JP2019183205A - Steel sheet and manufacturing method therefor - Google Patents
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 53
- 239000010959 steel Substances 0.000 title claims abstract description 53
- 238000004519 manufacturing process Methods 0.000 title claims description 14
- 239000000463 material Substances 0.000 claims abstract description 16
- 239000000203 mixture Substances 0.000 claims abstract description 13
- 238000005096 rolling process Methods 0.000 claims description 38
- 230000009467 reduction Effects 0.000 claims description 32
- 238000001816 cooling Methods 0.000 claims description 22
- 238000000034 method Methods 0.000 claims description 14
- 230000001186 cumulative effect Effects 0.000 claims description 11
- 230000009466 transformation Effects 0.000 claims description 10
- 239000012535 impurity Substances 0.000 claims description 4
- 229910052791 calcium Inorganic materials 0.000 abstract description 5
- 229910052799 carbon Inorganic materials 0.000 abstract description 4
- 229910052760 oxygen Inorganic materials 0.000 abstract description 4
- 229910052717 sulfur Inorganic materials 0.000 abstract description 4
- 229910052748 manganese Inorganic materials 0.000 abstract description 2
- 229910052759 nickel Inorganic materials 0.000 abstract description 2
- 229910052698 phosphorus Inorganic materials 0.000 abstract description 2
- 229910052757 nitrogen Inorganic materials 0.000 abstract 1
- 229910001566 austenite Inorganic materials 0.000 description 24
- 230000000694 effects Effects 0.000 description 23
- 238000012360 testing method Methods 0.000 description 20
- 238000003466 welding Methods 0.000 description 13
- 238000001953 recrystallisation Methods 0.000 description 11
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 8
- 239000010953 base metal Substances 0.000 description 6
- 230000006911 nucleation Effects 0.000 description 6
- 238000010899 nucleation Methods 0.000 description 6
- 229910000859 α-Fe Inorganic materials 0.000 description 6
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 5
- 238000005516 engineering process Methods 0.000 description 5
- 238000011156 evaluation Methods 0.000 description 5
- 238000005098 hot rolling Methods 0.000 description 5
- 230000006872 improvement Effects 0.000 description 5
- 238000005496 tempering Methods 0.000 description 5
- 239000002131 composite material Substances 0.000 description 4
- 239000006185 dispersion Substances 0.000 description 4
- 238000010438 heat treatment Methods 0.000 description 4
- 230000035882 stress Effects 0.000 description 4
- 239000000126 substance Substances 0.000 description 4
- XTQHKBHJIVJGKJ-UHFFFAOYSA-N sulfur monoxide Chemical compound S=O XTQHKBHJIVJGKJ-UHFFFAOYSA-N 0.000 description 4
- 238000009864 tensile test Methods 0.000 description 4
- 229910001563 bainite Inorganic materials 0.000 description 3
- 230000007423 decrease Effects 0.000 description 3
- 229910000734 martensite Inorganic materials 0.000 description 3
- 238000007670 refining Methods 0.000 description 3
- 239000002436 steel type Substances 0.000 description 3
- 239000000654 additive Substances 0.000 description 2
- 230000000996 additive effect Effects 0.000 description 2
- 229910045601 alloy Inorganic materials 0.000 description 2
- 239000000956 alloy Substances 0.000 description 2
- 230000000052 comparative effect Effects 0.000 description 2
- 239000000470 constituent Substances 0.000 description 2
- 238000010276 construction Methods 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 230000000593 degrading effect Effects 0.000 description 2
- 239000002245 particle Substances 0.000 description 2
- 238000010791 quenching Methods 0.000 description 2
- 230000000171 quenching effect Effects 0.000 description 2
- 230000001105 regulatory effect Effects 0.000 description 2
- 229920006395 saturated elastomer Polymers 0.000 description 2
- 238000005204 segregation Methods 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- -1 REM oxysulfides Chemical class 0.000 description 1
- 230000032683 aging Effects 0.000 description 1
- 238000005452 bending Methods 0.000 description 1
- 230000008901 benefit Effects 0.000 description 1
- 238000004364 calculation method Methods 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
- 239000011362 coarse particle Substances 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 230000001276 controlling effect Effects 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 238000006073 displacement reaction Methods 0.000 description 1
- 230000000459 effect on growth Effects 0.000 description 1
- 239000011159 matrix material Substances 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 238000005065 mining Methods 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 229910052758 niobium Inorganic materials 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 230000001376 precipitating effect Effects 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 238000004881 precipitation hardening Methods 0.000 description 1
- 230000005855 radiation Effects 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 230000004044 response Effects 0.000 description 1
- 239000006104 solid solution Substances 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
- RMLPZKRPSQVRAB-UHFFFAOYSA-N tris(3-methylphenyl) phosphate Chemical compound CC1=CC=CC(OP(=O)(OC=2C=C(C)C=CC=2)OC=2C=C(C)C=CC=2)=C1 RMLPZKRPSQVRAB-UHFFFAOYSA-N 0.000 description 1
- 229910052720 vanadium Inorganic materials 0.000 description 1
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- Heat Treatment Of Steel (AREA)
Abstract
Description
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 is not only excellent in low-temperature toughness as a base material, but also in CTOD characteristics of the joint part when forming a multilayer welded joint, as a steel plate used for ships, offshore structures, line pipes, pressure vessels, etc. In particular, the present invention relates to a steel plate excellent in the manufacturing method and a manufacturing method thereof.
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. For a thick steel plate having a thickness of 30 mm or more used as a structure, not only the toughness of the steel plate but also the toughness at the welded portion is required. In order to secure new resources in response to the increase in energy demand in recent years, construction areas such as offshore structures have spread to cold regions where resource mining has not been performed so far. Along with this, the guaranteed temperature of toughness required for steel sheets used in such structures has also been lowered. In 2016, the unified rules were revised by the IACS (International Classification Society Association), and in the future, the implementation of CTOD test (Crack Tip Opening Displacement Test) for each class will be required. Yes.
Here, the CTOD test refers to the resistance to occurrence of brittle fracture by performing a bending test at a low temperature on a specimen in which a fatigue crack has been introduced in the toughness evaluation part, and measuring the amount of opening (plastic deformation) of the crack immediately before 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 applying a steel plate to a structure, construction by multilayer welding is generally performed. In the weld heat affected zone of multilayer welding (hereinafter referred to as multilayer welding HAZ), a coarse structure (Coarse Grain Heat Affected Zone: hereinafter referred to as CGHAZ) in the vicinity of the weld line by the previous welding pass is welded to the next layer. It was reheated to two phases of ferrite + austenite by the pass, and the toughness was remarkably lowered due to the inclusion of island-like martensite (Martensite-Austenite Constituent: MA) in the coarse matrix. It is known that a region (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 ICCGHAZ structure is included in the evaluation area where fatigue precrack is introduced when multilayer welded HAZ is targeted. Will be. On the other hand, the joint CTOD characteristics obtained by joint CTOD tests are governed by the toughness of the most brittle region in the evaluation area, so the joint CTOD characteristics of multilayer welded HAZ reflect not only the CGHAZ structure but also the toughness of the ICCGHAZ structure. The Therefore, to improve the joint CTOD characteristics of multilayer welded HAZ, it is necessary to improve the toughness of the ICCGHAZ 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 HAZ toughness improvement technology, it is known to suppress the austenite grain coarsening of CGHAZ by the fine dispersion of TiN and to use the ferrite transformation nucleus of TiN.
Furthermore, a technique for suppressing grain growth of austenite grains due to dispersion of REM-based oxysulfide generated by adding REM, or for suppressing grain growth of austenite grains due to dispersion of Ca-based oxysulfide generated by adding Ca, A technique that combines the nucleation ability of BN with oxide dispersion has 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 the coarsening of the austenite structure of HAZ using REM and TiN particles. Patent Document 3 proposes a HAZ toughness improving technique 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。 Patent Document 4 proposes a technique for suppressing the generation of martensite by reducing C and Si, and further increasing the strength of the base metal by adding Cu, as a measure for reducing the toughness of ICCGHZ. Yes.
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 lower the hardness of the central segregation part, coarse particles are suppressed using TiN, and the balance of C, P, and Ni is optimized. A technique for satisfying the weld toughness of −80 ° C. and the CTOD characteristic at −60 ° C. by combining the improvement of toughness and the like has been proposed.
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。 Many technologies have been proposed to improve either the low-temperature toughness or CTOD characteristics of welds, but the technology to ensure both is not sufficient from the standpoint 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, regarding the technology for suppressing the coarsening of the austenite structure of HAZ using REM and TiN particles disclosed in Patent Document 1 and Patent Document 2, since TiN dissolves in the bond portion that reaches a high temperature during welding, austenite grains Insufficient effect on growth control.
REM oxysulfides and Ca oxysulfides are effective in suppressing austenite grain growth. However, joint CTOD characteristics at low service 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 large heat input welding where the cooling rate of the heat affected zone was slow and HAZ was a structure mainly composed of ferrite. However, in the case of a thick steel plate, the amount of alloy components contained in the base metal is relatively high, while the amount of heat input in multilayer 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 characteristic 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 uses precipitation hardening of Cu, so aging treatment after rolling is indispensable, and there is a problem that the manufacturing cost increases correspondingly. 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 and CTOD characteristics in the low temperature range are made compatible under TMCP conditions, but the Charpy toughness value at −80 ° C. and the CTOD value at −60 ° C. or lower are shown. There is room for improvement in terms of cost and realization with very expensive component systems.
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, a thick steel plate having a thickness of 30 to 100 mm and a yield stress of 480 MPa or more, which requires CTOD characteristics for shipbuilding, has been manufactured by a normal quenching and tempering method. However, in this method, it is necessary to add a lot of solute elements in order to ensure stable quality, and the toughness of the welded portion tends to be lowered.
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 is excellent in the toughness of the base metal and the welded part when multilayer welding is performed. Specifically, the joint part toughness value at −60 ° C. is 35 J or more and the CTOD value at −10 ° C. is 0.10 mm or more. The objective is to provide a low-cost steel sheet having a yield stress (YS) of 480 MPa or more and a tensile stress (TS) of 550 MPa or more and a thickness of 30 to 100 mm. Furthermore, it aims at proposing about the advantageous manufacturing method of this steel plate.
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］）
Inventors etc. earnestly examined about the method for solving the said problem, and acquired the following knowledge.
(I) When Ca, O, and S in steel are controlled within the range of atomic concentration ratio (ACR) shown by the following formula within a range of 0 to 1.0, the form of sulfide partially dissolves in Mn. It becomes a composite inclusion of Ca-based sulfide and Al-based oxide.
ACR = {[Ca] − (0.18 + 130 × [Ca]) × [O]} ÷ (1.25 × [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 making the inclusion form a composite inclusion composed 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 weld line. The austenite grain coarsening effect can be sufficiently exhibited. 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) Nucleation sites during cooling of HAZ are mainly austenite grain boundaries. By the presence of the above-mentioned composite inclusion having a nucleation effect in the austenite grains, nucleation starts from within the austenite grains in addition to the austenite grain boundaries, and the finally obtained HAZ structure becomes fine, and the toughness of HAZ 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, the toughness of the base structure of the multilayer welded HAZ can be improved.
（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) Usually, an alloy element is concentrated in the element segregation portion at the center of the plate thickness of the slab, thereby causing a problem that coarse inclusions are dispersed at a low density. However, if the rolling reduction ratio is 7% or more in each pass when the sheet thickness center temperature is 950 ° C or higher, adding a rolling reduction of 15% or more in all passes will increase the strain applied to the sheet thickness center, resulting in coarse interposition By extending and further dividing the object, fine inclusions can be dispersed with high density. In addition, the effect of improving the HAZ toughness by inclusions can be secured, 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 made based on the above findings and further studies. The gist of the present invention is as follows.
[1] By mass%
C: 0.01-0.10%,
Si: 0.6% or less,
Mn: 1.0-1.8%
P: 0.01% or less,
S: 0.0005-0.0050%,
Al: 0.001 to 0.060%,
Ni: 0.2-2.0%
Ti: 0.005-0.050%,
N: 0.0015-0.0065%,
O: 0.0010 to 0.0050% and
Ca: 0.0005 to 0.0060%
In the range where ACR defined by the following formula (1) exceeds 0 and 1.0 or less and Ceq defined by the following formula (2) is 0.45 or more and 0.53 or less, and the remaining Fe and inevitable impurity component group Steel sheet having
ACR = {[Ca] − (0.18 + 130 × [Ca]) × [O]} ÷ (1.25 × [S]) (1)
Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5) (2)
In the formulas (1) and (2), [] is the content (% by mass) of the element in the parentheses. However, no element is contained.
［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 in mass%,
Cu: 0.05-0.60%
Cr: 0.05 to 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% and
Mg: 0.0002 to 0.0060%
The steel plate according to [1], including one or more of them.
［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 described in [1] or [2] is heated to 1000 ° C. or more and 1200 ° C. or less, and the average rolling reduction / pass is 7% or more in a temperature range of 950 ° C. or more. Cumulative rolling reduction is 15% or more, and the average rolling reduction / pass is 3% or more in the temperature range below 900 ° C. Cumulative rolling reduction is 40% or more. The manufacturing method of the steel plate which performs the cooling from which the average cooling rate between 700-550 degreeC becomes 1.5-50 degrees C / s to 550 degrees 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 according to [3], wherein a 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。 ADVANTAGE OF THE INVENTION According to this invention, the steel plate which can obtain the toughness and CTOD characteristic which were excellent in the joint at the time of multilayer welding, and its manufacturing method can be provided, and it is very useful industrially.
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 the respective constituent requirements of the present invention will be described below.
1. About chemical composition First, the reason which prescribed | regulated the chemical composition of the steel of this invention is demonstrated. In addition, all “%” indications regarding the component composition mean “mass%”.
C: 0.01-0.10%
C is an element that improves the strength of the steel and needs to be contained in an amount of 0.01% or more. However, when C is contained excessively, the hardness of the concentrated portion is increased, and the toughness of the base material and the joint portion and the joint CTOD characteristic are deteriorated. For this reason, the upper limit of C is limited to a range of 0.10% or less that does not deteriorate the joint characteristics even if it is concentrated. In addition, Preferably it is 0.01 to 0.08%, More preferably, it is 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
When Si is excessively contained in excess of 0.6%, the joint CTOD characteristics deteriorate. For this reason, Si was limited to the range of 0.6% or less. In addition, Preferably it is 0.01% or more and 0.3% or less, More preferably, it is 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 the strength through improving the hardenability of steel. However, when added in excess, joint CTOD properties are significantly reduced. For this reason, Mn was limited to the range of 1.0 to 1.8%. In addition, Preferably it is 1.1 to 1.7% of range.
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 it is desirable to reduce it as much as possible in order to reduce the toughness of steel. In particular, it is necessary to manage more strictly than usual in order to ensure joint toughness at low temperatures. Therefore, 0.01% or less, which starts to lower the low temperature toughness. Preferably it is 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: 0.0005-0.0050%
S is an element necessary for inclusions for improving the toughness of multilayer welded HAZ, and it is necessary to contain 0.0005% or more. However, the content exceeding 0.0050% conversely decreases the toughness and CTOD characteristics of the joint, so it is limited to 0.0050% or less. Preferably it is 0.003% or less, More preferably, it is 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 necessary for inclusions to improve the toughness of multilayer welded HAZ, and it is necessary to contain 0.001% or more. On the other hand, the content exceeding 0.060% is limited to 0.060% or less in order to reduce the joint CTOD characteristics. 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 without greatly degrading the toughness of both the base material and the joint. For that purpose, it is 0.2% or more. However, if it exceeds 2.0%, the effect of increasing the strength is saturated, and the increase in cost becomes a problem. Therefore, the upper limit was made 2.0%. In addition, from the viewpoint of obtaining the effect more effectively, 1.8% or less immediately before the occurrence of saturation of strength increase 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: 0.005-0.050%
Ti precipitates as TiN and is an element effective in suppressing austenite grain coarsening of HAZ, refining the HAZ structure, and improving toughness. In order to acquire such an effect, 0.005% or more of content is required. On the other hand, when it contains excessively exceeding 0.050%, HAZ toughness will fall by precipitation of solid solution Ti or coarse TiC. For this reason, Ti was limited to 0.005 to 0.050% of range. Preferably it is 0.005-0.040%, More preferably, it is 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 effective in improving the toughness by suppressing the coarsening of austenite grains of HAZ by precipitating as TiN and by refining the HAZ structure. In order to acquire such an effect, 0.0015% or more needs to be contained. On the other hand, when it exceeds 0.0065% and it contains excessively, HAZ toughness will come to fall. For this reason, it limited to 0.0015 to 0.0065% of range. Preferably it is 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 necessary for inclusions for improving the toughness of the multilayer welded HAZ, and should be contained in an amount of 0.0010% or more. On the other hand, if the content exceeds 0.0050%, the joint CTOD characteristics are deteriorated. Therefore, in the present invention, the content is limited to the range of 0.0010 to 0.0050%. Preferably it is 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 to 0.0060%
Ca is an element necessary for inclusions to improve the toughness of multilayer welded HAZ, and it is necessary to contain 0.0005% or more. On the other hand, if the content exceeds 0.0060%, the joint CTOD characteristics are rather deteriorated. Therefore, in the present invention, the content is limited to the range of 0.0005 to 0.0060%. Preferably it is 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: More than 0 and 1.0 or less ACR according to the above-described formula (1) is an atomic concentration ratio of Ca, O and S in steel. In principle, when the value in the formula is 0 or less, the main form of the sulfide inclusion is MnS. Since MnS has a low melting point and dissolves in the vicinity of the weld line during welding, the effect of suppressing the austenite grain coarsening near the weld line and the transformation nucleus effect during cooling after welding cannot be obtained. On the other hand, if the value of the above formula exceeds 1.0, the main form of sulfide inclusions is CaS, and the transformation nucleus effect cannot be obtained because the Mn dilute layer necessary to become a transformation nucleus around CaS is not formed. . Therefore, the contents of Ca, O and S are regulated so that ACR exceeds 0 and is 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 more the amount of the additive element increases, 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 inferior toughness such as island martensite and bainite in the HAZ structure. The condition for maintaining the effect of the HAZ toughness improvement technology while ensuring YS ≧ 480 MPa was set to 0.53 or less. On the other hand, when Ceq is less than 0.45, it is difficult to obtain the target strength. 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 the component composition in which the balance other than the above components is Fe and inevitable 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%.
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 makes it possible to increase the strength without greatly degrading the toughness of the base material and the joint. For this purpose, it is preferable to add 0.05% or more. On the other hand, if added too much, it leads to a decrease in toughness, and there is a problem of steel plate cracking due to the Cu concentrated layer generated immediately below the scale. In order to satisfy the target characteristics of this time, the content 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 through the improvement of the hardenability of steel. However, if added excessively, the joint CTOD characteristic is lowered, so when added, the content is made 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 by improving the hardenability of steel, but if added in excess, the joint CTOD characteristics are lowered. For this reason, when adding, it is 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: 0.005-0.035%
Nb is an element that widens the non-recrystallization temperature region of the austenite phase, and is an effective element for efficiently performing non-recrystallization region rolling and obtaining a fine structure. In order to obtain the effect, a content of 0.005% or more is required. However, if it exceeds 0.035%, the toughness of the joint part and CTOD characteristics are deteriorated. Therefore, when added, the content is made 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: 0.01-0.10%
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 is lowered, so when added, the content is made 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: 0.01-0.50%
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 is lowered, so when added, the content is made 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: 0.0005-0.0020%
B is an element effective for improving the hardenability and thereby improving the strength of the steel sheet by containing a very small amount thereof. To obtain such an effect, B is preferably contained at 0.0005% or more. However, if the content exceeds 0.0020%, the HAZ toughness decreases, so when added, the content is made 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 HAZ austenite grain growth and improves HAZ toughness by forming oxysulfide inclusions. In order to obtain such an effect, the content is preferably 0.0020% or more. However, an excessive content exceeding 0.0200% lowers the toughness of the base metal and HAZ, so when added, the content is made 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 weld heat affected zone toughness by suppressing the growth of austenite grains in the weld heat affected zone by forming oxide inclusions. In order to obtain such an effect, the content is preferably 0.0002% or more. However, if the content exceeds 0.0060%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when added, the content is made 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. About a manufacturing method The limitation reason of each condition is described below about the manufacturing method of a steel plate. In addition, unless otherwise indicated, the following temperature shall be the thickness center temperature of a steel raw material or a 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 condition of steel material: 1000 ℃ or more and 1200 ℃ or less]
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 sufficient effects 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. For this reason, heating temperature is limited to 1000 degreeC or more and 1200 degrees C or less. In addition, Preferably it is 1050 degreeC or more and 1180 degrees C or less.
［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 define pass conditions in the recrystallization temperature range and pass conditions in the non-recrystallization 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 reduction rate of passes having an average reduction rate / pass of 7% or more is 15% or more. By recrystallization by this rolling, the subsequent structure is refined and coarse inclusions are refined and dispersed. In addition, since recrystallization hardly occurs in rolling in a temperature range of less than 950 ° C. and the austenite grains are insufficiently refined, it is necessary to define a reduction ratio in rolling at 950 ° C. or higher. That is, when the average rolling reduction / pass is less than 7%, uniform rolling is not applied to the entire rolled material. Further, if the cumulative rolling reduction is less than 15%, sufficient recrystallization is not performed. In addition, as for the preferable range of each condition, the cumulative reduction ratio is 20% or more, and the reduction ratio / 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, “average rolling reduction / pass” is an average value of rolling reduction per pass. The “average rolling reduction / cumulative rolling reduction of pass having a pass of 7% or more” indicates the overall rolling reduction in which rolling is performed so that the average value of the rolling reduction per pass is 7% or more. Specifically, the reduction ratio (A) obtained from the sheet thickness (A) immediately before rolling at which the average value of the reduction ratio per pass is 7% or more is determined from the sheet thickness (B) when the above rolling is completed ( [A−B] / 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 recrystallization temperature range, in the non-recrystallization temperature range, in the temperature range of less than 900 ° C., the average reduction rate / pass is 3% or more, and the cumulative reduction rate of the pass is 40% or more. Do. That is, the steel according to the present invention is less likely to recrystallize during rolling in a temperature range below 900 ° C., and strain introduced in the rolling is accumulated without being consumed by recrystallization, and acts as a transformation nucleus during cooling after rolling. The final structure becomes finer.
If the cumulative rolling reduction here is less than 40%, the effect of refining the crystal grains of the entire steel sheet becomes insufficient. In addition, if the average reduction ratio / pass is less than 3%, sufficient reduction is not applied to the central portion of the plate thickness, particularly the effect of crystal grain refinement at the central portion of the plate thickness is insufficient, and the non-uniformity of properties due to the plate thickness position is further increased. It becomes prominent. In addition, the preferable range of each condition is that the cumulative rolling reduction is 50% or more and the rolling reduction / pass is 4% or more. The final structure is refined by these series of rolling and cooling, and YS ≧ 480 MPa can be secured even when the amount of additive elements is lower than when subjected to quenching and tempering production, and the toughness of the welded portion 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]
In 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 less. 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 generated in the base material structure. ICCGHAZ CTOD characteristics 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 the strength of the base metal, so the average cooling rate from 700 ° C to 550 ° C is 1.5 to 50 ° C / s. Limited. Further, if the cooling stop temperature exceeds 550 ° C, transformation strengthening due to cooling becomes insufficient and the strength is 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。 In addition, when improving the toughness by reducing the strength of the steel sheet, tempering may be performed at or below the Ac 1 transformation point after stopping the cooling described above. When 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。 Test steels having the respective components shown in Table 1 were melted and formed into steel slabs by continuous casting. Steel types A to G are invention examples whose component compositions satisfy the scope of the present invention, and steel types H to N are comparative examples whose component compositions are outside the scope of the present invention. Steel plates were produced using these steel slabs under the production conditions shown in Table 2. The steel sheet thus obtained was subjected to a tensile test and the mechanical properties were measured as follows. Furthermore, for each steel plate obtained, a multi-layer welded joint was prepared, and the welded joint was evaluated as follows. Table 2 shows these measurement and evaluation results.
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。 Tensile test is a round bar tensile test piece with a parallel part diameter of 14mm and a parallel part length of 70mm parallel to the plate width direction at 1/4 position of the plate thickness (t) from the steel sheet surface, and tensile test according to EN10002-1 Went. The yield strength shown in Table 2 indicates the upper yield stress when the upper yield point appears, and the 0.2% yield strength 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 CTOD test of the joint was produced by submerged arc welding (multilayer welding) with a K groove shape and a heat input of 5.0 kJ / mm.
In the Charpy test, a test piece having a cross section of 10 × 10 mm in which a 2 mmV notch was made in a weld 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 conforms to BS standard EN10225 (2009), and the cross-sectional shape is t (plate thickness) x 2t (plate thickness) up to 50mm thick, and t (plate thickness) x t (plate thickness) up to 50mm thick. The piece was used to evaluate the CTOD value (δ) at a test temperature of −10 ° C. Three steels were tested for each steel type at each notch position, and the lowest CTOD value among the CTOD value of CGHAZ and the CTOD value at the SC / ICHAZ boundary is shown in Table 2. After the test, it was confirmed on the fracture surface of the specimen that the tip of the fatigue precrack was at the CGHAZ defined in EN10225 (2009) and the SC / ICHAZ boundary. In the case of multi-layer welded joint CTOD test, even if the notch position is CGHAZ, a certain amount of ICCGHAZ is included, so the toughness of both CGHAZ and ICCGHAZ is reflected in the test results.
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 invention examples satisfying the scope of the present invention in terms of both the chemical composition and the production conditions of the present invention, and showed the tensile strength of the base material and excellent joint CTOD characteristics.
On the other hand, No. 14 to 28 are comparative examples in which the chemical components or production conditions deviate from the present invention, and the base material characteristics and joint characteristics are inferior to those of the inventive examples.
Claims (4)
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。 % By mass
C: 0.01-0.10%,
Si: 0.6% or less,
Mn: 1.0-1.8%
P: 0.01% or less,
S: 0.0005-0.0050%,
Al: 0.001 to 0.060%,
Ni: 0.2-2.0%
Ti: 0.005-0.050%,
N: 0.0015-0.0065%,
O: 0.0010 to 0.0050% and
Ca: 0.0005 to 0.0060%
In the range where the ACR defined by the following formula (1) exceeds 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 remaining Fe and inevitable impurity component composition Steel sheet having
ACR = {[Ca] − (0.18 + 130 × [Ca]) × [O]} ÷ (1.25 × [S]) (1)
Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5) (2)
In the formulas (1) and (2), [] is the content (% by mass) of the element in the parentheses. However, no element is contained.
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 mass%,
Cu: 0.05-0.60%
Cr: 0.05 to 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% and
Mg: 0.0002 to 0.0060%
The steel plate according to claim 1, comprising one or more of them.
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