JPH06287682A - High strength uoe line pipe excellent in sulfide stress cracking resistance in weld zone - Google Patents

Abstract

PURPOSE:To obtain a high strength UOE line pipe excellent in sulfide stress cracking resistance in the weld zone by specifying the compsn. constituted of C, Si, Mn, P, S, Nb, Al, Ca, Ti and Fe. CONSTITUTION:This line pipe has a compsn. contg., by weight, 0.03 to 0.09% C, 0.1 to 0.6% Si, 1.1 to 1.5% Mn, <=0.015% P, <=0.0010% S, 0.010 to 0.050% Nb, 0.005 to 0.03% Al, 0.002 to 0.004% Ca and 0.005 to 0.025% Ti and furthermore contg., at need, one or more kinds among 0.01 to 0.1% V, 0.1 to 1.0% Ni, 0.1 to 1.0% Cu, 0.1 to 1.0% Cr and 0.1 to 0.5% Mo, and the balance iron with inevitable impurities and also satisfying the inequalities: Ca(1-980)/S>1.7 and 0.07<Ca/Al<0.17, and in which the form of oxides contg. Ca causing the generation of sulfide stress cracking is controlled, which sulfide stress cracking resistance is obtd. even in the softened zone.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloy having excellent resistance to sulfide stress cracking of welds in a wet hydrogen sulfide environment.
It relates to PI grades X60 to X70 linepipes.

[0002]

2. Description of the Related Art Since oil and natural gas produced in recent years contain hydrogen sulfide very often, the line pipes for transporting these oil and natural gas are hydrogen sulfide in the presence of water such as seawater. It is more likely to be exposed to the environment (sour environment). In the sour environment, hydrogen penetration into the steel due to corrosion of the steel surface is promoted by the catalytic action of hydrogen sulfide. On the other hand, a stress in the circumferential direction, that is, a hoop stress acts on the line pipe during operation by the pressure of the fluid passing through the inside.

[0003] Sulfide stress cracking is known as a cause of line pipe failure that occurs under such environmental conditions. In particular, sulfide stress cracking that occurs in the high hardness portion of the weld has been frequently experienced. Therefore, as a method for preventing sulfide stress cracking, it is general to regulate the maximum hardness of the welded portion, and it is usually required to set the Vickers hardness to 248 or less. In order to meet this demand, steel pipe manufacturers reduce the carbon equivalent of the steel pipe material and manufacture the material by a so-called TMCP, which is a thermomechanical treatment method including controlled rolling and accelerated cooling, in order to compensate for the strength reduction. The introduction of TMCP technology has improved the sulfide stress cracking resistance of the steel pipe base material by controlling the metallographic structure in addition to preventing the hardening of the weld.

However, in the TMCP steel, a softening region having a lower strength than the base metal is inevitably formed in the weld heat affected zone (HAZ) because of the low carbon equivalent. In particular, a UOE steel pipe having a high welding heat input has a high degree of softening. As a method for evaluating the sulfide stress crack resistance of line pipes, NACE standard TM0177 is used.
-90, but in this case, the setting criterion for the load stress is determined based on the strength of the base material, so TMC including the softened portion is included.
It is known that in P steel, cracking occurs preferentially in the softened portion where the stress condition becomes severe. In addition, recently, mainly in the United Kingdom, a test method for performing a sulfide stress cracking resistance evaluation test of a line pipe, a so-called CAPCIS test, is being standardized. In the CAPCIS test, the HAZ softened portion is included in the evaluation, and since ultrasonic flaw detection is adopted as an evaluation means, minute internal cracks generated in the softened portion can be regarded as a problem.

As a method for preventing the sulfide stress cracking of the softened portion, the following countermeasures for directly preventing the softening have been disclosed. (1) A method of preventing the HAZ from lowering its strength by precipitation hardening of a nitride or the like (for example, JP-A-62-284043). (2) A method of heat-treating a welded portion after welding to improve softening (for example, JP-A-4-168220). Although these means are effective only in terms of preventing softening, they have the following drawbacks when actually applied to the production of line pipes.

[0006]

When the precipitation hardening is utilized as in the above (1), the degree of softening is certainly reduced, but the softened portion is not completely eliminated. Further, since the nitride of V is used as the precipitation element, the fracture toughness of the welded portion, which is one of the important characteristics in the line pipe, is reduced. Further, although the method (2) is certainly a means for preventing softening, a large heat treatment means is required for application to the production of UOE steel pipes, and productivity is impaired, resulting in an increase in cost. Not a realistic method.

As described above, as a method for obtaining the sulfide stress cracking resistance of the welded portion, improving softening has few means, and even if possible, it is not an industrially advantageous means. Therefore, assuming the presence of HAZ softening, ensuring the sulfide stress cracking resistance of the softened part is an issue in manufacturing a high strength UOE line pipe having excellent sulfide stress cracking resistance of the welded part.

[0008]

The present invention advantageously solves the above-mentioned problems, and Ca, which is necessary for controlling the morphology of sulfides, is provided.
In addition to controlling the morphology of Ca-containing oxide, which is the starting point of sulfide stress cracking, by adding Ca and Al, the sulfide stress cracking resistance can be obtained even in the softened part. is there.

That is, the gist of the present invention is that
% By weight, C: 0.03 to 0.09%, Si: 0.1
-0.6%, Mn: 1.1-1.5%, P: 0.01
5% or less, S: 0.0010% or less, Nb: 0.01
0-0.050%, Al: 0.005-0.03%, C
a: 0.002-0.004%, Ti: 0.005-
0.025%, or in addition to these, V: 0.01 to 0.1%, Ni: 0.1 to 1.0
%, Cu: 0.1 to 1.0%, Cr: 0.1 to 1.0
%, Mo: 0.1 to 0.5%, or a mixture of iron and inevitable impurities, and the following expressions (1) and (2) are satisfied. It is a high-strength UOE line pipe with excellent sulfide stress cracking resistance of the welded part.

[Equation 3]

[0010]

[Function] The inventors of the present invention have manufactured X60 to X7 manufactured on an actual machine.
Experiments were performed to reproduce sulfide stress cracks in the weld softened portion having high crack susceptibility for various 0 grade UOE line pipes, and the fracture surface of the initial crack generated in the softened portion was observed to observe the origin of the crack. As a result, when the sulfide morphology control by Ca is not sufficiently performed, relatively large cracks occur, and the cracks start from elongated MnS,
When the sulfide morphology is sufficiently controlled by Ca, relatively small cracks occur, and the origins of the cracks are often CaO, Al ₂ O ₃ existing in the form of groups or complex oxidation mainly composed of these. It turned out to be a thing. Therefore,
In order to prevent sulfide stress cracking of the softened portion, the present inventor first thought that it is necessary to sufficiently control the sulfide morphology. As a result of investigating various UOE line pipes of X60 to X70 grade manufactured in actual equipment, regarding the amount of Ca necessary to sufficiently control the morphology of sulfide,
We have found that it is necessary to satisfy equation (1).

[Equation 4]

After satisfying these conditions, it is presumed that the existence state of oxides varies depending on the added amounts of Ca and Al, and Ca / A
For various UOE line pipes with different l, NACE standard TM017 in the direction perpendicular to the weld line from the seam weld
A test piece according to 7-90 method A was prepared, a constant load test was carried out according to the same standard, and after 720 hours, a critical stress at which the test piece did not break, σth, was determined. By standardizing σth by the standard minimum yield stress of steel pipe, SMYS, and judging that σth / SMYS ≧ 0.8, it is judged to have excellent sulfide stress cracking resistance,
The occurrence of sulfide stress cracking at the weld was examined. As a result, as shown in FIG. 1, it has been found that sulfide-resistant stress cracking can be obtained even in the softened part if the following expression (2) is satisfied. 0.07 <(% Ca) / (% Al) <0.17 ……………… (2)

Based on the above facts, if the above conditions are satisfied after limiting the chemical components for the reasons described below, API grade X60 excellent in sulfide stress cracking resistance of the welded part is obtained.
It was concluded that it is possible to manufacture X70 high strength UOE line pipes.

Next, the reasons for limiting the components in the present invention will be described. Since C is a strengthening element, it is made 0.03% or more to obtain a desired strength. On the other hand, if added in a large amount, the hardness of the base material of the line pipe and the welded portion will increase, the toughness will decrease, and sulfide stress cracking will easily occur in a hydrogen sulfide environment, so 0.09% or less. To do.

Si is a deoxidizing element, and if it is less than 0.1%, sufficient deoxidizing power cannot be obtained, and if it exceeds 0.6%, the steel is embrittled to 0.1 to 0.1%. 6%.
Mn forms MnS, which is the starting point of sulfide stress cracking, and co-segregates with P that promotes embrittlement of steel to promote the propagation and propagation of cracks, so the amount of Mn added is as low as possible. Is preferable. However, Mn is an indispensable element for obtaining strength and toughness, so X60 to X70
In order to obtain the strength of the line pipe of No. 1, it is made 1.1 to 1.5%.

P is an element that facilitates the propagation of sulfide stress cracking due to segregation, and the lower the better, the better 0.015.
% Is the upper limit. Since S is combined with Mn to form MnS, which is the starting point of sulfide stress cracking, S is preferably as low as possible. From the viewpoint of stably obtaining crack resistance, 0.00
The upper limit is 10%.

Nb is added in an amount of 0.010% or more in order to make the rolling structure finer grain, improve hardenability and precipitation hardening.
Even if added over 0%, the effect of adding a large amount is small,
Rather, coarse carbides are formed to reduce hydrogen-induced cracking resistance, so the upper limit is 0.05%. Al is important as a deoxidizing element, but if added in a large amount, it contaminates the steel and lowers the toughness, so the content is made 0.005 to 0.04%.

Ca is added in an amount of 0.002% or more in order to control the shape of sulfide inclusions such as MnS. However, if added in a large amount, the steel is contaminated, so Ca is made 0.004% or less. The lower limit of 0.005% of the amount of Ti added is the minimum amount at which fine TiN is formed and micronization of the microstructure is expected, and the upper limit is 0.025% from the condition that the toughness does not decrease due to TiC. .

In the present invention, in addition to the above elements, V, Ni,
One or more of Cu, Cr and Mo are added. V
Is added as a strengthening element in an amount of 0.01% or more, and if added excessively, the toughness decreases, so the content is made 0.1% or less. Ni,
Cu, Cr, and Mo all add 0.1% or more when it is necessary to increase the hardenability of steel and increase the strength, but excessive addition forms a low-temperature transformation product, resulting in toughness and resistance. Since hydrogen-induced crackability is impaired,
The upper limits are 1.0%, 1.0%, 1.0%, and 0.5%.

The present invention adjusts the amount of Ca added to the line pipe having the above components according to the formulas (1) and (2) to impart excellent sulfide stress cracking resistance of the welded portion.

[Equation 5]

Formula (1) is a condition necessary for sufficiently controlling the morphology of sulfides in order to prevent the formation of elongated MnS that causes relatively large sulfide stress cracking. The formula (2) is a condition for preventing the formation of CaO, Al ₂ O ₃ or complex oxides containing them as a main component, which exist in the form of groups, which cause a relatively small sulfide stress cracking. These conditions are also sufficient for obtaining the sulfide stress cracking resistance of the base material of the steel pipe.

[0021]

[Examples] Steels having the chemical composition shown in Table 1-1 were melted, slabs were manufactured by continuous casting, and after thick plate rolling, they were formed into UOE steel pipes. The size of the steel pipe has an outer diameter of about 30 inches and a pipe thickness of about 20 mm, and each steel pipe satisfies API standards X60 to X70, although it depends on the composition. NACE standard TM0177-90method in the direction perpendicular to the weld line from the seam weld
A similar test piece was prepared from the base material part of the steel pipe which was 180 ° away from the seam welded part. A constant load test was carried out according to the same standard, and after 720 hours, the critical stress and σth at which the test piece did not break were determined. σth was standardized by the standard minimum yield stress of the steel pipe, SMYS, and it was determined that the sulfide stress cracking resistance was excellent when σth / SMYS ≧ 0.8.

As shown in Table 1-2, under the conditions according to the present invention, in both cases, both the welded portion and the base metal have σth / SMYS.
≧ 0.80%, and excellent sulfide stress cracking resistance was obtained. However, in Comparative Example 1, the value of Formula (1), in Comparative Example 2 the Ca amount and the value of Formula (1), in Comparative Example 3 the S amount and the value of Formula (1), and in Comparative Example 4, the formula (1) The value of 2) is the value of Comparative Example 5
Since the Al amount and the value of the formula (2) deviate from the ranges of the present invention in Comparative Example 6, the value of the formula (2) in Comparative Example 7, and the Ca amount and the value of the formula (2) in Comparative Example 7, respectively, Sulfide stress cracking property decreases.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

INDUSTRIAL APPLICABILITY According to the present invention, AP having excellent resistance to sulfide stress cracking of welds in a humid hydrogen sulfide environment
Since the I grade X60 to X70 line pipes are obtained, the industrial effect is remarkably large.

[Brief description of drawings]

FIG. 1 shows that when the Ca / Al ratio is in the range of 0.07 to 0.17, the sulfide stress crack resistance of the weld is high.

Claims (2)

[Claims] 1. By weight%, C: 0.03 to 0.09%, Si: 0.1 to 0.6%, Mn: 1.1 to 1.5%, P: 0.015% or less, S: 0.0010% or less, Nb: 0.010 to 0.050%, Al: 0.005 to 0.03%, Ca: 0.002 to 0.004%, Ti: 0.005 to 0.025. %, With the balance consisting of iron and unavoidable impurities, and satisfying the following formulas (1) and (2), which is a high strength UOE line excellent in sulfide stress cracking resistance of welded parts. pipe. [Equation 1] 2. By weight%, C: 0.03 to 0.09%, Si: 0.1 to 0.6%, Mn: 1.1 to 1.5%, P: 0.015% or less, S: 0.0010% or less, Nb: 0.010 to 0.050%, Al: 0.005 to 0.03%, Ca: 0.002 to 0.004%, Ti: 0.005 to 0.025. %, V: 0.01-0.1%, Ni: 0.1-1.0%, Cu: 0.1-1.0%, Cr: 0.1-1.0%, Mo A welded part containing 0.1 to 0.5% of one or more kinds, the balance consisting of iron and unavoidable impurities, and satisfying the following formulas (1) and (2). High strength UOE line pipe with excellent sulfide stress cracking resistance. [Equation 2]