JP2010111924A - Low yield ratio steel plate for building having excellent high heat input weld zone toughness and method for producing the same - Google Patents

Low yield ratio steel plate for building having excellent high heat input weld zone toughness and method for producing the same Download PDF

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JP2010111924A
JP2010111924A JP2008286179A JP2008286179A JP2010111924A JP 2010111924 A JP2010111924 A JP 2010111924A JP 2008286179 A JP2008286179 A JP 2008286179A JP 2008286179 A JP2008286179 A JP 2008286179A JP 2010111924 A JP2010111924 A JP 2010111924A
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JP5365145B2 (en
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Yasuhiro Murota
康宏 室田
Shinichi Suzuki
伸一 鈴木
Nobuo Shikauchi
伸夫 鹿内
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JFE Steel Corp
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a low yield ratio steel plate for building having high heat input weld heat affected zone toughness which has low surface hardness and excellent earthquake resistance, and to provide a method for producing the same. <P>SOLUTION: The steel plate has a composition comprising, by mass, 0.03 to 0.07% C, 0.05 to 0.5% Si, 0.6 to 2.0% Mn, ≤0.020% P, 0.0005 to 0.003% S, 0.005 to 0.03% Ti, 0.0003 to 0.0020% B, ≤0.1% Al, 0.0025 to 0.0070% N, 0.001 to 0.003% O, 0.0005 to 0.005% Ca, ≤0.01% Mo and ≤0.005% Nb, further comprising one or more selected from Cu, Ni, Cr, V, Mg and rare earth metals, satisfying 0.40 to 0.45% Ceq and 0.2 to 0.8 ACR, and the balance Fe with inevitable impurities, has a microstructure in which ferrite fraction is 10 to 40%, and has a surface hardness of ≤350 HV10: Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 and ACR=(Ca-(0.18+130×Ca)×O)/(1.25×S). <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

本発明は、建築ボックス柱を作製する際に施工される、入熱400kJ/cm以上のサブマージアーク溶接やエレクトロスラグ溶接による大入熱溶接熱影響部靭性に優れた建築用低降伏比鋼板およびその製造方法に関し、特に鋼板の表面硬度が低く耐震性に優れたものに関する。   The present invention relates to a low yield ratio steel sheet for construction excellent in toughness of heat-affected zone by large heat input welding by submerged arc welding or electroslag welding with heat input of 400 kJ / cm or more, which is applied when manufacturing an architectural box column. In particular, the present invention relates to a steel sheet having a low surface hardness and excellent earthquake resistance.

近年、建築構造物の大型化に伴い、使用鋼材の厚肉化、高強度化が進展し、更に、建築構造物の耐震性を確保するため、降伏比80%以下の低降伏比を備えていることも要求されている。   In recent years, with the increase in size of building structures, the use of steel materials has increased in thickness and strength, and in order to ensure the earthquake resistance of building structures, it has a low yield ratio of 80% or less. It is also required to be.

建築構造物は、溶接接合によって所定の形状に組み立てられる溶接構造物であるが、地震時の大きな負荷荷重を受けると、十分な塑性変形を生じる前に、溶接部から脆性破壊が発生する場合があり、溶接部においても良好な靭性を有することが求められている。   A building structure is a welded structure that is assembled into a predetermined shape by welding, but if a large load is applied during an earthquake, brittle fracture may occur from the weld before sufficient plastic deformation occurs. In addition, the welded portion is required to have good toughness.

溶接構造物を高能率に製造するため、ボックス柱の製作においては、角継手部のサブマージアーク溶接やダイヤフラム接合部のエレクトロスラグアーク溶接等入熱400kJ/cm以上の大入熱溶接が施工される。   In order to manufacture a welded structure with high efficiency, large heat input welding with a heat input of 400 kJ / cm or more such as submerged arc welding of corner joints and electroslag arc welding of diaphragm joints is performed in the production of box columns. .

一般に、このような大入熱溶接部は、溶接後の冷却速度が遅いため、溶融点付近にまで加熱された領域の高温域での滞留時間が長くなり、組織が粗大化したり、MAと呼ばれる硬質な脆化相が生成することにより脆化する。   In general, such a high heat input welded part has a slow cooling rate after welding, so that the residence time in the high temperature region of the region heated up to the vicinity of the melting point becomes long, the structure becomes coarse, or called MA. Brittleness is caused by the formation of a hard brittle phase.

溶接熱影響部は、合金元素量が多くなる高強度鋼ほど脆化しやすく、建築構造用鋼ではTS590MPa級鋼の場合に問題となることが多いため、優れた溶接熱影響部靭性が得られるTS590MPa級の建築用鋼が種々提案されていている。   The weld heat-affected zone is more likely to be brittle as the strength of the steel increases in the amount of alloying elements, and is often a problem in the case of TS590 MPa grade steel for building structural steels. Therefore, TS590 MPa provides excellent weld heat-affected zone toughness. Various grades of architectural steel have been proposed.

特許文献1、2は、入熱400kJ/cmを超える超大入熱溶接熱影響部靭性に優れる建築用高強度厚鋼板の製造方法に関し、Ca、O、Sからなる式で求められるACR値を規制して溶接時に生成させた微細な粒子をフェライト変態核として活用することにより、溶接熱影響部の組織を微細化し、靭性を改善することが報告されている。   Patent Documents 1 and 2 regulate the ACR value required by the formula consisting of Ca, O, and S, regarding the manufacturing method of high strength thick steel sheet for building with excellent heat input toughness of super high heat input welding exceeding 400 kJ / cm. It has been reported that by utilizing fine particles generated during welding as ferrite transformation nuclei, the structure of the heat affected zone is refined and the toughness is improved.

また、特許文献3は大入熱溶接熱影響部靭性の優れた建築用低降伏比600N/mm級鋼板の製造方法に関し、鋼組成を低CーB無添加系として溶接熱影響部における焼入れ性を低下させるとともに、Ti酸化物を活用し溶接熱影響部の組織を微細化させることが記載されている。母材強度はCuによる析出強化で600N/mm級を確保する。 Further, quenching in the weld heat affected zone relates Patent Document 3 the method of manufacturing a low yield ratio 600N / mm 2 class steel sheet construction which is excellent in high heat input welding heat affected zone toughness, the steel composition as the low C - B-free addition system It is described that the structure of the weld heat-affected zone is refined by using Ti oxide while reducing the properties. The strength of the base material is 600 N / mm 2 by securing precipitation with Cu.

特許文献4は、大入熱溶接熱影響部の靭性に優れた高張力鋼板に関し、極低C化と焼入性向上元素であるMn、Ni、Crなどを適宜含有させた成分組成とすることにより、MAの抑制および形態制御、および、変態組織のブロックサイズ微細化によって入熱500kJ/cmを超える溶接熱影響部の靭性改善を達成している。   Patent Document 4 relates to a high-tensile steel plate excellent in toughness of a high heat input welding heat-affected zone, and has a component composition appropriately containing Mn, Ni, Cr, etc. that are extremely low C and hardenability improving elements. Therefore, the toughness improvement of the weld heat affected zone exceeding 500 kJ / cm is achieved by suppressing and controlling the shape of the MA and making the block size of the transformed structure finer.

特許文献5は大入熱溶接靭性に優れた低降伏比高張力鋼板に関し、入熱250kJ/cm以上の溶接に対して母材成分組成を低炭素当量化するとともに、Tiの炭窒化物を活用して、熱影響部の組織を微細化し、靭性を改善することが記載されている。低降伏比化にはフェライト分率を調整し、高強度化にはNb炭窒化物を利用する。   Patent Document 5 relates to a low-yield-ratio high-tensile steel sheet with excellent high heat input welding toughness, and lowers the carbon equivalent of the base material composition for welding with heat input of 250 kJ / cm or more, and uses Ti carbonitride. Thus, it is described that the structure of the heat-affected zone is refined to improve toughness. The ferrite fraction is adjusted for lower yield ratio, and Nb carbonitride is used for higher strength.

特許文献6は、超大入熱溶接HAZ靭性に優れた低降伏比建築構造用厚鋼板およびその製造方法に関し、入熱400kJ/cm超えで溶接する板厚30mm以上の鋼板の成分組成においてCa、O、Sからなる式で求められるACR値を規制して、溶接時に生成させたCaS表面上にMnSが析出した複合硫化物(サルファイド)を含む微細な粒子をフェライト変態核として活用する際、その性状と分布状態を規定することが記載されている。   Patent Document 6 relates to a steel plate for low-yield-ratio building structures excellent in super high heat input welding HAZ toughness and a method for producing the same, and in the component composition of a steel plate having a thickness of 30 mm or more to be welded at a heat input exceeding 400 kJ / cm, When the fine particles containing composite sulfide (sulfide) in which MnS is deposited on the surface of CaS produced during welding is used as a ferrite transformation nucleus by regulating the ACR value obtained by the formula consisting of S And specifying the distribution state.

上述したように、既存の大入熱溶接部の靭性に優れた低降伏比鋼において、大入熱溶接部の靭性改善は、母材成分の調整により溶接熱影響部に微細な粒子(硫化物(サルファイド)や酸化物(オキシサイド))を析出分散させて組織を微細化し(特許文献1,2,3、5、6)、MAの生成を抑制して達成される(特許文献4)。   As described above, in low yield ratio steel with excellent toughness of existing high heat input welds, the improvement in toughness of high heat input welds is achieved by adjusting the base material components to fine particles (sulfides). (Sulfide) or oxide (oxycide) is precipitated and dispersed to refine the structure (Patent Documents 1, 2, 3, 5, 6), and the production of MA is suppressed (Patent Document 4).

低降伏比化は鋼中のフェライト組織分率を二相域熱処理(特許文献1,2,6)や低Ceqした成分系に焼入れ焼戻しを施して調整することにより達成される(特許文献5)。
特開2005−68519号公報 特開2005−68478号公報 特開平6−128635号公報 特開2007−126725号公報 特開2001−172736号公報 特開2003−183767号公報
Low yield ratio is achieved by adjusting the ferrite structure fraction in steel by performing two-phase region heat treatment (Patent Documents 1, 2, 6) or quenching and tempering a low Ceq component system (Patent Document 5). .
JP 2005-68519 A JP 2005-68478 A JP-A-6-128635 JP 2007-126725 A JP 2001-172736 A JP 2003-183767 A

しかしながら、二相域熱処理は製造工程が複雑で生産性が低下する。一方、特許文献5記載の鋼のように、二相域熱処理を施さずに焼入れ焼戻しを施す場合は、強度を確保するためCが0.12%以上の高C成分系:0.12%以上となるため、低降伏比が達成されたとしても表面の硬度が著しく高くなり、表層部付近の延性劣化が著しく、地震等による応力負荷時に表層付近に亀裂が発生し、ノッチ効果で破断が生じる場合がある。   However, the two-phase region heat treatment has a complicated manufacturing process and decreases productivity. On the other hand, in the case of quenching and tempering without performing a two-phase heat treatment, as in the steel described in Patent Document 5, a high C component system in which C is 0.12% or more to ensure strength: 0.12% or more Therefore, even if a low yield ratio is achieved, the surface hardness is extremely high, the ductility deterioration near the surface layer is significant, cracks occur near the surface layer when stress is applied due to earthquakes, etc., and fracture occurs due to the notch effect There is a case.

尚、二相域熱処理を行う特許文献1、2、6も、実施例は高強度を得るため、C添加量が0.07%超えと多く、特許文献5と同様に、表面の延性が低下することが懸念される。   In addition, in Patent Documents 1, 2, and 6 in which two-phase region heat treatment is performed, in order to obtain high strength in the examples, the amount of C added is as large as over 0.07%, and the surface ductility is reduced as in Patent Document 5. There is a concern to do.

そこで本発明は、製造工期面で不利な二相域熱処理を施すことなく、表層近傍の硬度を低下させ、表層付近の延性を改善することにより耐震性を改善した低降伏比鋼板およびその製造方法を提供することを目的とする。   Accordingly, the present invention provides a low yield ratio steel sheet having improved seismic resistance by reducing the hardness near the surface layer and improving the ductility near the surface layer without subjecting the two-phase region heat treatment, which is disadvantageous in terms of the production period, and a method for producing the same. The purpose is to provide.

本発明者らは、上記課題を達成するため鋭意検討を重ねた結果、Ca、O、Sからなる下記式で示されるACR値を0.2〜0.8とし、かつ、Ti、Nを適量添加し、C量を0.07%以下、Nbを0.005%以下、Moを0.01%以下とした成分組成の鋼に加速冷却を施すことにより、優れた大入熱溶接熱影響部の靭性を備え、表層付近の表面硬度が350HV10以下で、降伏比80%以下のTS590MPa超えの鋼板を二相域熱処理を施すことなく製造することが可能であることを見出した。   As a result of intensive studies to achieve the above-mentioned problems, the present inventors set the ACR value represented by the following formula consisting of Ca, O, and S to 0.2 to 0.8, and appropriate amounts of Ti and N. By adding accelerated cooling to steel having a component composition with C content of 0.07% or less, Nb of 0.005% or less, and Mo of 0.01% or less, an excellent high heat input heat affected zone It was found that a steel sheet having a surface hardness near the surface layer of 350 HV10 or less and having a yield ratio of 80% or less and exceeding TS590 MPa can be produced without performing a two-phase region heat treatment.

ACR=(Ca−(0.18+130×Ca)×O)/(1.25×S)、但し、Ca,O,Sは鋼中含有量。   ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S), where Ca, O and S are steel contents.

本発明は、得られた知見をもとに、更に検討を加えてなされたもので、すなわち、本発明の要旨は次の通りである。
1.質量%で、C:0.03〜0.07%、Si:0.05〜0.5%、Mn:0.6〜2.0%、P:0.020%以下、S:0.0005〜0.003%、Ti:0.005〜0.03%、B:0.0003〜0.0020%、Al:0.1%以下、N:0.0025〜0.0070、O:0.001〜0.003%、Ca:0.0005〜0.005%、Mo:0.01%以下、Nb:0.005%以下、更に、Cu:0.5%以下、Ni:1.0%以下、Cr:0.5%以下、V:0.08%以下の1種または2種以上を含み、下記(1)式によるCeqが0.40〜0.45%、下記(2)式によるACRが0.2〜0.8を満足し、残部Feおよび不可避的不純物からなる組成とフェライト分率が2〜30%であるミクロ組織を有し、表面硬度が350HV10以下であることを特徴とする降伏比80%以下、引張強度590MPa以上の大入熱溶接部靭性に優れた建築構造用低降伏比鋼板。
The present invention has been made by further study based on the obtained knowledge. That is, the gist of the present invention is as follows.
1. In mass%, C: 0.03-0.07%, Si: 0.05-0.5%, Mn: 0.6-2.0%, P: 0.020% or less, S: 0.0005 -0.003%, Ti: 0.005-0.03%, B: 0.0003-0.0020%, Al: 0.1% or less, N: 0.0025-0.0070, O: 0.0. 001 to 0.003%, Ca: 0.0005 to 0.005%, Mo: 0.01% or less, Nb: 0.005% or less, Cu: 0.5% or less, Ni: 1.0% Hereinafter, one or two or more of Cr: 0.5% or less and V: 0.08% or less are included, and Ceq according to the following formula (1) is 0.40 to 0.45%, according to the following formula (2) ACR satisfies 0.2 to 0.8, has a composition comprising the balance Fe and inevitable impurities and a microstructure with a ferrite fraction of 2 to 30% 80% yield ratio, wherein a surface hardness of 350HV10 less or less, a tensile strength 590MPa or more low yield ratio steel sheet excellent architectural structures in high heat input weld toughness.

Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 −−(1)
ACR=(Ca−(0.18+130×Ca)×O)/(1.25×S)−−−(2)
但し、(1)式、(2)式中の各元素は含有量(質量%)とする。
2.さらに、成分組成にMg:0.005%以下、REM:0.02%以下の1種または2種を含有することを特徴とする1に記載の降伏比80%以下、引張強度590MPa以上の大入熱溶接部靭性に優れた建築構造用低降伏比鋼板。
3.1または2に記載の組成を有する鋼素材を1000〜1200℃に加熱後、圧延終了温度をAr変態点以上とする圧延を施し、ついで、加速冷却を平均冷却速度3〜12℃/s、冷却停止後焼戻しを行わない場合は冷却停止温度:400〜200℃、焼戻しを行う場合は冷却停止温度:400〜50℃で行うことを特徴とする大入熱溶接部靭性に優れた建築構造用低降伏比鋼板の製造方法。
4.1または2に記載の組成を有する鋼素材を1000〜1200℃に加熱後、圧延終了温度をAr変態点以上とする圧延を施し、ついで、平均冷却速度15℃/s以上、冷却停止温度:650〜500℃とする加速冷却を行った後、空冷または焼戻しを行うことを特徴とする大入熱溶接部靭性に優れた板厚19〜40mmの建築構造用低降伏比鋼板の製造方法。
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 − (1)
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) −−− (2)
However, each element in the formulas (1) and (2) has a content (% by mass).
2. Further, the component composition contains one or two of Mg: 0.005% or less and REM: 0.02% or less, wherein the yield ratio is 80% or less and the tensile strength is 590 MPa or more. Low yield ratio steel sheet for building structures with excellent heat input weld toughness.
After the steel material having the composition described in 3.1 or 2 is heated to 1000 to 1200 ° C., rolling is performed so that the rolling end temperature is equal to or higher than the Ar 3 transformation point, and then accelerated cooling is performed at an average cooling rate of 3 to 12 ° C. / s, a construction that is excellent in toughness of high heat input welds, characterized in that when tempering is not performed after cooling stop, the cooling stop temperature is 400 to 200 ° C., and when tempering is performed, the cooling stop temperature is 400 to 50 ° C. A method for producing a structural low yield ratio steel sheet.
After heating the steel material having the composition described in 4.1 or 2 to 1000 to 1200 ° C., rolling is performed so that the rolling end temperature is equal to or higher than the Ar 3 transformation point, and then cooling is stopped at an average cooling rate of 15 ° C./s or higher. Temperature: 650 to 500 ° C. Accelerated cooling, air cooling or tempering is performed, and a method of manufacturing a low yield ratio steel sheet for building structures having a thickness of 19 to 40 mm excellent in toughness of large heat input welds .

本発明によれば、大入熱溶接部靭性に優れたTS590MPa超え、かつ、表面硬度が350HV10以下で表面の延性に優れた鋼板を、製造工期面で不利な二相域熱処理を施すことなく、経済的に製造することが可能で産業上極めて有用である。   According to the present invention, a steel plate having a high heat input weld toughness exceeding TS 590 MPa and having a surface hardness of 350 HV10 or less and excellent surface ductility without subjecting the two-phase region heat treatment, which is disadvantageous in terms of manufacturing construction period, It can be produced economically and is extremely useful industrially.

以下、本発明を詳細に説明する。
[成分組成]説明において%は質量%とする。
C:0.03〜0.07%
Cは、強度、表面硬度および靭性に影響を与える重要な元素で0.03〜0.07%とする。
Hereinafter, the present invention will be described in detail.
[Ingredient composition] In the description, “%” means “mass%”.
C: 0.03-0.07%
C is an important element affecting the strength, surface hardness and toughness, and is 0.03 to 0.07%.

図1〜4は強度、表面硬度および靭性に及ぼすC量の影響を示す図で、図1は大入熱溶接熱影響部靭性に及ぼすC量の影響、図2は表層硬度に及ぼすC量の影響、図3は延性(表層引張試験伸び)に及ぼすC量の影響、図4は強度(YS,TS)に及ぼすC量の影響を示す。   1 to 4 are views showing the influence of the C amount on the strength, surface hardness and toughness, FIG. 1 is the influence of the C amount on the high heat input welding heat affected zone toughness, and FIG. 2 is the amount of the C amount on the surface hardness. FIG. 3 shows the effect of C content on ductility (surface tensile test elongation), and FIG. 4 shows the effect of C content on strength (YS, TS).

C量:0.04〜0.12%とした場合、大入熱溶接熱影響部靭性は70J以上の高エネルギー値が得られ(図1)、1/4t、1/2t部のTSは590MPa以上、YSは450MPa以上、降伏比(図示しない)として80%以下が得られる(図4)。   When the C content is 0.04 to 0.12%, the high heat input weld heat-affected zone toughness has a high energy value of 70 J or more (FIG. 1), and the TS of 1 / 4t and 1 / 2t parts is 590 MPa. As described above, YS is 450 MPa or more and a yield ratio (not shown) of 80% or less is obtained (FIG. 4).

尚、1/4t部、1/2t部の強度(TS)差は0.07%以下のC量の場合に、60MPa未満と板厚方向の材質差も低減している(図4)。   The difference in strength (TS) between the 1 / 4t part and the 1 / 2t part is less than 60 MPa and the material difference in the plate thickness direction is reduced when the C amount is 0.07% or less (FIG. 4).

一方、表層付近の硬度は、C量が0.07%を超えると350HV10超となり(図2)、表層付近の延性もC量が0.07%を超えると低下している(図3)。   On the other hand, the hardness near the surface layer exceeds 350HV10 when the C content exceeds 0.07% (FIG. 2), and the ductility near the surface layer also decreases when the C content exceeds 0.07% (FIG. 3).

従って、鋼板表面付近の延性を改善するためには、表面付近の硬度を低減することが有効で、鋼組成中のC量を0.07%以下とすることが必要である。   Therefore, in order to improve the ductility near the steel sheet surface, it is effective to reduce the hardness near the surface, and the C content in the steel composition must be 0.07% or less.

したがって、本発明においてC量は0.03〜0.07%に規定する。望ましくは、0.04〜0.07%である。   Therefore, in the present invention, the amount of C is specified as 0.03 to 0.07%. Desirably, it is 0.04 to 0.07%.

尚、図1〜4に示す結果は、Mass%で、0.04〜0.12%Cを含み、Si、Mn、Cu、Ni、CrでCeq(=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5)を0.42〜0.43とほぼ一定とし、また、ACR(=(Ca−(0.18+130×Ca)×O)/(1.25×S))を0.4〜0.5とした組成を有する鋼素材を、1150℃に加熱後、850℃で圧延を終了し、60mmの鋼板とした後に、810℃から370℃まで平均冷却速度は、10℃/sの加速冷却を施した鋼板を供試鋼とする実験結果である。平均冷却速度は、板厚1/4t部での冷却速度とした。   The results shown in FIGS. 1 to 4 are Mass%, including 0.04 to 0.12% C, and Ceq (= C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) in Si, Mn, Cu, Ni, and Cr. ) / 5) is substantially constant at 0.42 to 0.43, and ACR (= (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S)) is 0.4 to 0. After heating the steel material having the composition of .5 to 1150 ° C. and finishing rolling at 850 ° C. to form a 60 mm steel plate, the average cooling rate from 810 ° C. to 370 ° C. is 10 ° C./s accelerated cooling. It is an experimental result which uses the steel plate which gave it as test steel. The average cooling rate was the cooling rate at a thickness of 1/4 t.

得られた鋼板について、表層下0.5mm位置の硬度測定、引張試験および大入熱溶接熱影響部靭性の調査を実施した。表層下の硬度は、ビッカ−ス硬度で、表層下0.5mm位置を20点測定し、その最大値を代表値とした。   About the obtained steel plate, the hardness measurement of the 0.5 mm position under the surface layer, the tensile test, and the investigation of the high heat input heat affected zone toughness were carried out. The hardness under the surface layer was Vickers hardness, 20 points were measured at 0.5 mm below the surface layer, and the maximum value was taken as the representative value.

引張試験は、JISZ2201に規定されるJIS4号試験片(採取位置:1/4t位置、1/2t位置)を用いてJISZ2241に準拠して行い、得られた降伏応力(YS)、引張強さ(TS)から降伏比(YR)を求めた。   The tensile test was performed according to JISZ2241 using JIS No. 4 test specimens (collection position: 1 / 4t position, 1 / 2t position) specified in JISZ2201, and the yield stress (YS) and tensile strength ( The yield ratio (YR) was determined from TS).

表層部の延性評価は、表層直下位置(0.5〜6.5mm位置)から、6mm×24mmの評点距離を有する小型の丸棒引張試験片を採取し、引張試験後の伸び値を測定した。   For evaluation of the ductility of the surface layer portion, a small round bar tensile test piece having a rating distance of 6 mm × 24 mm was taken from the position directly below the surface layer (0.5 to 6.5 mm position), and the elongation value after the tensile test was measured. .

大入熱溶接熱影響部靭性は、入熱1000kJ/cmのエレクトロスラグ溶接継手から、切欠き位置をBOND部から母材側へ1mmとするシャルピ−衝撃試験片を採取し、試験温度0℃で3本試験した平均値で評価した。   Large heat input weld heat-affected zone toughness is obtained by collecting Charpy impact test pieces with a notch position of 1 mm from the BOND part to the base metal side from an electroslag weld joint with a heat input of 1000 kJ / cm. The evaluation was based on the average value of three tests.

Si:0.05〜0.5%
Siは、脱酸元素として有効な元素であり、その効果を発揮するためには、0.05%以上必要である。また、0.5%を超えて添加すると大入熱溶接熱影響部のMAが増大し、熱影響部靭性が劣化する。そのため、0.05〜0.5%に規制する。望ましくは、0.05〜0.4%である。
Si: 0.05-0.5%
Si is an effective element as a deoxidizing element, and 0.05% or more is necessary to exert its effect. Moreover, when adding over 0.5%, MA of a high heat input welding heat affected zone will increase, and heat affected zone toughness will deteriorate. Therefore, it regulates to 0.05 to 0.5%. Desirably, it is 0.05 to 0.4%.

Mn:0.6〜2.0%
Mnは、固溶強化により強度確保のために有効な元素であり、その効果を発揮するためには、0.6%以上必要である。また、2.0%を超えて添加すると溶接性が劣化する。そのため、0.6〜2.0%に規制する。望ましくは0.6〜1.6%である。
Mn: 0.6 to 2.0%
Mn is an element effective for securing strength by solid solution strengthening, and 0.6% or more is necessary to exert its effect. Moreover, if it exceeds 2.0%, weldability deteriorates. Therefore, it regulates to 0.6 to 2.0%. Desirably, it is 0.6 to 1.6%.

P:0.020%以下
Pは、不純物元素として混入するものであり、その混入量が増加すると母材靭性が劣化する。そのため、0.020%以下に規制する。望ましくは、0.015%以下である。
P: 0.020% or less P is mixed as an impurity element. When the mixed amount increases, the base material toughness deteriorates. Therefore, it regulates to 0.020% or less. Desirably, it is 0.015% or less.

S:0.0005〜0.003%
Sは、ACR値制御のために必要な元素であり、MnSの生成核となるCaSを形成し、生成したMnSが大入熱溶接部の粒内フェライト生成および組織微細化に有効な作用を及ぼす。その効果を得るためには、0.0005%以上必要である。また、0.003%超えの添加は、MnS生成による板厚方向の材質を劣化させる。そのため0.0005〜0.003%に規制する。望ましくは、0.0010〜0.003%である。
S: 0.0005 to 0.003%
S is an element necessary for controlling the ACR value, and forms CaS which is a nucleus of MnS formation, and the produced MnS has an effective effect on the formation of intragranular ferrite and refinement of the microstructure of the high heat input weld. . In order to obtain the effect, 0.0005% or more is necessary. Addition exceeding 0.003% degrades the material in the thickness direction due to MnS generation. Therefore, it limits to 0.0005 to 0.003%. Desirably, it is 0.0010 to 0.003%.

Ti:0.005〜0.03%
Tiは、TiNを生成することにより、溶接熱影響部の組織微細化に有効である。この効果を発揮するためには、0.005%以上必要である。0.03%を超えて添加すると、TiC析出により、母材靭性および熱影響部靭性を劣化させる。そのため、0.005〜0.03%とする。望ましくは、0.008〜0.015%である。
Ti: 0.005 to 0.03%
Ti is effective in refining the structure of the weld heat affected zone by generating TiN. In order to exert this effect, 0.005% or more is necessary. If added over 0.03%, the base metal toughness and the heat-affected zone toughness are deteriorated by TiC precipitation. Therefore, it is set as 0.005 to 0.03%. Desirably, it is 0.008 to 0.015%.

B:0.0003〜0.0020%
Bは、焼入性を向上させ、母材強度を確保するのに有効な元素である。その効果を発揮するためには、0.0003%以上必要である。また、0.0020%を超えての添加は溶接性を劣化させる。そのため、0.0003〜0.0020%とする。望ましくは、0.0005〜0.0015%である。
B: 0.0003 to 0.0020%
B is an element effective in improving hardenability and ensuring the strength of the base material. In order to exhibit the effect, 0.0003% or more is necessary. Addition exceeding 0.0020% deteriorates weldability. Therefore, it is set as 0.0003 to 0.0020%. Preferably, it is 0.0005 to 0.0015%.

Al:0.1%以下
Alは、0.1%を超えると、Alを生成し、鋼の清状度を劣化させる。そのため、0.1%以下とする。なお、望ましくは、0.020〜0.060%である。
Al: 0.1% or less When Al exceeds 0.1%, Al 2 O 3 is generated and the cleanliness of the steel is deteriorated. Therefore, it is made 0.1% or less. Desirably, it is 0.020 to 0.060%.

N:0.0025〜0.0070
Nは、TiNを生成することにより、溶接熱影響部の組織微細化に有効である。この効果を発揮するためには、0.0025%以上必要である。また、0.0070%を超えて添加すると、溶接熱影響部の固溶Nが増大し、熱影響部靭性劣化を及ぼす。そのため、0.0025〜0.0070%とする。望ましくは、0.0030〜0.0060%である。
N: 0.0025 to 0.0070
N is effective in refining the structure of the heat affected zone by generating TiN. In order to exhibit this effect, 0.0025% or more is necessary. Moreover, when adding exceeding 0.0070%, the solid solution N of a welding heat affected zone will increase, and heat affected zone toughness deterioration will be exerted. Therefore, it is set as 0.0025 to 0.0070%. Desirably, it is 0.0030 to 0.0060%.

O:0.001〜0.003%
Oは、不純物元素として混入する元素であり、低いほうが望ましいが、過度に酸素を低減させることは、溶製工程での製造コスト上昇につながる。また、0.003%を超えて添加すると酸化物系介在物が増加し、鋼の清状度を劣化させる。そのため、0.001〜0.003%とする。望ましくは、0.001〜0.0025%である。
O: 0.001 to 0.003%
O is an element mixed as an impurity element, and it is desirable that O be low. However, excessively reducing oxygen leads to an increase in manufacturing cost in the melting process. Moreover, when it exceeds 0.003% and an oxide type inclusion increases, the cleanliness degree of steel will be degraded. Therefore, the content is set to 0.001 to 0.003%. Desirably, it is 0.001 to 0.0025%.

Ca:0.0005〜0.005%
Caは、ACR値制御のために必要な元素であり、MnSの生成核となるCaSを形成し、生成したMnSが大入熱溶接部の粒内フェライト生成および組織微細化に有効な作用を及ぼす。その効果を得るためには、0.0005%以上必要である。0.005%を超えての添加は、Ca系酸化物が増大し、鋼の清状度を劣化させる。そのため、0.0005〜0.005%とする。望ましくは、0.0005〜0.005%である。
Ca: 0.0005 to 0.005%
Ca is an element necessary for controlling the ACR value, and forms CaS as a nucleus of MnS formation. The generated MnS has an effective effect on intragranular ferrite formation and microstructure refinement in a high heat input weld. . In order to obtain the effect, 0.0005% or more is necessary. Addition in excess of 0.005% increases Ca-based oxides and degrades the steel cleanliness. Therefore, it is set as 0.0005 to 0.005%. Preferably, it is 0.0005 to 0.005%.

Mo:0.01%以下
Moは微量の添加により、溶接熱影響部の焼入性を増大させ、その結果、フェライト生成を抑制し、上部ベイナイト化させ、靭性を劣化させる。0.01%を超えると、このような作用を生じ、溶接熱影響部靭性を劣化させる。そのため、添加する場合は0.01%以下とする。実質、無添加とすることが望ましい。
Mo: 0.01% or less Mo is added in a small amount to increase the hardenability of the weld heat-affected zone. As a result, the formation of ferrite is suppressed, the upper bainite is formed, and the toughness is deteriorated. If it exceeds 0.01%, such an action is produced, and the weld heat affected zone toughness is deteriorated. Therefore, when adding, it is 0.01% or less. It is desirable to add substantially no additives.

Nb:0.005%以下
Nbは微量の添加により、溶接熱影響部の焼入性を増大させ、その結果、フェライト生成を抑制し、上部ベイナイト化させ、靭性を劣化させる。0.005%を超えると、このような作用を生じ、溶接熱影響部靭性を劣化させる。そのため、0.005%以下とする。実質、無添加とすることが望ましい。
Nb: 0.005% or less Nb increases the hardenability of the weld heat-affected zone by adding a small amount, and as a result, suppresses the formation of ferrite, causes upper bainite, and deteriorates toughness. When it exceeds 0.005%, such an effect is produced, and the weld heat-affected zone toughness is deteriorated. Therefore, it is made 0.005% or less. It is desirable to add substantially no additives.

更に、Cu,Ni,Cr,Vの一種または二種以上
Cu:0.5%以下
Cuは、固溶強化として有効な元素であるが、0.5%を超える添加は、熱間延性の劣化、あるいは、表面疵の増加など製造上問題を生ずる。そのため、添加する場合は0.5%以下に規制する。
Further, one or more of Cu, Ni, Cr and V Cu: 0.5% or less Cu is an element effective as a solid solution strengthening, but addition exceeding 0.5% deteriorates hot ductility. Or manufacturing problems such as an increase in surface defects. Therefore, when adding, it controls to 0.5% or less.

Ni:1.0%以下
Niは、固溶強化として有効な元素であるが、1.0%を超える添加は、合金コストが上昇し、製造コストが上昇する。そのため添加する場合は1.0%以下とする。
Ni: 1.0% or less Ni is an element effective as a solid solution strengthening, but addition exceeding 1.0% increases the alloy cost and the manufacturing cost. Therefore, when adding, it is 1.0% or less.

Cr:0.5%以下
Crは、固溶強化に有効な元素であるが、0.5%を超える添加は溶接性を低下させる。そのため、0.5%以下とする。
Cr: 0.5% or less Cr is an element effective for solid solution strengthening, but addition exceeding 0.5% lowers weldability. Therefore, it is 0.5% or less.

V:0.08%以下
Vは、固溶強化あるいは析出強化として有効な元素であるが、0.08%を超える添加は、合金コストが上昇し、製造コストが上昇する。そのため、0.08%以下とする。
V: 0.08% or less V is an effective element for solid solution strengthening or precipitation strengthening, but addition exceeding 0.08% increases the alloy cost and the manufacturing cost. Therefore, it is 0.08% or less.

Ceq:0.40〜0.45%
Ceq(=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5、但し、各元素は含有量(質量%))は強度および大入熱溶接熱影響部靭性の観点から規制する。0.40%未満では強度が確保できず、また、0.45%を超えると、大入熱溶接熱影響部の靭性が劣化する。そのため、0.40〜0.45%に規制する。
Ceq: 0.40 to 0.45%
Ceq (= C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5, where each element content (% by mass) is regulated from the viewpoints of strength and toughness of heat-affected zone heat-affected zone. If it is less than 0.40%, the strength cannot be ensured, and if it exceeds 0.45%, the toughness of the heat-affected zone affected by high heat input welding deteriorates. Therefore, it regulates to 0.40 to 0.45%.

ACR:0.2〜0.8
ACR(=(Ca−(0.18+130×Ca)×O)/(1.25×S)、但し、各元素は含有量(質量%))は、大入熱溶接熱影響部靭性の観点から規制する。0.2%未満の場合には、フェライト生成に必要なCa系硫化物の生成量が減少し、大入熱溶接熱影響部靭性改善効果を得られない。また、0.8%超えとなると、Ca系粒化物は生成しても、それを核としたMnSが生成せずに、フェライト生成による熱影響部微細化効果を得ることが出来ない。そのため、0.2〜0.8%に規制する。
ACR: 0.2-0.8
ACR (= (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S), where each element content (% by mass)) is from the viewpoint of high heat input welding heat affected zone toughness regulate. If it is less than 0.2%, the amount of Ca-based sulfide required for ferrite formation decreases, and the effect of improving the toughness of the high heat input heat affected zone cannot be obtained. On the other hand, when it exceeds 0.8%, even if a Ca-based granulated material is generated, MnS having the core as a core is not generated, and the heat-affected zone refinement effect due to ferrite generation cannot be obtained. Therefore, it regulates to 0.2 to 0.8%.

以上が基本成分組成であるが、更に溶接熱影響部の靭性を改善する場合は、Mg、REMのうち1種または2種を添加する。   The above is the basic component composition, but when further improving the toughness of the weld heat affected zone, one or two of Mg and REM are added.

Mgは、酸硫化物を生成し、溶接熱影響部でフェライト生成および組織微細化に有効な元素で溶接熱影響部の靭性を改善するが、過剰に添加すると鋼の清状度を劣化させる。そのため、添加する場合は0.005%以下とする。   Mg forms oxysulfide and improves the toughness of the weld heat affected zone with an element effective for ferrite formation and microstructure refinement in the weld heat affected zone. However, when added in excess, the quality of the steel deteriorates. Therefore, when adding, it is 0.005% or less.

REMは、酸硫化物を生成し、溶接熱影響部でフェライト生成および組織微細化に有効な元素で溶接熱影響部の靭性を改善するが、過剰に添加すると鋼の清状度を劣化させる。そのため、添加する場合は0.02%とする。
[ミクロ組織]
本発明に係る建築用低降伏比鋼板はフェライト分率が2〜30%のミクロ組織とする。フェライト分率は、低降伏比化の観点から規制し、フェライト分率が2%未満の場合には、降伏比80%以下が得られず、30%を超えると、強度確保が困難となるため、2〜30%とする。
[表面硬度]
本発明に係る建築用低降伏比鋼板は表面硬度を350HV10以下とする。表面硬度は、表面の延性改善の観点から規制し、前述した図2,3に示すごとく、C量が0.08%を超えて表面硬度が350HV10超えとなると、表層付近の延性は25%以下に低下し、耐震性が劣化する。そのため、350HV10以下とする。
REM generates oxysulfide and improves the toughness of the weld heat-affected zone with an element effective for ferrite formation and microstructure refinement in the weld heat-affected zone. Therefore, when adding, it is 0.02%.
[Microstructure]
The low yield ratio steel sheet for construction according to the present invention has a microstructure with a ferrite fraction of 2 to 30%. The ferrite fraction is regulated from the viewpoint of lowering the yield ratio. If the ferrite fraction is less than 2%, a yield ratio of 80% or less cannot be obtained, and if it exceeds 30%, it is difficult to ensure the strength. 2 to 30%.
[surface hardness]
The low yield ratio steel sheet for building according to the present invention has a surface hardness of 350 HV10 or less. The surface hardness is regulated from the viewpoint of improving the ductility of the surface. As shown in FIGS. 2 and 3 described above, when the C content exceeds 0.08% and the surface hardness exceeds 350 HV10, the ductility near the surface layer is 25% or less. And the earthquake resistance deteriorates. Therefore, it shall be 350HV10 or less.

以下、本発明に係る建築用低降伏比鋼板の製造条件について説明する。
加熱温度:1000〜1200℃
加熱温度が1000℃未満では、熱間変形抵抗が高くて圧延が困難で、一方、1200℃を超えると、加熱時の初期の組織が粗大化し、母材組織が粗大化して靭性が劣化する。そのため、1000〜1200℃とする。
圧延終了温度:Ar変態点以上
圧延終了温度がAr変態点未満となると、圧延中に生成したフェライトが微細化し、降伏比が上昇する。そのため、Ar変態点以上とする。
Hereinafter, the manufacturing conditions of the low yield ratio steel sheet for construction according to the present invention will be described.
Heating temperature: 1000-1200 ° C
If the heating temperature is less than 1000 ° C., the hot deformation resistance is high and rolling is difficult, whereas if it exceeds 1200 ° C., the initial structure during heating becomes coarse, the base material structure becomes coarse, and the toughness deteriorates. Therefore, it shall be 1000-1200 degreeC.
Rolling end temperature: Ar 3 transformation point or higher When the rolling end temperature is lower than the Ar 3 transformation point, the ferrite generated during rolling is refined and the yield ratio is increased. Therefore, the Ar 3 transformation point or higher is set.

加速冷却:冷却速度:3〜12℃/s、冷却停止温度:400〜50℃
加速冷却の冷却速度が3℃/s未満では、フェライト分率が30%を超え、強度確保が困難となる。また、12℃/s超えでは、フェライト分率が2%未満となり、低降伏比化が困難となる。そのため、3〜12℃/sとする。
Accelerated cooling: Cooling rate: 3-12 ° C / s, Cooling stop temperature: 400-50 ° C
If the cooling rate of accelerated cooling is less than 3 ° C./s, the ferrite fraction exceeds 30%, and it is difficult to ensure the strength. On the other hand, if it exceeds 12 ° C./s, the ferrite fraction becomes less than 2%, and it becomes difficult to reduce the yield ratio. Therefore, it is set as 3-12 degrees C / s.

加速冷却の冷却停止温度は焼戻しを実施しない場合は400〜200℃、実施する場合は400〜50℃とする。焼戻しの有無に拘わらず、冷却停止温度が400℃超えでは、強度確保が困難となり、焼戻しを行わない場合、200℃未満では、冷却歪などにより鋼板形状を確保することが困難となる。   The cooling stop temperature of accelerated cooling is 400 to 200 ° C. when tempering is not performed, and 400 to 50 ° C. when it is performed. Regardless of the presence or absence of tempering, when the cooling stop temperature exceeds 400 ° C., it is difficult to ensure the strength, and when tempering is not performed, it is difficult to ensure the shape of the steel sheet due to cooling strain or the like if it is less than 200 ° C.

焼戻しを実施する場合には、焼戻しの加熱時に鋼板形状を矯正することが可能となるため、より低温まで停止温度を拡大することができ、冷却停止温度を50℃まで拡大する。   When tempering is performed, the steel plate shape can be corrected during tempering heating, so that the stop temperature can be increased to a lower temperature and the cooling stop temperature is increased to 50 ° C.

焼戻しを実施する場合、焼戻し温度は450℃以下とする。焼戻し温度が450℃を超えると、強度が低下し、降伏比が上昇する。そのため、450℃以下とする。   When tempering is performed, the tempering temperature is 450 ° C. or lower. When the tempering temperature exceeds 450 ° C., the strength decreases and the yield ratio increases. Therefore, it shall be 450 degrees C or less.

上記成分組成と加速冷却条件の組み合わせにより、焼戻しを行わない場合であっても表面硬度は350HV10以下となり、焼戻しを行うと更に低下する。   Even if tempering is not performed, the surface hardness becomes 350 HV10 or less due to the combination of the above component composition and accelerated cooling conditions, and further decreases when tempering is performed.

鋼板の板厚が薄くなると、板厚が厚い場合より高冷却速度、かつ、冷却停止温度を高温化しても、板厚が厚い場合と同様の組織形態が得られる。板厚19〜40mmの鋼板については、以下の製造方法とすることが好ましい。   When the plate thickness of the steel plate is reduced, a structure similar to that when the plate thickness is thick can be obtained even if the cooling rate is increased and the cooling stop temperature is increased as compared with the case where the plate thickness is thick. About the steel plate of plate | board thickness 19-40mm, it is preferable to set it as the following manufacturing methods.

加速冷却:冷却速度:15℃/s以上、冷却停止温度:650〜500℃
加速冷却の冷却速度が15℃/s未満では、強度確保が困難である。そのため、15℃/s以上とする。
Accelerated cooling: Cooling rate: 15 ° C / s or more, Cooling stop temperature: 650-500 ° C
If the cooling rate of accelerated cooling is less than 15 ° C./s, it is difficult to ensure the strength. Therefore, it is set to 15 ° C./s or more.

冷却停止温度が650℃を超えると、フェライト分率が30%を超えるため強度確保が出来ず、また、500℃を下回ると、フェライトが生成せず、低降伏比化が達成されない。そのため、650〜500℃とする。加速冷却後、450℃以下の焼戻しを施しても良い。   If the cooling stop temperature exceeds 650 ° C., the ferrite fraction exceeds 30%, and thus strength cannot be ensured. If the cooling stop temperature is less than 500 ° C., ferrite is not generated, and a low yield ratio is not achieved. Therefore, it is set as 650-500 degreeC. After accelerated cooling, tempering at 450 ° C. or lower may be performed.

上記成分組成と加速冷却条件の組み合わせにより、焼戻しを行わない場合であっても表面硬度は350HV10以下となり、焼戻しを行うと更に低下する。   Even if tempering is not performed, the surface hardness becomes 350 HV10 or less due to the combination of the above component composition and accelerated cooling conditions, and further decreases when tempering is performed.

表1に示す組成の溶鋼を真空溶解炉にて溶製した後、スラブとし、熱間圧延により所定の板厚の鋼板とした。その後、得られた鋼板に加速冷却を施して試験材とした。一部の鋼板は加速冷却後、焼戻しを行って試験材とした。   The molten steel having the composition shown in Table 1 was melted in a vacuum melting furnace, then made into a slab, and a steel plate having a predetermined thickness was obtained by hot rolling. Thereafter, the obtained steel plate was subjected to accelerated cooling to obtain a test material. Some steel plates were tempered after accelerated cooling and used as test materials.

各試験材について、フェライト分率測定、表面硬度測定、引張試験、エレクトロスラグ溶接部の熱影響部のシャルピ−衝撃試験を実施した。   Each test material was subjected to ferrite fraction measurement, surface hardness measurement, tensile test, and Charpy impact test of the heat affected zone of the electroslag weld.

フェライト分率は、各鋼板の1/4部を光学顕微鏡で500倍、180μm×150μmの範囲を5視野観察し、その中のフェライト分率の平均値とした。   The ferrite fraction was obtained by observing ¼ part of each steel sheet 500 times with an optical microscope and 5 fields of 180 μm × 150 μm, and taking the average value of the ferrite fractions therein.

表面硬度は、JISZ2244に準拠した荷重10kgfのビッカ−ス硬度試験で、表層下0.5mm位置を20点測定し、その最大値を代表値とした。引張試験は、板厚40〜80mmの鋼板は、JIS Z2201に準拠して、JIS4号試験片を、1/4t位置、1/2t位置の2箇所から採取し、引張特性(降伏応力(YS)、引張強さ(TS)、降伏比(YR))を調査した。試験片はL方向採取とした。   The surface hardness was a Vickers hardness test with a load of 10 kgf in accordance with JISZ2244, 20 points were measured at 0.5 mm below the surface layer, and the maximum value was taken as the representative value. Tensile tests were conducted on steel sheets with a thickness of 40 to 80 mm in accordance with JIS Z2201 by taking JIS No. 4 test pieces from two locations, 1 / 4t position and 1 / 2t position, and tensile properties (yield stress (YS)). , Tensile strength (TS), yield ratio (YR)). The test piece was taken in the L direction.

板厚19〜40mmの鋼板は、JIS Z2201に準拠して、全厚のJIS5号試験片を、L方向で採取し、引張特性(降伏応力(YS)、引張強さ(TS)、降伏比(YR))を調査した。   For a steel sheet having a thickness of 19 to 40 mm, a full thickness JIS No. 5 test piece was taken in the L direction in accordance with JIS Z2201, and tensile properties (yield stress (YS), tensile strength (TS), yield ratio ( YR)) was investigated.

大入熱溶接熱影響部靭性は、板厚40〜80mmの鋼板は、入熱960kJ/cmのエレクトロスラグ溶接を実施し、BOND部から1mm離れた溶接熱影響部をノッチ位置とするシャルピ−衝撃試験により評価した。   Large heat input weld heat affected zone toughness: steel plate with thickness of 40-80mm is subjected to electroslag welding with heat input of 960kJ / cm and Charpy impact with weld heat affected zone 1mm away from BOND part as notch position It was evaluated by testing.

板厚19〜40mmの鋼板は、入熱400kJ/cmのエレクトロスラグ溶接を実施し、BOND部から1mm離れた溶接熱影響部をノッチ位置とするシャルピ−衝撃試験により評価した。いずれのシャルピ−衝撃試験も試験温度0℃、試験本数3本で行いその平均値を代表値として評価した。   Steel plates having a thickness of 19 to 40 mm were evaluated by a Charpy impact test in which electroslag welding with a heat input of 400 kJ / cm was performed and a weld heat affected zone 1 mm away from the BOND portion was a notch position. All Charpy impact tests were conducted at a test temperature of 0 ° C. and three test pieces, and the average value was evaluated as a representative value.

表2に板厚40〜80mmの試験材の、表3に板厚19〜40mmの試験材の製造条件(スラブ加熱温度、圧延条件、加速冷却条件)と上記試験結果を併せて示す。   Table 2 shows the test conditions with a thickness of 40 to 80 mm, and Table 3 shows the manufacturing conditions (slab heating temperature, rolling conditions, accelerated cooling conditions) of the test thickness with a thickness of 19 to 40 mm and the test results.

表2、3より、本発明例(鋼1〜3、鋼5,6、鋼8、鋼15、16、18)は、いずれも表面硬度は350HV10以下、引張り強さ590MPa以上、降伏比80%以下が得られ、大入熱溶接熱影響部靭性も112J以上であった。   From Tables 2 and 3, according to the present invention examples (steel 1 to 3, steel 5, 6, steel 8, steel 15, 16, and 18), the surface hardness is 350 HV10 or less, the tensile strength is 590 MPa or more, and the yield ratio is 80%. The following was obtained, and the high heat input heat affected zone toughness was 112 J or more.

一方、比較例(鋼4、7、鋼9〜14、鋼17)は表面硬度、引張り強さ、降伏比および大入熱溶接熱影響部靭性の複数またはいずれかが本発明例より劣る。   On the other hand, the comparative examples (Steel 4, 7, Steel 9-14, Steel 17) are inferior to those of the present invention in terms of surface hardness, tensile strength, yield ratio and / or high heat input heat affected zone toughness.

比較例鋼4は加速冷却の冷却停止温度が450℃と高く、フェライト分率が高くて、引張り強さが590MPa未満であった。比較例鋼7は加速冷却の冷却速度が16℃/sと高く、フェライト分率が低くて、引張り強さに対して降伏強さが高くYR80%超であった。   In Comparative Example Steel 4, the cooling stop temperature for accelerated cooling was as high as 450 ° C., the ferrite fraction was high, and the tensile strength was less than 590 MPa. Comparative Example Steel 7 had a high accelerated cooling rate of 16 ° C./s, a low ferrite fraction, a high yield strength with respect to the tensile strength, and was over YR 80%.

比較例鋼9〜14は、成分組成が本発明範囲外で、大入熱溶接熱影響部靭性が低く、更にC量が0.08%と高い比較例鋼9、10は表層硬度が高く、表層の延性が劣る。比較例鋼17は加速冷却の冷却停止温度が低く、YR80%超であった。   Comparative Example Steels 9 to 14 have component compositions outside the scope of the present invention, low large heat input heat affected zone toughness, and Comparative Examples Steels 9 and 10 having a high C content of 0.08% have high surface hardness, The ductility of the surface layer is inferior. In Comparative Example Steel 17, the cooling stop temperature for accelerated cooling was low, and it was over YR 80%.

大入熱溶接熱影響部靭性に及ぼすC量の影響を示す図。The figure which shows the influence of the amount of C exerted on the high heat input welding heat affected zone toughness. 表層硬度に及ぼすC量の影響を示す図。The figure which shows the influence of C amount which acts on surface layer hardness. 延性(表層引張試験伸び)に及ぼすC量の影響を示す図。The figure which shows the influence of C amount which affects ductility (surface layer tensile test elongation). 強度(YS,TS)に及ぼすC量の影響を示す図。The figure which shows the influence of the amount of C which acts on intensity | strength (YS, TS).

Claims (4)

質量%で、C:0.03〜0.07%、Si:0.05〜0.5%、Mn:0.6〜2.0%、P:0.020%以下、S:0.0005〜0.003%、Ti:0.005〜0.03%、B:0.0003〜0.0020%、Al:0.1%以下、N:0.0025〜0.0070、O:0.001〜0.003%、Ca:0.0005〜0.005%、Mo:0.01%以下、Nb:0.005%以下、更に、Cu:0.5%以下、Ni:1.0%以下、Cr:0.5%以下、V:0.08%以下の1種または2種以上を含み、下記(1)式によるCeqが0.40〜0.45%、下記(2)式によるACRが0.2〜0.8を満足し、残部Feおよび不可避的不純物からなる組成と、フェライト分率が2〜30%であるミクロ組織を有し、表面硬度が350HV10以下であることを特徴とする降伏比80%以下、引張強度590MPa以上の大入熱溶接部靭性に優れた建築用低降伏比鋼板。
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 −−(1)
ACR=(Ca−(0.18+130×Ca)×O)/(1.25×S)−−−(2)
但し、(1)式、(2)式中の各元素は含有量(質量%)とする。
In mass%, C: 0.03-0.07%, Si: 0.05-0.5%, Mn: 0.6-2.0%, P: 0.020% or less, S: 0.0005 -0.003%, Ti: 0.005-0.03%, B: 0.0003-0.0020%, Al: 0.1% or less, N: 0.0025-0.0070, O: 0.0. 001 to 0.003%, Ca: 0.0005 to 0.005%, Mo: 0.01% or less, Nb: 0.005% or less, Cu: 0.5% or less, Ni: 1.0% Hereinafter, one or two or more of Cr: 0.5% or less and V: 0.08% or less are included, and Ceq according to the following formula (1) is 0.40 to 0.45%, according to the following formula (2) ACR satisfies 0.2 to 0.8, has a composition comprising the balance Fe and inevitable impurities, and a microstructure with a ferrite fraction of 2 to 30%. 80% yield ratio, wherein a surface hardness of 350HV10 less or less, a tensile strength 590MPa or more low yield ratio steel sheet excellent architectural high heat input weld toughness.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 − (1)
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) −−− (2)
However, each element in the formulas (1) and (2) has a content (% by mass).
さらに、成分組成にMg:0.005%以下、REM:0.02%以下の1種または2種を含有することを特徴とする請求項1に記載の降伏比80%以下、引張強度590MPa以上の大入熱溶接部靭性に優れた建築用低降伏比鋼板。   Furthermore, the component composition contains one or two of Mg: 0.005% or less and REM: 0.02% or less. The yield ratio is 80% or less and the tensile strength is 590 MPa or more. Low yield ratio steel sheet for construction with excellent high heat input weld toughness. 請求項1または2に記載の組成を有する鋼素材を1000〜1200℃に加熱後、圧延終了温度をAr変態点以上とする圧延を施し、ついで、加速冷却を平均冷却速度3〜12℃/s、冷却停止後焼戻しを行わない場合は冷却停止温度:400〜200℃、焼戻しを行う場合は冷却停止温度:400〜50℃で行うことを特徴とする大入熱溶接部靭性に優れた建築構造用低降伏比鋼板の製造方法。 After the steel material having the composition according to claim 1 or 2 is heated to 1000 to 1200 ° C, rolling is performed such that the rolling end temperature is equal to or higher than the Ar 3 transformation point, and then accelerated cooling is performed at an average cooling rate of 3 to 12 ° C / s, a construction that is excellent in toughness of high heat input welds, characterized in that when tempering is not performed after cooling stop, the cooling stop temperature is 400 to 200 ° C., and when tempering is performed, the cooling stop temperature is 400 to 50 ° C. A method for producing a structural low yield ratio steel sheet. 請求項1または2に記載の組成を有する鋼素材を1000〜1200℃に加熱後、圧延終了温度をAr変態点以上とする圧延を施し、ついで、平均冷却速度15℃/s以上、冷却停止温度:650〜500℃とする加速冷却を行った後、空冷または焼戻しを行うことを特徴とする大入熱溶接部靭性に優れた板厚19〜40mmの建築用低降伏比鋼板の製造方法。 After the steel material having the composition according to claim 1 or 2 is heated to 1000 to 1200 ° C, rolling is performed so that the rolling end temperature is not lower than the Ar 3 transformation point, and then the average cooling rate is 15 ° C / s or higher and the cooling is stopped. Temperature: 650-500 degreeC After performing accelerated cooling, air cooling or tempering is performed, The manufacturing method of the low-yield-ratio steel plate for construction of the plate | board thickness of 19-40 mm excellent in the high heat input weld toughness characterized by the above-mentioned.
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