JP3764588B2 - Manufacturing method of high-strength steel pipe with excellent hydraulic formability with shaft push-in - Google Patents

Description

次に、図１にその主要部分を模式的に示す軸押し込みを伴う液圧成形装置を用いて、Ｔ型継手の成形試験に供した。軸押し込み量を片側３５ｍｍとし、軸押し込み量の変化に比例させて負荷する内圧を増加させる条件で予備試験を行い、試験体が破裂する圧力Ｐ_Bを各製造条件の各ｔ／Ｄ毎について求めた。次に本試験として、成形時の最終内圧をＰ_Bの９０％、その他の条件は予備試験と同じとし、一つの種類の素管について各々５０体の成形を行った。
【００１７】
試験後、成形が問題なく為されたか否かを判定し、成形の成功率、すなわち（５０−成形不良数）／５０×１００（％）を求めた。本試験での成形不良の形態は、全て、非膨出部における小さなしわの発生、または、座屈の発生であった。各製造条件毎の成功率をｔ／Ｄに対してプロットしたグラフが図２である。
【００１８】
このように上記の化学成分を有する鋼を適切な製造条件で造管すれば、ｔ／Ｄが０．０２程度まではほぼ問題なく成形可能であるが、それよりｔ／Ｄの小さい極薄肉管になると、そのほとんどが成形不良となるＣｕの析出処理が不適切と考えられる鋼管に比べれば高い成功率が得られるものの、その値は高々５０％程度であり、工業的な使用には不向きであると判断される水準に留まっている。
【００１９】
そこで、本発明者らは、こうした極薄肉管であっても更に高い成功率で問題なく成形出来る鋼管の開発を目指し、数々の冷間圧延鋼板を用いた鋼管についてこうした試験を繰り返し行い、如何なる場合に成形不良、すなわち座屈またはしわの発生が生じるのかを究明した。その結果、極薄肉管の場合には、成形前の鋼管外面の表面粗さが劣るほど成功率が低くなることを見出した。
【００２０】
従って成形前に鋼管外面を厳しく管理すれば成形の成功率を高め得ることが明らかとなった。
【００２１】
しかしながら、鋼管外面全体を全ての素管について管理することは自動車部品のような大量生産工程には不向きであり、徒に製造コストを引き上げるのみであるから、より低いコストで効果のある方法を見出すため更に検討を進めた。そして、調質圧延に用いるワークロールの表面粗さ（調質圧延を複数パスで行う場合にはその最終パスで用いるワークロールの表面粗さ）を適切に管理しておけば極めて高い成功率で極薄肉管の成形が可能であるとの結論を得るに至った。
【００２２】
図３は、先に述べたものと同一の化学成分を有する鋼を、同じく先に述べた製造条件Ａを用いて素管とする過程のうち、調質圧延に用いるワークロールの表面粗度のみを変化させ、その後それらについて同一の成形試験にて成功率を調べた結果である。
【００２３】
この図から、調質圧延の最終パスに用いるワークロール表面の算術平均粗さＲ_a（以下、単にＲ_aという）が１０μｍ以下であれば極めて高い成功率で極薄肉管の成形が可能であることが明らかとなった。
【００２４】
本発明はこのような過程を経て成されたものであり、更に鋭意検討を行って本発明を完成した。
【００２５】
まず鋼材の化学成分について述べる。
【００２６】
Ｃは、鋼板の成形性と密接に関わる元素であり、０．０１％を越えると成形性を劣化させるので、その上限を０．０１％とする。
【００２７】
Ｍｎは、０．０１％以上添加することにより鋼を強化する作用があり、所望の強度に応じて添加量を調整すればよいが、０．５％を越えると成形性に悪影響を及ぼすので、その上限を０．５％とする。
【００２８】
Ｓｉは、０．００５％以上添加することにより鋼を強化する作用があり、所望の強度に応じて添加量を調整すればよいが、１．０％を越えると成形性に悪影響を及ぼすので、その上限を１．０％とする。
【００２９】
Ｓは、少ない程成形性を高めるが、０．０３％以下（０％を含む）であれば特段問題とならないので０．０３％を上限とする。
【００３０】
Ｐは、不純物であり、粒界に偏析して粒界脆化を起こし二次加工割れに対する懸念を生じさせるので少ない程好ましい。ただし０．０５％以下（０％を含む）であれば許容出来るのでその上限を０．０５％とする。
【００３１】
Ｎは、少ない程成形性を高めるが、０．００５％以下（０％を含む）であればさほど悪影響を及ぼさないのでその上限を０．００５％とする。
【００３２】
Ａｌは、鋼の脱酸、脱窒を目的に添加するものであるが、ｓｏｌ．Ａｌの含有量が０．００５％未満ではその効果が得られず、また、０．１％を越えて含有させると延性の劣化をもたらすので、０．００５％以上０．１％以下とする。
【００３３】
Ｃｕは、本発明上最も重要な元素であり、適切な熱処理によって析出させることにより成形性を劣化させることなく強度を高める効果を有する。該効果は０．８％未満では十分発現されないので、０．８％を下限とする。一方、２．２％を越えて含有させてもその効果は飽和してしまうだけでなく、鋼板の表面品位を劣化させるので２．２％をその上限とする。
【００３４】
Ｔｉは、固溶Ｃ、および、固溶Ｎを低減させる働きを有し、成形性を高めるのに有利な元素である。しかし、０．０１％未満では効果がなく、一方０．２％を越えて含有させてもそれ以上の効果は得られず、鋼のコストを高めてしまう。そのため含有量を０．０１％以上、０．２％以下とする。
【００３５】
Ｎｂは、固溶Ｎを低減させ、また、熱間圧延の仕上げ圧延前の結晶粒径を微細化する働きを有し、成形性を高めるのに有効であるが、０．００５％未満では効果がなく、０．２％を越えて含有させてもそれ以上の効果が得られないばかりでなく、再結晶温度を上昇させてしまうので０．００５％以上０．２％以下とする。
【００３６】
Ｎｉは、Ｃｕが添加され、かつ、スケールが生成する条件下で加熱される場合には、鋼板の表面品位を保ち熱間脆化を防止するのに有効な元素である。必要に応じて０．１５％以上０．４５％以下の範囲で添加してもよい。
【００３７】
次に熱間圧延等の条件について述べる。
【００３８】
本発明に用いる鋼の熱間圧延は、連続鋳造後直接、または、冷却再加熱後のいずれで為されてもよい。その仕上圧延の終了温度がＡｒ₃点未満になると表層近傍を中心に剪断変形を受けた組織がそのまま冷延原板にもたらされるため高い成形性を有する鋼板を得ることが出来ない。このため仕上圧延はＡｒ₃点以上で終了する必要がある。仕上圧延の開始温度は特に限定する必要はなく、それに先立つ粗圧延を１０００℃前後で終了し、続いて仕上圧延を開始すればよい。
【００３９】
熱間圧延後の巻き取りを５００〜６５０℃で行うとＣｕの析出が起こり、これが冷間圧延板の再結晶を遅延させる。これを避けるために巻き取り温度は４５０℃以下もしくは７００℃以上が望ましい。
【００４０】
冷間圧延の合計圧延率は良好な成形性を得るためには５０％以上、９０％以下が望ましい。
【００４１】
冷間圧延材の熱処理と調質圧延、および、造管工程について述べる。
【００４２】
冷間圧延された鋼板は加工組織を有しているため再結晶させると共にＣｕを固溶させる。温度は７５０℃以上Ａｃ₃点以下とする。Ａｃ₃点を越えて加熱されると成形性に好ましい集合組織の形成が為されないからである。この熱処理に引き続き、４５０〜７００℃に冷却後、その温度域で０．１分以上熱処理しＣｕの析出を図る。熱処理が３０分を超えると析出したＣｕが粗大化し、更には母相との結晶学的整合性を失って鋼を強化する効果が減少するという問題が生じるので、上限を３０分とする。これらの熱処理は連続焼鈍により行うものとする。その後、該帯鋼に調質圧延を施す。その際どのような表面粗さのワークロールを用いるかが極めて重要であり、Ｒ_aが１０μｍ以下であることが必要である。調質圧延を複数パスで行う場合に、最終以前のパスに使用するワークロールについては特に規定しないが、同程度の粗さであることが望ましい。
【００４３】
調質圧延で加えられる伸び率は、鋼材の機械的性質や平坦度の修正などの必要量によって適宜決定されればよく、０．３〜５％の範囲とする。
【００４４】
Ｒ_aの下限は、本発明の目的上は特に設けない。なおＲ_aはＪＩＳ Ｂ ０６１０−１９９４の規定に基づき、ロールの軸方向の等間隔の１０箇所を全周に渡って測定し、それらのうちの最大値を採用した。
【００４５】
かくして得られた帯鋼を造管して所定の寸法の鋼管と為す。鋼管の製造方法はどのようなものでもよく、シーム溶接の方法も特に限定しない。
【００４６】
また、Ｃｕの析出を図るための熱処理を造管後に行ってもよい。この方式によれば、造管時の鋼板は低強度であるため造管機の負荷を軽減することが出来るとともに、Ｃｕの析出処理と造管後の歪除去処理を一つの工程で済ませることが出来る。
【００４７】
【実施例】
本発明の実施例を比較例とともに説明する。
【００４８】
表２に化学成分を示す鋼片を加熱して圧延し、９０２〜９１７℃で仕上げた。巻き取り温度は７５１〜７６０℃であった。これらの熱間圧延板を酸洗した後、８０％の冷間圧延を施した。次いで、表３の製造条件にて鋼管とした。造管はレーザービーム溶接により行った。そのうち調質圧延工程は、複数の、Ｒ_aの異なるワークロールを使用して行った。管の外径は６０．５ｍｍ、成形に用いる素管の長さは３００ｍｍ、軸押し込み量は片側３５ｍｍとした。複数のｔ／Ｄ毎に５０体のＴ型成形試験を行った結果を表４に示す。
【００４９】
【表２】

【００５０】
【表３】

【００５１】
【表４】

表２において、ａ〜ｄは本発明の範囲内の鋼であるが、ｅはＣｕ量が本発明の範囲外の鋼である。
【００５２】
また、表３において、Ｐ１〜Ｐ６は本発明の範囲内の製造条件であるが、Ｐ７およびＰ８はＣｕの析出条件が本発明の範囲外である。
【００５３】
更に表４において、Ｎｏ．８はＲ_aが本発明の範囲外である。
【００５４】
成形試験の結果、表４に見られるように、本発明の範囲内で製造された鋼管を用いれば、ｔ／Ｄが０．００５程度の極薄肉管であってもかなり高い成功率で成形可能であるのに対して、いずれかの条件でも本発明の範囲を外れる場合、成形の成功率ははるかに低くなり、工業的な生産には不適切であることが明らかとなった。
【００５５】
【発明の効果】
本発明の製造方法を用いれば、特段のコスト増もなく、軸押し込みを伴う液圧成形性に優れた高強度鋼管を得ることが出来る。
【図面の簡単な説明】
【図１】Ｔ型継手の成形試験に用いた軸押し込みを伴う液圧成形装置の主要部分を示す模式図である。
【図２】製造条件毎の、ｔ／Ｄと成形の成功率の関係を示すグラフである。
【図３】Ｒ_a毎の、ｔ／Ｄと成形の成功率の関係を示すグラフである。
【符号の説明】
１ 上金型
２ 成形前の鋼管の断面
３ 成形後の鋼管の断面
４ 下金型
５ 軸押し込み用シリンダー[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel pipe that exhibits excellent formability when used for hydraulic forming with axial pushing.
[0002]
[Prior art]
After holding a metal tube with a predetermined mold, the inside is filled with a liquid, the pressure of the liquid is controlled to a required value, and the end surface of the pipe is pushed in to expand to a desired shape. A processing method for extrusion molding is known as hydraulic bulging or hydroforming.
[0003]
Conventionally, this processing method has been used exclusively for the production of butt-welded joints for piping and connecting members for bicycle frames, but recently, it has been made into a shape with a closed cross-section by combining multiple parts such as bending a plate material. The application to automobile parts has been studied focusing on attempts to replace existing parts with parts produced from pipes by the processing method.
[0004]
As a result, since it is common to use the same material tube as the pipe to be connected in welding joints and bicycle frame members, the material of the bare pipe, which has hardly been studied, has become a problem. I came. For example, Japanese Patent Application Laid-Open No. 10-88278 describes a method of obtaining an electric-welded steel pipe excellent in hydraulic bulge formability by defining cooling after hot rolling and winding conditions and pipe making conditions. .
[0005]
Japanese Patent Application Laid-Open No. 10-137863 describes a steel pipe that has been surface-treated for hydraulic bulge processing.
[0006]
[Problems to be solved by the invention]
Typical elements required for automobile parts are light weight and high rigidity, and in order to meet these requirements, it is desirable that the material for parts, that is, the processing raw pipe, is a thin-walled pipe made of high-strength steel. However, in general, the thinning of the steel pipe tends to cause buckling when the shaft is pushed in. On the other hand, the use of a high-strength material often deteriorates the bulge formability. In other words, high strength steel pipes suitable for the processing method premised on application to automobile parts must have appropriate strength and formability. There is no example that examined pressure forming.
[0007]
The invention described in Japanese Patent Laid-Open No. 10-88278 is a steel pipe made from a hot-rolled steel sheet, and is not suitable for obtaining a thin-walled pipe, and its surface is a steel pipe made from a cold-rolled steel sheet. There is a problem that a molding defect may occur with a high probability because it is much rougher than the above. The invention described in JP-A-10-137863 relates to a steel pipe that has been surface-treated to prevent seizure between the molding die and the raw pipe and to improve lubricity. There has been no problem regarding the material, and there is a problem that it is not sufficient for the purpose of obtaining a steel pipe excellent in formability.
[0008]
In order to solve these problems, an object of the present invention is to provide a method of manufacturing a steel pipe that can be thinned while suppressing the occurrence of buckling and has excellent formability.
[0009]
[Means for Solving the Problems]
The present invention
(1) In mass%,
C: 0.01% or less,
Mn: 0.01 to 0.5%,
Si: 0.005 to 1.0%,
S: 0.03% or less,
P: 0.05% or less,
N: 0.005% or less,
sol. Al: 0.005 to 0.1%,
Cu: 0.8 to 2.2% included,
Hot finish rolling of steel composed of Fe and unavoidable impurities is finished at Ar3 point or higher, and after rewinding, pickling and cold rolling, recrystallization annealing is performed from 750 ° C to Ac3 point, followed by 450-700 heat-treated at 0.1 to 30 min at ° C., the final pass of the subsequent temper rolling performed arithmetic average roughness Ra of the following work rolls 10 [mu] m, then the thickness t and the outer diameter D A method for producing a high-strength steel pipe excellent in hydroformability accompanied by axial pressing, characterized in that pipe making is performed with a thickness-diameter ratio t / D being 0.005 to 0.02.
[0010]
(2) In mass%,
C: 0.01% or less,
Mn: 0.01 to 0.5%,
Si: 0.005 to 1.0%,
S: 0.03% or less,
P: 0.05% or less,
N: 0.005% or less,
sol. Al: 0.005 to 0.1%,
Cu: 0.8 to 2.2% included,
Finish the hot finish rolling of the steel composed of Fe and inevitable impurities at Ar ₃ point or higher, and after rewinding, pickling and cold rolling, recrystallization annealing at 750 ° C. to Ac ₃ point, The final pass of the subsequent temper rolling is performed with a work roll having an arithmetic average roughness _Ra of 10 μm or less, and then the thickness-diameter ratio t / D, which is the ratio of the plate thickness t to the outer diameter D, is 0.005. A method for producing a high-strength steel pipe excellent in hydroformability with axial pressing, characterized in that the pipe is formed as .about.0.02 and then heat-treated at 450 to 700.degree. C. for 0.1 to 30 minutes.
[0011]
(3) Further, the steel contains one or both of Ti: 0.01 to 0.2% and Nb: 0.005 to 0.2% by mass% (1). Or the manufacturing method of (2) description.
[0012]
(4) The manufacturing method according to any one of (1) to (3), further including Ni: 0.15 to 0.45% by mass%.
Is a summary.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors consider that a steel pipe manufactured using a steel plate having appropriate strength and workability is also suitable for the processing method, and disclosed in JP-A No. 64-4429. The high workability cold-rolled steel sheet using the disclosed precipitation strengthening of Cu was noticed. First, C: 0.01, Mn: 0.20, Si: 0.60, S: 0.005, P: 0.09, N: 0.0020, sol. A steel piece containing Al: 0.035, Cu: 1.2, the balance being Fe and inevitable impurities is heated and rolled at a temperature range of Ar ₃ or higher, wound at 750 ° C., and then pickled. 80% cold rolling was performed. At this time, a plurality of cold rolled materials having different plate thicknesses were produced by changing the finished thickness of the hot rolled plate.
[0014]
Thereafter, heat treatment, temper rolling and pipe making were performed under the production conditions shown in Table 1 to obtain a forming pipe. Pipe making was performed by laser beam welding.
[0015]
The heat treatment was performed under conditions A and B, where Cu precipitation is expected to be appropriately performed, and conditions C and D, where the Cu precipitation process is considered inappropriate for comparison. The diameter (outer diameter) and length of the raw tube are 60.5 mm and 300 mm, respectively. In addition, steel pipes having different thickness-diameter ratios t / D (hereinafter simply referred to as t / D) corresponding to the difference in plate thickness were produced.
[0016]
[Table 1]

Next, it used for the shaping | molding test of the T type | mold joint using the hydraulic forming apparatus with axial pushing which shows the principal part typically in FIG. Preliminary test is performed under the condition that the shaft push-in amount is 35 mm on one side and the internal pressure to be applied is increased in proportion to the change in the shaft push-in amount, and the pressure P _{B at} which the test body bursts is obtained for each t / D of each manufacturing condition. It was. Next, as a final test, the final internal pressure at the time of molding was 90% of P _B , the other conditions were the same as in the preliminary test, and 50 bodies were molded for each kind of raw pipe.
[0017]
After the test, it was determined whether or not molding was performed without any problem, and a success rate of molding, that is, (50−number of molding defects) / 50 × 100 (%) was obtained. Forms of molding defects in this test were all occurrences of small wrinkles in the non-bulged portions or occurrence of buckling. FIG. 2 is a graph in which the success rate for each manufacturing condition is plotted against t / D.
[0018]
In this way, if steel having the above chemical components is formed under appropriate manufacturing conditions, it can be formed with almost no problem until t / D is about 0.02, but an ultrathin tube having a smaller t / D than that. Then, although the success rate is higher than that of steel pipes that are considered to be improper for the Cu precipitation treatment, most of which are poorly formed, the value is about 50% at most, which is not suitable for industrial use. It remains at a level that is judged to be.
[0019]
Therefore, the present inventors have repeatedly conducted these tests on steel pipes using a number of cold-rolled steel sheets, aiming to develop steel pipes that can be formed without problems even with such ultra-thin wall pipes. It was investigated whether molding defects occurred, that is, occurrence of buckling or wrinkles. As a result, it was found that in the case of an ultrathin wall tube, the success rate decreases as the surface roughness of the outer surface of the steel tube before forming decreases.
[0020]
Therefore, it became clear that if the outer surface of the steel pipe is strictly controlled before forming, the success rate of forming can be increased.
[0021]
However, managing the entire outer surface of the steel pipe for all the raw pipes is not suitable for mass production processes such as automobile parts, and only increases the manufacturing cost. Therefore, further examination was advanced. And if the surface roughness of the work roll used for temper rolling is properly controlled (if the temper rolling is performed in multiple passes, the surface roughness of the work roll used in the final pass), the success rate will be extremely high. We came to the conclusion that ultra-thin wall pipes can be formed.
[0022]
FIG. 3 shows only the surface roughness of the work roll used for the temper rolling in the process of making the steel having the same chemical composition as described above into the raw tube using the manufacturing condition A described above. , And then the success rate was examined in the same molding test for them.
[0023]
From this figure, temper arithmetic mean roughness R _a of the work roll surface for use in the final pass rolling (hereinafter, simply referred to as R _a) is capable of very thin pipe forming an extremely high success rate equal to or less than 10μm is It became clear.
[0024]
The present invention has been made through such a process, and the present invention was completed through further intensive studies.
[0025]
First, the chemical composition of steel will be described.
[0026]
C is an element closely related to the formability of the steel sheet, and if it exceeds 0.01%, the formability deteriorates, so the upper limit is made 0.01%.
[0027]
Mn has the effect of strengthening steel by adding 0.01% or more, and the addition amount may be adjusted according to the desired strength, but if it exceeds 0.5%, the formability is adversely affected. The upper limit is 0.5%.
[0028]
Si has the effect of strengthening steel by adding 0.005% or more, and the addition amount may be adjusted according to the desired strength, but if it exceeds 1.0%, the formability is adversely affected. The upper limit is 1.0%.
[0029]
The smaller the S, the higher the moldability, but if it is 0.03% or less (including 0%), there is no particular problem, so 0.03% is made the upper limit.
[0030]
P is an impurity, and is preferably as small as possible because it segregates at the grain boundary to cause grain boundary embrittlement and raises concerns about secondary processing cracks. However, since 0.05% or less (including 0%) is acceptable, the upper limit is made 0.05%.
[0031]
N increases the formability as it is less, but if it is 0.005% or less (including 0%), it does not have a bad influence, so the upper limit is made 0.005%.
[0032]
Al is added for the purpose of deoxidation and denitrification of steel. If the Al content is less than 0.005%, the effect cannot be obtained. If the Al content exceeds 0.1%, ductility is deteriorated, so the content is made 0.005% to 0.1%.
[0033]
Cu is the most important element in the present invention, and has the effect of increasing the strength without deteriorating the formability by being precipitated by an appropriate heat treatment. The effect is not sufficiently exhibited when the content is less than 0.8%, so 0.8% is set as the lower limit. On the other hand, the content exceeding 2.2% not only saturates the effect, but also deteriorates the surface quality of the steel sheet, so 2.2% is made the upper limit.
[0034]
Ti has a function of reducing the solid solution C and the solid solution N, and is an element advantageous for improving the moldability. However, if it is less than 0.01%, there is no effect. On the other hand, if it exceeds 0.2%, no further effect is obtained and the cost of steel is increased. Therefore, the content is made 0.01% to 0.2%.
[0035]
Nb has a function of reducing the solid solution N and reducing the crystal grain size before the hot rolling finish rolling, and is effective in improving the formability, but it is effective at less than 0.005%. However, if the content exceeds 0.2%, not only the effect is not obtained, but also the recrystallization temperature is raised, so 0.005% or more and 0.2% or less.
[0036]
Ni is an element effective for maintaining the surface quality of a steel sheet and preventing hot embrittlement when Cu is added and heating is performed under conditions where scale is generated. You may add in 0.15% or more and 0.45% or less as needed.
[0037]
Next, conditions such as hot rolling will be described.
[0038]
The steel used in the present invention may be hot-rolled either directly after continuous casting or after cooling and reheating. When the finishing temperature of the finish rolling is less than the Ar ₃ point, a structure subjected to shear deformation centering on the vicinity of the surface layer is brought directly to the cold-rolled original sheet, so that a steel sheet having high formability cannot be obtained. For this reason, finishing rolling needs to be completed at Ar ₃ point or more. The start temperature of finish rolling is not particularly limited, and the rough rolling preceding that may be finished at around 1000 ° C., and then finish rolling may be started.
[0039]
When winding after hot rolling is performed at 500 to 650 ° C., precipitation of Cu occurs, which delays recrystallization of the cold rolled sheet. In order to avoid this, the winding temperature is preferably 450 ° C. or lower or 700 ° C. or higher.
[0040]
The total rolling ratio of cold rolling is preferably 50% or more and 90% or less in order to obtain good formability.
[0041]
The heat treatment and temper rolling of the cold rolled material and the pipe making process will be described.
[0042]
Since the cold-rolled steel sheet has a processed structure, it is recrystallized and Cu is dissolved. The temperature is 750 ° C. or higher and Ac ₃ points or lower. This is because if the structure is heated beyond the Ac ₃ point, formation of a texture preferable for moldability is not performed. Subsequent to this heat treatment, after cooling to 450 to 700 ° C., heat treatment is performed for 0.1 minutes or more in that temperature range to precipitate Cu. If the heat treatment exceeds 30 minutes, the deposited Cu becomes coarse, and further, the problem of losing the crystallographic consistency with the parent phase and reducing the effect of strengthening the steel arises, so the upper limit is made 30 minutes. These heat treatments are performed by continuous annealing. Thereafter, the strip steel is subjected to temper rolling. At that time or using any surface roughness of the work rolls is extremely important, it is necessary that R _a is 10μm or less. When temper rolling is performed in a plurality of passes, the work roll used for the pass before the final is not particularly specified, but it is desirable that the rolls have the same degree of roughness.
[0043]
The elongation applied by temper rolling may be determined as appropriate depending on the necessary amount such as correction of mechanical properties and flatness of the steel material, and is in the range of 0.3 to 5%.
[0044]
The lower limit of _Ra is not particularly provided for the purpose of the present invention. Note that _Ra was measured over the entire circumference at 10 equally spaced intervals in the axial direction of the roll based on the provisions of JIS B 0610-1994, and the maximum value among them was adopted.
[0045]
The strip steel obtained in this way is piped into a steel pipe of a predetermined size. Any method of manufacturing the steel pipe may be used, and the method of seam welding is not particularly limited.
[0046]
Moreover, you may perform the heat processing for aiming at precipitation of Cu after pipe forming. According to this method, since the steel plate at the time of pipe making has low strength, the load on the pipe making machine can be reduced, and the Cu precipitation treatment and the strain removal treatment after the pipe making can be completed in one step. I can do it.
[0047]
【Example】
Examples of the present invention will be described together with comparative examples.
[0048]
Steel pieces having chemical components shown in Table 2 were heated and rolled, and finished at 902 to 917 ° C. The winding temperature was 751 to 760 ° C. These hot-rolled sheets were pickled and then 80% cold-rolled. Subsequently, it was set as the steel pipe on the manufacturing conditions of Table 3. Pipe making was performed by laser beam welding. Among them, the temper rolling process was performed using _a plurality of work rolls having different _Ra . The outer diameter of the tube was 60.5 mm, the length of the raw tube used for forming was 300 mm, and the amount of axial pushing was 35 mm on one side. Table 4 shows the results of 50 T-shaped molding tests performed for each of a plurality of t / Ds.
[0049]
[Table 2]

[0050]
[Table 3]

[0051]
[Table 4]

In Table 2, a to d are steels within the scope of the present invention, but e is steel whose Cu content is outside the scope of the present invention.
[0052]
In Table 3, P1 to P6 are production conditions within the scope of the present invention, but P7 and P8 have Cu deposition conditions outside the scope of the present invention.
[0053]
Furthermore, in Table 4, No. 8, R _a is outside the scope of the present invention.
[0054]
As shown in Table 4, as a result of the forming test, if a steel pipe manufactured within the scope of the present invention is used, it can be formed with a very high success rate even if it is an ultra-thin pipe having a t / D of about 0.005. On the other hand, if any of the conditions is out of the scope of the present invention, the success rate of the molding becomes much lower, and it has become clear that it is not suitable for industrial production.
[0055]
【The invention's effect】
By using the production method of the present invention, it is possible to obtain a high-strength steel pipe excellent in hydroformability accompanied by axial pushing without any particular increase in cost.
[Brief description of the drawings]
FIG. 1 is a schematic view showing a main part of a hydraulic forming apparatus with shaft pushing used for a molding test of a T-shaped joint.
FIG. 2 is a graph showing the relationship between t / D and molding success rate for each manufacturing condition.
FIG. 3 is a graph showing the relationship between t / D and molding success rate for each _Ra .
[Explanation of symbols]
1 Upper mold 2 Cross section of steel pipe before forming 3 Cross section of steel pipe after forming 4 Lower mold 5 Cylinder for pushing shaft

Claims (4)

In mass%
C: 0.01% or less,
Mn: 0.01 to 0.5%,
Si: 0.005 to 1.0%,
S: 0.03% or less,
P: 0.05% or less,
N: 0.005% or less,
sol. Al: 0.005 to 0.1%,
Cu: 0.8 to 2.2% included,
Finish the hot finish rolling of the steel composed of Fe and inevitable impurities at Ar ₃ point or higher, and after rewinding, pickling and cold rolling, recrystallization annealing at 750 ° C. to Ac ₃ point, Subsequently, heat treatment is performed at 450 to 700 ° C. for 0.1 to 30 minutes, and the final pass of the subsequent temper rolling is performed by a work roll having an arithmetic average roughness _Ra of 10 μm or less, and then the sheet thickness t and the outer diameter A method for producing a high-strength steel pipe excellent in hydroformability with axial pushing, characterized in that pipe making is performed with a thickness-diameter ratio t / D as a ratio to D of 0.005 to 0.02. In mass%
C: 0.01% or less,
Mn: 0.01 to 0.5%,
Si: 0.005 to 1.0%,
S: 0.03% or less,
P: 0.05% or less,
N: 0.005% or less,
sol. Al: 0.005 to 0.1%,
Cu: 0.8 to 2.2% included,
Finish the hot finish rolling of the steel composed of Fe and unavoidable impurities at Ar ₃ point or higher, and after rewinding, pickling and cold rolling, recrystallization annealing at 750 ° C. to Ac ₃ point, The final pass of the subsequent temper rolling is performed with a work roll having an arithmetic average roughness _Ra of 10 μm or less, and then the thickness-diameter ratio t / D, which is the ratio of the sheet thickness t to the outer diameter D, is 0.005. and forming tube as 0.02, followed by 450-700 high-strength steel pipe manufacturing method with excellent hydroformed be accompanied by axial pushing you characterized by applying 0.1 to 30 minutes heat treatment at ° C.. Furthermore, steel contains one or both of Ti: 0.01-0.2% and Nb: 0.005-0.2% in the mass%, The Claim 1 or Claim 2 characterized by the above-mentioned. A method for producing a high-strength steel pipe excellent in hydroformability with axial push-in described. Furthermore, steel contains Ni: 0.15-0.45% in the mass%, It is excellent in the hydraulic formability with the axial pushing of any one of Claims 1-3 characterized by the above-mentioned. A method for manufacturing high strength steel pipes.

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