JP5900312B2 - High-strength thick steel plate with excellent toughness and brittle crack propagation stopping characteristics for high heat input welds and its manufacturing method - Google Patents
High-strength thick steel plate with excellent toughness and brittle crack propagation stopping characteristics for high heat input welds and its manufacturing method Download PDFInfo
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Description
本発明は、船舶、海洋構造物、低温貯蔵タンク、建築・土木構造物等の大型構造物に、板厚５０ｍｍを超える厚鋼板として使用して好適な、大入熱溶接部の靭性および脆性き裂伝播停止特性に優れた高強度厚鋼板およびその製造方法に関する。 The present invention is suitable for use in large structures such as ships, offshore structures, low-temperature storage tanks, building / civil engineering structures, etc. as thick steel plates having a plate thickness exceeding 50 mm. The present invention relates to a high-strength thick steel plate having excellent crack propagation stopping characteristics and a method for producing the same.
船舶、海洋構造物、低温貯蔵タンク、建築・土木構造物等の大型構造物においては、脆性破壊に伴う事故が経済や環境に及ぼす影響が大きいため、使用される鋼材に対して、不慮の事故等で構造物にき裂が発生した場合においても破壊に至ることを防止する観点から、低温における脆性き裂伝播停止特性が要求されている。 In large structures such as ships, offshore structures, low-temperature storage tanks, and construction / civil engineering structures, accidents resulting from brittle fractures have a large impact on the economy and the environment. From the viewpoint of preventing breakage even when a crack occurs in a structure due to the above, brittle crack propagation stopping characteristics at low temperature are required.
例えば、コンテナ船やバルクキャリアーなどの船舶で、船体外板に使用される高強度の厚肉材には、船舶の安全性確保の観点から優れた脆性き裂伝播停止特性が要求されるが、これらの船舶の大型化に伴い、高強度化、厚肉化が一層進展し、その要求も一段と高度化している。 For example, in a vessel such as a container ship or a bulk carrier, a high-strength thick material used for the hull outer plate is required to have excellent brittle crack propagation stopping characteristics from the viewpoint of ensuring the safety of the ship. Along with the increase in size of these vessels, the strength and thickness of the vessels have further increased, and the requirements have been further advanced.
脆性き裂伝播停止特性を向上させるため、従来からＮｉ含有量を増加させる方法が知られており、液化天然ガス（ＬＮＧ）の貯槽タンクにおいては、９％Ｎｉ鋼が商業規模で使用されている。しかし、Ｎｉ量の増加はコストの大幅な上昇を余儀なくさせるため、ＬＮＧ貯槽タンク以外の用途には適用が難しい。 In order to improve the brittle crack propagation stopping characteristics, a method of increasing the Ni content is conventionally known, and 9% Ni steel is used on a commercial scale in a liquefied natural gas (LNG) storage tank. . However, since the increase in the amount of Ni necessitates a significant increase in cost, it is difficult to apply to applications other than the LNG storage tank.
一方、ＬＮＧのような極低温まで至らない、船舶やラインパイプに使用される鋼板のように板厚が５０ｍｍ未満の比較的薄手の鋼材に対しては、ＴＭＣＰ法により細粒化を図り、低温靭性を向上させて、優れた脆性き裂伝播停止特性を付与することができる。 On the other hand, for thin steel materials with a thickness of less than 50 mm, such as steel plates used for ships and line pipes, which do not reach extremely low temperatures such as LNG, fine graining is attempted by the TMCP method. Toughness can be improved and excellent brittle crack propagation stopping properties can be imparted.
また、近年、合金コストを上昇させることなく、鋼材の表層部の組織を超微細化する技術が、脆性き裂伝播停止特性を向上させる手段として提案されている。 In recent years, a technique for making the structure of the surface layer portion of a steel material ultrafine without increasing the alloy cost has been proposed as a means for improving the brittle crack propagation stopping property.
例えば、特許文献１には、脆性き裂が伝播する際に、鋼材表層部に発生するシアリップ（塑性変形領域）が脆性き裂伝播停止特性の向上に効果があることに着目し、シアリップ部分の結晶粒を微細化させて、伝播する脆性き裂が有する伝播エネルギーを吸収させる方法が開示されている。 For example, Patent Document 1 focuses on the fact that a shear lip (plastic deformation region) generated in a steel surface layer portion when a brittle crack propagates is effective in improving the brittle crack propagation stopping property. A method is disclosed in which the crystal grains are refined to absorb the propagation energy of the propagating brittle crack.
熱間圧延後の制御冷却により表層部分をＡｒ３変態点以下に冷却し、その後制御冷却を停止して表層部分を変態点以上に復熱させる工程を１回以上繰り返して行い、この間に鋼材に圧下を加えることにより、繰り返し変態させ又は加工再結晶させて、表層部分に超微細なフェライト組織又はベイナイト組織を生成させるものである。 The process of cooling the surface layer portion below the Ar 3 transformation point by controlled cooling after hot rolling, and then stopping the controlled cooling to reheat the surface layer portion above the transformation point is repeated one or more times. By applying the reduction, the material is repeatedly transformed or recrystallized to form an ultrafine ferrite structure or bainite structure in the surface layer portion.
さらに、特許文献２では、フェライト−パーライトを主体のミクロ組織とする鋼材において脆性き裂伝播停止特性を向上させる場合、鋼材の表裏面の表面部を円相当粒径：５μｍ以下、アスペクト比：２以上のフェライト粒を有するフェライト組織を５０％以上有する層で構成し、仕上げ圧延中の１パス当りの最大圧下率を１２％以下として局所的な再結晶現象を抑制し、フェライト粒径のバラツキを抑えることが重要であることが開示されている。 Furthermore, in Patent Document 2, when improving the brittle crack propagation stopping characteristics in a steel material mainly composed of ferrite-pearlite, the surface portion of the front and back surfaces of the steel material has a circle-equivalent particle size of 5 μm or less and an aspect ratio of 2 Consists of 50% or more of the ferrite structure with the above ferrite grains, and the maximum reduction rate per pass during finish rolling is 12% or less to suppress local recrystallization phenomenon, and the variation in ferrite grain size It is disclosed that it is important to suppress.
しかし、特許文献１、２記載の製造方法は、鋼材表層部のみを一旦冷却した後に復熱させ、かつ復熱中に加工を加えることによって、脆性き裂伝播停止特性に効果のある組織を得るものであり、実生産規模では制御が容易ではないものと考えられるプロセスである。 However, the manufacturing methods described in Patent Documents 1 and 2 obtain a structure effective in brittle crack propagation stopping characteristics by cooling only the steel surface layer part and then recovering it, and adding processing during the recovery. It is a process that is considered to be difficult to control on an actual production scale.
特許文献３には、フェライト結晶粒の微細化のみならずフェライト結晶粒内に形成されるサブグレインを利用して脆性き裂伝播停止特性を向上させる、ＴＭＣＰ法を利用した技術が記載されている。 Patent Document 3 describes a technique using the TMCP method for improving brittle crack propagation stopping characteristics by utilizing subgrains formed in ferrite crystal grains as well as making ferrite crystal grains finer. .
具体的には、板厚３０〜４０ｍｍの鋼板を対象とし、鋼板表層の冷却および復熱などの複雑な温度制御を必要とせずに、（ａ）微細なフェライト結晶粒を確保する圧延条件、（ｂ）鋼材板厚の５％以上の部分に微細フェライト組織を生成する圧延条件、（ｃ）微細フェライトに集合組織を発達させるとともに加工（圧延）により導入した転位を熱的エネルギーにより再配置しサブグレインを形成させる圧延条件、（ｄ）形成した微細なフェライト結晶粒と微細なサブグレイン粒の粗大化を抑制する冷却条件によって脆性き裂伝播停止特性を向上させることが記載されている。 Specifically, for a steel sheet having a thickness of 30 to 40 mm, without requiring complicated temperature control such as cooling and recuperation of the steel sheet surface layer, (a) rolling conditions for securing fine ferrite crystal grains, ( b) Rolling conditions for generating a fine ferrite structure in a portion of 5% or more of the steel plate thickness, (c) A texture is developed in the fine ferrite, and dislocations introduced by processing (rolling) are rearranged by thermal energy and sub- It is described that brittle crack propagation stopping characteristics are improved by rolling conditions for forming grains, and (d) cooling conditions for suppressing coarsening of the formed fine ferrite crystal grains and fine subgrain grains.
また、制御圧延において、変態したフェライトに圧下を加えて集合組織を発達させることにより、脆性き裂伝播停止特性を向上させる方法も知られている。鋼材の破壊面上にセパレーションを板厚方向と平行な方向に生ぜしめ、脆性き裂先端の応力を緩和させることにより、脆性破壊に対する抵抗を高めるものである。 In addition, in controlled rolling, a method of improving the brittle crack propagation stopping property by developing a texture by reducing the transformed ferrite is also known. Separation occurs on the fracture surface of the steel material in a direction parallel to the plate thickness direction, and the stress at the brittle crack tip is relaxed, thereby increasing the resistance to brittle fracture.
例えば、特許文献４には、制御圧延により（１１０）面Ｘ線強度比を２以上とし、かつ円相当径２０μｍ以上の粗大粒の面積率を１０％以下とすることにより、耐脆性破壊特性を向上させた鋼板が記載されている。 For example, Patent Document 4 discloses that the (110) plane X-ray intensity ratio is 2 or more by controlled rolling, and the area ratio of coarse grains having a circle-equivalent diameter of 20 μm or more is 10% or less. An improved steel sheet is described.
特許文献５には、継手部の脆性き裂伝播停止性能の優れた溶接構造用鋼として、集合組織発達により応力負荷方向とき裂伝播方向をずらすため、板厚内部の圧延面での（１００）面のＸ線面強度比を１．５以上とした鋼板が開示されている。更に、特許文献６〜９には制御圧延における平均圧下率を規定することで板厚方向の各部（板厚の１／４部、板厚中央部など）において集合組織を発達させる脆性亀裂伝播停止性能の優れた溶接構造用鋼の製造方法が記載されている。 In Patent Document 5, as a steel for welded structure having excellent brittle crack propagation stopping performance at the joint, the stress propagation direction and the crack propagation direction are shifted by texture development. A steel sheet having a surface X-ray surface intensity ratio of 1.5 or more is disclosed. Furthermore, Patent Documents 6 to 9 stop brittle crack propagation that develops a texture in each part in the sheet thickness direction (1/4 part of sheet thickness, center part of sheet thickness, etc.) by defining the average rolling reduction in controlled rolling. A method for producing welded structural steel with excellent performance is described.
上述したように、脆性き裂伝播停止性能に優れた鋼板やその製造方法に関して種々の提案がなされているが、大型構造物に使用される鋼材には安全性の観点から、優れた溶接熱影響部の靭性、特にボンド部の靭性に優れることも同時に要求される。 As mentioned above, various proposals have been made regarding steel sheets with excellent brittle crack propagation stopping performance and methods for producing the same, but steel materials used in large structures have excellent welding heat effects from the viewpoint of safety. It is also required that the toughness of the part, particularly the toughness of the bond part, be excellent.
ボンド部は、大入熱溶接時の融点直下の高温にさられて、オーステナイト結晶粒が最も粗大化しやすく、その後の冷却によって脆弱な上部ベイナイト組織に変態し、更に、ウィドマンステッテン組織や島状マルテンサイトが生成して靭性が低下する。 The bond part is exposed to a high temperature just below the melting point during high heat input welding, the austenite crystal grains are most likely to be coarsened, and then transformed into a fragile upper bainite structure by cooling. Martensite is formed and toughness is reduced.
ボンド部の靭性向上に関しては種々の研究がなされ、例えば、ＴｉＮの微細分散によるオーステナイトの粗大化抑制やフェライト変態核としての利用のほか、希土類元素（ＲＥＭ）をＴｉと複合添加することにより、鋼中に微細粒子を分散させてオーステナイトの粒成長を防止し、溶接部の靭性向上を図る方法が提案されている（特許文献１０、１１）。 Various studies have been made on improving the toughness of the bond part. For example, by adding a rare earth element (REM) with Ti in addition to suppressing the austenite coarsening by fine dispersion of TiN and using it as a ferrite transformation nucleus, A method has been proposed in which fine particles are dispersed therein to prevent austenite grain growth and to improve the toughness of the welded portion (Patent Documents 10 and 11).
また、Ｔｉ酸化物やＭｇ酸化物を利用したり（特許文献１２、１３）、ＢＮによりフェライト核を生成したり、ＣａやＲＥＭを添加することで硫化物の形態を制御して、靭性を向上させることが提案されている。 Also, toughness is improved by using Ti oxide or Mg oxide (Patent Documents 12 and 13), generating ferrite nuclei with BN, or adding Ca or REM to control the form of sulfide. It has been proposed to let
また、Ｃａ、Ｏ、Ｓ量を制御し、ＣａおよびＭｎの複合硫化物をフェライト核とし微細に分散させることによって、靭性を向上させる方法が提案されている（特許文献１４）。 A method for improving toughness by controlling the Ca, O, and S amounts and finely dispersing Ca and Mn composite sulfides as ferrite nuclei has been proposed (Patent Document 14).
ところで、近年、建造される大型のコンテナ船やバルクキャリアーの強力甲板部構造においてハッチサイドコーミングに接合される甲板部材では、開口部の周辺に板厚５０ｍｍを超える厚鋼板を用いる設計が採用されるようになっている。 By the way, in recent years, in a deck member to be joined to hatch side combing in a large deck structure of a large container ship or a bulk carrier to be constructed, a design using a thick steel plate having a thickness of more than 50 mm around the opening is adopted. It is like that.
最近、このような厚肉材の脆性き裂伝播停止性能に問題があることが指摘され、例えば、板厚６５ｍｍの鋼板の脆性き裂伝播停止性能を評価すると、母材の大型脆性き裂伝播停止試験で脆性き裂が停止しない結果が報告されている（特許文献１）。 Recently, it has been pointed out that there is a problem in the brittle crack propagation stopping performance of such a thick material. For example, when evaluating the brittle crack propagation stopping performance of a steel plate having a thickness of 65 mm, a large brittle crack propagation of the base metal It has been reported that a brittle crack does not stop in a stop test (Patent Document 1).
このような試験結果により、５０ｍｍを超える板厚の鋼板を適用した船体構造の安全確保が大きな問題となり、（財）日本海事協会が中心となって「超大型コンテナ船の安全性評価に関する研究（脆性き裂アレスト設計関係）」（２００７〜２００８年度）が実施されている。 As a result of these test results, ensuring the safety of the hull structure using a steel plate with a thickness exceeding 50 mm has become a major issue, led by the Japan Maritime Association, “Research on Safety Evaluation of Extra Large Container Ships ( Brittle crack arrest design) ”(fiscal 2007-2008).
上述した特許文献１〜５には、板厚５０ｍｍを超える厚肉材に関する記載がなく、特許文献５記載の発明に係る鋼板は、最大板厚が５０ｍｍで、５０ｍｍを超える厚肉材への適用は生産性などの観点から困難が予想される。 In Patent Documents 1 to 5 described above, there is no description regarding a thick material exceeding 50 mm, and the steel sheet according to the invention described in Patent Document 5 has a maximum plate thickness of 50 mm and is applied to a thick material exceeding 50 mm. Is expected to be difficult from the viewpoint of productivity.
また、特許文献６〜９においては、板厚中央部の集合組織を発達させるために、圧延時に１パスあたりの圧下率を高く設定する必要があるので、製造条件や鋼板サイズなどの面で各種の制約が生じ、その改善が求められていた。 Moreover, in patent documents 6-9, in order to develop the texture of a sheet thickness center part, since it is necessary to set the rolling reduction per pass high at the time of rolling, it is various in terms of manufacturing conditions, steel plate size, etc. As a result, there was a need for improvement.
また、溶接施工において、板厚５０ｍｍ以上の厚鋼板を溶接する場合、入熱３００ｋＪ／ｃｍを超える大入熱溶接の適用が検討され、さらなる大入熱化が予想される。 Moreover, in welding construction, when welding a thick steel plate having a thickness of 50 mm or more, application of large heat input welding with a heat input exceeding 300 kJ / cm is studied, and further increase in heat input is expected.
しかしながら、特許文献１０、１１記載の、ＴｉＮを主体に利用する技術においてはＴｉＮが溶解する温度域に加熱される溶接部でその作用が消失し、また固溶ＴｉおよびＮにより組織が脆化して著しく靭性が低下するので、３００ｋＪ／ｃｍを超える大入熱溶接部では十分な靭性が得られないことが予想される。 However, in the techniques mainly using TiN described in Patent Documents 10 and 11, the action disappears in the weld zone heated to a temperature range where TiN dissolves, and the solid solution Ti and N cause the structure to become brittle. Since the toughness is remarkably lowered, it is expected that sufficient toughness cannot be obtained at a high heat input weld portion exceeding 300 kJ / cm.
さらに、特許文献１０、１１記載の技術のようにＴｉ酸化物やＭｇ酸化物を利用してＨＡＺ靭性を改善する場合、これらの酸化物を十分均質に微細分散することは容易でなく、またＣａやＲＥＭを添加する技術においても３００ｋＪ／ｃｍを超える大入熱溶接では溶接熱影響部の高靭性を確保することは困難であった。 Furthermore, when the HAZ toughness is improved by using Ti oxide or Mg oxide as in the techniques described in Patent Documents 10 and 11, it is not easy to finely and uniformly disperse these oxides. Even in the technique of adding REM or REM, it is difficult to ensure high toughness of the heat affected zone by high heat input welding exceeding 300 kJ / cm.
また、特許文献１４においては、ＣａおよびＭｎの複合硫化物を利用することで４００ｋＪ／ｃｍを超える溶接熱影響部靭性を確保しているが、脆性き裂伝播停止性能に関する検討はなされていない。 In Patent Document 14, the weld heat-affected zone toughness exceeding 400 kJ / cm is ensured by using a composite sulfide of Ca and Mn, but no investigation has been made on brittle crack propagation stopping performance.
そこで、本発明では板厚５０ｍｍを超える厚鋼板においても、脆性き裂伝播停止特性に優れ、かつ、大入熱溶接熱影響部のボンド部において高靭性を有する鋼板及び工業的に極めて簡易なプロセスで安定して製造し得るその製造方法を提供することを目的とする。 Therefore, in the present invention, a steel plate having excellent brittle crack propagation stopping characteristics even in a thick steel plate exceeding 50 mm in thickness, and having high toughness in the bond portion of the large heat input welding heat affected zone, and an industrially simple process It aims at providing the manufacturing method which can be manufactured stably by this.
本発明者らは、上記課題の達成に向けて鋭意研究を重ね、厚肉鋼板でも優れたき裂伝播停止特性を有し、かつ大入熱溶接部の靭性に優れる高強度厚鋼板および当該鋼板を安定して得る製造方法について以下の知見を得た。
１．板厚５０ｍｍを超える厚鋼板において脆性き裂伝播停止特性に及ぼす集合組織の影響を詳細に調べた結果、鋼板表面に平行な面における（２１１）面Ｘ線強度比が１．０以上となる集合組織を有する領域が、板厚中心部を含め板厚の１／３以上の領域において存在することにより、優れた脆性き裂伝播停止特性が得られる。
２．さらに、上記集合組織は、特定の化学成分と板厚中央部を特定の温度域で累積圧下率を４０％以上とする熱間圧延条件の組み合わせにて得られる。
３．上記特定の化学成分の鋼板の溶接ボンド部の靭性は脆化組織に影響され、この脆化組織の靭性は冷却時にフェライト変態を促進させる変態核の微細化を行う事で大きく向上する。変態核を微細に分散させるためには、添加量を下記の（１）式を満足するようにＣａ、Ｓ、Ｏ量を調節することが有効である。すなわち、鋼を溶製する際の凝固段階でＣａＳを晶出させるにあたり、（１）式を満足するようにＣａ、Ｓの添加量および添加時の溶鋼中の溶存酸素量を制御することによって、ＣａＳの晶出後の固溶Ｓ量を確保すれば、ＣａＳの表面上にＭｎＳが析出する。ＭｎＳはフェライト核生成能を有し、その周囲にＭｎの希薄帯が形成されるとフェライト変態が促進され、溶接熱影響部靭性を向上させる。ＭｎＳ上にＴｉＮ、ＢＮ、ＡｌＮ等のフェライト生成核が析出することによって、より一層、フェライト変態が促進される。
The inventors of the present invention have made extensive studies to achieve the above-mentioned problems, and have developed a high-strength thick steel plate and the steel plate that have excellent crack propagation stopping characteristics even in thick steel plates and are excellent in toughness of high heat input welds. The following knowledge was obtained about the manufacturing method obtained stably.
1. As a result of examining in detail the effect of texture on brittle crack propagation stopping characteristics in a thick steel plate exceeding 50 mm thick, the (211) plane X-ray intensity ratio in a plane parallel to the steel plate surface is 1.0 or more. When the region having the structure exists in a region of 1/3 or more of the plate thickness including the central portion of the plate thickness, excellent brittle crack propagation stopping characteristics can be obtained.
2. Further, the texture is obtained by a combination of hot rolling conditions in which a specific chemical component and a central portion of the plate thickness are in a specific temperature range and a cumulative reduction ratio is 40% or more.
3. The toughness of the weld bond portion of the steel plate having the specific chemical component is affected by the embrittlement structure, and the toughness of the embrittlement structure is greatly improved by refining the transformation nucleus that promotes the ferrite transformation during cooling. In order to finely disperse the transformation nuclei, it is effective to adjust the amounts of Ca, S, and O so as to satisfy the following formula (1). That is, in crystallization of CaS in the solidification stage when melting steel, by controlling the amount of Ca and S added and the amount of dissolved oxygen in the molten steel at the time of addition so as to satisfy the formula (1), If the amount of solid solution S after crystallization of CaS is secured, MnS will precipitate on the surface of CaS. MnS has a ferrite nucleation ability. When a Mn dilute band is formed around the MnS, ferrite transformation is promoted and the weld heat affected zone toughness is improved. Ferrite transformation is further promoted by precipitation of ferrite-forming nuclei such as TiN, BN, and AlN on MnS.
０＜（Ｃａ−（０．１８＋１３０×Ｃａ）×Ｏ）／１．２５／Ｓ＜１ （１）
本発明は得られた知見に更に検討を加えてなされたもので、すなわち、本発明は、
１．鋼の化学成分が、質量％で、Ｃ：０．０３〜０．２０％、Ｓｉ：１．０％以下、Ｍｎ：１．５〜２．５％、Ａｌ：０．００５〜０．０６％、Ｐ：０．０１５％以下、Ｓ：０．００５０％以下、Ｎｂ：０．００５〜０．０２０％、Ｔｉ：０．００５〜０．０２０％、Ｎ：０．００３５〜０．００７５％、Ｃａ：０．０００５〜０．００３０％、Ｂ：０．０００５〜０．００２０％、かつ、Ｃａ、Ｏ、Ｓが、下記（１）式を満たし、かつ、下記（２）式で示される炭素等量（Ｃｅｑ）が０．３６％以上０．４５％未満であり、残部がＦｅおよび不可避的不純物からなり、
板厚中心部を含め板厚全厚の１／３以上の領域において、鋼板表面に平行な面における（２１１）面Ｘ線強度比が１．０以上となる集合組織を有し、板厚の中央部におけるベイナイト分率が８０％以上であり、かつ板厚１／４部におけるシャルピー破面遷移温度が−４０℃以下であることを特徴とする大入熱溶接部の靭性および脆性き裂伝播停止特性に優れた高強度厚鋼板。
0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S <1 (1)
The present invention has been made by further studying the obtained knowledge, that is, the present invention,
1. The chemical composition of steel is% by mass, C: 0.03 to 0.20%, Si: 1.0% or less, Mn: 1.5 to 2.5%, Al: 0.005 to 0.06% , P: 0.015% or less, S: 0.0050% or less, Nb: 0.005 to 0.020%, Ti: 0.005 to 0.020%, N: 0.0035 to 0.0075%, Carbon: 0.0005 to 0.0030%, B: 0.0005 to 0.0020%, and Ca, O, and S satisfy the following formula (1) and are represented by the following formula (2) The equivalent amount (Ceq) is 0.36% or more and less than 0.45%, and the balance consists of Fe and inevitable impurities,
In the region of 1/3 or more of the total thickness including the thickness center, the (211) plane X-ray intensity ratio in the plane parallel to the steel plate surface has a texture of 1.0 or more, Toughness and brittle crack propagation in high heat input welds, characterized in that the bainite fraction at the center is 80% or more and the Charpy fracture surface transition temperature at ¼ part is -40 ° C or less. High strength thick steel plate with excellent stopping characteristics.
０＜（Ｃａ−（０．１８＋１３０×Ｃａ）×Ｏ）／１．２５／Ｓ＜１ （１）
Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｕ＋Ｎｉ）／１５＋（Ｖ＋Ｍｏ＋Ｃｒ）／５ （２）
ただし、各元素記号は各成分の含有量（質量％）を表し、含有しない場合は０とする。
２．鋼の化学成分が、さらに、質量％で、Ｃｕ：０．０１〜０．５％、Ｎｉ：０．０１〜１．０％、Ｃｒ：０．０１〜０．５％、Ｍｏ：０．０１〜０．５％、Ｖ：０．００１〜０．１０％の１種または２種以上を含有することを特徴とする１記載の大入熱溶接部の靭性および脆性き裂伝播停止特性に優れた高強度厚鋼板。
３．１または２のいずれかに記載の化学成分を有する鋼素材を、１０００〜１２００℃の温度に加熱し、熱間圧延における板厚中央部の温度が（Ａｒ３点＋４０）℃以下、Ａｒ３点以上の温度域で累積圧下率４０％以上、且つ、１パス当たりの圧下率の平均値が６．０％以上で、各パスの圧下率範囲が５．０〜２０．０％となる圧延を行った後、３．０℃／ｓ以上の冷却速度にて４００℃以下まで冷却することを特徴とする大入熱溶接部の靭性および脆性き裂伝播停止特性に優れた高強度厚鋼板の製造方法。
0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S <1 (1)
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (V + Mo + Cr) / 5 (2)
However, each element symbol represents the content (% by mass) of each component, and 0 when not contained.
2. Further, the chemical composition of the steel is, by mass, Cu: 0.01 to 0.5%, Ni: 0.01 to 1.0%, Cr: 0.01 to 0.5%, Mo: 0.01 The toughness and brittle crack propagation stopping property of the high heat input welded portion according to 1, characterized by containing at least one of -0.5% and V: 0.001-0.10% High strength thick steel plate.
The steel material having the chemical composition described in 3.1 or 2 is heated to a temperature of 1000 to 1200 ° C., and the temperature at the center of the plate thickness in hot rolling is (Ar 3 points + 40) ° C. or less, Ar Cumulative rolling reduction of 40% or more in the temperature range of 3 or more points, the average value of rolling reduction per pass is 6.0% or more, and the rolling reduction range of each pass is 5.0 to 20.0%. A high-strength thick steel plate excellent in toughness and brittle crack propagation stopping characteristics of a large heat input weld, characterized by cooling to 400 ° C. or lower at a cooling rate of 3.0 ° C./s or higher after rolling. Manufacturing method.
本発明により得られる厚鋼板は板厚５０ｍｍ超えであっても、板厚位置に応じて集合組織が適切に制御されるので、脆性き裂伝播停止特性に優れる。本発明を、板厚５０ｍｍ以上、好ましくは板厚５０ｍｍ超え、より好ましくは板厚５５ｍｍ以上、さらに好ましくは板厚６０ｍｍ以上の鋼板に適用することが、従来技術に係る鋼に対してより顕著な優位性を発揮するため、有効である。
また、大入熱溶接を施した場合、溶接熱影響部に、高温でも溶解しないフェライト変態生成核が微細分散して、微細溶接熱影響部組織が得られるので、大入熱溶接熱影響部の靭性に優れ、産業上極めて有用である。なかでも船舶用の構造部材として、例えば、造船分野ではコンテナ船、バルクキャリアーの強力甲板部構造においてハッチサイドコーミングに接合される甲板部材へ本鋼板を適用することにより船舶の安全性向上に寄与するところ大である。
Even if the thick steel plate obtained by the present invention has a plate thickness exceeding 50 mm, the texture is appropriately controlled according to the plate thickness position, so that it has excellent brittle crack propagation stopping characteristics. Applying the present invention to a steel plate having a plate thickness of 50 mm or more, preferably more than 50 mm, more preferably 55 mm or more, and even more preferably 60 mm or more is more prominent than steels according to the prior art. Effective because it demonstrates its superiority.
In addition, when high heat input welding is performed, the ferrite transformation formation nuclei that do not melt even at high temperatures are finely dispersed in the weld heat affected zone, and a fine weld heat affected zone structure is obtained. Excellent toughness and extremely useful in industry. In particular, as a structural member for ships, for example, in the shipbuilding field, this steel plate is applied to deck members that are joined to hatch side combing in the strong deck structure of container ships and bulk carriers. But it ’s big.
本発明では、１．鋼の化学成分、２．鋼板内部の集合組織、３．板厚中心部のミクロ組織、４．母材靭性、を規定する。
１．化学成分
説明において％は質量％とする。
In the present invention, 1. Chemical composition of steel 2. Texture inside the steel plate; 3. Microstructure at the center of the plate thickness Specifies the base material toughness.
1. In the description of chemical components,% is mass%.
Ｃ：０．０３〜０．２０％
Ｃは鋼の強度を向上する元素であり、本発明では、所望の強度を確保するためには０．０３％以上の含有を必要とするが、０．２０％を超えると、溶接性が劣化するばかりか靭性にも悪影響がある。このため、Ｃは、０．０３〜０．２０％の範囲に規定した。なお、好ましくは０．０５〜０．１５％である。
C: 0.03-0.20%
C is an element that improves the strength of steel. In the present invention, it is necessary to contain 0.03% or more in order to ensure a desired strength, but if it exceeds 0.20%, the weldability deteriorates. As well as adversely affecting toughness. For this reason, C was specified in the range of 0.03 to 0.20%. In addition, Preferably it is 0.05 to 0.15%.
Ｓｉ：１．０％以下
Ｓｉは脱酸元素として、また鋼の強化元素として有効であるが１．０％を超えると鋼の表面性状を損なうばかりか靭性が極端に劣化するため、１．０％以下とする。
Si: 1.0% or less Si is effective as a deoxidizing element and as a steel strengthening element, but if it exceeds 1.0%, not only the surface properties of the steel are impaired, but also the toughness is extremely deteriorated. % Or less.
Ｍｎ：１．５〜２．５％
Ｍｎは、強化元素として添加する。１．５％より少ないとその効果が十分でなく、２．５％を超えると溶接性が劣化し、鋼材コストも上昇するため、１．５〜２．５％とする。
Mn: 1.5 to 2.5%
Mn is added as a strengthening element. If it is less than 1.5%, the effect is not sufficient, and if it exceeds 2.5%, the weldability deteriorates and the steel material cost also rises, so 1.5 to 2.5%.
Ａｌ：０．００５〜０．０６％
Ａｌは、脱酸剤として作用し、このためには０．００５％以上の含有を必要とするが、０．０６％を超えて含有すると、母材靭性を低下させるとともに、溶接した場合に、溶接金属部の靭性を低下させる。このため、Ａｌは、０．００５〜０．０６％の範囲に規定した。なお、好ましくは、０．０２〜０．０４％である。
Al: 0.005-0.06%
Al acts as a deoxidizer, and for this purpose it needs to contain 0.005% or more, but if it contains more than 0.06%, the base material toughness is lowered, and when welding, Reduces the toughness of the weld metal. For this reason, Al was prescribed | regulated in the range of 0.005-0.06%. In addition, Preferably, it is 0.02 to 0.04%.
Ｐ：０．０１５％以下
Ｐは、０．０１５％を超えて添加すると、溶接部の靭性を劣化させる。
P: 0.015% or less When P exceeds 0.015%, the toughness of the welded portion is deteriorated.
Ｓ：０．００５０％以下
Ｓは、０．００５０％を超えて添加すると、母材および溶接部の靭性を劣化させる。
S: 0.0050% or less When S is added in excess of 0.0050%, the toughness of the base metal and the welded portion is deteriorated.
Ｎｂ：０．００５〜０．０２０％
Ｎｂは再結晶温度域に影響を与える、制御圧延に不可欠な元素であり、鋼の強化に有効に作用する。この効果を得るためには、０．００５％以上の添加が必要である。しかし、０．０２０％を超える含有は溶接部靭性を劣化させる。
Nb: 0.005 to 0.020%
Nb is an indispensable element for controlled rolling, which affects the recrystallization temperature range, and effectively acts to strengthen steel. In order to obtain this effect, 0.005% or more must be added. However, the content exceeding 0.020% deteriorates the weld zone toughness.
Ｔｉ：０．００５〜０．０２０％
Ｔｉは凝固時にＴｉＮとなって析出し、溶接部でのオーステナイトの粗大化抑制やフェライト変態核となって高靭性化に寄与する。０．００５％未満ではその効果が少なく、一方、０．０２％を超えるとＴｉＮ粒子の粗大化によってその効果が得られなくなるため、０．００５〜０．０２％とする。
Ti: 0.005-0.020%
Ti precipitates as TiN during solidification, and contributes to the suppression of coarsening of austenite at the welded portion and to ferrite transformation nuclei to increase toughness. If it is less than 0.005%, the effect is small. On the other hand, if it exceeds 0.02%, the effect cannot be obtained due to the coarsening of TiN particles, so 0.005 to 0.02%.
Ｎ：０．００３５〜０．００７５％
Ｎは、ＴｉＮの必要量を確保するために必要な元素で、０．００３５％未満では十分なＴｉＮ量が得られず、０．００７５％を超えると溶接熱サイクルによってＴｉＮが溶解する領域において固溶Ｎ量が増加して靭性を著しく低下させるため、０．００３５〜０．００７５％とする。
N: 0.0035 to 0.0075%
N is an element necessary for securing the necessary amount of TiN. If it is less than 0.0035%, a sufficient amount of TiN cannot be obtained, and if it exceeds 0.0075%, it is solidified in the region where TiN is dissolved by the welding heat cycle. In order to increase the amount of dissolved N and significantly reduce toughness, the content is made 0.0035 to 0.0075%.
Ｃａ：０．０００５〜０．００３０％
Ｃａは、Ｓの固定による靭性改善効果を有する元素である。このような効果を発揮させるには少なくとも０．０００５％は含有することが必要であるが、０．００３０％を超えて含有しても効果が飽和するため、０．０００５％〜０．００３０％とする。
Ca: 0.0005 to 0.0030%
Ca is an element having an effect of improving toughness by fixing S. In order to exert such an effect, it is necessary to contain at least 0.0005%, but even if it exceeds 0.0030%, the effect is saturated, so 0.0005% to 0.0030% And
Ｂ：０．０００５〜０．００２０％
Ｂは溶接熱影響部でＴｉＮの溶解によるＮをＢＮとして固定し、溶接部靭性の劣化を抑制する。また、焼入性を向上させ母材の強度確保に有効に寄与する。このような効果は０．０００５％以上の添加で発揮され、また、０．００２０％よりも多く添加してもその効果は飽和するため、０．０００５〜０．００２０％とする。
B: 0.0005 to 0.0020%
B is a weld heat affected zone, fixing N due to dissolution of TiN as BN, and suppressing deterioration of toughness of the weld zone. Moreover, it improves hardenability and contributes effectively to securing the strength of the base material. Such an effect is exhibited by addition of 0.0005% or more, and even if added in an amount of more than 0.0020%, the effect is saturated, so 0.0005 to 0.0020%.
炭素当量（Ｃｅｑ）：０．３６％以上、０．４５％未満
但し、Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｕ＋Ｎｉ）／１５＋（Ｖ＋Ｍｏ＋Ｃｒ）／５で、各合金元素は含有量（質量％）とする。
炭素当量は組織の強度、変態挙動等を予測するための重要な指標となる。炭素当量が０．３６％未満では板厚中心部において、後述するベイナイト分率が得難く、また０．４５％以上では靭性が劣化してしまうため、０．３６％以上、０．４５未満とする。
Carbon equivalent (Ceq): 0.36% or more and less than 0.45% However, Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (V + Mo + Cr) / 5, and each alloy element has a content (mass%).
The carbon equivalent is an important index for predicting the strength and transformation behavior of the structure. If the carbon equivalent is less than 0.36%, it is difficult to obtain the bainite fraction described later in the center of the plate thickness, and if it is 0.45% or more, the toughness is deteriorated. Therefore, 0.36% or more and less than 0.45. To do.
０＜（Ｃａ−（０．１８＋１３０×Ｃａ）×Ｏ）／１．２５／Ｓ＜１ 但し、Ｃａ、Ｏ、Ｓは各成分の含有量（質量％）
本パラメータ式は複合硫化物をＣａＳ上にＭｎＳが析出した形態とするため、鋼中のＣａ、Ｓ、Ｏの含有量（質量％）を規定するものである。
0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S <1 However, Ca, O, and S are the contents of each component (% by mass).
This parameter formula defines the content (mass%) of Ca, S, and O in the steel in order to make the composite sulfide into a form in which MnS is precipitated on CaS.
（Ｃａ−（０．１８＋１３０×Ｃａ）×Ｏ）／１．２５／Ｓの値（パラメータの値と言う場合がある）が、０超え、１未満の場合、鋼を溶製する際の凝固段階でＣａＳが晶出し、ＣａＳの晶出後に固溶Ｓ量が確保されて、ＣａＳの表面上にＭｎＳが析出する。 When the value of (Ca− (0.18 + 130 × Ca) × O) /1.25/S (sometimes referred to as a parameter value) exceeds 0 and is less than 1, the solidification stage when melting the steel Thus, CaS is crystallized, and after crystallization of CaS, the amount of solid solution S is secured, and MnS is deposited on the surface of CaS.
ＭｎＳはフェライト核生成能を有し、その周囲にＭｎの希薄帯を形成してフェライト変態を促進し、溶接熱影響部靭性を向上させる。ＭｎＳ上にＴｉＮ、ＢＮ、ＡｌＮ等のフェライト生成核が析出することによって、より一層、フェライト変態が促進される。 MnS has a ferrite nucleation ability, forms a Mn dilute zone around it, promotes ferrite transformation, and improves weld heat affected zone toughness. Ferrite transformation is further promoted by precipitation of ferrite-forming nuclei such as TiN, BN, and AlN on MnS.
パラメータの値が、０以下の場合には、ＣａＳが晶出せず、ＳはＭｎＳ単独の形態で析出し、溶接熱影響部において複合硫化物を微細分散させることができない。 When the parameter value is 0 or less, CaS does not crystallize, S precipitates in the form of MnS alone, and the composite sulfide cannot be finely dispersed in the weld heat affected zone.
一方、パラメータの値が１以上の場合には、ＳがＣａによって完全に固定され、フェライト生成核として作用するＭｎＳが、ＣａＳ上に析出しないため、溶接熱影響部において複合硫化物を微細分散させることができない。 On the other hand, when the parameter value is 1 or more, since S is completely fixed by Ca and MnS acting as a ferrite nucleation does not precipitate on CaS, the composite sulfide is finely dispersed in the weld heat affected zone. I can't.
なお、Ｏは不可避的不純物として鋼中に含有され、清浄度を低下させる。このため本発明ではできるだけ低減することが望ましい。特に、Ｏ含有量が０．００３０％を超えるとＣａＯ系介在物が粗大化して母材靭性を低下させてしまうため、好ましくは０．００３０％以下とする。 In addition, O is contained in steel as an unavoidable impurity and reduces cleanliness. For this reason, it is desirable to reduce as much as possible in the present invention. In particular, when the O content exceeds 0.0030%, CaO-based inclusions become coarse and lower the base material toughness, so the content is preferably 0.0030% or less.
また、本発明では、ＣａをＣａＳとして晶出させるために、Ｃａと結合力の強いＯ量をＣａ添加前に低減させておくことが必要であり、Ｃａ添加前の残存酸素量は、０．００３０％以下であることが好ましい。残存酸素量の低減方法としては、脱ガスを強化する、あるいは、脱酸剤を投入する、などの方法をとることができる。 Further, in the present invention, in order to crystallize Ca as CaS, it is necessary to reduce the amount of O having a strong binding force with Ca before addition of Ca. It is preferable that it is 0030% or less. As a method for reducing the amount of residual oxygen, a method such as enhancing degassing or introducing a deoxidizer can be employed.
以上が本発明の基本成分組成（残部はＦｅ及び不可避的不純物とする）であるが、更に特性を向上させるため、Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ、Ｖの一種または二種以上を含有することが可能である。 The above is the basic component composition of the present invention (the balance is Fe and inevitable impurities), but in order to further improve the characteristics, it may contain one or more of Cu, Ni, Cr, Mo, V. Is possible.
Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ、Ｖ
Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ、Ｖはいずれも鋼の焼入れ性を高める元素で、圧延後の強度向上に直接寄与するとともに、所望する、靭性、高温強度、あるいは耐候性などの機能向上のために一種または二種以上を添加することができる。これらの効果は、それぞれ、０．０１％以上を含有することにより発揮される。
Cu, Ni, Cr, Mo, V
Cu, Ni, Cr, Mo, and V are all elements that enhance the hardenability of the steel, and contribute directly to improving the strength after rolling, and for improving the desired functions such as toughness, high-temperature strength, and weather resistance. One kind or two or more kinds can be added. Each of these effects is exhibited by containing 0.01% or more.
Ｃｕは０．５％を超えて添加すると、靭性や溶接性が劣化するようになるので、Ｃｕを添加する場合には、上限を０．５％とすることが好ましい。 If Cu is added in excess of 0.5%, the toughness and weldability will deteriorate, so when Cu is added, the upper limit is preferably made 0.5%.
Ｎｉは１．０％を超えて添加すると靭性や溶接性を劣化するようになるので、Ｎｉを添加する場合には、上限を１．０％とすることが好ましい。また、Ｃｒ、Ｍｏは０．５％を超えて添加すると、靭性や溶接性を劣化するようになるので、Ｃｒ、Ｍｏを添加する場合には、それぞれ上限を０．５％とすることが好ましい。 When Ni is added in excess of 1.0%, toughness and weldability are deteriorated. Therefore, when Ni is added, the upper limit is preferably set to 1.0%. Further, if Cr and Mo are added in excess of 0.5%, the toughness and weldability are deteriorated. Therefore, when adding Cr and Mo, the upper limit is preferably set to 0.5%. .
Ｖは、Ｖ（ＣＮ）として析出強化することによって、鋼の強度を向上する元素であり、その効果は０．００１％以上含有することによって発揮される。一方、０．１０％を超えて含有すると、靭性を低下させる。このため、Ｖを添加する場合は、０．００１〜０．１０％の範囲で添加することが好ましい。 V is an element which improves the strength of steel by precipitation strengthening as V (CN), and the effect is exhibited by containing 0.001% or more. On the other hand, when it contains exceeding 0.10%, toughness will be reduced. For this reason, when adding V, it is preferable to add in 0.001 to 0.10% of range.
２．板内部の集合組織
本発明では、圧延方向または圧延直角方向など水平方向に進展するき裂に対してき裂伝播停止特性を向上させるため、圧延面すなわち鋼板表面に平行に（２１１）面を発達させ、鋼板表面に平行な面における（２１１）面Ｘ線強度比が１．０以上となる集合組織を有する領域が、板厚中心部を含め板厚の１／３以上の領域において存在することを規定する。
2. In the present invention, the (211) plane is developed in parallel with the rolling surface, that is, the surface of the steel sheet in order to improve the crack propagation stop property with respect to a crack that propagates in the horizontal direction such as the rolling direction or the direction perpendicular to the rolling direction. A region having a texture where the (211) plane X-ray intensity ratio in a plane parallel to the steel plate surface is 1.0 or more exists in a region of 1/3 or more of the plate thickness including the center portion of the plate thickness. Stipulate.
板厚中央部で圧延面に平行に（２１１）面を発達させると、き裂進展に先立ち微視的なクラックが発生し、き裂進展の抵抗となる。き裂進展に先立つ微視的なクラックを発生させるため、板厚中心部を含め板厚全厚の１／３以上の領域において、鋼板表面に平行な面における（２１１）面Ｘ線強度比が１．０以上となる集合組織を有するものとする。上述の、亀裂進展に先立ち微視的クラックが発生して亀裂進展の抵抗となる、という作用効果は当該集合組織を有する領域が板厚中心部を含め板厚全厚の１／３以上の領域であれば得られるので、上限は特に規定しない。当該集合組織を有する領域が多くなれば、上記作用効果は更に発揮されるものであるが、その領域を板厚全厚の３／４を超えて多くしても、上記作用効果の増加は飽和してくるため、当該集合組織を有する領域を板厚全厚の３／４を超えて多くする必要はない。ただし、板厚全厚が当該集合組織であっても上記作用効果は発揮されることは言うまでもない。 When the (211) plane is developed parallel to the rolling surface at the center of the plate thickness, a microscopic crack is generated prior to the crack propagation, which becomes resistance to crack propagation. In order to generate microscopic cracks prior to crack growth, the (211) plane X-ray intensity ratio in the plane parallel to the steel sheet surface is greater than 1/3 of the total thickness including the thickness center. It shall have a texture of 1.0 or more. The above-described effect that microscopic cracks are generated prior to crack growth and resistance to crack growth is that the region having the texture is a region that is 1/3 or more of the total thickness including the thickness center Therefore, the upper limit is not specified. If the area having the texture increases, the above-described effect can be further exhibited. However, even if the area is increased beyond 3/4 of the total thickness, the increase in the effect is saturated. Therefore, it is not necessary to increase the region having the texture beyond 3/4 of the total thickness. However, it goes without saying that the above-described effects are exhibited even when the total thickness of the plate is the texture.
ここで、（２１１）面Ｘ線強度比とは対象材の（２１１）結晶面の集積度を表す数値で、対象材の（２１１）反射のＸ線回折強度（Ｉ（２１１））と、集合組織のないランダムな標準試料の（２１１）反射のＸ線回折強度（Ｉ０（２１１））の比（Ｉ（２１１）／Ｉ０（２１１））を指す。 Here, the (211) plane X-ray intensity ratio is a numerical value representing the degree of integration of the (211) crystal plane of the target material, the (211) reflection X-ray diffraction intensity (I (211) ) of the target material, and the set It refers to the ratio (I (211) / I 0 (211) ) of X-ray diffraction intensity (I 0 (211) ) of (211) reflection of a random standard sample without tissue.
３．板厚中心部の組織
圧延面すなわち鋼板表面に平行な面における（２１１）面は、圧延時に加工されたオーステナイト組織がフェライトやベイナイト組織に変態することにより発達する。しかしながら、フェライト−セメンタイトの組織の場合は、回復などの影響により、板厚の広い範囲においてこの集合組織が（２１１）面が発達しない。変態後の組織をベイナイト組織とすることにより広範囲において最も高い（２１１）面Ｘ線強度比を保つことが可能となる。そこで鋼板の集合組織を発達させるため、板厚中心部のベイナイト分率を８０％以上と規定とする。本発明において板厚中央部のミクロ組織とは、板厚中心部を含む少なくとも板厚の１／３部分の領域のミクロ組織を意味する。本発明では板厚方向の全断面が当該ミクロ組織となることを妨げない。
3. Structure at the center of the plate thickness The (211) plane in the plane parallel to the rolled surface, that is, the surface of the steel plate, is developed by transformation of the austenite structure processed during rolling into a ferrite or bainite structure. However, in the case of a ferrite-cementite structure, the (211) plane of this texture does not develop over a wide range of plate thickness due to the effect of recovery or the like. By making the structure after the transformation a bainite structure, it is possible to maintain the highest (211) plane X-ray intensity ratio in a wide range. Therefore, in order to develop the texture of the steel sheet, the bainite fraction at the center of the sheet thickness is defined as 80% or more. In the present invention, the microstructure in the center portion of the plate thickness means a microstructure in the region of at least 1/3 portion of the plate thickness including the center portion of the plate thickness. In this invention, it does not prevent that the whole cross section of a plate | board thickness direction becomes the said microstructure.
４．母材靭性
母材靭性が、良好な特性を有することが脆性き裂の進展を抑制する前提となるので、本発明に係る鋼板では鋼板の材質を代表する位置として板厚の１／４位置から採取したシャルピー試験片によるシャルピー衝撃試験におけるシャルピー破面遷移温度を規定する。
4). Base material toughness Since the base material toughness has the premise of suppressing the progress of brittle cracks since it has good characteristics, in the steel sheet according to the present invention, the position representative of the material of the steel sheet is from 1/4 position of the plate thickness. Specifies the Charpy fracture surface transition temperature in the Charpy impact test using the collected Charpy specimens.
板厚５０ｍｍ以上の厚肉材で、構造安全性を確保する上で目標とされるＫｃａ（−１０℃）≧７０００Ｎ/ｍｍ３／２の脆性き裂伝播停止性能を得るため、板厚の１／４位置から採取した試験片によるシャルピー衝撃試験におけるシャルピー破面遷移温度を−４０℃以下と規定する。
以下、本発明における好ましい製造条件について説明する。
In order to obtain a brittle crack propagation stopping performance of Kca (−10 ° C.) ≧ 7000 N / mm 3/2 which is a target for ensuring structural safety with a thick material having a thickness of 50 mm or more, The Charpy fracture surface transition temperature in a Charpy impact test using a test piece taken from the / 4 position is defined as −40 ° C. or lower.
Hereinafter, preferable production conditions in the present invention will be described.
５．製造条件
製造条件はスラブ加熱条件、熱間圧延条件および熱間圧延後の冷却条件を規定する。
5). Manufacturing conditions Manufacturing conditions define slab heating conditions, hot rolling conditions, and cooling conditions after hot rolling.
まず、上記化学成分の溶鋼を、転炉等で溶製後、連続鋳造等で鋼素材（スラブ）とし、
ついで、１０００〜１２００℃の温度に加熱して熱間圧延を行う。
First, molten steel of the above chemical composition is melted in a converter or the like, and then made into a steel material (slab) by continuous casting, etc.
Subsequently, hot rolling is performed by heating to a temperature of 1000 to 1200 ° C.
スラブ加熱温度が１０００℃未満では、オーステナイト再結晶温度域における圧延を行う時間が十分に確保できず、また、加熱温度が１２００℃超えではオーステナイト粒が粗大化し、靭性の低下を招くばかりか、酸化ロスが顕著となり、歩留が低下するので、スラブ加熱温度は１０００〜１２００℃とする。 If the slab heating temperature is less than 1000 ° C., sufficient time for rolling in the austenite recrystallization temperature region cannot be secured, and if the heating temperature exceeds 1200 ° C., the austenite grains become coarse, leading to a decrease in toughness, and oxidation. Since the loss becomes remarkable and the yield decreases, the slab heating temperature is set to 1000 to 1200 ° C.
靭性の観点から好ましいスラブ加熱温度の範囲は１０５０〜１１５０℃であり、より好ましくは１０５０〜１１００℃である。 The range of the preferable slab heating temperature from a viewpoint of toughness is 1050-1150 degreeC, More preferably, it is 1050-1100 degreeC.
熱間圧延は、板厚中央部の温度が（Ａｒ３点＋４０）℃以下、Ａｒ３点以上の温度域において累積圧下率４０％以上、且つ、１パス当たりの圧下率の平均値が６．０％以上で、各パスの圧下率範囲が５．０〜２０．０％となる圧延を含めて所望の板厚が得られるように行う。後述の冷却条件との組み合わせにより、圧延面すなわち鋼板表面に平行に（２１１）面を発達させ、鋼板表面に平行な面における（２１１）面Ｘ線強度比が１．０以上となる集合組織を有する領域を、板厚中心部を含め板厚の１／３以上の領域において得ることができる。この温度域における累積圧下率が４０％未満では、鋼板の靭性が劣化する。また、（２１１）面Ｘ線強度比を１．０以上とするため、未再結晶オーステナイト域である（（Ａｒ３点＋４０）℃以下、Ａｒ３点以上の温度域において累積圧下率４０％以上とする。この温度域における累積圧下率は、５０％以上であることがさらに好ましい。１パスあたり圧下率の平均値が６.０％未満である場合、あるいは、各パス圧下率の最小値が５．０％未満である場合には、靭性が低下し、かつ（２１１）面Ｘ線強度比が１．０以上となる領域を板厚中心を含み板厚全厚の１／３以上の領域とすることができない。いっぽう、各パス圧下率の最大値が２０．０％を超えると、加工歪の影響で、かえって靭性が劣化する。この温度域における１パス当りの圧下率の平均値は６．５％以上であることがさらに好ましく、また、各パスの圧下率範囲は５．５〜１８．０％であることがさらに好ましい。 In the hot rolling, the temperature at the center of the sheet thickness is (Ar 3 points + 40) ° C. or less, the cumulative reduction rate is 40% or more in the temperature range of Ar 3 points or more, and the average value of the reduction rate per pass is 6. It is performed so as to obtain a desired plate thickness including rolling in which the rolling reduction range of each pass is 5.0 to 20.0% at 0% or more. By combining with the below-mentioned cooling conditions, a (211) plane is developed in parallel to the rolling surface, that is, the steel plate surface, and a texture where the (211) plane X-ray intensity ratio in the plane parallel to the steel plate surface is 1.0 or more The area | region which has can be obtained in the area | region more than 1/3 of board thickness including a board thickness center part. If the cumulative rolling reduction in this temperature range is less than 40%, the toughness of the steel sheet deteriorates. Further, in order to set the (211) plane X-ray intensity ratio to 1.0 or more, the cumulative reduction ratio is 40% or more in the non-recrystallized austenite region ((Ar 3 points + 40) ° C. or lower, Ar 3 point or higher temperature range). The cumulative rolling reduction in this temperature range is more preferably 50% or more, when the average value of rolling reduction per pass is less than 6.0%, or the minimum value of each pass rolling reduction is When it is less than 5.0%, the toughness is reduced, and the region where the (211) plane X-ray intensity ratio is 1.0 or more includes the center of the plate thickness and the region is 1/3 or more of the total thickness. On the other hand, if the maximum value of each pass rolling reduction exceeds 20.0%, the toughness deteriorates due to the effect of processing strain.The average value of the rolling reduction per pass in this temperature range is 6.5% or more is more preferable, and each pass It is further preferred reduction ratio range is 5.5 to 18.0%.
本発明ではＡｒ３点（℃）を下式で求める。
Ａｒ３点＝９１０−２７３Ｃ−７４Ｍｎ−５７Ｎｉ−１６Ｃｒ−９Ｍｏ−５Ｃｕ
式において各元素記号は鋼中含有量（質量％）で、含有しない場合は０とする。
In the present invention, the Ar 3 point (° C.) is obtained by the following equation.
Ar 3 points = 910-273C-74Mn-57Ni-16Cr-9Mo-5Cu
In the formula, each element symbol is the content (% by mass) in steel, and 0 if not contained.
なお、本発明は累積圧下率４０％以上の圧延を規定した温度域外での圧延を制限するものではなく、通常、スラブ加熱後の高温で実施する粗圧延を行うことが可能である。たとえば、上記、板厚中央部の温度が（Ａｒ３点＋４０）℃以下、Ａｒ３点以上の温度域における圧延の前に、オーステナイト再結晶温度域である板厚中央部が（Ａｒ３点＋１００）℃以上の温度域で累積圧下率が３０％以上、より好ましくは３５％以上の熱間圧延を実施すると、この温度域におけるオーステナイト組織が細粒化することにより最終のミクロ組織も細粒化し、母材靭性が向上するので好ましい。 In addition, this invention does not restrict | limit rolling outside the temperature range which prescribed | regulated rolling with a cumulative reduction ratio of 40% or more, and can perform rough rolling normally implemented at the high temperature after slab heating. For example, before rolling in the temperature range where the temperature at the central portion of the plate thickness is (Ar 3 points + 40) ° C. or lower and at the Ar 3 point or higher, the central portion of the plate thickness which is the austenite recrystallization temperature range is (Ar 3 points + 100 ) When hot rolling with a cumulative rolling reduction of 30% or more, more preferably 35% or more in a temperature range of ℃ or higher, the austenite structure in this temperature range becomes finer, and the final microstructure becomes finer. The base material toughness is improved, which is preferable.
圧延終了温度はＡｒ３点以上であることが好ましい。 The rolling end temperature is preferably Ar 3 points or more.
圧延が終了した鋼板は３．０℃／ｓ以上の冷却速度にて４００℃以下まで冷却する。冷却速度が３．０℃／未満では、ベイナイトへの変態が十分に進行しないため、板厚中心部を含め板厚全厚の１／３以上の領域において、鋼板表面に平行な面における（２１１）面Ｘ線強度比が１．０以上となる集合組織を得ることができず、さらに所望のミクロ組織、すなわち、板厚の中央部におけるベイナイト分率が８０％以上の組織も得られない。また、冷却停止温度が４００℃超えでは、ベイナイトへの変態が十分に進行しないため、やはり、所望のミクロ組織が得られない。 The rolled steel sheet is cooled to 400 ° C. or lower at a cooling rate of 3.0 ° C./s or higher. If the cooling rate is less than 3.0 ° C., the transformation to bainite does not proceed sufficiently, and therefore in a region parallel to the steel plate surface in the region of 1/3 or more of the total thickness including the thickness center (211 ) A texture having a plane X-ray intensity ratio of 1.0 or more cannot be obtained, and a desired microstructure, that is, a structure having a bainite fraction of 80% or more in the central portion of the plate thickness cannot be obtained. Further, when the cooling stop temperature exceeds 400 ° C., the transformation to bainite does not proceed sufficiently, so that a desired microstructure cannot be obtained.
上述の製造条件により製造された鋼板では、シャルピー試験における破面単位が微細化されるので、板厚１／４位置におけるシャルピー破面遷移温度−４０℃以下が達成される。 In the steel sheet manufactured under the above-described manufacturing conditions, the fracture surface unit in the Charpy test is refined, so that a Charpy fracture surface transition temperature of −40 ° C. or less at the ¼ thickness position is achieved.
以上の説明において、板厚中央部の温度は、放射温度計で測定した鋼板表面温度から、伝熱計算により求める。圧延後の冷却条件における温度条件も板厚中央部温度とする。 In the above description, the temperature at the center of the plate thickness is obtained by heat transfer calculation from the surface temperature of the steel plate measured with a radiation thermometer. The temperature condition in the cooling condition after rolling is also the sheet thickness center temperature.
種々の組成の溶鋼を、転炉で溶製し、連続鋳造法で鋼素材（スラブ：２８０ｍｍ厚）とした後、熱間圧延し、その後、冷却を行い、板厚５０〜７０ｍｍの厚鋼板を製造した。表１に供試鋼の化学成分を、表２に製造条件を示す。Ａｒ３点（℃）は、次式により計算した。
Ａｒ３点＝９１０−２７３Ｃ−７４Ｍｎ−５７Ｎｉ−１６Ｃｒ−９Ｍｏ−５Ｃｕ
ただし、各元素記号は鋼中含有量（質量％）で、含有しない場合は０とする。
Molten steels of various compositions are melted in a converter, made into a steel material (slab: 280 mm thick) by a continuous casting method, hot rolled, then cooled, and a thick steel plate having a thickness of 50 to 70 mm is obtained. Manufactured. Table 1 shows the chemical composition of the test steel, and Table 2 shows the production conditions. Ar 3 points (° C.) were calculated by the following equation.
Ar 3 points = 910-273C-74Mn-57Ni-16Cr-9Mo-5Cu
However, each element symbol is a steel content (mass%), and is 0 when not contained.
得られた厚鋼板について、板厚１／４部よりΦ１４のＪＩＳ１４Ａ号試験片を採取し、引張試験を行い、降伏強度（ＹＳ）、引張強さ（ＴＳ）を測定した。また、板厚１／４部よりＪＩＳ４号衝撃試験片を試験片の長手軸の方向が圧延方向と平行となるように採取して、シャルピー衝撃試験を行い、破面遷移温度（ｖＴｒｓ）を求めた。板厚１／４部におけるシャルピー破面遷移温度が−４０℃以下のものを本発明範囲内とした。 About the obtained thick steel plate, the JIS14A test piece of (PHI) 14 was extract | collected from plate | board thickness 1/4 part, the tensile test was done, and the yield strength (YS) and the tensile strength (TS) were measured. In addition, a JIS No. 4 impact test specimen was sampled from a thickness of 1/4 part so that the longitudinal axis direction of the test specimen was parallel to the rolling direction, and a Charpy impact test was conducted to determine the fracture surface transition temperature (vTrs). It was. The Charpy fracture surface transition temperature at ¼ part of the plate thickness was within the range of the present invention within −40 ° C.
板厚の中央部におけるベイナイト分率は、板厚の中央部の圧延長手方向と平行な板厚断面を鏡面研磨したあと、エッチングにより現出させた金属組織について光学顕微鏡写真（倍率４００倍）を複数枚撮影し、画像解析により測定を行って平均値を求めた。 The bainite fraction in the central part of the plate thickness is optical micrograph (400 times magnification) of the metal structure revealed by etching after mirror-polishing the plate thickness section parallel to the rolling longitudinal direction of the central part of the plate thickness. A plurality of images were taken and measured by image analysis to obtain an average value.
また、鋼板の集合組織を評価するため、鋼板の表面から裏面にかけて、１ｍｍごとに圧延面での（２１１）面Ｘ線強度比を測定し、（２１１）面Ｘ線強度比が１．０以上となる領域を求めた。 Further, in order to evaluate the texture of the steel sheet, the (211) plane X-ray intensity ratio at the rolled surface is measured every 1 mm from the front surface to the back surface of the steel sheet, and the (211) plane X-ray intensity ratio is 1.0 or more. The area which becomes becomes.
次に、脆性亀裂伝播停止特性を評価するため、温度勾配型ＥＳＳＯ試験を行い、Ｋｃａ（−１０℃）を求めた。 Next, in order to evaluate the brittle crack propagation stop characteristic, a temperature gradient type ESSO test was performed to obtain Kca (−10 ° C.).
さらに、各鋼板から採取した継手用試験板に、Ｖ開先加工を施し、市販の低温用鋼用エレクトロガスアーク溶接用ワイヤを使用してエレクトロガスアーク溶接（溶接入熱３００〜５００ｋＪ／ｃｍ）により大入熱溶接継手を作製した。得られた溶接継手から切欠位置をボンドとするＪＩＳ４号衝撃試験片を板厚１／４部より採取し、試験温度―４０℃でシャルピー衝撃試験を実施し、吸収エネルギー（ｖＥ−４０）（３本平均値）を求めた。 Furthermore, V-groove processing is performed on the joint test plates collected from each steel plate, and large by electrogas arc welding (welding heat input 300 to 500 kJ / cm) using a commercially available wire for electrogas arc welding for low temperature steel. A heat input welded joint was prepared. From the obtained welded joint, a JIS No. 4 impact test piece having a notch position as a bond was taken from 1/4 part of the plate thickness, and subjected to a Charpy impact test at a test temperature of −40 ° C., and absorbed energy (vE- 40 ) (3 This average value) was determined.
表３に強度、母材靱性、Ｘ線強度比、Ｋｃａ（−１０℃）の結果と溶接継手部のシャルピー衝撃試験結果を併せて示す。 Table 3 shows the results of strength, base metal toughness, X-ray strength ratio, Kca (−10 ° C.) and Charpy impact test results of welded joints.
供試鋼板（Ｎｏ．１〜１５）は成分組成、製造条件が本発明範囲内でミクロ組織、集合組織および板厚１／４部におけるシャルピー破面遷移温度が本発明の規定を満足し、Ｋｃａ（−１０℃）が７０００Ｎ／ｍｍ３／２以上と優れた脆性き裂伝播停止性能と吸収エネルギー（ｖＥ−４０）（３本平均値）が９６Ｊ以上の優れた溶接部ボンド部靭性が得られた。 The test steel plates (Nos. 1 to 15) have the composition and production conditions within the scope of the present invention, and the microstructure, texture, and Charpy fracture surface transition temperature at ¼ part of the sheet thickness satisfy the provisions of the present invention, and Kca An excellent brittle crack propagation stopping performance (−10 ° C.) of 7000 N / mm 3/2 or more and an excellent weld joint bond toughness of absorbed energy (vE −40 ) (average of 3) of 96 J or more are obtained. It was.
供試鋼板（Ｎｏ．１６〜１８）は成分組成、製造条件のうち、成分組成が本発明範囲外でミクロ組織、集合組織および板厚１／４部におけるシャルピー破面遷移温度が本発明の規定を満足し、Ｋｃａ（−１０℃）が７０００Ｎ／ｍｍ３／２以上と優れた脆性き裂伝播停止性能を示したが、溶接部ボンド部靭性が吸収エネルギー（ｖＥ−４０）（３本平均値）で１４Ｊと低い。 The test steel plates (Nos. 16 to 18) have a component composition and production conditions out of the scope of the present invention, and the microstructure, texture, and Charpy fracture surface transition temperature at 1/4 part of the sheet thickness are specified in the present invention. The Kca (−10 ° C.) showed excellent brittle crack propagation stopping performance of 7000 N / mm 3/2 or more, but the weld joint toughness was absorbed energy (vE −40 ) (average of 3) ) Is as low as 14J.
供試鋼板（Ｎｏ．１９〜２３）は成分組成、製造条件のうち、成分組成が本発明範囲外でミクロ組織、集合組織は本発明の規定を満足するが、板厚１／４部におけるシャルピー破面遷移温度が本発明の規定を満足しないため、Ｋｃａ（−１０℃）が７０００Ｎ／ｍｍ３／２未満、溶接部ボンド部靭性が吸収エネルギー（ｖＥ−４０）（３本平均値）で３８Ｊ以下と低い。 The test steel plates (Nos. 19 to 23) are component compositions and production conditions, the component composition is outside the scope of the present invention, and the microstructure and texture satisfy the provisions of the present invention. Since the fracture surface transition temperature does not satisfy the provisions of the present invention, the Kca (−10 ° C.) is less than 7000 N / mm 3/2 , and the weld bond toughness is 38 J in terms of absorbed energy (vE −40 ) (average of three). Less than or below.
供試鋼板（Ｎｏ．２４〜３２）は化学成分範囲が本発明範囲内のため溶接ボンド部の靭性は良好であるが、製造条件が本発明範囲外のため、ミクロ組織、集合組織および板厚１／４部におけるシャルピー破面遷移温度のいずれかまたは複数が本発明の規定を満足せず、Ｋｃａ（−１０℃）が７０００Ｎ／ｍｍ３／２未満と低く本発明例に及ばなかった。 The test steel plates (Nos. 24-32) have good chemical composition range within the scope of the present invention, so that the weld bond has good toughness, but the manufacturing conditions are outside the scope of the present invention. One or more of the Charpy fracture surface transition temperatures at ¼ part did not satisfy the provisions of the present invention, and Kca (−10 ° C.) was as low as less than 7000 N / mm 3/2 and did not reach the examples of the present invention.
Claims (4)
板厚中心部を含め板厚全厚の１／３以上の領域において、鋼板表面に平行な面における（２１１）面Ｘ線強度比が１．０以上となる集合組織を有し、板厚の中央部におけるベイナイト分率が８０％以上であり、かつ板厚１／４部におけるシャルピー破面遷移温度が−４０℃以下であることを特徴とする大入熱溶接部の靭性および脆性き裂伝播停止特性に優れた高強度厚鋼板。
０＜（Ｃａ−（０．１８＋１３０×Ｃａ）×Ｏ）／１．２５／Ｓ＜１ （１）
Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｕ＋Ｎｉ）／１５＋（Ｖ＋Ｍｏ＋Ｃｒ）／５ （２）
ただし、各元素記号は各成分の含有量（質量％）を表し、含有しない場合は０とする。 The chemical composition of steel is% by mass, C: 0.03 to 0.20%, Si: 1.0% or less, Mn: 1.5 to 2.5%, Al: 0.02 to 0.06% , P: 0.015% or less, S: 0.0050% or less, Nb: 0.005 to 0.020%, Ti: 0.005 to 0.020%, N: 0.0035 to 0.0075%, Carbon: 0.0005 to 0.0030%, B: 0.0005 to 0.0020%, and Ca, O, and S satisfy the following formula (1) and are represented by the following formula (2) The equivalent amount (Ceq) is 0.36% or more and less than 0.45%, and the balance consists of Fe and inevitable impurities,
In the region of 1/3 or more of the total thickness including the thickness center, the (211) plane X-ray intensity ratio in the plane parallel to the steel plate surface has a texture of 1.0 or more, Toughness and brittle crack propagation in high heat input welds, characterized in that the bainite fraction at the center is 80% or more and the Charpy fracture surface transition temperature at ¼ part is -40 ° C or less. High strength thick steel plate with excellent stopping characteristics.
0 <(Ca− (0.18 + 130 × Ca) × O) /1.25/S <1 (1)
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (V + Mo + Cr) / 5 (2)
However, each element symbol represents the content (% by mass) of each component, and 0 when not contained.
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