JP6226542B2 - Steel with excellent toughness in weld heat affected zone - Google Patents

Steel with excellent toughness in weld heat affected zone Download PDF

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JP6226542B2
JP6226542B2 JP2013060452A JP2013060452A JP6226542B2 JP 6226542 B2 JP6226542 B2 JP 6226542B2 JP 2013060452 A JP2013060452 A JP 2013060452A JP 2013060452 A JP2013060452 A JP 2013060452A JP 6226542 B2 JP6226542 B2 JP 6226542B2
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steel
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崇 杉谷
崇 杉谷
正樹 島本
正樹 島本
秀徳 名古
秀徳 名古
木村　世意
世意 木村
進佑 佐藤
進佑 佐藤
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Kobe Steel Ltd
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Priority to PCT/JP2014/057205 priority patent/WO2014148447A1/en
Priority to CN201480016899.4A priority patent/CN105051229B/en
Priority to KR1020157025566A priority patent/KR101718275B1/en
Priority to EP14767775.1A priority patent/EP2977479B1/en
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Description

本発明は、造船、建築、橋梁等の溶接構造物用として好適な厚鋼板に関し、特に大入熱溶接であっても優れたＨＡＺ靭性を実現する鋼材に関する。   The present invention relates to a thick steel plate suitable for use in welded structures such as shipbuilding, construction, and bridges, and more particularly to a steel material that realizes excellent HAZ toughness even in high heat input welding.

近年、橋梁、高層建造物、および大型船舶等、鋼材を用いた構造物は大型化する傾向にあり、この大型構造物を実現するためには、鋼材の高強度化・厚肉化が望まれている。これと共に、大型構造物の施工効率の向上と施工コストの低減を目的として、高強度・厚肉の鋼材を溶接する際の溶接効率の向上が求められている。
ところで、鋼材の溶接効率を向上させるには、同一箇所に対する溶接回数を減らすことが有効であり、小さな熱量（溶接熱）で鋼材に対して溶接を複数回繰り返すよりも、鋼材に対して大きな熱量（溶接熱）を加える大入熱の溶接を行って１回で溶接を完了させる大入熱で高能率の溶接が指向される。 In recent years, structures using steel materials, such as bridges, high-rise buildings, and large ships, have tended to increase in size. To realize such large structures, it is desirable to increase the strength and thickness of steel materials. ing. At the same time, for the purpose of improving the construction efficiency of large structures and reducing the construction cost, there is a demand for improvement in welding efficiency when welding high-strength and thick steel materials.
By the way, in order to improve the welding efficiency of steel materials, it is effective to reduce the number of times of welding to the same location, and a large amount of heat is applied to the steel material rather than repeating the welding to the steel material several times with a small amount of heat (welding heat). High-efficiency welding with high heat input that completes welding at a time by performing welding with high heat input to add (welding heat) is directed.

しかし、一般的に、熱量の大小を問わず溶接熱に晒される溶接熱影響部（以下、ＨＡＺともいう）は、溶接時に高温となって鋼材の結晶粒が粗大化しやすい。加えて、鋼材への入熱量が増大するに従って溶接熱影響部が高温となり冷却時間が長くなる。高温かつ長い冷却時間は、溶接熱影響部における脆弱な上部ベイナイト組織の形成や、島状マルテンサイト等の脆化組織の形成を促進する条件となるため、鋼材のＨＡＺ靭性を低下させる原因となることはすでに知られている。   However, in general, a weld heat affected zone (hereinafter also referred to as HAZ) that is exposed to welding heat regardless of the amount of heat becomes high temperature during welding, and the crystal grains of the steel material tend to be coarse. In addition, as the amount of heat input to the steel material increases, the weld heat affected zone becomes higher in temperature and the cooling time becomes longer. A high temperature and a long cooling time are conditions that promote the formation of a brittle upper bainite structure in the heat affected zone and the formation of an embrittled structure such as island martensite, and thus reduce the HAZ toughness of the steel material. That is already known.

上述のような溶接熱に起因するＨＡＺ靭性の低下に対処するために、特許文献１〜５に開示される技術が提案されている。
特許文献１は、溶接熱影響部靭性（ＨＡＺ靭性）に優れた鋼材および製造方法を提供することを目的としたものである。具体的に、特許文献１に開示の製造方法は、溶鋼中で強い硫化物生成能を持つＣａに加えてＭｇおよび／またはＲＥＭを添加し、微細酸化物を生成させることで微細硫化物を分散させて１４００℃以上に加熱されたＨＡＺ組織を細粒化し、２００ｋＪ／ｃｍ以上の大入熱溶接でも良好なＨＡＺ靭性を実現するとされている。 In order to cope with the decrease in the HAZ toughness caused by the welding heat as described above, techniques disclosed in Patent Documents 1 to 5 have been proposed.
Patent document 1 aims at providing the steel material excellent in the welding heat affected zone toughness (HAZ toughness), and a manufacturing method. Specifically, the manufacturing method disclosed in Patent Document 1 disperses fine sulfides by adding Mg and / or REM in addition to Ca having strong sulfide-forming ability in molten steel to produce fine oxides. The HAZ structure heated to 1400 ° C. or higher is refined and good HAZ toughness is achieved even with high heat input welding of 200 kJ / cm or higher.

特許文献２は、母材靭性と溶接部ＨＡＺ靭性に優れた高強度溶接構造用鋼およびその製造方法を提供することを目的としたものである。具体的に、特許文献２に開示の製造方法は、Ｔｉ添加後にＭｇ、Ｃａ、ＲＥＭの１種又は２種以上を添加するか、あるいはＴｉ添加とＭｇ、Ｃａ、ＲＥＭの１種又は２種以上とを同時に添加して酸化物や硫化物を微細分散させることで母材の加熱γ粒径を微細化し、さらに溶接入熱に関わらずＨＡＺの加熱γ粒径も微細化することを意図している。特許文献２は、この二つの微細化による効果として、良好な母材靭性と溶接部ＨＡＺ靱性を有する高強度溶接構造用鋼を製造可能であるとされている。   Patent Document 2 aims to provide a high-strength welded structural steel excellent in base metal toughness and welded portion HAZ toughness and a method for producing the same. Specifically, the manufacturing method disclosed in Patent Document 2 includes adding one or more of Mg, Ca, and REM after adding Ti, or adding one or more of Ti, Mg, Ca, and REM. Is intended to refine the heated γ grain size of the base material by finely dispersing oxides and sulfides at the same time, and to further refine the heated γ grain size of HAZ regardless of welding heat input. Yes. In Patent Document 2, as an effect of these two miniaturizations, high strength welded structural steel having good base metal toughness and welded portion HAZ toughness can be manufactured.

特許文献３は、超大入熱溶接の溶接熱影響部靭性に優れた厚鋼板およびその製造方法を提供することを目的としたものである。具体的に、特許文献３の製造方法は、溶鋼中での酸化物、硫化物等の粒子組成の調整に加えて、さらに凝固過程で形成されるデンドライトの形態制御を行う。これにより特許文献３は、鋼板中における分散粒子を、従来に比べて均一かつ微細に分散させて、入熱量が３００ｋＪ／ｃｍ以上となる超大入熱溶接時のＨＡＺにおいても、オーステナイト粒を微細化しＨＡＺ靭性を顕向上させることができるとされている。   Patent document 3 aims at providing the thick steel plate excellent in the welding heat affected zone toughness of super-high heat input welding, and its manufacturing method. Specifically, the production method of Patent Document 3 controls the form of dendrites formed in the solidification process in addition to adjusting the particle composition of oxides, sulfides, etc. in the molten steel. As a result, Patent Document 3 disperses dispersed particles in a steel sheet more uniformly and finely than in the past, and refines austenite grains even in HAZ during super-high heat input welding where the heat input is 300 kJ / cm or more. It is said that the HAZ toughness can be significantly improved.

特許文献４は、良好なＨＡＺ靭性を有するＡＰＩ規格Ｘ１００以上の高強度鋼板を提供することを目的としたものである。具体的に、特許文献４の高強度鋼板は、Ｔｉ，Ｍｇ，ＲＥＭ，Ａｌ，Ｓ，Ｎ量を限定することにより、（１）０.１μｍ以下のＭｇ系酸化物を含んだＴｉＮ系微細析出物を含有させて、溶融線近傍においてもγ粒の粗大化を抑制する。さらに、この高強度鋼板は、（２）０.１μｍ以上のＴｉ、Ｍｇ、ＲＥＭを主体とする酸化物とＭｎＳとの複合体を含有させて比較的小さなγ粒内からＩＧＦを生成させることで、ＨＡＺ全域にわたって組織を微細化しＨＡＺ靱性を向上させることができるとされている。   Patent Document 4 aims to provide a high-strength steel sheet of API standard X100 or higher having good HAZ toughness. Specifically, the high-strength steel sheet of Patent Document 4 is limited to the amount of Ti, Mg, REM, Al, S, and N, (1) TiN-based fine precipitation containing Mg-based oxides of 0.1 μm or less. The inclusion of the material suppresses the coarsening of γ grains even in the vicinity of the melting line. Furthermore, this high-strength steel sheet (2) contains a complex of an oxide mainly composed of Ti, Mg, and REM of 0.1 μm or more and MnS to generate IGF from a relatively small γ grain. It is said that the HAZ toughness can be improved by refining the structure throughout the entire HAZ.

特許文献５は、良好な母材靱性および溶接熱影響部靱性を兼ね備えた非調質高張力鋼材を提案することを目的としたものである。具体的に、特許文献５の非調質高張力鋼材は、最適な酸化物系介在物の最適組成範囲として、Ｔｉ酸化物：２０〜９０重量％、ＣａＯおよびＲＥＭ酸化物の合計：５〜５０重量％、Ａｌ２Ｏ３：７０重量％以下に制御する。これによって、非調質高張力鋼材は、ノズル詰まりや有害な介在物クラスターの生成を引き起こすことなく介在物の結晶粒粗大化抑制能（ピン止め効果）を有効に利用できるので、溶接熱影響部靭性を向上させることができ、さらに、ＴｉＮ、あるいはさらにＶＮを最適分散させることにより母材の靭性と強度を向上させることができるとされている。 Patent Document 5 aims to propose a non-tempered high-tensile steel material having both good base material toughness and weld heat-affected zone toughness. Specifically, the non-tempered high-tensile steel material of Patent Document 5 has an optimal composition range of the oxide-based inclusions as follows: Ti oxide: 20 to 90 wt%, CaO and REM oxide total: 5 to 50 Weight%, Al 2 O 3 : Controlled to 70% by weight or less. As a result, non-tempered high-tensile steel can effectively utilize the grain coarsening suppression ability (pinning effect) of inclusions without causing nozzle clogging or generation of harmful inclusion clusters. It is said that the toughness can be improved, and further, the toughness and strength of the base material can be improved by optimally dispersing TiN or VN.

特許第４２６１９６８号公報Japanese Patent No. 4261968 特許第４７６２４５０号公報Japanese Patent No. 4762450 特許第４０３９２２３号公報Japanese Patent No. 4039223 特開平１１−２６４０４８号公報Japanese Patent Laid-Open No. 11-264048 特許第４１４４１２１号公報Japanese Patent No. 4144121

上述のように、特許文献１〜５のそれぞれは、溶接熱に起因するＨＡＺ靭性の低下に対処することができると開示しているが、いずれの技術によっても、更なる大入熱溶接時においてＨＡＺ靭性を向上させることは困難である。
特許文献１〜３に開示の技術は、酸硫化物のピンニング効果によってＨＡＺ組織の微細化を図るものであるが、酸化物起因の粒内変態による組織微細化効果については言及しておらず、更なる大入熱化に対応できる技術であるとはいえない。 As described above, each of Patent Documents 1 to 5 discloses that it is possible to cope with a decrease in the HAZ toughness caused by welding heat. It is difficult to improve the HAZ toughness.
The techniques disclosed in Patent Documents 1 to 3 are intended to refine the HAZ structure by the pinning effect of oxysulfides, but do not mention the structure refinement effect due to intragranular transformation caused by oxides. It cannot be said that it is a technology that can cope with further increase in heat input.

また、特許文献４では、酸化物を起点とした組織変態に言及しているが、粗大酸化物等に対する対策が示されていないので、粗大酸化物の生成によってＨＡＺ靭性が低下する可能性を排除することができず、更なる大入熱化に対応できる技術であるとはいえない。
さらに、特許文献５に開示の技術は、酸硫化物のピン止め効果によってＨＡＺ組織を微細化する技術であるが、酸硫化物を起点とする組織変態制御を考慮した技術ではないので、更なる大入熱化に対応できる技術であるとはいえない。 In addition, Patent Document 4 mentions a structural transformation starting from an oxide, but does not show a countermeasure against the coarse oxide and the like, and eliminates the possibility that the HAZ toughness is lowered due to the formation of the coarse oxide. It cannot be said that it is a technology that can cope with further increase in heat input.
Furthermore, the technique disclosed in Patent Document 5 is a technique for refining the HAZ structure by the pinning effect of oxysulfide, but it is not a technique that takes account of the structure transformation control starting from oxysulfide. It cannot be said that this technology can cope with large heat input.

本発明は、上述の問題に鑑みてなされたものであり、大入熱溶接時における溶接熱影響部の靭性（ＨＡＺ靭性）に優れた鋼材を提供することを目的とする。   This invention is made | formed in view of the above-mentioned problem, and it aims at providing the steel material excellent in the toughness (HAZ toughness) of the welding heat affected zone at the time of high heat input welding.

本発明は、上記目的を達成するために、次の手段を講じた。
即ち、本発明における課題解決のための技術的手段は、Ｃ ：０.０２〜０.１３％（質量％（mass%）の意味。以下成分について同じ。）、Ｓｉ：０.０５〜０.５％以下、Ｍｎ：１.０〜２.５％、Ｐ ：０.０３％以下（０％を含まない）、Ｓ ：０.０１％以下（０％を含まない）、Ａｌ：０.００２〜０.０４０％以下、Ｔｉ：０.００５〜０.０４０％、Ｚｒ：０.０００３〜０.０２０％、ＲＥＭ：０.０００３〜０.０２０％、Ｃａ：０.０００３〜０.００８０％、Ｎ ：０.００３０〜０.０１０％以下（０％を含まない）、Ｏ ：０.０００３〜０.００５０％を含有し、残部が鉄および不可避不純物からなる鋼材であって、前記鋼材は、ＲＥＭ、Ｚｒ、Ｔｉ、Ａｌ、ＣａおよびＳを含有する複合酸化物を含み、前記鋼材中の複合酸化物について、円相当直径で３μｍ超の酸化物が１ｍｍ２あたり５.０個以下であって、かつ円相当直径が０.１〜３μｍの複合酸化物の全てが下記式（１）を満たすと共に、前記円相当直径が０．１〜３μｍの複合酸化物個数が１００個／ｍｍ２以上であって、さらに、下記式（１）を満たす０.１〜３μｍの全ての複合酸化物の平均組成が、Ａｌ２Ｏ３：２０％以下、ＴｉＯ２：３〜２０％、ＺｒＯ２：５〜５０％、ＲＥＭ酸化物：５〜５０％、ＣａＯ：５〜５０％、Ｓ：１〜１５％であることを特徴とする。 In order to achieve the above object, the present invention has taken the following measures.
That is, technical means for solving the problems in the present invention are: C: 0.02 to 0.13% (meaning mass%), Si: 0.05 to 0.00. 5% or less, Mn: 1.0 to 2.5%, P: 0.03% or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0.002 ~ 0.040% or less, Ti: 0.005 to 0.040%, Zr: 0.0003 to 0.020%, REM: 0.0003 to 0.020%, Ca: 0.0003 to 0.0080% , N: 0.0003 to 0.010% or less (excluding 0%), O: 0.0003 to 0.0050%, the balance being iron and inevitable impurities, wherein the steel is , REM, Zr, Ti, Al, Ca, and a composite oxide containing S, and for the composite oxide in the steel material, the equivalent circle diameter A is 3μm greater oxides of 1 mm 2 per 5.0 or less, and all equivalent circle diameter of the composite oxide of 0.1~3μm along with satisfying the following formula (1), the circle equivalent diameter 0 The number of composite oxides of 0.1 to 3 μm is 100 / mm 2 or more, and the average composition of all the composite oxides of 0.1 to 3 μm satisfying the following formula (1) is Al 2 O 3 : than 20%, TiO 2: 3~20%, ZrO 2: 5~50%, REM oxides: 5~50%, CaO: 5~50% , S: characterized in that 1 to 15%.

0.008 ≦ (1/d)×｛mass%Ｓ／(mass%ＣａＯ＋mass%ＲＥＭ２Ｏ３)｝ ≦ 0.289 ・・・（１）
（但し、dは個々の複合酸化物の円相当直径であって、０.１〜３μｍである）
ここで、Ｎｉ：０.０５〜１.５０％、Ｃｕ：０.０５〜１.５０％、Ｃｒ：０.０５〜１.５０％、Ｍｏ：０.０５〜１.５０％のうち、少なくとも１種を含有するとよい。 0.008 ≦ (1 / d) × {mass% S / (mass% CaO + mass% REM 2 O 3 )} ≦ 0.289 (1)
(Where d is the equivalent circle diameter of each composite oxide and is 0.1 to 3 μm)
Here, at least among Ni: 0.05 to 1.50%, Cu: 0.05 to 1.50%, Cr: 0.05 to 1.50%, Mo: 0.05 to 1.50% It is good to contain 1 type.

また、Ｎｂ：０.００２〜０.１０％、Ｖ：０.００２〜０.１０％のうち少なくともいずれか一方を含有するとよい。
さらに、Ｂ：０.０００５〜０.００５０％を含有するとよい。 Moreover, it is good to contain at least any one among Nb: 0.002-0.10% and V: 0.002-0.10%.
Furthermore, it is good to contain B: 0.0005-0.0050%.

本発明によれば、大入熱溶接時における溶接熱影響部の靭性（ＨＡＺ靭性）に優れた鋼材を得ることができる。   ADVANTAGE OF THE INVENTION According to this invention, the steel material excellent in the toughness (HAZ toughness) of the welding heat affected zone at the time of high heat input welding can be obtained.

本発明の実施形態による鋼材が有するＨＡＺ靭性値の測定結果を示す図である。It is a figure which shows the measurement result of the HAZ toughness value which the steel materials by embodiment of this invention have. 本実施形態による鋼材が有するＨＡＺ靭性値の測定結果を示す図である。It is a figure which shows the measurement result of the HAZ toughness value which the steel materials by this embodiment have. 本実施形態による鋼材が有するＨＡＺ靭性値の測定結果を示す図である。It is a figure which shows the measurement result of the HAZ toughness value which the steel materials by this embodiment have.

以下に、図面を参照しながら、本願発明の実施形態による溶接熱影響部の靭性に優れた鋼材（以下、単に鋼材という）について詳細に説明する。
本実施形態による鋼材は、例えば溶接入熱量が６０ｋＪ／ｍｍを超えるような、非常に大きな溶接エネルギーの影響を受けた溶接熱影響部（ＨＡＺ,Heat Affected Zone）において優れた靭性を発揮する鋼材である。以下の説明において、本実施形態による鋼材の溶接熱影響部をＨＡＺと記し、ＨＡＺにおける靭性をＨＡＺ靭性と記す。 Hereinafter, a steel material (hereinafter simply referred to as a steel material) excellent in toughness of a weld heat affected zone according to an embodiment of the present invention will be described in detail with reference to the drawings.
The steel material according to the present embodiment is a steel material that exhibits excellent toughness in a weld heat affected zone (HAZ, Heat Affected Zone) affected by a very large welding energy, for example, a welding heat input exceeding 60 kJ / mm. is there. In the following description, the welding heat-affected zone of the steel material according to the present embodiment is denoted as HAZ, and the toughness in the HAZ is denoted as HAZ toughness.

本実施形態による鋼材は、粒内変態の核となる複合酸化物（Ａｌ，Ｔｉ，Ｚｒ，ＲＥＭ，ＣａおよびＳを含有する酸硫化物）を、そのサイズとＳ濃度を適切に制御して所定量生成させることによって、大入熱溶接においても良好なＨＡＺ靭性を安定的に実現できる。具体的には、ＨＡＺ靭性向上に悪影響を及ぼす円相当直径が３μｍを超える粗大な複合酸化物の個数を有意に抑制すると共に、ＨＡＺ靭性向上に有用な円相当直径が０.１〜３μｍで組成と粒度が適切に制御された複合酸化物の個数を所定量以上含有することを特徴とする。この特徴によって、本実施形態による鋼材は、大きな入熱量で溶接を行っても安定的に優れたＨＡＺ靭性を発揮することができる。   The steel material according to the present embodiment is a composite oxide (an oxysulfide containing Al, Ti, Zr, REM, Ca, and S) that serves as a nucleus of intragranular transformation, with its size and S concentration being appropriately controlled. By generating a constant amount, good HAZ toughness can be stably realized even in high heat input welding. Specifically, the number of coarse composite oxides having a circle equivalent diameter exceeding 3 μm that adversely affects the improvement of HAZ toughness is significantly suppressed, and the circle equivalent diameter useful for improving HAZ toughness is 0.1 to 3 μm. And a predetermined amount or more of complex oxides whose particle sizes are appropriately controlled. With this feature, the steel material according to the present embodiment can stably exhibit excellent HAZ toughness even when welding is performed with a large heat input.

上述の特徴を有する本実施形態による鋼材は、例えば、溶鋼の二次精錬において、以下に説明する化学成分組成となるように各元素を添加することで得られる。
本実施形態による鋼材（以下、単に本鋼材という）は、炭素Ｃを０.０２〜０.１３％、ケイ素Ｓｉを０.０５〜０.５％以下、マンガンＭｎを１.０〜２.５％、リンＰを０.０３％以下（０％を含まない）、硫黄Ｓを０.０１％以下（０％を含まない）、アルミニウムＡｌを０.００２〜０.０４０％以下、チタンＴｉを０.００５〜０.０４０％、ジルコニウムＺｒを０.０００３〜０.０２０％、希土類金属ＲＥＭを０.０００３〜０.０２０％、カルシウムＣａを０.０００３〜０.００８０％、窒素Ｎを０.００３０〜０.０１０％以下（０％を含まない）、酸素Ｏを０.０００３〜０.００５０％を含有し、残部が鉄および不可避不純物からなる。さらに、本鋼材は、ＲＥＭ、Ｚｒ、Ｔｉ、Ａｌ、ＣａおよびＳを含有する複合酸化物を含み、本鋼材中の複合酸化物について、円相当直径で３μｍ超の酸化物が１ｍｍ２あたり５.０個以下であって、かつ円相当直径が０．１〜３μｍの複合酸化物について、下記式（１）を満たす複合酸化物の個数が１００個／ｍｍ２以上である。 The steel material by this embodiment which has the above-mentioned characteristic is obtained by adding each element so that it may become the chemical composition composition explained below in the secondary refining of molten steel, for example.
The steel material according to the present embodiment (hereinafter simply referred to as the present steel material) has carbon C of 0.02 to 0.13%, silicon Si of 0.05 to 0.5% and manganese Mn of 1.0 to 2.5. %, Phosphorus P 0.03% or less (excluding 0%), sulfur S 0.01% or less (excluding 0%), aluminum Al 0.002 to 0.040% or less, titanium Ti 0.005 to 0.040%, zirconium Zr 0.0003 to 0.020%, rare earth metal REM 0.0003 to 0.020%, calcium Ca 0.0003 to 0.0008%, nitrogen N 0 .0030 to 0.010% or less (excluding 0%), oxygen O is contained in 0.0003 to 0.0050%, and the balance is composed of iron and inevitable impurities. Further, the steel material includes a composite oxide containing REM, Zr, Ti, Al, Ca, and S. As for the composite oxide in the steel material, an oxide having an equivalent circle diameter of more than 3 μm is 5 per 1 mm 2 . For the composite oxide having 0 or less and an equivalent circle diameter of 0.1 to 3 μm, the number of composite oxides satisfying the following formula (1) is 100 / mm 2 or more.

0.008 ≦ (1/d)×｛mass%Ｓ／(mass%ＣａＯ＋mass%ＲＥＭ２Ｏ３)｝ ≦ 0.289 ・・・（１）
（但し、dは個々の複合酸化物の円相当直径であって、０.１〜３μｍである）
さらに、上記式（１）を満たす０.１〜３μｍの複合酸化物の平均組成は、Ａｌ２Ｏ３が２０％以下、ＴｉＯ２が３〜２０％、ＺｒＯ２が５〜５０％、ＲＥＭ酸化物が５〜５０％、ＣａＯが５〜５０％、Ｓが１〜１５％である。 0.008 ≦ (1 / d) × {mass% S / (mass% CaO + mass% REM 2 O 3 )} ≦ 0.289 (1)
(Where d is the equivalent circle diameter of each composite oxide and is 0.1 to 3 μm)
Furthermore, the average composition of the composite oxide of 0.1 to 3 μm satisfying the above formula (1) is 20% or less for Al 2 O 3, 3 to 20% for TiO 2 , 5 to 50% for ZrO 2 , REM oxidation The product is 5 to 50%, CaO is 5 to 50%, and S is 1 to 15%.

本実施形態において、元素および成分の含有量を単に百分率「％」を用いて記載しているが、これは、質量百分率「質量％（mass%）」を簡略化して記載したものであることに留意されたい。
続いて、上述した本鋼材の構成について詳細に説明する。
［炭素Ｃ：０.０２〜０.１３％］
炭素Ｃは、鋼材（母材）の強度を確保するために欠くことのできない元素である。しかし、Ｃの含有量が０.１３％を超えると、ＨＡＺに島状マルテンサイト（ＭＡ）が多く生成してＨＡＺの靱性低下を招くばかりでなく、ＣＯガスの発生などに起因して溶接性にも悪影響を及ぼす。従って、Ｃの含有量を０.０２質量％以上０.１３質量％以下に規定する。好ましくは０.０４質量％以上０.１質量％以下である。 In the present embodiment, the contents of elements and components are simply described using the percentage “%”, but this is a simplified description of the mass percentage “mass%”. Please keep in mind.
Then, the structure of this steel material mentioned above is demonstrated in detail.
[Carbon C: 0.02 to 0.13%]
Carbon C is an element indispensable for securing the strength of the steel material (base material). However, if the content of C exceeds 0.13%, not only does island martensite (MA) form in the HAZ, leading to a reduction in the toughness of the HAZ, but also weldability due to the generation of CO gas, etc. It also has an adverse effect. Therefore, the C content is specified to be 0.02 mass% or more and 0.13 mass% or less. Preferably they are 0.04 mass% or more and 0.1 mass% or less.

［ケイ素Ｓｉ：０.０５〜０.５％］
ケイ素Ｓｉは、脱酸作用を有すると共に、固溶強化により母材の強度向上に寄与する元素である。しかし、Ｓｉの含有量が０.５％を超えると、鋼材の溶接性や靱性が低下するので、Ｓｉの含有量の上限を０.５％とする。特にＨＡＺ靱性を高めるには、Ｓｉの含有量を０.３％以下とすることが推奨される。但し、Ｓｉの含有量を抑えるほどＨＡＺ靱性が向上するが、その一方で鋼材の強度が低下する場合がある。従って、Ｓｉの含有量の下限を０.０５％とし、Ｓｉの含有量を０.０５質量％以上０.５質量％以下に規定する。好ましくは０.０７質量％以上０.３５質量％以下、より好ましくは０.１質量％以上０.２５質量％以下である。 [Silicon Si: 0.05-0.5%]
Silicon Si is an element that has a deoxidizing action and contributes to improving the strength of the base material by solid solution strengthening. However, if the Si content exceeds 0.5%, the weldability and toughness of the steel material deteriorate, so the upper limit of the Si content is set to 0.5%. In particular, in order to increase the HAZ toughness, it is recommended that the Si content be 0.3% or less. However, as the Si content is reduced, the HAZ toughness is improved, while the strength of the steel material may be reduced. Therefore, the lower limit of the Si content is set to 0.05%, and the Si content is specified to be 0.05% by mass or more and 0.5% by mass or less. Preferably they are 0.07 mass% or more and 0.35 mass% or less, More preferably, they are 0.1 mass% or more and 0.25 mass% or less.

［マンガンＭｎ：１.０〜２.５％］
マンガンＭｎは、母材の強度向上に寄与する元素である。しかし、Ｍｎの含有量が２.５％を超えると、母材の溶接性が低下する。さらに、Ｍｎの含有量が１.０％を下回ると強度が低下してしまうため、Ｍｎの含有量を１.０質量％以上２.５質量％以下に規定する。好ましくは１.３質量％以上２.０質量％以下である。 [Manganese Mn: 1.0 to 2.5%]
Manganese Mn is an element that contributes to improving the strength of the base material. However, if the Mn content exceeds 2.5%, the weldability of the base material is degraded. Furthermore, since intensity | strength will fall when content of Mn is less than 1.0%, content of Mn is prescribed | regulated to 1.0 mass% or more and 2.5 mass% or less. Preferably they are 1.3 mass% or more and 2.0 mass% or less.

［リンＰ：０.０３％以下］
リンＰは、偏析し易い元素であり、特に鋼材中の結晶粒界に偏析してＨＡＺ靱性を低下させる元素である。Ｐは、通常、母材に不可避的に０.００１％程度含有されているので、Ｐの含有量を０.０３質量％以下に規定する。好ましくは０.０２質量％以下、より好ましくは０.０１質量％以下である。但し、本実施形態は、Ｐの含有量が０質量％である場合を含まない。 [Phosphorus P: 0.03% or less]
Phosphorus P is an element that easily segregates, and is an element that segregates at a grain boundary in a steel material and lowers the HAZ toughness. Usually, P is unavoidably contained in the base material in an amount of about 0.001%, so the content of P is specified to be 0.03% by mass or less. Preferably it is 0.02 mass% or less, More preferably, it is 0.01 mass% or less. However, this embodiment does not include the case where the P content is 0% by mass.

［硫黄Ｓ：０.０１％以下］
硫黄Ｓは、Ｍｎと結合して硫化物（ＭｎＳ）を生成し、母材の靱性や板厚方向における延性を低下させる元素である。例えば、Ｓが、ランタンＬａやセリウムＣｅなどのＲＥＭと結合してＲＥＭの硫化物（例えば、ＬａＳやＣｅＳ）を生成すると、ＲＥＭ酸化物の生成が阻害されるため、ＨＡＺ靱性が低下する。しかし、Ｓは、通常、母材に不可避的に０.０００５％程度含有されているので、Ｓの含有量を０.０１質量％以下に規定する。好ましくは０.００８質量％以下、より好ましくは０.００６質量％以下である。但し、本実施形態は、Ｓの含有量が０質量％である場合を含まない。 [Sulfur S: 0.01% or less]
Sulfur S is an element that combines with Mn to produce sulfide (MnS) and lowers the toughness of the base material and the ductility in the plate thickness direction. For example, when S is combined with REM such as lanthanum La or cerium Ce to generate a REM sulfide (for example, LaS or CeS), generation of REM oxide is inhibited, so that HAZ toughness is reduced. However, since S is normally unavoidably contained in the base material in an amount of about 0.0005%, the S content is specified to be 0.01% by mass or less. Preferably it is 0.008 mass% or less, More preferably, it is 0.006 mass% or less. However, this embodiment does not include the case where the S content is 0% by mass.

［アルミニウムＡｌ：０.００２〜０.０４０％］
アルミニウムＡｌは、脱酸剤として作用する元素である。しかし、Ａｌを母材に対して過剰に添加すると、添加されたＡｌは母材中の酸化物を還元して粗大なＡｌ酸化物を形成するので、ＨＡＺ靱性が低下する。一方で、Ａｌの含有量が少ないと溶鋼が酸素で汚染されやすくなるため、Ａｌの含有量は０．００２％が下限である。従って、Ａｌの含有量を０.００２質量％以上０.０４０質量％以下に規定する。好ましくは０.００４質量％以上０.０２５質量％以下、より好ましくは０.００５質量％以上０.０１５質量％以下である。 [Aluminum Al: 0.002 to 0.040%]
Aluminum Al is an element that acts as a deoxidizer. However, when Al is added excessively with respect to the base material, the added Al reduces oxides in the base material to form coarse Al oxide, so that the HAZ toughness decreases. On the other hand, when the Al content is small, the molten steel is easily contaminated with oxygen, so the lower limit of the Al content is 0.002%. Therefore, the Al content is specified to be 0.002 mass% or more and 0.040 mass% or less. Preferably it is 0.004 mass% or more and 0.025 mass% or less, More preferably, it is 0.005 mass% or more and 0.015 mass% or less.

［チタンＴｉ：０.００５〜０.０４０％］
チタンＴｉは、母材中にＴｉＮなどの窒化物や、Ｔｉを含む酸化物を生成し、ＨＡＺ靱性の向上に寄与する元素である。しかし、Ｔｉを母材に対して過剰に添加すると、Ｔｉの固溶強化によって母材自体が硬化しＨＡＺ靱性の低下に繋がるため、Ｔｉの含有量を０.０４％以下に抑えなくてはならない。従って、Ｔｉの含有量を０.００５質量％以上０.０４０質量％以下に規定する。好ましくは０.０１０質量％以上０.０３０質量％以下、より好ましくは０.０１４質量％以上０.０２５質量％以下である。 [Titanium Ti: 0.005 to 0.040%]
Titanium Ti is an element that contributes to the improvement of HAZ toughness by generating a nitride such as TiN or an oxide containing Ti in the base material. However, if Ti is excessively added to the base material, the base material itself is hardened by solid solution strengthening of Ti and leads to a decrease in HAZ toughness. Therefore, the Ti content must be suppressed to 0.04% or less. . Therefore, the Ti content is specified to be 0.005% by mass or more and 0.040% by mass or less. Preferably it is 0.010 mass% or more and 0.030 mass% or less, More preferably, it is 0.014 mass% or more and 0.025 mass% or less.

［ジルコニウムＺｒ：０.０００３〜０.０２０％］
ジルコニウムＺｒは、Ｚｒを含む複合酸化物を生成してＨＡＺ靱性の向上に寄与する元素である。しかし、Ｚｒを母材に対して過剰に添加すると、粗大なＺｒ酸化物（ＺｒＯ２）が生成してＨＡＺ靱性が低下する。また、粗大なＺｒ炭化物（ＺｒＣ）が生成することで、母材自体の靱性が低下する。従って、Ｚｒの含有量を０.０００３質量％以上０.０２０質量％以下に規定する。好ましくは０.０００５質量％以上０.０１５質量％以下、よりに好ましくは０.０００９質量％以上０.０１０質量％以下である。 [Zirconium Zr: 0.0003-0.020%]
Zirconium Zr is an element that contributes to the improvement of HAZ toughness by generating a complex oxide containing Zr. However, when Zr is added excessively with respect to the base material, coarse Zr oxide (ZrO 2 ) is generated and HAZ toughness is lowered. Further, the formation of coarse Zr carbide (ZrC) reduces the toughness of the base material itself. Therefore, the Zr content is specified to be 0.0003 mass% or more and 0.020 mass% or less. Preferably it is 0.0005 mass% or more and 0.015 mass% or less, More preferably, it is 0.0009 mass% or more and 0.010 mass% or less.

［希土類金属ＲＥＭ：０.０００３〜０.０２０％］
ＲＥＭ（希土類元素）は、酸化物の生成に必要な元素である。ＲＥＭによるこれらの酸化物を含有することで、酸化物が鋼材中に微細分散し易くなる。この微細分散した酸化物が、ＨＡＺの粒内αの生成核となりＨＡＺ靱性の向上に寄与する。しかし、ＲＥＭを過剰に添加すると、固溶ＲＥＭが生成して母材内で偏析するので、母材自体の靱性が劣化する。従って、ＲＥＭの含有量を０.０００３質量％以上０.０２０質量％以下に規定する。好ましくは０.０００５質量％以上０.０１５質量％以下、よりに好ましくは０.０００９質量％以上０.０１０質量％以下である。 [Rare earth metal REM: 0.0003-0.020%]
REM (rare earth element) is an element necessary for generation of oxides. By containing these oxides by REM, the oxides are easily finely dispersed in the steel material. This finely dispersed oxide serves as a production nucleus for intra-grain α of HAZ and contributes to improvement of HAZ toughness. However, when REM is added excessively, solid solution REM is generated and segregates in the base material, so that the toughness of the base material itself deteriorates. Therefore, the content of REM is specified to be 0.0003 mass% or more and 0.020 mass% or less. Preferably it is 0.0005 mass% or more and 0.015 mass% or less, More preferably, it is 0.0009 mass% or more and 0.010 mass% or less.

具体的にＲＥＭとは、ランタノイド元素（ＬａからＬｎまでの１５元素）およびＳｃ（スカンジウム）とＹ（イットリウム）を含む元素を意味する。本実施形態では、これらの元素の中でもＬａ、ＣｅおよびＹよりなる群から選ばれる少なくとも１種の元素を含有することが好ましく、より好ましくはＬａおよび／またはＣｅを含有するとよい。
［カルシウムＣａ：０.０００３〜０.００８０％］
カルシウムＣａは、酸化物の生成に必要な元素である。Ｃａも、ＨＡＺの粒内αの生成核となりＨＡＺ靱性の向上に寄与するので、０．０００３％以上含有させるとよい。しかし、Ｃａを過剰に添加すると、粗大なＣａ硫化物が生成して母材の靱性が劣化する。従って、Ｃａの含有量を０.０００３質量％以上０.００８０質量％以下に規定する。好ましくは０.０００５質量％以上０.００６０質量％以下、より好ましくは０.０００７質量％以上０.００３０質量％以下である。 Specifically, REM means an element containing a lanthanoid element (15 elements from La to Ln) and Sc (scandium) and Y (yttrium). In this embodiment, it is preferable to contain at least one element selected from the group consisting of La, Ce and Y among these elements, and more preferably La and / or Ce.
[Calcium Ca: 0.0003-0.0080%]
Calcium Ca is an element necessary for the production of oxides. Ca also becomes a production nucleus of intra-grain α of HAZ and contributes to the improvement of HAZ toughness. However, when Ca is added excessively, coarse Ca sulfide is generated and the toughness of the base material is deteriorated. Therefore, the Ca content is specified to be 0.0003 mass% or more and 0.0008 mass% or less. Preferably it is 0.0005 mass% or more and 0.0006 mass% or less, More preferably, it is 0.0007 mass% or more and 0.0003 mass% or less.

［窒素Ｎ：０.００３０〜０.０１０％以下］
窒素Ｎは、窒化物（例えば、ＺｒＮやＴｉＮなど）を析出する元素である。窒化物は、ピン止め効果によって溶接時のオーステナイト粒の粗大化を抑制することで、ＨＡＺ靱性の向上に寄与する。Ｎは、含有量が多いほど窒化物を形成してオーステナイト粒の微細化を促進するため、ＨＡＺ靱性の向上に有効に作用する。しかし、Ｎの含有量が０．０１０％を超えると、固溶Ｎの量が増大して母材自体の靱性が劣化し、ＨＡＺ靱性も低下する。従って、Ｎの含有量を０.００３０質量％以上０.０１０質量％以下に規定する。好ましくは０.００４０質量％以上０.００９０質量％以下、より好ましくは０.００５０質量％以上０.００８０質量％以下である。 [Nitrogen N: 0.0003 to 0.010% or less]
Nitrogen N is an element that precipitates nitrides (eg, ZrN, TiN, etc.). Nitride contributes to the improvement of HAZ toughness by suppressing the austenite grain coarsening during welding by the pinning effect. Since N forms a nitride and promotes refinement of austenite grains as the content increases, it effectively works to improve HAZ toughness. However, if the N content exceeds 0.010%, the amount of solute N increases, the toughness of the base metal itself deteriorates, and the HAZ toughness also decreases. Therefore, the N content is specified to be 0.0030% by mass or more and 0.010% by mass or less. Preferably it is 0.0040 mass% or more and 0.0009 mass% or less, More preferably, it is 0.0050 mass% or more and 0.0008 mass% or less.

［酸素Ｏ：０.０００３〜０.００５０％］
酸素Ｏは、酸化物の生成に必須の元素であり、含有量が０．０００３％より少ないと、母材中に十分な量の酸化物が得られない。また、含有量が０．００５０％より多いと、酸化物の粗大化によりＨＡＺ靭性の低下を招く。従って、Ｏの含有量を０．０００３質量％以上０.００５０質量％以下に規定する。好ましくは０.００１０質量％以上０.００４０質量％以下、より好ましくは０.００１５質量％以上０.００３５質量％以下である。 [Oxygen O: 0.0003 to 0.0050%]
Oxygen O is an essential element for the formation of oxides. If the content is less than 0.0003%, a sufficient amount of oxide cannot be obtained in the base material. On the other hand, when the content is more than 0.0050%, the HAZ toughness is lowered due to the coarsening of the oxide. Therefore, the O content is specified to be 0.0003 mass% or more and 0.0050 mass% or less. Preferably it is 0.0010 mass% or more and 0.0040 mass% or less, More preferably, it is 0.0015 mass% or more and 0.0033 mass% or less.

ここでＯの含有量は、トータル酸素量を示し、母材中の酸化物を形成しているＯと、母材中に固溶しているフリーのＯの合計量を意味している。
本鋼材は、上述の各元素を含有し、残部が鉄および不可避不純物からなる。
上述の各元素以外の残部の成分は、鉄および不可避不純物（例えば、ＭｇやＡｓ，Ｓｅなど）である。 Here, the content of O represents the total amount of oxygen, and means the total amount of O forming an oxide in the base material and free O dissolved in the base material.
This steel material contains the above-mentioned elements, and the balance consists of iron and inevitable impurities.
The remaining components other than the above-described elements are iron and inevitable impurities (for example, Mg, As, Se, etc.).

さて、上述の元素を含有する本鋼材は、ＲＥＭ、Ｚｒ、Ｔｉ、Ａｌ、ＣａおよびＳを含有する複合酸化物（酸化物および／または酸硫化物）を含む。本鋼材に含有される複合酸化物は、Ａｌ、Ｔｉ、Ｚｒ、ＲＥＭ、およびＣａの酸化物と硫化物を含むＡｌ・Ｔｉ・Ｚｒ・ＲＥＭ・Ｃａ・Ｓ系複合酸化物であるが、これら以外に、例えば、Ｍｎ、Ｓｉなどの
元素や、その他の成分元素を含んでいても良い。Ａｌ・Ｔｉ・Ｚｒ・ＲＥＭ・Ｃａ系の酸化物は、鋼材との格子整合性がよく、ＨＡＺにおいて粒内組織変態（粒内変態）を促進するので、ＨＡＺの組織を微細化するのに有効である。 Now, the steel material containing the above-described element includes a composite oxide (oxide and / or oxysulfide) containing REM, Zr, Ti, Al, Ca, and S. The composite oxides contained in this steel are Al, Ti, Zr, REM, and Al / Ti / Zr / REM / Ca / S based composite oxides containing oxides and sulfides of Ca. In addition, for example, elements such as Mn and Si and other component elements may be included. Al, Ti, Zr, REM, and Ca-based oxides have good lattice matching with steel materials, and promote intragranular structure transformation (intragranular transformation) in HAZ, so they are effective in refining the HAZ structure. It is.

本鋼材は、上述の複合酸化物について、円相当直径で３μｍ超の酸化物の個数は、本鋼材の断面において１ｍｍ２あたり５.０個以下である。円相当直径で３μmを超える複合酸化物は粗大であるため、入熱量が６０ｋＪ／ｍｍに達するような大入熱溶接においては、ＨＡＺ靭性をかえって低下させる。そのため、３μmを超える複合酸化物の個数は５.０個／ｍｍ２以下に抑える必要がある。 In the present steel material, the number of oxides having an equivalent circle diameter of more than 3 μm in the above-described composite oxide is 5.0 or less per 1 mm 2 in the cross section of the steel material. Since the complex oxide having an equivalent circle diameter exceeding 3 μm is coarse, in high heat input welding in which the heat input reaches 60 kJ / mm, the HAZ toughness is reduced. Therefore, the number of composite oxides exceeding 3 μm needs to be suppressed to 5.0 pieces / mm 2 or less.

一方、本鋼材は、上述の複合酸化物が、円相当直径で０．１μｍ以上３μｍ以下（以下、０.１〜３μｍと表記する）の大きさであり、下記式（１）を満たし、かつその個数が本鋼材の断面において１００個／ｍｍ２以上存在するように含有している。
0.008 ≦ (1/d)×｛mass%Ｓ／(mass%ＣａＯ＋mass%ＲＥＭ２Ｏ３)｝ ≦ 0.289 ・・・（１）
（但し、dは個々の複合酸化物の円相当直径であって、０.１〜３μｍである）
この円相当直径で０.１〜３μｍの複合酸化物は、ＨＡＺにおいて粒内組織変態（粒内変態）を促進してＨＡＺ靭性を向上させるためのものであるため、以下に、円相当直径で０.１〜３μｍの複合酸化物について検討する。なお、円相当直径で０．１μｍ未満の複合酸化物は、ＨＡＺ靱性の向上に殆ど寄与しないため、上記複合酸化物の個数には含めない。 On the other hand, in the present steel material, the above-mentioned composite oxide has a circle equivalent diameter of 0.1 μm to 3 μm (hereinafter referred to as 0.1 to 3 μm), satisfies the following formula (1), and It is contained so that the number thereof is 100 / mm 2 or more in the cross section of the steel material.
0.008 ≦ (1 / d) × {mass% S / (mass% CaO + mass% REM 2 O 3 )} ≦ 0.289 (1)
(Where d is the equivalent circle diameter of each composite oxide and is 0.1 to 3 μm)
This composite oxide having an equivalent circle diameter of 0.1 to 3 μm is for promoting intragranular structure transformation (intragranular transformation) in HAZ and improving HAZ toughness. A composite oxide having a thickness of 0.1 to 3 μm will be examined. Note that a complex oxide having an equivalent circle diameter of less than 0.1 μm hardly contributes to the improvement of HAZ toughness, and thus is not included in the number of complex oxides.

以下、円相当直径で０.１〜３μｍの複合酸化物が式（１）を満たすべき理由について説明する。
まず、ＲＥＭとＣａは酸化物も硫化物も形成しうる酸硫化物形成元素である。そこで、酸硫化物形成元素（ＲＥＭ２Ｏ３とＣａＯ）の濃度に対するＳ濃度（mass%Ｓ）が高すぎると、過剰に生成される硫化物が、酸化物とマトリックスとの整合を阻害してしまうため、複合酸化物が組織制御に寄与する能力(粒内変態能）が低下する。また、酸硫化物形成元素（ＲＥＭ２Ｏ３とＣａＯ）の濃度に対するＳ濃度（mass%Ｓ）が低すぎる場合、硫化物生成に伴うひずみエネルギーが得られず粒内変態にとって不利となるため、粒内変態能が低下する。さらに、複合酸化物そのもののサイズ（複合酸化物の円相当直径ｄ）に起因するひずみエネルギーが粒内変態に影響を与える。 Hereinafter, the reason why a complex oxide having an equivalent circle diameter of 0.1 to 3 μm should satisfy the formula (1) will be described.
First, REM and Ca are oxysulfide-forming elements that can form oxides and sulfides. Therefore, if the S concentration (mass% S) with respect to the concentration of the oxysulfide forming elements (REM 2 O 3 and CaO) is too high, the excessively generated sulfide inhibits the matching between the oxide and the matrix. Therefore, the ability (intragranular transformation ability) of the composite oxide to contribute to the structure control is reduced. Also, if the S concentration (mass% S) relative to the concentration of oxysulfide forming elements (REM 2 O 3 and CaO) is too low, strain energy associated with sulfide formation cannot be obtained, which is disadvantageous for intragranular transformation. The intragranular transformation ability decreases. Furthermore, the strain energy resulting from the size of the composite oxide itself (equivalent circle diameter d of the composite oxide) affects the intragranular transformation.

式（１）では、これら粒内変態に影響を及ぼすと考えられる条件を考慮した第二項が示されている。ところで、この式（１）の第二項において、酸硫化物形成元素（ＲＥＭ２Ｏ３とＣａＯ）に対するＳ濃度（mass%Ｓ）、および複合酸化物のサイズ（複合酸化物の円相当直径ｄ）には、粒内変態に貢献するひずみエネルギーを産み出すための最適範囲があり、式（１）の第二項に上限値および下限値が存在すると推察される。そこで、実験的に式（１）の第二項の上限値および下限値を求めた。 In Formula (1), the 2nd term which considered the conditions considered to influence these intragranular transformation is shown. By the way, in the second term of the formula (1), the S concentration (mass% S) with respect to the oxysulfide forming elements (REM 2 O 3 and CaO), and the size of the composite oxide (the equivalent circle diameter d of the composite oxide) ) Has an optimum range for producing strain energy that contributes to intragranular transformation, and it is presumed that an upper limit value and a lower limit value exist in the second term of Equation (1). Therefore, the upper limit value and the lower limit value of the second term of Expression (1) were experimentally obtained.

式（１）の第二項の上限値および下限値の求め方を説明する。
まず、試作材に対して入熱量６０ｋＪ／ｍｍの溶接のＨＡＺを模擬した入熱試験を実施した。その後、入熱試験後の試作材を鏡面研磨し腐食を行い、腐食により組織を顕にして複合酸化物に起因する粒内変態の有無を調査した。
続いて、ＥＰＭＡ（Electron Probe MicroAnalyzer）にて、試作材における複合酸化物の組成とサイズを測定し、０.１〜３μmの複合酸化物について式（１）の第二項の値を算出した。 A method for obtaining the upper limit value and the lower limit value of the second term of Expression (1) will be described.
First, a heat input test simulating welding HAZ with a heat input of 60 kJ / mm was performed on the prototype material. Thereafter, the prototype material after the heat input test was mirror-polished and corroded, and the presence of intragranular transformation caused by the composite oxide was investigated by revealing the structure by the corrosion.
Subsequently, the composition and size of the composite oxide in the prototype were measured by EPMA (Electron Probe MicroAnalyzer), and the value of the second term of the formula (1) was calculated for the composite oxide of 0.1 to 3 μm.

粒内変態の有無と算出した第二項の値を表１に示す結果としてまとめ、粒内変態の有無が○印で示された（粒内変態が有った）鋼材の第二項の値に基づいて、第二項の範囲を０．００８以上０.２８９以下に設定した。   The presence or absence of intragranular transformation and the calculated value of the second term are summarized as the results shown in Table 1, and the presence or absence of intragranular transformation is indicated by a circle (the value of the second term of the steel material having intragranular transformation). Based on the above, the range of the second term was set to 0.008 or more and 0.289 or less.

円相当直径０．１〜３μｍの複合酸化物は、式（１）を満たした上で１００個／ｍｍ２以上含有されるが、さらに、平均組成について、Ａｌ２Ｏ３が２０％以下、ＴｉＯ２が３％以上２０％以下（３〜２０％）、ＺｒＯ２が５％以上５０％以下（５〜５０％）、ＲＥＭ酸化物が５％以上５０％以下（５〜５０％）、ＣａＯが５％以上５０％以下（５〜５０％）、Ｓが１％以上１５％以下（１〜１５％）となる必要がある。 The complex oxide having an equivalent circle diameter of 0.1 to 3 μm is contained at 100 pieces / mm 2 or more after satisfying the formula (1). Furthermore, the average composition is 20% or less of Al 2 O 3 , TiO 2. 2 is 3% to 20% (3 to 20%), ZrO 2 is 5% to 50% (5 to 50%), REM oxide is 5% to 50% (5 to 50%), CaO is It is necessary to be 5% or more and 50% or less (5 to 50%) and S is 1% or more and 15% or less (1 to 15%).

これは、酸化物組成がＨＡＺにおける酸化物と鋼材との格子整合性に影響するので、酸化物組成を上記範囲の含有量に制御しなければ、円相当直径０．１〜３μｍの複合酸化物がＨＡＺにおける粒内変態に寄与できない、つまりＨＡＺの組織微細化に寄与できないからである。
以上のような化学成分組成を有する本鋼材は、板厚が約３．０ｍｍ以上の厚鋼板などを対象としており、小〜中入熱溶接はもとより、入熱量が５０ｋＪ／ｍｍ以上となる大入熱溶接においてもＨＡＺ靱性の低下を防ぐことができるので、例えば橋梁や高層建造物、船舶などの大型構造物の材料として使用できる。 This is because the oxide composition affects the lattice matching between the oxide and the steel material in the HAZ. Therefore, if the oxide composition is not controlled to the content in the above range, a composite oxide having a circle equivalent diameter of 0.1 to 3 μm. This is because it cannot contribute to the intragranular transformation in HAZ, that is, it cannot contribute to the refinement of the HAZ structure.
This steel material having the chemical composition as described above is intended for thick steel plates having a plate thickness of about 3.0 mm or more. In addition to small to medium heat input welding, the heat input is 50 kJ / mm or more. Since the HAZ toughness can be prevented from being lowered even in thermal welding, it can be used as a material for large structures such as bridges, high-rise buildings, and ships.

［ニッケルＮｉ、銅Ｃｕ、クロムＣｒ、モリブデンＭｏ：０.０５〜１.５０％］
本実施形態に係る鋼材は、上述の成分元素に加えて、ニッケルＮｉ、銅Ｃｕ、クロムＣｒ、モリブデンＭｏのうち少なくとも１種を０.０５％以上１.５０％以下（０.０５〜１.５０％）含有してもよい。
Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏは、いずれも本鋼材の靭性と強度を高めるのに寄与する元素であり、各々単独で、あるいは複合して添加できる。例えば、Ｃｕの添加によって靭性と強度を有効に向上させるには、Ｃｕを０．０５％以上含有させることが好ましい。しかし、Ｃｕの含有量が１．５０％を超えると、母材の強度を高め過ぎて却って母材の靱性を低下させるため、ＨＡＺ靱性も低下してしまう。従って、Ｃｕの含有量を０．０５質量％以上１．５０質量％以下に規定する。 [Nickel Ni, Copper Cu, Chromium Cr, Molybdenum Mo: 0.05 to 1.50%]
In the steel material according to the present embodiment, in addition to the above-described component elements, at least one of nickel Ni, copper Cu, chromium Cr, and molybdenum Mo is 0.05% to 1.50% (0.05 to 1.50). 50%) may be contained.
Cu, Ni, Cr, and Mo are all elements that contribute to increasing the toughness and strength of the steel material, and can be added alone or in combination. For example, in order to effectively improve toughness and strength by adding Cu, it is preferable to contain 0.05% or more of Cu. However, if the Cu content exceeds 1.50%, the strength of the base material is excessively increased and the toughness of the base material is decreased, so that the HAZ toughness is also decreased. Therefore, the Cu content is specified to be 0.05% by mass or more and 1.50% by mass or less.

Ｎｉ、Ｃｒ、ＭｏもＣｕと同様に、０．０５％以上含有させることが好ましいが、含有量が１．５０％を超えると、母材の強度を高め過ぎて却って母材の靱性を低下させるため、ＨＡＺ靱性も低下してしまう。従って、Ｎｉ、Ｃｒ、Ｍｏの含有量も０．０５質量％以上１．５０質量％以下に規定する。
［ニオブＮｂ、バナジウムＶ：０.００２〜０.１０％］
さらに、本実施形態に係る鋼材は、ニオブＮｂ、バナジウムＶのうち少なくともいずれか一方を０.００２％以上０.１０％以下（０.００２〜０.１０％）含有していてもよい。ＮｂとＶは、いずれも炭窒化物として析出する。この炭窒化物はピン止め効果を発揮するので、溶接時におけるオーステナイト粒の粗大化が抑止され、ＨＡＺ靱性の向上に寄与する。そこで、Ｎｂの添加によってＨＡＺ靭性を有効に向上させるには、Ｎｂを、０.００２％以上含有させるのが好ましい。しかし、Ｎｂの含有量が０.１０％を超えると、析出する炭窒化物が粗大化し、却ってＨＡＺ靱性を低下させてしまう。従って、Ｎｂの含有量を０.００２質量％以上０.１０％質量％以下に規定する。 Like Cu, Ni, Cr, and Mo are preferably contained in an amount of 0.05% or more. However, if the content exceeds 1.50%, the strength of the base material is excessively increased and the toughness of the base material is decreased. Therefore, the HAZ toughness is also lowered. Therefore, the contents of Ni, Cr, and Mo are also specified to be 0.05% by mass or more and 1.50% by mass or less.
[Niobium Nb, Vanadium V: 0.002 to 0.10%]
Furthermore, the steel material according to the present embodiment may contain at least one of niobium Nb and vanadium V in a range of 0.002% to 0.10% (0.002 to 0.10%). Nb and V are both precipitated as carbonitrides. Since this carbonitride exhibits a pinning effect, coarsening of austenite grains at the time of welding is suppressed, contributing to improvement of HAZ toughness. Therefore, in order to effectively improve the HAZ toughness by adding Nb, it is preferable to contain Nb of 0.002% or more. However, if the Nb content exceeds 0.10%, the precipitated carbonitrides become coarse, and on the contrary, the HAZ toughness is lowered. Therefore, the Nb content is specified to be 0.002% by mass or more and 0.10% by mass or less.

ＶもＮｂと同様に、０.００２％以上含有させるのが好ましい。しかし、Ｖの含有量が０.１０％を超えると、析出する炭窒化物が粗大化し、却ってＨＡＺ靱性を低下させてしまう。従って、Ｖの含有量を０.００２質量％以上０.１０％質量％以下に規定する。
［ホウ素Ｂ：０.０００５〜０.００５０％］
加えて、本実施形態に係る鋼材は、ホウ素Ｂを０.０００５％以上０.００５０％以下（０.０００５〜０.００５０％）含有していてもよい。Ｂは、粒界フェライトの生成を抑制して靱性を向上させる元素である。そこで、Ｂの添加によって本鋼材の靭性を向上させるには、Ｂを０.０００５％以上含有させるのが好ましい。しかし、Ｂの含有量が０.００５０％を超えると、オーステナイト粒界にＢＮとして析出し、靱性の低下を招く。従って、Ｂの含有量を０.０００５質量％以上０.００５０質量％以下に規定する。好ましくは０.００１０質量％以上０.００４０質量％以下、より好ましくは０.００１５質量％以上０.００３０質量％以下である。 V, like Nb, is preferably contained in an amount of 0.002% or more. However, when the content of V exceeds 0.10%, the precipitated carbonitrides are coarsened, and on the contrary, the HAZ toughness is lowered. Therefore, the V content is specified to be 0.002% by mass or more and 0.10% by mass or less.
[Boron B: 0.0005 to 0.0050%]
In addition, the steel material according to this embodiment may contain 0.0005% or more and 0.0050% or less (0.0005 to 0.0050%) of boron B. B is an element that suppresses the formation of grain boundary ferrite and improves toughness. Therefore, in order to improve the toughness of the present steel material by adding B, it is preferable to contain 0.0005% or more of B. However, if the B content exceeds 0.0050%, it precipitates as BN at the austenite grain boundaries, leading to a decrease in toughness. Therefore, the B content is specified to be 0.0005 mass% or more and 0.0050 mass% or less. Preferably it is 0.0010 mass% or more and 0.0040 mass% or less, More preferably, it is 0.0015 mass% or more and 0.0003 mass% or less.

[本実施形態に係る鋼材の製造]
本実施形態に係る鋼材は、例えば溶鋼の二次精錬において、上述した化学成分組成となるように各元素を添加することで得られるが、本鋼材の製造方法の一例として、後述する実施例に示す鋼の製造方法（製造条件）、つまり各元素の添加方法について説明する。
以下の説明では、真空溶解炉（容量１５０ｋｇ）を用いて鋼を溶製し、１５０ｋｇのインゴットに鋳造して冷却することで、後述する実施例および比較例に示す鋼を得た。 [Manufacture of steel materials according to this embodiment]
The steel material according to the present embodiment can be obtained by adding each element so as to have the above-described chemical component composition, for example, in the secondary refining of molten steel. The steel manufacturing method (manufacturing conditions) shown, that is, the method of adding each element will be described.
In the following description, steel was melted using a vacuum melting furnace (capacity 150 kg), cast into a 150 kg ingot, and cooled to obtain steels shown in examples and comparative examples described later.

［溶存酸素量の調整］
初めに、真空溶解炉で溶解された溶鋼において、複合酸化物を形成する元素（複合酸化物形成元素）を添加する前に溶存酸素量とＳ濃度を調整する。
まず、複合酸化物形成元素の添加前における溶存酸素量(mass%Of)を０.００５％以下となるように調整した。その後、溶存酸素濃度(mass%Of)と、溶鋼中のＳ濃度(mass%S)の比（mass%Of／mass%S）が、０.２ ≦ mass%Of／mass%S ≦ ９.６となるようにＳ濃度(mass%S)を調整した。 [Adjustment of dissolved oxygen content]
First, in a molten steel melted in a vacuum melting furnace, the amount of dissolved oxygen and the S concentration are adjusted before adding an element for forming a complex oxide (a complex oxide forming element).
First, the amount of dissolved oxygen (mass% Of) before the addition of the complex oxide-forming element was adjusted to 0.005% or less. Then, the ratio (mass% Of / mass% S) of dissolved oxygen concentration (mass% Of) and S concentration (mass% S) in molten steel is 0.2 ≦ mass% Of / mass% S ≦ 9.6. S concentration (mass% S) was adjusted so that

ここで、Ｓ濃度(mass%S)を調整するための脱硫方法は特に限定されないが、予めＳ濃度の低い溶鋼を用いても良い。
上述の溶存酸素量とＳ濃度の根拠は次の通りである。まず、溶存酸素量が０.００５％を超えると、溶鋼中で生成する酸化物が粗大化してしまう。その上で、（mass%Of/mass%S）の値が大きい場合、酸化物に対して必要な硫化物が十分に生成しない。また、（mass%Of/mass%S）の値が小さい場合、所望の酸化物を得ることができないだけでなく、Ｓ濃度が高すぎるため粒内変態を阻害する水準にまで硫化物が生成してしまう。 Here, the desulfurization method for adjusting the S concentration (mass% S) is not particularly limited, but molten steel having a low S concentration may be used in advance.
The grounds for the dissolved oxygen amount and the S concentration are as follows. First, when the amount of dissolved oxygen exceeds 0.005%, the oxide generated in the molten steel becomes coarse. In addition, when the value of (mass% Of / mass% S) is large, the sulfide necessary for the oxide is not sufficiently generated. Moreover, when the value of (mass% Of / mass% S) is small, not only the desired oxide cannot be obtained, but also the sulfide is generated to a level that inhibits intragranular transformation because the S concentration is too high. End up.

そのため、mass%Ofとmass%Sの間には適正なバランスが有り、（mass%Of/mass%S）の値には適正な範囲が存在する。その範囲を実験的に求め、０.２ ≦ mass%Of／mass%S ≦ ９．６とした。
［Ａｌの添加］
次に、Ａｌは酸硫化物構成元素の一つであり、Ｔｉ酸化物を確保するために、Ｔｉよりも先に溶鋼に添加した。 Therefore, there is an appropriate balance between mass% Of and mass% S, and an appropriate range exists for the value of (mass% Of / mass% S). The range was experimentally determined, and 0.2 ≦ mass% Of / mass% S ≦ 9.6.
[Addition of Al]
Next, Al is one of oxysulfide constituent elements, and was added to the molten steel before Ti in order to secure Ti oxide.

［Ｔｉの添加］
Ａｌの添加に続いて、ＲＥＭ、Ｚｒよりも先にＴｉを溶鋼に添加した。ＴｉをＡｌよりも先に添加すると、その後の工程でＴｉ酸化物が全てＡｌによって還元されてしまうので
、Ａｌを添加した後にＴｉを添加しなくてはならない。Ｔｉの添加後、２分以上１５分以下にわたって、他の元素を添加せずに溶鋼を保持した。 [Addition of Ti]
Following the addition of Al, Ti was added to the molten steel before REM and Zr. If Ti is added before Al, all Ti oxides are reduced by Al in the subsequent steps. Therefore, Ti must be added after Al is added. After the addition of Ti, the molten steel was held for 2 to 15 minutes without adding other elements.

これは、Ａｌ→Ｔｉの順で添加しても、その後の溶鋼の保持時間が２分未満では、十分にＡｌとＴｉの複合酸化物が形成されず、逆に１５分を超えれば、Ａｌによる還元が進みすぎてしまうからである。つまり、ＡｌおよびＴｉの添加順は上述の式(１)に影響する。
［ＲＥＭ・Ｚｒの添加］
溶鋼を２分〜１５分間保持した後に、ＲＥＭおよびＺｒを添加した。ＲＥＭとＺｒの添加順は、特に問わない。つまり、ＲＥＭ→Ｚｒでも良いし、Ｚｒ→ＲＥＭでも良いし、またＲＥＭとＺｒを同時に添加しても良い。 Even if it is added in the order of Al → Ti, if the holding time of the molten steel thereafter is less than 2 minutes, a complex oxide of Al and Ti is not sufficiently formed. This is because the reduction proceeds too much. That is, the order of addition of Al and Ti affects the above formula (1).
[Addition of REM / Zr]
After holding the molten steel for 2 to 15 minutes, REM and Zr were added. The order of addition of REM and Zr is not particularly limited. That is, REM → Zr, Zr → REM, or REM and Zr may be added simultaneously.

Ｚｒの添加量とＲＥＭの添加量について、ＺｒとＲＥＭのうちいずれか一方でも過剰であると、円相当直径で３μｍを超える粗大な複合酸化物が形成されてしまうからであり、また、いずれか一方の元素でも少な過ぎると、円相当直径で０.１〜３μｍの微細な複合酸硫化物が不足してしまう。つまり、ＺｒとＲＥＭの添加量は複合酸化物の粒度分布に影響する。 For the addition amount and the addition amount of REM and Zr, if an excessive even either one of the Zr and REM, it is because coarse complex oxides of more than 3μm in equivalent circle diameter will be formed, also, any If one of these elements is too small, a fine complex oxysulfide having an equivalent circle diameter of 0.1 to 3 μm is insufficient. That is, the amount of Zr and REM added affects the particle size distribution of the composite oxide.

これに加えて、ＲＥＭは酸化物も硫化物も形成しやすい性質を有する反面、Ｚｒは酸化物を形成するが硫化物は形成しないという性質を有する。従って、酸化物と硫化物のバランスを適正化するには、mass%Ofとmass%Sに応じてＺｒとＲＥＭを添加する必要がある。
そこで、以下の式（２）を満足するように、Zr添加量とREM添加量の比（add[Zr]／add[REM]）を決定する。 In addition to this, REM has the property of easily forming oxides and sulfides, while Zr has the property of forming oxides but not sulfides. Therefore, in order to optimize the balance between oxide and sulfide, it is necessary to add Zr and REM according to mass% Of and mass% S.
Therefore, the ratio (add [Zr] / add [REM]) of the Zr addition amount and the REM addition amount is determined so as to satisfy the following expression (2).

0.27×(mass%Of/mass%S)+0.21≦add[Zr]／add[REM]≦0.41×(mass%Of/mass%S)+0.77 ・・・（２）
式（２）に基づいて、(mass%Of/mass%S)の値が大きい、すなわち、オキサイドが生成しやすく硫化物が生成しにくい場合には、ＺｒをＲＥＭより多めに添加する（add[Zr]／add[REM]の値を大きくする）。また、(mass%Of/mass%S)の値が小さい、すなわち、オキサイドよりも硫化物が生成しやすい場合には、ＲＥＭをＺｒより多めに添加する（add[Zr]／add[REM]の値を小さくする）。この考え方に立脚し、add[Zr]/add[REM]の値の上限および下限を実験的に求め、式（２）を得た。 0.27 × (mass% Of / mass% S) + 0.21 ≦ add [Zr] / add [REM] ≦ 0.41 × (mass% Of / mass% S) +0.77 (2)
Based on the formula (2), when the value of (mass% Of / mass% S) is large, that is, when oxide is easily generated and sulfide is difficult to be generated, Zr is added more than REM (add [ Increase the value of Zr] / add [REM]. In addition, when the value of (mass% Of / mass% S) is small, that is, when sulfides are more easily generated than oxide, REM is added more than Zr (add [Zr] / add [REM] Decrease the value). Based on this concept, the upper and lower limits of the value of add [Zr] / add [REM] were experimentally determined to obtain Equation (2).

［Ｃａの添加と鍛造］
ＲＥＭおよびＺｒを添加した後、Ｃａを添加して鋳造した。Ｃａもオキサイドや硫化物を形成するが、それらオキサイドや硫化物の形態は、基本的に既に存在する介在物の形態に依存するので、Ｃａの添加前の介在物の形態に特に留意すべきである。
なお、脱酸元素であるＡｌ，ＲＥＭ，Ｚｒ，Ｃａは、全量を一度に溶鋼へ投入するのではなく、２回以上に分割して投入するか、少量ずつ連続的に投入するのが望ましい。 [Ca addition and forging]
After REM and Zr were added, Ca was added and cast. Ca also forms oxides and sulfides. However, since the form of these oxides and sulfides basically depends on the form of inclusions already present, special attention should be paid to the form of inclusions before the addition of Ca. is there.
The deoxidizing elements Al, REM, Zr, and Ca are preferably not added all at once to the molten steel, but divided into two or more times or continuously in small amounts.

なお、溶鋼へ添加するＲＥＭやＣａ，Ｚｒ，Ｔｉの形態は特に限定されず、例えば、ＲＥＭとして、純Ｌａ，純Ｃｅ，および純Ｙ、或いは純Ｃａ，純Ｚｒ，および純Ｔｉ、更にはＦｅ−Ｓｉ−Ｌａ合金、Ｆｅ−Ｓｉ−Ｃｅ合金、Ｆｅ−Ｓｉ−Ｃａ合金、Ｆｅ−Ｓｉ−Ｌａ−Ｃｅ合金、Ｆｅ−Ｃａ合金、およびＮｉ−Ｃａ合金などを添加すればよい。また、ミッシュメタルを溶鋼へ添加してもよい。ミッシュメタルとは、希土類元素の混合物であり、具体的には、Ｃｅを４０〜５０％程度、Ｌａを２０〜４０％程度含有している。但し、ミッシュメタルには不純物としてＣａを含むことが多いので、ミッシュメタルがＣａを含む場合は、本実施形態で規定するＣａ含有量の範囲を満足する必要がある。   In addition, the form of REM, Ca, Zr, Ti added to molten steel is not specifically limited, For example, as REM, pure La, pure Ce, and pure Y, pure Ca, pure Zr, pure Ti, and also Fe -Si-La alloy, Fe-Si-Ce alloy, Fe-Si-Ca alloy, Fe-Si-La-Ce alloy, Fe-Ca alloy, Ni-Ca alloy, etc. may be added. Moreover, you may add misch metal to molten steel. Misch metal is a mixture of rare earth elements, and specifically contains about 40 to 50% Ce and about 20 to 40% La. However, since misch metal often contains Ca as an impurity, when the misch metal contains Ca, it is necessary to satisfy the range of the Ca content defined in the present embodiment.

ここまでで説明した成分元素の組成（含有量）、成分元素の含有量に関する関係式及び製造条件などを、「本実施形態で規定する条件」とよぶ。
［鋳造・圧延］
上述のように成分調整された溶鋼をインゴットに鋳造した。鋳造されたインゴットを熱間圧延して加工し、厚さが３０ｍｍ〜８０ｍｍの厚鋼板を製造した。実際の操業においては、成分調整して得られた溶鋼を、常法に従って連続鋳造してスラブとした後、常法に従
って熱間圧延すればよい。 The composition (content) of the component elements described so far, the relational expression regarding the content of the component elements, the manufacturing conditions, and the like are referred to as “conditions defined in the present embodiment”.
[Casting / Rolling]
Molten steel whose components were adjusted as described above was cast into an ingot. The cast ingot was hot-rolled and processed to produce a thick steel plate having a thickness of 30 mm to 80 mm. In actual operation, the molten steel obtained by adjusting the components may be continuously cast according to a conventional method to form a slab and then hot rolled according to a conventional method.

［ＨＡＺ靭性値の測定］
得られた厚鋼板について、溶接熱の影響を受けたＨＡＺの靱性を評価するために、該厚鋼板から、溶接継手用試験片を採取してＶ先加工を施した後、大入熱溶接相当の６０ｋＪ／ｍｍの入熱量でのエレクトロガスアーク溶接を実施した。この溶接された試験片の表面から深さｔ／４（ｔ：試験片の板厚）の位置の溶接線（ボンド）近傍のＨＡＺに切欠きを加工したシャルピー衝撃試験片（ＪＩＳ Ｚ ２２０２のＶノッチ試験片）を３本採取した。これら３本のＶノッチ試験片の各々に対して、−４０℃でシャルピー衝撃試験を行い、吸収エネルギー（ｖＥ−４０）を測定し、３本のＶノッチ試験片の測定結果の平均値と最小値を求めた。 [Measurement of HAZ toughness value]
In order to evaluate the toughness of the HAZ affected by the welding heat for the obtained thick steel plate, a specimen for a welded joint was sampled from the thick steel plate and subjected to V pre-processing, followed by high heat input welding. Electrogas arc welding was performed at a heat input of 60 kJ / mm. Charpy impact test piece (V of JIS Z 2202) in which a notch is machined in the HAZ near the weld line (bond) at a depth t / 4 (t: thickness of the test piece) from the surface of the welded test piece. Three notch specimens) were collected. For each of these three V-notch test pieces, a Charpy impact test is performed at −40 ° C., the absorbed energy (vE-40) is measured, and the average value and the minimum value of the measurement results of the three V-notch test pieces are measured. The value was determined.

この測定結果において、ｖＥ−４０の平均値が１４０Ｊを超える試験片（厚鋼板）を、ＨＡＺ靭性に優れる鋼板であると評価した。
［０.１〜３μmの複合酸化物組成の測定方法］
厚鋼板の表面から深さｔ／４（ｔ：厚鋼板の板厚）の位置から試験片を切り出し（試験片の軸心が深さｔ／４の位置を通るように採取）、圧延方向および板厚方向に平行な断面を鏡面研磨し、日本電子データム製の電子線マイクロプローブＸ線分析計（Electron Probe X-ray Microanalyzer：EPMA，商品名JXA-8500F)を用いて、０.１〜３μmの複合酸化物組成を測定した。この時の観察条件は、加速電圧を２０ｋＶ、試料電流を０.０１μＡ、倍率５０００倍、観察面積を０.４ｍｍ２以上とし、複合酸化物の中央部での成分組成を特性Ｘ線の波長分散分光により定量分析した。 In this measurement result, a test piece (thick steel plate) having an average value of vE-40 exceeding 140 J was evaluated as a steel plate excellent in HAZ toughness.
[Method for measuring composite oxide composition of 0.1 to 3 μm]
A test piece is cut out from the surface of the thick steel plate at a depth t / 4 (t: thickness of the thick steel plate) (taken so that the axis of the test piece passes through the position of the depth t / 4), and the rolling direction and The cross section parallel to the plate thickness direction is mirror-polished and 0.1-3 μm using an electron probe X-ray Microanalyzer (EPMA, trade name JXA-8500F) manufactured by JEOL Datum. The composite oxide composition of was measured. The observation conditions at this time were an acceleration voltage of 20 kV, a sample current of 0.01 μA, a magnification of 5000 times, an observation area of 0.4 mm 2 or more, and the component composition at the center of the composite oxide with wavelength dispersion of characteristic X-rays. Quantitative analysis was performed by spectroscopy.

つまり、定量の対象となる元素をＳｉ、Ｍｎ、Ｓ、Ａｌ、Ｔｉ、Ｚｒ、Ｌａ、Ｃｅ、Ｃａ、およびＯ（酸素）とし、既知物質を用いて各元素のＸ線強度と元素濃度の関係を予め検量線として求めておき、分析対象とする複合酸化物から得られたＸ線強度と検量線に基づいて、その複合酸化物に含まれる元素量を定量した。それらの元素の存在を示すＸ線強度の比から、Ｓ以外の各元素を単独酸化物に換算して酸化物の組成を算出した。Ｓは単独の濃度のまま算出した。本実施形態では、このように単独酸化物およびＳ単体濃度として質量換算し、複数の複合酸化物について平均したものを複合酸化物の組成とした。   In other words, the elements to be quantified are Si, Mn, S, Al, Ti, Zr, La, Ce, Ca, and O (oxygen), and the relationship between the X-ray intensity and element concentration of each element using a known substance. Was previously obtained as a calibration curve, and the amount of elements contained in the composite oxide was quantified based on the X-ray intensity and calibration curve obtained from the composite oxide to be analyzed. From the X-ray intensity ratio indicating the presence of these elements, each element other than S was converted to a single oxide, and the composition of the oxide was calculated. S was calculated with a single concentration. In this embodiment, mass conversion is performed as the single oxide and S single element concentration in this way, and the average of a plurality of composite oxides is used as the composite oxide composition.

なお、ＲＥＭの酸化物は、ＲＥＭを記号Ｍで表すと、鋼材中にＭ２Ｏ３、Ｍ３Ｏ５、ＭＯ２などの形態で存在するが、ＲＥＭの酸化物をすべてＭ２Ｏ３に換算した。Ｔｉについても同様に、全てＴｉＯ２に換算した。
［複合酸化物の円相当直径と個数の測定方法］
上述したＥＰＭＡを用いた複合酸化物の組成測定において、複合酸化物の面積を測定すると共に複合酸化物を円と仮定して、測定した面積に対応する円の直径を円相当直径として算出した。円相当直径が５μmを超える複合酸化物の個数を測定する際には、倍率を２００倍、観察面積を５０ｍｍ２以上とし、それ以外の条件を、円相当直径が５μm以下の複合酸化物の個数を測定する場合と同じ条件に揃えて実施した。 In addition, when the REM is represented by the symbol M, the REM oxide is present in the steel material in the form of M 2 O 3 , M 3 O 5 , MO 2, etc., but all the REM oxide is converted to M 2 O 3 . Converted. Similarly, all Ti was converted to TiO 2 .
[Measurement method of equivalent circle diameter and number of composite oxide]
In the composition measurement of the composite oxide using the above-described EPMA, the area of the composite oxide was measured and the composite oxide was assumed to be a circle, and the diameter of the circle corresponding to the measured area was calculated as the equivalent circle diameter. When measuring the number of composite oxides whose equivalent circle diameter exceeds 5 μm, the magnification is 200 times, the observation area is 50 mm 2 or more, and the other conditions are the number of composite oxides whose equivalent circle diameter is 5 μm or less. The measurement was performed under the same conditions as in the measurement.

次に、本実施形態による鋼材の実施例を、具体的に説明する。
下の表２は、本実施形態による鋼材の実施例である鋼材Ｎｏ.１〜Ｎｏ.３１の化学成分組成を示している。鋼材Ｎｏ.１〜Ｎｏ.３１の成分組成は全て、本実施形態で規定する条件を満たしている。 Next, examples of the steel material according to the present embodiment will be specifically described.
Table 2 below shows the chemical composition of steel materials No. 1 to No. 31 which are examples of the steel material according to the present embodiment. All the component compositions of the steel materials No. 1 to No. 31 satisfy the conditions defined in this embodiment.

下の表３は、本鋼材の実施例である鋼材Ｎｏ.１〜Ｎｏ.３１の製造条件を示している。鋼材Ｎｏ.１〜Ｎｏ.３１の製造条件も全て、本実施形態で規定する条件を満たしている。成分元素の添加順序や添加方法などが複数ある場合は、備考の欄に、選択した添加順序や添加方法が示されている。   Table 3 below shows the manufacturing conditions of steel materials No. 1 to No. 31 which are examples of this steel material. All the manufacturing conditions of the steel materials No. 1 to No. 31 also satisfy the conditions specified in this embodiment. When there are a plurality of addition orders and addition methods of the component elements, the selected addition order and addition method are shown in the remarks column.

下の表４は、本鋼材の実施例である鋼材Ｎｏ.１〜Ｎｏ.３１についての、複合酸化物の
粒径および個数分布、複合酸化物の平均組成、およびＨＡＺ靭性の試験結果を示している。本鋼材の実施例である鋼材Ｎｏ.１〜Ｎｏ.３１では全て、円相当直径で３μｍを超える複合酸化物の個数は５.０個／ｍｍ２以下であり、円相当直径が０.１〜３μｍの複合酸化物の個数は１００個／ｍｍ２以上である。さらに鋼材Ｎｏ.１〜Ｎｏ.３１では全て、円相当直径０.１〜３μｍの複合酸化物の平均組成が、本実施形態で規定する条件を満たしている。その結果、鋼材Ｎｏ.１〜Ｎｏ.３１は全て、ＨＡＺ靭性の試験結果が１４０Ｊ以上であり、優れたＨＡＺ靭性を発揮すると評価することができる。 Table 4 below shows the test results of the composite oxide particle size and number distribution, the composite oxide average composition, and the HAZ toughness for steel materials No. 1 to No. 31, which are examples of this steel material. Yes. In all the steel materials No. 1 to No. 31, which are examples of the present steel material, the number of complex oxides having a circle equivalent diameter exceeding 3 μm is 5.0 pieces / mm 2 or less, and the circle equivalent diameter is 0.1 to 0.1. The number of composite oxides of 3 μm is 100 / mm 2 or more. Furthermore, in all the steel materials No. 1 to No. 31, the average composition of the composite oxide having an equivalent circle diameter of 0.1 to 3 μm satisfies the conditions defined in this embodiment. As a result, all the steel materials No. 1 to No. 31 have a HAZ toughness test result of 140 J or more, and can be evaluated as exhibiting excellent HAZ toughness.

ここで、下の表５は、本実施形態で規定する条件を満たさない比較例としての鋼材Ｎｏ.３２〜Ｎｏ.６７の成分組成を示している。鋼材Ｎｏ.３２は、Ａｌの含有量が本実施形態で規定する条件を満たしていない。鋼材Ｎｏ.３４,３５は、Ｔｉの含有量が本実施形態で規定する条件を満たしていない。鋼材Ｎｏ.４０,４１は、ＲＥＭの含有量が本実施形態で規定する条件を満たしていない。鋼材Ｎｏ.４４,４５は、Ｚｒの含有量が本実施形態で規定する条件を満たしていない。鋼材Ｎｏ.４８,４９は、Ｃａの含有量が本実施形態で規定する条件を満たしていない。鋼材Ｎｏ.５２,５３は、Ｓの含有量が本実施形態で規定する条件を満たしていない。その他の鋼材については、上述した成分組成を満たしている。   Here, Table 5 below shows component compositions of steel materials No. 32 to No. 67 as comparative examples that do not satisfy the conditions defined in the present embodiment. Steel material No. 32 does not satisfy the conditions defined by the present embodiment for the content of Al. Steel materials No. 34 and 35 do not satisfy the conditions defined by the present embodiment for the Ti content. Steel materials No. 40 and 41 do not satisfy the conditions specified in the present embodiment by the REM content. In the steel materials No. 44 and 45, the content of Zr does not satisfy the conditions specified in this embodiment. Steel materials No. 48 and 49 do not satisfy the conditions defined by the present embodiment for the Ca content. Steel materials No. 52 and 53 do not satisfy the conditions defined by the present embodiment for the S content. About other steel materials, the component composition mentioned above is satisfy | filled.

下の表６は、本実施形態で規定する条件を満たさない鋼材Ｎｏ.３２〜Ｎｏ.６７の製造条件を示している。鋼材Ｎｏ.３３,３６,３７,４２,４３,４６,４７,５０,５１では、「複合酸化物形成元素の添加順序」に「×」印が付されており、複合酸化物形成元素（Ａｌ
とＴｉ）が上述した順序とは異なった順序で添加されたことを示している。鋼材Ｎｏ.３８,３９は、Ｔｉ添加後の溶鋼の保持時間が、本実施形態で規定する条件を満たしていないことを示している。鋼材Ｎｏ.５２は、(mass%Of/mass%S)の値が本実施形態で規定する条件を満たしていないことを示している。鋼材Ｎｏ.５３は、mass%Ofの値、(mass%Of/mass%S)の値、および（add[Zr]／add[REM]）の実績が、本実施形態で規定する条件を満たしていないことを示している。鋼材Ｎｏ.５４,５５は、共に（add[Zr]／add[REM]）の実績が本実施形態で規定する条件を満たしていないことを示している。鋼材Ｎｏ.５６,５７は、mass%Ofの値が、本実施形態で規定する条件を満たしていないことを示している。鋼材Ｎｏ.５８,５９は、add[Zr]の添加量、および（add[Zr]／add[REM]）の実績が、本実施形態で規定する条件を満たしていないことを示している。鋼材Ｎｏ.６０,６１は、add[REM]の添加量、および（add[Zr]／add[REM]）の実績が、本実施形態で規定する条件を満たしていないことを示している。鋼材Ｎｏ.６２〜６７は、（add[Zr]／add[REM]）の実績が、本実施形態で規定する条件を満たしていないことを示している。 Table 6 below shows the manufacturing conditions of steel materials No. 32 to No. 67 that do not satisfy the conditions specified in the present embodiment. In the steel materials No. 33, 36, 37, 42, 43, 46, 47, 50, and 51, “addition order of complex oxide forming elements” is marked with “x”, and complex oxide forming elements (Al
And Ti) are added in an order different from that described above. Steel materials No. 38 and 39 show that the holding time of the molten steel after addition of Ti does not satisfy the conditions defined in this embodiment. Steel material No. 52 has shown that the value of (mass% Of / mass% S) does not satisfy the conditions specified in this embodiment. In Steel No. 53, the value of mass% Of, the value of (mass% Of / mass% S), and the results of (add [Zr] / add [REM]) satisfy the conditions specified in this embodiment. It shows no. Steel materials Nos. 54 and 55 both indicate that the results of (add [Zr] / add [REM]) do not satisfy the conditions defined in this embodiment. Steel materials No. 56 and 57 indicate that the value of mass% Of does not satisfy the conditions defined in the present embodiment. Steel Nos. 58 and 59 indicate that the addition amount of add [Zr] and the results of (add [Zr] / add [REM]) do not satisfy the conditions defined in this embodiment. Steel materials No. 60 and 61 indicate that the addition amount of add [REM] and the result of (add [Zr] / add [REM]) do not satisfy the conditions defined in this embodiment. Steel materials Nos. 62 to 67 indicate that the results of (add [Zr] / add [REM]) do not satisfy the conditions defined in this embodiment.

このように、鋼材Ｎｏ.３２〜Ｎｏ.６７は、表５に示す成分組成および表６に示す製造条件の一方または両方において、本実施形態で規定する条件を満たしていない。   Thus, steel materials No. 32 to No. 67 do not satisfy the conditions defined in the present embodiment in one or both of the component composition shown in Table 5 and the manufacturing conditions shown in Table 6.

表７は、本実施形態で規定する条件を満たさない比較例としての鋼材Ｎｏ.３２〜Ｎｏ.６７の、複合酸化物の粒径および個数分布、複合酸化物の平均組成、およびＨＡＺ靭性の試験結果を示している。鋼材Ｎｏ.３２〜Ｎｏ.５５は、複合生成物の平均組成が本実施形態で規定する条件を満たしていない。鋼材Ｎｏ.５６,５７は、円相当直径で３μｍを超える複合酸化物の個数が５.０個／ｍｍ２を超えている。鋼材Ｎｏ.５８〜Ｎｏ.６７は、円相当直径３μｍを超える複合酸化物の個数および円相当直径０.１〜３μｍの複合酸化物の個数の一方または両方が、本実施形態で規定する条件を満たしていない。鋼材Ｎｏ.３２〜Ｎｏ.５５のそれぞれについて、本実施形態で規定する条件のうち満たしていない条件を「備考」に示されている。 Table 7 shows the particle size and number distribution of the composite oxide, the average composition of the composite oxide, and the HAZ toughness test of steel materials No. 32 to No. 67 as comparative examples that do not satisfy the conditions specified in this embodiment. Results are shown. Steel materials No. 32 to No. 55 do not satisfy the conditions defined by the average composition of the composite product in this embodiment. In the steel materials Nos. 56 and 57, the number of complex oxides having a circle equivalent diameter exceeding 3 μm exceeds 5.0 pieces / mm 2 . Steel materials No. 58 to No. 67 have a condition that one or both of the number of complex oxides having an equivalent circle diameter exceeding 3 μm and the number of complex oxides having an equivalent circle diameter of 0.1 to 3 μm are defined in this embodiment. not filled. About each of steel materials No.32-No.55, the conditions which are not satisfy | filled among the conditions prescribed | regulated by this embodiment are shown by "remarks."

その結果、鋼材Ｎｏ.３２〜Ｎｏ.６７は全て、ＨＡＺ靭性の試験結果が１４０Ｊ未満となり、本実施形態で規定する条件を１つ以上満たさない比較例において、ＨＡＺ靭性に優れた鋼材を得ることができなかった。   As a result, all of the steel materials No. 32 to No. 67 have a HAZ toughness test result of less than 140 J, and in a comparative example that does not satisfy one or more of the conditions specified in this embodiment, obtain a steel material having excellent HAZ toughness. I could not.

図１〜図３を参照しながら、表４に示す本実施形態による鋼材のＨＡＺ靭性と表７に示す比較例のＨＡＺ靭性を比較する。
図１は、表４に示す本実施形態による鋼材のＨＡＺ靭性と、表７に示す比較例の鋼材Ｎｏ.５９,６１〜６７のＨＡＺ靭性を示すグラフである。図１に示す全ての実施例および比較例において、円相当直径３μｍを超える複合酸化物の個数は５.０個／ｍｍ２未満であるが、比較例の鋼材Ｎｏ.５９,６１〜６７は、円相当直径０.１〜３μｍの複合酸化物の個数が１００個に満たなかった例であり、いずれにおいても、ＨＡＺ靭性の試験結果が１４０Ｊを大きく下回っている。 The HAZ toughness of the steel material according to this embodiment shown in Table 4 and the HAZ toughness of the comparative example shown in Table 7 will be compared with reference to FIGS.
FIG. 1 is a graph showing the HAZ toughness of steel materials according to the present embodiment shown in Table 4 and the HAZ toughness of steel materials Nos. 59 and 61 to 67 of comparative examples shown in Table 7. In all the examples and comparative examples shown in FIG. 1, the number of complex oxides having an equivalent circle diameter of more than 3 μm is less than 5.0 pieces / mm 2 . This is an example in which the number of composite oxides having an equivalent circle diameter of 0.1 to 3 μm is less than 100, and in any case, the HAZ toughness test result is significantly lower than 140J.

図２は、表４に示す本実施形態による鋼材のＨＡＺ靭性と、表７に示す比較例の鋼材Ｎｏ.３２〜５５のＨＡＺ靭性を示すグラフである。図１に示す全ての実施例および比較例において、円相当直径３μｍを超える複合酸化物の個数は５.０個／ｍｍ２未満であり、円相当直径０.１〜３μｍの複合酸化物の個数が１００個以上であるが、比較例の鋼材Ｎｏ.３２〜５５は、複合酸化物の平均組成が本実施形態で規定する条件を満たさなかった例であり、いずれにおいても、ＨＡＺ靭性の試験結果が１４０Ｊを大きく下回っている。 FIG. 2 is a graph showing the HAZ toughness of the steel materials according to the present embodiment shown in Table 4 and the HAZ toughness of steel materials Nos. 32-55 of the comparative examples shown in Table 7. In all the examples and comparative examples shown in FIG. 1, the number of complex oxides having a circle equivalent diameter of more than 3 μm is less than 5.0 / mm 2 , and the number of complex oxides having a circle equivalent diameter of 0.1 to 3 μm. However, the steel materials Nos. 32-55 of the comparative examples are examples in which the average composition of the composite oxide did not satisfy the conditions specified in the present embodiment, and in any case, the test results of the HAZ toughness Is far below 140J.

図３は、表４に示す本実施形態による鋼材のＨＡＺ靭性と、表７に示す比較例の鋼材Ｎｏ.５６，５７のＨＡＺ靭性を示すグラフである。図３に示す全ての実施例および比較例において、円相当直径０.１〜３μｍの複合酸化物の個数は１００個以上であるが、比較例の鋼材Ｎｏ.５６,５７は、円相当直径３μｍを超える複合酸化物の個数が５.０個／ｍｍ２以上となった例であり、いずれにおいても、ＨＡＺ靭性の試験結果が１４０Ｊを大きく下回っている。 FIG. 3 is a graph showing the HAZ toughness of the steel material according to the present embodiment shown in Table 4 and the HAZ toughness of steel materials Nos. 56 and 57 of the comparative examples shown in Table 7. In all the examples and comparative examples shown in FIG. 3, the number of complex oxides having a circle equivalent diameter of 0.1 to 3 μm is 100 or more, but the steel materials No. 56 and 57 of the comparative example have a circle equivalent diameter of 3 μm. This is an example in which the number of composite oxides exceeding 5.0 is 5.0 pieces / mm 2 or more, and in any case, the HAZ toughness test result is significantly lower than 140J.

上述のとおり、本実施形態で規定した条件を満たす構成の鋼材であれば、大入熱溶接においても優れたＨＡＺ靭性を発揮することができる。
なお、今回開示された実施形態はすべての点で例示であって制限的なものではないと考えられるべきである。特に、今回開示された実施形態において、明示的に開示されていない事項、例えば、運転条件や操業条件、各種パラメータ、構成物の寸法、重量、体積などは、当業者が通常実施する範囲を逸脱するものではなく、通常の当業者であれば、容易に想定することが可能な値を採用している。 As described above, if the steel material has a configuration that satisfies the conditions defined in the present embodiment, it can exhibit excellent HAZ toughness even in high heat input welding.
The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. In particular, in the embodiment disclosed this time, matters that are not explicitly disclosed, for example, operating conditions and operating conditions, various parameters, dimensions, weights, volumes, and the like of a component deviate from a range that a person skilled in the art normally performs. Instead, values that can be easily assumed by those skilled in the art are employed.

例えば、本鋼材を二次精錬において製造すると説明したが、転炉や電気炉を用いて製造しても、同様のＨＡＺ靭性を発揮する鋼材を得ることができる。従って、転炉や電気炉を用いた本鋼材の製造も本発明の技術的範囲に包含される。   For example, although it has been described that the present steel material is manufactured by secondary refining, a steel material that exhibits the same HAZ toughness can be obtained even when manufactured using a converter or an electric furnace. Therefore, the production of the present steel material using a converter or an electric furnace is also included in the technical scope of the present invention.

Claims (4)

Ｃ ：０.０２〜０.１３％（質量％（mass%）の意味。以下成分について同じ。）、
Ｓｉ：０.０５〜０.５％以下、
Ｍｎ：１.０〜２.５％、
Ｐ ：０.０３％以下（０％を含まない）、
Ｓ ：０.０１％以下（０％を含まない）、
Ａｌ：０.００２〜０.０４０％以下、
Ｔｉ：０.００５〜０.０４０％、
Ｚｒ：０.０００３〜０.０２０％、
ＲＥＭ：０.０００３〜０.０２０％、
Ｃａ：０.０００３〜０.００８０％、
Ｎ ：０.００３０〜０.０１０％以下（０％を含まない）、
Ｏ ：０.０００３〜０.００５０％を含有し、残部が鉄および不可避不純物からなる鋼材であって、
前記鋼材は、ＲＥＭ、Ｚｒ、Ｔｉ、Ａｌ、ＣａおよびＳを含有する複合酸化物を含み、
前記鋼材中の複合酸化物について、
円相当直径で３μｍ超の酸化物が１ｍｍ２あたり５.０個以下であって、
かつ円相当直径が０.１〜３μｍの複合酸化物の全てが下記式（１）を満たすと共に、前記円相当直径が０．１〜３μｍの複合酸化物個数が１００個／ｍｍ２以上であって、
さらに、下記式（１）を満たす０.１〜３μｍの全ての複合酸化物の平均組成が、Ａｌ２Ｏ３：２０％以下、ＴｉＯ２：３〜２０％、ＺｒＯ２：５〜５０％、ＲＥＭ酸化物：５〜５０％、ＣａＯ：５〜５０％、Ｓ：１〜１５％であることを特徴とする溶接熱影響部の靭性に優れた鋼材。
0.008 ≦ (1/d)×｛mass%Ｓ／(mass%ＣａＯ＋mass%ＲＥＭ２Ｏ３)｝ ≦ 0.289 ・・・（１）
（但し、dは個々の複合酸化物の円相当直径であって、０.１〜３μｍである） C: 0.02 to 0.13% (meaning mass% (mass%); the same applies to the following components)),
Si: 0.05-0.5% or less,
Mn: 1.0-2.5%,
P: 0.03% or less (excluding 0%),
S: 0.01% or less (excluding 0%),
Al: 0.002 to 0.040% or less,
Ti: 0.005 to 0.040%,
Zr: 0.0003-0.020%,
REM: 0.0003-0.020%,
Ca: 0.0003-0.0080%,
N: 0.0003 to 0.010% or less (excluding 0%),
O: a steel material containing 0.0003 to 0.0050%, the balance being iron and inevitable impurities,
The steel material includes a composite oxide containing REM, Zr, Ti, Al, Ca and S,
About the complex oxide in the steel material,
There are no more than 5.0 oxides with an equivalent circle diameter of more than 3 μm per mm 2 ,
In addition , all the complex oxides having an equivalent circle diameter of 0.1 to 3 μm satisfy the following formula (1), and the number of complex oxides having an equivalent circle diameter of 0.1 to 3 μm is 100 / mm 2 or more. And
Furthermore, the average composition of all composite oxides of 0.1 to 3 μm satisfying the following formula (1) is Al 2 O 3 : 20% or less, TiO 2 : 3 to 20%, ZrO 2 : 5 to 50%, REM oxide: 5 to 50%, CaO: 5 to 50%, S: 1 to 15%, a steel material excellent in toughness of a weld heat affected zone.
0.008 ≦ (1 / d) × {mass% S / (mass% CaO + mass% REM 2 O 3 )} ≦ 0.289 (1)
(Where d is the equivalent circle diameter of each composite oxide and is 0.1 to 3 μm) Ｎｉ：０.０５〜１.５０％、
Ｃｕ：０.０５〜１.５０％、
Ｃｒ：０.０５〜１.５０％、
Ｍｏ：０.０５〜１.５０％
のうち、少なくとも１種を含有することを特徴とする請求項１に記載の溶接熱影響部の靭性に優れた鋼材。 Ni: 0.05 to 1.50%,
Cu: 0.05 to 1.50%,
Cr: 0.05 to 1.50%,
Mo: 0.05 to 1.50%
The steel material excellent in toughness of the weld heat affected zone according to claim 1, comprising at least one of them. Ｎｂ：０.００２〜０.１０％、
Ｖ ：０.００２〜０.１０％
のうち少なくともいずれか一方を含有することを特徴とする請求項１または２に記載の溶接熱影響部の靭性に優れた鋼材。 Nb: 0.002 to 0.10%,
V: 0.002 to 0.10%
The steel material excellent in the toughness of the welding heat affected zone according to claim 1 or 2, characterized by containing at least one of the above. Ｂ ：０.０００５〜０.００５０％
を含有することを特徴とする請求項１〜３のいずれかに記載の溶接熱影響部の靭性に優れた鋼材。 B: 0.0005 to 0.0050%
The steel material excellent in the toughness of the weld heat affected zone according to any one of claims 1 to 3, characterized by comprising:
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