JP2011195944A - Method for producing steel excellent in fatigue-crack propagation resistant characteristic - Google Patents
Method for producing steel excellent in fatigue-crack propagation resistant characteristic Download PDFInfo
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本発明は、船舶、海洋構造物、橋梁、建築物、タンク、ラインパイプなどの鋼構造物に用いられる鋼材の製造方法に関し、特に繰り返し荷重を受けた場合の耐疲労き裂伝播特性に優れた鋼材の製造方法に関する。 The present invention relates to a method for producing steel materials used in steel structures such as ships, offshore structures, bridges, buildings, tanks, line pipes, etc., and is particularly excellent in fatigue crack propagation characteristics when subjected to repeated loads. The present invention relates to a method for manufacturing a steel material.
船舶、海洋構造物、橋梁、建築物、タンク、ラインパイプなどの鋼構造物に用いられる鋼材は、強度、靭性等の機械的性質や溶接性に優れていることに加え、安全性、信頼性確保の観点から耐疲労特性に優れていることが重要である。 Steel materials used in steel structures such as ships, offshore structures, bridges, buildings, tanks, and line pipes have excellent mechanical properties such as strength and toughness and weldability, as well as safety and reliability. From the viewpoint of ensuring, it is important to have excellent fatigue resistance.
構造物の疲労破壊を抑制するためには、応力集中を軽減する等の構造物の設計上の工夫や、溶接止端部の形状の改善や圧縮残留応力の付与等の対策がとられることが多いが、これらによっても十分な効果が得られない場合があり、鋼材自体の疲労き裂伝播を抑制すること、すなわち耐疲労き裂伝播特性の向上が望まれている。 In order to suppress fatigue failure of the structure, measures such as reducing the stress concentration, designing the structure, improving the shape of the weld toe, and applying compressive residual stress may be taken. In many cases, however, sufficient effects may not be obtained, and it is desired to suppress the fatigue crack propagation of the steel material itself, that is, to improve the fatigue crack propagation resistance.
鋼材の耐疲労き裂伝播特性を向上させる方法は、例えば特許文献１、特許文献２等に開示されている。特許文献１には、９００℃以下での圧延に引き続き７５０〜６００℃で圧延、もしくは一旦５００℃以下まで冷却した鋼をＡｃ１点〜Ａｃ３点に再加熱して圧延することにより、細粒フェライトとベイナイトもしくはマルテンサイトの混合組織として疲労強度を向上させることが記載されている。 Methods for improving the fatigue crack propagation characteristics of steel materials are disclosed in, for example, Patent Document 1, Patent Document 2, and the like. Patent Document 1 discloses that fine rolling is performed by rolling at 750 to 600 ° C. after rolling at 900 ° C. or lower, or by reheating steel once cooled to 500 ° C. or lower to Ac 1 point to Ac 3 points. It describes that fatigue strength is improved as a mixed structure of ferrite and bainite or martensite.
また、特許文献２には、Ｎｂ、Ｔｉを添加した鋼を熱間圧延後に、添加元素量から計算される冷却開始温度から５５０℃までを加速冷却することにより疲労き裂伝播抵抗性を向上させることが記載されている。 In Patent Document 2, fatigue crack propagation resistance is improved by accelerating cooling from a cooling start temperature calculated from the amount of added elements to 550 ° C. after hot rolling of steel added with Nb and Ti. It is described.
しかしながら、特許文献１記載の技術は鋼を熱間圧延・冷却した後、再加熱し圧延するものであり、製造工程が煩雑であることから、製造コストが多大でありまた生産能率が劣る。また、特許文献２記載の技術は冷却速度が鋼組成で規定されていないため、冷却条件、成分によっては表面はマルテンサイト、内面はフェライトもしくはベイナイトとなり、板厚方向に強度不均質な鋼材となるため、曲げ加工性などが劣化することが懸念される。 However, the technique described in Patent Document 1 involves hot rolling and cooling of steel, followed by reheating and rolling. Since the manufacturing process is complicated, the manufacturing cost is great and the production efficiency is inferior. In the technique described in Patent Document 2, the cooling rate is not defined by the steel composition, and depending on the cooling conditions and components, the surface becomes martensite and the inner surface becomes ferrite or bainite, resulting in a steel material with non-uniform strength in the thickness direction. Therefore, there is a concern that bending workability and the like deteriorate.
そこで、本発明は、上記問題点を解消し、耐疲労き裂伝播特性に優れた鋼材を生産性良く製造する方法を提供することを目的とする。 Then, this invention aims at solving the said problem and providing the method of manufacturing the steel material excellent in the fatigue crack propagation characteristics with sufficient productivity.
本発明者らは、上記課題を解決すべく実験と検討を重ねた結果、特定量の化学成分を有する鋼を熱間圧延後に、加速冷却してから急速再加熱、加速冷却を適正な条件で実施することにより、鋼組織が微細なフェライトとベイナイトとの混合組織となり、耐疲労き裂伝ぱ特性が向上することを見出した。 As a result of repeated experiments and studies to solve the above-mentioned problems, the present inventors have accelerated and cooled steel having a specific amount of chemical components after hot rolling, followed by rapid reheating and accelerated cooling under appropriate conditions. As a result, it was found that the steel structure becomes a mixed structure of fine ferrite and bainite, and the fatigue crack propagation resistance is improved.
図１は、表１に示す鋼種Ａについて加速冷却後の再加熱温度を変化させて疲労き裂伝播速度を調査したものである。再加熱温度がＡｃ１＋２０℃未満、およびＡｃ３−２０℃を超える場合は、Ａｃ１＋２０℃以上、Ａｃ３−２０℃以下の場合に比べ疲労き裂伝播速度が大きくなっている。なお、このときのミクロ組織は、再加熱温度がＡｃ１＋２０℃以上、Ａｃ３−２０℃以下では微細なフェライトとベイナイトの混合組織であるのに対し、再加熱温度がＡｃ１＋２０℃未満、およびＡｃ３−２０℃を超える場合は比較的粗いフェライトとベイナイトの混合組織であった。 FIG. 1 is an investigation of the fatigue crack propagation rate for steel type A shown in Table 1 by changing the reheating temperature after accelerated cooling. When the reheating temperature is less than Ac 1 + 20 ° C. and exceeds Ac 3 −20 ° C., the fatigue crack propagation rate is higher than that in the case of Ac 1 + 20 ° C. or more and Ac 3 −20 ° C. or less. The microstructure at this time is a mixed structure of fine ferrite and bainite when the reheating temperature is Ac 1 + 20 ° C. or more and Ac 3 −20 ° C. or less, whereas the reheating temperature is less than Ac 1 + 20 ° C., and when it exceeds Ac 3 -20 ° C. was mixed structure of relatively coarse ferrite and bainite.
本発明はこのような知見に基づきなされたものであって、すなわち、本発明は、
１．質量％で、Ｃ：０．０２〜０．２０％、Ｓｉ：０．０５〜０．５％、Ｍｎ：０．６〜２．０％、Ｐ：０．０５％以下、Ｓ：０．０３％以下、Ａｌ：０．０７％以下を含有し、残部Ｆｅと不可避不純物からなる鋼を、１０００℃以上に加熱して熱間圧延を行った後、Ａｒ３以上から６００℃以下４００℃以上の温度域まで２℃／秒以上の冷却速度で加速冷却し、引き続いてＡｃ１＋２０℃以上、Ａｃ３−２０℃以下の温度域まで２℃／秒以上の昇温速度で加熱してから６００℃以下、４００℃以上の温度域まで２℃／秒以上の冷却速度で加速冷却することを特徴とする、耐疲労き裂伝播特性に優れた鋼材の製造方法。
２．鋼組成に、質量％で、さらにＣｕ：０．０５〜１．０％、Ｎｉ：０．０５〜０．８％、Ｃｒ：０．０５〜１．０％、Ｍｏ：０．０１〜１．０％、Ｎｂ：０．００５〜０．１％、Ｖ：０．００５〜０．１％、Ｔｉ：０．００５〜０．０３％の１種または２種以上を含有することを特徴とする、１に記載の、耐疲労き裂伝播特性に優れた鋼材の製造方法。
The present invention has been made based on such knowledge, that is, the present invention
1. In mass%, C: 0.02 to 0.20%, Si: 0.05 to 0.5%, Mn: 0.6 to 2.0%, P: 0.05% or less, S: 0.03 %: Less than, Al: 0.07% or less, steel comprising the balance Fe and inevitable impurities, heated to 1000 ° C. or higher and hot rolled, Ar 3 or higher to 600 ° C. or lower 400 ° C. or higher Accelerated cooling to a temperature range at a cooling rate of 2 ° C./second or higher, and subsequently heating to a temperature range of Ac 1 + 20 ° C. or higher and Ac 3 -20 ° C. or lower at a temperature rising rate of 2 ° C./second or higher, then Hereinafter, a method for producing a steel material having excellent fatigue crack propagation characteristics, characterized by accelerated cooling to a temperature range of 400 ° C. or higher at a cooling rate of 2 ° C./second or higher.
2. In steel composition, in mass%, Cu: 0.05-1.0%, Ni: 0.05-0.8%, Cr: 0.05-1.0%, Mo: 0.01-1. 1 type or 2 types or more of 0%, Nb: 0.005-0.1%, V: 0.005-0.1%, Ti: 0.005-0.03%, It is characterized by the above-mentioned 2. A method for producing a steel material having excellent fatigue crack propagation characteristics according to 1.
本発明によれば、鋼材のミクロ組織が微細なフェライトとベイナイトの混合組織となり、耐疲労き裂伝播特性に優れた鋼材の製造が可能で、産業上極めて有用である。 According to the present invention, the microstructure of the steel material is a mixed structure of fine ferrite and bainite, and it is possible to produce a steel material having excellent fatigue crack propagation characteristics, which is extremely useful industrially.
以下、本発明の化学成分の限定理由、及び製造条件の限定理由について説明する。
（１）成分組成範囲
Ｃ：０．０２〜０．２０％
Ｃは、鋼の強度を確保するために０．０２％以上添加するが、０．２０％を超えて多量に含有させると靭性あるいは溶接性が劣化するため、その範囲を０．０２〜０．２０％とする。
Hereinafter, the reasons for limiting the chemical components of the present invention and the reasons for limiting the production conditions will be described.
(1) Component composition range C: 0.02 to 0.20%
C is added in an amount of 0.02% or more in order to ensure the strength of the steel, but if included in a large amount exceeding 0.20%, the toughness or weldability deteriorates, so the range is 0.02 to 0.00. 20%.
Ｓｉ：０．０５〜０．５％
Ｓｉは、脱酸のために０．０５％以上の添加が必要であるが、０．５％を超えるとＨＡＺ靭性及び溶接性が劣化するため、その範囲を０．０５〜０．５％とする。
Si: 0.05-0.5%
Si needs to be added in an amount of 0.05% or more for deoxidation, but if it exceeds 0.5%, the HAZ toughness and weldability deteriorate, so the range is 0.05-0.5%. To do.
Ｍｎ：０．６〜２．０％
Ｍｎは、鋼材の強度・靭性の向上ならびにＦｅＳの生成抑制のため０．６％以上は必要であるが、２．０％を超える多量の添加は鋼の焼き入れ性の増加を引き起こし、溶接時に硬化層が生成して割れ感受性が高くなるため、その範囲を０．６〜２．０％とする。
Mn: 0.6 to 2.0%
Mn is required to be 0.6% or more in order to improve the strength and toughness of the steel material and to suppress the formation of FeS. However, a large amount of addition exceeding 2.0% causes an increase in the hardenability of the steel. Since a hardened layer is generated and cracking sensitivity is increased, the range is set to 0.6 to 2.0%.
Ｐ：０．０５％以下
Ｐは鋼の靭性を劣化させるため、その含有量はできるだけ低いことが望ましい。このため上限を０．０５％とする。
P: 0.05% or less Since P deteriorates the toughness of steel, its content is desirably as low as possible. For this reason, the upper limit is made 0.05%.
Ｓ：０．０３％以下
Ｓは鋼の靭性を劣化させるため、その含有量はできるだけ低いことが望ましい。このため、上限を０．０３％とする。
S: 0.03% or less Since S deteriorates the toughness of steel, its content is desirably as low as possible. For this reason, the upper limit is made 0.03%.
Ａｌ：０．０７％以下
Ａｌは、脱酸剤として作用し、鋼材の溶鋼脱酸プロセスに於いて、もっとも汎用的に使われる。脱酸のためには０．０１５％以上の添加が好ましいが、０．０７％を超えて含有すると、母材の靭性が低下するとともにＨＡＺ靭性及び溶接性が劣化するため、０．０７％以下に限定する。
Al: 0.07% or less Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process of steel. Addition of 0.015% or more is preferable for deoxidation, but if it exceeds 0.07%, the toughness of the base material decreases and the HAZ toughness and weldability deteriorate, so 0.07% or less Limited to.
本発明は以上を基本成分とし、以下の選択成分群の１種または２種以上を添加する。
（選択成分群）
Ｃｕ：０．０５〜１．０％
Ｃｕは、強度上昇および靭性改善に非常に有効な元素であるが、含有量が０．０５％未満では十分な効果が発揮されず、１．０％を越えると析出硬化が著しくまた鋼材表面に割れが生じやすいため、Ｃｕを添加する場合にはその範囲を０．０５〜１．０％とする。
In the present invention, the above is the basic component, and one or more of the following selected component groups are added.
(Selected ingredient group)
Cu: 0.05 to 1.0%
Cu is an element that is very effective for increasing the strength and improving toughness. However, if the content is less than 0.05%, sufficient effects cannot be exhibited. Since cracking is likely to occur, the range is set to 0.05 to 1.0% when Cu is added.
Ｎｉ：０．０５〜０．８％
Ｎｉは、母材の強度ならびに靭性を向上させる効果を有するが、その含有量が０．０５％未満では十分な効果が得られず、０．８％を超える添加は製品のコストアップにつながるため、Ｎｉを添加する場合にはその範囲を０．０５〜０．８％とする。
Ni: 0.05-0.8%
Ni has the effect of improving the strength and toughness of the base material, but if its content is less than 0.05%, a sufficient effect cannot be obtained, and addition over 0.8% leads to an increase in product cost. When adding Ni, the range is made 0.05 to 0.8%.
Ｃｒ：０．０５〜１．０％
Ｃｒは、焼入性向上に有効な元素であるが、その含有量が０．０５％未満では効果が小さく、１．０％を超えると溶接性やＨＡＺ靭性を劣化させるため、Ｃｒを添加する場合にはその範囲を０．０５〜１．０％とする。
Cr: 0.05-1.0%
Cr is an element effective for improving hardenability, but if its content is less than 0.05%, the effect is small, and if it exceeds 1.0%, weldability and HAZ toughness are deteriorated, so Cr is added. In such a case, the range is 0.05 to 1.0%.
Ｍｏ：０．０１〜１．０％
Ｍｏは、焼入性を高めるとともに焼戻し軟化抵抗を高め、強度上昇に有効であるが、その含有量が０．０１％未満ではその効果が十分に発揮されず、１．０％を超えると溶接性を劣化させるため、Ｍｏを添加する場合にはその範囲を０．０１〜１．０％とする。
Mo: 0.01 to 1.0%
Mo increases hardenability and resistance to temper softening and is effective for increasing the strength. However, if its content is less than 0.01%, its effect is not fully exhibited. When Mo is added, the range is set to 0.01 to 1.0%.
Ｎｂ：０．００５〜０．１％
Ｎｂは、微細炭窒化物の析出効果により強度上昇、靭性向上に有効に作用する元素であるが、その含有量が０．００５％未満では効果が発揮されず、０．１％以上の添加はＨＡＺ靭性の劣化を招くため、Ｎｂを添加する場合にはその範囲を０．００５〜０．１％とする。
Nb: 0.005 to 0.1%
Nb is an element that effectively acts to increase strength and improve toughness due to the precipitation effect of fine carbonitrides, but if its content is less than 0.005%, the effect is not exhibited, and addition of 0.1% or more In order to cause deterioration of HAZ toughness, the range is made 0.005 to 0.1% when Nb is added.
Ｖ：０．００５〜０．１％
Ｖは、少量の添加により焼入性を向上させ、焼戻し軟化抵抗を高める元素であるが、その含有量が０．００５％未満ではその効果が十分に発揮されず、０．１％を超えて添加すると溶接性を劣化させるため、Ｖを添加する場合にはその範囲を０．００５〜０．１％とする。
V: 0.005 to 0.1%
V is an element that improves hardenability by adding a small amount and enhances temper softening resistance, but if its content is less than 0.005%, its effect is not sufficiently exhibited, exceeding 0.1%. When added, the weldability deteriorates, so when adding V, the range is made 0.005 to 0.1%.
Ｔｉ：０．００５〜０．０３％
Ｔｉは、ＴｉＮとして析出し溶接ＨＡＺ部の組織粗大化を抑制してＨＡＺ靱性の向上に寄与する元素である。０．００５％未満のＴｉ添加ではＨＡＺ靱性向上効果が発揮されない。０．０３％を越えて添加すると、溶接の冷却過程でＴｉＣが析出し、ＨＡＺ靱性の劣化を招くため、Ｔｉを添加する場合にはその範囲を０．００５％〜０．０３％の範囲とする。上記成分以外の残部は、Ｆｅ及び不可避不純物である。
Ti: 0.005 to 0.03%
Ti is an element that precipitates as TiN and contributes to the improvement of HAZ toughness by suppressing the coarsening of the welded HAZ part. When Ti is added in an amount of less than 0.005%, the effect of improving the HAZ toughness is not exhibited. If added over 0.03%, TiC precipitates during the cooling process of welding, leading to deterioration of HAZ toughness. Therefore, when adding Ti, the range is 0.005% to 0.03%. To do. The balance other than the above components is Fe and inevitable impurities.
（２）製造方法
上記の成分組成範囲に調整した鋼を、１０００℃以上に加熱して熱間圧延を行った後、Ａｒ３以上から６００℃以下、４００℃以上まで２℃／秒以上の冷却速度で加速冷却し、引続きＡｃ１＋２０℃以上、Ａｃ３−２０℃以下の温度域まで２℃／秒以上の昇温速度で加熱してから２℃／秒以上の冷却速度で加速冷却を行う。
(2) Manufacturing method After the steel adjusted to the above component composition range is heated to 1000 ° C or higher and hot-rolled, cooling is performed at 2 ° C / second or higher from Ar 3 or higher to 600 ° C or lower and 400 ° C or higher. Accelerate cooling at a speed, and then heat up to a temperature range of Ac 1 + 20 ° C. or higher and Ac 3 −20 ° C. or lower at a temperature rising rate of 2 ° C./second or higher, and then perform accelerated cooling at a cooling rate of 2 ° C./second or higher. .
ａ．鋼の加熱温度：１０００℃以上
１０００℃未満の加熱では、良好な熱間加工性が得られない。よって、鋼の加熱温度を１０００℃以上とする。
a. Heating temperature of steel: 1000 ° C. or more and less than 1000 ° C., good hot workability cannot be obtained. Therefore, the heating temperature of steel shall be 1000 degreeC or more.
ｂ．加速冷却開始温度：Ａｒ３以上
加速冷却の開始温度がＡｒ３未満では、加速冷却開始前に粗大なフェライトが生成し、最終的に所望の組織が得られないことに加え、靭性が劣化する。また、冷却待ち時間を要し生産性も低下する。よって、加速冷却の開始温度はＡｒ３以上とする。
b. Accelerated cooling start temperature: Ar 3 or more If the start temperature of accelerated cooling is less than Ar 3 , coarse ferrite is generated before the start of accelerated cooling, and finally a desired structure cannot be obtained, and toughness deteriorates. In addition, a cooling waiting time is required, and productivity is reduced. Therefore, the start temperature of accelerated cooling is set to Ar 3 or higher.
ｃ．Ａｒ３以上からの加速冷却速度：２℃／秒以上
Ａｒ３以上からの加速冷却速度が２℃／秒未満では、加速冷却中に粗大なフェライトが生成し、靭性が劣化することに加え、ベイナイトも生成しにくくなる。よって、Ａｒ３以上からの加速冷却速度は２℃／秒以上とする。
c. Accelerated cooling rate from Ar 3 or higher: 2 ° C./second or higher If the accelerated cooling rate from Ar 3 or higher is lower than 2 ° C./second, coarse ferrite is generated during accelerated cooling, and toughness deteriorates. Is also difficult to generate. Therefore, the accelerated cooling rate from Ar 3 or higher is set to 2 ° C./second or higher.
ｄ．加速冷却停止温度：６００℃以下、４００℃以上
加速冷却の停止温度が６００℃を超えると組織がフェライトとパーライトの混合組織となり、また４００℃未満になるとマルテンサイトが生成し、いずれの場合も再加熱前にフェライトとベイナイトの２相組織が得られない。これらの組織を再加熱−加速冷却しても微細なフェライトとベイナイトの混合組織とならないため、耐疲労き裂伝播特性に優れた鋼材が得られない。よって、加速冷却の停止温度は６００℃以下、４００℃以上とする。
d. Accelerated cooling stop temperature: 600 ° C. or lower, 400 ° C. or higher When the accelerated cooling stop temperature exceeds 600 ° C., the structure becomes a mixed structure of ferrite and pearlite, and when it is lower than 400 ° C., martensite is generated. A two-phase structure of ferrite and bainite cannot be obtained before heating. Even if these structures are reheated-accelerated cooled, a mixed structure of fine ferrite and bainite is not obtained, so that a steel material having excellent fatigue crack propagation characteristics cannot be obtained. Therefore, the accelerated cooling stop temperature is set to 600 ° C. or lower and 400 ° C. or higher.
ｅ．再加熱温度：Ａｃ１＋２０℃以上、Ａｃ３−２０℃以下
本発明は加速冷却後の再加熱により加速冷却ままよりも微細なフェライトとベイナイトの混合組織を得ることをポイントとし、Ａｃ１とＡｃ３の間の温度域に再加熱することにより、加速冷却で得られたフェライトとベイナイト組織の一部が逆変態して一旦オーステナイトになることにより達成される。
e. Reheating temperature: Ac 1 + 20 ° C. or higher, Ac 3 −20 ° C. or lower The point of the present invention is to obtain finer mixed structure of ferrite and bainite than with accelerated cooling by reheating after accelerated cooling. Ac 1 and Ac By reheating to a temperature range between 3 , the ferrite and part of the bainite structure obtained by accelerated cooling are reversely transformed to once become austenite.
再加熱の温度がＡｃ１＋２０℃未満では逆変態の分率が不十分であり、またＡｃ３−２０℃を超えると逆変態の分率が多くなりすぎ、いずれも適正な微細フェライトとベイナイトの混合組織が得られない。なお、逆変態の分率の好ましい範囲は、１５〜８５％である。再加熱温度がＡｃ１＋２０℃未満、あるいはＡｃ３−２０℃を超える場合は、いずれも適正な微細フェライトとベイナイトの混合組織が得られず、前述の図１に示したように、Ａｃ１＋２０℃以上、Ａｃ３−２０℃以下の場合に比べ疲労き裂伝播速度が大きく、耐疲労き裂伝播特性が劣る。よって、再加熱温度はＡｃ１＋２０℃以上、Ａｃ３−２０℃以下とする。 When the reheating temperature is less than Ac 1 + 20 ° C., the fraction of reverse transformation is insufficient, and when it exceeds Ac 3 -20 ° C., the fraction of reverse transformation increases too much. A mixed tissue cannot be obtained. In addition, the preferable range of the fraction of reverse transformation is 15 to 85%. When the reheating temperature is less than Ac 1 + 20 ° C. or more than Ac 3 −20 ° C., an appropriate mixed structure of fine ferrite and bainite cannot be obtained, and as shown in FIG. 1 described above, Ac 1 +20 ° C. or higher, the fatigue crack propagation rate is greater than that of the Ac 3 -20 ° C. or less, fatigue crack propagation characteristics are inferior. Therefore, the reheating temperature is set to Ac 1 + 20 ° C. or higher and Ac 3 −20 ° C. or lower.
ｆ． 再加熱時の加熱速度：２℃／秒以上
再加熱時の加熱速度が２℃／秒未満では、逆変態時に生成するオーステナイトが成長し、最終的に微細なフェライトとベイナイトの混合組織が得られない。したがって、再加熱時の加熱速度は２℃／秒以上とする。
f. Heating rate during reheating: 2 ° C / second or more When the heating rate during reheating is less than 2 ° C / second, austenite generated during reverse transformation grows and finally a fine mixed structure of ferrite and bainite is obtained. Absent. Therefore, the heating rate at the time of reheating is set to 2 ° C./second or more.
ｇ． 再加熱後の加速冷却速度：２℃／秒以上
再加熱後の加速冷却速度が２℃／秒未満ではベイナイトが生成しにくく、冷却後に微細なフェライトとベイナイトの混合組織が得られない。したがって、再加熱後の加速冷却速度は２℃／秒以上とする。
g. Accelerated cooling rate after reheating: 2 ° C./second or more If the accelerated cooling rate after reheating is less than 2 ° C./second, bainite is difficult to form, and a fine mixed structure of ferrite and bainite cannot be obtained after cooling. Therefore, the accelerated cooling rate after reheating is set to 2 ° C./second or more.
ｈ． 再加熱後の加速冷却停止温度：６００℃以下、４００℃以上
再加熱後の加速冷却停止温度が６００℃を超えると組織がフェライトとパーライトの混合組織となり、また４００℃未満になるとマルテンサイトが生成し、いずれの場合も微細なフェライトとベイナイトの混合組織が得られない。したがって、再加熱後の加速冷却停止温度は６００℃以下４００℃以上とする。以下、本発明の作用効果を、実施例をもちいて具体的に説明する。
h. Accelerated cooling stop temperature after reheating: 600 ° C. or lower, 400 ° C. or higher If the accelerated cooling stop temperature after reheating exceeds 600 ° C., the structure becomes a mixed structure of ferrite and pearlite, and if it is lower than 400 ° C., martensite is generated. In either case, a fine mixed structure of ferrite and bainite cannot be obtained. Therefore, the accelerated cooling stop temperature after reheating is set to 600 ° C. or lower and 400 ° C. or higher. Hereinafter, the operation and effect of the present invention will be specifically described with reference to examples.
成分系ならびに圧延、加速冷却、再加熱、および再加熱後の加速冷却条件を変えて製造した鋼材の機械的性質を調べた。表１に供試鋼の化学成分およびＡｃ１、Ａｃ３およびＡｒ３各変態点を示す。表２には、供試鋼の製造条件、逆変態の分率および引張強度、疲労き裂伝播速度を示す。なお、逆変態の分率は、各供試鋼のＡｃ１、Ａｃ３温度と再加熱温度より以下の式にて求めた。 The mechanical properties of steel materials produced by changing the component system and the rolling, accelerated cooling, reheating, and accelerated cooling conditions after reheating were investigated. Table 1 shows the chemical composition of the test steel and the transformation points of Ac 1 , Ac 3 and Ar 3 . Table 2 shows the production conditions of the test steel, the fraction of reverse transformation, the tensile strength, and the fatigue crack propagation rate. Incidentally, the fraction of the reverse transformation is obtained in Ac 1, Ac 3 temperature and the following equation from the reheating temperature of each test steel.
逆変態の分率（％）＝（再加熱温度−Ａｃ１）／（Ａｃ３−Ａｃ１）×１００
疲労き裂伝播速度は、き裂が圧延方向と直交する方向に進展するＬ−Ｔ方向に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。なお、試験片採取にあたり、板厚が２５ｍｍを超えるものは２５ｍｍまで両面減厚した。
Fraction of reverse transformation (%) = (reheating temperature−Ac 1 ) / (Ac 3 −Ac 1 ) × 100
Fatigue crack propagation rate is obtained by taking a CT specimen in the LT direction where the crack propagates in the direction perpendicular to the rolling direction, and conforming to ASTM E647 in a stress ratio of 0.1, frequency of 20 Hz, and room temperature atmosphere. went. In collecting the test piece, the thickness of the plate having a thickness exceeding 25 mm was reduced to 25 mm on both sides.
鋼種Ａ〜Ｈのいずれの成分系も本発明の範囲内である。製造条件が本発明の範囲内である鋼番１〜８はいずれも５×１０−８ｍ／ｃｙｃｌｅ以下の優れた疲労き裂伝播速度を示す。これに対し、加速冷却開始温度、加速冷却速度、加速冷却停止温度、再加熱速度、再加熱温度、再加熱後の加速冷却速度、再加熱後の加速冷却停止温度のいずれかが本発明の範囲外である鋼番９〜１８は本発明例に対し、疲労き裂伝播速度が劣っている。 Any component system of steel types A to H is within the scope of the present invention. Steel Nos. 1 to 8 whose production conditions are within the scope of the present invention all exhibit an excellent fatigue crack propagation rate of 5 × 10 −8 m / cycle or less. On the other hand, any of accelerated cooling start temperature, accelerated cooling rate, accelerated cooling stop temperature, reheating rate, reheating temperature, accelerated cooling rate after reheating, and accelerated cooling stop temperature after reheating is within the scope of the present invention. Steel numbers 9 to 18 which are outside are inferior in fatigue crack propagation rate to the examples of the present invention.
なお、供試鋼は、いずれも、強度、靭性、延性、溶接性、溶接継手特性等が構造用鋼として優れた特性を備えていることが確認された。 In addition, it was confirmed that all of the test steels had excellent properties as structural steel in terms of strength, toughness, ductility, weldability, weld joint characteristics, and the like.
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2015029873A1 (en) | 2013-08-26 | 2015-03-05 | 株式会社神戸製鋼所 | Thick steel sheet having excellent fatigue properties, and method for producing same |
| KR20160144439A (en) | 2014-05-22 | 2016-12-16 | 가부시키가이샤 고베 세이코쇼 | Thick steel plate |
| KR20190028770A (en) | 2016-08-19 | 2019-03-19 | 가부시키가이샤 고베 세이코쇼 | After-treatment steel sheet and its manufacturing method |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2015029873A1 (en) | 2013-08-26 | 2015-03-05 | 株式会社神戸製鋼所 | Thick steel sheet having excellent fatigue properties, and method for producing same |
| KR20160144439A (en) | 2014-05-22 | 2016-12-16 | 가부시키가이샤 고베 세이코쇼 | Thick steel plate |
| KR20180115352A (en) | 2014-05-22 | 2018-10-22 | 가부시키가이샤 고베 세이코쇼 | Thick steel plate |
| KR20190028770A (en) | 2016-08-19 | 2019-03-19 | 가부시키가이샤 고베 세이코쇼 | After-treatment steel sheet and its manufacturing method |
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