JP2012047037A - Method for manufacturing stainless steel oil well pipe with superior extensibility - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a stainless steel oil well pipe that has both good corrosion resistance and extensibility and can also prevent leakage from joint parts.SOLUTION: A stainless steel pipe is used as an original pipe. After the original pipe is subjected to heat treatment for quenching and tempering, both end face sides of the pipe are expanded at an expansion rate of preferably 3% or more, and a groove for welding is formed at the expanded parts. Consequently, the steel pipe acquires a composition of tempered martensite as the main phase and 5% or more in volume ratio of austenite as the second phase, or further 5% or less of ferrite as another phase. The steel pipe particularly enables prevention of leakage at weld joint parts, improves in extensibility while being inserted in an oil well, shows superior corrosion resistance against CO, and possesses a high strength with the yield strength of 350 MPa or more. If the steel pipe's n value is 0.08 or more and the relation between the n value and the uniform stretch u-El satisfies n>0.007×(25-u-El), then it contributes to further increase in extensibility.

Description

  The present invention relates to a method for producing oil well stainless steel pipes suitable for oil well pipes used for oil wells and gas wells for crude oil or natural gas, and more particularly to improvement of pipe expandability.

  In order to lay an oil well pipe from the ground surface to an underground oil field, first, excavation from the ground surface to a predetermined depth is carried out, and a steel pipe called a casing is buried therein to prevent the collapse of the wall. Thereafter, the basement is further excavated from the tip of the casing to form a deeper well, and a new casing is buried through the previously buried casing. By repeating this operation, an oil well pipe (tubing) that finally reaches the oil field is laid. When excavating deep wells, many types of casings with different diameters are required. Since the diameter of the oil well pipe (tubing) through which crude oil and gas pass is determined, when drilling deep wells, it is necessary to increase the drilling area in the radial direction, and the cost required for drilling will increase become. For this reason, it is strongly desired to reduce the drilling cost of oil wells.

  In response to such a request, for example, Patent Document 1 and Patent Document 2 describe a method of expanding a casing (steel pipe) in a well by means of expansion or the like. According to the techniques described in Patent Document 1 and Patent Document 2, the diameter of each casing having a multistage structure can be kept small by expanding the casing (steel pipe) in the radial direction in the well. It is said that it is possible to reduce the well drilling cost by keeping the upper casing size small.

However, casings (steel pipes) using the techniques described in Patent Literature 1 and Patent Literature 2 are exposed to crude oil and gas while being subjected to processing by pipe expansion. Therefore, oil well steel pipes used for various purposes are required to have excellent corrosion resistance while being cold worked.
In recent years, in order to cope with soaring crude oil prices and the depletion of petroleum resources expected in the near future, deep oil fields that have not been excluded in the past, or highly corrosive sour gas that was once abandoned. Developments for rice fields etc. are flourishing on a global scale. Such oil fields and gas fields are generally extremely deep, and the atmosphere is high in temperature and has a severe corrosive environment containing CO ₂ , Cl ^−, and the like. Therefore, as steel pipes for oil wells used for mining such oil fields and gas fields, steel pipes having high strength and corrosion resistance are required. Conventionally, 13% Cr martensitic stainless steel pipes excellent in CO ₂ corrosion resistance have been used as oil well steel pipes used in such an environment.

However, due to further severe oil well environment, the conventional 13% Cr martensitic stainless steel pipe has a problem of insufficient corrosion resistance. For such a problem, Patent Document 3 proposes a stainless steel pipe for oil wells having excellent CO ₂ corrosion resistance even in a high-temperature corrosive atmosphere exceeding 180 ° C. containing CO ₂ , Cl ^{− and the} like. . The stainless steel pipe described in Patent Document 3 includes Cr: 14-18%, Ni: 5.0-8.0%, Mo: 1.5-3.5%, Cu: 0.5-3.5%, and Cr, Ni, Mo, Cu Steel pipe with a composition that satisfies the specific relationship consisting of C, and the specific relationship consisting of Cr, Mo, Si, C, Mn, Ni, Cu, and N, ensuring a high strength of yield strength of 654 MPa or more. It is said to be excellent in CO ₂ corrosion resistance.

JP 7-507610 International Publication No. WO98 / 00626 Republished patent WO2004 / 001082

However, the 13% Cr martensitic stainless steel pipe manufactured by ordinary quenching and tempering treatment has high strength and does not have sufficient pipe expansion as required for deep oil field development. There was a problem. For this reason, in order to apply the new technology of expanding pipes in oil wells, there has been a strong demand for stainless steel pipes for oil wells that are excellent in both CO ₂ corrosion resistance and pipe expanding properties.

  Furthermore, in an oil well, for example, stainless steel pipes for oil wells as described in Patent Document 3 are connected by a threaded joint. Recently, instead of threaded joints, attempts have been made to connect and expand oil well steel pipes with welded joints. However, the welded part may have reduced ductility and toughness due to the heat cycle during welding, and there is a concern that gas or crude oil leaks from the welded joint part due to cracking during pipe expansion.

The present invention has been made in view of the above-described problems of the prior art, and has excellent CO ₂ corrosion resistance even in a severe corrosive environment containing carbon dioxide (CO ₂ ), chlorine ions (Cl ⁻ ), and the like. Providing an inexpensive stainless steel pipe for oil wells that combines high performance and excellent pipe expansion that can withstand severe pipe expansion, and that prevents leakage of gas, crude oil, etc. from the joint after pipe expansion. The purpose is to do.

  In order to achieve the above-described object, the present inventors first focused on connecting oil well stainless steel pipes with a welded joint and intensively studied means for preventing leakage from the welded joint. As a result, it has been thought that the expansion of the welded joint portion is not required or the amount of the expanded pipe is reduced as compared with a portion other than the welded joint portion (the mother pipe portion). And, in order to make the welded joint part a smaller amount than the mother pipe part, it is a stainless steel pipe (mother pipe) with excellent pipe expandability, and after the production of the steel pipe, a stainless steel pipe that is a part to be welded in advance. It was conceived that both ends of the tube were expanded, and then a welding groove was formed on each of the both ends.

Furthermore, in order to obtain a stainless steel pipe (base pipe) with improved pipe expandability, the present inventors have intensively studied the influence of various factors on the pipe pipe expandability using 13% Cr steel as a basic composition. As a result, in order to secure further excellent tube expandability, it has been found that it is important to have a yield strength within a desired range and an n value equal to or greater than a predetermined value as a material factor.
Further, in order to ensure desired strength, corrosion resistance (CO ₂ corrosion resistance, etc.) and toughness, C, Si, Mn, Cr, or even Cu, Ni, Mo, V, Nb, and / or Ti, By adjusting Zr, B, W, and / or Ca within the appropriate content range, and further forming a proper structure, it has the desired strength, excellent corrosion resistance, particularly excellent CO ₂ corrosion resistance, It has been found that it can be a stainless steel pipe for oil wells that combines excellent pipe expandability.

Here, “excellent tube expansion” refers to the case where the limit tube expansion rate is 25% or more, and “excellent CO ₂ corrosion resistance” refers to 100 ° C. or more containing CO ₂ of 0.1 MPa or more. The case where it can be used without any problem in a severe corrosive environment.
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) Using a stainless steel pipe as a raw pipe, the pipe is subjected to quenching treatment and tempering treatment or tempering treatment as heat treatment, and then subjected to pipe expansion processing on the end face side of the raw pipe, and then the pipe expansion processing. The manufacturing method of the stainless steel pipe for oil wells excellent in pipe expandability characterized by performing the groove processing for welding to the site | part which gave.
(2) In (1), the quenching process is a process of reheating to a heating temperature of 800 ° C. or higher followed by cooling at a cooling rate of air cooling or higher, and the tempering process to a temperature exceeding the Ac ₁ transformation point. A method for producing a stainless steel pipe for oil wells, characterized by heating and cooling.

(3) In the oil well stainless steel pipe according to (1) or (2), the tempering treatment is a treatment of heating and cooling to a two-phase temperature range exceeding the Ac ₁ transformation point and 700 ° C. or less. Production method.
(4) In any one of (1) to (3), the amount of the tube expansion processing is the following equation (1): tube expansion rate = [{(plug outer diameter) − (element tube inner diameter)} / (element tube inner diameter) ] X 100 (%) (1)
(Here, the outer diameter of the plug: the outer diameter (mm) of the tool for expanding the pipe (plug), the inner diameter of the raw pipe: the inner diameter of the steel pipe end face before processing (mm))
A method for producing a stainless steel pipe for oil wells, characterized in that the expansion ratio defined by is 3% or more.

(5) In any one of (1) to (4), the stainless steel pipe is mass%, C: 0.25% or less, Si: 1.0% or less, Mn: 0.10 to 2.50%, P: 0.05% or less, S : 0.005% or less, Al: 0.05% or less, Cr: 10.5 to 18.0%, N: 0.09% or less, and having a composition comprising the balance Fe and inevitable impurities, a method for producing a stainless steel pipe for oil wells .
(6) In (5), in addition to the above composition, the following groups A to D: A group: Cu: 3.5% or less
Group B: Ni: 7.0% or less, Mo: 3.0% or less, V: 0.20% or less, Nb: one or more selected from 0.20% or less,
Group C: Ti: 0.30% or less, Zr: 0.20% or less, B: 0.0005-0.01%, W: 1.0% or less selected from 1.0% or less,
Group D: Ca: 0.0005 to 0.01%
The manufacturing method of the stainless steel pipe for oil wells characterized by setting it as the composition containing 1 group or 2 groups or more selected from these.

According to the present invention, even in a high temperature severe corrosive environment containing CO ₂ , Cl ⁻ , it has sufficient corrosion resistance, particularly excellent CO ₂ corrosion resistance, and includes oil pipes and joints in oil wells. Stainless steel pipes for oil wells with excellent pipe expandability that can withstand severe pipe expansion processing can be manufactured at low cost, and there are remarkable industrial effects.

It is sectional drawing which shows schematic shape of the stainless steel pipe for oil wells of this invention.

  The stainless steel pipe for an oil well of the present invention is a steel pipe as shown in FIG. 1 having both ends expanded and having a welding groove on the end face of the expanded portion (expanded portion). In the present invention, plugs having various outer diameters larger than the inner diameter of the pipe are respectively pushed into both end surfaces of the stainless steel pipe (element pipe), and expanded in advance so as to obtain a predetermined expansion ratio, thereby forming the expanded section. Further, a groove processing for welding is performed on the end face side of the portion to form a groove processing portion. Thereby, the amount of pipe expansion in the oil well in the weld joint can be reduced, and the pipe expansion work in the oil well can be reduced, and the characteristic deterioration of the welded joint due to the pipe expansion in the oil well can be prevented. Or it can reduce, and the leak from a welded joint part can be prevented.

  In addition, let the area | region where a pipe expansion process is performed be predetermined length from the end surface of a steel pipe to a pipe-axis direction. Here, the “predetermined length” means a length that allows a groove having a dimension and shape necessary for appropriately welding and joining the ends of steel pipes to the end face side of the expanded portion. In addition, the shape of the groove formed on both end faces does not need to be particularly limited, but depending on the welding method, it should be a shape that can form a welded joint so that the contents (crude oil, gas, etc.) do not leak Is preferred. In addition, the welding (circumferential welding) method for connecting the steel pipes is not particularly limited, and any of the usual circumferential welding methods such as the GMAW method, the SMAW method, and the butt welding can be applied.

Moreover, it is preferable that the pipe expansion process in the both end surface side of a stainless steel pipe shall be 3% or more by a pipe expansion rate. The expansion ratio is the following formula (1). Expansion ratio = [{(plug outer diameter)-(element tube inner diameter)} / (element tube inner diameter)] x 100 (%) (1)
(Here, the outer diameter of the plug: the outer diameter (mm) of the tube expansion tool (plug), the inner diameter of the raw pipe: the inner diameter of the steel pipe end face before processing (mm))
Defined by If the pipe expansion rate is less than 3%, it is difficult to cope with high pipe expansion in the oil well. As used herein, “high expansion in the oil well” refers to expansion with a pipe expansion rate of 25% or more in the oil well.

  The oil well stainless steel pipe of the present invention is preferably a steel pipe having a yield strength of 350 MPa or more. The pipe expansion performed in a state where the oil well steel pipe is inserted into the oil well is usually performed through the pipe for expansion into the stainless steel pipe. If the yield strength of the stainless steel pipe for oil wells is less than 350 MPa, problems such as buckling may occur when passing through the pipe for pipe expansion, and proper pipe expansion may not be performed. For this reason, in order to ensure sufficient pipe expandability, in the present invention, it is preferable to limit the yield strength of the oil well stainless steel pipe to 350 MPa or more. More preferably, it is 350 to 550 MPa.

  In addition, the oil well stainless steel pipe of the present invention is preferably a steel pipe having an n value of 0.08 or more in order to ensure high pipe expandability. The n value is an important material factor that affects the pipe expandability of the steel pipe according to the study by the present inventors. In order to ensure excellent pipe expandability, the n value is limited to 0.08 or more in the present invention. Is preferred. If the n value is less than 0.08, it is difficult to ensure the desired tube expandability. In addition, Preferably it is 0.10 or more. The “n value” here refers to a tensile test piece with the tube axis direction as the tensile direction, sampled in accordance with API regulations or JIS regulations, and using these tensile test specimens (JIS Z 2253) The value measured in accordance with

In addition, if the uniform elongation u-El of the oil well steel pipe is sufficiently large, it is possible to expand the tube with a high expansion ratio even if the n value is low. However, if the uniform elongation u-El is small, sufficient expansion is possible. Cannot be secured. According to the study by the present inventors, in order to ensure excellent tube expandability, it has an n value in the above range and a predetermined value related to the uniform elongation u-El, that is, the following equation (2):
n> 0.007 × (25−u-El) (2)
(Where n: n value, u-El: uniform elongation (%))
It has been found that it is preferable to have an n value that satisfies the above. If the n value cannot satisfy the formula (2), it becomes difficult to ensure the desired excellent tube expandability. Uniform elongation u-El is determined by API regulations or JIS regulations (JIS Z 2241) using tensile test specimens collected according to API regulations or JIS regulations (JIS Z 2241) with the tube axis direction as the tensile direction. The value measured by carrying out a tensile test according to the above shall be used.

Next, the preferable composition range of the stainless steel pipe for oil well of the present invention and the reason for limitation thereof will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.25% or less C is an important element related to the strength of the martensitic stainless steel pipe, but if it is contained in a large amount exceeding 0.25%, there is an increased risk of causing cracking during the production of the steel pipe. In addition, a large amount of C decreases the corrosion resistance. For this reason, C was limited to 0.25% or less. In addition, Preferably it is 0.01 to 0.20% of range.

Si: 1.0% or less
Si is an element useful as a deoxidizer in a normal steelmaking process. In order to acquire such an effect, it is desirable to contain 0.05% or more, but inclusion exceeding 1.0% reduces hot workability and further toughness. For this reason, it is preferable to limit Si to 1.0% or less. In addition, More preferably, it is 0.05 to 0.50%.

Mn: 0.10-2.50%
Mn is an element that has the effect of increasing the steel pipe strength by solid solution and contributes effectively to the improvement of the n value. In order to ensure the desired strength as a martensitic stainless steel pipe for oil wells, it is preferable to contain 0.10% or more, but a large content exceeding 2.50% adversely affects toughness and causes cracking during steel pipe production. Increase fear. For this reason, it is preferable to limit Mn to the range of 0.10 to 2.50%. In addition, More preferably, it is 0.10 to 1.00%.

P: 0.05% or less P is an element that decreases hot workability and deteriorates both CO ₂ corrosion resistance, CO ₂ stress corrosion cracking resistance, pitting corrosion resistance and sulfide stress corrosion cracking resistance. In the present invention, the content is desirably as small as possible, but an extreme reduction leads to an increase in manufacturing cost. Therefore, in the present invention, P can be implemented industrially at a relatively low cost and is hot workability, CO ₂ corrosion resistance, CO ₂ stress corrosion crack resistance, pitting corrosion resistance and sulfide stress corrosion crack resistance. It is preferable to limit the amount to 0.05% or less, which is a range that does not reduce the amount of water. In addition, More preferably, it is 0.02% or less.

S: 0.005% or less S is an element that remarkably deteriorates the hot workability in the pipe making process. In the present invention, its content is preferably as low as possible, but extreme reduction increases the manufacturing cost. Invite. Therefore, in the present invention, S is preferably limited to 0.005% or less, which is a range in which pipe production in a normal process is possible. More preferably, it is 0.003% or less.

Al: 0.05% or less
Al is an element that acts as a strong deoxidizer and also has an effect of combining with N to refine crystal grains. In order to ensure such an effect stably, it is desirable to contain 0.005% or more, but inclusion exceeding 0.05% adversely affects toughness. For this reason, Al is preferably limited to 0.05% or less. In addition, More preferably, it is 0.005-0.03%.

Cr: 10.5 to 18.0%
Cr is an important element for maintaining desired CO ₂ corrosion resistance and CO ₂ stress corrosion cracking resistance, and is contained in an amount of 10.5% or more from the viewpoint of ensuring corrosion resistance under the environment targeted by the present invention. However, if the content exceeds 18.0%, the ferrite becomes stable and the desired steel pipe strength cannot be secured. For this reason, it is preferable to limit Cr to the range of 10.5 to 18.0%. In addition, More preferably, it is 11.0 to 15.0%.

N: 0.09% or less N is an austenite generating element and contributes effectively to the improvement of steel pipe strength. It also has the effect of improving pitting corrosion resistance. In order to acquire such an effect, it is desirable to contain 0.005% or more. However, if it contains more than 0.09%, a large amount of various nitrides such as Cr nitrides are formed to reduce toughness and corrosion resistance. For this reason, it is preferable to limit N to 0.09% or less. In addition, More preferably, it is 0.01 to 0.06%.

Although the above-mentioned components are basic components, in addition to this basic composition, one or more groups selected from the following groups A to D can be selected and contained.
Group A: Cu: 3.5% or less Group A: Cu is an element having an action of strengthening a protective film to suppress the penetration of hydrogen into steel and improving the resistance to sulfide stress corrosion cracking. Can be contained depending on the case. Such an effect becomes remarkable when the content is 0.2% or more. However, when the content exceeds 3.5%, CuS precipitates at the grain boundary at a high temperature, thereby reducing the hot workability. For this reason, it is preferable to limit Cu to 3.5% or less. In addition, More preferably, it is 0.5 to 2.5%.

Group B: Ni: 7.0% or less, Mo: 3.0% or less, V: 0.20% or less, Nb: One or more selected from 0.20% or less Group B: Any of Ni, Mo, V, and Nb Has the effect of increasing the strength of the steel pipe, and can be selected as necessary and contained in one or more kinds.
Ni is an element that contributes effectively to improving toughness. However, when it contains a large amount of Cr, it stabilizes the martensite phase and contributes effectively to an increase in steel pipe strength. In order to acquire such an effect, it is desirable to contain 0.1% or more. However, if it exceeds 7.0%, the transformation point is excessively lowered, and the steel pipe strength is significantly reduced. For this reason, Ni is preferably 7.0% or less. In addition, More preferably, it is 0.1 to 6.5%.

  Mo is an element that contributes to an increase in steel pipe strength through improvement of hardenability, but is also an element that improves resistance to sulfide stress corrosion cracking in an environment where hydrogen sulfide is present. In order to acquire such an effect, it is desirable to contain 0.5% or more, but even if it contains more than 3.0%, the effect is saturated and an effect commensurate with the content cannot be expected. For this reason, it is preferable to limit Mo to 3.0% or less. In addition, More preferably, it is 0.5 to 2.5%.

V is an element that contributes to an increase in steel pipe strength through an improvement in hardenability, but is also an element that improves the resistance to sulfide stress corrosion cracking. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.20% reduces toughness. For this reason, it is preferable to limit V to 0.20% or less. In addition, More preferably, it is 0.02 to 0.15%.
Nb is an element that contributes effectively to increasing the strength and toughness of steel. Such an effect becomes remarkable when the content is 0.01% or more, but when the content exceeds 0.20%, the toughness is lowered. For this reason, Nb is preferably 0.20% or less. In addition, More preferably, it is 0.02 to 0.12%.

Group C: Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less, W: 3.0% or less selected from one or more types C Group: Ti, Zr, B, W are any Is an element having an action of increasing the strength of the steel pipe and improving the resistance to stress corrosion cracking, and can be selected as necessary and contained in one or more kinds. Such an effect becomes significant when Ti: 0.01% or more, Zr: 0.01% or more, B: 0.0005% or more, and W: 0.1% or more. On the other hand, the contents exceeding Ti: 0.3%, Zr: 0.2%, B: 0.01%, W: 3.0% respectively deteriorate the toughness. For this reason, it is preferable to limit to Ti: 0.3% or less, Zr: 0.2% or less, B: 0.01% or less, and W: 3.0% or less, respectively.

Group D: Ca: 0.0005 to 0.01%
Group D: Ca is an element having an effect of reducing the trapping ability of hydrogen by reducing the lattice strain of the matrix around the inclusions by fixing S as CaS and spheroidizing the S-based inclusions. Such an effect becomes remarkable when the content is 0.0005% or more. However, when the content exceeds 0.01%, CaO increases, and the resistance to CO ₂ corrosion and pitting corrosion decreases. For this reason, it is preferable to limit Ca to 0.0005 to 0.01% of range. In addition, More preferably, it is 0.001 to 0.005%.

The balance other than the components described above consists of Fe and inevitable impurities.
Next, a preferred structure of the stainless steel pipe of the present invention will be described.
The stainless steel pipe of the present invention has the above-described composition, and has a structure containing a tempered martensite phase as a main phase and a volume ratio of 5% or more of an austenite phase as a second phase, or further containing 5% or less of a ferrite phase. It is preferable to have. The “main phase” here refers to a phase having a structure fraction of 50% or more by volume.

  A desired high strength can be ensured by using a tempered martensite phase as the main phase. If the tempered martensite phase is less than 50% by volume, the steel pipe strength is lowered and the desired steel pipe strength cannot be ensured. In addition to the main phase, the second phase is preferably limited to a structure containing an austenite phase. It is preferable to adjust the content of the austenite phase so that the volume ratio is 5% or more. Thereby, desired high tube expansion property can be combined. The austenite phase content can be adjusted by adjusting the composition and heat treatment. Moreover, in addition to the austenite phase described above as the second phase, a ferrite phase of 0 to 5% by volume may be contained. When the ferrite phase exceeds 5% and becomes a large amount, it causes a decrease in tube expandability. For this reason, the tempered martensite phase is the main phase, and the second phase is limited to a structure containing an austenite phase of 5% or more by volume, or a ferrite phase of 5% or less. Is preferred. In addition, as the second phase other than that, quenched martensite having a volume ratio of 20% or less is allowed.

By having the above-described composition and the above-described structure, it becomes an oil well stainless steel pipe excellent in corrosion resistance, particularly excellent CO ₂ corrosion resistance, and pipe expandability.
Next, a preferred method for producing the stainless steel pipe of the present invention will be described using a seamless steel pipe as an example. In the present invention, the steel pipe is not limited to a seamless steel pipe, and needless to say, it may be a welded steel pipe (electrically welded steel pipe) made of a hot-rolled steel sheet.

  Molten steel having the above composition is melted by a normal melting method such as a converter, an electric furnace, a vacuum melting furnace, etc., and a steel pipe such as a billet by a normal method such as a continuous casting method or an ingot-bundling rolling method. It is preferable to use a raw material. Subsequently, these steel pipe materials are heated and hot-worked and piped using a normal Mannesmann-plug mill system or Mannesmann-Mandrel mill system manufacturing process to obtain seamless steel pipes of desired dimensions. After the pipe making, the seamless steel pipe is preferably cooled to a temperature of about room temperature at a cooling rate of about air cooling, as in the normal process.

  If it is a seamless steel pipe (stainless steel pipe) having the composition of the present invention as described above, after hot working, it is cooled to a temperature of about room temperature at a cooling rate of about air cooling, and a structure mainly composed of a martensite phase; However, in the present invention, it is preferable to further heat-treat the as-made stainless steel pipe (base pipe). As the heat treatment, it is preferable to perform quenching and tempering treatment or tempering treatment.

The quenching process is preferably a process of reheating to a heating temperature of 800 ° C. or higher followed by cooling to a temperature of 200 ° C. or lower, preferably room temperature, at a cooling rate of air cooling or higher. The holding at the heating temperature is preferably 5 min or more. When the heating temperature is less than 800 ° C., the structure cannot have a structure whose main phase is a tempered martensite phase.
Further, the tempering treatment is preferably performed by heating to a temperature exceeding the Ac ₁ transformation point and preferably cooling at a cooling rate of about the air cooling or air cooling. By setting the tempering temperature to a temperature exceeding the Ac ₁ transformation point, precipitation of the austenite phase or formation of a quenched martensite phase occurs. In place of the quenching and tempering process, the above-described tempering process alone may be used. By applying such heat treatment to the steel pipe (stainless steel pipe), the above-described structure can be stably secured.

Further, the tempering process described above may be a process of heating and cooling to a temperature in a two-phase region exceeding the Ac ₁ transformation point and 700 ° C. or less.
As described above, the seamless steel pipe (base pipe) that is preferably subjected to heat treatment is then subjected to pipe expansion on both end face sides. The pipe expansion is usually performed through a pipe expansion pipe in the steel pipe. In the present invention, plugs having various outer diameters larger than the inner diameter of the raw tube are respectively pushed into the both end surfaces of the raw tube with a press or the like, and are preferably expanded in advance to obtain a predetermined expansion rate of 3% or more. Forming part. In addition, let the area | region (expansion part) to which a pipe expansion process is performed be the length which can process the groove for welding of a desired dimension shape from the end surface of a base pipe (stainless steel pipe) to a pipe-axis direction.

Next, a welding groove having an appropriate length and an appropriate size and shape is processed in the expanded portion to form a groove processed portion, thereby obtaining the oil well stainless steel pipe of the present invention.
The present invention will be further described in detail based on examples.

Molten steel with the composition shown in Table 1 was melted in a vacuum melting furnace, fully degassed, and then made into a 100 kg steel ingot, which was then piped by a model seamless rolling mill for research, and seamless steel pipe (outer diameter 73 mmφ x meat The thickness was 7.0 mm. In addition, it was made to air-cool to room temperature after pipe making.
Next, each steel pipe was subjected to heat treatment under the conditions shown in Table 2. In some steel pipes, hot rolling was performed (as it was).

Next, a specimen for observing the structure was collected from the steel pipe described above, and the cross section in the tube axis direction was polished and corroded to be used for observing the structure. Tissue observation was performed using a scanning electron microscope. The tissue was imaged (5 visual fields or more), and the tissue fraction (volume%) of each phase was calculated using an image analyzer. The structural fraction of the austenite phase was measured by X-ray diffraction.
In addition, a tensile test piece (arc-shaped test piece: GL: 25.4mm) with the pipe axis direction as the tensile direction is cut out from the above steel pipe in accordance with the API regulations, and a tensile test is conducted in accordance with the API regulations. The tensile properties (yield strength YS, tensile strength TS, uniform elongation u-El) were determined. At the same time, the n value was determined in accordance with JIS Z 2253.

Further, a pipe expansion test piece (steel pipe: length 300 mm) was collected from the above steel pipe. Plugs having various outer diameters larger than the inner diameter of the expanded test specimen (steel pipe) are sequentially pushed into these expanded test specimens (steel pipe) by pressing, and the plug diameter at the time when the crack occurs is obtained. {(Plug outer diameter when crack occurred)-(Test material inner diameter)} / (Test material inner diameter)] × 100 (%)
The critical expansion rate was calculated at In addition, the outer diameter of the used plug was considered so that the expansion ratio might be 5%.

Moreover, the test material (tubular) was extract | collected from the above-mentioned steel pipe, and the pipe expansion process of 30% was given. A corrosion test piece (thickness 3 mm × width 30 mm × length 40 mm) was sampled from the test piece (tubular) subjected to the tube expansion process by machining. The corrosion test was performed by immersing the corrosion test piece in a test solution: 20% NaCl aqueous solution (liquid temperature: 100 ° C., CO ₂ gas atmosphere of 30 atm) held in the autoclave, and setting the immersion period to 2 weeks. The weight of the test piece after the corrosion test was measured, the weight loss of the test piece by the corrosion test was determined, and the corrosion rate was calculated. In addition, the surface of the test piece after the corrosion test was observed with a magnifying glass 10 times, and the presence or absence of pitting corrosion was investigated. The CO ₂ corrosion resistance was evaluated based on the obtained corrosion rate and the presence or absence of pitting corrosion.

  The obtained results are shown in Table 2.

All of the steel pipes satisfying the preferred range of the present invention have a high yield strength of 350 MPa or more, an excellent tube expansion ratio of 25% or more, and a corrosion rate of 0.127 mm / y or less. The steel pipe has excellent CO ₂ corrosion resistance, no pitting corrosion, and excellent CO ₂ corrosion resistance. On the other hand, a steel pipe that is outside the preferred range of the present invention has a yield strength of less than 350 MPa, a low critical expansion rate and a slight decrease in expandability, or a corrosion rate greater than 0.127 mm / y. Or pitting corrosion has occurred, and the CO ₂ corrosion resistance is slightly reduced.

Next, plugs having various outer diameters were pressed into the both end faces of the seamless steel pipe (elementary pipe) having the characteristics shown in Table 2 with a press so as to achieve the pipe expansion ratio shown in Table 3, and subjected to end face side pipe expansion processing. . This end face side pipe expansion process was performed on at least two steel pipes under the same conditions so that the steel pipes could be screwed together. The expansion ratio is the following equation (1). Expansion ratio = [{(plug outer diameter)-(element tube inner diameter)} / (element tube inner diameter)] x 100 (%) (1)
(Here, the outer diameter of the plug: the outer diameter (mm) of the tube expansion tool (plug), the inner diameter of the raw pipe: the inner diameter of the steel pipe end face before processing (mm))
It calculated using.

Next, the weld groove processing was applied to the end face of the pipe expansion processed portion subjected to the pipe expansion processing. The processed groove was formed into an I-shaped groove so that butt welding was possible between the steel pipes to be connected, and was processed into one end and the other end of the steel pipe, respectively.
The ends of the obtained steel pipe under the same conditions were butted together and welded. Next, including pipe expansion processing on the end face side, pipe expansion is performed so that the total expansion ratio is 25%, and then a water pressure test (pressure: 8 MPa) is performed to check for leaks from the welded joint. did.

  The obtained results are shown in Table 3.

  In any of the inventive examples, no leakage from the joint portion was observed. On the other hand, the steel pipe (comparative example) outside the scope of the present invention was found to leak from the welded joint.

Claims (6)

  A stainless steel pipe is used as a raw pipe, and the pipe is subjected to a quenching process and a tempering process or a tempering process as a heat treatment, and then subjected to a pipe expansion process on the end face side of the raw pipe, and then the pipe expansion process is performed. A method for producing a stainless steel pipe for oil wells with excellent pipe expandability, characterized by subjecting a welded portion to a groove for welding. The quenching process is a process of reheating to a heating temperature of 800 ° C. or higher and subsequently cooling at a cooling rate of air cooling or higher, and the tempering process is a process of heating and cooling to a temperature exceeding the Ac ₁ transformation point. The manufacturing method of the stainless steel pipe for oil wells of Claim 1 characterized by these. 3. The method for producing a stainless steel pipe for oil wells according to claim 1, wherein the tempering treatment is a treatment of heating and cooling to a two-phase temperature range of more than Ac ₁ transformation point and 700 ° C. or less. 4. The method for producing a stainless steel pipe for an oil well according to claim 1, wherein an amount of the pipe expansion processing is 3% or more as a pipe expansion rate defined by the following formula (1).
Tube expansion rate = [{(plug outer diameter)-(element tube inner diameter)} / (element tube inner diameter)] x 100 (%) (1)
Here, plug outer diameter: outer diameter of pipe expansion tool (plug) (mm)
Base tube inner diameter: Inner diameter before processing of steel pipe end face (mm) The stainless steel pipe is in mass%,
C: 0.25% or less, Si: 1.0% or less,
Mn: 0.10 to 2.50%, P: 0.05% or less,
S: 0.005% or less, Al: 0.05% or less,
The stainless steel pipe for oil wells according to any one of claims 1 to 4, comprising Cr: 10.5 to 18.0%, N: 0.09% or less, and having a composition comprising the balance Fe and inevitable impurities. Production method. The stainless steel pipe for oil wells according to claim 5, wherein, in addition to the composition, the composition further contains one group or two or more groups selected from the following groups A to D in mass%. Manufacturing method.
Group A: Cu: 3.5% or less,
Group B: Ni: 7.0% or less, Mo: 3.0% or less, V: 0.20% or less, Nb: one or more selected from 0.20% or less,
Group C: Ti: 0.30% or less, Zr: 0.20% or less, B: 0.0005-0.01%, W: 1.0% or less selected from 1.0% or less,
Group D: Ca: 0.0005 to 0.01%

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