JP2003201535A - Steel sheet and steel pipe for electron beam welding, and pipeline having excellent low temperature toughness in weld metal zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet for an electron beam welding, a steel pipe for an electron beam welding, and a pipeline in which the variation of toughness at low temperatures in a weld metal zone welded by an electron beam can be reduced, and to provide an electron beam welding method for the steel pipe. <P>SOLUTION: The steel sheet has a composition containing, by mass, 0.03 to 0.06% C, 0.1 to 0.4% Si, 0.8 to 2.0% Mn, ≤0.015% P, 0.005 to 0.025% Ti, 0.020 to 0.050% Al, and ≤0.0030% O, and also satisfying [O]<0.019×[Si]+0.009×[Al], and in which the number of inclusions of ≥3 μm is ≤5.0 pieces/mm<SP>2</SP>. The steel pipe is obtained by using the steel sheet. As for the pipeline, in the weld metal zone in the circumferential direction of the steel pipe obtained by welding the respective steel pipes in the circumferential direction by electron beam welding, the number of inclusions of ≥3 μm is ≤3.0 pieces/mm<SP>2</SP>, or the ratio of bainite packets with dimensions of ≥30,000 μm<SP>2</SP>occupied in the whole weld metal structure is ≤10%. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate, a steel pipe, and an electron beam welding method for a steel pipe, which are excellent in low temperature toughness of a weld metal by welding with an electron beam.

[0002]

2. Description of the Related Art High-strength, excellent low-temperature toughness and field weldability are required for large-diameter pipelines for transporting crude oil and natural gas in cold regions and offshore. In particular, in recent years, from the viewpoint of welding work efficiency, the application of electron beam welding or laser welding has been studied as a welding method capable of performing welding even with a thick steel plate in one pass and simultaneously improving both welding efficiency and quality.

Unlike conventional welding methods (MIG welding, TIG welding, covered arc welding, submerged arc welding, etc.), these welding methods do not use a welding material, but a material to be welded, for example, a steel pipe itself in a pipeline. Are melted, solidified and joined. For this reason, it is difficult to adjust the components of the weld metal (structure control) in the weld metal that has melted and then solidified, and the components of the weld metal are mostly determined by the material to be welded. The low temperature toughness of the metal part is greatly affected by the composition of the material to be welded.

Developed for use in conventional welding methods,
Steels with excellent low temperature toughness in the heat affected zone (HAZ) are not able to improve the toughness in the weld metal at all when applied to electron beam welding, or to improve the low temperature toughness in the weld metal. Had to limit the welding conditions to a narrow range. On the other hand, as a method for improving the low temperature toughness of an electron beam or laser welded portion, Japanese Patent Laid-Open No. 64-15321
Japanese Unexamined Patent Application Publication No. 2-22418 has been disclosed. However, although these methods say that Ti ₂ O ₃ and TiN are finely dispersed in the weld heat-affected zone (HAZ) and the weld metal portion, the structure can be refined and good low temperature toughness can be obtained. It was not possible to reduce the variation in low temperature toughness of the weld metal portion welded by the beam.

Further, Japanese Patent Laid-Open No. 2001-207242 discloses a steel pipe which is excellent in low-temperature toughness of a weld metal portion welded by an electron beam or a laser, and in which hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance are excellent. It is disclosed for pipelines. However, even in such steel pipes and pipelines, it has not been possible to reduce the toughness variation of the electron beam circumferential weld metal portion at low temperatures.

In particular, almost no development has been made on a steel plate for electron beam welding or a steel pipe for electron beam welding capable of reducing the toughness variation of the weld metal portion at low temperature. The steel pipes are welded along the circumferential direction by electron beam welding. In a welded pipeline, it has been a serious problem that stable good low temperature toughness cannot be secured in the circumferential weld metal portion of the steel pipe.

[0007]

SUMMARY OF THE INVENTION Therefore, the present invention advantageously solves the above-mentioned problems, and it is possible to reduce the toughness variation of the weld metal portion welded by the electron beam at a low temperature and at a low temperature. A steel plate for electron beam welding, a steel pipe for electron beam welding, and a pipeline having excellent toughness are provided together with an electron beam welding method for a steel pipe.

[0008]

The inventors of the present invention have conducted various investigations and studies to solve the above problems, and as a result, have found that a certain size of a weld metal portion welded by an electron beam has a certain size. It was found that when inclusions are present at a certain frequency or more, the toughness value in the welding line direction varies in the low temperature region in the Charpy impact test result, and the present invention was completed.

Further, the inventors of the present invention have found that when a certain proportion of bainite packets of a specific size are present in the weld metal welded by the electron beam, the welding line direction in the low temperature region in the Charpy impact test results is shown. The inventors have found that variations in toughness are likely to occur, and have completed the present invention. That is, a steel sheet is manufactured so that the amount of inclusions of a size considered to be a starting point of fracture in the Charpy impact test is reduced as much as possible, and the variation in the toughness value in the low temperature region in the Charpy impact test result is controlled by setting the frequency to a certain frequency or less. It has been clarified that it can be reduced. Furthermore, by adjusting the steel plate composition and optimizing the welding conditions, the bainite packet of a size that is considered to reduce the propagation resistance of fracture is kept below a certain frequency, so that the toughness value variation in the low temperature region in the Charpy impact test results is small. It has been determined that this can be reduced. The low temperature is 0 here.
Indicates a temperature range of ℃ or less. In particular, in the region where the Charpy impact value required for the pipeline is close to the test temperature, variation becomes a problem.

In the Charpy impact test result, the above-mentioned variation in the toughness value is such that the absorbed energy measured at least three times with the temperature range of 20 ° C. or more is 200 J or more, and the difference between the maximum value and the minimum value is 150 J. If it is within the range, it is assumed that there is no variation. The present invention has been made based on the above findings, and the gist thereof is as follows.

% By mass, C: 0.03 to 0.06%, Si: 0.10 to
0.40%, Mn: 0.80 to 2.0%, P: 0.015% or less, Ti: 0.
005 to 0.025%, Al: 0.020 to 0.050%, O: 0.0030%
It contains the following, preferably S: 0.002% or less, Nb: 0.
01-0.06%, N: 0.0010-0.0050%, and
A steel plate for electron beam welding, which has a composition satisfying [O] <0.019 × [Si] + 0.009 × [Al], and has 5.0 inclusions / mm ² or less in steel of 3 μm or more. Bainite packets with electron beam welding steel pipes or welded metal parts of steel pipes obtained by electron beam welding of these steel pipes with 3.0 μm / mm ² or less of inclusions in steel of 3 μm or more, or 30000 μm ² or more Of the weld metal structure of 10% or less. Further, it is an electron beam welding method for steel pipes, characterized in that the steel pipes are electron beam welded at a vacuum degree of 10 Pa or less.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION 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, in order to clarify the cause of the difference in the absorbed energy of each test piece at the same test temperature exceeding 150 J, the Charpy impact test with low toughness was conducted. The fractured surface of the piece was observed by an electron microscope. As a result,
It was found that the origin of fracture in the Charpy impact test was oxide-based inclusions, and all of them were inclusions of 3 μm or more. As a result of the analysis, those inclusions are Ca, Si,
It was an oxide mainly composed of Ti, Al, etc., and partly was a complex with a sulfide containing Ca, Mn, etc.

Therefore, an experiment was conducted focusing on the influence of the inclusion size on the variation in the low temperature toughness of the electron beam weld metal. That is, steel plates A and B each having a plate thickness of 19 mm, which differ only in the amount of oxygen in the steel shown in Table 1, are used, and steel pipes A formed using the steel plates A and steel pipes B formed using the steel plates B are used.
Each of them was butt-circumferentially welded by electron beam welding, and the relationship between the inclusions in the steel plate and the circumferential weld metal portion and the toughness of the circumferential weld metal portion was investigated.

In the inclusion investigation, test pieces were sampled from a plurality of locations where the entire circumference of the steel plate and the circumferential weld metal portion were equally divided at a pitch of 45 ° or less,
The cross section perpendicular to the circumferential direction was observed with an optical microscope, and the number of inclusions of 3 μm or more per unit cross-sectional area was determined. The number of inclusions of 3 μm or more per unit cross-sectional area was determined by image analysis processing as follows. First, the area of each inclusion is determined from the observed image, the diameter of a circle is calculated as the size of the inclusion, and the number of inclusions whose diameter is 3 μm or more is calculated. It was

In this way, the number of inclusions having a size of inclusions of 3 μm or more per unit area was obtained, but the method of obtaining is not limited to the method of measuring by image analysis processing. Further, as shown in FIG. 1, the low temperature toughness of the circumferential weld metal portion of the steel pipe welded by the electron beam is the thickness t of the circumferential weld portion 2 of the test piece used for the Charpy impact test.
After the V-notch Charpy impact test piece is machined so that the tip of the V-notch is along the center of the width B of the circumferential welded portion 2 by machining, the impact direction becomes the welding direction θ. (See Fig. 2), other conditions are JI
A low temperature Charpy impact test was performed according to the regulations of SZ 2242 (Metallic material impact test method). The shape of the Charpy impact test piece is length L = 55mm, height H = 10mm, width W =
10mm, V notch depth D = 2mm, V notch angle α = 45 °,
The V notch tip R was 0.25 mm. A two-dot chain line in FIG. 1 is a perspective view showing a main part of a circumferential weld metal portion 2 of a steel pipe (steel pipe base material) 1, and z is a length direction of the steel pipe base material 1.

However, as the test piece used for the Charpy impact test of the circumferential weld metal part 2 and the inclusion observing test piece of the circumferential weld metal part 2, a plurality of test pieces are taken along the circumferential direction. , And subjected to each test. Table 2 shows the results of the number of inclusions of 3 μm or more and the low temperature toughness value of the circumferential weld metal portion per unit cross-sectional area in the steel sheet and the circumferential weld metal portion.

[0017]

[Table 1]

[0018]

[Table 2]

From the results shown in Table 2, when the amount of oxygen in the steel sheet is different and the number of inclusions in the steel sheet and weld metal portion used in steel pipes having the same other components is also different, it is observed in the circumferential weld metal portion. It was found that the low-temperature toughness of the steels differed in the dispersion state. The reason why the influence of inclusions with a size of 3 μm or more on the toughness variation is large in the electron beam welding method is because electron beam welding involves welding in a vacuum. Without increasing the amount of oxygen in the metal and oxide-based inclusions,
Unlike weld metal of other welding methods (MIG welding, TIG welding, covered arc welding, submerged arc welding, etc.), the number of inclusions in the weld metal is extremely small. When the number of inclusions in the weld metal is small, the influence of the size of inclusions, which is the starting point of the fracture, is particularly noticeable in tests such as the Charpy impact test that evaluate the notch toughness of steel and weld metal. Is estimated to have become very large.

The inclusions were observed this time on a cross section perpendicular to the circumferential direction, which was taken from a plurality of locations where the entire circumference in the circumferential direction of the steel plate and the circumferential weld metal was equally divided at a pitch of 45 ° or less. Therefore, this result is considered to sufficiently represent the average state of inclusions in each of the steel plate and the circumferential weld metal portion. The observation of inclusions was performed with an optical microscope, but an electron microscope may be used. The observation magnification of the inclusions is preferably 400 times or less with an optical microscope and 400 to 1000 times with an electron microscope.
Preparation of test pieces and test methods (measurement area, etc.) are JIS G
Based on 0555 (Microscopic test method for non-metallic inclusions in steel), the samples were adjusted so as to prevent polishing flaws and rust.

Here, the value obtained by the above-mentioned image analysis,
Inclusions with a size of inclusions of 3 μm or more per unit area include all non-ferrous inclusions in steel, as is clear from the measurement method, and precipitates such as sulfides and AIN. Needless to say, it includes a complex containing them. The present invention reduces the low temperature toughness variation of the weld metal part of the steel pipe welded by the electron beam by positively controlling the size of inclusions in the steel on the basis of the above knowledge, and it is stable and good. The toughness of the welded metal part can be stably obtained without cleaning the steel pipe, compared to the conventional method of producing a clean steel pipe that reduces the amount of inclusions to increase the toughness. It became clear.

Further, the inventors have made a closer examination on the toughness variation in the low temperature region of the electron beam welded metal part. As a result, a new amount of Si, Al, O in the steel sheet and a certain value or more are newly added. Bainite packet (for bainite packet, see, for example, “Amano et al .: Materia, Vol. 39 (2000), 185”), if the proportion of the weld metal structure is adjusted, the electron beam weld metal It was revealed that the toughness variation in the low temperature region of the part can be reduced.

Therefore, an experiment was conducted focusing on the influence of the amounts of Si, Al and O in the steel sheet on the toughness variation of the electron beam welded metal portion. That is, steel pipes having the composition shown in Table 3 having a plate thickness of 15 mm are butt-circumferentially welded by electron beam welding, and the amounts of Si, Al, and O in the steel sheet are steel sheet, inclusions in the weld metal part, and electron beam The influence on the toughness of the weld metal in the circumferential direction was investigated. At that time, the diameter per unit area is 3 μm
The number of the above inclusions was obtained by the image analysis processing as described above. Further, the low temperature toughness of the electron beam circumferential welded metal portion was measured in the same manner as in the above case.

Further, the proportion of bainite packets having a size of 30,000 μm ² or more in the circumferentially welded metal portion of electron beam welding in the entire weld metal structure was determined. As in the inclusions survey, collect specimens from multiple locations were equally divided circumferentially entire circumference at a pitch of 45 ° or less, perform optical microscopy of a cross-section perpendicular to the circumferential direction, 30000 [mu] m ² The ratio of the bainite packet having the above size to the entire weld metal structure was determined. First, the area of each bainite packet was calculated by image processing of the observed image, and 30000 μm ²
The ratio of the bainite packet having the above size to the entire weld metal structure was determined.

The results are shown in Table 4.

[0026]

[Table 3]

[0027]

[Table 4]

From the results shown in Table 4, not only the number of inclusions in the weld metal part but also the amount of Si, Al, O in the steel sheet and the proportion of bainite packet having a certain size or more in the entire weld metal structure are It was found that the low-temperature toughness values in the electron-beam welded metal part have different variations. That is, the number of inclusions of 3 μm or more in the weld metal is 3.0
Pieces / mm ² or less and steel plate composition is [O] <0.019 × [S
In the case of the steel pipe E satisfying i] + 0.009 × [Al], the variation of the toughness value in the electron beam weld metal part is extremely small, and the low temperature toughness is excellent. However, in the case of the steel pipe E, the proportion of bainite packets having a size of 30,000 μm ² or more in the entire weld metal structure exceeds 10%. On the other hand, in the weld metal part, the number of inclusions of 3 μm or more is 3.0 pieces / mm ² or less, but the steel plate composition does not satisfy [O] <0.019 × [Si] + 0.009 × [Al] and 30000 μm ^In the case of steel pipes C and D in which the ratio of bainite packets having a size of ² or more to the entire weld metal structure is more than 10%, the toughness value in the electron beam weld metal part varies greatly and the low temperature toughness is poor. .
In addition, in the weld metal part, the number of inclusions of 3 μm or more
Although it does not satisfy 3.0 pieces / mm ² or less, the steel plate composition in the weld metal part is [O] <0.019 × [Si] +0.0
In the case of the steel pipe F in which the proportion of bainite packets having a size of 09 × [Al] and 30,000 μm ² or more in the entire weld metal structure is 10% or less, the toughness variation in the electron beam weld metal portion is uneven. Has been improved.

The reason why the amounts of Si, Al, and O in the steel sheet greatly affect the toughness variation is [O] <0.01
When 9 × [Si] + 0.009 × [Al] is satisfied, due to the presence of solid solution Si and Al in the electron beam weld metal, the bainite structure in the weld metal with a very fast cooling rate It is presumed that the formation of island martensite was suppressed and the structure became less susceptible to cracking. It is considered that in the steel sheet that does not contain a sufficient amount of Si and Al with respect to the O content in the steel sheet, coarse island-shaped martensite was generated in the electron beam weld metal, and the toughness value varied.

The reason why the ratio of the bainite packet having a size of 30,000 μm ² or more to the total weld metal structure has a great influence on the toughness variation is that the bainite packet, which is a unit of propagation of fracture in the Charpy impact test, is large. It is considered that the smaller the size, the higher the propagation resistance of fracture and the smaller the toughness variation. It is considered that the weld metal in which the bainite packet having a size of 30,000 μm ² or more accounts for a large proportion of the entire weld metal structure has a low fracture propagation resistance and is likely to cause variations in toughness values.

The ratio of the bainite packet having a size of 30,000 μm ² or more obtained this time to the entire weld metal structure is sufficiently representative of the ratio of the circumferential weld metal structure to the average weld metal structure. It is considered to be a thing. Also,
The proportion of bainite packets having a size of 30,000 μm ² or more in the entire weld metal structure was determined by image analysis processing, but this determination method is not limited to the method of measurement by image analysis processing. Although the bainite structure was observed with an optical microscope, it may be observed with an electron microscope.
The observation magnification of the bainite structure is preferably 200 times or less with an optical microscope and 400 times or less with an electron microscope. A test piece was prepared in the same manner as the test piece used for observing inclusions, and etching was performed using nital as a corrosive liquid.

The reasons for limitation in the present invention will be described below. First, the reason for limiting the number of inclusions in the steel having a size of 3 μm or more in the steel plate or the steel pipe using the steel plate according to the present invention will be described. That is, in a steel plate or a steel pipe using the steel plate (hereinafter referred to as a steel pipe),
The number of inclusions in steel with a diameter of 3 μm or more is 5.0 /
If it exceeds 3 mm ² in the steel, it becomes the starting point of the Charpy impact test fracture in the low temperature region as described above, and causes the low temperature toughness variation of the weld metal part in the electron beam circumferential direction. The number of inclusions in the size of steel was 5.0 / mm ² or less.

Further, in the circumferential weld metal portion of the steel pipe which is electron beam welded in vacuum, it is considered that even oxide-based inclusions are melted during welding, and a part thereof becomes slag and is discharged out of the weld metal. However, some of them are reprecipitated during solidification cooling and become inclusions in the weld metal. Therefore, the size and the number of inclusions in the circumferential weld metal portion change with respect to the inclusions in the steel plate. The number of inclusions with a size of 3 μm or more is 3.0 / mm ^{2 in the} electron beam circumferential welded metal part.
If it exists in the steel in excess of the above, it becomes the starting point of the Charpy impact test fracture in the low temperature region as described above, and causes the low temperature toughness variation of the circumferential weld metal part. The number of inclusions in the steel with a size of 3 μm or more was set to 3.0 / mm ² or less.

Here, in the steel plate or steel pipe, 3 μm
In order to reduce the number of inclusions in the steel having the above size to 5.0 pieces / mm ² or less, impurity elements such as S,
Mass% of N and O, S: 0.002% or less, N: 0.
[0010] It is important to control in the range of 0010 to 0.0050%, O: 0.0030% or less, and to improve desulfurization and deoxidation efficiency or extend degassing treatment time. For example, desulfurization treatment at the time of manufacturing a steel sheet may be performed using desulfurization flux containing Ca or the like, or REM and the desulfurization flux. Further, in the rolling process, it is important to suppress the size of inclusions in the steel by lowering the slab heating temperature and shortening the slab heating time.

Further, in order to reduce the amount of island martensite produced in the electron beam weld metal, O, S
Not only limiting the amount of i, Al, but also Si, Al, O in the steel of the steel plate.
Balance the amount in mass% [O] <0.019 x [Si] + 0.00
The range was limited to 9 x [Al]. If the amounts of Si, Al, and O in the steel sheet are in mass% and do not satisfy [O] <0.019 x [Si] + 0.009 x [Al], the electron beam circumferential weld metal part will have a highly crack-sensitive structure. However, even when a steel sheet is manufactured by controlling inclusions in the steel, it causes variation in low temperature toughness in the low temperature region of the electron beam circumferential direction weld metal part. % [O] <0.019 x [S
i] + 0.009 × [Al] is satisfied.

At that time, in order to increase the fracture propagation resistance in the Charpy impact test, the bainite packet which is the propagation unit of the fracture in the electron beam weld metal is
It is preferable that the proportion of bainite packets having a size of 000 μm ² or more in the entire weld metal structure be 10% or less because the toughness value variation of the electron beam weld metal portion at low temperatures is improved. (F in Table 4) When the ratio of bainite packets having a size of 30,000 μm ² or more in the electron beam weld metal to the entire weld metal structure exceeds 10%, the fracture propagation resistance of the electron beam circumferential direction weld metal portion is increased. In some cases, the toughness value becomes low and the toughness value variation easily occurs in the low temperature region. (C in Table 4,
D).

In the electron beam weld metal, 3000
In order to make the proportion of bainite packets having a size of 0 μm ² or more in the entire weld metal structure 10% or less, the cooling rate after electron beam welding is adjusted, preheating is performed,
Post-welding treatment may be performed. In addition, it is important to adjust the steel plate or steel pipe composition that suppresses bainite transformation in the electron beam weld metal.

The reasons for limiting the composition of the steel sheet according to the present invention and the steel pipe using the steel sheet will be described below. All components are values in mass%. O: 0.0030% or less The upper limit of O is 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. C: 0.03 to 0.06% C requires at least 0.03% to secure the strength, but when it exceeds 0.06%, the maximum circumferential hardness of the weld metal in the circumferential direction exceeds Hv300 in Vickers hardness, and the steel pipe Since the low temperature toughness of the base metal and the weld metal in the circumferential direction deteriorates, C in steel
The amount was in the range of 0.03 to 0.06%.

In the case where the strength difference between the circumferential weld metal portion and the steel pipe base metal portion is large and the width B of the circumferential weld metal portion is narrow, as in the electron beam circumferential weld metal portion, V is V. It is well known that in the notch Charpy test, since the crack curves from the weld metal to the base metal side, the toughness of the circumferential weld metal part cannot be grasped. By setting the C content in the steel to 0.06% or less, the strength difference between the circumferential weld metal part and the steel pipe base metal part becomes small, and the low temperature toughness of the electron beam circumferential weld metal part is accurately determined by the V-notch Charpy test. It can also be evaluated. Si: 0.10 to 0.40% Si is an element that is effective not only for deoxidation during steel sheet production but also for strength improvement, but is an element that greatly affects the toughness of the electron beam circumferential direction weld metal part, At least 0.10% or more is required to obtain the effect. If it exceeds 0.40%, the toughness of the steel pipe base material and the weld heat affected zone deteriorates, so the upper limit was made 0.40%. Preferably, the amount of Si in steel is 0.20-
The range is 0.40%. Mn: 0.80-2.0% Mn is an essential element for securing strength and toughness,
Needs at least 0.80%, but above 2.0%,
In order to reduce the weldability and the toughness of the weld metal part in the electron beam circumferential direction, the Mn content in the steel was set in the range of 0.80% to 2.0%. Ti: 0.005 to 0.025% Ti forms fine TiN in the heat affected zone of the electron beam circumferential direction weld metal, controls the coarsening of austenite grains, and refines the microstructure to make the steel pipe base metal and welded. Improve the toughness of the heat affected zone. If the Ti content in the steel is less than 0.005%, its effect is small, and if it exceeds 0.025%, the low temperature toughness of the weld metal in the circumferential direction deteriorates.
The range was 5 to 0.025%. Preferably, the Ti content in steel is 0.
It is 005 to 0.020%. Al: 0.020 to 0.050% Al is an element used for deoxidation during steel sheet production, and in the present invention, plays an important role in stabilizing the toughness of the electron beam circumferential weld metal portion in the low temperature region. The amount of Al in steel
If it is less than 0.020%, the effect of stabilizing the toughness of the circumferential weld metal in the low temperature region cannot be expected. Therefore, the Al content in steel is 0.020% or more. On the other hand, when the Al content in the steel exceeds 0.050%, the toughness of the weld heat affected zone of the circumferential welded portion deteriorates, so the upper limit of the Al content in the steel was set to 0.050%. P: 0.015% or less P is an element that exists in 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 content in steel should be
It was set to 0.015%.

In the present invention, the following compositions may be further limited. S: 0.002% or less S, like P, is an element that is present in steel as an impurity and reduces the toughness of the steel pipe base material, and the lower the S, the better.
Further, by setting the S content in the steel to 0.002% or less, solidification cracking that occurs in the weld metal in the electron beam circumferential direction can be prevented, so the upper limit of the S content in the steel was set to 0.002%. Preferably, the S content in steel is 0.001% or less. Nb: 0.01 to 0.06% Nb has the effect of improving strength by precipitation strengthening.
If the addition amount is less than 0.01%, the effect is small, 0.06%
%, The toughness of the weld metal in the electron beam circumferential direction deteriorates, so the Nb content in the steel was set to 0.01 to 0.06%. N: 0.0010 to 0.0050% N deteriorates the toughness of the electron beam weld metal, so it is better to reduce it as much as possible. However, if it is less than 0.001%, precipitation of TiN and AIN will not occur and the steel pipe matrix will not be generated. Since the toughness of the material is impaired, the lower limit was made 0.001%. On the other hand, in the present invention, if the content is 0.0050% or less, sufficient toughness of the weld metal in the circumferential direction can be secured.
The amount was in the range of 0.0010 to 0.0050%. Preferably, the N content in steel is 0.0010 to 0.0040%.

Next, the reason why it is preferable to contain one kind or two or more kinds selected from the elements of Ca, Ni, Cu, Cr, Mo, V and REM will be explained. Ca: 0.0030% or less Ca is a useful element that controls the morphology of sulfide (MnS) and is effective in improving the toughness and anisotropy of the steel pipe base material and hydrogen-induced cracking resistance. If the amount of Ca in the steel exceeds 0.0030%, a large amount of CaO and CaS is generated and becomes large inclusions, which adversely affects not only the toughness of the steel pipe base metal but also the weldability. Therefore, the amount of Ca in the steel is set to 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 that improves the strength and toughness of the base metal without impairing the toughness of the weld metal in the electron beam circumferential direction. To obtain the target characteristics, 0.1% or more is required. is necessary. 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 steel is
The range was 0.1 to 1.0%. Cu: 0.1 to 1.0% Cu has the effect of improving strength and toughness as well as corrosion resistance and hydrogen-induced cracking resistance. If it is less than 0.1%, the effect is small, and if it exceeds 1.0%, the toughness of the steel pipe base material and the weld heat affected zone deteriorates, so the Cu content in the steel is 0.1 to 1.0.
The range is%. 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 the strength and toughness of the steel pipe base material. However, if the addition amount 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, 0.25% or less is preferable. V: 0.01 to 0.10% V is an element that 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 is deteriorated, so the V content in the steel is set to the range of 0.01 to 1.10.

REM: 0.0005 to 0.0050% 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, similar to Ca. If the REM content in steel is less than 0.0005%, its effect is poor. On the other hand, if the REM content in steel exceeds 0.0050%, the toughness of the base metal part of the steel pipe deteriorates.
The amount was in the range of 0.0005 to 0.0050%.

By the way, in order to butt-circularly weld steel pipes together by electron beam welding to form a pipeline having a good circumferential weld metal portion, the degree of vacuum is important and needs to be 10 Pa or less. The reason for this is that during electron beam welding,
This is because if the degree of vacuum exceeds 10 Pa, a circumferential weld metal portion having no welding defects such as undercut cannot be obtained.

In the above description, the steel sheet according to the present invention is used as a steel pipe and is applied to a pipeline. However, the steel sheet according to the present invention is not limited to a steel pipe and is used as a steel member using a steel sheet, Needless to say that it can be applied to welded steel structures such as pressure vessels other than pipelines and marine structures because the low temperature toughness of the weld metal part of steel members welded by beam welding is good. .

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 should 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). For electron beam welding, all-position circumferential welding is performed under the welding conditions of accelerating voltage of 30 to 80 kV, beam current of 10 to 600 mA, and welding speed of 300 to 900 mm / min. It is not necessary to particularly limit the number of welding passes, the welding posture, and the like, including the groove shape, welding conditions, and remelting of the weld metal.

[0047]

Example 1 A steel plate having a thickness of 14 mm and having the components shown in Table 5 was manufactured by a converter-continuous casting-thick plate process. Using each of the obtained steel plates, UOE is formed and the diameter is 610 mm.
It was a steel pipe. In Examples (No. 1 to 9), in steel plate production, in the converter-continuous casting step, various components were adjusted by converter blowing to enhance desulfurization, deoxidation and degassing treatment,
In the thick plate process, the number of inclusions having a size of 3 μm or more was set to 5.0 / mm ² or less by lowering the slab heating temperature and shortening the slab heating time. The UOE molding conditions were normal.

UOE steel pipes (having the same No.) of the same composition are butt-circumferentially welded by electron beam welding to form a pipeline, and a sample for inclusion measurement and a sample for Charpy impact test piece are prepared from the welded metal portion in the circumferential direction. Multiple samples were taken along the circumferential direction. After subjecting the collected samples to predetermined treatments, inclusion measurement and Charpy impact test were performed as described above. At that time, a sample for measuring inclusions was sampled from the steel pipe base material of the pipeline, and the inclusions were measured to obtain inclusions in the steel of each steel plate.

The sampling position for the inclusion measuring test piece is
Both the steel pipe base metal and the weld metal part are perpendicular to the circumferential direction,
The test piece used for the Charpy impact test is the one shown in FIG.
The same 2 mm V-notch test piece was sampled (Charpy impact test piece size: test piece length L = 55 mm, test piece height H = 10 mm, test piece width W = 10 mm, V notch depth D = 2 m).
m, V notch angle α = 45 °, V notch tip R = 0.25m
m).

At the time of electron beam welding, the degree of vacuum is 10 Pa.
Below, the electron beam circumferential movement speed is 600 mm / min,
Electron beam acceleration voltage 60kV, electron beam current 200mA,
Electron beam focusing condition (objective distance / focal length) 0.9 ~
It was 1.1. On the other hand, in Comparative Examples (Nos. 10 to 14), the steel plates shown in Table 5 were prepared by adjusting the components. The electron beam welding conditions were the same as in the example, and the pipeline was used, and the same investigation as in the example was performed.

Table 6 shows the evaluation results of the number of inclusions in the steel having a size of 3 μm or more and the low temperature toughness of the circumferential weld metal portion in the examples and the comparative examples. Here, the evaluation of the low-temperature toughness of the circumferential weld metal part, in the Charpy impact test results, the difference between the maximum value and the minimum value of the individual absorbed energy of each test piece at the same test temperature is in the temperature range exceeding 150J. If it occurred, x: there was variation, otherwise o: there was no variation.

[0052]

[Table 5]

[0053]

[Table 6]

The circumferentially welded metal portion of the steel pipe obtained by welding the steel pipes according to the present invention to each other by electron beam welding in the circumferential direction has good toughness characteristics at a low temperature and has no variation in absorbed energy. On the other hand, the steel pipe base material using the steel sheet not according to the present invention has an unsuitable chemical composition and cannot stably obtain good low temperature toughness. Further, in Comparative Example 14, the amount of S in the steel plate was out of the range of the present invention, so that cracks occurred in the electron beam circumferential direction weld metal portion. (Example 2) A steel plate G having the composition shown in Table 7 and having a plate thickness of 19 mm was manufactured by a converter-continuous casting-steel plate process.

[0055]

[Table 7]

Using the obtained steel sheet G, UOE molding was performed to obtain a UOE steel pipe having an outer diameter of 711 mm. The steel sheet was manufactured in the same manner as in Example 1. The UOE molding conditions were normal. UOE steel pipes are welded together by electron beam welding and welded circumferentially to form a welded joint. Samples for measuring inclusions, samples for structure observation and samples for Charpy impact test pieces are circumferentially welded from the circumferentially welded metal. Multiple samples were collected. After subjecting each of the collected samples to a predetermined treatment, inclusions were measured, the proportion of bainite packets having a size of 30,000 μm ² or more in the entire weld metal structure and the Charpy impact test were carried out as described above. . At that time, a sample for measuring inclusions was sampled from the steel pipe base material of the pipeline, and the inclusions were measured to obtain the number of inclusions in the steel plate.

The sampling positions of the inclusion measuring test piece and the tissue observing test piece were the same as in Example 1. Further, the test piece used in the Charpy impact test is also described in Example 1.
Same as For electron beam welding, set the vacuum to 10
Pa less than or equal to, electron beam circumferential movement speed is 400-700
The electron beam acceleration voltage was 60 kV, the electron beam current was 240 mA, and the electron beam focusing condition (objective distance / focal length) was 0.9 to 1.1. Welded joints G1 and G2 were produced by changing the cooling rate of the weld metal portion after welding by changing the electron beam welding conditions.

The number of inclusions in the steel having a size of 3 μm or more,
Table 8 shows the ratio of bainite packets having a size of 30,000 μm ² or more to the entire weld metal structure and the low temperature toughness evaluation results of the circumferential weld metal portion.

[0059]

[Table 8]

In the circumferential welded portion of the steel pipe in which the steel pipes according to the present invention are welded in the circumferential direction by electron beam melting, good toughness characteristics are exhibited even at a low temperature of −40 ° C., and there is no toughness variation. In addition, the proportion of bainite packets having a size of 30,000 μm ² or more in the entire weld is 10%.
In the welded joint having the weld metal portion exceeding the above range, the toughness varied at a very low test temperature (test temperature −60 ° C. or less) which does not pose a problem in use as a pipeline.

[0061]

According to the present invention, variations in low temperature toughness of a weld metal portion welded by an electron beam can be extremely reduced. Further, good toughness can be stably obtained in the Charpy absorbed energy low temperature region. As a result, it is possible to enhance the safety at low temperature in a pipeline used in a particularly severe environment among welded steel structures.

[Brief description of drawings]

FIG. 1 is a partial perspective view showing a sampling position of a Charpy impact test specimen in an electron beam circumferential weld metal portion.

FIG. 2 is a diagram showing the relationship between the shape of a V-notch Charpy impact test piece and the impact direction during the Charpy impact test, and the shape of the 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 of weld metal in circumferential direction θ welding direction z Steel pipe 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

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) B23K 103: 04 B23K 103: 04 (72) Inventor Koichi Yasuda 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Manufacturing Co., Ltd. Technical Research Institute F-term (reference) 4E066 BE04 CA02 CB00

Claims (7)

[Claims] 1. In mass%, C: 0.03 to 0.06%, Si: 0.10 to 0.40% Mn: 0.80 to 2.0%, P: 0.015% or less Ti: 0.005 to 0.025%, Al: 0.020 to 0.050% O: 0.0030 % Or less, and [O] <0.019 × [Si] + 0.009 ×
A steel sheet for electron beam welding, which has a composition satisfying [Al] and has 5.0 inclusions / mm ² or less in the steel of 3 μm or more. Here, [O], [Si] and [Al] represent the contents (mass%) of O, Si and Al, respectively. 2. In mass%, C: 0.03 to 0.06%, Si: 0.10 to 0.40% Mn: 0.80 to 2.0%, P: 0.015% or less S: 0.002% or less, Nb: 0.01 to 0.06% Ti: 0.005 to 0.025%, Al: 0.020 to 0.050% N: 0.0010 to 0.0050%, O: 0.0030% or less, and [O] <0.019 x [Si] + 0.009 x
A steel sheet for electron beam welding, which has a composition satisfying [Al] and has 5.0 inclusions / mm ² or less in the steel of 3 μm or more. Here, [O], [Si] and [Al] represent the contents (mass%) of O, Si and Al, respectively. 3. A steel pipe for electron beam welding, comprising the steel plate according to claim 1 or 2. 4. A pipeline characterized in that, in the weld metal portion of the steel pipes obtained by electron beam welding the steel pipes according to claim 3, the number of inclusions in the steel of 3 μm or more is 3.0 / mm ² or less. 5. In the weld metal portion of the steel pipe obtained by electron beam welding the steel pipes according to claim 3, the proportion of bainite packets having a size of 30,000 μm ² or more in the weld metal structure is 10% or less. Pipeline. 6. The weld metal portion of the steel pipe produced by electron beam welding the steel pipes according to claim 3 has 3.0 μm / mm ² or less of steel inclusions of 3 μm or more and a size of 30000 μm ² or more. The ratio of the bainite packet having a thickness to the weld metal structure is 10% or less, a pipeline. 7. An electron beam welding method for steel pipes, characterized in that the steel pipes according to claim 3 are electron beam welded at a vacuum degree of 10 Pa or less.