JP6807690B2 - Square steel pipe - Google Patents
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- JP6807690B2 JP6807690B2 JP2016187750A JP2016187750A JP6807690B2 JP 6807690 B2 JP6807690 B2 JP 6807690B2 JP 2016187750 A JP2016187750 A JP 2016187750A JP 2016187750 A JP2016187750 A JP 2016187750A JP 6807690 B2 JP6807690 B2 JP 6807690B2
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- 229910000831 Steel Inorganic materials 0.000 title claims description 120
- 239000010959 steel Substances 0.000 title claims description 120
- 229910000859 α-Fe Inorganic materials 0.000 claims description 43
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- 239000000203 mixture Substances 0.000 claims description 18
- 229910001567 cementite Inorganic materials 0.000 claims description 11
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 claims description 11
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- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 3
- 230000007797 corrosion Effects 0.000 description 3
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- RMLPZKRPSQVRAB-UHFFFAOYSA-N tris(3-methylphenyl) phosphate Chemical compound CC1=CC=CC(OP(=O)(OC=2C=C(C)C=CC=2)OC=2C=C(C)C=CC=2)=C1 RMLPZKRPSQVRAB-UHFFFAOYSA-N 0.000 description 3
- VTYYLEPIZMXCLO-UHFFFAOYSA-L Calcium carbonate Chemical class [Ca+2].[O-]C([O-])=O VTYYLEPIZMXCLO-UHFFFAOYSA-L 0.000 description 2
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Description
本発明は、角形鋼管に関するものである。 The present invention relates to a square steel pipe.
冷間プレス成形で製造される角形鋼管は、一般に、鋼板を冷間加工によりコの字形状に曲げて２つの部材を形成、または1枚の鋼板を箱型に曲げた後、端部同士を突き合わせて溶接することにより製造する。鋼板を９０°に曲げて形成した角部の曲率半径を小さくすれば、角形鋼管の幅や厚さを変えることなく、断面積や断面係数、断面二次モーメントを大きくすることができ、角形鋼管の荷重支持能力や変形抵抗能力を向上させることができる。曲げ荷重に対しては、単位質量当たりの荷重支持能力と変形抵抗能力も増加させることができる。さらには、角部の曲率が冷間プレス成形で製造される角形鋼管よりも小さい冷間ロール成形で製造される角形鋼管との接合も容易になる。 Square steel pipes manufactured by cold press forming generally have two members formed by bending a steel plate into a U shape by cold working, or by bending one steel plate into a box shape and then joining the ends together. Manufactured by butt welding. If the radius of curvature of the corner formed by bending the steel plate to 90 ° is reduced, the cross-sectional area, section coefficient, and moment of inertia of area can be increased without changing the width and thickness of the square steel pipe. It is possible to improve the load bearing capacity and deformation resistance capacity of. For bending load, the load bearing capacity and deformation resistance capacity per unit mass can also be increased. Furthermore, joining with a square steel pipe manufactured by cold roll forming, in which the curvature of the corner portion is smaller than that of the square steel pipe manufactured by cold press forming, becomes easy.
一方、プレスによる曲げ成形で角形鋼管を製造する場合は、冷間ロール成形に比べて、角部の曲率半径を小さくすることが難しい。更に、高強度鋼板を用いて角形鋼管を製造する場合、９０°曲げを行うと角部に表面疵が発生したり、角部の靱性が劣化する等の問題がある。これらの問題に対し、例えば、特許文献１では、所定の化学組成を有する鋼に適切な焼き入れ及び焼き戻し処理を施すことにより、角形鋼管に加工する際の表面疵の発生を抑制する鋼板及びその製造方法が提案されている。 On the other hand, when a square steel pipe is manufactured by bending by pressing, it is difficult to reduce the radius of curvature of the corner portion as compared with cold roll forming. Further, when a square steel pipe is manufactured using a high-strength steel plate, there are problems that surface flaws occur at the corners and the toughness of the corners deteriorates when bent by 90 °. In response to these problems, for example, in Patent Document 1, a steel sheet that suppresses the occurrence of surface defects when processing a square steel pipe by performing appropriate quenching and tempering treatment on steel having a predetermined chemical composition and The manufacturing method has been proposed.
また、特許文献２では、鋼板の化学成分組成を適正に調整すると共に、ミクロ組織中のベイナイトの面積分率を適切に制御し、且つ各部に応じた特性を規定することによって、冷間加工ままの状態で所定の強度（３５５ＭＰａ以上の降伏強度、５２０ＭＰａ以上の引張強度）と、低降伏比の両立を達成すると共に、角部における高い衝撃吸収特性と組成変形能を確保する角形鋼管が提案されている。 Further, in Patent Document 2, by appropriately adjusting the chemical composition of the steel sheet, appropriately controlling the area fraction of bainite in the microstructure, and defining the characteristics according to each part, the cold working state remains. A square steel pipe has been proposed that achieves both a predetermined strength (yield strength of 355 MPa or more) and a low yield ratio of 520 MPa or more in the above state, and secures high impact absorption characteristics and composition deformability at the corners. ing.
また、特許文献３では、鋼板の熱間圧延をＡｒ３点以上の温度で行い、その後室温以上７００℃以下まで、冷間成形後の辺部に対応する部位に対しては板厚中心の冷却速度が１℃／ｓ以上５０℃／ｓ以下の加速冷却を行い、角部に対応する部位に対しては、前記辺部よりも３０％以上遅い冷却速度に調整することにより、各部位の材質差を低減する角形コラム用鋼板の製造方法が開示されている。 Further, in Patent Document 3, hot rolling of a steel sheet is performed at a temperature of Ar 3 points or more, and then cooling is performed at the center of the plate thickness for a portion corresponding to a side portion after cold forming from room temperature to 700 ° C. or more. Accelerated cooling with a speed of 1 ° C / s or more and 50 ° C / s or less is performed, and the material of each part is adjusted to a cooling rate that is 30% or more slower than the side part for the part corresponding to the corner part. A method for manufacturing a steel plate for a square column that reduces the difference is disclosed.
また、特許文献４では、炭素当量、溶接割れ感受性組成、溶接熱影響部靭性指標が所定の範囲内になるように鋼板の化学成分組成を調整して鋼板の金属組織及び結晶粒の粒径を制御している。更に、降伏強さ、引張強さ、降伏比、および一様伸びと、更に２ｍｍＶノッチシャルピー値、特に−４０℃および−１０℃でのシャルピー値を一定の水準に設定している。特許文献４では、これらの条件によって、角形鋼管の端部と相手材との溶接部の靭性を保持し、大きな曲げモーメントが発生した際に応力集中に伴う溶接部からの脆性破壊のおそれを少なくし得る角形鋼管用厚鋼板が提案されている。 Further, in Patent Document 4, the chemical composition of the steel sheet is adjusted so that the carbon equivalent, the weld crack susceptibility composition, and the toughness index of the weld heat affected zone are within predetermined ranges, and the metal structure of the steel sheet and the particle size of the crystal grains are adjusted. I'm in control. Further, the yield strength, the tensile strength, the yield ratio, and the uniform elongation, and the 2 mm V notch Charpy value, particularly the Charpy value at −40 ° C. and −10 ° C. are set to a constant level. In Patent Document 4, the toughness of the welded portion between the end portion of the square steel pipe and the mating material is maintained under these conditions, and the risk of brittle fracture from the welded portion due to stress concentration is reduced when a large bending moment is generated. A possible thick steel plate for square steel pipes has been proposed.
特許文献１〜４に記載の発明によれば、所定の強度を備え且つ角形鋼管に加工する際の表面疵の発生を抑制する角形鋼管を製造することは可能である。しかし、特許文献１〜４には、角部の外面の曲率半径を鋼板の板厚ｔの２．５倍未満に形成することによって、角形鋼管の断面係数と断面二次モーメントを大きくして、曲げ荷重に対する単位質量当たりの荷重支持能力と変形抵抗能力を向上させるという技術的思想が開示も示唆も一切なされていない。 According to the inventions described in Patent Documents 1 to 4, it is possible to manufacture a square steel pipe having a predetermined strength and suppressing the occurrence of surface defects when processing the square steel pipe. However, in Patent Documents 1 to 4, the radius of curvature of the outer surface of the corner portion is formed to be less than 2.5 times the plate thickness t of the steel plate, thereby increasing the cross-sectional coefficient and the moment of inertia of area of the square steel pipe. No disclosure or suggestion has been made of the technical idea of improving the load bearing capacity and deformation resistance capacity per unit mass with respect to the bending load.
また、角形鋼管の角部の曲率半径を小さくすると、頂部が大きく変形するので、角部の表面疵の発生や靱性の劣化がする割れが問題になる。特に、鋼板の板厚が増加すると靱性が低下するため、板厚が５０ｍｍ程度になると角形鋼管の角部の靱性低下が顕著になる。 Further, if the radius of curvature of the corner portion of the square steel pipe is reduced, the top portion is greatly deformed, so that cracks that cause surface flaws at the corner portion and deterioration of toughness become problems. In particular, since the toughness decreases as the plate thickness of the steel plate increases, the toughness of the corner portion of the square steel pipe becomes remarkable when the plate thickness reaches about 50 mm.
本発明が解決しようとする課題は、角部の外面の曲率半径が鋼板の板厚ｔの２．５倍未満であって、角部の靱性が良好な降伏強度で２９５Ｎ／ｍｍ２級や３２５Ｎ／ｍｍ２級などの角形鋼管を提供することである。 The problem to be solved by the present invention is that the radius of curvature of the outer surface of the corner portion is less than 2.5 times the plate thickness t of the steel plate, and the toughness of the corner portion is good yield strength of 295 N / mm class 2 or 325 N. It is to provide a square steel pipe such as / mm 2nd grade.
また、本発明が解決しようとする課題は、角部の外面の曲率半径が２．５ｔ未満になるように鋼板にプレス曲げ加工を行って製造しても、外気温０℃のような厳しい環境下において角形鋼管の角部の外面に割れを生じない単位質量当たりの断面係数と断面二次モーメントが高い角形鋼管を提供することである。 Further, the problem to be solved by the present invention is a severe environment such as an outside temperature of 0 ° C. even if the steel sheet is press-bent so that the radius of curvature of the outer surface of the corner portion is less than 2.5 tons. It is to provide a square steel pipe having a high curvature of curvature and a moment of inertia of area per unit mass that does not cause cracks on the outer surface of the corner portion of the square steel pipe below.
本発明者らは、厚板の熱加工制御プロセス（THERMO-MECHANICAL CONTROL PROCESS、以下、「ＴＭＣＰ」という。）及び造船用高延性鋼の厚板の伸び改善技術に着目し、鋼板の金属組織の硬質相及びフェライトの平均結晶粒径を制御することにより、鋼板の曲げ加工時の製造歩留まりや曲げ部の機械的性質を向上できることを見出した。 The present inventors have focused on the thermomechanical control process for thick plates (THERMO-MECHANICAL CONTROL PROCESS, hereinafter referred to as "TMCP") and the technology for improving the elongation of thick plates of high-conductivity steel for shipbuilding. It has been found that by controlling the average crystal grain size of the hard phase and ferrite, the manufacturing yield at the time of bending the steel sheet and the mechanical properties of the bent portion can be improved.
本発明はこのような知見に基づいてなされたものであり、その要旨は以下のとおりである。 The present invention has been made based on such findings, and the gist thereof is as follows.
［１］平板部分と角部とからなる角形鋼管において、成分組成は、質量％で、Ｃ：０．０５〜０．２０％、Ｓｉ：０．０５〜１．００％、Ｍｎ：０．５〜２．０％、Ａｌ：０．００１〜０．１０％を含有し、Ｎ：０．００５％以下に制限し、残部がＦｅ及び不純物からなり、下記式（１）によって求められる炭素当量Ｃｅｑが０．４２以下であり、金属組織は、面積率で４０％以上のフェライトと残部が硬質相からなり、前記硬質相は、パーライト、島状マルテンサイト、セメンタイト及びフェライトサイドプレートの１種又は２種以上からなり、前記フェライトの平均結晶粒径は２０μｍ以下であり、前記硬質相の板厚方向の平均結晶粒径は１０μｍ以下であり、前記平板部分の板厚ｔが１５〜５０ｍｍ、全伸びが２１％以上であり、前記角部は、外面の曲率半径が２．５ｔ未満であり、２５０℃で１時間保持する時効処理を施した後の０℃における角部のシャルピー吸収エネルギーが７０Ｊ以上であることを特徴とする角形鋼管。
Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｕ＋Ｎｉ）/１５＋（Ｖ＋Ｍｏ＋Ｃｒ）／５ ・・・（１）
［２］前記硬質相のうち、パーライトの面積率が２０〜４０％であることを特徴とする上記［１］に記載の角形鋼管。
［３］成分組成は、質量％で、Ｐ：０．０２０％以下、Ｓ：０．００５％以下に制限することを特徴とする上記［１］又は［２］に記載の角形鋼管。
［４］成分組成は、更に、質量％で、Ｎｂ：０．１０％以下、Ｖ：０．５０％以下、Ｔｉ：０．０５％以下、Ｚｒ：０．０５％以下、Ｔａ：０．１０％以下、Ｃｒ：１．０％以下、Ｎｉ：１．０％以下、Ｍｏ：１．０％以下、Ｃｕ：１．０％以下、Ｂ：０．００５０％以下の１種又は２種以上を含有することを特徴とする上記［１］〜［３］の何れかに記載の角形鋼管。
［５］成分組成は、更に、質量％で、Ｃａ：０．０１％以下、Ｍｇ：０．０１％以下、ＲＥＭ：０．０５％以下の１種又は２種以上を含有することを特徴とする［１］〜［４］の何れかに記載の角形鋼管。
[1] In a square steel pipe composed of a flat plate portion and a corner portion, the component composition is C: 0.05 to 0.20%, Si: 0.05 to 1.00%, Mn: 0.5 in mass%. Contains ~ 2.0%, Al: 0.001 to 0.10%, N: limited to 0.005% or less, the balance consists of Fe and impurities, and the carbon equivalent Ceq calculated by the following formula (1). The metal structure is 0.42 or less, and the metal structure is composed of ferrite having an area ratio of 40% or more and a hard phase in the balance, and the hard phase is one or 2 of pearlite, island martensite, cementite and ferrite side plates. It is composed of seeds or more, the average crystal grain size of the ferrite is 20 μm or less, the average crystal grain size of the hard phase in the plate thickness direction is 10 μm or less, the plate thickness t of the flat plate portion is 15 to 50 mm, and the total elongation. 21% or more, the radius of curvature of the outer surface of the corner is less than 2.5t, and the charpy absorption energy of the corner at 0 ° C. after being subjected to the aging treatment held at 250 ° C. for 1 hour is 70J or more. A square steel pipe characterized by being.
Ceq = C + Mn / 6 + (Cu + Ni) / 15+ (V + Mo + Cr) / 5 ... (1)
[2] The square steel pipe according to the above [1], wherein the area ratio of pearlite in the hard phase is 20 to 40%.
[3] The square steel pipe according to the above [1] or [2], wherein the component composition is limited to P: 0.020% or less and S: 0.005% or less in mass%.
[4] The component composition further includes, in mass%, Nb: 0.10% or less, V: 0.50% or less, Ti: 0.05% or less, Zr: 0.05% or less, Ta: 0.10. % Or less, Cr: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, B: 0.0050% or less The square steel pipe according to any one of the above [1] to [3], which is characterized by containing.
[5] The component composition is further characterized by containing one or more types of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.05% or less in mass%. The square steel pipe according to any one of [1] to [4].
本発明によれば、角部の外面の曲率半径が２．５ｔ未満になるように、鋼板のプレス曲げ加工を行うことにより角形鋼管を製造することができる。また、本発明の角形鋼管は、外気温０℃のような厳しい環境下であってもその角部の外面に割れを生じないので、製造時期や予熱などを検討する必要が無く、生産効率が向上する。 According to the present invention, a square steel pipe can be manufactured by press-bending a steel plate so that the radius of curvature of the outer surface of the corner portion is less than 2.5 tons. Further, since the square steel pipe of the present invention does not crack on the outer surface of the corner even in a harsh environment such as an outside air temperature of 0 ° C., it is not necessary to consider the manufacturing time and preheating, and the production efficiency is improved. improves.
本発明の角形鋼管は、平板部分の全伸びが２１％以上であり、角部の外面の曲率半径が２．５ｔ未満であり、２５０℃で１時間保持する時効処理を施した後の０℃におけるシャルピー吸収エネルギーが７０Ｊ以上である。本発明の角形鋼管は、鋼板をコの字状、または箱型に冷間曲げ加工し、端部同士を突き合わせて溶接して製造され、断面形状は略正方形であり、４つの角部を有し、角部を繋ぐ平坦部のうち、一辺と対向する他片とに溶接部を有している。
まず、本発明の角形鋼管の組成限定理由について説明する。なお、特に断わらない限り質量％は、単に％で記す。
In the square steel pipe of the present invention, the total elongation of the flat plate portion is 21% or more, the radius of curvature of the outer surface of the corner portion is less than 2.5 tons, and the temperature is 0 ° C. after being subjected to an aging treatment of holding at 250 ° C. for 1 hour. The charpy absorption energy in the above is 70 J or more. The square steel pipe of the present invention is manufactured by cold-bending a steel plate into a U-shape or a box shape and welding the ends abutting each other. The cross-sectional shape is substantially square and has four corners. However, among the flat portions connecting the corner portions, a welded portion is provided on the other piece facing one side.
First, the reason for limiting the composition of the square steel pipe of the present invention will be described. Unless otherwise specified, mass% is simply expressed as%.
Ｃ：０．０５〜０．２０％
Ｃは、強度の向上に有効である。Ｃ量を増やすことによって、鋼の強度を高めることができるため、Ｃの含有量の下限を０．０５％とする。一方、Ｃ量が０．２０％を超えると、硬質相が増加して鋼の強度が高くなりすぎ、また、例えば、２５０℃で１時間保持する時効処理を施した後など、ひずみ時効による影響が大きくなり、靭性を劣化させるので、上限を０．２０％とする。また、強度を確保する観点から、Ｃ量の下限を好ましくは０．１０％以上にするとよい。強度を過剰に上昇させず、靱性を確保する観点からは、Ｃ量の上限を好ましくは０．１７％、より好ましくは０．１６％、確実に靭性を確保するためには０．１４％以下にすることが好ましい。
C: 0.05 to 0.20%
C is effective in improving the strength. Since the strength of steel can be increased by increasing the amount of C, the lower limit of the content of C is set to 0.05%. On the other hand, when the amount of C exceeds 0.20%, the hard phase increases and the strength of the steel becomes too high, and the influence of strain aging, for example, after aging treatment of holding at 250 ° C. for 1 hour. Will increase and the toughness will deteriorate, so the upper limit is set to 0.20%. Further, from the viewpoint of ensuring strength, the lower limit of the amount of C is preferably 0.10% or more. From the viewpoint of ensuring toughness without excessively increasing the strength, the upper limit of the amount of C is preferably 0.17%, more preferably 0.16%, and 0.14% or less to ensure toughness. Is preferable.
Ｓｉ：０．０５〜１．０％
Ｓｉは、脱酸剤として有効である。脱酸剤としての効果を得るためには、Ｓｉ量は０．０５％以上が好ましい。また、Ｓｉは固溶強化によって強度を高める元素であるので、Ｓｉ量は０．１０％以上が好ましい。Ｓｉ量は、１．０％を超えると、靭性を損なうので、上限を１．０％とする。靱性を確保する観点からは、Ｓｉ量を０．５０％以下にすることが好ましく、０．３０％以下がより好ましい。
Si: 0.05-1.0%
Si is effective as an antacid. In order to obtain the effect as a deoxidizer, the amount of Si is preferably 0.05% or more. Further, since Si is an element whose strength is increased by strengthening the solid solution, the amount of Si is preferably 0.10% or more. If the amount of Si exceeds 1.0%, the toughness is impaired, so the upper limit is set to 1.0%. From the viewpoint of ensuring toughness, the amount of Si is preferably 0.50% or less, more preferably 0.30% or less.
Ｍｎ：０．５〜２．０％
Ｍｎは、鋼の焼入れ性を高める元素である。本発明では、強度及び靱性を確保するために、０．５％以上のＭｎを含有させる。しかし、Ｍｎ量が過剰になると、強度が過度に高くなり、靱性が劣化するので、上限を２．０％とする。強度を確保する観点からは、Ｍｎ量を１．０％以上にすることが好ましい。靱性を確保する観点からは、Ｍｎ量を１．５％以下にすることが好ましい。
Mn: 0.5-2.0%
Mn is an element that enhances the hardenability of steel. In the present invention, 0.5% or more of Mn is contained in order to secure strength and toughness. However, if the amount of Mn is excessive, the strength becomes excessively high and the toughness deteriorates, so the upper limit is set to 2.0%. From the viewpoint of ensuring strength, the amount of Mn is preferably 1.0% or more. From the viewpoint of ensuring toughness, the amount of Mn is preferably 1.5% or less.
Ａｌ：０．００１〜０．１０％
Ａｌは、脱酸剤として有効である。脱酸剤としての効果を得るためには、０．００１％以上のＡｌを含有させることが好ましい。脱酸の効果を高めるためには、Ａｌ量は０．００５％以上が好ましく、０．０１％以上がより好ましい。Ａｌ量は、０．１０％を超えると、介在物が増加して、延性や靭性を損なうので、０．１０％以下に制限する。靱性を確保する観点からは、Ａｌ量を０．０６％以下にすることが好ましい。
Al: 0.001 to 0.10%
Al is effective as an antacid. In order to obtain the effect as a deoxidizer, it is preferable to contain 0.001% or more of Al. In order to enhance the deoxidizing effect, the Al amount is preferably 0.005% or more, more preferably 0.01% or more. If the amount of Al exceeds 0.10%, inclusions increase and ductility and toughness are impaired, so the amount is limited to 0.10% or less. From the viewpoint of ensuring toughness, the amount of Al is preferably 0.06% or less.
Ｎ：０．００５％以下
Ｎは不可避的に鋼中に存在するが、Ｎ量が多すぎると、ＴｉＮやＡｌＮが過度に増大して表面疵、角部形成後の靱性劣化等の弊害が生じるおそれがある。これに加えて、例えば、２５０℃で１時間保持する時効処理を施した後など、ひずみ時効後の靭性を劣化させるので、上限を０．００５％とする。さらに、介在物の生成を抑制する観点から、Ｎ量の上限は、好ましくは０．００４％である。下限は特に設定しないが、脱Ｎのコストや経済性を考慮し、０．００１％とすることが好ましい。
N: 0.005% or less N is inevitably present in steel, but if the amount of N is too large, TiN and AlN will increase excessively, causing adverse effects such as surface flaws and deterioration of toughness after formation of corners. There is a risk. In addition to this, the upper limit is set to 0.005% because the toughness after strain aging is deteriorated, for example, after the aging treatment of holding at 250 ° C. for 1 hour. Further, from the viewpoint of suppressing the formation of inclusions, the upper limit of the amount of N is preferably 0.004%. Although the lower limit is not particularly set, it is preferably 0.001% in consideration of the cost and economic efficiency of de-N.
Ｐ：０．０２％以下
Ｐは、不純物であり、含有量の上限を０．０２％とする。Ｐ量の低減により、靭性が向上することから、Ｐ量は０．０１５％以下が好ましく、０．０１０％以下がより好ましい。Ｐ量は少ない方が好ましいので、下限は設けない。特性とコストのバランスから、通常は、０．００１％以上が含有される。
P: 0.02% or less P is an impurity, and the upper limit of the content is 0.02%. Since the toughness is improved by reducing the amount of P, the amount of P is preferably 0.015% or less, more preferably 0.010% or less. Since it is preferable that the amount of P is small, no lower limit is set. Due to the balance between characteristics and cost, 0.001% or more is usually contained.
Ｓ：０．００５％以下
Ｓは、不純物であり、含有量の上限を０．００５％とする。Ｓ量の低減により、熱間圧延によって延伸化するＭｎＳを低減し、靭性を向上させることができることから、Ｓ量は０．００３％以下が好ましく、０．００２％以下がより好ましい。Ｓ量は少ない方が好ましいので、下限は設けない。特性とコストのバランスから、通常は、０．０００１％以上が含有される。
S: 0.005% or less S is an impurity, and the upper limit of the content is 0.005%. By reducing the amount of S, MnS stretched by hot rolling can be reduced and the toughness can be improved. Therefore, the amount of S is preferably 0.003% or less, more preferably 0.002% or less. Since it is preferable that the amount of S is small, no lower limit is set. Due to the balance between characteristics and cost, 0.0001% or more is usually contained.
本発明においては、さらに、炭窒化物の形成、鋼の焼入れ性の向上や介在物の形態制御により、強度や靱性を高めるために、Ｎｂ、Ｖ、Ｔｉ、Ｚｒ、Ｔａ、Ｃｒ、Ｎｉ、Ｍｏ、Ｃｕ、Ｂ、Ｃａ、Ｍｇ、ＲＥＭの１種又は２種以上を含有させることができる。以下の説明において好ましい下限値を記載するが、各元素の含有量が好ましい下限値未満であっても、鋼の特性に悪影響は及ぼさない。 In the present invention, in order to further increase the strength and toughness by forming a carbonitride, improving the hardenability of steel, and controlling the morphology of inclusions, Nb, V, Ti, Zr, Ta, Cr, Ni, Mo , Cu, B, Ca, Mg, REM can be contained alone or in combination of two or more. Although the preferable lower limit value is described in the following description, even if the content of each element is less than the preferable lower limit value, the characteristics of the steel are not adversely affected.
Ｎｂ：０．１０％以下
Ｎｂは、再結晶温度を低下させる元素であり、熱間圧延を行う際に、オーステナイトの再結晶を抑制して組織の微細化に寄与するので、０．００１％以上を含有させてもよい。Ｎｂ量が０．１０％を超えると粗大な析出物によって靭性が劣化することがあるので、０．１０％以下が好ましい。靭性確保の観点から、上限は０．０７％にすることが好ましく、より好ましい上限は、０．０５％である。一方、下限は組織微細化効果を確実にするため、下限は、好ましくは０．００５％、より好ましくは０．０１％、さらに好ましくは０．０２％とするとよい。
Nb: 0.10% or less Nb is an element that lowers the recrystallization temperature, and when hot rolling is performed, it suppresses the recrystallization of austenite and contributes to the miniaturization of the structure, so 0.001% or more. May be contained. If the amount of Nb exceeds 0.10%, the toughness may deteriorate due to coarse precipitates, so 0.10% or less is preferable. From the viewpoint of ensuring toughness, the upper limit is preferably 0.07%, and a more preferable upper limit is 0.05%. On the other hand, the lower limit is preferably 0.005%, more preferably 0.01%, and even more preferably 0.02% in order to ensure the effect of microstructuring.
Ｖ：０．５０％以下
Ｖは、炭化物、窒化物を生成し、析出強化によって鋼の強度を向上させる元素であり、強度を効果的に上昇させるために、０．０１％以上を含有させることが好ましい。Ｖを過剰に添加すると、炭化物及び窒化物が粗大化し、靭性の劣化をもたらすことがあるため、Ｖ量の上限は０．５０％が好ましく、さらに好ましくは０．１０％とする。
V: 0.50% or less V is an element that produces carbides and nitrides and improves the strength of steel by precipitation strengthening, and contains 0.01% or more in order to effectively increase the strength. Is preferable. If V is added in excess, carbides and nitrides may become coarse and the toughness may be deteriorated. Therefore, the upper limit of the amount of V is preferably 0.50%, more preferably 0.10%.
Ｔｉ：０．０５％以下
Ｔｉは、Ｎと結合してＴｉＮとしてスラブ中に微細に析出し、加熱時のオーステナイト粒の粗大化を抑制するので、圧延組織の微細化に有効である。また、ＴｉＮが鋼中に存在すると、溶接時に熱影響部の粗大化を抑制する。このため、Ｔｉは母材及び溶接部の靭性を改善する上で有用な元素である。これらの効果を得るためにＴｉの含有量を０．００５％以上にすることが好ましい。より好ましくはＴｉ量を０．０１０％以上とする。一方、０．０５％を超えてＴｉを含有させると溶接部の靭性を劣化させため、Ｔｉ含有量は０．０５％以下が好ましい。より好ましくはＴｉ量を０．０３０％以下、さらに好ましくは０．０２０％以下とする。
Ti: 0.05% or less Ti binds to N and finely precipitates as TiN in the slab, and suppresses the coarsening of austenite grains during heating, which is effective for finer rolling structure. Further, when TiN is present in the steel, it suppresses the coarsening of the heat-affected zone during welding. Therefore, Ti is a useful element for improving the toughness of the base metal and the welded portion. In order to obtain these effects, the Ti content is preferably 0.005% or more. More preferably, the amount of Ti is 0.010% or more. On the other hand, if Ti is contained in excess of 0.05%, the toughness of the welded portion is deteriorated. Therefore, the Ti content is preferably 0.05% or less. The amount of Ti is more preferably 0.030% or less, still more preferably 0.020% or less.
Ｚｒ：０．０５％以下
Ｚｒは、ＮｂやＴｉなどと同様に炭窒化物を形成してＣｒ炭窒化物の形成を抑制し耐食性を向上させるため、必要に応じて０．０１％以上で添加する。また、０．０５％を超えて含有させてもその効果は飽和し、大型酸化物の形成により表面疵の原因になることがあるため、０．０５％以下を添加することが好ましい。Ｔｉ，Ｎｂに較べると高価な元素でありため製造コストを考慮すると、Ｚｒの下限を０．０２％とし、上限を０．０５％とすることが望ましい。
Zr: 0.05% or less Zr is added at 0.01% or more as necessary in order to form carbonitrides like Nb and Ti, suppress the formation of Cr carbonitrides, and improve corrosion resistance. To do. Further, even if the content exceeds 0.05%, the effect is saturated and the formation of a large oxide may cause surface defects. Therefore, it is preferable to add 0.05% or less. Since it is an element that is more expensive than Ti and Nb, it is desirable that the lower limit of Zr is 0.02% and the upper limit is 0.05% in consideration of the manufacturing cost.
Ｔａ：０．１０％以下
Ｔａは、Ｚｒ等と同様に炭窒化物を形成してＣｒ炭窒化物の形成を抑制し耐食性を向上させる効果を有しており、必要に応じて０．０１％以上で添加する。また、０．０５％を超えて含有させてもその効果は飽和し、大型酸化物の形成により表面疵の原因になることがあるため、０．０５％以下を添加することが好ましい。Ｔｉ，Ｎｂに較べると高価な元素でありため製造コストを考慮すると、Ｔａの下限を０．０２％とし、上限を０．１０％とすることが望ましい。
Ta: 0.10% or less Ta has the effect of forming a carbonitride in the same manner as Zr and the like to suppress the formation of Cr carbonitride and improve the corrosion resistance, and 0.01% if necessary. Add with the above. Further, even if the content exceeds 0.05%, the effect is saturated and the formation of a large oxide may cause surface defects. Therefore, it is preferable to add 0.05% or less. Since it is an element that is more expensive than Ti and Nb, it is desirable that the lower limit of Ta is 0.02% and the upper limit is 0.10% in consideration of the manufacturing cost.
Ｃｒ：１．０％以下
Ｃｒは、強度の向上に有効な元素であり、０．０５％以上を含有させることが好ましい。Ｃｒを過度に含有させると、溶接性が劣化することがあるので、１．０％を上限とすることが好ましく、より好ましくは０．５％以下である。
Cr: 1.0% or less Cr is an element effective for improving strength, and preferably contains 0.05% or more. If Cr is excessively contained, the weldability may be deteriorated. Therefore, the upper limit is preferably 1.0%, more preferably 0.5% or less.
Ｎｉ：１．０％以下
Ｎｉは、鋼の焼入れ性を向上させる元素であり、靭性の向上にも寄与する。強度を向上させるためには、Ｎｉ量を０．０５％以上にすることが好ましい。また、Ｎｉは高価な元素であるため、上限は１．０％以下とすることが好ましく、０．３０％以下とすることがより好ましい。
Ni: 1.0% or less Ni is an element that improves the hardenability of steel and also contributes to the improvement of toughness. In order to improve the strength, the amount of Ni is preferably 0.05% or more. Further, since Ni is an expensive element, the upper limit is preferably 1.0% or less, and more preferably 0.30% or less.
Ｍｏ：１．０％以下
Ｍｏは、鋼の高強度化に寄与する元素であり、０．０５％以上を含有させることが好ましい。ただし、Ｍｏは高価な元素であり、１．０％を上限とすることが好ましい。より好ましいＭｏ量の上限は０．３０％以下であり、さらに好ましくは０．１０％以下とする。
Mo: 1.0% or less Mo is an element that contributes to increasing the strength of steel, and preferably contains 0.05% or more. However, Mo is an expensive element, and the upper limit is preferably 1.0%. The upper limit of the more preferable amount of Mo is 0.30% or less, and more preferably 0.10% or less.
Ｃｕ：１．０％以下
Ｃｕは、鋼の焼入れ性を向上させる元素であり、固溶強化にも寄与するので、０．０５％以上を含有させても良い。Ｃｕを過度に添加すると鋼板の表面性状を損なうことがあるため、上限は１．０％以下とすることが好ましい。経済性の観点から、Ｃｕ量のより好ましい上限は０．３０％以下である。
Cu: 1.0% or less Cu is an element that improves the hardenability of steel and contributes to solid solution strengthening. Therefore, 0.05% or more may be contained. Excessive addition of Cu may impair the surface properties of the steel sheet, so the upper limit is preferably 1.0% or less. From the viewpoint of economy, the more preferable upper limit of the amount of Cu is 0.30% or less.
Ｂ：０．００５０％以下
Ｂは鋼管の焼入れ性及び強度を向上する効果を有する元素であるので、０．０００３％以上を含有させても良い。より好ましくはＢ量を０．０００５％以上とする。但し、一定量以上のＢを添加しても効果が飽和するため、上限を０．００５０％とすることが好ましい。より好ましくはＢ量を０．００３０％以下、さらに好ましくは０．００２０％以下とする。
B: 0.0050% or less Since B is an element having an effect of improving the hardenability and strength of the steel pipe, 0.0003% or more may be contained. More preferably, the amount of B is 0.0005% or more. However, since the effect is saturated even if a certain amount or more of B is added, the upper limit is preferably 0.0050%. The amount of B is more preferably 0.0030% or less, still more preferably 0.0020% or less.
Ｃａ：０．０１％以下；Ｍｇ：０．０１％以下、
Ｃａ、Ｍｇは、硫化物系介在物の形態を制御し、靭性を向上させ、さらに、溶接部の酸化物を微細化して溶接部の靭性を向上させるので、一方、又は双方を０．００１％以上含有させることが好ましい。Ｃａ、Ｍｇを過剰に含有させると、酸化物や硫化物が大きくなり靭性に悪影響を及ぼすので、Ｃａ量及びＭｇ量のそれぞれの上限は０．０１％以下が好ましく、０．００５％がより好ましい。
Ca: 0.01% or less; Mg: 0.01% or less,
Ca and Mg control the morphology of sulfide-based inclusions, improve toughness, and further refine the oxide in the weld to improve the toughness of the weld. Therefore, one or both of them are 0.001%. It is preferable to contain the above. If Ca and Mg are excessively contained, oxides and sulfides become large and adversely affect the toughness. Therefore, the upper limits of the Ca amount and the Mg amount are preferably 0.01% or less, more preferably 0.005%. ..
ＲＥＭ：０．０５％以下
ＲＥＭも、前記したＣａ、Ｍｇと同様に硫化物系介在物の形態を制御し、靭性を向上させ、さらに、溶接部の酸化物を微細化して溶接部の靭性を向上させるので、Ｃａ及び／又はＭｇとともに、或いは単独で０．００１％以上含有させることが好ましい。ＲＥＭを過剰に含有させると、酸化物や硫化物が大きくなり靭性に悪影響を及ぼすので、ＲＥＭ量の上限は０．０５％とすることが好ましい。なお、ＲＥＭのうちＹ、Ｃｅが特に好ましい。
REM: 0.05% or less REM also controls the morphology of sulfide-based inclusions and improves toughness in the same manner as Ca and Mg described above, and further refines the oxide in the weld to improve the toughness of the weld. Since it improves, it is preferable to contain 0.001% or more together with Ca and / or Mg or alone. If an excessive amount of REM is contained, oxides and sulfides become large and adversely affect toughness. Therefore, the upper limit of the amount of REM is preferably 0.05%. Of the REMs, Y and Ce are particularly preferable.
Ｃｅｑ：０．４２以下
炭素当量Ｃｅｑは、Ｃ、Ｍｎ、Ｃｕ、Ｎｉ、Ｖ、Ｍｏ、Ｃｒの含有量［質量％］から、下記式（１）によって求める。炭素当量Ｃｅｑは焼入れ性の指標であり、強度の指標としても使用されることがあるが、靱性を確保するため０．４２以下にすることが必要である。Ｃｅｑが０．４２を超えると、硬質相が増加して、角形鋼管の平板部分の全伸びが２１％未満となり、また、角部の０℃におけるシャルピー吸収エネルギーが７０Ｊ未満になるおそれがある。また、Ｃｅｑが０．４２を超えると、本発明の角形鋼管を製造する目的でサブマージアーク溶接した際に、当該溶接部分の強度が維持できない場合がある。これらの効果を確実にするため、Ｃｅｑは０．４０以下が好ましく、０．３８以下がより好ましいく、０．３５以下がさらに好ましい。一方、強度を確保するために、Ｃｅｑは０．２８以上が好ましい。
Ceq: 0.42 or less The carbon equivalent Ceq is calculated by the following formula (1) from the content [mass%] of C, Mn, Cu, Ni, V, Mo, and Cr. The carbon equivalent Ceq is an index of hardenability and is sometimes used as an index of strength, but it is necessary to set it to 0.42 or less in order to secure toughness. If Ceq exceeds 0.42, the hard phase may increase, the total elongation of the flat plate portion of the square steel pipe may be less than 21%, and the Charpy absorption energy of the corner portion at 0 ° C. may be less than 70J. Further, if Ceq exceeds 0.42, the strength of the welded portion may not be maintained when submerged arc welding is performed for the purpose of manufacturing the square steel pipe of the present invention. In order to ensure these effects, Ceq is preferably 0.40 or less, more preferably 0.38 or less, and even more preferably 0.35 or less. On the other hand, in order to secure the strength, Ceq is preferably 0.28 or more.
Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｕ＋Ｎｉ）/１５＋（Ｖ＋Ｍｏ＋Ｃｒ）／５ ・・・（１） Ceq = C + Mn / 6 + (Cu + Ni) / 15+ (V + Mo + Cr) / 5 ... (1)
ここで、Ｃ、Ｍｎ、Ｃｒ、Ｍｏ、Ｖ、Ｎｉ、Ｃｕは各元素の含有量［質量％］である。なお、Ｃｒ、Ｍｏ、Ｖ、Ｎｉ、Ｃｕは、本発明においては選択的に含有させる元素であり、これらの元素を含まない場合は、上記式（１）ではその元素を０として計算する。 Here, C, Mn, Cr, Mo, V, Ni, and Cu are the contents [mass%] of each element. In addition, Cr, Mo, V, Ni, and Cu are elements to be selectively contained in the present invention, and when these elements are not contained, the element is calculated as 0 in the above formula (1).
本発明に係る角形鋼管の成分組成を上述したとおり、一定の範囲に制御し、且つＣｅｑを０．４２以下とすることによって、降伏強度として３２５Ｎ／ｍｍ２を確保することができる。 As described above, by controlling the composition of the square steel pipe according to the present invention within a certain range and setting the Ceq to 0.42 or less, a yield strength of 325 N / mm 2 can be secured.
本発明に係る角形鋼管の成分組成の、以上説明した以外の残部は、鉄、及び不純物である。ここで、不純物とは、原材料に含まれる、あるいは製造の過程で混入する成分であり、意図的に鋼に含有させたものではない成分のことをいう。 The rest of the composition of the square steel pipe according to the present invention other than those described above is iron and impurities. Here, the impurity is a component contained in the raw material or mixed in the manufacturing process, and is not intentionally contained in the steel.
具体的には、Ｐ、Ｓ、Ｏ、Ｓｂ、Ｓｎ、Ｗ、Ｃｏ、Ａｓ、Ｐｂ、Ｂｉ及びＨがあげられる。このうち、Ｐ及びＳは、上述のとおり、それぞれ、０．０２０％以下、０．００５％以下となるように制御する必要がある。Ｏは０．００６％以下となるように制御することが好ましい。 Specific examples thereof include P, S, O, Sb, Sn, W, Co, As, Pb, Bi and H. Of these, P and S need to be controlled to be 0.020% or less and 0.005% or less, respectively, as described above. It is preferable to control O to be 0.006% or less.
その他の元素については、通常、Ｓｂ、Ｓｎ、Ｗ、Ｃｏ、及びＡｓは０．１％以下、Ｐｂ及びＢｉは０．００５％以下、Ｈは０．０００４％以下の不純物としての混入があり得るが、通常の範囲であれば、特に制御する必要はない。 For other elements, Sb, Sn, W, Co, and As are usually mixed as impurities of 0.1% or less, Pb and Bi are 0.005% or less, and H is 0.0004% or less. However, if it is in the normal range, there is no need to control it.
次に、本発明の角形鋼管の金属組織について説明する。 Next, the metal structure of the square steel pipe of the present invention will be described.
本発明に係る角形鋼管は、フェライトを主体し、残部を硬質相とする組織構造を有し、降伏強度を確保するためフェライトを微細化させる。硬質相とは、フェライト以外の相であり、パーライト、島状マルテンサイト、セメンタイト及びフェライトサイドプレートの１種又は２種以上からなる。特に、角部における靱性を確保するために、フェライトの面積率、フェライト及び硬質相の平均結晶粒径は重要である。 The square steel pipe according to the present invention has a structure in which ferrite is the main component and the balance is the hard phase, and the ferrite is miniaturized in order to secure the yield strength. The hard phase is a phase other than ferrite, and is composed of one or more of pearlite, island martensite, cementite and ferrite side plates. In particular, in order to ensure toughness at the corners, the area ratio of ferrite and the average crystal grain size of ferrite and the hard phase are important.
本発明の角形鋼管を構成するフェライトの平均結晶粒径は２０μｍ以下である。フェライトの平均結晶粒径が２０μｍを超えると、角形鋼管の平板部分の全伸びが２１％未満となり、角部の外面の曲率半径が２．５ｔ未満になるように角形鋼管を製造する際、当該角部の外面に表面割れが生じるおそれがある。好ましくはフェライトの平均結晶粒径を１５μｍ以下とする。フェライトの平均結晶粒径の下限は特に限定しないが、製造コストの観点から、１μｍ以上であってもよい。 The average crystal grain size of the ferrite constituting the square steel pipe of the present invention is 20 μm or less. When the average crystal grain size of ferrite exceeds 20 μm, the total elongation of the flat plate portion of the square steel pipe becomes less than 21%, and when manufacturing the square steel pipe so that the radius of curvature of the outer surface of the corner portion becomes less than 2.5 t, the said. Surface cracks may occur on the outer surface of the corners. Preferably, the average crystal grain size of ferrite is 15 μm or less. The lower limit of the average crystal grain size of ferrite is not particularly limited, but may be 1 μm or more from the viewpoint of manufacturing cost.
また、フェライトの面積率は、靭性を確保し、降伏比を向上させるために、４０％以上が必要となる。フェライトの面積率が４０％未満の場合、角形鋼管の平板部分の全伸びが２１％未満となり、角部の外面の曲率半径が２．５ｔ未満になるように角形鋼管を製造する際、当該角部の外面に表面割れが生じるおそれがある。フェライトの面積率の上限は、降伏比を低下させるために、８０％以下が好ましく、７０％以下がより好ましい。 Further, the area ratio of ferrite is required to be 40% or more in order to secure toughness and improve the yield ratio. When the area ratio of ferrite is less than 40%, the total elongation of the flat plate portion of the square steel pipe is less than 21%, and when manufacturing the square steel pipe so that the radius of curvature of the outer surface of the corner is less than 2.5 tons, the angle is concerned. Surface cracks may occur on the outer surface of the part. The upper limit of the area ratio of ferrite is preferably 80% or less, more preferably 70% or less in order to reduce the yield ratio.
また、本発明の角形鋼管は、フェライトを除いた残部がパーライト、島状マルテンサイト、セメンタイト及びフェライトサイドプレートのうち少なくとも１種を含む硬質相で構成される。ここで、島状マルテンサイトは、オーステナイト−マルテンサイト混成物（ＭＡ）とも称される非常に硬質の相であり、フェライトサイドプレートは、ベイナイト及びウィッドマンステッテンフェライトの総称である。ベイナイトとウィッドマンステッテンフェライトとは、光学顕微鏡では判別が難しいが、セメンタイトの析出形態が異なるため、電子顕微鏡によって判別することができる。ベイナイトはセメンタイトが粒内に析出しており、ウィッドマンステッテンフェライトは粒内にはセメンタイトは析出していない点で相違している。 Further, the square steel pipe of the present invention is composed of a hard phase in which the balance excluding ferrite contains at least one of pearlite, island martensite, cementite and ferrite side plates. Here, island-like martensite is a very hard phase also referred to as an austenite-martensite hybrid (MA), and ferrite side plates are a general term for bainite and widmann stetten ferrite. Although it is difficult to distinguish between bainite and Widmann stetten ferrite with an optical microscope, they can be distinguished with an electron microscope because the precipitation morphology of cementite is different. Bainite differs in that cementite is precipitated in the grains, and Widmann stetten ferrite has no cementite precipitated in the grains.
前記硬質相は角形鋼管の平板部分の全伸び率を低下させ、角部の靱性を低下させるため、できるだけ少なく、平均結晶粒径は小さい方がよい。前記硬質相の板厚方向の平均結晶粒径が１０μｍ超の場合、角形鋼管の平板部分の全伸びが２１％未満となり、また、冷間加工によるプレス曲げ加工成形ままあるいは２５０℃で１時間保持する時効処理を施した後の角部の０℃におけるシャルピー吸収エネルギーが７０Ｊ未満になるおそれがある。 The hard phase reduces the total elongation rate of the flat plate portion of the square steel pipe and lowers the toughness of the corner portion. Therefore, it is preferable that the hard phase is as small as possible and the average crystal grain size is small. When the average crystal grain size of the hard phase in the plate thickness direction exceeds 10 μm, the total elongation of the flat plate portion of the square steel pipe is less than 21%, and it is held as it is by press bending by cold working or at 250 ° C. for 1 hour. The Charpy absorption energy at 0 ° C. at the corners after the aging treatment may be less than 70J.
フェライト及び硬質相の平均結晶粒径、フェライトの面積率は、加工前の母材或いは加工後の平板部分の表面から所定の深さ（例えば、６ｍｍ）の断面を、圧延方向に平行方向に採取し、光学顕微鏡を用いて当該断面を測定することにより求めることができる。フェライト及び硬質相の平均結晶粒径、フェライトの面積率の測定は、ＪＩＳ Ｇ ０５５１：２０１３に従って、或いは当該規格に準じた方法により測定される。例えば、加工前の母材或いは加工後の平板部分の表面から所定の深さ（例えば、６ｍｍ或いは板厚の１／４）から組織観察用試験片を採取し、当該試験片の圧延方向断面を研磨、ナイタール腐食を施した後、当該断面の組織を、光学顕微鏡を用いて観察し、１視野以上について撮像し、画像処理して、切断法でフェライト及び硬質相の平均結晶粒径、フェライトの面積率を算出するものとする。 The average crystal grain size of ferrite and the hard phase and the area ratio of ferrite are obtained by collecting a cross section of a predetermined depth (for example, 6 mm) from the surface of the base metal before processing or the flat plate portion after processing in a direction parallel to the rolling direction. However, it can be obtained by measuring the cross section using an optical microscope. The average crystal grain size of ferrite and the hard phase and the area ratio of ferrite are measured according to JIS G 0551: 2013 or by a method according to the standard. For example, a test piece for microstructure observation is collected from a predetermined depth (for example, 6 mm or 1/4 of the plate thickness) from the surface of the base material before processing or the flat plate portion after processing, and the cross section in the rolling direction of the test piece is obtained. After polishing and nital corrosion, the structure of the cross section is observed using an optical microscope, an image is taken for one or more visual fields, image processing is performed, and the average crystal grain size of ferrite and hard phase and ferrite are processed by cutting method. The area ratio shall be calculated.
ここで、フェライトの形態はほぼ粒状であり、観察する方向によって平均結晶粒径はほとんど変化しないので、単に平均結晶粒径と称する。これに対して、硬質相の形態は粒状でない場合、例えば、板状、針状などの形態であることが多いので、圧延方向に平行な板厚断面で板厚方向の平均結晶粒径を測定する。 Here, since the form of ferrite is almost granular and the average crystal grain size hardly changes depending on the observation direction, it is simply referred to as the average crystal grain size. On the other hand, when the form of the hard phase is not granular, for example, it is often in the form of a plate or a needle, so the average crystal grain size in the plate thickness direction is measured with a plate thickness cross section parallel to the rolling direction. To do.
硬質相のパーライト、島状マルテンサイト、セメンタイト及びフェライトサイドプレートは光学顕微鏡で判別することができるので、それぞれの面積率は画像処理による算出が可能である。なお、硬質相のうち、パーライトは、島状マルテンサイト、セメンタイト及びフェライトサイドプレートに比べて軟質であり、靱性に及ぼす悪影響が小さい。パーライトの面積率は、強度を高めるために、２０％以上であってもよい。一方、靱性を確保するためには、パーライトの面積率は４０％以下が好ましい。 Since the hard phase pearlite, island-shaped martensite, cementite and ferrite side plates can be discriminated by an optical microscope, the area ratio of each can be calculated by image processing. Of the hard phases, pearlite is softer than island martensite, cementite and ferrite side plates, and has a small adverse effect on toughness. The area ratio of pearlite may be 20% or more in order to increase the strength. On the other hand, in order to secure toughness, the area ratio of pearlite is preferably 40% or less.
次に、本発明の角形鋼管の製造方法について説明する。 Next, the method for manufacturing the square steel pipe of the present invention will be described.
まず、上記に示した本発明範囲の組成を有する溶湯から連続鋳造により熱間圧延素材となるスラブを製造する。 次いで、このようにして得られた素材（スラブ）を用いてＴＭＣＰにより鋼板を製造し、冷間成形及び溶接を施して、本発明の角形鋼管を製造することができる。 First, a slab to be a hot-rolled material is produced by continuous casting from the molten metal having the composition within the range of the present invention shown above. Next, a steel plate can be produced by TMCP using the material (slab) thus obtained, and cold-formed and welded to produce the square steel pipe of the present invention.
具体的には、まず、前記スラブについて、１０００〜１３００℃に再加熱後、９００℃以下での累積圧下率を３５％以上、圧延仕上温度を８７０℃未満とする。再加熱の温度が１３００℃超であったり、９００℃以下での累積圧下率が３５％未満であったり、圧延仕上温度が８７０℃以上であると、フェライト相、硬質相の微細化が不十分になる。なお、後述の冷却開始温度がＡｒ３温度以上であることが好ましいので、これに対応して、圧延仕上温度もＡｒ３温度以上であることが好ましい。より好ましくは８２０℃以上である。 Specifically, first, the slab is reheated to 1000 to 1300 ° C., then the cumulative reduction rate at 900 ° C. or lower is 35% or more, and the rolling finish temperature is less than 870 ° C. If the reheating temperature is higher than 1300 ° C, the cumulative reduction rate at 900 ° C or lower is less than 35%, or the rolling finish temperature is 870 ° C or higher, the ferrite phase and hard phase are not sufficiently refined. become. Since the cooling start temperature described later is preferably Ar 3 temperature or higher, it is preferable that the rolling finish temperature is also Ar 3 temperature or higher. More preferably, it is 820 ° C. or higher.
圧延後、鋼板は、平均冷却速度５℃／ｓ以上の冷却速度で６００℃未満の温度まで、圧延に引き続いて加速冷却を行う。好ましくは加速冷却の停止温度を５９０℃以下、より好ましくは５８０℃以下とする。加速冷却の停止温度は、フェライトの面積率を４０％以上とし、靱性を確保するために、５００℃超が好ましい。好ましくは５１０℃以上、より好ましくは５２０℃以上とする。なお、冷却開始温度については特に規定しないが、Ａｒ３温度以上でかつ、圧延が終了して速やかに冷却を開始することが望ましい。圧延に引き続いて加速冷却された鋼板は、その後、３５０〜５００℃の温度域で焼戻し処理を行うことで強靱化の調整を行ってもよい。 After rolling, the steel sheet is subjected to accelerated cooling following rolling to a temperature of less than 600 ° C. at a cooling rate of 5 ° C./s or more on average. The stop temperature of accelerated cooling is preferably 590 ° C. or lower, more preferably 580 ° C. or lower. The stop temperature for accelerated cooling is preferably more than 500 ° C. in order to make the area ratio of ferrite 40% or more and to ensure toughness. The temperature is preferably 510 ° C. or higher, more preferably 520 ° C. or higher. Although the cooling start temperature is not particularly specified, it is desirable that the temperature is Ar 3 or higher and the cooling is started immediately after the rolling is completed. The toughness of the steel sheet that has been accelerated and cooled after rolling may be adjusted by performing a tempering treatment in a temperature range of 350 to 500 ° C.
さらに、鋼板を角形状に冷間成形し、端部同士をサブマージアーク溶接して鋼管とする。鋼板の成形する方法にはＵＯＥ法、ＪＣＯ法等が適用される。また、溶接法にはアーク溶接、レーザー溶接等が使用可能である。現状サブマージドアーク溶接が最も一般的である。 Further, the steel plate is cold-formed into a square shape, and the ends are submerged arc welded to form a steel pipe. The UOE method, the JCO method, etc. are applied to the method for forming the steel sheet. Further, arc welding, laser welding and the like can be used as the welding method. Currently, submerged arc welding is the most common.
上述した成分組成と製造条件により、本発明の角形鋼管の金属組織は、フェライトの面積率が４０％以上、残部がパーライト、島状マルテンサイト、セメンタイト、フェライトサイドプレートのうち少なくとも１種を含む硬質相で構成される。また、前記フェライトの平均結晶粒径は２０μｍ以下であり、前記硬質相の板厚方向の平均結晶粒径は１０μｍ以下になる。そのため、２１％以上の全伸びと３２５Ｎ／ｍｍ２以上の降伏強度を確保することができ、外面の曲率半径が２．５ｔ未満になるように角部を形成しても、当該角部は十分な靱性を有しており、当該外面にひび割れが生じない。 According to the above-mentioned composition and manufacturing conditions, the metal structure of the square steel pipe of the present invention is hard including at least one of pearlite, island-shaped martensite, cementite, and ferrite side plate, with a ferrite area ratio of 40% or more. Consists of phases. Further, the average crystal grain size of the ferrite is 20 μm or less, and the average crystal grain size of the hard phase in the plate thickness direction is 10 μm or less. Therefore, a total elongation of 21% or more and a yield strength of 325 N / mm 2 or more can be secured, and even if a corner portion is formed so that the radius of curvature of the outer surface is less than 2.5 t, the corner portion is sufficient. It has a high toughness and does not crack on the outer surface.
また、冷間ロール成形で製造される本発明の角形鋼管と、プレスによる曲げ成形で製造された本発明の角形鋼管との継手部分を平滑にして応力集中を回避すれば、溶接部及びその近傍における破断の発生を抑制することができる。 Further, if the joint portion between the square steel pipe of the present invention manufactured by cold roll forming and the square steel pipe of the present invention manufactured by bending by pressing is smoothed to avoid stress concentration, the welded portion and its vicinity thereof. It is possible to suppress the occurrence of breakage in.
表１に示す化学組成Ａ〜Ｒを有する鋼をそれぞれ真空溶解炉にて溶製し、得られたスラブを表２の条件にてＴＭＣＰにより鋼素材とした。尚、表１の炭素当量Ｃｅｑは、上記式（１）で規定される値であり、Ｃｅｑのパラメータとなる元素成分が添加されていない場合、当該元素成分の含有量を０として計算する。また、表２において、加熱温度は加熱炉の設定温度、圧延終了温度、水冷開始温度、水冷停止温度及び焼戻温度は鋼板の表面温度を測定した値である。 The steels having the chemical compositions A to R shown in Table 1 were each melted in a vacuum melting furnace, and the obtained slab was used as a steel material by TMCP under the conditions shown in Table 2. The carbon equivalent Ceq in Table 1 is a value defined by the above formula (1), and when an elemental component that is a parameter of Ceq is not added, the content of the elemental component is calculated as 0. Further, in Table 2, the heating temperature is the set temperature of the heating furnace, the rolling end temperature, the water cooling start temperature, the water cooling stop temperature, and the tempering temperature are the measured values of the surface temperature of the steel sheet.
表２の条件にて製造された鋼Ｎｏ．Ａ〜Ｐをそれぞれ冷間加工してコの字状の形状とし、端部を溶接して角形鋼管Ｎｏ．１〜３２を製造した。このとき、角部は、当該外側の曲率半径が板厚ｔ（ｍｍ）に対して２．４倍或いは２倍になるようにプレス曲げ加工を行った。 Steel No. manufactured under the conditions shown in Table 2. Each of A to P is cold-worked to form a U-shape, and the ends are welded to form a square steel pipe No. 1-32 were manufactured. At this time, the corner portion was press-bent so that the radius of curvature on the outer side was 2.4 times or twice the plate thickness t (mm).
得られた各角形鋼管について、鋼管のフェライト相及び硬質相の面積分率、角部の靱性ｖＥ０及び表面割れの有無を下記の方法で評価するとともに、平坦部の降伏強度、引張強度、全伸びを下記の方法で評価した。 For each of the obtained square steel pipes, the area fraction of the ferrite phase and the hard phase of the steel pipe, the toughness vE 0 of the corners and the presence or absence of surface cracks are evaluated by the following methods, and the yield strength and tensile strength of the flat portion are all. The elongation was evaluated by the following method.
［鋼管のフェライト相及び硬質相の面積分率、並びに、平均結晶粒径の測定方法］
加工後の平板部分の表面から６ｍｍの断面を、圧延方向に平行方向に採取し、光学顕微鏡を用いて当該断面を観察し、当該断面を画像解析することにより、フェライト相及び硬質相の面積分率及び平均結晶粒径を測定した。その結果を表３−１に示す。尚、表３−１中、「Ｐ」はパーライト相を、「ＭＡ」は島状マルテンサイト相を、「Ｆｅ３Ｃ」はセメンタイト相を意味する。
[Measurement method of area fraction of ferrite phase and hard phase of steel pipe and average crystal grain size]
A cross section of 6 mm from the surface of the processed flat plate portion is sampled in a direction parallel to the rolling direction, the cross section is observed using an optical microscope, and the cross section is image-analyzed to determine the area of the ferrite phase and the hard phase. The rate and average crystal grain size were measured. The results are shown in Table 3-1. In Table 3-1 "P" means a pearlite phase, "MA" means an island-like martensite phase, and "Fe 3 C" means a cementite phase.
［平坦部及び角部の靭性ｖＥ0の測定方法］
平坦部の衝撃試験片はＪＩＳ Ｚ ２２４２のＶノッチ試験片とし、これにより０℃のシャルピー吸収エネルギーｖＥ０の評価を行った。これらの試験片を採取する場合、試験片の中心が外表面から１／４となるようにした（側面ノッチ）。ただし、試験片の中心が外表面から１／４となるように採取できない場合には、なるべくこれに近い位置から採取した。角部の靭性ｖＥ0は、ＪＩＳ Ｚ ２２４２に従って測定した。角部の靭性ｖＥ0も同様に、ＪＩＳ Ｚ ２２４２に従って測定した。
[Measurement method of toughness vE 0 of flat part and corner part]
The impact test piece of the flat portion was a V-notch test piece of JIS Z 2242, and the Charpy absorbed energy vE 0 at 0 ° C. was evaluated by this. When collecting these test pieces, the center of the test pieces was set to 1/4 from the outer surface (side notch). However, when it was not possible to collect the test piece so that the center of the test piece was 1/4 of the outer surface, the test piece was collected from a position as close to this as possible. The toughness vE 0 of the corner was measured according to JIS Z 2242. The toughness vE 0 of the corner was similarly measured according to JIS Z 2242.
すなわち、まず、冷間加工によるプレス曲げ加工成形ままおよび２５０℃で１時間保持する時効処理を施した後の、図１に示す鋼管角部４５°の位置の外面側から図２に示すように曲げ半径Ｒ（ｍｍ）を有する角部外面側から内側に６ｍｍの箇所を中心として、一辺が１０ｍｍのシャルピー試験片を管軸方向に３本採取し、鋼管の厚さ方向にＶノッチの切り込み（断面ノッチ）を施した。これらのシャルピー試験片を用いてシャルピー衝撃試験を行ない、温度０℃下でのシャルピー吸収エネルギーｖＥ0を測定し、その平均値を求めた。合格基準は７０Ｊ以上とした。 That is, first, as shown in FIG. 2, from the outer surface side at the position of the steel pipe corner portion 45 ° shown in FIG. 1, after the press bending process by cold working and the aging treatment of holding at 250 ° C. for 1 hour. Three Charpy test pieces with a side of 10 mm are collected in the pipe axis direction, centering on a portion 6 mm inward from the outer surface side of the corner having a bending radius R (mm), and a V notch is cut in the thickness direction of the steel pipe ( Cross-section notch) was applied. A Charpy impact test was performed using these Charpy test pieces, and the Charpy absorption energy vE 0 at a temperature of 0 ° C. was measured, and the average value was obtained. The passing standard was 70J or higher.
［平坦部及び角部の降伏強度、引張強度及び降伏比の評価方法］
図１に示す角形鋼管の平坦部について外面側から鋼板の１／４ｔ（ｔ：板厚）の位置における管軸方向に、ＪＩＳ Ｚ ２２４１の１Ａ号試験片を採取してＪＩＳ Ｚ ２２４１の要領で引張試験を行ない（測定温度：２５℃）、鋼管平坦部の降伏強度、引張強度及び降伏比を測定した。
[Evaluation method of yield strength, tensile strength and yield ratio of flat and corner parts]
Regarding the flat portion of the square steel pipe shown in FIG. 1, a 1A test piece of JIS Z 2241 was collected in the direction of the pipe axis at a position of 1/4 t (t: plate thickness) of the steel plate from the outer surface side, and in the same manner as JIS Z 2241. A tensile test was performed (measurement temperature: 25 ° C.), and the yield strength, tensile strength and yield ratio of the flat portion of the steel pipe were measured.
図1に示す角部についても、外面側から鋼板の１／４ｔ（ｔ：板厚）の位置における管軸方向に、ＪＩＳ Ｚ ２２４１ ４号試験片を採取してＪＩＳ Ｚ ２２４１の要領で引張試験を行ない（測定温度：２５℃）、鋼管平坦部の降伏強度、引張強度及び降伏比を測定した。ただし、試験片の中心が外表面から１／４となるように採取できない場合には、なるべくこれに近い位置から採取した。合格基準は、平坦部、角部とも、３２５Ｎ／ｍｍ２以上の降伏強度とした。 For the corners shown in FIG. 1, JIS Z 2241 No. 4 test pieces were collected in the direction of the pipe axis at the position of 1/4 t (t: plate thickness) of the steel plate from the outer surface side, and a tensile test was performed in the same manner as JIS Z 2241. (Measurement temperature: 25 ° C.), the yield strength, tensile strength and yield ratio of the flat portion of the steel pipe were measured. However, when it was not possible to collect the test piece so that the center of the test piece was 1/4 of the outer surface, the test piece was collected from a position as close to this as possible. The acceptance criteria were a yield strength of 325 N / mm 2 or more for both flat and corner parts.
また、鋼管角部の表面性状を評価するために、浸透探法によって表面割れの有無を確認した。これらの結果を下記表３−２及び表３−３に示す。 In addition, in order to evaluate the surface properties of the corners of the steel pipe, the presence or absence of surface cracks was confirmed by a penetration search method. These results are shown in Table 3-2 and Table 3-3 below.
これらの結果から、本発明で規定する条件で製造した鋼管Ｎｏ．１〜１６のものは、本発明で規定する要件を満足するものであり、いずれも角部におけるシャルピー吸収エネルギーｖＥ0が、２５０℃で１時間保持する時効処理を施した後においても７０Ｊを上回っており、良好な靭性を発揮している。また、角部の外面の曲率半径が板厚の２倍になる曲げ加工を行っても、当該外面に表面割れが生じておらず、良好な曲げ加工が可能であることが示された。 From these results, the steel pipe No. manufactured under the conditions specified in the present invention. Those 1 to 16 satisfy the requirements specified in the present invention, and the Charpy absorption energy vE 0 at the corners exceeds 70 J even after the aging treatment of holding at 250 ° C. for 1 hour. It has good toughness. Further, it was shown that even if the outer surface of the corner is bent so that the radius of curvature is twice the plate thickness, the outer surface is not cracked and good bending is possible.
これに対して、鋼管Ｎｏ．１７〜３２の比較例では、成分が本発明の範囲外であり、平坦部の降伏点又は耐力が不足したり、角部の外面の曲率半径が板厚の２．５倍未満（２．４倍及び２倍）になる曲げ加工を行った場合、当該外面に表面割れが生じたり、角部におけるシャルピー吸収エネルギーｖＥ0が７０Ｊを下回る等の問題が生じた。 On the other hand, the steel pipe No. In the comparative examples of 17 to 32, the components are outside the range of the present invention, the yield point or proof stress of the flat portion is insufficient, and the radius of curvature of the outer surface of the corner portion is less than 2.5 times the plate thickness (2.4). When the bending process was performed to double and double), there were problems such as surface cracking on the outer surface and Charpy absorption energy vE 0 at the corners being less than 70J.
本発明の角形鋼管は、外面の曲率半径を２．５ｔ未満とするプレス曲げ加工によって製造することができる。また、本発明の角形鋼管は、外気温０℃のような厳しい環境下であっても、その角部の外面に割れを生じない。本発明によれば、角形鋼管の製造において板厚の削減を行うことができるので、高い製造歩留まり性にて角形鋼管を製造することができ、産業界における価値はきわめて高い。 The square steel pipe of the present invention can be manufactured by press bending so that the radius of curvature of the outer surface is less than 2.5 tons. Further, the square steel pipe of the present invention does not crack on the outer surface of the corner portion even in a harsh environment such as an outside air temperature of 0 ° C. According to the present invention, since the plate thickness can be reduced in the production of the square steel pipe, the square steel pipe can be produced with a high production yield, and the value in the industry is extremely high.
Claims (5)
Ｃ：０．０５〜０．２０％、
Ｓｉ：０．０５〜１．０％、
Ｍｎ：０．５〜２．０％、
Ａｌ：０．００１〜０．１％
を含有し、
Ｐ：０．０２０％以下、
Ｓ：０．００５％以下、
Ｎ：０．００５％以下
に制限し、残部がＦｅ及び不純物からなり、
下記式（１）によって求められる炭素当量Ｃｅｑが０．４２以下であり、
金属組織は、面積率で４０％以上のフェライトと残部が硬質相からなり、
前記硬質相は、面積率が２０〜４０％であるパーライトと、島状マルテンサイト、セメンタイト及びフェライトサイドプレートの１種又は２種以上とからなり、前記フェライトの平均結晶粒径は２０μｍ以下であり、前記硬質相の板厚方向の平均結晶粒径は１０μｍ以下であり、
前記平板部分の板厚ｔが１５〜５０ｍｍ、全伸びが２１％以上であり、前記角部は、外面の曲率半径が２．５ｔ未満であり、
２５０℃で１時間保持する時効処理を施した後の０℃における角部のシャルピー吸収エネルギーが７０Ｊ以上であることを特徴とする角形鋼管。
Ｃｅｑ＝Ｃ＋Ｍｎ／６＋（Ｃｕ＋Ｎｉ）/１５＋（Ｖ＋Ｍｏ＋Ｃｒ）／５ ・・・（１） In a square steel pipe composed of a flat plate portion and a corner portion, the component composition is mass%.
C: 0.05 to 0.20%,
Si: 0.05-1.0%,
Mn: 0.5-2.0%,
Al: 0.001 to 0.1%
Contains,
P: 0.020% or less,
S: 0.005% or less,
N: Limited to 0.005% or less, the balance consists of Fe and impurities,
The carbon equivalent Ceq obtained by the following formula (1) is 0.42 or less.
The metallographic structure consists of ferrite with an area ratio of 40% or more and the rest of the hard phase.
Said hard phase includes a pearlite area ratio is 20-40%, consists of island martensite, one cementite and ferrite side plate or two or more, the average crystal grain size of the ferrite is at 20μm or less The average crystal grain size of the hard phase in the plate thickness direction is 10 μm or less.
The plate thickness t of the flat plate portion is 15 to 50 mm, the total elongation is 21% or more, and the radius of curvature of the outer surface of the corner portion is less than 2.5 t.
A square steel pipe characterized in that the Charpy absorption energy at the corners at 0 ° C. after being subjected to an aging treatment held at 250 ° C. for 1 hour is 70 J or more.
Ceq = C + Mn / 6 + (Cu + Ni) / 15+ (V + Mo + Cr) / 5 ... (1)
Ｎｂ：０．１０％以下、
Ｖ：０．５０％以下、
Ｔｉ：０．０５％以下、
Ｚｒ：０．０５％以下、
Ｔａ：０．１０％以下、
Ｃｒ：１．０％以下、
Ｎｉ：１．０％以下、
Ｍｏ：１．０％以下、
Ｃｕ：１．０％以下、
Ｂ：０．００５０％以下
の１種又は２種以上を含有することを特徴とする請求項１に記載の角形鋼管。 Ingredient composition is further in mass%,
Nb: 0.10% or less,
V: 0.50% or less,
Ti: 0.05% or less,
Zr: 0.05% or less,
Ta: 0.10% or less,
Cr: 1.0% or less,
Ni: 1.0% or less,
Mo: 1.0% or less,
Cu: 1.0% or less,
B: The square steel pipe according to claim 1 , wherein one type or two or more types of 0.0050% or less are contained.
Ｃａ：０．０１％以下、
Ｍｇ：０．０１％以下、
ＲＥＭ：０．０５％以下
の１種又は２種以上を含有することを特徴とする請求項１又は２に記載の角形鋼管。 Ingredient composition is further in mass%,
Ca: 0.01% or less,
Mg: 0.01% or less,
REM: The square steel pipe according to claim 1 or 2 , which contains 1 type or 2 or more types of 0.05% or less.
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