JP4924047B2 - Manufacturing method of steel material having excellent fatigue crack propagation characteristics with absolute value of surface residual stress of 150 N / mm 2 or less - Google Patents

Manufacturing method of steel material having excellent fatigue crack propagation characteristics with absolute value of surface residual stress of 150 N / mm 2 or less Download PDF

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JP4924047B2
JP4924047B2 JP2007006654A JP2007006654A JP4924047B2 JP 4924047 B2 JP4924047 B2 JP 4924047B2 JP 2007006654 A JP2007006654 A JP 2007006654A JP 2007006654 A JP2007006654 A JP 2007006654A JP 4924047 B2 JP4924047 B2 JP 4924047B2
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JP2008174766A (en
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圭治 植田
章夫 大森
茂 遠藤
伸夫 鹿内
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JFE Steel Corp
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本発明は,耐疲労亀裂伝播特性に優れた鋼材に関し、特に残留応力が小さく、船体,海洋構造物,橋梁,建築構造物,建設機械,産業機械等の素材に供する厚鋼板に好適なものに関する。   TECHNICAL FIELD The present invention relates to a steel material having excellent fatigue crack propagation resistance, and particularly to a steel plate having a small residual stress and suitable for a thick steel plate used for materials such as a hull, an offshore structure, a bridge, a building structure, a construction machine, and an industrial machine. .

近年,鋼構造物の大型化に伴い,加速冷却型の高強度鋼板の適用が拡大している.一般に,鋼構造物へ鋼板を適用する際には,条切り,あるいは型切りを実施した後,溶接施工により所望の構造に組み立てられる。この,条切り,型切りを加速冷却型の高強度鋼板に適用した際には,反り等の変形が生じることが問題となっている.これは,加速冷却過程に発生する残留応力に起因すると考えられる.
従来,残留応力を抑制するための手法として,加速冷却時の水量調節や冷間レベラ−矯正などが実施されているが,完全に残留応力を解消するには至っていない.
一方,溶接構造物が,使用環境下で繰返し応力を受ける場合には,溶接止端部などの大きな形状不連続部に応力が集中し,疲労亀裂の発生・進展を生じ,最終的には貫通・破断に至り大事故を引き起こす場合がある。
In recent years, with the increase in size of steel structures, the application of accelerated cooling type high-strength steel sheets is expanding. Generally, when a steel plate is applied to a steel structure, it is assembled into a desired structure by welding after cutting or die cutting. When these slitting and die cutting are applied to high-speed steel sheets of accelerated cooling type, deformation such as warpage is a problem. This is thought to be due to the residual stress generated during the accelerated cooling process.
Conventionally, methods for suppressing residual stress include water volume adjustment and cold leveler correction during accelerated cooling, but they have not completely eliminated residual stress.
On the other hand, when a welded structure is subjected to repeated stresses in the operating environment, the stress concentrates on large discontinuities such as weld toes, causing fatigue cracks to develop and eventually penetrate.・ It may break and cause a major accident.

溶接構造物の寿命は疲労の進行により決定される場合が多く,ライフサイクルコスト低減の観点から,疲労破壊の抑制が要望されている。また,船体,海洋構造物,橋梁等の溶接構造物の破壊は,人命が危険に晒されことから,安全上の観点からも疲労亀裂の発生・進展を抑制することが求められている。   In many cases, the life of a welded structure is determined by the progress of fatigue, and from the viewpoint of reducing the life cycle cost, suppression of fatigue fracture is desired. In addition, the destruction of welded structures such as hulls, offshore structures, bridges, etc., exposes human lives to danger, and from the viewpoint of safety, it is required to suppress the occurrence and progression of fatigue cracks.

従来から,疲労亀裂の発生を抑制するための手段として,溶接止端部の形状を不連続なく,滑らかな形状とし,応力集中を避けるための溶接施工法の工夫がなされているが、溶接施工に多大な時間を要し,施工能率の低下や製造コスト上昇が問題となる。   Conventionally, as a means to suppress the occurrence of fatigue cracks, the welding toe has a smooth shape with no discontinuities, and a welding method has been devised to avoid stress concentration. Therefore, it takes a lot of time, which causes problems such as reduced construction efficiency and increased manufacturing costs.

また,構造物の設計が複雑な場合には,施工の工夫だけでは応力集中が避けがたく,必ずしも,疲労亀裂の発生を抑制するために有効な対策が講じられないのが現状である。   In addition, when the design of a structure is complicated, stress concentration is unavoidable only by construction measures, and it is not always possible to take effective measures to suppress the occurrence of fatigue cracks.

このような問題に対し,鋼材に疲労亀裂が発生した場合,疲労亀裂の伝播速度を遅くすることにより,疲労亀裂の進展を抑制することが有効である。疲労亀裂伝播速度が遅ければ,疲労亀裂が発生しても,構造物の破壊を生じる前に,定期点検等で亀裂を発見し補修することができる。   For such problems, when fatigue cracks occur in steel, it is effective to suppress the growth of fatigue cracks by slowing the propagation speed of fatigue cracks. If the fatigue crack propagation speed is slow, even if a fatigue crack occurs, the crack can be found and repaired by periodic inspections before the structure breaks down.

また,鋼材の疲労亀裂伝播速度を遅くすることができれば,定期点検の頻度,すなわち補修の頻度を低減することができ,鋼材のライフサイクルコスト的にも有利になる.
このような要望に対して、特許文献1〜5には,疲労亀裂伝播速度を遅くするための鋼板と製造方法が提案されている。
If the fatigue crack propagation rate of steel can be slowed, the frequency of periodic inspections, that is, the frequency of repairs can be reduced, which is advantageous in terms of the life cycle cost of the steel.
In response to such demands, Patent Documents 1 to 5 propose steel sheets and manufacturing methods for reducing the fatigue crack propagation rate.

特許文献1および特許文献2には,フェライト母相中にベイナイト,マルテンサイト等の硬質第2相を分散させる方法が記載されている。   Patent Document 1 and Patent Document 2 describe a method of dispersing a hard second phase such as bainite and martensite in a ferrite matrix.

特許文献3には,フェライトの結晶方位を制御することによって,板厚方向の亀裂伝播速度を低減する方法が記載されている。   Patent Document 3 describes a method for reducing the crack propagation speed in the plate thickness direction by controlling the crystal orientation of ferrite.

特許文献4には,フェライト粒径を1〜3μmに微細化することによって疲労特性を向上させる方法が記載されている。   Patent Document 4 describes a method of improving fatigue characteristics by reducing the ferrite grain size to 1 to 3 μm.

結晶粒を微細化することによって同時に靭性も向上することができるが,通常の熱間圧延温度よりも低温域となるオーステナイト/フェライト2相域において50%以上の大きな累積圧下率の圧延を行う必要がある。   Although the toughness can be improved at the same time by refining the crystal grains, it is necessary to perform rolling with a large cumulative reduction ratio of 50% or more in the austenite / ferrite two-phase region, which is lower than the normal hot rolling temperature. There is.

特許文献5には,SiまたはAlの含有量を高めることによって鋼中に残留オーステナイトを含有させて疲労亀裂伝播特性を向上させる技術が記載されている。
特開平10−60575号公報 特開平11−310846号公報 特開平8−199286号公報 特開2002−363644号公報 特開2004−76156号公報
Patent Document 5 describes a technique for improving fatigue crack propagation characteristics by increasing the content of Si or Al so that retained austenite is contained in the steel.
Japanese Patent Laid-Open No. 10-60575 JP-A-11-310846 JP-A-8-199286 JP 2002-363644 A JP 2004-76156 A

しかしながら,特許文献1および特許文献2に記載された技術は,残留応力が高いために,条切りおよび型切り後の変形が問題となる。さらに,疲労亀裂の伝播を十分に抑制できない場合があり,また,靭性の劣化を招くことが懸念される。   However, since the techniques described in Patent Document 1 and Patent Document 2 have high residual stress, deformation after line cutting and die cutting becomes a problem. In addition, the propagation of fatigue cracks may not be sufficiently suppressed, and there is a concern that toughness may be deteriorated.

特許文献3に記載された技術は,残留応力が高いために,条切りおよび型切り後の変形が問題となるとともに,板厚方向以外に進展する疲労亀裂伝播特性を向上することができない懸念がある。   Since the technique described in Patent Document 3 has a high residual stress, there is a concern that deformation after line cutting and die cutting becomes a problem and fatigue crack propagation characteristics that propagate in directions other than the sheet thickness direction cannot be improved. is there.

特許文献4に記載された技術は、圧延機の負荷が大きくなることや,圧延機の占有時間が長くなり,圧延能率が低下することが懸念される。特許文献5に記載された技術は,SiやAlの含有量を高めるので,母材および溶接熱影響部の靭性が劣化することが懸念される。   The technique described in Patent Document 4 has a concern that the load on the rolling mill is increased, the occupation time of the rolling mill is increased, and the rolling efficiency is reduced. Since the technique described in Patent Document 5 increases the content of Si and Al, there is a concern that the toughness of the base material and the weld heat affected zone deteriorates.

そこで、本発明は上述した従来技術の問題を解決する、残留応力が小さく、耐疲労亀裂伝播特性に優れた鋼板およびその製造方法を提供することを目的とする。   Accordingly, an object of the present invention is to provide a steel sheet having a small residual stress and excellent fatigue crack propagation characteristics and a method for manufacturing the same, which solve the above-described problems of the prior art.

本発明者らは,上記した課題を達成するために,残留応力、耐疲労亀裂伝播特性および機械的特性に及ぼす各種要因について鋭意研究し、以下の知見を得た。
(1)耐疲労亀裂伝播特性の向上には、鋼板の構成組織を,軟質相として硬さの上限を規定したフェライト相と、硬質相として硬さの下限を規定した焼もどしマルテンサイト相とし、さらに混合組織の面積分率を制御することが重要である。
(2)この混合組織制御による耐疲労亀裂伝播特性を最大限に発揮するためには,厳格な成分調整が必須であり,フェライト相の硬さを上昇させることなく,オーステナイト域からの焼入れ時にはマルテンサイト生成を促進するCrを添加することが肝要である。
(3)更には,焼もどし軟化抵抗の高いMoあるいはVのうち少なくとも1種の添加と組み合わせると,なお良い結果が得られる。
(4)また,上記のように成分調整した鋼素材に熱間圧延を施した後,加熱温度および冷却速度を適正化した二相域再加熱処理と,加熱温度および保持時間を適正化した焼もどし処理を実施することにより,上記のミクロ組織要件を達成し,優れた疲労亀裂伝播特性と機械的特性を得、更に残留応力の低減を兼備することができる。
In order to achieve the above-mentioned problems, the present inventors diligently studied various factors affecting residual stress, fatigue crack propagation resistance and mechanical characteristics, and obtained the following knowledge.
(1) In order to improve fatigue crack propagation resistance, the steel sheet is composed of a ferrite phase that defines the upper limit of hardness as a soft phase and a tempered martensite phase that defines the lower limit of hardness as a hard phase. Furthermore, it is important to control the area fraction of the mixed tissue.
(2) In order to maximize the fatigue crack propagation characteristics by this mixed structure control, strict component adjustment is indispensable, and it does not increase the hardness of the ferrite phase. It is important to add Cr that promotes site formation.
(3) Furthermore, when it is combined with the addition of at least one of Mo or V having a high tempering and softening resistance, even better results can be obtained.
(4) In addition, after hot rolling the steel material whose components have been adjusted as described above, a two-phase reheating process in which the heating temperature and the cooling rate are optimized, and the heating temperature and the holding time are optimized. By performing the reversion treatment, the above-mentioned microstructure requirement can be achieved, excellent fatigue crack propagation characteristics and mechanical characteristics can be obtained, and further, the residual stress can be reduced.

本発明は、上記知見に基づき、さらに検討を加えて完成されたもので、すなわち、本発明は、
1.鋼組成が,質量%で,
C:0.05〜0.30%,
Si:0.03〜0.35%,
Cr:0.05〜2.0%,
P:0.03%以下
S:0.003%以下
Al:0.1%以下
を含有し,残部がFeおよび不可避的不純物からなる鋳片または鋼片を,熱間圧延後,再加熱して焼入れ処理後、Ac変態点+10℃〜790℃の2相域温度範囲に再加熱し、平均冷却速度5〜60℃/sで焼入れ後,400〜650℃で10分以上保持して焼もどしすることを特徴とする、金属組織がビッカース硬さで85以上130以下のフェライト相と,面積分率が15〜85%のビッカース硬さで340以上440以下の焼もどしマルテンサイト相の混合組織である、表面残留応力の絶対値が150N/mm 以下の耐疲労亀裂伝播特性に優れた鋼材の製造方法
2.鋼組成が,質量%でさらに,
Mo:0.05〜1.0%,
V:0.01〜0.3%
の1種または2種を含有する1に記載した、金属組織がビッカース硬さで85以上130以下のフェライト相と,面積分率が15〜85%のビッカース硬さで340以上440以下の焼もどしマルテンサイト相の混合組織である、表面残留応力の絶対値が150N/mm 以下の耐疲労亀裂伝播特性に優れた鋼材の製造方法
3.鋼組成が,質量%でさらに,
Mn:1.2%以下
Cu:0.8%以下
Ni:1.0%以下
Nb:0.1%以下
Ti:0.03%以下
B:0.005%以下
Ca:0.005%以下
REM:0.02%以下
Mg:0.005%以下
の1種または2種以上を含有する1または2に記載した、金属組織がビッカース硬さで85以上130以下のフェライト相と,面積分率が15〜85%のビッカース硬さで340以上440以下の焼もどしマルテンサイト相の混合組織である、表面残留応力の絶対値が150N/mm 以下の耐疲労亀裂伝播特性に優れた鋼材の製造方法
The present invention has been completed based on the above findings and further studies, that is, the present invention,
1. Steel composition is mass%,
C: 0.05-0.30%,
Si: 0.03 to 0.35%,
Cr: 0.05 to 2.0%,
P: 0.03% or less S: 0.003% or less Al: A slab or steel slab containing 0.1% or less, the balance being Fe and inevitable impurities, after hot rolling, and reheating After quenching treatment, reheat to 2-phase region temperature range of Ac 1 transformation point + 10 ° C-790 ° C, quench at average cooling rate 5-60 ° C / s, hold at 400-650 ° C for 10 minutes or more and temper A mixed structure of a ferrite structure having a Vickers hardness of 85 to 130 and a tempered martensite phase having a Vickers hardness of 15 to 85% and a 340 to 440 or less Vickers hardness. A method for producing a steel material having excellent fatigue crack propagation characteristics with an absolute value of surface residual stress of 150 N / mm 2 or less .
2. When the steel composition is mass%,
Mo: 0.05-1.0%,
V: 0.01 to 0.3%
Tempering of 340 to 440 or less with a Vickers hardness of 15 to 85% Vickers hardness and a ferrite structure having a Vickers hardness of 85 to 130 and a metal structure described in 1 containing 1 type or 2 types of A method for producing a steel material that is a mixed structure of martensite phase and has excellent fatigue crack propagation resistance with an absolute value of surface residual stress of 150 N / mm 2 or less .
3. When the steel composition is mass%,
Mn: 1.2% or less Cu: 0.8% or less Ni: 1.0% or less Nb: 0.1% or less Ti: 0.03% or less B: 0.005% or less Ca: 0.005% or less REM : 0.02% or less Mg: 0.005% or less of 1 or 2 containing one or more , the metal structure is a Vickers hardness of 85 to 130 ferrite phase, and the area fraction A method for producing a steel material excellent in fatigue crack propagation resistance having an absolute value of surface residual stress of 150 N / mm 2 or less, which is a mixed structure of tempered martensite phase of 340 to 440 with a Vickers hardness of 15 to 85% .

本発明によれば,耐疲労亀裂伝播特性に優れた厚鋼板を安定して製造することができ,鋼構造物の信頼性向上,ライフサイクルコストの低減に大きく寄与し,産業上格段の効果を奏する。   According to the present invention, it is possible to stably produce a thick steel plate having excellent fatigue crack propagation resistance, greatly contributing to the improvement of the reliability of the steel structure and the reduction of the life cycle cost, and a remarkable industrial effect. Play.

本発明では金属組織、成分組成及び製造条件を規定する。以下、それらの限定理由について具体的に説明する.
[金属組織]
本発明では,硬質相の硬さ、分散量だけでなく,軟質相の硬さを規定する。優れた耐疲労亀裂伝播特性と機械的特性を安定して達成するため,金属組織における硬質相を面積分率が15〜85%でビッカース硬さHV340以上の焼きもどしマルテンサイト相とするとともに、さらに,軟質相のビッカース硬さをHV130以下に制限したフェライト相とした混合組織とする。
In the present invention, the metal structure, component composition and production conditions are defined. The reasons for these limitations will be explained in detail below.
[Metal structure]
In the present invention, not only the hardness and dispersion amount of the hard phase but also the hardness of the soft phase is specified. In order to stably achieve excellent fatigue crack propagation characteristics and mechanical properties, the hard phase in the metal structure is made into a tempered martensite phase with an area fraction of 15 to 85% and a Vickers hardness of HV340 or more, and The mixed structure is a ferrite phase in which the Vickers hardness of the soft phase is limited to HV130 or less.

フェライト相の硬さの低減は,疲労亀裂先端の歪領域拡大,および繰返し負荷歪時の加工硬化抑制により,疲労亀裂進展速度を低下させる。また,フェライト相を進展した主亀裂が焼もどしマルテンサイト相のごく近傍に到達したとき,主亀裂から微小亀裂を発生させ,主亀裂を屈曲、分岐させて疲労亀裂進展速度を低下させる。   Reducing the hardness of the ferrite phase decreases the fatigue crack growth rate by expanding the strain region at the tip of the fatigue crack and suppressing work hardening during repeated load strain. In addition, when the main crack that has propagated through the ferrite phase is tempered and reaches very close to the martensite phase, a microcrack is generated from the main crack, and the main crack is bent and branched to reduce the fatigue crack growth rate.

この効果を得るため,フェライト相のビッカース硬さHV85以上130以下の範囲に限定する。なお,好ましくは,HV95以上120以下とする。   In order to obtain this effect, the Vickers hardness of the ferrite phase is limited to the range of HV85 to 130. In addition, Preferably, it is set as HV95 or more and 120 or less.

また,焼もどしマルテンサイト相のビッカース硬さHVは340以上440以下に限定する。なお,好ましくは,HV350以上420以下とする。焼入れままのマルテンサイト相の場合は,母材の延性および靭性が劣化するため,焼もどし処理を行って焼き戻しマルテンサイト相とする。   Further, the Vickers hardness HV of the tempered martensite phase is limited to 340 or more and 440 or less. In addition, Preferably, it is set as HV350-420. In the case of an as-quenched martensite phase, the ductility and toughness of the base metal deteriorate, so tempering is performed to obtain a tempered martensite phase.

焼もどしマルテンサイト相の面積分率が15%未満、もしくは85%より多い場合には,上記のような,疲労亀裂伝播の遅延効果が得られないため,焼もどしマルテンサイト相の面積分率は15〜85%の範囲に限定する。なお,好ましくは,20〜80%である。   If the area fraction of the tempered martensite phase is less than 15% or more than 85%, the effect of delaying fatigue crack propagation as described above cannot be obtained, so the area fraction of the tempered martensite phase is It is limited to a range of 15 to 85%. In addition, Preferably, it is 20 to 80%.

なお,硬さは,硬さ試験片のフェライト相および焼もどしマルテンサイト相を,微小ビッカース硬さ計を用いて,荷重:0.098N(10gf)〜0.98N(100gf)の範囲、好ましくは0.49N(50gf)で得られた値で規定する。本条件の場合、試験条件による誤差を無視することが可能である。   In addition, the hardness is a range of 0.098 N (10 gf) to 0.98 N (100 gf), preferably a ferrite phase and a tempered martensite phase of a hardness test piece using a micro Vickers hardness meter, preferably It is defined by the value obtained at 0.49 N (50 gf). In the case of this condition, it is possible to ignore errors due to test conditions.

フェライト相,焼もどしマルテンサイト相それぞれについて,少なくとも10個の粒について硬さ測定を行い,その平均値を各相の硬さとする。   For each of the ferrite phase and the tempered martensite phase, the hardness of at least 10 grains is measured, and the average value is taken as the hardness of each phase.

なお,本発明鋼は、耐疲労亀裂伝播特性を劣化させない範囲で、フェライト相と焼もどしマルテンサイト相以外の第3相として,ベイナイトおよびパーライト等の組織を少量混在することは許容する。   In the steel of the present invention, it is allowed to mix a small amount of a structure such as bainite and pearlite as the third phase other than the ferrite phase and the tempered martensite phase within a range not deteriorating the fatigue crack propagation characteristics.

第3相の面積分率は少ない方が良く、ベイナイトおよびパーライト等の組織は面積分率で5%以下とすることが好ましい。   The area fraction of the third phase is preferably small, and the structure of bainite and pearlite is preferably 5% or less in terms of area fraction.

次に,本発明鋼の成分組成の限定理由について具体的に説明する。
[成分組成]
成分に関する「%」表示は特に断らない限り質量%を意味するものとする.
C:0.05〜0.30%
Cは,鋼の強度を増加させ,構造用鋼材として必要な強度を確保するのに有用な元素である。また,上記したビッカース硬さが340以上の焼もどしマルテンサイト相の第2相組織を得るためには,0.05%以上の含有を必要とする。
Next, the reasons for limiting the component composition of the steel of the present invention will be specifically described.
[Ingredient composition]
Unless otherwise specified, the “%” label for ingredients means mass%.
C: 0.05-0.30%
C is an element useful for increasing the strength of steel and ensuring the strength necessary for structural steel. Further, in order to obtain the second phase structure of the tempered martensite phase having a Vickers hardness of 340 or more, the content of 0.05% or more is required.

一方,0.30%を超える含有は,HAZ靭性,耐溶接割れ性を劣化させるとともに,母材の靭性を劣化させる。このため,Cは0.05〜0.30%の範囲に限定する。なお,好ましくは,0.08〜0.25%である。   On the other hand, if the content exceeds 0.30%, the HAZ toughness and weld crack resistance deteriorate, and the toughness of the base material deteriorates. For this reason, C is limited to a range of 0.05 to 0.30%. In addition, Preferably, it is 0.08 to 0.25%.

Si:0.03〜0.35%
Siは,脱酸材として作用し,また,セメンタイトの生成を抑制することにより,オーステナイト中へCを濃縮し,焼入れ時のマルテンサイト生成を促進する。さらに,焼もどし時の,マルテンサイト相の焼もどし軟化抵抗を高めるため,0.03%以上とする。
Si: 0.03-0.35%
Si acts as a deoxidizer and suppresses the formation of cementite, thereby concentrating C into austenite and promoting martensite formation during quenching. Furthermore, in order to increase the temper softening resistance of the martensite phase during tempering, the content should be 0.03% or more.

一方,0.35%を超えて含有すると,フェライト相の硬さを上昇させて,耐疲労亀裂伝播特性を低下させる。また,母材の靭性が劣化するとともに,溶接性,HAZ靭性が顕著に劣化する。このため,Siは0.03〜0.35%の範囲に限定する。なお,好ましくは,0.05〜0.25%である。   On the other hand, if the content exceeds 0.35%, the hardness of the ferrite phase is increased, and the fatigue crack propagation resistance is decreased. In addition, the toughness of the base material deteriorates, and the weldability and HAZ toughness deteriorate significantly. For this reason, Si is limited to the range of 0.03 to 0.35%. In addition, Preferably, it is 0.05 to 0.25%.

Cr:0.05〜2.0%
Crは本発明において重要な合金元素であり,多量に添加してもAr変態点に対する影響が小さく,またα-Feと同じ体心立方構造で原子半径がFeに近いため固溶強化能が極めて小さく、フェライト相の硬さを上昇させない。一方,オーステナイト域からの焼入れ時には,オーステナイトの焼入れ性を増大させ、第2相組織としてマルテンサイト相の生成を促進する。
Cr: 0.05-2.0%
Cr is an important alloying element in the present invention. Even if it is added in a large amount, the effect on the Ar 3 transformation point is small, and the same body-centered cubic structure as α-Fe has an atomic radius close to that of Fe. Very small and does not increase the hardness of the ferrite phase. On the other hand, at the time of quenching from the austenite region, the hardenability of austenite is increased and the formation of a martensite phase as a second phase structure is promoted.

さらに,焼もどし時には,マルテンサイト相の焼もどし軟化抵抗を高める作用があり,疲労亀裂伝播速度の低減に有効である。本発明では,この効果を得るために,0.1%以上の含有を必要とする。   Furthermore, during tempering, it has the effect of increasing the temper softening resistance of the martensite phase and is effective in reducing the fatigue crack propagation rate. In this invention, in order to acquire this effect, 0.1% or more of content is required.

一方,2.0%を超えて含有すると,耐溶接割れ性およびHAZ靭性が著しく劣化する。このため,Crは0.05〜2.0%の範囲に限定する。なお,好ましくは,0.1〜1.5%である。   On the other hand, if the content exceeds 2.0%, the weld crack resistance and the HAZ toughness deteriorate significantly. For this reason, Cr is limited to a range of 0.05 to 2.0%. In addition, Preferably, it is 0.1 to 1.5%.

P:0.03%以下
Pは,鋼の強度を増加させ靭性を劣化させる元素であり,とくに溶接部の靭性を劣化させるので,できるだけ低減することが望ましい。Pが0.03%を超えて含有されると,この傾向が顕著となるため,上限とする。なお,過度のP低減は精錬コストを高騰させ経済的に不利となるため,0.003%以上とすることが望ましい。
P: 0.03% or less P is an element that increases the strength of steel and deteriorates toughness, and particularly deteriorates the toughness of welds. Therefore, it is desirable to reduce it as much as possible. If P is contained in excess of 0.03%, this tendency becomes remarkable, so the upper limit is set. In addition, excessive P reduction raises the refining cost and is economically disadvantageous, so 0.003% or more is desirable.

S:0.0050%以下
Sは母材および溶接部の靭性を劣化させる元素であり,できるだけ低減することが望ましい。Sが0.0050%を超えて含有されると,この傾向が顕著となるため,上限とする。
S: 0.0050% or less S is an element that deteriorates the toughness of the base metal and the welded portion, and is desirably reduced as much as possible. If S is contained in excess of 0.0050%, this tendency becomes remarkable, so the upper limit is set.

Al:0.1%以下
Alは,脱酸剤として作用し,高張力鋼の溶鋼脱酸プロセスに於いて,もっとも汎用的に使われる。また,鋼中のNをAlNとして固定し,母材の靭性向上に寄与する.一方,0.1%を超える含有は,母材の靭性が低下するとともに,溶接時に溶接金属部に混入して,靭性を劣化させる.このため,Alは0.1%以下に限定する。なお、好ましくは0.01〜0.07%である。
Al: 0.1% or less Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process of high-strength steel. Also, N in the steel is fixed as AlN, which contributes to improvement of the toughness of the base metal. On the other hand, if the content exceeds 0.1%, the toughness of the base metal decreases, and it is mixed into the weld metal during welding to deteriorate the toughness. For this reason, Al is limited to 0.1% or less. In addition, Preferably it is 0.01 to 0.07%.

本発明では,上記した基本成分系に加えて,所望する特性に応じ,Mo:0.05〜1.0%,V:0.01〜0.3%、Mn:1.2%以下、Cu:0.8%以下,Ni:1.0%以下, Nb:0.1%以下,Ti:0.03%以下,B:0.005%以下,Ca:0.005%以下,REM:0.02%以下およびMg:0.005%以下のうちから選ばれた1種または2種以上を含有することができる。   In the present invention, in addition to the basic component system described above, Mo: 0.05 to 1.0%, V: 0.01 to 0.3%, Mn: 1.2% or less, Cu, depending on the desired characteristics : 0.8% or less, Ni: 1.0% or less, Nb: 0.1% or less, Ti: 0.03% or less, B: 0.005% or less, Ca: 0.005% or less, REM: 0 One or two or more selected from 0.02% or less and Mg: 0.005% or less can be contained.

Mo:0.05〜1.0%
Moは焼入れ時に,オーステナイトの焼入れ性を増大させ、第2相組織としてマルテンサイト相の生成を促進するとともに、焼きもどし時には炭化物を生成することにより、マルテンサイト相の焼きもどし軟化を顕著に抑制し、疲労亀裂伝播速度の低減に有効である。この効果を発揮するためには0.05%以上の添加が必要である。一方、1.0%を超えて添加すると、靭性に悪影響を及ぼす。このため、Moを添加する場合は、0.05〜1.0%の範囲に限定する。
Mo: 0.05-1.0%
Mo enhances the hardenability of austenite during quenching, promotes the formation of martensite phase as a second phase structure, and significantly reduces temper softening of the martensite phase by generating carbides during tempering. It is effective in reducing the fatigue crack propagation rate. In order to exhibit this effect, addition of 0.05% or more is necessary. On the other hand, if added over 1.0%, the toughness is adversely affected. For this reason, when adding Mo, it limits to 0.05 to 1.0% of range.

V:0.01〜0.3%
Vは焼入れ時に,オーステナイトの焼入れ性を増大させ、第2相組織としてマルテンサイト相の生成を促進するとともに、焼きもどし時には炭化物を生成することにより、マルテンサイト相の焼きもどし軟化を顕著に抑制し、疲労亀裂伝播速度の低減に有効である。
V: 0.01 to 0.3%
V increases the hardenability of austenite during quenching, promotes the formation of martensite phase as a second phase structure, and significantly reduces temper softening of the martensite phase by generating carbides during tempering. It is effective in reducing the fatigue crack propagation rate.

この効果を得るためには0.01%以上の添加が必要である。一方、0.3%を超えて添加すると、靭性に悪影響を及ぼす。このため、Vを添加する場合は、0.01〜0.3%の範囲に限定する。   In order to obtain this effect, addition of 0.01% or more is necessary. On the other hand, if added over 0.3%, the toughness is adversely affected. For this reason, when adding V, it limits to 0.01 to 0.3% of range.

Mn:1.2%以下
Mnは,鋼の強度を増加させる効果を有している。一方,1.2%を超えて含有すると,フェライト相の硬さが上昇し、疲労亀裂伝播の遅延効果が劣化する.このため,Mnを添加する場合は、1.2%以下に限定する。
Mn: 1.2% or less Mn has the effect of increasing the strength of steel. On the other hand, if the content exceeds 1.2%, the ferrite phase hardness increases and the fatigue crack propagation delay effect deteriorates. For this reason, when adding Mn, it is limited to 1.2% or less.

Cu:0.8%以下
Cuは,高靭性を保ちつつ強度を増加させることが可能な元素であり,HAZ靭性への影響も小さく,高強度化のために有用で,必要に応じ選択して含有できる。
Cu: 0.8% or less Cu is an element that can increase strength while maintaining high toughness, has little influence on HAZ toughness, is useful for increasing strength, and can be selected as necessary. Can be contained.

一方、含有量が0.8%を超えると熱間脆性を生じて鋼板の表面性状を劣化させるとともに、フェライト相の硬さが上昇し、疲労亀裂伝播の遅延効果が低下する。このため、Cuを添加する場合は、0.8%以下に限定する。   On the other hand, when the content exceeds 0.8%, hot brittleness is caused to deteriorate the surface properties of the steel sheet, the hardness of the ferrite phase is increased, and the fatigue crack propagation delaying effect is reduced. For this reason, when adding Cu, it limits to 0.8% or less.

Ni:1.0%以下
Niは,高靭性を保ちつつ強度を増加させることが可能な元素であり,HAZ靭性への影響も小さく,高強度化のために有用で,必要に応じ選択して含有できる。しかし、1.0%を超えて含有しても,効果が飽和し,含有量に見合う効果が得られず,経済的に不利になるとともに、フェライト相の硬さが上昇し、疲労亀裂伝播の遅延効果が低下する。このため,Niを添加する場合は1.0%以下に限定する。
Ni: 1.0% or less Ni is an element that can increase strength while maintaining high toughness, has little effect on HAZ toughness, is useful for increasing strength, and can be selected as necessary. Can be contained. However, even if the content exceeds 1.0%, the effect is saturated, an effect commensurate with the content cannot be obtained, it becomes economically disadvantageous, the hardness of the ferrite phase increases, and fatigue crack propagation Delay effect is reduced. For this reason, when adding Ni, it limits to 1.0% or less.

Nb:0.1%以下
Nbは,強度向上に寄与する元素であるが,0.1%を超える含有は,母材靭性およびHAZ靭性を劣化させる。このため,Nbを添加する場合は0.1%以下に限定する。
Nb: 0.1% or less Nb is an element that contributes to strength improvement, but inclusion exceeding 0.1% degrades the base metal toughness and the HAZ toughness. For this reason, when adding Nb, it limits to 0.1% or less.

Ti:0.03%以下
Tiは,強度向上に寄与し,また,Nとの親和力が強く凝固時にTiNとして析出し,HAZでのオーステナイト粒の粗大化抑制してHAZの高靭化に寄与する。一方,0.03%を超えて含有すると,母材靭性を劣化させる。このため,Tiを添加する場合は、0.03%以下に限定することが望ましい。
Ti: 0.03% or less Ti contributes to strength improvement, and also has a strong affinity with N, precipitates as TiN during solidification, and suppresses coarsening of austenite grains in HAZ, thereby contributing to toughening of HAZ. . On the other hand, if it exceeds 0.03%, the toughness of the base metal is deteriorated. For this reason, when adding Ti, it is desirable to limit to 0.03% or less.

B:0.0050%以下
Bは,焼入れ性の向上を介して,鋼の強度を増加させる作用を有する.一方,0.0050%を超える含有は焼入れ性を著しく増加させ,母材の靭性,延性の劣化をもたらす.このため,Bを添加する場合は、0.0050%以下に限定する。
B: 0.0050% or less B has an effect of increasing the strength of steel through improving hardenability. On the other hand, if the content exceeds 0.0050%, the hardenability is remarkably increased and the toughness and ductility of the base metal are deteriorated. For this reason, when adding B, it limits to 0.0050% or less.

Ca:0.005%以下
Caは,結晶粒の微細化を介して靭性を向上させる有用な元素であるが、0.005%を超えて含有しても効果が飽和するため,添加する場合は0.005%を上限とする。
Ca: 0.005% or less Ca is a useful element that improves toughness through refinement of crystal grains, but if added over 0.005%, the effect is saturated. The upper limit is 0.005%.

REM:0,02%以下
REMは,靭性向上に寄与する元素であるが、0.02%を超えて含有しても効果が飽和するため,添加する場合は、0.02%を上限とする。
REM: 0.02% or less REM is an element that contributes to the improvement of toughness, but the effect is saturated even if contained over 0.02%, so when added, the upper limit is 0.02% .

Mg
Mgは,結晶粒の微細化を介して靭性を向上させる有用な元素であるが、0.005%を超えて含有しても効果が飽和するため,添加する場合は、0.005%を上限とする。
Mg
Mg is a useful element that improves toughness through refinement of crystal grains, but the effect is saturated even if contained over 0.005%, so when added, the upper limit is 0.005% And

なお,上記した成分以外の残部は,Feおよび不可避的不純物である。次に,本発明の製造条件について説明する。
[製造条件]
説明において、温度に関する「℃」表示は特に断らない限り板厚1/2t部の温度を意味するものとする。
再加熱温度
本発明に係る鋼は,上記組成の溶鋼を,転炉,電気炉,真空溶解炉等,通常公知の方法で溶製し,得られた鋼素材を好ましくは、1000℃〜1300℃に再加熱する。
The balance other than the components described above is Fe and inevitable impurities. Next, the manufacturing conditions of the present invention will be described.
[Production conditions]
In the description, “° C.” related to the temperature means a temperature of 1/2 t part thickness unless otherwise specified.
Reheating temperature The steel according to the present invention is obtained by melting a molten steel having the above composition by a generally known method such as a converter, an electric furnace, a vacuum melting furnace or the like, and the obtained steel material is preferably 1000 ° C to 1300 ° C. Reheat to.

再加熱温度が1000℃未満では,熱間圧延での変形抵抗が高くなり,1パス当たりの圧下量が大きく取れなくなることから,圧延パス数が増加し,圧延能率の低下を招くとともに,鋼素材(スラブ)中の鋳造欠陥を圧着することができない場合がある。   If the reheating temperature is less than 1000 ° C, the deformation resistance in hot rolling becomes high and the amount of rolling reduction per pass cannot be made large. Therefore, the number of rolling passes increases and the rolling efficiency decreases, and the steel material The casting defect in (slab) may not be crimped.

一方,再加熱温度が1300℃を超えると,加熱時のスケールによって表面疵が生じやすく,圧延後の手入れ負荷が増大する。このため,鋼素材の再加熱温度は1000〜1300℃の範囲とするのが好ましい。
熱間圧延条件
再加熱された鋼素材を、所望の板厚および形状が満足できるように熱間圧延する。熱間圧延は、圧延終了温度をAr変態点以上とすることが好ましい。熱間圧延後の冷却は、空冷でも加速冷却でもよい。
On the other hand, when the reheating temperature exceeds 1300 ° C., surface flaws are likely to occur due to the scale during heating, and the maintenance load after rolling increases. For this reason, it is preferable to make the reheating temperature of a steel raw material into the range of 1000-1300 degreeC.
Hot rolling conditions The reheated steel material is hot rolled so that a desired plate thickness and shape can be satisfied. In the hot rolling, it is preferable that the rolling end temperature is not less than the Ar 3 transformation point. The cooling after hot rolling may be air cooling or accelerated cooling.

板厚が80mmを超える極厚鋼板の場合には,ザク圧着のために1パスあたりの圧下率が15%以上となる圧延パスを少なくとも1パス以上確保することが望ましい。圧延終了温度がAr変態点未満では,変形抵抗が高くなりすぎて,圧延荷重が増大し,圧延機への負担が大きくなる。 In the case of an extremely thick steel plate having a plate thickness exceeding 80 mm, it is desirable to secure at least one rolling pass at which the rolling reduction per pass is 15% or more for zaku pressure bonding. When the rolling end temperature is less than the Ar 3 transformation point, the deformation resistance becomes too high, the rolling load increases, and the burden on the rolling mill increases.

なお,Ar点は化学組成との相関が概ね次(1)式で整理できる.
Ar=868−396C+25Si−68Mn−21Cu−36Ni−
25Cr−30Mo (1)
(ただし,C,Si,Mn,Cu,Ni,Cr,Mo:各合金元素の含有量(質量%))
熱処理
本発明で、熱間圧延後の2相域再加熱焼入れ処理および焼もどし処理は,残留応力の低下と耐疲労き裂伝播特性を両立するために重要なプロセスである。
The correlation with the chemical composition of Ar 3 points can be roughly organized by the following equation (1).
Ar 3 = 868-396C + 25Si-68Mn-21Cu-36Ni-
25Cr-30Mo (1)
(However, C, Si, Mn, Cu, Ni, Cr, Mo: content of each alloy element (mass%))
Heat treatment In the present invention, the two-phase region reheating quenching treatment and tempering treatment after hot rolling is an important process in order to achieve both a reduction in residual stress and fatigue crack propagation resistance.

2相域再加熱焼入れ処理は,Ac変態点+10℃〜790℃の2相域温度範囲に再加熱保持後,5〜60℃/sの平均冷却速度で水冷する。 In the two-phase region reheating and quenching treatment, water-cooling is performed at an average cooling rate of 5 to 60 ° C./s after reheating and holding in the two-phase region temperature range of Ac 1 transformation point + 10 ° C. to 790 ° C.

2相域再加熱焼入れ処理は、Ac変態点+10℃未満の場合,オーステナイト相の分率が低いため,焼入れ後の組織中のマルテンサイト分率が少なく耐疲労き裂伝播特性が劣化するため、Ac変態点+10℃以上とする。 In the case of two-phase reheating quenching, if the Ac 1 transformation point is less than + 10 ° C, the fraction of austenite phase is low, so the martensite fraction in the microstructure after quenching is small and the fatigue crack propagation characteristics deteriorate. , Ac 1 transformation point + 10 ° C. or higher.

一方,790℃超えの温度で保持後焼きいれると,焼もどし処理を施しても,十分に残留応力が低下しないため,2相域再加熱は,Ac変態点+10℃〜790℃、好ましくは,Ac変態点+15℃〜785℃の範囲とする。なお,Ac点は化学組成との相関が概ね(2)式で整理できる.
Ac=723−14Mn+22Si−14.4Ni+23.3Cr (2)
(ただし,C,Si,Mn, Ni,Cr:各合金元素の含有量(mass%))
当該温度域での保持時間は,鋼板内の温度均一化を図り,特性のばらつきを抑えるため,5min.以上が好ましい。保持時間が1hr以上になるとオーステナイト粒の粗大化により,母材の靭性が劣化するので,1hr以内が好ましい。
On the other hand, when not tempered after holding at a temperature of greater than 790 ° C., be subjected to a tempering treatment, since sufficient residual stress is not reduced, it reheated the two-phase region, Ac 1 transformation point + 10 ℃ ~790 ℃, preferably , Ac 1 transformation point + 15 ° C to 785 ° C. In addition, the Ac 1 point can be roughly correlated with the chemical composition by the equation (2).
Ac 1 = 723-14Mn + 22Si-14.4Ni + 23.3Cr (2)
(However, C, Si, Mn, Ni, Cr: Content of each alloy element (mass%))
The holding time in this temperature range is 5 min. In order to make the temperature uniform in the steel sheet and to suppress variation in characteristics. The above is preferable. When the holding time is 1 hr or longer, the toughness of the base material deteriorates due to the coarsening of the austenite grains, so that it is preferably within 1 hr.

上述した2相域再加熱焼入れ処理により、Cのオーステナイト地への濃化と,フェライト地の希釈軟化が効率的に促進されることにより,焼入れ後の組織が,硬質のマルテンサイトと軟質のフェライトの混合組織となり,耐疲労き裂伝播特性を向上させる。   The above-described two-phase reheating quenching process effectively promotes the concentration of C to austenite and the dilution softening of the ferrite ground, resulting in a hard martensite and soft ferrite. This improves the fatigue crack propagation characteristics.

2相域再加熱焼入れ処理における加熱後の平均冷却速度は5℃/s以下の場合,硬質相としてパーライトあるいはベイナイト相が生成し、目標のマルテンサイト組織を達成できず,耐疲労き裂伝播特性が劣化する。   When the average cooling rate after heating in a two-phase reheating quenching process is 5 ° C / s or less, a pearlite or bainite phase is formed as a hard phase, and the target martensite structure cannot be achieved, and fatigue crack propagation resistance characteristics Deteriorates.

一方,60℃/s以上になると焼もどし処理を施しても,十分に残留応力が低下しないため,平均冷却速度は5〜60℃/s、好ましくは,8〜55℃/sの範囲に限定する。尚、平均冷却速度とは,板厚方向1/2t部において,保持温度−50℃〜400℃までの冷却速度を意味するものとする。   On the other hand, when the temperature is 60 ° C./s or higher, the residual stress is not sufficiently reduced even if tempering is performed, so the average cooling rate is limited to the range of 5 to 60 ° C./s, preferably 8 to 55 ° C./s. To do. In addition, an average cooling rate shall mean the cooling rate to holding temperature -50 degreeC-400 degreeC in a board thickness direction 1 / 2t part.

本発明では,鋼板を2相域再加熱焼入れ処理後,再加熱焼もどし処理を施す。再加熱温度400〜650℃で10分以上保持してから空冷する焼もどし処理により,耐疲労き裂伝播特性を劣化させずに、残留応力の低減が可能で,母材の靭性および延性も向上する。   In the present invention, the steel sheet is subjected to a reheat tempering treatment after the two-phase region reheating quenching treatment. Residual stress can be reduced without deteriorating fatigue crack propagation characteristics by tempering by holding at reheating temperature of 400-650 ° C for 10 minutes or more and then air cooling, improving toughness and ductility of base metal To do.

焼もどし温度が400℃未満では,2相域焼入れで生じた残留応力が十分に解消されず,また,母材の靭性および延性が劣化する場合がある。   When the tempering temperature is less than 400 ° C., the residual stress generated by the two-phase quenching is not sufficiently eliminated, and the toughness and ductility of the base material may be deteriorated.

一方、650℃を超えると焼もどしマルテンサイト相の硬さが低下し、疲労き裂伝播の遅延効果が低下するため、焼もどし温度は,400℃〜650℃、好ましくは,420℃〜630℃の範囲とする。   On the other hand, when the temperature exceeds 650 ° C., the hardness of the tempered martensite phase decreases and the effect of delaying fatigue crack propagation decreases, so the tempering temperature is 400 ° C. to 650 ° C., preferably 420 ° C. to 630 ° C. The range.

保持時間は,10分未満の場合、残留応力の低減効果が得られないので,10分以上、好ましくは12分以上とする。保持時間の上限は規定しないが、1hr以上になると、焼もどしマルテンサイト相の硬さが低下し、疲労き裂伝播の遅延効果が低下するので、1hr以内とすることが好ましい。   If the holding time is less than 10 minutes, the effect of reducing the residual stress cannot be obtained. The upper limit of the holding time is not specified, but if it is 1 hour or more, the hardness of the tempered martensite phase is lowered and the effect of delaying fatigue crack propagation is lowered.

本発明では熱間圧延と2相域再加熱焼入れ処理の間で,再加熱して、焼きならし、もしくは焼入れ処理を施してもよい。   In the present invention, reheating and normalizing or quenching may be performed between hot rolling and two-phase region reheating and quenching.

厚鋼板の場合、Ac変態点以上の温度域に再加熱して保持することにより,厚鋼板内の組織がより均質化、微細化される。加熱温度の上限については規定していないが,1100℃以上になると鋼板表面性状が劣化するために,好ましくは1100℃以下とする。 In the case of a thick steel plate, the structure in the thick steel plate is further homogenized and refined by being reheated and held in a temperature range above the Ac 3 transformation point. Although the upper limit of the heating temperature is not specified, the surface property of the steel sheet deteriorates when it is 1100 ° C. or higher.

また,保持時間も規定しないが,1hr以上になるとオーステナイト粒の粗大化により,母材の靭性が劣化するので1hr以内が望ましい。   Further, although the holding time is not specified, it is preferably within 1 hr since the toughness of the base material deteriorates due to coarsening of austenite grains when it is 1 hr or more.

転炉−取鍋精錬−連続鋳造法で,表1に示す組成に調製された鋼素材を,表2に示す条件の熱間圧延−再加熱焼入れ−焼もどしにより表2に示す板厚の厚鋼板とした。   Thickness of the plate thickness shown in Table 2 by hot rolling-reheating quenching-tempering of steel materials prepared in the composition shown in Table 1 by the converter-ladle refining-continuous casting method. A steel plate was used.

組織分率の調査は,得られた各鋼板の圧延方向と平行な断面について,ミクロ組織観察用サンプルを採取し,ナイタール腐食の後,光学顕微鏡組織を撮影し,画像解析装置を用いて組織分率を求めた。   For the investigation of the texture fraction, a sample for microstructural observation was taken on the cross section parallel to the rolling direction of each steel plate obtained, and after taking Nital corrosion, the optical microstructure was photographed, and the texture fraction was analyzed using an image analyzer. The rate was determined.

本発明範囲は,引張強さ440N/mm2以上,全伸び22%以上を有する鋼材で靭性は,JISZ2242(2006)に準拠した2mmVノッチ試験片を用いたシャルピー衝撃試験により,−20℃での吸収エネルギーが100J以上有する鋼材とする。   The scope of the present invention is a steel material having a tensile strength of 440 N / mm 2 or more and a total elongation of 22% or more, and the toughness is absorbed at −20 ° C. by a Charpy impact test using a 2 mm V notch test piece according to JISZ2242 (2006). The steel material has energy of 100 J or more.

得られた各鋼板の板厚1/2位置からJIS4号引張試験片,あるいはJIS5号全厚引張試験片を採取し,JISZ2241(2006)の既定に準拠して引張試験を実施し,引張特性を調査した。   JIS No. 4 tensile test piece or JIS No. 5 full-thickness tensile test piece is taken from 1/2 position of the obtained steel plate thickness, and tensile test is performed according to the standard of JISZ2241 (2006). investigated.

また,得られた各鋼板の板厚1/2位置から,JISZ2202(2006)の規定に準拠してVノッチ試験片を採取し,JISZ2242(2006)の規定に準拠してシャルピー衝撃試験を実施し,−20℃における吸収エネルギー(vE−20)を求め,母材靭性を調査した。 In addition, V-notch test specimens were collected from the obtained steel plate thickness 1/2 position in accordance with JISZ2202 (2006), and Charpy impact test was conducted in accordance with JISZ2242 (2006). , The absorbed energy (vE -20 ) at -20 ° C was obtained, and the base metal toughness was investigated.

疲労亀裂伝播特性の調査は,各鋼板から,荷重負荷方向が圧延方向と平行になるようASTME647に準拠したCT試験片を採取し,クラックゲージ法で疲労亀裂伝播試験を実施し,伝播速度を求めた。   To investigate the fatigue crack propagation characteristics, CT specimens based on ASTM E647 are collected from each steel plate so that the load direction is parallel to the rolling direction, and the fatigue crack propagation test is performed by the crack gauge method to determine the propagation speed. It was.

本発明における優れた疲労亀裂伝播特性の鋼材とは,応力拡大係数ΔK:15MPa/√mにおける疲労亀裂伝播速度(da/dN)が8×10−9(m/cycle)以下を有する鋼材とする。 The steel material having excellent fatigue crack propagation characteristics in the present invention is a steel material having a fatigue crack propagation rate (da / dN) of 8 × 10 −9 (m / cycle) or less at a stress intensity factor ΔK: 15 MPa / √m. .

残留応力の調査は,各鋼板の表面部についてX線応力測定法を用い求めた。本発明における残留応力の小さい鋼板とは,表面残留応力の絶対値が150N/mm以下を有する鋼材とする。 The residual stress was investigated using the X-ray stress measurement method for the surface portion of each steel plate. The steel sheet having a small residual stress in the present invention is a steel material having an absolute value of surface residual stress of 150 N / mm 2 or less.

得られた結果を表3に示す。本発明例は,いずれも,応力拡大係数ΔK:15MPa/√mにおける疲労き裂伝播速度(da/dN)が8×10−9(m/cycle)以下と極めて遅く,優れた耐疲労き裂伝播特性を有する。 The obtained results are shown in Table 3. In all of the examples of the present invention, the fatigue crack propagation rate (da / dN) at a stress intensity factor ΔK: 15 MPa / √m is extremely slow as 8 × 10 −9 (m / cycle) or less, and excellent fatigue crack resistance. Has propagation characteristics.

また,鋼板表面の残留応力の絶対値が150N/mm以下と極めて残留応力が小さい。さらに,引張強さ440N/mm以上,全伸び22%以上,および−20℃での吸収エネルギーvE−20>100Jの高強度,高延性,高靭性の母材特性を有することが確認された。 In addition, the absolute value of the residual stress on the steel sheet surface is 150 N / mm 2 or less, and the residual stress is extremely small. Furthermore, it was confirmed that it has a base material characteristic of tensile strength of 440 N / mm 2 or more, total elongation of 22% or more, and high strength, high ductility, and high toughness of absorbed energy vE-20> 100 J at −20 ° C. .

一方,本発明の範囲を外れる比較例は,疲労き裂伝播特性,残留応力および機械的特性のうち,いずれか,あるいは複数の特性が目標値を満足していない。   On the other hand, in a comparative example that is out of the scope of the present invention, one or more of the fatigue crack propagation characteristics, residual stress, and mechanical characteristics do not satisfy the target value.

Claims (3)

鋼組成が,質量%で,
C:0.05〜0.30%,
Si:0.03〜0.35%,
Cr:0.05〜2.0%,
P:0.03%以下
S:0.003%以下
Al:0.1%以下
を含有し,残部がFeおよび不可避的不純物からなる鋳片または鋼片を,熱間圧延後,再加熱して焼入れ処理後、Ac変態点+10℃〜790℃の2相域温度範囲に再加熱し、平均冷却速度5〜60℃/sで焼入れ後,400〜650℃で10分以上保持して焼もどしすることを特徴とする、金属組織がビッカース硬さで85以上130以下のフェライト相と,面積分率が15〜85%のビッカース硬さで340以上440以下の焼もどしマルテンサイト相の混合組織である、表面残留応力の絶対値が150N/mm 以下の耐疲労亀裂伝播特性に優れた鋼材の製造方法
Steel composition is mass%,
C: 0.05-0.30%,
Si: 0.03 to 0.35%,
Cr: 0.05 to 2.0%,
P: 0.03% or less S: 0.003% or less Al: A slab or steel slab containing 0.1% or less, the balance being Fe and inevitable impurities, after hot rolling, and reheating After quenching treatment, reheat to 2-phase region temperature range of Ac 1 transformation point + 10 ° C-790 ° C, quench at average cooling rate 5-60 ° C / s, hold at 400-650 ° C for 10 minutes or more and temper A mixed structure of a ferrite structure having a Vickers hardness of 85 to 130 and a tempered martensite phase having a Vickers hardness of 15 to 85% and a 340 to 440 or less Vickers hardness. A method for producing a steel material having excellent fatigue crack propagation characteristics with an absolute value of surface residual stress of 150 N / mm 2 or less .
鋼組成,質量%でさらに,
Mo:0.05〜1.0%,
V:0.01〜0.3%
の1種または2種を含有する請求項1に記載した、金属組織がビッカース硬さで85以上130以下のフェライト相と,面積分率が15〜85%のビッカース硬さで340以上440以下の焼もどしマルテンサイト相の混合組織である、表面残留応力の絶対値が150N/mm 以下の耐疲労亀裂伝播特性に優れた鋼材の製造方法
When the steel composition is mass%,
Mo: 0.05-1.0%,
V: 0.01 to 0.3%
The metal structure described in claim 1 containing one or two of the above, a ferrite phase having a Vickers hardness of 85 or more and 130 or less, and a Vickers hardness of 15 to 85% in a Vickers hardness of 340 or more and 440 or less. A method for producing a steel material that is a mixed structure of a tempered martensite phase and has excellent fatigue crack propagation characteristics with an absolute value of surface residual stress of 150 N / mm 2 or less .
鋼組成,質量%でさらに,
Mn:1.2%以下
Cu:0.8%以下
Ni:1.0%以下
Nb:0.1%以下
Ti:0.03%以下
B:0.005%以下
Ca:0.005%以下
REM:0.02%以下
Mg:0.005%以下
の1種または2種以上を含有する請求項1または請求項2に記載した、金属組織がビッカース硬さで85以上130以下のフェライト相と,面積分率が15〜85%のビッカース硬さで340以上440以下の焼もどしマルテンサイト相の混合組織である、表面残留応力の絶対値が150N/mm 以下の耐疲労亀裂伝播特性に優れた鋼材の製造方法
When the steel composition is mass%,
Mn: 1.2% or less Cu: 0.8% or less Ni: 1.0% or less Nb: 0.1% or less Ti: 0.03% or less B: 0.005% or less Ca: 0.005% or less REM : 0.02% or less Mg: 0.005% or less of one type or two or more types , wherein the metal structure has a Vickers hardness of 85 to 130 ferrite phase, Excellent fatigue crack propagation resistance with an absolute value of surface residual stress of 150 N / mm 2 or less, which is a mixed structure of tempered martensite phase of 340 to 440 with a Vickers hardness of 15 to 85% . Steel manufacturing method .
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