JP3303647B2 - Welded steel pipe with excellent sour resistance and carbon dioxide gas corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a hydrogen-induced hydrogen source which can be stably used for a long time in a line pipe for transporting crude oil or a pressure pipe for a petroleum refining unit exposed to a wet hydrogen sulfide environment or a wet carbon dioxide gas environment. The present invention relates to a welded steel pipe having excellent cracking resistance, sulfide stress cracking resistance, and carbon dioxide gas corrosion resistance.

[0002]

2. Description of the Related Art FIG. 1 shows the problems encountered in the environment encountered during the extraction, transportation and refining of crude oil or gas, and the conventional countermeasures that have been taken against the steel plate which is the base material of the welded steel pipe in order to address them. It is a summarized drawing. A conventional technique will be described with reference to FIG.

[0003] Hydrogen induced cracking (hereinafter referred to as HIC) occurs in line pipes for transporting crude oil or gas containing hydrogen sulfide, or piping used for tank towers for refining crude oil or gas containing hydrogen sulfide.
Cracking :) or sulfide stress cracking (hereinafter, SSC: Sulfide Stress Cr).
acking) is a problem.

[0004] HIC is a crack that occurs in a steel material without external stress, and SSC is a crack that occurs under a static stress. HIC and SSC are both cracks or damage caused by the penetration of hydrogen into the steel that occurs when the steel corrodes in a wet hydrogen sulfide environment (referred to as a sour environment). The combined performance of both the HIC resistance and the SSC resistance is called sour resistance.

The following methods are known as methods for suppressing HIC and SSC in a sour environment.

[0006] Suppression of intrusion of hydrogen into steel by addition of Cu.

[0007] Morphological control of sulfide by addition of Ca, ie, suppression of action as a starting point of sulfide cracking (Japanese Patent Publication No. Sho 60
-35982).

High HIC and SSC at the center segregation part
Reduction of segregation for reducing sensitivity, that is, reduction of Mn and P segregation at the center segregation part of the continuous casting slab by diffusion annealing.

[0009] Prevention of formation of a hardened structure by controlled rolling and accelerated cooling after rolling (Japanese Patent Publication No. 63-1369).

[0010] Mn reduction and Cr increase to reduce center segregation to increase strength and maintain HIC resistance
50967 and JP-B-3-68101).

While these sour resistances have been improved, the depletion of high-quality petroleum resources has led to the development of oil and gas fields in harsh environments. The demand for steel pipes that can withstand use in high pressure environments (again, also in sour environments) has increased.

[0012] Further, a steel having high resistance to carbon dioxide corrosion simultaneously with or independently of sour resistance has been desired. As a steel material having a high resistance to carbon dioxide gas corrosion that meets this demand, a low alloy steel with an increased Cr is disclosed (Japanese Patent Application Laid-Open Nos. 3-100711 and 4-341540). In particular, Japanese Patent Application Laid-Open No. H3-110071 discloses an invention of a welded steel pipe itself in which not only the Cr of the base material is restricted but also the Cr of the weld metal is at least a certain amount with respect to the Cr of the base material.

However, an increase in the amount of Cr suppresses corrosion in a wet carbon dioxide gas environment, but has an effect of promoting corrosion in a low pH wet hydrogen sulfide environment. In particular, the corrosion rate and local corrosion depth of the weld metal increase. Therefore, it is preferable to reduce Cr in a low pH hydrogen sulfide environment in order to suppress corrosion.

In the present description, the "quantity" of an alloy element means the "concentration" of the alloy element, and the two are used without distinction. Further, when the alloy element itself, for example, Cr, is used, it may mean the amount of Cr or the concentration of Cr.

JP-A-7-216500 discloses a low pH
X80 class strength (API (American Petroleum Institute) standard: Yield strength of the lower limit of the standard: 80 ksi = 55) that can be used in both sour and carbon dioxide environments including hydrogen sulfide environment
As a welded steel pipe base material covering 1 MPa), a ferrite-bainite two-phase structure steel composed of low C-low Mn-low Cr-trace Ti-N in N is presented. However, this is an invention for a steel plate of a steel pipe base material, not for a welded steel pipe itself.

A large-diameter pipe having a diameter exceeding 16 inches is generally a welded steel pipe, and a large-diameter steel pipe having a diameter exceeding 24 inches is welded by submerged arc welding (SAW). No technique is disclosed that simultaneously considers both the sour resistance and the carbon dioxide gas corrosion resistance of an entire SAW welded steel pipe in an environment including low pH hydrogen sulfide. In particular, there is no invention of a welded steel pipe having a sour resistance and a carbon dioxide corrosion resistance of X80 class or higher.

[0017]

DISCLOSURE OF THE INVENTION The present invention is directed to HIC resistance and SS resistance in a sour environment including low pH hydrogen sulfide.
It is an object of the present invention to provide a welded steel pipe having excellent C properties and also having carbon dioxide gas corrosion resistance, particularly an X80 class high strength welded steel pipe.

The specific goals of each performance are as follows. The welded portion refers to a weld metal, and the HAZ is treated as a base metal because its hardness and structure are substantially determined by the chemical composition of the base material. However, when referring to the “base material of the two-phase structure of ferrite and bainite”,
It refers to the base metal excluding the heat affected zone (HAZ: Heat Affected Zone). HAZ is subject to the heat cycle of welding,
This is because it is no longer a “two-phase structure of ferrite and bainite”.

(A-1) HIC resistance of base material: crack area ratio (CAR: crack area ratio) 2% or less.

Here, CAR = (HIC area) / (test piece width × test piece length).

(A-2) SSC resistance of base metal and welded part:
The “crack initiation limit stress σ _th, which is the maximum stress that does not generate SSC”, is set to 80% or more of the actual yield strength. The “actual yield strength” at the crack initiation critical stress refers to the actual yield strength of the base material.

(A-3) Carbon dioxide corrosion resistance of the base metal and the welded portion: The corrosion rate is set to 1/2 or less of that of the untreated material.

(A-4) Strength level: In particular, in the case of [Invention 3] and [Invention 4] described below, which limit the lower limit of yield strength, X80 class (yield strength of 551 MPa as welded steel pipe)
Above).

However, in the case of [Invention 1] and [Invention 2], the strength is not particularly limited.

[Invention 3] is the same as [Invention 1],
[Invention 4] is an example of the limited embodiment of [Invention 2].

[0026]

Means for solving the problems Conventional knowledge and means for solving the problems which the present inventors have newly confirmed this time are summarized as follows.

(A) In order to improve the σ _{th of the} SAW weld by lowering the corrosion rate of the SAW weld in a low pH hydrogen sulfide environment, the Cr content of the weld metal is set to a certain range or less.

FIG. 2 shows NA, which is a low pH hydrogen sulfide environment.
It is a drawing showing the influence of the amount of Cr of the base material on the corrosion rate of the base material in the CE TM0177 bath. It can be seen from the figure that the corrosion rate increases as the Cr content of the base material increases.

FIG. 3 is a drawing showing the influence of the Cr content and (Cu + Ni) content of the weld metal on the selective corrosion depth of the weld metal. It is clear that the selective corrosion depth increases with increasing Cr in the weld metal. Also, Cu
It can also be seen that the inclusion of Ni and Ni suppresses the selective corrosion of the weld. As described later, the SSC resistance is deteriorated by the increase in the selective corrosion depth. Therefore, the Cr of the weld metal must be low as long as the carbon dioxide corrosion resistance described below is not deteriorated.

(B) In order to prevent corrosion of the Cr-reduced weld metal described in (a) above in a carbon dioxide gas environment, appropriate amounts of Cu and Ni in the weld metal are increased.

FIG. 4 is a graph showing the effects of the amount of Cr in the base material and the contents of Cu and Ni on the corrosion rate of the base material in a wet carbon dioxide gas environment. According to the figure, when Cr of the base material is 0.2% or less and Cu and Ni are both zero, the corrosion rate becomes extremely high. On the other hand, even if the base material Cr
Is less than 0.2%, if the proper amount of Cu and Ni is contained, the corrosion rate is zero compared to the case where the base material Cr is zero and both Cu and Ni are zero. The corrosion rate can be reliably reduced to １ ／ or less.

FIG. 5 shows the effect of the selected corrosion depth of the weld metal in wet carbon dioxide gas [Cr content of the weld metal−C of the base metal.
[r amount]. The lower the Cr content of the weld metal compared to the base metal (the more the horizontal axis is on the minus side), the greater the selected corrosion depth, but when the weld metal contains appropriate amounts of Cu and Ni, the same as in a sour environment In addition, the selective corrosion depth is suppressed.

(C) When the above invention is applied to a high-strength steel of X80 class or higher, Mn is used to maintain the sour resistance of the base metal.
Rather, it reduces the center segregation of the continuously cast slab. A chemical composition that simultaneously satisfies sour resistance and carbon dioxide gas corrosion resistance by compensating for a decrease in strength due to a decrease in Mn with an appropriate amount of C. A two-phase structure of ferrite and bainite is obtained by applying a manufacturing method combining controlled rolling and accelerated cooling to the steel whose chemical composition has been adjusted. By simultaneously satisfying these chemical compositions and structures, it is possible to obtain a strength of X80 class or higher, and at the same time, to have sufficient sour resistance and carbon dioxide gas corrosion resistance.

(D) In the case of a high-strength steel, the hardenability of the steel is inevitably increased, so that the hardness of the HAZ increases and the SSC resistance deteriorates. Therefore, it is necessary to add a measure for reducing the HAZ hardness. is there.

For this purpose, Ti, more preferably T
Appropriate amounts of i and N are added to refine the HAZ structure and suppress the increase in hardness, so that σ _th of the welded portion is set to 80% or more of the actual yield strength.

FIG. 6 is a drawing summarizing the gist of the present invention in a base material and a weld metal of a welded steel pipe integrating these. In the figure, the measures for HAZ are listed as measures for the base material.

The low p used in experiments confirming these facts
The sour environment for H was so-called NACE TM0177 bath, which is currently the most severe test bath, and was [0.5% acetic acid + 5% saline solution at 25 ° C. saturated with 1 atm of hydrogen sulfide]. The criterion is "CA for HIC resistance.
R2% or less ”and“ SSC resistance is NACET
Those showing a crack initiation limit stress σ _th of 80% or more of the actual yield strength in the M0177 bath were accepted. The latter is conventionally known as a standard minimum yield strength (SMYS: Specified).
(Minimum Yield Strength). Usually, the strength of the steel pipe base material is lower than that of the welded portion, and therefore, the yield strength of the welded steel pipe base material is adopted as the actual yield strength. It goes without saying that the yield strength of the base material is higher than the specified minimum yield strength.

The present invention is a synthesis of the above-mentioned improvements, and has a gist of an X80 grade welded steel pipe having the following chemical composition and structure.

(1) C: 0.02 to 0.15% by weight
%, Si: 0.01 to 0.5%, Mn: 0.1 to 2%,
P: 0.015% or less, S: 0.002% or less, Cr:
0.2-1%, Cu: 0-0.5%, Ni: 0-0.7
%, Mo: 0 to 0.3%, Nb: 0 to 0.1%, V: 0
-0.1%, Ti: 0-0.05%, Al: 0.005
To 0.1% and Ca: 0.0005 to 0.005%, the base material having the chemical composition of the balance Fe and unavoidable impurities, and the base material having the following Cr (A) or ( A welded steel pipe comprising a weld metal in the range of b) and having excellent sour resistance and carbon dioxide gas corrosion resistance (referred to as [Invention 1]).

When Cr ^O is the Cr content of the base material and Cr ^W is the Cr content of the weld metal, (a) When Cr ^O is more than 0.4% and 1% or less: Cr ^W (%) ≦ (1) / 2) × Cr 2 ^O (%) (b) When Cr 2 ^O is 0.2% or more and 0.4% or less: Cr ^W (%) <Cr 2 ^O (%) − 0.2 (2) wt% C: 0.02 to 0.15%, Si:
0.01 to 0.5%, Mn: 0.1 to 2%, P: 0.0
15% or less, S: 0.002% or less, Cr: 0.2 to 1
%, Cu: 0 to 0.5%, Ni: 0 to 0.7%, Mo:
0 to 0.3%, Nb: 0 to 0.1%, V: 0 to 0.1
%, Ti: 0 to 0.05%, Al: 0.005 to 0.1
% And Ca: 0.0005 to 0.005%, and a base material having a chemical composition of balance Fe and unavoidable impurities;
Sour resistance and carbonic acid resistance, characterized in that the sum of Cr, Cu and Ni in the base metal is made of a weld metal whose sum of Cr, Cu and Ni is in the range of (a) or (b) below. A welded steel pipe excellent in gas corrosion (referred to as [Invention 2]).

Cr ^{O 2} , Cu ^O and Ni ^O are replaced with the base material C
r amount, the Cu content and Ni ^content, the Cr W, Cu ^W and Ni ^W, Cr content in the weld metal, when the Cu amount and Ni amount, (b) Cr ^O is less 1% greater than 0.4% ^{If: Cr W (%) ≦ (} 1/2) × Cr O (%) Cu O (%) + Ni O (%) + (1/10) × {Cr
^{O (%) - Cr W (} %)} ≦ Cu W (%) + Ni W (%) ≦ Cu O (%) + Ni O (%) + 0.5 ( b) Cr ^O is 0.2% or more 0.4 % the following ^{cases: Cr W (%) <Cr} O (%) - 0.2 Cu O (%) + Ni O (%) + (1/10) × {Cr
^O (%) ^- with ^{Cr W (%)} ≦ Cu} W (%) + Ni W (%) ≦ Cu O (%) + Ni O (%) + 0.5 (3) wt%, C: from 0.03 to 0 0.07%, Si:
0.01-0.5%, Mn: 0.7-1.3%, P:
0.015% or less, S: 0.002% or less, Cr: 0.
2-1%, Cu: 0-0.5%, Ni: 0-0.7%,
Mo: 0 to 0.3%, Nb: 0 to 0.1%, V: 0
0.1%, Ti: 0-0.05%, Al: 0.005-
0.1% and Ca: 0.0005 to 0.005%, with the balance being Fe and unavoidable impurities, and having a chemical composition of a two-phase structure of ferrite and bainite; The Cr is composed of a weld metal in the range of (A) or (B) below, and has a yield strength of 551 MPa.
A welded steel pipe excellent in sour resistance and carbon dioxide gas corrosion resistance characterized by the above (referred to as [Invention 3]).

When Cr ^O is the Cr content of the base material and Cr ^W is the Cr content of the weld metal, (a) When Cr ^O is more than 0.4% and 1% or less: Cr ^W (%) ≦ (1) / 2) × Cr 2 ^O (%) (b) When Cr 2 ^O is 0.2% or more and 0.4% or less: Cr ^W (%) <Cr 2 ^O (%) − 0.2 (4) wt% C: 0.03-0.07%, Si:
0.01-0.5%, Mn: 0.7-1.3%, P:
0.015% or less, S: 0.002% or less, Cr: 0.
2-1%, Cu: 0-0.5%, Ni: 0-0.7%,
Mo: 0 to 0.3%, Nb: 0 to 0.1%, V: 0
0.1%, Ti: 0-0.05%, Al: 0.005-
0.1% and Ca: 0.0005 to 0.005%, the balance has a chemical composition of Fe and unavoidable impurities, a base material composed of a two-phase structure of ferrite and bainite, and a base material of Cr and Cu. Cr and C to the sum of Ni
It is made of a weld metal having a sum of u and Ni in the following range (a) or (b) and has a yield strength of 551 MPa or more, and is excellent in sour resistance and carbon dioxide gas corrosion resistance. Welded steel pipe (referred to as [Invention 4]).

Cr ⁰ , Cu ^0, and Ni ⁰ are used as base material Cr
Amount, Cu amount and Ni amount, and Cr ^W , Cu ^W and N
When i ^W is the Cr amount, Cu amount and Ni amount of the weld metal, (a) When Cr ⁰ is more than 0.4% and 1% or less: Cr ^W (%) ≦ (1/2) × Cr ⁰ ( %) Cu
⁰ (%) + Ni ⁰ (%) + (1/10) × ｛Cr
⁰ (%)-Cr ^W (%)｝ ≦ Cu ^W (%) + Ni
^W (%) ≦ Cu ⁰ (%) + Ni ⁰ (%) + 0.5 (b) When Cr ⁰ is 0.2% or more and 0.4% or less: Cr ^W (%) <Cr ⁰ (%) − 0 .2 Cu ⁰ (%) + Ni ⁰ (%) + (1/10) × ΔCr
⁰ (%)-Cr ^W (%)｝ ≦ Cu ^W (%) + Ni
^W (%) ≦ Cu ⁰ (%) + Ni ⁰ (%) + 0.5 The above “weld metal” is not limited to so-called vertical seam welds (single seam and double seam) of large-diameter pipes. Alternatively, the present invention is also applied to a circumferential weld, which is a joint between these welded pipes. The “base material having a two-phase structure of ferrite and bainite” refers to a base material excluding HAZ.

FIG. 7 shows the present invention ([Invention 1], [Invention 2],
Cr of the weld metal in [Invention 3] and [Invention 4])
3 is a drawing showing a range of the amount (Cr ^W ) and the amount of Cr (Cr ⁰ ) of the base material. In the figure, the range 1 indicates the range of the present invention. The difference between the Cr content of the base metal and the Cr content of the weld metal, [Cr
⁰ (%) − Cr ^W (%)] is the maximum when the base material Cr is 1% and the weld metal Cr is zero, the maximum value is 1%, and the minimum value is the base value. Cr of the material is 0.
The minimum value is 0.2% when 4% and Cr of the weld metal is 0.2%.

It is desirable that other alloying elements of the weld metal other than Cr be within the range of the chemical composition set for the base metal except Ca and S. This is because the corrosion resistance and the like can be determined by the chemical composition regardless of the base metal and the weld metal. However, it is desirable that only the C amount is lower than that of the base material while being within the range set for the base material.

The amount of Ca that oxidizes during welding and remains in the weld metal is smaller than that of the welding material, and generally tends to be lower than that of the base metal. However, as will be described later, S in the weld metal may contain MnS even if MnS is formed.
As described above, since the degree of deterioration of HIC resistance is small because the HIC resistance is not expanded by rolling, the amount of Ca in the weld metal may be lower than the range set for the base metal.

[0047] For the same reason, S is also Mn in the weld metal.
Since S is not rolled even if formed, the degree of deterioration of HIC resistance is small, and may be larger than the range set for the base material.

FIG. 8 shows [(Cu ^W + Ni ^W ) (%)] and [(Cu ⁰ + N) in [Invention 2] and [Invention 4].
i ⁰ ) (%)]. The upper limit of the amount in which the sum of Cu and Ni in the weld metal is larger than that of the base metal is 0.5%
(Straight line 12 in FIG. 7). However, the lower limit is not determined unless both Cr of the weld metal and the base metal are determined. Based on the maximum value and the minimum value of the difference between Cr of the weld metal and the base metal described in FIG. 7, (1/10) × [Cr ⁰ (%) −
[Cr ^W (%)] has a maximum value of 0.1% and a minimum value of 0.02%. Therefore, the weld metal Cu and Ni
As shown in FIG. 8, the lower limit of the sum of
3 and the minimum is a straight line 14.

The alloying elements in the weld metal other than Cr and Cu + Ni are the same as in [Invention 1] and [Invention 3].

[0050]

BEST MODE FOR CARRYING OUT THE INVENTION

 1. Chemical composition of steel pipe base material In the following description, "%" indicates "% by weight".

C: In the case of [Invention 1] and [Invention 2],
C must be added at 0.02% or more to ensure strength. However, when the content exceeds 0.15%, center segregation is remarkable and the SSC resistance of the welded part is impaired.
15% or less. X8 of [Invention 3] and [Invention 4]
For high-strength steel pipes of class 0 or higher, it is necessary to limit C more narrowly. If the content is less than 0.03%, the required yield strength cannot be obtained even by making full use of the production method described later, but if it exceeds 0.07%, the hardness of the HAZ increases, and the HA is increased.
A pearlite band is formed in the vicinity of the outermost Z line to prevent S
Since the SC property decreases, the content is set to 0.03 to 0.07%.
In [Invention 3] and [Invention 4], in order to obtain a proper balance between appropriate strength and good SSC resistance, the content is further desirably 0.04 to 0.06%.

Si: Si is used as a deoxidizing agent at the time of steel making. If it is less than 0.01%, deoxidation is not sufficiently performed, and the yield of Al described below is lowered. Since it deteriorates, it is made 0.01 to 0.5%.

Mn: In [Invention 1] and [Invention 2], Mn is effective for obtaining high strength at low cost. 0.1
%, The required yield strength cannot be obtained.
%, Center segregation is remarkably generated and HAZ
Is significantly hardened and SSC resistance is impaired,
2%.

In [Invention 3] and [Invention 4], this M
The amount of n is further limited. 0.7% or more is required to secure the yield strength of the X80 class. However, if the content exceeds 1.3%, an abnormal structure (hardened structure) due to co-segregation of Mn and P occurs in the central segregation portion as described above, and the HIC resistance and the SSC resistance are deteriorated. To 1.3%.

P: P is preferably low. However, reducing P involves an increase in cost. Therefore, the P is set to a value below the range where performance is not significantly deteriorated. If it exceeds 0.015%, it segregates with Mn in the central part of the continuous casting slab with Mn, resulting in an abnormal structure and significantly deteriorating the HIC resistance.

S: S is desirably low. However, lowering the value of S also increases the cost, so that the value is set to an allowable range or less. If the content exceeds 0.002%, MnS is generated in the center segregation portion even if the shape control of the sulfide by Ca is performed, and the resistance to H
Since the IC property is impaired, the content is made 0.002% or less.

Cr: 0.2% or more is required in order to enhance the corrosion resistance to carbon dioxide gas and to secure the X80-class yield strength and obtain good SSC resistance. However, if it exceeds 1%, the SSC resistance of the base material deteriorates. In order to simultaneously secure a higher base material strength, for example, a strength of X80 class or higher, SSC resistance and carbon dioxide gas corrosion resistance, the content is preferably 0.4 to 0.7%.

Cu: Cu need not be added. However, Cu can enhance the carbon dioxide gas corrosion resistance, and at the same time, prevents the invasion of hydrogen, thereby improving the HIC resistance and the SSC resistance under a low pH hydrogen sulfide environment. If it is to be added, it is added. However, even if it is added, if it exceeds 0.5%, the surface of the continuous casting slab will be cracked and the product yield will be significantly reduced.

Ni: Ni may not be added. However, Ni enhances toughness and at the same time enhances carbon dioxide gas corrosion resistance. Therefore, Ni is added when these properties are particularly improved. However, if it exceeds 0.7%, the SSC resistance and the HIC resistance are deteriorated.
% Or less. Improved toughness, SSC resistance and H resistance
0.2 to 0.5% as a range that does not deteriorate the IC property
Is desirable.

Mo: Mo may not be added. However, by adding, improve the strength and toughness,
Further, in an environment having a low pH, such as a NACE TM0177 bath, synergistic action with Ni suppresses hydrogen intrusion and reduces HIC resistance.
It is added when these properties are to be further improved because the properties are improved. Even if it is added, if it exceeds 0.3%, the toughness and the SSC resistance of the welded part decrease.
% Or less. In order to ensure better strength and toughness and maintain the SSC resistance of the weld, 0.05 to 0.15
% Is desirable.

Nb: Nb may not be added. However, Nb improves the strength and toughness by grain refinement and precipitation of carbides, and improves SSC resistance by grain refinement. Therefore, Nb is added when further improving them. However, even in the case where it is added, if it exceeds 0.1%, the toughness is remarkably reduced rather than 0.1%. To maintain a better balance of strength and toughness, 0.02
It is desirable to set it to 0.05%.

V: V may not be added. But V
Improves the strength and toughness by grain refinement and carbide precipitation similarly to Nb, and improves the SSC resistance by grain refinement. Therefore, when these properties are further improved, they are added. However, even if it is added, if it exceeds 0.1%, the toughness is remarkably reduced. In order to obtain a more appropriate balance of strength and toughness, it is necessary to use 0.1%.
It is desirably set to 02 to 0.07%.

Al: Al is effective as a deoxidizing agent in steel making. If the content is less than 0.005%, deoxidation is not sufficiently performed, and pinholes are generated during solidification in continuous casting.
% Or more. However, if it exceeds 0.1%, the cleanliness and toughness of the steel deteriorate, so the content is made 0.005 to 0.1%. In order to suppress the generation of pinholes and obtain good toughness, the content is desirably 0.01 to 0.03%.

Ca: Ca is an effective element for controlling the morphology of sulfide inclusions, but if it is less than 0.0005%, MnS is formed by rolling and the HIC resistance is impaired. 0005% or more. However, 0.0
If it exceeds 0.05%, excessive Ca forms aggregates of oxides and degrades HIC resistance.
%. To obtain even higher HIC resistance, 0.00
It is desirable to set it to 1 to 0.003%.

Ti: In [Invention 1] and [Invention 2], Ti may not be added. However, since the HAZ hardness is lowered by bonding with N and precipitating TiN, the hardness of the HAZ of the high-strength steel pipe of X80 class which is the object of [Invention 3] and [Invention 4] is suppressed, and the SSC resistance is reduced. Therefore, in [Invention 3] and [Invention 4], the content must be 0.005% or more. 0.00
If it is less than 5%, a clear effect cannot be obtained. However, when the content exceeds 0.05%, the toughness of the base material and the welded portion is deteriorated regardless of the high strength and the low strength. [Invention 1] to [Invention 4]
In order to obtain sufficient SSC resistance while securing toughness, the content is preferably 0.01 to 0.02%.

N: N is not particularly limited.
When adding Ti, it is slightly higher, for example, 0.004 to
The content is set to 0.01%, and when Ti is not added, it is desirably slightly lower, for example, 0.002 to 0.006%. When Ti is added, T is used to soften the HAZ.
In order to actively generate iN, and when Ti is not added, HAZ hardly hardens in a low-strength steel, and toughness is improved by reducing N and decreasing the amount of solute N in HAZ. It is.

2. Structure of Base Material In [Invention 1] and [Invention 2], the structure of the base material is not particularly limited. In the case of [Invention 3] and [Invention 4], the structure of the base material is limited as follows. The structure of the base material of the high-strength welded steel pipe of X80 class or higher is “two-phase structure of ferrite and bainite”, that is, “structure in which ferrite and bainite are uniformly mixed”. This is because such a structure ensures excellent HIC resistance and SSC resistance of the base material. Such a structure cannot be obtained unless it is manufactured by the chemical composition of the base material and a manufacturing method described later. When the amount of alloying elements is increased to increase the strength, the “structure containing residual austenite enriched with C”, the “hard martensite structure in the center segregation part”, the “block-like bainite structure” or the “structure containing pearlite” are obtained. Although it is likely to occur, such a structure does not provide good HIC resistance and SSC resistance. In order to avoid such a structure, a steel plate as a base material of a welded steel pipe is manufactured by a manufacturing method described later. Here, the “block-like bainite structure” refers to bainite formed from austenite in which C is concentrated after proeutectoid ferrite has sufficiently grown from austenite grain boundaries.

3. Weld metal As described above, in a low pH hydrogen sulfide environment, the higher the Cr concentration, the higher the corrosion rate of both the base metal and the weld metal. In addition, regardless of the Cr concentration and the corrosive environment, the corrosion rate of the weld metal is generally higher than that of the base metal. Therefore, when the weld metal and the base material having the same Cr amount are exposed to low pH wet hydrogen sulfide, The weld metal is selectively corroded compared to the base metal. Such an imbalance of corrosion is caused by the SSC resistance of the weld metal.
Therefore, the following components are limited for the weld metal (see FIGS. 6 to 8).

Cr: (a) When the Cr content (Cr ⁰ ) of the base metal is in the range of more than 0.4% and 1% or less, the Cr content (Cr ^W ) of the weld metal is [(1/2) × Cr ⁰ (%)] or less. When the Cr content of the weld metal exceeds [(1/2) × Cr ⁰ (%)] and approaches or equals the Cr content of the base material, or exceeds the Cr content of the base material, FIGS. As shown in (1), the corrosion of the base metal and the weld metal under a low pH hydrogen sulfide environment increases.
As a result, when the corrosion rate of the weld metal and the base metal becomes unbalanced, the SSC resistance of the welded portion is deteriorated, and the crack initiation limit stress σ _{th of} the welded portion does not become 80% or more of the actual yield strength.

In order for the weld metal to have good corrosion resistance and SSC resistance in a low pH hydrogen sulfide environment and at the same time to have good carbon dioxide gas corrosion resistance, the Cr content (Cr ^W ) of the weld metal must be as described above. It is desirable to make the height as high as possible within the range. For example, the amount of Cr (Cr ⁰ ) of the base material is set to 0.4 to
When the content is 0.7%, the Cr content (Cr ^W ) of the weld metal is
After satisfying [(1/2) × Cr ⁰ (%)] or less, it is desirable to be as high as possible, that is, 0.15 to 0.3%.

(B) Cr content of base material (Cr ⁰ ) is 0.2%
When it is in the range of not less than 0.4% and not more than 0.4%, the Cr content (Cr ^W ) of the weld metal is less than [Cr ⁰ (%)-0.2 (%)]. The Cr content (Cr ^W ) of the weld metal is [Cr ⁰ (%)
-0.2 (%)] or more, and approaching or equal to or exceeding the Cr content of the base material, the absolute value of the corrosion rate in a low pH hydrogen sulfide environment is small, Thus, an imbalance in the corrosion rate between the weld metal and the base metal occurs, which degrades the SSC resistance of the weld metal. Further, as described above, in order to simultaneously provide carbon dioxide corrosion resistance, the Cr content (Cr ^W ) of the weld metal is [Cr ⁰ (%) −
0.2 (%)], and preferably as high as possible.

In [Invention 1] and [Invention 3],
Only the Cr content (Cr ^W ) of the above weld metal is limited. C
The elements of the weld metal other than r, except for Ca and S, are preferably the same as the chemical composition ranges set for the base material in both [Invention 1] and [Invention 3]. However, the C content of the weld metal is preferably set to be lower than that of the base metal, while satisfying the chemical composition range of the base metal. This is because, in the rapidly solidified as-welded state, if the C content is the same as that of the base metal, the hardness becomes too high and the SSC resistance is deteriorated.

The Ca of the weld metal may be lower than the range set for the base metal for the reason described above, and the S of the weld metal may be higher than the range of S set for the base metal.

In [Invention 2] and [Invention 4],
In addition to the above limitation on Cr, the following limitation is added on Cu and Ni in order to further improve the carbon dioxide corrosion resistance of the weld metal.

Cu and Ni: Cu and Ni are shown in FIG.
As shown in (1), the carbon dioxide gas corrosion resistance is improved, and in a wet hydrogen sulfide environment having a high pH, hydrogen intrusion is suppressed and the HIC resistance is also improved. Further, increasing Cu and Ni of the weld metal as compared with the base metal has an effect of simultaneously improving SSC resistance in a low pH hydrogen sulfide environment.

In [Invention 2] and [Invention 4],
Since an appropriate amount of Cu and Ni in the weld metal can be increased to improve the carbon dioxide corrosion resistance, there is also an advantage that the Cr of the weld metal can be sufficiently reduced in order to improve the sour resistance under a low pH hydrogen sulfide environment. .

The sum of Cu and Ni of the weld metal (Cu ^W + Ni
^W ) is [(1/1) than that of the base material (Cu ⁰ + Ni ⁰ ).
10) × {Cr ⁰ (%) − Cr ^W (%)}] or more. If the amount is less than this, the carbon dioxide corrosion resistance of the weld metal is not sufficient, and the weld metal undergoes selective corrosion and the SSC resistance of the weld metal deteriorates. As described above, this amount cannot be determined unless both the Cr of the weld metal and the base metal are determined, but the maximum value is 0.1% (the straight line 13 in FIG. 8) and the minimum value is 0.02%. (Line 14 in FIG. 8).
However, the sum of Cu and Ni of the weld metal (Cu ^W + Ni ^W )
Is 0.5% higher than that of the base material (Cu ⁰ + Ni ⁰ ) (FIG. 8).
When the amount exceeds the straight line 12), not only the carbon dioxide gas corrosion resistance is saturated, but also the SCC resistance is reduced, so the amount of the base material exceeding 0.5% is set to 0.5% or less.

In [Invention 2] or [Invention 4],
In addition to the limitation of Cr of [Invention 1] or [Invention 3], respectively, the limitation of Cu and Ni is performed. Other elements are the same as [Invention 1] or [Invention 3].

4. In the following, in [Invention 3] and [Invention 4],
The structure of the steel sheet, which is the material of the welded steel pipe, is referred to as a “two-phase structure of ferrite and bainite”, that is, a “structure in which ferrite and bainite are uniformly mixed”, and has a yield strength of 551 MPa.
A method for securing the above will be described.

(1) Rolling The slab heating temperature is set to 1,050 to 1,250 ° C. 105
If the temperature is less than 0 ° C., the carbide does not form a solid solution and remains coarse, so that the strength does not increase and it is difficult to secure a predetermined rolling temperature. On the other hand, when the temperature exceeds 1250 ° C., the structure becomes coarse and toughness cannot be secured.

At the same time, the SSC resistance also deteriorates.
50 ° C. or less. In order to obtain high strength with a fine structure, the temperature is desirably 1100 to 1200 ° C.

Rolling of the slab to a steel sheet, which is a material of the welded steel pipe, is performed at a reduction rate of 950 ° C. or less and a reduction rate of 50% or more (reduction rate =
[(Slab thickness-steel plate thickness) / slab thickness]). If the rolling reduction at 950 ° C. or less is less than 50%,
Austenite grains cannot be refined by recrystallization, so that fine ferrite grains cannot be obtained.
SC property becomes insufficient. In order to further improve the SSC resistance, it is desirable that the rolling reduction at 950 ° C. or less is 60% or more.

The finishing temperature of the rolling is set to _three or more Ar points.
If the finishing temperature is lower than the Ar ₃ point, the accelerated cooling starts from the two-phase region of ferrite and austenite in which a large amount of pro-eutectoid ferrite is formed, and “C-enriched residual austenite structure” and “central segregation part” The structure becomes "hard martensite structure" or "block-like bainite structure", and the sour resistance decreases. In order to further suppress such a hardened structure, it is desirable to finish with [high temperature of 30 ° C. or more from the Ar ₃ point].

(2) Accelerated Cooling The accelerated cooling is started at a temperature lower than the Ar ₃ point by 30 ° C. or higher. When cooling is started from a temperature lower than [the temperature lower by 30 ° C. from the Ar ₃ point], cooling takes place from the two-phase region of ferrite and austenite in which a large amount of pro-eutectoid ferrite is formed, and “C-enriched residual austenite structure” The center segregated portion becomes a hard martensite structure or a block-like bainite structure, and the sour resistance is reduced. In order to further suppress the generation of such a structure even in the center segregation portion, it is desirable that the accelerated cooling start temperature be set to the Ar ₃ point or higher. The upper limit of the accelerated cooling start temperature is the rolling finish temperature.

The accelerated cooling stop temperature is 400 ° C. or more and 550
It should be below ° C. If the temperature exceeds 550 ° C., untransformed austenite remains during accelerated cooling, so that C is concentrated in the segregated portion and transforms into pearlite or the like that impairs the SSC resistance of the base material, so that the HIC resistance decreases. If the temperature is lower than 400 ° C., hardened block bainite is likely to be generated, and the sour resistance of the base material is reduced. In order to completely avoid the hardened structure, the temperature is desirably 450 ° C. or more and 500 ° C. or less.

After the accelerated cooling is stopped, it is allowed to cool or cool slowly. After this, tempering may or may not be performed.

The steel plate as the base material of the welded steel pipe manufactured under the above conditions may be a thick steel plate or a hot-rolled steel plate, that is, a so-called hot coil. In the case of a hot coil, the coil is wound after the accelerated cooling is stopped, and is gradually cooled in the state of the coil, and is usually not tempered. Hot coil is narrower than thick steel plate, so when processing and welding to large diameter steel pipe,
Spiral welded steel pipe or double seam welded steel pipe is often used.

5. Pipe Making The steel sheet manufactured by the above method is processed into a tubular shape, and then is made into a welded steel pipe by SAW. The pipe making method is to apply U press and O press to a thick steel plate for processing.
O method, spiral method of welding hot-rolled steel sheets in a spiral shape, and processing of narrow-width hot-rolled steel sheets or thick steel sheets
Any of the double seam method in which a steel pipe having two vertical seams is used.

FIG. 9A is a view showing a shape of a groove before welding, and FIG. 9B is a view showing a vertical section of a welding line after welding. In the figure, it can be seen that the weld metal 21 is constituted by the base metal melted during welding and the part transferred from the welding material near the groove. The transition from the welding material to the welding metal is particularly called "weld metal". HAZ2
In No. 2, the chemical composition is the same as that of the base material 23 except for a special element such as hydrogen, which is affected by heat due to welding.

In order to make the Cr of the weld metal lower than that of the base metal, it is necessary to use a welding material having a Cr concentration suitable for it. In SAW, the contribution of the base metal is 40% to 60%, and the contribution of the "weld metal" is 60% to 40%. Therefore, [Cr concentration of weld metal (%)] = [Cr concentration of base metal (%)] × (0.4 to 0.6) + [Cr concentration of weld material (%)] × (0.6 In the formula (A), if the Cr concentration of the base metal and the weld metal is set, the Cr concentration of the welding material to be used can be calculated and used. Welding material can be selected. That is,
Since the specific gravities of the base metal and the welding material may be considered to be the same, 40 to 60% of the weld metal is shifted from the base material, and 60 to 40% of the weld metal is "weld metal". Cr or Cu and Ni described later do not disappear into the atmosphere during welding.

To increase Cu and Ni in the weld metal,
40-60% of both Cu and Ni are from the base metal, and 6-40%
Assuming that the ratio shifts from the welding material, the amount of Cu and the amount of Ni of the welding material to be used are calculated by the same method, and a welding material having an appropriate standard can be selected.

The yield strength of the welded steel pipe refers to the yield strength of the portion including the welded portion. Usually, the welding material is selected so that the welded portion has a strength higher than that of the base material. The yield strength is adopted as the yield strength of the welded steel pipe. [Invention 3]
And [Invention 4], the yield strength is 551 MPa (AP
(Specific minimum yield strength of I standard X80).

It is desirable to increase the heat input of welding from the viewpoint of the welding efficiency and also from the viewpoint of reducing the hardness of the HAZ. However, if the heat input is too large, the toughness of the HAZ deteriorates.
It is good to be 150 kJ / cm.

The above description is for the case of seam welding of a welded steel pipe in the case of SAW. When using welded steel pipes, circumferential welding for welding the welded steel pipes must be performed. Circumferential welding is usually performed by gas metal arc welding (GMA
W). GMAW usually has a heat input of 15-50k
Since the melting is performed at J / cm, the melting of the base material is small. Therefore, in the above equation (A), the contribution of the base material is 0.3 to 0.5 (S
0.4 to 0.6 for AW), contribution of welding material is 0.7 to
It is calculated as 0.5 (0.6 to 0.4 in SAW).

[0095]

EXAMPLES Tables 1 to 4 are lists of the chemical compositions of the base materials of the welded steel pipes classified to show the effects of the implementation of [Invention 1] to [Invention 4]. That is, for example, Table 1
Is a list of welded steel pipe preforms used to show the effects of the [Invention 1].

Table 5 is a table showing the rolling and heat treatment conditions for producing these welded steel pipe base materials. The steel I of the comparative example falls within the range of [Invention 3] with respect to the chemical composition of the base material, but has a two-phase structure of ferrite and martensite, and thus has a base material structure outside the range of [Invention 3]. Is the base material. Further, although the structure of steels A to E is not a two-phase structure of ferrite and bainite, the structure is not particularly limited in [Invention 1], and therefore the structure is not out of the range of [Invention 1].

[0097]

[Table 1]

[0098]

[Table 2]

[0099]

[Table 3]

[0100]

[Table 4]

[0101]

[Table 5]

Tables 6 to 9 show the composition of the welding wire used for longitudinal seam SAW of the edge portion of the above-mentioned welded steel pipe base material that was abutted through the U press and the O press.
5 is a list showing the composition of the weld metal formed as a result of the above. Tables 6 to 9 correspond to weld metals for describing the effects of the implementation of [Invention 1] to [Invention 4], respectively. In these tables, only the alloy elements of the weld metal limited in each invention are shown, but the other alloy elements are almost the same as the corresponding steel (base metal) of each trial number. SAW forming a vertical seam has a welding heat input of 40k
J / cm single-sided, single-sided welding. The dimensions of the manufactured welded steel pipe are 36 inches in diameter (914.4 mm) x 1 inch in wall thickness (25.4 mm).
It is. In the columns of “weld metal-base metal” in Tables 6 to 9,
The difference between the composition of the weld metal and the base metal for Cr and Cu + Ni is shown.

[0103]

[Table 6]

[0104]

[Table 7]

[0105]

[Table 8]

[0106]

[Table 9]

FIG. 10A shows a test piece used for evaluating SSC resistance, and FIG. 10B shows a sampling position. As shown in the figure, the shape of the test piece was a round bar, and the test piece was sampled from the inner surface of the tube so that the weld metal portion was located at the center of the parallel portion of the test piece. The applied stress is the actual material Y
80% of S (yield strength). Test solution is NACET
M0177-90 (Test Method by National Associat
ion of Corrosion Engineering)
A TM0177 bath (0.5% acetic acid + 5% saline, 1 atm hydrogen sulfide saturation, 25 ° C.) was used. Those which did not break during the test period of 720 h were regarded as having good SSC resistance.

FIG. 11A shows a test piece used for evaluating corrosion resistance, and FIG. 11B shows a sampling position. The test piece was a plate-shaped test piece having a thickness of 3 mm, a width of 10 mm, and a length of 40 mm. Similar to the SSC-resistant test piece, a test piece was taken from the inner surface of the pipe so that the weld metal was located at the center from the welded portion. The evaluation was made based on the overall corrosion rate of the base material and the selected corrosion depth of the weld. The overall corrosion rate of the base material portion was obtained by converting the corrosion loss after descaling into a unit area per unit time. The selected corrosion depth of the weld was determined from the thickness difference between the weld metal and the base metal using a micrometer for measuring the pit depth after descaling. If the pit depth of the base metal is deeper than that of the weld metal,
In Tables 10 to 13 described below, the values are shown as minus (-) values. Test solution is NACE as hydrogen sulfide environment
The TM0177 bath was set to 1 atmosphere CO ₂ as a carbon dioxide gas environment.
And two types of artificial seawater at 50 ° C. saturated with In addition,
The test solution was constantly stirred during the 96 h test period.

Tables 10 to 13 are lists showing the results of the evaluation of the SSC resistance and corrosion resistance described above. Table 10 to Table 1
No. 3 explains the effect of the implementation of [Invention 1] to [Invention 4]. The steel pipe strengths described in these tables were obtained by a tensile test of a welded steel pipe base material. The stress value of the yield strength “551 MPa” is “79.9 ks
i ".

[0110]

[Table 10]

[0111]

[Table 11]

[0112]

[Table 12]

[0113]

[Table 13]

Test No. 9 (Table 10) and Test No. 20 to 22 using Steel E (Table 1), Steels I to K (Table 3) and Steel N (Table 4)
(Table 12) and test number 30 (Table 13) show the S
SC property was not good. Steel E and steel N have a Cr content of the base material of more than 1%, steel I for high-strength welded steel pipe is a ferritic-martensitic two-phase structure steel (see Table 5), and steel J is a base metal. This is because the amount of C is too high, and the amount of Mn in the base material of steel K is too high.

Test Nos. 7 and 8 using steels C and D (Table 1) and steel G (Table 2) in which the Cr content of the base material is less than the range of the present invention.
Table 10 and Test No. 13 (Table 11) show that the SSC resistance and the corrosion resistance in the NACE TM0177 bath are good, but the overall corrosion rate of the base material in the CO ₂ environment is 4%.
mm / year or more, significantly higher than other examples.

On the other hand, when the amount of Cr in the base material is 0.2 to 1
% Steels A, B (Table 1), steel F (Table 2) within the scope of the invention
No. 1 to 6 using steel H (Table 3) to steel M (Table 4)
(Table 10), 10 to 12 (Table 11) and 14 (Table 1)
2) to 29 (Table 13) show that the higher the Cr content in carbon dioxide gas, the smaller the overall corrosion rate is, that is, 2 mm / year or less. In particular, trial numbers 4 to 6, 10 to 12, and 14 to using steels B, F, and H to M having a Cr content of about 0.5% or more.
No. 29 has a good overall corrosion rate of about 1 mm / year or less. Further, regardless of the amount of Cr, steel F containing Cu and Ni,
Test numbers 10 to 12, 23 to 28, and 29 using L and M are test numbers 1 to 6 using steels A, B, and H to K containing neither Cu nor Ni as shown in FIG. And 1
Corrosion rate is lower than 4 to 22.

In the weld metal, the amount of Cr is the same as that of the base metal.
番 of the amount or less than (Cr amount of base material−0.2)%, and at the same time, Cr of base material is 0.2 to 1%. 12 (Table 7, Table 11), 14-2
2 (Table 8, Table 12) and 23 to 29 (Table 9, Table 13)
Are sample numbers 2, 5 (Tables 6 and 10) in which the (Cu + Ni) amount of the weld metal is 0.1 to 0.5% higher than that of the base metal portion.
0 (Table 7, Table 11), 16, 17 (Table 8, Table 12), 2
In 3, 24 and 29 (Tables 9 and 13), the selected corrosion depth of the weld is less than 0.1 mm in a CO ₂ environment,
In a NACE TM0177 bath, it can be suppressed to 0.11 mm or less. On the other hand, samples containing neither Cu nor Ni, such as test numbers 1, 4 (Table 6, Table 10), 14, 15 (Table 8, Table 12) and 20 to 22 (Table 9, Table 12) is selected corrosion depth of weld is suppressed below 0.10mm in NACE TM0177 bath, CO ₂
More than 0.1 mm in the environment.

It is better to make the Cr amounts of the base metal and the weld metal uniform.
It is easy to suppress selective corrosion in CO ₂ environment, but Cu and Ni
It is also effective to contain an appropriate amount of. Therefore, considering the sour environment, Cu and Ni
It is necessary to increase the sum of

In the NACE TM0177 bath,
The higher the Cr of the base material, the greater the overall corrosion of the base material. However, those containing Cu and Ni have a low corrosion rate similarly to the tendency shown in FIG.

In addition, test numbers 3, 6 (Tables 6 and 10), 12
(Tables 7, 11), 18 (Tables 8, 12) and 27
As shown in Tables 9 and 13, in the NACE TM0177 bath, the higher the amount of Cr in the weld metal, the greater the selective corrosion depth. This is considered to be one of the reasons why the SSC resistance of the weld is not good. Also, test numbers 11 and 12 (Tables 7 and 11)
Also, as shown in Tables 26 and 27 (Tables 9 and 13), those having a low (Cu + Ni) content of the weld metal and falling outside the range of the present invention ([Invention 2] or [Invention 4]) also have SSC resistance of the welded portion.
Sex is not good. Conversely, test numbers 19 (Table 8, Table 12) and 28 (Table 9, Table 13) show that the amount of (Cu + Ni) in the weld metal is more than 0.5% higher than that of the base metal, so that the SSC
Rupture. Test numbers 2, 5 (Table 6, Table 10), 1
0 (Table 7, Table 11), 16, 17 (Table 8, Table 12), 2
3 to 25 (Table 9, Table 13) and 29 (Table 9, Table 13)
In the case where the amount of (Cu + Ni) of the weld metal is higher than that of the base metal, the selective corrosion is suppressed. Therefore, from the viewpoint of SSC resistance, the (Cu + Ni) amount of the weld metal is
It must be higher than the base material within a range of 0.5% or less.

[0121]

Industrial Applicability The present invention provides a welded steel pipe excellent in all of HIC resistance, SSC resistance and carbon dioxide corrosion resistance, particularly a high-strength welded steel pipe of API standard X80 class. It is extremely useful for natural gas related industries.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a drawing summarizing the problems encountered in the environment encountered during the extraction, transportation and refining of crude oil or gas, and the conventional countermeasures taken against the steel plate as a welded steel pipe base material in order to deal with them. is there.

FIG. 2 shows a low pH hydrogen sulfide environment, NACE.
It is a drawing showing the influence of the amount of Cr (Cr ⁰ ) of the base material on the corrosion rate of the base material in the TM0177 bath.

FIG. 3 is a drawing showing the effect of Cr ^W on the selective corrosion depth in a weld metal.

FIG. 4 is a drawing showing the influence of Cr ⁰ , Cu ⁰ and Ni ⁰ on the corrosion rate of a base material in a wet carbon dioxide gas environment.

FIG. 5 is a drawing showing the effect of (Cr ^W −Cr ⁰ ) on the selective corrosion depth of the weld metal in wet carbon dioxide gas.

FIG. 6 is a drawing summarizing the gist of the present invention in a base material and a weld metal of a welded steel pipe.

FIG. 7 is a graph showing the Cr content (Cr ^W ) of the weld metal and the Cr content (Cr) of the base metal in the present invention ([Invention 1] to [Invention 4]).
It is a drawing showing the range of ⁰ ).

FIG. 8 shows the range of the sum of Cu and Ni of the weld metal (Cu ^W + Ni ^W ) and the sum of Cu and Ni of the base material (Cu ⁰ + Ni ⁰ ) in [Invention 2] and [Invention 4]. FIG.

FIG. 9 (a) shows the shape of a groove before welding;
FIG. 9B is a drawing showing a vertical cross section of a welding line after welding.

FIG. 10 (a) is a drawing showing the shape of an SSC test piece, and FIG. 10 (b) is a drawing showing the sampling position of the same test piece.

11 (a) is a drawing showing a shape of a corrosion test piece, and FIG. 11 (b) is a drawing showing a sampling position of the test piece.

[Description of Signs] 1 ... Range of Cr content of weld metal and base metal of welded steel pipe of the present invention, 2 ... Straight line expressing relationship of Cr ^W (%) = (1/2) × Cr ⁰ (%) 3 ... A straight line (dotted line) representing the relationship of Cr ^W (%) = Cr ^W (%) − 0.2 4 ... A straight line representing Cr ⁰ (%) = 1 5 ... Cr ^W (%) = Cr ⁰ (%) Straight line (dashed-dotted line) 11: range of the sum of Cu and Ni of the weld metal and the base metal of the welded steel pipe of the present invention (that is, Cu ^W (%) + Ni)
^W (%) and Cu ⁰ (%) + Ni ⁰ (%) range 12... Cu ^W (%) + Ni ^W (%) = Cu ⁰ (%) + N
a straight line representing i ⁰ (%) + 0.5 13... Cu ^W (%) + Ni ^W (%) = Cu ⁰ (%) + N
i ⁰ (%) + (1/10) × ｛(Cr ⁰ (%) − Cr ^W
(%)) 最大 = Cu ⁰ (%) + Ni ⁰ (%) +
Straight line (dotted line) representing 0.1 14 ... Cu ^W (%) + Ni ^W (%) = Cu ⁰ (%) + N
i ⁰ (%) + (1/10) × ｛(Cr ⁰ (%) − Cr ^W
(%)) = Cu ⁰ (%) + Ni ⁰ (%) +
Straight line (broken line) representing 0.02 15 Cu ^W (%) + Ni ^W (%) = Cu ⁰ (%) + N
Straight line (dot-dash line) representing i ⁰ (%) 21: weld metal, 22: heat affected zone (HAZ), 23: base material (steel plate)

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-110071 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] (1) C: 0.02 to 0.15% by weight, S
i: 0.01 to 0.5%, Mn: 0.1 to 2%, P:
0.015% or less, S: 0.002% or less, Cr: 0.
2-1%, Cu: 0-0.5%, Ni: 0-0.7%,
Mo: 0 to 0.3%, Nb: 0 to 0.1%, V: 0
0.1%, Ti: 0-0.05%, Al: 0.005-
0.1% and Ca: 0.0005 to 0.005%, the base material having the chemical composition of the balance Fe and unavoidable impurities, and the base material having the following Cr (a) or (b) A welded steel pipe excellent in sour resistance and carbon dioxide gas corrosion resistance, comprising a weld metal in the range of (1). Cr ^O is defined as the amount of Cr in the base metal, and Cr ^W is defined as C in the weld metal.
When the r volume, (b) Cr if ^O is less than 1% greater than ^{0.4%: Cr W (%)} ≦ (1/2) × Cr O (%) ( b) Cr ^O is 0.2% In the case of not less than 0.4% or less: Cr ^W (%) <Cr ^O (%) − 0.2 2. C: 0.02 to 0.15% by weight, S
i: 0.01 to 0.5%, Mn: 0.1 to 2%, P:
0.015% or less, S: 0.002% or less, Cr: 0.
2-1%, Cu: 0-0.5%, Ni: 0-0.7%,
Mo: 0 to 0.3%, Nb: 0 to 0.1%, V: 0
0.1%, Ti: 0-0.05%, Al: 0.005-
A base material containing 0.1% and Ca: 0.0005 to 0.005% and having a chemical composition of the balance Fe and unavoidable impurities; and a C of the base material with respect to the sum of Cr and Cu and Ni.
A welded steel pipe excellent in sour resistance and carbon dioxide gas corrosion resistance, characterized by being composed of a weld metal in which the sum of r, Cu and Ni is in the following range (a) or (b). Cr ^O, C
The u ^O and Ni ^O, Cr amount of the base material, Cu amount and Ni
And Cr ^W , Cu ^W and Ni ^W are the
When the r weight, Cu content and Ni content, if (i) Cr ^O is 0.4% more than 1% or ^{less: Cr W (%) ≦ (} 1/2) × Cr O (%) Cu O (% ) + Ni 2 ^O (%) + (1/10) × ΔCr
^{O (%) - Cr W (} %)} ≦ Cu W (%) + Ni W (%) ≦ Cu O (%) + Ni O (%) + 0.5 ( b) Cr ^O is 0.2% or more 0.4 % the following ^{cases: Cr W (%) <Cr} O (%) - 0.2 Cu O (%) + Ni O (%) + (1/10) × {Cr
^{O (%) - Cr W (} %)} ≦ Cu W (%) + Ni W (%) ≦ Cu O (%) + Ni O (%) + 0.5 3. C: 0.03 to 0.07% by weight, S
i: 0.01 to 0.5%, Mn: 0.7 to 1.3%,
P: 0.015% or less, S: 0.002% or less, Cr:
0.2-1%, Cu: 0-0.5%, Ni: 0-0.7
%, Mo: 0 to 0.3%, Nb: 0 to 0.1%, V: 0
-0.1%, Ti: 0-0.05%, Al: 0.005
~ 0.1% and Ca: 0.0005-0.005%, with the balance being Fe and unavoidable impurities,
A base material having a two-phase structure of ferrite and bainite, and the base material Cr has the following Cr (a) or (b)
And a yield strength of 551MP
a. A welded steel pipe excellent in sour resistance and carbon dioxide gas corrosion resistance, which is not less than a. When Cr ^O is the Cr content of the base material and Cr ^W is the Cr content of the weld metal, (a) When Cr ^O is more than 0.4% and 1% or less: Cr ^W (%) ≦ (1/2) × ^Cr O (%) (b) Cr if ^O is less than 0.4% or more ^{0.2%: Cr W (%)} <Cr O (%) - 0.2 4. C: 0.03 to 0.07% by weight, S
i: 0.01 to 0.5%, Mn: 0.7 to 1.3%,
P: 0.015% or less, S: 0.002% or less, Cr:
0.2-1%, Cu: 0-0.5%, Ni: 0-0.7
%, Mo: 0 to 0.3%, Nb: 0 to 0.1%, V: 0
-0.1%, Ti: 0-0.05%, Al: 0.005
~ 0.1% and Ca: 0.0005-0.005%, with the balance being Fe and unavoidable impurities,
A base metal having a two-phase structure of ferrite and bainite, and a weld metal in which the sum of Cr, Cu, and Ni is in the following range (a) or (b) with respect to the sum of Cr, Cu, and Ni in the base material A welded steel pipe excellent in sour resistance and carbon dioxide gas corrosion resistance, characterized by having a yield strength of 551 MPa or more. When Cr ^O , Cu ^O and Ni ^O are the amounts of Cr, Cu and Ni in the base material, and Cr ^W , Cu ^W and Ni ^W are the amounts of Cr, Cu and Ni in the weld metal, ) Cr if ^O is less than 1% greater than ^{0.4%: Cr W (%)} ≦ (1/2) × Cr O (%) Cu O (%) + Ni O (%) + (1/10) × { Cr
^{O (%) - Cr W (} %)} ≦ Cu W (%) + Ni W (%) ≦ Cu O (%) + Ni O (%) + 0.5 ( b) Cr ^O is 0.2% or more 0.4 % the following ^{cases: Cr W (%) <Cr} O (%) - 0.2 Cu O (%) + Ni O (%) + (1/10) × {Cr
^{O (%) - Cr W (} %)} ≦ Cu W (%) + Ni W (%) ≦ Cu O (%) + Ni O (%) + 0.5