JP2008255469A - Fatigue crack propagation delayed steel and its manufacturing method - Google Patents
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Description
本発明は、橋梁、船舶、建築物、海洋構造物、タンク、パイプなど構造安全性が強く求められる溶接構造物や建設、輸送、採掘、掘削などの機器・機械に好適な疲労き裂進展を抑制する鋼材、およびその製造方法に関する。 The present invention provides fatigue crack growth suitable for welded structures such as bridges, ships, buildings, offshore structures, tanks, pipes, and other equipment / machines such as construction, transportation, mining, and excavation. The present invention relates to a steel material to be suppressed and a manufacturing method thereof.
橋梁、船舶、建築物、海洋構造物、タンク、パイプなどの構造物や機器に使用される鋼材は、強度、靭性などの機械的性質や溶接性に優れていることに加え、常時稼動における繰返し荷重や風、地震等による震動に起因する繰返しに対して構造物の構造安全性を確保しなければならない。 Steel materials used in structures and equipment such as bridges, ships, buildings, offshore structures, tanks, pipes, etc. are excellent in mechanical properties such as strength and toughness and weldability, and are also repeatedly operated at all times. The structural safety of the structure must be ensured against repetitive vibration caused by loads, winds, earthquakes, etc.
繰返し荷重は疲労破壊をもたらすため、上記用途に用いられる構造用鋼に対しては疲労特性に優れていることが要求される。一般的に、疲労き裂は溶接部位の止端やルートあるいはスカラップなどの応力集中部から発生し、それが鋼材へと進展して、部材の終局的な破断へと至る。 Since repeated loading causes fatigue failure, the structural steel used in the above applications is required to have excellent fatigue characteristics. In general, a fatigue crack is generated from a stress concentrated portion such as a toe, a root or a scallop at a welded site, and it progresses to a steel material, leading to a final fracture of the member.
疲労き裂の発生に対しては、応力集中を低減することが重要であり、そのような手法としては溶接止端形状の改善（付加溶接、ピーニング処理など）が効果的であることが広く知られている。 It is important to reduce stress concentration for the occurrence of fatigue cracks, and it is widely known that improvement of the weld toe shape (additional welding, peening treatment, etc.) is effective as such a method. It has been.
しかし、数百あるいは数千の溶接部にそのような処理を工業的な規模で実施することは施工時間やコストの観点から非現実的で、そのため、新設された溶接構造物は定期的に検査が行われ、疲労き裂が検出された際には、補修を繰り返して構造安全性を保持していくことが行われるが、このような検査や補修の手間、コストは莫大である。 However, it is impractical to perform such treatment on hundreds or thousands of welds on an industrial scale from the standpoint of construction time and cost, so new welded structures are regularly inspected. When a fatigue crack is detected, repairs are repeated to maintain structural safety. However, the labor and cost of such inspections and repairs are enormous.
そこで、疲労き裂が発生したとしてもそれが部材の破壊をもたらさぬように鋼材自身に疲労き裂進展を抑制する効果を持たせることが、検査や補修の観点からも極めて重要と考えられる。 Thus, it is considered that it is extremely important from the viewpoint of inspection and repair that the steel itself has an effect of suppressing the growth of fatigue cracks so that it does not cause destruction of the member even if a fatigue crack occurs.
一方、構造物の使用される環境には、風雨や飛来塩分に曝されたり、機器・機械の運転により温度上昇が生じることもある。例えば、前者は山間部や海岸付近で敷設された橋梁に相当し、後者はオイルサンドよりスチームインジェクション法によって原油を採取する機器・機械に相当する。 On the other hand, the environment in which the structure is used may be exposed to wind and rain or flying salt, or the temperature may increase due to the operation of equipment / machines. For example, the former corresponds to a bridge laid in a mountainous area or near the coast, and the latter corresponds to a device / machine that collects crude oil from the oil sand by the steam injection method.
このような構造物および機器の構成部材は疲労き裂進展抵抗だけでなく、耐候性や耐熱性も具備していることが望ましい。 It is desirable that such structural members and components of equipment have not only fatigue crack propagation resistance but also weather resistance and heat resistance.
非特許文献１は限られた成分の鋼でラボスケールの特殊な熱処理を繰り返して製造した２種類の鋼材の疲労き裂伝播挙動を論じたものである。 Non-Patent Document 1 discusses the fatigue crack propagation behavior of two types of steel materials manufactured by repeating special heat treatments on a lab scale with limited steel components.
軟質相（ビッカース硬度：１４８）中に硬質相（ビッカース硬度：５６５、分率：３６．４％、平均サイズ：１４９μｍ）を均一分散させた鋼材：Ａと硬質相（ビッカース硬度：５４６、分率：３９．２％）で軟質相（ビッカース硬度：１４９）を網目状に取り囲んだ鋼材：Ｂの疲労き裂伝播特性を調べた結果、鋼材：Ｂの方が疲労き裂伝播速度が大きく低減することが詳細な考察とともに述べられている。 Steel material in which a hard phase (Vickers hardness: 565, fraction: 36.4%, average size: 149 μm) is uniformly dispersed in a soft phase (Vickers hardness: 148): A and a hard phase (Vickers hardness: 546, fraction) : 39.2%) Steel material surrounding the soft phase (Vickers hardness: 149) in a mesh shape: As a result of investigating the fatigue crack propagation characteristics of B: Steel material: B significantly reduces the fatigue crack propagation rate With detailed considerations.
特許文献１にはミクロ組織を硬質部の素地とこの素地に分散した軟質部とで構成し、両者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き裂進展抑制効果を有する鋼板が記載されている。 In Patent Document 1, the microstructure is composed of a base of a hard part and a soft part dispersed in the base, and the hardness difference between the two has a Vickers hardness of 150 or more, and has a fatigue crack growth suppressing effect characterized by A steel sheet is described.
特許文献２にはミクロ組織が軟質相とそれを網目状に囲む硬質第二相からなり、軟質相はフェライト、焼戻しベイナイト、焼戻しマルテンサイトの一種または二種以上で構成され、ビッカース硬度１５０以下であり、硬質第二相は、ベイナイト、マルテンサイト、焼戻しベイナイト、焼戻しマルテンサイトの一種または二種以上で構成され、ビッカース硬度２５０以上であり、硬質第二相の粒界占有率（硬質第二相が占めている粒界長さの総和／総粒界長さ）が０．５以上であることを特徴とする耐疲労き裂伝播特性に優れた厚鋼材が記載されている。 In Patent Document 2, the microstructure is composed of a soft phase and a hard second phase surrounding it in a network, and the soft phase is composed of one or more of ferrite, tempered bainite, and tempered martensite, and has a Vickers hardness of 150 or less. Yes, the hard second phase is composed of one or more of bainite, martensite, tempered bainite, and tempered martensite, has a Vickers hardness of 250 or more, and the grain boundary occupancy of the hard second phase (hard second phase) Describes a thick steel material excellent in fatigue crack propagation resistance, characterized in that the sum of the grain boundary lengths occupied by (3 / total grain boundary length) is 0.5 or more.
特許文献３にはミクロ組織がフェライトと硬質第二相とを含む組織からなり、かつ、鋼板表面に平行な断面組織における硬質第二相が、面積分率：２０〜８０％、ビッカース硬度：２５０〜８００、平均円相当径：１０〜２００μｍで、且つ、硬質第二相間の最大間隔：５００μｍ以下であることを特徴とする疲労強度に優れた厚鋼板が記載されている。
しかしながら、非特許文献１に記載された鋼は５段階の熱処理を必要とするものであり、工場・製品規模で工程生産を行うにはコストや期間の観点から不可能に近い。また、疲労き裂伝播特性と相反して延性が低下しており、このような鋼を構造物へと適用することはできない。 However, the steel described in Non-Patent Document 1 requires five stages of heat treatment, and it is almost impossible to perform process production at a factory / product scale in terms of cost and time. In addition, the ductility decreases in contrast to the fatigue crack propagation characteristics, and such steel cannot be applied to structures.
特許文献１、２記載の発明は非特許文献１と類似する内容で、特許文献１には詳細な製造条件が記載されておらず、特許文献１記載の発明に係る鋼板を製造することは困難を伴う。特許文献２記載の発明に係る鋼の硬質相の粒界占有率は非特許文献１に記述のマルテンサイト組織の連結性（ｃｏｎｎｅｃｔｉｖｉｔｙ）と同意語で、非特許文献１記載の鋼と同様の問題を有している。 The inventions described in Patent Documents 1 and 2 are similar to Non-Patent Document 1, and detailed manufacturing conditions are not described in Patent Document 1, and it is difficult to manufacture the steel sheet according to the invention described in Patent Document 1. Accompanied by. The grain boundary occupancy of the hard phase of the steel according to the invention described in Patent Document 2 is synonymous with the connectivity of the martensite structure described in Non-Patent Document 1, and the same problem as that of the steel described in Non-Patent Document 1 have.
特許文献３は実施例で示しているように鋼材の板厚方向のみでの疲労き裂進展特性を抑制するものであり、鋼板長手方向や幅方向での疲労き裂進展特性の劣化が懸念される。 Patent Document 3 suppresses fatigue crack growth characteristics only in the plate thickness direction of steel as shown in the examples, and there is concern about deterioration of fatigue crack growth characteristics in the longitudinal direction and width direction of the steel sheet. The
そこで、本発明は、上記課題を解決する、疲労き裂進展の抑制に優れる鋼およびその製造方法を提供することを目的とする。 Then, an object of this invention is to provide the steel which is excellent in suppression of fatigue crack growth which solves the said subject, and its manufacturing method.
本発明者等は、疲労き裂進展特性におよぼすミクロ組織形態の影響について鋭意詳細に検討した。 The present inventors diligently examined the influence of the microstructure morphology on the fatigue crack growth characteristics.
図１は、種々の成分および方法で製造した鋼材において、硬質相の平均ビッカース硬度が２５０以上の鋼材を抽出し、硬質相面積分率と圧延直角方向での疲労き裂伝播速度を示したものである（成分、製造方法、試験方法等の詳細については実施例にて記述）。図より、硬質相の硬度と面積分率では、疲労き裂伝播速度は一義的に整理されない。 Fig. 1 shows steel materials produced with various components and methods, and steel materials with an average Vickers hardness of 250 or more in the hard phase extracted, and the hard phase area fraction and fatigue crack propagation rate in the direction perpendicular to the rolling direction are shown. (Details of components, production methods, test methods, etc. are described in the examples). From the figure, the fatigue crack propagation rate is not uniquely organized by the hardness and area fraction of the hard phase.
図２に、さらに図１の試験結果より硬質相と軟質相の平均ビッカース硬度差が１００以上の鋼材を抽出し、硬質相の面積分率で整理を試みた結果を示す。図より、伝播速度は硬質相分率で５０％をピークに凹型の傾向を示すことが認められるが、同一の組織分率においても疲労き裂伝播速度は大きくばらつき、このような指標をもって工程製造することは品質保証の観点からも好ましくない。 FIG. 2 shows a result of further extracting the steel materials having an average Vickers hardness difference of 100 or more between the hard phase and the soft phase from the test results of FIG. 1 and arranging them by the area fraction of the hard phase. From the figure, it can be seen that the propagation velocity shows a concave-shaped tendency with a peak of 50% in the hard phase fraction, but the fatigue crack propagation velocity varies greatly even with the same structure fraction. It is not preferable from the viewpoint of quality assurance.
そこで、本発明者等は、更に検討を進め、硬質相の面積分率［％］と硬質相／軟質相の硬度差：ΔＨｖとの積を用いた場合、疲労き裂伝播特性が整理されることを見出した。 Therefore, the present inventors have further studied and, when the product of the area fraction [%] of the hard phase and the hardness difference of the hard phase / soft phase: ΔHv is used, the fatigue crack propagation characteristics are arranged. I found out.
更に、上記知見を満足する鋼は成分組成を調整した場合、耐候性や耐熱性を具備できること、工業規模で工程生産しうることを見いだした。 Furthermore, it has been found that a steel satisfying the above-described knowledge can have weather resistance and heat resistance when the component composition is adjusted, and can be produced on an industrial scale.
本発明は、得られた知見に更に検討を加えてなされたもので、すなわち、本発明は、
１．質量％で、Ｃ：０．０２〜０．２５％、Ｓｉ：０．０１〜０．５０％、Ｍｎ：０．５〜２．０％、Ｐ：０．１％以下、Ｓ：０．０５％以下、残部が実質的に鉄および不可避的不純物からなり、更に、鋼成分としてＣｕ：０．０１〜１．０％、Ｎｉ：０．０１〜５.０％、Ｃｒ：０．０１〜３．０％、Ｍｏ：０．０１〜１．０％の一種または二種以上を含有し、ミクロ組織が硬質相と軟質相から構成され、（１）式による組織分率パラメータ：ＶＰと、（２）式による硬質相／軟質相の硬度差：ΔＨｖとの積が５０以上であることを特徴とする疲労き裂進展抑制に優れる鋼材。
The present invention has been made by further studying the obtained knowledge, that is, the present invention,
1. In mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, P: 0.1% or less, S: 0.05 %, The balance being substantially composed of iron and inevitable impurities, and further steel components of Cu: 0.01 to 1.0%, Ni: 0.01 to 5.0%, Cr: 0.01 to 3 0.0%, Mo: 0.01 to 1.0%, or one or two or more of them, the microstructure is composed of a hard phase and a soft phase, and the structure fraction parameter according to the formula (1): VP, ( 2) Hard material / soft phase hardness difference according to the formula: A steel material excellent in fatigue crack growth suppression, characterized in that the product of ΔHv is 50 or more.
ＶＰ＝ＶＦＨ／５０ （１）
但し、（１）式は０＜ＶＦＨ≦５０の場合で、５０＜ＶＦＨ＜１００の場合は、
ＶＰ＝（１００−ＶＦＨ）／５０を（１）式とする。ここで、ＶＦＨ：硬質相
の面積分率［％］。
VP = VFH / 50 (1)
However, equation (1) is for 0 <VFH ≦ 50, and for 50 <VFH <100,
Let VP = (100−VFH) / 50 be the expression (1). Where VFH: hard phase
The area fraction of [%].
ΔＨｖ＝ＨｖＨ−ＨｖＳ （２）
但し、ＨｖＨ：硬質相の平均ビッカース硬度、ＨｖＳ：軟質相の平均ビッカー
ス硬度
２．更に、質量％で、Ｎｂ：０．１％以下、Ｖ：０．１％以下、Ｔｉ：０．１％以下、Ｂ：０．００５％以下の一種または二種以上を含有する１に記載の疲労き裂進展抑制に優れる鋼材。
３．１又は２に記載の成分組成と、フェライトと、パーライト、ベイナイト、マルテンサイトの一種または二種以上からなるミクロ組織を有する鋼を、Ａｃ１点以上Ａｃ３点未満に加熱した後に５℃／ｓ以上でＭｓ点以下まで冷却することを特徴とする疲労き裂進展抑制に優れる鋼材の製造方法。
４．冷却した後に、更にＡｃ１点未満で焼戻すことを特徴とする３に記載の疲労き裂進展抑制に優れる鋼材の製造方法。
ΔHv = HvH−HvS (2)
However, HvH: Average Vickers hardness of hard phase, HvS: Average Vicker of soft phase
1. Hardness Further, according to 1 which contains one or two or more of Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, and B: 0.005% or less in mass%. Steel material with excellent fatigue crack growth control.
3. After heating the steel having a microstructure composed of one or more of component composition, ferrite, pearlite, bainite, and martensite to Ac 1 point or more and less than Ac 3 point at 5 ° C. A method for producing a steel material that is excellent in suppressing fatigue crack growth, characterized by cooling to / Ms or less and not more than Ms point.
4). 3. The method for producing a steel material excellent in fatigue crack growth suppression according to 3, wherein the steel is further tempered at less than 1 Ac after cooling.
本発明によれば、疲労き裂進展を抑制した鋼材が得られ、構造物に用いて応力集中部や溶接部等から疲労き裂が発生したとしても、使用過程での疲労き裂進展を遅らせることが可能で、鋼構造物や機械・機器の寿命延伸や補修工程の省力化に繋がる点で、産業上極めて有用である。 According to the present invention, a steel material in which fatigue crack growth is suppressed can be obtained, and even if fatigue cracks are generated from a stress-concentrated part or a welded part in a structure, the fatigue crack propagation in the use process is delayed. This is extremely useful industrially in terms of extending the life of steel structures and machinery / equipment and saving labor in the repair process.
また、鋼成分を適切に調整することで耐候性や耐熱性を付与することも可能であり、鋼構造物や機械・機器の安全性をさらに高めることができる。また、そのような鋼材を、加熱速度、冷却速度、冷却温度、焼戻し温度を適切に制御することで工業規模で生産できる点で、産業発展への寄与が大きい。 In addition, weather resistance and heat resistance can be imparted by appropriately adjusting the steel components, and the safety of steel structures, machines and equipment can be further enhanced. Moreover, the contribution to industrial development is great in that such steel materials can be produced on an industrial scale by appropriately controlling the heating rate, cooling rate, cooling temperature, and tempering temperature.
以下、本発明の鋼材の成分組成と組織形態について詳細に説明する。
１ 成分組成（含有量％は質量％とする。）
Ｃ
Ｃは強度ならびに疲労特性を向上させるための硬質相の面積率を確保するため０．０２％以上添加する。０．２５％を超えて添加すると溶接性が阻害され、延性や曲げ加工性に劣る。好ましくは０．０５％以上０．２０％以下を添加する。
Hereinafter, the component composition and structure of the steel material of the present invention will be described in detail.
1 component composition (content% is mass%)
C
C is added in an amount of 0.02% or more in order to ensure the area ratio of the hard phase for improving strength and fatigue characteristics. If added over 0.25%, weldability is hindered and the ductility and bending workability are poor. Preferably 0.05% or more and 0.20% or less are added.
Ｓｉ
Ｓｉは脱酸と強度を確保するため０．０１％以上添加する。０．５０％を超えて添加すると溶接性、靭性が劣化する。好ましくは０．０５％以上０．４０％以下を添加する。
Si
Si is added in an amount of 0.01% or more to ensure deoxidation and strength. If added over 0.50%, weldability and toughness deteriorate. Preferably 0.05% or more and 0.40% or less are added.
Ｍｎ
Ｍｎは焼入れ性の増加により、強度、靭性を確保するため、０．５％以上添加する。２．０％を超えると溶接性が劣化する。好ましくは０．５％以上１．８％以下を添加する。
Mn
Mn is added in an amount of 0.5% or more in order to ensure strength and toughness by increasing hardenability. If it exceeds 2.0%, the weldability deteriorates. Preferably, 0.5% to 1.8% is added.
Ｐ
Ｐは耐候性の向上に寄与する。しかし、極度の添加は靭性劣化に繋がるため、上限を０．１％とする。好ましくは０．０５％以下とする。
P
P contributes to improvement of weather resistance. However, extreme addition leads to toughness deterioration, so the upper limit is made 0.1%. Preferably it is 0.05% or less.
Ｓ
Ｓは靭性を劣化させるため、上限を０．０５％とする。好ましくは０．０３％以下とする。
S
Since S deteriorates toughness, the upper limit is made 0.05%. Preferably it is 0.03% or less.
Ｃｕ
Ｃｕは固溶を通じて強度を上昇させるとともに耐候性を向上させるので、０．０１％以上添加する。１．０％を超えると溶接性が損なわれ、鋼材製造時に疵が生じやすくなる。好ましくは、０．０１％以上０．８％以下を添加する。
Cu
Cu increases the strength through solid solution and improves the weather resistance, so 0.01% or more is added. If it exceeds 1.0%, the weldability is impaired, and flaws are likely to occur during the production of steel. Preferably, 0.01% or more and 0.8% or less are added.
Ｎｉ
Ｎｉは低温靭性や耐候性を向上させるとともに、Ｃｕを添加した場合の熱間脆性を改善するため、０．０１％以上添加する。添加量が５.０％を超えると溶接性が損なわれ、鋼材コストも上昇する。好ましくは、０．０１％以上４.０％以下とする。
Ni
Ni is added in an amount of 0.01% or more in order to improve low temperature toughness and weather resistance and to improve hot brittleness when Cu is added. If the added amount exceeds 5.0%, the weldability is impaired, and the steel material cost also increases. Preferably, the content is 0.01% or more and 4.0% or less.
Ｃｒ
Ｃｒは焼入れ性の増加や焼戻し軟化抵抗を通じて強度を上昇させ、また耐候性や耐熱性を向上させるので、０．０１％以上添加する。添加する場合、３．０％を超えると溶接性と靭性が損なわれる。好ましくは、０．０１％以上２．５％以下とする。
Cr
Cr increases strength through hardenability and resistance to temper softening and improves weather resistance and heat resistance, so 0.01% or more is added. When adding, if it exceeds 3.0%, weldability and toughness are impaired. Preferably, it is 0.01% or more and 2.5% or less.
Ｍｏ
Ｍｏは焼入れ性の増加や焼戻し軟化抵抗を通じて強度を上昇させ、耐候性、耐熱性を向上させるので、０．０１％以上添加する。添加量が１．０％を超えると溶接性と靭性が損なわれる。好ましくは、０．０１％以上０．８％以下とする。
Mo
Mo increases strength through an increase in hardenability and resistance to temper softening and improves weather resistance and heat resistance, so 0.01% or more is added. If the added amount exceeds 1.0%, weldability and toughness are impaired. Preferably, it is 0.01% or more and 0.8% or less.
以上が本発明に係る鋼の基本成分組成であるが、更に強度、靭性、溶接性の向上などの目的でＮｂ，Ｖ，Ｔｉ，Ｂの一種または二種以上を添加する。 The above is the basic component composition of the steel according to the present invention, and one or more of Nb, V, Ti and B are added for the purpose of improving the strength, toughness and weldability.
Ｎｂ
Ｎｂは圧延・焼入れ時のオーステナイトの細粒化を図ると同時に、焼戻し時に析出し強度を上昇させるので、必要に応じて添加する。添加する場合、０．１％を超えると靭性が損なわれる。好ましくは０．０５％以下とする。
Nb
Nb is intended to make austenite fine during rolling and quenching, and at the same time, precipitates during tempering and increases the strength. Therefore, Nb is added as necessary. When added, if it exceeds 0.1%, the toughness is impaired. Preferably it is 0.05% or less.
Ｖ
Ｖは、圧延・焼入れ時のオーステナイトの細粒化を図るとともに、焼戻し時の析出を通じて強度上昇が図れるため、必要に応じて添加する。０．１％を超えて添加すると溶接性と靭性が損なわれる。好ましくは０．０５％以下とする。
V
V is intended to be fine as austenite during rolling and quenching, and can increase the strength through precipitation during tempering, so V is added as necessary. If added over 0.1%, weldability and toughness are impaired. Preferably it is 0.05% or less.
Ｔｉ
Ｔｉは、強度を上昇させ、溶接部靭性を向上させるので、必要に応じて添加する。添加量が０．１％を超えると鋼材コストの上昇や靱性が劣化する。好ましくは０．０５％以下とする。
Ti
Ti increases strength and improves weld toughness, so it is added as necessary. If the added amount exceeds 0.1%, the steel material cost increases and the toughness deteriorates. Preferably it is 0.05% or less.
Ｂ
Ｂは焼入れ性を高め、強度を上昇させるので、必要に応じて添加する。添加する場合、０．００５％を超えると溶接性が低下する。好ましくは０．００３％以下とする。
B
B increases hardenability and increases strength, so it is added as necessary. When adding, if it exceeds 0.005%, weldability will fall. Preferably it is 0.003% or less.
２．ミクロ組織形態
構成組織
鋼材のミクロ組織は、構成組織を硬質相と軟質相の複合組織とする。鋼材組織が硬質相単相あるいは軟質相単相の場合、疲労き裂進展を抑制することができない。
2. Microstructure Morphology Structure The microstructure of the steel material is a composite structure of a hard phase and a soft phase. When the steel structure is a hard phase single phase or a soft phase single phase, fatigue crack growth cannot be suppressed.
軟質相中に疲労き裂先端が存在し、その前方に硬質相が存在すると、塑性域の拘束などを通じ、疲労き裂が硬質相を避けて屈曲や分岐し進展するようになる。 If a fatigue crack tip is present in the soft phase and a hard phase is present in front of it, the fatigue crack will bend and branch away from the hard phase and propagate through restraint of the plastic region.
このようなき裂の屈曲や分岐は破面粗さ誘起き裂閉口や応力遮蔽効果をもたらして疲労き裂進展駆動力を低下させる。 Such bending or branching of the crack brings about fracture surface roughness-induced crack closure and a stress shielding effect, thereby reducing the fatigue crack growth driving force.
軟質相はフェライト、焼戻しベイナイト、焼戻しマルテンサイトのうち一種または二種以上である。硬質相はパーライト、焼戻しベイナイト、焼戻しマルテンサイト、ベイナイト、マルテンサイトのうち一種または二種以上である。 The soft phase is one or more of ferrite, tempered bainite, and tempered martensite. The hard phase is one or more of pearlite, tempered bainite, tempered martensite, bainite, and martensite.
組織分率および硬質相／軟質相硬度差
本発明では、組織分率パラメータ：ＶＰと硬質相／軟質相の硬度差：ΔＨｖとの積を５０以上とする。
In the present invention, the product of the tissue fraction parameter: VP and the hardness difference between the hard phase / soft phase: ΔHv is 50 or more.
組織分率パラメータ：ＶＰは、前出の図２の傾向を基に以下のように定義し、硬質相の面積分率が５０％に近づくほど１に近づく。
ＶＰ＝ＶＦＨ／５０・・・・・・・・・０＜ＶＦＨ≦５０の場合
＝（１００−ＶＦＨ）／５０・・・５０＜ＶＦＨ＜１００の場合
但し、ＶＦＨ：硬質相の面積分率［％］
また、硬質相／軟質相の硬度差：ΔＨｖを次式で定義する。
ΔＨｖ ＝ＨｖＨ−ＨｖＳ
但し、ＨｖＨ：硬質相の平均ビッカース硬度
ＨｖＳ：軟質相の平均ビッカース硬度
図３に、組織分率パラメータ：ＶＰと硬質相／軟質相の硬度差：ΔＨｖとの積をとり、疲労き裂伝播速度を整理した結果を示す。試験データは前出の図１、図２のものを用いた。
The tissue fraction parameter: VP is defined as follows based on the above-mentioned tendency of FIG. 2, and approaches 1 as the area fraction of the hard phase approaches 50%.
VP = VFH / 50... 0 <VFH ≦ 50 = (100−VFH) / 50... 50 <VFH <100 However, VFH: hard phase area fraction [ %]
Further, the hardness difference: ΔHv between the hard phase and the soft phase is defined by the following equation.
ΔHv = HvH−HvS
Where HvH: average Vickers hardness of the hard phase
HvS: Average Vickers Hardness of Soft Phase FIG. 3 shows the results of organizing the fatigue crack propagation rate by taking the product of the texture fraction parameter: VP and the hardness difference between the hard phase / soft phase: ΔHv. The test data shown in FIGS. 1 and 2 were used.
図より、ＶＰ×ΔＨｖにて疲労き裂伝播速度は一義的に整理可能で、ＶＰ×ΔＨｖが５０以上で安定的に疲労き裂伝播速度が低くなっていることが認められる。 From the figure, it can be seen that the fatigue crack propagation rate can be uniquely arranged by VP × ΔHv, and that the fatigue crack propagation rate is stably reduced when VP × ΔHv is 50 or more.
上記指標で疲労き裂伝播速度が整理された原因としては、疲労き裂進展速度には（１）硬質相に遭遇する頻度と（２）硬質相に遭遇したときに局所的に伝播速度が低下する度合いが相乗的に関与していることが考えられる。 The reasons why fatigue crack propagation speed is organized by the above-mentioned index are as follows: (1) frequency of encountering the hard phase and (2) local decrease in propagation speed when the hard phase is encountered. It is thought that the degree to do is synergistically involved.
そして、それらの組織学的な特徴がそれぞれ（１）組織分率パラメータ：ＶＰと（２）硬質相／軟質相の硬度差：ΔＨｖにより表現されたため、疲労き裂進展速度がこれまでにないほど整理できたものと考えられる。なお、鋼材組織が３相以上からなる場合、ＶＦＨ、ＨｖＨは最も硬い相の分率と硬度、ＨｖＳは最も軟らかい相の硬度である。 Since these histological characteristics are expressed by (1) tissue fraction parameter: VP and (2) hardness difference between hard phase / soft phase: ΔHv, the fatigue crack growth rate is unprecedented. It is thought that it was organized. When the steel structure is composed of three or more phases, VFH and HvH are the fraction and hardness of the hardest phase, and HvS is the hardness of the softest phase.
２．製造方法
本発明に係る鋼材は上記に記載の成分の鋼を、熱間圧延や熱処理で調整し、前組織として、フェライトと、パーライト、ベイナイト、マルテンサイトの１種もしくは２種以上からなる鋼材とした後、Ａｃ１点以上Ａｃ３点未満に加熱し、５℃／ｓ以上でＭｓ点以下まで冷却することにより得られる。
2. Manufacturing method The steel material according to the present invention is prepared by adjusting the steel of the above-described components by hot rolling or heat treatment, and as a pre-structure, a steel material composed of one or more of ferrite, pearlite, bainite, and martensite. Then, it is obtained by heating to Ac 1 point or more and less than Ac 3 point and cooling to 5 ° C./s or more and Ms point or less.
強度、靭性を調整する場合は、更に、冷却後、Ａｃ１点未満で焼戻す。なお、上記温度は鋼材表面温度とし、冷却速度は鋼材の厚さ方向での平均値とする。以下、それらの詳細について記述する。 When adjusting the strength and toughness, the steel is further tempered after cooling at less than 1 Ac. The temperature is the steel surface temperature, and the cooling rate is the average value in the thickness direction of the steel. The details will be described below.
前組織
前組織はフェライトと硬質相からなる組織とし、その硬質相はパーライト、ベイナイト、マルテンサイトの１種もしくは２種以上とする。
Prestructure The prestructure is composed of ferrite and a hard phase, and the hard phase is one or more of pearlite, bainite, and martensite.
このような組織を有する鋼板を、Ａｃ１点以上Ａｃ３点未満に加熱することで、硬質相部分のみをオーステナイトへと逆変態させ、その後の冷却によって、フェライト部は残存したまま、オーステナイト部分のみをベイナイト、マルテンサイト等の硬質相へと変態させ、軟質相と硬質相との硬度差を大きくすることが可能となる。 By heating the steel sheet having such a structure to Ac 1 point or more and less than Ac 3 point, only the hard phase portion is reversely transformed into austenite, and the ferrite portion remains by the subsequent cooling, and only the austenite portion remains. Can be transformed into a hard phase such as bainite or martensite, and the hardness difference between the soft phase and the hard phase can be increased.
なお、本発明では前組織を得るための製造条件は特に規定しない。常法の熱間圧延やノルマ等の熱処理によって調整される。 In the present invention, the production conditions for obtaining the previous structure are not particularly defined. It is adjusted by heat treatment such as normal hot rolling or normal.
加熱・焼入れ
前記前組織を有する鋼をＡｃ１点以上Ａｃ３点未満に加熱した後に５℃／ｓ以上でＭｓ点以下まで冷却する。
Heating and quenching After heating the steel having the previous structure to Ac 1 point or more and less than Ac 3 point, it is cooled to 5 ° C./s or more and Ms point or less.
前組織としてフェライトと硬質相からなる鋼材をＡｃ１点以上に加熱することによって、硬質相をオーステナイトへと変態させる。 By heating a steel material composed of ferrite and a hard phase as a pre-structure to Ac 1 point or more, the hard phase is transformed into austenite.
加熱温度がＡｃ３点を超えると、組織全体がオーステナイトに変態するため、その後の冷却によって硬質相単一組織となる。加熱後にオーステナイト相を硬質相とするために５℃／ｓ以上でＭｓ点以下まで冷却する。 When the heating temperature exceeds Ac 3 points, the entire structure is transformed into austenite, and thus a hard phase single structure is formed by subsequent cooling. In order to make the austenite phase a hard phase after heating, it is cooled to a temperature not lower than 5 ° C./s and not higher than the Ms point.
冷却速度が５℃／ｓ未満の場合、フェライトなどの軟質相の生成が多くなるとともに、硬質相が低硬度となり、疲労進展抑制に対し十分な効果が得られない。 When the cooling rate is less than 5 ° C./s, the generation of a soft phase such as ferrite increases and the hard phase has a low hardness, so that a sufficient effect for suppressing fatigue progress cannot be obtained.
また、冷却停止温度がＭｓ点を上回る場合、硬質相が低硬度となり、疲労進展抑制効果が得られない。 Further, when the cooling stop temperature exceeds the Ms point, the hard phase has a low hardness, and the fatigue progress suppressing effect cannot be obtained.
なお、Ａｃ３点、Ａｃ１点、Ｍｓ点は例えば、Ａｃ３（℃）＝８５４−１８０Ｃ＋４４Ｓｉ−１４Ｍｎ−１７．８Ｎｉ−１．７Ｃｒ、Ａｃ１（℃）＝７２３−１４Ｍｎ＋２２Ｓｉ−１４．４Ｎｉ＋２３．３Ｃｒ、Ｍｓ（℃）＝５１７−３００Ｃ−３３Ｍｎ−２２Ｃｒ−１７Ｎｉ−１１Ｍｏ−１１Ｓｉ（但し、元素記号は鋼材中の各元素の質量％での含有量を表す）で表される関係式により鋼材の成分組成に基づいて導くことが出来る。 Ac 3 point, Ac 1 point, and Ms point are, for example, Ac 3 (° C.) = 854-180C + 44Si-14Mn-17.8Ni-1.7Cr, Ac 1 (° C.) = 723-14Mn + 22Si-14.4Ni + 23.3Cr , Ms (° C.) = 517-300C-33Mn-22Cr-17Ni-11Mo-11Si (wherein the symbol represents the content in mass% of each element in the steel) It can be derived based on the composition.
上記熱処理の後、鋼材の形状補正や延性、靱性の向上の観点から、必要に応じて、冷却後にＡｃ１点未満で焼戻すことができる。但し、焼戻し温度がＡｃ１点を超えると島状マルテンサイトが生成し、延性、靭性が劣化するためＡｃ１点以下とする。 After the heat treatment, from the viewpoint of correcting the shape of the steel material and improving ductility and toughness, it can be tempered after cooling at less than 1 point of Ac if necessary. However, when the tempering temperature exceeds Ac 1 point, island-shaped martensite is generated, and ductility and toughness deteriorate, so that it is set to Ac 1 point or less.
表１に示す成分組成の鋼片にて、表２に示す条件にて板厚１２〜１００ｍｍの供試鋼板を作成し、組織観察、硬さ試験、強度・靭性試験、疲労き裂伝播試験を実施した。尚、一部の供試鋼については耐候性試験、高温引張試験を実施した。 A steel sheet having a thickness of 12 to 100 mm was prepared using the steel pieces having the composition shown in Table 1 under the conditions shown in Table 2, and the structure observation, hardness test, strength / toughness test, and fatigue crack propagation test were performed. Carried out. Some of the test steels were subjected to a weather resistance test and a high temperature tensile test.
組織観察は任意の箇所から採取した試料を研磨したサンプルを用いて、２％ナイタール腐食液によりエッチングした圧延方向に平行な断面の板厚／４位置にて実施した。光学顕微鏡観察により硬質相の面積分率を求めた。面積分率は５視野で実施し、それら総視野での平均値として求めた。 The structure observation was carried out at a thickness of / 4 position of the cross section parallel to the rolling direction etched with a 2% nital etchant using a sample obtained by polishing a sample collected from an arbitrary location. The area fraction of the hard phase was determined by observation with an optical microscope. The area fraction was measured with 5 fields of view, and was determined as an average value in these total fields of view.
硬さ試験は、軟質相と硬質相のビッカース硬度を、上記５視野の観察位置において、各相１０点を荷重０．０９８Ｎ（１０ｇｆ）にて測定した。それら測定値を平均して、軟質相および硬質相の平均ビッカース硬度とした。 In the hardness test, the Vickers hardness of the soft phase and the hard phase was measured at the observation position of the five visual fields with 10 points for each phase at a load of 0.098 N (10 gf). The measured values were averaged to obtain the average Vickers hardness of the soft phase and the hard phase.
強度は圧延方向に直角方向に採取したＪＩＳ Ｚ２２０１ １Ａ号の全厚試験片（板厚５０ｍｍ以上は板厚／４位置でのＪＩＳ Ｚ２２０１ ４号丸棒試験片）により評価した。引張強度（σＴＳ）で４９０ＭＰａ以上、かつ破断伸び（Ｅｌ）で１５％以上を合格とした。 The strength was evaluated by a full thickness test piece of JIS Z2201 1A collected in a direction perpendicular to the rolling direction (JIS Z2201 No. 4 round bar test piece at a thickness of 4 mm for a plate thickness of 50 mm or more). The tensile strength (σ TS ) was 490 MPa or more, and the elongation at break (El) was 15% or more.
靭性は板厚／４位置（板厚２５ｍｍ未満は板厚／２位置）で圧延方向と平行方向に採取したＪＩＳ Ｚ ２２０２のＶノッチシャルピー衝撃試験片により評価した。延性・脆性破面遷移温度（ｖＴｓ）で−２０℃以下を合格とした。 Toughness was evaluated by a V-notch Charpy impact test piece of JIS Z 2202 taken in a direction parallel to the rolling direction at a plate thickness / 4 position (plate thickness / 2 position is less than 25 mm). A ductile / brittle fracture surface transition temperature (vTs) of −20 ° C. or lower was regarded as acceptable.
疲労き裂伝播速度はき裂が圧延直角方向および圧延方向に進展する全厚（板厚２５ｍｍを超えるものは２５ｍｍｔまで片面減厚）のＣＴ試験片を採取し、応力比０．１、周波数２０Ｈｚ、室温大気中でＡＳＴＭ Ｅ６４７に準拠して行った。応力拡大係数範囲（ΔＫ）で２０ＭＰａ√ｍの時の疲労き裂伝播速度が５．０×１０−８ｍ／Ｃｙｃｌｅ以下の場合を合格とした。 Fatigue crack propagation rate was obtained by taking CT specimens with full thickness (thickness reduced to 25mmt when the thickness exceeds 25mm) where the crack propagates in the direction perpendicular to the rolling direction and the rolling direction, stress ratio 0.1, frequency 20Hz. In accordance with ASTM E647 in room temperature atmosphere. The case where the fatigue crack propagation rate when the stress intensity factor range (ΔK) was 20 MPa√m was 5.0 × 10 −8 m / cycle or less was regarded as acceptable.
また、板厚方向への疲労き裂進展速度は全厚（板厚２５ｍｍを超えるものは２５ｍｍｔまで片面減厚）の三点曲げ試験片により、応力比０．１、周波数１０Ｈｚ、室温大気中にて実施した。応力拡大係数範囲（ΔＫ）で２０ＭＰａ√ｍの時の疲労き裂伝播速度が５．０×１０−８ｍ／Ｃｙｃｌｅ以下の場合を合格とした。 In addition, the fatigue crack growth rate in the plate thickness direction was measured at a stress ratio of 0.1, frequency of 10 Hz, and room temperature in the atmosphere using a three-point bending test piece of full thickness (thickness on one side was reduced to 25 mm when the plate thickness exceeded 25 mm). Carried out. The case where the fatigue crack propagation rate when the stress intensity factor range (ΔK) was 20 MPa√m was 5.0 × 10 −8 m / cycle or less was regarded as acceptable.
また、耐候性を向上させることを狙いとして製造した鋼材については、川崎湾岸工業地帯にて大気暴露試験を行った。試験片は１５０ｍｍ角×全厚の板状とし、南向きに３０°傾斜させて暴露した。暴露期間は２年とし、回収した試験片の腐食減量を調査した。腐食減量はＳＳ４００のそれを１とした時に０．５以下である場合を合格とした。 In addition, steel products manufactured with the aim of improving weather resistance were subjected to an atmospheric exposure test in the Kawasaki Bay Industrial Area. The test piece was in the form of a plate of 150 mm square × total thickness, and was exposed by inclining 30 ° southward. The exposure period was 2 years, and the corrosion weight loss of the collected specimens was investigated. Corrosion weight loss was determined to be 0.5 or less when SS400 was taken as 1.
また、耐熱性を向上させることを狙いとして製造した鋼材については、６００℃、大気中において引張試験を行い、耐力（０．２％オフセット耐力）を室温のそれと比較した。６００℃の耐力／室温の耐力が０．５以上の場合を合格とした。 Moreover, about the steel materials manufactured aiming at improving heat resistance, the tension test was performed in 600 degreeC and the air | atmosphere, and the yield strength (0.2% offset yield strength) was compared with that of room temperature. A case where the proof stress at 600 ° C./proof stress at room temperature was 0.5 or more was regarded as acceptable.
組織観察結果を表２、引張、靭性、疲労き裂伝播試験結果を表３に示す。また、大気暴露試験結果・高温引張試験結果を表３にそれぞれ示す。 Table 2 shows the structure observation results, and Table 3 shows the tensile, toughness, and fatigue crack propagation test results. Table 3 shows the atmospheric exposure test results and the high temperature tensile test results.
成分、製造方法、組織を本発明規定範囲内とした板番Ｎｏ．１〜Ｎｏ．１０の鋼板はいずれの方向においても優れた耐疲労き裂進展抵抗を示し、かつ、強度、延性、靭性にも優れていることが確認される。 The plate number No. in which the components, the production method and the structure are within the scope of the present invention. 1-No. It is confirmed that No. 10 steel sheet exhibits excellent fatigue crack growth resistance in any direction and is excellent in strength, ductility, and toughness.
また、Ｎｏ．３、４、９の鋼板については優れた耐候性を兼ね備えている。さらに、Ｎｏ．６、７、８、１０の鋼板については耐熱性に優れていることがわかる。 No. The steel plates 3, 4, and 9 have excellent weather resistance. Furthermore, no. It can be seen that the steel sheets of 6, 7, 8, and 10 are excellent in heat resistance.
これに対し、Ｃ、Ｃｕ、Ｎｉ、Ｃｒが本発明範囲を下回るＮｏ．１１の鋼板は、前組織、熱処理後の組織がともにフェライト単相組織となり、低強度、かつ疲労き裂伝播速度が高い。Ｃ、Ｐ、Ｓが本発明範囲を超えるＮｏ．１２の鋼板は延性、靱性が低い。 On the other hand, C, Cu, Ni, and Cr are lower than the scope of the present invention. In Steel No. 11, both the pre-structure and the structure after heat treatment are ferrite single-phase structures, and have low strength and high fatigue crack propagation rate. No. C, P and S exceeding the scope of the present invention. No. 12 steel sheet has low ductility and toughness.
加熱温度がＡｃ３点を超えるＮｏ．１３の鋼板は焼戻しベイナイト単相組織となり、疲労き裂進展抵抗が劣る。 No. with heating temperature exceeding Ac3 point. Steel plate No. 13 has a tempered bainite single-phase structure and is inferior in fatigue crack growth resistance.
加熱温度がＡｃ１点を下回るＮｏ．１４の鋼板、焼入れ時の冷却速度が本発明下限値を下回るＮｏ．１５の鋼板、焼入れ時の停止温度が本発明上限値を上回るＮｏ．１６の鋼板は硬質相の硬度が低く、結果としてＶＰ×ΔＨｖが５０を下回る。このため、いずれも疲労き裂伝播速度が高い。 No. with heating temperature below Ac1 point. No. 14, the cooling rate during quenching is lower than the lower limit of the present invention. No. 15 steel plate, No. in which the stop temperature during quenching exceeds the upper limit of the present invention. Steel plate No. 16 has a low hardness of the hard phase, and as a result, VP × ΔHv is less than 50. For this reason, all have a high fatigue crack propagation rate.
焼戻し温度が本発明上限値を上回るＮｏ．１７の鋼板は島状マルテンサイトが生じたため延性および靱性が低く、さらにはＶＰ×ΔＨｖが５０を下回るために疲労き裂進展抵抗性に劣る。加熱前の組織がベイナイト単相であるＮｏ．１８の鋼板は硬質相と軟質相の硬度差がほとんどなく、ＶＰ×ΔＨｖが５０を下回るために疲労き裂伝播速度が高い。 No. whose tempering temperature exceeds the upper limit of the present invention. Steel plate No. 17 has low ductility and toughness due to the formation of island-like martensite. Furthermore, since VP × ΔHv is less than 50, it is inferior in fatigue crack growth resistance. No. in which the structure before heating is a bainite single phase. Steel No. 18 has almost no hardness difference between the hard phase and the soft phase, and VP × ΔHv is less than 50, so that the fatigue crack propagation rate is high.
Claims (4)
ＶＰ＝ＶＦＨ／５０ （１）
但し、（１）式は０＜ＶＦＨ≦５０の場合で、５０＜ＶＦＨ＜１００の場合は、
ＶＰ＝（１００−ＶＦＨ）／５０を（１）式とする。ここで、ＶＦＨ：硬質相
の面積分率［％］。
ΔＨｖ＝ＨｖＨ−ＨｖＳ （２）
但し、ＨｖＨ：硬質相の平均ビッカース硬度、ＨｖＳ：軟質相の平均ビッカー
ス硬度 In mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, P: 0.1% or less, S: 0.05 %, The balance being substantially composed of iron and inevitable impurities, and further steel components of Cu: 0.01 to 1.0%, Ni: 0.01 to 5.0%, Cr: 0.01 to 3 0.0%, Mo: 0.01 to 1.0%, or one or two or more of them, the microstructure is composed of a hard phase and a soft phase, and the structure fraction parameter according to the formula (1): VP, ( 2) Hard material / soft phase hardness difference according to the formula: A steel material excellent in fatigue crack growth suppression, characterized in that the product of ΔHv is 50 or more.
VP = VFH / 50 (1)
However, equation (1) is for 0 <VFH ≦ 50, and for 50 <VFH <100,
Let VP = (100−VFH) / 50 be the expression (1). Where VFH: hard phase
The area fraction of [%].
ΔHv = HvH−HvS (2)
However, HvH: Average Vickers hardness of hard phase, HvS: Average Vicker of soft phase
Hardness
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| WO2015099214A1 (en) * | 2013-12-23 | 2015-07-02 | 주식회사 포스코 | Quenched steel sheet having excellent strength and ductility and method for manufacturing same |
| CN105849293A (en) * | 2013-12-23 | 2016-08-10 | Posco公司 | Quenched steel sheet having excellent strength and ductility and method for manufacturing same |
| CN105849293B (en) * | 2013-12-23 | 2017-10-17 | Posco公司 | Intensity and the excellent heat treatment hardening shape steel plate of ductility and its manufacture method |
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| RU2630916C1 (en) * | 2016-10-28 | 2017-09-14 | Открытое акционерное общество по проектированию строительства мостов "Институт Гипростроймост" | Iron-based alloy |
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