JP3319358B2 - Method for producing welded steel pipe for line pipe with excellent hydrogen-induced cracking resistance, sulfide stress cracking resistance and low-temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel pipe for a line pipe for transporting a crude oil or a natural gas containing hydrogen sulfide, and more particularly to an AP.
Manufacture of welded steel pipes for line pipes with excellent weld strength, hydrogen-induced cracking resistance and sulfide stress cracking resistance suitable for high-strength line pipes of X42 class or higher specified by I standard (American Petroleum Institute standard) About the method.

[0002]

2. Description of the Related Art As a method for manufacturing a line pipe, UOE is used.
A method in which a steel sheet is formed into a tube by pressing and the seam is welded by submerged arc welding (hereinafter referred to as SAW method)
And steel strip are continuously formed into a tube, and the seam is welded by electric resistance welding or high frequency resistance welding (hereinafter referred to as ER)
W method). The ERW method can obtain a product with high efficiency because the welding speed is high, but has the following problems.

That is, in the ERW method, even when welding in the atmosphere or welding in an inert gas shield, the shield is incomplete and the oxygen partial pressure is relatively high. Welding easily occurs when it is mixed between welding surfaces. In addition, when the high-frequency input power is low, welding defects due to insufficient melting occur frequently. On the contrary, when the high-frequency input power is high, the unstable phenomenon of molten steel due to strong electromagnetic force occurs, and penetrator defects frequently occur. However, it is extremely difficult to fine-tune the high-frequency input power to prevent the occurrence of these welding defects and penetrator defects.

[0004] When a welded steel pipe manufactured by such an ERW method is used as a line pipe, the following problems may occur. That is, the inner surface of the pipe is wet H ₂ S
When exposed to the environment, hydrogen generated by corrosion penetrates into steel and accumulates as hydrogen gas at weld defects,
Hydrogen-induced cracking (hereinafter referred to as HIC) occurs on the abutment weld surface, and this propagates in the thickness direction along the metal flow formed by the upset during the abutment welding. In this case, the resistance to hydrogen-induced cracking (hereinafter referred to as HIC resistance) is poor. In a line pipe, since hoop stress due to internal pressure is applied perpendicularly to the metal flow, sulfide stress cracking (hereinafter referred to as SSC property) is likely to occur at a welded portion, and sulfide stress cracking resistance (hereinafter, referred to as SSC resistance). SSC resistance) is not always satisfactory.

On the other hand, in the SAW method, welding defects are less likely to occur, and even if defects occur, they can be found and repaired by non-destructive inspection, so that the reliability of the welded steel pipe obtained is low. Very high, almost no occurrence of HIC or SSC in the weld.

However, the welding speed of the SAW method is lower than that of the ERW method, and the productivity is significantly inferior. Therefore, there is a long-felt need for a welded steel pipe having a productivity comparable to the ERW method and free from HIC and SSC, and a manufacturing process.

[0007] The causes of HIC and SSC are roughly divided into inclusions and hardened structures, and welding defects in the ERW method are mainly oxide-based inclusions, which are caused by HIC.
Therefore, there is a need for a method for preventing the incorporation of a defect-inducing substance and the occurrence of a penetrator defect and a welding defect at the welding contact surface.

Japanese Patent Application Laid-Open No. 2-70379 discloses that a laser beam whose welding speed is 1/5 to 1/10 of that of the ERW method is applied to a welded portion following high-frequency heating of both edges of a steel strip. There has been proposed a method for obtaining a welded steel pipe having a welded portion performance equivalent to that of arc welding such as the SAW method.

Further, Japanese Patent Application Laid-Open No. HEI 8-118050 discloses a laser welding condition for setting a lower limit of a laser output according to a preheating temperature before laser welding to secure HIC resistance and SSC resistance. .

[0010]

However, in Japanese Patent Application Laid-Open No. Hei 8-118050, only the lower limit of the laser output when the preheating and the welding speed are determined is set, and the upper limit of the laser output is not particularly set. Not. But,
If the laser output becomes excessive, the HIC resistance and S resistance
There is a problem that not only does the SC property deteriorate, but also the low-temperature toughness decreases. The mechanism for deteriorating the characteristics when the laser output becomes excessive is considered as follows.

As the laser output increases, the weld pool naturally becomes longer in the welding direction. That is, the time during which the weld metal is molten increases. By the way, an oxide film is formed on the end face of the steel sheet by preheating. When the end faces are butted by laser welding, the oxide present on the end faces is dispersed in the molten metal. In laser welding, since the welding speed is high, melting occurs rapidly, and oxides are dispersed very finely immediately after melting.

However, the oxide particles agglomerate and coarsen over time. Fine oxides (about 0.4-0.3 μm in diameter) in laser weld metal are resistant to HIC and SSC
It does not adversely affect the toughness and low-temperature toughness, but has an adverse effect when coarsened to about 1-2 μm. This is HIC or SSC
This is because the coarse oxide acts as a starting point of cracking and also acts as a starting point of brittle fracture at low temperatures.

Further, in Japanese Patent Application Laid-Open No. HEI 8-118050, the lower limit of the laser output is set by a relational expression according to the welding speed and the preheating temperature. Unlike the case where the laser beam penetrates and a bead is formed to the back side, a defect such as a penetrator seen in the ERW does not occur.

Therefore, the lower limit of the laser output in laser welding is sufficient if the laser welding condition is monitored from the back surface to confirm the penetration of the laser beam, and the accuracy is high. The present invention has been made based on the above findings,
It is an object of the present invention to provide a method for producing a welded steel pipe for a line pipe having excellent heat resistance, SSC resistance and low-temperature toughness.

[0015]

According to the present invention, there are provided: (1) C: 0.01 to 0.2% by weight, and Si:
0.03 to 0.8%, Mn: 0.4 to 2.0%,
P: 0.025% or less, S: 0.002% or less, so
l. Al: 0.01 to 0.1%, Cu: 0 to 0.5%,
Ni: 0 to 0.5%, Cr: 0 to 1.2%, Mo: 0 to 0%
1.0%, Nb: 0 to 0.15%, V: 0 to 0.15
%, Ti: 0 to 0.15%, Zr: 0 to 0.15%, and B: 0 to 0.005%, Ca: 0 to 0.005%, and REM : 0
A steel strip containing one or two selected from 0.01% is continuously formed into an open pipe shape through a group of forming rolls, and the open pipe is pressed with a squeeze roll to form both edges of the steel strip. Irradiating a laser beam to the butt portion to form a welded steel pipe by butt welding to form a welded steel pipe, and irradiating with a laser beam under conditions satisfying the following formula (1) to perform welding. A method for producing a welded steel pipe for line pipes having excellent cracking properties, sulfide stress cracking resistance, and low-temperature toughness.

T _m ≦ 1.8 (1) where t _m = 1.57.2 a / v ² [Q / {2πλk (T _m −T)}] ² (2) a: thermal diffusivity of the steel strip ( m ² / s) v: Welding speed (m / s) Q: Laser power (J / s) λ: Thermal conductivity of the strip (J / m · s · k) k: Strip thickness (m) T _m: steel melting point (K) T: preheating temperature (K) t _m: the method according to melting time (s) (2) (1 ), after welding, and heating at least the weld than Ac ₃ transformation point A method for producing a welded steel pipe for a line pipe having excellent resistance to hydrogen-induced cracking and resistance to sulfide stress cracking, characterized by allowing the steel pipe to cool afterwards.

(3) In the method described in (1), after welding, at least the welded portion is heated to the Ac ₃ transformation point or more, and then from the temperature range of (Ar ₃ transformation point −30 ° C.) or more and 1000 ° C. or less. A method for producing a welded steel pipe for a line pipe having excellent hydrogen-induced cracking resistance and sulfide stress cracking resistance characterized by accelerated cooling.

(4) The method according to (3), characterized in that tempering is performed in a temperature range of 500 to 750 ° C. following accelerated cooling, and the method is excellent in hydrogen-induced cracking resistance and sulfide stress cracking resistance. This is a method for manufacturing a welded steel pipe for a line pipe.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the method of the present invention will be described in detail. First, as a material, excellent HIC resistance
(Hot rolled steel sheet) having heat resistance, SSC resistance and low temperature toughness is used. After the pipe production, at least a predetermined heat treatment is applied to the welded seam portion, preferably the entire pipe, to obtain a base material having excellent HIC resistance, SSC resistance and low-temperature toughness regardless of the history of the production of the material strip. It is preferred that the Hereinafter, the reasons for containing each component of the steel strip and the reasons for limiting the addition range will be described.

C: 0.01 to 0.2% C is given a predetermined strength (strength of grade X42 or higher) by adding C. If the content is less than 0.01%, the above strength cannot be obtained, and if the content exceeds 0.2%, the toughness is deteriorated. Therefore, the C content is 0.01 to 0.2.
%. However, when the post-heat treatment is not performed on the welded seam after welding, if the content exceeds 0.12%, the effect of the welded portion is caused and both the HIC resistance and the SSC resistance are reduced. In this case, the C content is 0.01 to 0.1.
It is desirable to be 12%. Further, when obtaining a high-tensile steel pipe of X70 to X80 grade, the content thereof is 0.01 to 0.0 irrespective of the presence or absence of post heat treatment of the welded seam portion.
It is desirable to be 7%.

Si: 0.03 to 0.8% Si needs to secure a content of 0.03% or more to deoxidize steel. On the other hand, if the content exceeds 0.8%, toughness is deteriorated and tempering embrittlement is caused. Therefore, the Si content is set to 0.03 to 0.8%, preferably 0.05%.
It is preferable to set it to 0.30%.

Mn: 0.4 to 2.0% A predetermined strength (strength of grade X42 or higher) is ensured in the steel pipe by adding Mn. If the content is less than 0.4%, the desired strength cannot be secured, and if the content exceeds 2.0%, the S
Since the decrease in SC property is caused, the Mn content is 0.4 to 2.0.
%. However, when attempting to obtain a high-strength steel pipe of less than X70 grade, if the content exceeds 1.8%, the alloy element concentration in the base metal segregated portion becomes high, and particularly, the C content is 0.08%.
% Or more, the HIC resistance and the SSC resistance of the base material are both deteriorated. In this case, the Mn content is 0.4 to
It is desirably 1.8%. When the post-heat treatment is not performed on the weld seam after welding, the content is 1.
If it exceeds 4%, the weld is hardened, and HIC resistance and S resistance
In this case, the Mn content is desirably set to 0.4 to 1.4% because the SC property is lowered. In addition, X70
In order to obtain a high tensile steel pipe of up to X80 grade, the content thereof is desirably 0.4 to 2.0% regardless of the presence or absence of post heat treatment of the welded seam.

P: not more than 0.025% P is an unavoidable impurity. In particular, if P is contained in excess of 0.025%, the alloy element concentration in the base material segregated portion becomes high, and the SSC resistance of the base material decreases. Is notable, and the tempering embrittlement also has an adverse effect. Therefore, the P content is preferably 0.025% or less, and more preferably 0.015% or less.

S: 0.002% or less S is an unavoidable impurity. In particular, when S is contained in an amount exceeding 0.002%, a sulfide-based material whose form cannot be controlled by Ca or REM (rare earth element). Inclusion (MnS)
Is generated, and the HIC resistance and the SSC resistance are significantly reduced. Therefore, the S content is set to 0.002% or less. Preferably, the content is 0.001% or less.

Sol.Al: 0.01 to 0.1% Al needs to secure 0.01% or more in sol.Al content to deoxidize steel. If the content exceeds 0.1%, it becomes difficult to ensure the cleanliness of the steel, so the sol.Al content is 0.01 to 0.1%, preferably 0.02 to 0.0%.
It is good to make it 5%.

[0026] In addition to the above components, the steel strip of the present invention comprises:
One or more selected from Cu, Ni, Cr, Mo, Nb, V, Ti, Zr and B, and one or two selected from Ca and REM (rare earth element)
Species can also be included.

Cu, Ni, Cr, Mo, Nb, V, T
i, Zr and B The addition of these elements all improves the strength and toughness of the steel pipe, so that Cu, Ni, Cr,
One, two or more of Mo, Nb, V, Ti, Zr and B can be contained. Cu ≦ 0.5%, Ni ≦ 0.5% Cu and Ni have an effect of improving strength and toughness.
If the content exceeds 5%, the hot workability is reduced and it becomes difficult to produce a hot-rolled coil as a material.
5%, preferably 0.05 to 0.5%.

Cr ≦ 1.2% Cr has an effect of improving strength and toughness, but if contained in excess of 1.2%, the toughness and SSC resistance are reduced. And preferably 0.05 to
It is better to be 1.2%. Mo ≦ 1.0% Mo has the effect of improving the strength and toughness, but if it is contained in excess of 1.0%, the toughness and SSC resistance decrease, so the upper limit is set to 1.0%. Preferably 0.05 to
It is good to make it 1.0%. Nb ≦ 0.15%, V ≦ 0.15%, Ti ≦ 0.15
%, Zr ≦ 0.15% Nb, V, Ti, and Zr have an effect of improving the strength and toughness, but if contained in excess of 0.15%, the toughness is reduced, so the upper limit is 0.15%. And preferably 0.01
The content is preferably set to 〜0.15%. B ≦ 0.005% B has the effect of improving strength and toughness, but 0.005%
If the content exceeds 0.005%, the toughness is reduced, so the upper limit is made 0.005%, preferably 0.0005 to 0.005%.
It is good to make it 5%. Ca ≦ 0.005%, REM (rare earth element) ≦ 0.01
% These components exhibit the effect of improving SSC resistance through morphological control of sulfide-based inclusions. 0.0 for Ca
If the content exceeds 0.05%, Ca-based inclusions increase, leading to deterioration of HIC resistance and SSC resistance.
In the case of M, if the content exceeds 0.01%, an increase in oxide inclusions causes deterioration of HIC resistance and SSC resistance, so the upper limits are 0.005% and 0.01%, respectively.
Preferably, the Ca content is 0.0005 to 0.0
It is good to make it 05% and REM content 0.0005-0.01%. The material strip having the above-mentioned composition is produced, for example, under the following conditions. However, the present invention is not limited to this condition.

[Slab Heating Temperature] The slab heating temperature is preferably set within a temperature range in which the following hot rolling is possible. [Hot rolling finish temperature] In order to improve the HIC resistance and the SSC resistance, it is preferable to complete the hot rolling at an Ar ₃ transformation point or higher and secure the following accelerated cooling start temperature. The temperature is preferably equal to or higher than the temperature of (Ar ₃ transformation point + 30 ° C.).

[Accelerated Cooling Start Temperature] If the accelerated cooling start temperature after hot rolling is low, C
Is concentrated to form a hardened structure during accelerated cooling, which causes a decrease in HIC resistance and SSC resistance.
It is difficult to obtain a high strength of 0 to X80 class. In order to prevent these, the accelerated cooling start temperature is preferably equal to or higher than the temperature of (Ar ₃ transformation point−30 ° C.), and particularly preferably equal to or higher than the Ar ₃ transformation point without proeutectoid ferrite.

[Average Cooling Rate During Accelerated Cooling] If the average cooling rate during accelerated cooling is low, the two-phase separation of ferrite / pearlite proceeds, and the HIC resistance and SSC resistance at the center segregation portion are increased.
Inferior band-like structure is formed, and X70-X8
It is difficult to obtain a zero-grade high strength. Therefore, the average cooling rate is preferably set to 5 ° C./s or more. However, when the value exceeds 20 ° C./s, hardened block-like bainite is easily formed, which is not preferable. In the case of a hot-rolled steel sheet for X70 to X80 grade high strength, approximately 30 ° C./s is suitable.

[Accelerated Cooling Stop Temperature] When accelerated cooling is stopped in a temperature range exceeding 600 ° C., untransformed austenite remains at the time of the stop, so that pearlite is generated, and the center segregated portion is hardened by the concentration of C and hardened. HIC and SSC resistance tend to decrease. Also, X70
It is difficult to obtain a high strength of up to X80 class. Meanwhile, 4
When accelerated cooling is stopped in a temperature range lower than 00 ° C., hardened block-like bainite is easily generated, and the HIC resistance and the SSC resistance decrease. Therefore, the accelerated cooling stop temperature is preferably set to 600 to 400 ° C. X70 ~
X80 class high strength hot rolled steel sheet: 600-200 ° C
Is preferred.

[Take-up temperature] Take-up is performed following the stop of the accelerated cooling. [Laser Welding Conditions] In the present invention, the steel strip having the above-mentioned composition and manufactured under the above-mentioned conditions is passed through a group of forming rolls as usual and continuously roll-formed to form an open pipe. A laser beam is applied vertically from above to a position where the two edges of the steel strip abut on each other by a pair of left and right squeeze rolls provided at the rear stage of the forming roll group, that is, the butted portion of the open pipe, to perform abutment welding. At this time, conditions satisfying the following equation (1) are set.

T _m ≦ 1.8 (1) where t _m = 1.57.2 a / v ² [Q / {2πλk (T _m -T)}] ² (2) a: Thermal diffusivity (m ² / s) v: Welding speed (m / s) Q: Laser power (J / s) λ: Thermal conductivity of steel strip (J / m · s · k) k: Steel strip thickness (M) T _m : melting point of steel (K) T: preheating temperature (K) t _m : melting time (s) Here, the above formulas (1) and (2) are intensively studied, studied, and experimented by the present inventors. Is a relational expression found as a result of (1)
Defines the time during which the weld metal is melted in the laser welding, and indicates the condition under which the oxide inclusions in the weld metal are not coarsened as described above. Also (2)
In the equation, the melting time is obtained from parameters such as the preheating temperature, the welding speed, and the laser output, and was derived by the inventor from consideration of heat transfer theory. FIG. 1 shows strip steel thickness t, welding speed V,
The preheating temperature T and laser output P at both edges of the steel strip are SS
FIG. 3 is a diagram showing the effect on C generation, and FIG. 1 to t showing the relationship between T _m calculated from the equation (2) and the HIC occurrence rate.
_It can be seen that when _m > 2.1, SSC occurs. FIG. 2 shows the relationship between t _m calculated from the equation (2) and the HIC occurrence rate. FIG. 2 shows that HIC occurs when t _m > 1.8. Further, FIG. 3 shows the relationship between absorbed energy at -50 ° C. for t _m and Charpy calculated from equation (2). FIG. 3 shows that when T _m > 2.0, the absorbed energy sharply decreases. From the above, t _m ≦
It can be seen that a value of 1.8 satisfies all of HIC resistance, SSC resistance and low-temperature toughness. The welded steel pipe obtained by welding the butt portion of the open pipe in this way can exhibit sufficient performance as it is, but in order to further improve the performance, perform the following heat treatment on the welded seam after welding. Is preferred.

[Heating temperature of welding seam portion] It is preferable to accelerate the temperature to a temperature range not lower than the transformation point of the welding seam portion Ac ₃ . This is to destroy the coarse-grained structure of the welded portion as an austenitic single phase during heating to obtain a fine-grained structure. However, if the heating temperature is lower than this temperature, the effect cannot be obtained. In addition,
The upper limit does not need to be particularly defined. However, if the temperature exceeds 1100 ° C., the crystal grains become coarse again. Particularly, in a material having a C content of more than 0.12%, the HIC resistance, SSC resistance and toughness are adversely affected. The temperature is preferably set to 1100 ° C. or lower.

[Cooling after heating] After heating to the above temperature range,
If left to cool as it is (air cooling), the hardness of the welded portion is kept low, and the HIC resistance and SSC resistance are improved. However,
If only the weld seam is heated to the above temperature range and allowed to cool, the strength of the weld may be lower than that of the base material. In this case, if accelerated cooling is performed immediately after heating, a decrease in the strength of the welded portion can be prevented. Therefore, it is preferable to perform accelerated cooling when it is desired to prevent a decrease in strength. However, if the accelerated cooling start temperature is lower than the temperature of (Ar ₃ transformation point −30 ° C.), the residual austenite in which C is concentrated with the growth of pro-eutectoid ferrite hardens during accelerated cooling, and the HIC resistance and SSC resistance are reduced. Will be reduced. On the other hand, when the accelerated cooling start temperature exceeds 1000 ° C., a martensite structure or a bainite structure having high hardness is generated, which is not desirable for the HIC resistance and the SSC resistance. Therefore, the cooling start temperature when performing accelerated cooling is (Ar ₃ transformation point −30 ° C.) to
The temperature is preferably set to 1000 ° C. Further, the cooling rate in the case of performing the accelerated cooling is preferably 5 to 30 ° C./s, which is the same as the condition at the time of manufacturing the base material steel strip (hot-rolled steel sheet).

[Tempering after Accelerated Cooling] When tempering is performed after accelerated cooling to soften the welded portion, the HIC resistance and toughness of the welded portion are further improved. If further improvement is desired, a tempering treatment is preferably performed after accelerated cooling. However, if the tempering temperature is lower than 500 ° C., the material does not soften, so that the tempering effect cannot be obtained. On the other hand, when the temperature exceeds 750 ° C., not only the austenite transformation occurs partially but a predetermined strength cannot be obtained, but also a retained austenite and a martensite phase which is not tempered occur, which is undesirable for HIC resistance and SSC resistance. The temperature when performing the return process is 50
It is preferably set to 750 ° C.

[0038]

EXAMPLES As a base material for pipe making, SSC resistance and HI resistance were used.
Nine types of steel strips (hot-rolled steel sheets) having chemical compositions shown in Table 1 of X42 to X80 grades having excellent C properties were prepared. API grade, strength, HIC resistance and SSC resistance of these base materials
Table 1 also shows the results of the examination of the properties. In addition, H resistance of the base material
The IC property is determined by NACE (American Corrosion Association Standard) -TM-02
96 h in a NACE bath (0.5% acetic acid, 5% saline, 25 ° C., 1 atm H ₂ S saturation) based on the method specified in −84.
It was evaluated by the crack length ratio (CLR) when immersed. S resistance
SC property is NACE-TM-01-77-METHOD
Σ when evaluated by the uniaxial tensile test method specified in -A
th (the minimum stress that causes a fracture by SSC) and σys (yield stress).
The HIC resistance satisfies CLR ≦ 15% under NACE conditions, which is one standard, and the SSC resistance satisfies σth / σys ≧ 72% under NACE conditions, which is one standard.

[0039]

[Table 1]

The raw steel strip (hot rolled steel sheet) is formed into an open pipe by passing it through a group of forming rolls according to a conventional method, and both edges thereof are butted together by a squeeze roll. When performing abutment welding by vertically irradiating a laser beam using helium gas having a high removal effect, welding was performed under the conditions shown in Table 2. As the laser source, the beam diameter D before condensing is 51 mm, and the mirror (parabolic mirror) focal length f is 381 mm.
A carbon dioxide laser oscillator having an output of 25 kW was used. Note that the focal position was set to 3 mm inward from the outer surface of the open pipe at the butting portion. Furthermore, the condition of the welding bead on the back surface was monitored by a CCD camera installed on the inner surface of the tube, and the penetration of the laser beam was secured. For comparison, a welded steel pipe as-welded by a conventional ERW method using a steel strip was also prepared.

The HIC resistance and SSC resistance of the as-welded welded portion of the obtained welded steel pipe (for some, the welded portion after the post-heat treatment of the welded seam under the conditions shown in Table 2)
Properties and low temperature toughness were evaluated by the following methods.

After incision in the pipe axis direction so that the welded seam portion is located at the center and development in the circumferential direction, from each position shown in FIG. 4, the HIC test piece has the shape and dimensions shown in FIG. 6 from different positions in the longitudinal direction of the tube axis. For the SSC test piece, a test piece of either (X) or (Y) shown in FIG. In the Charpy test, three test pieces each having the shape and dimensions shown in FIG. 9 were cut out from three different positions in the longitudinal direction of the tube axis and sampled. Since the SSC test piece lacked a screw portion due to the thickness of the material, a Teflon tape or the like was wound around the screw portion and sealed during the test before the test.

In the HIC test, the above test pieces (each test number 3
Pieces) in a NACE bath (0.5% acetic acid, 5% saline, 25%).
° C., and after 96 hours immersion in 1 atm H ₂ S saturated), to detect the bonding surface cracking area by the ultrasonic flaw detection method as shown in FIG. 7 for each specimen, an average crack area ratio (CAR) HI
C sensitivity was evaluated. Note that the triple value of the CAR value is the above C value.
This corresponds to the LR value, and if CRA ≦ 5%, it indicates that the HIC resistance is excellent.

In the SSC test, each of the above test pieces (each test number, two) was subjected to NACE using an SSC test apparatus shown in FIG.
Bath (0.5% acetic acid, 5% saline, 25 ° C., 1 atm H ₂ S
(Saturated), a tensile stress of 72% of SMYS (standard minimum yield stress) was applied, and the presence or absence of fracture during the 720 hour test period was examined.

The low-temperature toughness was evaluated by a Charpy test.
It evaluated by the absorption energy at 50 degreeC. If the absorbed energy is 65 J or more, sufficient low-temperature toughness is obtained. Table 2 shows the test results.

[0046]

[Table 2]

In Table 2, trial numbers 1, 6, and 11 are conventional examples based on ERW, and 2, 3, 7, 8, 12, 13, 14, 17, 1
8, 19, 21 to 30 are examples of the present invention, and the others are comparative examples by laser welding with heat input exceeding the conditions of the present invention.

As is apparent from the results of these Examples and Comparative Examples, according to the present invention, EHIC property and SSC resistance
A welded pipe having excellent heat resistance and low-temperature toughness can be obtained. Further, the HIC resistance, SSC resistance and low-temperature toughness of the welded portion were good even when the post-heat treatment was not performed on the welded seam portion.

[0049]

As described above in detail, according to the method of the present invention, the ER sufficiently satisfies all the strengths, the HIC resistance and the SSC resistance of the weld in a wet H ₂ S environment.
It is possible to stably manufacture a welded steel pipe for a line pipe having a production size by the W method with high efficiency, and has an extremely useful effect in industry.

[Brief description of the drawings]

FIG. 1 is a view showing the influence of a correlation (Equation 1) of a sheet thickness, an output, a welding speed, and a preheating temperature on SSC generation.

FIG. 2 is a diagram showing the influence of the correlation (Equation 1) of the sheet thickness, output, welding speed, and preheating temperature on HIC generation.

FIG. 3 is a view showing the influence of the correlation (Equation 1) of the sheet thickness, output, welding speed, and preheating temperature on the low-temperature toughness.

FIG. 4 is a conceptual diagram showing a sampling site of a test piece for HIC and SSC investigation.

FIG. 5 is a view showing the shape and dimensions of an HIC test piece.

FIG. 6 is a view showing the shape and dimensions of an SSC test piece.

FIG. 7 is a conceptual diagram illustrating a state in which HIC is evaluated by an ultrasonic flaw detection method.

FIG. 8 is a conceptual diagram of an SSC test apparatus.

FIG. 9 is a diagram showing the shape and dimensions of a Charpy test piece.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/00 301 C22C 38/00 301Z 38/50 38/50 38/58 38/58 // B23K 103: 04 B23K 103: 04 (56) References JP-A-8-120346 (JP, A) JP-A-5-228660 (JP, A) JP-A-5-15986 (JP, A) JP-A-8-155563 (JP, A) ( 58) Field surveyed (Int.Cl. ⁷ , DB name) B23K 26/00-26/30 B21C 37/08

Claims (4)

(57) [Claims] (1) C: 0.01 to 0.2% by weight.
Si: 0.03 to 0.8% and Mn: 0.4 to 2.0%
, P: 0.025% or less, S: 0.002% or less, sol. Al: 0.01 to 0.1%, Cu: 0 to 0.5%, Ni: 0 to 0.5 %, Cr: 0
1.2%, Mo: 0 to 1.0%, Nb: 0 to 0.15
%, V: 0 to 0.15%, Ti: 0 to 0.15%, Z
One or more selected from r: 0 to 0.15% and B: 0 to 0.005%, Ca: 0 to 0.005%, and REM: 0 to 0.01%
The steel strip containing one or two types selected from the following is continuously formed into an open pipe shape through a group of forming rolls, and the open pipe is pressed with a squeeze roll to butt the two edges of the steel strip. A method for producing a welded steel pipe for a line pipe by irradiating a butt portion with a laser beam to form a welded steel pipe by abutment welding, wherein the welding is performed by irradiating the laser beam under a condition satisfying the following expression (1). Method for producing welded steel pipes for line pipes having excellent resistance to hydrogen-induced cracking, sulfide stress cracking and low-temperature toughness. t _m ≦ 1.8 (1) where t _m = 1.57.2a / v ² [Q / {2πλk (T _m −T)}] ² (2) a: heat diffusion of the steel strip Rate (m ² / s) v: Welding speed (m / s) Q: Laser output (J / s) λ: Thermal conductivity of the strip (J / m · s · k) k: Strip thickness (m) ) T _m : melting point of steel (K) T: preheating temperature (K) t _m : melting time (s) 2. The hydrogen-induced cracking resistance and the sulfide stress cracking resistance according to claim 1, wherein after welding, at least the weld is heated to at least the Ac ₃ transformation point and then left to cool. For manufacturing welded steel pipes for line pipes. 3. After welding, at least the weld is heated to at least the Ac ₃ transformation point and then (Ar ₃ transformation point −30 ° C.) or more.
The method for producing a welded steel pipe for a line pipe excellent in hydrogen-induced cracking resistance and sulfide stress cracking resistance according to claim 1, wherein the cooling is performed at an accelerated temperature from a temperature range of 1000 ° C or lower. 4. Following accelerated cooling, the temperature is 500 to 750 ° C.
The method for producing a welded steel pipe for a line pipe having excellent resistance to hydrogen-induced cracking and resistance to sulfide stress cracking according to claim 3, wherein the tempering is performed in a temperature range of: