JP2001115238A - Martensitic stainless steel welded steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thick martensitic stainless steel welded steel pipe of a large diameter excellent in corrosion resistance, particularly, in the stress corrosion cracking resistance of the weld zone. SOLUTION: In this welded steel pipe 1, the size of the projection of a welding bead 3 in the seam zone at the inside face is limited small based on the conditional formula in consideration of the width W and height H of the bead and the proof stress of the base material and weld metal. Therefore, the welded steel pipe is excellent in corrosion resistance, particularly, in stress corrosion cracking resistance(SCC resistance) in the base material part and the seam zone at the inside face of the pipe. By the selection of the chemical composition, its corrosion resistance such as sulfide stress cracking resistance (sour resistance) and gaseous carbon dioxide corrosion resistance can moreover be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welded steel pipe made of martensitic stainless steel having excellent corrosion resistance, especially excellent resistance to stress corrosion cracking. More specifically, welded steel pipes used in pipelines, particularly trunk lines, for transporting fluids that readily corrode metals such as oil and natural gas, with an outer diameter exceeding 20 inches and a wall thickness exceeding 0.5 inches Such large diameter and thick wall martensitic stainless steel welded steel pipes.

[0002]

2. Description of the Related Art Pipelines, especially trunk lines, for transporting fluids that easily corrode metals such as oil and natural gas have an outer diameter exceeding 20 inches and a wall thickness exceeding 0.5 inches. Large diameter and thick steel pipes are widely used.

[0003] Such a large-diameter thick-walled steel pipe is usually manufactured by bending a thick steel plate or a hot-rolled steel strip into an open pipe shape or a spiral shape, and then welding a butt portion. The steel pipe manufactured by such a method is called a UO steel pipe, a spiral steel pipe, or the like.

[0004] UO steel pipe is manufactured by a forming process represented by alphabetical letters (U and O) added to its name. In this step, a method is employed in which a thick steel plate is bent into an open pipe shape by using a U-press, then the end surfaces are butted by an O-press into a pipe shape, and the butted portion is welded.

A spiral steel pipe is manufactured by sequentially bending a hot-rolled steel strip in a spiral shape, forming the pipe shape, and welding a butt portion between the end faces of the steel strip.

[0006] As a method of manufacturing a large-diameter thick-walled steel pipe, there is a manufacturing method other than the above. For example, there is a method in which a thick steel plate is bent into a pipe shape using a three-roll type roll forming machine called a roll bender, and then the butted portions of the end surfaces of the thick steel plate are seam-welded.

[0007] A submerged arc welding method (hereinafter, referred to as SAW method) is widely used for producing these large-diameter and thick-walled welded steel pipes. When welding a large-diameter thick-walled steel pipe, one layer is usually welded from the inner surface side and the outer surface side of the pipe. In addition, when the thickness of the material is large, multi-layer welding of three or more layers in which two or more layers are welded on one side may be performed.

Conventionally, large-diameter, thick-walled welded steel pipes used for transporting fluids that easily corrode metals such as petroleum and natural gas have been produced using carbon steel or low alloy steel containing at most about 1% by mass of Cr. Base steel sheet and welding materials were used. Large-diameter, thick-wall welded steel pipes made of carbon steel having poor corrosion resistance are used because carbon steel is more economically advantageous. However, carbon steel has poor corrosion resistance. Therefore, in a pipeline composed of carbon steel welded steel pipes, before transportation,
Dehydration of crude oil and natural gas collected from oil wells,
Operations have been taken to reduce the corrosive effects of the fluid.

[0009] However, the construction cost of the dewatering treatment equipment and the platform for installing the equipment is high. For this reason, a method of omitting dehydration treatment equipment and using a material having good corrosion resistance for a steel pipe for a pipeline has begun to be adopted. Stainless steel is considered as a material having excellent corrosion resistance.

This method is extremely advantageous for the development of small-scale oil fields and gas fields, such as horizontal oil wells, which conventionally could not be drilled, because dewatering equipment and a platform therefor are not required at the mining site. Specifically, there is an advantage that the crude oil and the like can be transported to an existing platform by a pipeline, and can be collectively and efficiently dewatered there.

In a high latitude region where latitudes of 70 ° or more north are to be studied in the future, such as the North Sea Oil Field, it is difficult to construct the platform itself due to the waves on the sea. In such a case, it is necessary to transport crude oil and the like that is not subjected to dehydration treatment by pipeline.

[0012] From such a background, a large-diameter thick-walled welded steel pipe for transporting a fluid which easily corrodes metal, which has been omitted from the dehydration treatment, has been required.

Some proposals have been made for stainless steel having good corrosion resistance suitable for transporting fluids as described above, a seamless steel pipe made of the steel, or an electric resistance welded steel pipe or a laser welded steel pipe having a relatively small diameter and a thin wall. I have. As for a large-diameter and thick-walled welded steel pipe made of stainless steel, for example, JP-A-10-60599 and JP-A-12-8144 disclose a welded steel pipe and a base material thereof.

For the above-mentioned applications, martensitic stainless steel containing 9 to 13% by mass of Cr is mainly used from the viewpoint of economy. Martensitic stainless steel is economical, has sufficient corrosion resistance under the above-mentioned environment, and has excellent hot workability, so it manufactures thick steel plates and hot rolled steel plates, which are materials for welded steel pipes. Because it is easy to do. These stainless steel base metals and seam weld metals were all considered to be excellent in stress corrosion cracking resistance, carbon dioxide gas corrosion resistance, and sulfide stress cracking resistance.

However, when a welded steel pipe made of martensitic stainless steel is applied to a pipeline for transporting a fluid having a corrosive effect, for which dehydration treatment has been omitted, stress corrosion cracking occurs at the welded portion on the inner surface of the pipe. Has been found to easily occur. In particular, in a welded steel pipe manufactured by the SAW method, there is a tendency for SCC to occur in a large-diameter thick-walled welded steel pipe in which the weld beads on the inner and outer surfaces of the pipe are not removed by cutting. Furthermore, even if it has SCC resistance,
It has been found that the base metal and the weld metal have poor sour resistance, and the weld metal may also have poor hot cracking resistance.

[0016]

DISCLOSURE OF THE INVENTION The present invention has excellent corrosion resistance, particularly stress corrosion cracking resistance (hereinafter, referred to as SCC resistance) of a base material portion and a seam portion of a pipe inner surface, and further has sulfide stress cracking resistance (hereinafter referred to as SCC resistance). It is an object of the present invention to provide a large-diameter and thick-walled martensitic stainless steel welded steel pipe having excellent sour resistance and carbon dioxide corrosion resistance (hereinafter referred to as CO ₂ resistance).

[0017]

The gist of the present invention is that the mass%
And C: 0.05% or less, Cr: 9 to 20%, and a base material of a stainless steel mainly composed of a martensite phase containing a single phase of martensite or a ferrite phase, and a mass% of C : 0.1% or less, Cr: including 7 to 20%,
The metal structure is composed of a weld metal of a seam portion of stainless steel mainly composed of a martensite phase including an austenite phase,
Further, the weld bead of the seam portion on the inner surface of the pipe is a martensitic stainless steel welded steel pipe satisfying the following expression (1).

L ≦ 0.2 × W (1) where L is a weld bead at the seam portion, and
Bead length h = {1+ (2 × H / W)} × (YS _B100 / YS _W100 ) (2) where H is the distance from the inner surface of the bead. Height (mm) W: Width of bead (mm) YS _B100 : _Yield strength of base metal at 100 ° C (MPa) YS _W100 : _Yield strength of weld metal at 100 ° C (MPa) In the base material, the martensite phase is 55 to 90%, and the ferrite phase is 10 to 4% by volume.
5%, 70-95% martensite phase in weld metal,
It is desirable that the austenitic phase is composed of 5 to 30%.

The content of chemical components other than C and Cr in the base metal of the welded steel pipe of the present invention and the weld metal of the seam portion is as follows:
The following ranges are desirable.

The base material is represented by mass%, Si: 0.01 to 1%, Mn: 0.05 to 2%, Ni: 9% or less, Mo: 5% or less, W: 6% or less, Cu: 5% Zr: 0.5% or less, V: 0.5% or less, Ca: 0.05% or less, Mg: 0.05% or less, Ti: 0.1% or less, sol.Al: 0.001 or less 0.1%, balance: mass of Fe, impurities, and weld metal, Si: 0.01 to 1%, Mn: 0.05 to 2%, Ni: 10% or less, Mo: 5% or less, W: 3% Cu: 3% or less, Zr: 0.3% or less, V: 0.3% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.1% or less, sol. Al: 0.001 to 0.1%, balance: Fe, impurities. P, S and O (oxygen) in the impurities are P: 0.025% or less in both the base metal and the weld metal. : 0.01%
Hereinafter, it is preferable that O: 0.01% or less, N: 0.02% or less of base metal, and 0.05% or less of weld metal. The above welded steel pipe is a basic welded steel pipe of the present invention, and the welded steel pipe of the present invention satisfying the above conditions is particularly resistant to SCC.
Excellent in nature. The base material of this steel pipe may be hot rolled and used for pipe production. Further, even after the heat treatment of the heat-affected zone of the base material and the heat treatment of the weld metal portion are omitted after welding, the steel pipe has sufficient SCC resistance and CO ₂ resistance.

In the basic welded steel pipe according to the present invention, the content of Cr, Ni and Ti in the base material is expressed by
It is preferable that the content is 11 to 20%, the content of Ni is 3 to 7%, and the content of Ti is 0.05% or less, and the following formulas (3) and (4) are satisfied. In this case, even if the base material is hot rolled,
In addition to SCC resistance, it has excellent sour resistance, and satisfies these corrosion resistances even if heat treatment of the weld after welding is omitted.

Cr + 1.5Mo-Ni-0.4Cu-14 ≧ 0 (3) Cr + 1.5Mo-2Ni-0.8Cu-12.5 ≦ 0 (4) In the above basic welded steel pipe, the base metal Cr,
The content of Ni, Mo and Ti in mass% is Cr: 15
-20%, Ni: 4-7%, Mo: 1.5-4%, T
i: 0.015% or less, Cr, Ni, Mo,
The content of W and Ti is expressed in mass%, and Cr: 11 to 18
%, Ni: 5 to 10%, Mo: 1.5 to 4%, W: 4%
Hereinafter, it is desirable that Ti: 0.03% or less and that the chemical composition of the weld metal satisfies the following formulas (5) and (6). In this case, even if the heat treatment after welding is omitted, the base metal and the weld metal are excellent in toughness and strength, and the weld metal portion is excellent in sour resistance and weld hot cracking resistance.

−1 ≦ Cr + Mo−1.7Ni ≦ 13−220 × O (oxygen) (5) 25 ≦ Cr + Mo + 1.8Ni ≦ 30 (6) The above welded steel pipe of the present invention can be obtained from a crude oil from which dehydration treatment is omitted. It is most suitable for use as a steel pipe for pipelines for transporting natural gas.

The present invention is directed to a large-diameter, thick-walled welded steel pipe. The large diameter means about 20 inches (508 mm) or more, and the thick wall means about 0.5 inches (12.7 mm) or more. . Further, “mainly composed of a martensite phase” means that the proportion of the martensite phase exceeds 50% by volume.
Further, the term “weld metal part” means a weld metal side that does not include a transition region existing at a boundary between the base metal and the molten metal part.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, crude oil and natural gas from which dehydration treatment has been omitted have a function of corroding metals. In pipelines for conveying such fluids,
When a large-diameter, thick-walled welded steel pipe of martensitic stainless steel manufactured by the SAW method is applied, stress corrosion cracking (SCC) usually tends to occur at a seam portion on the inner surface of the pipe.

The present inventors have clarified that the above-mentioned SCC mainly occurs at the toe of the weld bead and the cause thereof,
The present invention has been completed.

FIG. 1 is a schematic diagram for explaining a cross-sectional structure of a seam portion of a general large-diameter and thick-walled welded steel pipe.

In the welded steel pipe 1 of the present invention, as described above,
The width and height of the weld bead 3 on the inner surface of the pipe, the base metal and the weld metal 2
Was limited by considering the proof stress at a temperature of 100 ° C. Therefore, stress concentration on the weld bead toe 4 due to the shape of the weld bead is suppressed, and
It has become possible to prevent the occurrence of cracks due to the difference in strength between the base metal and the weld metal at high temperatures up to 00 ° C. In particular, the effect of preventing the occurrence of SCC due to the synergistic effect of the two is great. Therefore, the welded steel pipe of the present invention exhibits good SCC resistance even in a high-temperature fluid up to about 100 ° C. containing a high concentration of chloride and CO ₂ .

At the toe 4 of the weld bead 3 on the inner surface of the pipe,
The reason why is likely to occur is more specifically described as follows.

When a welded steel pipe of martensitic stainless steel is manufactured, the welding material (welding wire) generally includes a duplex stainless steel comprising a ferrite phase containing 22% by mass or more of Cr and an austenite phase, and 25% by mass. % Of Cr is used. Among the above welding materials, duplex stainless steel has better corrosion resistance than the base metal, and super duplex stainless steel has better corrosion resistance and strength than the base metal. On the other hand, when a martensitic welding material having the same composition as the base metal is used as the welding material, a welding material having a chemical composition that generates an austenite phase in the weld metal is selected. The reason is,
Martensitic stainless steel has high welding cold cracking susceptibility.
This is for suppressing low-temperature cracking. In the case where a ferrite phase is present in the weld metal, high temperature cracking easily occurs in the weld metal. Therefore, the metal structure of the weld metal is a martensite phase and an austenite phase. Such a weld metal having a two-phase structure including an austenite phase has a remarkable decrease in strength at high temperatures up to about 100 ° C., in particular, a decrease in proof strength, as compared with a base material having substantially a martensite single phase structure. Therefore, in a high temperature environment during use, the strength of the weld metal is lower than the strength of the base metal. In particular, when applied to a pipeline, a tensile hoop stress is generated during use, so that a weld metal having a low proof stress is deformed more than a base metal.
The reason why SCC is likely to occur at the toe of the weld bead is that the tensile stress and the stress concentration on the weld bead are superimposed, and the strain on the weld metal side of the toe becomes excessively large. In the welded steel pipe of the present invention, distortion is prevented from occurring at the toe of the weld bead on the inner surface of the pipe by means described below. Therefore, SCC does not easily occur.

Hereinafter, the welded steel pipe of the present invention will be described in detail.
Will be explained. In addition,% indication of the content of chemical components is
Means% by volume. The shape of the weld bead on the inner surface of the pipe:
The height index h of the weld bead is specified by the following equation (2).
You. h = {1+ (2 × H / W)} × (YS_B100/ YS_W100(2) H (mm) in the equation (2) is a weld bead on the inner surface side of the pipe.
Height from the inner surface of pipe 3 (from the boundary between the weld bead and the base metal)
Height), W (mm) is the width of the weld bead 3, YS
_B100(MPa) is the proof stress of the base material at 100 ° C., YS
_W100(MPa) indicates the proof stress at 100 ° C. of the weld metal.

In the welded steel pipe of the present invention, the index h = 1.25 represented by the above equation (2) is used as a reference index. And
The length L in the direction of the weld bead of the welded steel pipe of the convex portion where h exceeds 1.25 is determined, and L satisfies the following expression (1). L ≦ 0.2 × W (1) The reason why the SCC resistance is evaluated by the combination of h = 1.25 and the equation (1) is that this combination can most accurately grasp the SCC resistance.

Here, in the case of a spiral welded steel pipe, the weld bead direction means the direction of the weld line of the spiral seam. In addition, the above criteria are particularly effective for evaluating the SCC resistance of a welded steel pipe applied to the transportation of high-temperature crude oil or the like containing a large amount of chloride and carbon dioxide gas in a fluid that has not been subjected to dehydration treatment. . In the above formula (2), the proof stress of the base metal and the weld metal is specified by the value at 100 ° C. in a pipeline in which the welded steel pipe of the present invention is mainly used, even if the temperature in the steel pipe is usually high. 100 ℃
Because it is about.

Metal Structure of Base Metal and Weld Metal: The metal structure of the base metal is a structure mainly composed of a martensite phase including a ferrite phase. The base material may be martensite single phase, but in the case of single phase, the strength of thick steel plate or hot rolled steel plate, which is the material for manufacturing welded steel pipes, is too high, making it difficult to process.
Use conditions are limited, for example, it is difficult to use a welded steel pipe as it is welded without heat treatment after manufacturing. Therefore, in order to obtain appropriate strength, the metal structure of the base material is a structure in which the martensite phase exceeds 50% by volume and includes a ferrite phase. However, the ferrite phase may be 0%. A desirable volume ratio of the martensite phase in the metal structure is 55 to 55.
90%. The balance is a ferrite phase, and its desirable volume ratio is 10 to 45%.

The strength of the base material is desirably a strength equivalent to X80 grade (withstand strength of 551 MPa or more) specified in the API standard as it is in a hot rolled state or a welded pipe. In order to obtain such strength, it is necessary to have a structure containing a soft ferrite phase, that is, a δ ferrite phase, as the second phase. 90% martensite phase
If the volume percentage is exceeded, the strength becomes too high, and local deformation may occur in the welded portion at the time of constructing the pipeline. For this purpose, the martensite phase is 90% by volume.
Below, that is, the ratio of the ferrite phase is 10% or more,
It is desirable to suppress the increase in strength. On the other hand, when the ratio of the martensite phase is less than 55% by volume, the proof stress equivalent to X-80 grade may not be secured. Therefore, the volume ratio of the martensite phase is desirably 55 to 90%.
The volume ratio of the martensite phase and the ferrite phase in the metal structure can be determined by a point calculation method. In this method, microscopic photographs (7.3 cm × 9.5 cm) of a metal structure are first taken at five times at a magnification of 1000 times,
Print twice as large. Next, squares are drawn on the photograph at a pitch of 5 mm, and if the lattice point is in the martensite phase, one point, if the lattice point is in the ferrite phase, 0 point, if the lattice point is at the boundary between the martensite phase and the ferrite phase, 0.5 point For all grids. Further, the total score is calculated, and the score of each phase is obtained by dividing the score of each phase by the total score. This value represents the volume ratio of each phase. The metal structure of the weld metal is a structure mainly containing a martensite phase and containing an austenite phase. The structure containing the austenitic phase is for ensuring the strength, toughness, cold workability and weld hot cracking resistance of the weld metal. The volume fraction of the austenite phase is 5
-30%, and the volume ratio of the martensite phase is 70-95.
% Is desirable. In particular, in the case of a fusion welding method in which the O (oxygen) content of the weld metal tends to be high, a structure containing an austenitic phase is effective from the viewpoint of securing toughness.

The volume ratio (%) of the austenite phase in the martensite structure can be determined by the following method. For a steel pipe or steel plate, a longitudinal section in the rolling direction and a direction perpendicular to the rolling direction,
The intensity ratio of the {220} diffraction line of the austenite phase to the {211} diffraction line of the martensite phase is measured by X-ray diffraction using Co-Kα radiation as the primary X-ray for the two cross sections. Then, the volume ratio of the austenite phase is calculated based on the measured values of the three cross sections, and the average value is obtained.

However, since the intensity of the diffraction line is different between the martensite phase and the austenite phase, errors occur due to the characteristics of each device. For this reason, it is necessary to perform strength correction using a standard sample of martensitic stainless steel and austenitic stainless steel mixed at a predetermined phase ratio.

Chemical composition of base metal and weld metal: C: When the C content is large, the hardness of both the base metal and the weld metal increases. If the hardness is high, the SCC resistance and the sour resistance decrease, so that the C content is preferably low. In particular, as for the base material, it is better that the C content is low, and the C content is 0.0
If it exceeds 5%, hardening in the weld heat affected zone becomes remarkable. In that case, the difference in proof stress between the base metal and the weld metal portion increases, and strain tends to concentrate on the weld metal side at the boundary between the weld metal portion and the weld heat affected zone at the weld bead toe. When such a welded steel pipe is not subjected to a dehydration treatment and is applied to transport a high-temperature fluid containing a large amount of chloride in a carbon dioxide gas environment, stress corrosion cracking is likely to occur.
Further, in an environment containing a small amount of hydrogen sulfide, the sour resistance is significantly reduced.

Therefore, the C content of the base material is 0.05%
It was as follows. The upper limit of the C content is preferably higher than that of the base metal, and 0.1% or less so that the weld metal can obtain higher strength than the base metal. Desirable C contents are 0.03% or less for the base material and 0.05% or less for the weld metal.

Cr: Cr is an element that improves corrosion resistance, especially CO ₂ resistance. If the Cr content of the base metal is less than 9% and the Cr content of the weld metal is less than 7%, transport of fluids that are likely to corrode metals such as carbon dioxide-containing natural gas or crude oil, which has not been subjected to dehydration treatment. the steel pipe use, is not sufficient CO ₂ resistance. Therefore, the lower limits of the Cr content are 9% for the base metal and 7% for the weld metal.

The upper limit of the Cr content is set to 20% for both the base metal and the weld metal. If the Cr content of the base material exceeds 20%, the martensite transformation start temperature (Ms point) decreases, and it is difficult to obtain a metal structure mainly composed of a martensite phase. In particular, when a steel sheet in a hot-rolled state is used as a base material for manufacturing a welded steel pipe, it is difficult to secure a desirable lower limit of the martensite phase ratio of 55% by volume.

When the Cr content of the weld metal exceeds 20%, a ferrite phase is easily formed. In this case, the metal structure of the weld metal portion does not become a two-phase structure mainly containing a martensite phase and including an austenite phase.

Therefore, the Cr content of the base material is 9-20.
%, And the Cr content of the weld metal was 7 to 20%. In addition,
The CO ₂ resistance improves as the Cr content increases. Therefore, the desirable range of the Cr content is 11 to 2 in the base material.
0%, more preferably 15-20%, for weld metal,
It is 11 to 18%, more preferably 11 to 15%.

The content of Cr also has an important effect on the sour resistance of the base metal and the weld metal, the weld hot cracking property of the weld metal, and the like. The corrosion resistance is influenced by the content of other elements such as Ni, which is an austenite phase forming element. Therefore, as described below, the Cr content is
It is preferable to select in consideration of the contents of these elements. The elements described below may not be added, but may be contained in the base material as needed.

Si: Si is an element effective in deoxidizing molten steel when manufacturing a base material or deoxidizing a molten portion during welding.
However, if the Si content exceeds 1%, the toughness tends to decrease. Therefore, when Si is used as a deoxidizing agent,
It is desirable that both the base metal and the weld metal be 1% or less.
A more desirable upper limit is 0.5%.

In order to obtain the deoxidizing effect of Si, the content of Si is desirably 0.01% or more. A more desirable lower limit is 0.05%. Therefore, when Si is contained, a desirable content is 0.01 to 1%, more preferably 0.05 to 0.5%.

Mn: Mn is an element effective for deoxidizing molten steel at the time of manufacturing a base material or deoxidizing a molten portion during welding and improving the hot workability of steel. In order to obtain these effects, it is desirable that both the base metal and the weld metal be contained at 0.05% or more.

The upper limit of the Mn content is preferably 2% for both the base metal and the weld metal. If the Mn content of the base material exceeds 2%, segregation of Mn is likely to occur inside the slab for manufacturing the base material, and the toughness may be reduced due to the segregation of Mn, and the SCC resistance may also be reduced. Tend to. The upper limit of the desirable Mn content of the base material is 1%. In addition, Mn of the weld metal
If the content exceeds 2%, toughness, SCC resistance and the like are likely to be reduced.

Therefore, the Mn content when added is 0.05 to 2%, preferably 0.05 to 1%, for both the base metal and the weld metal.

Ni: With respect to the base material, Ni is an element effective for securing the toughness of the heat affected zone in large heat input welding. Therefore, it is included when it is necessary to obtain the effect. If the content is less than 3%, the amount of ferrite in the weld heat affected zone increases, and sufficient strength and toughness of the weld heat affected zone may not be obtained. On the other hand, when the Ni content of the base material exceeds 9%, the Ms point tends to decrease and the strength tends to decrease. A more desirable upper limit is 7%.

In the weld metal, Ni has the effect of increasing the ratio of the austenite phase, and is included when it is necessary to obtain the effect. To get that effect,
The Ni content is desirably 2% or more, or 5% or more. If the Ni content of the weld metal exceeds 10%, the effect of Ni is saturated and the cost increases, so the upper limit of the Ni content of the weld metal is preferably set to 10%.

Therefore, when the base material contains Ni, the preferable content is 3 to 9%, more preferably 3 to 7%, and still more preferably 4 to 7%. Also, if the weld metal is Ni
Is preferably 2 to 10%, more preferably 5 to 10%. Since Co has almost the same action as Ni, a part of Ni may be replaced with Co.

Mo, W, Cu: Since these elements have the effect of improving the sour resistance of the base metal and the weld metal, when the welded steel pipe is used for transporting a fluid containing hydrogen sulfide, It is preferable to include at least one element. Mo and Cu are particularly effective in improving sour resistance.

In order to obtain the effects of these elements, it is preferable that all of the elements are contained in the base metal and the weld metal in an amount of 0.5% or more. However, with respect to the base material, when the Mo content exceeds 5%, the toughness of the weld heat affected zone decreases, and when the W content exceeds 6% and the Cu content exceeds 5%, Workability decreases. When the content of Mo exceeds 5%, the content of W and Cu exceeds 6% and 3%, respectively.

Therefore, when these elements are added, the content of Mo and Cu is 0.5 to 5 with respect to the base material.
% And W are preferably 0.5 to 6%. A desirable range of the Mo content is 1.5 to 4%, and a desirable range of the Cu content is 1 to 3%.

For the weld metal, Mo is 0.5
-5%, and W and Cu are preferably 0.5-3%. Mo
A desirable range of the content is 1.5 to 4%, and a desirable range of the W and Cu content is 1 to 3%. sol. Al: Al is an element effective for deoxidizing molten steel at the time of manufacturing a base material or molten metal at the time of welding. In order to obtain the deoxidizing effect of Al, both the base metal and the weld metal are so
l. The Al content is preferably 0.001% or more. On the other hand, sol. If the Al content exceeds 0.1%, alumina clusters are likely to remain in the base metal or the weld metal, causing a decrease in toughness.

Therefore, when Al is added, both the base metal and the weld metal are sol. 0.001 in Al content
It is preferable to set it to 0.1%. A desirable content range is 0.001 to 0.04%, more preferably 0.001 to 0.04%.
0.01%.

V, Zr: All of these elements have the effect of fixing C and N in steel as carbides or nitrides and reducing the variation in strength such as proof stress of the base metal and the weld metal. When it is necessary to obtain this effect, it is preferable to include at least one element. The effect is
All elements become significant at 0.001% or more. However, if the content of these elements exceeds 0.5% in the base metal and 0.3% in the weld metal, both the base material and the weld metal tend to have reduced toughness and corrosion resistance.

Therefore, when these elements are added, the preferable contents are 0.001 to 0.5%, more preferably 0.001 to 0.3% for the base material, and 0.1 to 0.3% for the weld metal.
001 to 0.3%, more preferably 0.001 to 0.
2%.

Ca, Mg: These elements have the effect of improving the hot workability of the base material. It also has the effect of preventing nozzle clogging when casting a base material slab in continuous casting. Therefore, when it is necessary to obtain these effects, it is preferable to add at least one element. The effects of these elements become significant at 0.0005% or more. However, each element is 0.05%
Exceeds, coarse oxides tend to remain in the steel,
In some cases, the toughness of the base material is easily reduced, and the corrosion resistance is reduced as a starting point of pitting corrosion.

Therefore, when these elements are contained in the base material, the contents thereof are set to 0.1% for both Ca and Mg.
It is better to be 0005 to 0.05%. A desirable range is from 0.0005% to 0.03%, and a more desirable range is from 0.005% to 0.03%.
0005-0.01%.

[0062] Ca and Mg have the effect of fixing S in the weld metal and improving the resistance to hot cracking during welding.
In order to obtain the effects of these elements, it is preferable to contain 0.0003% or more. However, when the content of these elements exceeds 0.03%, coarse oxides tend to remain in the weld metal, and in such a case, the toughness is reduced and the coarse oxides cause pitting corrosion. As a starting point, corrosion resistance may be reduced. Therefore, when Ca is added,
Desirable content of Mg, 0.0003 ~
0.03%.

Ti: In the case of a base material whose metal structure is composed of a martensite phase and a ferrite phase, the presence of Ti lowers the toughness of the base material. In particular, when the content of Ti is 0.1.
If it exceeds 1%, the toughness significantly decreases. Therefore, the content of Ti in the base material is preferably 0.1% or less. Desirable Ti content is 0.05% or less, more preferably 0.015% or less.

On the other hand, Ti is usually added to the welding material in order to stabilize the welding arc. Therefore, Ti remains in the weld metal. The effect of Ti on the toughness of the weld metal is relatively small, but if it exceeds 0.1%, the toughness tends to decrease. Therefore, the Ti content of the weld metal is desirably 0.1% or less, more desirably 0.05%.
%, More preferably 0.03% or less.

P, S, N, and O in impurities: All elements are elements existing as impurities, and it is preferable that the content of both the base metal and the weld metal is low.

If the P content exceeds 0.025%, the corrosion resistance and toughness of the base metal and the weld metal tend to decrease.
Therefore, the P content is preferably set to 0.025% or less. It is more preferably at most 0.015%, further preferably at most 0.01%.

If the S content exceeds 0.01%, the hot workability, corrosion resistance and toughness of the base metal, the hot crack resistance, the corrosion resistance and the toughness of the weld metal tend to decrease. Therefore,
The S content of both the base metal and the weld metal is preferably 0.01% or less. More preferably, it is 0.005% or less, and still more preferably 0.002% or less.

When the N content exceeds 0.02% in the base metal and 0.05% in the weld metal, the toughness and corrosion resistance of the base metal in a hot-rolled or welded state, and the weld metal Tends to decrease toughness and corrosion resistance. In particular, the toughness of the heat affected zone of the base metal and the sour resistance of the base metal and the weld metal tend to decrease. Therefore, the base material N
Content is 0.02% or less, N content of weld metal is 0.0
It is good to make it 5% or less. More desirably, all are 0.01% or less.

If the O (oxygen) content exceeds 0.01%, the toughness and corrosion resistance of the base metal and the weld metal tend to decrease. Therefore, the O content is preferably set to 0.01% or less. More preferably, it is 0.005 or less.

Preferred Embodiment of Chemical Composition: As described above, the welded steel pipe of the present invention has excellent SCC resistance when applied to transport of crude oil or the like that has not been subjected to dehydration treatment. Further, as described in the section of “Means for Solving the Problems”, when the base material or the weld metal satisfies the chemical composition described below, the welded steel pipe of the present invention has still other useful properties. Prepare.

As described above, the metal structure of the base material is desirably constituted by a volume percentage of 55 to 90% of a martensite phase and 10 to 45% of a ferrite phase. In addition to ensuring such a metallographic structure, even if the heat treatment after hot rolling or the heat treatment after welding is omitted in the production of the base steel sheet, the base material has excellent strength, toughness, and also has corrosion resistance such as sour resistance. In order to obtain a material, the following conditions should be satisfied.

The contents of Cr, Ni and Ti in the base material were
Cr: 11 to 20%, Ti: 0.05% or less,
Ni: 3 to 7%. When the base material contains Mo, the content including these elements is set to satisfy the following expressions (3) and (4) at the same time. Formula (3) is a condition suitable for obtaining a ferrite phase ratio of 10% by volume or more and a martensite phase ratio of 90% by volume or less.
The formula is a suitable condition for obtaining a martensite phase ratio of 55% by volume or more and a ferrite phase ratio of 45% by volume or less.

The element symbols in the equations (3) and (4) represent the contents (% by mass) of each element contained in the steel.

Cr + 1.5Mo-Ni-0.4Cu-14 ≧ 0 (3) Cr + 1.5Mo-2Ni-0.8Cu-12.5 ≦ 0 (4) As for the weld metal, 5 to 30% by volume of austenite phase It has been already mentioned that it is desirable to be composed of a martensite organization including. Such a metal structure is desirable in order to prevent cracks (called welding hot cracking) from occurring during the process of solidification from the molten state during welding and to improve the strength and toughness of the weld metal.
In order to prevent this cracking, an appropriate amount of ferrite phase exists in the solidification process and the cooling process after solidification,
The chemical composition should be chosen such that the ferrite phase disappears while the temperature drops to about 200 ° C. Considering the presence of the ferrite phase in such a solidification process,
The weld metal portion preferably has a martensite structure containing an austenite phase of 30% by volume. For this purpose, it is desirable to select an appropriate amount that simultaneously considers the contents of Cr and Mo, which are ferrite-forming elements, and the Ni content, which is an austenite-forming element. In the case of the welded steel pipe of the present invention, the contents of Cr, Ni, Mo, and Ti in the base material are respectively set to Cr: 15 to 20%, Ni: 4 to 7%, and Mo:
1.5 to 4%, Ti: 0.015% or less, Cr, Ni, Mo, W, and Ti of the weld metal are each Cr: 1.
1 to 18%, Ni: 5 to 10%, Mo: 1.5 to 4%,
It is preferable that W: 4% or less and Ti: 0.03% or less, and satisfy the following expressions (5) and (6). −1 ≦ Cr + Mo−1.7Ni ≦ 13−220 × O (oxygen) (5) 25 ≦ Cr + Mo + 1.8Ni ≦ 30 (6) Equation (5) is an equation taking into account the ferrite phase in the solidification process. The term “−1.7 × Ni” is an empirical formula representing the tendency to form a ferrite phase. The smaller the value of this term, the smaller the amount of ferrite phase formed. However, when the value of this item is less than -1, since a ferrite phase does not exist at a high temperature immediately after solidification, high-temperature welding cracks are likely to occur. On the other hand, if the value of this term is too large, the amount of ferrite phase formed will be excessive, and the toughness will decrease.

The toughness of the weld metal is greatly affected by the O (oxygen) content. The formula “Cr + Mo” considering the O content
If “−1.7 × Ni ≦ 13−220 × O” is satisfied, a weld metal having sufficient toughness can be obtained.

In the equation (6), “Cr + Mo + 1.
The term “8 × Ni” is an empirical formula representing the tendency to form an austenite phase. When the value of this term is less than 25, sufficient toughness cannot be obtained because the generation amount of the austenite phase is small. On the other hand, when the value of this item exceeds 30, the amount of austenite phase formed is too large, and sufficient tensile strength and proof stress cannot be secured.

Here, the sufficient tensile strength of the weld metal means
In the joint tensile test, it means that a fracture occurs in the base metal part and does not fracture in the weld metal part. In the case of the welded steel pipe of the present invention, the tensile strength of the weld metal is 650 MPa in order to make the strength X80 grade or more (proof strength 551 MPa or more).
It is better to be at least a.

In the welded steel pipe of the present invention, the base material is the above (3).
In the case where the combination of formulas (4) and (4) and the weld metal satisfy formulas (5) and (6), a welded steel pipe having more excellent properties can be obtained.

In addition to the above welded steel pipes, the chemical composition of the base metal is as follows: C: 0.05% or less, Si: 0.5% or less, Mn:
1% or less, Cr: 9 to 17%, Ni: 9% or less, W:
1% or less, Cu: 3% or less, V: 0.3% or less, Ca:
0.01% or less, Ti: 0.1% or less, balance: Fe and impurities, and the chemical composition of the weld metal is C: 0.05
%, Si: 0.5% or less, Mn: 1% or less, Cr:
9-20%, Ni: 9% or less, W: 3% or less, Cu: 3
%, V: 0.2% or less, Ca: 0.01% or less,
B: 0.01% or less, Ti: 0.1% or less, balance: F
e, when the projections of the weld bead satisfy the above-mentioned formula (1), the SCC resistance is the most excellent.

It is most effective to use the welded steel pipe of the present invention as a pipeline steel pipe for transporting crude oil or natural gas from which dehydration treatment has been omitted. In particular, when the end faces of the steel pipes are connected by welding at the time of laying the pipeline, the welded portion and the heat-affected zone of the base material can be used as welded without post-heat treatment. For welding the end faces of steel pipes, SAW method, TIG method, MIG method
MAG method or the like can be applied.

Method for Producing Welded Steel Pipe: The welded steel pipe of the present invention can be produced by a conventional method for producing a welded steel pipe. The general manufacturing method is as follows. However, since the shape of the weld bead needs to satisfy the conditions defined in the present invention, it is good to consider the welding conditions described below.

First, a hot-rolled steel plate or a thick steel plate is cut into a width substantially equal to the outer peripheral length of a steel pipe of a product. Next, C press,
It is formed into a cylindrical shape using a UOE method formed by a U press and an O press. The butted portion is subjected to SAW welding to form a welded steel pipe. At the molding stage, a UO method, a spiral method, a roll bend method, or the like may be employed.

The welding conditions, the flux and the welding wire to be used may be selected in consideration of the chemical composition of the weld metal and the like. If fine adjustment of the hardness is required, a tempering treatment or the like may be performed.

The chemical composition of the base material to be used may be appropriately selected in consideration of the above conditions, depending on the use of the welded steel pipe.

The method of manufacturing the hot rolled steel plate and the thick steel plate as the base material may be a generally adopted method.
A hot-rolled steel plate or a thick steel plate may be manufactured by a method of hot-rolling a continuously cast slab or a method of bulk-rolling and hot-rolling a steel ingot.

The ferrite phase has a volume ratio of 10 to 45.
% (Martensite phase 55 to 90%), the heating temperature of the raw steel before hot rolling is 1100
C. to 1250.degree. C. is desirable. Heating temperature is 125
If the temperature exceeds 0 ° C., a large amount of ferrite phase is precipitated, and the metal structure of the product may not be mainly composed of the martensite phase. If the temperature is lower than 1100 ° C., the deformation resistance of the raw steel is large, and it is difficult to perform hot rolling.

The shape of the weld bead must satisfy the conditions specified in the present invention. Therefore, the following points should be considered when selecting welding conditions.

First, it is preferable to provide a groove on the end face of the base material to be butt-welded. The angle of the groove is preferably large as long as productivity is not impaired. Also, qualitatively,
It is better that welding heat input is large and welding speed is low. However,
Since these conditions differ in optimum conditions depending on the thickness of the base material, the chemical composition, the chemical composition of the welding material, and the like, it is preferable to determine them in consideration of the conditions of the base material and the welding material.

[0089]

The test method common to Examples 1 to 3 is as follows. FIG. 3 shows Embodiments 1 and 2 described later.
It is a schematic diagram which shows the cross-sectional structure of the welding part of the single-sided welding member 7 used in FIG. SCC resistance test: The size of the test piece is 5 mm in thickness and 2 in width
One test piece in Example 1 and two test pieces in Examples 2 and 3 were prepared for each test condition with a length of 5.4 mm and a length of 165 mm. The bending stress is shown in FIG.
As shown in (b), the test piece 5 was set on a bending application jig 6 and applied so that the deflection amount was Y (mm).
Y is the amount of deflection when a stress equal to the proof stress (YS) of the base material at room temperature is applied as σ (bending applied stress) in the equation shown in FIG. 2B. The test piece in the state where the amount of deflection was imparted was autoclaved. The treatment conditions were as follows: CO ₂ gas in an atmosphere of 3 MPa; test solution at a temperature of 100 ° C .; The test piece after the treatment was visually observed to check for the occurrence of cracks. If the presence or absence of cracks could not be clearly confirmed only by visual observation, the cross section was polished and then observed with an optical microscope to determine whether or not cracks occurred.

When the number of test pieces was one, the case where no crack was observed was evaluated as good (良好), and the case where cracks were generated was evaluated as poor (x). When the number of test pieces was two, the case where no cracks were found was good (○), the case where one crack was slightly bad (△), the case where both were cracked Indicates poor “x”. CO ₂ resistance test: The size of the test piece was 2 mm in thickness and 2 in width
0 mm and length 50 mm. The treatment of the test piece is an autoclave treatment, and the treatment conditions are the same as in the case of the SCC resistance test. The corrosion rate in the treatment was investigated.

The CO ₂ resistance was evaluated as good (」) when the corrosion thinning was 0.1 mm or less per year, and as poor (x) when the corrosion thinning was 0.1 mm or less per year. Sour resistance test: The shape and number of test pieces are the same as in the SCC resistance test. SCC resistance to bending stress
Same as the sex test. Autoclave treatment is performed in a state where the deflection amount Y (mm) is given. The conditions of the autoclave treatment were as follows: CO ₂ gas having an atmosphere of 3 MPa and H ₂ S gas having a partial pressure of 0.003 MPa.
C., pH 4.5, aqueous solution with NaCl concentration of 10% by mass, immersion time: 720 hours. The test piece after the test was visually observed to check for occurrence of cracks. If the presence or absence of cracks could not be clearly confirmed only by visual observation, the cross section was polished and then observed with an optical microscope to confirm the presence or absence of cracks.

The evaluation is the same as in the above-described SCC resistance test. (Example 1) In Example 1, the shape of the weld bead and the SC resistance
The relationship with C sex was investigated. Further, a combination of five types of base materials and two types of welding welding materials having different chemical compositions, metal structures, and tensile properties was also investigated.

Table 1 shows five types of thick steel plates (reference A)
To E), the chemical composition of the welding material used (codes F and G) and the chemical composition of the weld metal obtained by combining these base materials and the weld metal with the metal structures (codes AF) To EG). The base material is a martensitic stainless steel containing a ferrite phase. The strength of each thick steel plate is adjusted to the X80 grade (proof strength of 80 ksi (551 MPa) or more) specified by the API standard. The thickness of the thick steel plate for the welding test is 12.7 to 25.4 mm in thickness, 300 mm in width, and 1 m in length.

As for the thick steel plates B, D, and E, in addition to the thick steel plates for the welding test described above, a steel plate for a real machine pipe test having a width of 1920 mm and a length of 6 m (symbol B is two sheets) is prepared. Then, four large-diameter thick welded steel pipes having an outer diameter of 610 mm and a length of 6 m were produced by the SAW method. In addition, the chemical composition of the welding material used for this pipe making is denoted by reference symbol G in Table 1. The test for the welded steel pipe corresponds to test numbers 33 to 36 in Table 4.

[0095]

[Table 1] For a thick steel plate for a welding test with a width of 300 mm, after performing a groove process on one end surface in the width direction according to the plate thickness,
The grooved portions were butted and welded on one side by the SAW method. FIG. 3 shows a cross-sectional structure of a weld bead portion of a test piece (hereinafter also referred to as a welded joint) after this welding. Reference numeral 7 denotes a thick steel plate (base material), and reference numeral 2 denotes a weld metal. From the position shown in FIG. 4 of this welded joint, a thickness of 5 mm and a width of 25.4
Two 4-point bending test pieces 5 each having a welding bead having a length of 165 mm and a length of 165 mm were sampled two by two and subjected to a corrosion resistance test. In addition, about the welding steel pipe of 6 m in length, the corrosion resistance test piece 5 was sampled from the position shown in FIG. 5 in the region where the h of the weld bead portion was determined to be the largest. For the weld metal, 2
Table 1 also shows the chemical compositions of the welding materials for the above-mentioned welded joints.

Tables 2 to 4 show the size of the test material and the shape of the weld bead of the test piece for each test number for 32 types of single-sided welding members and 4 types of test pieces of welded steel pipe.

[0097]

[Table 2]

[Table 3]

[Table 4] In addition, among the single-sided welding members shown in Tables 2 to 4, trial numbers 3 to
Reference numeral 7 denotes the same single-sided welding member as in Test No. 2, and reference numerals 18 to 21 denote the same single-sided welding member as in Test No. 17. However, both sides other than the length L are removed by cutting to the same height as the surface of the base material so that the value of L becomes the length shown in the table.

Each of the obtained single-sided welded members and the welded steel pipe was subjected to the following tests without any welding, that is, without performing heat treatment after welding.

For each test piece, a tensile test, a weld bead
Shape measurement, SCC resistance, CO resistance _TwoAnd sour resistant
Sex was investigated.

The conditions of the tensile test in Example 1 are as follows.

For both the single-sided welded member and the welded steel pipe, the sampling position of the test piece is in the direction perpendicular to the rolling direction of the steel sheet for the base metal portion and in the seam welding line direction for the weld metal portion. Outer diameter 8mm, parallel part length 60m
m. The test temperature was 100 ° C. and room temperature.
At 00 ° C., the yield strength (YS) of the base metal and the weld metal was examined, and at room temperature, the tensile strength (TS) and yield strength (YS) of the base metal and the weld metal were examined.

The results of the above tests are also shown in Tables 2 to 4.

The following items are clear from the results shown in Tables 2 to 4.

A single-sided welded member (test numbers 6 to 8, 10, 11, 1) in which the shape of the weld bead satisfies the conditions specified in the present invention.
3, 15, 16, 20 to 23, 25, 26, 28, 2
9, 31, 32) and the welded steel pipes (test numbers 33 to 36) all had good SCC resistance. Furthermore, the CO ₂ resistance was also excellent.

Regarding the sour resistance, except for the single-sided welded members Nos. 20 to 23, 28, 29, 31 and 32 and the No. 36 welded steel pipe, which have low Mo content in the base metal steel,
All were good. From these results, when sour resistance is required, it is desirable that the shape of the weld bead satisfy the conditions specified in the present invention and that other measures for improving the sour resistance be used together. confirmed.

On the other hand, a single-sided welding member (test numbers 1 to 5, 9, 1 and 1) whose weld bead shape does not satisfy the conditions specified in the present invention.
2, 14, 17 to 19, 24, 27, and 30) all had poor SCC resistance.

Note that the test numbers 20 to 23, 28, 29, 3
As for Nos. 1, 32 and 36, the shape of the weld bead satisfied the conditions specified in the present invention, so that they were excellent in SCC resistance and CO ₂ resistance. However, the sour resistance was not sufficient. (Example 2) In Example 2, for a welded steel pipe that satisfies the shape of the weld bead specified by the present invention, that is, a welded steel pipe having good SCC resistance, the desirable conditions of the chemical composition of the base metal and the metal structure were determined. investigated. In particular, the condition of a base material having excellent sour resistance and toughness was confirmed even with welding.

Table 5 shows the 26 types of steel (base metal) subjected to the test.
Shows the chemical composition of Steel No. 24 of au and xz
For the type, cast slabs produced in a small melting furnace
After heating to 0 ° C., it was hot forged into a billet, and further hot-rolled under the conditions of 5 passes and a finishing temperature of 980 ° C.
It processed into the steel plate of thickness 25mm, width 120mm, and length 400mm. In addition, steel No. The steels s to u are conventional steels. In addition, steel No. “v” and “w” are thick steel plates for pipe production having a thickness of 19 mm manufactured by an actual machine.

[0109]

[Table 5] As the welding material, C: 0.01%, Cr:
A welding wire having a diameter of 4 mm containing 12%, Ni: 9%, and Mo: 3% and a high basicity bond type flux were prepared.

Then, the production of an actual welded steel pipe was simulated,
At the end face parallel to the rolling direction of the prepared steel sheet, an angle of 60 degrees,
A Y-groove having a root face height of 13 mm was machined, and the machined parts were butt-welded. The welding method was a single-sided welding by the SAW method, with a welding heat input of 7.5 kJ / mm.

Next, the following test specimens were taken from the base material after welding and the butt welded parts, and the mechanical properties (proof strength,
Toughness) and sour resistance.

The tensile test piece for measuring the proof stress has a sampling position in a direction perpendicular to the rolling direction of the base material and a shape of 4 m in diameter.
m, a round bar shape with a gauge length of 20 mm.

The Charpy impact test specimen for measuring toughness has a sampling position in a direction perpendicular to the rolling direction and a shape according to JIS.
No. 4 test piece specified in Z 2202 (1980) (length: 55 mm, width: 10 mm, notch shape: 2 mm
V type). The Charpy impact test specimen was cut from both the base metal and the butt weld. The notch of the No. 4 test piece cut out from the butt weld was located at the weld bond (the boundary between the melted portion and the unmelted portion). The toughness was evaluated by vTrs.

[0114] The test piece for the sour resistance test was a sample having a thickness of 2 mm and a width of 10 m in a direction perpendicular to the rolling direction.
m, 75 mm in length, for a four-point bending test with a V notch. The number of samples is two for each of the base metal and the butt weld from the center in the thickness direction. The notch of the four-point bending test piece cut from the butt weld was
Like the Charpy impact test specimen described above, it was located at the weld bond.

The bending stress applied during the sour resistance test is as described above. The processing conditions for the test piece were
The atmosphere is carbon dioxide gas with a partial pressure of 3 MPa and 0.001 MPa
H ₂ S gas, the solution was an aqueous solution having a temperature of 25 ° C. and a NaCl concentration of 5% by mass, and immersion time was 200 hours. The method for evaluating sour resistance is as described above.

Table 6 summarizes the results of the investigation of the material properties and the calculated values of the equations (3) and (4), which are the desirable chemical composition of the base material specified in the present invention.

[0117]

[Table 6] The following items are apparent from the results shown in Table 6.

Test Nos. 1 to 15 and 22 to 26 have the chemical compositions satisfying the formulas (3) and (4), and the ferrite phase ratio is 12 to 43% by volume (the martensite phase is 57 to 8%).
8% by volume). Therefore, both the base metal and the weld heat affected zone near the toe of the weld bead had good sour resistance and toughness. In addition, the proof stress was large, satisfying 551 MPa or more corresponding to X80 grade specified by the API.

On the other hand, in the case of test numbers 16, 17, 19 to 21 which do not satisfy either one of the equations (3) and (4),
At least one of sour resistance and toughness was poor. Among these, the conventional steels of Test Nos. 19 to 21 all had a single-phase martensite phase and no ferrite phase because the chemical composition did not satisfy the formula (3). Therefore, although the toughness is good, the proof stress is 820 to 820.
It was unusually high at 843 MPa.

Therefore, when sour resistance and toughness are required in addition to SCC resistance, the base material satisfies the equations (3) and (4) and the ferrite phase ratio is 45% by volume or less. Was found to be desirable. (Example 3) In Example 3, for a welded steel pipe that satisfies the shape of the weld bead specified in the present invention, that is, a welded steel pipe having good SCC resistance, the desirable conditions of the chemical composition and the metal structure of the weld metal were determined. investigated. In particular, the conditions of a weld metal having excellent sour resistance and toughness were confirmed even in the as-welded state. In Example 3, welding hot cracking properties were also investigated.

In Example 3, in order to confirm a desirable chemical composition and a metal structure of a weld metal, an investigation was performed on a welded portion of a welded steel pipe. Therefore, the steel No. prepared in Example 2 was used. The v and w thick steel plates were used as a base material, and after forming this base material into an open pipe, the butted portions were welded by the SAW method to produce a welded steel pipe having an outer diameter of 608 mm (24 inches) and a wall thickness of 19 mm.

In order to form weld metal parts having different chemical compositions, twelve kinds of wires having a different chemical composition shown in Table 7 and having a diameter of 4 mm were used as a welding material during pipe production.
Further, two types of molten fluxes having different basicities were combined and welded to prepare 19 types of welded steel pipes having different chemical compositions of weld metals. The welding conditions were a three-electrode SAW welding method and a heat input of 4.5 kJ / mm. Table 8 shows the chemical composition of the weld metal.

[0123]

[Table 7]

[Table 8] The resistance of the molten metal to high-temperature cracking by welding was investigated based on the presence or absence of cracks in the welded metal portion of the above welded steel pipe. In the evaluation of the resistance to high temperature cracking in welding, a case where no crack was observed was indicated as “good”, and a case where cracks were observed was indicated as “bad”.

The test specimen for the tensile test was JIS Z220.
This is a No. 5 tensile test piece specified in 1, wherein the length of the parallel portion was 110 mm and the distance between the evaluation points was 100 mm. The test piece was sampled in a direction perpendicular to the welding line so that the parallel portion includes the weld metal, the heat affected zone, and the base metal. The test temperature is room temperature. In the test results, those broken at the base metal portion were indicated as good “○”, and those broken at the weld metal portion were indicated as poor “×”.

A Charpy impact test specimen for toughness measurement was taken from each of the welded joints in a direction perpendicular to the weld bead. The shape of the test piece was the same as that of Example 2, and the notch portion of the test piece was worked so as to match the center of the weld metal. The test temperature was −30 ° C., and the toughness was evaluated by the absorbed energy vE _{−30 ° C.} at that temperature. The evaluation of toughness is good when the absorbed energy is 50 J or more, "O",
A case of less than 50 J was indicated as defective “x”.

The test piece for the sour resistance test was a plate having a thickness of 5 mm, a width of 20 mm, and a parallel portion length of 165 mm. The test piece was sampled from a direction perpendicular to the weld line such that the weld line (the center of the weld bead) was located at the center in the longitudinal direction of the test piece. The excess weld bead was left on one side as it was, and the back side was ground smoothly. The method for applying the bending stress and the autoclave treatment conditions are as described above.

The workability was evaluated by a test in which a butt-welded steel sheet was bent at a radius of curvature twice the sheet thickness. The test results are indicated as good workability when no cracking or buckling occurs, and as poor workability otherwise.

Table 9 summarizes the test results.

[0129]

[Table 9] Test numbers 4, 6 to 8, 14 and 16 to 18 satisfying both the formulas (5) and (6) defined in the present invention have sour resistance, toughness, weld hot cracking property, workability and tensile strength. All of the properties were good. Test No. 13 satisfies the expressions (5) and (6), but was poor in sour resistance because the Cr content of the weld metal was low at 10.4%. From these results, it was confirmed that, in order to impart sour resistance to the weld metal, it is preferable to satisfy the expressions (5) and (6) and to set the Cr content to 11% or more.

For the test numbers other than the above, formula (5)
Since at least one of the expressions (6) is not satisfied, the test number 5
Except for the above, at least one of the characteristics of sour resistance and toughness was inferior. In Test No. 5, the sour resistance and toughness were good when the austenite phase in the weld metal portion was as large as 40% by volume, but the tensile strength was too low.

[0131]

The martensitic stainless steel welded steel pipe of the present invention has excellent corrosion resistance, particularly SCC resistance, at the welded portion on the inner surface of the pipe, despite being a large-diameter thick-walled welded steel pipe. Also, depending on the choice of chemical composition, sour resistance, C resistance
O ₂ properties, toughness and weld hot cracking properties can also be improved. Therefore, it is very suitable as a steel pipe for a pipeline for transporting crude oil or natural gas which has a strong effect of corroding metals, without dehydration treatment.

[Brief description of the drawings]

[0132]

FIG. 1 is a schematic view for explaining a cross-sectional structure of a seam portion of a general large-diameter thick-walled welded steel pipe.

[0133]

FIG. 2 is a diagram for explaining a method of applying a bending stress for SCC resistance and sour resistance tests in a test of an example. FIG. 2A is a cross section of a jig for applying a bending stress. FIG. 2B is a diagram showing a state where a four-point bending stress is applied.

[0134]

FIG. 3 is a schematic view showing a cross-sectional structure of a welded portion of a single-sided welding member used in Examples 1 and 2.

[0135]

FIG. 4 is a diagram illustrating a sampling position of a test piece in a test of an example, and is a diagram illustrating a position where a four-point bending test piece for SCC resistance and sour resistance tests is sampled from a single-sided welded member.

[0136]

FIG. 5 is a diagram showing a sampling position of a test piece in a test of an example, and is a diagram showing a position where a four-point bending test piece for SCC resistance test and sour resistance test is sampled from a welded steel pipe.

[0137]

[Brief description of reference numerals]

 REFERENCE SIGNS LIST 1 welded steel pipe 2 molten metal 3 weld bead 4 weld bead toe end 5 four-point bending test piece 6 bending application jig 7 single-sided welding member

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Kunio Kondo, Inventor Sumitomo Metal Industries Co., Ltd. 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka (72) Kazuhiro Ogawa 4-5-Kitahama, Chuo-ku, Osaka, Osaka No. 33 Sumitomo Metal Industries, Ltd. (72) Inventor Masahiko Hamada 4-33, Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd.

Claims (4)

[Claims] (1) In mass%, C: 0.05% or less, Cr: 9
A stainless steel base material mainly composed of a martensite phase containing a martensite phase or a ferrite phase, and a C: 0.1% or less, Cr:
7-20%, and the metal structure is composed of a weld metal of a seam portion of stainless steel mainly composed of a martensite phase including an austenite phase, and a weld bead of a seam portion on an inner surface satisfies the following formula (1). Series stainless steel welded steel pipe. L ≦ 0.2 × W (1) where L: bead length of a portion of the seam portion where the value h obtained by the following equation (2) exceeds 1.25: h = ｛1+ (2 × H / W)｝ × (YS _B100 / YS _W100 ) (2) where H: height of the bead from the inner surface of the pipe (mm) W: width of the bead (mm) YS _B100 : yield strength of base material at 100 ° C. (MPa) ) YS _W100 : Strength of weld metal at 100 ° C (MPa) 2. The metal structure of the base metal and the weld metal is expressed by volume%
The base material is a martensite phase of 55 to 90%, a ferrite phase is 10 to 45%, and the weld metal is a martensite phase of 70 to 9%.
2. The welded steel pipe according to claim 1, wherein the chemical composition of the base metal and the weld metal is 5% and the austenite phase is 5 to 30% as follows. 3. By mass%, base material Si: 0.01 to 1%, Mn: 0.05 to 2%, Ni: 9% or less, Mo: 5% or less, W: 6% or less, Cu: 5% or less, Zr: 0.5% or less, V: 0.5% or less, Ca: 0.05% or less, Mg: 0.05% or less, Ti: 0.1% or less, sol.Al: 0.001 to 0.1% The balance: Fe, impurities, weld metal Si 0.01 to 1%, Mn: 0.05 to 2%, Ni: 10% or less, Mo: 5% or less, W: 3% or less, Cu: 3% or less, Zr : 0.3% or less, V: 0.3% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.1% or less, sol.Al: 0.001 to 0.1 %, Balance: Fe, impurities, where the P, S and O (oxygen) contents in the impurities are:
P: 0.025% or less for both base metal and weld metal;
01% or less, O: 0.01% or less, N content is 0.02% or less for base metal and 0.05% or less for weld metal. 3. The base material has Cr, Ni and Ti contents in mass% of Cr: 11 to 20%, Ni: 3 to 7%, Ti:
The welded steel pipe according to claim 1, which is 0.05% or less and satisfies the following expressions (3) and (4). Cr + 1.5Mo-Ni-0.4Cu-14 ≧ 0 (3) Cr + 1.5Mo-2Ni-0.8Cu-12.5 ≦ 0 (4) 4. The base material has Cr, Ni, Mo and Ti contents in mass% of Cr: 15 to 20%, Ni: 4 to 7%,
Mo: 1.5 to 4%, Ti: 0.015% or less, the content of Cr, Ni, Mo, W and Ti in the weld metal is, in mass%, Cr: 11 to 18%, Ni: 5 to 10 %, Mo:
4. The steel composition according to claim 1, wherein 1.5 to 4%, W: 4% or less, Ti: 0.03% or less, and the chemical composition of the weld metal satisfies the following formulas (5) and (6). A welded steel pipe according to claim 1. −1 ≦ Cr + Mo−1.7Ni ≦ 13−220 × O (oxygen) (5) 25 ≦ Cr + Mo + 1.8Ni ≦ 30 (6)