JP2003342673A - Steel material having excellent resistance to fatigue crack propagation and its manufacturing method - Google Patents
Steel material having excellent resistance to fatigue crack propagation and its manufacturing methodInfo
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- JP2003342673A JP2003342673A JP2002157915A JP2002157915A JP2003342673A JP 2003342673 A JP2003342673 A JP 2003342673A JP 2002157915 A JP2002157915 A JP 2002157915A JP 2002157915 A JP2002157915 A JP 2002157915A JP 2003342673 A JP2003342673 A JP 2003342673A
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- fatigue crack
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Abstract
Description
【０００１】[0001]
【発明の属する技術分野】本発明は、船体、土木建設
物、建設機械、水圧鉄管、海洋構造物、ラインパイプな
どの構造用材料として使用される厚鋼板の分野における
疲労亀裂進展抵抗性に優れた鋼材およびその製造方法に
関する。TECHNICAL FIELD The present invention is excellent in fatigue crack growth resistance in the field of thick steel plates used as structural materials for hulls, civil engineering constructions, construction machinery, hydraulic iron pipes, marine structures, line pipes and the like. Steel material and manufacturing method thereof.
【０００２】[0002]
【従来の技術】近年、溶接構造物が大型化される傾向が
顕著になってきており、高強度化と軽量化が望まれてい
る。しかしながら高強度鋼を使用する際には設計応力が
上昇するため、溶接部から疲労破壊が発生しやすくな
り、その改善が重要な問題となっている。構造用鋼材な
どの厚鋼板では一般に溶接施工が施されるため、溶接部
から疲労亀裂が発生する可能性がある。従って、溶接部
から発生、進展する疲労亀裂を鋼材で滞留させることが
できれば、構造物の疲労寿命の延長に有効である。この
ため、疲労亀裂進展抑制効果を有する鋼板が種々提案さ
れている。2. Description of the Related Art In recent years, the tendency for welded structures to become large has become remarkable, and there is a demand for higher strength and lighter weight. However, when high-strength steel is used, the design stress increases, so that fatigue fracture easily occurs from the welded portion, and its improvement is an important issue. Since thick steel plates such as structural steels are generally welded, fatigue cracks may occur at the welds. Therefore, if the fatigue cracks generated and propagated from the weld can be retained in the steel material, it is effective in extending the fatigue life of the structure. Therefore, various steel sheets having a fatigue crack growth suppressing effect have been proposed.
【０００３】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が開示されている。
この鋼板は、圧延方向に延在する縞状の硬質な第二相
が、軟質な母相内に面積率で 5〜50％の割合で散在した
組織を有するものである。Japanese Unexamined Patent Publication (Kokai) No. 7-90478 discloses a steel sheet having good resistance to fatigue crack growth and a method for producing the steel sheet.
This steel sheet has a structure in which striped hard second phases extending in the rolling direction are scattered in the soft matrix at an area ratio of 5 to 50%.
【０００４】また、特開平6-271985号公報には、組織が
主にフェライト、パーライト、ベイナイトの１種または
２種以上で構成され、さらに平均存在間隔20μm 以下で
かつ平均扁平比５以上の形状をした島状マルテンサイト
を体積率で0.5 〜５％の割合で存在させた耐疲労亀裂伝
播特性の優れた鋼板が示されている。Further, in Japanese Patent Laid-Open No. 6-271985, the structure is mainly composed of one or more of ferrite, pearlite and bainite, and further, the average existence interval is 20 μm or less and the average flatness ratio is 5 or more. A steel sheet having excellent fatigue crack propagation resistance in which the island-shaped martensite formed in the above manner is present in a volume ratio of 0.5 to 5% is shown.
【０００５】特開平7-242992号公報には、組織が硬質部
の素地と、この素地に分散した軟質部とからなり、この
２部分の硬度差がビッカース硬度で150 以上であること
を特徴とする疲労亀裂進展抑制効果を有する鋼板が開示
されている。Japanese Unexamined Patent Publication (Kokai) No. 7-242992 is characterized in that the structure is composed of a base of a hard part and a soft part dispersed in the base, and the hardness difference between these two parts is 150 or more in Vickers hardness. A steel sheet having a fatigue crack growth suppressing effect is disclosed.
【０００６】[0006]
【発明が解決しようとする課題】しかしながら特開平7-
90478号公報で提案された方法では、疲労亀裂の進展抑
制効果が発揮されるのは板厚方向のみであり、その他の
方向での進展抑制効果は小さい。特開平6-271985号公報
で提案された方法では、高強度鋼の場合、平均扁平比の
大きな島状マルテンサイトが存在すると、靱性が劣化し
やすいという問題がある。また、特開平7-242992号公報
にもあるように、硬質部（ベイナイト、マルテンサイ
ト、パーライト、焼戻マルテンサイト）と軟質部（フェ
ライト）からなる混合組織では、強度低下が生じる場合
があるため、高強度鋼材を得る方法としては必ずしも満
足な方法ではない。[Problems to be Solved by the Invention] However, JP-A-7-
In the method proposed in Japanese Patent No. 90478, the effect of suppressing the growth of fatigue cracks is exhibited only in the plate thickness direction, and the effect of suppressing the growth in other directions is small. In the method proposed in JP-A-6-271985, in the case of high-strength steel, the toughness is apt to deteriorate in the presence of island martensite having a large average flatness ratio. Further, as disclosed in Japanese Patent Laid-Open No. 7-242992, a mixed structure consisting of a hard part (bainite, martensite, pearlite, tempered martensite) and a soft part (ferrite) may cause strength reduction. However, it is not always a satisfactory method for obtaining high strength steel.
【０００７】本発明はこれらの課題を解決するためにな
されたものであり、その目的とするところは、船体、土
木建設物、建設機械、水圧鉄管、海洋構造物、ラインパ
イプなど構造用材料として使用される鋼材であって、主
として硬質相からなる耐疲労亀裂進展抑制特性に優れた
鋼材およびその製造方法を提供することにある。The present invention has been made to solve these problems, and its object is to provide a structural material such as a hull, civil engineering construction, construction machinery, hydraulic iron pipes, marine structures, and line pipes. It is an object of the present invention to provide a steel material used, which is mainly composed of a hard phase and is excellent in fatigue crack growth resistance, and a manufacturing method thereof.
【０００８】[0008]
【問題を解決するための手段】本発明者らは鋼の結晶組
織と疲労亀裂進展抵抗性との関係について種々研究を重
ねた結果、以下の知見を得た。[Means for Solving the Problem] As a result of various studies on the relationship between the crystal structure of steel and fatigue crack growth resistance, the present inventors have obtained the following findings.
【０００９】すなわち、硬質相（ベイナイト、マルテン
サイト、焼戻マルテンサイトなど）の組織中の転位密度
が疲労亀裂進展速度に影響し、特に、前記転位密度が十
分に高い場合には、硬質相を主体とする組織を有する鋼
においても、疲労亀裂進展抵抗性が高く耐疲労性に優れ
た強度の高い鋼材を得ることができる。このような硬質
相を主体とする組織を有する鋼においても疲労亀裂進展
抵抗性が優れるのは、鋼が繰り返し変形を受ける過程
で、転位密度が高い硬質相の硬度が低減し、これにより
疲労亀裂先端での開口荷重が低下することが寄与してい
るものと考えられた。That is, the dislocation density in the structure of the hard phase (bainite, martensite, tempered martensite, etc.) affects the fatigue crack growth rate, and particularly when the dislocation density is sufficiently high, the hard phase Even in steel having a structure as a main component, a steel material having high fatigue crack growth resistance and excellent fatigue resistance can be obtained. Fatigue crack growth resistance is excellent even in steel having a structure mainly composed of such a hard phase, the hardness of the hard phase having a high dislocation density is reduced in the process of repeated deformation of the steel, which results in fatigue cracking. It was considered that the decrease in the opening load at the tip contributed to this.
【００１０】このような転位密度が高い組織は低温で変
態した組織で得られる。さらに、低温で変態し転位密度
の高い組織は格子歪を多く含むため、Ｘ線回折試験を行
った際の回折強度分布においてピークの幅が広くなる。
従って、十分な疲労亀裂進展抵抗性は、Ｘ線回折試験で
得られる回折強度の半価幅（強度がピーク強度の1/2に
おける分布幅、単位は「度」）がある一定値以上である
場合に得ることができる。Such a structure having a high dislocation density is obtained as a structure transformed at a low temperature. Further, since the structure transformed at a low temperature and having a high dislocation density contains a large amount of lattice strain, the peak width becomes wide in the diffraction intensity distribution when the X-ray diffraction test is performed.
Therefore, the sufficient fatigue crack growth resistance is a certain value or more with a half width of the diffraction intensity (the intensity is a distribution width at 1/2 of the peak intensity, the unit is "degree") obtained in the X-ray diffraction test. You can get in case.
【００１１】本発明はこれらの知見を基にして完成され
たものであり、その要旨は下記 (1)〜(5) に記載の疲労
亀裂進展抵抗性に優れた鋼材、および(6) に記載のその
製造方法にある。The present invention has been completed based on these findings, and the gist thereof is described in (1) to (5) below, which are steel materials excellent in fatigue crack growth resistance, and (6). Is in its manufacturing method.
【００１２】(1) 組織が、主として、ベイナイトおよび
マルテンサイト、またはベイナイトもしくはマルテンサ
イトで構成され、かつ、（200)面からのＸ線回折強度の
半価幅が0.20度以上である疲労亀裂進展抵抗性に優れた
鋼材。(1) Fatigue crack growth in which the structure is mainly composed of bainite and martensite, or bainite or martensite, and the half width of the X-ray diffraction intensity from the (200) plane is 0.20 degrees or more. Steel material with excellent resistance.
【００１３】(2) 鋼の化学組成が、質量％で、Ｃ：0.01
％以上、0.15％以下、Si：0.03％以上、0.6 ％以下、M
n：0.5 ％以上、2.0 ％以下、sol.Al:0.005％超、0.10
％以下、Ｎ：0.0005％以上、0.008 ％以下を含み、残部
がFeおよび不可避的不純物からなり、かつ、下記式で計
算されるFt値が4.0 ％以上、7.5 ％以下を満足する上記
(1) 記載の疲労亀裂進展抵抗性に優れた鋼材。
Ft＝3Mn(％)+Cu( ％)+1.5Cr(％)+1.8Ni(％)+1.5Mo(％)
(3) 鋼の化学組成が、質量％で、Ｃ：0.01％以上、0.15
％以下、Si：0.03％以上、0.6 ％以下、Mn：0.3 ％以
上、2.0 ％以下、Ｂ：0.0003％以上、0.0030％以下、so
l.Al:0.005％超、0.10％以下、Ｎ：0.0005％以上、0.00
8 ％以下を含み、残部がFeおよび不可避的不純物からな
り、かつ、下記式で計算されるFt値が3.0％以上、7.0
％以下を満足する上記(1) に記載の疲労亀裂進展抵抗性
に優れた鋼材。
Ft＝3Mn(％)+Cu (％)+1.5Cr(％)+1.8Ni(％)+1.5Mo(％)
。(2) The chemical composition of steel is C: 0.01 in mass%.
% Or more, 0.15% or less, Si: 0.03% or more, 0.6% or less, M
n: 0.5% or more, 2.0% or less, sol.Al: more than 0.005%, 0.10
% Or less, N: 0.0005% or more and 0.008% or less, the balance consisting of Fe and unavoidable impurities, and satisfying the Ft value calculated by the following formula of 4.0% or more and 7.5% or less.
(1) A steel material having excellent fatigue crack growth resistance as described. Ft = 3Mn (%) + Cu (%) + 1.5Cr (%) + 1.8Ni (%) + 1.5Mo (%) (3) Chemical composition of steel is% by mass, C: 0.01% or more, 0.15
% Or less, Si: 0.03% or more, 0.6% or less, Mn: 0.3% or more, 2.0% or less, B: 0.0003% or more, 0.0030% or less, so
l.Al: more than 0.005%, 0.10% or less, N: 0.0005% or more, 0.00
8% or less, the balance Fe and unavoidable impurities, and the Ft value calculated by the following formula is 3.0% or more, 7.0
% Or less, the steel material excellent in fatigue crack growth resistance according to the above (1). Ft = 3Mn (%) + Cu (%) + 1.5Cr (%) + 1.8Ni (%) + 1.5Mo (%)
.
【００１４】(4) 鋼の化学組成が、さらに質量％で、C
u：0.7 ％未満、Ni：3.0 ％以下、Cr：1.0 ％未満、M
o：0.8 ％以下からなる群の内の１種または２種以上を
含有する上記(2) または(3) に記載の疲労亀裂進展抵抗
性に優れた鋼材。(4) If the chemical composition of steel is further mass%, C
u: less than 0.7%, Ni: less than 3.0%, Cr: less than 1.0%, M
o: A steel material excellent in fatigue crack growth resistance according to (2) or (3) above, which contains one or more members selected from the group consisting of 0.8% or less.
【００１５】(5) 鋼の化学組成が、さらに質量％で、N
b：0.005 ％以上、0.08％以下、Ti：0.005 ％以上、0.0
3％以下、Ｖ：0.005 ％以上、0.08％以下からなる群の
内の１種または２種以上を含有する上記(2) 〜(4) のい
ずれかに記載の疲労亀裂進展抵抗性に優れた鋼材。(5) The chemical composition of steel is N% by mass.
b: 0.005% or more, 0.08% or less, Ti: 0.005% or more, 0.0
3% or less, V: 0.005% or more, 0.08% or less, and excellent in fatigue crack growth resistance according to any one of the above (2) to (4), containing one or more of the following: Steel material.
【００１６】(6) 上記(2) 〜(5) のいずれかに記載の化
学組成を有する鋳造スラブを1000℃〜1250℃に加熱する
加熱工程と、加熱された前記スラブに熱間圧延を施す熱
間圧延工程と、前記熱間圧延をされた鋼に冷却を施す冷
却工程とを備え、前記冷却工程においては、650 ℃〜50
0 ℃の間の平均冷却速度を５〜25℃/sとする加速冷却を
施し、該加速冷却を500 ℃〜350 ℃の間で停止し、その
後、復熱温度幅が70℃以下となるようにして冷却を終了
することを特徴とする疲労亀裂進展抵抗性に優れた鋼材
の製造方法。(6) A heating step of heating the cast slab having the chemical composition according to any one of the above (2) to (5) to 1000 ° C. to 1250 ° C., and hot rolling of the heated slab. A hot rolling step and a cooling step of cooling the hot-rolled steel are provided, and in the cooling step, 650 ° C to 50 ° C.
Accelerated cooling is performed at an average cooling rate of 5 to 25 ° C / s between 0 ° C, the accelerated cooling is stopped between 500 ° C and 350 ° C, and then the recuperation temperature range is 70 ° C or less. A method for manufacturing a steel material having excellent fatigue crack growth resistance, characterized by terminating the cooling.
【００１７】ここで、上記の「主として」との意味は、
鋼の結晶組織において、ベイナイトおよびマルテンサイ
ト、またはベイナイトもしくはマルテンサイトの構成比
率（２種からなる場合はその合計の構成比率）が面積率
にて95％以上であることを意味する。Here, the meaning of "mainly" is as follows.
In the crystal structure of steel, it means that the composition ratio of bainite and martensite, or the composition ratio of bainite or martensite (the composition ratio of the total of two kinds) is 95% or more in area ratio.
【００１８】また、本発明にかかる疲労亀裂進展抵抗性
に優れた鋼材の性能は、特に限定するものではないが、
望ましい性能としては、疲労亀裂進展速度が３×10-5mm
/cycle以下である。さらには、靱性が重要視される鋼材
の場合には、衝撃試験における吸収エネルギがvE-20 で
100J以上の特性を兼ね備えているものである。The performance of the steel material excellent in fatigue crack growth resistance according to the present invention is not particularly limited,
The desirable performance is a fatigue crack growth rate of 3 × 10 -5 mm.
Below / cycle. Furthermore, in the case of steel materials where toughness is important, the absorbed energy in the impact test is vE -20 .
It has the characteristics of 100J or more.
【００１９】[0019]
【発明の実施の形態】本発明に係る鋼材の組織や化学組
成を限定する理由は次のとうりである。組織：本発明の
鋼は、容易に高強度を得るために、その組織は、主とし
て、ベイナイトおよびマルテンサイト、またはベイナイ
トもしくはマルテンサイトで主に構成される。上記ベイ
ナイトは上部ベイナイト、下部ベイナイト、アシキュラ
ーフェライト、グラニュラーベイナイトなどの組織を含
むものであり、上記マルテンサイトはほとんどの場合、
ラスマルテンサイトである。BEST MODE FOR CARRYING OUT THE INVENTION The reason for limiting the structure and chemical composition of the steel material according to the present invention is as follows. Structure: The steel of the present invention is mainly composed of bainite and martensite, or bainite or martensite in order to easily obtain high strength. The bainite is an upper bainite, a lower bainite, acicular ferrite, including a structure such as granular bainite, the martensite in most cases,
This is Ras Martensite.
【００２０】「主として」との意味は、鋼の組織におい
てこれらの組織の構成比率が合計で面積率にて95％以上
であることを意味する。残りの組織は特に限定するもの
ではなく、粒界フェライト組織、粒状フェライト組織、
パーライト組織など、通常観察される組織で構わない。The term "mainly" means that the composition ratio of these structures in the steel structure is 95% or more in total in area ratio. The remaining structure is not particularly limited, grain boundary ferrite structure, granular ferrite structure,
A commonly observed structure such as a pearlite structure may be used.
【００２１】Ｘ線回折の半価幅：半価幅は、Ｘ繰回折強
度の分布において、回折強度がピーク強度の1/2 となる
部分の分布幅を回折角度で示した値である。高温で生成
し、転位密度の小さな組織ほど半価幅は小さいものとな
る。半価幅の大きな組織ほど転位密度が大きく、疲労亀
裂進展抵抗性が優れる。Full width at half maximum of X-ray diffraction: The full width at half maximum is a value in which the distribution width of the portion where the diffraction intensity is 1/2 of the peak intensity in the distribution of the X-ray diffraction intensity is shown by the diffraction angle. The full width at half maximum becomes smaller as the structure is generated at a higher temperature and the dislocation density is smaller. The larger the half-value width, the higher the dislocation density and the better the fatigue crack growth resistance.
【００２２】Ｘ線回折をおこなう結晶面は、最も一般的
に用いられる理由から、（200)面を対象とした。本発明
で規定する半価幅は、良好な疲労亀裂進展抵抗性を得る
ために、（200)面での回折強度の半価幅が0.20度以上の
ものとする。なお、結晶面は(110) でも良いが、この場
合は0.14度以上のものとなる。The crystal plane for X-ray diffraction is the (200) plane because it is most commonly used. The half width defined in the present invention is such that the half width of the diffraction intensity on the (200) plane is 0.20 degrees or more in order to obtain good fatigue crack growth resistance. The crystal plane may be (110), but in this case it is 0.14 degrees or more.
【００２３】図１は、Ｘ線回折における半価幅の解析法
を説明する模式図である。図１(a)、(b) は、それぞれ
(200) 面における回折強度を示すグラフである。図１に
示すように、半価幅は回折強度のピークにおいて、回折
強度が最も高い強度値の1/2のところでの分布の幅を角
度で表したものである。ピークが２つに分かれている場
合には、高い方のピークの1/2 の値をとる。FIG. 1 is a schematic diagram for explaining a method of analyzing a half width in X-ray diffraction. Figures 1 (a) and (b) are respectively
3 is a graph showing the diffraction intensity on the (200) plane. As shown in FIG. 1, the full width at half maximum represents the width of the distribution at the peak of the diffraction intensity at a half of the intensity value with the highest diffraction intensity, which is represented by an angle. When the peak is divided into two, take the value of 1/2 of the higher peak.
【００２４】上記半価幅は、回折パターンでＫα1 とＫ
α2 のピークが独立して現れる時は、Ｋα1 の値を、Ｋ
α1 とＫα2 の値が重なって現れる時は合計の幅で測定
する。なお、上記半価幅の測定は、厚さ方向で鋼材表面
から1 mm内部にはいった部位において、圧延面と平行な
面でおこなうものとする。The above-mentioned half width is Kα 1 and K in the diffraction pattern.
When the peak of α 2 appears independently, change the value of K α 1 to K
When the values of α 1 and K α 2 overlap, measure with the total width. The measurement of the full width at half maximum shall be performed on a surface parallel to the rolling surface at a portion 1 mm inside from the surface of the steel material in the thickness direction.
【００２５】鋼の化学組成は、以下のものとするのが望
ましい。
Ｃ：鋼の強度を高めるのに有効な元素であり、鋼の強度
を得るために、0.01％以上含有させる。しかしながら0.
15％を超えて含有させると靱性が劣化するので、これを
避けるためにＣ含有量は0.15％以下とする。より望まし
くは0.10％以下である。The chemical composition of steel is preferably as follows. C: An element effective in increasing the strength of steel, and is contained in an amount of 0.01% or more in order to obtain the strength of steel. However, 0.
If the content exceeds 15%, the toughness deteriorates. Therefore, in order to avoid this, the C content is set to 0.15% or less. More preferably, it is 0.10% or less.
【００２６】Si：鋼の脱酸に有効な元素であり、その効
果を得るために0.03％以上含有させる。しかしながら0.
6 ％を超えて含有させると、Ｍ−Ａ組織の形成が促進さ
れる。Ｍ−Ａ組織は、ベイナイト組織中に形成される島
状マルテンサイトの一種で、残留オーステナイトを含む
Ｍ−Ａ変態生成物である。Ｍ−Ａ組織は非常に硬度が高
く、容易に靱性を劣化させることが知られている。従っ
て靱性劣化を避けるためにSi含有量は0.6 ％以下とす
る。より望ましくは0.3 ％以上、0.5 ％以下である。Si: An element effective for deoxidizing steel, and in order to obtain the effect, 0.03% or more is contained. However, 0.
If the content exceeds 6%, the formation of the M-A structure is promoted. The M-A structure is a type of island martensite formed in the bainite structure and is a M-A transformation product containing retained austenite. It is known that the M-A structure has extremely high hardness and easily deteriorates toughness. Therefore, the Si content should be 0.6% or less to avoid deterioration of toughness. More preferably, it is 0.3% or more and 0.5% or less.
【００２７】Mn：焼入性向上に有効な元素であり、強度
上昇と疲労亀裂進展抵抗性を向上させるために、0.5 ％
以上含有させる。他方、2.0 ％を超えると靱性が劣化す
るので、Mn含有量は2.0 ％以下とする。Mn: an element effective in improving hardenability, and 0.5% in order to improve strength and fatigue crack growth resistance.
The above is contained. On the other hand, if it exceeds 2.0%, the toughness deteriorates, so the Mn content is made 2.0% or less.
【００２８】ただし、後述するようにＢを含有する場合
にはMn：0.3 ％以上、2.0 ％以下としてもよい。
sol.Al：AlはSiとともに脱酸に必要な元素であり、その
効果を得るために0.005 ％超のsol.Alを含有させる。他
方、sol.Al含有量が0.10％を超えるとＭ−Ａ比率 (Ｍ−
Ａ組織の存在比率) が増加し靱性が劣化する。これを避
けるためにsol.Al含有量は0.10％以下とする。However, as described later, when B is contained, Mn may be 0.3% or more and 2.0% or less. sol.Al: Al is an element necessary for deoxidation together with Si, and in order to obtain its effect, sol.Al of more than 0.005% is contained. On the other hand, when the sol.Al content exceeds 0.10%, the M-A ratio (M-
The existence ratio of A structure) increases and the toughness deteriorates. To avoid this, the sol.Al content is set to 0.10% or less.
【００２９】Ｎ：AlやTiと結合して析出物となり、オー
ステナイト粒の細粒化に寄与し靱性を改善する作用があ
る。この効果を得るために、Ｎは0.0005％以上含有させ
る。他方Ｎ含有量が0.008 ％を超えるとＭ−Ａ比率が増
加し靱性が劣化する。これを避けるためにため、Ｎ含有
量は0.008 ％以下とする。N: Combined with Al or Ti to form a precipitate, which has the function of contributing to the refinement of austenite grains and improving the toughness. To obtain this effect, N is contained at 0.0005% or more. On the other hand, when the N content exceeds 0.008%, the M-A ratio increases and the toughness deteriorates. In order to avoid this, the N content should be 0.008% or less.
【００３０】Ｂ：必須元素ではないが、Ｂは焼入性を著
しく高める作用があり、強度上昇と疲労亀裂進展抵抗性
を向上させるのに有効である。従ってさらにこれらの効
果を得るために含有させても構わない。上記効果を得る
には、0.0003％以上含有させるのが有効である。しかし
ながらＢを0.0030％を超えて含有させると靱性が劣化す
るため、その上限は0.0030％とするのが望ましい。B: Although it is not an essential element, B has the effect of remarkably increasing the hardenability, and is effective in increasing the strength and improving the fatigue crack growth resistance. Therefore, it may be contained in order to further obtain these effects. In order to obtain the above effect, it is effective to contain 0.0003% or more. However, if B is contained in an amount exceeding 0.0030%, the toughness deteriorates, so the upper limit is preferably made 0.0030%.
【００３１】Cu：必須元素ではないが、鋼の強度を高め
る作用があるので、その目的で含有させても構わない。
その効果を得るには0.3 ％以上含有させるのが望まし
い。しかしながらその含有量が0.7 ％以上になると鋼の
靱性が劣化するので、含有させる場合でもその上限は0.
7 ％未満とする。望ましくは0.5 ％未満である。Cu: Although it is not an essential element, it has the effect of increasing the strength of steel, so Cu may be contained for that purpose.
In order to obtain the effect, it is desirable to contain 0.3% or more. However, if its content exceeds 0.7%, the toughness of the steel deteriorates, so even if it is contained, the upper limit is 0.
It is less than 7%. It is preferably less than 0.5%.
【００３２】Ni：必須元素ではないが、鋼の強度を高め
る作用があり、また、疲労亀裂進展抑制にも有効であ
る。従ってこれらの効果を得るために含有させても構わ
ない。その効果を得るには0.2 ％以上含有させるのが望
ましい。しかしながらその含有量が3.0 ％を超えるとコ
スト上昇に見合う高強度化と疲労亀裂進展抑制効果が見
られないので、含有させる場合でもその上限は3.0 ％と
する。Ni: Although not an essential element, it has the effect of increasing the strength of steel and is also effective in suppressing fatigue crack growth. Therefore, it may be contained to obtain these effects. In order to obtain the effect, it is desirable to contain 0.2% or more. However, if its content exceeds 3.0%, the effect of increasing the strength and suppressing the fatigue crack growth commensurate with the cost increase cannot be seen, so even if it is contained, the upper limit is 3.0%.
【００３３】Cr：必須元素ではないが、鋼の強度を高め
る作用があり、また、疲労亀裂進展抑制にも有効であ
る。従ってこれらの効果を得るために含有させても構わ
ない。その場合には0.3 ％以上含有させるのが望まし
い。しかしながら過剰に含有させると靱性が劣化するの
で、含有させる場合でも1.0 ％未満とするのが望まし
い。Cr: Although not an essential element, it has the effect of increasing the strength of steel and is also effective in suppressing fatigue crack growth. Therefore, it may be contained to obtain these effects. In that case, it is desirable to contain 0.3% or more. However, if it is contained excessively, the toughness deteriorates, so even if it is contained, it is desirable to make it less than 1.0%.
【００３４】Mo：必須元素ではないが、鋼の強度を高め
る作用があり、また、疲労亀裂進展抑制にも有効であ
る。従ってこれらの効果を得るためにMoを含有させても
構わない。その場合には0.15％以上含有させるのが望ま
しい。しかしながら過剰に含有させると靱性が劣化する
ので、含有させる場合でもその上限は0.8 ％とするのが
望ましい。Mo: Although it is not an essential element, it has the effect of increasing the strength of steel and is also effective in suppressing fatigue crack growth. Therefore, Mo may be contained to obtain these effects. In that case, it is desirable to contain 0.15% or more. However, if it is contained excessively, the toughness deteriorates. Therefore, even if it is contained, the upper limit is preferably 0.8%.
【００３５】Nb：必須元素ではないが、細粒化作用を通
じて靱性を向上させる作用がある。また、焼入性を増す
ので強度向上と疲労亀裂進展抑制に有効である。従って
これらの効果を得るために含有させても構わない。その
場合、Nbは0.005 ％以上含有させるのが望ましい。他方
その含有量が0.08％を超えると靱性が劣化するので、そ
の上限は0.08％とする。より好ましくは0.06％以下であ
る。Nb: Although not an essential element, it has an effect of improving toughness through a grain refining effect. Further, since it increases the hardenability, it is effective for improving the strength and suppressing the growth of fatigue cracks. Therefore, it may be contained to obtain these effects. In that case, it is desirable to contain Nb in an amount of 0.005% or more. On the other hand, if its content exceeds 0.08%, the toughness deteriorates, so its upper limit is made 0.08%. It is more preferably 0.06% or less.
【００３６】Ti：必須元素ではないが、強度向上と疲労
亀裂進展抑制に有効であるので、これらの効果を得るた
めに含有させても構わない。上記効果を得るには0.005
％以上含有させるのが望ましい。他方、0.03％を超える
と靱性が劣化するので、その上限は0.03％とするのが望
ましい。Ti: Although it is not an essential element, it is effective for improving strength and suppressing fatigue crack growth, so Ti may be contained to obtain these effects. 0.005 to obtain the above effect
% Or more is desirable. On the other hand, if it exceeds 0.03%, the toughness deteriorates, so the upper limit is preferably made 0.03%.
【００３７】Ｖ：必須元素ではないが、強度向上に有効
であるので、これらの効果を得るために含有させても構
わない。含有させる場合には、上記効果を得るために0.
005％以上含有させるのが望ましい。他方、0.08％を超
えると靱性が劣化するので、その上限は0.08％とするの
が望ましい。V: Although it is not an essential element, it is effective for improving strength, so it may be contained in order to obtain these effects. When it is contained, in order to obtain the above effect, 0.
It is desirable to contain 005% or more. On the other hand, if it exceeds 0.08%, the toughness deteriorates, so the upper limit is preferably made 0.08%.
【００３８】Ft値：Mn、Cu、Cr、NiおよびMoは、いずれ
もベイナイトまたはマルテンサイト変態の変態温度に影
響して変態温度を低下させて、転位密度を上昇させる作
用を有しており、これらの元素を含有させることで鋼の
疲労亀裂進展抵抗性を改善する効果が得られる。この効
果は元素の種類により差異があり、Mn、Ni、Cr、Moが大
きい。この関係はこれらの元素の含有量（質量％）か
ら、下記式で計算されるFt値で表すことができ、Ft値が
大きいほど疲労亀裂進展抵抗性が向上する。Ft values: Mn, Cu, Cr, Ni and Mo all have the effect of affecting the transformation temperature of bainite or martensite transformation, lowering the transformation temperature, and increasing the dislocation density. By containing these elements, the effect of improving the fatigue crack growth resistance of steel can be obtained. This effect varies depending on the type of element, and Mn, Ni, Cr, and Mo are large. This relationship can be expressed by the Ft value calculated by the following formula from the contents (mass%) of these elements. The larger the Ft value, the more the fatigue crack growth resistance improves.
【００３９】Ft＝3Mn(％)+Cu (％)+1.5Cr(％)+1.8Ni
(％)+1.5Mo(％)
ただし、上記式において対象とする鋼の化学組成に含ま
れない元素については「ゼロ」としてFt値を計算する。Ft = 3Mn (%) + Cu (%) + 1.5Cr (%) + 1.8Ni
(%) + 1.5Mo (%) However, for the elements not included in the chemical composition of the target steel in the above formula, the Ft value is calculated as “zero”.
【００４０】しかしながらFt値が過度に大きい場合は鋼
の強度が過剰となり靱性が劣化するうえ、溶接割れも生
じやすくなる。従って、強度と靱性のバランスを良好に
保ちつつ疲労亀裂進展速度を小さくするには、Ft値が特
定範囲に収まるように、これらの合金元素の含有量を調
整するのが有効である。However, if the Ft value is excessively large, the strength of the steel becomes excessive, the toughness deteriorates, and weld cracking easily occurs. Therefore, in order to reduce the fatigue crack growth rate while maintaining a good balance between strength and toughness, it is effective to adjust the content of these alloying elements so that the Ft value falls within a specific range.
【００４１】Ft値は、鋼の焼入性に大きく影響するＢを
含有するか否かにより変化させる必要がある。すなわ
ち、鋼がＢを含有しないものである場合のFt値は、4.0
％以上、7.5 ％以下とする。望ましくは4.5 ％以上、6.
0 ％以下である。鋼がＢを含有するものである場合のFt
値は、3.0 ％以上、7.0 ％以下である。望ましくは3.5
％以上、5.5 ％以下である。The Ft value needs to be changed depending on whether or not B is contained, which greatly affects the hardenability of steel. That is, the Ft value when the steel does not contain B is 4.0
% Or more and 7.5% or less. Desirably 4.5% or more, 6.
It is 0% or less. Ft when the steel contains B
The value is 3.0% or more and 7.0% or less. Preferably 3.5
% Or more and 5.5% or less.
【００４２】残部は、Feおよび不可避的不純物である。
製造方法：本発明に係る疲労亀裂進展抵抗性に優れた鋼
材を製造する手段は特に限定するものではなく、公知の
熱間圧延設備、または公知の熱間圧延設備と公知の熱処
理設備を使用して、容易に製造することができる。その
製造条件は以下に述べる方法が好適である。The balance is Fe and inevitable impurities. Manufacturing method: Means for manufacturing a steel material having excellent fatigue crack growth resistance according to the present invention is not particularly limited, and known hot rolling equipment, or known hot rolling equipment and known heat treatment equipment are used. And can be easily manufactured. The method described below is suitable for the manufacturing conditions.
【００４３】本発明にかかる化学組成を有する鋳造スラ
ブを1000℃〜1250℃に加熱した後に熱間圧延を施す。つ
いでこれを冷却するに際し、その冷却工程において、65
0 ℃〜500 ℃の間の平均冷却速度を５℃/s以上、好まし
くは５〜25℃/sとする加速冷却を施し、500 ℃以下、好
ましくは500 〜350 ℃の温度で前記加速冷却を停止し、
その後、復熱温度幅が70℃以下となるようにして冷却を
終了する。ここで復熱温度幅とは冷却を停止した時の到
達温度と、冷却停止後鋼板内部の熱で表面の温度が上昇
し、安定した時の温度の差を意味する。A cast slab having the chemical composition according to the present invention is heated to 1000 ° C. to 1250 ° C. and then hot rolled. Then, when cooling this, in the cooling process, 65
Accelerated cooling is performed so that the average cooling rate between 0 ° C and 500 ° C is 5 ° C / s or more, preferably 5-25 ° C / s, and the accelerated cooling is performed at a temperature of 500 ° C or less, preferably 500-350 ° C. Stop and
Then, the cooling is completed by setting the recuperation temperature range to 70 ° C or less. Here, the recuperation temperature width means the difference between the temperature reached when cooling is stopped and the temperature when the temperature of the surface is stable due to the heat inside the steel sheet after cooling is stopped.
【００４４】鋳造スラブの加熱温度が1000℃に満たない
場合にはフェライト率が高くなり進展速度が大きくな
る。1250℃を超える場合には組織が粗大になり、靱性が
劣化する。冷却過程の内の650 ℃〜500 ℃の間の平均冷
却速度が５℃/sに満たない場合にはフェライト率が高く
なり進展速度が大きくなる。好ましくは25℃/s以下であ
る。加速冷却停止後冷却終了までの間の復熱温度幅が70
℃を超える場合には転位密度が減少して進展速度が大き
くなる。加速冷却停止温度が500 ℃超になる場合にはフ
ェライト率が高くなり、進展速度が大きくなる。好まし
くは350 ℃以上である。When the heating temperature of the cast slab is less than 1000 ° C., the ferrite rate becomes high and the growth rate becomes high. If it exceeds 1250 ° C, the structure becomes coarse and the toughness deteriorates. If the average cooling rate between 650 ° C and 500 ° C in the cooling process is less than 5 ° C / s, the ferrite rate increases and the growth rate increases. It is preferably 25 ° C / s or less. The recuperation temperature range between the accelerated cooling stop and the cooling end is 70.
When the temperature exceeds ° C, the dislocation density decreases and the growth rate increases. If the accelerated cooling stop temperature exceeds 500 ° C, the ferrite rate increases and the growth rate increases. The temperature is preferably 350 ° C or higher.
【００４５】[0045]
【実施例】表１に示す化学組成の鋼を実験室的に真空溶
解し、厚さ100 〜160mm のスラブとし、種々の条件で熱
間圧延を施した後、種々の条件で冷却して厚さが12〜48
mmの厚鋼板とした。熱間圧延条件と冷却条件を表２に示
す。EXAMPLE Steels having chemical compositions shown in Table 1 were vacuum-melted in a laboratory to form slabs having a thickness of 100 to 160 mm, hot-rolled under various conditions, and then cooled under various conditions to obtain a thick plate. 12 to 48
mm thick steel plate. Table 2 shows hot rolling conditions and cooling conditions.
【００４６】[0046]
【表１】 [Table 1]
【００４７】[0047]
【表２】 [Table 2]
【００４８】得られた鋼板の組織、Ｘ線回折の半価幅、
引張強度、靱性および疲労亀裂進展速度を以下の方法で
調査した。鋼の組織は、板厚の1/4 に相当する部分から
採取した試料の断面を研磨し、２％ナイタール腐食液に
よりエッチングを施した面について、光学顕微鏡観察に
よりベイナイトとマルテンサイトの合計面積率を測定し
た。１試料について10視野測定し、10個の測定値の平均
を当該鋼板の面積率とした。Structure of the obtained steel sheet, half width of X-ray diffraction,
The tensile strength, toughness and fatigue crack growth rate were investigated by the following methods. The structure of steel is the total area ratio of bainite and martensite observed by an optical microscope on the surface of a sample taken from a portion corresponding to 1/4 of the plate thickness, which was polished by 2% Nital etchant. Was measured. Ten visual fields were measured for one sample, and the average of ten measured values was defined as the area ratio of the steel sheet.
【００４９】Ｘ線回折の半価幅は、25mm角の試験片を採
取し、厚さ方向で表面から１mm内側の圧延面と平行な面
を電解研磨して測定面とした。引張試験片は板厚の中心
部からJIS 14A 号引張試験片を庄延方向に平行に採取し
て、引張試験に供した。靱性は、JIS-Z2202 に規定され
る４号のシャルピー衝撃試験片を板厚中心部から庄延方
向に平行に採取してシャルピー衝撃試験を行い、衝撃吸
収エネルギ（vE-20 、単位はＪ）を求めた。The half-value width of X-ray diffraction was measured by taking a 25 mm square test piece and electrolytically polishing a surface parallel to the rolling surface 1 mm inward from the surface in the thickness direction. For the tensile test piece, a JIS 14A tensile test piece was sampled from the center of the plate thickness in parallel with the stretching direction and subjected to a tensile test. For toughness, a Charpy impact test of No. 4 specified in JIS-Z2202 was taken from the center of the plate thickness in parallel with the stretching direction, and a Charpy impact test was performed. Impact absorption energy (vE -20 , unit: J) I asked.
【００５０】疲労亀裂進展速度は、図２(a) に示すサー
ボパルサ装置と、図２(b) に示すCT試験片１を用いる疲
労試験法により測定した。図２(a) に示す装置で、参照
番号１はCT試験片、２は試験溶液槽、３は溶液循環ポン
プ、４はロードセル、５は油圧シリンダ、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、
10a および10b は負荷棒をそれぞれ示す。図２(b) に示
すCT試験片には2.5 mmの切り込みが施してあり、その上
下の穴部に負荷棒10a および10b を装着する。本装置に
より、試験溶液槽２中で試験片１に油圧シリンダ５より
負荷棒10a および10b を経由して切り込み先端部に繰り
返し応力を負荷する。試験片は厚さ方向で板厚中心の部
分から切り込みの長手方向が圧延垂直方向に平行になる
ように採取した。The fatigue crack growth rate was measured by the fatigue test method using the servo pulser device shown in FIG. 2 (a) and the CT test piece 1 shown in FIG. 2 (b). In the apparatus shown in FIG. 2 (a), reference numeral 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, 8 is a waveform generator, 9 is a load controller,
10a and 10b indicate load rods, respectively. The CT test piece shown in Fig. 2 (b) has a notch of 2.5 mm, and load rods 10a and 10b are attached to the upper and lower holes. With this device, the test piece 1 in the test solution tank 2 is repeatedly stressed at the tip end of the notch by the hydraulic cylinder 5 via the load rods 10a and 10b. The test piece was sampled from the center of the plate thickness in the thickness direction such that the longitudinal direction of the cut was parallel to the vertical direction of rolling.
【００５１】疲労試験条件は次のとおりとした。 ｆ（繰り返し速度）＝20Hz Ｒ（応力比）＝0.1 Ｔ（試験温度）＝室温 試験雰囲気は大気中。The fatigue test conditions were as follows. f (repeating speed) = 20Hz R (stress ratio) = 0.1 T (test temperature) = room temperature The test atmosphere is air.
【００５２】疲労き裂進展試験の結果、いずれの試験片
の場合も、中ΔＫ領域（ΔＫ：応力拡大係数範囲で最大
応力拡大係数と最小応力拡大係数との差) における疲労
き裂進展速度が評価された。本試験での中ΔＫ領域は
(15〜30MPa √m)疲労き裂進展の第II領域に相当した。As a result of the fatigue crack growth test, the fatigue crack growth rate in the medium ΔK region (ΔK: difference between the maximum stress intensity factor and the minimum stress intensity factor in the stress intensity factor range) was found to be high for all test pieces. Was evaluated. The middle ΔK region in this test is
(15 to 30 MPa √m) Corresponding to the second region of fatigue crack growth.
【００５３】Paris則〔Trans.ASTM,Ser.D.85.523(1963)
〕、すなわち
da/dN＝C(ΔK)m 、ただしΔK ：kN/mm3/2、
da/dN：mm/cycle
が成り立つことが判明した。Paris Rule [Trans.ASTM, Ser.D.85.523 (1963)
], That is, da / dN = C (ΔK) m , where ΔK: kN / mm 3/2 and da / dN: mm / cycle are established.
【００５４】このことから、本発明では、疲労亀裂進展
特性はこの中ΔK 領域のΔK ＝20MPa √m における、亀
裂進展速度da/dn(mm/cycle）で評価した。表３に上記の
調査、測定および疲労試験の結果を示す。表３で、主体
となる組織（面積比で95％以上を占めた組織）欄の符号
Ｂはベイナイト、Ｍはマルテンサイト、Ｆはフェライ
ト、Ｐはパーライトを意味する。Therefore, in the present invention, the fatigue crack growth characteristics were evaluated by the crack growth rate da / dn (mm / cycle) in ΔK = 20 MPa√m in the ΔK region. Table 3 shows the results of the above investigations, measurements and fatigue tests. In Table 3, the sign B in the main structure (structure occupying 95% or more in area ratio) means bainite, M means martensite, F means ferrite, and P means pearlite.
【００５５】[0055]
【表３】 [Table 3]
【００５６】表３に示すように、組織と半価幅が本発明
が規定する条件を満足する試験No.1〜22は、疲労亀裂進
展速度が４×10-5mm/cycle以下と遅く、極めて優れた疲
労亀裂進展抵抗性を有していた。また、鋼の化学組成が
好適範囲にあった試験No.1〜13では、靱性も極めて良好
であった。これに対し、試験No.23 〜27の鋼は、主体と
なる組織やＸ線回折の半価幅が本発明の規定する範囲を
はずれていたものは、疲労亀裂進展速度が３×10-5mm/c
ycleを超えており、所望の疲労亀裂進展抵抗性が得られ
なかった。As shown in Table 3, in the test Nos. 1 to 22 in which the structure and the half width satisfy the conditions defined by the present invention, the fatigue crack growth rate is slow as 4 × 10 -5 mm / cycle or less, It had extremely excellent fatigue crack growth resistance. Further, in Test Nos. 1 to 13 in which the chemical composition of steel was within the suitable range, the toughness was also extremely good. On the other hand, in the steels of Test Nos. 23 to 27, the fatigue crack growth rate was 3 × 10 −5 when the main structure and the half-value width of X-ray diffraction were out of the range specified by the present invention. mm / c
ycle was exceeded, and the desired fatigue crack growth resistance was not obtained.
【００５７】[0057]
【発明の効果】本発明に係る鋼材は、疲労亀裂進展抵抗
性が良好であるうえ、硬質な組織を主体とするものであ
るので、鋼の強度を高めるのが容易である。また、化学
組成の調整により優れた靱性を備えさせることもでき
る。従って船体、土木建設物、建設機械、水圧鉄管、海
洋構造物、ラインパイプなど構造用材料として使用され
る任意の厚さの厚鋼板に好適である。また、本発明の鋼
材は熱間圧延後の冷却制御により容易に製造できるの
で、工業上の価値が大きい。EFFECTS OF THE INVENTION The steel material according to the present invention is excellent in fatigue crack growth resistance and mainly has a hard structure, so that the strength of the steel can be easily increased. Further, it is possible to provide excellent toughness by adjusting the chemical composition. Therefore, it is suitable for thick steel plates of any thickness used as structural materials such as hulls, civil engineering constructions, construction machinery, penstocks, marine structures, and line pipes. Further, the steel material of the present invention can be easily manufactured by controlling the cooling after hot rolling, and thus has great industrial value.
【図１】Ｘ線回折の半価幅測定法を説明するための模式
図であり、図１(a) 、(b) はそれぞれ(200) 面における
回折強度を示すグラフである。FIG. 1 is a schematic diagram for explaining a half width measurement method of X-ray diffraction, and FIGS. 1 (a) and 1 (b) are graphs showing diffraction intensity on a (200) plane, respectively.
【図２】図２(a) はサーボパルサ疲労試験装置の概要を
示す模式図、図２(b) は疲労試験片の形状を示す模式図
である。FIG. 2A is a schematic diagram showing an outline of a servo pulser fatigue test apparatus, and FIG. 2B is a schematic diagram showing the shape of a fatigue test piece.
1：ＣＴ試験片、 2：試験溶液槽、 3：溶液循環ポン
プ、 4：ロードセル、5：油圧シリンダ、 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 秀治 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 (72)発明者 有持 和茂 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 (72)発明者 勝元 弘 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 (72)発明者 長谷川 和彦 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 (72)発明者 瀬田 一郎 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 Ｆターム(参考） 4K032 AA01 AA02 AA11 AA14 AA16 AA19 AA21 AA22 AA23 AA24 AA31 AA35 AA36 BA01 CA02 CA03 CB02 CC03 CC04 CD03 ─────────────────────────────────────────────────── ─── Continued front page (72) Inventor Shuji Okaguchi 4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Inventor Kamo Shige 4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Inventor Hiroshi Katsumoto 4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Inventor Kazuhiko Hasegawa 4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Inventor Ichiro Seta 4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. F-term (reference) 4K032 AA01 AA02 AA11 AA14 AA16 AA19 AA21 AA22 AA23 AA24 AA31 AA35 AA36 BA01 CA02 CA03 CB02 CC03 CC04 CD03
Claims (6)
ルテンサイト、またはベイナイトもしくはマルテンサイ
トで構成され、かつ、（200)面からのＸ線回折強度の半
価幅が0.20度以上である疲労亀裂進展抵抗性に優れた鋼
材。1. A fatigue crack growth resistance whose structure is mainly composed of bainite and martensite, or bainite or martensite, and the half-value width of X-ray diffraction intensity from (200) plane is 0.20 degrees or more. Steel material with excellent properties.
以上、0.15％以下、Si：0.03％以上、0.6 ％以下、Mn：
0.5 ％以上、2.0 ％以下、sol.Al:0.005％超、0.10％以
下、Ｎ：0.0005％以上、0.008 ％以下を含み、残部がFe
および不可避的不純物からなり、かつ、下記式で計算さ
れるFt値が4.0 ％以上、7.5 ％以下を満足する請求項１
に記載の疲労亀裂進展抵抗性に優れた鋼材。 Ft＝3Mn(％)+Cu( ％)+1.5Cr(％)+1.8Ni(％)+1.5Mo(％)2. The chemical composition of steel, in mass%, C: 0.01%
Or more, 0.15% or less, Si: 0.03% or more, 0.6% or less, Mn:
0.5% or more, 2.0% or less, sol.Al: more than 0.005%, 0.10% or less, N: 0.0005% or more, 0.008% or less, the balance Fe
And an unavoidable impurity, and the Ft value calculated by the following formula satisfies 4.0% or more and 7.5% or less.
A steel material having excellent fatigue crack growth resistance described in. Ft = 3Mn (%) + Cu (%) + 1.5Cr (%) + 1.8Ni (%) + 1.5Mo (%)
以上、0.15％以下、Si：0.03％以上、0.6 ％以下、Mn：
0.3 ％以上、2.0 ％以下、Ｂ：0.0003％以上、0.0030％
以下、sol.Al:0.005％超、0.10％以下、Ｎ：0.0005％以
上、0.008 ％以下を含み、残部がFeおよび不可避的不純
物からなり、かつ、下記式で計算されるFt値が3.0 ％以
上、7.0 ％以下を満足する請求項１に記載の疲労亀裂進
展抵抗性に優れた鋼材。 Ft＝3Mn(％)+Cu( ％)+1.5Cr(％)+1.8Ni(％)+1.5Mo(％)3. The chemical composition of steel, in mass%, C: 0.01%
Or more, 0.15% or less, Si: 0.03% or more, 0.6% or less, Mn:
0.3% or more, 2.0% or less, B: 0.0003% or more, 0.0030%
Below, sol.Al: more than 0.005%, 0.10% or less, N: 0.0005% or more, 0.008% or less, the balance consists of Fe and inevitable impurities, and the Ft value calculated by the following formula is 3.0% or more. , 7.0% or less, the steel material excellent in fatigue crack growth resistance according to claim 1. Ft = 3Mn (%) + Cu (%) + 1.5Cr (%) + 1.8Ni (%) + 1.5Mo (%)
0.7 ％未満、Ni：3.0 ％以下、Cr：1.0 ％未満、Mo：0.
8 ％以下からなる群の内の１種または２種以上を含有す
る請求項２または３に記載の疲労亀裂進展抵抗性に優れ
た鋼材。4. The chemical composition of steel is Cu:
Less than 0.7%, Ni: 3.0% or less, Cr: less than 1.0%, Mo: 0.
The steel material excellent in fatigue crack growth resistance according to claim 2 or 3, containing one or more members selected from the group consisting of 8% or less.
0.005 ％以上、0.08％以下、Ti：0.005 ％以上、0.03％
以下、Ｖ：0.005 ％以上、0.08％以下からなる群の内の
１種または２種以上を含有する請求項２〜４のいずれか
に記載の疲労亀裂進展抵抗性に優れた鋼材。5. The chemical composition of steel is Nb:
0.005% or more, 0.08% or less, Ti: 0.005% or more, 0.03%
Hereinafter, the steel material excellent in fatigue crack growth resistance according to any one of claims 2 to 4, which contains one or more members selected from the group consisting of V: 0.005% or more and 0.08% or less.
成を有する鋳造スラブを1000℃〜1250℃に加熱する加熱
工程と、加熱された前記スラブに熱間圧延を施す熱間圧
延工程と、前記熱間圧延をされた鋼に冷却を施す冷却工
程とを備え、前記冷却工程においては、650 ℃〜500 ℃
の間の平均冷却速度を５〜25℃/sとする加速冷却を施
し、該加速冷却を500 ℃〜350 ℃の間で停止し、その
後、復熱温度幅が70℃以下となるようにして冷却を終了
することを特徴とする疲労亀裂進展抵抗性に優れた鋼材
の製造方法。6. A heating step of heating a cast slab having the chemical composition according to claim 2 to 1000 ° C. to 1250 ° C., and a hot rolling step of performing hot rolling on the heated slab. And a cooling step of cooling the hot-rolled steel, in the cooling step, 650 ℃ ~ 500 ℃
The accelerated cooling is performed at an average cooling rate of 5 to 25 ° C / s, stopped at 500 ° C to 350 ° C, and then the recuperation temperature range is 70 ° C or less. A method for producing a steel material having excellent fatigue crack growth resistance, characterized by terminating cooling.
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