JP5942572B2 - ERW welded steel pipe for automobile parts having excellent fatigue resistance and method for producing the same - Google Patents
ERW welded steel pipe for automobile parts having excellent fatigue resistance and method for producing the same Download PDFInfo
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- 239000010959 steel Substances 0.000 title claims description 94
- 238000004519 manufacturing process Methods 0.000 title claims description 14
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- 239000000463 material Substances 0.000 claims description 32
- 238000005096 rolling process Methods 0.000 claims description 30
- 230000006698 induction Effects 0.000 claims description 27
- 239000000203 mixture Substances 0.000 claims description 27
- 229910052802 copper Inorganic materials 0.000 claims description 24
- 230000009466 transformation Effects 0.000 claims description 23
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- 230000009467 reduction Effects 0.000 claims description 22
- 229910052759 nickel Inorganic materials 0.000 claims description 20
- 239000010953 base metal Substances 0.000 claims description 11
- 238000003466 welding Methods 0.000 claims description 9
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- 239000012535 impurity Substances 0.000 claims description 7
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- 229910001566 austenite Inorganic materials 0.000 description 6
- 238000009661 fatigue test Methods 0.000 description 6
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- 230000007423 decrease Effects 0.000 description 4
- 150000001247 metal acetylides Chemical class 0.000 description 4
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- 230000000087 stabilizing effect Effects 0.000 description 4
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- 230000033228 biological regulation Effects 0.000 description 1
- 229910001567 cementite Inorganic materials 0.000 description 1
- 238000010622 cold drawing Methods 0.000 description 1
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- 230000000052 comparative effect Effects 0.000 description 1
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- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
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Description
本発明は、ドライブシャフト、スタビライザー、ラックバーなどの駆動系の自動車部品用として好適な電縫溶接鋼管に係り、とくに耐疲労特性の向上に関する。 The present invention relates to an electric resistance welded steel pipe suitable for use in automobile parts of a drive system such as a drive shaft, a stabilizer, and a rack bar, and more particularly to improvement of fatigue resistance.
近年、地球環境の保全という観点から、自動車の排気ガス規制が厳しくなり、自動車燃費向上のため、自動車車体の軽量化が進められている。自動車車体を構成する部品、例えば、剛性を要求され、従来から中実素材が利用されてきた、ドライブシャフト、スタビライザー、ラックバーなどの部品にも、軽量化が要求されている。
このような自動車車体を軽量化するという要求と剛性の確保を両立させるために、ドライブシャフトやスタビライザーなどの自動車部品を、例えば素材として継目無鋼管を用いて中空化することが進められている。
In recent years, automobile exhaust gas regulations have become stricter from the viewpoint of protecting the global environment, and the weight of automobile bodies has been reduced in order to improve automobile fuel efficiency. Parts such as drive shafts, stabilizers, rack bars, and the like that are required to be rigid and have conventionally used solid materials are also required to be lightweight.
In order to satisfy both the demand for reducing the weight of an automobile body and ensuring rigidity, it has been promoted to hollow out automobile parts such as a drive shaft and a stabilizer using, for example, a seamless steel pipe as a material.
例えば、特許文献１には、Ｃ：0.30〜0.47％、Si：0.5％以下、Mn：0.3〜2.0％、Cr：0.15〜1.0％、Al：0.001〜0.05％、Ti：0.005〜0.05％、Ca：0.004％以下、Ｎ：0.01％以下、Ｂ：0.0005〜0.005％を含み、Ｂeffが0.0001以上で、オーステナイト結晶粒度番号が９以上である高周波焼入れ中空駆動軸が記載されている。特許文献１に記載された高周波焼入れ中空駆動軸は、継目無鋼管を素材としており、優れた冷間加工性、焼入れ性、靭性、およびねじり疲労強度を兼ね備え、安定した疲労寿命を発揮するとしている。しかし、特許文献１に記載された技術では、継目無鋼管を素材としており、その製法上、表面脱炭や表面疵が多く、十分な疲労特性を確保するためには、表面を研磨、研削しなければならないうえ、偏心・偏肉があり、回転物として使用するには、必ずしも適さないという問題があった。 For example, in Patent Document 1, C: 0.30 to 0.47%, Si: 0.5% or less, Mn: 0.3 to 2.0%, Cr: 0.15 to 1.0%, Al: 0.001 to 0.05%, Ti: 0.005 to 0.05%, Ca : Induction hardened hollow drive shaft including 0.004% or less, N: 0.01% or less, B: 0.0005 to 0.005%, Beff of 0.0001 or more, and austenite grain size number of 9 or more. The induction-quenched hollow drive shaft described in Patent Document 1 is made of a seamless steel pipe, and has excellent cold workability, hardenability, toughness, and torsional fatigue strength, and exhibits a stable fatigue life. . However, in the technique described in Patent Document 1, a seamless steel pipe is used as a raw material, and due to its manufacturing method, there are many surface decarburization and surface flaws, and in order to ensure sufficient fatigue characteristics, the surface is polished and ground. In addition, there is a problem that it has eccentricity and thickness deviation and is not necessarily suitable for use as a rotating object.
このような問題に対し、例えば特許文献２には、自動車シャフト等の用途に用いることが可能な、優れた強度、耐遅れ破壊性、疲労強度を有する高強度鋼管が記載されている。特許文献２に記載された鋼管は、質量％で、Ｃ：0.30％超0.50％以下、Si：1.0％以下、Mn：1.5％以下、Ti：0.1％以下、Mo：0.3〜0.5％、Ｂ：0.0005〜0.01％を含む鋼で、好ましくは鋼帯を所定形状に成形し電縫溶接して製造した鋼管に、焼入れ焼戻を施し、旧オーステナイト粒径が10μｍ以下となる硬化部が鋼管のＣ断面面積の30％以上形成された鋼組織を有し、耐遅れ破壊特性および耐疲労特性に優れた高強度鋼管である。 For such a problem, for example, Patent Document 2 describes a high-strength steel pipe having excellent strength, delayed fracture resistance, and fatigue strength that can be used for applications such as an automobile shaft. The steel pipe described in Patent Document 2 is in mass%, C: more than 0.30%, 0.50% or less, Si: 1.0% or less, Mn: 1.5% or less, Ti: 0.1% or less, Mo: 0.3 to 0.5%, B: Steel containing 0.0005% to 0.01%, preferably steel pipes formed by forming a steel strip into a predetermined shape and welded by electro-welding, quenching and tempering, and the hardened part where the prior austenite grain size is 10 μm or less is C It is a high-strength steel pipe having a steel structure formed with a cross-sectional area of 30% or more and excellent delayed fracture resistance and fatigue resistance.
また、特許文献３には、冷間鍛造加工性と転造加工性に優れた高炭素鋼管が記載されている。特許文献３に記載された技術では、質量％で、Ｃ：0.2〜0.6％、Si：1％以下、Mn：0.4〜３％を含む組成の、好ましくは電縫鋼管である鋼管素材を、加熱したのち、仕上圧延温度を700〜800℃として縮径率：30％以上の縮径圧延を行い、ついで所定の寸法の鋼管となるように、冷間引抜きしたのち、Ａc1変態点〜（Ａc1変態点＋50℃）の範囲の温度で熱処理し、アスペクト比が３以上であるセメンタイトを分散させた組織を有し、引張強さを焼準材相当強度の90％以下とする高炭素鋼管が得られる。特許文献３に記載された技術によれば、スウェージ加工性、スウェージ加工後の平転造加工性に優れた高炭素鋼管を容易に製造でき、ドライブシャフト等の中空部品の高強度化、軽量化に貢献できるとしている。 Patent Document 3 describes a high carbon steel pipe excellent in cold forging workability and rolling workability. In the technique described in Patent Document 3, a steel pipe material having a composition containing C: 0.2 to 0.6%, Si: 1% or less, and Mn: 0.4 to 3%, preferably an electric resistance steel pipe, is heated. After that, the final rolling temperature is set to 700 to 800 ° C., the diameter reduction ratio is reduced to 30% or more, and then the steel pipe is cold-drawn so as to obtain a steel pipe having a predetermined size, and then the Ac1 transformation point to (Ac1 transformation). A high-carbon steel pipe having a structure in which cementite having an aspect ratio of 3 or more is dispersed and having a tensile strength of 90% or less of the equivalent strength of the normalizing material is obtained. . According to the technique described in Patent Document 3, a high carbon steel pipe excellent in swaging workability and flat rolling workability after swaging can be easily manufactured, and the strength and weight reduction of hollow parts such as drive shafts can be achieved. Can contribute to.
しかしながら、特許文献２に記載された技術では、鋼管が、例えば電縫溶接鋼管である場合には、電縫溶接工程において接合部の中心に白色層とよばれる領域が生じ、この白色層では、周りの部分に比べてＣ量が低く焼入れ性が不足して、焼入れ処理を施したのちにおいても、硬度が低く、局部的な低硬度領域が生じるという問題がある。しかも、白色層は、管の長手方向全長にわたり存在するため、例えばドライブシャフトのように繰返し剪断応力（ねじり疲労）が付与される用途においては、白色層（低硬度域）が起点となり割れが発生する危険性が高く、電縫鋼管の信頼性を著しく低下させることになるという問題がある。 However, in the technique described in Patent Document 2, when the steel pipe is, for example, an electric resistance welded steel pipe, an area called a white layer is generated at the center of the joint in the electric resistance welding process. There is a problem in that the amount of C is low compared with the surrounding portion, the hardenability is insufficient, and the hardness is low even after the quenching treatment, and a local low hardness region is generated. In addition, since the white layer exists over the entire length in the longitudinal direction of the tube, cracks are generated starting from the white layer (low hardness region) in applications where repeated shear stress (torsional fatigue) is applied, such as drive shafts. There is a problem that the reliability of the ERW steel pipe is remarkably lowered.
また、特許文献３に記載された技術では、冷間引抜き−二相域加熱処理を行うため、工程が複雑になるという問題がある。また、特許文献３に記載された技術では、白色層を狭くすることの必要性や、焼入れ焼戻処理後の耐疲労特性を向上させることについてまでの言及はない。
本発明は、かかる従来技術の問題を有利に解決し、ドライブシャフト等の駆動系の自動車部品用として好適な、高周波焼入れ焼戻処理を施した後に、優れた耐疲労特性を有する電縫溶接鋼管およびその製造方法を提供することを目的とする。
Moreover, in the technique described in Patent Document 3, there is a problem that the process becomes complicated because the cold drawing-two-phase region heat treatment is performed. Moreover, in the technique described in patent document 3, there is no mention about the necessity of narrowing a white layer and improving the fatigue resistance after a quenching and tempering process.
The present invention advantageously solves the problems of the prior art, and is suitable for use as an automotive part of a drive system such as a drive shaft, and is subjected to induction hardening and tempering treatment, and then has an electric resistance welded steel pipe having excellent fatigue resistance. And it aims at providing the manufacturing method.
本発明者らは、上記した目的を達成するために、電縫鋼管の耐疲労特性に影響する各種要因について鋭意検討した。その結果、耐疲労特性を改善するためには、管円周方向に均一な硬度分布を持たせることが重要で、そのためには、まず、素材である電縫鋼管の段階で、白色層の幅を狭くし、かつ電縫溶接部の白色層とその周りの母材部との硬度差を小さくしておくことが肝要であることに想到した。 In order to achieve the above-mentioned object, the present inventors diligently studied various factors that affect the fatigue resistance characteristics of an electric resistance welded steel pipe. As a result, in order to improve fatigue resistance, it is important to have a uniform hardness distribution in the pipe circumferential direction. First, at the stage of the ERW steel pipe, which is the material, the width of the white layer It was thought that it was important to reduce the hardness difference between the white layer of the ERW welded part and the surrounding base material part.
そして、更なる検討の結果、焼入れ後に白色層と母材部との硬度差を小さくするためには、焼入れ時の加熱中に母材部からＣ量の低い白色層へＣを拡散させて、Ｃ量の均一化を図る必要があることに思い至った。しかし、ドライブシャフトの場合には通常、焼入れ時の加熱は、高周波加熱を用いており、加熱時間は非常に短時間である。そのため、Ｃ量の均一化は、短時間で達成する必要があることになる。 And as a result of further examination, in order to reduce the hardness difference between the white layer and the base material part after quenching, C is diffused from the base material part to the white layer with a low amount of C during heating during quenching, I came up with the need to make the C amount uniform. However, in the case of a drive shaft, generally, heating during quenching uses high-frequency heating, and the heating time is very short. Therefore, it is necessary to achieve the uniform amount of C in a short time.
Ｃ量の均一化を図るには、白色層をAc3変態点以上のオーステナイト域まで加熱し、Ｃの拡散を促進させる必要があるが、Ｃ量が低い白色層は周辺の母材部に比べてAc3変態点が高くなっているため、加熱温度を高くする必要がある。しかし、高周波加熱の際に過剰に温度を上昇させると管表面近傍でオーステナイト粒が粗大化し、靭性の低下を招くという問題がある。また、高周波加熱の場合、加熱時間が短いため、厚肉鋼管では、肉厚中央と表層とで温度差が生じ、肉厚中央がAc3変態点に到達せず、肉厚中央部でＣの拡散が進行しない恐れがある。 In order to make the C content uniform, it is necessary to heat the white layer to the austenite region above the Ac3 transformation point to promote the diffusion of C. However, the white layer with a low C content is compared to the surrounding base metal part. Since the Ac3 transformation point is high, it is necessary to increase the heating temperature. However, if the temperature is raised excessively during high-frequency heating, there is a problem that austenite grains become coarse in the vicinity of the tube surface, leading to a decrease in toughness. In addition, in the case of high-frequency heating, since the heating time is short, in a thick steel pipe, a temperature difference occurs between the center of the wall and the surface layer, the center of the wall does not reach the Ac3 transformation point, and C diffuses at the center of the wall. May not progress.
そこで、本発明者らは、白色層周辺でのＣの拡散を十分に促進させるために、オーステナイト安定化元素であるMn、Cr、Cu、Niを適正量含有させる一方、フェライト安定化元素であるAl、Ｐを極力少なくすることを思い付いた。これにより、Ac3変態点を低下でき、加熱温度を低く抑えることができる。とくに、Cu、Cr、Niは、白色層に残留し、白色層のAc3変態点を低下させることに有効に作用するため、次（１）式
2Cu＋1.52Ni＋1.1Cr≧0.5 ‥‥（１）
（ここで、Cu、NI、Cr：各元素の含有量（質量％）
を満足するように調整すれば、Ac3変態点がより低下し、より低い加熱温度においてもＣの拡散が促進され、白色層におけるＣ量分布の均一化がより容易になることを見出した。
Therefore, the present inventors include a proper amount of austenite stabilizing elements Mn, Cr, Cu, Ni in order to sufficiently promote the diffusion of C around the white layer, while being ferrite stabilizing elements. I came up with the idea of reducing Al and P as much as possible. Thereby, the Ac3 transformation point can be lowered and the heating temperature can be kept low. In particular, Cu, Cr and Ni remain in the white layer and effectively act to lower the Ac3 transformation point of the white layer.
2Cu + 1.52Ni + 1.1Cr ≧ 0.5 (1)
(Where Cu, NI, Cr: content of each element (mass%)
It has been found that if the adjustment is made to satisfy the above, the Ac3 transformation point is further lowered, the diffusion of C is promoted even at a lower heating temperature, and the C content distribution in the white layer is more uniform.
さらに、本発明者は、縮径圧延により幾何学的に白色層の幅を狭くすることにより、より低温、より短時間の加熱においても、硬さの均一化を容易に達成できることを見出した。
このようなことから、Al、P、Cr、Cu、Ni含有量を適正範囲に調整した電縫溶接鋼管を素材として、この素材に、縮径圧延加熱温度、縮径圧延終了温度、縮径率をそれぞれ所定の範囲に調整した熱間縮径圧延を施すことにより、高周波焼入れ焼戻処理後の耐疲労特性の低下に影響しない程度に、焼入れ処理後の硬さ分布が均一になるように、電縫溶接部の白色層幅を狭くすることができることを見出した。
Furthermore, the present inventor has found that uniform hardness can be easily achieved even at a lower temperature and in a shorter time by geometrically reducing the width of the white layer by diameter reduction rolling.
For this reason, the ERW welded steel pipe with the Al, P, Cr, Cu, and Ni contents adjusted to the appropriate range is used as a raw material. By applying hot reduction rolling adjusted to each predetermined range, so that the hardness distribution after the quenching treatment is uniform to the extent that it does not affect the deterioration of fatigue resistance after induction hardening and tempering treatment, It has been found that the width of the white layer of the ERW weld can be reduced.
まず、本発明の基礎になった実験結果について説明する。
管円周方向に測定した、白色層の周辺におけるＣ量分布を模式的に図１に示す。白色層では、母材部に比較してＣ量が低下している。ここで、母材部のＣ量（平均値）に対し、低下した距離（幅）を白色層幅と定義する。
母材部が、質量％で、0.42％Ｃ−0.26％Si−1.52％Mn−0.030％Al−0.0025％Ｎ−0.20％Cr−0.200％Cuを含有する組成を有し、白色層幅の異なる電縫溶接鋼管に、高周波加熱（加熱温度：900℃、保持時間：１ｓ）を施し急冷する焼入れ処理を施したのち、EPMA（Electron Probe Micro-Analyzer）を用いて、溶接部（白色層）周辺のＣ量分布を測定した。なお、ここでは、白色層の幅は、電縫溶接鋼管に冷間引抜きで縮径加工を施し、幾何学的に白色層の幅を変化させた。
First, the experimental results on which the present invention is based will be described.
FIG. 1 schematically shows the C amount distribution around the white layer measured in the tube circumferential direction. In the white layer, the amount of C is lower than that of the base material portion. Here, the reduced distance (width) with respect to the C amount (average value) of the base material portion is defined as the white layer width.
The base metal part has a composition containing 0.42% C-0.26% Si-1.52% Mn-0.030% Al-0.0025% N-0.20% Cr-0.200% Cu in mass%, and different white layer widths. A sewn welded pipe is subjected to high-frequency heating (heating temperature: 900 ° C, holding time: 1 s) and quenched, and then the EPMA (Electron Probe Micro-Analyzer) is used to surround the welded part (white layer). The C amount distribution was measured. Here, the width of the white layer was geometrically changed by applying a diameter reduction process to the ERW welded steel pipe by cold drawing.
そして、得られた高周波焼入れ処理後のＣ量分布から、母材部Ｃ量と焼入れ処理後の白色層のＣ量(最低Ｃ量)との差ΔＣ（図１参照）を算出し、白色層幅との関係で図２に示す。また、高周波焼入れ処理を施された電縫溶接鋼管について、母材部（白色層から管円周方向に20mm離れた位置）と白色層について、マイクロビッカース硬さHV0.5（測定荷重500gf）を各５点測定し、その平均値を求め、母材部と白色層と硬さの差ΔHVを白色層幅との関係で図３に示す。 Then, a difference ΔC (see FIG. 1) between the base material C amount and the white layer C amount (minimum C amount) after the quenching process is calculated from the obtained C amount distribution after induction hardening, and the white layer FIG. 2 shows the relationship with the width. In addition, for ERW welded steel pipes that have been induction hardened, micro Vickers hardness HV0.5 (measurement load 500gf) is applied to the base metal part (position 20mm away from the white layer in the pipe circumferential direction) and the white layer. Three points are measured, the average value is obtained, and the difference ΔHV in hardness between the base material portion and the white layer is shown in FIG. 3 in relation to the width of the white layer.
図２、図３から、白色層幅を40μm以下と狭くすることにより、Ｃ量分布が短時間で均一化することがわかる。また、白色層幅を40μm以下と狭くすることにより、高周波焼入れ後のΔHVが25ポイント以下となり硬度差が小さく抑えられ、硬さ分布の均一化が短時間で達成できることがわかる。なお、高周波焼入れ後のΔHVが25ポイント以下であれば、ねじり疲労試験により、白色層起点で割れが発生しないことを確認している。 2 and 3, it can be seen that the C amount distribution becomes uniform in a short time by narrowing the white layer width to 40 μm or less. It can also be seen that by reducing the width of the white layer to 40 μm or less, ΔHV after induction hardening becomes 25 points or less, the hardness difference is kept small, and uniform hardness distribution can be achieved in a short time. If ΔHV after induction hardening is 25 points or less, it has been confirmed by a torsional fatigue test that cracks do not occur at the origin of the white layer.
本発明は、かかる知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨は次のとおりである。
（１）母材部と電縫溶接部とを有する電縫溶接鋼管であって、前記母材部が、質量％で、Ｃ：0.41〜0.55％、Si：0.01〜0.5％、Mn：1.0〜2.0％、Al：0.05％以下、Ｐ：0.02％以下、Ｓ：0.01％以下、Cr：1.0％以下、Cu：1.0％以下、Ni：1.0％以下を含み、かつCu、Ni、Crが次（１）式
２Cu＋1.52Ni＋1.1Cr ≧ 1.0 ‥‥（１）
（ここで、Cu、Ni、Cr：各元素の含有量（質量％））
を満足するように含有し、残部Feおよび不可避的不純物からなる組成を有し、前記電縫溶接部が白色層を有し、縮径圧延ままで該白色層の幅が、40μm以下であり、高周波焼入れ後にビッカース硬さHV0.5で測定した前記白色層の硬さと前記母材部との硬さの差ΔHVの絶対値が、25ポイント以下であることを特徴とする耐疲労特性に優れた自動車部品用電縫溶接鋼管。
（２）（１）において、前記組成に加えてさらに、質量％で、Ｂ：0.0003〜0.0050％を含むことを特徴とする自動車部品用電縫溶接鋼管。
（３）（１）または（２）において、前記組成に加えてさらに、質量％で、Mo：0.1〜1.0％、Ｗ：0.1〜1.0％、Nb：0.1％以下、Ｖ：0.1〜1.0％のうちから選ばれた１種または２種以上を含むことを特徴とする自動車部品用電縫溶接鋼管。
（４）（１）ないし（３）のいずれかにおいて、前記組成に加えてさらに、質量％で、Ca：0.02％以下、REM：0.02％以下のうちから選ばれた１種または２種を含むことを特徴とする自動車部品用電縫溶接鋼管。
（５）鋼帯を連続的にロール成形して略円筒状の成形体としたのち、該成形体の円周方向端部同士を突き合わせ電縫溶接して電縫溶接鋼管とし、さらに該電縫溶接鋼管を素材として、該素材に縮径圧延を施して、自動車部品用電縫溶接鋼管とする自動車部品用電縫溶接鋼管の製造方法であって、前記鋼帯を、質量％で、Ｃ：0.41〜0.55％、Si：0.01〜0.5％、Mn：1.0〜2.0％、Al：0.05％以下、Ｐ：0.02％以下、Ｓ：0.01％以下、Cr：1.0％以下、Cu：1.0％以下、Ni：1.0％以下を含み、かつCu、Ni、Crが次（１）式
２Cu＋1.52Ni＋1.1Cr ≧ 1.0 ‥‥（１）
（ここで、Cu、Ni、Cr：各元素の含有量（質量％））
を満足するように含有し、残部Feおよび不可避的不純物からなる組成を有する鋼帯とし、前記縮径圧延を、Ａc３変態点以上の温度に加熱・均熱したのち、圧延終了温度：800℃超950℃以下で、累積縮径率：35〜70％である縮径圧延とすることを特徴とする耐疲労特性に優れた自動車部品用電縫溶接鋼管の製造方法。
（６）（５）において、前記組成に加えてさらに、質量％で、Ｂ：0.0003〜0.0050％を含むことを特徴とする自動車部品用電縫溶接鋼管の製造方法。
（７）（５）または（６）において、前記組成に加えてさらに、質量％で、Mo：0.1〜1.0％、Ｗ：0.1〜1.0％、Nb：0.1％以下、Ｖ：0.1〜1.0％のうちから選ばれた１種または２種以上を含むことを特徴とする自動車部品用電縫溶接鋼管の製造方法。
（８）（５）ないし（７）のいずれかにおいて、前記組成に加えてさらに、質量％で、Ca：0.02％以下、REM：0.02％以下のうちから選ばれた１種または２種を含むことを特徴とする自動車部品用電縫溶接鋼管の製造方法。
The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1 ) An electric resistance welded steel pipe having a base metal part and an ERW weld part, wherein the base metal part is in mass%, C: 0.41 to 0.55%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.05% or less, P: 0.02% or less, S: 0.01% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, and Cu, Ni and Cr are the following ( 1) Formula
2Cu + 1.52Ni + 1.1Cr ≧ 1.0 (1)
(Here, Cu, Ni, Cr: content of each element (mass%))
Containing the remainder Fe and unavoidable impurities, the ERW weld has a white layer, and the width of the white layer is 40 μm or less as it is in reduced diameter rolling, Excellent fatigue resistance, characterized in that the absolute value of the difference ΔHV between the hardness of the white layer and the hardness of the base material measured at Vickers hardness HV0.5 after induction hardening is 25 points or less ERW welded steel pipe for automobile parts.
(2) In (1), in addition to the said composition, B: 0.0003-0.0050% is further included by the mass%, The electric-welding steel pipe for motor vehicle parts characterized by the above-mentioned.
(3) In (1) or (2), in addition to the above composition, in terms of mass%, Mo: 0.1 to 1.0%, W: 0.1 to 1.0%, Nb: 0.1% or less, V: 0.1 to 1.0% An electric resistance welded steel pipe for automobile parts, comprising one or more selected from among them.
(4) In any one of (1) to (3), in addition to the above composition, the composition further includes one or two kinds selected from Ca: 0.02% or less and REM: 0.02% or less by mass%. An electric resistance welded steel pipe for automobile parts.
(5) A steel strip is continuously roll-formed into a substantially cylindrical shaped body, and then the circumferential ends of the shaped body are butted together and electro-welded to form an electric-welded steel pipe. A method of manufacturing an electric resistance welded steel pipe for automobile parts by using a welded steel pipe as a raw material and reducing the diameter of the raw material to make an electric resistance welded steel pipe for automobile parts, wherein the steel strip is in mass%, C: 0.41 to 0.55%, Si: 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.05% or less, P: 0.02% or less, S: 0.01% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni : 1.0% or less, and Cu, Ni, and Cr are the following formula (1)
2Cu + 1.52Ni + 1.1Cr ≧ 1.0 (1)
(Here, Cu, Ni, Cr: content of each element (mass%))
And a steel strip having a composition composed of the remaining Fe and inevitable impurities, the reduced diameter rolling is heated and soaked to a temperature equal to or higher than the Ac 3 transformation point, and the rolling finish temperature is 800 ° C. A method for producing an electric-welded steel pipe for automobile parts having excellent fatigue resistance, characterized by reducing rolling at a super-950 ° C. or lower and a cumulative diameter reduction ratio of 35 to 70%.
(6) In (5), in addition to the said composition, B: 0.0003-0.0050% is further included by the mass%, The manufacturing method of the electric-welding steel pipe for motor vehicle parts characterized by the above-mentioned.
(7) In (5) or (6), in addition to the above composition, in terms of mass%, Mo: 0.1 to 1.0%, W: 0.1 to 1.0%, Nb: 0.1% or less, V: 0.1 to 1.0% A method for producing an electric resistance welded steel pipe for automobile parts, comprising one or more selected from among them.
(8) In any one of (5) to (7), in addition to the above composition, the composition further includes one or two selected from Ca: 0.02% or less and REM: 0.02% or less by mass%. A method for producing an electric-welded welded steel pipe for automobile parts.
本発明によれば、焼入れ焼戻処理後に円周方向の硬さ分布が均一化され、耐疲労特性が顕著に向上した電縫溶接鋼管を容易に製造でき、産業上格段の効果を奏する。なお、本発明によれば、焼入れ焼戻処理以外の熱処理を施す場合、あるいは熱処理を施さない場合においても、耐疲労特性が顕著に向上するという効果もある。 ADVANTAGE OF THE INVENTION According to this invention, the hardness distribution of the circumferential direction is equalized after hardening and tempering processing, the electric resistance welded steel pipe which improved the fatigue resistance remarkably can be manufactured easily, and there exists a remarkable effect on an industry. In addition, according to this invention, even when heat processing other than a quenching tempering process is performed, or when not performing a heat processing, there also exists an effect that a fatigue resistance improves notably.
本発明電縫溶接鋼管は、鋼帯を管状（ほぼ円筒状）に成形し電縫溶接してなり、母材部と電縫溶接部とから構成される鋼管で、母材部が、質量％で、Ｃ：0.41〜0.55％、Si；0.01〜0.5％、Mn：1.0〜2.0％、Al：0.05％以下、Ｐ：0.02％以下、Ｓ：0.01％以下、Cr：1.0％以下、Cu：1.0％以下、Ni：1.0％以下を含み、かつCu、Ni、Crが次（１）式
２Cu＋1.52Ni＋1.1Cr ≧ 1.0 ‥‥（１）
（ここで、Cu、Ni、Cr：各元素の含有量（質量％））
を満足するように含有し、残部Feおよび不可避的不純物からなる組成を有する。
The electric resistance welded steel pipe of the present invention is a steel pipe formed by forming a steel strip into a tubular shape (substantially cylindrical) and performing electric resistance welding, and is composed of a base material portion and an electric resistance welded portion. C: 0.41 to 0.55%, Si; 0.01 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.05% or less, P: 0.02% or less, S: 0.01% or less, Cr: 1.0% or less, Cu: 1.0 %, Ni: 1.0% or less, and Cu, Ni, Cr is the following formula (1)
2Cu + 1.52Ni + 1.1Cr ≧ 1.0 (1)
(Here, Cu, Ni, Cr: content of each element (mass%))
And has a composition composed of the remaining Fe and unavoidable impurities.
まず、本発明電縫溶接鋼管の母材部組成について、その限定理由を説明する。以下、とくに断わらない限り、質量％は単に％と記す。
Ｃ：0.41〜0.55％
Ｃは、強度（硬さ）増加に寄与する元素であり、高周波焼入れ処理後に自動車駆動系部品として必要な硬さを確保するためには、0.41％以上の含有を必要とする。Ｃ量が0.41％未満では、焼入れ処理を施しても十分な硬さを確保できないため、所望の耐疲労特性を確保できなくなる。一方、0.55％を超える多量の含有は、溶接性が低下し、安定して所望の電縫溶接部品質を確保できなくなる。このため、Ｃは0.41〜0.55％の範囲に限定した。なお、電縫溶接性を安定して確保するという観点からＣは0.41〜0.50％とすることが好ましい。
First, the reason for limitation of the base material composition of the ERW welded steel pipe of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply referred to as%.
C: 0.41-0.55%
C is an element that contributes to an increase in strength (hardness), and needs to be contained in an amount of 0.41% or more in order to ensure the hardness required for automobile drive system parts after induction hardening. If the amount of C is less than 0.41%, sufficient hardness cannot be ensured even if quenching is performed, and thus desired fatigue resistance characteristics cannot be ensured. On the other hand, if the content exceeds 0.55%, the weldability decreases, and the desired quality of the ERW weld cannot be secured stably. For this reason, C was limited to the range of 0.41 to 0.55%. In addition, it is preferable to set C to 0.41 to 0.50% from the viewpoint of stably securing the electric resistance weldability.
Si：0.01〜0.5％
Siは、脱酸剤として作用するとともに、固溶して鋼の強度を増加させる固溶強化元素である。このような効果を得るためには、0.01％以上の含有を必要とする。0.01％未満の含有では、十分な脱酸効果が得られない。一方、0.5％を超える含有は、溶接性が低下し、安定した電縫溶接部品質を確保できなくなる。このため、Siは0.01〜0.5％の範囲に限定した。なお、好ましくは0.01〜0.3％である。
Si: 0.01-0.5%
Si is a solid solution strengthening element that acts as a deoxidizer and increases the strength of the steel by solid solution. In order to acquire such an effect, 0.01% or more of content is required. If the content is less than 0.01%, a sufficient deoxidizing effect cannot be obtained. On the other hand, if the content exceeds 0.5%, the weldability is lowered, and stable quality of the ERW weld cannot be secured. For this reason, Si was limited to the range of 0.01 to 0.5%. In addition, Preferably it is 0.01 to 0.3%.
Mn：1.0〜2.0％
Mnは、焼入れ性向上を介して鋼の強度増加に寄与する元素である。このような効果を得るためには、1.0％以上の含有を必要とする。一方、2.0％を超える含有は、電縫溶接部品質を低下させ、さらに残留オーステナイト（γ）量が増加し耐疲労特性が低下する。このため、Mnは1.0〜2.0％の範囲に限定した。なお、好ましくは1.0〜1.7％である。
Mn: 1.0-2.0%
Mn is an element that contributes to increasing the strength of steel through improving hardenability. In order to obtain such an effect, a content of 1.0% or more is required. On the other hand, if the content exceeds 2.0%, the quality of the ERW weld is deteriorated, the amount of retained austenite (γ) is increased, and the fatigue resistance is lowered. For this reason, Mn was limited to the range of 1.0 to 2.0%. In addition, Preferably it is 1.0 to 1.7%.
Al：0.05％以下
Alは、脱酸剤として有効な元素であり、また、焼入れ時のγ粒の成長を抑制し、焼入れ処理後の強度確保に寄与する元素である。このような効果を得るためには0.001％以上含有する必要がある。一方、0.05％を超える含有は、その効果が飽和するだけでなく、Al系介在物の増加を伴い、疲労強度が低下する場合がある。さらに、Alは、フェライト安定化元素であり、Ac3変態点を上昇させるため、加熱時に白色層でのＣ量均一化が不十分となる場合がある。このため、Alは0.05％以下に限定した。
Al: 0.05% or less
Al is an element that is effective as a deoxidizer, and is an element that suppresses the growth of γ grains during quenching and contributes to securing strength after quenching. In order to acquire such an effect, it is necessary to contain 0.001% or more. On the other hand, the content exceeding 0.05% not only saturates the effect but also increases the amount of Al-based inclusions, and may reduce the fatigue strength. Furthermore, Al is a ferrite stabilizing element and raises the Ac3 transformation point, so that the C amount in the white layer may be insufficiently uniform during heating. For this reason, Al was limited to 0.05% or less.
Ｐ：0.02％以下
Ｐは、粒界等に偏析しやすく、靭性を低下させる元素であり、また、Alと同様に、フェライト安定化元素であり、本発明ではできるだけ低減することが好ましいが、0.02％までは許容できる。このようなことから、Ｐは0.02％以下に限定した。なお、好ましくは0.018％以下である。
P: 0.02% or less P is an element that easily segregates at grain boundaries and reduces toughness, and, like Al, is a ferrite stabilizing element and is preferably reduced as much as possible in the present invention. % Is acceptable. Therefore, P is limited to 0.02% or less. In addition, Preferably it is 0.018% or less.
Ｓ：0.01％以下
Ｓは、鋼中では硫化物系介在物として存在し、加工性、耐疲労特性および靭性を低下させる元素であり、本発明では、できるだけ低減することが好ましいが、0.01％までは許容できる。このようなことから、Ｓは0.01％以下に限定した。なお、好ましくは0.008％以下である。
S: 0.01% or less S is an element that exists as sulfide inclusions in steel and reduces workability, fatigue resistance, and toughness. In the present invention, S is preferably reduced as much as possible, but up to 0.01%. Is acceptable. For these reasons, S is limited to 0.01% or less. In addition, Preferably it is 0.008% or less.
Cr：1.0％以下
Crは、焼入れ性を高める元素であり、また、γ安定化元素でもあり、本発明では、Ac3変態点を低下させ、白色層近傍のＣ量均一化に有効に寄与する。このような効果を確保するためには、0.1％以上含有することが望ましい。一方、1.0％超えて含有すると、酸化物を形成しやすくなり、電縫溶接性が低下する。このようなことから、Crは1.0％以下に限定した。なお、好ましくは0.1〜0.5％である。
Cr: 1.0% or less
Cr is an element that enhances hardenability and is also a γ-stabilizing element, and in the present invention, it lowers the Ac3 transformation point and effectively contributes to the uniform C content in the vicinity of the white layer. In order to ensure such an effect, it is desirable to contain 0.1% or more. On the other hand, when it contains exceeding 1.0%, it becomes easy to form an oxide and electro-welding weldability falls. For these reasons, Cr is limited to 1.0% or less. In addition, Preferably it is 0.1 to 0.5%.
Cu：1.0％以下
Cuは、焼入れ性を高める元素であり、また、γ安定化元素でもあり、本発明では、Ac3変態点を低下させ、白色層近傍のＣ量均一化に有効に寄与する。このような効果を得るためには、0.1％以上含有することが望ましい。一方、1.0％を超えて含有すると、加工性が著しく低下する。このようなことから、Cuは1.0％以下に限定した。なお、好ましくは0.1〜0.8％である。
Cu: 1.0% or less
Cu is an element that enhances hardenability and is also a γ-stabilizing element. In the present invention, the Ac3 transformation point is lowered and contributes effectively to the uniform C content in the vicinity of the white layer. In order to acquire such an effect, it is desirable to contain 0.1% or more. On the other hand, when it contains exceeding 1.0%, workability will fall remarkably. Therefore, Cu is limited to 1.0% or less. In addition, Preferably it is 0.1 to 0.8%.
Ni：1.0％以下
Niは、焼入れ性を高める元素であり、またγ安定化元素でもあり、本発明では、Ac3変態点を低下させ、白色層近傍のＣ量均一化に有効に寄与する。このような効果を得るためには、0.1％以上含有することが望ましい。一方、1.0％を超えて含有すると、加工性が著しく低下する。このようなことから、Niは1.0％以下に限定した。なお、好ましくは0.1〜0.8％である。
Ni: 1.0% or less
Ni is an element that enhances hardenability and is also a γ-stabilizing element. In the present invention, the Ac3 transformation point is lowered and effectively contributes to uniform C content in the vicinity of the white layer. In order to acquire such an effect, it is desirable to contain 0.1% or more. On the other hand, when it contains exceeding 1.0%, workability will fall remarkably. For these reasons, Ni is limited to 1.0% or less. In addition, Preferably it is 0.1 to 0.8%.
なお、本発明では、Cu、Cr、Niは、上記した範囲内でかつ、次（１）式
２Cu＋1.52Ni＋1.1Cr ≧ 1.0 ‥‥（１）
（ここで、Cu、Ni、Cr：各元素の含有量（質量％））
を満足するように調整して含有する。
Cu、Cr、Niは、上記したようにいずれも、Ac3変態点を低下させる元素であるが、その効果は各元素でそれぞれで異なる。このため、本発明では、各元素の効果を係数で表し、（１）式を満足するように、各元素の含有量を調整する。Cu、Cr、Niの含有量が、（１）式を満足しない場合には、十分なAc3変態点低減効果が認められない。このため、白色層でのＣの拡散量の不足に繋がり、硬さの均一化が不十分となる。
In the present invention, Cu, Cr, and Ni are within the above-mentioned range and the following formula (1)
2Cu + 1.52Ni + 1.1Cr ≧ 1.0 (1)
(Here, Cu, Ni, Cr: content of each element (mass%))
The content is adjusted so as to satisfy.
As described above, Cu, Cr, and Ni are all elements that lower the Ac3 transformation point, but the effect differs depending on each element. For this reason, in this invention, the effect of each element is represented by a coefficient, and the content of each element is adjusted so as to satisfy the expression (1). When the contents of Cu, Cr, and Ni do not satisfy the formula (1), a sufficient effect of reducing the Ac3 transformation point is not recognized. For this reason, it leads to the lack of the amount of C diffusion in the white layer, and the uniformity of the hardness becomes insufficient.
上記した成分が基本の成分であり、本発明では基本成分に加えてさらに、Ｂ：0.0003〜0.0050％、および／または、Mo：0.1〜1.0％、Ｗ：：0.1〜1.0％、Nb：0.1％以下、Ｖ：0.1〜1.0％のうちから選ばれた１種または２種以上、および／または、Ca：0.02％以下、REM：0.02％以下のうちから選ばれた１種または２種、を必要に応じて選択した含有できる。 The above components are basic components. In the present invention, in addition to the basic components, B: 0.0003 to 0.0050%, and / or Mo: 0.1 to 1.0%, W :: 0.1 to 1.0%, Nb: 0.1% Hereinafter, one or two or more selected from V: 0.1 to 1.0% and / or one or two selected from Ca: 0.02% or less and REM: 0.02% or less are required. Depending on the content.
Ｂ：0.0003〜0.0050％
Ｂは、微量含有で焼入れ性を向上させる元素である。このような効果を得るためには、0.0003％以上の含有を必要とする。0.0003％未満では、焼入れ性向上効果が十分に発揮されない。一方、0.0050％を超えて含有しても、その効果が飽和するとともに、粒界に偏析して粒界破壊を促進し耐疲労特性を低下させる。このため、含有する場合には、Ｂは0.0003〜0.0050％の範囲に限定することが好ましい。
B: 0.0003-0.0050%
B is an element that improves hardenability when contained in a trace amount. In order to acquire such an effect, 0.0003% or more needs to be contained. If it is less than 0.0003%, the effect of improving hardenability is not sufficiently exhibited. On the other hand, if the content exceeds 0.0050%, the effect is saturated and segregates at the grain boundaries to promote grain boundary fracture and reduce fatigue resistance. For this reason, when it contains, it is preferable to limit B to 0.0003 to 0.0050% of range.
Mo：0.1〜1.0％、Ｗ：0.1〜1.0％、Nb：0.1％以下、Ｖ：0.1〜1.0％のうちから選ばれた１種または２種以上
Mo、Ｗ、Nb、Ｖはいずれも、鋼の強度を高め、疲労強度の増加に寄与する元素であり、必要に応じて選択して１種または２種以上含有できる。
Moは、焼入れ性向上を介して鋼の強度を増加させる。このような効果を確保するためには、0.1％以上の含有を必要とする。一方、1.0％を超えて含有すると、加工性が著しく低下するうえ、コスト高を招く。このため、含有する場合には、Moは0.1〜1.0％の範囲に限定することが好ましい。
One or more selected from Mo: 0.1 to 1.0%, W: 0.1 to 1.0%, Nb: 0.1% or less, V: 0.1 to 1.0%
Mo, W, Nb, and V are all elements that increase the strength of steel and contribute to an increase in fatigue strength, and can be selected as necessary and contained in one or more.
Mo increases the strength of the steel through improving hardenability. In order to ensure such an effect, the content of 0.1% or more is required. On the other hand, if the content exceeds 1.0%, the workability is remarkably lowered and the cost is increased. For this reason, when it contains, it is preferable to limit Mo to 0.1 to 1.0% of range.
Ｗは、炭化物を形成し、析出強化を介して鋼の強度増加に有効に寄与する。このような効果を得るためには、0.1％以上の含有を必要とする。一方、1.0％を超えて含有すると、炭化物が析出しすぎて、耐疲労特性を低下させ、加工性をも低下させる。このため、含有する場合には、Ｗは0.1〜1.0％の範囲に限定することが好ましい。
Nbは、焼入れ性を向上させ、さらに、炭化物を形成し、鋼の強度増加に有効に寄与する。このような効果を得るためには、0.002％以上含有する必要がある。一方、0.1％を超えて含有すると、上記した効果が飽和するとともに、加工性が低下する。このため、含有する場合には、Nbは0.1％以下に限定することが好ましい。
W forms carbides and contributes effectively to increasing the strength of steel through precipitation strengthening. In order to obtain such an effect, the content of 0.1% or more is required. On the other hand, if the content exceeds 1.0%, carbides are precipitated too much, reducing fatigue resistance and workability. For this reason, when it contains, it is preferable to limit W to 0.1 to 1.0% of range.
Nb improves hardenability, further forms carbides, and contributes effectively to increasing the strength of steel. In order to acquire such an effect, it is necessary to contain 0.002% or more. On the other hand, if the content exceeds 0.1%, the above-described effects are saturated and processability is lowered. For this reason, when it contains, it is preferable to limit Nb to 0.1% or less.
Ｖは、炭化物を形成し、析出強化を介して鋼の強度増加に有効に寄与するとともに、焼戻軟化抵抗の増加に寄与する元素である。このような効果を得るためには、0.1％以上含有する必要がある。一方、1.0％を超えて含有すると、上記した効果が飽和するとともに、加工性が低下する。このため、含有する場合には、Ｖは0.1〜1.0％の範囲に限定することが好ましい。 V is an element that forms carbides and contributes effectively to increasing the strength of steel through precipitation strengthening and contributes to an increase in temper softening resistance. In order to acquire such an effect, it is necessary to contain 0.1% or more. On the other hand, if the content exceeds 1.0%, the above-described effects are saturated and workability is lowered. For this reason, when it contains, it is preferable to limit V to 0.1 to 1.0% of range.
Ca：0.02％以下、REM：0.02％以下のうちから選ばれた１種または２種
Ca、REMはいずれも、酸化物等の介在物形態制御に寄与する元素であり、必要に応じて選択して含有できる。
Caは、介在物の形態を制御する作用を有し、加工性および靭性の向上に寄与する元素である。このような効果を得るためには0.0005％以上含有する必要がある。一方、0.02％を超えて含有すると、介在物量が増大し、延性、靭性がかえって低下する。このため、含有する場合には、Caは0.02％以下に限定することが好ましい。
One or two types selected from Ca: 0.02% or less, REM: 0.02% or less
Both Ca and REM are elements that contribute to the control of inclusion morphology such as oxides, and can be selected and contained as necessary.
Ca is an element that has the effect of controlling the form of inclusions and contributes to the improvement of workability and toughness. In order to acquire such an effect, it is necessary to contain 0.0005% or more. On the other hand, if the content exceeds 0.02%, the amount of inclusions increases, and ductility and toughness are reduced. For this reason, when it contains, it is preferable to limit Ca to 0.02% or less.
REMは、介在物の形態を制御する作用を有し、加工性および靭性の向上に寄与するとともに、耐食性をも向上させる元素である。このような効果を得るためには0.0005％以上含有する必要がある。一方、0.02％を超えて含有すると、磁気特性を低下させる。このため、含有する場合には、REMは0.02％以下に限定することが好ましい。
上記した成分以外の残部は、Feおよび不可避的不純物からなる。不可避的不純物としては、Ｏ：0.01％以下、Ｎ：0.01％以下が許容できる。
REM is an element that has a function of controlling the form of inclusions, contributes to improvement of workability and toughness, and also improves corrosion resistance. In order to acquire such an effect, it is necessary to contain 0.0005% or more. On the other hand, if the content exceeds 0.02%, the magnetic properties are lowered. For this reason, when it contains, it is preferable to limit REM to 0.02% or less.
The balance other than the components described above consists of Fe and inevitable impurities. As unavoidable impurities, O: 0.01% or less and N: 0.01% or less are acceptable.
また、本発明電縫溶接鋼管は、上記した組成を有する母材部と、幅：40μm以下の白色層を有し、高周波焼入れ後に白色層の硬さが母材部との硬さの差ΔHVの絶対値で、25ポイント以下である電縫溶接部から構成される。
電縫溶接部の白色層幅：40μm以下
白色層では局部的にＣ量が低いため、焼入れ硬さの局部的低下域が生じ、割れ発生の原因となる。このため、白色層のＣ量を均一化し、焼入れ硬さの局部的低下域の発生を抑制する必要がある。高周波焼入れなどの短時間加熱において、白色層のＣ量をほぼ均一化するためには、図２に示したように、白色層の幅を40μm以下とする必要がある。白色層の幅が40μmを超えて大きくなると、高周波焼入れなどの短時間加熱では白色層Ｃ量の均一化は達成できない。このようなことから、電縫溶接部の白色層幅を40μm以下に限定した。なお、好ましくは35μm以下である。
The ERW welded steel pipe of the present invention has a base material portion having the above composition and a white layer having a width of 40 μm or less, and the hardness of the white layer after induction hardening is a difference ΔHV in hardness from the base material portion. It is composed of ERW welds with an absolute value of 25 points or less.
White layer width of ERW weld: 40 μm or less Since the C content is locally low in the white layer, a locally reduced region of quenching hardness is generated, which causes cracking. For this reason, it is necessary to equalize the amount of C in the white layer and suppress the occurrence of a locally reduced region of quenching hardness. In order to make the amount of C in the white layer substantially uniform in short-time heating such as induction hardening, the width of the white layer needs to be 40 μm or less as shown in FIG. If the width of the white layer exceeds 40 μm, uniform white layer C content cannot be achieved by short-time heating such as induction hardening. For this reason, the width of the white layer of the ERW weld is limited to 40 μm or less. In addition, Preferably it is 35 micrometers or less.
高周波焼入れを実施したのちの白色層と母材部のマイクロビッカース硬さの差ΔHVの絶対値：25ポイント以下
高周波焼入れ後に白色層の硬さが、母材部に比較して低いと、白色層を起点として亀裂が発生しやすく、耐疲労特性が低下する。本発明では、マイクロビッカース硬さHV0.5（試験荷重：500gf）で、白色層と母材部との硬さ差の絶対値ΔHVを、25ポイント以下に限定した。ΔHVが25ポイントを超えて大きい場合には、耐疲労性が低下するため、ΔHVを25ポイント以下に限定した。
Difference in micro Vickers hardness between the white layer and the base metal after induction hardening ΔHV: 25 points or less If the hardness of the white layer is lower than the base material after induction hardening, the white layer As a starting point, cracks are likely to occur, and the fatigue resistance is reduced. In the present invention, the absolute value ΔHV of the hardness difference between the white layer and the base material is limited to 25 points or less at a micro Vickers hardness of HV0.5 (test load: 500 gf). When ΔHV is greater than 25 points, fatigue resistance decreases, so ΔHV is limited to 25 points or less.
つぎに、本発明電縫溶接鋼管の製造方法について説明する。
上記した組成の鋼帯を連続的にロール成形して略円筒状の成形体としたのち、該成形体の円周方向端部同士を突き合わせ電縫溶接して電縫溶接鋼管とする。なお、使用する鋼帯は、熱延鋼帯あるいは冷延鋼帯のいずれもが適用可能である。
本発明では、このようにして製造された電縫溶接鋼管を素材とする。なお、素材として鍛接鋼管をもちいても何ら問題はない。そして、本発明では、これら素材に、好ましくは加熱処理または均熱処理を施したのち、縮径圧延を施して、自動車部品用、好ましくは自動車駆動系部品用電縫溶接鋼管とする。
Below, the manufacturing method of this invention electric resistance welded steel pipe is demonstrated.
The steel strip having the above composition is continuously roll-formed to form a substantially cylindrical shaped body, and then the circumferential ends of the shaped body are butted together by electro-welding to obtain an electro-welded steel pipe. In addition, as a steel strip to be used, either a hot rolled steel strip or a cold rolled steel strip can be applied.
In the present invention, the electric resistance welded steel pipe manufactured as described above is used as a material. It should be noted that there is no problem in using a forged steel pipe as a material. In the present invention, these materials are preferably subjected to heat treatment or soaking, and then subjected to diameter reduction rolling to obtain an electric resistance welded steel pipe for automobile parts, preferably for automobile drive system parts.
なお、加熱処理または均熱処理は、Ac3変態点以上の加熱温度に加熱・均熱する処理とすることが好ましい。加熱温度がAc3変態点未満では、組織等の均質化が不十分となる。このため、加熱処理または均熱処理の加熱温度はAc3変態点以上に限定することが好ましい。なお、保持時間は３s以上とすることが好ましい。保持時間が３s未満では、均質化の目的を達成できない。なお、加熱処理または均熱処理は、必要に応じて適宜、適用することが望ましい。 Note that the heat treatment or soaking is preferably a heating and soaking treatment at a heating temperature equal to or higher than the Ac3 transformation point. When the heating temperature is less than the Ac3 transformation point, the homogenization of the structure and the like becomes insufficient. For this reason, it is preferable that the heating temperature of the heat treatment or soaking is limited to the Ac3 transformation point or higher. The holding time is preferably 3 seconds or longer. If the holding time is less than 3 s, the purpose of homogenization cannot be achieved. Note that heat treatment or soaking is preferably applied as appropriate.
素材である電縫溶接鋼管に施す縮径圧延は、Ac3変態点以上の温度に加熱したのち、累積縮径率が35〜70％、圧延終了温度が800℃超950℃以下となる圧延とする。
縮径圧延のための加熱温度：Ac3変態点以上
加熱温度がAc3変態点未満では、電縫溶接部の靭性が低下するとともに、白色層でのＣ量の均一化の進行が遅くなる。このため、縮径圧延のための加熱温度はAc3変態点以上に限定した。好ましくは900℃以上である。一方、1100℃を超えて高温とすると、製品の表面性状が低下するため、1100℃以下とすることが望ましい。なお、加熱は誘導加熱とすることが生産性の観点から好ましい。また、縮径圧延のための加熱保持時間は３s以上とすることが材質の均質化、電縫溶接部のＣの拡散を促進するという観点から好ましい。
Reduced diameter rolling applied to the material ERW welded pipe is heated to a temperature above the Ac3 transformation point, and then the cumulative diameter reduction ratio is 35 to 70% and the rolling end temperature is over 800 ° C to 950 ° C or lower. .
Heating temperature for diameter reduction rolling: Ac3 transformation point or more When the heating temperature is less than Ac3 transformation point, the toughness of the ERW welded part is lowered and the progress of uniformizing the C content in the white layer is delayed. For this reason, the heating temperature for diameter reduction rolling was limited to the Ac3 transformation point or higher. Preferably it is 900 degreeC or more. On the other hand, if the temperature is higher than 1100 ° C., the surface properties of the product deteriorate, so it is desirable that the temperature be 1100 ° C. or lower. The heating is preferably induction heating from the viewpoint of productivity. In addition, it is preferable that the heating and holding time for diameter reduction rolling be 3 s or more from the viewpoint of promoting homogenization of the material and diffusion of C in the ERW weld.
縮径圧延の圧延終了温度：800℃超950℃以下
縮径圧延の圧延終了温度が800℃以下と低温では、加工歪が残留しやすく延性が低下し、さらに管表面に残留応力が発生し、その影響で耐疲労特性が低下する。一方、950℃を超えて高温では、管の表面性状が低下し、生産性も低下する。このため、縮径圧延の圧延終了温度は800℃超950℃以下に限定した。なお、好ましくは900℃以下である。
Rolling end temperature of reduced diameter rolling: Over 800 ° C and 950 ° C or lower Rolling end temperature of reduced diameter rolling is as low as 800 ° C or lower, processing strain tends to remain, ductility decreases, and residual stress occurs on the tube surface. As a result, the fatigue resistance is reduced. On the other hand, when the temperature is higher than 950 ° C., the surface properties of the pipe are lowered and the productivity is also lowered. For this reason, the rolling end temperature of the reduced diameter rolling is limited to more than 800 ° C. and 950 ° C. or less. In addition, Preferably it is 900 degrees C or less.
縮径圧延の累積縮径率：35〜70％
電縫溶接鋼管では、通常、白色層幅は50〜100μm程度である。そのため、高周波焼入れ時の加熱で、白色層のＣ量均一化を達成するために必須な白色層幅40μm以下を確保するには、幾何学的に25〜33％程度以上の縮径率を確保することが必要となるため、本発明では累積縮径率を35％以上に限定した。一方、70％を超えると、素材全体の加工硬化が大きくなり、延性が低下し、生産性も低下する。このため、縮径圧延の累積縮径率は35〜70％の範囲に限定した。なお、好ましくは35〜65％である。
Cumulative reduction ratio of reduction rolling: 35-70%
In an electric resistance welded steel pipe, the white layer width is usually about 50 to 100 μm. Therefore, in order to ensure the white layer width of 40 μm or less, which is essential for achieving uniform C content in the white layer by heating during induction hardening, a reduction ratio of about 25 to 33% or more is secured geometrically. Therefore, in the present invention, the cumulative diameter reduction ratio is limited to 35% or more. On the other hand, if it exceeds 70%, work hardening of the whole material becomes large, ductility is lowered, and productivity is also lowered. For this reason, the cumulative diameter reduction ratio of the diameter reduction rolling is limited to a range of 35 to 70%. In addition, Preferably it is 35 to 65%.
上記した組成を有する電縫溶接鋼管を素材とし、この素材に上記した条件で縮径圧延を施すことにより、白色層の幅が40μm以下で、高周波焼入れ後にビッカース硬さHV0.5で測定した白色層と母材部との硬さの差ΔHVの絶対値が、25ポイント以下となる、電縫溶接鋼管が得られる。
以下、実施例に基づき、さらに本発明について説明する。
Using the ERW welded steel pipe having the above composition as a raw material, and by subjecting this raw material to reduction rolling under the above-mentioned conditions, the white layer width is 40 μm or less, and the white measured by Vickers hardness HV0.5 after induction hardening An electric resistance welded steel pipe is obtained in which the absolute value of the hardness difference ΔHV between the layer and the base metal part is 25 points or less.
Hereinafter, based on an Example, this invention is demonstrated further.
表１に示す組成を有する鋳片を、加熱し、熱間圧延して肉厚7.9mmの鋼帯とした。ついで、これら熱延鋼帯を用いて、連続してロール成形して略円筒状の成形体としたのち、該成形体の円周方向端部同士を突き合わせ電縫溶接して電縫溶接鋼管（外径88mmφ×肉厚7.9mm）とした。
これら電縫溶接鋼管を、加熱処理として表２に示す加熱温度に誘導加熱した後、表２に示す条件で累積縮径率を変化させて縮径圧延して、縮径圧延鋼管とした。ついで、これら縮径圧延鋼管に、高周波焼入れ処理、焼戻処理を施した。比較のために、同一組成で同一サイズに製造した電縫溶接鋼管（縮径圧延なし）を作製し、同様の高周波焼入れ焼戻処理を実施した。なお、高周波焼入れ処理は、高周波誘導加熱装置を使用して、加熱温度：950℃、保持時間：１ｓの条件で加熱し、水冷する処理とした。また、焼戻処理は、高周波誘導加熱装置を用い、加熱温度：180℃に加熱後、空冷する処理とした。
A slab having the composition shown in Table 1 was heated and hot-rolled to form a steel strip having a thickness of 7.9 mm. Next, using these hot-rolled steel strips, rolls are continuously formed into a substantially cylindrical shaped body, and then the circumferential ends of the shaped body are butted together by electro-welding and welded by electric-welded steel pipes ( The outer diameter was 88 mmφ × thickness 7.9 mm).
These electric resistance welded steel pipes were induction-heated to the heating temperature shown in Table 2 as heat treatment, and then subjected to diameter reduction rolling under the conditions shown in Table 2 to obtain reduced diameter rolled steel pipes. Next, induction-quenching treatment and tempering treatment were performed on these reduced diameter rolled steel pipes. For comparison, ERW welded steel pipes having the same composition and the same size (no reduction rolling) were produced and subjected to the same induction hardening and tempering treatment. In addition, the induction hardening process was set as the process which heats on the conditions of heating temperature: 950 degreeC and holding time: 1 second using a high frequency induction heating apparatus, and water-cools. Further, the tempering treatment was performed by heating to a heating temperature of 180 ° C. and then air cooling using a high frequency induction heating apparatus.
なお、高周波焼入れ処理を行う前に、電縫溶接鋼管の白色層を含んで電縫溶接部近傍から試験片を採取し、ＥＰＭＡを用いて、白色層を含み円周方向にＣ量分析を行い、白色層幅を測定した。
また、高周波焼入れ焼戻処理後の電縫溶接鋼管から試験片を採取し、電縫溶接部の白色層と、白色層から円周方向に20mm離れた母材部について、ビッカース硬さ計（荷重：500gf（試験力：4.9Ｎ））を用いて、マイクロビッカース硬さHV0.5を測定し、各位置で５点測定し、その算術平均値を、それぞれの平均硬さHV0.5とし、それら平均値を用いて、白色層と母材部との差ΔHVを算出した。
Before induction hardening, a specimen is taken from the vicinity of the ERW weld including the white layer of the ERW welded pipe, and the amount of C is analyzed in the circumferential direction including the white layer using EPMA. The white layer width was measured.
In addition, specimens were collected from the ERW welded steel pipe after induction hardening and tempering treatment, and the Vickers hardness tester (load) was applied to the white layer of the ERW weld and the base metal part 20 mm away from the white layer in the circumferential direction. : 500gf (test force: 4.9N)), measure the micro Vickers hardness HV0.5, measure 5 points at each position, the arithmetic average value of each average hardness HV0.5, those Using the average value, the difference ΔHV between the white layer and the base material was calculated.
また、得られた高周波焼入れ焼戻処理後の電縫溶接鋼管から、管状のねじり疲労試験片（長さ：450mm） を採取し、ねじり疲労試験を実施した。ねじり疲労試験は、負荷応力：330MPa、応力比：−１（両振り）、周波数２Hz、波形：正弦波の条件で行い、破断時までの繰返し回数を測定し、耐疲労特性を評価した。なお、ねじり疲労試験片は、管状の試験片の外面側を研削し、平行部径：32.5mmφ、肉厚5.1mmの平行部を形成した。平行部以外は径：36.8mmφ、肉厚7.2mmとした。そして、繰返し回数が100万回以上である場合を耐ねじり疲労性に優れると評価した。なお、目視で亀裂の起点も調査した。 In addition, a tubular torsional fatigue test piece (length: 450 mm) was collected from the obtained ERW welded steel pipe after induction hardening and tempering treatment, and a torsional fatigue test was performed. The torsional fatigue test was performed under the conditions of load stress: 330 MPa, stress ratio: −1 (both swing), frequency 2 Hz, waveform: sine wave, and the number of repetitions until breakage was measured to evaluate fatigue resistance. In the torsional fatigue test piece, the outer surface side of the tubular test piece was ground to form a parallel part having a parallel part diameter of 32.5 mmφ and a wall thickness of 5.1 mm. Except for the parallel part, the diameter was 36.8 mmφ and the wall thickness was 7.2 mm. The case where the number of repetitions was 1 million times or more was evaluated as being excellent in torsional fatigue resistance. The starting point of the crack was also examined visually.
得られた結果を表２に示す。 The obtained results are shown in Table 2.
本発明例はいずれも、白色層が40μm以下と狭く、白色層と母材部との硬さ差ΔHVの絶対値が25ポイント以下と均一な硬さ分布となり、高周波焼入れ焼戻処理後の耐疲労特性が顕著に向上した電縫溶接鋼管となっている。一方、本発明の範囲を外れる比較例は、白色層が所望値より広くなっており、ねじり疲労試験時に白色層部から早期に、疲労亀裂が発生し、耐ねじり疲労特性が低下している。 In all of the examples of the present invention, the white layer is as narrow as 40 μm or less, and the hardness difference ΔHV between the white layer and the base material part has a uniform hardness distribution of 25 points or less, and resistance to resistance after induction hardening and tempering treatment. This is an electric resistance welded steel pipe with significantly improved fatigue characteristics. On the other hand, in the comparative example that is outside the scope of the present invention, the white layer is wider than the desired value, fatigue cracks occur early from the white layer portion during the torsional fatigue test, and the torsional fatigue resistance is reduced.
Claims (8)
前記母材部が、質量％で、
Ｃ：0.41〜0.55％、 Si：0.01〜0.5％、
Mn：1.0〜2.0％、 Al：0.05％以下、
Ｐ：0.02％以下、 Ｓ：0.01％以下、
Cr：1.0％以下、 Cu：1.0％以下、
Ni：1.0％以下
を含み、かつCu、Ni、Crが下記（１）式を満足するように含有し、残部Feおよび不可避的不純物からなる組成を有し、前記電縫溶接部が白色層を有し、縮径圧延ままで該白色層の幅が40μm以下で、高周波焼入れ後にビッカース硬さHV0.5で測定した前記白色層の硬さと前記母材部との硬さの差ΔHVの絶対値が、25ポイント以下であることを特徴とする耐疲労特性に優れた自動車部品用電縫溶接鋼管。
記
２Cu＋1.52Ni＋1.1Cr ≧ 1.0 ‥‥（１）
ここで、Cu、Ni、Cr：各元素の含有量（質量％） A electric resistance welding steel pipe having a base metal and the electric-resistance welded portion,
The base material part is mass%,
C: 0.41-0.55%, Si: 0.01-0.5%
Mn: 1.0 to 2.0%, Al: 0.05% or less,
P: 0.02% or less, S: 0.01% or less,
Cr: 1.0% or less, Cu: 1.0% or less,
Ni: 1.0% or less and Cu, Ni, Cr are contained so as to satisfy the following formula (1), and the composition is composed of the remaining Fe and inevitable impurities, and the ERW weld has a white layer. a width of the white color layer remain contraction径圧rolling is at 40μm or less, the absolute value of the difference ΔHV of the hardness of the hardness and the base metal of the white layer measured by Vickers hardness HV0.5 after induction hardening Is an electric resistance welded steel pipe for automobile parts with excellent fatigue resistance, characterized by being 25 points or less.
Record
2Cu + 1.52Ni + 1.1Cr ≧ 1.0 (1)
Here, Cu, Ni, Cr: Content of each element (mass%)
前記鋼帯を、質量％で、
Ｃ：0.41〜0.55％、 Si；0.01〜0.5％、
Mn：1.0〜2.0％、 Al：0.05％以下、
Ｐ：0.02％以下、 Ｓ：0.01％以下、
Cr：1.0％以下、 Cu：1.0％以下、
Ni：1.0％以下
を含み、かつCu、Ni、Crが下記（１）式を満足するように含有し、残部Feおよび不可避的不純物からなる組成を有する鋼帯とし、
加熱処理または均熱処理を、Ａc３変態点以上の温度に加熱・均熱する処理とし、
前記縮径圧延を、圧延終了温度：800℃超950℃以下で、累積縮径率：35〜70％である縮径圧延とし、
前記自動車部品用電縫溶接鋼管が母材部と電縫溶接部とから構成され、
前記電縫溶接部における白色層の幅が40μm以下で、かつ、ビッカース硬さHV0.5で測定した、高周波焼入れ後の前記白色層の硬さと前記母材部との硬さの差ΔHVの絶対値が、25ポイント以下であることを特徴とする耐疲労特性に優れた自動車部品用電縫溶接鋼管の製造方法。
記
２Cu＋1.52Ni＋1.1Cr ≧ 1.0 ‥‥（１）
ここで、Cu、Ni、Cr：各元素の含有量（質量％） After the steel strip is continuously roll-formed into a substantially cylindrical shaped body, the circumferential ends of the shaped body are butted together and welded into an ERW steel pipe, and the ERW steel pipe as a material , A method for producing an electric resistance welded steel pipe for automobile parts by subjecting the material to reduction rolling, and making an electric resistance steel pipe for automobile parts,
The steel strip in mass%,
C: 0.41 to 0.55%, Si; 0.01 to 0.5%,
Mn: 1.0 to 2.0%, Al: 0.05% or less,
P: 0.02% or less, S: 0.01% or less,
Cr: 1.0% or less, Cu: 1.0% or less,
Ni: A steel strip containing 1.0% or less and containing Cu, Ni, Cr so as to satisfy the following formula (1), and having a composition comprising the balance Fe and inevitable impurities ,
The pressurized heat treatment or soaking, the heating and soaking treatment Ac 3 transformation point or above the temperature,
The diameter reduction rolling is a rolling reduction temperature: a rolling end temperature: more than 800 ° C. and 950 ° C. or less, and a cumulative diameter reduction ratio: 35 to 70%,
The electric parts welded steel pipe for automobile parts is composed of a base material part and an electric resistance welded part,
In less than the width of the white layer is 40μm in the electric-resistance welded portion, and was measured by bi Vickers hardness HV0.5, the difference in hardness between the hardness and the base metal of the white layer after induction hardening ΔHV A method for producing an electric resistance welded steel pipe for automobile parts having excellent fatigue resistance, characterized in that the absolute value of is 25 points or less.
Record
2Cu + 1.52Ni + 1.1Cr ≧ 1.0 (1)
Here, Cu, Ni, Cr: Content of each element (mass%)
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