JP2669178B2 - High toughness and high strength seamless steel pipe - Google Patents
High toughness and high strength seamless steel pipeInfo
- Publication number
- JP2669178B2 JP2669178B2 JP3102371A JP10237191A JP2669178B2 JP 2669178 B2 JP2669178 B2 JP 2669178B2 JP 3102371 A JP3102371 A JP 3102371A JP 10237191 A JP10237191 A JP 10237191A JP 2669178 B2 JP2669178 B2 JP 2669178B2
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- Prior art keywords
- steel pipe
- less
- content
- strength
- toughness
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
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Description
【０００１】[0001]
【産業上の利用分野】本発明は、熱間圧延ままで引張強
さで150 kgf/mm 2 以上の高い強度と優れた低温靱性を有
し、自動車のドア補強材などに用いるのに好適な継目無
鋼管に関する。BACKGROUND OF THE INVENTION The present invention is applicable to tensile strength as hot rolled.
The present invention relates to a seamless steel pipe having a high strength of 150 kgf / mm 2 or more and an excellent low temperature toughness, which is suitable for use as a vehicle door reinforcing material.
【０００２】[0002]
【従来の技術】近年、自動車の軽量化の動向に対応して
鋼管製ドア補強材の開発が進んでいる。このドア補強材
は、側面衝突時の乗員の保護を目的としてドアの内部に
設置される。従来、このような用途に使用される鋼管と
しては、例えば、電縫鋼管を高周波加熱後水焼入れし、
必要に応じて 150〜250 ℃程度で焼き戻しを行った 150
kgf/mm2 級の熱処理鋼管や、熱間圧延ままの140kgf/mm2
級の継目無鋼管が用いられている。しかし、前者の電縫
鋼管では熱処理のコストが高く、後者の継目無鋼管は靱
性が低い。そのため、150kgf/mm2級の高強度を有し、さ
らに、衝突時の突発的な応力負荷（曲げ変形）に対して
も脆性的な折損が生じないような十分な靱性を有し、し
かも、製造コストが低い高靱性高強度継目無鋼管が強く
求められている。2. Description of the Related Art In recent years, steel pipe door reinforcing materials have been developed in response to the trend of reducing the weight of automobiles. This door reinforcement is installed inside the door for the purpose of protecting the occupant in the event of a side collision. Conventionally, as a steel pipe used for such an application, for example, an electric resistance welded steel pipe is induction-heated and then water-quenched,
If necessary, tempered at 150-250 ℃ 150
kgf / mm 2 grade heat treatment steel and, in as-hot-rolled 140 kgf / mm 2
Grade seamless steel pipe is used. However, the former ERW pipe has a high heat treatment cost, and the latter seamless steel pipe has low toughness. Therefore, a high strength of 150 kgf / mm 2 grade, further comprising a sufficient toughness as brittle breakage does not occur even for sudden stress loads (bending deformation) at the time of collision, moreover, There is a strong demand for high-toughness, high-strength seamless steel pipes with low manufacturing costs.
【０００３】[0003]
【発明が解決しようとする課題】本発明は、熱間圧延ま
まで引張強さが150kgf/mm2以上の高い強度を有するとと
もに、寒冷地での使用においても脆性的な折損を生じな
い優れた低温靱性を有する継目無鋼管を提供することを
目的とする。DISCLOSURE OF THE INVENTION The present invention has a high tensile strength of 150 kgf / mm 2 or more as hot rolled and does not cause brittle breakage even when used in cold regions.
And to provide a seamless steel pipe having had excellent low-temperature toughness.
【０００４】[0004]
【課題を解決するための手段】本発明は、比較的多量の
Crを含有するCr−Mo鋼に主として適量のNbを含有させ、
Ｏ (酸素) 含有量を極力低く抑えることにより、熱間圧
延ままで高強度ならびに優れた低温靱性を有する継目無
鋼管を得ることができるという知見に基づいてなされた
もので、その要旨は下記およびの高靱性継目無鋼管
にある（以下、「％」はすべて重量％を意味する）。SUMMARY OF THE INVENTION The present invention provides a relatively large amount of
A Cr-Mo steel containing Cr mainly contains an appropriate amount of Nb,
It is based on the finding that by keeping the O (oxygen) content as low as possible, a seamless steel pipe having high strength and excellent low-temperature toughness can be obtained as hot rolled. High toughness seamless steel pipe (hereinafter, "%" means "% by weight").
【０００５】 重量％で、Ｃ：0.15〜0.40％、Si：0.
05〜0.50％、Mn：0.50〜2.00％、Cr：1.00％を超えて3.
00％まで、Mo：0.10〜1.00％、Al： 0.005〜0.050 ％、
Nb： 0.005〜0.050 ％を含有し、残部がFeおよび不可避
不純物からなり、不純物中のＰが 0.025％以下、Ｓが
0.015％以下、Ｏが0.0010％未満である低温靱性に優れ
た高強度継目無鋼管。% By weight, C: 0.15 to 0.40%, Si: 0.
05-0.50%, Mn: 0.50-2.00%, Cr: over 1.00% 3.
Up to 00% , Mo: 0.10-1.00%, Al: 0.005-0.050%,
Nb: 0.005 to 0.050%, with the balance being Fe and unavoidable impurities, P in the impurities is 0.025% or less, and S is
Excellent low temperature toughness with 0.015% or less and O less than 0.0010%
High strength seamless steel pipe .
【０００６】 前記に記載の成分に加え、さらに、
重量％で、Ni：1.00〜3.00％、Ｖ：0.02〜0.10％、Ti：
0.010〜0.100 ％およびＢ：0.0010〜0.0050％のうちの
１種以上を含有し、残部がFeおよび不可避不純物からな
り、不純物中のＰが0.025 ％以下、Ｓが 0.015％以下、
Ｏが0.0010％未満である低温靱性に優れた高強度継目無
鋼管。[0006] In addition to the above ingredients,
% By weight, Ni: 1.00 to 3.00%, V: 0.02 to 0.10%, Ti:
0.010 to 0.100% and B: one or more of 0.0010 to 0.0050%, the balance being Fe and unavoidable impurities, P in the impurities is 0.025% or less, S is 0.015% or less,
High strength seamless with excellent low temperature toughness with O less than 0.0010%
Steel pipe .
【０００７】[0007]
【作用】以下に、本発明の鋼管を構成する鋼に含まれる
各成分の作用効果とそれらの含有量の限定理由について
述べる。The function and effect of each component contained in the steel constituting the steel pipe of the present invention and the reason for limiting the content will be described below.
【０００８】Ｃは鋼の強度を高める元素である。しか
し、その含有量が0.15％未満では強度の確保が不十分で
あり、一方、0.40％を超えると強度が高くなり過ぎ、靱
性に乏しくなるので、その含有量は0.15〜0.40％とす
る。C is an element that increases the strength of steel. However, if the content is less than 0.15%, the strength is insufficiently secured, while if it exceeds 0.40%, the strength becomes too high and the toughness is poor, so the content is made 0.15 to 0.40%.
【０００９】Siは製鋼時の脱酸剤として必要不可欠の元
素であるが、0.05％未満では十分な効果が得られず、一
方、含有量が0.50％を超えると効果が飽和するので、そ
の含有量は0.05〜0.50％とする。[0009] Si is an indispensable element as a deoxidizing agent in steel making, but if it is less than 0.05%, a sufficient effect cannot be obtained. On the other hand, if the content exceeds 0.50%, the effect is saturated. The amount is 0.05-0.50%.
【００１０】Mnは鋼の強度を高める元素であるが、含有
量が0.50％未満では機械構造用部材としての強度が不十
分であり、一方、2.00％を超えると強度が高くなり過ぎ
て靱性に悪影響を及ぼす。従って、Mn含有量は0.50〜2.
00％とする。[0010] Mn is an element that increases the strength of steel. If the content is less than 0.50%, the strength as a member for machine structure is insufficient, while if it exceeds 2.00%, the strength becomes too high and the toughness is reduced. Adversely affect. Therefore, the Mn content is 0.50-2.
00%
【００１１】CrおよびMoは鋼の強度を向上させる元素で
あるが、それらの含有量がCrについては1.00％以下、Mo
については0.10％未満ではその効果は小さい。一方、Cr
の含有量が3.00％、Moの含有量が1.00％を超えると効果
は飽和する。従って、Cr含有量は1.00％を超えて3.00％
まで、Mo含有量は0.10〜1.00％とする。[0011] Cr and Mo are elements that improve the strength of steel, but their content is 1.00% or less for Cr,
If the content is less than 0.10%, the effect is small. On the other hand, Cr
The effect is saturated when the content of Al exceeds 3.00% and the content of Mo exceeds 1.00%. Therefore, the Cr content exceeds 1.00% and 3.00%
Up to the Mo content is 0.10 to 1.00%.
【００１２】Alは鋼の脱酸に用いる元素であるが、含有
量が 0.005％未満では脱酸効果は少なく、一方、 0.050
％を超えると効果が飽和する。Al is an element used for deoxidizing steel. When the content is less than 0.005%, the deoxidizing effect is small.
If it exceeds%, the effect is saturated.
【００１３】Nbは微量含有させることにより鋼の強度と
低温靱性を向上させる効果を有するが、含有量が 0.005
％未満ではその効果は十分ではなく、一方、 0.050％を
超えると効果が飽和するので、その含有量は 0.005〜0.
050 ％とする。[0013] Nb is contained in a small amount to improve the strength of steel.
It has the effect of improving low temperature toughness, but its content is 0.005
%, The effect is not sufficient, while if it exceeds 0.050%, the effect saturates, so its content is 0.005-0.
050%
【００１４】前記の発明の鋼は、上記の成分以外、残
部がFeと不可避の不純物からなるものである。不純物と
しては、Ｐ、Ｓ、およびＯ (酸素) の上限を抑えること
が重要である。In the steel of the above invention, the balance is Fe and inevitable impurities in addition to the above components. It is important to suppress the upper limits of P, S, and O 2 (oxygen) as impurities.
【００１５】Ｐ、Ｓは鋼の熱間での加工性を向上させる
ために極力低減させることが望ましく、その上限を、Ｐ
については 0.025％、Ｓについては 0.015％とする。P and S are preferably reduced as much as possible in order to improve the hot workability of steel.
0.025% for S and 0.015% for S.
【００１６】Ｏは 150〜180kgf/mm2級の高強度材の靱性
に大きく影響する。特に低温靱性を向上させるために
は、その含有量を極力低減させる必要があり、−40〜−
60℃程度の寒冷地での使用を考慮して、その上限を0.00
10％未満とする。O greatly affects the toughness of high-strength materials of 150 to 180 kgf / mm 2 class. In particular, in order to improve the low temperature toughness, it is necessary to reduce the content as much as possible.
Considering use in cold regions around 60 ° C, its upper limit is 0.00
It is less than 10%.
【００１７】の発明の鋼は、の発明の鋼に、さらに
強度向上において同じ作用効果を持つNi、Ｖ、Tiおよび
Ｂのうち１種以上をそれぞれ所定量加えた鋼である。[0017] The steel of the invention of the invention is
It is a steel to which one or more kinds of Ni, V, Ti and B, which have the same action and effect in improving strength , are added in predetermined amounts.
【００１８】Niは鋼の強度を向上させる元素であるとと
もに低温靱性改善の効果もある。その含有量が1.00未満
ではその効果は小さく、一方、3.00％を超えると効果は
飽和する。従って、Niの含有量は1.00〜3.00％とする。[0018] Ni and is an element that improves the strength of the steel and
It also has the effect of improving low-temperature toughness. If the content is less than 1.00, the effect is small, while if it exceeds 3.00%, the effect is saturated. Therefore, the content of Ni is set to 1.00 to 3.00%.
【００１９】Ｖは鋼の強度を向上させる元素であるが、
0.02％未満ではその効果は小さく、一方、0.10％を超え
ると靱性が劣化するので、その含有量は0.02〜0.10％と
する。V is an element that improves the strength of steel,
If it is less than 0.02%, the effect is small. On the other hand, if it exceeds 0.10%, the toughness deteriorates, so the content is made 0.02 to 0.10%.
【００２０】TiはNbと同様に微量含有させることにより
鋼の強度を上昇させる効果を有する。しかし、その含有
量が 0.010％未満では効果は小さく、一方、 0.100％を
超えると低温靱性が劣化するので、その含有量は 0.010
〜0.100 ％とする。Ti, like Nb, has the effect of increasing the strength of steel by containing a trace amount. However, if the content is less than 0.010%, the effect is small. On the other hand, if it exceeds 0.100%, the low-temperature toughness deteriorates.
~ 0.100%
【００２１】ＢはNbおよびTiと同様に微量含有させるこ
とにより鋼の強度を上昇させる効果を有する。しかし、
含有量が0.0010％未満ではその効果は小さく、一方、0.
0050％を超えると効果は飽和する。従って、Ｂ含有量は
0.0010〜0.0050％とする。B, like Nb and Ti, has the effect of increasing the strength of the steel by containing a trace amount. But,
When the content is less than 0.0010%, the effect is small, while on the other hand, the content is less than 0.1%.
If it exceeds 0050%, the effect is saturated. Therefore, the B content is
0.0010 to 0.0050%.
【００２２】本発明の鋼管を製造するには、上記の各成
分を含有する鋼をマンドレルミル法など通常の方法によ
り熱間圧延を行い、継目無鋼管とすればよい。この熱間
圧延のままの継目無鋼管は、引張強さが約 150〜180kgf
/mm2であり、−40〜−60℃程度の使用条件でも十分な靱
性を有している。しかも、熱処理を行う必要がないの
で、従来の熱処理鋼管に比べて製造コストが安い。In order to manufacture the steel pipe of the present invention, the steel containing each of the above components may be hot-rolled by an ordinary method such as a mandrel mill method to obtain a seamless steel pipe. This hot-rolled seamless steel pipe has a tensile strength of approximately 150 to 180 kgf.
/ mm 2 and has sufficient toughness even under operating conditions of about -40 to -60 ° C. Moreover, since it is not necessary to perform heat treatment, the manufacturing cost is lower than that of the conventional heat treated steel pipe.
【００２３】[0023]
【実施例】表１に示す化学組成を有する鋼を、通常のマ
ンネスマン・マンドレルミルを用いて熱間圧延により継
目無鋼管（いずれも外径31.8mm、肉厚 2.6mm）に圧延
し、これらの鋼管について、引張試験（降伏点および引
張強さを測定）と低温落重試験を行った。EXAMPLE A steel having the chemical composition shown in Table 1 was rolled into a seamless steel pipe (both an outer diameter of 31.8 mm and a wall thickness of 2.6 mm) by hot rolling using a normal Mannesmann mandrel mill. The steel pipe was subjected to a tensile test (measurement of yield point and tensile strength) and a low temperature drop weight test.
【００２４】低温落重試験では、前記の継目無鋼管から
切り出した長さ１ｍの鋼管を試験片とし、これらの試験
片を所定の温度に冷却し、支点間距離を 800mmとして水
平に支持し、その中央部に高さ３ｍから重さ 150kgの重
錘を落下させることにより瞬間的 (衝撃的) に応力を負
荷して試験片が脆性的な折損を生ずるか否かを判定し
た。In the low-temperature drop test, a 1-m-long steel pipe cut out of the seamless steel pipe was used as a test piece, and the test piece was cooled to a predetermined temperature, and supported horizontally with a distance between supports of 800 mm. By dropping a weight having a weight of 150 kg from a height of 3 m at the center thereof, an instantaneous (shock) stress was applied to determine whether or not the test piece caused brittle breakage.
【００２５】試験結果を表２に示す。低温落重試験の欄
において、○印は鋼管試験片が延性変形したことを、×
印は鋼管試験片が脆性的に折損したことをあらわす。Table 2 shows the test results. In the column of low temperature drop weight test, ○ indicates that the steel pipe test piece was ductile deformed, ×
The mark indicates that the steel pipe test piece was brittlely broken.
【００２６】表２の結果から、本発明の鋼管は、150kgf
/mm2を超える高強度を有するとともに、−100 ℃でも脆
性的な折損を生ずることなく、優れた低温靱性を示すこ
とがわかる。From the results shown in Table 2, the steel pipe of the present invention is 150 kgf.
It can be seen that the material has high strength exceeding / mm 2 and exhibits excellent low-temperature toughness without brittle breakage even at -100 ° C.
【００２７】[0027]
【表１】 [Table 1]
【００２８】[0028]
【表２】 [Table 2]
【００２９】[0029]
【発明の効果】本発明の継目無鋼管は、高い強度と優れ
た低温靱性を有し、特に、近年開発が進みつつある自動
車の鋼管製ドア補強材として、あるいはその他の機械構
造用部材として好適な鋼管であり、寒冷地での使用にも
対応できるものである。The seamless steel pipe of the present invention has high strength and excellent low-temperature toughness, and is particularly suitable as a door reinforcing material for steel pipes of automobiles, which is being developed in recent years, or as a member for other mechanical structures. Steel pipe, suitable for use in cold regions
It can be dealt with .
Claims (2)
〜0.50％、Mn：0.50〜2.00％、Cr：1.00％を超えて3.00
％まで、Mo：0.10〜1.00％、Al： 0.005〜0.050 ％、N
b： 0.005〜0.050 ％を含有し、残部がFeおよび不可避
不純物からなり、不純物中のＰが 0.025％以下、Ｓが
0.015％以下、Ｏが0.0010％未満である低温靱性に優れ
た高強度継目無鋼管。1. By weight%, C: 0.15 to 0.40%, Si: 0.05
~ 0.50%, Mn: 0.50 ~ 2.00%, Cr: 1.00% over 3.00
% , Mo: 0.10 to 1.00%, Al: 0.005 to 0.050%, N
b: 0.005 to 0.050%, with the balance being Fe and unavoidable impurities, P in the impurities is 0.025% or less, and S is
Excellent low temperature toughness with 0.015% or less and O less than 0.0010%
High strength seamless steel pipe .
量％で、Ni：1.00〜3.00％、Ｖ：0.02〜0.10％、Ti：
0.010〜0.100 ％およびＢ：0.0010〜0.0050％のうちの
１種以上を含有し、残部がFeおよび不可避不純物からな
り、不純物中のＰが0.025 ％以下、Ｓが 0.015％以下、
Ｏが0.0010％未満である低温靱性に優れた高強度継目無
鋼管。2. In addition to the components described in claim 1, Ni: 1.00 to 3.00%, V: 0.02 to 0.10%, Ti:
0.010 to 0.100% and B: one or more of 0.0010 to 0.0050%, the balance being Fe and unavoidable impurities, P in the impurities is 0.025% or less, S is 0.015% or less,
High strength seamless with excellent low temperature toughness with O less than 0.0010%
Steel pipe .
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP3102371A JP2669178B2 (en) | 1991-05-08 | 1991-05-08 | High toughness and high strength seamless steel pipe |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP3102371A JP2669178B2 (en) | 1991-05-08 | 1991-05-08 | High toughness and high strength seamless steel pipe |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPH0551699A JPH0551699A (en) | 1993-03-02 |
| JP2669178B2 true JP2669178B2 (en) | 1997-10-27 |
Family
ID=14325605
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP3102371A Expired - Lifetime JP2669178B2 (en) | 1991-05-08 | 1991-05-08 | High toughness and high strength seamless steel pipe |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JP2669178B2 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN103556069A (en) * | 2013-11-04 | 2014-02-05 | 洛阳双瑞特种装备有限公司 | Large-diameter seamless steel tube for high-pressure gas cylinders and manufacturing method thereof |
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|---|---|---|---|---|
| JP4475440B1 (en) * | 2008-11-26 | 2010-06-09 | 住友金属工業株式会社 | Seamless steel pipe and manufacturing method thereof |
| CN113846262B (en) * | 2020-06-28 | 2022-12-16 | 宝山钢铁股份有限公司 | Seamless steel tube for automobile integral hollow transmission half shaft and manufacturing method thereof |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5949285B2 (en) * | 1977-08-26 | 1984-12-01 | 川崎製鉄株式会社 | Manufacturing method of steel plate with excellent unstable fracture properties |
| JPS6037680B2 (en) * | 1977-09-08 | 1985-08-27 | 日本電気株式会社 | Encoder/decoder for narrowband color television signals |
| JPS5940203B2 (en) * | 1978-08-18 | 1984-09-28 | 新日本製鐵株式会社 | Manufacturing method of high cleanliness, low yield ratio hot rolled steel sheet |
| JPS5871354A (en) * | 1981-10-20 | 1983-04-28 | Daido Steel Co Ltd | Unnormalized structural steel and its manufacture |
| JPH02101145A (en) * | 1988-10-06 | 1990-04-12 | Sumitomo Metal Ind Ltd | Water-cooled-type tmcp steel stock and large-diameter steel pipe and welding executing method therefor |
-
1991
- 1991-05-08 JP JP3102371A patent/JP2669178B2/en not_active Expired - Lifetime
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103556069A (en) * | 2013-11-04 | 2014-02-05 | 洛阳双瑞特种装备有限公司 | Large-diameter seamless steel tube for high-pressure gas cylinders and manufacturing method thereof |
| CN103556069B (en) * | 2013-11-04 | 2016-02-17 | 洛阳双瑞特种装备有限公司 | A kind of gas cylinder seamless large-diameter pipes and manufacture method thereof |
Also Published As
| Publication number | Publication date |
|---|---|
| JPH0551699A (en) | 1993-03-02 |
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