JPH11172374A - Bent pipe with high strength and high toughness, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve toughness at low temperatures by a specific composition which consists of C, Si, Mn, P, S, Ni, Mo, Nb, Ti, Al, N, O, and the balance Fe with inevitable impurities and in which respective component values satisfy a specific relation and further providing a microstructure containing specific amounts of bainite. SOLUTION: This bent pipe has a composition consisting of, by weight, 0.03-0.10% C, <=0.6% Si, 1.8-2.5% Mn, <=0.015% P, <=0.003% S, 0.20-1.0% Ni, 0.25 0.60% Mo, 0.01-0.10% Nb, 0.005-0.030% Ti, <=0.06% Al, 0.001-0.006% N, <=0.005% O, and the balance Fe with inevitable impurities. Moreover, the value of P1 defined by equation is regulated to 2.5-3.8. Further, this bent pipe has a microstructure containing bainite of <=10 μm average grain size, transformed from austenite, by >=70% by volume fraction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the American Petroleum Institute (A)
PI) standard X100 or more (yield strength about 690 N / mm
Bend pipe (bent pipe) having high strength and high toughness ( ² or more) and a method for producing the same.

[0002]

2. Description of the Related Art Line pipes (straight pipes) and deformed pipes (bend pipes, elbow pipes, T-shaped pipes, etc.) used in pipelines for transporting crude oil and natural gas over long distances are: (1) High pressure In order to improve the transport efficiency due to heat treatment and (2) to improve the welding efficiency in the field by reducing the thickness, the tension tends to be higher and higher. Up to now, practical application of line pipes up to X80 according to the API standard is in progress, but there is a need for higher strength line pipes and deformed pipes.

Conventionally, the mechanical properties (strength, low-temperature toughness, etc.) of a steel pipe are deteriorated in a bent pipe or the like as compared with a straight pipe.
913, JP-A-7-3330, JP-A5-
Various methods for improving the mechanical properties of a bend tube, such as 279743 and JP-A-59-232225, are disclosed.

[0004] For example, Japanese Patent Application Laid-Open No. Sho 62-10212,
JP-A-4-154913, JP-A-7-3330 and JP-A-5-279743 disclose a method in which a steel pipe is heated, quenched while being bent, and then tempered within a specific range after cooling. Is disclosed. However, these methods have a problem from the viewpoint of productivity and manufacturing cost because a tempering treatment is essential.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 59-232225 discloses a bend pipe for securing high strength and good low-temperature toughness by omitting a tempering treatment in order to improve productivity and reduce manufacturing costs. Are described. However, in these methods, at most X70 (yield strength 490)
N / mm ² ) Bend tube production is considered to be the limit.

[0006] In order to satisfy the strength of X100 or more, it is necessary to further add an alloy element. In the above-mentioned method, coarse upper bainite or MA (Martensite-Austenite) is contained in the structure after heating, bending, and water cooling. Constituen
t) Since a structure in which so-called martensite and austenite coexist is generated, it is considered impossible to ensure stable low-temperature toughness. Against this background,
Ultra-high strength bend pipe (X10) with excellent toughness at low temperature
(0 or more) was strongly demanded.

[0007]

SUMMARY OF THE INVENTION The present invention provides a low-temperature toughness with a yield strength of 690 N / mm ² or more (API standard X100).
It is an object of the present invention to provide a high-strength, high-toughness bend pipe and a method for producing the same.

[0008]

Means for Solving the Problems The first invention bend pipe of the present invention which achieves the above object, has a C content of 0.03 to 0.03% by weight.
0.10%, Si: 0.6% or less, Mn: 1.8 to 2.
5%, P: 0.015% or less, S: 0.003% or less,
Ni: 0.20 to 1.0%, Mo: 0.25 to 0.60
%, Nb: 0.01 to 0.10%, Ti: 0.005 to
0.030%, Al: 0.06% or less, N: 0.001
0.006%, O: 0.005% or less, the balance being Fe and unavoidable impurities, and the P ₁ value defined by the following formula (1) is in the range of 2.5 to 3.8. A high-strength and high-toughness bend pipe having a component composition and a microstructure containing 70% or more by volume fraction of bainite transformed from austenite having an average particle size of 10 μm or less. P ₁ = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V (1)

The bend tube according to the second aspect of the present invention has a C:
0.03 to 0.10%, Si: 0.6% or less, Mn:
1.7 to 2.2%, P: 0.015% or less, S: 0.0
03% or less, Ni: 0.10 to 1.0%, Mo: 0.1
5 to 0.50%, Nb: 0.01 to 0.10%, Ti:
0.005 to 0.030%, B: 0.0003 to 0.0
020%, Al: 0.06% or less, N: 0.001 to
0.006%, O: 0.005% or less, with the balance being F
e and unavoidable impurities, and has a P ₂ value defined by the following formula (2) in the range of 2.5 to 4.0, and is further transformed from austenite having an average particle size of 10 μm or less. Is a high-strength and high-toughness bend pipe characterized by having a microstructure containing 70% or more by volume fraction. P ₂ = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + 2Mo (2)

In the first invention bend tube and the second invention bend tube, Cu: 0.1 to
1.0%, Cr: 0.1 to 1.0%, V: 0.01 to
0.10%, Ca: 0.001 to 0.005% 1
It is preferable to contain one or more species.

Further, according to the method of the present invention, a steel pipe having the component composition described in the first invention bend pipe and the second invention bend pipe is heated to 780 to 950 ° C., and then bent and immediately heated to 10 ° C./sec or more. This is a method for producing a high-strength, high-toughness bend pipe characterized by cooling with water at a cooling rate.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, extremely low carbon-high Mn
-Nb- (Mo, Cr) - trace Ti steel, after heating above Ac ₃ temperature, by quenching with bending, to be able to ensure a high strength and good low temperature toughness of about X70, JP No. 59-232225. However, in order to obtain a high strength of X100 or more, it is necessary to further add an alloy element. And the low temperature toughness of the base material becomes insufficient due to the addition of the alloy element. Accordingly, as a result of intensive studies to improve the low-temperature toughness of the bent ultrahigh-strength bend pipe, the present invention has been achieved.

[0013] The bend tube of the present invention can be formed into a fine austenite state and be immediately cooled with water as in the method of the present invention described below, whereby the effective crystal grains of the transformed bainite can be remarkably refined. , Low temperature toughness can be improved.
Bainite transformed from austenite having an average grain size of 10 μm or less has good low-temperature toughness due to a small effective crystal grain size. At this time, the bainite structure was changed to 7
If the content is not 0% or more, the required strength of X100 or more cannot be obtained.

The low-temperature toughness of low alloy steel is governed by various metallurgical factors such as (1) the size of crystal grains and (2) the dispersed state of the hardened phase such as MA and upper bainite. In particular, the higher the strength, the greater the amount of alloying elements necessarily added, and the structure during quenching becomes a structure mainly composed of upper bainite.
Low-temperature toughness is deteriorated in combination with the formation of grains and coarsening of crystal grains. However, as described above, even if the microstructure of the steel material is strictly controlled, a steel material having desired characteristics cannot be obtained. For this purpose, it is necessary to limit the chemical composition simultaneously with the microstructure.

Therefore, the first invention bend pipe of the present invention is:
(1) Low C- at high Mn-Ni-Mo-Nb- Ti -based limited ingredients of, (2) above (1) P ₁ value defined by equation 2.
(3) A microstructure containing 70% or more by volume of bainite transformed from austenite having an average particle size of 10 μm or less in a volume fraction of 5% to 3.8. Thus, it has high strength of X100 or more and good low-temperature toughness.

The reasons for limiting the component composition in the first invention bend tube will be described below. C is 0.03-0.10
%. C is an element effective for improving the strength of the base metal and the welded portion, and at least 0.03% is necessary for obtaining a desired strength in a structure mainly composed of bainite. This amount is also the minimum amount for the precipitation hardening due to the addition of Nb and V and the manifestation of the effect of refining the crystal grains. But C
If the amount is too large, the low-temperature toughness of the base material and the HAZ (welding heat affected zone) and the remarkable deterioration of on-site weldability are caused. Therefore, the upper limit is set to 0.10%.

[0017] Si is an element added for deoxidation and for improving the strength, but if added in a large amount, the toughness and on-site weldability of HAZ are remarkably deteriorated, so the upper limit was made 0.6%. Steel can be sufficiently deoxidized with Ti or Al, and Si need not always be added. Mn is an element indispensable for securing strength and low-temperature toughness.
8%. However, if the Mn content is too large, the hardenability of the steel increases and not only deteriorates the toughness and on-site weldability of the HAZ, but also promotes the center segregation of the continuously cast steel slab and deteriorates the low-temperature toughness of the base material. Therefore, the upper limit was set to 2.5%.

The purpose of adding Ni is to improve the strength of the low carbon component system of the present invention without deteriorating low-temperature toughness and on-site weldability. Ni addition is Mn, Cr, Mo
It hardly forms a hardened structure harmful to low-temperature toughness in the rolled structure (especially the center segregation zone of the slab) as compared with the addition.
It has been found that the addition of a small amount of Ni is also effective for improving the HAZ toughness. To achieve this effect, 0.20
% Or more is required. However, if the addition amount is too large, not only economic efficiency but also HAZ toughness and on-site weldability are degraded, so the upper limit was made 1.0%.

The reason for adding Mo is to improve the hardenability of steel and obtain the desired bainite-based structure. In order to obtain such an effect, Mo should be at least 0.1.
25% is required. However, excessive Mo addition degrades HAZ toughness and on-site weldability, so the upper limit is 0.60.
%.

In the first invention bend pipe, Nb: 0.01 to 0.10% and Ti: 0.005 to 0.005% are essential elements.
Contains 0.030%. Nb coexists with Mo and contributes to refinement of crystal grains and precipitation hardening, and has an effect of toughening steel. As the minimum amount for exhibiting this effect, the lower limit was set to 0.01%. However, if Nb exceeds 0.10%, the HAZ toughness and on-site weldability are adversely affected, so the upper limit was made 0.10%.

On the other hand, the addition of Ti forms fine TiN,
It suppresses coarsening of austenite grains in a heated and welded HAZ, refines the microstructure, and improves the low-temperature toughness of the base material and the HAZ. When the amount of Al is small (for example, 0.005% or less), Ti forms an oxide, acts as an intragranular ferrite generation nucleus in the HAZ, and has an effect of refining the HAZ structure. In order to exert such a Ti addition effect, at least 0.005% of Ti must be added. However, if the amount of Ti is too large, coarsening of TiN and precipitation hardening due to TiC occur, and low-temperature toughness deteriorates. Therefore, the upper limit is set to 0.030%.

Al is an element usually contained in steel as a deoxidizing material, and has an effect on refining the structure. However, if the Al content exceeds 0.06%, Al-based nonmetallic inclusions increase and impair the cleanliness of the steel, so the upper limit was made 0.06%. Deoxidation can be performed with Ti or Si, and Al need not always be added.

N forms TiN so that it is heated and welded H
It suppresses coarsening of austenite grains in AZ and improves low-temperature toughness of the base material and HAZ. The minimum required for this is 0.001%. However, if the amount is too large, it causes deterioration of HAZ toughness due to slab surface flaws and solid solution N, so the upper limit must be suppressed to 0.006%.

The contents of P, S, and O as impurity elements are set to 0.015% or less, 0.003% or less, and 0.05% or less, respectively.
It shall be not more than 05%. The main reason for this is that the base material and HA
This is for further improving the low-temperature toughness of Z. The reduction of the P content reduces the center segregation of the continuously cast slab, prevents grain boundary fracture, and improves low temperature toughness. Further, the reduction of the S content has the effect of reducing the stretched MnS and improving the ductility. Reduction of the amount of O is effective in improving low-temperature toughness by reducing oxides in steel.

Next, preferable conditions are Cu, Cr,
The reason for adding one or more of V and Ca will be described. The main purpose of adding these elements in addition to the above-mentioned basic components is to increase the thickness of the plate that can be manufactured and the strength and strength of the base material without impairing the excellent features of the present invention.
This is for the purpose of improving properties such as toughness. Therefore, the amount of addition is of a nature that should be restricted.

Cu has almost the same effect as Ni, and also has an effect on improving corrosion resistance and resistance to hydrogen-induced cracking. Also, the strength is greatly increased by Cu precipitation hardening. In order to exhibit this effect, it is necessary to add 0.1% or more. However, if added in excess, precipitation hardening lowers the toughness of the base material and HAZ and causes Cu cracks during hot rolling, so the upper limit was made 1.0%.

Cr has the effect of increasing the strength of the base metal and the welded portion. To exhibit this effect, 0.1%
The above addition is necessary. However, if it is too large, the HAZ toughness and the on-site weldability are remarkably deteriorated. Therefore, the upper limit of the Cr content is set to 0.6%. V has almost the same effect as Nb, but the effect is weaker than Nb. However, the effect of V addition on high strength steel is significant. In order to exhibit this effect, 0.01% or more must be added. The upper limit is allowable up to 0.10% from the viewpoint of HAZ toughness and on-site weldability.

Ca controls the form of sulfide (MnS),
Improve low-temperature toughness (increase in absorbed energy in Charpy test, etc.). However, if the Ca content is 0.001% or less, there is no practical effect, and if the Ca content exceeds 0.005%, CaO—CaS is generated in a large amount to form clusters,
It becomes a large inclusion, which not only impairs the cleanliness of the steel, but also adversely affects on-site weldability. For this reason, the amount of Ca added was limited to 0.001 to 0.005%.

In addition to the above-mentioned limitation of the individual additive elements, it is necessary to further limit the P ₁ value defined by the above formula (1) to 2.5 or more and 3.8 or less. This is HAZ toughness,
This is to achieve the target strength-low temperature toughness balance without impairing the on-site weldability. _1. Lower limit of P ₁ value
The reason for setting the value to 5 is to obtain a strength of X100 or more and excellent low-temperature toughness. The upper limit of the P ₁ value is set to 3.8 in order to maintain excellent HAZ toughness and on-site weldability.

Next, the second invention bend tube is composed of (1) a low C-high Mn-Mo-Nb-trace B-trace Ti system limited component as described above, and (2) a formula (2). The base metal is a steel having a component composition having a defined P ₂ value in the range of 2.5 to 4.0, and (3) bainite transformed from austenite having an average grain size of 10 μm or less by 70% by volume fraction. %, It has high strength of X100 or more and good low-temperature toughness.

The component system in the second invention bend tube contains B. B is to significantly improve the hardenability of steel in a trace amount and obtain the desired bainite-based structure,
The second invention is an indispensable element in the bend tube. There is an effect corresponding to 1 in the P ₂ value described later, that is, 1% Mn. Further, B enhances the hardenability improving effect of Mo, and synergistically increases the hardenability together with Nb. In order to obtain such an effect, B must be at least 0.0003%. On the other hand, if added in excess,
Since not only the low-temperature toughness is deteriorated but also the effect of improving the hardenability of B may be lost, the upper limit is set to 0.0020%.

As a result of the addition of B, Mn is 1.
7% or more and 2.2% or less, Ni is 0.10% or more and 1.0% or less
Hereinafter, Mo is set to 0.15% or more and 0.50% or less, and the amounts of other elements C, Si, Nb, Ti, Al, N, P, S and O are the same as those in the first invention bend tube. The reasons for limiting the amounts of these elements are the same as in the first invention bend tube.

As preferable conditions, Cu, Cr,
The reason for adding one or more of V and Ca, and the reason for limiting the amount of addition are the same as in the first invention bend tube. Further, in the second invention bend pipe, it is necessary to limit the P ₂ value defined by the above equation (2) to 2.5 or more and 4.0 or less, for the same reason as in the first invention bend pipe. is there.

Such a first invention bend pipe and a second invention bend pipe are manufactured by melting a steel having the above-mentioned composition, forming a steel pipe by a known method, and then bending the steel pipe by the method of the present invention described later. can do. When forming a steel pipe, it is desirable to perform controlled rolling or controlled rolling to accelerated cooling as a method for rolling steel. By doing so, the microstructure of the steel pipe becomes finer, and fine austenite is generated during heating to bending, and the microstructure after quenching becomes a fine bainite-based structure. As a result, predetermined strength and low-temperature toughness of the bend pipe can be secured.

Next, the method of the present invention will be described. According to the method of the present invention, a steel pipe having each component composition shown in the first invention bend pipe and the second invention bend pipe is heated to a so-called γ / α2 phase region of 780 to 950 ° C., and then bent and immediately processed.
This is a method of cooling with water at a cooling rate of at least ° C / sec.

The reason why the heating temperature of the steel pipe is set to 780 ° C. or higher is to make the structure finer by partially austenitizing during heating and to obtain a predetermined strength by water cooling after bending. When the heating temperature of the steel pipe is lower than 780 ° C., austenite is formed only at the former austenite grain boundaries before heating, and when C is diffused and concentrated in this region, coarse and row-like MA is generated during water cooling after bending. Low temperature toughness deteriorates. Therefore, the lower limit of the heating temperature was set to 780 ° C.
However, when the heating temperature exceeds 950 ° C., austenite grains grow during heating, the structure after transformation becomes coarse, and the low-temperature toughness is deteriorated. Therefore, the upper limit of the heating temperature is 9
50 ° C.

After heating, the steel pipe was bent and immediately heated to 10 ° C. /
It is necessary to perform water cooling at a cooling rate of at least seconds. This is because a fine bainite structure of 70% or more is formed by cooling with water to obtain high strength and excellent low-temperature toughness. If the cooling rate is less than 10 ° C./sec, a fine bainite structure cannot be obtained, and high strength and good low-temperature toughness cannot be obtained at the same time. For this reason, the lower limit of the cooling rate during water cooling was set to 10 ° C./sec. If the steel pipe is not cooled immediately after the bending, the temperature of the steel pipe decreases, and high strength cannot be achieved due to the formation of ferrite and the like. The water cooling may be started before the completion of the bending process as long as the range does not hinder the bending process.

[0038]

EXAMPLES Example 1 Bend pipes were manufactured from steel pipes having various steel components shown in Table 1, and the results of examination of various properties are shown in Table 2. The mechanical properties were investigated in the direction perpendicular to the rolling. Invention Example No. 1 to No. The first invention bend tube of No. 6 has a predetermined high strength and good low-temperature toughness. Comparative example
No. 7-No. The 19 bend tubes are not suitable in component composition or microstructure, and are inferior in either property.

In the present invention example No. 1 to No. No. 6 is composed of the component composition of the bend tube of the first invention and satisfies the conditions of the method of the present invention, so that it has an appropriate microstructure, a predetermined high strength and good low-temperature toughness. On the other hand, Comparative Example N
o. 17-No. No. 19 is inferior in characteristics because the composition of the component is appropriate but out of the conditions of the method of the present invention.

No. Sample No. 7 has an inferior low-temperature toughness because the C content is too large. No. No. 8 has a low Mn content and thus has a low low-temperature toughness. N
o. No. 9 has a low Mn content because of an excessive amount of Mn. No.
Sample No. 10 has low Ni toughness and thus low toughness at low temperature. No. No. 11 has low Mo toughness due to low Mo content. No. In No. 12, the low temperature toughness is poor because the Mo content is too large. No. _No. 13 cannot obtain an intensity of X100 or more because the P1 value is small. No. _No. 14 has poor low-temperature toughness because the P ₁ value is too high. No. No. 15 has poor low-temperature toughness due to a large prior austenite grain size. No. 1
No. 6 does not satisfy the strength of X100 or more because the bainite fraction is low.

No. In No. 17, since the heating temperature at the time of bending is low, the prior austenite grain size is large and the low-temperature toughness is poor. No. Sample No. 18 had a high heating temperature, so the prior austenite grain size was large, and the low-temperature toughness was poor. No. In No. 19, the cooling rate immediately after bending was low, so that the bainite fraction was low and the low-temperature toughness was poor.

[0042]

[Table 1]

[0043]

[Table 2]

Example 2 Bend pipes were manufactured from steel pipes of various steel components shown in Table 3 and the properties were investigated. Table 4 shows the results. The mechanical properties were investigated in the direction perpendicular to the rolling.
Invention Example No. 21 to No. The 26nd invention bend tube has predetermined high strength and good low-temperature toughness. In contrast, Comparative Example No. 27-No. The 41 bend tube is not suitable in component composition or microstructure, and is inferior in either property.

In the present invention example No. 21 to No. 26 is the second
Since it is composed of the component composition of the invention bend tube and satisfies the conditions of the method of the invention, it has an appropriate microstructure, and has a predetermined high strength and good low-temperature toughness. In contrast, Comparative Example No. 39-No. 41 has inferior properties because the component composition is appropriate but out of the conditions of the method of the present invention.

No. Sample No. 27 has too low a C content, so that the low-temperature toughness is poor. No. Sample No. 28 has a low low-temperature toughness due to a small amount of Mn. No. No. 29 has too low a Mn content, so that the low-temperature toughness is poor. No. No. 30 has low low temperature toughness due to low Ni content. N
o. No. 31 has low Mo toughness due to low Mo content. No. 3
No. 2 has a low Mo toughness because the Mo content is too large. No. No. 33 has low strength and low-temperature toughness due to low B content. No. 3
No. 4 has low temperature toughness due to too much B content. No. 35 can not be obtained X100 more strength for P ₂ value is small.
No. 36 has poor low-temperature toughness for P ₂ value is too high. No.
No. 37 has poor low-temperature toughness due to a large austenite grain size. No. No. 38 does not satisfy the strength of X100 or more because the bainite fraction is low.

No. 39 has a low heating temperature at the time of bending, so that the prior austenite grain size is large and the low-temperature toughness is poor. No. In No. 40, since the heating temperature is high, the prior austenite grain size is large and the low-temperature toughness is poor. No. 41 has a low cooling rate immediately after bending, so that the bainite fraction decreases and the low-temperature toughness is poor.

[0048]

[Table 3]

[0049]

[Table 4]

[0050]

The bend pipe of the present invention has an API specification of X10.
It has high strength of 0 or more (yield strength of about 690 N / mm ² or more) and excellent low-temperature toughness. According to the method of the present invention, such a high-strength and high-toughness bend pipe can be stably manufactured. As a result, the safety of the pipeline has been significantly improved, and the transportation efficiency of the pipeline has been improved.

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/58 C22C 38/58 (72) Inventor Naoki Maruyama 3-35-1, Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Nippon Steel Corporation (72) Inventor Hitoshi Asahi 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Headquarters

Claims (4)

[Claims] C: 0.03 to 0.10%, Si: 0.6% or less, Mn: 1.8 to 2.5%, P: 0.015% or less, S: 0.003% or less, Ni: 0.20 to 1.0%, Mo: 0.25 to 0.60%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.030%, Al: 0.06% or less, N: 0.001 to 0.006%, O: 0.005% or less, the balance being Fe and unavoidable impurities, and P defined by the following formula (1). ₁ is a component composition in the range of 2.5 to 3.8, and further has a microstructure containing 70% or more by volume fraction of bainite transformed from austenite having an average particle size of 10 μm or less. High strength and high toughness bend tube. P ₁ = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V (1) 2. In% by weight, C: 0.03 to 0.10%, Si: 0.6% or less, Mn: 1.7 to 2.2%, P: 0.015% or less, S: 0.003% or less, Ni: 0.10 to 1.0%, Mo: 0.15 to 0.50%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.030%, B: 0.0003% to 0.0020%, Al: 0.06% or less, N: 0.001% to 0.006%, O: 0.005% or less, with the balance being Fe and unavoidable impurities, and the following (2) P ₂ value defined by formula of component composition in the range of 2.5 to 4.0, 70% more bainite that transformation from the austenite 10μm in average particle diameter at a volume fraction A high-strength, high-toughness bend pipe characterized by having a microstructure contained therein. P ₂ = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + 2Mo (2) 3. In% by weight, Cu: 0.1 to 1.0%, Cr: 0.1 to 1.0%, V: 0.01 to 0.10%, Ca: 0.001 to The high-strength and high-toughness bend pipe according to claim 1 or 2, wherein one or more of 0.005% is contained. 4. A steel pipe having the composition described in claim 1, 2 or 3, which is heated to 780 to 950 ° C., bent, and immediately cooled with water at a cooling rate of 10 ° C./sec or more. To manufacture high strength and high toughness bend pipes.