JPH08118050A - Manufacture of welded steel tube for line pipe excellent in hydrogen induced crack resistance and sulfide stress crack resistance - Google Patents

Abstract

PURPOSE: To provide the method for manufacturing a welded steel pipe capable of executing a laser welding at a welding speed equal to ERW method and excellent in the strength of a welded zone, HIC resistance and SSC resistance. CONSTITUTION: When a steel strip having a prescribed composition is formed continuously into the state of an open pipe through a group of forming rolls, the open pipes are pressurized by a squeezing roll and both edges are butted to each other, and the butting part is irradiated with a laser beam for butt welding to form a welded tube, the welding is performed by the irradiation of laser beam in the condition where the following formula I, II are satisfied. Furthermore, in this method, it is preferable that the welded zone after welding is heated to Ac3 transformation point or above, then is left standing for cooling is subjected to a post heat treatment being acceleratedly cooled and further a tempering treatment suceedingly to the accelerated cooling. V>=2...I. P>=0.4Vt/e<a(> T<-> T<0)> ...II. Wherein, a=0.0006, P: laser output (kW), V: welding speed (m/min), t: strip thickness (mm), T: preheating temperature ( deg.C) in both edges of strip, and T0: room temperature ( deg.C).

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 crude oil or natural gas containing hydrogen sulfide, and more specifically to X prescribed in API standard (American Petroleum Institute standard).
The present invention relates to a method for producing a welded steel pipe for a line pipe, which has excellent weld strength, hydrogen-induced cracking resistance, and sulfide stress cracking resistance suitable for high-strength line pipes of 42 grade or higher.

[0002]

2. Description of the Related Art When a method of manufacturing a line pipe, which occupies a large weight in the field of use of welded steel pipe, is classified according to the welding method, the outer diameter is usually 508 mm or more and the wall thickness is 6.35.
The submerged arc welding method (hereinafter referred to as SAW method) is applied to medium- and large-diameter thick-walled pipes of mm or more, and this large-diameter thick-walled pipe is formed into a tubular shape by the UOE press method. Because it is difficult to use the ERW method (hereinafter,
The ERW method) is applied.

However, the ERW method applied to the production of the small- and medium-sized thin-walled pipes has a high welding speed and thus can obtain a product with high efficiency, but has the following problems.

That is, in the ERW method, even if welding is performed in the atmosphere or in an inert gas shield, the shield is incomplete and the oxygen partial pressure is relatively high. Mixing between the welding surfaces is likely to cause many welding defects. In addition, when the high frequency input power is low, cold welding defects frequently occur due to insufficient melting, and conversely, when the high frequency input power is high, the instability phenomenon of molten steel due to strong electromagnetic force occurs and penetrator defects frequently occur. Also, it is extremely difficult to finely adjust the high-frequency input power to prevent the occurrence of penetrator defects.

When a welded steel pipe manufactured by such an ERW method is used as a line pipe, when the inner surface of the pipe is exposed to a wet H ₂ S environment, hydrogen generated by corrosion penetrates into the steel and welds. Hydrogen-induced cracking (hereinafter referred to as HIC) occurs on the abutting weld surface due to the accumulation of hydrogen gas in the defect portion, and this occurs in the thickness direction along the metal flow formed by the upset during the abutting welding. It may propagate, and is inferior in hydrogen-induced cracking resistance (hereinafter referred to as HIC resistance). Further, in the line pipe, since the hoop stress due to the internal pressure is applied perpendicularly to the metal flow, sulfide stress cracking (hereinafter referred to as SSC) is likely to occur in the welded portion, and sulfide stress cracking resistance (hereinafter referred to as resistance to sulfide stress cracking). SSC property) 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, and therefore the reliability of the welded steel pipe obtained is high. It is very high and almost no HIC or SSC is found in the weld.

However, as described above, the outer diameter to which the SAW method cannot be applied is 508 mm or less and the wall thickness is 6.
Since the only method for producing small- and medium-diameter thin-walled pipes of 35 mm or less is by the ERW method, it is possible to give the performance required as a line pipe to the welded portion while taking advantage of the ERW method that high-speed pipe production is possible. is necessary.

By the way, the causes of HIC and SSC are roughly classified into inclusions and hardened structure. However, the welding defects in the ERW method are mainly oxide inclusions, which is HIC.
Since it becomes the starting point of SSC and SSC, it is necessary to prevent the mixing of defect-inducing substances on the weld abutting surface and the generation of penetrator defects and cold welding defects.

For this reason, in Japanese Unexamined Patent Publication No. 3-810972, a reverse polarity consumable electrode wire is used to collide ions with the surface of the base material to remove and clean the surface oxide near both edges, and the upset amount is opened. A method for manufacturing a welded steel pipe has been proposed in which a defect-inducing substance is extruded from between welded surfaces to a thickness of ⅕ or more of the pipe wall thickness, and welding defects are reduced to almost zero to improve SSC resistance. . However, although this method can reduce welding defects, it has a drawback in that a metal flow is formed by the upset and SSC easily occurs along the metal flow.

Further, Japanese Patent Laid-Open No. 2-70379 discloses that
Following high frequency heating of both edges of the steel strip, the welding speed is E
By irradiating the welded portion with a laser beam that is 1/5 to 1/10 of that of the RW method, a method aimed at obtaining a welded steel pipe having a welded portion performance equivalent to arc welding such as the SAW method has been proposed. ing. However, in this method, since laser welding, which is fusion welding that is essentially different from the ERW method, is used together, the occurrence of weld defects can be eliminated, but the welding speed is at most as high as that of the laser-only welding pipe manufacturing method. Two
It has a drawback that it is only about double.

Further, in JP-A-5-228660, as shown in FIG. 1, a butt cross-sectional shape of both edges of the strip steel 1 immediately before welding is formed on the outer surface side of the open pipe with a predetermined width a and depth. The V-groove 2 having a thickness b is formed into a Y shape, and the bottom of the V-groove 2 is focused and irradiated with a laser beam to obtain a manufacturing size (external size) by the ERW method excellent in HIC resistance and SSC resistance. Diameter 19-610 mm,
A method of manufacturing a welded steel pipe for a line pipe having a wall thickness of 1 to 19 mm) by a laser welding method alone at a welding speed almost equal to that of the ERW method has been proposed. But with this method,
It is necessary to make the depth b of the V-groove 2 deeper in order to apply it to a thin-walled tube up to about 6 to 8 mm, and to apply it to a higher-speed tube or a thick-walled tube exceeding 8 to 13 mm. In this case, the welding progresses faster than the V groove 2 is filled with the molten metal. Therefore, as shown in FIG. 2, the undercut 6 and a bead such as an underbead whose surface is recessed from the surface of the base metal strip. Defects with poor shape frequently occur. When the welding speed is slowed to avoid the occurrence of this bead shape defect, the cooling rate of the molten metal portion after welding is reduced and the welded shape of the welded portion becomes a wine cup shape as shown in FIG. Not only does HIC resistance and SSC resistance deteriorate as the crystal grains in the weld become coarser to form columnar crystals in the thickness direction, but the weld heat affected zone (HAZ) becomes wider and HA
It has a drawback that the Z softening becomes remarkable and the strength decreases.

The above-mentioned wine cup-shaped melted structure is usually a locally heatable high frequency induction used for post heat treatment of the welded part after welding from the viewpoint of economy in manufacturing a welded steel pipe for a line pipe. Even if heat treatment is performed using a heating means, it is impossible to eliminate this. That is, in order to eliminate the wine cup-shaped melted structure generated in the welded portion by the induction heating means capable of local heating, it is necessary to perform wide heating corresponding to the wine cup-shaped melted width. Since the temperature of the central part is too high, the structure becomes coarse and the SSC resistance decreases,
This is because if the temperature control is performed to prevent this, the HAZ is insufficiently heated, the HAZ softening cannot be eliminated, and the strength decreases. The above problem can be solved by subjecting the entire tube to a post heat treatment, but it is difficult to adopt because the productivity is remarkably reduced and the economical efficiency is impaired.

[0013]

The object of the present invention has been made in view of the above-mentioned circumstances, and it is possible to obtain ER even in a thick tube having a manufacturing size of the above-mentioned ERW method and exceeding 6 to 8 mm.
The efficiency is almost equal to W, and the above-mentioned JP-A-5-228
It is possible to perform welding at a welding speed higher than that of the single laser welding method in which the V groove disclosed in Japanese Patent No. 660 is provided, the strength of the welded portion,
It is an object of the present invention to provide a method for producing a welded steel pipe for a line pipe using a laser welding method which is excellent in HIC resistance and SSC resistance.

[0014]

The gist of the present invention is a method for producing a welded steel pipe for a line pipe, which is excellent in hydrogen-induced cracking resistance and sulfide stress cracking resistance as described in the following items.

% By weight, C: 0.01 to 0.20
%, Si: 0.03 to 0.80%, Mn: 0.40
2.00%, P: 0.025% or less, S: 0.002%
Hereinafter, sol-Al: 0.01 to 0.10% is included, further Cu: 0 to 0.50%, Ni: 0 to 0.50%, C
r: 0 to 1.20%, Mo: 0 to 1.00%, Nb: 0
~ 0.15%, V: 0 ~ 0.15%, Ti: 0 ~ 0.1
5%, Zr: 0 to 0.15% and B: 0 to 0.005
One or more of 0%, and Ca: 0
A strip steel containing 0.0050% and REM: 0 to 0.01% of 1 or 2 and the balance of Fe and unavoidable impurities is continuously formed into an open pipe shape through a forming roll group, When this open pipe is pressed by a squeeze roll, both edges of the steel strip are butted against each other, and the butted portion is irradiated with a laser beam to be abutted and welded to form a welded steel pipe, the following formulas (1) and (2) are satisfied. A method for producing a welded steel pipe for a line pipe, which is excellent in hydrogen-induced cracking resistance and sulfide stress cracking resistance, characterized by irradiating a laser beam for welding under the conditions.

V ≧ 2 (1) P ≧ 0.4 Vt / e ^{a (T-T0)} (2) where a = 0.006 P: laser output (kW) ) V: Welding speed (m / min) t: Strip steel wall thickness (mm) T: Preheating temperature (° C) of both edges of the strip T0: Room temperature (° C) After welding, at least the weld point is Ac ₃ transformation point or higher The method for producing a welded steel pipe for a line pipe, which is excellent in hydrogen-induced cracking resistance and sulfide stress cracking resistance, according to claim 1, wherein the method is allowed to cool after heating.

After welding, after heating at least the welded portion to the Ac ₃ transformation point or higher, (Ar ₃ transformation point −30 ° C.) or higher, 1
The method for producing a welded steel pipe for a line pipe having excellent hydrogen-induced cracking resistance and sulfide stress cracking resistance according to claim 1, wherein accelerated cooling is performed from a temperature range of 000 ° C. or less.

Following the accelerated cooling, 500-750
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 3, wherein the tempering is performed in a temperature range of ° C.

In the above-mentioned methods of the present invention, Cu, Ni, Cr, Mo, Nb, V, Ti, Zr and B, and Ca and REM of the material strip steel may be added without addition. When these are positively added, 0.05% or more for Cu, Ni, Cr and Mo, 0.01% or more for Nb, V, Ti and Zr, B, Ca and REM.
It is desirable to contain 0.0005% or more.

As a result of various experimental studies, the present inventors have found that
The present invention was made based on the findings of (a) to (d).

(A) When the laser welding method is used, if the welding speed is 2 m / min or more, that is, if the above expression (1) is satisfied, the V groove adopted by the method disclosed in the above-mentioned JP-A-5-228660 is adopted. Irrespective of the presence / absence of the laser beam, the laser output, the material thickness, etc., it is possible to prevent the melted shape of the welded portion from becoming a wine cup shape and coarsening of the crystal grains of the welded portion to form columnar crystals in the thickness direction.

(B) In the single laser welding method, the V groove is provided as disclosed in Japanese Unexamined Patent Publication No. 5-228660, so that a through bead in the thickness direction can be formed even with a low power laser beam and high speed welding is possible. This is to enable 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
The preheating effect is almost the upper limit temperature because the energy density is lower than that of the laser beam.
Even if preheating is performed, there is at most a double effect in terms of laser output ratio compared to the case without preheating, that is, there is only the effect of the denominator in the right term of the above equation (2). Insufficient, high-power laser oscillator, for example, the conventional 5kW was 25kW
It is necessary to use a high-power laser oscillator.

(C) 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 strip. When the preheating temperature of both edges of the steel is T (° C.) and the welding is performed while satisfying the relationship of the above formula (2) according to the strip steel wall thickness t, a defect-free weld without oxides or the like is obtained. thing.

(D) After the welding, when the post-heat treatment is applied to the weld using the high frequency induction heating means capable of local heating, the weld is once heated to the Ac ₃ transformation point or higher and then allowed to cool (Ar ₃ When accelerated cooling is carried out from the temperature range of (transformation point −30 ° C.) to 1000 ° C., the structure of the weld metal part does not become a coarse-grained structure, and further the accelerated cooling is followed by tempering in the temperature range of 500 to 700 ° C. to increase the hardness. When lowered, HIS resistance and S resistance of welded parts
SC will be further improved.

[0025]

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

In order to obtain a welded steel pipe for line pipes having excellent HIC resistance and SSC resistance, it is naturally necessary to use a material strip steel (hot rolled steel sheet) having excellent HIC resistance and SSC resistance. There is. After pipe production, at least the weld seam, preferably the entire pipe, is subjected to a prescribed heat treatment to obtain HIC resistance and S resistance irrespective of the history during the production of the material strip steel.
It is preferable to obtain the base material performance excellent in SC property. For this reason, the composition of the strip steel (hot rolled steel sheet) used as a material is specified, but the content of each component is limited to a specific range for the following reasons.

C: 0.01 to 0.20% C has an effect of imparting a predetermined strength (strength of X42 grade or higher) to the steel pipe, but if the content is less than 0.01%, the above-mentioned strength is increased. Guarantee becomes difficult, while its content is 0.
If it exceeds 20%, toughness is deteriorated, so the C content is set to 0.01 to 0.20%.

However, in the case where the post-heat treatment is not applied to the weld seam after welding, if the content exceeds 0.12%, hardening of the weld will be caused and both the HIC resistance and the SSC resistance will decrease. , And the C content in this case is 0.01-0.
12% is preferable.

When an X70 to X80 grade high-strength steel pipe is to be obtained, its content is preferably 0.01 to 0.07% regardless of the presence or absence of post heat treatment of the weld seam. .

Si: 0.03 to 0.80% Si is required to secure a content of 0.03% or more for deoxidation of steel. On the other hand, when the content exceeds 0.80%. Since it causes toughness deterioration and temper embrittlement, Si
The content was set to 0.03 to 0.80%. Preferably,
It is 0.05 to 0.30%.

Mn: 0.40 to 2.00% Mn has a function of ensuring a predetermined strength (strength of X42 grade or higher) in the steel pipe, but if the content thereof is less than 0.40%, the desired strength is obtained. However, the content of Mn exceeds 2.00%, the SSC resistance is deteriorated.
The content was set to 0.40 to 2.00%.

However, in order to obtain a high-strength steel pipe of less than X70 grade, if the content exceeds 1.80%, the alloy element concentration in the base metal segregated portion becomes high, and particularly the C content is less than 0.
Since the HIC resistance and the SSC resistance of the base material both deteriorate with a base material of 08% or more, the Mn content in this case is 0.40.
It is desirable to be set to 1.80%. Further, when the post-heat treatment is not applied to the weld seam portion after welding, if the content exceeds 1.40%, hardening of the weld portion is caused and both the HIC resistance and the SSC resistance decrease, so in this case The Mn content of is preferably 0.40 to 1.40%. Further, when an X70 to X80 grade high-strength steel pipe is to be obtained, the content thereof is preferably 0.40 to 2.00% regardless of the presence or absence of post heat treatment of the weld seam.

P: 0.025% or less P is an unavoidable impurity, and its content is preferably low. In particular, when P is contained in excess of 0.025%, the concentration of alloying elements in the base metal segregated portion becomes high, the SSC resistance of the base material is significantly reduced, and adverse effects also appear in terms of temper embrittlement. Therefore, the P content is determined to be 0.025% or less. It is preferably 0.015% or less.

S: 0.002% or less S is an unavoidable impurity, and its content is preferably low. In particular, when S is contained in excess of 0.002%, Ca
Alternatively, REM (rare earth elements) also produces sulfide-based inclusions (MnS) whose morphology cannot be controlled, and is resistant to HI.
Since the deterioration of the C property and SSC resistance becomes remarkable, S
The content was set to 0.002% or less. Preferably, 0.
It is 001% or less.

Sol-Al: 0.01 to 0.10% Al is 0.01 in sol-Al content for deoxidizing steel.
% Must be secured, but the content is 0.10
%, It becomes difficult to secure the cleanliness of steel.
The ol-Al content was set to 0.01 to 0.10%. Preferably, it is 0.02 to 0.05%.

The material strip steel of the present invention, in addition to the above components,
One or two of the following Ca and REM (rare earth elements)
Seeds, Cu, Ni, Cr, Mo, Nb, V, Ti,
It may be made of steel containing one or more of Zr and B.

Ca and REM (rare earth elements): The upper limits are 0.0050% and 0.01%, respectively. These components exert an action of improving SSC resistance through controlling the morphology of sulfide inclusions. If desired, one or two of Ca and REM can be contained. However, if the content of any element is less than 0.0005%, the desired effect due to the above action can be obtained. In the case of Ca, on the other hand, if the content of Ca exceeds 0.005%, HIC resistance and SSC resistance are deteriorated due to the increase of Ca-based inclusions. In some cases, if the content exceeds 0.01%, HIC resistance and SSC resistance are increased due to an increase in oxide inclusions.
If it is contained, the Ca content is 0.0005 to 0.0050%, and the REM content is 0.
It was defined as 0005 to 0.01%.

Cu, Ni, Cr, Mo, Nb, V, T
i, Zr, and B All of these components have the effect of improving the strength and toughness of the steel pipe. Therefore, if these effects are desired,
Cu, Ni, Cr, Mo, Nb, V, T as required
One or more of i, Zr, and B can be selected and contained. However, the content of each component is defined as follows for the following reasons.

Cu, Ni: The upper limits are both 0.50%. If the content of any element is less than 0.05%, the effect of improving strength and toughness is insufficient, while 0.5%
If the content exceeds 0%, the hot workability is deteriorated and it becomes difficult to manufacture the hot-rolled coil used as the raw material. Therefore, the content of each element is 0.05 to 0.50%. I decided.

Cr: The upper limit is 1.20%. If the Cr content is less than 0.05%, the effect of improving the strength and toughness is insufficient, while if it is more than 1.20%, the toughness is improved. Since it causes deterioration in the SSC resistance and the SSC resistance, the content in the case of containing it is set to 0.05 to 1.20%.

Mo: The upper limit is 1.00%. If the Mo content is less than 0.05%, the effect of improving the strength and toughness is insufficient, while if it exceeds 1.00%, the toughness is increased. And SSC resistance are deteriorated.
When it is contained, the content is defined as 0.05 to 1.00%.

Nb, V, Ti and Zr: The upper limits are all 0.15%. If the content of any element is less than 0.01%, the effect of improving strength and toughness is insufficient. , 0.1
If it is contained in excess of 5%, the toughness will be deteriorated. Therefore, the content of each element is 0.01 to 0.
It was set at 15%.

B: The upper limit is 0.0050%, and if the B content is less than 0.0005%, the effect of improving strength and toughness is insufficient, while if it exceeds 0.0050%. Since the toughness is deteriorated, the content in the case of containing it is set to 0.0005 to 0.0050%.

The material strip steel having the above composition is preferably produced under the conditions described below, but the invention is not limited thereto.

[Slab heating temperature] The slab heating temperature is preferably in a temperature range that enables the following hot rolling.

[Hot Rolling Finishing Temperature] HIC resistance and SS resistance
In order to improve C property, it is preferable to finish hot rolling at the Ar ₃ transformation point or higher and secure the following accelerated cooling start temperature. If possible, the hot rolling finishing temperature is (Ar ₃ transformation point + 30 ° C). It is better to be above the temperature.

[Accelerated Cooling Start Temperature] When the accelerated cooling start temperature after hot rolling is low, C is formed in the segregated portion as the pro-eutectoid ferrite grows.
Is concentrated and a hardened structure is generated at the time of accelerated cooling, resulting in deterioration of HIC resistance and SSC resistance. Also, X7
It becomes difficult to obtain high strength of 0 to X80 grade. Therefore, the accelerated cooling start temperature is preferably set to a temperature higher than (Ar ₃ transformation point −30 ° C.), and if possible, Ar ₃ without proeutectoid ferrite.
A transformation point or higher is desirable.

[Average cooling rate during accelerated cooling] When the average cooling rate during accelerated cooling is slow, two-phase separation of ferrite / pearlite progresses, and HIC resistance and SSC resistance at the center segregation portion occur.
A band-like structure with poor properties is formed. Also, X70 to X
It becomes difficult to obtain high strength of 80 grade. Therefore, the average cooling rate is preferably 5 ° C./s or more. However, if the value exceeds 20 ° C./s, hardened block bainite is likely to be formed, which is not preferable. In the case of the high-strength hot-rolled steel sheets of X70 to X80 grade, it is 30 ° C / s.

[Accelerated Cooling Stop Temperature] When accelerated cooling is stopped in a temperature range exceeding 600 ° C., untransformed austenite remains at the time of stopping, so that pearlite is generated thereafter, and the central segregation portion is hardened by the concentration of C and hardened. The HIC property and SSC resistance tend to decrease. Also, X70
~ It becomes difficult to obtain high strength of X80 grade. On the other hand, 4
If accelerated cooling is stopped in the temperature range below 00 ° C, hardened block-shaped bainite is likely to be formed, and the H-resistant
IC property and SSC resistance decrease. Therefore, the accelerated cooling stop temperature is preferably 600 to 400 ° C. X
In the case of 70-X80 grade hot rolled steel sheet for high strength, 600-2
00 ° C is desirable.

[Winding temperature] Winding may be carried out after the accelerated cooling is stopped.

In the present invention, first, the raw steel strip having the above component composition and produced under the above conditions is continuously roll-formed through a forming roll group in the usual manner to form an open pipe shape, The abutting welding is performed by vertically irradiating the laser beam to the joint point where both edges of the steel strip contact each other by the action of a pair of right and left squeeze rolls provided at the end of the group, that is, the abutting portion of the open pipe, At this time, the laser output P (kW),
When welding speed V (m / min), strip steel wall thickness t (mm), preheating temperature T (° C) of both edges of the strip steel, and room temperature T 0 (° C), the following equations (1) and (2) are used. Butt welding is performed under the condition that satisfies

V ≧ 2 ··· (1) P ≧ 0.4Vt / e ^{a (T-T0)} ··· (2) However, a = 0.0006, that is, the above (1) 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. And the welding speed V is 2 m / m regardless of the strip steel wall thickness (strip steel wall thickness) t, etc.
If it is less than in, the melting shape of the molten metal portion due to the decrease in the cooling rate after welding becomes a wine cup shape and the crystal grains of the welding portion become coarse and become columnar crystals in the thickness direction,
Not only the toughness, HIC resistance, and SSC resistance of the welded part deteriorate, but also the HAZ becomes wider, so that HAZ softening becomes remarkable and the strength of the pipe also decreases, but the welding speed V is 2 m / mi.
When pipe manufacturing welding is performed with n or more, the welded part does not have a wine cup shape, and the structure of the welded part can be prevented from becoming a columnar crystal structure in the thickness direction in which the crystal grains become coarse. As a result, it is possible to prevent the HIC resistance and the like from being deteriorated due to the coarse columnar crystal structure of the crystal grains of the welded portion. This is also clear from the results of Examples described later.

The above equation (2) shows the conditions for performing defect-free welding in which no oxides or the like are 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 becomes equal to or higher than the value obtained by the equation (2) and welding, the HIC resistance due to the presence of foreign matters such as oxides in the welded portion. And deterioration of SSC resistance can be prevented. That is, when the laser output P is less than the value obtained on the right side of the equation (2), the welding speed is 2 m / min.
Even if the structure of the welded portion is not the columnar crystal structure in the thickness direction in which the crystal grains are coarsened as described above, welding defects due to foreign matters such as oxides occur in the welded portion and HIC resistance and SSC resistance Is meant to decrease.

FIG. 4 is a view showing 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 the SSC generation. It is the figure which showed the difference of the value calculated | required by "0.4Vt / ea ^(T-T0) " on the right-hand side in a formula, and the laser output P by taking the SSC generation rate on the vertical axis.

As is apparent from FIG. 4, the laser output P
Is less than the value obtained by "0.4Vt / ea ^(T-T0) ", SSC occurs and the laser output P is "0.4Vt.
/ E ^{a (T−T0)} ”or more, the SSC does not occur, which indicates that it is necessary to satisfy the above equation (2).

In order to satisfy the above equations (1) and (2) at the same time, as described above, the low output laser oscillator of about 5 kW which is generally used in the past is not sufficient. It is necessary to use a high-power laser oscillator of 25 kW or more.

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 JP-A-5-228660 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, when the strip steel 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, the width a and the depth b are It is desirable to form and weld a V groove satisfying the following expressions (3) to (6).

A / b> D / f (3) a × b ≦ 2 × (P / V) (4) t−b ≦ 4 × (P / V) 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

Expression (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 strip continuously existing at the bottom of the V groove are completely abutted with 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".

Expression (6): Condition for preventing undercut 6 from being generated 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”.

The welded steel pipe in which the butt portion of the open pipe is thus welded exhibits sufficient performance as it is, but in order to further improve the performance, the next heat treatment is performed on the welded seam portion after welding. Is desirable.

[Heating Temperature of Weld Seam] It is desirable to heat the weld seam to a temperature range not lower than the A _C3 transformation point by destroying the coarse grain structure of the weld as austenite single phase at the time of heating to obtain fine grain. This is to obtain a tissue, but if the heating temperature is lower than this temperature, the effect cannot be obtained. The upper limit is not particularly specified, but when the temperature exceeds 1100 ° C., the crystal grains become coarse again, and particularly in a material having a C content of more than 0.12%, HIC resistance, SSC resistance and toughness are adversely affected. Therefore, it is desirable to set the temperature to 1100 ° C. or lower.

[Cooling after heating] After heating to the above temperature range,
If it is left to cool (air cool) as it is, the hardness of the welded portion is suppressed to a low level, and the HIC resistance and SSC resistance are improved.

However, if only the weld seam portion is heated to the above temperature range and allowed to cool, the strength of the welded portion may be lower than that of the base metal portion. In that case, accelerated cooling is performed immediately after heating. Then, since it is possible to prevent the strength of the welded portion from decreasing, it is desirable to carry out accelerated cooling if it is desired to prevent the strength from decreasing. However, if the accelerated cooling start temperature is lower than the temperature of (Ar ₃ transformation point −30 ° C.), the retained austenite in which C is concentrated is hardened during the accelerated cooling as the pro-eutectoid ferrite grows, and HIC resistance and SSC resistance are increased. Will lead to a decrease in. On the other hand, when the accelerated cooling start temperature exceeds 1000 ° C., a martensite structure or bainite structure with high hardness is generated, which is not desirable for HIC resistance and SSC resistance. Therefore, the cooling start temperature in the case of performing accelerated cooling is preferably (Ar ₃ transformation point −30 ° C.) to 1000 ° C. In addition, the cooling rate in the case of performing accelerated cooling is the same as the condition at the time of manufacturing the base material strip steel (hot rolled steel sheet), which is the base material.
It is desirable to set it to -30 ° C / s.

[Tempering After Accelerated Cooling] When the tempering process is performed after the above accelerated cooling to soften the welded portion, the HIC resistance and toughness of the welded portion are further improved. If further improvement is desired, it is desirable to perform tempering treatment after accelerated cooling. However, if the tempering temperature is lower than 500 ° C, the material is not softened, so that the tempering effect cannot be obtained. On the other hand, if the tempering temperature exceeds 750 ° C, some austenite transformation occurs and the predetermined strength cannot be obtained, and the residual strength cannot be obtained. Since austenite and an untempered martensite phase are generated, which is not desirable for HIC resistance and SSC resistance, the tempering temperature is 50.
The temperature is preferably 0 to 750 ° C.

[0068]

[Example] SSC resistance and HI resistance as a molten base material for pipe manufacturing
Nine types of raw material steel strips (hot rolled steel sheets) having chemical compositions shown in Table 1 of X42 to X80 grade having excellent C property were prepared. API grade, strength, HIC resistance and SSC resistance of these base materials
The results of the sex investigation are also shown in Table 1. The H resistance of the base material
IC property is NACE (American Corrosion Association standard) -TM-02
Based on the method specified in -84, 96 hours in NACE bath (0.5% acetic acid, 5% saline solution, 25 ° C, 1 atm H ₂ S saturated).
As a result of evaluation by a crack length ratio (CLR) when immersed, SSC resistance is NACE-TM-01-77-METHO.
Σth (minimum stress that causes fracture by SSC) and σys when evaluated by the uniaxial tensile test method specified in DA
The evaluation results are based on the ratio of (yield stress). For all the base materials, one criterion for HIC resistance is CLR ≦ 15% under NACE conditions and one criterion for SSC resistance. It satisfies the NACE condition of σth / σys ≧ 72%.

[0069]

[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 with 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 with helium gas having a high removal effect, welding was performed under the conditions shown in Tables 2, 3, and 4. As a laser source, the beam diameter D before condensing is 51 mm, a mirror (parabolic mirror)
A carbon dioxide gas laser oscillator with a focal length f of 381 mm and an output of 25 kW and a carbon dioxide gas laser oscillator with an output of 5 kW and a beam diameter D before focusing of 30 mm and a mirror (parabolic mirror) focal length f of 150 mm were used. The focal position is
All of them were set on the outer surface of the open pipe of the butted portion, but some of the butted portions having V grooves were set at the bottom of the V groove. For comparison, an as-welded steel pipe welded by the conventional ERW method using a strip steel was also prepared.

[0071]

[Table 2]

[0072]

[Table 3]

[0073]

[Table 4]

The resistance of the as-welded welded part of the obtained welded steel pipe (for some, the welded part after post-heat treatment of the weld seam under the conditions shown in Table 2, Table 3 and Table 4) H
The IC property and SSC resistance were evaluated by the methods described below.

After incising in the pipe axial direction so that the weld seam portion is located at the center and deploying in the circumferential direction, from each position shown in FIG. 6, the HIC test piece has the shape and size shown in FIG. 3 pieces from different positions in the pipe axis length direction, and for the SSC test piece, a test piece of either shape (X) or (Y) shown in FIG. 8 according to the thickness of the material strip steel is used. Three pieces were cut out from different positions in the tube axis length direction and collected. The SSC test piece lacked the threaded portion due to the thickness of the material, so a Teflon tape or the like was wrapped around the threaded portion during the test and sealed before use. In addition, prior to the test, a room temperature tensile test was performed on each SSC test piece of each trial number to determine the yield stress (YS) and tensile strength (TS). For the test number with a decreased value, both the HIC test and the SSC test were omitted.

The HIC test was conducted by using the above-mentioned test pieces (each trial number 3 for each).
NACE bath (0.5% acetic acid, 5% saline solution, 25 sheets)
After dipping for 96 hours in (atmospheric pressure, H ₂ S saturated at 1 atmosphere), the crack generation area of the joint surface was detected by ultrasonic flaw detection for each test piece as shown in FIG. 9, and the average crack area ratio (CAR) was calculated. HIC susceptibility was evaluated. It should be noted that a triple value of the CAR value corresponds to the CLR value, and CRA ≦ 5% indicates that the HIC resistance is excellent.

In the SSC test, each of the above test pieces (two for each test number) was subjected to NAC using the SSC test apparatus shown in FIG.
E bath (0.5% acetic acid, 5% saline, 25 ° C, 1 atm H ₂
72% of SMYS (standard minimum yield stress) in S saturation)
Was applied and the presence or absence of breakage was examined during the test period of 720 hours.

The results of these tests are also shown in Tables 2, 3 and 4. In Table 2, Table 3 and Table 4, trial numbers 1, 18 and 35 are conventional examples by the ERW method, trial number 2,
3, 4, 19 to 21, 43, and 44 are conventional examples of laser-only welding in which a V groove is formed, and others are examples of the present invention and comparative examples.

As is clear from Test Nos. 1, 18 and 35, the conventional ERW method is inferior in both HIC resistance and SSC resistance of the welded portion.

Also, trial numbers 4, 8, 12, 13, 21, 2
As is clear from 5, 30, 34 and 40, when the welding speed is less than 2 m / min, the thickness of the material strip steel, the size of the laser output, the presence or absence of V-groove formation and the edges of both strips of the steel strip. In all cases, regardless of whether preheating is performed, the welded shape of the weld becomes a wine cup shape, and the strength of the weld is significantly reduced, or even if there is no strength reduction, the SSC resistance is inferior. It can be seen that the expression (1) is a necessary condition for preventing the strength of the welded portion from decreasing and the occurrence of SSC.

For I steel (X80 grade) having a wall thickness of 11.1 mm, if the laser output is 5 kW, trial number 2 and trial number 3
As is clear from the comparison of the above, the maximum welding speed at which a product having a V-groove-formed conventional weld with good HIC resistance and SSC resistance is 2.2 m / min at most. Also, as is clear from trial number 4, when the welding speed is as low as 1.8 m / min even if the V groove is formed, the welded shape of the weld becomes a wine cup shape and the strength of the welded part is the base metal. Is significantly inferior to.

Further, when the V groove is not formed, trial number 5 is used.
As is clear from the above, even if the welding speed is 2 m / min, if both edges of the steel strip are not preheated, the melting will be insufficient and welding will be impossible. However, as is clear from trial Nos. 6 and 7, it is possible to weld at a welding speed of 2 m / min or more without forming V-grooves when both edges of the steel strip are preheated at a high temperature. So that 0.4 Vt / e
^{Since the} value of ^{a (T-T0)} is out of the range defined by the formula (2) defined in the present invention, welding defects occur and the SSC resistance is poor.

On the other hand, in the case of the laser output of 25 kW, as is clear from the comparison of trial Nos. 9 to 13, even when the both edges of the strip steel are not preheated and the V groove is not formed, The maximum welding speed that can obtain a product with good SSC resistance is 5 m / min, and the laser output with V-groove is 5
Welding is possible at a welding speed that is more than twice that of the kW type (Trial No. 3). Further, as is clear from the comparison between trial No. 14 and trial No. 15, when both edges of the steel strip were preheated to a high temperature of 1250 ° C., the maximum welding speed at which the product with good SSC resistance was obtained was 10 m / min. Has gone up. Further, as is clear from the comparison between trial No. 16 and trial No. 17, when the high-power laser with an output of 25 kW and V-groove formation are combined, the same dimensions can be obtained without preheating both edges of the steel strip. 5k laser output with V groove
A product with good SSC resistance is obtained even at a welding speed as high as 8 m / min, which is about four times that of the W type (Trial No. 3).

Further, E steel (X65 with a thickness of 7.1 mm
Grade), the wall thickness is smaller than that of the I steel, so when the laser output is 5 kW, the maximum welding speed at which a good product such as SSC resistance can be obtained is without preheating both edges of the strip steel. V
4 m / min when the groove is formed (see trial numbers 19-21)
Is 3 m / min when both edges of the steel strip are preheated to 1200 ° C. without forming V-grooves (see trial numbers 22 to 25), but good SSC resistance and the like when the laser output is 25 kW. The maximum welding speed that can be obtained with various products is 12 when both edges of the steel strip are preheated to 900 ° C without forming V-grooves.
When it is preheated to 1200 ° C., the speed is increased to 16 m / min (see trial numbers 26 to 30).

Furthermore, the B steel (X52
Class), when the laser output is 5 kW, when both edges of the steel strip are preheated to a high temperature of 1250 ° C. without forming V-grooves, a product with good SSC resistance at a welding speed of 3 m / min is Although not obtained (see Trial No. 36), the welding speed was 12 m / mi under the same conditions when the laser output was 25 kW.
Even with n, products with good SSC resistance and the like have been obtained (see trial numbers 37 to 40).

Furthermore, in the thickest F steel (X70 grade) having a wall thickness of 12.7 mm, when the laser output is 5 kW, V-grooves are formed and both edges of the strip steel are 1100 ° C.
When it was preheated, the product with good SSC resistance was only obtained at the welding speed of 2 m / min (see trial Nos. 43 and 44), but when the laser output was 25 kW, both edges of the strip steel were Even if pre-heated to the same temperature without forming V-grooves, triple welding speed 6m / min, strength, HIC resistance, S resistance
Products with excellent SC properties have been obtained (Trial No. 4
5).

The above is also clear from the present invention examples of trial numbers 44 to 48 for A, C, D, G and H steels. Further, the HIC resistance and SSC resistance of the welded portion were good even when the post-heat treatment was not applied to the welded seam portion (see trial numbers 49 to 53).

The average welding speed in the ERW method is
15 mm / min with 7.1 mm thick E steel, thickness 11.1
It is 10 m / min for A steel of mm, the welding speed when the high power laser of the present invention example is used is equivalent to the welding speed in this ERW method, and when using the conventional low power laser, V
It can be seen that this is about twice as much as the method of forming a groove and welding.

[0089]

EFFECT OF THE INVENTION According to the method of the present invention, for a line pipe having a manufacturing size according to the ERW method, which sufficiently satisfies all the performances of the weld strength, HIC resistance and SSC resistance in a wet H ₂ S environment. It is possible to manufacture welded steel pipes with high efficiency and stability, and it has an extremely useful effect in industry.

[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 a bead shape that occurs when the welding speed is low.

FIG. 4 is a diagram showing the influence of correlation (Equation 2) of plate thickness, output, welding speed, and preheating temperature on SSC generation.

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

FIG. 6 is a conceptual diagram showing a collection site of a test piece for HIC and SSC investigation.

FIG. 7 is a diagram showing the shape and dimensions of a HIC test piece.

FIG. 8 is a diagram showing the shape and dimensions of an SSC test piece.

FIG. 9 is a conceptual diagram for explaining how to evaluate HIC by an ultrasonic flaw detection method.

FIG. 10 is a conceptual diagram of an SSC test apparatus.

[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 indication C22C 38/04

Claims (4)

[Claims] 1. By weight%, C: 0.01 to 0.20%, S
i: 0.03 to 0.80%, Mn: 0.40 to 2.00
%, P: 0.025% or less, S: 0.002% or less, s
ol-Al: 0.01 to 0.10% inclusive, and further C
u: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0
˜1.20%, Mo: 0 to 1.00%, Nb: 0 to 0.
15%, V: 0 to 0.15%, Ti: 0 to 0.15%,
One or more of Zr: 0 to 0.15% and B: 0 to 0.0050%, and Ca: 0 to 0.00
50% and REM: 0 to 0.01% of one or two kinds, and the balance consisting of Fe and inevitable impurities is continuously formed into an open pipe shape through a forming roll group, and this open When pressing the pipe with a squeeze roll, butting both edges of the steel strip together, irradiating the butted portion with a laser beam and performing abutting welding to form a welded steel pipe, under the conditions that satisfy the following formulas (1) and (2). A method for producing a welded steel pipe for a line pipe, which is excellent in hydrogen-induced cracking resistance and sulfide stress cracking resistance, characterized by irradiating a laser beam for welding. V ≧ 2 (1) P ≧ 0.4 Vt / e ^{a (T-T0)} (2) where a = 0.006 P: laser output (kW) V: Welding speed (m / min) t: Wall thickness of steel strip (mm) T: Preheating temperature of both edges of steel strip (° C) T0: Room temperature (° C) 2. The method according to claim 1, wherein after welding,
A method for producing a welded steel pipe for a line pipe, which is excellent in hydrogen-induced cracking resistance and sulfide stress cracking resistance, characterized in that at least a welded portion is heated to an Ac ₃ transformation point or higher and then allowed to cool. 3. The method according to claim 1, wherein after welding,
After heating at least the weld to the Ac ₃ transformation point or higher, (A
A method for producing a welded steel pipe for a line pipe, which is excellent in hydrogen-induced cracking resistance and sulfide stress cracking resistance, characterized by accelerated cooling from a temperature range of r ₃ transformation point −30 ° C.) to 1000 ° C. inclusive. 4. The line pipe excellent in hydrogen-induced cracking resistance and sulfide stress cracking resistance, which is characterized in that, in the method according to claim 3, it is tempered in a temperature range of 500 to 750 ° C. following accelerated cooling. For manufacturing welded steel pipes for automobiles.