JP2009242858A - High-strength steel pipe and manufacturing method therefor - Google Patents
High-strength steel pipe and manufacturing method therefor Download PDFInfo
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Abstract
Description
本発明は、衝撃曲げ靭性に優れる高強度鋼管およびその製造方法に関する。 The present invention relates to a high-strength steel pipe excellent in impact bending toughness and a method for producing the same.
自動車、自転車等の構造部材に使用する鋼管には高強度と高疲労特性が要求される。最近では構造部材に対する軽量化の要求が高くなり、構造用鋼管には薄肉化に伴う更なる高強度化が求められる状況にある。ただし、高強度化に必要なコスト上昇をできるだけ抑えることが強く望まれるところである。 Steel pipes used for structural members such as automobiles and bicycles are required to have high strength and high fatigue characteristics. Recently, there is an increasing demand for weight reduction of structural members, and structural steel pipes are required to have further increased strength accompanying thinning. However, it is strongly desired to suppress as much as possible the cost increase necessary for increasing the strength.
鋼材の強化手法としては、固溶強化、変態組織強化、加工強化が挙げられる。このうち固溶強化および変態組織強化の場合には、それらの機構を効果的に生じさせるために特殊な合金成分を含有する鋼を採用することが必要となり、必然的に素材コストの上昇を伴う。この点、加工強化によれば、特殊な元素を添加することなく高強度化が可能であり、コスト上昇を抑える上で有利となる。特許文献１、２には、加工強化された素材鋼板を使用することによって電縫鋼管の高強度化を図る手法が開示されている。 Examples of methods for strengthening steel materials include solid solution strengthening, transformation structure strengthening, and process strengthening. Among these, in the case of solid solution strengthening and transformation structure strengthening, it is necessary to adopt steel containing special alloy components in order to effectively generate these mechanisms, which inevitably increases the material cost. . In this respect, according to the processing strengthening, it is possible to increase the strength without adding a special element, which is advantageous in suppressing an increase in cost. Patent Documents 1 and 2 disclose a technique for increasing the strength of an electric resistance welded steel pipe by using a work-strengthened material steel sheet.
自動車等の乗り物に使用する構造用鋼管では、高強度化が重要であるとともに、衝突時の衝撃を受けた際に、湾曲部に裂け疵が発生しにくい性質、すなわち「衝撃曲げ靭性」に優れることが望まれる。しかし従来の高強度鋼管においては、衝撃曲げ靭性を向上させることまでは十分考慮されておらず、例えば特許文献１、２には高強度を付与しながら衝撃曲げ靭性に優れたものを得る手法は開示されていない。 For structural steel pipes used in vehicles such as automobiles, it is important to increase the strength, and when subjected to impact during collision, the curved portion is less prone to tearing, that is, excellent in impact bending toughness. It is desirable. However, in conventional high-strength steel pipes, sufficient consideration has not been given to improving impact bending toughness. For example, Patent Documents 1 and 2 provide a method of obtaining high impact bending toughness while imparting high strength. Not disclosed.
本発明は、強度レベルが高く、かつ衝撃曲げ靭性に優れた溶接鋼管を、安価な手法により提供しようというものである。 The present invention is to provide a welded steel pipe having a high strength level and excellent impact bending toughness by an inexpensive method.
発明者らは詳細な検討の結果、フェライト結晶粒を十分に展伸させ繊維状の組織状態とした加工硬化鋼板を溶接造管して得られる高強度鋼管において、衝撃曲げ靭性が顕著に改善されることを見出した。 As a result of detailed investigations, the impact bending toughness is significantly improved in a high-strength steel pipe obtained by welding and pipe-forming a work-hardened steel sheet that has been sufficiently stretched with ferrite crystal grains to have a fibrous structure. I found out.
すなわち本発明では、質量％で、Ｃ：０.０１〜０.２％、Ｓｉ：１.５％以下、Ｍｎ：２％以下、Ｐ：０.０５％以下、Ｓ：０.０２％以下、酸可溶Ａｌ：０.００５〜０.１％であり、必要に応じてさらにＴｉ：０.１５％以下、Ｎｂ：０.１５％以下、Ｎｉ：１％以下、Ｃｒ：１％以下、Ｍｏ：０.３％以下、Ｖ：０.３％以下、Ｚｒ：０.３％以下の１種以上を含有し、残部Ｆｅおよび不可避的不純物からなる組成を有し、下記（１）式により定まるフェライト結晶粒展伸度ＥFが５.０以上の未焼鈍冷延鋼板を圧延方向が長手方向となるように溶接造管してなる引張強さ９８０Ｎ／ｍｍ2以上の高強度鋼管が提供される。
ＥF＝Ｎ1／Ｎ2 ……（１）
ここで、
ＥF：圧延板のフェライト結晶粒展伸度、
Ｎ1：圧延方向および板厚方向に平行な断面（Ｌ断面）の顕微鏡視野において板厚方向の一定長さＸの線分によって切断される結晶粒の数、ただし顕微鏡視野および線分の長さＸはＮ1が１０以上となるように設定する、
Ｎ2：前記視野において圧延方向の前記長さＸの線分によって切断される結晶粒の数
That is, in the present invention, in mass%, C: 0.01 to 0.2%, Si: 1.5% or less, Mn: 2% or less, P: 0.05% or less, S: 0.02% or less, Acid-soluble Al: 0.005 to 0.1%, Ti: 0.15% or less, Nb: 0.15% or less, Ni: 1% or less, Cr: 1% or less, Mo if necessary : Contains at least one of 0.3% or less, V: 0.3% or less, Zr: 0.3% or less, and has a composition comprising the balance Fe and inevitable impurities, and is determined by the following formula (1) A high-strength steel pipe having a tensile strength of 980 N / mm 2 or more formed by welding an unannealed cold-rolled steel sheet having a ferrite grain extension E F of 5.0 or more so that the rolling direction is the longitudinal direction is provided. The
E F = N 1 / N 2 (1)
here,
E F : Ferrite grain extension elongation of rolled sheet,
N 1 : Number of crystal grains cut by a line segment of a certain length X in the plate thickness direction in a microscope view of a cross section (L cross section) parallel to the rolling direction and the plate thickness direction, but the length of the microscope view and the line segment X is set so that N 1 is 10 or more.
N 2 : Number of crystal grains cut by the line segment of the length X in the rolling direction in the visual field
上記のＮ1およびＮ2は、ＪＩＳ Ｇ０５５１：２００５の附属書２（旧ＪＩＳ Ｇ０５５２）の切断法に規定される測定方法に準拠して求めることができる。ただし、１本の線分で切断されるフェライト結晶粒の数を１０個以上とする点については、Ｎ1の測定にのみ適用する。「未焼鈍冷延鋼板」は、冷間圧延後に焼鈍を受けていない状態の鋼板である。 The above N 1 and N 2 can be determined in accordance with the measurement method defined in the cutting method of Annex 2 (former JIS G 0552) of JIS G 0551: 2005. However, the point that the number of ferrite crystal grains cut by one line segment is 10 or more applies only to the measurement of N 1 . An “unannealed cold rolled steel sheet” is a steel sheet that has not been annealed after cold rolling.
この高強度鋼管は、例えば管の肉厚が０.６〜２.４ｍｍ、外径が２２〜７０ｍｍである。 This high-strength steel pipe has, for example, a pipe thickness of 0.6 to 2.4 mm and an outer diameter of 22 to 70 mm.
また、本発明では、上記の高強度鋼管の製造方法として、熱間圧延、スケール除去処理（例えば酸洗）、冷間圧延、溶接造管の工程により鋼管を製造するに際し、熱間圧延において熱延仕上げ温度を（Ａｒ3点−２０℃）〜（Ａｒ3点＋５０℃）、仕上げ圧延最終パス終了後、巻取までの平均冷却速度を２０〜１００℃／秒、巻取温度を４５０〜６００℃として前記（１）式により定まるフェライト結晶粒展伸度ＥFが２.０以上の熱延鋼板を作り、冷間圧延において圧延率を４０〜８０％として同フェライト結晶粒展伸度ＥFが５.０以上の冷延鋼板を作り、溶接造管において未焼鈍のままの冷延鋼板をその圧延方向が管の長手方向となるように造管する手法が提供される。 In the present invention, as a method for producing the above-described high-strength steel pipe, when producing a steel pipe by the steps of hot rolling, descaling (for example, pickling), cold rolling, and welded pipe, The final finishing temperature is (Ar 3 point−20 ° C.) to (Ar 3 point + 50 ° C.), after the final pass of the finish rolling, the average cooling rate until winding is 20 to 100 ° C./second, and the winding temperature is 450 to 600. ferrite grain elongation rate E F determined by the equation (1) as ℃ is made 2.0 or more hot-rolled steel sheet, the ferrite grain elongation rate E F a reduction ratio as 40% to 80% in cold rolling A method is provided in which a cold-rolled steel sheet having a thickness of 5.0 or more is made, and the cold-rolled steel sheet that has not been annealed in the welded pipe making is piped so that the rolling direction is the longitudinal direction of the pipe.
本発明によれば、引張強さ９８０Ｎ／ｍｍ2以上の高強度を有しながら、衝撃曲げ靭性を顕著に向上させた溶接鋼管を、安価な手法で安定して提供することが可能になった。この鋼管は、変態組織強化のＤＰ鋼やＴＲＩＰ鋼を素材とした電縫鋼管と比較して衝撃曲げ靭性に優れ、衝突時の衝撃を受けた際に湾曲部の側面に裂け疵が極めて発生しにくい。したがって、特に自動車や自転車などの乗り物に使用する構造部材に適している。 According to the present invention, it has become possible to stably provide a welded steel pipe having a high tensile strength of 980 N / mm 2 or more and a significantly improved impact bending toughness by an inexpensive method. . This steel pipe is superior in impact bending toughness compared to ERW steel pipe made of transformation-strengthened DP steel or TRIP steel. Hateful. Therefore, it is particularly suitable for structural members used for vehicles such as automobiles and bicycles.
〔成分組成〕
本発明の高強度鋼管は加工強化の手法を採用するものであるから、特殊な元素を添加しないシンプルな成分組成とすることができる。ただし、必要に応じて種々の合金元素を選択的に含有させても構わない。以下、成分元素について説明する。成分組成における「％」は「質量％」を意味する。
(Component composition)
Since the high-strength steel pipe of the present invention employs a process strengthening method, it can have a simple component composition without adding a special element. However, various alloy elements may be selectively contained as necessary. Hereinafter, the component elements will be described. “%” In the component composition means “mass%”.
Ｃは、鋼の高強度化に有効な元素であり、冷間圧延率を４０％としたときに最終的な鋼管において引張強さ９８０Ｎ／ｍｍ2以上の強度レベルを得るためには、０.０１％以上のＣ含有量を確保することが望ましい。しかし、多量のＣ含有は延性および溶接部の靭性を低下させる。種々検討の結果、Ｃ含有量は０.２％以下とする必要がある。 C is an element effective for increasing the strength of steel, and in order to obtain a strength level of 980 N / mm 2 or more in the final steel pipe when the cold rolling rate is set to 40%, it is set to 0. It is desirable to ensure a C content of 01% or more. However, a large amount of C content decreases ductility and weld toughness. As a result of various studies, the C content needs to be 0.2% or less.
Ｓｉは、鋼の強度上昇に有効な元素であり、０.０５％以上の含有量を確保することがより効果的である。ただし、１.５％を超えて多量にＳｉを含有させると、強度は上昇するが、冷間加工性および表面性状が劣化する。したがってＳｉ含有量は１.５％以下とする。 Si is an effective element for increasing the strength of steel, and it is more effective to secure a content of 0.05% or more. However, if Si is contained in a large amount exceeding 1.5%, the strength increases, but the cold workability and the surface properties deteriorate. Accordingly, the Si content is 1.5% or less.
Ｍｎは、鋼の強度上昇に有効な元素である。しかし、Ｍｎ含有量が２％を超えると、添加量に伴い強度は上昇するものの、溶接性が著しく劣化する。また、Ｍｎが多量に含まれると溶接部で焼入れ硬化が生じやすくなり、溶接部の加工性を劣化させ割れの原因にもなる。したがってＭｎ含有量は２％以下とする。 Mn is an element effective for increasing the strength of steel. However, if the Mn content exceeds 2%, the weldability is remarkably deteriorated although the strength increases with the addition amount. In addition, if Mn is contained in a large amount, quench hardening is likely to occur in the welded portion, which deteriorates the workability of the welded portion and causes cracking. Therefore, the Mn content is 2% or less.
Ｐは、高強度化に有効な合金元素であるが、０.０５％を超えて含有させると低温靭性が劣化する。
Ｓは、多量に含有すると熱間加工性や冷間加工性を劣化させるが、０.０２％以下の含有量であれば特に問題はない。
P is an alloy element effective for increasing the strength, but if it exceeds 0.05%, the low temperature toughness deteriorates.
When S is contained in a large amount, hot workability and cold workability are deteriorated, but there is no particular problem as long as the content is 0.02% or less.
Ａｌは、脱酸剤として添加される元素であり、十分な脱酸効果を得るためには酸可溶Ａｌとして０.００５％以上の添加が必要である。Ａｌ脱酸の効果は０.１％程度で飽和しそれ以上の添加は鋼材のコストの上昇を招く。したがって本発明では酸可溶Ａｌの含有量を０.００５〜０.１％の範囲とする。 Al is an element added as a deoxidizer, and 0.005% or more of acid-soluble Al is required to obtain a sufficient deoxidation effect. The effect of Al deoxidation is saturated at about 0.1%, and addition beyond that causes an increase in the cost of the steel material. Therefore, in the present invention, the content of acid-soluble Al is set in the range of 0.005 to 0.1%.
Ｔｉは、Ｃ、Ｓ、Ｎと析出物を形成し、析出強化により鋼材の高強度化に寄与する元素である。また、これらの析出物により溶接熱影響部の加工歪の回復が抑制されるとともに溶接加熱時の固溶、再析出により熱影響部の軟化が防止できる。このため必要に応じてＴｉを添加することができる。Ｔｉ添加量は０.０１％以上とすることがより効果的である。ただし０.１５％を超えて添加しても上記効果が飽和するとともに製造コストの上昇を招く。したがってＴｉを添加する場合は０.１５％以下の範囲で行う。 Ti is an element that forms precipitates with C, S, and N and contributes to increasing the strength of steel by precipitation strengthening. In addition, recovery of processing strain in the weld heat affected zone is suppressed by these precipitates, and softening of the heat affected zone can be prevented by solid solution and reprecipitation during welding heating. For this reason, Ti can be added as needed. It is more effective to make the Ti addition amount 0.01% or more. However, even if added over 0.15%, the above effect is saturated and the manufacturing cost is increased. Therefore, when adding Ti, it is performed within a range of 0.15% or less.
Ｎｂは、Ｔｉと同様に炭化物を形成し、析出強化により鋼材の高強度化に寄与する。また、鋼板の金属組織を微細化して強度を向上させる。さらに溶接部においてはＴｉの効果と同様に析出物により溶接熱影響部の加工歪の回復が抑制されるとともに固溶、再析出により熱影響部の軟化が防止できる。このため必要に応じてＮｂを添加することができる。Ｎｂ添加量は０.０１％以上とすることがより効果的である。ただし０.１５％を超えて添加しても上記効果が飽和するとともに製造コストの上昇を招く。したがってＮｂを添加する場合は０.１５％以下の範囲で行う。 Nb forms carbides like Ti, and contributes to increasing the strength of steel by precipitation strengthening. Moreover, the metal structure of the steel sheet is refined to improve the strength. Further, in the welded portion, similarly to the effect of Ti, recovery of work strain of the weld heat affected zone is suppressed by the precipitate, and softening of the heat affected zone can be prevented by solid solution and reprecipitation. For this reason, Nb can be added as needed. It is more effective to add Nb to 0.01% or more. However, even if added over 0.15%, the above effect is saturated and the manufacturing cost is increased. Therefore, when adding Nb, it is performed within a range of 0.15% or less.
Ｎｉは、高強度化と溶接部の靭性向上に有効な元素であり、必要に応じて添加することができる。その効果を十分に得るためには０.０５％以上のＮｉ含有量を確保することがより好ましい。ただし、ただし１％を超えて添加しても上記効果が飽和するとともに製造コストの上昇を招く。したがってＮｉを添加する場合は１％以下の範囲で行う。 Ni is an element effective for increasing the strength and improving the toughness of the welded portion, and can be added as necessary. In order to sufficiently obtain the effect, it is more preferable to secure a Ni content of 0.05% or more. However, even if added in excess of 1%, the above effect is saturated and the manufacturing cost is increased. Therefore, when adding Ni, it is performed within a range of 1% or less.
Ｍｏ、Ｖは、ＴｉやＮｂと同様に炭化物を形成し、析出強化により鋼材の高強度化に有効な元素である。さらに溶接部においてはＴｉ、Ｎｂの効果と同様に熱影響部の軟化防止に有効である。このためＭｏ、Ｖの一方または双方を必要に応じて添加することができる。Ｍｏ、Ｖとも、それぞれ０.０１％以上の含有量を確保することがより効果的である。ただし、過剰に添加しても上記効果が飽和するとともに製造コストの上昇を招く。またＣｒの多量添加は焼入れ性を高めて溶接部の加工性を劣化させる要因となる。種々検討の結果、Ｍｏ、Ｖを添加する場合はそれぞれ０.３％以下の範囲で行う。 Mo and V are elements that form carbides similarly to Ti and Nb and are effective in increasing the strength of steel by precipitation strengthening. Further, in the welded portion, it is effective for preventing the heat affected zone from being softened in the same manner as Ti and Nb. For this reason, one or both of Mo and V can be added as needed. It is more effective to secure a content of 0.01% or more for both Mo and V. However, even if it adds excessively, the said effect will be saturated and the raise of manufacturing cost will be caused. Addition of a large amount of Cr increases the hardenability and deteriorates the workability of the weld. As a result of various investigations, when adding Mo and V, each is performed within a range of 0.3% or less.
Ｃｒ、Ｚｒは、高強度化と溶接部の靭性向上に有効な元素である。このためＣｒ、Ｚｒの一方または双方を必要に応じて添加することができる。Ｃｒの場合は０.０５％以上、Ｚｒの場合は０.０１％以上の含有量を確保することがより効果的である。ただし、過剰に添加しても上記効果が飽和するとともに製造コストの上昇を招く。またＣｒの多量添加は焼入れ性を高めて溶接部の加工性を劣化させる要因となる。種々検討の結果、Ｃｒを添加する場合は１％以下の範囲で行い、Ｚｒを添加する場合は０.３％以下の範囲で行う。 Cr and Zr are effective elements for increasing the strength and improving the toughness of the welded portion. Therefore, one or both of Cr and Zr can be added as necessary. It is more effective to secure a content of 0.05% or more in the case of Cr and 0.01% or more in the case of Zr. However, even if it adds excessively, the said effect will be saturated and the raise of manufacturing cost will be caused. Addition of a large amount of Cr increases the hardenability and deteriorates the workability of the weld. As a result of various studies, when Cr is added, it is performed within a range of 1% or less, and when Zr is added, it is performed within a range of 0.3% or less.
〔繊維状組織〕
本発明の鋼管は、加工硬化した組織を有し、かつフェライト結晶粒が長手方向に伸びた繊維状組織を呈するものである。具体的には、前記（１）式により定まるフェライト結晶粒展伸度ＥFが５.０以上の未焼鈍冷延鋼板を造管用の素材として用いたものである。発明者らの検討によれば、前述の成分組成を有するとともに、このような繊維状組織を長手方向に持つ鋼管において、衝撃曲げ靭性が顕著に改善されることがわかった。なお、造管に供する未焼鈍冷延鋼板は、マトリクスがフェライト単相であるか、またはフェライト相＋５０体積％以下の第２相からなるものである。第２相の量が多くなりすぎるとフェライト結晶粒展伸度を５.０以上とすることによる衝撃曲げ靱性の改善効果が十分に発揮できない場合がある。第２相は、マルテンサイト、ベイナイト、パーライト等であるが、パーライトが多量に生成すると強度および衝撃曲げ靭性の低下を招くので、マトリクス中のパーライトの存在量は１０体積％以下に抑えられていることが望ましい。
[Fibrous structure]
The steel pipe of the present invention has a work-hardened structure and exhibits a fibrous structure in which ferrite crystal grains extend in the longitudinal direction. Specifically, one in which the (1) ferrite grain elongation rate E F determined by the equation with 5.0 or more unannealed cold-rolled steel sheet as a material for pipe production. According to the study by the inventors, it has been found that impact bending toughness is remarkably improved in a steel pipe having the above-described component composition and having such a fibrous structure in the longitudinal direction. In addition, the unannealed cold-rolled steel sheet used for pipe making has a matrix of a ferrite single phase or a second phase of ferrite phase + 50% by volume or less. If the amount of the second phase is too large, the effect of improving the impact bending toughness by setting the ferrite crystal grain extension to 5.0 or more may not be sufficiently exhibited. The second phase is martensite, bainite, pearlite, etc., but if a large amount of pearlite is generated, the strength and impact bending toughness are reduced, so the amount of pearlite in the matrix is suppressed to 10% by volume or less. It is desirable.
〔強度レベル〕
本発明の鋼管は、鋼管としての長手方向の引張強さが９８０Ｎ／ｍｍ2以上の高強度を有するものである。これより強度レベルが低いと自動車を初めとする各種構用鋼管として使用されている従来の鋼管の代替として使用することが難しい場合がある。この強度レベルは、熱延巻取温度、および冷間圧延率を調整することによって実現できる。
[Strength level]
The steel pipe of the present invention has a high strength with a tensile strength in the longitudinal direction of the steel pipe of 980 N / mm 2 or more. If the strength level is lower than this, it may be difficult to use as an alternative to conventional steel pipes used as various structural steel pipes including automobiles. This strength level can be achieved by adjusting the hot rolling coiling temperature and the cold rolling rate.
〔製造工程〕
本発明の高強度鋼管は、上記のように成分組成が調整された鋼を溶製し、熱間圧延、スケール除去処理（例えば酸洗）、冷間圧延、溶接造管の工程により製造することができる。
〔Manufacturing process〕
The high-strength steel pipe of the present invention is manufactured by melting the steel whose component composition has been adjusted as described above, and performing the steps of hot rolling, descaling (for example, pickling), cold rolling, and welded pipe making. Can do.
〔熱間圧延〕
熱間圧延工程では、熱延仕上げ温度を（Ａｒ3点−２０℃）〜（Ａｒ3点＋５０℃）とする。（Ａｒ3点−２０℃）よりも低温になると熱間変形抵抗の変動が大きくなり、熱延鋼帯の板厚精度が低下しやすい。この場合、後工程での冷延鋼帯の板厚精度が低下する要因ともなる。一方、（Ａｒ3点＋５０℃）より高温で熱延最終パスを通すと、動的再結晶が生じやすく、熱延板でのフェライト結晶粒の展伸度を安定して２.０以上とすることが難しくなる。
(Hot rolling)
In the hot rolling step, the hot rolling finishing temperature is set to (Ar 3 point−20 ° C.) to (Ar 3 point + 50 ° C.). When the temperature is lower than (Ar 3 point−20 ° C.), the variation of hot deformation resistance becomes large, and the thickness accuracy of the hot-rolled steel strip tends to decrease. In this case, the sheet thickness accuracy of the cold-rolled steel strip in the subsequent process is also a factor. On the other hand, when the hot rolling final pass is passed at a temperature higher than (Ar 3 point + 50 ° C.), dynamic recrystallization is likely to occur, and the elongation of the ferrite crystal grains on the hot rolled plate is stably set to 2.0 or more. It becomes difficult.
仕上げ圧延最終パス終了後、巻取までの平均冷却速度を２０〜１００℃／秒とする。平均冷却速度が２０℃／秒未満ではパーライトが生成しやすくなり、目的とする強度および衝撃曲げ靭性を得ることが難しくなる。一方、１００℃／秒を超えるとベイナイトの生成量が増大してフェライト相が少なくなり、フェライト結晶粒の展伸による本発明の効果が享受できないばかりか高強度となり冷延が困難となる。 After the final pass of the final rolling, the average cooling rate until winding is 20 to 100 ° C./second. When the average cooling rate is less than 20 ° C./second, pearlite is easily generated, and it becomes difficult to obtain the intended strength and impact bending toughness. On the other hand, if it exceeds 100 ° C./second, the amount of bainite generated increases and the ferrite phase decreases, and the effect of the present invention due to the expansion of ferrite crystal grains cannot be enjoyed, and the strength becomes high and cold rolling becomes difficult.
巻取温度を４５０〜６００℃とする。巻取温度が高くなるほど鋼材の強度レベルは低下する。特に６００℃を超えるとパーライトの生成が著しくなり、強度が低下するだけでなく所定の展伸度が得られなくなって衝撃曲げ靭性の低下を招く。一方、巻取温度が４５０℃を下回ると変態組織強化により強度は著しく上昇するが、次工程の冷間圧延において冷間圧延率、板厚、強度のバランス調整が難しくなる。 The coiling temperature is set to 450 to 600 ° C. The higher the coiling temperature, the lower the strength level of the steel material. In particular, when the temperature exceeds 600 ° C., pearlite is remarkably generated and not only the strength is lowered, but also a predetermined elongation cannot be obtained, and the impact bending toughness is lowered. On the other hand, when the coiling temperature is lower than 450 ° C., the strength is remarkably increased by strengthening the transformation structure, but it is difficult to adjust the balance of the cold rolling rate, the plate thickness, and the strength in the cold rolling of the next step.
以上の熱延条件の範囲で、前記（１）式により定まるフェライト結晶粒展伸度ＥFが２.０以上の熱延鋼板を作る。この段階で展伸度を２.０以上としておかないと、冷間圧延後に展伸度５.０以上を得るためには高い冷間圧延率を余儀なくされ、所望の製品板厚において展伸度５.０以上に調整することが難しくなりやすい。 Within the range of the above hot rolling conditions, a hot rolled steel sheet having a ferrite grain extension E F determined by the above formula (1) of 2.0 or more is made. If the elongation is not set to 2.0 or more at this stage, a high cold rolling rate is required to obtain a degree of elongation of 5.0 or more after cold rolling, and the extension is achieved at a desired product thickness. It tends to be difficult to adjust to 5.0 or higher.
〔冷間圧延〕
冷間圧延工程では、冷間圧延率を４０〜８０％とする。冷延率４０％未満では鋼管にしたときの引張強さ９８０Ｎ／ｍｍ2以上を安定して得ることが難しい。また、フェライト結晶粒の展伸度を５.０以上とするうえでも不利となる。一方、冷延率が８０％を超えると製造コストの増大を招き、好ましくない。
(Cold rolling)
In the cold rolling step, the cold rolling rate is 40 to 80%. If the cold rolling rate is less than 40%, it is difficult to stably obtain a tensile strength of 980 N / mm 2 or more when formed into a steel pipe. Further, it is disadvantageous when the elongation of ferrite crystal grains is set to 5.0 or more. On the other hand, if the cold rolling rate exceeds 80%, the manufacturing cost increases, which is not preferable.
上記冷間圧延率の範囲で前記（１）式により定まるフェライト結晶粒展伸度ＥFが５.０以上の冷延鋼板を得る。すなわち、予め判っている製品板厚に応じて、冷間圧延率４０〜８０の範囲でフェライト結晶粒展伸度が５.０以上が得られるように、前記熱間圧延条件と冷間圧延率の組み合わせを設定することが重要である。この条件の組み合わせは、成分組成に応じて予め予備実験を行うことなどによって定めることができる。 The range of the cold-rolling reduction (1) ferrite grain elongation rate E F determined by the equation to obtain 5.0 or more cold-rolled steel sheet. That is, the hot rolling conditions and the cold rolling rate so that a ferrite grain extension of 5.0 or more can be obtained in the range of the cold rolling rate of 40 to 80, depending on the product plate thickness that is known in advance. It is important to set a combination. This combination of conditions can be determined by conducting a preliminary experiment in advance according to the component composition.
〔造管〕
フェライト結晶粒展伸度が５.０以上の冷延鋼板をそのまま未焼鈍の状態で溶接造管に供する。その際、圧延方向すなわちフェライト結晶粒が伸びている方向が管の長手方向となるように造管する。造管は、所定幅の鋼帯をロールフォーミングによって円筒状に成形していき、鋼帯の両エッジ部同士を付き合わせて溶接する一般的な溶接造管方法が適用できる。溶接は高周波溶接、プラズマ溶接、ＴＩＧ溶接などが挙げられるが、高周波溶接による電縫鋼管とすることが好ましい。このようにして得られた鋼管は、引張強さ９８０Ｎ／ｍｍ2以上の高強度を有し、かつ衝突曲げ靱性にも優れたものである。
[Pipe making]
A cold-rolled steel sheet having a ferrite grain extension degree of 5.0 or more is used as it is in an unannealed state for welding. At that time, the pipe is formed such that the rolling direction, that is, the direction in which the ferrite crystal grains are extended becomes the longitudinal direction of the pipe. For pipe making, a general welding pipe making method in which a steel strip having a predetermined width is formed into a cylindrical shape by roll forming, and both edge portions of the steel strip are attached to each other and welded can be applied. Examples of the welding include high-frequency welding, plasma welding, TIG welding, and the like, but it is preferable to use an ERW steel pipe by high-frequency welding. The steel pipe thus obtained has a high strength with a tensile strength of 980 N / mm 2 or more and is excellent in impact bending toughness.
表１に示す組成の鋼スラブを１２４０℃に加熱したのち抽出して熱間圧延した。熱延条件は表２に示すとおりとした。表２中「巻取までの冷却速度」は、熱延仕上げ温度、仕上げ圧延最終パス終了後、巻取までの平均冷却速度を意味する。表２中には各鋼のＡｒ3点も示してある。次いで酸洗によりスケールを除去した後、表２に示す冷間圧延率にて冷間圧延を行った。冷延鋼板を一部の比較例を除き未焼鈍の鋼帯の状態で造管ラインに通板し、高周波溶接にて造管して肉厚１.０ｍｍ、外径３１.８ｍｍの電縫鋼管を得た。 A steel slab having the composition shown in Table 1 was heated to 1240 ° C., extracted and hot-rolled. The hot rolling conditions were as shown in Table 2. In Table 2, “Cooling rate until winding” means the average cooling rate until the winding after the hot rolling finish temperature and finish rolling final pass. Table 2 also shows the Ar 3 point of each steel. Subsequently, after removing the scale by pickling, cold rolling was performed at the cold rolling rate shown in Table 2. Cold-rolled steel sheets are passed through a pipe making line in the state of an unannealed steel strip except for some comparative examples, piped by high-frequency welding, and have a thickness of 1.0 mm and an outer diameter of 31.8 mm. Got.
酸洗後の熱延鋼板および冷延鋼板（冷延後に焼鈍を行った例では焼鈍後の鋼板）からサンプルを採取し、板厚方向に垂直な断面について顕微鏡観察を行い、前述の方法で熱延鋼板および冷延鋼板のフェライト結晶粒展伸度を調べた。その際、圧延材のＬ断面を研磨およびエッチングし、板厚中心部近傍を観察した。
なお、本発明例のものはいずれもマトリクスはフェライト相が７０体積％以上を占めており、第２相はベイナイトまたはマルテンサイトからなり、パーライトの量は１０体積％以下であった。
Samples were taken from hot-rolled steel sheets after pickling and cold-rolled steel sheets (steel sheets after annealing in the case of annealing after cold rolling), and the sections perpendicular to the thickness direction were observed with a microscope, and heat was applied by the method described above. The ferrite crystal grain elongation of the rolled steel sheet and the cold rolled steel sheet was examined. At that time, the L cross section of the rolled material was polished and etched, and the vicinity of the center of the plate thickness was observed.
In all of the examples of the present invention, the matrix comprised 70% by volume or more of the ferrite phase, the second phase was composed of bainite or martensite, and the amount of pearlite was 10% by volume or less.
得られた鋼管からＪＩＳ Ｚ２２０１の１１号引張試験片を作製し、引張試験を行い、降伏応力ＹＳおよび引張強さＴＳを測定した。また、得られた鋼管について、以下の方法で落下衝突試験および低温落重試験を実施した。 A No. 11 tensile test piece of JIS Z2201 was produced from the obtained steel pipe, a tensile test was performed, and a yield stress YS and a tensile strength TS were measured. Moreover, about the obtained steel pipe, the drop impact test and the low temperature drop test were implemented with the following method.
〔落下衝突試験〕
鋼管から長さ８００ｍｍの試料を採取し、１７５ｋｇの重錘を試料に取り付けた。その際、試料が水平になるように重錘を試料の両端付近に固定バンドにて取り付けた。固定バンドの間隔は６００ｍｍであり、２箇所の固定部には重錘の荷重が均等に掛かるようにしてある。溶接ビード位置は上端から１８０°の位置になるようにした。この重錘を取り付けた試料を、高さ１１ｍの位置からガイドレールに沿って自由落下させ、Ｒ＝１５０ｍｍの逆Ｕ字型断面を持つ固定治具に試料中央部を衝突させた。試料の鋼管が衝突により曲がった際、重錘本体と試料とが接触しないように、重錘本体と試料の間には十分な間隔を設けてある。荷重−変位曲線の面積を測定することにより、衝突によって試料鋼管が吸収した吸収エネルギーを求めた。この吸収エネルギーが０.６ｋＪ以上であれば、自動車等の構造部材に適した衝突吸収エネルギー（動的強度）を有していると判定される。
(Drop collision test)
A sample having a length of 800 mm was taken from the steel pipe, and a 175 kg weight was attached to the sample. At that time, weights were attached to both ends of the sample with fixing bands so that the sample was horizontal. The interval between the fixing bands is 600 mm, and the weight is equally applied to the two fixing portions. The position of the weld bead was 180 ° from the upper end. The sample to which the weight was attached was freely dropped along the guide rail from a height of 11 m, and the center of the sample was collided with a fixing jig having an inverted U-shaped cross section of R = 150 mm. A sufficient space is provided between the weight body and the sample so that the weight body and the sample do not contact when the steel pipe of the sample is bent by the collision. By measuring the area of the load-displacement curve, the absorbed energy absorbed by the sample steel pipe by collision was determined. If the absorbed energy is 0.6 kJ or more, it is determined that the absorbed energy (dynamic strength) is suitable for a structural member such as an automobile.
〔低温落重試験〕
鋼管から長さ９００ｍｍの試料を採取し、これを−３０℃に冷却した後、直ちに、３００ｍｍの間隔で固定された２箇所の支持治具の上に水平に置き、１１４ｋｇの重錘を落下距離３００ｍｍにて試料鋼管の支持治具間の中央位置に落下・衝突させた。支持治具はＲ＝３０ｍｍの逆Ｕ字型断面を有し、重錘は衝突部に先端Ｒ＝２５ｍｍのポンチを有している。溶接ビード位置は上端から０°の位置になるようにした。試験後、湾曲した試験片の側面に裂け疵が発生しておらず、かつ溶接部に割れが認められないものを○（衝突曲げ靭性；良好）、それ以外のものを×（衝突曲げ靭性；不良）と評価した。
これらの結果を表２に示す。
[Low temperature drop test]
A sample with a length of 900 mm was taken from the steel pipe, cooled to −30 ° C., and immediately placed horizontally on two supporting jigs fixed at intervals of 300 mm, and a weight of 114 kg was dropped. It was dropped and collided at a central position between the support jigs of the sample steel pipe at 300 mm. The support jig has an inverted U-shaped cross section with R = 30 mm, and the weight has a punch with a tip R = 25 mm at the collision part. The weld bead position was 0 ° from the upper end. After the test, no cracks occurred on the side surface of the curved specimen, and no cracks were observed in the welded portion, ○ (impact bending toughness; good), and other specimens × (impact bending toughness; Bad).
These results are shown in Table 2.
表２からわかるように、本発明例の鋼管はフェライト結晶粒展伸度が５.０以上である冷延鋼板を素材に用いたことにより、低温落重試験において優れた衝突曲げ靭性を有することが確認された。鋼管の引張強さは９８０Ｎ／ｍｍ2以上、落下衝突試験による吸収エネルギーも０.６ｋＪ以上が確保された。 As can be seen from Table 2, the steel pipe of the present invention has excellent impact bending toughness in a low temperature drop test by using a cold rolled steel sheet having a ferrite grain elongation of 5.0 or more as a material. Was confirmed. The tensile strength of the steel pipe was ensured to be 980 N / mm 2 or more, and the absorbed energy by the drop impact test was 0.6 kJ or more.
これに対し、比較例Ｎｏ.１０は熱延巻取温度が高すぎたことにより、熱延鋼板でのフェライト結晶粒展伸度が２.０未満となり、冷延鋼板での展伸度は５.０に達しなかった。その結果、強度レベル（引張強さ、衝突吸収エネルギー）は高いものの、低温落重試験では湾曲部に割れ疵が発生し、衝撃曲げ靭性に劣った。Ｎｏ.１１は熱延仕上げ温度が高く、Ｎｏ.１２は巻取までの冷却速度が遅すぎたことにより、いずれも強度レベルが不十分であった。Ｎｏ.１３は冷間圧延率が低かったことにより、強度レベルに劣った。また、冷延鋼板での展伸度は５.０に達しなかったことから衝撃曲げ靭性にも劣った。Ｎｏ.１４〜１６は鋼組成が本発明規定範囲を外れており、低温落重試験では溶接部に割れが発生した。Ｎｏ.１７は冷延後に焼鈍を行って、変態組織強化によって作り込んだ例であり、強度レベルは高いものの、繊維状組織となっていないために衝撃曲げ靭性に劣った。Ｎｏ.１８は熱延仕上げ温度が高すぎたことにより、熱延鋼板でのフェライト結晶粒展伸度が２.０未満となり、冷延鋼板での展伸度は５.０に達しなかった。冷間圧延率６０％で引張強さは９６０Ｎ／ｍｍ2程度となったが、衝撃曲げ靭性に劣った。 On the other hand, in Comparative Example No. 10, the hot rolling coiling temperature was too high, so the ferrite crystal grain elongation in the hot rolled steel sheet was less than 2.0, and the elongation in the cold rolled steel sheet was 5 It did not reach 0.0. As a result, although the strength level (tensile strength and impact absorption energy) was high, cracks occurred in the curved portion in the low temperature drop test, and the impact bending toughness was poor. No. 11 had a high hot rolling finishing temperature, and No. 12 had an insufficient strength level because the cooling rate until winding was too slow. No. 13 was inferior in strength level because the cold rolling rate was low. Further, the degree of elongation in the cold-rolled steel sheet did not reach 5.0, so that the impact bending toughness was inferior. In Nos. 14 to 16, the steel composition was out of the scope of the present invention, and cracks occurred in the welded part in the low temperature drop test. No. 17 is an example in which annealing was performed after cold rolling and the structure was made by strengthening the transformation structure, and although the strength level was high, it was inferior in impact bending toughness because it was not a fibrous structure. In No. 18, the hot rolling finish temperature was too high, so the ferrite crystal grain elongation in the hot rolled steel sheet was less than 2.0, and the elongation in the cold rolled steel sheet did not reach 5.0. The tensile strength was about 960 N / mm 2 at a cold rolling rate of 60%, but the impact bending toughness was poor.
Claims (4)
ＥF＝Ｎ1／Ｎ2 ……（１）
ここで、
ＥF：圧延板のフェライト結晶粒展伸度、
Ｎ1：圧延方向および板厚方向に平行な断面（Ｌ断面）の顕微鏡視野において板厚方向の一定長さＸの線分によって切断される結晶粒の数、ただし顕微鏡視野および線分の長さＸはＮ1が１０以上となるように設定する、
Ｎ2：前記視野において圧延方向の前記長さＸの線分によって切断される結晶粒の数 In mass%, C: 0.01 to 0.2%, Si: 1.5% or less, Mn: 2% or less, P: 0.05% or less, S: 0.02% or less, acid-soluble Al: 0.005% to 0.1%, having a composition the balance being Fe and inevitable impurities, the following (1) rolling a ferrite grain elongation rate E F is 5.0 or more unannealed cold-rolled steel sheet defined by formula A high-strength steel pipe having a tensile strength of 980 N / mm 2 or more, which is formed by welding so that the direction is the longitudinal direction.
E F = N 1 / N 2 (1)
here,
E F : Ferrite grain extension elongation of rolled sheet,
N 1 : Number of crystal grains cut by a line segment of a certain length X in the plate thickness direction in a microscope view of a cross section (L cross section) parallel to the rolling direction and the plate thickness direction, but the length of the microscope view and the line segment X is set so that N 1 is 10 or more.
N 2 : Number of crystal grains cut by the line segment of the length X in the rolling direction in the visual field
ＥF＝Ｎ1／Ｎ2 ……（１）
ここで、
ＥF：圧延板のフェライト結晶粒展伸度、
Ｎ1：圧延方向および板厚方向に平行な断面（Ｌ断面）の顕微鏡視野において板厚方向の一定長さＸの線分によって切断される結晶粒の数、ただし顕微鏡視野および線分の長さＸはＮ1が１０以上となるように設定する、
Ｎ2：前記視野において圧延方向の前記長さＸの線分によって切断される結晶粒の数 When manufacturing a steel pipe by the processes of hot rolling, descaling, cold rolling, and welded pipe making, the hot rolling finish temperature in hot rolling is (Ar 3 point−20 ° C.) to (Ar 3 point + 50 ° C.), after finish rolling final pass finish, the average cooling rate to the winding 20 to 100 ° C. / sec, below the coiling temperature as 450 to 600 ° C. (1) ferrite grain elongation rate E F determined by the equation 2.0 make more hot-rolled steel sheet, the ferrite grain elongation rate E F a reduction ratio as 40% to 80% in the cold rolling to make a 5.0 or more cold-rolled steel sheet, the remains unannealed in welding pipe The method for producing a high-strength steel pipe according to any one of claims 1 to 3, wherein the cold-rolled steel sheet is formed so that a rolling direction thereof is a longitudinal direction of the pipe.
E F = N 1 / N 2 (1)
here,
E F : Ferrite grain extension elongation of rolled sheet,
N 1 : Number of crystal grains cut by a line segment of a certain length X in the plate thickness direction in a microscope view of a cross section (L cross section) parallel to the rolling direction and the plate thickness direction, but the length of the microscope view and the line segment X is set so that N 1 is 10 or more.
N 2 : Number of crystal grains cut by the line segment of the length X in the rolling direction in the visual field
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| JP2010235964A (en) * | 2009-03-30 | 2010-10-21 | Nisshin Steel Co Ltd | High strength steel pipe excellent in corrosion resistance and shock bending toughness and method for manufacturing the same |
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| JP2012246550A (en) * | 2011-05-30 | 2012-12-13 | Jfe Steel Corp | Electric resistance welded steel pipe having excellent formability, low-temperature toughness and fatigue resistance characteristic in electric resistance welded part, and method for manufacturing the same |
| JP2012246548A (en) * | 2011-05-30 | 2012-12-13 | Jfe Steel Corp | Electric resistance welded steel pipe having excellent hic resistance and low-temperature toughness in electric resistance welded part, and method for manufacturing the same |
| JP2015209560A (en) * | 2014-04-25 | 2015-11-24 | 新日鐵住金株式会社 | Full hard cold rolled steel sheet |
| JP2018062712A (en) * | 2017-12-15 | 2018-04-19 | 新日鐵住金株式会社 | Full hard cold rolled steel sheet |
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| JP2010235964A (en) * | 2009-03-30 | 2010-10-21 | Nisshin Steel Co Ltd | High strength steel pipe excellent in corrosion resistance and shock bending toughness and method for manufacturing the same |
| JP2012246550A (en) * | 2011-05-30 | 2012-12-13 | Jfe Steel Corp | Electric resistance welded steel pipe having excellent formability, low-temperature toughness and fatigue resistance characteristic in electric resistance welded part, and method for manufacturing the same |
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