JP4132178B2 - PC steel wire or bar with good delayed fracture resistance and manufacturing method thereof - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、耐遅れ破壊特性の優れたＰＣ鋼線または鋼棒およびその製造方法に関するものである。
【０００２】
【従来の技術】
プレストレストコンクリートはコンクリートと鋼材の長所を組み合わせた複合材料であり、ポールやパイルなど多くの用途に利用されている。プレストレストコンクリート用鋼材（以下ＰＣ鋼材と記す）の高強度化に伴い、耐遅れ破壊特性の向上が強く求められている。これまで、高強度で、かつ耐遅れ破壊特性の良いＰＣ鋼材としては、特開昭４８−８８０１８号公報、特開昭５８−１５７９２１号公報、特開平３−２６００１２号公報などに報告されている。これらは主に、Ｓｉ、Ｎｉ、Ｔｉ、Ｂなどの合金元素を添加して強度と耐遅れ破壊特性のバランスを向上させたものであるが、合金添加の分、製造コストの高騰を招くものである。さらに過度の合金添加はスポット溶接部の靱性を低下させるという危惧もある。
【０００３】
一方、特に合金添加をせずに耐遅れ破壊特性を向上させる目的で高温で歪みを与える方法が特公平５−５９９６７号公報に報告されているが、製造法に難点がある。また、金属組織を制御して耐遅れ破壊特性を向上させる技術として、特開平５−２８７４５０号公報、特開平７−３００６５２号公報等の報告があるが、前者はスポット溶接部の表層硬度の上昇を抑える技術に限られており、後者はマルテンサイト１相組織のため効果に限界がる。
【０００４】
【発明が解決しようとする課題】
本発明は、過度の合金添加に頼ることなく、これまでにない金属組織を造り込むことにより、高強度で、かつ耐遅れ破壊特性が良好なＰＣ鋼線または鋼棒とその製造方法を提供することを目的とするものである。
【０００５】
【課題を解決するための手段】
本発明は、上記のような従来のＰＣ鋼材の欠点を有利に排除し得る、高強度で、かつ耐遅れ破壊特性が良好なＰＣ鋼線または鋼棒とその製造方法であり、その要旨とするところは下記のとおりである。
（１）重量％で、
Ｃ：０．２０〜０．４０％、
Ｓｉ：０．０５〜１．５％、
Ｍｎ：０．２〜１．５％、
Ａｌ：０．００２〜０．１０％、
Ｎｂ：０．０１〜０．０７％
を含有し、残部がＦｅおよび不可避的不純物からなる直径４ｍｍ以上１２ｍｍ以下の鋼線または鋼棒で、金属組織の８０％以上がマルテンサイトまたは下部ベイナイトからなる母相中に、焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織が５０％以上を占める帯状の組織が平均で厚み１μｍ以上３０μｍ以下でかつ間隔が平均で１０μｍ以上２００μｍ以内で鋼線または鋼棒の長さ方向と平行に層状に配列されていることを特徴とする耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。
【０００６】
（２）重量％で、
Ｔｉ：０．００２〜０．１０％
を含有することを特徴とする前記（１）記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。
（３）重量％で、
Ｖ：０．００５〜０．１％
を含有することを特徴とする前記（１）または（２）記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。
【０００７】
（４）重量％で、
Ｃｕ：０．０５〜０．５％、
Ｎｉ：０．０５〜１．０％、
Ｃｒ：０．０５〜０．５％、
Ｍｏ：０．０５〜０．５％、
Ｃｏ：０．０５〜０．５％、
Ｗ：０．０５〜０．５％
の１種または２種以上を含有することを特徴とする前記（１）〜（３）のいずれかに記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。
【０００８】
（５）重量％で、
Ｂ：０．０００２〜０．００２５％
を含有することを特徴とする前記（１）〜（４）のいずれかに記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。
（６）重量％で、
Ｒｅｍ：０．００２〜０．１０％、
Ｃａ：０．０００３〜０．００３０％
の１種または２種を含有することを特徴とする前記（１）〜（５）のいずれかに記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。
【０００９】
（７）重量％で、
Ｍｇ：０．０００３〜０．０１％
を含有することを特徴とする前記（１）〜（６）のいずれかに記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。
（８）前記（１）〜（７）のいずれかに記載の化学成分からなる鋼を直径４ｍｍ以上１２ｍｍ以下の鋼線または鋼棒へ熱間圧延する際に、最終線径に至るまでの真歪みが０．８以上となる線径から以後の圧延を８５０℃以下で行い、圧延終了後に１０℃／ｓ以上の冷却速度でＭｓ点以下まで冷却し、その後に加熱速度１０℃／ｓ以上でＡｃ1 点以上Ａｃ1 点＋８０℃以下に加熱し、焼き入れることを特徴とする前記（１）〜（７）のいずれかに記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒の製造方法。
【００１０】
ここで、真歪みεは、ε＝ｌｎ（ｄ0 ／ｄ1 ）で定義される。ただし、ｌｎは自然対数、ｄ0 、ｄ1 はそれぞれ圧延途中の所定の線径と圧延最終線径。
（９）熱間圧延後の直接冷却後、付加的な加工工程、形状調整工程等を経た後に、加熱速度１０℃／ｓ以上でＡｃ1 点以上Ａｃ1 点＋８０℃以下に加熱し、焼き入れることを特徴とする前記（８）記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒の製造方法。
【００１１】
以下、本発明について詳細に説明する。
本発明の根幹をなす技術思想は以下のとおりである。
遅れ破壊は水素に起因して生ずるため、その防止には水素の侵入を抑制することと、水素のトラッブサイトとなる欠陥を低減することなどが有効である。前者のためには、３．５％以上のＮｉを添加して鋼材表面に安定な被膜を生成させ、水素の侵入を抑制する方法が知られている。一方、後者のためには、Ｓｉのような固溶強化元素を多量に添加して焼き入れ時点での強度を上昇させ、その強度上昇量に見合う分を焼戻温度の上昇により低下させることにより、同一強度でかつ種々の格子欠陥が少ない鋼材を得る方法が知られている。
【００１２】
本発明は、このような合金元素の添加に頼るものではなく、金属組織を制御することにより遅れ破壊の亀裂の伝播を遅らせることを意図したものである。一般に遅れ破壊は鋼材の断面方向に亀裂が進み、最終的に破断する破壊現象であり、焼き入れ前の旧オーステナイト結晶粒界が破壊経路となるいわゆる粒界破壊の様相を呈する場合が多い。そのため、鋼材の断面に垂直な方向に組織が配列するいわゆる異方性を有する組織であれば、遅れ破壊の亀裂の鋼材断面方向への進展は阻止され、遅れ破壊自体を抑制することが可能である。特に、通常の母相の組織より高い硬度を有する組織が断面に垂直な方向に配列する場合に、この効果は著しいことを見出した。
【００１３】
以下に、本発明の限定理由について述べる。
Ｃは鋼を強化するのに有効な元素であり、０．２０％未満ではＰＣ鋼線または鋼棒としての十分な強度が得られない。一方、その含有量が０．４０％を超えるとスポット溶接部が硬化し過ぎて割れやすくなる。
Ｓｉは脱酸元素として、また鋼の強化元素として有効であるが、０．０５％未満の含有量ではその効果がない。一方、１．５％を超えるとスポット溶接部の靱性を損なう。
【００１４】
Ｍｎは鋼の強化に有効な元素であり、０．２％未満では十分な効果が得られない。一方、その含有量が１．５％を超えると鋼の加工性を劣化させる。
Ａｌは脱酸元素として添加される。０．００２％未満の含有量ではその効果がなく、０．１０％を超えると鋼の表面性状を損なう。
Ｎｂは熱間圧延においてオーステナイトの再結晶の抑制に有効であることは良く知られているため、０．０１％以上添加する。Ｎｂの効果は０．０７％を超えると飽和するため、添加量は０．０７％以下とする。
【００１５】
Ｔｉは微量の添加で結晶粒の微細化と析出硬化の面で有効に機能するが、過度に添加すると析出脆化を起こす。このため、その添加量の上限を０．１０％とする。一方、添加量が少な過ぎると効果がないため、添加量の下限を０．００２％とする。
Ｖは微量の添加で析出強化をもたらすが、過度に添加すると析出脆化を起こす。このため、その添加量の上限を０．１０％とする。一方、添加量が少な過ぎると効果がないため、添加量の下限を０．００５％とする。
【００１６】
Ｃｕ、Ｎｉ、Ｃｒ、Ｍｏ、Ｃｏ、Ｗはいずれも固溶強化により鋼を強化させる元素である。本発明における場合、その添加により鋼の強度を高めることができるが、過度の量の添加は鋼を硬化させ割れやすくするため、Ｃｕ≦０．５％、Ｎｉ≦１．０％、Ｃｒ≦０．５％、Ｍｏ≦０．５％、Ｃｏ≦０．５％、Ｗ≦０．５％に限定する。また、添加量が少な過ぎると効果がないため、添加量の下限をいずれの元素とも０．０５％とする。
【００１７】
Ｂは鋼の焼入れ性を向上させる元素である。本発明における場合、その添加により鋼の強度を高めることができるが、過度の添加はＢの析出物を増加させて鋼の靱性を損なうため、その添加量の上限を０．００２５％とする。また、添加量が少な過ぎると効果がないため、添加量の下限を０．０００２％とする。
ＲｅｍとＣａはＳの無害化に有効であるが、添加量が少ないとＳが有害のまま残り、過度の添加は酸化物を多く生成せしめて靱性を損なうため、Ｒｅｍ：０．００２〜０．１０％、Ｃａ：０．０００３〜０．００３０％の範囲で添加する。
【００１８】
Ｍｇは微細な酸化物となり鋼の組織を微細化して靱性を向上させる。０．０００３％未満ではその効果がなく、また０．０１％を超えると酸化物を起点とした割れが生じやすくなるため、含有量を０．０００３〜０．０１％の範囲とする。
次に、本発明における鋼線または鋼棒の金属組織の特徴について述べる。
ＰＣ鋼線または鋼棒の強度はＪＩＳにより類別されており、最も高強度のものは１４２０ＭＰａ以上のＴＳが規定されている。このような高強度を得るためには、金属組織の８０％以上をマルテンサイトあるいは下部ベイナイトとする必要がある。
【００１９】
一般に遅れ破壊は亀裂がいわゆる旧オーステナイト結晶粒界を伝播し、破壊に至る場合が多い。そのため、棒・線の軸方向に直角な断面を考えた場合に、断面の直径方向と平行にオーステナイト結晶粒界が向いていれば、亀裂は容易に断面を貫通して最終破断を起こす。逆に、断面の直径方向と直角に、すなわち棒・線の軸方向に平行に亀裂の進展を抑制し得る高硬度の組織が配列していれば、棒・線の表面から発生した亀裂は棒・線の軸方向へ分岐しやすくなり、断面を貫通して最終破断を起こす確率は低くなる。
【００２０】
ＰＣ鋼線または鋼棒の場合は、上述のように母相自体が高強度の焼戻しマルテンサイトであるため、これより高硬度を有する組織は変態ままのマルテンサイトである必要がある。また、残留オーステナイトは、ＰＣ鋼線または鋼棒の使用環境下で応力を負荷された場合に歪み誘起変態によりマルテンサイトヘ変態するため、結果的にはマルテンサイトと同様の効果をもたらす。耐遅れ破壊特性の向上のためには、このように焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織が主体である帯状の組織が母相中に配列する必要があるが、帯状の組織内部で焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織が面積率で５０％未満であると、亀裂の進展抑制効果が小さい。また、その帯状の組織が平均で厚み１μｍ未満であると、やはり亀裂の進展抑制効果が小さい。しかし、この厚みが３０μｍを超えるとＰＣ鋼線または鋼棒自体の脆化を招く。また、帯状の組織の間隔が平均で１０μｍ未満であると、やはり硬質相が多過ぎてＰＣ鋼線または鋼棒自体の脆化を招く。一方、間隔が平均で２００μｍを超えると、亀裂の進展抑制効果が小さい。
【００２１】
図１に本発明鋼の金属組織の例を示す。また、図２には帯状組織内部での焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織の面積率、帯状組織の厚み（板厚方向の幅）と間隔（板厚方向）の測定法の例を示す。
次に、本発明におけるＰＣ鋼線または鋼棒の製造方法の特徴について述べる。
【００２２】
直径を４ｍｍ以上１２ｍｍ以下としたのは、通常使用されるＰＣ鋼線または鋼棒のサイズがほぼこの範囲に収まるためである。
また、最終線径に至るまでの真歪みが０．８以上となる線径から以後の熱間圧延温度を８５０℃以下としたのは、Ｎｂ添加と低温域での圧延の相乗効果によりオーステナイトの動的再結晶および静的再結晶を共に抑制し、かつ０．８以上の歪みを与えることにより十分に変態前の旧オーステナイト粒を延伸せしめ、引き続いて行われる直接冷却過程での変態により圧延方向（鋼線の長さ方向）と平行に延伸した組織を得るためである。
【００２３】
熱間圧延後に直接冷却するが、その際の冷却速度が１０℃／ｓ未満ではマルテンサイトまたは下部ベイナイトを面積率にして８０％以上得ることは困難である。冷却は、十分な量のマルテンサイトまたは下部ベイナイトを得るためにＭｓ点以下までとする。
冷却後に次の工程である熱処理までの間に、必要に応じて伸線、表面加工、形状調整等の付加的な加工工程を経ることは本発明の目的である耐遅れ破壊特性の向上にとって何ら障害となるものではない。
【００２４】
熱処理過程においては、加熱速度１０℃／ｓ以上でＡｃ1 点以上Ａｃ1 点＋８０℃以下に加熱し、焼き入れるが、加熱速度が１０℃／ｓ未満では加熱中に組織の粗大化が起こり、耐遅れ破壊特性が劣化する。また、最高加熱温度がＡｃ1 点未満では逆変態オーステナイトが得られず、加熱後の焼き入れによっても焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織が得られない。また、加熱温度がＡｃ1 点＋８０℃を超えると逆変態オーステナイトが多過ぎて焼入れ性が低下し、焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織が十分に得られない。そこで、加熱温度の範囲をＡｃ1 点以上Ａｃ1 点＋８０℃以下と限定する。
【００２５】
加熱後の焼き入れは可及的速やかに行うことが好ましく、かくして十分な量の焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織を確保できる。
以上の工程を経ることにより、本発明のＰＣ鋼線または鋼棒の金属組織を得ることが可能となる。
【００２６】
【発明の実施の形態】
次に、本発明を実施例に基づいて詳細に説明する。
先ず、表１に示す化学成分の鋼を表２に示す製造条件で同じく表２中に示す線径の鋼線とした。この鋼線の金属組織、硬質相の厚み、間隔、帯状組織内部の焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織の面積率および引張試験結果および遅れ破壊試験結果（ＦＩＰ遅れ破壊試験におけるｆ50時間）を併せて表３、表４（表３のつづき）に示す。
【００２７】
【表１】
【表２】
【表３】
【表４】
【発明の効果】
表３および表４から明らかなように、本発明の鋼線は、いずれもマルテンサイトまたは下部ベイナイトが８０％以上の組織を有し、母相中に、焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織が５０％以上を占める帯状の組織が平均で厚み１μｍ以上３０μｍ以下でかつ間隔が平均で１０μｍ以上２００μｍ以内で鋼線の長さ方向と平行に層状に配列している。このような組織を有する鋼線は十分な強度と伸びを有し、さらに遅れ破壊試験の破断時間も顕著に長い。表２から、このような組織を得るための製造方法は、８５０℃以下の低温域で０．８以上の真歪みで圧延され、さらに圧延終了後に１０℃／ｓ以上の冷却速度でＭｓ点以下まで冷却され、その後に加熱速度１０℃／ｓ以上でＡｃ1 点以上Ａｃ1 点＋８０℃以下に加熱し、焼き入れされたものであることがわかる。また、熱間圧延後の直接冷却と熱処理の間に付加的な伸線工程、表面加工、形状調整工程等を入れても、本発明鋼の特性に何ら悪影響がないこともわかる。
【図面の簡単な説明】
【図１】本発明鋼の金属組織写真の例を示す図である。
【図２】帯状組織内部での焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織の面積率、帯状組織の厚みと間隔の測定法を示す模式図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a PC steel wire or steel rod having excellent delayed fracture resistance and a method for producing the same.
[0002]
[Prior art]
Prestressed concrete is a composite material that combines the advantages of concrete and steel, and is used in many applications such as poles and piles. With the increase in strength of steel materials for prestressed concrete (hereinafter referred to as PC steel materials), improvement in delayed fracture resistance is strongly demanded. Up to now, PC steel materials having high strength and good delayed fracture resistance have been reported in JP-A-48-88018, JP-A-58-157921, JP-A-3-260012, and the like. . These are mainly made by adding alloy elements such as Si, Ni, Ti, B, etc. to improve the balance between strength and delayed fracture resistance. is there. Furthermore, there is a concern that excessive alloy addition may reduce the toughness of the spot weld.
[0003]
On the other hand, a method of imparting strain at a high temperature for the purpose of improving delayed fracture resistance without adding an alloy has been reported in Japanese Patent Publication No. 5-59967, but there is a difficulty in the production method. In addition, as a technique for improving the delayed fracture resistance by controlling the metal structure, there are reports of JP-A-5-287450, JP-A-7-300652 and the like, but the former is an increase in the surface hardness of the spot welded portion. The latter is limited to a technique that suppresses the effect, and the latter has a limited effect because of the martensite single-phase structure.
[0004]
[Problems to be solved by the invention]
The present invention provides a PC steel wire or steel rod having high strength and good delayed fracture resistance and a method for producing the same by building a metal structure unprecedented without relying on excessive alloy addition. It is for the purpose.
[0005]
[Means for Solving the Problems]
The present invention relates to a PC steel wire or steel rod having high strength and good delayed fracture resistance, and a method for producing the same, which can advantageously eliminate the disadvantages of the conventional PC steel materials as described above, and its summary. The place is as follows.
(1) By weight%
C: 0.20 to 0.40%,
Si: 0.05 to 1.5%,
Mn: 0.2 to 1.5%
Al: 0.002 to 0.10%,
Nb: 0.01 to 0.07%
In the parent phase consisting of martensite or lower bainite with a balance of 4 to 12 mm in diameter and comprising a balance of Fe and unavoidable impurities and having a diameter of 4 mm to 12 mm. A band-like structure in which sites or residual austenite or a mixed structure thereof account for 50% or more has an average thickness of 1 μm to 30 μm and an interval of 10 μm to 200 μm on average, and is layered in parallel to the length direction of the steel wire or steel rod PC steel wire or steel rod with good delayed fracture resistance, characterized in that
[0006]
(2) By weight%
Ti: 0.002-0.10%
The PC steel wire or steel bar having good delayed fracture resistance according to (1) above.
(3) By weight%
V: 0.005 to 0.1%
The PC steel wire or steel bar having good delayed fracture resistance according to the above (1) or (2).
[0007]
(4) By weight%
Cu: 0.05 to 0.5%,
Ni: 0.05 to 1.0%,
Cr: 0.05 to 0.5%,
Mo: 0.05-0.5%
Co: 0.05-0.5%
W: 0.05-0.5%
The PC steel wire or the steel rod having good delayed fracture resistance according to any one of the above (1) to (3), comprising one or more of the following.
[0008]
(5)% by weight
B: 0.0002 to 0.0025%
The PC steel wire or steel bar having good delayed fracture resistance according to any one of the above (1) to (4), wherein
(6)% by weight
Rem: 0.002 to 0.10%,
Ca: 0.0003 to 0.0030%
The PC steel wire or steel bar having good delayed fracture resistance according to any one of the above (1) to (5), characterized by containing one or two of the following.
[0009]
(7)% by weight
Mg: 0.0003 to 0.01%
The PC steel wire or steel bar having good delayed fracture resistance according to any one of the above (1) to (6), comprising:
(8) When the steel comprising the chemical component according to any one of the above (1) to (7) is hot-rolled to a steel wire or a steel rod having a diameter of 4 mm or more and 12 mm or less, the truth until the final wire diameter is reached. The subsequent rolling is performed at a temperature of 850 ° C. or less from the wire diameter at which the strain becomes 0.8 or more, and after the rolling is completed, the cooling is performed at a cooling rate of 10 ° C./s or less to the Ms point or less, and then the heating rate is 10 ° C./s or more. The method for producing a PC steel wire or a steel rod with good delayed fracture resistance according to any one of (1) to (7), wherein the steel is heated to Ac 1 point or more and Ac 1 point + 80 ° C. or less and quenched. .
[0010]
Here, the true strain ε is defined by ε = ln (d 0 / d 1 ). Here, ln is a natural logarithm, and d 0 and d 1 are a predetermined wire diameter during rolling and a final rolling wire diameter, respectively.
(9) After direct cooling after hot rolling, after additional processing steps, shape adjustment steps, etc., heat at a heating rate of 10 ° C / s or higher to Ac 1 point or higher and Ac 1 point + 80 ° C or lower and quench. The method for producing a PC steel wire or steel bar having good delayed fracture resistance according to (8) above.
[0011]
Hereinafter, the present invention will be described in detail.
The technical idea forming the basis of the present invention is as follows.
Since delayed fracture is caused by hydrogen, it is effective to prevent hydrogen from entering and to reduce defects that become hydrogen trap sites. For the former, a method is known in which 3.5% or more of Ni is added to form a stable film on the surface of the steel material, thereby suppressing hydrogen intrusion. On the other hand, for the latter, a large amount of a solid solution strengthening element such as Si is added to increase the strength at the time of quenching, and the amount corresponding to the strength increase amount is reduced by increasing the tempering temperature. A method of obtaining a steel material having the same strength and few various lattice defects is known.
[0012]
The present invention does not rely on the addition of such alloy elements, but is intended to delay the propagation of delayed fracture cracks by controlling the metallographic structure. In general, delayed fracture is a fracture phenomenon in which a crack progresses in the cross-sectional direction of the steel material and eventually breaks, and often exhibits a so-called grain boundary fracture mode in which the prior austenite grain boundaries before quenching become fracture paths. Therefore, if the structure has so-called anisotropy in which the structure is arranged in a direction perpendicular to the cross section of the steel material, the progress of the delayed fracture crack in the steel material cross section direction is prevented, and the delayed fracture itself can be suppressed. is there. In particular, it has been found that this effect is remarkable when a structure having a hardness higher than that of a normal matrix structure is arranged in a direction perpendicular to the cross section.
[0013]
The reasons for limiting the present invention will be described below.
C is an element effective for strengthening steel, and if it is less than 0.20%, sufficient strength as a PC steel wire or steel rod cannot be obtained. On the other hand, when the content exceeds 0.40%, the spot welded portion is excessively hardened and easily cracks.
Si is effective as a deoxidizing element and as a strengthening element for steel, but if its content is less than 0.05%, it has no effect. On the other hand, if it exceeds 1.5%, the toughness of the spot weld will be impaired.
[0014]
Mn is an element effective for strengthening steel, and if it is less than 0.2%, a sufficient effect cannot be obtained. On the other hand, when the content exceeds 1.5%, the workability of steel is deteriorated.
Al is added as a deoxidizing element. If the content is less than 0.002%, the effect is not obtained. If the content exceeds 0.10%, the surface properties of the steel are impaired.
Nb is well known to be effective in suppressing recrystallization of austenite during hot rolling, so 0.01% or more is added. Since the effect of Nb is saturated when it exceeds 0.07%, the addition amount is set to 0.07% or less.
[0015]
Ti functions effectively in terms of crystal grain refinement and precipitation hardening when added in a small amount, but excessive addition causes precipitation embrittlement. For this reason, the upper limit of the addition amount is set to 0.10%. On the other hand, since there is no effect if the addition amount is too small, the lower limit of the addition amount is set to 0.002%.
V causes precipitation strengthening when added in a small amount, but excessive addition causes precipitation embrittlement. For this reason, the upper limit of the addition amount is set to 0.10%. On the other hand, since there is no effect if the addition amount is too small, the lower limit of the addition amount is set to 0.005%.
[0016]
Cu, Ni, Cr, Mo, Co, and W are all elements that strengthen the steel by solid solution strengthening. In the case of the present invention, the strength of the steel can be increased by the addition, but an excessive amount of addition hardens the steel and makes it easy to crack, so Cu ≦ 0.5%, Ni ≦ 1.0%, Cr ≦ 0. .5%, Mo ≦ 0.5%, Co ≦ 0.5%, W ≦ 0.5%. Moreover, since there will be no effect if there is too little addition amount, the minimum of addition amount shall be 0.05% with any element.
[0017]
B is an element that improves the hardenability of steel. In the present invention, the addition can increase the strength of the steel, but excessive addition increases the precipitates of B and impairs the toughness of the steel. Therefore, the upper limit of the addition amount is set to 0.0025%. Moreover, since there will be no effect if there is too little addition amount, the minimum of addition amount shall be 0.0002%.
Rem and Ca are effective for detoxification of S, but if the addition amount is small, S remains harmful, and excessive addition produces a large amount of oxides and impairs toughness. 10%, Ca: Add in the range of 0.0003-0.0030%.
[0018]
Mg becomes a fine oxide and refines the structure of steel to improve toughness. If it is less than 0.0003%, the effect is not obtained, and if it exceeds 0.01%, cracks starting from oxides tend to occur, so the content is made 0.0003 to 0.01%.
Next, the characteristics of the metal structure of the steel wire or steel bar in the present invention will be described.
The strength of PC steel wire or steel bar is classified according to JIS, and TS of 1420 MPa or more is defined for the highest strength. In order to obtain such high strength, 80% or more of the metal structure needs to be martensite or lower bainite.
[0019]
In general, delayed fracture often causes cracks to propagate through so-called prior austenite grain boundaries, leading to fracture. Therefore, when a cross section perpendicular to the axial direction of the bar / line is considered, if the austenite grain boundary faces in parallel to the diameter direction of the cross section, the crack easily penetrates the cross section and causes the final fracture. Conversely, if a hard structure that can suppress the growth of cracks is arranged perpendicular to the diameter direction of the cross-section, that is, parallel to the axial direction of the rod / line, the crack generated from the surface of the bar / line is -It becomes easy to branch in the axial direction of the line, and the probability of causing the final break through the cross section is low.
[0020]
In the case of a PC steel wire or a steel bar, since the matrix phase itself is tempered martensite having a high strength as described above, the structure having a higher hardness than this needs to be martensite as transformed. Moreover, since retained austenite is transformed into martensite by strain-induced transformation when stress is applied in an environment where a PC steel wire or a steel rod is used, the same effect as martensite is obtained as a result. In order to improve the delayed fracture resistance, it is necessary to arrange a band-like structure mainly composed of as-quenched martensite or retained austenite or a mixed structure thereof in the matrix. If the martensite or retained austenite or their mixed structure as quenched inside is less than 50% in terms of area ratio, the crack growth suppressing effect is small. In addition, if the band-like structure has an average thickness of less than 1 μm, the effect of suppressing crack growth is also small. However, when the thickness exceeds 30 μm, the PC steel wire or the steel rod itself is brittle. In addition, if the interval between the strip-like structures is less than 10 μm on average, the hard phase is too much to cause embrittlement of the PC steel wire or the steel rod itself. On the other hand, if the interval exceeds 200 μm on average, the crack growth suppressing effect is small.
[0021]
FIG. 1 shows an example of the metal structure of the steel of the present invention. FIG. 2 shows a method of measuring the area ratio of as-quenched martensite or retained austenite or a mixed structure thereof, the thickness (width in the plate thickness direction) and the interval (in the plate thickness direction) of the band structure. An example of
Next, the features of the method for producing a PC steel wire or steel bar in the present invention will be described.
[0022]
The reason why the diameter is set to 4 mm or more and 12 mm or less is that the size of the PC steel wire or the steel rod that is usually used is almost within this range.
Moreover, the subsequent hot rolling temperature was set to 850 ° C. or less from the wire diameter at which the true strain until the final wire diameter reaches 0.8 or more because of the synergistic effect of Nb addition and rolling in the low temperature region. Both the dynamic recrystallization and the static recrystallization are suppressed, and by applying a strain of 0.8 or more, the prior austenite grains before the transformation are sufficiently stretched, and the rolling direction by the transformation in the subsequent direct cooling process is performed. This is to obtain a structure stretched parallel to (the length direction of the steel wire).
[0023]
Although it cools directly after hot rolling, if the cooling rate at that time is less than 10 ° C./s, it is difficult to obtain martensite or lower bainite in an area ratio of 80% or more. Cooling should be below the Ms point to obtain a sufficient amount of martensite or lower bainite.
It is necessary for the improvement of the delayed fracture resistance, which is the object of the present invention, to pass through additional processing steps such as wire drawing, surface processing, and shape adjustment as needed before the heat treatment which is the next step after cooling. It is not an obstacle.
[0024]
In the heat treatment process, heating is performed at a heating rate of 10 ° C./s or more to Ac 1 point or more and Ac 1 point + 80 ° C. or less, and quenching is performed, but if the heating rate is less than 10 ° C./s, the structure becomes coarse during heating, Delayed fracture resistance deteriorates. Further, when the maximum heating temperature is less than Ac 1 point, reverse-transformed austenite cannot be obtained, and as-quenched martensite or residual austenite or a mixed structure thereof cannot be obtained even by quenching after heating. On the other hand, if the heating temperature exceeds Ac 1 point + 80 ° C., the amount of reverse-transformed austenite is too much and the hardenability is lowered, and as-quenched martensite or residual austenite or a mixed structure thereof cannot be obtained sufficiently. Therefore, the range of the heating temperature is limited to Ac 1 point or more and Ac 1 point + 80 ° C. or less.
[0025]
It is preferable that quenching after heating is performed as quickly as possible, and thus a sufficient amount of as-quenched martensite, retained austenite, or a mixed structure thereof can be secured.
By passing through the above process, it becomes possible to obtain the metal structure of the PC steel wire or steel bar of the present invention.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be described in detail based on examples.
First, the steel having the chemical composition shown in Table 1 was made into a steel wire having the diameter shown in Table 2 under the production conditions shown in Table 2. The metal structure of this steel wire, the thickness of the hard phase, the spacing, the area ratio of the as-quenched martensite or retained austenite inside the band structure or their mixed structure, tensile test results and delayed fracture test results (in the FIP delayed fracture test) f 50 hours) are also shown in Tables 3 and 4 (continued in Table 3).
[0027]
[Table 1]
[Table 2]
[Table 3]
[Table 4]
【The invention's effect】
As is apparent from Tables 3 and 4, the steel wire of the present invention has a structure in which martensite or lower bainite is 80% or more, and in the matrix, as-quenched martensite or residual austenite or The band-like structures in which the mixed structure occupies 50% or more have an average thickness of 1 μm to 30 μm and an interval of 10 μm to 200 μm on average, and are arranged in layers in parallel with the length direction of the steel wire. A steel wire having such a structure has sufficient strength and elongation, and further, the fracture time of the delayed fracture test is remarkably long. From Table 2, the manufacturing method for obtaining such a structure is rolled at a true strain of 0.8 or higher in a low temperature region of 850 ° C. or lower, and further, at the cooling rate of 10 ° C./s or higher after the rolling, the Ms point or lower. It was found that the sample was cooled to Ac 1 point to Ac 1 point + 80 ° C. at a heating rate of 10 ° C./s or higher and then quenched. It can also be seen that there is no adverse effect on the properties of the steel of the present invention even if an additional wire drawing step, surface treatment, shape adjustment step, etc. are inserted between the direct cooling after the hot rolling and the heat treatment.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of a metallographic photograph of steel of the present invention.
FIG. 2 is a schematic diagram showing a method for measuring the area ratio of as-quenched martensite or retained austenite or a mixed structure thereof, the thickness of the band structure, and the interval inside the band structure.
Claims (9)
Ｃ：０．２０〜０．４０％、
Ｓｉ：０．０５〜１．５％、
Ｍｎ：０．２〜１．５％、
Ａｌ：０．００２〜０．１０％、
Ｎｂ：０．０１〜０．０７％
を含有し、残部がＦｅおよび不可避的不純物からなる直径４ｍｍ以上１２ｍｍ以下の鋼線または鋼棒で、金属組織の８０％以上がマルテンサイトまたは下部ベイナイトからなる母相中に、焼き入れままのマルテンサイトまたは残留オーステナイトまたはそれらの混合組織が５０％以上を占める帯状の組織が平均で厚み１μｍ以上３０μｍ以下でかつ間隔が平均で１０μｍ以上２００μｍ以内で鋼線または鋼棒の長さ方向と平行に層状に配列されていることを特徴とする耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。% By weight
C: 0.20 to 0.40%,
Si: 0.05 to 1.5%,
Mn: 0.2 to 1.5%
Al: 0.002 to 0.10%,
Nb: 0.01 to 0.07%
In the parent phase consisting of martensite or lower bainite with a balance of 4 to 12 mm in diameter and comprising a balance of Fe and unavoidable impurities and having a diameter of 4 mm to 12 mm. A band-like structure in which sites or residual austenite or a mixed structure thereof account for 50% or more has an average thickness of 1 μm to 30 μm and an interval of 10 μm to 200 μm on average, and is layered in parallel to the length direction of the steel wire or steel rod PC steel wire or steel rod with good delayed fracture resistance, characterized in that
Ｔｉ：０．００２〜０．１０％
を含有することを特徴とする請求項１記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。% By weight
Ti: 0.002-0.10%
The PC steel wire or steel bar having good delayed fracture resistance according to claim 1.
Ｖ：０．００５〜０．１％
を含有することを特徴とする請求項１または２記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。% By weight
V: 0.005 to 0.1%
3. A PC steel wire or a steel bar having good delayed fracture resistance according to claim 1 or 2, characterized by comprising:
Ｃｕ：０．０５〜０．５％、
Ｎｉ：０．０５〜１．０％、
Ｃｒ：０．０５〜０．５％、
Ｍｏ：０．０５〜０．５％、
Ｃｏ：０．０５〜０．５％、
Ｗ：０．０５〜０．５％
の１種または２種以上を含有することを特徴とする請求項１〜３のいずれか１項に記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。% By weight
Cu: 0.05 to 0.5%,
Ni: 0.05 to 1.0%,
Cr: 0.05 to 0.5%,
Mo: 0.05-0.5%
Co: 0.05-0.5%
W: 0.05-0.5%
The PC steel wire or the steel bar having good delayed fracture resistance according to any one of claims 1 to 3, wherein the PC steel wire or the steel rod has good delayed fracture resistance.
Ｂ：０．０００２〜０．００２５％
を含有することを特徴とする請求項１〜４のいずれか１項に記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。% By weight
B: 0.0002 to 0.0025%
The PC steel wire or steel bar having good delayed fracture resistance according to any one of claims 1 to 4.
Ｒｅｍ：０．００２〜０．１０％、
Ｃａ：０．０００３〜０．００３０％
の１種または２種を含有することを特徴とする請求項１〜５のいずれか１項に記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。% By weight
Rem: 0.002 to 0.10%,
Ca: 0.0003 to 0.0030%
The PC steel wire or steel bar having good delayed fracture resistance according to any one of claims 1 to 5, wherein the PC steel wire or the steel rod has good delayed fracture resistance.
Ｍｇ：０．０００３〜０．０１％
を含有することを特徴とする請求項１〜６のいずれか１項に記載の耐遅れ破壊特性の良いＰＣ鋼線または鋼棒。% By weight
Mg: 0.0003 to 0.01%
The PC steel wire or steel bar having good delayed fracture resistance according to any one of claims 1 to 6.
ここで、真歪みεは、ε＝ｌｎ（ｄ0 ／ｄ1 ）で定義される。ただし、ｌｎは自然対数、ｄ0 、ｄ1 はそれぞれ圧延途中の所定の線径と圧延最終線径。When the steel comprising the chemical component according to any one of claims 1 to 7 is hot-rolled to a steel wire or a steel rod having a diameter of 4 mm to 12 mm, the true strain until the final wire diameter is 0. carried from the line diameter of 8 or more subsequent rolling at 850 ° C. or less, rolling end after cooling to Ms point or lower at 10 ° C. / s or more cooling rate, then the heating rate 10 ° C. / s or higher at Ac 1 or more points The method for producing a PC steel wire or a steel rod with good delayed fracture resistance according to any one of claims 1 to 7, wherein the steel is heated to Ac 1 point + 80 ° C or lower and quenched.
Here, the true strain ε is defined by ε = ln (d 0 / d 1 ). Here, ln is a natural logarithm, and d 0 and d 1 are a predetermined wire diameter during rolling and a final rolling wire diameter, respectively.
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| JP6034632B2 (en) | 2012-03-26 | 2016-11-30 | 株式会社神戸製鋼所 | Boron-added steel for high strength bolts and high strength bolts with excellent delayed fracture resistance |
| CN102644036B (en) * | 2012-04-28 | 2013-08-21 | 江苏天舜金属材料集团有限公司 | High-strength reinforcing steel bar with spiral fins and processing method of high-strength reinforcing steel bar |
| CN103060715B (en) | 2013-01-22 | 2015-08-26 | 宝山钢铁股份有限公司 | A kind of ultra-high strength and toughness steel plate and manufacture method thereof with low yielding ratio |
| CN104745940A (en) * | 2015-04-03 | 2015-07-01 | 甘肃酒钢集团宏兴钢铁股份有限公司 | Rust-resistant hot rolled ribbed bar and production process thereof |
| CN113966404B (en) * | 2020-02-24 | 2023-09-15 | 浦项股份有限公司 | Non-heat-treated wire rod having excellent drawability and impact toughness and method for manufacturing the same |
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1998
- 1998-02-18 JP JP03608398A patent/JP4132178B2/en not_active Expired - Fee Related
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