STEEL MATERIAL SUITABLE FOR USE IN SOUR ENVIRONMENTS

Abstract

To provide a steel material with a yield strength of 110 ksi and excellent SSC strength. A steel material according to the present description has a chemical composition consisting of, 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 0.100%, Cr: 0.55 to 1.10%, Mo: 0.70 to 1.00%, Ti: 0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, the remainder being Fe and impurities, and satisfying formula (1) described in the description of the invention. The grain diameter of a preaustenite grain is 15.0 µm or less, and the average precipitate area at a preaustenite grain boundary is 12.5 × 10⁻³ µm² or less. The yield strength is 758 to 862 MPa.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0013] Fig. 1 is a view showing the relationship between Mo content and pre-austenite (pre-γ) grain diameter. Figure 2 is a view showing the relationship between Fl(=Mo / Cr) and the average area of ​​the precipitates 7Q1 Qnn / ίΖΠΖ / Β / ΥΙΛΙ specific. DESCRIPTION OF THE MODALITY

[0014] The present inventors have conducted research and studies regarding a method for obtaining excellent SSC strength from a steel material expected to be used in an acidic environment while maintaining a yield strength of 758 to 862 MPa (grade 110 ksi). As a result, they obtained the following findings.

[0015] Increasing the dislocation density in steel increases its yield strength (YS). However, there is also the possibility of hydrogen occlusion within the steel. Therefore, increasing the dislocation density can increase the amount of hydrogen occluded. While this increased hydrogen concentration can lead to higher strength, the steel's structural strength (SSC) is reduced. Consequently, increasing the strength by increasing the dislocation density is not preferable for achieving both a 110 ksi yield strength and excellent SSC.

[0016] In view of the foregoing, the present inventors considered that when the yield strength of a steel material is increased using a method other than an increase in the dislocation density of the steel material, excellent SSC strength can be obtained even if the yield strength of the steel material is increased to grade 110 ksi.

[0017] Specifically, the present inventors have considered that a steel material having a chemical composition that includes, in % by mass: C: from 0.15 to 0.45%, Si: from 0.05 to 1.00%, Mn: from 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: from 0.005 to 0.100%, Cr: from 0.55 to 1.10%, Ti: from 0.002 to 0.020%, V: from 0.05 to 0.30%, Nb: from 0.002 to 0.100%, B: from 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, Ca: from 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50% and W: 0 to 0.50% can achieve both a yield strength of 110 degrees ksi and SSC strength.

[0018] The present inventors further considered that when Mo is present, in addition to the aforementioned chemical composition, alloy carbide is formed, and therefore the yield strength can be increased without excessively increasing the dislocation density. Accordingly, the present inventors produced various steel materials in which Mo is added to the aforementioned chemical composition and investigated the characteristics of the steel materials. As a result, the present inventors recently discovered that, in the steel material having the aforementioned chemical composition, the Mo content and the grain diameter of the pre-austenite grain (hereafter also referred to as the “pre-γ grain”) are dependent on each other.

[0019] Specifically, the relationship between Mo content and a pre-γ grain diameter will be described with reference to a figure. Figure 1 is a view showing the relationship between Mo content and a pre-γ grain diameter. Figure 1 is formed using Mo contents (% by mass) and pre-γ grain diameters (pm) acquired by observing the microstructure, described later, with respect to steel materials having a chemical composition other than the Mo content that satisfies the range of the chemical composition mentioned above and produced by a preferred production method, described later in an example that will be described later. In the present description, “pre-γ grain diameter” means the grain diameter of a pre-γ grain, obtained by a method in accordance with a comparison method defined in ASTM E12-10.

[0020] With reference to Fig. 1, when the Mo content increases, the pre-γ grain diameter is drastically reduced. It became evident that, in the steel material having the aforementioned chemical composition, when the Mo content becomes 0.70% or more, a notable advantageous effect is obtained in reducing the pre-γ grain diameter to 15.0 pm or less. Furthermore, when the pre-γ grain is fine, the steel material can increase both the yield strength and the SSC strength. Accordingly, the chemical composition of the steel material, according to the present embodiment, contains 0.70% or more Mo in addition to the chemical composition mentioned above. In this case, the pre-γ grain diameter in the steel material becomes 15.0 pm or less.

[0021] The present inventors consider the reason as follows. In the case where the steel material having the aforementioned chemical composition contains 0.70% or more Mo, there is a possibility that the Mo dissolved in the steel material will segregate at the austenite grain boundaries during heating in a quenching process. Consequently, the dissolved Mo segregated at the austenite grain boundaries suppresses the movement of the grain boundaries. As a result, the austenite grain is prevented from readily thickening during heating in a quenching process, and therefore the pre-γ grain on which the tempering is performed is considered to be made thin.

[0022] Therefore, the steel material, according to the present modality, has a chemical composition consisting of, 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 0.100%, Cr: 0.55 to 1.10%, Mo: 0.70 to 1.00%, Ti: 0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50%, W: 0 to 0.50%, and the remainder is Fe and impurities. Furthermore, in the microstructure of the steel material, according to the present modality, a pre-γ grain diameter is 15.0 pm or less. ζα / οηη / ίζηζ / Β / Υΐ

[0023] However, in steel material having the aforementioned chemical composition and a pre-γ grain diameter of 15.0 pm or less, when attempting to achieve a yield strength of 110 ksi, a large amount of coarse carbide may precipitate in the steel material. Further research by the present inventors revealed that when a large amount of coarse carbide precipitates in steel material having the aforementioned chemical composition, the steel material may not achieve excellent SSC strength in an acidic environment.

[0024] Accordingly, the present inventors discussed in more detail the carbide that reduces SSC strength in steel material having the chemical composition mentioned above. As a result, they acquired the following findings. Coarse carbide is prone to forming stress concentrators and promotes the propagation of SSC-induced cracks. Therefore, it has been considered that reducing coarse carbide increases the SSC strength of a steel material.

[0025] However, as a result of detailed discussion by the present inventors, they found that coarse carbides, particularly coarse carbides precipitating at pre-γ grain boundaries, can lead to a reduction in the SSC strength of a steel material. That is, the present inventors discovered that the SSC strength of a steel material can be increased not simply by reducing coarse carbides but by reducing coarse carbides precipitating at pre-γ grain boundaries.

[0026] In steel material of the present embodiment, having the aforementioned chemical composition, most of the precipitates that precipitate at the pre-γ grain boundaries are carbides. Therefore, reducing the coarse precipitates that precipitate at the pre-γ grain boundaries can reduce the coarse carbides that precipitate at the pre-γ grain boundaries. In the present description, the precipitates that precipitate at the pre-γ grain boundaries are also referred to as “specific precipitates.”

[0027] The present inventors discussed in further detail the relationship between steel material having the aforementioned chemical composition and a pre-γ grain diameter of 15.0 pm or less and the specific precipitates. Specifically, the present inventors then produced various types of steel materials having the aforementioned chemical composition and a pre-γ grain diameter of 15.0 pm or less and investigated the average area of ​​the specific precipitates.

[0028] As a result, the present inventors found that, in steel material having the above-mentioned chemical composition and a pre-γ grain diameter of 15.0 pm or less, the ratio of Mo content to Cr content (Mo / Cr) affects the average area of ​​the specific precipitates.

[0029] F1 is defined as Mo / Cr. Fig. 2 is a view showing the relationship between F1 and the average area ζο / οηη / ίζηζ / Β / Υΐ of the specific precipitates. Fig. 2 is formed using F1 and the average area of ​​the specific precipitates (xlO3μη2) acquired by microstructure observation, described later, with respect to steel materials having the chemical composition and SSC strength mentioned above, evaluated by a method in accordance with “Method A” specified in NACE TM0177-2005 and produced by a preferred production method, described later in an example. Note that the symbol “O” in Fig. 2 indicates a steel material for which the SSC strength test result was good. On the other hand, the symbol “·” in Fig. 2 indicates a steel material for which the SSC strength test result was not good.

[0030] With reference to Fig. 2, as F1 increases, the average area of ​​the specific precipitates decreases dramatically. Specifically, in the steel material having the chemical composition mentioned above and a pre-γ grain diameter of 15.0 pm or less, when F1 is 0.90 or greater, the average area of ​​the specific precipitates is 12.5 x 10³ pm² or less. In this case, the steel material exhibits excellent SSC strength. On the other hand, when F1 is less than 0.90, the average area of ​​the specific precipitates is greater than 12.5 x 10³ pm², and the steel material does not exhibit excellent SSC strength.

[0031] The detailed reason has not been clarified. However, in steel material having the chemical composition mentioned above and a pre-γ grain diameter of 15.0 pm or less, when F1 is 0.90 or more, the average area of ​​the specific precipitates is 12.5 x 10³ μη² or less, and therefore the SSC strength of the steel material can be increased. The effect is proven by the present example, which is described below.

[0032] Accordingly, the steel material, according to the present modality, has the chemical composition mentioned, F1 is 0.90 or greater, a pre-γ grain diameter of 15.0 pm or less, and, furthermore, the average area of ​​the specific precipitates is 12.5 x 10.3 pm² or less. As a result, the steel material, according to the present modality, can achieve both a yield strength of 758 to 862 MPa (grade 110 ksi) and excellent SSC strength in a sour environment.

[0033] The steel material, according to the present modality, completed based on the findings mentioned above, has the chemical composition consisting of, 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 0.100%, Cr: 0.55 to 1.10%, Mo: 0.70 to 1.00%, Ti: 0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, Ca: from 0 to 0.0100%, Mg: from 0 to 0.0100%, Zr: from 0 to 0.0100%, rare earth metal: from 0 to 0.0100%, Cu: from 0 to 0.50%, Ni: from 0 to 0.50%, Co: from 0 to 0.50% and W: from 0 to 0.50%, the remainder being Fe and impurities and satisfying formula (1). In the microstructure of the steel material, the grain diameter of a pre-austenite grain is 15.0 ζα / οηη / ίζηζ / Β / Υΐ μιη or less. In steel material, the average area of ​​precipitates that precipitate at the pre-austenite grain boundary is 12.5x103pm2 or less.The elastic limit of steel material is 758 to 862 MPa. Mo / Cr>0.90 (1) where the content (% by mass) of a corresponding element is substituted for each element symbol in formula (1).

[0034] In the present description, the steel material is not particularly limited. However, the steel material may be a steel tube or a steel plate, for example.

[0035] The steel material, according to the present modality, exhibits a yield strength of 758 to 862 MPa (grade 110 ksi) and excellent SSC strength.

[0036] The above-mentioned chemical composition may contain one or more types of elements selected from the group consisting of Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100% and rare earth metal: 0.0001 to 0.0100%.

[0037] The above-mentioned chemical composition may contain one or more types of elements selected from the group consisting of Cu: 0.02 to 0.50% and Ni: 0.02 to 0.50%.

[0038] The above-mentioned chemical composition may contain one or more types of elements selected from the group consisting of Co: 0.02 to 0.50% and W: 0.02 to 0.50%.

[0039] The steel material referred to above may be oil well steel pipe.

[0040] In this description, oil well steel pipe may be steel pipe used for a pipeline or steel pipe used for petroleum tubular products (OCTG). The shape of the oil well steel pipe is not limited, and, for example, oil well steel pipe may be seamless steel pipe or welded steel pipe. Petroleum tubular products are, for example, steel pipes used for casing or pipeline applications.

[0041] The steel material mentioned above may be a seamless steel tube. When the steel material, according to the present modality, is a seamless steel tube, even if the wall thickness is 15 mm or more, the steel material has a yield strength of 758 to 862 MPa (grade 110 ksi) and also has a more stable SSC strength in an acidic environment.

[0042] The excellent SSC strength mentioned above can be specifically evaluated by a method in accordance with “Method A” specified in NACE TM0177-2005 and a four-point bending test. In the method in accordance with “Method A” specified in NACE TM0177-2005, a mixed aqueous solution containing 5.0% by mass sodium chloride and 0.5% by mass acetic acid (NACE Solution A) at 24°C is used as a test bath. A stress equivalent to 90% of the true yield strength is applied to the test specimen taken from the steel material, in accordance with the present modality, and the test specimen is immersed in the test bath. The test bath is degassed, then H2S gas is blown into it at 1 atm to saturate it. The test bath where H2S saturation occurs is maintained at 24 °C for 720 hours.

[0043] Meanwhile, in the four-point bend test, tension is applied to a test specimen taken from the steel material by four-point bending in accordance with ASTM G39-99 (2011), such that the tension applied to the test specimen is set at 90% of the actual yield strength of the steel material. A 5.0% by mass aqueous sodium chloride solution at 24°C is used as a test bath. The tensioned test specimen is immersed in the test bath in the autoclave. The test bath is degassed, and then H2S gas at 15 atm is introduced, and the autoclave is pressure sealed. After the autoclave is sealed, the test bath is agitated for 720 hours at 24°C.

[0044] In steel material, according to the present modality, cracking is not confirmed after 720 hours have elapsed, both in the above-mentioned method in accordance with “Method A” and in the four-point bending test.

[0045] A steel material will now be described in detail, in accordance with this modality. Unless otherwise specified, “%” with respect to an item means % by mass.

[0046] Chemical composition The chemical composition of the steel material, according to the present modality, contains the following elements.

[0047] C: from 0.15 to 0.45% Carbon (C) increases the hardenability of steel, which in turn increases its yield strength. Furthermore, C promotes carbide spheroidization during tempering in a production process, thus increasing the steel's surface hardness (SSC). When the carbides disperse, the steel's yield strength increases even further. If the C content is too low, these advantageous effects cannot be achieved. Conversely, if the C content is too high, the toughness of the steel is reduced, making quenching cracks more likely. Therefore, the C content is typically between 0.15% and 0.45%. A preferred lower limit for C content is 0.18%, more preferably 0.20%, even more preferably 0.22%, and still more preferably 0.25%. A preferred upper limit for the C content is 0.40%, more preferably 0.38%, and even more preferably 0.35%.

[0048] Yes: from 0.05 to 1.00% Silicon (Si) deoxidizes steel. When the Si content is too low, this advantageous effect cannot be achieved. Conversely, when the Si content is too high, the tensile strength of a steel material is reduced. Therefore, the Si content is within a range of 0.05 to 1.00%. A preferred lower limit for the Si content is 0.10%, and more preferably 0.15%. A preferred upper limit for the Si content is 0.85%, more preferably 0.70%, and even more preferably 0.60%.

[0049] Mn: from 0.01 to 1.00% Manganese (Mn) deoxidizes steel. Mn also improves the hardenability of steel, thus increasing its yield strength. When the Mn content is too low, these beneficial effects cannot be achieved. Conversely, when the Mn content is too high, the Mn segregates at the grain boundaries along with impurities such as phosphorus (P) and sulfur (S). In this case, the steel's hardness, strain, and cross-section (SSC) is reduced. Therefore, the Mn content is within a range of 0.01 to 1.00%. A preferred lower limit for Mn content is 0.02%, more preferably 0.03%, and even more preferably 0.10%. A preferred upper limit for Mn content is 0.80%, more preferably 0.70%, even more preferably 0.65%, even more preferably less than 0.60%, and still more preferably 0.55%.

[0050] 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 reduces the SSC strength of a steel material. Therefore, the P content is 0.030% or less. A preferred upper limit for P content is 0.025%, and more preferably 0.020%. Preferably, the P content is as low as possible. However, when the P content is reduced excessively, the cost of production increases significantly. Consequently, for industrial production purposes, a preferred lower limit for P content is 0.0001%, more preferably 0.0003%, even more preferably 0.001%, and still more preferably 0.002%.

[0051] 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 reduces the SSC strength of a steel material. Therefore, the S content is 0.0050% or less. A preferred upper limit for the S content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%. Preferably, the S content is as low as possible. However, when the S content is reduced excessively, the production cost increases significantly. Therefore, for industrial production purposes, a preferred lower limit for the S content is 0.0001%, and more preferably 0.0003%.

[0052] Al: from 0.005 to 0.100% Aluminum (Al) deoxidizes steel. When the Al content is too low, this advantageous effect cannot be achieved, and the steel's hardness and toughness (SSC) is reduced. Conversely, when the Al content is too high, coarse inclusion-based oxide forms, and the steel's SSC is reduced. 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 0.080%, and more preferably 0.060%. In this description, "Al" refers to the acid-soluble Al content, i.e., the Al sol content.

[0053] Cr: from 0.55 to 1.10% Chromium (Cr) increases the hardenability and yield strength of steel. Furthermore, Cr enhances temper softening resistance and allows for high-temperature tempering. As a result, the steel's supercritical strength (SCS) increases. These advantageous effects cannot be achieved when the Cr content is too low. Conversely, when the Cr content is too high, coarse carbides form at the pre-γ grain boundaries in the steel. In this case, the steel's SSC is reduced. Therefore, the Cr content is within a range of 0.55 to 1.10%. A preferred lower limit for the Cr content is 0.57%, more preferably 0.60%, even more preferably 0.65%, still more preferably 0.67%, and most preferably 0.70%. A preferred upper limit for Cr content is 1.05%, more preferably 1.00%, and even more preferably less than 1%.00%, even more preferably is 0.95% and even more preferably is 0.90%.

[0054] Mo: from 0.70 to 1.00% Molybdenum (Mo) increases the hardenability and yield strength of steel. Mo dissolves in the steel, and some of this dissolved Mo segregates at the austenite grain boundaries during quenching. As a result, the pre-γ grain diameter in the tempered steel is reduced through a locking effect. This increases the steel's hardness, strain, and cross-sectional area (SSC). These beneficial effects cannot be achieved when the Mo content is too low. Conversely, when the Mo content is too high, coarse carbides form at the pre-γ grain boundaries. This reduces the steel's SSC. Therefore, the Mo content should be within the range of 0.70 to 1.00%. A preferred lower limit is 0.72%, and 0.00% is more preferable.75%, even more preferably 0.78%, even more preferably 0.80%, and even more preferably 0.82%. A preferred upper limit for the Mo content is less than 1.00%, more preferably 0.97%, even more preferably 0.95%, even more preferably 0.90%, and even more preferably 0.87%.

[0055] Ti: from 0.002 to 0.020% Titanium (Ti) forms nitrides and refines the microstructure of steel through a fixative effect. As a result, the steel's supercritical strength (SCS) increases. This advantageous effect cannot be achieved when the Ti content is too low. Conversely, when the Ti content is too high, a large amount of Ti nitrides forms, reducing the steel's SCS. 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%, and even more preferably 0.015%.

[0056] V: from 0.05 to 0.30% Vanadium (V) combines with carbon (C) and / or nitrogen (N) to form carbides, nitrides, or carbonitrides (hereafter referred to as “carbonitrides and the like”). Carbonitrides and the like refine the microstructure of the steel material through a fixing effect. As a result, the steel's supercritical strength (SCS) increases. V also combines with C to form fine carbides. As a result, the yield strength of the steel material increases. When the V content is too low, these advantageous effects cannot be achieved. On the other hand, when the V content is too high, carbonitrides and the like form in excess, and the SSC strength of the steel material is reduced. Therefore, the V content is within a range of 0.05 to 0.30%. A preferred lower limit for the V content is greater than 0.05%, more preferably 0.06%, even more preferably 0.07%, and still more preferably 0.09%.A preferred upper limit of the V content is 0.25%, more preferably 0.20%, and even more preferably 0.15%.

[0057] 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 microstructure of steel through a fixing effect. As a result, the steel's supercritical strength (SCS) increases. Nb also combines with carbon to form fine carbides. This increases the yield strength of steel. When the Nb content is too low, these advantageous effects cannot be achieved. Conversely, when the Nb content is too high, carbonitrides and similar compounds form in excess, reducing the steel's SSC. Therefore, the Nb content is within a range of 0.002 to 0.100%. A preferred lower limit for Nb content is 0.005%, more preferably 0.010%, even more preferably 0.012%, and still more preferably 0.015%. A preferred upper limit for Nb content is 0.080%, more preferably 0.060%, even more preferably 0.050%, and even more preferably 0.030%.

[0058] B: from 0.0005 to 0.0040%. Boron (B) dissolves in steel, increasing its hardenability and yield strength. This advantageous effect cannot be achieved when the B content is too low. Conversely, excessively high B content leads to the formation of coarse nitrides and a reduction in the steel's surface hardness (SSC). Therefore, the B content is typically within a range of 0.0005% to 0.0040%. A preferred lower limit for the B content is 0.0007%, more preferably 0.0010%, and even more preferably 0.0012%. A preferred upper limit for the B content is 0.0035%, more preferably 0.0030%, and even more preferably 0.0025%.

[0059] N: 0.0100% or less Nitrogen (N) is unavoidably present. That is, the N content is greater than 0%. N combines with Ti to form fine nitrides, thereby refining the microstructure of the steel material through a fixation effect. As a result, the steel's SSC strength increases. On the other hand, when the N content is too high, coarse nitrides form, reducing the steel's SSC strength. Therefore, the N content is 0.0100% or less. A preferred upper limit for the N content is 0.0080%, and more preferably 0.0070%. A preferred lower limit for the N content to effectively achieve the aforementioned advantageous effects is 0.0020%, more preferably 0.0025%, even more preferably 0.0030%, even more preferably 0.0035%, and still more preferably 0.0040%.

[0060] O: less than 0.0020% Oxygen (O) is an impurity. That is, the O content is greater than 0%. O forms coarse oxides and reduces the SSC strength of the steel material. Therefore, the O content is less than 0.0020%. A preferred upper limit for the O content is 0.0018%, and more preferably 0.0015%. Preferably, the O content is as low as possible. However, when the O content is reduced excessively, the production cost increases significantly. Consequently, considering industrial production, a preferred lower limit for the O content is 0.0001%, and more preferably 0.0003%.

[0061] The remainder of the chemical composition of the steel material, according to the present modality, is Fe and impurities. In the present modality, “impurities” means materials that are mixed with the steel material from ore or scrap as raw material, a production environment, or the like in the industrial production of the steel material and that are permitted within a range where the impurities do not adversely affect the steel material of the present modality.

[0062] Optional elements The chemical composition of the aforementioned steel material 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, controlling the sulfide morphology in the steel material and increasing its SSC strength. Ca: from 0 to 0.0100%

[0063] Calcium (Ca) is an optional element and may not be present. That is, the Ca content may be 0%. When present, Ca renders the sulfur in the steel material harmless by forming sulfides, thus increasing the steel's SSC strength. Even a small amount of Ca can achieve this advantageous effect to some extent. However, when the Ca content is too high, the oxides in the steel material become coarse, reducing its SSC strength. Therefore, the Ca content is typically within the range of 0 to 0.0100%. A preferred lower limit for the Ca content is greater than 0%, more preferably 0.0001%, even more preferably 0.0003%, even more preferably 0.0006%, and even more preferably 0.0010%. A preferred upper limit for the Ca content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0001%.0.0025% and even more preferably 0.0020%.

[0064] Mg: from 0 to 0.0100% Magnesium (Mg) is an optional element and may not be present. That is, the Mg content can be 0%. When present, Mg renders the sulfur (S) in the steel material harmless by forming sulfides, thus increasing the steel's surface corrosion resistance (SCR). Even a small amount of Mg can provide this advantageous effect to some extent. However, when the Mg content is too high, the oxides in the steel material become thicker, reducing its SCR. 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%, even more preferably 0.0006%, and still more preferably 0.0010%. A preferred upper limit for Mg content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0.0025% and even more preferably 0.0020%.

[0065] Zr: from 0 to 0.0100% Zirconium (Zr) is an optional element and may not be present. That is, the Zr content can be 0%. When Zr is present, it renders the sulfur (S) in the steel material harmless by forming sulfides, thus increasing the steel's shock-resistant properties (SRP). Even a small amount of Zr can provide this advantage to some extent. However, when the Zr content is too high, the oxides in the steel material become thicker, reducing its SRP. Therefore, the Zr content is typically within a range of 0 to 0.0100%. A preferred lower limit for Zr content is less than 0%, more preferably 0.0001%, even more preferably 0.0003%, even more preferably 0.0006%, and still more preferably 0.0010%. A preferred upper limit for the Zr content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0.0025% and even more preferably 0.0020%.

[0066] Rare earth metal (REM): 0 to 0.0100% Rare earth metals (REMs) are optional elements and may not be present. That is, the REM content can be 0%. When present, REMs render sulfur (S) in the steel material harmless by forming sulfides, thus increasing the steel's surface hardness (SSC). REMs also combine with phosphorus (P) in the steel material and suppress P segregation at grain boundaries. Therefore, a reduction in low-temperature toughness and SSC attributable to P segregation is suppressed. Even a small amount of REM can produce these advantageous effects to some extent. However, when the REM content is too high, the oxides in the steel material become coarse, reducing the steel's low-temperature toughness and SSC. Therefore, the REM content is within a range of 0 to 0.0100%.A preferred lower limit for REM content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, even more preferably 0.0006%, and even more preferably 0.0010%. A preferred upper limit for REM content is 0.0040%, more preferably 0.0030%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0067] It is noted that, in the present description, the term “REM” refers to one or more types of elements selected from a group consisting of scandium (Se), the element with atomic number 21, yttrium (Y), the element with atomic number 39, and the elements lanthanum (La), with atomic number 57, through lutetium (Lu), with atomic number 71, which are lanthanides. Furthermore, in this description, the term “REM content” refers to the total content of these items.

[0068] The chemical composition of the steel material mentioned above may also contain one or more types of elements selected from the group consisting of Cu and Ni instead of a portion of Fe. Each of these elements is optional and increases the hardenability of the steel material. Cu: 0 to 0.50%.

[0069] Copper (Cu) is an optional element and may not be present. That is, the Cu content may be 0%. When Cu is present, it increases the hardenability of the steel material and thus increases its yield strength. Even a small amount of Cu can provide this advantageous effect to some extent. However, when the Cu content is too high, the hardenability of the steel material becomes excessively high, and its SSC strength is reduced. Therefore, the Cu content is within a range of 0 to 0.50%. A preferred lower limit for the Cu 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 Cu content is 0.35%, and more preferably 0.25%.

[0070] Ni: from 0 to 0.50% Nickel (Ni) is an optional element and may not be present. That is, the Ni content may be 0%. When Ni is present, it increases the hardenability of the steel and thus raises its yield strength. Even a small amount of Ni can provide this advantageous effect to some extent. However, excessively high Ni content promotes localized corrosion, thereby reducing the steel's surface hardness (SSC). Consequently, the Ni content is typically between 0 and 0.50%. A preferred lower Ni content limit is greater than 0%, more preferably 0.02%, even more preferably 0.03%, and still more preferably 0.05%. A preferred upper Ni content limit is 0.35%, and more preferably 0.25%.

[0071] The chemical composition of the aforementioned steel material may further 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 optional, forming an anti-corrosive coating that provides protection in a hydrogen sulfide environment and thus suppresses hydrogen penetration. With this configuration, these elements increase the SSC resistance of the steel material.

[0072] Co:de0a0.50% Cobalt (Co) is an optional element and may not be present. That is, the Co content may be 0%. When present, Co forms an anti-corrosive coating that provides protection in a hydrogen sulfide environment and thus suppresses hydrogen penetration. As a result, the steel's SSC resistance increases. Even a small amount of Co can provide this advantageous effect to some extent. However, when the Co content is too high, the hardenability of the steel is reduced, lowering its yield strength. Therefore, the Co content is within a range of 0 to 0.50%. A preferred lower limit for the Co 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 Co content is 0.45%, and more preferably 0.40%.

[0073] W: from 0 to 0.50% Tungsten (W) is an optional element and may not be present. That is, the W content may be 0%. When W is present, it forms an anti-corrosive coating that provides protection in a hydrogen sulfide environment and thus suppresses hydrogen penetration. As a result, the steel's corrosion resistance (CSR) increases. Even a small amount of W can provide this advantageous effect to some extent. However, when the W content is too high, coarse carbides form in the steel, reducing its CSR. 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%.

[0074] Formula (1) The chemical composition of the steel material, according to the present modality, also satisfies formula (1). ζο / οηη / ίζηζ / Β / γι Mo / Cr>0.90 (1) where the content (% by mass) of a corresponding element is substituted for each element symbol in formula (1).

[0075] F1 (=Mo / Cr) is an index that shows the average area of ​​specific precipitates in steel material with the chemical composition mentioned above. With reference to Fig. 2, when F1 is 0.90 or greater, the notable advantageous effect of reducing the average area of ​​specific precipitates to 12.5 x 10³ pm² or less is obtained. Also with reference to Fig. 2, when the average area of ​​specific precipitates is 12.5 x 10³ pm² or less, the steel material has excellent SSC strength.

[0076] On the other hand, when F1 is less than 0.90, the average area of ​​the specific precipitates is too large. As a result, the steel material does not have excellent SSC strength. Therefore, the steel material, according to the present modality, satisfies the chemical composition mentioned above and, in addition, has an F1 of 0.90 or more.

[0077] A preferred lower limit of F1 is 0.92, more preferably 0.96, and even more preferably 1.00. A preferred upper limit of F1 is not particularly limited. However, for steel material, according to the present embodiment, having the chemical composition mentioned above, the upper limit of F1 may be 1.67, for example. A preferred upper limit of F1 is 1.60, more preferably 1.55, even more preferably 1.50, even more preferably 1.45, and even more preferably 1.40.

[0078] Pre-austenite grain diameter In the microstructure of the steel material, according to the present embodiment, the pre-austenite grain diameter (pre-γ grain diameter) is 15.0 pm or less. As described above, in this embodiment, the pre-austenite grain diameter (pre-γ grain diameter) means the pre-austenite grain diameter obtained in accordance with a comparison method of ASTM I12-10. When the pre-γ grain of a steel material is fine, the yield strength and SSC strength increase steadily. In view of the above, in the present embodiment, the steel material contains 0.70% or more Mo to fineen the pre-γ grain of the steel material.

[0079] When the pre-γ grain diameter in the steel material, according to the present modality, is 15.0 pm or less, both a grade 110 ksi yield strength and excellent SSC strength can be achieved, provided that the other specifications of the steel material, according to the present modality, have been met.

[0080] A preferred upper limit of the pre-γ grain diameter in the steel material, according to the present modality, is less than 15.0 pm, more preferably 14.5 pm, more preferably 14.0 pm, and most preferably 13.5 pm. A preferred lower limit of the pre-γ grain diameter in the steel material, according to the present modality, is not particularly limited. However, the lower limit of the pre-γ grain diameter in the steel material, according to the present modality, may be 4.5 pm, for example.

[0081] As described above, the pre-γ grain diameter can be obtained in accordance with a comparison method of ASTM I12-10. More specifically, the pre-γ grain diameter can be acquired by the following method. In the case where the steel material is a steel plate, a test specimen is cut having an observation surface perpendicular to the rolling direction of the center portion of the thickness. In the case where the steel material is a steel tube, a test specimen is cut having an observation surface perpendicular to the axial direction of the steel tube from the center portion of the wall thickness. The observation surface is mirror-polished and subsequently embedded in a resin. The test specimen is then immersed in a 2% nital chemical etching reagent for approximately 10 seconds to develop pre-γ grain boundaries by chemical etching.

[0082] The chemically etched observation surface is subjected to 10-field observation in a secondary electronic image using a scanning electron microscope (SEM) to form a photographic image. The observation magnification is 200x, for example. By comparing the formed photographic image with a standard view of the grain size number, which is defined in ASTM I12-10, the grain size number is evaluated. The average grain diameter of the pre-γ grain in each field of view is acquired from the evaluated grain size number. The arithmetic mean value of the average pre-γ grain diameter acquired in the 10 fields of view is defined as the pre-γ grain diameter (pm).

[0083] Precipitates that precipitate at pre-γ grain boundaries In steel material according to this specification, the average area of ​​precipitates that form at pre-austenite grain boundaries (pre-γ grain boundaries) is 12.5 x 10³ μm² or less. As described above, in this specification, precipitates that form at pre-γ grain boundaries are also referred to as “specific precipitates.” When the average area of ​​specific precipitates is 12.5 x 10³ μm² or less, both a Grade 110 ksi yield strength and excellent SSC strength can be achieved, provided that the other specifications for steel material, according to this specification, are met.

[0084] As described above, in steel material having the aforementioned chemical composition and a pre-γ grain diameter of 15.0 pm or less, when attempting to achieve a yield strength of 110 ksi, a large amount of coarse carbides may precipitate within the steel material. Furthermore, of the coarse carbides in the steel material, those that precipitate at the pre-γ grain boundaries reduce the steel material's SSC strength. In the steel material, according to the present modality, the majority of the precipitates that form at the pre-γ grain boundaries are carbides.

[0085] In view of the above, in steel material, according to the present modality, the average area of ​​the precipitates (specific precipitates) that precipitate at the pre-γ grain boundaries is set at 12.5 x 10³ pm² or less. When the average area of ​​the specific precipitates is greater than 12.5 x 10³ pm², a reduction in the SSC strength of a steel material may occur. When the average area of ​​the specific precipitates is greater than 12.5 x 10³ pm², a yield strength of 758 to 862 MPa (grade 110 ksi) may not be obtainable.

[0086] Accordingly, in the steel material, according to the present modality, the average area of ​​the specific precipitates is 12.5x103pm2 or less. A preferred upper limit of the average area of ​​the specific precipitates is 12.0x10'3pm2, more preferably 11.5x10'3pm2, even more preferably 11.0x103pm2, and still more preferably 10.0x103pm2.

[0087] The lower limit of the average area of ​​the specific precipitates is not particularly limited and may be 0.0x103pm2. However, in the steel material, according to the present modality, having the chemical composition mentioned, the lower limit of the average area of ​​the specific precipitates may be 3.0x103pm2, for example.

[0088] The average area of ​​the specific precipitates can be obtained by the following method. A test sample of the steel material is cut in a manner similar to the previously mentioned method for determining the grain diameter of prc-γ. Specifically, in the case where the steel material is a steel plate, a test sample is cut having an observation surface perpendicular to the rolling direction of the central portion of the thickness. In the case where the steel material is a steel tube, a test sample is cut having an observation surface perpendicular to the axial direction of the steel tube of the central portion of the wall thickness. The observation surface is polished to a mirror finish and subsequently embedded in resin.Next, the test sample is immersed in a 2% nital chemical etching reagent for approximately 10 seconds to develop pre-γ grain boundaries through chemical etching. The chemically etched observation surface is then subjected to 10-field observation in a secondary electron image using a SEM to form a photographic image. The observation magnification is 1000x (ten thousand), for example.

[0089] The pre-γ grain boundaries are specified from the contrast-based photographic image. Precipitates are also specified from the contrast-based photographic image. It should be noted that, as described above, the observation magnification is 1000x, for example. Furthermore, precipitates can be identified based on contrast when they have an equivalent circular diameter of 50 nm or more. On the other hand, in the present modality, the upper limit of the equivalent circular diameter of the identified precipitates is not specifically limited. In the steel material having the aforementioned chemical composition, the upper limit of the equivalent circular diameter of the identified precipitates is 1000 nm, for example.Therefore, in the present modality, the equivalent circular diameter of the identified precipitates is within a range of 50 to 1000 nm, for example.

[0090] Precipitates that overlap with and / or contact specified pre-γ grain boundaries are specified as “specific precipitates.” That is, specific precipitates (precipitates that precipitate at pre-γ grain boundaries) mean precipitates that partially overlap and / or contact the pre-γ grain boundary. The average area (pm2) of the specified specific precipitates is acquired by performing image analysis.

[0091] Microstructure The microstructure of the steel material, according to the present modality, is composed primarily of tempered martensite and tempered bainite. More specifically, in the microstructure, the sum of the volume ratio of tempered martensite to the volume ratio of tempered bainite is 90% or more. The remainder of the microstructure consists of ferrite or pearlite, for example.

[0092] When the microstructure of the steel material having the above-mentioned chemical composition contains tempered martensite and tempered bainite such that the sum of the volume ratio of tempered martensite and the volume ratio of tempered bainite is 90% or more, the steel material has a yield strength of 758 to 862 MPa (grade 110 ksi), provided that the other specifications of this modality are satisfied.

[0093] The sum of the tempered martensite volume ratio and the tempered bainite volume ratio can be obtained by observing the microstructure. When observing the microstructure, the aforementioned photographic image formed during the acquisition of the pre-γ grain diameter is used. In each field of view, the tempered martensite and tempered bainite can be distinguished from other phases (ferrite or pearlite, for example) based on contrast. Consequently, in each field of view, the tempered martensite and tempered bainite are specified based on contrast.

[0094] The sum of the specified tempered martensite area fraction and the specified tempered bainite area fraction is acquired. In the present mode, the arithmetic mean of the sum of the tempered martensite area fraction and the tempered bainite area fraction, acquired in all fields of view, is assumed to be the tempered martensite-to-bainite volume ratio.

[0095] ζο / οηη / ίζηζ / Β / Υΐ Elastic limit of the steel material The yield strength of the steel material, according to this specification, is 758 to 862 MPa (grade 110 ksi). The yield strength in this description means 0.7% of the elongation stress (0.7% of the yield strength) achieved in a tensile test. Even if the yield strength of the steel material, according to this specification, is grade 110 ksi, the steel material, according to this specification, has excellent SSC strength provided it meets the aforementioned chemical composition, pre-γ grain diameter, and average precipitate area.

[0096] The yield strength of the steel material, according to this modality, may be determined by the following method. The tensile test is performed using a method in accordance with ASTM E8 / E8M (2013). A test specimen is taken from the round bar of the steel material, according to this modality. In the case where the steel material is a steel plate, a test specimen is taken from the center portion of the thickness of the round bar. In the case where the steel material is a steel tube, a test specimen is taken from the center portion of the wall thickness of the round bar. The size of the test specimen of the round bar is such that the diameter of a parallel portion is 8.9 mm and the length of the parallel portion is 35.6 mm, for example. The axial direction of the test specimen of the round bar is parallel to the rolling direction of the steel material.The tensile test is performed using the round bar of the test specimen in the atmosphere at normal temperature (25 °C) and 0.7% of the elongation stress is defined as the elastic limit (MPa).

[0097] SSC resistance of the steel material The SSC strength of the steel material, according to the present modality, can be evaluated by a method in accordance with the “Method A” specified in NACE TM0177-2005 and a four-point bending test.

[0098] In the method conforming to “Method A” specified in NACE TM0177-2005, a test sample round bar is taken from the steel material, according to the present method. In the case where the steel material is a steel plate, a test sample is taken from the center portion of the thickness of the round bar. In the case where the steel material is a steel tube, a test sample is taken from the center portion of the wall thickness of the round bar. The size of the test sample round bar is such that, for example, the diameter is 6.35 mm and the length of a parallel portion is 25.4 mm. The axial direction of the test sample round bar is parallel to the rolling direction of the steel material.

[0099] A mixed aqueous solution containing 5.0% by mass sodium chloride and 0.5% by mass acetic acid (NACE solution A) at 24 °C is used as a test solution. A stress equivalent to 90% of the actual yield stress is applied to the round bar test specimen. The test solution at 24 °C is poured into a test vessel so that the stressed round bar test specimen is submerged in it, and this is adopted as a test bath. After degassing the test bath, H₂S gas at 1 atm pressure is blown into the test bath and saturated. The test bath, where H₂S saturation is achieved, is maintained for 720 hours at 24 °C.

[0100] On the other hand, in the four-point bending test, a test sample is taken from the steel material, according to the present procedure. In the case where the steel material is a steel plate, the test sample is taken from a central portion of the thickness. In the case where the steel material is a steel tube, the test sample is taken from a central portion of the wall thickness. The size of the test sample is such that the thickness is 2 mm, the width is 10 mm, and the length is 75 mm, for example. The length direction of the test sample is parallel to the rolling direction of the steel material.

[0101] An aqueous solution containing 5.0% by mass sodium chloride at 24°C is used as the test solution. In accordance with ASTM G39-99 (2011), stress is applied to the test specimens by four-point bending such that the stress applied to each test specimen becomes 90% of the true yield stress. The stressed test specimen is placed in an autoclave, along with the test template. The test solution is poured into the autoclave so that a vapor-phase portion is left and used as the test bath. After degassing the test bath, it is pressure-sealed with H₂S gas at 15 atm in the autoclave, and the test bath is agitated to saturate the H₂S gas. After the autoclave is hermetically sealed, the test bath is agitated for 720 hours at 24°C.

[0102] In steel material, according to the present modality, cracking is not confirmed after 720 hours in both the method in accordance with “Method A” and the four-point bending test. It should be noted that, in the present description, the term “cracking is not confirmed” means that cracking is not confirmed in a test specimen in a case where the test specimen after the test is observed to the naked eye.

[0103] Shape of the steel material The form of the steel material, according to this modality, is not particularly limited. The steel material may be a steel pipe or a steel plate, for example. In the case where the steel material is a steel pipe for oil wells, a preferred wall thickness is 9 to 60 mm. More preferably, the steel material, according to this modality, is suitable for use as a thick-walled seamless steel pipe. More specifically, even when the steel material, according to this modality, is a seamless steel pipe having a wall thickness of 15 mm or more, or, in addition, 20 mm or more, the steel material exhibits a yield strength of 110 ksi and excellent SSC strength.

[0104] Production method A method for producing steel material in accordance with this specification will be described. The production method described below is a method for producing a seamless steel tube, which is an example of steel material in accordance with this specification. It should be noted that the method for producing steel material in accordance with this specification is not limited to the production method described below.

[0105] Preparation process In a preparation process, an intermediate steel material is prepared that has the aforementioned chemical composition. Provided the intermediate steel material has the aforementioned chemical composition, there is no particular limitation on the method for producing it. In the present embodiment, if the final product is a steel plate, the intermediate steel material is a steel plate. Whereas, if the final product is a steel tube, the intermediate steel material is a hollow casing.

[0106] The preparation process may preferably include a process for preparing a starting material (starting material preparation process) and a process for producing an intermediate steel material by performing hot working on the starting material (hot working process). The case where the preparation process includes the starting material preparation process and the hot working process will be described in detail below.

[0107] Process of preparing the starting material In the process of preparing the starting material, a starting material is produced using molten steel with the aforementioned chemical composition. Specifically, a casting (plate, billet, or bar) is produced through a continuous casting process using molten steel. An ingot can be produced through an ingot-making process using molten steel. A bar can be produced through a bar-forming process applied to a plate, billet, or ingot as needed. The starting material (plate, billet, or bar) is produced through the processes described above.

[0108] ζο / οηη / ίζηζ / Β / γι Hot work process In the hot working process, the heat is applied to the prepared starting material, producing an intermediate steel product. If the starting material is a steel tube, the intermediate steel product is a hollow shell. First, a bar is heated in a heating furnace. The heating temperature is not specifically limited; for example, it can range from 1100 to 1300 °C. The hot working is then performed on the bar removed from the heating furnace to produce a hollow shell (seamless steel tube).

[0109] For example, the Mannesmann process can be performed for hot working to produce a hollow shell. In this case, a round bar is subjected to perforated rolling using a perforating machine. When perforated rolling is performed, the perforation ratio is not particularly limited; for example, the perforation ratio can be from 1.0 to 4.0. The perforated round bar is then subjected to further hot rolling using a mandrel mill, a reducer, a sizing mill, or similar equipment, thus forming a hollow shell. The cumulative area reduction in the hot working process is, for example, from 20 to 70%.

[0110] A hollow shell can be produced from a bar stock using another hot working method. For example, in the case of a short-length, thick-walled steel material, such as a coupling, a hollow shell can be produced by forging using the Ehrhardt method or a similar process. The hollow shell is produced through the processes mentioned above. The wall thickness of the hollow shell to be produced is not particularly limited; for example, the wall thickness can be from 9 to 60 mm.

[0111] The hollow shell produced by hot working may be air-cooled (while being rolled). The hollow shell produced by hot working may be subjected to direct tempering after hot working without being cooled to normal temperature or may be subjected to tempering after undergoing additional heating (reheating) after hot working.

[0112] In cases where direct hardening or hardening after supplementary heating is performed, the cooling process can be stopped or slow-cooled during hardening. This can prevent hardening cracks from forming in the hollow shell. In cases where direct hardening or hardening after supplementary heating is performed, stress-relieving treatment (SR treatment) can be carried out after hardening and before the subsequent heat treatment (tempering or similar). This eliminates residual stress in the hollow shell.

[0113] As described above, the intermediate steel material is prepared in the preparation process. The intermediate steel material may be produced by the aforementioned preferred process or may be an intermediate steel material produced by a third party or an intermediate steel material produced in a different factory from the one where the quenching and tempering processes described below are carried out, or in a different operation.

[0114] Heat treatment process In the heat treatment process, the heat treatment is performed on the prepared intermediate steel material. Specifically, quenching and tempering are performed on the prepared intermediate steel material. In this description, “quenching” means rapidly cooling an intermediate steel material to the temperature of point As or higher. In this description, “tempering” means reheating and holding the quenched intermediate steel material at the temperature of point Aci or lower.

[0115] In the heat treatment process according to the present method, it is preferable to perform the quenching and tempering multiple times. Specifically, it is preferable to perform each of the quenching and tempering two or more times. More specifically, it is preferable to perform the quenching and then the tempering on the prepared intermediate steel material. In addition, the quenching and then the tempering are performed on the prepared intermediate steel material.

[0116] It should be noted that, in the heat treatment process according to the present method, quenching and tempering can be performed three or more times. However, even if quenching and tempering are performed four or more times, the advantageous effects obtained from the heat treatment become saturated. Therefore, in the heat treatment process according to the present method, it is preferable to perform quenching and tempering two or three times. Quenching and tempering will be described in detail below.

[0117] Tempered The tempering is performed on the prepared intermediate steel material (hollow shell) and / or the intermediate steel material on which the annealing is performed. In the heat treatment process according to this embodiment, a preferred tempering temperature is 800 to 1000 °C. In this description, "temperature" refers to the surface temperature of the intermediate steel material as measured by a thermometer installed on the outlet side of an apparatus performing final hot working when tempering is performed directly after hot working. The tempering temperature also refers to the temperature of a supplementary heating furnace or a heat treatment furnace when tempering is performed using the holding furnace or the heat treatment furnace after hot working.

[0118] That is, in the heat treatment process according to the present embodiment, tempering can be carried out by rapidly cooling the intermediate steel material from 800 to 1000 °C after hot working. Tempering can be carried out such that the intermediate steel material on which hot working is performed is heated to 800 to 1000 °C using the supplementary heating furnace or the heat treatment furnace and then rapidly cooled. Alternatively, tempering can be carried out such that the intermediate steel material on which tempering is performed is heated to 800 to 1000 °C using the heat treatment furnace and then rapidly cooled.

[0119] When the quenching temperature is too high, the pre-γ grain may become coarser, thus reducing the SSC strength of a steel material. Accordingly, the quenching temperature is preferably set between 800 and 1000 °C. A more preferable upper limit for the quenching temperature is 950 °C.

[0120] In the heat treatment process according to the present modality, where quenching is performed using the supplementary heating furnace or the heat treatment furnace after hot working, a preferred quenching time is 5 to 20 minutes. In the present description, “quenching time” means the time from when an intermediate steel material is loaded into the supplementary heating furnace or the heat treatment furnace until the intermediate steel material is removed.

[0121] In cases where tempering is performed using a supplementary heating furnace or a heat treatment furnace after hot working, if the tempering time is too long, the pre-γ grain size may become coarser after the final tempering. Therefore, in cases where tempering is performed using a supplementary heating furnace or a heat treatment furnace after hot working in the heat treatment process described herein, it is preferable to set the tempering time between 5 and 20 minutes.

[0122] For example, a tempering method may be adopted where a hollow shell is continuously cooled from a starting temperature to continuously reduce the temperature of the hollow shell. The method for a continuous cooling process is not particularly limited, and any known method may be adopted. The method for the continuous cooling process may be one where a hollow shell is immersed in a water tank for cooling, or a method where a hollow shell is cooled with a water bath or mist for accelerated cooling.

[0123] When the cooling rate during quenching is too low, a microstructure consisting primarily of martensite and bainite cannot be obtained, and therefore the mechanical property defined in this embodiment cannot be achieved. Accordingly, in the method for producing a steel material according to this embodiment, an intermediate steel material (hollow shell) is rapidly cooled during quenching. Specifically, in the quenching process, an average cooling rate when the temperature of the intermediate steel material (hollow shell) during quenching falls within a range of 800 to 500 °C is defined as a quenching cooling rate CRgoo soo (°C / s). More specifically, the quenching cooling rate CRsoo soo is determined from a temperature measured at the surface of the quenched intermediate steel material.

[0124] A preferred cooling rate during CRsoo-soo quenching is 8 °C / s or more. In this case, the microstructure of an intermediate steel material (hollow shell) in which the quenching is performed is composed mainly of martensite and bainite in a stable manner. A preferred lower limit of the cooling rate during CRsoo-soo quenching is 10 °C / s. A preferred upper limit of the cooling rate during CRsoo-soo quenching is 500 °C / s.

[0125] Revenido Tempering is performed on the intermediate steel material that undergoes the previously mentioned quenching process. When tempering steel intended for use in an acidic environment, the tempering temperature and time are adjusted according to the steel's chemical composition and the expected yield strength. In this case, only the final tempering step is monitored, and it is generally considered sufficient to establish a tempering temperature at or below the Aci point during any tempering step other than the final one.

[0126] On the other hand, in the steel material, according to the present embodiment, the pre-γ grain size is made finer by increasing the Mo content. With respect to this mechanism, as described above, it is considered that the Mo dissolved in the steel material segregates at the austenite grain boundaries during heating in a quenching process, thus making the pre-γ grain size fine after tempering due to a fixation effect. In the present embodiment, the Mo can form M2C carbide in the steel material having the aforementioned chemical composition. Furthermore, in the steel material having the aforementioned chemical composition, the M2C carbide is susceptible to precipitation during tempering.

[0127] In view of the above, in the heat treatment process, according to the present embodiment, a sufficient amount of Mo is dissolved in a steel material in which the penultimate tempering is performed.Specifically, in the heat treatment process, according to the present modality, a tempering parameter TMP2 (= (tempering temperature (°C)+273)x(log (tempering time (min) / 60)+20)) is controlled during the penultimate tempering and, therefore, it is possible to reduce the amount of Mo that precipitates as M2C carbide.

[0128] More specifically, in the steel material having the above-mentioned chemical composition, when the tempering parameter TMP2 during the penultimate tempering is 15000 to 19000, it is possible to make the pre-γ grain diameter fine in the steel material in which the last tempering is performed. ζο / οηη / ίζηζ / Ε / Υΐ When the tempering parameter TMP2 during the penultimate tempering is less than 15000, the beneficial effects of tempering may not be sufficiently achieved, resulting in quenching cracks or station cracks in the steel material. Conversely, when the tempering parameter TMP2 during the penultimate tempering is greater than 19000, insufficient dissolved Mo may be obtained during heating in the final tempering, potentially leading to coarser grain size in the pre-γ stage during the final tempering.

[0129] Accordingly, in the heat treatment process, according to the present modality, a preferred tempering parameter TMP2 during the penultimate tempering is from 15000 to 19000. A more preferred lower limit of the tempering parameter TMP2 during the penultimate tempering is 15500 and even more preferably 16000. A more preferred upper limit of the tempering parameter TMP2 during the penultimate tempering is 18500 and even more preferably 18000.

[0130] In the penultimate tempering step, a preferred tempering temperature is 500 to less than 700 °C. In the penultimate tempering step, a more preferred tempering time (holding time) is 10 to 60 minutes. That is, in the present embodiment, in the penultimate tempering step, the tempering temperature is set from 500 to less than 700 °C and the tempering time is set from 10 to 60 minutes, and furthermore, the tempering parameter TMP2 is set from 15000 to 19000.

[0131] It should be noted that, in this description, “tempering temperature” refers to the temperature of a heat treatment furnace at the time of heating and holding an intermediate steel material undergoing tempering. In this description, tempering time (holding time) means the time from when the intermediate steel material is loaded into the heat treatment furnace for heating and holding until a predetermined time when the intermediate steel material is removed.

[0132] Furthermore, in this description, “penultimate tempering” means the tempering performed before the final quenching and tempering. That is, if each quenching and tempering step is performed twice in the heat treatment process, the penultimate tempering step means the first tempering step. If each quenching and tempering step is performed three times in the heat treatment process, the penultimate tempering step means the second tempering step.

[0133] The steel material, according to the present embodiment, further reduces the coarse specific precipitates from the precipitates that precipitate at the pre-γ grain boundaries (specific precipitates). As described above, most of the specific precipitates are carbides. Therefore, most of the specific precipitates precipitate in the last tempering. Consequently, in the heat treatment process, according to the present embodiment, not only is the tempering parameter TMP2 controlled during the penultimate tempering, but a tempering parameter TMPi is also controlled during the last tempering (=(tempering temperature (°C)+273)x(log(tempering time (min) / 60)+20)). ζο / οηη / ίζηζ / Β / Υΐ

[0134] More specifically, in steel material with the aforementioned chemical composition, provided the tempering parameter TMPi during the final tempering is between 19100 and 19600, the coarse specific precipitates in the steel material undergoing the final tempering can be reduced. When the tempering parameter TMPi during the final tempering is less than 19100, the advantageous effects of tempering may not be sufficiently achieved, and the yield strength of the tempered steel material may be too high. A large amount of coarse specific precipitates may also be precipitated when the tempering parameter TMPi during the final tempering is less than 19100.

[0135] Furthermore, when the tempering parameter TMPi during the last tempering is greater than 19600, it may be the case that the yield strength of the steel material being tempered is too low. When the tempering parameter TMPi during the last tempering is greater than 19600, it may also be the case that a large amount of coarse precipitates form.

[0136] Accordingly, in the heat treatment process, according to the present modality, a preferred tempering parameter TMPi during the last tempering is from 19100 to 19600. A more preferred lower limit of the tempering parameter TMPi during the last tempering is 19200 and even more preferably is 19300. A more preferred upper limit of the tempering parameter TMPi during the last tempering is 19570 and even more preferably is 19500.

[0137] In the final tempering, a preferred tempering temperature is 650 to 730 °C. In the final tempering, a preferred tempering time (holding time) is 10 to 90 minutes. That is, in the present embodiment, in the final tempering, the tempering temperature is set from 650 to 730 °C and the tempering time is set from 10 to 90 minutes, and furthermore, the tempering parameter TMPi is set from 19100 to 19600.

[0138] When the steel material is a steel tube, variations in temperature are likely to occur during tempering compared to other shapes. Therefore, when the steel material is a steel tube, a preferred tempering time is 15 to 90 minutes. It is sufficiently possible for those skilled in the art to establish the yield strength of 758 to 862 MPa (grade 110 ksi) by appropriately adjusting the aforementioned tempering temperature and tempering time for steel material having the chemical composition described herein.

[0139] The steel material, according to the present modality, can be produced by the production method mentioned above. In the production method mentioned above, the method for producing a seamless steel tube has been described as an example. However, the steel material, according to the present modality, can be a steel plate or can have another form. Similarly to the production method mentioned above, the method for producing a steel plate or a product having another form also includes a preparation process and a heat treatment process, for example. Furthermore, the production method mentioned above is merely an example, and the steel material can be produced by another production method. EXAMPLES

[0140] Molten steels were produced that had the chemical composition shown in Table 1. The F1 of each steel was also obtained from the chemical composition described in Table 1. It is observed that in Table 1 it means that the content of each element is at the level of an impurity. ζα / οηη / ίζηζ / Β / γι > α rch

[0141] lip Maple Chemical composition (the unit is % in mass, the rest are Fe and impurezas) F1 C Si Mn P s Cr Mo Al N Ti Nb VB 0 Ca Mg Zr REM Cu Ni Co w A 0.24 0.27 0.42 0.012 0.0007 0.98 0.92 0.034 0.0054 0.009 0.035 0.10 0.0011 0.0015 - - - - - 0.94 B 0.26 0.25 0.43 0.013 0.0008 0.76 0.85 0.033 0.0059 0.007 0.036 0.10 0.0011 0.0019 0.0012 - - - - - 1.12 C 0.25 0.25 0.42 0.012 0.0006 0.74 0.95 0.035 0.0060 0.006 0.029 0.10 0.0010 0.0019 0.0012 - - - - - 1.28 D 0.25 0.26 0.37 0.012 0.0005 1.01 0.97 0.035 0.0057 0.006 0.029 0.10 0.0009 0.0017 0.0050 - - - - 0.96 E 0.25 0.27 0.43 0.011 0.0006 0.90 0.91 0.035 0.0046 0.010 0.030 0.10 0.0011 0.0013 - - 0.0030 0.03 0.04 - - 1.01 F 0.27 0.26 0.42 0.011 0.0006 0.71 0.75 0.033 0.0041 0.010 0.040 0.11 0.0010 0.0010 • - 0.0070 - - - - 1.06 G 0.27 0.27 0.41 0.011 0.0005 0.62 0.95 0.032 0.0047 0.010 0.030 0.12 0.0010 0.0010 - 0.03 - 0.20 - 1.53 H 0.26 0.26 0.41 0.010 0.0007 0.62 0.95 0.038 0.0046 0.010 0.040 0.11 0.0011 0.0011 - 0.03 0.04 - 0.25 1.53 I 0.24 0.25 0.07 0.012 0.0006 0.85 0.82 0.030 0.0051 0.010 0.030 0.11 0.0012 0.0015 0.0012 - 0.03 0.04 - - 0.96 J 0.25 0.32 0.41 0.011 0.0009 1.02 0.81 0.029 0.0044 0.008 0.029 0.11 0.0010 0.0018 - - - - - 0.79 K 0.25 0.26 0.41 0.011 0.0005 1.02 0.77 0.038 0.0039 0.005 0.027 0.10 0.0011 0.0013 0.0015 - - - - - - - 0.75 L 0.26 0.25 0.40 0.010 0.0005 1.01 0.89 0.039 0.0037 0.005 0.026 0.10 0.0012 0.0010 0.0017 - - - - - 0.88 M 0.24 0.25 0.42 0.010 0.0008 0.50 1.16 0.031 0.0043 0.006 0.026 0.09 0.0012 0.0019 - - - - - - - - 2.32 N 0.27 0.30 0.45 0.008 0.0012 1.05 0.30 0.035 0.0035 0.006 0.027 0.10 0.0012 0.0009 - - - - - 0.29 0 0.27 0.25 0.41 0.011 0.0007 1.20 0.90 0.037 0.0047 0.010 0.040 0.12 0.0010 0.0011 - - - - - 0.04 - - 0.75 P 0.26 0.25 0.42 0.011 0.0006 0.42 0.80 0.031 0.0042 0.010 0.030 0.11 0.0010 0.0011 - 0.03 - - - 1.90 Q 0.25 0.25 0.42 0.012 0.0008 0.61 0.56 0.032 0.0041 0.009 0.026 0.11 0.0009 0.0016 - - - - - 0.92 R 0.24 0.26 0.43 0.011 0.0007 0.63 0.60 0.039 0.0052 0.008 0.029 0.10 0.0010 0.0015 - - - - - - 0.95.

[0142] The bars were produced using the aforementioned cast steels by a continuous casting process. The bars produced of the respective trial numbers were held for one hour at 1250 °C and subsequently hot-rolled (hot-worked) on the bars using the Mannesmann mandrel method to produce hollow casings (seamless steel tubes) of the respective trial numbers.

[0143] Heat treatment (quenching and tempering) was performed twice on each of the hollow shells of the respective test numbers on which hot work was performed. Specifically, the heat treatment was performed on the hollow shells of the respective test numbers using the following method.

[0144] The hollow shells of the respective test numbers produced by hot working were held for 5 minutes in a supplemental heating furnace at 950 °C, and then a direct quench (i.e., first quench) was performed. All cooling rates during the CRgoo 500 quench in the first quench of the respective test numbers were within a range of 8 to 500 °C / s. It should be noted that the cooling rate during the CRgoo 500 quench was obtained by measuring the surface temperature of the hollow shell of each test number.

[0145] Subsequently, the first tempering, i.e., the penultimate tempering, was carried out on the hollow shells of the respective test numbers. Specifically, in the hollow shell of each test number, a tempering was performed where each hollow shell was held at the tempering temperature (°C) for the tempering time (min) described in the “penultimate tempering” column in Table 2. The tempering parameters TMP2 (= (tempering temperature (°C)+273)x(log (tempering time (min) / 60)+20)) during the penultimate tempering are also shown in Table 2. ζα / οηη / ίζηζ / Β / γι

[0146] Table 2 Test No. Steel Penultimate tempering Last tempering Last tempering YS (MPa) TS (MPa) Pre-y grain diameter (pm) Average area of ​​specific precipitates (xl0'3 pm2) SSC resistance Temp. Temp. Temp. (°C) Temp. tempering time (min) TMP] Temp. tempering time (°C) Temp. Tempering (°C) Tempering time (min) TMPi H2S 1 atm H2S 15 atm 1 A 550 30 16212 920 15 700 60 19460 831 901 13.5 8.9 EE 2 B 550 30 16212 920 15 700 60 19460 837 903 13.1 7.6 EE 3 C 550 30 16212 920 15 700 60 19460 832 904 13.9 7.2 EE 4 D 550 30 16212 920 15 700 60 19460 832 904 14.1 9.9 EE 5 E 550 30 16212 920 15 700 60 19460 845 916 12.7 7.7 EE 6 F 550 30 16212 920 15 700 60 19460 832 910 11.9 6.9 EE 7 G 550 30 16212 920 15 700 60 19460 832 912 12.9 8.3 EE 8 H 550 30 16212 920 15 700 60 19460 841 917 13.1 9.2 EE 9 I 550 30 16212 920 15 700 60 19460 843 917 13.2 9.4 EE 10 J 550 30 16212 920 15 700 60 19460 819 896 12.5 14.2 NA E 11 K 550 30 16212 920 15 700 60 19460 839 913 12.5 13.5 NA E 12 L 550 30 16212 920 15 700 60 19460 825 907 12.4 12.9 NA E 13 M 550 30 16212 920 15 700 60 19460 804 874 13.5 6.5 E NA 14 N 600 30 17197 920 15 695 60 19360 793 874 20.6 13.0 NA NA 15 0 550 30 16212 920 15 700 60 19460 836 904 12.6 7.8 NA NA 16 P 550 30 16212 920 15 700 60 19460 849 905 13.9 14.2 NA E 17 Q 550 30 16212 920 15 700 60 19460 826 901 17.1 9.5 NA NA 18 R 550 30 16212 920 15 700 60 19460 835 903 16.3 8.5 NA NA 19 B 700 30 19167 920 15 700 60 19460 798 887 20.5 11.5 NA NA 20 C 700 30 19167 920 15 700 60 19460 789 867 22.9 12.2 NA NA 21 D 700 30 19167 920 15 690 30 18970 875 935 14.5 13.2 NA NA 22 D 700 30 19167 920 15 700 100 19676 798 874 14.3 13.4 NA E.

[0147] The second, or final, tempering was performed in the hollow shells of the respective test numbers in which the first tempering, mentioned above, was performed. Specifically, the hollow shell of each test number was held at the tempering temperature (°C) for the tempering time (min) described in the “final tempering” column of Table 2, and then tempering was carried out in the hollow shell. All cooling rates during the second tempering of the respective test numbers were within the range of 8 to 500 °C / s.

[0148] In addition to the above, a second tempering, i.e., the final tempering, was performed on the hollow shells of the respective test numbers in which the final tempering was carried out. Specifically, in the hollow shell of each test number, a tempering was performed where each hollow shell was held at the tempering temperature (°C) for the tempering time (min) described in the “final tempering” column in Table 2. The tempering parameters TMPi during the final tempering (=(tempering temperature (°C) +273)x(log (tempering time (min) / 60) +20)) are shown in Table 2.

[0149] It is observed that, in the present example, the temperature of the supplementary heating furnace or heat treatment furnace used for heating in tempering corresponds to the “tempering temperature (°C)”. Furthermore, the temperature of the heat treatment furnace used in annealing corresponds to the “annealing temperature (°C)”. Additionally, the time from when the hollow shell is loaded into the holding furnace or heat treatment furnace during the tempering process until the hollow shell is removed corresponds to the “tempering time (min)”. The time from when the hollow shell is loaded into the heat treatment furnace during annealing until the hollow shell is removed corresponds to the “annealing time (min)”.

[0150] Assessment test The microstructure observation, tensile test, and SSC strength evaluation test, which will be described below, were performed on the seamless steel tubes of the respective test numbers on which the tempering treatment was performed.

[0151] Observation of the microstructure The pre-γ grain diameter was measured in the seamless steel tube of each test number using the method described above. The pre-γ grain diameters (pm) of the seamless steel tubes of the respective test numbers are shown in Table 2. For the seamless steel tube of each test number, the average area of ​​the precipitates that formed at the pre-γ grain boundaries (specific precipitates) was also obtained using the method described above. The average areas of the specific precipitates (xlO3pm2) in the seamless steel tubes of the respective test numbers are shown in Table 2.

[0152] Traction test The yield strength of the seamless steel tube for each test number was measured using the method described above. Specifically, a tensile test was performed in accordance with ASTM E8 / E8M (2013). More specifically, a tensile test specimen was prepared from the round bar, with a parallel portion measuring 8.9 mm in diameter and 35.6 mm in length, extending from the center of the wall thickness of the seamless steel tube for each test number. The axial direction of the tensile test specimen was parallel to the axial direction of the seamless steel tube.

[0153] A tensile test was performed using the round test bar of each test number in the atmosphere at normal temperature (25 °C) to acquire the yield strength (MPa) of the seamless steel tube of each test number. Note that, in the present example, the stress at 0.7% elongation acquired in the tensile test was defined as the yield strength of each test number. The acquired yield strength YS (MPa) and tensile strength TS (MPa) are shown in Table 2.

[0154] SSC strength evaluation test of steel material Using the seamless steel tubing of the respective test numbers, a conformity test was performed according to “Method A” specified in NACE TM0177-2005, and a four-point bending test was performed to evaluate the SSC strength. Specifically, the conformity test according to “Method A” specified in NACE TM0177-2005 was performed using the following method.

[0155] Three test samples of round bars were taken, each with a diameter of 6.35 mm and a parallel portion 25.4 mm long, from the center portion of the wall thickness of the seamless steel tube for each test number. Each test sample of the round bar was taken so that its axial direction was parallel to the axial direction of the seamless steel tube. A tensile stress was applied in the axial direction of the test sample of the round bar for each test number. At this operating point, the adjustment was made so that the stress to be applied was 90% of the actual yield strength of the seamless steel tube for each test number.

[0156] A mixed aqueous solution containing 5.0% by mass sodium chloride and 5.0% by mass acetic acid (NACE solution A) was used as the test solution. The test solution at 24 °C was poured into three test vessels, which were then used as test baths. The three round bar test specimens subjected to stress were individually immersed in different test vessels. After each test bath was degassed, H2S gas at 1 atm was blown into the respective test baths and brought to saturation. The test baths in which the H2S gas at 1 atm was saturated were maintained at 24 °C for 720 hours.

[0157] Meanwhile, the four-point bending test was performed using the following method. Three test specimens, each 2 mm thick, 10 mm wide, and 75 mm long, were taken from the central portion of the wall thickness of the seamless steel pipe of each test number. The test specimen was taken so that its longitudinal direction was parallel to the axial direction of the seamless steel pipe. Tension was applied to the test specimens of each test number by four-point bending, in accordance with ASTM G39-99 (2011), such that the tension applied to each test specimen was 90% of the true yield strength of the seamless steel pipe of each test number. The stressed test specimen was sealed in an autoclave along with a test template.

[0158] An aqueous solution containing 5.0% by mass sodium chloride was used as the test solution. The test solution was poured into the autoclave while maintaining a portion of the gas phase, thus preparing the test bath. After degassing the test bath, H2S gas at 15 atm was pressure sealed, and the test bath was agitated to saturate it with H2S gas. After sealing the autoclave, the test bath was agitated for 720 hours at 24 °C.

[0159] In each of the aforementioned tests, in accordance with Method A specified in NACE TM0177-2005 and the four-point bending test, the test samples of the respective test numbers were observed for the presence or absence of sulfide stress cracking (SSC) after being held for 720 hours. Specifically, the test samples held for 720 hours were observed visually. As a result of the observation, a test number in which cracking was not confirmed in all test samples was determined to be "E" (Excellent). On the other hand, a test number in which cracking was confirmed in at least one test sample was determined to be "NA" (Not Acceptable).

[0160] Test results Table 2 shows the test results. Regarding the SSC strength test, the test results, in accordance with “Method A” specified in NACE TM0177-2005, are shown in the “H2S 1 atm” column, and the results of the four-point bending test are shown in the “H2S 15 atm” column.

[0161] With reference to Table 1 and Table 2, in the seamless steel tubes of test numbers 1 to 9, the chemical composition was appropriate, the yield strength was 758 to 862 MPa, the pre-γ grain diameter was 15.0 pm or less, and the average area of ​​the specific precipitates was 12.5 x 10³ pm² or less. As a result, excellent SSC strength was shown in both the test in accordance with “Method A” specified in NACE TM0177-2005 and the four-point bending test.

[0162] On the other hand, in the seamless steel tubes of test numbers 10 to 12, the F1 was too low. Therefore, the average area of ​​the specific precipitates was greater than 12.5 x 10³ pm². As a result, excellent SSC strength was not shown in the test in accordance with “Method A” specified in NACE TM0177-2005.

[0163] In the seamless steel tube of test number 13, the Cr content was too low. Also, the Mo content was too high. As a result, excellent SSC strength was not shown in the four-point bending test.

[0164] In the seamless steel tube of test number 14, the Mo content was too low. In addition, F1 was too low. Therefore, the pre-γ grain diameter was greater than 15.0 pm. Consequently, the average area of ​​the specific precipitates was also greater than 12.5 x 10.3 pm². As a result, excellent SSC strength was not shown in either the test in accordance with “Method A” specified in NACE TM0177-2005 or the four-point bending test.

[0165] In the seamless steel tube of test number 15, the Cr content was too high. In addition, F1 was too low. As a result, excellent SSC strength was not shown in either the test conforming to the “Method A” specified in NACE TM0177-2005 or the four-point bending test.

[0166] In the seamless steel pipe of test number 16, the Cr content was too low. Therefore, the average area of ​​the specific precipitates was also greater than 12.5 x 10³ pm². As a result, excellent SSC strength was not shown in the test in accordance with “Method A” specified in NACE TM0177-2005.

[0167] In the seamless steel tubes of test numbers 17 and 18, the Mo content was too low. Therefore, the pre-γ grain diameter was greater than 15.0 pm. As a result, excellent SSC strength was not shown in either the test conforming to the “Method A” specified in NACE TM0177-2005 or the four-point bending test.

[0168] In the seamless steel tubes of test numbers 19 and 20, the tempering parameter TMP2 during the penultimate tempering was too high. Therefore, the pre-γ grain diameter was greater than 15.0 pm. As a result, excellent SSC strength was not shown in either the test conformity to the “Method A” specified in NACE TM0177-2005 or in the four-point bend test.

[0169] In the seamless steel tube of test number 21, the tempering parameter TMP2 during the penultimate tempering was too high. In addition, the tempering parameter TMPi during the penultimate tempering was too low. Therefore, the average area of ​​the specific precipitates was also greater than 12.5x103pm2. As a result, the yield strength was greater than 862 MPa, so a yield strength of grade 110 ksi was not obtained. As a result, excellent SSC strength was not shown in either the test in accordance with the “Method A” specified in NACE TMO177-2005 or the four-point bending test.

[0170] In the seamless steel tube of test number 22, the tempering parameter TMP2 during the penultimate tempering was too high. In addition, the tempering parameter TMPi during the penultimate tempering was too high. Therefore, the average area of ​​the specific precipitates was greater than 12.5x103pm2. As a result, excellent SSC strength was not shown in the test in accordance with “Method A” specified in NACE TM0177-2005.

[0171] 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 modified and implemented appropriately within a range that does not depart from the essence of the present invention. INDUSTRIAL APPLICABILITY The steel material according to the present invention is widely applicable in steel materials used in harsh environments, such as polar regions. It is preferable that the steel material according to the present invention can be used as a steel material in oil well environments. It is even more preferable that the steel material according to the present invention can be used as a steel material, such as casing, tubing, or pipe.

Claims

1. A steel material characterized in that it comprises: a chemical composition consisting of, in % by mass: C: from 0.15 to 0.45%, Si: from 0.05 to 1.00%, Mn: from 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: from 0.005 to 0.100%, Cr: from 0.55 to 1.10%, Mo: from 0.70 to 1.00%, Ti: from 0.002 to 0.020%, V: from 0.05 to 0.30%, Nb: from 0.002 to 0.100%, B: from 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Co: 0 to 0.50% and W: 0 to 0.50%, the remainder being Fe and impurities and satisfying formula (1), wherein in the steel material, a grain diameter of a pre-austenite grain is 15.0 pm or less, an average precipitate area that precipitates at a pre-austenite grain boundary is 12.5xl03 pm2 or less and a yield strength is 758 to 862 MPa: Mo / Cr>0.90 (1) ζο / οηη / ίζηζ / Β / γι where the content (% by mass) of a corresponding element is substituted for each element symbol in formula (1).

2. The steel material according to claim 1, characterized in that the chemical composition 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 steel material, according to claim 1, characterized in that the chemical composition 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%.

4. The steel material, according to claim 2, characterized in that the chemical composition 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%.

5. The steel material, in accordance with any of claim 1 to claim 4, characterized in that the chemical composition 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%.

6. The steel material, according to claim 1, characterized in that the steel material is a steel pipe for oil wells.

7. The steel material, according to claim 1, characterized in that the steel material is a seamless steel tube.