JP2003064442A - Steel sheet having excellent fatigue crack propagation resistance - Google Patents
Steel sheet having excellent fatigue crack propagation resistanceInfo
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- JP2003064442A JP2003064442A JP2001250089A JP2001250089A JP2003064442A JP 2003064442 A JP2003064442 A JP 2003064442A JP 2001250089 A JP2001250089 A JP 2001250089A JP 2001250089 A JP2001250089 A JP 2001250089A JP 2003064442 A JP2003064442 A JP 2003064442A
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Abstract
Description
【０００１】[0001]
【発明の属する技術分野】本発明は、土木建築構造物、
船体、海洋構造物およびラインパイプなどの材料として
好適な、大気中および腐食環境中において耐疲労亀裂進
展性に優れた鋼板に関する。TECHNICAL FIELD The present invention relates to a civil engineering building structure,
The present invention relates to a steel sheet which is suitable as a material for a hull, an offshore structure, a line pipe and the like and has excellent fatigue crack growth resistance in the atmosphere and in a corrosive environment.
【０００２】[0002]
【従来の技術】近年、溶接鋼構造物が大型化される傾向
が顕著となっており、高強度化と軽量化が望まれてい
る。土木建築構造物や船体用鋼板においても大型化が要
求されており、そのため高張力鋼の使用が拡大されつつ
ある。しかし、高強度鋼を使用する場合、当然設計応力
が上昇するために疲労破壊が発生し易くなり重要な問題
となる。2. Description of the Related Art In recent years, the tendency for welded steel structures to become large has become remarkable, and there is a demand for higher strength and lighter weight. Larger sizes are also required for civil engineering structures and steel plates for hulls, and therefore the use of high-strength steel is being expanded. However, when using high-strength steel, the design stress naturally rises, and fatigue fracture easily occurs, which is an important problem.
【０００３】一方、溶接構造用の鋼板では、溶接が施工
されるため、溶接部から疲労亀裂が発生することが多
く、亀裂の進展を抑制することができなければ重大な事
故に発展する恐れがある。疲労亀裂進展速度は、亀裂長
さが数ｍｍ以上のいわゆる巨視亀裂が、繰り返し荷重下
で進展する速度を意味する。疲労亀裂は、応力が集中す
る材料欠陥から発生し、構造物中を進展し構造物が耐え
得る亀裂長さ（数ｍｍから数十ｍｍ）まで進展し、それ
を超えれば破断に至る。On the other hand, with steel sheets for welded structures, since welding is carried out, fatigue cracks often occur at the welded portions, and if the crack propagation cannot be suppressed, a serious accident may occur. is there. The fatigue crack growth rate means the rate at which a so-called macroscopic crack having a crack length of several mm or more propagates under repeated loading. Fatigue cracks are generated from material defects in which stress is concentrated, propagate in a structure, and propagate to a crack length (several mm to several tens of mm) that the structure can withstand, and if the crack length is exceeded, fracture occurs.
【０００４】溶接部から発生、進展する疲労亀裂を溶接
部近傍で停留させることができれば、重大な事故を防止
することができて構造物の疲労寿命を延長することが可
能である。したがって、大型溶接構造物では亀裂の発生
の抑制よりも亀裂進展の抑制が重要視されつつある。疲
労亀裂の進展を抑制する方法としては、特開平５−１４
８５４１号公報に開示されているように、疲労亀裂先端
にマイクロクラックを発生させる方法がある。しかし、
この方法による効果は、低△Ｋ（△Ｋ：最大応力拡大係
数と最小応力拡大係数の差）領域、すなわち、亀裂が長
くなく応力レベルが低い場合に限られ、溶接部から発生
してある程度の長さを有している中△Ｋ領域にある亀裂
に対しての効果は小さい。If the fatigue cracks generated and propagated from the weld can be retained near the weld, a serious accident can be prevented and the fatigue life of the structure can be extended. Therefore, in large-scale welded structures, suppression of crack growth is becoming more important than suppression of crack generation. As a method for suppressing the development of fatigue cracks, JP-A-5-14
As disclosed in Japanese Patent No. 8541, there is a method of generating microcracks at the tips of fatigue cracks. But,
The effect of this method is limited to a low ΔK (ΔK: difference between the maximum stress intensity factor and the minimum stress intensity factor) region, that is, when the crack is not long and the stress level is low, and a certain degree of occurrence occurs from the weld. The effect on cracks in the medium ΔK region having a length is small.
【０００５】特開平７−９０４７８号公報には、耐疲労
亀裂進展性の良好な鋼板およびその製造法の発明が開示
されている。同公報には、面積率で５〜５０％の軟質相
が母相として存在しており、かつ硬質の第二相は縞状で
あり鋼板圧延方向に延在すると記載されている。疲労亀
裂の進展が硬質相の存在によって抑制されるわけである
が、硬質相が鋼板圧延方向に縞状に延在するのであれ
ば、圧延方向に進展する亀裂は抑制することができな
い。Japanese Unexamined Patent Publication (Kokai) No. 7-90478 discloses a steel sheet having excellent fatigue crack growth resistance and a method for producing the same. The publication describes that a soft phase having an area ratio of 5 to 50% is present as a matrix phase, and the hard second phase is striped and extends in the steel sheet rolling direction. Although the growth of fatigue cracks is suppressed by the presence of the hard phase, if the hard phase extends in a striped shape in the steel sheet rolling direction, cracks that propagate in the rolling direction cannot be suppressed.
【０００６】本発明者らは、先に金属組織の硬質部と軟
質部の硬度差をビッカース硬度で１５０以上とした疲労
亀裂進展性の優れた鋼板を開発した（特開平７−２４２
９９２号公報）。軟質相と硬質相の硬度差を１５０以上
にすることは疲労亀裂進展速度を抑制するのに有効であ
るが、溶接部の靱性が低下するので溶接施工した場合に
用途が限定されることが分かった。The inventors of the present invention previously developed a steel sheet having excellent fatigue crack propagation properties, in which the hardness difference between the hard portion and the soft portion of the metal structure is 150 or more in Vickers hardness (Japanese Patent Laid-Open No. 7-242).
992 publication). Setting the hardness difference between the soft phase and the hard phase to 150 or more is effective in suppressing the fatigue crack growth rate, but the toughness of the welded portion decreases, so it is found that the application is limited when welding is performed. It was
【０００７】このように、これまでに提案された耐疲労
亀裂進展性に優れた鋼板は溶接部の靱性に乏しく、また
十分な耐疲労亀裂進展性を備えているとは言えず、より
優れた溶接後の靱性および耐疲労亀裂進展性に優れた鋼
板の開発が要望されていた。As described above, the steel sheets which have been proposed so far and have excellent fatigue crack growth resistance are poor in the toughness of the welded portion and cannot be said to have sufficient fatigue crack growth resistance, and thus are more excellent. Development of a steel sheet having excellent toughness after welding and fatigue crack growth resistance has been demanded.
【０００８】[0008]
【発明が解決しようとする課題】本発明の課題は、優れ
た耐疲労亀裂進展性を有し、かつ溶接施工しても溶接部
の靱性に優れている鋼板を提供することにある。SUMMARY OF THE INVENTION An object of the present invention is to provide a steel sheet having excellent fatigue crack growth resistance and excellent weld toughness even after welding.
【０００９】[0009]
【課題を解決するための手段】本発明者らは、先に開発
した上記の特開平７−２４２９９２号公報に開示されて
いる鋼板について、耐疲労亀裂進展性の改善と溶接部の
靱性の改善を図るため種々の実験を繰り返して検討した
結果、以下の知見を得るに至った。DISCLOSURE OF THE INVENTION The inventors of the present invention have improved the fatigue crack growth resistance and the weld toughness of the steel sheet disclosed in the above-mentioned JP-A-7-242992. As a result of repeating and examining various experiments to achieve the above, the following findings were obtained.
【００１０】ａ）金属組織の硬質部と軟質部の硬度差を
ビッカース硬度で１５０未満と小さくして溶接部の靱性
を高めても、金属組織およびＣとＳｉ含有量を下記のよ
うに規制すれば耐疲労亀裂進展性を改善することができ
る。A) Even if the hardness difference between the hard and soft portions of the metal structure is reduced to less than 150 in Vickers hardness to enhance the toughness of the weld, the metal structure and the C and Si contents should be restricted as follows. For example, fatigue crack growth resistance can be improved.
【００１１】パーライト以外のフェライトとベイナイ
トの混合組織のアスペクト比が１．７以下の組織と１．
７を超える組織の存在比率については、アスペクト比が
１．７以下の組織が１５〜４５％となるようにする。A structure having an aspect ratio of 1.7 or less in the mixed structure of ferrite and bainite other than pearlite, and
Regarding the existence ratio of the structure exceeding 7, the ratio of the structure having the aspect ratio of 1.7 or less is set to 15 to 45%.
【００１２】下記式を満足するようにＣおよびＳｉ含
有量を調整する。The C and Si contents are adjusted so as to satisfy the following formula.
【００１３】1.3×C(%)+0.04≦Si(%)≦6.4×C(%)+0.1
ｂ）さらに、疲労亀裂進展を安定して抑制するには、旧
オーステナイト（以下、旧γとも記す）粒界の存在比率
を０〜５本／100μmとし、組織のマクロ的な均一性を増
大させるのがよい。1.3 × C (%) + 0.04 ≦ Si (%) ≦ 6.4 × C (%) + 0.1 b) Further, in order to stably suppress fatigue crack growth, old austenite (hereinafter also referred to as old γ) ) It is preferable that the existence ratio of grain boundaries is 0 to 5 grains / 100 μm to increase the macroscopic homogeneity of the structure.
【００１４】本発明はこのような知見に基づきなされた
もので、その要旨は以下の通りである。The present invention has been made on the basis of such findings, and the gist thereof is as follows.
【００１５】（１）質量％で、Ｃ：０．０３〜０．２
％、Ｓｉ：０．０５〜０．６％、Ｍｎ：０．３〜２％、
sol.Ａｌ：０．００１〜０．１％を含有し、残部はＦｅ
および不純物からなり、かつ下記（１）式を満足し、金
属組織が面積率で５％以下のパーライト組織と８５％以
上のフェライトとベイナイトの混合組織を含み、この混
合組織のうち、アスペクト比が１．７以下の組織の占め
る割合が面積率で１５〜４５％であり、アスペクト比が
１．７以下の組織と１．７超の組織との硬度差がビッカ
ース硬度で３０〜１５０未満である鋼板。(1) C: 0.03 to 0.2 in mass%
%, Si: 0.05 to 0.6%, Mn: 0.3 to 2%,
sol.Al: 0.001-0.1%, balance Fe
And the following formula (1) is satisfied, and the metal structure includes a pearlite structure having an area ratio of 5% or less and a mixed structure of ferrite and bainite of 85% or more, and the aspect ratio of the mixed structure is The ratio of the structure of 1.7 or less is 15 to 45% in area ratio, and the difference in hardness between the structure having an aspect ratio of 1.7 or less and the structure of more than 1.7 is 30 to less than 150 in Vickers hardness. steel sheet.
【００１６】 1.3×C(%)+0.04≦Si(%)≦6.4×C(%)+0.1 ・・・・（１） ここで、元素記号は各元素の含有量（質量％）を示す。[0016] 1.3 × C (%) + 0.04 ≦ Si (%) ≦ 6.4 × C (%) + 0.1 ・ ・ ・ ・ (1) Here, the element symbol indicates the content (mass%) of each element.
【００１７】（２）Ｆｅの１部に替えて、質量％でＣ
ｕ：０．１〜１．５％、Ｎｉ：０．１〜１．５％、Ｃ
ｒ：０．１〜１．５％、Ｍｏ：０．１〜１．０％の１種
以上を含有する上記（１）に記載の鋼板。(2) In place of 1 part of Fe, C in mass%
u: 0.1 to 1.5%, Ni: 0.1 to 1.5%, C
The steel sheet according to (1) above, which contains at least one of r: 0.1 to 1.5% and Mo: 0.1 to 1.0%.
【００１８】（３）Ｆｅの１部に替えて、質量％でＶ：
０．０１〜０．１％、Ｎｂ：０．０１〜０．１％、Ｔ
ｉ：０．０１〜０．０５％の１種以上を含有する上記
（１）または（２）に記載の鋼板。(3) V in mass% in place of 1 part of Fe:
0.01-0.1%, Nb: 0.01-0.1%, T
i: The steel sheet according to (1) or (2), which contains at least one of 0.01 to 0.05%.
【００１９】（４）Ｆｅの１部に替えて、質量％でＢ：
０．０００５〜０．００３％を含む上記（１）〜（３）
のいずれかに記載の鋼板。(4) B in mass% in place of 1 part of Fe:
The above (1) to (3) including 0.0005 to 0.003%
The steel plate according to any one of 1.
【００２０】（５）金属組織における旧γ粒界の存在密
度ρbが０〜５である上記（１）〜（４）のいずれかに
記載の鋼板。(5) The steel sheet according to any one of (1) to (4) above, wherein the existing density ρb of the old γ grain boundaries in the metal structure is 0 to 5.
【００２１】ここで、旧オーステナイト粒界の存在密度
ρb とは、光学顕微鏡の５００倍の視野において、鋼板
断面の２０ヶ所に板厚方向に長さ１００μm の仮想的な
線を引き、この線と交差する旧オーステナイト粒界の数
を測定し、２０箇所の測定値の平均値とする。Here, the existence density ρb of the prior austenite grain boundaries means a virtual line having a length of 100 μm in the plate thickness direction at 20 points of the steel plate cross section in the field of view of 500 times the optical microscope. The number of former austenite grain boundaries that intersect is measured, and the average value of the measured values at 20 points is used.
【００２２】また、アスペクト比は結晶粒の長径と短径
の比であり、鋼板の断面の金属組織を光学顕微鏡で観察
して求めることができる。The aspect ratio is the ratio of the major axis to the minor axis of the crystal grains, and can be determined by observing the metal structure of the cross section of the steel sheet with an optical microscope.
【００２３】[0023]
【発明の実施の形態】以下、本発明で規定した化学組成
および金属組織について詳しく説明する。なお、各元素
の含有量の「％」表示は「質量％」である。
１）鋼板の化学組成
Ｃ：０．０３〜０．２％
Ｃは、鋼の強度を高めるのに有効な成分であり、特にア
スペクト比が１．７を超える組織の強度を高める働きが
ある。Ｃ含有量が０．０３％未満では、アスペクト比が
１．７を超える組織の硬度が十分な大きさとならず、本
発明の組織を確保することができない。一方、溶接施工
した場合の溶接部の靱性を確保し、溶接割れの発生を防
止するためにＣ含有量の上限は０．２％とする必要があ
る。望ましいＣ含有率の上限は０．１２％であり、さら
に望ましい上限は０．０９％である。BEST MODE FOR CARRYING OUT THE INVENTION The chemical composition and metal structure defined in the present invention will be described in detail below. The "%" display of the content of each element is "mass%". 1) Chemical composition C of steel sheet: 0.03 to 0.2% C is an effective component for increasing the strength of steel, and particularly has the function of increasing the strength of a structure having an aspect ratio of more than 1.7. If the C content is less than 0.03%, the hardness of the structure having an aspect ratio of more than 1.7 is not sufficiently large, and the structure of the present invention cannot be secured. On the other hand, the upper limit of the C content needs to be 0.2% in order to secure the toughness of the welded portion when welding is performed and to prevent the occurrence of weld cracks. The upper limit of the desirable C content is 0.12%, and the more desirable upper limit is 0.09%.
【００２４】Ｓｉ：０．０５〜０．６％
Ｓｉは、鋼の脱酸のために必要な成分である。Ｓｉ含有
量が０．０５％未満ではこの効果が期待できない。一
方、０．６％を超えると鋼の靱性が損なわれる。望まし
くは０．３〜０．４％である。Si: 0.05 to 0.6% Si is a component necessary for deoxidizing steel. If the Si content is less than 0.05%, this effect cannot be expected. On the other hand, if it exceeds 0.6%, the toughness of steel is impaired. It is preferably 0.3 to 0.4%.
【００２５】Ｍｎ：０．３〜２％
Ｍｎは、鋼の強度を向上させる成分であり、アスペクト
比が１．７を超える金属組織の生成を容易にし、かつそ
の組織の硬度を高める働きを有している。これらの効果
を得るにはＭｎ含有量を０．３％以上にする必要があ
る。しかし、ＭｎもＣと同様、溶接熱影響部を硬化さ
せ、溶接部の靱性劣化や溶接割れをもたらすことから、
上限は２％とする必要がある。望ましくは０．８〜１．
７％である。Mn: 0.3-2% Mn is a component for improving the strength of steel, and has the function of facilitating the formation of a metal structure having an aspect ratio of more than 1.7 and increasing the hardness of the structure. is doing. In order to obtain these effects, the Mn content needs to be 0.3% or more. However, Mn, like C, also hardens the heat-affected zone of the weld, causing deterioration of toughness and weld cracking of the weld,
The upper limit must be 2%. Desirably 0.8-1.
7%.
【００２６】sol.Ａｌ：０．００１〜０．１％
Ａｌは、脱酸作用があり鋼を脱酸するためにはsol.Ａｌ
として含有量が０．００１％以上となるように調整する
必要がある。また、Ａｌは酸化物となってγ粒径の微細
化に寄与し、母材や溶接部の靱性の確保に有効な元素で
ある。しかし、sol.Ａｌが０．１％を超えると、鋼の清
浄度および靱性が著しく損なわれるため、上限を０．１
％とした。Sol.Al: 0.001 to 0.1% Al has a deoxidizing action, and in order to deoxidize steel, sol.Al.
Therefore, it is necessary to adjust the content to be 0.001% or more. Further, Al is an element which becomes an oxide and contributes to the refinement of the γ grain size, and is an element effective for ensuring the toughness of the base material and the welded portion. However, if sol.Al exceeds 0.1%, the cleanliness and toughness of the steel are significantly impaired, so the upper limit is set to 0.1.
%.
【００２７】以上の元素の他に下記の元素を必要により
含有させることができる。In addition to the above elements, the following elements can be contained if necessary.
【００２８】Ｃｕ、Ｎｉ、ＣｒおよびＭｏの１種以上：
これらの元素は、腐食環境下での耐疲労亀裂進展性の改
善およびアスペクト比が１．７を超える組織の転位構造
の制御と微視的塑性変形の抑制に有効な元素で、必要に
より下記の範囲で１種以上含有させる。
Ｃｕ：０．１〜１．５％、Ｎｉ：０．１〜１．５％、Ｃ
ｒ０．１〜１．５％、Ｍｏ：０．１〜１％
各元素の含有量が、０．１％未満では上記の効果を十分
得ることができないので下限をそれぞれ０．１％とし
た。一方、Ｃｕ、Ｎｉ、Ｃｒは１．５％を、Ｍｏは１％
をそれぞれ超えると、上記効果が飽和する上に鋼の強度
が過剰に上昇し靱性が損なわれる。各元素の望ましい範
囲は以下の通りである。One or more of Cu, Ni, Cr and Mo:
These elements are effective in improving the fatigue crack growth resistance in a corrosive environment, controlling the dislocation structure of a structure with an aspect ratio of more than 1.7, and suppressing microscopic plastic deformation. One or more kinds are contained in the range. Cu: 0.1-1.5%, Ni: 0.1-1.5%, C
r0.1 to 1.5%, Mo: 0.1 to 1% If the content of each element is less than 0.1%, the above effects cannot be sufficiently obtained, so the lower limits were made 0.1%. On the other hand, Cu, Ni, and Cr are 1.5%, and Mo is 1%.
When each of the above values is exceeded, the above effect is saturated and the strength of the steel is excessively increased to impair the toughness. The desirable range of each element is as follows.
【００２９】Ｃｕ：０．１〜０．５％、Ｎｉ：０．２〜
０．４％、Ｃｒ：０．３〜１％、Ｍｏ：０．１〜０．３
％
Ｖ、ＮｂおよびＴｉの１種以上：これらの元素は、いず
れも炭化物を生成することにより、アスペクト比が１．
７以下の組織を細粒化して強化するため、腐食環境下で
の耐疲労亀裂進展性の改善に有効な元素で、必要により
下記の範囲で１種以上含有させる。
Ｖ：０．０１〜０．１％、Ｎｂ：０．０１〜０．１％、
Ｔｉ：０．０１〜０．０５％
これらの元素は、いずれも０．０１％以上で上記の効果
が得られる。しかし含有量がＶ、Ｎｂについてはそれぞ
れ０．１％、Ｔｉについては０．０５％を超えると、上
記効果が飽和するだけでなく、鋼の強度が過剰に上昇し
すぎ、靱性が損なわれる。各元素の望ましい範囲は以下
の通りである。Cu: 0.1-0.5%, Ni: 0.2-
0.4%, Cr: 0.3-1%, Mo: 0.1-0.3
% V, one or more of Nb and Ti: All of these elements have an aspect ratio of 1.
It is an element effective for improving the fatigue crack growth resistance in a corrosive environment in order to strengthen the structure of 7 or less by refining it, and if necessary, one or more elements are contained in the following range. V: 0.01 to 0.1%, Nb: 0.01 to 0.1%,
Ti: 0.01 to 0.05% The above effects can be obtained when the content of each of these elements is 0.01% or more. However, if the contents of V and Nb exceed 0.1% and that of Ti exceeds 0.05%, the above effect is not only saturated, but also the strength of the steel excessively increases and the toughness is impaired. The desirable range of each element is as follows.
【００３０】Ｖ：０．０２〜０．０３％、Ｎｂ：０．０
１〜０．０２％、Ｔｉ：０．０１〜０．０２％
Ｂ：０．００３％以下
Ｂは、アスペクト比が１．７以下の組織の生成を抑制
し、アスペクト比が１．７を超える組織の生成を促進す
る元素として有効であり、鋼板の製造プロセスにおいて
前者の組織だけが急激に生成するのを抑制し、後者の組
織とその組織の占有比率をある特定の範囲に制御するの
に有効である。そのためには、Ｂを含有させる場合に
は、０．０００５％以上とするのがよい。しかし含有量
が０．００３％を超えるとアスペクト比が１．７以下の
組織が生成しないようになり、また旧γ粒界の存在比率
も適正値を超え、亀裂進展速度を劣化させるので含有量
は０．００３％以下とする必要がある。より望ましい含
有量は０．０００５〜０．００１５％である。V: 0.02-0.03%, Nb: 0.0
1 to 0.02%, Ti: 0.01 to 0.02% B: 0.003% or less B suppresses the formation of a structure having an aspect ratio of 1.7 or less, and the aspect ratio exceeds 1.7. It is effective as an element that promotes the formation of microstructure, suppresses the rapid formation of only the former microstructure in the steel plate manufacturing process, and controls the occupancy ratio of the latter microstructure and its microstructure within a certain range. It is valid. For that purpose, when B is contained, the content is preferably 0.0005% or more. However, if the content exceeds 0.003%, a structure with an aspect ratio of 1.7 or less will not be generated, and the existing ratio of old γ grain boundaries will exceed the appropriate value, which will deteriorate the crack growth rate. Needs to be 0.003% or less. A more desirable content is 0.0005 to 0.0015%.
【００３１】1.3×C(%)+0.04≦Si(%)≦6.4×C(%)+0.1：
アスペクト比が１．７以下の組織と１．７超の組織との
硬度差がビッカース硬度で１５０以下であっても、優れ
た耐疲労亀裂進展性を得るためには、ＣとＳｉの含有量
をそれぞれ前記した範囲内で、かつ上記式を満足する量
とする必要がある。1.3 × C (%) + 0.04 ≦ Si (%) ≦ 6.4 × C (%) + 0.1: The difference in hardness between the structure having an aspect ratio of 1.7 or less and the structure having an aspect ratio of more than 1.7 is Vickers hardness. Even if it is 150 or less, in order to obtain excellent fatigue crack growth resistance, it is necessary to set the contents of C and Si within the ranges described above and to satisfy the above formulas.
【００３２】上記式で、Ｓｉ含有量が１．３×Ｃ(％)＋
０．０４＞Ｓｉ(％)となる場合は、アスペクト比が１．
７以下の組織の強度が必要以上に低下し、疲労亀裂の進
展抑制効果が十分に発揮されない。一方、Ｓｉ(％)＞
６．４×Ｃ(％)＋０．１となる場合は、溶接部の靱性が
劣化する。In the above formula, the Si content is 1.3 × C (%) +
When 0.04> Si (%), the aspect ratio is 1.
The strength of the structure of 7 or less decreases unnecessarily, and the effect of suppressing the growth of fatigue cracks is not sufficiently exerted. On the other hand, Si (%)>
If 6.4 × C (%) + 0.1, the toughness of the welded portion deteriorates.
【００３３】金属組織および硬度：本発明の鋼板の金属
組織は複合組織であり、面積率で５％以下のパーライト
組織と８５％以上のフェライトとベイナイトの混合組織
とを含み、その他１５％以下の割合でマルテンサイトや
島状マルテンサイトを含む。Metal structure and hardness: The metal structure of the steel sheet of the present invention is a composite structure, which includes a pearlite structure having an area ratio of 5% or less, a mixed structure of ferrite and bainite of 85% or more, and other than 15%. Includes martensite and island martensite in proportion.
【００３４】パーライト組織を５％以下に規定したの
は、本発明で規定する化学組成においては５％を超える
と耐疲労亀裂進展性が劣化するからであり、パーライト
組織はなくてもよい。。The pearlite structure is defined to be 5% or less because the fatigue crack growth resistance is deteriorated if the chemical composition defined in the present invention exceeds 5%, and the pearlite structure is not necessary. .
【００３５】フェライトとベイナイトの混合組織を８５
％以上と規定したのは、８５％未満では、耐疲労亀裂進
展性が劣化するからである。The mixed structure of ferrite and bainite is 85
% Or more is defined because if it is less than 85%, the fatigue crack growth resistance deteriorates.
【００３６】フェライトとベイナイトの混合組織は、ア
スペクト比によりほぼ分類ができ、アスペクト比が１．
７以下の組織は初析フェライトを中心とする組織で軟質
であり、その組織の占める割合は面積率で１５〜４５％
とする。アスペクト比が１．７を超える組織はベイナイ
トを中心とした組織で硬質である。The mixed structure of ferrite and bainite can be roughly classified according to the aspect ratio, and the aspect ratio is 1.
The structure of 7 or less is a structure centering on proeutectoid ferrite and is soft, and the ratio of the structure is 15 to 45% in area ratio.
And The structure having an aspect ratio of more than 1.7 is a structure mainly composed of bainite and is hard.
【００３７】ラス状組織をアスペクト比により分類した
上記２種類の組織を複合形成させるのは、その界面近傍
において亀裂進展の停留効果を得るためである。アスペ
クト比が１．７以下の組織と１．７超の組織の硬度差が
大きいほど亀裂進展抑制効果は大きくなるが、靱性の低
下も顕著になる。亀裂進展抑制効果が大きくなるのは、
亀裂先端の転位の移動が両組織の界面で阻止されるとと
もに、バーガースベクトルが界面に直交する転位が両組
織の界面近傍のアスペクト比が１．７以下の組織内に配
列するため、傾角粒界が形成されるからである。この傾
角粒界は粒界一次転位のみにより構成されるため、粒界
凝集力が高く、破壊の抵抗となりやすい。さらに形成さ
れた傾角粒界には転位が突入しにくいため、引き続き繰
り返し応力が作用する場合には、粒界に隣接するアスペ
クト比が１．７以下の組織側に新しい傾角粒界が形成さ
れる。このようなステップを繰り返すことにより、大き
な体積を有する傾角粒界の集合部が形成される。この集
合部は亀裂進展の抵抗となり、鋼板の亀裂進展抑制効果
を向上させるのである。The reason why the above-mentioned two types of structures in which the lath-like structure is classified according to the aspect ratio is formed in a composite manner is to obtain the effect of retaining crack growth near the interface. The greater the difference in hardness between the structure having an aspect ratio of 1.7 or less and the structure having an aspect ratio of more than 1.7, the greater the effect of suppressing crack growth, but the remarkable reduction in toughness. The effect of suppressing crack growth is
The movement of dislocations at the crack tip is blocked at the interface between both tissues, and the dislocations in which the Burgers vector is orthogonal to the interface are arranged in the tissues with an aspect ratio of 1.7 or less near the interface between both tissues. Is formed. Since this tilt grain boundary is composed of only grain boundary primary dislocations, the grain boundary cohesive force is high and resistance to fracture tends to occur. Further, since dislocations are less likely to rush into the formed tilt boundaries, a new tilt boundary is formed on the side of the structure adjacent to the grain boundary with an aspect ratio of 1.7 or less when the stress is repeatedly applied. . By repeating such steps, a cluster of tilt grain boundaries having a large volume is formed. This gathering portion becomes a resistance against crack growth and improves the crack growth suppressing effect of the steel sheet.
【００３８】しかしながら、両組織の硬度差をビッカー
ス硬度で１５０以上と大きくしすぎると、溶接部の靱性
が劣化する。また、硬度差を小さくすれば靱性は良好と
なるが、耐亀裂進展抑性が低下する。耐亀裂進展抑性を
確保するためには硬度差は３０以上とする必要がある。
したがって、両組織の硬度差はビカース硬度で３０〜１
５０未満とした。However, if the hardness difference between the two structures is too large as Vickers hardness of 150 or more, the toughness of the welded portion deteriorates. Further, if the hardness difference is reduced, the toughness is improved, but the crack growth resistance is lowered. The hardness difference must be 30 or more in order to secure crack growth resistance.
Therefore, the difference in hardness between the two structures is 30 to 1 in terms of Vickers hardness.
It was less than 50.
【００３９】上記のような亀裂先端と両組織の界面との
干渉効果をより大きくするためには、両組織の硬度差を
上記のように規定する必要があり、アスペクト比１．７
以下の組織の占有比率は、面積率で１５〜４５％とする
必要がある。In order to further increase the effect of interference between the crack tip and the interface between the two tissues, it is necessary to define the hardness difference between the two tissues as described above, and the aspect ratio is 1.7.
The occupation ratio of the following tissues needs to be 15 to 45% in terms of area ratio.
【００４０】本発明で規定する化学組成を有し、かつフ
ェライトとベイナイトの混合組織を適切な前記組織分率
にすることで、靱性を損なうことなく疲労亀裂進展抑制
に有効な硬度差を得ることができる。アスペクト比１．
７以下の組織の占有比率は、１５％未満でも、４５％を
超えても耐疲労亀裂進展性が劣化する。By having the chemical composition defined by the present invention and making the mixed structure of ferrite and bainite have an appropriate structure fraction, it is possible to obtain a hardness difference effective for suppressing fatigue crack growth without impairing toughness. You can Aspect ratio 1.
If the occupation ratio of the structure of 7 or less is less than 15% or more than 45%, the fatigue crack growth resistance deteriorates.
【００４１】さらに、旧γ粒界の存在を示すパラメータ
ρbを０〜５本／100μmとすれば、マクロ的に均一な組
織となってより安定した亀裂進展抑制効果が得られる。Furthermore, if the parameter ρb indicating the existence of the old γ grain boundaries is set to 0 to 5 grains / 100 μm, a macroscopically uniform structure is obtained and a more stable crack growth suppressing effect can be obtained.
【００４２】パラメータρbの算出方法は前記のとおり
である。旧γ粒界が数多く残った組織ほどパラメータρ
bの値は大きくなるが、本発明ではこのパラメータを０
〜５にすることで、より安定した亀裂進展抑制効果が得
られる。本発明の鋼板は以下のようにして製造すること
ができる。The method of calculating the parameter ρb is as described above. The structure with many old γ grain boundaries remains
Although the value of b becomes large, this parameter is set to 0 in the present invention.
By setting it to -5, a more stable crack growth suppressing effect can be obtained. The steel sheet of the present invention can be manufactured as follows.
【００４３】すなわち、本発明で規定する化学組成の鋼
を溶製し、連続鋳造により鋳片にするか、またはインゴ
ットケースに鋳込んで鋼塊とし、それらを熱間加工して
熱処理を施すことにより製造できる。本発明で規定する
金属組織にするには、鋼塊を１０６０〜１２６０℃の温
度範囲に加熱し、１２００〜１０００℃から圧延を開始
し、８２０〜９７０℃で圧延を仕上げる。適切な仕上げ
温度は、化学組成により異なる。さらに、熱間圧延後、
８４０〜７５０℃まで空冷し、その後水冷する。水冷停
止温度は、５００℃以下であり、低い方が望ましい。し
かし、２００℃以下になると靱性が劣化するので、２０
０℃を超える温度で水冷を停止するのが好ましい。別の
方法として、熱間圧延後空冷し、８００〜１１００℃の
温度域に再加熱し、水冷してもよい。That is, the steel having the chemical composition specified in the present invention is melted and cast into a slab by continuous casting, or cast into an ingot case to obtain a steel ingot, which is hot worked and heat treated. Can be manufactured by In order to obtain the metallographic structure defined in the present invention, the steel ingot is heated to a temperature range of 1060 to 1260 ° C, rolling is started from 1200 to 1000 ° C, and rolling is finished at 820 to 970 ° C. The appropriate finishing temperature depends on the chemical composition. Furthermore, after hot rolling,
Air cool to 840-750 ° C, then water cool. The water cooling stop temperature is 500 ° C. or lower, and the lower one is desirable. However, when the temperature is 200 ° C. or lower, the toughness deteriorates.
Water cooling is preferably stopped at temperatures above 0 ° C. As another method, it may be air-cooled after hot rolling, reheated to a temperature range of 800 to 1100 ° C., and water-cooled.
【００４４】８２０〜９７０℃の温度範囲で圧下率３０
％以上で熱間圧延することにより、旧γ粒界の存在密度
ρbを０〜５にすることができる。９００℃以下で５０
％以上の圧下率で圧延すればρbは０となる。本発明で
規定する化学組成の鋼では、上記の条件で熱間圧延後空
冷および水冷することにより、パーライトが５％以下、
フェライトとベイナイトの混合組織を８５％以上とする
ことができる。また、水冷開始温度を８４０〜７５０℃
の範囲とすることにより、フェライトとベイナイトの混
合組織のうちアスペクト比が１．７以下の組織の占める
割合を面積率で１５〜４５％にすることができる。アス
ペクト比が１．７以下の組織と１．７超の組織との硬度
差がビッカース硬度で３０〜１５０未満に調整するに
は、水冷開始温度を８４０〜７５０℃の範囲とし、かつ
水冷停止温度を２００℃以上とすればよい。The rolling reduction is 30 in the temperature range of 820 to 970 ° C.
By hot rolling at a content of at least%, the existing density ρb of the old γ grain boundaries can be made 0 to 5. 50 below 900 ° C
If rolled at a rolling reduction of not less than%, ρb becomes 0. In the steel having the chemical composition specified in the present invention, pearlite is 5% or less by hot-rolling and air-cooling and water-cooling under the above conditions,
The mixed structure of ferrite and bainite can be 85% or more. In addition, the water cooling start temperature is 840 to 750 ° C.
By setting the range to, the ratio of the structure having an aspect ratio of 1.7 or less in the mixed structure of ferrite and bainite can be made 15 to 45% in area ratio. To adjust the hardness difference between the structure having an aspect ratio of 1.7 or less and the structure having a ratio of more than 1.7 to a Vickers hardness of 30 to less than 150, the water cooling start temperature is set to a range of 840 to 750 ° C., and the water cooling stop temperature is set. May be 200 ° C. or higher.
【００４５】[0045]
【実施例】表１に示す２４種の化学組成を有する鋼を真
空溶解により溶製してインゴットとし、鍛造してスラブ
を製造した。EXAMPLE Steels having 24 chemical compositions shown in Table 1 were melted by vacuum melting into ingots and forged to manufacture slabs.
【００４６】[0046]
【表１】 [Table 1]
【００４７】これらのスラブは、表２の製造番号Ａ〜Ｆ
に示すような加熱−熱間圧延−制御冷却−熱処理の工程
で各工程の条件を種々変えて厚さ１０〜６５ｍｍの鋼板
を製造した。These slabs are manufactured under the serial numbers A to F in Table 2.
In the steps of heating-hot rolling-controlled cooling-heat treatment as shown in (4), the steel plate having a thickness of 10 to 65 mm was manufactured by variously changing the conditions of each step.
【００４８】[0048]
【表２】 [Table 2]
【００４９】それらの鋼板から顕微鏡組織観察用の試
料、ＪＩＳ Ｚ ２２０１に規定の５号引張試験片を採取
した。また、鋼板に入熱５ｋＪ／ｃｍにてサブマージア
ーク溶接を施し、そのＦＬ部からＪＩＳ Ｚ ２２０２に
規定のＶノッチのシャルピー衝撃試験片および後述する
疲労試験片とを採取した。A sample for microscopic observation, a No. 5 tensile test piece prescribed in JIS Z 2201, was taken from these steel plates. Further, the steel plate was subjected to submerged arc welding at a heat input of 5 kJ / cm, and a Charpy impact test piece having a V notch specified in JIS Z 2202 and a fatigue test piece described later were collected from the FL portion.
【００５０】鋼板の組織調査は、サンプルをエポキシ樹
脂に埋め込み、断面を研磨し、エッチングを施して顕微
鏡観察および微小領域の硬度測定をおこなった。The structure of the steel sheet was examined by embedding the sample in an epoxy resin, polishing the cross section, etching, and observing with a microscope and measuring the hardness of a minute area.
【００５１】強度は、引張試験により引張り強さ（Ｔ
Ｓ）を測定した。圧延熱処理条件と成分を適切に組み合
わせることにより、引張強度はいずれも５００〜６５０
ＭＰａの範囲となった。The strength is the tensile strength (T
S) was measured. By appropriately combining the rolling heat treatment conditions and components, the tensile strength is 500 to 650 in all cases.
It became the range of MPa.
【００５２】シャルピー衝撃試験は、−２０℃での吸収
エネルギーで評価した。The Charpy impact test was evaluated by the absorbed energy at -20 ° C.
【００５３】耐疲労亀裂進展性の調査は、図１に示すＣ
Ｔ試験片とサーボパルサ装置を用いる疲労試験法により
おこなった。The fatigue crack growth resistance was investigated by using C shown in FIG.
It was performed by a fatigue test method using a T test piece and a servo pulser device.
【００５４】図１（ａ）は、サーボパルサ装置の概念
図、図１（ｂ）はＣＴ試験片の平面図である。FIG. 1A is a conceptual view of the servo pulser device, and FIG. 1B is a plan view of a CT test piece.
【００５５】図１で、１はＣＴ試験片、２は試験溶液
槽、３は溶液循環ポンプ、４はロ−ドセル、５は油圧シ
リンダー、６は油圧源、７はサーボバルブ、８は波形発
生器、９は負荷制御器であり、この装置は、試験溶液２
中で試験片１に油圧シリンダー５により繰り返し応力を
負荷するようになっている。疲労試験の条件は下記の通
りであた。In FIG. 1, 1 is a CT test piece, 2 is a test solution tank, 3 is a solution circulation pump, 4 is a load cell, 5 is a hydraulic cylinder, 6 is a hydraulic source, 7 is a servo valve, and 8 is a waveform generator. And 9 is a load controller.
In the test piece 1, a hydraulic cylinder 5 repeatedly applies a stress. The conditions of the fatigue test were as follows.
【００５６】ｆ(繰り返し速度)＝30Ｈｚ、
Ｒ(応力比)＝0.1、
Ｔ(試験温度)＝室温
試験雰囲気：大気中
疲労試験結果によれば、中△Ｋ領域(本試験では約50〜3
00kgf/mm３／２)における第２領域でParis則[Trans.ASM
E,Ser.D.85.523(1963)参照]、すなわち、da/dN＝C(△
Ｋ)ｍが成り立つ。F (repetition rate) = 30 Hz, R (stress ratio) = 0.1, T (test temperature) = room temperature test atmosphere: According to the fatigue test results in the air, the medium ΔK region (about 50 to 3 in this test)
00kgf / mm 3/2 ) in the second region, Paris law [Trans.ASM
E, Ser.D.85.523 (1963)], that is, da / dN = C (△
K) m holds.
【００５７】ここで、[△Ｋ]：kgf/mm３／２、[da/d
N]：mm/cycle、Ｃ：1×10−５〜2×10 −５、m：2〜4
したがって、疲労亀裂進展性は、この中△Ｋ領域の△Ｋ
＝50および100kgf/mm３ ／２における、亀裂進展速度da/
dN(mm/cycle)の平均値で評価することとした。Here, [ΔK]: kgf / mm3/2, [Da / d
N]: mm / cycle, C: 1 x 10-5~ 2 x 10 -5, M: 2 to 4
Therefore, the fatigue crack growth rate is
= 50 and 100 kgf / mmThree / 2Crack growth rate da /
The average value of dN (mm / cycle) was used for evaluation.
【００５８】表３に試験結果を示す。Table 3 shows the test results.
【００５９】[0059]
【表３】 [Table 3]
【００６０】表３から明らかなうように、本発明例の全
てが耐疲労亀裂進展性は目標値であるda/dN≦3×10−６
mm/cycleを満たしている。また、溶接部の靱性は、本発
明例では全てｖＥ-20が５０Ｊを超えており優れた靱性
を示している。As is clear from Table 3, the fatigue crack growth resistance of all the examples of the present invention is the target value da / dN ≦ 3 × 10 −6.
Meets mm / cycle. In addition, the toughness of the welded portion in all of the examples of the present invention is vE-20 of more than 50 J, showing excellent toughness.
【００６１】[0061]
【発明の効果】本発明の鋼板は、中△Ｋ領域においても
耐疲労亀裂進展性に優れており、また溶接部における靱
性にも優れており、土木建築構造物、船体、海洋構造物
や装置およびラインパイプ等に用いて優れた効果を奏す
る。INDUSTRIAL APPLICABILITY The steel sheet of the present invention is excellent in fatigue crack growth resistance even in the middle ΔK region, and is also excellent in toughness in the welded portion. Also, it has excellent effects when used in line pipes and the like.
【図１】サーボパルサ装置および試験片を示す図であ
る。FIG. 1 is a diagram showing a servo pulser device and a test piece.
１：ＣＴ試験片 ２：試験溶液槽 ３：溶液循環ポンプ ４：ロ−ドセル ５：油圧シリンダー ６：油圧源 ７：サーボバルブ ８：波形発生器 ９：負荷制御器 1: CT test piece 2: Test solution tank 3: Solution circulation pump 4: load cell 5: hydraulic cylinder 6: hydraulic source 7: Servo valve 8: Waveform generator 9: Load controller
───────────────────────────────────────────────────── フロントページの続き (72)発明者 岡口 秀冶 大阪府大阪市中央区北浜４丁目５番33号 住友金属工業株式会社内 ─────────────────────────────────────────────────── ─── Continued front page (72) Inventor Shuji Okaguchi 4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd.
Claims (5)
ｉ：０．０５〜０．６％、Ｍｎ：０．３〜２％、sol.Ａ
ｌ：０．００１〜０．１％を含有し、残部はＦｅおよび
不純物からなり、かつ下記（１）式を満足し、金属組織
が面積率で５％以下のパーライト組織と８５％以上のフ
ェライトとベイナイトの混合組織を含み、この混合組織
のうち、アスペクト比が１．７以下の組織の占める割合
が面積率で１５〜４５％であり、アスペクト比が１．７
以下の組織と１．７超の組織との硬度差がビッカース硬
度で３０〜１５０未満であることを特徴とする鋼板。 1.3×C(%)+0.04≦Si(%)≦6.4×C(%)+0.1 ・・・・（１） ここで、元素記号は各元素の含有量（質量％）を示す。1. In mass%, C: 0.03 to 0.2%, S
i: 0.05 to 0.6%, Mn: 0.3 to 2%, sol.A
1: 0.001 to 0.1%, the balance consisting of Fe and impurities, satisfying the following formula (1), and having a pearlite structure having an area ratio of 5% or less and a ferrite structure of 85% or more. And a bainite mixed structure, and the ratio of the structure having an aspect ratio of 1.7 or less is 15 to 45% in area ratio, and the aspect ratio is 1.7.
A steel sheet having a hardness difference of 30 to less than 150 in Vickers hardness between the following structure and a structure exceeding 1.7. 1.3 × C (%) + 0.04 ≦ Si (%) ≦ 6.4 × C (%) + 0.1 (1) Here, the element symbol indicates the content (mass%) of each element.
１〜１．５％、Ｎｉ：０．１〜１．５％、Ｃｒ：０．１
〜１．５％、Ｍｏ：０．１〜１％の１種以上を含有する
ことを特徴とする請求項１に記載の鋼板。2. Cu: 0 ..% in mass% instead of 1 part of Fe.
1 to 1.5%, Ni: 0.1 to 1.5%, Cr: 0.1
The steel sheet according to claim 1, wherein the steel sheet contains at least one of 0.1 to 1.5% and Mo: 0.1 to 1%.
１〜０．１％、Ｎｂ：０．０１〜０．１％、Ｔｉ：０．
０１〜０．０５％の１種以上を含有することを特徴とす
る請求項１または２に記載の鋼板。3. V: 0.0 in mass% in place of 1 part of Fe
1 to 0.1%, Nb: 0.01 to 0.1%, Ti: 0.
The steel sheet according to claim 1 or 2, containing at least one of 01 to 0.05%.
００５〜０．００３％を含むことを特徴とする請求項１
〜３のいずれかに記載の鋼板。4. B: 0.0 in mass% in place of 1 part of Fe
The content of 005 to 0.003% is included.
The steel plate according to any one of to 3.
存在密度ρbが０〜５であることを特徴とする請求項１
〜４のいずれかに記載の鋼板。ここで、旧オーステナイ
ト粒界の存在密度ρb とは、光学顕微鏡の５００倍の視
野において、鋼板断面の２０ヶ所に板厚方向に長さ１０
０μm の仮想的な線を引き、この線と交差する旧オース
テナイト粒界の数を測定し、２０箇所の測定値の平均値
とする。5. The existing density ρb of prior austenite grain boundaries in the metal structure is 0-5.
The steel plate according to any one of to 4. Here, the existence density ρb of the former austenite grain boundaries means the length 10 in the plate thickness direction at 20 places in the cross section of the steel plate in a field of view of 500 times of an optical microscope.
A virtual line of 0 μm is drawn, the number of prior austenite grain boundaries intersecting this line is measured, and the average value of the measured values at 20 points is set.
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| CN100420764C (en) * | 2003-12-01 | 2008-09-24 | 住友金属工业株式会社 | Steel product excellent in fatigue characteristics and method for production thereof |
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| JP2007009322A (en) * | 2005-05-30 | 2007-01-18 | Jfe Steel Kk | High strength hot rolled sheet having excellent elongation property, stretch flange formability and tensile fatigue property, and method for producing the same |
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