step after the rough rolling cooling step, rough rolling is performed in a non-recrystallized austenite region of 930 ° C. or lower by 1 pass or more and 5 passes or less to obtain a sheet bar. In the present invention, since the rough rolling cooling step of performing predetermined rapid cooling is performed before the second rough rolling step, in the second rough rolling step, rough rolling is performed in an unrecrystallized austenite region of 930 ° C. or less. Can be. In the second rough rolling step, if rough rolling is performed for 6 passes or more, the temperature drop is large, the steel sheet warps, and the sheet passing property deteriorates. Further, if the second rough rolling step is not performed, that is, if the cooling step is performed between the rough rolling and the finish rolling as in the related art, there is a possibility that a surface defect may be generated due to scale biting during the finish rolling.
粗圧延工程後、すなわち、第二粗圧延工程後に行う仕上圧延工程では、粗圧延工程で得られたシートバーに仕上圧延を施して熱延板を得る。例えば、１パス当たりの圧下率が１５〜５０％の複数パスの仕上圧延を施すことが好ましい。１パス当たりの圧下率が１５％未満では、所望の延性亀裂伝播停止特性および低温靱性が期待できなくなるうえ、反り等の形状不良が発生する恐れがある。一方、１パス当たりの圧下率が５０％を超えると、圧延時に割れが発生し、また、圧延機に多大な負荷がかかり、好ましくない。仕上圧延での１パス当たりの圧下率は、より好ましくは１５〜３０％である。１パス当たりの圧下率は、仕上圧延工程で圧延する各パス毎の圧下率であり、圧下率は、（パス入側の板厚−パス出側の板厚）／パス入側の板厚で求めることができる。なお、仕上圧延工程では、圧延パス間の時間は１０ｓ以内とすることが望ましい。というのは、仕上圧延は、複数の圧延機を直列で並べた仕上げ圧延機で連続的に圧延を行うが、圧延機間では被圧延材（シートバー）が高温であるため、圧延パス間の時間が１０ｓを超えると、転位の回復、再結晶、結晶粒の粗大化等が生じ、延性亀裂伝播停止特性および低温靱性が低下する恐れがあるためである。 In the finish rolling step performed after the rough rolling step, that is, after the second rough rolling step, the sheet bar obtained in the rough rolling step is subjected to finish rolling to obtain a hot-rolled sheet. For example, it is preferable to perform finish rolling in a plurality of passes in which the rolling reduction per pass is 15 to 50%. If the rolling reduction per pass is less than 15%, desired ductile crack propagation stopping characteristics and low-temperature toughness cannot be expected, and shape defects such as warpage may occur. On the other hand, if the rolling reduction per pass exceeds 50%, cracks occur during rolling, and a large load is applied to the rolling mill, which is not preferable. The rolling reduction per pass in the finish rolling is more preferably 15 to 30%. The rolling reduction per pass is the rolling reduction for each pass to be rolled in the finish rolling process, and the rolling reduction is (thickness at the entrance of the pass−thickness at the exit of the pass) / thickness at the entrance of the pass. You can ask. In the finish rolling step, the time between rolling passes is desirably 10 s or less. This is because finish rolling is performed continuously by a finishing rolling mill in which a plurality of rolling mills are arranged in series. However, since the material to be rolled (sheet bar) is at a high temperature between the rolling mills, the finish rolling is performed between rolling passes. If the time exceeds 10 s, recovery of dislocations, recrystallization, coarsening of crystal grains, etc. may occur, and the ductile crack propagation arrest property and low-temperature toughness may be reduced.
そして、本発明では、第二粗圧延工程および仕上圧延工程における、９３０℃以下の未再結晶オーステナイト域での全圧下率は、６５％以上である。延性亀裂伝播停止特性および低温靱性の向上に有効な温度域、すなわち、９３０℃以下の未再結晶オーステナイト域での全圧下率が６５％未満では、所望の延性亀裂伝播停止特性および低温靭性を達成できない。 In the present invention, the total draft in the unrecrystallized austenite region at 930 ° C. or lower in the second rough rolling step and the finish rolling step is 65% or more. In a temperature range effective for improving ductile crack propagation arrestability and low-temperature toughness, that is, when the total rolling reduction in an unrecrystallized austenite region at 930 ° C. or less is less than 65%, desired ductile crack propagation arrestability and low-temperature toughness are achieved. Can not.
なお、仕上圧延の圧延終了温度は、鋼板靭性、鋼板強度、圧延負荷等の観点から、７２０〜８５０℃とすることが好ましい。仕上圧延の圧延終了温度が８５０℃を超えて高温となると、９３０℃以下の未再結晶オーステナイト域での全圧下率を６５％以上とするために、１パス当たりの圧下量を大きくする必要があり、圧延荷重の増加を招く。一方、７２０℃未満と低温となると、圧延中にフェライトが生成し、組織、析出物の粗大化を招き、低温靭性、強度が低下する。 In addition, it is preferable that the rolling end temperature of finish rolling is 720-850 degreeC from a viewpoint of steel plate toughness, steel plate strength, rolling load, and the like. When the finishing temperature of the finish rolling exceeds 850 ° C. and becomes high, the rolling reduction per pass needs to be increased in order to make the total rolling reduction in the unrecrystallized austenite region of 930 ° C. or less 65% or more. Yes, causing an increase in rolling load. On the other hand, when the temperature is as low as less than 720 ° C., ferrite is generated during rolling, causing the structure and precipitates to be coarsened, and the low-temperature toughness and strength are reduced.
粗圧延工程後の仕上圧延後冷却工程では、熱延板を板厚中心で冷却開始から冷却停止までの平均冷却速度が２０℃／ｓ以上となる冷却速度で冷却する。２０℃／ｓ未満では平均粒径５．０μｍ以下の微細なベイニティックフェライト組織が得られない。好ましくは平均冷却速度：３０℃／ｓ以上、さらに好ましくは平均冷却速度：４０℃／ｓ以上である。板厚中心の平均冷却速度は、（（冷却開始時の板厚中心の温度−冷却停止時の板厚中心の温度）／冷却時間）で求められる。板厚中心の温度は、伝熱解析により鋼板断面内の温度分布を計算し、その結果を実際の外面および内面の温度によって補正することにより求める。 In the cooling step after finish rolling after the rough rolling step, the hot rolled sheet is cooled at the center of the sheet thickness at a cooling rate at which the average cooling rate from the start of cooling to the stop of cooling is 20 ° C./s or more. If it is less than 20 ° C./s, a fine bainitic ferrite structure having an average particle size of 5.0 μm or less cannot be obtained. Preferably, the average cooling rate is 30 ° C / s or more, and more preferably, the average cooling rate is 40 ° C / s or more. The average cooling rate at the center of the sheet thickness is determined by ((temperature at center of sheet thickness at start of cooling−temperature at center of sheet thickness at stop of cooling) / cooling time). The temperature at the center of the sheet thickness is obtained by calculating the temperature distribution in the cross section of the steel sheet by heat transfer analysis and correcting the result by the actual temperatures of the outer surface and the inner surface.
仕上圧延後冷却工程後の巻取工程では、仕上圧延後冷却工程で冷却された熱延板を、コイル状に巻取る。巻取り温度は３５０〜７００℃とすることが好ましい。巻取り温度が３５０℃未満では、鋼板各位置での温度ばらつきが大きくなり、材質のばらつきや形状のばらつきが生じ、さらには、コイラー能力によっては巻き取ることができない場合も生ずる。一方、巻取り温度が７００℃を超えると、結晶粒が粗大化し、延性亀裂伝播停止特性および低温靱性が低下する場合がある。このようなことから、巻取り温度は３５０〜７００℃とすることが好ましい。そして、巻取り完了後、コイル中央部で平均冷却速度が５〜２０℃／ｈで室温まで冷却することが好ましい。コイル状に巻き取った後のコイル中央部の平均冷却速度が５℃／ｈ未満では、結晶粒が粗大化し延性亀裂伝播停止特性および低温靱性が低下する場合がある。一方、該平均冷却速度が２０℃／ｈを超えると、コイル中央部と外周、内周部との温度差が大きくなり、材質の不均一を招きやすい。なお、２０℃／ｈを超える冷却速度は通常の巻取り設備では不可能で、冷却能力を向上させるための新たな設備投資が必要となる。コイル中央部の平均冷却速度は、（（冷却開始時のコイル中央部の温度−冷却停止時のコイル中央部の温度）／冷却時間）で求められる。コイル中央部の温度は、放射温度計により測温して求める。 In the winding step after the finish-rolling cooling step, the hot-rolled sheet cooled in the finish-rolling cooling step is wound into a coil. The winding temperature is preferably from 350 to 700 ° C. If the winding temperature is lower than 350 ° C., the temperature variation at each position of the steel sheet becomes large, the material and the shape vary, and further, depending on the capacity of the coiler, it may not be possible to wind. On the other hand, if the winding temperature exceeds 700 ° C., the crystal grains become coarse, and the ductile crack propagation stopping characteristics and low-temperature toughness may decrease. For this reason, the winding temperature is preferably set to 350 to 700 ° C. Then, after the winding is completed, it is preferable to cool to a room temperature at an average cooling rate of 5 to 20 ° C./h at the center of the coil. If the average cooling rate at the center of the coil after winding into a coil is less than 5 ° C./h, the crystal grains may become coarse, and the ductile crack propagation stopping characteristics and low-temperature toughness may decrease. On the other hand, if the average cooling rate exceeds 20 ° C./h, the temperature difference between the center part of the coil, the outer periphery, and the inner periphery becomes large, and the material tends to be uneven. Note that a cooling rate exceeding 20 ° C./h is impossible with ordinary winding equipment, and new equipment investment for improving the cooling capacity is required. The average cooling rate at the center of the coil is determined by ((temperature of the center of the coil at the start of cooling−temperature of the center of the coil at the time of cooling stop) / cooling time). The temperature at the center of the coil is determined by measuring the temperature with a radiation thermometer.
このようにして製造された熱延鋼板は、引張強さＴＳが６４０ＭＰａ以上で、深さ１００μｍ以上の表面割れがなく表面品質に優れ、ＣＴＯＤ試験で−１０℃での限界開口変位量δｃが０．２５ｍｍ以上であり延性亀裂伝播停止特性に優れ、ＤＷＴＴ８５％ＳＡＴＴが−２５℃以下であり低温靱性に優れる。 The hot-rolled steel sheet manufactured in this manner has a tensile strength TS of 640 MPa or more, has no surface cracks with a depth of 100 μm or more, has excellent surface quality, and has a critical opening displacement δc at −10 ° C. of 0 in the CTOD test. 0.25 mm or more, excellent in ductile crack propagation arrestability, DWTT 85% SATT is -25 ° C or less, and excellent in low-temperature toughness.
以下に、実施例を用いて説明するが、実施例はなんら本発明を限定するものではない。 Hereinafter, the present invention will be described using examples, but the examples do not limit the present invention in any way.
表１に示す組成を有するスラブ（鋼素材）（肉厚：２５０ｍｍ）に、表２に示す条件で粗圧延工程、仕上圧延工程、仕上圧延後冷却工程および巻取工程を順に施し、熱延鋼板を得た。得られた熱延鋼板の板厚は、表２に示す仕上圧延工程で得られた熱延板の板厚と同じである。熱延鋼板Ｎｏ．１〜４、７、９〜１１、１３、１５、１６、１８〜２０については、仕上げ圧延工程後の冷却（仕上圧延後冷却工程）に加えて、粗圧延途中で粗圧延終了５パス前以降に、表２に示す急冷（粗圧延冷却工程）を行ない、粗圧延途中の冷却停止後、表２に示す温度となるまで復熱させた、すなわち、粗圧延工程として、表２に示す第一粗圧延工程、粗圧延冷却工程および第二粗圧延工程をこの順に行った。Ａｒ３変態点およびＡｃ３変態点を、表２に記載する。Ａｒ３変態点およびＡｃ３変態点は、それぞれ下記式で求めた。下記式中の元素記号は、鋼中の各成分量を示す（質量％）。
Ａｒ３（℃）＝９１０−３１０Ｃ−８０Ｍｎ−２０Ｃｕ−１５Ｃｒ−５５Ｎｉ−８０Ｍｏ
Ａｃ３（℃）＝９６１．６−３１１．９Ｃ＋４９．５Ｓｉ−３６．４Ｍｎ−５１Ｃｕ−２９Ｎｉ−８．７Ｃｒ＋１３．５Ｍｏ＋３０８．１Ｎｂ−１４０Ｎｂ＋３１８．９Ｔｉ＋１２．７Ａｌ＋４３８．１Ｐ−２８１８Ｓ＋６１１．２Ｂ−９６９Ｎ
得られた熱延鋼板から、圧延方向に平行な断面が観察面となるよう切り出し、該断面を３％ナイタールで腐食現出し、走査電子顕微鏡で２０００倍に拡大して、板厚１／２ｔ位置（ｔは鋼板の厚さ（板厚）である。）を観察中心として撮影した。得られた熱延鋼板の鋼組織は、熱延鋼板Ｎｏ．２０はベイニティックフェライトの面積率が９０％およびパーライトの面積率が１０％の混合相であり、その他の比較例および発明例は、ベイニティックフェライトの面積率が９５％以上であるベイニティックフェライト単相であった。また、ＪＩＳ Ｇ ０５５１ ２０１３ 鋼−結晶粒度の顕微鏡試験方法に記載の切断法により、ベイニティックフェライトの平均粒径を求めた結果を表２「平均粒径」欄に示す。
A slab (steel material) (thickness: 250 mm) having the composition shown in Table 1 was subjected to a rough rolling step, a finish rolling step, a cooling step after finish rolling, and a winding step in order under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. Got. The thickness of the obtained hot-rolled steel sheet is the same as the thickness of the hot-rolled sheet obtained in the finish rolling step shown in Table 2. Hot rolled steel sheet No. For 1-4, 7, 9-11, 13, 15, 16, 18-20, in addition to cooling after the finish rolling step (cooling step after finish rolling), 5 passes before and after the end of rough rolling in the course of rough rolling Then, rapid cooling (rough rolling cooling step) shown in Table 2 was performed, and after cooling was stopped during the rough rolling, the heat was recovered to the temperature shown in Table 2, that is, the first step shown in Table 2 was performed as the rough rolling step. The rough rolling step, the rough rolling cooling step, and the second rough rolling step were performed in this order. The Ar3 transformation point and the Ac3 transformation point are described in Table 2. The Ar3 transformation point and the Ac3 transformation point were determined by the following equations, respectively. The symbol of the element in the following formula indicates the amount of each component in the steel (% by mass).
Ar3 (° C) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
Ac3 (C) = 961.6-311.9C + 49.5Si-36.4Mn-51Cu-29Ni-8.7Cr + 13.5Mo + 308.1Nb-140Nb + 318.9Ti + 12.7Al + 438.1P-2818S + 611.2B-969N
From the obtained hot-rolled steel sheet, a cross section parallel to the rolling direction was cut out so as to be an observation surface, the cross section was exposed with 3% nital, and the cross section was magnified 2000 times with a scanning electron microscope. (T is the thickness (plate thickness) of the steel plate). The steel structure of the obtained hot-rolled steel sheet is as follows. Reference numeral 20 denotes a mixed phase in which the area ratio of bainitic ferrite is 90% and the area ratio of pearlite is 10%, and in other comparative examples and invention examples, the bainic ferrite has an area ratio of bainitic ferrite of 95% or more. It was a tick ferrite single phase. In addition, Table 2 “Average particle size” shows the result of obtaining the average particle size of bainitic ferrite by the cutting method described in JIS G 0551 2013 Steel—Microscopic Test Method for Grain Size.
また、得られた熱延鋼板について、表面品質試験、引張試験、衝撃試験、ＣＴＯＤ試験、ＤＷＴＴ試験を実施した。得られた結果を表３に示す。試験方法は次のとおりである。
（１）表面品質試験
得られた熱延鋼板について、鋼板の全域にわたり表面を目視で観察し、割れの有無を調査し、表面品質を評価した。１００μｍ以上の割れ等の表面欠陥が観察された場合を×、観察されなかった場合を○として評価した。
（２）引張試験
得られた熱延鋼板から、圧延方向に直交する方向（Ｃ方向）が長手方向（引張方向）となるように採取した全厚の矩形試験片を用い、ＪＩＳ Ｚ ２２４１の規定に準拠して、引張試験を実施し、引張強さＴＳを求めた。
（３）衝撃試験
得られた熱延鋼板の板厚中央部から、圧延方向に直交する方向（Ｃ方向）が長手方向となるようにＶノッチ試験片を採取し、ＪＩＳ Ｚ ２２４２の規定に準拠してシャルピー衝撃試験を実施し、破面遷移温度ｖＴｒｓ、および、試験温度：−８０℃での吸収エネルギーｖＥ−８０（Ｊ）を求めた。板厚中央部は、板厚をｔとしたときの、板厚１／２ｔ位置である。なお、試験片は３本とし、得られた破面遷移温度ｖＴｒｓ（℃）の算術平均および吸収エネルギー値の算術平均をそれぞれ求め、その鋼板の破面遷移温度ｖＴｒｓ（℃）および吸収エネルギー値ｖＥ−８０（Ｊ）とした。
（４）ＣＴＯＤ試験
得られた熱延鋼板から、圧延方向に直交する方向（Ｃ方向）が長手方向となるようにＣＴＯＤ試験片を採取し、ＢＳ ７４４８：Ｐａｒｔ１ １９９１の規定に準拠して、試験温度：−１０℃でＣＴＯＤ試験を行い、−１０℃での限界開口変位量δｃ（ｍｍ）を求めた。
（５）ＤＷＴＴ試験
得られた熱延鋼板から、圧延方向に直交する方向（Ｃ方向）が長手方向となるようにＤＷＴＴ試験片を採取し、ＡＳＴＭ Ｅ４３６の規定に準拠して、ＤＷＴＴ試験を実施し、８５％ＳＡＴＴ温度（℃）（：延性破面率が８５％となる最低温度）を求めた。
Further, a surface quality test, a tensile test, an impact test, a CTOD test, and a DWTT test were performed on the obtained hot-rolled steel sheet. Table 3 shows the obtained results. The test method is as follows.
(1) Surface quality test The surface of the obtained hot-rolled steel sheet was visually observed over the entire area of the steel sheet, checked for cracks, and the surface quality was evaluated. The case where surface defects such as cracks of 100 μm or more were observed was evaluated as x, and the case where they were not observed was evaluated as ○.
(2) Tensile test From the obtained hot-rolled steel sheet, a rectangular test piece of full thickness collected so that the direction (direction C) perpendicular to the rolling direction is the longitudinal direction (tensile direction) is used, and is specified in JIS Z 2241. , A tensile test was performed to determine a tensile strength TS.
(3) Impact test From the central part of the thickness of the obtained hot-rolled steel sheet, a V-notch test piece was sampled such that the direction perpendicular to the rolling direction (C direction) was the longitudinal direction, and was compliant with the provisions of JIS Z 2242. Then, a Charpy impact test was performed to determine a fracture surface transition temperature vTrs and an absorbed energy vE-80 (J) at a test temperature of −80 ° C. The plate thickness center is a position where the plate thickness is t, where t is the plate thickness. The number of test pieces was three, and the arithmetic mean of the obtained fracture surface transition temperature vTrs (° C.) and the arithmetic average of the absorbed energy value were obtained, respectively, and the fracture surface transition temperature vTrs (° C.) and the absorbed energy value vE of the steel sheet were obtained. -80 (J).
(4) CTOD test A CTOD test piece was sampled from the obtained hot-rolled steel sheet so that the direction perpendicular to the rolling direction (C direction) was the longitudinal direction, and tested in accordance with the rules of BS 7448: Part1 1991. Temperature: A CTOD test was performed at −10 ° C., and a critical opening displacement δc (mm) at −10 ° C. was obtained.
(5) DWTT test A DWTT test piece is sampled from the obtained hot-rolled steel sheet so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction, and a DWTT test is performed in accordance with the provisions of ASTM E436. Then, an 85% SATT temperature (° C.) (: the lowest temperature at which the ductile fracture ratio becomes 85%) was determined.
本発明例である熱延鋼板Ｎｏ．７、９、１０、１３、１５、１６、１９はいずれも、引張強さが６４０ＭＰａ以上で、表面割れの発生等の表面欠陥もなく、表面品質に優れ、かつＤＷＴＴ試験の８５％ＳＡＴＴが−２５℃以下と低温靭性に優れ、−１０℃におけるδｃが０．２５ｍｍ以上と延性亀裂伝播停止特性に優れた高強度熱延鋼板となっていた。一方、本発明を外れる比較例は、表面割れが発生していたか、靭性が低下し延性亀裂伝播停止特性が低下していたか、あるいは表面割れが発生し、低温靭性が低下し延性亀裂伝播停止特性が低下していた。 Hot-rolled steel sheet No. 7 , 9, 10, 13, 15, 16 and 19 all have a tensile strength of 640 MPa or more, have no surface defects such as generation of surface cracks, are excellent in surface quality, and have an 85% SATT in the DWTT test. A high-strength hot-rolled steel sheet excellent in low-temperature toughness of 25 ° C. or less and having a δc at −10 ° C. of 0.25 mm or more and excellent in ductile crack propagation stopping characteristics was obtained. On the other hand, in the comparative examples deviating from the present invention, surface cracks were generated, ductility crack propagation arresting properties were reduced due to reduced toughness, or surface cracks occurred, and low temperature toughness was reduced to cause ductile crack propagation arrested properties. Had declined.
Claims (4)
Ｃ：０．０２〜０．０８％、Ｓｉ：０．５％以下、
Ｍｎ：０．８〜２．０％、Ｐ：０．０２５％以下、
Ｓ：０．００５％以下、Ａｌ：０．００５〜０．１０％、
Ｎ：０．００５％以下、Ｎｂ：０．０６６〜０．１０％、
Ｔｉ：０．００５〜０．０５％、Ｃａ：０．０００５〜０．００５％、
Ｏ：０．００５％以下を含有し、残部Ｆｅおよび不可避的不純物からなる成分組成を有し、
鋼組織が、平均粒径５．０μｍ以下のベイニティックフェライト単相からなり、
引張強さＴＳが６４０ＭＰａ以上であり、ＣＴＯＤ試験で−１０℃での限界開口変位量δｃが０．２５ｍｍ以上であることを特徴とする高強度熱延鋼板。 In mass%,
C: 0.02-0.08%, Si: 0.5% or less,
Mn: 0.8 to 2.0%, P: 0.025% or less,
S: 0.005% or less, Al: 0.005 to 0.10%,
N: 0.005% or less, Nb: 0.066 to 0.10%,
Ti: 0.005 to 0.05%, Ca: 0.0005 to 0.005%,
O: contains 0.005% or less, has a component composition consisting of the balance Fe and inevitable impurities,
The steel structure is composed of a bainitic ferrite single phase having an average grain size of 5.0 μm or less,
Tensile strength TS is der least 640MPa is, high-strength hot-rolled steel sheet, wherein the limiting aperture displacement δc at -10 ° C. in CTOD test is the 0.25mm or more.
前記鋼素材を、請求項１または２に記載の成分組成を有する鋼素材とし、
前記粗圧延工程が、鋼素材を加熱した後、粗圧延を施す第一粗圧延工程と、第一粗圧延工程後に、表面温度で平均冷却速度：５０℃／ｓ以上となる冷却速度でＡｒ３変態点以下の温度に達するまで急冷した後、該急冷を停止する粗圧延冷却工程と、粗圧延冷却工程後に、９３０℃以下の未再結晶オーステナイト域で粗圧延を１パス以上５パス以下施しシートバーとする第二粗圧延工程とを有し、
前記第二粗圧延工程および前記仕上圧延工程における９３０℃以下の未再結晶オーステナイト域での全圧下率が６５％以上であり、
前記仕上圧延後冷却工程が、前記熱延板を板厚中心で平均冷却速度：２０℃／ｓ以上となる冷却速度で冷却することを特徴とする、鋼組織が、平均粒径５．０μｍ以下のベイニティックフェライト単相からなり、引張強さＴＳが６４０ＭＰａ以上であり、ＣＴＯＤ試験で−１０℃での限界開口変位量δｃが０．２５ｍｍ以上である高強度熱延鋼板の製造方法。 A rough rolling step of subjecting the steel material to rough rolling to form a sheet bar, a finish rolling step of subjecting the sheet bar to finish rolling to form a hot rolled sheet, a finishing rolling step of cooling the hot rolled sheet, and a cooling step after finish rolling; In performing a winding process and a winding process after the cooling process after rolling in this order to obtain a hot-rolled steel sheet,
The steel material, a steel material having the component composition according to claim 1 or 2,
In the rough rolling step, a steel material is heated and then rough rolling is performed, and after the first rough rolling step, an Ar3 transformation is performed at a cooling rate at which the average cooling rate at the surface temperature is 50 ° C./s or more. After the quenching is performed until the temperature reaches the temperature equal to or lower than the point, the quenching is stopped, and after the rough rolling cooling step, the rough rolling is performed in the unrecrystallized austenite region at 930 ° C. or less by 1 pass or more and 5 passes or less. And a second rough rolling step,
The total rolling reduction in the unrecrystallized austenite region of 930 ° C. or less in the second rough rolling step and the finish rolling step is 65% or more;
The steel structure has an average grain size of 5.0 μm or less , wherein the cooling step after the finish rolling is to cool the hot-rolled sheet at a cooling rate of 20 ° C./s or more at the center of the thickness of the hot-rolled sheet. A method for producing a high-strength hot-rolled steel sheet comprising a bainitic ferrite single phase having a tensile strength TS of 640 MPa or more and a critical opening displacement δc at −10 ° C. of 0.25 mm or more in a CTOD test .
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