JP2001262273A - Weather resistant steel tube excellent in weldability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel tube good in weldability and also exhibiting excellent weather resistance in a seaside district. SOLUTION: In this welded steel tube, as a base metal, Ni-Cu series components are applied, and fine TiN including oxides composed of Mg and Al is contained, and as a weld metal, from the viewpoints of weather resistance and the prevention of high temperature cracking in a seaside district, the contents of C, P, S and Ni are limited to proper ranges. The steel tube good in weldablilty and showing excellent weather resistance in a seaside district can thus be provided, and, consequently, the production cost of a steel structure can remarkably be reduced, and further, the maintenance and management cost can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to weldability used in steel structures such as bridges and steel towers in an atmospheric environment where salt damage is likely to occur in a high flying sea salt particle environment, such as a beach area or an area where snow melting salt is sprayed. The present invention relates to a good weather-resistant steel pipe.

[0002]

2. Description of the Related Art Conventionally, as corrosion protection of steel structural members used in places where salt damage occurs, such as in coastal areas, painting and coating of ordinary steel, surface coating such as plated steel sheets, thermal spraying and mortar lining, and high corrosion resistance such as stainless steel and titanium. Alloy high corrosion resistant materials are used. Furthermore, as a technique for reducing the maintenance cost of steel structures, there is a non-painting use of weathering steel (JIS G3141 weathering steel for welded structures).

In the case of painting, repainting is necessary,
Maintenance costs are high. Further, in the case of plating, hot-dip plating of a structure has deformation and peeling of plating due to thermal stress. Also in the case of thermal spraying and mortar lining, there are problems such as peeling and deterioration of the anticorrosion film. Further, in the case of a high-alloy corrosion-resistant material, there is a problem that the material cost is high and cannot be widely used as a main structural member.

[0004] In the case of weather-resistant steel, if it is used without coating, a dense stable rust with excellent corrosion resistance is formed on the surface of the steel several years to about 10 years after use, and this stable rust causes the progress of corrosion of the steel thereafter. Is a steel material that prevents For steel structures, weather-resistant welded structural steels that take weldability into account mainly for bridges and buildings have been used in many cases. However, as shown in the “Design and Construction Procedure for Unpainted Weatherproof Bridges (Revised): Ministry of Construction, Public Works Research Institute, Steel Club, Japan Bridge Construction Association, March 1993,” In areas where there are a lot of incoming sea salt particles, such as in areas where it is sprayed, there is a problem in that the formation of stable rust with excellent protection properties is hindered by the salt attached to the surface of the steel material, making it unsuitable for unpainted use.

[0005] With regard to the improvement of weather resistance of weathering steel in the seaside area, for example, Japanese Patent Publication No. 56-9356 discloses P
A steel material which contains 0.03 to 0.20%, has excellent weldability, and has excellent weather resistance in a corrosive environment involving seawater or a general atmospheric environment. In addition, Japanese Patent Application Laid-Open No. H2-1258
No. 39 describes that the addition of a composite oxide of Ca and Al is effective for the addition of a low Si-P-Cu-Ni composite. JP-A-5-51668 discloses that it is effective to finely disperse an oxide in a steel material to suppress a decrease in pH of the steel material surface. As described above, steel materials having excellent weather resistance in the seaside area, which is a drawback of conventional weather resistant steel, have been developed.

[0006] In recent years, it has been proposed to use steel pipes for bridge girders in order to omit welding of members occupying a large specific gravity in bridge construction costs. A steel pipe used as a bridge girder is a large-diameter welded steel pipe manufactured mainly by UOE or BR (bending roll). These pipes are manufactured by submerged arc welding (SAW),
When a large amount of i is contained, hot cracking (solidification cracking) occurs during welding. In addition, the hardness of the welding heat affected zone (HAZ) increases,
Cold cracking occurs due to hydrogen. For this reason, steel pipes having good weldability and excellent weather resistance in the seaside area are strongly desired.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a steel pipe having good weldability and excellent weather resistance in a beach area.

[0008]

The gist of the present invention is that the mass%
C: 0.03 to 0.15, Si: 0.6 or less, Mn: 0.3 to 2.0, P: 0.03 or less, S: 0.005 or less, Cu: 0.3 to 1 0.0, Ni: more than 1.0 to 5.5, Cr: 0.1 or less, Ti: 0.005 to 0.03, Al: 0.001 to 0.050, Mg: 0.0001 to 0.0050 , N: 0.001 to 0.006, O: 0.001 to 0.006, and if necessary, Mo: 0.1 to 1.0, Nb: 0.005 to 0.1, V: 0.01 to 0.1, B: 0.003 to 0.002, Ca: 0.001 to 0.005, containing one or more kinds, the balance being iron and inevitable impurities,
0.01 to 0.5 containing an oxide composed of Mg and Al
A base material containing 10,000 μm / mm ² or more of TiN, C: 0.02 to 0.07, Si: 0.6 or less, Mn: 0.3 to 2.0, P: 0. 01 or less, S: 0.005 or less, Cu: 0.1 to 1.0, Ni: more than 1.0 to 3.5, Cr: 0.1 or less, Ti: 0.005 to 0.03, Al: 0.09 or less, N: 0.001-0.006, O: 0.02-0.06, Mo: 0.1-1.0, Nb: 0.005- 0.1, V: 0.01 to 0.1, B: 0.0003 to 0.002, Ca: 0.001 to 0.005, Mg: 0.0001 to 0.0050 One or more of them , The balance consisting of iron and unavoidable impurities, and a Ni content of 2.0% of the base material.
It is a weather-resistant steel pipe excellent in weldability, characterized in that it has a weld metal part in a range from a low% region to a high 0.2% region.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the weldability of the present invention is good.
In addition, a steel pipe having excellent weather resistance in the seaside area will be described in detail. Conventionally, in inland areas where there are few incoming sea salt particles,
It is known that Cr is effective for improving weather resistance. However, it has been found that Cr has an adverse effect on weather resistance in a high flying salt particle environment such as a beach area or a region where snow melting salt is sprayed. In addition, as a result of investigations for improving the weather resistance of various alloy elements, Cu
It has been clarified that the combined addition of -1% or more of Ni is effective for the formation of stable rust in the beach area. Furthermore, Cu-
It has been found that the application limit of Ni-based (the upper limit of the annual average amount of sea salt particles that can form stable rust) can be substantially controlled by the amount of Ni added.

On the other hand, as a result of intensive studies on the weld metal components of the welded steel pipe having excellent weather resistance in the seaside area and excellent weld quality, the Ni content of the weld metal is at least 2.0% less than that of the base metal. It has been found that if it is equal to or more than the region, good weather resistance can be obtained. Furthermore, a large amount of Ni is contained by limiting the Ni content of the weld metal to the base metal Ni + 0.2% or less and 3.5% or less and by limiting the amounts of C, P and S in the weld metal. It has been found that hot cracking does not occur even in this case.

Further, when a large amount of Ni is contained in the base material, the hardness of the HAZ is significantly increased, and low-temperature cracking is likely to occur. If fine TiN containing an oxide composed of Mg and Al is contained, the growth of the γ grain size is suppressed even in a region reheated to a high temperature near the melting line, and the fine γ
, The γ → α transformation temperature rises and H
The inventors have found that the AZ hardness is reduced, and have reached the present invention.

That is, the feature of the present invention is that a Ni-Cu based component is applied as a steel pipe base material, fine TiN containing an oxide composed of Mg and Al is contained, and weather resistance in a seaside area is used as a weld metal. C from the viewpoint of heat resistance and prevention of hot cracking
By limiting the amount, the amount of P, the amount of S, and the amount of Ni to appropriate ranges, it is possible to provide a steel pipe having good weldability and excellent weatherability in a beach area.

[0013] First, HA by suppressing γ grain growth during reheating
The reduction in Z hardness will be described. In the HAZ near the melting line, the temperature becomes 1400 ° C. or higher, so that the carbonitride such as TiN dissolves and coarsens, and the ability to suppress the movement of the γ grain boundary is remarkably reduced, and the growth of γ grains cannot be avoided. . For this reason, during cooling after welding, the coarsened γ
Due to the transformation, the transformation temperature was lowered and the hardness was significantly increased. In particular, in the steel of the present invention containing a large amount of Ni, the hardness is significantly increased and low-temperature cracking occurs.

Since the amount of Ni cannot be reduced from the viewpoint of securing the weather resistance, γ → α due to the refinement of the γ particle size during γ → α transformation.
As a method of reducing hardness by increasing the transformation temperature, 1
The effect of oxides that are chemically stable even at a high temperature of 400 ° C. or more was examined for the effect of suppressing grain growth. As a result, they have found that the inclusion of trace amounts of Mg and Al generates a large amount of unprecedented ultrafine (Mg, Al) oxide having a size of 0.01 to 0.1 μm. Furthermore, 0.01 or more.
Fine TiN having a size of less than 5 μm is
1) It has been clarified that the compound precipitates on the oxide and exhibits an unprecedented and extremely strong effect of suppressing grain growth at a high temperature of 1400 ° C. or higher. At this time, the content of 0.01 to 0.
When 5 μm of TiN is less than 10,000 / mm ² ,
The effect of suppressing the coarsening of γ grains becomes insufficient, and good HAZ toughness cannot be obtained. In addition, TiN of 0.5 μm or more
Is less effective in suppressing the growth of γ grains,
The upper limit of the size was 0.5 μm. Note that the lower limit of the size of TiN is set to 0.01 μm or more from the viewpoint of analysis accuracy. As described above, based on the findings described above, it has been clarified that the HAZ hardness near the melting line is reduced, and low-temperature cracking can be prevented.

Next, the reasons for limiting the components of the steel pipe base material will be described. C is an element necessary for securing the strength of the base material and the welded portion, and the lower limit is 0.03% (mass%, the same applies hereinafter). However, if the C content is too large, the low-temperature toughness and the weldability are deteriorated.
%.

[0016] Si is an element added for deoxidation and to improve the strength, but if added in a large amount, the toughness of the heat affected zone (HAZ) is deteriorated, so the upper limit was made 0.6%. Al and Ti alone are sufficient for deoxidizing steel, and Si need not always be added.

Mn is an indispensable element for securing strength and low-temperature toughness, and its lower limit is 0.3%. But M
If n is too large, not only does the hardenability of the steel increase, thereby deteriorating the weldability, but also promotes the center segregation of the continuously cast steel slab and lowers the low-temperature toughness, so the upper limit was made 2.0%.

P is an element for improving the weather resistance.
If the content exceeds 0.03%, the low-temperature toughness and weldability deteriorate, so the upper limit value is set to 0.03%. In particular, in order to prevent hot cracking of the welded portion, 0.015% or less is preferable.

Since S is an unavoidable impurity that deteriorates the toughness and weather resistance of the steel pipe, it is preferable that the content of S is as small as possible. In particular,
If it exceeds 0.01%, the low-temperature toughness deteriorates.
It is preferably at most 05%.

Cu forms rust particle crystals when forming the rust layer.
It is an element that improves the weather resistance in an environment with a lot of incoming sea salt particles in order to suppress coarsening and maintain the denseness of rust,
In order to exhibit this effect, it is necessary to add 0.3% or more. However, if the content exceeds 1.0%, precipitation hardening causes deterioration of the base material and HAZ in toughness and Cu-cracks occur during hot rolling. Therefore, the upper limit value is set to 1.0%.

Ni suppresses the penetration of Cl ions adhering to the surface of the rust layer as flying sea salt particles to the interface between the rust layer and the ferrous iron, and makes the rust layer have a low Cl environment to promote crystallization and coarse growth of the rust particles. The suppression has the effect of maintaining the denseness of the rust layer. To achieve this effect,
Addition of more than 1.0% is required. However, the addition of more than 5.5% causes cracking on the steel sheet surface, so the upper limit was made 5.5%.

[0022] Since Cr is a metal lower than iron, several percent
In addition, since the addition of C impairs the weather resistance and weldability in an environment with a large amount of sea salt particles, the smaller the amount, the better. In particular,
If it exceeds 0.1%, the weather resistance deteriorates, so the upper limit value was set to 0.1%.

Ti forms fine TiN, suppresses coarsening of γ grains in the HAZ during reheating of the slab, and refines the microstructure, thereby improving the low-temperature toughness and HAZ hardness of the base material and the HAZ, It is an essential element in the present invention. In order to exhibit this effect, 0.005% or more must be added. If the content is too large, coarsening of TiN and precipitation hardening due to TiC occur to deteriorate low-temperature toughness. Therefore, the upper limit value is limited to 0.03%.

Al is important for controlling the (Mg, Al) oxide of 0.01 to 0.1 μm, which is the precipitation nucleus of the composite precipitate TiN, which is the particle that suppresses the γ grain growth. A
When l is less than 0.001%, the number of (Mg, Al) oxides is less than 10,000 / mm ² ,
When the number of N is insufficient, γ grains are not sufficiently refined,
No reduction in HAZ hardness can be obtained. On the other hand, when the Al content is 0.0
If it exceeds 50%, Al-based nonmetallic inclusions increase and the cleanliness of the steel deteriorates, so the upper limit was made 0.050%.

Mg is an element having an important role in the present invention. By containing an appropriate amount of Mg, the dispersed state of the oxide in the present invention can be achieved. Mg is 0.
If it is less than 0001%, it is a precipitation nucleus of TiN (Mg,
Al) Oxide shortage. On the other hand, 0.0050% of Mg consumed as an oxide is sufficient. Since Mg is a very active element having a high vapor pressure and a strong oxidizing power, it is not preferable to include Mg in steel more than necessary, because it increases the production cost.

N forms TiN and improves the low-temperature toughness of the base material and the HAZ by suppressing the re-heating of the slab and the coarsening of the γ grains of the HAZ. The minimum amount required for this is 0.00
1%. However, if the amount of N is too large, it causes the HAZ toughness to be deteriorated due to slab surface flaws or solid solution N, so the upper limit value must be suppressed to 0.006%.

O is a precipitation nucleus of TiN (Mg, Al)
It is necessary to secure the number of oxides. O is 0.0
If it is less than 01%, the number of fine oxides is insufficient, and the effect of suppressing the growth of γ grains is not exhibited. On the other hand, if O exceeds 0.006%, the cleanliness of the steel decreases, and mechanical properties such as toughness deteriorate.

Next, Mo, Nb,
The reason for adding V, B, and Ca will be described. The main purpose of adding these elements to the basic components as needed is to improve properties such as strength and low-temperature toughness without impairing the features of the steel of the present invention. Therefore, the amount of addition is of a nature that should be restricted by itself.

Mo is an element that increases the strength of the base material and the welded portion, but if it exceeds 1.0%, the toughness and weldability of the base material and HAZ deteriorate. The effect is weak when the addition is less than 0.1%.

Nb not only suppresses the recrystallization of γ during controlled rolling to refine the crystal grains, but also contributes to the precipitation hardening and hardenability, and has the effect of toughening the steel. To obtain this effect, at least 0.005% of Nb is required. However, if the Nb content is too large, the HAZ toughness deteriorates, so the upper limit value is limited to 0.1%.

V has almost the same effect as Nb, but the effect is much weaker than Nb. In order to exert the effect, it is necessary to add 0.01% or more. Further, the upper limit is allowable up to 0.1% from the viewpoint of on-site weldability and HAZ toughness.

B has an extremely small amount to dramatically improve the hardenability of steel, and provides good strength and toughness. In order to exhibit this effect, 0.0003% or more must be added. If the amount is too large, the HAZ toughness is deteriorated. Therefore, the upper limit value is limited to 0.002%.

Ca controls the form of sulfide (MnS),
Improve low-temperature toughness (eg, increase absorbed energy in Charpy test). If less than 0.001%, the effect is small, and if it exceeds 0.005%, Ca
O-CaS is generated in large amounts to form clusters and large inclusions, which not only impairs the cleanliness of the steel but also adversely affects on-site weldability. Therefore, the amount of Ca added is 0.001 to
Limited to 0.005%.

Next, the reasons for limiting the components of the weld metal will be described. When a steel having good weather resistance and containing a large amount of Ni is welded with large heat input, hot cracking occurs. Therefore, in order to prevent high-temperature cracking of the weld metal and secure good weather resistance, C: 0.02 to 0.07%, P: 0.01%
Hereinafter, S: 0.005% or less, Ni: more than 1.0 to 3.5.
% And a range in which the Ni content is 2.0% less than the Ni content of the base material to a range in which the Ni content is 0.2% higher.
Hot cracking can be prevented by limiting the amounts of C, P, S and Ni. Therefore, the upper limits of the amounts of C, P and S are set to 0.07%, 0.01% and 0.00, respectively.
Limited to 5%.

Further, the Ni content of the weld metal is equal to the base metal Ni content +
If it is 0.2% or less and 3.5% or less, hot cracking can be prevented. In addition, if the Ni content of the weld metal is equal to or more than a region less than 2.0% less than the Ni content of the base metal, good weather resistance can be obtained. 2.0% less. Further, C is an element necessary for securing the strength of the welded portion, and the lower limit value is limited to 0.02% in order to exert its effect.

[0036] Si is an element added for deoxidation and strength improvement, but when added in a large amount, the low-temperature toughness is deteriorated.
The upper limit was set to 0.6%.

Mn is an indispensable element for securing strength and low-temperature toughness, and its lower limit is 0.3%. But,
If the Mn content is too large, the hardenability of the steel increases and the low-temperature toughness deteriorates, so the upper limit was made 2.0%.

Cu forms rust particle crystals when forming the rust layer.
It is an element that improves the weather resistance in an environment with a lot of incoming sea salt particles in order to suppress coarsening and maintain the denseness of rust,
In order to exhibit this effect in the weld metal, it is necessary to use 0.1.
Addition of 1% or more is required. However, if it exceeds 1.0%, the toughness deteriorates due to precipitation hardening, so the upper limit was made 1.0%.

[0039] Since Cr is a metal lower than iron, several%
In addition, since the addition of harmful to the weather resistance in an environment with many sea salt particles, the smaller the amount, the better. In particular, 0.1%
If it exceeds the value, the weather resistance deteriorates.

Ti forms fine TiN and TiO, refines the microstructure, and improves the low-temperature toughness of the weld metal.
In order to exhibit this effect, 0.005% or more must be added. If the content is too large, precipitation hardening due to TiC occurs and the low-temperature toughness is deteriorated.
Limited to 03%.

Al usually has an effect as a deoxidizing element.
However, if the amount of Al exceeds 0.09%, the amount of Al-based nonmetallic inclusions increases to deteriorate the cleanliness of the steel, so the upper limit was made 0.09%. Ti alone is sufficient for deoxidizing steel, and Al need not always be added.

N forms TiN, refines the microstructure, and improves low-temperature toughness. The minimum required for this is 0.001%. However, if the amount of N is too large, it causes deterioration of toughness due to solid solution N. Therefore, the upper limit value must be suppressed to 0.006%.

O forms an oxide in the weld metal,
It acts as a nucleus for intragranular transformed ferrite and is effective in refining the structure. However, if the amount is too large, the low-temperature toughness of the weld metal deteriorates and welding defects such as slag entrainment occur. Therefore, the lower limit of the O content is 0.02% and the upper limit is 0.0%.
6%.

Next, Mo, Nb,
The reason for adding V, B, Ca, and Mg will be described.
The main purpose of adding these elements, if necessary, to the basic components is to improve properties such as strength and low-temperature toughness of the weld metal without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount of addition is of a nature that should be restricted.

Mo is an element which increases the strength of the welded portion, but if it exceeds 1.0%, the low-temperature toughness deteriorates. The effect is weak when the addition is less than 0.1%.

Nb not only refines crystal grains, but also contributes to precipitation hardening and hardenability, and has the effect of toughening steel. At least 0.005% to achieve this effect
Of Nb is required. However, if the Nb content is too large, the HAZ toughness deteriorates.
Limited to.

V has almost the same effect as Nb, but the effect is much weaker than Nb. In order to exert the effect, it is necessary to add 0.01% or more. Also, the upper limit is allowable up to 0.1% from the viewpoint of low temperature toughness.

B has an extremely small amount and dramatically improves the hardenability of steel, and provides good strength and toughness. In order to exhibit this effect, 0.0003% or more must be added. If the content is too large, the low-temperature toughness deteriorates. Therefore, the upper limit value is limited to 0.002%.

Ca controls the form of sulfide (MnS),
Improve low-temperature toughness (eg, increase absorbed energy in Charpy test). If less than 0.001%, the effect is small, and if it exceeds 0.005%, Ca
O-CaS is generated in large amounts to form clusters and large inclusions, which impair the cleanliness of the steel. For this reason, the amount of Ca added is set to 0.1.
001 to 0.005%.

Mg forms an oxide in the weld metal, acts as a finer solidification structure and acts as a core of intragranular transformed ferrite, and is effective in reducing the structure. However, if too large, the low-temperature toughness of the weld metal deteriorates. For this reason, the lower limit of the amount of Mg was made 0.0001%, and the upper limit was made 0.0050%.

[0051]

An embodiment of the present invention will be described. Using steel plates of various steel components, steel pipes were manufactured and their properties were investigated.
The quality of the steel pipe weld (the presence or absence of welding defects) is determined by the S
After performing AW (submerged-arc welding), evaluation was carried out using an ultrasonic probe. Each of the prototype steel pipes has an offshore distance (average amount of incoming sea salt particles) of 5 m (1.3 md).
d), 50 m (0.8 mdd), 200 m (0.5 md)
d) At the point of 800 m (0.2 mdd), it was subjected to an exposure test. The weather resistance in a high sea salt particle environment was evaluated from the average sheet thickness reduction obtained from the ion permeation resistance and corrosion amount of rust. In the rust ion permeation resistance measurement, the rust ion permeation resistance value was measured by an AC impedance method, and those having a resistance of 3 kΩ or more were determined to be dense stable rust formation. The average reduction in thickness is calculated from the corrosion-time curves at four locations, and the estimated reduction in thickness after 50 years is calculated. And

Table 1 shows the components of the steel pipe of the present invention. Table 2 (Table 2-1) and Table 3 (Table 2-2) show the components of the comparative steel pipe.
The measured values are shown in Table 4 (Table 3-1) and Table 5 (Table 3-2). Steel No. of the welded steel pipe of the present invention. Nos. 1 to 10 have excellent weldability and weather resistance in the base material and the welded portion.

On the other hand, the comparative steel No. 11-27
Are not suitable for the chemical components and are inferior in either property. steel
In No. 11, the low-temperature toughness of the base material is inferior because the C content of the base material is too large.
In steel 12, the low-temperature toughness of the base material is inferior because the P content of the base material is too large. Steel 13 is inferior in low-temperature toughness of the base material because the S content of the base material is too large. Steel 14 has poor weather resistance of the base material due to the small amount of Cu in the base material. Steel 15 has poor weather resistance of the base material because the amount of Ni in the base material is small. Steel 16 had cracks in the base metal because the base material had too much Ni content. In steel 17, the weather resistance of the base material is inferior because the amount of Cr in the base material is too large. Steel 18 had low-temperature cracking due to the small amount of Mg in the base metal.

In steel 19, low-temperature cracking occurred because the number of TiN particles containing (Mg, Al) oxide included in the base metal was 0.01 or more and less than 0.5 μm. In steel 20, high temperature cracking occurred in the weld metal because the amount of C in the weld metal was too large.
In steel 21, hot cracking occurred in the weld metal because the P content of the weld metal was too large. In steel 22, hot cracking occurred in the weld metal because the S content of the weld metal was too large. In steel 23, the weather resistance of the weld metal is poor because the amount of Ni in the weld metal is small. steel
In No. 24, the Ni content of the weld metal was too large, and hot cracking occurred in the weld metal. In Steel 25, the weather resistance of the weld metal is poor because the amount of Cr in the weld metal is too large. Steel 26 is weld metal Ni
Since the amount is more than 2.0% and less than the Ni amount of the base material, the weather resistance of the weld metal is inferior. In steel 27, the Ni content of the weld metal is more than 0.2% greater than the Ni content of the base metal,
Hot cracking occurs in the weld metal.

[0055]

[Table 1]

[0056]

[Table 2]

[0057]

[Table 3]

[0058]

[Table 4]

[0059]

[Table 5]

[0060]

According to the present invention, the steel pipe having excellent weldability and weather resistance in the seaside area according to the present invention is adopted as a member of a steel structure such as a bridge, so that the manufacturing cost of the steel structure is reduced and the maintenance cost is reduced. Management has been simplified and management costs have been significantly reduced.

Claims (3)

[Claims] C .: 0.03 to 0.15; Si: 0.6 or less; Mn: 0.3 to 2.0; P: 0.03 or less; S: 0.005 or less; Cu: 0.3 to 1.0, Ni: more than 1.0 to 5.5, Cr: 0.1 or less, Ti: 0.005 to 0.03, Al: 0.001 to 0.050, Mg: 0.0001 to 0.0050, N: 0.001 to 0.006, O: 0.001 to 0.006, the balance consisting of iron and unavoidable impurities.
0.01 to 0.5 μg containing an oxide consisting of g and Al
A base material containing 10,000 m / m ² or more of mN and C: 0.02 to 0.07, Si: 0.6 or less, Mn: 0.3 to 2.0, P: 0 by mass%. 0.01 or less, S: 0.005 or less, Cu: 0.1 to 1.0, Ni: more than 1.0 to 3.5, Cr: 0.1 or less, Ti: 0.005 to 0.03, Al : 0.09 or less, N: 0.001 to 0.006, O: 0.02 to 0.06, the balance consisting of iron and inevitable impurities, and the Ni content of the base material is 2.0% or less. 0.2% less area
A weldable steel pipe having excellent weldability, characterized by having a weld metal portion in the range of a% higher region. 2. The base material component further contains Mo: 0.1 to 1.0, Nb: 0.005 to 0.1, V: 0.01 to 0.1, and B: 0.0003 to 100% by mass. The weatherable steel pipe excellent in weldability according to claim 1, wherein the steel pipe contains one or more of 0.002 and Ca: 0.001 to 0.005. 3. The weld metal portion further includes Mo: 0.1 to 1.0, Nb: 0.005 to 0.1, V: 0.01 to 0.1, and B: 0.0003 to 100% by mass. 3. Excellent weldability according to claim 1 or 2, characterized by containing one or more of 0.002, Ca: 0.001 to 0.005, and Mg: 0.0001 to 0.0050. Weathered steel pipe.