A 90 ksi grade alloy steel for extra-high pressure wellhead equipment and a method of manufacturing the same

Abstract

The application relates to the technical field of alloy steel, in particular to a 90 ksi steel grade alloy steel for extra-high voltage wellhead equipment and a preparation method thereof. The raw material components of the alloy steel include, by mass percentage, C: 0.25-0.35%, Si: 0.15-0.35%, Mn: 0.40-0.90%, P<=0.020%, S<=0.010%, Cr: 1.90-2.50%, Ni<=0.10%, Mo: 1.00-1.50%, V: 0.01-0.10%, Nb: 0.01-0.10%, Al: 0.01-0.10%, B: 30-60 ppm, and the rest is Fe and inevitable impurities. The 90 ksi steel grade alloy steel for extra-high voltage wellhead equipment provided by the application has high strength and good hydrogen sulfide corrosion resistance as a wellhead forging material.

Description

Technical Field

[0001] This invention relates to the field of alloy steel technology, and in particular to a 90 ksi grade alloy steel for ultra-high pressure wellhead equipment and its preparation method. Background Technology

[0002] Finding forging materials that meet the 90ksi steel grade requirement (yield strength ≥ 621 MPa) and can be used in hydrogen sulfide environments is a crucial engineering challenge that must be addressed in the design and manufacture of ultra-high pressure wellhead equipment. US Patent US20080017284A1 discloses a high-strength oil well pipe steel with a strength of 125 ksi (862 MPa) or higher, exhibiting both high strength and excellent sulfur resistance.

[0003] Currently, large forgings used in the base materials of EE-grade and HH-grade wellhead equipment mainly utilize low-alloy Cr-Mo steels with good resistance to hydrogen sulfide stress corrosion, such as 4130 steel. The strength at 1 / 4 diameter of a 254mm equivalent QTC specimen can reach the highest strength grade of 75 ksi (yield strength ≥ 517 MPa) specified by API Spec 6A. If improved F22 steel is used, the strength at 1 / 4 diameter of the QTC specimen can reach 85 ksi (yield strength ≥ 586 MPa). However, when manufacturing large-section forgings, the Cr-Mo alloy steel core is not easily hardened, resulting in insufficient strength and a significant risk of core embrittlement. Therefore, materials with high strength and good hardenability have become one of the candidate materials for large-section components in ultra-high pressure wellhead equipment.

[0004] Therefore, there is an urgent need to develop a new high hardenability 90 ksi grade Cr-Mo alloy steel for forgings of large-size wellhead equipment. Summary of the Invention

[0005] To address the above problems, this invention provides 90 ksi grade alloy steel for ultra-high pressure wellhead equipment and its preparation method. The invention provides the following technical solution: This invention provides a 90 ksi grade alloy steel for ultra-high pressure wellhead equipment. The raw material composition of the alloy steel includes, by mass percentage: C: 0.25%~0.35%, Si: 0.15%~0.35%, Mn: 0.40%~0.90%, P≤0.020%, S≤0.010%, Cr: 1.90%~2.50%, Ni≤0.10%, Mo: 1.00%~1.50%, V: 0.01%~0.10%, Nb: 0.01%~0.10%, Al: 0.01%~0.10%, B: 30 ppm~60 ppm, with the remainder being Fe and unavoidable impurities.

[0006] Furthermore, the microstructure of the surface and core of the alloy steel consists of bainite and dispersed carbides.

[0007] Furthermore, the alloy steel has a tensile strength greater than or equal to 724 MPa, a yield strength of 621–724 MPa, an impact energy of greater than or equal to 27 J at -46°C, and an elongation greater than or equal to 17%.

[0008] A method for preparing 90 ksi grade alloy steel for ultra-high pressure wellhead equipment is also provided, the method comprising the following steps: The raw material of 90 ksi grade alloy steel for the above-mentioned ultra-high pressure wellhead equipment is smelted to obtain ingots; The ingot is hot-worked to obtain a blank; The blank was subjected to normalizing, quenching and two tempering processes in sequence to obtain alloy steel for ultra-high pressure wellhead equipment.

[0009] Furthermore, the raw materials for 90 ksi grade alloy steel used in ultra-high voltage wellhead equipment are smelted using electric furnaces and ladle refining.

[0010] Furthermore, during the normalizing process, the normalizing temperature is 900℃~960℃, and after holding at that temperature for 4-12 hours, it is air-cooled to room temperature.

[0011] Furthermore, during the quenching process, the quenching temperature is 900℃~960℃, and after holding at that temperature for 4-12 hours, it is water-cooled to room temperature.

[0012] Furthermore, the two tempering processes include: after the first tempering process, air cooling to room temperature, followed by a second tempering process.

[0013] Furthermore, the first tempering treatment temperature is 650℃~700℃, and after holding at this temperature for 4-12 hours, it is air-cooled to room temperature.

[0014] Furthermore, the second tempering treatment is carried out at a temperature of 650℃~700℃, and after holding at this temperature for 4-12 hours, it is air-cooled to room temperature.

[0015] The technical effects and advantages of this invention are as follows: The 90 ksi grade alloy steel for ultra-high pressure wellhead equipment provided by this invention is a high hardenability Cr-Mo steel used for large-section forgings. Compared with traditional 4130 steel, the Cr and Mo content is appropriately increased, and appropriate amounts of V, Nb, Al, and B elements are added to significantly improve hardenability. Using a 254mm standard QTC sample, after heat treatment, the entire cross section can achieve the mechanical properties of 90 ksi grade steel, while also having good resistance to sulfide stress cracking.

[0016] Compared with existing technologies, this invention uses a combination of Cr, Mo, V, Nb, Al, and B in its composition design, all of which can improve the hardenability of the material. This makes the steel grade of this invention more suitable for manufacturing wellhead equipment forgings with large cross-sectional dimensions. Through normalizing pretreatment + quenching + two tempering heat treatments, a uniform single-phase bainitic structure can be obtained at a cooling rate of not less than 5℃ / min. After tempering, a bainitic (ferrite) matrix + carbide structure is formed. This structure not only meets the strength and toughness requirements of 90 ksi steel grade, but also has good resistance to sulfide stress corrosion. It is suitable for manufacturing large cross-sectional forgings for wellhead equipment of EE grade and HH grade.

[0017] The 90 ksi grade alloy steel used in the UHV wellhead equipment provided by this invention is not only high in strength, but also has good resistance to hydrogen sulfide corrosion: (1) After two high-temperature temperings, the matrix of Cr-Mo steel is bainitic (ferrite) with finely dispersed second-phase carbides, which can capture hydrogen atoms and prevent local stress concentration, thus having good resistance to sulfide stress cracking. (2) In sections exceeding the hardenability limit, low-alloy Cr-Mo steel forms a coarse lamellar isothermal transformation structure or precipitates a coarse second phase along the grain boundaries, which significantly reduces the impact toughness or resistance to sulfide stress cracking. Therefore, this application performs microalloying modification on the basis of Cr-Mo to improve both strength and hardenability.

[0018] 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

[0019] Figure 1 This is a flowchart of the method for preparing alloy steel for ultra-high pressure wellhead equipment provided in the embodiments of this application; Figure 2a This is a scanning electron microscope image of the outer diameter of the alloy steel QTC used in ultra-high voltage wellhead equipment provided in Embodiment 1 of this application; Figure 2b This is a scanning electron microscope image of the alloy steel QTC for ultra-high pressure wellhead equipment at 1 / 2 radius provided in Embodiment 1 of this application; Figure 2c This is a scanning electron microscope image of the core of the alloy steel QTC for ultra-high pressure wellhead equipment provided in Embodiment 1 of this application; Figure 3a This is a scanning electron microscope image of the outer circle of the alloy steel QTC used for ultra-high voltage wellhead equipment provided in Embodiment 2 of this application; Figure 3b This is a scanning electron microscope image of the alloy steel QTC for ultra-high pressure wellhead equipment at 1 / 2 radius provided in Embodiment 2 of this application; Figure 3c This is a scanning electron microscope image of the core of the alloy steel QTC for ultra-high pressure wellhead equipment provided in Embodiment 2 of this application; Figure 4a This is a scanning electron microscope image of the outer diameter of the alloy steel QTC used for ultra-high voltage wellhead equipment provided in Embodiment 3 of this application; Figure 4b This is a scanning electron microscope image of the alloy steel QTC for ultra-high pressure wellhead equipment at 1 / 2 radius provided in Embodiment 3 of this application; Figure 4c This is a scanning electron microscope image of the core of the alloy steel QTC for ultra-high pressure wellhead equipment provided in Embodiment 3 of this application; Figure 5a This is a scanning electron microscope image of the outer diameter of the alloy steel QTC used in ultra-high pressure wellhead equipment provided in Comparative Example 1 of this application; Figure 5b This is a scanning electron microscope image of the alloy steel QTC for ultra-high pressure wellhead equipment at 1 / 2 radius provided in Comparative Example 1 of this application; Figure 5c This is a scanning electron microscope image of the core of the alloy steel QTC for ultra-high pressure wellhead equipment provided in Comparative Example 1 of this application; Figure 6a This is a high-magnification scanning electron microscope image of the outer diameter of the alloy steel QTC used in ultra-high voltage wellhead equipment provided in Embodiment 1 of this application; Figure 6b This is a high-magnification scanning electron microscope image of the alloy steel QTC for ultra-high pressure wellhead equipment at 1 / 2 radius provided in Embodiment 1 of this application; Figure 6c This is a high-magnification scanning electron microscope image of the core of the alloy steel QTC for ultra-high pressure wellhead equipment provided in Embodiment 1 of this application. Detailed Implementation

[0020] 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.

[0021] To address the shortcomings of existing technologies, this invention discloses a 90 ksi grade alloy steel for ultra-high pressure wellhead equipment. The raw material composition of the alloy steel, by mass percentage, includes: C (carbon): 0.25%~0.35%, Si (silicon): 0.15%~0.35%, Mn (manganese): 0.40%~0.90%, P (phosphorus): ≤0.020%, S (sulfur): ≤0.010%, Cr (chromium): 1.90%~2.50%, Ni (nickel): ≤0.10%, Mo (molybdenum): 1.00%~1.50%, V (vanadium): 0.01%~0.10%, Nb (niobium): 0.01%~0.10%, Al (aluminum): 0.01%~0.10%, B (boron): 30 ppm~60 ppm, with the remainder being Fe (iron) and unavoidable impurities.

[0022] The technical solution of this invention controls the Cr (chromium) content to 1.90%~2.50%, the Mo (molybdenum) content to 1.00%~1.50%, adds 0.01%~0.10% V (vanadium), 0.01%~0.10% Nb (niobium), and 0.01%~0.10% Al (aluminum) for microalloying reinforcement, and adds 30 ppm~60 ppm B (boron) to improve hardenability, while reducing the accumulation of hydrogen at grain boundaries and improving the resistance to sulfide stress cracking.

[0023] Compared with the raw material composition of traditional 4130 steel, the raw material composition of the steel grade of this invention shows that: (1) The raw material composition of the steel grade of the present invention has a high Cr and Mo content: the Cr content is 1.90%~2.50%, which can improve the resistance to uniform corrosion in acidic atmosphere and improve the flow hardenability, thus improving both corrosion resistance and strength; the Mo content is increased to 1.00%~1.50%, which can improve the resistance to pitting corrosion and hardenability, and also reduce the temper brittleness of low alloy Cr-Mo steel, and improve the margin of strength and impact toughness.

[0024] (2) The steel raw material composition of this invention uses V, Nb, and Al composite microalloying reinforcement: 0.01%~0.10% V, 0.01%~0.10% Nb, and 0.01%~0.10% Al are added. Among them, the combined effect of V and Nb can enhance the formation of nanoscale second-phase precipitation during high-temperature tempering, pinning dislocations, balancing the decrease in yield strength caused by the recovery softening of the matrix structure, enabling the material to meet the strength requirements of 90 ksi steel grade and maintain a high yield strength. The nanoscale precipitated phase can also act as a "hydrogen trap," improving the resistance to sulfide stress cracking. An appropriate amount of Al can refine the grains and improve the strength and toughness of the material. Excessive Al content will significantly reduce processing performance and plasticity, so the upper limit is no more than 0.10%.

[0025] (3) The steel raw material composition of the present invention uses B element microalloying: adding 30 ppm~60 ppm of B element can improve hardenability and ensure the core hardenability of large cross section forgings. After adding B element, hardenability can be improved by not significantly increasing C content, avoiding banded structure defects caused by excessive C content. B element can also play a beneficial role in improving the resistance to sulfide stress cracking.

[0026] In one specific embodiment of the present invention, the microstructure of the surface and core of the alloy steel is bainite and dispersed carbide precipitates. The alloy steel achieves the strength and low-temperature toughness requirements of 90 ksi steel grade: tensile strength greater than or equal to 724 MPa, yield strength of 621–724 MPa, impact energy at -46°C greater than or equal to 27 J; elongation greater than or equal to 17%, and also exhibits good resistance to sulfide stress cracking.

[0027] This invention also provides a method for preparing 90 ksi grade alloy steel for ultra-high pressure wellhead equipment, such as... Figure 1 As shown, the method includes the following steps: Step 1: Smelt the raw materials to obtain ingots; the raw materials include, by mass percentage: C: 0.25%~0.35%, Si: 0.15%~0.35%, Mn: 0.40%~0.90%, P≤0.020%, S≤0.010%, Cr: 1.90%~2.50%, Ni≤0.10%, Mo: 1.00%~1.50%, V: 0.01%~0.10%, Nb: 0.01%~0.10%, Al: 0.01%~0.10%, B: 30 ppm~60 ppm, with the remainder being Fe and unavoidable impurities.

[0028] Step 2: Hot-dry the ingot to obtain a blank; preferably, the blank is a cylindrical QTC specimen with an equivalent circular size of less than or equal to 254 mm, which has performance equivalent to the blank. For example, a cylindrical QTC specimen with an equivalent circular size of 254 mm as specified in API Spec 6A is used as the blank.

[0029] Step 3: The blank is subjected to normalizing, quenching, and two tempering treatments in sequence to obtain alloy steel for ultra-high pressure wellhead equipment. Among them, During the normalizing process, the normalizing temperature is 900℃~960℃, and after holding at this temperature for 4-12 hours, it is air-cooled to room temperature.

[0030] During the quenching process, the quenching temperature is 900℃~960℃, and after holding at that temperature for 4-12 hours, it is cooled to room temperature by water.

[0031] The two tempering processes consist of: a first tempering treatment followed by air cooling to room temperature, and then a second tempering treatment. The first tempering treatment is conducted at 650℃~700℃, held for 4-12 hours, and then air-cooled to room temperature. The second tempering treatment is also conducted at 650℃~700℃, held for 4-12 hours, and then air-cooled to room temperature. For example, the temperature range for both tempering processes is the same, 50℃~700℃; the actual temperatures may differ, but should remain within this range.

[0032] The alloy steel prepared by the above method has good hardenability and is suitable for manufacturing large cross-section forgings. The test material is processed by cylindrical QTC specimens with an equivalent circle size of 254 mm as specified in API Spec 6A standard as the blank. After heat treatment, the QTC specimens can obtain a single bainitic matrix structure from the core to the surface, and the mechanical properties can meet the requirements of 90 ksi steel grade.

[0033] The alloy steel prepared by the above method exhibits good resistance to sulfide stress cracking. The specimens were processed using cylindrical QTC samples with an equivalent circle size of 254 mm as specified in API Spec 6A. After heat treatment, samples were taken at 1 / 4 of the specimen diameter and subjected to full-area SSC evaluation according to GB / T 20972.2 standard. No cracking was observed after 720 h of testing with solution A using the four-point bending method. The test procedure meets the requirements of GB / T 4157-2017 standard. Solution A is an acidified saturated H2S salt solution containing 5.0% NaCl, 0.5% CH3COOH, and deionized water.

[0034] For example, using the above-mentioned steel raw material composition, a cylindrical QTC sample with an equivalent circle size of 254 mm was used as a blank. After normalizing at 900~960℃, quenching at 900~960℃ and tempering at 650~700℃, the yield strength of the QTC sample on the surface and in the core was higher than 640MPa, the tensile strength was higher than 800MPa, the elongation was higher than 20%, and the impact energy at -46℃ was higher than 45J. The entire cross section met the technical requirements of 90 ksi steel grade.

[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 In this embodiment, the raw material composition of the alloy steel includes, by mass percentage: C: 0.29%, Si: 0.22%, Mn: 0.74%, P<0.005%, S: 0.0040%, Cr: 2.04%, Ni: 0.01%; Mo: 1.24%, V: 0.087%, Nb: 0.041%, Al: 0.02%, B: 30 ppm.

[0038] The preparation process of alloy steel is as follows: Step 1: The above raw materials are smelted using a non-vacuum furnace and ladle refining to obtain ingots; Step 2: Perform hot working on the ingot to obtain a blank with an equivalent circle size of 254 mm as specified in API Spec 6A.

[0039] Step 3: The blank sample was subjected to normalizing, quenching, and two tempering treatments sequentially. During normalizing, the temperature was 950℃, held for 5 hours, and then air-cooled to room temperature. During quenching, the temperature was 930℃, held for 4 hours, and then rapidly transferred to circulating water for cooling. The two tempering treatments included: a first tempering at 660℃, held for 6 hours, and then air-cooled to room temperature; and a second tempering at 660℃, held for 4 hours, and then air-cooled to room temperature, yielding the alloy steel for UHV wellhead equipment. The microstructure of this alloy steel for UHV wellhead equipment is as follows... Figures 2a-2c As shown in the figure, the full cross-section of the alloy steel QTC used for UHV wellhead equipment is bainitic with carbides, indicating that it is fully hardened.

[0040] Example 2 In this embodiment, the raw material composition of the alloy steel includes, by mass percentage: C: 0.32%, Si: 0.22%, Mn: 0.70%, P<0.005%, S: 0.0043%, Cr: 2.29%, Ni: 0.01%; Mo: 1.25%, V: 0.086%, Nb: 0.038%, Al: 0.03%, B: 37 ppm.

[0041] The preparation process of alloy steel is as follows: Step 1: The above raw materials are smelted using a non-vacuum furnace and ladle refining to obtain ingots; Step 2: Perform hot working on the ingot to obtain a blank with an equivalent circle size of 254 mm as specified in API Spec 6A.

[0042] Step 3: The blank sample was subjected to normalizing, quenching, and two tempering treatments sequentially. During normalizing, the temperature was 950℃, held for 5 hours, and then air-cooled to room temperature. During quenching, the temperature was 930℃, held for 4 hours, and then rapidly transferred to circulating water for cooling. The two tempering treatments included: a first tempering at 660℃, held for 6 hours, and then air-cooled to room temperature; and a second tempering at 660℃, held for 4 hours, and then air-cooled to room temperature, yielding the alloy steel for UHV wellhead equipment. The microstructure of this alloy steel for UHV wellhead equipment is as follows... Figures 3a-3c As shown in the figure, the full cross-section of the alloy steel QTC used for UHV wellhead equipment is bainitic with carbides, indicating that it is fully hardened.

[0043] Example 3 In this embodiment, the raw material composition of the alloy steel includes, by mass percentage: C: 0.29%, Si: 0.21%, Mn: 0.73%, P<0.005%, S: 0.0039%, Cr: 2.48%, Ni: 0.01%; Mo: 1.25%, V: 0.085%, Nb: 0.04%, Al: 0.02%, B: 41 ppm.

[0044] The preparation process of alloy steel is as follows: Step 1: The above raw materials are smelted using a non-vacuum furnace and ladle refining to obtain ingots; Step 2: Perform hot working on the ingot to obtain a blank with an equivalent circle size of 254 mm as specified in API Spec 6A.

[0045] Step 3: The blank sample is subjected to normalizing, quenching, and two tempering treatments in sequence to obtain alloy steel for UHV wellhead equipment. During the normalizing process, the normalizing temperature is 950℃, and after holding at that temperature for 5 hours, it is air-cooled to room temperature.

[0046] During the quenching process, the quenching temperature is 930℃, and after holding at that temperature for 4 hours, it is quickly transferred to circulating water for water cooling.

[0047] The two tempering treatments included: the first tempering treatment at 660℃, held for 6 hours, and then air-cooled to room temperature; the second tempering treatment at 660℃, held for 4 hours, and then air-cooled to room temperature. The microstructure of the alloy steel used in this ultra-high pressure wellhead equipment is as follows: Figures 4a-4c As shown in the figure, the full cross-section of the alloy steel QTC used for UHV wellhead equipment is bainitic with carbides, indicating that it is fully hardened.

[0048] Comparative Example 1 The material is smelted using an electric furnace and ladle refining process, and then machined into a blank with an equivalent circular size of 254 mm as specified in API Spec 6A. The specific dimensions are as follows: diameter 254 mm, height 254 mm. Its chemical composition by mass percentage is: C: 0.29%, Si: 0.27%, Mn: 0.60%, P: 0.0086%, S: 0.0020%, Cr: 1.13%, Mo: 0.23%, Ni: 0.23%, Nb: 0.0012%, V: 0.0069%, Ti: 0.0057%, Al: 0.023%, with the balance being Fe and unavoidable impurities.

[0049] Then, the blank is placed in the heating furnace, and the following steps are performed: Normalizing + Quenching: After heating to 900℃ in the furnace and holding for 4.5 h, air cool; then heat to 870℃ in the furnace and hold for 4.5 h, followed by water cooling.

[0050] Tempering: The furnace temperature is raised to 660℃ and held for 7.5 hours, then air-cooled to obtain the improved -4130 steel.

[0051] Comparative Example 2 (F22 steel - improved version) The material is smelted using an electric furnace and ladle refining process, and then machined into a blank with an equivalent circular size of 254 mm as specified in API Spec 6A. The specific dimensions are as follows: diameter 254 mm, height 254 mm. Its chemical composition by mass percentage is: C: 0.15%, Si: 0.17%, Mn: 0.54%, P: 0.0055%, S: 0.0007%, Cr: 2.36%, Mo: 1.03%, Ni: 0.26%, Nb: 0.0036%, V: 0.023%, Al: 0.032%, Cu: 0.080%, with the balance being Fe and unavoidable impurities.

[0052] Then, the blank is placed in the heating furnace, and the following steps are performed: Normalizing + Quenching: After heating to 950℃ and holding for 5 hours, air cool; then heat to 940℃ and hold for 5 hours, followed by water cooling.

[0053] Tempering: Heat the furnace to 650℃ and hold for 10 hours, then air cool to obtain F22 steel - improved type.

[0054] Comparative Example 3 The raw material composition of the steel in this comparative example includes, by mass percentage: C: 0.26%, Si: 0.16%, Mn: 0.69%, P<0.005%, S: 0.0041%, Cr: 1.72%, Ni: 0.01%; Mo: 0.90%, V: 0.077%, Nb: 0.03%.

[0055] The preparation process is as follows: Step 1: The above raw materials are smelted using a non-vacuum furnace and ladle refining to obtain ingots; Step 2: Perform hot working on the ingot to obtain a blank with an equivalent circle size of 254 mm as specified in API Spec 6A.

[0056] Step 3: The blank sample was subjected to normalizing, quenching, and two tempering treatments sequentially. During normalizing, the temperature was 950℃, held for 5 hours, and then air-cooled to room temperature. During quenching, the temperature was 930℃, held for 4 hours, and then rapidly transferred to circulating water for cooling. The two tempering treatments included: a first tempering at 660℃, held for 6 hours, and then air-cooled to room temperature; and a second tempering at 660℃, held for 4 hours, and then air-cooled to room temperature. This yielded the alloy steel for UHV wellhead equipment. The microstructure of this alloy steel for UHV wellhead equipment is shown below. Figures 5a-5c As shown in the figure, the surface microstructure of the QTC alloy steel is bainitic with carbides. α-ferrite is present in 1 / 4 of the diameter and the core, indicating that an isothermal transformation has occurred, suggesting that it was not fully hardened.

[0057] The alloy steels prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to mechanical property tests. The performance comparison of the steels in Examples 1-3 and Comparative Examples 1-3 is shown in Table 1.

[0058] Table 1

[0059] As shown in Table 1, the mechanical properties of the surface area, quarter diameter, and core of the alloy steel QTC samples prepared in Examples 1-3 all meet the mechanical property requirements of 90 ksi steel grade. However, the strength at the quarter diameter position of the steels prepared in Comparative Examples 1 and 2 is lower than the yield strength requirement specified for 90 ksi steel grade. The core of the alloy steel prepared in Comparative Example 3 is lower than the yield strength and low-temperature impact performance requirements specified for 90 ksi steel grade.

[0060] In addition, the sulfur resistance of the alloy steels prepared in Examples 1-3 was tested. The test conditions were as follows: samples were taken from a 1 / 4 diameter position, and the SSC resistance test was conducted according to GB / T 20972.2. The test type was the four-point bending method (FPB), and the test solution was solution A specified in NACE™ 0177 (acidified saturated H2S salt solution, with an applied stress of 80% of the yield strength, the solution consisting of 5.0% NaCl / 0.5% CH3COOH and deionized water), the H2S partial pressure was 0.10 MPa, and the test procedure met the requirements of GB / T 4157-2017. After 720 hours of SSC testing, no cracks were observed in the four-point bending samples of the alloy steels prepared in Examples 1-3. The test results show that the alloy steel of the present invention has good resistance to sulfide stress cracking.

[0061] Figures 6a-6c The image shows the surface region → 1 / 4 diameter → core microstructure of the alloy steel prepared in Example 1. According to... Figures 6a-6c As shown, the entire cross-section maintains the bainitic matrix structure, and the grain boundary carbides in the core region are not coarse. It maintains the microstructure characteristics of bainitic + dispersed carbide precipitates, ensuring that the strength, toughness and sulfur resistance meet the requirements.

[0062] Based on the performance tests of the above embodiments and comparative examples, it can be seen that the full cross-section of the alloy steel QTC large-section sample prepared in this application meets the 90 ksi performance requirement. However, existing improved 4130 steel, F22 steel-improved steel, and the CrMo steel of Comparative Example 3 cannot achieve the 90 ksi performance across the entire cross-section. Simultaneously, the steel grade of this invention, while improving strength, also exhibits better resistance to sulfide stress cracking. According to the comparison results of the alloy steels prepared in Example 1 and Comparative Example 3, the amount of B in this invention is controlled between 30 ppm and 60 ppm. The alloy steel prepared in Comparative Example 3 did not contain added B, resulting in a decrease in the core impact resistance of the alloy steel prepared in Comparative Example 3, below 27 (KV2) / J, and the strength did not meet the 90 ksi requirement; the QTC sample was not fully hardened. Therefore, adding B to alloy steel has the benefits of improving hardenability and increasing strength and toughness.

[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 90 ksi grade alloy steel for ultra-high pressure wellhead equipment, characterized in that, The raw material composition of the alloy steel includes, by mass percentage: C: 0.25%~0.35%, Si: 0.15%~0.35%, Mn: 0.40%~0.90%, P≤0.020%, S≤0.010%, Cr: 1.90%~2.50%, Ni≤0.10%, Mo: 1.00%~1.50%, V: 0.01%~0.10%, Nb: 0.01%~0.10%, Al: 0.01%~0.10%, B: 30 ppm~60 ppm, with the remainder being Fe and unavoidable impurities.

2. The 90 ksi grade alloy steel for ultra-high pressure wellhead equipment according to claim 1, characterized in that, The microstructure of the surface and core of the alloy steel consists of bainite and dispersed carbides.

3. The 90 ksi grade alloy steel for ultra-high pressure wellhead equipment according to claim 1, characterized in that, The alloy steel has a tensile strength greater than or equal to 724 MPa, a yield strength of 621–724 MPa, an impact energy of greater than or equal to 27 J at -46℃, and an elongation greater than or equal to 17%.

4. A method for preparing 90 ksi grade alloy steel for ultra-high pressure wellhead equipment, characterized in that, The method includes the following steps: The raw material of the 90 ksi grade alloy steel for the ultra-high pressure wellhead equipment according to any one of claims 1-3 is smelted to obtain an ingot; The ingot is hot-worked to obtain a blank; The blank was subjected to normalizing, quenching and two tempering processes in sequence to obtain alloy steel for ultra-high pressure wellhead equipment.

5. The method for preparing 90 ksi grade alloy steel for ultra-high pressure wellhead equipment according to claim 4, characterized in that, The raw materials of 90 ksi grade alloy steel for ultra-high voltage wellhead equipment are smelted using electric furnace and ladle refining.

6. The method for preparing 90 ksi grade alloy steel for ultra-high pressure wellhead equipment according to claim 4, characterized in that, During the normalizing process, the normalizing temperature is 900℃~960℃, and after holding at this temperature for 4-12 hours, it is air-cooled to room temperature.

7. The method for preparing 90 ksi grade alloy steel for ultra-high pressure wellhead equipment according to claim 4, characterized in that, During the quenching process, the quenching temperature is 900℃~960℃, and after holding at that temperature for 4-12 hours, it is cooled to room temperature by water.

8. The method for preparing 90 ksi grade alloy steel for ultra-high pressure wellhead equipment according to claim 4, characterized in that, The two tempering processes consist of: after the first tempering process, air cooling to room temperature, followed by a second tempering process.

9. The method for preparing 90 ksi grade alloy steel for ultra-high pressure wellhead equipment according to claim 8, characterized in that, The first tempering treatment is performed at a temperature of 650℃~700℃, and the temperature is maintained for 4-12 hours before air cooling to room temperature.

10. The method for preparing 90 ksi grade alloy steel for ultra-high pressure wellhead equipment according to claim 8, characterized in that, The second tempering treatment is carried out at a temperature of 650℃~700℃, and after holding at this temperature for 4-12 hours, it is air-cooled to room temperature.

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