A 175 ksi high toughness oil well pipe steel and method of making

Abstract

The present application relates to a kind of 175ksi high tough oil well pipe steel and its preparation method, belong to steel technical field for oil, natural gas exploitation drilling engineering, to solve at least one of the problems such as the existing process cannot realize 175ksi steel grade low-cost stable production, poor strength and toughness matching, complex heat treatment process and poor controllability.The strength and toughness of the present application are synergistically improved by controlling each element and the ratio, Nb-V complex precipitation is used, the NbC precipitation temperature is about 1180 DEG C, the austenite grain boundary is pinned during rolling, the grain coarsening is inhibited, and the austenite dynamic recrystallization is blocked, the cumulative deformation energy storage is used to refine the subsequent phase transformation organization, and the steel strength is simultaneously improved by fine-grain strengthening and precipitation strengthening.Effective cost reduction is realized by selecting low content Mo and not adding Ni and other valuable alloy elements, the comprehensive mechanical properties of 175ksi steel grade are realized, and the limitation of traditional composition-performance matching is broken through.

Description

Technical Field

[0001] This invention relates to the field of steel technology for drilling engineering in oil and gas extraction, and in particular to a 175ksi high-strength and high-toughness oil well pipe steel and its preparation method. Background Technology

[0002] As global oil and gas exploration and development continues to extend into ultra-deep, extra-deep, and high-temperature, high-pressure, and highly corrosive media extreme conditions, drilling depths are constantly breaking through the 10,000-meter mark. Downhole loads and environments are becoming increasingly demanding, placing higher requirements on the strength grade, load-bearing capacity, and wellbore adaptability of oil well tubing. Currently, 150ksi-grade oil well tubing remains the mainstream product in ultra-deep well drilling and production. However, its strength reserves are limited. To meet the requirements of ultra-high pressure resistance and crush-resistant safety design, a large wall thickness is required, resulting in insufficient effective wellbore diameter and difficulty in adapting to the efficient construction needs of small wellbores, ultra-long horizontal sections, and complex well structures.

[0003] Against this backdrop, 165ksi grade ultra-high strength oil well tubing, with its higher yield strength and comprehensive service performance, can reduce wall thickness and increase working space to a certain extent, showing broad prospects for engineering applications. However, with the continuous increase in well depth and further deterioration of downhole conditions, 165ksi grade steel still has limitations in terms of strength margin, wall thickness optimization space, and well diameter adaptability. Under the conditions of small-diameter, ultra-high-density drilling and completion, there are still problems such as difficulty in running the tubing string, limited working space, and low drilling and production efficiency, which cannot fully meet the long-term needs of future development of deeper and more extreme oil and gas resources.

[0004] Compared to 150ksi and 165ksi steel grades, 175ksi ultra-high strength oil well tubing boasts higher strength and structural safety. Under the same design standards, it allows for greater wall thickness reduction, significantly increasing the effective downhole diameter. This effectively addresses common industry challenges such as limited drilling and production operations, difficult tool insertion, and high construction risks associated with small-diameter ultra-deep wells. Furthermore, the lightweight tubing resulting from thinner walls reduces tubing weight and friction, significantly improving drilling, completion, and production enhancement efficiency in ultra-deep and small-diameter wells. Therefore, it has broader application prospects and higher engineering value than 165ksi tubing in deep and ultra-deep oil and gas development.

[0005] Currently, the existing ultra-high strength oil well tubing is basically stagnant at the 165ksi steel grade, with no 175ksi steel grade available. Furthermore, the gripping force of all ultra-high strength oil well tubing relies on high-carbon, high-alloy systems, resulting in poor strength-toughness matching. Some tubing uses complex heat treatment processes, leading to low industrial controllability and difficulty in mass production. Moreover, the synergistic design of low cost, high stability, and high performance has not been achieved in most cases.

[0006] Existing patent CN120158687A discloses a 165ksi steel grade drill pipe product. This product achieves a good balance of strength and toughness through composition and process optimization, meeting the "one-tenth" design principle for the safe service of oil and gas well pipes. Its impact toughness index can stably reach more than 1 / 10 of the yield strength value, providing certain technical reference value in terms of mechanical property adaptability. However, the core heat treatment process adopted by this patent is three-stage induction heating cycle quenching. The investment, energy consumption, and operating condition matching threshold of induction heating equipment are extremely high. It has stringent requirements for equipment control precision, power load stability, and heating sequence coordination. The three-stage induction heating cycle quenching is difficult to control, has a narrow production process window, and is difficult to replicate. Summary of the Invention

[0007] Based on the above analysis, the present invention aims to provide a 175ksi high-strength and high-toughness oil well pipe steel and its preparation method, in order to solve at least one of the following problems: the inability of existing processes to achieve low-cost and stable production of 175ksi steel grade, poor strength and toughness matching, and complex and poor controllability of heat treatment processes.

[0008] In a first aspect, the present invention provides a 175ksi high-strength and high-toughness oil well pipe steel, wherein the high-strength and high-toughness oil well pipe steel is composed of the following components by mass percentage: C: 0.2~0.3%, Si: 0.2~0.3%, Mn: 0.3~0.5%, P≤0.008%, S≤0.003%, Cr: 0.5~0.8%, Mo: 0.5~0.6%, V: 0.15~0.2%, Nb: 0.04~0.06%, with the balance being Fe and unavoidable impurities.

[0009] Secondly, the present invention provides a method for preparing the above-mentioned 175ksi high-strength and high-toughness oil well pipe steel, comprising the following steps: S1: Prepare the raw materials according to the proportions of each component, smelt, and forge into round billets; S2: The forging billet is subjected to homogenization and heat preservation treatment. The homogenized round billet is then pierced and rolled to obtain a rough tube. The rolled rough tube is then sized and straightened to obtain a rolled tube. S3: The rolled pipe is subjected to induction heating quenching + induction heating preheating + high temperature tempering heat treatment to obtain the high strength and toughness oil well pipe steel.

[0010] Furthermore, in S2, four rolling passes are used, with rolling reductions of 22-30%, 18-25%, 8-15%, and 5-8%, respectively.

[0011] Furthermore, the rolling temperature for the first three passes is in the austenite recrystallization region, while the rolling temperature for the last pass is in the non-recrystallization region, with a final rolling temperature of 850~880℃.

[0012] Furthermore, in S2, the 180mm round billet is pierced on a 180mm two-roll skew rolling mill with a roll inclination angle of 14~16° and a piercing temperature of 1100~1120℃ to pierce the 180mm round billet into a 220mm rough tube.

[0013] Furthermore, in S2, the sizing parameters include: inlet size Φ194×8.0mm; outlet size Φ194×8.0mm; single-stand diameter reduction rate of 1~3%; and sizing temperature controlled above 850℃ to avoid crystal mixing. After sizing, the sample is straightened and then air-cooled to room temperature.

[0014] Furthermore, in S2, after the heat soaking and insulation treatment, high-pressure water is used to remove the iron oxide scale from the surface of the tube blank.

[0015] Furthermore, in S3, the induction heating quenching involves induction heating the rolled tube to 900~950℃, followed by immediate water quenching to room temperature.

[0016] Furthermore, in S3, the temperature of the induction heating preheating is 600~650℃.

[0017] Furthermore, in S3, the high-temperature tempering heat treatment temperature is 630~680℃.

[0018] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: 1. This invention achieves a synergistic improvement in strength and toughness by controlling the elements and their proportions. It employs Nb-V composite precipitation, with NbC precipitation at approximately 1180℃. During rolling, this stabilizes austenite grain boundaries, inhibits grain coarsening, and hinders dynamic austenite recrystallization, accumulating deformation energy to refine the subsequent phase transformation structure. The steel's strength is simultaneously enhanced through fine-grain strengthening and precipitation strengthening. Furthermore, the use of low-content Mo and the absence of expensive alloying elements like Ni effectively reduces costs, achieving the comprehensive mechanical properties of 175ksi steel and overcoming the limitations of traditional composition-performance matching.

[0019] 2. The method of this invention employs induction heating quenching, which features rapid heating, short quenching cycle, and high production efficiency. It yields a quenched microstructure that is finer and more uniform than the rolled microstructure, resulting in more significant grain refinement and toughening effects. Leveraging the strong grain refinement effect brought about by rapid induction quenching, the amount of expensive alloying elements such as Mo and Ni can be reduced while meeting the service requirements of the same steel grade, effectively lowering alloy costs. This achieves the comprehensive mechanical properties of 175ksi steel grade, breaking through the limitations of traditional composition-performance matching.

[0020] 3. This invention uses induction rapid heating quenching, which can raise the temperature of the rolled pipe to above the complete austenitization temperature in a very short time. Due to the short holding and quenching time, vanadium carbide (VC) in the steel cannot be fully dissolved, and some VC dispersed phases are retained and pinned to the grain boundaries, which effectively inhibits the high-temperature growth of austenite grains, achieves further ultra-fine grains, and significantly improves the strength and toughness matching of the pipe.

[0021] 4. This invention employs a four-pass rolling process. The first three passes are rough rolling, with rolling temperatures in the austenite recrystallization zone, and the final pass is finish rolling, with rolling temperatures in the non-recrystallization zone. Reasonable control of the piercing rolling reduction ensures uniform piercing deformation and reduces internal defects such as central porosity and delamination in the pipe wall. A multi-stand continuous rolling mill is used for wall thickness reduction and extension rolling, with segmented temperature control. Two-stage controlled rolling is performed in the austenite recrystallization and non-recrystallization zones: rolling in the recrystallization zone achieves repeated recrystallization of austenite to refine the grains; rolling with a large reduction in the non-recrystallization zone flattens and elongates the austenite grains, accumulating deformation dislocations and deformation energy storage, providing the microstructure conditions for subsequent phase transformation and heat treatment to further refine the grains, simultaneously matching the strength and toughness requirements of ultra-high strength oil well pipes. Micro-deformation sizing and reduction are carried out in the low-temperature austenitic range to precisely control the outer diameter and roundness of the steel pipe, eliminate residual rolling stress, stabilize the geometric dimensions and microstructure uniformity of the pipe, and finally form a rough pipe with a specification of Φ194×8.0mm.

[0022] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0023] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0024] Figure 1 The microstructure of the oil well pipe steel prepared in Example 1 of the present invention is shown in the figure. Figure 2 The microstructure of the oil well pipe steel prepared in Example 4 of the present invention is shown in the figure. Figure 3 This is a microstructure diagram of the oil well pipe steel prepared in Comparative Example 1 of the present invention. Detailed Implementation

[0025] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0026] A specific embodiment of the present invention discloses a 175ksi high-strength and high-toughness oil well pipe steel, wherein the high-strength and high-toughness oil well pipe steel is composed of the following components by mass percentage: C: 0.2~0.3%, Si: 0.2~0.3%, Mn: 0.3~0.5%, P≤0.008%, S≤0.003%, Cr: 0.5~0.8%, Mo: 0.5~0.6%, V: 0.15~0.2%, Nb: 0.04~0.06%, with the balance being Fe and unavoidable impurities.

[0027] This invention achieves a synergistic improvement in strength and toughness by controlling the elements and their proportions. Carbon (C), the most important solid solution strengthening element in steel, ensures the matrix hardness required for the ultra-high strength of the oil well pipe. This invention controls the C content at 0.2-0.3%, avoiding insufficient strength due to low carbon content (failing to meet the 175ksi strength requirement) and preventing excessive carbon content from causing grain boundary segregation, which would damage the steel's impact toughness and corrosion resistance. Si, a residual deoxidizing element from the steelmaking process, also helps improve the matrix solid solution strength. Si is controlled at 0.2-0.3%, avoiding surface decarburization and toughness deterioration caused by excessive silicon content. Mn also plays a solid solution strengthening role, but excessive content can easily cause central segregation. Therefore, this invention controls Mn at 0.3-0.5%, balancing strengthening effect and microstructure uniformity. P and S are harmful impurity elements in steel; strictly controlling their levels within the lower limits can prevent grain boundary embrittlement and improve the overall toughness of the steel. Cr and Mo are the main precipitation strengthening and hardenability guaranteeing elements in this invention system. Cr can improve the hardenability of steel and help improve corrosion resistance, while Mo can improve strength by precipitating fine carbides and improve temper brittleness. The synergistic effect of the two ensures the overall hardenability and strength uniformity of thick-walled oil well pipes. V and Nb are microalloying elements that improve strength and toughness through the dual effects of grain refinement strengthening and precipitation strengthening. Fine V and Nb carbonitrides can hinder austenite grain growth during rolling and heat treatment, and at the same time, they precipitate and strengthen in the matrix, improving strength without significantly sacrificing toughness. This invention uses Nb-V composite precipitation, with NbC precipitation temperature of approximately 1180℃. During rolling, it pins austenite grain boundaries, inhibits grain coarsening, and hinders dynamic recrystallization of austenite, accumulating deformation energy to refine the subsequent phase transformation structure. The strength of the steel is improved by simultaneously relying on grain refinement strengthening and precipitation strengthening. At the same time, by selecting low Mo content and not adding precious alloying elements such as Ni, the cost is effectively reduced, achieving the comprehensive mechanical properties of 175ksi steel grade, breaking through the limitations of traditional composition-performance matching.

[0028] The following provides a detailed explanation of the function and dosage selection of the components contained in this invention.

[0029] Carbon (C): As a core strengthening element of the matrix, it ensures the quenching and hardening ability and matrix strength of oil well pipe steel, meeting the mechanical properties requirements of ultra-high strength steel. Appropriate control of carbon content can avoid the precipitation of continuous network carbides at grain boundaries, reduce the tendency of grain boundary embrittlement, and balance the plasticity and impact toughness of steel. In this invention, C is controlled at 0.2~0.3%, for example, 0.2%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, and 0.3%.

[0030] Silicon (Si): It has both deoxidation and purification functions as well as solid solution strengthening functions, improving the yield strength and tempering stability of the matrix; it inhibits the precipitation of temper brittle phases, improves the plasticity and low-temperature toughness of steel, avoids the problem of low toughness under high strength of ultra-high strength oil well pipes, and ensures the long-term service safety of the pipe. In this invention, the Si content is controlled at 0.2~0.3%, for example, 0.2%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, and 0.3%.

[0031] Manganese (Mn): It broadens the temperature range of the non-recrystallization region of austenite, inhibits the coarsening of austenite grains during rolling and heating; at the same time, it improves the hardenability of steel, exerts solid solution strengthening effect, balances strength and plasticity, and can fix sulfur element, weaken the harmful effect of sulfide inclusions, and improve the overall forming processability and microstructure uniformity of oil well pipe. In this invention, the Mn content is controlled at 0.3~0.5%, for example, 0.3%, 0.32%, 0.34%, 0.36%, 0.38%, 0.40%, 0.42%, 0.44%, 0.46%, 0.48%, and 0.5%.

[0032] Phosphorus (P) and sulfur (S): The content of P and S impurities is strictly limited. P is prone to segregation along grain boundaries, inducing cold embrittlement, while S is prone to forming coarse sulfide inclusions, causing stress concentration and reducing toughness and fatigue resistance. Through the purification design of low P and low S, the sources of grain boundary embrittlement and internal defects are reduced, and the uniformity of oil well pipe structure, impact toughness and long-term service reliability are improved. The P and S contents of this invention are controlled at P≤0.008% and S≤0.003%, respectively.

[0033] Chromium (Cr): Significantly improves the hardenability of steel, promotes the formation of continuously hardened structures, and ensures the uniformity and stability of the mechanical properties of thick-walled oil well pipe sections; it can refine the morphology and distribution of carbides, inhibit the intergranular precipitation of coarse carbides, and reduce temper brittleness; at the same time, it significantly improves the steel's resistance to hydrogen sulfide corrosion, downhole media corrosion, and oxidation resistance, making it suitable for the harsh corrosive service environment of deep and ultra-deep wells; it also helps to moderately broaden the non-recrystallization rolling process range, taking into account strength, toughness, and corrosion resistance. In this invention, the Cr content is controlled at 0.5~0.8%, for example, 0.5%, 0.52%, 0.54%, 0.56%, 0.58%, 0.60%, 0.62%, 0.64%, 0.66%, 0.68%, 0.70%, 0.72%, 0.74%, 0.76%, 0.78%, and 0.8%.

[0034] Molybdenum (Mo): Mo is a high-value alloying element with a high cost. However, for ultra-high-strength steel-grade oil well pipes, high-temperature tempering is necessary to meet the strength and toughness requirements while ensuring safe service performance. High-temperature tempering easily leads to softening of the matrix structure and a significant decrease in strength. Mo can significantly improve the hardenability and resistance to tempering softening of steel, effectively inhibiting grain coarsening and precipitation of brittle grain boundary structures during high-temperature tempering. Simultaneously, it can moderately increase the temperature of the non-recrystallization zone of austenite, reducing grain boundary element segregation and improving the pipe's resistance to stress corrosion and low-temperature toughness. In this invention, the Mo content is controlled at 0.5~0.6%, for example, 0.5%, 0.51%, 0.52%, 0.53%, 0.54%, 0.55%, 0.56%, 0.57%, 0.58%, 0.59%, and 0.6%.

[0035] Vanadium (V): As an important microalloying element, it can disperse and precipitate V (C,N) nano-precipitates, producing a significant precipitation strengthening effect and effectively improving the strength of the oil well tubing matrix. Simultaneously, it can help increase the temperature of the austenite non-recrystallization zone, inhibit austenite grain coarsening, and refine the rolled and tempered microstructure. During the high-temperature tempering process of ultra-high strength oil well tubing, V precipitates can effectively pin grain boundaries and dislocations, stabilize the matrix structure, improve tempering toughness and plasticity, and alleviate the contradiction between strength and toughness in high-strength materials. Furthermore, V can optimize the carbide precipitation morphology, weaken the tendency for brittle grain boundary phase precipitation, and, in conjunction with the alloy system, achieve a synergistic match between strength and toughness in 175ksi steel-grade oil well tubing, meeting the comprehensive performance requirements for safe service in deep and ultra-deep wells. In this invention, the V content is controlled at 0.15~0.2%, for example, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, and 0.2%.

[0036] Niobium (Nb): This invention adds trace amounts of Nb, which can form highly stable nanoscale NbC precipitates in steel. During high-temperature rolling, it pins austenite grain boundaries, inhibits grain coarsening, and hinders dynamic recrystallization of austenite. It accumulates deformation energy to refine the subsequent phase transformation structure, and improves the strength of steel by simultaneously strengthening through fine grains and precipitation. At the same time, it fixes interstitial C and N atoms in the matrix, regulates phase transformation behavior, optimizes the morphology of the martensite matrix, significantly improves tempering stability, and inhibits tempering strength decay and grain boundary brittleness. It achieves a synergistic improvement in the strength and toughness of ultra-high-strength steel in a low-carbon composition system. In this invention, the Nb content is controlled at 0.04~0.06%, for example, 0.04%, 0.042%, 0.044%, 0.046%, 0.048%, 0.05%, 0.052%, 0.054%, 0.056%, 0.058%, and 0.06%.

[0037] Another specific embodiment of the present invention discloses a method for preparing the above-mentioned 175ksi high-strength and high-toughness oil well pipe steel, comprising the following steps: S1: Prepare the raw materials according to the proportions of each component, smelt, and forge into round billets; S2: The forging billet is subjected to homogenization and heat preservation treatment. The homogenized round billet is then pierced and rolled to obtain a rough tube. The rolled rough tube is then sized and straightened to obtain a rolled tube. S3: The rolled pipe is subjected to induction heating quenching + induction heating preheating + high temperature tempering heat treatment to obtain the high strength and toughness oil well pipe steel.

[0038] The method of this invention employs induction heating quenching, which features rapid heating, short quenching cycle, and high production efficiency. It yields a quenched microstructure that is finer and more uniform than the rolled microstructure, with more significant grain refinement and toughening effects. Leveraging the strong grain refinement effect brought about by rapid induction quenching, the amount of expensive alloying elements such as Mo and Ni can be reduced while meeting the service requirements of the same steel grade, effectively lowering alloy costs. This achieves the comprehensive mechanical properties of 175ksi steel grade, breaking through the limitations of traditional composition-performance matching.

[0039] This invention employs induction rapid heating quenching, which allows the rolled tube to reach a temperature above the complete austenitization temperature in a very short time. Due to the short holding and quenching time, vanadium carbide (VC) in the steel cannot be fully dissolved, and some VC dispersed phases are retained and pinned to the grain boundaries, effectively inhibiting the high-temperature growth of austenite grains, achieving further ultra-fine grains, and significantly improving the strength and toughness matching of the tube.

[0040] Specifically, in S1, the size of the round blank is Φ180mm.

[0041] Specifically, in S2, the temperature of the heat equalization and heat preservation treatment is 1120~1140℃, for example, 1120℃, 1122℃, 1124℃, 1126℃, 1128℃, 1130℃, 1132℃, 1134℃, 1136℃, 1138℃, 1140℃, and the heat preservation time is 1~2h, for example, 1h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h, 2h.

[0042] After heat treatment, high-pressure water is used to remove the iron oxide scale from the surface of the tube blank, so as to avoid the iron oxide scale being pressed into the tube wall during the rolling process and forming surface defects, thus ensuring the surface quality of the seamless tube and subsequent heat treatment. Hollow piercing of the solid round blank is achieved by two-roll skew rolling, forming a hollow tube with uniform wall thickness and good inner and outer surface quality.

[0043] Preferably, the 180mm round billet is pierced on a 180mm two-roll skew rolling mill with a roll inclination angle of 14~16°, for example, 14°, 14.5°, 15°, 15.5°, 16°, and the piercing temperature is 1100~1120℃, for example, 1100℃, 1102℃, 1104℃, 1106℃, 1108℃, 1110℃, 1112℃, 1114℃, 1116℃, 1118℃, 1120℃, to pierce the 180mm round billet into a 220mm rough tube.

[0044] Specifically, in S2, four rolling passes are used, with rolling reductions of 22-30% (e.g., 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%), 18-25% (e.g., 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%), 8-15% (e.g., 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%), and 5-8% (e.g., 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%).

[0045] Specifically, the rolling temperatures for the first three passes are in the austenite recrystallization region (above 880℃, preferably 880~1100℃, for example, 880℃, 885℃, 890℃, 895℃, 900℃, 950℃, 1000℃, 1050℃, 1100℃), and the rolling temperature for the last pass is in the non-recrystallization region, with a final rolling temperature of 850~880℃, for example, 850℃, 855℃, 860℃, 865℃, 870℃, 875℃, 880℃.

[0046] It should be noted that the first three passes are rough rolling, with rolling temperatures in the austenite recrystallization zone, while the last pass is finish rolling, with rolling temperatures in the non-recrystallization zone. Reasonable control of the piercing rolling reduction ensures uniform piercing deformation and reduces internal defects such as central porosity and delamination in the pipe wall. A multi-stand continuous rolling mill is used for wall thickness reduction and extension rolling, with segmented temperature control. Two-stage controlled rolling is performed in the austenite recrystallization and non-recrystallization zones: rolling in the recrystallization zone achieves repeated recrystallization of austenite to refine the grains; rolling with a large reduction in the non-recrystallization zone flattens and elongates the austenite grains, accumulating deformation dislocations and deformation energy, providing the microstructure conditions for subsequent phase transformation and heat treatment to further refine the grains, simultaneously matching the strength and toughness requirements of ultra-high strength oil well pipes. Micro-deformation sizing and reduction are carried out in the low-temperature austenitic range to precisely control the outer diameter and roundness of the steel pipe, eliminate residual rolling stress, stabilize the geometric dimensions and microstructure uniformity of the pipe, and finally form a rough pipe with a specification of Φ194×8.0mm.

[0047] Specifically, in S2, the sizing parameters include: inlet size Φ194×8.0mm; outlet size Φ194×8.0mm; single-stand diameter reduction rate of 1~3%, for example, 1%, 1.5%, 2%, 2.5%, 3%; and sizing temperature controlled above 850℃ to avoid crystal mixing. After sizing, the sample is straightened and then air-cooled to room temperature.

[0048] Specifically, in S3, the induction heating quenching involves induction heating the rolled tube to 900~950℃, for example, 900℃, 910℃, 920℃, 930℃, 940℃, or 950℃, followed by immediate water quenching to room temperature.

[0049] This invention employs induction heating quenching, which features rapid heating, short quenching cycles, and high production efficiency. It yields a quenched microstructure that is finer and more uniform than the rolled microstructure, resulting in more significant grain refinement and toughening effects. Leveraging the strong grain refinement effect of rapid induction quenching, the amount of expensive alloying elements such as Mo can be reduced while meeting the service requirements of the same steel grade, effectively lowering alloy costs. It achieves the comprehensive mechanical properties of 175ksi steel, breaking through the limitations of traditional composition-performance matching.

[0050] This invention employs induction rapid heating quenching, which allows the rolled tube to reach a temperature above the complete austenitization temperature in a very short time. Due to the short holding and quenching time, vanadium carbide (VC) in the steel cannot be fully dissolved, and some VC dispersed phases are retained and pinned to the grain boundaries, effectively inhibiting the high-temperature growth of austenite grains, achieving further ultra-fine grains, and significantly improving the strength and toughness matching of the tube.

[0051] Specifically, in S3, the induction heating preheating temperature is 600~650℃, for example, 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, and the high-temperature tempering heat treatment temperature is 630~680℃, for example, 630℃, 640℃, 650℃, 660℃, 670℃, 680℃.

[0052] This invention performs induction preheating treatment on the rolled tube after quenching, accurately controlling the tube temperature within the range of 600-650℃. After preheating, the tube immediately enters the conventional heat treatment furnace without the need for long-term heat treatment and heat preservation, which greatly shortens the residence time of the tube in the heat treatment furnace, simplifies the process, and significantly improves the overall production line efficiency.

[0053] The advantages of precise control over the composition and process parameters of the high-strength and tough oil well tubing steel of the present invention are demonstrated below with specific embodiments and comparative examples. The mass percentage (%) of the composition of the high-strength and tough oil well tubing steel in the embodiments and comparative examples is shown in Table 1, with the balance being Fe and unavoidable impurity elements. The specific process parameters are shown in Table 2, and the performance parameters are shown in Table 3.

[0054] Example A method for preparing the above-mentioned 175ksi high-strength and high-toughness oil well pipe steel includes the following steps: S1: Prepare the raw materials according to the proportions of each component, smelt, and forge into round billets with a size of Φ180mm; S2: The forging billet is subjected to homogenization and heat preservation treatment at a temperature of 1120~1140℃ for 1~2 hours. The homogenized round billet is pierced and rolled to obtain a rough tube. The rolled rough tube is sized and straightened, and then air-cooled to room temperature to obtain a rolled tube. The process involves piercing a Φ180mm round billet into a Φ220mm rough tube using a 14-16° roll inclination mill at a piercing temperature of 1100-1120℃. Four passes are used, with rolling reductions of 22-30%, 18-25%, 8-15%, and 5-8% respectively. The first three passes are rolled at 880-1100℃ in the austenite recrystallization zone, while the final pass is rolled in the non-recrystallization zone at a final rolling temperature of 850-880℃. The sizing parameters include: an inlet size of Φ194×8.0mm; an outlet size of Φ194×8.0mm; a single-stand reduction rate of 1-3%; and a sizing temperature controlled above 850℃.

[0055] S3: The rolled pipe is induction heated to 900~950℃, immediately quenched in water to room temperature, preheated by induction heating to 600~650℃, and immediately put into a conventional heat treatment furnace for high-temperature tempering heat treatment at a temperature of 630~680℃ to obtain the high-strength and tough oil well pipe steel.

[0056] Comparative Example 1 The preparation method of the oil well pipe steel in this comparative example is similar to that in Example 4, except that the piercing temperature in S2 is 1112℃. In S3, the rolled pipe is conventionally quenched at a temperature of approximately 904℃ for 1 hour, and then immediately water-quenched to room temperature. The quenched pipe is then subjected to high-temperature tempering at a temperature of approximately 675℃.

[0057] Comparative Example 2 The preparation method of the oil well pipe steel in this comparative example is similar to that in Example 4, except that the piercing temperature in S2 is 1112℃. In S3, the rolled pipe is conventionally quenched at a temperature of approximately 862℃ for 1 hour, and then immediately water-quenched to room temperature. The quenched pipe is then subjected to high-temperature tempering at a temperature of approximately 610℃.

[0058] Comparative Example 3 The preparation method of the oil well pipe steel in this comparative example is similar to that in Example 4. The differences in parameters in the preparation method are shown in Table 2.

[0059] Table 1 Chemical composition of each embodiment and comparative example

[0060] Table 2 shows the process parameters involved in each embodiment and comparative example.

[0061] Table 3 Performance Parameters

[0062] The oil well pipe steel prepared by this invention has a yield strength ≥1233MPa, tensile strength ≥1299MPa, elongation after fracture ≥18%, reduction of area ≥70%, and a strength of 0℃K. V2 ≥147J, -20℃K V2 ≥124J.

[0063] As shown in Tables 1-3, compared with the comparative example, the present invention effectively achieves grain refinement by rationally designing the composition and combining controlled rolling and cooling with induction rapid heating and quenching processes. Under the premise of adding fewer precious alloying elements, the pipe material simultaneously meets the requirements of ultra-high strength, high elongation and good low-temperature impact toughness, and stably reaches the performance index of 175ksi steel grade. The strength and toughness matching effect is far superior to that of the comparative example, and it has both cost and performance advantages.

[0064] The microstructure of the oil well pipe steel prepared in Example 1 is as follows: Figure 1 As shown, the microstructure of the oil well pipe steel prepared in Example 4 is as follows. Figure 2 As shown, the microstructure of the oil well tubing steel prepared in Comparative Example 1 is as follows. Figure 3 As shown. (Through) Figure 1-3 As shown, at the same multiple, induction heating and the addition of Nb elements significantly refine the grains and significantly improve the strength.

[0065] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A 175ksi high-strength and high-toughness steel for oil well tubing, characterized in that, The high-strength and tough oil well pipe steel is composed of the following components by mass percentage: C: 0.2~0.3%, Si: 0.2~0.3%, Mn: 0.3~0.5%, P≤0.008%, S≤0.003%, Cr: 0.5~0.8%, Mo: 0.5~0.6%, V: 0.15~0.2%, Nb: 0.04~0.06%, with the balance being Fe and unavoidable impurities.

2. A method for preparing the 175ksi high-strength and high-toughness oil well pipe steel according to claim 1, characterized in that, Includes the following steps: S1: Prepare the raw materials according to the proportions of each component, smelt, and forge into round billets; S2: The forging billet is subjected to homogenization and heat preservation treatment. The homogenized round billet is then pierced and rolled to obtain a rough tube. The rolled rough tube is then sized and straightened to obtain a rolled tube. S3: The rolled pipe is subjected to induction heating quenching + induction heating preheating + high temperature tempering heat treatment to obtain the high strength and toughness oil well pipe steel.

3. The preparation method according to claim 2, characterized in that, In S2, four rolling passes are used, with rolling reductions of 22-30%, 18-25%, 8-15%, and 5-8%, respectively.

4. The preparation method according to claim 3, characterized in that, The rolling temperature for the first three passes is in the austenite recrystallization zone, and the rolling temperature for the last pass is in the non-recrystallization zone, with a final rolling temperature of 850~880℃.

5. The preparation method according to claim 2, characterized in that, In S2, a 180mm two-roll skew rolling mill is used for piercing. The roll inclination angle is 14~16° and the piercing temperature is 1100~1120℃. The 180mm round billet is pierced into a 220mm rough tube.

6. The preparation method according to claim 2, characterized in that, In S2, the sizing parameters include: inlet size Φ194×8.0mm; outlet size Φ194×8.0mm; single-stand diameter reduction rate 1~3%; and sizing temperature controlled above 850℃ to avoid crystal mixing. After sizing, the sample is straightened and then air-cooled to room temperature.

7. The preparation method according to claim 2, characterized in that, In S2, after the heat soaking and heat preservation treatment, the process also includes using high-pressure water to remove the iron oxide scale from the surface of the tube blank.

8. The preparation method according to claim 2, characterized in that, In S3, the induction heating quenching involves induction heating the rolled tube to 900~950℃, followed by immediate water quenching to room temperature.

9. The preparation method according to claim 2, characterized in that, In S3, the temperature of the induction heating preheating is 600~650℃.

10. The preparation method according to claim 2, characterized in that, In S3, the high-temperature tempering heat treatment temperature is 630~680℃.

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