JP3160452B2 - Welded steel pipe with excellent carbon dioxide gas corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a submerged arc welded steel pipe used for transporting a fluid containing carbon dioxide gas such as natural gas or crude oil. The present invention relates to a welded steel pipe having excellent carbon dioxide corrosion resistance suitable for use in a steel pipe to be subjected to heat treatment.

[0002]

2. Description of the Related Art Conventionally, in pipelines for transporting carbon dioxide-containing fluids such as natural gas and crude oil containing carbon dioxide, an inhibitor is generally added to the transport fluid in order to prevent carbon dioxide corrosion. ing. However, the use of this inhibitor has not always been able to be said to work efficiently at all parts of the pipe. Moreover,
When a trouble occurs in the inhibitor injection device, there is also a problem that it is not possible to sufficiently cope with carbon dioxide gas corrosion.

[0003] In order to improve the corrosion resistance of the steel pipe material itself, such a problem has been conventionally dealt with by, for example, forming the pipe from stainless steel or high alloy steel. However, the use of such a corrosion-resistant material is expensive, and sulfide stress corrosion cracking is likely to occur when hydrogen sulfide is mixed, and workability of circumferential welding at the time of pipeline construction is poor. In particular, there is a problem that the heat-affected zone is easily cured.

[0004] As a technique for newly solving such a problem, Japanese Patent Laid-Open No. 3-110071 discloses a technique.
"C: 0.25% or less, Si: 0.01 to 0.50%, Mn: 0.1 to 2.0
%, P: 0.03% or less, S: 0.01% or less, Cr: 0.25 to 1.0
%, Al: 0.01 to 0.10%, with the balance being from 0.2% less Cr to 1.0% more Cr based on the base metal consisting of Fe and unavoidable impurities Steel pipe for pipelines with excellent carbon dioxide gas corrosion resistance. "
Has been proposed. However, when the steel pipe according to the conventional technique is subjected to heat treatment after pipe forming, such as a hot-bent pipe, there is a problem that the toughness of the weld metal is deteriorated due to the quench hardening property of Cr. Was.

[0005]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Laid-Open Publication No. Hei.
The steel pipe for pipeline disclosed in Japanese Patent No. 110071 has the disadvantage that the toughness of the weld metal at the weld joint is deteriorated by heat treatment, so that solving this problem further improves the line pipe for fluid transportation. I can do it.
Therefore, the present invention reduces both general corrosion and local corrosion in a submerged arc welded portion of a steel pipe for a pipeline used in an environment containing carbon dioxide gas, and has sufficient toughness regardless of the presence or absence of heat treatment. An object of the present invention is to provide a welded steel pipe excellent in carbon dioxide gas corrosion resistance.

[0006]

According to the present invention, the content of Cr is set to be less than 0.1.
Use a base material of 25-1.0 wt%, and
In a welded steel pipe obtained by applying one or more layers of submerged arc welding to the outer surface, the thickness of the welded metal pipe from the inner surface of the pipe to the thickness corresponding to the design corrosion allowance to 50% or less of the plate thickness Contains Cr whose Cr content is in the range of -0.2 to +1.0 wt% with respect to the Cr content of the base metal, and the Cr of the weld metal in the remaining thickness portion
The object has been achieved by making the content within the range of less than the Cr content of the base metal and the inner surface side weld metal.

It is desirable to adjust the Cr content in the weld metal by changing the composition of the filler used for each layer of the submerged arc welding performed from the inner and outer surfaces. This adjustment can also be made by selecting a groove shape, welding conditions, and the like, and the Cr content in the weld metal is adjusted by employing one or more of these means.

[0008]

According to the research conducted by the inventor on a material for a line pipe that sends a fluid containing carbon dioxide gas, the inner surface of the pipe,
In particular, it was found that the Cr content had a great influence on the material properties of the welded joint, and the relative proportion of the Cr content in the base metal and the weld metal was particularly important. That is,
If the Cr content of the submerged arc weld metal is smaller than the Cr content of the base metal by 0.2 wt% or more, local corrosion is likely to occur in the weld metal. On the other hand, even when the Cr content of the weld metal is at least 1.0 wt% greater than the Cr content of the base metal,
Local corrosion is likely to occur in the weld heat affected zone. Therefore,
In order to improve the carbon dioxide gas corrosion resistance, the Cr content of the submerged arc weld metal at this joint should be within the range of -0.2 to +1.0 wt% with respect to the Cr content of the base metal. There is a need.

Further, since Cr is a quench hardening element,
When heat treatment is performed, the toughness tends to deteriorate. In particular, the tendency is remarkable for a weld metal having a high oxygen content. FIG. 1 shows the results of investigating the effect of heat treatment on the absorbed energy by the Cr content in the weld metal by the Charpy impact test. As is apparent from this figure, when the heat treatment was not performed, no decrease in energy was observed even when the Cr content was 1 wt%, but when the heat treatment was performed, the higher the Cr content, the higher the Cr content. It was found that the energy reduction was remarkable.

From the above results, regarding the weld joint,
When observing in the thickness direction, it is necessary to consider separately the part where carbon dioxide gas corrosion resistance must be ensured and the part where toughness must be ensured. Both corrosion resistance and toughness can be achieved. For that purpose, the former part (corrosion resistance) is designed corrosion allowance (for example, when the plate thickness is 25 mm, 3 to
(The range of 10 mm), it is necessary to set the Cr content within the required range (−0.2 to +1.0 wt%) with respect to the Cr content of the base material. 50% of
Therefore, for the latter purpose, the Cr content of the remaining thickness portion, that is, at least a portion exceeding 50% of the plate thickness, is made lower than the Cr content of the base metal and the inner surface side weld metal. It is necessary.

[0011] For the above reasons, the Cr content of the weld metal in the portion of the thickness region not less than the design corrosion allowance and 50% or less of the plate thickness from the inner surface in the thickness direction of the steel pipe is determined by the Cr content of the base metal.
The Cr content of the weld metal having the remaining thickness was set to a range of not more than the Cr content of the base metal and the Cr content of the inner surface side weld metal.

The present invention is also effective for steel pipes not subjected to heat treatment, and has a cost advantage by reducing the Cr content in the outer surface weld metal.

[0013]

In the first embodiment of the present invention, a single layer of submerged arc welding was performed on both the inner and outer surfaces of a 20 mm thick base material (API standard 5L × 60) containing 1 wt% of Cr. The Cr content of the outer weld metal is about 0.6%, the Cr content of the inner weld metal is about 1.1%, and the proportion of each in the thickness direction is 3%.
Welded steel pipes with different stages were produced. As a comparative example, a welded steel pipe was prepared in which the Cr content of the weld metal was about 0.6% on both the inner and outer surfaces. For these, a corrosion test and a Charpy impact test of the weld metal were performed. Adjustment of the Cr content in the weld metal was performed by adjusting the groove shape and using a filler having a different Cr content.

Table 1 shows the chemical composition of the base material, and Table 2 shows the groove shape and welding conditions. Collect a test piece from the weld of the steel pipe,
After heat treatment was performed under the conditions shown in Table 3, an impact test and a corrosion test were performed under the conditions shown in Tables 4 and 5, respectively. Table 6 shows the Cr content of the weld metal and the welding wire, the results of the Charpy impact test, and the results of the corrosion test. For the corrosion test, a rotary immersion test apparatus shown in FIG. 2 was used. In order to investigate the state of local corrosion, the sample after the test was 5 mm x 25 mm x
The sample was cut into 100 mm strips, and eight sections were observed. The average value was obtained.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

[0018]

[Table 4]

[0019]

[Table 5]

[0020]

[Table 6]

Table 6 shows the relationship between each sample and the results of the impact test and the local corrosion allowance. The local corrosion allowance shown in Table 6 represents the difference between the corrosion allowance (thickness) of the weld metal and the corrosion allowance (thickness) of the base metal, and a positive value indicates that the base metal has a larger corrosion allowance than the weld metal. Conversely, a negative value indicates that the corrosion allowance of the weld metal is larger than that of the base metal. As is clear from this table, the joint C in which the inner weld metal having a high Cr content accounts for 76% of the plate thickness (Comparative Example 1)
Has almost no local corrosion, but has low impact test results. On the other hand, the inner surface weld metal with high Cr content
The joint A (Example 1 of the present invention) having 27% has a high impact test result. In addition, joint B in which the inner surface weld metal is about 50% of the plate thickness
Also in (Example 2 of the present invention), the Charpy absorbed energy is high, which is a general design energy of a line pipe.
It turned out that 40J or more can be fully satisfied. In Examples 1 and 2 of the present invention and Comparative Example 1, although local corrosion hardly occurred during the test period of 4 weeks, the joint D in which the Cr content of the weld metal was about 0.4 wt% lower than that of the base metal was obtained. (Comparative Example 2)
Has about 10 times more corrosion allowance on the weld metal side than the other three types of joints. Based on the above, if corrosion progresses in a corrosive environment for a long time and the high Cr portion on the inner surface disappears, local corrosion may suddenly occur, leading to leakage of the pipe, and it is considered that the corrosion rate may exceed the design corrosion allowance. Is Cr equivalent to the base metal
Can be said to be essential.

Next, a second embodiment of the present invention will be described. In this example, a single-layer submerged arc welding was performed on both the inner and outer surfaces of a 25 mm thick base material (API standard 5L × 60) containing 0.6 wt% Cr, and the Cr content of the outer surface weld metal was measured. Welded steel pipes were prepared in which was changed in five stages from about 0.4 wt% to 1.6 wt%. Further, as comparative examples, welded steel pipes in which the Cr content of the weld metal was equal to about 0.6 wt% and about 1.6 wt% on both the inner and outer surfaces were produced. These were subjected to a Charpy impact test of the weld metal. Table 7 shows the chemical composition of the base material, and Table 8 shows the groove shape and welding conditions. A test piece was prepared from the welded portion of the steel pipe, and the test temperature was set to -30 ° C for those subjected to heat treatment under the conditions shown in Table 3 and those not subjected to heat treatment.
The Charpy impact test was performed under the conditions shown in Table 4 except that the temperature was changed to -10 ° C.

Table 9 shows the relationship between the Cr content of the inner and outer surface side weld metals of each sample and the results of the Charpy impact test. In Examples 3 to 7 of the present invention, the higher the Cr content of the inner surface weld metal, the lower the impact energy. The reason for this is that when the inner surface weld metal is thermally affected by the successive outer surface weld metal, This is because part of the inner surface weld metal was embrittled by the quench hardening property of the steel. However, in this range, it is possible to secure 40 J, which is a general design standard for line pipes, and it is understood that the impact resistance is improved after the heat treatment. On the other hand, in Comparative Examples 3 and 4, since the Cr content of the outer surface weld metal was relatively high, the impact strength in the as-welded state was lower than that of Inventive Example 4 or 7 in which the Cr content of the inner surface weld metal was equivalent. Although relatively high, it is extremely low after heat treatment.

[0024]

[Table 7]

[0025]

[Table 8]

[0026]

[Table 9]

As described above, in order to obtain a balanced toughness both in the as-welded state and after the heat treatment, the Cr content of the outer side weld metal is made lower than that of the base metal and the inner side weld metal. The superiority of the present invention is understood.

[0028]

As described above, the welded steel pipe of the present invention is excellent in toughness even if it is a heat-treated steel pipe, has little overall corrosion and local corrosion in a fluid transportation line containing carbon dioxide gas, and has a long service life. Can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between Cr content and Charpy impact performance.

FIG. 2 is an explanatory diagram of a CO ₂ corrosion test apparatus.

FIG. 3 is a diagram of a test piece collection capacity.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B23K 9/23 C22C 38/00 C22C 38/28

Claims (1)

(57) [Claims] 1. A welded steel pipe obtained by using a base metal having a Cr content of 0.25 to 1.0 wt% and performing one or more layers of submerged arc welding on each of the inner and outer surfaces of the welded joint. In the range from the thickness corresponding to the design corrosion allowance to the thickness corresponding to 50% or less of the plate thickness, the Cr content of the weld metal is -0.2 to +1.0 wt% with respect to the Cr content of the base metal. The Cr content of the weld metal in the remaining thickness portion is within the range of less than the Cr content of the base metal and the Cr on the inner surface side weld metal. Excellent welded steel pipe.