JP4765283B2 - Method for producing martensitic stainless steel pipe circumferential welded joint - Google Patents

Description

  The present invention relates to martensitic stainless steel pipes suitable for use in natural gas and petroleum pipelines, etc., and in particular, improves intergranular stress corrosion cracking resistance of weld heat affected zones in martensitic stainless steel pipe circumferential welded joints. About.

  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, and highly corrosive sour gas fields that were once abandoned. Development on the world is thriving on a global scale. In such oil and gas fields, the steel pipes used are required to have high corrosion resistance.

  Conventionally, for example, in an environment containing a large amount of carbon dioxide, an inhibitor has been added as a means for preventing corrosion. However, the addition of an inhibitor not only increases the cost, but may not be sufficiently effective at high temperatures, so recently there has been a tendency to use a steel pipe with excellent corrosion resistance without using an inhibitor. .

As a material for line pipes, 12% Cr martensitic stainless steel with reduced C content is defined in the API standard, and recently, martensitic stainless steel pipes are often used as line pipes for natural gas containing CO _2. It has come to be. However, the martensitic stainless steel pipe has problems that it requires preheating and post-heating at the time of circumferential welding and has poor weld toughness.

For such a problem, for example, in Patent Document 1, C: 0.02% or less and N: 0.07% or less are reduced, and the Cr, Ni, and Mo amounts are related to the C amount, and Cr, Ni There has been proposed a martensitic stainless steel in which the Mo amount is adjusted to C and N amounts and the Ni and Mn amounts are adjusted to appropriate amounts in relation to the C and N amounts. The martensitic stainless steel pipe manufactured by the technique described in Patent Document 1 is said to be a steel pipe having excellent carbon dioxide corrosion resistance, stress corrosion cracking resistance, weldability, high temperature strength, and weld toughness. .
JP-A-9-316611

However, recently, cracks have occurred in the weld heat affected zone (hereinafter also referred to as HAZ) of the circumferential welded part where the martensitic stainless steel pipes are butted together in multiple welding passes in an environment containing CO _2. This is a new problem in martensitic stainless steel pipes.

Conventionally, as the corrosion that occurs in an environment containing CO ₂ , so-called carbon dioxide gas corrosion accompanied by thinning of the base material, or stress corrosion cracking of the base material is known. However, the crack that has recently become a problem is characterized by occurring only in the HAZ of the circumferential weld, and also in a mild environment in which so-called carbon dioxide corrosion is not a problem at all. Moreover, since this crack exhibits a grain boundary crack, it is estimated that the crack is an intergranular stress corrosion cracking (hereinafter also referred to as IGSCC).

In order to prevent IGSCC occurring in the HAZ of such a circumferential weld, it has been proved that a short post-weld heat treatment is effective for holding at 600 to 650 ° C. for 3 to 5 minutes. However, the post-weld heat treatment has problems that it complicates the pipeline laying process, lengthens the construction period, and increases the laying cost, even for a short time. For these reasons, there is a demand for a method for manufacturing a martensitic stainless steel pipe circumferential welded joint that can prevent IGSCC of HAZ that occurs in an environment containing CO ₂ without performing post-weld heat treatment.

  The present invention has been made in view of such a demand, and an object of the present invention is to propose a method for manufacturing a martensitic stainless steel pipe circumferential welded joint having a weld heat affected zone with excellent intergranular stress corrosion cracking resistance. And

  In order to achieve the above-described problems, the present inventors have intensively studied the cause of IGSCC occurring in the HAZ of a martensitic stainless steel pipe circumferential weld. As a result, as shown in FIG. 1 (a), the carbide dispersed in the base is once dissolved in the base by the welding heat cycle at the time of circumferential welding, and Cr carbide is formed in the prior austenite grain boundaries in the subsequent welding heat cycle. As a result of precipitation, a Cr-deficient layer was formed in the vicinity of the prior austenite grain boundary, and it was found that IGSCC was generated.

  Stress corrosion cracking due to such a mechanism has been known in austenitic stainless steel, but was not considered to occur in martensitic stainless steel. This is because the diffusion rate of Cr in the martensite structure is much higher than that in the austenite structure, so in martensitic stainless steel, Cr is continuously supplied even if Cr carbide is generated. This is because it was thought that a Cr-deficient layer was not formed. However, the present inventors have found for the first time that even in martensitic stainless steel, a Cr-deficient layer is formed under specific welding conditions, leading to intergranular stress corrosion cracking even in a mild corrosive environment.

  For this reason, the present inventors have received at least one welding heat cycle in which the carbide dispersed in the matrix once dissolves in the matrix, that is, a welding heat cycle in which the peak temperature becomes 950 ° C. or higher. The heat cycle that the HAZ undergoes in the subsequent welding pass can prevent the formation of Cr carbide at the prior austenite grain boundaries and improve the resistance to intergranular stress corrosion cracking. It came to the mind that the generation of IGSCC in HAZ can be prevented.

  And the present inventors, among the HAZ of the circumferential weld, heat cycle in which Cr carbide is re-dissolved in HAZ of the steel pipe inner layer heated to 950 ° C. or higher, thermal cycle in which Cr carbide does not precipitate, or Cr Welding after adjusting the heat input (welding conditions) of the subsequent welding pass so that a thermal cycle that improves intergranular stress corrosion cracking resistance, such as a thermal cycle that diffuses and extinguishes the Cr-deficient layer, can be applied. Found good.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) After marching the end portions of martensitic stainless steel pipes, perform multi-pass welding consisting of a plurality of welding passes in the circumferential direction along the end portions to form a circumferential welded portion, thereby forming martensitic stainless steel When manufacturing a steel pipe circumferential welded joint, the peak temperature is heated to 950 ° C. or more by a welding heat cycle of at least one of the plurality of welding passes among the weld heat affected zone in the circumferential welded portion. The final welding pass of the subsequent welding passes of the one welding pass is applied so that the heat cycle that improves the intergranular stress corrosion cracking resistance is given to the weld heat affected zone of the surface layer in the steel pipe. The peak temperature of the weld heat-affected zone on the surface layer of the steel pipe is completely quenched by hardening the martensitic stainless steel pipe to a 100% by volume martensite structure, and then heated to a predetermined temperature and held for 20 seconds. As the welding path 1 vol% or more austenite phase is generated by a predetermined temperature at which A1 Tenkoe a and 950 ° C. below the temperature of the lower limit of the temperature when the, intergranular, characterized in that the welding A method of manufacturing a martensitic stainless steel pipe circumferential welded joint with excellent stress corrosion cracking properties.
(2) In (1), the martensitic stainless steel pipe is mass%, C: 0.015% or less, N: 0.015% or less, Cr: 10-14%, Ni: 3-8%, Si: 1.0% Hereinafter, Mn: 2.0% or less, P: 0.03% or less, S: 0.010% or less, Al: 0.10% or less, further Cu: 1-4%, Co: 1-4%, Mo: 1-4%, W: A method for producing a martensitic stainless steel pipe circumferential welded joint, comprising one or more selected from 1 to 4% and having a composition comprising the balance Fe and inevitable impurities .
(3) In (2), in addition to the above composition, it is further mass%, Ti: 0.15% or less, Nb: 0.10% or less, V: 0.10% or less, Zr: 0.10% or less, Hf: 0.20% or less, Ta : A method for producing a martensitic stainless steel pipe circumferential welded joint, comprising a composition containing one or more selected from 0.20% or less.
(4) In (2) or (3), in addition to the above composition, it is further selected from mass%, Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.010% or less, B: 0.010% or less The manufacturing method of the martensitic stainless steel pipe circumference welded joint characterized by containing 1 type, or 2 or more types.

  According to the present invention, the martensitic system has excellent intergranular stress corrosion cracking resistance, which is excellent in carbon dioxide gas corrosion resistance in the weld heat affected zone, and further can prevent IGSCC in the weld heat affected zone without performing post-weld heat treatment Stainless steel pipe circumferential welded joints can be provided at a low cost, and have a remarkable industrial effect.

  In the present invention, the end portions of the martensitic stainless steel pipes are butted together and then subjected to multi-layer welding consisting of a plurality of welding passes in the circumferential direction along the end portions to form a circumferential welded portion. Construct stainless steel pipe circumferential weld joint. At that time, it is formed on the surface layer in the steel pipe of the weld heat affected zone (hereinafter also referred to as HAZ) in the circumferential weld zone, and is heated to 950 ° C. or more by at least one welding pass of the multipass welding pass. The subsequent welding pass of the HAZ (hereinafter also referred to as C solid solution HAZ) and a welding pass (hereinafter also referred to as C solid solution pass, for example, Nth layer pass) in which the HAZ was finally heated to 950 ° C. or higher. (Pass after (N + 1) layer) Adjusting welding conditions such as peak temperature, cooling rate, etc., thermal cycle in which Cr carbide is remelted, thermal cycle in which Cr carbide is not precipitated, or Cr is diffused and Cr-depleted layer The thermal cycle which improves the intergranular stress corrosion cracking resistance, such as a thermal cycle that eliminates, is imparted.

As shown in FIG. 1 (b), the application of a thermal cycle in which Cr carbide re-dissolves is performed so that the peak temperature of C solid solution HAZ in the steel pipe inner surface layer exceeds the A1 point and is 950 ° C. or less. In addition, this can be achieved by using a welding pass in which the peak temperature, the cooling rate, etc. are adjusted. If the peak temperature by the subsequent welding pass of C solid solution HAZ on the surface layer in the steel pipe is A1 point or less, solid solution C may precipitate at the prior austenite grain boundary, and Cr deficient layer may be formed in the vicinity of the prior austenite grain boundary There is. Beyond the peak temperature point A1 by welding path of the final and the to Rukoto and 950 ° C. or less, precipitated carbides are redissolved part. As a result, the Cr-depleted layer near the prior austenite grain boundary is recovered.

In the present invention, the point A1 indicates that an austenite phase of 1% by volume or more is generated when a martensitic stainless steel pipe is completely quenched to obtain a 100% by volume martensite structure and then rapidly heated to a predetermined temperature and held for 20s. It is defined as the lower limit of the predetermined temperature that (lower limit temperature).

  In addition, as shown in FIG. 1C, the application of a thermal cycle in which Cr carbide is re-dissolved includes at least one welding pass (for example, M-th layer pass) among the welding passes after the C solid solution pass, The welding pass in which the peak temperature of C solid solution HAZ on the surface layer in the steel pipe exceeds the A1 point and becomes a temperature of 950 ° C. or less, and all subsequent welding passes following the welding pass (passes after the (M + 1) th layer) Can also be achieved by using a welding pass in which the peak temperature of C solute HAZ on the surface layer in the steel pipe is A1 or less. If the welding pass where the peak temperature of C solute HAZ on the surface layer in the steel pipe exceeds the A1 point and the temperature is 950 ° C. or less is the welding pass before the final layer, the subsequent welding pass is the C on the surface layer in the steel pipe. A welding pass in which the peak temperature of the solid solution HAZ is A1 or lower is used. By adopting such a welding pass, even if crystal grains are refined and carbides are precipitated, formation of a Cr-deficient layer in the vicinity of the prior austenite grain boundary can be prevented.

In addition, as shown in FIG. 1 (d), all the welding passes (subsequent welding passes) after the C solid solution pass (the Nth layer pass) are applied to the surface layer in the steel pipe as shown in FIG. 1 (d). The peak temperature of C solid solution HAZ is set to A1 point or less, and the following equation (1) of those welding passes
Ptotal = (Tp + 273) (20 + logΣtp) (1)
Here, Ptotal: the total heat input parameter received by the weld heat affected zone of the steel pipe inner layer by the subsequent welding pass,
Tp: Peak temperature (° C) of the welding heat cycle that the weld heat affected zone of the surface layer in the steel pipe undergoes the subsequent welding pass,
Σtp: the sum of tp from the start time ts to the end time tf of the welding heat cycle,
tp: Conversion holding time (h) at the peak temperature Tp of the subsequent welding pass
Can be achieved by adjusting the total heat input parameter Ptotal defined in (1) to be 12,500 or less. Here, the total heat input parameter Ptotal is a parameter serving as an index of the total heat input received by the C solid solution HAZ of the surface layer in the steel pipe through a welding pass (subsequent welding pass) after the C solid solution pass. Tp is the maximum temperature (° C.) of the peak temperature of the welding heat cycle that the C solid solution HAZ of the steel pipe inner layer receives in the subsequent welding pass, Σtp is the total time (h) of tp, and tp is It is the conversion holding time (h) at the peak temperature Tp of the subsequent welding pass.

  In the present invention, the welding heat cycle in which the temperature changes with time is divided at a time interval Δt along the welding heat cycle curve as shown in FIG. Then, it is assumed that the welding heat cycle by the subsequent welding pass to be processed is equivalent to repeated step heating in which heating and holding at temperature: T (t) for time: Δt is repeated stepwise. FIG. 2 shows a case where the subsequent welding passes are two passes, but it goes without saying that the present invention is not limited to this.

For example, the step heating at the time t is a heating that keeps the time: Δt at the temperature: T (t). The influence of the step heating on the HAZ characteristic is expressed by the following formula P ( t) = (T (t) +273) (20+ (log Δt / 3600))
The heat input parameter defined by (1) is evaluated using P (t) as an index. If this heat input parameter: P (t) is used, the heating temperature or heating holding time can be converted into an equivalent heating holding time or heating temperature at the same P (t).
In the present invention, P (t) is calculated for each step heating divided along the welding heat cycle curve. By using this heat input parameter: P (t), the influence on the HAZ characteristics due to the difference in the heating temperature and holding time of the welding heat cycle can be evaluated in an integrated manner.
About each calculated P (t), it converts into the retention time tp in the highest peak temperature: Tp (degreeC) of the welding heat cycle made into object. Peak temperature: The retention time tp at Tp (° C.) is expressed by the following formula: tp = 10 ^{{P (t) / (Tp + 273) −20}}
Calculate with The obtained tp in each step heating is summed up for the step heating from the start time ts to the end time tf of the welding heat cycle to obtain Σtp. Based on the calculated Σtp and the peak temperature Tp of the target welding heat cycle, the total heat input parameter: Ptotal, which is an index of the total heat input by the target welding heat cycle, is calculated using the equation (1). .

  In the present invention, the welding pass after the C solid solution pass (the subsequent welding pass) is adjusted so that the Ptotal is 12500 or less. When Ptotal of the subsequent welding pass exceeds 12500, Cr carbide precipitates on the prior austenite grain boundaries, and a possibility that a Cr-deficient layer is formed increases, and the risk of occurrence of IGSCC increases. For this reason, in this invention, it is preferable to make Ptotal 12500 or less.

  Also, the welding pass after the C solid solution pass (the subsequent weld pass) is a welding pass in which the peak temperature of the C solid solution HAZ of the surface layer in the steel pipe is A1 point or less and the Ptotal of those weld passes is 14500 or more. Also good. Thereby, Cr can be diffused, the Cr-deficient layer can be eliminated, and the Cr-deficient layer can be prevented from being formed in the vicinity of the prior austenite grain boundary.

  Next, the composition of the martensitic stainless steel pipe preferably used in the present invention will be described. Hereinafter, mass% in the composition is simply referred to as%.

C: 0.015% or less C is an element that dissolves in steel and contributes to increasing the strength of the steel. However, if contained in a large amount, HAZ is hardened to cause weld cracks or toughness of the weld heat affected zone. Therefore, in the present invention, it is desirable to reduce as much as possible. In the present invention, in order to prevent the generation of IGSCC in HAZ, it is preferable to limit C, which precipitates as Cr carbide and causes Cr deficient layer formation, to 0.015% or less. If the C content exceeds 0.015%, it will be difficult to prevent the generation of IGSCC in HAZ. More preferably, it is 0.010% or less.

N: 0.015% or less N, like C, is an element that dissolves in steel and contributes to an increase in the strength of the steel. If a large amount is contained, it will harden HAZ, cause weld cracking, or affect the heat of welding. Deteriorates toughness. In addition, N combines with Ti, Nb, Zr, V, Hf, and Ta to form nitrides. Therefore, the amount of Ti, Nb, Zr, V, Hf, and Ta that can form carbides and prevent the formation of Cr carbides is reduced. This substantially reduces the effect of suppressing the formation of a Cr-deficient layer of these elements and reducing the IGSCC. For this reason, it is desirable to reduce N as much as possible. Since the adverse effect of N described above is acceptable if it is 0.015% or less, in the present invention, N is preferably limited to 0.015% or less. More preferably, it is 0.010% or less.

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 is desirably contained in an amount of 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 preferably limited to a 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. Moreover, it is an austenite forming element, and acts effectively in order 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, it is preferable to limit Ni to the range of 3-8%. In addition, More preferably, it is 4 to 7%.

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 preferably contained in an amount of 0.1% 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, it is preferable to limit Si to 1.0% or less. In addition, More preferably, it is 0.1 to 0.5%.

Mn: 2.0% or less
Mn dissolves and contributes to increasing the strength of the steel, and is an austenite-generating element, and suppresses ferrite formation to improve the base material and the weld heat-affected zone toughness. In order to acquire such an effect, it is preferable to contain 0.2% or more. On the other hand, even if the content exceeds 2.0%, the effect is saturated. For this reason, it is preferable to limit Mn to 2.0% or less. In addition, More 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, it is preferable to limit P to 0.03% or less. In view of hot workability, it is more preferably 0.02% or less.

S: 0.010% or less S is an element that forms sulfides such as MnS and reduces workability. In the present invention, S is preferably reduced as much as possible, but 0.010% is acceptable. For this reason, it is preferable to limit S to 0.010% or less.

Al: 0.10% or less
Al acts as a deoxidizer and is preferably contained in an amount of 0.01% or more. However, if it exceeds 0.10%, the toughness deteriorates. For this reason, it is preferable to limit Al to 0.10% or less.

  In addition, More preferably, it is 0.01 to 0.04%.

One or more selected from Cu: 1-4%, Co: 1-4%, Mo: 1-4%, W: 1-4%
Cu, Co, Mo, and W are all elements that improve carbon dioxide corrosion resistance, which is a characteristic required for steel pipes for line pipes that transport natural gas containing CO ₂ , and are selected in the present invention. It is preferable to contain 1 type or 2 types or more with Cr and Ni.

Cu: 1-4%
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 1% or more. On the other hand, if the content exceeds 4%, 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 the range of 1-4%. In addition, More preferably, it is 1.5 to 2.5%.

Co: 1-4%
Co, like Cu, improves the corrosion resistance of carbon dioxide gas and is an austenite forming element, and effectively acts to stably secure a martensite structure in a low carbon region. In order to acquire such an effect, it is preferable to contain 1% or more. On the other hand, if the content exceeds 4%, 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 Co to the range of 1-4%. In addition, More preferably, it is 1.5 to 2.5%.

Mo: 1-4%
Mo is an element that improves the resistance to stress corrosion cracking, further resistance to sulfide stress corrosion cracking, and pitting corrosion resistance. In order to obtain the effect, Mo is preferably contained in an amount of 1% or more. On the other hand, if the content exceeds 4%, ferrite is easily generated, and the effect of improving the 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, it is preferable to limit Mo to the range of 1-4%. In addition, More preferably, it is 1.5 to 3.0%.

W: 1-4%
W, like Mo, is an element that improves stress corrosion cracking resistance, further sulfide stress corrosion cracking resistance, and pitting corrosion resistance. In order to obtain the effect, W is preferably contained in an amount of 1% or more. On the other hand, if the content exceeds 4%, ferrite is easily generated, and the effect of improving the 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, it is preferable to limit W to the range of 1-4%. In addition, More preferably, it is 1.5 to 3.0%.

One or more selected from Ti: 0.15% or less, Nb: 0.10% or less, V: 0.10% or less, Zr: 0.10% or less, Hf: 0.20% or less, Ta: 0.20% or less
Ti, Nb, V, Zr, Hf, and Ta are all carbide-forming elements, and it is preferable that one or two or more are selected and contained. Ti, Nb, V, Zr, Hf, and Ta all have a stronger carbide forming ability than Cr, and suppress the precipitation of C dissolved in welding heat as prior to austenite grain boundaries as Cr carbide during cooling. , HAZ has the effect of improving intergranular stress corrosion cracking resistance. In addition, Ti, Nb, V, Zr, Hf, and Ta carbides are difficult to dissolve even when heated to high temperatures by welding heat, and the formation of solute C is suppressed, which suppresses the formation of Cr carbides. There is also an effect of improving the intergranular stress corrosion cracking resistance of HAZ. In order to obtain such an effect, Ti: 0.03% or more, Nb: 0.03% or more, V: 0.02% or more, Zr: 0.03% or more, Hf: 0.03% or more, Ta: 0.03% or more are contained. It is preferable. On the other hand, the content exceeding Ti: 0.15%, Nb: 0.10%, V: 0.10%, Zr: 0.10%, Hf: 0.20%, Ta: 0.20% deteriorates weld crack resistance and toughness. Therefore, Ti is preferably limited to 0.15% or less, Nb: 0.10% or less, V: 0.10% or less, Zr: 0.10% or less, Hf: 0.20% or less, and Ta: 0.20% or less. More preferably, they are Ti: 0.03-0.12%, Nb: 0.03-0.08%, V: 0.02-0.08%, Zr: 0.03-0.08%, Hf: 0.10-0.18%, Ta: 0.10-0.18%.

One or more selected from Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.010% or less, B: 0.010% or less
Ca, Mg, REM, and B are all elements that effectively work to improve hot workability and stable manufacturability in continuous casting, and can be selected and contained as necessary. In order to acquire such an effect, it is preferable to contain Ca: 0.0005% or more, Mg: 0.0010% or more, REM: 0.0010% or more, and B: 0.0005% or more. On the other hand, if Ca exceeds 0.010%, Mg: 0.010%, REM: 0.010%, B: more than 0.010%, it tends to exist as coarse inclusions, so that the corrosion resistance is deteriorated and the toughness is significantly reduced. For this reason, it is preferable to limit to Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.010% or less, and B: 0.010% or less. Ca is the most effective from the viewpoints of quality stability and economical efficiency because the quality stability of the steel pipe is high and the manufacturing cost can be kept low. A more preferable range of Ca is 0.0005 to 0.0030%.

  The balance other than the above components is preferably Fe and inevitable impurities.

  The martensitic stainless steel pipe suitably used in the present invention is prepared by melting the molten steel having the above composition by a normal melting method such as a converter, electric furnace, vacuum melting furnace, etc. In a known method such as a lump rolling method, a steel pipe material such as a billet is used, and then the steel pipe material is heated and hot-worked using a manufacturing facility such as a normal Mannesmann-plug mill method or Mannesman-Mandrel mill method It is preferable to produce a seamless steel pipe having a desired dimension by pipe making. In addition, it is preferable that the obtained seamless steel pipe is cooled to room temperature at a cooling rate equal to or higher than air cooling. In addition, there is no problem even if a steel pipe raw material is used as a seamless steel pipe using a press type hot extrusion equipment.

If it is a seamless steel pipe having the above composition, it can be made into a martensite structure if it is cooled at a cooling rate higher than air cooling after hot working, but it is cooled to room temperature after hot working and subjected to tempering treatment. Is preferred. In addition, after hot working, after cooling to room temperature, reheating to a temperature not lower than the Ac ₃ transformation point and then cooling at a cooling rate not lower than air cooling may be performed. The seamless steel pipe subjected to the quenching treatment is preferably subjected to a tempering treatment at a temperature not higher than the Ac ₁ transformation point. In addition, the steel pipe suitably used in the present invention is not limited to the seamless steel pipe as described above, and using a steel pipe material having the above composition, according to a normal process, an electric resistance steel pipe, a UOE steel pipe, A welded steel pipe such as a spiral steel pipe may be used.

From a martensitic stainless steel seamless pipe (outer diameter 219φ x wall thickness 11mm) with the composition shown in Table 1, a test material with a thickness of 4mm x width 15mm x length 115mm was sampled and placed in the center of the test material. A welding heat cycle was applied. The applied welding heat cycle consists of a multi-layered GMAW method with a heat input of 10 kJ / cm, consisting of 5 layers of welding passes in the circumferential direction along the ends after abutting the ends of martensitic stainless steel pipes. When the circumferential weld was formed by performing the prime welding, a multiple welding thermal cycle simulating the welding thermal cycle received by the HAZ on the surface layer in the steel pipe was used. The applied multiple welding thermal cycle is schematically shown in FIG. In the applied multiple welding thermal cycle, the peak temperature of the thermal cycle after the third layer was variously changed. The cooling conditions for each pass were _t800-500 = 9s. The interpass temperature was 100 ° C. In addition, in the given multiple welding heat cycle, the total heat input parameter Ptotal by the welding pass after the third layer is divided into steps in the welding heat cycle curve by the method shown in FIG. 2, and P (t) in each divided section. was calculated, the conversion retention times from those P (t) at the maximum peak temperature Tp t _p were calculated and calculated Ptotal using equation (1). In addition, each steel pipe is heated and then cooled with water to obtain a 100% by volume martensite structure, and then rapidly heated to each temperature of 400 to 700 ° C., and the time change of the shrinkage amount when held is measured. The amount of formation was calculated, and the lower limit temperature among the temperatures at which 1 volume% or more of the austenite phase was formed when held for 20 s was defined as A1 point.

  Next, a test piece having a thickness of 2 mm, a width of 15 mm, and a length of 75 mm was cut out from the center of the test piece material having been subjected to the welding heat cycle, and a U-bending stress corrosion cracking test was performed.

  The U bending stress corrosion cracking test was a test in which a test piece was bent into a U shape with an inner radius of 8 mm using a jig as shown in FIG. 4 and immersed in a corrosive environment. The test period was 168 hours. The corrosive environment used was a 5% NaCl solution having a liquid temperature of 100 ° C., a CO2 pressure of 0.1 MPa, and a pH of 2.0. After the test, the cross section of the test piece was observed for cracking with a 100 × optical microscope to evaluate the intergranular stress corrosion cracking resistance. The case where there was a crack was rated as x, and the case where there was no crack was marked as ◯.

  The obtained results are shown in Table 2.

  It can be seen that all of the examples of the present invention can prevent IGSCC in the weld heat affected zone without performing post-weld heat treatment, and are excellent in intergranular stress corrosion cracking resistance of the weld heat affected zone. On the other hand, in the comparative example outside the scope of the present invention, IGSCC is generated in the HAZ, and the intergranular stress corrosion cracking resistance of the HAZ is insufficient.

It is explanatory drawing shown typically which shows an example of the welding heat cycle provided to a circumference welding part. It is explanatory drawing which shows typically an example of the step-like division | segmentation of a welding heat cycle used at the time of calculation of the total heat input parameter Ptotal. It is explanatory drawing which shows typically the welding thermal cycle used in the Example. It is explanatory drawing which shows typically the bending condition of the test piece for U bending stress corrosion cracking tests used in the Example.

Claims (4)

After the ends of martensitic stainless steel pipes are butted together, multi-pass welding consisting of a plurality of welding passes is performed in the circumferential direction along the ends to form a circumferential welded portion, and the martensitic stainless steel pipe circumference In producing a welded joint, a steel pipe heated to a peak temperature of 950 ° C. or higher by a welding heat cycle of at least one welding pass among the plurality of welding passes among the weld heat affected zone in the circumferential weld. The final weld pass of the subsequent weld passes of the one weld pass is set to the steel pipe so that a heat cycle that improves the intergranular stress corrosion cracking resistance is given to the weld heat affected zone of the inner surface layer. The peak temperature of the weld heat-affected zone on the inner surface layer was completely quenched by quenching the martensitic stainless steel pipe to a 100% by volume martensite structure, and then heated to a predetermined temperature and held for 20 s. Intergranular stress, wherein the weld path 1 vol% or more austenite phase is generated by a predetermined temperature at which A1 Tenkoe a and 950 ° C. below the temperature of the lower limit of the temperature to come, the welding A method of manufacturing a martensitic stainless steel pipe circumferential welded joint with excellent corrosion cracking properties. The martensitic stainless steel pipe is mass%,
C: 0.015% or less under, N: 0.015% or less,
Cr: 10-14%, Ni: 3-8%,
Si: 1.0% or less, Mn: 2.0% or less,
P: 0.03% or less, S: 0.010% or less,
Including Al: 0.10% or less, further containing Cu: 1-4%, Co: 1-4%, Mo: 1-4%, W: 1-4% selected from 1 to 4% And the manufacturing method of the martensitic stainless steel pipe circumference welded joint of Claim 1 which has a composition which consists of remainder Fe and an unavoidable impurity.   In addition to the above composition, mass%, Ti: 0.15% or less, Nb: 0.10% or less, V: 0.10% or less, Zr: 0.10% or less, Hf: 0.20% or less, Ta: 0.20% or less The manufacturing method of the martensitic stainless steel pipe circumference welded joint according to claim 2, wherein the composition contains one or more kinds.   In addition to the above-described composition, it may further contain at least one selected from mass: Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.010% or less, B: 0.010% or less. The manufacturing method of the martensitic stainless steel pipe circumference welded joint of Claim 2 or 3 characterized by these.

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