JP2011214053A - Low-yield-ratio thick steel plate for building structure superior in toughness at ultrahigh-heat-input weld zone, and method for manufacturing the same - Google Patents

Low-yield-ratio thick steel plate for building structure superior in toughness at ultrahigh-heat-input weld zone, and method for manufacturing the same Download PDF

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JP2011214053A
JP2011214053A JP2010082401A JP2010082401A JP2011214053A JP 2011214053 A JP2011214053 A JP 2011214053A JP 2010082401 A JP2010082401 A JP 2010082401A JP 2010082401 A JP2010082401 A JP 2010082401A JP 2011214053 A JP2011214053 A JP 2011214053A
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Yasuhiro Murota
康宏 室田
Misao Ishikawa
操 石川
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Jfe Steel Corp
Jfeスチール株式会社
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PROBLEM TO BE SOLVED: To provide a high-strength thick steel plate superior in earthquake resistance and toughness at a heat-affected zone in ultrahigh-heat-input weld.SOLUTION: A base steel material has a composition including, by mass%, 0.03-0.07% C, 0.05-0.5% Si, 0.6-2.0% Mn, 0.020% or less P, 0.0005-0.003% S, 0.005-0.03% Ti, 0.0003-0.0020% B, 0.0005-0.005% Ca, 0.0070% or less N and 0.003% or less O so that Ceq satisfies 0.40-0.45% and ACR satisfies 0.2-0.8. The manufacturing method includes: a rolling step of heating the above base steel material and rolling the base steel material so as to finish the rolling at the Artransformation temperature or higher; and an accelerated cooling step. The accelerated cooling step includes: a first cooling step of starting cooling within 60 seconds after the finish of the rolling step, cooling the steel plate at a cooling rate of 100°C/s or more and stopping the cooling at 700°C or lower in terms of a temperature of the surface layer part; and a second cooling step of holding the steel plate for 30-180 seconds after the first cooling step, cooling the steel plate, and stopping the cooling at 400-200°C in terms of a temperature in a center part of the plate thickness. Furthermore, the manufacturing method also may include a tempering step.

Description

本発明は、建築等の溶接構造物用として好適な、高強度厚鋼板に係り、とくに建築ボックス柱の施工に際し適用されるような、入熱400 kJ/cm以上のサブマージアーク溶接あるいはエレクトロスラグ溶接のような超大入熱溶接を施されても、溶接熱影響部靭性に優れた低降伏比高強度厚鋼板に関する。ここでいう「厚鋼板」は、板厚19mm以上の鋼板をいうものとする。   The present invention relates to a high-strength thick steel plate suitable for use in welded structures such as buildings, and more particularly to submerged arc welding or electroslag welding with a heat input of 400 kJ / cm or more as applied in the construction of building box columns. The present invention relates to a high strength thick steel sheet having a low yield ratio that is excellent in weld heat-affected zone toughness even if super-high heat input welding is applied. The “thick steel plate” here refers to a steel plate having a thickness of 19 mm or more.

近年、建築等の鋼構造物の大型化に伴い、使用する鋼材の高強度化や厚肉化が進められている。また、建築鋼構造物では、耐震性の向上が要求され、鋼材自体に、塑性変形能確保のために、降伏比YR(降伏強さYS/引張強さTS)を80%以下とする低降伏比を有することが要求されてきた。さらに、鋼構造物は溶接接合により組み立てられるため、溶接部を含めて、良好な靭性を保持することが要求されている。溶接鋼構造物では、地震時のような大きな負荷荷重を受けると、塑性変形が生じる前に、溶接部から脆性破壊が発生する場合があり、近年、とくに溶接継手部において高い靱性が要求されるようになっている。   In recent years, with the increase in size of steel structures such as buildings, the strength and thickness of steel materials to be used have been increased. Building steel structures are also required to have improved earthquake resistance, and the yield ratio YR (yield strength YS / tensile strength TS) is 80% or less in order to secure plastic deformability in the steel itself. It has been required to have a ratio. Furthermore, since the steel structure is assembled by welding, it is required to maintain good toughness including the welded portion. In welded steel structures, when subjected to a large load such as during an earthquake, brittle fracture may occur from the weld before plastic deformation occurs. In recent years, particularly high toughness is required in welded joints. It is like that.

しかも、最近では、構造物の施工能率向上と施工コストの低減という要望から、溶接効率の向上が求められ、大入熱溶接の適用範囲が拡大されている。例えば、高層建築物に用いられるボックス柱では、サブマージアーク溶接やエレクトロスラグ溶接などの溶接入熱が400kJ/cmを超えるような超大入熱溶接が適用されている。このような超大入熱溶接を適用する部位としては、例えば、角継手部のサブマージアーク溶接やダイヤフラム接合部のエレクトロスラグ溶接などが、挙げられる。   Moreover, recently, due to the demand for improving the construction efficiency of construction and reducing the construction cost, the improvement of welding efficiency is required, and the application range of large heat input welding has been expanded. For example, for box columns used in high-rise buildings, super-high heat input welding, such as submerged arc welding and electroslag welding, in which welding heat input exceeds 400 kJ / cm is applied. Examples of the part to which such super large heat input welding is applied include submerged arc welding of corner joints and electroslag welding of diaphragm joints.

一般に、このような大入熱溶接部では、溶接熱影響部(以下、HAZともいう)の靭性劣化が問題となる。これは、大入熱溶接により融点近傍まで加熱された領域では、冷却が遅いため高温域での滞留時間が長く、オーステナイト粒が粗大化しやすいうえ、さらにその後の冷却の際に、MA(島状マルテンサイトともいう)等の硬質な脆化相が生じやすいことに起因する。このようなHAZの靭性劣化は、鋼材の強度が増加するにしたがい、顕著となり、とくに、TS590MPa級鋼材で問題となることが多い。   In general, in such a high heat input welded portion, deterioration of the toughness of the weld heat affected zone (hereinafter also referred to as HAZ) becomes a problem. This is because, in the region heated to near the melting point by high heat input welding, since the cooling is slow, the residence time in the high temperature region is long, and the austenite grains are likely to be coarsened. This is because hard brittle phases such as martensite are easily generated. Such toughness degradation of HAZ becomes more prominent as the strength of the steel increases, and in particular, it is often a problem with TS590 MPa grade steel.

このような問題に対し、例えば特許文献1には、C:0.05〜0.11%、Si:0.5%以下、Mn:0.6〜1.6%を含み、P、Sを適正範囲内に調整し、さらに、Cu:0.80〜1.60%、Ni:0.30〜1.0%を含み、Nb:0.005〜0.02%、Ti:0.005〜0.025%、N:0.001〜0.004%、O:0.001〜0.006%を含む鋼を熱間圧延後、再加熱焼入れし、さらに二相域に再加熱し焼入れ、焼戻する大入熱溶接熱影響部靭性の優れた建築用低降伏比600N/mm級鋼板の製造方法が記載されている。特許文献1に記載された技術では、低Cとし、B無添加でTi酸化物を利用して大入熱溶接熱影響部靭性を向上させるとともに、二相域加熱焼入れとCuによる析出硬化を利用して、低降伏比で、600N/mm級の高強度を有する鋼板の製造が可能になるとしている。 For such a problem, for example, Patent Document 1 includes C: 0.05 to 0.11%, Si: 0.5% or less, Mn: 0.6 to 1.6%, and P and S are adjusted within an appropriate range. : Hot-rolled steel containing 0.80 to 1.60%, Ni: 0.30 to 1.0%, Nb: 0.005 to 0.02%, Ti: 0.005 to 0.025%, N: 0.001 to 0.004%, O: 0.001 to 0.006% A method for producing a low yield ratio 600 N / mm grade 2 steel sheet for construction with excellent high heat input weld heat affected zone toughness by reheating and quenching and then reheating, quenching and tempering in a two-phase region is described. In the technique described in Patent Document 1, low C is added, B oxide is added and Ti oxide is used to improve the high heat input heat affected zone toughness, and two-phase region heating quenching and precipitation hardening by Cu are used. Thus, it is possible to manufacture a steel sheet having a high yield of 600 N / mm 2 with a low yield ratio.

また、特許文献2には、C:0.03〜0.15%、Si:0.05〜0.5%、Mn:0.5〜3.0%を含み、Al、P、Sを適正範囲に調整して含有し、さらに、Ti:0.004〜0.03%、B:0.0005〜0.0030%、Ca:0.0005〜0.0030%、N:0.0020〜0.0070%、O:0.0050%以下を含み、さらに、Cu:1.5%以下、Ni:2.0%以下のうちから選ばれた1種または2種を、炭素当量Ceqが0.35%以上、後述の(2)式で表わされるACRが0.3〜0.8%を満足する範囲で含む鋼素材に、熱間圧延を施し厚鋼板とし、該厚鋼板に再加熱焼入れ工程と、ついで、二相域の温度に再加熱したのち焼入れ、焼戻する、超大入熱溶接熱影響部靭性に優れる低降伏比高強度厚鋼板の製造方法が提案されている。特許文献2に記載された技術では、超大入熱溶接部靭性を向上するために、TiNを利用してHAZでのオーステナイト粒の粗大化を抑制しつつ、ACRを0.3〜0.8を満足するようにCa、O、Sを調整して、CaS上にMnSが析出した複合硫化物を析出させ、フェライト変態核として作用させ、粒内フェライトの核生成を促進させてHAZ組織の微細化を図り、超大入熱溶接部靭性を向上させるとしている。さらに、特許文献2に記載された技術では、固溶強化に有効なCu、Ni量を適正化して、二相域加熱し、焼入れる処理により、引張強さTS590MPa以上の高強度化と、80%以下の低降伏比を、超大入熱溶接HAZ靭性の劣化を招くことなく、達成できるとしている。   Patent Document 2 includes C: 0.03-0.15%, Si: 0.05-0.5%, Mn: 0.5-3.0%, and contains Al, P, and S adjusted to an appropriate range, and further contains Ti: 0.004 to 0.03%, B: 0.0005 to 0.0030%, Ca: 0.0005 to 0.0030%, N: 0.0020 to 0.0070%, O: 0.0050% or less, Cu: 1.5% or less, Ni: 2.0% or less Thick steel plates that are hot-rolled into steel materials that contain one or two selected materials within a range where the carbon equivalent Ceq is 0.35% or more and the ACR represented by the formula (2) described below is 0.3 to 0.8%. And reheating and quenching the thick steel plate, followed by quenching and tempering after reheating to a temperature in the two-phase region, and a method for producing a low yield ratio high strength thick steel plate having excellent super-high heat input heat affected zone toughness Has been proposed. In the technique described in Patent Document 2, in order to improve super tough heat input weld toughness, TiN is used to suppress coarsening of austenite grains in HAZ, while satisfying ACR of 0.3 to 0.8. Adjusting Ca, O, and S to precipitate a composite sulfide with MnS deposited on CaS, causing it to act as a ferrite transformation nucleus, promoting nucleation of intragranular ferrite, and miniaturizing the HAZ structure. The heat input weld toughness is improved. Furthermore, in the technique described in Patent Document 2, the strength of Cu and Ni effective for solid solution strengthening is optimized, the two-phase region heating and quenching are performed to increase the tensile strength of TS590 MPa or more, and 80 % Yield ratio can be achieved without causing deterioration of the super large heat input welding HAZ toughness.

また、特許文献3には、C:0.05〜0.15%、Si:0.05〜0.50%、Mn:0.6〜1.6%を含み、P,S,Alを適正範囲に調整して含有し、さらに、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、ACRが0.2〜0.8%、Ceqが0.47%以下となる範囲で含む鋼素材を、熱間圧延後、加速冷却を施し厚鋼板とし、さらに二相域の温度に再加熱したのち焼入れ、焼戻する、超大入熱溶接熱影響部靭性に優れる低降伏比高強度厚鋼板の製造方法が提案されている。特許文献3に記載された技術では、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、超大入熱溶接部靭性を向上するために重要であるとして、TiNの利用と、Ca、O、Sの含有量をACRが適正範囲となるように調整して形態を最適化したCaの酸化物または硫化物を鋼中に分散して粒内フェライトの核生成を促進させてHAZ組織を微細化し、超大入熱溶接部靭性を向上させるとしている。   Patent Document 3 includes C: 0.05 to 0.15%, Si: 0.05 to 0.50%, Mn: 0.6 to 1.6%, and contains P, S, and Al adjusted to an appropriate range, and 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%, ACR Steel material containing 0.2 to 0.8% and Ceq in the range of 0.47% or less, after hot rolling, accelerated cooling to thick steel plate, and further re-heating to two-phase temperature, followed by quenching and tempering. There has been proposed a method for producing a low yield ratio high strength thick steel plate that is excellent in heat input weld heat affected zone toughness. In the technique described in Patent Document 3, the austenite grain coarsening is suppressed in a region heated to a high temperature, and the fine dispersion of transformation nuclei that promote ferrite transformation during cooling improves the toughness of the super high heat input weld zone. It is important to use TiN and adjust the Ca, O, and S contents so that the ACR is in the proper range and optimize the form to disperse the Ca oxide or sulfide in the steel. It promotes nucleation of internal ferrite to refine the HAZ structure and improve the toughness of super large heat input welds.

しかしながら、特許文献1〜3に記載された技術はいずれも、二相域熱処理を行うため、工程が複雑となり、製造期間が長期化し、生産性に問題を残している。
一方、二相域熱処理を行なうことなく、優れた超大入熱溶接部靭性と低降伏比とを両立させることができる技術が、特許文献4に記載されている。特許文献4に記載された技術は、C:0.03〜0.15%、Si:0.05〜0.50%、Mn:0.5〜2.0%を含み、P,S,Alを適正範囲に調整して含有し、さらに、Ti:0.004〜0.02%、Ca:0.0005〜0.0030%、N:0.0020〜0.0070%を、ACRが0.3〜0.8%となる範囲で含む鋼素材を、圧延終了温度をAr変態点以上とする熱間圧延を施し、1℃/s以上の冷却速度で600〜250℃の範囲まで冷却し、空冷する加速冷却を施す、超大入熱溶接熱影響部靭性に優れる低降伏比高強度厚鋼板の製造方法である。特許文献4に記載された技術では、熱間圧延条件および圧延終了後の加速冷却条件を調整して、母材厚鋼板の低降伏比化を図るとともに、Ca、O、S含有量からなる関係式であるACRを適正範囲となるように調整して、溶接時にフェライト変態核となる微細な粒子を多数生成して、HAZ組織を微細化し、超大入熱溶接熱影響部靭性を改善するとしている。
However, since all the techniques described in Patent Documents 1 to 3 perform the two-phase region heat treatment, the process becomes complicated, the manufacturing period becomes longer, and a problem remains in productivity.
On the other hand, Patent Document 4 describes a technique that can achieve both excellent super high heat input weld toughness and low yield ratio without performing two-phase region heat treatment. The technique described in Patent Document 4 includes C: 0.03 to 0.15%, Si: 0.05 to 0.50%, Mn: 0.5 to 2.0%, and contains P, S, and Al adjusted to an appropriate range. Ti: 0.004~0.02%, Ca: 0.0005~0.0030 %, N: a 0.0020 to 0.0070%, the steel material containing a range of ACR is 0.3 to 0.8% hot to the rolling end temperature than the Ar 3 transformation point A method for producing a high strength thick steel sheet having a low yield ratio and excellent in toughness of super-high heat input welding heat-affected zone, which is rolled, cooled to a range of 600 to 250 ° C. at a cooling rate of 1 ° C./s or more, and subjected to accelerated cooling by air cooling. It is. In the technique described in Patent Document 4, the hot rolling conditions and the accelerated cooling conditions after the end of rolling are adjusted to reduce the yield ratio of the base steel plate, and the relationship consisting of Ca, O, and S contents. ACR, which is the formula, is adjusted to be in the proper range, many fine particles that become ferrite transformation nuclei are generated at the time of welding, the HAZ structure is refined, and the super-high heat input heat affected zone toughness is improved. .

また、特許文献5には、C:0.12〜0.17%、Si:0.1〜0.5%、Mn:2%以下、Ti:0.005〜0.035%、Nb:0.005〜0.075%、N:0.002〜0.01%を含み、Ceq:0.38〜0.43%を満たすように含み、25〜500nmのTi炭窒化物を0.01〜1.0体積%と、75nm以下のNb炭窒化物を0.01〜0.8体積%とを含み、フェライト組織を2%以上を占める組織とする、引張強さ590MPa以上で大入熱溶接部靭性に優れた低降伏比高張力鋼板が記載されている。特許文献5に記載された技術では、微細なTi炭窒化物を活用して、HAZ組織を微細化しHAZ靭性を向上させ、また、微細なNb炭窒化物を活用し、Ceqを低く維持したまま母材および溶接継手部の強度を所望の値以上とし、さらにフェライト相分率を制御し、80%以下の低降伏比を確保している。   Patent Document 5 includes C: 0.12 to 0.17%, Si: 0.1 to 0.5%, Mn: 2% or less, Ti: 0.005 to 0.035%, Nb: 0.005 to 0.075%, N: 0.002 to 0.01% Ceq: 0.38 to 0.43%, 25 to 500 nm Ti carbonitride is 0.01 to 1.0% by volume, 75 nm or less Nb carbonitride is 0.01 to 0.8% by volume, and the ferrite structure is 2 % Low tensile strength steel sheet having a tensile strength of 590 MPa or more and excellent high heat input weld toughness. In the technique described in Patent Document 5, the fine Ti carbonitride is utilized to refine the HAZ structure and improve the HAZ toughness, and the fine Nb carbonitride is utilized to keep Ceq low. The strength of the base metal and the welded joint is set to a desired value or more, and the ferrite phase fraction is controlled to ensure a low yield ratio of 80% or less.

また、特許文献6には、降伏比についての言及はないが、C:0.02〜0.05%、Mn:1.0〜2.5%、Ti:0.005〜0.025%、Ni:0.2〜2.0%、Cr:0.5〜2.0%を含み、Mn、Ni、Crの特定関係式が所定の範囲内となるように調整した組成を有し、引張強さが590MPa以上で、大入熱溶接熱影響部の靭性に優れた高張力鋼板が記載されている。特許文献5に記載された技術では、焼入れ焼戻処理を施し、所定の高強度を確保するとともに、TiNを利用してHAZのγ(オーステナイト)粒を微細化し、さらにMn、Ni、Crの特定関係式を所定の範囲に調整することにより、針状MA(島状マルテンサイトともいう)の生成や、γ粒界における粗大な組織が抑制でき、さらにγ粒内の変態組織のブロックサイズを微細化して、HAZの高靭性化を達成するとしている。   Patent Document 6 does not mention the yield ratio, but C: 0.02 to 0.05%, Mn: 1.0 to 2.5%, Ti: 0.005 to 0.025%, Ni: 0.2 to 2.0%, Cr: 0.5 to 2.0 With a composition adjusted so that the specific relational expression of Mn, Ni, and Cr is within the specified range, with a tensile strength of 590 MPa or more and high toughness in the heat-affected zone of high heat input welding Tensile steel sheets are described. In the technique described in Patent Document 5, quenching and tempering treatment is performed to ensure a predetermined high strength, and γ (austenite) grains of HAZ are refined using TiN, and further, Mn, Ni, and Cr are specified. By adjusting the relational expression within a predetermined range, the generation of acicular MA (also called island martensite) and the coarse structure at the γ grain boundary can be suppressed, and the block size of the transformed structure in the γ grain can be reduced. To achieve high toughness of HAZ.

特開平06-128635号公報Japanese Patent Laid-Open No. 06-128635 特開2005-68478号公報JP 2005-68478 特開2005-68519号公報JP 2005-68519 JP 特開2003−183767号公報JP2003-183767 特開2001−172736号公報JP 2001-1772736 特開2007−126725号公報JP 2007-126725 A

しかしながら、特許文献4に記載された技術は、二相域熱処理を施すことはないが、引張強さTSが490MPa以上の強度を有する厚鋼板を対象としており、更なる高強度化のためには、合金元素の多量含有を必要とし、その場合、鋼板表面硬さが高くなり、表層の延性が低下し、構造体としての変形性能が低下して、耐震性が問題となる。また、特許文献5に記載された技術で製造された鋼板は、C含有量が高く、表面硬さが著しく高くなり、表層の延性が低下して、使用時の耐震性に問題を残していた。というのは、表層付近の延性が低下した鋼板では、地震等による応力負荷に際し、表層付近に亀裂が発生し、その亀裂によるノッチ効果で、鋼材(構造物)が破断に至る場合があるからである。   However, the technique described in Patent Document 4 does not perform a two-phase heat treatment, but targets a thick steel plate having a tensile strength TS of 490 MPa or more. In this case, the steel sheet surface hardness is increased, the ductility of the surface layer is lowered, the deformation performance as a structure is lowered, and the earthquake resistance becomes a problem. In addition, the steel sheet produced by the technique described in Patent Document 5 has a high C content, a surface hardness that is remarkably high, and the ductility of the surface layer is reduced, leaving a problem in the earthquake resistance during use. . This is because in steel sheets with reduced ductility near the surface, cracks occur near the surface when stress is applied due to earthquakes, etc., and the steel material (structure) may break due to the notch effect due to the crack. is there.

また、特許文献6に記載された技術では、焼入れ性を高める元素を多量含有し、焼入れ処理を施すため、表層部が高硬度化して延性が低下し、構造体としての変形性能が低下するため、また降伏比が高くなり、耐震性に問題を残していた。
本発明は、上記した従来技術の問題を解決し、二相域熱処理を用いることなく、建築構造用として好適な、引張強さTS:590MPa以上、降伏比YR:80%以下を有し、かつ表層近傍の延性低下が抑制され、構造体としての変形性能に優れ、さらに、超大入熱溶接熱影響部靭性に優れた低降伏比建築構造用厚鋼板およびその製造方法を提供することを目的とする。
Moreover, in the technique described in Patent Document 6, since a large amount of elements that enhance hardenability is contained and subjected to quenching treatment, the surface layer portion becomes harder, ductility is reduced, and deformation performance as a structure is reduced. Also, the yield ratio was high, leaving problems with earthquake resistance.
The present invention solves the above-mentioned problems of the prior art, suitable for building structures without using a two-phase heat treatment, has a tensile strength TS: 590 MPa or more, a yield ratio YR: 80% or less, and An object of the present invention is to provide a low yield ratio thick steel plate for building structures and a method for producing the same, in which ductility deterioration in the vicinity of the surface layer is suppressed, the deformation performance as a structure is excellent, and the super high heat input welding heat affected zone toughness is excellent. To do.

なお、ここでいう「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を超える超大入熟溶接部のボンド部近傍の熱影響部(ボンド部から1mm)において、シャルピー衝撃試験の0℃における吸収エネルギー(vEo)が 70J以上を示す場合をいうものとする。   The term “excellent toughness of the super-high heat input welding heat-affected zone” as used herein refers to the heat-affected zone (1 mm from the bond zone) in the vicinity of the bond portion of the super-high-ripening weld zone where the welding heat input exceeds 400 kJ / cm. The case where the absorbed energy (vEo) at 0 ° C in the Charpy impact test shows 70J or more.

本発明者らは、上記した目的を達成するために、合金元素の種類とその含有量について鋭意検討した。その結果、まず、本発明では、所望の超大入熱溶接熱影響部靭性を保持させるために、Ca、O、Sを、下記式
ACR=(Ca−(0.18+130×Ca)×O)/(1.25×S)
(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の範囲内となるように調整し、さらに、Ti,Nを適量含有させ、不純物元素(?)としてNb:0.005%以下、Mo:0.01%以下と極力低減したうえで、C:0.07%以下に調整することにより、所定の熱間圧延と、熱間圧延後に途中での保持を含む二段階の加速冷却を施すことにより、二相域熱処理を施すことなく、表層硬さが280HV10以下で、引張強さTSが590MPa以上となる、超大入熱溶接熱影響部靭性に優れた低降伏比厚鋼板を安価に製造できることを知見した。
In order to achieve the above-mentioned object, the present inventors diligently studied the types of alloy elements and their contents. As a result, first, in the present invention, in order to maintain the desired super-high heat input welding heat-affected zone toughness, Ca, O, and S are represented by the following formulae.
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S)
(Where Ca, O, S: content of each element (mass%))
The ACR defined in the above is adjusted to be within the range of 0.2 to 0.8, and further, Ti and N are contained in an appropriate amount, and Nb: 0.005% or less and Mo: 0.01% or less are reduced as much as possible as impurity elements (?). In addition, by adjusting to C: 0.07% or less, by performing two-stage accelerated cooling including predetermined hot rolling and holding in the middle after hot rolling, without performing two-phase region heat treatment, It was found that a low yield specific thickness steel plate with super high heat input welding heat-affected zone toughness with surface hardness of 280HV10 or less and tensile strength TS of 590MPa or more can be produced at low cost.

まず、本発明者らが行った、本発明の基礎となる実験結果について説明する。
質量%で、C:0.04〜0.12%を含み、さらにSi、Mn、Cu、Ni、Crを炭素当量Ceqが0.42〜0.43となるように、また、Ca、O、SをACRが0.4〜0.5となるようにそれぞれ含み、さらにTi:0.02%以下を含む組成の鋼素材に、1150℃に加熱し、圧延終了温度:850℃とする熱間圧延を施し、60mm厚の厚鋼板とし、熱間圧延終了後、60s以内に冷却を開始し、鋼板表面温度で120℃/sの冷却速度(平均)で600℃まで冷却する一次冷却と、その後、40s間保持したのち、再冷却を開始し、板厚中心部温度で8℃/sの冷却速度(平均)で、350℃となるまで、冷却する二次冷却を施した。
First, the experimental results, which are the basis of the present invention, conducted by the present inventors will be described.
C: 0.04 to 0.12% by mass%, Si, Mn, Cu, Ni, and Cr, so that the carbon equivalent Ceq is 0.42 to 0.43, and Ca, O, and S are ACR 0.4 to 0.5 Each steel material with a composition containing Ti: 0.02% or less is heated to 1150 ° C and hot-rolled to a rolling end temperature of 850 ° C to form a 60mm thick steel plate. After the completion of cooling, start cooling within 60 s, primary cooling at a steel sheet surface temperature of 120 ° C / s to 600 ° C at a cooling rate (average), and then hold for 40 s. Secondary cooling was performed until the temperature reached 350 ° C. at a cooling rate (average) of 8 ° C./s at the thickness center temperature.

得られた厚鋼板について、表層の硬さ測定、表層部の引張試験、および板厚方向1/4位置の引張試験を実施した。表層の硬さ測定は、ビッカース硬さ計(荷重:10kgf(試験力:98N))を用いて、表面下0.5mm位置の硬さHVを20点測定し、その最大値をその鋼板の表層硬さHV10とした。また、表層部の引張試験は、表面下0.5〜6.5mm位置(表層)から小型丸棒引張試験片(平行部:6mmφ×24mm(GL))を採取し、引張試験を実施し、伸びElsを求め、表層の延性低下の度合を評価した。また、鋼板の板厚方向1/4位置から、JIS Z 2201の規定に準拠してJIS4号引張試験片を採取し、JIS Z 2241に準拠して引張試験を実施し、引張特性(降伏強さYS、引張強さTS)を求め、降伏比YRを算出した。   About the obtained thick steel plate, the hardness measurement of the surface layer, the tensile test of the surface layer part, and the tensile test at a 1/4 position in the plate thickness direction were performed. The surface hardness is measured using a Vickers hardness tester (load: 10 kgf (test force: 98 N)), measuring 20 points of hardness HV at 0.5 mm below the surface, and the maximum value is the surface hardness of the steel sheet. HV10. In addition, a tensile test of the surface layer part is carried out by taking a small round bar tensile test piece (parallel part: 6mmφ x 24mm (GL)) from the position 0.5 to 6.5mm below the surface (surface layer), conducting a tensile test, and measuring the elongation Els. The degree of surface ductility reduction was evaluated. In addition, JIS No. 4 tensile test specimens were collected from ¼ position in the thickness direction of the steel sheet in accordance with JIS Z 2201, and tensile tests were conducted in accordance with JIS Z 2241 to obtain tensile properties (yield strength). YS and tensile strength TS) were determined, and the yield ratio YR was calculated.

得られた結果を、C含有量との関係で図1に示す。
図1から、実験したC量範囲では、板厚方向1/4位置の引張強さTSが590MPa以上で、降伏比YRが80%以下と、所望の高強度と低降伏比が達成されている。しかし、C量が、質量%で0.07%を超えると、表層の硬さが280HV10を超えて、表層の延性Elsが30%未満と、表層の延性が低下している。図1から、表層の延性低下を抑制するためには、Cを、質量%で0.07%以下に限定する必要があることがわかる。
The obtained results are shown in FIG. 1 in relation to the C content.
From FIG. 1, in the C amount range that was experimentally tested, the tensile strength TS at 1/4 position in the thickness direction was 590 MPa or more, the yield ratio YR was 80% or less, and the desired high strength and low yield ratio were achieved. . However, when the amount of C exceeds 0.07% by mass%, the hardness of the surface layer exceeds 280HV10, and the ductility of the surface layer is less than 30%, which decreases the ductility of the surface layer. From FIG. 1, it can be seen that C must be limited to 0.07% by mass or less in order to suppress a decrease in ductility of the surface layer.

本発明は、かかる知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨はつぎのとおりである。
(1)質量%で、C:0.03〜O.07%、Si:0.05〜0.5%、Mn:0.6〜2.0%、P:0.020%以下、S:0.0005〜0.003%、Ti:0.005〜0.03%、B:0.0003〜0.0020%、Ca:0.0005〜0.005%、N:0.0070%以下、O:0.003%以下を含み、不純物としてMo、Nbを、Mo:0.01%以下、Nb:0.005%以下に調整し、次(1)式
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ‥‥(1)
(ここで、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0.40〜0.45%、次(2)式
ACR=(Ca−(0.18+130×Ca)×O)/(1.25×S) ‥‥(2)
(ここで、V、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Feおよび不可避的不純物からなる組成と、フェライト分率が体積率で10〜40%である組織を有し、引張強さTS590MPa以上、降伏比80%以下で、表層硬さHVが280 HV10以下であることを特徴とする超大入熱溶接熱影響部靭性に優れた低降伏比建築構造用厚鋼板。
The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.03-0.07%, Si: 0.05-0.5%, Mn: 0.6-2.0%, P: 0.020% or less, S: 0.0005-0.003%, Ti: 0.005-0.03%, B: 0.0003 to 0.0020%, Ca: 0.0005 to 0.005%, N: 0.0070% or less, O: 0.003% or less as impurities, Mo and Nb are adjusted to Mo: 0.01% or less, Nb: 0.005% or less, Next (1) Formula Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
(Here, C, Mn, Cu, Ni, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by is 0.40 to 0.45%, the following formula (2)
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) (2)
(Where V, Ca, O, S: content of each element (mass%))
ACR defined by 0.2 to 0.8 satisfies the composition consisting of the balance Fe and inevitable impurities, and a structure with a ferrite fraction of 10 to 40% by volume, tensile strength of TS590 MPa or more, yield ratio A steel plate for building structures with a low yield ratio and excellent in heat-affected zone toughness of super high heat input welding, characterized by a surface hardness HV of 280 HV10 or less and 80% or less.

(2)(1)において、前記組成に加えてさらに、質量%で、Cu:0.5%以下、Ni:1.0%以下、Cr:0.5%以下、V:0.08%以下のうちから選ばれた1種または2種以上を含有することを特徴とする低降伏比建築構造用厚鋼板。
(3)(1)または(2)において、前記組成に加えてさらに、質量%で、Al:0.1%以下を含有することを特徴とする低降伏比建築構造用厚鋼板。
(2) In (1), in addition to the above-mentioned composition, by mass%, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, V: 0.08% or less Or the steel plate for low yield ratio building structures characterized by containing 2 or more types.
(3) The low yield ratio thick steel plate for building structure according to (1) or (2), further comprising, in addition to the above composition, Al: 0.1% or less by mass%.

(4)(1)ないし(3)のいずれかにおいて、前記組成に加えてさらに、質量%で、Mg:0.005%以下、REM:0.02%以下のうちから選ばれた1種または2種を含有することを特徴とする低降伏比建築構造用厚鋼板。
(5)鋼素材に、加熱し熱間圧延を施す圧延工程と、該圧延工程終了後に加速冷却を施す加速冷却工程とを行う、厚鋼板の製造方法であって、前記鋼素材が、質量%で、C:0.03〜O.07%、Si:0.05〜0.5%、Mn:0.6〜2.0%、P:0.020%以下、S:0.0005〜0.003%、Ti:0.005〜0.03%、B:0.0003〜0.0020%、Ca:0.0005〜0.005%、N:0.0070%以下、O:0.003%以下を含み、不純物としてMo、Nbを、Mo:0.01%以下、Nb:0.005%以下に調整し、次(1)式
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ‥‥(1)
(ここで、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0.40〜0.45%、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Feおよび不可避的不純物からなる組成の鋼素材であり、前記圧延工程が、前記鋼素材を1000〜1200℃に加熱したのち、圧延終了温度をAr変態点以上とする熱間圧延を施し厚鋼板とする工程であり、前記加速冷却工程が、前記圧延工程終了後60s以内に冷却を開始する工程であり、前記冷却を、前記厚鋼板の表層部の温度で、平均冷却速度:100℃/s以上で、冷却停止温度:700℃以下まで冷却する一次冷却と、該一次冷却後、30〜180s間冷却を停止する保持と、該保持終了後、前記厚鋼板の板厚中央部の温度で、平均冷却速度が3℃/s以上で冷却停止温度:400〜200℃の範囲の温度まで冷却する二次冷却とからなる加速冷却を施す工程であり、前記加速冷却工程後、空冷することを特徴とする大入熱溶接部靭性に優れた低降伏比建築構造用厚鋼板の製造方法。
(4) In any one of (1) to (3), in addition to the above-described composition, the composition further contains one or two kinds selected from Mg: 0.005% or less and REM: 0.02% or less by mass%. A steel plate for building structures with a low yield ratio.
(5) A method for producing a thick steel plate, comprising: a rolling process in which a steel material is heated and subjected to hot rolling; and an accelerated cooling process in which accelerated cooling is performed after completion of the rolling process, wherein the steel material has a mass%. C: 0.03-0.07%, Si: 0.05-0.5%, Mn: 0.6-2.0%, P: 0.020% or less, S: 0.0005-0.003%, Ti: 0.005-0.03%, B: 0.0003-0.0020 , Ca: 0.0005 to 0.005%, N: 0.0070% or less, O: 0.003% or less, and Mo and Nb as impurities are adjusted to Mo: 0.01% or less and Nb: 0.005% or less. Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
(Here, C, Mn, Cu, Ni, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by is 0.40 to 0.45%, the following formula (2)
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) (2)
(Where Ca, O, S: content of each element (mass%))
Is a steel material having a composition consisting of the balance Fe and unavoidable impurities, ACR defined by the following, and after the rolling process has heated the steel material to 1000-1200 ℃, the rolling end temperature is A step of hot rolling at Ar 3 transformation point or more to form a thick steel plate, wherein the accelerated cooling step is a step of starting cooling within 60 s after the end of the rolling step, and the cooling is performed on the thick steel plate. At the surface layer temperature, average cooling rate: 100 ° C / s or more, cooling stop temperature: primary cooling to cool to 700 ° C or less, holding to stop cooling for 30 to 180 seconds after the primary cooling, and holding end Then, the process of giving the accelerated cooling which consists of the secondary cooling which cools to the temperature of the range of the cooling stop temperature: 400-200 degreeC with an average cooling rate of 3 degree-C / s or more at the temperature of the plate | board thickness center part of the said thick steel plate. The high heat input welded portion is air-cooled after the accelerated cooling step A method for producing a steel plate for building structures with low yield ratio and excellent toughness.

(6)鋼素材に、加熱し圧延を施す圧延工程と、該圧延工程終了後に加速冷却を施す加速冷却工程と、焼戻工程とを行う、厚鋼板の製造方法であって、前記鋼素材が、質量%で、C:0.03〜O.07%、Si:0.05〜0.5%、Mn:0.6〜2.0%、P:0.020%以下、S:0.0005〜0.003%、Ti:0.005〜0.03%、B:0.0003〜0.0020%、Ca:0.0005〜0.005%、N:0.0070%以下、O:0.003%以下を含み、不純物としてMo、Nbを、Mo:0.01%以下、Nb:0.005%以下に調整し、次(1)式
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ‥‥(1)
(ここで、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0.40〜0.45%、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Feおよび不可避的不純物からなる組成の鋼素材であり、前記圧延工程が、前記鋼素材を1000〜1200℃に加熱したのち、圧延終了温度をAr変態点以上とする熱間圧延を施し厚鋼板とする工程であり、前記加速冷却工程が、前記圧延工程終了後60s以内に冷却を開始する工程であり、前記冷却を、前記厚鋼板の表層部の温度で、平均冷却速度が100℃/s以上で、冷却停止温度:700℃以下となるまで冷却する一次冷却と、該一次冷却後、30〜180s間冷却を停止する保持と、該保持終了後、前記厚鋼板の板厚中央部の温度で、平均冷却速度:3℃/s以上で、冷却停止温度:400〜50℃の範囲の温度まで冷却する二次冷却とからなる加速冷却を施す工程であり、前記焼戻工程が、前記加速冷却工程を経た厚鋼板を焼戻温度:450℃以下の温度で焼戻す工程である、ことを特徴とする大入熱溶接部靭性に優れた低降伏比建築構造用厚鋼板の製造方法。
(6) A method for manufacturing a thick steel plate, comprising: a rolling process in which a steel material is heated and rolled, an accelerated cooling process in which accelerated cooling is performed after completion of the rolling process, and a tempering process. , In mass%, C: 0.03-0.07%, Si: 0.05-0.5%, Mn: 0.6-2.0%, P: 0.020% or less, S: 0.0005-0.003%, Ti: 0.005-0.03%, B: Containing 0.0003-0.0020%, Ca: 0.0005-0.005%, N: 0.0070% or less, O: 0.003% or less, and adjusting impurities Mo and Nb to Mo: 0.01% or less and Nb: 0.005% or less 1) Formula Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
(Here, C, Mn, Cu, Ni, Cr, Mo, V: content of each element (mass%))
The carbon equivalent Ceq defined by the formula is 0.40 to 0.45%, the following (2) Formula ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) (2)
(Where Ca, O, S: content of each element (mass%))
Is a steel material having a composition consisting of the balance Fe and unavoidable impurities, and the rolling step heats the steel material to 1000 to 1200 ° C. A step of hot rolling at Ar 3 transformation point or more to form a thick steel plate, wherein the accelerated cooling step is a step of starting cooling within 60 s after the end of the rolling step, and the cooling is performed on the thick steel plate. Primary cooling at which the average cooling rate is 100 ° C./s or higher at the temperature of the surface layer portion, and cooling stop temperature: 700 ° C. or lower, holding for stopping cooling for 30 to 180 seconds after the primary cooling, After the end of the holding, accelerated cooling consisting of secondary cooling that cools to a temperature in the center of the thickness of the steel plate at an average cooling rate of 3 ° C./s or higher and a cooling stop temperature of 400 to 50 ° C. A thick steel plate in which the tempering step has undergone the accelerated cooling step Tempering temperature: a 450 ° C. tempered at temperatures below steps, low yield ratio method for manufacturing a building structure for thick steel sheet excellent in high heat input weld toughness, characterized in that.

(7)(5)または(6)において、前記組成に加えてさらに、質量%で、Cu:0.5%以下、Ni:1.0%以下、Cr:0.5%以下、V:0.08%以下のうちから選ばれた1種または2種以上を含有することを特徴とする低降伏比建築構造用厚鋼板の製造方法。
(8)(5)ないし(7)のいずれかにおいて、前記組成に加えてさらに、質量%で、Al:0.1%以下を含有することを特徴とする低降伏比建築構造用厚鋼板の製造方法。
(7) In (5) or (6), in addition to the above-mentioned composition, it is further selected by mass% from Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, V: 0.08% or less The manufacturing method of the steel plate for low yield ratio building structures characterized by containing 1 type, or 2 or more types.
(8) In any one of (5) to (7), in addition to the above-described composition, the method further comprises a mass% of Al: 0.1% or less, and a method for producing a thick steel sheet for building structure having a low yield ratio .

(9)(5)ないし(8)のいずれかにおいて、前記組成に加えてさらに、質量%で、Mg:0.005%以下、REM:0.02%以下のうちから選ばれた1種または2種を含有することを特徴とする低降伏比建築構造用厚鋼板の製造方法。   (9) In any one of (5) to (8), in addition to the above composition, the composition further contains one or two selected from the group consisting of Mg: 0.005% or less and REM: 0.02% or less by mass%. A method of manufacturing a thick steel plate for building structures having a low yield ratio.

本発明によれば、建築構造用として好適な、引張強さTS:590MPa以上、降伏比YR:80%以下を有し、さらに表層硬さが280HV10以下で表層の延性が高く耐震性に優れ、かつ超大入熱溶接熱影響部靭性に優れた高強度厚鋼板を、安価にしかも生産性高く製造できるという、産業上格段の効果を奏する。また、本発明は、鋼構造物の大型化や、耐震性の向上、施工効率の向上などに、大きく寄与するという効果もある。   According to the present invention, suitable for building structure, tensile strength TS: 590 MPa or more, yield ratio YR: 80% or less, surface hardness is 280HV10 or less, surface ductility is high and excellent in earthquake resistance, In addition, a high-strength thick steel plate excellent in super-high heat input welding heat-affected zone toughness can be produced at low cost and with high productivity. Moreover, this invention also has the effect of making a big contribution to the enlargement of a steel structure, the improvement of earthquake resistance, the improvement of construction efficiency, etc.

表層硬さ、表層延性、および板厚方向1/4位置の引張特性に及ぼすC量の影響を示すグラフである。It is a graph which shows the influence of the amount of C exerted on the surface layer hardness, the surface layer ductility, and the tensile properties at 1/4 position in the sheet thickness direction.

まず、本発明高強度厚鋼板の組成限定理由について説明する。以下、とくに断わらない限り、質量%は単に%と記す。
C:0.03〜O.07%
Cは、鋼の強度を増加させ、構造用鋼材として所望の高強度を確保するのに有用であり、本発明では、0.03%以上の含有を必要とする。また、Cは、表面硬さに影響する元素であり、表面硬さ(表層硬さ)の低減や表層延性の低下抑制のため、本発明では、0.07%以下に限定した。なお、好ましくは0.04〜0.07%である。
First, the reason for limiting the composition of the high-strength thick steel plate of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply referred to as%.
C: 0.03 ~ O.07%
C is useful for increasing the strength of steel and ensuring a desired high strength as a structural steel material. In the present invention, C is required to be contained in an amount of 0.03% or more. Further, C is an element that affects the surface hardness, and is limited to 0.07% or less in the present invention in order to reduce the surface hardness (surface layer hardness) and suppress the decrease in surface layer ductility. In addition, Preferably it is 0.04 to 0.07%.

Si:0.05〜0.5%
Siは、脱酸剤として作用するとともに、母材の強度を高める元素であり、本発明では0.05%以上の含有を必要とする。一方、0.5%を超える含有は、HAZでのMAの生成が促進され、HAZ靭性の低下が著しくなる。このため、本発明ではSiは、0.05〜0.5%に限定した。なお、好ましくは0.05〜0.4%である。
Si: 0.05-0.5%
Si is an element that acts as a deoxidizer and increases the strength of the base material, and in the present invention, it needs to be contained in an amount of 0.05% or more. On the other hand, if the content exceeds 0.5%, the formation of MA in HAZ is promoted, and the reduction in HAZ toughness becomes significant. For this reason, Si was limited to 0.05 to 0.5% in the present invention. In addition, Preferably it is 0.05 to 0.4%.

Mn:0.6〜2.0%
Mnは、固溶強化により、鋼の強度を増加させる作用を有する元素であり、所望の高強度を確保するために、0.6%以上の含有を必要とする。一方、2.0%を超えて含有すると、溶接性の低下が著しくなる。このため、Mnは0.6〜2.0%の範囲に限定した。なお、好ましくは0.6〜1.6%である。
Mn: 0.6-2.0%
Mn is an element having an action of increasing the strength of steel by solid solution strengthening, and needs to be contained in an amount of 0.6% or more in order to ensure a desired high strength. On the other hand, when it contains exceeding 2.0%, the weldability will fall remarkably. For this reason, Mn was limited to the range of 0.6 to 2.0%. In addition, Preferably it is 0.6 to 1.6%.

P:0.020%以下
Pは、不純物として混入する元素であり、靭性を低下させるため、本発明ではできるだけ低減することが望ましいが、0.020%程度までは許容できる。このため、Pは0.020%以下に限定した。なお、好ましくは0.015%以下である。
S:0.0005〜0.003%
Sは、Ca、Mnと結合してCaS、MnSを形成する元素である。CaSは、MnSの生成核として作用し、CaSを核として生成したMnSが、超大入熱溶接部の、旧オーステナイト粒内で粒内フェライトの生成サイトとして機能し、HAZ組織の微細化に寄与し、HAZ靱性を向上させる効果を有する。このような効果を得るためには、0.0005%以上の含有を必要とする。一方、0.003%を超える含有は、MnSの多量生成を生じ、MnS生成に起因した板厚方向(Z方向)の材質劣化を誘起させる。このため、本発明では、Sは0.0005〜0.003%に限定した。なお、好ましくは0.0010〜0.003%である。
P: 0.020% or less P is an element mixed as an impurity and reduces toughness. Therefore, it is desirable to reduce it as much as possible in the present invention, but it is acceptable up to about 0.020%. For this reason, P was limited to 0.020% or less. In addition, Preferably it is 0.015% or less.
S: 0.0005-0.003%
S is an element that combines with Ca and Mn to form CaS and MnS. CaS acts as a production nucleus of MnS, and MnS produced using CaS as a nucleus functions as a production site of intragranular ferrite in the prior austenite grains of the super-high heat input weld, contributing to refinement of the HAZ structure. , HAZ has the effect of improving toughness. In order to acquire such an effect, 0.0005% or more needs to be contained. On the other hand, if the content exceeds 0.003%, a large amount of MnS is generated, and material deterioration in the plate thickness direction (Z direction) due to MnS generation is induced. For this reason, in this invention, S was limited to 0.0005 to 0.003%. In addition, Preferably it is 0.0010 to 0.003%.

Ti:0.005〜0.03%
Tiは、Nとの親和力が強くTiNとして析出し、HAZでのオーステナイト粒の粗大化を抑制し、あるいはフェライト変態核としてHAZの高靱化に寄与する元素である。このような効果を得るためには、0.005%以上の含有を必要とする。一方、0.03%を超える含有は、TiCが析出し、母材靭性、HAZ靭性が低下する。このため、Tiは0.005〜0.03%の範囲に限定した。なお、好ましくは、0.O08〜0.015%である。
Ti: 0.005-0.03%
Ti is an element that has a strong affinity for N and precipitates as TiN, suppresses the coarsening of austenite grains in HAZ, or contributes to high toughness of HAZ as a ferrite transformation nucleus. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, if the content exceeds 0.03%, TiC precipitates and the base metal toughness and the HAZ toughness are lowered. For this reason, Ti was limited to the range of 0.005 to 0.03%. In addition, Preferably, it is 0.00 to 0.015%.

B:0.0003〜0.0020%
Bは、少量の含有で焼入れ性を向上させ、母材強度を増加させる元素である。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、0.0020%を超える含有は、溶接性を低下させる。このため、Bは0.0003〜0.0020%の範囲に限定した。なお、好ましくは0.0005〜0.0015%である。
B: 0.0003-0.0020%
B is an element that improves the hardenability and increases the strength of the base metal when contained in a small amount. In order to acquire such an effect, 0.0003% or more needs to be contained. On the other hand, the content exceeding 0.0020% lowers the weldability. For this reason, B was limited to the range of 0.0003 to 0.0020%. In addition, Preferably it is 0.0005 to 0.0015%.

Ca:0.0005〜0.005%
Caは、硫化物を形成し、MnSの生成核として作用する元素である。CaSを核として生成したMnSが、超大入熱溶接部の、旧オーステナイト粒内で粒内フェライトの生成サイトとして機能し、HAZ組織の微細化に寄与し、HAZ靱性を向上させる効果を有する。このような効果を得るためには、Caは0.0005%以上の含有を必要とする。一方、0.005%を超える含有は、Ca系酸化物が増加し、鋼の清浄度を低下させる。このため、Caは0.0005〜0.005%の範囲に限定した。なお、好ましくは0.0005〜0.0020%である。
Ca: 0.0005 to 0.005%
Ca is an element that forms sulfides and acts as a production nucleus of MnS. MnS produced using CaS as a nucleus functions as a site for formation of intragranular ferrite in the prior austenite grains of the super high heat input weld, contributing to refinement of the HAZ structure and improving HAZ toughness. In order to acquire such an effect, Ca needs to contain 0.0005% or more. On the other hand, if the content exceeds 0.005%, Ca-based oxides increase and the cleanliness of the steel decreases. For this reason, Ca was limited to 0.0005 to 0.005% of range. In addition, Preferably it is 0.0005 to 0.0020%.

N:0.0070%以下
Nは、Tiと結合しTiNを形成し、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の組織の微細化に有効に作用し、HAZの高靱化に寄与する。このような効果は、0.0025%以上含有することが望ましいが、0.0070%を超えると、固溶N量が増加し、HAZ靭性を低下させる。このため、Nは0.0070%以下に限定した。なお、好ましくは0.0060%以下である。
N: 0.0070% or less N combines with Ti to form TiN, suppresses the coarsening of austenite grains, or effectively acts as a ferrite transformation nucleus to refine the structure of the HAZ, thereby increasing the toughness of the HAZ Contribute. Such an effect is desirably contained in an amount of 0.0025% or more. However, if it exceeds 0.0070%, the amount of solute N increases and the HAZ toughness decreases. For this reason, N was limited to 0.0070% or less. In addition, Preferably it is 0.0060% or less.

O:0.003%以下
Oは、不純物として混入する元素であり、できるだけ低減することが望ましいが、過度の低減は、溶製コストの高騰を招くため、0.0030%程度以下に限定した。0.003%を超えて含有すると、酸化物系介在物が増加し、鋼の清浄度を低下させる。なお、好ましくは0.0025%以下である。
O: 0.003% or less O is an element mixed as an impurity, and it is desirable to reduce it as much as possible. However, excessive reduction causes an increase in melting cost, so it is limited to about 0.0030% or less. If the content exceeds 0.003%, oxide inclusions increase and the cleanliness of the steel decreases. In addition, Preferably it is 0.0025% or less.

Mo:0.01%以下、Nb:0.005%以下
Mo、Nbはいずれも、微量の含有でもHAZの焼入れ性を増大させ、HAZにおけるフェライトの生成を抑制し、HAZ組織を上部ベイナイト化し、HAZ靭性を低下させる。このため、本発明では、不可避的不純物として、できるだけ低減することが好ましく、Moは0.01%以下に、Nbは0.005%以下に調整する。
Mo: 0.01% or less, Nb: 0.005% or less
Both Mo and Nb increase the hardenability of HAZ even when contained in a trace amount, suppress the formation of ferrite in HAZ, turn the HAZ structure into upper bainite, and lower the HAZ toughness. For this reason, in this invention, it is preferable to reduce as much as possible as an inevitable impurity, Mo is adjusted to 0.01% or less, and Nb is adjusted to 0.005% or less.

炭素当量Ceq:0.40〜0.45%
本発明では、Ceqは、次(1)式で定義されるものを使用する。なお、(1)式に記載された元素が含有されない場合には、その元素を零として計算するものとする。
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ‥‥(1)
(ここで、C、Mn、Cu、Ni、Cr、Mo、V:各元素の含有量(質量%))
Ceqは、所望の強度確保、超大入熱溶接HAZ靭性確保の観点から0.40〜0.45%に調整する。Ceqが、0.40未満では、所望の高強度を確保できない。一方、0.45%を超えると、HAZ靭性が低下し、所望の超大入熱溶接HAZ靭性を確保できなくなる。このため、炭素当量Ceqは0.40〜0.45%の範囲に限定した。
Carbon equivalent Ceq: 0.40-0.45%
In the present invention, Ceq is defined by the following formula (1). In addition, when the element described in Formula (1) is not contained, the element is calculated as zero.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
(Here, C, Mn, Cu, Ni, Cr, Mo, V: content of each element (mass%))
Ceq is adjusted to 0.40 to 0.45% from the viewpoint of securing a desired strength and super large heat input welding HAZ toughness. If Ceq is less than 0.40, the desired high strength cannot be secured. On the other hand, if it exceeds 0.45%, the HAZ toughness decreases, and the desired super-high heat input welding HAZ toughness cannot be ensured. For this reason, the carbon equivalent Ceq was limited to the range of 0.40 to 0.45%.

ACR:0.2〜0.8
ACRは、次(2)式で定義される。
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%))
ACRは、超大入熱溶接HAZ靭性を確保するという観点から0.2〜0.8に調整する。ACRが0.2未満では、粒内フェライトの核生成に必要なCa系硫化物の生成量が少なく、所望の超大入熱溶接HAZ靭性を確保できなくなる。一方、ACRが0.8を超えて大きくなると、Ca系硫化物が生成しても、それを核としてMnSが生成せず、粒内フェライト生成による超大入熱溶接HAZ組織の微細化が達成できないため、所望の超大入熱溶接HAZ靭性を確保できなくなる。このため、ACRは0.2〜0.8の範囲に限定した。
ACR: 0.2-0.8
ACR is defined by the following equation (2).
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) (2)
(Where Ca, O, S: content of each element (mass%))
The ACR is adjusted to 0.2 to 0.8 from the viewpoint of securing super large heat input welding HAZ toughness. If the ACR is less than 0.2, the amount of Ca-based sulfide required for nucleation of intragranular ferrite is small, and the desired super-high heat input welding HAZ toughness cannot be ensured. On the other hand, when ACR exceeds 0.8, even if Ca-based sulfide is generated, MnS is not generated using it as a nucleus, and refinement of the super high heat input welding HAZ structure due to intragranular ferrite generation cannot be achieved. Desired super high heat input welding HAZ toughness cannot be secured. For this reason, ACR was limited to the range of 0.2-0.8.

上記した成分が基本の成分であるが、本発明では、これら基本の組成に加えて、選択成分として、さらにCu:0.5%以下、Ni:1.0%以下、Cr:0.5%以下、V:0.08%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、Al:0.1%以下、および/または、Mg:0.005%以下、REM:0.02%以下のうちから選ばれた1種または2種、を選択して含有してもよい。   The above-mentioned components are basic components, but in the present invention, in addition to these basic compositions, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, V: 0.08% One or more selected from the following, and / or Al: 0.1% or less, and / or Mg: 0.005% or less, REM: 0.02% or less , May be selected and contained.

Cu:0.5%以下、Ni:1.0%以下、Cr:0.5%以下、V:0.08%以下のうちから選ばれた1種または2種以上
Cu、Ni、Cr、Vはいずれも、鋼の強度増加に寄与する元素であり、必要に応じて選択して1種または2種以上を含有できる。
Cuは、固溶強化を介して、強度増加に有効に寄与する元素である。このような効果を得るためには、0.1%以上含有することが望ましいが、0.5%を超える含有は、熱間延性の低下、表面疵の増加など、鋼板の製造性に問題を生じる。このため、含有する場合には、Cuは0.5%以下に限定することが好ましい。
One or more selected from Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, V: 0.08% or less
Cu, Ni, Cr, and V are all elements that contribute to increasing the strength of the steel, and can be selected as necessary to contain one or more.
Cu is an element that contributes effectively to an increase in strength through solid solution strengthening. In order to acquire such an effect, it is desirable to contain 0.1% or more. However, the content exceeding 0.5% causes problems in the productivity of the steel sheet such as a decrease in hot ductility and an increase in surface defects. For this reason, when it contains, it is preferable to limit Cu to 0.5% or less.

Niは、固溶強化を介して、強度増加に有効に寄与する元素である。このような効果を得るためには、0.1%以上含有することが望ましいが、高価なNiの1.0%を超える含有は、材料コストの高騰を招く。このため、含有する場合には、Niは1.0%以下に限定することが好ましい。
Crは、固溶強化を介して、強度増加に有効に寄与する元素である。このような効果を得るためには、0.1%以上含有することが望ましいが、0.5%を超える含有は、溶接性の低下を招く。このため、含有する場合には、Crは0.5%以下に限定することが好ましい。
Ni is an element that contributes effectively to an increase in strength through solid solution strengthening. In order to acquire such an effect, it is desirable to contain 0.1% or more. However, containing more than 1.0% of expensive Ni causes an increase in material cost. For this reason, when it contains, it is preferable to limit Ni to 1.0% or less.
Cr is an element that contributes effectively to an increase in strength through solid solution strengthening. In order to acquire such an effect, it is desirable to contain 0.1% or more, but inclusion exceeding 0.5% causes a decrease in weldability. For this reason, when contained, Cr is preferably limited to 0.5% or less.

Vは、固溶強化さらには析出強化を介して、強度増加に有効に寄与する元素である。このような効果を得るためには、0.02%以上含有することが望ましいが、0.08%を超える多量の含有は、材料コストの高騰を招く。このため、含有する場合には、Vは0.08%以下に限定することが好ましい。
Al:0.1%以下
Alは、脱酸剤として作用する元素であり、また、鋼中のNをAlNとして固定し,Nによる靭性低下や割れ発生を防止する作用も有する。このような効果を得るためには0.015%以上含有することが望ましいが、0.1%を超える多量の含有は、多量のアルミナ(介在物)を形成し鋼の清浄度を低下させる。このため、含有する場合には、Alは0.1%以下に限定することが好ましい。
V is an element that contributes effectively to an increase in strength through solid solution strengthening and precipitation strengthening. In order to obtain such an effect, it is desirable to contain 0.02% or more. However, if the content exceeds 0.08%, the material cost increases. For this reason, when it contains, it is preferable to limit V to 0.08% or less.
Al: 0.1% or less
Al is an element that acts as a deoxidizer, and also has an effect of fixing N in steel as AlN and preventing toughness reduction and cracking due to N. In order to obtain such an effect, it is desirable to contain 0.015% or more. However, if it contains more than 0.1%, a large amount of alumina (inclusions) is formed and the cleanliness of the steel is lowered. For this reason, when it contains, it is preferable to limit Al to 0.1% or less.

Mg:0.005%以下、REM:0.02%以下のうちから選ばれた1種または2種
Mg、REMはいずれも、酸化物や硫化物を形成し、HAZの微細フェライトの生成を促進し、HAZの組織微細化に有効に寄与する元素であり、必要に応じて選択して1種または2種を含有できる。このような効果を得るためには、Mg:0.0005%以上、REM:0.0005%以上、それぞれ含有することが望ましいが、Mg:0.005%、REM:0.02%を、それぞれ超える含有は、鋼の清浄度を低下させる。このため、含有する場合には、Mg:0.005%以下、REM:0.02%以下に限定することが好ましい。
One or two selected from Mg: 0.005% or less and REM: 0.02% or less
Mg and REM are both elements that form oxides and sulfides, promote the formation of fine ferrite of HAZ, and contribute effectively to the refinement of the HAZ structure. Two types can be contained. In order to obtain such effects, it is desirable to contain Mg: 0.0005% or more and REM: 0.0005% or more, respectively, but Mg: 0.005% and REM: 0.02%, respectively, the contents exceeding respectively, cleanliness of steel Reduce. For this reason, when it contains, it is preferable to limit to Mg: 0.005% or less and REM: 0.02% or less.

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。不可避的不純物としては、Sn:0.005%以下、Sb:0.005%以下が許容できる。
また、本発明の厚鋼板は、上記した組成を有し、フェライト分率が体積率で10〜40%である組織を有する。
フェライト分率が10%未満では、降伏比80%以下を確保できない。一方、40%を超えると、所望の高強度が確保できなくなる。このため、フェライト分率は10〜40%に限定した。なお、フェライト相以外の組織はとくに限定する必要はないが、所望の高強度を確保するという観点からは、靭性に優れた低温変態相(ベイナイト相、マルテンサイト相)とすることが好ましい。
The balance other than the above components is Fe and inevitable impurities. As unavoidable impurities, Sn: 0.005% or less and Sb: 0.005% or less are acceptable.
The thick steel plate of the present invention has the above-described composition and has a structure in which the ferrite fraction is 10 to 40% by volume.
If the ferrite fraction is less than 10%, a yield ratio of 80% or less cannot be secured. On the other hand, if it exceeds 40%, a desired high strength cannot be secured. For this reason, the ferrite fraction was limited to 10 to 40%. The structure other than the ferrite phase is not particularly limited. However, from the viewpoint of ensuring a desired high strength, a low-temperature transformation phase (bainite phase, martensite phase) excellent in toughness is preferable.

本発明の厚鋼板は、上記した組成と上記した組織とを有し、表層硬さHVが280 HV10以下の鋼板である。表層硬さHVが280 HV10を超えると、図1に示すように表層延性が低下し、耐震性が低下するため、本発明では、表層硬さHVを280 HV10以下に限定した。なお、ビッカース硬さ計(荷重10kgf(試験力98N))を用いて、表面下0.5mm位置の硬さHVを20点測定し、その最大値をその鋼板の表層硬さHV10とする。   The thick steel plate of the present invention is a steel plate having the above composition and the above structure and having a surface hardness HV of 280 HV10 or less. When the surface hardness HV exceeds 280 HV10, the surface layer ductility decreases and the earthquake resistance decreases as shown in FIG. 1, and therefore the surface hardness HV is limited to 280 HV10 or less in the present invention. In addition, using a Vickers hardness tester (load 10 kgf (test force 98 N)), measure the hardness HV at a position 0.5 mm below the surface at 20 points, and set the maximum value as the surface hardness HV10 of the steel sheet.

つぎに、本発明厚鋼板の好ましい製造方法について説明する。
本発明では、上記した組成を有する鋼素材に、加熱し熱間圧延を施す圧延工程と、該圧延工程終了後に加速冷却を施す加速冷却工程と、あるいはさらに焼戻工程を行って、所定の板厚の厚鋼板とする。
使用する鋼素材は、上記した組成を有していれば、その製造方法はとくに限定する必要はないが、上記した組成の溶鋼を、転炉、電気炉、真空溶解炉等の常用の溶製方法で溶製し、必要に応じてさらに脱酸処理や脱ガスプロセス等を経て、連続鋳造法等の鋳造法によりスラブ等の鋼素材とすることが好ましい。
Below, the preferable manufacturing method of this invention steel plate is demonstrated.
In the present invention, a steel plate having the above-described composition is subjected to a rolling process for heating and hot rolling, an accelerated cooling process for performing accelerated cooling after completion of the rolling process, or a tempering process, and a predetermined plate. Thick steel plate.
As long as the steel material to be used has the above-described composition, the production method is not particularly limited. However, the molten steel having the above-described composition is used for ordinary melting such as a converter, an electric furnace, a vacuum melting furnace, and the like. It is preferable that the steel material is melted by a method, and further subjected to a deoxidation treatment, a degassing process, or the like as necessary, and a steel material such as a slab is obtained by a casting method such as a continuous casting method.

圧延工程は、上記した組成の鋼素材を1000〜1200℃に加熱したのち、圧延終了温度をAr変態点以上とする熱間圧延を施し厚鋼板とする工程とする。
加熱温度:1000〜1200℃
加熱温度が1000℃未満では、圧延における変形抵抗が高くなりすぎて、厚板圧延の圧延負荷が高くなり、圧延が困難となる場合がある。一方、1200℃を超えて高温になると、組織が粗大化するため、それを引き継いだ母材(厚鋼板)の靭性が低下する。このため、加熱温度は1000〜1200℃に限定した。
The rolling process is a process in which a steel material having the above composition is heated to 1000 to 1200 ° C., and then subjected to hot rolling with a rolling end temperature equal to or higher than the Ar 3 transformation point to form a thick steel plate.
Heating temperature: 1000 ~ 1200 ℃
When the heating temperature is less than 1000 ° C., the deformation resistance in rolling becomes too high, the rolling load of thick plate rolling becomes high, and rolling may be difficult. On the other hand, when the temperature exceeds 1200 ° C., the structure becomes coarse, so that the toughness of the base material (thick steel plate) that inherits it decreases. For this reason, heating temperature was limited to 1000-1200 degreeC.

圧延終了温度:Ar変態点以上
圧延終了温度がAr変態点未満では、圧延中にフェライトが生成し、微細化して、降伏比を高くする。このため、圧延終了温度はAr変態点以上に限定した。なお、好ましくは800℃以上である。
圧延工程終了後に施す、加速冷却工程は、途中に保持を含み、一次冷却と二次冷却からなる二段階冷却とする。一次冷却では、圧延工程終了後60s以内に冷却を開始し、厚鋼板の表層部の温度で、平均冷却速度が100℃/s以上で、冷却停止温度:700℃以下となるまで冷却する。
Rolling end temperature: Ar 3 transformation point or higher If the rolling end temperature is less than the Ar 3 transformation point, ferrite is generated during rolling, and is refined to increase the yield ratio. For this reason, the rolling end temperature is limited to the Ar 3 transformation point or higher. In addition, Preferably it is 800 degreeC or more.
The accelerated cooling process performed after the rolling process is completed is a two-stage cooling process including primary cooling and secondary cooling including holding in the middle. In the primary cooling, cooling is started within 60 s after the end of the rolling process, and cooling is performed until the average cooling rate is 100 ° C./s or higher and the cooling stop temperature is 700 ° C. or lower at the surface layer portion temperature of the thick steel plate.

冷却開始時間:60s以内
冷却開始時間が圧延工程終了後60sを超えて遅くなると、表面近傍で粗大なフェライト相が生成し、強度が低下し、靭性の劣化が生じる。このため、冷却(一次冷却)の開始は圧延工程終了後60s以内に限定した。
一次冷却の平均冷却速度:100℃/s以上
一次冷却は、とくに表層部にフェライト相を生成させ、表層部硬さを所望の硬さ(280HV10)以下に調整することを目的とする。なお、一次冷却は、表層温度を基準として調整する。一次冷却の平均冷却速度は、得られる表層部の結晶粒径と関連し、冷却速度が速ければ速いほど、過冷オーステナイトを得やすく、得られるフェライト相の粒径も細かくなる。一次冷却の平均冷却速度が100℃/s未満では、フェライト相の粒径が5μmを超えて大きくなり、表層部の靭性が低下する。このため、一次冷却の平均冷却速度を100℃/s以上に限定した。なお、一次冷却の冷却速度の上限はとくに限定する必要はない。
Cooling start time: within 60 s When the cooling start time is over 60 s after the end of the rolling process, a coarse ferrite phase is generated near the surface, the strength is lowered, and the toughness is deteriorated. For this reason, the start of cooling (primary cooling) was limited to within 60 s after the end of the rolling process.
Average cooling rate of primary cooling: 100 ° C./s or more Primary cooling is aimed at adjusting the hardness of the surface layer part to a desired hardness (280 HV10) or less, particularly by forming a ferrite phase in the surface layer part. The primary cooling is adjusted based on the surface layer temperature. The average cooling rate of the primary cooling is related to the crystal grain size of the obtained surface layer portion, and the faster the cooling rate, the easier it is to obtain supercooled austenite and the smaller the ferrite phase grain size obtained. When the average cooling rate of primary cooling is less than 100 ° C./s, the grain size of the ferrite phase exceeds 5 μm, and the toughness of the surface layer portion decreases. For this reason, the average cooling rate of primary cooling was limited to 100 ° C./s or more. In addition, the upper limit of the cooling rate of primary cooling does not need to be specifically limited.

一次冷却の冷却停止温度:700℃以下
一次冷却の冷却停止温度が、700℃を超えて高くなると、表層部にフェライト相の生成が見られず、所望の低い表層硬さが得られず、さらに表層の延性の改善が得られない。一次冷却の冷却停止温度を700℃以下とすることにより、その後の保持過程で表層部が復熱し、復熱過程で表層部にフェライト相が生成する。このため、一次冷却の冷却停止温度は700℃以下に限定した。
Cooling stop temperature for primary cooling: 700 ° C or less If the cooling stop temperature for primary cooling exceeds 700 ° C, the formation of ferrite phase in the surface layer is not observed, and the desired low surface hardness cannot be obtained. Improve surface ductility. By setting the cooling stop temperature of primary cooling to 700 ° C. or lower, the surface layer portion recuperates in the subsequent holding process, and a ferrite phase is generated in the surface layer portion in the reheating process. For this reason, the cooling stop temperature of the primary cooling is limited to 700 ° C. or lower.

一次冷却後、本発明では30〜180s間、冷却を停止してそのまま保持し、鋼板の復熱を図る。
保持時間:30〜180s
保持時間が30s未満と短いと、復熱が不十分で、この復熱過程で表層部にフェライト相の生成が認められず、所望の低い表層硬さを確保できなくなる。一方、180sを超えて長くなると、フェライト相の成長が著しくなり、強度低下が生じる。このため、一次冷却後の保持時間は30〜180sの範囲に限定した。
After the primary cooling, in the present invention, the cooling is stopped and held as it is for 30 to 180 s to reheat the steel sheet.
Holding time: 30-180s
When the holding time is as short as less than 30 s, recuperation is insufficient, and the formation of a ferrite phase in the surface layer portion is not recognized during this recuperation process, and a desired low surface hardness cannot be ensured. On the other hand, if the length exceeds 180 s, the ferrite phase grows remarkably and the strength is reduced. For this reason, the holding time after primary cooling was limited to the range of 30 to 180 s.

一次冷却後の保持を終了したのち、二次冷却を行う。二次冷却は、所望の高強度を確保するために実施する。そのため、軟質のフェライト相の生成を所定の範囲内とし、かつ低温変態生成相の生成を促進させる冷却とする。二次冷却では、平均冷却速度:3℃/s以上で、冷却停止温度:400〜200℃または400〜50℃の範囲の温度まで冷却する。なお、二次冷却は、板厚中心部の温度で調整する。   After completing the holding after the primary cooling, the secondary cooling is performed. Secondary cooling is performed to ensure the desired high strength. Therefore, the cooling is performed so that the generation of the soft ferrite phase is within a predetermined range and the generation of the low-temperature transformation generation phase is promoted. In the secondary cooling, the cooling is performed at an average cooling rate of 3 ° C./s or more and a cooling stop temperature of 400 to 200 ° C. or a temperature in the range of 400 to 50 ° C. Secondary cooling is adjusted by the temperature at the center of the plate thickness.

二次冷却の平均冷却速度:3℃/s以上
二次冷却の平均冷却速度:3℃/s未満では、フェライト相の生成が促進され、しかも粗大化しやすく、所望の高強度を確保できなくなり、靭性も低下する。このため、二次冷却の平均冷却速度:3℃/s以上に限定した。
二次冷却の冷却停止温度:400〜200℃または400〜50℃
二次冷却の冷却停止温度が、400℃を超えて高温となると、所望の高強度を確保できなくなる。一方、200℃未満では、冷却時の歪等の影響で鋼板形状が乱れる場合が多い。このため、二次冷却の冷却停止温度は400〜200℃の範囲に限定した。なお、加速冷却工程後に焼戻工程を施す場合には、焼戻時に鋼板形状の矯正を行うことができるため、二次冷却の冷却停止温度はより低温まで拡大することができる。このようなことから、焼戻工程を実施する場合には、二次冷却の冷却停止温度は400〜50℃の範囲としてもよい。
Average cooling rate of secondary cooling: 3 ° C./s or more Average cooling rate of secondary cooling: less than 3 ° C./s, the formation of ferrite phase is promoted, and it tends to be coarsened, making it impossible to secure a desired high strength. Toughness also decreases. For this reason, it was limited to the average cooling rate of secondary cooling: 3 ° C./s or more.
Secondary cooling stop temperature: 400-200 ° C or 400-50 ° C
When the cooling stop temperature of the secondary cooling is higher than 400 ° C., the desired high strength cannot be secured. On the other hand, when the temperature is lower than 200 ° C., the shape of the steel sheet is often disturbed due to the influence of distortion during cooling. For this reason, the cooling stop temperature of the secondary cooling is limited to a range of 400 to 200 ° C. In addition, when performing a tempering process after an accelerated cooling process, since a steel plate shape can be corrected at the time of tempering, the cooling stop temperature of secondary cooling can be extended to a lower temperature. For this reason, when performing the tempering step, the cooling stop temperature of the secondary cooling may be in the range of 400 to 50 ° C.

加速冷却工程終了後は、空冷とする。なお、加速冷却工程終了後に、さらに焼戻工程を施しても良い。焼戻温度は450℃以下とすることが好ましい。
焼戻温度:450℃以下
焼戻温度が450℃を超えて高温となると、鋼板強度が低下し、また降伏比も上昇する。このため、焼戻温度は450℃以下に限定することが好ましい。
After the accelerated cooling process, air cooling is performed. In addition, you may perform a tempering process after completion | finish of an accelerated cooling process. The tempering temperature is preferably 450 ° C. or lower.
Tempering temperature: 450 ° C. or less When the tempering temperature exceeds 450 ° C. and becomes high, the strength of the steel sheet decreases and the yield ratio also increases. For this reason, it is preferable to limit the tempering temperature to 450 ° C. or lower.

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

表1に示す組成の溶鋼を転炉、取鍋精錬で溶製し、連続鋳造法でスラブ(肉厚:250mm)とし鋼素材とした。これら鋼素材に、表2に示す条件で熱間圧延を行う圧延工程と、圧延工程後、表2に示す条件で二段階の加速冷却を行う加速冷却工程と、あるいはさらに表2に示す条件で焼戻を行う焼戻工程と、を施し、表2に示す板厚の厚鋼板とした。
得られた厚鋼板から、試験片を採取して、母材の組織観察、引張試験、硬さ試験、溶接熱影響部靭性試験を実施した。試験方法は次のとおりとした。
Molten steel having the composition shown in Table 1 was smelted by a converter and ladle refining and made into a slab (wall thickness: 250 mm) by a continuous casting method as a steel material. These steel materials are subjected to a rolling process in which hot rolling is performed under the conditions shown in Table 2, an accelerated cooling process in which two-stage accelerated cooling is performed under the conditions shown in Table 2 after the rolling process, or further in conditions shown in Table 2. A tempering step for tempering was performed to obtain a thick steel plate having a thickness shown in Table 2.
A test piece was collected from the obtained thick steel plate and subjected to a structure observation of the base material, a tensile test, a hardness test, and a weld heat affected zone toughness test. The test method was as follows.

(1)母材の組織観察
得られた厚鋼板の板厚1/4位置から組織観察用試験片を採取し、圧延方向断面を研磨し、ナイタール液で腐食し、光学顕微鏡(倍率:400倍)または走査型電子顕微鏡(倍率:1000倍)を用いて、フェライト相等の組織を同定し、画像解析装置を用いて、各相の組織分率(体積率)を測定した。
(1) Microstructure observation of base material A specimen for microstructural observation was taken from the 1/4 thickness position of the obtained thick steel plate, the cross section in the rolling direction was polished, corroded with a nital solution, and an optical microscope (magnification: 400 times) ) Or a scanning electron microscope (magnification: 1000 times), a structure such as a ferrite phase was identified, and a structure fraction (volume ratio) of each phase was measured using an image analysis apparatus.

(2)引張試験
得られた厚鋼板の板厚1/4位置から、圧延方向と直交する方向が引張方向となるように、J1S4号引張試験片を採取し、J1S Z 2241の規定に準拠して引張試験を実施し、引張特性(YS,TS)を測定し、降伏比YRを算出した。
(3)硬さ試験
得られた厚鋼板の表層部から硬さ測定用試験片を採取し、ビッカース硬さ計(荷重:10kgf(試験力:98N))を用いて、表面下0.5mm位置の硬さHVを20点測定し、その最大値をその鋼板の表層硬さHV10とした。
(2) Tensile test J1S No. 4 tensile test specimen was taken from the position of 1/4 thickness of the obtained steel plate so that the direction perpendicular to the rolling direction was the tensile direction, and in accordance with the provisions of J1S Z 2241 Tensile tests were carried out, the tensile properties (YS, TS) were measured, and the yield ratio YR was calculated.
(3) Hardness test Take a test piece for hardness measurement from the surface layer of the obtained thick steel plate and use a Vickers hardness tester (load: 10kgf (test force: 98N)) at a position 0.5mm below the surface. The hardness HV was measured at 20 points, and the maximum value was defined as the surface hardness HV10 of the steel sheet.

(4)溶接熱影響部靭性試験
得られた厚鋼板からダイヤフラム厚60mmとし、エレクトロスラグ溶接ESW(溶接入熱量:960kJ/cm)により溶接継手(ESW継手)を作製した。
得られた溶接継手から、試験片の切欠き位置をボンド部から1mm離れた位置のHAZとするVノッチ試験片を採取し,JlS Z 2242の規定に準拠して、シャルピー衝撃試験を実施し、試験温度:0℃における吸収エネルギー(vEo)を求め、超大入熱溶接HAZ靱性を評価した。なお、吸収エネルギー値は、試験片3本の平均値とした。
(4) Weld heat affected zone toughness test A diaphragm thickness of 60 mm was obtained from the obtained steel plate, and a welded joint (ESW joint) was produced by electroslag welding ESW (welding heat input: 960 kJ / cm).
From the obtained welded joint, a V-notch test piece with HAZ at a position 1 mm away from the bond portion is taken from the notch of the test piece, and a Charpy impact test is performed in accordance with the provisions of JlS Z 2242. Test temperature: Absorbed energy (vEo) at 0 ° C. was determined, and super large heat input welding HAZ toughness was evaluated. The absorbed energy value was an average value of three test pieces.

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

本発明例はいずれも、引張強さ:590 MPa以上で、降伏比:80%以下と、所望の高強度で低降伏比を有し、しかも280HV10以下の低い表層硬さを有する厚鋼板となっている。しかも、本発明例はいずれも、溶接入熱量が400kJ/cmを超える960 kJ/cmの超大入熟溶接HAZ部において、vEoが70J以上を示しており、超大入熱溶接熱影響部靭性に優れた低降伏比厚鋼板となっている。   Each of the inventive examples is a steel plate having a tensile strength of 590 MPa or more, a yield ratio of 80% or less, a desired high strength and low yield ratio, and a low surface hardness of 280HV10 or less. ing. In addition, in all of the examples of the present invention, vEo is 70 J or more in the super-high-ripening weld HAZ part where the welding heat input exceeds 400 kJ / cm and is 960 kJ / cm, and the super-high heat input welding heat-affected zone toughness is excellent. It has a low yield specific thickness steel plate.

一方、本発明の範囲を外れる比較例は、所望の強度を満足できていないか、降伏比が80%以上と高いか、表層硬さが280HV10を超えて高いか、あるいはvEoが70J未満と溶接熱影響部靭性が低下している。   On the other hand, comparative examples that are out of the scope of the present invention are those that do not satisfy the desired strength, the yield ratio is as high as 80% or higher, the surface hardness is higher than 280HV10, or vEo is less than 70J. Heat affected zone toughness is reduced.

Claims (9)

質量%で、
C:0.03〜O.07%、 Si:0.05〜0.5%、
Mn:0.6〜2.0%、 P:0.020%以下、
S:0.0005〜0.003%、 Ti:0.005〜0.03%、
B:0.0003〜0.0020%、 Ca:0.0005〜0.005%、
N:0.0070%以下、 O:0.003%以下
を含み、不純物としてMo、Nbを、Mo:0.01%以下、Nb:0.005%以下に調整し、下記(1)式で定義される炭素当量Ceqが0.40〜0.45%、下記(2)式で定義されるACRが0.2〜0.8を満足し、残部Feおよび不可避的不純物からなる組成と、フェライト分率が体積率で10〜40%である組織を有し、引張強さTS590MPa以上、降伏比80%以下で、表層硬さHVが280HV10以下であることを特徴とする超大入熱溶接熱影響部靭性に優れた低降伏比建築構造用厚鋼板。

Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ‥‥(1)
ACR=(Ca−(0.18+130×Ca)×O)/(1.25×S) ‥‥(2)
ここで、C、Mn、Cu、Ni、Cr、Mo、V、Ca、O、S:各元素の含有量(質量%)
% By mass
C: 0.03-0.07%, Si: 0.05-0.5%,
Mn: 0.6 to 2.0%, P: 0.020% or less,
S: 0.0005-0.003%, Ti: 0.005-0.03%,
B: 0.0003 to 0.0020%, Ca: 0.0005 to 0.005%,
N: 0.0070% or less, O: 0.003% or less, Mo and Nb as impurities are adjusted to Mo: 0.01% or less, Nb: 0.005% or less, and the carbon equivalent Ceq defined by the following formula (1) is 0.40 ~ 0.45%, ACR defined by the following formula (2) satisfies 0.2 ~ 0.8, and has a composition consisting of the balance Fe and inevitable impurities, and a structure in which the ferrite fraction is 10 to 40% by volume. A steel plate for building structures with a low yield ratio and excellent heat-affected zone toughness with super high heat input welding, characterized by a tensile strength of TS590MPa or more, a yield ratio of 80% or less, and a surface hardness HV of 280HV10 or less.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) (2)
Here, C, Mn, Cu, Ni, Cr, Mo, V, Ca, O, S: Content of each element (mass%)
前記組成に加えてさらに、質量%で、Cu:0.5%以下、Ni:1.0%以下、Cr:0.5%以下、V:0.08%以下のうちから選ばれた1種または2種以上を含有することを特徴とする請求項1に記載の低降伏比建築構造用厚鋼板。   In addition to the above composition, the composition further contains one or more selected from Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, and V: 0.08% or less in terms of mass%. The thick steel sheet for building structure according to claim 1, characterized in that: 前記組成に加えてさらに、質量%で、Al:0.1%以下を含有することを特徴とする請求項1または2に記載の低降伏比建築構造用厚鋼板。   In addition to the said composition, the steel plate for low-yield-ratio building structures of Claim 1 or 2 characterized by further containing Al: 0.1% or less by the mass%. 前記組成に加えてさらに、質量%で、Mg:0.005%以下、REM:0.02%以下のうちから選ばれた1種または2種を含有することを特徴とする請求項1ないし3のいずれかに記載の低降伏比建築構造用厚鋼板。   4. The composition according to claim 1, further comprising one or two selected from Mg: 0.005% or less and REM: 0.02% or less in mass% in addition to the composition. Low steel plate for building structure with low yield ratio. 鋼素材に、加熱し熱間圧延を施す圧延工程と、該圧延工程終了後に加速冷却を施す加速冷却工程とを行う、厚鋼板の製造方法であって、
前記鋼素材が、質量%で、
C:0.03〜O.07%、 Si:0.05〜0.5%、
Mn:0.6〜2.0%、 P:0.020%以下、
S:0.0005〜0.003%、 Ti:0.005〜0.03%、
B:0.0003〜0.0020%、 Ca:0.0005〜0.005%、
N:0.0070%以下、 O:0.003%以下
を含み、不純物としてMo、Nbを、Mo:0.01%以下、Nb:0.005%以下に調整し、下記(1)式で定義される炭素当量Ceqが0.40〜0.45%、下記(2)式で定義されるACRが0.2〜0.8を満足し、残部Feおよび不可避的不純物からなる組成の鋼素材であり、
前記圧延工程が、前記鋼素材を1000〜1200℃に加熱したのち、圧延終了温度をAr変態点以上とする熱間圧延を施し厚鋼板とする工程であり、
前記加速冷却工程が、前記圧延工程終了後60s以内に冷却を開始する工程であり、前記冷却を、前記厚鋼板の表層部の温度で、平均冷却速度:100℃/s以上で、冷却停止温度:700℃以下まで冷却する一次冷却と、該一次冷却後、30〜180s間冷却を停止する保持と、該保持終了後、前記厚鋼板の板厚中央部の温度で、平均冷却速度が3℃/s以上で冷却停止温度:400〜200℃の範囲の温度まで冷却する二次冷却とからなる加速冷却を施す工程であり、前記加速冷却工程後、空冷することを特徴とする大入熱溶接部靭性に優れた低降伏比建築構造用厚鋼板の製造方法。

Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ‥‥(1)
ACR=(Ca−(0.18+130×Ca)×O)/(1.25×S) ‥‥(2)
ここで、C、Mn、Cu、Ni、Cr、Mo、V、Ca、O、S:各元素の含有量(質量%)
A steel sheet is heated by hot rolling and an accelerated cooling process for performing accelerated cooling after completion of the rolling process.
The steel material is mass%,
C: 0.03-0.07%, Si: 0.05-0.5%,
Mn: 0.6 to 2.0%, P: 0.020% or less,
S: 0.0005-0.003%, Ti: 0.005-0.03%,
B: 0.0003 to 0.0020%, Ca: 0.0005 to 0.005%,
N: 0.0070% or less, O: 0.003% or less, Mo and Nb as impurities are adjusted to Mo: 0.01% or less, Nb: 0.005% or less, and the carbon equivalent Ceq defined by the following formula (1) is 0.40 -0.45%, ACR defined by the following formula (2) satisfies 0.2-0.8, and is a steel material having a composition comprising the balance Fe and inevitable impurities,
The rolling step is a step of heating the steel material to 1000 to 1200 ° C., and then performing hot rolling with a rolling end temperature equal to or higher than the Ar 3 transformation point to obtain a thick steel plate,
The accelerated cooling step is a step of starting cooling within 60 s after the end of the rolling step, and the cooling is performed at the temperature of the surface layer portion of the thick steel plate, at an average cooling rate of 100 ° C./s or more, and at a cooling stop temperature. : Primary cooling to 700 ° C or lower, holding after 30 to 180 s after the primary cooling, and temperature at the center of the plate thickness of the steel plate after the holding, the average cooling rate is 3 ° C / S or more, cooling stop temperature: a step of performing an accelerated cooling comprising secondary cooling to a temperature in the range of 400 to 200 ° C., and a high heat input welding characterized by air cooling after the accelerated cooling step A method for manufacturing a steel plate for building structures with low yield ratio and excellent toughness.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) (2)
Here, C, Mn, Cu, Ni, Cr, Mo, V, Ca, O, S: Content of each element (mass%)
鋼素材に、加熱し圧延を施す圧延工程と、該圧延工程終了後に加速冷却を施す加速冷却工程と、焼戻工程とを行う、厚鋼板の製造方法であって、
前記鋼素材が、質量%で、C:0.03〜O.07%、Si:0.05〜0.5%、Mn:0.6〜2.0%、P:0.020%以下、S:0.0005〜0.003%、Ti:0.005〜0.03%、B:0.0003〜0.0020%、Ca:0.0005〜0.005%、N:0.0070%以下、O:0.003%以下を含み、不純物としてMo、Nbを、Mo:0.01%以下、Nb:0.005%以下に調整し、下記(1)式で定義される炭素当量Ceqが0.40〜0.45%、下記(2)式で定義されるACRが0.2〜0.8を満足し、残部Feおよび不可避的不純物からなる組成の鋼素材であり、
前記圧延工程が、前記鋼素材を1000〜1200℃に加熱したのち、圧延終了温度をAr変態点以上とする熱間圧延を施し厚鋼板とする工程であり、
前記加速冷却工程が、前記圧延工程終了後60s以内に冷却を開始する工程であり、前記冷却を、前記厚鋼板の表層部の温度で、平均冷却速度が100℃/s以上で、冷却停止温度:700℃以下となるまで冷却する一次冷却と、該一次冷却後、30〜180s間冷却を停止する保持と、該保持終了後、前記厚鋼板の板厚中央部の温度で、平均冷却速度:3℃/s以上で、冷却停止温度:400〜50℃の範囲の温度まで冷却する二次冷却とからなる加速冷却を施す工程であり、
前記焼戻工程が、前記加速冷却工程を経た厚鋼板を焼戻温度:450℃以下の温度で焼戻す工程である、
ことを特徴とする大入熱溶接部靭性に優れた低降伏比建築構造用厚鋼板の製造方法。

Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ‥‥(1)
ACR=(Ca−(0.18+130×Ca)×O)/(1.25×S) ‥‥(2)
ここで、C、Mn、Cu、Ni、Cr、Mo、V、Ca、O、S:各元素の含有量(質量%)
A steel material, a rolling process for heating and rolling, an accelerated cooling process for performing accelerated cooling after completion of the rolling process, and a tempering process, and a method for producing a thick steel plate,
The steel material is mass%, C: 0.03-0.07%, Si: 0.05-0.5%, Mn: 0.6-2.0%, P: 0.020% or less, S: 0.0005-0.003%, Ti: 0.005-0.03 %, B: 0.0003 to 0.0020%, Ca: 0.0005 to 0.005%, N: 0.0070% or less, O: 0.003% or less, and Mo and Nb as impurities are adjusted to Mo: 0.01% or less and Nb: 0.005% or less In addition, a steel material having a carbon equivalent Ceq defined by the following formula (1) of 0.40 to 0.45% and an ACR defined by the following formula (2) of 0.2 to 0.8, the balance being Fe and inevitable impurities. And
The rolling step is a step of heating the steel material to 1000 to 1200 ° C., and then performing hot rolling with a rolling end temperature equal to or higher than the Ar 3 transformation point to obtain a thick steel plate,
The accelerated cooling step is a step of starting cooling within 60 s after the end of the rolling step, and the cooling is performed at a temperature of a surface layer portion of the thick steel plate, an average cooling rate is 100 ° C./s or more, and a cooling stop temperature. : Primary cooling for cooling to 700 ° C. or lower, holding for stopping cooling for 30 to 180 s after the primary cooling, and temperature at the center of the thickness of the thick steel plate after the holding, average cooling rate: It is a step of performing accelerated cooling consisting of secondary cooling which is cooled to a temperature in the range of 400 to 50 ° C. at a cooling stop temperature of 3 ° C./s or more,
The tempering step is a step of tempering the steel plate that has undergone the accelerated cooling step at a tempering temperature of 450 ° C. or lower.
A method of manufacturing a thick steel plate for a low-yield-ratio building structure excellent in toughness of a large heat input weld.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
ACR = (Ca− (0.18 + 130 × Ca) × O) / (1.25 × S) (2)
Here, C, Mn, Cu, Ni, Cr, Mo, V, Ca, O, S: Content of each element (mass%)
前記組成に加えてさらに、質量%で、Cu:0.5%以下、Ni:1.0%以下、Cr:0.5%以下、V:0.08%以下のうちから選ばれた1種または2種以上を含有することを特徴とする請求項5または6に記載の低降伏比建築構造用厚鋼板の製造方法。   In addition to the above composition, the composition further contains one or more selected from Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, and V: 0.08% or less in terms of mass%. The manufacturing method of the steel plate for low yield ratio building structures of Claim 5 or 6 characterized by these. 前記組成に加えてさらに、質量%で、Al:0.1%以下を含有することを特徴とする請求項5ないし7のいずれかに記載の低降伏比建築構造用厚鋼板の製造方法。   The method for producing a thick steel sheet for building structures with a low yield ratio according to any one of claims 5 to 7, further comprising Al: 0.1% or less by mass% in addition to the composition. 前記組成に加えてさらに、質量%で、Mg:0.005%以下、REM:0.02%以下のうちから選ばれた1種または2種を含有することを特徴とする請求項5ないし8のいずれかに記載の低降伏比建築構造用厚鋼板の製造方法。   9. The composition according to claim 5, further comprising one or two selected from Mg: 0.005% or less and REM: 0.02% or less by mass% in addition to the composition. The manufacturing method of the steel plate for low yield ratio building structures as described.
JP2010082401A 2010-03-31 2010-03-31 Low-yield-ratio thick steel plate for building structure superior in toughness at ultrahigh-heat-input weld zone, and method for manufacturing the same Pending JP2011214053A (en)

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