SEAMLESS STEEL PIPE SUITABLE FOR USE IN A SOUR ENVIRONMENT

Abstract

A seamless steel tube is provided that has excellent SSC resistance in an acidic environment and can achieve excellent ultrasonic inspection accuracy. The seamless steel tube, as described herein, includes a base material and a decarburized layer formed on the surface of the base material. The chemical composition of the base material consists of, in % by mass, C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.01 to 1.00%, P: 0.0300% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.30 to 1.20%, Mo: 0.30 to 1.50%, Ti: 0.002 to 0.050%, V: 0.01 to 0.30%, Nb: 0.002 to 0.100%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0050% or less, and the remainder being Fe and Impurities. The base material has a yield strength of 655 MPa or more and a yield ratio of 85.0% or more. The decarburized layer has a depth of 150 µm or less.

Description

SEAMLESS STEEL PIPE SUITABLE FOR USE IN A SOUR ENVIRONMENT TECHNICAL FIELD The present description refers to a seamless steel pipe and, more particularly, to a seamless steel pipe suitable for use in an acidic environment. BACKGROUND OF THE TECHNIQUE Due to the deepening of oil and gas wells (hereafter, oil and gas wells will be collectively referred to as "oil wells"), there is a demand to improve the strength of steel pipes in oil wells. Specifically, 80 ksi grade (yield strength or elastic limit is 80 to less than 95 ksi, i.e., 552 to less than 655 MPa) and 95 ksi grade (yield strength or elastic limit is 95 to less than 110 ksi, i.e., 655 to less than 758 MPa) oil well steel pipes are widely used, and recently, 110 ksi grade (yield strength is 110 to less than 125 ksi, i.e., 758 to less than 862 MPa) and 125 ksi grade (yield strength is 125 to 140 ksi, i.e., 862 to 965 MPa) oil well steel pipes are also beginning to be requested. Most deep wells are located in a sour environment containing corrosive hydrogen sulfide. In this description, the term “acid environment” means an environment containing hydrogen sulfide and that is acidified. It should be noted that a sour environment may contain carbon dioxide. Steel oil well pipes for use in such sour environments are required not only to have high strength but also to have resistance to sulfide stress cracking (hereafter referred to as “SSC resistance”). The technology for improving the SSC strength of steel materials, typified by petroleum steel pipes, is described in Japanese patent application publication no. 62-253720 (patent literature 1), Japanese patent application publication no. 59-232220 (patent literature 2), Japanese patent application publication no. 6-322478 (patent literature 3), Japanese patent application publication no. 8-311551 (patent literature 4), Japanese patent application publication no. 2000-256783 (patent literature 5), Japanese patent application publication no. 2000-297344 (patent literature 6), Japanese patent application publication no. 2005-350754 (patent literature 7), and the national publication of international patent applications. Q / zL / n / Lznz / q / Yi No. 2012-519238 (patent literature 8) and Japanese patent application publication No. 2012 26030 (patent literature 9). Patent literature 1 proposes a method for improving the SSC strength of oil well steel by reducing impurities such as Mn and P. Patent literature 2 proposes a method for improving the SSC strength of steel by performing a double tempering to refine the grains. Patent literature 3 proposes a method for improving the SSC strength of a 125 ksi grade steel material by refining the steel's microstructure through induction heating heat treatment. Patent literature 4 proposes a method for improving the SSC strength of 110 to 140 ksi grade steel tubes by improving the steel's hardenability through a direct quenching process and by increasing the tempering temperature. Patent literature 5 and patent literature 6 each propose a method for improving the SSC strength of a low-alloy steel for tubular products for the petroleum industry, grade 110 to 140 ksi, by controlling the carbide shapes. Patent literature 7 proposes a method for improving the SSC strength of steel materials of grade 125 ksi or higher by controlling the dislocation density and hydrogen diffusion coefficient to desired values. Patent literature 8 proposes a method for improving the SSC strength of 125 ksi grade steel by subjecting a low-alloy steel containing 0.3 to 0.5% C to repeated tempering. Patent literature 9 proposes a method for controlling the shapes or number of carbides using a tempering process consisting of a two-stage heat treatment. More specifically, patent literature 9 proposes a method that improves the SSC strength of 125 ksi grade steel by suppressing the number density of M3C particles or large M2C particles. APPOINTMENT LIST PATENT LITERATURE Patent Literature 1: Japanese Patent Application Publication No. 62-253720 Patent Literature 2: Japanese Patent Application Publication No. 59-232220 Patent Literature 3: Japanese Patent Application Publication No. 6-322478 Patent Literature 4: Japanese Patent Application Publication No. 8-311551 Patent Literature 5: Japanese Patent Application Publication No. 2000-256783 Patent Literature 6: Japanese Patent Application Publication No. 2000-297344 Q / zL / n / Lznz / q / Yi Patent Literature 7: Japanese Patent Application Publication No. 2005-350754 Patent Literature 8: National Publication of International Patent Application No. 2012-519238 Patent Literature 9: Japanese Patent Application Publication No. 2012-26030 SUMMARY OF THE INVENTION TECHNICAL PROBLEM Each of the patent literatures 1 through 9 above describes a technique for improving the SSC strength of a steel material. Meanwhile, in a seamless steel pipe production process, ultrasonic inspection can be performed on the pipe to detect surface and / or internal defects during a final refining stage of production. In this case, the seamless steel pipe is required to have excellent ultrasonic inspection accuracy. However, patent literatures 1 through 9 above do not describe the accuracy of ultrasonic inspection on a seamless steel pipe. One objective of the present description is to provide a seamless steel tube that has a yield strength of 655 MPa or more (95 ksi or more) and excellent SSC strength in a sour environment and that can achieve excellent ultrasonic inspection accuracy. SOLUTION TO THE PROBLEM The seamless steel tube, according to the present description, includes a base material and a decarburized layer formed on the surface of the base material. The chemical composition of the base material consists of, in % by mass, C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.01 to 1.00%, P: 0.0300% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.30 to 1.20%, Mo: 0.30 to 1.50%, Ti: 0.002 to 0.050%, V: 0.01 to 0.30%, Nb: 0.002 to 0.100%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0050% or less, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Cu: 0 to 0.50%, and Ni: 0 to 0.50%, the remainder being Fe and impurities. The yield strength of the base material is 655 MPa or higher, and the yield ratio of the base material is 85.0% or higher. The decarburized layer is 150 pm or less in depth. ADVANTAGEOUS EFFECTS OF THE INVENTION Q / zL / n / Lznz / q / Yi The seamless steel tube, according to the present description, has a yield strength of 655 MPa or more (95 ksi or more) and excellent SSC resistance in an acidic environment and can achieve excellent ultrasonic inspection accuracy. DESCRIPTION OF THE MODALITIES The present inventors conducted research and studies regarding a method for obtaining excellent SSC resistance and also achieving excellent ultrasonic inspection accuracy in a seamless steel tube that is supposed to be used in an acidic environment and obtained the following findings. The present inventors first studied a seamless steel tube that has a yield strength of 655 MPa or more (95 ksi or more) and excellent SSC strength. As a result, they came to consider that if a seamless steel tube has a chemical composition consisting of, in % by mass, C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.01 to 1.00%, P: 0.0300% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.30 to 1.20%, Mo: 0.30 to 1.50%, Ti: 0.002 to 0.050%, V: 0.01 to 0.30%, Nb: 0.002 to 0.100%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0050% or less, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Cu: 0 to 0.50% and Ni: 0 to 0.50%, the remainder being Fe and impurities and a yield strength ratio of 85.0% or more; there is a possibility that it may have a yield strength of 655 MPa or more and excellent SSC strength. Accordingly, the present inventors have produced several seamless steel tubes having the chemical composition and mechanical properties described above (i.e., a yield strength of 655 MPa or higher and a yield strength of 85.0% or higher) and have conducted research and studies on the accuracy of ultrasonic inspection thereof. Specifically, a seamless steel tube is irradiated with ultrasonic waves using the method described below to determine the signal height ratio between an artificial defect and noise, known as the signal-to-noise ratio (SN ratio). When the SN ratio is low, it becomes difficult to distinguish a defect from noise. In other words, it can be determined that excellent ultrasonic inspection accuracy can be achieved with a seamless steel tube exhibiting a high SN ratio during ultrasonic inspection. In seamless steel tubing with the chemical composition and mechanical properties described above, a decrease in the SN ratio was observed, preventing the achievement of excellent ultrasonic inspection accuracy. Therefore, the present inventors have conducted research and studies on the factors causing the deterioration in ultrasonic inspection accuracy in seamless steel tubing with the described chemical composition. Q / zL / n / Lznz / q / Yi above and the mechanical properties described above. As a result, they have discovered that in the seamless steel tube, which has the chemical composition described above, a decarburized layer forms on an outer layer of the seamless steel tube when trying to obtain the mechanical properties described above. The decarburized layer is a layer with a reduced carbon content that forms as a result of the oxidation and removal of carbon (C) gas from a portion of the surface of a seamless steel tube. As described above, the chemical composition of the seamless steel tube, according to the present embodiment, has a relatively high carbon content of 0.20 to 0.50%. For this reason, the seamless steel tube, according to the present embodiment, has a large difference in carbon content at the surface of the base material (i.e., the interface between the decarburized layer and the base material). As a result, the seamless steel tube, according to the present embodiment, is likely to reflect and diffract ultrasonic waves at the interface between the decarburized layer and the base material. When ultrasonic waves are reflected and / or diffracted at the interface between the decarburized layer and the base material, the intensity of the ultrasonic wave reaching a defect decreases. As a result, the SN ratio decreases during ultrasonic inspection, which impairs the accuracy of the ultrasonic inspection of seamless steel pipe. Thus, in seamless steel pipe, according to the present method, when a decarburized layer forms, the accuracy of the ultrasonic inspection is likely to deteriorate due to the relatively high carbon content. Based on the findings described above, the present inventors concluded that if a seamless steel tube could be produced in which a decarburized layer does not form on the outer layer, the accuracy of ultrasonic inspection of the seamless steel tube could be improved. However, as a result of detailed studies conducted by the present inventors, it became clear that the depth of the decarburized layer formed on the outer layer of a seamless steel tube is affected by the chemical composition of the base material and the manufacturing process of the seamless steel tube. Specifically, when a steel material contains a high proportion of Cr, the decarburized layer tends to be shallow. Similarly, when a steel material contains a high proportion of Si, the decarburized layer tends to be deep. Thus, the depth of the decarburized layer is affected by the chemical composition of the base material. Furthermore, as described earlier, the decarburized layer forms through the oxidation of C on a portion of the surface of a seamless steel tube. Therefore, when seamless steel tubes are produced, especially if high-temperature heating is involved, it is likely that decarburized layers will form. Q / zL / n / Lznz / q / Yi forms the deep decarburized layer. In this way, the depth of the decarburized layer is also affected according to the production process. Furthermore, in a seamless steel tube with the chemical composition described above, a decarburized layer will have formed due to the nature of the production process when attempting to achieve the aforementioned mechanical properties. In other words, to consistently achieve excellent ultrasonic inspection accuracy in a seamless steel tube with the chemical composition and mechanical properties described above, it is satisfactory that excellent ultrasonic inspection accuracy can be achieved even if a decarburized layer is present on the outer layer. Accordingly, the present inventors carried out a detailed study on a technique for improving the accuracy of ultrasonic inspection of a seamless steel pipe comprising a base material having the chemical composition and mechanical properties described above, and a decarburized layer formed on the surface of the base material. As a result, the present inventors have discovered that it is possible to improve the accuracy of the ultrasonic inspection of the seamless steel pipe, even if a decarburized layer forms, by decreasing the depth of the decarburized layer. Regarding the reason for this, the present inventors consider the following. As described above, a decarburized layer has a reduced carbon content. For this reason, the microstructure of the decarburized layer is composed primarily of ferrite. Here, if a shallow carburized layer forms, ferrite grains with a small grain diameter can form within the microstructure of the decarburized layer. When the ferrite grain diameter is small, compared to the case where the ferrite grain diameter is large, the ultrasonic waves that reflect and / or diffract at the ferrite grain boundaries are reduced. Therefore, it becomes less likely that ultrasonic waves will diffract within a decarburized layer, so the intensity of the ultrasonic waves that do diffract increases.As a result, the SN ratio in ultrasonic inspection is considered to increase, thus improving the accuracy of ultrasonic inspection of seamless steel pipe. Therefore, the seamless steel tube, according to the present embodiment, includes a base material having the chemical composition and mechanical properties described above, and a decarburized layer formed on the surface of the base material, wherein the depth of the decarburized layer is 150 pm or less. If the depth of the decarburized layer is 150 pm or less, it is possible to improve the accuracy of the ultrasonic inspection of a seamless steel tube, even if a decarburized layer forms on the outer layer of the seamless steel tube. It should be noted that an elasticity ratio here means a ratio of the yield strength to the yield strength. Q / zL / n / Lznz / q / Yi to the tensile strength (i.e., elasticity ratio YR (%) = elastic limit YS / tensile strength TS). The seamless steel tube, according to the present modality, which has been completed based on the findings described so far, includes a base material and a decarburized layer formed on the surface of the base material. The chemical composition of the base material consists of, in % by mass, C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.01 to 1.00%, P: 0.0300% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.30 to 1.20%, Mo: 0.30 to 1.50%, Ti: 0.002 to 0.050%, V: 0.01 to 0.30%, Nb: 0.002 to 0.100%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0050% or less, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Cu: 0 to 0.50%, and Ni: 0 to 0.50%, the remainder being Fe and impurities. The yield strength of the base material is 655 MPa or higher, and the yield ratio of the base material is 85.0% or higher. The decarburized layer is 150 pm or less in depth. The seamless steel tube, according to the present modality, has excellent SSC strength and can achieve excellent ultrasonic inspection accuracy. The aforementioned chemical composition may contain one or more types of elements selected from the group consisting of Ca: from 0.0001 to 0.0100%, Mg: from 0.0001 to 0.0100%, Zr: from 0.0001 to 0.0100% and rare earth metal: from 0.0001 to 0.0100%. The aforementioned chemical composition may contain one or more types of elements selected from the group consisting of Co: from 0.02 to 0.50% and W: from 0.02 to 0.50%. The aforementioned chemical composition may contain one or more types of elements selected from the group consisting of Cu: from 0.02 to 0.50% and Ni: from 0.02 to 0.50%. The aforementioned decarburized layer may have a depth of 130 pm or less. The seamless steel pipe mentioned above may be an oil well steel pipe. In this description, "steel pipe for oil wells" can refer to steel pipe used for pipeline construction or steel pipe used for petroleum tubular products (OCTG). Petroleum tubular products include, for example, seamless steel pipes used as casing or pipelines. The seamless steel tube, in accordance with this specification, shall be described in detail. The symbol “%” associated with an item means “percentage by mass” unless specifically stated otherwise. Q / zL / n / Lznz / q / Yi Seamless steel pipe configuration A seamless steel tube, according to the present embodiment, includes a base material and a decarburized layer formed on the surface of the base material. Chemical composition of the base material The chemical composition of the base material that constitutes the seamless steel tube, according to the present modality, contains the following elements. C: from 0.20 to 0.50% Carbon (C) improves the hardenability of steel and increases its yield strength. C also promotes carbide spheroidization during tempering in the production process, increasing the steel's suture hardness (SSC). If the carbides disperse, the steel's yield strength increases even further. If the C content is too low, these effects may not be achieved. Conversely, if the C content is too high, the steel's toughness will decrease, and quench cracking is likely. Therefore, the C content is typically within a range of 0.20 to 0.50%. A preferred lower limit for C content is 0.22%, more preferably 0.24%, and even more preferably 0.25%. A preferred upper limit for C content is 0.48%, more preferably 0.45%, and even more preferably 0.40%. Yes: from 0.05 to 0.50% Silicon (Si) deoxidizes steel. If the Si content is too low, the desired effect cannot be achieved. Conversely, if the Si content is too high, the steel's SSC resistance decreases. If the Si content is too high, in some cases a deep decarburized layer forms, compromising the accuracy of ultrasonic inspection. Therefore, the Si content should be within a range of 0.05 to 0.50%. A preferred lower limit for the Si content is 0.08%, and more preferably 0.10%. A preferred upper limit for the Si content is 0.48%, more preferably 0.46%, and even more preferably 0.44%. Mn: from 0.01 to 1.00% Manganese (Mn) deoxidizes steel. Mn also improves the hardenability of steel and increases its yield strength. If the Mn content is too low, these effects cannot be achieved. On the other hand, if the Mn content is too high, Mn segregates at the grain boundaries, along with impurities such as phosphorus (P) and sulfur (S). In such a case, the steel's surface hardness (SSC) will decrease. Therefore, the Mn content should be within a range of 0.01 to 1.00%. A preferable lower limit for Mn content is 0.02%. The preferred Q / zL / n / Lznz / q / Yi content is 0.03%, and even more preferably 0.10%. A preferred upper limit for the Mn content is 0.98%, and even more preferably 0.95%. P: 0.0300% or less Phosphorus (P) is an impurity. In other words, the P content is greater than 0%. P segregates at grain boundaries and decreases the SSC strength of the steel material. Therefore, the P content is 0.0300% or less. A preferred upper limit for the P content is 0.0250%, and more preferably 0.0200%. Ideally, the P content is as low as possible. However, if the P content is reduced excessively, the production cost increases significantly. Therefore, when considering industrial production, a preferred lower limit for the P content is 0.0001%, and more preferably 0.0003%. S: 0.0100% or less Sulfur (S) is an impurity. In other words, the S content is greater than 0%. S segregates at grain boundaries and decreases the SSC strength of the steel material. Therefore, the S content is 0.0100% or less. A preferred upper limit for the S content is 0.0095%, more preferably 0.0090%, and even more preferably 0.0085%. Preferably, the S content is as low as possible. However, if the S content is reduced excessively, the production cost increases significantly. Therefore, when considering industrial production, a preferred lower limit for the S content is 0.0001%, and more preferably 0.0003%. Al: from 0.005 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, the effect cannot be achieved. On the other hand, if the Al content is too high, coarse oxide-based inclusions form, and the steel's SSC strength decreases. Therefore, the Al content is within a range of 0.005 to 0.100%. A preferred lower limit for the Al content is 0.015%, and more preferably 0.020%. A preferred upper limit for the Al content is 0.095%, more preferably 0.090%, and even more preferably 0.085%. In this description, "A" refers to "acid-soluble Al," i.e., the content of "Al sol." Cr: from 0.30 to 1.20% Chromium (Cr) increases temper softening resistance and allows for high-temperature tempering, thereby increasing the steel's surface hardness (SSC) strength. If the Cr content is too low, this effect cannot be achieved. If the Cr content is too low, in some cases a deep decarburized layer forms, impairing the accuracy of ultrasonic inspection. Conversely, if the Cr content is too high, thick carbides form in the steel, reducing its SSC strength. Therefore, the Cr content should ideally be between 0.30 and 1.20%. A preferred lower limit is 0.32%, with a higher value being preferable. Q / zL / n / Lznz / q / Yi is 0.35% and even more preferably is 0.40%. A preferred upper limit of the Cr content is 1.15%, more preferably is 1.10% and even more preferably is 1.05%. Mo: from 0.30 to 1.50% Molybdenum (Mo) increases temper softening resistance and allows for high-temperature tempering, thereby increasing the steel's hardness, toughness, and strength (STS). If the Mo content is too low, this effect cannot be achieved. Conversely, if the Mo content is too high, coarse carbides form in the steel, reducing its STS. Therefore, the Mo content is typically between 0.30% and 1.50%. A preferred lower limit is 0.35%, with 0.40% being more preferable. A preferred upper limit is 1.45%, with 1.40% being more preferable. Ti: from 0.002 to 0.50% Titanium (Ti) forms nitrides and refines crystal grains through a fixative effect. As a result, it increases the yield strength of the steel. If the Ti content is too low, this effect cannot be achieved. Conversely, if the Ti content is too high, a large amount of Ti nitrides forms, and the steel's SSC strength decreases. Therefore, the Ti content is within a range of 0.002 to 0.050%. A preferred lower limit for the Ti content is 0.003%, and more preferably 0.004%. A preferred upper limit for the Ti content is 0.040%, more preferably 0.030%, and even more preferably 0.020%. V: from 0.01 to 0.30% Vanadium (V) increases temper softening resistance and allows for high-temperature tempering, thereby increasing the steel's hardness, toughness, and strength (STS). Furthermore, V combines with C and / or N to form carbides, nitrides, or carbonitrides (hereafter referred to as "carbonitrides and the like"). Carbonitrides and the like refine the steel's substructure through a locking effect and increase its STS. Additionally, V combines with C to form fine carbides. As a result, the steel's yield strength increases. If the V content is too low, these effects cannot be achieved. Conversely, if the V content is too high, the steel's toughness decreases. Therefore, the V content is typically within a range of 0.01 to 0.30%. A preferred lower limit of the V content is 0.03%, more preferably 0.05%, and even more preferably 0.07%.A preferred upper limit of the V content is 0.25%, more preferably 0.20%, and even more preferably 0.15%. Nb: from 0.002 to 0.100% Niobium (Nb) combines with carbon and / or nitrogen to form carbonitrides and similar compounds. Carbonitrides and similar compounds refine the substructure of steel material through a locking effect and improve the steel's structural strength. Nb also combines with carbon to form carbides. Q / zL / n / Lznz / q / Yi fine. As a result, the yield strength of the steel material increases. If the Nb content is too low, the desired effects cannot be achieved. On the other hand, if the Nb content is too high, carbonitrides and similar compounds form in excess, and the steel's SSC strength decreases. Therefore, the Nb content is within a range of 0.002 to 0.100%. A preferred lower limit for the Nb content is 0.003%, more preferably 0.005%, and even more preferably 0.010%. A preferred upper limit for the Nb content is 0.095%, more preferably 0.090%, and even more preferably 0.080%. B: from 0.0001 to 0.0050% Boron (B) dissolves in steel to increase its hardenability and yield strength. If the B content is too low, this effect cannot be achieved. Conversely, if the B content is too high, coarse nitrides form in the steel, reducing its SSC strength. Therefore, the B content is typically within a range of 0.0001% to 0.0050%. A preferred lower limit is 0.0003%, a more preferred limit is 0.0005%, and an even more preferred limit is 0.0012%. A preferred upper limit is 0.0045%, a more preferred limit is 0.0040%, and an even more preferred limit is 0.0035%. N: 0.0100% or less Nitrogen (N) is unavoidably present. In other words, the N content is greater than 0%. N combines with Ti to form nitrides and refines the crystal grains through a fixation effect. As a result, the yield strength of the steel material increases. On the other hand, if the N content is too high, coarse nitrides form, and the steel material's SSC strength decreases. Therefore, the N content is 0.0100% or less. A preferred upper limit for the N content is 0.0090%, and more preferably, 0.0080%. A preferred lower limit for the N content to effectively achieve the above effect is 0.0005%, more preferably 0.0010%, even more preferably 0.0015%, and still more preferably 0.0020%. O: 0.0050% or less Oxygen (O) is an impurity. In other words, the O content is greater than 0%. O forms coarse oxides and decreases the SSC strength of the steel material. Therefore, the O content is 0.0050% or less. A preferred upper limit for the O content is 0.0048%, and more preferably 0.0045%. Preferably, the O content is as low as possible. However, if the O content is reduced excessively, the production cost increases significantly. Therefore, when considering industrial production, a preferred lower limit for the O content is 0.0001%, and more preferably 0.0003%. Q / zL / n / Lznz / q / Yi The remainder of the chemical composition of the steel material, according to this modality, consists of Fe and impurities. Herein, the term “impurities” refers to elements that, during the industrial production of the steel material, are mixed in from ore or scrap used as raw material for the steel material, or from the production environment or similar sources, and that are permitted within a range that does not adversely affect the steel material according to this modality. Regarding the optional elements The chemical composition of the steel material described above may also contain one or more elements selected from the group consisting of Ca, Mg, Zr, and rare earth metals (REM) instead of a portion of Fe. Each of these elements is optional and renders the S in the steel material harmless by preventing the formation of sulfides and increases the steel's SSC resistance. Ca: from 0 to 0.0100% Calcium (Ca) is an optional element and does not need to be present. In other words, the Ca content can be 0%. If present, Ca renders the sulfur (S) in the steel material harmless by forming sulfides and increases the steel's shock-resistant sulfide (SSC) strength. Even a small amount of Ca can produce a noticeable effect. However, if the Ca content is too high, the oxides in the steel material become thicker, and the steel's SSC strength decreases. Therefore, the Ca content is within a range of 0 to 0.0100%. A preferred lower limit for Ca content is greater than 0%, more preferably 0.0001%, even more preferably 0.0003%, and still more preferably 0.0006%. A preferred upper limit for Ca content is 0.0060%, more preferably 0.0050%, and still more preferably 0.0030%. Mg: from 0 to 0.0100% Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content can be 0%. If present, Mg renders the sulfur in the steel material harmless by forming sulfides and increases the steel's SSC strength. Even a small amount of Mg can produce a noticeable effect. However, if the Mg content is too high, the oxides in the steel material become thicker, and the steel's SSC strength decreases. Therefore, the Mg content is within a range of 0 to 0.0100%. A preferred lower limit for the Mg content is less than 0%, more preferably 0.0001%, even more preferably 0.0003%, and most preferably 0.0006%. A preferred upper limit for Mg content is 0.0060%, more preferably 0.0050%, and even more preferably 0.0030%. Q / zL / n / Lznz / q / Yi Zr: from 0 to 0.0100% Zirconium (Zr) is an optional element and does not need to be present. In other words, the Zr content can be 0%. If present, Zr renders the sulfur in the steel material harmless by forming sulfides and increases the steel's surface hardness (SSC) strength. Even a small amount of Zr can produce a noticeable effect. However, if the Zr content is too high, the oxides in the steel material become thicker, and the steel's SSC strength decreases. Therefore, the Zr content is within a range of 0 to 0.0100%. A preferred lower limit for the Zr content is greater than 0%, more preferably 0.0001%, even more preferably 0.0003%, and still more preferably 0.0006%. A preferred upper limit for Zr content is 0.0060%, more preferably 0.0050%, and even more preferably 0.0030%. Rare earth metal (REM): 0 to 0.0100% Rare earth metals (REMs) are optional and do not need to be included. In other words, the REM content can be 0%. If included, REMs render sulfur (S) in the steel material harmless by forming sulfides and increase the steel's shock-resistant crystal structure (SSC) strength. REMs also combine with phosphorus (P) in the steel material and suppress P segregation at the crystal grain boundaries. Therefore, a decrease in the steel material's low-temperature toughness and SSC strength attributable to P segregation is suppressed. Even a small amount of REM can produce some effect. However, if the REM content is too high, the oxides become coarser, and the steel material's low-temperature toughness and SSC strength decrease. Therefore, the REM content is typically within a range of 0 to 0.0100%.A preferred lower limit for REM content is greater than 0%, more preferably 0.0001%, even more preferably 0.0003%, and still more preferably 0.0006%. A preferred upper limit for REM content is 0.0060%, more preferably 0.0050%, and still more preferably 0.0030%. It should be noted that, in this description, the term “REM” refers to one or more types of elements selected from the group consisting of scandium (Se), the element with atomic number 21, yttrium (Y), the element with atomic number 39, and the lanthanides from lanthanum (La), with atomic number 57, to lutetium (Lu), with atomic number 71. Furthermore, in this description, the term “REM content” refers to the total content of these elements. The chemical composition of the steel material described above may also contain one or more elements selected from the group consisting of Co and W instead of Fe. Each of these elements is an optional component that forms a protective layer against corrosion in an acidic environment and suppresses hydrogen penetration. In this way, each of these elements increases the SSC resistance of the steel material. Co: from 0 to 0.50% Cobalt (Co) is an optional element and does not need to be included. In other words, the Co content can be 0%. If included, Co forms a protective layer against corrosion in an acidic environment and suppresses hydrogen penetration. As a result, the steel's SSC strength increases. Even a small amount of Co can produce a certain effect. However, if the Co content is too high, the steel's hardenability and yield strength will decrease. Therefore, the Co content is within a range of 0 to 0.50%. A preferred lower limit for Co content is greater than 0%, more preferably 0.02%, more preferably 0.03%, and most preferably 0.05%. A preferred upper limit for Co content is 0.45%, and more preferably 0.40%. W: from 0 to 0.50% Tungsten (W) is an optional element and its presence is not required. In other words, the W content can be 0%. If present, W forms a protective layer against corrosion in acidic environments and suppresses hydrogen penetration. As a result, the steel's surface corrosion resistance (SCR) increases. Even a small amount of W can produce a certain effect. However, if the W content is too high, coarse carbides form in the steel, and its SCR decreases. Therefore, the W content is within a range of 0 to 0.50%. A preferred lower limit for the W content is less than 0%, more preferably 0.02%, even more preferably 0.03%, and still more preferably 0.05%. A preferred upper limit for the W content is 0.45%, and more preferably 0.40%. The chemical composition of the steel material described above may also contain one or more elements selected from the group consisting of copper and nickel instead of a portion of iron. Each of these elements is optional and improves the hardenability of the steel material and increases its SSC strength. Cu: from 0 to 0.50% Copper (Cu) is an optional element and may not be present. In other words, the Cu content can be 0%. If present, Cu improves the hardenability of the steel material, increasing its yield strength. Even a small amount of Cu can produce a noticeable effect. On the other hand, if the Cu content is too high, the hardenability of the steel material will be excessively high, and its SSC strength will decrease. Therefore, the Cu content is between 0 and 0.50%. A preferred lower limit for the Cu content is greater than 0%, more preferably 0.02%, even more preferably 0.03%, and still more preferably 0.05%. A preferred upper limit for the Cu content is 0.40%, more preferably 0.30%, even more preferably 0.20%, and still more preferably 0.15%. Ni: from 0 to 0.50% Nickel (Ni) is an optional element and may not be present. In other words, the Ni content can be 0%. If present, Ni improves the hardenability of the steel and increases its yield strength. Even a small amount of Ni can produce a noticeable effect. However, if the Ni content is too high, it will promote localized corrosion and decrease the steel's surface hardness (SSC). Therefore, the preferred Ni content is between 0% and 0.50%. A lower preferred Ni content is less than 0%, more preferably 0.02%, even more preferably 0.05%, and still more preferably 0.10%. An upper preferred Ni content is 0.40%, more preferably 0.30%, even more preferably 0.20%, and still more preferably 0.15%. Elastic limit and elasticity ratio of the base material A base material constituting a seamless steel pipe, according to this embodiment, has a yield strength of 655 MPa or more and a base material elasticity ratio of 85.0% or more. In summary, the seamless steel pipe, according to this embodiment, has a yield strength of 95 ksi or more. As a result of the base material constituting the seamless steel pipe, according to this embodiment, having the chemical composition described above, a yield strength of 655 MPa or more, and an elasticity ratio of 85.0% or more, the seamless steel pipe of this embodiment has a yield strength of 655 MPa or more and excellent SSC strength. It should be noted that the upper limit of the yield strength of a seamless steel pipe, according to this specification, is not specifically limited. However, at least when the yield strength is within the range of 655 to 965 MPa, it has been demonstrated by the examples described below that a seamless steel pipe, according to this specification, has excellent SSC strength and excellent ultrasonic inspection accuracy. Accordingly, the yield strength of a seamless steel pipe, according to this specification, includes at least 655 to 965 MPa (95 to 140 ksi). In other words, the yield strength of a seamless steel pipe, according to this specification, includes at least 655 to less than 758 MPa (95 ksi grade), 758 to less than 862 MPa (110 ksi grade), and 862 to 965 MPa (125 ksi grade). In this description, the yield strength of the base material of the seamless steel pipe, according to this modality, is defined in accordance with API 5CT (2011). Specifically, in a case where the base material, according to this modality, has a yield strength Q / zL / n / Lznz / q / Yi of elasticity within the range of 655 to less than 758 MPa (grade 95 ksi), the elastic limit means a stress or strain at a moment of 0.5% of total elongation (0.5% of the proof stress or strain) obtained in a tensile test. In a case where the base material, according to the present modality, has an elastic limit within the range of 758 MPa or more (110 ksi or more), the elastic limit means a stress (a strain) at a moment of 0.7% of total elongation (0.7% of the proof stress or strain) obtained in a tensile test. Furthermore, in the present description, an elastic ratio is defined as a ratio between the tensile strength and the elastic limit. In the present document, the upper limit of the elastic ratio, according to the present modality, is not particularly limited and, for example, may be 100.0%.It is noted that, in the present description, the tensile strength of the base material is defined as the maximum stress during uniform elongation in a tensile test. The yield strength and yield ratio of the base material constituting the seamless steel pipe, according to this modality, can be determined by the following method. A tensile test is performed using a method in accordance with ASTM E8 / E8M (2013). A round bar test specimen is taken from a seamless steel pipe, according to this modality. Specifically, a round bar test specimen is taken from a central portion of the wall thickness. The size of a round bar test specimen is, for example, 4 mm in diameter and 35 mm in length. It should be noted that the axial direction of the round bar test specimen is parallel to the axis of the seamless steel pipe. A tensile test is carried out at normal temperature (25 °C) in the atmosphere using the round bar test specimen. In a case where the stress obtained at 0.5% of total elongation (0.5% of the test stress) is within the range of 655 to less than 758 MPa (95 ksi grade), 0.5% of the test stress is considered the yield strength (MPa). In a case where the stress obtained at 0.7% elongation (0.7% of the test stress) is within the range of 758 MPa or more (110 ksi or more), 0.7% of the test stress is used as the yield strength (MPa). The maximum stress obtained during uniform elongation in the tensile test is also considered the tensile strength (MPa). Additionally, a ratio of tensile strength (TS) to elastic limit (YS) is used as the elasticity ratio (YR) (%) (elasticity ratio YR = elastic limit YS / tensile strength TS). Q / zL / n / Lznz / q / Yi Microstructure of the base material In the microstructure of the base material, the total volume proportions of tempered martensite and tempered bainite are 90% or more. The remainder of the microstructure is, for example, ferrite or pearlite. In the present embodiment, if the yield strength of the base material satisfying the chemical composition described above is 655 MPa or more (95 ksi or more) and the yield strength is 85.0% or more, the microstructure of the base material is considered to contain 90% or more of the total volume proportions of tempered martensite and tempered bainite. Preferably, the microstructure of the base material consists solely of tempered martensite and / or tempered bainite. In the field of alloys, mechanical properties depend on the microstructure (phases, precipitates, and inclusions) and / or the crystalline structure of the metallic crystal and / or the atomic arrangement of a seamless steel tube, as well as the balance of these properties. Therefore, a person skilled in the technique can identify the microstructure from the chemical composition, yield strength, and yield ratio of a seamless steel tube. It should be noted that the total volume proportions of tempered martensite and tempered bainite can also be determined by observing the microstructure. A test specimen with an observation surface measuring 10 mm along the tube axis and 10 mm radially is cut from a central portion of the wall thickness of a seamless steel tube. After polishing to a mirror finish, the observation surface is immersed in a 2% Nital chemical etching reagent for approximately 10 seconds to reveal the structure through chemical etching. The chemically etched observation surface is then viewed in 10 fields of view in a bright-field image using an optical microscope. The area of ​​each field of view is, for example, 2000 pm² (500x magnification). In each field of view, tempered martensite and tempered bainite can be distinguished from other phases (e.g., ferrite or pearlite) based on contrast. Therefore, tempered martensite and tempered bainite are identified in each field of view. A total of the identified tempered martensite and tempered bainite area fractions is determined. In this modality, an arithmetic mean value of the total tempered martensite and tempered bainite area fractions, determined in all fields of view, is defined as a volume ratio (%) of tempered martensite and tempered bainite. decarburized layer The seamless steel tube, according to the present embodiment, further includes a decarburized layer formed on the surface of the base material described above. The decarburized layer constituting the seamless steel tube, according to the present embodiment, has a Q / zL / n / Lznz / q / Yi depth of 150 pin or less. As described so far, the decarburized layer that constitutes the seamless steel tube, according to the present embodiment, is a layer with a reduced C content formed as a result of the oxidation of a portion of the surface of the seamless steel tube. As described above, the decarburized layer has a reduced carbon content. For this reason, the microstructure of the decarburized layer is composed primarily of ferrite. Specifically, the microstructure of the decarburized layer is composed of 90% or more ferrite by volume. The remainder of the decarburized layer's microstructure consists of, for example, precipitates and inclusions. On the other hand, as described above, the chemical composition of the base material that constitutes the seamless steel tube, according to the present embodiment, has a carbon content of 0.20 to 0.50%. Therefore, there is a difference in carbon content between the decarburized layer and the base material in their chemical compositions.As a result, in seamless steel tubing, according to the present method, ultrasonic waves are likely to be reflected and / or diffracted at an interface between the base material and the decarburized layer during ultrasonic inspection. In other words, in seamless steel tubing, according to the present method, the accuracy of the ultrasonic inspection is likely to be compromised. As described above, the chemical composition of seamless steel tubing, as described herein, is likely to decrease the accuracy of ultrasonic inspection. For this reason, the depth of the decarburized layer formed on the outer surface of seamless steel tubing, as described herein, is kept small. When the decarburized layer of seamless steel tubing, as described herein, becomes deeper than 150 pm, the ultrasonic waves diffracted within the decarburized layer during ultrasonic inspection increase. As a result, the accuracy of the ultrasonic inspection deteriorates. Therefore, the decarburized layer of seamless steel tubing, as described herein, is kept to a depth of 150 pm or less. A preferred upper limit for the depth of the decarburized layer is 140 µm, more preferably 130 µm, even more preferably 120 µm, and still more preferably 110 µm. If the depth of the decarburized layer is 130 µm or less, the accuracy of the ultrasonic inspection is further improved. On the other hand, the lower limit for the depth of the decarburized layer will not be particularly limited. However, in seamless steel tubing, according to the present modality, the decarburized layer will form to be approximately 15 µm thick, due to the nature of its production process. For that reason, the lower limit for the depth of the layer Q / zL / n / Lznz / q / Yi decarburized from the seamless steel tube, according to the present modality, is substantially 15 pm. The depth of the decarburized layer of the seamless steel pipe, according to this modality, may be determined by a method conforming to JIS G 0558 (2007). A test sample is taken for observation of the decarburized layer of the seamless steel pipe, according to this modality. Specifically, a test sample is taken that includes the surface of the seamless steel pipe, according to this modality, and has an observation area with dimensions of 10 mm in the direction of the pipe axis and 10 mm in the radial direction of the pipe. The surface of the seamless steel pipe may be either the outer or inner surface.It should be noted that, in cases where the seamless steel tube has a wall thickness of less than 10 mm, a test sample is taken with an observation surface measuring 10 mm along the tube axis and the wall thickness of the seamless steel tube along the tube's radial direction. After polishing the observation surface of the test sample to a mirror finish, the sample is immersed in a 2% Nital chemical etching reagent for approximately 10 seconds to reveal its microstructure through chemical etching. The chemically etched observation surface is then viewed in 10 fields of view in a bright-field image using an optical microscope. The area of ​​each field of view is, for example, 0.1 mm² (200x magnification). As previously described, in this modality, the microstructure of the base material is composed, by volume, of 90% or more tempered martensite and / or tempered bainite. On the other hand, the microstructure of the decarburized layer is composed, by volume, of 90% or more ferrite. Furthermore, as previously described, in each field of view, the tempered martensite and tempered bainite can be distinguished from the ferrite based on contrast. Therefore, the ferrite is identified in each field of view based on contrast. In each field of view, a region containing 90% or more of the identified ferrite is defined as the decarburized layer. In each field of view, a location where the decarburized layer forms at its deepest point is identified. The depth of the decarburized layer at this identified location is then determined.An arithmetic average value of the decarburized layer depths determined in all fields of view is defined as the depth (pm) of the decarburized layer. SSC resistance of seamless steel pipe In a seamless steel tube, according to the present embodiment, an excellent SSC strength is defined for each yield strength interval. It is noted that the SSC strength of the seamless steel tube, according to the present embodiment, can be evaluated by a method Q / zL / n / Lznz / q / Yi in accordance with Method A specified in NACE TMO177-2005, without regard to the elastic limit range. SSC strength when the yield strength is from 655 to less than 758 MPa In cases where the yield strength is from 655 to less than 758 MPa (95 to less than 110 ksi, grade 95 ksi), the SSC strength of the seamless steel pipe can be evaluated using the following method. A mixed aqueous solution containing 5.0% by mass of sodium chloride and 0.5% by mass of acetic acid (NACE Solution A) is used as the test solution. A round bar test sample is taken from a central portion of the wall thickness of the seamless steel pipe, according to this method. The size of the round bar test sample is, for example, 6.35 mm in diameter and 25.4 mm in length of the parallel portion. It should be noted that the axial direction of the round bar test sample is parallel to the axis of the seamless steel pipe. The stress or strain corresponding to 90% of the actual tensile strength of the base material is applied to the round bar test specimen. A test solution at 24 °C is poured into a test vessel, so that the stressed round bar test specimen is submerged, thus providing a test bath. After degassing the test bath, 1 atm of gaseous H₂S is introduced to create a corrosive environment. The test bath in which the round bar test specimen is immersed is maintained at 24 °C for 720 hours. In this modality, in a case where the yield strength is from 655 to less than 758 MPa, if no cracking is confirmed after 720 hours under the conditions described above, the seamless steel tube is determined to have excellent SSC strength.In other words, in a seamless steel tube, according to the present specification, in a case where the yield strength is from 655 to less than 758 MPa, no cracking is confirmed after 720 hours under the condition described above. It is noted that “no cracking is confirmed,” as used herein, means that no cracking is confirmed in the test specimen when the test specimen is visually inspected after the test. SSC strength when the yield strength is from 758 to less than 862 MPa In a case where the yield strength is from 758 to less than 862 MPa (110 to less than 125 ksi, grade 110 ksi), the SSC strength of the seamless steel pipe can be evaluated using the following method. A mixed aqueous solution containing 5.0% by mass of sodium chloride and 0.5% by mass of acetic acid (NACE Solution A) is used as the test solution. A round bar test sample is taken from a central portion of the wall thickness of the seamless steel pipe, according to the present method. The size of the round bar test sample is, for example, Q / zL / n / Lznz / q / Yi The parallel portion is 6.35 mm in diameter and 25.4 mm in length. It should be noted that the axial direction of the round bar test specimen is parallel to the axis direction of the seamless steel tube. The stress or strain corresponding to 90% of the actual tensile strength of the base material is applied to the round bar test specimen. A test solution at 24 °C is poured into a test vessel, so that the stressed round bar test specimen is submerged, thus providing a test bath. After degassing the test bath, 1 atm of gaseous H₂S is introduced to create a corrosive environment. The test bath in which the round bar test specimen is immersed is maintained at 24 °C for 720 hours. In this modality, in a case where the yield strength is from 758 to less than 862 MPa, if no cracking is confirmed after 720 hours under the conditions described above, the seamless steel tube is determined to have excellent SSC strength.In other words, in a seamless steel tube, according to the present specification, in a case where the yield strength is from 758 to less than 862 MPa, no cracking is confirmed after 720 hours under the condition described above. It is noted that “no cracking is confirmed,” as used herein, means that no cracking is confirmed in the test specimen when the test specimen is visually inspected after the test. SSC strength when the yield strength is 862 MPa or more In cases where the yield strength is 862 MPa or higher (125 ksi or higher), the SSC strength of the seamless steel pipe can be evaluated using the following method. A mixed aqueous solution containing 5.0% by mass of sodium chloride, 0.41% by mass of sodium acetate, and 2.5% by mass of acetic acid (NACE Solution B) is used as the test solution. A round bar test sample is taken from a central portion of the wall thickness of the seamless steel pipe, according to this method. The size of the round bar test sample is, for example, 6.35 mm in diameter and 25.4 mm in length of the parallel portion. It should be noted that the axial direction of the round bar test sample is parallel to the axis direction of the seamless steel pipe. A stress corresponding to 90% of the actual tensile strength of the base material is applied to the round bar test specimen. A test solution at 24 °C is poured into a test vessel so that the stressed round bar test specimen is submerged, thus providing a test bath. After degassing the test bath, a gas mixture of 0.1 atm of gaseous H₂S and 0.9 atm of gaseous CO₂ is introduced to create a corrosive environment. The test bath in which the round bar test specimen is immersed is maintained at 24 °C for 720 hours. In the present embodiment, in a case where the yield strength is 862 MPa or higher, if no cracking is confirmed after 720 hours, the test is deemed to have failed. Under the conditions described above, the seamless steel pipe is determined to have excellent SSC strength. In other words, in a seamless steel pipe, according to the present modality, where the yield strength is at least 862 to 965 MPa, no cracking is confirmed after 720 hours under the conditions described above. It is noted that “no cracking is confirmed,” as used herein, means that no cracking is confirmed in the test specimen when the test specimen is visually inspected after the test. Production method The following describes a method for producing seamless steel tubing according to the present embodiment. An example of the method for producing seamless steel tubing according to the present embodiment includes a process for preparing a hollow casing (preparation process) and a process for quenching and tempering the hollow casing to make a seamless steel tubing (quenching process and tempering process). The preparation process may include a process for preparing a starting material (starting material preparation process) and a process for hot working the starting material to produce the hollow casing (hot working process). Each process is described in detail below. Preparation process In the preparation process, a hollow shell with the aforementioned chemical composition is prepared. If the hollow shell has the aforementioned chemical composition, the preparation process of the hollow shell will not be particularly limited. In other words, the preparation process may include the process of preparing a starting material (starting material preparation process) and the process of hot working the starting material to produce the hollow shell (hot working process). A case where the preparation process includes both the starting material preparation process and the hot working process is described in detail below. Initial material preparation process In the process of preparing the starting material, a starting material is produced using molten steel with the chemical composition mentioned above. Specifically, a casting (plate, billet, or bar) is produced by a continuous casting process using the molten steel with the aforementioned chemical composition. An ingot can also be produced by an ingot-making process using the molten steel with the composition Q / zL / n / Lznz / q / Yi chemistry mentioned above. If necessary, the plate, billet, or ingot can undergo a billet forming process to produce a bar. Through the processes described above, a starting material (plate, billet, bar, or ingot) is produced. Hot work process In the hot working process, the prepared starting material is subjected to hot working to produce a hollow shell. Specifically, the bar is first heated in a heating furnace. Although the heating temperature is not particularly limited, for example, it is typically within a range of 1100 to 1300 °C. The bar removed from the heating furnace is then hot worked to produce a hollow shell (seamless steel tube). It's worth noting that the heating furnace used in the hot working process is not subject to any particular limitations. This is because, given the high heating temperature in the heating furnace used in hot working, scale formation is more likely than decarburized layer formation. For example, a hot rolling process, such as the Mannesmann process, can be performed as the hot working process to produce the hollow shell. In this case, a round bar is first perforated using a perforating machine. When performing perforated rolling, although the perforation ratio is not particularly limited, it is, for example, within a range of 1.0 to 4.0. The perforated round bar is then further hot-rolled to form a hollow shell using a mandrel mill, a reducer, a sizing mill, or similar equipment. The cumulative area reduction in the hot working process is, for example, 20 to 70%. A hollow shell can also be produced from the starting material using another hot working method. For example, in the case of a short, thick-walled steel material, such as a coupling, a hollow shell can be produced by hot forging, such as the Ehrhardt process. The hollow shell is produced from the starting material using the process described above. Although not specifically limited, the wall thickness of the hollow shell is, for example, 9 to 60 mm. The hollow shell produced by hot working can be air-cooled (freshly rolled). The hollow shell produced by hot working can be tempered directly after hot working without being cooled to room temperature, or it can be tempered after being reheated following hot working. However, in the case of direct tempering, it is preferable to stop the cooling halfway through the tempering process and perform a slow cooling in order to suppress temper cracking. Q / zL / n / Lznz / q / Yi In a case where direct hardening is performed after hot working, in order to eliminate residual stress, it is preferable to perform a stress relief annealing (SR) at a time that is after hardening and before a heat treatment (tempering or similar) of the next process. As described above, a hollow shell is prepared in the preparation process. The hollow shell may be produced by the preferred process mentioned above, or it may be a hollow shell that was produced by a third party, or a hollow shell that was produced in a factory other than the one where the tempering and annealing processes described below are carried out, or in a different operation. Tempering process In the tempering process, the prepared hollow shell is tempered. In this description, “tempering” means rapidly cooling a hollow shell to a temperature no lower than point A3. In this description, after hot working, rapidly cooling a hollow shell to a temperature no lower than point A3, either directly or with supplementary heating, is referred to as “direct tempering.” In this description, rapidly cooling a hollow shell to a temperature no lower than point A3 after reheating is referred to as “offline tempering.” Hereafter, offline tempering and offline annealing, described below, are collectively referred to as “offline heat treatment.” A preferred tempering temperature is 800 to 1000 °C.When direct tempering is performed after hot working, the tempering temperature corresponds to the surface temperature of the hollow shell, measured by a thermometer placed on the delivery side of the apparatus used for the final hot working. When tempering is performed using a supplementary heating furnace or a heat treatment furnace after hot working, the tempering temperature also corresponds to the temperature of the supplementary heating furnace or the heat treatment furnace (hereafter, the supplementary heating furnace or the heat treatment furnace used for pre-tempering is also referred to simply as the “tempering furnace”). When the tempering temperature is too high, the pre-formed austenite crystal grain will become coarser in the seamless steel tube after tempering, and the seamless steel tube's SSC strength may decrease. Therefore, the tempering temperature is preferably between 800 and 1000 °C. A more preferable upper limit for the tempering temperature is 950 °C. In the tempering process, according to the present method, when tempering is carried out using a tempering furnace after hot working, the temperature inside the furnace The tempering process is controlled by the combustion of a flammable gas. In this description, the flammable gas includes, for example, hydrocarbon gas (methane, ethane, propane, butane, ethylene, or similar), hydrogen, carbon monoxide, or similar. Here, when the flammable gas is burned, a greater quantity of oxygen is supplied than is required for complete combustion. This is to prevent incomplete combustion. On the other hand, in the tempering process, according to the present method, the atmospheric gas in the tempering furnace is the post-combustion gas obtained from the combustion of a flammable gas. In other words, as a result of supplying an excessive amount of oxygen to burn the flammable gas, a certain amount of oxygen will remain in the tempering furnace. As described above, the chemical composition of the hollow shell, according to the present embodiment, has a high carbon content. Furthermore, the temperature inside the tempering furnace is 800 to 1000 °C. As a result, the oxygen remaining in the tempering furnace combines with carbon on the surface portion of the hollow shell to generate gaseous carbon dioxide. According to this mechanism, a decarburized layer forms on the surface portion of the hollow shell, according to the present embodiment. For this reason, in the tempering process, according to the present embodiment, the oxygen concentration in the atmospheric gas inside the tempering furnace, which is generated as a result of the combustion of a flammable gas, is reduced.More specifically, in the tempering process, according to the present modality, the oxygen concentration in the atmospheric gas in the tempering furnace is adjusted to be from 1 to 8% in the volume fraction, not considering water vapor. If the oxygen concentration of the atmospheric gas in the quenching furnace exceeds 8% by volume, excluding water vapor, excessive decarburization may occur during quenching, resulting in an excessively large decarburized layer on the seamless steel tube after quenching and tempering. Conversely, if the oxygen concentration of the atmospheric gas in the quenching furnace falls below 1% by volume, excluding water vapor, incomplete combustion of the flammable gas may occur. Therefore, in the quenching process described herein, the oxygen concentration of the atmospheric gas in the quenching furnace is set within a range of 1% to 8% by volume, excluding water vapor.A preferred upper limit for the oxygen concentration of the atmospheric gas in the tempering furnace, expressed as a volume fraction excluding water vapor, is 7%. A preferred lower limit for the oxygen concentration of the atmospheric gas in the tempering furnace is 2%, and more preferably 3%. Q / zL / n / Lznz / q / Yi The oxygen concentration of the atmospheric gas in the tempering furnace can be controlled, for example, by mixing and burning a flammable gas with air. Specifically, the amount of oxygen required to completely burn the flammable gas can be determined by calculation using the flammable gas's chemical formula. Furthermore, the oxygen concentration before combustion can be adjusted by regulating the mixing ratio between the flammable gas and air. In this way, the oxygen concentration of the atmospheric gas in the tempering furnace is controlled. It should be noted that there is no particular limitation on the amount of remaining atmospheric gas in the tempering furnace. However, when, as described above, the flammable gas and air are mixed and burned, the remaining atmospheric gas in the tempering furnace is an inert gas composed primarily of nitrogen. Subsequently, the hollow shell at the tempering temperature is rapidly cooled. In the tempering method, for example, the hollow shell is continuously cooled from the tempering temperature so that its temperature decreases continuously. The method for carrying out the continuous cooling treatment is not particularly limited, and any known method may be used. The method for carrying out the continuous cooling treatment is, for example, a method that cools the hollow shell by immersing it in a water bath or a method that cools the hollow shell rapidly by means of shower cooling or mist cooling. If the cooling rate during quenching is too slow, a microstructure composed primarily of martensite and bainite will not be obtained. In this case, the seamless steel tube, after quenching and tempering, will not achieve excellent SSC strength. Therefore, in the method for producing a seamless steel tube according to the present embodiment, the hollow shell is cooled rapidly during quenching. Specifically, in the quenching process, an average cooling rate when the temperature of the hollow shell is within the range of 800 to 500 °C during quenching is defined as the quenching cooling rate CRsoo-soo (°C / s). The quenching cooling rate CRsoo-soo is determined from the surface temperature of the hollow shell and the quenching time. A preferred cooling rate during CRsoo-soo tempering is 8 °C / s or more. In this case, the microstructure of the hollow shell after tempering will consist mainly of martensite and bainite in a stable manner. A preferred lower limit for the cooling rate during CRsoo-soo tempering is 10 °C / s. A preferred upper limit for the cooling rate during CRsoo-soo tempering is 500 °C / s. It is observed that the tempering process, according to the present method, can be performed only once. On the other hand, the tempering process, according to the present method, can be performed several times. If the tempering process is performed several times, the austenite grains will Q / zL / n / Lznz / q / Yi will refine and thus further improve the SSC strength of the seamless steel tube. However, when the tempering process is performed several times, it may happen that the depth of a decarburized layer on the seamless steel tube after tempering and annealing is excessively deep. Therefore, in the tempering process according to the present method, the number of tempering cycles is limited. Specifically, the tempering process, according to the present method, is performed from 1 to 4 times in total. If the number of tempering cycles is zero, the microstructure of the hollow shell will not develop into one composed primarily of martensite and bainite. In this case, the seamless steel tube after tempering and annealing cannot achieve excellent SSC strength. On the other hand, if the number of tempering cycles is 5 or more, the decarburized layer of the seamless steel tube after tempering and annealing will exceed 150 µm. In this case, the seamless steel tube cannot achieve excellent ultrasonic inspection accuracy. Therefore, the tempering process, according to the present method, is performed a total of 1 to 4 times. It should be noted that whenever the tempering process is to be repeated, an offline tempering can be performed. Alternatively, between the times the tempering process is to be repeated, a direct tempering can be performed during the first tempering process. In this case, the decarburized layer of the seamless steel tube becomes shallower, and even better ultrasonic inspection accuracy can be achieved. Furthermore, in this case, the pre-austenite grain of the seamless steel tube is refined, and even better SSC strength can be achieved. A preferred upper limit for the number of times the tempering process should be repeated is 3 times in total. In this case, even better ultrasonic inspection accuracy can be achieved. In other words, in the tempering process according to the present embodiment, the atmospheric gas in the tempering furnace is controlled as described above, and the tempering process is performed from one to four times in total. This will allow the depth of the decarburized layer constituting the seamless steel tube, according to the present embodiment, to be within a range of 150 µm or less. It should be noted that the tempering method described above is an example, and the depth of the decarburized layer can be controlled by another method. The tempering process will now be described in detail. Tempering process In the tempering process, the tempering is performed on the hollow shell that has undergone the aforementioned quenching process. As used in this description, the term “tempering” means reheating and holding the hollow shell after quenching at a temperature no higher than Q / zL / n / Lznz / q / Yi that the Aci point. In the present description, an tempering process followed by offline quenching is referred to as “offline tempering.” The tempering temperature is appropriately adjusted according to the chemical composition of the steel material and the yield strength to be achieved. In other words, the yield strength of a seamless steel tube is adjusted by setting the tempering temperature for a hollow shell having the chemical composition of the present model. In other words, the seamless steel tube is adjusted to have a yield strength of 655 MPa or more (95 ksi or more) by setting the tempering temperature for a hollow shell having the chemical composition of the present model. In this description, the term “tempering temperature” refers to the temperature of the furnace (tempering furnace) at which the hollow shell is heated and held after quenching. In the tempering process according to this embodiment, the tempering temperature is adjusted when the yield strength is to be achieved. Specifically, when a yield strength of 655 to less than 758 MPa (95 ksi) is to be achieved, a preferred tempering temperature is within the range of 650 to 740 °C. In this case, a lower preferred tempering temperature is 670 °C, and more preferably 680 °C. In this case, a higher preferred tempering temperature is 730 °C, and more preferably 720 °C. Also, in a case where a yield strength of 758 to less than 862 MPa (110 ksi grade) is desired, a preferred tempering temperature is within the range of 650 to 720 °C. In this case, a more preferred lower tempering temperature is 660 °C, and more preferably 670 °C. In this case, a more preferred upper tempering temperature is 715 °C, and more preferably 710 °C. Furthermore, in a case where a yield strength of 862 to 965 MPa (125 ksi grade) is desired, a preferred tempering temperature is within the range of 650 to 720 °C. In this case, a more preferred lower tempering temperature is 660 °C, and more preferably 670 °C. In this case, a more preferable upper limit of the tempering temperature is 715 °C and more preferably 710 °C. In this description, the term “tempering furnace time (tempering time)” means the period of time from when the hollow shell is loaded into the furnace (tempering furnace), which is used to heat and hold the hollow shell after quenching, until the hollow shell is removed from the furnace. If the tempering time is too short, the total volume ratio of tempered martensite to tempered bainite may be less than 90% in some cases. In this case, the seamless steel tube may not achieve excellent SSC strength. On the other hand, if the tempering time is too long, the aforementioned effect is saturated. Furthermore, if the tempering time is too long, the desired yield strength may not be achieved in some cases. Q / zL / n / Lznz / q / Yi Therefore, in the tempering process of the present embodiment, the tempering time is preferably set within a range of 10 to 180 minutes. A more preferred lower limit of the tempering time is 15 minutes. A more preferred upper limit of the tempering time is 120 minutes, and more preferably 90 minutes. In a seamless steel tube having the chemical composition of the present embodiment, it is certainly possible for a person skilled in the art to adjust the yield strength to a desired value by appropriately adjusting the tempering temperature and tempering time. A seamless steel tube can be produced according to the present embodiment using the production method described above. It should be noted that the aforementioned production method is an example, and the steel material, according to the present embodiment, can be produced using another production method. EXAMPLE Molten steels were produced containing the chemical compositions shown in Table 1. Q / zL / n / Lznz / q / Yi Table 1 Steel Chemical composition (in the unit of mass %, the rest being Fe and impurities) C Si Mn PS Al Cr Mo Ti V Nb BNO Ca Mg Zr REM Co w Cu Ni A 0.088 0.0019 0.0032 0.0016 - B 0.33 0.11 0.19 0.0210 0.0079 0.081 0.57 0.46 0.004 0.12 0.0002 0.002 0.004 0.0058 - - - - - - C 0.40 0.50 0.42 0.0039 0.0028 0.091 1.01 0.59 0.002 0.07 0.060 0.0049 0-00-48 0.0048 - D 0.26 0.10 0.06 0.0244 0.0002 0.094 0.39 0.86 0.005 0.13 0.097 0.0038 0.0011 0.0009 - - 40 0.2.001 0.0009 0.0289 0.0081 0.085 0.32 0.48 0.003 0.10 0.070 0.0025 0.0095 0.0044 - - - 0.0045 - F 0.28 0.074 0.0.0 0.099 0.48 1.49 0.003 0.21 0.051 0.0021 0.0098 0.0037 - - - - 0.34 - - - G 0.25 0.14 0.04 0.070 0.0091 0.40 0.004 0.26 0.040 0.0011 0.0044 0.0020 - - - - 0.50 - H 0.50 0.16 0.27 0.0149 0.0029 0.101.10 1.0 0.074 0.0045 0.0038 0.0003 - - - - - 0.05 - I 0.32 0.26 0.27 0.0167 0.0062 0.030 0.66 0.65 0.00306 0.036 0.0041 0.0006 0.0035 - - - - - - 0.10 J 0.38 0.29 0.23 0.0010 0.0073 0.054 1.18 0.30 0.005 0.07 0.089 0.0002 0.0095 0.0017 0.0002 - 0.0016 - - - K 0.38 0.19 0.85 0.0234 0.0023 0.006 0.71 0.58 0.002 0.12 0.023 0.0036 0.0044 0.0020 0.0009 - - 0.0020 - - - L 0.22 0.44 0.64 0.0236 0.0062 0.013 0.68 0.83 0.003 0.09 0.017 0.0034 0.0066 0.0013 - - - - 0.34 0.18 M 0.26 0.20 0.95 0.0038 0.0070 0.080 0.48 1.22 0.003 0.05 0.089 0.0014 0.0030 0.0014 - N 0.30 0.41 0.58 0.0004 0.0047 0.028 0.20 0.93 0.004 0.04 0.048 0.0015 0.0075 0.0021 - 0 0.35 0.38 0.60 0.0197 0.0070 0.010 1.40 0.48 0.004 0.08 0.093 0.0008 0.0003 0.0004 - P 0.26 0.46 0.90 0.0008 0.0071 0.066 0.40 0.14 0.003 0.09 0.016 0.0046 0.0032 0.0032 - Q 0.47 0.44 0.27 0.0027 0.0087 0.062 0.40 1.62 0.002 0.04 0.018 0.0042 0.0059 0.0006 - R 0.24 0.41 0.98 0.0215 0.0074 0.021 0.40 0.43 0.003 0.08 0.066 0.0020 0.0076 0.0080 - S 0.39 0.21 0.52 0.0075 0.0500 0.085 1.04 1.22 0.004 0.11 0.081 0.0007 0.0062 0.0025 - T 0.37 0.40 0.54 0.0094 0.0002 0.250 0.40 0.89 0.004 0.12 0.017 0.0048 0.0008 0.0050 -. A bar with an outside diameter of 310 mm was produced using the same cast steel. This bar was heated to 1250 °C and then hot-rolled to produce a hollow shell with an outside diameter of 273.5 mm and a wall thickness of 17.07 mm. The steels used for the hollow shells of each test number are shown in Tables 2 to 7. Q / zL / n / Lznz / q / Yi Table 2 Test No. Direct Hardened Steel Offline Heat Treatment Total Number of Hardening Times Decarburized Layer Depth (µm) YS (MPa) TS (MPa) YR (%) SSC Strength Ultrasonic Inspection Accuracy Number of Times Gas Used Hardening Process Tempering Process Hardening Temperature (°C) Hardening Time (min) Tempering Temperature (°C) Tempering Time (min) 1-1 A Performed 1 A 900 30 730 55 2 69 726 803 90.4 EA 1-2 A Performed 2 A 900 30 730 55 3 105 744 829 89.7 EA 1-3 A Performed 3 A 900 30 730 55 4 134 738 816 90.4 EB 1-4 A Completed 4 A 900 30 730 55 5 179 705 786 89.7 EC 1-5 A Not performed 2 A 900 30 730 55 2 95 658 725 90.7 EA 1-6 A Completed 1 B 900 30 730 55 2 85 744 818 90.9 EA 1-7 A Completed 2 B 900 30 730 55 3 104 656 735 89.3 EA 1-8 A Completed 3 B 900 30 730 55 4 145 756 862 87.7 EB 1-9 A Completed 4 B 900 30 730 55 5 174 686 789 87.0 EC 1-10 A Completed 1 B 900 30 730 110 2 108 750 847 88.5 E A 1-11 A Not performed 9 B 900 30 730 55 9 í. 100 732 820 89.3 E A 1-12 B Performed 1 A 900 30 730 55 2 73 674 755 89.3 E A 1-13 B Performed 2 A 900 30 730 55 3 112 665 737 90.2 E A 1-14 C Performed 1 A 900 30 730 55 2 79 691 760 90.9 E A 1-15 C Performed 9 A 900 30 730 55 3 103 679 755 89.9 E A. Table 3 Test No. Direct Hardened Steel Offline Heat Treatment Total Number of Hardening Times Decarburized Layer Depth (mp) YS (MPa) TS (MPa) YR (%) SSC Strength Ultrasonic Inspection Accuracy Number of Times Gas Used Hardening Process Tempering Process Hardening Temperature (°C) Hardening Time (min) Tempering Temperature (°C) Tempering Time (min) 1-16 D Performed 1 A 900 30 730 55 2 70 726 804 90.3 EA 1-17 D Performed 9 A 900 30 730 55 3 105 660 747 88.4 EA 1-18 E Performed 1 A 900 30 730 55 2 71 658 739 89.0 EA 1-19 E Performed 2 A 900 30 730 55 3 94 663 753 88.1 EA 1-20 F Completed 1 A 900 30 730 55 9 70 693 783 88.5 EA 1-21 F Completed 2 A 900 30 730 55 3 99 715 814 87.8 EA 1-22 G Completed 1 A 900 30 730 55 2 73 706 808 87.4 EA 1-23 G Completed 9 AA 900 30 730 55 3 109 699 792 88.3 EA 1-24 H Completed 1 A 900 30 730 55 9 Yo. 71 684 769 88.9 EA 1-25 I Completed 1 A 900 30 730 55 2 82 734 835 87.9 E A 1-26 J Performed 1 A 900 30 730 55 9 59 672 758 88.6 E A 1-27 K Performed 1 A 900 30 730 55 2 62 749 850 88.1 E A 1-28 L Performed 1 A 900 30 730 55 2 67 722 829 87.1 E A 1-29 M Performed 2 C 900 30 730 55 3 156 723 823 87.9 EC 1-30 M Performed 3 c 900 30 730 55 4 183 692 765 90.4 E C. > Pi l\ c l\ c l\ o Table 4 Test No. Steel Direct Hardened Offline Heat Treatment Total Number of Hardening Times Decarburized Layer Depth (mp) YS (MPa) TS (MPa) YR (%) SSC Strength Ultrasonic Inspection Accuracy Number of Times Gas Used Hardening Process Tempering Process Hardening Temperature (°C) Hardening Time (min) Tempering Temperature (°C) Tempering Time (min) 2-1 A Performed 1 A 900 30 700 55 2 66 800 893 89.6 EA 2-2 A Performed 9 A 900 30 700 55 3 100 805 896 89.8 EA 2-3 A Performed 3 A 900 30 700 55 4 131 820 905 90.6 EB 2-4 A Performed 4 A 900 30 700 55 5 172 816 902 90.5 EC 2-5 A Not done 2 A 900 30 700 55 2 96 797 890 89.6 EA 2-6 A Done 1 B 900 30 700 55 2 75 788 877 89.9 EA 2-7 A Completed 2 B 900 30 700 55 3 101 809 890 90.9 EA 2-8 A Completed 3 B 900 30 700 55 4 147 816 894 91.3 EB 2-9 A Completed 4 B 900 30 700 55 5 175 815 892 91.4 EC 2-10 A Completed 1 B 900 30 690 110 2 110 779 863 90.3 E A 2-11 A Not completed 2 B 900 30 700 55 9 99 780 870 89.7 E A 2-12 B Completed 1 A 900 30 700 55 2 71 795 872 91.2 E A 2-13 B Completed 2 A 900 30 700 55 3 108 814 883 922 E A 2-14 C Completed 1 A 900 30 700 55 2 77 783 871 89.9 E A 2-15 C Completed 2 A 900 30 700 55 3 101 799 882 90.6 E A 2-16 D Completed 1 A 900 30 700 55 9 í. 64 794 882 90.0 E A 2-17 D Performed 9 AA 900 30 700 55 3 105 812 896 90.6 E A 2-18 E Performed 1 A 900 30 700 55 9 í. 79 778 863 90.2 E A 2-19 E Performed 9 AA 900 30 700 55 3 98 797 874 91.2 E A. > Pi l\ c l\ c l\ o Table 5 Test No. Direct Hardened Steel Offline Heat Treatment Total Number of Hardening Times Decarburized Layer Depth (mp) YS (MPa) TS (MPa) YR (%) SSC Strength Ultrasonic Inspection Accuracy Number of Times Gas Used Hardening Process Tempering Process Hardening Temperature (°C) Hardening Time (min) Tempering Temperature (T) Tempering Time (min) 2-20 F Performed 1 A 900 30 700 55 2 77 788 873 90.3 EA 2-21 F Performed 9 A 900 30 700 55 3 100 793 877 90.4 EA 2-22 G Performed 1 A 900 30 700 55 2 72 790 873 90.5 EA 2-23 G Performed 2 A 900 30 700 55 3 108 808 880 91.8 EA 2-24 H Completed 1 A 900 30 700 55 9 78 782 863 90.6 EA 2-25 I Completed 1 A 900 30 700 55 2 74 788 873 90.3 EA 2-26 J Completed 1 A 900 30 700 55 2 61 791 876 90.3 EA 2-27 K Completed 1 A 900 30 700 55 9 63 810 885 91.5 EA 2-28 L Completed 1 A 900 30 700 55 9 Yo. 64 795 872 91.2 EA 2-29 M Completed 9 AC 900 30 700 55 3 156 816 905 90.2 EC 2-30 M Performed 3 c 900 30 700 55 4 181 821 904 90.8 EC 2-31 N Performed 1 A 900 30 700 55 9 76 798 885 90.2 NA A 2-32 0 Performed 1 A 900 30 700 55 2 67 787 868 90.7 NA A 2-33 P Performed 1 A 900 30 700 55 9 64 797 877 90.9 NA A 2-34 Q Performed 1 A 900 30 700 55 2 73 788 873 90.3 NA A 2-35 R Performed 1 A 900 30 700 55 9 í. 66 804 893 90.0 NA A 2-36 S Performed 1 A 900 30 700 55 9 74 785 871 90.1 NA A 2-37 T Performed 1 A 900 30 700 55 9 í. 69 794 873 91.0 NA A. > Pi l\ c l\ c l\ o Table 6 Test No. Steel Direct Quenched Offline Heat Treatment Total Number of Quenching Times Decarburized Layer Depth (pm) YS (MPa) TS (MPa) YR (%) SSC Strength Ultrasonic Inspection Accuracy Number of Times Gas Used Quenching Process Tempering Process Quenching Temperature (°C) Quenching Time (min) Tempering Temperature (°C) Tempering Time (min) 3-1 A Performed 1 A 900 30 670 55 2 63 922 1024 90.0 EA 3-2 A Performed 9 A 900 30 670 55 3 96 892 980 91.0 EA 3-3 A Performed 3 A 900 30 670 55 4 132 895 1002 89.3 EB 3-4 A Performed 4 A 900 30 670 55 5 171 898 991 90.6 EC 3-5 A Not done 2 A 900 30 670 55 2 92 884 982 90.0 EA 3-6 A Done 1 B 900 30 670 55 2 74 876 973 90.0 EA 3-7 A Completed 2 B 900 30 670 55 3 99 879 981 89.6 EA 3-8 A Completed 3 B 900 30 670 55 4 144 920 1020 90.2 EB 3-9 A Completed 4 B 900 30 670 55 5 168 905 1004 90.1 EC 3-10 A Completed 1 B 900 30 670 110 2 107 888 982 90.4 E A 3-11 A Not completed 2 B 900 30 670 55 9 92 885 974 90.9 E A 3-12 B Completed 1 A 900 30 670 55 2 63 909 1016 89.5 E A 3-13 B Completed 2 A 900 30 670 55 3 101 880 968 90.9 E A 3-14 C Completed 1 A 900 30 670 55 2 74 864 965 89.5 E A 3-15 C Completed 2 A 900 30 670 55 3 98 928 1024 90.6 E A 3-16 D Completed 1 A 900 30 670 55 9 í. 59 892 985 90.6 E A 3-17 D Performed 9 AA 900 30 670 55 3 102 899 989 90.9 E A 3-18 E Performed 1 A 900 30 670 55 9 í. 69 870 978 89.0 E A 3-19 E Performed 9 AA 900 30 670 55 3 93 926 1021 90.7 E A. > Pi l\ c l\ c l\ o Tabla? Test No. Direct Hardened Steel Offline Heat Treatment Total Number of Hardening Cycles Decarburized Layer Depth (µm) YS (MPa) TS (MPa) YR (%) SSC Strength Ultrasonic Inspection Accuracy Number of Cycles Gas Used Hardening Process Tempering Process Hardening Temperature (°C) Hardening Time (min) Tempering Temperature (°C) Tempering Time (min) 3-20 F Performed 1 A 900 30 670 55 2 67 893 1002 89.1 EA 3-21 F Performed 2 A 900 30 670 55 3 92 876 981 89.3 EA 3-22 G Performed 1 A 900 30 670 55 2 66 892 991 90.0 EA 3-23 G Completed 2 A 900 30 670 55 3 103 922 1031 89.4 EA 3-24 H Completed 1 A 900 30 670 55 2 70 922 1018 90.6 EA 3-25 I Completed 1 A 900 30 670 55 2 71 886 990 89.5 EA 3-26 J Completed 1 A 900 30 670 55 2 56 925 1027 90.1 EA 3-27 K Completed 1 A 900 30 670 55 0 i. 55 915 1014 90.2 EA 3-28 L Completed 1 A 900 30 670 55 2 62 890 978 91.0 EA 3-29 M Completed 2 C 900 30 670 55 3 151 904 1011 89.4 EC 3-30 M Performed 3 C 900 30 670 55 4 180 910 1010 90.1 EC 3-31 N Performed 1 A 900 30 670 55 2 74 904 1006 89.9 NA A 3-32 0 Performed 1 A 900 30 670 55 2 59 883 985 89.6 NA A 3-33 P Performed 1 A 900 30 670 55 2 57 887 977 90.8 NA A 3-34 Q Performed 1 A 900 30 670 55 2 63 924 1018 90.8 NA A 3-35 R Performed 1 A 900 30 670 55 2 59 922 1034 89.2 NA A 3-36 S Performed 1 A 900 30 670 55 2 66 921 1028 89.6 NA A 3-37 T Performed 1 A 900 30 670 55 2 62 881 968 91.0 NA A. The “Direct Quenching” column in Tables 2–7 shows whether or not direct quenching was performed on a hollow shell of each test number after hot rolling. Specifically, for test numbers other than 1–5, 1–11, 2–5, 2–11, 3–5, and 3–11, direct quenching was performed. This involved supplemental heating of a hollow shell after hot rolling for 10 minutes in a supplemental heating furnace at 920 °C and then water-cooling. In the examples presented, the same atmospheric gas used in the supplemental heating furnace as in the quenching furnace described below was used (see the “Gas Used” column in Tables 2–7). Subsequently, each hollow shell was tempered at an annealing temperature of 550 °C for 45 minutes.On the other hand, in test numbers 1-5, 1-11, 2-5, 2-11, 3-5 and 3-11 the hollow shells after hot rolling were allowed to cool to room temperature instead of performing direct tempering. Subsequently, the hollow shell of each test number was repeatedly subjected to offline heat treatment (offline quenching and offline tempering) the number of times shown in Tables 2 to 7. The total number of quenching cycles (total number of direct quenching and offline quenching cycles) performed on the hollow shell of each test number was as shown in Tables 2 to 7. The atmospheric gas in the quenching furnace during offline quenching is shown in the “Gas Used” column in Tables 2 to 7. In this document, “Gas A” means a gas obtained by burning a gas mixture containing a total of 11 to 15% by volume fraction of flammable gas (methane, ethylene, hydrogen gas, and carbon monoxide), the remainder being air. The oxygen concentration of Gas A was 2 to 7% by volume fraction, excluding water vapor. Gas “B” means a gas obtained by the combustion of a gas mixture containing a total of 5 to 8% by volume fraction of flammable gas (methane, ethylene, propane, and butane), the remainder being air. The oxygen concentration in Gas B was 2 to 7% by volume fraction, excluding water vapor. Gas “C” means a gas obtained by the combustion of a gas mixture containing a total of 5 to 8% by volume fraction of flammable gas (methane, ethylene, hydrogen gas, and carbon monoxide), the remainder being air. The oxygen concentration in Gas C was 12 to 15% by volume fraction, excluding water vapor. In the tempering treatment performed on the hollow shell of each test number, the tempering temperature (°C) and tempering time (time spent in the tempering furnace) (min) are shown in Tables 2 to 7. In addition, for the hollow shell of each test number, the cooling rate during CRsoo soo tempering was determined from the surface temperature of the hollow shell of each test number and a period of time from the inlet side Q / zL / n / Lznz / q / Yi to the delivery side of the tempering equipment. All cooling rates thus determined during the tempering of each test number were 30 °C / so plus. In addition, for the tempering treatment performed in the hollow shell of each test number, the tempering temperature (°C) and tempering time (time in the tempering furnace) (min) at the tempering temperature are shown in Tables 2 to 7. In the present example, the tempering temperature (°C) was brought to the temperature of the furnace in which the reheating was carried out in the tempering treatment. Similarly, the tempering temperature (°C) was brought to the temperature of the furnace in which the reheating was carried out in the tempering treatment. It should be noted that the tempering temperature and tempering time were adjusted so that the yield strength of the hollow shell of each test number was 655 MPa or higher (95 ksi or higher). Specifically, the tempering temperature and tempering time were adjusted so that the yield strength of the hollow shell of test numbers 1-1 to 1-30 was 655 to less than 758 MPa (95 ksi grade). The tempering temperature and tempering time were adjusted so that the yield strength of the hollow shell of test numbers 2-1 to 2-37 was 758 to less than 862 MPa (110 ksi grade). The tempering temperature and tempering time were adjusted so that the yield strength of the hollow shell of test numbers 3-1 to 3-37 was 862 to 965 MPa (grade 125 ksi). The seamless steel tube of each test number was produced by the production process described above. Assessment test The seamless steel tube of each test number, after the tempering mentioned above, was subjected to a decarburized layer depth measurement test, a tensile test, an SSC strength evaluation test, and an ultrasonic inspection test. Measurement of decarburized layer depth The depth of a decarburized layer was measured using the method described above, utilizing the seamless steel tube from each test number. Specifically, a test sample was taken, comprising an internal surface of the seamless steel tube from each test number, with an observation surface measuring 10 mm along the tube axis and 10 mm radially. After polishing to a mirror finish, the observation surface was immersed in a 2% Nital chemical etching reagent for approximately 10 seconds to perform the chemical etching. The etched observation surface Q / zL / n / Lznz / q / Yi was chemically observed in 10 fields of view in a bright-field image using an optical microscope. The area of ​​each field of view was 0.1 mm2 (200x magnification). In each field of view of each test number, ferrite was identified based on contrast and defined as a decarburized layer. The deepest position of the decarburized layer in each identified field of view and its depth from the surface of the seamless steel tube were determined. An arithmetic mean of the decarburized layer depths in 10 fields of view was defined as the decarburized layer depth (pm). The decarburized layer depth (pm) of the seamless steel tube for each test number is shown in Tables 2 through 7. Traction test A tensile test was performed using the method described above on the seamless steel tube of each test number to measure the yield strength, tensile strength, and yield ratio. Specifically, a tensile test was performed in accordance with ASTM E8 / E8M (2013). A round bar tensile test specimen with a parallel portion diameter of 4 mm and a parallel portion length of 35 mm was made from a central portion of the wall thickness of the seamless steel tube of each test number. The axial direction of the round bar tensile test specimen was parallel to the axis direction of the seamless steel tube. The tensile tests were conducted at room temperature (25 °C) in the atmosphere using the round bar test specimen of each test number to obtain the yield strength (MPa) of the seamless steel tube of each test number. It should be noted that, in this example, the stress obtained at a total elongation of 0.5% (0.5% of the test stress) in the tensile test is less than 758 MPa; therefore, 0.5% of the test stress was used as the yield strength. Similarly, in this example, the stress obtained at a total elongation of 0.7% (0.7% of the test stress) in the tensile test is 758 MPa or greater; therefore, 0.7% of the test stress was used as the yield strength. Furthermore, the maximum stress obtained during uniform elongation in the tensile test was used as the tensile strength. The ratio between the tensile strength and the yield strength is used as the yield ratio. The yield strength (MPa), tensile strength (MPa), and yield ratio (%) obtained are shown in Tables 2 through 7 as YS (MPa), TS (MPa), and YR (%). SSC Strength Assessment Test The SSC strength was evaluated using a method in accordance with Method A specified in NACE TM0177-2005, using seamless steel pipes of each number of Q / zL / n / Lznz / q / Yi test. Specifically, each of the SSC strength evaluation tests of seamless steel tubes will be described for test numbers 1-1 to 1-30 having a yield strength of 655 to less than 758 MPa (grade 95 ksi), test numbers 2-1 to 2-37 having a yield strength of 758 to less than 862 MPa (grade 110 ksi), and test numbers 3-1 to 3-37 having a yield strength of 862 to 965 MPa (grade 125 ksi). SSC strength of seamless steel tubes of test numbers 1-1 to 1-30 A round bar test specimen with a diameter of 6.35 mm and a parallel portion length of 25.4 mm was taken from a central wall thickness portion of the seamless steel pipe of test numbers 1-1 through 1-30. The axial direction of the round bar test specimen was parallel to the tube axis direction of the seamless steel pipe. The tensile test was applied in the axial direction of the test specimens of test numbers 1-1 through 1-30. At this time, the stress to be applied to each test specimen was adjusted to 90% of the actual tensile strength of the seamless steel pipe of each test number 1-1 through 1-30, in accordance with Method A specified in NACE TM0177-2005. A mixed aqueous solution containing 5.0% by mass of sodium chloride and 0.5% by mass of acetic acid (NACE solution A) was used as the test solution. The test solution at 24 °C was poured into three separate test vessels to provide test baths. Each of the three round-bar test specimens, to which stress was applied, was immersed in a different test bath. After the test bath was degassed, 1 atm of gaseous H₂S was introduced to saturate it. The test bath was maintained at 24 °C for 720 hours. The round bar test specimens from test numbers 1-1 to 1-30 were observed after being left for 720 hours to determine if sulfide stress cracking (SSC) had occurred. Specifically, the test specimens were visually inspected after being immersed for 720 hours. As a result of the observation, a test number in which no cracking was confirmed in the test specimens was determined to be “E” (Excellent). On the other hand, a test number in which cracking was confirmed in at least one test specimen was determined to be “NA” (Not Acceptable). The results of the SSC strength evaluation test for test numbers 1-1 to 1-30 are shown in Tables 2 and 3. SSC strength of seamless steel pipe from test numbers 2-1 to 2-37 A test sample of round bar having a diameter of 6.35 mm and a parallel portion length of 25.4 mm was taken from a central portion of the wall thickness of the steel tube Q / zL / n / Lznz / q / Yi seamless test specimens 2-1 through 2-37. The axial direction of the round bar test specimen was parallel to the axis direction of the seamless steel tube. Tensile stress was applied in the axial direction of the test specimens 2-1 through 2-37. At this time, the stress to be applied to each test specimen was adjusted to be 90% of the actual yield strength of the seamless steel tube of each test specimen 2-1 through 2-37, in accordance with Method A specified in NACE TM0177-2005. A mixed aqueous solution containing 5.0% by mass of sodium chloride and 0.5% by mass of acetic acid (NACE solution A) was used as the test solution. The test solution at 24 °C was poured into three separate test vessels to provide test baths. Each of the three test samples of the round bar under stress was immersed in a different test bath. After degassing the test bath, 1 atm of gaseous H₂S was introduced to saturate it. The test bath was maintained at 24 °C for 720 hours. The round bar test specimens from test numbers 2-1 to 2-37 were observed after being held for 720 hours to determine whether sulfide stress cracking (SSC) had occurred. Specifically, the test specimens were visually inspected after being submerged for 720 hours. As a result of this observation, a test number for which no cracking was confirmed in any of the three test specimens was determined to be “E” (Excellent). Conversely, a test number for which cracking was confirmed in at least one test specimen was determined to be “NA” (Not Acceptable). The results of the SSC strength evaluation test for test numbers 2-1 to 2-37 are shown in Tables 4 and 5. SSC strength of seamless steel pipe from test numbers 3-1 to 3-37 A round bar test specimen with a diameter of 6.35 mm and a parallel portion length of 25.4 mm was taken from a central portion of the wall thickness of seamless steel pipe from test numbers 3-1 to 3-37. The axial direction of the round bar test specimen was parallel to the axis direction of the seamless steel pipe. Tensile stress was applied in the axial direction of the test specimen from test numbers 3-1 to 3-37. At this time, the stress to be applied to each test specimen was adjusted to be 90% of the actual yield strength of the seamless steel pipe from each test number 3-1 to 3-37, in accordance with Method A specified in NACE TM0177-2005. A mixed aqueous solution containing 5.0% by mass of sodium chloride, 0.41% by mass of sodium acetate, and 2.5% by mass of acetic acid (NACE solution B) was used as the test solution. The test solution at 24 °C was poured into three separate test containers for Q / zL / n / Lznz / q / Yi provided test baths. Each of the three round bar test specimens subjected to stress was immersed in a different test bath. After degassing the test bath, a gas mixture of 0.1 atm of gaseous H2S and 0.9 atm of gaseous CO2 was introduced to saturate the test bath. The test bath was maintained at 24 °C for 720 hours. The round bar test specimens of test numbers 3-1 through 3-37 were observed after being held for 720 hours to determine whether sulfide stress cracking (SSC) had occurred. Specifically, the test specimens were visually inspected after being immersed for 720 hours. As a result of this observation, a test number for which no cracking was confirmed in any of the three test specimens was determined to be “E” (Excellent). Conversely, a test number for which cracking was confirmed in at least one test specimen was determined to be “NA” (Not Acceptable). The results of the SSC strength evaluation test for test numbers 3-1 through 3-37 are shown in Tables 6 and 7. Ultrasonic inspection test Using the seamless steel tube of each test number, an ultrasonic inspection test was performed as follows. Specifically, using a local immersion-type ultrasonic inspection apparatus, an ultrasonic inspection test was performed by detecting oblique defects in the L direction to determine the SN ratios. The sensitivity adjustment of the ultrasonic inspection apparatus was performed using an artificial defect, which was provided at a depth of 3% of the wall thickness in the direction of the tube axis from the inner surface of the seamless steel tube of each test number. The artificial defect was a notch formed by electrical discharge machining and had dimensions of 0.275 mm deep, 1 mm wide, and 50.8 mm long. The longitudinal direction of the artificial defect was parallel to the circumferential direction of the tube. Using an ultrasonic inspection device with adjusted sensitivity, the SN ratio was measured for each test sample as follows. By passing ultrasonic waves 10 times, a defect height and a maximum noise height were determined. The angle of incidence of the ultrasonic waves incident on a seamless steel tube from its outer surface during flaw detection was set at 45°. The average of 10 defect heights was defined as the S value. The average of 10 maximum noise heights was defined as the N value. Using the obtained S and N values, the SN ratio (= S / N) was determined. Q / zL / n / Lznz / q / Yi When the determined SN ratio was greater than 4, even more excellent ultrasonic inspection accuracy was observed (indicated by “A” in Tables 2 through 7). When the determined SN ratio was between 3 and 4, excellent ultrasonic inspection accuracy was observed (indicated by “B” in Tables 2 through 7). Conversely, when the SN ratio was less than 3, non-excellent ultrasonic inspection accuracy was observed (indicated by “C” in Tables 2 through 7). The ultrasonic inspection test results for each test number are shown in Tables 2 through 7. Test results With reference to Table 1, Table 2, and Table 3, in the seamless steel tubes of test numbers 1-1 to 1-3, 1-5 to 1-8, and 1-10 to 1-28, the chemical composition of the base material was appropriate, the yield strength of the base material was 655 to less than 758 MPa (95 ksi grade), the yield ratio of the base material was 85.0% or higher, and the depth of the decarburized layer was 150 µm or less. As a result, they exhibited excellent SSC strength in the SSC strength test. Furthermore, as a result, they exhibited excellent ultrasonic inspection accuracy in the ultrasonic inspection test. In the seamless steel tubes of test numbers 1-1, 1-2, 1-5 to 1-7, and 1-10 to 128, the depth of the decarburized layer was 130 µm or less. As a result, they exhibited even more excellent ultrasonic inspection accuracy in the ultrasonic inspection test. On the other hand, in the inclined seamless steel tubes of test numbers 1-4 and 1-9, the number of tempering cycles was too high. For this reason, the depth of the decarburized layer was greater than 150 µm. As a result, excellent ultrasonic inspection accuracy was not observed. In the seamless steel tubes of test numbers 1-29 and 30, the atmospheric gas in the quenching furnace was unsuitable. For this reason, the depth of the decarburized layer was greater than 150 pm. As a result, excellent ultrasonic inspection accuracy was not achieved. Furthermore, with reference to Table 1, Table 4, and Table 5, in the seamless steel tubes of test numbers 2-1 to 2-3, 2-5 to 2-8, and 2-10 to 2-28, the chemical composition of the base material was appropriate, the yield strength of the base material was 758 to less than 862 MPa (grade 110 ksi), the yield ratio of the base material was 85.0% or higher, and the depth of the decarburized layer was 150 µm or less. As a result, they exhibited excellent SSC strength in the SSC strength test. Additionally, they exhibited excellent ultrasonic inspection accuracy in the ultrasonic inspection test. Q / zL / n / Lznz / q / Yi In the seamless steel tubes of test numbers 2-1, 2-2, 2-5 to 2-7, and 2-10 to 228, the depth of the decarburized layer was 130 µm or less. As a result, they exhibited excellent ultrasonic inspection accuracy in the ultrasonic inspection test. On the other hand, in the seamless inclined tubes of test numbers 2-4 and 2-9, the number of tempers was too high. For this reason, the depth of the decarburized layer was greater than 150 pm. As a result, excellent ultrasonic inspection accuracy was not observed. In the seamless steel tubes of test numbers 2-29 and 2-30, the atmospheric gas in the quenching furnace was unsuitable. For this reason, the depth of the decarburized layer was greater than 150 µm. As a result, excellent ultrasonic inspection accuracy was not achieved. In the seamless steel tube of test number 2-31, the Cr content was too low. As a result, excellent SSC strength was not observed. In the seamless steel tube of test number 2-32, the Cr content was too high. As a result, excellent SSC strength was not observed. In the seamless steel tube of test number 2-33, the Mo content was too low. As a result, excellent SSC strength was not observed. In the seamless steel tube of test number 2-34, the Mo content was too high. As a result, excellent SSC strength was not observed. In the seamless steel tube of test number 2-35, the oxygen content was too high. As a result, excellent SSC strength was not observed. In the seamless steel tube of test number 2-36, the sulfur content was too high. As a result, excellent SSC strength was not observed. In the seamless steel tube of test number 2-37, the Al content was too high. As a result, excellent SSC strength was not observed. Furthermore, with reference to Table 1, Table 6, and Table 7, in the seamless steel tubes of test numbers 3-1 to 3-3, 3-5 to 3-8, and 3-10 to 3-28, the chemical composition of the base material was appropriate, the yield strength of the base material was 862 to 965 MPa (grade 125 ksi), the yield ratio of the base material was 85.0% or higher, and the depth of the decarburized layer was 150 pm or less. As a result, they exhibited excellent SSC strength in the SSC strength test. Additionally, they exhibited excellent ultrasonic inspection accuracy in the ultrasonic inspection test. In the seamless steel tubes of test numbers 3-1, 3-2, 3-5 to 3-7, and 3-10 to 328, the depth of the decarburized layer was 130 µm or less. As a result, they exhibited excellent ultrasonic inspection accuracy in the ultrasonic inspection test. Q / zL / n / Lznz / q / Yi On the other hand, in the seamless steel tubes of test numbers 3-4 and 3-9, the number of temperings was too high. For this reason, the depth of the decarburized layer was greater than 150 µm. As a result, excellent ultrasonic inspection accuracy was not achieved. In the seamless steel tubes of test numbers 3-29 and 3-30, the atmospheric gas in the tempering furnace was unsuitable. For this reason, the depth of the decarburized layer was greater than 150 µm. As a result, excellent ultrasonic inspection accuracy was not achieved. In the seamless steel tube of test number 3-31, the Cr content was too low. As a result, excellent SSC strength was not observed. In the seamless steel tube of test number 3-32, the Cr content was too high. As a result, excellent SSC strength was not observed. In the seamless steel tube of test number 3-33, the Mo content was too low. As a result, excellent SSC strength was not observed. In the seamless steel tube of test number 3-34, the Mo content was too high. As a result, excellent SSC strength was not observed. In the seamless steel tube of test number 3-35, the oxygen content was too high. As a result, excellent SSC strength was not observed. In the seamless steel tube of test number 3-36, the sulfur content was too high. As a result, excellent SSC strength was not observed. In the seamless steel tube of test number 3-37, the Al content was too high. As a result, excellent SSC strength was not observed. One modality of the present description has been described above. However, the modality described above is merely an example for implementing the present description. Consequently, the present description is not limited to the above modality, and the above modality may be modified and implemented within an appropriate range that does not deviate from the essence of the present description. INDUSTRIAL APPLICABILITY The seamless steel pipe, according to the present description, is widely applicable to steel materials that will be used in a severe environment, such as a polar region, and preferably can be used as a seamless steel pipe that is used in an oil well environment and more preferably can be used as a seamless steel pipe for pipe wrapping, tubing, or conduit pipes or the like.

Claims

1. A seamless steel tube, characterized in that it comprises a base material and a decarburized layer formed on a surface of the base material, wherein the chemical composition of the base material consists, in % by mass, of: C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.01 to 1.00%, P: 0.0300% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.30 to 1.20%, Mo: 0.30 to 1.50%, Ti: 0.002 to 0.050%, V: 0.01 to 0.30%, Nb: 0.002 to 0.100%, B: 0.0001 to 0.0050%, N: 0.0100% or less, O: 0.0050% or less, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and the remainder being Fe and impurities, wherein the yield strength of the base material is 655 MPa or more, the yield ratio of the base material is 85.0% or more, and the decarburized layer has a depth of 150 pm or less.

2. The seamless steel tube, according to claim 1, characterized in that the chemical composition of the base material contains one or more types of elements selected from the group consisting of: Ca: from 0.0001 to 0.0100%, Mg: from 0.0001 to 0.0100%, Zr: from 0.0001 to 0.0100%, and rare earth metal: from 0.0001 to 0.0100%.

3. The seamless steel tube, according to claim 1, characterized in that the chemical composition of the base material contains one or more types of elements selected from the group consisting of: Co: from 0.02 to 0.50%, and W: from 0.02 to 0.50%.

4. The seamless steel tube, according to claim 2, characterized in that the chemical composition of the base material contains one or more types of element selected from the group consisting of: Co: from 0.02 to 0.50%, and W: from 0.02 to 0.50%.

5. The seamless steel tube, according to claim 1, characterized in that the chemical composition of the base material contains one or more types of elements selected from the group consisting of: Cu: from 0.02 to 0.50%, and Ni: from 0.02 to 0.50%.

6. The seamless steel tube, according to claim 2, characterized in that the chemical composition of the base material contains one or more types of elements selected from the group consisting of: Cu: from 0.02 to 0.50%, and Ni: from 0.02 to 0.50%.

7. The seamless steel tube, according to claim 3, characterized in that the chemical composition of the base material contains one or more types of elements selected from the group consisting of: Cu: from 0.02 to 0.50%, and Ni: from 0.02 to 0.50%.

8. The seamless steel tube, according to claim 4, characterized in that the chemical composition of the base material contains one or more types of elements selected from the group consisting of: Q / zL / n / Lznz / q / Yi > Π Ν C Ν -j Cu: from 0.02 to 0.50%, and É Ni: from 0.02 to 0.50%. * σ 9. The seamless steel tube, according to claim 1, characterized in that the decarburized layer has a depth of 130 pm or less.

10. The seamless steel tube, according to claim 2, wherein the decarburized layer has a depth of 130 pm or less.

11. The seamless steel tube, according to claim 3, characterized in that the decarburized layer has a depth of 130 pm or less.

12. The seamless steel tube, according to claim 4, characterized in that the decarburized layer has a depth of 130 pm or less.

13. The seamless steel tube, according to claim 5, characterized in that the decarburized layer has a depth of 130 pm or less.

14. The seamless steel tube, according to claim 6, characterized in that the decarburized layer has a depth of 130 pm or less.

15. The seamless steel tube, according to claim 7, characterized in that the decarburized layer has a depth of 130 pm or less.

16. The seamless steel tube, according to claim 8, characterized in that the decarburized layer has a depth of 130 pm or less.

17. The seamless steel pipe, according to any of claims 1 to 16, characterized in that the seamless steel pipe is an oil well steel pipe