JP6515340B2 - Oil well tube - Google Patents

Description

  The present disclosure relates to an oil well pipe, and more particularly, to an oil well pipe connected directly or via a coupling with another oil well pipe.

Conventionally, martensitic stainless steel has been widely used in oil well environments. Conventional well environments contain carbon dioxide (CO ₂ ) and / or chloride ions (Cl ⁻ ). Martensitic stainless steel (hereinafter referred to as 13% Cr steel) containing about 13% by mass of Cr has excellent corrosion resistance in such a conventional oil well environment.

In recent years, deep oil wells have been developed due to soaring crude oil prices. Deep wells are deep. And deep oil wells are highly corrosive and hot. More specifically, deep wells contain high temperature corrosive gases. The corrosive gas contains CO ₂ and / or Cl ⁻ and may further contain hydrogen sulfide gas. The corrosion reaction at high temperature is more severe than the corrosion reaction at normal temperature. Therefore, oil well steels used in deep wells are required to have higher strength and corrosion resistance than 13% Cr steels.

  Here, duplex stainless steel has a higher Cr content than 13% Cr steel. Therefore, duplex stainless steel has higher corrosion resistance than 13% Cr steel. The duplex stainless steel is, for example, 22% Cr steel containing 22% Cr or 25% Cr steel containing 25% Cr. However, duplex stainless steel is expensive because it contains many alloying elements. Therefore, there is a need for a stainless steel that has higher corrosion resistance than 13% Cr steel and is cheaper than duplex stainless steel.

  In response to this demand, stainless steels have been proposed that contain 15.5-18% Cr and have high corrosion resistance in hot oil well environments. JP 2005-336595 A (patent document 1) proposes a stainless steel pipe having high strength and having carbon dioxide gas corrosion resistance in a high temperature environment of 230 ° C. The chemical composition of this steel pipe contains 15.5 to 18% of Cr, 1.5 to 5% of Ni, and 1 to 3.5% of Mo, and Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 19.5, and further Cr + Mo + 0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N> 11.5. The metallographic structure of this steel pipe contains 10 to 60% of a ferrite phase and 30% or less of an austenite phase, with the balance being a martensite phase.

  WO 2010/050519 (Patent Document 2) has corrosion resistance in a high temperature carbon dioxide gas environment of 200 ° C., and further, the oil or gas well environmental temperature by temporarily stopping the recovery of crude oil or gas Proposes a stainless steel pipe having high resistance to sulfide stress corrosion cracking even when The chemical composition of this steel pipe contains more than 16% to 18% Cr, more than 2% to 3% Mo, 1 to 3.5% Cu, and 3 to 5% less Ni. Mn] × ([N]-0.0045) 0.001 0.001 is satisfied. The metal structure of this steel pipe contains 10 to 40% of a ferrite phase and 10% or less of a retained austenite phase by volume ratio, and the balance is a martensitic phase.

  WO 2010/134498 (patent document 3) proposes a high-strength stainless steel having excellent corrosion resistance in a high temperature environment and excellent SSC resistance at normal temperature. The chemical composition of this steel is more than 16% to 18% Cr, 1.6 to 4.0% Mo, 1.5 to 3.0 Cu, and more than 4.0 It contains Ni and satisfies Cr + Cu + Ni + Mo ≧ 25.5, and satisfies −8 ≦ 30 (C + N) +0.5 Mn + Ni + Cu / 2 + 8.2−1.1 (Cr + Mo) ≦ -4. The metallographic structure of this steel contains a martensite phase, 10 to 40% of a ferrite phase, and a retained austenite phase, and the ferrite phase distribution rate is higher than 85%.

  By the way, in the high Cr stainless steel containing 15.5 to 18% of Cr disclosed in these documents, low temperature toughness may be inadequate. Unexamined-Japanese-Patent No. 2010-209402 (patent document 4) proposes the high strength stainless steel pipe for oil wells excellent in low temperature toughness. This steel pipe contains 15.5 to 17.5% of Cr, and in the largest of the crystal grains in the microstructure, the distance between any two points in the crystal grain is 200 μm or less (in other words, Crystal grain size is 200 μm or less). Moreover, in WO 2013/179667 (patent document 5), the GSI value defined as the number of ferrite-martensitic grain boundaries existing per unit length of the line segment drawn in the thickness direction is the thickness It is described that it is possible to combine excellent corrosion resistance and low temperature toughness by having a structure of 120 or more in the center.

  A dual phase high Cr stainless steel containing 15.5 to 18% Cr has high strength in a high temperature environment as compared to 22% Cr steel and 25% Cr steel. For example, in a high temperature environment of 250 ° C., the strength of duplex stainless steel containing 15.5 to 18% of Cr is about 8% higher than the strength of 22% Cr steel or 25% Cr steel. Threaded joints used for oil well pipe connection are subjected to performance tests at loads as high as 90 to 95% of the yield strength of the pipe body. For this reason, the difference in strength of 8% is extremely large.

  The types of threaded joints used for oil well pipe connection are roughly classified into integral type and coupling type. In integral type, the oil well pipes are directly connected to each other. Specifically, an external thread provided on the outer periphery of the end of the other well is screwed into the internal thread provided on the inner periphery of the end of the one well, and the oil wells are connected with each other. In the coupling type, oil well pipes are connected via a coupling. Specifically, the oil well pipes are connected with each other by screwing an external thread portion provided on the outer periphery of the end of the oil well pipe on each of the female screw portions provided on the inner circumferences of both ends of the coupling.

  Generally, the end of the wellbore having an externally threaded portion is referred to as a pin because it includes an element to be inserted into the internally threaded portion. The end of the well bore or coupling in which the internally threaded portion is formed is referred to as a box because it includes an element for receiving the externally threaded portion.

  In deep oil wells, the well bottom is at a high temperature of more than 250 ° C., and the temperature difference between the well bottom and the well port is large. For this reason, high strength in a high temperature environment is required for the threaded joint of the oil well pipe used in deep oil wells. Even in the case of a threaded joint having high strength in a normal temperature environment, if the strength in a high temperature environment is low, eventually it is necessary to design the oil well according to the lower strength, resulting in waste.

  In addition, the production target of deep well is often natural gas. Therefore, the well joint of an oil well pipe used in a deep well is also required to have excellent sealing performance against high pressure gas.

  In order to ensure excellent sealing performance, there is known a technique in which a nose portion forming a tip portion of a pin is provided to a threaded joint in addition to a seal portion by metal-metal contact. In the metal-to-metal seal portion, the diameter of the pin seal surface is slightly larger than the diameter of the box seal surface (hereinafter this difference in diameter is referred to as an interference amount), and the seal surfaces are fitted together by fastening. Sometimes, the contact pressure is generated on each seal surface by the elastic recovery force that each seal surface tries to return to the original diameter, and the seal surfaces are in close contact with each other. The nose portion is configured not to interfere with the box, and amplifies the elastic recovery force of the pin seal surface. The amplification effect of the elastic recovery force improves the sealing performance of the threaded joint.

  Corrosion resistance, strength in high-temperature environment, and sealing by selecting a high-phase stainless steel containing 15.5 to 18% Cr as a material for oil well pipes and applying a nose to a threaded joint Performance can be secured.

JP 2005-336595 A International Publication No. 2010/050519 WO 2010/134498 JP, 2010-209402, A International Publication No. 2013/179667

  By the way, the sealing performance of the threaded joint of the oil well pipe, that is, the contact force of the seal portion is rapidly reduced as long as the periphery of the seal portion is macroscopically elastic if the temperature is uniform even under high temperature environment There is nothing to do. That is, the sealing performance does not rapidly decrease unless the area around the seal portion is plastically deformed macroscopically due to the decrease in yield stress due to high temperature. If the temperature around the threaded joint is uniform, thermal expansion and / or thermal contraction and change in strength are almost the same for the pin and the box, and the amount of interference that generates the contact force of the seal and the rigidity of the nose are almost the same. It is because it does not change.

  The temperature of deep well production is basically equal to the ground temperature at the bottom of the well. The temperature of the product decreases somewhat as the product is pumped up to the ground, but is still quite high when the product reaches the wellhead. As this high temperature product passes through the inside of the connected plurality of oil well tubes, a temperature difference occurs between the inner surface and the outer surface of the oil well tubes, particularly in the upper part of the oil wells. That is, in each oil well pipe, the phenomenon that the inner surface becomes hot due to the high temperature product passing inside, while the outer surface remains cold.

  Due to the above-mentioned temperature difference, in the threaded joint, a difference in strength and a degree of thermal expansion and / or thermal contraction may occur between the pin and the box. If these differences are large, the contact force of the seal may be significantly reduced and the sealing performance may be lost. The temperatures of the inner and outer surfaces of each oil well tube gradually become uniform as time passes, but once the contact force of the seal portion decreases and a leak occurs, a leak path is formed. The leak path does not completely close even if the seal portion comes in contact again. For this reason, it will be in the state which a leak produces continuously.

  An object of the present disclosure is to provide an oil well pipe capable of maintaining high sealing performance with other oil well pipes or couplings even when the inner surface becomes hot.

The oil well pipe according to the present disclosure is made of stainless steel. The oil well pipe is connected to another oil well pipe directly or via a coupling. In the stainless steel, the chemical composition is, by mass, C: 0.001 to 0.06%, Si: 0.05 to 0.5%, Mn: 0.01 to 2.0%, P: 0.03 % Or less, S: less than 0.005%, Cr: 15.5 to 18.0%, Ni: 2.5 to 6.0%, V: 0.005 to 0.25%, Al: 0.05% Hereinafter, N: 0.06% or less, O: 0.01% or less, Cu: 0 to 3.5%, Co: 0 to 1.5%, Nb: 0 to 0.25%, Ti: 0 to 0 .25%, Zr: 0 to 0.25%, Ta: 0 to 0.25%, B: 0 to 0.005%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, and REM: contains 0 to 0.05%. The stainless steel further contains one or two selected from the group consisting of Mo: 0 to 3.5% and W: 0 to 3.5% in a range satisfying the formula (1). The balance of stainless steel is Fe and impurities. The matrix structure has a volume fraction of 40 to 70% of a tempered martensite phase, 10 to 50% of a ferrite phase, and 1 to 15% of an austenite phase. A 1 mm × 1 mm microstructure image obtained by photographing a matrix tissue at a magnification of 100 × is arranged in an xy coordinate system with the thickness direction as the x axis and the length direction as the y axis, 1024 × 1024 When each pixel is expressed in grayscale, β defined by Expression (2) is 1.55 or more.
1.0 ≦ Mo + 0.5 W ≦ 3.5 (1)

Here, Mo and W are content (mass%) of Mo and W.

However, in Formula (2), Su is defined by Formula (3) and Sv is defined by Formula (4).

In Formula (3) and Formula (4), F (u, v) is defined by Formula (5).

  In equation (5), f (x, y) represents the gradation of the pixel at coordinates (x, y).

An oil well tube according to the present disclosure comprises a tube body and a pin. The pin is formed continuously on at least one end of the tube body. The pin is inserted into the box of the other oil well tube or the box of the coupling. The pin includes a nose portion, an external thread portion, a pin seal surface, and a pin shoulder surface. The nose portion constitutes the tip of the pin. The nose portion is the same as the inner diameter of the box in the fastened state (the adjacent box seal surface is expanded and deformed by the interference amount) in the fastened state (the adjacent pin seal surface is reduced and deformed by the interference amount) It has a smaller outer diameter. The male screw portion is formed on the outer periphery of the pin on the tube main body side than the nose portion. The pin seal surface is formed on the outer periphery of the pin between the nose portion and the male screw portion. The pin shoulder surface is formed on the tip end surface of the nose portion. The pin shoulder surface is inclined so that the outer peripheral side is positioned closer to the tip end of the pin than the inner peripheral side. The volume V (mm ³ ) of the gap between the nose portion and the box is L (mm) of the length in the tube axis direction of the nose portion, and D (mm) in the outer diameter and thickness at the center in the tube axis direction of the nose portion. Formula (6) is satisfied as and T (mm). The angle between the pin shoulder surface and the plane perpendicular to the tube axis is 8 ° or more and 21 ° or less when D, L, and T satisfy the equation (7).
5πL ² /T<V<0.4πLD (6)
TD / L> 50 (7)

  According to the present disclosure, it is possible to maintain high sealing performance between the wellbore and another wellbore or coupling even when the inner surface of the well is hot.

FIG. 1 is a microstructure image showing an example of a microstructure of stainless steel for oil well tubes according to the embodiment. FIG. 2 is a logarithmic frequency spectrum diagram obtained by two-dimensional discrete Fourier transform of the microstructure image of FIG. FIG. 3 is a photograph showing an example of a microstructure of stainless steel as a comparative example. FIG. 4 is a logarithmic frequency spectrum diagram obtained by performing two-dimensional discrete Fourier transform on the microstructure image of FIG. FIG. 5 is a microstructure image showing an example of the microstructure of the stainless steel for oil well tubes according to the embodiment. FIG. 6 is a logarithmic frequency spectrum diagram obtained by two-dimensional discrete Fourier transform of the microstructure image of FIG. FIG. 7 is a photograph showing an example of the microstructure of stainless steel as a comparative example. FIG. 8 is a logarithmic frequency spectrum diagram obtained by performing two-dimensional discrete Fourier transform on the microstructure image of FIG. FIG. 9 is a graph showing the relationship between β and the transition temperature of ductility and brittleness. FIG. 10 is a partial cross-sectional view showing the oil well pipe according to the embodiment. FIG. 11 is an enlarged sectional view of an end portion in the axial direction of the oil well pipe shown in FIG. 12 is a partial cross-sectional view of an oil well pipe having a different structure from the oil well pipe shown in FIG. FIG. 13 is an enlarged cross-sectional view of the axial end of an oil well pipe having a structure different from the structure shown in FIG. FIG. 14 is an enlarged cross-sectional view of the axial end of the oil well pipe having a structure different from the structure shown in FIGS. 11 and 13.

<1. About the material of oil well tube>
The oil well pipe according to the embodiment is made of stainless steel. Hereinafter, stainless steel used as a material of an oil well pipe concerning an embodiment is explained.

  The matrix structure of stainless steel includes a ferrite phase, and a tempered martensite phase and an austenite phase (hereinafter, referred to as a substantially martensitic phase). In the matrix structure, when the ferrite phase and the substantially martensitic phase extend along the rolling direction (longitudinal direction) and are arranged in layers, stainless steel is excellent in low temperature toughness. On the other hand, in the matrix structure, when the ferrite phase is irregularly distributed in a mesh shape, the low temperature toughness of the stainless steel is low. When stainless steel is a steel plate, the central axis of the steel plate extended by rolling is taken as the rolling direction. When stainless steel is a steel pipe, the central axis of the steel pipe is taken as the rolling direction.

  Here, the present inventors characterized that the ferrite phase and the substantially martensitic phase of stainless steel are elongated in the longitudinal direction, and two-dimensional discrete Fourier transform of the microstructure layer degree, the microstructure image. It has been found that both thickness direction and length direction can be evaluated and quantified. Hereinafter, this point will be described in detail.

  From a cross section perpendicular to any plate width direction of stainless steel, a 1 mm × 1 mm microstructure image at an observation magnification of 100 × is obtained by photographing in gray scale (256 gradations) using an optical microscope. An example of a microstructure image is shown in FIG. In FIG. 1, the microstructure image is arranged in the xy coordinate system. The y-axis in FIG. 1 is the length direction, and the x-axis is the thickness direction perpendicular to the length direction. In FIG. 1, the gray part is the substantial martensitic phase, and the white part located between the grains of the substantial martensitic phase is the ferrite phase. The microstructure image has M = 1024 pixels in the x-axis direction and N = 1024 pixels in the y-axis direction. That is, the microstructure image has the number of pixels of M × N = 1024 × 1024.

Two-dimensional data f (x, y) of each pixel (x, y) (x = 0 to M-1, y = 0 to N-1) are obtained from the microstructure image. f (x, y) represents the grayscale of the pixel at the coordinates (x, y). A two-dimensional discrete Fourier transform (2D DFT) defined by equation (5) is performed on the obtained two-dimensional data. M-1 = 1023 and N-1 = 1023.

  Here, F (u, v) is a two-dimensional frequency spectrum after two-dimensional discrete Fourier transform of the two-dimensional data f (x, y). The frequency spectrum F (u, v) is generally a complex number, and includes information on periodicity and regularity of the two-dimensional data f (x, y). In other words, the frequency spectrum F (u, v) contains information on the periodicity and regularity of the structure of the ferrite phase and the substantially martensitic phase in the microstructure image as shown in FIG.

  FIG. 2 is a logarithmic frequency spectrum view of the microstructure image shown in FIG. The horizontal axis in FIG. 2 is the v axis, and the vertical axis is the u axis. The frequency spectrum diagram of FIG. 2 is a black and white gradation image (gray scale image), and the maximum value of the frequency spectrum is white and the minimum value is black. The high part (white part in FIG. 2) of the frequency spectrum has, for example, a shape extending in the u-axis in the case of FIG. 2 and the boundary is not clear.

Here, in the frequency spectrum F (u, v) of the frequency spectrum diagram, the sum Su of the absolute values of the spectrum on the u-axis is defined by the equation (3). In the frequency spectrum F (u, v), the sum Sv of the absolute values of the spectrum on the v axis is defined by the equation (4). Furthermore, the ratio of Su to Sv is β defined by equation (2). Note that Su and Sv do not include the spectral intensity at coordinates (0, 0) in (u, v) space.

  Also, in the same manner, microstructure images of stainless steel shown in FIGS. Further, logarithmic frequency spectrum diagrams are obtained from each of the microstructure images shown in FIGS. 4 is a logarithmic frequency spectrum diagram of the microstructure image shown in FIG. 3, FIG. 6 is a logarithmic frequency spectrum diagram of the microstructure image shown in FIG. 5, and FIG. 8 is a diagram of the microstructure image shown in FIG. It is a logarithmic frequency spectrum figure. Hereinafter, the microstructure shown in FIG. 1 is referred to as “tissue 1”, the microstructure shown in FIG. 3 is referred to as “tissue 2”, the microstructure shown in FIG. 5 is referred to as “tissue 3”, and the microstructure shown in FIG. It is called four.

  Comparing the image of the structure 1 (FIG. 1) with the image of the structure 2 (FIG. 3), the structure 1 has a shape in which the ferrite phase and the substantially martensitic phase extend in the rolling direction (length direction) . Further, the structure 1 has a shorter lamination cycle (period aligned in the thickness direction) of the ferrite phase and the substantially martensitic phase than the structure 2 and is regular. Comparing the image of the tissue 1 with the image of the tissue 3 (FIG. 5), each of the tissue 1 and the tissue 3 has a shape in which each phase extends in the longitudinal direction. Furthermore, the tissue 3 has a short stacking cycle and is regular like the tissue 1. Comparing the image of the tissue 3 with the image of the tissue 4 (FIG. 7), the tissue 3 has a shape in which each phase extends in the longitudinal direction more than the tissue 4. Furthermore, the tissue 3 has a shorter stacking cycle than the tissue 4 and is regular.

  Also, in each of the logarithmic frequency spectrum diagrams of each of the tissues 1 to 4, the white portion extends along the u-axis. However, in the tissue 1 and the tissue 4, the width in the v-axis direction of the white portion is narrower than that of the tissue 2 and the tissue 4. For β, Tissue 1 is 2.024, Tissue 2 is 1.458, Tissue 3 is 2.183, and Tissue 4 is 1.395. In short, as the β is lower, the white part becomes shorter in the u-axis direction and spreads in the v-axis direction.

The transition temperature of ductility and brittleness is -82 ° C for structure 1, -12 ° C for structure 2, -109 ° C for structure 3, and -19 ° C for structure 4. In addition, transition temperature is a result on the same conditions as the below-mentioned Example. FIG. 9 is a diagram showing the relationship between β and transition temperature (° C.). FIG. 9 was obtained by the following method. The chemical composition was within the scope of the present embodiment described later, and a plurality of stainless steels different in β were manufactured. A low temperature toughness evaluation test described later was conducted on each stainless steel to obtain a transition temperature, and FIG. 9 was created. The straight line in FIG. 9 is a line obtained by the least squares method from all the plots in FIG. 9, and R ² is a correlation function.

  Thus, it was found that as the value of β increases, the low temperature toughness tends to be excellent. From the above, β can be considered to be an index of the stratification degree.

  The present inventors have completed stainless steel used for the oil well pipe according to the embodiment based on the above-mentioned findings. Hereinafter, the stainless steel will be described.

The stainless steel for oil well tubes according to the embodiment has a chemical composition of, by mass%, C: 0.001 to 0.06%, Si: 0.05 to 0.5%, Mn: 0.01 to 2.0 %, P: 0.03% or less, S: less than 0.005%, Cr: 15.5 to 18.0%, Ni: 2.5 to 6.0%, V: 0.005 to 0.25% Al: 0.05% or less, N: 0.06% or less, O: 0.01% or less, Cu: 0 to 3.5%, Co: 0 to 1.5%, Nb: 0 to 0.25 %, Ti: 0 to 0.25%, Zr: 0 to 0.25%, Ta: 0 to 0.25%, B: 0 to 0.005%, Ca: 0 to 0.01%, Mg: 0 -0.01% and REM: containing 0-0.05%. Furthermore, one or two selected from the group consisting of Mo: 0 to 3.5% and W: 0 to 3.5% are contained in the range satisfying the formula (1). The balance consists of Fe and impurities. The matrix structure has a volume fraction of 40 to 70% of a tempered martensite phase, 10 to 50% of a ferrite phase, and 1 to 15% of an austenite phase. A 1 mm × 1 mm microstructure image obtained by photographing a matrix tissue at a magnification of 100 × is arranged in an xy coordinate system with the thickness direction as the x axis and the length direction as the y axis, 1024 × 1024 When each pixel is expressed in grayscale, β defined by Expression (2) is 1.55 or more.
1.0 ≦ Mo + 0.5 W ≦ 3.5 (1)

Here, Mo and W are content (mass%) of Mo and W.

However, in Formula (2), Su is defined by Formula (3) and Sv is defined by Formula (4).

In Formula (3) and Formula (4), F (u, v) is defined by Formula (5).

  In equation (5), f (x, y) represents the gradation of the pixel at coordinates (x, y).

  In this stainless steel, the transition temperature of ductility and brittleness is −30 ° C. or less because β is 1.55 or more. As a result, this stainless steel is excellent in low temperature toughness. Furthermore, this stainless steel has high strength, and is excellent in SCC resistance at high temperature and SSC resistance at normal temperature.

  The chemical composition of the above stainless steel contains one or two selected from the group consisting of Cu: 0.2 to 3.5% and Co: 0.05 to 1.5% by mass%. It is also good.

  The chemical composition of the above stainless steel is, in mass%, Nb: 0.01 to 0.25%, Ti: 0.01 to 0.25%, Zr: 0.01 to 0.25%, and Ta: 0. You may contain 1 type, or 2 or more types selected from the group which consists of 01 to 0.25%.

  The chemical composition of the above stainless steel is, in mass%, B: 0.0003 to 0.005%, Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, and REM: 0. You may contain 1 type, or 2 or more types selected from the group which consists of 0005-0.05%.

[Chemical composition]
The stainless steel for oil well tubes according to the embodiment has the following chemical composition. Hereinafter, “%” relating to an element means mass%.

C: 0.001 to 0.06%
Carbon (C) enhances the strength of the steel. However, if the C content is too high, the hardness after tempering becomes too high, and the SSC resistance decreases. Furthermore, in the chemical composition of the present embodiment, the Ms point decreases as the C content increases. Therefore, as the C content increases, austenite tends to increase and the yield strength tends to decrease. Therefore, the C content is 0.06% or less. The C content is preferably 0.05% or less, more preferably 0.03% or less. Further, in view of the cost of decarburizing treatment in the steel making process, the C content is 0.001% or more. The C content is preferably 0.003% or more, more preferably 0.005% or more.

Si: 0.05 to 0.5%
Silicon (Si) deoxidizes the steel. However, if the Si content is too high, the toughness and hot workability of the steel are reduced. If the Si content is too high, the amount of ferrite formed further increases, and the yield strength tends to decrease. Therefore, the Si content is 0.05 to 0.5%. The Si content is preferably less than 0.5%, more preferably 0.4% or less. The Si content is preferably 0.06% or more, more preferably 0.07% or more.

Mn: 0.01 to 2.0%
Manganese (Mn) deoxidizes and desulfurizes the steel to enhance hot workability. If the Mn content is too low, the above effects can not be obtained effectively. On the other hand, if the Mn content is too high, austenite tends to remain excessively during quenching, making it difficult to secure the strength of the steel. Therefore, the Mn content is 0.01 to 2.0%. The Mn content is preferably 1.0% or less, more preferably 0.6% or less. The Mn content is preferably 0.02% or more, more preferably 0.04% or more.

P: 0.03% or less Phosphorus (P) is an impurity. P reduces the SSC resistance of the steel. Therefore, the P content is preferably as small as possible. P content is 0.03% or less. The P content is preferably 0.028% or less, more preferably 0.025% or less. Also, although it is preferable to reduce the P content as much as possible, the extreme reduction leads to an increase in steelmaking costs. Therefore, the P content is preferably 0.0005% or more, and more preferably 0.0008% or more.

S: less than 0.005% Sulfur (S) is an impurity. S reduces the hot workability of the steel. Therefore, it is preferable that the S content be as small as possible. The S content is less than 0.005%. The S content is preferably 0.003% or less, more preferably 0.0015% or less. Also, although it is preferable to reduce the S content as much as possible, the extreme reduction leads to an increase in steelmaking costs. Therefore, the S content is preferably 0.0001% or more, more preferably 0.0003% or more.

Cr: 15.5 to 18.0%
Chromium (Cr) enhances the corrosion resistance of the steel. Specifically, Cr lowers the corrosion rate and enhances the SCC resistance of the steel. If the C content is too low, the above effect can not be obtained effectively. On the other hand, if the Cr content is too high, the volume fraction of ferrite phase in the steel increases and the strength of the steel decreases. Therefore, the Cr content is 15.5 to 18.0%. The Cr content is preferably 17.8% or less, more preferably 17.5% or less. The Cr content is preferably 16.0% or more, more preferably 16.3% or more.

Ni: 2.5 to 6.0%
Nickel (Ni) increases the toughness of the steel. Ni further enhances the strength of the steel. If the Ni content is too low, the above effect can not be obtained effectively. On the other hand, when the Ni content is too high, austenite is generated in a large amount, and as a result, the strength of the steel is reduced. Therefore, the Ni content is 2.5 to 6.0%. The Ni content is preferably less than 6.0%, more preferably 5.9% or less. The Ni content is preferably 3.0% or more, more preferably 3.5% or more.

V: 0.005 to 0.25%
Vanadium (V) increases the strength of the steel. However, if the V content is too high, the toughness decreases. Therefore, the V content is made 0.005 to 0.25%. The V content is preferably 0.20% or less, more preferably 0.15% or less. The V content is preferably 0.008% or more, more preferably 0.01% or more.

Al: 0.05% or less Aluminum (Al) deoxidizes the steel. However, if the Al content is too high, inclusions in the steel increase and the toughness of the steel decreases. Therefore, the upper limit is made 0.05%. The Al content is preferably 0.048% or less, more preferably 0.045% or less. The Al content is preferably 0.0005% or more, more preferably 0.001% or more.

N: 0.06% or less Nitrogen (N) enhances the strength of steel. However, if the N content is too high, austenite is excessively formed and inclusions in the steel also increase. As a result, the toughness of the steel is reduced. Therefore, the N content is 0.06% or less. The N content is 0.05% or less, more preferably 0.03% or less. Although it is preferable to reduce the N content as much as possible, extreme reduction leads to an increase in steelmaking costs. Therefore, the N content is preferably 0.001% or more, more preferably 0.002% or more.

O: 0.01% or less Oxygen (O) is an impurity. O reduces the toughness and corrosion resistance of the steel. Therefore, the O content is 0.01% or less. The O content is preferably less than 0.01%, more preferably 0.009% or less, still more preferably 0.006% or less. It is preferable to reduce the O content as much as possible, but extreme reduction leads to an increase in steelmaking costs. Therefore, the O content is preferably 0.0001% or more, more preferably 0.0003% or more.

Mo: 0 to 3.5%, W: 0 to 3.5%
Molybdenum (Mo) and tungsten (W) are mutually substitutable elements, and may contain both or only one. It is essential that Mo and W contain at least one. These elements enhance the SCC resistance of the steel. On the other hand, if the content of these elements is too high, the effect is saturated. Therefore, the Mo content is 0 to 3.5%, the W content is 0 to 3.5%, and one or two selected from the group consisting of Mo and W satisfy the formula (1) It is necessary to contain in the range. The Mo content is preferably 3.3% or less, more preferably 3.0% or less. The Mo content is preferably 0.01% or more, more preferably 0.03% or more. The W content is preferably 3.3% or less, more preferably 3.0% or less. The W content is preferably 0.01% or more, more preferably 0.03% or more.
1.0 ≦ Mo + 0.5 W ≦ 3.5 (1)

  The chemical composition of the stainless steel according to the present embodiment may contain the following selective elements. That is, the following elements may not be contained in the stainless steel according to the present embodiment. Moreover, only a part may be contained.

Cu: 0 to 3.5%, Co: 0 to 1.5%
Copper (Cu) and cobalt (Co) are mutually substitutable elements. These elements are selective elements. These elements increase the volume fraction of the tempered martensitic phase and increase the strength of the steel. Furthermore, Cu precipitates as Cu particles during tempering to further increase its strength. If the content of these elements is too low, the above effect can not be obtained effectively. On the other hand, if there is too much content of these elements, the hot workability of steel will fall. Therefore, the Cu content is 0 to 3.5%, and the Co content is 0 to 1.5%. Furthermore, in order to sufficiently obtain the above effects, one or two selected from the group consisting of Cu: 0.2 to 3.5% and Co: 0.05 to 1.5% may be contained. preferable. The Cu content is preferably 3.3% or less, more preferably 3.0% or less. The Cu content is preferably 0.3% or more, more preferably 0.5% or more. The Co content is preferably 1.0% or less, more preferably 0.8% or less. The Co content is preferably 0.08% or more, more preferably 0.1% or more.

Nb: 0 to 0.25%, Ti: 0 to 0.25%, Zr: 0 to 0.25% and Ta: 0 to 0.25%
Niobium (Nb), titanium (Ti), zirconium (Zr) and tantalum (Ta) are mutually substitutable elements. These elements are selective elements. These elements increase the strength of the steel. These elements improve the pitting resistance and SCC resistance of the steel. The above effect can be obtained if any of these elements is contained. However, if the content of these elements is too high, the toughness of the steel decreases. Therefore, the Nb content is 0 to 0.25%, the Ti content is 0 to 0.25%, the Zr content is 0 to 0.25%, and the Ta content is 0 to 0.25. %. Furthermore, to sufficiently obtain the above effects, Nb: 0.01 to 0.25%, Ti: 0.01 to 0.25%, Zr: 0.01 to 0.25%, and Ta: 0.. It is preferable to contain 1 type or 2 types selected from the group which consists of 01 to 0.25%. The Nb content is preferably 0.23% or less, more preferably 0.20% or less. The Nb content is preferably 0.02% or more, more preferably 0.05% or more. The Ti content is preferably 0.23% or less, more preferably 0.20% or less. The Ti content is preferably 0.02% or more, more preferably 0.05% or more. The Zr content is preferably 0.23% or less, more preferably 0.20% or less. The Zr content is preferably 0.02% or more, more preferably 0.05% or more. The Ta content is preferably 0.24% or less, more preferably 0.23% or less. The Ta content is preferably 0.02% or more, more preferably 0.05% or more.

Ca: 0 to 0.01%, Mg: 0 to 0.01%, REM: 0 to 0.05% and B: 0 to 0.005%
Calcium (Ca), magnesium (Mg), rare earth elements (REM) and boron (B) are mutually substitutable elements. These elements are selective elements. These elements improve the hot workability during production. If any of these elements is contained, the above effect can be obtained to some extent. However, if the content of Ca, Mg and REM is too high, it combines with oxygen to significantly reduce the cleanliness of the alloy and degrade the SSC resistance. Moreover, if there is too much B content, the toughness of steel will be reduced. Therefore, the Ca content is 0 to 0.01%, the Mg content is 0 to 0.01%, the REM content is 0 to 0.05%, and the B content is 0 to 0.005. %. In addition, in order to sufficiently obtain the above effects, Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, REM: 0.0005 to 0.05% and B: 0.0003. It is preferable to contain 1 type or 2 types selected from the group which consists of-0.005%. The Ca content is preferably 0.008% or less, more preferably 0.005% or less. The Ca content is preferably 0.0008% or more, more preferably 0.001% or more. The Mg content is preferably 0.008% or less, more preferably 0.005% or less. The Mg content is preferably 0.0008% or more, more preferably 0.001% or more. The REM content is preferably 0.045% or less, more preferably 0.04% or less. The REM content is preferably 0.0008% or more, more preferably 0.001% or more. The B content is preferably 0.0045% or less, more preferably 0.004% or less. The B content is preferably 0.0005% or more, more preferably 0.0008% or more.

  REM is a generic term for a total of 17 elements of scandium (Sc), yttrium (Y) and lanthanoids. In the present embodiment, the REM content means the total content of one or more of the 17 elements described above.

  The balance of the chemical composition of the stainless steel according to the present embodiment is Fe and impurities. The term "impurity" as used herein means an element mixed from ore or scrap used as a raw material when industrially manufacturing stainless steel, or an element mixed from environment of production process or the like.

[Microstructure]
The matrix structure of the stainless steel according to the present embodiment has a volume fraction of 40 to 70% of a tempered martensite phase, 10 to 50% of a ferrite phase, and 1 to 15% of an austenite phase. Hereinafter,% with respect to these volume fractions (fractions) of matrix tissue means volume%.

  The volume fraction of ferrite phase (ferrite fraction:%), the volume fraction of austenite phase (austenite fraction:%), and the volume fraction of tempered martensite phase (martensite fraction:%) in the matrix structure are as follows: taking measurement.

[Measuring method of ferrite fraction]
Take a sample from any position of stainless steel. The surface of the sample corresponding to the cross section of stainless steel (hereinafter referred to as the observation surface) is polished. The polished observation surface is etched using a mixed solution of aqua regia and glycerin. The portion white-corroded by etching is a ferrite phase, and the area ratio of this ferrite phase is measured by a point calculation method in accordance with JIS G0555 (2003). The area fraction measured is considered to be equal to the volume fraction of the ferrite phase, so this is defined as the ferrite fraction (%).

[Method of measuring austenite fraction]
The austenite fraction is determined using X-ray diffraction. A sample of 15 mm × 15 mm × 2 mm is taken from any position of stainless steel. Using samples, the X-ray intensities of the (200) and (211) planes of the ferrite phase (α phase), the (200) plane of the austenitic phase (γ phase), the (220) plane and the (311) plane Measure and calculate the integrated intensity of each surface. After the calculation, the volume fraction Vγ is determined using the following equation (8) for each combination (total 6 sets) of each face of the α phase and each face of the γ phase. An average value of the volume fraction Vγ of each surface is defined as an austenite fraction (%).
Vγ = 100 / {1+ (Iα × Rγ) / (Iγ × Rα)} (8)

  Here, Iα is the integral intensity of the α phase, Rγ is the crystallographic theoretical calculation value of the γ phase, Iγ is the integral intensity of the γ phase, and Rα is the crystallographic theoretical calculation value of the α phase .

[Measuring method of martensite fraction]
Of the matrix structure, the balance other than the ferrite phase and the austenite phase is defined as the volume fraction (martensite fraction) of the tempered martensite phase. That is, the martensite fraction (%) is a value obtained by subtracting the ferrite fraction (%) and the austenite fraction (%) from 100%.

[Β]
In the stainless steel of the present embodiment, β defined by the equation (2) is 1.55 or more. β is determined by the following method. The matrix structure is photographed at a magnification of 100 times from a cross section perpendicular to an arbitrary plate width direction of stainless steel (in the case of a steel pipe, a thick cross section parallel to the pipe axis). The obtained 1 mm × 1 mm microstructure image is disposed in an xy coordinate system in which the thickness direction is the x axis and the length direction is the y axis, and each pixel of 1024 × 1024 is represented in gray scale. Therefore, a microstructure image represented in gray scale (256 gradations) is obtained from a cross section of stainless steel in a plane including the thickness direction and the length direction. Further, using a two-dimensional discrete Fourier transform, β defined by equation (2) is determined from the microstructure image represented in gray scale.

However, in Formula (2), Su is defined by Formula (3) and Sv is defined by Formula (4).

In Formula (3) and Formula (4), F (u, v) is defined by Formula (5).

  In equation (5), f (x, y) represents the gradation of the pixel at coordinates (x, y).

  As described above, β and low temperature toughness have the relationship shown in FIG. In the stainless steel according to one embodiment of the present invention, when β determined from the matrix structure is 1.55 or more, the transition temperature of ductility and brittleness becomes −30 ° C. or less as shown in FIG. Thus, the stainless steel according to one embodiment of the present invention exhibits excellent low temperature toughness at the normally required -10 ° C. β is preferably 1.6 or more, more preferably 1.65 or more.

  From the above, the stainless steel according to the present embodiment has high strength, is excellent in SCC resistance at high temperature and SSC resistance at normal temperature, and has excellent low temperature toughness.

[Production method]
An example of a method of manufacturing stainless steel according to the present embodiment will be described. By hot rolling a steel material (slab, bloom, billet or other slab or billet) having the above-mentioned chemical composition in an appropriate temperature range as high as possible, a matrix structure with a β of 1.55 or more is obtained. can get. In this example, a method of manufacturing a stainless steel plate will be described as an example of a method of manufacturing a stainless steel.

  A steel material having the above-described chemical composition is prepared. The material may be a slab produced by continuous casting, or may be a plate produced by hot working a slab or an ingot.

The prepared material is charged into a heating furnace or a soaking furnace and heated. The heated material is hot-rolled to produce an intermediate material (steel material after hot rolling). At this time, the rolling reduction in the hot rolling process is 40% or more. Here, the rolling reduction (r:%) is defined by the following equation (9).
r = {1- (thickness of steel material after hot rolling / thickness of steel material before hot rolling)} × 100 (9)

  The steel material temperature (rolling start temperature) at the time of hot rolling is set to 1200 to 1300 ° C. The steel material temperature here means the surface temperature of the material. The surface temperature of the material is measured, for example, at the start of hot rolling. The surface temperature of the material is the average of the surface temperatures measured along the axial direction of the material. When the material is homogenized in a heating furnace, for example, at a heating temperature of 1250 ° C., the steel material temperature becomes substantially equal to the heating temperature, and becomes 1250 ° C. Furthermore, the steel material temperature (rolling completion temperature) at the end of hot rolling is preferably 1100 ° C. or more.

  In the case where a plurality of hot rolling processes exist during the manufacturing process, the rolling reduction means the cumulative rolling ratio of the hot rolling processes continuously performed on the steel material temperature of 1100 to 1300 ° C.

  When the steel material temperature is lower than 1100 ° C. during hot rolling, a large amount of wrinkles may be generated on the surface of the steel material due to the decrease in hot workability. Therefore, the one where the heating temperature of steel materials is higher is preferable. On the other hand, in order to increase the degree of layering, rolling at a high rolling rate is preferable.

  Quenching and tempering are performed on the sheet (intermediate material) after hot rolling. By performing hardening and tempering on the intermediate material, the yield strength of the stainless steel sheet can be made 758 MPa or more. Furthermore, the matrix structure has a tempered martensitic phase.

  Preferably, in the quenching step, the intermediate material is once cooled to a temperature near normal temperature. Then, the cooled intermediate material is heated to a temperature range of 850 to 1050.degree. The heated intermediate material is cooled with water or the like and quenched to produce a stainless steel plate. Preferably, in the tempering step, the intermediate material after quenching is heated to a temperature of 650 ° C. or less. That is, the tempering temperature is preferably 650 ° C. or less. When the tempering temperature exceeds 650 ° C., austenite increases in the steel and the strength tends to decrease. Preferably, in the tempering step, the intermediate material after quenching is heated to a temperature exceeding 500 ° C. That is, the tempering temperature is preferably a temperature above 500 ° C.

  According to the above manufacturing process, a stainless steel plate having β of 1.55 or more is manufactured. The stainless steel is not limited to the steel plate, and may have other shapes other than the steel plate. Preferably, the material is soaked at a temperature of 1200 to 1250 ° C. for a predetermined time, and then hot rolling is performed at a rolling reduction rate of 50% or more and a rolling finish temperature of 1100 ° C. or more. In this case, it is possible to obtain a stainless steel material having a high degree of layering while suppressing the occurrence of surface defects.

<2. About the structure of oil well pipe>
The present inventors repeated studies on the structure of the oil well tube and obtained the following findings.

  As described above, when the temperature difference between the inner surface and the outer surface of the oil well tube occurs and the degree of thermal expansion and / or thermal contraction and the difference in strength between the pin and the box occur, the sealing performance of the threaded joint May not be maintained. The nose is an effective technique for improving the sealing performance, but even if the pin is provided with a nose, the difference in the degree of thermal expansion and / or thermal contraction between the pin and the box and / or the strength If the difference is large, situations may occur where the sealing performance of the threaded joint is significantly impaired.

  Specifically, when the inner surface of the pin is heated, the nose portion provided on the pin expands due to thermal expansion. The nose portion also tries to expand in the axial direction of the tube, but the end face abuts against the end face of the box and can hardly expand in the axial direction of the tube. For this reason, the nose portion only expands in the radial direction, and the expansion due to the original thermal expansion is promoted. When the outer peripheral surface of the nose comes into contact with the inner peripheral surface of the box due to thermal expansion, the amplification effect of the elastic recovery force of the pin seal surface by the nose is drastically reduced, and the interference amount of the seal is substantially reduced. It will The thermal expansion of the pin seal surface seems to increase the contact force of the seal at first glance, but in fact the contact force of the seal is significantly reduced and the sealing performance of the threaded joint is impaired.

  The present inventors thought that it is sufficient to define the gap between the nose portion and the box so as not to contact the box even when the nose portion of the pin is thermally expanded significantly in the radial direction. The present inventors have found that the amount of diameter expansion is the largest near the tip of the nose portion, and the diameter of the outer diameter of the nose portion, the length in the axial direction of the tube, and the thickness most greatly affect this amount of diameter expansion. I found out. Then, as a result of intensive investigations, the inventors of the present invention have found that the volume of the gap between the nose portion and the box is contained within a certain range defined by the outer diameter of the nose portion, the length in the tube axis direction, and the thickness. It was found that contact between the nose and the box due to inflation could be prevented.

In an oil well pipe according to an embodiment, the oil well pipe includes a pipe body and a pin. The pin is formed continuously on at least one end of the tube body. The pin is inserted into the box of the other oil well tube or the box of the coupling. The pin includes a nose portion, an external thread portion, a pin seal surface, and a pin shoulder surface. The nose portion constitutes the tip of the pin. The nose portion is the same as the inner diameter of the box in the fastened state (the adjacent box seal surface is expanded and deformed by the interference amount) in the fastened state (the adjacent pin seal surface is reduced and deformed by the interference amount) It has a smaller outer diameter. The male screw portion is formed on the outer periphery of the pin on the tube main body side than the nose portion. The pin seal surface is formed on the outer periphery of the pin between the nose portion and the male screw portion. The pin shoulder surface is formed on the tip end surface of the nose portion. The pin shoulder surface is inclined so that the outer peripheral side is positioned closer to the tip end of the pin than the inner peripheral side. The volume V (mm ³ ) of the gap between the nose portion and the box is L (mm) of the length in the tube axis direction of the nose portion, and D (mm) in the outer diameter and thickness at the center in the tube axis direction of the nose portion. Formula (6) is satisfied as and T (mm). The angle between the pin shoulder surface and the plane perpendicular to the tube axis is 8 ° or more and 21 ° or less when D, L, and T satisfy the equation (7).
5πL ² /T<V<0.4πLD (6)
TD / L> 50 (7)

In the above oil well pipe, the volume V (mm ³ ) of the gap between the nose portion and the other oil well pipe or coupling box is specified to be larger than 5πL ² / T and smaller than 0.4πLD. By defining the volume V in this manner, it is possible to prevent the contact between the nose and the box even when the inner surface of the well is heated and the nose thermally expands in the radial direction. Therefore, in addition to the fact that the amplification effect of the elastic recovery force of the pin seal surface is reduced can be prevented even when the inner surface of the oil well tube becomes high temperature, the interference amount of the seal portion is also substantially reduced. This can be prevented and as a result, high sealing performance can be maintained between the well and the other well or coupling.

  When the pin shoulder surface provided on the tip end face of the nose portion abuts on the innermost end face of the box, the radial component of the reaction force received from the innermost end face of the box amplifies the contact force of the pin seal face. The effect of the pin shoulder surface is exerted by securing the rigidity of the nose portion. That is, the effect of the pin shoulder surface is closely related to the length and thickness of the nose portion in the tube axis direction.

  In the above oil well tube, under the condition that the nose portion satisfies TD / L> 50, the angle between the pin shoulder surface and the surface perpendicular to the tube axis is specified to be 8 ° or more and 21 ° or less. Thereby, the pin shoulder surface sufficiently exerts an effect of amplifying the contact force of the pin seal surface.

  Hereinafter, the structure of the oil well pipe will be described in more detail with reference to FIGS. 10 to 14. The same or corresponding components in the drawings have the same reference characters allotted, and the same description will not be repeated. For convenience of explanation, in each drawing, the configuration may be simplified or schematically illustrated, or a part of the configuration may be omitted.

  FIG. 10 is a partial cross-sectional view showing a schematic configuration of an oil well pipe according to an embodiment. FIG. 10 shows a state in which one oil well pipe 10 is connected to another oil well pipe 10. The oil well tubes 10, 10 are connected to each other via a tubular coupling 20. The oil well pipes 10 and 10 and the coupling 20 are made of the above-described stainless steel.

  The oil well pipe 10 comprises a pin 11 and a pipe body 12. The pin 11 is formed continuously at one end of the tube body 12 in the tube axial direction. Although illustration is omitted, the pin 11 is continuously formed also at the other end of the tube body 12 in the tube axial direction. That is, both ends of the oil well pipe 10 are respectively constituted by the pins 11.

  The pin 11 is inserted into the box 21 of the coupling 20 and fastened with the box 21. The pin 11 includes a nose portion 111, a male screw portion 112, a pin seal surface 113, and a pin shoulder surface 114.

  The nose portion 111 is disposed closer to the tip end of the pin 11 than the male screw portion 112 and the pin seal surface 113. The nose portion 111 constitutes the tip of the pin 11. The nose portion 111 is in a fastened state (a state in which the adjacent pin seal surface is reduced in diameter due to the interference amount) and a box 21 in a similar fastened state (a state in which the adjacent box seal surface is expanded due to the interference amount). Have an outer diameter smaller than the inner diameter of For example, the outer peripheral surface of the nose portion 111 is formed of a concave surface which is recessed toward the inner peripheral side of the pin 11.

  The male screw portion 112 is formed on the outer periphery of the pin 11. The male screw portion 112 is disposed closer to the tube main body 12 than the nose portion 111 in the pin 11. A pin seal surface 113 is formed between the nose portion 111 and the male screw portion 112 on the outer periphery of the pin 11.

  The pin seal surface 113 is a generally tapered surface whose diameter decreases from the male screw portion 112 side to the nose portion 111 side. The pin seal surface 113 is formed, for example, by combining one or two or more types of circumferential surfaces of a rotary body obtained by rotating an arc around a tube axis CL or a circumferential surface of a truncated cone having the tube axis CL as an axis.

  The pin shoulder surface 114 is an annular surface formed on the tip end surface of the nose portion 111. The pin shoulder surface 114 is inclined such that the outer peripheral side is disposed closer to the tip end of the pin 11 than the inner peripheral side. That is, viewed from the cross section of the oil well pipe 10 cut at a plane including the pipe axis CL, the pin shoulder surface 114 has a shape in which the outer peripheral side is inclined in the screwing advancing direction of the pin 11 than the inner peripheral side.

  The coupling 20 has a box 21 at each end in the axial direction. Each box 21 is inserted with the pin 11 of the oil well pipe 10 and fastened with the pin 11. The oil well pipes 10 are connected by fastening one box 21 to the pin 11 of one oil well pipe 10 and fastening the other box 21 to the pin 11 of the other oil well pipe 10.

  Each box 21 comprises an internal thread 212, a box sealing surface 213 and a box shoulder surface 214.

  The female screw portion 212 is formed on the inner periphery of the box 21 corresponding to the male screw portion 112 of the pin 11. The female screw portion 212 is configured by a screw that engages with a screw constituting the male screw portion 112.

  The box seal surface 213 is formed on the inner periphery of the box 21 corresponding to the pin seal surface 113. The box seal surface 213 contacts the pin seal surface 113 when the pin 11 and the box 21 are fastened.

  The pin seal surface 113 and the box seal surface 213 have an interference amount. That is, the pin seal surface 113 has an outer diameter slightly larger than the inner diameter of the box seal surface 213. For this reason, the pin seal surface 113 and the box seal surface 213 contact with each other as the pin 11 is screwed into the box 21. In the fastened state, the pin seal surface 113 and the box seal surface 213 are closely fitted and form an interference fit. Thereby, the pin seal surface 113 and the box seal surface 213 form a seal portion by metal-metal contact.

  The box shoulder surface 214 corresponds to the pin shoulder surface 114 and is formed on the end surface of the box 21 in the tube axial direction. The box shoulder surface 214 contacts the pin shoulder surface 114 in the fastened state.

  The pin shoulder surface 114 and the box shoulder surface 214 are pressed in contact with each other by screwing in the pin 11 to the box 21. The pin shoulder surface 114 and the box shoulder surface 214 form a shoulder by such mutual pressure contact.

  FIG. 11 is a view showing a schematic configuration of an end portion in the axial direction of the well 10. As shown in FIG. 11, when the pin 11 is fastened to the box 21, the pin seal surface 113 and the box seal surface 213, and the pin shoulder surface 114 and the box shoulder surface 214 contact each other. The outer diameter of the nose portion 111 is smaller than the inner diameter of the portion of the box 21 facing the nose portion 111 in the fastened state, the outer peripheral surface of the nose portion 111 does not contact the inner peripheral surface of the box 21. Therefore, in the fastened state, a gap is generated between the outer peripheral surface of the nose portion 111 and the inner peripheral surface of the box 21.

The volume V (mm ³ ) of the gap between the nose portion 111 and the box 21 is D (mm) for the outer diameter of the nose portion 111, L (mm) for the tube axial length of the nose portion 111, Formula (6) is satisfy | filled by setting thickness to T (mm).
5πL ² /T<V<0.4πLD (6)

Further, when the outer diameter D, the length L, and the thickness T of the nose portion 111 satisfy the following equation (7), the angle θ formed between the pin shoulder surface 114 and the plane perpendicular to the tube axis CL satisfies the equation (10) Fulfill.
TD / L> 50 (7)
8 ° ≦ θ ≦ 21 ° (10)

As a precondition to derive the above equation (6), the thermal expansion coefficient is assumed to be 16 × 10 ^-6 which is a general value of steel, and the temperature difference between the inner and outer surfaces of the pin 11 is 300 ° C. at maximum. The lower limit value of the volume V, 5πL ² / T, is a volume calculation formula using (a prediction of) a change in the diameter of the tip of the nose portion 111 based on elastic shell theory, and is approximately described within the practical range ing. The upper limit value of the volume V, 0.4πLD, is the volume of the gap between the nose portion 111 and the box 21 when the average gap is 0.4 mm.

  The reason for setting the upper limit value to the volume V is that if the gap between the nose portion 111 and the box 21 is unnecessarily increased under limited dimensional constraints, other components are sacrificed. For example, by increasing the gap, it is necessary to reduce the thickness of the nose portion 111 or the box 21 or to make the pin seal surface 113 and the box seal surface 213 sharp. This not only results in a wasteful design but also reduces the sealing performance, strength and the like, so the average gap is set to 0.4 mm. More preferably, the average gap is less than 0.3 mm and the volume V is less than 0.3π LD.

  The volume V of the gap between the nose portion 111 and the box 21 is derived from the dimensions of each portion before the pin 11 and the box 21 are fastened. The volume V can be defined in the following manner.

  For example, using a three-dimensional shape measuring instrument (manufactured by Mitutoyo Co., Ltd.), the three-dimensional contour shape and dimensions of the pin lip portion before fastening are measured. The pin lip portion is a portion of the pin 11 other than the male screw portion 112, and includes a nose portion 111, a pin seal surface 113, and a pin shoulder surface 114. Also, using the same three-dimensional shape measuring device, the three-dimensional contour shape and dimensions of the box housing portion before fastening are measured. The box housing portion is a portion for receiving the pin lip portion in the box 21 and includes a portion corresponding to the nose portion 111, a box sealing surface 213, and a box shoulder surface 214.

  Based on the measurement results described above, for the pin lip portion and the box housing portion, a measured contour (shape / dimension) on a longitudinal cross section cut along a plane including the pipe axis CL is created. The pin seal surface 113, the box seal surface 213, and the pin shoulder surface are obtained by translating the measured profile of the pin lip portion inward in the radial direction by an amount equivalent to half of the seal interference amount and the measured profile of the box housing portion. A gap formed between the contour of the pin lip portion and the contour of the box housing portion when the 114 and the box shoulder surface 214 are arranged to be in contact with each other (that is, the deformation before the fastening) The volume of the rotation of the cross section of the gap between the nose portion 111 and the box 21), which is geometrically calculated without taking into account the angle around the pipe axis CL, is determined. This volume can be defined as the volume V of the gap between the nose portion 111 and the box 21.

  The length L of the nose portion 111 in the tube axial direction is the end of the tip end side (nose portion 111 side) of the pin lip portion at the contact portion Ac between the pin seal surface 113 and the box seal surface 213 obtained by the above geometric calculation. It is defined as the length along the tube axis direction from the tip to the tip of the pin lip portion (pin 11). The outer diameter D and the thickness T are defined as the outer diameter and the thickness at the center of the nose portion 111 in the tube axis direction (the position L / 2 from the tip of the pin 11), respectively.

  In the pin 11, components other than the nose portion 111 are not provided between the pin seal surface 113 and the pin shoulder surface 114.

As described above, in the oil well pipe 10 according to the present embodiment, the volume V of the gap between the nose portion 111 and the box 21 is 5πL, where D, L, and T are the outer diameter, length, and thickness of the nose portion 111, respectively. It is specified to be larger than ² / T and smaller than 0.4πLD. By setting the volume V of the gap within this range, it is possible to prevent the nose portion 111 and the box 21 from coming into contact when the pin 11 is thermally expanded in the radial direction due to the temperature rise of the inner surface of the wellbore 10 Can. Therefore, in addition to the fact that the amplification effect of the elastic recovery force of the pin seal surface 113 by the nose portion 111 is reduced, the interference amount of the seal portion can also be prevented from being substantially reduced. The high sealing performance between the well 10 and the coupling 20 is maintained.

  In the oil well pipe 10 according to the present embodiment, the rigidity of the nose portion 111 is sufficiently ensured by the outer diameter D, the length L, and the thickness T of the nose portion 111 satisfying TD / L> 50. Under this condition, an angle θ between the pin shoulder surface 114 and a plane perpendicular to the tube axis CL is set to 8 ° or more and 21 ° or less. According to this configuration, the pin shoulder surface 114 can sufficiently maintain the effect of amplifying the contact force of the pin seal surface 113 even when a temperature difference occurs between the inner surface and the outer surface of the wellbore 10. Therefore, the sealing performance between the oil well pipe 10 and the coupling 20 can be secured.

  The structure of the oil well tube according to the present disclosure is not limited to the above. For example, in FIG. 10, although one oil well pipe 10 is connected to another oil well pipe 10 via a coupling 20, the oil well pipes may be directly connected to each other.

  FIG. 12 is a partial cross-sectional view showing the oil well pipes 10A, 10A directly connected. Each well 10A has a pin 11 at one end in the axial direction. Each well 10A has a box 21 at the other end in the axial direction. The pin 11 of one oil well pipe 10A is inserted into the box 21 of the other oil well pipe 10A and fastened with the box 21. Thus, the oil well pipes 10A and 10A are directly connected without the coupling 20 (FIG. 10).

  The pin 11 may further have a pin seal surface 115 at the outer periphery at the end on the tube body 12 side. In this case, a box seal surface 215 corresponding to the pin seal surface 115 is provided on the inner periphery of the box 21. In the fastened state of the pin 11 and the box 21, the pin seal surface 115 and the box seal surface 215 form a metal-to-metal seal.

  The pin 11 may further have a pin shoulder surface 116 at the end face on the tube body 12 side. In this case, the box 21 is provided with a box shoulder surface 216 corresponding to the pin shoulder surface 116. In the fastened state, pin shoulder surface 116 and box shoulder surface 216 are in pressure contact with each other to form a shoulder.

  13 and 14 show a pin and / or a box having a structure other than the above. In the pin 11 shown in FIG. 11, the outer peripheral surface of the nose portion 111 is configured by the concave surface recessed toward the inner peripheral side of the pin 11. On the other hand, in the pin 11A shown in FIG. 13, the outer peripheral surface of the nose portion 111A is formed by the peripheral surface of a truncated cone whose axis is the pipe axis CL. That is, the outer peripheral surface of the nose portion 111A is substantially tapered and gradually reduces in diameter toward the tip end of the pin 11A. The outer peripheral surface of the nose portion 111A is not limited to that shown, but is a peripheral surface of a convex rotary body or a combination of a truncated cone and a convex rotary body which can be produced by rotating a convex curved surface on the outer diameter side around the pipe axis CL. May be configured by Even in this case, the outer peripheral surface of the nose portion 111A gradually reduces in diameter toward the tip end side of the pin 11A.

  In the pin 11B shown in FIG. 14, a pin sub-shoulder surface 117 is provided on the nose portion 111B. The pin sub-shoulder surface 117 is a tapered surface which reduces in diameter toward the tip of the pin 11B and has a relatively large inclination. The boundary between the pin sub-shoulder surface 117 and the pin shoulder surface 114 constitutes the tip of the pin 11B.

  The box 21 B has a box sub-shoulder surface 217 corresponding to the pin sub-shoulder surface 117. In the fastened state of the pin 11B and the box 21B, the pin sub-shoulder surface 117 and the box sub-shoulder surface 217 may or may not be in contact with each other. However, when the shape and size before fastening and deformation are not taken into consideration (when the pins before deformation and the shoulder surfaces of the box and the seal surfaces are geometrically matched), the pin sub-shoulder surface 117 It does not touch the box sub-shoulder surface 217.

  As mentioned above, although embodiment was described, this indication is not limited to the said embodiment, A various change is possible unless it deviates from the meaning.

  Hereinafter, the present disclosure will be described in more detail by way of examples. However, the present disclosure is not limited to the following examples.

<1. About the material of oil well tube>
Steels of steel types A to V having the chemical compositions shown in Table 1 were melted to produce ingots. The chemical compositions of steel types A to V are within the scope of the present embodiment. Each ingot was hot forged to produce a plate having a width of 100 mm and a height of 30 mm. The manufactured board | plate material was prepared as a steel material of numbers 1-36. In the chemical composition shown in Table 1, the content of each element is mass%, and the balance is Fe and impurities.

  The prepared plurality of materials were heated in a heating furnace. The heated material was extracted from the heating furnace, and hot rolling was performed immediately after the extraction, to produce intermediate materials of Nos. 1 to 36. The steel material temperature of each material at the time of hot rolling is shown in Table 2. In the present example, since the material was heated in a heating furnace for a sufficient time, the steel material temperature was equal to the heating temperature. The rolling ratio in hot rolling of each number is shown in Table 2.

  Quenching and tempering were performed on each of the intermediate materials of Nos. 1 to 36. The quenching temperature was 950.degree. The holding time (heat treatment time) at the quenching temperature was 15 minutes. The intermediate material was quenched by water cooling. The tempering temperatures were 550 ° C. for the intermediate materials of Nos. 1, 23 to 30, 32, and 33, and 600 ° C. for the intermediate materials of Nos. 2 to 22, 31, and 34 to 36. The holding time at the tempering temperature was 30 minutes. The steel plate of each number was manufactured by the above manufacturing process.

[Microstructure observation test]
The steel plates of Nos. 1 to 36 were cut in the longitudinal direction at the width center. The sample for microstructure observation was extract | collected from the center part of the steel plate among the cut surfaces (A longitudinal direction is made into y-axis and a thickness direction is made into x-axis). From the collected samples, the area ratio was measured by the method described above, and defined as the volume ratio of the ferrite phase. Furthermore, the volume fraction of the austenite phase was determined by the above-mentioned X-ray diffraction method. Furthermore, the volume fraction of the tempered martensite phase was determined by the above-mentioned method using the volume fraction of the ferrite phase and the volume fraction of the austenite phase.

  Furthermore, from an arbitrary position in the observation plane, a 1 mm × 1 mm microstructure image (for example, an image as shown in FIG. 1) at an observation magnification of 100 × was obtained. Using the obtained microstructure image, β of the steel plate of each number was calculated by the method described above.

[Yield strength evaluation test]
A round bar for tensile test was taken from the center in the thickness direction of each of the steel plates No. 1 to 36. The longitudinal direction of the round bar was a direction (L direction) parallel to the rolling direction of the steel plate. The diameter of the parallel part of the round bar was 6 mm, and the distance between marks was 40 mm. Based on JIS Z2241 (2011), a tensile test was performed on the collected round bars at room temperature to determine the yield strength (0.2% proof stress).

[Low temperature toughness evaluation test]
Charpy impact test was conducted as a low temperature toughness evaluation test. Full-size test pieces in accordance with ASTM E23 were collected from the central part in the thickness direction of steel plates No. 1 to 36. The longitudinal direction of the test piece was parallel to the plate width direction. The Charpy impact test was carried out in the temperature range of 20 ° C. to −120 ° C. using the collected test pieces, the absorbed energy (J) was measured, and the ductile brittle fracture surface transition temperature was determined.

[High temperature SCC resistance evaluation test]
The 4-point bending test piece was extract | collected from the steel plate of number 1-36. The length of the test piece was 75 mm, the width 10 mm, and the thickness 2 mm. The test piece was given a deflection due to 4-point bending. At this time, in accordance with ASTM G39, the amount of deflection of the test piece was determined so that the stress applied to the test piece was equal to the 0.2% offset resistance of the test piece. A 200 ° C. autoclave in which 30 bar (3.0 MPa) of CO ₂ and 0.01 bar (1 kPa) of H ₂ S were pressure sealed was prepared for each of the numbers 1 to 36. The deflection test piece was placed in an autoclave. The test piece was immersed in a 25 mass% NaCl solution for 720 hours in an autoclave. The solution was adjusted to pH 4.5 with a CH ₃ COONa + CH ₃ COOH buffer system containing 0.41 g / l CH ₃ COONa. The presence or absence of occurrence of stress corrosion cracking (SCC) was observed on the test specimen after immersion. Specifically, with respect to the test piece, the cross section of the portion to which the tensile stress was applied was observed using an optical microscope at a magnification of 100 times to determine the presence or absence of a crack. In Table 3, the absence of cracking is ○, the presence of cracking is x, and the case of ○ is more excellent in SCC resistance than the case of x. Furthermore, the corrosion loss was determined for the test pieces based on the weight before the test and the change in weight after the immersion. The annual corrosion rate (mm / Year) was calculated from the obtained corrosion weight loss.

[SSC resistance evaluation test at normal temperature]
Round bar specimens for NACE TM0177 METHOD A were collected from the steel plates numbered 1 to 36. The diameter of the test piece was 6.35 mm, and the length of the parallel portion was 25.4 mm. A tensile stress was applied in the axial direction of the test piece. At this time, in accordance with NACA TM0177-2005, the stress applied to the test piece was adjusted to be 90% of the measured yield stress of the test material. The test pieces were immersed for 720 hours in a 25 mass% NaCl solution saturated with 0.01 bar (1 kPa) of H ₂ S and 0.99 bar (0.099 MPa) of CO ₂ . The solution was adjusted to pH 4.0 with a CH ₃ COONa + CH ₃ COOH buffer system containing 0.41 g / l CH ₃ COONa. Furthermore, the temperature of the solution was adjusted to 25 ° C. The presence or absence of occurrence of sulfide stress cracking (SSC) was observed for the test specimen after immersion. Specifically, for each of the test pieces broken during the test and the test piece not broken among the test pieces of Nos. 1 to 36, the parallel portion is observed with the naked eye, and the crack or pitting corrosion is observed. It was judged whether or not it occurred. In Table 3, the case where no crack or pitting occurs is ○, the case where crack or pitting is occurring is x, and the case of ○ is superior to SSC in the case of x.

[Test results]
Table 3 shows the test results. In all the steel plates of Nos. 1 to 36, the volume fraction of the ferrite phase (α fraction), the volume fraction of the austenite phase (γ fraction) and the volume fraction of the tempered martensite phase (M fraction) It was in the range. The steel materials of Nos. 1 to 36 all had a yield strength of 758 MPa or more, an annual corrosion amount of 0.01 mm / Year or less, and were excellent in SCC resistance and SSC resistance.

  Each steel materials of number 1, 4, 7, 10, 12-16, and 19-36 had β of 1.55 or more. These steel materials have a transition temperature of −30 ° C. or less, and are excellent in low temperature toughness.

  Moreover, as for each steel materials of No. 2, 3, 5, 6, 8, 9, 11, 17, 18, β was less than 1.5 and transition temperature exceeded −30 ° C. These steels are inferior in low temperature toughness.

<2. About the structure of oil well pipe>
In order to confirm the effect by a structure mainly about the oil well pipe | tube which concerns on this indication, the numerical simulation analysis by the elastic plastic finite element method was implemented.

  Multiple models are prepared for oil well pipes with the basic structure shown in Fig. 10 and Fig. 11 and dimensions of 9-5 / 8 "53.5 # (outside diameter: 244.5 mm, thickness: 13.8 mm) did.

  Each model was tightened with a predetermined amount of interference and a tightening turn (+1/100 turn after shouldering) at room temperature (25 ° C.) using an axisymmetric elasto-plastic finite element method. After that, the series A test of 2011 version ISO 13679 is simulated, tensile and cyclic load are applied cyclically while applying internal pressure and external pressure alternately at normal temperature, then high temperature (250 ° C) is applied and the same load is applied. Then, the temperature was returned to normal temperature, and a finite element analysis was performed to load the same load, and the contact force of the seal portion (Ac) was evaluated (analysis 1).

  In addition, after fastening each model with a prescribed amount of interference and tightening turns (+1/100 turns after shouldering) at room temperature (25 ° C) using the axisymmetric elasto-plastic finite element method, production start / restart time Assuming rapid heating of the inner surface, only the pin (11) was heated to 250 ° C., and the contact force of the seal portion (Ac) was evaluated (analysis 2).

  Table 4 shows the dimensions and materials of each model, and the contact force of the seal portion (Ac) (hereinafter referred to as seal contact force).

Among the examples shown in Table 4 and Comparative Examples 1 to 4, in the examples, models satisfying all of the following formulas (6), (7) and (10) were used. In each equation, D, L, and T are the outer diameter, length, and thickness (in mm, respectively) of the nose portion (111), and V is the volume of the gap between the nose portion (111) and the box (21) The unit is mm ³ ) and θ is the angle between the pin shoulder surface (114) and the plane perpendicular to the tube axis (CL). D and T were the outer diameter and thickness of the nose portion (111) at a position moved by L / 2 in the axial direction of the tube from the tip of the pin (11).
5πL ² /T<V<0.4πLD (6)
TD / L> 50 (7)
8 ° ≦ θ ≦ 21 ° (10)

The material of the model according to the example is a stainless steel having a yield strength of 110 ksi and having the same chemical composition and matrix structure as the steel material of the above-mentioned No. 7 (Steel class C). The 0.2% yield stress of the stainless steel is about 760 N / mm ² , and the thermal expansion coefficient is 16.0 × 10 ⁻⁶ / ° C. In Table 4, for convenience, the stainless steel is described as 17Cr.

In the comparative example 1, although Formula (6), (7), and (10) are satisfy | filled, the material used the different model from the said Example. The material of the model according to Comparative Example 1 is a two-phase 25% Cr steel. The 25% Cr steel also has a 0.2% yield stress of about 760 N / mm ² and a thermal expansion coefficient of 16.0 × 10 ⁻⁶ / ° C.

In Comparative Examples 2 to 4, a model was used in which the material was the same as that of the above example but one of the formulas (6), (7) and (10) was not satisfied. In Comparative Examples 2 and 3, V is 5πL ² / T or less and does not satisfy Formula (6). The comparative examples 2 and 3 do not satisfy the formula (7). In the comparative example 4, θ is 0 °, which does not satisfy the equation (10).

  Among the results of the process of Analysis 1, each seal contact force is performed when (a) simultaneously applying internal pressure load and compression load at the last normal temperature and (b) simultaneously applying external pressure load and compression load. The relative values when the example is 1.00 are shown in Table 4. From these results, it is understood that only the examples maintain high seal contact force in any of the cases (a) and (b), and the examples have the best sealing performance.

  The seal contact force in Analysis 2 is shown in Table 4 as a relative value when the example is 1.00. As a result of analysis 2, it is understood that the example has the highest seal contact force at high temperature and the most excellent sealing performance in a high temperature environment.

10, 10A: oil well tube 11, 11A, 11B: pin 111, 111A, 111B: nose portion 112: male screw portion 113: pin seal surface 114: pin shoulder surface 20: coupling 21, 21B: box

Claims (4)

An oil well pipe made of stainless steel and connected directly or via a coupling to another oil well pipe,
The stainless steel is
The chemical composition is in mass%,
C: 0.001 to 0.06%,
Si: 0.05 to 0.5%,
Mn: 0.01 to 2.0%,
P: 0.03% or less,
S: less than 0.005%,
Cr: 15.5 to 18.0%,
Ni: 2.5 to 6.0%,
V: 0.005 to 0.25%,
Al: 0.05% or less,
N: 0.06% or less,
O: 0.01% or less,
Cu: 0 to 3.5%,
Co: 0 to 1.5%,
Nb: 0 to 0.25%,
Ti: 0 to 0.25%,
Zr: 0 to 0.25%,
Ta: 0 to 0.25%,
B: 0 to 0.005%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01% and REM: 0 to 0.05% are further contained,
Mo: 0 to 3.5% and W: 1 or 2 selected from the group consisting of 0 to 3.5% are contained in a range satisfying the formula (1),
The balance consists of Fe and impurities,
The matrix structure has, by volume fraction, 40 to 70% of a tempered martensite phase, 10 to 50% of a ferrite phase, and 1 to 15% of an austenite phase,
A 1 mm × 1 mm microstructure image obtained by photographing the matrix tissue at a magnification of 100 × is arranged in an xy coordinate system with the thickness direction as the x axis and the length direction as the y axis, 1024 × 1024 When each pixel of is expressed in gray scale, β defined by the equation (2) is 1.55 or more,
1.0 ≦ Mo + 0.5 W ≦ 3.5 (1)
Here, Mo and W are content (mass%) of Mo and W.

However, in Formula (2), Su is defined by Formula (3) and Sv is defined by Formula (4).

In Formula (3) and Formula (4), F (u, v) is defined by Formula (5).

In equation (5), f (x, y) represents the gradation of the pixel at coordinates (x, y).
The oil well pipe is
Tube body,
Pins continuously formed on at least one end of the pipe body and inserted into the box of the other oil well pipe or the box of the coupling;
Equipped with
The pin is
A nose portion which constitutes a tip portion and has an outer diameter smaller than the inner diameter of the box at the time of fastening;
An externally threaded portion formed on the outer periphery of the tube main body side than the nose portion;
A pin seal surface formed on an outer periphery between the nose portion and the male screw portion;
A pin shoulder surface formed on the tip end surface of the nose portion and inclined such that the outer peripheral side is positioned closer to the tip end of the pin than the inner peripheral side;
Including
The volume V (mm ³ ) of the gap between the nose portion and the box is L (mm) of the length in the tube axis direction of the nose portion, and the outer diameter and thickness at the center in the tube axis direction of the nose portion Formula (6) is satisfied as D (mm) and T (mm),
The oil well pipe whose angle which the said pin shoulder surface and a surface perpendicular | vertical to a pipe axis make is 8 degrees or more and 21 degrees or less, when D, L, T satisfy Formula (7).
5πL ² /T<V<0.4πLD (6)
TD / L> 50 (7) The oil well pipe according to claim 1, wherein
The stainless steel is
The chemical composition is, in mass%,
An oil well pipe containing one or two selected from the group consisting of Cu: 0.2 to 3.5%, and Co: 0.05 to 1.5%. The oil well pipe according to claim 1 or 2, wherein
The stainless steel is
The chemical composition is, in mass%,
Nb: 0.01 to 0.25%,
Ti: 0.01 to 0.25%,
An oil well pipe containing one or more selected from the group consisting of Zr: 0.01 to 0.25% and Ta: 0.01 to 0.25%. An oil well pipe according to any one of claims 1 to 3, which is:
The stainless steel is
The chemical composition is, in mass%,
B: 0.0003 to 0.005%,
Ca: 0.0005 to 0.01%,
An oil well pipe containing one or more selected from the group consisting of Mg: 0.0005 to 0.01%, and REM: 0.0005 to 0.05%.