JP2005068478A - Method for manufacturing thick steel plate with low yield ratio and high tension superior in toughness at heat-affected zone in super heavy-heat-input welding - Google Patents

Method for manufacturing thick steel plate with low yield ratio and high tension superior in toughness at heat-affected zone in super heavy-heat-input welding Download PDF

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JP2005068478A
JP2005068478A JP2003298250A JP2003298250A JP2005068478A JP 2005068478 A JP2005068478 A JP 2005068478A JP 2003298250 A JP2003298250 A JP 2003298250A JP 2003298250 A JP2003298250 A JP 2003298250A JP 2005068478 A JP2005068478 A JP 2005068478A
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JP4096839B2 (en
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Keiji Ueda
圭治 植田
Tatsumi Kimura
達巳 木村
Toshiyuki Hoshino
俊幸 星野
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JFE Steel Corp
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<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a thick steel plate with a low yield ratio and high tension, which has a TS of 590 MPa or higher, a superior toughness in a base metal, and a superior toughness at a heat-affected zone by welding with a super heavy heat input exceeding 400 kJ/cm. <P>SOLUTION: The base material of steel has a composition comprising 0.03-0.15% C, 0.05-0.5% Si, 0.5-3.0% Mn, 0.005-0.1% Al, 0.004-0.03% Ti, 0.0020-0.0070% N, 0.030% or less P, 0.0005-0.0030% S, 0.0005-0.0030% Ca, 0.0005-0.0030% B, 0.0050% or less O, and further one or more of 1.5% or less Cu and 2.0% or less Ni, while having Ceq so as to satisfy 0.35 or more and ACR satisfy 0.3 to 0.8. The manufacturing method comprises heating the base material to 1,000 to 1,300°C, hot rolling it, then air-cooling it to make a thick steel plate, subsequently reheating it to Ac3 or higher, quenching it at an average cooling rate of 1°C/s or higher, then heating it to the temperature of a two-phase region between (Ac1+10°C) and (Ac1+50°C), holding it, quenching it at an average cooling rate of 1°C/s or higher, and tempering it. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

この発明は、建築、土木および橋梁等の使途に好適な、厚鋼板に係り、とくに溶接熱影響部の靱性劣化を招くことなしに、溶接入熱が400kJ/cmを超える超大入熱溶接の実施が可能な、引張強さ(TS)が590MPa以上で、降伏比が80%以下を有する低降伏比高張力厚鋼板に関する。なお、この発明でいう、「厚鋼板」とは板厚30mm以上の鋼板を言うものとする。   The present invention relates to a thick steel plate suitable for use in construction, civil engineering, and bridges, and in particular, performs super-high heat input welding with a welding heat input exceeding 400 kJ / cm without causing toughness deterioration in the weld heat affected zone. The present invention relates to a low yield ratio high tensile steel plate having a tensile strength (TS) of 590 MPa or more and a yield ratio of 80% or less. In the present invention, the “thick steel plate” refers to a steel plate having a thickness of 30 mm or more.

建築、土木および橋梁等の各分野で使用される鋼材は、一般に溶接接合によって所望形状の鋼構造物に仕上げられている。このような鋼構造物においては、安全性の観点から、使用される鋼材には、母材靱性は勿論のこと、溶接熱影響部靱性に優れることが要求される。   Steel materials used in various fields such as architecture, civil engineering, and bridges are generally finished into steel structures having a desired shape by welding. In such a steel structure, from the viewpoint of safety, the steel material used is required to have excellent weld heat affected zone toughness as well as base metal toughness.

近年、建築構造物の大型化に伴い、構造物の施工能率向上と施工コストの低減の観点から、溶接効率の向上が求められ、大入熱溶接の適用範囲が拡大している。例えば、建築構造用ボックス柱では、サブマージアーク溶接やエレクトロスラグ溶接などの溶接入熱が400kJ/cmを超えるような超大入熱溶接が適用されている。   In recent years, with the increase in the size of building structures, improvement in welding efficiency is required from the viewpoint of improving the construction efficiency of the structure and reducing the construction cost, and the application range of large heat input welding is expanding. For example, for box columns for building structures, super-high heat input welding, such as submerged arc welding and electroslag welding, in which welding heat input exceeds 400 kJ / cm is applied.

また、近年、建築構造物の耐震性向上が求められ、建築構造物の溶接継手部においても、高い靱性を有することが要求されるようになっている。例えば、柱−梁接合部については、0℃におけるシャルピー吸収エネルギーが70Jを超えるような、高い靱性を有することが要求されている。また、ボックス柱の溶接部にも、同様の要求がある。   In recent years, improvement in earthquake resistance of building structures has been demanded, and the welded joints of building structures are also required to have high toughness. For example, the column-beam joint is required to have high toughness such that the Charpy absorbed energy at 0 ° C. exceeds 70 J. Moreover, the same request | requirement also exists in the welding part of a box pillar.

一般に、鋼材に大入熱溶接を適用した際に、最も問題となるのは、溶接熱影響部(以下、HAZともいう)のボンド部における靱性劣化である。ボンド部では、大入熱溶接時に融点直下の高温に晒されて、オーステナイト粒が最も粗大化し易く、また引き続く冷却に際し、脆弱な上部ベイナイト組織に変態し易く、さらには、ウィッドマンステッテン組織や島状マルテンサイト組織といった脆化組織が生成し易い。このような組織形成が、ボンド部における靱性低下の原因となっている。   In general, when high heat input welding is applied to a steel material, the most serious problem is toughness deterioration in the bond portion of the weld heat affected zone (hereinafter also referred to as HAZ). In the bond part, the austenite grains are most likely to be coarsened when exposed to a high temperature immediately below the melting point during high heat input welding, and are easily transformed into a fragile upper bainite structure upon subsequent cooling. Brittle structures such as island martensite structures are easily generated. Such a structure formation causes a decrease in toughness in the bond portion.

このような問題に対し、例えば、特許文献1には、TiNとREMオキシサルファイドを複合して鋼中に微細分散させてオーステナイト粒の粗大化を抑制し、大入熱溶接HAZの靭性を改善する技術が提案されている。また、特許文献2には、希土類元素(REM )をTiと複合添加することにより、鋼中に微細粒子を分散させてオーステナイトの粒成長を防止し、入熱量:230kJ/cmの溶接ボンド部靱性の向上を図る技術が開示されている。   For such a problem, for example, in Patent Document 1, TiN and REM oxysulfide are combined and finely dispersed in steel to suppress coarsening of austenite grains, and toughness of high heat input welding HAZ is improved. Technology has been proposed. Patent Document 2 discloses that rare earth elements (REM) and Ti are added in combination to disperse fine particles in steel and prevent austenite grain growth, and toughness of weld bond part with heat input of 230 kJ / cm. A technique for improving the above is disclosed.

また、特許文献3には、Ti酸化物を微細分散させ、大入熱溶接HAZの高靱化を図る技術が提案されている。また、特許文献4には、Ti窒化物の微細分散と、固溶B量を低減したうえでBNの析出を組み合わせて、大入熱溶接HAZの高靱化を図る技術が提案されている。   Patent Document 3 proposes a technique for finely dispersing Ti oxide to increase the toughness of high heat input welding HAZ. Patent Document 4 proposes a technique for increasing the toughness of high heat input welding HAZ by combining fine dispersion of Ti nitride and BN precipitation after reducing the amount of dissolved B.

また、特許文献5、特許文献6には、Tiの酸化物を微細分散させ、フェライト変態の核生成サイトとして利用し、大入熱溶接HAZの靱性を改善する技術が提案されている。   Patent Documents 5 and 6 propose a technique for improving the toughness of high heat input welding HAZ by finely dispersing Ti oxide and using it as a nucleation site for ferrite transformation.

また、特許文献7には、溶接時の冷却過程でTiN などの上に析出するBNをフェライト変態の核として利用し、大入熱溶接HAZの靱性を改善する技術が提案されている。   Patent Document 7 proposes a technique for improving the toughness of high heat input welding HAZ by using BN precipitated on TiN or the like in the cooling process during welding as a core of ferrite transformation.

また、特許文献8には、固溶Nを徹底的に低減するため、Tiと十分なAl量(0.05〜0.10質量%)を含有させ、さらに微細酸化物としてCa酸化物を活用して、超大入熱溶接におけるHAZ靱性を向上させる技術が提案されている。   Further, Patent Document 8 contains Ti and a sufficient amount of Al (0.05 to 0.10% by mass) in order to thoroughly reduce solid solution N, and further utilizes Ca oxide as a fine oxide, which is extremely large. Techniques for improving HAZ toughness in heat input welding have been proposed.

さらには、特許文献9には、Caを添加することで硫化物の形態を制御することにより、大入熱溶接HAZの靱性を改善する技術が提案されている。また、特許文献10には、REM を添加し硫化物の形態を制御することにより、大入熱溶接HAZの靱性を改善する技術が提案されている。   Furthermore, Patent Document 9 proposes a technique for improving the toughness of the high heat input welding HAZ by controlling the form of sulfide by adding Ca. Patent Document 10 proposes a technique for improving toughness of high heat input welding HAZ by adding REM and controlling the form of sulfide.

しかしながら、上記したTi酸化物を用いる従来技術では、酸化物を均一かつ微細に分散させることがかなりの困難を伴い、このため、酸化物の複合化等によりその分散能を改善すべく種々の検討がなされているが、入熱が400kJ/cmを超える大入熱溶接においてはオーステナイト粒の成長を十分に制御することが現在までのところ難しく、超大入熱溶接HAZを安定して高靱性とすることが困難であった。   However, the conventional technology using the Ti oxide described above involves considerable difficulty in uniformly and finely dispersing the oxide. For this reason, various studies have been made to improve the dispersibility by combining oxides. However, in high heat input welding where the heat input exceeds 400 kJ / cm, it has been difficult to control the growth of austenite grains so far, and the super high heat input welding HAZ is made stable and highly tough. It was difficult.

また、上記したTiN を主体に利用する従来技術で製造された鋼材に、400kJ/cmを超える大入熱溶接を適用した場合、HAZがTiN の溶解する高温域に長時間曝されるため、TiN による結晶粒粗大化制御の効果が無くなり、超大入熱溶接HAZを高靱化することができなくなるという問題があった。また、上記した従来技術では、固溶Tiおよび固溶Nの増加に起因して、脆化組織が生成し、著しくHAZ靱性が低下する場合もあるという問題があった。   In addition, when high heat input welding exceeding 400 kJ / cm is applied to the steel materials manufactured by the prior art mainly using TiN, the HAN is exposed to the high temperature region where TiN dissolves for a long time. There is a problem that the effect of controlling the coarsening of crystal grains is lost, and the super-high heat input welding HAZ cannot be toughened. In addition, the above-described conventional technique has a problem that an embrittled structure is generated due to an increase in solute Ti and solute N, and the HAZ toughness may be significantly reduced.

また、特許文献8に記載された技術は、靱性に悪影響を及ぼす固溶N量の低減と融点近傍の高温域でも結晶粒粗大化制御効果を有する酸化物を活用することで、超大入熱溶接におけるHAZ靱性の向上させたものであり、過剰にAlを含有させることが特徴である。しかし、多量のAl添加は、溶接金属に混入して脱酸反応に影響し、溶接金属部靱性を低下させるという問題があった。
特公平3-53367号公報 特開平6-184663号公報 特開昭57-51243号公報 特開昭62-170459号公報 特開昭60-245768号公報 特開昭61-79745号公報 特開昭61-253344号公報 特開2001-107177号公報 特開昭60-204863号公報 特公平4-14180号公報
In addition, the technique described in Patent Document 8 uses ultra-high heat input welding by utilizing an oxide having a reduction effect of solid solution N that adversely affects toughness and a grain coarsening control effect even in a high temperature region near the melting point. HAZ toughness is improved and is characterized by containing Al excessively. However, a large amount of Al added to the weld metal affects the deoxidation reaction, resulting in a problem that the weld metal part toughness is lowered.
Japanese Patent Publication No. 3-53367 JP-A-6-184663 JP 57-51243 A JP-A-62-170459 JP-A-60-245768 JP 61-79745 JP 61-253344 JP Japanese Patent Laid-Open No. 2001-107177 JP 60-204863 A Japanese Patent Publication No. 4-14180

最近では、建築構造物の更なる大型化に伴い、鋼材の一層の厚肉化、高強度化が進められており、引張強さ:590MPa以上の厚鋼板が要望されている。一方、鋼構造物の安全性、すなわち脆化破壊防止の観点からは、降伏比が低い鋼材が要求されている。降伏比を低減することにより、降伏点以上の応力が付加されても破壊までに許容される応力が大きくなり、また、一様伸びが大きくなるため、塑性変形能に優れた鋼材となる。   Recently, with further increase in size of building structures, steel materials have been made thicker and stronger, and steel plates with a tensile strength of 590 MPa or more are being demanded. On the other hand, steel materials having a low yield ratio are required from the viewpoint of safety of steel structures, that is, prevention of embrittlement failure. By reducing the yield ratio, even if a stress higher than the yield point is applied, the stress allowed until failure increases, and the uniform elongation increases, so that the steel material is excellent in plastic deformability.

この発明が対象とする厚鋼板は、いわゆる溶接用低合金鋼と呼ばれるもので、例えば、建築構造用590MPa級高張力鋼(SA440)としては、化学成分が質量%で、C:0.18%以下、Si:0.55%以下、Mn:1.60%以下、P:0.035%以下、S:0.008%以下で、引張強さが590〜740MPa、降伏比が80%以下が、鋼構造評定委員会の基準強度として要求されている。   The steel plate targeted by the present invention is a so-called low alloy steel for welding. For example, as a 590 MPa class high strength steel (SA440) for building structures, the chemical composition is mass%, C: 0.18% or less, Si: 0.55% or less, Mn: 1.60% or less, P: 0.035% or less, S: 0.008% or less, tensile strength of 590 to 740 MPa, yield ratio of 80% or less is the standard strength of the steel structure evaluation committee It is requested.

しかしながら、一般に、鋼板の強度を増加するにしたがって、降伏比は上昇する傾向にあるため、低降伏比で高い強度を有する厚鋼板を製造することは極めて難しいという問題があった。さらに、現在までのところ、溶接入熱が400kJ/cmを超える超大入熱溶接部を安定して高靱性とする技術が確立されているとは言い難い。   However, in general, the yield ratio tends to increase as the strength of the steel sheet is increased, and thus there is a problem that it is extremely difficult to manufacture a thick steel sheet having a high strength at a low yield ratio. Furthermore, to date, it is difficult to say that a technology has been established that makes a super high heat input weld with a welding heat input exceeding 400 kJ / cm stable and tough.

このようなことから、引張強さ:590MPa以上で、80%以下の低降伏比を有し、かつ400kJ/cmを超える超大入熱溶接を施した場合でもHAZ靱性に優れた低降伏比高張力厚鋼板の開発が望まれていた。   Therefore, tensile strength: 590 MPa or more, low yield ratio of 80% or less, and low yield ratio and high tensile strength with excellent HAZ toughness even when super high heat input welding exceeding 400 kJ / cm is applied. The development of thick steel plates has been desired.

例えば、特開昭59-211528号公報には、熱間圧延終了後、徐冷により一部フェライト析出させた後、急冷して、フェライト+硬質相の2相混合組織を得る技術が提案されている。   For example, Japanese Patent Application Laid-Open No. 59-211528 proposes a technique for obtaining a two-phase mixed structure of ferrite and hard phase by precipitating a part of ferrite by slow cooling after hot rolling is completed and then rapidly cooling. Yes.

また、特開平5-125438号公報には、熱間圧延終了後、 Ac3変態点以上の温度から550〜700℃の温度範囲まで5℃/s以上の速度で制御冷却することにより、制御冷却途中のフェライト析出を抑制し、その後の空冷過程で変態させて低降伏比に好適なフェライト+パーライト+ベイナイト組織を得る技術が提案されている。   JP-A-5-125438 discloses that after the hot rolling is completed, controlled cooling is performed at a rate of 5 ° C./s or higher from the temperature above the Ac3 transformation point to a temperature range of 550 to 700 ° C. Has been proposed in which ferrite precipitation is suppressed and transformed in the subsequent air cooling process to obtain a ferrite + pearlite + bainite structure suitable for a low yield ratio.

これらの技術は、フェライトのような軟質相とベイナイトのような硬質相との混合組織にすることにより、軟質相によって低い降伏強さを、硬質相によって高い引張強さを確保しようとするものである。   These technologies attempt to secure a low yield strength by the soft phase and a high tensile strength by the hard phase by making a mixed structure of a soft phase such as ferrite and a hard phase such as bainite. is there.

しかしながら、上記した技術では、いずれも厚鋼板の板厚方向の温度差が大きくなり、材質のばらつきが大きくなるという問題があった。   However, each of the above-described techniques has a problem that the temperature difference in the plate thickness direction of the thick steel plate becomes large and the variation in material becomes large.

この発明は、上記した従来技術の問題点を有利に解決し、板厚方向の機械的特性の不均一を解消し、降伏比80%以下の低降伏比で、590MPa以上の引張強さ(TS)を有し、かつ母材靭性に優れ、さらに溶接入熱が400kJ/cmを超えるような超大入熱溶接を施した場合であっても溶接熱影響部の靱性に優れた、超大入熱溶接熱影響部靱性に優れた低降伏比高張力厚鋼板の製造方法を提案することを目的とする。   The present invention advantageously solves the above-mentioned problems of the prior art, eliminates the non-uniformity of the mechanical properties in the thickness direction, has a low yield ratio of 80% or less, and a tensile strength (TS of 590 MPa or more). ), Excellent toughness of the base metal, and super high heat input welding with excellent weld heat affected zone toughness even when super high heat input welding with a heat input exceeding 400 kJ / cm is applied. It aims at proposing the manufacturing method of the low yield ratio high tension thick steel plate excellent in the heat affected zone toughness.

本発明者らは、上記した課題を達成するために鋭意研究し、その結果、厳格な成分調整により、TiN の微細分散形態の制御によるオーステナイト粒の粗大化を効果的に抑制するとともに、溶接時の冷却過程にフェライト変態を促進する変態核を微細分散させることにより、溶接入熱が400kJ/cmを超える大入熱溶接を施した場合の溶接熱影響部で、優れた靱性を安定して確保できることを知見した。また、さらに本発明者らは、固溶強化に有効な元素であるCuおよびNiを適正量含有し、(Ac1 変態点+30℃)程度の2相域温度に加熱し、保持してから焼入れる熱処理を施すことにより、超大入熱溶接HAZ靱性の劣化を招くことなく、引張強さ(TS)が590MPa以上の高強度化と、80%以下の低降伏比を達成できることを知見した。   The inventors of the present invention have intensively studied to achieve the above-mentioned problems, and as a result, it is possible to effectively suppress the coarsening of austenite grains by controlling the finely dispersed form of TiN by strict component adjustment, and at the time of welding. By stably dispersing transformation nuclei that promote ferrite transformation during the cooling process of steel, excellent toughness can be stably secured in the heat affected zone when high heat input welding with a heat input exceeding 400 kJ / cm is applied. I found out that I can do it. Furthermore, the present inventors contain appropriate amounts of Cu and Ni, which are effective elements for solid solution strengthening, and are heated to a two-phase temperature of about (Ac1 transformation point + 30 ° C.), held, and quenched. It has been found that by performing heat treatment, the tensile strength (TS) can be increased to 590 MPa or more and the yield ratio can be reduced to 80% or less without causing deterioration of the super high heat input welding HAZ toughness.

この発明は、上記した知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨はつぎのとおりである。
(1)質量%で、C:0.03〜0.15%、Si:0.05〜0.5%、Mn:0.5〜3.0%、Al:0.005〜0.1%、Ti:0.004〜0.03%、N:0.0020〜0.0070%、P:0.030%以下、S:0.0005〜0.0030%、Ca:0.0005〜0.0030%、B:0.0005〜0.0030%、O:0.0050%以下を含み、さらに、Cu:1.5%以下、Ni:2.0%以下のうちから選ばれた1種または2種を含有し、かつ次(1)式
Ceq =C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 ………(1)
ここで、Ceq :炭素当量(%)
C、Mn、Ni、Cu、Cr、Mo、V:各元素の含有量(質量%)
で定義される炭素当量Ceq が0.35以上を満足し、かつ次(2)式
ACR ={Ca−(0.18+130Ca)×0}/(1.25×S)………(2)
ここで、Ca、O、S:各元素の含有量(質量%)
で定義されるACRが0.3〜0.8を満足する組成を有する鋼素材に、1000〜1300℃に加熱し熱間圧延した後、空冷する熱延工程を施し厚鋼板としたのち、該厚鋼板に、Ac3変態点以上の加熱温度に再加熱し、該加熱温度で保持してから1℃/s以上の平均冷却速度で焼入れする再加熱焼入れ工程と、ついで、(Ac1変態点+10℃)〜(Ac1 変態点+50℃ )の二相域の温度に加熱し、該温度で保持してから1℃/s以上の平均冷却速度で焼入れする二相域加熱焼入れ工程と、さらに、Ac1 変態点以下の温度で焼戻しする焼戻し工程とを、順次施すことを特徴とする、引張強さ:590MPa以上、降伏比:80%以下を有する超大入熱溶接熱影響部靱性に優れた低降伏比高張力厚鋼板の製造方法。
(2)(1)において、前記組成に加えてさらに、質量%で、Cr:0.7%以下、Mo:0.7%以下、Nb:0.05%以下、V:0.2%以下のうちから選ばれた1種または2種以上を含有することを特徴とする低降伏比高張力厚鋼板の製造方法。
(3)(1)または(2)において、前記組成に加えてさらに、REM :0.02%以下、Mg:0.005%以下のうちから選ばれた1種または2種を含有することを特徴とする低降伏比高張力厚鋼板の製造方法。
The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.03-0.15%, Si: 0.05-0.5%, Mn: 0.5-3.0%, Al: 0.005-0.1%, Ti: 0.004-0.03%, N: 0.0020-0.0070%, P : 0.030% or less, S: 0.0005 to 0.0030%, Ca: 0.0005 to 0.0030%, B: 0.0005 to 0.0030%, O: 0.0050% or less, Cu: 1.5% or less, Ni: 2.0% or less Contains one or two selected, and the following formula (1)
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
Where Ceq: carbon equivalent (%)
C, Mn, Ni, Cu, Cr, Mo, V: Content of each element (% by mass)
The carbon equivalent Ceq defined by is satisfied with 0.35 or more, and the following formula (2)
ACR = {Ca− (0.18 + 130Ca) × 0} / (1.25 × S) (2)
Here, Ca, O, S: Content of each element (mass%)
A steel material having a composition satisfying an ACR defined by 0.3 to 0.8 is heated to 1000 to 1300 ° C. and hot-rolled, and then subjected to a hot rolling step of air cooling to obtain a thick steel plate. A reheating and quenching step of reheating to a heating temperature above the Ac3 transformation point, holding at that heating temperature and quenching at an average cooling rate of 1 ° C / s or higher, then (Ac1 transformation point + 10 ° C) to (Ac1) A two-phase region heating and quenching process in which the steel is heated to a temperature in the two-phase region (transformation point + 50 ° C.) and held at the temperature, and then quenched at an average cooling rate of 1 ° C./s or more, and a temperature below the Ac1 transformation point. A low-yield ratio high-tensile steel plate with excellent tensile strength: 590 MPa and yield ratio: 80% or less Production method.
(2) In (1), in addition to the above composition, in addition to mass, one selected from Cr: 0.7% or less, Mo: 0.7% or less, Nb: 0.05% or less, V: 0.2% or less Or the manufacturing method of the low yield ratio high-tensile thick steel plate characterized by containing 2 or more types.
(3) In the above (1) or (2), in addition to the above composition, the composition further comprises one or two selected from REM: 0.02% or less and Mg: 0.005% or less. Yield ratio high tension steel plate manufacturing method.

この発明によれば、引張強さ590MPa以上で、降伏比80%以下の低降伏比を有し、400kJ/cmを超える超大入熱溶接の溶接熱影響部靱性に優れた高張力厚鋼板を、材質ばらつき無く安定して製造することができ、鋼構造物の大型化や、鋼構造物の耐震性の向上や施工能率向上に、大きく寄与し、産業上格段の効果を奏する。   According to the present invention, a high-tensile thick steel plate having a tensile strength of 590 MPa or more, a low yield ratio of 80% or less, and an excellent weld heat-affected zone toughness of super-high heat input welding exceeding 400 kJ / cm, It can be manufactured stably without material variations, greatly contributes to increasing the size of steel structures, improving the earthquake resistance of steel structures and improving construction efficiency, and has a remarkable industrial effect.

まず、本発明で使用する鋼素材の組成限定理由について説明する。なお、以下、組成に関する%表示は、特に断らない限り質量%を意味するものとする。   First, the reasons for limiting the composition of the steel material used in the present invention will be described. In addition, hereinafter, the “%” regarding the composition means “% by mass” unless otherwise specified.

C:0.03〜0.15%
Cは、鋼の強度を増加させ、構造用鋼材として必要な強度を確保するのに有用な元素であり、この発明では、上記した効果を得るため、0.03%以上の含有を必要とする。一方、0.15%を超える含有は、HAZの靱性、耐溶接割れ性を劣化させる.このため、Cは0.03〜0.15%の範囲に限定した。なお、好ましくは、0.03〜0.12%である。
C: 0.03-0.15%
C is an element useful for increasing the strength of steel and ensuring the strength required as a structural steel material. In the present invention, it is necessary to contain 0.03% or more in order to obtain the effects described above. On the other hand, a content exceeding 0.15% deteriorates the toughness and weld crack resistance of HAZ. For this reason, C was limited to the range of 0.03-0.15%. In addition, Preferably, it is 0.03 to 0.12%.

Si:0.05〜0.50%
Siは、脱酸剤として作用し、製鋼上、少なくとも0.05%必要であるが、0.50%を超えて含有すると、母材の靱性が劣化するとともに、超大入熱溶接HAZ靱性において島状マルテンサイトが生成して、HAZ靱性が顕著に劣化する。このため、Siは0.05〜0.50%の範囲に限定した。なお、好ましくは、0.05〜0.40%である。
Si: 0.05-0.50%
Si acts as a deoxidizer and needs to be at least 0.05% for steelmaking. However, if it exceeds 0.50%, the toughness of the base metal deteriorates, and island martensite is present in the super large heat input welding HAZ toughness. And HAZ toughness is significantly degraded. For this reason, Si was limited to the range of 0.05 to 0.50%. In addition, Preferably, it is 0.05 to 0.40%.

Mn:0.5〜3.0%
Mnは、鋼の強度を増加させる作用を有しており、本発明では、引張強さ590MPa以上を確保するために、0.5%以上の含有を必要とする。一方、3.0%を超えて含有すると、母材靭性および溶接熱影響部靭性が著しく劣化する。このため、Mnは0.5〜3.0%の範囲に限定した。なお、好ましくは0.6〜1.6%である。
Mn: 0.5-3.0%
Mn has the effect of increasing the strength of the steel, and in the present invention, it is necessary to contain 0.5% or more in order to ensure a tensile strength of 590 MPa or more. On the other hand, if the content exceeds 3.0%, the base material toughness and the weld heat affected zone toughness deteriorate significantly. For this reason, Mn was limited to the range of 0.5 to 3.0%. In addition, Preferably it is 0.6 to 1.6%.

Al:0.005〜0.1%
Alは、脱酸剤として作用し、製鋼上、0.005%以上の含有を必要とするが、0.1%を超えて含有すると、母材の靱性が低下するとともに、溶接時に溶接金属部に混入して、溶接金属靱性を劣化させる。このため、Alは0.005〜0.1%に限定した。なお、好ましくは、0.005〜0.05%である。
Al: 0.005-0.1%
Al acts as a deoxidizer and requires 0.005% or more in steelmaking. However, if it exceeds 0.1%, the toughness of the base metal is reduced and mixed into the weld metal during welding. , Deteriorate the weld metal toughness. For this reason, Al was limited to 0.005 to 0.1%. In addition, Preferably, it is 0.005-0.05%.

Ti:0.004〜0.03%
Tiは、Nとの親和力が強く凝固時にTiN として析出し、HAZでのオーステナイト粒の粗大化を抑制し、あるいはフェライト変態核としてHAZの高靱化に寄与する。このような効果は、0.004%以上の含有で認められるが、0.03%を超えて含有すると、TiN 粒子が粗大化し、上記した効果が期待できなくなる。このため、Tiは0.004〜0.03%の範囲に限定した。なお、好ましくは、0.005〜0.020%である。
Ti: 0.004 to 0.03%
Ti has a strong affinity for N and precipitates as TiN during solidification, thereby suppressing the coarsening of austenite grains in the HAZ or contributing to the toughening of the HAZ as a ferrite transformation nucleus. Such an effect is recognized when the content is 0.004% or more. However, if the content exceeds 0.03%, the TiN particles become coarse, and the above-described effects cannot be expected. For this reason, Ti was limited to the range of 0.004 to 0.03%. In addition, Preferably, it is 0.005-0.020%.

N:0.0020〜0.0070%
Nは、Tiと結合してTiN として析出して、HAZでのオーステナイト粒の粗大化を抑制し、あるいはフェライト変態核としてHAZの高靱化に寄与する。このような効果を有するTiN の必要量を確保するために、Nは0.0020%以上含有する必要がある。一方、0.0070%を超えて含有すると、溶接時にTiN が溶解する温度まで加熱される領域では、固溶N量が増加し、靱性が著しく低下する。このため、Nは0.0020〜0.0070%に限定した。
N: 0.0020-0.0070%
N combines with Ti and precipitates as TiN to suppress coarsening of austenite grains in the HAZ, or contributes to high toughness of the HAZ as a ferrite transformation nucleus. In order to secure the necessary amount of TiN having such an effect, N needs to be contained in an amount of 0.0020% or more. On the other hand, if the content exceeds 0.0070%, in the region heated to a temperature at which TiN dissolves during welding, the amount of solute N increases and the toughness significantly decreases. For this reason, N was limited to 0.0020-0.0070%.

P:0.030%以下
Pは、鋼の強度増加させ靭性を劣化させる元素であり、とくに溶接部の靭性を劣化させるため、できるだけ低減することが望ましい。Pが0.030%を超えて含有されるとこの傾向が顕著となるため、上限とした。なお、過度のPの低減は、精錬コストを高騰させ経済的に不利となるため、0.005%以上とすることが好ましい。
P: 0.030% or less P is an element that increases the strength of steel and degrades toughness. In particular, P is desirably reduced as much as possible in order to degrade the toughness of welds. Since this tendency becomes conspicuous when P exceeds 0.030%, it was set as the upper limit. In addition, since excessive reduction of P raises refining costs and is economically disadvantageous, it is preferable to make it 0.005% or more.

S:0.0005〜0.0030%
Sは、Caと結合してCaS 粒子として凝固段階で微細に晶出する。Sは、さらに溶接時にCaS 粒子上にMnS として析出し、フェライト変態核として作用し、溶接部靱性を向上させる効果を有する。このような効果は、0.0005%以上の含有で認められる。一方、0.0030%を超えて含有すると、母材および溶接部の靱性を劣化させる。このため、Sは0.0005〜0.0030%に限定した。
S: 0.0005-0.0030%
S combines with Ca and crystallizes finely as CaS particles in the solidification stage. S further precipitates as MnS on the CaS particles during welding, acts as a ferrite transformation nucleus, and has an effect of improving weld toughness. Such an effect is recognized when the content is 0.0005% or more. On the other hand, when it contains exceeding 0.0030%, the toughness of a base material and a welding part will deteriorate. For this reason, S was limited to 0.0005 to 0.0030%.

Ca:0.0005〜0.0030%
Caは、硫化物の形態を制御して鋼の延性向上に寄与する元素である。このような効果を発揮させるには、少なくとも0.0005%含有することが必要であるが、0.0030%を超えて含有しても効果が飽和する。このため、本発明では、Caは0.0005〜0.0030%の範囲に限定した。なお、本発明では、後述するように、Ca添加直前の溶存酸素量を0.0050%以下に調整した後、Caを添加して、Ca酸化物を抑制してCaS を晶出させる。CaS は、溶鋼中で酸化物に比べて低温で晶出するため、鋼中で微細かつ均一な分散が可能となる。このような微細なCaS 粒子はMnS と複合して溶接時にフェライト変態核として作用し、HAZ靱性の向上に寄与する。
Ca: 0.0005 to 0.0030%
Ca is an element that contributes to improving the ductility of steel by controlling the form of sulfide. In order to exhibit such an effect, it is necessary to contain at least 0.0005%, but even if it exceeds 0.0030%, the effect is saturated. For this reason, in this invention, Ca was limited to 0.0005 to 0.0030% of range. In the present invention, as will be described later, after adjusting the amount of dissolved oxygen immediately before Ca addition to 0.0050% or less, Ca is added to suppress Ca oxide and crystallize CaS. Since CaS crystallizes in molten steel at a lower temperature than oxides, it enables fine and uniform dispersion in the steel. Such fine CaS particles are combined with MnS and act as ferrite transformation nuclei during welding, contributing to the improvement of HAZ toughness.

B:0.0005〜0.0030%
Bは、焼入れ性の向上を介して微量で鋼の高強度化に寄与する元素である。このような効果を得るためには、0.0005%以上の含有を必要とするが、0.0030%を超える含有は、溶接熱影響部の靱性を劣化させる。このため、Bは0.0005〜0.0030%の範囲に限定した。
B: 0.0005-0.0030%
B is an element that contributes to increasing the strength of steel in a small amount through improvement of hardenability. In order to obtain such an effect, the content of 0.0005% or more is required, but the content exceeding 0.0030% deteriorates the toughness of the weld heat affected zone. For this reason, B was limited to the range of 0.0005 to 0.0030%.

O:0.0050%以下
Oは、不可避的不純物として含有され、鋼中では酸化物として存在し、清浄度を低下させる。このため、本発明では、できるだけ低減することが好ましい。O含有量が0.0050%を超えると、CaO系介在物が粗大化して靭性に悪影響を及ぼす。また、本発明では、CaをCaS として晶出させるために、Caとの結合力が強いOはCa添加前に、脱ガスを強化するか、脱酸剤を投入して、溶鋼中のOを0.0050%以下に低減しておくことが好ましい。
O: 0.0050% or less O is contained as an unavoidable impurity and exists as an oxide in steel, which lowers the cleanliness. For this reason, in this invention, it is preferable to reduce as much as possible. When the O content exceeds 0.0050%, CaO inclusions become coarse and adversely affect toughness. Further, in the present invention, in order to crystallize Ca as CaS, O having a strong binding force with Ca reinforces degassing or introduces a deoxidizer before adding Ca to add O in molten steel. It is preferable to reduce it to 0.0050% or less.

Cu:1.5%以下、Ni:2.00%以下のうちから選ばれた1種または2種
Cu、Niは、本発明における重要な合金元素であり、いずれも母材の高強度化に寄与する元素であり、選択して1種または2種を含有する。
One or two selected from Cu: 1.5% or less, Ni: 2.00% or less
Cu and Ni are important alloying elements in the present invention, both of which are elements that contribute to increasing the strength of the base material, and optionally contain one or two kinds.

Cuは、固溶強化により母材強化に寄与するとともに、オーステナイト安定化元素であり、2相域の温度に加熱保持中に、オーステナイト相中に優先的に濃化し、オーステナイト相の焼入れ性を向上させる。一方、フェライト相中のCu濃度は低下するため、フェライト相がより軟化する。このため、2相域の温度から急冷(焼入れ)することにより、フェライト+マルテンサイト混合組織となり、高強度化と低降伏比を両立させることが可能となる。このような効果は、Cuの0.1%以上の含有により顕著となる。しかし、Cu含有量が1.5%を超えると熱間脆性が生じ、鋼板の表面性状を劣化させる。このため、Cuは1.5%以下に限定した。なお、好ましくは0.1〜1.0%である。   Cu contributes to the strengthening of the base metal by solid solution strengthening and is an austenite stabilizing element. It is preferentially concentrated in the austenite phase during heating and holding at a temperature in the two-phase region, improving the hardenability of the austenite phase. Let On the other hand, since the Cu concentration in the ferrite phase decreases, the ferrite phase becomes softer. For this reason, by rapidly cooling (quenching) from the temperature in the two-phase region, a mixed structure of ferrite and martensite is obtained, and it is possible to achieve both high strength and a low yield ratio. Such an effect becomes remarkable when the Cu content is 0.1% or more. However, when the Cu content exceeds 1.5%, hot brittleness occurs and the surface properties of the steel sheet are deteriorated. For this reason, Cu was limited to 1.5% or less. In addition, Preferably it is 0.1 to 1.0%.

Niは、Cuと同様、固溶強化により母材の高強度化に寄与するとともに、母材の低温靱性を向上させる、有用な元素である。さらに、Niは、オーステナイト安定化元素であり、2相域の温度に加熱、保持中に、オーステナイト相中に優先的に濃化し、オーステナイト相の焼入れ性を増加させる。これに伴い、フェライト相中のNi濃度が低下し、フェライト相がより軟化する。このため、2相域の温度から急冷(焼入れ)することにより、フェライト+マルテンサイト混合組織となり、高強度化と低降伏比とを両立させることが可能となる。このような効果は、Niの0.1%以上の含有により顕著となる。しかし、Ni含有量が2.0%を超えて含有しても効果が飽和するため、Niは高価であり経済的に不利となる。このため、Niは2.0%以下に限定した。なお、好ましくは0.1〜1.5%である。   Ni, like Cu, is a useful element that contributes to increasing the strength of the base metal by solid solution strengthening and improves the low temperature toughness of the base material. Further, Ni is an austenite stabilizing element, and is preferentially concentrated in the austenite phase during heating and holding at a temperature in a two-phase region, thereby increasing the hardenability of the austenite phase. Along with this, the Ni concentration in the ferrite phase decreases and the ferrite phase becomes softer. For this reason, by rapidly cooling (quenching) from the temperature in the two-phase region, a mixed structure of ferrite and martensite is obtained, and it is possible to achieve both high strength and a low yield ratio. Such an effect becomes remarkable by containing 0.1% or more of Ni. However, even if the Ni content exceeds 2.0%, the effect is saturated, so Ni is expensive and economically disadvantageous. For this reason, Ni was limited to 2.0% or less. In addition, Preferably it is 0.1 to 1.5%.

Ceq :0.35以上
本発明では、上記した成分組成範囲内で、さらに、次(1)式
Ceq =C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 ………(1)
ここで、Ceq :炭素当量(%)
C、Mn、Ni、Cu、Cr、Mo、V:各元素の含有量(質量%)
で定義される炭素当量Ceqが0.35%以上となるように、各成分の含有量を調整する。Ceqが0.35%未満では、所望の引張強さ:590MPa以上を確保できなくなる。なお、CeqはHAZ靭性確保の観点から0.50%以下とすることが好ましい。(1)式におけるCeq の計算は、含有しない元素がある場合には、当該元素の含有量を零として計算するものとする。
Ceq: 0.35 or more In the present invention, within the above component composition range, the following formula (1)
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
Where Ceq: carbon equivalent (%)
C, Mn, Ni, Cu, Cr, Mo, V: Content of each element (% by mass)
The content of each component is adjusted so that the carbon equivalent Ceq defined by is 0.35% or more. If Ceq is less than 0.35%, the desired tensile strength: 590 MPa or more cannot be secured. Ceq is preferably 0.50% or less from the viewpoint of securing HAZ toughness. In the calculation of Ceq in equation (1), when there is an element not contained, the content of the element is assumed to be zero.

ACR:0.3〜0.8
この発明では、Ca添加時の溶鋼中の溶存酸素量を0.0050%以下に調整したうえで、次(2)式
ACR ={Ca−(0.18+130Ca)×0}/(1.25×S)………(2)
ここで、Ca、O、S:各元素の含有量(質量%)
で定義されるACRが0.3〜0.8を満足するように、Ca、Sを添加、調整する。ACRが0.3未満では、CaS が晶出しないために、SはMnS 単独の形態で析出する。このMnS は鋼板製造時の圧延で伸長されて均一且つ微細に分散しないため、母材の靱性低下を招くとともに、溶接HAZ靱性向上にも寄与しない。一方、ACRが0.8を超えると、SがCaによって固定されMnSとなるSが不足し、MnSがフェライト生成核として働くCaS 上へ析出しないことから、HAZ靱性の向上が期待できない。ACRが、0.3〜0.8を満足してはじめて、CaS 上にMnS が析出した複合硫化物の形態となる。この複合硫化物の存在により、フェライト変態核として機能し、HAZ組織が微細化され、HAZ靱性が向上する。
ACR: 0.3-0.8
In this invention, after adjusting the amount of dissolved oxygen in molten steel at the time of Ca addition to 0.0050% or less, the following formula (2)
ACR = {Ca− (0.18 + 130Ca) × 0} / (1.25 × S) (2)
Here, Ca, O, S: Content of each element (mass%)
Ca and S are added and adjusted so that the ACR defined by the above satisfies 0.3 to 0.8. When ACR is less than 0.3, CaS does not crystallize, so S precipitates in the form of MnS alone. This MnS is stretched by rolling at the time of manufacturing the steel sheet and does not disperse uniformly and finely, so that the toughness of the base metal is reduced and does not contribute to the improvement of the welded HAZ toughness. On the other hand, when the ACR exceeds 0.8, S is fixed by Ca and S which becomes MnS is insufficient, and MnS does not precipitate on CaS which functions as a ferrite nuclei. Therefore, improvement in HAZ toughness cannot be expected. Only when the ACR satisfies 0.3 to 0.8 is a composite sulfide in which MnS is deposited on CaS. The presence of this composite sulfide functions as a ferrite transformation nucleus, refines the HAZ structure, and improves the HAZ toughness.

本発明では、上記した基本成分に加えて、必要に応じ、Cr:0.7%以下、Mo:0.7%以下、Nb:0.05%以下、V:0.2%以下のうちから選ばれた1種または2種以上、および/または、REM :0.02%以下および Mg:0.005%以下のうちから選ばれた1種または2種を含有することができる。   In the present invention, in addition to the basic components described above, one or two selected from Cr: 0.7% or less, Mo: 0.7% or less, Nb: 0.05% or less, and V: 0.2% or less as necessary. One or two selected from the above and / or REM: 0.02% or less and Mg: 0.005% or less can be contained.

Cr:0.7%以下、Mo:0.7%以下、Nb:0.05%以下、V:0.2%以下のうちから選ばれた1種または2種以上
Cr、Mo、Nb、Vは、いずれも鋼の強度向上に寄与する元素であり、必要に応じ選択して含有できる。
One or more selected from Cr: 0.7% or less, Mo: 0.7% or less, Nb: 0.05% or less, V: 0.2% or less
Cr, Mo, Nb, and V are all elements that contribute to improving the strength of steel, and can be selected and contained as necessary.

Crは、0.05%以上含有することが好ましいが、0.7%を超える含有は、HAZ靱性を劣化させる。このため、Crは0.7%以下に限定することが好ましい。   Cr is preferably contained in an amount of 0.05% or more, but the content exceeding 0.7% deteriorates the HAZ toughness. For this reason, it is preferable to limit Cr to 0.7% or less.

Moは、0.05%以上含有することが好ましいが、0.7%を超える含有は、母材靱性およびHAZ靭性に悪影響を及ぼす。このため、Moは0.7%以下に限定することが好ましい。   Mo is preferably contained in an amount of 0.05% or more, but the content exceeding 0.7% adversely affects the base material toughness and the HAZ toughness. For this reason, it is preferable to limit Mo to 0.7% or less.

Nbは、0.005%以上含有することが好ましいが、0.05%を超える含有は、母材靱性およびHAZ靭性を劣化させる。このため、Nbは0.05%以下に限定することが好ましい。   Nb is preferably contained in an amount of 0.005% or more, but the content exceeding 0.05% deteriorates the base metal toughness and the HAZ toughness. For this reason, it is preferable to limit Nb to 0.05% or less.

Vは、0.01%以上含有することが好ましいが、0.2%を超える含有は、靱性を劣化させる。このため、Vは0.2%以下に限定することが好ましい。   V is preferably contained in an amount of 0.01% or more, but containing over 0.2% deteriorates toughness. For this reason, it is preferable to limit V to 0.2% or less.

REM :0.02%以下および Mg:0.005%以下のうちから選ばれた1種または2種
REM 、Mgは、いずれも靭性向上に寄与する元素であり、必要に応じ選択して含有できる。
One or two selected from REM: 0.02% or less and Mg: 0.005% or less
REM and Mg are both elements that contribute to the improvement of toughness, and can be selected and contained as necessary.

REM は、0.002%以上含有することが好ましいが、0.02%を超えて含有しても効果が飽和するため、0.02%を上限とした。   REM is preferably contained in an amount of 0.002% or more, but the effect is saturated even if it exceeds 0.02%, so 0.02% was made the upper limit.

Mgは、結晶粒の微細化を介して靭性を向上させる有用な元素であり、0.001%以上含有することが好ましいが、0.005%を超えて含有しても効果が飽和するため、0.005%を上限とした。   Mg is a useful element that improves toughness through refinement of crystal grains, and it is preferable to contain 0.001% or more, but the effect is saturated even if it exceeds 0.005%, so 0.005% is the upper limit. It was.

なお、上記した成分以外の残部は、Feおよび不可避的不純物である。   The balance other than the components described above is Fe and inevitable impurities.

転炉、電気炉、真空溶解炉等の公知の方法で溶製した溶鋼に、さらに脱酸処理や脱ガスプロセスを用いて、ガス成分の制御を行った後、CaSiワイヤの添加による介在物制御したうえで、連続鋳造法などの鋳造方法で、上記した組成の鋼素材(スラブ)とする。なお、溶製時に、CaをCaS として晶出させるために、Caとの結合力の強いOはCa添加前に脱ガスを強化するか、あるいは脱酸剤を投入して、溶鋼中のOを0.0050%以下に低減しておくことが好ましい。   Control of inclusions by adding CaSi wire after controlling the gas components of molten steel melted by known methods such as converters, electric furnaces, vacuum melting furnaces, etc. using deoxidation and degassing processes In addition, the steel material (slab) having the above composition is obtained by a casting method such as a continuous casting method. In addition, in order to crystallize Ca as CaS during melting, O having a strong binding force with Ca reinforces degassing before adding Ca, or a deoxidizer is added, so that O in molten steel is removed. It is preferable to reduce it to 0.0050% or less.

ついで、好ましくは上記した工程を経て得られた鋼素材(スラブ)に、1000〜1300℃に加熱し熱間圧延した後、空冷する熱延工程を施し、所定の板厚の厚鋼板とする。   Next, preferably, the steel material (slab) obtained through the above-described steps is heated to 1000 to 1300 ° C., hot-rolled, and then subjected to a hot rolling step of air cooling to obtain a thick steel plate having a predetermined thickness.

鋼素材の加熱温度が、1000℃未満では、鋼素材の変形抵抗が高く、1パス当たりの圧下量が大きく取れなくなるため、熱間圧延のパス数が増加し、圧延能率の低下を招く。また、鋼素材の加熱温度が低い場合には、1パス当たりの圧下量が大きく取れなくなるため、鋼素材中の鋳造欠陥を圧着することができない場合があり、内部欠陥発生の危険性が増大する。一方、加熱温度が1300℃を超えて高くなると、凝固過程で析出したTiN がオストワルド成長により粗大化し、超大入熱溶接時の溶接接合部近傍におけるオーステナイト粒の粗大化抑制効果が失われるため、超大入熱溶接HAZの靱性が低下する。このため、鋼素材の加熱温度は1000〜1300℃の範囲の温度とする。   If the heating temperature of the steel material is less than 1000 ° C., the deformation resistance of the steel material is high, and a large amount of reduction per pass cannot be obtained. Therefore, the number of hot rolling passes increases, and the rolling efficiency decreases. In addition, when the heating temperature of the steel material is low, the amount of reduction per pass cannot be increased, and thus casting defects in the steel material may not be able to be crimped, increasing the risk of occurrence of internal defects. . On the other hand, when the heating temperature is higher than 1300 ° C, TiN precipitated during the solidification process becomes coarse due to Ostwald growth, and the effect of suppressing the coarsening of austenite grains near the weld joint during super-high heat input welding is lost. The toughness of the heat input weld HAZ decreases. For this reason, the heating temperature of a steel raw material shall be the temperature of the range of 1000-1300 degreeC.

熱延工程では、熱間圧延終了後は、空冷される。なお、熱間圧延の終了温度は、圧延能率および母材の低降伏比化の観点からAc3変態点以上とすることが好ましい。熱間圧延の終了温度が Ac3変態点未満の低い温度では、フェライトが加工されるため、降伏点が上昇し、降伏比が増加するため、好ましくない。   In the hot rolling step, air cooling is performed after the hot rolling is completed. In addition, it is preferable that the completion | finish temperature of hot rolling shall be more than Ac3 transformation point from a viewpoint of rolling efficiency and the yield ratio reduction of a base material. When the hot rolling end temperature is lower than the Ac3 transformation point, since ferrite is processed, the yield point rises and the yield ratio increases, which is not preferable.

上記した熱延工程により得られた厚鋼板に、ついで、再加熱焼入れ工程を施す。
再加熱焼入れ工程では、厚鋼板はまず、Ac3変態点以上の加熱温度に再加熱される。厚鋼板を、Ac3変態点以上の加熱温度に再加熱して保持することにより、厚鋼板の内部まで均一なオーステナイト相となる。加熱温度の上限についてはとくに規定しないが、1100℃以下とすることが好ましい。1100℃を超えて高くなると、鋼板の表面性状が劣化する。なお、保持時間はオーステナイトの粗大化による母材靭性劣化の観点から1h以下とすることが好ましい。また、熱処理炉内の均熱が良ければ、10min程度の短時間の保持でもかまわない。なお、Ac3変態点は、概ね次式
Ac3(℃)=910−273C+25Si−74Mn−56Ni−16Cr−9Mo−5Cu−1620Nb
(ただし、C、Mn、Ni、Cr、Mo、Cu、Nb:各元素の含有量(mass%))
で整理できる。
The thick steel plate obtained by the hot rolling process is then subjected to a reheating and quenching process.
In the reheating and quenching step, the thick steel plate is first reheated to a heating temperature equal to or higher than the Ac3 transformation point. By reheating and holding the thick steel plate at a heating temperature equal to or higher than the Ac3 transformation point, a uniform austenite phase is formed even inside the thick steel plate. The upper limit of the heating temperature is not particularly specified, but is preferably 1100 ° C. or lower. If the temperature exceeds 1100 ° C., the surface properties of the steel sheet deteriorate. The holding time is preferably 1 h or less from the viewpoint of deterioration of the toughness of the base metal due to coarsening of austenite. Further, if the soaking in the heat treatment furnace is good, it may be held for a short time of about 10 minutes. The Ac3 transformation point is approximately the following formula: Ac3 (° C.) = 910−273C + 25Si−74Mn−56Ni−16Cr−9Mo−5Cu−1620Nb
(However, C, Mn, Ni, Cr, Mo, Cu, Nb: Content of each element (mass%))
Can be organized.

Ac3変態点以上の加熱温度に再加熱され保持された厚鋼板は、その後、1℃/s以上の平均冷却速度で焼入れされる。これにより、厚鋼板内の組織の均質化および微細化が達成できる。焼入れの冷却速度が1℃/s未満では、上記した効果が期待できなくなる。また、焼入れ冷却速度の上限はとくに限定されないが、母材の靭性の観点から40℃/s以下とすることが好ましい。なお、焼入れにおける冷却速度は、厚鋼板の1/2T位置における300℃までの平均冷却速度をいうものとする。   The thick steel plate reheated and held at the heating temperature above the Ac3 transformation point is then quenched at an average cooling rate of 1 ° C./s or higher. Thereby, homogenization and refinement | miniaturization of the structure | tissue in a thick steel plate can be achieved. When the quenching cooling rate is less than 1 ° C./s, the above-described effects cannot be expected. The upper limit of the quenching cooling rate is not particularly limited, but is preferably 40 ° C./s or less from the viewpoint of the toughness of the base material. In addition, the cooling rate in hardening shall mean the average cooling rate to 300 degreeC in the 1 / 2T position of a thick steel plate.

再加熱焼入れ工程を施された厚鋼板に、ついで、二相域加熱焼入れ工程を施す。   Next, the two-phase region heat quenching step is performed on the thick steel plate subjected to the reheating quenching step.

二相域加熱焼入れ工程では厚鋼板は、まず、(Ac1変態点+10℃)〜(Ac1 変態点+50℃)の二相域の温度に加熱される。(Ac1変態点+10℃)〜(Ac1 変態点+50℃)の二相域の温度に加熱することにより、C、Mn、Cu、Niなどの強化元素がオーステナイト相へ濃化し、フェライト相が希釈軟化する現象が効率的に促進される。これにより、焼入れ後の組織が、硬質のマルテンサイト相と軟質のフェライト相との混合組織となり、高強度と低降伏比とを兼備した厚鋼板とすることができる。一方、加熱温度が、(Ac1変態点+10℃)未満では、オーステナイト相分率が低く、焼入れ後組織中のマルテンサイト相分率が少なくフェライト主体の組織となり、所望の高強度を確保できなくなる。また、加熱温度が(Ac1 変態点+50℃)を超えると、オーステナイト相分率が高くなりすぎて、オーステナイト相へのCu、Ni等の合金元素の濃化が希釈され、焼入れ後の組織がベイナイト主体の組織となり、所望の高強度と低降伏比を確保できなくなる。このため、二相域加熱焼入れ工程における加熱温度は、(Ac1変態点+10℃)〜(Ac1 変態点+50℃)の範囲の温度に限定した。なお、加熱温度における保持時間についてとくに規定しないが、鋼板内の温度均一化を図り、特性のばらつきを抑える意味で、10min以上、1h以内とすることが好ましい。   In the two-phase region heating and quenching process, the thick steel plate is first heated to a temperature in the two-phase region of (Ac1 transformation point + 10 ° C.) to (Ac1 transformation point + 50 ° C.). By heating to a temperature in the two-phase region from (Ac1 transformation point + 10 ° C) to (Ac1 transformation point + 50 ° C), strengthening elements such as C, Mn, Cu and Ni are concentrated in the austenite phase, and the ferrite phase is diluted and softened. This phenomenon is efficiently promoted. Thereby, the structure after quenching becomes a mixed structure of a hard martensite phase and a soft ferrite phase, and a thick steel plate having both high strength and a low yield ratio can be obtained. On the other hand, when the heating temperature is less than (Ac1 transformation point + 10 ° C.), the austenite phase fraction is low, the martensite phase fraction in the structure after quenching is small, and the structure is mainly composed of ferrite, and the desired high strength cannot be secured. In addition, when the heating temperature exceeds (Ac1 transformation point + 50 ° C), the austenite phase fraction becomes too high, the concentration of alloy elements such as Cu and Ni in the austenite phase is diluted, and the structure after quenching becomes bainite. It becomes a main structure, and the desired high strength and low yield ratio cannot be secured. For this reason, the heating temperature in the two-phase region heating and quenching step is limited to a temperature in the range of (Ac1 transformation point + 10 ° C.) to (Ac1 transformation point + 50 ° C.). The holding time at the heating temperature is not particularly specified, but it is preferably 10 min or more and 1 h or less in order to make the temperature uniform in the steel sheet and to suppress variation in characteristics.

上記した二相域の加熱温度に加熱され保持された厚鋼板は、ついで、1℃/s以上の平均冷却速度で焼入れされる。焼入れ冷却速度が、平均で1℃/s未満では、オーステナイト相がマルテンサイト相以外の相に変態し、所望の高強度が確保できない。なお、焼入れ冷却速度の上限については特に規定しないが、母材の靭性という観点からは40℃/s以下とすることが望ましい。   The thick steel plate heated and held at the above-described two-phase heating temperature is then quenched at an average cooling rate of 1 ° C./s or more. When the quenching cooling rate is less than 1 ° C./s on average, the austenite phase is transformed into a phase other than the martensite phase, and a desired high strength cannot be ensured. The upper limit of the quenching cooling rate is not particularly defined, but is preferably 40 ° C./s or less from the viewpoint of the toughness of the base material.

二相域加熱焼入れ工程を施された厚鋼板は、ついで焼戻し工程を施される。   The thick steel plate that has been subjected to the two-phase region heating and quenching step is then subjected to a tempering step.

焼戻し工程では厚鋼板は、Ac1 変態点以下の温度で焼戻しされる。Ac1 変態点以下の焼戻しにより、焼入れにより生成した脆い硬質相の靭化を達成できる。なお、焼戻し温度は、300〜650℃とすることが好ましい。焼戻し温度が300℃未満では、長時間の熱処理を必要とし効率が悪く、650℃を超えると、母材の強度低下が顕著となる。焼戻し温度での保持時間は、焼入れにより生成した脆い硬質相の靱化を達成でき所望の強度が確保できればよく、とくに限定する必要はないが、概ね60min程度とすることが好ましい。なお、Ac1 変態点、Ac3変態点は、変態膨張曲線を測定することにより求めるものとする。 In the tempering process, the thick steel plate is tempered at a temperature below the Ac1 transformation point. By tempering below the Ac1 transformation point, toughening of the brittle hard phase produced by quenching can be achieved. In addition, it is preferable that tempering temperature shall be 300-650 degreeC. If the tempering temperature is less than 300 ° C, a long-time heat treatment is required and the efficiency is poor, and if it exceeds 650 ° C, the strength of the base material is significantly reduced. The holding time at the tempering temperature is not particularly limited as long as it can achieve toughening of the brittle hard phase generated by quenching and ensure a desired strength, but it is preferably about 60 min. Incidentally, Ac1 transformation point, Ac 3 transformation point shall be determined by measuring the transformation expansion curve.

以下、実施例に基づき、さらにこの発明について詳細に説明する。   Hereinafter, the present invention will be described in more detail based on examples.

転炉−取鍋精錬−連続鋳造法で、表1に示す組成に調整された鋼素材(スラブ)に、表2に示す条件の熱延工程により表2に示す板厚の厚鋼板とした。ついで、これら厚鋼板に表2に示す条件の、再加熱焼入れ工程、二相域加熱焼入れ工程、焼戻し工程を施した。なお、一部の厚鋼板には焼戻し工程を施さなかった。   A steel plate (slab) adjusted to the composition shown in Table 1 by a converter-ladder refining-continuous casting method was used to form a thick steel plate having a thickness shown in Table 2 by a hot rolling process under the conditions shown in Table 2. Subsequently, these thick steel plates were subjected to a reheating quenching process, a two-phase region heating quenching process, and a tempering process under the conditions shown in Table 2. In addition, the tempering process was not given to some thick steel plates.

得られた各厚鋼板の板厚1/4位置から、JIS4号引張試験片を採取し、JIS Z 2241の規定に準拠して引張試験を実施し、引張特性を調査した。また、得られた各厚鋼板の板厚1/4位置から、JIS Z 2202の規定に準拠してVノッチ試験片を採取し、JIS Z 2242の規定に準拠してシャルピー衝撃試験を実施し、0℃における吸収エネルギー(vE0 )を求め、母材靭性を評価した。   JIS No. 4 tensile test specimens were sampled from the position of the thickness ¼ of each thick steel plate obtained, and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to investigate the tensile characteristics. In addition, V-notch test specimens were collected from ¼ position of each thick steel plate obtained according to JIS Z 2202, and Charpy impact test was conducted according to JIS Z 2242. The absorbed energy (vE0) at 0 ° C. was determined, and the base metal toughness was evaluated.

また、得られた各厚鋼板から、継手用試験板(大きさ:400×600mm)を採取し、図1に示すような開先形状としたエレクトロスラグ溶接(溶接入熱量:1000kJ/cm)により、溶接継手を作製した。   In addition, a test plate for joints (size: 400 x 600 mm) was sampled from each thick steel plate obtained, and electroslag welding (welding heat input: 1000 kJ / cm) with a groove shape as shown in Fig. 1 was performed. A welded joint was prepared.

得られた溶接継手から、図2に示すように切欠き位置をボンド部とするVノッチ試験片を採取し、JIS Z 2242の規定に準拠して試験温度:0℃でシャルピー衝撃試験を行って、溶接継手ボンド部の0℃における吸収エネルギー(vE0 )を求め、継手靭性を評価した。   From the obtained welded joint, a V-notch test piece having a notch position as a bond portion as shown in FIG. 2 is collected and subjected to a Charpy impact test at a test temperature of 0 ° C. in accordance with the provisions of JIS Z 2242. Then, the absorbed energy (vE0) at 0 ° C. of the welded joint bond part was obtained and the joint toughness was evaluated.

得られた結果を表3に示す。   The obtained results are shown in Table 3.

本発明例はいずれも、590MPa以上の引張強さ、降伏比80%以下、vE0 が100J以上の高強度、低降伏比で、高靭性の母材特性を有するとともに、溶接入熱:1000kJ/cmの超大入熱溶接施工を施した場合であっても、ボンド部のvE0が100J以上と優れた溶接熱影響部靱性を有している。一方、本発明の範囲を外れる比較例は、母材強度、母材降伏比、母材靭性、溶接熱影響部靭性のうちのいずれか、あるいは複数の特性が目標値を満足していない。   All of the examples of the present invention have a tensile strength of 590 MPa or more, a yield ratio of 80% or less, a high strength of vE0 of 100 J or more, a low yield ratio, and a tough base material, and a heat input of welding of 1000 kJ / cm. Even when super-high heat input welding is applied, vE0 of the bond portion is 100 J or more and has excellent weld heat affected zone toughness. On the other hand, in a comparative example that is out of the scope of the present invention, any of the base material strength, base material yield ratio, base material toughness, weld heat affected zone toughness, or a plurality of characteristics do not satisfy the target value.

実施例で用いたエレクトロスラグ溶接継手の開先形状を示す説明図である。It is explanatory drawing which shows the groove shape of the electroslag welded joint used in the Example. 実施例におけるエレクトロスラグ溶接継手部からのシャルピー衝撃試験片の採取要領を示す説明図である。It is explanatory drawing which shows the extraction | collection point of the Charpy impact test piece from the electroslag welded joint part in an Example.

Claims (3)

質量%で、
C:0.03〜0.15%、 Si:0.05〜0.5%、
Mn:0.5〜3.0%、 Al:0.005〜0.1%、
Ti:0.004〜0.03%、 N:0.0020〜0.0070%、
P:0.030%以下、 S:0.0005〜0.0030%、
Ca:0.0005〜0.0030%、 B:0.0005〜0.0030%、
O:0.0050%以下
を含み、さらに、Cu:1.5%以下、Ni:2.0%以下のうちから選ばれた1種または2種を含有し、かつ下記(1)式で定義される炭素当量Ceq が0.35以上を満足し、かつ下記(2)式で定義されるACRが0.3〜0.8を満足する組成を有する鋼素材に、1000〜1300℃に加熱し熱間圧延した後、空冷する熱延工程を施し厚鋼板としたのち、該厚鋼板に、Ac3変態点以上の加熱温度に再加熱し、該加熱温度で保持してから1℃/s以上の平均冷却速度で焼入れする再加熱焼入れ工程と、ついで、(Ac1変態点+10℃)〜(Ac1変態点+50℃)の二相域の温度に加熱し、該温度で保持してから1℃/s以上の平均冷却速度で焼入れする二相域加熱焼入れ工程と、さらに、Ac1変態点以下の温度で焼戻しする焼戻し工程とを、順次施すことを特徴とする、引張強さ:590MPa以上、降伏比:80%以下を有する超大入熱溶接熱影響部靱性に優れた低降伏比高張力厚鋼板の製造方法。

Ceq =C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 ………(1)
ここで、Ceq :炭素当量(%)
C、Mn、Ni、Cu、Cr、Mo、V:各元素の含有量(質量%)
ACR ={Ca−(0.18+130Ca)×0}/(1.25×S) ………(2)
ここで、Ca、O、S:各元素の含有量(質量%)
% By mass
C: 0.03-0.15%, Si: 0.05-0.5%,
Mn: 0.5-3.0%, Al: 0.005-0.1%,
Ti: 0.004 to 0.03%, N: 0.0020 to 0.0070%,
P: 0.030% or less, S: 0.0005 to 0.0030%,
Ca: 0.0005 to 0.0030%, B: 0.0005 to 0.0030%,
O: 0.0050% or less, Cu: 1.5% or less, Ni: 2.0% or less, one or two selected from the following, and the carbon equivalent Ceq defined by the following formula (1) A steel material having a composition satisfying 0.35 or more and having an ACR defined by the following formula (2) satisfying 0.3 to 0.8 is heated to 1000 to 1300 ° C., hot-rolled, and then air-cooled. A reheating and quenching step in which the thick steel plate is reheated to a heating temperature equal to or higher than the Ac 3 transformation point and held at the heating temperature and then quenched at an average cooling rate of 1 ° C./s or higher. Then, the mixture is heated to a temperature in the two-phase region of (Ac 1 transformation point + 10 ° C.) to (Ac 1 transformation point + 50 ° C.), held at the temperature, and then quenched at an average cooling rate of 1 ° C./s or more. A phase region heating and quenching step, and a tempering step of further tempering at a temperature below the Ac 1 transformation point are sequentially performed, A method for producing a high-tensile steel plate with a low yield ratio and high tensile strength that has excellent tensile strength: 590 MPa or more, yield ratio: 80% or less, and excellent heat-affected zone toughness.
Record
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
Where Ceq: carbon equivalent (%)
C, Mn, Ni, Cu, Cr, Mo, V: Content of each element (% by mass)
ACR = {Ca− (0.18 + 130Ca) × 0} / (1.25 × S) (2)
Here, Ca, O, S: Content of each element (mass%)
前記組成に加えてさらに、質量%で、Cr:0.7%以下、Mo:0.7%以下、Nb:0.05%以下、V:0.2%以下のうちから選ばれた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に記載の低降伏比高張力厚鋼板の製造方法。 In addition to the above composition, the composition further contains one or more selected from Cr: 0.7% or less, Mo: 0.7% or less, Nb: 0.05% or less, and V: 0.2% or less in terms of mass%. The method for producing a low-yield ratio high-tensile thick steel plate according to claim 1. 前記組成に加えてさらに、質量%で、REM :0.02%以下、Mg:0.005%以下のうちから選ばれた1種または2種を含有することを特徴とする請求項1または2に記載の低降伏比高張力厚鋼板の製造方法。 3. The low content according to claim 1, further comprising one or two kinds selected from REM: 0.02% or less and Mg: 0.005% or less in mass% in addition to the composition. Yield ratio high tension steel plate manufacturing method.
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