JP2003306748A - Pipeline having excellent low temperature toughness in weld metal part and electron beam circumferential welding method for steel pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pipeline in which the low temperature toughness of a circumferential weld metal part is excellent in a circumferential direction, and to provide a circumferential electron beam welding method of steel pipes. <P>SOLUTION: In the pipeline, the circumferential weld metal part of a steel pipe obtained by welding steel pipes in a circumferential direction by electron beam welding contains, by mass, 0.005 to 0.020% solid solution Ti and 0.0015 to 0.0060% solid solution N. In the electron beam circumferential welding method for steel pipes, as for the cooling rate of the circumferential weld metal part, the cooling time from 800 to 500°C is controlled to ≤5 s, and the cooling time from 500 to 300°C is controlled to 2 to 10 s; and the content of solid solution Ti in the circumferential weld metal part is controlled to, by mass, 0.005 to 0.020%, and the content of solid solution N is controlled to 0.0015 to 0.0060%. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a pipeline excellent in low temperature toughness of a weld metal portion and an electron beam welding method for a steel pipe.

[0002]

2. Description of the Related Art For steel pipes used for large-diameter linen pipes for transporting crude oil and natural gas in cold regions and offshore, they have high strength, low temperature toughness and field weldability, especially weldability in the circumferential direction. Is required to be excellent. Especially in recent years, from the viewpoint of welding work efficiency, the application of electron beam welding or laser welding has been advanced as a welding method that can weld thick-walled steel pipes in one pass and improve both welding efficiency and quality at the same time. There is. Unlike conventional welding methods (MIG welding, TIG welding, covered arc welding, submerged arc welding, etc.), these welding methods do not use welding materials, but the materials to be welded, for example, the metal part of the steel pipe itself in a pipeline. Are melted, solidified and joined.

Therefore, in electron beam welding, since the material to be welded, which is the metal portion of the steel pipe itself, is melted and then solidified, it is difficult to adjust the composition (structure control) of the weld metal portion, and the weld metal portion The composition depends mostly on the steel pipe material. Therefore, the composition of the steel pipe has a great influence on the low temperature toughness of the circumferential weld metal portion of the pipeline.

[0004]

The steel having excellent low-temperature toughness in the welding heat shadow zone (HAZ) developed for use in the conventional welding method is toughness in the weld metal portion when applied to electron beam welding. Could not be improved. On the other hand, methods for improving the low temperature toughness of a weld metal portion welded by an electron beam or the like are disclosed in JP-A-64-15321 and JP-A-2-22418. However, these methods
Ti in the weld heat shadow (HAZ) and in the weld metal
₂ 0 _3, TiN was finely dispersed, but tissue is miniaturized, there is a problem that variation in the absorbed energy of the weld metal obtained by performing Charpy impact test at low temperature is large.

Further, in Japanese Patent Laid-Open No. 2001-20742, a steel pipe excellent in low temperature toughness of a weld metal portion welded to an electron beam or a laser, and excellent in hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance. And pipelines. However, the problem that the low temperature toughness is inferior in a part of the weld metal part where the steel pipes are welded in the circumferential direction by electron beam welding using such a steel pipe, and the variation in absorbed energy is large cannot be solved. . If such a portion with low low temperature toughness is present in a part of the pipeline, variations in absorbed energy occur in a temperature range of 0 ° C. or less, particularly in a temperature range close to a test temperature at which a pipeline performance test (Charpy impact test) is performed. Easy to do is a problem.

The present invention advantageously solves the above-mentioned problems, and an object of the present invention is to provide a pipeline and an electron beam welding method for steel pipes, in which the low temperature toughness of the weld metal portion is excellent in the circumferential direction. .

[0007]

The present invention is as follows. 1. The weld metal part where steel pipes are welded together is in mass% and forms a solid solution.
Ti amount: 0.005 to 0.020%, solute N amount: 0.0015 to 0.0060%
A pipeline excellent in low-temperature toughness of a welded metal portion, which is characterized by containing. 2. The steel pipe according to claim 1 has a mass% of C: 0.03 to 0.
06%, Si: 0.1 to 0.6%, Mn: 0.80 to 2.0%, P: 0.01
5% or less, Ti: 0.005 to 0.025%, Al: 0.010 to 0.050
%, O: 0.0030% or less, and a steel pipe having the balance of iron and inevitable impurities welded to each other. A pipeline excellent in low-temperature toughness of a weld metal part. 3. The steel pipe according to claim 2, further comprising:
, S: 0.002% or less, Nb: 0.01 to 0.06%, and N:
The above-mentioned 2. characterized in that it contains 0.0010 to 0.0060%.
Pipeline with excellent low temperature toughness of the circumferential electron beam welded metal part described in. 4. In the electron beam welding method of steel pipes, the cooling time from 800 ℃ to 500 ℃ of the weld metal is within 5 seconds, 50
The cooling time from 0 ° C to 300 ° C is set within 2 to 10 seconds, the content of the components in the weld metal part is% by mass, and the amount of solid solution Ti: 0.00
A welding method for a steel pipe, characterized in that the content of S is 5 to 0.020% and the amount of dissolved N is 0.0015 to 0.0060%. 5. The steel pipe according to claim 4, in mass%, C: 0.03 to 0.
06%, Si: 0.10 to 0.60%, Mn: 0.80 to 2.0%, P: 0.01
5% or less, Ti: 0.005 to 0.025%, Al: 0.010 to 0.050
%, O: 0.0030% or less. The method for welding a steel pipe according to. 6. The steel pipe according to claim 5 is further added in mass%,
S: 0.002% or less, Nb: 0.01 to 0.06%, and N: 0.00
5. The content according to the above 5, characterized by containing 10 to 0.0060%. The method for welding a steel pipe according to.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the circumferential weld metal portion of a steel pipe in which steel pipes are welded in the circumferential direction by electron beam welding is mass%, and the amount of solid solution Ti: 0.005 to 0.020%, solid solution N
It has been clarified that the low temperature toughness of the circumferential electron beam welded metal portion can be improved over the circumferential direction by containing the amount of 0.0015 to 0.0060%. Unless otherwise specified,% in the following sizes is% by mass.
Shall be indicated.

The operation of the present invention will be described below. First, the background of the invention will be described based on experimental results. First, in the Charpy impact test result at low temperature,
15 for each absorbed energy of each test piece at the same test temperature
In order to elucidate the cause of the difference in absorbed energy exceeding 0 J, the fracture surface of the Charpy impact test piece having a low absorbed energy was observed with an electron microscope. As a result, it was found that the origin of the fracture in the Charpy impact test was oxide inclusions.

Therefore, an experiment was conducted paying attention to the influence of the amount of oxide-based inclusions, which is the starting point of fracture, on the variation in absorbed energy obtained by the Charpy impact test. That is, each of the three types of steel plates shown in Table 1 having a plate thickness of 19 mm is used to perform UOE molding using steel plate A and UO using steel plate B.
E forming, UOE forming using steel plate C, the produced steel pipes are butt-circle welded by electron beam welding,
The relationship between the inclusions in the metal of the steel pipe base metal and the inclusions in the circumferential weld metal part and the low temperature toughness of the circumferential weld metal part was investigated. Note that the three types of steel plates were steel plate A having a different oxygen content in the steel and steel plate C having a different Ti content relative to steel plate B.

[0011]

[Table 1]

(Evaluation of Inclusions in Metal of Steel Pipe Base Material and Inclusion in Circumferential Weld Metal) For measuring the amount of inclusions, the circumference of the steel pipe is measured for each of the steel pipe base metal and the circumferential weld metal. Specimens were sampled from a plurality of locations that were equally divided over the entire circumference in a pitch of 45 ° or less, and observed under an optical microscope in a cross section perpendicular to the circumferential direction. The number of inclusions of 3 μm or more per unit cross-sectional area was measured. I asked. Specifically, the diameter of a circle having the area of the inclusion equivalent to the size of the inclusion was determined from the observed image, and the number of inclusions having a diameter of the circle of 3 μm or more was determined. The number of inclusions having a size of 3 μm or more per unit cross-sectional area was obtained by the image analysis process, but this obtaining method is not limited to the method of measuring by the image analysis process. (Evaluation of low temperature toughness of circumferential weld metal part) As shown in FIG.
From the center of the thickness t of the steel pipe 1 such that the Z direction of the steel pipe 1 is the length direction L of the Charpy impact test piece, and the test pipes are sampled from a plurality of locations where the entire circumference of the steel pipe 1 is equally divided at a pitch of 45 ° or less. After machining, the Charpy impact test was conducted at a low temperature, and the absorbed energy of the weld metal portion at a plurality of positions in the circumferential direction obtained and its variation were evaluated.

In this case, the absorbed energies measured three times at the respective test temperatures of 0, -20 and -40 ° C. are all 200 J
When the difference is 150 J or less between the maximum value and the minimum value, the low temperature toughness is excellent (marked with O), and in other cases, the low temperature toughness is poor (marked with X). The Charpy impact test uses a Charpy impact test piece machined so that the position of the tip of the V notch coincides with the center of the width B of the weld metal part 2 in the circumferential direction. The measurement was performed at a low temperature, and the absorbed energy in the welding line direction of the circumferential weld metal part 2 was measured. The two-dot chain line in FIG. 1 is the main part of the steel pipe (steel pipe base material) 1, and the B dimension is the width of the circumferential weld metal part 2. For other conditions, the Charpy impact test was performed at a low temperature in accordance with JIS Z 2242 (Metallic material impact test method). The shape of the Charpy impact test piece has a length L =
55mm, height H = 10mm, width W = 10mm, V notch depth D = 2
mm, V notch angle α = 45 °, and V notch tip R = 0.25 mm.

Number of inclusions of 3 μm or more per unit cross-sectional area in the steel pipe base material and the circumferential weld metal portion, solid solution Ti, N content in the circumferential weld metal portion, and circumferential weld metal Table 2 also shows the results of the Charpy impact test for each part.

[0015]

[Table 2]

The amount of solid solution Ti and the amount of solid solution N in the weld metal are values obtained by subtracting the amount of precipitated Ti and the amount of precipitated N from the amount of Ti and N contained in the metal. The amount of precipitated Ti in the weld metal part welded in the circumferential direction by the electron beam is 10 mass% AA (acetylacetone) for the sample taken from the weld metal part.
The residue was dissolved by electrolysis in a system electrolytic solution, and the obtained residue was analyzed by emission spectrometry. The amount of precipitated N in the weld metal is 10
The residue obtained by electrolysis in a mass% AA (acetylacetone) -based electrolytic solution was further dissolved in a bromine-methanol solution, and the residue was analyzed. The observation of inclusions was performed with an optical microscope, but an electron microscope may be used. The observation magnification of inclusions is 400 times or less with an optical microscope and with an electron microscope.
It is desirable to carry out 400 to 1000 times. The preparation of test pieces and the test method (measurement area, etc.) were based on JIS G0555 (microscopic test method for non-metallic inclusions in steel), and the samples were adjusted so as not to produce polishing flaws and rust.

From the results shown in Table 2, in the case of the steel pipe C, the number of inclusions of 3 μm or more per unit cross-sectional area in the weld metal portion in the circumferential direction is more than 3 and is large, but at a plurality of locations in the circumferential direction. It was revealed that the absorbed energy of the weld metal part is sufficient and its variation is small, and the low temperature toughness of the weld metal part is excellent. Furthermore, a detailed study was conducted on the relationship between the variation in absorbed energy of the circumferential weld metal part obtained by performing the Charpy impact test at low temperature in the circumferential position and the composition and structure of the weld metal part. In addition, it was clarified that adjusting the cooling rate after welding reduces variations in absorbed energy in the circumferential weld metal and is effective in stabilizing toughness.

Therefore, an experiment was conducted paying attention to the influence of the cooling rate after welding on the dispersion of the absorbed energy of the circumferential weld metal portion. That is, a steel plate having a composition shown in Table 3 with a plate thickness of 15 mm is subjected to butt welding by electron beam welding by subjecting it to electron beam irradiation conditions, electron beam welding speed, preheating, and post-welding treatment, and after welding. The effect on the low temperature toughness of the weld metal part was investigated by changing the cooling rate of the weld metal part. The results are shown in Table 4.

Since the steel plates were electron-beam welded to each other, the Charpy impact test specimens were sampled from a plurality of weld metal portions in the welding line direction in the same manner as above.
Machining was performed as in the above case and the Charpy impact test was performed at low temperature as in the above case. The cooling time after welding was measured by using a recorder or the like with a thermocouple set in advance near the position where electron beam welding was performed. The cooling time in the vicinity of the electron beam welded portion was set as the cooling time after welding.

[0020]

[Table 3]

[0021]

[Table 4]

From the results shown in Table 4, it was found that the low temperature toughness varies depending on not only the amount of solid solution Ti and the amount of solid solution N in the weld metal but also the cooling rate of the electron beam weld metal. That is, the amount of solid solution Ti in the weld metal part is 0.005 to 0.020%, and the amount of solid solution N is
Content of 0.0015 to 0.0060%, cooling time after electron beam welding from 800 ℃ to 500 ℃ within 5 seconds, from 500 ℃
It was revealed that the low temperature toughness of the welded metal portion was excellent when the temperature up to 300 ° C was within 2 to 10 seconds. On the other hand, in the electron beam weld metal, the solid solution Ti content is 0.005 to 0.020%,
Even if the amount of solute N satisfies 0.0015 to 0.0060%, it was revealed that the low temperature toughness of the weld metal part is inferior when the cooling rate is out of the above range.

The reason why the cooling time after welding has a great influence on the toughness variation is that the temperature is from 800 ° C to 500 ° C.
When the cooling time is within 5 seconds up to ℃ and within 2-10 seconds within 500 ℃ to 300 ℃, even in weld metal containing a large amount of solid solution Ti and solid solution N such as electron beam weld metal. It is presumed that due to the island-like martensite (MA) structure and the upper bainite structure in which the formation of cementite was suppressed, the propagation resistance of fracture increased and the toughness was stabilized. Within 5 seconds from 800 ℃ to 500 ℃, 500 ℃
If the temperature does not satisfy the cooling time within 2 to 10 seconds from 1 to 300 ° C, the amount of island martensite (MA) structure or cementite in the upper bainite structure in the weld metal increases, and the coarsening causes It is considered that the propagation resistance of destruction was lowered.

The structure was observed by observing the mirror-polished observation surface with Nital and observing it with an optical microscope at a magnification of 400 times.
The structure observed when the toughness is stabilized is allowed to include the following structure as a minor structure while mainly having the upper bainite structure. That is,
Continuous bainite microstructure similar to upper bainite microstructure, lower bainite of continuous cooling transformation structure, Acicular ferrite, Widmanstaetten ferrite, pseudopolygonal ferrite, granular Zw ferrite, intermediate transformation (Zw) structure, etc. . Also, island martensite and cementite structures are allowed up to an area ratio of about 10%.

Such a toughness stabilizing method is different from the above-mentioned conventional method in that fine precipitates containing Ti are used as nuclei to generate fine acicular ferrite, and fine toughening of the structure results in good toughness. It is a toughening mechanism. The reasons for limitation in the present invention will be described below. First, in the pipeline according to the present invention, the amount of solid solution Ti in the weld metal part: 0.005 to 0.020%, the amount of solid solution N: 0.0015 to 0.0
Explain the reason why it is limited to 060%. Hereinafter, the amount of each component is% by mass. (Amount of solid solution Ti: 0.005 to 0.020%) In the weld metal of pipeline, when the amount of solid solution Ti exceeds 0.020% and is present in the weld metal, destruction of Charpy impact test in the low temperature region as described above. The solid solution Ti content was set to 0.020% or less because it causes a decrease in the propagation resistance of the weld metal and causes a large variation in the absorbed energy of the weld metal part by the Charpy impact test. Weld metal that is electron beam welded in a vacuum is thought to dissolve even oxide-based inclusions during welding, and part of it is discharged outside the weld metal as slag, but part of it is solidified during cooling. Reprecipitated and present in the weld metal. However, the cooling rate of the electron beam weld metal is very fast, and most of the weld metal Ti exists in a solid solution state. If the solid solution Ti content is 0.005% or less, the Ti content of the steel pipe base material decreases and the toughness is impaired.
The lower limit of the amount was set to 0.005%. (Solute N content: 0.0015% to 0.0060%) N is an element that deteriorates the toughness of the weld metal part, and Ti is the electron beam weld metal.
Similarly, most of N in the weld metal exists in a solid solution state, and if it exceeds 0.0060%, the weld metal portion has low toughness, so the upper limit was made 0.0060%. Further, if the amount of solute N is less than 0.0015%, the N content of the steel pipe base material decreases, the precipitation amount of TiN, AIN, etc. decreases, and the toughness of the steel pipe base material deteriorates.
The lower limit was made 0.0015%.

Here, solid solution Ti in the electron beam weld metal part
Content and solid solution N content of 0.005% to 0.020% and
In order to set the range of 0.0015% to 0.0060%, it is important to adjust the composition of the steel plate or steel pipe and also adjust the cooling time after electron beam welding. In order to adjust the composition of the steel plate or steel pipe, not only the amount of Ti and N in the steel plate, but also Si, Mn, Al, etc. which act as deoxidizing elements, and S, N, etc. which form precipitates and inclusions with these elements, etc. It is important to make adjustments. Further, in order to satisfy the hardness regulation in the electron beam circumferential direction weld metal, adjustment of these components is very important.

The cooling rate of the weld metal portion after electron beam welding is from 500 ° C. to 800 ° C. within 5 seconds.
The cooling time from ℃ to 300 ℃ was within 2-10 seconds. If this range is not satisfied, the island martensite (MA) structure in the upper bainite structure in the weld metal or the amount of cementite generated increases and coarsens, which reduces the propagation resistance of fracture and results in a Charpy impact test. Large variations occur in the absorbed energy of the weld metal. The cooling rate of the weld metal portion can be adjusted by adjusting the electron beam irradiation conditions and the electron beam welding rate, performing preheating, and performing post-welding treatment.
Microstructure control in the upper bainite structure is very important to satisfy the strength and toughness required for electron beam circumferential weld metal.

The reasons for limiting the composition of the steel pipe will be described below. C: 0.03 to 0.06% C requires at least 0.03% to secure the strength, but if it exceeds 0.06%, the circumferential maximum weld metal part hardness exceeds Hv 300 in Vickers hardness, and steel pipe In order to deteriorate the low temperature toughness of the base metal and the weld metal in the circumferential direction,
The amount was in the range of 0.03 to 0.06%.

When there is a large difference in strength between the circumferentially welded metal portion and the steel pipe base metal and the width B of the circumferentially welded metal portion is narrow, as in the electron beam circumferentially welded portion, the V notch Charpy test is performed. It is well known that since the crack curves from the weld metal to the base metal side, the toughness of the weld metal in the circumferential direction cannot be grasped. By setting the C content in the steel to 0.06% or less, the low temperature toughness of the electron beam circumferential weld metal portion can be accurately evaluated by the V-notch Charpy test.

Si: 0.10 to 0.60% Si is an element which is effective not only for deoxidizing but also for improving strength at the time of manufacturing a steel sheet, but at least 0.10% or more is required to obtain the effect. . If it exceeds 0.60%, the toughness of the steel pipe base material and the weld heat affected zone deteriorates, so the upper limit is 0.60.
%. Preferably, the Si content in steel is in the range of 0.20 to 0.50%.

Mn: 0.80 to 2.0% or less Mn is an essential element for ensuring strength and toughness,
At least 0.80 is required, but if it exceeds 2.0%, the weldability and the toughness of the electron beam welded metal part are deteriorated, so the range was made 0.80 to 2.0%. P: 0.015% or less P is an element present in the steel as an impurity and reduces the toughness of the steel pipe. It is better to be as low as possible, and the upper limit of P in the steel is 0.
015% or less.

Ti: 0.005 to 0.025% Ti forms fine TiN in the heat-affected zone of the weld, suppresses coarsening of austenite grains, refines the microstructure, and improves the toughness of the steel pipe base material and the heat-affected zone of the weld. Improve. If the Ti content in the steel is less than 0.005%, its effect is small, 0.025%.
Since the low temperature toughness of the weld metal in the circumferential direction is deteriorated if it exceeds 0.1%, the Ti content in the steel is set in the range of 0.005 to 0.025%.

Al: 0.010 to 0.050% Al is an element used for deoxidation at the time of manufacturing a steel sheet, and also plays an important role in the toughness of the heat affected zone of welding. Al in steel
If the amount is less than 0.010%, the effect of improving the toughness of the heat affected zone cannot be expected. Therefore, the Al content in steel is set to 0.010% or more. On the other hand, if the Al content in the steel exceeds 0.050%, the toughness of the weld heat affected zone deteriorates, so the upper limit of the Al content in the steel was set to 0.050%.

O: 0.0030% The upper limit of O is set to 0.0030% or less from the viewpoint of deterioration of toughness of the weld metal portion in the electron beam circumferential direction due to oxide inclusions. It is preferably 0.0020% or less. In the present invention, it is preferable to further limit the following compositions. S: 0.002% or less S, like P, is an element present in the steel as an impurity and deteriorates the toughness of the steel pipe base material, and the lower the S, the better.
Further, by setting the S content in the steel sheet to 0.002% or less, solidification cracking that occurs in the electron beam circumferential weld metal portion can be prevented, so the upper limit of the S content in the steel was made 0.002% or less.
Preferably, the S content in steel is 0.001% or less.

Nb: 0.01 to 0.06% Nb has the effect of improving the strength of the steel pipe base material by precipitation strengthening. If the addition amount is less than 0.01%, the effect is small, and if it exceeds 0.06%, the toughness of the electron beam circumferential weld metal portion is deteriorated, so the Nb content in the steel is set to 0.01 to 0.06%. N: 0.0010 to 0.0060% N deteriorates the toughness of the electron beam weld metal, so it is better to reduce it as much as possible, but if it is less than 0.0010%, precipitation of TiN and AIN will not occur and the steel pipe matrix Since the toughness of the material is impaired, the lower limit was made 0.0010%. On the other hand, in the present invention, when the content is 0.0060% or less, sufficient toughness of the electron beam circumferential direction weld metal portion is secured, so the N content in steel is set to the range of 0.0010 to 0.0060%. Preferably, it is 0.0010 to 0.0050%.

As appropriate elements, Ca, Ni, Cu, Cr, Mo,
The reason why it is preferable to contain one or more selected from the elements of V and REM will be described. Ca: 0.0030% or less Ca is a useful element that suppresses the morphology of sulfide (MnS) and improves the toughness and anisotropy of the steel pipe base material and hydrogen-induced cracking resistance. The amount of Ca in the steel was 0.0030% or less. Preferably, the amount of Ca in steel is 0.0025% or less.

Ni: 0.1 to 1.0% Ni is a useful element for improving the strength and toughness of the base metal without impairing the toughness of the weld metal part in the electron beam circumferential direction. 0.1% or more is required. However, even if added over 1.0%, the effect of improving the properties is small, and since it is an expensive element, the Ni content in the steel is 0.
The range was 1 to 1.0%.

Cu: 0.1-1.0% Cu has the effect of improving the strength and toughness as well as the corrosion resistance and hydrogen-induced cracking resistance. If it is less than 0.1%, its effect is small, and if it exceeds 1.0%, the toughness of the steel pipe base material and the heat-affected zone deteriorates.
And the range. Cr: 0.1 to 1.0% Cr is a useful element that improves hardenability and enhances the strength of the steel pipe base material and the electron beam circumferential weld metal part. 0.
If it is less than 1%, its effect is small, and if it exceeds 1.0%, the weldability and the toughness of the weld metal in the circumferential direction are deteriorated.
The Cr content was in the range of 0.1 to 1.0%.

Mo: 1.0% or less Mo is an element that improves both strength and toughness of the steel pipe base material. However, if the amount of addition of Mo exceeds 1.0%, the toughness of the weld metal in the electron beam circumferential direction is deteriorated.
% Or less. From the viewpoint of reducing the toughness variation of the weld metal portion in the circumferential direction in the low temperature region, 0.50% or less is preferable.

V: 0.01 to 0.10% V is an element which improves strength by precipitation strengthening. 0.
If it is less than 01%, its effect is small, and if it exceeds 0.10%, the toughness of the electron-beam circumferential direction welded metal portion deteriorates, so the V content in the steel was made 0.01 to 0.10%. REM (rare earth metal): 0.0005 to 0.0050% Like Ca, REM effectively contributes to morphology control of sulfide (MnS), improvement of toughness and anisotropy of steel pipe base metal, and improvement of hydrogen-induced cracking resistance. If the REM content in the steel is less than 0.0005%, the effect is poor. On the other hand, if it exceeds 0.0050%, the toughness of the base material of the steel pipe deteriorates, so the REM content in the steel was set to a range of 0.0005 to 0.0050%.

The plate thickness of the above-mentioned steel plate, steel pipe or the like to be subjected to electron beam welding is about 40 mm or less, which allows through-hole welding by electron beam welding, and is not particularly limited. Further, the diameter of the steel pipe is not particularly limited as long as it is a size that allows electron beam welding. An example of the welding conditions for electron beam welding used in the present invention is shown below.

As the electron beam power source, the rated maximum output is 5 to 5
It may be about 50kW. Electron beam welding is performed after the above-described steel plate, steel pipe, and the like are subjected to groove processing of a predetermined shape (mainly I groove). Electron beam welding in the present invention, acceleration voltage 30 ~ 80kV, beam current 100 ~ 600mA, welding speed 300 ~ 9
Performs all-position circumferential welding under the welding condition of 00mm / min. The groove shape, welding conditions, welding position, etc. are not particularly limited.

The degree of vacuum of the welded portion is preferably 10 Pa or less from the viewpoint of ensuring the convergence of the electron beam. The weld bead width of electron beam weld metal shall be 3 mm or less.

[0044]

[Example] Converter-Continuous casting-Thick plate manufacturing process to manufacture a steel plate with a plate thickness of 19 mm, and this steel plate was UOE-molded to obtain a diameter of 610 mm.
UOE steel pipes (steel pipes with the same composition) were butt-welded and circumferentially welded by electron beam welding to form a pipeline. A plurality of samples for the Charpy impact test were taken along the circumferential direction from the circumferential weld metal portion in the pipeline. The Charpy impact test piece was sampled, processed and machined into a 2 mm V notch test piece in the same manner as in FIGS. 1 and 2 (Charpy impact test piece size: test piece length L = 55 m.
m, test piece height H = 10mm, test piece width W = 10mm, V notch depth D
= 2mm, V notch angle α = 45 ℃, V notch tip R = 0.25m
m).

Regarding electron beam welding, the degree of vacuum is 10 Pa.
The electron beam circumferential speed is 400 to 600 m
m / min, electron beam acceleration voltage 60 kV, electron beam current
200mA, electron beam focusing condition (objective distance / focal length)
It was set to 0.9 to 1.1. At that time, the cooling time after electron beam welding is 80 in the entire circumference of the circumferential weld metal part.
It was within 5 seconds from 0 ° C to 500 ° C, and within 2 to 10 seconds from 500 ° C to 300 ° C.

Table 5 shows the composition of the above-mentioned steel pipe and the amount of solid solution Ti and the amount of solid solution N in the circumferential weld metal portion in the pipeline.
Table 6 shows the low-temperature toughness evaluation results of the electron beam circumferential weld metal portion. In this case, as the low temperature toughness, the absorbed energy measured three or more times by the Charpy impact test is
It was considered that the low temperature toughness was excellent when it was 200 J or more and the difference between the maximum value and the minimum value was 150 J or less.

[0047]

[Table 5]

[0048]

[Table 6]

From the results shown in Table 6, the absorbed energy is 2 in the circumferential weld metal portion of the steel pipe in which the steel pipes according to the present invention are welded in the circumferential direction by electron beam welding.
00J or more, and the variation in absorbed energy is small,
Excellent low temperature toughness. On the other hand, the steel pipe base material using the steel sheet not according to the present invention has an unsuitable chemical composition, the absorbed energy may be less than 200 J, the absorbed energy varies widely, and the low temperature toughness is poor.

[0050]

EFFECTS OF THE INVENTION According to the present invention, the low temperature toughness of the circumferential electron beam welded metal portion in which the absorbed energy in the circumferential welded portion is large and the variation in the absorbed energy in the circumferential position is small. It can be a pipeline.

[Brief description of drawings]

FIG. 1 is a partial perspective view showing an example of a low temperature toughness evaluation method for an electron beam circumferential direction weld metal portion.

FIG. 2 is an explanatory diagram showing the impact direction of a Charpy impact test on a V-notch Charpy impact test piece.

[Explanation of symbols]

1 Steel pipe (steel pipe base material) 2 Circumferentially welded metal part B Width of weld metal in circumferential direction t thickness θ Circumferential direction (welding direction) Z length direction L Charpy impact test piece length H Charpy impact test piece height W Charpy impact test piece width α V notch angle R V Notch tip radius DV notch depth

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Claims (6)

[Claims] 1. A mass% of a weld metal part in which steel pipes are welded to each other.
Then, the amount of solid solution Ti: 0.005 to 0.020%, the amount of solid solution N: 0.0015 to
A pipeline with excellent low-temperature toughness in the weld metal, which is characterized by containing 0.0060%. 2. The steel pipe according to claim 1, in mass%, C:
0.03 to 0.06%, Si: 0.10 to 0.60%, Mn: 0.80 to 2.0%,
P: 0.015% or less, Ti: 0.005 to 0.025%, Al: 0.010
The steel line containing 0 to 0.050% and O: 0.0030% or less is welded to each other, and the pipeline excellent in low temperature toughness of the weld metal portion according to claim 1. 3. The steel pipe according to claim 2, further comprising S: 0.002% or less, Nb: 0.01 to 0.06%, and N: 0.0010 to 0.0060% in mass%. Item 2. A pipeline excellent in low temperature toughness of a weld metal part. 4. In the electron beam welding method for steel pipes, the cooling time from 800 ° C. to 500 ° C. of the weld metal is 5
Within 2 seconds, the cooling time from 500 ℃ to 300 ℃ shall be within 2 to 10 seconds, and the weld metal part is in mass% and the solid solution Ti content is 0.005
~ 0.020%, amount of solid solution N: 0.0015 to 0.0060%, a method for welding a steel pipe. 5. The steel pipe according to claim 4, wherein C:
0.03 to 0.06%, Si: 0.10 to 0.60%, Mn: 0.80 to 2.0%,
P: 0.015% or less, Ti: 0.005 to 0.025%, Al: 0.010
5. The method for welding a steel pipe according to claim 4, wherein the composition has a content of .about.0.050% and O: 0.0030% or less. 6. The steel pipe according to claim 5, further comprising S: 0.002% or less, Nb: 0.01 to 0.06%, and N: 0.0010 to 0.0060% by mass%. 5. The method for welding a steel pipe according to item 5.