JPH09194991A - Welded steel tube excellent in sour resistance and carbon dioxide gas corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an API standard X80 class high strength steel tube. SOLUTION: This welded steel tube is composed of a base material, having a chemical composition consisting of, by weight, 0.02-0.15% C, 0.01-0.5% Si, 0.1-2% Mn, 0.2-1% Cr, 0.005-0.1% Al, 0.0005-0.005% Ca, <=0.015% P, <=0.002% S, <=0.5% Cu, <=0.7% Ni, <=0.3% Mo, <=0.1% Nb, <=0.1% V, <=0.05% Ti, and the balance Fe with inevitable impurities, and a weld metal. At this time, Cr content in the weld metal satisfies, in relation with Cr content in the base material, either of the following (a) and (b): (a) in the case where Cr<0> is >0.4 to 1, CrW(%)<=(1/2)×Cr<0> (%); (b) in the case where Cr<0> is 0.2 to 0.4%, CrW (%)<(1/2)×Cr<0> (%)-0.2, where Cr<0> and CrW represent Cr content in the base material and Cr content in the weld metal, respectively.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to hydrogen resistance induction that can be stably used for a long period of time in crude oil transportation line pipes exposed to wet hydrogen sulfide environments and wet carbon dioxide gas environments, pressure pipes for petroleum refining equipment, and the like. 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 measures taken for the steel sheet which is the base material of the welded steel pipe in order to cope with them. It is the summarized drawing. The conventional technique will be described with reference to FIG.

Hydrogen-induced cracking (hereinafter referred to as HIC: Hydrogen Induced) is used in line pipes for transporting crude oil or gas containing hydrogen sulfide or pipes used for tank towers for purifying crude oil or gas containing hydrogen sulfide.
Cracking: or sulfide stress cracking (hereinafter, SSC: Sulfide Stress Cr)
acking :) becomes a problem.

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

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

Suppression of hydrogen penetration into steel by addition of Cu.

-Control of sulfide morphology by addition of Ca, that is, suppression of action as a starting point of sulfide cracking (Japanese Patent Publication No. 60)
-35982).

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

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

To reduce the center segregation and increase the amount of Cr and increase the amount of Cr to increase the strength and maintain the HIC resistance (Japanese Patent Publication No.
50967 and Japanese Patent Publication No. 3-68101).

While these sour resistances have been improved, the development of oil fields and gas fields in harsh environments has been promoted with the exhaustion of high-quality petroleum resources. This has led to an increasing demand for steel pipes that can withstand use in high-pressure environments (again, so-called sour environments).

Further, it has been desired to develop a steel having a high resistance to carbon dioxide corrosion at the same time as the sour resistance or regardless of the sour resistance. As a steel material having high resistance to carbon dioxide gas corrosion in response to this demand, low alloy steel containing higher Cr has been disclosed (JP-A-3-110071 and JP-A-4-341540). In particular, Japanese Patent Application Laid-Open No. 3-110071 discloses an invention of a welded steel pipe itself which not only limits the Cr of the base metal but also makes the Cr of the weld metal a certain amount or more with respect to the Cr of the base metal.

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

In the present description, the term "amount" of an alloy element means the "concentration" of the alloy element, and both are used without distinction. In addition, the alloy element itself, for example, Cr, may refer to the Cr amount or Cr concentration.

Japanese Patent Application Laid-Open No. 7-216500 discloses a low pH.
X80 class strength (API (American Petroleum Institute) standard: lower limit of yield strength 80 ksi = 55) that can be used in both sour environment and carbon dioxide environment including hydrogen sulfide environment
As a welded steel pipe base material covering 1 MPa), a ferrite-bainite dual phase steel composed of low C-low Mn-low Cr-trace Ti-medium N is proposed. However, this is an invention for the steel plate of the steel pipe base material, not for the welded steel pipe itself.

Large diameter pipes having a diameter of more than 16 inches are generally welded steel pipes, and particularly large diameter pipes having a diameter of more than 24 inches are welded by submerged arc welding (SAW). No technology is disclosed that simultaneously considers both sour resistance and carbon dioxide corrosion resistance of the entire SAW welded steel pipe in an environment including low pH hydrogen sulfide. In particular, no invention of a welded steel pipe having sour resistance and carbon dioxide corrosion resistance of X80 grade or higher is found.

[0017]

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

Specific goals of each performance are as follows. The welded portion refers to the weld metal, and the weld heat affected zone (HAZ) is treated as the base metal because its hardness and structure are almost determined by the chemical composition of the base metal. However, when we say "base material of two-phase structure of ferrite and bainite",
Refers to the base metal excluding the heat affected zone (HAZ). HAZ undergoes the heat cycle of welding,
This is because the "two-phase structure of ferrite and bainite" is lost.

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

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

(A-2) SSC resistance of base material and welded portion:
The “cracking 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 base material and welded part: Corrosion rate of 1/2 or less compared to non-measured material.

(A-4) Strength level: Especially in the case of [Invention 3] and [Invention 4] which limit the lower limit of the yield strength, in the case of X80 grade (yield strength 551 MPa as a welded steel pipe)
And above).

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

[Invention 3] corresponds to [Invention 1],
[Invention 4] is an example of a limited embodiment of [Invention 2].

[0026]

[Means for Solving the Problems] The existing knowledge and the means for solving the problems that the present inventor newly confirmed this time are summarized as follows.

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

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. In the figure, it can be seen that the corrosion rate increases as the amount of Cr in the base metal 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 in the weld metal. It is clear that the selective corrosion depth also 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 welded portion. As will be described later, the SSC resistance deteriorates as the selective corrosion depth increases. Therefore, the Cr content of the weld metal must be lowered as long as the carbon dioxide corrosion resistance described below is not deteriorated.

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

FIG. 4 is a drawing showing the influence of the Cr amount of the base material and 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
Even if is less than 0.2%, if Cu and Ni are contained in appropriate amounts, the corrosion rate is as compared with that of the base material where Cr is zero and Cu and Ni are both zero. The corrosion rate can be reliably reduced to 1/2 or less.

Further, FIG. 5 shows the effect on the selective corrosion depth of the weld metal in wet carbon dioxide [Cr content of weld metal-C of base metal]
It is a figure showing the influence of r amount. The smaller the Cr content of the weld metal compared to the base metal (the more negative the horizontal axis), the greater the selective corrosion depth, but when the weld metal contains appropriate amounts of Cu and Ni, it is the same as in the sour environment. In addition, the selective corrosion depth is suppressed.

(C) When the above invention is applied to high-strength steel of X80 grade or higher, Mn is added to maintain the sour resistance of the base metal.
Rather reduces the segregation of the center of the continuously cast slab. By supplementing the strength decrease due to Mn decrease with an appropriate amount of Cr, the chemical composition is made to satisfy sour resistance and carbon dioxide corrosion resistance at the same time. By applying a manufacturing method in which controlled rolling and accelerated cooling are combined, a two-phase structure of ferrite and bainite is applied to the steel whose chemical composition is adjusted. By simultaneously satisfying these chemical compositions and structures, it is possible to obtain strength of X80 grade or higher and at the same time, sufficient sour resistance and carbon dioxide corrosion resistance.

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

For this purpose, appropriate amounts of N and Ti are added to refine the HAZ structure and suppress the increase in hardness so that the σ _th of the welded portion becomes 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 in which these are integrated. In the same figure, the measures for HAZ are listed as the measures for the base metal.

The low p used in the experiments confirming these points
As the sour environment for H, a NACE TM0177 bath, which is the most severe test bath at present, is used [0.5% acetic acid + 5% saline solution at 25 ° C. saturated with hydrogen sulfide at 1 atm]. The criterion is "CA for HIC resistance
R2% or less "or" NACET for SSC resistance "
The one showing a crack generation limit stress σ _th of 80% or more of the actual yield strength in the M0177 bath was regarded as acceptable. The latter is conventionally the standard minimum yield strength (SMYS: Specified).
80% or more of the Minimum Yield Strength). Since the strength of the steel pipe base material is usually lower than that of the welded portion, the yield strength of the welded steel pipe base material is used as the actual yield strength. It goes without saying that the yield strength of the base metal is higher than the specified minimum yield strength.

The present invention is a synthesis of the above improvements, and the gist thereof is 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: 0.5% or less, Ni: 0.7% or less, Mo: 0.3% or less, Nb: 0.1% or less, V:
0.1% or less, Ti: 0.05% or less, Al: 0.00
5 to 0.1% and Ca: 0.0005 to 0.005%
And a base material having the chemical composition of the balance Fe and unavoidable impurities, and Cr of the base material with respect to the following (a)
Alternatively, a welded steel pipe having excellent sour resistance and carbon dioxide gas corrosion resistance (hereinafter referred to as [invention 1]), which is composed of a weld metal in the range of (b).

When Cr ⁰ is the Cr content of the base metal and Cr ^W is the Cr content of the weld metal, (a) When Cr ⁰ is in the range of 0.4% to 1%: Cr ^W (%) ≦ (1 / 2) × Cr ⁰ (%) (b) When Cr ⁰ is 0.2% or more and 0.4% or less: Cr ^W (%) <(1/2) × Cr ⁰ (%)-0.2 ( 2) C: 0.02 to 0.15% by weight, Si:
0.01-0.5%, Mn: 0.1-2%, P: 0.0
15% or less, S: 0.002% or less, Cr: 0.2 to 1
%, Cu: 0.5% or less, Ni: 0.7% or less, Mo:
0.3% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.05% or less, Al: 0.005 to 0.1
% And Ca: 0.0005 to 0.005% and a base material having the chemical composition of the balance Fe and unavoidable impurities,
Sour resistance and carbon dioxide resistance, characterized by comprising a weld metal having a sum of Cr, Cu and Ni in the range of (a) or (b) below with respect to the sum of Cr, Cu and Ni of the base material. Welded steel pipe with excellent gas corrosiveness (referred to as [Invention 2]).

Cr ⁰ , Cu ⁰ and Ni ⁰ are replaced with C of the base material.
When the amount of r, the amount of Cu and the amount of Ni are Cr, and the amounts of Cr ^W , Cu ^W and Ni ^W are the amount of Cr, the amount of Cu and the amount of Ni of the weld metal, (a) Cr ⁰ is more than 0.4% and less than 1%. Case: 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) × {Cr
⁰ (%)-Cr ^W (%)} ≤ Cu ^W (%) + Ni
^W (%) ≦ Cu ⁰ (%) + Ni ⁰ (%) + 0.5 (3)% by weight, C: 0.03 to 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: 0.5% or less, Ni: 0.7% or less,
Mo: 0.3% or less, Nb: 0.1% or less, V: 0.1
% Or less, Ti: 0.005 to 0.05%, Al: 0.0
05-0.1% and Ca: 0.0005-0.005
%, The balance of Fe and the unavoidable impurities, and a base material consisting of a two-phase structure of ferrite and bainite, and Cr of the base material with respect to the range of (a) or (b) below. It has a yield strength of 55.
A welded steel pipe excellent in sour resistance and carbon dioxide corrosion resistance (referred to as [invention 3]) characterized by having a pressure of 1 MPa or more.

When Cr ⁰ is the Cr content of the base metal and Cr ^W is the Cr content of the weld metal, (a) When Cr ⁰ is in the range of 0.4% to 1%: Cr ^W (%) ≦ (1 / 2) × Cr ⁰ (%) (b) When Cr ⁰ is 0.2% or more and 0.4% or less: Cr ^W (%) <(1/2) × Cr ⁰ (%)-0.2 ( 4)% by weight, C: 0.03 to 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: 0.5% or less, Ni: 0.7% or less,
Mo: 0.3% or less, Nb: 0.1% or less, V: 0.1
% Or less, Ti: 0.005 to 0.05%, Al: 0.0
05-0.1% and Ca: 0.0005-0.005
%, The base material having a chemical composition of the balance Fe and unavoidable impurities, and consisting of a two-phase structure of ferrite and bainite, and Cr of the base material and the sum of Cu and Ni.
And a weld metal in which the sum of Cu and Ni is within the range of (a) or (b) below, and has a yield strength of 551 MPa or more, which is excellent in sour resistance and carbon dioxide corrosion resistance. Welded steel pipe (referred to as [Invention 4]).

Cr ⁰ , Cu ⁰ and Ni ⁰ are used as the 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 not more than 1%: Cr ^W (%) ≦ (1/2) × Cr ⁰ ( %) Cu
⁰ (%) + Ni ⁰ (%) + (1/10) x {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) × {Cr
⁰ (%)-Cr ^W (%)} ≤ Cu ^W (%) + Ni
^W (%) ≤ Cu ⁰ (%) + Ni ⁰ (%) + 0.5 The above "welded metal" is not limited to so-called vertical seam welds (single seam and double seam) of large diameter pipes, but also spiral welds, Alternatively, it is also applied to a circumferential weld which is a joint between these welded pipes. Further, 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],
[Invention 3] and [Invention 4])
Is a diagram representing the range of the amount (Cr ^W) and the Cr content of the base metal (Cr ^0). In the figure, the range of 1 represents 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 maximum when the base metal Cr is 1% and the weld metal Cr is zero, the maximum value is 1%, and the minimum is the base metal. Cr of the material is 0.
The minimum value is 0.2% when the Cr content of the weld metal is 0.2%.

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

The amount of Ca oxidized and staying in the weld metal during welding becomes 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, since S in the weld metal does not spread by rolling like MnS in the base metal even if MnS is formed, it does not deteriorate the HIC resistance to a small extent. It may be lower than the specified range.

For the same reason, S also contains 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 more 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 of the sum of Cu and Ni of the weld metal is larger than that of the base metal is 0.5%
(Line 12 in FIG. 7). However, the lower limit cannot be determined unless both the weld metal and Cr of the base metal are determined. Based on the maximum value and the minimum value of the difference between the weld metal and the base material Cr 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 metals Cu and Ni
As shown in FIG. 8, the lower limit of the sum of
3 and, in the minimum case, the 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, "%" represents "% by weight".

C: In the case of [Invention 1] and [Invention 2],
C must be added in an amount of 0.02% or more in order to secure the strength. However, if it exceeds 0.15%, center segregation occurs remarkably and the SSC resistance of the welded portion is impaired.
15% or less. X8 of [Invention 3] and [Invention 4]
For high-strength steel pipes of grade 0 or higher, it is necessary to limit C more narrowly. If it is less than 0.03%, the required yield strength cannot be obtained even by making full use of the manufacturing method described later, but if it exceeds 0.07%, the HAZ hardness increases, and HA
A pearlite band is generated near the Z outermost line and is resistant to S
Since the SC property deteriorates, it 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, it is preferable that the content be 0.04 to 0.06%.

Si: Si is used as a deoxidizer during steel making, but if it is less than 0.01%, deoxidation is not sufficiently carried out and the yield of Al, which will be described later, is reduced, but if it exceeds 0.5%, toughness is increased. Since it deteriorates, it is set to 0.01 to 0.5%.

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

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

P: P is preferably low. However, since reducing P causes an increase in cost, the range is set to be below the range where performance deterioration is not significant. If it exceeds 0.015%, it is densely segregated together with Mn in the central portion of the continuous cast slab, and an abnormal structure is generated, which significantly deteriorates the HIC resistance. Therefore, the content must be 0.015% or less.

S: It is desirable that S is low. However, lowering S also increases the cost, so the value is set within the allowable range. If it exceeds 0.002%, MnS will be generated in the center segregated portion even if the sulfide shape control by Ca is performed, and the resistance to H
The IC property is impaired, so the content is made 0.002% or less.

Cr: 0.2% or more is required to enhance the carbon dioxide corrosion resistance and to secure the X80 grade yield strength and to obtain good SSC resistance. However, if it exceeds 1%, the SSC resistance of the base material deteriorates, so it is set to 0.2 to 1%. In order to secure higher base metal strength, for example, strength of X80 grade or higher and SSC resistance and carbon dioxide corrosion resistance at the same time, 0.4 to 0.7% is preferable.

Cu: Cu may not be added. However, Cu can enhance the corrosion resistance to carbon dioxide gas, and at the same time, prevent the intrusion of hydrogen to improve the HIC resistance and the SSC resistance in a low pH hydrogen sulfide environment. If added, add. However, even if added, if it exceeds 0.5%, hexagonal cracks are generated on the surface of the continuously cast slab and the product yield is remarkably reduced, so the content is made 0.5% or less.

Ni: Ni may not be added. However, Ni enhances toughness and at the same time enhances carbon dioxide corrosion resistance, so 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
The range that does not deteriorate the IC property is 0.2 to 0.5%
Is desirable.

Mo: Mo may not be added. However, by adding it, the strength and toughness are improved,
In addition, in an environment of low pH such as NACE TM0177 bath, hydrogen invasion is suppressed by the synergistic action with Ni, and HIC resistance is high.
Since it improves the properties, it is added when these properties are further improved. Even if added, if it exceeds 0.3%, the toughness and the SSC resistance of the welded part deteriorate, so 0.3
% Or less. To secure better strength and toughness and maintain the SSC resistance of the weld, 0.05-0.15
% Is desirable.

Nb: Nb may not be added. However, Nb improves strength and toughness due to grain refinement and carbide precipitation, and improves SSC resistance due to grain refinement. Therefore, Nb is added to further improve these. However, even if it is added, if it exceeds 0.1%, the toughness is rather deteriorated, so the content is made 0.1% or less. 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
Like Nb, it improves the strength and toughness by grain refinement and carbide precipitation, and improves the SSC resistance by grain refinement. Therefore, if these properties are further improved, they are added. However, even if it is added, if it exceeds 0.1%, the toughness is remarkably lowered, so the content is made 0.1% or less. In order to obtain a more appropriate balance of strength and toughness, 0.
It is desirable to set it to 02 to 0.07%.

Al: Al is effective as a deoxidizer during steel making. If 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, it is desirable to set the content to 0.01 to 0.03%.

Ca: Ca is an element effective for controlling the morphology of sulfide inclusions, but if it is less than 0.0005%, MnS that is stretched by rolling is produced, and the HIC resistance is impaired. 0005% or more. However, 0.0
If it exceeds 05%, excessive Ca forms an aggregate of oxides and deteriorates the HIC resistance, so 0.0005 to 0.005.
%. To obtain further 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, by combining with N and precipitating TiN, the HAZ hardness is lowered, so the hardness of the HAZ of the X80-grade high-strength steel pipe, which is the subject of [Invention 3] and [Invention 4], is suppressed and SSC resistance is reduced. Therefore, in [Invention 3] and [Invention 4], the content must be 0.005% or more. 0.00
This is because if it is less than 5%, a clear effect cannot be obtained. However, if the content exceeds 0.05%, the toughness of the base material and the welded portion deteriorates regardless of high strength and low strength, so the content is made 0.05% or less. [Invention 1] to [Invention 4]
In order to secure sufficient toughness and to obtain sufficient SSC resistance, 0.01 to 0.02% is desirable.

N: N is not particularly limited,
When Ti is added, it is slightly higher, for example, 0.004 to
It is desirable to set the content to 0.01% and, if Ti is not added, to slightly lower the content, for example, 0.002 to 0.006%. When Ti is added, T is added to soften the HAZ.
This is because iN is positively generated, and in the case where Ti is not added, HAZ does not harden so much in low-strength steel, and toughness is improved when N is reduced and the amount of solid solution N in HAZ is reduced. 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 grade or higher is "a two-phase structure of ferrite and bainite", that is, "a 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 above-described chemical composition of the base material and the manufacturing method described later. Increasing the amount of alloying elements to increase the strength results in a "structure in which residual austenite mixed with C is concentrated", a "hard martensite structure in the central segregation part", a "blocky bainite structure" or a "structure containing pearlite". Although such a structure is likely to occur, good HIC resistance and SSC resistance cannot be obtained with such a structure. In order to avoid such a structure, a steel plate which is a welded steel pipe base material is manufactured by a manufacturing method described later. Here, the "block-shaped bainite structure" refers to bainite formed from austenite enriched with C after proeutectoid ferrite is 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, since the corrosion rate of the weld metal is generally higher than that of the base metal, when the weld metal and the base metal having the same Cr content are exposed to low-pH wet hydrogen sulfide, The weld metal is selectively corroded as compared with the base metal. Such an imbalance of corrosion causes the SSC resistance of the weld metal.
Therefore, the following components are limited to the weld metal because it deteriorates the property (see FIGS. 6 to 8).

Cr: (b) When the Cr content (Cr ⁰ ) of the base metal is in the range of 0.4% to 1%, 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 becomes equal to the Cr content of the base metal, or exceeds the Cr content of the base metal, FIGS. As shown in, the corrosion of the base material and the weld metal under a low pH hydrogen sulfide environment becomes large.
As a result, when the corrosion rate of the weld metal and the base metal becomes imbalanced, the SSC resistance of the welded portion deteriorates, and the crack initiation critical stress σ _{th of} the welded portion does not exceed 80% 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 have good carbon dioxide corrosion resistance, the Cr content (Cr ^W ) of the weld metal is set to the above value. It is desirable to make it as high as possible within the range. For example, the Cr amount (Cr ⁰ ) of the base material is 0.4 to
When the content is 0.7%, the Cr content (Cr ^W ) of the weld metal is
It is desirable to satisfy the condition of [(1/2) × Cr ⁰ (%)] or less and be as high as possible, that is, 0.15 to 0.3%.

(B) The Cr content (Cr ⁰ ) of the base material is 0.2%
When in the range of 0.4% or less, Cr content in the weld metal (Cr ^{W) is, [Cr 0 (%) -} 0.2 (%)] is less than. The Cr content (Cr ^W ) of the weld metal is [Cr ⁰ (%)
-0.2 (%)] or more, approaching, equalizing, or exceeding the Cr amount of the base metal, the absolute value of the corrosion rate under a low pH hydrogen sulfide environment is small, but the above-mentioned As described above, an imbalance in the corrosion rate between the weld metal and the base metal occurs, which causes the SSC resistance of the weld metal to deteriorate. Further, as described above, in order to have the carbon dioxide gas corrosion resistance at the same time, the Cr content (Cr ^W ) of the weld metal is [Cr ⁰ (%)-
It is desirable to be as high as possible while satisfying that it is less than 0.2 (%).

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

The Ca of the weld metal may be lower than the range set for the base metal for the reasons 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 limitations are added to 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, the carbon dioxide gas corrosion resistance is improved, and in a wet hydrogen sulfide environment with high pH, hydrogen invasion is suppressed and the HIC resistance is also improved. Further, making Cu and Ni of the weld metal higher than that of the base metal has the effect of simultaneously improving SSC resistance in a low pH hydrogen sulfide environment.

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

The sum of Cu and Ni of the weld metal (Cu ^W + Ni
^W ) is larger than that of the base metal (Cu ⁰ + Ni ⁰ ) by [(1 /
10) × {Cr ⁰ (%) − Cr ^W (%)}] or more. If the amount is less than that, the carbon dioxide corrosion resistance of the weld metal is not sufficient, and the weld metal undergoes selective corrosion to deteriorate the SSC resistance of the weld metal. As described above, this amount is not determined unless both the weld metal and Cr of the base metal are determined, but the maximum value is 0.1% (line 13 in FIG. 8) and the minimum value is 0.02%. (Line 14 in Figure 8).
However, the sum of Cu and Ni of the weld metal (Cu ^W + Ni ^W )
Is 0.5% than that of the base material (Cu ⁰ + Ni ⁰ ).
12), the carbon dioxide corrosion resistance is saturated and the SCC resistance is adversely decreased. Therefore, the amount of the base material exceeding 0.5% is 0.5% or less.

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

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

(1) The heating temperature of the rolling slab is set to 1050 to 1250 ° C. 105
If the temperature is lower than 0 ° C, the carbide does not form a solid solution and remains coarse, so that the strength does not increase and it becomes difficult to secure a predetermined rolling temperature. On the other hand, if it exceeds 1250 ° C, the structure becomes coarse and the toughness cannot be secured.

At the same time, the SSC resistance deteriorates, so that
It shall be 50 ° C or lower. In order to obtain high strength with a fine structure, it is desirable to set the temperature to 1100 to 1200 ° C.

Rolling from a slab to a steel plate, which is a material for welded steel pipe, is performed at a temperature of 950 ° C. or lower at a reduction ratio of 50% or more (reduction ratio =
[(Slab thickness-steel plate thickness) / slab thickness]). If the rolling reduction at 950 ° C or lower is less than 50%,
No refinement due to recrystallization of austenite grains was obtained, and as a result, fine ferrite grains were not obtained, resulting in toughness and S resistance.
SC property becomes insufficient. In order to further improve the SSC resistance, the rolling reduction at 950 ° C. or lower is preferably 60% or higher.

The rolling finishing temperature is set to Ar ₃ point or higher.
When the finishing temperature is less than Ar ₃ point, accelerated cooling starts from the two-phase region of ferrite and austenite where a large amount of pro-eutectoid ferrite is generated, and “C-enriched retained austenite structure” and “central segregation part” occur. It becomes a "hard martensite structure" or "block-shaped bainite structure" and the sour resistance decreases. In order to further suppress such a hardened structure, it is desirable to finish by [high temperature from Ar ₃ point to 30 ° C. or higher].

(2) Accelerated Cooling Accelerated cooling is started from [temperature lower than Ar ₃ point by 30 ° C.] or higher. When cooling is started from a temperature lower than [temperature lower than Ar ₃ point by 30 ° C.], cooling is performed from the two-phase region of ferrite and austenite in which a large amount of proeutectoid ferrite is generated, and “C-enriched retained austenite structure” and The center segregation part becomes a hard martensite structure or a blocky bainite structure, and sour resistance decreases. In order to further suppress the formation of such a structure even in the central segregation portion, it is desirable that the accelerated cooling start temperature is 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 higher 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 transformed 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 formed, and the sour resistance of the base material is deteriorated. In order to avoid the hardened structure more completely, it is desirable that the temperature is 450 ° C. or higher and 500 ° C. or lower.

After the accelerated cooling is stopped, it is cooled or gradually cooled. In addition, tempering may or may not be performed thereafter.

The steel plate which is 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, after the accelerated cooling is stopped, the hot coil is wound and gradually cooled in the state of the coil, and tempering is not normally performed. The hot coil is narrower than the thick steel plate, so when processing and welding large diameter steel pipe,
Spiral welded steel pipes or double seam welded steel pipes are often used.

5. Pipe Making The steel plate produced by the above method is processed into a tubular shape and then made into a welded steel pipe by SAW. The pipe manufacturing method is U in which thick steel plate is subjected to U press and O press for processing.
The O method, the spiral method of welding a hot rolled steel sheet in a spiral shape, and the processed hot rolled steel sheet or thick steel sheet having a narrow width are 2
Either of the double seam method in which the steel pipes are aligned and two vertical seams are used.

FIG. 9 (a) is a drawing showing the shape of the groove before welding, and FIG. 9 (b) is a drawing showing a vertical cross section of the welding line after welding. In the figure, it can be seen that the weld metal 21 is composed of the base material near the groove melted during welding and the portion transferred from the welding material. The amount of the transition from the welding material to the welding metal is called "welding metal". HAZ2
Although No. 2 is affected by heat due to welding, the chemical composition is the composition itself of the base material 23 except for special elements such as hydrogen.

In order to lower the Cr of the weld metal as compared with 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 "deposited metal" is 60% to 40% in the element amount of the weld metal. Therefore, [Cr concentration of weld metal (%)] = [Cr concentration of base metal (%)] x (0.4 to 0.6) + [Cr concentration of weld material (%)] x (0.6 ~ 0.4) ... (A) If the Cr concentrations of the base metal and weld metal are set in the formula (A), the Cr concentration of the welding material to be used can be calculated and should be used. Welding material can be selected. That is,
Since the base metal and the welding material can be considered to have the same specific gravity, 40 to 60% of the weld metal is transferred from the base metal, and 60 to 40% of the weld metal is "welded metal". Cr or Cu and Ni described later do not disappear in the atmosphere during welding.

To enhance the Cu and Ni of the weld metal,
For both Cu and Ni, 40 to 60% is from the base metal, and 6 to 4
It is possible to calculate the Cu content and Ni content of the welding material to be used by a similar method, assuming that the proportion of the welding material is shifted from the welding material, and to select the welding material of an appropriate standard.

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

A higher heat input for welding is desirable from the viewpoint of welding efficiency and reduction of the hardness of the HAZ, but if it is made too high, the toughness of the HAZ deteriorates.
It is good to set it to 150 kJ / cm.

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

[0095]

EXAMPLES Tables 1 to 4 are lists of chemical compositions of welded steel pipe base materials, which are classified to show the effects of carrying out [Invention 1] to [Invention 4]. That is, for example, Table 1
[Fig. 3] is a list of welded steel pipe base materials used to show the effect of [Invention 1].

Table 5 is a list showing rolling and heat treatment conditions for producing these welded steel pipe base materials. Steel I of Comparative Example falls within the range of [Invention 3] with respect to the chemical composition of the base material, but since the structure is a two-phase structure of ferrite and martensite, it is outside the range of [Invention 3] with respect to the base material structure. Is the base material. Further, although the structures of Steels A to E are not the two-phase structure of ferrite and bainite, the structure is not particularly limited in [Invention 1], and therefore the structure is not outside 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 in the longitudinal seam SAW of the edges of the above-mentioned welded steel pipe base materials that have been butted through U press and O press, SAW
3 is a list showing the composition of the weld metal formed as a result of the above. These Tables 6 to 9 correspond to weld metals for explaining the effects of carrying out [Invention 1] to [Invention 4], respectively. In these tables, only the alloying elements of the weld metal limited in each invention are shown, but the other alloying elements are almost the same as the corresponding steel (base material) of each trial number. SAW forming a vertical seam has a welding heat input of 40k
It is a J / cm double-sided single layer weld, and the dimensions of the welded steel pipe produced are as follows: diameter 36 inches (914.4 mm) x wall thickness 1 inch (25.4 mm)
It is. In the columns of "Welded metal-base material" in Tables 6 to 9,
The difference in composition between the weld metal and the base metal is shown for Cr and Cu + Ni.

[0103]

[Table 6]

[0104]

[Table 7]

[0105]

[Table 8]

[0106]

[Table 9]

FIG. 10 (a) is a drawing showing the test piece used for evaluating the SSC resistance, and FIG. 10 (b) is a drawing showing the 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 pipe so that the weld metal part was located at the center of the parallel part of the test piece. Load stress is the actual Y of the base metal
It was set to 80% of S (yield strength). Test solution is NACET
M0177-90 (Test Method by National Associat
NACE stipulated in ion of Corrosion Engineering)
The TM0177 bath (0.5% acetic acid + 5% saline, 1 atm hydrogen sulfide saturated, 25 ° C.) was used. Those that did not break during the test period of 720 h were regarded as good SSC resistance.

FIG. 11A is a drawing showing the test piece used for evaluating the corrosion resistance, and FIG. 11B is a drawing showing the sampling position. The test piece was a plate-shaped test piece of 2t × 10w × 40l. Similar to the SSC resistance test piece, the test piece was sampled from the inner surface of the pipe so that the weld metal was located at the center of the weld. The evaluation was based on the general corrosion rate of the base metal and the selective corrosion depth of the weld. The general corrosion rate of the base metal is
The corrosion weight loss after descaling was calculated by converting per unit time per unit area. The selective corrosion depth of the welded portion was obtained from the difference in wall thickness between the weld metal portion and the base metal portion with a micrometer for measuring the pitting corrosion depth after descaling. When the pitting depth of the base metal is deeper than that of the weld metal, Table 10 described later is used.
It was displayed as a value of minus (-) in 13. The test solution was saturated with NACE TM0177 bath as a hydrogen sulfide environment and 1 atmosphere CO ₂ as a carbon dioxide gas environment.
Two types of artificial seawater at ℃ were used. The test solution was constantly stirred during the test period of 96h.

Tables 10 to 13 are tables showing the results of evaluating the above SSC resistance and corrosion resistance. Table 10 to Table 1
3 explains the effect of implementation of each of [Invention 1] to [Invention 4]. The steel pipe strengths described in these tables are obtained by the tensile test of the welded steel pipe base material. The stress value of the yield strength of "551 MPa" is "79.9 ks."
It corresponds to i ".

[0110]

[Table 10]

[0111]

[Table 11]

[0112]

[Table 12]

[0113]

[Table 13]

Steel E (Table 10), Steel I (Table 12), Steel J (Table 1)
2), Steel K (Table 12) and Steel N (Table 13) were not good in SSC resistance of the base material. Steel E and Steel N are base materials of Cr
Is 1% or more, steel I for high-strength welded steel pipe is a ferrite-martensite dual-phase steel, steel J has an excessively high C content, and steel K has an excessively high Mn content.

Steel C whose Cr content is less than the range of the present invention (Table
10), Steel D (Table 10) and Steel G (Table 11) Trial No. 7
(Table 10), 8 (Table 10) and 13 (Table 11) are SSC resistant
And the corrosion resistance in the NACE TM0177 bath are good, but the general corrosion rate of the base metal part in the CO ₂ environment is 4 mm / year or more, which is remarkably higher than other examples.

On the other hand, steel A (Table 10), steel B (Table 10), steel F (Table 1) having a Cr content of 0.2 to 1% and within the scope of the present invention.
1) and steels H (Table 12) to steel M (Table 13) used in carbon dioxide gas, the higher the Cr, the smaller the general corrosion rate, and the better that it was 2 mm / year or less. In particular, Cr is 0.
Steels with 5% or more (steels B, F and H to M) have a good general corrosion rate of 1 mm / year or less. Further, regardless of the amount of Cr, the steel containing Cu and Ni has a lower corrosion rate than the steel containing neither Cu nor Ni, as shown in FIG.

In the weld metal, if the Cr content is 1/2 or less of the base metal portion and the base metal content is 0.2 to 0.4% at the same time,
The amount of (Cu + Ni) in the weld is 0.1 to 0.1% that of the base metal.
0.5% higher trial number 2 (Table 10), 5 (Table 10), 10 (Table 1
1), 16 (Table 12), 17 (Table 12), 19 (Table 12), 2
3 (Table 13), 24 (Table 13), 25 (Table 13), 28 (Table 1
In 3), 29 (Table 13) and 30 (Table 13), the selective corrosion depth is suppressed to less than 0.1 mm. On the other hand, trial number 1 (Table 10), 4 (Table 10), 9 (Table 10), 11 (Table 1
1), 14 (Table 12), 15 (Table 12), 20 (Table 12), 2
As shown in Nos. 1 (Table 12), 22 (Table 12), and 26 (Table 13), which do not contain both Cu and Ni, the selective corrosion depth of the welded portion exceeds 0.1 mm in the CO ₂ environment.

It is better to make the amounts of Cr in the base metal and weld metal uniform.
It is easy to suppress selective corrosion in a 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 in the weld metal
It is necessary to increase the sum of the above than that of the base metal.

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

Trial Nos. 3 (Table 10), 6 (Table 10), 12
As in (Table 11), 18 (Table 12) and 27 (Table 13),
In the NACE TM0177 bath, the higher the Cr content in the weld metal, the larger the selective corrosion depth, which is considered to be one of the reasons why the SSC resistance is poor. Also, trial number 11 (Table 11)
And 16 (Table 12) of the weld metal (Cu + Ni)
SSC resistance is not good even if the amount is low and does not fall within the range of the present invention ([Invention 2] or [Invention 4]). On the other hand, trial Nos. 19 (Table 12) and 28 (Table 13) are weld metal (Cu
+ Ni) is higher than the base metal by more than 0.5%, so SS
A fracture due to C occurred. Trial No. 2 (Table 10), 5 (Table 10),
10 (Table 11), 16 (Table 12), 17 (Table 12), 23 (Table
13), 24 (Table 13), 25 (Table 13), 29 (Table 13) and 30 (Table 13), where the amount of (Cu + Ni) in the weld metal is higher than that of the base metal, selective corrosion is suppressed. Has been.
Therefore, from the viewpoint of SSC resistance, the (Cu
The amount of + Ni) needs to be higher than that of the base material in the 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 gas corrosion resistance, in particular, a high strength welded steel pipe of API standard X80 grade, and oil and It is extremely useful for natural gas related industries.

[Brief description of the drawings]

FIG. 1 is a drawing summarizing the problems that occur in the environment encountered during the extraction, transportation, and refining of crude oil or gas, and the conventional measures taken for the steel plate that is the base material of the welded steel pipe to cope with them. is there.

FIG. 2 is a low pH hydrogen sulfide environment, NACE.
It is a drawing showing the influence of the Cr amount (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 influence of Cr ^W on the selective corrosion depth in 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 weld metal in wet carbon dioxide.

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 figure showing the range of ⁰ ).

FIG. 8 shows the ranges 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 metal (Cu ⁰ + Ni ⁰ ) in [Invention 2] and [Invention 4]. It is a drawing showing.

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

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

[Explanation of symbols]

1 ... Range of Cr content of weld metal and base material of welded steel pipe of the present invention, 2 ... Straight line showing relation of Cr ^W (%) = (1/2) × Cr ⁰ (%) 3 ... Cr ^W (%) = A straight line representing the relationship of Cr ^W (%)-0.2 (dotted line) 4 ... a straight line representing Cr ⁰ (%) = 1 5 ... a straight line representing Cr ^W (%) = Cr ⁰ (%) (dashed line) 11 ... Range of sum of Cu and Ni of weld metal and base material of 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
(%)) Maximum value)} = Cu ⁰ (%) + Ni ⁰ (%) +
Straight line representing 0.1 (dotted line) 14 ... Cu ^W (%) + Ni ^W (%) = Cu ⁰ (%) + N
i ⁰ (%) + (1/10) × {(Cr ⁰ (%) − Cr ^W
Minimum value of (%)} = Cu ⁰ (%) + Ni ⁰ (%) +
Straight line representing 0.02 (broken line) 15 ... Cu ^W (%) + Ni ^W (%) = Cu ⁰ (%) + N
Straight line (dashed line) representing i ⁰ (%) 21 ... Weld metal, 22 ... Weld heat affected zone (HAZ), 23 ... Base metal (steel plate)

Claims (4)

[Claims] 1. By weight%, C: 0.02 to 0.15%, 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: 0.5% or less, Ni: 0.7% or less,
Mo: 0.3% or less, Nb: 0.1% or less, V: 0.1
% Or less, Ti: 0.05% or less, Al: 0.005-
0.1% and Ca: 0.0005 to 0.005% and a base material having the chemical composition of the balance Fe and unavoidable impurities, and the Cr of the base material with respect to the following (a) or (b). ) A welded steel pipe having excellent sour resistance and carbon dioxide corrosion resistance, which is characterized by comprising a weld metal within the range. Cr ⁰ is the amount of Cr in the base metal and Cr ^W is C in the weld metal.
(b) When Cr ⁰ is more than 0.4% and not more than 1%: Cr ^W (%) ≦ (1/2) × Cr ⁰ (%) (b) Cr ⁰ is 0.2% If 0.4% or ^{less: Cr W (%) <(} 1/2) × Cr 0 (%) - 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: 0.5% or less, Ni: 0.7% or less,
Mo: 0.3% or less, Nb: 0.1% or less, V: 0.1
% Or less, Ti: 0.05% or less, Al: 0.005-
0.1% and Ca: a base material having a chemical composition of the balance Fe and inevitable impurities containing 0.0005 to 0.005%, and C of the base material Cr and the sum of Cu and Ni.
A welded steel pipe excellent in sour resistance and carbon dioxide gas corrosion resistance, characterized by comprising a weld metal in which the sum of r and Cu and Ni is within the following range (a) or (b). Cr ⁰ , C
u ⁰ and Ni ⁰ are the Cr content, Cu content and Ni of the base material.
The amount of Cr ^W , Cu ^W and Ni ^W is C of the weld metal.
When the amount of r, the amount of Cu, and the amount of Ni are used, (a) When Cr ⁰ is more than 0.4% and not more than 1%: 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) × {Cr
⁰ (%)-Cr ^W (%)} ≤ Cu ^W (%) + Ni
^W (%) ≤ Cu ⁰ (%) + Ni ⁰ (%) + 0.5 3. In weight%, C: 0.03 to 0.07%, 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: 0.5% or less, Ni: 0.7% or less, Mo: 0.3% or less, Nb: 0.1% or less, V:
0.1% or less, Ti: 0.005 to 0.05%, Al:
0.005-0.1% and Ca: 0.0005-0.
A base material having a chemical composition of Fe and unavoidable impurities containing 005%, and having a two-phase structure of ferrite and bainite, and the Cr of the base material being less than (a) or (b) below. A welded steel pipe excellent in sour resistance and carbon dioxide corrosion resistance, which is made of a weld metal in a range and has a yield strength of 551 MPa or more. When Cr ⁰ is the Cr content of the base metal and Cr ^W is the Cr content of the weld metal, (a) When Cr ⁰ is in the range of 0.4% to 1%: Cr ^W (%) ≦ (1/2) × Cr ⁰ (%) (b) When Cr ⁰ is 0.2% or more and 0.4% or less: Cr ^W (%) <(1/2) × Cr ⁰ (%)-0.2 4. In weight%, C: 0.03 to 0.07%, 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: 0.5% or less, Ni: 0.7% or less, Mo: 0.3% or less, Nb: 0.1% or less, V:
0.1% or less, Ti: 0.005 to 0.05%, Al:
0.005-0.1% and Ca: 0.0005-0.
A base material having a chemical composition of Fe and unavoidable impurities containing 005% and having a two-phase structure of ferrite and bainite; and the sum of Cr and Cu and Ni of the base material, The sum is (a) or (b) below
Made of weld metal within the range of yield strength 551MP
A welded steel pipe excellent in sour resistance and carbon dioxide corrosion resistance characterized by being a or more. Cr ⁰ , Cu ⁰ and Ni
⁰ is the Cr content, Cu content and Ni content of the base metal, and Cr ^W , Cu ^W and Ni ^W are the Cr content of the weld metal, Cu
(A) When Cr ⁰ is more than 0.4% and not more than 1%: 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) × {Cr
⁰ (%)-Cr ^W (%)} ≤ Cu ^W (%) + Ni
^W (%) ≤ Cu ⁰ (%) + Ni ⁰ (%) + 0.5