Steel for planetary screw used in oil and gas exploitation and manufacturing method therefor

Abstract

The present invention relates to a steel for a planetary screw used in oil and gas exploitation, comprising the following chemical components in mass percentage: C: 0.25-0.50%; Si: 1.50-2.00%; Mn: 1.20-1.60%; Cr: 0.80-1.20%; S≤0.002%; P≤0.025%; Nb: 0.02-0.06%; W: 0.50-1.30%; Al≤0.05%; Ca: 0.002-0.006%; Ti≤0.003%; O≤0.0010%; As≤0.04%; Sn≤0.03%; Sb≤0.005%; Pb≤0.002%; and the balance being Fe and inevitable impurities. Requirements for a steel pipe in a delivery state regarding high-temperature tensile performance at 350°C and low-temperature impact performance at -20°C are: yield strength≥1350 MPa, tensile strength≥1580 MPa, elongation≥5%, and Charpy impact energy AKU2 at -20°C≥27 J. The end hardenability is tested by using the JIS G0561 method, and the hardness of J9 mm is required to be greater than or equal to 58 HRC.

Description

A steel for planetary screws used in oil and gas extraction and its manufacturing method Technical Field

[0001] This invention belongs to the field of special steel smelting technology, specifically relating to a steel for planetary screws used in oil and gas extraction and its manufacturing method. Background Technology

[0002] With the development of the oil and gas extraction industry, traditional "nodding donkeys" are being replaced by planetary screws. Because the service environment often involves high temperatures of 200-350℃, the materials themselves must first be heat-resistant. Secondly, oil and gas extraction requires downhole components to have resistance to hydrogen sulfide corrosion. This necessitates steel with higher strength, toughness, wear resistance, resistance to hydrogen sulfide corrosion, and resistance to heat treatment deformation.

[0003] Traditional ball screws use high-carbon chromium bearing steels such as GCr15 and GCr15SiMn. These materials lack sufficient resistance to tempering softening at high temperatures, and heat treatment deformation of high-carbon bearing steel is difficult to control, especially when manufacturing hollow, thin-walled parts like the outer nut of a planetary ball screw. This deformation problem is a major factor leading to substandard grinding precision in the final planetary ball screw. Furthermore, high-carbon chromium bearing steel has a very high susceptibility to hydrogen embrittlement, making it unsuitable for use in the oil and gas extraction industry. Technical issues

[0004] The technical problem to be solved by the present invention is to provide a steel for planetary screws used in oil and gas extraction and a method for manufacturing the same, so that the processed planetary screws can maintain ultra-high hardness, strength, wear resistance and resistance to hydrogen sulfide corrosion under high temperature service conditions, while also having good dimensional stability during processing and use, thus ensuring the precision and lifespan of the final screw service process. Technical solutions

[0005] The requirements for non-metallic inclusions in the steel of this invention are shown in Table 1 below:

[0006]

[0007] The mechanical properties of this invention are tested under the delivery condition. The requirements for tensile properties at 350℃ and impact properties at -20℃ are shown in Table 2 below:

[0008]

[0009] The end hardenability is tested using the JIS G 0561 method, and the hardness of J9mm is required to be ≥58HRC.

[0010] The technical solution adopted by the present invention to solve the above problems is as follows: a steel for planetary screws used in oil and gas extraction, wherein the chemical composition of the steel, by mass percentage, is C: 0.25-0.50%, Si: 1.50-2.00%, Mn: 1.20-1.60%, Cr: 0.80-1.20%, S: ≤0.002%, P≤0.025%, Nb: 0.02-0.06%, W: 0.50-1.30%, Al≤0.05%, Ca: 0.002-0.006%, Ti≤0.003%, O≤0.0010%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, with the balance being Fe and unavoidable impurities.

[0011] The chemical composition of the steel used in the planetary lead screw of this invention is designed based on the following:

[0012] 1) Determination of C content

[0013] Carbon (C) is an essential element for ensuring wear resistance. In steel, carbon increases hardness and strength by enhancing martensitic transformation, thereby improving wear resistance. However, a C content exceeding 0.50% increases the susceptibility of pipes to penetration and heat treatment cracking. This invention controls its content to 0.25–0.50%.

[0014] 2) Determination of Si content

[0015] Si is a deoxidizer in the steelmaking process, and it improves the hardness, strength, elastic limit, and yield strength ratio of steel through solid solution strengthening. It reduces the diffusion rate of carbon in ferrite, making it less likely for carbides precipitated during tempering to aggregate, thus improving the steel's resistance to tempering softening. In addition, Si reduces oxidation during frictional heating and increases the cold work hardening rate of steel, thereby improving the material's wear resistance. However, excessively high Si content will reduce the toughness of the steel. This invention controls the Si content to be 1.50–2.00%.

[0016] 3) Determination of Mn content

[0017] Mn, as a deoxidizing element in the steelmaking process, is an effective strengthening element for steel, playing a solid solution strengthening role to compensate for the strength loss caused by the reduction of carbon content in the steel. Furthermore, Mn can improve the hardenability and hot working properties of steel. Mn can eliminate the influence of sulfur: in steelmaking, Mn can combine with sulfur to form high-melting-point MnS, thereby weakening and eliminating the adverse effects of sulfur. An Mn content higher than 1.60% will significantly reduce the toughness of the steel. In this invention, the Mn content is controlled between 1.20% and 1.60%.

[0018] 4) Determination of Cr content

[0019] Cr is a carbide-forming element that improves the hardenability, wear resistance, and corrosion resistance of steel. In steel, some Cr replaces iron to form alloyed cementite, improving the tempering stability of the steel; others dissolve into ferrite, resulting in solid solution strengthening and increasing the strength and hardness of the ferrite. However, excessively high Cr content can lead to the formation of large carbides by combining with carbon in the steel. These large carbides reduce the contact fatigue life of the steel. Based on the above analysis, the Cr content range in this invention is determined to be 0.80-1.20%.

[0020] 5) Determination of Al content

[0021] Al is a deoxidizer used in the smelting process. Besides reducing dissolved oxygen in molten steel, Al combines with N to form finely dispersed aluminum nitride inclusions, which can refine the grain size. However, when the Al content exceeds 0.05%, the fluidity of the molten steel decreases significantly, increasing casting difficulty. In this invention, the Al content is defined as ≤0.05%.

[0022] 6) Determination of Nb content

[0023] Nitrogen (Nb) is both a strong ferrite-forming element and a strong carbonitride-forming element. Under prolonged heating, it readily forms metallic compounds. Besides its inherent solid solution strengthening, NbC and Nb(CN) exhibit grain boundary movement and grain refinement during hot working through the "pinning" principle of precipitated material points. To control costs and achieve the desired results, the Nb content in this invention is determined to be in the range of 0.02% to 0.06%.

[0024] 7) Determination of W content

[0025] Tungsten (W) improves the tempering resistance of steel, and its carbides are very hard, thus improving the wear resistance of the steel and giving it a certain degree of hot hardness. It also enhances the steel's creep resistance at high temperatures. During quenching, carbides such as (FeW6)C are difficult to dissolve, thus refining the grain size. During tempering at 500–600°C, tungsten carbides (W2C) mainly precipitate, dispersed in the martensitic matrix, providing secondary hardening. However, due to the high melting point of W, and to minimize stress concentration caused by tungsten inclusions while ensuring high-temperature strength, and considering smelting cost control, the W content in this invention is determined to be in the range of 0.50–1.30%.

[0026] 8) Determination of Ca content

[0027] Ca can form dot-like sulfides with S, altering the morphology of elongated sulfides and thus achieving excellent resistance to hydrogen sulfide. The optimal calcium-to-sulfur ratio is 2–2.5. However, excessive Ca content increases the number and size of dot-like oxides in the steel. Furthermore, because dot-like oxides have high hardness and poor plasticity, they do not deform during steel deformation and easily form voids at the interface, degrading the steel's properties. In this invention, the Ca content is determined to be in the range of 0.002–0.006%.

[0028] 9) Determination of Ti content

[0029] Ti detrimental to steel by remaining as titanium nitride and titanium carbonitride inclusions. These inclusions are hard and angular, severely affecting the fatigue life of the material, especially when the purity is significantly improved and the amount of other oxide inclusions is very small, the harm caused by titanium inclusions is particularly prominent. In conjunction with smelting cost control, the Ti content range of this invention is determined to be ≤0.003%.

[0030] 10) Determination of O content

[0031] Oxygen content represents the total amount of oxide inclusions. The brittle inclusions of oxides significantly affect the service life of the finished product. Numerous experiments have shown that reducing oxygen content is significantly beneficial for improving steel purity, especially reducing the content of brittle oxide inclusions. This is also considered in conjunction with smelting cost control. Therefore, the oxygen content range in this invention is defined as ≤0.0010%.

[0032] 11) Determination of P and S content

[0033] Phosphorus (P) in steel causes severe segregation during solidification. P dissolves in ferrite, leading to grain distortion and coarsening, and increasing cold brittleness. Considering both smelting cost control and other factors, the P content range in this invention is determined to be ≤0.025%. Sulfur (S) causes hot brittleness in steel, reducing its ductility and toughness. S content in steel resistant to hydrogen sulfide corrosion needs strict control, also considering smelting cost control. Therefore, the S content range in this invention is determined to be ≤0.002%.

[0034] 12) Determination of the contents of As, Sn, Sb and Pb

[0035] Trace elements such as As, Sn, Sb, and Pb are all low-melting-point non-ferrous metals. Their presence in steel causes soft spots and uneven hardness on the surface of parts. Therefore, they are considered harmful elements in steel, and this is also considered in conjunction with smelting cost control. The present invention defines the content range of these elements as As ≤ 0.04%, Sn ≤ 0.03%, Sb ≤ 0.005%, and Pb ≤ 0.002%.

[0036] The manufacturing process for the steel used in the planetary screws is as follows: KR pretreatment—electric furnace or converter—ladle refining—VD or RH vacuum degassing—continuous casting—continuous rolling—shearing or sawing—slow cooling—spheroidizing annealing—finishing—parts packaging and warehousing.

[0037] The manufacturing method mainly includes the following steps:

[0038] 1) Steel is smelted using molten iron, scrap steel, and raw materials. Deep desulfurization of the molten iron is achieved through KR pretreatment, enhancing deoxidation during refining to ensure sufficient residual aluminum content in the steel. Centralized pre-deoxidation and VD or RH vacuum degassing are performed using the kinetics of the molten steel to ensure sufficient flotation of non-metallic inclusions and control the low content of gaseous elements in the molten steel. After vacuum degassing, prolonged soft argon blowing is performed to ensure sufficient flotation of inclusions. Simultaneously, anti-oxidation protection is implemented throughout the continuous casting process to reduce the number of inclusions in the steel.

[0039] 2) The continuous casting process employs electromagnetic stirring and light reduction technology, along with low superheat casting, which effectively improves and reduces compositional segregation in the continuously cast billet. In particular, with the addition of advanced equipment such as electromagnetic stirring and light reduction at the end of solidification, the density of the solidified structure of the billet is improved, the porosity and shrinkage cavities in the center of the billet are effectively controlled, the spacing between secondary dendrite arms is significantly improved, the equiaxed crystal ratio in the center is significantly increased, and the grains are refined, thereby significantly improving the quality of the billet and reducing compositional segregation.

[0040] 3) The raw materials are sequentially smelted in an electric furnace or converter, refined in LF, degassed in RH or VD vacuum, and continuously cast to produce continuously cast square billets with a size of 390×510mm or larger that match the chemical composition of the finished steel product. The continuously cast billets should be slowly cooled in a pit to prevent cracking, and the slow cooling time should not be less than 72 hours. Subsequently, the continuously cast billets are electroslag remelted again to form φ600mm electroslag ingots, and slowly cooled for 72 hours. They are then sent to a heating furnace with a neutral or weakly oxidizing atmosphere for heating and then slabd into intermediate billets of 200×200mm-300×300mm. The heating temperature is 1020-1270℃, and the heating time is greater than 5 hours. During the slab rolling, the initial rolling temperature is 1000℃-1250℃, the final rolling temperature is ≥850℃, and the slab rolling compression ratio is greater than 5. The intermediate billets should be slowly cooled in a pit with a pit temperature ≥500℃, and the slow cooling time should not be less than 72 hours.

[0041] 4) The intermediate billet is then sent to a heating furnace for rolling into the target steel pipe. The specific rolling process is as follows: the preheating zone temperature is controlled at 500-800℃, the heating zone temperature is controlled at 1050-1250℃, and the soaking zone temperature is controlled at 1050-1250℃. To ensure that the billet is heated fully and evenly, the total heating time is more than 8 hours. The initial rolling temperature is controlled at 1000℃-1200℃, and the final rolling temperature is controlled at above 850℃. After rolling, the billet is cooled in a pile.

[0042] To ensure that the steel pipes used in planetary screw manufacturing only require machining followed by laser hardening of the roller surface to achieve the required hardness, stable microstructure, and dimensional accuracy while meeting service strength requirements, the steel pipe production innovatively employs an online normalizing + sub-temperature quenching + secondary tempering process.

[0043] During hot-rolled steel pipe production, online water cooling is performed after final rolling to ensure the upper cooling bed temperature range is 600~750℃. This results in a mixed state of bainite and martensite in the hot-rolled state, providing distortion energy for subsequent spheroidizing annealing. Heating at 820±10℃ for 7 hours ensures that some cementite dissolves in the austenite within the ferrite-austenite two-phase region, while the matrix retains cementite particles for subsequent nucleation, achieving dynamic equilibrium. Subsequently, salt bath quenching followed by holding at 880±10℃ for 3 hours yields a martensite-ferrite dual-phase structure. This is then followed by tempering at 710±10℃ for 3.5 hours with water cooling, and then tempering at 610±10℃ for 5 hours with air cooling, resulting in uniform, dotted cementite (typically 0.1~0.5μm) while fully utilizing the tempering strengthening function of tungsten. The final delivered tensile strength can reach over 1580MPa. Beneficial effects

[0044] Compared with the prior art, the advantages of the present invention are as follows:

[0045] 1) Unlike traditional GCr15 bearing steel, its chemical composition has been optimized, significantly improving the steel's hardenability, yield strength, tensile strength, resistance to hydrogen sulfide corrosion, and resistance to high-temperature softening, while also reducing its tendency to crack. It meets the machining strength requirements for hollow materials.

[0046] 2) Compared to the coarse spherical cementite (typically 1~3μm) of traditional GCr15 bearing steel, the cementite in the steel of this invention exists in a uniform and finer spheroidized state (typically 0.1~0.5μm) with a spheroidization rate of over 95%, and the remaining microstructure is ferrite. This results in low microstructural distortion energy, minimal heat treatment deformation during the machining of ball screws, and high dimensional accuracy, meeting the precision requirements of planetary ball screws.

[0047] 3) Traditional GCr15 bearing steel has poor tempering resistance. Long-term high temperature environment will reduce the strength of the steel and cause deformation and loss of precision. The planetary screw steel of this invention has yield strength (≥1350MPa) and tensile strength (≥1580MPa) at 350℃. At the same time, in order to cope with the low temperature environment that high-altitude equipment is often in, this invention has low temperature toughness far exceeding that of bearing steel (Charpy impact energy AKU2≥27J at -20℃). Attached Figure Description

[0048] Figure 1 is an organizational diagram of the delivery status of Embodiment 1 of the present invention.

[0049] Figure 2 is an organizational diagram of the delivery status in Embodiment 2 of the present invention.

[0050] Figure 3 is an organizational diagram of the delivery status of Embodiment 3 of the present invention. The best embodiment of the present invention

[0051] The technical solution of the present invention will be described in more detail with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the invention.

[0052] Examples 1-3 illustrate the chemical composition and manufacturing method of the steel used in the planetary screw of the present invention, and compare them with the commercially available GCr15 bearing steel.

[0053] The chemical composition (wt%) of each embodiment is shown in Tables 3 and 4.

[0054]

[0055]

[0056] Inclusions in the steel of each embodiment are shown in Table 5.

[0057]

[0058] A comparison of the mechanical properties (delivery condition) of each embodiment, including tensile properties at 350°C and impact properties at -20°C, is shown in Table 6.

[0059]

[0060] The end hardenability data of the steel in each embodiment are shown in Table 7.

[0061]

[0062] The microstructure of the steel in each embodiment is shown in Figures 1-3. Unlike the coarse spheroidal cementite of traditional GCr15 bearing steel, the cementite in the steel of this invention exists in a uniform and finer (generally 0.1~0.5μm) spheroidized state in the delivery state, with a spheroidization rate of over 95%, and the remaining microstructure is ferrite. The microstructure has low distortion energy, resulting in minimal heat treatment deformation during the machining of ball screw products, high dimensional accuracy, and the ability to meet the precision requirements of planetary ball screws.

[0063] The manufacturing process of the planetary screw steel in each embodiment is as follows: electric furnace or converter — ladle refining — VD or RH vacuum degassing — continuous casting — electroslag remelting — squaring electroslag ingots into intermediate billets — heating and rolling intermediate billets into finished products — heat treatment — finishing — forming and warehousing.

[0064] In the specific smelting process, molten iron, scrap steel, and raw and auxiliary materials are selected, along with high-quality deoxidizers and refractory materials. During the electric furnace / converter production process, the tapping endpoint C in the three embodiments is controlled at 0.05-0.35%, the endpoint P is required to be ≤0.025%, and the continuous casting superheat is controlled within 15-35℃.

[0065]

[0066] The intermediate billet is sent to a heating furnace for rolling into the target steel pipe. The specific rolling process is as follows: the preheating zone temperature is controlled at 500-800℃, the heating zone temperature is controlled at 1050-1250℃, and the soaking zone temperature is controlled at 1050-1250℃. To ensure that the billet is fully and uniformly heated, the total heating time is 2 hours or more. The initial rolling temperature is controlled at 1000-1200℃, and the final rolling temperature is controlled above 850℃. When hot-rolling the steel pipe, it is quenched online after the final rolling to ensure that the upper cooling bed temperature range is 600~750℃. After rolling, it is stacked and cooled to make the hot-rolled stacked and cooled microstructure a mixed state of bainite + martensite. The rolled, online normalized finished steel pipes undergo sub-critical quenching followed by a second tempering treatment. Heating at 825±10℃ for 7 hours ensures that some cementite dissolves in the austenite within the ferrite-austenite two-phase region, while the matrix retains cementite particles for subsequent nucleation, achieving dynamic equilibrium. This is followed by salt bath quenching at 880±10℃ for 3 hours to obtain a martensitic-ferrite two-phase microstructure. This is then followed by tempering at 680±10℃ for 3.5 hours with water cooling, and then tempering at 580±10℃ for 5 hours with air cooling. After tempering, the pipes undergo subsequent straightening and flaw detection to obtain the target steel pipe product, which is then packaged and stored.

[0067] As shown in Tables 3, 4, 5, 6, and 7, the planetary screw steel pipe for oil and gas drilling environments in the embodiments of the present invention exhibits significantly better control of harmful elements such as oxygen, titanium, and non-metallic inclusions compared to traditional GCr15 bearing steel. Particularly in terms of mechanical properties at high temperatures, the yield strength, tensile strength, low-temperature impact resistance, resistance to hydrogen sulfide corrosion, and resistance to high-temperature softening of the present invention are significantly superior to traditional GCr15 bearing steel. The yield strength is increased by nearly 500 MPa, the tensile strength by 500 MPa, the low-temperature impact performance by nearly 60 J, and the hardness by nearly 15 HRC. The hardenability is also significantly better than that of traditional GCr15 bearing steel.

[0068] Although preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention. Embodiments of the present invention

[0069] Type the description paragraph of embodiments of the present invention here. Industrial applicability

[0070] Type the industrial utility description paragraph here. Sequence List Free Content

[0071] Type the free content description paragraph for the sequence list here.

Claims

1. A type of steel for planetary screws used in oil and gas extraction, characterized in that: The chemical composition of the steel, by mass percentage, is: C: 0.25–0.50%, Si: 1.50–2.00%, Mn: 1.20–1.60%, Cr: 0.80–1.20%, S: ≤0.002%, P≤0.025%, Nb: 0.02–0.06%, W: 0.50–1.30%, Al≤0.05%, Ca: 0.002–0.006%, Ti≤0.003%, O≤0.0010%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, with the balance being Fe and unavoidable impurities.

2. The steel for planetary screws used in oil and gas extraction according to claim 1, characterized in that: The steel shall meet the following requirements for high-temperature tensile properties at 350℃ and low-temperature impact properties at -20℃: yield strength ≥1350MPa, tensile strength ≥1580MPa, elongation ≥5%, Charpy impact energy AKU2 ≥27J at -20℃, end hardenability tested by JIS G 0561 method, and hardness ≥58HRC for J9mm.

3. The steel for planetary screws used in oil and gas extraction according to claim 1, characterized in that: The cementite in the delivered steel is in a spheroidized state of 0.1~0.5μm with a spheroidization rate of over 95%, and the remaining microstructure is ferrite.

4. A method for manufacturing steel for planetary screws used in oil and gas extraction as described in claim 1, characterized in that: The method includes the following steps: 1) The raw materials are smelted in sequence by electric furnace or converter, LF refining, RH or VD vacuum degassing and continuous casting, and continuous casting is carried out to produce continuous casting square billets with a specification of 390×510mm and above that are consistent with the chemical composition of the finished steel product. 2) The continuously cast billet is slowly cooled in the pit for no less than 72 hours. Then the continuously cast billet is electroslag remelted again to form a φ600mm electroslag ingot, which is then slowly cooled for 72 hours. 3) After being heated in a furnace with a neutral or weak oxidizing atmosphere, the billets are cut into intermediate billets of 200×200mm-300×300mm. The intermediate billets are then slowly cooled in a pit at a temperature ≥500℃ for a period of not less than 72 hours. 4) The intermediate billet is sent to the heating furnace and rolled into the target steel pipe. The temperature of the preheating section is controlled at 500-800℃, the temperature of the heating section is controlled at 1050-1250℃, the temperature of the soaking section is controlled at 1050-1250℃, the total heating time is more than 8 hours, the rolling start temperature is controlled at 1000℃-1200℃, the final rolling temperature is controlled at more than 850℃, and the billet is cooled after rolling. 5) The online normalizing + sub-temperature quenching + secondary tempering process is adopted to obtain steel pipes in which cementite exists in a uniform and finer spheroidized state in the delivery state, with a spheroidization rate of over 95%, and the rest of the structure is ferrite. 6) After tempering, the steel pipe undergoes subsequent straightening and flaw detection to obtain the target steel pipe product.

5. A method for manufacturing steel for planetary screws in oil and gas extraction according to claim 4, characterized in that: During the electric arc furnace / converter production process, the tapping endpoint C is controlled at 0.05-0.35%, the endpoint P is required to be ≤0.025%, and the continuous casting superheat is controlled within 15-35℃.

6. A method for manufacturing steel for planetary screws in oil and gas extraction according to claim 4, characterized in that: In step 3), the heating temperature is 1020-1270℃, the heating time is greater than 5 hours, and the rolling compression ratio is greater than 5.

7. A method for manufacturing steel for planetary screws in oil and gas extraction according to claim 4, characterized in that: In step 5), during the hot rolling of the steel pipe, water is applied online after the final rolling to ensure that the temperature range of the upper cooling bed is 600~750℃, so that the hot-rolled cold-pressed structure is a mixed state of bainite and martensite.

8. A method for manufacturing steel for planetary screws in oil and gas extraction according to claim 4, characterized in that: Step 5) The online normalizing + sub-temperature quenching + secondary tempering process is as follows: the steel pipe is heated to 820±10℃ for 7 hours; then salt bath quenching is performed, followed by quenching at 880±10℃ for 3 hours to obtain a martensitic-ferrite dual-phase structure; then tempering and water cooling are performed at 710±10℃ for 3.5 hours, followed by tempering and air cooling at 610±10℃ for 5 hours to obtain uniform dotted cementite.

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