JPH09209079A - Welded steel pipe excellent in corrosion resistance to carbon dioxide gas and toughness in weld zone, pipe, line, and girth welding method for steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a welded steel pipe excellent in toughness in a weld zone as well as in corrosion resistance to carbon dioxide gas by regulating, in a welded steel pipe containing specific weight percentages of specific elements, the proportion of the amount of a specific element, in wt.%, in a weld metal to that in a base material to a value in a specific range and also regulating the ratio, by wt.%, between other specific elements to a value in a specific range. SOLUTION: In this welded steel pipe, a base material part has a chemical composition which consists of, by weight, <=0.25% C, 0.01-0.5% Si, 0.1-2% Mn, 0.25-1% Cr, 0.0005-0.02% Al, 0.005-0.02% Ti, 0.003-0.010% N, 0-1% Cu, 0-1% Ni, 0-0.5% Mo, 0-0.3% V, 0-0.3% Nb, 0-0.002% B, O-0.006% Ca, and the balance Fe with inevitable impurities and in which the amounts of P and S among the impurities are limited to <=0.03% and <=0.03%, respectively. Moreover, the proportion of Cr, in wt.%, in a weld metal in a weld zone to Cr in a base material is regulated to 0.5-2, and further, the ratio, by wt.%, between Al and O in the weld metal is regulated to 0.2-1.2. By this method, the welded steel pipe, excellent in toughness in weld zone as well as in corrosion resistance to carbon dioxide gas, can be produced.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a welded steel pipe and pipeline excellent in carbon dioxide gas corrosion resistance and weld toughness, and a method for circumferentially welding a steel pipe.

[0002]

2. Description of the Related Art Welded steel pipes have been mainly used for the production of pipelines for transporting crude oil and natural gas, as well as towers and tanks for refining crude oil and natural gas. However, in recent years, with the depletion of high-quality petroleum resources, the environment in which the steel pipe or the like as the base material is used, there is hydrogen sulfide or carbon dioxide gas, or the concentration of these gases is higher than conventional,
It has become more severe. In such an environment,
Sufficient countermeasures against carbon dioxide corrosion and sulfide stress corrosion cracking are required. Further, from the viewpoint of ensuring the safety of the welded structure of the pipeline, it is essential that the welded portion (weld metal and weld heat affected zone) has sufficient toughness.

The addition of Cr is effective for improving the carbon dioxide corrosion resistance of the base material itself of a steel pipe for pipelines. For example, Japanese Patent Application Laid-Open No. 54-124817 discloses C addition.
A steel pipe for a line pipe to which r is added is disclosed. Further, in Japanese Patent Application Laid-Open No. 3-110071, carbon dioxide corrosion is prevented by defining the Cr content of the base metal (hereinafter, the “content” is also simply referred to as “amount”) and the Cr content in the weld metal. A method for preventing selective corrosion of a welded part under a generated environment, Japanese Patent Laid-Open No. 4-66648, describes C in weld metal.
A method for improving the selective corrosion resistance of a welded portion is disclosed by defining the amount of r, or the amount of Ni and the amount of Cu.

However, from the viewpoint of ensuring the toughness of the welded portion, C
Addition of r is not preferable, and if the amount of Cr is increased, the hardenability is increased and the toughness of the welded portion is deteriorated. Therefore, it is more difficult to secure the weld toughness of the Cr-added steel as compared with the Cr-free steel, and in the Cr-added steel, a welded steel pipe excellent in carbon dioxide gas corrosion resistance and also in the weld toughness has not yet been obtained. The current situation is that it has not been developed.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of such a situation, and is a welded steel pipe and pipeline excellent in both carbon dioxide gas corrosion resistance and weld toughness, and circumferential welding of the steel pipe. It is intended to provide a way.

[0006]

Means for Solving the Problems In order to obtain a welded steel pipe excellent in both carbon dioxide corrosion resistance and weld zone toughness, the present inventors have conducted an examination focusing on the following points. 2)
And 3), the Cr-added steel was mixed with various Cr
Submerged arc welding was carried out using a certain amount of welding wire, and the selective corrosion behavior and toughness of the weld were investigated.

1) Effect of Cr Content on Carbon Dioxide Corrosion Resistance of Steel Pipe Base Material 2) Selective Corrosion Behavior in Weld of Cr-Added Steel 3) Effect of Cr Content on Weld Toughness and Method of Improving Weld Toughness As a result, the following findings were obtained.

1) The addition of Cr is effective for improving the carbon dioxide corrosion resistance of the steel pipe base material, and 0.25 wt% or more of Cr is used.
By containing the above, the carbon dioxide gas corrosion resistance can be dramatically improved.

2) If the Cr content (wt%) in the weld metal is controlled to be 0.5 times or more and less than 2 times the Cr content (wt%) of the base metal, the selective corrosion of the welded portion can be significantly suppressed.

3) As the amount of Cr increases, the toughness of the heat-affected zone and the weld metal deteriorate. In order to prevent the deterioration of the toughness of the heat-affected zone of welding, the Ti amount and N amount of the base material are specified, the microstructure is made fine by utilizing fine precipitates, and
It is essential to limit the amount to suppress the formation of hardened tissue. Further, in order to secure the toughness of the weld metal, balance the amount of Al (aluminum) and the amount of O (oxygen) in the weld metal, that is, the ratio of the amount of Al (wt%) to the amount of O (wt%). It is necessary to limit (Al (wt%) / O (wt%), hereinafter simply referred to as “Al / O”) to 0.2 or more and 1.2 or less. When Al / O is controlled within the above range, the structure of the weld metal is remarkably refined and high toughness is obtained.

The present invention has been made based on the above findings, and its gist resides in the following (1) welded steel pipe and (2) pipeline, and (3) circumferential welding method of steel pipe. In addition, "%" of the chemical components in the base metal portion, the weld metal, and the welding wire means "% by weight".

(1) The base material portion is C: 0.25% or less, S
i: 0.01 to 0.5%, Mn: 0.1 to 2%, Cr:
0.25 to 1%, Al: 0.0005 to 0.02%, T
i: 0.005-0.02%, N: 0.003-0.0
10%, Cu: 0 to 1%, Ni: 0 to 1%, Mo: 0
0.5%, V: 0 to 0.3%, Nb: 0 to 0.3%,
Chemical composition containing B: 0 to 0.002% and Ca: 0 to 0.006%, the balance being Fe and inevitable impurities, P in the impurities being 0.03% or less and S being 0.03% or less. It is a welded steel pipe having a composition, and the Cr content (%) in the weld metal of the welded portion is 0.5 times or more of the Cr content (%) in the base metal 2
Double or less, and the amount of Al (%) and the amount of O (%) in the weld metal
Ratio (Al (%) / O (%)), hereinafter simply referred to as “Al /
O)) is 0.2 or more and 1.2 or less. Welded steel pipe excellent in carbon dioxide corrosion resistance and weld toughness.

(2) A pipeline in which the welded steel pipe described in (1) above or the seamless steel pipe having the chemical composition of the base material of the welded steel pipe described in (1) above is connected by circumferential welding. The Cr content (%) in the weld metal of the connection portion is 0.5 times or more and 2 times or less than the Cr content (%) in the base material of the welded steel pipe or the Cr content (%) in the seamless steel pipe. And the Al / O in the weld metal is 0.2 or more and 1.2 or less, a pipeline excellent in carbon dioxide corrosion resistance and weld toughness.

(3) When the welded steel pipe described in (1) above or the seamless steel pipe having the chemical composition of the base material of the welded steel pipe described in (1) above is connected by circumferential welding, the amount of Cr is (%) Is 0.5 times or more and 2 times or less with respect to the Cr amount (%) in the base material of the welded steel pipe or the Cr amount (%) of the seamless steel pipe, and a welding wire satisfying the following formula is used, A method for circumferentially welding a steel pipe, wherein the gas metal arc welding is performed so that Al / O in the weld metal of the connection portion is 0.2 or more and 1.2 or less.

Ti (%) + 3 × Al (%) ≦ 0.5

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

First, in the welded steel pipe of the present invention (the welded steel pipe of the above (1)), the effect of the chemical composition of the steel as the base material and the reason for limiting the content thereof will be described.

C: 0.25% or less C is necessary as an element for increasing the strength of steel, but 0.2
If the content exceeds 5%, the toughness of the base material and the weld heat affected zone deteriorates. Therefore, the content of C is set to 0.25% or less. The lower limit is not particularly limited, but it is desirable to contain 0.03% or more from the viewpoint of ensuring the strength of the steel.

Si: 0.01 to 0.5% Si is an element that is useful for deoxidizing molten steel and is also effective for securing the strength of steel, but if the content is less than 0.01%, it has no effect. On the other hand, if the content exceeds 0.5% and is excessively contained, the toughness of the base material and the weld heat affected zone deteriorates. Therefore, Si
Content of 0.01 to 0.5%.

Mn: 0.1 to 2% Mn is an element effective for ensuring the strength of steel. However, if its content is less than 0.1%, it is not effective, while if it exceeds 2%, the toughness and weldability of the base material are significantly deteriorated. Therefore, the Mn content is set to 0.1 to 2%.

Cr: 0.25 to 1% Cr is an essential element for improving the strength of steel and for improving the carbon dioxide corrosion resistance in the present invention. However, the effect is not remarkable when it is less than 0.25%. On the other hand, even if the content exceeds 1%, the effect corresponding to that is not improved, and the toughness of the base material and the weld heat affected zone deteriorates. Therefore, the content of Cr is 0.
It was set to 25 to 1%.

Al: 0.0005 to 0.02% Al is useful for deoxidizing molten steel, and for that purpose 0.00
It is necessary that the content be at least 05%. However, 0.
If it is contained in excess of 02%, the formation of a hardened structure (island martensite) in the heat-affected zone of welding is promoted,
Since the toughness of the heat-affected zone of welding is significantly deteriorated, the upper limit is set to 0.
02%. From the viewpoint of ensuring the toughness of the heat affected zone
It is desirable that the amount of 1 be 0.01% or less.

Ti: 0.005 to 0.02% Ti is stable and forms fine carbonitrides even at high temperatures, and suppresses the coarsening of crystal grains in the weld heat affected zone by the pinning effect. Has the effect of significantly increasing the toughness of In order to obtain such effects, it is necessary to contain 0.005% or more. On the other hand, when the content exceeds 0.02%, the toughness of the heat-affected zone is rather decreased due to the decrease in the effect of suppressing the coarsening of the crystal grains due to the coarsening of the carbonitride to be formed and the increase in the solid solution Ti. Deteriorates.

N: 0.003 to 0.010% N has a great influence on the formation of Ti carbonitrides. However, if the N content is less than 0.003%, a sufficient amount of Ti
Carbonitrides are not formed, and the desired effect of improving the toughness of the weld heat affected zone cannot be obtained. On the other hand, when the content exceeds 0.010%, the deterioration of toughness due to an increase in solute N is predominant rather than the effect of improving the toughness of the weld heat affected zone due to the refinement of the structure. In addition, particularly in a welding method in which the base metal is highly diluted, such as submerged arc welding, excessive addition of N to the base metal causes an increase in the amount of N in the weld metal, which deteriorates the toughness of the weld metal. Therefore, the N amount is 0.003 to
It was set to 0.010%.

Cu: 0 to 1% Ni: 0 to 1% Mo: 0 to 0.5% V: 0 to 0.3% Nb: 0 to 0.3% B: 0 to 0.002% These elements All have the effect of increasing the strength of the steel, so they are added as necessary. However, Cu and N
1% for i, 0.5% for Mo,
If the content of V and Nb exceeds 0.3% and the content of B exceeds 0.002%, the toughness of the heat-affected zone of the weld decreases, so the respective contents were defined as above. In order to sufficiently exert the above-mentioned strength improving effect, Cu and Ni are each 0.1 to 0.1%.
1%, 0.1-0.5% for Mo, V and Nb
Is preferably 0.01 to 0.3%, and B is 0.0003 to 0.002%.

Ca: 0 to 0.006% or less Since Ca has a function of controlling the morphology of inclusions and improving the hydrogen-induced cracking resistance (HIC resistance) of the base material, it is added if necessary. However, if the content exceeds 0.006%, coarse inclusions are formed, the toughness of the base material and the weld heat affected zone deteriorates, and the weldability also deteriorates. Stipulated. In order to fully exert the above effects, the content is preferably 0.001 to 0.006%.

The welded steel pipe of the above (1) is a steel pipe in which the base material portion is composed of Fe and the unavoidable impurities with the balance other than the above components. As impurities, it is necessary to suppress the upper limits of P and S.

P: 0.03% or less S: 0.03% or less Both P and S are unavoidable impurity elements contained in steel, and the smaller the amount, the better. However, reduction of these elements causes an increase in manufacturing cost. Therefore, considering economic efficiency, the upper limit was set to 0.03% for both P and S.

Next, the reason why the composition of the weld metal (ie, seam weld metal) in the welded steel pipe of the above (1) is limited as described above will be described.

The Cr content (%) in the weld metal is the Cr content in the base metal.
If it is less than 0.5 times the amount (%), selective corrosion occurs in the weld metal, while if it exceeds 2 times, selective corrosion occurs in the weld heat affected zone. That is, it is important to balance the Cr amount between the base metal and the weld metal, and the Cr amount (%) in the weld metal is 0.5 times or more than the Cr amount (%) in the base metal.
The selective corrosion can be prevented by adjusting so as to be equal to or less than twice.

Furthermore, Al / O in the weld metal must be 0.2 or more and 1.2 or less. If this ratio is less than 0.2 or exceeds 1.2, the structure of the weld metal is Becomes coarse and the toughness deteriorates significantly.

The pipeline of (2) above is based on (1) above.
The welded steel pipe of, or a seamless steel pipe having the chemical composition of the base material portion of the welded steel pipe is a pipeline connected by circumferential welding. Also in this pipeline, the composition of the weld metal (that is, the weld metal of the circumferential weld portion) is the same as that of the seam weld metal of the above welded steel pipe, and the Cr content (%) in the weld metal is the base metal ( It is 0.5 times or more and 2 times or less with respect to the Cr amount (%) in the base material portion of the welded steel pipe or the seamless steel pipe), and the Al / O in the weld metal is 0.2 or more and 1.
It must be 2 or less.

Next, a method (invention of the above (3)) for circumferentially welding the welded steel pipe of the above (1) or a seamless steel pipe having the same chemical composition as the base material of the steel pipe will be described.

This welding method is carried out by using the welded steel pipe of the above (1),
Alternatively, when connecting a seamless steel pipe having the chemical composition of the base metal portion of the welded steel pipe by circumferential welding, the Cr content (%) becomes the Cr content (%) of the base metal portion of the welded steel pipe or the seamless steel pipe. 0.5 times or more and 2 times or less, and "Ti amount (%)
+ 3 × Al amount (%) ”is 0.5 or less, and Al / O in the weld metal of the connection part (circumferential weld part) is used.
Is a method of performing gas metal arc welding so that the value is 0.2 or more and 1.2 or less.

The gas metal arc welding method is adopted.
This is to improve the welding work efficiency. Further, since the gas metal arc welding performed at the time of circumferential welding is generally performed with a single electrode, it is possible to adjust the Cr amount in the weld metal to a desired range by defining the Cr amount in the welding wire. This is possible.

The reason why the Cr content of the welding wire is limited as described above is for the same reason as in the case of the seam weld metal of the above welded steel pipe. The Cr content (%) of the welding wire is defined as the base metal of the welded steel pipe. If the amount of Cr in the weld metal is 0.5 times or more and 2 times or less with respect to the Cr content (%) of the section or seamless steel pipe, the Cr content (%) in the weld metal is %) To 0.5 times or more and twice or less, so that selective corrosion can be prevented.

The “Ti amount (%) + 3 × Al amount (%)” of the welding wire is set to 0.5 or less in order to perform the welding operation smoothly and without causing defects. If these elements are excessively contained in the welding wire, the amount of dissolved oxygen in the molten metal will decrease, the surface tension will increase and it will be difficult to separate the droplets, and the droplets will increase and the amount of spatter will increase. As a result, welding workability is reduced. Therefore, the “Ti amount (%) + 3 × Al amount (%)” of the welding wire is defined as 0.5 or less. The lower limit is not particularly specified, but it is preferably 0.025 in order to secure the toughness of the weld metal.

Al / O in the weld metal of the connection
Welding is carried out so that
As described above, Al / O is less than 0.2, or 1.
When it exceeds 2, the structure of the weld metal is coarsened and the toughness is significantly deteriorated.

[0039]

【Example】

Example 1 A steel having the composition shown in Table 1 and Table 1
After melting in a 50 kg vacuum melting furnace, lumping and rolling, appropriate heat treatment was performed to finish a steel plate with a plate thickness of 30 mm to manufacture a base material.

Welded joints were prepared by welding one layer on both sides to these steel sheets by three-electrode submerged arc welding, and the corrosion resistance and weld toughness of the base metal portion and the welded portion were investigated.

At the time of welding, a commercially available molten flux, a wire (diameter 4 mm), and a trial wire with various amounts of Cr and Al were used, and the welding heat input was 60 k.
It was set to J / cm. Table 1 and Table 2 show the amount of Cr in the obtained weld metal, the ratio of the amount of Cr in the weld metal to the amount of Cr in the base metal (indicated as "Cr _WM / Cr _BM " in the table) and Al / O. .

The welding method used is the same as the welding method used for vertical seam welding of UO steel pipes and the like. UO
In the vertical seam welding of steel pipes, generally, the inner surface is welded and then the outer surface is welded.Therefore, in the welded joint produced in this example, for convenience, the steel plate surface previously welded is `` inner surface '' and later welded. The surface of the steel plate is referred to as "outer surface".

The corrosion resistance was evaluated by immersing the test piece in a test solution (artificial seawater) saturated with carbon dioxide (CO ₂ ) at 1 atm, and after 96 hours, the general corrosion rate of the base metal and the welding were performed. It was carried out by measuring the selective corrosion depth of the parts (welded metal and weld heat affected zone). The test solution was at 50 ° C., and the solution was constantly stirred during the test. For the measurement of the general corrosion rate of the base metal part, a test piece (thickness 2 mm x width 10 mm x length 40 mm) which does not include the welded part, which was taken from the base material part, was used.
The corrosion weight loss was calculated from the weight difference before and after the test, and the corrosion rate was converted to a corrosion rate (mm / y) per year. Further, as shown in FIG. 1, the selective corrosion depth of the welded portion is determined by the welded portion corrosion resistance evaluation test piece 3 (thickness of the welded portion obtained from the inner surface of the steel plate 1 so that the weld metal 2 is located at the center of the test piece). 2m
m × width 10 mm × length 40 mm) and the unevenness of the surface (welding metal and welding heat affected zone) after the test was measured by measuring with a surface roughness meter.

The weld toughness was evaluated by the transition temperature obtained by the Charpy impact test. The test piece used is shown in FIG.
As shown in Fig. 3, from the 1/4 part in the plate thickness direction on the outer surface side of the steel plate 1 to the position where the notch position corresponds to the bond part (more accurately, the weld metal and the weld heat effect indicated by the arrow in the figure). Charpy test piece 4 sampled so that the part becomes 1: 1 in the plate thickness direction) and Charpy test piece sampled so that the notch position corresponds to the center of the weld metal (not shown) Is. The toughness of the weld heat affected zone was evaluated by the transition temperature obtained from the test piece 4 having a notch in the bond portion, and the toughness of the weld metal was evaluated by the transition temperature obtained from the test piece having a notch in the center of the weld metal. .

[0045]

[Table 1]

[0046]

[Table 2]

Tables 3 and 4 collectively show the evaluation results of corrosion resistance and weld toughness. In these tables, the measured values in the column of selective corrosion depth are shown with a minus sign when the corrosion of the weld metal is large with respect to the heat affected zone, and in the opposite case, without a sign. .

In Test Nos. 1 and 2 (hereinafter abbreviated as test numbers), the Cr content of the base metal is out of the lower limit defined by the present invention, and the corrosion rate of the base metal is remarkably high. As the Cr content increases, the corrosion rate of the base metal decreases, and the Cr content is 0.2
The corrosion rate of the base metal was 5 mm / y or less in steel types of 5% or more (trial numbers 3 to 7). However, when Cr is contained in an amount of more than 1%, the corrosion rate decreases slightly with an increase in the Cr amount, which is not preferable from an economical point of view.
Also, as the Cr content increases, the welding heat affected zone (in the table,
HAZ) (indicated as HAZ) tends to decrease, and in Sample No. 7 in which the Cr content exceeds 1%, the toughness of the weld heat affected zone is -35.
It has deteriorated to ℃.

Trial Nos. 8 to 13 are results obtained by using a welded joint in which a base material having almost the same composition as Trial No. 5 was used and the amount of Cr in the weld metal was changed. In trial No. 8 in which the Cr content of the weld metal is less than 0.5 times the Cr content of the base metal,
Selective corrosion exceeding 0.1 mm in depth occurred in the weld metal. On the other hand, in trial No. 13 in which the amount of Cr in the weld metal exceeds twice the amount of Cr in the base metal, the depth of the weld heat affected zone was 0.1 m.
Selective corrosion exceeding m occurred.

In the case of trial Nos. 9 to 12 in which the ratio of the Cr content of the weld metal to that of the base metal satisfies the requirements of the present invention, the selective corrosion depth is 0.1.
It is found that the thickness is less than or equal to mm, and that it has excellent selective corrosion resistance.

Regarding the toughness of the weld heat-affected zone of trial Nos. 8 to 13, it was -50.degree. C. or less in trial Nos. 8 to 10, whereas it was -42 to -45.degree. C. in trial Nos. 11 to 13. However, a slight deterioration in the toughness of the heat affected zone was observed. Such a change in the toughness of the heat-affected zone is due to A in the base metal.
It is preferable that the Al content be 0.01% or less from the viewpoint of ensuring the toughness of the weld heat affected zone, because of the difference in the 1 content.

Test Nos. 14 to 20 are results obtained by using the same base material as in Test No. 4 and using a welded joint in which Al / O in the weld metal was changed. In sample numbers 15 to 17 in which Al / O is within the range defined in the present invention, the toughness of the weld metal is excellent at a transition temperature of −60 ° C. or less, whereas in the sample numbers out of the rule of the present invention. At 14, 18, 19 and 20,
The toughness of the weld metal deteriorated. In addition, trial number 19 and 20
For, the toughness of the weld heat affected zone also deteriorated, but because the toughness of the weld heat affected zone was evaluated using a test piece with a notch in the bond, it was affected by the deterioration of the toughness of the weld metal. It is due to.

In Test No. 21, since the amount of Ti in the base material was insufficient, the coarsening of the structure in the weld heat affected zone was not sufficiently suppressed, and the toughness of the weld heat affected zone was significantly deteriorated. In sample No. 22, the toughness of the heat-affected zone of welding was deteriorated because the amount of Ti in the base material was excessive.

Test Nos. 23 to 28 have the same composition system and have N as the base material.
It is the result obtained by the welded joint with varying amounts. In sample No. 23, the amount of N was insufficient and the toughness of the heat-affected zone of welding was deteriorated. In sample No. 28, the solid solution N increases because the N content is excessive, and the toughness of the heat-affected zone of the weld tends to deteriorate slightly. Furthermore, due to the increase in the N content in the weld metal due to dilution of the base metal, the weld metal Toughness was significantly deteriorated.

Trial Nos. 29 to 33 have the same composition system and A of the base metal
It is the result obtained by the welded joint in which the amount of l was changed.
In sample Nos. 32 and 33 having an excessive amount of Al, the toughness of the weld heat affected zone deteriorated. Also in trial numbers 29 to 31 in which the Al amount is within the range defined by the present invention, it is recognized that the toughness of the weld heat affected zone is improved with the reduction of the Al amount. In Nos. 29 and 30, the toughness of the heat-affected zone was -50 ° C or less, which was extremely good.

As described above, in a welded joint deviating from the regulations of the present invention, at least one of the corrosion resistance of the base metal, the selective corrosion resistance, the toughness of the weld metal, and the toughness of the weld heat affected zone is deteriorated. In contrast, it is clear that welded joints meeting the requirements of the present invention have excellent performance.

Trial Nos. 34 to 44 are Cu, Ni, Mo,
It is an example corresponding to a welded steel pipe containing one or more of V, Nb, B and Ca (all of which are components added as necessary), and any welded joint has corrosion resistance and selective corrosion resistance of the base metal. It is clear that the toughness of the weld metal and the toughness of the weld heat affected zone are excellent.

[0058]

[Table 3]

[0059]

[Table 4]

Example 2 A seamless steel pipe having a composition shown in Table 5 (diameter: 357 mm × wall thickness: 18 mm), and a welding wire having a Cr content, a Ti content, and an Al content shown in Table 6 (diameter: 1.2).
mm) as a prototype, and gas metal arc welding (shield gas: Ar-20% CO ₂ , welding conditions: 280A-33)
V, 20 cm / min) was used for circumferential welding to produce a welded joint, and corrosion resistance and weld toughness were investigated by the same method as in Example 1.

The corrosion resistance was evaluated from the inside of the steel pipe in Example 1.
Using the corrosion resistance test piece for welded portion collected by the same method as above, a corrosion resistance test was performed in the same manner, and the depth of selective corrosion generated in the welded portion was measured.

To evaluate the toughness of the weld zone, a transition temperature was determined by a Charpy impact test using a test piece taken from the center of the plate thickness of the steel pipe so that the notch position corresponded to the center of the weld metal. The toughness was evaluated.

Table 6 shows the evaluation results. Further, Table 6 also shows the results of evaluation of the weldability based on the stability of the welding arc and the occurrence of spatter. As in the case of Example 1, the measured value in the column of the selective corrosion depth is displayed with a minus sign when the corrosion of the weld metal is large with respect to the weld heat affected zone, and in the opposite case, the sign. Expressed without. In this example, since the steel pipe main pipe was the same in all the joints used in the test, only in trial number B, the general corrosion rate of the base metal and the toughness of the weld heat affected zone were measured. as a result,
It was confirmed that the general corrosion rate was 1.22 mm / y and the toughness of the weld heat affected zone in the circumferential weld was −65 ° C., which was excellent in both general corrosion resistance and toughness of the weld heat affected zone.

Trial Nos. A to E are the results when the Cr content in the weld metal was changed by using the welding wire in which the Cr content was changed. In the trial Nos. A and E, the amount of Cr in the wire did not satisfy the conditions defined in the present invention, and therefore the amount of Cr in the weld metal was 0.5 times or more and 2 times or less of the Cr amount of the base metal. Out of the range, selective corrosion occurred in the weld.

In trial Nos. B, C and D satisfying the conditions defined in the present invention, the occurrence of selective corrosion was remarkably suppressed, and the toughness of the welded portion was excellent, and the welding workability was also good.

Test Nos. E to J are the results when the Al amount of the welding wire was changed and the Al / O in the weld metal was changed. In sample Nos. E and J, Al / O in the weld metal was out of the range defined by the present invention, and deterioration of the toughness of the weld metal was recognized.

In trial numbers L and M, "Ti (%) + 3 × A"
The index represented by "l (%)" deviates from the range defined by the present invention, and therefore, the welding work could not be stably performed. In trial number L, Al / O was also outside the range defined by the present invention, and deterioration of the toughness of the weld metal was observed.

As described above, when the circumferential welding is performed under the condition out of the range defined by the present invention, the selective corrosion resistance,
At least one of the toughness and welding workability of the weld metal was poor.

[0069]

[Table 5]

[0070]

[Table 6]

[0071]

EFFECTS OF THE INVENTION The welded steel pipe and pipeline of the present invention are excellent in carbon dioxide gas corrosion resistance and also in weld zone toughness, and are highly safe. With the circumferential welding method of the present invention,
A welded steel pipe or a seamless steel pipe can be connected to manufacture the pipeline, or a welded structure including the welded steel pipe can be constructed, and welding can be efficiently performed.

[Brief description of drawings]

FIG. 1 is a diagram showing a method of collecting a test piece from a welded portion.

[Explanation of reference symbols] 1: Steel plate 2: Weld metal 3: Test piece for evaluating corrosion resistance of weld zone 4: Charpy test piece

Claims (3)

[Claims] 1. A base material part, in% by weight, C: 0.25% or less, Si: 0.01 to 0.5%, Mn: 0.1 to 2%,
Cr: 0.25 to 1%, Al: 0.0005 to 0.02
%, Ti: 0.005 to 0.02%, N: 0.003 to
0.010%, Cu: 0 to 1%, Ni: 0 to 1%, M
o: 0 to 0.5%, V: 0 to 0.3%, Nb: 0 to 0.
3%, B: 0 to 0.002% and Ca: 0 to 0.00
A welded steel pipe containing 6%, the balance being Fe and unavoidable impurities, P in the impurities being 0.03% or less, and S being 0.03% or less in the weld metal of the weld metal. Carbon dioxide corrosion resistance, characterized in that the Cr content is 0.5 times or more and 2 times or less by weight% of the Cr content in the base metal, and the Al content and O content in the weld metal satisfy the following formula And welded steel pipe with excellent weld toughness. 0.2 ≦ Al (wt%) / O (wt%) ≦ 1.2 2. A pipeline in which the welded steel pipe according to claim 1 or the seamless steel pipe having the chemical composition of the base material of the welded steel pipe according to claim 1 is connected by circumferential welding, The amount of Cr in the weld metal of the connection portion is 0.5 times or more and 2 times or less in weight% with respect to the amount of Cr in the base material of the welded steel pipe or the amount of Cr in the seamless steel pipe, and the amount of Al in the weld metal. And O content satisfy the following formula, a pipeline excellent in carbon dioxide corrosion resistance and weld toughness. 0.2 ≦ Al (wt%) / O (wt%) ≦ 1.2 3. The welded steel pipe according to claim 1, or the seamless steel pipe having the chemical composition of the base material of the welded steel pipe according to claim 1, when the welded steel pipe is connected by circumferential welding, the amount of Cr is the above-mentioned weld. Using a welding wire having a Cr content in the base material of the steel pipe or a Cr content in the seamless steel pipe in an amount of 0.5 times or more and 2 times or less by weight% and satisfying the following formula, Al in the weld metal of the connection part is used.
Welding method for steel pipes, characterized in that gas metal arc welding is performed so that the amount of oxygen and the amount of O satisfy the following formula. 0.2 ≦ Al (wt%) / O (wt%) ≦ 1.2 ... Ti (wt%) + 3 × Al (wt%) ≦ 0.5