JP5303571B2 - Steel for welded structures including weld joints with excellent CTOD characteristics - Google Patents
Steel for welded structures including weld joints with excellent CTOD characteristics Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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Description
本発明は溶接構造物に使用される亀裂先端開口変位（ＣＴＯＤ）特性に優れた溶接継ぎ部に関し、より詳細には、船舶、建築、橋梁、海洋構造物、鋼管、ラインパイプなどに行うサブマージアーク溶接（ＳＡＷ）時に、大入熱溶接継ぎ部のＣＴＯＤ特性を改善することができる溶接構造用鋼に関する。 The present invention relates to a weld joint having excellent crack tip opening displacement (CTOD) characteristics used in a welded structure, and more particularly, a submerged arc performed on a ship, an architecture, a bridge, an offshore structure, a steel pipe, a line pipe, and the like. The present invention relates to a welded structural steel capable of improving the CTOD characteristics of a high heat input weld joint during welding (SAW).
最近、国際原油価格の持続的な上昇と建築技術の多変化により、海洋構造物の建設はさらに極限な環境で行われている傾向にある。このような海洋構造物のうち、特に、寒冷地に建設される構造物の素材は、高強度化及び亀裂先端開口変位（ＣＴＯＤ）特性が大きく求められるのが現実である。しかし、一般的な高強度厚物鋼材を溶接して構造物を与えられた工期内に製作するためには、高能率の溶接が必須である。このような傾向に合わせて、厚肉化された鋼材を溶接するために大入熱溶接方法が現われた。特に、このような方法の中でも最も広く用いられる溶接技術が、サブマージアーク溶接（ＳＡＷ）である。 Recently, construction of offshore structures tends to be carried out in a more extreme environment due to the continuous rise in international crude oil prices and many changes in building technology. Among such marine structures, in particular, materials for structures constructed in cold regions are required to have high strength and large crack tip opening displacement (CTOD) characteristics. However, high-efficiency welding is indispensable in order to produce a structure within a given construction period by welding a general high-strength thick steel material. In response to this trend, a large heat input welding method has appeared to weld thickened steel. In particular, the most widely used welding technique among such methods is submerged arc welding (SAW).
一般的にサブマージアーク溶接法は、溶着量が大きくて溶接パス数が減少する。このため、ＳＡＷは、一般的なガスメタルアーク溶接（ＧＭＡＷ）より生産性の側面で遥かに有利であるという長所がある。現在用いられているＳＡＷ技術の場合、入熱の範囲は約２５〜４５ｋＪ／ｃｍである。 Generally, in the submerged arc welding method, the amount of welding is large and the number of welding passes is reduced. For this reason, SAW has an advantage that it is far more advantageous in productivity than general gas metal arc welding (GMAW). For currently used SAW technology, the heat input range is about 25-45 kJ / cm.
しかし、このような大入熱溶接時には、粗大な柱状晶組織が形成されることができることにより、溶接金属（Ｗｅｌｄ Ｍｅｔａｌ）が凝固し、かつ粗大な結晶粒内にオーステナイト結晶粒界に沿って粗大な粒界フェライト及びＷｉｄｍａｎｓｔａｔｔｅｎ フェライトなどが形成されることができる。すなわち、溶接継ぎ部は溶接部で衝撃靭性が最も劣化する部位であると言える。従って、このような大入熱溶接で形成される溶接構造物の安全性を確保するためには、溶接金属部の微細組織を制御し、溶接金属部のＣＴＯＤ特性を確保する必要がある。 However, during such high heat input welding, a coarse columnar crystal structure can be formed, so that the weld metal (Weld Metal) is solidified and coarsened along the austenite grain boundaries in the coarse crystal grains. Grain boundary ferrite and Widmanstatten ferrite can be formed. That is, it can be said that the weld joint is a portion where the impact toughness is most deteriorated in the weld. Therefore, in order to ensure the safety of a welded structure formed by such high heat input welding, it is necessary to control the microstructure of the weld metal part and ensure the CTOD characteristics of the weld metal part.
これを解決するために、従来は溶接材料の合金成分を規定して、またはスラグ生成剤を用いて、衝撃靭性を向上させる技術があった。しかしながら、これらの技術は溶接金属の微細組織、粒径などを制御せず、また溶接金属内の酸素や窒素含量に対する制御をしない。このため、ＳＡＷ溶接のような大入熱溶接時に溶接継ぎ部の衝撃靭性を確保することが困難である。 In order to solve this problem, conventionally, there has been a technique for improving impact toughness by specifying an alloy component of a welding material or using a slag forming agent. However, these techniques do not control the microstructure, grain size, etc. of the weld metal and do not control the oxygen and nitrogen content in the weld metal. For this reason, it is difficult to ensure the impact toughness of the weld joint at the time of high heat input welding such as SAW welding.
従って、本発明は、上述の問題点を解決するためのものであり、従って本発明の一態様は、ＳＡＷ溶接のような大入熱溶接時に溶接継ぎ部のＣＴＯＤ特性を向上させるための成分及び微細組織を有する溶接継ぎ部並びにこのような溶接継ぎ部を有する溶接構造用鋼材を提供することである。 Therefore, the present invention is for solving the above-mentioned problems, and therefore, one aspect of the present invention is to provide a component for improving the CTOD characteristics of a weld joint at the time of high heat input welding such as SAW welding, and It is to provide a welded joint having a microstructure and a welded structural steel material having such a welded joint.
本発明は、重量％で、Ｃ：０．０１〜０．２％、Ｓｉ：０．１〜０．５％、Ｍｎ：１．０〜３．０％、Ｔｉ：０．０１〜０．１％、Ｎｉ：０．５〜３．０％、Ｂ：０．００１０〜０．０１％、Ｎ：０．００３〜０．００６％、Ｐ：０．０３０％以下、Ａｌ：０．００５〜０．０５％、Ｓ：０．０３０％以下、及びＯ：０．０５％以下、並びにその他不可避な不純物及び残部Ｆｅを含む。上記Ｔｉ、Ｏ、Ｎ及びＢは０．２≦Ｔｉ／Ｏ≦０．５、２≦Ｔｉ／Ｎ≦５、５≦Ｏ／Ｂ≦１０、及び０．７≦（Ｔｉ＋４Ｂ）／Ｏ≦１．５の関係が成り立つ。上記溶接継ぎ部は、Ｃｕ：０．１〜２．０％、Ｎｂ：０．０００１〜０．１％、Ｖ：０．００５〜０．１％、Ｃｒ：０．０５〜１．０％、Ｍｏ：０．０５〜１．０％、Ｗ：０．０５〜０．５％、及びＺｒ：０．００５〜０．５％からなる群より選択される１種以上の元素をさらに含むことができる。上記溶接継ぎ部は、Ｃａ：０．０００５〜０．０５％、ＲＥＭ：０．００５〜０．０５％またはＣａ：０．０００５〜０．０５％及びＲＥＭ：０．００５〜０．０５％をさらに含むことができる。 In the present invention, by weight, C: 0.01 to 0.2%, Si: 0.1 to 0.5%, Mn: 1.0 to 3.0%, Ti: 0.01 to 0.1 %, Ni: 0.5 to 3.0%, B: 0.0010 to 0.01%, N: 0.003 to 0.006%, P: 0.030% or less, Al: 0.005 to 0 0.05%, S: 0.030% or less, and O: 0.05% or less, and other inevitable impurities and the remaining Fe. Ti, O, N and B are 0.2 ≦ Ti / O ≦ 0.5, 2 ≦ Ti / N ≦ 5, 5 ≦ O / B ≦ 10, and 0.7 ≦ (Ti + 4B) / O ≦ 1. The relationship of 5 holds. The weld joint is Cu: 0.1-2.0%, Nb: 0.0001-0.1%, V: 0.005-0.1%, Cr: 0.05-1.0%, It further includes one or more elements selected from the group consisting of Mo: 0.05 to 1.0%, W: 0.05 to 0.5%, and Zr: 0.005 to 0.5%. it can. The weld joint is made of Ca: 0.0005 to 0.05%, REM: 0.005 to 0.05%, or Ca: 0.0005 to 0.05% and REM: 0.005 to 0.05%. Further can be included.
また、上記溶接構造用鋼の溶接継ぎ部の微細組織は、組織分率で針状フェライトが８５％以上であり、かつ残部はポリゴナールフェライト及びその他粒界フェライト組織である溶接継ぎ部を含む。また、上記溶接継ぎ部は、組織内にＴｉＯ酸化物が０．５μｍ以下の間隔で均一に分散される。ＴｉＯ酸化物の粒径は０．０１〜０．１μｍ（マイクロメーター）である。ＴｉＯ酸化物の粒子個数は、１．０×１０７個／ｍｍ３以上である。 The microstructure of the welded joint of the steel for welded structure includes a welded joint that is 85% or more of acicular ferrite in the structure fraction, and the balance includes polygonal ferrite and other grain boundary ferrite structures. In the weld joint, the TiO oxide is uniformly dispersed in the structure at intervals of 0.5 μm or less. The particle diameter of the TiO oxide is 0.01 to 0.1 μm (micrometer). The number of TiO oxide particles is 1.0 × 10 7 particles / mm 3 or more.
本発明による溶接継ぎ部を有する溶接構造用鋼は、優れた強度とＣＴＯＤ特性を同時に有するため、酷寒地においても優れた安定性を示すことができる。 Since the welded structural steel having a welded joint according to the present invention has excellent strength and CTOD characteristics at the same time, it can exhibit excellent stability even in extremely cold regions.
以下、本発明を詳しく説明する。 The present invention will be described in detail below.
本発明者は、溶接金属部のＣＴＯＤに効果的であると知られている針状フェライトに影響を及ぼす酸化物の種類及びサイズなどに対して研究を重ねた結果、ＴｉＯ及び固溶Ｂの存在の有無により、溶接金属部の粒界フェライト及び針状フェライトの量が変化し、溶接金属部のＣＴＯＤ値が変化するということが分かった。 As a result of repeated studies on the types and sizes of oxides that affect acicular ferrite, which is known to be effective for CTOD of weld metal parts, the present inventors have found that TiO and solute B exist. It was found that the amount of grain boundary ferrite and acicular ferrite in the weld metal part changes depending on the presence or absence of, and the CTOD value of the weld metal part changes.
このような研究に基づいて完成した本発明の溶接構造用鋼は、
１）ＳＡＷ溶接のような大入熱溶接のために、金属にＴｉＯ酸化物を使用し、
２）酸化物の分布を１．０×１０７個／ｍｍ３以上で、酸化物のサイズを０．０１〜０．１μｍ（マイクロメーター）に制御し、
３）針状フェライトの変態を促進させるため、溶接継ぎ部内にＴｉＯ及び固溶Ｂを確保することによって、８５％以上の針状フェライトを確保し、溶接部の靭性を向上させる
ことによって特徴付けられる。
The welded structural steel of the present invention completed based on such research,
1) For high heat input welding such as SAW welding, TiO oxide is used for the metal,
2) The oxide distribution is 1.0 × 10 7 pieces / mm 3 or more, and the oxide size is controlled to 0.01 to 0.1 μm (micrometer).
3) In order to promote the transformation of acicular ferrite, by securing TiO and solute B in the weld joint, it is characterized by ensuring at least 85% acicular ferrite and improving the toughness of the weld. .
１．ＴｉＯ酸化物の管理
溶接金属内にＴｉ／Ｏ、Ｏ／Ｂの比を適切に維持すると、ＴｉＯ酸化物が適切に分布する。この適切な分布は、ＴｉＯ酸化物から針状フェライトへの変態を促進することができ、かつ溶接金属の凝固過程におけるオーステナイト結晶粒の粗大化を防止することができる。温度が減少すると、オーステナイト結晶粒内に適切に分布するＴｉＯ酸化物は、針状フェライトの不均一の核生成位置として作用する。したがって、結晶粒界における粒界フェライトの形成より先に、針状フェライトを形成させることができる。このような多量の針状フェライトの生成により、溶接金属部の靭性を画期的に改善することができる。
1. Management of TiO oxide When the ratio of Ti / O and O / B is appropriately maintained in the weld metal, the TiO oxide is appropriately distributed. This appropriate distribution can promote transformation from TiO oxide to acicular ferrite, and can prevent coarsening of austenite grains during the solidification process of the weld metal. When the temperature decreases, the TiO oxide appropriately distributed in the austenite grains acts as a non-uniform nucleation position of the acicular ferrite. Therefore, acicular ferrite can be formed prior to the formation of grain boundary ferrite at the crystal grain boundary. By producing such a large amount of acicular ferrite, the toughness of the weld metal part can be dramatically improved.
そのためには、先ずＴｉＯ酸化物を微細、且つ均一に分布させることが重要である。この点において、本発明者は、Ｔｉ／Ｏ及びＯ／Ｂの比を最適化させることにより、所望のＴｉＯ酸化物のサイズと量及び分布が得られることが分かった。本発明では、Ｔｉ／Ｏ及びＯ／Ｂをそれぞれ０．２〜０．５及び５〜１０に限定し、この場合、０．０１〜０．１μｍ（マイクロメーター）サイズのＴｉＯ酸化物が、１．０×１０７個／ｍｍ３以上得られることが分かった。 For that purpose, first, it is important to distribute the TiO oxide finely and uniformly. In this regard, the inventors have found that by optimizing the ratio of Ti / O and O / B, the desired size, amount and distribution of TiO oxide can be obtained. In the present invention, Ti / O and O / B are limited to 0.2 to 0.5 and 5 to 10, respectively. In this case, 0.01 to 0.1 μm (micrometer) size TiO oxide is 1 0.0 × 10 7 pieces / mm 3 or more were found to be obtained.
２．溶接継ぎ部の微細組織
上述のようにして得られた多量のＴｉＯ酸化物が溶接継ぎ部内に適切に分布すると、溶接金属部の冷却過程において、結晶粒界より優先的に、結晶粒内に針状フェライトの変態が促進される。従って、本発明ではこのような針状フェライトを多量に確保することで、溶接継ぎ部に針状フェライトを８５％以上形成させることを特徴とする。
2. Microstructure of the weld joint When the large amount of TiO oxide obtained as described above is properly distributed in the weld joint, the needle in the crystal grain is preferentially given to the crystal grain boundary in the cooling process of the weld metal part. The transformation of the shaped ferrite is promoted. Therefore, the present invention is characterized in that 85% or more of acicular ferrite is formed in the weld joint by securing a large amount of such acicular ferrite.
３．溶接継ぎ部内の固溶ホウ素の役割
本発明者は、溶接継ぎ部に均一に分散されている酸化物とは別個に、溶接継ぎ部に存在する固溶ホウ素は結晶粒界に拡散して結晶粒界のエネルギーを低め、かつ結晶粒界で粒界フェライトの形成を抑制することを知った。このような粒界フェライトの抑制は結晶粒内に針状フェライトの変態を促進するため、溶接継ぎ部のＣＴＯＤ特性の向上に寄与する。
3. The role of the solid solution boron in the weld seam The present inventor believes that the solid solution boron present in the weld seam diffuses to the crystal grain boundaries separately from the oxide uniformly dispersed in the weld seam. I learned that it lowers the energy of the boundary and suppresses the formation of grain boundary ferrite at the grain boundary. Such suppression of the grain boundary ferrite promotes the transformation of the acicular ferrite in the crystal grains, and thus contributes to the improvement of the CTOD characteristics of the weld joint.
以下、本発明を溶接構造用鋼の成分係を通じてさらに詳しく説明する（以下重量％）。 Hereinafter, the present invention will be described in more detail through the components of the welded structural steel (hereinafter referred to as weight%).
Ｃの含量は、０．０１〜０．２％の範囲である。
Ｃは、溶接金属の強度を確保し、溶接硬化性を確保するために０．０１％以上を添加する。しかし、Ｃの含量が０．２％を超えると、溶接性及び大入熱衝撃靭性が大きく低下するため、溶接継ぎ部に低温亀裂が発生することがある。このため、Ｃの含量は、０．０１〜０．２％の範囲に限定する。
The content of C is in the range of 0.01 to 0.2%.
C is added in an amount of 0.01% or more in order to secure the strength of the weld metal and ensure weld curability. However, if the C content exceeds 0.2%, the weldability and the large heat input impact toughness are greatly deteriorated, so that a low temperature crack may occur in the weld joint. For this reason, the content of C is limited to a range of 0.01 to 0.2%.
Ｓｉの含量は、０．１〜０．５％の範囲である。
Ｓｉは、脱酸効果のために添加する元素である。Ｓｉの含量が０．１％未満では、溶接金属内の脱酸効果が不十分である。また、Ｓｉが少なすぎると、溶接金属の流動性を低下させ、望ましくない結果となる。一方、Ｓｉの含量が０．５％を超えると、溶接金属内に島状マルテンサイト（Ｍ−Ａ ｃｏｎｓｔｉｔｕｅｎｔ）組織の変態を促進させることができるため、低温衝撃靭性が急に低下することがあり、溶接亀裂の感受性に悪影響を及ぼす。このため、Ｓｉの含量は、０．１〜０．５％に限定する。
The content of Si is in the range of 0.1 to 0.5%.
Si is an element added for the deoxidation effect. If the Si content is less than 0.1%, the deoxidation effect in the weld metal is insufficient. On the other hand, if the amount of Si is too small, the fluidity of the weld metal is lowered, resulting in an undesirable result. On the other hand, if the Si content exceeds 0.5%, the transformation of island-like martensite (MA-constituent structure) can be promoted in the weld metal, so that the low temperature impact toughness may suddenly decrease. Adversely affects the sensitivity of weld cracks. For this reason, the content of Si is limited to 0.1 to 0.5%.
Ｍｎの含量は１．０〜３．０％の範囲である。
Ｍｎは、脱酸作用及び強度を向上させる合金元素である。本発明では、ＭｎはＴｉＯ酸化物の周りにＭｎＳの形態で析出され、Ｔｉ複合酸化物が溶接金属部の靭性改善に有利な針状フェライトを生成することを促進させる。しかし、Ｍｎの含量が多すぎると低温変態組織の形成をもたらすことがある。このため、Ｍｎを３．０％以下で添加する。
The Mn content is in the range of 1.0 to 3.0%.
Mn is an alloy element that improves deoxidation and strength. In the present invention, Mn is precipitated around the TiO oxide in the form of MnS, and the Ti composite oxide promotes the formation of acicular ferrite advantageous for improving the toughness of the weld metal part. However, if the Mn content is too high, it may lead to the formation of a low temperature transformation structure. For this reason, Mn is added at 3.0% or less.
Ｔｉの含量は、０．０１〜０．１％の範囲である。
Ｔｉは、Ｏと結合して微細なＴｉ酸化物及び微細ＴｉＮ析出物を形成することができる。このため、Ｔｉは本発明では非常に重要な元素である。このような微細なＴｉＯ酸化物及びＴｉＮ複合析出物の効果を得るためには、Ｔｉを０．０１％以上添加する必要がある。しかし、過量のＴｉが添加されると、粗大なＴｉＯ酸化物及び粗大なＴｉＮ析出物が形成されることがあり、溶接部の物性に悪影響を及ぼす。このため、Ｔｉを０．１％以下で添加する。
The Ti content is in the range of 0.01 to 0.1%.
Ti can combine with O to form fine Ti oxides and fine TiN precipitates. For this reason, Ti is a very important element in the present invention. In order to obtain the effect of such fine TiO oxide and TiN composite precipitates, it is necessary to add 0.01% or more of Ti. However, when an excessive amount of Ti is added, coarse TiO oxides and coarse TiN precipitates may be formed, which adversely affects the physical properties of the weld. For this reason, Ti is added at 0.1% or less.
Ｎｉの含量は、０．５〜３．０％の範囲である。
Ｎｉは、固溶強化効果を通じてマトリックスの強度と靭性を向上させる有効な元素である。このため、Ｎｉは０．５％以上添加する。しかし、Ｎｉの含量が３．０％を超えると、焼入性、及び高温亀裂が発生するリスクが増大する。このため、Ｎｉを３．０％以下で添加する。
The content of Ni is in the range of 0.5 to 3.0%.
Ni is an effective element that improves the strength and toughness of the matrix through the solid solution strengthening effect. For this reason, Ni is added at 0.5% or more. However, if the Ni content exceeds 3.0%, the hardenability and the risk of high temperature cracking increase. For this reason, Ni is added at 3.0% or less.
Ｂの含量は、０．００１０〜０．０１％の範囲である。
Ｂは、焼入性を向上させる元素である。粒界に偏析されて粒界フェライトの変態を抑制するためには、０．００１０％以上のＢが必要である。しかし、Ｂの量が０．０１％以上を超えると、さらなる効果は保障されず、かつ溶接硬化性が大きく増加するため、Ｍ−Ａ構造の変態が促進される。これは、溶接低温亀裂の発生及び靭性を低下させることがある。このため、Ｂを０．０１％以下で添加する。
The content of B is in the range of 0.0010 to 0.01%.
B is an element that improves hardenability. In order to suppress segregation at the grain boundaries and suppress the transformation of the grain boundary ferrite, 0.0010% or more of B is necessary. However, if the amount of B exceeds 0.01% or more, further effects are not guaranteed, and the weld curability is greatly increased, so that the transformation of the MA structure is promoted. This can reduce the occurrence of weld cold cracks and toughness. For this reason, B is added at 0.01% or less.
Ｎの含量は、０．００３〜０．００６％の範囲である。
Ｎは、ＴｉＮのような析出物を形成する元素であって、微細ＴｉＮ析出物の量を増加させる。特に、ＮはＴｉＮ析出物のサイズ、析出物の間隔、析出物の分布、酸化物との複合析出頻度数、析出物そのものの高温安全性などに著しい影響を及ぼす。このため、Ｎの含量は０．００３％以上に設定する。しかし、Ｎの含量０．００６％を超えると、さらなる効果は保障されず、溶接金属内に存在する固溶Ｎ量の増加により、靭性が低下する。このため、Ｎの含量は、０．００３〜０．００６％の範囲に制限する。
The N content is in the range of 0.003 to 0.006%.
N is an element that forms precipitates such as TiN, and increases the amount of fine TiN precipitates. In particular, N significantly affects the size of TiN precipitates, the interval between precipitates, the distribution of precipitates, the frequency of composite precipitation with oxides, the high-temperature safety of the precipitates themselves, and the like. Therefore, the N content is set to 0.003% or more. However, if the N content exceeds 0.006%, further effects are not guaranteed, and the toughness decreases due to an increase in the amount of solute N present in the weld metal. For this reason, the N content is limited to a range of 0.003 to 0.006%.
Ｐの含量は、０．０３０％以下である。
Ｐは、溶接時に高温亀裂を助長する不純物元素である。このため、Ｐの含量は可能な限り低く制御することが好ましい。特に、靭性の向上及び亀裂の低減のためには、Ｐを０．０３％以下添加する。
The P content is 0.030% or less.
P is an impurity element that promotes high temperature cracks during welding. For this reason, it is preferable to control the P content as low as possible. In particular, 0.03% or less of P is added to improve toughness and reduce cracks.
Ａｌの含量は、０．００５〜０．０５％の範囲である。
Ａｌは、脱酸剤であって、溶接金属内の酸素量を減少させる。Ａｌは、固溶窒素と結合して微細なＡｌＮ析出物を形成する。このため、Ａｌを０．００５％以上添加する。しかし、過量のＡｌが添加されると粗大なＡｌ２Ｏ３を形成し、靭性の改善に必要なＴｉＯ酸化物の形成を却って妨害する。このため、Ａｌを０．０５％以下で添加する。
The Al content is in the range of 0.005 to 0.05%.
Al is a deoxidizer and reduces the amount of oxygen in the weld metal. Al combines with solute nitrogen to form fine AlN precipitates. Therefore, 0.005% or more of Al is added. However, when an excessive amount of Al is added, coarse Al 2 O 3 is formed, which obstructs the formation of TiO oxide necessary for improving toughness. For this reason, Al is added at 0.05% or less.
Ｓの含量は０．０３０％以下である。
Ｓは、ＭｎＳ形成のために必要な元素である。ＭｎＳ複合析出物の析出のために、Ｓを０．０３％以下添加する。Ｓの含量が０．０３０％を超えると、高温亀裂の原因となるＦｅＳなどの低融点化合物を形成する。
The S content is 0.030% or less.
S is an element necessary for forming MnS. In order to precipitate the MnS composite precipitate, 0.03% or less of S is added. When the content of S exceeds 0.030%, a low melting point compound such as FeS that causes high temperature cracks is formed.
Ｏの含量は、０．０５％以下である。
Ｏは、溶接金属部の凝固中にＴｉと反応してＴｉ酸化物を形成する元素であって、このようなＴｉ酸化物は溶接金属内で針状フェライトの変態を促進させる役割をする。しかし、Ｏの含有量が多すぎると粗大なＴｉ酸化物及びその他ＦｅＯなどの酸化物が生成され、溶接金属部に悪影響を及ぼす。このため、Ｏを０．０５％以下で添加する。
The O content is 0.05% or less.
O is an element that forms Ti oxide by reacting with Ti during solidification of the weld metal part, and such Ti oxide plays a role of promoting the transformation of acicular ferrite in the weld metal. However, when there is too much content of O, coarse Ti oxide and other oxides, such as FeO, will be produced and it will have a bad influence on a weld metal part. For this reason, O is added at 0.05% or less.
Ｔｉ／Ｏは、０．２〜０．５の範囲である。
Ｔｉ／Ｏ値が０．２未満では溶接金属内でのオーステナイト結晶粒の成長抑制及び針状フェライトの変態に求められるＴｉＯ酸化物の個数が不十分となる。特に、ＴｉＯ酸化物内に含有されたＴｉの割合が低くなり、針状フェライト核生成の役割を失う。このため、溶接熱影響部の靭性改善に有効な針状フェライトの相分率が低下する。一方、Ｔｉ／Ｏ値が０．５を超えると、溶接金属内でのオーステナイト結晶粒の成長を抑制するさらなる効果は示されず、酸化物内に含有される合金成分の割合が却って小さくなり、針状フェライトの核生成位置としての機能を失う。このため、Ｔｉ／Ｏの割合は、０．２〜０．５の範囲に制御する。
Ti / O is in the range of 0.2 to 0.5.
If the Ti / O value is less than 0.2, the number of TiO oxides required for suppressing the growth of austenite crystal grains in the weld metal and for the transformation of acicular ferrite becomes insufficient. In particular, the proportion of Ti contained in the TiO oxide is reduced, and the role of acicular ferrite nucleation is lost. For this reason, the phase fraction of acicular ferrite effective in improving the toughness of the weld heat affected zone decreases. On the other hand, if the Ti / O value exceeds 0.5, no further effect of suppressing the growth of austenite crystal grains in the weld metal is shown, and the proportion of the alloy component contained in the oxide becomes rather small. Loses its function as a nucleation position of ferrite. For this reason, the ratio of Ti / O is controlled in the range of 0.2 to 0.5.
Ｔｉ／Ｎは、２〜５の範囲である。
Ｔｉ／Ｎ比が２未満では、ＴｉＯ酸化物に形成されるＴｉＮ析出物の量が減少し靭性改善に効果的な針状フェライトの変態が促進されにくくなる。一方、５を超えるＴｉ／Ｎ値ではさらなる効果は保障されず、また固溶Ｎ量が増加し、衝撃靭性が低下する。このため、Ｔｉ／Ｎは２〜５の範囲に限定する。
Ti / N is in the range of 2-5.
When the Ti / N ratio is less than 2, the amount of TiN precipitates formed in the TiO oxide is reduced, and it becomes difficult to promote the transformation of acicular ferrite effective for improving toughness. On the other hand, when the Ti / N value exceeds 5, further effects are not guaranteed, the amount of solute N increases, and impact toughness decreases. For this reason, Ti / N is limited to the range of 2-5.
Ｏ／Ｂは、５〜１０の範囲である。
Ｏ／Ｂの値が５未満であれば、溶接後冷却過程中にオーステナイト結晶粒界に拡散され、粒界フェライトの変態を抑制する固溶Ｂの量が不十分となる。一方、Ｏ／Ｂの値が１０を超えるとさらなる効果は保障されず、固溶窒素量が増加し溶接熱影響部の靭性が低下する。従って、Ｏ／Ｂは、５〜１０の範囲に制御する。
O / B is in the range of 5-10.
If the value of O / B is less than 5, the amount of the solid solution B that is diffused to the austenite grain boundaries during the post-weld cooling process and suppresses the transformation of the grain boundary ferrite becomes insufficient. On the other hand, when the value of O / B exceeds 10, the further effect is not guaranteed, the amount of solute nitrogen increases, and the toughness of the weld heat affected zone decreases. Therefore, O / B is controlled in the range of 5-10.
（Ｔｉ＋４Ｂ）／Ｏは、０．７〜１．５の範囲である。
本発明で（Ｔｉ＋４Ｂ）／Ｏの値が０．７未満の場合は、固溶Ｎ量が増加するため、溶接金属部の靭性改善に効果的でない。一方、１．５を超える（Ｔｉ＋４Ｂ）／Ｏの値は、ＴｉＮ、ＢＮなどの析出物の個数が不足する。
(Ti + 4B) / O is in the range of 0.7 to 1.5.
In the present invention, when the value of (Ti + 4B) / O is less than 0.7, the amount of solute N increases, which is not effective in improving the toughness of the weld metal part. On the other hand, the value of (Ti + 4B) / O exceeding 1.5 is insufficient for the number of precipitates such as TiN and BN.
本発明では、上記のように組成される鋼に機械的性質をさらに向上させるために、Ｎｂ、Ｖ、Ｃｕ、Ｍｏ、Ｃｒ、Ｗ、Ｚｒからなる群より選択される１種以上の元素をさらに添加する。 In the present invention, in order to further improve the mechanical properties of the steel having the above composition, one or more elements selected from the group consisting of Nb, V, Cu, Mo, Cr, W, and Zr are further added. Added.
Ｃｕの含量は、０．１〜２．０％の範囲である。
Ｃｕは、マトリックスに固溶されて、固溶強化の効果による強度を向上させる。このため、Ｃｕは強度及び靭性の向上に有用な元素である。このため、Ｃｕは０．１％以上添加される。しかしながら、Ｃｕの含量が２．０％を超えると、溶接金属部で硬化性を増加させて靭性を低下させ、溶接金属で高温亀裂を助長する。このため、Ｃｕの含量は、０．１〜２．０％の範囲に限定する。
The content of Cu is in the range of 0.1 to 2.0%.
Cu is dissolved in the matrix to improve the strength due to the effect of solid solution strengthening. For this reason, Cu is an element useful for improving strength and toughness. For this reason, Cu is added 0.1% or more. However, if the Cu content exceeds 2.0%, the hardenability is increased at the weld metal portion to lower the toughness, and high temperature cracks are promoted at the weld metal. For this reason, the content of Cu is limited to a range of 0.1 to 2.0%.
また、ＣｕとＮｉを複合添加する場合は、これらの合計を３．５％以下に限定する。ＣｕとＮｉの添加量の合計が３．５％を超えると、焼入性が大きくなりすぎて、靭性及び溶接性を損なう。 Further, when Cu and Ni are added in combination, the total of these is limited to 3.5% or less. When the total amount of Cu and Ni added exceeds 3.5%, the hardenability becomes too high, and the toughness and weldability are impaired.
Ｎｂの含量は、０．０００１〜０．１％の範囲である。
Ｎｂは焼入性を向上させるための元素である。特に、ＮｂはＡｒ３温度を低めて冷却速度の低い範囲でも針状フェライト組織の生成範囲を広めるという効果があり、針状フェライト組織を効率的に得るのに役立つ。従って、このような強度向上の効果を期待するためには、Ｎｂは０．０００１％以上添加することができる。しかしながら、Ｎｂの含量が０．１％を超えると、溶接時に、溶接金属部に溶接金属部の靭性を低下させるＭ−Ａ構造の形成が促進される。このため、Ｎｂの含量は０．０００１〜０．１％の範囲に限定する。
The content of Nb is in the range of 0.0001 to 0.1%.
Nb is an element for improving hardenability. In particular, Nb has the effect of lowering the Ar 3 temperature and widening the generation range of the acicular ferrite structure even in a range where the cooling rate is low, and helps to obtain the acicular ferrite structure efficiently. Therefore, in order to expect such an effect of improving strength, Nb can be added in an amount of 0.0001% or more. However, when the content of Nb exceeds 0.1%, formation of an MA structure that reduces the toughness of the weld metal part in the weld metal part is promoted during welding. For this reason, the Nb content is limited to a range of 0.0001 to 0.1%.
Ｖの含量は、０．００５〜０．１％の範囲である。
Ｖは、ＶＮ析出物を形成させて、フェライトの変態を促進する元素である。Ｖは０．００５％以上添加した方が良い。しかしながら、過量のＶは、溶接金属部にカーバイドのような硬化相を形成させて、溶接金属部の靭性を損なう。このため、Ｖの含量を０．００５〜０．１％の範囲に制限する。
The content of V is in the range of 0.005 to 0.1%.
V is an element that promotes the transformation of ferrite by forming VN precipitates. V should be added in an amount of 0.005% or more. However, an excessive amount of V causes a hardened phase such as carbide to be formed in the weld metal part and impairs the toughness of the weld metal part. For this reason, the V content is limited to a range of 0.005 to 0.1%.
Ｃｒの含量は、０．０５〜１．０％の範囲である。
Ｃｒは、焼入性及び強度を向上させる。Ｃｒの含有量が０．０５％未満ではその効果が微々たるものである一方、１．０％を越えると溶接金属部の靭性劣化をもたらすことがある。
The content of Cr is in the range of 0.05 to 1.0%.
Cr improves hardenability and strength. If the Cr content is less than 0.05%, the effect is insignificant. On the other hand, if it exceeds 1.0%, the toughness of the weld metal part may be deteriorated.
Ｍｏの含量は、０．０５〜１．０％の範囲である。
Ｍｏは、焼入性及び強度を向上させる元素である。Ｍｏは強度確保のために０．０５％以上添加することができる。しかしながら、溶接金属部の硬化及び溶接低温亀裂の発生を抑制するために、Ｍｏ含量の上限を１．０％に制限する。
The Mo content is in the range of 0.05 to 1.0%.
Mo is an element that improves hardenability and strength. Mo can be added in an amount of 0.05% or more to ensure strength. However, the upper limit of the Mo content is limited to 1.0% in order to suppress the hardening of the weld metal part and the occurrence of welding low temperature cracks.
Ｗの含量は、０．０５〜０．５％の範囲である。
Ｗは、高温強度を向上させ析出強化に効果的な元素として作用する。このため、Ｗは０．０５％以上添加することができる。しかし、Ｗの含量が０．５％を超えると溶接金属部の靭性を損なう。このため、Ｗの含量を０．０５〜０．５％の範囲に制限する。
The content of W is in the range of 0.05 to 0.5%.
W acts as an element that improves high-temperature strength and is effective for precipitation strengthening. For this reason, W can be added by 0.05% or more. However, if the W content exceeds 0.5%, the toughness of the weld metal part is impaired. For this reason, the content of W is limited to a range of 0.05 to 0.5%.
Ｚｒの含量は、０．００５〜０．５％の範囲である。
Ｚｒは、強度の上昇に効果があるため、０．００５％以上添加することができる。しかしながら、Ｚｒの含量が０．５％を超えると溶接金属部の靭性に悪影響を及ぼす。このため、Ｚｒの含量は、０．００５〜０．５％の範囲に制限する。
The content of Zr is in the range of 0.005 to 0.5%.
Zr is effective in increasing strength, so 0.005% or more can be added. However, if the Zr content exceeds 0.5%, the toughness of the weld metal part is adversely affected. For this reason, the content of Zr is limited to a range of 0.005 to 0.5%.
また、本発明では一次オーステナイトの結晶粒成長を抑制するために、Ｃａ及び／またはＲＥＭをさらに添加することができる。 In the present invention, Ca and / or REM can be further added in order to suppress primary austenite crystal grain growth.
Ｃａ及び／またはＲＥＭは、溶接時に、アークを安定させ、溶接金属部で酸化物を形成させることができる元素として機能する。また、Ｃａ及び／またはＲＥＭは、冷却過程でオーステナイト結晶粒の成長を抑制し、粒内フェライトの変態を促進させて、溶接金属部の靭性を向上させる。そのために、Ｃａは０．０００５％以上、ＲＥＭは０．００５％以上添加することができる。しかしながら、Ｃａが０．０５％、ＲＥＭが０．０５％を超えると大型酸化物を形成し靭性に悪影響を及ぼし得る。上記ＲＥＭで使用することができる元素は、Ｃｅ、Ｌａ、Ｙ、Ｈｆなどである。 Ca and / or REM function as an element capable of stabilizing the arc and forming an oxide in the weld metal part during welding. Further, Ca and / or REM suppresses the growth of austenite crystal grains during the cooling process, promotes the transformation of intragranular ferrite, and improves the toughness of the weld metal part. Therefore, Ca can be added by 0.0005% or more, and REM can be added by 0.005% or more. However, if Ca exceeds 0.05% and REM exceeds 0.05%, a large oxide may be formed and the toughness may be adversely affected. Elements that can be used in the REM are Ce, La, Y, Hf, and the like.
以下で、本発明の溶接構造用鋼を構成する微細組織及び酸化物に関して詳しく説明する。 Hereinafter, the microstructure and oxides constituting the welded structural steel of the present invention will be described in detail.
主組織：針状フェライト組織の相分率８５％以上
本発明でＳＡＷ溶接後に形成される溶接金属部の微細組織は、針状フェライトで構成され、その相分率は８５％以上である。上記針状フェライト組織は、フェライト＋ベイナイト組織のようにＣＴＯＤには有利であるが、溶接金属部の強度が低い組織や、Ｍ−Ａ構造＋ベイナイト混合組織のように溶接金属部の強度は高いが溶接金属部のＣＴＯＤなどの機械的性質が悪く、低温亀裂の感受性が高い組織とは異なり、高強度と低温ＣＴＯＤを同時に得ることができる組織として機能することができる。針状フェライトを除いた残部組織は多角形フェライト及び少量の粒界フェライトで構成されることができる。
Main structure: 85% or more phase fraction of acicular ferrite structure The microstructure of the weld metal portion formed after SAW welding in the present invention is composed of acicular ferrite, and the phase fraction is 85% or more. The above-mentioned acicular ferrite structure is advantageous for CTOD like ferrite + bainite structure, but the strength of the weld metal part is high such as the structure with low strength of the weld metal part or the mixed structure of MA and bainite. However, unlike a structure having poor mechanical properties such as CTOD of the weld metal part and high sensitivity to low temperature cracks, it can function as a structure capable of simultaneously obtaining high strength and low temperature CTOD. The remaining structure excluding acicular ferrite can be composed of polygonal ferrite and a small amount of grain boundary ferrite.
酸化物：ＴｉＯ酸化物を０．５μｍ（マイクロメーター）以下の間隔で均一に分散させ、その粒径及び臨界個数は０．０１〜０．１μｍ（マイクロメーター）及び１．０×１０７個／ｍｍ３以上である。 Oxide: TiO oxide is uniformly dispersed at intervals of 0.5 μm (micrometer) or less, and its particle size and critical number are 0.01 to 0.1 μm (micrometer) and 1.0 × 10 7 / mm 3 or more.
一般的に、溶接金属部に存在する酸化物の種類、サイズ、個数などは、溶接後の溶接金属部の微細組織の変態に大きな影響を及ぼす。特に、ＳＡＷ溶接金属部の場合、凝固過程で結晶粒が粗大化し、結晶粒界から粗大な粒界フェライト、Ｗｉｄｍａｎｓｔａｔｔｅｎフェライト、ベイナイトなどの組織が形成されることがあり、溶接金属部の物性が大きく低下することがある。これを防止するために、本発明では溶接金属内にＴｉＯ酸化物を０．５μｍ以下の間隔で均一に分散させ、ＴｉＯ酸化物の粒径及び臨界個数を０．０１〜０．１μｍ及び１．０×１０７個／ｍｍ３以上に限定する。もし、酸化物の粒径が０．０１μｍ（マイクロメーター）より小さいと、ＳＡＷ溶接金属部で針状フェライトの変態を促進させられない。一方、０．１μｍを超えるとオーステナイト結晶粒に対するピンニング（ｐｉｎｎｉｎｇ、結晶粒の成長抑制）効果が少なくなり、粗大な非金属介在物のような挙動をし、溶接金属部のＣＴＯＤ特性に悪影響を及ぼし得る。 In general, the type, size, number, etc. of oxides present in the weld metal part have a great influence on the transformation of the microstructure of the weld metal part after welding. In particular, in the case of a SAW weld metal part, crystal grains coarsen during the solidification process, and coarse grain boundary ferrite, Widmanstatten ferrite, bainite, and other structures may be formed from the crystal grain boundary, and the physical properties of the weld metal part are large. May decrease. In order to prevent this, in the present invention, TiO oxide is uniformly dispersed in the weld metal at intervals of 0.5 μm or less, and the particle size and critical number of TiO oxide are 0.01 to 0.1 μm and 1. It is limited to 0 × 10 7 pieces / mm 3 or more. If the particle size of the oxide is smaller than 0.01 μm (micrometer), the transformation of acicular ferrite cannot be promoted in the SAW weld metal part. On the other hand, if it exceeds 0.1 μm, the pinning effect on austenite crystal grains will be reduced, and it will behave like coarse non-metallic inclusions, adversely affecting the CTOD characteristics of weld metal parts. obtain.
本発明のような溶接構造用鋼は、ＳＡＷ溶接方法以外の溶接方法にも十分に応用することができる。特に、本発明は溶接金属部の冷却速度が速いと酸化物を微細分散させ組織が微細であるため、冷却速度が速い大入熱溶接工程が好ましい。また、同様の理由で溶接部の冷却速度を向上させるために鋼材冷却及びＣｕ−バッキング（ｂａｃｋｉｎｇ）法にも有利である。しかし、このように公知の技術を本発明に適用しても、それは本発明の単純な変更であり、実質的に本発明の技術思想の範囲内であると解釈するのは当然である。 The welded structural steel as in the present invention can be sufficiently applied to welding methods other than the SAW welding method. In particular, in the present invention, when the cooling rate of the weld metal part is high, the oxide is finely dispersed and the structure is fine. Therefore, a high heat input welding process with a high cooling rate is preferable. Also, for the same reason, it is advantageous to the steel material cooling and Cu-backing method in order to improve the cooling rate of the weld. However, even if such a known technique is applied to the present invention, it is a matter of course that it is a simple modification of the present invention and is substantially within the scope of the technical idea of the present invention.
以下、本発明の任意の／例示的な態様を、実施例及び表を通じて具体的に説明する。 In the following, arbitrary / exemplary aspects of the present invention will be specifically described through examples and tables.
表１の下部に示す成分組成を有する溶接金属部を、３０〜４５ｋＪ／ｃｍ以上の溶接入熱量を適用してＳＡＷ溶接により製造した。本発明の効果を示すための、溶接金属部の合金成分元素間の構成比を、表２に示した。 The weld metal part which has a component composition shown in the lower part of Table 1 was manufactured by SAW welding by applying a welding heat input of 30 to 45 kJ / cm or more. Table 2 shows the composition ratios between the alloy component elements of the weld metal part in order to show the effects of the present invention.
上述のように溶接された溶接金属部の機械的性質を評価するための試験片を溶接金属部の中央部で採取した。引張試験片はＫＳ規格（ＫＳ Ｂ ０８０１）４号試験片を利用した。引張試験はクロスヘッドスピード（ｃｒｏｓｓ ｈｅａｄ ｓｐｅｅｄ）１０ｍｍ／分で行った。ＣＴＯＤ試験片はＢＳ７４４８−１規格に準じて製造し、疲労亀裂はＳＡＷ溶接金属部の中央に位置させた。 A test piece for evaluating the mechanical properties of the weld metal part welded as described above was collected at the center of the weld metal part. As the tensile test piece, a KS standard (KS B 0801) No. 4 test piece was used. The tensile test was performed at a cross head speed of 10 mm / min. The CTOD test piece was manufactured in accordance with the BS7448-1 standard, and the fatigue crack was positioned at the center of the SAW weld metal part.
溶接金属部のＣＴＯＤに重要な影響を及ぼす酸化物のサイズと個数及び間隔は画像分析機（ｉｍａｇｅ ａｎａｌｙｚｅｒ）と電子顕微鏡を利用したポイントカウンティング（ｐｏｉｎｔ ｃｏｕｎｔｉｎｇ）法で測定した。このとき、被検面は１００ｍｍ２を基準にして評価した。また、ＳＡＷ溶接金属部のＣＴＯＤ評価はＳＡＷ溶接後のＣＴＯＤ試験片で加工し−１０℃でＣＴＯＤ試験機器を通じて評価した。 The size, number, and spacing of oxides that have an important effect on the CTOD of the weld metal were measured by a point counting method using an image analyzer and an electron microscope. At this time, the test surface was evaluated based on 100 mm 2 . Moreover, CTOD evaluation of the SAW weld metal part was processed with a CTOD test piece after SAW welding, and evaluated through a CTOD test apparatus at −10 ° C.
表３に示したように、本発明により製造された溶接金属部はＴｉＯ酸化物の個数は２×１０８個／ｍｍ３以上の範囲を有するが、比較鋼は４．３×１０６個／ｍｍ３以下の範囲を示した。このため、比較鋼に比べて発明鋼がかなり均一、且つ微細なサイズの複合析出物でありながら、その個数も著しく増加したことが分かる。本発明の微細組織の場合、針状フェライトの相分率は８５％以上の高い分率で存在していた。ＳＡＷ溶接時において、本発明鋼は粒内針状フェライト及び多角形フェライトを含み、針状フェライトの相分率は８５％以上であった。従って、本発明鋼は、比較鋼より優れた溶接金属部のＣＴＯＤ特性を示す。 As shown in Table 3, the number of TiO oxides in the weld metal part manufactured according to the present invention is in the range of 2 × 10 8 pieces / mm 3 or more, but the comparative steel is 4.3 × 10 6 pieces / A range of mm 3 or less was shown. For this reason, it can be seen that the number of the invention steels was remarkably increased while the invented steels were considerably uniform and fine composite precipitates as compared with the comparative steels. In the case of the microstructure of the present invention, the phase fraction of acicular ferrite was present at a high fraction of 85% or more. At the time of SAW welding, the steel of the present invention contained intragranular acicular ferrite and polygonal ferrite, and the phase fraction of acicular ferrite was 85% or more. Therefore, this invention steel shows the CTOD characteristic of the weld metal part superior to comparative steel.
Claims (7)
上記Ｔｉ、Ｏ、Ｎ及びＢは、０．２≦Ｔｉ／Ｏ≦０．５、２≦Ｔｉ／Ｎ≦５、５≦Ｏ／Ｂ：≦１０、及び０．７≦（Ｔｉ＋４Ｂ）／Ｏ≦１．５の関係を満たし、及び、
ＴｉＯ酸化物を含み、該ＴｉＯ酸化物の粒子の個数は、１．０×１０ ７ 個／ｍｍ ３ 以上である、
溶接継ぎ部を含む、溶接構造用鋼。 By weight, C: 0.01 to 0.2%, Si: 0.1 to 0.5%, Mn: 1.0 to 3.0%, Ti: 0.01 to 0.1%, Ni: 0.5 to 3.0%, B: 0.0010 to 0.01%, N: 0.003 to 0.006%, P: 0.030% or less, Al: 0.005 to 0.05%, S: 0.030% or less, and O: 0.05% or less, and other inevitable impurities and the balance Fe,
Ti, O, N and B are 0.2 ≦ Ti / O ≦ 0.5, 2 ≦ Ti / N ≦ 5, 5 ≦ O / B: ≦ 10, and 0.7 ≦ (Ti + 4B) / O ≦. It meets the relationship of 1.5, and,
TiO oxide is included, and the number of particles of the TiO oxide is 1.0 × 10 7 particles / mm 3 or more.
Steel for welded structures, including welded joints.
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