A 125 ksi grade ferritic / martensitic dual phase oil well pipe steel and a method of manufacturing the same

Abstract

The application belongs to the technical field of high-strength low-alloy oil well pipe steel, and particularly relates to a 125ksi grade ferrite / martensite dual-phase oil well pipe steel and a preparation method thereof. The chemical composition of the oil well pipe steel comprises, in percentage by weight, C 0.10-0.15%, V 0.1-0.3%, Mo 0.80-1.0%, Mn 0.50-0.80%, Cr 0.8-1.0%, Si 0.20-0.30%, Al 0.01-0.03%, P≤0.005%, S≤0.005%, Cu 0.80-1.5%, and the percentage by weight of Ni and Cu satisfies 0.6≤Ni / Cu≤1, and the balance is iron. The preparation method comprises vacuum smelting and pouring, forging, and a heat treatment process of 'quenching + two-phase zone quenching + tempering' in sequence. The application effectively solves the contradiction between the strength and the sulfide stress cracking (SSC) resistance of high-strength oil well pipe steel, and realizes the good matching between high strength and excellent SSC resistance.

Description

Technical Field

[0001] This invention belongs to the technical field of high-strength low-alloy oil well pipe steel, specifically a 125ksi grade ferritic / martensitic dual-phase oil well pipe steel and its preparation method. Background Technology

[0002] As oil and gas resource development gradually moves towards deep wells, ultra-deep wells, and high-sulfur oil and gas fields, the service environment for oil well tubing is becoming increasingly demanding. Oil well tubing not only needs to possess high strength and high toughness, but also needs to have excellent resistance to sulfide stress cracking (SSC) in acidic environments containing H2S.

[0003] Currently, high-strength oil well tubing steel typically employs quenching followed by high-temperature tempering to obtain a tempered martensitic structure and achieve higher strength. However, while high strength of 125 ksi can be obtained by adjusting the tempering temperature, the resistance to sputtering stress (SSC) does not meet the requirements of NACE TM0177 standard Method A. Increasing the tempering temperature can significantly reduce dislocation density and optimize SSC resistance, but the reduction in dislocation density also prevents the strength from reaching the 125 ksi level. The main reason for this is that the high-density dislocations introduced during quenching are both the primary source of strengthening in oil well tubing steel and a key factor inducing SSC susceptibility. Furthermore, the residual stress generated during quenching has a significant impact on SSC, as residual tensile stress is one of the necessary conditions for SSC to occur.

[0004] Patent CN115141972A discloses a 125ksi grade low-alloy oil well pipe steel resistant to sulfide stress cracking and its preparation method. It adopts a tempered martensite and a small amount of granular austenite microstructure and obtains its properties through a three-step heat treatment. Its shortcomings are: the carbon content is still relatively high (0.20-0.40%), and the dislocation density and residual stress after quenching cannot be reduced to the ideal level; the Cr content is only 0.5-0.6%, which cannot form enough Cr-rich carbides as hydrogen traps and has insufficient corrosion resistance; the Cu content is only 0.5-0.7%, and the precipitation of nano-Cu-rich phase is insufficient, resulting in limited precipitation strengthening and hydrogen trapping effects; the microstructure is tempered martensite and a small amount of granular austenite (4-6%), and the austenite content is too low, resulting in limited hydrogen capacity and a lack of ferrite phase to disperse stress and reduce dislocation density.

[0005] Patent CN114395696A discloses a type of steel for oil well pipes, its preparation method, and the oil well pipe itself, providing two technical solutions: high-carbon and low-carbon. However, both solutions have significant drawbacks: the high-carbon solution (C 0.4-0.6%) has an excessively high carbon content, which easily leads to the precipitation of coarse argyrophosphate particles at grain boundaries. 23C6 carbides become the preferred site for SSC crack initiation, resulting in unstable SSC resistance. The low-carbon scheme (C 0.05-0.15%) has a single bainite microstructure, which lacks martensite phase, leading to insufficient strength. Furthermore, the dislocation density in the bainite microstructure is still relatively high, limiting the improvement in SSC resistance.

[0006] Patent CN110616366A discloses a 125ksi grade sulfur-resistant oil well pipe and its manufacturing method. It employs a high-carbon composition design (C 0.20-0.30%) and adds various precious microalloying elements such as Nb, Ti, and W. The microstructure is a single tempered sorbite. Performance is achieved through a single quenching and high-temperature tempering at 680-700℃. Its main drawbacks are: the excessively high carbon content leads to a significant increase in dislocation density and residual stress after quenching, necessitating high-temperature tempering to reduce SSC susceptibility; however, high-temperature tempering severely reduces strength, making it difficult to consistently meet the 125ksi grade requirement; the addition of various precious alloying elements significantly increases production costs; and the single tempered sorbite microstructure lacks a ferrite phase to disperse stress and reduce dislocation density, resulting in limited improvement in SSC resistance.

[0007] The patent with publication number CN112522622A proposes a high-strength oil well pipe and its preparation method. The steel grade of the oil well pipe is only 590MPa, which is far below the requirement of 125ksi (862MPa). Its main shortcomings are: insufficient strength to meet the high strength requirements of deep wells and ultra-deep wells; and the microstructure is granular bainite and retained austenite. The retained austenite is prone to phase transformation and hydrogen embrittlement under stress.

[0008] Therefore, the contradiction between high strength and excellent SSC resistance in 125ksi grade oil well tubing steel is difficult to reconcile. Developing a 125ksi grade oil well tubing steel with high strength, excellent toughness and excellent SSC resistance and its preparation method is a major problem that oil well tubing manufacturers at home and abroad urgently need to solve. Summary of the Invention

[0009] To address the problem of the incompatibility between strength and SSC resistance in existing high-strength oil well tubing steels, this invention provides a 125ksi grade ferritic / martensitic dual-phase oil well tubing steel and its preparation method. This invention, through rational alloy composition design and a "quenching + two-phase zone quenching + tempering" heat treatment process, forms a ferritic / martensitic dual-phase structure in the steel, effectively reducing initial dislocation density and quenching residual stress. Due to the low initial dislocation density and quenching residual stress, subsequent tempering does not require high temperatures, thus enabling the formation of high-density and dispersed nanoscale Cu-rich phases, VC, and Cr-rich carbides, preventing coarsening. These second-phase particles act as irreversible hydrogen traps, generating a strong hydrogen capture effect, and also hinder dislocation movement, enhancing strength. This achieves a good match between high strength and excellent SSC resistance.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows:

[0011] A 125ksi grade ferritic / martensitic duplex oil well pipe steel, with the following chemical composition by weight percentage: C 0.10~0.15%, V 0.1~0.3%, Mo 0.80~1.0%, Mn 0.50~0.80%, Cr 0.8~1.0%, Si 0.20~0.30%, Al 0.01~0.03%, P≤0.005%, S≤0.005%, Cu 0.80~1.5%, and Ni and Cu weight percentages satisfying 0.6≤Ni / Cu≤1, with the balance being iron; the steel's Ac1 temperature is 750℃±20℃, and its Ac3 temperature is 880℃±20℃.

[0012] The 125ksi grade ferritic / martensitic dual-phase oil well pipe steel has a microstructure consisting of ferrite and tempered martensite dual-phase microstructure, wherein the volume fraction of ferrite is 10-15% and the volume fraction of tempered martensite is 85-90%.

[0013] The 125ksi grade ferritic / martensitic dual-phase oil well pipe steel contains dispersed nanoscale Cu-rich phase, VC and Cr-rich carbides; wherein the average size of the Cu-rich phase is 10-20nm, the average size of the VC is 20-30nm, and the average size of the Cr-rich carbides is 30-40nm.

[0014] The 125ksi grade ferritic / martensitic dual-phase oil well pipe steel has a C content of 0.12-0.15%, a V content of 0.15-0.20%, a Cu content of 0.8-1.0%, and a Ni content of 0.60-0.65%.

[0015] The preparation method of the 125ksi grade ferritic / martensitic dual-phase oil well pipe steel includes the following steps:

[0016] (1) Vacuum induction smelting and casting: Mix raw materials according to chemical composition requirements, and then perform vacuum induction smelting and casting to obtain steel ingots;

[0017] (2) Homogenization treatment and forging: The steel ingot is homogenized and then hot forged. The homogenization temperature is 1100-1150℃ and the homogenization time is 6-8h. Forging includes primary forging and final forging. The primary forging temperature is 1100-1150℃ and the primary forging time is 30-60min. The final forging temperature is 950-1000℃ and the final forging time is 30-60min. The forging ratio is 10-20. After forging, the steel ingot is air-cooled to room temperature at a cooling rate of 0.5℃ / s-5℃ / s.

[0018] (3) Quenching: The forged steel pipe is quenched for the first time. The quenching temperature is 30-50℃ above Ac3, and the holding time is 60-90min. Water cooling is used for quenching.

[0019] (4) Two-phase quenching: The steel pipe after the first quenching is subjected to two-phase quenching. The two-phase quenching temperature is 80-120℃ above Ac1, and the holding time is 60-90min. Water cooling is used for quenching.

[0020] (5) Tempering: Temper the steel pipe after quenching in the two-phase region. The tempering temperature is 150-200℃ below Ac1 and the tempering time is 60-90min. Then air cool to room temperature at a rate of 0.5-5℃ / s.

[0021] In the preparation method of the 125ksi grade ferritic / martensitic dual-phase oil well pipe steel, in step (3), the water cooling rate of the first quenching is 30-50℃ / s, and the first quenching temperature is 900-920℃.

[0022] In the preparation method of the 125ksi grade ferritic / martensitic dual-phase oil well pipe steel, in step (4), the water cooling rate of the two-phase quenching is 30-50℃ / s, and the quenching temperature of the two-phase region is 850-870℃.

[0023] In the preparation method of the 125ksi grade ferritic / martensitic dual-phase oil well pipe steel, the tempering temperature in step (5) is 550-560℃.

[0024] The method for preparing 125ksi grade ferritic / martensitic dual-phase oil well pipe steel has a yield strength ≥862MPa, tensile strength ≥931MPa, and room temperature Charpy V-notch impact energy ≥150J.

[0025] The preparation method of the 125ksi grade ferritic / martensitic dual-phase oil well pipe steel refers to NACE TM0177 standard Method A. Under constant load of 85% yield strength, the fracture time of the oil well pipe steel in the SSC test shall not be less than 720 hours.

[0026] The design concept of this invention is as follows:

[0027] In existing technologies, the high dislocation density and residual stress in tempered martensite structures lead to poor resistance to precipitates (SSC), and high-temperature tempering causes coarsening of nanoprecipitates, resulting in severe strength loss. Duplex microstructure oil well tubing steels suffer from insufficient strength or unstable SSC resistance. This invention organically combines microstructure control, precipitation strengthening, and hydrogen trap design, as detailed below:

[0028] (1) By introducing an appropriate amount of ferrite through a three-step heat treatment process of "quenching + two-phase quenching + tempering", and precisely controlling the ferrite volume fraction at 10-15%, a ferrite / martensite dual-phase structure is obtained. The ferrite phase, as a soft phase, can effectively disperse stress, reduce dislocation density and quenching residual stress, thereby reducing SSC sensitivity; the martensite phase, as a hard phase, ensures the high strength of the steel. When the ferrite content is <10%, the reduction in residual stress is small, and the improvement in SSC resistance is not significant; when the ferrite content is >15%, the yield strength will drop below 862MPa, which cannot meet the 125ksi grade requirement.

[0029] (2) By optimizing the composition design (low carbon, high Cr, high Cu, appropriate amounts of V and Mo), the hardenability and tempering stability of the steel are improved by alloying Mo with Cr, and the formation of fine Cr-rich carbides is promoted. Under medium-temperature tempering conditions of 150-200℃ below Ac1, high-density, dispersed nanoscale Cu-rich phases, VC and Cr-rich carbides are precipitated, improving the precipitation strengthening effect. They also act as irreversible hydrogen traps to capture hydrogen atoms, strongly capturing hydrogen atoms that have entered the steel and preventing hydrogen atoms from accumulating at grain boundaries, dislocations and other defects, thereby significantly improving the steel's resistance to SSC. At the same time, these nanoscale precipitates act as obstacles to dislocation movement, significantly improving the strength of the steel and compensating for the strength loss caused by the ferrite phase.

[0030] (3) By controlling the Ni / Cu ratio between 0.6 and 1, Ni and Cu work together to suppress Cu hot brittleness, ensuring uniform precipitation of Cu-rich phase, and improving the toughness and SSC resistance of steel.

[0031] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0032] 1. This invention, through the design of a ferrite / martensite dual-phase microstructure, precisely controls the volume fraction of ferrite, obtaining a dual-phase microstructure with a volume fraction of 10-15% ferrite and 85-90% tempered martensite, reducing the initial dislocation density and residual stress, and effectively reducing the probability of SSC (substrate stress concentration).

[0033] 2. This invention utilizes Cu-rich phase, VC and Cr-rich carbides to form a large number of irreversible hydrogen traps. Referring to NACETM0177 standard Method A, under constant load of 85% yield strength, the fracture time in the SSC resistance test is not less than 720 hours, which significantly improves the SSC resistance performance and meets the requirements for use in high-sulfur oil and gas fields.

[0034] 3. The present invention has a yield strength ≥862MPa, tensile strength ≥931MPa, and room temperature Charpy impact energy ≥150J, fully meeting the strength and toughness requirements of 125ksi grade oil well tubing. While ensuring high strength at the 125ksi grade, it still possesses excellent toughness, effectively solving the technical challenge of reconciling strength and SSC resistance in 125ksi grade oil well tubing steel.

[0035] 4. This invention does not add expensive microalloying elements such as Nb, Ti, and W, but achieves performance requirements only through reasonable composition design and heat treatment process, making it suitable for large-scale industrial production. Attached Figure Description

[0036] Figure 1 This is a microscopic image of the tissue morphology under an optical microscope from Example 1.

[0037] Figure 2 The residual stress distribution curves along the depth direction are for Example 1 and Comparative Example 1.

[0038] Figure 3 The image shows a comparison of the full width at half maximum (FWHM) of the XRD diffraction peaks of different BCC crystal planes in Example 1 and Comparative Example 1.

[0039] Figure 4 The image shows the scanning electron microscope (SEM) microstructure of Comparative Example 5. In the image, ferrite represents the ferrite phase, martensite represents the martensite phase, and the white particles pointed to by the arrows represent the carbide precipitate phase. Detailed Implementation

[0040] In its specific implementation, this invention proposes a 125ksi grade ferritic / martensitic duplex oil well pipe steel and its preparation method. The chemical composition of this oil well pipe steel, by weight percentage, includes: C 0.10–0.15%, V 0.1–0.3%, Mo 0.80–1.0%, Mn 0.50–0.80%, Cr 0.8–1.0%, Si 0.20–0.30%, Al 0.01–0.03%, P ≤ 0.005%, S ≤ 0.005%, Cu 0.80–1.5%, and the weight percentages of Ni and Cu satisfy 0.6 ≤ Ni / Cu ≤ 1, with the balance being iron. The Ac1 temperature of the steel is 750℃ ± 20℃, and the Ac3 temperature is 880℃ ± 20℃.

[0041] In this invention, the mechanism of action of the main chemical components of the steel used for oil well pipes is as follows:

[0042] C 0.10~0.15% (preferably 0.12~0.15%)

[0043] Carbon is a stable element in austenite, directly determining the Ac1 and Ac3 phase transformation temperatures and the volume fraction of ferrite after two-phase quenching. This invention, by controlling the carbon content to 0.10–0.15% and combining it with a two-phase quenching temperature of 80–120°C above Ac1, can precisely obtain 10–15% ferrite and 85–90% tempered martensite. Compared to the commonly used high-carbon design of 0.20–0.40% in existing technologies, the low-carbon composition of this invention reduces the dislocation density in quenched martensite from the source, significantly reducing SSC susceptibility.

[0044] Cu 0.80–1.5% (preferably 0.8–1.0%)

[0045] During tempering at 550–560℃, a 10–20 nm Cu-rich phase precipitates, acting as a barrier to dislocation movement and compensating for the strength loss caused by the ferrite phase, thus increasing the yield strength by approximately 150–200 MPa. The interface between the nano-Cu-rich phase and the matrix is ​​a strong irreversible hydrogen trap, which can capture hydrogen atoms entering the steel, preventing their accumulation at grain boundaries, dislocations, and other defects, significantly improving resistance to solid-state corrosion cracking (SSC). In an H₂S environment, Cu can form a dense Cu₂S protective film on the material surface, hindering further hydrogen atom intrusion into the matrix.

[0046] V 0.1~0.3% (preferably 0.15~0.20%)

[0047] When combined with carbon, VC forms 20–30 nm VC nanoprecipitates with extremely high thermal stability. It does not coarsen during tempering at 550–560 °C and can enhance precipitation strengthening by approximately 100–150 MPa. VC has a high degree of mismatch with the matrix, acting as a stronger irreversible hydrogen trap than Cu-rich phases, effectively fixing hydrogen atoms and reducing the hydrogen diffusion coefficient. During high-temperature forging and heat treatment, undissolved VC particles can pin austenite grain boundaries, preventing grain growth and improving the toughness of the steel.

[0048] Cr 0.8~1.0%

[0049] During the tempering process, a 30-40 nm M-type structure is formed. 23 C6 Cr-rich carbides supplement the precipitation strengthening effect and also act as hydrogen traps to capture hydrogen atoms. Cr dissolved in the matrix increases the electrode potential of the steel, reduces the corrosion rate in the H2S environment, and decreases the generation of hydrogen atoms. This ensures that large-size steel pipes can completely transform into martensite during quenching, avoiding the formation of soft phases such as pearlite or bainite.

[0050] Mo 0.80~1.0%

[0051] It significantly increases the coarsening temperature of nano-precipitates, ensuring that Cu-rich phases, VC, and Cr-rich carbides maintain a fine and dispersed distribution during tempering at 550–560℃, thus avoiding the problem of coarsening of precipitates. It prevents the segregation of impurity elements such as P and Sn at grain boundaries, improving the low-temperature toughness and SSC resistance of the steel. Working synergistically with Cr, it ensures the hardenability of thick-walled steel pipes, resulting in a uniform microstructure.

[0052] Ni and Cu are controlled synergistically to satisfy 0.6≤Ni / Cu≤1 (preferably, Cu 0.8~1.0%, Ni 0.60~0.65%).

[0053] Cu is prone to grain boundary melting above 1100℃, leading to hot embrittlement. Ni can form an infinite solid solution with Cu, increasing Cu's melting point and ensuring smooth hot working. Ni is an austenite stabilizing element, which can refine martensite laths and improve the low-temperature impact toughness of steel. A small amount of Ni can improve the hydrogen tolerance of the matrix and reduce hydrogen embrittlement sensitivity.

[0054] Mn 0.50~0.80%

[0055] It improves hardenability, strengthens the matrix through solid solution, and combines with S to form MnS, reducing the harmful effects of S; it also prevents Mn segregation from forming banded structures, which would worsen the anti-SSC properties.

[0056] Si 0.20~0.30%

[0057] It strengthens the matrix through solid solution, inhibits cementite precipitation, promotes uniform precipitation of nano-Cu-rich phases, avoids excessive Si content which increases the steel's tendency to become brittle, and can also be used as a deoxidizer.

[0058] Al 0.01~0.03%

[0059] It refines grains, combines with N to form AlN, and prevents austenite grain growth; it can also act as a deoxidizer, ensuring deoxidation effect while avoiding the formation of coarse Al2O3 inclusions.

[0060] The preparation method includes sequential vacuum smelting and casting, forging, and a heat treatment process of "quenching + two-phase quenching + tempering". The first quenching temperature is 900-920℃, the two-phase quenching temperature is 80-120℃ (850-870℃) above Ac1, corresponding to a ferrite volume fraction of 10-15%; the tempering temperature is 150-200℃ (550-560℃) below Ac1, which is the peak temperature for the precipitation of nano-Cu-rich phase and VC. Temperatures above 600℃ will cause the precipitated phase to coarsen to more than 50nm, losing its hydrogen trapping function.

[0061] The present invention will be further described in detail below with reference to specific embodiments.

[0062] The preparation methods of some embodiments of the present invention are as follows, and the differences in preparation methods between different embodiments will be explained in the corresponding embodiment content.

[0063] The materials are mixed according to the chemical composition described in this invention, and then vacuum induction melting and casting are performed to obtain 25kg steel ingots.

[0064] The steel ingot was homogenized at 1120℃ for 8 hours, followed by primary forging and final forging. The primary forging temperature was 1120℃ and the primary forging time was 30 minutes; the final forging temperature was 1000℃ and the final forging time was 40 minutes; the forging ratio was 12, and the ingot was air-cooled after final forging. Subsequently, longitudinal samples were taken from the forged round bar for heat treatment preparation according to different embodiments.

[0065] Mechanical property test specimens and SSC resistance test specimens were cut from steel samples after different heat treatments. Tensile specimens were 5 mm in diameter with a gauge length of 25 mm, and tested at room temperature. Impact specimens were 10 mm × 10 mm × 55 mm with a V-notch, and tested at room temperature. The SSC resistance test was performed according to NACE TM0177 standard method A, using solution A (5 wt% NaCl, 0.5 wt% CH3COOH, water balance), saturated with H2S, at a test temperature of 24 ± 3 °C, with a applied stress of 733 MPa, and the fracture time was recorded.

[0066] The following are different embodiments, which are for illustrative purposes only, and the present invention is not limited to these embodiments.

[0067] Example 1

[0068] The chemical composition of the oil well tubing steel in this embodiment, by weight percentage, includes: C 0.10%, V 0.20%, Mo 0.80%, Mn 0.54%, Cr 1.0%, Si 0.20%, Al 0.03%, P 0.002%, S 0.001%, Cu 0.93%, Ni 0.63%, with the balance being iron. Actual measurements show that the Ac1 temperature of this steel composition is 748℃, and the Ac3 temperature is 876℃.

[0069] The preparation process is as follows:

[0070] (1) Vacuum induction smelting and casting: The raw materials are mixed according to the above chemical composition, and after vacuum induction melting, 25kg steel ingots are cast.

[0071] (2) Homogenization and forging: The steel ingot was homogenized at 1120℃ for 8 hours; then hot forging was carried out, with an initial forging temperature of 1120℃ and an initial forging time of 30 min; the final forging temperature was 1000℃ and the final forging time was 40 min; the forging ratio was 12; after forging, it was air-cooled to room temperature at a rate of 2℃ / s.

[0072] (3) First quenching: The forged steel sample was heated to 910℃ and held for 60 min, and then water-cooled to room temperature at a rate of 40℃ / s.

[0073] (4) Two-phase quenching: The steel sample after the first quenching is heated to 870℃ and held for 60 min, and then water-cooled to room temperature at a rate of 40℃ / s.

[0074] (5) Tempering: Heat the steel sample after quenching in the two-phase region to 560℃ and hold for 60 min, then air cool to room temperature at a rate of 2.0℃ / s.

[0075] In this embodiment, the ferrite volume fraction in the obtained 125ksi grade ferrite / martensite dual-phase oil well pipe steel is approximately 12%, and the tempered martensite volume fraction is approximately 88%. Microstructure photographs are attached. Figure 1 .Depend on Figure 1 It can be seen that ferrite is uniformly dispersed in the tempered martensite matrix, without obvious segregation or banding. The average ferrite grain size is approximately 5–10 μm, and the tempered martensite laths are fine and uniform, without coarse primary austenite grain boundaries. The soft ferrite phase effectively disperses the internal stress generated during quenching, reducing the overall dislocation density; the hard tempered martensite phase ensures the high strength foundation of the steel. The synergistic effect of these two phases achieves a good match between strength and resistance to susceptibility stress (SSC). Nanoscale Cu-rich phases, VC, and Cr-rich carbides are dispersed in the steel. The average size of the Cu-rich phase is approximately 12 nm, the VC phase is approximately 25 nm, and the Cr-rich carbides are approximately 35 nm.

[0076] Example 2

[0077] The chemical composition of the oil well tubing steel in this embodiment, by weight percentage, includes: C 0.15%, V 0.13%, Mo 0.81%, Mn 0.54%, Cr 0.96%, Si 0.20%, Al 0.03%, P 0.003%, S 0.002%, Cu 0.87%, Ni 0.61%, with the balance being iron. Actual measurements show that the Ac1 temperature of this steel composition is 755℃, and the Ac3 temperature is 882℃.

[0078] The preparation process is as follows:

[0079] (1) Vacuum induction smelting and casting: The raw materials are mixed according to the above chemical composition, and after vacuum induction melting, 25kg steel ingots are cast.

[0080] (2) Homogenization and forging: The steel ingot was homogenized at 1120℃ for 8 hours; then hot forging was carried out, with an initial forging temperature of 1120℃ and an initial forging time of 30 min; the final forging temperature was 1000℃ and the final forging time was 40 min; the forging ratio was 12; after forging, it was air-cooled to room temperature at a rate of 2.0℃ / s.

[0081] (3) First quenching: The forged steel sample is heated to 900℃ and held for 60 min, then water-cooled to room temperature at a rate of 40℃ / s;

[0082] (4) Two-phase quenching: The steel sample after the first quenching is heated to 850℃ and held for 60 min, and then water-cooled to room temperature at a rate of 40℃ / s.

[0083] (5) Tempering: Heat the steel sample after quenching in the two-phase region to 550℃ and hold for 60 min, then air cool to room temperature at a rate of 2.0℃ / s.

[0084] In this embodiment, the ferrite volume fraction in the obtained 125ksi grade ferrite / martensitic dual-phase oil well pipe steel is about 10%, and the tempered martensite volume fraction is about 90%.

[0085] Example 3

[0086] By weight percentage, the chemical composition of the oil well tubing steel in this embodiment includes: C 0.12%, V 0.15%, Mo 0.84%, Mn 0.53%, Cr 0.99%, Si 0.21%, Al 0.03%, P 0.002%, S 0.002%, Cu 0.93%, Ni 0.64%, with the balance being iron. Actual measurements show that the Ac1 temperature of this steel composition is 752℃, and the Ac3 temperature is 879℃.

[0087] The preparation process is as follows:

[0088] (1) Vacuum induction smelting and casting: The raw materials are mixed according to the above chemical composition, and after vacuum induction melting, 25kg steel ingots are cast.

[0089] (2) Homogenization and forging: The steel ingot was homogenized at 1120℃ for 8 hours; then hot forging was carried out, with an initial forging temperature of 1120℃ and an initial forging time of 30 min; the final forging temperature was 1000℃ and the final forging time was 40 min; the forging ratio was 12; after forging, it was air-cooled to room temperature at a rate of 2.0℃ / s.

[0090] (3) First quenching: The forged steel sample was heated to 920℃ and held for 60 min, and then water-cooled to room temperature at a rate of 40℃ / s;

[0091] (4) Two-phase quenching: The steel sample after the first quenching is heated to 860℃ and held for 60 min, and then water-cooled to room temperature at a rate of 40℃ / s.

[0092] (5) Tempering: Heat the steel sample after quenching in the two-phase region to 555℃ and hold for 60 min, then air cool to room temperature at a rate of 2.0℃ / s.

[0093] In this embodiment, the ferrite volume fraction in the obtained 125ksi grade ferrite / martensitic dual-phase oil well pipe steel is about 11%, and the tempered martensite volume fraction is about 89%.

[0094] Comparative Example 1

[0095] The composition of the steel in Comparative Example 1 is the same as that in Example 1.

[0096] The preparation process is as follows:

[0097] (1) Vacuum induction smelting and casting: Same as in Example 1;

[0098] (2) Homogenization treatment and forging: Same as in Example 1;

[0099] (3) First quenching: Same as in Example 1;

[0100] (4) Tempering: Heat the steel sample after the first quenching to 690℃ and hold for 60 min, then air cool to room temperature at a rate of 2.0℃ / s.

[0101] Due to the lack of two-phase quenching and the absence of ferrite phase, the residual stress after quenching is high. High-temperature tempering leads to coarsening of the precipitates, a decrease in hydrogen trap density, and a significant increase in SSC susceptibility.

[0102] Comparative Example 2

[0103] The composition of the steel in Comparative Example 2 is the same as that in Example 1.

[0104] The preparation process is as follows:

[0105] (1) Vacuum induction smelting and casting: Same as in Example 1;

[0106] (2) Homogenization treatment and forging: Same as in Example 1;

[0107] (3) Two-phase quenching: The homogenized and forged steel sample was heated to 870℃ and held for 60 min, and then water-cooled to room temperature at a rate of 40℃ / s.

[0108] (4) Tempering: Same as in Example 1.

[0109] The lack of a first quenching to eliminate the forging structure, and the direct two-phase quenching, results in coarse austenite grains and a decrease in both strength and toughness.

[0110] Comparative Example 3

[0111] The composition of the steel in Comparative Example 3 was the same as that in Example 1.

[0112] The preparation process is as follows:

[0113] (1) Vacuum induction smelting and casting: Same as in Example 1;

[0114] (2) Homogenization treatment and forging: Same as in Example 1;

[0115] (3) First quenching: Same as in Example 1;

[0116] (4) Two-phase quenching: The steel sample after the first quenching is heated to 890℃ and held for 60 min, and then water-cooled to room temperature at a rate of 40℃ / s.

[0117] (5) Tempering: Same as in Example 1.

[0118] Because the quenching temperature in the two-phase region is too high, the amount of ferrite precipitation is insufficient, which cannot effectively disperse stress and reduce dislocation density, and the SSC sensitivity remains high.

[0119] Comparative Example 4

[0120] The composition of the steel in Comparative Example 4 is the same as that in Example 1.

[0121] The preparation process is as follows:

[0122] (1) Vacuum induction smelting and casting: Same as in Example 1;

[0123] (2) Homogenization treatment and forging: Same as in Example 1;

[0124] (3) First quenching: Same as in Example 1;

[0125] (4) Two-phase quenching: The steel sample after the first quenching is heated to 810℃ and held for 60 min, and then water-cooled to room temperature at a water cooling rate of 40℃ / s;

[0126] (5) Tempering: Same as in Example 1.

[0127] Due to the excessively low quenching temperature in the two-phase region, excessive ferrite precipitation and a high proportion of soft phase lead to a significant decrease in strength, failing to meet the 125ksi grade (yield strength ≥862MPa) requirement; although the toughness is good, the insufficient martensite content weakens the precipitation strengthening effect and reduces the hydrogen trap density.

[0128] Comparative Example 5

[0129] The composition of the steel in Comparative Example 5 was the same as that in Example 1.

[0130] The preparation process is as follows:

[0131] (1) Vacuum induction smelting and casting: Same as in Example 1;

[0132] (2) Homogenization treatment and forging: Same as in Example 1;

[0133] (3) First quenching: Same as in Example 1;

[0134] (4) Two-phase quenching: Same as in Example 1;

[0135] (5) Tempering: Heat the steel sample after quenching in the two-phase region to 600℃ and hold for 60 min, then air cool to room temperature at a rate of 2.0℃ / s.

[0136] Because the tempering temperature exceeds the coarsening critical temperature of the nano-precipitates, the size of the precipitates increases dramatically; the loss of precipitation strengthening effect leads to a yield strength of less than 862 MPa, while the hydrogen trap density decreases and the SSC resistance significantly declines.

[0137] Comparative Example 6

[0138] The composition of steel in Comparative Example 6 is the same as that in Example 1.

[0139] The preparation process is as follows:

[0140] (1) Vacuum induction smelting and casting: Same as in Example 1;

[0141] (2) Homogenization treatment and forging: Same as in Example 1;

[0142] (3) First quenching: Same as in Example 1;

[0143] (4) Two-phase quenching: Same as in Example 1;

[0144] (5) Tempering: Heat the steel sample after quenching in the two-phase region to 530℃ and hold for 60 min, then air cool to room temperature at a rate of 2.0℃ / s.

[0145] Due to the low tempering temperature, the atomic diffusion ability is insufficient, and the nanoprecipitated phase cannot fully nucleate and grow; although the precipitation strengthening effect is strong, the hydrogen trap density is insufficient, while the residual stress and dislocation density are high, and the SSC sensitivity increases significantly.

[0146] Table 1 shows the mechanical properties and SSC fracture time of the steels in different embodiments and comparative examples.

[0147]

[0148] As can be seen from Table 1, the yield strength of the oil well pipe steel (Examples 1-3) using the composition and preparation method of the present invention is not less than 862 MPa, the tensile strength is not less than 931 MPa, the Charpy impact energy is not less than 150 J, and it can withstand 720 h without fracture in the NACE TM0177 standard A method test, which meets the various index requirements of 125 ksi grade oil well pipe.

[0149] Even if the chemical composition is within the range specified in this invention, if the heat treatment scheme is not carried out in accordance with this invention, it is still impossible to obtain oil well pipe steel with high strength and excellent SSC resistance. For example, Comparative Example 1 yields a fully tempered martensitic structure with relatively high internal dislocation density and residual stress. Although its conventional mechanical properties such as strength meet the requirements of 125ksi grade oil well pipe steel, its SSC susceptibility is relatively high.

[0150] like Figure 2 As shown, the residual stress test results for Example 1 and Comparative Example 1 reveal that the residual tensile stress at the sample surface in Comparative Example 1 is as high as approximately 420 MPa, while that in Example 1 is only about 205 MPa. The residual stress level in Comparative Example 1 is significantly higher than that in Example 1. This invention, through a dual-phase microstructure design, reduces the surface residual tensile stress to below 200 MPa, fundamentally reducing the driving force for the initiation and propagation of SSC cracks.

[0151] Furthermore, the full width at half maximum (FWHM) of XRD diffraction peaks reflects the dislocation density in the microstructure; the higher the dislocation density, the larger the FWHM. For example... Figure 3 As shown, the XRD diffraction peak full width at half maximum (FWHM) test results for Example 1 and Comparative Example 1 demonstrate a positive correlation between the FWHM and the dislocation density within the material; a larger FWHM indicates a higher dislocation density. On all tested BCC crystal planes, the FWHM of each XRD diffraction peak in Example 1 was significantly higher than that in Comparative Example 1, indicating that the dislocation density in Example 1 was significantly higher than that in Comparative Example 1. This invention significantly reduces dislocation density through a two-phase microstructure design, both reducing the diffusion paths of hydrogen atoms and decreasing the concentration of hydrogen atoms at defects, thereby substantially reducing SSC susceptibility.

[0152] For example, in Comparative Example 2, direct two-phase quenching retains coarse austenite grains formed during forging, resulting in an uneven final microstructure. Because there is no quenching treatment before two-phase quenching, the undesirable microstructure from hot forging remains in the two-phase microstructure, significantly reducing strength and worsening SSC resistance. In Comparative Examples 3 and 4, improper selection of the two-phase quenching temperature results in excessively low (7%) and excessively high (50%) ferrite contents, respectively. Excessively low ferrite content cannot effectively reduce dislocation density and residual stress, worsening SSC resistance, while excessively high ferrite content cannot effectively improve strength. Comparative Examples 5 and 6 are examples of improper tempering temperature selection. Excessively high tempering temperature (Example 5) causes coarsening of the precipitates, failing to provide more irreversible hydrogen traps and strengthening, while excessively low tempering temperature (Example 6) fails to further reduce residual stress and dislocation density; although strength is high, SSC susceptibility is also high.

[0153] The chemical composition of Comparative Example 5 is exactly the same as that of Example 1, but the tempering temperature is 600°C, which is 150-200°C (550-560°C) higher than the Ac1 specified in this invention. Figure 4 As shown, the carbide precipitates in Comparative Example 5 exhibit significant coarsening, with an average size of approximately 50–100 nm, far exceeding the 10–40 nm nanoscale precipitates in the embodiments of this invention. Some carbides aggregate and grow at the ferrite / martensite phase boundaries and martensite lath boundaries, resulting in uneven distribution. This coarsening of the carbides eliminates the irreversible hydrogen trapping and dislocation pinning effects of the nanoscale precipitates, leading to a decrease in hydrogen trap density and a significant reduction in precipitation strengthening effect. This directly results in a yield strength of 812 MPa for Comparative Example 5 (below the 862 MPa required for the 125 ksi grade), and a SSC fracture time of only 369 hours, far below the 720 hours required by this invention.

[0154] The results show that this invention significantly reduces dislocation density and quenching residual stress by forming a ferrite / martensite dual-phase structure in the oil well tubing steel. It also synergistically precipitates high-density, dispersed nanoscale copper-rich phases, vanadium carbide, and chromium-rich carbides, creating numerous irreversible hydrogen traps. This invention effectively resolves the contradiction between the strength and sulfide stress cracking (SSC) resistance of high-strength oil well tubing steel, achieving a good match between high strength and excellent SSC resistance.

Claims

1. A 125ksi grade ferritic / martensitic dual-phase oil well pipe steel, characterized in that, The chemical composition of oil well pipe steel by weight percentage includes: C 0.10~0.15%, V 0.1~0.3%, Mo 0.80~1.0%, Mn 0.50~0.80%, Cr 0.8~1.0%, Si 0.20~0.30%, Al 0.01~0.03%, P≤0.005%, S≤0.005%, Cu 0.80~1.5%, and the weight percentage of Ni and Cu satisfies 0.6≤Ni / Cu≤1, with the balance being iron; the Ac1 temperature of the steel is 750℃±20℃, and the Ac3 temperature is 880℃±20℃.

2. The 125ksi grade ferritic / martensitic duplex oil well tubing steel according to claim 1, characterized in that, The microstructure of steel consists of a dual-phase structure of ferrite and tempered martensite, with the volume fraction of ferrite being 10-15% and the volume fraction of tempered martensite being 85-90%.

3. The 125ksi grade ferritic / martensitic dual-phase oil well pipe steel according to claim 1, characterized in that, The steel contains dispersed nanoscale Cu-rich phases, VC, and Cr-rich carbides; the average size of the Cu-rich phase is 10–20 nm, the average size of the VC is 20–30 nm, and the average size of the Cr-rich carbides is 30–40 nm.

4. The 125ksi grade ferritic / martensitic dual-phase oil well pipe steel according to claim 1, characterized in that, The C content is 0.12–0.15%, the V content is 0.15–0.20%, the Cu content is 0.8–1.0%, and the Ni content is 0.60–0.65%.

5. A method for preparing 125ksi grade ferritic / martensitic duplex oil well pipe steel as described in any one of claims 1 to 4, characterized in that, Includes the following steps: (1) Vacuum induction smelting and casting: Mix raw materials according to chemical composition requirements, and then perform vacuum induction smelting and casting to obtain steel ingots; (2) Homogenization treatment and forging: The steel ingot is homogenized and then hot forged. The homogenization temperature is 1100-1150℃ and the homogenization time is 6-8h. Forging includes primary forging and final forging. The primary forging temperature is 1100-1150℃ and the primary forging time is 30-60min. The final forging temperature is 950-1000℃ and the final forging time is 30-60min. The forging ratio is 10-20. After forging, the steel ingot is air-cooled to room temperature at a cooling rate of 0.5℃ / s-5℃ / s. (3) Quenching: The forged steel pipe is quenched for the first time. The quenching temperature is 30-50℃ above Ac3, and the holding time is 60-90min. Water cooling is used for quenching. (4) Two-phase quenching: The steel pipe after the first quenching is subjected to two-phase quenching. The two-phase quenching temperature is 80-120℃ above Ac1, and the holding time is 60-90min. Water cooling is used for quenching. (5) Tempering: Temper the steel pipe after quenching in the two-phase region. The tempering temperature is 150-200℃ below Ac1 and the tempering time is 60-90min. Then air cool to room temperature at a rate of 0.5-5℃ / s.

6. The method for preparing 125ksi grade ferritic / martensitic duplex oil well pipe steel according to claim 5, characterized in that, In step (3), the water cooling rate for the first quenching is 30-50℃ / s, and the temperature for the first quenching is 900-920℃.

7. The method for preparing 125ksi grade ferritic / martensitic duplex oil well pipe steel according to claim 5, characterized in that, In step (4), the water cooling rate for quenching the two-phase region is 30-50℃ / s, and the quenching temperature for the two-phase region is 850-870℃.

8. The method for preparing 125ksi grade ferritic / martensitic duplex oil well pipe steel according to claim 5, characterized in that, In step (5), the tempering temperature is 550-560℃.

9. The method for preparing 125ksi grade ferritic / martensitic dual-phase oil well pipe steel according to claim 5, characterized in that, The yield strength of the steel used for oil well pipes is ≥862MPa, the tensile strength is ≥931MPa, and the room temperature Charpy V-notch impact energy is ≥150J.

10. The method for preparing 125ksi grade ferritic / martensitic dual-phase oil well pipe steel according to claim 5, characterized in that, According to NACE TM0177 standard Method A, under constant load conditions of 85% yield strength, the fracture time of oil well pipe steel in the SSC test shall not be less than 720 hours.

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