JP6816739B2 - Steel plate and its manufacturing method - Google Patents

Steel plate and its manufacturing method Download PDF

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JP6816739B2
JP6816739B2 JP2018073016A JP2018073016A JP6816739B2 JP 6816739 B2 JP6816739 B2 JP 6816739B2 JP 2018073016 A JP2018073016 A JP 2018073016A JP 2018073016 A JP2018073016 A JP 2018073016A JP 6816739 B2 JP6816739 B2 JP 6816739B2
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佳子 竹内
佳子 竹内
克行 一宮
克行 一宮
長谷 和邦
和邦 長谷
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JFE Steel Corp
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本発明は、船舶や海洋構造物、ラインパイプ、圧力容器等に使用される鋼板、特に、母材としての低温靭性に優れるだけでなく、多層溶接継手を形成した際の当該継手部のCTOD特性にも優れる鋼板およびその製造方法に関するものである。 The present invention not only has excellent low temperature toughness as a base material for steel plates used for ships, marine structures, line pipes, pressure vessels, etc., but also has CTOD characteristics of the joint when a multi-layer welded joint is formed. It also relates to an excellent steel sheet and its manufacturing method.

鋼の靭性の評価基準として、主にシャルピー試験が用いられており、構造物として用いられる厚さが30mm以上の厚鋼板については鋼板の靱性のみならず溶接部での靱性が要求される。近年のエネルギー需要の増加に対応し新たな資源を確保するために、海洋構造物等の建造地域がこれまで資源採掘を行っていなかった寒冷域に及んでいる。それに伴って、かような構造物に使用する鋼板に求められる靱性の保証温度も低温化してきている。また、2016年にIACS(国際船級協会連合)において統一規則が改定され、今後は船級毎にCTOD試験(Crack Tip Opening Displacement Test)の実施が要求されるようになる、可能性が高くなってきている。
ここで、CTOD試験とは、靭性評価部に疲労予き裂を導入した試験片を低温で曲げ試験し、破壊直前のき裂の開口量(塑性変形量)を測定して脆性破壊の発生抵抗を評価するものである。
The Charpy test is mainly used as an evaluation standard for the toughness of steel, and for thick steel sheets with a thickness of 30 mm or more used as structures, not only the toughness of the steel sheet but also the toughness at the welded part is required. In order to secure new resources in response to the increase in energy demand in recent years, the construction areas such as marine structures extend to cold regions where resources have not been mined so far. Along with this, the guaranteed toughness temperature required for steel sheets used in such structures has also been lowered. In addition, the unified rules were revised by the IACS (International Association of Classification Societies) in 2016, and it is highly possible that CTOD tests (Crack Tip Opening Displacement Test) will be required for each ship class in the future. There is.
Here, the CTOD test is a bending test at a low temperature of a test piece in which a fatigue pre-fracture is introduced into the toughness evaluation unit, and the opening amount (plastic deformation amount) of the crack immediately before fracture is measured to prevent brittle fracture. Is to evaluate.

鋼板を構造物に適用する場合は、多層溶接による施工が行われるのが一般的である。多層溶接の溶接熱影響部(以下、多層溶接HAZと称する)には、先行の溶接パスによる溶接線近傍の粗大な組織(Coarse Grain Heat Affected Zone:以下、CGHAZと称する)が、次層の溶接パスによりフェライト+オーステナイトの2相域に再加熱されて、粗大な基地組織中に島状マルテンサイト(Martensite-Austenite Constituent:以下、MAと称する)組織が混在したことによって、著しく靭性が低くなった領域(Inter Critically Coarse Grain Heat Affected Zone:以下、ICCGHAZと称する)が含まれることが知られている。ここで、特に、継手CTOD試験は、基本的に板の全厚にわたる試験となるため、多層溶接HAZを対象とする場合、疲労予き裂を導入する評価領域には、上記したICCGHAZ組織が含まれることになる。一方、継手CTOD試験により得られる継手CTOD特性は、評価領域で最も脆化する領域の靭性に支配されるため、多層溶接HAZの継手CTOD特性は、CGHAZ組織だけでなくICCGHAZ組織の靭性も反映される。このため、多層溶接HAZの継手CTOD特性の向上にはICCGHAZ組織の靭性向上も必要になる。 When a steel plate is applied to a structure, it is generally constructed by multi-layer welding. In the weld heat-affected zone of multi-layer welding (hereinafter referred to as multi-layer welding HAZ), a coarse structure (Coarse Grain Heat Affected Zone: hereinafter referred to as CGHAZ) near the welding line due to the preceding welding path is formed in the next layer welding. It was reheated to the two-phase region of ferrite + austenite by the path, and the toughness was significantly reduced due to the mixture of island-like martensite (Martensite-Austenite Constituent: hereinafter referred to as MA) structure in the coarse matrix structure. It is known that an area (Inter Critically Coarse Grain Heat Affected Zone: hereinafter referred to as ICCGHAZ) is included. Here, in particular, since the joint CTOD test is basically a test over the entire thickness of the plate, the above-mentioned ICCG HAZ structure is included in the evaluation area for introducing fatigue pre-crack when targeting multi-layer welded HAZ. Will be. On the other hand, the joint CTOD characteristics obtained by the joint CTOD test are governed by the toughness of the region most embrittled in the evaluation region, so the joint CTOD characteristics of multi-layer welded HAZ reflect not only the toughness of the CGHAZ structure but also the toughness of the ICCGHAZ structure. To. Therefore, in order to improve the joint CTOD characteristics of multi-layer welded HAZ, it is also necessary to improve the toughness of the ICCG HAZ structure.

従来、HAZの靭性向上技術として、TiNの微細分散によるCGHAZのオーステナイト粒粗大化の抑制や、TiNのフェライト変態核利用が知られている。
さらに、REMを添加して生成したREM系酸硫化物の分散によるオーステナイト粒の粒成長抑制またはCaを添加して生成したCa系酸硫化物の分散によるオーステナイト粒の粒成長抑制に係る技術や、BNのフェライト核生成能と酸化物分散とを組み合わせる技術も用いられてきた。
Conventionally, as a technique for improving the toughness of HAZ, suppression of austenite grain coarsening of CGHAZ by fine dispersion of TiN and utilization of ferrite transformed nuclei of TiN are known.
Furthermore, techniques for suppressing grain growth of austenite grains by dispersing REM-based acid sulfides produced by adding REM, or suppressing grain growth of austenite grains by dispersing Ca-based acid sulfides generated by adding Ca, and Techniques that combine the ability of BN to form ferrite nuclei with oxide dispersion have also been used.

例えば、特許文献1および特許文献2には、REMとTiN粒子によるHAZのオーステナイト組織の粗大化抑制技術が提案されている。また、特許文献3には、CaS利用によるHAZ靭性向上技術と熱間圧延による母材靭性向上技術が提案されている。 For example, Patent Document 1 and Patent Document 2 propose a technique for suppressing coarsening of the austenite structure of HAZ by using REM and TiN particles. Further, Patent Document 3 proposes a HAZ toughness improving technique by using CaS and a base metal toughness improving technique by hot rolling.

また、特許文献4には、ICCGHZの靭性低下対策として、低C化および低Si化することによりマルテンサイトの生成を抑制し、さらにCuを添加することにより母材強度を高める技術が提案されている。 Further, Patent Document 4 proposes a technique for suppressing the formation of martensite by lowering C and Si by lowering C and Si, and further increasing the strength of the base metal by adding Cu as a countermeasure for lowering the toughness of ICCGHZ. There is.

なお、特許文献5には、偏析しやすい元素の量を制限して中心偏析部の硬度を下げること、TiNを用いて粗大粒を抑制すること、C,P,Ni量のバランスを最適化して靱性を改善すること、などを組み合わせて、−80℃の溶接部靱性および−60℃におけるCTOD特性を満足させる技術が提案されている。 In Patent Document 5, the amount of elements that are easily segregated is limited to reduce the hardness of the central segregation portion, coarse grains are suppressed by using TiN, and the balance of C, P, and Ni amounts is optimized. A technique has been proposed that satisfies the weld toughness at -80 ° C and the CTOD characteristics at -60 ° C in combination with improving the toughness.

特公平03−053367号公報Special Fair 03-0533367 特開昭60−184663号公報Japanese Unexamined Patent Publication No. 60-184663 特許第5177310号Patent No. 5177310 特許第3045856号Patent No. 3045856 特開2017−2349号JP-A-2017-2349

溶接部の低温靱性およびCTOD特性のどちらか一方を向上させる技術は、数多く提案されているものの、その両方を確保する技術としては特性もしくは製造性の観点から決して十分とは言えず検討の余地があった。 Although many techniques for improving either the low temperature toughness of the welded part or the CTOD property have been proposed, it cannot be said that the technique for ensuring both is sufficient from the viewpoint of characteristics or manufacturability, and there is room for consideration. there were.

例えば、特許文献1および特許文献2に開示の、REMとTiN粒子によるHAZのオーステナイト組織の粗大化抑制技術については、TiNは溶接時に高温に達するボンド部では溶解してしまうため、オーステナイト粒の粒成長抑制に対して十分な効果を発揮できない。
また、REM系酸硫化物やCa系酸硫化物はオーステナイト粒成長抑制には有効である。しかしながら、HAZのオーステナイト粒粗大化抑制による靭性向上の効果のみでは低い使用温度での継手CTOD特性を満足することはできない。BNのフェライト核生成能は、大入熱溶接で溶接熱影響部の冷却速度が遅く、HAZがフェライト主体となる組織の場合には有効であった。しかしながら、厚鋼板の場合、母材に含有される合金成分量が比較的高くなる一方で、多層溶接は入熱量が比較的小さいので、HAZ組織がベイナイト主体となり、その効果が得られない。
For example, with respect to the technique for suppressing the coarsening of the austenite structure of HAZ by REM and TiN particles disclosed in Patent Document 1 and Patent Document 2, TiN melts in the bond portion that reaches a high temperature during welding, so that the austenite particles are grains. It cannot exert a sufficient effect on growth suppression.
In addition, REM-based acid sulfide and Ca-based acid sulfide are effective in suppressing austenite grain growth. However, the CTOD characteristics of joints at low operating temperatures cannot be satisfied only by the effect of improving toughness by suppressing the coarsening of austenite grains in HAZ. The ferrite nucleation ability of BN was effective in the case of a structure in which HAZ is mainly ferrite because the cooling rate of the weld heat affected zone is slow in high heat input welding. However, in the case of a thick steel sheet, the amount of alloy components contained in the base metal is relatively high, while the amount of heat input in multi-layer welding is relatively small, so the HAZ structure is mainly bainite, and the effect cannot be obtained.

特許文献3に記載の技術では、−10℃での継手CTOD特性を満足するものの、−60℃という低温での継手靱性を確保する方途は示されていない。 Although the technique described in Patent Document 3 satisfies the joint CTOD characteristics at −10 ° C., there is no way to ensure the joint toughness at a low temperature of −60 ° C.

特許文献4に記載の技術は、低温靱性およびCTOD値を満足するが、Cuの析出硬化を利用するため、圧延後の時効処理が必須であり、その分製造時のコストが増加することが問題であった。 The technique described in Patent Document 4 satisfies low temperature toughness and CTOD value, but since it utilizes precipitation hardening of Cu, aging treatment after rolling is indispensable, and there is a problem that the manufacturing cost increases accordingly. Met.

特許文献5には、TMCP条件の下で低温域での継手靱性値とCTOD特性を両立させることが示されているものの、−80℃でのシャルピー靱性値および−60℃以下でのCTOD値を非常に高価な成分系で実現する点、コスト面からの改善の余地があった。 Patent Document 5 shows that the joint toughness value in the low temperature range and the CTOD characteristic are compatible under the TMCP condition, but the Charpy toughness value at -80 ° C and the CTOD value at -60 ° C or less are obtained. There was room for improvement in terms of cost, as it was realized with a very expensive component system.

従来、造船用としてCTOD特性が要求される、板厚が30mmから100mmで降伏応力が480MPa以上の厚鋼板は、通常の焼入焼戻し法で製造されていた。しかし、この方法では安定した品質を確保するために溶質元素を多く添加する必要があり、溶接部の靱性が低くなる傾向があった。 Conventionally, thick steel sheets with a thickness of 30 mm to 100 mm and a yield stress of 480 MPa or more, which require CTOD characteristics for shipbuilding, have been manufactured by a normal quenching and tempering method. However, in this method, it is necessary to add a large amount of solute elements in order to ensure stable quality, and the toughness of the welded portion tends to be low.

そこで、本発明は、多層溶接を行った場合に母材並びに溶接部の靱性に優れる、具体的には−60℃における継手部靱性値:35J以上および−10℃におけるCTOD値:0.10mm以上を満足する、降伏応力(YS):480MPa以上かつ引張応力(TS):550MPa以上で板厚が30〜100mmの鋼板を低コストで提供することを目的とする。さらには、該鋼板の有利な製造方法について提案することを目的とする。 Therefore, the present invention has excellent toughness of the base metal and the welded portion when multi-layer welding is performed, specifically, a joint portion toughness value at -60 ° C: 35 J or more and a CTOD value at -10 ° C: 0.10 mm or more. An object of the present invention is to provide a satisfactory steel sheet having a yield stress (YS) of 480 MPa or more and a tensile stress (TS) of 550 MPa or more and a plate thickness of 30 to 100 mm at low cost. Furthermore, it aims to propose an advantageous manufacturing method of the steel sheet.

発明者等は、上記問題点を解決するための手法について鋭意検討を行い、以下の知見を得た。
(i)鋼中のCa、OおよびSを、下式で示される原子濃度比ACR(Atomic Concentration Ratio)を0〜1.0の範囲内に制御すると、硫化物の形態がMnの一部固溶したCa系硫化物とAl系酸化物との複合介在物となる。
ACR={[Ca]−(0.18+130×[Ca])×[O]}÷(1.25×[S])
The inventors have diligently studied a method for solving the above problems and obtained the following findings.
(I) When Ca, O and S in steel were controlled in the range of 0 to 1.0 in the atomic concentration ratio ACR (Atomic Concentration Ratio) represented by the following formula, the sulfide morphology was partially dissolved in Mn. It is a composite inclusion of Ca-based sulfide and Al-based oxide.
ACR = {[Ca]-(0.18 + 130 x [Ca]) x [O]} ÷ (1.25 x [S])

(ii)介在物形態をCaおよびMnを含む硫化物とAlを含む酸化物とからなる複合介在物とすることによって、溶接線近傍の高温まで昇温される領域においても安定的に存在できるためオーステナイト粒粗大化効果を十分に発揮できる。さらに、複合介在物の周囲にMn希薄層が形成されるため、ベイナイトやアシキュラーフェライトの核生成効果を期待できる。 (Ii) By setting the inclusion form to a composite inclusion consisting of a sulfide containing Ca and Mn and an oxide containing Al, it can exist stably even in a region where the temperature is raised to a high temperature near the welding line. The austenite grain coarsening effect can be fully exerted. Furthermore, since a Mn dilute layer is formed around the composite inclusions, the nucleation effect of bainite and acicular ferrite can be expected.

(iii)HAZの冷却時の核生成サイトは主にオーステナイト粒界である。このオーステナイト粒内に核生成効果を有する上記複合介在物を存在させることによって、オーステナイト粒界に加えオーステナイト粒内からも核生成が開始し、最終的に得られるHAZ組織が微細となり、HAZの靭性および継手CTOD特性が向上する。 (Iii) The nucleation site during cooling of HAZ is mainly the austenite grain boundary. By the presence of the above-mentioned complex inclusions having a nucleation effect in the austenite grains, nucleation is started not only in the austenite grain boundaries but also in the austenite grains, and the finally obtained HAZ structure becomes fine and the HAZ toughness And joint CTOD characteristics are improved.

(iv)炭素当量Ceqを0.45〜0.53の範囲に制御することにより、多層溶接HAZの基地組織の靭性向上が可能である。 (Iv) By controlling the carbon equivalent Ceq in the range of 0.45 to 0.53, it is possible to improve the toughness of the matrix structure of the multilayer welded HAZ.

(v)通常、スラブの板厚中心の元素偏析部には合金元素が濃化することで粗大な介在物が低密度で分散してしまう問題が生じる。しかしながら、板厚中心温度が950℃以上における圧下率が各パスで7%以上の全パスの累積圧下率が15%以上となる圧下を加えれば、板厚中心に加わる歪みを増加させ、粗大介在物を伸長、さらには分断させることにより、細かな介在物を高密度に分散させることができる。また、介在物によるHAZ靭性向上効果を確保することができるとともに、良好なCTOD特性を実現することができる。 (V) Normally, there is a problem that coarse inclusions are dispersed at a low density due to the concentration of alloying elements in the element segregation portion at the center of the plate thickness of the slab. However, if the reduction rate at a plate thickness center temperature of 950 ° C or higher is 7% or more in each pass and the cumulative reduction rate of all passes is 15% or more, the strain applied to the plate thickness center is increased and coarse intervention is performed. By extending and further dividing the object, fine inclusions can be dispersed at high density. In addition, the HAZ toughness improving effect due to inclusions can be ensured, and good CTOD characteristics can be realized.

本発明は、以上の知見を基に、更に検討を加えてなされたものであり、本発明の要旨構成は次のとおりである。
[1]質量%で、
C:0.01〜0.10%、
Si:0.6%以下、
Mn:1.0〜1.8%、
P:0.01%以下、
S:0.0005〜0.0050%、
Al:0.001〜0.060%、
Ni:0.2〜2.0%、
Ti:0.005〜0.050%、
N:0.0015〜0.0065%、
O:0.0010〜0.0050%および
Ca:0.0005〜0.0060%
を、下記(1)式で定義されるACRが0を超え1.0以下および下記(2)式で定義されるCeqが0.45以上0.53以下となる範囲で含有し、残部Feおよび不可避的不純物の成分組を有する鋼板。
ACR={[Ca]−(0.18+130×[Ca])×[O]}÷(1.25×[S])…(1)
Ceq=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5)…(2)
(1)式および(2)式において、[ ]は該括弧内の元素の含有量(質量%)である。但し、含有されない元素はゼロとする。
The present invention has been further studied based on the above findings, and the gist structure of the present invention is as follows.
[1] By mass%
C: 0.01 to 0.10%,
Si: 0.6% or less,
Mn: 1.0-1.8%,
P: 0.01% or less,
S: 0.0005 to 0.0050%,
Al: 0.001 to 0.060%,
Ni: 0.2-2.0%,
Ti: 0.005 to 0.050%,
N: 0.0015-0.0065%,
O: 0.0010-0.0050% and
Ca: 0.0005-0.0060%
Is contained in the range where the ACR defined by the following formula (1) is more than 0 and 1.0 or less and the Ceq defined by the following formula (2) is 0.45 or more and 0.53 or less, and the component set of the balance Fe and the unavoidable impurities. Steel plate with.
ACR = {[Ca]-(0.18 + 130 x [Ca]) x [O]} ÷ (1.25 x [S]) ... (1)
Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5) ... (2)
In the formulas (1) and (2), [] is the content (mass%) of the element in the parentheses. However, the elements that are not contained are zero.

[2]前記成分組成は、更に、質量%で、
Cu:0.05〜0.60%、
Cr:0.05〜0.50%、
Mo:0.05〜0.50%、
Nb:0.005〜0.035%、
V:0.01〜0.10%、
W:0.01〜0.50%、
B:0.0005〜0.0020%、
REM:0.0020〜0.0200%および
Mg:0.0002〜0.0060%
のうちの1種または2種以上を含む前記[1]に記載の鋼板。
[2] The component composition is further increased by mass%.
Cu: 0.05-0.60%,
Cr: 0.05-0.50%,
Mo: 0.05-0.50%,
Nb: 0.005 to 0.035%,
V: 0.01 to 0.10%,
W: 0.01-0.50%,
B: 0.0005 to 0.0020%,
REM: 0.0020-0.0200% and
Mg: 0.0002 to 0.0060%
The steel sheet according to the above [1], which comprises one or more of the above.

[3]前記[1]または[2]に記載の成分組成の鋼素材を1000℃以上1200℃以下に加熱し、950℃以上の温度域における、平均圧下率/パスが7%以上のパスの累積圧下率が15%以上であり、かつ900℃未満の温度域における、平均圧下率/パスが3%以上のパスの累積圧下率が40%以上である、熱間圧延後、板厚中心での700−550℃間の平均冷却速度が1.5〜50℃/sとなる冷却を550℃以下まで行う鋼板の製造方法。 [3] The steel material having the composition according to the above [1] or [2] is heated to 1000 ° C. or higher and 1200 ° C. or lower, and the average rolling reduction / pass is 7% or higher in the temperature range of 950 ° C. or higher. Cumulative reduction rate is 15% or more, and the cumulative reduction rate of paths with an average reduction rate / pass of 3% or more is 40% or more in a temperature range of less than 900 ° C. After hot rolling, at the center of plate thickness. A method for manufacturing a steel sheet in which cooling is performed so that the average cooling rate between 700 and 550 ° C is 1.5 to 50 ° C / s to 550 ° C or less.

[4]前記[3]に記載の方法において、前記冷却後にさらに、Ac1変態点以下の温度で焼戻し処理を行うことを特徴とする鋼板の製造方法。 [4] The method for producing a steel sheet according to the method according to [3], wherein the tempering treatment is further performed at a temperature equal to or lower than the Ac 1 transformation point after the cooling.

本発明によれば、多層溶接した際の継手において優れた靱性並びにCTOD特性が得られる鋼板およびその製造方法を提供することができ、産業上極めて有用である。 According to the present invention, it is possible to provide a steel sheet and a method for producing the same, which can obtain excellent toughness and CTOD characteristics in a joint when multi-layer welding is performed, which is extremely useful in industry.

以下に本発明の各構成要件の限定理由について説明する。
1.化学成分について
はじめに、本発明の鋼の化学成分を規定した理由を説明する。なお、成分組成に関する「%」表示は全て「質量%」を意味する。
C:0.01〜0.10%
Cは、鋼の強度を向上させる元素であり、0.01%以上の含有を必要とする。しかし、Cを過剰に含有すると濃化した部分の硬度が高くなってしまい、母材および継手部の靱性、継手CTOD特性が低下する。このため、Cの上限は、濃化しても継手特性を劣化させない0.10%以下の範囲に限定した。なお、好ましくは0.01〜0.08%、より好ましくは0.01〜0.50%である。
The reasons for limiting each constituent requirement of the present invention will be described below.
1. 1. Chemical Composition First, the reason for defining the chemical composition of the steel of the present invention will be described. In addition, all "%" indications about a component composition mean "mass%".
C: 0.01 to 0.10%
C is an element that improves the strength of steel and requires a content of 0.01% or more. However, if C is excessively contained, the hardness of the concentrated portion becomes high, and the toughness of the base material and the joint portion and the joint CTOD characteristics deteriorate. Therefore, the upper limit of C is limited to the range of 0.10% or less, which does not deteriorate the joint characteristics even if it is thickened. It is preferably 0.01 to 0.08%, more preferably 0.01 to 0.50%.

Si:0.6%以下
Siは0.6%を超えて過剰に含有すると、継手CTOD特性が低下する。このため、Siは0.6%以下の範囲に限定した。なお、好ましくは0.01%以上0.3%以下、さらに好ましくは0.2%未満である。
Si: 0.6% or less
If Si is contained in excess of more than 0.6%, the CTOD characteristics of the joint will deteriorate. Therefore, Si was limited to the range of 0.6% or less. It is preferably 0.01% or more and 0.3% or less, and more preferably less than 0.2%.

Mn:1.0〜1.8%
Mnは、鋼の焼入れ性の向上を介して強度を向上させる元素である。しかしながら、過剰に添加すると、継手CTOD特性を著しく低下させる。このため、Mnは1.0〜1.8%の範囲に限定した。なお、好ましくは1.1〜1.7%の範囲である。
Mn: 1.0-1.8%
Mn is an element that improves strength through improved hardenability of steel. However, excessive addition significantly reduces joint CTOD characteristics. Therefore, Mn was limited to the range of 1.0 to 1.8%. It is preferably in the range of 1.1 to 1.7%.

P:0.01%以下
Pは、不純物として鋼中に不可避的に含有される元素であり、鋼の靭性を低下させるため、できるだけ低減することが望ましい。特に、低温における継手靱性を確保するために通常より厳しく管理する必要がある。従って、低温靱性を低下させはじめる、0.01%以下とする。好ましくは0.080%以下である。
P: 0.01% or less P is an element unavoidably contained in steel as an impurity and reduces the toughness of steel, so it is desirable to reduce it as much as possible. In particular, it is necessary to control it more strictly than usual in order to ensure the toughness of the joint at low temperature. Therefore, the temperature should be 0.01% or less, which starts to reduce the low temperature toughness. It is preferably 0.080% or less.

S:0.0005〜0.0050%
Sは、多層溶接HAZの靭性を向上させるための介在物に必要な元素であり、0.0005%以上の含有が必要である。しかしながら、0.0050%を超える含有は、逆に継手部の靱性およびCTOD特性を低下させるため、0.0050%以下に限定した。好ましくは0.003%以下、より好ましくは0.002%以下である。
S: 0.0005 to 0.0050%
S is an element required for inclusions for improving the toughness of multi-layer welded HAZ, and must contain 0.0005% or more. However, the content of more than 0.0050% was limited to 0.0050% or less because it deteriorates the toughness and CTOD characteristics of the joint. It is preferably 0.003% or less, more preferably 0.002% or less.

Al:0.001〜0.060%
Alは、多層溶接HAZの靭性を向上させるための介在物に必要な元素であり、0.001%以上の含有が必要である。一方、0.060%を超える含有は、継手CTOD特性を低下させるため、0.060%以下に限定した。好ましくは、0.050%以下である。
Al: 0.001 to 0.060%
Al is an element required for inclusions for improving the toughness of multi-layer welded HAZ, and must contain 0.001% or more. On the other hand, the content exceeding 0.060% was limited to 0.060% or less in order to reduce the CTOD characteristics of the joint. Preferably, it is 0.050% or less.

Ni:0.2〜2.0%
Niは、母材と継手の両方の靭性を大きく劣化させることなく高強度化が可能な有用な元素である。そのためには、0.2%以上とする。しかし、2.0%を超えると強度上昇の効果が飽和すること、またコスト増加が問題となる。そのため、上限を2.0%とした。なお、より効果的に効果を得られるという観点から、強度上昇の飽和が発生する直前の1.8%以下が好ましい範囲である。
Ni: 0.2-2.0%
Ni is a useful element that can increase the strength of both the base metal and the joint without significantly deteriorating the toughness. For that purpose, it should be 0.2% or more. However, if it exceeds 2.0%, the effect of increasing the strength is saturated and the cost increase becomes a problem. Therefore, the upper limit is set to 2.0%. From the viewpoint that the effect can be obtained more effectively, 1.8% or less immediately before the saturation of the intensity increase occurs is a preferable range.

Ti:0.005〜0.050%
Tiは、TiNとして析出することでHAZのオーステナイト粒粗大化を抑制し、HAZ組織を微細化し、靭性を向上するのに有効な元素である。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、0.050%を超えて過剰に含有すると、固溶Tiや粗大TiCの析出によりHAZ靭性が低下するようになる。このため、Tiは0.005〜0.050%の範囲に限定した。好ましくは0.005〜0.040%、より好ましくは0.030%以下である。
Ti: 0.005 to 0.050%
Ti is an element that is effective in suppressing the coarsening of austenite grains in HAZ by precipitating as TiN, refining the HAZ structure, and improving toughness. In order to obtain such an effect, a content of 0.005% or more is required. On the other hand, if it is excessively contained in excess of 0.050%, HAZ toughness will be lowered due to precipitation of solid solution Ti and coarse TiC. Therefore, Ti was limited to the range of 0.005 to 0.050%. It is preferably 0.005 to 0.040%, more preferably 0.030% or less.

N:0.0015〜0.0065%
Nは、TiNとして析出することでHAZのオーステナイト粒粗大化を抑制し、HAZ組織の微細化により、靭性向上に有効な元素である。このような効果を得るためには0.0015%以上の含有を必要とする。一方、0.0065%を超えて過剰に含有すると、HAZ靭性が低下するようになる。このため、0.0015〜0.0065%の範囲に限定した。好ましくは0.0015〜0.0055%である。
N: 0.0015-0.0065%
N is an element that is effective in improving toughness by suppressing the coarsening of austenite grains in HAZ by precipitating as TiN and by refining the HAZ structure. In order to obtain such an effect, a content of 0.0015% or more is required. On the other hand, if it is excessively contained in excess of 0.0065%, HAZ toughness will decrease. Therefore, it was limited to the range of 0.0015 to 0.0065%. It is preferably 0.0015 to 0.0055%.

O:0.0010〜0.0050%
Oは、多層溶接HAZの靭性を向上させるための介在物に必要な元素であり、0.0010%以上の含有が必要である。一方、0.0050%を超える含有は、継手CTOD特性が低下するようになるため、本発明では0.0010〜0.0050%の範囲に限定した。好ましくは0.0010〜0.0045%、より好ましくは0.0040%以下である。
O: 0.0010 to 0.0050%
O is an element required for inclusions for improving the toughness of multi-layer welded HAZ, and must contain 0.0010% or more. On the other hand, if the content exceeds 0.0050%, the CTOD characteristics of the joint will deteriorate, so in the present invention, the content is limited to the range of 0.0010 to 0.0050%. It is preferably 0.0010 to 0.0045%, more preferably 0.0040% or less.

Ca:0.0005〜0.0060%
Caは、多層溶接HAZの靭性を向上させるための介在物に必要な元素であり、0.0005%以上の含有が必要である。一方、0.0060%を超える含有は、かえって継手CTOD特性が低下するため、本発明では0.0005〜0.0060%の範囲に限定した。好ましくは0.0007〜0.0050%である。
Ca: 0.0005-0.0060%
Ca is an element required for inclusions for improving the toughness of multi-layer welded HAZ, and must contain 0.0005% or more. On the other hand, if the content exceeds 0.0060%, the CTOD characteristics of the joint are deteriorated, so that the content is limited to 0.0005 to 0.0060% in the present invention. It is preferably 0.0007 to 0.0050%.

ACR:0を超え1.0以下
上記した(1)式に従うACRは、鋼中のCa、OおよびSの原子濃度比である。原理上、式中の値が0以下では硫化物系介在物の主要形態がMnSとなる。MnSは、融点が低く溶接時の溶接線近傍では溶解してしまうため、溶接線近傍でのオーステナイト粒粗大化抑制効果および溶接後の冷却時の変態核効果も得られない。一方で、上式の値が1.0を超えると、硫化物系介在物の主要形態はCaSとなり、CaS周囲に変態核となるために必要なMn希薄層が形成されないため変態核効果が得られない。従って、ACRが0を超え1.0以下となる範囲に、Ca、OおよびSの含有量を規制する。好ましくは、0.1以上0.9以下とする。
ACR: Exceeding 0 and 1.0 or less ACR according to the above equation (1) is the atomic concentration ratio of Ca, O and S in steel. In principle, when the value in the equation is 0 or less, the main form of the sulfide-based inclusions is MnS. Since MnS has a low melting point and dissolves in the vicinity of the welding line during welding, the effect of suppressing austenite grain coarsening in the vicinity of the welding line and the effect of transformation nucleation during cooling after welding cannot be obtained. On the other hand, when the value of the above equation exceeds 1.0, the main form of the sulfide-based inclusions becomes CaS, and the transformation nucleus effect cannot be obtained because the Mn dilute layer necessary for becoming transformation nuclei is not formed around CaS. .. Therefore, the contents of Ca, O and S are regulated within the range where the ACR exceeds 0 and becomes 1.0 or less. Preferably, it is 0.1 or more and 0.9 or less.

Ceq:0.45以上0.53以下
一般に、高強度になるほど添加元素の量が増し、上記した(2)式に従うCeqが増加する傾向にある。しかしながら、Ceqが増加すると、HAZ組織中の島状マルテンサイトやベイナイトといった靭性の劣る組織量の増加によりHAZ靭性が劣化してしまう。YS≧480MPaを確保しつつ、HAZ靭性向上技術の効果を維持させるための条件として0.53以下とした。一方、Ceqが0.45未満になると、目標としている強度を得ることが困難となるため、0.45以上とする。好ましくは、0.46以上0.51以下とする。
Ceq: 0.45 or more and 0.53 or less Generally, the higher the strength, the greater the amount of added elements, and the Ceq according to the above equation (2) tends to increase. However, when Ceq increases, HAZ toughness deteriorates due to an increase in the amount of tissue with inferior toughness such as island-like martensite and bainite in the HAZ tissue. The condition for maintaining the effect of HAZ toughness improvement technology while ensuring YS ≥ 480 MPa was set to 0.53 or less. On the other hand, if Ceq is less than 0.45, it becomes difficult to obtain the target strength, so it is set to 0.45 or more. Preferably, it is 0.46 or more and 0.51 or less.

本発明に係る鋼板は、上記成分以外の残部はFeおよび不可避的不純物である、成分組成を基本とする。さらに、強度および靭性の調整や、継手靭性向上を目的として、Cu:0.05〜0.60%、Cr:0.05〜0.50%、Mo:0.05〜0.50%、Nb:0.005〜0.035%、V:0.01〜0.10%、W:0.01〜0.50%、B:0.0005〜0.0020%、REM:0.0020〜0.0200%、Mg:0.0002〜0.0060%の1種または2種以上を含有できる。 The steel sheet according to the present invention is based on a component composition in which the balance other than the above components is Fe and unavoidable impurities. Furthermore, for the purpose of adjusting strength and toughness and improving joint toughness, Cu: 0.05 to 0.60%, Cr: 0.05 to 0.50%, Mo: 0.05 to 0.50%, Nb: 0.005 to 0.035%, V: 0.01 to 0.10% , W: 0.01 to 0.50%, B: 0.0005 to 0.0020%, REM: 0.0020 to 0.0200%, Mg: 0.0002 to 0.0060%, which can contain one or more kinds.

Cu:0.05〜0.60%
Cuは、母材および継手の靭性を大きく劣化させることなく高強度化を可能とする元素であり、そのためには0.05%以上で添加することが好ましい。一方、添加しすぎると靱性の低下につながり、またスケール直下に生成するCu濃化層起因の鋼板割れが問題となる。今回の目標とする特性を満足させるためには、0.60%以下とすることが好ましい。さらに好ましくは0.50%以下である。
Cu: 0.05-0.60%
Cu is an element that enables high strength without significantly deteriorating the toughness of the base metal and the joint, and for that purpose, it is preferable to add it at 0.05% or more. On the other hand, if it is added too much, the toughness will be lowered, and cracking of the steel plate due to the Cu concentrated layer generated immediately under the scale will be a problem. In order to satisfy the characteristics targeted this time, it is preferably 0.60% or less. More preferably, it is 0.50% or less.

Cr:0.05〜0.50%
Crは、鋼の焼入れ性の向上を介して強度を向上させる元素であるが、過剰に添加すると継手CTOD特性を低下させるため、添加する場合は、0.05〜0.50%とする。
Cr: 0.05-0.50%
Cr is an element that improves the strength by improving the hardenability of steel, but if it is added excessively, the CTOD characteristics of the joint will deteriorate. Therefore, when it is added, it should be 0.05 to 0.50%.

Mo:0.05〜0.50%
Moは、鋼の焼入れ性の向上を介して強度を向上させる元素であるが、過剰に添加すると継手CTOD特性を低下させる。このため、添加する場合は0.05〜0.50%とする。
Mo: 0.05-0.50%
Mo is an element that improves the strength through the improvement of hardenability of steel, but when added in excess, it lowers the CTOD characteristics of the joint. Therefore, when it is added, it should be 0.05 to 0.50%.

Nb:0.005〜0.035%
Nbは、オーステナイト相の未再結晶温度域を広げる元素であり、未再結晶域圧延を効率的に行い、微細組織を得るために有効な元素である。その効果を得るためには0.005%以上の含有を必要とする。しかしながら、0.035%を超えると、継手部の靱性およびCTOD特性の低下を招くため、添加する場合は、0.005〜0.035%とする。
Nb: 0.005 to 0.035%
Nb is an element that widens the unrecrystallized temperature range of the austenite phase, and is an effective element for efficiently performing unrecrystallized rolling and obtaining a fine structure. In order to obtain the effect, the content of 0.005% or more is required. However, if it exceeds 0.035%, the toughness of the joint and the CTOD characteristics will deteriorate. Therefore, when it is added, it should be 0.005 to 0.035%.

V:0.01〜0.10%
Vは、母材の強度を向上させる元素であり、0.01%以上の添加で効果を発揮する。しかし、0.10%を超えるとHAZ靭性の低下を招くため、添加する場合は、0.01〜0.10%とする。さらに好ましくは、0.02〜0.05%である。
V: 0.01 to 0.10%
V is an element that improves the strength of the base material, and is effective when added in an amount of 0.01% or more. However, if it exceeds 0.10%, the HAZ toughness will decrease, so when adding it, it should be 0.01 to 0.10%. More preferably, it is 0.02 to 0.05%.

W:0.01〜0.50%
Wは、母材の強度を向上させる元素であり、0.01%以上の添加で効果を発揮する。しかし、0.50%を超えるとHAZ靭性の低下を招くため、添加する場合は、0.01〜0.50%とする。より好ましくは、0.05〜0.35%である。
W: 0.01 to 0.50%
W is an element that improves the strength of the base material, and is effective when added in an amount of 0.01% or more. However, if it exceeds 0.50%, the HAZ toughness will decrease, so when adding it, it should be 0.01 to 0.50%. More preferably, it is 0.05 to 0.35%.

B:0.0005〜0.0020%
Bは、極微量の含有で焼入れ性を向上させ、それにより鋼板の強度を向上させるのに有効な元素であり、このような効果を得るには0.0005%以上で含有することが好ましい。しかし、0.0020%を超えて含有すると、HAZ靭性が低下するようになるため、添加する場合は、0.0005〜0.0020%とする。
B: 0.0005 to 0.0020%
B is an element effective for improving hardenability by containing a very small amount and thereby improving the strength of the steel sheet, and it is preferable to contain B in an amount of 0.0005% or more in order to obtain such an effect. However, if it is contained in excess of 0.0020%, the HAZ toughness will decrease. Therefore, when it is added, it should be 0.0005 to 0.0020%.

REM:0.0020〜0.0200%
REMは、酸硫化物系介在物を形成することでHAZのオーステナイト粒成長を抑制しHAZ靭性を向上させる。このような効果を得るためには、0.0020%以上で含有することが好ましい。しかし、0.0200%を超える過剰の含有は、母材およびHAZの靭性を低下させるようになるため、添加する場合は0.0020〜0.0200%とする。
REM: 0.0020-0.0200%
REM suppresses the growth of austenite grains in HAZ by forming acid sulfide-based inclusions and improves HAZ toughness. In order to obtain such an effect, it is preferably contained in an amount of 0.0020% or more. However, an excess content of more than 0.0200% will reduce the toughness of the base metal and HAZ, so when added, it should be 0.0020 to 0.0200%.

Mg:0.0002〜0.0060%
Mgは、酸化物系介在物を形成することで溶接熱影響部においてオーステナイト粒の成長を抑制し、溶接熱影響部靭性の改善に有効な元素である。このような効果を得るには0.0002%以上で含有することが好ましい。しかし、0.0060%を超える含有は、効果が飽和して含有量に見合う効果が期待できずに経済的に不利となるため、添加する場合は0.0002〜0.0060%とする。
Mg: 0.0002 to 0.0060%
Mg is an element effective in improving the toughness of the weld heat-affected zone by suppressing the growth of austenite grains in the weld heat-affected zone by forming oxide-based inclusions. In order to obtain such an effect, it is preferably contained in an amount of 0.0002% or more. However, if the content exceeds 0.0060%, the effect is saturated and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when it is added, it should be 0.0002 to 0.0060%.

2.製造方法について
鋼板の製造方法について、各条件の限定理由を以下に述べる。なお、以下の温度は特に断らない限り鋼素材または鋼板の厚み中心温度とする。厚み中心部の温度は、放射温度計で測定した鋼素材または鋼板の表面温度から、伝熱計算により求める。
2. 2. Manufacturing method Regarding the manufacturing method of steel sheet, the reasons for limiting each condition are described below. Unless otherwise specified, the following temperatures are the center temperature of the thickness of the steel material or steel plate. The temperature at the center of the thickness is obtained by heat transfer calculation from the surface temperature of the steel material or steel plate measured with a radiation thermometer.

[鋼素材の加熱条件:1000℃以上1200℃以下]
鋼素材は連続鋳造によるものとし、1000℃以上1200℃以下に加熱する。加熱温度が1000℃より低くなると後述する熱間圧延条件を満足できず、十分な効果が得られない。一方、加熱温度が1200℃よりも高くなると、オーステナイト粒が粗大になり制御圧延後に所望の細粒組織が得られなくなる。このため、加熱温度を1000℃以上1200℃以下に限定する。なお、好ましくは1050℃以上1180℃以下である。
[Heating conditions for steel materials: 1000 ° C or higher and 1200 ° C or lower]
The steel material shall be continuously cast and heated to 1000 ° C or higher and 1200 ° C or lower. If the heating temperature is lower than 1000 ° C., the hot rolling conditions described later cannot be satisfied, and a sufficient effect cannot be obtained. On the other hand, when the heating temperature is higher than 1200 ° C., the austenite grains become coarse and a desired fine grain structure cannot be obtained after controlled rolling. Therefore, the heating temperature is limited to 1000 ° C or higher and 1200 ° C or lower. The temperature is preferably 1050 ° C or higher and 1180 ° C or lower.

[熱間圧延条件]
熱間圧延は、再結晶温度域のパス条件と未再結晶温度域のパス条件とを規定することが肝要である。まず、再結晶温度域である950℃以上の温度域において、平均圧下率/パスが7%以上のパスの累積圧下率が15%以上となる、圧延を行う。この圧延により再結晶させることにより、その後の組織を細かくするとともに、粗大な介在物を微細化・分散させる。なお、950℃未満の温度域での圧延では再結晶が起こり難くなり、オーステナイト粒の微細化が不十分となるため、950℃以上の圧延における圧下率を規定する必要がある。すなわち、平均圧下率/パスが7%未満では、圧延材全体に均一な圧下が加わらないためである。また、累積圧下率が15%未満では、充分に再結晶が行われないためである。なお、それぞれの条件の好ましい範囲は、累積圧下率が20%以上であり、圧下率/パスが8%以上である。
[Hot rolling conditions]
In hot rolling, it is important to specify the pass conditions in the recrystallization temperature range and the pass conditions in the unrecrystallized temperature range. First, in a temperature range of 950 ° C. or higher, which is a recrystallization temperature range, rolling is performed so that the cumulative rolling reduction of passes having an average reduction rate / pass of 7% or more is 15% or more. By recrystallizing by this rolling, the subsequent structure is made finer and coarse inclusions are made finer and dispersed. It should be noted that recrystallization is unlikely to occur in rolling in a temperature range of less than 950 ° C., and the miniaturization of austenite grains becomes insufficient. Therefore, it is necessary to specify the rolling reduction ratio in rolling at 950 ° C. or higher. That is, if the average reduction rate / pass is less than 7%, uniform reduction is not applied to the entire rolled material. Further, if the cumulative reduction rate is less than 15%, recrystallization is not sufficiently performed. The preferable range of each condition is that the cumulative reduction rate is 20% or more, and the reduction rate / pass is 8% or more.

ここで、「平均圧下率/パス」とは、1パス当たりの圧下率の平均値のことである。この「平均圧下率/パスが7%以上のパスの累積圧下率」とは、1パス当たりの圧下率の平均値が7%以上となる圧延を行った全体の圧下率を示す。具体的には、1パス当たりの圧下率の平均値が7%以上となる圧延を行う直前の板厚(A)から、上述の圧延を終了したときの板厚(B)から求める圧下率([A−B]/A×100)である。 Here, the "average reduction rate / pass" is the average value of the reduction rate per pass. The "average reduction rate / cumulative reduction rate of passes having a pass of 7% or more" indicates the total reduction rate of rolling in which the average value of the reduction rate per pass is 7% or more. Specifically, the rolling reduction (B) obtained from the plate thickness (A) immediately before rolling at which the average value of the rolling reduction per pass is 7% or more, and the plate thickness (B) at the end of the above-mentioned rolling ( [AB] / A × 100).

前記再結晶温度域での圧延に引き続く、未再結晶温度域では、900℃未満の温度域において、平均圧下率/パスが3%以上のパスの累積圧下率が40%以上となる、圧延を行う。すなわち、本発明鋼は900℃未満の温度域における圧延では再結晶が起こり難くなり、圧延で導入された歪みは再結晶に消費されずに蓄積され、圧延後の冷却時に変態核として作用する結果、最終組織が微細化する。
ここでの累積圧下率が40%未満では、鋼板全体の結晶粒の微細化効果が不十分になる。また、平均圧下率/パスが3%未満では板厚中央部に十分な圧下が加わらず、特に板厚中央部の結晶粒微細化効果が不十分となり、板厚位置による特性の不均一がより顕著になってしまう。なお、それぞれの条件の好ましい範囲は、累積圧下率が50%以上であり、圧下率/パスが4%以上である。これら一連の圧延、冷却により最終組織が微細化し、焼入、焼戻し製造に供する場合よりも低い添加元素量でもYS≧480MPaを確保できるとともに、溶接部靱性も向上する。
Following the rolling in the recrystallized temperature range, in the unrecrystallized temperature range, in the temperature range of less than 900 ° C., the rolling is performed so that the cumulative rolling reduction of the pass having an average reduction rate / pass of 3% or more is 40% or more. Do. That is, the steel of the present invention is less likely to recrystallize when rolled in a temperature range of less than 900 ° C., and the strain introduced in rolling is accumulated without being consumed by recrystallization, and as a result, it acts as a transformation nucleus during cooling after rolling. , The final structure becomes finer.
If the cumulative reduction rate here is less than 40%, the effect of refining the crystal grains of the entire steel sheet becomes insufficient. Further, if the average reduction rate / pass is less than 3%, sufficient reduction is not applied to the central portion of the plate thickness, and in particular, the effect of grain refinement in the central portion of the plate thickness becomes insufficient, and the characteristics become more uneven depending on the plate thickness position. It becomes noticeable. In the preferable range of each condition, the cumulative reduction rate is 50% or more, and the reduction rate / pass is 4% or more. By these series of rolling and cooling, the final structure becomes finer, and YS ≧ 480 MPa can be secured even with a lower amount of added elements than in the case of quenching and tempering, and the toughness of the welded part is also improved.

[冷却条件]
前記熱間圧延後の冷却は、700℃から550℃までの平均冷却速度を1.5〜50℃/sとし、この冷却を550℃以下まで行う。すなわち、700℃から550℃までの平均冷却速度が1.5℃/s未満になると、母材組織に粗大なフェライト相が生じるため、Subcritically reheated coarse-grain heat-affected zone (以下、SCCGHAZと称する)およびICCGHAZのCTOD特性が劣化する。一方、平均冷却速度が50℃/sよりも速くなると、母材強度の増加によりSCCGHAZおよびICCGHAZのCTOD特性が劣化するため、700℃から550℃までの平均冷却速度を1.5〜50℃/sに限定した。また、冷却停止温度が550℃を超えると、冷却による変態強化が不十分になり強度が不足するため、冷却停止温度は550℃以下とする。
[Cooling conditions]
For the cooling after the hot rolling, the average cooling rate from 700 ° C. to 550 ° C. is 1.5 to 50 ° C./s, and this cooling is performed to 550 ° C. or lower. That is, when the average cooling rate from 700 ° C to 550 ° C is less than 1.5 ° C / s, a coarse ferrite phase is formed in the base metal structure, so that the subcritically reheated coarse-grain heat-affected zone (hereinafter referred to as SCCGHAZ) and The CTOD characteristics of ICCG HAZ deteriorate. On the other hand, if the average cooling rate is faster than 50 ° C / s, the CTOD characteristics of SCCGHAZ and ICCGHAZ deteriorate due to the increase in base metal strength, so the average cooling rate from 700 ° C to 550 ° C is reduced to 1.5 to 50 ° C / s. Limited. If the cooling stop temperature exceeds 550 ° C, the transformation strengthening by cooling becomes insufficient and the strength becomes insufficient, so the cooling stop temperature is set to 550 ° C or less.

なお、鋼板の強度を低下させて靭性を向上させる場合は、上記した冷却停止後、Ac1変態点以下で焼戻しを行ってもよい。焼戻し温度が変態点を超えてしまうと、オーステナイト組織が発生してしまい、圧延で得られた組織がキャンセルされてしまい諸特性が劣化することになる。 When the strength of the steel sheet is lowered to improve the toughness, tempering may be performed below the Ac 1 transformation point after the cooling is stopped. If the tempering temperature exceeds the transformation point, an austenite structure is generated, the structure obtained by rolling is canceled, and various properties are deteriorated.

表1に示す各成分の供試鋼を溶製し連続鋳造によって鋼スラブとした。鋼種A〜Gは成分組成が本発明の範囲を満足する発明例であり、鋼種H〜Nは成分組成が本発明の範囲外の比較例である。これらの鋼スラブを用いて表2に示す製造条件により鋼板を製造した。かくして得られた鋼板について、引張試験を行って機械的特性を次のように測定した。さらに、得られた鋼板毎に、多層盛溶接継手を作製し、溶接継手について次のように評価を行った。これら測定および評価結果を表2に示す。 The test steels of each component shown in Table 1 were melted and continuously cast to obtain steel slabs. Steel types A to G are examples of inventions in which the component composition satisfies the range of the present invention, and steel types H to N are comparative examples in which the component composition is outside the range of the present invention. Using these steel slabs, steel sheets were manufactured under the manufacturing conditions shown in Table 2. The steel sheet thus obtained was subjected to a tensile test and its mechanical properties were measured as follows. Further, a multi-layer welded joint was prepared for each of the obtained steel plates, and the welded joint was evaluated as follows. The results of these measurements and evaluations are shown in Table 2.

引張試験は、鋼板表面から板厚(t)の1/4位置において、板幅方向と平行に平行部直径14mmおよび平行部長さ70mmの丸棒引張試験片を採取し、EN10002−1に従って引張試験を行った。なお、表2に示す降伏強度は、上降伏点が現れた場合は上降伏応力を、上降伏点が現れなかった場合は0.2%耐力を示している。 In the tensile test, a round bar tensile test piece having a parallel portion diameter of 14 mm and a parallel portion length of 70 mm was sampled at a position 1/4 of the plate thickness (t) from the surface of the steel plate in parallel with the plate width direction, and a tensile test was performed in accordance with EN10002-1. Was done. The yield strength shown in Table 2 shows the upper yield stress when the upper yield point appears, and 0.2% proof stress when the upper yield point does not appear.

継手のシャルピー試験およびCTOD試験に使用する溶接継手は、K開先形状、入熱量5.0kJ/mmのサブマージアーク溶接(多層溶接)を用いて作製した。
シャルピー試験は、溶接線(Fusion Line;FL)に2mmVノッチを入れた10×10mm断面の試験片を作製し、−60℃でシャルピー試験を行った。
The welded joint used for the Charpy test and the CTOD test of the joint was manufactured by submerged arc welding (multilayer welding) having a K groove shape and a heat input of 5.0 kJ / mm.
For the Charpy test, a test piece having a cross section of 10 × 10 mm having a 2 mm V notch in the fusion line (FL) was prepared, and the Charpy test was performed at −60 ° C.

CTOD試験は、BS規格EN10225(2009)に準拠し、板厚50mmまではt(板厚)×2t(板厚)、50mm超えはt(板厚)×t(板厚)の断面形状の試験片を用い、試験温度−10℃においてCTOD値(δ)を評価した。各鋼種に対し切欠位置ごとに3本ずつ試験し、CGHAZのCTOD値と、SC/ICHAZ境界のCTOD値のうち、最も低いCTOD値を表2に記載した。試験後、試験片破面で、疲労予亀裂の先端が同EN10225(2009)で規定するCGHAZと、SC/ICHAZ境界のそれぞれにあることを確認した。なお、多層溶接の継手CTOD試験の場合、切欠位置がCGHAZであっても、一定量のICCGHAZが含まれるため、試験結果には、CGHAZとICCGHAZの両方の靭性が反映される。 The CTOD test complies with BS standard EN10225 (2009), and is a cross-sectional shape test of t (plate thickness) x 2t (plate thickness) up to a plate thickness of 50 mm and t (plate thickness) x t (plate thickness) over 50 mm. The CTOD value (δ) was evaluated at a test temperature of −10 ° C. using the pieces. Three pieces were tested for each notch position for each steel type, and the lowest CTOD value among the CTOD value of CGHAZ and the CTOD value of the SC / ICHAZ boundary is shown in Table 2. After the test, it was confirmed on the fracture surface of the test piece that the tip of the fatigue pre-crack was at the CGHAZ and SC / ICHAZ boundaries specified in EN10225 (2009). In the case of the multi-layer welded joint CTOD test, even if the notch position is CGHAZ, a certain amount of ICCGHAZ is included, so the test results reflect the toughness of both CGHAZ and ICCGHAZ.

表2に試験結果を示す。No.1〜13は本発明の化学成分および製造条件ともに本発明の範囲を満足する発明例であり、母材の引張強度および優れた継手CTOD特性を示していた。
一方、No.14〜28は化学成分もしくは製造条件が本発明から外れる比較例であり、発明例と比較して母材特性および継手特性が劣位である。
Table 2 shows the test results. Nos. 1 to 13 are examples of inventions that satisfy the scope of the present invention in terms of both the chemical composition and the production conditions of the present invention, and show the tensile strength of the base metal and the excellent CTOD characteristics of the joint.
On the other hand, No. 14 to 28 are comparative examples in which the chemical composition or the manufacturing conditions deviate from the present invention, and the base material characteristics and the joint characteristics are inferior to those of the invention examples.

Claims (4)

質量%で、
C:0.01〜0.10%、
Si:0.6%以下、
Mn:1.0〜1.8%、
P:0.01%以下、
S:0.0005〜0.0050%、
Al:0.001〜0.060%、
Ni:0.2〜2.0%、
Ti:0.005〜0.050%、
N:0.0015〜0.0065%、
O:0.0010〜0.0050%および
Ca:0.0005〜0.0060%
を、下記(1)式で定義されるACRが0を超え1.0以下および下記(2)式で定義されるCeqが0.45以上0.53以下となる範囲で含有し、残部Feおよび不可避的不純物の成分組成を有し、−60℃における継手部靱性値:35J以上および−10℃におけるCTOD値:0.10mm以上を満足し、降伏応力(YS):480MPa以上かつ引張応力(TS):550MPa以上で板厚が30〜100mmである、鋼板。
ACR={[Ca]−(0.18+130×[Ca])×[O]}÷(1.25×[S])…(1)
Ceq=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5)…(2)
(1)式および(2)式において、[ ]は該括弧内の元素の含有量(質量%)である。但し、含有されない元素はゼロとする。
By mass%
C: 0.01 to 0.10%,
Si: 0.6% or less,
Mn: 1.0-1.8%,
P: 0.01% or less,
S: 0.0005 to 0.0050%,
Al: 0.001 to 0.060%,
Ni: 0.2-2.0%,
Ti: 0.005 to 0.050%,
N: 0.0015-0.0065%,
O: 0.0010-0.0050% and
Ca: 0.0005-0.0060%
Is contained in the range where the ACR defined by the following formula (1) is more than 0 and 1.0 or less and the Ceq defined by the following formula (2) is 0.45 or more and 0.53 or less, and the component composition of the balance Fe and the unavoidable impurities. have a joint portion toughness value at -60 ° C.: 35 J or higher and CTOD value at -10 ° C.: satisfied than 0.10 mm, the yield stress (YS): 480 MPa or more and a tensile stress (TS): sheet thickness at least 550MPa Is a steel plate with a diameter of 30 to 100 mm .
ACR = {[Ca]-(0.18 + 130 x [Ca]) x [O]} ÷ (1.25 x [S]) ... (1)
Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5) ... (2)
In the formulas (1) and (2), [] is the content (mass%) of the element in the parentheses. However, the elements that are not contained are zero.
前記成分組成は、更に、質量%で、
Cu:0.05〜0.60%、
Cr:0.05〜0.50%、
Mo:0.05〜0.50%、
Nb:0.005〜0.035%、
V:0.01〜0.10%、
W:0.01〜0.50%、
B:0.0005〜0.0020%、
REM:0.0020〜0.0200%および
Mg:0.0002〜0.0060%
のうちの1種または2種以上を含む請求項1に記載の鋼板。
The component composition is further increased by mass%.
Cu: 0.05-0.60%,
Cr: 0.05-0.50%,
Mo: 0.05-0.50%,
Nb: 0.005 to 0.035%,
V: 0.01 to 0.10%,
W: 0.01-0.50%,
B: 0.0005 to 0.0020%,
REM: 0.0020-0.0200% and
Mg: 0.0002 to 0.0060%
The steel sheet according to claim 1, which comprises one or more of the above.
請求項1または2に記載の成分組成の鋼素材を1000℃以上1200℃以下に加熱し、950℃以上の温度域における、平均圧下率/パスが7%以上のパスの累積圧下率が15%以上であり、かつ900℃未満の温度域における、平均圧下率/パスが3%以上のパスの累積圧下率が40%以上である、熱間圧延を施し、その後、700℃から550℃までの平均冷却速度が1.5〜50℃/sとなる冷却を550℃以下まで行う、−60℃における継手部靱性値:35J以上および−10℃におけるCTOD値:0.10mm以上を満足し、降伏応力(YS):480MPa以上かつ引張応力(TS):550MPa以上で板厚が30〜100mmである、鋼板の製造方法。 The steel material having the composition according to claim 1 or 2 is heated to 1000 ° C. or higher and 1200 ° C. or lower, and the cumulative rolling reduction rate of the pass having an average reduction rate / pass of 7% or more in a temperature range of 950 ° C. or higher is 15%. Hot rolling is performed in a temperature range of more than 900 ° C and less than 900 ° C, with an average reduction rate / pass of 3% or more and a cumulative reduction rate of 40% or more, and then from 700 ° C to 550 ° C. Cooling to an average cooling rate of 1.5 to 50 ° C / s to 550 ° C or less , satisfying joint toughness value at -60 ° C: 35J or more and CTOD value at -10 ° C: 0.10mm or more, yield stress (YS) ): 480 MPa or more and tensile stress (TS): 550 MPa or more and a plate thickness of 30 to 100 mm, a method for manufacturing a steel sheet. 請求項3に記載の方法において、前記冷却後にさらに、Ac1変態点以下の温度で焼戻し処理を行う鋼板の製造方法。

The method for producing a steel sheet according to claim 3, wherein after the cooling, the steel sheet is further tempered at a temperature equal to or lower than the Ac 1 transformation point.

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