JP4635708B2 - Non-tempered high-tensile welded steel pipe for automotive structural members with excellent formability and low-temperature toughness and excellent torsional fatigue resistance after cross-section forming processing - Google Patents
Non-tempered high-tensile welded steel pipe for automotive structural members with excellent formability and low-temperature toughness and excellent torsional fatigue resistance after cross-section forming processing Download PDFInfo
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Description
本発明は、溶接鋼管に係り、とくにトーションビーム、アクスルビーム、トレーリングアーム、サスペンションアーム、スタビライザーなどの自動車構造部材用として好適な、自動車構造部材用高張力溶接鋼管に関する。 The present invention relates to a welded steel pipe, and more particularly to a high-tensile welded steel pipe for automobile structural members suitable for automobile structural members such as torsion beams, axle beams, trailing arms, suspension arms, and stabilizers.
近年、地球環境の保全という観点から、自動車の燃費低減が指向され、そのため、自動車車体の軽量化が熱望されている。車体の軽量化のために、トーションビーム、スタビライザーなどの自動車構造部材についても例外ではなく、従来、棒鋼を製品形状に加工した中実材、あるいは溶接した板材が使用されていたこれら自動車構造部材への、閉断面高剛性の中空材(鋼管)の適用が考えられるようになってきている。 In recent years, from the viewpoint of conservation of the global environment, reduction in fuel consumption of automobiles has been aimed at, and therefore, weight reduction of automobile bodies has been eagerly desired. To reduce the weight of the vehicle body, automotive structural members such as torsion beams and stabilizers are no exception. Conventionally, solid materials in which steel bars are processed into product shapes or welded plate materials have been used. The application of hollow materials (steel pipes) having a closed section and high rigidity has been considered.
例えば、特許文献1には、中空スタビライザー用として、C、Si、Mn、Cr、Mo、Bを含有し、さらにTiをTi/Nが所定の関係を満足するように含み、あるいはさらに化学組成を理想臨界直径が1.0 in以上となるように調整し、好ましくは平均フェライト結晶粒径が3〜40μmで、アスペクト比が0.5〜3.0のフェライト結晶が面積率で90%以上となる組織を有する電縫溶接鋼管が提案されている。特許文献1に記載された電縫溶接鋼管は、母鋼管を高周波誘導加熱により所定の温度に加熱したのち、縮径圧延することで製造でき、電縫溶接部および母材部の金属組織が均一で、加工性に優れた鋼管であるとされる。しかし、この技術では、スタビライザー形状に成形したのち、強度確保のため、焼入れ‐焼戻処理を施すことが必須であり、製造コストが高くなるという問題に加えて、焼入れ処理時に歪(焼入れ歪)が発生するため、小径・厚肉部材に限定されるという問題がある。 For example, Patent Document 1 includes C, Si, Mn, Cr, Mo, and B for hollow stabilizers, and further includes Ti so that Ti / N satisfies a predetermined relationship, or further includes a chemical composition. Adjusting so that the ideal critical diameter is 1.0 in or more, preferably having a structure in which ferrite crystals having an average ferrite crystal grain size of 3 to 40 μm and an aspect ratio of 0.5 to 3.0 have an area ratio of 90% or more Welded steel pipes have been proposed. The ERW welded steel pipe described in Patent Document 1 can be manufactured by heating the mother steel pipe to a predetermined temperature by high-frequency induction heating and then rolling it down, and the metal structure of the ERW welded part and the base metal part is uniform. Therefore, it is said that the steel pipe has excellent workability. However, with this technology, after forming into a stabilizer shape, it is indispensable to perform quenching and tempering in order to ensure strength. In addition to the problem of increased manufacturing costs, distortion (quenching strain) occurs during quenching. Since this occurs, there is a problem that it is limited to small diameter and thick members.
また、特許文献2には、C含有量を0.05〜0.2質量%の範囲とし、目標特性に応じて、Mn含有量を1.5超え〜5.0質量%の範囲に変化して、引張強さ:780MPa以上、n値とr値の積(n×r)が0.15以上としたハイドロフォーミング性に優れた溶接鋼管が提案されている。特許文献2に記載された溶接鋼管は自動車の構造や足回り部材などの使途に好適な鋼管であり、上記した組成を有する鋼管を加熱後、圧延終了温度500〜900℃、累積縮径率35%以上で絞り圧延を施すことにより製造できるとしている。 Patent Document 2 discloses that the C content is in the range of 0.05 to 0.2% by mass, the Mn content is changed in the range of more than 1.5 to 5.0% by mass according to the target characteristics, and the tensile strength is 780 MPa or more. A welded steel pipe excellent in hydroforming properties in which the product of n value and r value (n × r) is 0.15 or more has been proposed. The welded steel pipe described in Patent Document 2 is a steel pipe suitable for the use of automobile structures, suspension members, and the like. After heating the steel pipe having the above-described composition, the rolling end temperature is 500 to 900 ° C., and the cumulative diameter reduction ratio is 35. It is said that it can be manufactured by drawing at a ratio of at least%.
また、特許文献3には、特定組成を有する電縫溶接してなる鋼管を縮径圧延するに際し、γ域ないしγ+αの二相域で縮径率:10%以上で縮径圧延する前段圧延と、γ域に昇温する中間再加熱と、γ域で縮径率:5%以上で縮径圧延する後段圧延とを行い、複合二次加工性に優れた引張強さ:550MPa以上の高張力鋼管とする高張力鋼管の製造方法が提案されている。特許文献3に記載された鋼管は、曲げ加工、縮径加工、管端偏平加工を組み合わせた複合二次加工でも割れ発生がなく、足回り部材等の自動車構造部材用に好適な鋼管であるとされる。 Patent Document 3 discloses a pre-rolling method in which when a steel pipe having a specific composition is subjected to diameter reduction rolling, the diameter reduction ratio is 10% or more in a γ range or a γ + α two-phase range. , Intermediate reheating to raise the temperature to the γ region, and subsequent rolling to reduce the diameter reduction ratio: 5% or more in the γ region, and excellent tensile strength of composite secondary workability: high tension of 550 MPa or more A method for producing a high-strength steel pipe as a steel pipe has been proposed. The steel pipe described in Patent Document 3 is a steel pipe suitable for automobile structural members such as undercarriage members without cracking even in composite secondary processing combining bending processing, diameter reduction processing, and tube end flattening processing. Is done.
また、特許文献4には、C:0.10〜0.20%、Mn:1.3〜2.5%を含み、さらにTi、B、Cr、Mo、N等を適正量含有する鋼スラブを、950℃〜Ar3変態点以上で熱間圧延し、450〜700℃で巻き取った熱延コイルを電縫溶接し、溶接鋼管とすることにより、引張強さ:100〜130kgf/mm2 で溶接熱影響部が軟化しにくい高強度電縫鋼管とする、高強度電縫鋼管の製造方法が提案されている。特許文献4に記載された技術で製造された電縫鋼管は、引張強さが高く、疲労強度が高くなるとしている。 Patent Document 4 includes a steel slab containing C: 0.10 to 0.20%, Mn: 1.3 to 2.5%, and further containing appropriate amounts of Ti, B, Cr, Mo, N, and the like, from 950 ° C to Ar3 transformation point. The hot-rolled coil that has been hot-rolled as described above and wound at 450 to 700 ° C. is electro-welded and made into a welded steel pipe, so that the weld heat-affected zone is difficult to soften at a tensile strength of 100 to 130 kgf / mm 2. A method for producing a high-strength ERW steel pipe has been proposed. The electric resistance welded steel pipe manufactured by the technique described in Patent Document 4 has high tensile strength and high fatigue strength.
しかしながら、特許文献2、特許文献3に記載された技術により製造された高張力鋼管は、成形性に優れるものの、トーションビーム、アクスルビーム、トレーリングアーム、サスペンションアーム、スタビライザーなどの自動車構造用部材として必要な、断面形状成形加工後に高い捻り疲労強度を確保することができないという問題があった。また、特許文献4に記載された技術では、強度が高すぎて加工性に問題があり、液圧加工などの異形断面化に適応できないという問題がある。 However, high-tensile steel pipes manufactured by the techniques described in Patent Document 2 and Patent Document 3 are excellent in formability, but are necessary as automotive structural members such as torsion beams, axle beams, trailing arms, suspension arms, and stabilizers. In addition, there is a problem that high torsional fatigue strength cannot be secured after the cross-sectional shape forming process. Further, the technique described in Patent Document 4 has a problem that the strength is too high and there is a problem in workability, and it cannot be applied to a modified cross-section such as hydraulic processing.
このような問題に対し、特許文献5には、C:0.035〜0.185%、Mn:0.75〜1.95%含み、さらにTi:0.010〜0.145%、Mo:0.01〜0.49%を含み、さらに粒径:10nm以下、原子比でMo/(Ti+Mo)が0.33〜0.77である(Ti,Mo)複合炭化物が析出したフェライト組織を面積率で60〜100%とする高張力溶接鋼管が提案されている。特許文献5に記載された溶接鋼管は、引張強さ590MPa以上を有し、曲げ、液圧、拡管、縮管、およびこれらを複合した成形等に必要な加工性と、曲げ加工後の優れた疲労特性を兼備した高張力溶接鋼管であるとされ、サスペンションアーム、サスペンションメンバー、アクスルビーム、スタビライザー、フレーム、シャフトなどの自動車構造用部材として好適であるとされる。しかしながら、特許文献5に記載された溶接鋼管は、得られる伸びレベルがJIS12号試験片で18パーセント未満、JIS11号試験片で30%未満と低く、成形できる形状に制約があり成形性に問題を残しているうえ、曲げ加工ではなく断面形状成形加工を行い捻り疲労試験を行うと、剪断応力により約1/√3疲労強度が低下するため、所望の疲労強度が得られなくなる場合があり、断面形状成形加工後の耐捻り疲労特性に問題を残していた。
本発明は、上記した従来技術の問題を有利に解決し、焼入れ焼戻処理を施すことなく、トーションビーム、アクスルビーム、トレーリングアーム、サスペンションアーム、スタビライザーなどの自動車構造部材用として好適な、引張強さ:700MPa以上の高強度(高張力)を有し、成形性、低温靭性に優れ、かつ断面成形加工後の耐捩り疲労特性に優れる高張力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。なお、本発明が目的とする高張力溶接鋼管は、焼入れ焼戻処理を施す必要のない非調質型の鋼管とする。 The present invention advantageously solves the problems of the prior art described above, and is suitable for use in automobile structural members such as torsion beams, axle beams, trailing arms, suspension arms, and stabilizers without quenching and tempering. The purpose is to provide a high-strength welded steel pipe having a high strength (high tension) of 700 MPa or more, excellent formability and low-temperature toughness, and excellent torsional fatigue resistance after cross-section forming, and a method for producing the same. To do. The high-tensile welded steel pipe intended by the present invention is a non-tempered steel pipe that does not require quenching and tempering.
ここで、本発明でいう「成形性に優れる」とは、JIS Z 2201の規定に準拠したJIS12号試験片を用い、JIS Z 2241の規定に準拠して行った引張試験での伸びElが18%以上(JIS11号試験片では30%以上)を示す場合をいうものとする。また、「低温靭性に優れる」とはJIS Z 2202の規定に準拠したC方向の展開Vノッチシャルピー試験片(1/4サイズ)を用い、JIS Z 2242の規定に準拠して行ったシャルピー衝撃試験により得られた破面遷移温度が、母材部で−60℃以下およびシーム溶接部(電縫溶接部)で−20℃以下をともに満足する場合をいうものとする。 Here, "excellent in formability" as used in the present invention means that an elongation El in a tensile test conducted in accordance with the JIS Z 2241 standard is 18 using a JIS No. 12 test piece compliant with the JIS Z 2201 standard. % Or more (30% or more for JIS11 test piece). “Excellent low-temperature toughness” means a Charpy impact test conducted in accordance with the provisions of JIS Z 2242 using a C-direction developed V-notch Charpy test piece (1/4 size) in accordance with the provisions of JIS Z 2202. When the fracture surface transition temperature obtained by the above conditions satisfies both −60 ° C. or lower at the base metal part and −20 ° C. or lower at the seam welded portion (electro-sealed welded portion).
また、「断面成形加工後の耐捩り疲労特性に優れる」とは、図5に示すように、鋼管の長手中央部分をV字形状に断面を成形加工したのち、両端部をチャッキングにより固定して捻り疲労試験を、1Hz、両振りの条件で行い5×105繰返し疲れ限度σBを求め、得られた5×105繰返し疲れ限度σBと鋼管引張強さTSとの比、(σB/TS)が0.35以上である場合をいうものとする。 “Excellent torsional fatigue resistance after cross-section forming” means that, as shown in FIG. 5, after the cross-section is formed into a V-shaped cross section at the longitudinal center of the steel pipe, both ends are fixed by chucking. the torsional fatigue test Te, 1 Hz, seek is performed 5 × 10 5 repeated fatigue limit sigma B of both swing conditions resulting 5 × 10 5 repeated fatigue limit sigma B and the ratio of the steel pipe tensile strength TS, (sigma B / TS) is 0.35 or more.
本発明者らは、上記した課題を達成するため、強度、成形性、低温靭性、断面成形加工後の耐捻り疲労特性といった相反する特性に影響する要因、とくに鋼管の化学成分、製造条件について系統的に鋭意検討した。その結果、C含有量を低くかつ狭い範囲に限定したうえで、高Mn含有量とし、さらにC/Mn比を所定値以下に調整した特定鋼管組成とし、さらに特定の条件で縮径圧延することにより、所望の高強度を有し、かつ成形性、低温靭性および断面成形加工後の耐捻り疲労特性がともに優れた高張力溶接鋼管が得られることを見出した。 In order to achieve the above-mentioned problems, the present inventors have established a system for factors affecting conflicting properties such as strength, formability, low-temperature toughness, and torsional fatigue resistance after cross-section forming processing, particularly the chemical composition and manufacturing conditions of steel pipes. We studied earnestly. As a result, after limiting the C content to a low and narrow range, a high Mn content, a specific steel pipe composition with the C / Mn ratio adjusted to a predetermined value or less, and further reduction rolling under specific conditions Thus, it has been found that a high-tensile welded steel pipe having desired high strength and excellent in formability, low-temperature toughness and torsional fatigue resistance after cross-section forming can be obtained.
本発明は、このような知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨はつぎのとおりである。 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)鋼帯を連続成形してオープン管とし、該オープン管を電縫溶接して管体としたのち、該管体に縮径圧延を施し製品管とする溶接鋼管の製造方法において、前記鋼帯を、質量%で、C:0.035〜0.099%、Si:0.10〜0.45%、Mn:2.05〜2.8%、Ti:0.001〜0.04%、Nb:0.001〜0.04%、B:0.0001〜0.0035%、Cr:0.001〜0.29%、Al:0.01〜0.08%を含み、かつCとMnを、C含有量とMn含有量との比、C/Mnが0.040以下を満足するように含有し、不純物としてのPを0.019%以下、Sを0.003%以下、Nを0.005%以下、Oを0.0025%以下に制限し、残部Feおよび不可避的不純物からなる組成を有する鋼スラブを加熱し、仕上圧延温度:980〜750℃、巻取り温度:700〜350℃とする熱間圧延により得られた熱延鋼帯とし、前記縮径圧延が、前記管体を1000〜850℃に加熱したのち、該管体に縮径圧延終了温度:800〜620℃、縮径率:25〜75%とする縮径圧延を施す圧延であり、該縮径圧延終了後、620〜420℃までを平均冷却速度:0.5〜50℃/sで冷却し、前記製品管を、内面および外面の表面粗さが算術平均粗さRa:2μm以下、最大高さ粗さRz:30μm以下、十点平均粗さRzJIS:20μm以下である製品管とすることを特徴とする成形性と低温靭性に優れ、かつ断面成形加工後の耐ねじり疲労特性に優れた自動車構造部材用非調質高張力溶接鋼管の製造方法。 ( 1 ) In the method for producing a welded steel pipe, the steel strip is continuously formed into an open pipe, and the open pipe is electro-welded to form a pipe, and then the pipe is subjected to diameter reduction rolling to obtain a product pipe. Steel strip in mass%, C: 0.035 to 0.099%, Si: 0.10 to 0.45%, Mn: 2.05 to 2.8%, Ti: 0.001 to 0.04%, Nb: 0.001 to 0.04%, B: 0.0001 to 0.0035%, Cr: 0.001 to 0.29%, Al: 0.01 to 0.08%, and C and Mn are contained so that the ratio of C content to Mn content, C / Mn satisfies 0.040 or less, A steel slab having a composition composed of the balance Fe and unavoidable impurities is heated by limiting P to 0.019% or less, S to 0.003% or less, N to 0.005% or less, O to 0.0025% or less, and finish rolling temperature: 980 to A hot-rolled steel strip obtained by hot rolling at 750 ° C. and a winding temperature of 700 to 350 ° C., and after the diameter reduction heating the tube to 1000 to 850 ° C., the tube is reduced to the tube. Radial pressure End temperature: 800 to 620 ° C., reduction ratio: 25 to 75% reduction rolling, average cooling rate from 0.5 to 50 ° C./s from 620 to 420 ° C. after completion of the reduction rolling. The product tube is cooled with a product tube, and the surface roughness of the inner surface and the outer surface is arithmetic mean roughness Ra: 2 μm or less, maximum height roughness Rz: 30 μm or less, and ten-point average roughness Rz JIS : 20 μm or less A method for producing a non-tempered high-tensile welded steel pipe for automobile structural members that has excellent formability and low-temperature toughness, and has excellent torsional fatigue resistance after cross-section forming.
(2)(1)において、前記鋼スラブが、前記組成に加えてさらに、質量%で、V:0.001〜0.04%、W:0.001〜0.04%のうちから選ばれた1種または2種を含有する組成を有することを特徴とする自動車構造部材用非調質高張力溶接鋼管の製造方法。
(3)(1)または(2)において、前記鋼スラブが、前記組成に加えてさらに、質量%で、Mo:0.001〜0.2%を含有する組成を有することを特徴とする自動車構造部材用非調質高張力溶接鋼管の製造方法。
( 2 ) In ( 1 ), in addition to the composition, the steel slab further contains one or two kinds selected from V: 0.001 to 0.04% and W: 0.001 to 0.04% in mass%. The manufacturing method of the non-tempered high tension welded steel pipe for automotive structural members characterized by having the composition to do.
( 3 ) In ( 1 ) or ( 2 ), the steel slab further has a composition containing Mo: 0.001 to 0.2% by mass% in addition to the above composition. Manufacturing method of tempered high-tensile welded steel pipe.
(4)(1)ないし(3)のいずれかにおいて、前記鋼スラブが、前記組成に加えてさらに、質量%で、Cu:0.001〜0.2%、Ni:0.001〜0.2%のうちから選ばれた1種または2種を含有する組成を有することを特徴とする自動車構造部材用非調質高張力溶接鋼管。
(5)(1)ないし(4)のいずれかにおいて、前記鋼スラブが、前記組成に加えてさらに、質量%で、Ca:0.0001〜0.003%を含有する組成を有することを特徴とする自動車構造部材用非調質高張力溶接鋼管の製造方法。
( 4 ) In any one of ( 1 ) to ( 3 ), the steel slab is further selected from Cu: 0.001 to 0.2% and Ni: 0.001 to 0.2% by mass% in addition to the composition. A non-tempered high-tensile welded steel pipe for automobile structural members, characterized by having a composition containing one or two kinds.
( 5 ) The automobile structure according to any one of ( 1 ) to ( 4 ), wherein the steel slab further has a composition containing Ca: 0.0001 to 0.003% by mass% in addition to the composition. Manufacturing method of non-tempered high strength welded steel pipe for parts.
(6)(1)ないし(5)のいずれかにおいて、前記製品管の内面あるいは外面の一部又は全部に、さらに研削加工、研磨加工あるいはホーニング加工を施し、該製品管の表面粗さを、算術平均粗さRa:1μm以下、最大高さ粗さRz:15μm以下、十点平均粗さRzJIS:10μm以下に調整することを特徴とする自動車構造部材用非調質高張力溶接鋼管の製造方法。 ( 6 ) In any one of ( 1 ) to ( 5 ), part or all of the inner surface or outer surface of the product pipe is further subjected to grinding, polishing or honing, and the surface roughness of the product pipe is the arithmetic mean roughness Ra: 1 [mu] m or less, the maximum height roughness Rz: 15 [mu] m or less, ten point average roughness Rz JIS: you and adjusting to 10μm or less automobile structural member for a non-tempered high tensile welded steel pipe Manufacturing method.
(7)(1)ないし(6)いずれかにおいて、前記製品管に、さらに、100〜750℃の範囲の温度に加熱し、該温度で1〜3600sの範囲の時間、保持する歪取り焼鈍を施すことを特徴とする自動車構造部材用非調質高張力溶接鋼管の製造方法。
(8)(1)ないし(7)ずれかにおいて、前記製品管の内面あるいは外面の一部または全部に、さらにショットブラスト処理、サンドブラスト処理、ショットピーニング処理のうちのいずれかの処理を施すことを特徴とする自動車構造部材用非調質高張力溶接鋼管の製造方法。
( 7 ) In any one of ( 1 ) to ( 6 ), the product tube is further heated to a temperature in the range of 100 to 750 ° C., and is subjected to strain relief annealing that is maintained for a time in the range of 1 to 3600 s. A method for producing a non-tempered high-tensile welded steel pipe for automobile structural members.
( 8 ) In any of ( 1 ) to ( 7 ), a part or all of the inner surface or outer surface of the product pipe is further subjected to any one of shot blasting, sandblasting, and shot peening. A method for producing a non-tempered high-tensile welded steel pipe for automobile structural members.
本発明によれば、引張強さ:700MPa以上の高強度を有し、自動車構造部材用として好適な、成形性、低温靭性および断面成形加工後の耐捻り疲労特性に優れた高張力溶接鋼管を、非調質で、容易に安価に製造でき、産業上格段の効果を奏する。本発明になる高張力溶接鋼管は、トーションビーム、アクスルビーム、トレーリングアーム、サスペンションアーム、スタビライザーなど、断面成形加工後の耐捩り疲労特性をとくに必要とする自動車構造部材向けとして好適である。 According to the present invention, a high-strength welded steel pipe having a high tensile strength of 700 MPa or more and excellent in formability, low-temperature toughness and torsional fatigue resistance after cross-section forming processing is suitable for automobile structural members. It is non-tempered, can be easily manufactured at low cost, and has a remarkable industrial effect. The high-tensile welded steel pipe according to the present invention is suitable for automobile structural members that particularly require torsional fatigue resistance after cross-section forming processing, such as torsion beams, axle beams, trailing arms, suspension arms, and stabilizers.
まず、本発明鋼管の組成限定理由について説明する。なお、以下、組成における質量%は単に%で示す。
C:0.035〜0.099%
Cは、強度、成形性、低温靭性をバランスよく向上させる、本発明において重要な元素である。引張強さ700MPa以上の高強度を確保するために、0.035%以上の含有を必要とする。一方、0.099%を超える含有は、組織中にマルテンサイトなど、Cの濃化した硬質第二相が増加するため、優れた低温靭性を確保できなくなる。とくにシーム溶接部の靭性劣化が顕著となる。このため、Cは0.035〜0.099%の範囲に限定した。なお、好ましくは0.045〜0.075%である。
First, the reasons for limiting the composition of the steel pipe of the present invention will be described. Hereinafter, the mass% in the composition is simply indicated by%.
C: 0.035 to 0.099%
C is an important element in the present invention that improves the strength, formability, and low-temperature toughness in a well-balanced manner. In order to secure a high strength of 700 MPa or more, it is necessary to contain 0.035% or more. On the other hand, if the content exceeds 0.099%, the hard second phase enriched with C, such as martensite, increases in the structure, so that excellent low temperature toughness cannot be secured. In particular, the deterioration of toughness of seam welds becomes significant. For this reason, C was limited to the range of 0.035 to 0.099%. In addition, Preferably it is 0.045 to 0.075%.
Mn:2.05%〜2.8%
Mnは、Cと同様に強度、成形性、低温靭性をバランスよく向上させる、本発明において重要な元素である。引張強さ700MPa以上の高強度を確保するために、2.05%以上の含有を必要とする。一方、2.8%を超えて含有すると、組織が全般に硬質化し、延性が低下し、さらに低温靭性、とくにシーム溶接部の低温靭性が顕著に低下する。このため、Mnは2.05%〜2.8%の範囲に限定した。なお、好ましくは2.3〜2.6%である。
Mn: 2.05% to 2.8%
Mn, like C, is an important element in the present invention that improves the strength, formability, and low temperature toughness in a well-balanced manner. In order to secure a high strength of 700 MPa or more, it is necessary to contain 2.05% or more. On the other hand, if the content exceeds 2.8%, the structure is generally hardened, the ductility is lowered, and the low-temperature toughness, particularly the low-temperature toughness of the seam welded portion is significantly lowered. For this reason, Mn was limited to the range of 2.05% to 2.8%. In addition, Preferably it is 2.3 to 2.6%.
C/Mn比:0.040以下
C/Mn比は、縮径圧延−冷却過程での組織形成に影響を及ぼす。C/Mn比が低下するに従い、硬質第二相の生成量が低下し、軟質母相であるフェライト相主体の均一組織となる。C、Mn量と強度、成形性、低温靭性の関係を図1に示す。上記したC、Mnの範囲内で、C/Mn比を0.040以下に調整することにより、引張強さ:700MPa以上の高強度を維持しつつ、伸びElが増加し、シャルピー破面遷移温度(低温靭性)が低温となり、高強度においても成形性と低温靭性がともに向上する。C/Mn比が0.040を超えると、母相と硬質第二相との硬度差が増大し、低温靭性が低下する。このようなことから、C/Mnは0.040以下に限定した。なお、好ましくは0.015〜0.030である。
C / Mn ratio: 0.040 or less
The C / Mn ratio affects the structure formation in the diameter reduction rolling-cooling process. As the C / Mn ratio decreases, the amount of hard second phase generated decreases, resulting in a uniform structure mainly composed of a ferrite phase that is a soft matrix. The relationship between the amount of C and Mn and strength, formability, and low temperature toughness is shown in FIG. By adjusting the C / Mn ratio to 0.040 or less within the range of C and Mn described above, tensile strength: while maintaining a high strength of 700 MPa or more, the elongation El increases and the Charpy fracture surface transition temperature (low temperature) (Toughness) becomes low temperature, and both formability and low temperature toughness are improved even at high strength. When the C / Mn ratio exceeds 0.040, the hardness difference between the parent phase and the hard second phase increases, and the low temperature toughness decreases. For this reason, C / Mn is limited to 0.040 or less. In addition, Preferably it is 0.015-0.030.
Si:0.10〜0.45%
Siは、フェライト変態を促進する元素であり、所定量以上のフェライト相を生成させ、
成形性を確保するために0.10%以上の含有を必要とする。一方、0.45%を超える含有は、電縫溶接性が劣化するとともに、低温靭性が劣化する。このため、Siは0.10〜0.45%の範囲に限定した。なお、好ましくは0.10〜0.30%である。
Si: 0.10 to 0.45%
Si is an element that promotes ferrite transformation, generates a ferrite phase of a predetermined amount or more,
In order to ensure moldability, the content of 0.10% or more is required. On the other hand, if the content exceeds 0.45%, ERW weldability deteriorates and low temperature toughness deteriorates. For this reason, Si was limited to the range of 0.10 to 0.45%. In addition, Preferably it is 0.10 to 0.30%.
Ti:0.001〜0.04%
Tiは、Nと結合して固溶Nを低減させることにより、成形性向上に寄与する元素である。また、Nと結合した以外のTiは、炭化物として析出し、縮径圧延−冷却工程での回復・再結晶粒の粒成長を抑制して組織を微細化し、低温靭性向上に寄与する。このような効果は0.001%以上の含有で顕著となる。一方、0.04%を超える含有は、析出炭窒化物による強度上昇、延性・靭性低下が顕著となる。このため、Tiは0.001〜0.04%の範囲に限定した。なお、好ましくは0.005〜0.025%である。
Ti: 0.001 to 0.04%
Ti is an element that contributes to improving the formability by combining with N to reduce solid solution N. Further, Ti other than that combined with N precipitates as a carbide, suppresses recovery / recrystallized grain growth in the reduced diameter rolling-cooling step, refines the structure, and contributes to improvement of low temperature toughness. Such an effect becomes remarkable when the content is 0.001% or more. On the other hand, if the content exceeds 0.04%, the strength increase and the ductility / toughness decrease due to the precipitated carbonitrides become remarkable. For this reason, Ti was limited to the range of 0.001 to 0.04%. In addition, Preferably it is 0.005-0.025%.
Nb:0.001〜0.04%
Nbは、炭化物として析出し、縮径圧延−冷却工程での回復・再結晶粒の粒成長を抑制し組織を微細化し、低温靭性向上に寄与する。このような効果は、0.001%以上の含有で認められる。一方、0.04%を超える含有は、析出炭窒化物による強度上昇、延性・靭性低下が顕著となる。このため、Nbは0.001〜0.04%の範囲に限定した。なお、好ましくは0.005〜0.025%である。
Nb: 0.001 to 0.04%
Nb precipitates as a carbide, suppresses recovery / recrystallized grain growth in the reduced diameter rolling-cooling process, refines the structure, and contributes to improvement of low temperature toughness. Such an effect is recognized when the content is 0.001% or more. On the other hand, if the content exceeds 0.04%, the strength increase and the ductility / toughness decrease due to the precipitated carbonitrides become remarkable. For this reason, Nb was limited to 0.001 to 0.04% of range. In addition, Preferably it is 0.005-0.025%.
B:0.0001〜0.0035%
Bは、縮系圧延−冷却工程での組織の2相分離を抑制し、低温靭性に悪影響を及ぼす硬質第2相の生成を抑制する作用を有する重要な元素である。このような効果は0.0001%以上の含有で認められるが、0.0035%を超えて含有しても、上記した効果が飽和するうえ、BNの析出による延性低下が顕著となる。このため、Bは0.0001〜0.0035%の範囲に限定した。なお、好ましくは0.0005〜0.0025%である。
B: 0.0001-0.0035%
B is an important element having an effect of suppressing the two-phase separation of the structure in the reduced rolling-cooling step and suppressing the formation of a hard second phase that adversely affects the low temperature toughness. Such an effect is recognized when the content is 0.0001% or more. However, even if the content exceeds 0.0035%, the above-described effect is saturated and the ductility decrease due to precipitation of BN becomes remarkable. For this reason, B was limited to the range of 0.0001 to 0.0035%. In addition, Preferably it is 0.0005 to 0.0025%.
Cr:0.001〜0.29%
Crは、強度を増加させる元素であるとともに、鋼中Cの易動度を抑制し、Bと同様に、縮径圧延−冷却工程での組織の2相分離を抑制して、低温靭性向上に有利なフェライト相主体の均質組織とすることに寄与する元素である。このような効果は、0.001%以上の含有で顕著となる。一方、0.29%を超える含有は、電縫溶接性を劣化させるとともに、成形性を低下させる。このためにCrは0.001〜0.29%の範囲に限定した。なお、好ましくは0.03〜0.25%である。
Cr: 0.001 to 0.29%
Cr is an element that increases the strength, suppresses the mobility of C in the steel, and, like B, suppresses the two-phase separation of the structure in the diameter reduction rolling-cooling process, thereby improving the low temperature toughness. It is an element that contributes to an advantageous homogeneous structure mainly composed of a ferrite phase. Such an effect becomes remarkable when the content is 0.001% or more. On the other hand, if the content exceeds 0.29%, the weldability is deteriorated and the formability is lowered. For this reason, Cr is limited to the range of 0.001 to 0.29%. In addition, Preferably it is 0.03-0.25%.
Al:0.01〜0.08%
Alは、製鋼時の脱酸剤として作用するとともに、縮径圧延工程でのオーステナイト粒の成長を抑制し結晶粒を微細化して、低温靭性向上に寄与する。このような効果は、0.001%以上の含有で認められる。一方、0.08%を越える含有は、効果が飽和するとともに、酸化物系介在物が増加し、耐疲労特性が低下する。このため、Alは0.01〜0.08%の範囲に限定した。
Al: 0.01-0.08%
Al acts as a deoxidizer at the time of steelmaking, and suppresses the growth of austenite grains in the diameter reduction rolling process and refines the crystal grains, thereby contributing to improvement of low temperature toughness. Such an effect is recognized when the content is 0.001% or more. On the other hand, if the content exceeds 0.08%, the effect is saturated, oxide inclusions increase, and the fatigue resistance decreases. For this reason, Al was limited to the range of 0.01 to 0.08%.
また、本発明では、不純物としてのP、S、N、Oをそれぞれ所定値以下に低減する。
P:0.019%以下
Pは、Mnとの凝固共偏析、縮径圧延前の再加熱時のオーステナイト粒界への偏析などを介し、低温靭性を低下させる元素である。このような悪影響は、0.019%を超える含有で顕著となる。このため、Pは0.019%以下に限定した。
In the present invention, P, S, N, and O as impurities are each reduced to a predetermined value or less.
P: 0.019% or less
P is an element that lowers low temperature toughness through solidification co-segregation with Mn, segregation to austenite grain boundaries during reheating before diameter reduction rolling, and the like. Such an adverse effect becomes significant when the content exceeds 0.019%. For this reason, P was limited to 0.019% or less.
S:0.003%以下
Sは、MnSなど鋼中介在物として存在し、鋼の電縫溶接性、耐疲労特性、成形性を低下させる。このような悪影響は、0.003%を超える含有で顕著となる。このため、Sは0.003%以下に限定した。
N:0.005%以下
Nは,固溶Nとして残存すると転位の易動度を低下させ、成形性を低下させる。このような悪影響は、0.005%を超える含有で顕著となる。このため、Nは0.005%以下に限定した。
S: 0.003% or less
S exists as inclusions in steel such as MnS, and lowers the electric resistance weldability, fatigue resistance, and formability of steel. Such an adverse effect becomes significant when the content exceeds 0.003%. For this reason, S was limited to 0.003% or less.
N: 0.005% or less
If N remains as solute N, it lowers the mobility of dislocations and lowers the formability. Such an adverse effect becomes significant when the content exceeds 0.005%. For this reason, N was limited to 0.005% or less.
O:0.0025%以下
Oは、鋼中では酸化物系介在物として存在し、鋼の耐疲労特性を低下させる。このような悪影響は、0.0025%を超える含有で顕著となる。このため、Oは0.0025%以下に限定した。
本発明では、上記した基本成分に加えてさらに、必要に応じ、V:0.001〜0.04%、W:0.001〜0.04%のうちから選ばれた1種または2種、および/または、Mo:0.001〜0.2%、および/または、Cu:0.001〜0.2%、Ni:0.001〜0.2%のうちから選ばれた1種または2種、および/または、Ca:0.0001〜0.003%、をそれぞれ単独または複合して含有することができる。
O: 0.0025% or less
O exists as oxide inclusions in steel and reduces the fatigue resistance of the steel. Such an adverse effect becomes significant when the content exceeds 0.0025%. For this reason, O was limited to 0.0025% or less.
In the present invention, in addition to the basic components described above, one or two selected from V: 0.001 to 0.04%, W: 0.001 to 0.04%, and / or Mo: 0.001 to 0.2% and / or Cu: 0.001 to 0.2%, Ni: 0.001 to 0.2% selected from one or two, and / or Ca: 0.0001 to 0.003%, each alone or in combination Can be contained.
V:0.001〜0.04%、W:0.001〜0.04%のうちから選ばれた1種または2種
V、Wは、いずれも炭化物として析出し、縮径延圧−冷却工程での回復・再結晶粒の粒成長を抑制し組織を微細化し、低温靭性確保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。このような効果は、V:0.001%、W:0.001%以上の含有で発現するが、0.04%を超える含有は、析出炭化物による強度上昇、延性・靭性低下が顕著となる。このため、Vは0.001〜0.04%、Wは0.001〜0.04%の範囲にそれぞれ限定することが好ましい。
V: One or two selected from 0.001 to 0.04%, W: 0.001 to 0.04%
V and W are both elements that precipitate as carbides, suppress recovery and recrystallization grain growth in the diameter reduction and cooling process, refine the structure, and contribute to ensuring low temperature toughness. Can be selected and contained. Such effects are manifested when the content is V: 0.001% and W: 0.001% or more. However, when the content exceeds 0.04%, the strength increase and the ductility / toughness decrease due to the precipitated carbides become remarkable. For this reason, it is preferable to limit V to 0.001 to 0.04% and W to 0.001 to 0.04%, respectively.
Mo:0.001〜0.2%
Moは、鋼中のCの易動度を抑制する作用を有する元素であり、縮径延圧−冷却工程での組織の2相分離を抑制する効果を有し、Crの効果を補完する働きがあり、必要に応じて含有できる。このような効果は、0.001%以上の含有で発現するが、0.2%を超える含有は成形性を低下させる。このため、Moは0.001〜0.2%の範囲に限定することが好ましい。
Mo: 0.001-0.2%
Mo is an element that has the effect of suppressing the mobility of C in steel, has the effect of suppressing the two-phase separation of the structure in the reduced diameter rolling and cooling process, and complements the effect of Cr. And can be contained as necessary. Such an effect is manifested at a content of 0.001% or more, but a content exceeding 0.2% lowers the moldability. For this reason, it is preferable to limit Mo to the range of 0.001 to 0.2%.
Cu:0.001〜0.2%、Ni:0.001〜0.2%のうちから選ばれた1種または2種
Cu、Niは、Crの効果を補完する働きと、鋼材の耐食性を向上させる効果があり、必要に応じて選択して含有できる。これらの効果はCu、Niともに 0.001%以上の含有で発現するが、それぞれ 0.2%を超える含有は成形性を低下させる。このために、Cuは0.001〜0.2%、Niは0.001〜0.2%の範囲に限定することが好ましい。
One or two selected from Cu: 0.001 to 0.2%, Ni: 0.001 to 0.2%
Cu and Ni have the effect of complementing the effect of Cr and the effect of improving the corrosion resistance of the steel material, and can be selected and contained as necessary. These effects are manifested when Cu and Ni are contained in amounts of 0.001% or more, but inclusions exceeding 0.2% each reduce formability. For this reason, it is preferable to limit Cu to 0.001 to 0.2% and Ni to 0.001 to 0.2%.
Ca:0.0001〜0.003%
Caは、展伸したMnSを粒状のCaSとする、所謂、形態制御作用を有し、シーム溶接部の低温靭性を向上させる効果を有し、本発明では必要に応じて含有できる。このような効果は、0.0001%以上の含有で発現するが、0.003%を超える含有は、非金属介在物が増加し、耐疲労特性が低下する。このため、Caは0.0001〜0.003%の範囲に限定することが好ましい。
Ca: 0.0001 to 0.003%
Ca has a so-called shape control action in which expanded MnS is granular CaS, and has an effect of improving the low temperature toughness of the seam welded portion. In the present invention, Ca can be contained as necessary. Such an effect is manifested with a content of 0.0001% or more. However, when the content exceeds 0.003%, non-metallic inclusions increase and fatigue resistance is reduced. For this reason, it is preferable to limit Ca to 0.0001 to 0.003% of range.
上記した成分以外の残部は、Feおよび不可避的不純物である。
本発明の高張力溶接鋼管は、上記した組成を有し、さらに面積率で60%以上のフェライト相を有し、該フェライト相の円周方向断面の平均結晶粒径が0.5〜5.5μmである組織を有する溶接鋼管である。つぎに、組織限定理由について説明する。
フェライト相:面積率で60%以上
優れた成形性と優れた断面成形加工後の耐捻り疲労特性を確保するためには、面積率で60%以上のフェライト相からなる組織とすることが重要となる。フェライト相が面積率で60%未満では、JIS12号試験片を用いた引張試験での伸びElが18%未満となり、延性が低下し、所望の優れた成形性が確保できなくなる。このため、本発明ではフェライト相を面積率で60%以上に限定した。なお、好ましくは面積率で80%以上である。なお、本発明でいう「フェライト相」は、硬質相であるセメンタイト、パーライト、高炭素ベイナイト、マルテンサイト、残留オーステナイト以外の軟質相を意味し、具体的には、ポリゴナルフェライト、擬ポリゴナルフェライト、アシキュラーフェライト、ベイニティックフェライトを含み、その形態は問わない。したがって、フェライト相以外の残部(第二相)は、セメンタイト相、パーライト相、ベイナイト相、マルテンサイト相、残留オーステナイト相のうちの1種以上から構成される。
The balance other than the above components is Fe and inevitable impurities.
The high-tensile welded steel pipe of the present invention has the above-described composition, and further has a ferrite phase of 60% or more in area ratio, and the average crystal grain size in the circumferential cross section of the ferrite phase is 0.5 to 5.5 μm. It is a welded steel pipe having a structure. Next, the reason for organization limitation will be described.
Ferrite phase: 60% or more in area ratio In order to ensure excellent formability and torsional fatigue resistance after cross-section forming, it is important to have a structure composed of ferrite phase with an area ratio of 60% or more. Become. If the ferrite phase is less than 60% in area ratio, the elongation El in a tensile test using a JIS No. 12 test piece is less than 18%, the ductility is lowered, and the desired excellent formability cannot be secured. For this reason, in the present invention, the ferrite phase is limited to 60% or more by area ratio. The area ratio is preferably 80% or more. The term “ferrite phase” as used in the present invention means a soft phase other than cementite, pearlite, high carbon bainite, martensite, and retained austenite, which are hard phases. Specifically, polygonal ferrite, pseudopolygonal ferrite , Including acicular ferrite and bainitic ferrite. Therefore, the remainder (second phase) other than the ferrite phase is composed of one or more of a cementite phase, a pearlite phase, a bainite phase, a martensite phase, and a retained austenite phase.
フェライト相の円周方向断面の平均結晶粒径:0.5〜5.5μm
優れた成形性と優れた断面成形加工後の耐捻り疲労特性を確保するために、本発明ではさらに、フェライト相の円周方向断面の平均結晶粒径を0.5〜5.5μmに限定する。フェライト相の円周方向断面の平均結晶粒径が5.5μmを超えると、JIS12号試験片を用いた引張試験での伸びElが18%未満となり、強度−延性バランスが低下するとともに、低温靭性も低下する。一方、平均結晶粒径が0.5μm未満では、降状強さが増加し、JIS12号試験片を用いた引張試験での伸びElが18%未満となり延性が低下する。このため、フェライト相の円周方向断面の平均結晶粒径を0.5〜5.5μmに限定した。なお、好ましくは1.5〜4.5μmである。
Average crystal grain size in the circumferential section of the ferrite phase: 0.5 to 5.5 μm
In order to ensure excellent formability and excellent torsional fatigue resistance after cross-sectional forming, the present invention further limits the average crystal grain size of the ferrite phase in the circumferential direction to 0.5 to 5.5 μm. If the average grain size of the ferrite phase in the circumferential section exceeds 5.5 μm, the elongation El in the tensile test using the JIS No. 12 test piece will be less than 18%, the strength-ductility balance will be lowered, and the low temperature toughness will be low. descend. On the other hand, when the average crystal grain size is less than 0.5 μm, the yield strength increases, and the elongation El in the tensile test using the JIS No. 12 test piece is less than 18% and the ductility is lowered. For this reason, the average crystal grain size in the circumferential cross section of the ferrite phase is limited to 0.5 to 5.5 μm. In addition, Preferably it is 1.5-4.5 micrometers.
なお、本発明におけるフェライト相の結晶粒径は、大傾角粒界、小傾角亜粒界、異相で囲まれたフェライト相の平均円相当径で、亜粒界も結晶粒界とみなすものとする。
また、本発明の高張力溶接鋼管では、疲労初期亀裂の発生を抑制し、鋼管の断面成形加工後の耐捻り疲労特性を向上させるために、上記した組成、組織に加えて、さらに鋼管内面および外面の表面粗さを限定する。内面および外面の表面粗さは、算術平均粗さRa:2μm以下、最大高さ粗さRz:30μm以下、十点平均粗さRzJIS:20μm以下に限定する。なお、表面粗さの測定は、JIS B 0601−2001の規定に準拠して行うものとする。
The crystal grain size of the ferrite phase in the present invention is the average equivalent circle diameter of the ferrite phase surrounded by the large-angle grain boundary, the small-angle sub-grain boundary, and the different phase, and the sub-grain boundary is also regarded as the crystal grain boundary. .
In addition, in the high-tensile welded steel pipe of the present invention, in addition to the above composition and structure, in addition to the above composition and structure, the inner surface of the steel pipe and Limit the surface roughness of the outer surface. The surface roughness of the inner and outer surfaces is limited to arithmetic average roughness Ra: 2 μm or less, maximum height roughness Rz: 30 μm or less, and ten-point average roughness Rz JIS : 20 μm or less. The surface roughness is measured in accordance with JIS B 0601-2001.
算術平均粗さRa:2μm以下、最大高さ粗さRz:30μm以下、十点平均粗さRzJIS:20μm以下
算術平均粗さRaが2μmを超えるかあるいは、最大高さ粗さRzが30μmを超えるかあるいは、十点平均粗さRzJIS が20μmを超える場合には、断面成形加工後の耐捻り疲労特性が低下し、5×105繰返し疲れ限度σBと鋼管引張強さTSとの比、(σB/TS)が0.35未満となる。算術平均粗さRaが2μm以下、最大高さ粗さRz:30μm以下、十点平均粗さRzJIS:20μm以下を満足する表面粗さの場合には、鋼管の断面成形加工後、(σB/TS)が0.35以上を満足することができるようになる。なお、好ましくは算術平均粗さRa:1.5μm以下、最大高さ粗さRz:20μm以下、十点平均粗さRzJIS:15μm以下である。さらに好ましくは算術平均粗さRa:1μm以下、最大高さ粗さRz:15μm以下、十点平均粗さRzJIS:10μm以下である。この表面粗さは、内面あるいは外面の一部又は全部に、さらに研削加工、研磨加工あるいはホーニング加工を施すことにより達成できる。
Arithmetic average roughness Ra: 2 μm or less, maximum height roughness Rz: 30 μm or less, 10-point average roughness Rz JIS : 20 μm or less Arithmetic average roughness Ra exceeds 2 μm, or maximum height roughness Rz is 30 μm Or when the ten-point average roughness Rz JIS exceeds 20 μm, the torsional fatigue resistance after the cross-section forming process decreases, and the ratio between the 5 × 10 5 repeated fatigue limit σ B and the steel pipe tensile strength TS , (Σ B / TS) is less than 0.35. In the case of surface roughness satisfying arithmetic average roughness Ra of 2 μm or less, maximum height roughness Rz: 30 μm or less, and ten-point average roughness Rz JIS : 20 μm or less, (σ B / TS) can satisfy 0.35 or more. The arithmetic average roughness Ra is preferably 1.5 μm or less, the maximum height roughness Rz is 20 μm or less, and the ten-point average roughness Rz JIS is 15 μm or less. More preferably, the arithmetic average roughness Ra is 1 μm or less, the maximum height roughness Rz is 15 μm or less, and the ten-point
次に、上記した溶接鋼管の好ましい製造方法について説明する。
本発明の溶接鋼管は、鋼帯を連続成形してオープン管とし、該オープン管を電縫溶接して管体としたのち、該管体に縮径圧延を施し製品管とする。本発明では、使用する鋼帯は、上記した組成を有する鋼スラブを加熱し、仕上圧延温度:980〜750℃、巻取り温度:700〜350℃とする熱間圧延を施して得られた熱延鋼帯とすることが好ましい。
Next, the preferable manufacturing method of the above-mentioned welded steel pipe will be described.
In the welded steel pipe of the present invention, a steel strip is continuously formed into an open pipe, and the open pipe is electro-welded to form a pipe, and then the pipe is subjected to reduction rolling to obtain a product pipe. In the present invention, the steel strip to be used is a heat obtained by heating a steel slab having the above-described composition and performing hot rolling at a finish rolling temperature of 980 to 750 ° C. and a winding temperature of 700 to 350 ° C. It is preferable to use a rolled steel strip.
熱間圧延条件の限定理由はつぎのとおりである。なお、熱間圧延における加熱温度はとくに限定する必要はないが、上記した仕上圧延温度、巻取り温度を確保できるように、980℃以上1300℃以下とすることが好ましい。
仕上圧延温度:980〜750℃
熱間圧延における仕上圧延温度は、鋼管の断面成形加工後の捻り疲労特性を良好にするために重要である。仕上圧延温度が980℃を超えると鋼帯の表面粗さが粗くなり、鋼管の疲労強度が低下する。一方、750℃を下回ると熱間変形抵抗が上昇し圧延が困難となるとともに、鋼帯段階での異方性が増し、製品管の成形性が低下する。このため、仕上圧延温度は980〜750℃の範囲に限定することが好ましい。なお、さらに好ましくは880〜800℃である。
The reasons for limiting the hot rolling conditions are as follows. The heating temperature in the hot rolling is not particularly limited, but is preferably 980 ° C. or higher and 1300 ° C. or lower so as to ensure the above finish rolling temperature and winding temperature.
Finish rolling temperature: 980-750 ° C
The finish rolling temperature in the hot rolling is important for improving the torsional fatigue characteristics after the cross-section forming process of the steel pipe. When the finish rolling temperature exceeds 980 ° C., the surface roughness of the steel strip becomes rough, and the fatigue strength of the steel pipe decreases. On the other hand, when the temperature is lower than 750 ° C., the hot deformation resistance increases and rolling becomes difficult, and the anisotropy at the steel strip stage increases, and the formability of the product pipe decreases. For this reason, it is preferable that the finish rolling temperature is limited to a range of 980 to 750 ° C. In addition, it is 880-800 degreeC more preferably.
巻取り温度:700〜350℃
熱間圧延における巻取り温度は、仕上げ圧延温度と並んで鋼管の断面成形加工後の耐捻り疲労特性を良好にするために重要である。巻取り温度が700℃を超えると鋼帯の表面粗さが粗くなり、鋼管の疲労強度が低下する。一方、350℃を下回ると鋼帯の強度が上昇し、巻き形状が悪化し、造管時に均一なカリバー形状(断面曲率)が得られず製品管の成形性が低下し、また、シーム部の低温靭性も低下する。このため、巻取り温度は700〜350℃の範囲に限定することが好ましい。なお、さらに好ましくは650〜550℃である。
Winding temperature: 700 ~ 350 ℃
The coiling temperature in the hot rolling is important for improving the torsional fatigue resistance after the cross-section forming process of the steel pipe along with the finish rolling temperature. When the coiling temperature exceeds 700 ° C, the surface roughness of the steel strip becomes rough, and the fatigue strength of the steel pipe decreases. On the other hand, when the temperature is lower than 350 ° C, the strength of the steel strip increases, the winding shape deteriorates, a uniform caliber shape (cross-sectional curvature) cannot be obtained during pipe making, and the formability of the product pipe decreases. Low temperature toughness also decreases. For this reason, the winding temperature is preferably limited to a range of 700 to 350 ° C. In addition, it is 650-550 degreeC more preferably.
管体に施す縮径圧延は、管体を1000〜850℃に加熱したのち、該管体に縮径圧延終了温度:800〜620℃、縮径率:21〜75%とする圧延とすることが好ましい。
縮径圧延の圧延条件の限定理由はつぎのとおりである。
縮径圧延時の加熱温度:1000〜850℃
縮径圧延時の加熱温度は、製品管の延性と断面成形加工後の耐捻り疲労特性を良好にするために重要である。縮径圧延時の加熱温度と、引張試験により得られた伸びEl(延性)および断面成形加工後の捻り疲労試験により得られた5×105繰返し疲れ限度σBと鋼管引張強さTSとの比、(σB/TS)との関係を図2に示す。加熱温度が1000℃を超えると鋼帯表面粗さが粗くなり、(σB/TS)が低下する。一方、加熱温度が900℃未満であると加熱時の組織が完全にオーステナイト化されず不均一組織となり、フェライト相の円周方向断面の平均結晶粒径が5.5μmを越え、伸びElが低下する。このため、縮径圧延時の加熱温度範囲を1000〜850℃に限定することが好ましい。なお、さらに好ましくは960〜880℃である。
The diameter reduction rolling applied to the tube body is a rolling method in which the tube body is heated to 1000 to 850 ° C. and then the diameter reduction rolling finish temperature of the tube body is 800 to 620 ° C. and the diameter reduction rate is 21 to 75%. Is preferred.
The reasons for limiting the rolling conditions for reduced diameter rolling are as follows.
Heating temperature during reduction rolling: 1000-850 ° C
The heating temperature at the time of diameter reduction rolling is important for improving the ductility of the product tube and the torsional fatigue resistance after the cross-section forming process. Heating temperature during reduction rolling, elongation El (ductility) obtained by tensile test, and 5 × 10 5 repeated fatigue limit σ B obtained by torsional fatigue test after section forming and steel pipe tensile strength TS The relationship with the ratio, (σ B / TS) is shown in FIG. When heating temperature exceeds 1000 degreeC, the steel strip surface roughness will become coarse and ((sigma) B / TS) will fall. On the other hand, when the heating temperature is less than 900 ° C., the structure at the time of heating is not completely austenite and becomes a non-uniform structure, the average crystal grain size in the circumferential section of the ferrite phase exceeds 5.5 μm, and the elongation El decreases. . For this reason, it is preferable to limit the heating temperature range at the time of diameter reduction rolling to 1000-850 degreeC. In addition, it is 960-880 degreeC more preferably.
縮径圧延終了温度:800〜620℃
縮径圧延時の縮径圧延終了温度は、製品管の断面成形加工後の耐捻り疲労特性や延性、低温靭性を良好に保つために重要である。縮径圧延終了温度と伸びEl(延性)および(σB/TS)との関係、および縮径圧延終了温度とシーム部のシャルピー衝撃試験における破面遷移温度(低温靭性)との関係を図3に示す。縮径圧延終了温度が800℃を超えると製品管の表面粗さが粗くなり、断面成形加工後の耐捻り疲労特性が低下するとともに、延性、低温靭性が低下する。一方、縮径圧延終了温度が620℃を下回ると圧延歪が残存し、延性、低温靭性および断面成形加工後の耐捻り疲労特性が低下する。このため、縮径圧延終了温度を800〜620℃の範囲に限定することが好ましい。なお、さらに好ましくは750〜650℃である。
Diameter reduction end temperature: 800 ~ 620 ℃
The temperature at which the diameter reduction finishes at the time of diameter reduction rolling is important for maintaining good torsional fatigue resistance, ductility and low temperature toughness after the cross-section forming of the product pipe. FIG. 3 shows the relationship between the diameter reduction rolling end temperature and elongation El (ductility) and (σ B / TS), and the relationship between the diameter reduction rolling end temperature and the fracture surface transition temperature (low temperature toughness) in the Charpy impact test of the seam. Shown in When the diameter reduction rolling finish temperature exceeds 800 ° C., the surface roughness of the product pipe becomes rough, the torsional fatigue resistance after the cross-section forming process is lowered, and the ductility and the low temperature toughness are also lowered. On the other hand, when the diameter reduction rolling finish temperature is lower than 620 ° C., rolling strain remains, and ductility, low-temperature toughness, and torsional fatigue resistance after cross-section forming process deteriorate. For this reason, it is preferable to limit the temperature at which the diameter reduction finishes to a range of 800 to 620 ° C. In addition, More preferably, it is 750-650 degreeC.
縮径率:21〜75%
縮径圧延時の縮径率は、製品管の延性、低温靭性を良好に保つために重要である。縮径率が21%未満では、フェライト相の円周方向断面の平均結晶粒径が5.5μmを超え、所定の伸び特性が確保できなくなる。一方、75%を超えると縮径歪により表面粗さが粗くなり、成形加工後の耐捻り疲労特性が低下する。このため、縮径率を21〜75%に限定することが好ましい。なお、さらに好ましくは30〜55%である。
Reduction ratio: 21-75%
The diameter reduction ratio during diameter reduction rolling is important for maintaining good ductility and low temperature toughness of the product pipe. If the reduction ratio is less than 21%, the average crystal grain size of the ferrite phase in the circumferential cross section exceeds 5.5 μm, and the predetermined elongation characteristics cannot be secured. On the other hand, when it exceeds 75%, the surface roughness becomes rough due to the reduced diameter strain, and the torsional fatigue resistance after the molding process is lowered. For this reason, it is preferable to limit the diameter reduction rate to 21 to 75%. More preferably, it is 30 to 55%.
縮径圧延後の冷却は、620〜420℃までの平均冷却速度で0.5〜50℃/sとすることが好ましい。
縮径圧延後の620〜420℃までの平均冷却速度:0.5〜50℃/s
縮径圧延後の冷却は、所望の組織を得るために重要となる。620〜420℃までの平均冷却速度が0.5℃/s未満では、フェライト相の円周方向断面の平均結晶粒径が5.5μmを超え、所望の延性が確保できなくなる。一方、620〜420℃までの平均冷却速度が50℃/sを超えると、フェライト相の面積率が60%未満となり、所望の延性が確保できなくなる。このため、縮径圧延後の冷却は、620〜420℃までの平均冷却速度を0.5〜50℃/sに限定することが好ましい。なお、さらに好ましくは0.7〜10℃/sである。
The cooling after the reduction rolling is preferably 0.5 to 50 ° C./s at an average cooling rate of 620 to 420 ° C.
Average cooling rate from 620 to 420 ° C after reduction rolling: 0.5 to 50 ° C / s
Cooling after reduction rolling is important for obtaining a desired structure. If the average cooling rate from 620 to 420 ° C. is less than 0.5 ° C./s, the average grain size of the ferrite phase in the circumferential section exceeds 5.5 μm, and the desired ductility cannot be ensured. On the other hand, when the average cooling rate from 620 to 420 ° C. exceeds 50 ° C./s, the area ratio of the ferrite phase becomes less than 60%, and the desired ductility cannot be ensured. For this reason, as for cooling after diameter reduction rolling, it is preferable to limit the average cooling rate to 620-420 degreeC to 0.5-50 degreeC / s. More preferably, it is 0.7 to 10 ° C./s.
上記した製造方法で得られた溶接鋼管(製品管)には、特に高い疲れ限度が必要とされる場合、さらに管の内面あるいは外面の一部又は全部に、研削加工、研磨加工あるいはホーニング加工を施してもよい。これにより、管内面または外面の表面粗さを、算術平均粗さRa:1μm以下、最大高さ粗さRz:15μm以下、十点平均粗さRzJIS:10μm以下に調整することができ、5×105疲れ限度応力を約10%増加できる。 When a particularly high fatigue limit is required for the welded steel pipe (product pipe) obtained by the above manufacturing method, grinding, polishing or honing is further applied to part or all of the inner or outer surface of the pipe. You may give it. As a result, the surface roughness of the tube inner surface or outer surface can be adjusted to arithmetic average roughness Ra: 1 μm or less, maximum height roughness Rz: 15 μm or less, and ten-point average roughness Rz JIS : 10 μm or less. × 10 5 The fatigue limit stress can be increased by about 10%.
また、得られた溶接鋼管(製品管)には、残留応力低減を目的にした歪取り焼鈍を施してもよい。造管等に伴う残留応力を熱処理により低減させることは、疲労初期亀裂の抑制を通じて、鋼管の断面成形加工後の耐捻り疲労特性を向上させる効果がある。歪取り焼鈍としては、100〜750℃×1〜3600sの条件とすることが好ましい。歪取り焼鈍における温度・時間が、100℃未満あるいは1s未満では残留応力低減効果が得られず、一方、750℃超えあるいは3600s超えでは鋼管強度の低下が著しくなる。このため、歪取り焼鈍は、100〜750℃×1〜3600sの範囲に限定することが好ましい。歪取り焼鈍温度と、歪取り焼鈍後の鋼管の引張強さTS、歪取り焼鈍後の残留応力、および断面成形加工後の耐捻り疲労強度(σB/TS)との関係を図4に示す。なお、図4はV字断面成形後に、歪取り焼鈍温度に600s保持した後の値である。焼鈍温度が、100〜750℃の範囲で(σB/TS)が向上する。なお、歪取り焼鈍温度は、耐疲労特性向上の観点からは420〜650℃の範囲とすることが望ましい。 The obtained welded steel pipe (product pipe) may be subjected to strain relief annealing for the purpose of reducing residual stress. Reducing the residual stress associated with pipe making or the like by heat treatment has the effect of improving the torsional fatigue resistance after cross-section forming of the steel pipe through the suppression of initial fatigue cracks. The strain relief annealing is preferably performed at 100 to 750 ° C. × 1 to 3600 s. If the temperature and time in the stress relief annealing is less than 100 ° C. or less than 1 s, the effect of reducing the residual stress cannot be obtained, whereas if it exceeds 750 ° C. or more than 3600 s, the steel pipe strength is significantly reduced. For this reason, it is preferable to limit the strain relief annealing to a range of 100 to 750 ° C. × 1 to 3600 s. FIG. 4 shows the relationship between the stress relief annealing temperature, the tensile strength TS of the steel pipe after the stress relief annealing, the residual stress after the stress relief annealing, and the torsional fatigue strength (σ B / TS) after the cross-section forming process. . FIG. 4 shows values after holding the strain relief annealing temperature for 600 s after forming the V-shaped cross section. (Σ B / TS) is improved when the annealing temperature is in the range of 100 to 750 ° C. The strain relief annealing temperature is preferably in the range of 420 to 650 ° C. from the viewpoint of improving fatigue resistance.
また、得られた溶接鋼管(製品管)には、管内外面の粗さ調整、加工硬化による疲労寿命延長を目的として、管内外面の一部あるいは全部への鋼球や砂の吹付け処理(ショットブラスト処理、サンドブラストッ処理、ショットピーニング処理)を施してもよい。これにより、5×105疲れ限度応力を最大約15%向上することができる。なお、吹き付ける鋼球や砂のサイズ、材質については特に限定する必要はないが、耐疲労特性向上効果を最大限に発揮するために、粒径:0.05〜2mm、硬さHV:250以上とすることが望ましい。 In addition, the obtained welded steel pipe (product pipe) is sprayed with steel balls and sand (shot) on part or all of the inner and outer surfaces of the pipe for the purpose of adjusting the roughness of the inner and outer surfaces of the pipe and extending the fatigue life by work hardening. (Blasting process, sandblasting process, shot peening process) may be performed. This can improve the 5 × 10 5 fatigue limit stress by up to about 15%. The size and material of the steel balls and sand to be sprayed are not particularly limited, but in order to maximize the effect of improving fatigue resistance, the particle size is 0.05 to 2 mm and the hardness HV is 250 or more. It is desirable.
またさらには、製品管に、上記したホーニング加工処理および鋼球や砂の吹き付け処理を組合わせた、すなわち、ホーニング加工−ショットブラスト処理、ホーニング加工−サンドブラスト処理、ホーニング加工−ショットピーニング処理のうちのいずれかの処理を施してもよい。これにより、更に耐疲労特性向上効果が顕著となる。
本発明では、上記した高張力溶接鋼管を用いて、所定の寸法形状の成形体に成形加工し、自動車構造部材用成形体とすることができる。成形加工方法は、プレス加工、液圧加工、ハイドロフォーミング加工等通常の成形加工方法がいずれも適用できる。なお、成形体としたのち、該成形体に、上記した、研削加工、研磨加工あるいはホーニング加工や、歪取り焼鈍、鋼球や砂の吹付け処理を施してもよい。なお、とくに歪取り焼鈍は、成形体に施すことにより耐捻り疲労特性向上に顕著な効果を示す。また、成形体に、上記した歪取り焼鈍および鋼球や砂の吹き付け処理を組合わせた、すなわち、歪取り焼鈍−ショットブラスト処理、歪取り焼鈍−サンドブラスト処理、歪取り焼鈍−ショットピーニング処理のうちのいずれかの処理を施してもよい。これにより、更に耐疲労特性向上効果が顕著となる。
Still further, the product pipe is combined with the above honing process and steel ball or sand spraying process, that is, among the honing process-shot blast process, honing process-sand blast process, honing process-shot peening process. Either treatment may be performed. Thereby, the fatigue resistance improvement effect becomes more remarkable.
In the present invention, the above-described high-tensile welded steel pipe can be used to form a molded body having a predetermined size and shape to obtain a molded body for automobile structural members. As the forming method, any of normal forming methods such as press processing, hydraulic processing, and hydroforming can be applied. In addition, after forming a molded body, the above-described grinding process, polishing process, or honing process, strain relief annealing, and a steel ball or sand spraying process may be performed on the molded body. In particular, the strain relief annealing has a remarkable effect in improving the torsional fatigue resistance by being applied to the molded body. Moreover, the above-described strain relief annealing and steel ball or sand spraying treatment are combined with the molded body, that is, among strain relief annealing-shot blasting treatment, strain relief annealing-sand blasting treatment, strain relief annealing-shot peening treatment. Any of these processes may be performed. Thereby, the fatigue resistance improvement effect becomes more remarkable.
(実施例1)
表1に示す組成の鋼スラブを約1200℃に加熱し、仕上げ圧延温度:約900℃、巻取り温度:約600℃とする熱間圧延を施し、熱間圧延鋼帯(板厚:約3mm)とした。ついでこれら熱間圧延鋼帯に、酸洗を施したのち、所定の幅寸法にスリット加工し、連続成形してオープン管とし、該オープン管を高周波抵抗溶接により電縫溶接して管体(外径:約150mmφ×肉厚:約3mm)とした。
Example 1
A steel slab having the composition shown in Table 1 is heated to about 1200 ° C, hot-rolled to a finish rolling temperature of about 900 ° C and a winding temperature of about 600 ° C, and a hot-rolled steel strip (sheet thickness: about 3 mm) ). Next, these hot-rolled steel strips are pickled, slitted to a predetermined width, continuously formed into an open pipe, and the open pipe is electro-sealed by high-frequency resistance welding to form a tubular body (outside Diameter: about 150 mmφ × wall thickness: about 3 mm).
ついで、これら管体に、加熱温度:約900℃に再加熱し、縮径圧延終了温度:約650℃、縮径率:約40%とする縮径圧延を施したのち、620〜420℃までの平均冷却速度:約5℃/sとする冷却を施し、外径φ89.1mm、肉厚約3mmの溶接鋼管(製品管)を得た。
これら製品管から、試験片を採取し、組織観察試験、引張試験、低温靭性試験、表面粗さ試験、断面成形加工後の捻り疲労試験を実施した。試験方法はつぎのとおりとした。
Next, these tubes are reheated to a heating temperature of about 900 ° C., and subjected to reduction rolling at a diameter reduction end temperature of about 650 ° C. and a diameter reduction ratio of about 40%, and then to 620 to 420 ° C. The average cooling rate was about 5 ° C./s, and a welded steel pipe (product pipe) having an outer diameter of φ89.1 mm and a wall thickness of about 3 mm was obtained.
Test pieces were collected from these product pipes, and subjected to a structure observation test, a tensile test, a low temperature toughness test, a surface roughness test, and a torsional fatigue test after cross-section forming. The test method was as follows.
(1)組織観察試験
これら製品管(溶接鋼管)の円周方向断面が観察面となるように、組織観察用試験片を採取し、研磨、ナイータール腐食して走査型電子顕微鏡(3000倍)で組織を観察し、撮像して、画像解析装置を用いて、フェライト相の面積率、フェライト平均結晶粒径(円相当径)を測定した。
(1) Microstructure observation test Samples for microstructural observation were collected so that the circumferential section of these product tubes (welded steel pipes) would be the observation surface, polished, and subjected to Naytar corrosion, and scanned with a scanning electron microscope (3000x). The structure was observed and imaged, and the area ratio of the ferrite phase and the ferrite average crystal grain size (equivalent circle diameter) were measured using an image analyzer.
(2)引張試験
これら製品管(溶接鋼管)から、L方向が引張方向となるように、JIS Z 2201の規定に準拠してJIS12号試験片(一部11号試験片)を切出し、JIS Z 2241の規定に準拠して引張試験を実施し、引張特性(引張強さTS、El)を求めた。なお、JIS12号試験片のElの値ElJIS12とJIS11号試験片でのElの値ElJIS11間にはおよそ、ElJIS11=(5/3) ElJIS12の相関関係がある。本発明ではJIS12号のElの値を延性の代表値とした。
(2) Tensile test From these product pipes (welded steel pipes), JIS No. 12 test pieces (part No. 11 test pieces) are cut out in accordance with the provisions of JIS Z 2201 so that the L direction is the tensile direction. A tensile test was performed in accordance with the provisions of 2241 to determine tensile properties (tensile strength TS, El). Incidentally, approximately correlates of El JIS11 = (5/3) El JIS12 between the value El JIS11 of El with a value El JIS12 and JIS11 test piece No. of El of JIS12 No. specimen. In the present invention, the El value of JIS No. 12 is used as a representative value of ductility.
(3)低温靭性試験
これら製品管(溶接鋼管)の母材およびシーム溶接部より、管円周方向(C方向)が試験片長さ方向となるように展開し、JIS Z 2202の規定に準拠してシャルピー試験片(2mmVノッチ、1/4サイズ)を切出し、JIS Z 2242の規定に準拠してシャルピー衝撃試験を実施し、破面遷移温度を求め、低温靭性を評価した。
(3) Low temperature toughness test Developed from the base metal and seam weld of these product pipes (welded steel pipes) so that the pipe circumferential direction (C direction) is the test piece length direction, and conforms to the provisions of JIS Z 2202 A Charpy test piece (2 mm V notch, 1/4 size) was cut out, a Charpy impact test was performed in accordance with the provisions of JIS Z 2242, the fracture surface transition temperature was determined, and the low temperature toughness was evaluated.
(4)表面粗さ試験
これら製品管(溶接鋼管)の内外表面の表面粗さを、触針式粗度計を用いてJIS B 0601−2001の規定に準拠して、粗さ曲線を測定し、粗さパラメータとして、算術平均粗さRa、最大高さ粗さRz、十点平均粗さRzJISを求めた。なお、粗さ曲線の測定方向は、管の円周方向(C方向)とし、低減カットオッフ値0.8mm、評価長さ4mmとした。代表値としては、内表面又は外表面のうち、値の大きい方を採用した。
(4) Surface roughness test The surface roughness of the inner and outer surfaces of these product pipes (welded steel pipes) was measured using a stylus type roughness meter in accordance with the provisions of JIS B 0601-2001. As the roughness parameters, arithmetic average roughness Ra, maximum height roughness Rz, and ten-point average roughness Rz JIS were determined. The measurement direction of the roughness curve was the circumferential direction (C direction) of the tube, the reduced cut-off value was 0.8 mm, and the evaluation length was 4 mm. As the representative value, the larger one of the inner surface and the outer surface was adopted.
(5)断面成形加工後の捻り疲労試験
これら製品管(溶接鋼管)から、試験用管材(長さ:1500mm)を採取し、該試験用管材の中央部約1000mmLに、図5(特開2001-321846号公報の図11)に示すように円周方向断面がV字形状となるように断面成形加工し、捻り疲労試験用管材とした。
捻り疲労試験は、1Hz、両振りの条件で応力水準を種々変化させた捩り疲労試験を行い、負荷応力Sにおける破断までの繰返し回数Nを求めた。得られたS−N線図より5×105繰返し疲れ限度σB(MPa)を求めた。なお、負荷応力は最初にダミー片で捩り試験を行い、疲労亀裂位置(最大応力位置)を確認し、その位置に3軸歪ゲージを貼付けて実測した。
(5) Torsional fatigue test after cross-section forming processing Pipes for test (length: 1500 mm) were collected from these product pipes (welded steel pipes), and the center of the test pipes was about 1000 mmL. As shown in FIG. 11) of Japanese Patent No. -321846, a cross section was formed so that the circumferential cross section was V-shaped, and a tube material for a torsional fatigue test was obtained.
In the torsional fatigue test, a torsional fatigue test in which the stress level was variously changed under the conditions of 1 Hz and double swing was performed, and the number of repetitions N until the fracture at the load stress S was obtained. A 5 × 10 5 repeated fatigue limit σ B (MPa) was determined from the obtained SN diagram. The load stress was measured by first conducting a torsion test with a dummy piece, confirming the fatigue crack position (maximum stress position), and attaching a triaxial strain gauge at that position.
得られた結果を表2に示す。 The obtained results are shown in Table 2.
本発明例(製品管No.1〜10)はいずれも、フェライト相が面積率で60%以上、フェライト相の平均結晶粒径が0.5〜5.5μmの組織を呈し、引張強さTSが700MPa以上の高強度を有し、かつ伸びElが18%以上の延性を有し優れた成形性と、母材部およびシーム溶接部のC方向シャルピー破面遷移温度がそれぞれ−60℃以下、−20℃以下と優れた低温靭性を有し、さらにV字断面成形加工後の5×105サイクルの繰返し疲れ限度σBと引張強度TSとの比、(σB/TS)が0.35以上と優れた断面成形加工後の耐捻り疲労特性を有している。 Examples of the present invention (product pipe Nos. 1 to 10) all have a structure in which the ferrite phase has an area ratio of 60% or more, the average crystal grain size of the ferrite phase is 0.5 to 5.5 μm, and the tensile strength TS is 700 MPa or more. With high strength and elongation of 18% or more, and excellent formability, and transition temperature of C direction Charpy fracture surface of base material and seam weld is -60 ℃ or less, -20 ℃ Excellent low-temperature toughness, and the ratio of the 5 × 10 5 cycle repeated fatigue limit σ B and tensile strength TS after V-shaped cross-section forming processing, (σ B / TS) is 0.35 or more and excellent cross section Has torsional fatigue resistance after molding.
一方、鋼成分、C/Mn比のいずれかが本発明の範囲を外れるNo.11〜No.31は、強度、成形性、低温靭性、疲労強度のいずれかが低下している。
CまたはMnが本発明の範囲を下回る比較例(No.12、No.16)はTSが700MPa未満であり、C、Mn、Ti、Nb、Crのいずれかが本発明の範囲を超える比較例No.13、No.17、No.19、No.21、No.25はTSが高く、Elが18%未満と低く、低温靭性も劣化している。SiまたはCrが本発明の範囲を下回るか、BまたはNが本発明の範囲を超える比較例No.14、No.23、No.24、No.30はいずれもElが18%未満と低い。Ti、Nb、B、Alのいずれかが本発明の範囲を下回るか、C/Mn比、Si、Al、P、S、Oのいずれかが本発明の範囲を超える比較例No.11、No.15、No.18、No.20、No.22、No.26、No.27、No.28、No.29、No.31はElが18%以上であるが、低温靭性が低い。さらに、比較例No.13、No.17、No.20、No.25、No.27、No.28、No.29、No.31は、σB/TSが0.35未満と断面成形加工後の耐捻り疲労特性が劣化している。
On the other hand, in No. 11 to No. 31, in which either the steel component or the C / Mn ratio is outside the scope of the present invention, any of strength, formability, low temperature toughness, and fatigue strength is reduced.
Comparative examples (No. 12, No. 16) in which C or Mn is less than the range of the present invention are those in which TS is less than 700 MPa, and any of C, Mn, Ti, Nb, Cr exceeds the range of the present invention No.13, No.17, No.19, No.21 and No.25 have high TS, low El of less than 18% and low temperature toughness. In Comparative Examples No. 14, No. 23, No. 24, and No. 30 in which Si or Cr is less than the range of the present invention or B or N exceeds the range of the present invention, El is as low as less than 18%. Comparative Examples No. 11 and No. in which any of Ti, Nb, B, and Al falls below the range of the present invention, or any of C / Mn ratio, Si, Al, P, S, and O exceeds the range of the present invention .15, No.18, No.20, No.22, No.26, No.27, No.28, No.29 and No.31 have El of 18% or more, but low temperature toughness. Further, Comparative Examples No. 13, No. 17, No. 20, No. 25, No. 27, No. 28, No. 29, and No. 31 have a σ B / TS of less than 0.35 and after cross-section molding processing. The torsional fatigue resistance is degraded.
なお、製品管No.1〜31はいずれも、表面粗さが、算術平均粗さRa:1.2〜1.9μm、最大高さ粗さRz:16〜26μm、十点平均粗さRzJIS :11〜18μmの範囲にあり、良好であった。
(実施例2)
表1に示す鋼No.A、F、Hの組成を有する鋼スラブに、表3に示す条件の熱間圧延を施し熱延鋼帯とした。ついでこれら熱延鋼帯に酸洗を施したのち、該熱延鋼帯を所定の幅寸法にスリット加工し、連続成形加工を施してオープン管とし、該オープン管を高周波抵抗溶接により電縫溶接し、ついで、表3に示す条件の縮径圧延を施したのち、表3に示す条件で冷却し、溶接鋼管(製品管)(外径:外径31.8〜130mmφ×肉厚1.0〜8.0mm)とした。なお、縮径圧延の際、縮径圧延工程での温度降下が大きい特に薄肉材では、一部縮径圧延の途中で再加熱を行い、縮径圧延終了温度を確保した。縮径圧延後は、酸洗処理を施した。なお、一部は黒皮ままとした。
In addition, as for all the product pipes No.1-31, the surface roughness is arithmetic average roughness Ra: 1.2-1.9 μm, maximum height roughness Rz: 16-26 μm, ten-point average roughness Rz JIS : 11- It was in the range of 18 μm and was good.
(Example 2)
A steel slab having the composition of steel Nos. A, F, and H shown in Table 1 was hot-rolled under the conditions shown in Table 3 to obtain a hot-rolled steel strip. Next, after pickling these hot-rolled steel strips, the hot-rolled steel strips are slit to a predetermined width, subjected to continuous forming to form an open pipe, and the open pipe is electro-welded by high-frequency resistance welding. Next, after performing diameter-reduction rolling under the conditions shown in Table 3, it is cooled under the conditions shown in Table 3, and welded steel pipe (product pipe) (outer diameter: outer diameter 31.8 to 130 mmφ x wall thickness 1.0 to 8.0 mm) It was. In the case of diameter-reducing rolling, particularly for a thin-walled material having a large temperature drop in the diameter-reducing rolling process, reheating was performed partially during the diameter-reducing rolling to secure the temperature reduction end temperature. After the diameter reduction rolling, pickling treatment was performed. Some of the skin was left black.
なお、一部の製品管では、さらに管内面及び/又は外面の一部又は全部に、表3に示すように、ホーニング加工またはショットブラスト処理、あるいは歪取り焼鈍を施した。また、一部の製品管では、V字形状への断面成形加工を施し成形体としたのちさらに、成形体の内面及び/又は外面の一部又は全部に、ホーニング加工、ショットブラスト処理を行い、または成形体に歪取り焼鈍処理,めっき処理を施した。 In some product pipes, as shown in Table 3, honing or shot blasting or strain relief annealing was performed on part or all of the pipe inner surface and / or outer surface. Further, in some product pipes, after forming a molded body by performing cross-sectional molding into a V shape, honing and shot blasting are performed on part or all of the inner surface and / or outer surface of the molded body, Alternatively, the compact was subjected to strain relief annealing and plating.
得られた製品管あるいはさらにホーニング加工、ショットブラスト処理、または歪取り焼鈍処理あるいはめっき処理を行った製品管から、実施例1と同様に試験片を採取し、実施例1と同様に組織観察試験、引張試験、低温靭性試験、表面粗さ試験、断面成形加工後の捻り疲労試験を実施した。なお、製品管の肉厚が2.5mm未満のものについては、管厚ままのシャルピー衝撃試験を実施した。また、V字形状への断面成形加工後に、さらにホーニング加工、ショットブラスト処理を行った成形体では、ホーニング加工あるいはショットブラスト処理後にその部位で表面粗さを測定し、その値を代表値として採用した。 From the obtained product tube or a product tube subjected to further honing, shot blasting, or strain relief annealing or plating, a test piece was collected in the same manner as in Example 1, and the structure observation test was performed in the same manner as in Example 1. Then, a tensile test, a low temperature toughness test, a surface roughness test, and a torsional fatigue test after cross-section forming were performed. For products with a wall thickness of less than 2.5 mm, a Charpy impact test was conducted with the tube thickness unchanged. For molded products that have undergone honing and shot blasting after V-shaped cross-section molding, measure the surface roughness at the site after honing or shot blasting, and use that value as a representative value. did.
得られた結果を表4に示す。 Table 4 shows the obtained results.
本発明例(No.33〜37、No.40〜45、No.48〜51、No.54〜57、No.60〜64、No.67〜82)はいずれも、フェライト相の面積率が60%以上、フェライト相の円周方向断面の平均結晶粒径が0.5〜5.5μmである組織を呈し、TS:700MPa以上の高強度と、伸びEl:18%以上の高成形性、母材部およびシーム溶接部のC方向シャルピー破面遷移温度がそれぞれ−60℃以下、−20℃以下の高低温靭性、V字断面成形加工後の5×105サイクルの繰返し疲れ限度σBと引張強さTSとの比、σB/TSが0.35以上の優れた耐捻り疲労特性を示している。 In all of the inventive examples (No. 33 to 37, No. 40 to 45, No. 48 to 51, No. 54 to 57, No. 60 to 64, No. 67 to 82), the area ratio of the ferrite phase is Exhibits a structure with 60% or more and an average crystal grain size of 0.5 to 5.5 μm in the circumferential section of the ferrite phase, TS: High strength of 700 MPa or more, Elongation El: High formability of 18% or more, Base material part And C-direction Charpy fracture surface transition temperature of seam welds of -60 ° C or less, -20 ° C or less, low temperature toughness, cyclic fatigue limit σ B and tensile strength of 5 × 10 5 cycles after V-shaped section forming It shows excellent torsional fatigue resistance with a ratio to TS, σ B / TS of 0.35 or more.
なお、とくに、ホーニング加工、ショットブラスト処理、歪取り焼鈍を施した本発明例(No.35、No.42、No.50、No.56、No.62、No.63、No.70〜76)はいずれも、σB/TSが0.40以上と、特に優れた断面成形加工後の耐捩り疲労特性を示している。
一方、鋼帯の熱間圧延条件、あるいは管体の縮径圧延条件が本発明の範囲を外れる比較例(No.32、No.38、No.39、No.46、No.47、No.52、No.53、No.58、No.59、No.65、No.66、No.83)は強度、成形性、低温靭性、疲労強度のいずれかが低下している。
In particular, examples of the present invention (No. 35, No. 42, No. 50, No. 56, No. 62, No. 63, No. 70 to 76) subjected to honing, shot blasting, and strain relief annealing. ) Shows a particularly excellent torsional fatigue resistance after cross-section forming with σ B / TS of 0.40 or more.
On the other hand, comparative examples (No.32, No.38, No.39, No.46, No.47, No. 47) in which the hot rolling conditions of the steel strip or the reduced diameter rolling conditions of the tube are out of the scope of the present invention. No. 52, No. 53, No. 58, No. 59, No. 65, No. 66, No. 83) are reduced in strength, formability, low temperature toughness, and fatigue strength.
鋼帯の熱間圧延の仕上圧延温度、巻取り温度、管体の縮径圧延時の再加熱温度、縮径圧延終了温度、縮径率、縮径圧延後の冷却速度が本発明の範囲を下回る比較例(No.38、No.46、No.52、No.58、No.59、No.66)はいずれもElが18%未満と低く、成形性が低下している。また、縮径率、縮径圧延後の冷却速度が本発明の範囲を超えた比較例(No.65、No.83)では、Elが18%未満と低く、成形性が低下している。 The finish rolling temperature, coiling temperature, reheating temperature at the time of pipe diameter reduction rolling, temperature reduction end temperature, diameter reduction ratio, and cooling rate after diameter reduction are within the scope of the present invention. The lower comparative examples (No. 38, No. 46, No. 52, No. 58, No. 59, No. 66) all have a low El of less than 18%, and the moldability is low. Further, in the comparative examples (No. 65, No. 83) in which the diameter reduction rate and the cooling rate after the diameter reduction rolling exceeded the range of the present invention, El was as low as less than 18%, and the formability was lowered.
鋼帯の熱間圧延の仕上圧延温度、巻取り温度、管体の縮径圧延時の再加熱温度、縮径圧延終了温度が本発明の範囲を超える比較例(No.32、No.39、No.47、No.53)は、鋼管内外面の表面粗さが粗く、いずれもσB/TSが0.35未満と疲労強度が低下している。また、比較例No.39、No.46、No.58、No.83では、母材あるいはシーム溶接部の低温靭性のいずれかが低下している。 Comparative examples (No. 32, No. 39, No. 32, No. 39, No. 32, No. 39, the finish rolling temperature of the hot rolling of the steel strip, the coiling temperature, the reheating temperature at the time of the diameter reduction rolling of the tube, and the end temperature of the diameter reduction rolling exceed the scope of the present invention. No. 47 and No. 53) have a rough surface roughness on the inner and outer surfaces of the steel pipe, and in both cases, σ B / TS is less than 0.35 and the fatigue strength is lowered. In Comparative Examples No. 39, No. 46, No. 58, and No. 83, either the base metal or the low temperature toughness of the seam welded portion is lowered.
一部の製品管では、亜鉛メッキ、アルミメッキを施したが、鋼帯の熱間圧延条件、管体の縮径圧延条件が本発明の範囲にあれば、優れた成形性、優れた低温靭性、優れた断面成形加工後の耐捩り疲労特性を示している。
製品管No.34、No.41、No.49、No.55、No.61、No.69では、縮径圧延後酸洗した場合と黒皮ままとを比較したが、特に特性差が認められないことを確認している。
Some product pipes are galvanized and aluminized. However, if the hot rolling conditions of the steel strip and the reduced diameter rolling conditions of the pipe are within the scope of the present invention, excellent formability and excellent low temperature toughness It shows excellent torsional fatigue resistance after cross-section forming.
Product pipe No. 34, No. 41, No. 49, No. 55, No. 61, No. 69 were compared between pickling after diameter reduction rolling and black skin as-is, but there was a particular difference in characteristics. It is confirmed that it is not possible.
Claims (8)
C:0.035〜0.099%、 Si:0.10〜0.45%、
Mn:2.05〜2.8%、 Ti:0.001〜0.04%、
Nb:0.001〜0.04%、 B:0.0001〜0.0035%、
Cr:0.001〜0.29%、 Al:0.01〜0.08%
を含み、かつCとMnを、C含有量とMn含有量との比、C/Mnが0.040以下を満足するように含有し、不純物としてのPを0.019%以下、Sを0.003%以下、Nを0.005%以下、Oを0.0025%以下に制限し、残部Feおよび不可避的不純物からなる組成を有する鋼スラブを加熱し、仕上圧延温度:980〜750℃、巻取り温度:700〜350℃とする熱間圧延により得られた熱延鋼帯とし、
前記縮径圧延が、前記管体を1000〜850℃に加熱したのち、該管体に縮径圧延終了温度:800〜620℃、縮径率:25〜75%とする縮径圧延を施す圧延であり、該縮径圧延終了後、620〜420℃までを平均冷却速度:0.5〜50℃/sで冷却し、前記製品管を、内面および外面の表面粗さが算術平均粗さRa:2μm以下、最大高さ粗さRz:30μm以下、十点平均粗さRzJIS:20μm以下である製品管とすることを特徴とする成形性と低温靭性に優れ、かつ断面成形加工後の耐ねじり疲労特性に優れた自動車構造部材用非調質高張力溶接鋼管の製造方法。 In a method of manufacturing a welded steel pipe, a steel strip is formed by continuously forming a steel strip into an open pipe, and the open pipe is electro-welded and formed into a pipe body. In mass%
C: 0.035 to 0.099%, Si: 0.10 to 0.45%,
Mn: 2.05-2.8%, Ti: 0.001-0.04%,
Nb: 0.001 to 0.04%, B: 0.0001 to 0.0035%,
Cr: 0.001 to 0.29%, Al: 0.01 to 0.08%
And C and Mn in a ratio of C content to Mn content so that C / Mn satisfies 0.040 or less, P as an impurity is 0.019% or less, S is 0.003% or less, N Is limited to 0.005% or less, O is limited to 0.0025% or less, and a steel slab having a composition composed of the remaining Fe and inevitable impurities is heated to a finish rolling temperature of 980 to 750 ° C. and a winding temperature of 700 to 350 ° C. A hot-rolled steel strip obtained by hot rolling,
After the diameter reduction rolling, the tube body is heated to 1000 to 850 ° C., and then the tube body is subjected to diameter reduction rolling at a temperature reduction end temperature of 800 to 620 ° C. and a diameter reduction ratio of 25 to 75%. After the diameter reduction rolling, the product is cooled to 620 to 420 ° C. at an average cooling rate of 0.5 to 50 ° C./s, and the surface roughness of the inner and outer surfaces of the product tube is arithmetic average roughness Ra: 2 μm The maximum height roughness Rz: 30 μm or less and the 10-point average roughness Rz JIS : 20 μm or less product pipe, characterized by excellent formability and low-temperature toughness, and torsional fatigue resistance after cross-section forming A method for producing a non-tempered high-tensile welded steel pipe for automobile structural members having excellent characteristics.
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JP2000178688A (en) * | 1998-12-11 | 2000-06-27 | Nisshin Steel Co Ltd | Resistance welded tube for hollow stabilizer, excellent in fatigue endurance |
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