Sulfur-resistant alloy steel for pressure-bearing piece of wellhead equipment and preparation method of sulfur-resistant alloy steel

Abstract

The invention relates to the technical field of alloy steel, in particular to sulfur-resistant alloy steel for a wellhead equipment pressure-bearing part and a preparation method of the sulfur-resistant alloy steel. The sulfur-resistant alloy steel is prepared from the following raw material components in percentage by mass: 0.28 percent to 0.33 percent of C, 0.15 percent to 0.35 percent of Si, 0.60 percent to 0.80 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.010 percent of S, 1.20 percent to 1.39 percent of Cr, 0.025 percent to 0.060 percent of Ni, 0.50 percent to 0.90 percent of Mo, 0.020 percent to 0.050 percent of Nb, 0.030 percent to 0.10 percent of V, 0.010 percent to 0.10 percent of Ti, 0.020 percent to 0.050 percent of Al and the balance of Fe and inevitable impurities. The tensile strength of the prepared sulfur-resistant alloy steel is larger than or equal to 655 MPa, the yield strength is larger than or equal to 586 MPa, and the impact energy at-46 DEG C is larger than or equal to 27 J; the ductility is not less than 17%, and the hardness is not more than 22 HRC.

Description

Technical Field

[0001] This invention relates to the field of alloy steel technology, and in particular to a sulfur-resistant alloy steel for wellhead equipment pressure-bearing components and its preparation method. Background Technology

[0002] Ultra-deep oil and gas exploration and development faces severe challenges from "ultra-high pressure and extremely high temperature" operating conditions, making the demand for ultra-high pressure wellhead equipment with higher pressure levels increasingly urgent. However, the highest strength grade of wellhead equipment materials currently specified in the API Spec 6A standard is only 75 ksi (yield strength ≥ 517 MPa). Considering the lightweight design and manufacturing requirements of ultra-high pressure wellhead equipment, developing forging materials with a strength of 85 ksi steel grade, while also being able to be used in high hydrogen sulfide environments, is an engineering and technical challenge that urgently needs to be solved in the research and development of ultra-high pressure wellhead equipment.

[0003] Currently, the main material used for pressure-bearing components of high-pressure wellhead equipment is low-alloy Cr-Mo steel forgings, which have good resistance to hydrogen sulfide stress corrosion. Typical materials include standard 4130 steel (Cr 0.80% ~ 1.10%, Mo 0.15% ~ 0.25%), with a strength grade of generally 75 ksi, but poor hardenability. For pressure-bearing components of wellhead equipment with a strength grade of 85 ksi, improved 4130 steel (with higher Ni and Mn content added to 4130 steel) or improved F22 steel (Cr 2.00% ~ 2.50%, Mo 0.87% ~ 1.13%, and Ni added) are often used to improve hardenability and increase the strength grade to close to 85 ksi steel grade. However, increased strength also increases the environmental fracture sensitivity limit, and the addition of Ni and Mn elements can lead to a decrease in its resistance to sulfide stress cracking (SSC). Therefore, the sulfur resistance of such materials is mostly specified by hardness (GB / T 20972.2 standard specifies that the Rockwell hardness of carbon steel and alloy steel serving in SSC Zone 3 should be ≤22 HRC), without specifying SSC laboratory evaluation requirements. For forging materials used in wellhead equipment serving in acidic environments, while improving the material strength grade, it is also necessary to ensure excellent SSC resistance, and strictly control the material hardness level to ensure long-term service safety in high sulfur content environments.

[0004] Chinese patent CN 110284062 A discloses a large-diameter round steel with high strength and high toughness. This patent uses a higher carbon content and increases the alloying element V, while reducing the content of precious metals Mo and Ni to ensure hardenability and strength. It uses the micro-alloying element Nb to refine the grain of the rolled material and the final quenched and tempered bar to compensate for the decrease in plasticity and toughness caused by the increase of C and V. However, this material pays attention to improving the strength level, and the excessively high C content can easily lead to a decrease in overall corrosion resistance and sulfur resistance.

[0005] Therefore, in order to improve the service performance of Cr-Mo steel large cross-section components in acidic environments, it is urgent to break through the bottleneck of traditional Cr-Mo steel large forging process control (poor hardenability and uneven performance), and develop pressure-bearing component materials that have both high strength (≥85 ksi) and high sulfur resistance (hardness ≤22HRC, evaluated by SSC test), while avoiding the significant increase in cost caused by the addition of a large amount of alloying elements. Summary of the Invention

[0006] To address the above problems, this invention provides a sulfur-resistant alloy steel for pressure-bearing components of wellhead equipment and its preparation method. The invention provides the following technical solution: This invention provides a sulfur-resistant alloy steel for pressure-bearing components of wellhead equipment, wherein the raw material composition of the sulfur-resistant alloy steel, by mass percentage, comprises: C: 0.28%~0.33%, Si: 0.15%~0.35%, Mn: 0.60%~0.80%, P≤0.015%, S≤0.010%, Cr: 1.20%~1.39%, Ni: 0.025%~0.060%, Mo: 0.50%~0.90%, Nb: 0.020%~0.050%, V: 0.030%~0.10%, Ti: 0.010%~0.10%, Al: 0.020%~0.050%, with the remainder being Fe and unavoidable impurities.

[0007] Furthermore, the raw material components of the sulfur-resistant alloy steel, by mass percentage, include: C: 0.28%~0.33%, Si: 0.15%~0.35%, Mn: 0.60%~0.80%, P≤0.015%, S≤0.010%, Cr: 1.30%~1.39%, Ni: 0.035%~0.055%, Mo: 0.50%~0.90%, Nb: 0.020%~0.050%, V: 0.030%~0.10%, Ti: 0.010%~0.10%, Al: 0.020%~0.050%, with the remainder being Fe and unavoidable impurities.

[0008] Furthermore, the sum of the mass percentages of Nb, V, and Ti is 0.09% to 0.25%.

[0009] Furthermore, the surface and 1 / 4 diameter microstructure of the sulfur-resistant alloy steel consists of a bainitic matrix and dispersed carbide precipitates.

[0010] Furthermore, the sulfur-resistant alloy steel has a tensile strength ≥655MPa, a yield strength ≥586MPa, an impact energy of -46℃ ≥27J, an elongation ≥17%, and a hardness ≤22 HRC.

[0011] A method for preparing sulfur-resistant alloy steel for pressure-bearing components of wellhead equipment is also provided, the method comprising the following steps: The raw material of sulfur-resistant alloy steel for the pressure-bearing components of the wellhead equipment mentioned above is smelted to obtain ingots; The ingot is hot-worked to obtain a blank; The blank is subjected to normalizing, quenching, first tempering and second tempering in sequence to obtain sulfur-resistant alloy steel for pressure bearing parts of wellhead equipment.

[0012] Furthermore, during the normalizing process, the furnace temperature is raised to 630℃~660℃, held for 1.5 h~3 h, then raised to 900℃~950℃, held for 4 h~10 h, and air-cooled to room temperature.

[0013] Furthermore, during the quenching process, the furnace temperature is raised to 630℃~660℃ and held for 1.5 h~3 h, then raised to 890℃~940℃ and held for 4 h~7 h, and then water-cooled to room temperature.

[0014] Furthermore, the first tempering treatment: after holding at 670℃~690℃ for 5 h~10 h, air cool to room temperature.

[0015] Further, a second tempering treatment: hold at 670℃~690℃ for 5 h~10 h, then air cool to room temperature.

[0016] The technical effects and advantages of this invention are as follows: The sulfur-resistant alloy steel for wellhead equipment pressure-bearing components provided by this invention, compared with standard 4130 steel, 4130 improved steel, or F22 improved steel, optimizes the element content of Cr, Mo, and Ni, limits the addition of Ni element, combines Nb, V, and Ti element micro-alloying, and a secondary tempering heat treatment process. It effectively improves the strength of large-section materials while also having excellent anti-SSC performance. It solves the contradictory goal of achieving "high strength, high sulfur resistance, and low cost" for large-size alloy steel pressure-bearing components, and can effectively support the selection and manufacturing of materials for ultra-high pressure wellhead equipment in acidic environments.

[0017] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures pointed out in the description and the drawings. Attached Figure Description

[0018] Figure 1 This is a flowchart illustrating the method for preparing sulfur-resistant alloy steel for wellhead equipment pressure-bearing components provided in this application embodiment; Figure 2 This is a microscopic morphology diagram of the sulfur-resistant alloy steel used for pressure-bearing components of wellhead equipment provided in Embodiment 1 of this application. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] To address the shortcomings of existing technologies, this invention discloses a sulfur-resistant alloy steel for pressure-bearing components of wellhead equipment. The raw material composition of the sulfur-resistant alloy steel, by mass percentage, includes: C (carbon): 0.28%~0.33%, Si (silicon): 0.15%~0.35%, Mn (manganese): 0.60%~0.80%, P (phosphorus): ≤0.015%, S (sulfur): ≤0.010%, Cr (chromium): 1.20%~1.39%, Ni (nickel): 0.025%~0.060%, Mo (molybdenum): 0.50%~0.90%, Nb (niobium): 0.020%~0.050%, V (vanadium): 0.030%~0.10%, Ti (titanium): 0.010%~0.10%, and Al (aluminum): 0.020%~0.050%. % is Fe (iron) and unavoidable impurities.

[0021] The raw material composition of this invention has the following advantages compared with existing CrMo alloy steels (such as 4130 steel, 4130 steel-improved, and F22 steel-improved): (1) The Cr content is precisely controlled within the equilibrium range. Compared with high-Cr steel (e.g., F22 steel-improved type), it effectively avoids the risk of grain boundary Cr depletion and significant cost increase caused by high Cr content while ensuring sufficient hardenability and corrosion resistance. Compared with low-Cr steel (e.g., 4130 steel, 4130 steel-improved type), it improves the hardenability and strength of the matrix. For example, keeping other components unchanged, the Cr content is controlled at 1.30%~1.39% by mass percentage.

[0022] (2) Adding a small amount of Ni element improves the hardenability of large cross-section materials, but the content of Ni is strictly limited, which effectively avoids the risk of deterioration of anti-sulfide stress cracking performance due to excessive Ni content. For example, keeping other components unchanged, the Ni content is controlled at 0.035%~0.055% by mass percentage.

[0023] (3) By jointly adding Nb, V, and Ti, a synergistic microalloying effect is formed. The carbonitrides of Nb, V, and Ti are dispersed and precipitated at different heat treatment stages, strongly pinning the austenite grain boundaries, achieving grain refinement, and providing significant precipitation strengthening, which compensates for the strength loss that may be caused by the low Ni content. For example, keeping other components unchanged, the sum of the mass percentages of the microalloying elements Nb, V, and Ti is 0.09%~0.25%.

[0024] (4) The deoxidation effect of Al element is brought into play, and fine AlN particles are formed, which refines the austenite grains and further improves the impact toughness of the material.

[0025] Table 1. Chemical composition (wt.%) of the steel grade of this invention.

[0026] In one specific embodiment of the present invention, the surface and 1 / 4 diameter microstructure of the sulfur-resistant alloy steel is a bainitic matrix and dispersed carbide precipitates.

[0027] The sulfur-resistant alloy steel has a tensile strength greater than or equal to 655 MPa, a yield strength greater than or equal to 586 MPa, an impact energy at -46℃ greater than or equal to 27 J, an elongation greater than or equal to 17%, and a hardness ≤22 HRC.

[0028] This invention provides an anti-sulfur alloy steel for pressure-bearing components of wellhead equipment. Compared to standard 4130 steel, improved 4130 steel, or improved F22 steel, this alloy steel optimizes the content of major elements such as Cr and Mo, limits the addition of Ni, combines microalloying of Nb, V, and Ti elements, and employs a secondary tempering heat treatment process. This effectively improves the strength of large-section materials while also exhibiting excellent anti-sulfur (SSC) performance, resolving the contradictory goal of achieving "high strength, high sulfur resistance, and low cost" in large-size alloy steel pressure-bearing components. It effectively supports the selection and manufacturing of materials for ultra-high pressure wellhead equipment used in acidic environments.

[0029] This invention also provides a method for preparing sulfur-resistant alloy steel for pressure-bearing components of wellhead equipment, such as... Figure 1 As shown, the method includes the following steps: The raw material of sulfur-resistant alloy steel for pressure-bearing components of wellhead equipment is smelted to obtain ingots; The ingot is hot-worked to obtain a blank; preferably, the blank is a cylindrical QTC specimen with an equivalent circle size of 254 mm or less, whose performance is equivalent to that of the blank. For example, a cylindrical QTC specimen with an equivalent circle size of 254 mm as specified in API Spec 6A is used.

[0030] The blank was subjected to normalizing, quenching, first tempering, and second tempering treatments in sequence to obtain sulfur-resistant alloy steel for pressure-bearing components of wellhead equipment. In one specific embodiment of the present invention, during the normalizing process, the furnace temperature is raised to 630℃~660℃ and held for 1.5 h~3 h, then raised to 900℃~950℃ and held for 4 h~10 h before air cooling to room temperature. During the quenching process, the furnace temperature is raised to 630℃~660℃ and held for 1.5 h~3 h, then raised to 890℃~940℃ and held for 4 h~7 h, followed by water cooling to room temperature. The first tempering process involves holding at 670℃~690℃ for 5 h~10 h, followed by air cooling to room temperature. The second tempering process involves holding at 670℃~690℃ for 5 h~10 h, followed by air cooling to room temperature.

[0031] For example, the composition is designed as Cr-Mo steel, specifically C: 0.28%~0.33%, Si: 0.15%~0.35%, Mn: 0.60%~0.80%, P≤0.015%, S≤0.010%, Cr: 1.20%~1.39%, Ni: 0.025%~0.060%, Mo: 0.50%~0.90%, Nb: 0.020%~0.050%, V: 0.030%~0.10%, Ti: 0.010%~0.10%, Al: 0.020%~0.050%, with the remainder being Fe and unavoidable impurities. (Example) In one specific embodiment of the present invention, a cylindrical QTC specimen with an equivalent circle size of 254 mm or less is used to replace the blank for performance testing. Exemplarily, a cylindrical QTC specimen with an equivalent circle size of 254 mm as specified in API Spec 6A is used. Exemplarily, the QTC specimen with an equivalent circle size of 254 mm is prepared using the steel composition of this technology, and then heated in the furnace to 630℃~660℃, held for 1.5 h~3 h, then further heated to 900℃~950℃, held for 4 h~10 h, and air-cooled; then heated in the furnace to 630℃~660℃, held for 1.5 h~3 h, then further heated to 890℃~940℃, held for 4 h~7 h, using water as the medium; then heated to 670℃~690℃, held for 5 h~10 h, and air-cooled; then heated to 670℃~690℃, held for 5 h~10 h, and air-cooled. The yield strength from the surface to a depth of 1 / 4 of the diameter is higher than 586 MPa, the tensile strength is higher than 655 MPa, the elongation is higher than 17%, the impact energy at -46℃ is higher than 27 J, and the hardness is ≤22 HRC. The material strength meets the technical requirements of steel grade 85 ksi and above. It was evaluated in the full area of ​​SSC according to GB / T 20972.2 standard. No cracking was observed after 720 hours of uniaxial tensile testing with solution A.

[0032] The sulfur-resistant alloy steel used for wellhead equipment pressure bearing components can achieve the strength and low-temperature toughness requirements of 85 ksi steel grade (yield strength ≥586 MPa, tensile strength ≥655 MPa, -46℃ low-temperature impact energy ≥27J) from the surface to 1 / 4 diameter depth, and also has excellent resistance to sulfide stress cracking.

[0033] The method of this invention employs a two-stage tempering process. The first tempering aims to fully precipitate carbides and eliminate internal stress; the second tempering further stabilizes the microstructure, eliminates residual austenite to the maximum extent, and prevents large forgings from becoming brittle due to the transformation of residual austenite during service.

[0034] The method of this invention, through the design of the above-mentioned components and the optimization of the heat treatment process, enables the 254 mm equivalent circular thick section forging to achieve a yield strength ≥85 ksi and uniform performance from the surface to a depth of 1 / 4 of the diameter, with a hardness ≤22 HRC. This meets the requirements of GB / T 20972.2 standard and passes the SSC full-area evaluation of GB / T 20972.2 standard. It successfully solves the industry problem of traditional pressure-bearing materials being unable to simultaneously achieve "high strength, high sulfur resistance (low hardness and passing the SSC evaluation), and low cost". Without significantly increasing the cost of the alloy, it achieves excellent comprehensive performance and market competitiveness.

[0035] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0036] In all embodiments and comparative examples described in the specific implementation, cylindrical QTC specimens with an equivalent circle size of 254 mm or less are used to replace the blanks for performance testing. For example, the cylindrical QTC specimen is a cylindrical QTC specimen with an equivalent circle size of 254 mm as specified in API Spec 6A.

[0037] Example 1 Cylindrical QTC specimens with an equivalent circle size of 254 mm as specified in API Spec 6A were produced through electric furnace smelting, refining, degassing, forging, and machining. The specific dimensions are as follows: diameter 254 mm, height 254 mm. The chemical composition by mass percentage is: C: 0.30%, Si: 0.23%, Mn: 0.64%, P: 0.012%, S: 0.0020%, Cr: 1.37%, Mo: 0.68%, Ni: 0.044%, Nb: 0.024%, V: 0.054%, Ti: 0.016%, Al: 0.025%, with the balance being Fe and unavoidable impurities.

[0038] Then, the cylindrical QTC sample is placed in the heating furnace, and the following steps are performed sequentially: The furnace temperature was raised to 650℃ and held for 3 hours, then raised to 900℃ and held for 4 hours before being air-cooled. The furnace temperature was raised to 650℃ and held for 3 hours, then raised to 890℃ and held for 6 hours. The medium was water. The furnace temperature was raised to 680℃, held for 6 hours, and then air-cooled. The furnace temperature was raised to 680℃, held for 6 hours, and then air-cooled to obtain sulfur-resistant alloy steel for wellhead equipment pressure-bearing components. The microstructure of this sulfur-resistant alloy steel for wellhead equipment pressure-bearing components is as follows: Figure 2 As shown, according to Figure 2 It can be seen that the microstructure of the sulfur-resistant alloy steel surface consists of a bainitic matrix and dispersed carbide precipitates.

[0039] Comparative Example 1 Cylindrical QTC samples with an equivalent circle size of 254 mm as specified in API Spec 6A were prepared using the electric furnace smelting, refining, degassing, forging and machining processes described in Example 1. The chemical composition design was the same as that in Example 1.

[0040] Then, the cylindrical QTC sample is placed in the heating furnace, and the following steps are performed sequentially: The furnace temperature was raised to 630℃ and held for 3 hours, then raised to 900℃ and held for 4 hours before being air-cooled. The furnace temperature was raised to 630℃ and held for 3 hours, then raised to 930℃ and held for 6 hours. The medium was water. The furnace temperature was raised to 660℃, held for 6 hours, and then air-cooled. The temperature was raised to 660℃ in the furnace, held for 6 hours, and then air-cooled to obtain sulfur-resistant alloy steel for pressure-bearing components of wellhead equipment.

[0041] Comparative Example 2 (4130 steel - improved version) Cylindrical QTC specimens with an equivalent circle size of 254 mm as specified in API Spec 6A were produced through electric furnace smelting, refining, degassing, forging, and machining. The specific dimensions are as follows: diameter 254 mm, height 254 mm. The chemical composition by mass percentage is: C: 0.31%, Si: 0.29%, Mn: 0.72%, P: 0.0070%, S: 0.0027%, Cr: 1.03%, Mo: 0.21%, Ni: 0.45%, Nb < 0.0001%, V: 0.012%, Ti: 0.0016%, Al: 0.018%, with the balance being Fe and unavoidable impurities.

[0042] Then, the cylindrical QTC sample is placed in the heating furnace, and the following steps are performed sequentially: Normalizing + Quenching: After heating to 900℃ and holding for 6 hours in the furnace, air cool; then heat to 880℃ and hold for 6 hours in the furnace, followed by water cooling.

[0043] Tempering: Heat the furnace to 675℃ and hold for 10 hours, then air cool to obtain 4130 steel - improved type.

[0044] Comparative Example 3 (F22 steel - improved version) Cylindrical QTC specimens with an equivalent circle size of 254 mm as specified in API Spec 6A were produced through electric furnace smelting, refining, degassing, forging, and machining. The specific dimensions are as follows: diameter 254 mm, height 254 mm. The chemical composition by mass percentage is: C: 0.15%, Si: 0.19%, Mn: 0.57%, P: 0.0076%, S: 0.0020%, Cr: 2.58%, Mo: 1.10%, Ni: 0.24%, Nb: 0.0040%, V: 0.024%, Ti: 0.0015%, Al: 0.031%, Cu: 0.032%, with the balance being Fe and unavoidable impurities.

[0045] Then, the cylindrical QTC sample is placed in the heating furnace, and the following steps are performed sequentially: Normalizing + Quenching: After heating the furnace to 920℃ and holding for 3 hours, air cool. Then, after heating the furnace to 900℃ and holding for 3 hours, water cool.

[0046] Tempering: Heat the furnace to 650℃ and hold for 4 hours, then air cool to obtain F22 steel - improved type.

[0047] Comparative Example 4 (Cr content higher than the range) Cylindrical QTC specimens with an equivalent circle size of 254 mm as specified in API Spec 6A were produced through electric furnace smelting, refining, degassing, forging, and machining. The specific dimensions are as follows: diameter 254 mm, height 254 mm. The chemical composition by mass percentage is: C: 0.31%, Si: 0.26%, Mn: 0.69%, P: 0.012%, S: 0.0015%, Cr: 1.50%, Mo: 0.66%, Ni: 0.043%, Nb: 0.025%, V: 0.050%, Ti: 0.013%, Al: 0.024%, with the balance being Fe and unavoidable impurities.

[0048] Then, the cylindrical QTC sample is placed in the heating furnace, and the following steps are performed sequentially: The furnace temperature was raised to 650℃ and held for 3 hours, then raised to 900℃ and held for 4 hours before being air-cooled. The furnace temperature was raised to 650℃ and held for 3 hours, then raised to 890℃ and held for 6 hours. The medium was water. The furnace temperature was raised to 680℃, held for 6 hours, and then air-cooled. The furnace temperature was raised to 680℃, held for 6 hours, and then air-cooled to obtain sulfur-resistant alloy steel.

[0049] Comparative Example 5 (Cr content below the range) Cylindrical QTC specimens with an equivalent circle size of 254 mm as specified in API Spec 6A were produced through electric furnace smelting, refining, degassing, forging, and machining. The specific dimensions are as follows: diameter 254 mm, height 254 mm. The chemical composition by mass percentage is: C: 0.30%, Si: 0.25%, Mn: 0.70%, P: 0.013%, S: 0.0012%, Cr: 1.15%, Mo: 0.58%, Ni: 0.040%, Nb: 0.021%, V: 0.035%, Ti: 0.020%, Al: 0.020%, with the balance being Fe and unavoidable impurities.

[0050] Then, the cylindrical QTC sample is placed in the heating furnace, and the following steps are performed sequentially: The furnace temperature was raised to 650℃ and held for 3 hours, then raised to 900℃ and held for 4 hours before being air-cooled. The furnace temperature was raised to 650℃ and held for 3 hours, then raised to 890℃ and held for 6 hours. The medium was water. The furnace temperature was raised to 680℃, held for 6 hours, and then air-cooled. The furnace temperature was raised to 680℃, held for 6 hours, and then air-cooled to obtain sulfur-resistant alloy steel.

[0051] Comparative Example 6 (Ni content higher than the range) Cylindrical QTC specimens with an equivalent circle size of 254 mm as specified in API Spec 6A were produced through electric furnace smelting, refining, degassing, forging, and machining. The specific dimensions are as follows: diameter 254 mm, height 254 mm. The chemical composition by mass percentage is: C: 0.33%, Si: 0.24%, Mn: 0.70%, P: 0.013%, S: 0.005%, Cr: 1.35%, Mo: 0.50%, Ni: 0.10%, Nb: 0.030%, V: 0.053%, Ti: 0.015%, Al: 0.022%, with the balance being Fe and unavoidable impurities.

[0052] Then, the cylindrical QTC sample is placed in the heating furnace, and the following steps are performed sequentially: The furnace temperature was raised to 650℃ and held for 3 hours, then raised to 900℃ and held for 4 hours before being air-cooled. The furnace temperature was raised to 650℃ and held for 3 hours, then raised to 890℃ and held for 6 hours. The medium was water. The furnace temperature was raised to 680℃, held for 6 hours, and then air-cooled. The furnace temperature was raised to 680℃, held for 6 hours, and then air-cooled to obtain sulfur-resistant alloy steel.

[0053] Comparative Example 7 (Ni content below the range) Cylindrical QTC specimens with an equivalent circle size of 254 mm as specified in API Spec 6A were produced through electric furnace smelting, refining, degassing, forging, and machining. The specific dimensions are as follows: diameter 254 mm, height 254 mm. The chemical composition by mass percentage is: C: 0.28%, Si: 0.26%, Mn: 0.65%, P: 0.012%, S: 0.004%, Cr: 1.30%, Mo: 0.68%, Ni: 0.010%, Nb: 0.021%, V: 0.043%, Ti: 0.016%, Al: 0.024%, with the balance being Fe and unavoidable impurities.

[0054] Then, the cylindrical QTC sample is placed in the heating furnace, and the following steps are performed sequentially: The furnace temperature was raised to 650℃ and held for 3 hours, then raised to 900℃ and held for 4 hours before being air-cooled. The furnace temperature was raised to 650℃ and held for 3 hours, then raised to 890℃ and held for 6 hours. The medium was water. The furnace temperature was raised to 680℃, held for 6 hours, and then air-cooled. The furnace temperature was raised to 680℃, held for 6 hours, and then air-cooled to obtain sulfur-resistant alloy steel.

[0055] The sulfur resistance test conditions were as follows: samples were taken from the 1 / 4 diameter position, and the SSC resistance test was conducted according to GB / T 20972.2. All three parallel samples showed no cracking after 720 hours, meeting the performance requirements. The test type used was the uniaxial tensile method (Method A) of GB / T 4157-2017. The uniaxial tensile specimen was taken from the surface of the submitted specimen or from the 1 / 4 diameter envelope. A stress of 80% of the tensile strength (AYS) was applied. The test solution was the A solution environment specified in GB / T 4157-2017, with an H2S partial pressure of 0.10 MPa. Solution A was an acidified saturated H2S salt solution composed of 5.0% NaCl, 0.5% CH3COOH, and deionized water.

[0056] The following are the performance test results of the examples and comparative examples. It can be seen that the mechanical properties (yield strength, tensile strength, elongation, hardness, and impact energy at -46℃) of the QTC specimens prepared in the examples of the present invention on the surface and at 1 / 4 of the diameter all meet the technical requirements of 85 ksi steel grade. Moreover, the anti-SSC test was conducted by uniaxial tensile method, solution A, and 80% yield strength loading force. The test procedure meets the requirements of GB / T 4157-2017 standard. None of the three parallel specimens cracked after 720h, which meets the performance index requirements.

[0057] Table 2 Performance test results of the examples and comparative examples

[0058] As shown in Table 2, comparing Example 1 with Comparative Example 1, it can be seen that the temperature for the secondary tempering treatment of the cylindrical QTC sample must be controlled above 670℃. If the temperature is lower than this, the hardness of the sulfur-resistant alloy steel will increase, resulting in the alloy steel failing to meet the hardness requirements of GB / T 20972.2 as a material for the pressure-bearing components of wellhead equipment in SCC 3 area. Long-term service in high-sulfur blocks carries a risk of failure. As shown in Table 2, comparing Example 1 with Comparative Examples 2 and 3, it can be seen that the steel grade of this invention has excellent resistance to hydrogen sulfide stress cracking, significantly superior to existing steel grades 4130 steel and F22 steel. The strength of the steel grade of this technology meets the technical requirements of 85 ksi steel grade, significantly superior to low-Cr sulfur-resistant steel.

[0059] Comparison of Example 1 and Comparative Example 4 shows that, by controlling the Cr content to be between 1.20% and 1.39%, the yield strength and tensile strength of the sulfur-resistant alloy steel prepared by this invention are increased when the Cr content is higher than the range of this invention. However, its hardness increases beyond 22 HRC, failing to meet the requirements of GB / T 20972.2 standard for SSC 3 zone service alloy steel. Comparison of Example 1 and Comparative Example 5 shows that, by controlling the Cr content to be between 1.20% and 1.39%, the yield strength of the sulfur-resistant alloy steel prepared by this invention is reduced to below 586 MPa when the Cr content is lower than the range of this invention, failing to meet the strength requirements of 85 ksi steel grade.

[0060] According to the comparison results of Example 1 and Comparative Example 6, the present invention controls the amount of Ni to be between 0.025% and 0.060%. However, when the Ni content is higher than the range of the present invention, although the yield strength, tensile strength and hardness of the prepared sulfur-resistant alloy steel meet the technical requirements of 85 ksi steel grade, the anti-SSC performance of the prepared sulfur-resistant alloy steel does not meet the technical requirements.

[0061] According to the comparison results between Example 1 and Comparative Example 7, the present invention controls the amount of Ni to be between 0.025% and 0.060%. However, when the Ni content is lower than the range of the present invention, although the sulfur-resistant alloy steel prepared has sulfur resistance, tensile strength and hardness that meet the technical requirements of 85 ksi steel grade, the yield strength of the prepared sulfur-resistant alloy steel is reduced to less than 586 MPa, which does not meet the strength requirements of 85 ksi steel grade.

[0062] Therefore, this application requires simultaneous control of the amounts of Ni, Cr, and Mo, so that the sulfur-resistant alloy steel can not only meet the mechanical properties of 85 ksi grade alloy steel, but also have excellent sulfur resistance, while achieving low hardness control.

[0063] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A sulfur-resistant alloy steel for pressure-bearing components of wellhead equipment, characterized in that, The raw material components of the sulfur-resistant alloy steel, by mass percentage, include: C: 0.28%~0.33%, Si: 0.15%~0.35%, Mn: 0.60%~0.80%, P≤0.015%, S≤0.010%, Cr: 1.20%~1.39%, Ni: 0.025%~0.060%, Mo: 0.50%~0.90%, Nb: 0.020%~0.050%, V: 0.030%~0.10%, Ti: 0.010%~0.10%, Al: 0.020%~0.050%, with the remainder being Fe and unavoidable impurities.

2. The sulfur-resistant alloy steel for wellhead equipment pressure-bearing components according to claim 1, characterized in that, The raw material components of the sulfur-resistant alloy steel, by mass percentage, include: C: 0.28%~0.33%, Si: 0.15%~0.35%, Mn: 0.60%~0.80%, P≤0.015%, S≤0.010%, Cr: 1.30%~1.39%, Ni: 0.035%~0.055%, Mo: 0.50%~0.90%, Nb: 0.020%~0.050%, V: 0.030%~0.10%, Ti: 0.010%~0.10%, Al: 0.020%~0.050%, with the remainder being Fe and unavoidable impurities.

3. The sulfur-resistant alloy steel for wellhead equipment pressure-bearing components according to claim 1, characterized in that, The sum of the mass percentages of Nb, V, and Ti is 0.09% to 0.25%.

4. The sulfur-resistant alloy steel for wellhead equipment pressure-bearing components according to claim 1, characterized in that, The surface and 1 / 4 diameter microstructure of the sulfur-resistant alloy steel consists of a bainitic matrix and dispersed carbide precipitates.

5. The sulfur-resistant alloy steel for wellhead equipment pressure-bearing components according to claim 1, characterized in that, The sulfur-resistant alloy steel has a tensile strength ≥655MPa, a yield strength ≥586MPa, an impact energy of -46℃ ≥27J, an elongation ≥17%, and a hardness ≤22 HRC.

6. A method for preparing sulfur-resistant alloy steel for pressure-bearing components of wellhead equipment, characterized in that, The method includes the following steps: The raw material of the sulfur-resistant alloy steel for the pressure-bearing components of wellhead equipment according to any one of claims 1-5 is smelted to obtain an ingot; The ingot is hot-worked to obtain a blank; The blank is subjected to normalizing, quenching, first tempering and second tempering in sequence to obtain sulfur-resistant alloy steel for pressure bearing parts of wellhead equipment.

7. The method for preparing sulfur-resistant alloy steel for wellhead equipment pressure-bearing components according to claim 6, characterized in that, During the normalizing process, the furnace temperature is raised to 630℃~660℃ and held for 1.5 h~3 h. Then, the temperature is raised to 900℃~950℃ and held for 4 h~10 h before being air-cooled to room temperature.

8. The method for preparing sulfur-resistant alloy steel for wellhead equipment pressure-bearing components according to claim 6, characterized in that, During the quenching process, the furnace temperature is raised to 630℃~660℃ and held for 1.5 h~3 h, then raised to 890℃~940℃ and held for 4 h~7 h, and then cooled to room temperature with water.

9. The method for preparing sulfur-resistant alloy steel for wellhead equipment pressure-bearing components according to claim 6, characterized in that, First tempering treatment: Hold at 670℃~690℃ for 5 h~10 h, then air cool to room temperature.

10. The method for preparing sulfur-resistant alloy steel for wellhead equipment pressure-bearing components according to claim 6, characterized in that, Second tempering treatment: Hold at 670℃~690℃ for 5 h~10 h, then air cool to room temperature.

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