JP2011089159A - Method for manufacturing martensitic stainless steel welded tube excellent in intergranular stress corrosion cracking resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a martensitic stainless steel welded tube having a seam weld excellent in intergranular stress corrosion cracking resistance. <P>SOLUTION: The method for manufacturing the martensitic stainless steel welded tube having the seam weld excellent in intergranular stress corrosion cracking resistance uses a martensitic stainless steel strip which contains, by mass%, <0.0200% C, <0.0200% N, 10-14% Cr, 3-8% Ni, 1-4% Mo, 0.02-0.10% V, 0.0005-0.010% Ca. further contains proper amounts of Si, Mn, P, S and Al or further contains one or more kinds selected from among Ti, Nb, and Zr and/or one or two kinds selected from among Cu and W. In the metod, the martensitic stainless steel strip is welding-jointed to obtain the welded tube 12 and thereafter, post-welding heat treatment for the seam weld for holding 1-20 min in the temperature range of 550-700°C, is applied to the seam weld of the welded tube. The welding-joint is desirable to be a high frequency resistance welding or high density energy beam welding. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a martensitic stainless steel welded tube suitable for use in natural gas or petroleum pipelines containing corrosive gases such as carbon dioxide gas, and in particular, grain resistance of a welded portion of a martensitic stainless steel welded tube. It relates to improvement of field stress corrosion cracking.

In recent years, in order to cope with the rise in crude oil prices and the expected depletion of oil resources in the near future, deep oil fields that have not been excluded in the past, highly corrosive gas fields that were once abandoned, etc. Development on the world is thriving on a global scale. In such oil and gas fields, oil well pipes and steel pipes for line pipes used are required to have high corrosion resistance.
Conventionally, duplex stainless steel pipes have been mainly used as such steel pipes having excellent corrosion resistance. However, in the late 1990s, martensitic stainless steel pipes having lower cost and appropriate corrosion resistance were developed. As a material for line pipes, 12% Cr martensitic stainless steel with a reduced carbon content is defined in the API standard, and it has come to be used in large quantities as a line pipe for natural gas containing carbon dioxide. It is coming.

Until now, martensitic stainless steel seamless pipes have mainly been used for such applications, but recently, demand for martensitic stainless steel welded pipes has increased from the viewpoint of reducing material costs. ing.
However, recently, when used in an environment containing carbon dioxide, intergranular stress corrosion cracking occurs in the weld heat-affected zone formed when the martensitic stainless steel seamless pipe is circumferentially welded. ) (Hereinafter also referred to as IGSCC).

  For such a problem, for example, in Patent Document 1, mass%, C: less than 0.0100%, N: less than 0.0100%, Cr: 10-14%, Ni: 3-8%, effective C solid solution Csol = C-1 / 3Cpre (where Cpre: C amount precipitated by combining with Ti, Nb, Zr, V, Hf, Ta) is contained so as to satisfy less than 0.0050% There has been proposed a martensitic stainless steel pipe having a composition that exhibits excellent resistance to intergranular stress corrosion cracking in the weld heat affected zone. According to the technique described in Patent Document 1, the cause of IGSCC is that a Cr-depleted layer is formed when carbon dissolves in the thermal cycle during welding and precipitates at grain boundaries as Cr carbide in the subsequent thermal cycle. For this reason, IGSCC can be prevented by reducing the C content or containing an appropriate amount of carbide forming elements and adjusting the effective solid solution C amount Csol during welding to less than a predetermined value.

However, it is pointed out that even the technique described in Patent Document 1 cannot completely prevent the reprecipitation of Cr carbide at the time of welding, and therefore cannot completely eliminate the risk of IGSCC occurrence. Moreover, in order to reduce the amount of C to the level described in Patent Document 1, there is also a problem that the refining cost increases.
As a practical measure that can prevent IGSCC occurring in the weld heat affected zone HAZ of such a circumferential weld, application of post welding heat treatment (hereinafter also referred to as PWHT) is considered. This is based on the idea that after welding, PWHT is applied to the welded part under appropriate conditions to diffuse Cr into the Cr-deficient layer that causes IGSCC and to extinguish the Cr-deficient layer.

However, when PWHT is applied to the weld zone, if the conditions are inappropriate, it is not only possible to prevent the desired generation of IGSCC from occurring, but there is a concern that HAZ may be cured.
For such a problem, for example, Patent Document 2 proposes a method for manufacturing a martensitic stainless steel pipe circumferential welded joint. In the technique described in Patent Document 2, when end portions of martensitic stainless steel pipes are attached to each other, multilayer build-up welding is performed along the end portions, and a circumferential weld portion is formed, at least once. Subsequent weld passes so that a heat cycle that improves the intergranular stress corrosion cracking resistance is given to the weld heat affected zone heated to 950 ° C or higher at the peak temperature of the surface layer in the steel pipe by the weld heat cycle by the weld pass. We are going to adjust and weld. As a heat cycle that improves the intergranular stress corrosion cracking resistance, for example, by adjusting the welding conditions such as the peak temperature of the subsequent welding pass, the cooling rate, etc. Cycles or thermal cycles where Cr diffuses and the Cr-deficient layer disappears are considered. This makes it possible to provide a martensitic stainless steel seamless pipe circumferential welded joint that can prevent IGSCC in the weld heat-affected zone and has excellent intergranular stress corrosion cracking resistance without performing post-weld heat treatment.

  Patent Document 3 proposes a method for forming a martensitic stainless steel material weld. In the technique described in Patent Document 3, the value defined by the specific relational expression between the temperature of the heat treatment after welding and the holding time is not less than a predetermined value in the welded portion of the martensitic stainless steel material having a specific composition, and after welding. The value defined by the specific relational expression between the heat treatment temperature and the lower limit temperature of the temperature at which 1 volume% or more of the austenite phase is formed when heated and held for 20 seconds after forming a 100 volume% martensite structure is a predetermined value. By performing post-weld heat treatment that satisfies the following conditions and the holding time is in the range of 60 to 1000 s, the weld heat-affected zone has excellent IGSCC resistance, excellent hydrogen embrittlement resistance, and excellent toughness. It is said that a circumferential welded joint of a martensitic stainless steel seamless pipe can be formed.

JP 2005-336601 A JP 2006-068757 A JP 2007-321181

  According to the techniques described in Patent Document 2 and Patent Document 3, IGSCC occurring in a circumferential welded joint of a martensitic stainless steel seamless pipe can be prevented. Recently, however, instead of expensive martensitic stainless steel seamless pipes, it is possible to manufacture thinner steel pipes and use cheap martensitic stainless steel welded pipes that can reduce the material cost per pipe length. The desire to do is high.

  In view of such a demand, the present inventors diligently studied how to prevent the occurrence of IGSCC in a martensitic stainless steel welded pipe. As a result, in order to prevent IGSCC generated in a corrosive environment containing carbon dioxide, post-weld heat treatment is applied to the circumferential welds of martensitic stainless steel welded pipe circumferential welded joints in the same way as seamless pipes. As shown in FIG. 1, a region A that is sensitized to IGSCC is generated in the seam welded portion, and the phenomenon that there is a risk of occurrence of IGSCC was found.

  The present invention solves the above-mentioned problems, has a seam welded portion excellent in IGSCC resistance, and is suitable as a steel pipe for a flow line used in oil fields and gas fields containing carbon dioxide gas, martensitic stainless steel welding It aims at providing the manufacturing method of the martensitic stainless steel welded pipe which can manufacture a pipe | tube.

  In order to achieve the above-mentioned object, the present inventors diligently studied various factors that affect the occurrence of IGSCC in a martensitic stainless steel welded pipe. As a result, the region A sensitized to IGSCC, which is formed by PWHT after circumferential welding in the seam weld, is heated to the temperature range where C is dissolved in the thermal cycle during seam welding, and the circumference It was found that the PWHT after welding was a region heated to a low temperature that did not reach the target PWHT heating temperature. Therefore, as a result of further studies, the present inventors have found that the temperature at which C dissolves in the seam welded portion, that is, the thermal cycle during seam welding, in order to prevent the formation of a sensitized region of the seam welded portion. It was conceived that it was necessary to heat-treat the region heated to the region sufficiently to precipitate the solid solution C as carbides after the end of seam welding. And by such heat treatment, after welding the ends of the welded pipes, it is possible to prevent the area heated to low temperature during PWHT from becoming sensitized, and to prevent the occurrence of IGSCC in martensitic stainless steel welded pipes I got the knowledge that I can do it.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) In a method of manufacturing a welded pipe in which a steel strip is continuously formed to form an open pipe having a substantially cylindrical cross section, both end faces of the open pipe are butted and welded to form a welded pipe. In mass%, C: less than 0.0200%, N: less than 0.0200%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.010% or less, Al: 0.10% or less, Cr: A martensitic system containing 10-14%, Ni: 3-8%, Mo: 1-4%, V: 0.02-0.10%, Ca: 0.0005-0.010%, and having the balance Fe and inevitable impurities After making the stainless steel strip and welding, the seam welded part of the welded pipe is subjected to post-heat treatment of the seam welded part held at a temperature in the range of 550 to 700 ° C for 1 to 20 minutes, and the intergranular stress corrosion cracking resistance A method for producing a martensitic stainless steel welded pipe, characterized in that the welded pipe has a seam welded portion with excellent properties.

(2) In (1), in addition to the above composition, the composition further contains, in mass%, one or more selected from Ti: 0.15% or less, Nb: 0.10% or less, Zr: 0.10% or less The manufacturing method of the martensitic stainless steel welded pipe characterized by the above-mentioned.
(3) In (1) or (2), in addition to the above composition, the composition further contains, by mass%, one or two selected from Cu: 4.0% or less and W: 4.0% or less A method for producing a martensitic stainless steel welded pipe, characterized in that:

(4) The method for producing a martensitic stainless steel welded pipe according to any one of (1) to (3), wherein the weld joint is a weld joint using a high-density energy beam welding method.
(5) In (4), after the welded joint, and before the heat treatment after the seam welded portion, 0.5 mm or more in the thickness direction from the inner surface of the seam welded portion to the seam welded portion of the welded pipe A method for manufacturing a martensitic stainless steel welded pipe, comprising repairing a seam weld that is remelted and solidified.

  ADVANTAGE OF THE INVENTION According to this invention, the martensitic stainless steel welded pipe which has the seam weld part excellent in the intergranular stress cracking resistance can be manufactured easily and cheaply, and there is a remarkable industrial effect.

It is explanatory drawing which shows typically the formation position of the IGSCC sensitization area | region of the martensitic stainless steel welded pipe resulting from PWHT after circumferential welding. It is explanatory drawing which shows typically the bending condition of the test piece in a 4-point bending stress corrosion cracking test. It is explanatory drawing which shows typically an example of the manufacturing equipment of a welded pipe.

In the present invention, as shown in FIG. 3, the steel strip is continuously formed with various forming rolls to form an open tube having a substantially cylindrical cross section, and the open tube is pressed with a squeeze roll. Both end faces are butted together and welded to form a welded pipe. In the present invention, the steel strip used is a steel strip having a martensitic stainless steel composition.
First, the reasons for limiting the composition of the steel strip used will be described. Hereinafter, the mass% in the composition is simply expressed as% unless otherwise specified.

C: less than 0.0200%,
C is an element that dissolves in steel and contributes to increasing the strength of the steel. However, if contained in a large amount, the HAZ is hardened to cause weld cracking or deteriorate the weld heat-affected zone toughness. Then, it is desirable to reduce as much as possible. In the present invention, in order to prevent IGSCC particularly in the seam welded portion, C that precipitates as Cr carbide and causes formation of a Cr-deficient layer is limited to less than 0.0200%. When C is contained in an amount of 0.0200% or more, it becomes difficult to prevent IGSCC in the seam weld. In addition, Preferably it is 0.0100% or less.

N: less than 0.0200%,
N, like C, is an element that dissolves in steel and contributes to an increase in the strength of the steel, and if contained in a large amount, it hardens the welded portion and causes weld cracking or deteriorates the toughness of the welded portion. In addition, N combines with Ti, Nb, Zr, V, Hf, Ta, etc. to form nitrides. Therefore, Ti can form carbides and prevent formation of Cr carbides. Ti, Nb, Zr, V, Hf, Ta amount Thus, the effect of suppressing the IGSCC by reducing the formation of Cr-deficient layers of these elements is reduced. For this reason, in the present invention, it is desirable to reduce N as much as possible. Since the adverse effect of N described above is acceptable if it is less than 0.0200%, in the present invention, N is limited to less than 0.0200%. In addition, Preferably it is 0.0100% or less.

Si: 1.0% or less,
Si is an element that acts as a deoxidizer and contributes to an increase in strength by solid solution. In the present invention, Si is desirably contained in an amount of 0.05% or more. However, Si is also a ferrite-forming element, and a large content exceeding 1.0% deteriorates the base material and the HAZ toughness. For this reason, Si was limited to 1.0% or less. In addition, Preferably it is 0.1 to 0.5%.

Mn: 2.0% or less,
Mn is an austenite-generating element while contributing to an increase in the strength of steel by solid solution,
Suppresses ferrite formation and improves the base metal and HAZ toughness. In order to acquire such an effect, it is desirable to contain 0.1% or more. On the other hand, even if the content exceeds 2.0%, the effect is saturated. For this reason, Mn was limited to 2.0% or less. In addition, Preferably it is 0.2 to 1.2%.

P: 0.03% or less,
P is an element that segregates at the grain boundary to lower the grain boundary strength and adversely affects the stress corrosion cracking resistance, and is preferably reduced as much as possible, but is acceptable up to 0.03%. For this reason, P was limited to 0.03% or less. From the viewpoint of hot workability, it is preferably 0.02% or less.

S: 0.010% or less,
S is an element that forms sulfides such as MnS and reduces hot workability. In the present invention, S is preferably reduced as much as possible, but is acceptable up to 0.010%. For this reason, S was limited to 0.010% or less. In addition, from the viewpoint of securing more stable hot workability, it is preferably 0.004% or less.

Al: 0.10% or less,
Al is an element that acts as a deoxidizer, and such an effect is recognized when contained in an amount of 0.001% or more, but inclusion exceeding 0.10% degrades toughness. For this reason, Al was limited to 0.10% or less. In addition, Preferably it is 0.01 to 0.04%.
Cr: 10-14%
Cr is a basic element for improving corrosion resistance such as carbon dioxide corrosion resistance, pitting corrosion resistance, and sulfide stress corrosion cracking resistance, and it is necessary to contain 10% or more in the present invention. On the other hand, if the content exceeds 14%, a ferrite phase tends to be formed, and a large amount of alloying element is required to stably secure a martensite structure, leading to an increase in material cost. For this reason, in the present invention, Cr is limited to the range of 10 to 14%.

Ni: 3-8%,
Ni is an element that improves the corrosion resistance of carbon dioxide gas, contributes to an increase in strength by solid solution, and improves toughness. Ni is an austenite-forming element and acts effectively to stably secure a martensite structure in a low carbon region. In order to obtain such an effect, the content of 3% or more is required. On the other hand, if the content exceeds 8%, the transformation point is excessively lowered, and the tempering treatment for securing the desired characteristics takes a long time, and the material cost increases. For this reason, Ni was limited to the range of 3 to 8%. In addition, Preferably it is 4 to 7%.

Mo: 1.0-4.0%
Mo is an element that improves stress corrosion cracking resistance, further sulfide stress corrosion cracking resistance, and pitting corrosion resistance. In order to obtain such effects, it is necessary to contain 1.0% or more. On the other hand, if the content exceeds 4.0%, ferrite is easily generated and the effect of improving resistance to sulfide stress corrosion cracking is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, Mo was limited to 1.0-4.0. In addition, Preferably it is 1.5 to 3.0%.

V: 0.02 to 0.10%,
V is a carbide forming element, and has a stronger carbide forming ability than Cr, and suppresses reprecipitation of C, which is solid-solved by welding heat, as Cr carbide at the prior austenite grain boundaries, and is resistant to IGSCC in the weld zone. Has the effect of improving. In order to acquire such an effect, it is necessary to contain 0.02% or more. A large content exceeding 0.10% deteriorates weld crack resistance and toughness. For this reason, V was limited to 0.10% or less. In addition, Preferably it is 0.025 to 0.075%.

Ca: 0.0005 to 0.010%
Ca is an element that contributes to the improvement of hot workability through the form control of inclusions. In order to obtain such an effect, it is desirable to contain 0.0005% or more. However, if it exceeds 0.010%, it tends to exist as coarse inclusions, so that the corrosion resistance and toughness are significantly reduced. For this reason, Ca was limited to 0.0005 to 0.010% of range. In addition, Preferably it is 0.0005 to 0.0030%.

The above-mentioned components have a basic composition. In addition to the basic composition, one or two selected from Ti: 0.15% or less, Nb: 0.10% or less, and Zr: 0.10% or less as required. One or more selected from Cu or more and / or Cu: 4.0% or less, W: 4.0% or less can be selected and contained.
One or more selected from Ti: 0.15% or less, Nb: 0.10% or less, Zr: 0.10% or less
Ti, Nb, and Zr are all carbide-forming elements like V, and can be selected from one or more as required. Ti, Nb, and Zr all have a stronger carbide forming ability than Cr, and suppress the re-precipitation of C, which is solid-solved by welding heat, as Cr carbide at the prior austenite grain boundaries. Has the effect of improving. In addition, Ti, Nb and Zr carbides are difficult to dissolve even when heated to high temperatures with welding heat, and the formation of solute C is suppressed, which suppresses the formation of Cr carbides and suppresses the IGSCC resistance of welds. There is also an effect of improving the performance. In order to obtain such an effect, it is preferable to contain Ti: 0.03% or more, Nb: 0.03% or more, and Zr: 0.03% or more. On the other hand, the content exceeding Ti: 0.15%, Nb: 0.10% and Zr: 0.10% respectively deteriorates weld crack resistance and toughness. For this reason, it is preferable to limit to Ti: 0.15% or less, Nb: 0.10% or less, and Zr: 0.10% or less, respectively. More preferably, they are Ti: 0.03-0.12%, Nb: 0.03-0.08%, Zr: 0.03-0.08%.

One or two selected from Cu: 4.0% or less, W: 4.0% or less
Both Cu and W are elements that improve the corrosion resistance of carbon dioxide gas, which is a characteristic required for steel pipes for line pipes that transport natural gas containing CO ₂ , and one or two of them can be selected as necessary. Can be selected and contained.
Cu is an austenite forming element as well as improving the carbon dioxide gas corrosion resistance, and effectively acts to secure a stable martensite structure in a low carbon region. In order to acquire such an effect, it is preferable to contain 0.5% or more. On the other hand, even if the content exceeds 4.0%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, it is preferable to limit Cu to 4.0% or less. In addition, More preferably, it is 1.0 to 3.0%.

  W is an element that improves the corrosion resistance of carbon dioxide gas, and further improves the resistance to stress corrosion cracking, further resistance to sulfide stress corrosion cracking, and pitting corrosion resistance. It is preferable to contain more than 4.0%, but if the content exceeds 4.0%, it is easy to produce ferrite, and the effect of improving resistance to sulfide stress corrosion cracking is saturated, and an effect commensurate with the content cannot be expected. Become. For this reason, it is preferable to limit W to 4.0% or less. In addition, More preferably, it is 1.0 to 3.0%.

The balance other than the above components is composed of Fe and inevitable impurities.
The martensitic stainless steel strip used in the present invention is obtained by melting the molten steel having the above composition by a normal melting method such as a converter, an electric furnace, a vacuum melting furnace, and the like. It is preferable to use a known method such as a lump rolling method as a product material such as a slab and apply a known hot working to the product material to obtain a hot-rolled steel sheet having a predetermined size. After hot working, these hot-rolled steel sheets cooled to room temperature were further reheated to a temperature above the Ac ₃ transformation point, and then subjected to quenching treatment at a cooling rate above air cooling, and then below the Ac ₁ transformation point. It is preferable to perform a tempering treatment at a temperature. Note that the steel having the above composition can be made into a martensite structure if it is cooled at a cooling rate equal to or higher than air cooling after hot working, so that the quenching treatment is omitted and the steel is cooled to room temperature after hot working. Thereafter, direct tempering treatment may be performed.

  For example, the steel strip 1 is continuously formed in a continuous forming facility shown in FIG. 3, for example, in which a plurality of various forming rolls (breakdown roll 3, cage roll 4, fin pass roll 5, etc.) are arranged in series. The open tube 11 has a substantially cylindrical cross section. Next, while pressing the open pipe 11 with the squeeze roll 7, both end faces of the open pipe are brought into contact with each other, and the seam portion is welded and joined by the welding means 6 to obtain a welded pipe 12. In addition, as for the manufacturing equipment to be used, any of the conventional equipment for manufacturing welded pipes can be applied.

  The welding joining is preferably performed using a conventional high-frequency resistance welding method (electro-sealing welding method) as a welding means, but may be a high-density energy beam welding method instead of the high-frequency resistance welding method. The high-density energy beam is preferably a laser beam. Needless to say, when welding and bonding using high-density energy beam welding, the irradiation condition of the high-density energy beam is a condition in which both end portions of the open tube are melted over the entire thickness. In addition, by pressurizing a welded part at the time of welding, the molten metal is extruded and solidified on the outer surface and the inner surface of the welded pipe obtained by welding joining to form a raised part (bead). These beads are preferably removed and smoothed by cutting or the like on both the inner and outer surfaces.

  Stainless steel containing a large amount of alloy components such as Cr tends to generate a large amount of high-melting-point oxide during welding, and the generated high-melting-point oxide remains in the seam weld and easily causes welding defects called penetrators. . In order to suppress the formation of the high melting point oxide, it is conceivable to shield the molten metal with an inert gas, but the generation of the high melting point oxide cannot be completely prevented. Therefore, it is preferable to perform welding joining using high-density energy beam welding in which the molten metal generated during welding is prevented from coming into contact with the atmosphere. However, depending on the irradiation condition of the high-density energy beam, the irradiation amount may be insufficient, and an unmelted part may remain on the inner surface side of the welded pipe. There is a problem that welding defects are likely to occur.

  Assuming the occurrence of such a weld defect on the inner surface of the pipe, it is preferable to perform a seam weld repair process for remelting and solidifying the seam weld on the inner surface of the pipe by 0.5 mm or more in the thickness direction from the inner surface of the pipe. In addition, the seam welded part repair process specifically has a width of the melted part of 2 to 5 mm, that is, a weld defect of about 2 to 5 times or less the width of the melted part of the seam welded part. From the viewpoint of preventing the occurrence of Thereby, generation | occurrence | production of the welding defect of the seam welding part inner surface side surface can be prevented. The remelting depth (distance in the thickness direction) is preferably 40% or less of the tube thickness from the inner surface, specifically, 5 mm or less. The seam weld repair process is not particularly limited as long as it is a heating method capable of controlling the heating position and temperature. Any heating / melting method using arc, plasma, laser or the like can be used.

  In the present invention, after being welded or welded by high-density energy beam welding, and further subjected to seam weld repair on the inner surface of the pipe, the seam welded portion of the welded pipe is subjected to 550 to 700 ° C. A post-heat treatment of the seam weld is held at a temperature in the range for 1-20 min. Thereby, even if PWHT for IGSCC prevention is given after pipe circumference welding, it can prevent IGSCC generating in a seam welded part. The post-seam weld post-heat treatment may be performed on the entire tube, but is heated to a relatively low temperature by, for example, circumferential welding or subsequent PWHT, and the temperature at which the solid solution C reprecipitates as a Cr carbide at the old γ grain boundary. Only the seam weld where there is a concern of becoming a zone, that is, the seam weld near the ends of the tube, is sufficient.

  In addition, when the heating temperature of the post-seam weld post-heat treatment is less than 550 ° C, stable carbides are less likely to be generated, and the possibility of IGSCC occurring during the subsequent PWHT treatment increases, and the IGSCC resistance of the seam weld tends to decrease. Get higher. On the other hand, when it becomes high temperature exceeding 700 degreeC, the hardness of a welded part will increase and toughness will fall. For this reason, the heating temperature of the seam weld post-heat treatment was set to 550 to 700 ° C. In addition, Preferably it is 600-650 degreeC. Moreover, the heating method of the post-seam weld post-heat treatment is not particularly limited, but is preferably induction heating in which the heating position and temperature can be easily controlled.

Further, if the holding time is less than 1 min in the heating temperature range of the heat treatment described above, the holding time is too short and stable IGSCC resistance cannot be ensured. On the other hand, when the holding time is longer than 20 min, the productivity is lowered. For this reason, the heat treatment holding time was set to 1 to 20 minutes. In addition, Preferably it is 3-10min.
In addition, the post heat treatment of the seam welded portion is at least about 1 to 2 times the combined width of the seam welded portion and the heat affected zone, or the combined width of the seam welded portion repair process and the heat affected zone. 1 times or more, preferably 2 times or less. Here, “the heat affected zone of the seam weld zone” and “the heat affected zone of the seam weld zone repair process” refer to a region heated to the Ac ₁ transformation point or higher by the process.

A martensitic stainless steel strip (thickness: 6 mm) having the composition shown in Table 1 is used to produce a welded steel pipe (outside) using a welded pipe manufacturing facility consisting of continuous roll forming, squeeze roll, welding means, etc. shown in FIG. Diameter: 219 mmφ). In addition, the steel strip used was air-cooled to room temperature after hot rolling, and then tempered at 580 to 620 ° C. The welding means used was a high frequency resistance welding method and a laser welding method. The welding conditions of the high frequency resistance welding method were a welding speed: 20 m / min and in an N ₂ atmosphere. The laser welding method was performed using a fiber laser, laser output: 20 kW, welding speed: 8 m / min, in an Ar gas atmosphere.

After seam welding, the beads on the inner and outer surfaces of the seam welded portion were each smoothed by cutting. In addition, some steel pipes were subjected to seam weld repair treatment under the conditions shown in Table 2 from the inner surface side of the seam weld.
In order to make the pipe circumferential direction of the obtained welded pipe the tensile direction, a tensile test piece (marking point: 50.4 mm) is taken from the base material according to the API-5LX standard, and a tensile test is performed. Tensile properties (yield strength YS, tensile strength TS, elongation El) of the base material were determined. The obtained tensile properties are shown in Table 2.

Further, a test material (wall thickness: 3 mm × width 15 mm × 115 mm length) including a seam weld was collected from the obtained welded pipe. The test material is subjected to a post-seam weld heat treatment under the conditions shown in Table 3, and then a PWHT equivalent heat treatment of the circumferential weld under the conditions shown in Table 3 in a region of 10 mm width orthogonal to the seam weld of the test piece. Carried out.
The test material subjected to the above treatment was polished to obtain a test piece for a four-point bending stress corrosion cracking test, and a four-point bending stress corrosion cracking test was performed.

The 4-point bending stress corrosion cracking test was a test in which a test piece was held using a jig 20 as shown in FIG. 2 and immersed in a corrosive environment. The test period was 720 hours. The corrosive environment was a 5% NaCl aqueous solution having a liquid temperature of 100 ° C., a CO ₂ pressure of 0.1 MPa, and a pH of 2.0. The load strain was 0.6%.
After the test, the cross section of the test piece was visually observed and observed for cracking with a 100 × optical microscope, and the intergranular stress corrosion cracking resistance (IGSCC resistance) was evaluated. No cracking was indicated by ○, and cracking was indicated by ×.

  The obtained results are shown in Table 3.

  Each of the examples of the present invention is a welded steel pipe that can prevent IGSCC in the seam welded portion and has excellent IGSCC resistance in the seam welded portion. On the other hand, in the comparative example outside the scope of the present invention, the occurrence of cracks was observed, and the IGSCC resistance was lowered.

DESCRIPTION OF SYMBOLS 1 Steel strip 2 Edge mirror 3 Breakdown roll 4 Cage roll 5 Fin pass roll 6 Welding means 7 Squeeze roll 8 Sizing roll 9 Cutting machine
11 open tube
12 Welded pipe
20 Jig

Claims (5)

In the manufacturing method of a welded pipe in which a steel strip is continuously formed and formed into an open pipe having a substantially cylindrical cross section, both end faces of the open pipe are butted and welded to form a welded pipe, %so,
C: Less than 0.0200%, N: Less than 0.0200%,
Si: 1.0% or less, Mn: 2.0% or less,
P: 0.03% or less, S: 0.010% or less,
Al: 0.10% or less, Cr: 10-14%,
Ni: 3-8%, Mo: 1-4%
V: 0.02 to 0.10%, Ca: 0.0005 to 0.010%
And a martensitic stainless steel strip having a composition comprising the balance Fe and unavoidable impurities, and after the welding joint,
To the seam weld of the welded pipe, a post-heat treatment for the seam weld that is held for 1 to 20 minutes at a temperature in the range of 550 to 700 ° C.
A method for producing a martensitic stainless steel welded pipe, comprising a welded pipe having a seam welded portion having excellent intergranular stress corrosion cracking resistance.   In addition to the above composition, the composition further comprises, in mass%, one or more selected from Ti: 0.15% or less, Nb: 0.10% or less, Zr: 0.10% or less. The manufacturing method of the martensitic stainless steel welded pipe of Claim 1.   The composition according to claim 1 or 2, further comprising one or two kinds selected from Cu: 4.0% or less and W: 4.0% or less by mass% in addition to the composition. Manufacturing method of martensitic stainless steel welded pipe.   The method for manufacturing a martensitic stainless steel welded pipe according to any one of claims 1 to 3, wherein the weld joint is a weld joint using a high-density energy beam welding method.   A seam welded part that is remelted and solidified by 0.5 mm or more in the thickness direction from the inner surface of the seam welded part to the seam welded part of the welded pipe after the welded joint and before the post-seam welded part heat treatment The method for manufacturing a martensitic stainless steel welded pipe according to claim 4, wherein repair treatment is performed.

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