JP2005320624A - Thick high-strength steel plate having excellent low-temperature toughness in weld heat-affected zone effected by large heat input welding - Google Patents

Thick high-strength steel plate having excellent low-temperature toughness in weld heat-affected zone effected by large heat input welding Download PDF

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JP2005320624A
JP2005320624A JP2005102041A JP2005102041A JP2005320624A JP 2005320624 A JP2005320624 A JP 2005320624A JP 2005102041 A JP2005102041 A JP 2005102041A JP 2005102041 A JP2005102041 A JP 2005102041A JP 2005320624 A JP2005320624 A JP 2005320624A
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toughness
haz
heat input
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steel
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Toshinaga Hasegawa
Minoru Ito
Akihiko Kojima
Masanori Minagawa
Jun Otani
Yoichi Tanaka
実 伊藤
明彦 児島
潤 大谷
洋一 田中
昌紀 皆川
俊永 長谷川
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Nippon Steel Corp
新日本製鐵株式会社
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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/08Ferrous alloys, e.g. steel alloys containing nickel
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/14Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/001Ferrous alloys, e.g. steel alloys containing N
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    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/002Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/005Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese
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    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/06Ferrous alloys, e.g. steel alloys containing aluminium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/12Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick high-strength steel plate by which excellent weld HAZ toughness can be achieved even when a welding is applied with an amount of welding heat input of 20 to 100 kJ/mm to a steel plate having a plate thickness of 50 to 80 mm and a base-material tensile strength of 490 to 570 MPa class. <P>SOLUTION: The steel plate has a composition which consists of, by mass, 0.03 to 0.14% C, ≤0.30% Si, 0.8 to 2.0% Mn, ≤0.02% P, ≤0.005% S, 0.8 to 4.0% Ni, 0.003 to 0.040% Nb, 0.001 to 0.040% Al, 0.0010 to 0.0100% N, 0.005 to 0.030% Ti and the balance iron with inevitable impurities and in which Ni and Mn satisfy inequality Ni/Mn≥10×Ceq-3 (where 0.36<Ceq<0.42 is satisfied), wherein Ceq is represented by Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

本発明は、船舶、海洋構造物、中高層ビル、橋梁などに使用される溶接熱影響部(Heat Affected Zone、以後、HAZと称す。)の低温靭性に優れた厚手高強度鋼板に関するもので、特に、板厚50mm以上、母材引張強度490〜570MPa級の鋼板で、溶接入熱量が20〜100kJ/mmの溶接を行った場合においても優れた溶接継手部を有する鋼板に関するものである。   The present invention relates to a thick high-strength steel sheet excellent in low-temperature toughness of a heat-affected zone (hereinafter referred to as HAZ) used in ships, offshore structures, mid-to-high-rise buildings, bridges, etc. Further, the present invention relates to a steel plate having an excellent weld joint even when welding with a welding heat input of 20 to 100 kJ / mm is performed on a steel plate having a thickness of 50 mm or more and a base metal tensile strength of 490 to 570 MPa.
近年、船舶、海洋構造物、中高層ビル、橋梁などの大型構造物に使用される溶接用鋼材の材質特性に対する要望は厳しさを増している。特に、これら構造物の中では、板厚50mmを超える厚手で母材の引張強度が570MPa級である鋼板の使用も多くなっている。また、溶接の効率化を促進するため、このような厚手高強度鋼板の溶接には、エレクトロガス溶接法、エレクトロスラグ溶接法などに代表されるような大入熱溶接法による1パス溶接が検討されており、母材そのものの靭性と同様に、HAZ靭性の要求も厳しさを増している。   In recent years, demands for material properties of steel materials for welding used in large structures such as ships, offshore structures, high-rise buildings, and bridges have increased. In particular, in these structures, the use of steel plates having a thickness exceeding 50 mm and a tensile strength of the base material of 570 MPa class is increasing. Also, in order to promote the efficiency of welding, one-pass welding by high heat input welding methods such as electrogas welding method and electroslag welding method is considered for welding such thick high strength steel plates. As with the toughness of the base material itself, the demand for HAZ toughness is becoming stricter.
大入熱溶接法が適用される鋼材のHAZ靭性に注目した提案は、これまで数多くなされてきた。例えば、特許文献1では、微細なTi窒化物を鋼中に確保することによって、HAZのオーステナイト粒を小さくし、靭性を向上させる発明が開示されている。また、特許文献2では、Ti窒化物とMnSとの複合析出物をフェライトの変態核として活用し、HAZの靭性を向上させる発明が提案されている。さらに、特許文献3では、Ti窒化物とBNとの複合析出物を粒界フェライトの析出核として活用し、HAZ靭性を向上させる発明が提案されている。   Many proposals that focus on the HAZ toughness of steel materials to which the high heat input welding method is applied have been made so far. For example, Patent Document 1 discloses an invention that reduces the austenite grains of HAZ and secures toughness by securing fine Ti nitride in steel. Patent Document 2 proposes an invention in which a composite precipitate of Ti nitride and MnS is utilized as a ferrite transformation nucleus to improve the toughness of HAZ. Furthermore, Patent Document 3 proposes an invention in which a composite precipitate of Ti nitride and BN is utilized as a precipitation nucleus of grain boundary ferrite to improve HAZ toughness.
しかしながら、このTi窒化物は、HAZのうち最高到達温度が1400℃を超える溶接金属との境界(以下、溶接ボンド部とも称する。)近傍ではほとんど固溶してしまうので、靭性向上効果が低下してしまうという問題がある。そのため、上記のようなTi窒化物を利用した鋼材では、近年のHAZ靭性に対する厳しい要求や、超大入熱溶接におけるHAZ靭性の必要特性を達成することが困難である。   However, since this Ti nitride is almost dissolved in the vicinity of the boundary (hereinafter also referred to as a weld bond portion) with a weld metal having a maximum temperature exceeding 1400 ° C. of HAZ, the effect of improving toughness is reduced. There is a problem that it ends up. For this reason, in steel materials using Ti nitride as described above, it is difficult to achieve the strict requirements for recent HAZ toughness and the required characteristics of HAZ toughness in super-high heat input welding.
この溶接ボンド部近傍の靭性を改善する方法として、Ti酸化物を含有した鋼が厚板、形鋼などの様々な分野で使用されている。例えば、厚鋼板の分野では、特許文献4や特許文献5に記載された発明のように、Ti酸化物を含有した鋼が大入熱溶接部靭性向上に非常に有効であり、高張力鋼への適用が有望である。この原理は、鋼の融点においても安定なTi酸化物を析出サイトとして、溶接後の温度低下途中にTi窒化物、MnS等が析出し、さらにそれらをサイトとして微細フェライトが生成し、その結果、靭性に有害な粗大フェライトの生成が抑制されて、靭性の劣化が防止できるというものである。   As a method for improving the toughness in the vicinity of the weld bond portion, steel containing Ti oxide is used in various fields such as thick plates and section steel. For example, in the field of thick steel plates, as in the inventions described in Patent Document 4 and Patent Document 5, steel containing Ti oxide is very effective in improving the toughness of large heat input welds. Application is promising. This principle is based on the fact that Ti oxide, which is stable even at the melting point of steel, is used as a precipitation site, Ti nitride, MnS, etc. are precipitated in the middle of the temperature drop after welding, and further, fine ferrite is generated using them as a site. The generation of coarse ferrite harmful to toughness is suppressed, and deterioration of toughness can be prevented.
しかしながら、このようなTi酸化物は、鋼中へ分散される個数をあまり多くすることができないという問題がある。その原因は、Ti酸化物の粗大化や凝集合体であり、Ti酸化物の個数を増加させようとすれば5μm以上の粗大なTi酸化物、いわゆる介在物が増加してしまうためと考えられる。この5μm以上の介在物は、構造物の破壊の起点となったり、靭性の低下を引き起こしたりして、有害であるため回避すべきものである。そのため、さらなるHAZ靭性の向上を達成するためには、粗大化や凝集合体が起こりにくく、Ti酸化物よりも微細に分散する酸化物を活用する必要があった。   However, such a Ti oxide has a problem that the number dispersed in steel cannot be increased too much. The cause of this is thought to be the coarsening or aggregation of Ti oxides, and if the number of Ti oxides is increased, coarse Ti oxides of 5 μm or more, so-called inclusions, increase. This inclusion of 5 μm or more should be avoided because it is harmful because it becomes a starting point of destruction of the structure or causes a decrease in toughness. Therefore, in order to achieve further improvement in HAZ toughness, it is necessary to utilize an oxide that is less likely to be coarsened or aggregated and is more finely dispersed than Ti oxide.
また、このようなTi酸化物の鋼中への分散方法としては、Al等の強脱酸元素を実質的に含まない溶鋼中へのTi添加によるものが多い。しかしながら、単に溶鋼中にTiを添加するだけでは鋼中のTi酸化物の個数、分散度を制御することは困難であり、さらには、TiN、MnS等の析出物の個数、分散度を制御することも困難である。そのため、Ti脱酸のみによってTi酸化物を分散させた鋼においては、例えば、Ti酸化物の個数が充分に得られない、あるいは、厚板の板厚方向の靭性が変動するといった問題があった。   Further, as a method of dispersing such Ti oxide in steel, there are many methods by adding Ti to molten steel which does not substantially contain a strong deoxidizing element such as Al. However, it is difficult to control the number of Ti oxides and the degree of dispersion in the steel simply by adding Ti to the molten steel. Further, the number and the degree of dispersion of precipitates such as TiN and MnS are controlled. It is also difficult. Therefore, in steel in which Ti oxide is dispersed only by Ti deoxidation, for example, the number of Ti oxides cannot be obtained sufficiently, or the toughness in the thickness direction of the thick plate varies. .
このような問題に対して、特許文献6や特許文献7では、Ti添加直後のAl添加、あるいはAl、Ca複合添加で、生成するTi−Al複合酸化物やTi、Al、Caの複合酸化物を活用する発明が開示されている。このような発明により、大入熱溶接HAZ靭性を大幅に向上させることが可能となった。
特公昭55−026164号公報 特開平03−264614号公報 特開平04−143246号公報 特開昭61−079745号公報 特開昭62−103344号公報 特開平06−293937号公報 特開平10−183295号公報
With respect to such a problem, in Patent Document 6 and Patent Document 7, Ti—Al composite oxide or composite oxide of Ti, Al, and Ca that is generated by Al addition immediately after Ti addition or Al / Ca composite addition is disclosed. An invention utilizing the above is disclosed. By such an invention, it became possible to greatly improve the high heat input welding HAZ toughness.
Japanese Patent Publication No. 55-026164 Japanese Patent Laid-Open No. 03-264614 Japanese Patent Laid-Open No. 04-143246 JP 61-079745 A JP-A-62-103344 Japanese Patent Laid-Open No. 06-293937 JP-A-10-183295
しかし、HAZのオーステナイト粒を小さくしたり、析出物をフェライトの変態核としてフェライトを生成したりする上記の従来手段では、板厚50mm以上で母材強度を引張強度で490MPa以上確保するためには、合金元素を増加させる必要があり、この場合、溶接HAZの硬さが上昇することとともに、靭性を劣化させるMA(Martensite-Austenite constituent)の生成が顕在化するため、例えば、造船分野でのEグレード(−20℃保証)のような十分なHAZ靭性を安定して確保することができない。まして母材強度が引張強度で570MPa以上になると必要なHAZ靭性を得ることができない。   However, in the above conventional means for reducing the HAZ austenite grains or generating ferrite using precipitates as ferrite transformation nuclei, in order to ensure a base material strength of 490 MPa or more with a plate thickness of 50 mm or more. It is necessary to increase the alloy elements. In this case, the hardness of the welded HAZ is increased, and the generation of MA (Martensite-Austenite constituent) that deteriorates toughness becomes obvious. Sufficient HAZ toughness such as grade (guaranteed at −20 ° C.) cannot be secured stably. Furthermore, if the base material strength is 570 MPa or more in terms of tensile strength, the required HAZ toughness cannot be obtained.
そこで、本発明は、板厚50〜80mm、母材引張強度490〜570MPa級の鋼板で、溶接入熱量が20〜100kJ/mmの溶接を行った場合においても優れた溶接HAZ靭性を実現できる、大入熱溶接による溶接熱影響部の低温靭性に優れた厚手高強度鋼板を提供することを目的とするものである。   Therefore, the present invention is a steel plate having a thickness of 50 to 80 mm and a base material tensile strength of 490 to 570 MPa, and can achieve excellent weld HAZ toughness even when welding heat input is 20 to 100 kJ / mm. It is an object of the present invention to provide a thick high strength steel plate excellent in low temperature toughness of a weld heat affected zone by high heat input welding.
本発明者らは、Ni添加量およびNi/Mnを規定することによって、上記課題を有利に解決し得ることを知見し、さらに検討を加えて初めて本発明を完成させたものであり、その要旨は、以下の通りである。
(1)質量%で、
C :0.03〜0.14%、 Si:0.30%以下、
Mn:0.8〜2.0%、 P :0.02%以下、
S :0.005%以下、 Al:0.001〜0.040%、
N :0.0010〜0.0100%、Ni:0.8〜4.0%、
Ti:0.005〜0.030%、 Nb:0.003〜0.040%
を含有し、NiとMnが式[1]を満たし、残部が鉄および不可避不純物であることを特徴とする、大入熱溶接による溶接熱影響部の低温靭性に優れた厚手高強度鋼板。
Ni/Mn≧10×Ceq−3 (0.36<Ceq<0.42) ・・[1] 但し、Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
(2)さらに、質量%で、
Ca:0.0003〜0.0050%、Mg:0.0003〜0.0050%、
REM:0.001〜0.030%
のうちの1種または2種以上を含有し、かつ、
O :0.0010〜0.0050%
を含有し、円相当径が0.005〜0.5μmの酸化物を、100個/mm2以上含有することを特徴とする、上記(1)に記載の大入熱溶接による溶接熱影響部の低温靭性に優れた厚手高強度鋼板。
(3)さらに、質量%で、
B :0.0005〜0.0050%
を含有することを特徴とする、上記(1)または(2)に記載の大入熱溶接による溶接熱影響部の低温靭性に優れた厚手高強度鋼板。
(4)さらに、質量%で、
Cr:0.1〜0.5%、 Mo:0.01〜0.5%、
V :0.005〜0.10%、 Cu:0.1〜1.0%
のうちの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)ないし(3)のいずれか1項に記載の大入熱溶接による溶接熱影響部の低温靭性に優れた厚手高強度鋼板。
The present inventors have found that the above problem can be advantageously solved by defining the amount of Ni added and Ni / Mn, and the present invention has been completed for the first time after further studies. Is as follows.
(1) In mass%,
C: 0.03-0.14%, Si: 0.30% or less,
Mn: 0.8 to 2.0%, P: 0.02% or less,
S: 0.005% or less, Al: 0.001-0.040%,
N: 0.0010 to 0.0100%, Ni: 0.8 to 4.0%,
Ti: 0.005-0.030%, Nb: 0.003-0.040%
A thick high-strength steel sheet excellent in low-temperature toughness of the weld heat-affected zone by high heat input welding, characterized in that Ni and Mn satisfy the formula [1], and the balance is iron and inevitable impurities.
Ni / Mn ≧ 10 × Ceq-3 (0.36 <Ceq <0.42) [1] where Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15
(2) Furthermore, in mass%,
Ca: 0.0003 to 0.0050%, Mg: 0.0003 to 0.0050%,
REM: 0.001 to 0.030%
One or more of these, and
O: 0.0010 to 0.0050%
The weld heat-affected zone by high heat input welding according to (1) above, wherein the oxide having an equivalent circle diameter of 0.005 to 0.5 μm is contained at 100 pieces / mm 2 or more. Thick high-strength steel sheet with excellent low-temperature toughness.
(3) Furthermore, in mass%,
B: 0.0005 to 0.0050%
A thick high-strength steel sheet excellent in low-temperature toughness of the weld heat-affected zone by high heat-input welding as described in (1) or (2) above.
(4) Furthermore, in mass%,
Cr: 0.1-0.5%, Mo: 0.01-0.5%,
V: 0.005-0.10%, Cu: 0.1-1.0%
Thickness excellent in low temperature toughness of weld heat affected zone by high heat input welding according to any one of (1) to (3), characterized by containing one or more of High strength steel plate.
本発明は、船舶、海洋構造物、中高層ビルなどの破壊に対する厳しい靭性要求を満足する厚手鋼板を供給するものであり、この種の産業分野にもたらす効果は極めて大きく、さらに構造物の安全性の意味から社会に対する貢献も非常に大きい。   The present invention supplies thick steel plates that satisfy severe toughness requirements for the destruction of ships, offshore structures, mid-to-high-rise buildings, etc., and the effects brought to this type of industrial field are extremely great. Contributing to society is also very significant.
以下に本発明について詳細に説明する。
これまでHAZ靭性の向上手段として、前述のとおり、高温でのオーステナイト粒の成長を抑制することが考えられてきた。その手段として最も有効な方法は、分散粒子によりオーステナイトの粒界をピンニングし、粒界の移動を止める方法である。これは、溶接入熱が20〜100kJ/mmと大入熱である場合においても、HAZの再加熱オーステナイト粒はピンニングにより極めて有効に細粒化する。しかし、母材強度を高めるために合金添加量を増加させていき、鋼材の溶接性と同時に化学成分的な焼入性を示す炭素当量(Ceq)が0.36以上となる鋼材ではHAZの硬さがより高くなるため、再加熱オーステナイト粒がピンニングにより細粒化した場合であっても十分なHAZ靭性が得られないという問題が新たに生じた。このようにHAZ部の硬さが高くなる場合では、地鉄そのものの靭性を向上させることが必要である。
The present invention is described in detail below.
So far, as described above, as a means for improving the HAZ toughness, it has been considered to suppress the growth of austenite grains at a high temperature. The most effective method for this is to pin the austenite grain boundaries with dispersed particles and stop the movement of the grain boundaries. This is because even when the welding heat input is as large as 20 to 100 kJ / mm, the HAZ reheated austenite grains are refined very effectively by pinning. However, in order to increase the strength of the base metal, the alloy addition amount is increased, and in the steel material in which the carbon equivalent (Ceq) showing the chemical component hardenability simultaneously with the weldability of the steel material is 0.36 or more, the hardness of HAZ Therefore, there is a new problem that sufficient HAZ toughness cannot be obtained even when reheated austenite grains are refined by pinning. Thus, when the hardness of a HAZ part becomes high, it is necessary to improve the toughness of the base iron itself.
そこで、発明者らは、課題としている厚手高強度鋼に必要となる、Ceqが0.36以上0.42以下と高い場合でのHAZ靭性改善に、地鉄そのものの靭性を改善する最適成分系を鋭意検討した。マトリックスの靭性を高める元素としては従来からNiが有効であることが知られている。しかし、今回のようにCeqが0.36以上0.42以下と高いHAZの靭性改善に有効かどうか、また有効である場合はどのような成分条件であれば有効かは知られていない。そこでまず、Ni添加量の影響を検討した。検討にあたっては、母材強度確保に有効なNb量を0.003%以上添加することを前提とした。HAZ靭性の評価には、図1で示されるエレクトロガス溶接(入熱45kJ/mm)相当の熱サイクルを付与した時のシャルピー衝撃試験での延性・脆性遷移温度(vTrs)を採用した。   Therefore, the inventors are the optimum component system for improving the toughness of the base iron itself in order to improve the HAZ toughness when Ceq is as high as 0.36 or more and 0.42 or less, which is necessary for the thick high-strength steel in question. We have studied earnestly. It has been conventionally known that Ni is effective as an element for increasing the toughness of the matrix. However, as in this case, it is not known whether it is effective for improving the toughness of HAZ having a high Ceq of 0.36 to 0.42, and if it is effective, what component conditions are effective. Therefore, first, the influence of the Ni addition amount was examined. In the examination, it was assumed that 0.003% or more of Nb amount effective for securing the strength of the base material was added. For the evaluation of HAZ toughness, the ductile / brittle transition temperature (vTrs) in the Charpy impact test when a thermal cycle corresponding to electrogas welding (heat input 45 kJ / mm) shown in FIG. 1 was applied was adopted.
Ni添加量の影響を検討した結果、まず、Niが0.8%より少ない場合では必要な靭性が得られないことが判明した。また、Niを0.8%以上添加した場合であっても、HAZ靭性が改善されないものと、逆にHAZ靭性が低下するものも見られた。そこで、さらに他の添加元素やCeqとの関係を含め鋭意検討した結果、このようにCeqが0.36以上0.42以下の場合では、図2に示すように、HAZ靭性は、CeqとNi/Mnとによって関係付けられることを見出した。図2は、検討に用いた鋼材の再現HAZ靭性(vTrs)を、Ceq毎に層別し、Ni/Mn比を横軸としてプロットしたものである。図2から、
Ni/Mn≧10×Ceq−3 ・・・・・・・・・・・・[1]の関係が成立つ鋼材において、vTrsで−15℃以下の良好な靭性が得られた。式[1]を満たさない鋼材が十分なHAZ靭性が得られない理由としては、Niの添加量が十分
ではなくマトリックス高靭化効果が小さいため、あるいは、Niを多く含む場合であってもMnの過剰添加によりHAZ中にMA生成し、Niの高靭化効果が消失されるためと考えられる。なお、上記検討で用いた鋼材を入熱100kJ/mm相当の熱サイクルにて同様の検討を行なった結果、入熱100kJ/mmの場合においても、式[1]の関係にある鋼材においては良好な再現HAZ靭性が得られることを確認している。
As a result of examining the influence of the amount of Ni added, it was first found that the necessary toughness could not be obtained when Ni was less than 0.8%. In addition, even when Ni was added in an amount of 0.8% or more, there was a case where the HAZ toughness was not improved and a case where the HAZ toughness was decreased. Therefore, as a result of intensive studies including the relationship with other additive elements and Ceq, as shown in FIG. 2, when Ceq is 0.36 or more and 0.42 or less, the HAZ toughness is as follows. / Mn and found to be related. FIG. 2 is a plot of the reproducible HAZ toughness (vTrs) of the steel used in the study, stratified for each Ceq, with the Ni / Mn ratio as the horizontal axis. From FIG.
Ni / Mn ≧ 10 × Ceq-3... In steel materials satisfying the relationship [1], good toughness of −15 ° C. or less was obtained in vTrs. The reason why the steel material that does not satisfy the formula [1] cannot obtain sufficient HAZ toughness is that the addition amount of Ni is not sufficient and the effect of toughening the matrix is small, or even if it contains a large amount of Ni, Mn It is considered that MA is formed in the HAZ by excessive addition of Ni and the toughening effect of Ni disappears. In addition, as a result of carrying out the same examination with the heat cycle equivalent to heat input of 100 kJ / mm for the steel material used in the above examination, even in the case of heat input of 100 kJ / mm, the steel material having the relationship of the formula [1] is good. It has been confirmed that a reproducible HAZ toughness can be obtained.
上述の検討により、HAZ靭性は、式[1]を満たす、0.8%以上のNi添加により改善されることを見出したが、さらに発明者らは、一層のHAZ靭性改善を検討した。HAZ靭性を改善させる方法として以下の3つを検討した。第一に、大入熱溶接では高温滞留時間が長期化するためオーステナイト粒が粗大化し、これがHAZ靭性を低下させることから、高温滞留時のオーステナイトの粗大化を抑制させる方法である。第二に、大入熱溶接では溶接後の冷却時間が長いためオーステナイト粒界から生成するフェライトの粗大化し、この粗大な粒界フェライトがHAZ靭性低下の原因になることから、粒界フェライトの粗大化を抑制する方法である。第三に、HAZ組織そのものを微細にさせる方法である。   As a result of the above study, the HAZ toughness was found to be improved by adding 0.8% or more of Ni that satisfies the formula [1], but the inventors further studied the improvement of the HAZ toughness. The following three methods were examined as methods for improving HAZ toughness. First, in high heat input welding, since the high temperature residence time is prolonged, the austenite grains are coarsened, which reduces the HAZ toughness, and is therefore a method of suppressing the austenite coarsening during high temperature residence. Secondly, in high heat input welding, since the cooling time after welding is long, the ferrite formed from the austenite grain boundaries becomes coarse, and this coarse grain boundary ferrite causes the HAZ toughness to decrease. This is a method of suppressing the conversion. Third, the HAZ structure itself is made fine.
第一のオーステナイト粒の粗大化を抑制する方法に関しては、例えば、特許文献7に記載されているように、微細酸化物を分散させる方法が有効である。特許文献7では、微細酸化物の分散に、脱酸工程で溶鋼の溶存酸素量をSiとの平衡反応で調整し、さらにその後Ti、Al、Caの順序で脱酸するとしている。そして、この方法により、粒子径が0.01〜1.0μmの酸化物を5×103〜1×105個/mm2で分散させるとしている 。 As a method for suppressing the coarsening of the first austenite grains, for example, as described in Patent Document 7, a method of dispersing a fine oxide is effective. In Patent Document 7, the amount of dissolved oxygen in the molten steel is adjusted by an equilibrium reaction with Si in the deoxidation step to disperse the fine oxides, and then deoxidized in the order of Ti, Al, and Ca. And by this method, an oxide having a particle diameter of 0.01 to 1.0 μm is dispersed at 5 × 10 3 to 1 × 10 5 particles / mm 2 .
そこで、発明者らは、Ceqが0.36以上0.42以下と高い場合で、Nbを0.003%含み、かつNiを0.8%以上添加した系において、微細酸化物を分散させHAZ靭性を更に向上させる方法を鋭意検討した。まず、微細酸化物を分散させる方法であるが、このような系においては、脱酸工程で溶鋼の溶存酸素量を0.0010〜0.0050%に調整し、その後、まずTiで脱酸し、引き続きAlで脱酸した後、さらに、Ca、Mg、REMのうち1種類以上添加することで、円相当径が0.005〜0.5μmの微細酸化物を100個/mm2以上分散させることが可能であることを見出した。 Therefore, the inventors dispersed HAS in a system in which Ceq is as high as 0.36 or more and 0.42 or less, and Nb is contained in an amount of 0.003% and Ni is added in an amount of 0.8% or more. A method for further improving the toughness was intensively studied. First, a method of dispersing fine oxides. In such a system, the amount of dissolved oxygen in the molten steel is adjusted to 0.0010 to 0.0050% in the deoxidation step, and then first deoxidized with Ti. Subsequently, after deoxidizing with Al, one or more of Ca, Mg, and REM are further added to disperse 100 oxides / mm 2 or more of fine oxides having an equivalent circle diameter of 0.005 to 0.5 μm. I found that it was possible.
また、この微細酸化物分散により、溶接での高温滞留時のオーステナイト粒粗大化が抑制されHAZ靭性を更に改善させることできた。一例として、Niを適正添加したのみのHAZ靭性と比較した結果を図3に示す。なお、生成される酸化物は、Niの量が多いほど細かく、個数も多くなり、Ni量が1.5%以上の場合では1000個/mm2以上と なる。これは今回発見したものである。さらに、溶鋼中のSi量については、Si量が多い場合では酸化物ができにくくなるため、Si量は0.30%以下、さらには0.20%以下とすることが好ましいことが今回の検討から明らかとなった。 Moreover, this fine oxide dispersion could suppress the austenite grain coarsening at the time of high temperature residence in welding and further improve the HAZ toughness. As an example, the result compared with the HAZ toughness which added only Ni appropriately is shown in FIG. Note that the generated oxide is finer and more numerous as the amount of Ni increases, and when the amount of Ni is 1.5% or more, it is 1000 / mm 2 or more. This was discovered this time. Furthermore, with regard to the amount of Si in the molten steel, since it is difficult to form an oxide when the amount of Si is large, it is preferable that the amount of Si is 0.30% or less, more preferably 0.20% or less. It became clear from.
他方、Ti脱酸の前の溶存酸素量が0.0050%を超える場合や脱酸元素の順番が異なる場合では、酸化物が粗大化し微細酸化物が十分に得られないため、オーステナイト粒の粗大化の抑制効果は殆ど得られない。   On the other hand, when the amount of dissolved oxygen before Ti deoxidation exceeds 0.0050% or when the order of deoxidation elements is different, the oxide becomes coarse and fine oxide cannot be obtained sufficiently. Almost no inhibitory effect is obtained.
なお、円相当径0.005〜0.5μmの酸化物の個数は、母材となる鋼板から抽出レプリカを作製し、それを電子顕微鏡にて10000倍で100視野以上(観察面積にして10000μm2以上)を観察し、0.1μm未満 の粒子に関しては適宜倍率を高めて観察した。観察された0.005〜0.5μm径の各粒子において元素分析を行い、酸化物であるものカウントした。 In addition, the number of oxides having an equivalent circle diameter of 0.005 to 0.5 μm was obtained by producing an extraction replica from a steel plate as a base material, and 10000 times with an electron microscope at least 100 fields of view (observation area 10000 μm 2 The above was observed, and the particles of less than 0.1 μm were observed with the magnification increased appropriately. Elemental analysis was performed on the observed particles having a diameter of 0.005 to 0.5 μm, and the number of oxide particles was counted.
次に、発明者らは、HAZ靭性向上方法として、上述で第二の方法、および、第三の方法として記した、粒界フェライトの粗大化抑制、および、HAZ組織の微細化を鋭意検討した。その結果、Ceqが0.36以上0.42以下と高い場合で、かつNiを0.8%以上添加した系で、特に今回のような20〜100kJ/mm相当の大入熱溶接をする場合においては、Bの添加が有効であることが判明した。その理由は、粒界フェライトの粗大化抑制の点では、再加熱オーステナイト粒界に固溶Bが偏析することにより粒界フェライトの生成が抑制されるためである。   Next, the inventors diligently studied the coarsening suppression of grain boundary ferrite and the refinement of the HAZ structure described as the second method and the third method as the HAZ toughness improving method. . As a result, when Ceq is as high as 0.36 or more and 0.42 or less, and in a system to which Ni is added at 0.8% or more, particularly when performing high heat input welding equivalent to 20 to 100 kJ / mm like this time In addition, it was found that the addition of B was effective. This is because, in terms of suppressing the coarsening of the grain boundary ferrite, the formation of grain boundary ferrite is suppressed by the segregation of the solid solution B at the reheated austenite grain boundary.
また、HAZ組織の微細化の点では、今回のような大入熱溶接で冷却速度が遅い場合では、B添加によりオーステナイト粒界、および、オーステナイト粒内の介在物にB窒化物が析出し、それを核とする数μmの微細なフェライトがオーステナイト粒界および粒内に多数生成することによりHAZ組織が微細化されるためである。B添加によるHAZ靭性の改善を、Niを適正添加したのみのHAZ靭性と比較した結果を図3に示す。B添加によりHAZ靭性がさらに向上していることが判る。さらに、図3には、上述の微細酸化物を分散させる方法に加えB添加させた場合でのHAZ靭性を示しているが、微細酸化物分散とB添加によりHAZ靭性が一層向上している。これは、BNの析出サイトとなる酸化物が増えたことによって、そのBNを核するフェライトが増えHAZ組織がより微細化したためと考えられる。   In addition, in terms of HAZ microstructure refinement, when the cooling rate is slow in this large heat input welding, B nitride precipitates on the austenite grain boundaries and inclusions in the austenite grains due to the addition of B, This is because the HAZ structure is refined by forming a large number of fine ferrite having a diameter of several μm at the austenite grain boundaries and in the grains. FIG. 3 shows the result of comparing the improvement of HAZ toughness by addition of B with the HAZ toughness of only adding Ni appropriately. It can be seen that the addition of B further improves the HAZ toughness. Further, FIG. 3 shows the HAZ toughness when B is added in addition to the above-described method of dispersing the fine oxide, but the HAZ toughness is further improved by the fine oxide dispersion and addition of B. This is thought to be due to the fact that the number of oxides serving as BN precipitation sites increased, the ferrite that nucleates the BN increased, and the HAZ structure further refined.
また、強度確保や耐食性の向上の観点から、上記条件に加え、Cu、Cr、Mo、Vを添加した場合でのHAZ靭性も検討した。その結果、それぞれ、0.1〜0.4%、0.1〜0.5%、0.03〜0.2%、0.005〜0.050%の範囲での添加であれば、HAZ靭性を大きく低下しないことが判明した。   In addition to the above conditions, HAZ toughness when Cu, Cr, Mo, and V were added was also examined from the viewpoint of securing strength and improving corrosion resistance. As a result, if the addition is within the range of 0.1 to 0.4%, 0.1 to 0.5%, 0.03 to 0.2%, and 0.005 to 0.050%, HAZ It has been found that the toughness is not significantly reduced.
なお、この発明の鋼板の製造方法は、特に制限されることはなく、公知の方法に従って製造すれば良い。例えば、上記の好適成分組成に調整した溶鋼を連続鋳造法でスラブとしたのち、1000〜1250℃に加熱してから、熱間圧延を施せばよい。   In addition, the manufacturing method of the steel plate of this invention is not restrict | limited in particular, What is necessary is just to manufacture according to a well-known method. For example, after the molten steel adjusted to the above-mentioned preferred component composition is made into a slab by a continuous casting method, it is heated to 1000 to 1250 ° C. and then hot rolled.
次に、本発明で使用する鋼素材の成分組成の限定理由について説明する。以下、組成における質量は単に%で記す。
Cは、鋼の強度を向上させる有効な成分として下限を0.03%とし、また過剰の添加は、炭化物やMAを多量に生成しHAZ靭性を著しく低下させるので、上限を0.14%とした。
Next, the reason for limiting the component composition of the steel material used in the present invention will be described. Hereinafter, the mass in the composition is simply expressed as%.
C is an effective component for improving the strength of steel. The lower limit is 0.03%, and excessive addition generates a large amount of carbides and MA and significantly reduces the HAZ toughness. Therefore, the upper limit is 0.14%. did.
Siは、母材の強度確保、脱酸などに必要な成分であるが、HAZの硬化により靭性が低下するのを防止するため上限を0.30%とした。さらに酸化物を利用する場合では溶鋼中の酸素濃度の減少を防ぐために上限を0.20%以下とするのが好ましい。   Si is a component necessary for securing the strength of the base material, deoxidation, etc., but the upper limit was made 0.30% in order to prevent the toughness from being lowered by the hardening of the HAZ. Further, in the case of using an oxide, the upper limit is preferably made 0.20% or less in order to prevent a decrease in oxygen concentration in the molten steel.
Mnは、母材の強度、靭性の確保に有効な成分として0.8%以上の添加が必要であるが、溶接部の靭性、割れ性などの許容できる範囲で上限を2.0%とした。さらに、Mnに上限に関しては、Ceq、Mn量、およびNi量との関係を示す式[1]を満たす必要がある。これは、今回の検討で新たに見出された、Ceqが高い場合でMnの増加がHAZ組織中にMAを多量に生成させる原因となりNiによるHAZ靭性の向上効果を消失
せるということに基づく。
Ni/Mn≧10×Ceq−3 ・・・・・・・・・・・・[1]
Mn needs to be added in an amount of 0.8% or more as an effective component for securing the strength and toughness of the base material, but the upper limit is set to 2.0% within an allowable range such as toughness and crackability of the welded portion. . Further, regarding the upper limit of Mn, it is necessary to satisfy the formula [1] indicating the relationship between Ceq, Mn content, and Ni content. This is based on the fact that when the Ceq is high, an increase in Mn causes a large amount of MA to be generated in the HAZ structure, and the effect of improving the HAZ toughness by Ni is lost, which is newly found in this study.
Ni / Mn ≧ 10 × Ceq-3 [1]
Pは、含有量が少ないほど望ましいが、これを工業的に低減させるためには多大なコストがかかることから、含有範囲を0.02以下とした。   The content of P is preferably as low as possible. However, in order to reduce this industrially, it takes a great deal of cost, so the content range was set to 0.02 or less.
Sは、含有量が少ないほど望ましいが、これを工業的に低減させるためには多大なコストがかかることから、含有範囲を0.005以下とした。   The content of S is preferably as low as possible, but since it takes a great deal of cost to reduce this industrially, the content range is set to 0.005 or less.
Niは、本発明で重要な元素であり、少なくとも0.8%の添加が必要である。さらに、Niに下限に関しては、Ceq、Mn量、およびNi量との関係を示す式[1]を満たす必要がある。上限に関しては、製造コストの観点から4.0%とした。
Ni/Mn≧10×Ceq−3 ・・・・・・・・・・・・[1]
Ni is an important element in the present invention and needs to be added at least 0.8%. Further, regarding the lower limit of Ni, it is necessary to satisfy the equation [1] indicating the relationship between Ceq, the amount of Mn, and the amount of Ni. The upper limit was set to 4.0% from the viewpoint of manufacturing cost.
Ni / Mn ≧ 10 × Ceq-3 [1]
Nbは、焼き入れ性を向上させることにより母材の強度を向上させるために有効な元素であることから0.003%以上添加する。しかし、Nbを多く添加するとNi/Mn比に関係なくHAZ中にMAが生成しやすくなり、0.040%より多く添加した場合ではHAZ中に長径が5μm以上の粗大なMAが多数生成しHAZ靭性を大きく低下させることがあることからNbの上限を0.040%とした。なお、より高い靭性を得るためには、上述の式[1]を満たすNi/Mn比の場合で長径が5μm以上の粗大なMAが殆ど生成しない0.020%以下にNb量を抑えることが好ましい。さらにより高い靭性をより安定的に得るためには、上述の式[1]を満たすNi/Mn比の場合で長径が3μm以上のMAが殆ど生じない0.010%以下にNb量を抑えることが好ましい。   Nb is an effective element for improving the strength of the base material by improving the hardenability, so 0.003% or more is added. However, when a large amount of Nb is added, MA tends to be generated in the HAZ regardless of the Ni / Mn ratio, and when it is added more than 0.040%, a large number of coarse MA having a major axis of 5 μm or more is generated in the HAZ. Since the toughness may be greatly reduced, the upper limit of Nb is set to 0.040%. In order to obtain higher toughness, the Nb content should be suppressed to 0.020% or less, in which the coarse MA having a major axis of 5 μm or more is hardly generated in the case of the Ni / Mn ratio satisfying the above formula [1]. preferable. In order to obtain higher toughness more stably, the amount of Nb should be suppressed to 0.010% or less where MA with a major axis of 3 μm or more hardly occurs in the Ni / Mn ratio satisfying the above formula [1]. Is preferred.
Alは、重要な脱酸元素であり、下限値を0.001%とした。また、Alが多量に存在すると、鋳片の表面品位が劣化するため、上限を0.040%とした。   Al is an important deoxidizing element, and the lower limit was set to 0.001%. Further, when a large amount of Al is present, the surface quality of the slab deteriorates, so the upper limit was made 0.040%.
Tiは、再加熱オーステナイト粒の粗大化抑制のために必要なピンニング粒子となるTi窒化物やTi含有酸化物を生成させるため、0.005%以上添加する。しかし、過剰の添加は固溶Ti量を増加させHAZ靭性の低下を招くことから、0.030%を上限とした。   Ti is added in an amount of 0.005% or more in order to generate Ti nitrides and Ti-containing oxides that serve as pinning particles necessary for suppressing coarsening of reheated austenite grains. However, excessive addition increases the amount of dissolved Ti and causes a decrease in HAZ toughness, so 0.030% was made the upper limit.
Nは、溶接後の冷却中にオーステナイト粒界および粒内にTi窒化物やB窒化物を生成させるために必要に応じて添加量を調整する。Bと結合してB窒化物を形成させるためには0.0010%以上添加が必要であるが、過剰の添加は固溶N量を増大させHAZ靭性の低下を招くことから、0.0100%を上限とした。   N adjusts the addition amount as necessary in order to generate Ti nitride and B nitride in the austenite grain boundaries and grains during cooling after welding. Addition of 0.0010% or more is necessary to form B nitride by combining with B. However, excessive addition increases the amount of dissolved N and causes a decrease in HAZ toughness. Was the upper limit.
Caは、再加熱オーステナイト粒の粗大化抑制のために必要なピンニング粒子となるCa系酸化物を生成させるために必要に応じて0.0003%以上の添加する。しかし、過剰の添加は粗大介在物を生成させることから、0.0050%を上限とした。   Ca is added in an amount of 0.0003% or more as necessary in order to generate Ca-based oxides that become pinning particles necessary for suppressing the coarsening of reheated austenite grains. However, excessive addition generates coarse inclusions, so 0.0050% was made the upper limit.
Mgは、再加熱オーステナイト粒の粗大化抑制のために必要なピンニング粒子となるMg系酸化物を生成させるために必要に応じて0.0003%以上の添加する。しかし、過剰の添加は粗大介在物を生成させることから、0.0050%を上限とした。   Mg is added in an amount of 0.0003% or more as necessary in order to generate Mg-based oxides that become pinning particles necessary for suppressing the coarsening of reheated austenite grains. However, excessive addition generates coarse inclusions, so 0.0050% was made the upper limit.
REMは、再加熱オーステナイト粒の粗大化抑制のために必要なピンニング粒子となるREM系酸化物を生成させるために必要に応じて0.0001%以上の添加する。しかし、過剰の添加は粗大介在物を生成させることから、0.030%を上限とした。また、ここで述べるREMとは、CeおよびLaであり、添加量は両者の総量である。   REM is added in an amount of 0.0001% or more as necessary in order to generate REM-based oxides that become pinning particles necessary for suppressing the coarsening of reheated austenite grains. However, excessive addition generates coarse inclusions, so 0.030% was made the upper limit. Moreover, REM described here is Ce and La, and the addition amount is the total amount of both.
Bは、固溶Bとして溶接後の冷却中にオーステナイト粒界に偏析させ粒界フェライトの生成を抑制するため、また、オーステナイト粒界や粒内でBNを生成させるために、必要に応じて0.0005%以上添加する。しかし、過剰の添加は固溶B量を増大させ、HAZ硬さを大きく上昇させてHAZ靭性の低下を招くことから、0.0050%を上限とした。   B is solute B and segregates at the austenite grain boundaries during cooling after welding to suppress the formation of intergranular ferrite. Also, B is produced as necessary to produce BN within the austenite grain boundaries and grains. Add 0005% or more. However, excessive addition increases the amount of solute B, greatly increases the HAZ hardness and causes a decrease in HAZ toughness, so 0.0050% was made the upper limit.
Cuは、鋼材の強度および耐食性を向上させるために必要に応じて0.1%以上添加する。その効果は、1.0%で飽和するので上限を1.0としたが、0.4%を超えるとMAが生成しやすくなりHAZ靭性が低下することから、好ましくは0.4%以下が良い。   Cu is added in an amount of 0.1% or more as necessary to improve the strength and corrosion resistance of the steel material. The effect is saturated at 1.0%, so the upper limit was set to 1.0. However, if it exceeds 0.4%, MA tends to be formed and the HAZ toughness decreases, so 0.4% or less is preferable. good.
Crは、鋼材の耐食性を向上させるために必要に応じて0.1%以上添加するが、過剰の添加はMA生成によるHAZ靭性の低下を招くことから、0.5%を上限とした。   Cr is added in an amount of 0.1% or more as necessary in order to improve the corrosion resistance of the steel material, but excessive addition causes a reduction in HAZ toughness due to MA formation, so 0.5% was made the upper limit.
Moは、母材の強度および耐食性を向上させるために有効な元素であり必要に応じて0.01%以上添加する。その効果は、0.5%で飽和するので上限を0.5%としたが、過剰の添加はMA生成によるHAZ靭性の低下を招くことから、好ましくは0.2%以下が良い。   Mo is an effective element for improving the strength and corrosion resistance of the base material, and is added in an amount of 0.01% or more as necessary. The effect is saturated at 0.5%, so the upper limit was made 0.5%. However, excessive addition causes a decrease in HAZ toughness due to MA formation, so 0.2% or less is preferable.
Vは、母材の強度を向上させるために有効な元素であり必要に応じて0.005%添加する。その効果は、0.5%で飽和するので上限を0.10%としたが、過剰の添加はMA生成によるHAZ靭性の低下を招くことから、好ましくは0.050%以下が良い。   V is an effective element for improving the strength of the base material, and is added in an amount of 0.005% as necessary. The effect is saturated at 0.5%, so the upper limit was made 0.10%. However, excessive addition causes a reduction in HAZ toughness due to MA formation, so 0.050% or less is preferable.
表1に示した化学成分の溶鋼を連続鋳造して綱片を作製した。D23〜D31、D46〜D49に関してはTi投入前に溶鋼の溶存酸素をSiで0.0010%〜0.0050%に調整し、その後、まずTiで脱酸し、引き続きAlで脱酸した後、Ca、Mg,REMのいずれかを添加し脱酸した。これらを1100〜1250℃で再加熱したあと、以下の2種類の圧延方法により板厚50〜80mmの鋼板を製造した。一つは、表面温度が750〜900℃の温度範囲で圧延したあと、水冷後の板表面の温度が200〜400℃の温度範囲内で復熱するまで水冷する方法(表2ではTMCPと記載)であり、もう1つは、熱間圧延したのち室温まで水冷し、500〜600℃の範囲で焼戻す製造方法(表2ではDQ−Tと記載)である。   A steel piece having a chemical composition shown in Table 1 was continuously cast to produce a rope piece. For D23 to D31 and D46 to D49, the dissolved oxygen in the molten steel was adjusted to 0.0010% to 0.0050% with Si before introducing Ti, and then first deoxidized with Ti and subsequently deoxidized with Al. Any of Ca, Mg, and REM was added for deoxidation. After reheating these at 1100 to 1250 ° C., steel plates having a thickness of 50 to 80 mm were manufactured by the following two types of rolling methods. One is a method in which the surface temperature is rolled in a temperature range of 750 to 900 ° C., and then the plate surface after water cooling is cooled in water until it is reheated within a temperature range of 200 to 400 ° C. (described as TMCP in Table 2). The other is a manufacturing method (indicated as DQ-T in Table 2) after hot rolling, water cooling to room temperature, and tempering in the range of 500-600 ° C.
表2に鋼板の製造条件、板厚、機械的性質を示す。また、D23〜D31、D46〜D49に関しては、鋼板の任意の箇所において測定した、円相当径0.005〜0.5μmの微細酸化物の個数を併記した。酸化物の個数は、鋼板の任意の箇所から抽出レプリカを作製し、それを電子顕微鏡にて10000倍で100視野以上(観察面積にして10000μm2以上)を観察し、0.1μm未満の粒子に関しては適宜倍率を高めて観察した。観察される0.005〜0.5μm径の各粒子において元素分析を行い、酸化物であるものカウントすることにより求めた。D23〜D31、D46〜D49のどの鋼材も、円相当径で0.01〜0.5μmの微細酸化物が本発明範囲の100個/mm2以上分散させている。なお、Si以外の元素がほぼ同等であるD46、D47およびD48、D49の比較から、Si量は0.20%以下と少ない方が酸化物の量が多いことが分かる。 Table 2 shows the manufacturing conditions, plate thickness, and mechanical properties of the steel plate. Moreover, regarding D23 to D31 and D46 to D49, the number of fine oxides having an equivalent circle diameter of 0.005 to 0.5 μm, which was measured at an arbitrary portion of the steel sheet, was also shown. Regarding the number of oxides, an extraction replica is prepared from an arbitrary part of the steel plate, and it is observed with an electron microscope at a magnification of 10,000 at 100 fields or more (observation area of 10,000 μm 2 or more). Were observed with appropriately increased magnification. Elemental analysis was performed on each of the particles having a diameter of 0.005 to 0.5 μm to be observed, and the number of oxide particles was counted. In each of the steel materials D23 to D31 and D46 to D49, fine oxides having an equivalent circle diameter of 0.01 to 0.5 μm are dispersed in a range of 100 / mm 2 or more within the range of the present invention. From the comparison of D46, D47 and D48, D49 in which elements other than Si are almost the same, it can be seen that the smaller the amount of Si is 0.20% or less, the larger the amount of oxide.
これら鋼板に、溶接入熱量が20〜100kJ/mmであるエレクトロガス溶接(EGW)あるいはエレクトロスラグ溶接(ESW)を用いて、鋼板を突き合せて立て向き1パス溶接を行った。そして、板厚中央部(t/2)に位置するHAZにおいて、FLから1mm離れたHAZとFLの2箇所にノッチを入れ、−40℃でシャルピー衝撃試験を行った。表2に溶接条件とHAZ靭性を示す。ここでのシャルピー衝撃試験では、JIS4号の2mmVノッチのフルサイズ試験片を用いた。また、表2には、FL〜HAZ1mm間の旧オーステナイト粒径を併記した。ここで記載しているFL〜HAZ1mm間の旧オーステナイト粒径は、板厚中央部(2/t)を中心とした板厚方向2mmと、FL〜HAZ1mmを含む面に含まれる旧オーステナイト粒の粒径を断面法により測定した平均粒径である。なお、ここではネット状につながっている塊状フェライトを旧オーステナイト粒の粒界として測定を行なった。   These steel plates were subjected to vertical one-pass welding by butting the steel plates using electrogas welding (EGW) or electroslag welding (ESW) with a welding heat input of 20 to 100 kJ / mm. And in HAZ located in a plate | board thickness center part (t / 2), notch was put into two places, 1 mm away from FL, HAZ and FL, and the Charpy impact test was done at -40 degreeC. Table 2 shows welding conditions and HAZ toughness. In this Charpy impact test, a full size test piece of JIS No. 2 mmV notch was used. Table 2 also shows the prior austenite grain size between FL and HAZ of 1 mm. The prior austenite grain size between FL and HAZ of 1 mm described here is the grain size of the prior austenite grains included in the plane including 2 mm in the plate thickness direction centered on the plate thickness center (2 / t) and FL to HAZ of 1 mm. It is an average particle diameter measured by a cross-sectional method. Here, the measurement was performed using the massive ferrite connected in a net shape as the grain boundary of the prior austenite grains.
D1〜D49は本発明鋼である。鋼の化学成分が適正に制御されているために、所定の母材性能を満たしつつ、−40℃での大入熱HAZ靭性が良好である。また、微細酸化物を分散させたD23〜D31、D46〜D49はFL〜HAZ1mm間の旧オーステナイト粒径が200μm以下と他のものより細粒になっており、−40℃での大入熱HAZ靭性が一層高くなっている。また、Bの添加しHAZ組織の微細化を図ったD20は、Bを添加していない、B以外の添加元素が同量であるD19に比べてHAZ靭性が良好であり、−40℃での大入熱HAZ靭性も高い値を示している。
一方、比較鋼のC1〜17は、式[1]を満たすための十分なNiが含まれていない、もしくは、鋼の化学成分が適正に制御されているために、大入熱HAZ靭性が不充分である。
D1 to D49 are steels of the present invention. Since the chemical composition of steel is appropriately controlled, the high heat input HAZ toughness at −40 ° C. is satisfactory while satisfying a predetermined base material performance. Further, D23 to D31 and D46 to D49 in which fine oxides are dispersed have a prior austenite grain size between FL and HAZ of 1 mm of 200 μm or less, which is finer than others, and a large heat input HAZ at −40 ° C. The toughness is even higher. In addition, D20 with the addition of B and refinement of the HAZ structure has better HAZ toughness than D19 with no addition of B and the same amount of additive elements other than B, at −40 ° C. The high heat input HAZ toughness also shows a high value.
On the other hand, C1-17 of the comparative steel does not contain sufficient Ni to satisfy the formula [1], or the chemical composition of the steel is appropriately controlled, so that the high heat input HAZ toughness is not good. It is enough.
45kJ/mm相当の溶接熱サイクルを示す図である。It is a figure which shows the welding heat cycle equivalent to 45 kJ / mm. Ni/MnとCeqと再現HAZ靭性との関係を示す図である。It is a figure which shows the relationship between Ni / Mn, Ceq, and reproduction HAZ toughness. 微細酸化物分散またはB活用による再現HAZ靭性向上効果を示す図であ る。It is a figure which shows the reproduction HAZ toughness improvement effect by fine oxide dispersion | distribution or B utilization.

Claims (4)

  1. 質量%で、
    C :0.03〜0.14%、
    Si:0.30%以下、
    Mn:0.8〜2.0%、
    P :0.02%以下、
    S :0.005%以下、
    Al:0.001〜0.040%、
    N :0.0010〜0.0100%、
    Ni:0.8〜4.0%、
    Ti:0.005〜0.030%、
    Nb:0.003〜0.040%
    を含有し、NiとMnが式[1]を満たし、残部が鉄および不可避不純物であることを特徴とする、大入熱溶接による溶接熱影響部の低温靭性に優れた厚手高強度鋼板。
    Ni/Mn≧10×Ceq−3 (0.36<Ceq<0.42) ・・[1] 但し、Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
    % By mass
    C: 0.03-0.14%,
    Si: 0.30% or less,
    Mn: 0.8 to 2.0%,
    P: 0.02% or less,
    S: 0.005% or less,
    Al: 0.001 to 0.040%,
    N: 0.0010 to 0.0100%,
    Ni: 0.8 to 4.0%,
    Ti: 0.005 to 0.030%,
    Nb: 0.003-0.040%
    A thick, high-strength steel sheet excellent in the low temperature toughness of the weld heat-affected zone by high heat input welding, wherein Ni and Mn satisfy the formula [1], and the balance is iron and inevitable impurities.
    Ni / Mn ≧ 10 × Ceq-3 (0.36 <Ceq <0.42) [1] where Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15
  2. さらに、質量%で、
    Ca:0.0003〜0.0050%、
    Mg:0.0003〜0.0050%、
    REM:0.001〜0.030%
    のうちの1種または2種以上を含有し、かつ
    O :0.0010〜0.0050%
    を含有し、円相当径が0.005〜0.5μmの酸化物を、100個/mm2以上含有す ることを特徴とする、請求項1に記載の大入熱溶接による溶接熱影響部の低温靭性に優れた厚手高強度鋼板。
    Furthermore, in mass%,
    Ca: 0.0003 to 0.0050%,
    Mg: 0.0003 to 0.0050%,
    REM: 0.001 to 0.030%
    1 type or 2 types or more, and O 2: 0.0010 to 0.0050%
    The weld heat-affected zone by high heat input welding according to claim 1, characterized in that it contains 100 oxides / mm 2 or more of oxides having an equivalent circle diameter of 0.005 to 0.5 µm. Thick high-strength steel sheet with excellent low-temperature toughness.
  3. さらに、質量%で、
    B :0.0005〜0.0050%
    を含有することを特徴とする、請求項1または請求項2に記載の大入熱溶接による溶接熱影響部の低温靭性に優れた厚手高強度鋼板。
    Furthermore, in mass%,
    B: 0.0005 to 0.0050%
    A thick high-strength steel sheet excellent in low-temperature toughness of a weld heat affected zone by high heat input welding according to claim 1 or 2, characterized by comprising:
  4. さらに、質量%で、
    Cr:0.1〜0.5%、
    Mo:0.01〜0.5%、
    V :0.005〜0.10%、
    Cu:0.1〜1.0%
    のうちの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ないし請求項3のいずれか1項に記載の大入熱溶接による溶接熱影響部の低温靭性に優れた厚手高強度鋼板。
    Furthermore, in mass%,
    Cr: 0.1 to 0.5%,
    Mo: 0.01 to 0.5%,
    V: 0.005-0.10%,
    Cu: 0.1 to 1.0%
    4. Thickness excellent in low temperature toughness of weld heat-affected zone by high heat input welding according to claim 1, characterized by containing at least one of Strength steel plate.
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