JP2014155949A - Welded steel pipe for line pipe with excellent low-temperature toughness, and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welded steel pipe for a line pipe with excellent low-temperature toughness, which is molded in a tubular shape in any one of UOE, JCO, bend roll and spiral steps, and a method of manufacturing the same.SOLUTION: When a butt part of a steel plate molded in a tubular shape has one layer of each of inner and outer surfaces welded by submerged arc welding, a weld metal sectional area S1 on an inner surface side and a weld metal sectional area S2 on an outer surface side in a weld zone are set to satisfy inequality (1): 0.5≤S2/S1≤1.0.

Description

  The present invention relates to a welded steel pipe for line pipe excellent in low temperature toughness and a method for producing the same. In particular, the present invention relates to a submerged arc welding method for steel materials, which is suitable for use in tube-forming welding of welded steel pipes for line pipes formed into a tubular shape by any one of UOE, JCO, bend roll, and spiral processes.

Currently, the main pipeline material for long-distance transportation of crude oil and natural gas is the American Petroleum Institute (API) standard X70 (tensile strength of 570 MPa) or higher, and also X80 (tensile strength of 625 MPa or higher) welded steel pipes for line pipes. Has been put to practical use.
In recent years, in order to further improve transportation efficiency, increasing the internal pressure of welded steel pipes for line pipes has been studied. Accordingly, API standard X70 (hereinafter referred to as X70) or higher, further API standard X80 (hereinafter referred to as There is a demand for increasing the thickness of welded steel pipes for high-strength line pipes (referred to as X80). In addition, the future drilling area for crude oil and natural gas is expected to extend to extremely cold regions such as the Arctic Circle, and weld steel pipes for high-strength thick-walled line pipes are required to guarantee low temperature toughness at -40 ° C or lower. It is expected to be. In particular, when manufacturing a steel pipe, a thick steel plate is formed into a tubular shape by any one of UO, JCO, and bend rolls, then the ends are butted together and the seam is welded by arc welding. Since the heat input by welding becomes a large heat input and the particle size of the weld heat affected zone (HAZ) becomes coarse, a decrease in low temperature toughness becomes an important problem.

  Submerged arc welding with two or more electrodes is applied to pipe-forming welding (seam welding) of welded pipes for line pipes. From the viewpoint of improving pipe production efficiency, both inner and outer layers are welded with one pass on the inner side and one pass on the outer side. High-efficiency welding is performed (for example, Patent Documents 1 and 2).

  In double-sided single-layer welding, it is necessary to ensure a sufficient penetration depth so that the inner and outer weld metals overlap and there are no unmelted parts. It becomes high and the toughness of the heat affected zone tends to deteriorate. In consideration of welding efficiency and workability, the penetration depth of the inner weld metal to be welded first is set to the penetration depth of the outer weld metal in order to avoid melting down of the molten metal during submerged arc welding. The welding heat input on the outer surface side is generally higher than the welding heat input on the inner surface side. Furthermore, generally, the toughness of the outer surface heat affected zone having a large heat input is lower than the toughness of the inner surface heat affected zone.

  To increase the toughness of the weld heat affected zone, it is effective to reduce the heat input of welding, but if the heat input is not significantly reduced compared to the heat input of the usual seam welding, the toughness is improved. The effect is not clear. However, if the heat input is significantly reduced, the welding amount is also reduced, so that it is necessary to reduce the groove cross-sectional area in accordance with the amount of welding reduction. Therefore, if penetration welding having a large weld metal cross-sectional area is not performed, the weld metals on the inner and outer surfaces do not overlap, increasing the risk of insufficient penetration.

  Therefore, increasing the toughness of the weld heat-affected zone requires both a significant reduction in input heat input and an increase in penetration depth, and various proposals have been made so far, but this is extremely difficult to achieve. . In addition, when the thickness is increased, the toughness of the inner-surface weld heat-affected zone, which is welded with a lower heat input than that of the normal outer-surface weld heat-affected zone, becomes noticeable.

  For example, Patent Document 2 proposes a submerged arc welding method in which the current density is increased in accordance with the electrode diameter and the penetration depth is increased. However, the current and current density are insufficient for recent specifications. It is difficult to achieve both a significant reduction in heat input and an increase in penetration depth.

  Patent Document 3 proposes a submerged arc welding method with a high current and a further high current density. By supplying arc energy in the plate thickness direction as much as possible, only the necessary penetration depth is secured, and the steel width By suppressing the melting of the base material in the direction, excessive welding heat input is omitted, and both heat input reduction and deep penetration are achieved.

  In Patent Document 4, the heat input method is reduced while securing the penetration depth by controlling the current density and the power supply method to the wires and electrodes used during submerged arc welding, and the toughness is improved at the weld heat affected zone. ing.

  In Patent Documents 5 and 6, by controlling the ratio of the bead width of the plate thickness surface layer to the bead width in the vicinity of the penetration tip and the steel plate thickness, it is possible to improve toughness in the heat affected zone while suppressing slag entrainment. It has been.

  In Patent Documents 7 and 8, by controlling the weld metal cross-sectional area of the inner and outer surfaces according to the plate thickness, the bead width on the steel plate surface is increased while obtaining sufficient penetration, and the toughness is improved in the weld heat affected zone. ing.

JP 11-138266 A JP-A-10-109171 JP 2006-272377 A JP 2007-260684 A JP 2009-214127 A JP 2010-274276 A JP 2009-233679 A JP 2010-274275 A

  However, in the submerged arc welding method described in Patent Document 3, although both heat input reduction and deep penetration can be achieved, the bead width on the steel sheet surface is small and tends to be a substantially uniform bead width from the steel sheet surface to the penetration tip. That is, since the fusion line (also referred to as Fusion Line, FL) is oriented in the thickness direction, brittle fracture tends to progress in the thickness direction, and the toughness value tends to be lowered despite low heat input welding. there were.

Further, in the submerged arc welding method described in Patent Document 4, although heat input reduction and deep penetration can be achieved at the same time, the bead width near the overlapping position (meeting portion) of the inner and outer surface weld metal is reduced, so that the weld metal on the inner and outer surfaces is reduced. In order to overlap, there has been a problem that each steel pipe axial welding position must be strictly controlled.
In the submerged arc welding methods described in Patent Documents 5 to 8, although the bead width with respect to the plate thickness or the bead cross-sectional area with respect to the plate thickness is mentioned, the relative shapes of the inner surface side welded portion and the outer surface side welded portion are referred to. There is no mention of the relationship, and furthermore, since a high toughness is achieved mainly by the shape of the entire welded part, there is a problem that a significant heat input reduction effect cannot be obtained particularly at the outer surface welded part.

The present invention has been made in view of such circumstances, and by controlling the ratio of the cross-sectional area of the weld metal on the inner and outer surfaces appropriately, excellent toughness can be obtained in the weld heat affected zone on both the inner and outer surfaces. Is.
In particular, in the present invention, when submerged arc welding is performed on the butt portion of the steel sheet formed into a tubular shape from the inner and outer surfaces, the heat input to the outer surface is greatly reduced and the low temperature toughness of the outer surface welding heat affected zone is improved. On the other hand, although the inner surface heat input is increased in order to sufficiently overlap the inner and outer surface weld metals, the toughness of the inner surface heat affected zone is improved by tempering during outer surface welding. In other words, the outer surface welding heat affected zone is improved by lowering heat input, and the inner surface welding heat affected zone uses tempering during outer surface welding to obtain sufficient penetration while obtaining excellent low temperature in both the inner and outer surface welding heat affected zones. It is an object of the present invention to provide a welded steel pipe for line pipe that can obtain toughness and a method for producing the same.

The present inventors produced welded steel pipes having inner and outer surface welded joints of steel plates using various welding conditions in submerged arc welding, and investigated the weld metal cross-sectional shape, heat input, and toughness of the weld heat affected zone.
As a result, by properly controlling the ratio of the weld metal cross-sectional area of each inner and outer surface side, the toughness of the heat-affected zone on the outer surface is improved and the toughness of the heat-affected zone on the inner surface is improved by tempering during outer surface welding. As a result, it has been found that excellent toughness can be obtained at the weld heat-affected zone on both the inner and outer surfaces while obtaining sufficient penetration. The present invention has been made by further study based on the obtained knowledge, and the gist thereof is as follows.

[1] A welded steel pipe welded to a steel plate formed into a tubular shape, and a butt portion of the steel plate formed into a tubular shape is welded to each of the inner and outer surfaces by submerged arc welding. A welded steel pipe for line pipes having excellent low-temperature toughness, characterized in that a cross-sectional area S1 (mm ² ) and a weld metal cross-sectional area S2 (mm ² ) on the outer surface side satisfy the formula (1).
0.5 ≦ S2 / S1 ≦ 1.0 (1)
[2] The welded steel pipe for line pipes with excellent low-temperature toughness according to [1] above, wherein the tensile strength of the steel sheet is 570 to 825 MPa when the circumferential direction of the steel pipe is the tensile direction.
[3] When submerged arc welding is performed on a butt portion of a steel sheet formed into a tubular shape, the groove shape is an X groove whose inner surface groove depth d1 and outer surface groove depth d2 satisfy the expression (2). The method for producing a welded steel pipe for a line pipe excellent in low temperature toughness according to the above [1] or [2], wherein the obtained X groove is subjected to submerged arc welding.
d2 / d1 ≦ 1.0 (2)
[4] In the submerged arc welding, the heat input λ1 on the inner surface side is 3.5 to 16.0 kJ / mm, the heat input λ2 on the outer surface side is 2.5 to 11.0 kJ / mm, and (3) The method for producing a welded steel pipe for line pipe excellent in low temperature toughness according to the above [3], wherein the formula is satisfied.
0.1 ≦ λ2 / λ1 ≦ 2.5 (3)

  According to the present invention, by appropriately controlling the ratio of the weld metal cross-sectional area of the inner and outer surfaces, the outer surface heat input is greatly reduced and the toughness of the outer surface weld heat affected zone is improved. As a result of improving the toughness by tempering during outer surface welding, a welded steel pipe having excellent low-temperature toughness in both the inner and outer surface of the heat affected zone can be obtained while obtaining sufficient penetration.

It is a figure explaining the welding part shape in this embodiment. It is a figure explaining the groove shape in this embodiment. It is a figure explaining the sampling position of the Charpy impact test piece in a present Example. It is a figure explaining the relationship of Charpy absorbed energy and S2 / S1 in this embodiment.

Hereinafter, the welded steel pipe for line pipes of the present invention and the manufacturing method thereof will be described.
A welded steel pipe for a line pipe according to the present invention is a welded steel pipe welded to a steel plate formed into a tubular shape, and the butt portion of the steel plate formed into a tubular shape is welded to each inner and outer surfaces by submerged arc welding, and welded. The weld metal cross-sectional area S1 on the inner surface side and the weld metal cross-sectional area S2 on the outer surface side satisfy the expression (1).
In the submerged arc welding method for a steel sheet according to the present invention, the inner and outer surfaces of the steel sheet are each welded by submerged arc welding, and the weld metal cross-sectional area S1 on the inner surface side and the weld metal cross-sectional area S2 on the outer surface side satisfy the expression (1). Select welding conditions as follows.
0.5 ≦ S2 / S1 ≦ 1.0 (1)
Here, S1: weld metal cross-sectional area (mm ² ) on the inner surface side, S2: weld metal cross-sectional area (mm ² ) on the outer surface side.

As described above, in consideration of welding efficiency and workability, in order to avoid melting of the molten metal during melt welding (meltdown), the depth of penetration of the weld metal on the inner surface side to be welded first, In general, the depth is smaller than the penetration depth of the weld metal on the outer surface side, and the welding heat input on the outer surface side is higher than the welding heat input on the inner surface side. However, in order to achieve high toughness, it is necessary to achieve a significant reduction in the amount of heat input input during welding, but at the same time, it is sufficient that the weld metals on the inner and outer surfaces overlap and no unmelted part occurs. Unless the penetration depth was ensured, it was not possible to obtain a steel pipe having a sound welded joint.
Therefore, the present inventors have found that the low temperature toughness of the weld heat affected zone of both the inner and outer surfaces can be improved by appropriately controlling the ratio of the cross sectional areas of the weld metals on the inner and outer surfaces of the welded steel pipe.
That is, according to the present invention, the welding heat input on the outer surface side, which has been considered difficult in the past, is greatly reduced to improve the low temperature toughness of the outer surface side heat affected zone, and the inner surface heat input Although the weld metal is increased in order to overlap sufficiently, the toughness of the inner-surface weld heat-affected zone is improved by tempering during outer-surface welding, so that excellent low-temperature toughness can be obtained without causing weld defects.

Hereinafter, the reason which limited the said Formula (1) is demonstrated in detail.
First, the relationship between Charpy absorbed energy and (outer surface weld metal S2) / (inner surface weld metal S1) will be described. FIG. 4 shows the relationship between Charpy absorbed energy and S2 / S1 when tested at −40 ° C. When S2 / S1 increased, the Charpy absorbed energy of the outer surface welding heat affected zone decreased due to an increase in outer surface heat input. On the other hand, when S2 / S1 is decreased, the Charpy absorbed energy of the outer surface welding heat affected zone is increased due to the reduction of the heat input to the outer surface, but the annealing effect on the inner surface weld heat affected zone is impaired, and the Charpy absorption of the inner surface weld heat affected zone is reduced. Energy decreased.
Therefore, the cross-sectional area S2 (mm ²⁾ is larger divided by the (S2 / S1) by the cross-sectional area S1 of the weld metal of the inner surface side (inner surface weld metal) (mm ²⁾ of the weld metal of the outer surface side (outer surface weld metal) In other words, if the penetration amount of the weld metal on the outer surface side is relatively larger than the penetration amount of the weld metal on the inner surface side, the effect of greatly reducing the heat input to the outer surface cannot be obtained, and the toughness deteriorates. The upper limit of S2 / S1 is 1.0. In order to further obtain a significant effect of reducing the heat input to the outer surface, the upper limit is preferably set to 0.95, and more preferably set to 0.86.

On the other hand, the smaller the S2 / S1, the greater the effect of reducing the heat input to the outer surface. However, when the ratio is less than 0.5, the effect of improving the toughness of the inner surface heat affected zone by annealing the inner surface heat affected zone during outer surface welding. Is not sufficiently obtained, and the toughness of the inner-surface weld heat-affected zone deteriorates, so the lower limit is made 0.5. Further, as S2 / S1 decreases, excessive inner surface heat input is required, and the welding speed decreases. Therefore, a more preferable lower limit is set to 0.6.
In this way, by defining the cross-sectional area ratio of the weld metal on each of the inner and outer surfaces, the heat input on the outer surface is greatly reduced, and the inner surface weld affected part uses tempering during outer surface welding, so Excellent low temperature toughness can be obtained at the weld heat affected zone.

Here, in the present invention, the weld metal cross-sectional area S1 on the inner surface side is defined as follows.
Usually, the outer surface welding is performed after the inner surface welding is performed, but since a part of the inner surface welding metal is melted during the outer surface welding, the cross-sectional area of the inner surface welding metal of the steel pipe can be measured as it is after the inner and outer surface welding. Impossible. Therefore, the area of the portion of the inner surface weld metal excluding the portion overlapping with the outer surface weld metal is measured as the inner surface weld metal cross-sectional area S1.

  Moreover, it is preferable that the tensile strength of the steel plate used as a base material is 570-825 MPa when the circumferential direction of the steel pipe which concerns on this invention is made into a tension direction.

  Moreover, the present invention is a welded steel pipe for line pipes including the above-described welded joint, and by appropriately controlling the ratio of the cross-sectional area of the weld metal on the inner and outer surfaces of the welded steel pipe, Low temperature toughness can be improved.

Next, the manufacturing method of the welded steel pipe for line pipes which concerns on this invention is demonstrated.
In the method for manufacturing a welded steel pipe according to the present invention, when the butt portion of a steel plate formed into a tubular shape is welded to the inner and outer surfaces by submerged arc welding, the groove shape may be an X groove as shown in FIG. The shape of the X groove has an inner groove depth d1 and an outer groove depth d2 as shown in FIG. 2 in order to make the weld metal sectional area on the outer surface side smaller than the weld metal sectional area on the inner surface side. You may employ | adopt the shape which satisfies (2) Formula.
d2 / d1 ≦ 1.0 (2)
Thus, by submerging arc welding the X groove having the inner groove depth d1 longer than the outer groove depth d2, it is possible to satisfy the relationship of S2 / S1 as described above, and tougher. Can be improved. In addition, although it does not specifically limit about the lower limit of d2 / d1, It is preferable to set it as 0.2 or more.

In the present invention, after the steel plate is formed into a tubular shape, the butt portion is subjected to submerged arc welding from the inner and outer surfaces to form a welded steel pipe. The process at the time of forming into a tubular shape is UOE in which the steel plate is C-pressed, U-pressed, and O-pressed. It may be a process, a JCO process, or a bend roll.
In the submerged arc welding according to the present invention, the heat input on the inner surface side is preferably 3.5 to 16.0 kJ / mm, and the heat input on the outer surface side is preferably 2.5 to 11.0 kJ / mm. It is more preferable to satisfy the formula (3).
0.1 ≦ λ2 / λ1 ≦ 2.5 (3)

  Further, a length dn (root) obtained by subtracting the inner surface groove depth d1 and the outer surface groove depth d2 from the steel plate thickness is not particularly defined. However, in order not to cause meltdown during inner surface welding, the lower limit of dn is set. The thickness is preferably 3 mm, more preferably 5 mm. In addition, about the upper limit of dn, it is preferable to set it as 10 mm, and it is more preferable to set it as 8 mm.

  Hereinafter, the effects of the present invention will be described with reference to examples, but the present invention is not limited to the conditions used in the following examples.

In this example, the base steel plate to be subjected to submerged arc welding was a steel plate for a line pipe having a tensile strength of 570 to 625 MPa and a thickness of 26 mm. The steel sheet has a carbon equivalent Ceq calculated by the following (formula 3) of 0.39.
Next, after the groove shape of the groove shape shown in FIG. 2 is applied to the butt portion of the base steel plate to be welded, it is formed into a tubular shape, and a multi-electrode submerged of inner and outer surface single layer welding under the welding conditions shown in Table 2 Arc welding was performed to produce a welded joint. Table 1 shows the groove dimensions. In addition, the UOE process was employ | adopted as the process at the time of shape | molding in a tubular shape.
In addition, the tensile strength of the base material steel plate in Table 1 is the tensile strength when the circumferential direction of the steel pipe is the tensile direction. Production No. In any of 1-4, the tensile strength was 570-625 MPa.
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5
... (Formula 3)
Here, C, Mn, Ni, Cu, Cr, Mo, and V are contents [mass%] of each element.

A Charpy impact test piece 2 is taken from the produced joint, and subjected to a Charpy impact test (notch position: melt line, test temperature: −40 ° C.) in accordance with the metal material impact test method of JIS Z 2242. Absorbed energy (vE- ₄₀ ) was determined. In addition, HAZ toughness vE- ₄₀ in Table 3 is an average value of absorbed energy in three test pieces. Moreover, about evaluation of the low temperature toughness in HAZ, HAZ toughness vE- ₄₀ evaluated 100J or more as favorable.

FIG. 3 shows the sampling position of the Charpy impact test piece 2. With the melt line 5 of the welded portion 4 as a notch position, the notch 3 is parallel to the plate thickness direction, and for each of the inner surface welding and the outer surface welding, the position 7 mm below the surface of the base steel plate 1 is the center of the Charpy impact test piece 2 It collected so that it might become. Table 4 shows the HAZ toughness vE- ₄₀ (upper: inner surface, lower: outer surface) obtained as a result of the Charpy impact test, the observation results of the weld metal cross-sectional shape, and the groove dimensions.

  In the present invention example (production Nos. 1 to 3), the value S2 / S1 obtained by dividing the outer surface weld metal cross-sectional area S2 by the inner surface weld metal cross-sectional area S1 is in the range of 0.5 to 1.0, and the outer surface weld metal breakage By reducing the area S2, the heat input to the outer surface was greatly reduced, and the HAZ toughness on the outer surface side was greatly improved, and a high Charpy absorbed energy of 100 J or more was exhibited even at −40 ° C. Further, by increasing the inner surface weld metal cross-sectional area S1, the weld metals on the inner and outer surfaces are sufficiently overlapped, and a sound welded joint is obtained. Furthermore, the HAZ toughness on the inner surface side maintains a high Charpy absorption energy of 100 J or more even at -40 ° C. due to the annealing effect during outer surface welding by increasing the outer surface weld metal cross-sectional area S2 even if the heat input is increased. Excellent toughness was obtained in the heat affected zone of both the inner and outer surfaces.

On the other hand, the comparative example (Production No. 4) is within the range of the above-mentioned Patent Document 7, but the outer surface weld metal cross-sectional area S2 is greatly reduced. This is an example in which the area S2 is too small to sufficiently obtain the tempering effect in the inner surface welding heat affected zone and the toughness of the inner surface welding heat affected zone is lowered.
Further, the comparative example (condition No. 5) is also within the range of Patent Document 7, but the outer surface weld metal cross-sectional area S2 is larger than the inner surface weld metal S1 and the inner surface heat-affected zone has good toughness, but outer surface welding. This is an example in which the metal cross-sectional area is too large and the toughness of the heat-affected zone on the outer surface is reduced.

DESCRIPTION OF SYMBOLS 1 Base material steel plate 2 Charpy impact test piece 3 Notch 4 Welded part 5 Melt line

Claims (4)

A welded steel pipe welded to a tubular steel plate,
The butt portion of the steel sheet formed into a tubular shape is welded to the inner and outer surfaces respectively by submerged arc welding,
In welds, for line pipe superior in low temperature toughness characterized by satisfying the weld metal cross sectional area S1 of the inner surface (mm ²⁾ and the outer surface weld metal cross sectional area S2 (mm ²⁾ is a (1) Welded steel pipe.
0.5 ≦ S2 / S1 ≦ 1.0 (1)   The welded steel pipe for line pipe excellent in low temperature toughness according to claim 1, wherein the tensile strength of the steel sheet is 570 to 825 MPa when the circumferential direction of the steel pipe is a tensile direction. When submerged arc welding is performed on a butt portion of a steel sheet formed into a tubular shape, the groove shape is an X groove where the inner surface groove depth d1 and the outer surface groove depth d2 satisfy the expression (2). The method for producing a welded steel pipe for a line pipe according to claim 1 or 2, wherein the X groove is processed and submerged arc welding is performed.
d2 / d1 ≦ 1.0 (2) In the submerged arc welding, the heat input λ1 on the inner surface side is 3.5 to 16.0 kJ / mm, the heat input λ2 on the outer surface side is 2.5 to 11.0 kJ / mm, and further satisfies Equation (3). The manufacturing method of the welded steel pipe for line pipes excellent in low-temperature toughness of Claim 3 characterized by performing.
0.1 ≦ λ2 / λ1 ≦ 2.5 (3)

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