Economical corrosion-resistant 5Cr80 rare earth oil well pipe steel and preparation method thereof

Abstract

The invention relates to economical corrosion-resistant 5Cr80 rare earth oil well pipe steel and a preparation method thereof, belongs to the technical field of corrosion-resistant oil well pipe steel for petroleum engineering, and aims to solve the problems that the cost is high due to addition of precious metals such as Mo and Ni in existing medium Cr steel, pitting corrosion is induced by inclusion of MnS, and a rare earth treatment process is unstable. The oil well pipe steel is prepared from the following chemical components in percentage by mass: 0.05 percent to 0.08 percent of C, 0.3 percent to 0.50 percent of Si, 0.80 percent to 1.5 percent of Mn, less than or equal to 0.01 percent of P, less than or equal to 0.0020 percent of S, less than or equal to 0.0020 percent of O, 0.0050 percent to 0.0350 percent of Ce, 4.5 percent to 5.5 percent of Cr, less than or equal to 0.035 percent of Al, 0.020 percent to 0.030 percent of V and the balance of Fe and inevitable impurities. The yield strength of the oil well pipe steel is larger than or equal to 580 MPa. The economical corrosion-resistant 5Cr80 rare earth oil well pipe steel which is low in cost and free of a Mo-Ni component system is used for replacing a traditional high-alloy pipe in a harsh oil and gas well environment containing high Cl <-> and CO2, and the economical corrosion-resistant 5Cr80 rare earth oil well pipe steel is suitable for batch production of corrosion-resistant oil well pipes of deep wells, ultra-deep wells and sulfur-containing and CO2-containing complex working conditions.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of corrosion-resistant oil well pipe steel for petroleum engineering, and particularly relates to an economic corrosion-resistant 5Cr80 rare earth oil well pipe steel and a preparation method thereof. BACKGROUND

[0002] At present, oil and gas exploitation is faced with serious corrosion problems of oil well pipes. Ordinary carbon steel oil well pipes (such as N80) are frequently failed due to insufficient corrosion resistance, resulting in huge economic losses. Although corrosion-resistant 3Cr oil well pipes have low cost, the Cr content is insufficient, the corrosion resistance is limitedly improved, and it is difficult to meet the demand of high corrosion environment. Although 13Cr oil well pipes have excellent corrosion resistance, the cost is high due to the addition of a large amount of expensive alloy elements (such as Mo, Ni, etc.), and it is difficult to be applied on a large scale. The existing medium Cr oil well pipes (such as 5Cr series) mostly rely on high-priced elements such as Mo and Cu to improve corrosion resistance, resulting in high material cost and limiting the popularization in economically sensitive fields.

[0003] In addition, the traditional corrosion-resistant oil well pipe still has deficiencies in inhibiting local corrosion (such as pitting corrosion and stress corrosion cracking), especially the MnS inclusions are easy to become corrosion initiation points, and the existing rare earth treatment process has unstable inclusion modification effect and large fluctuation of Ce yield, which affects the uniformity of corrosion resistance. Therefore, it is urgent to develop a low-cost and high-corrosion-resistant oil well pipe material and manufacturing process to meet the economic and reliability requirements of oil and gas exploitation. SUMMARY

[0004] In view of the above analysis, the embodiments of the present application aim to provide an economic corrosion-resistant 5Cr80 rare earth oil well pipe steel and a preparation method thereof, to solve the technical problems that the existing medium Cr oil well pipe has high cost due to the addition of noble metals such as Mo and Ni, MnS inclusions induce pitting corrosion and stress corrosion cracking, and the rare earth treatment has unstable Ce yield and poor inclusion modification effect.

[0005] In one aspect, the embodiments of the present application provide an economic corrosion-resistant 5Cr80 rare earth oil well pipe steel, and the components of the 5Cr80 rare earth oil well pipe steel include, by mass percentage: C: 0.05% to 0.08%, Si: 0.30% to 0.50%, Mn: 0.80% to 1.5%, P: ≤0.01%, S: ≤0.0020%, O: ≤0.0020%, Ce: 0.0050% to 0.0350%, Cr: 4.5% to 5.5%, Al: ≤0.035%, V: 0.020% to 0.030%, and the balance is Fe and inevitable impurities; the yield strength of the oil well pipe steel is ≥580 MPa.

[0006] Further, the microstructure of the 5Cr80 rare earth oil well pipe steel is tempered sorbite, and the average size of rare earth inclusions in the microstructure is not more than 5 μm.

[0007] Further, the mass ratio of Ce / S in the 5Cr80 rare earth oil well pipe steel is greater than or equal to 5, and the mass ratio of Ce / O is greater than or equal to 5.

[0008] Further, the mass ratio of Cr / C in the 5Cr80 rare earth oil well pipe steel is controlled to be 60-90:1.

[0009] Further, the tensile strength of the 5Cr80 rare earth oil well pipe steel is greater than or equal to 689 MPa, and the elongation after fracture is greater than or equal to 23%.

[0010] Further, the 0℃ impact energy KV2 of the 5Cr80 rare earth oil well pipe steel is greater than or equal to 80J.

[0011] Further, the 5Cr80 rare earth oil well pipe steel has a high-pressure autoclave corrosion rate of less than or equal to 0.37mm / a under the conditions of CO2 partial pressure 1Mpa, Cl - content 25000mg / L and 70℃ for 168 hours.

[0012] In another aspect, the present application provides a method for preparing the above-mentioned economical corrosion-resistant 5Cr80 rare earth oil well pipe steel, comprising the following steps:

[0013] S1. The continuous casting blank is obtained by smelting and off-furnace refining to obtain a steel blank;

[0014] S2. After the steel blank is broken down and forged, it is hot-rolled, the heating temperature is 1150-1200℃, the holding time is 1-2h, the starting rolling temperature is 1100-1150℃, and the final rolling temperature is 800-850℃, to obtain the hot-rolled steel blank;

[0015] S3. The hot-rolled steel blank is subjected to quenching and tempering treatment, first quenched at 850-900℃ and held for 40min-1h, and then tempered at 610-660℃ and held for 1-2h;

[0016] S4. The quenched and tempered steel blank is air-cooled to obtain the oil well pipe steel.

[0017] Further, in S1, the off-furnace refining includes LF refining, calcium treatment and RH vacuum refining, and cerium-iron alloy is added for rare earth treatment during the RH vacuum refining process, and the Ce recovery rate is controlled to be 45%-55%.

[0018] Further, the cerium-iron alloy is added in two times: 60%-70% of the total mass is added first and stirred for greater than or equal to 5min; the remaining cerium-iron alloy is added 3min before the RH is broken.

[0019] Compared with the prior art, the present application can at least achieve one of the following beneficial effects:

[0020] 1) The present application replaces the traditional high-cost design containing Mo, Ni and Cu with the 5Cr-Ce alloy system (Cr: 4.5%~5.5%, Ce: 0.0050%~0.0350%) innovatively, completely abandoning high-priced alloy elements, significantly reducing the material cost (about 1000 yuan / ton lower than the traditional 5Cr-Mo oil well pipe), while the corrosion resistance reaches or even exceeds the existing 5Cr-Mo oil well pipe level (CO2 partial pressure 1 MPa, Cl - 25000 mg / L environment, corrosion rate ≤0.37 mm / a), achieving the best balance between cost and performance.

[0021] 2) By strictly controlling the mass ratio of Ce / S≥5 and Ce / O≥5, it is ensured that rare earth Ce reacts with S and O preferentially, converting harmful MnS inclusions into stable Ce2O2S or Ce2S3 compounds (size ≤5 μm), completely eliminating the risk of MnS as a local corrosion starting point, so that the oil well pipe performs excellent pitting corrosion resistance and sulfide stress corrosion cracking (SSCC) resistance in high-corrosion environments containing H2S / Cl - , and the corrosion resistance uniformity is significantly improved.

[0022] 3) Optimizing the Cr / C ratio to 60~90:1 (C≤0.08%, Cr: 4.5%~5.5%) ensures sufficient solid solution Cr content in the matrix to form a dense Cr2O3 passivation film, and avoids the intergranular corrosion tendency caused by the precipitation of carbides such as Cr 23 C6, so that the material has excellent plasticity and toughness (elongation after fracture ≥23%, 0℃ impact energy ≥80 J) while maintaining high yield strength (≥580 MPa), fully meeting the requirements of deep wells, ultra-deep wells and other harsh working conditions.

[0023] 4) Using LF+RH double refining combined with batch cerium-iron alloy addition process (Ce recovery rate 45%~55%), combined with calcium treatment (wire feeding amount ≥400 m) and deep deoxidation (activity oxygen ≤1.0 ppm), the purity of the molten steel and the uniformity of rare earth distribution are significantly improved, solving the problem of unstable modification effect of traditional rare earth treatment, and greatly improving the batch consistency of product performance.

[0024] 5) A combined process of "large reduction controlled temperature rolling" and optimized laminar cooling is developed for the alloy characteristics of the 5Cr-Ce system. By precisely controlling the rough rolling reduction rate (18%~25%), the finish rolling temperature (800~850℃) and the cooling rate (8~12℃ / s to 550~600℃), the solid solution and precipitation of Cr and Ce in the composition are matched, the original austenite grains are refined and the harmful carbide precipitation is inhibited, and finally a uniform tempered sorbite structure is obtained. This process is expected to significantly improve the material's resistance to hydrogen-induced cracking (HIC) and fatigue performance.

[0025] 6) For the stress sensitivity of high Cr steel caused by the composition characteristics, a low temperature straightening process (150-200 DEG C preheating, straightening deformation 0.5%-1.0%) is developed, through the accurate control of straightening temperature and deformation, the risk of stress corrosion caused by cold straightening is avoided, at the same time, the microstructure characteristics caused by the composition are matched, the size precision (straightness deviation ≤0.5mm / m) of the oil well pipe is ensured to reach high standard, which provides reliable guarantee for subsequent thread processing and downhole safe service.

[0026] The above technical solutions in the application can also be combined with each other to realize more preferred combination solutions. Other features and advantages of the application will be described in the subsequent description, and some advantages will become apparent from the description, or will be understood by implementing the application. The purposes and other advantages of the application can be realized and obtained from the contents specifically pointed out in the description and the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0027] The accompanying drawings are included to provide a further understanding of the application, and are incorporated in and constitute a part of this application, illustrate embodiments of the application, and together with the description serve to explain the principles of the application, and do not limit the scope of the application.

[0028] Figure 1 Typical microstructure diagram of the oil well pipe steel of the application. DETAILED DESCRIPTION

[0029] The preferred embodiments of the application will be described in detail below with reference to the drawings, wherein the drawings constitute a part of this application, and are used to explain the principles of the embodiments of the application, and are not used to limit the scope of the application.

[0030] In the prior art, the general carbon steel oil well pipe has insufficient corrosion resistance and frequently fails; the corrosion-resistant 3Cr oil well pipe has limited improvement in corrosion resistance although the cost is low; the 13Cr oil well pipe has good corrosion resistance but the cost is too high; the medium Cr oil well pipe relies on high-priced elements to improve corrosion resistance and the cost is high. In addition, the traditional corrosion-resistant oil well pipe has insufficient local corrosion resistance, especially the MnS inclusions are easy to become the corrosion starting point, and the existing rare earth treatment process has unstable modification effect on the inclusions, so that the uniformity of corrosion resistance is difficult to guarantee.

[0031] In view of these problems, the application provides an oil well pipe material with low cost and high corrosion resistance and a manufacturing process, which provides an innovative solution for the oil well pipe material system.

[0032] In one aspect, the application discloses an economical corrosion-resistant 5Cr80 rare earth oil well pipe steel, and the 5Cr80 rare earth oil well pipe steel comprises the following components in percentage by mass: C: 0.05%-0.08%, Si: 0.30%-0.50%, Mn: 0.80%-1.5%, P: <=0.01%, S: <=0.0020%, O: <=0.0020%, Ce: 0.0050%-0.0350%, Cr: 4.5%-5.5%, Al: <=0.035%, V: 0.020%-0.030%, and the balance is Fe and inevitable impurities; and the yield strength of the oil well pipe steel is >=580 MPa.

[0033] In order to improve the corrosion resistance of the oil well pipe, a certain amount of alloy elements, such as Cr, Mo, Cu, Ni and RE, are usually added in the prior art. Among them, the content of Cr is generally about 3%, the content of Mo is usually 0.3%-0.5%, the content of Cu is about 0.2%-0.4%, and the content of Ni is between 0.5% and 1.0%. The application considers that the oil well pipe is large in quantity and sensitive to price, and therefore the content of Cr is increased to 4.5%-5.5% to enhance the corrosion resistance, and Mo, Cu and Ni alloy elements (among which Mo and Ni are expensive) are no longer added. This is because the Cr element is not only low in price, but also has a significant effect on improving the corrosion resistance and the highest cost performance.

[0034] In addition, for the problem of local corrosion of the oil well pipe, a small amount of Ce element (usually less than 0.0050%) is usually added in the prior art, but it is difficult to effectively inhibit the local corrosion. The content of Ce is increased to 0.0050%-0.0350% in the application, which effectively inhibits the local corrosion, and the advantage of the low price of Ce is used to replace the expensive Mo element, thereby reducing the material cost. Taking the addition of 0.3% Mo as an example, the cost of the oil well pipe will increase by about 1000 yuan, and the application significantly reduces the cost of the corrosion-resistant oil well pipe by not adding Mo element.

[0035] The oil well pipe steel of the application adopts a 5Cr-Ce system, the content of Cr is about 5% (4.5%-5.5%), the content of Ce is increased to replace the expensive Mo element, so that the corrosion resistance of the oil well pipe is ensured while the production cost is significantly reduced. The corrosion resistance of the oil well pipe is between that of the corrosion-resistant 3Cr oil well pipe and that of the corrosion-resistant 13Cr oil well pipe, the corrosion resistance reaches or exceeds the level of the existing 5Cr-Mo oil well pipe, and the oil well pipe has a good market application prospect.

[0036] The types, functions and content selection of the alloy elements in the oil well pipe steel of the application and the synergistic effect are described below.

[0037] Cr (Chromium) is a key corrosion resistant element, which can form stable passive film (such as Cr2O3) to significantly improve the overall corrosion resistance of steel in CO2 environment. Meanwhile, Cr as carbide forming element can also enhance the hardenability and comprehensive mechanical properties of steel. In the prior art, 3Cr and 13Cr oil well pipe balance corrosion resistance and cost by adjusting the content of Cr respectively, but 3Cr has insufficient corrosion resistance, and 13Cr has too high cost. The present application optimizes the content of Cr to 4.5%~5.5%, which balances the excellent corrosion resistance and economy.

[0038] Ce (Cerium) as a rare earth element exists in steel in the form of solid solution, oxide or sulfide. In the corrosion process, Ce 3+ ions are released and migrated to the corrosion area to form precipitated film such as Ce2(CO3)3, which effectively inhibits the Cl - induced local corrosion (such as pitting corrosion, crevice corrosion). The present application controls the content of Ce to 0.005%~0.035%, which fully plays the inhibiting effect on local corrosion, while avoiding the adverse effects of excessive addition on mechanical properties.

[0039] Synergistic effect of Cr and Ce:

[0040] Cr and Ce have good synergistic and complementary effect: Cr can significantly improve the overall corrosion resistance of steel, especially for CO2 corrosion; while Ce provides additional protection for local corrosion (such as Cl - induced pitting corrosion), which makes up for the deficiency of Cr in local corrosion resistance. The combination of the two makes the oil well pipe steel exhibit excellent corrosion resistance in complex corrosion environment (such as high CO2, high Cl - working conditions), and its corrosion resistance is about 4 times that of ordinary N80 oil well pipe, while the cost is much lower than that of high Cr steel (such as 13Cr), achieving the best balance between performance and economic benefit.

[0041] C (Carbon) is an element that expands the austenite phase region, has strong solid solution strengthening effect, and can significantly improve the hardness and strength of steel, and to some extent, helps to improve the corrosion resistance. However, too high C content will damage the plasticity, toughness and weldability of steel, and promote the precipitation of Cr 23 C6, Cr7C3 and other carbides, consume Cr elements in the matrix, reduce its passivation ability, and thus weaken the corrosion resistance of the material. Therefore, the present application strictly controls the content of C to 0.05%~0.08% to balance the strength, toughness and corrosion resistance of the material.

[0042] S (Sulfur) is a harmful impurity element in steel, which can significantly deteriorate corrosion resistance and toughness, especially in high corrosive environment (such as containing H2S or Cl -Sulfide stress corrosion cracking (SSCC) in the medium. In addition, the presence of S will form inclusions such as MnS, the starting point of localized corrosion. To minimize its negative impact, the present application will limit the content of S to ≤0.002%, to ensure the high purity of the material and excellent corrosion resistance.

[0043] O (oxygen) in steel mainly in the form of oxide inclusions, not only will reduce the mechanical properties and corrosion resistance of the material, but also with rare earth Ce combined to form stable rare earth oxides (such as Ce2O3), thus consuming the effective Ce content, weaken the inhibition of Ce on localized corrosion. Because the rare earth oxides are difficult to dissolve in the corrosion process, can not release Ce 3+ ion formation protective precipitate film. Therefore, the present application will strictly control the oxygen content in ≤0.0020%, to optimize the corrosion resistance of Ce and ensure the high purity of the material, while taking into account the economy of deoxidization process.

[0044] Si (silicon) is a common deoxidizer in steelmaking process, can be solid solution in ferrite and austenite, improve the strength, hardness and wear resistance of steel. However, too high Si content will damage the welding performance and plasticity of steel. But considering the demand for higher strength and wear resistance of 80 ksi grade oil well pipe, the present application will control the Si content in 0.30% ~ 0.50%, which can not only play its beneficial effect, but also avoid the adverse effects on the processing performance of the material.

[0045] Mn (manganese) as an austenite forming element, can significantly improve the hardenability of steel and stabilize the austenite structure, help to improve the strength and plasticity of the material. The appropriate amount of Mn content can optimize the comprehensive mechanical properties of steel, but excessive addition will increase the hardenability, and have adverse effects on the weldability and toughness. Since Mn has little effect on corrosion resistance, the present application will control the Mn content in 0.80% ~ 1.50% based on the demand for strength and toughness when designing 80 ksi grade oil well pipe, to ensure good plasticity and toughness while avoiding the deterioration of process performance.

[0046] P (phosphorus) is a harmful impurity element in steel, which will seriously damage the plasticity and toughness of the material, especially in low temperature environment, easy to cause cold brittleness, significantly reduce the impact performance of steel. In addition, P will also deteriorate the welding performance, promote the formation of cracks. In order to minimize its negative impact, the present application will strictly control the P content in ≤0.01%, to ensure the excellent low temperature toughness and welding suitability of the material.

[0047] Al (aluminum) in steel mainly as a deoxidizer, but excessive Al will form coarse inclusions, affect the purity and mechanical properties of steel. The present application will control the Al content in ≤0.035%, to ensure effective deoxidization while avoiding adverse effects on the performance of the material. This control range not only ensures the stability of the smelting process, but also maintains the good comprehensive performance of the steel.

[0048] V (vanadium) produces significant precipitation strengthening effect by refining grain size and forming carbide (VC), and improves the high-temperature tempering resistance of the steel. The fine VC particles precipitated during tempering can also act as hydrogen traps, effectively improving the hydrogen-induced cracking resistance of the material. However, too high V content will damage the toughness of the steel. Based on the performance balance consideration, the V content is precisely controlled at 0.020% to 0.030% in the present application, so as to fully exert its strengthening and hydrogen-induced cracking resistance effect, while avoiding the adverse effect on toughness.

[0049] Further, the mass ratio of Ce / S in the 5Cr80 rare earth oil well pipe steel is greater than or equal to 5, and the mass ratio of Ce / O is greater than or equal to 5.

[0050] The present application realizes the optimal utilization of rare earth element Ce by strictly controlling the mass ratio of Ce / S and Ce / O to be greater than or equal to 5. This control strategy has a dual mechanism: first, sufficient Ce content can ensure that it reacts with S and O in the steel first, converting harmful MnS inclusions into thermodynamically more stable Ce2O2S or Ce2S3 rare earth compounds, effectively eliminating the risk of MnS as a local corrosion starting point; second, this ratio control can prevent the over-consumption of rare earth elements by S and O, so that the yield of Ce is stably maintained above 45%, ensuring the stability and reproducibility of the rare earth modification effect.

[0051] Compared with the uneven inclusion modification caused by insufficient Ce / S ratio (usually < 3) in the traditional process, the present application realizes significant performance improvement by optimizing the ratio of Ce / S and Ce / O: on the one hand, the inclusion size is stably controlled below 5 microns, significantly improving the purity of the material; on the other hand, the formed rare earth compounds can play a cathodic protection role, effectively delaying the expansion of pitting corrosion by inhibiting the anodic dissolution process. This dual control strategy not only solves the corrosion problem caused by MnS inclusions, but also ensures the batch stability of product performance by stabilizing the yield of rare earth.

[0052] Further, the mass ratio of Cr / C in the 5Cr80 rare earth oil well pipe steel is controlled in the range of 60 to 90:1.

[0053] The present application realizes the optimal improvement of material performance by precisely controlling the mass ratio of Cr / C in the range of 60 to 90:1 by adopting a design of low carbon (≤0.08%) with medium chromium (≈5%). This ratio control has three key roles: first, sufficient Cr content ensures the formation of a continuous and dense Cr2O3 passivation film on the surface of the steel, significantly enhancing the stability of the material in a corrosive environment; second, this ratio effectively inhibits the precipitation of Cr carbides in the steel, which can cause embrittlement of the material; third, the Cr content is sufficient to form a continuous and dense Cr2O3 passivation film on the surface of the steel, significantly enhancing the stability of the material in a corrosive environment. 23The precipitation of coarse carbides such as C6 is avoided, and the low-carbon design reduces the tendency of M3C-type carbide formation and avoids the risk of intergranular corrosion caused by local chromium element depletion; thirdly, the optimized Cr / C ratio ensures more Cr element to be solid-solved in the matrix, which strengthens the material and maintains good toughness.

[0054] Further, the microstructure of the 5Cr80 rare earth oil well pipe steel is shown in the figure, which is a uniform tempered sorbite structure, and the average size of the rare earth inclusions in the structure is not more than 5 μm. Figure 1

[0055] This microstructure gives the material excellent performance. The tempered sorbite structure has high strength and good toughness, which can ensure that the oil well pipe does not break or deform under high stress and complex working conditions. The uniform distribution of the structure effectively reduces stress concentration and significantly improves the fatigue resistance of the material. The fine and dispersed rare earth inclusions (rare earth oxides, rare earth oxysulfides, and rare earth sulfides) have a size of ≤5 μm, which avoids stress concentration and reduces the starting point of local corrosion, and further strengthens the material by refining the grains, significantly improves the corrosion resistance, and is especially suitable for harsh environments with high chloride concentration and high temperature and high pressure.

[0056] The 5Cr80 rare earth oil well pipe steel developed by the application has excellent comprehensive mechanical properties: yield strength ≥ 580 MPa, tensile strength ≥ 689 MPa, elongation after fracture ≥ 23%, and impact energy KV2 at 0℃ ≥ 80 J. These indicators fully meet the performance requirements of 80 ksi grade oil well pipes, which have good plasticity and toughness while ensuring high strength, and are especially suitable for harsh working conditions such as deep wells and super deep wells. Compared with traditional oil well pipe materials, the steel grade has significantly improved in strength-toughness matching.

[0057] In the 168-hour autoclave test simulating harsh corrosion environment (CO2 partial pressure 1 Mpa, Cl - content 25000 mg / L, 70℃), the corrosion rate of the oil well pipe steel is ≤0.37 mm / a. Its corrosion resistance is outstanding: it is 4 times higher than that of ordinary N80 oil well pipe, and it is better than that of conventional 5Cr-Mo oil well pipe. This excellent corrosion resistance is mainly due to the optimized Cr / Ce synergistic mechanism, as well as the precise control of the Cr / C mass ratio and the Ce / S, Ce / O ratio, which makes the material exhibit excellent stability in complex corrosion environment containing CO2 and high Cl - .

[0058] On the other hand, the application proposes a method for preparing the above-mentioned economical corrosion-resistant 5Cr80 rare earth oil well pipe steel, comprising the following steps:

[0059] S1. The continuous casting blank is obtained by smelting and secondary refining to obtain a steel blank; ​

[0060] S2. The billet is subjected to hot rolling after being open-die forged, and before the hot rolling, the billet is subjected to heating, the heating temperature is 1150-1200℃, and the holding time is 1-2h; in the hot rolling process, the open rolling temperature is 1100-1150℃, and the finish rolling temperature is 800-850℃, to obtain the hot-rolled billet;

[0061] S3. The hot-rolled billet is subjected to quenching and tempering treatment, first quenching at 850-900℃ and holding for 40min-1h, and then tempering at 610-660℃ and holding for 1-2h;

[0062] S4. The quenched and tempered billet is subjected to air cooling to obtain the oil well pipe steel.

[0063] Further, the off-furnace refining in S1 includes LF refining, calcium treatment and RH vacuum refining, and cerium-iron alloy is added for rare earth treatment in the RH vacuum refining process, and the Ce yield is controlled to be 45%-55%.

[0064] The prior art usually adopts single LF refining or simple wire feeding method to add rare earth, and the Ce yield is low (<30%) and unevenly distributed. The present application adopts LF+RH double refining, optimizes the inclusion morphology in combination with calcium treatment, and adds cerium-iron alloy in batches in the RH vacuum refining stage, so that the Ce yield is increased to 45%-55%. This process not only reduces the oxidation loss of Ce, but also avoids the formation of coarse oxide inclusions, thereby significantly improving the toughness and corrosion resistance of the steel, and providing a stable foundation for subsequent rare earth micro-alloying.

[0065] Further, the LF refining satisfies the following conditions: the power supply parameters and time are controlled according to the preset outgoing molten steel temperature, the molten steel is deeply deoxidized with aluminum; the top slag white slag time is ≥20min; the outgoing molten steel activity oxygen is ≤1.0ppm; the top slag (FeO+MnO) of the ladle at the time of outgoing is ≤0.5%; the top slag thickness is 130-140mm (or the top slag amount: 4.5t-5.0t); the outgoing molten steel temperature is ≥1590℃; and the sulfur content of the outgoing molten steel is ≤0.002%.

[0066] Compared with the traditional LF refining which only controls the oxygen content to be ≤3ppm, the white slag time is short (10-15min) and the (FeO+MnO) content requirement is relatively loose (≤1.0%), the present application reduces the outgoing molten steel activity oxygen to ≤1.0ppm by deep deoxidization with aluminum, prolongs the white slag time to ≥20min, and strictly controls the top slag (FeO+MnO) content to be ≤0.5%. This improvement significantly improves the purity of the molten steel, reduces the number of inclusions, makes the steel have better hydrogen induced cracking (HIC) and sulfide stress corrosion (SSCC) resistance, and at the same time creates a low-oxygen environment for subsequent rare earth treatment, reducing the Ce burning loss.

[0067] Further, the calcium treatment satisfies the following conditions: calcium treatment feeding line amount ≥ 400 m, soft blowing time 6-8 min; the molten steel temperature after calcium treatment ≥ 1590℃.

[0068] On the basis of the deep deoxidation of the molten steel in the LF refining stage, the high melting point deoxidation product (mainly aluminum oxide) is changed into a low melting point product by calcium treatment and soft blowing, so as to create conditions for its efficient floating removal. The molten steel surface is avoided to be "bare" during calcium treatment and soft blowing, so as to avoid the secondary oxidation of the molten steel. Compared with the prior art, the present application adopts high feeding line amount (≥ 400 m) and long soft blowing time (6-8 min), so as to ensure that Ca fully modifies inclusions, promotes the formation of CaS and CaS and CaO-Al2O3 composite inclusions, and reduces harmful MnS inclusions, thereby significantly improving the local corrosion resistance of the steel. In addition, the molten steel temperature after calcium treatment is strictly controlled to be ≥ 1590℃, so as to avoid that the temperature drop is too large to affect the subsequent RH refining effect.

[0069] Further, the molten steel enters the RH vacuum refining stage after LF refining and calcium treatment. Since the inclusions generated in the previous deep deoxidation process may not be completely removed by floating, it is necessary to rely on RH vacuum treatment to achieve the sufficient floating and removal of fine inclusions. The vacuum degree of this stage is required to be not more than 100 Pa, and the vacuum cycle time is required to be not less than 16 minutes, so as to promote the floating of fine deoxidation inclusions in the molten steel and be captured by the top slag, thereby minimizing the total oxygen content of the molten steel. The rare earth cerium iron alloy is added in two times: 60% to 70% of the total mass of the first time is added and stirred for not less than 5 minutes, and the remaining cerium iron alloy is added 3 minutes before the RH breaks the vacuum, and continues to vacuum cycle for 3 minutes after the addition. Finally, the total oxygen content of the RH outgoing molten steel is not more than 15 ppm.

[0070] Compared with the traditional RH refining (mainly used for dehydrogenation, and the rare earth addition is mainly carried out in the LF stage), the present application adds the cerium iron alloy in batches in the RH stage, so as to ensure the uniform distribution of Ce and reduce the oxidation loss. The process not only improves the utilization rate of rare earth, but also further optimizes the corrosion resistance of the steel through the synergistic effect of vacuum deoxidation and rare earth micro-alloying.

[0071] Further, the hot rolling in S2 includes: the rough rolling stage: each pass reduction rate 18% to 25%, inter-pass air cooling temperature drop 30 to 50℃ / pass; the finishing rolling stage: the last 3 passes single pass reduction rate 12% to 18%, inter-pass air cooling temperature drop ≤ 30℃ / pass; the total deformation amount is controlled to be 70% to 85%.

[0072] The existing high-Cr steel hot rolling usually adopts a lower reduction ratio (10%-15%) to avoid cracking, but is prone to cause coarse structure. The present application adopts a large reduction ratio of 18%-25% per pass in the rough rolling stage, and the temperature is reduced by 30-50 DEG C between passes; the single-pass reduction ratio is 12%-18% in the last three passes in the finish rolling stage, the temperature is reduced by no more than 30 DEG C between passes, and the total deformation is controlled to be 70%-85%. The process promotes dynamic recrystallization by "large reduction + controlled temperature rolling", refines austenite grains, and at the same time avoids the rolling cracking problem caused by the hot brittleness of high-Cr steel, to provide a uniform and refined structure basis for subsequent quenching and tempering treatment.

[0073] Further, the S2 hot rolling is followed by laminar cooling, and the cooling speed is 8-12 DEG C / s to the final cooling temperature of 550-600 DEG C.

[0074] The traditional hot rolling is usually followed by air cooling or slower cooling (3-5 DEG C / s), which is prone to cause the carbide in the high-Cr steel to precipitate along the grain boundaries. The present application adopts laminar cooling (8-12 DEG C / s) to the final cooling temperature of 550-600 DEG C, which quickly inhibits the precipitation of Cr 23 The harmful phases such as C6 gather at the grain boundaries, significantly improve the intergranular corrosion resistance, and at the same time obtain a uniform bainite / martensite structure, which is convenient for subsequent quenching and tempering process to accurately control the performance.

[0075] Further, the S4 is followed by S5: straightening treatment is performed on the steel material, the straightening temperature is 150-200 DEG C, the straightening deformation is controlled to be 0.5%-1.0%, and 150-180 DEG C preheating is performed before straightening.

[0076] The existing straightening is usually performed at room temperature, and the high-Cr steel is prone to cause excessive residual stress due to cold deformation. The present application adopts preheating (150-180 DEG C) before straightening and low-temperature straightening (150-200 DEG C), and controls the deformation to be 0.5%-1.0%, which effectively reduces the cold work hardening and residual stress, avoids the risk of stress corrosion cracking (SCC), and at the same time improves the straightness and size accuracy of the pipe.

[0077] In summary, the present application creatively proposes an economical corrosion-resistant 5Cr80 rare earth oil well pipe steel and a preparation process thereof, which innovatively adopts "high Cr (5%) + high Ce" alloy design to replace the traditional Mo / Ni / Cu alloying route, cooperates with the key process breakthroughs such as LF+RH double refining and batch rare earth addition technology (Ce recovery rate 45%-55%), large reduction controlled temperature rolling (rough rolling 18%-25% reduction ratio), and optimized laminar cooling (8-12 DEG C / s), successfully solves the technical bottlenecks such as the easy cracking of high-Cr steel during hot working, uneven distribution of rare earth, and local corrosion control difficulty, and in the case of completely abandoning high-priced alloy elements, not only reduces the material cost, but also realizes the corrosion resistance better than the traditional 5Cr steel, and provides an innovative solution with economy and reliability for the development of high-corrosion oil and gas fields.

[0078] The application will be described in more detail by specific examples. The examples are only a description of the best mode of the application and do not have any limitation on the scope of the application.

[0079] Examples 1-4

[0080] Examples 1-4 provide an economical corrosion-resistant 5Cr80 rare earth oil well pipe steel and a preparation method thereof.

[0081] The economical corrosion-resistant 5Cr80 rare earth oil well pipe steel of Examples 1-4 includes, by mass percentage, C: 0.05%-0.08%, Si: 0.30%-0.50%, Mn: 0.80%-1.5%, P: ≤0.01%, S: ≤0.0020%, O: ≤0.0020%, Ce: 0.0050%-0.0350%, Cr: 4.5%-5.5%, Al: ≤0.035%, V: 0.020%-0.030%, and the balance of Fe and unavoidable impurities, wherein the mass ratio of Ce / S is ≥5, the mass ratio of Ce / O is ≥5, and the mass ratio of Cr / C is controlled to be 60-90:1. The specific components are shown in Table 1.

[0082] The preparation method of the oil well pipe steel of Example 1 includes the following steps:

[0083] S1. The continuous casting billet is obtained by smelting and off-furnace refining, wherein the off-furnace refining includes LF refining, calcium treatment, and RH vacuum refining, specifically:

[0084] LF refining: the power supply parameters and time are controlled according to the preset outgoing molten steel temperature, and the molten steel is deeply deoxidized with aluminum. The outgoing molten steel activity oxygen is controlled to be ≤1.0 ppm, the top slag white slag time is 20 min, the top slag (FeO+MnO) content is 0.4%, the top slag thickness is 130 mm (top slag amount: 4.5 t), the outgoing molten steel temperature is 1590°C, and the sulfur content is 0.0015%.

[0085] Calcium treatment: the calcium treatment wire feeding amount is 400 m, and the soft blowing time is 6 min. The molten steel temperature after calcium treatment is 1590°C.

[0086] RH vacuum refining: the vacuum degree is 80 Pa, and the vacuum circulation time is 16 min. The cerium-iron alloy is added in two times, 60% of the total mass is added in the first time, 0.33 kg / t of 30% cerium-iron alloy is added, and stirring is performed for 5 min. The remaining part is added 3 min before the RH breaks the vacuum, vacuum circulation is continued for 3 min, 0.22 kg / t of 30% cerium-iron alloy is added, vacuum circulation is continued for 3 min, and the total oxygen content of the outgoing molten steel of the RH is ensured to be ≤15 ppm.

[0087] S2. The billet is heated to 1150°C for 1 h, and then hot-rolled at an initial rolling temperature of 1100°C and a final rolling temperature of 800°C. The reduction ratio of each pass is 20% in the rough rolling stage, and the inter-pass air cooling temperature is reduced by 40°C per pass. The reduction ratio of each pass is 15% in the finish rolling stage, and the inter-pass air cooling temperature is reduced by 20°C per pass in the last three passes. The total deformation is controlled to be 75%. After hot rolling, the billet is cooled at a cooling rate of 10°C / s to a final cooling temperature of 580°C.

[0088] S3. The hot-rolled billet is subjected to quenching and tempering treatment, i.e., the billet is quenched at 850°C for 40 min, and then tempered at 640°C for 1 h.

[0089] S4. The quenched and tempered billet is air-cooled to obtain the oil well pipe steel.

[0090] S5. The steel is subjected to straightening treatment at a straightening temperature of 150°C, and the straightening deformation is controlled to be 0.5%. In addition, the steel is preheated at 150°C before straightening.

[0091] The preparation method of the oil well pipe steel in Example 2 is different from that in Example 1 only in that:

[0092] LF refining: the white slag time of the top slag is prolonged to 22 min, the content of the top slag (FeO+MnO) is controlled to be 0.45%, the thickness of the top slag is 135 mm (the amount of the top slag: 4.75 t), the temperature of the molten steel is 1595°C, and the sulfur content is 0.0018%.

[0093] Calcium treatment: the wire feeding amount is increased to 420 m, and the soft blowing time is prolonged to 7 min. The temperature of the molten steel after calcium treatment is 1595°C.

[0094] RH vacuum refining: the vacuum degree is increased to 90 Pa, the vacuum circulation time is prolonged to 17 min, the proportion of the first addition of cerium-iron alloy is increased to 65%, 0.70 kg / t of 30% cerium-iron alloy is added, the stirring time is prolonged to 5.5 min, the remaining part is added 3 min before the RH breaks the vacuum, 0.37 kg / t of 30% cerium-iron alloy is added, and the vacuum circulation is continued for 3 min to ensure that the total oxygen content of the molten steel after the RH is controlled to be less than 15 ppm.

[0095] Hot rolling: the heating temperature is increased to 1175°C, the holding time is prolonged to 1.5 h, the initial rolling temperature is 1125°C, the final rolling temperature is 825°C, the reduction ratio of each pass is increased to 22% in the rough rolling stage, the inter-pass air cooling temperature is reduced by 45°C per pass, the reduction ratio of each pass is increased to 16% in the last three passes in the finish rolling stage, the inter-pass air cooling temperature is reduced by 25°C per pass, the total deformation is controlled to be 80%, the cooling rate is increased to 11°C / s, and the final cooling temperature is 575°C.

[0096] Tempering treatment: quenching temperature increased to 875℃, holding time extended to 50min, tempering temperature increased to 635℃, holding time extended to 1.5h.

[0097] Straightening treatment: straightening temperature increased to 175℃, straightening deformation controlled at 0.75%, preheating temperature before straightening increased to 165℃.

[0098] Example 3 oil well pipe steel preparation method, the difference only in example 1 is:

[0099] LF refining: top slag white slag time extended to 24min, top slag (FeO+MnO) content controlled at 0.5%, top slag thickness 140mm (top slag amount: 5.0t), outgoing molten steel temperature 1600℃, sulfur content 0.002%.

[0100] Calcium treatment: wire feeding amount increased to 440m, soft blowing time extended to 8min, molten steel temperature after calcium treatment 1600℃.

[0101] RH vacuum refining: vacuum degree increased to 100Pa, vacuum cycle time extended to 18min, the first time the proportion of cerium iron alloy increased to 70%, 1.12kg / t of 30% cerium iron alloy was added, stirring time extended to 6min, the remaining part was added 3min before RH breaking vacuum, 0.48kg / t of 30% cerium iron alloy was added, and vacuum cycle was continued for 3min, ensuring that the total oxygen content of RH outgoing molten steel was controlled below 15ppm.

[0102] Hot rolling: heating temperature increased to 1200℃, holding time extended to 2h, rolling temperature 1150℃, finishing temperature 850℃, each pass reduction rate increased to 25% in rough rolling stage, interpass air cooling temperature drop 50℃ / pass, single pass reduction rate increased to 18% in the last 3 passes in finishing rolling stage, interpass air cooling temperature drop 30℃ / pass, total deformation controlled at 85%, cooling speed increased to 12℃ / s, final cooling temperature 600℃.

[0103] Tempering treatment: quenching temperature increased to 900℃, holding time extended to 1h, tempering temperature increased to 660℃, holding time extended to 2h.

[0104] Straightening treatment: straightening temperature increased to 200℃, straightening deformation controlled at 1.0%, preheating temperature before straightening increased to 180℃.

[0105] Example 4 oil well pipe steel preparation method, the difference only in example 1 is:

[0106] LF refining: top slag white slag time extended to 26 min, top slag (FeO + MnO) content controlled at 0.5%, top slag thickness 140 mm (top slag amount: 5.0 t), outgoing molten steel temperature 1605℃, sulfur content 0.002%.

[0107] Calcium treatment: calcium wire feeding amount increased to 460 m, soft blowing time extended to 9 min, molten steel temperature after calcium treatment 1605℃.

[0108] RH vacuum refining: vacuum degree increased to 100 Pa, vacuum circulation time extended to 19 min, first time cerium-iron alloy addition ratio increased to 70%, 1.55 kg / t 30% cerium-iron alloy added, stirring time extended to 6.5 min, second time remaining part added 3 min before RH breaking vacuum, 0.65 kg / t 30% cerium-iron alloy added, vacuum circulation continued for 3 min, ensuring RH outgoing molten steel total oxygen content controlled below 15 ppm.

[0109] Hot rolling: heating temperature increased to 1210℃, holding time extended to 2.5 h, rolling start temperature 1160℃, finishing temperature 860℃, rough rolling stage each pass reduction ratio increased to 25%, interpass air cooling temperature drop 50℃ / pass, last 3 passes of finishing rolling stage single pass reduction ratio increased to 18%, interpass air cooling temperature drop 30℃ / pass, total deformation controlled at 85%, cooling speed increased to 12℃ / s, final cooling temperature 600℃.

[0110] Quenching and tempering treatment: quenching temperature increased to 910℃, holding time extended to 1 h, tempering temperature increased to 650℃, holding time extended to 2 h.

[0111] Straightening treatment: straightening temperature increased to 210℃, straightening deformation controlled at 1.0%, preheating temperature before straightening increased to 185℃.

[0112] Comparative Examples 1-3

[0113] The present application also provides three comparative examples, the chemical composition of the steel plates of which is shown in Table 1. Comparative Example 1 is a common oil well pipe N80, Comparative Example 2 is a rare earth-free Ce oil well pipe steel. Comparative Example 3 is a 5Cr-Mo oil well pipe.

[0114] Comparative Example 1

[0115] Preparation method of common oil well pipe N80 (the composition of which is shown in Table 1):

[0116] LF refining: traditional LF refining is adopted.

[0117] Calcium treatment: calcium wire is used to modify inclusions.

[0118] RH refining: high-pressure nitrogen gas is used as lifting gas to further complete degassing, desulfurization and nitrogen addition.

[0119] Hot rolling: continuous casting billet heating temperature 1150-1220℃, finishing temperature 920-950℃, coiling temperature 500-600℃.

[0120] Comparative Example 2

[0121] The preparation method of the rare earth-free Ce oil well pipe steel differs from that of Example 1 only in that: RH vacuum refining: no rare earth Ce is added. The remaining steps are the same as those of Example 1.

[0122] Comparative Example 3

[0123] The preparation method of the 5Cr-Mo oil well pipe differs from that of Example 1 only in that: RH vacuum refining: no rare earth Ce is added, but 0.18% Mo is added. The remaining steps are the same as those of Example 1.

[0124] Table 1 Chemical composition, wt%

[0125] Steel No. Comparative Example 1 Comparative Example 2 Comparative Example 3 Example 1 Example 2 Example 3 Example 4 C 0.35 0.071 0.070 0.060 0.064 0.071 0.073 Si 0.14 0.35 0.35 0.45 0.40 0.40 0.35 Mn 1.51 1.21 1.20 1.40 1.35 1.30 1.21 P 0.015 0.010 0.010 0.010 0.010 0.010 0.010 S 0.002 0.0019 0.0019 0.0017 0.018 0.0020 0.0019 Cr / 5.05 5.01 5.01 5.02 5.04 5.02 Mo / / 0.18 / / / / V 0.11 0.024 0.024 0.025 0.025 0.025 0.025 Al 0.030 0.020 0.022 0.023 0.027 0.029 0.023 Ce / / / 0.075 0.016 0.024 0.033 O / 0.0019 0.0019 0.0017 0.0019 0.0018 0.0017

[0126] The mechanical properties of the example are shown in Table 2, the yield strength is ≥580 MPa, the tensile strength is ≥689 MPa, the elongation after fracture is ≥23%, the 0℃ impact energy KV2 is ≥80 J, all of which meet the requirements of the API SPEC 5CT standard of N80 oil well pipe steel.

[0127] Table 2 Mechanical properties

[0128]

[0129] In order to illustrate the corrosion resistance of the steel plate of the present application, the corrosion rate data of the steel of the example and the comparative example in the 168-hour autoclave accelerated corrosion test (test conditions: 70℃, CO2 partial pressure 1Mpa, Cl - content 25000mg / L, total pressure 6MPa) were used to measure the corrosion resistance, and the corrosion resistance of the steel plate of the present application was evaluated by this test. The results of the corrosion test are shown in Table 3.

[0130] Table 3 168-hour autoclave accelerated corrosion test corrosion rate data

[0131] Steel No. 168 hour autoclave corrosion test corrosion rate (mm / a) Relative N80 corrosion rate Comparative Example 1 (N80) 1.510 100％ Comparative Example 2 0.400 26.5％ Comparative Example 3 0.374 24.8％ Example 1 0.370 24.5％ Example 2 0.353 23.4％ Example 3 0.346 22.9％ Example 4 0.340 22.5％

[0132] From Table 1 and Table 3, it can be seen that the corrosion rate of Comparative Example 1 is about 3.8 times that of Comparative Example 2, and it can be seen that the corrosion resistance of the oil well pipe steel can be greatly improved after adding only 5% Cr.

[0133] From Table 1 and Table 3, it can be seen that the corrosion rate of Comparative Example 3 is further improved compared to Comparative Example 2, which indicates that Mo can improve the corrosion resistance of the steel. The corrosion rate of Comparative Example 3 is significantly higher than that of Example 2, although the addition amount of Mo is much larger than that of Ce, but Example 2 containing Ce still shows better corrosion resistance, which indicates that Ce has a greater effect on improving the corrosion resistance of the steel.

[0134] From Table 1 and Table 3, it can be seen that as the rare earth content increases, the corrosion resistance of the test steel also increases, and the corrosion resistance of the oil well pipe with the highest addition of 330 ppm Ce (Example 4) is improved by 15% compared to that without Ce (Comparative Example 2).

[0135] From Table 1 and Table 3, it can be seen that the corrosion resistance of Examples 1-4 is more than 4 times that of Comparative Example 1, and the corrosion resistance of Example 4 (330 ppm Ce) is 4.44 times that of Comparative Example 1.

[0136] From the above analysis, it can be seen that the present application is to develop an economical corrosion-resistant 5Cr80 rare earth oil well pipe steel. Corrosion of oil well pipes always exists a big problem, and the use of oil well pipes is large, which is very sensitive to price. The present application improves the corrosion resistance of oil well pipes by selecting inexpensive corrosion-resistant metal Cr and high-abundance rare earth Ce, which ensures that the oil well pipe has good corrosion resistance and is inexpensive. The economical corrosion-resistant 5Cr80 rare earth oil well pipe steel has good corrosion resistance and is inexpensive, which will have good market competitiveness and application prospect.

[0137] The above description is only a preferred embodiment of the present application, but the protection scope of the present application is not limited thereto, and any changes or replacements within the technical range disclosed by the present application can be easily thought by those skilled in the art, which should be covered within the protection scope of the present application.

Claims

1. An economical corrosion-resistant 5Cr80 rare earth oil well pipe steel, characterized in that: The 5Cr80 rare earth oil well pipe steel comprises, by mass percentage, C: 0.05%-0.08%, Si: 0.30%-0.50%, Mn: 0.80%-1.5%, P: ≤0.01%, S: ≤0.0020%, O: ≤0.0020%, Ce: 0.0050%-0.0350%, Cr: 4.5%-5.5%, Al: ≤0.035%, V: 0.020%-0.030%, and the balance is Fe and unavoidable impurities. The oil well pipe steel has a yield strength of ≥580 MPa.

2. The economical corrosion-resistant 5Cr80 rare earth oil well pipe steel according to claim 1, characterized in that: The microstructure of the 5Cr80 rare earth oil well pipe steel is a tempered bainite structure, and the average size of rare earth inclusions in the structure does not exceed 5 μm.

3. The economical corrosion-resistant 5Cr80 rare earth oil well pipe steel according to claim 1, characterized in that: The mass ratio of Ce / S in the 5Cr80 rare earth oil well pipe steel is ≥5, and the mass ratio of Ce / O is ≥5.

4. The economical corrosion-resistant 5Cr80 rare earth oil well pipe steel according to claim 1, characterized in that: The mass ratio of Cr / C in the 5Cr80 rare earth oil well pipe steel is controlled at 60-90:

1.

5. The economical corrosion-resistant 5Cr80 rare earth oil well pipe steel according to claim 1, characterized in that: The tensile strength of the 5Cr80 rare earth oil well pipe steel is ≥689 MPa, and the elongation after fracture is ≥23%.

6. The economical corrosion-resistant 5Cr80 rare earth oil well pipe steel according to claim 1, characterized in that: The 5Cr80 rare earth oil well pipe steel has an impact energy KV2 of 80J or more at 0°C.

7. The economical corrosion-resistant 5Cr80 rare earth oil well pipe steel according to claim 1, characterized in that: The 5Cr80 rare earth oil well pipe steel has a 168-hour autoclave corrosion rate of ≤0.37 mm / a under conditions of a CO2 partial pressure of 1 MPa, a Cl- content of 25,000 mg / L, and 70°C.

8. A method for preparing the economical corrosion-resistant 5Cr80 rare earth oil well pipe steel according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1. The continuous casting billet is obtained by smelting and refining outside the furnace; S2 billet forging after hot rolling, heating temperature of 1150 ~ 1200 ℃, holding time 1 ~ 2h, rolling temperature of 1100 ~ 1150 ℃, the final rolling temperature of 800 ~ 850 ℃, to obtain hot-rolled billet; S3. The hot-rolled steel billet is quenched and tempered at 850-900°C and held for 40 min to 1 h, then tempered at 610-660°C and held for 1 to 2 h. S4. Air-cooling the quenched and tempered steel billet to produce oil well pipe steel.

9. The preparation method according to claim 8, characterized in that The off-furnace refining in S1 includes LF refining, calcium treatment and RH vacuum refining, and ferrocerium alloy is added during the RH vacuum refining process for rare earth treatment, and the Ce yield is controlled to be 45% to 55%.

10. The preparation method according to claim 9, characterized in that The ferrocerium alloy is added in two steps: the first step is to add 60% to 70% of the total mass and stir for ≥5 minutes; the second step is to add the remaining ferrocerium alloy 3 minutes before RH breaking.

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