JP2005220379A - Method for producing non-heat treated high strength thick steel plate excellent in toughness at extra-large heat input weld heat-affected zone - Google Patents

Method for producing non-heat treated high strength thick steel plate excellent in toughness at extra-large heat input weld heat-affected zone Download PDF

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JP2005220379A
JP2005220379A JP2004027046A JP2004027046A JP2005220379A JP 2005220379 A JP2005220379 A JP 2005220379A JP 2004027046 A JP2004027046 A JP 2004027046A JP 2004027046 A JP2004027046 A JP 2004027046A JP 2005220379 A JP2005220379 A JP 2005220379A
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JP4539100B2 (en
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Keiji Ueda
圭治 植田
Tatsumi Kimura
達巳 木村
Toshiyuki Hoshino
俊幸 星野
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JFE Steel Corp
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<P>PROBLEM TO BE SOLVED: To produce a high strength thick steel plate excellent in toughness at an extra-large heat input weld heat-affected zone, which has ≥590 MPa tensile strength, ≤80% low yield ratio and excellent HAZ toughness even in an extra-large heat input welding of >400 KJ/cm, in a non-heat treatment. <P>SOLUTION: A steel block having the composition containing 0.05-0.13% C, and the suitable contents of Si, Mn, P, S and Al and further, 0.005-0.030% Ti, 0.0030-0.0070% N, 0.0005-0.0050% Ca, 0.0010-0.0030% O and 0.35-0.44% carbon equivalent Ceq and satisfying 0.2-0.8 ACR, is heated into the range of 1000-1300°C, and after applying hot-rolling at not lower than Ar<SB>3</SB>transformation point to the rolling-finish temperature, accelerating cooling treatment is applied till ≤600°C at 1-20°C/s average cooling speed. In this way, the thick steel plate having ≥590 MPa TS, ≤80% YR and showing the excellent HAZ toughness even in the case of such welding heat affected part of the extra-large heat input welding as to exceed 400 KJ/cm welding heat input, can be obtained with the non-heat treatment. <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、とくに引張強さ(TS)が590MPa以上で、降伏比80%以下を有し、かつ超大入熱溶接熱影響部靭性に優れた非調質高強度厚鋼板に関する。なお、本発明でいう厚鋼板とは、板厚20mm以上の鋼板を言うものとする。また、本発明でいう「超大入熱溶接」とは、溶接入熱が400kJ/cmを超える溶接をいうものとする。   The present invention relates to a thick steel plate suitable for use in a building structure, and in particular, it has a tensile strength (TS) of 590 MPa or more, a yield ratio of 80% or less, and an excellent adjustment of super-high heat input heat affected zone toughness. It relates to a high-quality, high-strength steel plate. In the present invention, the thick steel plate refers to a steel plate having a thickness of 20 mm or more. Further, “super large heat input welding” in the present invention refers to welding in which the welding heat input exceeds 400 kJ / cm.

近年、建築構造物の大型化と大スパン化に伴い、使用鋼材の厚肉化、高強度化が要望されている。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。   In recent years, with the increase in size and span of building structures, there has been a demand for thicker and higher strength steel used. 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.

このような建築構造物は、溶接接合によって所望形状の構造物に仕上げられる。しかし、兵庫県南部地震に際して指摘されているように、溶接構造物では、地震時のような急激でかつ大きな負荷荷重を受けると、十分な塑性変形が生じる前に、溶接部を主体に脆性破壊を生じる場合がある。このため、近年、溶接構造用鋼材には、溶接部をも含めて良好な靭性を具備することが求められている。例えば、柱−梁の大入熱溶接部については、0℃におけるシャルピー吸収エネルギーが70Jを超えるような、高い靭性を有することが要求されている。また、ボックス柱の溶接部にも同様の要求がある。   Such a building structure is finished into a desired shape structure by welding. However, as pointed out during the Hyogoken-Nanbu Earthquake, if a welded structure is subjected to an abrupt and large load such as during an earthquake, brittle fracture occurs mainly in the weld area before sufficient plastic deformation occurs. May occur. For this reason, in recent years, steel materials for welded structures are required to have good toughness including welds. For example, a column-beam large heat input weld is required to have high toughness such that Charpy absorbed energy at 0 ° C. exceeds 70 J. There are similar requirements for the welded portion of the box column.

一方、構造物の施行能率向上と施行コストの低減の観点から、溶接効率の向上が求められ、高能率の大入熱溶接の適用範囲が拡大している。例えば、建築構造用ボックス柱では、溶接入熱が400kJ/cmを超えるような超大入熱のサブマージアーク溶接やエレクトロスラグ溶接などが適用されるようになっている。   On the other hand, from the viewpoint of improving the efficiency of construction and reducing the cost of implementation, improvement in welding efficiency is required, and the application range of high-efficiency large heat input welding is expanding. For example, for box columns for building structures, supermerged heat input submerged arc welding, electroslag welding, or the like with a welding heat input exceeding 400 kJ / cm is applied.

一般に、鋼材に大入熱溶接を適用した際に、最も問題となるのは、溶接熱影響部(以下HAZともいう)のボンド部における靭性劣化である。このボンド部は、大入熱溶接時に溶融点直下の高温に曝されて、オーステナイトの結晶粒が最も粗大化し易く、また引き続く冷却によって、脆化な上部ベイナイト組織に変態し易い。さらに、このボンド部では、ウッドマンステッテン組織や島状マルテンサイトといった脆化組織が生成し易く、このことも靭性低下の原因となっている。とくに、引張強さが590MPaを超える高強度厚鋼板では、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り、HAZ靭性も低くなる。このため、低降伏比と優れたHAZ靭性とを兼備した高強度厚鋼板の開発が要望されている。   Generally, when high heat input welding is applied to a steel material, the most serious problem is toughness deterioration in the bond portion of the weld heat affected zone (hereinafter also referred to as HAZ). This bond portion is exposed to a high temperature just below the melting point during high heat input welding, and the austenite crystal grains are most likely to be coarsened, and the subsequent cooling is likely to transform into a brittle upper bainite structure. Further, in this bond portion, a brittle structure such as a woodman-stetten structure or island martensite is easily generated, which also causes a decrease in toughness. In particular, in a high-strength thick steel plate having a tensile strength exceeding 590 MPa, it is common to add a large amount of alloy to ensure the strength, so the yield ratio tends to increase and the HAZ toughness also decreases. For this reason, development of the high strength thick steel plate which combines the low yield ratio and the outstanding HAZ toughness is desired.

このような要望に対して、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、低降伏比高強度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、直ちに焼入れする直接焼入れ法であり、圧延後の冷却開始を遅らせ、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+硬質相の2相組織としている。これにより、高強度化と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保持させた後に急冷し、フェライト+硬質相の2相組織とすることにより、高強度化と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、焼入れした後、焼戻しを行い、高強度化と低降伏比化を達成している。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、高強度化と低降伏比化を達成できても、十分なHAZ靭性を有するまでには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-strength steel. The techniques described in Patent Document 1 and Patent Document 2 are both direct quenching methods in which quenching is performed immediately after rolling, the cooling start after rolling is delayed, and about 5 to 60% of ferrite is precipitated, followed by rapid cooling. Thus, it has a two-phase structure of ferrite phase + hard phase. As a result, high strength and low yield ratio are realized. On the other hand, the technique described in Patent Document 3 achieves high strength and low yield ratio by quenching after being kept in the ferrite precipitation temperature range and by forming a two-phase structure of ferrite + hard phase. . Moreover, in the technique described in Patent Document 4, after hot-rolling the steel sheet, it is heated again to the two-phase region of ferrite + austenite, and after quenching, tempering is performed to increase the strength and lower the yield ratio. Has achieved. However, with the techniques described in Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4, it is possible to achieve sufficient HAZ toughness even if high strength and low yield ratio can be achieved. There is a problem that has not been reached.

このような問題に対して、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靭性を向上させる技術が提案されている。特許文献5には、100 kJ/cmの溶接ボンド部靭性の改善を目指し、希土類元素とTiとを複合添加して、鋼中に微細粒子を分散させてオーステナイト粒成長を抑制し、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HAZの高靭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強度鋼板が提案されている。   For such problems, for example, Patent Literature 5, Patent Literature 6, Patent Literature 7, and Patent Literature 8 propose techniques for improving the HAZ toughness of high heat input welding. In Patent Document 5, aiming to improve the weld bond toughness of 100 kJ / cm, a rare earth element and Ti are added in combination, and fine particles are dispersed in the steel to suppress austenite grain growth. Techniques for improving the toughness of steel have been proposed. Patent Document 6 proposes a technique for finely dispersing Ti oxide to increase the toughness of the high heat input HAZ. Patent Document 7 proposes a technique for improving the toughness of high heat input 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 Al amount are contained, and further, Ca oxide is used as a fine oxide, and HAZ toughness in super-high heat input welding. High-strength steel sheets that improve the strength have been proposed.

しかし、特許文献5、特許文献6、特許文献7、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 Document 5, Patent Document 6, Patent Document 7, and Patent Document 8, the tensile strength is a high strength of 590 MPa or more and the base material yield ratio is a low yield ratio of 80% or less. Furthermore, it has been difficult to stably maintain excellent HAZ toughness even in super-high heat input welding in which the heat input of welding exceeds 400 kJ / cm.
Japanese Patent Publication No.58-10442 JP-A 62-77419 JP-A-2-34721 Japanese Patent Laid-Open No. 4-99817 JP-A-60-184663 JP-A-57-51243 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靭性を有する、超大入熱溶接熱影響部靭性に優れた非調質高強度厚鋼板の製造方法を提案することを目的とする。本発明でいう、「超大入熱溶接熱影響部靭性に優れた」とは、溶接入熱量が400kJ/cmを超える超大入熱溶接熱影響部の、0℃におけるシャルピー吸収エネルギーvE0が70J以上を有する場合をいうものとする。 The present invention solves the above-mentioned problems of the prior art, and has a tensile strength of 590 MPa or more and a low yield ratio of 80% or less, and also in super-high heat input welding in which the welding heat input exceeds 400 kJ / cm. It aims at proposing the manufacturing method of the non-tempered high-strength thick steel plate which was excellent in the super-high heat-input welding heat affected zone toughness which has the outstanding HAZ toughness. In the present invention, “excellent toughness of the super-high heat input welding heat-affected zone” means that the Charpy absorbed energy vE 0 at 0 ° C. of the super-high heat input welding heat-affected zone having a welding heat input exceeding 400 kJ / cm is 70 J or more. The case where it has.

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HAZ靭性に及ぼす各種要因について鋭意研究した。その結果、溶接入熱量が400kJ/cmを超える超大入熱HAZにおいて高靭性を確保するためには、溶接時に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分散が重要であることを知見した。そのためには、厳格な成分調整により、TiNを微細分散させることおよび、溶製時Ca添加の際の溶存酸素量を0.0010〜0.0050%に調整したうえで、Ca、S、Oの添加量をACRが0.2〜0.8%を満足するように調整することが、オーステナイト粒の粗大化を抑制しフェライト変態を促進するために肝要であることを知見した。また、母材の引張強さを590MPa以上とし、かつ母材強度と靭性を両立させるためには、厳格な成分調整とともに、炭素当量Ceqを0.35〜0.44%の範囲とすることが肝要であることも知見した。   In order to achieve the above-mentioned problems, the present inventors have intensively studied various factors affecting strength, yield ratio, and HAZ toughness. As a result, in order to ensure high toughness in the super-high heat input HAZ with a welding heat input exceeding 400 kJ / cm, the austenite grain coarsening is suppressed in the region heated to a high temperature during welding, and ferrite transformation is promoted during cooling We found that the dispersion of transformation nuclei is important. To achieve this, fine dispersion of TiN by strict component adjustment and adjustment of the amount of dissolved oxygen at the time of melting Ca addition to 0.0010 to 0.0050%, and the addition amount of Ca, S, O is ACR It has been found that adjusting to satisfy 0.2 to 0.8% is essential to suppress coarsening of austenite grains and promote ferrite transformation. In addition, in order to make the tensile strength of the base metal to be 590 MPa or more and to achieve both the base material strength and toughness, it is important to adjust the carbon equivalent Ceq in the range of 0.35 to 0.44% in addition to strict component adjustment. Also found out.

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を施すことにより、上記した優れた超大入熱HAZ靭性と、引張強さで590MPa以上を有し、かつ80%以下の低降伏比を有し、かつ高靭性の母材特性とを兼備させることができることを知見した。   Furthermore, after performing hot rolling on the steel material whose components are adjusted as described above, it is subjected to accelerated cooling treatment in which the cooling rate and the cooling stop temperature are optimized, so that the excellent super heat input HAZ toughness and the tensile strength described above are obtained. It has been found that it has a strength of 590 MPa or more, a low yield ratio of 80% or less, and a high toughness base material characteristic.

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、本発明は、mass%で、C:0.05〜0.13%、Si:0.05〜0.50%、Mn:0.5〜2.0%、P:0.02%以下、S:0.0050%以下、Al:0.1%以下、Ti:0.005〜0.030%、N:0.0030〜0.0070%、Ca:0.0005〜0.0050%、O:0.0010〜0.0030%を含有し、次(1)式
Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 ………(1)
(ここで、C、Mn、Ni、Cu、Cr、Mo、V:各元素の含有量(質量%))
で定義される炭素当量Ceqが0.35〜0.44%で、かつ次(2)式
ACR={Ca−(0.18+130×Ca)×O}/(1.25×S) …………………(2)
(ここで、Ca、O、S:各元素の含有量(質量%))
で定義されるACRが0.2〜0.8を満足し、残部がFeおよび不可避的不純物からなる組成を有する鋼素材を、1000℃〜1300℃の範囲に加熱し、圧延終了温度がAr変態点以上となる熱間圧延を施した後、1〜20℃/sの平均冷却速度で600℃以下の温度まで冷却する加速冷却処理を施すことを特徴とする引張強さ:590MPa以上、降伏比:80%以下を有する超大入熱溶接熱影響部靭性に優れた非調質高強度厚鋼板の製造方法であり、また、本発明では、前記組成に加えてさらに、mass%で、Cu:1.0%以下、Ni:2.0%以下のうちから選ばれた1種または2種を含有する組成とすることが好ましく、また、本発明では、前記各組成に加えてさらに、mass%で、Cr:0.7%以下、Mo:1.0%以下、Nb:0.05%以下、V:0.2%以下のうちから選ばれた1種または2種以上を含有する組成とすることが好ましく、また、本発明では、前記各組成に加えてさらに、mass%で、REM:0.02%以下、Mg:0.005%以下のうちから選ばれた1種または2種を含有する組成とすることが好ましい。
The present invention has been completed based on the above findings and further studies. That is, the present invention is mass%, C: 0.05 to 0.13%, Si: 0.05 to 0.50%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.0050% or less, Al: 0.1% or less, Ti : 0.005 to 0.030%, N: 0.0030 to 0.0070%, Ca: 0.0005 to 0.0050%, O: 0.0010 to 0.0030%, the following formula (1)
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
(Here, C, Mn, Ni, Cu, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by is 0.35-0.44%, and the following formula (2)
ACR = {Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) ………………… (2)
(Where Ca, O, S: content of each element (mass%))
A steel material having a composition defined by ACR of 0.2 to 0.8 and the balance consisting of Fe and inevitable impurities is heated to a range of 1000 ° C to 1300 ° C, and the rolling end temperature is Ar 3 transformation point or higher. After being subjected to hot rolling, it is subjected to accelerated cooling treatment to cool to a temperature of 600 ° C. or lower at an average cooling rate of 1 to 20 ° C./s, tensile strength: 590 MPa or more, yield ratio: 80% In addition to the above composition, in addition to the above composition, Cu is 1.0% or less, and Cu: 1.0% or less. Ni: It is preferable to set it as the composition containing 1 type or 2 types chosen from 2.0% or less. Moreover, in this invention, in addition to each said composition, it is mass%, Cr: 0.7% or less, Contains one or more selected from Mo: 1.0% or less, Nb: 0.05% or less, V: 0.2% or less Further, in the present invention, in addition to each of the above-mentioned compositions, the composition further contains one or two selected in mass% from REM: 0.02% or less and Mg: 0.005% or less. A composition is preferred.

本発明によれば、引張強さが590MPa以上で、降伏比が80%以下の低降伏比を有し、
400kJ/cmを超える超大入熱溶接HAZ靱性に優れた高強度厚鋼板を安価に製造することができ、鋼構造物の大型化や、鋼構造物の耐震性の向上、施工効率の向上に大きく寄与し、産業上格段の効果を奏する。
According to the present invention, the tensile strength is 590 MPa or more, and the yield ratio is 80% or less.
Super high heat input welding exceeding 400kJ / cm High strength thick steel plate with excellent HAZ toughness can be manufactured at low cost, greatly increasing the size of steel structures, improving the earthquake resistance of steel structures, and improving construction efficiency Contributes and has a remarkable industrial effect.

まず、本発明で使用する鋼素材の成分限定理由について具体的に説明する。なお、成分に関する%表示は、特に断らない限りmass%を意味するものとする。   First, the reasons for limiting the components of the steel material used in the present invention will be specifically described. In addition, unless otherwise indicated, the% display regarding a component shall mean mass%.

C:0.05〜0.13%
Cは、鋼の強度を増加させ、構造用鋼材として必要な強度を確保するのに有用な元素であり、この発明では、上記した効果を得るために、0.05%以上の含有を必要とする。一方、0.13%を超える含有は、HAZ靱性、耐溶接割れ性を劣化させる。このため、Cは0.05〜0.13%の範囲に限定した。なお、好ましくは、0.05〜0.12%である。
C: 0.05-0.13%
C is an element useful for increasing the strength of steel and ensuring the strength required as a structural steel material. In the present invention, the content of 0.05% or more is required to obtain the above-described effects. On the other hand, the content exceeding 0.13% degrades the HAZ toughness and weld crack resistance. For this reason, C was limited to the range of 0.05 to 0.13%. 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、少なくとも0.05%必要であるが、0.50%を超えて含有すると、母材の靱性が劣化するとともに、超大入熱HAZにおいて島状マルテンサイトが生成し、HAZ靱性が顕著に劣化する。このため、Siは0.05〜0.50%の範囲に限定した。なお、好ましくは、0.05〜0.35%である。
Si: 0.05-0.50%
Si acts as a deoxidizer and needs to be at least 0.05% for steelmaking. However, if it exceeds 0.50%, the toughness of the base material deteriorates and island martensite is generated in the super-high heat input HAZ. , HAZ toughness is significantly deteriorated. 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:0.5〜2.0%
Mnは、鋼の強度を増加させる作用を有しており、この発明では、引張強さ590MPa以上を確保するために、0.5%以上の含有を必要とする。一方、2.0%を超えて含有すると、母材の靱性およびHAZ靱性が著しく劣化する。このため、Mnは0.5〜2.0%の範囲に限定した。なお、好ましくは、0.6〜1.5%である。
Mn: 0.5-2.0%
Mn has the effect of increasing the strength of steel, and in the present invention, it is necessary to contain 0.5% or more in order to ensure a tensile strength of 590 MPa or more. On the other hand, if the content exceeds 2.0%, the toughness and the HAZ toughness of the base material deteriorate significantly. For this reason, Mn was limited to the range of 0.5 to 2.0%. In addition, Preferably, it is 0.6 to 1.5%.

P:0.02%以下
Pは、鋼の強度を増加させ靱性を劣化させる元素であり、とくに溶接部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。Pが0.02%を超えて含有されるとこの傾向が顕著となるため、0.02%を上限とした。なお、過度のP低減は、精錬コストを高騰させ経済的に不利となるため、0.005%以上とすることが望ましい。
P: 0.02% or less P is an element that increases the strength of steel and deteriorates toughness. In particular, P is desirably reduced as much as possible in order to deteriorate the toughness of welds. This tendency becomes remarkable when P exceeds 0.02%, so 0.02% was made the upper limit. In addition, since excessive P reduction raises refining cost and becomes economically disadvantageous, it is desirable to set it as 0.005% or more.

S:0.0050%以下
Sは、Caと結合してCaS粒子として凝固段階で微細に晶出する。これらの微細粒子は熱間圧延後の冷却時に、フェライト変態核として作用し、加速冷却後の組織を軟質のフェライト相と硬質のベイナイト相の二相組織とし、母材の低降伏比化に寄与する。さらに、溶接時には、CaS粒子上にMnSとして析出し、フェライト変態核として作用し、溶接部靱性を向上させる効果を有する。このような効果は、0.0005%以上の含有で顕著に認められる。一方、0.0050%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0.0050%以下に限定した。
S: 0.0050% or less S is finely crystallized at the solidification stage as CaS particles by combining with Ca. These fine particles act as ferrite transformation nuclei during cooling after hot rolling, and the structure after accelerated cooling is made into a two-phase structure of soft ferrite phase and hard bainite phase, contributing to lower yield ratio of the base metal To do. Furthermore, at the time of welding, it precipitates as MnS on CaS particles, acts as a ferrite transformation nucleus, and has the effect of improving the weld zone toughness. Such an effect is noticeable when the content is 0.0005% or more. On the other hand, if the content exceeds 0.0050%, the toughness of the base metal and the welded portion is deteriorated. For this reason, S was limited to 0.0050% or less.

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おいて、もっとも汎用的に使われる。また、鋼中のNをAlNとして固定し、結晶粒の粗大化を防止し母材の靭性向上に寄与する。このような効果は0.005%以上の含有で顕著に認められる。一方、0.1%を超える含有は、母材の靱性が低下するとともに、溶接時に溶接金属部に混入して、靱性を劣化させる。このため、Alは0.1%以下に限定した。なお、好ましくは、0.01〜0.07%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 in the steel is fixed as AlN, which prevents coarsening of crystal grains and contributes to improvement of the toughness of the base material. Such an effect is noticeable when the content is 0.005% or more. On the other hand, when the content exceeds 0.1%, the toughness of the base material is lowered, and the toughness is deteriorated by being mixed into the weld metal part during welding. For this reason, Al was limited to 0.1% or less. In addition, Preferably, it is 0.01 to 0.07%.

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、HAZでのオーステナイト粒の粗大化抑制、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。また、熱間圧延後の加速冷却時においては、溶製段階で形成されたTiNがフェライト相を生成する変態核として作用し、加速冷却後の組織を軟質のフェライト相と硬質のベイナイト相の二相組織として、母材の低降伏比化に寄与する。このような効果を得るためには、0.005%以上の含有が必要である。一方、0.030%を超えて含有すると、TiN粒子が粗大化し、上記した効果が期待できなくなる。このため、Tiは0.005〜0.030%の範囲に限定した。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, and contributes to the suppression of coarsening of austenite grains in HAZ, or to strengthening HAZ as a ferrite transformation nucleus. Also, during accelerated cooling after hot rolling, TiN formed in the melting stage acts as a transformation nucleus that generates a ferrite phase, and the structure after accelerated cooling is divided into a soft ferrite phase and a hard bainite phase. As phase structure, it contributes to lower yield ratio of the base metal. In order to acquire such an effect, 0.005% or more needs to be contained. On the other hand, if the content exceeds 0.030%, the TiN particles become coarse and the above-described effects cannot be expected. For this reason, Ti was limited to 0.005 to 0.030% of range. In addition, Preferably, it is 0.010 to 0.030%.

N:0.0030〜0.0070%
Nは、Tiと結合してTiNとして析出して、HAZでのオーステナイト粒の粗大化を抑制し、あるいはフェライト変態核としてHAZの高靱化に寄与する。また、熱間圧延後の加速冷却時においては、溶製段階に形成されたTiNがフェライト相を生成する変態核として作用し、加速冷却後の組織を軟質のフェライト相と硬質のベイナイト相の二相組織として、母材の低降伏比化に寄与する。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、0.0070%を超えて含有すると、TiNが溶解する温度まで加熱されるHAZでは、固溶N量が増加し、HAZ靱性が著しく低下する。このため、Nは0.0030〜0.0070%に限定する。
N: 0.0030-0.0070%
N combines with Ti and precipitates as TiN to suppress the coarsening of the austenite grains in the HAZ or contribute to the toughening of the HAZ as a ferrite transformation nucleus. Also, during accelerated cooling after hot rolling, TiN formed in the melting stage acts as a transformation nucleus that produces a ferrite phase, and the structure after accelerated cooling is divided into a soft ferrite phase and a hard bainite phase. As phase structure, it contributes to lower yield ratio of the base metal. In order to secure the necessary amount of TiN having such an effect, N needs to be contained by 0.0030% or more. On the other hand, when the content exceeds 0.0070%, in HAZ heated to a temperature at which TiN is dissolved, the amount of solid solution N increases and the HAZ toughness significantly decreases. For this reason, N is limited to 0.0030-0.0070%.

Ca:0.0005〜0.0050%
Caは、硫化物の形態制御を介して、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%の範囲に限定した。なお、本発明では、後述するように、Ca添加直前の溶存酸素量を0.0050%以下に調整した後、Caを添加して、Ca酸化物の生成を抑制して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、鋼中で微細かつ均一な分散が可能となる。このような微細なCaS粒子はMnSと複合して溶接時にフェライト変態核として作用し、HAZ靱性の向上に寄与する。さらに、微細なCaS粒子は、熱間圧延後の加速冷却時に、フェライト変態核として作用し、加速冷却後の組織を、軟質のフェライト相と硬質のベイナイト相の二相組織とし、母材の低降伏化に寄与する。
Ca: 0.0005 to 0.0050%
Ca is an element that contributes to improving the ductility of steel through the form control 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% of range. In the present invention, as will be described later, after adjusting the dissolved oxygen content immediately before the addition of Ca to 0.0050% or less, Ca is added to suppress the formation of Ca oxide and to crystallize CaS. CaS crystallizes in molten steel at a temperature lower than that of oxides, so that fine and uniform dispersion is possible in the steel. Such fine CaS particles are combined with MnS and act as ferrite transformation nuclei during welding, contributing to the improvement of HAZ toughness. Furthermore, fine CaS particles act as ferrite transformation nuclei during accelerated cooling after hot rolling, and the structure after accelerated cooling is made into a two-phase structure of a soft ferrite phase and a hard bainite phase. Contributes to yielding.

O:0.0010〜0.0030%
Oは、不可避的不純物として含有され、鋼中では酸化物として存在し、清浄度を低下させる。このため、本発明では、できるだけ低減することが好ましいが、過度の低減は精錬コストの高騰を招くため、0.0010%以上に限定した。一方、酸素含有量が0.0030%を超えると、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、本発明ではOを0.0010〜0.0030%の範囲に限定した。なお、本発明では、CaをCaSとして晶出させるために、Caとの結合力が強いOは、Ca添加前に、脱ガスを強化するか脱酸剤を投入して、溶鋼中のOを0.0050%以下に低減しておくことが好ましい。
O: 0.0010 to 0.0030%
O is contained as an unavoidable impurity and is present as an oxide in the steel, reducing the cleanliness. For this reason, in the present invention, it is preferable to reduce as much as possible. However, since excessive reduction leads to an increase in refining cost, it is limited to 0.0010% or more. On the other hand, when the oxygen content exceeds 0.0030%, CaO inclusions are coarsened and adversely affect toughness. For this reason, in this invention, O was limited to 0.0010 to 0.0030% of range. In the present invention, in order to crystallize Ca as CaS, O having a strong binding force with Ca strengthens degassing or introduces a deoxidizing agent before adding Ca, so that O in molten steel is It is preferable to reduce it to 0.0050% or less.

炭素当量Ceq:0.35〜0.44%
本発明では、上記した成分範囲内で、さらに、炭素当量Ceqが0.35〜0.44%となるように、各成分の含有量を調整する。本発明で使用する炭素当量Ceqは、次(1)式
Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 ………(1)
(ここで、C、Mn、Ni、Cu、Cr、Mo、V:各元素の含有量(mass%))
で定義される。
Carbon equivalent Ceq: 0.35-0.44%
In the present invention, the content of each component is adjusted so that the carbon equivalent Ceq is 0.35 to 0.44% within the above-described component range. The carbon equivalent Ceq used in the present invention is represented by the following formula (1):
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
(Here, C, Mn, Ni, Cu, Cr, Mo, V: content of each element (mass%))
Defined by

Ceqが0.35%未満では、圧延後加速冷却時の焼入れ性が不足し、加速冷却後の組織がフェライト主体組織となるために、所望の引張強さ590MPa以上を確保できなくなる。一方、Ceqが0.44%を超えると、圧延後加速冷却後の母材靭性が著しく劣化する。このため、本発明では、Ceqは0.35〜0.44%の範囲に限定した。   If Ceq is less than 0.35%, the hardenability at the time of accelerated cooling after rolling becomes insufficient, and the structure after accelerated cooling becomes a ferrite main structure, so that a desired tensile strength of 590 MPa or more cannot be secured. On the other hand, if Ceq exceeds 0.44%, the base metal toughness after accelerated cooling after rolling deteriorates significantly. For this reason, in the present invention, Ceq is limited to a range of 0.35 to 0.44%.

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0.0010〜0.0050%と調整した上で、Ca、SおよびOを次(2)式
ACR={Ca−(0.18+130×Ca)×O}/(1.25×S) (2)
(ここで、Ca、O、S:各元素の含有量(mass%))
で定義されるACRを0.2〜0.8を満足するように調整する。
ACR: 0.2 to 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 0.0050%, and then Ca, S and O are expressed by the following formula (2):
ACR = {Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) (2)
(Ca, O, S: content of each element (mass%))
The ACR defined by is adjusted to satisfy 0.2 to 0.8.

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か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靱性の向上にも寄与しない。一方、ACRが0.8を超えると、SがCaによって固定されMnSとなるSが不足し、MnSがフェライト生成核として働くCaS上へ析出しないことから、HAZ靱性の向上が期待できない。ACRが、0.2〜0.8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組織が微細化されてHAZ靱性が向上する。   When ACR is less than 0.2, Ca does not crystallize, so S precipitates in the form of MnS alone. Since this MnS is elongated by rolling during the production of the steel sheet and does not disperse uniformly and finely, it causes a decrease in the toughness of the base metal and does not contribute to an improvement in the weld HAZ toughness. On the other hand, when the ACR exceeds 0.8, S is fixed by Ca and S which becomes MnS is insufficient, and MnS does not precipitate on CaS that acts as a ferrite nuclei. Therefore, 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.

本発明では、上記した基本成分に加えてさらに、Cu:1.0%以下、Ni:2.0%以下のうちから選ばれた1種または2種、および/または、Cr:0.7%以下、Mo:1.0%以下、Nb:0.05%以下、V:0.2%以下のうちから選ばれた1種または2種以上、および/または、REM:0.02%以下、Mg:0.005%以下のうちから選ばれた1種または2種を含有することができる。   In the present invention, in addition to the basic components described above, one or two selected from Cu: 1.0% or less, Ni: 2.0% or less, and / or Cr: 0.7% or less, Mo: 1.0% Hereinafter, Nb: 0.05% or less, V: One or more selected from 0.2% or less, and / or REM: 0.02% or less, Mg: 0.005% or less Two types can be contained.

Cu:1.0%以下、Ni:2.0%以下のうちから選ばれた1種または2種
Cu、Niはいずれも、高靱性を保ちつつ強度を増加させることが可能で、しかもHAZ靱性への影響も小さいため、高強度化のために有用な元素であり、必要に応じ選択して含有できる。このような効果は、Cu:0.1%以上、Ni:0.1%以上の含有で顕著となる。一方、Cu:1.0%を超える含有は、熱間脆性が生じ鋼板の表面性状が劣化する。また、Ni: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、Cuは1.0%以下、Niは2.0%以下に限定することが好ましい。なお、より好ましくは、Cu:0.2〜0.7%、Ni:0.2〜1.7%である。
One or two selected from Cu: 1.0% or less, Ni: 2.0% or less
Both Cu and Ni can increase strength while maintaining high toughness, and also have a small effect on HAZ toughness. Therefore, Cu and Ni are useful elements for increasing strength. it can. Such an effect becomes remarkable when Cu: 0.1% or more and Ni: 0.1% or more are contained. On the other hand, if the Cu content exceeds 1.0%, hot brittleness occurs and the surface properties of the steel sheet deteriorate. Moreover, even if it contains Ni exceeding 2.0%, an effect will be saturated and the effect corresponding to content will not be expectable, but it becomes economically disadvantageous. For this reason, it is preferable to limit Cu to 1.0% or less and Ni to 2.0% or less. More preferably, Cu is 0.2 to 0.7% and Ni is 0.2 to 1.7%.

Cr:0.7%以下、Mo:1.0%以下、Nb:0.05%以下、V:0.2%以下のうちから選ばれた1種または2種以上
Cr、Mo、Nb、Vは、いずれも鋼の強度向上に寄与する元素であり、必要に応じ選択して含有できる。このような効果は、Cr:0.05%以上、Mo:0.05%以上、Nb:0.005%以上、V:0.01%以上の含有で顕著となる。一方、Cr:0.7%を超える含有はHAZ靱性を劣化させ、Mo:1.0%を超える含有は、母材靭性およびHAZ靱性を劣化させ、Nb:0.05%を超える含有は、母材靭性およびHAZ靱性を劣化させ、V:0.2%を超える含有は、HAZ靱性を劣化させる。このため、Crは0.7%以下、Moは1.0%以下、Nbは0.05%以下、Vは0.2%以下に限定することが望ましい。
One or more selected from Cr: 0.7% or less, Mo: 1.0% or less, Nb: 0.05% or less, V: 0.2% or less
Cr, Mo, Nb, and V are all elements that contribute to improving the strength of steel, and can be selected and contained as necessary. Such an effect becomes remarkable when Cr: 0.05% or more, Mo: 0.05% or more, Nb: 0.005% or more, and V: 0.01% or more. On the other hand, when Cr exceeds 0.7%, HAZ toughness deteriorates. When Mo exceeds 1.0%, the base metal toughness and HAZ toughness deteriorate. When Nb exceeds 0.05%, the base metal toughness and HAZ toughness deteriorate. When the content exceeds V: 0.2%, the HAZ toughness deteriorates. Therefore, it is desirable to limit Cr to 0.7% or less, Mo to 1.0% or less, Nb to 0.05% or less, and V to 0.2% or less.

REM:0.02%以下、Mg:0.005%以下のうちから選ばれた1種または2種
REMおよびMgは、いずれも靱性向上に寄与する元素であり、必要に応じて選択して含有できる。このような効果は、REM:0.002%以上の含有で顕著となる。一方、0.02%を超えて含有しても効果が飽和するため、REMは0.02%を上限とした。Mgは、結晶粒の微細化を介して靱性を向上する有用な元素であり、このような効果はMg:0.001%以上の含有で顕著となるが、0.005%を超えて含有しても効果が飽和するため、0.005%を上限とした。
One or two selected from REM: 0.02% or less, Mg: 0.005% or less
REM and Mg are both elements that contribute to toughness improvement, and can be selected and contained as necessary. Such an effect becomes remarkable when the content of REM is 0.002% or more. On the other hand, even if the content exceeds 0.02%, the effect is saturated, so REM has an upper limit of 0.02%. Mg is a useful element that improves toughness through refinement of crystal grains, and this effect becomes significant when Mg is contained in an amount of 0.001% or more, but even if contained in excess of 0.005%, the effect is significant. Because of saturation, 0.005% was made the upper limit.

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

本発明で使用する鋼素材の製造方法は、とくに限定する必要はなく、通常の溶製方法、鋳造方法がいずれも適用できる。   The method for producing the steel material used in the present invention is not particularly limited, and any ordinary melting method and casting method can be applied.

上記した組成の溶鋼を、通常の溶製方法である転炉、電気炉、真空溶解炉等で溶製し、脱酸処理や脱ガスプロセスを用いて、ガス成分の制御を行った後、CaSiワイヤの添加により介在物制御を行ったうえで、連続鋳造法などの通常の鋳造方法で鋼素材(スラブ)とすることが好ましい。   The molten steel with the above composition is melted in a normal melting method such as a converter, electric furnace, vacuum melting furnace, etc., and after controlling the gas components using a deoxidation treatment or degassing process, the CaSi It is preferable to use a steel material (slab) by a normal casting method such as a continuous casting method after controlling inclusions by adding wires.

なお、溶製時に、CaをCaSとして晶出させるために、Caとの結合力の強いOはCa添加前に、脱ガスを強化するか、脱酸剤を投入して、溶鋼中のOを0.0050%以下に低減しておくことが好ましい。また、本発明では、Ca添加時の溶鋼酸素量を0.0050%以下に調整したうえで、ACRが0.2〜0.8を満足するように、Ca、Sを添加、調整することが好ましい。   In addition, in order to crystallize Ca as CaS at the time of melting, O having a strong binding force with Ca strengthens degassing or introduces a deoxidizing agent before adding Ca, thereby adding O in molten steel. It is preferable to reduce it to 0.0050% or less. Moreover, in this invention, after adjusting the amount of molten steel oxygen at the time of Ca addition to 0.0050% or less, it is preferable to add and adjust Ca and S so that ACR may satisfy 0.2-0.8.

ついで、これらの鋼素材を1000℃〜1300℃に再加熱する。再加熱温度が1000℃未満では、熱間圧延での変形抵抗が高くなり、1パス当たりの圧下率が大きく取れなくなることから、圧延パス数が増加し、圧延能率の低下を招くとともに、鋼素材(スラブ)中の鋳造欠陥を圧着することができない場合がある。一方、再加熱温度が1300℃を超えると、加熱時のスケールによって表面疵が生じやすく、圧延後の手入れ負荷が増大する。このため、鋼素材の再加熱温度は1000〜1300℃の範囲に限定した。   Next, these steel materials are reheated to 1000 ° C to 1300 ° C. If the reheating temperature is less than 1000 ° C., the deformation resistance in hot rolling becomes high and the rolling reduction per pass cannot be made large. Therefore, the number of rolling passes increases, and the rolling efficiency decreases, and the steel material The casting defect in (slab) may not be crimped. On the other hand, when the reheating temperature exceeds 1300 ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. For this reason, the reheating temperature of the steel material was limited to a range of 1000 to 1300 ° C.

再加熱された鋼素材は、圧延終了温度がAr変態点以上となる熱間圧延を施され、所定の板厚の厚鋼板とされる。熱間圧延条件は、圧延終了温度をAr変態点とする以外には、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 reheated steel material is hot-rolled so that the rolling end temperature is equal to or higher than the Ar 3 transformation point, thereby obtaining a thick steel plate having a predetermined thickness. The hot rolling conditions are not particularly limited as long as a predetermined plate thickness and shape can be satisfied except that the rolling end temperature is the Ar 3 transformation point. 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Ar変態点未満では、変形抵抗が高くなり圧延荷重が増大し、圧延機への負荷が大きくなる。また、厚肉材をAr変態点未満まで圧延終了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Ar変態点以上に限定した。 When the rolling end temperature is less than the Ar 3 transformation point, the deformation resistance increases, the rolling load increases, and the load on the rolling mill increases. Further, in order to lower the rolling end temperature of the thick material to less than the Ar 3 transformation point, it is necessary to wait in the middle of rolling, which greatly hinders productivity. For this reason, in the present invention, the rolling end temperature is limited to the Ar 3 transformation point or higher.

なお、Ar変態点は、概ね次式
Ar=910−273C+25Si−74Mn−56Ni−16Cr−9Mo−5Cu−1620Nb
(ここで、C、Si、Mn、Ni、Cr、Mo、Cu、Nb:各元素の含有量(mass%))
で整理できる。
The Ar 3 transformation point is approximately the following formula: Ar 3 = 910-273C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb
(Here, C, Si, Mn, Ni, Cr, Mo, Cu, Nb: content of each element (mass%))
Can be organized.

熱間圧延終了後、厚鋼板は、直ちに1〜20℃/sの平均冷却速度で、600℃以下まで加速冷却処理を施される。加速冷却処理の冷却速度が1℃/s未満では、加速冷却処理後のミクロ組織がフェライト主体組織となるため、目標の引張強さ590MPa以上を満足することができない。一方、冷却速度が20℃/sを超えると、加速冷却処理後のミクロ組織がベイナイト単相組織となり、目標の降伏比80%以下を満足できない。熱間圧延後の加速冷却処理の冷却速度が1〜20℃/sの範囲を満足してはじめて、加速冷却処理後のミクロ組織がベイナイト主体の組織中に、TiN等の微細介在物を核としてフェライト相が生成したベイナイト+フェライト二相組織となる。このベイナイト+フェライト二相組織が、高強度化と低降伏比に有効に作用する。なお、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 hot rolling is finished, the thick steel plate is immediately subjected to accelerated cooling to 600 ° C. or less at an average cooling rate of 1 to 20 ° C./s. When the cooling rate of the accelerated cooling process is less than 1 ° C./s, the microstructure after the accelerated cooling process becomes a ferrite main structure, and thus the target tensile strength of 590 MPa or more cannot be satisfied. On the other hand, if the cooling rate exceeds 20 ° C./s, the microstructure after the accelerated cooling treatment becomes a bainite single-phase structure, and the target yield ratio of 80% or less cannot be satisfied. Only after the cooling rate of the accelerated cooling treatment after hot rolling satisfies the range of 1 to 20 ° C./s, the microstructure after the accelerated cooling treatment is mainly composed of bainite and other fine inclusions such as TiN. It becomes a bainite + ferrite two-phase structure in which a ferrite phase is generated. This bainite + ferrite two-phase structure effectively acts on high strength and low yield ratio. In addition, the cooling rate as used in the field of this invention shall mean the average cooling rate from the cooling start to 600 degreeC in the plate | board thickness 1/4 position of a thick steel plate.

本発明では、加速冷却処理の冷却停止温度を、600℃以下とする。冷却停止温度が600℃を超えると、目標の引張強さ590MPa以上を確保することができない。なお、冷却停止温度は室温以上とすることが好ましい。厚鋼板は、加速冷却処理の冷却停止温度から、室温まで放冷される。   In the present invention, the cooling stop temperature of the accelerated cooling process is set to 600 ° C. or lower. If the cooling stop temperature exceeds 600 ° C, the target tensile strength of 590 MPa or more cannot be secured. The cooling stop temperature is preferably room temperature or higher. The thick steel plate is allowed to cool to the room temperature from the cooling stop temperature of the accelerated cooling process.

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と、熱間圧延後の加速冷却処理を行うことにより、引張強さ590MPa以上の高強度と、降伏比80%以下の低降伏比とを兼備し、かつ超大入熱溶接時のHAZ靱性に優れた厚鋼板を容易に製造することができる。   By using the steel material having the above composition, hot rolling under the above conditions and accelerated cooling treatment after hot rolling, high strength with a tensile strength of 590 MPa or more and low yield with a yield ratio of 80% or less. It is possible to easily manufacture a thick steel plate that combines the ratio and has excellent HAZ toughness during super-high heat input welding.

転炉−取鍋精錬−連続鋳造法で、表1に示す組成に調製された鋼素材(スラブ:板厚310mm)を、表2に示す条件の熱間圧延、加速冷却処理により表2に示す板厚の厚鋼板とした。   A steel material (slab: plate thickness 310 mm) prepared in the converter shown in Table 1 by the converter-ladder refining-continuous casting method is shown in Table 2 by hot rolling and accelerated cooling treatment under the conditions shown in Table 2. A thick steel plate was used.

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

また、得られた各厚鋼板から、継手用試験板(大きさ:400×600mm)を採取し、図1に示すような開先形状としたエレクトロスラグ溶接(溶接入熱量:1000kJ/cm)により、溶接継手を作製した。なお、供給ワイヤは、JIS Z 3353 YES62相当品、フラックスはJIS Z 3353 FS−FG3相当品を使用した。   In addition, a test plate for joints (size: 400 × 600 mm) was collected from each thick steel plate obtained, and electroslag welding (welding heat input: 1000 kJ / cm) in a groove shape as shown in FIG. A welded joint was prepared. The supply wire was JIS Z 3353 YES62 equivalent, and the flux was JIS Z 3353 FS-FG3 equivalent.

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

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

本発明はいずれも、引張強さ590MPa以上、降伏比80%以下の高強度、低降伏比で、かつ0℃での吸収エネルギー(vE0)100J以上の高靱性の母材特性を有するとともに、溶接入熱:1000kJ/cmの超大入熱溶接施工を施した場合であっても、ボンド部のvE0が100J以上と優れたHAZ靱性を示している。一方、本発明の範囲を外れる比較例は、母材強度、降伏比、母材靱性、HAZ靱性のうち、いずれか、あるいは複数の特性が目標値を満足していない。 Each of the present invention has a base material characteristic of a tensile strength of 590 MPa or more, a yield strength of 80% or less, a low yield ratio, and a high toughness of absorbed energy (vE 0 ) of 100 J or more at 0 ° C. Weld heat input: Even when super-high heat input welding is performed at 1000 kJ / cm, vE 0 of the bond portion is 100 J or more, indicating excellent HAZ toughness. On the other hand, in a comparative example that is out of the scope of the present invention, one or more of the base material strength, yield ratio, base material toughness, and HAZ toughness do not satisfy the target value.

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

Claims (4)

mass%で、
C:0.05〜0.13%、 Si:0.05〜0.50%、
Mn:0.5〜2.0%、 P:0.02%以下、
S:0.0050%以下、 Al:0.1%以下、
Ti:0.005〜0.030%、 N:0.0030〜0.0070%、
Ca:0.0005〜0.0050%、 O:0.0010〜0.0030%
を含有し、下記(1)式で定義される炭素当量Ceqが0.35〜0.44%で、かつ下記(2)式で定義されるACRが0.2〜0.8を満足し、残部がFeおよび不可避的不純物からなる組成を有する鋼素材を、1000℃〜1300℃の範囲に加熱し、圧延終了温度がAr3変態点以上となる熱間圧延を施した後、1〜20℃/sの平均冷却速度で600℃以下の温度まで冷却する加速冷却処理を施すことを特徴とする引張強さ:590MPa以上、降伏比:80%以下を有する超大入熱溶接熱影響部靭性に優れた非調質高強度厚鋼板の製造方法。

Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 ………(1)
ここで、C、Mn、Ni、Cu、Cr、Mo、V:各元素の含有量(mass%)
ACR={Ca−(0.18+130×Ca)×O}/(1.25×S) …………………(2)
ここで、Ca、O、S:各元素の含有量(mass%)
mass%
C: 0.05 to 0.13%, Si: 0.05 to 0.50%,
Mn: 0.5 to 2.0%, P: 0.02% or less,
S: 0.0050% or less, Al: 0.1% or less,
Ti: 0.005-0.030%, N: 0.0030-0.0070%,
Ca: 0.0005 to 0.0050%, O: 0.0010 to 0.0030%
The carbon equivalent Ceq defined by the following formula (1) is 0.35 to 0.44%, the ACR defined by the following formula (2) is 0.2 to 0.8, and the balance is Fe and inevitable impurities. A steel material having a composition as follows is heated to a range of 1000 ° C. to 1300 ° C. and subjected to hot rolling at a rolling end temperature equal to or higher than the Ar 3 transformation point, and then an average cooling rate of 1 to 20 ° C./s is 600. Non-tempered high-strength thick steel plate with excellent tensile strength: 590MPa and yield ratio: 80% or less Manufacturing method.
Record
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (1)
Here, C, Mn, Ni, Cu, Cr, Mo, V: Content of each element (mass%)
ACR = {Ca− (0.18 + 130 × Ca) × O} / (1.25 × S) ………………… (2)
Here, Ca, O, S: Content of each element (mass%)
前記組成に加えてさらに、mass%で、Cu:1.0%以下、Ni:2.0%以下のうちから選ばれた1種または2種を含有する組成とすることを特徴とする請求項1に記載の非調質高強度厚鋼板の製造方法。   2. The composition according to claim 1, further comprising, in addition to the composition, at least 1% or 2 types selected from Cu: 1.0% or less and Ni: 2.0% or less in mass%. Manufacturing method of non-tempered high strength thick steel plate. 前記組成に加えてさらに、mass%で、Cr:0.7%以下、Mo:1.0%以下、Nb:0.05%以下、V:0.2%以下のうちから選ばれた1種または2種以上を含有する組成とすることを特徴とする請求項1または2に記載の非調質高強度厚鋼板の製造方法。   In addition to the above composition, the composition further contains at least one selected from mass: Cr: 0.7% or less, Mo: 1.0% or less, Nb: 0.05% or less, and V: 0.2% or less. The method for producing a non-tempered high strength thick steel plate according to claim 1 or 2. 前記組成に加えてさらに、mass%で、REM:0.02%以下、Mg:0.005%以下のうちから選ばれた1種または2種を含有する組成とすることを特徴とする請求項1ないし3のいずれかに記載の非調質高強度厚鋼板の製造方法。   The composition according to any one of claims 1 to 3, wherein, in addition to the composition, the composition further includes one or two kinds selected from mass%, REM: 0.02% or less, and Mg: 0.005% or less. The manufacturing method of the non-tempered high strength thick steel plate in any one.
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