JP2005068519A - Method for producing high strength thick steel plate for building structure, having excellent toughness to super-large heat input welding-affected zone - Google Patents

Method for producing high strength thick steel plate for building structure, having excellent toughness to super-large heat input welding-affected zone Download PDF

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JP2005068519A
JP2005068519A JP2003302709A JP2003302709A JP2005068519A JP 2005068519 A JP2005068519 A JP 2005068519A JP 2003302709 A JP2003302709 A JP 2003302709A JP 2003302709 A JP2003302709 A JP 2003302709A JP 2005068519 A JP2005068519 A JP 2005068519A
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JP4120531B2 (en
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Tatsuki Kimura
達己 木村
Keiji Ueda
圭治 植田
Toshiyuki Hoshino
俊幸 星野
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Jfe Steel Kk
Jfeスチール株式会社
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high strength thick steel plate for building structure having excellent toughness in a super-large heat input welding-affected zone. <P>SOLUTION: This production method employs a steel blank having a composition containing, by mass%: 0.05 to 0.15% C, 0.05 to 0.50% Si, 0.6 to 1.6% Mn, ≤0.005% S, and an appropriate amount of P, Al, Cu, and Ni. The steel blank further contains 0.005 to 0.030% Ti, 0.0003 to 0.0050% B, 0.0005 to 0.0050% Ca, 0.0030 to 0.0060% N, 0.0010 to 0.0030% O, and has 0.2 to 0.8 ACR defined by ACR=äCa-(0.18+130Ca)×O}/(1.25×S) and ≤0.47 Ceq. The steel blank is heated to a temperature in the range of 1,000 to 1,300°C, and subjected to hot-rolling having ≥900°C rolling finish temperature and then to accelerated cooling to ≤600°C at ≥1°C/s cooling speed. Further, this rolled steel plate is heated to a reheating temperature in two-phase range of (Ac1+10°C) to (Ac1+70°C) and desirably held for 5 to 60 min and thereafter rapidly cooled, subjected to reheating quenching and then tempering. Then, this steel blank may contain one or more kinds of Cr, V and Mo. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

本発明は、建築構造用として好適な、厚鋼板に係り、とくに、建築構造物の柱および梁等の、溶接入熱が400kJ/cmを超える超大入熱溶接を施される使途に好適な、引張強さが590MPaを超える建築構造用高強度厚鋼板に関する。   The present invention relates to a thick steel plate suitable for use in a building structure, and particularly suitable for a use in which super-high heat input welding with a welding heat input exceeding 400 kJ / cm, such as columns and beams of a building structure, is performed. The present invention relates to a high-strength steel plate for building structures having a tensile strength exceeding 590 MPa.

近年、建築構造物の大型化と大スパン化に伴い、使用される鋼材には厚肉化、高強度化が要求されている。一方、鋼構造物の安全性の観点から、使用される鋼材の降伏比の低減が要求されている。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。さらに、兵庫県南部地震において指摘されているとおり、溶接鋼構造物では、地震時のような急激でかつ大きな荷重負荷を受けると、十分な塑性変形を生じる前に、溶接部を主体に脆性破壊が生じる場合がある。このため、近年、溶接構造物用鋼材には、溶接部も含めて良好な靱性を具備することが求められている。   In recent years, with the increase in size and span of building structures, the steel materials used are required to be thicker and stronger. On the other hand, from the viewpoint of the safety of steel structures, reduction of the yield ratio of the steel material used is required. 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. Furthermore, as pointed out in the Hyogoken-Nanbu Earthquake, welded steel structures are subject to brittle fracture mainly in the weld zone before undergoing sufficient plastic deformation when subjected to a sudden and large load load, such as during an earthquake. May occur. For this reason, in recent years, steel materials for welded structures are required to have good toughness including welds.

一方、構造物の施工効率の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、大入熱の高能率溶接が指向されている。例えば、高層および超高層建築物に主としてに適用されている箱型四面ボックス柱(あるいはコンクリート充填箱型四面ボックス柱)では、角溶接部で溶接入熱が600kJ/cmの多電極1パスサブマージアーク溶接が、ダイアフラム部で溶接入熱が1000kJ/cmを超えるようなエレクトロスラグ溶接が適用されている。このような溶接部では、溶接時に高温に晒される滞留時間が増大するとともにその冷却速度が低下する。このため、溶接熱影響部(以下、HAZともいう)の組織は粗大化しやすく、一般的に、良好なHAZ靱性が得られがたい。   On the other hand, from the viewpoint of improving the construction efficiency of the structure and reducing the construction cost, improvement in welding efficiency is required, and high-efficiency welding with large heat input is directed. For example, in a box-type four-sided box column (or concrete-filled box-type four-sided box column) mainly applied to high-rise and super-high-rise buildings, a multi-electrode one-pass submerged arc with a welding heat input of 600 kJ / cm at the corner weld. Electroslag welding is applied so that the welding heat input exceeds 1000 kJ / cm at the diaphragm. In such a weld, the residence time exposed to high temperature during welding increases and the cooling rate thereof decreases. For this reason, the structure of the weld heat-affected zone (hereinafter also referred to as HAZ) is likely to be coarse, and it is generally difficult to obtain good HAZ toughness.

とくに、引張強さが590MPaを超える高張力鋼では、強度確保のために合金を多量に添加することが一般的であるため、降伏比が上昇する傾向にあるとともに、HAZ靱性も低くなる。このため、低降伏比と優れたHAZ靭性とを兼備した高張力鋼板が要望されている。   In particular, in a high-strength steel having a tensile strength exceeding 590 MPa, it is common to add a large amount of an alloy for securing the strength, so that the yield ratio tends to increase and the HAZ toughness also decreases. For this reason, a high-tensile steel sheet having both a low yield ratio and excellent HAZ toughness is desired.

このような要望に対し、例えば、特許文献1、特許文献2、特許文献3、特許文献4には、低降伏比高張力鋼の製造方法が提案されている。特許文献1、特許文献2に記載された技術は、いずれも圧延後、直に焼入れする直接焼入れ法であり、圧延後の冷却の開始を遅らせ、5〜60%程度のフェライトを析出させた後、急冷して、フェライト相+硬化相の2相組織としている。これにより、低降伏比化を実現している。一方、特許文献3に記載された技術では、フェライト析出温度域にて保持させた後に冷却し、フェライト+硬化相の2相組織とすることによって低降伏比化を達成している。また、特許文献4に記載された技術では、熱間圧延後の鋼板を焼入れした後、再度フェライト+オーステナイトの2相域まで加熱し、焼入れしたのち、焼戻しを行い、低降伏比化を実現している。しかしながら、特許文献1、特許文献2、特許文献3、特許文献4に記載された技術では、このように低降伏比化は実現できても、十分なHAZ靭性を有するまでに至っていないという問題がある。   In response to such a demand, for example, Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4 propose a method for producing a low-yield ratio high-tensile steel. The techniques described in Patent Document 1 and Patent Document 2 are direct quenching methods in which quenching is performed directly after rolling, after the start of cooling after rolling is delayed, and about 5 to 60% of ferrite is precipitated. , Rapidly cooled to a two-phase structure of ferrite phase + hardened phase. This achieves a low yield ratio. On the other hand, in the technique described in Patent Document 3, a low yield ratio is achieved by cooling after holding in the ferrite precipitation temperature range to form a two-phase structure of ferrite and hardened phase. Moreover, in the technique described in Patent Document 4, after quenching the hot-rolled steel sheet, the steel is again heated to a ferrite + austenite two-phase region, quenched, and tempered to achieve a low yield ratio. ing. However, the techniques described in Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4 have a problem that even if the yield ratio can be reduced in this way, sufficient HAZ toughness is not achieved. is there.

このような問題に対し、例えば、特許文献5、特許文献6、特許文献7、特許文献8には、大入熱溶接のHAZ靱性を向上させる技術が提案されている。特許文献5には、100kJ/cmの溶接ボンド部靭性の改善をめざし、希土類元素とTiとを複合添加して、鋼中微細粒子を分散させてオーステナイトの粒成長を抑制し、溶接ボンド部の靭性向上を図る技術が提案されている。また、特許文献6には、Ti酸化物を微細分散させ、大入熱溶接HAZの高靭性化を図る技術が提案されている。また、特許文献7には、Tiの酸化物を微細分散させて、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の靭性を改善する技術が提案されている。また、特許文献8には、固溶Nを徹底的に低減するために、Tiと十分なAl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靭性を向上させる高張力鋼板が提案されている。   For such problems, for example, Patent Document 5, Patent Document 6, Patent Document 7, and Patent Document 8 propose techniques for improving the HAZ toughness of high heat input welding. In Patent Document 5, with the aim of improving the weld bond toughness of 100 kJ / cm, a rare earth element and Ti are added in combination to disperse fine particles in the steel and suppress the grain growth of austenite. Techniques for improving toughness have been proposed. Patent Document 6 proposes a technique for finely dispersing Ti oxide to increase the toughness of the high heat input welding HAZ. Patent Document 7 proposes 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. In Patent Document 8, in order to thoroughly reduce solid solution N, Ti and a sufficient amount of Al are contained, and further, Ca oxide is used as a fine oxide, and HAZ toughness in super large heat input welding. High-tensile steel sheets that improve the strength have been proposed.

しかし、特許文献5〜特許文献8に記載された技術によっても、引張強さが590MPa以上の高強度で、かつ母材降伏比を80%以下の低降伏比とし、さらに溶接入熱量が400kJ/cmを超えるような超大入熱溶接においても優れたHAZ靭性を安定して保持させることは困難なことであった。
特公昭58-10442号公報 特開昭62-77419号公報 特開平2-34721号公報 特開平4-99817号公報 特開昭60-184663号公報 特開昭57-51243号公報 特開昭60-245768号公報 特開2001-107177号公報
However, even with the techniques described in Patent Documents 5 to 8, the tensile strength is high strength of 590 MPa or more, the base material yield ratio is 80% or less, and the heat input of welding is 400 kJ / It was difficult to stably maintain excellent HAZ toughness even in super-high heat input welding exceeding cm.
Japanese Patent Publication No.58-10442 JP 62-77419 A JP-A-2-34721 Japanese Patent Laid-Open No. 4-99817 JP-A-60-184663 JP 57-51243 A JP-A-60-245768 Japanese Patent Laid-Open No. 2001-107177

本発明は、上記した従来技術の問題を解決し、590MPa以上の引張強さと、80%以下の低降伏比とを有し、さらに溶接入熱量が400kJ/cmを超えるような超大入熱溶接においても優れたHAZ靱性を有する、超大入熱溶接部靭性に優れる建築構造用高強度厚鋼板の製造方法を提案することを目的とする。ここでいう、「400kJ/cmを超える超大入熱溶接においても優れたHAZ靱性」とは、0℃におけるシャルピー吸収エネルギーVが70J以上を有する場合をいうものとする。 The present invention solves the above-described problems of the prior art, has a tensile strength of 590 MPa or more, a low yield ratio of 80% or less, and a super high heat input welding in which the welding heat input exceeds 400 kJ / cm. Another object of the present invention is to propose a method for producing a high-strength thick steel sheet for building structures having excellent HAZ toughness and excellent toughness in super-high heat input welds. Here, “excellent HAZ toughness even in super high heat input welding exceeding 400 kJ / cm” refers to a case where the Charpy absorbed energy V E 0 at 0 ° C. is 70 J or more.

本発明者らは、上記した課題を達成するために、強度、降伏比、および入熱400kJ/cmを超える超大入熱溶接HAZ靭性におよぼす各種要因について研究、検討した。その結果、
入熱が400kJ/cmを超える超大入熱溶接HAZにおいて、高靭性を得るためには、高温に加熱された領域におけるオーステナイト粒の粗大化抑制と、冷却時にフェライト変態を促進する変態核の微細分散が重要であり、そのために、TiNの適用と、Ca添加時の溶存酸素量を0.0010〜0.0030%に調節したうえで、Ca、S、Oの添加量をACRが0.2〜0.8%を満足するように調整し、さらにBを添加して炭素当量Ceqが0.47%以下となるようにすることが肝要であることを知見した。さらに、上記のように成分調整した鋼素材に熱間圧延を施した後、冷却速度と冷却停止温度を適正化した加速冷却処理と、さらに二相域に再加熱し焼入れ、焼戻しする熱処理とを組み合わせることにより、板厚:50mmを超える厚鋼板においても、上記した優れた超大入熱溶接HAZ靭性と、引張強さ:590MPa以上を有し、0.80%以下の低降伏比を有する母材特性とを兼備させることができることを知見した。
In order to achieve the above-mentioned problems, the present inventors have studied and examined various factors affecting strength, yield ratio, and super large heat input welding HAZ toughness exceeding 400 kJ / cm. as a result,
In order to obtain high toughness in super high heat input welding HAZ with a heat input exceeding 400 kJ / cm, it is necessary to suppress coarsening of austenite grains in a region heated to a high temperature and to finely disperse transformation nuclei that promote ferrite transformation during cooling. Therefore, after adding TiN and adjusting the amount of dissolved oxygen at the time of Ca addition to 0.0010 to 0.0030%, the addition amount of Ca, S, and O should satisfy ACR of 0.2 to 0.8%. It was found that it is important to adjust the carbon equivalent Ceq to 0.47% or less by further adding B. Furthermore, after subjecting the steel material whose components are adjusted as described above to hot rolling, an accelerated cooling process in which the cooling rate and the cooling stop temperature are optimized, and a heat treatment in which the two-phase region is reheated, quenched, and tempered. By combining, even with thick steel plates exceeding 50 mm, the above-mentioned excellent super high heat input welding HAZ toughness and the base metal properties with tensile strength: 590 MPa or more and low yield ratio of 0.80% or less It was found that both can be combined.

本発明は、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、本発明の要旨は次のとおりである。
(1)質量%で、C:0.05〜0.15%、Si:0.05〜0.50%、Mn:0.6〜1.6%、P:0.018%以下、S:0.005%以下、Al:0.1%以下、Cu:0.1〜1.0%、Ni:0.1〜2.0%、Ti:0.005〜0.030%、B:0.0003〜0.0050%、Ca:0.0005〜0.0050%、N:0.0030〜0.0060%、O:0.0010〜0.0030%を、次(1)式
ACR={Ca−(0.18+130Ca)×O}/(1.25×S) ………(1)
(ここで、Ca、O、S:各元素の含有量(質量%))
で定義されるACRが0.2〜0.8、かつ次(2)式
Ceq =C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5 ………(2)
(ここで、Ceq:炭素当量(%)、C、Mn、Cu、Ni、Cr、Mo、V:各元素の含有量(質量%))
で定義される炭素当量Ceqが0.47%以下となる範囲で含み、残部がFeおよび不可避的不純物からなる組成の鋼素材を、1000〜1300℃の範囲の温度に加熱し、圧延終了温度を900℃以上とする熱間圧延を施した後、1℃/s以上の冷却速度で600℃以下まで加速冷却を行い、厚鋼板としたのち、さらに該厚鋼板に、 (Ac1変態点+10℃)〜 (Ac1変態点+70℃)の2相域の再加熱温度に加熱したのち、急冷する再加熱焼入れを行い、ついで焼戻しを行う再加熱焼入れ−焼戻し処理を施すことを特徴とする、超大入熱溶接熱影響部靱性に優れる建築構造用低降伏比高強度厚鋼板の製造方法。
(2)(1)において、前記組成に加えて質量%で、Cr:0.05〜0.50%、V:0.005〜0.01%、Mo:0.01〜0.30%のうちの1種または2種以上含有することを特徴とする建築構造用低降伏比高強度厚鋼板の製造方法。
(3)(1)または(2)において、前記再加熱温度で、5〜60min間保持することを特徴とする建築構造用低降伏比高強度厚鋼板の製造方法。
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: 0.05 to 0.15%, Si: 0.05 to 0.50%, Mn: 0.6 to 1.6%, P: 0.018% or less, S: 0.005% or less, Al: 0.1% or less, Cu: 0.1 to 1.0%, Ni: 0.1-2.0%, Ti: 0.005-0.030%, B: 0.0003-0.0050%, Ca: 0.0005-0.0050%, N: 0.0030-0.0060%, O: 0.0010-0.0030%, (1) Formula ACR = {Ca− (0.18 + 130Ca) × O} / (1.25 × S) (1)
(Where Ca, O, S: content of each element (mass%))
The ACR defined by the formula is 0.2 to 0.8, and the following equation (2): Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (2)
(Where Ceq: carbon equivalent (%), C, Mn, Cu, Ni, Cr, Mo, V: content of each element (mass%))
A steel material having a composition including a carbon equivalent Ceq defined by the formula of 0.47% or less and the balance consisting of Fe and inevitable impurities is heated to a temperature in the range of 1000 to 1300 ° C, and the rolling end temperature is 900 ° C. After performing the above hot rolling, accelerated cooling to 600 ° C. or less is performed at a cooling rate of 1 ° C./s or more to form a thick steel plate, and further to the thick steel plate, (Ac1 transformation point + 10 ° C.) to ( Super high heat input welding heat, characterized in that after heating to the reheating temperature in the two-phase region (Ac1 transformation point + 70 ° C), reheating and quenching is performed, followed by tempering. A method for producing a low-yield-ratio high-strength thick steel sheet for building structures having excellent toughness at the affected area.
(2) In (1), in addition to the above composition, it is contained by mass%, Cr: 0.05 to 0.50%, V: 0.005 to 0.01%, Mo: 0.01 to 0.30%. A method for producing a low-yield-ratio, high-strength thick steel sheet for building structures.
(3) In (1) or (2), the reheating temperature is maintained for 5 to 60 minutes, and the method for producing a low yield ratio high strength thick steel sheet for building structures.

まず、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、組成におけるmass%は単に%と記す。   First, the reasons for limiting the composition of the steel material used will be described. Hereinafter, mass% in the composition is simply referred to as%.

C:0.05〜0.15%
Cは、鋼の強度を増加させる元素であり、本発明では建築構造用厚鋼板として必要な強度(TS590MPa以上)を確保するために0.05%以上の含有が必要である。一方、0.15%を超えて含有すると、溶接部の靱性低下や低温溶接割れ感受性の増大をもたらす。このため、本発明ではCは0.05〜0.15%の範囲に限定した。なお、好ましくは0.05〜0.12%である。
C: 0.05-0.15%
C is an element that increases the strength of steel. In the present invention, C is required to be contained in an amount of 0.05% or more in order to ensure the strength (TS590 MPa or more) required for a thick steel plate for building structures. On the other hand, if the content exceeds 0.15%, the toughness of the weld is reduced and the sensitivity to low-temperature weld cracking is increased. For this reason, in the present invention, C is limited to a range of 0.05 to 0.15%. In addition, Preferably it is 0.05 to 0.12%.

Si:0.05〜0.50%
Siは、脱酸剤として作用し、製鋼上0.05%以上の含有を必要とするが、0.50%を超える含有は、母材靱性を低下させるとともに、超大入熱溶接HAZにおいて島状マルテンサイトを生成し、HAZ靱性を低下させる。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、0.05〜0.35%である。
Si: 0.05-0.50%
Si acts as a deoxidizer and requires 0.05% or more in steelmaking. However, if it exceeds 0.50%, it lowers the toughness of the base metal and generates island martensite in super high heat input welding HAZ. And reduces the HAZ toughness. For this reason, Si was limited to the range of 0.05 to 0.50%. In addition, Preferably, it is 0.05 to 0.35%.

Mn:0.6〜1.6%
Mnは、鋼の強度を向上させる元素であり、本発明では建築構造用厚鋼板として必要な強度(TS590MPa以上)を確保するために0.6%以上の含有を必要とする。一方、1.6%を超える含有は、HAZ靱性を著しく劣化させる。このため、Mnは0.6〜1.6%の範囲に限定した。なお、好ましくは、0.8〜1.6%である。
Mn: 0.6-1.6%
Mn is an element that improves the strength of steel. In the present invention, Mn needs to be contained in an amount of 0.6% or more in order to ensure the strength (TS590 MPa or more) required for a thick steel plate for building structures. On the other hand, the content exceeding 1.6% significantly deteriorates the HAZ toughness. For this reason, Mn was limited to the range of 0.6 to 1.6%. In addition, Preferably, it is 0.8 to 1.6%.

P:0.018%以下
Pは、不純物として鋼中に不可避的に含有される元素であり、鋼の靱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。特に、0.018%を超えるPの含有は、高強度鋼のHAZ靱性を著しく低下させる。このため、本発明では、Pは0.018%以下に限定した。なお、好ましくは0.015%以下である。
P: 0.018% or less P is an element unavoidably contained in steel as an impurity, and is desirably reduced as much as possible in order to deteriorate the toughness of the steel. In particular, the P content exceeding 0.018% significantly reduces the HAZ toughness of high strength steel. For this reason, in this invention, P was limited to 0.018% or less. In addition, Preferably it is 0.015% or less.

S:0.005%以下
Sは、Caを含有する鋼素材を用いる本発明では、Caと結合してCaS 粒子として凝固過程で微細に晶出し、さらに溶接時にCaS 粒子上にMnS として析出して、フェライト変態核として作用し、HAZ、特に、融合部(ボンド部)近傍の粗粒域HAZ、の靱性を向上させる効果を有する。このような効果は、S:0.0005%以上の含有で認められる。一方、0.005%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0.005%以下に限定した。なお、好ましくは0.0005〜0.0030%である。
S: 0.005% or less In the present invention using a Ca-containing steel material, S combines with Ca to form fine crystals as CaS particles during the solidification process, and further precipitates as MnS on the CaS particles during welding. It acts as a transformation nucleus and has the effect of improving the toughness of the HAZ, particularly the coarse grain region HAZ near the fusion part (bond part). Such an effect is recognized when the content of S is 0.0005% or more. On the other hand, if the content exceeds 0.005%, the toughness of the base metal and the welded portion is deteriorated. For this reason, S was limited to 0.005% or less. In addition, Preferably it is 0.0005 to 0.0030%.

Al:0.1%以下
Alは、脱酸剤として作用し、高張力鋼の溶鋼脱酸プロセスに於いてもっとも汎用的に使われる。また、熱処理時にNをAIN として固定し、Bの焼入れ性を維持する効果も有する。このような効果はAl:0.005%以上の含有で認められる。一方、0.1%を超える含有は、超大入熱溶接時に溶接金属部に混入して溶接金属部の靱性を低下させる。このため、本発明では、Alは0.1%以下に限定した。なお、好ましくは、0.010〜0.070%である。
Al: 0.1% or less
Al acts as a deoxidizer and is most commonly used in the high-strength steel deoxidation process. In addition, N is fixed as AIN during heat treatment, and the effect of maintaining the hardenability of B is also obtained. Such an effect is recognized when Al: 0.005% or more is contained. On the other hand, if the content exceeds 0.1%, the weld metal part is mixed during super-high heat input welding and the toughness of the weld metal part is lowered. For this reason, in this invention, Al was limited to 0.1% or less. In addition, Preferably, it is 0.010 to 0.070%.

Cu:0.1〜1.0%
Cuは、高靱性を保ちつつ強度を増加させることが可能な元素であり、HAZ靱性への悪影響も小さいため、高強度化のために有用な元素である。このような効果を得るためには、0.1%以上含有することが必要となる。一方、1.0%を超える含有は、熱間脆性を生じ、鋼板の表面性状を低下させる。このため、Cuは0.1〜1.0%の範囲に限定した。なお、好ましくは、0.2〜0.7%である。
Cu: 0.1-1.0%
Cu is an element that can increase the strength while maintaining high toughness, and has a small adverse effect on the HAZ toughness. Therefore, Cu is a useful element for increasing the strength. In order to acquire such an effect, it is necessary to contain 0.1% or more. On the other hand, the content exceeding 1.0% causes hot brittleness and lowers the surface properties of the steel sheet. For this reason, Cu was limited to the range of 0.1 to 1.0%. In addition, Preferably, it is 0.2 to 0.7%.

Ni:0.1〜2.0%
Niは、Cuと同様に、高靱性を保ちつつ強度を増加させることが可能な元素であり、HAZ靱性への悪影響も小さいため、高強度化のために有用な元素である。このような効果を得るためには0.1%以上の含有を必要とする。一方、2.0%を超えて含有しても、効果が飽和し、含有量に見合う効果が期待できなくなり、経済的に不利になる。このため、Niは0.1〜2.0%に限定した。なお、好ましくは、0.2〜1.7%である。
Ni: 0.1-2.0%
Ni, like Cu, is an element that can increase strength while maintaining high toughness, and has a small adverse effect on HAZ toughness, and is therefore an element useful for increasing strength. In order to obtain such an effect, a content of 0.1% or more is required. On the other hand, if the content exceeds 2.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, Ni was limited to 0.1 to 2.0%. In addition, Preferably, it is 0.2 to 1.7%.

Ti:0.005〜0.030%
Ti は、Nとの親和力が強く凝固時に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もHAZの高靱化に寄与する。このような効果を得るためには、Tiは、0.005%以上の含有を必要とする。一方、0.030%を超える含有は、逆にTiN が粗大化するため、上記した効果が期待できなくなる。このため、Tiは0.005〜0.030%に限定した。なお、好ましくは、0.010〜0.030%である。
Ti: 0.005-0.030%
Ti has a strong affinity for N and precipitates as TiN during solidification, thereby suppressing the austenite grain coarsening in the HAZ or contributing to the toughening of the HAZ as a ferrite transformation nucleus. In order to acquire such an effect, Ti needs to contain 0.005% or more. On the other hand, if the content exceeds 0.030%, TiN coarsens on the contrary, and the above effect cannot be expected. For this reason, Ti was limited to 0.005 to 0.030%. In addition, Preferably, it is 0.010 to 0.030%.

B:0.0003〜0.0050%
Bは、焼入れ性の向上を介して、鋼の強度を増加させる作用を有するとともに、TiN が固溶するような高温に晒される溶接融合部(ボンド部)近傍の粗粒域HAZではBNを形成して、固溶Nの低減とフェライト変態核としてHAZ靱性向上に寄与する。このような効果を得るためには、0.0003%以上の含有を必要とする。一方、0.0050%を超える含有は焼入れ性を著しく増加させ、母材の靱性、延性の劣化をもたらすとともに、降伏比の制御が困難となる。このため、本発明ではBは0.0003〜0.0050%の範囲に限定した。なお、好ましくは、0.0003〜0.0020%である。
B: 0.0003-0.0050%
B has the effect of increasing the strength of the steel through the improvement of hardenability, and forms BN in the coarse-grained area HAZ near the weld fusion part (bond part) exposed to a high temperature at which TiN dissolves. Therefore, it contributes to the reduction of solid solution N and the improvement of HAZ toughness as a ferrite transformation nucleus. In order to acquire such an effect, 0.0003% or more needs to be contained. On the other hand, if the content exceeds 0.0050%, the hardenability is remarkably increased, the toughness and ductility of the base material are deteriorated, and the yield ratio is difficult to control. For this reason, in the present invention, B is limited to the range of 0.0003 to 0.0050%. In addition, Preferably, it is 0.0003 to 0.0020%.

Ca:0.0005〜0.0050%
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、0.0050%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0.0005〜0.0050%に限定した。なお、本発明では、後述するように、好ましくはCa添加直前の溶存酸素量を0.0030%以下に調整した後、Caを添加して、Ca酸化物の生成を抑制して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に鋼中で微細且つ均一な分散が可能となる。このCaS 微細粒子はMnS 微細粒子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 0.0050%
Ca is an element that contributes to improving the ductility of steel by controlling the form of sulfide. In order to exert such an effect, it is necessary to contain at least 0.0005%, but even if it exceeds 0.0050%, the effect is saturated. For this reason, in this invention, Ca was limited to 0.0005 to 0.0050%. In the present invention, as will be described later, preferably, the amount of dissolved oxygen immediately before the addition of Ca is adjusted to 0.0030% or less, and then Ca is added to suppress the formation of Ca oxide to crystallize CaS. CaS crystallizes in molten steel at a lower temperature than oxides, so that fine and uniform dispersion is possible in the steel. These CaS fine particles are combined with MnS fine particles and act as ferrite transformation nuclei during welding, contributing to the improvement of HAZ toughness.

N:0.0030〜0.0060%
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0.0030%以上含有する必要がある。一方、0.0.0060%を超えて含有すると、溶接金属の靱性を低下させる。このため、Nは0.0030〜0.0.0060%に限定した。
N: 0.0030-0.0060%
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 0.0030% or more. On the other hand, if the content exceeds 0.0.0060%, the toughness of the weld metal is lowered. For this reason, N was limited to 0.0030-0.0.0060%.

O:0.0010〜0.0030%
Oは、不可避的不純物として含有し、鋼中では酸化物として存在して、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が、0.0010%未満とするためには精錬コストが多大となる。一方、0.0030%を超える含有は、CaO 系介在物が粗大化して、靱性に悪影響を及ぼす。このため、Oは0.0010〜0.0030%に限定した。
O: 0.0010 to 0.0030%
O is contained as an unavoidable impurity and is present as an oxide in the steel to lower the cleanliness. For this reason, although it is preferable to reduce as much as possible in this invention, in order to make it less than 0.0010%, refining cost becomes large. On the other hand, if the content exceeds 0.0030%, CaO inclusions become coarse, which adversely affects toughness. For this reason, O was limited to 0.0010 to 0.0030%.

ACR:0.2〜0.8
本発明では、Ca添加時の溶鋼中の溶存酸素量を0.0010〜0.0030%と調整した上で、Ca、SおよびOを次(1)式で定義されるACRを0.2〜0.8を満足するように添加、調整する。
ACR: 0.2-0.8
In the present invention, the amount of dissolved oxygen in molten steel at the time of Ca addition is adjusted to 0.0010 to 0.0030%, and the ACR defined by the following formula (1) is satisfied for Ca, S and O to satisfy 0.2 to 0.8. Add and adjust.

ACR={Ca−(0.18+130Ca)×O}/(1.25×S) ………(1)
ここで、Ca、O、S:各元素の含有量(質量%)
ACRが0.2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CaS 上へ析出しないことから、フェライト生成核として働かずHAZ靱性向上が期待できない。ACRが、0.2〜0.8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の組織が微細化され、HAZ靱性が向上する。
ACR = {Ca− (0.18 + 130Ca) × O} / (1.25 × S) (1)
Here, Ca, O, S: Content of each element (mass%)
When ACR is less than 0.2, 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 ACR exceeds 0.8, S which is fixed by Ca and becomes MnS is insufficient, and MnS does not precipitate on CaS, so that it does not work as a ferrite formation nucleus, and improvement in HAZ toughness cannot be expected. Only when the ACR satisfies 0.2 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, the HAZ structure is refined, and the HAZ toughness is improved.

Ceq:0.47%以下
本発明では、上記した成分組成範囲内で、さらに、次(2)式で定義される炭素当量Ceqが0.47%以下となるように、各成分の含有量を調整する。
Ceq: 0.47% or less In the present invention, the content of each component is adjusted so that the carbon equivalent Ceq defined by the following formula (2) is 0.47% or less within the above-described component composition range.

Ceq=C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5 ………(2)
ここで、Ceq:炭素当量(%)、
C、Mn、Cu、Ni、Cr、Mo、V:各元素の含有量(質量%)
次に、HAZ靭性におよぼすCeq の影響について、本発明者らが行った実験結果について、説明する。
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (2)
Where Ceq: carbon equivalent (%),
C, Mn, Cu, Ni, Cr, Mo, V: Content of each element (% by mass)
Next, the results of experiments conducted by the present inventors on the influence of Ceq on HAZ toughness will be described.

Ti:0.005〜0.025%、B:0.0004〜0.0025%を含有し、さらにACRを0.28〜0.73とし、Ceqを0.353〜0.495%に変化した厚鋼板から採取した再現熱サイクル試験片に、溶接入熱1000kJ/cmのエレクトロスラブ溶接のHAZの熱履歴を想定した再現熱サイクル(1400℃加熱、800〜500℃の冷却時間:1000s)を付与した。これら試験片からVノッチシャルピー衝撃試験片を採取し、0℃でのシャルピー吸収エネルギー(vE0)を求め、再現HAZ靱性を評価した。得られた結果を、vE0―Ceqの関係で図1に示す。   Ti: 0.005-0.025%, B: 0.0004-0.0025%, ACR was 0.28-0.73, Ceq was changed from 0.353-0.495% Reproduced thermal cycle test piece, welding heat input 1000kJ A reproducible thermal cycle (1400 ° C. heating, 800 to 500 ° C. cooling time: 1000 s) assuming a thermal history of HAZ of / scm electroslab welding was applied. V-notch Charpy impact test specimens were collected from these test specimens, Charpy absorbed energy (vE0) at 0 ° C. was determined, and reproducible HAZ toughness was evaluated. The obtained results are shown in FIG. 1 in the relationship of vE0-Ceq.

図1から、Ceqの増加により再現HAZ靱性(vE0)は低下する。これは、Ceqの増加により、組織が上部ベイナイト組織となり、島状マルテンサイトが増加するためと考えられる。また、図1から、400kJ/cmを超える超大入熱溶接において、0℃におけるシャルピー吸収エネルギーVで70J以上となる、優れたHAZ靭性を確保するためには、炭素当量Ceqを0.47%以下とする必要があることがわかる。 From FIG. 1, the reproducible HAZ toughness (vE0) decreases with increasing Ceq. This is presumably because the structure becomes an upper bainite structure due to an increase in Ceq, and island martensite increases. Also, from Fig. 1, in order to ensure excellent HAZ toughness with Charpy absorbed energy V E 0 at 0 ° C of 70 J or more in super high heat input welding exceeding 400 kJ / cm, carbon equivalent Ceq is 0.47% or less. It is understood that it is necessary to.

上記した基本組成に加えて、本発明では、さらに強度増加の目的で、必要に応じて、Cr:0.05〜0.50%、V:0.005〜0.01%、Mo:0.01〜0.30%のうちの1種または2種以上を含有することができる。Cr、V、Moはいずれも鋼の強度を増加させる元素であり、母材強度、溶接継手部強度の確保のため、必要に応じ選択して含有することができる。   In addition to the basic composition described above, in the present invention, for the purpose of further increasing the strength, one of Cr: 0.05 to 0.50%, V: 0.005 to 0.01%, Mo: 0.01 to 0.30% or Two or more kinds can be contained. Cr, V, and Mo are all elements that increase the strength of steel, and can be selected and contained as necessary to ensure the strength of the base material and the strength of the welded joint.

Crは、鋼の強度を増加させる元素であり、このような効果は、0.05%以上の含有で認められる。一方、0.50%を超える含有はHAZ靭性を劣化させる。このようなことから、Crは0.05〜0.50%の範囲に限定することが好ましい。   Cr is an element that increases the strength of steel, and such an effect is recognized with a content of 0.05% or more. On the other hand, the content exceeding 0.50% deteriorates the HAZ toughness. For these reasons, Cr is preferably limited to a range of 0.05 to 0.50%.

Vは、Crと同様に鋼の強度を増加させる元素であり、このような効果は、0.005%以上の含有で認められる。一方、0.01%を超える含有はHAZ靭性を劣化させる。このようなことから、Vは0.005〜0.01%の範囲に限定することが好ましい。   V, like Cr, is an element that increases the strength of steel, and such an effect is recognized with a content of 0.005% or more. On the other hand, the content exceeding 0.01% deteriorates the HAZ toughness. For these reasons, V is preferably limited to a range of 0.005 to 0.01%.

Moは、Cr,Vと同様に鋼の強度を増加させる元素であり、このような効果は、0.01%以上の含有で認められる。一方、0.30%を超える含有はHAZ靭性に悪影響を及ぼす。このため、Moは0.005〜0.01%の範囲に限定することが好ましい。   Mo, like Cr and V, is an element that increases the strength of steel, and such an effect is recognized with a content of 0.01% or more. On the other hand, the content exceeding 0.30% adversely affects the HAZ toughness. For this reason, it is preferable to limit Mo to the range of 0.005-0.01%.

本発明で使用する鋼素材は、上記した組成の溶鋼を、転炉等を用いる公知の溶製方法で溶製し、あるいはさらに、取鍋精錬等の精錬工程を経て、好ましくは連続鋳造法でスラブ等とすることが好ましい。   The steel material used in the present invention is a molten steel having the above composition, which is melted by a known melting method using a converter or the like, or further through a refining process such as ladle refining, preferably by a continuous casting method. A slab or the like is preferable.

得られた鋼素材を、1000〜1300℃の範囲の温度に加熱し、圧延終了温度を900℃以上とする熱間圧延を施し、所定の寸法、形状の厚鋼板とする。   The obtained steel material is heated to a temperature in the range of 1000 to 1300 ° C. and subjected to hot rolling at a rolling end temperature of 900 ° C. or higher to obtain a thick steel plate having a predetermined size and shape.

鋼素材の加熱温度が1000℃未満では、鋼素材の変形抵抗が高いために強圧することができず、板厚中心部まで十分圧下することが困難となる。特に、板厚が80mmを超える極厚鋼板の場合には、UT欠陥(ザク)が残存する恐れがある。一方、加熱温度が1300℃を超えると、加熱時のスケールによって表面疵を生じやすく、圧延後の手入れ負荷が増大する。そのため、鋼素材の加熱温度は1000〜1300℃の範囲の温度に限定した。   If the heating temperature of the steel material is less than 1000 ° C., the steel material has a high deformation resistance, so it cannot be strongly pressed, and it is difficult to sufficiently reduce it to the center of the plate thickness. In particular, in the case of an extremely thick steel plate having a plate thickness exceeding 80 mm, UT defects (zaku) may remain. On the other hand, if the heating temperature exceeds 1300 ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. Therefore, the heating temperature of the steel material was limited to a temperature in the range of 1000 to 1300 ° C.

鋼素材に施される熱間圧延は、圧延終了温度を900℃以上とする以外には、所定の板厚および形状を満足できればよく、その条件はとくに限定されない。なお、板厚が80mmを超える極厚鋼板の場合には、ザク圧着のために1パスあたりの圧下率が15%以上となる圧延パスを少なくとも1パス以上確保することが望ましい。   The hot rolling applied to the steel material is not particularly limited as long as the predetermined plate thickness and shape can be satisfied except that the rolling end temperature is 900 ° C. or higher. In the case of a very thick steel plate having a plate thickness exceeding 80 mm, it is desirable to secure at least one rolling pass at which the rolling reduction per pass is 15% or more for zaku pressure bonding.

圧延終了温度が、900℃未満では、変形抵抗が高くなりすぎて、圧延荷重が増大し圧延機への負荷が大きくなる。また、厚肉材を900℃未満まで圧延温度を低下させるためには、圧延途中で待機する必要があり、生産性を大きく阻害する。このため、本発明では、圧延終了温度を900℃以上とした。   If the rolling end temperature is less than 900 ° C., the deformation resistance becomes too high, the rolling load increases, and the load on the rolling mill increases. Moreover, in order to reduce the rolling temperature to less than 900 ° C., it is necessary to wait in the middle of rolling, which greatly impedes productivity. For this reason, in this invention, rolling completion temperature was 900 degreeC or more.

圧延終了後、厚鋼板は、直ちに1℃/s以上の冷却速度で、600℃以下まで加速冷却される。圧延終了後の冷却速度が1℃/s未満では、目標の引張強さ:590MPa以上の高強度を確保することができない。このため、本発明では、熱間圧延終了後の冷却速度を1℃/s以上とした。なお、熱間圧延終了後の冷却速度の上限は、冷却設備の能力により決定されるが、形状の観点から50℃/s程度とすることが好ましい。また、本発明でいう冷却速度は、厚鋼板の板厚1/4位置における、冷却開始から600℃までの平均冷却速度をいうものとする。   After the end of rolling, the thick steel plate is immediately accelerated to 600 ° C. or lower at a cooling rate of 1 ° C./s or higher. If the cooling rate after rolling is less than 1 ° C./s, it is not possible to secure a high strength of a target tensile strength of 590 MPa or more. For this reason, in this invention, the cooling rate after completion | finish of hot rolling was 1 degree-C / s or more. In addition, although the upper limit of the cooling rate after completion | finish of hot rolling is determined by the capability of cooling equipment, it is preferable to set it as about 50 degrees C / s from a viewpoint of a shape. Moreover, the cooling rate as used in the field of this invention shall mean the average cooling rate from the start of cooling to 600 degreeC in the 1/4 thickness position of a thick steel plate.

本発明では、加速冷却の冷却停止温度を、600℃以下とする。冷却停止温度が600℃を超えて高くなると、目標の引張強さ:590MPa以上の高強度を確保することができない。加速冷却終了後は、厚鋼板を大気中で室温まで放冷させるか、あるいは室温まで冷却することなく直に連続して、再加熱焼入れ−焼戻し処理を施してもよい。   In the present invention, the cooling stop temperature of accelerated cooling is set to 600 ° C. or lower. If the cooling stop temperature exceeds 600 ° C, a target tensile strength of 590 MPa or higher cannot be secured. After completion of accelerated cooling, the thick steel plate may be allowed to cool to room temperature in the atmosphere, or may be subjected to reheating quenching and tempering directly and continuously without cooling to room temperature.

加速冷却された厚鋼板は、ついで、再加熱焼入れ−焼戻し処理を施される。   The accelerated steel plate is then subjected to a reheating quenching-tempering treatment.

再加熱焼入れ処理は、フェライト+オーステナイトの2相域である、(Ac1変態点+10℃)〜(Ac1変態点+70℃)の範囲の再加熱温度に加熱したのち、急冷(焼入れ)する処理とする。この再加熱焼入れ処理は、硬質相と軟質相の強度差を十分与え、高強度で且つ低降伏比とするために重要なプロセスである。再加熱温度が、(Ac1変態点+10℃)未満では、再加熱時に生成するオーステナイト量が少なく、高強度を確保することが困難となる。一方、(Ac1変態点+70℃)を超えると、生成するオーステナイト量が多くなり、生成したオーステナイト相への炭素の濃化が希釈され、焼入れ性が低下する。このため、高強度および低降伏比が得られない。このようなことから、再加熱温度は(Ac1変態点+10℃)〜(Ac1変態点+70℃)の範囲の温度に限定した。なお、好ましくは、(Ac1変態点+10℃)〜(Ac1変態点+50℃)の範囲である。   The reheating quenching process is a process of heating to a reheating temperature in the range of (Ac1 transformation point + 10 ° C.) to (Ac1 transformation point + 70 ° C.), which is a two-phase region of ferrite and austenite, and then quenching (quenching). . This reheating quenching process is an important process in order to give a sufficient strength difference between the hard phase and the soft phase, and to achieve a high strength and a low yield ratio. When the reheating temperature is less than (Ac1 transformation point + 10 ° C.), the amount of austenite generated during reheating is small, and it is difficult to ensure high strength. On the other hand, if it exceeds (Ac1 transformation point + 70 ° C.), the amount of austenite to be generated increases, the concentration of carbon in the generated austenite phase is diluted, and the hardenability is lowered. For this reason, high strength and low yield ratio cannot be obtained. For this reason, the reheating temperature was limited to a range of (Ac1 transformation point + 10 ° C.) to (Ac1 transformation point + 70 ° C.). In addition, Preferably, it is the range of (Ac1 transformation point +10 degreeC)-(Ac1 transformation point +50 degreeC).

また、再加熱温度からの急冷(焼入れ)は、0.1℃/s以上の冷却速度で冷却することが好ましい。再加熱時に生成したオーステナイト相は炭素が濃化しているため焼入れ性が高く、0.1℃/s以上の冷却速度で冷却すれば、容易にマルテンサイト変態を生起させることができ、高強度化できる。   Further, the rapid cooling (quenching) from the reheating temperature is preferably performed at a cooling rate of 0.1 ° C./s or more. The austenite phase produced during reheating has high hardenability due to the concentration of carbon, and if it is cooled at a cooling rate of 0.1 ° C./s or more, martensitic transformation can easily occur and the strength can be increased.

上記した再加熱温度での保持時間は、5〜60min間とすることが好ましい。保持時間が5min未満では、オーステナイト相への炭素濃化が十分に行えない。一方、保持時間が60minを超えて長くなると、オーステナイト相への炭素の濃化が希釈され焼入性の低下による強度低下という不具合が生じる。このようなことから、再加熱温度における保持時間は、5〜60minとすることが望ましい。   The holding time at the above reheating temperature is preferably between 5 and 60 minutes. If the holding time is less than 5 minutes, carbon cannot be sufficiently concentrated in the austenite phase. On the other hand, if the holding time is longer than 60 min, the concentration of carbon in the austenite phase is diluted, resulting in a problem of strength reduction due to a decrease in hardenability. For this reason, the holding time at the reheating temperature is desirably 5 to 60 minutes.

再加熱後、急冷(焼入れ)された厚鋼板は、ついで焼戻し処理を施される。焼戻し温度は、板厚に応じて、すなわち、圧延後の加速冷却や再加熱焼入れ処理時の冷却速度の違いに応じて、決定され、とくに限定されないが、600℃以下で行うことが望ましい。焼戻し温度が600℃を越えると、強度低下が著しくなり、目標強度を満足することができなくなる。   After reheating, the rapidly cooled (quenched) thick steel plate is then tempered. The tempering temperature is determined according to the plate thickness, that is, according to the difference in cooling rate during the accelerated cooling after rolling or the reheating quenching process, and is not particularly limited, but is preferably performed at 600 ° C. or less. When the tempering temperature exceeds 600 ° C., the strength is remarkably reduced and the target strength cannot be satisfied.

上記した組成の鋼素材を用いて、上記した条件の熱間圧延、再加熱焼入れ−焼戻し処理を施すことにより、板厚100mmを超える極厚鋼板においても、超大入熱溶接時のHAZ靱性に優れ、かつ引張強さ:590MPa級で降伏比:80%以下の低降伏比を有する低降伏比高強度厚鋼板を容易に製造することができる。   By using the steel material having the composition described above, hot rolling under the above-mentioned conditions, reheating quenching-tempering treatment, it is excellent in HAZ toughness at the time of super-high heat input welding even in extra-thick steel plates exceeding 100 mm in thickness. In addition, it is possible to easily produce a high yield thick steel plate having a low yield ratio and a tensile strength: 590 MPa class and a yield ratio: 80% or less.

表1に示す組成の溶鋼を転炉で溶製し、取鍋で脱ガス処理を施したのち、連続鋳造法でスラブ(鋼素材:厚み310mm)とした。これら鋼素材に、表2に示す条件(加熱温度、圧延終了温度)の熱間圧延を施したのち、表2に示す条件(冷却速度、冷却停止温度)の加速冷却(冷却速度、冷却停止温度)を施し表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−焼戻し処理を施した。   Molten steel having the composition shown in Table 1 was melted in a converter, degassed with a ladle, and then made into a slab (steel material: thickness 310 mm) by a continuous casting method. These steel materials were subjected to hot rolling under the conditions shown in Table 2 (heating temperature, rolling end temperature), and then accelerated cooling (cooling speed, cooling stop temperature) under the conditions shown in Table 2 (cooling rate, cooling stop temperature). ) To obtain a thick steel plate having the thickness shown in Table 2. Subsequently, these thick steel plates were subjected to reheating quenching-tempering treatment under the conditions shown in Table 2.

得られた厚鋼板の1/2T部より、圧延方向に、JIS Z 2201の規定に準拠したJIS 4号引張試験片およびJIS Z 2202の規定に準拠したVノッチシャルピー衝撃試験片を採取し、母材の引張特性および0℃における吸収エネルギー(vE0 )(J)を求めた。 From the 1 / 2T part of the obtained thick steel plate, in the rolling direction, JIS No. 4 tensile test piece according to JIS Z 2201 and V-notch Charpy impact test piece according to JIS Z 2202 were collected. The tensile properties of the material and the absorbed energy (vE 0 ) (J) at 0 ° C. were determined.

得られた厚鋼板の一部から、さらに、再現熱サイクル試験片を採取した。これら再現熱サイクル試験片に、入熱1000kJ/cmのエレクトロスラグ溶接でのHAZの熱履歴をシミュレートした熱サイクル(1400℃加熱、800〜500℃の冷却時間:Δt800−500=1000s)を付与した。このような熱サイクルを付与した試験片から、JIS Z 2202の規定に準拠したVノッチシャルピー衝撃試験片(10mm厚)を採取し、0℃で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 )(J)を求め、再現HAZ靱性を調べた。 From a part of the obtained thick steel plate, a reproducible thermal cycle test piece was further collected. A thermal cycle (1400 ° C heating, 800-500 ° C cooling time: Δt 800-500 = 1000 s ) simulating the thermal history of HAZ in electroslag welding with a heat input of 1000 kJ / cm was applied to these reproduced thermal cycle test pieces. Granted. A V-notch Charpy impact test piece (10 mm thick) conforming to the provisions of JIS Z 2202 is collected from the test piece that has been subjected to such a thermal cycle, and a Charpy impact test is performed at 0 ° C. vE 0 ) (J) was determined and the reproduced HAZ toughness was examined.

また、得られた厚鋼板の一部から継手用試験板(板厚:60mm)を採取し、この継手用試験板をスキンプレート側に配し、ダイアフラム側に板厚:60mmの厚鋼板(60キロ鋼)を配し、ギャップ:25mmとしてエレクトロスラグ溶接(入熱900kJ/cm)を実施しT型溶接継手を作製した。なお、エレクトロスラグ溶接用ワイヤはJIS Z 3353 YES62相当品を、フラックスはJIS Z 3353 FS-FG3相当品を使用した。   In addition, a joint test plate (thickness: 60 mm) was sampled from a portion of the resulting thick steel plate, and this joint test plate was placed on the skin plate side, and a 60 mm thick steel plate (60 mm on the diaphragm side). (Kilo steel) was arranged, and electroslag welding (heat input 900 kJ / cm) was performed with a gap of 25 mm to produce a T-type welded joint. The electroslag welding wire was JIS Z 3353 YES62 equivalent, and the flux was JIS Z 3353 FS-FG3 equivalent.

得られた溶接継手の、スキンプレート側の各位置(溶接金属部溶接ボンド部、HAZ1mm、HAZ中央部)から、シャルピー衝撃試験片(Vノッチ試験片)を採取し、0℃で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 )(J)をもとめ、継手靭性を評価した。 Charpy impact test specimens (V-notch test specimens) are taken from each position on the skin plate side of the welded joint obtained (welded metal part weld bond part, HAZ 1 mm, HAZ center part) and subjected to a Charpy impact test at 0 ° C. The joint toughness was evaluated by calculating the absorbed energy (vE 0 ) (J) at 0 ° C.

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、TS:590MPa以上の十分な高強度と、降伏比80%以下の低降伏比とを有する優れた母材特性を有するとともに、再現HAZ靱性もvE0 で70J以上の優れた超大入熱溶接HAZ靭性を有する厚鋼板となっている。さらに、継手靱性も、溶接金属部、ボンド部およびHAZ全ての部位において、vE0 が70Jを上回る優れた超大入熱溶接継手靱性を示している。 The example of the present invention has excellent base material characteristics having a sufficiently high strength of TS: 590 MPa or more and a low yield ratio of 80% or less, and a reproducible HAZ toughness of vJ 0 of 70 J or more. It is a thick steel plate having heat input welding HAZ toughness. Furthermore, the joint toughness also shows excellent super-high heat input welded joint toughness with vE 0 exceeding 70 J in all parts of the weld metal part, bond part and HAZ.

一方、本発明範囲を逸脱する比較例では、母材性能が目標を満足していないか、あるいは再現HAZ靱性、溶接継手靱性が低く、目標の超大入熱溶接HAZ靭性を満足していない。   On the other hand, in the comparative example that departs from the scope of the present invention, the base material performance does not satisfy the target, or the reproduced HAZ toughness and the weld joint toughness are low, and the target super high heat input welding HAZ toughness is not satisfied.

図1は、再現HAZ靱性に及ぼす炭素当量(CeqW)の影響をグラフである。FIG. 1 is a graph of the effect of carbon equivalent (CeqW) on reproducible HAZ toughness.

Claims (3)

質量%で、
C:0.05〜0.15%、 Si:0.05〜0.50%、
Mn:0.6〜1.6%、 P:0.018%以下、
S:0.005%以下、 Al:0.1%以下、
Cu:0.1〜1.0%、 Ni:0.1〜2.0%、
Ti:0.005〜0.030%、 B:0.0003〜0.0050%、
Ca:0.0005〜0.0050%、 N:0.0030〜0.0060%、
O:0.0010〜0.0030%
を、下記(1)式で定義されるACRが0.2〜0.8%、かつ下記(2)式で定義される炭素当量Ceqが0.47%以下、となる範囲で含み、残部がFeおよび不可避的不純物からなる組成の鋼素材を、1000〜1300℃の範囲の温度に加熱し、圧延終了温度を900℃以上とする熱間圧延を施した後、1℃/s以上の冷却速度で600℃以下まで加速冷却を行い、厚鋼板としたのち、さらに該厚鋼板に、 (Ac1変態点+10℃)〜 (Ac1変態点+70℃)の2相域の再加熱温度に加熱したのち、急冷する再加熱焼入れを行い、ついで焼戻しを行う再加熱焼入れ−焼戻し処理を施すことを特徴とする、超大入熱溶接熱影響部靱性に優れる建築構造用低降伏比高強度厚鋼板の製造方法。

ACR={Ca−(0.18+130Ca)×O}/(1.25×S) ………(1)
ここで、Ca、O、S:各元素の含有量(質量%)
Ceq =C+Mn/6+Cu/15+Ni/15+Cr/5+Mo/5+V/5 ………(2)
ここで、Ceq:炭素当量(%)、
C、Mn、Cu、Ni、Cr、Mo、V:各元素の含有量(質量%)
% By mass
C: 0.05 to 0.15%, Si: 0.05 to 0.50%,
Mn: 0.6 to 1.6%, P: 0.018% or less,
S: 0.005% or less, Al: 0.1% or less,
Cu: 0.1-1.0%, Ni: 0.1-2.0%,
Ti: 0.005-0.030%, B: 0.0003-0.0050%,
Ca: 0.0005 to 0.0050%, N: 0.0030 to 0.0060%,
O: 0.0010 to 0.0030%
In the range where the ACR defined by the following formula (1) is 0.2 to 0.8% and the carbon equivalent Ceq defined by the following formula (2) is 0.47% or less, and the balance is Fe and inevitable impurities. The steel material with the composition is heated to a temperature in the range of 1000 to 1300 ° C, subjected to hot rolling to a rolling end temperature of 900 ° C or higher, and then accelerated to 600 ° C or lower at a cooling rate of 1 ° C / s or higher. After cooling to a thick steel plate, the steel plate is further heated to a reheating temperature in the two-phase region from (Ac1 transformation point + 10 ° C) to (Ac1 transformation point + 70 ° C), and then rapidly recooled and quenched. A method for producing a low yield ratio, high strength thick steel sheet for building structures, which is excellent in super-high heat input heat affected zone toughness, characterized by performing reheating quenching-tempering treatment that is then performed.
ACR = {Ca− (0.18 + 130Ca) × O} / (1.25 × S) (1)
Here, Ca, O, S: Content of each element (mass%)
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (2)
Where Ceq: carbon equivalent (%),
C, Mn, Cu, Ni, Cr, Mo, V: Content of each element (% by mass)
前記組成に加えてさらに、質量%で、Cr:0.05〜0.50%、V:0.005〜0.01%、Mo:0.01〜0.30%のうちの1種または2種以上を含有することを特徴とする請求項1に記載の建築構造用低降伏比高強度厚鋼板の製造方法。 In addition to the above composition, the composition further contains one or more of Cr: 0.05 to 0.50%, V: 0.005 to 0.01%, and Mo: 0.01 to 0.30% in mass%. The manufacturing method of the low yield ratio high-strength thick steel plate for building structures of 1. 前記再加熱温度で、5〜60min間保持することを特徴とする請求項1または2に記載の建築構造用低降伏比高強度厚鋼板の製造方法。 The method for producing a low yield ratio high strength thick steel sheet for building structure according to claim 1 or 2, wherein the reheating temperature is maintained for 5 to 60 minutes.
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