JP4876972B2

JP4876972B2 - Thick steel plate for welded structure having excellent fatigue crack propagation characteristics in the thickness direction and method for producing the same

Info

Publication number: JP4876972B2
Application number: JP2007049178A
Authority: JP
Inventors: 章夫大森; 浩文大坪; 智之横田; 眞司三田尾; 伸夫鹿内
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2007-02-28
Filing date: 2007-02-28
Publication date: 2012-02-15
Anticipated expiration: 2027-02-28
Also published as: JP2008214646A

Description

本発明は、建築、橋梁、船舶、海洋構造物、圧力容器（タンク）等の溶接鋼構造物用として好適な、溶接構造用厚鋼板に係り、とくに、板厚方向の疲労亀裂の伝播を抑制し、板厚方向の耐疲労亀裂伝播特性の向上に関する。 The present invention relates to a welded steel plate suitable for welded steel structures such as buildings, bridges, ships, offshore structures, pressure vessels (tanks), and in particular, suppresses the propagation of fatigue cracks in the thickness direction. And improving fatigue crack propagation characteristics in the thickness direction.

建築、橋梁、船舶、海洋構造物、圧力容器（タンク）等の溶接鋼構造物に使用される鋼材は、強度、靭性などの機械的特性および溶接性に優れていることはもちろん、構造安全性を確保するため、近年とくに、耐疲労特性に優れていることが要求されている。
疲労特性は、一般に、疲労亀裂発生特性と、疲労亀裂伝播特性とに分けて評価されている。溶接鋼構造物では、溶接止端部が応力集中部となりやすく、また引張の残留応力も作用するため、溶接止端部が疲労亀裂の発生源となることが多い。このような疲労亀裂発生の防止策として、止端部形状の改善や圧縮の残留応力の導入などの方策が知られている。しかし、溶接鋼構造物には多数の溶接止端部があり、上記した疲労亀裂発生の防止策を実行するには、多くの労力と時間とを必要とし、施工工数の増加や、施工コストの高騰を招き、溶接鋼構造物の製造コストを著しく増大させる。そのため、溶接鋼構造物の耐疲労特性向上対策として、使用される鋼材自体の耐疲労亀裂伝播特性の向上が要望されている。鋼材自体の耐疲労亀裂伝播特性を向上させることにより、疲労亀裂の初期成長が抑制され、鋼構造物の寿命を延長することが可能となる。 Steel materials used in welded steel structures such as buildings, bridges, ships, offshore structures and pressure vessels (tanks) are not only excellent in mechanical properties such as strength and toughness and weldability, but also structural safety. In recent years, in particular, it has been required to have excellent fatigue resistance.
Fatigue properties are generally evaluated by dividing them into fatigue crack initiation properties and fatigue crack propagation properties. In welded steel structures, the weld toe tends to be a stress concentration part, and tensile residual stress also acts, so the weld toe often becomes a source of fatigue cracks. As measures for preventing the occurrence of such fatigue cracks, measures such as improvement of the shape of the toe portion and introduction of compressive residual stress are known. However, there are many weld toes in welded steel structures, and it takes a lot of labor and time to execute the above-mentioned measures to prevent the occurrence of fatigue cracks. Incurs a surge and significantly increases the manufacturing cost of welded steel structures. Therefore, as a measure for improving the fatigue resistance of welded steel structures, improvement of the fatigue crack propagation characteristics of the steel material used is desired. By improving the fatigue crack propagation characteristics of the steel material itself, the initial growth of fatigue cracks can be suppressed and the life of the steel structure can be extended.

厚鋼板を溶接して構成される溶接鋼構造物では、溶接止端部である鋼板表面から疲労亀裂が発生し、板厚方向に伝播して疲労破壊が進展していく場合が多い。このため、疲労破壊の進展を防止し溶接構造物の耐疲労特性を向上させるには、板厚方向の疲労亀裂伝播速度を低減することが肝要となる。このようなことから、溶接鋼構造物用として、板厚方向の疲労亀裂伝播を抑制できる板厚方向の耐疲労亀裂伝播特性に優れた厚鋼板が要望されている。 In a welded steel structure constructed by welding thick steel plates, fatigue cracks are often generated from the surface of the steel plate, which is the weld toe, and propagates in the plate thickness direction and progresses in fatigue failure in many cases. For this reason, in order to prevent the progress of fatigue fracture and improve the fatigue resistance of the welded structure, it is important to reduce the fatigue crack propagation rate in the thickness direction. For these reasons, there is a demand for a thick steel plate excellent in fatigue crack propagation characteristics in the plate thickness direction that can suppress the fatigue crack propagation in the plate thickness direction for use in welded steel structures.

このような要望に対し、例えば、特許文献１には、耐疲労亀裂進展特性の良好な鋼板およびその製造方法が記載されている。特許文献１に記載された技術は、Ｃ：0.02〜0.2％を含み、Si、Mn、Ｐ、Alを適正量含有し、さらにTi、Nb、Ｂ、Cu、Niのうちの１種以上、およびCr：１％以下、Mo：１％以下を含有する溶接構造用鋼に、Ar_３変態点以上の未再結晶域で圧下率60％以上の圧延を施した後、圧延終了温度から（Ar_３変態点−50℃）までを３℃/ｓ以下の冷却速度で冷却し、さらに５℃/ｓ以上の冷却速度で冷却することにより、硬さが母相の硬さより30％以上高く、さらにはアスペクト比が４以上、長さ20μm以上の、圧延方向に延在する縞状の第二相を母相中に５〜50％散在する組織の鋼板を得る技術である。特許文献１に記載された技術では、硬質第二相と母相(フェライト相)との硬度差を大きくし、硬質第二相のアスペクト比を調整して、耐疲労亀裂伝播特性を向上させている。特許文献１に記載された技術では、かかる鋼板を大型構造物に使用することにより、大型構造物に高い疲労亀裂伝播阻止性能を付与することができるとしている。 In response to such a demand, for example, Patent Document 1 describes a steel plate having good fatigue crack growth resistance and a method for manufacturing the same. The technology described in Patent Document 1 includes C: 0.02 to 0.2%, contains an appropriate amount of Si, Mn, P, and Al, and further includes at least one of Ti, Nb, B, Cu, and Ni, and A welded structural steel containing Cr: 1% or less and Mo: 1% or less is subjected to rolling at a reduction rate of 60% or more in an unrecrystallized region above the Ar ₃ transformation point, and then from the rolling end temperature (Ar ₃ By cooling at a cooling rate of 3 ° C./s or less, and further at a cooling rate of 5 ° C./s or more, the hardness is 30% or more higher than the hardness of the parent phase, This is a technique for obtaining a steel sheet having an aspect ratio of 4 or more and a length of 20 μm or more and having a structure in which a striped second phase extending in the rolling direction is dispersed in the matrix by 5 to 50%. In the technique described in Patent Document 1, the hardness difference between the hard second phase and the parent phase (ferrite phase) is increased, the aspect ratio of the hard second phase is adjusted, and the fatigue crack propagation resistance is improved. Yes. In the technique described in Patent Document 1, high fatigue crack propagation prevention performance can be imparted to a large structure by using such a steel plate for the large structure.

また、特許文献２には、Ｃ：0.02〜0.20％を含み、Si、Mn、Ｐ、Ｓ、Al、Ｎを適正量含有し、あるいはさらにCu、Ni、Cr、Mo、Nb、Ｖ、Tiのうちの１種以上を含む鋼塊または鋼片を所定範囲内の温度に加熱し、Ar_３変態点以上の温度で粗圧延を終了したのち、フェライト分率が60％以上となる温度域における累積圧下率が40％以上となる仕上圧延を行うことにより、板厚方向の断面組織が60〜90面積％のフェライト母相と第二相からなり、第二相が、フェライト相の硬さとの比が特定の関係式で定義される所定値を超える硬さと、3.42超えのアスペクト比を有する厚鋼板を得る技術が記載されている。特許文献２に記載された技術では、硬質第二相と母相(フェライト相)との硬度差を大きくし、硬質第二相のアスペクト比を調整して、耐疲労亀裂伝播特性を向上させている。かかる厚鋼板は従来材に比べて疲労寿命を改善することができ、かかる厚鋼板を溶接鋼構造物に使用すれば、溶接構造物における溶接構造部位の疲労設計限度を向上させることができるとしている。 Patent Document 2 includes C: 0.02 to 0.20%, contains appropriate amounts of Si, Mn, P, S, Al, and N, or further includes Cu, Ni, Cr, Mo, Nb, V, and Ti. The steel ingot or steel slab containing one or more of them is heated to a temperature within the specified range, and after rough rolling is completed at a temperature above the Ar ₃ transformation point, accumulation in a temperature range where the ferrite fraction becomes 60% or more By performing finish rolling with a rolling reduction of 40% or more, the cross-sectional structure in the plate thickness direction is composed of a ferrite matrix phase and a second phase of 60 to 90 area%, and the second phase is a ratio of the hardness of the ferrite phase. Describes a technique for obtaining a thick steel plate having a hardness exceeding a predetermined value defined by a specific relational expression and an aspect ratio exceeding 3.42. In the technique described in Patent Document 2, the hardness difference between the hard second phase and the parent phase (ferrite phase) is increased, the aspect ratio of the hard second phase is adjusted, and the fatigue crack propagation resistance is improved. Yes. Such a thick steel plate can improve the fatigue life compared to conventional materials, and if such a thick steel plate is used in a welded steel structure, the fatigue design limit of the welded structure portion in the welded structure can be improved. .

また、特許文献３には、Ｃ：0.04〜0.3％を含み、Si、Mn、Ｐ、Ｓ、Al、Ｎを適正量に調整して含有し、あるいはさらにCu、Ni、Cr、Mo、Ｗ、Nb、Ｖ、Ti、Zr、Ta、Ｂのうちの１種以上を含む鋼片を所定範囲内の温度に加熱し、圧延開始温度：850℃以下、圧延終了温度：Ar_３変態点以上で、累積圧下率が30％以上の圧延を含み、全圧下比：５以上の熱間圧延を行い、500℃以下まで５℃/s以下の冷却速度で冷却したのち、（Ac_１変態点＋30℃）〜（Ac_３変態点−50℃）の温度に再加熱し、制御冷却を施す厚鋼板の製造方法が記載されている。特許文献３に記載された技術によれば、鋼板圧延方向に平行な板厚断面の組織が、平均粒径20μm以下のフェライト相と、組織分率が10〜70％で、アスペクト比が10以上、硬さが230HV以上の硬質第二相と、からなる複合組織を有し、引張強さ：400MPa以上の強度と、優れた疲労強度を有する厚鋼板が得られるとしている。特許文献３に記載された技術では、特殊な合金元素や複雑な製造プロセスを利用することなく、高い疲労強度を有する厚鋼板を製造できるとしている。特許文献３に記載された技術では、硬質第二相と母相(フェライト相)との硬度差を大きくし、硬質第二相のアスペクト比を調整するとともに、フェライト相を微細化して、耐疲労亀裂伝播特性を向上させている。 Patent Document 3 contains C: 0.04 to 0.3%, contains Si, Mn, P, S, Al, N adjusted to an appropriate amount, or further contains Cu, Ni, Cr, Mo, W, A steel slab containing one or more of Nb, V, Ti, Zr, Ta, and B is heated to a temperature within a predetermined range, rolling start temperature: 850 ° C. or lower, rolling end temperature: Ar ₃ transformation point or higher, Including rolling with a cumulative reduction ratio of 30% or more, hot rolling with a total reduction ratio of 5 or more, cooling to 500 ° C or less at a cooling rate of 5 ° C / s or less (Ac ₁ transformation point + 30 ° C) ~ reheated to a temperature of (Ac ₃ transformation point -50 ° C.), a manufacturing method of a steel plate subjected to controlled cooling is described. According to the technique described in Patent Document 3, the structure of the plate thickness cross section parallel to the rolling direction of the steel sheet is a ferrite phase having an average grain size of 20 μm or less, the structure fraction is 10 to 70%, and the aspect ratio is 10 or more. It is said that a thick steel plate having a composite structure composed of a hard second phase having a hardness of 230 HV or more, a tensile strength of 400 MPa or more, and an excellent fatigue strength can be obtained. In the technique described in Patent Document 3, a thick steel plate having high fatigue strength can be manufactured without using a special alloy element or a complicated manufacturing process. In the technique described in Patent Document 3, the hardness difference between the hard second phase and the parent phase (ferrite phase) is increased, the aspect ratio of the hard second phase is adjusted, the ferrite phase is refined, and fatigue resistance is improved. The crack propagation characteristics are improved.

また、特許文献４には、Ｃ：0.04〜0.3％を含み、Si、Mn、Ｐ、Ｓ、Al、Ｎを適正量に調整して含有し、あるいはさらにCu、Ni、Cr、Mo、Ｗ、Nb、Ｖ、Ti、Zr、Ta、Ｂのうちの１種以上を含む鋼片を所定範囲内の温度に加熱し、圧下比が２以上の熱間圧延を行い、熱間圧延終了後、フェライト分率が10％以上となる温度まで0.1〜２℃/sの冷却速度で冷却したのち、500℃以下まで５〜100℃/sで急冷することにより、鋼板表面に平行な断面組織が、フェライト相と、組織分率が20〜80％で、硬さが250〜800HV、平均円相当径が10〜200μm、硬質第二相間の最大間隔：500μm以下の硬質第二相とからなる複合組織を有し、優れた耐疲労特性を有する厚鋼板を得る技術が記載されている。特許文献４に記載された技術では、硬質第二相と母相(フェライト相)との硬度差を大きくし、硬質第二相の分率を大きくするとともに、硬質第二相の分布状態を適正に調整して、耐疲労亀裂伝播特性を向上させている。 Patent Document 4 contains C: 0.04 to 0.3%, contains Si, Mn, P, S, Al, N adjusted to an appropriate amount, or further Cu, Ni, Cr, Mo, W, A steel slab containing one or more of Nb, V, Ti, Zr, Ta, and B is heated to a temperature within a predetermined range, hot rolled with a reduction ratio of 2 or more, and after hot rolling is completed, ferrite After cooling at a cooling rate of 0.1 to 2 ° C / s to a temperature at which the fraction becomes 10% or more, by rapidly cooling to 5 to 100 ° C / s to 500 ° C or less, the cross-sectional structure parallel to the steel sheet surface becomes ferrite. A composite structure comprising a phase and a hard second phase having a structure fraction of 20 to 80%, a hardness of 250 to 800 HV, an average equivalent circle diameter of 10 to 200 μm, and a maximum interval between the hard second phases: 500 μm or less A technique for obtaining a thick steel plate having excellent fatigue resistance is described. In the technique described in Patent Document 4, the hardness difference between the hard second phase and the parent phase (ferrite phase) is increased, the fraction of the hard second phase is increased, and the distribution state of the hard second phase is appropriately set. To improve fatigue crack propagation characteristics.

また、特許文献５には、板厚方向の疲労亀裂伝播速度が低い厚鋼板およびその製造方法が記載されている。特許文献５に記載された技術は、Ｃ：0.015〜0.20％を含み、Si、Mn、Ｐ、Ｓを適正量含有し、あるいはさらにCu、Ni、Cr、Mo、Nb、Ｖのうちの１種以上、および/または、Ti、ＮをTi/Ｎが2.0〜3.4となるように含む鋼塊を、所定範囲内の温度に加熱し、再結晶温度域で20〜90％の累積圧下率で圧延し、引続きAr_３変態点以上の未再結晶温度域で10〜80％の累積圧下率で圧延し、さらにAr_３変態点〜600℃で40〜90％の累積圧下率で仕上圧延し、圧延終了後、30〜300ｓ放冷し、しかる後に５〜100℃/sで室温〜600℃に制御冷却することにより、回復または再結晶フェライト粒の面積率が15〜40％で、板厚方向の（２００）面の回折強度比が2.0〜15.0である組織を有する厚鋼板を得る技術である。特許文献５に記載された技術により製造された鋼板は、板厚方向の疲労亀裂伝播速度が低く、かかる鋼板を溶接鋼構造物に適用することにより、溶接鋼構造物の疲労破壊に対する信頼性を向上させることができるとしている。 Patent Document 5 describes a thick steel plate having a low fatigue crack propagation rate in the thickness direction and a method for manufacturing the same. The technique described in Patent Document 5 includes C: 0.015 to 0.20%, contains an appropriate amount of Si, Mn, P, and S, or is further one of Cu, Ni, Cr, Mo, Nb, and V The above and / or a steel ingot containing Ti and N so that Ti / N is 2.0 to 3.4 is heated to a temperature within a predetermined range, and rolled at a cumulative reduction ratio of 20 to 90% in the recrystallization temperature range. Then, rolling is performed at an unrecrystallized temperature range above the Ar ₃ transformation point at a cumulative reduction of 10 to 80%, and finish rolling at a cumulative reduction of 40 to 90% at an Ar ₃ transformation point to 600 ° C. After the completion, it is allowed to cool for 30 to 300 s, and then is controlled and cooled at room temperature to 600 ° C. at 5 to 100 ° C./s, so that the area ratio of recovered or recrystallized ferrite grains is 15 to 40% in the thickness direction. This is a technique for obtaining a thick steel plate having a structure in which the diffraction intensity ratio of the (200) plane is 2.0 to 15.0. The steel plate manufactured by the technique described in Patent Document 5 has a low fatigue crack propagation rate in the thickness direction, and by applying such a steel plate to a welded steel structure, the reliability against fatigue fracture of the welded steel structure is improved. It can be improved.

また、特許文献６には、Ｃ：0.02〜0.2％を含み、Si、Mn、Ｐ、Ｓ、Al、Ｎを適正量含有し、あるいはさらにCu、Ni、Cr、Moのうちの１種以上、および／または、Nb、Ｖ、Tiのうちの１種以上を含む鋼片を所定範囲内の温度に加熱し、粗圧延において少なくともオーステナイト未再結晶域の圧下率を10％以上とし、さらにフェライトが70％を超えるα−γ二相域またはα単相域で累積圧下率を75％以上とする仕上圧延を施すことにより、フェライト相が70％以上で、鋼板表面に平行な測定面で鋼板内部の（１１１）面のＸ線回折強度比と（１００）面のＸ線回折強度比との比が1.25〜2.0である組織を有する鋼板を得る技術が記載されている。特許文献６に記載された技術では、異方位の境界で亀裂の分岐が生じ、（１１１）面のＸ線回折強度比と（１００）面のＸ線回折強度比との比が適切なときに、とくに耐疲労特性が向上するとしている。
特開平７−90478号公報特開平11−1742号公報特開2003−3229号公報特開2003−239036号公報特開平８−199286号公報特開2000−17379号公報 Patent Document 6 includes C: 0.02 to 0.2%, contains an appropriate amount of Si, Mn, P, S, Al, N, or one or more of Cu, Ni, Cr, Mo, And / or a steel slab containing one or more of Nb, V, and Ti is heated to a temperature within a predetermined range, and the rolling reduction of at least the austenite non-recrystallized region is 10% or more in rough rolling, By applying finish rolling to a cumulative reduction ratio of 75% or more in the α-γ two-phase region or α single-phase region exceeding 70%, the ferrite phase is 70% or more and the measurement surface parallel to the steel plate surface is inside the steel plate. Describes a technique for obtaining a steel sheet having a structure in which the ratio of the X-ray diffraction intensity ratio of the (111) plane to the X-ray diffraction intensity ratio of the (100) plane is 1.25 to 2.0. In the technique described in Patent Document 6, when a branch of a crack occurs at a boundary of different orientations, the ratio of the (111) plane X-ray diffraction intensity ratio to the (100) plane X-ray diffraction intensity ratio is appropriate. In particular, fatigue resistance is improved.
JP-A-7-90478 Japanese Patent Laid-Open No. 11-1742 Japanese Patent Laid-Open No. 2003-3229 Japanese Patent Laid-Open No. 2003-239036 JP-A-8-199286 Japanese Unexamined Patent Publication No. 2000-17379

特許文献１〜４に記載された技術は、いずれも鋼板組織を、軟質のフェライト相と、鋼板の圧延方向に伸長した硬質第二相とからなる複相組織とし、軟質相と硬質相との界面で疲労亀裂の進展が遮られ、疲労亀裂が停滞したり屈曲するという現象を利用して耐疲労亀裂伝播特性を向上させる技術である。しかし、これら硬質第二相を利用した技術においては、疲労亀裂伝播速度を十分に低減させるために、硬質第二相の組織分率と硬質第二相の硬さを高くすることが必要となる。このため、鋼板の靭性や延性が低下するという問題があった。この鋼板の靭性や延性の低下は、多量の合金元素の含有で防止できる。しかし、多量の合金元素の含有は、材料コストの高騰を招くという問題が避けられない。 In the techniques described in Patent Documents 1 to 4, the steel sheet structure is a multiphase structure composed of a soft ferrite phase and a hard second phase elongated in the rolling direction of the steel sheet. This is a technique for improving the fatigue crack propagation characteristics by utilizing the phenomenon that fatigue cracks are blocked at the interface and the fatigue cracks stagnate or bend. However, in the technology using these hard second phases, it is necessary to increase the structure fraction of the hard second phase and the hardness of the hard second phase in order to sufficiently reduce the fatigue crack propagation rate. . For this reason, there existed a problem that the toughness and ductility of a steel plate fell. This reduction in toughness and ductility of the steel sheet can be prevented by containing a large amount of alloy elements. However, the inclusion of a large amount of alloy elements inevitably increases the material cost.

また、特許文献５、６に記載された技術は、特定の結晶方位を有するフェライト粒を発達させ、集合組織を活用して耐疲労亀裂伝播特性を向上させる技術である。
疲労亀裂先端の塑性変形挙動がフェライトの結晶方位に大きく依存することは広く知られている。例えば、疲労亀裂が、｛１００｝面が板厚方向に垂直（｛１００｝面が板面に平行）な方位を有するフェライト中を板厚方向に伝播する際には、疲労亀裂先端において、様々なすべり系が活動して転位同士の干渉が生じ、亀裂の伝播が抑制される。一方、疲労亀裂が、｛１１１｝面が板厚方向に垂直（｛１１１｝面が板面に平行）なフェライト中を板厚方向に伝播する際には、亀裂先端において活動するすべり系が限定されるため、他のすべり系の干渉を受けることがなく、塑性変形が容易で疲労亀裂が進展し、疲労亀裂の伝播は抑制されず、疲労破壊が進行する。 The techniques described in Patent Documents 5 and 6 are techniques for developing ferrite grains having a specific crystal orientation and utilizing the texture to improve fatigue crack propagation resistance.
It is well known that the plastic deformation behavior at the tip of a fatigue crack depends greatly on the crystal orientation of ferrite. For example, when a fatigue crack propagates in a ferrite in which the {100} plane is perpendicular to the plate thickness direction ({100} plane is parallel to the plate plane) in the plate thickness direction, The slipping system is activated, causing interference between dislocations and suppressing the propagation of cracks. On the other hand, when a fatigue crack propagates in the thickness direction of ferrite in which the {111} plane is perpendicular to the plate thickness direction ({111} plane is parallel to the plate surface), the slip system that is active at the crack tip is limited. Therefore, there is no interference from other slip systems, plastic deformation is easy, fatigue cracks progress, fatigue crack propagation is not suppressed, and fatigue fracture proceeds.

特許文献５、６に記載された技術では、フェライト＋オーステナイト二相域あるいはフェライト単相域で圧延を行い、フェライトを加工し、｛１００｝面が板面に平行に揃った集合組織を発達させている。しかし、加工され硬化したフェライト相の増加は、鋼板靭性を顕著に低下させる。このため、特許文献５、６に記載された技術では、優れた耐疲労亀裂伝播特性と優れた靭性とを両立させることが難しいという問題を残していた。 In the techniques described in Patent Documents 5 and 6, rolling is performed in a ferrite + austenite two-phase region or a ferrite single-phase region, the ferrite is processed, and a texture in which {100} planes are aligned parallel to the plate surface is developed. ing. However, the increase in the processed and hardened ferrite phase significantly reduces the steel sheet toughness. For this reason, the techniques described in Patent Documents 5 and 6 have left a problem that it is difficult to achieve both excellent fatigue crack propagation characteristics and excellent toughness.

本発明は、かかる従来技術の問題を有利に解決し、靭性に優れ、かつ板厚方向の耐疲労亀裂伝播特性に優れた溶接構造用厚鋼板および該溶接構造用厚鋼板を工業的に安定しかつ容易に製造できる、溶接構造用厚鋼板の製造方法を提供することを目的とする。なお、ここでいう「靭性に優れた」とは、シャルピー衝撃試験における破面遷移温度vTrs（℃）が−60℃以下である場合をいう。また、ここでいう「耐疲労亀裂伝播特性に優れた」とは、ASTM E647の規定に準拠した疲労亀裂伝播試験において、応力拡大係数範囲ΔＫが10MPa√ｍの場合に疲労亀裂伝播速度が0.60×10^-9ｍ/cycle未満、および、ΔＫが15MPa√ｍの場合に疲労亀裂伝播速度が5.0×10^-9ｍ/cycle未満である場合をいう。 The present invention advantageously solves the problems of the prior art, and industrially stabilizes the steel plate for welded structure and the welded steel plate for welded structure that have excellent toughness and excellent fatigue crack propagation characteristics in the thickness direction. Another object of the present invention is to provide a method for producing a thick steel plate for welded structure that can be easily produced. Here, “excellent toughness” refers to the case where the fracture surface transition temperature vTrs (° C.) in the Charpy impact test is −60 ° C. or lower. The term “excellent in fatigue crack propagation characteristics” as used herein means that the fatigue crack propagation rate is 0.60 × when the stress intensity factor range ΔK is 10 MPa√m in a fatigue crack propagation test in accordance with the provisions of ASTM E647. less than 10 ^-9 m / cycle, and, [Delta] K means a case fatigue crack growth rate in the case of 15MPa√m is less than 5.0 × 10 ^-9 m / cycle.

発明者らは、上記した目的を達成するために、靭性および板厚方向の耐疲労亀裂伝播特性に影響する各種要因、とくにミクロ組織の影響について鋭意研究した。その結果、発明者らは、鋼板組織を、
(イ)板厚方向に垂直な面（板面に平行な面）において、｛１００｝集合組織または｛１１０｝集合組織を発達させた組織とする、さらに、
(ロ)板面（圧延面）にほぼ平行な結晶面を共有するフェライト粒の集団（フェライト粒コロニー）の板厚方向の厚さを薄くした組織とする
ことにより、特別に硬さを増加させた硬質な第二相を含まなくとも、優れた板厚方向の耐疲労亀裂伝播特性を確保でき、優れた靭性と優れた板厚方向の耐疲労亀裂伝播特性を兼備した厚鋼板とすることができることを知見した。 In order to achieve the above-mentioned object, the inventors diligently studied various factors affecting the toughness and fatigue crack propagation characteristics in the thickness direction, particularly the influence of the microstructure. As a result, the inventors have changed the steel sheet structure to
(A) In a plane perpendicular to the plate thickness direction (plane parallel to the plate surface), a {100} texture or a {110} texture is developed.
(B) By increasing the thickness of the group of ferrite grains (ferrite grain colonies) that share a crystal plane almost parallel to the plate surface (rolled surface) in the thickness direction, the hardness is increased specially Even if it does not contain a hard second phase, it is possible to secure excellent fatigue crack propagation characteristics in the thickness direction, and to make a thick steel sheet that has excellent toughness and excellent fatigue crack propagation characteristics in the thickness direction. I found out that I can do it.

また、発明者らは、｛１００｝集合組織を発達させるための圧延を、500℃以上のフェライト単相温度域、あるいはオーステナイト＋フェライト二相温度域で、かつ圧延歪速度を通常より低下させるか、圧延パス間時間を比較的に長くすることにより、加工されたフェライトの回復または再結晶を十分に進行させ、回復または再結晶により平均硬さが150HV未満となったフェライトの組織分率を60体積％以上とすることが、更に優れた靭性と優れた板厚方向の耐疲労亀裂伝播特性とを兼備させるという観点から重要であることを新規に知見した。 In addition, the inventors can perform rolling to develop a {100} texture in a ferrite single-phase temperature range of 500 ° C. or higher, or austenite + ferrite two-phase temperature range, and lower the rolling strain rate than usual. By making the time between rolling passes relatively long, recovery or recrystallization of the processed ferrite proceeds sufficiently, and the structural fraction of ferrite whose average hardness is less than 150 HV by recovery or recrystallization is reduced to 60 The present inventors have newly found out that it is important to set the volume% or more to be more excellent in toughness and excellent fatigue crack propagation characteristics in the thickness direction.

さらに、発明者らは、上記した500℃以上のフェライト単相温度域、あるいはオーステナイト＋フェライト二相温度域で行う圧延の、直前の素材組織を微細化しておくことが、上記したフェライト粒コロニーの板厚方向厚さが薄い組織を形成するうえで重要であることを知見した。
本発明は、かかる知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨はつぎのとおりである。
（１）質量％で、Ｃ：0.06〜0.20％、Si：1.0％以下、Mn：2.0％以下、Ｐ：0.10％以下、Ｓ：0.006％以下、Al：0.10％以下を含み、残部Feおよび不可避的不純物からなる組成を有する溶接構造用の厚鋼板であって、該厚鋼板の組織を、フェライト相とそれ以外の第二相からなり、平均で150HV未満の硬さを有するフェライト相を組織全体に対する体積率で60％以上含み、前記第二相が240HV未満の硬さを有する相であり、さらに該厚鋼板の板厚中央位置および板厚1/4位置における（２００）面のＸ線回折強度比が2.0以上または（１１０）面のＸ線回折強度比が2.5以上で、かつ｛１００｝面、｛１１０｝面、｛１１１｝面、｛２１１｝面のうちのいずれかの面が、圧延面に対して５°以内に揃ったフェライト粒コロニーの板厚方向の厚さが、板厚中央位置および板厚1/4位置において平均で５μm以下である組織とすることを特徴とする板厚方向の耐疲労亀裂伝播特性に優れた溶接構造用厚鋼板。
（２）（１）において、前記組成に加えてさらに、質量％で、Cu：1.0％以下、Ni：1.5％以下、Cr：1.0％以下、Mo：1.0％以下、Ｂ：0.010％以下のうちから選ばれた１種または２種以上を含有することを特徴とする溶接構造用厚鋼板。
（３）（１）または（２）において、前記組成に加えてさらに、質量％で、Ｖ：0.10％以下、Nb：0.10％以下、Ti：0.05％以下のうちから選ばれた１種または２種以上を含有することを特徴とする溶接構造用厚鋼板。
（４）（１）ないし（３）のいずれかにおいて、前記組成に加えてさらに、質量％で、Ca：0.010％以下、REM：0.010％以下のうちから選ばれた１種または２種を含有することを特徴とする溶接構造用厚鋼板。
（５）質量％で、Ｃ：0.06〜0.20％、Si：1.0％以下、Mn：2.0％以下、Ｐ：0.10％以下、Ｓ：0.006％以下、Al：0.10％以下を含み、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、加熱工程と、圧延工程とを施し厚鋼板とする溶接構造用厚鋼板の製造方法であって、前記圧延工程が、少なくとも500℃以上のフェライト単相域またはフェライトの体積率が60％以上となる二相域での累積圧下率が50％以上で、かつ次（１）式
圧延速度Ｓ＝ln(di/df)／t ‥‥‥（１）
（ここで、di：温間圧延前の板厚(mm)、df：温間圧延後の板厚(mm)、ｔ：温間圧延に要した時間(ｓ)）
で定義される圧延速度Ｓが2.0×10^-2/ｓ以下となる圧延を施す温間圧延を含む工程であり、前記鋼素材を、該温間圧延前に、方位差15゜以上の大角境界に囲まれた領域の平均大きさが15μm以下である組織を有するように調整したのち、前記温間圧延を行うことを特徴とする板厚方向の耐疲労亀裂伝播特性に優れた溶接構造用厚鋼板の製造方法。
（６）質量％で、Ｃ：0.06〜0.20％、Si：1.0％以下、Mn：2.0％以下、Ｐ：0.10％以下、Ｓ：0.006％以下、Al：0.10％以下を含み、残部Feおよび不可避的不純物からなる組成を有し、方位差15゜以上の大角境界に囲まれた領域の平均大きさが15μm以下である組織を有する鋼素材に、500℃以上Ac_３変態点未満の温度域の温度に加熱する加熱工程と、ついで、該加熱工程を経た鋼素材に、（Ac_３変態点−40℃）以下500℃以上の温度域での累積圧下率が50％以上で、かつ次（１）式
圧延速度Ｓ＝ln(di/df)／t ‥‥‥（１）
（ここで、di：温間圧延前の板厚(mm)、df：温間圧延後の板厚(mm)、ｔ：温間圧延に要した時間(ｓ)）
で定義される圧延速度Ｓが2.0×10^-2/ｓ以下となる温間圧延を行う圧延工程と、を施すことを特徴とする板厚方向の耐疲労亀裂伝播特性に優れた溶接構造用厚鋼板の製造方法。
（７）質量％で、Ｃ：0.06〜0.20％、Si：1.0％以下、Mn：2.0％以下、Ｐ：0.10％以下、Ｓ：0.006％以下、Al：0.10％以下を含み、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、加熱工程と、圧延工程とを施し厚鋼板とする溶接構造用厚鋼板の製造方法であって、前記加熱工程が、前記鋼素材をAc_３変態点以上950℃未満の温度域の温度に加熱する工程であり、前記圧延工程が、900℃未満（Ar_３変態点−20℃）以上の温度域での累積圧下率が30％以上である一次圧延と、（Ar_３変態点−20℃）未満500℃以上の温度域での累積圧下率が50％以上で、かつ次（１）式
圧延速度Ｓ＝ln(di/df)／t ‥‥‥（１）
（ここで、di：温間圧延前の板厚(mm)、df：温間圧延後の板厚(mm)、ｔ：温間圧延に要した時間(ｓ)）
で定義される圧延速度Ｓが2.0×10^-2/ｓ以下となる温間圧延と、を行う工程であることを特徴とする板厚方向の耐疲労亀裂伝播特性に優れた溶接構造用厚鋼板の製造方法。
（８）（７）において、前記加熱工程が、前記鋼素材を950〜1180℃の温度域の温度に加熱する工程であり、前記圧延工程が、前記一次圧延前に、1050〜900℃の温度域での累積圧下率が30％以上である前段圧延を行うことを特徴とする溶接構造用厚鋼板の製造方法。
（９）（７）または（８）において、前記一次圧延と前記温間圧延との間で、1℃/s以上の冷却速度で冷却する加速冷却を施すことを特徴とする溶接構造用厚鋼板の製造方法。
（１０）（８）において、前記前段圧延と前記温間圧延との間で前記一次圧延に代えて、1℃/s以上の冷却速度で冷却する加速冷却を施すことを特徴とする溶接構造用厚鋼板の製造方法。
（１１）（５）ないし（１０）のいずれかにおいて、前記圧延工程を施したのち、400℃以上Ac_１変態点以下の温度に加熱する焼戻工程を施すことを特徴とする溶接構造用厚鋼板の製造方法。 Furthermore, the inventors have refined the material structure immediately before rolling in the ferrite single-phase temperature range of 500 ° C. or higher, or in the austenite + ferrite two-phase temperature range, It was found that the thickness in the plate thickness direction is important in forming a thin structure.
The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.06-0.20%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.006% or less, Al: 0.10% or less, the remaining Fe and inevitable Steel plate for welded structure having a composition consisting of mechanical impurities, wherein the structure of the steel plate is composed of a ferrite phase and another second phase, and the ferrite phase having an average hardness of less than 150 HV X-ray diffraction of (200) plane at a central position and a 1/4 position of the thickness of the thick steel plate, the second phase being a phase having a hardness of less than 240 HV The intensity ratio is 2.0 or more, or the (110) plane X-ray diffraction intensity ratio is 2.5 or more, and any one of {100} plane, {110} plane, {111} plane, {211} plane is The thickness in the thickness direction of the ferrite grain colonies aligned within 5 ° with respect to the rolling surface is the thickness center position and the plate. A thick steel plate for welded structure having excellent fatigue crack propagation resistance in the thickness direction, characterized by having a structure having an average of 5 μm or less at a 1/4 thickness position.
(2) In (1), in addition to the above composition, by mass%, Cu: 1.0% or less, Ni: 1.5% or less, Cr: 1.0% or less, Mo: 1.0% or less, B: 0.010% or less A thick steel plate for welded structure, comprising one or more selected from the group consisting of:
(3) In (1) or (2), in addition to the above-mentioned composition, in addition to mass%, one or two selected from V: 0.10% or less, Nb: 0.10% or less, Ti: 0.05% or less A thick steel plate for welded structure containing at least a seed.
(4) In any one of (1) to (3), in addition to the above composition, the composition further contains one or two kinds selected from Ca: 0.010% or less and REM: 0.010% or less by mass%. A thick steel plate for welded structure.
(5) In mass%, C: 0.06-0.20%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.006% or less, Al: 0.10% or less, the remainder Fe and inevitable A steel material having a composition composed of mechanical impurities is subjected to a heating step and a rolling step to produce a thick steel plate for welded structure, wherein the rolling step has a ferrite single-phase region of at least 500 ° C or higher. Or the cumulative rolling reduction in the two-phase region where the volume fraction of ferrite is 60% or more is 50% or more, and the following equation (1)
Rolling speed S = ln (di / df) / t (1)
(Where di: thickness before warm rolling (mm), df: thickness after warm rolling (mm), t: time required for warm rolling (s))
Including a warm rolling process in which rolling is performed at a rolling speed S of 2.0 × 10 ⁻² / s or less, and the steel material is subjected to a large-angle boundary with an orientation difference of 15 ° or more before the warm rolling. The thickness for the welded structure is excellent in fatigue crack propagation resistance in the thickness direction, characterized by performing the above-mentioned warm rolling after adjusting to have a structure in which the average size of the region surrounded by 15 μm or less A method of manufacturing a steel sheet.
(6) In mass%, C: 0.06-0.20%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.006% or less, Al: 0.10% or less, the remainder Fe and inevitable In a steel material having a composition consisting of mechanical impurities and having a structure with an average size of 15 μm or less in the region surrounded by a large angle boundary with an orientation difference of 15 ° or more, a temperature range of 500 ° C. or more and less than the Ac ₃ transformation point A heating step of heating to a temperature, and then, to the steel material that has undergone the heating step, an accumulative rolling reduction in a temperature range of 500 ° C. or higher (Ac ₃ transformation point −40 ° C.) is 50% or higher and the following (1 )formula
Rolling speed S = ln (di / df) / t (1)
(Where di: thickness before warm rolling (mm), df: thickness after warm rolling (mm), t: time required for warm rolling (s))
The thickness of the welded structure is excellent in fatigue crack propagation resistance in the thickness direction, characterized by performing a warming process in which the rolling speed S defined by is a warm rolling at 2.0 × 10 ⁻² / s or less. A method of manufacturing a steel sheet.
(7) In mass%, C: 0.06-0.20%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.006% or less, Al: 0.10% or less, the remainder Fe and inevitable A method for producing a thick steel plate for welded structure, which is obtained by subjecting a steel material having a composition composed of impurities to a heating step and a rolling step to obtain a thick steel plate, wherein the heating step is performed at an Ac ₃ transformation point or higher. A primary rolling process in which the rolling process is performed at a temperature in a temperature range of less than 950 ° C., and the rolling step has a cumulative rolling reduction in a temperature range of less than 900 ° C. (Ar ₃ transformation point−20 ° C.) , (Ar ₃ transformation point -20 ° C) less than 500 ° C cumulative rolling reduction is 50% or more, and the following formula (1)
Rolling speed S = ln (di / df) / t (1)
(Where di: thickness before warm rolling (mm), df: thickness after warm rolling (mm), t: time required for warm rolling (s))
A steel plate for a welded structure with excellent fatigue crack propagation resistance in the plate thickness direction, characterized in that it is a step of performing a warm rolling with a rolling speed S defined by ≦ 2.0 × 10 ⁻² / s. Manufacturing method.
(8) In (7), the heating step is a step of heating the steel material to a temperature in a temperature range of 950 to 1180 ° C, and the rolling step is performed at a temperature of 1050 to 900 ° C before the primary rolling. A method for producing a thick steel plate for welded structure, characterized by performing pre-rolling in which the cumulative rolling reduction in the region is 30% or more.
(9) The thick steel plate for welded structure according to (7) or (8), wherein accelerated cooling is performed between the primary rolling and the warm rolling at a cooling rate of 1 ° C./s or more. Manufacturing method.
(10) In the welding structure according to (8), accelerated cooling that cools at a cooling rate of 1 ° C./s or more is performed instead of the primary rolling between the preceding rolling and the warm rolling. Manufacturing method of thick steel plate.
(11) In any one of (5) to (10), after performing the rolling step, a tempering step of heating to a temperature not lower than 400 ° C. and not higher than the Ac ₁ transformation point is performed. Manufacturing method of steel sheet.

本発明によれば、優れた靭性と、優れた板厚方向の耐疲労亀裂伝播特性とを兼備する溶接構造用厚鋼板を容易にかつ工業的に安定して製造でき、産業上格段の効果を奏する。また、本発明の溶接構造用厚鋼板を溶接構造物に適用することにより、溶接構造物の疲労破壊に対する耐久性を顕著に向上させることができるという効果もある。 According to the present invention, it is possible to easily and industrially stably manufacture a thick steel plate for welded structure that has both excellent toughness and excellent fatigue crack propagation characteristics in the thickness direction. Play. Further, by applying the thick steel plate for welded structure of the present invention to a welded structure, there is an effect that the durability against fatigue fracture of the welded structure can be remarkably improved.

まず、本発明厚鋼板の組成限定理由について説明する。以下、とくに断らない限り、質量％は単に％と記す。
Ｃ：0.06〜0.20％
Ｃは、固溶してあるいは硬質第二相を生成させることにより鋼の強度を増加させる元素であり、溶接構造用として所望の強度を確保するために含有する必要がある。また、（α＋γ）２相域および／またはα単相域での圧延に際し、急激な粒成長や再結晶の進行を防ぎ、所望の｛１００｝または｛１１０｝集合組織を発達させ、フェライト粒コロニー厚さの減少を達成するために、Ｃを0.06％以上含有させる必要がある。Ｃが0.06％未満では、鋼中のセメンタイト等の炭化物粒子が少ないため、圧延中に急激な粒成長や再結晶が生じ、集合組織の発達が妨げられたり組織が粗大化して靭性が低下する。また、0.20％を超える含有は、溶接性が急激に低下する。このようなことから、Ｃは0.06〜0.20％の範囲に限定した。なお、好ましくは0.06〜0.15％である。 First, the reasons for limiting the composition of the steel plate of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply referred to as%.
C: 0.06-0.20%
C is an element that increases the strength of the steel by forming a solid solution or forming a hard second phase, and needs to be contained in order to ensure a desired strength for a welded structure. In addition, when rolling in the (α + γ) two-phase region and / or α single-phase region, rapid progress of grain growth and recrystallization is prevented, and a desired {100} or {110} texture is developed, and ferrite grain colonies are developed. In order to achieve a reduction in thickness, it is necessary to contain 0.06% or more of C. If C is less than 0.06%, there are few carbide particles such as cementite in the steel, so rapid grain growth and recrystallization occur during rolling, and the development of the texture is hindered or the structure becomes coarse and the toughness decreases. On the other hand, if the content exceeds 0.20%, the weldability decreases rapidly. For these reasons, C is limited to the range of 0.06 to 0.20%. In addition, Preferably it is 0.06 to 0.15%.

Si：1.0％以下
Siは、脱酸剤として作用するとともに、固溶強化により鋼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、0.1％以上含有することが好ましいが、1.0％を超える含有は、表面性状を損なううえ、靭性が極端に低下する。このため、Siは1.0％以下に限定した。なお、好ましくは0.1〜0.5％である。 Si: 1.0% or less
Si is an element that acts as a deoxidizer and has the effect of increasing the strength of steel by solid solution strengthening. In order to obtain such an effect, the content is preferably 0.1% or more. However, if the content exceeds 1.0%, the surface properties are impaired and the toughness is extremely lowered. For this reason, Si was limited to 1.0% or less. In addition, Preferably it is 0.1 to 0.5%.

Mn：2.0％以下
Mnは、鋼中では強化元素として作用する元素である。このような効果を得るためには、0.5％以上含有することが望ましいが、2.0％を超える多量の含有は、溶接性を低下させるとともに、材料コストの高騰を招く。このため、Mnは2.0％以下に限定した。なお、好ましくは0.5〜1.8％である。 Mn: 2.0% or less
Mn is an element that acts as a strengthening element in steel. In order to acquire such an effect, it is desirable to contain 0.5% or more, but if it contains more than 2.0%, the weldability is lowered and the material cost is increased. For this reason, Mn was limited to 2.0% or less. In addition, Preferably it is 0.5 to 1.8%.

Ｐ：0.10％以下
Ｐは、不純物元素として含まれ、靭性等に悪影響を及ぼすため、できるだけ低減することが望ましいが、実用上、0.10％までは許容できる。このため、Ｐは0.10％以下に限定した。なお、好ましくは0.020％以下である。
Ｓ：0.006％以下
Ｓは、鋼中では介在物として存在し、延性、靭性等に悪影響を及ぼすため、できるだけ低減することが望ましいが、実用上、0.006％までは許容できる。このため、Ｓは0.006％以下に限定した。なお、好ましくは0.003％以下である。 P: 0.10% or less P is contained as an impurity element and adversely affects toughness and the like, so it is desirable to reduce it as much as possible. However, practically, up to 0.10% is acceptable. For this reason, P was limited to 0.10% or less. In addition, Preferably it is 0.020% or less.
S: 0.006% or less S is present as an inclusion in steel and adversely affects ductility, toughness, etc., so it is desirable to reduce it as much as possible, but in practice it is acceptable up to 0.006%. For this reason, S was limited to 0.006% or less. In addition, Preferably it is 0.003% or less.

Al：0.10％以下
Alは、脱酸剤として作用する元素であるが、このような効果を得るためには、0.01％以上含有することが望ましい。一方、0.10％を超える含有は、介在物量を増加させるとともに、靭性をも低下させる。このため、Alは0.10％以下に限定した。なお、好ましくは0.05％以下である。 Al: 0.10% or less
Al is an element that acts as a deoxidizer, but in order to obtain such an effect, it is desirable to contain 0.01% or more. On the other hand, the content exceeding 0.10% increases the amount of inclusions and decreases toughness. For this reason, Al was limited to 0.10% or less. In addition, Preferably it is 0.05% or less.

上記した成分が基本成分であるが、本発明では、基本の組成に加えてさらに、Cu：1.0％以下、Ni：1.5％以下、Mo：1.0％以下、Cr：1.0％以下、Ｂ：0.010％以下のうちから選ばれた１種または２種以上、および／または、Ｖ：0.10％以下、Nｂ：0.10％以下、Ti：0.05％以下のうちから選ばれた１種または２種以上、および／または、Ca：0.010％以下、REM：0.010％以下のうちから選ばれた１種または２種を含有できる。 In the present invention, in addition to the basic composition, Cu: 1.0% or less, Ni: 1.5% or less, Mo: 1.0% or less, Cr: 1.0% or less, B: 0.010% One or more selected from the following, and / or V: 0.10% or lower, Nb: 0.10% or lower, Ti: 0.05% or lower, and / or Alternatively, one or two selected from Ca: 0.010% or less and REM: 0.010% or less can be contained.

Cu：1.0％以下、Ni：1.5％以下、Mo：1.0％以下、Cr：1.0％以下、Ｂ：0.010％以下のうちから選ばれた１種または２種以上
Cu、Ni、Mo、Cr、Ｂはいずれも、鋼の焼入れ性を高め、強度向上に直接寄与するとともに、靭性をも向上させる元素であり、必要に応じて選択して1種または2種以上を含有できる。このような効果を得るためには、Cu：0.01％以上、Ni：0.01％以上、Mo：0.01％以上、Cr：0.01％以上、Ｂ：0.0005％以上、含有することが望ましいが、Cu：1.0％、Ni：1.5％、Mo：1.0％、Cr：1.0％、Ｂ：0.010％をそれぞれ超える過度の含有は、靭性、溶接性を低下させる。このため、含有する場合は、Cu：1.0％以下、Ni：1.5％以下、Mo：1.0％以下、Cr：1.0％以下、Ｂ：0.010％以下に、それぞれ限定することが好ましい。 One or more selected from Cu: 1.0% or less, Ni: 1.5% or less, Mo: 1.0% or less, Cr: 1.0% or less, B: 0.010% or less
Cu, Ni, Mo, Cr, and B are all elements that increase the hardenability of steel, contribute directly to improving strength, and also improve toughness. Select one or more as required. Can be contained. In order to obtain such an effect, Cu: 0.01% or more, Ni: 0.01% or more, Mo: 0.01% or more, Cr: 0.01% or more, B: 0.0005% or more are desirable, but Cu: 1.0 %, Ni: 1.5%, Mo: 1.0%, Cr: 1.0%, B: Excessive content exceeding 0.010% decreases toughness and weldability. For this reason, when it contains, it is preferable to limit to Cu: 1.0% or less, Ni: 1.5% or less, Mo: 1.0% or less, Cr: 1.0% or less, B: 0.010% or less, respectively.

Ｖ：0.10％以下、Nｂ：0.10％以下、Ti：0.05％以下のうちから選ばれた１種または２種以上
Ｖ、Nb、Tiはいずれも、窒化物、炭化物、あるいは炭窒化物を形成し、結晶粒を微細化し、鋼を強化する効果を有する元素であり、必要に応じて選択して1種または2種以上を含有できる。このような効果を得るためには、Ｖ、Nb、Tiを、それぞれ0.003％以上含有することが望ましい。一方、Ｖ：0.10％、Nb：0.10％、Ti：0.05％を超えて多量に含有すると、鋳片に割れを生じ、製造コストの高騰を招く。このため、Ｖ：0.10％以下、Nb：0.10％以下、Ti：0.05％以下、の範囲にそれぞれ限定することが好ましい。 One or more selected from V: 0.10% or less, Nb: 0.10% or less, Ti: 0.05% or less V, Nb, and Ti all form nitrides, carbides, or carbonitrides. It is an element that has the effect of refining crystal grains and strengthening steel, and it can be selected as necessary and can contain one or more. In order to acquire such an effect, it is desirable to contain 0.003% or more of V, Nb, and Ti, respectively. On the other hand, if it contains more than V: 0.10%, Nb: 0.10%, Ti: 0.05%, the slab is cracked and the manufacturing cost is increased. For this reason, it is preferable to limit to the ranges of V: 0.10% or less, Nb: 0.10% or less, and Ti: 0.05% or less.

Ca：0.010％以下、REM：0.010％以下のうちから選ばれた１種または２種
Ca、REMはいずれも、介在物の形状制御を介して、延性、靭性向上に寄与する元素であり、必要に応じて選択して1種または2種を含有できる。このような効果を得るためには、Ca：0.001％以上、REM：0.001％以上含有することが好ましいが、Ca：0.010％、REM：0.010％を超える多量の含有は、靭性を低下させる。このため、Ca：0.010％以下、REM：0.010％以下に限定することが好ましい。 One or two selected from Ca: 0.010% or less, REM: 0.010% or less
Both Ca and REM are elements that contribute to the improvement of ductility and toughness through the shape control of inclusions, and can be selected as necessary to contain one or two kinds. In order to obtain such an effect, it is preferable to contain Ca: 0.001% or more and REM: 0.001% or more. However, a large content exceeding Ca: 0.010% and REM: 0.010% reduces toughness. For this reason, it is preferable to limit to Ca: 0.010% or less and REM: 0.010% or less.

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。
本発明厚鋼板は、上記した組成を有しさらに、フェライト相とそれ以外の第二相からなり、平均で150HV未満の硬さを有するフェライト相を組織全体に対する体積率で60％以上含み、第二相が240HV未満の硬さを有する組織を有する。
本発明では、｛１００｝集合組織を発達させるために、鋼素材に、少なくとも（フェライト＋オーステナイト）２相域あるいはフェライト単相域で圧延を行う温間圧延を施す。しかし、生成したフェライト相のうち、圧延で加工されて硬化した、加工フェライト相の組織分率が組織全体に対する体積率で40％を超えると、鋼板靭性が顕著に低下する。そのため、優れた鋼板靭性を確保するために、本発明では、回復あるいは再結晶を促進し、加工フェライト相内の転位密度が低減して平均で150HV未満の硬さを有する、回復あるいは再結晶したフェライト相の組織分率を、組織全体に対する体積率で60％以上に限定した。 The balance other than the components described above consists of Fe and inevitable impurities.
The thick steel plate of the present invention has the above-described composition, and further includes a ferrite phase and a second phase other than the ferrite phase, and includes a ferrite phase having an average hardness of less than 150 HV in a volume ratio of 60% or more with respect to the entire structure. The two phases have a texture with a hardness of less than 240 HV.
In the present invention, in order to develop the {100} texture, the steel material is subjected to warm rolling in which rolling is performed at least in the (ferrite + austenite) two-phase region or the ferrite single-phase region. However, when the structure fraction of the processed ferrite phase that has been processed by rolling and hardened among the formed ferrite phases exceeds 40% in terms of the volume ratio with respect to the entire structure, the toughness of the steel sheet is significantly reduced. Therefore, in order to ensure excellent steel sheet toughness, in the present invention, recovery or recrystallization is promoted, the dislocation density in the processed ferrite phase is reduced, and the average hardness is less than 150 HV. The structure fraction of the ferrite phase was limited to 60% or more by volume ratio with respect to the entire structure.

また、本発明では、フェライト相以外の第二相は、ビッカース硬さで240HV未満の硬さを有する相とする。240HV以上の硬さを有する硬質な第二相の存在は、鋼板靭性を著しく低下させる場合がある。このため、本発明では第二相の硬さを、ビッカース硬さで240HV未満に限定した。なお、好ましくは200HV以上240HV以下である。
さらに本発明厚鋼板は、板厚中央位置および板厚1/4位置における（２００）面のＸ線回折強度比が2.0以上または（１１０）面のＸ線回折強度比が2.5以上である集合組織を有する。 In the present invention, the second phase other than the ferrite phase is a phase having a Vickers hardness of less than 240 HV. The presence of a hard second phase having a hardness of 240 HV or more may significantly reduce the steel sheet toughness. For this reason, in the present invention, the hardness of the second phase is limited to less than 240 HV in terms of Vickers hardness. In addition, Preferably it is 200HV or more and 240HV or less.
Furthermore, the thick steel plate of the present invention has a texture in which the X-ray diffraction intensity ratio of the (200) plane is 2.0 or more or the X-ray diffraction intensity ratio of the (110) plane is 2.5 or more at the center position and 1/4 position Have

本発明厚鋼板では、｛１００｝集合組織または｛１１０｝集合組織を発達させる。これにより、板厚方向の疲労亀裂伝播が抑制され、板厚方向の耐疲労亀裂伝播特性を向上する。というのは、疲労亀裂が、｛１００｝面が板厚方向に垂直（｛１００｝面が板面に平行）な方位を有するフェライト中を板厚方向に伝播する場合には、疲労亀裂先端において様々なすべり系が活動して転位同士の干渉が生じる。これにより、疲労亀裂の伝播が抑制される。なお、｛１１０｝面は、｛１００｝面に比べてその効果は若干弱いものの、｛１００｝面と同様の効果を有する。 In the steel plate of the present invention, a {100} texture or a {110} texture is developed. Thereby, fatigue crack propagation in the plate thickness direction is suppressed, and the fatigue crack propagation characteristics in the plate thickness direction are improved. This is because when a fatigue crack propagates in a ferrite having a {100} plane perpendicular to the plate thickness direction ({100} plane is parallel to the plate plane) in the plate thickness direction, Various slip systems act and interference between dislocations occurs. Thereby, propagation of fatigue cracks is suppressed. The {110} plane has the same effect as the {100} plane, although its effect is slightly weaker than that of the {100} plane.

これらの集合組織の発達の程度は、（２００）面のＸ線回折強度比または（１１０）面のＸ線回折強度比で表される。ここでいう特定面の「Ｘ線回折強度比」は、鋼板圧延面に平行に採取した試料を用いて、Ｘ線回折により得られた特定面からの回折強度と、粉末試料を用いて作製したランダム方位サンプル材の同一特定面からの回折強度との比である。このＸ線回折強度比が大きいほど特定の結晶方位が集中した集合組織が発達していることを示す。 The degree of development of these textures is represented by the (200) plane X-ray diffraction intensity ratio or the (110) plane X-ray diffraction intensity ratio. The “X-ray diffraction intensity ratio” of the specific surface referred to here was prepared using a sample collected in parallel with the steel sheet rolling surface, using the diffraction intensity from the specific surface obtained by X-ray diffraction, and a powder sample. It is a ratio with the diffraction intensity from the same specific surface of a random orientation sample material. A larger X-ray diffraction intensity ratio indicates that a texture in which a specific crystal orientation is concentrated is developed.

板厚方向の耐疲労亀裂伝播特性を十分に向上させるために、本発明では、板厚中央位置および板厚1/4位置における（２００）面のＸ線回折強度比が2.0以上または（１１０）面のＸ線回折強度比が2.5以上に限定した。板厚中央位置および板厚1/4位置における（２００）面のＸ線回折強度比が2.0未満または（１１０）面のＸ線回折強度比が2.5未満では、｛１００｝集合組織または｛１１０｝集合組織の発達が不十分で、板厚方向の疲労亀裂伝播を十分に抑制することができない。このため、本発明では、板厚中央位置および板厚1/4位置における（２００）面のＸ線回折強度比を2.0以上または（１１０）面のＸ線回折強度比を2.5以上に限定した。 In order to sufficiently improve the fatigue crack propagation characteristics in the thickness direction, in the present invention, the X-ray diffraction intensity ratio of the (200) plane at the thickness center position and the thickness 1/4 position is 2.0 or more or (110) The X-ray diffraction intensity ratio of the surface was limited to 2.5 or more. When the X-ray diffraction intensity ratio of the (200) plane is less than 2.0 or the X-ray diffraction intensity ratio of the (110) plane is less than 2.5 at the plate thickness center position and the plate thickness 1/4 position, {100} texture or {110} The texture development is insufficient, and fatigue crack propagation in the thickness direction cannot be sufficiently suppressed. For this reason, in the present invention, the X-ray diffraction intensity ratio of the (200) plane at the plate thickness center position and the 1/4 thickness position is limited to 2.0 or more, or the X-ray diffraction intensity ratio of the (110) plane is limited to 2.5 or more.

またさらに、本発明厚鋼板は、｛１００｝面、｛１１０｝面、｛１１１｝面、｛２１１｝面のうちのいずれかの面が、圧延面に対して５°以内に揃ったフェライト粒コロニーの板厚方向の厚さを、板厚中央位置および板厚1/4位置において平均で５μm以下とする組織を有する。
｛１００｝、｛１１０｝などの集合組織が発達した鋼板の圧延方向に平行な断面（Ｌ断面）について、EBSP（Electoron Back Scattering Pattern）法で結晶方位分布を解析すると、図１に示すように、圧延面に平行に伸長し、圧延面（板面）にほぼ平行な結晶面を共有するフェライト粒やサブグレインの集団（フェライト粒コロニーという）が認められる。 Furthermore, the thick steel plate of the present invention is a ferrite grain in which any one of {100} plane, {110} plane, {111} plane, and {211} plane is aligned within 5 ° with respect to the rolling plane. The colony has a structure in which the thickness in the plate thickness direction is 5 μm or less on average at the plate thickness center position and the plate thickness 1/4 position.
When the crystal orientation distribution is analyzed by an EBSP (Electoron Back Scattering Pattern) method for a cross section (L cross section) parallel to the rolling direction of the steel sheet having a texture such as {100}, {110} developed, as shown in FIG. A group of ferrite grains and subgrains (referred to as ferrite grain colonies) extending parallel to the rolling surface and sharing a crystal plane substantially parallel to the rolling surface (plate surface) is observed.

板厚方向に進展する疲労亀裂は、これらフェライト粒コロニーを分断しながら伝播していく。フェライト粒コロニーの板厚方向の厚さを薄くすることにより、｛１００｝および｛１１０｝集合組織の発達による疲労亀裂伝播の抑制効果はさらに顕著となる。というのは、フェライト粒コロニーの板厚方向の厚さが、疲労亀裂先端の塑性変形領域の大きさと同程度以下、すなわち５μm以下になると、異なる塑性変形挙動を有する積層したフェライト粒コロニー同士が相互に干渉、拘束しあって、塑性変形が抑制され、その結果、疲労亀裂の伝播速度が低下する。この効果は、｛１００｝および｛１１０｝集合組織が発達している場合に顕著となる。フェライト粒コロニーの板厚方向の厚さが５μmを超えて厚くなると、そのような効果は生じなくなる。なお、フェライト粒コロニーの厚さを薄くすることは、組織微細化にも繋がり、鋼板靭性を向上させることにもなる。 Fatigue cracks that propagate in the plate thickness direction propagate while dividing these ferrite grain colonies. By reducing the thickness of the ferrite grain colony in the plate thickness direction, the effect of suppressing fatigue crack propagation due to the development of {100} and {110} textures becomes even more remarkable. This is because, when the thickness of the ferrite grain colonies in the plate thickness direction is less than or equal to the size of the plastic deformation region at the tip of the fatigue crack, that is, 5 μm or less, the laminated ferrite grain colonies having different plastic deformation behaviors The plastic deformation is suppressed by interfering with and constraining, and as a result, the propagation speed of fatigue cracks decreases. This effect is significant when {100} and {110} textures are developed. If the thickness of the ferrite grain colony in the plate thickness direction exceeds 5 μm, such an effect does not occur. In addition, reducing the thickness of the ferrite grain colony leads to refinement of the structure and also improves the steel sheet toughness.

このようなことから、フェライト粒コロニーの板厚方向の厚さを、板厚中央位置および板厚1/4位置において平均で５μm以下に限定した。なお、本発明でいう「フェライト粒コロニー」とは、｛１００｝面、｛１１０｝面、｛１１１｝面、｛２１１｝面のうちのいずれかの面が、圧延面に対して５°以内（ほぼ平行）に揃ったフェライト粒およびフェライト粒の集団（サブグレインの集団も含む）をいう。なお、フェライト粒コロニーの板厚方向の厚さは、EBSP法で結晶方位分布を解析することにより求めることができる。 For this reason, the thickness of the ferrite grain colony in the plate thickness direction was limited to 5 μm or less on average at the plate thickness center position and the plate thickness 1/4 position. The “ferrite grain colony” referred to in the present invention means that any one of {100} plane, {110} plane, {111} plane, and {211} plane is within 5 ° with respect to the rolling plane. It refers to a ferrite grain and a group of ferrite grains (including a subgrain group) aligned in (substantially parallel). The thickness of the ferrite grain colony in the plate thickness direction can be obtained by analyzing the crystal orientation distribution by the EBSP method.

つぎに、本発明厚鋼板の製造方法について説明する。
上記した組成の溶鋼を転炉等の常用の溶製方法で溶製し、連続鋳造法等の常用の鋳造方法でスラブ等の鋼素材とすることが好ましいが、鋼素材の製造方法はこれに限定されるものではない。
得られた鋼素材は、ついで加熱工程と、圧延工程とを施され厚鋼板とされる。 Below, the manufacturing method of this invention thick steel plate is demonstrated.
It is preferable that the molten steel having the above composition is melted by a conventional melting method such as a converter and used as a steel material such as a slab by a conventional casting method such as a continuous casting method. It is not limited.
The obtained steel material is then subjected to a heating step and a rolling step to obtain a thick steel plate.

本発明の圧延工程では、少なくとも下記に示す温間圧延を含む圧延を行う。なお、ここでいう「温間圧延」とは、温度域が500℃以上のフェライト単相域またはフェライトの体積率が60％以上となる二相域での圧延をいうものとする。フェライトの体積率が60％以上の温度としては、加熱温度がAc_３変態点未満の場合には、概ねAc₁変態点とAc_３変態点の中間温度より10℃以上低い温度が、加熱温度がAc_３変態点以上の場合には、概ね（Ar_３変態点−60℃）以下の温度が該当する。 In the rolling process of the present invention, at least rolling including the following warm rolling is performed. Here, “warm rolling” refers to rolling in a ferrite single-phase region where the temperature range is 500 ° C. or higher or in a two-phase region where the volume fraction of ferrite is 60% or higher. The temperature volume fraction of 60% or more of ferrite, if the heating temperature is Ac less than ₃ transformation point, generally a temperature lower 10 ° C. or higher than the intermediate temperature of Ac ₁ transformation point and Ac ₃ transformation point, the heating temperature is When the temperature is equal to or higher than the Ac ₃ transformation point, the temperature generally falls below (Ar ₃ transformation point −60 ° C.).

温間圧延の温度域が500℃未満と低すぎると、フェライトの回復が十分に進行しないため、靭性が低下する。また、温間圧延の温度域がフェライトの体積率が60％未満の二相域を超えて高温となると、フェライトの再結晶により疲労亀裂伝播特性の観点から望ましくない方位のフェライト粒が生成し、耐疲労亀裂伝播特性が低下したり、あるいはフェライト粒の粗大化が生じ、靭性が低下する。 If the temperature range of the warm rolling is too low as less than 500 ° C., the recovery of ferrite does not proceed sufficiently, so that the toughness decreases. In addition, when the temperature range of the warm rolling is higher than the two-phase region where the volume fraction of ferrite is less than 60%, ferrite grains with undesirable orientation are generated from the viewpoint of fatigue crack propagation characteristics due to recrystallization of the ferrite, Fatigue crack propagation characteristics are reduced, or ferrite grains are coarsened, resulting in a decrease in toughness.

本発明の圧延工程で行う温間圧延は、上記した温度域で、かつ累積圧下率が50％以上で、かつ次（１）式
圧延速度Ｓ＝ln(di/df)／t ‥‥‥（１）
（ここで、di：温間圧延前の板厚(mm)、df：温間圧延後の板厚(mm)、ｔ：温間圧延に要した時間(ｓ)）
で定義される圧延速度Ｓが2.0×10^-2/ｓ以下となる圧延とする。 The warm rolling performed in the rolling process of the present invention is the above temperature range, the cumulative rolling reduction is 50% or more, and the following formula (1)
Rolling speed S = ln (di / df) / t (1)
(Where di: thickness before warm rolling (mm), df: thickness after warm rolling (mm), t: time required for warm rolling (s))
It is assumed that the rolling speed S defined by the above is 2.0 × 10 ⁻² / s or less.

上記した温度域での累積圧下率を50％以上とすることにより、板厚中央位置および板厚1/4位置における（２００）面のＸ線回折強度比が2.0以上または（１１０）面のＸ線回折強度比が2.5以上となる集合組織を発達させることができる。累積圧下率が50％未満では、所望の集合組織の発達が不十分となり、板厚方向の疲労亀裂伝播を十分に抑制することができなくなる。 By setting the cumulative rolling reduction in the above temperature range to 50% or more, the X-ray diffraction intensity ratio of the (200) plane at the plate thickness center position and the plate thickness 1/4 position is 2.0 or more, or the (110) plane X Textures with a line diffraction intensity ratio of 2.5 or more can be developed. If the cumulative rolling reduction is less than 50%, the development of a desired texture becomes insufficient, and fatigue crack propagation in the thickness direction cannot be sufficiently suppressed.

また、本発明では、上記した温間圧延において、前記（１）式で定義される圧延速度Ｓを2.0×10^-2/ｓ以下に限定する。これにより、平均で150HV未満の硬さを有する、回復または再結晶したフェライト相を組織全体に対する体積率で60％以上含む組織とすることができ、優れた鋼板靭性を確保できる。圧延速度Ｓが2.0×10^-2/ｓを超えて大きくなると、加工フェライトの回復が十分に進行しないため、フェライト粒内の転位密度が高くなり、鋼板靭性が低下する。 In the present invention, in the above-described warm rolling, the rolling speed S defined by the equation (1) is limited to 2.0 × 10 ⁻² / s or less. Thereby, it is possible to obtain a structure including a recovered or recrystallized ferrite phase having a hardness of less than 150 HV on average and having a volume ratio of 60% or more with respect to the entire structure, and excellent steel sheet toughness can be ensured. When the rolling speed S exceeds 2.0 × 10 ⁻² / s, the recovery of the processed ferrite does not proceed sufficiently, so that the dislocation density in the ferrite grains increases and the steel sheet toughness decreases.

（１）式で定義される圧延速度Ｓは、温間圧延によって被圧延材に導入される塑性歪量(ln(di/df))を圧延に要した時間tで除した値であり、塑性歪の付加速度、をあらわし、Ｓが小さいほど、加工フェライトの回復あるいは再結晶が生じやすいことを意味する。
なお、本発明では、上記した組成の鋼素材が、上記した温間圧延直前に、方位差15゜以上の大角境界に囲まれた領域の平均大きさが15μm以下である組織を有するように調整する。平均大きさとは、その領域と等しい面積を有する正方形の一辺長さを指す。このような組織を有する素材（被圧延材）に温間圧延を施すことにより、｛１００｝面、｛１１０｝面、｛１１１｝面、｛２１１｝面のうちのいずれかの面が、圧延面に対して５°以内に揃ったフェライト粒コロニーの板厚方向の厚さを、少なくとも板厚中央位置および板厚1/4位置において平均で５μm以下とすることができる。フェライト粒コロニーの板厚方向の厚さを平均で５μm以下とすることにより、｛１００｝および｛１１０｝集合組織の発達による疲労亀裂伝播の抑制効果がさらに顕著となる。フェライト粒コロニーの板厚方向の厚さが５μmを超えて厚くなると、そのような効果は生じなくなる。 The rolling speed S defined by the equation (1) is a value obtained by dividing the amount of plastic strain (ln (di / df)) introduced into the material to be rolled by warm rolling by the time t required for rolling. The strain addition speed is expressed, and the smaller the S, the easier the recovery or recrystallization of the processed ferrite.
In the present invention, the steel material having the above composition is adjusted so that the average size of the region surrounded by the large angle boundary with an orientation difference of 15 ° or more is 15 μm or less immediately before the above warm rolling. To do. The average size refers to the length of one side of a square having the same area as that region. By subjecting a material (rolled material) having such a structure to warm rolling, any one of {100} plane, {110} plane, {111} plane, and {211} plane is rolled. The thickness in the plate thickness direction of the ferrite grain colonies aligned within 5 ° with respect to the surface can be 5 μm or less on average at least at the plate thickness central position and the plate thickness 1/4 position. By controlling the thickness of the ferrite grain colony in the plate thickness direction to 5 μm or less on average, the effect of suppressing the propagation of fatigue cracks due to the development of {100} and {110} textures becomes even more pronounced. If the thickness of the ferrite grain colony in the plate thickness direction exceeds 5 μm, such an effect does not occur.

本発明における加熱工程は、上記した組成を有する鋼素材を、少なくとも上記した温間圧延が可能な温度、すなわち500℃以上の温度、に加熱する工程とする。なお、加熱温度が500℃以上Ac_３変態点未満の場合には、加熱工程後で温間圧延前に、方位差15゜以上の大角境界に囲まれた領域の平均大きさが15μm以下である組織となるように、鋼素材に必要に応じて予め加工等の前処理等を施して組織調整を行うことが好ましい。なお、500℃以上Ac_３変態点未満に加熱されただけで、上記した組織となる場合にはその限りではない。 The heating step in the present invention is a step of heating a steel material having the above-described composition to at least a temperature at which the above-described warm rolling is possible, that is, a temperature of 500 ° C. or higher. When the heating temperature is 500 ° C. or higher and lower than the Ac ₃ transformation point, the average size of the region surrounded by the large-angle boundary with an orientation difference of 15 ° or more is 15 μm or less after the heating step and before warm rolling. It is preferable to adjust the structure by pre-processing such as processing in advance to the steel material as necessary so as to form a structure. Note that this is not the case when the above-described structure is obtained only by heating to 500 ° C. or higher and lower than the Ac ₃ transformation point.

このような前処理等としては、上記した組成を有する鋼素材に、例えば、オーステナイト域に加熱したのち、おおむね1050℃〜900℃の温度範囲のオーステナイト再結晶温度域で少なくとも累積圧下率：30％以上となる圧延を施し、平均粒径：50μm程度以下に微細化したオーステナイト粒とし、しかるのちに放冷、あるいは加速冷却、あるいは焼入れ等の冷却処理を施す処理とすることが好ましい。これにより、平均フェライト粒径が15μm以下、あるいはベイナイトまたはマルテンサイト内部の方位差15゜以上の大角境界に囲まれた領域の大きさが平均で15μm以下となる圧延用素材を得ることができる。このような素材であれば、Ac_３変態点未満の温度に加熱する加熱工程後で温間圧延前に、方位差15゜以上の大角境界に囲まれた領域の平均大きさが15μm以下となる組織を有するようになる。 As such a pretreatment, the steel material having the above composition is heated to an austenite region, for example, and then at least a cumulative reduction ratio of 30% in an austenite recrystallization temperature range of approximately 1050 ° C to 900 ° C. It is preferable that the above-described rolling is performed to obtain austenite grains refined to an average grain size of about 50 μm or less, and then cooling treatment such as standing cooling, accelerated cooling, or quenching is performed. As a result, it is possible to obtain a rolling material having an average ferrite grain size of 15 μm or less, or an area surrounded by a large-angle boundary with an orientation difference of 15 ° or more inside bainite or martensite having an average of 15 μm or less. With such a material, the average size of the region surrounded by the large-angle boundary with an orientation difference of 15 ° or more becomes 15 μm or less after the heating step of heating to a temperature below the Ac ₃ transformation point and before warm rolling. Come to have an organization.

加熱工程における加熱温度がAc_３変態点以上950℃未満の場合には、圧延工程において、温間圧延の前に、900℃未満（Ar_３変態点−20℃）以上のオーステナイト未再結晶温度域での累積圧下率が30％以上である一次圧延を施すことが好ましい。
また、加熱工程における加熱温度が950℃以上1180℃以下の場合には、圧延工程において、温間圧延の前に、1050〜900℃のオーステナイト再結晶温度域での累積圧下率が30％以上である前段圧延によってある程度オーステナイト粒を細粒化した後、900℃未満（Ar_３変態点−20℃）以上のオーステナイト未再結晶温度域での累積圧下率が30％以上である一次圧延を施すことが好ましい。 When the heating temperature in the heating process is higher than the Ac ₃ transformation point and lower than 950 ° C., the austenite non-recrystallization temperature range of less than 900 ° C. (Ar ₃ transformation point−20 ° C.) before the warm rolling in the rolling process. It is preferable to perform primary rolling with a cumulative rolling reduction of 30% or more.
In addition, when the heating temperature in the heating process is 950 ° C. or higher and 1180 ° C. or lower, the cumulative rolling reduction in the austenite recrystallization temperature region of 1050 to 900 ° C. is 30% or higher in the rolling process before warm rolling. Austenite grains are refined to some extent by certain pre-rolling, and then primary rolling is performed with a cumulative reduction ratio of 30% or more in the austenite non-recrystallization temperature range of less than 900 ° C (Ar ₃ transformation point – 20 ° C) or more. Is preferred.

上記した一次圧延、または前段圧延および一次圧延を、加熱工程を経た鋼素材に施すことにより、組織が微細化され、方位差15°以上の大角境界に囲まれる領域の大きさ（オーステナイト粒）が15μm以下となり、その後の温間圧延で、平均５μm以下の厚さを有するフェライト粒コロニーを容易に形成することができるようになる。
なお、一次圧延と温間圧延の間で、圧延を中断して、１℃/s以上の冷却速度で冷却する加速冷却を施し、加速冷却を停止したのち、温間圧延を行ってもよい。また、前段圧延と温間圧延との間で、一次圧延に代えて、加速冷却を施してもよい。また、前段圧延と一次圧延との間で、加速冷却を施してもよいことは言うまでもない。圧延途中で、圧延を中断して、加速冷却を施すことにより、結晶粒の成長を防止することができ、温間圧延前の方位差15°以上の大角境界に囲まれる領域の大きさをより微細化でき、温間圧延により所望の集合組織をより容易に形成できるようになる。 By applying the above-described primary rolling, or pre-rolling and primary rolling to the steel material that has undergone the heating process, the structure is refined, and the size of the region surrounded by a large-angle boundary with an orientation difference of 15 ° or more (austenite grains) It becomes 15 μm or less, and a ferrite grain colony having an average thickness of 5 μm or less can be easily formed by subsequent warm rolling.
In addition, after rolling is interrupted between primary rolling and warm rolling, accelerated cooling which cools at a cooling rate of 1 ° C./s or more is performed, and after accelerated cooling is stopped, warm rolling may be performed. Further, accelerated cooling may be performed between the pre-rolling and the warm rolling instead of the primary rolling. Needless to say, accelerated cooling may be performed between the preceding rolling and the primary rolling. During rolling, by interrupting rolling and applying accelerated cooling, the growth of crystal grains can be prevented, and the size of the region surrounded by a large angle boundary with an orientation difference of 15 ° or more before warm rolling can be further increased. The desired texture can be more easily formed by warm rolling.

なお、圧延工程後の冷却は、とくに限定する必要はなく、徐冷、空冷、加速冷却等、必要に応じて適正な冷却条件を選択することが望ましい。また、圧延工程の後に、焼戻工程を施してもよい。焼戻工程を施すことにより、フェライトの回復が進行して、さらに靭性が向上する場合もある。焼戻工程は、400℃以上Ac_１変態点以下の温度に加熱する工程とすることが好ましい。焼戻の加熱温度が400℃未満では、温度が低すぎて焼戻の所期の目的を達成できない。一方、Ac_１変態点を超えて高温とすると、オーステナイトが生成し所望の組織を確保できなくなる。 The cooling after the rolling process is not particularly limited, and it is desirable to select appropriate cooling conditions such as slow cooling, air cooling, accelerated cooling, and the like. Moreover, you may give a tempering process after a rolling process. By performing the tempering process, the recovery of ferrite proceeds and the toughness may be further improved. The tempering step is preferably a step of heating to a temperature not lower than 400 ° C. and not higher than the Ac ₁ transformation point. If the tempering heating temperature is less than 400 ° C, the temperature is too low to achieve the intended purpose of tempering. On the other hand, when the temperature is higher than the Ac ₁ transformation point, austenite is generated and a desired structure cannot be secured.

表1に示す組成の溶鋼を真空溶解炉で溶製し、小型鋼塊としたのち、粗圧延により120mm厚のスラブ（鋼素材）とした。得られた鋼素材に、表２に示す条件で加熱工程および圧延工程を施し、板厚20mmの鋼板とした。なお、加熱工程での加熱温度をAc_３変態点未満とする一部の鋼素材には、1200℃加熱、圧下率30％の熱間圧延および急冷からなる前処理を施す組織調整を行ったのち、加熱工程に供した。 Molten steel having the composition shown in Table 1 was melted in a vacuum melting furnace to form a small steel ingot, and then slab (steel material) having a thickness of 120 mm was obtained by rough rolling. The obtained steel material was subjected to a heating step and a rolling step under the conditions shown in Table 2 to obtain a steel plate having a thickness of 20 mm. Note that some steel materials whose heating temperature in the heating process is less than the Ac ₃ transformation point are subjected to a structure adjustment in which pretreatment including heating at 1200 ° C., hot rolling at a reduction rate of 30% and rapid cooling is performed. And subjected to a heating step.

また、加熱温度をAc_３変態点未満とする鋼素材については、加熱工程終了後、急冷した試験片を作製し、EBSP法による結晶方位解析を行い、方位差15°以上の大角境界に囲まれる領域の大きさを調査し、温間圧延前の組織を確認した。
なお、表１に各鋼の、Ac_１、Ac_３、Ar_３変態点を併記したが、これら変態点は次の各式により算出した。 For steel materials whose heating temperature is less than the Ac ₃ transformation point, a rapidly cooled specimen is prepared after the heating process, and crystal orientation analysis is performed by the EBSP method, which is surrounded by a large-angle boundary with an orientation difference of 15 ° or more. The size of the region was investigated and the structure before warm rolling was confirmed.
In Table 1, the Ac ₁ , Ac ₃ , and Ar ₃ transformation points of each steel are shown together, and these transformation points were calculated by the following equations.

Ac_１(℃)＝751−26.6Ｃ＋17.6Si−11.6Mn−22.9Cu−23Ni＋24.1Cr＋22.5Mo−39.7Ｖ
−5.7Ti＋233Nb−169Al−895Ｂ
Ac_３(℃)＝937−476.5Ｃ＋56Si−19.7Mn−16.3Cu−26.6Ni−4.9Cr＋38.1Mo＋124.8Ｖ
＋136.3Ti−19Nb＋198Al＋3315Ｂ
Ar_３(℃)＝868−396Ｃ＋25Si−68Mn−21Cu−36Ni−25Cr−30Mo
（ここで、Ｃ、Si、Mn、Cu、Ni、Cr、Mo、Ｖ、Ti、Nb、Al、Ｂ：各元素の含有量（質量％））
得られた厚鋼板について、組織観察、引張試験、シャルピー衝撃試験、疲労亀裂伝播試験を実施し、組織、強度、靭性、耐疲労亀裂伝播特性を評価した。試験方法は次のとおりとした。
（１）組織観察
得られた厚鋼板から、圧延面に平行に、板状試験片（大きさ：25×25mm）を、測定面が板厚1/4位置および板厚中央位置となるように、採取した。そして、板状試験片の測定面を、600番のエメリー紙研磨により仕上げたのち、電解研磨または化学研磨により測定面の残留歪を除去し、測定用試験片とした。得られた測定用試験片を用いて、Ｘ線回折測定を行い、（２００）面および（１１０）面のＸ線回折強度比を求めた。この各特定面のＸ線回折強度比は、測定用試験片を用いて得られた各特定面からの回折強度と、粉末試料を用いて作製したランダム方位サンプル材の同一特定面からの回折強度との比である。このＸ線回折強度比が大きいほど特定の結晶方位が集中した集合組織が発達していることを示す。 _{Ac 1 (℃) = 751-26.6C +} 17.6Si-11.6Mn-22.9Cu-23Ni + 24.1Cr + 22.5Mo-39.7V
-5.7Ti + 233Nb-169Al-895B
Ac ₃ (° C) = 937-476.5C + 56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr + 38.1Mo + 124.8V
+ 136.3Ti-19Nb + 198Al + 3315B
Ar ₃ (° C) = 868−396C + 25Si−68Mn−21Cu−36Ni−25Cr−30Mo
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al, B: content of each element (mass%))
The obtained thick steel plate was subjected to a structure observation, a tensile test, a Charpy impact test, and a fatigue crack propagation test, and the structure, strength, toughness, and fatigue crack propagation characteristics were evaluated. The test method was as follows.
(1) Microstructure observation From the obtained thick steel plate, a plate-shaped test piece (size: 25 x 25 mm) is placed in parallel with the rolling surface so that the measurement surface is at the 1/4 thickness position and the central thickness position. It was collected. And after finishing the measurement surface of a plate-shaped test piece by No. 600 emery paper grinding | polishing, the residual distortion of the measurement surface was removed by electrolytic polishing or chemical polishing, and it was set as the test piece for a measurement. X-ray diffraction measurement was performed using the obtained test specimen, and the X-ray diffraction intensity ratio between the (200) plane and the (110) plane was determined. The X-ray diffraction intensity ratio of each specific surface is the diffraction intensity from each specific surface obtained using the test specimen for measurement and the diffraction intensity from the same specific surface of the random orientation sample material prepared using the powder sample. And the ratio. A larger X-ray diffraction intensity ratio indicates that a texture in which a specific crystal orientation is concentrated is developed.

また、同一の測定用試験片を用いて、ＥＢＳＰ法による結晶方位解析を行い、｛１００｝面、｛２１１｝面、｛１１０｝面、｛１１１｝面のうちのいずれかの面が、圧延面に対し５°以内に揃った領域である、フェライト粒コロニーを定めた。そして、ＥＢＳＰ解析結果を紙面に印刷し、各フェライト粒コロニーの板厚方向の厚さの最大値を測定した。得られたフェライト粒コロニー厚さの最大値を平均し、その値を、各厚鋼板におけるフェライト粒コロニーの板厚方向の平均厚さとした。 Further, the crystal orientation analysis by the EBSP method is performed using the same test specimen for measurement, and any one of {100} plane, {211} plane, {110} plane, and {111} plane is rolled. Ferrite grain colonies, which are regions aligned within 5 ° with respect to the surface, were determined. And the EBSP analysis result was printed on the paper surface, and the maximum value in the thickness direction of each ferrite grain colony was measured. The maximum value of the obtained ferrite grain colony thickness was averaged, and the value was defined as the average thickness in the thickness direction of the ferrite grain colony in each thick steel sheet.

また、同一試験片を用いて、フェライト相およびフェライト相以外の第二相について、ビッカース硬度計（試験力：0.098N）を用いて、ビッカース硬さHVを測定し、その平均値をその鋼板のフェライト相および第二相の硬さHVとした。なお、各鋼板で測定した第二相の粒数は10個以上とした。
（２）引張試験
得られた厚鋼板から、JIS Z 2201の規定に準拠して、引張方向が圧延方向となるように、JIS 1B号引張試験片（平行部:幅25mm×長220mm）を採取し、JIS Z 2241の規定に準拠して引張試験を実施し、引張特性（降伏強さＹＳ、引張強さＴＳ）を求めた。
（３）シャルピー衝撃試験
得られた厚鋼板の板厚中央部から、JIS Z 2242の規定に準拠して、圧延方向（Ｌ方向）にＶノッチ試験片を採取し、シャルピー衝撃試験を実施し、０℃における吸収エネルギーvE_０（Ｊ）、破面遷移温度vＴrs（℃）を求めた。なお、vE_０は３本の平均値とした。
（４）疲労亀裂伝播試験
得られた厚鋼板から、疲労亀裂伝播方向が板厚方向となるように、図２に示すASTM E647の規定に準拠して片側切欠引張試験片を採取し、試験した。なお、試験片には予め疲労予亀裂を導入した。試験条件は、周波数：15Hz、応力比0.1とし、ΔＫ＝10MPa√m、15 MPa√mにおける疲労亀裂伝播速度da/dN（m/cycle）を求めた。
得られた結果を表３に示す。 Also, using the same specimen, measure the Vickers hardness HV for the ferrite phase and the second phase other than the ferrite phase using a Vickers hardness tester (test force: 0.098 N), and calculate the average value of the steel sheet. The hardness of the ferrite phase and the second phase was HV. The number of grains of the second phase measured on each steel plate was 10 or more.
(2) Tensile test JIS 1B tensile test specimen (parallel part: width 25mm x length 220mm) is taken from the obtained thick steel plate so that the tensile direction is the rolling direction in accordance with the provisions of JIS Z 2201. Then, a tensile test was performed in accordance with the provisions of JIS Z 2241 to determine tensile properties (yield strength YS, tensile strength TS).
(3) Charpy impact test V-notch specimens were collected in the rolling direction (L direction) from the central part of the obtained thick steel plate in accordance with the provisions of JIS Z 2242, and the Charpy impact test was conducted. Absorption energy vE ₀ (J) at 0 ° C. and fracture surface transition temperature vTrs (° C.) were determined. Note that vE ₀ was an average of three.
(4) Fatigue crack propagation test From the obtained thick steel plate, a one-side notch tensile test piece was sampled and tested in accordance with the provisions of ASTM E647 shown in FIG. 2 so that the fatigue crack propagation direction was the plate thickness direction. . A fatigue precrack was introduced in advance in the test piece. The test conditions were a frequency: 15 Hz, a stress ratio of 0.1, and a fatigue crack propagation rate da / dN (m / cycle) at ΔK = 10 MPa√m and 15 MPa√m was obtained.
The obtained results are shown in Table 3.

本発明例はいずれも、150HV未満の回復または再結晶したフェライト相を組織全体に対する体積率で60％以上含み、フェライト相以外の第二相が240HV未満の硬さを有し、さらに板厚中央位置および板厚1/4位置における（２００）面のＸ線回折強度比が2.0以上または（１１０）面のＸ線回折強度比が2.5以上である集合組織が発達し、さらにフェライト粒コロニーの板厚方向の厚さが、平均で５μm以下である組織を有し、vE_０が100J以上、vTrsが-60℃以下という優れた靭性と、応力拡大係数範囲ΔKが10MPa√mのときに疲労亀裂伝播速度が0.60×10^-9m/cycle未満、ΔKが15MPa√mのときに疲労亀裂伝播速度が5.0×10^-9m/cycle未満と、極めて優れた板厚方向の耐疲労亀裂伝播特性を有する厚鋼板となっている。

Each of the inventive examples includes a recovered or recrystallized ferrite phase of less than 150 HV in a volume ratio of 60% or more with respect to the entire structure, the second phase other than the ferrite phase has a hardness of less than 240 HV, and the center of the plate thickness. A texture having an X-ray diffraction intensity ratio of (200) plane of 2.0 or higher or a (110) plane X-ray diffraction intensity ratio of 2.5 or higher at a position and a thickness of 1/4 position is developed; It has a microstructure with an average thickness of 5 μm or less, excellent toughness with vE ₀ of 100 J or more, vTrs of −60 ° C. or less, and fatigue crack when the stress intensity factor range ΔK is 10 MPa√m The fatigue crack propagation speed is less than 5.0 × 10 ^-9 m / cycle when the propagation speed is less than 0.60 × 10 ^-9 m / cycle and ΔK is 15 MPa√m. It has a thick steel plate.

一方、本発明範囲を外れた比較例は、靭性、耐疲労亀裂伝播特性のいずれかあるいは両方が低下している。 On the other hand, in the comparative example out of the scope of the present invention, either or both of toughness and fatigue crack propagation characteristics are deteriorated.

フェライト粒コロニーの分布状態の一例を示す説明図である。It is explanatory drawing which shows an example of the distribution state of a ferrite grain colony. 実施例で疲労亀裂伝播試験に用いた試験片の寸法形状を示す説明図である。It is explanatory drawing which shows the dimension shape of the test piece used for the fatigue crack propagation test in the Example.

Claims

% By mass
C: 0.06-0.20%, Si: 1.0% or less,
Mn: 2.0% or less, P: 0.10% or less,
S: 0.006% or less, Al: 0.10% or less, a thick steel plate for a welded structure having a composition consisting of the balance Fe and unavoidable impurities, wherein the structure of the thick steel plate is divided into a ferrite phase and the other second A ferrite phase having a hardness of less than 150 HV on average and including a volume ratio of 60% or more with respect to the entire structure, and the second phase is a phase having an average hardness of less than 240 HV, The X-ray diffraction intensity ratio of the (200) plane at the plate thickness center position and the 1/4 thickness position is 2.0 or more, or the X-ray diffraction intensity ratio of the (110) plane is 2.5 or more, and the {100} plane, {110} plane The thickness in the plate thickness direction of the ferrite grain colony in which any one of the plane, {111} plane, and {211} plane is aligned within 5 ° with respect to the rolling plane is the plate thickness center position and plate thickness. Thickness characterized by an average structure of 5 μm or less at 1/4 position Steel plate for welded structures with excellent fatigue crack propagation characteristics in the direction.

In addition to the above composition, one or two selected in terms of mass%, Cu: 1.0% or less, Ni: 1.5% or less, Cr: 1.0% or less, Mo: 1.0% or less, B: 0.010% or less The thick steel plate for welded structure according to claim 1, comprising at least a seed.

In addition to the composition, the composition further contains one or more selected from V: 0.10% or less, Nb: 0.10% or less, and Ti: 0.05% or less in terms of mass%. The thick steel plate for welded structures according to 1 or 2.

4. In addition to the composition, the composition further comprises one or two selected from Ca: 0.010% or less and REM: 0.010% or less by mass%. The steel plate for welded structure as described.

% By mass
C: 0.06-0.20%, Si: 1.0% or less,
Mn: 2.0% or less, P: 0.10% or less,
S: 0.006% or less, Al: 0.10% or less, a steel material having a composition comprising the balance Fe and inevitable impurities, a heating process and a rolling process are performed to produce a thick steel sheet for welded structure. In the rolling process, the cumulative rolling reduction in a ferrite single phase region at least 500 ° C. or higher or a two-phase region in which the volume fraction of ferrite is 60% or more is 50% or more, and the following formula (1): A process including warm rolling in which a rolling speed S defined is 2.0 × 10 ⁻² / s or less, and the steel material is placed on a large angle boundary with an orientation difference of 15 ° or more before the warm rolling. Thick steel plate for welded structure excellent in fatigue crack propagation resistance in the thickness direction, characterized by performing the warm rolling after adjusting the structure so that the average size of the enclosed region is 15 μm or less Manufacturing method.
Record
Rolling speed S = ln (di / df) / t (1)
Where, di: plate thickness before warm rolling (mm),
df: plate thickness after warm rolling (mm),
t: Time required for the warm rolling (s)

% By mass
C: 0.06-0.20%, Si: 1.0% or less,
Mn: 2.0% or less, P: 0.10% or less,
S: 0.006% or less, Al: 0.10% or less, a composition comprising the balance Fe and inevitable impurities, and an average size of a region surrounded by a large angle boundary with an orientation difference of 15 ° or more is 15 μm or less a steel material having a heating step of heating to a temperature of the temperature range below 500 ° C. or higher Ac ₃ transformation point, and then, the steel material which has undergone the heating step, (Ac ₃ transformation point -40 ° C.) or less 500 ° C. or higher A rolling process in which the hot rolling is performed so that the rolling reduction rate in the temperature range is 50% or more and the rolling speed S defined by the following formula (1) is 2.0 × 10 ⁻² / s or less. A method of manufacturing a thick steel plate for welded structure having excellent fatigue crack propagation characteristics in the thickness direction, characterized by
Record
Rolling speed S = ln (di / df) / t (1)
Where, di: plate thickness before warm rolling (mm),
df: plate thickness after warm rolling (mm),
t: Time required for the warm rolling (s)

% By mass
C: 0.06-0.20%, Si: 1.0% or less,
Mn: 2.0% or less, P: 0.10% or less,
S: 0.006% or less, Al: 0.10% or less, a steel material having a composition comprising the balance Fe and inevitable impurities, a heating process and a rolling process are performed to produce a thick steel sheet for welded structure. a is, the heating step is a step of heating the steel material to a temperature of the temperature range of 950 less ° C. or higher Ac ₃ transformation point, the rolling process is less than 900 ° C. (Ar ₃ transformation point -20 ° C.) Primary rolling in which the cumulative rolling reduction in the above temperature range is 30% or more, the cumulative rolling reduction in the temperature range of 500 ° C. or more below (Ar ₃ transformation point−20 ° C.), and the following (1 For the welded structure with excellent fatigue crack propagation resistance in the plate thickness direction, which is a process of performing a warm rolling with a rolling speed S defined by the formula of 2.0 × 10 ⁻² / s or less. Manufacturing method of thick steel plate.
Record
Rolling speed S = ln (di / df) / t (1)
Where, di: plate thickness before warm rolling (mm),
df: plate thickness after warm rolling (mm),
t: Time required for the warm rolling (s)

The heating step is a step of heating the steel material to a temperature in a temperature range of 950 to 1180 ° C., and the rolling step has a cumulative reduction ratio in a temperature range of 1050 to 900 ° C. before the primary rolling is 30. The manufacturing method of the thick steel plate for welded structures according to claim 7, wherein the pre-stage rolling is at least%.

The method for producing a thick steel plate for welded structure according to claim 7 or 8, wherein accelerated cooling is performed between the primary rolling and the warm rolling at a cooling rate of 1 ° C / s or more.

9. The thick steel plate for welded structure according to claim 8, wherein accelerated cooling is performed to cool at a cooling rate of 1 ° C./s or more instead of the primary rolling between the preceding rolling and the warm rolling. Manufacturing method.

After subjected to the rolling process, the production method of welding structural steel plate according to any one of claims 5 to 10, characterized by applying tempering step of heating to a temperature of less than ₁ transformation point 400 ° C. or higher Ac .