JP5842314B2 - High heat input welding steel - Google Patents
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- 229910000831 Steel Inorganic materials 0.000 title claims description 44
- 239000010959 steel Substances 0.000 title claims description 44
- 238000003466 welding Methods 0.000 title claims description 44
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 claims description 16
- 239000000203 mixture Substances 0.000 claims description 14
- XTQHKBHJIVJGKJ-UHFFFAOYSA-N sulfur monoxide Chemical compound S=O XTQHKBHJIVJGKJ-UHFFFAOYSA-N 0.000 claims description 11
- 229910052748 manganese Inorganic materials 0.000 claims description 10
- 229910052804 chromium Inorganic materials 0.000 claims description 7
- 229910052759 nickel Inorganic materials 0.000 claims description 7
- 229910052802 copper Inorganic materials 0.000 claims description 6
- 229910052750 molybdenum Inorganic materials 0.000 claims description 6
- 229910052726 zirconium Inorganic materials 0.000 claims description 6
- 239000012535 impurity Substances 0.000 claims description 5
- 229910052698 phosphorus Inorganic materials 0.000 claims description 5
- 229910052799 carbon Inorganic materials 0.000 claims description 4
- 229910052758 niobium Inorganic materials 0.000 claims description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims description 4
- 229910052720 vanadium Inorganic materials 0.000 claims description 3
- 229910000859 α-Fe Inorganic materials 0.000 description 32
- 229910000734 martensite Inorganic materials 0.000 description 21
- 229910001566 austenite Inorganic materials 0.000 description 16
- 230000009466 transformation Effects 0.000 description 16
- 230000015572 biosynthetic process Effects 0.000 description 12
- 239000000463 material Substances 0.000 description 11
- 230000000694 effects Effects 0.000 description 9
- 238000000034 method Methods 0.000 description 9
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 7
- 238000001816 cooling Methods 0.000 description 7
- 229910001563 bainite Inorganic materials 0.000 description 5
- 238000009863 impact test Methods 0.000 description 5
- 239000002245 particle Substances 0.000 description 5
- 239000010953 base metal Substances 0.000 description 4
- 239000013078 crystal Substances 0.000 description 4
- 230000008018 melting Effects 0.000 description 4
- 238000002844 melting Methods 0.000 description 4
- 230000008569 process Effects 0.000 description 4
- 238000005098 hot rolling Methods 0.000 description 3
- 230000006872 improvement Effects 0.000 description 3
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- 229910052751 metal Inorganic materials 0.000 description 3
- 230000009467 reduction Effects 0.000 description 3
- 238000005096 rolling process Methods 0.000 description 3
- 239000006104 solid solution Substances 0.000 description 3
- 238000007711 solidification Methods 0.000 description 3
- 230000008023 solidification Effects 0.000 description 3
- 150000004763 sulfides Chemical class 0.000 description 3
- 238000005496 tempering Methods 0.000 description 3
- 229910000746 Structural steel Inorganic materials 0.000 description 2
- 239000006185 dispersion Substances 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
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Description
本発明は、造船、建築、土木等の各種構造物で使用される溶接用鋼に関し、特に入熱量が３００ｋＪ／ｃｍを超える大入熱溶接で優れた溶接熱影響部靭性を有する大入熱溶接用鋼に関する。 The present invention relates to a steel for welding used in various structures such as shipbuilding, construction, and civil engineering, and particularly has a high heat input welding having excellent heat-affected zone toughness in high heat input with a heat input exceeding 300 kJ / cm. Related to steel.
溶接構造物が大型化し、使用される鋼材の高強度化・厚肉化に伴い、溶接施工でサブマージアーク溶接、エレクトロガス溶接およびエレクトロスラグ溶接などの高能率な大入熱溶接の適用が増加し、溶接熱影響部の靱性確保が課題となっている。 As the welded structure becomes larger and the strength and thickness of the steel used increases, the application of highly efficient high heat input welding such as submerged arc welding, electrogas welding, and electroslag welding has increased. Securing the toughness of the weld heat affected zone is an issue.
溶接金属と熱影響部（Ｈｅａｔ Ａｆｆｅｃｔｅｄ Ｚｏｎｅ；ＨＡＺ）との境界部は、一般に「ボンド部」と称されている。このボンド部近傍の熱影響部（ＨＡＺ）は、熱影響部の中でも特に溶融点付近の高温に加熱され、その後、急冷されるため、硬さが最高硬さを示すことが多い。また、上記溶接熱影響部（ＨＡＺ）は、溶接時の入熱量が大きくなると、結晶粒が粗大化し、靱性が著しく低下することが知られている。 A boundary portion between a weld metal and a heat affected zone (HAZ) is generally referred to as a “bond portion”. The heat affected zone (HAZ) in the vicinity of the bond portion is heated to a high temperature particularly in the vicinity of the melting point among the heat affected zone, and then rapidly cooled, so that the hardness often exhibits the highest hardness. Further, it is known that the weld heat affected zone (HAZ) has a large crystal grain size and a significant decrease in toughness when the heat input during welding increases.
大入熱溶接による靱性の低下に対して、これまでにも多くの対策が提案され、特許文献１では、大入熱溶接部の靭性改善技術を、（１）鋼中に分散する粒子（介在物）によるピンニング効果に基づく結晶粒の粗大化防止（結晶粒の微細化）、（２）オーステナイト結晶粒内のフェライト変態促進に基づく変態組織及び有効結晶粒の微細化、（３）Ｍ−Ａ（Ｍａｒｔｅｎｓｉｔｅ−Ａｕｓｔｅｎｉｔｅ ｃｏｎｓｔｉｔｕｅｎｔ）に代表される局所的な脆化相の生成抑制、（４）地組織の靭性改善等の４点に大別し、これらを組み合わせたものとして固溶Nの徹底的低減と酸化物による粒径微細化効果による靭性改善を提案している。 Many countermeasures have been proposed for the reduction of toughness due to large heat input welding. In Patent Document 1, the technique for improving the toughness of large heat input welds is as follows: (1) Particles dispersed in steel (intervening) Prevention of crystal grain coarsening based on the pinning effect of the product) (2) refinement of the transformation structure and effective crystal grain based on the ferrite transformation promotion in the austenite grains, (3) MA It is roughly divided into four points, such as suppression of formation of local embrittlement phase represented by (Martensite-Austenite constituent), and (4) improvement of toughness of geological structure, and thorough reduction of solid solution N as a combination of these. And toughness improvement by the effect of grain refinement by oxides.
近年、適用されることが多くなった４００ｋＪ／ｃｍを超える大入熱溶接の場合、従来の３００ｋＪ／ｃｍ程度までの入熱量であれば高靱性の確保が可能な鋼材、特許文献２に記載のＣａを添加する技術や特許文献３に記載のＲＥＭを添加する技術による鋼材であっても靭性確保は困難となっている。 In the case of high heat input welding exceeding 400 kJ / cm, which has been frequently applied in recent years, a steel material capable of ensuring high toughness with a heat input up to about 300 kJ / cm, disclosed in Patent Document 2 It is difficult to secure toughness even with steel materials based on the technology of adding Ca and the technology of adding REM described in Patent Document 3.
特許文献４は、４００ｋＪ／ｃｍを超える大入熱溶接でも良好な溶接熱影響部靱性を確保する鋼材に関し、高温領域でのオーステナイトの粗大化を抑制するとともに、その後の冷却過程におけるフェライト変態も促進させるように、変態核となる硫化物の形態制御に必要なＣａを適正に含有させて、変態核を微細に分散させることを特徴とする。 Patent Document 4 relates to a steel material that ensures good weld heat-affected zone toughness even with high heat input welding exceeding 400 kJ / cm, and suppresses austenite coarsening in a high temperature region and promotes ferrite transformation in the subsequent cooling process. As described above, the transformation nucleus is finely dispersed by appropriately containing Ca necessary for the shape control of the sulfide serving as the transformation nucleus.
特許文献５は、建築用ＢＯＸ柱で適用される５００ｋＪ／ｃｍを超える大入熱溶接でも良好な溶接熱影響部靱性を確保する鋼材に関し、Ｍｇ含有酸化物を内包するＴｉＮとＣａを含有するＭｎ硫化物を分散させることで、高温領域でのオーステナイトの粗大化を抑制するとともに、その後の冷却過程における粒内フェライト変態も促進させるようにしたものである。 Patent Document 5 relates to a steel material that ensures good weld heat affected zone toughness even with a high heat input welding exceeding 500 kJ / cm applied in a BOX column for building, and Mn containing TiN and Ca containing Mg-containing oxides. By dispersing sulfides, coarsening of austenite in a high temperature region is suppressed, and intragranular ferrite transformation in the subsequent cooling process is also promoted.
しかしながら、特許文献４や特許文献５に記載の発明鋼であっても、比較的Ｃ量や合金添加量が多く添加された鋼成分、すなわち比較的強度の高くなる成分系においては、溶接入熱量が４００ｋＪ／ｃｍを超える大入熱溶接を施したときのボンド部組織に、島状マルテンサイトと呼ばれる硬質の脆化組織が数％形成し、靭性のさらなる向上が困難となっている。また、特許文献５記載の発明鋼においてはＭｇ添加により、粒内フェライト変態に寄与するＭｎ硫化物が析出しにくい傾向にあり、造船用など−４０℃でのシャルピー衝撃特性が求められる場合、さらなる組織改善が必要となる。 However, even in the invention steels described in Patent Document 4 and Patent Document 5, in a steel component to which a relatively large amount of C or alloy is added, that is, in a component system having a relatively high strength, the amount of welding heat input. However, a hard embrittlement structure called island martensite is formed in the bond portion structure when large heat input welding exceeding 400 kJ / cm is formed, making it difficult to further improve toughness. In addition, in the invention steel described in Patent Document 5, when Mg is added, Mn sulfide that contributes to intragranular ferrite transformation tends to be difficult to precipitate, and when Charpy impact characteristics at −40 ° C. are required, such as for shipbuilding, further Organizational improvement is required.
そこで、本発明は、特許文献４の高靭化技術に加え、さらにボンド部組織における島状マルテンサイトを低減して靭性を向上させた大入熱溶接用鋼を提供することを目的とする。 Therefore, an object of the present invention is to provide a steel for high heat input welding in which, in addition to the toughening technique of Patent Document 4, the island-like martensite in the bond structure is reduced to improve the toughness.
本発明者らは、大入熱溶接を施したときのボンド部組織に形成する島状マルテンサイトの低減に対して有効な措置を鋭意検討し、以下の知見を得た。
１．ボンド部の旧オーステナイト粒の粒内組織をアシキュラーフェライトとすることが極めて有効である。
The inventors diligently studied effective measures for reducing island martensite formed in the bond structure when high heat input welding was performed, and obtained the following knowledge.
1. It is extremely effective to use acicular ferrite as the intragranular structure of the prior austenite grains in the bond portion.
アシキュラーフェライトは、従来知られているように、有効結晶粒径の微細化にも有効である。アシキュラーフェライトは、いわば粒内から核生成したベイナイトである。粒内アシキュラーフェライトが生成する場合、旧オーステナイト粒界よりベイナイトが生成するときと比較して核生成サイトが多くなり、未変態オーステナイトへのＣの濃化が軽減される結果、島状マルテンサイトが形成しにくくなるものと考えられる。 As known conventionally, acicular ferrite is also effective in reducing the effective crystal grain size. Acicular ferrite is so-called bainite nucleated from within the grains. When intragranular acicular ferrite is formed, the number of nucleation sites is greater than when bainite is formed from the prior austenite grain boundaries, and the concentration of C in the untransformed austenite is reduced, resulting in island martensite. Is considered to be difficult to form.
２．粒内アシキュラーフェライトの生成促進には、適正量のＣａ添加に加え、Ｍｎ添加量を１．５〜２．６質量％と高めることが有効で、ＭｎとＣａとを質量比でＭｎ／（Ｍｎ＋Ｃａ）：０．１〜０．７の範囲で含む硫化物：（ＣａＭｎ）Ｓが、鋼板を溶製する際の凝固段階で晶析出されるようになる。 2. In order to promote the formation of intragranular acicular ferrite, it is effective to increase the Mn addition amount to 1.5 to 2.6% by mass in addition to an appropriate amount of Ca addition. Mn + Ca): sulfide contained in the range of 0.1 to 0.7: (CaMn) S is crystallized in the solidification stage when melting the steel sheet.
（ＣａＭｎ）Ｓは多くの場合、酸化物の周囲の一部に付着したような形で析出し、粒子は球状である。また、大入熱溶接時の高温下で部分的に溶解し、冷却時にＭｎＳがその表面に再析出するが、ＭｎＳは、それ自身がフェライト核生成能をもっているほか、周囲に形成されるＭｎの希薄帯がフェライト変態、ベイナイト変態を促進する。更に、ＭｎＳ上にＴｉＮ、ＢＮ、ＡｌＮ、ＶＮ等のフェライト生成核が析出することによって、より一層フェライト変態が促進される。 In many cases, (CaMn) S is deposited in the form of adhering to a part of the periphery of the oxide, and the particles are spherical. In addition, MnS partially dissolves at high temperature during high heat input welding and reprecipitates on the surface during cooling. MnS itself has the ability to form ferrite nuclei, and MnS formed around The dilute band promotes ferrite transformation and bainite transformation. Furthermore, ferrite transformation nuclei such as TiN, BN, AlN, and VN are deposited on MnS, thereby further promoting ferrite transformation.
一方、Ｍｎ添加量が少ないと、溶製する際の凝固段階でＣａＳが形成されるようになるが、ＣａＳは大入熱溶接時の高温下で安定で、Ｍｎを含む硫化物の溶解・再析出を生じないため、粒内アシキュラーフェライトの生成に寄与しない。 On the other hand, when the amount of Mn added is small, CaS is formed in the solidification stage when melting, but CaS is stable at high temperatures during high heat input welding, and dissolution / re-generation of sulfides containing Mn. Since no precipitation occurs, it does not contribute to the formation of intragranular acicular ferrite.
３．粒内をアシキュラーフェライト組織とするために、旧オーステナイト粒界に沿った初析フェライトを析出させ、生成速度の大きい、旧オーステナイト粒界からのベイナイト生成をできるだけ抑えることも有効である。 3. In order to obtain an acicular ferrite structure in the grains, it is also effective to precipitate pro-eutectoid ferrite along the prior austenite grain boundaries and suppress bainite formation from the prior austenite grain boundaries having a high production rate as much as possible.
４．鋼材成分組成において、溶接熱影響部における島状マルテンサイトの生成量は概ね、Ｃｅｑ（ＩＩＷ）の値で整理され、Ｃｅｑ（ＩＩＷ）（＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５、各元素記号は含有量（質量％））：０．３３〜０．４５で島状マルテンサイトの生成量を抑制することが可能である。 4). In the steel composition, the amount of island martensite generated in the weld heat affected zone is roughly arranged by the value of Ceq (IIW), Ceq (IIW) (= C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, Each element symbol is content (mass%)): 0.33 to 0.45, and the generation amount of island martensite can be suppressed.
５．以上の方策をとることによって、大入熱溶接熱影響部において粒内アシキュラーフェライト変態を促進し、島状マルテンサイトの形成が抑制され、その高靱性化を達成することが可能である。 5. By taking the above measures, it is possible to promote intragranular acicular ferrite transformation in the high heat input welding heat-affected zone, suppress formation of island martensite, and achieve high toughness.
本発明は、上記知見をもとに、さらに検討をくわえてなされたもので、すなわち、本発明は、
１．Ｃｅｑ（ＩＩＷ）（＝Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５、各元素記号は含有量（質量％））：０．３３〜０．４５を満足する鋼であって、質量％で、Ｃ：０．０３〜０．０８％、Ｓｉ：０．０１〜０．１５％、Ｍｎ：１．５〜２．６％、Ｐ：０．０３％以下、Ｓ：０．０００５〜０．００４０％、Ａｌ：０．００５〜０．１％、Ｎｂ：０．００３〜０．０５％、Ｔｉ：０．００３〜０．０３％、Ｎ：０．００２５〜０．００７０％、Ｂ：０．０００３〜０．００２５％、Ｃａ：０．０００５〜０．００３０％、残部Ｆｅおよび不可避的不純物からなる成分組成と、鋼中に、ＭｎとＣａとを質量比でＭｎ／（Ｍｎ＋Ｃａ）：０．１〜０．７の範囲で含む硫化物あるいは酸硫化物が、０．１〜５μｍの大きさで１ｍｍ２中に５０〜１０００個が分散して存在していることを特徴とする大入熱溶接用鋼。
２．更に、質量％で、Ｃｕ：１．０％以下、Ｎｉ：１．０％以下、Ｃｒ：０．４％以下、Ｍｏ：０．４％以下、Ｖ：０．２％以下のうちから選ばれる１種または２種以上を含有することを特徴とする１に記載の大入熱溶接用鋼。
３．更に、質量％で、Ｍｇ：０．０００５〜０．００５０％、Ｚｒ：０．００１〜０．０２％、ＲＥＭ：０．００１〜０．０２％のうちから選ばれる１種または２種以上を含有することを特徴とする１または２に記載の大入熱溶接用鋼。
The present invention has been made based on the above findings and further studies, that is, the present invention
1. Ceq (IIW) (= C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, each element symbol is content (mass%)): steel satisfying 0.33 to 0.45, in mass% C: 0.03-0.08%, Si: 0.01-0.15%, Mn: 1.5-2.6%, P: 0.03% or less, S: 0.0005-0. 0040%, Al: 0.005-0.1%, Nb: 0.003-0.05%, Ti: 0.003-0.03%, N: 0.0025-0.0070%, B: 0 .0003-0.0025%, Ca: 0.0005-0.0030%, the component composition consisting of the balance Fe and inevitable impurities, and Mn / Ca in the mass ratio of Mn / (Mn + Ca): 0 .1 to 0.7 m in size of sulfide or oxysulfide contained in the range of 1 to 0.7 High heat input welding steel 50 to 1000 in 2, characterized in that it is present dispersed.
2. Further, in terms of mass%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.4% or less, Mo: 0.4% or less, V: 0.2% or less are selected. The steel for high heat input welding according to 1, which contains one kind or two or more kinds.
3. Furthermore, by mass%, Mg: 0.0005 to 0.0050%, Zr: 0.001 to 0.02%, REM: 0.001 to 0.02%, or one or more selected from The steel for high heat input welding according to 1 or 2, characterized by containing.
本発明によれば、サブマージアーク溶接、エレクトロガス溶接、エレクトロスラグ溶接などの３００ｋＪ／ｃｍを超える大入熱溶接で優れた溶接熱影響部靱性を有する鋼が得られ、産業上極めて有用である。 According to the present invention, steel having excellent weld heat affected zone toughness can be obtained by high heat input welding exceeding 300 kJ / cm such as submerged arc welding, electrogas welding, electroslag welding, and the like, which is extremely useful industrially.
本発明では成分組成とミクロ組織を規定する。
［成分組成］以下の説明において％は質量％とする。
In the present invention, the component composition and the microstructure are defined.
[Component Composition] In the following description, “%” means “mass%”.
Ｃｅｑ（ＩＩＷ）：０．３３〜０．４５
Ｃｅｑ（ＩＩＷ）は、０．３３未満であると必要な母材強度が得られない。また０．４５以下とすると島状マルテンサイトの面積分率を概ね３％未満に抑えることができるが０．４５を超えると、溶接熱影響部とくにボンド部近傍において島状マルテンサイトの生成が顕著となって靭性が劣化するため、０．３３〜０．４５の範囲とする。Ｃｅｑ（ＩＩＷ）は、Ｃ＋Ｍｎ／６＋（Ｃｒ＋Ｍｏ＋Ｖ）／５＋（Ｃｕ＋Ｎｉ）／１５、各元素記号は含有量（質量％）とする。
Ceq (IIW): 0.33 to 0.45
If Ceq (IIW) is less than 0.33, the required base material strength cannot be obtained. If the area ratio is 0.45 or less, the area fraction of island martensite can be suppressed to less than about 3%, but if it exceeds 0.45, the formation of island martensite is remarkable in the heat affected zone, particularly in the vicinity of the bond. Since the toughness deteriorates, the range of 0.33 to 0.45 is set. Ceq (IIW) is C + Mn / 6 + (Cr + Mo + V) / 5 + (Cu + Ni) / 15, and each element symbol is a content (mass%).
以下、各成分の限定理由について説明する。
Ｃ：０．０３〜０．０８％
Ｃは、構造用鋼として必要な強度を得るために０．０３％以上とし、一方、０．０８％を超えると島状マルテンサイトの生成が顕著となるため、０．０３〜０．０８％とする。
Hereinafter, the reason for limitation of each component is demonstrated.
C: 0.03-0.08%
C is not less than 0.03% in order to obtain the strength required for structural steel, and on the other hand, if it exceeds 0.08%, the formation of island martensite becomes significant, so 0.03 to 0.08% And
Ｓｉ：０．０１〜０．１５％
Ｓｉは、製鋼上０．０１％以上が必要で、一方、０．１５％を超えると、母材の靱性を劣化させるほか、大入熱溶接熱影響部に島状マルテンサイトを生成して溶接部の靱性を劣化させるようになるので、０．０１〜０．１５％とする。
Si: 0.01 to 0.15%
Si needs to be 0.01% or more in terms of steelmaking. On the other hand, if it exceeds 0.15%, it degrades the toughness of the base metal and also generates island martensite in the heat-affected zone of high heat input welding. The toughness of the part is deteriorated, so 0.01 to 0.15%.
Ｍｎ：１．５〜２．６％
Ｍｎは、母材の強度を確保するとともに、アシキュラーフェライトの生成を促進するために１．５％以上を必要とし、一方、２．６％を超えると溶接部の靱性を劣化させるようになるため、１．５〜２．６％とする。
Mn: 1.5 to 2.6%
Mn needs to be 1.5% or more in order to secure the strength of the base material and promote the formation of acicular ferrite. On the other hand, if it exceeds 2.6%, the toughness of the weld will deteriorate. Therefore, the content is set to 1.5 to 2.6%.
Ｐ：０．０３％以下、Ｓ：０．０００５〜０．００４０％
本発明においてＰ，Ｓは不可避的不純物で、Ｐは、０．０３％を超えると溶接部の靱性を劣化させるため、０．０３％以下とする。Ｓは、アシキュラーフェライトの生成に必要な（Ｃａ，Ｍｎ）Ｓを生成するために０．０００５％以上が必要で、一方、０．００４０％を超えると母材の靱性を劣化させるようになるため、０．０００５〜０．００４０％とする。
P: 0.03% or less, S: 0.0005 to 0.0040%
In the present invention, P and S are inevitable impurities. If P exceeds 0.03%, the toughness of the welded portion is deteriorated. S needs to be 0.0005% or more in order to generate (Ca, Mn) S necessary for the generation of acicular ferrite. On the other hand, if it exceeds 0.0040%, the toughness of the base material deteriorates. Therefore, the content is set to 0.0005 to 0.0040%.
Ａｌ：０．００５ 〜０．１ ％
Ａｌは、鋼の脱酸上０．００５％以上、好ましくは０．０１％以上を必要とし、一方、０．１％を超えて含有すると母材の靱性を低下させると同時に溶接金属の靱性を劣化させるようになるため、０．００５ 〜０．１ ％とする。
Al: 0.005 to 0.1%
Al needs to be 0.005% or more, preferably 0.01% or more in terms of deoxidation of steel. On the other hand, if it exceeds 0.1%, the toughness of the base metal is lowered and the toughness of the weld metal is reduced. In order to deteriorate, it is made 0.005 to 0.1%.
Ｎｂ：０．００３〜０．０５％以下
Ｎｂは、母材の強度・靱性および継手の強度を確保するのに有効な元素であるが、０．００３％未満ではその効果が小さく、０．０５％を超えて含有すると溶接熱影響部の靱性が
劣化するようになるため、０．００３〜０．０５％以下とする。
Nb: 0.003 to 0.05% or less Nb is an element effective for ensuring the strength and toughness of the base material and the strength of the joint, but if less than 0.003%, the effect is small. If the content exceeds 50%, the toughness of the weld heat-affected zone will deteriorate, so 0.003 to 0.05% or less.
Ｔｉ：０．００３ 〜０．０３％
Ｔｉは、凝固時にＴｉＮとなって析出し、溶接熱影響部でのオーステナイトの粗大化抑制やフェライト変態核となって高靱性化に寄与するが０．００３％に満たないとその効果が少なく、一方、０．０３％を超えるとＴｉＮ粒子の粗大化によって期待する効果が得られなくなるため、０．００３ 〜０．０３％とする。
Ti: 0.003-0.03%
Ti precipitates as TiN during solidification and contributes to the increase in toughness by suppressing the coarsening of austenite and ferrite transformation nuclei in the weld heat-affected zone. On the other hand, if it exceeds 0.03%, the expected effect cannot be obtained due to the coarsening of TiN particles, so 0.003 to 0.03%.
Ｎ：０．００２５〜０．００７０％
Ｎは、溶接熱影響部でのオーステナイトの粗大化抑制やフェライト変態核となって高靱性化に寄与するＴｉＮの必要量を確保するうえで必要な元素で、０．００２５％未満では十分なＴｉＮ量が得られず、一方、０．００７０％を超えると溶接熱サイクルによってＴｉＮが溶解する領域では固溶Ｎ量が増加するので靱性が著しく低下するため、０．００２５〜０．００７０％とする。
N: 0.0025 to 0.0070%
N is an element necessary to suppress the austenite coarsening in the weld heat affected zone and to secure the necessary amount of TiN that contributes to high toughness by becoming a ferrite transformation nucleus. If less than 0.0025%, TiN is sufficient. On the other hand, if the amount exceeds 0.0070%, the amount of solid solution N increases in the region where TiN is dissolved by the welding heat cycle, so the toughness is remarkably reduced, so 0.0025 to 0.0070%. .
Ｂ：０．０００３〜０．００２５％
Ｂは、溶接熱影響部でＢＮを生成して、固溶Ｎを低減するとともにフェライト変態核として作用させるため０．０００３％以上を必要とし、一方、０．００２５％を超えて添加すると焼入れ性が増して溶接部の靱性が劣化するようになるため、０．０００３〜０．００２５％とする。
B: 0.0003 to 0.0025%
B needs to be 0.0003% or more in order to generate BN in the weld heat affected zone to reduce the solid solution N and to act as a ferrite transformation nucleus. On the other hand, when added over 0.0025%, hardenability is required. Increases and the toughness of the welded portion deteriorates, so 0.0003 to 0.0025%.
Ｃａ：０．０００５〜０．００３０％
Ｃａは、アシキュラーフェライトの生成に必要な（Ｃａ，Ｍｎ）Ｓを生成するために０．０００５％以上を必要とし、一方、０．００３０％を超えて含有しても効果が飽和するため、０．０００５〜０．００３０％とする。
Ca: 0.0005 to 0.0030%
Ca needs 0.0005% or more in order to generate (Ca, Mn) S necessary for the generation of acicular ferrite, and on the other hand, even if contained over 0.0030%, the effect is saturated, 0.0005 to 0.0030%.
以上が本発明の基本成分組成であるが、さらに特性を向上させる場合、Ｃｕ、Ｎｉ、Ｃｒ、ＭｏおよびＶから選ばれる少なくとも１種または２種以上を含有させることができる。
Ｃｕ：１．０％以下、Ｎｉ：１．０％以下、Ｃｒ：０．４％以下およびＭｏ：０．４％以下
Ｃｕ，Ｎｉ，ＣｒおよびＭｏは、母材の高強度化に有効な元素であるが、その効果を得るためにはＣｕ，Ｎｉは０．０５％以上、Ｃｒ，Ｍｏは０．０２％以上の添加が必要である。しかし、いずれの元素も多量に添加し過ぎると、靱性に悪影響を及ぼすため、添加する場合には、Ｃｕ，Ｎｉは１．０％以下、Ｃｒ，Ｍｏは０．４％以下とするのが望ましい。
Ｖ：０．２％以下
Ｖは、母材の強度・靱性の向上および溶接熱影響部においてＶＮを形成してフェライト生成核として働くが、０．２％を超えると靱性の低下を招くようになるので、含有させる場合は、０．２％以下とする。
The above is the basic component composition of the present invention, but when further improving the characteristics, at least one or more selected from Cu, Ni, Cr, Mo and V can be contained.
Cu: 1.0% or less, Ni: 1.0% or less, Cr: 0.4% or less, and Mo: 0.4% or less Cu, Ni, Cr and Mo are effective elements for increasing the strength of the base material. However, in order to obtain the effect, it is necessary to add 0.05% or more of Cu and Ni and 0.02% or more of Cr and Mo. However, if any of these elements is added too much, the toughness is adversely affected. Therefore, when added, it is desirable that Cu and Ni be 1.0% or less and Cr and Mo be 0.4% or less. .
V: 0.2% or less V increases the strength and toughness of the base metal and forms VN in the heat affected zone of the base metal to act as ferrite nuclei. However, if it exceeds 0.2%, the toughness is reduced. Therefore, when it is contained, the content is made 0.2% or less.
本発明では、さらに溶接熱影響部の靭性を向上させる場合、Ｍｇ、Ｚｒ、ＲＥＭから選ばれる少なくとも１種または２種以上を含有させることができる。 In this invention, when improving the toughness of a welding heat affected zone further, at least 1 sort (s) or 2 or more types chosen from Mg, Zr, and REM can be contained.
Ｍｇ：０．０００５〜０．００５０％、Ｚｒ：０．００１〜０．０２％、ＲＥＭ：０．００１〜０．０２％
Ｍｇ、Ｚｒ、ＲＥＭは、酸化物の分散による靱性改善効果を有し、このような効果を発揮させるため、含有させる場合は、Ｍｇ：０．０００５〜０．００５０％、Ｚｒ：０．００１〜０．０２％、ＲＥＭ：０．００１〜０．０２％とする。上限はいずれも効果が飽和するため規定する。
［硫化物あるいは酸硫化物］
本発明に係る鋼は、鋼中に、ＭｎとＣａとを質量比でＭｎ／（Ｍｎ＋Ｃａ）：０．１〜０．７の範囲で含む硫化物あるいは酸硫化物が、０．１〜５μｍの大きさで１ｍｍ２中に５０〜１０００個を分散して存在させ、大入熱溶接時に最も高温にさらされるボンド部近傍において、アシキュラーフェライトの生成を促進する。前記硫化物は（Ｃａ、Ｍｎ）Ｓとする。また、前記硫化物が酸化物と複合した物質である酸硫化物として存在する場合にも、硫化物単独の場合と同様、上述のアシキュラーフェライトの生成が促進される。
Mg: 0.0005-0.0050%, Zr: 0.001-0.02%, REM: 0.001-0.02%
Mg, Zr, and REM have an effect of improving toughness due to dispersion of oxides. In order to exert such an effect, when contained, Mg: 0.0005 to 0.0050%, Zr: 0.001 0.02%, REM: 0.001 to 0.02%. The upper limit is specified because the effect is saturated.
[Sulphides or oxysulfides]
In the steel according to the present invention, the sulfide or oxysulfide containing Mn and Ca in a mass ratio of Mn / (Mn + Ca): 0.1 to 0.7 is 0.1 to 5 μm in the steel. 50 to 1000 particles are dispersed in 1 mm 2 in size, and the formation of acicular ferrite is promoted in the vicinity of the bond portion that is exposed to the highest temperature during high heat input welding. The sulfide is (Ca, Mn) S. In addition, when the sulfide is present as an oxysulfide that is a substance combined with an oxide, the formation of the above-mentioned acicular ferrite is promoted as in the case of the sulfide alone.
Ｃｅｑ（ＩＩＷ）を０．４５以下とすれば、島状マルテンサイトの面積分率を概ね３％未満に抑えることができるが、靭性向上の観点からは、２％未満、更には１％未満に抑えることが望ましいため、ミクロ組織をアシキュラーフェライト組織主体として、島状マルテンサイトの低減を達成する。 If Ceq (IIW) is 0.45 or less, the area fraction of island martensite can be suppressed to less than 3%, but from the viewpoint of improving toughness, it is less than 2%, and further less than 1%. Since it is desirable to suppress, the microstructure is mainly composed of the acicular ferrite structure, and the reduction of island martensite is achieved.
島状マルテンサイトの生成量は上述のとおり主としてＣｅｑ（ＩＩＷ）によるが、ベイナイト変態機構にも左右され、アシキュラーフェライト組織が多くなるほど、未変態オーステナイトが細かく分断されることで、Ｃの濃化が軽減されるので、島状マルテンサイトの生成量も減少する。 The amount of island-like martensite produced is mainly due to Ceq (IIW) as described above, but it depends on the bainite transformation mechanism. Is reduced, so that the amount of island martensite produced is also reduced.
Ｃｅｑ（ＩＩＷ）を０．４５以下とし、更に、上記硫化物あるいは酸硫化物の組成および分散状態を規定する本発明によれば、旧オーステナイト粒界から析出した粒界フェライトを除いた旧オーステナイト粒内組織の大きさが１０μｍ以下となるとともに、島状マルテンサイトの形成が抑制されて２％未満となり、溶接熱影響部の靭性が向上する。 According to the present invention which defines Ceq (IIW) of 0.45 or less and further defines the composition and dispersion state of the sulfide or oxysulfide, the prior austenite grains excluding the grain boundary ferrite precipitated from the prior austenite grain boundaries While the size of the inner structure is 10 μm or less, the formation of island martensite is suppressed to less than 2%, and the toughness of the weld heat affected zone is improved.
なお、旧オーステナイト粒内組織の大きさとは、旧オーステナイト粒界に存在する初析フェライトを除いた粒内フェライトの大きさを評価したもので、ＥＢＳＤ（電子線後方散乱回折）で測定した１５°以上の傾角を有する結晶粒界組織において、線分法で測定した平均切片長さのことである。 The size of the prior austenite intragranular structure is an evaluation of the size of intragranular ferrite excluding proeutectoid ferrite existing in the prior austenite grain boundaries, and is measured by EBSD (electron beam backscatter diffraction). The average intercept length measured by the line segment method in the grain boundary structure having the above inclination.
本発明に係る鋼材は、構造用鋼の常法による製造条件で製造可能である。例えば、まず溶銑を転炉で精錬して鋼とした後、ＲＨ脱ガスを行い、連続鋳造または造塊−分塊工程を経て鋼片とする。これを再加熱し、熱間圧延後放冷するか、あるいはまた、前記熱間圧延後に、加速冷却、直接焼入れ−焼戻し、再加熱焼入れ−焼戻し、再加熱焼準−焼戻しなどの工程で製造される。以下、本発明の作用効果を実施例に基づいて説明する。 The steel material according to the present invention can be manufactured under manufacturing conditions according to a conventional method for structural steel. For example, the hot metal is first refined in a converter to form steel, and then RH degassing is performed to obtain a steel slab through a continuous casting or ingot-bundling process. This can be reheated and allowed to cool after hot rolling, or alternatively, can be produced by such processes as accelerated cooling, direct quenching-tempering, reheating quenching-tempering, reheating normalization-tempering after the hot rolling. The Hereinafter, the operation and effect of the present invention will be described based on examples.
１５０ｋｇの高周波溶解炉にて、表１に示す組成の鋼を溶製し、熱間圧延により厚さ７０ｍｍのスラブとした。得られたスラブを１１５０℃に２時間加熱後、板厚中心温度で８５０℃において板厚３０ｍｍの鋼板に仕上げた後、７℃／ｓの冷却速度で加速冷却した。当該冷却速度は板厚６０ｍｍの鋼板の板厚１／４位置の冷速を、板厚３０ｍｍの板厚中心でシミュレートしたものである。 In a 150 kg high-frequency melting furnace, steel having the composition shown in Table 1 was melted and formed into a slab having a thickness of 70 mm by hot rolling. The obtained slab was heated to 1150 ° C. for 2 hours, finished to a steel plate having a thickness of 30 mm at a center thickness of 850 ° C., and then accelerated and cooled at a cooling rate of 7 ° C./s. The cooling rate is a simulation of the cooling rate at the position of 1/4 of the thickness of a steel plate having a thickness of 60 mm at the center of the thickness of 30 mm.
圧延した３０ｍｍの鋼板を５００℃で１０分保持して焼戻した後、試験片長手方向が板幅方向と一致するように、平行部１４φ×８５ｍｍ、標点間距離７０ｍｍの丸棒引張試験片を採取し、母材強度（降伏応力ＹＳ，引張強さＴＳ）を測定した。また、上記厚鋼板から、試験片長手方向が圧延方向と一致するように、２ｍｍＶノッチシャルピー試験片を採取し、−１００〜４０℃の範囲で適宜シャルピー衝撃試験を行い、脆性破面率５０％となる破面遷移温度ｖＴｒｓを求め、靭性を評価した。 After rolling and rolling a 30 mm steel plate at 500 ° C. for 10 minutes, a round bar tensile test piece with a parallel part 14φ × 85 mm and a distance between gauge points of 70 mm so that the longitudinal direction of the test piece coincides with the plate width direction. The base material strength (yield stress YS, tensile strength TS) was measured. Further, a 2 mm V notch Charpy test piece was taken from the thick steel plate so that the longitudinal direction of the test piece coincided with the rolling direction, and an appropriate Charpy impact test was conducted in the range of −100 to 40 ° C., and the brittle fracture surface ratio was 50%. The fracture surface transition temperature vTrs to be obtained was determined and the toughness was evaluated.
また、得られた鋼板について、大きさ０．１〜５μｍの酸硫化物の個数密度、硫化物組成を調査した。酸硫化物の個数密度は、倍率：５００倍または１０００倍の光学顕微鏡を用いて、ミクロ組織観察用試料を観察することにより計測した。硫化物中の質量比：Ｍｎ／（Ｍｎ＋Ｃａ）は、分散した酸硫化物粒子に含まれる硫化物に着目し、ＳＥＭ−ＥＤＸ（ＳＥＭ：走査型電子顕微鏡、ＥＤＸ：エネルギー分散型Ｘ線分析器）装置にて分析して求めた。 The obtained steel sheet was examined for the number density and sulfide composition of oxysulfides having a size of 0.1 to 5 μm. The number density of oxysulfides was measured by observing a sample for observing a microstructure using an optical microscope with a magnification of 500 times or 1000 times. Mass ratio in sulfide: Mn / (Mn + Ca) pays attention to sulfide contained in dispersed oxysulfide particles, SEM-EDX (SEM: scanning electron microscope, EDX: energy dispersive X-ray analyzer) It was determined by analysis with an apparatus.
さらに、これらの鋼板の大入熱溶接熱影響部の靭性を評価するため、再現溶接熱サイクル試験を行った。幅８０ｍｍ×長さ８０ｍｍ×厚み１５ｍｍの再現熱サイクル試験片を採取し、１４５０℃に加熱後８００〜５００℃を２７０ｓで冷却する再現溶接熱サイクル（板厚３０ｍｍの鋼板のエレクトロガス溶接での入熱量４００ｋＪ／ｃｍの溶接熱影響部に相当）を付与し、２ｍｍＶノッチシャルピー試験にてｖＴｒｓ（℃）を求めて評価した。 Furthermore, in order to evaluate the toughness of the high heat input welding heat-affected zone of these steel plates, a reproducible welding heat cycle test was conducted. A reproducible heat cycle test piece of width 80 mm x length 80 mm x thickness 15 mm was collected, heated to 1450 ° C and then cooled to 800-500 ° C in 270 s (recycled welding heat cycle (input in electrogas welding of 30 mm thick steel plate) (Corresponding to a welding heat-affected zone having a heat quantity of 400 kJ / cm) was applied, and vTrs (° C.) was obtained and evaluated by a 2 mmV notch Charpy test.
再現溶接熱サイクル付与部（２ｍｍＶノッチのノッチ底相当部）における、アシキュラーフェライト生成の指標として、粒内組織平均切片長さを求めた。粒内組織平均切片長さはＥＢＳＤで測定した１５°以上の傾角を有する粒界組織において、旧オーステナイト粒界に存在する初析フェライトを除いて線分法で評価した平均切片長さのことである。 The average grain length of the intragranular structure was determined as an index for the formation of acicular ferrite in the reproducible welding heat cycle application part (corresponding to the notch bottom of 2 mmV notch). Intragranular structure average section length is the average section length evaluated by the line segment method with the exception of proeutectoid ferrite present in the prior austenite grain boundaries in grain boundary structures with an inclination of 15 ° or more measured by EBSD. is there.
再現溶接熱サイクル付与部（２ｍｍＶノッチのノッチ底相当部）における島状マルテンサイトの面積分率（以下、島状マルテンサイト分率（％））は、２段エッチング法により島状マルテンサイトを現出したのち、ＳＥＭの２０００倍の写真をトレースしたうえで画像解析により算出した。 The area fraction of island martensite (hereinafter referred to as island martensite fraction (%)) in the reproducible welding heat cycle imparted part (corresponding to the notch bottom of the 2 mmV notch) is expressed as island martensite by a two-step etching method. After taking out, it was calculated by image analysis after tracing a photograph of 2000 times SEM.
表２に、母材鋼板の酸硫化物個数密度（個／ｍｍ２）、硫化物組成（Ｍｎ／（Ｍｎ＋Ｃａ），各元素記号は含有量（質量％））と機械的性質（ＹＳ（ＭＰａ）、ＴＳ（ＭＰａ）），靭性（シャルピー衝撃試験結果、ｖＴｒｓ（℃））ならびに再現溶接熱サイクル付与部（２ｍｍＶノッチのノッチ底相当部）における粒内組織平均切片長さ（μｍ）および島状マルテンサイト分率（％）と再現溶接熱サイクルシャルピー衝撃試験結果（ｖＴｒｓ（℃））を示す。 Table 2 shows the oxysulfide number density (pieces / mm 2 ), sulfide composition (Mn / (Mn + Ca), each element symbol is content (% by mass)) and mechanical properties (YS (MPa)) of the base steel sheet. , TS (MPa)), toughness (Charpy impact test result, vTrs (° C.)), average grain length of the intragranular structure (μm) and repetitive welding heat cycle imparted part (corresponding notch bottom of 2 mmV notch) The site fraction (%) and reproducible welding thermal cycle Charpy impact test results (vTrs (° C.)) are shown.
表２中、鋼番１、２、８〜１０はいずれも、本発明範囲内の化学成分、ミクロ組織の規定を満足する本発明例で、再現溶接熱サイクル付与部（２ｍｍＶノッチのノッチ底相当部）における、粒内組織平均切片長さが１０μｍ未満、島状マルテンサイト面積分率が２％未満で、再現溶接熱サイクルシャルピー衝撃試験結果（ｖＴｒｓ（℃））は−５５℃以下であった。 In Table 2, steel numbers 1, 2 and 8 to 10 are all examples of the present invention that satisfy the chemical composition and microstructure specifications within the scope of the present invention. In part), the intragranular tissue average section length was less than 10 μm, the island-like martensite area fraction was less than 2%, and the reproduced welding thermal cycle Charpy impact test result (vTrs (° C.)) was −55 ° C. or less. .
一方、鋼番１１〜２３は成分組成および／またはミクロ組織の規定が本発明範囲外となった比較例である。これらは、本発明例の場合に比べ、粒内組織平均切片長さ（μｍ）が長い、および／または島状マルテンサイト分率（％）が大きいため、再現溶接熱サイクルシャルピー衝撃試験結果（ｖＴｒｓ（℃））は−４５℃以上と劣っていた。 On the other hand, Steel Nos. 11 to 23 are comparative examples in which the composition of the components and / or the microstructure is out of the scope of the present invention. Compared to the case of the present invention example, the average intra-granular length (μm) and / or the island-like martensite fraction (%) are large, so that the results of the repeated welding thermal cycle Charpy impact test (vTrs (° C.) was inferior to −45 ° C. or higher.
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