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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations

Definitions

• the present invention relates to steel for oil wells and oil country tubular goods.
• oil wells for extracting crude oil, natural gas, and other resources, which contain hydrogen sulfide (H 2 S), sulfide stress cracking (hereafter referred to as "SSC") of steel in wet hydrogen sulfide environments poses a problem.
• Oil country tubular goods with excellent SSC resistance are therefore needed.
• Low-alloy oil country tubular goods with excellent SSC resistance and high strength are needed for use in casing applications.
• SSC is a kind of hydrogen embrittlement in which hydrogen generated on the surface of steel in a corrosive environment diffuses into the steel material. The steel consequently ruptures due to a synergetic effect with under applied stress. In steel that is highly susceptible to SSC, cracks are easily generated by a load stress that is lower than the yield stress.
• tempered martensite has a body-centered cubic (hereafter referred to as "BCC") structure.
• BCC body-centered cubic
• Tempered martensite and ferrite which also has a BCC structure, are inherently susceptible to hydrogen embrittlement. Therefore, it is very difficult to completely prevent SSC for steel whose main microstructure is tempered martensite or ferrite.
• SSC susceptibility increases with increasing material strength, obtaining low-alloy steel with high strength and excellent SSC resistance is difficult.
• Patent Documents 1 to 4 describe high-strength steels that have a high Mn (an austenite-stabilizing element) content to give them excellent SSC resistance
• Patent Document 5 describes steel that has high toughness in addition to excellent SSC resistance.
• Non-Patent Document 1 A. Saeed-Akbari et al.: Metall. Mater. Trans. 40A (2009), 3076-3090 .
• Patent Documents 1 to 5 excellent SSC resistance and high strength are realized by increasing the content of austenite-stabilizing elements, such as C and Mn, and achieving precipitation hardening using elements such as Cr and V.
• austenite-stabilizing elements such as C and Mn
• precipitation hardening using elements such as Cr and V.
• a different approach is required to obtain materials that can withstand harsh wet hydrogen sulfide environments.
• the present invention solves the above problem.
• the proposed steel for oil wells has a yield stress of 125 ksi (862 MPa) or more and has SSC resistance that is higher than that of conventional steel. Oil country tubular goods made of this steel for oil wells are also described.
• the effective amount of C, defined in Formula (i) below, is between 0.55 and 1.54.
• the total volume fraction of ⁇ ' martensite and ferrite is less than 0.1%
• the volume fraction of ⁇ martensite, which has an HCP structure is 10% or less
• the balance is austenite.
• the number density of carbonitrides with an equivalent circular diameter of 5 to 100 nm is 100 particles/ ⁇ m 2 or more, the yield stress is 862 MPa or more, the stacking fault energy (SFE) at 25°C is 30 mJ/m 2 or more, and the austenite grain size is 100 ⁇ m or smaller.
• SFE stacking fault energy
• austenite which has excellent SSC resistance
• aging treatment with the addition of V improves the strength by precipitating fine carbonitrides as well as by refining the austenite grains.
• the C and Mn content can serve as an index of austenite stability.
• austenite is stabilized, if the austenitic structure easily changes from the FCC structure to a transformation phase (for example, the ⁇ phase with a hexagonal close-packed, hereafter referred to as "HCP", structure, or the ⁇ phase with a BCC structure) due to applied stress in a corrosive environment, the hydrogen embrittlement susceptibility will increase, making it difficult to completely prevent SSC.
• HCP hexagonal close-packed
• the present inventors focused on the SFE for austenite.
• the SFE is the energy associated with plane defects, which are a type of lattice defect in the FCC structure. Increasing the SFE suppresses the occurrence of twin deformation, the localization of dislocations (planarization), the stress concentration at the periphery of austenite grain boundaries, and the precipitation of the transformation phase, even under applied stress in a corrosive environment.
• the addition of Al increases the SFE, which suppresses the stress concentration at austenite grain boundaries and precipitation of the transformation phase. Furthermore, the addition of Al leads to the formation of a stable passivation layer that contains Al 2 O 3 on the surface, which suppresses hydrogen intrusion into the steel.
• the present inventors conducted research that confirmed that the addition of several percent Al to steel does not cause any particular production problems.
• the present inventors also found that increasing the Al content significantly improves the hydrogen embrittlement resistance compared to that for conventional steel.
• the present invention is based on the above findings.
• the requirements of steel for one embodiment of the present invention are described in detail below.
• the strength of the steel is improved by performing an aging treatment that causes carbonitrides to precipitate. C is consumed by this precipitation process. It is thus necessary to adjust the C content. Note that it is assumed that carbides are also included in the carbonitrides.
• C content is varied between 0.60 and 2.00%.
• the lower limit of C content is preferably more than 0.70%, and more preferably is 0.80% or more.
• the upper limit of C content is preferably 1.60% or less, and more preferably is 1.30% or less.
• Si Silicon
• the lower limit of Si content is preferably 0.10% or more, and more preferably is 0.20% or more.
• the upper limit of Si content is preferably 1.00% or less, and more preferably is 0.80% or less.
• Manganese (Mn) stabilizes austenite at low cost.
• the Mn content must be 16.0% or more. Mn dissolves preferentially in wet hydrogen sulfide environments, and thus it is difficult for stable corrosion products to form on the material surface. Consequently, the corrosion resistance may decrease with increasing Mn content. If the Mn content is more than 30.0%, the corrosion rate may exceed the standard corrosion rate for low-alloy oil country tubular goods. Therefore, the Mn content is varied in the range of 16.0 to 30.0%. The Mn content should preferably be between 17.0% (ideally 19.0%) and 25.0%.
• Aluminum (Al) is necessary for the deoxidation of steel. It markedly increases the SFE for steel.
• Al significantly improves the SSC resistance.
• the Al content must be 0.07% or more. If the Al content is more than 6.00%, the hot workability of the steel will markedly decrease and the ductility at room temperature will decrease. Therefore, the Al content is varied from 0.07 to 6.00%.
• the lower limit of Al content is preferably 0.50% or more, more preferably is more than 1.00%, and further preferably is 2.00% or more.
• the upper limit of Al content is preferably 5.00% or less, and more preferably is 4.00% or less.
• Al is in the form of acid-soluble Al (sol. Al).
• Vanadium (V) can cause fine carbonitrides to precipitate in steel, which increases the strength. This can be achieved by performing a heat treatment at an appropriate temperature for an appropriate period of time. Excessive V content will saturate the aforementioned effect and consume a large amount of C, which stabilizes austenite. Therefore, the V content is varied from 0.50 to 3.00%.
• the lower limit of V content is preferably 0.60% or more, and more preferably is 0.70% or more.
• the upper limit of V content is preferably 2.00% or less, and more preferably is 1.80% or less.
• N Nitrogen
• the N content is required to be 0.500% or less.
• the N content should preferably be 0.100% or less (ideally 0.050% or less). Note that there is no need to perform denitrification from the viewpoint of refining cost.
• the N content should preferably be 0.001% or more.
• Phosphorus (P) is unavoidably present in steel as an impurity. If the P content is more than 0.030%, P will segregate at grain boundaries, decreasing the SSC resistance. Therefore, the P content is set to 0.030% or less. Note that the P content should be minimized, preferably to 0.020% or less (ideally 0.012% or less). However, because obtaining a very low P content will increase the production cost, the P content should preferably be 0.001% or more (ideally 0.005% or more).
• S Sulfur
• S is unavoidably present in steel as an impurity. If the S content is more than 0.030%, S will segregate at grain boundaries, leading to the formation of sulfide-based inclusions, and thus reduce the SSC resistance. Therefore, the S content is set to 0.030% or less. Note that the S content should be minimized, preferably to 0.015% or less (ideally 0.010% or less). However, because obtaining a very low S content will increase the production cost, the S content should preferably be 0.001% or more (ideally 0.002% or more).
• one or more other elements i.e., Cr, Mo, Cu, Ni, Ti, Nb, Zr, Ta, Hf, W, B, Ca, and Mg
• Cr, Mo, Cu, Ni, Ti, Nb, Zr, Ta, Hf, W, B, Ca, and Mg may be present in the proposed steel for oil wells according to the present embodiment.
• the chromium (Cr) content can be as high as necessary because it improves the corrosion resistance of the steel. However, if the Cr content is excessive, Cr will decrease the SSC resistance and the stress corrosion cracking (SCC) resistance. Cr may also precipitate as carbonitrides during aging heat treatment and consume C in the base metal and thereby hinder the stabilization of austenite. In addition, when the Cr content is high, a higher solution heat treatment temperature is required, leading to economic disadvantages. Therefore, the Cr content is set to 10.00% or less. The Cr content is preferably 5.00% or less, and ideally 1.00% or less. Note that the Cr content should preferably be set to 0.05% or more (ideally 0.50% or more) to achieve the aforementioned effect.
• the molybdenum (Mo) content can be as high as necessary because it stabilizes corrosion products in wet hydrogen sulfide environments and improves the corrosion resistance. However, if the Mo content is more than 3.00%, it may decrease the SSC resistance and the SCC resistance. Furthermore, Mo is very expensive. Therefore, the Mo content is set to 3.00% or less. Note that the Mo content should preferably be set to 0.10% or more (ideally 0.50% or more) to achieve the aforementioned effects.
• Copper (Cu) stabilizes austenite and thus can be present in a small amount.
• Cu when the influence of Cu on corrosion resistance is taken into consideration (Cu promotes local corrosion and is liable to form a stress-concentrated zone on the steel surface), excessive Cu content may decrease the SSC resistance and SCC resistance. For this reason, the Cu content is set to 3.00% or less.
• the Cu content should preferably be 1.00% or less. Note that the Cu content should preferably be set to 0.10% or more (ideally 0.20% or more) to stabilize austenite.
• Nickel (Ni) stabilizes austenite and thus can be present in a small amount.
• Ni when the influence on corrosion resistance is taken into consideration (Ni promotes local corrosion and is liable to form a stress-concentration zone on the steel surface), excessive Ni content may decrease the SSC resistance and SCC resistance.
• Ni is expensive. Therefore, the Ni content is set to 20.00% or less.
• the Ni content should preferably be 10.00% or less (ideally 5.00% or less). Note that the Ni content should preferably be set to 0.10% or more (ideally 0.50% or more) to stabilize austenite.
• Titanium (Ti), niobium (Nb), and zirconium (Zr) strengthen steel by combining with C or N to form fine carbonitrides; the content of these elements can thus be as high as necessary.
• this strengthening effect is limited compared to that of V.
• the content of each of these elements is set to 3.00% or less (preferably 2.00% or less). Note that the content of one or more of these elements should preferably be 0.005% or more (ideally 0.05% or more) to achieve the aforementioned effect.
• Tantalum (Ta), hafnium (Hf), and tungsten (W) strengthen steel by combining with C or N to form minute carbonitrides; their content can thus be as high as necessary.
• this strengthening effect is limited compared to that of V.
• the content of each of these elements is set to 6.00% or less (preferably 3.00% or less). Note that one or more of these elements should preferably be 0.005% or more (ideally 0.05% or more) to achieve the aforementioned effect.
• Calcium (Ca) and magnesium (Mg) concentrations can be as high as necessary because they improve the toughness and corrosion resistance by controlling the morphology of inclusions and they enhance casting properties by suppressing nozzle clogging.
• the content of each element is set to 0.0050% or less (preferably 0.0030% or less).
• the total content of these elements should preferably be 0.0050% or less.
• the total content of one or more of these elements should preferably be 0.0003% or more (ideally 0.0005% or more).
• B Boron mainly strengthens grain boundaries and thus its content can be as high as necessary. However, if the B content is high, low-melting-point compounds may form and hot workability may decrease. There are cases where hot workability decreases markedly when the B content is more than 0.0150%. Therefore, the B content is set to 0.0150% or less. Note that the B content should preferably be set to 0.0001% or more to achieve the aforementioned effect.
• the proposed steel for oil wells according to the present embodiment has a chemical composition that consists of the elements described above (the balance is Fe and impurities).
• impurities refers to components which, during industrial production of the steel, are mixed in from raw materials such as ore, scrap, or other materials introduced during the production process, at a concentration that does not adversely affect the present embodiment.
• the C content is between 0.55 and 1.54.
• C is the most consumed when all V is precipitated as the carbonitrides.
• Cr, Mo, Ti, Nb, Zr, Ta, Hf, or W is present in the steel, C will be consumed by precipitation of carbonitrides of these elements.
• the content of C, V, Cr, Mo, Ti, Nb, Zr, Ta, Hf, and W it is necessary to adjust the content of C, V, Cr, Mo, Ti, Nb, Zr, Ta, Hf, and W so that the effective C content given in Formula (i) is 0.55 or more. If the effective C content is 1.54 or more, the microstructure will become inhomogeneous and the hot workability will decrease, accompanying the formation of compounds such as cementite. It is thus necessary to adjust the content of C, V, Cr, Mo, Ti, Nb, Zr, Ta, Hf, and W so that the effective C content is less than 1.54.
• the lower limit of effective amount of C is preferably 0.65 or more, and more preferably is 0.70 or more.
• the upper limit of effective amount of C is preferably 1.40 or less, more preferably is 1.30 or less, and further preferably is 1.20 or less.
• Mo and B in combination to satisfy Formula (ii).
• the value of Mo-200B in Formula (ii) should preferably be 0.10 or more (ideally 0.20 or more).
• the upper limit value of Mo-200B in Formula (ii) for the steel is 2.98.
• Mo ⁇ 200 B ⁇ 0 where each symbol represents the content (mass%) of the corresponding element contained in the steel material. If the element is not contained, 0 shall be substituted for the relevant element symbol.
• the Ti and Mo content should satisfy Formula (iii).
• the lower limit value of Ti/Mo in Formula (iii) is more preferably 0.45 or more, and further preferably is 0.48 or more.
• the upper limit value of Ti/Mo in Formula (iii) is more preferably 0.55 or less. 0.40 ⁇ Ti / Mo ⁇ 0.60 where each symbol represents the content (mass%) of the corresponding element in the steel. If the element is not contained, 0 shall be substituted for the relevant element symbol.
• the steel in the present embodiment has a microstructure in which the total volume fraction of ⁇ ' martensite and ferrite is less than 0.1%, the volume fraction of ⁇ martensite is 10% or less, and the balance is austenite. If ⁇ ' martensite, ferrite, and other phases, which have a BCC structure, are intermixed in the steel microstructure, SSC resistance will be reduced. However, as the matrix of the steel material, ⁇ ' martensite and ferrite are permitted to be present as long as the total volume fraction of ⁇ ' martensite and ferrite is less than 0.1%.
• the volume fraction of ⁇ martensite is set to 10% or less (ideally 2% or less).
• ⁇ ' martensite, ferrite, ⁇ martensite, and other phases are present in the steel microstructure as fine crystals, measurement of their volume fractions using methods such as X-ray diffraction and microscopic observation is difficult.
• the total volume fraction of microstructures with BCC and HCP structures can be measured using a ferrite meter and electron backscattering diffraction pattern analysis.
• the strength of steel with the microstructure described above is generally lower than that of steel that is mainly composed of ferrite. Therefore, in the present embodiment, the steel is strengthened by causing carbonitrides to precipitate, thus suppressing dislocation movement. However, if the equivalent circular diameter of the carbonitride precipitates is less than 5 nm, little strengthening will occur. Also, if the precipitates are too coarse (equivalent circular diameter of more than 100 nm) and the content of each element is within the range of the present embodiment, the number of precipitates will greatly decrease, and the strengthening effect will also be small. Therefore, the precipitate size should be 5 to 100 nm (ideally 15 to 50 nm).
• carbonitride precipitates with a size of 5 to 100 nm need to be present at a number density of 100 particles/ ⁇ m 2 or more. If the number density is more than 500 particles/ ⁇ m 2 , the strengthening effect will be saturated. Prolonged aging treatment after this saturation point would unnecessarily increase the production cost. Therefore, the upper limit is 500 particles/ ⁇ m 2 (ideally 400 particles/ ⁇ m 2 ).
• the number density of carbonitride precipitates is measured using the following method.
• a thin film with a thickness of 100 nm is prepared from the center of the wall thickness or that of the plate thickness of the steel. This thin film is observed using transmission electron microscopy (TEM) and the number of carbonitride precipitates with an equivalent circular diameter of 5 to 100 nm in the visual field (1 ⁇ m ⁇ 1 ⁇ m) is counted.
• TEM transmission electron microscopy
• the number density measurement should be carried out using various visual fields and the average value should be used.
• carbonitride precipitates include V carbonitride precipitates and that when the steel contains Cr, Mo, Ti, Nb, Zr, Ta, Hf, or W, carbonitrides of these elements may be included. There may also be other complex carbonitrides present.
• the SFE should preferably be 40 mJ/m 2 or more (ideally 50 mJ/m 2 or more). The upper limit should be 100 mJ/m 2 . Note that the SFE is calculated in conformity with Non-Patent Document 1.
• the SSC resistance is improved by making the steel have a fine-grained microstructure.
• the effect is particularly large for steel with a microstructure mainly composed of ferrite.
• the microstructure is mainly composed of austenite. Even though the aforementioned effect is less than that for steel mainly composed of ferrite, the SSC resistance is greatly improved when the grain size is 100 ⁇ m or smaller.
• the grain size of the austenite should preferably be 80 ⁇ m or smaller (ideally 60 ⁇ m or smaller). If it is smaller than 1 ⁇ m, grain boundary sliding will occur and the strength will markedly decrease. Therefore, the lower grain size limit is 1 ⁇ m.
• the steel sample is cut to reveal a cross section that is parallel to the rolling and thickness directions. From this cross section, a sample is taken in a manner such that a position at a depth of 1/4 t, where t is the wall or plate thickness, from the surface becomes the observation surface. After the observation surface is etched with a nital etching reagent, observation is performed using optical microscopy at a magnification of 100 to 1000 ⁇ . The grain size of austenite is then determined using the intercept method defined in JIS G 0551 (2013).
• the steel in the present embodiment can achieve both a high yield stress of 862 MPa or more and excellent SSC resistance that exceeds that of prior art. Further, in the proposed steel for oil wells according to the present invention, the SSC resistance is stable even at a yield stress of 965 MPa or more.
• the upper limit of the yield stress is 1275 MPa (preferably 1241 MPa and ideally 1206 MPa).
• the steel in the present embodiment can be produced using the following method. Other methods can also be used.
• General austenitic steel materials can be produced using melting and casting (ingot or continuous casting). To produce seamless pipes, the steel may be cast into the shape of a round billet for pipe-making by round continuous casting.
• hot working e.g., forging, piercing, and rolling
• hot working e.g., forging, piercing, and rolling
• a round billet is cast to produce seamless pipes using round continuous casting
• processes such as forging and blooming for forming a round billet are unnecessary.
• rolling is performed using a mandrel mill or a plug mill.
• the process includes performing finishing rolling after a slab has been rough-rolled. The desirable conditions for hot working (e.g., piercing and rolling) are described below.
• the billet may be heated to an extent such that hot piercing can be performed on a piercing-rolling mill.
• the desirable heating temperature range is 1000 to 1250°C.
• the heating time should preferably be set to 0.5 to 10 h.
• a finishing temperature of 1100°C or lower is desirable.
• the heating temperature of the slab can be set within a temperature range in which hot rolling can be performed (e.g., 1000 to 1250°C).
• the heating time should preferably be set to 0.5 to 10 h.
• the pass schedule of hot rolling can be arbitrarily set, considering hot workability (reducing the occurrence of surface defects, edge cracks, etc.) in the product, it is desirable to set the finishing temperature to 900°C or more.
• the maximum finishing temperature should be 1100°C (as in the case of the seamless pipe).
• the steel After the steel has been subjected to hot working, it is heated to a temperature that is sufficient to completely dissolve carbonitrides and other precipitates and then rapidly cooled. In this case, after being held at 1000 to 1200°C for 10 min or more, the steel is rapidly cooled. If the solution heat treatment temperature is lower than 1000°C, carbonitrides will not be completely dissolved and precipitation strengthening will be insufficient, making it difficult to obtain a yield stress of 852 MPa or more. If the solution heat treatment temperature is more than 1200°C, phases such as ferrite, which easily cause SSC, may be precipitated. Furthermore, if the holding time is less than 10 min, the effect of the solution heat treatment will be insufficient, and the target strength (i.e., a yield stress of 862 MPa or more) may not be obtained.
• the target strength i.e., a yield stress of 862 MPa or more
• the upper limit of the holding time depends on the size and shape of the steel sample and cannot be determined unconditionally. Although it is necessary to have a holding time sufficient to allow the entire steel sample to be soaked, to decrease production cost, an excessive holding time is undesirable. It is usually appropriate to set the holding time to within 1 h. To prevent precipitation of carbonitrides and other intermetallic compounds during cooling, it is desirable to cool the steel at a rate that is equal to or higher than the cooling rate for oil cooling.
• the minimum holding time is that for the case where the steel material is reheated at 1000 to 1200°C after it had cooled once to less than 1000°C after hot working.
• the end temperature of hot working finishing temperature
• the lower limit of the holding time in the present embodiment includes the case where the end temperature of hot working (finishing temperature) is 1000 to 1200°C and supplementary heating is performed at that temperature for around 5 min or more.
• the steel After the steel has undergone the solution heat treatment, it is subjected to an aging treatment to precipitate fine carbonitrides to enhance the strength.
• the effect of the aging treatment depends on the temperature and the holding time at the set temperature. A shorter (longer) holding time is required for a higher (lower) temperature. Therefore, an appropriately temperature and holding time should be selected so that the predetermined target strength is obtained.
• the steel material should be heated to a temperature of 600 to 800°C and held at that temperature for 30 min or more.
• the heating temperature for the aging treatment is lower than 600°C, precipitation of carbonitrides will be insufficient and it will be difficult to obtain a yield stress of 862 MPa or more. If the heating temperature is higher than 800°C, carbonitrides will easily dissolve and it will be difficult for carbonitrides to precipitate, making it difficult to obtain the aforementioned yield stress.
• the holding time for the aging treatment is less than 30 min, the precipitation of carbonitrides will be insufficient and it will be difficult to obtain the aforementioned yield stress.
• the upper limit of the holding time it is usually appropriate to set the upper limit to within 72 h. Holding the steel at the heating temperature after precipitation hardening has saturated wastes energy and thus increases the production cost. After the aging treatment ends, the steel should be allowed to cool.
• the proposed steel for oil wells according to the present embodiment can be used to make oil country tubular goods for use in wet hydrogen sulfide environments.
• the SFE was calculated in the manner described above.
• the austenite grain size was determined using the intercept method.
• the presence or absence of ⁇ martensite was determined by X-ray diffraction measurements, and the volume ratio of ⁇ ' martensite and ferrite was measured using a ferrite meter. This volume ratio was determined to 0.1% or more for Test Nos. 24, 26, and 27, and the presence of ⁇ martensite was confirmed. Neither ⁇ martensite, ⁇ ' martensite, nor ferrite was detected for Test Nos. 1 to 23, 25, and 28 to 32.
• a thin film with a thickness of 100 nm was prepared from the center of the plate thickness of each test material. Each thin film was observed using transmission electron microscopy (TEM). The number of carbonitride precipitates with an equivalent circular diameter of 5 to 100 nm within a visual field of 1 ⁇ m ⁇ 1 ⁇ m was measured. In addition, a round bar tensile test specimen with a parallel portion that had an external diameter of 6 mm and a length of 40 mm was taken from the aforementioned central portion of the plate thickness for each test material, and a tensile test was performed at room temperature (25°C). The yield stress YS (0.2% proof stress) (MPa) was determined.
• the proposed steel has excellent SSC resistance because it has an austenitic structure. It has a high yield stress of 862 MPa or more, which is obtained by precipitation strengthening. Therefore, the proposed steel for oil wells according to the present invention can be used to make oil country tubular goods for use in wet hydrogen sulfide environments.

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• 2020
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