JP5950045B2 - Steel sheet and manufacturing method thereof - Google Patents

Steel sheet and manufacturing method thereof Download PDF

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JP5950045B2
JP5950045B2 JP2015524556A JP2015524556A JP5950045B2 JP 5950045 B2 JP5950045 B2 JP 5950045B2 JP 2015524556 A JP2015524556 A JP 2015524556A JP 2015524556 A JP2015524556 A JP 2015524556A JP 5950045 B2 JP5950045 B2 JP 5950045B2
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steel
steel sheet
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steel plate
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克行 一宮
克行 一宮
茂樹 木津谷
茂樹 木津谷
長谷　和邦
和邦 長谷
遠藤　茂
茂 遠藤
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JFE Steel Corp
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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Description

本発明は、海洋構造物や船舶、圧力容器、ペンストックなど鉄鋼構造物に用いられる高張力の鋼板およびその製造方法に関し、特に、降伏応力（ＹＳ）が４６０ＭＰａ以上で、鋼板の強度・靭性に優れるだけでなく、多層溶接部の低温靭性（ＣＴＯＤ特性）および溶接施工時の熱処理（ＰＷＨＴ）後の強度と靭性（ＰＷＨＴ特性）にも優れる厚肉高張力の鋼板とその製造方法に関するものである。   TECHNICAL FIELD The present invention relates to a high-tensile steel plate used for steel structures such as marine structures, ships, pressure vessels, and penstocks, and a method for producing the same. In particular, the yield stress (YS) is 460 MPa or more, and the strength and toughness of the steel plate is increased. The present invention relates to a high-thickness and high-tensile steel sheet that is not only excellent, but also excellent in low-temperature toughness (CTOD characteristics) of multi-layer welds and strength and toughness (PWHT characteristics) after heat treatment during welding (PWHT characteristics), and a manufacturing method thereof. .

船舶や海洋構造物、圧力容器などに用いられる鋼板は、溶接接合されて、所望の形状の構造物として仕上げられる。そのため、これらの鋼板には、構造物の安全性の観点から、強度が高く、靭性が優れていることはもちろんのこと、溶接を施した際の溶接継手部（溶接金属や熱影響部）の靭性に優れていることも要求される。   Steel plates used for ships, offshore structures, pressure vessels, etc. are welded and finished as structures of a desired shape. Therefore, these steel sheets have high strength and excellent toughness from the viewpoint of the safety of the structure, as well as welded joints (welded metals and heat-affected zones) when welding is performed. It is also required to have excellent toughness.

従来、鋼板の靭性の評価基準としては、主にシャルピー衝撃試験による吸収エネルギーが用いられてきたが、近年では、より信頼性を高めるために、き裂開口変位試験（Ｃｒａｃｋ Ｔｉｐ Ｏｐｅｎｉｎｇ Ｄｉｓｐｌａｃｅｍｅｎｔ Ｔｅｓｔ、以下、ＣＴＯＤ試験と称する）が用いられることが多い。この試験は、靭性を評価する部位に疲労予き裂を施した試験片を３点曲げし、破壊直前のき裂の口開き量（塑性変形量）を測定して脆性破壊の発生抵抗を評価するものである。   Conventionally, absorbed energy by Charpy impact test has been mainly used as an evaluation standard for the toughness of steel sheets, but in recent years, in order to further improve the reliability, a crack opening displacement test (hereinafter referred to as a Crack Tip Opening Displacement Test, hereinafter). Often referred to as CTOD test). This test evaluates the resistance to brittle fracture by bending a specimen with fatigue cracks at the site where toughness is evaluated and measuring the amount of crack opening (plastic deformation) just before fracture. To do.

ＣＴＯＤ試験では疲労予き裂を用いるので、極めて微小な領域が靭性評価部となる。そのため、鋼板中に局所脆化域が存在すると、シャルピー衝撃試験で良好な靭性が得られたとしても、ＣＴＯＤ試験では低い値を示す場合がある。   Since the CTOD test uses a fatigue precrack, a very small region becomes the toughness evaluation part. Therefore, if a local embrittlement region exists in the steel sheet, even if good toughness is obtained in the Charpy impact test, the CTOD test may show a low value.

この局所脆化域は、板厚が厚い鋼板などにおいて、多層盛溶接によって複雑な熱履歴を受ける溶接熱影響部（以下、ＨＡＺとも称する）で発生しやすく、加えて、ボンド部（溶接金属と母材の境界）や、２相域にボンド部が再加熱される部分（１サイクル目の溶接で粗粒となり、後続の溶接パスによりフェライトとオーステナイトの２相域に加熱される領域、以下、２相域再加熱部と称する）が局所脆化域となりやすい。   This local embrittlement region is likely to occur in a welding heat-affected zone (hereinafter also referred to as HAZ) that is subjected to a complex thermal history by multi-layer welding in a thick steel plate or the like. The boundary of the base material) and the part where the bond part is reheated in the two-phase region (coarse grains in the first cycle welding, the region heated to the two-phase region of ferrite and austenite by the subsequent welding pass, (Referred to as a two-phase region reheating part) tends to be a local embrittlement region.

ここで、ボンド部は、溶接時に融点直下の高温にさらされるため、オーステナイト粒が粗大化し、引続く冷却によって靭性の低い上部ベイナイト組織に変態しやすいことから、マトリクス自体の靭性が低くなりやすい。さらに、ボンド部は、ウッドマンステッテン組織や島状マルテンサイト（以下、Ｍ−Ａとも称する）などの脆化組織が生成しやすく、この脆化組織が生成すると、鋼板の靭性はさらに低下しやすくなる。   Here, since the bond portion is exposed to a high temperature just below the melting point during welding, the austenite grains are coarsened and are easily transformed into an upper bainite structure having low toughness by subsequent cooling, so that the toughness of the matrix itself tends to be low. Further, the bond portion is liable to generate an embrittled structure such as a woodman-stetten structure or island martensite (hereinafter also referred to as MA), and when this embrittled structure is generated, the toughness of the steel sheet is likely to further decrease. Become.

ここに、溶接熱影響部の靭性を向上させるため、例えば鋼板中にＴｉＮを微細分散させて、オーステナイト粒の粗大化を抑制したりフェライト変態核として利用したりする技術が実用化されている。しかしながら、ボンド部においては、ＴｉＮが溶解する温度域にまで加熱されることがあり、溶接部の低温靭性要求が厳しいほど加熱温度が高いので、上述のＴｉＮを微細分散させた作用効果が発現しにくくなる。   Here, in order to improve the toughness of the weld heat-affected zone, for example, a technique of finely dispersing TiN in a steel sheet to suppress coarsening of austenite grains or use it as a ferrite transformation nucleus has been put into practical use. However, the bonding part may be heated to a temperature range where TiN dissolves, and the heating temperature is higher as the requirement for low temperature toughness of the welded part becomes more severe. Therefore, the effect of finely dispersing TiN described above appears. It becomes difficult.

これらの問題を解決するために、特許文献１や特許文献２には、希土類元素（ＲＥＭ）をＴｉと共に複合添加して、鋼板中に微細粒子を分散させることにより、オーステナイトの粒成長を抑制して、溶接部靭性を向上させる技術が開示されている。
併せて、特許文献１や特許文献２には、Ｔｉの酸化物を分散させる技術や、ＢＮのフェライト核生成能と酸化物分散を組み合わせる技術、さらにはＣａやＲＥＭを添加して硫化物の形態を制御することにより、靭性を高める技術などが提案されている。In order to solve these problems, Patent Document 1 and Patent Document 2 suppress the austenite grain growth by adding rare earth elements (REM) together with Ti and dispersing fine particles in the steel sheet. Thus, a technique for improving the toughness of the weld zone is disclosed.
In addition, Patent Document 1 and Patent Document 2 include a technique for dispersing an oxide of Ti, a technique for combining ferrite nucleation ability of BN and oxide dispersion, and further adding Ca and REM to form a sulfide. Techniques for increasing toughness by controlling the thickness have been proposed.

また、特許文献３には、Ｔｉ酸化物を鋼中に分散させて、ＨＡＺ靭性を向上させる技術ついて開示されている。   Patent Document 3 discloses a technique for improving the HAZ toughness by dispersing Ti oxide in steel.

さらに、２相域再加熱部について言えば、２相域再加熱によってオーステナイトに逆変態した領域に炭素が濃化し、冷却中に島状マルテンサイトを含む脆弱なベイナイト組織が生成されて、鋼の靭性は低下するが、この靭性低下を防ぐために、鋼板成分を低Ｃ、低Ｓｉ化し、島状マルテンサイトの生成を抑制して靭性を向上し、Ｃｕを添加することによって母材強度を確保する技術が開示されている（例えば、特許文献４および５）。   Further, regarding the two-phase region reheating part, carbon is concentrated in a region reversely transformed into austenite by the two-phase region reheating, and a brittle bainite structure including island martensite is generated during cooling. Although the toughness is reduced, in order to prevent this toughness reduction, the steel plate component is made low C and low Si, the formation of island martensite is suppressed to improve the toughness, and the base material strength is ensured by adding Cu. Techniques are disclosed (for example, Patent Documents 4 and 5).

ここで、特許文献４に記載の技術は、圧延後の冷却速度を０．１℃／ｓ以下とし、この過程でＣｕ粒子を析出させる方法を取っているが、製造安定性に課題がある。   Here, the technique described in Patent Document 4 uses a method in which the cooling rate after rolling is set to 0.1 ° C./s or less and Cu particles are precipitated in this process, but there is a problem in manufacturing stability.

また、特許文献５に記載の技術では、Ｎ／Ａｌ比を０．３〜３．０とすることでＡｌＮの粗大化や固溶Ｎの悪影響による靭性劣化を抑制しているが、固溶ＮはＴｉによる制御の方がより簡便である。
なお、ＹＳが４６０ＭＰａを超える厚肉材においては、溶接施工時に後熱処理（ＰＷＨＴ）を施される場合がある。このとき母材も同時に加熱されるため、ＰＷＨＴ処理を受けても母材特性を保持しなければならないが、従来、熱を受けた場合の強度低下を抑えるためには、その温度で析出物を形成する元素を添加することが一般的であった。Moreover, in the technique described in Patent Document 5, the N / Al ratio is set to 0.3 to 3.0 to suppress the deterioration of toughness due to the coarsening of AlN or the adverse effect of solid solution N. Is more easily controlled by Ti.
In addition, in the thick material whose YS exceeds 460 MPa, post-heat treatment (PWHT) may be performed at the time of welding. At this time, since the base material is also heated at the same time, it is necessary to maintain the base material characteristics even when subjected to the PWHT treatment. It has been common to add elements to form.

特公平０３−０５３３６７号公報Japanese Patent Publication No. 03-053367 特開昭６０−１８４６６３号公報JP 60-184663 A 特許第３６９７２０２号公報Japanese Patent No. 3697202 特許第３０４５８５６号公報Japanese Patent No. 3045856 特許第４４３２９０５号公報Japanese Patent No. 4432905

ここで、特許文献１および２に記載された技術は、比較的低強度で合金元素量の少ない鋼材が対象であるところ、より高強度で合金元素量の多い厚肉材の場合は、ＨＡＺ組織がフェライトを含まない組織となるために適用することができないという問題が有る。   Here, the techniques described in Patent Documents 1 and 2 are intended for steel materials having relatively low strength and a small amount of alloy elements. In the case of thick materials having higher strength and a large amount of alloy elements, the HAZ structure is used. However, since it becomes a structure | tissue which does not contain a ferrite, there exists a problem that it cannot apply.

また、特許文献３に記載された技術では、Ｔｉ酸化物を鋼中に安定して微細分散させることが困難であるという問題が有った。   Further, the technique described in Patent Document 3 has a problem that it is difficult to stably and finely disperse Ti oxide in steel.

さらに、Ｃｕ析出物による強度確保は、靭性の低下を招くことが多く、鋼板の低温靭性確保に課題があった。また、特許文献５に記載されるＣｕ析出強化を用いた鋼材では、ＰＷＨＴ処理の過程でＣｕ粒子が大きく成長し、強度が低下しやすいという問題があった。   Furthermore, securing strength with Cu precipitates often causes a decrease in toughness, and there is a problem in securing low temperature toughness of the steel sheet. Further, in the steel material using Cu precipitation strengthening described in Patent Document 5, there is a problem that Cu particles grow greatly in the process of PWHT treatment and the strength tends to decrease.

加えて、近年では、船舶や海洋構造物、圧力容器、ペンストックなど、鉄鋼構造物においてはその大型化に伴い、いっそうの高強度化が要望されている。
これら鉄鋼構造物に用いられる鋼材は、例えば、板厚が３５ｍｍ以上の厚肉材が多いので、降伏応力４６０ＭＰａ級やそれ以上の強度を確保するためには、添加する合金元素を多く含有する鋼成分系が有利となっている。In addition, in recent years, steel structures such as ships, offshore structures, pressure vessels, and penstocks have been required to have higher strength as their size has increased.
Steel materials used for these steel structures are, for example, many thick materials having a plate thickness of 35 mm or more. Therefore, in order to ensure a yield stress of 460 MPa class or higher, steel containing a large amount of alloying elements to be added. The component system is advantageous.

しかしながら、合金元素量の多い高強度鋼材を対象とするボンド部や２相域再加熱部の靭性向上については、十分検討されているとは言難かった。そして、ＰＷＨＴ後の鋼板特性の確保は、従来の単純な析出元素の添加では強度と靭性の維持が困難であった。   However, it has been difficult to say that sufficient consideration has been given to improving the toughness of the bond portion and the two-phase region reheated portion targeting high-strength steel materials having a large amount of alloying elements. And securing of the steel sheet characteristics after PWHT has been difficult to maintain strength and toughness with the conventional simple addition of precipitation elements.

本発明は、上記した問題を有利に解決するもので、海洋構造物や船舶、圧力容器、ペンストックなど鉄鋼構造物に用いて好適な、降伏応力（ＹＳ）が４６０ＭＰａ以上（本発明ではこのＹＳを満足することを高張力という）で、多層溶接部の溶接熱影響部の低温靭性（ＣＴＯＤ特性）と溶接施工時の熱処理後の強度および靭性（ＰＷＨＴ特性）に優れる厚肉高張力鋼とその製造方法を提供することを目的とする。   The present invention advantageously solves the above-mentioned problems, and has a yield stress (YS) of 460 MPa or more suitable for use in steel structures such as offshore structures, ships, pressure vessels, and penstocks. Is a high-strength steel with excellent strength and toughness (PWHT characteristics) after heat treatment during welding and its low temperature toughness (CTOD characteristics). An object is to provide a manufacturing method.

発明者らは、上記した問題を解決すべく鋭意研究を重ね、以下の知見を得た。
（ａ）ＣＴＯＤ特性は鋼板全厚の試験片で評価されるため、成分の濃化する中心偏析部が破壊の起点となる。
従って、溶接熱影響部のＣＴＯＤ特性を向上するためには、鋼板の中心偏析として濃化しやすい元素を適正量に制御し、中心偏析部の硬化を抑制することが効果的である。また、溶鋼が凝固する際に最終凝固部となるスラブの中心において、Ｃ、Ｍｎ、Ｐ、ＮｉおよびＮｂが他の元素に比べて濃化度が高いため、これらの元素の添加量を中心偏析部の硬さ指標により制御して、中心偏析部での硬さを抑制することが効果的である。Inventors repeated earnest research in order to solve an above-mentioned problem, and acquired the following knowledge.
(A) Since the CTOD characteristic is evaluated with a test piece having a full thickness of the steel sheet, the central segregation portion where the components are concentrated becomes the starting point of the fracture.
Therefore, in order to improve the CTOD characteristics of the weld heat affected zone, it is effective to control the elements that are easily concentrated as the center segregation of the steel sheet to an appropriate amount and to suppress the hardening of the center segregation zone. In addition, since C, Mn, P, Ni and Nb have a higher concentration than other elements at the center of the slab, which becomes the final solidified part when the molten steel solidifies, the amount of addition of these elements is centrally segregated. It is effective to control the hardness at the center segregation part by controlling by the hardness index of the part.

（ｂ）溶接熱影響部の靭性を向上させるためには、ＴｉＮを有効利用して、溶接ボンド部近傍でのオーステナイト粒の粗大化を抑制することが効果的である。特に、Ｔｉ／Ｎ比を適正量に制御すれば、鋼中にＴｉＮを均一微細に分散することができる。   (B) In order to improve the toughness of the weld heat affected zone, it is effective to effectively use TiN to suppress the austenite grain coarsening in the vicinity of the weld bond portion. In particular, if the Ti / N ratio is controlled to an appropriate amount, TiN can be uniformly and finely dispersed in the steel.

（ｃ）硫化物の形態制御を目的として添加しているＣａの化合物（ＣａＳ）の晶出を溶接熱影響部の靭性向上に利用することが効果的である。
ＣａＳは、酸化物に比べて低温で晶出するため、均一に微細分散することができる。そして、Ｃａの添加量および添加時の溶鋼中の溶存酸素量を適正範囲に制御することによって、ＣａＳ晶出後でも固溶Ｓが確保されるので、ＣａＳの表面上にＭｎＳが析出して複合硫化物を形成する。このＭｎＳの周囲には、Ｍｎの希薄帯が形成されるので、フェライト変態がより促進される。(C) It is effective to use the crystallization of the Ca compound (CaS) added for the purpose of controlling the form of the sulfide to improve the toughness of the weld heat affected zone.
Since CaS crystallizes at a lower temperature than the oxide, it can be uniformly finely dispersed. And by controlling the addition amount of Ca and the amount of dissolved oxygen in molten steel at the time of addition to an appropriate range, solid solution S is ensured even after crystallization of CaS, so that MnS is precipitated on the surface of CaS and combined. Forms sulfides. Since a thin Mn band is formed around the MnS, the ferrite transformation is further promoted.

（ｄ）析出物を形成するＮｂに加えて、ＴｉやＭｏを必須添加とすることで、ＰＷＨＴ（おおよそ５５０〜６５０℃、２〜４ｈの範囲で実施）による加熱でも粗大化しないＭｏ，Ｔｉ，Ｎｂの複合炭窒化物を厚鋼板の製造段階で微細析出させることができる。
従来、ＹＳ：４６０ＭＰａ超級の鋼板では、ＰＷＨＴ後に強度の低下が顕著であったが、開発鋼板では、微細なＭｏ，Ｔｉ，Ｎｂ複合析出物（炭化物、窒化物または炭窒化物）が安定して存在することで、析出強化を維持することができ、鋼板の強度低下を抑制することが可能であることが分かった。また、微細なＭｏ，Ｔｉ，Ｎｂ複合析出物の存在により、鋼板の靭性も併せて維持できることが分かった。(D) In addition to Nb that forms precipitates, Mo and Ti, which are not coarsened even by heating with PWHT (implemented in the range of about 550 to 650 ° C. for 2 to 4 hours), are essential additions of Ti and Mo. Nb composite carbonitride can be finely precipitated in the manufacturing stage of thick steel plates.
Conventionally, in steel sheets of YS: 460 MPa or higher, the strength was significantly reduced after PWHT, but in the developed steel sheets, fine Mo, Ti, Nb composite precipitates (carbides, nitrides or carbonitrides) are stable. It has been found that the presence of precipitation strengthening can be maintained and the strength reduction of the steel sheet can be suppressed. It was also found that the toughness of the steel sheet can be maintained due to the presence of fine Mo, Ti, Nb composite precipitates.

本発明は上記した知見に基づき完成したものであって、本発明の要旨構成は次のとおりである。
１．質量％で、
Ｃ：０．０２０〜０．０９０％
Ｓｉ：０．０１〜０．３５％
Ｍｎ：１．４０〜２．００％
Ｐ：０．００８％以下
Ｓ：０．００３５％以下
Ａｌ：０．０１０〜０．０６０％
Ｎｉ：０．４０〜２．００％
Ｍｏ：０．０５〜０．５０％
Ｎｂ：０．００５〜０．０４０％
Ｔｉ：０．００５〜０．０２５％
Ｎ：０．００２０〜０．００５０％
Ｃａ：０．０００５〜０．００５０％および
Ｏ：０．００３５％以下
を含有し、下記（１）式で規定されるＣｅｑが０．４２０〜０．５２０％の範囲であって、下記（２）式、（３）式および（４）式を満たすと共に、Ｂを０．０００３％未満に抑制し、残部がＦｅおよび不可避的不純物からなる鋼板成分と、
Ｔｉ、ＮｂおよびＭｏを、Ｔｉ量（〔Ｔｉ〕）、Ｎｂ量（〔Ｎｂ〕）およびＭｏ量（〔Ｍｏ〕）が、〔Ｎｂ〕/（〔Ｔｉ〕+〔Ｎｂ〕+〔Ｍｏ〕）≧０．３の関係を満足する範囲で含み、かつ、析出物粒径が２ｎｍ以上である析出物を測定対象にして算出した平均粒子径が２０ｎｍ以下の析出物を有し、さらに、ＹＳ≧４６０ＭＰａ、ＴＳ≧５７０ＭＰａおよびｖＥ-40℃ ≧２００Ｊであり、溶接部の靭性が、ｖＥ-40℃ ≧１５０Ｊ、ＣＴＯＤ値（δ−１０℃）の最小値が０．５ｍｍ以上、また、強度、靭性について、ＰＷＨＴ前後の変化が、ΔＴＳが５〜−１５ＭＰａ、ΔｖＴｒｓが１０〜−５℃の範囲を満足する鋼板。
記
Ｃｅｑ＝［Ｃ］＋［Ｍｎ］／６＋（［Ｃｕ］＋［Ｎｉ］）／１５＋（［Ｃｒ］＋［Ｍｏ］＋［Ｖ］）／ ５・・・（１）
１．５≦［Ｔｉ］／［Ｎ］≦４．０ ・・・（２）
０＜｛［Ｃａ］−（０．１８＋１３０×［Ｃａ］）×［Ｏ］｝／１．２５／［Ｓ］＜１．５ ・・・（３）
５．５［Ｃ］(4/3)＋１５［Ｐ］＋０．９０［Ｍｎ］＋０．１２［Ｎｉ］＋７．９［Ｎｂ］(1/2)＋０．５３［Ｍｏ］ ≦３．７０ ・・・（４）
但し、［Ｍ］は、鋼板中の元素Ｍの含有量（質量％）を表す。 The present invention has been completed based on the above-described knowledge, and the gist of the present invention is as follows.
1. % By mass
C: 0.020 to 0.090%
Si: 0.01 to 0.35%
Mn: 1.40 to 2.00%
P: 0.008% or less S: 0.0035% or less Al: 0.010 to 0.060%
Ni: 0.40 to 2.00%
Mo: 0.05 to 0.50%
Nb: 0.005 to 0.040%
Ti: 0.005-0.025%
N: 0.0020 to 0.0050%
Ca: 0.0005 to 0.0050% and O: 0.0035% or less, and Ceq defined by the following formula (1) is in the range of 0.420 to 0.520%, and the following (2 ), (3) and (4) are satisfied, B is suppressed to less than 0.0003%, and the balance is a steel plate component composed of Fe and inevitable impurities,
Ti, Nb, and Mo are changed into Ti amount ([Ti]), Nb amount ([Nb]), and Mo amount ([Mo]), [Nb] / ([Ti] + [Nb] + [Mo]) ≧ 0.3 in the range that satisfies the relationship of 0.3 , and having a precipitate having a particle size of 2 nm or more and having an average particle size of 20 nm or less calculated as a measurement target; 460 MPa, TS ≧ 570 MPa and vE −40 ° C. ≧ 200 J, weld toughness is vE −40 ° C. ≧ 150 J, minimum value of CTOD value (δ−10 ° C.) is 0.5 mm or more, strength, toughness In the steel sheet, the change before and after PWHT satisfies the ranges of ΔTS of 5 to −15 MPa and ΔvTrs of 10 to −5 ° C.
Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 (1)
1.5 ≦ [Ti] / [N] ≦ 4.0 (2)
0 <{[Ca] − (0.18 + 130 × [Ca]) × [O]} / 1.25 / [S] <1.5 (3)
5.5 [C] (4/3) +15 [P] +0.90 [Mn] +0.12 [Ni] +7.9 [Nb] (1/2) +0.53 [Mo] ≦ 3.70・ (4)
However, [M] represents content (mass%) of the element M in a steel plate.

２．前記鋼板成分に、さらに質量％で、Ｃｕ：０．７％未満、Ｃｒ：０．１〜１．０％およびＶ：０．００５〜０．０５％のうちから選ばれる１種または２種以上を含有する前記１に記載の鋼板。 2. One or more selected from the steel sheet components, further, by mass, Cu: less than 0.7%, Cr: 0.1-1.0%, and V: 0.005-0.05% 2. The steel plate according to 1 above, comprising

３．前記鋼板成分に、さらに質量％で、Ｍｇ：０．０００２〜０．００５０％およびＲＥＭ：０．００１０〜０．０２００％のうちから選ばれる１種または２種を含有する前記１または２に記載の鋼板。 3. 3. Said 1 or 2 which contains 1 type or 2 types chosen from Mg: 0.0002-0.0050% and REM: 0.0010-0.0200% in the said steel plate component further by the mass%. Steel plate.

４．前記１〜３のいずれかに記載の鋼板を製造する方法であって、前記１〜３のいずれかに記載の鋼板成分を有する鋼に、９５０〜１１５０℃に加熱後、９００℃以上の温度域における累積圧下率が３０％以上、９００℃未満の温度域における累積圧下率が３０〜７０％となる熱間圧延を施し、その後、少なくとも５００℃までを冷却速度１．０℃／ｓ以上で冷却する鋼板の製造方法。 4). A method for producing the steel plate according to any one of 1 to 3 , wherein the steel having the steel plate component according to any one of 1 to 3 above is heated to 950 to 1150 ° C, and then a temperature range of 900 ° C or higher. Is subjected to hot rolling in which the cumulative reduction ratio is 30% or more and the cumulative reduction ratio in the temperature range below 900 ° C. is 30 to 70%, and then cooled to at least 500 ° C. at a cooling rate of 1.0 ° C./s or more. Steel sheet manufacturing method.

５．前記冷却後、さらに４５０〜６５０℃で焼戻し処理を施す前記４に記載の鋼板の製造方法。 5. 5. The method for producing a steel sheet according to 4, wherein the steel sheet is further tempered at 450 to 650 ° C. after the cooling.

本発明によれば、海洋構造物など大型の鉄鋼構造物に用いて好適な降伏応力（ＹＳ）が４６０ＭＰａ以上であって、多層溶接部のＣＴＯＤ特性およびＰＷＨＴ特性に優れる厚肉高張力鋼板とその製造方法が得られるので、産業上極めて有用である。   According to the present invention, a thick high-strength steel sheet having a yield stress (YS) suitable for use in large steel structures such as offshore structures of 460 MPa or more and excellent in CTOD characteristics and PWHT characteristics of multi-layer welds, and its Since a manufacturing method is obtained, it is very useful industrially.

ＰＷＨＴ熱処理における強度、靭性変化と析出物・サイズ組成の関係を示す図である。It is a figure which shows the relationship between the intensity | strength and toughness change in PWHT heat processing, and a precipitate and size composition. 鋼板中の析出物のＴＥＭレプリカ観察とＥＤＸ分析結果を示す図である。It is a figure which shows the TEM replica observation of the precipitate in a steel plate, and an EDX analysis result.

以下、本発明を具体的に説明する。
まず、本発明において、鋼板（以下、厚肉材ともいう）の成分組成（鋼成分）を上記の範囲に限定した理由について、成分毎に詳しく説明する。なお、以下に述べる鋼板の成分組成を示す％表示は特に断らない限り質量％を意味する。
Ｃ：０．０２０〜０．０９０％
Ｃは、高張力鋼板としての強度確保に必要な元素である。０．０２０％未満の添加では、焼入性が低下し、強度確保のために、Ｃｕ、Ｎｉ、ＣｒおよびＭｏなどの焼入性向上元素の多量添加が必要となって、コスト高を招く。一方、０．０９０％を超える添加は溶接部靭性を低下させる。従って、Ｃ量は０．０２０〜０．０９０％の範囲とする。好ましくは、０．０２０〜０．０８０％の範囲である。Hereinafter, the present invention will be specifically described.
First, in the present invention, the reason why the component composition (steel component) of a steel plate (hereinafter also referred to as a thick material) is limited to the above range will be described in detail for each component. In addition, unless otherwise indicated, the% display which shows the component composition of the steel plate described below means the mass%.
C: 0.020 to 0.090%
C is an element necessary for securing strength as a high-tensile steel plate. If the addition is less than 0.020%, the hardenability decreases, and a large amount of hardenability improving elements such as Cu, Ni, Cr, and Mo are required to ensure strength, resulting in high costs. On the other hand, addition exceeding 0.090% lowers the weld zone toughness. Therefore, the C content is in the range of 0.020 to 0.090%. Preferably, it is 0.020 to 0.080% of range.

Ｓｉ：０．０１〜０．３５％
Ｓｉは、脱酸元素として、また、鋼板強度を得るために添加する成分であり、これらの効果を得るためには０．０１％以上の添加が必要である。一方、０．３５％を超える多量の添加は、溶接性の低下と溶接継手靭性の低下を招く。従って、Ｓｉ量は０．０１〜０．３５％の範囲とする必要がある。好ましくは、０．０１〜０．２３％である。Si: 0.01 to 0.35%
Si is a component added as a deoxidizing element and for obtaining steel plate strength. To obtain these effects, addition of 0.01% or more is necessary. On the other hand, a large amount of addition exceeding 0.35% causes a decrease in weldability and a decrease in weld joint toughness. Therefore, the amount of Si needs to be in the range of 0.01 to 0.35%. Preferably, it is 0.01 to 0.23%.

Ｍｎ：１．４０〜２．００％
Ｍｎは、鋼板強度および溶接継手強度を確保するため、１．４０％以上添加する必要がある。一方、２．００％を超える添加は、溶接性を低下させ、焼入性が過剰となって、鋼板靭性および溶接継手靭性を低下させる。従って、Ｍｎ量は１．４０〜２．００％の範囲とする。さらに好ましくは、１．４０〜１．９５％である。Mn: 1.40 to 2.00%
Mn needs to be added in an amount of 1.40% or more in order to ensure the strength of the steel plate and the welded joint. On the other hand, addition exceeding 2.00% reduces weldability, the hardenability becomes excessive, and the steel plate toughness and weld joint toughness are reduced. Therefore, the amount of Mn is set to 1.40 to 2.00%. More preferably, it is 1.40 to 1.95%.

Ｐ：０．００８％以下
不純物元素であるＰは、鋼板靭性および溶接部靭性を低下させ、特に溶接部における含有量が０．００８％を超えるとＣＴＯＤ特性が著しく低下するので、０．００８％以下とする。好ましくは、０．００６％以下である。なお、Ｐの含有量は、極力少ないほうが良いが、精錬コスト等の点から、その下限値は、０．００２％程度である。P: 0.008% or less P, which is an impurity element, lowers the steel sheet toughness and weld zone toughness, and in particular, when the content in the weld zone exceeds 0.008%, the CTOD characteristics are significantly lowered. The following. Preferably, it is 0.006% or less. Although the P content is preferably as small as possible, the lower limit is about 0.002% from the viewpoint of refining costs and the like.

Ｓ：０．００３５％以下
Ｓは、不純物元素であり、０．００３５％を超えて含有すると鋼板および溶接部靭性を低下させるため、０．００３５％以下とする。好ましくは、０．００３０％以下である。なお、Ｓの含有量は、極力少ないほうが良いが、精錬コスト等の点から、その下限値は、０．０００４％程度である。S: 0.0035% or less S is an impurity element, and if contained in excess of 0.0035%, the toughness of the steel sheet and welded portion is lowered, so the content is made 0.0035% or less. Preferably, it is 0.0030% or less. The S content is preferably as small as possible, but the lower limit is about 0.0004% from the viewpoint of refining costs and the like.

Ａｌ：０．０１０〜０．０６０％
Ａｌは、溶鋼を脱酸するために添加される元素であり、０．０１０％以上含有させる必要がある。一方、０．０６０％を超えて添加すると鋼板および溶接部靭性を低下させるとともに、溶接による希釈によって溶接金属部に混入し、靭性を低下させるので、０．０６０％以下に制限する。好ましくは、０．０１７〜０．０５５％である。なお、本発明においてＡｌ量は、酸可溶性Ａｌ（Ｓｏｌ．Ａｌなどとも称される）で規定するものとする。Al: 0.010 to 0.060%
Al is an element added for deoxidizing molten steel, and it is necessary to contain 0.010% or more. On the other hand, if added over 0.060%, the toughness of the steel sheet and welded portion is lowered and mixed into the welded metal portion by dilution by welding to lower the toughness, so the content is limited to 0.060% or less. Preferably, it is 0.017 to 0.055%. In the present invention, the amount of Al is defined by acid-soluble Al (also referred to as Sol.Al or the like).

Ｎｉ：０．４０〜２．００％
Ｎｉは、鋼板の強度と靭性の向上に有効な元素であり、溶接部ＣＴＯＤ特性の向上にも有効である。この効果を得るには０．４０％以上の添加が必要である。一方、Ｎｉは高価な元素であること、また過度の添加は鋳造時にスラブの表面にキズを発生しやすくするので、含有する上限は２．００％とする。Ni: 0.40 to 2.00%
Ni is an element effective for improving the strength and toughness of the steel sheet, and is also effective for improving the welded portion CTOD characteristics. To obtain this effect, 0.40% or more must be added. On the other hand, Ni is an expensive element, and excessive addition tends to cause scratches on the surface of the slab during casting, so the upper limit of content is 2.00%.

Ｍｏ：０．０５〜０．５０％
Ｍｏは、本発明において重要な役割を果たし、適量添加によって鋼板を高強度化するのに有効な元素である。これは、焼入れ性と、焼き戻し時の軟化抵抗性の向上による効果である。また、ＴｉやＮｂと形成する複合析出物を微細に維持し、厚肉材の強化と靭性低下抑制の効果がある。これらの効果を得るためには、Ｍｏを０．０５％以上含有する必要がある。一方、過剰に含有すると、厚肉材の靭性に悪影響を与えるので、Ｍｏ量の上限は０．５０％とする。なお、Ｍｏ量は０．０８〜０．４０％の範囲であることがより好ましい。また、０．１６〜０．３０％の範囲であることがさらに好ましい。Mo: 0.05 to 0.50%
Mo plays an important role in the present invention and is an effective element for increasing the strength of a steel sheet by adding an appropriate amount. This is the effect of improving hardenability and softening resistance during tempering. Moreover, the composite precipitate formed with Ti and Nb is maintained finely, and there is an effect of strengthening the thick material and suppressing toughness reduction. In order to acquire these effects, it is necessary to contain 0.05% or more of Mo. On the other hand, if contained excessively, the toughness of the thick material is adversely affected, so the upper limit of the Mo amount is 0.50%. The Mo amount is more preferably in the range of 0.08 to 0.40%. Moreover, it is more preferable that it is in the range of 0.16 to 0.30%.

Ｎｂ：０．００５〜０．０４０％
Ｎｂは、低温域で、オーステナイトの未再結晶域を形成するので、その温度域で圧延を施すことにより、鋼板の組織微細化や、高靭化を図ることができる。また、Ｎｂは、焼入れ性の向上効果を有するとともに、ＭｏやＴｉと複合添加することで、焼戻し時の軟化抵抗を高める効果を有し、鋼板強度の向上に有効な元素でもある。これらの効果を得るためには、Ｎｂを０．００５％以上含有する必要がある。一方、０．０４０％を超えて含有すると靭性を劣化させるので、Ｎｂ量の上限は、０．０４０％とし、好ましくは０．０３５％とする。Nb: 0.005 to 0.040%
Since Nb forms a non-recrystallized area of austenite at a low temperature range, the structure of the steel sheet can be refined and toughened by rolling in that temperature range. Further, Nb has an effect of improving hardenability and has an effect of increasing softening resistance at the time of tempering by being added in combination with Mo and Ti, and is also an element effective for improving the strength of the steel sheet. In order to acquire these effects, it is necessary to contain Nb 0.005% or more. On the other hand, if the content exceeds 0.040%, the toughness is deteriorated, so the upper limit of the Nb amount is 0.040%, preferably 0.035%.

Ｔｉ：０．００５〜０．０２５％
Ｔｉは、溶鋼が凝固する際にＴｉＮとなって析出し、溶接部におけるオーステナイトの粗大化を抑制して溶接部の靭性向上に寄与する。さらにＭｏ，Ｎｂと併せて複合添加することで焼戻し時の軟化抵抗を高める効果がある。しかし、０．００５％未満の含有ではその効果が小さい一方で、０．０２５％を超えて含有すると、ＴｉＮが粗大化し、鋼板や溶接部の靭性改善効果が得られないため、Ｔｉは、０．００５〜０．０２５％の範囲とする。Ti: 0.005-0.025%
Ti precipitates as TiN when the molten steel is solidified, and suppresses austenite coarsening in the welded portion, thereby contributing to improvement in the toughness of the welded portion. Furthermore, the combined addition of Mo and Nb has the effect of increasing the softening resistance during tempering. However, when the content is less than 0.005%, the effect is small. However, when the content exceeds 0.025%, TiN becomes coarse, and the effect of improving the toughness of the steel sheet or the welded portion cannot be obtained. The range is 0.005 to 0.025%.

Ｎ：０．００２０〜０．００５０％
Ｎは、ＴｉやＡｌと反応して析出物を形成することで、結晶粒を微細化し、鋼板靭性を向上させる。また、溶接部の組織の粗大化を抑制するＴｉＮを形成させるために必要な元素である。これらの作用を発揮するには、Ｎを０．００２０％以上含有することが必要である。一方、Ｎは、０．００５０％を超えて添加すると固溶Ｎが鋼板や溶接部の靭性を著しく低下させたり、ＴｉおよびＮｂの複合析出物生成による固溶Ｎｂ減少に伴う強度低下を招いたりするので、上限を０．００５０％とする。N: 0.0020 to 0.0050%
N reacts with Ti and Al to form precipitates, thereby refining crystal grains and improving steel sheet toughness. Moreover, it is an element required in order to form TiN which suppresses the coarsening of the structure | tissue of a welding part. In order to exert these effects, it is necessary to contain N 0.0020% or more. On the other hand, when N is added in excess of 0.0050%, the solid solution N significantly reduces the toughness of the steel sheet and the welded portion, or causes a decrease in strength due to the decrease in the solid solution Nb due to the formation of composite precipitates of Ti and Nb. Therefore, the upper limit is made 0.0050%.

Ｃａ：０．０００５〜０．００５０％
Ｃａは、Ｓを固定することによって靭性を向上させる元素である。この効果を得るためには、少なくとも０．０００５％の添加が必要である。一方、０．００５０％を超えて含有してもその効果は飽和するため、Ｃａは０．０００５〜０．００５０％の範囲で添加する。Ca: 0.0005 to 0.0050%
Ca is an element that improves toughness by fixing S. In order to obtain this effect, addition of at least 0.0005% is necessary. On the other hand, since the effect is saturated even if it contains exceeding 0.0050%, Ca is added in 0.0005 to 0.0050% of range.

Ｏ：０．００３５％以下
Ｏは、０．００３５％を超えると鋼板の靭性が劣化するため、０．００３５％以下、好ましくは０．００２８％以下とする。なお、Ｏの含有量は、極力少ないほうが良いが、精錬コスト等の点から、その下限値は、０．００１０％程度である。O: 0.0035% or less O exceeds 0.0035%, so that the toughness of the steel sheet deteriorates, so 0.0035% or less, preferably 0.0028% or less. The O content is preferably as low as possible, but the lower limit is about 0.0010% from the viewpoint of refining costs and the like.

Ｃｅｑ：０．４２０〜０．５２０％
以下の式で規定されるＣｅｑが０．４２０％未満の場合、４６０ＭＰａ級の厚肉材強度が得られない。一方、０．５２０％を超えると、厚肉材の溶接性や溶接部靭性が低下するため、０．５２０％以下とする。好ましくは、０．４４０〜０．５２０％の範囲である。なお、以下、［Ｍ］は元素Ｍの鋼中含有量（質量％）を表す。また、含有しない元素は０で計算する。
Ｃｅｑ＝［Ｃ］＋［Ｍｎ］／６＋（［Ｃｕ］＋［Ｎｉ］）／１５＋（［Ｃｒ］＋［Ｍｏ］＋［Ｖ］）／ ５Ceq: 0.420 to 0.520%
When Ceq defined by the following formula is less than 0.420%, a thick material strength of 460 MPa class cannot be obtained. On the other hand, if it exceeds 0.520%, the weldability and weld toughness of the thick-walled material are lowered, so the content is made 0.520% or less. Preferably, it is 0.440 to 0.520% of range. Hereinafter, [M] represents the content (mass%) of the element M in steel. Moreover, the element which does not contain is calculated by 0.
Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5

［Ｔｉ］／［Ｎ］：１．５〜４．０
［Ｔｉ］／［Ｎ］の値が１．５未満では生成するＴｉＮ量が減少し、ＴｉＮとならない固溶Ｎが溶接部の靭性を低下させてしまう。一方、［Ｔｉ］／［Ｎ］の値が４．０を超えると、ＴｉＮが粗大化し、溶接部靭性を低下させる。従って、［Ｔｉ］／［Ｎ］の値の範囲は１．５〜４．０、好ましくは、１．８〜３．５とする。[Ti] / [N]: 1.5 to 4.0
If the value of [Ti] / [N] is less than 1.5, the amount of TiN produced decreases, and solid solution N that does not become TiN reduces the toughness of the weld. On the other hand, when the value of [Ti] / [N] exceeds 4.0, TiN becomes coarse and the weld zone toughness is lowered. Therefore, the range of the value of [Ti] / [N] is 1.5 to 4.0, preferably 1.8 to 3.5.

０＜｛［Ｃａ］−（０．１８＋１３０×［Ｃａ］）×［Ｏ］｝／１．２５／［Ｓ］＜１．５
｛［Ｃａ］−（０．１８＋１３０×［Ｃａ］）×［Ｏ］｝／１．２５／［Ｓ］は、硫化物形態制御に有効なＣａとＳの原子濃度の比を示す値で、Ｃａの添加量および添加時の溶鋼中の溶存酸素量を適正範囲に制御することによって調整することができ、ＡＣＲ（Ａｔｏｍｉｃ Ｃｏｎｃｅｎｔｒａｔｉｏｎ Ｒａｔｉｏ）とも称される。このＡＣＲ値によって硫化物の形態を推定することができるが、本発明では、高温でも溶解しないフェライト変態生成核ＣａＳを微細分散させる指標として規定する。
ここで、ＡＣＲ値が０以下の場合、ＣａＳが晶出しない。そのため、Ｓは、ＭｎＳ単独の形態で析出するので、溶接熱影響部では容易に固溶してしまいフェライト生成核が得られない。また、単独で析出したＭｎＳは、圧延時に伸長されて、鋼板の靭性低下を引き起こしてしまう。従って、本発明では、ＡＣＲ値を０超とする必要がある。
一方、ＡＣＲ値が１．５以上の場合には、Ｃａ系介在物中の酸化物の割合が多くなって、変態核として機能する硫化物の割合が低下し、靭性向上効果が得られない。従って、本発明では、ＡＣＲ値を１．５未満とする必要がある。
従って、ＡＣＲ値を０超かつ１．５未満に制御すると、ＣａＳを主体とする複合硫化物が効果的に形成し、フェライト生成核として有効に機能させることができる。なお、ＡＣＲ値は、好ましくは０．１５〜１．３０の範囲である。より好ましくは、０．２０〜１．００の範囲である。0 <{[Ca] − (0.18 + 130 × [Ca]) × [O]} / 1.25 / [S] <1.5
{[Ca] − (0.18 + 130 × [Ca]) × [O]} / 1.25 / [S] is a value indicating the ratio of atomic concentrations of Ca and S effective for sulfide morphology control. It can be adjusted by controlling the amount of oxygen added and the amount of dissolved oxygen in the molten steel at the time of addition to an appropriate range, and is also referred to as ACR (Atomic Concentration Ratio). Although the form of sulfide can be estimated from this ACR value, in the present invention, it is defined as an index for finely dispersing the ferrite transformation nuclei CaS that does not dissolve even at high temperatures.
Here, when the ACR value is 0 or less, CaS does not crystallize. Therefore, since S precipitates in the form of MnS alone, it easily dissolves in the weld heat affected zone and ferrite formation nuclei cannot be obtained. In addition, MnS precipitated alone is elongated during rolling and causes a reduction in the toughness of the steel sheet. Therefore, in the present invention, the ACR value needs to exceed zero.
On the other hand, when the ACR value is 1.5 or more, the ratio of the oxide in the Ca-based inclusion increases, the ratio of the sulfide functioning as a transformation nucleus decreases, and the toughness improving effect cannot be obtained. Therefore, in the present invention, the ACR value needs to be less than 1.5.
Therefore, when the ACR value is controlled to be more than 0 and less than 1.5, a composite sulfide mainly composed of CaS can be effectively formed and function effectively as a ferrite forming nucleus. The ACR value is preferably in the range of 0.15 to 1.30. More preferably, it is in the range of 0.20 to 1.00.

５．５［Ｃ］（４／３）＋１５［Ｐ］＋０．９０［Ｍｎ］＋０．１２［Ｎｉ］＋７．９［Ｎｂ］（１／２）＋０．５３［Ｍｏ］ ≦３．７０
上記式の左辺（５．５［Ｃ］（４／３）＋１５［Ｐ］＋０．９０［Ｍｎ］＋０．１２［Ｎｉ］＋７．９［Ｎｂ］（１／２）＋０．５３［Ｍｏ］）は、中心偏析部に濃化しやすい成分で構成された中心偏析部の硬さ指標であり、以下の説明ではＣｅｑ＊値と称する。
ＣＴＯＤ試験は、鋼板全厚での試験のため、試験片は中心偏析を含み、中心偏析での成分濃化が顕著な場合には、溶接熱影響部に硬化域が生成するので、ＣＴＯＤ試験として、良好な結果が得られない。
そこで、本発明では、Ｃｅｑ＊値を適正範囲に制御することによって、中心偏析部における過度の硬度上昇を抑制し、板厚が厚い鋼板の溶接部においても優れたＣＴＯＤ特性が得られるのである。
Ｃｅｑ＊値の適正範囲は、実験的に求められたものであり、Ｃｅｑ＊値が３．７０を超えるとＣＴＯＤ特性が低下するので３．７０以下とする。好ましくは３．５０以下である。なお、Ｃｅｑ＊値の下限に特に制限はないが、生産性の観点などから２．２程度が好ましい。5.5 [C] (4/3) +15 [P] +0.90 [Mn] +0.12 [Ni] +7.9 [Nb] (1/2) +0.53 [Mo] ≦ 3.70
Left side of the above formula (5.5 [C] (4/3) +15 [P] +0.90 [Mn] +0.12 [Ni] +7.9 [Nb] (1/2) +0.53 [Mo]) Is a hardness index of the center segregation part composed of components that are easily concentrated in the center segregation part, and is referred to as a Ceq * value in the following description.
Since the CTOD test is a test at the full thickness of the steel sheet, the test piece includes center segregation, and when the concentration of components at the center segregation is significant, a hardened zone is generated in the weld heat affected zone. Good results cannot be obtained.
Therefore, in the present invention, by controlling the Ceq * value within an appropriate range, an excessive increase in hardness in the center segregation portion is suppressed, and excellent CTOD characteristics can be obtained even in a welded portion of a steel plate having a large plate thickness.
Appropriate range of Ceq * values are those determined experimentally, Ceq * value is to 3.70 or less because CTOD properties deteriorate exceeds 3.70. Preferably it is 3.50 or less. In addition, although there is no restriction | limiting in particular in the minimum of Ceq * value, About 2.2 is preferable from a viewpoint of productivity.

また、本発明では、上記した必須成分に加えて、焼入性を高めるために、Ｃｕ：０．７％未満、Ｃｒ：０．１〜１．０％およびＶ：０．００５〜０．０５％のうちから選ばれる１種または２種以上を含有することができる。   In the present invention, in addition to the above-described essential components, Cu: less than 0.7%, Cr: 0.1-1.0%, and V: 0.005-0.05 in order to enhance hardenability. 1 type or 2 types or more chosen from% can be contained.

Ｃｕ：０．７％未満
Ｃｕは添加することで、鋼板強度を向上させることができる。ただし、０．７％を超えての添加は熱間延性を低下させるので、０．７％以下に制限する。好ましくは、０．１〜０．６％である。Cu: Less than 0.7% By adding Cu, the steel plate strength can be improved. However, since addition exceeding 0.7% reduces hot ductility, it limits to 0.7% or less. Preferably, it is 0.1 to 0.6%.

Ｃｒ：０．１〜１．０％
Ｃｒは、鋼板を高強度化するのに有効な元素であり、この効果を発揮するには０．１％以上を含有する。しかし、過剰に含有すると靭性に悪影響を与えるので、含有する場合は０．１〜１．０％の範囲が好ましく、０．２〜０．８％の範囲であることがより好ましい。Cr: 0.1 to 1.0%
Cr is an element effective for increasing the strength of a steel sheet, and in order to exert this effect, it contains 0.1% or more. However, if contained in excess, the toughness is adversely affected, so when contained, the range of 0.1 to 1.0% is preferable, and the range of 0.2 to 0.8% is more preferable.

Ｖ：０．００５〜０．０５％
Ｖは、０．００５％以上の含有で鋼板の強度と靭性の向上に有効な元素であるが、含有量が０．０５％を超えると靭性低下を招くので、含有する場合は０．００５〜０．０５％であることが好ましい。V: 0.005-0.05%
V is an element effective for improving the strength and toughness of the steel sheet when contained in an amount of 0.005% or more. However, if the content exceeds 0.05%, the toughness is reduced. It is preferably 0.05%.

さらに本発明では、上記した必須成分に加えて、ＨＡＺ靭性を高めるために、Ｍｇ：０．０００２〜０．００５０％およびＲＥＭ：０．００１０〜０．０２００％のうちから選ばれる１種または２種を含有することができる。   Furthermore, in the present invention, in addition to the above-described essential components, one or two selected from Mg: 0.0002 to 0.0050% and REM: 0.0010 to 0.0200% in order to increase the HAZ toughness Seeds can be included.

ＭｇおよびＲＥＭは、酸化物の分散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現させるために、Ｍｇは０．０００２％以上、ＲＥＭは０．００１０％以上添加する。一方、Ｍｇは０．００５０％超、ＲＥＭは０．０２００％超を添加しても、その効果は飽和するだけである。よってこれらの元素を添加する場合は、それぞれ上記した範囲とするのが好ましい。より好ましくは、Ｍｇ：０．０００５〜０．００２０％、ＲＥＭ：０．００２０〜０．０１５０％である。   Mg and REM are elements having an effect of improving toughness due to oxide dispersion. In order to exhibit such effects, 0.0002% or more of Mg and 0.0010% or more of REM are added. On the other hand, even if Mg exceeds 0.0050% and REM exceeds 0.0200%, the effect is only saturated. Therefore, when adding these elements, it is preferable to set it as the above-mentioned range, respectively. More preferably, they are Mg: 0.0005-0.0020% and REM: 0.0020-0.0150%.

上記、鋼板成分以外の成分は、Ｆｅおよび不可避的不純物であるが、特にＢは、鋼板がオーステナイト域から冷却される際に、オーステナイト粒界に偏析してフェライト変態を抑制し、Ｍ−Ａを多量に含むベイナイト組織を生成させることで、特に溶接熱影響部の組織を脆化させる不利がある。よって、本発明において、鋼板中のＢ量は、０．０００３％未満に抑制する必要がある。   The above components other than the steel plate components are Fe and inevitable impurities, but particularly B segregates at the austenite grain boundaries when the steel plate is cooled from the austenite region, and suppresses the ferrite transformation. By generating a bainite structure containing a large amount, there is a disadvantage that the structure of the weld heat affected zone becomes brittle. Therefore, in this invention, it is necessary to suppress B amount in a steel plate to less than 0.0003%.

また、鋼板中、析出物はＰＷＨＴ前後でサイズの変化が少なく、鋼板の強度靭性を維持することが必要である。図１には、ＰＷＨＴ後の析出物サイズと析出物組成と、ＰＷＨＴ前後での強度・靭性変化（ΔＴＳ、ΔｖＴｒｓ）の関係を、また、図２には、鋼中析出物のＴＥＭレプリカ観察とＥＤＸ分析結果を示す。   In addition, in the steel sheet, the precipitate has little change in size before and after PWHT, and it is necessary to maintain the strength and toughness of the steel sheet. Fig. 1 shows the relationship between precipitate size and precipitate composition after PWHT, and changes in strength and toughness (ΔTS, ΔvTrs) before and after PWHT. Fig. 2 shows TEM replica observation of precipitates in steel. An EDX analysis result is shown.

強度、靭性について、ＰＷＨＴ前後の変化は、安定性の観点から、それぞれ、ΔＴＳが５〜−１５ＭＰａ、ΔｖＴｒｓが１０〜−５℃の範囲を満足することが必要である。そして、その範囲を満足するためには、析出物の平均サイズを２０ｎｍ以下に抑えつつ、析出物中のＴｉ量（〔Ｔｉ〕と表す）、Ｎｂ量（〔Ｎｂ〕と表す）およびＭｏ量（〔Ｍｏ〕と表す）が、〔Ｎｂ〕／（〔Ｔｉ〕＋〔Ｎｂ〕＋〔Ｍｏ〕）≧０．３の関係を満足することが必要であることが図１から分かる。   Regarding strength and toughness, changes before and after PWHT are required to satisfy the ranges of ΔTS of 5 to −15 MPa and ΔvTrs of 10 to −5 ° C., respectively, from the viewpoint of stability. And in order to satisfy the range, while suppressing the average size of the precipitates to 20 nm or less, the Ti amount (represented as [Ti]), the Nb amount (represented as [Nb]) and the Mo amount (represented by [Nb]). It can be seen from FIG. 1 that [Mo] is required to satisfy the relationship [Nb] / ([Ti] + [Nb] + [Mo]) ≧ 0.3.

また、上記した析出物は、図２中の鋼中析出物のＥＤＸ分析結果を示した表１から分かるように、Ｔｉ、ＮｂおよびＭｏの析出物であるが、析出物中のＴｉ量、Ｎｂ量およびＭｏ量が、〔Ｎｂ〕／（〔Ｔｉ〕＋〔Ｎｂ〕＋〔Ｍｏ〕）≧０．３の関係を満足すればよいので、上記析出物は少なくともＮｂの析出物であればよく、ＴｉおよびＭｏの析出物はこの関係を満足する範囲で含まれていれば良い。なお、本発明でＰＷＨＴ特性に優れるとは、ΔＴＳが５〜−１５ＭＰａ、ΔｖＴｒｓが１０〜−５℃の範囲を満足することである。また、本発明における析出物（複合析出物）とは、Ｍｏや，Ｔｉ，Ｎｂの析出物であって、具体的には、Ｍｏや，Ｔｉ，Ｎｂの、炭化物、窒化物または炭窒化物、あるいはこれらの混合物である。   Further, as can be seen from Table 1 showing the EDX analysis result of the precipitate in steel in FIG. 2, the above-mentioned precipitate is a precipitate of Ti, Nb and Mo, but the amount of Ti in the precipitate, Nb Since the amount and the amount of Mo only need to satisfy the relationship [Nb] / ([Ti] + [Nb] + [Mo]) ≧ 0.3, the precipitates should be at least Nb precipitates, Ti and Mo precipitates may be included within a range satisfying this relationship. In the present invention, “excellent PWHT characteristics” means that ΔTS satisfies a range of 5 to −15 MPa and ΔvTrs satisfies a range of 10 to −5 ° C. In addition, the precipitate (composite precipitate) in the present invention is a precipitate of Mo, Ti, Nb, specifically, a carbide, nitride, or carbonitride of Mo, Ti, Nb, Or a mixture of these.

〔析出物粒子径の求め方〕
本発明における析出物粒子径の求め方は、ＴＥＭレプリカ法に準拠する。すなわち、鋼中の、Ｔｉ、ＮｂおよびＭｏの炭化物の析出部を適宜採取したのち、１０万倍で４視野による観察から画像処理を用いて平均円相当径を求め、これを析出物の粒子径とする。なお、本発明では、析出物粒径の測定対象の下限値を２ｎｍとする。これ未満の析出物粒径の析出物では、測定が難しくなるからである。[How to determine the particle size of precipitates]
The method for obtaining the precipitate particle diameter in the present invention is based on the TEM replica method. That is, after appropriately collecting precipitates of carbides of Ti, Nb, and Mo in steel, an average equivalent circle diameter was obtained using image processing from observation with four fields of view at 100,000 times, and this was determined as the particle diameter of the precipitates. And In the present invention, the lower limit value of the measurement target of the precipitate particle size is 2 nm. This is because it is difficult to measure with precipitates having a particle size of less than this.

次に、本発明鋼の製造方法を説明する。本発明鋼は以下に説明する製造方法で製造することが好ましい。
前記した本発明範囲内の鋼板成分に調整した溶鋼を、転炉や、電気炉、真空溶解炉などを用いた通常の方法で溶製し、次いで、連続鋳造の工程を経てスラブとした後、熱間圧延により所望の板厚とし、その後冷却して必要に応じて焼戻し処理を施す。なお、本発明における熱間圧延ではスラブ加熱温度と、圧下率を規定する。
なお、本発明において、特に記載しない限り、鋼板の温度条件は、鋼板の板厚中心部の温度で規定するものとする。板厚中心部の温度は、板厚、表面温度および冷却条件などから、シミュレーション計算などにより求められる。たとえば、差分法を用い、板厚方向の温度分布を計算することによって、板厚中心部の温度を求めることができる。Next, a method for producing the steel of the present invention will be described. The steel of the present invention is preferably produced by the production method described below.
After the molten steel adjusted to the steel plate component within the scope of the present invention described above is melted by a usual method using a converter, an electric furnace, a vacuum melting furnace, etc., and then converted into a slab through a continuous casting process, A desired plate thickness is obtained by hot rolling, followed by cooling and tempering as necessary. In the hot rolling in the present invention, the slab heating temperature and the rolling reduction are specified.
In the present invention, unless otherwise specified, the temperature condition of the steel sheet is defined by the temperature at the center of the thickness of the steel sheet. The temperature at the center of the plate thickness is obtained by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, the temperature at the center of the plate thickness can be obtained by calculating the temperature distribution in the plate thickness direction using the difference method.

スラブ加熱温度：９５０〜１１５０℃
スラブ加熱温度は、スラブに存在する鋳造欠陥を熱間圧延によって着実に圧着させるため９５０℃以上とする。一方、スラブを、１１５０℃を超える温度に加熱するとオーステナイト結晶粒が粗大化して鋼板の靭性が低下するため、加熱温度の上限を１１５０℃とする。Slab heating temperature: 950-1150 ° C
The slab heating temperature is set to 950 ° C. or higher so that casting defects existing in the slab are steadily pressed by hot rolling. On the other hand, when the slab is heated to a temperature exceeding 1150 ° C., the austenite crystal grains become coarse and the toughness of the steel sheet decreases, so the upper limit of the heating temperature is set to 1150 ° C.

９００℃以上の温度域における熱間圧延の累積圧下率：３０％以上
鋳造欠陥の圧着による無害化と、オーステナイト粒を再結晶により微細なミクロ組織とするために、９００℃以上の温度域における熱間圧延の累積圧下率を３０％以上とする。３０％未満では、加熱時に生成した粗大粒が残存して、鋼板の靭性に悪影響を及ぼすからである。なお、９００℃以上の温度域における熱間圧延の累積圧下率の上限は特に限定されないが、工業的には９５％程度である。Cumulative rolling reduction of hot rolling in a temperature range of 900 ° C. or higher: 30% or higher Heat in a temperature range of 900 ° C. or higher in order to make the austenite grains finer by recrystallization by detoxification of casting defects. The cumulative rolling reduction of the hot rolling is set to 30% or more. If it is less than 30%, coarse grains generated during heating remain, which adversely affects the toughness of the steel sheet. In addition, although the upper limit of the cumulative rolling reduction of hot rolling in a temperature range of 900 ° C. or higher is not particularly limited, it is about 95% industrially.

９００℃未満の温度域における熱間圧延の累積圧下率：３０〜７０％
この温度域で圧延されたオーステナイト粒は十分に再結晶しないため、圧延後のオーステナイト粒は偏平に変形したままで、内部に変形帯などの欠陥を多量に含む内部歪の高い状態となる。これらは、フェライト変態の駆動力として働き、相変態を促進する。
しかし、累積圧下率が３０％未満では、内部歪による内部エネルギーの蓄積が十分でないためフェライト変態が起こりにくく鋼板靭性が低下する一方で、累積圧下率が７０％を超えると、逆にポリゴナルフェライトの生成が促進されて、高強度と高靭性が両立しない。従って、本発明では、９００℃未満の温度域における熱間圧延の累積圧下率を３０〜７０％の範囲とする。Cumulative rolling reduction of hot rolling in a temperature range below 900 ° C .: 30 to 70%
Since the austenite grains rolled in this temperature range are not sufficiently recrystallized, the austenite grains after rolling remain flatly deformed and have a high internal strain state including a large amount of defects such as deformation bands inside. These act as a driving force for the ferrite transformation and promote the phase transformation.
However, if the cumulative rolling reduction is less than 30%, the internal energy accumulation due to internal strain is not sufficient, so that the ferrite transformation hardly occurs and the steel sheet toughness is reduced. On the other hand, if the cumulative rolling reduction exceeds 70%, the polygonal ferrite Generation is promoted, and high strength and high toughness are not compatible. Therefore, in the present invention, the cumulative rolling reduction of hot rolling in the temperature range below 900 ° C. is set to a range of 30 to 70%.

少なくとも５００℃まで冷却速度：１．０℃／ｓ以上
熱間圧延後、冷却速度を１．０℃／ｓ以上として少なくとも５００℃まで加速冷却する。冷却速度が１．０℃／ｓ未満では十分な鋼板の強度が得られないからである。また、５００℃より高い温度で冷却を停止するとフェライト＋パーライト組織の分率が高くなって、厚肉材の高強度と高靭性とが両立しない。なお、加速冷却の停止温度の下限は特に限定されるものではなく、室温まで行っても良い。Cooling rate to at least 500 ° C .: 1.0 ° C./s or more After hot rolling, accelerated cooling to at least 500 ° C. with a cooling rate of 1.0 ° C./s or more. This is because if the cooling rate is less than 1.0 ° C./s, sufficient steel sheet strength cannot be obtained. Moreover, if cooling is stopped at a temperature higher than 500 ° C., the fraction of ferrite + pearlite structure increases, and the high strength and high toughness of the thick material are not compatible. In addition, the minimum of the stop temperature of accelerated cooling is not specifically limited, You may carry out to room temperature.

焼戻し温度：４５０〜６５０℃
本発明で焼戻し処理を行う場合、４５０℃未満の焼戻し温度では十分な焼戻しの効果が得られない。一方、６５０℃を超える温度で焼戻しを行うと、析出物が粗大になって靭性が低下したり、強度が低下したりすることもあるため好ましくない。
また、本発明の焼戻し処理は、誘導加熱を用いることにより、焼戻し時の炭化物の粗大化が抑制されるためより好ましい。その場合は、差分法などのシミュレーションによって計算される鋼板の中心温度が４５０〜６５０℃となるようにすることが望ましい。
なお、本発明において、ＴＭＣＰ鋼板等、鋼板の所望の性能が得られている場合には、上記焼戻し処理を行わなくても良い。Tempering temperature: 450-650 ° C
When the tempering treatment is performed in the present invention, a sufficient tempering effect cannot be obtained at a tempering temperature of less than 450 ° C. On the other hand, tempering at a temperature exceeding 650 ° C. is not preferable because precipitates may become coarse and the toughness may be reduced or the strength may be reduced.
Further, the tempering treatment of the present invention is more preferable because induction heating is used to suppress the coarsening of carbides during tempering. In that case, it is desirable that the center temperature of the steel sheet calculated by simulation such as a difference method is 450 to 650 ° C.
In the present invention, when the desired performance of a steel plate such as a TMCP steel plate is obtained, the tempering process may not be performed.

本発明の厚肉材は、厚みが１５ｍｍ以上である。従って、本発明において厚肉とは、鋼の厚みが１５ｍｍ以上であるが、最も本発明の効果が得られるのは、鋼の厚みが４０〜１００ｍｍの範囲である。なお、上記した厚肉高張力鋼の製造条件以外の製造条件は、常法に従えば良い。   The thick material of the present invention has a thickness of 15 mm or more. Therefore, in the present invention, the term “thick” means that the thickness of the steel is 15 mm or more, but the effect of the present invention is most obtained when the thickness of the steel is in the range of 40 to 100 mm. In addition, the manufacturing conditions other than the manufacturing conditions of the above-described thick high-strength steel may be in accordance with ordinary methods.

本発明に従う厚肉高張力鋼は、溶接熱影響部のオーステナイト粒の粗大化を抑制しつつ、高温でも溶解しないフェライト変態生成核を微細に分散させることで、溶接熱影響部の組織を微細化するので、高い靭性が得られる。また、多層溶接時の熱サイクルにより２相域に再加熱される領域においても、最初の溶接による溶接熱影響部の組織が微細化されているので２相域再加熱領域で未変態領域の靭性が向上し、再変態するオーステナイト粒も微細化して、靭性の低下度合いを小さくすることが可能である。加えて、Ｔｉ，Ｎｂ，Ｍｏの複合析出物を微細に生成させることによって、ＣＴＯＤ特性およびＰＷＨＴ特性に優れる厚肉高張力鋼板となる。   The thick high-strength steel according to the present invention refines the structure of the weld heat-affected zone by finely dispersing the ferrite transformation nuclei that do not melt even at high temperatures while suppressing the coarsening of austenite grains in the weld heat-affected zone. Therefore, high toughness can be obtained. Even in the region where reheating is performed in the two-phase region by the thermal cycle during multi-layer welding, the structure of the weld heat affected zone by the first welding is refined, so the toughness of the untransformed region in the two-phase region reheating region As a result, the austenite grains that are retransformed can be made finer, and the degree of decrease in toughness can be reduced. In addition, by forming fine composite precipitates of Ti, Nb, and Mo, a thick high-tensile steel sheet having excellent CTOD characteristics and PWHT characteristics can be obtained.

次に、本発明の実施例について説明する。
表２に示す成分組成を有する鋼記号Ａ〜Ｚの連続鋳造スラブを素材とした後、表３に示す熱間圧延と熱処理とを行い、厚さが５０〜１５０ｍｍの厚鋼板を製造した。鋼板の評価方法として、引張試験は鋼板の板厚の１／２位置より試験片の長手方向が鋼板の圧延方向と垂直になるようにＪＩＳ４号試験片を採取し、降伏応力（ＹＳ）および引張強さ（ＴＳ）を測定した。
また、シャルピー衝撃試験は、鋼板の板厚の１／２位置より試験片の長手方向が鋼板の圧延方向と垂直になるようにＪＩＳ４号２ｍｍＶノッチ試験片を採取し、−４０℃における吸収エネルギーｖＥ-40℃ を測定した。なお、本実施例では、ＹＳ≧４６０ＭＰａ、ＴＳ≧５７０ＭＰａおよびｖＥ-40℃ ≧２００Ｊの全てを満たすものを鋼板特性が良好であると評価した。
溶接部靭性の評価は、レ型開先を用いて、溶接入熱３５ｋＪ／ｃｍのサブマージアーク溶接による多層盛溶接継手を作製し、鋼板の板厚の１／２位置のストレート側の溶接ボンド部をシャルピー衝撃試験のノッチ位置として、−４０℃の温度における吸収エネルギーｖＥ-40℃ を測定した。そして、３本の平均がｖＥ-40℃ ≧１５０Ｊを満足するものを溶接部靭性が良好と判断した。
また、ストレート側の溶接ボンド部をＣＴＯＤ試験片のノッチ位置として、−１０℃におけるＣＴＯＤ値であるδ−１０℃を測定し、試験数量３本のうちＣＴＯＤ値（δ−１０℃）の最小値が０．５ｍｍ以上である場合を、溶接継手のＣＴＯＤ特性が良好と判断した。
さらに、鋼中の析出部をＴＥＭレプリカ法により採取し、１０万倍４視野による観察から画像処理により平均円相当径を求め、これを析出物サイズとした。また、ＥＤＸにより粒子径がほぼ平均に近い析出物を選び、その析出物組成を求め、３個の平均として〔Ｎｂ〕/（〔Ｔｉ〕+〔Ｎｂ〕+〔Ｍｏ〕）を求めた。
ＰＷＨＴ後の鋼板特性変化については、ΔＴＳ（＝ＴＳ（ＰＷＨＴ後）−ＴＳ（ＰＷＨＴ前））、ΔｖＴｒｓ（＝ｖＴｒｓ（ＰＷＨＴ後）−ｖＴｒｓ（ＰＷＨＴ前））を求めた。ＰＷＨＴ熱処理は、５８０℃で４ｈ保持とし、昇温、降温速度を７０℃／ｈとして行った。
表３に、熱間圧延条件、熱処理条件とともに、鋼板特性および上記溶接部のシャルピー衝撃試験結果とＣＴＯＤ試験結果、析出物サイズ・組成、ＰＷＨＴ後の鋼板特性変化を併記する。 Next, examples of the present invention will be described.
After using continuously cast slabs of steel symbols A to Z having the component compositions shown in Table 2, hot rolling and heat treatment shown in Table 3 were performed to produce a thick steel plate having a thickness of 50 to 150 mm. As a steel sheet evaluation method, a tensile test was conducted by taking a JIS No. 4 test piece from the position of 1/2 the thickness of the steel sheet so that the longitudinal direction of the test piece was perpendicular to the rolling direction of the steel sheet, yield stress (YS) and tensile stress. Strength (TS) was measured.
In the Charpy impact test, a JIS No. 2 mmV notch test piece was sampled so that the longitudinal direction of the test piece was perpendicular to the rolling direction of the steel sheet from 1/2 position of the thickness of the steel sheet, and the absorbed energy vE at −40 ° C. -40 ° C was measured. In this example, the steel sheet characteristics that satisfy all of YS ≧ 460 MPa, TS ≧ 570 MPa, and vE −40 ° C. ≧ 200 J were evaluated as good.
The weld toughness is evaluated by using a lathe groove to produce a multi-layer welded joint by submerged arc welding with a welding heat input of 35 kJ / cm, and a straight-side weld bond portion at half the plate thickness of the steel sheet. Was the notch position of the Charpy impact test, and the absorbed energy vE- 40 ° C at a temperature of -40 ° C was measured. Then, it was judged to be good weld toughness having an average of three satisfies vE -40 ℃ ≧ 150J.
Further, δ-10 ° C, which is the CTOD value at -10 ° C, is measured with the weld bond portion on the straight side as the notch position of the CTOD test piece, and the minimum value of the CTOD value (δ-10 ° C) among the three test quantities. Was 0.5 mm or more, it was judged that the CTOD characteristics of the welded joint were good.
Furthermore, the precipitation part in steel was extract | collected by the TEM replica method, the average equivalent circle diameter was calculated | required by image processing from the observation by 100,000 times 4 visual fields, and this was made into the precipitate size. Further, a precipitate having a particle size almost equal to the average was selected by EDX, and the precipitate composition was determined, and [Nb] / ([Ti] + [Nb] + [Mo]) was determined as the average of three particles.
Regarding changes in steel sheet characteristics after PWHT, ΔTS (= TS (after PWHT) −TS (before PWHT)) and ΔvTrs (= vTrs (after PWHT) −vTrs (before PWHT)) were obtained. The PWHT heat treatment was performed at 580 ° C. for 4 hours and at a temperature increase / decrease rate of 70 ° C./h.
Table 3 shows steel sheet characteristics, Charpy impact test results and CTOD test results, precipitate size / composition, and steel sheet characteristics changes after PWHT, along with hot rolling conditions and heat treatment conditions.

表２に示したように、鋼記号Ａ〜Ｅは、本発明の適合鋼で、鋼記号Ｆ〜Ｚは鋼成分のいずれかが本発明の範囲外の比較鋼である。また、表３の試料Ｎｏ．１、２、５、６、８および１１は、いずれも発明例であり、溶接ボンド部のシャルピー衝撃試験結果、溶接ボンド部の三点曲げＣＴＯＤ試験結果、鋼板中の析出物サイズ・組成およびＰＷＨＴ特性の全てにおいて目標を満足する結果が得られている。
一方、試料Ｎｏ．３、４、７、９、１０、１２〜３１は、鋼板成分、製造条件、析出物サイズ・組成の少なくとも一つが本発明の範囲外であり、鋼板特性や、溶接ボンド部のシャルピー衝撃試験結果、溶接ボンド部の三点曲げＣＴＯＤ試験結果、ＰＷＨＴ特性のいずれかが目標を満足しなかった。なお、表３中、ヨコ線の項目は、当該項目の測定ができなかったことを意味する。As shown in Table 2, steel symbols A to E are compatible steels of the present invention, and steel symbols F to Z are comparative steels whose steel components are outside the scope of the present invention. In addition, Sample No. 1, 2, 5, 6, 8, and 11 are all inventive examples, the Charpy impact test result of the weld bond part, the three-point bending CTOD test result of the weld bond part, the precipitate size and composition in the steel plate, and the PWHT A result that satisfies the target in all of the characteristics is obtained.
On the other hand, sample No. 3, 4, 7, 9, 10, 12 to 31 are at least one of steel plate components, production conditions, precipitate size and composition outside the scope of the present invention, and steel plate characteristics and Charpy impact test results of welded bonds. One of the three-point bending CTOD test results and PWHT characteristics of the weld bond part did not satisfy the target. In Table 3, a horizontal line item means that the item could not be measured.

また、本発明に従った発明例の鋼は、鋼板の降伏応力（ＹＳ）が４６０ＭＰａ以上で、シャルピー吸収エネルギー（ｖＥ-40℃ ）が２００Ｊ以上を有しており、鋼板の強度、靭性が共に優れていることと、更に溶接継手ボンド部についても、ｖＥ-40℃ が１５０Ｊ以上で、ＣＴＯＤ値が０．５ｍｍ以上であり、溶接熱影響部の靭性にも優れていることが分かる。また、析出物の平均粒子径が２０μｍ以下でかつ〔Ｎｂ〕/（〔Ｔｉ〕+〔Ｎｂ〕+〔Ｍｏ〕）≧０．３であれば、ＰＷＨＴ後の鋼板特性にも優れている。これに対し、本発明の範囲を外れる比較例では、上記の特性いずれかにおいて劣った鋼板しか得られていないことが分かる。 In addition, the steel of the inventive example according to the present invention has a yield stress (YS) of the steel plate of 460 MPa or more and a Charpy absorbed energy (vE -40 ° C ) of 200 J or more, and both the strength and toughness of the steel plate are both. It can be seen that the welded joint bond part is excellent, and that the vE -40 ° C is 150 J or more, the CTOD value is 0.5 mm or more, and the weld heat affected zone has excellent toughness. Further, if the average particle size of the precipitate is 20 μm or less and [Nb] / ([Ti] + [Nb] + [Mo]) ≧ 0.3, the steel sheet characteristics after PWHT are also excellent. On the other hand, in the comparative example outside the scope of the present invention, it can be seen that only a steel sheet inferior in any of the above characteristics is obtained.

Claims (5)

質量％で、
Ｃ：０．０２０〜０．０９０％
Ｓｉ：０．０１〜０．３５％
Ｍｎ：１．４０〜２．００％
Ｐ：０．００８％以下
Ｓ：０．００３５％以下
Ａｌ：０．０１０〜０．０６０％
Ｎｉ：０．４０〜２．００％
Ｍｏ：０．０５〜０．５０％
Ｎｂ：０．００５〜０．０４０％
Ｔｉ：０．００５〜０．０２５％
Ｎ：０．００２０〜０．００５０％
Ｃａ：０．０００５〜０．００５０％および
Ｏ：０．００３５％以下
を含有し、下記（１）式で規定されるＣｅｑが０．４２０〜０．５２０％の範囲であって、下記（２）式、（３）式および（４）式を満たすと共に、Ｂを０．０００３％未満に抑制し、残部がＦｅおよび不可避的不純物からなる鋼板成分と、
Ｔｉ、ＮｂおよびＭｏを、Ｔｉ量（〔Ｔｉ〕）、Ｎｂ量（〔Ｎｂ〕）およびＭｏ量（〔Ｍｏ〕）が、〔Ｎｂ〕/（〔Ｔｉ〕+〔Ｎｂ〕+〔Ｍｏ〕）≧０．３の関係を満足する範囲で含み、かつ、析出物粒径が２ｎｍ以上である析出物を測定対象にして算出した平均粒子径が２０ｎｍ以下の析出物を有し、さらに、ＹＳ≧４６０ＭＰａ、ＴＳ≧５７０ＭＰａおよびｖＥ-40℃ ≧２００Ｊであり、溶接部の靭性が、ｖＥ-40℃ ≧１５０Ｊ、ＣＴＯＤ値（δ−１０℃）の最小値が０．５ｍｍ以上、また、強度、靭性について、ＰＷＨＴ前後の変化が、ΔＴＳが５〜−１５ＭＰａ、ΔｖＴｒｓが１０〜−５℃の範囲を満足する鋼板。
記
Ｃｅｑ＝［Ｃ］＋［Ｍｎ］／６＋（［Ｃｕ］＋［Ｎｉ］）／１５＋（［Ｃｒ］＋［Ｍｏ］＋［Ｖ］）／ ５・・・（１）
１．５≦［Ｔｉ］／［Ｎ］≦４．０ ・・・（２）
０＜｛［Ｃａ］−（０．１８＋１３０×［Ｃａ］）×［Ｏ］｝／１．２５／［Ｓ］＜１．５ ・・・（３）
５．５［Ｃ］(4/3)＋１５［Ｐ］＋０．９０［Ｍｎ］＋０．１２［Ｎｉ］＋７．９［Ｎｂ］(1/2)＋０．５３［Ｍｏ］ ≦３．７０ ・・・（４）
但し、［Ｍ］は、鋼板中の元素Ｍの含有量（質量％）を表す。 % By mass
C: 0.020 to 0.090%
Si: 0.01 to 0.35%
Mn: 1.40 to 2.00%
P: 0.008% or less S: 0.0035% or less Al: 0.010 to 0.060%
Ni: 0.40 to 2.00%
Mo: 0.05 to 0.50%
Nb: 0.005 to 0.040%
Ti: 0.005-0.025%
N: 0.0020 to 0.0050%
Ca: 0.0005 to 0.0050% and O: 0.0035% or less, and Ceq defined by the following formula (1) is in the range of 0.420 to 0.520%, and the following (2 ), (3) and (4) are satisfied, B is suppressed to less than 0.0003%, and the balance is a steel plate component composed of Fe and inevitable impurities,
Ti, Nb, and Mo are changed into Ti amount ([Ti]), Nb amount ([Nb]), and Mo amount ([Mo]), [Nb] / ([Ti] + [Nb] + [Mo]) ≧ 0.3 in the range that satisfies the relationship of 0.3 , and having a precipitate having a particle size of 2 nm or more and having an average particle size of 20 nm or less calculated as a measurement target; 460 MPa, TS ≧ 570 MPa and vE −40 ° C. ≧ 200 J, weld toughness is vE −40 ° C. ≧ 150 J, minimum value of CTOD value (δ−10 ° C.) is 0.5 mm or more, strength, toughness In the steel sheet, the change before and after PWHT satisfies the ranges of ΔTS of 5 to −15 MPa and ΔvTrs of 10 to −5 ° C.
Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5 (1)
1.5 ≦ [Ti] / [N] ≦ 4.0 (2)
0 <{[Ca] − (0.18 + 130 × [Ca]) × [O]} / 1.25 / [S] <1.5 (3)
5.5 [C] (4/3) +15 [P] +0.90 [Mn] +0.12 [Ni] +7.9 [Nb] (1/2) +0.53 [Mo] ≦ 3.70・ (4)
However, [M] represents content (mass%) of the element M in a steel plate. 前記鋼板成分に、さらに質量％で、Ｃｕ：０．７％未満、Ｃｒ：０．１〜１．０％およびＶ：０．００５〜０．０５％のうちから選ばれる１種または２種以上を含有する請求項１に記載の鋼板。   One or more selected from the steel sheet components, further, by mass, Cu: less than 0.7%, Cr: 0.1-1.0%, and V: 0.005-0.05% The steel plate according to claim 1 containing. 前記鋼板成分に、さらに質量％で、Ｍｇ：０．０００２〜０．００５０％およびＲＥＭ：０．００１０〜０．０２００％のうちから選ばれる１種または２種を含有する請求項１または２に記載の鋼板。   The steel sheet component further includes one or two kinds selected from Mg: 0.0002 to 0.0050% and REM: 0.0010 to 0.0200% in mass%. The described steel sheet. 請求項１〜３のいずれか１項に記載の鋼板を製造する方法であって、請求項１〜３のいずれか１項に記載の鋼板成分を有する鋼に、９５０〜１１５０℃に加熱後、９００℃以上の温度域における累積圧下率が３０％以上、９００℃未満の温度域における累積圧下率が３０〜７０％となる熱間圧延を施し、その後、少なくとも５００℃までを冷却速度１．０℃／ｓ以上で冷却する鋼板の製造方法。   It is the method of manufacturing the steel plate of any one of Claims 1-3, Comprising: After heating to 950-1150 degreeC to the steel which has the steel plate component of any one of Claims 1-3, Hot rolling is performed so that the cumulative rolling reduction in a temperature range of 900 ° C. or higher is 30% or more and the cumulative rolling reduction in a temperature range of less than 900 ° C. is 30 to 70%. A method of manufacturing a steel sheet that is cooled at a temperature of ° C / s or higher. 前記冷却後、さらに４５０〜６５０℃で焼戻し処理を施す請求項４に記載の鋼板の製造方法。   The manufacturing method of the steel plate of Claim 4 which further performs a tempering process at 450-650 degreeC after the said cooling.
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