JP6390677B2 - Low carbon martensitic stainless steel welded pipe and method for producing the same - Google Patents

Description

  The present invention is applied to a low-carbon martensitic stainless steel welded pipe (hereinafter sometimes referred to as “welded pipe”) suitable for use in natural gas containing corrosive gas such as carbon dioxide or petroleum pipeline. Related. According to the welded pipe of the present invention, it is possible to ensure the resistance to sulfide stress corrosion cracking similar to martensitic stainless steel seamless steel pipe, and to improve the intergranular stress corrosion cracking resistance of the seam welded part.

  In recent years, the development of deep oil fields and highly corrosive gas fields that were once abandoned has been developed on a global scale in order to cope with soaring oil prices and the depletion of oil resources expected in the near future. It is flourishing. 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 mass% Cr martensitic stainless steel with a reduced carbon content is defined in the API standard, and a large amount of this steel is used as a material for line pipes for natural gas containing carbon dioxide gas. Is becoming used.

  In the past, low-carbon martensitic stainless steel seamless pipes have mainly been used for the use of pipelines, but recently, martensitic stainless steel has been used in this application from the viewpoint of reducing material costs. The demand for welded pipes is also increasing. The oil well environment in which this martensitic stainless steel pipe is used may be a humid environment containing a small amount of hydrogen sulfide gas in addition to carbon dioxide gas. Therefore, the steel pipe used for this application may be required to have resistance to sulfide stress corrosion cracking (hereinafter sometimes referred to as “SSC resistance”). Therefore, Patent Document 1, Patent Document 2 and Patent Document 3 disclose a method of manufacturing a low-carbon martensitic stainless steel welded tube having good SSC resistance.

  By the way, recently, when a martensitic stainless steel seamless pipe is used in an environment containing carbon dioxide gas, intergranular stress corrosion cracking occurs in a weld heat affected zone formed during circumferential welding (Intergranular Stress Corrosion Cracking). (Hereinafter also referred to as IGSCC) occurs. This problem may also occur in a seam weld of a martensitic stainless welded steel pipe. Patent Document 4 discloses a method for producing a martensitic stainless steel welded pipe excellent in IGSCC resistance. Patent Document 5 discloses a method for manufacturing a martensitic stainless steel welded pipe having a seam welded portion excellent in IGSCC resistance. According to the technique described in Patent Document 5, it is said that generation of IGSCC can be prevented by performing heat treatment on the seam portion to sufficiently precipitate solute C as carbides after completion of seam welding.

Japanese Patent No. 3077576 JP-A-11-343519 Japanese Patent Laid-Open No. 2000-226643 Japanese Patent Laid-Open No. 9-327721 JP 2011-89159 A

  The technique described in Patent Document 1 manufactures a low-carbon martensitic stainless steel welded pipe excellent in sulfide stress corrosion cracking resistance by laser welding. In Patent Document 1, there is a problem in that productivity is poor because it requires two heating / cooling operations after pipe making.

  The technology described in Patent Document 2 is based on the resistance to sulfuration by setting the annealing conditions of the steel sheet after hot rolling, which is a welded pipe material, and the conditions of post-heat treatment of the welded pipe after pipe making within a specific range. This is to produce a low carbon martensitic stainless steel welded pipe excellent in physical stress corrosion cracking. In the example of Patent Document 2, the tempering process is further performed after the post-heat treatment of the welded pipe, and the problem of poor productivity is not solved.

  Patent Document 3 specifies the austenite fraction in the steel sheet after hot rolling as a raw material, thereby avoiding unnecessarily high strength, and the low-carbon martensitic stainless steel welded pipe after pipe making. The SSC resistance is ensured. However, since austenite in the raw steel sheet is transformed into martensite due to strain applied during pipe making, the SSC resistance of the product cannot be ensured only by defining the austenite fraction of the raw material.

  Furthermore, in the methods for producing low-carbon martensitic stainless steel laser welded pipes described in Patent Documents 1 to 3, the test period is 336 hr, which is shorter than 720 hr of NACE TM0177 of the standard that is usually referred to, and still , SSC resistance is insufficient.

  When the literature described about the IGSCC resistance is seen, the technique described in Patent Document 4 is not evaluated for the SSC resistance. In addition, there is a problem in that productivity is poor because it requires two heating / cooling operations after pipe making. In addition, the method for manufacturing a martensitic stainless steel welded pipe described in Patent Document 5 generates IGSCC by performing heat treatment on the seam portion to sufficiently precipitate solute C as carbide after the end of seam welding. It is to prevent. However, the technique described in Patent Document 5 is a technique for ensuring the IGSCC resistance of the seam portion, and is not a technique for improving the SSC resistance.

  In addition, the tensile properties (yield strength YSL, YST) in the tube axis direction and the tube circumferential direction of the welded tube are set to less than 700 MPa for the following reason. In general, the higher the strength, the lower the SSC resistance, so it is necessary to fall within an appropriate range according to the use of the material and cost performance. An object of the present invention is to provide a low carbon martensitic stainless steel welded pipe excellent in SSC resistance and IGSCC resistance while satisfying the strength of API 5L standard and PSL2-X80 level, and a method for producing the same. Therefore, the above strength range was adopted.

  In view of the state of the prior art, the present invention has a sulfide stress corrosion cracking resistance comparable to that of a low carbon martensitic stainless steel seamless steel pipe, and is excellent in the intergranular stress corrosion cracking resistance of a seam weld. An object of the present invention is to provide a low-carbon martensitic stainless steel welded pipe having the following tensile properties and a method for producing the same.

  In order to achieve the above-mentioned object, the present inventors firstly have inferior SSC resistance of a low carbon martensitic stainless steel welded pipe compared to that of a low carbon martensitic stainless steel seamless pipe. We studied earnestly about the cause.

  As a result, the most effective method for improving the SSC resistance is to remove the residual pipe-forming strain in the low-carbon martensitic stainless steel welded pipe after pipe making and to secure a suitable amount of retained austenite fraction. It was found that.

  Next, the present inventors have also intensively studied various factors affecting the occurrence of IGSCC in a low-carbon martensitic stainless steel welded pipe.

  As a result, the highest risk of IGSCC occurrence is that the low-carbon martensitic stainless steel welded pipes are circumferentially welded and heated by a post-weld heat treatment (hereinafter also referred to as PWHT) performed immediately thereafter. It was found that this was the intersection of the surrounding area and the seam weld. This part is heated to a temperature range where C is dissolved in the thermal cycle during seam welding, and then reheated at a low temperature less than the PWHT temperature by conduction heat transfer from the PWHT application part after circumferential welding. Since it is received, solute C is precipitated as Cr carbide, and a Cr-deficient layer is easily formed around it.

  The present inventors perform heat treatment on the seam welded portion before the welded pipe is subjected to circumferential welding to sufficiently precipitate solute C as carbides and sufficiently diffuse Cr around the periphery. I found out that it was necessary to keep them.

  Furthermore, the present inventors conducted further research on the manufacturing conditions of the low carbon martensitic stainless steel welded pipe applying the above knowledge, and finally, the resistance to sulfidation was the same as the low carbon martensitic stainless steel seamless steel pipe. We have found out the conditions under which a low-carbon martensitic stainless steel welded pipe that can improve the resistance to intergranular stress corrosion cracking of seam welds at the same time can be manufactured economically and easily.

  The present invention has been completed based on the above findings. That is, the gist of the present invention is as follows.

  [1] By 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 0.010% or less, Al: 0.10% or less, Cr: 10-14%, Ni: 3-8%, Mo: 1-4%, V: 0.02-0.10%, Ca: 0.0005 Tensile test in the tube axis direction containing ~ 0.010%, having a component composition consisting of the balance Fe and inevitable impurities, the volume fraction of the retained austenite phase in the base metal part being 5% or more and less than 20% The yield stress YSL obtained by the above and the yield stress YST obtained by the tensile test in the pipe circumferential direction are both less than 700 MPa, and the YST / YSL is 0.81 or more and less than 1.00, Low carbon steel with excellent sulfide stress corrosion cracking resistance and intergranular stress corrosion cracking resistance Ten sites stainless steel welded pipe.

  [2] The component composition further includes one or more selected from the group consisting of Ti: 0.15% or less, Nb: 0.10% or less, and Zr: 0.10% or less. The martensitic stainless steel welded pipe according to [1], characterized in that the composition is contained.

  [3] The component composition further includes, by mass%, a composition containing one or two selected from Cu: 4.0% or less and W: 4.0% or less. The low-carbon martensitic stainless steel welded tube according to [1] or [2].

  [4] After annealing the low-carbon martensitic stainless hot-rolled steel sheet having the component composition according to any one of [1] to [3] at a temperature of 550 ° C. or more and less than Ac1 transformation point + 20 ° C., Low carbon formed into a tube, welded with a laser beam with both opposite edges butted together, or heated by applying a high-frequency current or heating with an electrical resistance method, and then welded with a laser beam with both edges butted A method for producing a martensitic stainless steel welded pipe, wherein the steel pipe after the pipe making is subjected to a single heat treatment for annealing the entire steel pipe at a temperature of 550 ° C. or higher and less than Ac1 transformation point + 50 ° C. Low coal with excellent resistance to sulfide stress corrosion cracking and intergranular stress corrosion cracking, characterized by adjusting the volume fraction of retained austenite phase of the material to 5% or more and less than 20% Method for manufacturing a martensitic stainless steel welded pipe.

  According to the present invention, low carbon martensite that can ensure the resistance to sulfide stress corrosion cracking of a low carbon martensitic stainless steel seamless steel pipe and at the same time can improve the intergranular stress corrosion cracking resistance of a seam weld. A stainless steel welded pipe is obtained.

  Moreover, according to this invention, the tensile characteristic of the pipe axial direction and pipe circumferential direction of a welded pipe can be adjusted to a desired range.

  In particular, according to the production method of the present invention, the low carbon martensitic stainless steel welded tube of the present invention can be obtained economically and easily. That is, the production method of the present invention can increase the productivity as compared with the above-described prior art method.

  As described above, the present invention has a remarkable industrial effect.

It is explanatory drawing which shows typically an example of the manufacturing equipment of a welded pipe.

  The low-carbon martensitic stainless steel welded pipe of the present invention, as will be described later, is obtained by annealing a hot-rolled steel sheet and continuously forming it with various forming rolls as shown in FIG. However, while the open pipe is pressurized with a squeeze roll, both end faces of the open pipe are butted together and welded to form a pipe, and the welded pipe is annealed after pipe making. In the present invention, the hot-rolled steel sheet to be used is a low carbon martensitic stainless hot-rolled steel sheet (sometimes referred to as “hot-rolled steel sheet” in this specification). Hereinafter, the low-carbon martensitic stainless hot-rolled steel sheet, annealing of the hot-rolled steel sheet, pipe forming, annealing of the welded pipe, and low-carbon martensitic stainless steel welded pipe will be described in this order.

<Low carbon martensitic stainless hot-rolled steel sheet>
First, the reasons for limiting the composition of the hot-rolled steel sheet used will be described. Hereinafter, unless otherwise specified, “%” indicating the content of a component means “% by mass”.

C: Less than 0.0200% When a large amount of C is contained, HAZ (welding heat affected zone) is hardened to cause weld cracking or to reduce weld heat affected zone toughness. In particular, since the hardening of HAZ is accompanied by a decrease in resistance to sulfide stress corrosion cracking, it is desirable to reduce the C content as much as possible in the present invention. Further, when C is precipitated as Cr carbide in the HAZ, a Cr-deficient layer is formed around it, causing intergranular stress corrosion cracking. From this viewpoint as well, it is desirable to reduce the C content as much as possible. From the above viewpoint, in the present invention, the C content is limited to less than 0.0200%. In addition, Preferably it is 0.0100% or less.

N: Less than 0.0200% When a large amount of N is contained, the welded portion is hardened to cause weld cracking or deteriorate the welded portion toughness. In particular, since the hardening of HAZ is accompanied by a decrease in resistance to sulfide stress corrosion cracking, it is desirable to reduce the N content as much as possible in the present invention. N combines with Ti, Nb, Zr, V, Hf, Ta and the like to form a nitride. As a result, the amount of Ti, Nb, Zr, V, Hf, and Ta that can form carbides and prevent the formation of Cr carbides is substantially reduced, and the effect of suppressing the formation of Cr-deficient layers by these elements, that is, intergranular stress corrosion cracking The effect of suppressing the occurrence is reduced. Also from this viewpoint, in the present invention, it is desirable to reduce the N content 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, the N content 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, it is desirable to contain 0.05% or more. However, Si is also a ferrite-forming element, and when the Si content exceeds 1.0%, the toughness of the base material and the HAZ is lowered. For this reason, Si content was limited to 1.0% or less. In addition, Preferably it is 0.1 to 0.5%.

Mn: 2.0% or less Mn is a solid solution that contributes to an increase in the strength of the steel and is an austenite-generating element that suppresses ferrite formation and improves the toughness of the base material and the HAZ. In order to obtain such an effect, the Mn content is desirably 0.1% or more. On the other hand, even if the Mn content exceeds 2.0%, the effect is saturated and hardly increased. For this reason, Mn content 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 sulfide stress corrosion cracking resistance and the intergranular stress corrosion cracking resistance. In the present invention, it is preferable to reduce the P content as much as possible. In the present invention, the P content is acceptable up to 0.03%. For this reason, the P content is limited to 0.03% or less. From the viewpoint of hot workability, the P content 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, it is preferable to reduce the S content as much as possible. In the present invention, the S content is acceptable up to 0.010%. For this reason, S content was limited to 0.010% or less. In addition, from a viewpoint of ensuring the more stable hot workability, it is preferable to set it as 0.004% or less.

Al: 0.10% or less Al is an element that acts as a deoxidizer, and such an effect is recognized by containing 0.001% or more of Al. When the Al content exceeds 0.10%, the toughness decreases. For this reason, Al content 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. In the present invention, the Cr content needs to be 10% or more. On the other hand, if the Cr content exceeds 14%, a ferrite phase is likely to be formed, and in order to ensure a stable martensite structure, a large amount of other alloy elements are required, resulting in an increase in material cost. . For this reason, in this invention, Cr content was limited to the range of 10-14%.

Ni: 3-8%
Ni is an element that improves the carbon dioxide gas corrosion resistance, 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 Ni content is set to 3% or more. On the other hand, when the Ni content exceeds 8%, the transformation point is excessively lowered, and the tempering process for ensuring desired characteristics takes a long time. On the other hand, if the Ni content exceeds 8%, the material cost increases. For this reason, Ni content was limited to 3 to 8% of range. In addition, Preferably it is 4 to 7%.

Mo: 1-4%
Mo is an element that improves resistance to stress corrosion cracking, further resistance to sulfide stress corrosion cracking, and pitting resistance. In order to obtain such an effect, the Mo content is set to 1% or more. On the other hand, if the Mo content exceeds 4%, ferrite tends to be 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, Mo was limited to 1-4%. In addition, Preferably it is 1.5 to 3.0%.

V: 0.02-0.10%
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 stress corrosion resistance of welds. It has the effect of improving crackability. In order to obtain such an effect, the V content is set to 0.02% or more. If the V content exceeds 0.10%, weld crack resistance and toughness deteriorate. For this reason, the V content is 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 improvement of hot workability through the form control of inclusions. In order to obtain such an effect, the Ca content is set to 0.0005% or more. On the other hand, when the Ca content exceeds 0.010%, Ca tends to exist as coarse inclusions, so that the corrosion resistance is deteriorated and the toughness is significantly reduced. For this reason, Ca content was limited to 0.0005 to 0.010% of range. In addition, Preferably it is 0.0005 to 0.0030%.

  The above-described components have a basic composition. In addition to the above basic composition, if necessary, Ti: 0.15% or less, Nb: 0.10% or less, Zr: 0.10% or less. One or two or more selected and / or Cu: 4.0% or less and 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 all form carbides in the same manner as V It is an element, and one or two or more can be selected and contained as necessary. Ti, Nb, and Zr all have strong carbide-forming ability compared to Cr, and suppress the re-precipitation of C as a Cr carbide on the prior austenite grain boundaries as a Cr carbide, resulting in a grain boundary in the weld zone. It has the effect of improving the stress corrosion cracking property. Further, Ti, Nb, and Zr carbides are difficult to dissolve even when heated to high temperatures by welding heat, and the formation of solute C is suppressed. There is also an effect of improving the field stress corrosion cracking property. In order to obtain such an effect, the Ti content is preferably 0.03% or more, the Nb content is 0.03% or more, and the Zr content is preferably 0.03% or more. On the other hand, when the Ti content exceeds 0.15%, the Nb content exceeds 0.10%, and the Zr content exceeds 0.10%, the weld crack resistance and toughness deteriorate. 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, the Ti content is 0.03 to 0.12%, the Nb content is 0.03 to 0.08%, and the Zr content is 0.03 to 0.08%.

One or two types selected from Cu: 4.0% or less and W: 4.0% or less Cu and W are both required for steel pipes for line pipes that transport natural gas containing CO _2. It is an element that improves the carbon dioxide gas corrosion resistance, which is a characteristic, and can be selected from one or two as required.

  Cu is an austenite forming element as well as improving the carbon dioxide gas corrosion resistance, and effectively acts to ensure a stable martensite structure in a low carbon region. In order to obtain such an effect, the Cu content is preferably 0.5% or more. On the other hand, even if the Cu 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 make Cu content 4.0% or less. In addition, More preferably, it is 1.0 to 3.0%.

  W is an element that improves the resistance to carbon dioxide gas corrosion, and further improves the resistance to stress corrosion cracking, further the resistance to sulfide stress corrosion cracking, and the resistance to pitting corrosion. In order to obtain these effects, the W content is preferably 0.5% or more. On the other hand, if the W content exceeds 4.0%, ferrite is likely to be 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 4.0% or less. In addition, More preferably, it is 1.0 to 3.0%.

  The balance other than the components described above consists of Fe and inevitable impurities.

  The hot-rolled steel sheet having the above component composition is obtained by hot rolling a steel material having the above component composition. Although the specific production method and production conditions for obtaining the hot-rolled steel sheet are not particularly limited, the molten steel having the above-described component composition is melted by a normal melting method such as a converter, an electric furnace, a vacuum melting furnace, It is preferable that a steel material such as a slab is formed by a known method such as a continuous casting method or an ingot-bundling rolling method, and a known hot working is applied to the steel material to obtain a hot-rolled steel plate having a predetermined size. Usually, the hot-rolled steel sheet obtained as described above is wound up and used in the next step.

<Annealing of hot-rolled steel sheet>
After the winding, the above hot-rolled steel sheet cooled to room temperature is subjected to an annealing treatment in which it is reheated to a temperature of 550 ° C. or higher and less than Ac1 transformation point + 20 ° C. and then cooled. The Ac1 transformation point is measured by a thermal expansion / shrinkage test using a cylindrical specimen taken from a hot-rolled steel sheet, and the temperature at which the gradient of sample expansion changes during the temperature rising process is the Ac1 point, that is, the austenite phase. Was defined as the temperature at which it begins to form.

  Since the steel of the component composition targeted in the present invention 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, the above annealing treatment is substantially a tempering treatment. Equivalent to.

  When reheated at a temperature of Ac1 transformation point + 20 ° C. or higher, austenite is partially formed, and martensite that is not tempered in the subsequent cooling process is formed, increasing the strength of the steel. Become. Further, if the reheating temperature is less than 550 ° C., a sufficient tempering effect cannot be obtained, and the strength of the steel is too high, so that it is difficult to make a pipe. For these reasons, the annealing temperature was set to 550 ° C. or higher and less than Ac1 transformation point + 20 ° C.

  About annealing holding time, it adjusts according to the plate | board thickness, coil outer diameter, etc. of a hot-rolled steel plate to be processed. It is usually sufficient to hold for 10 to 30 hours. Moreover, there is no restriction | limiting in particular also about the cooling method after annealing, Furnace cooling or standing to cool is enough.

  The hot-rolled steel sheet wound up after hot working is heated to a temperature of Ac3 point or higher, cooled to below 200 ° C at a cooling rate of air cooling or higher, and then reheated to a temperature of 550 ° C or higher and lower than the Ac1 transformation point. A process of heating and cooling at a cooling rate higher than air cooling, that is, a two-stage heat treatment corresponding to quenching and tempering may be performed. However, in the present invention, as described later, since the material characteristics can be adjusted by annealing of the entire pipe after pipe forming (annealing of the entire steel pipe), the heat treatment of the hot-rolled steel sheet is intended to enable pipe forming. Implementation of only one time is sufficient. Rather, if the heat treatment of the hot-rolled steel sheet is completed once, it is industrially advantageous from the viewpoint of manufacturing cost and lead time reduction.

<Pipe making>
Pipe making can be performed using, for example, a welded pipe manufacturing facility 1 shown in FIG. The hot-rolled steel sheet 2 continuously discharged by the uncoiler 10 is cut at the edge by an edge mirror 11 and passed through a breakdown roll 12, a cage roll 13, and a fin pass roll 14, and roll-formed into a tubular shape. An open tube 3 having a cross section is used. While pressurizing the open tube 3 with the squeeze roll 16, both end surfaces of the open tube 3 are brought into contact with each other, and the seam portion is penetrated and melted by a laser beam generated from a laser oscillator 15 which is a welding means to be welded and joined. The sizing roll 17 is used to make a constant-diameter roll to obtain a welded pipe 5. Thereafter, the welded tube is cut into a predetermined length by the cutting machine 18. In addition, the equipment used for manufacture of the welded pipe of the present invention may be a conventional welded pipe manufacturing equipment that is welded and joined by a laser beam.

  In the present invention, the reason for employing laser beam welding is as follows. Since the low carbon martensitic stainless steel targeted by the present invention contains a large amount of alloy components such as Cr, a large amount of high melting point oxide is likely to be generated in the molten pool during welding. For this reason, the produced high melting point oxide remains in the seam welded portion, and a welding defect called a penetrator tends to occur. In order to suppress the formation of the high melting point oxide, it is conceivable to shield the molten pool with an inert gas, but this method cannot completely prevent the formation of the high melting point oxide. Therefore, if laser beam welding is used, which has a high energy density and can minimize the weld pool region, the contact of the weld pool with the atmosphere is minimized, and the occurrence of the weld defect can be suppressed.

  In the above-mentioned tube forming method by laser beam welding, the open tube 3 is heated by heating means (induction heating coil or contact tip) using high frequency current applied in ERW (electric resistance welding) method or heating by electric resistance method. Alternatively, laser welding may be performed after preheating both ends in the width direction. In this case, the pipe making speed can be increased and the productivity is improved. Further, on the outer surface and the inner surface of the tube body 4, the weld is pressurized during welding to be extruded and solidified to form a raised portion (bead). It is preferable that the bead is smoothed by removing the inner and outer surfaces by cutting or the like.

<Annealing of welded pipe>
In the present invention, the entire pipe annealing treatment (treatment for annealing the entire steel pipe) is performed on the welded pipe (steel pipe). The purpose of annealing treatment of this welded pipe is to remove pipe-forming strain remaining in the welded pipe, to secure a suitable fraction of retained austenite, and to improve the intergranular stress corrosion cracking resistance of seam welds. It is in.

  Among the processing conditions, the temperature needs to be 550 ° C. or higher and less than Ac1 point + 50 ° C. If the temperature of the annealing treatment of the entire steel pipe is less than 550 ° C., the temperature is too low, and thus heating for a long time is required to remove the residual stress, which is not industrially useful. Further, when the treatment temperature is less than 550 ° C., the solid solution C in the seam welded portion is precipitated as carbides, and a Cr-deficient layer may be generated. On the other hand, when the processing temperature is Ac1 point + 50 ° C. or higher, most of the welded pipe is austenitized, and martensite that is not tempered in the subsequent cooling process is formed over almost the entire pipe. The mechanical characteristics cannot be obtained.

  Further, the holding time of the all-tube annealing treatment is not particularly limited, but is preferably 10 minutes or more from the viewpoint of homogenizing the entire tube. There is no restriction | limiting in particular about the cooling method after the said process, Any of hardening processing by water cooling, normalizing processing by standing_to_cool, and also furnace cooling may be sufficient. Moreover, you may perform the tempering process for the hardening process and the intensity | strength adjustment for the purpose of further refinement | miniaturization after performing the said process.

  It should be noted that the tensile properties in the pipe axis direction and the pipe circumferential direction of the welded pipe can be adjusted to a desired range by the above-described full pipe annealing.

<Low carbon martensitic stainless steel welded pipe>
In the low carbon martensitic stainless steel welded pipe of the present invention, the volume fraction of the retained austenite phase in the base metal part is 5% or more and less than 20%. When the volume fraction of the retained austenite phase is within this range, the SSC resistance is improved. Most of the remaining austenite phase is a martensite phase (tempered martensite phase), and the volume fraction of the martensite phase is more than 80%. In addition, you may include phases other than a retained austenite phase and a martensite phase in the range which does not impair the effect of this invention.

  Moreover, since the low carbon martensitic stainless steel welded pipe of the present invention is manufactured by the above-described manufacturing method, it is obtained by the yield stress YSL obtained by the tensile test in the pipe axis direction and the tensile test in the pipe circumferential direction. The yield stress YST is less than 700 MPa, and YST / YSL is 0.81 or more and less than 1.00.

  Eight types of slabs having the composition shown in Table 1 were hot-rolled and then air-cooled to room temperature to obtain a low-carbon martensitic stainless hot-rolled steel sheet (sheet thickness: 6 mm). This hot-rolled steel sheet was annealed under the conditions shown in Table 2. In order to make the rolling direction of the obtained hot-rolled steel sheet the tensile direction, a rectangular tensile test piece (Sheet type, 1/2 in. Wide, mark: 50 mm) is taken in accordance with ASTM standards, and a tensile test is performed. The tensile properties (tensile strength TS, elongation El) of the hot-rolled steel sheet were determined. If the total elongation value obtained in the tensile test was larger than the amount of strain expected to be applied in the subsequent pipe making, it was judged that pipe making was possible, and if it was smaller, pipe making was impossible. Table 2 shows the obtained tensile characteristics and the result of determination of whether or not pipe forming is possible.

  Next, the hot-rolled steel sheet determined to be pipe-made was used as a welded pipe (outer diameter: 219 mmφ) using the equipment shown in FIG. Laser welding was performed in a Ar gas atmosphere using a fiber laser, laser output: 20 kW, welding speed: 8 m / min. After seam welding using this laser, the beads on the inner and outer surfaces of the seam welded portion were each smoothed by cutting.

  The welded pipe was subjected to annealing treatment under the conditions shown in Table 2 and allowed to cool to obtain a product welded pipe. As a comparative example, it was set as the product pipe which performed annealing processing on conditions different from this invention, without implementing annealing processing with respect to one part welded pipe.

  From each of the obtained product pipes, the base material part is sub-compliant with the ASTM standard so that the pipe axis direction of the welded pipe is the tensile direction and the pipe circumferential direction of the welded pipe is the tensile direction. Size-size bar specimens (Small-size Specimen Proportional to Standard, standard point: 10.0 mm) are sampled and subjected to a tensile test. Tensile properties (yield strength) in the pipe axis direction and pipe circumferential direction of the base metal part YSL, YST). The obtained tensile properties are shown in Table 2.

  Further, a 10 mm square test piece was collected from the base part of the welded pipe of the product, and after grinding and polishing from the outer surface side until the test piece surface became flat, the austenite fraction was measured by an X-ray diffraction method.

  Further, test pieces were collected from the obtained product tubes as follows, and a sulfide stress corrosion cracking resistance test and an intergranular stress corrosion cracking test were performed.

First, with respect to the SSC resistance test, from each of the obtained product pipes, four points were set so that the longitudinal direction of the test piece was parallel to the pipe axis, and the base metal part or the seam welded part was located at the center in the width direction of the test piece. A bending stress corrosion test piece (thickness 2 mm × width 10 mm × length 75 mm) was collected and carried out. These test pieces are set on a 4-point bending jig and loaded with 100% stress on the axial yield stress YSL of the steel pipe base material, and immersed in a corrosive environment to check for cracks. Examined. Environmental conditions of the test, Atmosphere: 0.005MPa H ₂ S-CO ₂ balance _{(H 2} partial pressure of S is 0.005 MPa H ₂ S balance other than the gas _CO 2), the test solution: 25 wt% NaCl aqueous solution, The solution pH was 4.5, the solution temperature was 25 ° C., and the immersion time was 720 hours. The pH was adjusted by adding an acetic acid-sodium acetate aqueous solution. Evaluation was made by visually confirming that no cracking was observed in both the base metal part test piece and the seam welded part test piece, which had good SSC resistance, and cracking was observed in either one. The SSC resistance was determined to be poor “x”. Note that the pass / fail criterion was according to the method described in EFC17.

  Furthermore, for the IGSCC resistance test, from each of the obtained product tubes, the test piece (thickness 2 mm × width) was set so that the longitudinal direction of the test piece was parallel to the pipe axis and the seam weld was positioned in the center of the test piece width direction. 15 mm x 115 mm in length), and the test piece was subjected to heat treatment (heating at 450 ° C. for 600 seconds and then allowed to cool) simulating heating by heat transfer during circumferential PWHT, and then polished Thus, a test piece for a four-point bending stress corrosion cracking test was carried out.

The four-point bending stress corrosion cracking test was carried out by immersing in a corrosive environment after setting a 4-point bending imparting jig and applying a strain of 0.5%. The corrosive environment was a 5 mass% NaCl aqueous solution having a liquid temperature of 100 ° C., a CO ₂ partial pressure of 0.1 MPa, and a pH of 2.0. The test period was 720 hr. After the test, the cross section of the test piece was visually observed and observed for cracking with a 100-fold optical microscope, and the intergranular stress corrosion cracking resistance (IGGSCC resistance) was evaluated. The obtained results are shown in Table 2. “O” indicates that there is no crack, and “X” indicates that there is no crack. In addition, the judgment criteria of pass / fail was according to the method described in EFC 17.

  These results are also shown in Table 2. As is apparent from Table 2, the welded pipe of the present invention example made of low carbon martensitic stainless steel having the chemical composition defined in the present invention and manufactured under the heat treatment conditions defined in the present invention The moldability at the time was good, and the SSC resistance and IGSCC resistance were also good.

  In contrast, the chemical composition is within the range specified in the present invention, but when the heat treatment conditions of the hot-rolled steel sheet are out of the range of the present invention (No. 1-1, 1-2, 1-9). 2-1, 2-6, 4-6), and the strength of the steel strip was too high, making pipe making difficult.

  In addition, the chemical composition and the heat treatment conditions of the hot-rolled steel sheet are within the range defined in the present invention, but when the heat treatment conditions of the welded pipe are outside the scope of the present invention (No. 1-4, 1- 5, 1-8, 2-2, 2-3, 2-5, 4-2, 4-3, 4-5), insufficient removal of the tube-forming strain remaining in the welded pipe, lack of residual austenite, Since a Cr-depleted layer was formed in a part of the seam welded portion, the SSC resistance or the IGSCC resistance was not good due to any one or a plurality of factors. The removal of tube-forming strain and the formation of a Cr-deficient layer were confirmed as follows.

  Since the pipe is formed while being pulled in the L direction, the YS in the L direction is increased by work hardening after the pipe is formed. That is, the anisotropy of YS indicates tube-forming strain. If the anisotropy is reduced by annealing of the entire tube, that is, if YST / YSL is 0.81 or more, the tube-forming strain is reduced to the required level. It was determined that it was removed. On the other hand, when YST / YSL is 1.00 or more, when tensile stress is applied in the longitudinal direction of the steel pipe, thinning in the longitudinal direction has priority over reduced diameter in the circumferential direction, and buckling and fracture Cause. Therefore, YST / YSL is limited to less than 1.00.

  For some test pieces in which cracks were observed in the IGSCC resistance test, the prior austenite grain boundaries connected to the crack tips were observed and analyzed by FIB-TEM. From this analysis, it was confirmed that the Cr concentration was lowered in the vicinity of the grain boundary, that is, a Cr-deficient layer was formed.

DESCRIPTION OF SYMBOLS 1 Welded pipe manufacturing equipment 10 Uncoiler 11 Edge mirror 12 Breakdown roll 13 Cage roll 14 Fin pass roll 15 Laser oscillator 16 Squeeze roll 17 Sizing roll 18 Cutting machine 2 Hot-rolled steel sheet 3 Open pipe 4 Pipe body 5 Welded pipe

Claims (4)

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% Hereinafter, Al: 0.10% or less, Cr: 10-14%, Ni: 3-8%, Mo: 1-4%, V: 0.02-0.10%, Ca: 0.0005-0. Containing 010%, having a component composition consisting of the balance Fe and inevitable impurities,
The volume fraction of the retained austenite phase of the base metal part is 5% or more and less than 20%,
The yield stress YSL obtained by the tensile test in the pipe axis direction and the yield stress YST obtained by the tensile test in the pipe circumferential direction are both less than 700 MPa, and YST / YSL is 0.81 or more and less than 1.00. A low carbon martensitic stainless steel welded pipe with excellent resistance to sulfide stress corrosion cracking and intergranular stress corrosion cracking.   The component composition further includes, in mass%, one or more selected from Ti: 0.15% or less, Nb: 0.10% or less, Zr: 0.10% or less. The martensitic stainless steel welded pipe according to claim 1, wherein   The component composition further comprises, by mass%, a composition containing one or two selected from Cu: 4.0% or less and W: 4.0% or less. Item 3. The low-carbon martensitic stainless steel welded tube according to item 1 or 2. A method for producing a low-carbon martensitic stainless steel welded pipe according to any one of claims 1 to 3,
A low-carbon martensitic stainless hot-rolled steel sheet having the component composition according to any one of claims 1 to 3 is annealed once at a temperature of 550 ° C or higher and an Ac1 transformation point + 20 ° C, and then shaped into a tubular shape. Low carbon martensitic stainless steel that is welded with a laser beam with both opposing edges butted together, or heated by applying a high-frequency current or heating with an electric resistance method, and then welded with a laser beam with both edges butted together A method for manufacturing a welded pipe, comprising:
The steel pipe after the pipe forming is subjected to a single heat treatment for annealing the entire steel pipe at a temperature of 550 ° C. or higher and an Ac1 transformation point + 50 ° C., so that the volume fraction of the retained austenite phase in the base metal portion is 5% or higher. A method for producing a low-carbon martensitic stainless steel welded pipe excellent in sulfide stress corrosion cracking resistance and intergranular stress corrosion cracking resistance, characterized by adjusting to less than 20%.

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