JPH08141762A - Method for producing welded steel pipe containing high C-high Cr excellent in weld zone toughness - Google Patents

Abstract

(57) [Summary] (Revised) [Purpose] High C-high for oil wells or line pipes of L80 kg class or higher with excellent weld toughness, capable of laser welding at a welding speed equal to or higher than the ERW method. Provided is a method for manufacturing a Cr-containing welded steel pipe. [Structure] A band steel consisting of steel with a predetermined composition is passed through a group of forming rolls to be continuously formed into an open pipe shape. This open pipe is pressed by a squeeze roll to abut both edges, and a laser is applied to the abutting portion. When forming a welded steel pipe by irradiating a beam to form a welded steel pipe, laser pipe welding is performed under the conditions satisfying the following formulas (1) and (2), and then the weld seam portion is immediately or reheated to a predetermined temperature. After quenching or normalizing, tempering is performed at a predetermined temperature. V ≧ 6 ··· (1) P ≧ 0.4Vt / e ^{a (T-T0)} ··· (2) where a = 0.006, P: laser output (k
W), V: welding speed (m / min), t: strip steel wall thickness (mm) T: preheating temperature (° C) of both edges of the strip, T0
: Room temperature (℃)

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a welded steel pipe having a high C content and a high Cr content, and more specifically, it transports an oil well pipe for excavating and collecting crude oil or natural gas containing wet carbon dioxide. The present invention relates to a method for producing a high C-high Cr content welded steel pipe excellent in carbon dioxide gas corrosion resistance, which is suitable for use in a line pipe, particularly in a high strength oil well pipe of L80 class or higher prescribed in API standard (American Petroleum Institute standard).

[0002]

2. Description of the Related Art In recent years, due to deterioration of energy situation, oil wells and natural gas wells containing a large amount of carbon dioxide and H ₂ S have been developed. In a wet carbon dioxide gas environment that also contains a trace amount of H ₂ S, from the viewpoint of corrosion resistance, high strength and relatively inexpensive A
High C-high Cr content steels represented by ISI (American Iron and Steel Institute) -420 steel are mainly used.

0.2C-typified by AISI-420
High C such as 13Cr steel and 0.1C-9Cr-1Mo steel
-High Cr steel is relatively inexpensive because it only contains Cr as an expensive alloy essential component, and it is possible to obtain a martensitic structure by increasing C, and it is possible to easily obtain high strength by quenching-tempering. It is possible and easy to adjust the strength. On the other hand, such a high C steel has a high susceptibility to welding cracks, and it is necessary to reduce the welding heat input in order to prevent the welding cracks. However, if the welding heat input is reduced, the welding speed becomes slow and productivity decreases. In addition, the hardness of the welded part increases and the delayed fracture susceptibility also increases.
Steel pipes made of high Cr steel have been manufactured as seamless steel pipes.

Further, regardless of the above-mentioned high C-high Cr steel, a seamless steel pipe has been widely used as a steel pipe as an oil country tubular good.
The reason is that the oil country tubular goods are exposed to a corrosive environment in which a stress close to the yield stress of the material is added due to their own weight, and therefore sufficient resistance to sulfide stress cracking (hereinafter referred to as SSC) and soil This is because sufficient resistance to collapse due to pressure is required, but the reliability of these performances was superior in the seamless steel pipe to the welded steel pipe. In particular, the above high C-high Cr steel has high hardenability, high delayed fracture susceptibility due to high hardness after quenching, and welding defects due to inclusion of oxides during welding. Since the welded steel pipe lacks the reliability of the welded portion due to the problems such as the easiness of occurrence of the above, it has not been generally used as an oil country tubular good.

However, the seamless steel pipe has lower productivity, higher cost, and higher cost than the welded steel pipe, and is also inferior in roundness and straightness, so that smoothing of the fluid flow improves the oil collection efficiency. From the viewpoint of reducing the chance of galvanic corrosion due to mutual contact between casing pipes that are multiply loaded or between casing pipes and tubing pipes, inexpensive and highly reliable welded steel pipes with excellent roundness and straightness are strongly recommended. Is desired. Further, in the field of steel pipes for line pipes, welded steel pipes are strongly desired mainly from the viewpoint of economy.

The welding methods used in the production of welded steel pipes are roughly divided into submerged arc welding methods (hereinafter referred to as S
AW method), electric resistance welding method (hereinafter referred to as ERW method)
And laser beam welding method (hereinafter referred to as laser welding).

Among the above welding methods, the SAW method has the advantage that welding defects are unlikely to occur and even if defects occur, they can be found and repaired by nondestructive inspection or the like. However, it is difficult to apply the SAW method in the welding of high C-high Cr steel represented by AISI-420 steel due to the problem of weld cracking. In addition,
Even with the SAW method, it is possible to weld even such high C-high Cr steels without the occurrence of weld cracks if the welding heat input is reduced, but the welding speed is inevitably slowed and productivity is markedly impaired. Be done.

On the other hand, the ERW method has a high welding speed and can be manufactured with high efficiency, but even when welding is performed in the atmosphere or in an inert gas shield, the shield is incomplete and the oxygen partial pressure is low. If it is high, a defect-inducing substance such as oxide scale easily mixes into the joint to cause welding defects.If the high-frequency input power is lowered to eliminate this, cold welding defects frequently occur due to insufficient melting, and conversely, high-frequency input power. When the value is increased, the instability phenomenon of molten steel due to strong electromagnetic force occurs and penetrator defects occur frequently. Further, in the ERW method, an upset is provided so as to discharge the above-mentioned oxide scale, which is a substance that induces welding defects during welding, from between the welding surfaces, so that high-temperature reheating by post-heat treatment after welding is performed in the thickness direction of the welded portion. A metal flow that is extremely difficult to eliminate is formed, which adversely affects the mechanical properties of the weld, especially toughness. The metal flow can be eliminated by further heating at a higher temperature during the post heat treatment, but in this case, the structure becomes coarser and the toughness decreases.

Further, since the laser welding is a fusion welding method which is essentially different from the ERW method, it is possible to suppress the occurrence of welding defects in the welded portion and it is not necessary to give an excessive upset, which results from the metal flow. The problem of deterioration of toughness does not occur. However, with laser welding alone, the welding speed is extremely slow at 1/5 to 1/10 compared to the ERW method, and it is never economically advantageous considering the equipment cost of the laser oscillator and other incidental equipment. hard.

By the way, in Japanese Patent Laid-Open No. 6-25746, a high C-high Cr steel slab having a predetermined composition is heated for the purpose of inexpensively obtaining a welded pipe made of high C-high Cr steel for oil country tubular goods. A hot-rolled steel strip having a low rolling strength of 800 ° C. or higher, a coiling temperature of 780 ° C. or higher, and a cooling rate of 0.1 ° C./sec or lower up to at least 600 ° C., which has low strength at room temperature and is easily pipe-formed. There has been proposed a method in which the pipe is welded by ERW method to form a welded pipe, and immediately or after the welded seam portion is subjected to a predetermined post-heat treatment, the whole pipe is subjected to a predetermined post-heat treatment. However, the method using the ERW method proposed in Japanese Patent Laid-Open No. 6-25746 has a drawback that it is difficult to obtain a high C-high Cr content welded steel pipe having excellent weld toughness. That is, the ERW method is a welding method having the above-mentioned problems, and particularly high C
-High Cr steel easily produces oxide scale, and since the produced oxide scale is stable, welding defects are likely to occur, and the above-mentioned oxide scale, which is a substance that induces this welding defect, is discharged from between the welded joint surfaces. Since it is necessary to provide a high upset as much as possible, a significant metal flow is formed in the thickness direction of the welded part, and the toughness of the welded part is significantly inferior to that of the base metal part. are doing.

Further, recently, for the purpose of obtaining a welded steel pipe having a welding portion performance equivalent to that of the SAW method while further increasing the welding speed, the welding speed is 1/5 to that of the ERW method as described above. It has been studied to use a 1/10 carbon dioxide laser beam welding method together with a welding heat source. For example, Japanese Patent Laid-Open No. 2-70379 discloses ER.
A pipe-making welding method has been proposed, in which high-frequency heating of both edges of a steel strip by the W method is performed and then laser welding is performed by a laser beam. However, this method uses laser welding, which is fusion welding which is essentially different from the ERW method, and therefore does not require the occurrence of weld defects and does not require excessive upset, and therefore has the advantage of preventing metal flow formation. However, it has a drawback that the welding speed is only twice as high as that of the laser-only welding pipe manufacturing method.

Further, in Japanese Unexamined Patent Publication No. 6-116645, as shown in FIG. 1, a butt cross-sectional shape of both edges of the steel strip 1 immediately before welding is formed on the outer surface of the open pipe with a predetermined width a and depth. The V-groove 2 having the width b is formed into a Y shape, and the bottom of the V-groove 2 is focused and irradiated with a laser beam, so that the manufacturing size (outer diameter: about 21) is excellent in SSC resistance. A method of manufacturing a welded steel pipe for oil wells having a thickness of ˜610 mm and a wall thickness of about 2 to 15 mm) by a laser welding method alone at a welding speed almost comparable to the ERW method has been proposed. However, this method can be applied only to thin-walled pipes up to about 6 to 8 mm, and the depth b of the V-groove 2 can be deepened in order to further speed up or apply to thick-walled pipes exceeding 8 to 13 mm. In this case, since the welding progresses faster than the V groove 2 is filled with the molten metal, the undercut 6 and its surface are recessed from the surface of the base metal strip as shown in FIG. Frequent defects such as under-beads and poor bead shape. If the welding speed is slowed down in order to avoid the occurrence of the bead shape defect, the heat input for welding becomes high and the cooling speed of the molten metal becomes slow, so that high C-high C
In the case of r steel, martensitic transformation may not proceed sufficiently during post heat treatment performed using induction heating means on the weld seam after laser welding, resulting in insufficient softening of the weld and welding. It has a defect that delayed fracture (deposition crack) occurs before the seam portion is subjected to post heat treatment and before heat treatment.

It is conceivable to reduce the carbon content in order to improve the toughness of the welded portion, but in this case, it is necessary to maintain the martensitic structure during quenching, and only the amount commensurate with the reduction in carbon content is required. Since it is necessary to additionally contain an austenite-forming element such as Ni, the material cost becomes high and it is difficult to economically adopt.

[0014]

The object of the present invention has been made in view of the above-mentioned circumstances, and even if a thick-walled pipe having a manufacturing size of the above-mentioned ERW method and exceeding 8 to 13 mm is used,
The efficiency is almost equal to that of the RW method, and the above-mentioned Japanese Patent Laid-Open No. 6-1
Japanese Patent No. 16645 discloses an oil well made of an inexpensive high C-high Cr steel which is capable of laser welding at a welding speed higher than that of the single laser welding method in which a V groove is provided and which is excellent in toughness of a weld portion without fear of delayed fracture. A method for manufacturing a welded steel pipe for a pipe or a line pipe.

[0015]

SUMMARY OF THE INVENTION The gist of the present invention is to provide a high C-high Cr excellent in weld zone toughness of the following (1) and (2).
It is in the method of manufacturing the welded steel pipe containing.

(1) C: 0.08 to 0.25 in% by weight
%, Cr: 7.5 to 14.0%, with the balance being Fe and inevitable impurities, a high C-high Cr steel material strip steel is continuously formed into an open pipe shape through a forming roll group. When both edges of the steel strip of the open pipe are pressed against each other by a squeeze roll and the welded portions are irradiated with a laser beam to perform pipe welding, the laser output is P (kw) and the welding speed is V (m / min), the thickness of the material strip steel is t (mm), and the preheating temperature of both edges of the strip is T
(° C.), when the room temperature T0 (° C.), the following equation (1) and after laser pipe-making welding conditions satisfying the relationship (2), heating the welded seam to 600～A _C1 transformation point High C-high C with excellent weld toughness characterized by tempering
Method for producing r-containing welded steel pipe.

V ≧ 6 (1) P ≧ 0.4 Vt / e ^{a (T-T0)} (2) where a = 0.006 (2) wt% and C: 0 0.08 to 0.25%, Cr:
A raw steel strip of a high C-high Cr steel containing 7.5 to 14.0% and the balance being Fe and unavoidable impurities is continuously formed into an open pipe shape through a forming roll group. When both edges of the strip steel are pressed against each other by squeeze rolls and the butted portion is irradiated with a laser beam for pipe welding, the laser output is P
(Kw), welding speed V (m / min), material strip steel wall thickness t (mm), strip steel both edge preheating temperature T (° C),
When the room temperature is T0 (° C), laser pipe welding is performed under the conditions that satisfy the relations of the following formulas (1) and (2), and then the weld seam is heated to 880 ° C or higher and then quenched or standardized. , then the method of producing a high C- high Cr content welded steel pipe with excellent weld toughness, characterized in that tempering at 600～A _C1 transformation point.

V ≧ 6 ··· (1) P ≧ 0.4 Vt / e ^{a (T-T0)} ··· (2) However, a = 0.006 The inventors of the present invention conducted various experimental studies. As a result, the following findings were obtained and the present invention was made based on these findings.

When high C-high Cr steel is laser pipe-welded, delayed fracture may occur in the weld seam portion before post-heat treatment of the weld seam portion as described above. In order to completely prevent it, after the laser pipe welding, the post-heat treatment for softening is performed after the temperature of the weld seam during the post-heat treatment applied to the weld seam is 300 ° C. or less at which the martensitic transformation has sufficiently progressed. Specifically, it is necessary to perform tempering heat treatment, but for that purpose, the welding speed must be 6 m / min or more, that is, the above formula (1) must be satisfied.

In laser-only welding, Japanese Patent Laid-Open No.
The V groove is provided as disclosed in Japanese Patent Laid-Open No. 116645 so that a through bead in the thickness direction can be formed even with a low power laser beam and high speed welding can be performed. Therefore, it is impossible to obtain a penetrating bead with a low-power laser beam unless the V groove is provided. In this case, both edge portions of the steel strip are preheated in advance by a high-frequency heating means or the like and a post-laser beam is irradiated. Welding is effective, but
Since the preheating effect is that the energy density is lower than that of the laser beam, even if it is preheated to the upper limit temperature of 1250 ° C., it is at most twice as effective in terms of laser output ratio as compared to the case without preheating, that is, the above ( There is only the effect of the denominator in the right term of the formula (2), and even if preheating is used together, the effect is not sufficient, and a high output laser oscillator, for example, a conventional 5 kW laser is 25 kW.
It is necessary to use a high-power laser oscillator.

Further, it is not enough to simply use a high-power laser. The laser output is P (Kw), the strip steel wall thickness is t (mm), the welding speed is V (m / min), and the strip steel edges are both. When the preheating temperature of the part is T (° C.) and the welding is performed while satisfying the relationship of the above formula (2) depending on the band steel wall thickness t, a defect-free weld part free of oxides and the like can be obtained.

[0022]

The reason why the method of the present invention is limited as described above will be described in detail below.

First, the reason for limiting the C and Cr content ranges of the high C-high Cr steel targeted by the present invention will be explained.

C: 0.08 to 0.25% High Cr containing 7.5% or more of Cr targeted by the present invention
In steel, the C content needs to be 0.08% or more in order to have a martensitic structure. On the other hand, when the amount of C is 0.
If it exceeds 25%, the hardness during welding becomes too high, so
Weld cracking and delayed fracture problems occur. Therefore, the C content is set to 0.08 to 0.25%. Preferably 0.14
Is about 0.22%.

Cr: 7.5 to 14.0% In order to exert excellent corrosion resistance in the wet carbon dioxide environment targeted by the present invention, the amount of Cr must be 7.5% or more. On the other hand, when the Cr content exceeds 14.0%, not only the effect is saturated, but also it becomes difficult to maintain the martensitic structure without adding other alloying elements. Therefore,
The Cr content was determined to be 7.5 to 14.0%. It is preferably 8.5 to 13.5%.

The high C-high Cr steel targeted by the present invention is a steel having the above-mentioned composition of components, but contains the following amounts of the respective components as impurities in addition to the above-mentioned components. Good.

That is, in weight%, Si: 1.0% or less, Mn: 2.0%, Al: 0.2% or less, N: 0.0
5% or less, Mo: 2.5% or less, W: 4.0% or less, N
i: 2.5% or less, Cu: 2.0% or less, Co: 0.0
4% or less, V: 0.5% or less, Ti: 0.2% or less, N
b: 0.5% or less, Zr: 0.2% or less, Ta: 0.2
% Or less, Hf: 0.2% or less, B: 0.01% or less, C
a: 0.01% or less, REM (rare earth element): 0.02
% Or less, P: 0.025% or less and S: 0.003%
You may contain the following 1 type or 2 types or more.

Next, the reasons for limiting the laser welding conditions of the present invention will be described.

In the present invention, first, the raw material steel strip having the above-mentioned composition is passed through a forming roll group in an ordinary manner to be continuously roll-formed into an open pipe shape, and right and left provided at the end of the forming roll group. The open pipes are abutted by the action of the pair of squeeze rolls, and the abutting portion, that is, the joining point at which both end faces of the steel strip contact each other, is irradiated with a laser beam, preferably from the top vertically to perform abutting welding, At this time, the laser output P (k
W), welding speed V (m / min), strip steel wall thickness t (m
m), preheating temperature of both edges of steel strip T (° C), room temperature T0
When the temperature is set to (° C.), the butt welding is performed under the condition that the following expressions (1) and (2) are satisfied.

V ≧ 6 (1) P ≧ 0.4 Vt / e ^{a (T-T0)} (2) where a = 0.006, that is, (1) above The expressions (2) and (2) are relational expressions found by the present inventors as a result of various experimental studies. As described above, the expression (1) is the presence or absence of V-groove formation at the butt portion of the open pipe and the laser output P. Irrespective of the strip steel wall thickness t, etc., when the welding speed V is less than 6 m / min, the temperature of the weld seam portion exceeds 300 ° C. and the martensitic transformation occurs due to the decrease in the cooling rate of the molten metal portion after welding. Does not progress sufficiently and does not soften even if subjected to a tempering treatment described later, so delayed fracture may occur. However, when welding is performed with the welding speed V set to 6 m / min or more, after welding the molten metal part, The decrease in cooling rate is suppressed and the temperature of the weld seam is 300 ° C or higher. Martensitic transformation sufficiently proceeds become possible to prevent the delayed fracture occurs because sufficiently softened by applying a tempering treatment to be described later.

FIG. 3 shows the influence of the welding speed V on the delayed fracture occurring in the weld before the post heat treatment. The horizontal axis represents the welding speed V and the vertical axis represents the presence or absence of delayed fracture. is there.

As is apparent from FIG. 3, the welding speed V
Is less than 6 m / min, delayed fracture occurs, but 6 m / min
If it is more than min, delayed fracture does not occur, which means that it is necessary to satisfy the above equation (1).

The above formula (2) shows the conditions for performing defect-free welding in which no oxide or the like is present in the welded portion, and the welding speed V and the strip steel are both determined according to the strip steel wall thickness t. By adjusting and setting the preheating temperature T of the edge portion so that the laser output P is equal to or more than the value obtained by the right side of the equation (2) and welding is performed, due to the presence of foreign matter such as oxide in the welded portion. It is possible to prevent deterioration of SSC resistance. That is, even if the laser output P is less than the value obtained from the right side of the equation (2), the welding speed is 6 m / min or more, and the martensitic transformation of the welded portion is sufficiently advanced to prevent the occurrence of delayed fracture. It means that a welding defect due to the presence of foreign matter such as oxide occurs in the welded portion.

FIG. 4 shows the effects of the strip steel wall thickness t, the welding speed V, the preheating temperature T of both edges of the strip steel and the laser output P on whether or not defect-free welding is possible. It is the figure which showed the difference of the value calculated | required by "0.4Vt / ea ^(T-T0) " on the right side in the inside, and the laser output P, showing the propriety of defect-free welding on the vertical axis. As is apparent from FIG. 4, if the laser output P is less than the value obtained by "0.4 Vt / e ^{a (T-T0)} ", defect-free welding is impossible, but the laser output P is " When the value is equal to or more than 0.4 Vt / ea ^(T-T0) ", defect-free welding is possible, which shows that it is necessary to satisfy the above expression (2).

In the above equation (2), a is a constant and
From a number of experimental results, it was found that “0.0006” is appropriate. Further, in order to satisfy the above formulas (1) and (2) at the same time, as described above, the laser oscillator with a low output of about 5 kW, which is generally used in the past, is insufficient. It is necessary to use the above high-power laser oscillator.

Further, in the method of the present invention using a high power laser, the V groove 2 shown in FIG. 1 similar to that described in the above-mentioned Japanese Patent Laid-Open No. 6-116645 is formed at the abutting portion of the open pipe and welded. It is also possible to perform higher speed welding in this case. That is,
The steel strip thickness is t, the laser beam diameter before focusing is D, the focal length of the focusing optical system is f, the welding speed is V, and the laser output is P.
At this time, it is desirable to form and weld a V groove having a width a and a depth b satisfying the following expressions (3) to (6).

A / b> D / f (3) a × b ≦ 2 × (P / V) (4) tb ≦ 4 × (P / V) ···· (5) a ≦ 2 × (P / V) ···· (6) In addition, the meaning of the above formulas (3) to (6) is as follows. On the street.

Formula (3): Conditions for preventing the penetration depth from decreasing. That is, the beam diameter before focusing is D
When the optical axis of the laser beam 3 is aligned with the abutting portions of both edge ends of the material strip steel 1 and the focus position is aligned with the bottom portion 5 of the V groove 2, the a / b value is less than D / f. At times, as shown in FIG.
Hits the shoulder portion 4 of the V groove 2 and metal plasma is generated in the shoulder portion 4 and the laser beam 3 is absorbed by this metal plasma to lower the laser energy and the penetration depth is reduced, but the a / b value is When the D / f value is exceeded, sometimes FIG.
As shown in (b), since the laser beam 3 does not hit the shoulder portion 4, the laser beam 3 without energy reduction can be focused on the bottom portion 5 of the V groove 2 and a deep penetration depth can be efficiently obtained. You can

Formula (4): A condition for preventing under-beads from being generated on the outer surface side of the welded portion. That is, in order not to generate the under-beads 6 as shown in FIG. 2, it is necessary that the V groove is sufficiently filled with the molten metal, and the amount of the molten metal required for this is supplied with the cross-sectional area of the V groove. And the amount of molten metal. The cross-sectional area of the V groove is represented by 1/2 (a × b), while the molten metal amount is proportional to the heat input amount (P / V) at the laser output P and the welding speed V. The prevention condition for the under-bead 6 is [a × b
≦ k × (P / V)]. It should be noted that k is a proportional constant, and it has been found from a number of experimental results that it is appropriate to set it to “2”.

Formula (5): The wall thickness (t) of the portion where the two edges of the steel strip, which continuously exist at the bottom of the V groove, are completely butted against each other.
-Conditions for melt-through of b). That is, in order to increase the welding speed by forming a V groove in the butted portion and reducing the thickness that needs to be melted by laser,
It is necessary to supply laser energy sufficient to completely melt the wall thickness (tb) portion existing at the bottom of the V groove, but the depth of laser penetration is the heat input (P /
Since it has a proportional relationship with V), the condition for completely melting the thickness (t-b) portion is [(t-b) ≦ p × (P /
V)]. It should be noted that p is a proportional constant, and it was found from a number of experimental results that it is appropriate to set it to "4".

Formula (6): Condition for preventing undercut 6 from occurring in the welded portion. That is, as shown in FIG. 2, undercut 6 occurs when the width a of the V groove is excessively larger than the bead width Ba. Therefore, in order not to generate the undercut 6 at the welded portion, it is necessary to make the width a of the V groove smaller than the bead width Ba. now,
If the welding speed V is constant, the bead width Ba is proportional to the amount of heat input by the laser, so the relationship [a ≦ q × (P / V)] is established. It should be noted that q is a proportional constant, and it has been found from a number of experimental results that it is appropriate to set it to “2”.

After laser-welding the butt portion of the open pipe in this manner, at least the weld seam portion is subjected to a tempering treatment as a post heat treatment or reheating to a predetermined temperature range to perform quenching or normalizing treatment and then post tempering. The treatment is performed, but the heat treatment conditions are defined for the following reasons.

[Tempering Temperature] If the tempering temperature is lower than 600 ° C., the welded part is not softened, and the toughness, delayed fracture resistance and SSC resistance of the welded part deteriorate. On the other hand, if the temperature exceeds the A _C1 transformation point, not only a certain amount of austenite transformation occurs and a predetermined strength cannot be obtained, but also retained austenite and fresh martensite phase are produced and the toughness, delayed fracture resistance and SSC resistance of the welded portion are increased. to degrade.

[Reheating Temperature] Prior to quenching or normalizing, the weld seam is reheated to a temperature range of 880 ° C. or higher. This reheating causes the structure of the weld metal to once become an austenite single phase to form a fine grain structure. If the reheating temperature is less than 880 ° C., a mixed grain structure is formed, and the toughness, delayed fracture resistance and SSC resistance of the welded portion deteriorate. In addition,
The upper limit of the reheating temperature does not need to be particularly limited. But,
If it exceeds 1050 ° C, a coarse-grained structure is formed and the toughness and SSC resistance of the welded portion deteriorate, so it is desirable to set it to 1050 ° C or less.

[0045]

EXAMPLES The present invention will now be described in more detail based on examples.

Two types of raw material steel strips having the chemical components shown in Table 1 were prepared as the base materials for pipe production, which provided excellent toughness and high strength by tempering treatment or quenching or normalizing-tempering treatment. Table 1 also shows the A _C1 transformation point.

[0047]

[Table 1]

The above material strip steel (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 edge portions thereof are butted against each other by a squeeze roll, and a plasma is used as a shield gas from above at the butted portion. When performing butt welding by vertically irradiating a laser beam using helium gas having a high removal effect, welding was performed under each condition shown in Tables 2 and 3. As the laser source, two types of carbon dioxide gas laser oscillators having outputs of 5 kW and 25 kW were used. The beam diameter D of the laser beam with an output of 25 kW before focusing is 51 mm, the focal length f of the mirror (parabolic mirror) is 381 mm, the beam diameter D of the laser beam with an output of 5 kW before focusing is 30 mm, The focal length f of the mirror (parabolic mirror) was 150 mm, and the focal points were set on the outer surface of the open pipe at the abutting portion, but some of the V-grooves formed at the abutting portion were at the bottom of the V groove. Focused. For comparison, a welded steel pipe welded by the conventional ERW method using the same material strip steel was also prepared.

[0049]

[Table 2]

[0050]

[Table 3]

The welded steel pipe thus obtained was subjected to heat treatment under the conditions shown in Tables 2 and 3 at the weld seam portion. Regarding delayed fracture resistance, after the post-treatment (T1) of heating only the weld seam to 700 ° C. and leaving it for one day and night, the presence or absence of cracking in the weld was observed and the delayed fracture was observed. A mark “◯” indicates that the delayed fracture did not occur and was evaluated by a mark “◯”.

Further, the weld toughness was evaluated by cutting the pipe in the axial direction and deploying it for the ones in which delayed fracture did not occur and the bead shape was good as a result of the evaluation of delayed fracture resistance. The length of the weld seam is 75 mm from the position shown in FIG. 6 and is 2 mm in the thickness direction.
A test piece with a side of 10 mm square having mmV notches formed was sampled and evaluated by a Charpy impact test, and a fracture surface transition temperature (vTs) of 0 ° C or less was evaluated as good "○", and exceeded 0 ° C. What was evaluated as unsatisfactory "x".

The results of these tests are shown in Tables 2 and 3. In Tables 2 and 3, trial numbers 1 and 20 are conventional examples by the conventional ERW method, and trial numbers 2, 3, 21 and 23 are conventional examples by laser single welding in which a V groove is formed, trial number 4 And 22 are conventional methods by high frequency preheating combined laser welding, and others are examples of the present invention and comparative examples.

As is apparent from Test Nos. 1 and 20, the ERW method of the conventional example does not cause delayed fracture because the welding speed is high, but the toughness of the welded portion is poor.

Further, as is clear from the trial numbers 2 to 4 and 21 to 23, the welding speed is 2 to 3 m although it is laser welding.
/ Min, slower conventional example, and trial numbers 9, 12, 15, 2
As can be seen from 4, 28 and 32, the welding speed is 6
In the comparative examples of the present invention, which are slower than less than m / min, delayed fracture occurs in the welded portion, and trial number 2 without preheating is used.
In Nos. 21 and 21, the welding speed was high even though the V groove was formed, and the welding conditions were not appropriate, resulting in defective bead shape.

On the other hand, according to the method of the present invention, when both edge portions of the steel strip are preheated to a relatively high temperature, pipe forming welding can be performed at a welding speed equal to or higher than that of the ERW method without forming the V groove. A product having a good bead shape and excellent delayed fracture resistance and toughness of the welded portion is obtained.

[0057]

EFFECTS OF THE INVENTION According to the method of the present invention, an oil well having sufficient corrosion resistance in a wet carbon dioxide environment with excellent weld toughness without the risk of delayed fracture before tempering as a post heat treatment for welded seams. It is possible to efficiently and stably manufacture a welded steel pipe containing a high C-high Cr for a pipe or a line pipe, and it has an extremely useful effect industrially.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a V-groove shape before welding.

FIG. 2 is a view showing a weld bead shape that occurs when the V groove width is excessively large.

FIG. 3 is a diagram showing an influence of welding speed on delayed fracture of a welded portion.

FIG. 4 is a diagram showing influences of open pipe wall thickness, welding speed, preheating temperatures of both edges of a steel strip, and laser output on whether defect-free welding is possible.

FIG. 5 is an explanatory diagram showing a relationship between a V groove size and a laser beam path.

FIG. 6 is a view showing a sampling position of an impact test piece.

[Explanation of symbols]

 1: strip steel 2: V groove 3: laser beam 4: shoulder of V groove 5: bottom of V groove 6: undercut

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B23K 31/00 B C22C 38/00 302 Z 38/18

Claims (2)

[Claims] 1. C: 0.08 to 0.25% by weight, C
r: 7.5 to 14.0%, with the balance being Fe and unavoidable impurities, a high C-high Cr steel material strip steel is continuously formed into an open pipe shape through a forming roll group. Both edges of the open pipe strip steel are pressed against each other with a squeeze roll, and the welded portion is irradiated with a laser beam to weld a pipe, and the laser output is P (kw) and the welding speed is V (m / min). , The thickness of the material strip steel is t (mm), and the preheating temperature of both edges of the strip is T
(° C.), when the room temperature T0 (° C.), the following equation (1) and after laser pipe-making welding conditions satisfying the relationship (2), heating the welded seam to 600～A _C1 transformation point High C-high C with excellent weld toughness characterized by tempering
Method for producing r-containing welded steel pipe. V ≧ 6 ・ ・ ・ ・ (1) P ≧ 0.4Vt / e ^{a (T-T0)}・ ・ ・ (2) where a = 0.0006 2. C: 0.08 to 0.25% by weight, C
r: 7.5 to 14.0%, with the balance being Fe and unavoidable impurities, a high C-high Cr steel material strip steel is continuously formed into an open pipe shape through a forming roll group. Both edges of the open pipe strip steel are pressed against each other with a squeeze roll, and the welded portion is irradiated with a laser beam to weld a pipe, and the laser output is P (kw) and the welding speed is V (m / min). , The thickness of the material strip steel is t (mm), and the preheating temperature of both edges of the strip is T
(° C) and room temperature are T0 (° C), laser pipe welding is performed under the conditions that satisfy the relations of the following formulas (1) and (2), and then the weld seam is heated to 880 ° C or more and then quenched. or normalizing, and then 600～A _C1 high C- high Cr manufacturing method of containing welded steel pipe superior in weld zone toughness, characterized by tempering in transformation. V ≧ 6 ・ ・ ・ ・ (1) P ≧ 0.4Vt / e ^{a (T-T0)}・ ・ ・ (2) where a = 0.0006