JP2003129177A - High-strength pc steel bar superior in delayed fracture resistance, and manufacturing method therefor - Google Patents
High-strength pc steel bar superior in delayed fracture resistance, and manufacturing method thereforInfo
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- JP2003129177A JP2003129177A JP2001321990A JP2001321990A JP2003129177A JP 2003129177 A JP2003129177 A JP 2003129177A JP 2001321990 A JP2001321990 A JP 2001321990A JP 2001321990 A JP2001321990 A JP 2001321990A JP 2003129177 A JP2003129177 A JP 2003129177A
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- Prior art keywords
- delayed fracture
- strength
- bainite
- fracture resistance
- steel
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- Heat Treatment Of Strip Materials And Filament Materials (AREA)
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Abstract
Description
【0001】[0001]
【発明の属する技術分野】本発明は、耐遅れ破壊特性の
優れた鋼材、特に1300MPa以上の引張強度を有す
る、耐遅れ破壊特性の優れたPC鋼棒およびその製造方
法に関するものである。BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having excellent delayed fracture resistance, particularly a PC steel rod having a tensile strength of 1300 MPa or more and excellent delayed fracture resistance, and a method for producing the same.
【0002】[0002]
【従来の技術】PC鋼棒、橋梁用ワイヤ、レールに数多
く使われている高強度パーライト鋼は、引張り強度が1
300MPaを超えると遅れ破壊の危険性が高くなるこ
とが知られており、使用の範囲が限定されている。2. Description of the Related Art High strength pearlite steel, which is widely used for PC steel rods, bridge wires and rails, has a tensile strength of 1
It is known that the risk of delayed fracture increases when the pressure exceeds 300 MPa, and the range of use is limited.
【0003】パーライト鋼の耐遅れ破壊特性を改善する
方法として、例えば特開2000−37332号公報、
特開2000−337333号公報や特開2000−3
37334号公報では、耐遅れ破壊特性の改善にはパー
ライト鋼を強伸線加工することが有効であると提案して
いる。確かにパーライト鋼を強伸線加工すると耐遅れ破
壊特性が改善するが、強伸線加工によりコストが高くな
る問題があり、また強伸線加工が必要なことから、形状
およびサイズも限定される。As a method for improving the delayed fracture resistance of pearlite steel, for example, Japanese Patent Laid-Open No. 2000-37332,
JP-A-2000-337333 and JP-A-2000-3
In Japanese Patent No. 37334, it is proposed that hard drawing of pearlite steel is effective for improving delayed fracture resistance. Certainly, when ferritic steel is subjected to hard wire drawing, the delayed fracture resistance is improved, but there is a problem that high wire drawing cost increases, and the shape and size are also limited because hard wire drawing is required. .
【0004】[0004]
【発明が解決しようとする課題】本発明は上記のような
実情を考慮してなされたものであり、本発明が解決しよ
うとする課題は、遅れ破壊特性が優れた高強度PC鋼
棒、特に遅れ破壊特性が良好で、且つ強度が1300M
Pa以上のパーライト組織が主体のPC鋼棒を提供する
ことである。SUMMARY OF THE INVENTION The present invention has been made in consideration of the above circumstances, and the problem to be solved by the present invention is to provide a high strength PC steel bar excellent in delayed fracture characteristics, particularly Good delayed fracture characteristics and strength of 1300M
The purpose is to provide a PC steel rod mainly composed of a pearlite structure of Pa or more.
【0005】[0005]
【課題を解決するための手段】上記課題を解決するた
め、本発明者らは鋼材組成、組織形態を最適に選択すれ
ば、遅れ破壊特性に優れたPC鋼棒を実現できるという
結論に達し、本発明をなしたものである。In order to solve the above problems, the present inventors have reached the conclusion that a PC steel bar excellent in delayed fracture characteristics can be realized by optimally selecting the steel material composition and microstructure. It is the invention.
【0006】本発明の要旨とするところは下記の通りで
ある。The gist of the present invention is as follows.
【0007】(1)質量%で、C :0.55〜1.2
%を含有し、表面から少なくとも50μm深さまでベイ
ナイトの面積率が30%以上であり、その内部はパーラ
イトの面積率が70%以上であることを特徴とする耐遅
れ破壊特性に優れた高強度PC鋼棒。(1)% by mass, C: 0.55 to 1.2
%, The area ratio of bainite is 30% or more up to a depth of at least 50 μm from the surface, and the area ratio of pearlite is 70% or more in the inside thereof. Steel rod.
【0008】(2)表面から少なくとも50μm深さま
でのベイナイトの最大硬さがビッカース硬度で350〜
450であることを特徴とする前記(1)記載の耐遅れ
破壊特性に優れた高強度PC鋼棒。(2) The maximum hardness of bainite at a depth of at least 50 μm from the surface is 350-Vickers hardness.
The high-strength PC steel rod excellent in delayed fracture resistance as described in (1) above, which is 450.
【0009】(3)質量%でさらに、
Si:0.01〜3%、 Mn:0.1〜2%、
を含有し、さらに、
Cr:0.05〜1.5%、 Al:0.003〜0.1%、
Nb:0.001〜0.05%、Cu:0.01〜4%、
Ni:0.01〜4%、 B :0.0001〜0.005%、
Ti:0.005%以下、 V :0.01〜1.5%、
Mo:0.01〜3%
の1種又は2種以上を含有し、残部がFeおよび不可避
的不純物からなることを特徴とする前記(1)又は
(2)記載の耐遅れ破壊特性に優れた高強度PC鋼棒。(3) In mass%, Si: 0.01 to 3%, Mn: 0.1 to 2% are further contained, and Cr: 0.05 to 1.5%, Al: 0. 003 to 0.1%, Nb: 0.001 to 0.05%, Cu: 0.01 to 4%, Ni: 0.01 to 4%, B: 0.0001 to 0.005%, Ti: 0 0.005% or less, V: 0.01 to 1.5%, Mo: 0.01 to 3%, and one or more of them are contained, and the balance is Fe and inevitable impurities. A high-strength PC steel rod excellent in delayed fracture resistance as described in (1) or (2).
【0010】(4)限界拡散性水素量が0.2ppm以
上であることを特徴とする前記(1)〜(3)のいずれ
か1項に記載の耐遅れ破壊特性に優れた高強度PC鋼
棒。(4) A high-strength PC steel excellent in delayed fracture resistance as set forth in any one of (1) to (3) above, wherein the critical diffusible hydrogen content is 0.2 ppm or more. rod.
【0011】(5)前記(1)〜(4)のいずれか1項
に記載の耐遅れ破壊特性に優れた高強度PC鋼棒を製造
する方法であって、前記(1)又は(3)記載の成分を
有する鋼材を、圧延終了後又は再加熱後、Ac3以上か
らMs〜550℃まで1〜10℃/秒の冷却速度で冷却
することを特徴とする耐遅れ破壊特性に優れた高強度P
C鋼棒の製造方法。(5) A method for producing a high-strength PC steel rod excellent in delayed fracture resistance as set forth in any one of (1) to (4) above, which comprises (1) or (3) above. A steel material having the described components is cooled at a cooling rate of 1 to 10 ° C./sec from Ac 3 or higher to Ms to 550 ° C. after completion of rolling or after reheating, and is excellent in delayed fracture resistance. Strength P
C Steel bar manufacturing method.
【0012】[0012]
【発明の実施の形態】本発明者らは、圧延ままおよび圧
延後加速冷却した種々の強度レベルのPC鋼棒を用い
て、遅れ破壊挙動を詳細に解析した。遅れ破壊は外部環
境から鋼材中に侵入し、鋼材中を室温で拡散し得る拡散
性水素に起因して発生していることは既に明らかであ
る。BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have made detailed analyzes of delayed fracture behavior using PC steel rods with various strength levels that have been subjected to accelerated cooling as-rolled and after rolling. It is already clear that delayed fracture is caused by diffusible hydrogen that penetrates into the steel from the external environment and can diffuse in the steel at room temperature.
【0013】耐遅れ破壊特性は、遅れ破壊が発生しない
限界拡散性水素量を求めることにより評価した。この方
法は、電解水素チャージにより種々のレベルの拡散性水
素量を含有させた後、遅れ破壊試験中に試料中の水素が
大気中に抜けることを抑制するためにCdめっきを施
し、その後、大気中で所定の荷重を負荷し、遅れ破壊が
発生しなくなる拡散性水素量を評価するものである。The delayed fracture resistance was evaluated by determining the amount of critical diffusible hydrogen in which delayed fracture did not occur. In this method, various amounts of diffusible hydrogen are contained by electrolytic hydrogen charging, and then Cd plating is performed to suppress hydrogen in the sample from being released into the atmosphere during the delayed fracture test, and then the atmosphere Among them, the amount of diffusible hydrogen at which a prescribed load is applied and delayed fracture does not occur is evaluated.
【0014】図3に示した水素放出プロファイルは標準
的な鋼材の放出プロファイルであり、拡散性水素量と
は、100℃に見られるピークの面積を積分したものが
拡散性水素量である。The hydrogen release profile shown in FIG. 3 is a standard steel release profile, and the diffusible hydrogen content is the diffusible hydrogen content obtained by integrating the peak area observed at 100 ° C.
【0015】図4に、拡散性水素量と遅れ破壊が発生す
るまでの破断時間の関係について解析した一例を示す。
試料中に含まれる拡散性水素量が少なくなるほど遅れ破
壊が発生するまでの時間が長くなり、拡散性水素量があ
る値以下では遅れ破壊が発生しなくなる。この遅れ破壊
が発生しない上限の拡散性水素量を「限界拡散性水素
量」と定義する。この限界拡散性水素量が高いほど遅れ
破壊特性は良好である。FIG. 4 shows an example of analysis of the relationship between the amount of diffusible hydrogen and the fracture time until delayed fracture occurs.
The smaller the amount of diffusible hydrogen contained in the sample, the longer the time until delayed fracture occurs, and the delayed fracture does not occur when the amount of diffusible hydrogen is below a certain value. The upper limit amount of diffusible hydrogen at which this delayed fracture does not occur is defined as "limit diffusible hydrogen amount". The higher the critical diffusible hydrogen content, the better the delayed fracture property.
【0016】なお、この拡散性水素量は、ガスクロマト
グラフを用いた昇温分析によって測定することができ
る。The amount of diffusible hydrogen can be measured by temperature rising analysis using a gas chromatograph.
【0017】そこで、PC鋼棒の遅れ破壊特性を改善す
べく詳細に検討した結果、遅れ破壊の起点となる表層付
近を耐遅れ破壊特性の優れたベイナイト主体の組織にす
ることで、限界拡散性水素量が増加し耐遅れ破壊特性が
向上する条件を確立した。Therefore, as a result of a detailed study to improve the delayed fracture characteristics of PC steel rods, the critical diffusivity was improved by forming a bainite-based structure excellent in delayed fracture resistance near the surface layer which is the starting point of delayed fracture. The conditions for increasing the hydrogen content and improving the delayed fracture resistance were established.
【0018】まず、本発明の成分の限定理由について説
明する。First, the reasons for limiting the components of the present invention will be explained.
【0019】C:Cは本発明にとって重要な元素であ
り、鋼の強度確保およびパーライト組織形成のために必
須な元素であるが、0.55%未満であるとパーライト
組織形成は困難であり、1.2%を超えると延靭性の低
下が顕著に現れてくるため、0.55〜1.2%に限定
した。C: C is an important element for the present invention, and is an element essential for securing the strength of steel and forming a pearlite structure. However, if it is less than 0.55%, it is difficult to form a pearlite structure. If it exceeds 1.2%, the ductility is remarkably deteriorated, so the content is limited to 0.55 to 1.2%.
【0020】本発明では上記成分のほか、必要に応じて
以下の元素を添加することが好ましい。In the present invention, in addition to the above components, it is preferable to add the following elements, if necessary.
【0021】Si:Siは初析セメンタイトの生成を抑
え、焼入れ性を向上させる効果がある。また、パーライ
トラメラー間のフェライトに固溶し、固溶体硬化によっ
て強度を高める効果がある。しかし、0.01%未満で
はこの強化の効果を得られず、3%を超えると延靭性が
著しく低下するため、0.01〜3%の範囲に限定し
た。Si: Si has the effect of suppressing the formation of pro-eutectoid cementite and improving the hardenability. Further, it has a solid solution in ferrite between pearlite lamellas, and has the effect of increasing the strength by solid solution hardening. However, if it is less than 0.01%, this strengthening effect cannot be obtained, and if it exceeds 3%, the ductility is remarkably reduced, so the range is limited to 0.01-3%.
【0022】Mn:Mnは脱酸、脱硫のために必要であ
るばかりでなく、焼入れ性を向上させるために有効な元
素であるが、0.1%未満であると上記の効果は得られ
ず、2%を超えるとMnの偏析部にマルテンサイトなど
の過冷組織が生成して延靭性が劣化するため、0.1〜
2%に限定した。Mn: Mn is an element effective not only for deoxidizing and desulfurizing but also for improving hardenability, but if it is less than 0.1%, the above effect cannot be obtained. If it exceeds 2%, a supercooled structure such as martensite is generated in the segregated portion of Mn and the ductility is deteriorated.
Limited to 2%.
【0023】Cr:Crはパーライトのラメラー間隔を
微細化して強度を向上させる効果がある。しかし0.0
5%未満では上記効果は得られず、1.5%を超えると
変態終了時間が長くなりすぎて、マルテンサイトなどの
過冷組織が生成して延靭性が低下するため、0.05〜
1.5%の範囲に限定した。Cr: Cr has the effect of reducing the lamellar spacing of pearlite to improve the strength. But 0.0
If it is less than 5%, the above effect cannot be obtained, and if it exceeds 1.5%, the transformation end time becomes too long, and a supercooled structure such as martensite is formed to lower the ductility.
The range was limited to 1.5%.
【0024】Al:Alは脱酸およびAlNを形成する
ことによりオーステナイト粒の粗大化を防止する効果を
有しているが、0.003%未満では上記効果は発揮さ
れず、0.1%を超えると効果が飽和するため、0.0
03〜0.1%の範囲に限定した。
Nb:Nbは炭窒化物を生成し、遅れ破壊特性の向上を
もたらすが、0.001%未満であると上記効果は不十
分であり、0.05%を超えるとこの効果が飽和するた
め、0.001〜0.05%の範囲に限定した。Al: Al has an effect of preventing coarsening of austenite grains by deoxidizing and forming AlN, but if it is less than 0.003%, the above effect is not exhibited and 0.1% is used. If it exceeds, the effect will be saturated, so 0.0
It was limited to the range of 03-0.1%. Nb: Nb forms carbonitrides and improves delayed fracture characteristics, but if it is less than 0.001%, the above effect is insufficient, and if it exceeds 0.05%, this effect saturates. It was limited to the range of 0.001 to 0.05%.
【0025】Cu:Cuは焼入れ性の向上と析出効果に
よる高強度化が図れるが、0.01%未満だと上記効果
は得られず、4%を超えると粒界脆化を起こして耐遅れ
破壊特性を劣化させるため、0.01〜4%の範囲に限
定した。Cu: Cu improves the hardenability and enhances the strength due to the precipitation effect, but if it is less than 0.01%, the above effect cannot be obtained, and if it exceeds 4%, grain boundary embrittlement occurs and delay resistance is delayed. In order to deteriorate the fracture characteristics, the range is limited to 0.01 to 4%.
【0026】Ni:Niは延靭性を向上する効果がある
が、0.01%未満であると上記効果が得られず、4%
を超えるとCrと同様に変態終了時間が長くなりすぎ
て、マルテンサイトなどの過冷組織が生成して延靭性が
低下するため、0.01〜4%の範囲に限定した。Ni: Ni has an effect of improving ductility, but if it is less than 0.01%, the above effect cannot be obtained, and it is 4%.
If it exceeds 0.1%, the transformation end time becomes too long, and a supercooled structure such as martensite is generated to lower the ductility, as in the case of Cr. Therefore, the content is limited to the range of 0.01 to 4%.
【0027】B:Bは粒界破壊を抑制し、遅れ破壊特性
を向上させる効果がある。さらに、Bはオーステナイト
粒界に偏析することにより焼入れ性を著しく高めるが、
0.0001%未満であると上記効果が得られず、0.
005%を超えても効果が飽和するため、0.0001
〜0.005%の範囲に限定した。B: B has the effect of suppressing grain boundary fracture and improving delayed fracture characteristics. Further, B segregates at the austenite grain boundaries to remarkably enhance the hardenability,
If it is less than 0.0001%, the above effect cannot be obtained, and
Even if it exceeds 005%, the effect is saturated, so 0.0001
The range is limited to 0.005%.
【0028】V:Vは焼入れ性を向上させる元素である
と共に、炭化物または窒化物あるいはその複合析出物を
形成し、この析出物が水素トラップサイトとなり、遅れ
破壊特性が向上する。しかし、0.01%未満である
と、この水素トラップの効果はほとんど得られず、1.
5%を超えてもその添加量に見合う効果が得られないた
め、0.01〜1.5%の範囲に限定した。V: V is an element that improves the hardenability and forms carbides or nitrides or their composite precipitates, and these precipitates serve as hydrogen trap sites to improve delayed fracture characteristics. However, if it is less than 0.01%, the effect of this hydrogen trap is hardly obtained, and
Even if it exceeds 5%, the effect corresponding to the added amount cannot be obtained, so the range is set to 0.01 to 1.5%.
【0029】Mo:MoもVと同様に焼入れ性を向上さ
せる元素であると共に、炭化物または窒化物あるいはそ
の複合析出物を形成し、この析出物が水素トラップサイ
トとなり、遅れ破壊特性が向上する。しかし0.01%
未満であると、この水素トラップの効果はほとんど得ら
れず、3%を超えてもその添加量に見合う効果が得られ
ないため、0.01〜3%の範囲に限定した。Mo: Mo is an element that improves the hardenability similarly to V, and forms a carbide or a nitride or a composite precipitate thereof, which serves as a hydrogen trap site, and the delayed fracture property is improved. But 0.01%
If it is less than the above, the effect of the hydrogen trap is hardly obtained, and if it exceeds 3%, the effect corresponding to the added amount cannot be obtained, so the range is set to 0.01 to 3%.
【0030】不純物であるP,SおよびNについては特
に制限しないものの、遅れ破壊特性を向上させる観点か
ら、それぞれ0.015%以下が好ましい範囲である。Impurities P, S and N are not particularly limited, but from the viewpoint of improving delayed fracture characteristics, 0.015% or less is a preferable range.
【0031】次に、本発明の組織形態について説明す
る。Next, the structure of the present invention will be described.
【0032】棒鋼の長手方向に垂直なC断面において、
表層にベイナイト組織を形成させると遅れ破壊特性が向
上する。その深さが表層から50μm未満であると遅れ
破壊特性が向上の効果が無いため、表層ベイナイトの深
さを少なくとも表層から50μmに限定した。なお、ベ
イナイト組織の深さが500μmを超えても遅れ破壊特
性の向上の効果は飽和し、強度が低下してしまうため、
好ましくは500μm以下がよい。ベイナイトの面積率
は遅れ破壊特性向上のため、30%以上とする。この表
層の組織はベイナイトを100%としてもよいが、残部
は破壊靭性の確保のためにフェライト、パーライトの1
種又は2種を含有する組織で、パーライトの面積率が7
0%以上であることが表層硬度の確保の点で好ましい。In the C section perpendicular to the longitudinal direction of the steel bar,
When a bainite structure is formed in the surface layer, delayed fracture characteristics are improved. If the depth is less than 50 μm from the surface layer, there is no effect of improving the delayed fracture property, so the depth of the surface bainite was limited to at least 50 μm from the surface layer. Even if the bainite structure has a depth of more than 500 μm, the effect of improving the delayed fracture property is saturated and the strength is reduced.
It is preferably 500 μm or less. The area ratio of bainite is set to 30% or more in order to improve delayed fracture characteristics. The surface layer structure may be made of bainite at 100%, but the balance is 1% of ferrite or pearlite in order to secure fracture toughness.
The area ratio of pearlite is 7 in a tissue containing two or more species.
It is preferably 0% or more from the viewpoint of ensuring surface hardness.
【0033】尚、ベイナイトの面積率は表層〜500μ
mの間を光学顕微鏡により100〜500倍で連続的に
撮影し、この表層〜500μmの連続組織を5〜10視
野観察し、面積率により求めた値と定義する。The area ratio of bainite is from the surface layer to 500 μm.
The area between m is continuously photographed at 100 to 500 times with an optical microscope, the continuous structure of the surface layer to 500 μm is observed in 5 to 10 fields of view, and it is defined as the value obtained by the area ratio.
【0034】また、その内部は強度確保のため、パーラ
イトを面積率で70%以上とすることが必要である。内
部はパーライトを100%としてもよいが、残部の組織
はフェライト、ベイナイトの1種または2種を30%以
下含有することが遅れ破壊特性、破壊靭性確保の点で好
ましい。In order to secure the strength of the inside, it is necessary that the area ratio of pearlite is 70% or more. Although pearlite may be 100% in the interior, it is preferable that the remaining structure contains 30% or less of one or two kinds of ferrite and bainite in terms of delayed fracture characteristics and fracture toughness.
【0035】尚、フェライト、パーライトの面積率はベ
イナイトの面積率と同様の方法で測定した値と定義す
る。The area ratio of ferrite and pearlite is defined as a value measured by the same method as the area ratio of bainite.
【0036】また、表層をベイナイト組織にすると共に
このベイナイト組織中の強度レベルをビッカース硬さで
450以下に調整することによって、更に遅れ破壊特性
が向上するため、表層のベイナイト組織中の最大硬さを
ビッカース硬さで450以下とすることが好ましい。一
方、疲労特性確保のためには350以上とすることが好
ましい。Further, by making the surface layer a bainite structure and adjusting the strength level in this bainite structure to 450 or less by Vickers hardness, the delayed fracture property is further improved, so that the maximum hardness in the bainite structure of the surface layer is improved. Is preferably 450 or less in Vickers hardness. On the other hand, it is preferably 350 or more in order to secure fatigue characteristics.
【0037】尚、ベイナイトの最大硬さとは、ビッカー
ス硬さ100〜1000gで試料を5〜20点測定した
ときの平均値と定義する。The maximum hardness of bainite is defined as the average value when the sample is measured at 5 to 20 points with a Vickers hardness of 100 to 1000 g.
【0038】次に、限界拡散性水素量の限定理由につい
て説明する。Next, the reason for limiting the limit diffusible hydrogen amount will be described.
【0039】上記成分の鋼材の遅れ破壊を評価した結
果、遅れ破壊の起点となる表層をベイナイト主体の組織
とし、さらに好ましくは強度レベルを調整することで限
界拡散性水素量が著しく向上した。特に1300MPa
以上の高強度PC鋼棒では、上記の効果により0.2p
pm以上,好ましくは0.5ppm以上の限界拡散性水
素量にすることができる。次に、本発明の製造条件につ
いて説明する。As a result of evaluating the delayed fracture of the steel materials having the above-mentioned components, the surface layer which is the starting point of the delayed fracture has a structure mainly composed of bainite, and more preferably the strength level is adjusted, whereby the critical diffusible hydrogen content is remarkably improved. Especially 1300MPa
With the above high-strength PC steel bars, due to the above effects, 0.2p
The limit diffusible hydrogen content can be pm or more, preferably 0.5 ppm or more. Next, the manufacturing conditions of the present invention will be described.
【0040】熱間圧延終了後、もしくは再加熱によりオ
ーステナイト域の温度から加速冷却することにより、表
層がベイナイト主体の組織になり、内部がベイナイト変
態時の復熱によりパーライトもしくはフェライト・パー
ライト主体の組織になる。再加熱温度は特に定めない
が、オーステナイト化の点で750℃以上とし,オース
テナイト粒粗大化防止の点で1050℃以下とすること
が好ましい。冷却開始温度がAc3より低いと低強度の
フェライト・パーライト主体の組織になりベイナイトが
生成せず、遅れ破壊特性が低くなるため、冷却開始温度
はAc3以上に限定した。一方、冷却開始温度はの上限
は特に定めないが、加速冷却時間の生産性の関係から、
Ac3+300℃以下が望ましい。After the hot rolling is completed, or by accelerated heating from the temperature in the austenite region by reheating, the surface layer becomes a structure mainly composed of bainite, and the inside is a structure mainly composed of pearlite or ferrite / pearlite due to reheat during bainite transformation. become. The reheating temperature is not particularly specified, but it is preferably 750 ° C. or higher from the viewpoint of austenite formation and 1050 ° C. or lower from the viewpoint of preventing austenite grain coarsening. When the cooling start temperature is lower than Ac 3, the structure is mainly composed of low-strength ferrite / pearlite, bainite is not formed, and the delayed fracture property is deteriorated. Therefore, the cooling start temperature is limited to Ac 3 or higher. On the other hand, the upper limit of the cooling start temperature is not specified, but from the relationship of productivity of accelerated cooling time,
Ac 3 + 300 ° C. or lower is desirable.
【0041】尚、Ac3は小型熱膨張型変態測定装置
(フォーマスター)により求めた値と定義する。Ac 3 is defined as a value obtained by a small thermal expansion type transformation measuring device (Formaster).
【0042】加速冷却停止温度は、Ms点より低いとマ
ルテンサイトが生成し、550℃より高いとベイナイト
が生成しないため、Ms〜550℃に限定した。冷却速
度は、1℃/秒より低いとベイナイトが生成せず、10
℃/秒より早いと全断面にベイナイト主体の組織が生成
するため、1〜10℃/秒に限定した。The accelerated cooling stop temperature is limited to Ms to 550 ° C., since martensite is formed below the Ms point and bainite is not formed above 550 ° C. If the cooling rate is lower than 1 ° C / sec, bainite will not be formed and 10
If it is faster than ° C / sec, a bainite-based structure is generated on the entire cross section, so the rate was limited to 1-10 ° C / sec.
【0043】[0043]
【実施例】以下、実施例により本発明の効果を更に具体
的に説明する。EXAMPLES The effects of the present invention will be described more specifically below with reference to examples.
【0044】表1に示す化学組成を有する鋼を1200
℃に加熱後、熱間圧延でφ7mmの鋼棒を製造し、95
0℃に加熱し、表2に示す冷却条件で冷却した。その機
械的性質と、図1に示す遅れ破壊試験片を用いて図2に
示す遅れ破壊試験機で評価した遅れ破壊特性を表2に示
す。1200 steels having the chemical compositions shown in Table 1 were prepared.
After heating to ℃, by hot rolling to produce a steel rod φ7mm, 95
It was heated to 0 ° C. and cooled under the cooling conditions shown in Table 2. Table 2 shows the mechanical properties and the delayed fracture characteristics evaluated by the delayed fracture tester shown in FIG. 2 using the delayed fracture test piece shown in FIG.
【0045】遅れ破壊特性は、図4に示す100時間後
に遅れ破壊しない拡散性水素量、すなわち限界拡散性水
素量で評価を行い、負荷荷重は引張り強さの90%の条
件で実施した。なお水素チャージは、電解水素チャージ
法を用いて、チャージ電流を変えることにより水素レベ
ルを変化させた。The delayed fracture characteristics were evaluated by the amount of diffusible hydrogen which does not cause delayed fracture after 100 hours shown in FIG. 4, that is, the amount of limit diffusible hydrogen, and the load was applied under the condition of 90% of the tensile strength. For hydrogen charging, the hydrogen level was changed by changing the charging current using the electrolytic hydrogen charging method.
【0046】表1および表2の鋼種A〜F,符号1〜6
が本発明鋼であり、鋼種G〜L,符号7〜17が比較鋼
である。Steel types A to F in Tables 1 and 2 and symbols 1 to 6
Is a steel of the present invention, steel types G to L, and symbols 7 to 17 are comparative steels.
【0047】本発明鋼は、いずれも引張強度が1300
MPa以上であると共に、表層からのベイナイト層の深
さが50μm以上で、ベイナイトの最大硬さがビッカー
ス硬さで450以下であり、遅れ破壊試験の限界拡散性
水素量が0.2ppm以上である。Each of the steels of the present invention has a tensile strength of 1300.
In addition to being at least MPa, the depth of the bainite layer from the surface layer is at least 50 μm, the maximum hardness of bainite is 450 or less in Vickers hardness, and the critical diffusible hydrogen content in the delayed fracture test is at least 0.2 ppm. .
【0048】これに対して、比較鋼である符号7は、C
量が低いため内部のパーライト組織の面積率が低く、引
張強度が1300MPa未満と低い例である。符号8は
冷却開始温度が低く表層ベイナイトが生成していないた
め、限界拡散性水素量が0.2ppm未満と低く遅れ破
壊特性が悪くなった例である。符号9は、C量が多く表
層のベイナイト硬さがHv450超であるため、限界拡
散性水素量が0.2ppm未満と遅れ破壊特性が悪くな
った例である。符号10は、冷却停止温度が高く表層ベ
イナイトが生成していないため、限界拡散性水素量が
0.2ppm未満と遅れ破壊特性が悪くなった例であ
る。符号11はSiが3.10%と高く、Mnが0.0
5%と低いため、ベイナイト組織中の硬さが高くなり靭
性が低下して遅れ破壊特性が悪くなった例である。符号
12は冷却速度が遅く、引張強度が1300MPa未満
という強度が低い例である。符号13はSiが0.00
9%と低く、Mnが2.1%と高いため、ベイナイト組
織中の硬さが高くなり破壊靭性値が低低下して遅れ破壊
特性が悪い例である。符号14は、冷却速度が速く表層
にマルテンサイトが生成したため、限界拡散性水素量が
0.2ppm未満と遅れ破壊特性が悪くなった例であ
る。符号15は、Vが1.58%,Crが1.61%と
高く焼入れ性が非常に高くなったため、表層にマルテン
サイトを生成し、限界拡散性水素量が0.2ppm未満
と遅れ破壊特性が悪くなった例である。符号16は、冷
却停止温度が低く表層にマルテンサイト組織を生成し、
表層のベイナイト組織の面積率が低いため、著しく遅れ
破壊特性が悪くなった例である。符号17はMoが3.
05%と高いほかにCuおよびNiが高く焼入れ性が非
常に高いため、表層にマルテンサイトが生成し、ベイナ
イトの面積率が低いため遅れ破壊特性が悪い例である。On the other hand, reference numeral 7 which is a comparative steel is C
This is an example in which the area ratio of the internal pearlite structure is low because the amount is low, and the tensile strength is low at less than 1300 MPa. Reference numeral 8 is an example in which the cooling start temperature was low and surface bainite was not formed, so that the critical diffusible hydrogen content was as low as less than 0.2 ppm and the delayed fracture characteristics were poor. Reference numeral 9 is an example in which the amount of C was large and the bainite hardness of the surface layer was more than Hv450, so that the critical diffusible hydrogen amount was less than 0.2 ppm and the delayed fracture characteristics were deteriorated. Reference numeral 10 is an example in which the critical fracture diffusivity was less than 0.2 ppm and the delayed fracture characteristics were poor because the cooling stop temperature was high and surface bainite was not formed. Reference numeral 11 has a high Si content of 3.10% and a Mn content of 0.0.
Since it is as low as 5%, the hardness in the bainite structure is increased, the toughness is lowered, and the delayed fracture property is deteriorated. Reference numeral 12 is an example in which the cooling rate is slow and the tensile strength is less than 1300 MPa, which is low in strength. Reference numeral 13 is Si 0.00
It is as low as 9% and Mn is as high as 2.1%, so that the hardness in the bainite structure is high, the fracture toughness value is low, and the delayed fracture property is poor. Reference numeral 14 is an example in which the delayed fracture property was deteriorated with the critical diffusible hydrogen amount of less than 0.2 ppm because martensite was generated in the surface layer with a high cooling rate. Reference numeral 15 has a V content of 1.58% and a Cr content of 1.61% and has a very high hardenability, so martensite is generated in the surface layer, and the critical diffusible hydrogen content is less than 0.2 ppm, which is a delayed fracture property. Is a bad example. Reference numeral 16 has a low cooling stop temperature and generates a martensite structure in the surface layer,
This is an example in which the delayed fracture characteristics are significantly deteriorated due to the low area ratio of the bainite structure in the surface layer. Reference numeral 17 indicates Mo is 3.
This is an example in which delayed fracture characteristics are poor because martensite is generated in the surface layer and the area ratio of bainite is low because Cu and Ni are high and the hardenability is very high in addition to as high as 05%.
【0049】[0049]
【表1】 [Table 1]
【0050】[0050]
【表2】 [Table 2]
【0051】[0051]
【発明の効果】本発明によれば、表層に遅れ破壊特性の
優れたベイナイト層を生成させることにより、引張強さ
が1300MPa以上の耐遅れ破壊特性に優れたPC鋼
棒を製造することができる。According to the present invention, a PC steel bar having a tensile strength of 1300 MPa or more and excellent resistance to delayed fracture can be manufactured by forming a bainite layer having excellent delayed fracture characteristics on the surface layer. .
【図1】鋼材の遅れ破壊試験に用いた試験片平面図。FIG. 1 is a plan view of a test piece used for a delayed fracture test of a steel material.
【図2】遅れ破壊試験装置の説明図。FIG. 2 is an explanatory diagram of a delayed fracture test device.
【図3】水素分析の水素放出プロファイル。FIG. 3: Hydrogen release profile of hydrogen analysis.
【図4】限界拡散性水素量の説明図。FIG. 4 is an explanatory diagram of the limit diffusible hydrogen amount.
1 試験片 2 バランスウェイト 3 支点 1 test piece 2 balance weight 3 fulcrum
フロントページの続き Fターム(参考) 4K032 AA01 AA02 AA06 AA07 AA11 AA12 AA14 AA15 AA16 AA19 AA20 AA22 AA23 AA24 AA31 AA32 AA36 BA02 CD02 4K042 AA14 BA02 CA02 CA05 CA06 CA08 CA09 CA10 CA13 DA01 DE01 Continued front page F-term (reference) 4K032 AA01 AA02 AA06 AA07 AA11 AA12 AA14 AA15 AA16 AA19 AA20 AA22 AA23 AA24 AA31 AA32 AA36 BA02 CD02 4K042 AA14 BA02 CA02 CA05 CA06 CA08 CA09 CA10 CA13 DA01 DE01
Claims (5)
イトの面積率が30%以上であり、その内部はパーライ
トの面積率が70%以上であることを特徴とする耐遅れ
破壊特性に優れた高強度PC鋼棒。1. The content of C: 0.55 to 1.2% by mass% is such that the area ratio of bainite is 30% or more up to a depth of at least 50 μm from the surface, and the area ratio of pearlite is 70% inside. %, A high strength PC steel bar with excellent delayed fracture resistance.
ベイナイトの最大硬さがビッカース硬度で350〜45
0であることを特徴とする請求項1記載の耐遅れ破壊特
性に優れた高強度PC鋼棒。2. The maximum hardness of bainite at a depth of at least 50 μm from the surface is 350-45 in Vickers hardness.
The high-strength PC steel rod excellent in delayed fracture resistance according to claim 1, which is 0.
的不純物からなることを特徴とする請求項1又は2記載
の耐遅れ破壊特性に優れた高強度PC鋼棒。3. In mass%, further contains Si: 0.01 to 3%, Mn: 0.1 to 2%, and further Cr: 0.05 to 1.5%, Al: 0.003. -0.1%, Nb: 0.001-0.05%, Cu: 0.01-4%, Ni: 0.01-4%, B: 0.0001-0.005% V: 0.01 To 1.5%, Mo: 0.01 to 3%, one or more, and the balance consisting of Fe and unavoidable impurities, delayed fracture resistance according to claim 1 or 2. Excellent high strength PC steel rod.
あ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に優れた高強度PC鋼棒。4. A high-strength PC steel rod excellent in delayed fracture resistance as set forth in any one of claims 1 to 3, wherein the limit diffusible hydrogen content is 0.2 ppm or more.
遅れ破壊特性に優れた高強度PC鋼棒を製造する方法で
あって,請求項1又は請求項3記載の成分を有する鋼材
を、圧延終了後又は再加熱後、Ac3以上からMs〜5
50℃まで1〜10℃/秒の冷却速度で冷却することを
特徴とする耐遅れ破壊特性に優れた高強度PC鋼棒の製
造方法。5. A method for producing a high-strength PC steel rod excellent in delayed fracture resistance according to any one of claims 1 to 4, which comprises the components according to claim 1 or claim 3. After rolling the steel material or after reheating, the steel material is Ac 3 or more and Ms to 5
A method for producing a high-strength PC steel rod excellent in delayed fracture resistance, characterized by cooling to 50 ° C at a cooling rate of 1 to 10 ° C / sec.
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