SEAMLESS STEEL TUBE FOR USE IN BITTER ENVIRONMENTS

Abstract

A seamless steel tube is provided that has a yield strength in the range of 758 to 862 MPa (grade 110 ksi) and excellent resistance to HIC. The seamless steel tube according to the present invention has a chemical composition consisting of, by mass %, C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.070%, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, rare earth metal: 0.0001 to 0.0015%, Ca: 0.0001 to 0.0100%, N: 0.0100% or less, and O: 0.0020% or less, the remainder being Fe and impurities, and satisfying Formula (1) described in the description. A maximum predicted major axis of inclusions is 150 µm or less; the maximum predicted major axis is determined by extreme value statistical processing. The yield strength is within the range of 758 to 862 MPa.

Description

SEAMLESS STEEL TUBE FOR USE IN BITTER ENVIRONMENTS FIELD OF INVENTION

[0001] The present invention relates to a steel tube and, more particularly, to a seamless steel tube. STATE OF THE ART

[0002] 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 for improved strength in steel oil well pipes. Specifically, 80 ksi grade (yield strength 80 to less than 95 ksi, i.e., 552 to less than 655 MPa) and 95 ksi grade (yield strength 95 to less than 110 ksi, i.e., 655 to less than 758 MPa) steel oil well pipes are in widespread use, and recently, 110 ksi grade (yield strength 110 to 125 ksi, i.e., 758 to 862 MPa) steel oil well pipes are also beginning to be requested.

[0003] Most deep wells are located in a sour environment containing corrosive hydrogen sulfide. In this description, the term sour environment means an acidified environment containing hydrogen sulfide. Note that in some cases, a sour environment may also contain carbon dioxide. Steel oil well pipes for use in such sour environments are required to have not only high strength but also resistance to sulfide stress cracking (hereafter referred to as SSC resistance).

[0004] The technology for improving the SSC resistance of steel pipes for oil wells is described in Japanese patent application publication no. 2000-256783 (patent literature 1), Japanese patent application publication no. 2005-350754 (patent literature 2), Japanese patent application publication no. 2005-350754 (patent literature 3), Japanese patent application publication no. 2012-26030 (patent literature 4), and international application publication no. WO 2010 / 150915 (patent literature 5).

[0005] A high-strength oil well steel described in patent literature 1 contains, in wt. percent, C: 0.2 to 0.35%, Cr: 0.2 to 0.7%, Mo: 0.1 to 0.5%, and V: 0.1 to 0.3%. The amount of precipitating carbides is within the range of 2 to 5 percent by weight, and among the precipitating carbides, the proportion of MC-type carbides is within the range of 8 to 40 percent by weight, and the above austenite grain size is No. 11 or higher in terms of the grain size numbers defined in ASTM. Patent literature 1 describes the above-mentioned high-strength oil well steel as having excellent toughness and resistance to sulfide stress corrosion cracking. CRLenn / Lznz / E / Yii

[0006] An oil well steel described in patent literature 2 is a low-alloy steel containing, by mass percent, C: 0.15 to 0.3%, Cr: 0.2 to 1.5%, Mo: 0.1 to 1%, V: 0.05 to 0.3%, and Nb: 0.003 to 0.1%. The amount of precipitating carbides is within the range of 1.5 to 4% by mass, the proportion of MC-type carbides to the total amount of carbides is within the range of 5 to 45% by mass, and when the wall thickness of the product is taken as t (mm), the proportion of M23C6-type carbides is 200 / t or less by mass percent. Patent literature 2 describes the aforementioned oil well steel as having excellent toughness and resistance to sulfide stress corrosion cracking.

[0007] A low-alloy steel for oilfield tubular products described in patent literature 3 contains, in % by mass, C: 0.20 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.05 to 1.0%, P: 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.10%, Cr: 0.1 to 1.0%, Mo: 0.5 to 1.0%, Ti: 0.002 to 0.05%, V: 0.05 to 0.3%, B: 0.0001 to 0.005%, N: 0.01% or less, and O (oxygen): 0.01% or less. A medium-valued width H and a hydrogen diffusion coefficient D (10-6 cm2 / s) satisfy the expression (30H + D < 19.5). It is described in patent literature 3 that the aforementioned low-alloy steel for oilfield tubular products has excellent resistance to SSC even when the steel has a high strength with a yield strength (YS) of 861 MPa or more.

[0008] A steel oil well pipe described in patent literature 4 has a composition consisting, in % by mass, C: 0.18 to 0.25%, Si: 0.1 to 0.3%, Mn: 0.4 to 0.8%, P: 0.015% or less, S: 0.005% or less, Al: 0.01 to 0.1%, Cr: 0.3 to 0.8%, Mo: 0.5 to 1.0%, Nb: 0.003 to 0.015%, Ti: 0.002 to 0.05% and B: 0.003% or less, the remainder being Fe and unavoidable impurities. In the microstructure of the aforementioned oil well steel pipe, a tempered martensite phase is the main phase, the number of M3C or M2C included in a 20 pm x 20 pm region and having an aspect ratio of 3 or less and a major axis of 300 nm or more when the carbide shape is taken as elliptical is not more than 10, the M23C6 content is less than 1% by mass, acicular M2C precipitates within the grains, and the amount of Nb that precipitates as carbides having a size of 1 pm or more is less than 0.005% by mass.Patent literature 4 describes the aforementioned oil well steel pipe as being excellent in terms of resistance to sulfide stress cracking even when the yield strength is 862 MPa or more.

[0009] A seamless steel pipe for oil wells described in patent literature 5 has a composition consisting, in % by mass, of C: 0.15 to 0.50%, Si: 0.1 to 1.0%, Mn: 0.3 to 1.0%, P: 0.015% or less, S: 0.005% or less, Al: 0.01 to 0.1%, N: 0.01% or less, Cr: 0.1 to 1.7%, Mo: 0.4 to 1.1%, V: 0.01 to 0.12%, Nb: 0.01 to 0.08%, and B: 0.0005 to 0.003%, wherein the proportion of Mo contained as dissolved Mo is 0.40% or more, the remainder being Fe and unavoidable impurities. In the CRLrnn / Lznz / E / Yi The microstructure of the aforementioned oil well steel pipe shows that a tempered martensite phase is the main phase, the austenite grain size number is 8.5 or higher, and substantially particulate M2C-type precipitates are dispersed in an amount of 0.06% by mass or more. It is described in patent literature 5 that the aforementioned seamless oil well steel pipe has a high strength of 110 ksi and excellent resistance to sulfide stress cracking. LIST OF REFERENCES PATENT LITERATURE

[0010] Patent Literature 1: Japanese Patent Application Publication No. 2000-256783 Patent Literature 2: Japanese Patent Application Publication No. 2000-297344 Patent Literature 3: Japanese Patent Application Publication No. 2005-350754 Patent Literature 4: Japanese Patent Application Publication No. 2012-26030 Patent literature 5: International application publication no. WO 2010 / 150915 SUMMARY OF THE INVENTION TECHNICAL PROBLEM

[0011] As described above, steel oil well pipes conforming to a desired yield strength and exhibiting excellent SSC resistance are proposed in patent literature 1 through 5. In addition to SSC, hydrogen-induced cracking (hereafter referred to as HIC) can occur in some seamless steel pipes used in sour environments. HIC is cracking caused by hydrogen generated from a corrosion reaction in a sour environment that penetrates the seamless steel pipe. In short, unlike SSC, HIC can occur even when no stress is applied.

[0012] In other words, there is a possibility of HIC occurring in seamless steel pipe being used as oil well tubing. However, almost no studies have been conducted regarding HIC resistance for seamless steel pipe with a yield strength of 110 ksi (758 to 862 MPa).

[0013] An objective of the present invention is to provide a seamless steel tube having a yield strength of 758 to 862 MPa (110 to 125 ksi, grade 110 ksi) and also having excellent resistance to HIC. SOLUTION TO THE PROBLEM CRLenn / Lznz / E / Yii

[0014] A seamless steel tube according to the present invention has a chemical composition consisting of, % by mass, C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.070%, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, rare earth metal: 0.0001 to 0.0015%, Ca: 0.0001 to 0.0100%, N: 0.0100% or less, O: 0.0020% or less, V: 0 to 0.30%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 1.00%, W: 0 to 1.00%, Ni: 0 to 0.50%, and Cu: 0 to 0.50%, the remainder being Fe and impurities, and satisfies Formula (1). In the seamless steel tube according to the present invention, the maximum major axis of inclusions is 150 pm or less; the maximum major axis is predicted by extreme value statistical processing. The seamless steel tube according to the present invention has a yield strength within the range of 758 to 862 MPa. (Ca / 0+Ca / S+0.285xREM / 0+0.285xREM / S)x(Al / Ca) > 40.0 (1) where, a content (% by mass) of a corresponding element is substituted for each element symbol in Formula (1). ADVANTAGEOUS EFFECTS OF THE INVENTION

[0015] Seamless steel tubing according to the present description has a yield strength within a range of 758 to 862 MPa (grade 110 ksi) and has excellent resistance to HIC. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a view illustrating the relationship between a predicted maximum major axis of inclusions and the strength of HIC. Figure 2 is a schematic diagram showing the distribution of inclusions in the field of view when the predicted maximum major axis of inclusions is obtained according to the present modality. DESCRIPTION OF THE MODALITIES

[0017] The present inventors conducted research and studies regarding the resistance to HIC in seamless steel tubes having a yield strength within a range of 758 to 862 MPa (grade 110 ksi) that are supposedly to be used in a sour environment and obtained the following findings.

[0018] First, the present inventors considered increasing the yield strength of a seamless steel tube to a grade of 110 ksi by adjusting the chemical composition to consist of, by mass %, C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.070%, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, rare earth metal: 0.0001 to 0.0015%, Ca: 0.0001 to 0.0100%, N: 0.0100% or less, O: 0.0020% or less, V: 0 to 0.30%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 1.00%, W: 0 to 1.00%, Ni: 0 to 0.50%, and Cu: 0 to 0.50%, the remainder being Fe and impurities. The present inventors then produced several types of 110 ksi grade seamless steel tubes having the composition CRLrnn / Lznz / E / Yi chemistry mentioned above, and investigated and studied the HIC resistance of seamless steel tubes.

[0019] The presence of HIC was confirmed in some seamless steel tubes with the aforementioned chemical composition and a yield strength of 110 ksi. Therefore, the present inventors conducted detailed investigations into the seamless steel tubes in which HIC had occurred. As a result, the present inventors found that, in the seamless steel tubes where HIC had occurred, cracking originated from coarse inclusions as starting points.

[0020] The present inventors then conducted detailed studies on the relationship between coarse inclusions and HIC resistance. As a result, the present inventors obtained the following finding. Namely, when coarse inclusions are present in a seamless steel tube, stress concentration is likely to occur at the interface between the inclusions and the base metal. In such a case, HIC originates from the inclusions. Furthermore, among the coarse inclusions, stress concentration is likely to occur at the interface between, in particular, inclusions with a long major axis and the base metal. Therefore, in a case where inclusions with a long major axis are present in a seamless steel tube, the HIC resistance of the seamless steel tube decreases.In other words, to increase the HIC resistance of a seamless steel tube, it is good to reduce inclusions that have a long major axis and not simply reduce coarse inclusions.

[0021] As a result of further studies conducted by the present inventors, they clarified that, among the inclusions contained in a seamless steel tube, fine inclusions do not reduce the HIC strength. That is, it is considered that to increase the HIC strength of a seamless steel tube, requirements can be established that are appropriate to the actual situation if it is determined whether or not inclusions with a long major axis are present in the seamless steel tube, and these can be used as an index, rather than using an average value of inclusions, such as the average grain size of the inclusions.

[0022] On the other hand, conventionally, the grain size of inclusions obtained by microscopic observation (e.g., equivalent circular diameter or square root of area) or the major axis of inclusions has been used as an index of inclusion thickness. In conventional microscopic observation, although inclusions contained within a seamless steel tube can be observed, such observation is little more than the observation of an average distribution of inclusions, such as the numerical density in several fields of view. Furthermore, in conventional microscopic observation, to determine whether or not inclusions with a long major axis are present, it is necessary to increase the number of fields of view for microscopic observation and magnify the CRLenn / Lznz / E / Yii area of ​​the visual field. However, if the number of visual fields for microscopic observation is increased without careful consideration, the time and expense required to perform the microscopic observation will increase.

[0023] Therefore, the present inventors conceived of using statistical processing to predict the major axis of inclusions contained in a seamless steel tube. Specifically, the present inventors focused their attention on a technique called extreme value statistical processing. The term extreme value statistical processing refers to a technique that acquires an extreme value (e.g., a maximum major axis of inclusions) in the respective fields of view and estimates the probability distribution in a plurality of fields of view. By using extreme value statistical processing, the maximum major axis of inclusions present in a seamless steel tube can be predicted.Therefore, the present inventors investigated the relationship between the maximum major axis of inclusions contained in a seamless steel tube that is predicted by extreme value statistical processing (hereafter also referred to simply as the predicted maximum major axis of inclusions) and the resistance to HIC.

[0024] Specifically, the present inventors investigated in detail the relationship between a predicted maximum major axis of inclusions (Dmax) determined by extreme value statistical processing described below and the HIC resistance of seamless steel tubes having the aforementioned chemical composition and a grade 110 ksi yield strength. Figure 1 is a view illustrating the relationship between the predicted maximum major axis of inclusions and the HIC resistance. Figure 1 was created using a predicted maximum major axis of inclusions Dmax (qm) obtained by a method described below and a cracking area ratio CAR (%) obtained by an HIC test described below, with respect to seamless steel tubes which, among the seamless steel tubes of the examples described below, have the aforementioned chemical composition and a grade 110 ksi yield strength.

[0025] Note that the yield strength adjustment for each seamless steel tube shown in Figure 1 was performed by adjusting the tempering temperature. Furthermore, regarding HIC resistance, it was determined that HIC resistance was good if the crack area ratio (CAR) was less than 3.0%. The downward arrow in Figure 1 indicates that the CAR crack area ratio is less than the position shown on the graph.

[0026] With reference to Figure 1, in seamless steel tubes meeting the aforementioned chemical composition and having a grade 110 ksi yield strength, when the maximum predicted major axis of inclusions Dmax is greater than 150 qm, the CAR cracking area ratio is 3.0% or more and the HIC resistance decreases. On the other hand, when the maximum predicted major axis of inclusions Dmax is 150 qm or less, the CAR cracking area ratio is less than 3.0% and CRLrnn / Lznz / E / Yi increases HIC resistance. That is, in Figure 1, as a result of detailed studies carried out by the present inventors, the present inventors clarified that when the maximum predicted major axis of Drnax inclusions is 150 pm or less, the HIC resistance can be significantly increased.

[0027] Therefore, with reference to Figure 1, it was clarified as a result of studies conducted by the present inventors that in a seamless steel tube satisfying the aforementioned chemical composition and having a yield strength of 110 ksi, if the maximum predicted major axis of inclusions Dmax is 150 pm or less, there is the notable advantage that the crack area ratio (CAR) is less than 3.0%. Consequently, in the seamless steel tube according to the present embodiment, the aforementioned chemical composition is satisfied, the yield strength is 110 ksi, and the maximum predicted major axis of inclusions Dmax is 150 pm or less. As a result, the seamless steel tube according to the present embodiment exhibits excellent resistance to HIC, with a crack area ratio (CAR) of less than 3.0%.

[0028] A seamless steel tube according to the present embodiment, completed based on the foregoing findings, has a chemical composition consisting of, by mass %, C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.070%, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, rare earth metal: 0.0001 to 0.0015%, Ca: 0.0001 to 0.0100%, N: 0.0100% or less, O: 0.0020% or less, V: 0 to 0.30%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 1.00%, W: 0 to 1.00%, Ni: 0 to 0.50%, and Cu: 0 to 0.50%, the remainder being Fe and impurities, and satisfies Formula (1). In the seamless steel tube according to the present embodiment, a maximum major axis of inclusions in the seamless steel tube is 150 pm or less; the maximum major axis is predicted by extreme value statistical processing.The seamless steel tube according to the present modality has a yield strength within a range of 758 to 862 MPa. (Ca / 0+Ca / S+0.285xREM / 0+0.285xREM / S)x(Al / Ca) > 40.0 (1) CRLrnn / Lznz / B / Yu where, a content (% by mass) of the corresponding element is replaced by each element symbol in Formula (1).

[0029] The above-mentioned chemical composition may contain V in an amount of 0.01 to 0.30%.

[0030] The above-mentioned chemical composition may contain one or more types of elements selected from the group consisting of Mg: 0.0001 to 0.0100% and Zr: 0.0001 to 0.0100%.

[0031] The above-mentioned chemical composition may contain one or more types of elements selected from the group consisting of Co: 0.02 to 1.00% and W: 0.02 to 1.00%.

[0032] The above-mentioned chemical composition may contain one or more types of elements selected from a group consisting of Ni: 0.01 to 0.50% and Cu: 0.01 to 0.50%.

[0033] The aforementioned seamless steel pipe may be an oil well steel pipe.

[0034] In this description, oil well steel pipe or tubing may refer to oilfield tubular products. Oilfield tubular products include, for example, steel pipes used in casing or tubing.

[0035] If the seamless steel pipe according to the present modality is an oil well steel pipe, even when the wall thickness is 15 mm or more, the seamless steel pipe has a yield strength of 758 to 862 MPa (grade 110 ksi) and has excellent resistance to HIC in a sour environment.

[0036] The excellent resistance to HIC in a sour environment mentioned above can be evaluated by a method in accordance with NACE TM0284-2011. Specifically, HIC resistance can be evaluated by 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.

[0037] A test specimen prepared from the seamless steel tube is immersed in the test solution at 24 °C. After degassing the test solution, H2S is sealed in at 1 atm and used as a test bath. After 96 hours, while stirring the test bath, the test specimen is removed. The removed test specimen is subjected to an ultrasonic flaw detection test (C-scan), and the area of ​​the indication portions (portions of HIC occurrence) is determined.

[0038] The CAR cracking area ratio (%) is obtained from the following formula (2) based on the determined area of ​​the indicator portions and the projected area of ​​the test specimen during the ultrasonic crack detection test. CAR (%) = (area of ​​indicator portions / projected area) x 100 (2)

[0039] For seamless steel pipe according to the present modality, in the HIC resistance test, the cracking area ratio CAR (%) after 96 hours is less than 3.0%.

[0040] The seamless steel tube according to the present invention is described in detail below. The symbol % in relation to an element means percentage by mass unless specifically stated otherwise.

[0041] [Chemical composition] The chemical composition of the seamless steel tube according to the present invention contains the following elements.

[0042] C: 0.15 to 0.45% Carbon (C) improves the hardenability of steel and increases its yield strength. C also promotes carbide spheroidization during tempering in the production process, further increasing the steel's yield strength. These effects will not be achieved if the CRLrnn / Lznz / E / Yi The carbon content is too low. On the other hand, if the carbon content is too high, the toughness of the steel material will decrease, and rapid cooling cracking is likely to occur. Therefore, the carbon content is within the range of 0.15 to 0.45%. A preferred lower limit for the carbon content is 0.18%, more preferably 0.20%, more preferably 0.22%, and more preferably 0.24%. A preferred upper limit for the carbon content is 0.40%, more preferably 0.35%, more preferably 0.33%, and more preferably 0.30%.

[0043] Yes: 0.05 to 1.00% Silicon (Si) deoxidizes steel. If the Si content is too low, this effect is not achieved. Conversely, if the Si content is too high, the steel's resistance to steel corrosion cracking (SSC) decreases. Therefore, the Si content is within the range of 0.05 to 1.00%. A preferred lower limit for the Si content is 0.15%, and more preferably 0.20%. A preferred upper limit for the Si content is 0.85%, with 0.70% being more preferable, 0.60% more preferable, 0.50% more preferable, 0.45% more preferable, and 0.40% more preferable.

[0044] Mn: 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 are not achieved. On the other hand, if the Mn content is too high, the Mn segregates at the grain boundaries along with impurities such as phosphorus (P) and sulfur (S). As a result, the steel's high-intensity cross-link (HIC) resistance decreases. Furthermore, if the Mn content is too high, the amount of MnS, a readily extending inclusion, increases. Consequently, the predicted maximum major axis of inclusions lengthens, and the steel's HIC resistance decreases. Therefore, the Mn content is within a range of 0.01 to 1.00%. A preferred lower limit for Mn content is 0.02%, and a more preferable limit is 0.03%. A preferred upper limit for Mn content is 0.90%, more preferably 0.80%, more preferably 0.70%, more preferably 0.60%, more preferably 0.55% and more preferably 0.50%.

[0045] P: 0.030% or less Phosphorus (P) is an impurity. That is, the P content is greater than 0%. P segregates at grain boundaries and embrittles the steel material. As a result, the steel material's resistance to high-intensity corrosion (HIC) decreases. Therefore, the P content is 0.030% or less. A preferred upper limit for the P content is 0.025%, and more preferably 0.020%. Preferably, 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%, more preferably 0.0003%, more preferably 0.001%, and most preferably 0.002%. CRLenn / Lznz / E / Yii

[0046] S: 0.0050% or less Sulfur (S) is an impurity. That is, the S content is greater than 0%. S segregates at grain boundaries and embrittles the steel material. As a result, the HIC resistance of the steel material decreases. S also combines with Mn to form MnS. MnS is an easily spreading inclusion, and if the amount of MnS increases, the predicted maximum major axis of inclusions becomes longer. As a result, the HIC resistance of the steel material decreases. Therefore, the S content is 0.0050% or less. A preferred upper limit for the S content is 0.0045%, more preferably 0.0035%, more preferably 0.0030%, and most preferably 0.0025%. Preferably, the S content is as low as possible. However, if the S content is reduced excessively, the production cost increases significantly. Therefore, when taking industrial production into consideration, a preferable lower limit for the S content is 0.0001%, and more preferably 0.0003%.

[0047] Al: 0.005 to 0.070% Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect is not achieved. Conversely, if the Al content is too high, coarse inclusions form in the steel material, and the predicted maximum major axis of inclusions becomes longer. As a result, the HIC resistance of the steel material decreases. Therefore, the Al content is within a range of 0.005 to 0.070%. A preferred lower limit for the Al content is 0.010%, and more preferably 0.015%. A preferred upper limit for the Al content is 0.060%, more preferably 0.050%, more preferably 0.045%, more preferably 0.040%, and more preferably 0.035%. In this description, Al content refers to acid-soluble Al, i.e., the sol. Al content.

[0048] Cr: 0.30 to 1.50% Chromium (Cr) improves the hardenability of steel and increases its yield strength. If the Cr content is too low, this effect is not achieved. Conversely, if the Cr content is too high, coarse carbides form in the steel, and its SSC resistance decreases. Therefore, the Cr content is within a range of 0.30 to 1.50%. A preferred lower limit for the Cr content is 0.32%, more preferably 0.35%, more preferably 0.40%, more preferably 0.45%, and more preferably 0.50%. A preferred upper limit for the Cr content is 1.40%, more preferably 1.30%, more preferably 1.25%, and more preferably 1.10%.

[0049] Mo: 0.25 to 2.00% Molybdenum (Mo) improves the hardenability of steel and increases its yield strength. If the Mo content is too low, this effect is not achieved. On the other hand, if the content If the Mo content (CRLrnn / Lznz / E / Yi) is too high, the aforementioned effects become saturated. Therefore, the Mo content should be within a range of 0.25 to 2.00%. A preferred lower limit for the Mo content is 0.30%, more preferably 0.40%, more preferably 0.45%, more preferably 0.50%, more preferably 0.55%, and more preferably 0.60%. A preferred upper limit for the Mo content is 1.70%, more preferably 1.50%, more preferably 1.40%, and more preferably 1.30%.

[0050] Ti: 0.002 to 0.020 % Titanium (Ti) combines with nitrogen (N) to form fine nitrides and refines the crystal grains through a fixative effect. As a result, the yield strength of the steel material increases. If the Ti content is too low, this effect is not achieved. Conversely, if the Ti content is too high, coarse Ti nitrides form in the steel material, and its high-intensity crystallinity (HIC) resistance decreases. Therefore, the Ti content is within a range of 0.002 to 0.020%. 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 0.018%, more preferably 0.015%, more preferably 0.012%, and more preferably 0.010%.

[0051] Nb: 0.002 to 0.100% Niobium (Nb) combines with carbon to form fine carbides. As a result, the yield strength of the steel material increases. This effect is not achieved if the Nb content is too low. On the other hand, if the Nb content is too high, excessive carbides, nitrides, or carbonitrides (hereafter referred to as carbonitrides and the like) may form. In such cases, the high-intensity cracking (HIC) resistance of the steel material decreases. Therefore, the Nb content is within the range of 0.002 to 0.100%. A preferred lower limit for the Nb content is 0.003%, more preferably 0.007%, more preferably 0.010%, more preferably 0.015%, and most preferably 0.020%. A preferred upper limit for Nb content is 0.080%, more preferably 0.050%, more preferably 0.040%, and most preferably 0.030%

[0052] B: 0.0005 to 0.0040% Boron (B) dissolves in steel, improving its hardenability and increasing its yield strength. If the B content is too low, this effect is not achieved. Conversely, if the B content is too high, coarse B nitrides form, reducing the steel's high-intensity cracking (HIC) resistance. Therefore, the B content is within a range of 0.0005 to 0.0040%. A preferred lower limit for B content is 0.0008%, and more preferably 0.0010%. A preferred upper limit for B content is 0.0030%, more preferably 0.0025%, more preferably 0.0020%, more preferably 0.0018%, and more preferably 0.0015%.

[0053] Rare earth metal: 0.0001 to 0.0015% CRLrnn / Lznz / E / Yi Rare earth metals (REMs) reduce FeO. As a result, REMs suppress the formation of Al₁₂O₃ clusters, and Al₁₂O₃, X₂O₃, and X₂O₃ (X represents REMs) are formed. Consequently, the predicted maximum major axis of inclusions decreases, and the high-intensity crystallization (HIC) resistance of the steel material increases. REMs also combine with phosphorus (P) in the steel material and suppress P segregation at crystal grain boundaries. As a result, the HIC resistance of the steel material increases. These effects are not observed if the REM content is too low. Conversely, if the REM content is too high, coarse inclusions form in the steel material, and the predicted maximum major axis of inclusions becomes longer. As a result, the HIC resistance of the steel material decreases. Therefore, the REM content is typically within the range of 0.0001 to 0.0015%. A preferred lower limit for REM content is 0.0002%, more preferably 0.0.0003%, more preferably 0.0004%, more preferably 0.0005%, and more preferably 0.0006%. A preferred upper limit for REM content is 0.0012%, more preferably 0.0011%, more preferably 0.0010%, and more preferably 0.0009%.

[0054] Note that, in this description, the term REM refers to one or more types of elements selected from a group consisting of scandium (Se), which is the element with atomic number 21, yttrium (Y), which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, which are lanthanoids. Furthermore, in this description, the term REM content refers to the total content of these elements.

[0055] Ca: 0.0001 to 0.0100 % Calcium (Ca) spheroidizes inclusions within steel and reduces the predicted maximum major axis of inclusions. As a result, the steel's HIC strength increases. This effect is not achieved if the Ca content is too low. Conversely, if the Ca content is too high, coarse oxide-based inclusions form in the steel, and its HIC strength decreases. Therefore, the Ca content is within the range of 0.0001 to 0.0100%. A preferred lower limit for Ca content is 0.0002%, more preferably 0.0003%, more preferably 0.0005%, more preferably 0.0006%, more preferably 0.0008%, and most preferably 0.0010%. A preferred upper limit for Ca content is 0.0040%, more preferably 0.0030%, more preferably 0.0025%, more preferably 0.0020%, more preferably 0.0017%, and most preferably 0.0015%.

[0056] N: 0.0100% or less Nitrogen (N) is inevitably present. That is, the N content is greater than 0%. N combines with Ti to form fine 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 Ti nitrides form in the steel material, and the HIC resistance of the CRLrnn / Lznz / B / Yi steel material decreases. Therefore, the N content is 0.0100% or less. A preferred upper limit of the N content is 0.0050%, and more preferably 0.0045%. A preferred lower limit of the N content to more effectively obtain the aforementioned effect is 0.0015%, more preferably 0.0020%, more preferably 0.0025%, and more preferably 0.0030%.

[0057] O: 0.0020% or less Oxygen (O) is an impurity. That is, the O content is greater than 0%. O forms coarse oxide-based inclusions and lengthens the maximum predicted major axis of inclusions. As a result, the HIC strength of the steel material decreases. Therefore, the O content is 0.0020% or less. A preferred upper limit for the O content is 0.0019%, more preferably 0.0018%, more preferably 0.0016%, and most preferably 0.0015%. 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%.

[0058] The remainder of the chemical composition of the steel material according to the present modality is 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 the like, and are permitted within a range that does not adversely affect the steel material according to the present modality.

[0059] [Regarding optional elements] The chemical composition of the steel material described above may also contain V instead of a portion of Fe.

[0060] V:0a0.30% Vanadium (V) is an optional element and its presence is not required. That is, the V content can be 0%. If present, V forms fine carbides during tempering and increases the yield strength of the steel. Even a small amount of V produces this effect to some extent. However, if the V content is too high, the toughness of the steel decreases. Therefore, the V content is within the range of 0 to 0.30%. A preferred lower limit for the V content is less than 0%, more preferably 0.01%, more preferably 0.02%, more preferably 0.04%, more preferably 0.06%, and more preferably 0.08%. A preferred upper limit for the V content is 0.25%, more preferably 0.20%, more preferably 0.15%, and more preferably 0.12%. CRLrnn / Lznz / B / Yi

[0061] The chemical composition of the steel material described above may further contain one or more elements selected from the group consisting of Mg and Zr in place of a portion of Fe. Each of these elements is optional and increases the HIC resistance of the steel material.

[0062] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and does not need to be included. That is, the Mg content can be 0%. If present, Mg refines the sulfide-based inclusions in the steel material and shortens the predicted maximum major axis of inclusions. As a result, the steel material's HIC resistance increases. Even a small amount of Mg produces this effect to some extent. However, if the Mg content is too high, coarse inclusions form in the steel material, and the predicted maximum major axis of inclusions becomes longer. As a result, the steel material's HIC resistance decreases. Therefore, the Mg content is typically within the range of 0 to 0.0100%. A preferred lower limit for Mg content is more than 0%, more preferably 0.0001%, more preferably 0.0003%, more preferably 0.0006%, and more preferably 0.0010%.A preferred upper limit for Mg content is 0.0040%, more preferably 0.0030%, more preferably 0.0025%, and most preferably 0.0020%.

[0063] Zr: 0 to 0.0100%. Zirconium (Zr) is an optional element and does not need to be included. That is, the Zr content can be 0%. If present, Zr refines sulfide-based inclusions in the steel material, shortening the predicted maximum major axis of inclusions. As a result, the steel material's HIC resistance increases. Even a small amount of Zr produces this effect to some extent. However, if the Zr content is too high, coarse inclusions form in the steel material, and the predicted maximum major axis of inclusions becomes longer. Consequently, the steel material's HIC resistance decreases. Therefore, the Zr content is typically within the range of 0 to 0.0100%. A preferred lower limit of Zr content is more than 0%, more preferably 0.0001%, more preferably 0.0003%, more preferably 0.0006%, and more preferably 0.0010%.A preferred upper limit of Zr content is 0.0040%, more preferably 0.0030%, more preferably 0.0025%, and most preferably 0.0020%.

[0064] 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 a portion of Fe. Each of these elements is an optional element that forms a protective layer against corrosion in a sour environment and suppresses hydrogen penetration. In this way, each of these elements increases the HIC resistance of the steel material.

[0065] Co: 0 to 1.00% CRLrnn / Lznz / B / Yi Cobalt (Co) is an optional element and does not need to be included. That is, the Co content can be 0%. If included, Co forms a protective layer against corrosion in a sour environment and suppresses hydrogen penetration. As a result, Co increases the HIC resistance of the steel material. Even a small amount of Co produces this effect to some extent. However, if the Co content is too high, the hardenability and yield strength of the steel material will decrease. Therefore, the Co content is within the range of 0 to 1.00%. A preferred lower limit for the Co content is greater than 0%, more preferably 0.02%, more preferably 0.03%, and most preferably 0.05%. A preferred upper limit for the Co content is 0.90%, and more preferably 0.80%.

[0066] W: 0 to 1.00% Tungsten (W) is an optional element and does not need to be included. That is, the W content can be 0%. If present, W forms a protective layer against corrosion in a sour environment and suppresses hydrogen penetration. As a result, W increases the HIC resistance of the steel material. Even a small amount of W produces this effect to some extent. However, if the W content is too high, coarse carbides form in the steel material, weakening it. As a result, the HIC resistance of the steel material decreases. Therefore, the W content is within the range of 0 to 1.00%. A preferred lower limit for the W content is greater than 0%, more preferably 0.02%, more preferably 0.03%, and most preferably 0.05%. A preferred upper limit for the W content is 0.90%, and more preferably 0.80%.

[00067] The chemical composition of the steel material described above may also contain one or more elements selected from the group consisting of Ni and Cu instead of a portion of Fe. Each of these elements is optional, improves the hardenability of the steel material, and increases its yield strength.

[0068] Ni: 0 to 0.50% Nickel (Ni) is an optional element and its presence is not required. That is, 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 produces this effect to some extent. However, if the Ni content is too high, it will promote localized corrosion and decrease the steel's SSC resistance. Therefore, the Ni content is typically between 0 and 0.50%. A preferred lower limit for Ni content is greater than 0%, with 0.01% being more preferable and 0.02% being even more desirable. A preferred upper limit for Ni content is 0.10%, with 0.08% being more preferable and 0.06% being even more desirable.

[0069] Cu: 0 to 0.50% CRLrnn / Lznz / B / Yi Copper (Cu) is an optional element and its presence is not required. That is, the Cu content can be 0%. If present, Cu improves the hardenability of the steel material and increases its yield strength. Even a small amount of Cu produces this effect to some extent. However, if the Cu content is too high, the hardenability of the steel material will be excessively high, and its toughness will decrease. Therefore, the Cu content is within the range of 0 to 0.50%. A preferred lower limit for the Cu content is greater than 0%, more preferably 0.01%, more preferably 0.02%, and most preferably 0.05%. A preferred upper limit for the Cu content is 0.35%, and more preferably 0.25%.

[0070] [With regard to formula (1)] The chemical composition of the seamless steel tube according to the present embodiment also satisfies Formula (1). (Ca / 0+Ca / S+0.285xREM / 0+0.285xREM / S)x(Al / Ca) > 40.0 (1) where, a content (% by mass) of a corresponding element is substituted for each element symbol in Formula (1).

[0071] Fnl (= (Ca / O+Ca / S+0.285xREM / O+0.285xREM / S)x(Al / Ca)) is an index that indicates the shape of the inclusions produced by Ca and REM in a seamless steel tube having the aforementioned chemical composition and a yield strength of 110 ksi. The value 0.285 of Fnl is a coefficient in a case where the REM content is converted to Ca content by an approximate calculation. In Fnl, Ca / O+Ca / S+0.285xREM / O+0.285xREM / S is the sum of the proportions of Ca to O and S content obtained when the REM content is converted to Ca content. Al / Ca in Fnl is an index of the melting point of the inclusions.

[0072] If Fnl is too small, inclusions are likely to spread. Therefore, Fnl is 40.0 or greater. A preferred lower bound for Fnl is 41.0, and more preferably 42.0. A preferred upper bound for Fnl is 140.0, and more preferably 130.0.

[0073] [With respect to the maximum predicted major axis of inclusions] In seamless steel tubing according to this specification, the maximum major axis (predicted maximum major axis of inclusions) Dmax of inclusions contained in the seamless steel tubing is 150 pm or less. The maximum major axis is predicted by means of extreme value statistical processing. If the predicted maximum major axis of inclusions Dmax is greater than 150 pm, the CAR of the seamless steel tubing will be 3.0% or more, and the HIC resistance of the seamless steel tubing will decrease. Therefore, the predicted maximum major axis of inclusions Dmax is 150 pm or less.

[0074] A preferred upper limit of the predicted maximum major axis of inclusions Dmax is 148 pm, and more preferably 145 pm. The predicted maximum major axis of inclusions Dmax is preferably as small as possible. CALrnn / Lznz / E / Yi

[0075] The predicted maximum major axis of inclusions, Dmax, can be determined by the following method. A test specimen having an observation surface with dimensions of 10 mm in the direction of the tube axis and 10 mm in the radial direction of the tube is cut from a central portion of the wall thickness of the seamless steel tube according to the present embodiment. In addition, if the wall thickness of the seamless steel tube is less than 10 mm, a test specimen is cut having an observation surface with dimensions of 10 mm in the direction of the tube axis and a wall thickness equal to that of the seamless steel tube in the radial direction of the tube.After polishing the observation surface of the test specimen to obtain a mirror-like surface, the observation surface is observed by making an observation with respect to an fields of view (n represents a natural number) by means of a secondary electronic image obtained using a scanning electron microscope (SEM).

[0076] In this case, if the number of observation fields n is too small, accuracy in the statistical processing of extreme values ​​may not be achieved in some cases. Therefore, in the statistical processing of extreme values ​​according to the present modality, the number of observation fields n is 20 or more. The number of observation fields n is, for example, 108. Furthermore, if the gross area of ​​the observation fields (hereafter also referred to as the reference area SO) is too narrow, accuracy in the statistical processing of extreme values ​​may not be achieved in some cases. Therefore, in the statistical processing of extreme values ​​according to the present modality, the reference area SO is 20 mm² or more. The reference area SO is, for example, 196.5 mm².

[0077] A maximum major axis Lmax of inclusions is determined in each field of view. The maximum major axis Lmax of inclusions in each field of view can be determined by image analysis of an observational image. Note that in a case where the shortest distance between the plurality of inclusions is 40 pm or less in the direction of the tube axis and 15 pm or less in the radial direction of the tube, these inclusions are considered a single inclusion. This will be described with reference to the drawing.

[0078] Figure 2 is a schematic diagram showing the distribution of inclusions in the observation field 1 when the maximum predicted major axis of inclusions is obtained according to the present modality. Figure 2 is a diagram to describe whether two inclusions are considered one inclusion or not. The vertical direction in Figure 2 corresponds to the direction of the tube axis. The lateral direction in Figure 2 corresponds to the radial direction of the tube. The reference number 10 in Figure 2 denotes the inclusions in the observation field 1. With reference to Figure 2, the shortest distance in the direction of the tube axis between the inclusions 10 is dL, and the shortest distance in the radial direction of the tube between the inclusions 10 is dT. In a case where the shortest distance in the direction of the CRLrnn / Lznz / B / Yi tube dLsea of ​​40 µm or less and the shortest distance in the radial direction of the tube dTsea of ​​15 µm or less, these 10 inclusions are considered one inclusion. On the other hand, in the case where the shortest distance in the direction of the axis of the tube dLsea is greater than 40 µm, these 10 inclusions are considered distinct inclusions respectively. Furthermore, in a case where the shortest distance in the radial direction of the tube dTsea is greater than 15 µm, these 10 inclusions are also considered distinct inclusions respectively.

[0079] Note that the same determination is made as to whether three or more inclusions are considered a single inclusion. In this case, it is first determined, as described above, whether two adjacent inclusions are considered a single inclusion. In a case where two adjacent inclusions are considered a single inclusion, and the shortest distance between the inclusion considered as a single inclusion and the additional adjacent inclusion is 40 pm or less in the tube axis direction and 15 pm or less in the tube radial direction, these three or more inclusions are considered a single inclusion. As described above, whether three or more inclusions are considered a single inclusion can be determined by continuously applying the method described above.

[0080] The maximum major axis Lmax of the respective visual fields being determined is defined as Lmaxj (j = 1 an) in order from the smallest value. That is, the maximum major axes of the inclusions of the respective visual fields are assigned numbers such that Lmax ! <Lmax2<Lmax3< ... <Lmaxn.

[0081] Next, using the following formulas (3) and (4), a cumulative distribution function Fj and a standardized variable yj are determined for each value of j. Fj = j / (n+l) (3) and j = -ln{-ln(Fj)} (4) Note, “In” in formula (4) means natural logarithm.

[0082] A plot of the standardized variable yj (j = 1 an) is created with respect to the maximum major axis Lmaxj (j = 1 an). With respect to the plot created, a straight approximation line (maximum inclusion distribution straight line) is created using the least squares method. The created approximation straight line can be expressed by the following formula (5). yj = cxLmaxj+d (5) where, cyd are coefficients of a straight line determined by the least squares method.

[0083] Next, a recurrence period T is determined using the following formula (6). T = (S+S0) / S0 (6) where, S represents a virtual surface area (mm2) in the central part of the wall thickness of the seamless steel tube. Specifically, S can be determined using the following formula (7) S = (Rt)xKxL (7) CRLrnn / Lznz / B / Yi where, R represents the outside diameter (mm) of the seamless steel tube, t represents the wall thickness (mm) of the seamless steel tube and L represents the length (mm) in the axial direction of the seamless steel tube.

[0084] A predicted standardized variable “y” is determined using the determined recurrence period T and formula (8). y =-ln{-ln((Tl) / T)} (8) Note that In in formula (8) represents a natural logarithm, similarly to formula (4).

[0085] Based on the predicted standardized variable “y” that is determined and formula (5), Lmax is determined with respect to the predicted standardized variable y. The Lmax thus determined is defined as the predicted maximum major axis of inclusions Dmax (pm).

[0086] [Regarding the microstructure] The microstructure of the seamless steel tube according to the present modality is composed primarily of tempered martensite and tempered bainite. Specifically, the total volume proportions of tempered martensite and tempered bainite in the microstructure are 90% or more. The remainder of the microstructure is, for example, ferrite or pearlite. If the microstructure of the seamless steel tube having the aforementioned chemical composition contains tempered martensite and tempered bainite in an amount equivalent to a total volume proportion of 90% or more, and provided that the other requirements of the present modality are met, the yield strength of the seamless steel tube shall be in the range of 758 to 862 MPa (grade 110 ksi), and the yield strength of the seamless steel tube shall be 90.0% or more.

[0087] The ratio of the total volume of tempered martensite to tempered bainite can be determined by observing the microstructure. A test specimen having an observation surface with dimensions of 10 mm along the tube axis and 10 mm radially is cut from a central portion of the seamless steel tube wall thickness according to the present embodiment. Furthermore, if the wall thickness of the seamless steel tube is less than 10 mm, a test specimen is cut having an observation surface with dimensions of 10 mm along the tube axis and the wall thickness of the seamless steel tube radially. After polishing the observation surface to a mirror finish, the small piece is immersed for approximately 10 seconds in a 2% nital etching reagent to reveal the microstructure by etching.The recorded observation surface is viewed using a secondary electron image obtained with a scanning electron microscope (SEM) in relation to 10 fields of view. The field of view area is 400 pm² (5000x magnification). CRLenn / Lznz / E / Yii

[0088] In each field of view, tempered martensite and tempered bainite can be distinguished from other phases (ferrite or pearlite) based on contrast. Consequently, tempered martensite and tempered bainite are identified in each field of view. The total area ratios of the identified tempered martensite and tempered bainite are determined. In this modality, the arithmetic mean of the total area ratios of tempered martensite and tempered bainite determined in all fields of view is defined as the volume ratio of tempered martensite to tempered bainite.

[0089] [Uses of seamless steel tubing] In a case where the seamless steel pipe according to the present embodiment is an oil well steel pipe, a preferred wall thickness is in the range of 9 to 60 mm. More preferably, the seamless steel pipe according to the present embodiment is suitable for use as a heavy-walled oil well steel pipe. More specifically, even if the seamless steel pipe according to the present embodiment is an oil well steel pipe having a thick wall of 15 mm or more, or, moreover, 20 mm or more, a yield strength within the range of 758 to 862 MPa (grade 110 ksi) is obtained, and excellent resistance to high-intensity compression (HIC) is exhibited.

[0090] [Regarding the elastic limit and the elasticity index] The yield strength of the seamless steel pipe according to this specification is within the range of 758 to 862 MPa (grade 110 ksi). As used herein, yield strength means the stress at 0.7% of total elongation (0.7% proof stress) obtained in a tensile test. In summary, the yield strength of the seamless steel pipe according to this specification is 110 ksi.

[0091] In seamless steel tubing according to the present embodiment, the yield strength (YR) is 90.0% or more. The yield strength is the ratio of the yield strength (YS) to the tensile strength (TS) (YR = YS / TS). As described above, in seamless steel tubing according to the present embodiment, if the yield strength is 110 ksi and the yield strength is 90.0% or more, the total volume proportions of tempered martensite and tempered bainite in the microstructure are 90% or more. As a result, seamless steel tubing according to the present embodiment can achieve both a yield strength of 110 ksi and excellent resistance to high iron compression (HIC).

[0092] The yield strength and yield strength of the seamless steel pipe according to the present embodiment can be determined by the following method. A tensile test is performed in accordance with ASTM E8 / E8M (2013). A round bar test specimen is taken from a central portion of the wall thickness of the seamless steel pipe according to the present embodiment. With respect to the size of the round bar test specimen, for example, the round bar test specimen has a parallel portion diameter of 8.9 mm and a parallel portion length of 35.6 mm. Note in CRLenn / Lznz / E / Yii states that the axial direction of the round bar test specimen is parallel to the axis direction of the seamless steel tube. A tensile test is performed in the atmosphere at normal temperature (25 °C) using the round bar test specimen. The stress obtained at 0.7% of total elongation is defined as the yield strength (MPa). The greatest stress during uniform elongation is defined as the tensile strength (MPa). The ratio between the yield strength (YS) and the tensile strength (TS) (YR = YS / TS) is defined as the yield strength (YR) (%).

[0093] [Regarding strength at HIC] A hydrostatic index (HIC) resistance test for seamless steel pipe can be performed according to the present method, in accordance with NACE TM0284-2011. A test specimen for the HIC resistance test is prepared from the seamless steel pipe according to the present method. Specifically, a portion shaped like an arc in the circumferential direction of the pipe is taken from the seamless steel pipe according to the present method. Two curved surfaces of the portion (corresponding to the outer and inner surfaces of the seamless steel pipe) are machined in parallel planes. In this case, the thickness of the portion is reduced to the wall thickness of the seamless steel pipe minus 2 mm.In this way, a test specimen is prepared that has a rectangular cross-section and a width of 20 mm, a thickness of -2 mm of the wall thickness of the seamless steel tube, and a length of 100 mm. Note that the length of the test specimen is parallel to the axis of the seamless steel tube, and the thickness of the test specimen is parallel to the radial direction of the tube.

[0094] 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. The prepared test specimen is immersed in the test solution at 24°C. N2 gas is injected into the test solution for three hours to degas it. After degassing, H2S is injected at 1 atm to create a corrosive environment, and this is used as the test bath. The test specimen is kept in the test bath for 96 hours while the bath is agitated. The test specimen is removed from the test bath after 96 hours. After removal, an ultrasonic flaw detection test (C-scan) is performed to determine the area of ​​the indication portions (portions of HIC occurrence).

[0095] The CAR cracking area ratio (%) can be determined from the following formula (2) based on the area of ​​the indication portions that was determined and the projected area of ​​the test specimen during the ultrasonic flaw detection test. Note that, in the present modality, the projected area is, for example, 20 mm x 100 mm. CAR (%) = (area of ​​indication portions / projected area) x 100 (2) CRLenn / Lznz / E / Yii

[0096] For seamless steel pipe according to the present modality, in the HIC resistance test, the cracking area ratio CAR (%) after 96 hours is less than 3.0%.

[0097] [Production Method] The following describes a method for producing seamless steel tubing according to this embodiment. The production method described below is an example of a method for producing seamless steel tubing according to this embodiment. In other words, a method for producing seamless steel tubing according to this embodiment is not limited to the production method described below.

[0098] An example of the production method includes: a steelmaking process for refining and casting molten steel to produce a starting material (a casting, ingot, or bar); a hot working process of subjecting the starting material to hot working to produce a hollow shell; a quenching process for subjecting the hollow shell to quenching; and an tempering process for subjecting the quenched hollow shell to tempering.

[0099] [Steel manufacturing process] In the steelmaking process, the molten metal produced by a known method is first refined (primary refining) using a converter. The molten steel that has undergone primary refining then undergoes secondary refining. In secondary refining, alloying elements that were previously adjusted are added to the molten steel to produce a molten steel that meets the specified chemical composition.

[0100] Specifically, the molten steel extracted from the converter undergoes a deoxidation treatment. The deoxidation treatment is not particularly restricted, and it is sufficient that the deoxidation treatment be carried out using an element other than REM and Ca. The deoxidation treatment is performed, for example, by adding Al. If Al is added during the deoxidation treatment, the oxygen content in the molten steel can be reduced efficiently. Therefore, in the present embodiment, it is preferable to add Al during the deoxidation treatment. After the deoxidation treatment, a slag removal treatment is carried out. Following the slag removal treatment, secondary refining is performed.

[0101] In secondary refining, for example, a Ruhrstahl-Hausen (RH) vacuum degassing process is carried out. The final adjustment of the alloying elements is then performed. In secondary refining, a composite refining process may be performed. In such a case, prior to the Ruhrstahl-Hausen (RH) vacuum degassing process, for example, a refining treatment using a ladle furnace (LF) or vacuum arc degassing (VAD) is carried out.

[0102] In the final adjustment of the alloying elements, the adjustment of alloying elements other than REM and Ca is performed first. That is, alloying elements other than REM and Ca are adjusted in the CRLrnn / Lznz / E / Yi molten steel to obtain the aforementioned chemical composition. Subsequently, after adding at least one type of element among the REM elements, Ca is added, and the alloying elements in the molten steel are adjusted to obtain the aforementioned chemical composition. Note that, when adding REM to molten steel, the REM can be used as a simple substance and can also be used in the form of a mixture.

[0103] As described above, REM suppresses the formation of A12O3 clusters by reducing FeO. As a result, the inclusions A12O3, X2O3, and X2OS (X represents REM) form in the molten steel. In a case where Ca is added to the molten steel after these inclusions have formed, XCaAlOS (X represents REM), which are fine inclusions, are formed.

[0104] On the other hand, if Ca is added to molten steel before adding REM, calcium aluminates (kCaO-lAl2O3; where k and l are natural numbers) are formed, which are coarse inclusions. In this case, the formation of the fine XCaAlOS inclusions (X represents REM) mentioned above is prevented. Therefore, in a case where REM is added after adding Ca to molten steel, the reforming of the inclusions does not occur, and the effect of containing REM is not effectively achieved.

[0105] Furthermore, calcium aluminates also form even if Ca is added to the molten steel immediately after adding REM. Specifically, if the time from the addition of REM to the addition of Ca (hereafter also referred to as the molten steel holding time) is less than 15 seconds, calcium aluminates form, hindering the formation of XCaAlOS (X represents REM). As a result, the predicted maximum major axis of inclusions, Dmax, is greater than 150 pm, and the HIC resistance of the seamless steel tube decreases.

[0106] On the other hand, if the time between the addition of REM and the addition of Ca is too long, the reformatting of the inclusions does not occur in some cases. Specifically, if the holding time of the molten steel is greater than 600 seconds, the predicted maximum major axis of inclusions (Dmax) is greater than 150 pm, and the HIC resistance of the seamless steel tube decreases. Although the detailed reason has not been clarified, in a case where the holding time of the molten steel is too long, the inclusions X2O3 and X2OS (X represents REM) in the molten steel are considered to decrease, and XCaAlOS (X represents REM) is unlikely to form.

[0107] Therefore, in the steelmaking process according to the present method, the holding time of the molten steel is 15 to 600 seconds. If the holding time of the molten steel is 15 to 600 seconds, the formation of calcium aluminates is suppressed and the formation of XCaAlOS (X represents REM), which are fine inclusions, is accelerated. As a result, the maximum major axis of inclusions contained in a seamless steel tube, as predicted by extreme value statistical processing, can be 150 pm or less. CRLrnn / Lznz / E / Yi

[0108] The starting material is produced using molten steel produced by the method described above. Specifically, a casting (plate, billet, or bar) is produced by a continuous casting process using molten steel. An ingot can also be produced by an ingot-making process using molten steel. As required, the plate, billet, or ingot can be rough-rolled to produce a bar. The starting material (plate, billet, ingot, or bar) is produced by the process described above.

[0109] [Hot working process] In the hot working process, the prepared starting material is subjected to heat working to produce a hollow shell. First, the bar is 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, once removed from the heating furnace, is then hot worked to produce a hollow shell.

[0110] For example, the Mannesmann process is performed as hot working to produce the hollow shell. In this case, a round bar is punched with a punching machine. When punching is performed with rolling, although the punching ratio is not particularly limited, the punching ratio is, for example, within a range of 1.0 to 4.0. The round bar that has undergone punching with rolling is further hot-rolled to form a hollow shell using a mandrel mill, a reducer, a rolling mill, or similar equipment. The cumulative reduction of area in the hot working process is, for example, from 20 to 70%.

[0111] A hollow shell can also be produced from the bar by another hot working method. For example, in the case of short-length, thick-walled steel material, such as a coupling or coupling, a hollow shell can be produced by forging using the Ehrhardt process or a similar method. A hollow shell is produced by the process described above. Although not particularly limited, the wall thickness of the hollow shell is, for example, from 9 to 60 mm.

[0112] The hollow shell produced by hot working may be air-cooled (rolled). The hollow shell produced by hot working may be quenched directly after hot working without cooling to normal temperature or it may be quenched after further heating (reheating) after hot working. However, in the case of direct quenching or quenching after further heating, it is preferable to stop the cooling halfway through the quenching process and carry out slow cooling in order to suppress quench cracking.

[0113] In a case where direct quenching is performed after hot working, or quenching is performed after supplementary heating after hot rolling, in order to eliminate CRLrnn / Lznz / B / Yi If residual stress is present, it is preferable to perform stress relief (SR treatment) after quenching and before the heat treatment (quenching and similar) of the subsequent process.

[0114] [Quenching Process] In the tempering process, the hollow shell produced by hot working is subjected to quenching. In this description, the term quenching means rapidly cooling the hollow shell to a temperature no lower than point A3. The quenching can be carried out by any known method and is not particularly limited. A quenching temperature of 800 to 1000 °C is an example.

[0115] In a case where direct hardening is performed after hot working, the hardening temperature corresponds to the surface temperature of the hollow shell as measured by a thermometer placed on the outlet side of the apparatus performing the final hot working. Furthermore, in a case where hardening is performed using a supplementary heating furnace or a heat treatment furnace after hot working, the hardening temperature corresponds to the temperature of the supplementary heating furnace or the heat treatment furnace.

[0116] The quenching method, for example, continuously cools the hollow shell from the initial quenching temperature, continuously lowering its temperature. The method for performing the continuous quenching treatment is not particularly restricted, and any known method may be used. The method for performing the continuous quenching 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 water bath quenching or mist quenching.

[0117] If the cooling rate during quenching is too slow, the microstructure does not develop into one composed primarily of martensite and bainite, and the mechanical properties defined in the present embodiment cannot be obtained. Therefore, in the method for producing seamless steel tubing according to the present embodiment, the hollow shell is cooled rapidly during quenching.

[0118] Specifically, in the quenching process, the average cooling rate when the temperature of the hollow shell is within the range of 800 to 500 °C during quenching is defined as a quenching cooling rate CR800-500 (°C / s). More specifically, the quenching cooling rate CR800-500 is determined based on a temperature measured in a region that cools more slowly within a cross-section of the hollow shell being quenched (e.g., in the case of forced quenching of both the outer and inner surfaces of the hollow shell, the cooling rate is measured in the middle portion of the hollow shell wall thickness). CRLrnn / Lznz / E / Yii

[0119] A preferred cooling rate during CR8O0.5oo quenching is 8 °C / s or higher. In this case, the microstructure of the hollow shell after stable quenching becomes a microstructure composed mainly of martensite and bainite. A preferred lower limit of the cooling rate during CR800-5oo quenching is 10 °C / s. A preferred upper limit of the cooling rate during CR8O0-5oo quenching is 500 °C / s.

[0120] Preferably, quenching is performed after heating the hollow shell in the austenite zone a plurality of times. In this case, the SSC resistance and low-temperature toughness of the seamless steel tube are increased because the austenite grains are refined before quenching. Heating in the austenite zone can be repeated a plurality of times by performing multiple quenching cycles, or heating in the austenite zone can be repeated a plurality of times by performing normalizing and quenching.

[0121] [Annealing Process] In the tempering process, the hardened hollow shell is subjected to further tempering. In this description, tempering means reheating the hardened hollow shell to a temperature no higher than the Aci point and holding it at that temperature. The tempering temperature is appropriately adjusted according to the chemical composition of the seamless steel tube and the desired yield strength. Specifically, for the hollow shell with the chemical composition described herein, the tempering temperature is adjusted to achieve a yield strength of the seamless steel tube within the range of 758 to 862 MPa (grade 110 ksi).

[0122] The tempering temperature corresponds to the furnace temperature when the hollow shell, after quenching, is heated and held at the relevant temperature. In the tempering process according to the present embodiment, a preferred tempering temperature is from 650 to 720 °C. A more preferred lower limit of the tempering temperature is 655 °C, and more preferably 660 °C. A more preferred upper limit of the tempering temperature is 715 °C, and more preferably 710 °C.

[0123] The term tempering time means the period of time from when the hollow shell, after quenching, is inserted into the furnace to be heated and held, until when the hollow shell is removed from the furnace. If the tempering time is too short, in some cases a microstructure composed mainly of tempered martensite and tempered bainite will not be obtained. On the other hand, if the tempering time is too long, the aforementioned effects become saturated. Therefore, in the tempering process of the present modality, the tempering time is preferably set within the 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. CRLrnn / Lznz / E / Yi

[0124] Seamless steel tubing according to the present embodiment can be produced by the production method described above. Note that the production method mentioned above is an example, and seamless steel tubing according to the present embodiment can be produced by another production method. EXAMPLES

[0125] Cast steels were produced with the chemical compositions shown in Table 1. In addition, the Fnl values ​​obtained based on the chemical compositions shown in Table 1 and the aforementioned formula (1) are shown in Table 2. Note that, with respect to Fnl, in a case where a corresponding element is not contained, 0 is replaced by the symbol of the corresponding element 10. CRLrnn / Lznz / B / Yi I HEARD

[0126] [Table 1] LO LO «O CO Λ 1 O | | 1 < i 1 1 a O oo LO ,φ z 1 1 i <o t t ? 1 o 1 • 1 i 1 1 1 co o o o 5 ’ 6 1 3 1 t t o • 1 1 9Ό 1 1 3 co o co 1 | a a a a | O có LO OJ a o a o I 1 1 a a a o o o o LO Mg 1 1 1 i O o < 1 t • 1 • • 1 t 3 co 05 CD o O 05 Oí en 05 co 1 1 t < a t 1 O 1 1 0 O 0 O s O O o O 0 có 0 O LO co OI -r— OJ co ’St· co OJ LO LO CO OJ LO LO CL t— T_ W“· T— t— T“ T- V“ T“ ^lililí 1 T- LO X'·1· E O § 8 o o 00 o co 8 00 o o 00 8 8 8 .00 .00 .00 .00 8 Φ o θ o co o CO co co co o’ CO O co 0 0 0 0 Φ LL· co LO Oí LO LO co «Q OI LO co CO co LO OJ LO co co CO co co CO co co CO ”d- M- CO co z* o O o Q r v f1. o o O 0 0 0 o t 5 r < o c S co R o co o o o o O 0 0 X o O o - < co o o o o o o o CO 0 0 0 K Cxl co rv>~f^ LQ LO OI x— oo en OI 04 ov— Mr OJ kp.anrj <φ|>·!·· ap··· C™* a,····· o T- v— v- 05 ΓΊ o O CO ooooooo O 0 0 O C3 r- o co co o co o O c JAO co O oo / io O ooooo O co 0 0 05 wHUVDvmv e ,ϊ o LO co LQ LQ LO o 05 ^· LO LO co 05 φ OO o OJ, o O ooooo OO 0 0 O ÍH ooo co orooooo O 0 0 O (Y ooo x5 co co CO ooooo O o 0 O as O co o fí o co O ooo ICO o O 0 0 O TR ν~ OJ m'........ LO LO co Cxl CM co 'e oo en LQ LO OJ co ya..... Φ njaaea Hg·—roo V^aiM a¡..... ^* CQ O ooo O co ooooooo O 0 OMO o co co oo Λ co oo co o O 0 O soo ° oo co ooooooo O |O 0 O 5 LO co co LO CO LO o LO h- LO o 00 Ico LO LO ro JQ OJ OI OI OI OJ OI 04 OJ t S aull Cxl OJ OI ¡01 OJ OI z O o co o co co ca t. J ooo _'.l lO ¡o O co & ooooo co CO oo Q o ¡OO 0 o 'c co en co co Xf O· r*~ «Ό IO co ico en co a—— oo co co γ co or , 11 Ϊ ii* 1 o co co oooo co o A oo Io IO o 0 lo 0 0 σ c> ¡oooo co IO oo ICO IO o 0 0 0 0 íQ co LO co LO fx.I — LO CQ OI __ LO Ico |O4 co 00 co ^r OI ^J· r**~ en o> IOJ 1 co 1 Oí loo 105 co co & 2 cS or— co co co o ·<— 1 *“ i O o 0 > CO 0 E & OJ o un 04 LO LO LO 1 V— o OJ co LO co oo LO O co ico l J en 00 ICO 0 A 1 *— ° T~ V o O oo -»··- O co V“ T- co LO rO 1 LO / “* 1 co co 1 co r^|LO CO íO OI OI 04 OJ CQ OI OI co OI 1 CQ Oí IOI CO OJ OI OI < ooo AO oooooo O 0 0 0 0 o co ¡o O co o co co o* o |oo co 0 0 co OJ co o OI co Y— co 04 xt— OI en |Xa*.. LO co LO oo ap·^ o 'H;·· V ▼— Ί o 0 co CC O oo O CO ooooo 1 os 0 0 0 o lo ooc¿ ooooo C> o ¿5 0 0 0 d co co CO ° 1 co o IO oo 1 o I o id u 0 co co LO 1 r*— co o» loo r**> 1 O· co / <, n co en 00 Ico co o IO ooo ¡o IO IO o 1 o IO lO co O 0 Q_ o IO A o CO A IO IO oo IO 1 O 0 O 0 !C......í co 1 CO CO o co co co co 1 C2 oo co 1 co co LO OI LO ¡LO LO LO 1 'LO h- . <d co c xj vj co ^1* lo lio ««fr 1 'd1 ¡ oj ico o o ¡o io 10 'o 05 04 lq • l t v) oi en £*5 i oxi ° ío 0 yo τ'*- oo ir·- cm b" 00 u oí a ioi ¡o 'ó 2 0j íí rueb t" ¡co γ-. ’dv— 16 z clcr Lrnn / Lznz / E / YiAi

[0127] [Table 2] cr Lrnn / Lznz / E / YiAi Test Number Fn1 Molten Steel Holding Time Tempering Temperature (°C) Tempering Time (min) Dmax (gm) YS (MPa) TS (MPa) YR (%) CAR (%) 1 47.3 A 680 45 138 835 914 91.4 <3.0 2 74.2 A 705 45 78 798 868 91.9 <3.0 3 53.6 A 680 45 57 840 913 92.0 <3.0 4 52.7 A 680 45 dOQ IZO 832 923 90.2 <3.0 5 66.8 A 680 45 135 800 871 91.8 <3.0 6 55.3 A 680 45 120 840 918 91.5 <3.0 7 45.8 A 690 30 128 776 853 91.0 <3.0 8 54.8 A 690 30 113 769 850 90.5 <3.0 9 47.6 A 690 30 50 855 944 90.6 <3.0 10 75.8 A 690 oü 91 BU 891 91.4 <3.0 11 49.7 S 680 45 189 835 914 91.4 5.2 12 47.3 L 680 45 220 831 917 90.6 4.5 13 150.8 A 700 30 244 774 O&O 90.2 5.7 14 151.0 A 700 60 152 780 866 90 0 8.0 15 23.4 A 680 90 250 806 889 90.7 4.2 16 22.1 A 690 30 167 828 902 91.8 9.8 17 28.1 A 690 G8 779 865 90.1 9.6

[0128] The molten steels of the respective test numbers were produced by the following method. The hot metals produced by a known method were subjected to primary refining under the same conditions using a converter. After tapping from the converter, aluminum was added to the molten steel for deoxidation treatment, followed by deslag removal. Subsequently, after performing a vacuum RH degassing process, the composition of alloying elements other than REM and Ca in the molten steel was adjusted. REM was then added to the molten steel, followed by Ca, and the composition was adjusted.

[0129] For each of the test numbers, the time from the addition of REM to the addition of Ca (the molten steel holding time) is shown in Table 2. In a Molten Steel Holding Time column of Table 2, A (appropriate) means that the molten steel holding time is from 15 to 600 seconds. In a Molten Steel Holding Time column of Table 2, S (Short) means that the molten steel holding time is less than 15 seconds. In a Molten Steel Holding Time column of Table 2, L (long) means that the molten steel holding time is more than 600 seconds.

[0130] Bars with a cross-sectional diameter of 310 mm were produced by a continuous casting process using the molten steel from each trial number. The produced bars were hot-rolled to produce hollow shells (seamless steel tube) with an outside diameter of 244.48 mm, a wall thickness of 13.84 mm, and a length of 12000 mm. The hollow shell produced from each trial number was allowed to cool to bring the surface temperature of the hollow shell to normal temperature (25°C).

[0131] The hollow shell of each test number was subjected to tempering. Specifically, after cooling as described above, the hollow shell of each test number was held for 10 minutes in a tempering furnace at 920 °C. After being held for 10 minutes, the hollow shell of each test number was immersed in a water bath for water quenching. At this time, the cooling rate during the CR8000 tempering was at least 300 °C / min.

[0132] After water quenching, the hollow shell of each test number was tempered to produce a seamless steel tube of each test number. The tempering temperature was adjusted so that the hollow shell of each test number was grade 110 ksi (yield strength within the range of 758 to 862 MPa) in accordance with API standards. Specifically, the tempering temperature (°C) and tempering time (min) for tempering the hollow shell of each test number are shown in Table 2.

[0133] [Evaluation tests] A tensile test, a predicted maximum major axis measurement test of inclusions, and a HIC resistance evaluation test, described below, were performed on the seamless steel tube of each test number after the aforementioned tempering.

[0134] [Tensile test] A tensile test was performed in accordance with ASTM E8 / E8M (2013). Round bar specimens with a parallel portion diameter of 8.9 mm and a parallel portion length of 35.6 mm were prepared from the center portion of the wall thickness of the seamless steel pipe for each test number. The axial direction of the round bar test specimens was parallel to the axial direction of the seamless steel pipe. A tensile test was performed on each round bar test specimen in the atmosphere at normal temperature (25 °C), and the results were obtained from each pipe test number. CRLrnn / Lznz / E / Yi seamless steel yield strength YS (MPa), tensile strength TS (MPa), and elasticity index YR (%). Note that, in the examples presented, the stress at the time of full elongation 0.7% obtained in the tensile test was defined as the yield strength YS for each test number. Similarly, the highest stress during uniform elongation obtained in the tensile test was defined as the tensile strength TS for each test number. The ratio (YS / TS) between the yield strength YS and the tensile strength TS was taken as the elasticity index YR (%). The yield strength YS (MPa), tensile strength TS (MPa), and elasticity index YR (%) obtained are shown in Table 2.

[0135] With reference to Table 2, the yield strength of each test number was within the range of 758 to 862 MPa (grade 110 ksi). In addition, the yield strength of each test number was 90.0% or higher. Therefore, the microstructure of the seamless steel tube of each test number was 90% or higher of tempered martensite and tempered bainite by volume.

[0136] [Predicted major axis measurement test of inclusions] The predicted maximum major axis of inclusions Dmax (pm) was determined for the seamless steel tube of each test number using the method described above. Note that the number of observation fields n was 108 and the reference area SO was 196.5 mm². In addition, the virtual surface area S in the central portion of the wall thickness of the seamless steel tube was 8.69 × 10⁶ mm².

[0137] [Seamless Steel Pipe HIC Resistance Evaluation Test] A hydrostatic index (HIC) resistance evaluation test was performed using the method described above on the seamless steel pipe of each test number. Specifically, the method was performed in accordance with NACE TM0284-2011. A test specimen was prepared with a rectangular cross-section, a width of 20 mm, a thickness of -2 mm from the wall thickness of the seamless steel pipe, and a length of 100 mm from the seamless steel pipe of each test number. Note that the length of the test specimen was parallel to the axis of the seamless steel pipe, and the thickness of the test specimen was parallel to the radial direction of the pipe.

[0138] 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. Test samples of the respective test numbers were prepared and immersed in the test solution at 24 °C. The test solution of each test number was degassed by injecting N2 gas into the test bath for three hours.

[0139] The degassed test solution from each test number was converted into a corrosive environment by injecting H2S at 1 atm, and this was adopted as a test bath. The samples of The CRLrnn / Lznz / E / Yi test samples of the respective test numbers were kept in the test bath for each test number for 96 hours while the test bath was agitated. After being retained for 96 hours, the test samples were removed from the test baths. The test samples that were removed from the test baths were subjected to an ultrasonic flaw detection test (C-scan) to determine the area of ​​the indication portions (portions of occurrence of the HIC).

[0140] The CAR cracking area portion (%) was determined from the following formula (2) based on the area of ​​the indicator portions that was determined and the projected area of ​​the test specimen during the ultrasonic flaw detection test. Note that the projected area was 20 mm x 100 mm. CAR (%) = (area of ​​indication portions / projected area) x 100 (2)

[0141] [Test Results] The test results are shown in Table 2.

[0142] With reference to Table 1 and Table 2, for the respective seamless steel tubes of test numbers 1 to 10, the chemical composition was appropriate, Fnl was 40.0 or higher, and the yield strength YS was within the range of 758 to 862 MPa (grade 110 ksi). In addition, the predicted maximum major axis of inclusions Dmax was 150 pm or less. As a result, in the HIC resistance test, the CAR was less than 3.0%, and excellent HIC resistance was exhibited.

[0143] On the other hand, in the seamless steel tube of test number 11, the retention time of the molten steel was too short. Consequently, the predicted maximum major axis of inclusions Dmax was greater than 150 pm. As a result, in the HIC resistance test, the seamless steel tube of test number 11 did not exhibit excellent HIC resistance.

[0144] In the seamless steel tube of test number 12, the retention time of the molten steel was too long. Consequently, the predicted maximum major axis of inclusions Dmax was greater than 150 pm. As a result, in the HIC resistance test, the seamless steel tube of test number 12 did not exhibit excellent HIC resistance.

[0145] In the seamless steel tube of test number 13, the Al content was too high. Consequently, the predicted maximum major axis of inclusions Dmax was greater than 150 pm. As a result, in the HIC resistance test, the seamless steel tube of test number 13 did not exhibit excellent HIC resistance.

[0146] In the seamless steel tube of test number 14, the REM content was too high. Consequently, the predicted maximum major axis of inclusions Dmax was greater than 150 pm. As a result, in the HIC resistance test, the seamless steel tube of test number 14 did not exhibit excellent HIC resistance. CRLenn / Lznz / E / Yii

[0147] In the seamless steel tube of test number 15, the S content was too high. In addition, Fnl was less than 40.0. Consequently, the predicted maximum major axis of inclusions Dmax was greater than 150 pm. As a result, in the HIC resistance test, the seamless steel tube of test number 15 did not exhibit excellent HIC resistance.

[0148] In the seamless steel tube of test number 16, the O content was too high. In addition, Fnl was less than 40.0. Consequently, the predicted maximum major axis of inclusions Dmax was greater than 150 pm. As a result, in the HIC resistance test, the seamless steel tube of test number 16 did not exhibit excellent HIC resistance.

[0149] In the seamless steel tube of test number 17, Fnl was less than 40.0. Consequently, the predicted maximum principal axis of inclusions Dmax was greater than 150 pm. As a result, in the HIC resistance test, the seamless steel tube of test number 17 did not show excellent HIC resistance.

[0150] An embodiment of the present invention has been described above. However, the embodiment described above is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the above embodiment, and the above embodiment may be appropriately modified and implemented within a range that does not depart from the essence of the present invention. INDUSTRIAL APPLICABILITY

[0151] The seamless steel tube according to the present invention is widely applicable to seamless steel tubes to be used in a severe environment, such as a polar region, and preferably can be used as a seamless steel tube used in an oil well environment, and furthermore preferably can be used as oilfield tubular products for casings and pipes.< / d>

Claims

1. A seamless steel tube characterized in that it comprises: a chemical composition consisting, in % by mass, C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.070%, Cr: 0.30 to 1.50%, Mo: 0.25 to 2.00%, Ti: 0.002 to 0.020%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, rare earth metal: 0.0001 to 0.0015%, Ca: 0.0001 to 0.0100%, N: 0.0100% or less, O: 0.0020% or less, V: 0 to 0.30%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 1.00%, W: 0 to 1.00%, Ni: 0 to 0.50%, Cu: 0 to 0.50% and the remainder being Fe and impurities, and satisfying Formula (1), wherein a maximum major axis of inclusions in the seamless steel tube is 150 pm or less, the maximum major axis is predicted by extreme value statistical processing, and a yield strength is within a range of 758 to 862 MPa: (Ca / O + Ca / S + 0.285xREM / O + 0.285xREM / S) x (Al / Ca) x 40.0 (1) CALrnn / Lznz / E / Yi where, a content (% by mass) of a corresponding element is substituted for each element symbol in Formula (1).

2. The seamless steel tube according to claim 1, characterized in that the chemical composition contains: V: 0.01 to 0.30%.

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

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

5. The seamless steel tube according to claim 1, characterized in that the chemical composition contains one or more types of elements selected from the group consisting of: Co: 0.02 to 1.00% and W: 0.02 to 1.00%.

6. The seamless steel tube according to claim 2, characterized in that the chemical composition contains one or more types of elements selected from the group consisting of: Co: 0.02 to 1.00% and W: 0.02 to 1.00%.

7. The seamless steel tube according to claim 3, characterized in that the chemical composition contains one or more types of elements selected from the group consisting of: Co: 0.02 to 1.00% and W: 0.02 to 1.00%.

8. The seamless steel tube according to claim 4, characterized in that the chemical composition contains one or more types of elements selected from the group consisting of: Co: 0.02 to 1.00% and W: 0.02 to 1.00%. CRLrnn / Lznz / E / Yi 9. The seamless steel tube according to any of claims 1 to 8, characterized in that the chemical composition contains one or more types of elements selected from the group consisting of: Ni: 0.01 to 0.50%, and Cu: 0.01 to 0.50%.

10. The seamless steel pipe according to claim 1, characterized in that the seamless steel pipe is an oil well steel pipe.