JP4864297B2 - 490 MPa class high strength steel for welded structure excellent in high temperature strength and method for producing the same - Google Patents

490 MPa class high strength steel for welded structure excellent in high temperature strength and method for producing the same Download PDF

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JP4864297B2
JP4864297B2 JP2004213511A JP2004213511A JP4864297B2 JP 4864297 B2 JP4864297 B2 JP 4864297B2 JP 2004213511 A JP2004213511 A JP 2004213511A JP 2004213511 A JP2004213511 A JP 2004213511A JP 4864297 B2 JP4864297 B2 JP 4864297B2
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steel
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toughness
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JP2006028628A (en
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泰 水谷
義之 渡部
龍治 植森
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Nippon Steel Corp
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Priority to PCT/JP2005/013101 priority patent/WO2006009091A1/en
Priority to CNA2005800242067A priority patent/CN1989264A/en
Priority to EP05760159A priority patent/EP1790749A1/en
Priority to TW094124502A priority patent/TWI297732B/en
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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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • 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
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
  • Metal Rolling (AREA)

Description

本発明は、建築、土木、海洋構造物、造船、各種の貯槽タンクなどの一般的な溶接構造物に用いられ、６００℃以上８００℃以下の温度範囲において、１時間程度の比較的短時間における高温強度に優れた溶接構造用高張力鋼ならびにその製造方法に関するものである。以下の説明では、主として厚板（厚鋼板）を対象としているが、本発明は、鋼管や形鋼などにも幅広く適用できる。   INDUSTRIAL APPLICABILITY The present invention is used for general welded structures such as architecture, civil engineering, offshore structures, shipbuilding, various storage tanks, etc., and in a relatively short time of about 1 hour in a temperature range of 600 ° C. to 800 ° C. The present invention relates to a high-strength steel for welded structures excellent in high-temperature strength and a method for producing the same. In the following description, thick plates (thick steel plates) are mainly targeted, but the present invention can be widely applied to steel pipes, shaped steels, and the like.

一般的な溶接構造用鋼材の強度は、約３５０℃から強度が低下し、その許容温度は約５００℃とされている。このため、それらの鋼材をビルや事務所、住居、立体駐車場などの建築物に用いた場合には、火災における安全性を確保するため、十分な耐火被覆を施すことが義務付けられており、建築関連諸法令では、火災時に鋼材温度が３５０℃以上にならないよう規定されている。これは、前記鋼材では、耐力（降伏強度）が３５０℃程度で常温の２／３程度になり、必要な強度を下回るためである。このような耐火被覆は、建設コストに多大な影響を及ぼしているのが実態である。   The strength of a general welded structural steel material decreases from about 350 ° C., and its allowable temperature is about 500 ° C. For this reason, when these steel materials are used for buildings such as buildings, offices, residences, and multistory parking lots, it is obliged to apply sufficient fireproof coating to ensure safety in fire. Various laws related to construction stipulate that the temperature of steel materials does not exceed 350 ° C in the event of a fire. This is because the steel material has a yield strength (yield strength) of about 2/3 of room temperature at about 350 ° C., which is lower than the required strength. In fact, such a fireproof coating has a great influence on the construction cost.

このような課題を解決するため、高温時の耐力を備えた「耐火鋼」が開発されている（例えば、特許文献１、２参照）。それぞれ６００℃、７００℃での耐力が常温での規格最小耐力（降伏強度）の２／３以上を維持できるとするものである。しかし、いずれも特定の温度での耐力が示されるのみで、それより高温での耐力については、全く言及されていない。特に、７００℃超の温度は、鋼成分によっては部分的に変態を開始する温度領域に入るため、急激な耐力（降伏強度）低下が懸念されるなど、設計に反映できるような安定した実用鋼製造はきわめて困難であった。   In order to solve such a problem, “refractory steel” having a proof stress at high temperatures has been developed (for example, see Patent Documents 1 and 2). The proof stress at 600 ° C. and 700 ° C. can maintain 2/3 or more of the standard minimum proof strength (yield strength) at room temperature. However, in any case, the proof stress at a specific temperature is only shown, and the proof stress at a higher temperature is not mentioned at all. In particular, since a temperature exceeding 700 ° C. is in a temperature range in which transformation is partially started depending on the steel components, there is a concern that a sudden decrease in yield strength (yield strength) may occur and stable practical steel that can be reflected in the design. Manufacturing was extremely difficult.

先に本発明者らは、７００〜８００℃での高温強度を確保できる鋼およびその製造方法として、発明をなしている（例えば、特許文献３参照）。鋼成分的にはＢ添加を必須とするもので、組織制御を容易にし、特に建築構造用鋼としての低降伏比を達成し得るものである。しかし、一般的に知られているように、Ｂは焼入性を増大させるなど功罪相半ばする。例えば、小入熱溶接時には、溶接熱影響部が著しく硬化するため靭性に劣り、逆に溶接入熱が大きくなり過ぎるとオーステナイト粒界に析出し、Ｂの焼入性を有効利用できず組織が粗大となって靭性に劣るため、溶接入熱範囲が限られるという問題を孕んでいた。   The present inventors have made an invention as steel that can secure high-temperature strength at 700 to 800 ° C. and a method for producing the same (see, for example, Patent Document 3). As a steel component, addition of B is indispensable, and the structure control is facilitated, and in particular, a low yield ratio as a steel for building structures can be achieved. However, as is generally known, B is in the midst of merits and demerits, such as increasing hardenability. For example, at the time of small heat input welding, the weld heat-affected zone is significantly hardened, so that the toughness is inferior. Conversely, if the welding heat input becomes too large, it precipitates at the austenite grain boundaries, and the hardenability of B cannot be used effectively. Since it was coarse and inferior in toughness, the problem was that the welding heat input range was limited.

ところで、建築構造用鋼としては、耐震性の観点から低降伏比の要求があり、ＪＩＳにおける「建築構造用圧延鋼材」規格でも８０％以下と規定されている。本発明者らによる先の発明は、これらを念頭に置いたものであった。しかし、平成１２年６月に施行された改正建築基準法では、これまでの使用規定から性能規定に改正され、新しい技術、材料を早期に実用化することが含まれている。建築用鋼材に関しては、建築基準法３７条において、第１項：ＪＩＳ材で建築構造用として使用が許可されるもの、第２項：必要各種性能に応じて鋼材の性能を評価した上で国土交通大臣が認定したものが使用できることになった。そこで、本発明者らは、ＪＩＳ建築用鋼材における降伏比の規定にとらわれることなく、高温強度はもとより、溶接性、広い入熱範囲での溶接部靭性に優れる鋼材を鋭意検討し、本願発明に至った。   By the way, as steel for building structures, there is a demand for a low yield ratio from the viewpoint of earthquake resistance, and it is defined as 80% or less in the “rolled steel for building structures” standard in JIS. The previous invention by the present inventors was made with these in mind. However, the revised Building Standards Law, which came into effect in June 2000, includes revisions from previous usage rules to performance rules, and the early implementation of new technologies and materials. Regarding building steel materials, Article 37 of the Building Standards Law, Article 1: JIS materials that are allowed to be used for building structures, Item 2: National land after evaluating the performance of steel materials according to various required performances Those approved by the Minister of Transport can be used. Therefore, the present inventors diligently studied a steel material excellent in weldability and weld toughness in a wide heat input range as well as high-temperature strength without being bound by the definition of the yield ratio in JIS steel for construction. It came.

特開平２−７７５２３号公報Japanese Patent Laid-Open No. 2-77523 特開平１０−６８０４４号公報Japanese Patent Laid-Open No. 10-68044 特開２００４−４３９６１号公報JP 2004-43961 A

上述したように、建築物に鋼材を利用する場合、通常の鋼材では高温強度（耐力＝降伏応力）が低いため、無被覆や耐火被覆軽減で利用することができず、高価な耐火被覆を施さなければならなかった。また、新しく開発された鋼材でも、その耐火温度は６００〜７００℃までの保証が限界であり、７００〜８００℃での無耐火被覆使用およびこれによる耐火被覆工程の省略が可能となる鋼材の開発が望まれていた。   As described above, when steel is used in buildings, high-temperature strength (proof strength = yield stress) is low with ordinary steel, so it cannot be used for uncovered or fire-resistant coating reduction, and expensive fire-resistant coating is applied. I had to. In addition, even with newly developed steel materials, the fire resistance temperature is limited to a guarantee of 600 to 700 ° C., and the development of steel materials that enable the use of a non-refractory coating at 700 to 800 ° C. and the elimination of the fireproof coating process thereby. Was desired.

本発明の目的は、６００℃以上８００℃以下の温度範囲における高温強度に優れた溶接構造用高張力鋼ならびに当該鋼を工業的に安定して供給可能な製造方法を提供することにある。   An object of the present invention is to provide a high-strength steel for welded structures excellent in high-temperature strength in a temperature range of 600 ° C. or higher and 800 ° C. or lower, and a manufacturing method capable of supplying the steel stably industrially.

本発明は、前述の課題を克服するために、鋼成分やミクロ組織などを適正範囲に限定することで目的を達成したもので、その要旨は以下に示す通りである。   In order to overcome the above-mentioned problems, the present invention achieves the object by limiting steel components, microstructures, and the like to appropriate ranges, and the gist thereof is as follows.

（１）鋼成分が質量％で、
Ｃ：０．００５％以上０．０４０％未満、
Ｓｉ：０．５％以下、
Ｍｎ：０．１〜０．５％、
Ｐ：０．０２％以下、
Ｓ：０．０１％以下、
Ｍｏ：０．３〜１．５％、
Ｎｂ：０．０３〜０．１５％、
Ａｌ：０．０６％以下、
Ｎ：０．００６％以下、
かつ、
ＰCM＝Ｃ＋Ｓｉ／３０＋Ｍｎ／２０＋Ｃｕ／２０＋Ｎｉ／６０＋Ｃｒ／２０／Ｍｏ／１５＋Ｖ／１０＋５Ｂ
と定義する溶接割れ感受性組成ＰCMが０．１５％以下で、残部が鉄および不可避的不純物からなり、ミクロ組織がフェライトとベイナイトの混合組織主体であって、そのベイナイトの分率が２０〜９０％であることを特徴とする高温強度に優れた溶接構造用４９０ＭＰａ級高張力鋼。
(1) The steel component is mass%,
C: 0.005% or more and less than 0.040%,
Si: 0.5% or less,
Mn: 0.1 to 0.5%
P: 0.02% or less,
S: 0.01% or less,
Mo: 0.3 to 1.5%,
Nb: 0.03-0.15%,
Al: 0.06% or less,
N: 0.006% or less,
And,
P CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 / Mo / 15 + V / 10 + 5B
Definition welding crack susceptibility composition P CM is below 0.15% and the remaining part being iron and unavoidable impurities, the microstructure is a mixed structure mainly of ferrite and bainite, 20 to the fraction of the bainite 490 MPa class high strength steel for welded structures excellent in high temperature strength characterized by being 90%.

（２）質量％で、さらに、
Ｃｕ：０．０５〜１．０％、
Ｎｉ：０．０５〜１．０％、
Ｃｒ：０．０５〜１．０％、
Ｖ：０．０１〜０．１％、
Ｔｉ：０．００５〜０．０２５％
の範囲で１種または２種以上を含有することを特徴とする上記（１）に記載の高温強度に優れた溶接構造用４９０ＭＰａ級高張力鋼。 (2) In mass%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
Cr: 0.05 to 1.0%,
V: 0.01 to 0.1%
Ti: 0.005-0.025%
The 490 MPa class high-tensile steel for welded structures having excellent high-temperature strength as described in (1) above, which contains one or more kinds in the range of

（３）質量％で、さらに、
Ｃａ：０．０００５〜０．００４％、
ＲＥＭ：０．０００５〜０．００４％、
Ｍｇ：０．０００１〜０．００６％
のいずれか１種または２種以上を含有することを特徴とする上記（１）または（２）に記載の高温強度に優れた溶接構造用４９０ＭＰａ級高張力鋼。 (3) In mass%,
Ca: 0.0005 to 0.004%,
REM: 0.0005 to 0.004%,
Mg: 0.0001 to 0.006%
The 490 MPa class high-tensile steel for welded structures having excellent high-temperature strength as described in (1) or (2) above, comprising one or more of the above.

（４）板厚１／４厚位置の圧延方向と平行な断面の旧オーステナイト粒の平均円相当径が、１２０μｍ以下であることを特徴とする上記（１）〜（３）のいずれかに記載の高温強度に優れた溶接構造用４９０ＭＰａ級高張力鋼。   (4) The average equivalent circular diameter of the prior austenite grains having a cross section parallel to the rolling direction at a thickness of ¼ thickness is 120 μm or less, according to any one of (1) to (3) above 490MPa class high strength steel for welded structures with excellent high temperature strength.

（５）上記（１）〜（３）のいずれか１項に記載の鋼成分からなる鋼片または鋳片を１１００〜１２５０℃の温度範囲に再加熱後、１１００℃以下での累積圧下量を３０％以上として、８５０℃以上の温度で圧延し、その後８００℃以上の温度から６５０℃以下の温度まで加速冷却することを特徴とする高温強度に優れた溶接構造用４９０ＭＰａ級高張力鋼の製造方法。
(5) (1) above - cumulative reduction ratio of the billet or slab of steel components after reheating to a temperature range of 1100 to 1250 ° C., at 1100 ° C. or less according to any one of (3) Of 490 MPa class high-strength steel for welded structures excellent in high-temperature strength, characterized by being rolled at a temperature of 850 ° C. or higher at a temperature of 30% or higher, and then accelerated cooling from a temperature of 800 ° C. or higher to a temperature of 650 ° C. or lower. Production method.

本発明に基づいた鋼成分、製造方法で製造された鋼材は、ミクロ組織も本発明の限定範囲を満たし、高温強度はもとより、溶接性、溶接部靭性にも優れることが実施例で実証された。すなわち、従来の６００℃程度までの高温特性を保証した耐火鋼をはるかに凌ぐ高温特性を有する溶接構造用鋼が工業的に安定して大量生産できることが示され、例えば建築用途としては、適用建築物や完全無耐火被覆の大幅拡大が期待され、その利用価値は大である。   It was demonstrated in the Examples that the steel components produced by the steel components and the production method based on the present invention satisfy the limited range of the present invention, and are excellent not only in high temperature strength but also in weldability and weld zone toughness. . That is, it is shown that welded structural steel having high-temperature characteristics far exceeding the conventional refractory steel that guarantees high-temperature characteristics up to about 600 ° C. can be industrially mass-produced stably. The product and the complete fire-resistant coating are expected to greatly expand, and its utility value is great.

以下、本発明の詳細を説明する。   Details of the present invention will be described below.

高温強度は、ＭｏとＮｂの複合添加により、高温時に安定な炭窒化物の析出を促進するとともに、ミクロ組織のベイナイト化による転位密度を増大させ、さらには固溶ＭｏおよびＮｂにより転位回復を遅延させることが有効である。特に、本発明の目的とする７００〜８００℃というきわめて高温での強度発現のためには、従来知見の延長ではＭｏの多量添加が必須であるが、溶接構造用鋼としての優れた溶接性、溶接部靭性確保の観点とは相反し、高温強度との両立はきわめて困難である。   High-temperature strength promotes the precipitation of carbonitrides stable at high temperatures by the combined addition of Mo and Nb, increases the dislocation density by bainite of the microstructure, and further delays dislocation recovery by solid solution Mo and Nb It is effective to make it. In particular, in order to develop strength at an extremely high temperature of 700 to 800 ° C., which is the object of the present invention, a large amount of Mo is indispensable for extension of conventional knowledge, but excellent weldability as a steel for welded structures, Contrary to the viewpoint of ensuring the weld zone toughness, it is extremely difficult to achieve high temperature strength.

本発明者らの研究によれば、合金元素の適正化と組織制御、特に高温における母相組織の熱的安定性と適切な整合析出強化効果および転位回復遅延効果を得ることにより、優れた溶接性、溶接部靭性と高温強度とを両立することができることを見出した。   According to the study by the present inventors, it is possible to achieve excellent welding by optimizing the alloy elements and controlling the structure, in particular, obtaining the thermal stability of the matrix structure at high temperatures and the appropriate coherent precipitation strengthening effect and the dislocation recovery delay effect. It has been found that it is possible to satisfy both of the properties, weld toughness and high temperature strength.

まず、本発明が請求項の通りに鋼成分を限定した理由について説明する。   First, the reason why the present invention limits the steel components as claimed will be described.

Ｃは、鋼材の特性に最も顕著な効果を及ぼすもので、狭い範囲に制御されなければならず、０．００５％以上０．０４０％未満が限定範囲である。０．００５％未満のＣ量では強度が不足し、０．０４０％以上となるとＭｏ添加量が多い本発明においては溶接性、溶接部靭性を劣化させるとともに、圧延終了後の冷却速度が過大の場合はベイナイトの生成分率が増加し強度が超過する危険性が高まる。さらに、火災相当の高温加熱時に、ベイナイトとフェライトの混合母相組織を熱力学的に安定に保ち、Ｍｏ、Ｎｂ、Ｖ、Ｔｉの複合炭窒化析出物との整合性を維持して、強化効果を確保する上でもＣを０．０４０％未満とする必要がある。   C has the most remarkable effect on the properties of the steel material, and must be controlled in a narrow range, with 0.005% or more and less than 0.040% being the limited range. If the amount of C is less than 0.005%, the strength is insufficient, and if it is 0.040% or more, the amount of Mo added is large. In the present invention, weldability and weld zone toughness are deteriorated, and the cooling rate after rolling is excessive. In such a case, the bainite production rate increases and the risk of excess strength increases. Furthermore, during high-temperature heating equivalent to fire, the mixed matrix structure of bainite and ferrite is kept thermodynamically stable, maintaining consistency with the composite carbonitride precipitates of Mo, Nb, V, and Ti. In order to secure C, C must be less than 0.040%.

Ｓｉは、脱酸上鋼に含まれる元素であり、置換型の固溶強化作用をもつことから常温での母材強度向上に有効であるが、特に６００℃超の高温強度を改善する効果はない。また、多く添加すると溶接性、溶接部靭性が劣化するため、上限を０．５％に限定した。鋼の脱酸はＴｉ、Ａｌのみでも可能であり、溶接部靭性や焼入性などの観点から低いほど好ましく、必ずしも添加する必要はない。   Si is an element contained in deoxidized upper steel and is effective in improving the strength of the base metal at room temperature because it has a substitutional solid solution strengthening action. In particular, the effect of improving the high temperature strength above 600 ° C. is Absent. Moreover, since weldability and weld part toughness will deteriorate when adding much, the upper limit was limited to 0.5%. Deoxidation of steel can be performed only with Ti and Al, and is preferably as low as possible from the viewpoint of weld toughness and hardenability, and it is not always necessary to add it.

Ｍｎは、強度、靭性を確保する上で不可欠な元素ではあるが、置換型の固溶強化元素であるＭｎは、常温での強度上昇には有効であるが、特に６００℃超の高温強度にはあまり大きな改善効果はない。したがって、本発明のような比較的多量のＭｏを含有する鋼において溶接性向上すなわちＰCM低減の観点から０．５％以下とする必要がある。Ｍｎの上限を低く抑えることにより、連続鋳造スラブの中心偏析の点からも有利となる。なお、下限については、母材の強度、靭性調整上、０．１％以上の添加が必要である。 Mn is an indispensable element for ensuring strength and toughness. However, Mn, which is a substitutional solid solution strengthening element, is effective for increasing the strength at room temperature. There is not much improvement effect. Therefore, it is necessary to relatively viewpoint of weldability improving i.e. P CM reduced in steel containing a large amount of Mo such as in the present invention is 0.5% or less. By keeping the upper limit of Mn low, it is advantageous from the viewpoint of center segregation of the continuously cast slab. In addition, about a minimum, 0.1% or more of addition is required on the intensity | strength and toughness adjustment of a base material.

ＰおよびＳは、本発明鋼においては不純物であり、低いほど好ましい。Ｐは粒界に偏析して粒界破壊を助長し、ＳはＭｎＳに代表される硫化物を形成して母材および溶接部の靭性を劣化させるため、それぞれ上限を０．０２％、０．０１％とした。   P and S are impurities in the steel of the present invention, and the lower the better. P segregates at the grain boundaries to promote grain boundary fracture, and S forms sulfides typified by MnS to deteriorate the toughness of the base metal and the welded portion. 01%.

Ｍｏは、本発明鋼においては高温強度発現、維持の観点からＮｂと並んで必要不可欠の元素である。単に高温強度の観点からは多く添加するほど有利であるが、母材強度や溶接性、溶接部靭性をも考慮すれば制約すべきものである。Ｃを低く抑える本発明においては、後述するＰCMの範囲内（０．１６％以下）であれば、Ｍｏは１．５％まで許容される。下限は、Ｎｂとの複合添加、あるいはさらに後述する高温強度向上に有効なＶ、Ｔｉを添加したとしても安定して高温強度を確保するためには０．３％以上の添加が必要である。 Mo is an indispensable element along with Nb from the viewpoint of high-temperature strength development and maintenance in the steel of the present invention. From the standpoint of high-temperature strength, it is more advantageous to add more, but it should be constrained if the base metal strength, weldability, and weld toughness are also taken into consideration. In the present invention to reduce the C, as long as it is within the range of P CM which will be described later (0.16% or less), Mo is allowed up to 1.5%. As for the lower limit, 0.3% or more of addition is necessary in order to ensure high-temperature strength stably even when V and Ti which are effective for improving high-temperature strength, which will be described later, are added in combination with Nb.

Ｎｂは、Ｍｏとともに複合添加が必須の元素である。まず、Ｎｂの一般的な効果として、オーステナイトの再結晶温度を上昇させ、熱間圧延時の制御圧延の効果を最大限に発揮する上で有用な元素である。また、圧延に先立つ再加熱時の加熱オーステナイトの細粒化にも寄与する。さらに、析出強化および転位回復抑制による高温強度向上効果を有し、Ｍｏとの複合添加により、一層の高温強度向上に寄与する。０．０３％未満では７００℃および８００℃における析出硬化および転位回復抑制の効果が少なく、０．１５％を超えると添加量に対し硬化の度合いが減少し、経済的にも好ましくないばかりでなく、溶接部の靭性も劣化する。これらの理由により、Ｎｂは０．０３〜０．１５％の範囲に限定する。   Nb is an element that must be added in combination with Mo. First, as a general effect of Nb, it is an element useful for raising the recrystallization temperature of austenite and maximizing the effect of controlled rolling during hot rolling. Moreover, it contributes to the refinement of the heated austenite at the time of reheating prior to rolling. Furthermore, it has the effect of improving high temperature strength by precipitation strengthening and suppressing dislocation recovery, and contributes to further improvement of high temperature strength by the combined addition with Mo. If it is less than 0.03%, the effect of suppressing precipitation hardening and dislocation recovery at 700 ° C. and 800 ° C. is small, and if it exceeds 0.15%, the degree of curing decreases with respect to the amount added, which is not preferable economically. Also, the toughness of the welded portion deteriorates. For these reasons, Nb is limited to the range of 0.03 to 0.15%.

Ａｌは、一般に脱酸上鋼に含まれる元素であるが、脱酸はＳｉまたはＴｉだけでも十分であり、本発明においては、その下限は限定しない（０％を含む）。しかし、Ａｌ量が多くなると鋼の清浄度が悪くなるだけでなく、溶接部の靭性が劣化するので、上限を０．０６％とした。   Al is an element generally contained in deoxidized upper steel, but Si or Ti is sufficient for deoxidation, and the lower limit is not limited (including 0%) in the present invention. However, when the amount of Al increases, not only the cleanliness of the steel deteriorates but also the toughness of the welded portion deteriorates, so the upper limit was made 0.06%.

Ｎは、不可避的不純物として鋼中に含まれるものであるが、Ｎｂおよび後述するＴｉを添加した場合、Ｎｂと結合して炭窒化物を形成して強度を増加させたり、ＴｉＮを形成して鋼の性質を高める。このため、Ｎ量として最低０．００１％は必要である。しかしながら、Ｎ量の増加は溶接部靭性、溶接性に有害であり、本発明においてはその上限は０．００６％である。なお、この上限は必ずしも特性上の限界的な意味合いはなく、本発明者らが確認した範囲内で限定したものである。   N is contained in steel as an unavoidable impurity. However, when Nb and Ti described later are added, Nb is combined with Nb to form carbonitride to increase the strength, or TiN is formed. Enhance the properties of steel. For this reason, a minimum amount of 0.001% is necessary. However, an increase in the amount of N is detrimental to weld toughness and weldability, and in the present invention the upper limit is 0.006%. The upper limit is not necessarily limited in terms of characteristics, and is limited within the range confirmed by the present inventors.

次に、必要に応じて含有することができるＣｕ、Ｎｉ、Ｃｒ、Ｖ、ＴｉおよびＣａ、ＲＥＭ、Ｍｇの添加理由とその添加量範囲について説明する。   Next, the reason for the addition of Cu, Ni, Cr, V, Ti, Ca, REM, and Mg that can be contained as necessary, and the range of the amount added will be described.

基本となる成分に、さらにこれらの元素を添加する主たる目的は、本発明鋼の優れた特徴を損なうことなく、強度、靭性などの特性を向上させるためである。したがって、その添加量は自ずと制限されるべき性質のものである。   The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount of addition is naturally limited.

Ｃｕは、溶接性、溶接部靭性に顕著な悪影響を及ぼすことなく母材の強度、靭性を向上させる。これらの効果を発揮させるためには、少なくとも０．０５％以上の添加が必須である。一方、過剰な添加は溶接性劣化に加え、熱間圧延時にＣｕクラック発生の危険性を増大させることにもつながるので、上限を１．０％に限定した。なお、Ｃｕクラック自体は、Ｃｕ量に応じた適正なＮｉ添加で回避できることが知られており、溶接性もＣ量をはじめとする他の合金元素量とも関係するので、上限は必ずしも限界的な意味合いを持つものではない。   Cu improves the strength and toughness of the base material without significantly affecting the weldability and weld zone toughness. In order to exhibit these effects, addition of at least 0.05% is essential. On the other hand, excessive addition leads to an increase in the risk of Cu cracking during hot rolling in addition to deterioration of weldability, so the upper limit was limited to 1.0%. It is known that Cu cracks themselves can be avoided by adding appropriate Ni according to the amount of Cu, and since the weldability is also related to the amount of other alloy elements including the amount of C, the upper limit is not necessarily limited. It has no meaning.

Ｎｉは、Ｃｕとほぼ同様の効果を示し、特に母材の靭性向上には大きな効果がある。これらの効果を確実に享受するためには少なくとも０．０５％以上の添加が必須である。一方、過剰な添加は、Ｎｉといえども溶接性を劣化させるとともに、比較的高価な元素であるため、経済性を損ねることにもなるので、本発明においては４９０ＭＰａ級鋼をターゲットとしていることも考慮し、１．０％を上限とする。   Ni exhibits substantially the same effect as Cu, and is particularly effective in improving the toughness of the base material. Addition of at least 0.05% or more is indispensable for reliably enjoying these effects. On the other hand, excessive addition of Ni deteriorates weldability and is a relatively expensive element, and thus impairs economic efficiency. Therefore, in the present invention, 490 MPa class steel may be targeted. In consideration, 1.0% is made the upper limit.

Ｃｒは、母材の強度を向上させるため、必要に応じて添加することができる。スクラップなどからのトランプ・エレメントとしての微量混入と明確に区別でき、確実に効果を享受する上で、最低限０．０５％以上の添加が必要である。多すぎる添加は、他の元素同様、溶接性や溶接部靭性を劣化させるため、上限を１．０％に限定する。   Cr can be added as necessary to improve the strength of the base material. It can be clearly distinguished from a small amount of mixing as a playing card element from scrap or the like, and in order to enjoy the effect reliably, it is necessary to add 0.05% or more at least. Too much addition, like other elements, degrades weldability and weld toughness, so the upper limit is limited to 1.0%.

上記、Ｃｕ、Ｎｉ、Ｃｒは、母材の機械的特性上の観点のみならず、耐候性にも有効であり、そのような目的においては、溶接性、溶接部靭性を大きく損ねることのない範囲で積極的に添加することが好ましい。   Cu, Ni, and Cr are effective not only in terms of mechanical properties of the base material but also in weather resistance, and in such a purpose, the range in which weldability and weld toughness are not significantly impaired. It is preferable to add it positively.

Ｖは、高温強度向上も含めＮｂとほぼ同様の効果・作用を有するものであるが、Ｎｂに比べてその効果は小さい。また、ＶはＰCMの式にも入っていることから分かるように、焼入性、溶接性にも影響を及ぼす。したがって、Ｖ添加の効果を確実に享受する上で下限を０．０１％とするとともに、悪影響を排除するため上限を０．１％とする。 V has substantially the same effect and action as Nb including improvement in high-temperature strength, but its effect is smaller than that of Nb. Also, V is as can be seen from that contained in the formula of P CM, hardenability, also affects the weldability. Therefore, in order to enjoy the effect of V addition reliably, the lower limit is made 0.01%, and the upper limit is made 0.1% in order to eliminate adverse effects.

Ｔｉは、Ｎｂ、Ｖなどと同様、高温強度向上に有効である。それ以外にも、特に、母材および溶接部靭性に対する要求が厳しい場合には、添加することが好ましい。なぜならば、Ｔｉは、Ａｌ量が少ない時（例えば０．００３％以下）、Ｏと結合してＴｉ２Ｏ３を主成分とする析出物を形成、粒内変態フェライト生成の核となり、溶接部靭性を向上させる。また、ＴｉはＮと結合してＴｉＮとしてスラブ中に微細析出し、加熱時のオーステナイト粒の粗大化を抑え、圧延組織の微細化に有効であり、また鋼板中に存在する微細ＴｉＮは、溶接時に溶接熱影響部組織を細粒化する。これらの効果を得るためには、Ｔｉは最低０．００５％必要である。しかし、多すぎるとＴｉＣを形成し、低温靭性や溶接性を劣化させるので、その上限は０．０２５％である。   Ti, like Nb and V, is effective for improving the high temperature strength. In addition, it is preferable to add, especially when the requirements for the base material and weld zone toughness are severe. This is because when Ti has a small amount of Al (for example, 0.003% or less), it combines with O to form a precipitate mainly composed of Ti2O3, which becomes the nucleus of intragranular transformation ferrite formation and improves weld toughness. Let In addition, Ti combines with N and finely precipitates in the slab as TiN, suppresses the coarsening of austenite grains during heating, is effective for refining the rolling structure, and the fine TiN present in the steel sheet is welded. Sometimes the heat affected zone structure is refined. In order to obtain these effects, Ti needs to be at least 0.005%. However, if it is too much, TiC is formed and the low temperature toughness and weldability are deteriorated, so the upper limit is 0.025%.

Ｃａ、ＲＥＭは、不純物であるＳと結合し、靭性の向上や溶接部の拡散性水素による割れを抑制する働きを有するが、多すぎると粗大な介在物を形成し、靭性に悪影響を及ぼすので、いずれも０．０００５〜０．００４％の範囲に限定する。両元素は、ほぼ同等の効果を有するので、上述した効果を享受するためには少なくともいずれか一方を添加すればよい。   Ca and REM bind to impurity S and have the effect of improving toughness and suppressing cracking due to diffusible hydrogen in the weld, but if too much, coarse inclusions are formed, which adversely affects toughness. , Both are limited to the range of 0.0005 to 0.004%. Since both elements have substantially the same effect, at least one of them may be added in order to enjoy the above-described effect.

Ｍｇは、溶接熱影響部においてオーステナイト粒の成長を抑制し細粒化する作用があり、溶接部の強靭化が図れる。このような効果を享受するためには、Ｍｇは０．０００１％≧必要である。一方、添加量が増えると添加量に対する効果代が小さくなり、経済性を失するため、上限は０．００６％とした。   Mg has the effect of suppressing the growth of austenite grains in the weld heat-affected zone and making it finer, so that the weld zone can be strengthened. In order to enjoy such an effect, Mg needs to be 0.0001% ≧. On the other hand, as the amount added increases, the effect on the amount added decreases and the economy is lost, so the upper limit was made 0.006%.

なお、本発明においては、Ｂは意図的に添加することなく、製鋼工程におけるコンタミネーションとして含まれるレベルを超えては実質的に含有しないことがポイントである。Ｂは微量添加で焼入性を顕著に高めるため、高張力鋼に用いる場合には組織制御や強度向上の点で有利であるが、溶接性や溶接部靭性を劣化させる危険性をも同時に併せ持つ。本発明は、高温特性に加えて溶接構造用鋼としての使用性能を一段と高めることを目的として、意図的なＢ添加を忌避し、実質的にＢフリーとした。   In the present invention, B is not added intentionally and the point is that it is not substantially contained beyond the level included as a contamination in the steelmaking process. B is extremely advantageous in terms of microstructure control and strength improvement when used in high-strength steel because B significantly increases hardenability, but it also has the risk of degrading weldability and weld toughness. . The present invention avoids intentional B addition and makes it substantially B-free for the purpose of further enhancing the use performance as a welded structural steel in addition to the high temperature characteristics.

鋼の個々の成分を上述した通り限定しても、成分系全体が適切でないと、本発明の特徴である優れた特性は得られない。特に、本発明者らによる先の特許（特願２００４−４３９６１号）より、溶接性、溶接部靭性を大きく改善させることを意図したものであるため、ＰCMの値を０．１５％以下に限定する。一般に、ＰCMは低いほど溶接性に優れ、０．２２％以下であれば、溶接時の（溶接冷間割れ防止のための）予熱が不要といわれている。高張力鋼、なかでも本発明のような高温強度に優れた高張力鋼で、しかも焼入性を顕著に高める元素であるＢを含まない鋼において、ＰCMが０．１５％以下というのは、きわめて低い値である。

Even if the individual components of the steel are limited as described above, the excellent characteristics that are the characteristics of the present invention cannot be obtained unless the entire component system is appropriate. In particular, from the present inventors prior patent by (Japanese Patent Application No. 2004-43961), weldability, because it is intended to greatly improve the weld toughness, the value of P CM below 0.15% limit. In general, P CM is low enough good weldability, not more than 0.22%, the preheating (for preventing cracks between welded cold) during welding are said unnecessary. High tensile steel, among others a high-strength steel excellent in high temperature strength, such as in the present invention, moreover in B a that does not contain steel is an element to enhance significantly the hardenability, P CM is that 0.15% or less Is a very low value.

さらに、本発明においては、ミクロ組織をも限定する。鋼成分のみの限定では、溶接構造用鋼としての優れた溶接性や溶接部靭性は確保可能であるが、高温特性や何より４９０ＭＰａ級鋼としての基本特性（特に強度）を満足することはできない。このため、本発明の目的に適うものとして、ミクロ組織はフェライトとベイナイトの混合組織主体であって、そのうちのベイナイトの分率を２０〜９０％であることに限定する。これは、ベイナイト分率が低いと４９０ＭＰａ級の常温強度および高温強度の確保が困難であり、ベイナイト分率が高すぎるとＪＩＳなどで規定される４９０ＭＰａ級鋼の強度範囲を超過する危険性が増大するという、本発明者らの実験結果に基づいて本発明の特徴を明確にするために限定したもので、必ずしも限界的な意味を持つものではない。   Furthermore, in the present invention, the microstructure is also limited. By limiting only the steel components, excellent weldability and weld toughness as welded structural steel can be ensured, but high temperature characteristics and above all, basic characteristics (particularly strength) as 490 MPa class steel cannot be satisfied. For this reason, as suitable for the purpose of the present invention, the microstructure is mainly composed of a mixed structure of ferrite and bainite, and the fraction of bainite is limited to 20 to 90%. This is because if the bainite fraction is low, it is difficult to ensure the normal temperature strength and high temperature strength of the 490 MPa class, and if the bainite fraction is too high, the risk of exceeding the strength range of the 490 MPa class steel specified by JIS and the like increases. However, the present invention is limited to clarify the characteristics of the present invention based on the experimental results of the present inventors, and does not necessarily have a limiting meaning.

なお、これらのミクロ組織は板厚断面方向１／４厚位置を代表させるものとする。また、組織名としての「ベイナイト」の呼称は、当業者においては広く用いられているものであるが、バリエーションの多様さなどから分率測定に際しては、その領域の特定の点で不明確さが生じる可能性がある。その場合、組織構成上のもう一つの組織である「フェライト」で判定する方法もある。この場合のフェライト分率は１０〜８０％である。ここで呼ぶところのフェライトは、セメンタイトを含まないポリゴナルまたは擬ポリゴナルフェライトである（針状フェライトは含まない）。   These microstructures are representative of the 1/4 thickness position in the plate thickness cross-sectional direction. In addition, the name of “bainite” as an organization name is widely used by those skilled in the art. However, due to the variety of variations, etc., when measuring the fraction, there is ambiguity at certain points in the area. It can happen. In that case, there is also a method of determining by “ferrite” which is another structure in the structure. In this case, the ferrite fraction is 10 to 80%. The ferrite referred to here is polygonal or pseudopolygonal ferrite not containing cementite (not including acicular ferrite).

圧延後の変態前のオーステナイト粒径は、本発明のような比較的高いＭｏ添加鋼の靭性を制御（高靭化）する上で適正に限定する必要がある。該オーステナイトが細粒であるほど、最終変態組織も微細となり靭性を向上させる。通常のＭｏの低い鋼と遜色ない靭性を得るため、鋼板の最終圧延方向の板厚断面方向１／４厚位置における該オーステナイト粒径を平均円相当直径で１２０μｍ以下に限定する。板厚や鋼成分によっては、１２０μｍ超でも十分な靭性が得られるケースもあるが、確実に安定して靭性を確保できる粒径として限定したもので、必ずしも限界的意味合いはない。なお、オーステナイト粒径は、その判別が必ずしも容易でないケースも少なからずある。このような場合には、板厚１／４厚位置を中心として、鋼板の最終圧延方向と直角方向に採取した切り欠き付き衝撃試験片、例えば、ＪＩＳ Ｚ ２２０２ ２ｍｍＶノッチ試験片などを用い、十分低温で脆性破壊させた際の破面単位をオーステナイト粒径と読み替え得る有効結晶粒径と定義し、その平均円相当直径を測定することとし、この場合でも同様に１２０μｍ以下であることが必要である。   The austenite grain size before transformation after rolling needs to be appropriately limited in order to control (toughen) the toughness of the relatively high Mo-added steel as in the present invention. The finer the austenite, the finer the final transformation structure and the better the toughness. In order to obtain toughness comparable to that of normal steel with low Mo, the austenite grain size at the 1/4 thickness position in the sheet thickness cross-section direction in the final rolling direction of the steel sheet is limited to an average equivalent circle diameter of 120 μm or less. Depending on the plate thickness and steel component, sufficient toughness may be obtained even if it exceeds 120 μm, but it is limited as a particle size that can ensure the toughness stably and stably, and does not necessarily have a critical meaning. In many cases, it is not always easy to distinguish the austenite grain size. In such a case, an impact test piece with a notch sampled in a direction perpendicular to the final rolling direction of the steel sheet with a center of the thickness of the plate thickness 1/4, for example, a JIS Z 2202 2 mmV notch test piece is sufficient. The fracture surface unit at the time of brittle fracture at low temperature is defined as the effective crystal grain size that can be read as austenite grain size, and the average equivalent circle diameter is to be measured. Even in this case, it is necessary to be 120 μm or less. is there.

上記に限定した通りの組織（組織、組織分率、旧オーステナイト粒径など）と高温特性をはじめとする本発明が目的とする優れた諸特性は、製造方法を以下の通り限定することで容易に得ることができる。   The excellent properties aimed at by the present invention, including the structure as described above (structure, structure fraction, prior austenite grain size, etc.) and high-temperature properties, can be easily achieved by limiting the production method as follows. Can get to.

所定の鋼成分を有する鋼片または鋳片の再加熱は、１１００〜１２５０℃の温度範囲に限定する。下限の１１００℃は、Ｍｏ、Ｎｂおよび必要に応じて添加するＶ、Ｔｉを高温特性確保を第一の目的として固溶状態とするためである。この目的のためには、再加熱温度は高いほど好ましいが、加熱オーステナイト粒が粗大化し、母材靭性の観点から好ましくないため、上限を１２５０℃に限定する。   Reheating of a steel slab or slab having a predetermined steel component is limited to a temperature range of 1100 to 1250 ° C. The lower limit of 1100 ° C. is to make Mo, Nb, and V and Ti added as necessary, in a solid solution state for the first purpose of ensuring high temperature characteristics. For this purpose, the higher the reheating temperature, the better. However, since the heated austenite grains become coarse and not preferable from the viewpoint of the base material toughness, the upper limit is limited to 1250 ° C.

圧延条件の限定は、直接的には圧延後変態前のオーステナイト粒径を上述の通りに比較的細粒に制御するためであり、主として靭性確保のためである。このため、圧延は１１００℃以下での累積圧下量を３０％以上とする必要がある。圧延終了温度は、低温域の圧下でＭｏ、Ｎｂ、あるいは必要に応じて添加するＶ、Ｔｉが炭化物として析出するための下限温度として８５０℃以上に限定する。   The limitation of the rolling conditions is to directly control the austenite grain size before transformation after rolling to be relatively fine as described above, mainly for securing toughness. For this reason, rolling needs to make 30% or more the cumulative reduction amount in 1100 degrees C or less. The rolling end temperature is limited to 850 ° C. or more as the lower limit temperature for precipitation of Mo, Nb, or V and Ti added as needed as carbides under a low temperature pressure.

圧延後の冷却も、組織制御の観点から限定すべきものである。鋼成分にもよるが、比較的薄手においては、放冷程度の冷速でも所定の組織を得ることができるが、厚手になると放冷では冷速が遅くなり、加速冷却が必要となる場合がある。この場合の加速冷却は、厚鋼板製造においては水冷が最も一般的であるが、必ずしも水冷である必要はない。また、加速冷却は組織制御のため変態域の冷速を上げることが目的であるので、８００℃以上の温度から６５０℃以下の温度まで行う必要がある。   Cooling after rolling should also be limited from the viewpoint of structure control. Although it depends on the steel components, a relatively thin material can obtain a predetermined structure even at a cooling rate of about the degree of cooling, but if it is thicker, the cooling rate will be slowed by cooling, and accelerated cooling may be necessary. is there. The accelerated cooling in this case is most commonly water cooling in the production of thick steel plates, but it is not necessarily required to be water cooling. In addition, since accelerated cooling is intended to increase the cooling speed in the transformation region for structure control, it must be performed from a temperature of 800 ° C. or higher to a temperature of 650 ° C. or lower.

なお、本発明においては、高温強度とは６００℃から８００℃までをターゲットとしており、その定量的目標は、高温時の降伏応力の常温降伏応力に対する比ｐ（＝高温降伏応力／常温降伏応力）が、鋼材温度Ｔ（℃）が６００℃以上８００℃以下の範囲で、ｐ≧−０．００３３×Ｔ＋２．８０である。   In the present invention, the high temperature strength is targeted from 600 ° C. to 800 ° C., and the quantitative target is the ratio p of the yield stress at high temperature to the normal temperature yield stress (= high temperature yield stress / normal temperature yield stress). However, it is p> =-0.0033 * T + 2.80 in the range whose steel material temperature T (degreeC) is 600 degreeC or more and 800 degrees C or less.

転炉−連続鋳造−厚板工程で、種々の鋼成分の鋼板（厚さ１２〜８０ｍｍ）を製造し、その機械的性質および溶接性、溶接部靭性評価としてＪＩＳに準拠した斜めｙ形溶接割れ試験におけるルート割れの有無および溶接再現熱サイクルによる小入熱と超大入熱溶接相当の再現ＨＡＺ靭性を調査した。   In the converter-continuous casting-thick plate process, steel plates with various steel components (thickness 12-80mm) are manufactured, and their mechanical properties, weldability, and oblique y-type weld cracks in accordance with JIS for evaluating weld toughness The presence or absence of root cracks in the test and the reproduced HAZ toughness corresponding to the small heat input and super large heat input welding by the welding reproduction heat cycle were investigated.

表１に比較例とともに本発明例の鋼成分を、表２に製造条件、表３に組織および諸特性の調査結果を示す。   Table 1 shows the steel components of the present invention together with the comparative example, Table 2 shows the manufacturing conditions, and Table 3 shows the results of the investigation of the structure and various properties.

本発明例では、いずれも本発明の限定範囲を満足し、高温強度、再現ＨＡＺ靭性を含めた各種特性も極めて良好である。これに対し、比較例は鋼成分や製造条件、組織などの少なくとも一つ以上が本発明の限定範囲を逸脱しているために、本発明例に対し、特性が劣っていることが分かる。すなわち、比較例１９は、Ｃ量が低いためベイナイト分率が低く、常温強度、高温強度（比）とも低い。比較例２０は、Ｃ量が高いためベイナイト分率が高く、常温強度が高い。また、母材靭性、再現ＨＡＺ靭性にも劣る。比較例２１は、Ｍｏ量が低く、加速冷却開始温度も低いため、ベイナイト分率が低いこともあって、高温強度（比）が低い。比較例２２は、Ｎｂ量が低く、加熱温度、圧延終了温度も低いことに加え、加速冷却停止温度が高いため、常温強度、高温強度（比）が低い。比較例２３は、Ｂが添加されているため、加速冷却を適用した場合、ベイナイト分率が高く、母材靭性に劣る。また、再現ＨＡＺ靭性にも劣る。比較例２４は、Ｍｎ量が高く、ＰCMも高いのに加え、１１００℃以下での累積圧下量も低いため、ベイナイト分率が高くなって４９０ＭＰａ級鋼として母材強度が過剰となり、母材靭性、再現ＨＡＺ靭性にも劣る。 In the examples of the present invention, all satisfy the limited range of the present invention, and various properties including high temperature strength and reproducible HAZ toughness are very good. On the other hand, it can be seen that the comparative example is inferior in characteristics to the inventive example because at least one of the steel components, production conditions, structure, etc. departs from the limited range of the present invention. That is, Comparative Example 19 has a low C content and thus a low bainite fraction and a low normal temperature strength and high temperature strength (ratio). Since the comparative example 20 has a high C content, the bainite fraction is high and the room temperature strength is high. Moreover, it is inferior to a base material toughness and reproduction HAZ toughness. Since the comparative example 21 has a low Mo amount and a low accelerated cooling start temperature, the high-temperature strength (ratio) is low due to the low bainite fraction. In Comparative Example 22, the Nb amount is low, the heating temperature and the rolling end temperature are low, and the accelerated cooling stop temperature is high. Therefore, the normal temperature strength and the high temperature strength (ratio) are low. In Comparative Example 23, since B is added, when accelerated cooling is applied, the bainite fraction is high and the base material toughness is poor. Also, the reproduced HAZ toughness is inferior. Comparative Example 24 has a high Mn content, in addition to high P CM, since lower cumulative reduction ratio at 1100 ° C. or less, the base material strength is excessive as 490MPa class steel becomes higher fraction of bainite, preform Inferior toughness and reproducible HAZ toughness.

なお、斜めｙ形溶接割れ試験におけるルート割れは、比較例２４がＰCMが本発明の限定範囲より高いとはいえ０．１８５％程度であり、いずれのケースでも発生しなかった。 Incidentally, the root cracks in the oblique y-groove weld cracking test, Comparative Example 24 is P CM is about 0.185% although higher than the limited range of the present invention, did not occur in any case.

Figure 0004864297
Figure 0004864297

Claims (5)

鋼成分が質量％で、
Ｃ：０．００５％以上０．０４０％未満、
Ｓｉ：０．５％以下、
Ｍｎ：０．１〜０．５％、
Ｐ：０．０２％以下、
Ｓ：０．０１％以下、
Ｍｏ：０．３〜１．５％、
Ｎｂ：０．０３〜０．１５％、
Ａｌ：０．０６％以下、
Ｎ：０．００６％以下、
かつ、
ＰＣＭ＝Ｃ＋Ｓｉ／３０＋Ｍｎ／２０＋Ｃｕ／２０＋Ｎｉ／６０＋Ｃｒ／２０／Ｍｏ／１５＋Ｖ／１０＋５Ｂ
と定義する溶接割れ感受性組成ＰＣＭが０．１５％以下で、残部が鉄および不可避的不純物からなり、ミクロ組織がフェライトとベイナイトの混合組織主体であって、そのベイナイトの分率が２０〜９０％であることを特徴とする高温強度に優れた溶接構造用４９０ＭＰａ級高張力鋼。 Steel component is mass%,
C: 0.005% or more and less than 0.040%,
Si: 0.5% or less,
Mn: 0.1 to 0.5%
P: 0.02% or less,
S: 0.01% or less,
Mo: 0.3 to 1.5%,
Nb: 0.03-0.15%,
Al: 0.06% or less,
N: 0.006% or less,
And,
P CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 / Mo / 15 + V / 10 + 5B
Weld crack susceptibility composition P CM be defined as the below 0.15%, the balance being iron and unavoidable impurities, the microstructure is a mixed structure mainly of ferrite and bainite, the fraction of the bainite 20-90 % 490 MPa class high strength steel for welded structures excellent in high temperature strength. 質量％で、さらに、
Ｃｕ：０．０５〜１．０％、
Ｎｉ：０．０５〜１．０％、
Ｃｒ：０．０５〜１．０％、
Ｖ：０．０１〜０．１％、
Ｔｉ：０．００５〜０．０２５％
の範囲で１種または２種以上を含有することを特徴とする請求項１に記載の高温強度に優れた溶接構造用４９０ＭＰａ級高張力鋼。 In mass%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
Cr: 0.05 to 1.0%,
V: 0.01 to 0.1%
Ti: 0.005-0.025%
The 490 MPa class high-tensile steel for welded structures excellent in high-temperature strength according to claim 1, comprising one or more kinds in a range of 質量％で、さらに、
Ｃａ：０．０００５〜０．００４％、
ＲＥＭ：０．０００５〜０．００４％、
Ｍｇ：０．０００１〜０．００６％
のいずれか１種または２種以上を含有することを特徴とする請求項１または２に記載の高温強度に優れた溶接構造用４９０ＭＰａ級高張力鋼。 In mass%,
Ca: 0.0005 to 0.004%,
REM: 0.0005 to 0.004%,
Mg: 0.0001 to 0.006%
The 490 MPa class high-tensile steel for welded structures excellent in high-temperature strength according to claim 1 or 2, characterized in that any one or more of these are contained. 板厚１／４厚位置の圧延方向と平行な断面の旧オーステナイト粒の平均円相当径が、１２０μｍ以下であることを特徴とする請求項１〜３のいずれかに記載の高温強度に優れた溶接構造用４９０ＭＰａ級高張力鋼。   The average equivalent circular diameter of the prior austenite grains having a cross section parallel to the rolling direction at a thickness of 1/4 thickness is 120 µm or less, and excellent in high temperature strength according to any one of claims 1 to 3 490 MPa class high strength steel for welded structures. 請求項１〜３のいずれか１項に記載の鋼成分からなる鋼片または鋳片を１１００〜１２５０℃の温度範囲に再加熱後、１１００℃以下での累積圧下量を３０％以上として、８５０℃以上の温度で圧延し、その後８００℃以上の温度から６５０℃以下の温度まで加速冷却することを特徴とする高温強度に優れた溶接構造用４９０ＭＰａ級高張力鋼の製造方法。 After reheating steel slabs or slab made of steel component according to any one of 請 Motomeko 1-3 in a temperature range of 1100 to 1250 ° C., the cumulative reduction ratio at 1100 ° C. or less as 30% or more, A method for producing a 490 MPa class high-strength steel for welded structures excellent in high-temperature strength, characterized by rolling at a temperature of 850 ° C. or higher and then accelerated cooling from a temperature of 800 ° C. or higher to a temperature of 650 ° C. or lower.
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