JPWO2011108764A1 - High-strength seamless steel pipe for machine structure with excellent toughness and its manufacturing method - Google Patents
High-strength seamless steel pipe for machine structure with excellent toughness and its manufacturing method Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
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- B21C23/00—Extruding metal; Impact extrusion
- B21C23/002—Extruding materials of special alloys so far as the composition of the alloy requires or permits special extruding methods of sequences
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
- C21D9/085—Cooling or quenching
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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Abstract
低コストで製造可能な、加速冷却ままで、高強度、高靭性を有するシームレス鋼管であって、質量％で、Ｃ：０．０３〜０．２０％、Ｓｉ：０．０１〜０．５０％、Ｍｎ：０．８０〜３．００％を含有し、Ｐ：０．０２０％以下、Ｓ：０．００８０％以下、Ａｌ：０．０５０％以下、Ｎ：０．００８０％以下、Ｏ：０．００５０％以下に制限し、残部がＦｅ及び不純物からなり、β＝２．７Ｃ＋０．４Ｓｉ＋Ｍｎ＋０．４５Ｎｉ＋Ｍｏ（元素記号は各元素の含有量［質量％］）が２．５０〜４．００、Ｐｃｍ＝Ｃ＋Ｓｉ／３０＋（Ｍｎ＋Ｃｕ）／２０＋Ｎｉ／６０＋Ｍｏ／１５＋Ｖ／１０が０．１５〜０．３０であり、組織がフレッシュマルテンサイトからなり、旧オーステナイトの平均粒径が５０〜２００μｍであることを特徴とする。A seamless steel pipe with high strength and high toughness that can be manufactured at a low cost, with accelerated cooling, and in mass%, C: 0.03 to 0.20%, Si: 0.01 to 0.50% , Mn: 0.80 to 3.00%, P: 0.020% or less, S: 0.0080% or less, Al: 0.050% or less, N: 0.0080% or less, O: 0 .0050% or less, the balance is Fe and impurities, β = 2.7C + 0.4Si + Mn + 0.45Ni + Mo (element symbol is the content [mass%] of each element) is 2.50 to 4.00, Pcm = C + Si / 30 + (Mn + Cu) / 20 + Ni / 60 + Mo / 15 + V / 10 is 0.15 to 0.30, the structure is fresh martensite, and the average grain size of the prior austenite is 50 to 200 μm .
Description
本発明は、特にシリンダー、ブッシュ、ブーム等の構造部材及びシャフト等の機械用部材に好適なシームレス鋼管及びその製造方法に関する。 The present invention relates to a seamless steel pipe particularly suitable for structural members such as cylinders, bushes, and booms, and mechanical members such as shafts, and a method for manufacturing the same.
自動車や産業機械等に使用される機械部品の多くは、棒鋼を鍛造、切削加工して所定の形状とした後、調質熱処理により、所定の機械的性質が付与される。
近年では、部品の製造コスト低減のため、また、機械等の軽量化のため、部品に要求される機械的性質を有する鋼管を素材として中空形状部品を製造し、鍛造工程の短縮及び熱処理工程を省略する場合も増えている。
しかし、一般に、鋼管は、棒鋼よりも高価であり、特にシームレス鋼管は、製造コストが高い。そのため、鋼管を中空形状部品の素材として用いても、コストダウンの効果が十分ではない。
これまでに、要求される機械的性質を有し、かつ、製造コストを低減した安価な鋼管を提供するために、さまざまな検討がされている。
特許文献１には、特定の組成の素管に、特定の温度域で絞り圧延と傾斜圧延とを組み合わせた加工を施すことにより、ミクロ組織をフェライト粒径２μｍ以下の微細かつ均一なフェライト、セメンタイト組織とし、高強度でかつ延性・靭性に優れる鋼管を製造する技術が開示されている。
特許文献２には、外表面からのみの加速冷却により、外面、内面の冷却速度の違いが生じる環境であっても、板厚方向全面に渡って、高強度、高靭性を両立できる最適な組織を生成する技術が開示されている。
特許文献３には、ＡｌとＴｉの添加量を最適化して粒内変態を活用し、さらに、シームレス圧延後の加速冷却により製造する、高強度、高靭性を両立できる微細な金属組織を有する鋼管が開示されている。しかし、この技術では、粒内変態を活用するためにＡｌ量を低減する必要があり、脱酸のコストが高くなる。
特許文献４には、機械構造部材用鋼管を安価に製造することを目的として、主にＣｒ添加鋼で、金属組織が、自己焼戻しマルテンサイト単独組織、又は、下部ベイナイトとの混合組織である鋼管が開示されている。自己焼戻しマルテンサイトとは、加速冷却中にオーステナイト相がマルテンサイト変態し、加速冷却停止後の放冷で微細なセメンタイトがラス内に析出した組織である。
近年の、機械構造用部品の用途拡大や、環境問題に対応した排気ガス削減の要請に伴い、要求される機械的性質を保ちつつ、さらに低コストなシームレス鋼管が求められている。しかし、従来の技術では、高強度、高靭性を維持し、かつ、低コスト化するには限界があった。
また、鋼管の焼入れ性を向上させるためには、通常、Ｃｒを添加するが、Ｃｒを添加すると、圧延時のロールやプラグとの焼付に起因する表面疵が発生するという問題があった。Many mechanical parts used in automobiles, industrial machines, and the like are given predetermined mechanical properties by tempering heat treatment after forging and cutting steel bars into a predetermined shape.
In recent years, in order to reduce the manufacturing cost of parts and to reduce the weight of machines, etc., hollow parts are manufactured using steel pipes with the mechanical properties required for parts as raw materials, shortening the forging process and heat treatment process. More cases are omitted.
However, in general, steel pipes are more expensive than bar steel, and seamless steel pipes are particularly expensive to manufacture. Therefore, even if the steel pipe is used as a material for the hollow part, the effect of cost reduction is not sufficient.
Various studies have been made so far to provide an inexpensive steel pipe having the required mechanical properties and reduced manufacturing costs.
Patent Document 1 discloses that a micro-structure of fine and uniform ferrite and cementite having a ferrite grain size of 2 μm or less is obtained by subjecting a raw tube having a specific composition to a combination of drawing and tilt rolling in a specific temperature range. A technique for producing a steel pipe having a structure and high strength and excellent ductility and toughness is disclosed.
Patent Document 2 discloses an optimum structure capable of achieving both high strength and high toughness across the entire plate thickness direction even in an environment where the cooling rate of the outer surface and the inner surface is different due to accelerated cooling only from the outer surface. Techniques for generating are disclosed.
Patent Document 3 discloses a steel pipe having a fine metal structure that can achieve both high strength and high toughness by using intragranular transformation by optimizing the addition amount of Al and Ti, and further producing by accelerated cooling after seamless rolling. Is disclosed. However, in this technique, it is necessary to reduce the amount of Al in order to utilize intragranular transformation, and the cost of deoxidation increases.
Patent Document 4 discloses a steel pipe mainly made of Cr-added steel and having a metal structure of a self-tempered martensite single structure or a mixed structure with lower bainite for the purpose of inexpensively manufacturing a steel pipe for machine structural members. Is disclosed. Self-tempered martensite is a structure in which austenite phase has undergone martensitic transformation during accelerated cooling, and fine cementite is precipitated in the lath by cooling after stopping the accelerated cooling.
With the recent expansion of applications of machine structural parts and demands for exhaust gas reduction corresponding to environmental problems, there is a need for a low-cost seamless steel pipe while maintaining the required mechanical properties. However, the conventional techniques have limitations in maintaining high strength and high toughness and reducing the cost.
In order to improve the hardenability of the steel pipe, Cr is usually added. However, when Cr is added, there is a problem that surface flaws are generated due to seizure with a roll or a plug during rolling.
本発明は、上記のような現状に鑑みてなされたものであり、特にシリンダー、ブッシュ、ブーム等の構造部材及びシャフト等の機械用部材に好適な、高強度、高靭性で、溶接性に優れ、かつ、表面疵の発生を抑えられる機械構造用シームレス鋼管の提供、及び、適正な熱処理によって機械構造用シームレス鋼管を安価に製造する方法の提供を課題とする。 The present invention has been made in view of the above situation, and is particularly suitable for structural members such as cylinders, bushes, booms, and mechanical members such as shafts, and has high strength, high toughness, and excellent weldability. Further, it is an object of the present invention to provide a seamless steel pipe for machine structure that can suppress the occurrence of surface flaws, and a method for inexpensively manufacturing the seamless steel pipe for machine structure by appropriate heat treatment.
本発明者らは、表面疵の発生を防ぐために、Ｃｒを添加しない成分組成の鋼管について、鋼管の熱処理工程の省略によるコスト低減の検討を行った。具体的には、加速冷却まま鋼管（加速冷却後、熱処理を施さずに製造される鋼管）に注目し、検討した。
加速冷却まま鋼管の組織の旧オーステナイトの粒径は１００μｍ程度であり、焼入れ、焼戻し処理（以下「ＱＴ処理」という）を施した鋼管（以下「ＱＴ鋼管」という）では２０〜３０μｍ程度である。
なお、脱酸元素であるＡｌ量を０．０１０％以下に低減して、Ｔｉを添加すれば、粒内変態を活用して粒径を微細にすることが可能である。しかし、本発明では、製造コストを低減するため、通常、脱酸に必要とされる０．０１０％超のＡｌを添加する。
したがって、従来、加速冷却まま鋼管の組織の粒径は、ＱＴ鋼管の組織の粒径と比較して粗大であり、ＱＴ鋼管と同等又は同等以上の強度及び靭性は確保できないと考えられていた。
また、表面疵の発生を防ぐためにＣｒを添加しないと、焼入れ性が低下するので、強度確保はさらに困難であり、強度確保のためにＣｒ以外の金属を添加すると、コストが上昇すると考えられていた。
しかし、本発明者らが鋭意検討した結果、鋼管の成分組成を適正にすることで、靭性に有害な組織である上部ベイナイトの生成を抑制することができ、Ｃｒを添加しない加速冷却まま鋼管でも、溶接性を損なうことなく、ＱＴ鋼管と同等の強度及び靭性が得られることを見出した。
本発明は、上記知見に基づきなされたものであって、その要旨は以下のとおりである。
（１）質量％で、
Ｃ ：０．０３〜０．２０％、
Ｓｉ：０．０１〜０．５０％、
Ｍｎ：０．８０〜３．００％、
Ａｌ：０．０１０％超、０．０５０％以下
を含有し、
Ｐ ：０．０２０％以下、
Ｓ ：０．００８０％以下、
Ｎ ：０．００８０％以下、
Ｏ ：０．００５０％以下
に制限し、残部がＦｅ及び不可避的不純物からなり、下記式（１）によって求められるβが２．５０〜４．００、下記式（２）によって求められるＰｃｍが０．１５〜０．３０であり、組織がフレッシュマルテンサイトからなり、旧オーステナイトの粒径が５０〜２００μｍであることを特徴とする靭性に優れた機械構造用高強度シームレス鋼管。
β＝２．７Ｃ＋０．４Ｓｉ＋Ｍｎ＋０．４５Ｎｉ
＋Ｍｏ …（１）
Ｐｃｍ＝Ｃ＋Ｓｉ／３０＋（Ｍｎ＋Ｃｕ）／２０
＋Ｎｉ／６０＋Ｍｏ／１５＋Ｖ／１０ …（２）
ここで、Ｃ、Ｓｉ、Ｍｎ、Ｎｉ、Ｃｕ、Ｍｏ、Ｖは各元素の含有量［質量％］である。
（２）さらに、前記鋼管が、質量％で、
Ｂ ：０．０００１〜０．００３０％
を含有し、前記式（１）に代えて下記式（３）よって求められるβが２．５０〜４．００、前記式（２）に代えて下記式（４）によって求められるＰｃｍが０．１５〜０．３０であることを特徴とする前記（１）の靭性に優れた機械構造用高強度シームレス鋼管。
β＝２．７Ｃ＋０．４Ｓｉ＋Ｍｎ＋０．４５Ｎｉ
＋２Ｍｏ …（３）
Ｐｃｍ＝Ｃ＋Ｓｉ／３０＋（Ｍｎ＋Ｃｕ）／２０
＋Ｎｉ／６０＋Ｍｏ／１５＋Ｖ／１０＋５Ｂ …（４）
ここで、Ｃ、Ｓｉ、Ｍｎ、Ｎｉ、Ｃｕ、Ｍｏ、Ｖ、Ｂは各元素の含有量［質量％］である。
（３）さらに、前記鋼管が、質量％で、
Ｎｉ：１．００％以下、
Ｃｕ：１．００％以下、
Ｍｏ：１．５０％以下
の１種又は２種以上を含有することを特徴とする前記（１）又は（２）の靭性に優れた機械構造用高強度シームレス鋼管。
（４）さらに、前記鋼管が、質量％で、
Ｔｉ：０．０５０％以下、
Ｎｂ：０．０５０％以下、
Ｖ ：０．０５０％以下
の１種又は２種以上を含有することを特徴とする前記（１）〜（３）の靭性に優れた機械構造用高強度シームレス鋼管。
（５）さらに、前記鋼管が、質量％で、
Ｃａ：０．００４０％以下、
Ｍｇ：０．００１０％以下、
ＲＥＭ：０．００５％以下
の１種又は２種以上を含有することを特徴とする前記（１）〜（４）の靭性に優れた機械構造用高強度シームレス鋼管。
（６）前記（１）〜（５）のいずれかの機械構造用高強度シームレス鋼管の製造方法であって、
前記（１）〜（５）のいずれかの成分を有する鋼をシームレス圧延し、その後、
開始温度７５０〜９５０℃で、冷却速度が１０〜５０℃／秒の加速冷却を施す
ことを特徴とする靭性に優れた機械構造用高強度シームレス鋼管の製造方法。In order to prevent the occurrence of surface flaws, the present inventors have studied cost reduction by omitting the heat treatment step of the steel pipe for the steel pipe having a component composition to which Cr is not added. Specifically, we focused on and examined steel pipes with accelerated cooling (steel pipes manufactured without heat treatment after accelerated cooling).
The grain size of the prior austenite in the structure of the steel pipe with accelerated cooling is about 100 μm, and about 20 to 30 μm in a steel pipe (hereinafter referred to as “QT steel pipe”) subjected to quenching and tempering treatment (hereinafter referred to as “QT steel pipe”).
If the amount of Al, which is a deoxidizing element, is reduced to 0.010% or less and Ti is added, the grain size can be made fine by utilizing intragranular transformation. However, in the present invention, in order to reduce the manufacturing cost, more than 0.010% Al required for deoxidation is usually added.
Therefore, conventionally, the grain size of the structure of the steel pipe with accelerated cooling is coarse compared to the grain size of the structure of the QT steel pipe, and it has been considered that strength and toughness equal to or higher than that of the QT steel pipe cannot be secured.
Further, if Cr is not added to prevent the occurrence of surface flaws, the hardenability is deteriorated, so that it is further difficult to ensure the strength, and it is believed that the cost increases if a metal other than Cr is added to ensure the strength. It was.
However, as a result of intensive studies by the present inventors, it is possible to suppress the formation of upper bainite, which is a structure harmful to toughness, by making the component composition of the steel pipe appropriate, and even in the steel pipe with accelerated cooling without adding Cr. The inventors have found that the same strength and toughness as those of the QT steel pipe can be obtained without impairing the weldability.
The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) In mass%,
C: 0.03-0.20%,
Si: 0.01 to 0.50%,
Mn: 0.80 to 3.00%,
Al: more than 0.010%, 0.050% or less,
P: 0.020% or less,
S: 0.0080% or less,
N: 0.0080% or less,
O: Limited to 0.0050% or less, the balance being Fe and unavoidable impurities, β calculated from the following formula (1) is 2.50 to 4.00, and Pcm calculated from the following formula (2) is 0 A high-strength seamless steel pipe for machine structures having excellent toughness, characterized in that it has a texture of .15 to 0.30, the structure is fresh martensite, and the particle size of the prior austenite is 50 to 200 μm.
β = 2.7C + 0.4Si + Mn + 0.45Ni
+ Mo (1)
Pcm = C + Si / 30 + (Mn + Cu) / 20
+ Ni / 60 + Mo / 15 + V / 10 (2)
Here, C, Si, Mn, Ni, Cu, Mo, and V are contents [mass%] of each element.
(2) Furthermore, the said steel pipe is mass%,
B: 0.0001 to 0.0030%
The β calculated from the following formula (3) instead of the formula (1) is 2.50 to 4.00, and the Pcm calculated by the following formula (4) instead of the formula (2) is 0.00. The high-strength seamless steel pipe for machine structures having excellent toughness as described in (1) above, which is 15 to 0.30.
β = 2.7C + 0.4Si + Mn + 0.45Ni
+ 2Mo (3)
Pcm = C + Si / 30 + (Mn + Cu) / 20
+ Ni / 60 + Mo / 15 + V / 10 + 5B (4)
Here, C, Si, Mn, Ni, Cu, Mo, V, and B are content [mass%] of each element.
(3) Furthermore, the said steel pipe is mass%,
Ni: 1.00% or less,
Cu: 1.00% or less,
Mo: High-strength seamless steel pipe for machine structure excellent in toughness of (1) or (2) above, containing one or more of 1.50% or less.
(4) Furthermore, the said steel pipe is mass%,
Ti: 0.050% or less,
Nb: 0.050% or less,
V: The high-strength seamless steel pipe for machine structures excellent in toughness according to the above (1) to (3), comprising one or more of 0.050% or less.
(5) Furthermore, the said steel pipe is mass%,
Ca: 0.0040% or less,
Mg: 0.0010% or less,
REM: The high-strength seamless steel pipe for machine structures excellent in toughness according to the above (1) to (4), characterized by containing one or more of 0.005% or less.
(6) A method for producing a high-strength seamless steel pipe for machine structure according to any one of (1) to (5),
Seamless rolling the steel having any one of the components (1) to (5),
A method for producing a high-strength seamless steel pipe for machine structures excellent in toughness, characterized by performing accelerated cooling at a starting temperature of 750 to 950 ° C and a cooling rate of 10 to 50 ° C / second.
本発明によれば、加速冷却まま鋼管において、加速冷却時に上部ベイナイトの生成を抑制することが可能となる。その結果、ＱＴ処理を施すことなく、低コストで、ＱＴ鋼管と同等の靭性をもつ、加速冷却まま鋼管を製造することが可能となる。 ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to suppress the production | generation of an upper bainite at the time of accelerated cooling in a steel pipe with accelerated cooling. As a result, it is possible to manufacture a steel pipe having a toughness equivalent to that of a QT steel pipe with accelerated cooling without performing QT treatment.
本発明のシームレス鋼管は、加速冷却ままで、上部ベイナイトの生成を抑制して靭性を改善するために、焼入れ性の指標であるβを適正な範囲に制御し、かつ、溶接性を確保するために、溶接性の指標であるＰｃｍを適正な範囲に制御したものである。以下、本発明について詳細に説明する。
まず、本発明で、鋼管の化学成分を限定した理由を述べる。以下、「％」は、「質量％」を意味するものとする。
Ｃは、強度向上に極めて有効な元素である。目標とする強度を得るためには、０．０３％以上のＣを添加する必要がある。一方、０．２０％超のＣを添加すると、低温靭性が低下し、溶接時に割れを生じる。したがって、Ｃ量は、０．０３〜０．２０％に限定する。強度を高めるには、Ｃ量は、０．０７％以上が好ましい。一方、靭性を確保するためには、Ｃ量は、０．１５％以下が好ましい。
Ｓｉは、脱酸元素であり、強度の向上に寄与する元素である。添加の効果を得るためには、０．０１％以上のＳｉの添加が必要である。強度を向上させるには、Ｓｉ量は、０．１０％以上が好ましい。一方、Ｓｉを、０．５０％を超えて添加すると、上部ベイナイトが生成し、低温靭性を損なうので、Ｓｉ量の上限は０．５０％に制限する。Ｓｉ量の好ましい上限は０．２５％である。
Ｍｎは、低温変態組織の生成を促進する元素であり、強度と低温靭性のバランスを向上させるために有効である。この効果を得るには、０．８０％以上のＭｎを添加することが必要である。しかし、Ｍｎ量が３．００％よりも多いと、低温靭性を損なうことがあるので、３．００％を上限とする。強度と低温靭性のバランスを向上させるために好ましいＭｎ量の範囲は、１．５０〜２．４０％である。
Ｐ及びＳは不純物であり、過剰に含有すると靭性が低下し、溶接性が低下する。よって、Ｐ及びＳの含有量の上限は、それぞれ、０．０２０％及び０．００８０％とする。Ｐ及びＳの含有量は、靭性を確保するために、添加量が少ない方が好ましく、それぞれ、０．０１５％以下及び０．００５０％以下がより好ましい。Ｐ及びＳは含有しないことが好ましいので、下限値は規定しない。ただし、Ｐ及びＳの含有量を０．００１０％未満にすると製造コストが増大するので、０．００１０％を下限とすることが好ましい。
Ａｌは、強力な脱酸元素であり、脱酸のコストの観点から、０．０１０％超を添加する。Ａｌを過剰に添加すると粗大なＡｌ酸化物が生成し、低温靭性が劣化するので、上限を０．０５０％に制限する。靭性を高めるには、Ａｌ量の上限を０．０３５％にすることがより好ましい。
Ｎは、不純物であり、０．００８０％を超えると、粗大なＴｉＮが生成し、靭性が低下するので上限を０．００８０％に制限する。Ｎの含有量は、０．００６０％未満が好ましく、０．００５０％以下であれば、さらに好ましい。Ｎは含有しないことが好ましいので、下限は特に規定しない。ただし、Ｎの含有量を０．００１０％未満とすると製造コストが増大するので、０．００１０％を下限とすることが好ましい。
Ｏは、０．００５０％を超えて含有すると、粗大な酸化物が生成し、低温靭性を損なうので、上限を０．００５０％とする。Ｏは含有しないことが好ましいので、下限は特に規定しない。ただしＯの含有量を０．００１０％未満とすると製造コストが増大するので、０．００１０％を下限とすることが好ましい。
本発明の鋼には、さらにＢを添加してもよい。Ｂは、焼入れ性を高め、鋼の強靭化に寄与する元素である。その効果を得るためには、０．０００１％以上のＢを添加することが好ましい。一方、Ｂの添加量が０．００３０％より多いとＢＮなどの析出物を生じ、焼入れ性が低下することがある。より好ましいＢ量の範囲は、０．００１０〜０．００２０％である。
さらに、Ｎｉ、Ｃｕ、Ｍｏの１種又は２種以上を添加してもよい。これらは、焼入れ性を高める元素であり、本発明の鋼の強靱化に寄与する。
Ｎｉは、低温靭性を劣化させることなく強度を向上させる元素でもあり、添加の効果を得るためには０．０５％以上添加することが好ましい。１．００％を超えてＮｉを添加すると、偏析して組織が不均一になり、靭性が劣化することがあるため、Ｎｉ量の上限は１．００％とすることが好ましい。Ｎｉの添加量は０．８０％未満が好ましく、０．５０％以下がより好ましい。さらに好ましい範囲は、０．２５〜０．４５％である。
Ｃｕ、及びＭｏは、強度を向上させる効果を得るために、それぞれ、０．０５％以上を添加することが好ましい。Ｃｕ及びＭｏは、添加量がそれぞれ、１．００％、及び１．５０％を超えると溶接性を損なうことがある。また、Ｃｕを単独で添加すると表面疵が発生することがあるので、Ｃｕは、Ｎｉと同時に添加することが好ましい。
さらに、Ｔｉ、Ｎｂ、Ｖの１種又は２種以上を添加してもよい。これらは、析出強化によって鋼の強度を向上させる元素である。
Ｔｉは、鋼を析出強化させるために、０．００５％以上含有させることが好ましい。また、不純物であるＮを固定し、靭性を高めるには、０．０１０％以上のＴｉを添加することが好ましい。Ｔｉ量が、０．０５０％を超えると、粗大なＴｉ酸化物の析出によって靭性が低下することがあるので、上限を０．０５０％とすることが好ましい。また、ＴｉＮの粗大化を防止して低温靭性を向上させるには、Ｔｉ量の上限を０．０３５％以下とすることが好ましい。
Ｎｂは、炭化物、窒化物等の析出物を生成し、圧延時のオーステナイトの再結晶を抑制し組織を微細化するだけでなく、焼入れ性を増大させ、鋼の強靱化に有効な元素である。Ｎｂ量が０．０５０％を超えると、粗大なＮｂの析出物が生成し、靭性が劣化することがあるので、上限を０．０５０％とする。Ｎｂ添加の効果を得るには、０．００５％以上添加することが好ましい。
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、焼入れ性を高める効果も有する。Ｖ量が０．０５０％を超えると炭化物、窒化物が粗大化し、靭性を損なうことがあるので、Ｖ量の上限を０．０５０％とすることが好ましい。Ｖ添加の効果を得るには、０．００５％以上添加することが好ましい。
さらに、Ｃａ、Ｍｇ、ＲＥＭの１種又は２種以上を添加してもよい。これらは、介在物の形状を調整し加工性を向上させる元素であり、さらに、硫化物、酸化物又は硫酸化物として析出し、鋼管の接合部の硬化を防止する作用をもつ。
Ｃａ、Ｍｇ、及びＲＥＭの含有量は、それぞれ、０．００４０％、０．００１０％、及び０．００５％を超えると、介在物が多くなりすぎ、延性が劣化するので、上限をそれぞれ、０．００４０％、０．００１０％、及び０．００５％とする。熱間加工性を高めるには、Ｃａ、Ｍｇ、及びＲＥＭの含有量の下限を、それぞれ、０．０００５％、０．０００５％、及び０．０００１％とすることが好ましい。
上記の元素の残部は、Ｆｅ及び不可避的不純物である。不可避的不純物としては、スクラップから混入するＳｎ、Ｂｉ等が挙げられる。また、脱酸時に必要に応じて添加される、Ｚｒ、Ｔａなどを、本発明の特性を損なわない範囲で含有してもよい。
本発明のシームレス鋼管は、上部ベイナイトの生成を抑制し、靭性を改善した点が特徴である。上部ベイナイトの生成を抑制するためには、鋼材の焼入れ性を高めることが必要である。そこで、本発明では、個々の元素の組成の限定に加えて、さらに、下記式（１）で求められるβを、２．５０〜４．００の範囲に限定する。βは鋼の焼入れ性の指標であり、式（１）の元素記号は、各元素の含有量（質量％）を表す。
β＝２．７Ｃ＋０．４Ｓｉ＋Ｍｎ＋０．４５Ｎｉ
＋Ｍｏ … （１）
βが２．５０よりも小さくなると、低温靭性が劣化する。また、βが４．００を超えると、ＨＡＺ靭性、溶接性が悪化する。
さらに、下記式（２）で求められるＰｃｍを０．１５〜０．３０に限定する。式（２）の元素記号は、各元素の含有量（質量％）を表す。
Ｐｃｍ＝Ｃ＋Ｓｉ／３０＋（Ｍｎ＋Ｃｕ）／２０
＋Ｎｉ／６０＋Ｍｏ／１５＋Ｖ／１０ … （２）
Ｐｃｍが０．１５よりも小さくなると、必要な強度が得られなくなり、さらに、溶接性が悪化する。Ｐｃｍが０．３０を超えると、低温靭性が劣化し、さらに、溶接性が悪化する。より好ましいＰｃｍの範囲は、０．２０超〜０．３０である。
式（１）、式（２）に含まれる選択元素を添加しないときは、その元素の含有量は０として計算する。
鋼管がＢを含有するときは、上記式（１）、式（２）に代えて、下記式（３）、式（４）を用いて、β及びＰｃｍを求める。式（３）及び式（４）の元素記号は、各元素の含有量（質量％）を表す。
β＝２．７Ｃ＋０．４Ｓｉ＋Ｍｎ＋０．４５Ｎｉ
＋２Ｍｏ … （３）
Ｐｃｍ＝Ｃ＋Ｓｉ／３０＋（Ｍｎ＋Ｃｕ）／２０
＋Ｎｉ／６０＋Ｍｏ／１５＋Ｖ／１０＋５Ｂ … （４）
式（３）、式（４）に含まれる選択元素を添加しないときは、式（１）、式（２）の場合と同様に、その元素の含有量は０として計算する。
式（１）及び式（３）によって定義されるβは、鋼の焼入れ性に及ぼす各元素の影響を重み付けした経験式である。式（１）と式（３）との相違点は、Ｍｏの係数である。これは、Ｍｏを単独で含有する場合と比較して、ＭｏとＢとを同時に含有する場合は、相乗効果によって、焼入れ性を高めるＭｏの効果が高まることを意味する。
本発明のシームレス鋼管の金属組織は、フレッシュマルテンサイトからなる。フレッシュマルテンサイトは、ラス状の組織であり、光学顕微鏡による観察ではセメンタイトが見られない点で、焼戻しマルテンサイトやベイナイトとは異なる。本発明のシームレス鋼管は、冷却ままで製造されるので、焼戻しマルテンサイトは含まれない。また、本発明のシームレス鋼管は、靭性を低下させる相の生成を抑制したものであり、上部ベイナイトも含まれない。
フレッシュマルテンサイトは、冷却によってオーステナイトが変態した組織であり、旧オーステナイトの粒径が大きいほど、靭性が低下する。しかし、シームレス鋼管を製造する場合、一般に、鋼片の加熱温度が高く、穿孔及び圧延によって導入される累積ひずみ量を確保できないので、旧オーステナイトを微細にすることが困難である。
本発明では、旧オーステナイトの粒径を５０μｍ以上とする。これは、ＱＴ処理を施すことなく旧オーステナイトの粒径を５０μｍ未満にするには、低温で穿孔及び圧延を行う必要があり、製造コストが高くなるためである。
一方、旧オーステナイトの粒径が粗大であると靭性が低下するので、本発明では、靭性を確保するために、旧オーステナイトの粒径を２００μｍ以下とする。
旧オーステナイトの粒径は、ＪＩＳ Ｇ ０５５１に準拠して測定することができる。
本発明のシームレス鋼管は、成分、特に、焼入れ性指標β及び溶接性指標Ｐｃｍを適正な範囲とすることにより、金属組織をフレッシュマルテンサイトとし、旧オーステナイトの粒径を５０〜２００μｍとするものである。そして、本発明によれば、特に製造コストが上昇することなく、強度と靭性のバランスに優れたシームレス鋼管を提供することができる。なお、本発明の機械構造用シームレス鋼管の好ましい特性は、例えば、引張強度が７８０ＭＰａ以上、さらに好ましくは９８０ＭＰａ以上であり、かつ−２０℃でのシャルピー吸収エネルギーが１００Ｊ以上である。
次に、本発明のシームレス鋼管の製造方法について説明する。
本発明の鋼管は、熱間で約１１００〜１３００℃で加熱した鋼片を穿孔し、圧延して製造されるシームレス鋼管であり、シームレス圧延後に延伸工程を経ることもある。結晶粒微細化による高靭性化の観点からは、穿孔、圧延で累積ひずみ量を増加させることが好ましい。
シームレス圧延後、必要に応じて、磨管、定型工程を経て、その後、所定の温度まで再加熱する。なお、再加熱前に、鋼管の温度が６００℃未満になると、部分的に変態が生じ、再加熱後、異常粒成長によって、局所的に粗大な結晶粒を生じることがある。また、再加熱前の冷却時に析出物を生じ、焼入れ性を高める元素の固溶量が減少して焼入れ性が低下することがあるので、鋼管の温度を６００℃未満にすることなく再加熱することが好ましい。
再加熱後、必要に応じて絞り圧延を施し、鋼管を加速冷却する。加速冷却を開始する際の鋼管の温度は、高すぎるとオーステナイト粒が粗大化し、靭性が低下することがあるので、９５０℃以下とし、９００℃以下が好ましい。また、結晶粒界からのフェライト変態を抑制するため、加速冷却を開始する際の鋼管の温度は７５０℃以上とする。
加速冷却の冷却速度が遅すぎると、上部ベイナイトが生成する。上部ベイナイトは、比較的高温で生じるベイナイトであり、局部的な脆化相である島状マルテンサイトを多く含有するので、靭性の低下を招く。また、冷却速度が速すぎると、均一な冷却が困難となり、冷却後、鋼管が大きく変形する原因となる。したがって、加速冷却の速度は１０〜５０℃／秒とする。加速冷却の速度とは、加速冷却の開始から冷却停止までの、平均の冷却速度をいう。
冷却停止温度が高すぎても上部ベイナイトが生成することがあり、４００℃以下で加速冷却を停止することが好ましい、より好ましくは２５０℃以下である。
冷却方法は、水を鋼管の外表面に直接当てる方法、鋼管外周の接線方向に当てる方法、ミスト冷却などから任意に選定できる。
本発明の成分組成を有する鋼管を、適切な冷却速度で加速冷却することにより、上部ベイナイトの生成が抑制された、フレッシュマルテンサイトからなる金属組織が得られる。また、本発明の成分組成を有する鋼管は、旧オーステナイトの粒径が５０〜２００μｍであるから、圧延温度を低くしたり、粒内変態を活用したりする必要がない。したがって、本発明によれば、コストを上昇させることなく、強度及び靭性に優れた鋼管を製造することが可能である。In order to improve the toughness by suppressing the formation of upper bainite while maintaining the accelerated cooling, the seamless steel pipe of the present invention controls β, which is an index of hardenability, in an appropriate range, and ensures weldability. Furthermore, Pcm, which is an index of weldability, is controlled within an appropriate range. Hereinafter, the present invention will be described in detail.
First, the reason why the chemical components of the steel pipe are limited in the present invention will be described. Hereinafter, “%” means “% by mass”.
C is an element that is extremely effective for improving the strength. In order to obtain the target strength, it is necessary to add 0.03% or more of C. On the other hand, if more than 0.20% of C is added, the low temperature toughness decreases and cracks occur during welding. Therefore, the amount of C is limited to 0.03 to 0.20%. In order to increase the strength, the C content is preferably 0.07% or more. On the other hand, in order to ensure toughness, the C content is preferably 0.15% or less.
Si is a deoxidizing element and is an element that contributes to improvement in strength. In order to obtain the effect of addition, it is necessary to add 0.01% or more of Si. In order to improve the strength, the amount of Si is preferably 0.10% or more. On the other hand, if Si is added in excess of 0.50%, upper bainite is generated and low temperature toughness is impaired, so the upper limit of Si content is limited to 0.50%. A preferable upper limit of the amount of Si is 0.25%.
Mn is an element that promotes the formation of a low-temperature transformation structure, and is effective for improving the balance between strength and low-temperature toughness. In order to obtain this effect, it is necessary to add 0.80% or more of Mn. However, if the amount of Mn is more than 3.00%, the low temperature toughness may be impaired, so 3.00% is made the upper limit. In order to improve the balance between strength and low temperature toughness, the preferable range of the amount of Mn is 1.50 to 2.40%.
P and S are impurities, and if contained excessively, toughness is lowered and weldability is lowered. Therefore, the upper limits of the P and S contents are 0.020% and 0.0080%, respectively. In order to ensure toughness, the content of P and S is preferably less, and more preferably 0.015% or less and 0.0050% or less, respectively. Since it is preferable not to contain P and S, a lower limit is not prescribed | regulated. However, if the content of P and S is less than 0.0010%, the production cost increases, so 0.0010% is preferably set as the lower limit.
Al is a strong deoxidizing element, and more than 0.010% is added from the viewpoint of deoxidation cost. If Al is added excessively, coarse Al oxide is generated and low temperature toughness deteriorates, so the upper limit is limited to 0.050%. In order to increase toughness, the upper limit of the Al content is more preferably 0.035%.
N is an impurity, and if it exceeds 0.0080%, coarse TiN is generated and the toughness is lowered, so the upper limit is limited to 0.0080%. The N content is preferably less than 0.0060%, more preferably 0.0050% or less. Since it is preferable not to contain N, the lower limit is not particularly defined. However, if the N content is less than 0.0010%, the production cost increases, so 0.0010% is preferably set as the lower limit.
If O is contained in an amount exceeding 0.0050%, a coarse oxide is generated and the low temperature toughness is impaired, so the upper limit is made 0.0050%. Since it is preferable not to contain O, the lower limit is not particularly defined. However, if the O content is less than 0.0010%, the production cost increases, so 0.0010% is preferably set as the lower limit.
B may be further added to the steel of the present invention. B is an element that enhances hardenability and contributes to toughening of steel. In order to obtain the effect, 0.0001% or more of B is preferably added. On the other hand, when the addition amount of B is more than 0.0030%, precipitates such as BN are generated, and the hardenability may be lowered. A more preferable range of the amount of B is 0.0010 to 0.0020%.
Furthermore, you may add 1 type, or 2 or more types, Ni, Cu, and Mo. These are elements that enhance the hardenability and contribute to the toughening of the steel of the present invention.
Ni is an element that improves the strength without deteriorating the low-temperature toughness, and it is preferable to add 0.05% or more in order to obtain the effect of addition. When Ni is added in excess of 1.00%, segregation occurs and the structure becomes non-uniform, and the toughness may deteriorate, so the upper limit of the Ni content is preferably 1.00%. The amount of Ni added is preferably less than 0.80%, more preferably 0.50% or less. A more preferable range is 0.25 to 0.45%.
In order to obtain the effect of improving strength, Cu and Mo are each preferably added in an amount of 0.05% or more. Cu and Mo may impair weldability when the addition amount exceeds 1.00% and 1.50%, respectively. Moreover, since Cu may be generated when Cu is added alone, Cu is preferably added simultaneously with Ni.
Further, one or more of Ti, Nb, and V may be added. These are elements that improve the strength of steel by precipitation strengthening.
Ti is preferably contained in an amount of 0.005% or more in order to precipitate and strengthen steel. Moreover, in order to fix N which is an impurity and improve toughness, it is preferable to add 0.010% or more of Ti. If the amount of Ti exceeds 0.050%, the toughness may be reduced by the precipitation of coarse Ti oxides, so the upper limit is preferably made 0.050%. Moreover, in order to prevent coarsening of TiN and improve low temperature toughness, it is preferable to make the upper limit of Ti amount 0.035% or less.
Nb is an element that produces precipitates such as carbides and nitrides, suppresses recrystallization of austenite during rolling and refines the structure, and also increases hardenability and is effective in strengthening steel. . If the amount of Nb exceeds 0.050%, coarse Nb precipitates are produced and the toughness may be deteriorated, so the upper limit is made 0.050%. In order to obtain the effect of Nb addition, 0.005% or more is preferably added.
V is an element that generates carbides and nitrides and improves the strength of the steel by precipitation strengthening, and also has an effect of improving hardenability. If the V amount exceeds 0.050%, carbides and nitrides are coarsened and the toughness may be impaired. Therefore, the upper limit of the V amount is preferably 0.050%. In order to obtain the effect of V addition, 0.005% or more is preferably added.
Furthermore, you may add 1 type (s) or 2 or more types of Ca, Mg, and REM. These elements are elements that adjust the shape of inclusions to improve workability, and further precipitate as sulfides, oxides, or sulfates, and have an action of preventing hardening of the joint portion of the steel pipe.
When the contents of Ca, Mg, and REM exceed 0.0040%, 0.0010%, and 0.005%, respectively, inclusions increase excessively and ductility deteriorates. .0040%, 0.0010%, and 0.005%. In order to improve hot workability, it is preferable that the lower limits of the contents of Ca, Mg, and REM are 0.0005%, 0.0005%, and 0.0001%, respectively.
The balance of the above elements is Fe and inevitable impurities. Inevitable impurities include Sn, Bi and the like mixed from scrap. Moreover, you may contain Zr, Ta, etc. which are added as needed at the time of deoxidation in the range which does not impair the characteristic of this invention.
The seamless steel pipe of the present invention is characterized by suppressing the formation of upper bainite and improving toughness. In order to suppress the formation of upper bainite, it is necessary to improve the hardenability of the steel material. Therefore, in the present invention, in addition to limiting the composition of each element, β obtained by the following formula (1) is further limited to a range of 2.50 to 4.00. β is an index of the hardenability of steel, and the element symbol of the formula (1) represents the content (% by mass) of each element.
β = 2.7C + 0.4Si + Mn + 0.45Ni
+ Mo (1)
When β is smaller than 2.50, the low temperature toughness deteriorates. Moreover, when β exceeds 4.00, the HAZ toughness and weldability deteriorate.
Furthermore, Pcm calculated | required by following formula (2) is limited to 0.15-0.30. The element symbol of Formula (2) represents content (mass%) of each element.
Pcm = C + Si / 30 + (Mn + Cu) / 20
+ Ni / 60 + Mo / 15 + V / 10 (2)
If Pcm is smaller than 0.15, the required strength cannot be obtained, and the weldability deteriorates. When Pcm exceeds 0.30, the low temperature toughness deteriorates, and further, the weldability deteriorates. A more preferable range of Pcm is more than 0.20 to 0.30.
When the selective element included in the formulas (1) and (2) is not added, the content of the element is calculated as 0.
When the steel pipe contains B, β and Pcm are obtained using the following formulas (3) and (4) instead of the above formulas (1) and (2). The element symbols in the formulas (3) and (4) represent the content (% by mass) of each element.
β = 2.7C + 0.4Si + Mn + 0.45Ni
+ 2Mo (3)
Pcm = C + Si / 30 + (Mn + Cu) / 20
+ Ni / 60 + Mo / 15 + V / 10 + 5B (4)
When the selective element included in the formulas (3) and (4) is not added, the content of the element is calculated as 0 as in the case of the formulas (1) and (2).
Β defined by the equations (1) and (3) is an empirical equation that weights the influence of each element on the hardenability of steel. The difference between Equation (1) and Equation (3) is the coefficient of Mo. This means that when Mo and B are simultaneously contained, the effect of Mo that enhances the hardenability is enhanced by a synergistic effect as compared with the case where Mo is contained alone.
The metal structure of the seamless steel pipe of the present invention consists of fresh martensite. Fresh martensite is a lath-like structure and is different from tempered martensite and bainite in that cementite is not observed by observation with an optical microscope. Since the seamless steel pipe of the present invention is manufactured while being cooled, tempered martensite is not included. Moreover, the seamless steel pipe of this invention suppresses the production | generation of the phase which reduces toughness, and does not contain an upper bainite.
Fresh martensite is a structure in which austenite is transformed by cooling, and the toughness decreases as the particle size of prior austenite increases. However, when producing a seamless steel pipe, generally, the heating temperature of the steel slab is high, and the accumulated strain amount introduced by drilling and rolling cannot be secured, so it is difficult to make the prior austenite fine.
In the present invention, the particle size of the prior austenite is 50 μm or more. This is because in order to reduce the grain size of the prior austenite to less than 50 μm without performing QT treatment, it is necessary to perform piercing and rolling at a low temperature, which increases the manufacturing cost.
On the other hand, if the particle size of the prior austenite is coarse, the toughness is lowered. Therefore, in the present invention, the particle size of the prior austenite is set to 200 μm or less in order to ensure toughness.
The particle size of prior austenite can be measured according to JIS G 0551.
The seamless steel pipe of the present invention has a metal structure of fresh martensite and a prior austenite particle size of 50 to 200 μm by adjusting the components, particularly the hardenability index β and the weldability index Pcm. is there. And according to this invention, the seamless steel pipe excellent in the balance of intensity | strength and toughness can be provided, without manufacturing cost rising especially. In addition, as for the preferable characteristic of the seamless steel pipe for machine structures of this invention, tensile strength is 780 Mpa or more, More preferably, it is 980 Mpa or more, and the Charpy absorbed energy in -20 degreeC is 100 J or more.
Next, the manufacturing method of the seamless steel pipe of this invention is demonstrated.
The steel pipe of the present invention is a seamless steel pipe manufactured by punching and rolling a steel piece heated at about 1100 to 1300 ° C., and may undergo a drawing process after seamless rolling. From the viewpoint of increasing toughness by refining crystal grains, it is preferable to increase the cumulative strain amount by drilling and rolling.
After seamless rolling, if necessary, it is subjected to a polishing tube and a standard process, and then reheated to a predetermined temperature. If the temperature of the steel pipe is less than 600 ° C. before reheating, partial transformation occurs, and after reheating, locally coarse crystal grains may be generated due to abnormal grain growth. In addition, precipitates are formed during cooling before reheating, and the solid solution amount of elements that enhance the hardenability may decrease and hardenability may decrease. Therefore, reheating is performed without reducing the temperature of the steel pipe to less than 600 ° C. It is preferable.
After reheating, the steel pipe is accelerated and cooled as necessary, and the steel pipe is accelerated. If the temperature of the steel pipe at the start of accelerated cooling is too high, the austenite grains become coarse and the toughness may be lowered. Therefore, the temperature is 950 ° C. or less, and preferably 900 ° C. or less. Moreover, in order to suppress the ferrite transformation from a crystal grain boundary, the temperature of the steel pipe at the time of starting accelerated cooling shall be 750 degreeC or more.
If the cooling rate of accelerated cooling is too slow, upper bainite is generated. The upper bainite is a bainite that is generated at a relatively high temperature and contains a large amount of island martensite, which is a local embrittlement phase, resulting in a decrease in toughness. On the other hand, when the cooling rate is too high, uniform cooling becomes difficult, and the steel pipe is greatly deformed after cooling. Accordingly, the accelerated cooling rate is 10 to 50 ° C./second. The accelerated cooling rate means an average cooling rate from the start of accelerated cooling to the stop of cooling.
Even if the cooling stop temperature is too high, upper bainite may be generated, and it is preferable to stop the accelerated cooling at 400 ° C. or lower, more preferably 250 ° C. or lower.
The cooling method can be arbitrarily selected from a method in which water is directly applied to the outer surface of the steel pipe, a method in which water is applied in a tangential direction of the outer periphery of the steel pipe, and mist cooling.
By accelerating and cooling the steel pipe having the component composition of the present invention at an appropriate cooling rate, a metal structure composed of fresh martensite in which the formation of upper bainite is suppressed is obtained. Moreover, since the steel tube which has a component composition of this invention has the particle size of prior austenite 50-200 micrometers, it is not necessary to make a rolling temperature low or to utilize an intragranular transformation. Therefore, according to the present invention, it is possible to manufacture a steel pipe excellent in strength and toughness without increasing costs.
表１に記載の成分組成を有する鋼を溶製し、転炉、連続鋳造プロセスにより直径１００〜１７０ｍｍの鋼片を鋳造した。これらの鋼片を１１００〜１２５０℃に加熱し、マンネスマン−プラグミル方式により穿孔、圧延し、９００〜１０００℃に再加熱後、表２に示す条件で加速冷却を行った。加速冷却は、水を鋼管の外表面に直接当てる方法によって行った。製造後、鋼管の表面疵の有無を目視で確認した。
旧オーステナイトの粒径とは、マルテンサイトに変態する前の組織（高温での組織）の粒径のことをいう。旧オーステナイトの粒径は、マルテンサイトに変態した後も変わらないので、変態後でも測定可能である。
引張試験は円弧状のＪＩＳ１２号引張試験片を用いて行い、降伏強度と引張強度を測定した。靭性の評価は、ＪＩＳ Ｚ ２２４２に準拠し、２ｍｍＶノッチフルサイズ試験片を用いて、−２０℃でシャルピー試験を実施し、吸収エネルギーを測定した。
表３に結果を示す。
Ｍ：フレッシュマルテンサイト，ＢＬ：下部ベイナイト，ＢＵ：上部ベイナイト，
Ｐ：パーライト，Ｆ：フェライト
下線は本発明の範囲外であることを意味する。
本発明で既定する成分組成、β、及びＰｃｍを満たす鋼Ａ〜Ｉを用いて、本発明で規定する製造方法で製造した鋼管は、組織がフレッシュマルテンサイトであり、旧オーステナイトの粒径が５０〜２００μｍとなる。
その結果、機械構造用鋼管として必要な強度を有し、さらに、シャルピー衝撃試験の−２０℃における吸収エネルギー（ｖＥ−２０）が１１５Ｊ以上と高い値を示しており、靭性も優れている。
鋼Ｇを用いて、本発明の範囲内で冷却開始温度や冷却速度を変えて製造した鋼管（Ｎｏ．７、１０〜１３）は、いずれも適正な金属組織を有し、また、機械構造用鋼管として必要な強度を有する。さらに、シャルピー衝撃試験の−２０℃における吸収エネルギー（ｖＥ−２０）が１１５Ｊ以上と高い値を示しており、靭性も優れている。
Ｎｏ．１４は、Ｃｒを含有するため、シームレス圧延によって鋼管に表面疵が発生した。
Ｎｏ．１５は、Ｃが本発明で規定する下限を下回っており、βが２．２９と焼入れ性が劣っている。そのため、加速冷却後の金属組織は上部ベイナイトと下部ベイナイトとなり、その結果、強度が弱く、靭性も劣っている。
Ｎｏ．１６は、Ｂが本発明で規定する上限を超えており、βが２．１１と焼入れ性が劣っている。加速冷却後の金属組織は上部ベイナイトとパーライトとなり、その結果、強度が弱く、靭性も劣っている。
Ｎｏ．１７は、Ｃが本発明で規定する上限を超えているので、加速冷却後の金属組織はフレッシュマルテンサイトであるが、強度が高くなりすぎ、靭性が低下した。
Ｎｏ．１８は、個々の元素の組成は本発明の範囲内であるが、βが４．０１と大きい。その結果、加速冷却後の金属組織はフレッシュマルテンサイトであるが、強度が高くなりすぎ、靭性が低下した。
Ｎｏ．１９は、成分組成、β、及びＰｃｍが本発明の範囲内である鋼Ｇを用いて製造した鋼管であるが、冷却開始温度が高いので、金属組織が上部ベイナイトとフェライトとなり、強度が弱く、靭性も劣っている。
Ｎｏ．２０は、鋼Ｇを用いて製造した鋼管であるが、冷却開始温度が高いので、旧オーステナイトの平均粒径が大きくなり、その結果、靭性が低下した。
Ｎｏ．２１は、鋼Ｇを用いて製造した鋼管であるが、冷却速度が遅いので、金属組織がパーライトとフェライトとなり、さらに、旧オーステナイトの平均粒径が小さくなり、その結果、強度が弱く、靭性もやや劣っている。
以上示したとおり、本発明で規定する成分組成、β、及びＰｃｍを満たす鋼を、本発明の製造方法で製造することにより、ＱＴ鋼管と同等の靭性をもつ、ＱＴ処理を施さない加速冷却まま鋼管を製造することが可能となる。Steel having the composition shown in Table 1 was melted, and steel pieces having a diameter of 100 to 170 mm were cast by a converter and a continuous casting process. These steel pieces were heated to 1100 to 1250 ° C., punched and rolled by the Mannesmann-plug mill method, reheated to 900 to 1000 ° C., and then subjected to accelerated cooling under the conditions shown in Table 2. Accelerated cooling was performed by applying water directly to the outer surface of the steel pipe. After the production, the presence or absence of surface flaws on the steel pipe was visually confirmed.
The grain size of prior austenite refers to the grain size of the structure (transformation at high temperature) before transformation to martensite. The particle size of the prior austenite does not change even after transformation to martensite, so it can be measured even after transformation.
The tensile test was performed using an arc-shaped JIS No. 12 tensile test piece, and the yield strength and tensile strength were measured. Evaluation of toughness was based on JIS Z 2242, and a Charpy test was performed at −20 ° C. using a 2 mmV notch full-size test piece, and the absorbed energy was measured.
Table 3 shows the results.
P: pearlite, F: ferrite underline means outside the scope of the present invention.
The steel pipe manufactured by the manufacturing method specified by the present invention using steels A to I satisfying the composition defined by the present invention, β, and Pcm has a structure of fresh martensite, and the grain size of the prior austenite is 50. ˜200 μm.
As a result, it has the necessary strength as a steel pipe for machine structural use, and the absorbed energy (vE-20) at −20 ° C. in the Charpy impact test is as high as 115 J or more, and the toughness is also excellent.
Steel pipes (Nos. 7 and 10 to 13) manufactured by using steel G and changing the cooling start temperature and the cooling rate within the scope of the present invention all have an appropriate metal structure, and are used for mechanical structures. It has the necessary strength as a steel pipe. Furthermore, the absorbed energy (vE-20) at −20 ° C. of the Charpy impact test shows a high value of 115 J or more, and the toughness is also excellent.
No. Since No. 14 contains Cr, surface flaws occurred in the steel pipe by seamless rolling.
No. No. 15 is lower than the lower limit defined by C in the present invention, and β is 2.29, which is inferior in hardenability. Therefore, the metal structure after accelerated cooling is an upper bainite and a lower bainite. As a result, the strength is weak and the toughness is inferior.
No. No. 16 is inferior in hardenability, with B exceeding the upper limit specified in the present invention and β being 2.11. The metal structure after accelerated cooling becomes upper bainite and pearlite. As a result, the strength is weak and the toughness is inferior.
No. In No. 17, since C exceeds the upper limit defined in the present invention, the metal structure after accelerated cooling is fresh martensite, but the strength is too high and the toughness is lowered.
No. No. 18, although the composition of each element is within the scope of the present invention, β is as large as 4.01. As a result, the metal structure after accelerated cooling was fresh martensite, but the strength was too high and the toughness was lowered.
No. 19 is a steel pipe manufactured using steel G whose component composition, β, and Pcm are within the scope of the present invention, but because the cooling start temperature is high, the metal structure becomes upper bainite and ferrite, and the strength is weak. Toughness is also poor.
No. Although 20 is a steel pipe manufactured using steel G, since the cooling start temperature is high, the average grain size of prior austenite was increased, and as a result, toughness was reduced.
No. 21 is a steel pipe manufactured using steel G, but because the cooling rate is slow, the metal structure becomes pearlite and ferrite, and the average grain size of the prior austenite becomes small, resulting in low strength and high toughness. Somewhat inferior.
As described above, steel that satisfies the component composition, β, and Pcm defined in the present invention is manufactured by the manufacturing method of the present invention, so that it has the same toughness as a QT steel pipe and is not cooled by QT treatment. It becomes possible to manufacture a steel pipe.
本発明によれば、特にシリンダー、ブッシュ、ブーム等の構造部材及びシャフト等の機械用部材に好適な、ＱＴ鋼管と同等又は同等以上の靭性をもつシームレス鋼管を、低コストで、製造することが可能となるので、自動車産業、機械産業等への貢献は大きい。 According to the present invention, a seamless steel pipe having a toughness equivalent to or equal to or better than a QT steel pipe, which is particularly suitable for structural members such as cylinders, bushes, booms, and mechanical members such as shafts, can be manufactured at low cost. Because it becomes possible, the contribution to the automobile industry, the machine industry, etc. is great.
Claims (6)
Ｃ ：０．０３〜０．２０％、
Ｓｉ：０．０１〜０．５０％、
Ｍｎ：０．８０〜３．００％、
Ａｌ：０．０１０％超、０．０５０％以下
を含有し、
Ｐ ：０．０２０％以下、
Ｓ ：０．００８０％以下、
Ｎ ：０．００８０％以下、
Ｏ ：０．００５０％以下
に制限し、残部がＦｅ及び不可避的不純物からなり、下記式（１）によって求められるβが２．５０〜４．００、下記式（２）によって求められるＰｃｍが０．１５〜０．３０であり、組織がフレッシュマルテンサイトからなり、旧オーステナイトの粒径が５０〜２００μｍであることを特徴とする靭性に優れた機械構造用高強度シームレス鋼管。
β＝２．７Ｃ＋０．４Ｓｉ＋Ｍｎ＋０．４５Ｎｉ
＋Ｍｏ …（１）
Ｐｃｍ＝Ｃ＋Ｓｉ／３０＋（Ｍｎ＋Ｃｕ）／２０
＋Ｎｉ／６０＋Ｍｏ／１５＋Ｖ／１０ …（２）
ここで、Ｃ、Ｓｉ、Ｍｎ、Ｎｉ、Ｃｕ、Ｍｏ、Ｖは各元素の含有量［質量％］である。% By mass
C: 0.03-0.20%,
Si: 0.01 to 0.50%,
Mn: 0.80 to 3.00%,
Al: more than 0.010%, 0.050% or less,
P: 0.020% or less,
S: 0.0080% or less,
N: 0.0080% or less,
O: Limited to 0.0050% or less, the balance being Fe and unavoidable impurities, β calculated from the following formula (1) is 2.50 to 4.00, and Pcm calculated from the following formula (2) is 0 A high-strength seamless steel pipe for machine structures having excellent toughness, characterized in that it has a texture of .15 to 0.30, the structure is fresh martensite, and the particle size of the prior austenite is 50 to 200 μm.
β = 2.7C + 0.4Si + Mn + 0.45Ni
+ Mo (1)
Pcm = C + Si / 30 + (Mn + Cu) / 20
+ Ni / 60 + Mo / 15 + V / 10 (2)
Here, C, Si, Mn, Ni, Cu, Mo, and V are contents [mass%] of each element.
Ｂ ：０．０００１〜０．００３０％
を含有し、前記式（１）に代えて下記式（３）よって求められるβが２．５０〜４．００、前記式（２）に代えて下記式（４）によって求められるＰｃｍが０．１５〜０．３０であることを特徴とする請求項１に記載の靭性に優れた機械構造用高強度シームレス鋼管。
β＝２．７Ｃ＋０．４Ｓｉ＋Ｍｎ＋０．４５Ｎｉ
＋２Ｍｏ …（３）
Ｐｃｍ＝Ｃ＋Ｓｉ／３０＋（Ｍｎ＋Ｃｕ）／２０
＋Ｎｉ／６０＋Ｍｏ／１５＋Ｖ／１０＋５Ｂ …（４）
ここで、Ｃ、Ｓｉ、Ｍｎ、Ｎｉ、Ｃｕ、Ｍｏ、Ｖ、Ｂは各元素の含有量［質量％］である。Furthermore, the said steel pipe is mass%,
B: 0.0001 to 0.0030%
The β calculated from the following formula (3) instead of the formula (1) is 2.50 to 4.00, and the Pcm calculated by the following formula (4) instead of the formula (2) is 0.00. It is 15-0.30, The high-strength seamless steel pipe for machine structures excellent in toughness of Claim 1 characterized by the above-mentioned.
β = 2.7C + 0.4Si + Mn + 0.45Ni
+ 2Mo (3)
Pcm = C + Si / 30 + (Mn + Cu) / 20
+ Ni / 60 + Mo / 15 + V / 10 + 5B (4)
Here, C, Si, Mn, Ni, Cu, Mo, V, and B are content [mass%] of each element.
Ｎｉ：１．００％以下、
Ｃｕ：１．００％以下、
Ｍｏ：１．５０％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に記載の靭性に優れた機械構造用高強度シームレス鋼管。Furthermore, the said steel pipe is mass%,
Ni: 1.00% or less,
Cu: 1.00% or less,
The high-strength seamless steel pipe for machine structures having excellent toughness according to claim 1 or 2, characterized by containing Mo: 1.50% or less.
Ｔｉ：０．０５０％以下、
Ｎｂ：０．０５０％以下、
Ｖ ：０．０５０％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１〜３のいずれか１項に記載の靭性に優れた機械構造用高強度シームレス鋼管。Furthermore, the said steel pipe is mass%,
Ti: 0.050% or less,
Nb: 0.050% or less,
The high-strength seamless steel pipe for machine structure excellent in toughness according to any one of claims 1 to 3, wherein V: 1 type or 2 types or more of 0.050% or less is contained.
Ｃａ：０．００４０％以下、
Ｍｇ：０．００１０％以下、
ＲＥＭ：０．００５％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１〜４のいずれか１項に記載の靭性に優れた機械構造用高強度シームレス鋼管。Furthermore, the said steel pipe is mass%,
Ca: 0.0040% or less,
Mg: 0.0010% or less,
REM: 0.005% or less of 1 type or 2 types or more, The high-strength seamless steel pipe for machine structures excellent in toughness of any one of Claims 1-4 characterized by the above-mentioned.
請求項１〜５のいずれか１項に記載の成分を有する鋼をシームレス圧延し、その後、
開始温度７５０〜９５０℃で、冷却速度が１０〜５０℃／秒の加速冷却を施す
ことを特徴とする靭性に優れた機械構造用高強度シームレス鋼管の製造方法。It is a manufacturing method of the high strength seamless steel pipe for machine structures given in any 1 paragraph of Claims 1-5,
Seamless rolling the steel having the component according to any one of claims 1 to 5,
A method for producing a high-strength seamless steel pipe for machine structures excellent in toughness, characterized by performing accelerated cooling at a starting temperature of 750 to 950 ° C and a cooling rate of 10 to 50 ° C / second.
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| JP2011532386A JP4860786B2 (en) | 2010-03-05 | 2011-03-03 | High-strength seamless steel pipe for machine structure with excellent toughness and its manufacturing method |
| PCT/JP2011/055562 WO2011108764A1 (en) | 2010-03-05 | 2011-03-03 | High-strength seamless steel pipe for mechanical structure which has excellent toughness, and process for production of same |
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| KR (1) | KR101471730B1 (en) |
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| CN104911475B (en) * | 2015-06-25 | 2017-05-10 | 东北大学 | Preparation method for low-carbon medium-manganese high-toughness super-thick steel plate |
| WO2017006144A1 (en) * | 2015-07-09 | 2017-01-12 | Arcelormittal | Steel for press hardening and press hardened part manufactured from such steel |
| CN106555045A (en) * | 2015-09-24 | 2017-04-05 | 宝山钢铁股份有限公司 | A kind of seamless steel pipe press quenching cooling technique and manufacture method of utilization waste heat |
| US11168375B2 (en) | 2016-09-21 | 2021-11-09 | Jfe Steel Corporation | Steel pipe or tube for pressure vessels, method of producing steel pipe or tube for pressure vessels, and composite pressure vessel liner |
| CN108393355A (en) * | 2018-03-26 | 2018-08-14 | 天津商业大学 | A kind of manufacturing method of oil/gas well novel seamless steel tube |
| CN114107794B (en) * | 2020-08-31 | 2023-08-11 | 宝山钢铁股份有限公司 | 980 MPa-grade ultra-low carbon martensite and residual austenite ultra-high hole-enlarging steel and manufacturing method thereof |
| CN114318128A (en) * | 2020-09-30 | 2022-04-12 | 宝山钢铁股份有限公司 | Self-tempering martensite type high-strength and high-toughness seamless steel pipe and manufacturing method thereof |
| CN113528954B (en) * | 2021-06-29 | 2022-06-14 | 鞍钢股份有限公司 | Seamless steel tube for cold-drawing hydraulic cylinder and manufacturing method thereof |
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| JPH10140250A (en) * | 1996-11-12 | 1998-05-26 | Sumitomo Metal Ind Ltd | Production of steel tube for air bag, having high strength and high toughness |
| JP4608739B2 (en) * | 2000-06-14 | 2011-01-12 | Ｊｆｅスチール株式会社 | Manufacturing method of steel pipe for automobile door reinforcement |
| JP3975852B2 (en) * | 2001-10-25 | 2007-09-12 | Ｊｆｅスチール株式会社 | Steel pipe excellent in workability and manufacturing method thereof |
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| JP4959471B2 (en) * | 2007-08-28 | 2012-06-20 | 新日本製鐵株式会社 | High strength seamless steel pipe with excellent toughness for machine structure and manufacturing method thereof |
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| CN103924155B (en) | 2018-10-26 |
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| JP4860786B2 (en) | 2012-01-25 |
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