A high-strength, high-corrosion-resistant, economical oil well pipe steel and its preparation method

Abstract

The present invention discloses a high-strength, high-corrosion-resistant, economical oil-well pipe steel and a preparation method thereof, belonging to the field of corrosion-resistant steel technology. It solves the problems of high cost and difficulty matching high strength and high corrosion resistance in the prior art for oil-well pipe steel. The preparation method of the oil-well pipe steel comprises: step 1: melting metal raw materials into molten steel; step 2: smelting the molten steel into a continuous casting billet or ingot; step 3: forging the continuous casting billet or ingot into a forging billet; hot-rolling the forging billet into a hot-rolled plate; step 4: solution-treating the hot-rolled plate, and subjecting the solution-treated hot-rolled plate to a quantitative cold rolling with a deformation of 5% to 25%, ultimately producing an oil-well pipe steel with a three-phase mixed structure of ferrite, austenite, and martensite. The oil-well pipe steel prepared by the preparation method of the present invention has high strength, good plasticity and toughness, and excellent corrosion resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of corrosion-resistant steel, and in particular to a high-strength, high-corrosion-resistant, economical oil well pipe steel and a preparation method thereof. Background Art

[0002] Oil well pipes are important consumables in the exploration and development of oil and natural gas. Due to the flow of corrosive liquids in oil well pipes, various corrosion-resistant products must be selected according to the corrosion conditions. Currently, the main material for oil well pipes that resist CO2 corrosion is 13Cr martensitic stainless steel, but martensitic steel cannot be used in environments containing H2S. Although the use of 22Cr (or 25Cr, 27Cr, the higher the Cr content, the better the corrosion resistance) duplex stainless steel with a higher alloy content can meet the requirements for oil well pipe use, the high raw material cost is not suitable for large-scale industrial applications in oil and gas extraction scenarios. In view of the actual situation of China's oil and gas extraction and the limitations of mineral resources, it is of great significance to develop an economical high-strength and corrosion-resistant oil well pipe material with high strength, high corrosion resistance and controllable costs.

[0003] At present, there are many patents at home and abroad on the varieties of oil well pipe steel with good comprehensive performance and the corresponding process flow, but most of them have problems such as high cost or complex process and cannot meet the needs of large-scale production and practical application. Summary of the Invention

[0004] In view of the above situation, the present invention aims to provide a high-strength, high-corrosion-resistant, economical oil well pipe steel and a preparation method thereof, so as to solve the problems of high cost and difficulty in matching high strength and high corrosion resistance of existing oil well pipe steel.

[0005] The purpose of the present invention is mainly achieved through the following technical solutions:

[0006] In one aspect, the present invention provides a method for preparing high-strength, high-corrosion-resistant, economical oil well pipe steel, comprising:

[0007] Step 1: Melt the metal raw materials into molten steel;

[0008] Step 2: smelting the molten steel into continuous casting billets or ingots;

[0009] Step 3: Forging the continuous casting billet or ingot into a forging billet; hot rolling the forging billet into a hot-rolled plate;

[0010] Step 4: subjecting the hot-rolled plate to a solution treatment, and subjecting the hot-rolled plate after the solution treatment to a quantitative cold rolling with a deformation amount of 5% to 25%, to finally produce an oil well pipe steel with a three-phase mixed structure of ferrite + austenite + martensite.

[0011] In a possible design, in step 3, the forging temperature is 1150-1250°C.

[0012] In one possible design, in step 3, the hot rolling is followed by water cooling to room temperature.

[0013] In one possible design, in step 4, the solution treatment process includes: keeping the temperature at 1000-1150° C. for 1-3 hours and then cooling with water.

[0014] On the other hand, the present invention also provides a high-strength, high-corrosion-resistant, economical oil well pipe steel, which is prepared by the above-mentioned preparation method.

[0015] In one possible design, the composition of the high-strength, high-corrosion-resistant, economical oil well pipe steel includes, by mass percentage, C: ≤0.03%, Cr: 17.4% to 18.4%, Mo: 0.5% to 2.5%, Ni: 1.0% to 2.8%, Mn: 1.0% to 3.5%, Si: 0.1% to 0.5%, N: 0.1% to 0.25%, P: ≤0.03%, S: ≤0.01%, with the balance being Fe and unavoidable impurities.

[0016] In a possible design, the composition of the high-strength, high-corrosion-resistant economical oil well pipe steel is Cr+3.3Mo+16N≥26.

[0017] In a possible design, the composition of the high-strength, high-corrosion-resistant economical oil well pipe steel is 50Ni+17Mn-12Mo-800N≤0.

[0018] In one possible design, the composition of the high-strength, high-corrosion-resistant, economical oil well pipe steel includes, by mass percentage, C: 0.01% to 0.03%, Cr: 17.4% to 18.3%, Mo: 1.1% to 2.2%, Ni: 1.0% to 2.4%, Mn: 1.2% to 3.2%, Si: 0.1% to 0.5%, N: 0.1% to 0.2%, P: ≤0.03%, S: ≤0.01%, with the balance being Fe and unavoidable impurities.

[0019] In a possible design, the composition of the high-strength, high-corrosion-resistant economical oil well pipe steel is 50Ni+17Mn-12Mo-800N≤-3.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] a) The high-strength, highly corrosion-resistant, economical oil-well tubular steel of the present invention precisely controls the mass percentages of Cr, Ni, Mn, Mo, and N in the steel. Combining a hot rolling, solutionizing, and cold rolling process, the steel achieves a mixed structure of ferrite, austenite, and martensite after solutionizing. Furthermore, through quantitative cold deformation, a portion of the austenite that has not undergone the quenching martensitic transformation undergoes a deformation-induced martensitic transformation, resulting in a mixed structure primarily composed of ferrite and martensite, with a certain amount of austenite. Because the martensite content of the present invention is close to 50%, the tensile strength of the oil-well tubular steel can reach grades 110 and 125. This multiphase structure creates a synergistic effect, resulting in a high-strength oil-well tubular steel comparable to martensitic stainless steel, but at a lower material cost. Compared to existing martensitic stainless steel, the multiphase structure of the present oil-well tubular steel exhibits superior resistance to intergranular corrosion and H2S stress corrosion. In addition, the stainless steel of the present invention has a certain austenite content and has better plasticity while having strength similar to that of martensitic stainless steel.

[0022] b) The steel of the present invention not only ensures high strength and good ductility, but also has excellent corrosion resistance. The yield strength of the steel of the present invention is greater than 800 MPa (e.g., 812-1020 MPa), the tensile strength is greater than 1000 MPa (e.g., 1095-1300 MPa), and the elongation is greater than 10% (e.g., 11%-20%). (180°C) CO2 corrosion rate is 0.035 g / m 2 h or less (e.g. 0.0289 to 0.0324 g / m 2 h); (200℃) CO2 corrosion rate 0.12g / m 2 h or less (e.g. 0.104 to 0.112 g / m 2 h); pitting corrosion rate 2.6g / m 2 h or less (e.g. 2.138 to 2.521 g / m 2 h); crevice corrosion rate 6.1g / m 2 h or less (e.g. 5.421 to 6.023 g / m 2 h); H2S stress corrosion test passed.

[0023] c) The steel of the present invention has excellent strength, toughness, and corrosion resistance, combining the high strength of super martensitic stainless steel oil well pipe with the stress corrosion, intergranular corrosion, and low-level H2S stress corrosion resistance of duplex stainless steel. Due to its low Cr and Ni content, the raw material cost of the steel of the present invention is comparable to or slightly lower than that of existing super 13Cr martensitic stainless steel. Furthermore, the steel of the present invention exhibits excellent hot workability, resulting in a low overall cost, economical operation, and excellent performance.

[0024] Other features and advantages of the present invention will be described in the following description, and in part will become apparent from the description or be understood by practicing the present invention. The purposes and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description, claims, and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like parts throughout the drawings.

[0026] Figure 1 This is a microstructure diagram of the steel of Example 1 of the present invention;

[0027] Figure 2 EBSD two-phase distribution diagram of the final structure of the steel of Example 1 of the present invention;

[0028] Figure 3 This is a microstructure diagram of the steel of Example 2 of the present invention;

[0029] Figure 4 This is a microstructure diagram of the steel of Example 3 of the present invention;

[0030] Figure 5 This is a diagram showing the H2S stress corrosion test results of Example 1 of the present invention. DETAILED DESCRIPTION

[0031] The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used to explain the principles of the present invention together with the embodiments of the present invention.

[0032] The present invention provides a method for preparing high-strength, high-corrosion-resistant, economical oil well pipe steel, comprising:

[0033] Step 1: Melt the metal raw materials into molten steel;

[0034] Step 2: smelting the molten steel into continuous casting billets or ingots by continuous casting or mold casting;

[0035] Step 3: After heating the continuous casting billet or ingot at 1150-1250°C and keeping the temperature, forging the billet to make a forging billet; heating the forging billet at 1050-1250°C and hot rolling it into a hot-rolled plate through multiple passes;

[0036] Step 4: Solution treat the hot-rolled plate, then cold-roll the solution-treated hot-rolled plate by a 5% to 25% deformation, ultimately producing an economical oil-well tubular steel with both high strength and corrosion resistance. The high-strength, high-corrosion-resistant economical oil-well tubular steel has a three-phase structure of ferrite, austenite, and martensite, with a volume percentage of 30% to 50% ferrite, 25% to 50% martensite, and 10% to 30% austenite.

[0037] Specifically, in the above steps 1 and 2, a converter or electric furnace is used for smelting, LF refining, and casting into ingots.

[0038] Specifically, in step 3 above, the forging temperature is 1150-1250°C. This is because the material of the present invention is a two-phase system consisting of ferrite and austenite at temperatures above 800°C. The ratio of the two phases varies with the heating temperature: the higher the heating temperature, the greater the ferrite content, and the corresponding decrease in austenite content. Selecting this temperature range for heating simultaneously ensures the full dissolution of harmful precipitated phases and increases the ferrite content in the material to greater than 70%. This allows the steel to be forged in a near-single-phase region with better thermoplasticity, avoiding forging cracking caused by the uncoordinated deformation of the two phases during the forging process due to the different thermal deformation capacities of the ferrite and austenite phases.

[0039] Specifically, in the above step 3, the hot rolling is followed by water cooling to room temperature to avoid precipitation of harmful phases.

[0040] Specifically, in step 4 above, solution treatment is performed before cold rolling to eliminate microstructural differences between different regions of the plate caused by the rolling temperatures (higher rolling temperature at the head end and lower rolling temperature at the tail end) during hot rolling. The solution treatment process includes holding at 1000-1150°C for 1-3 hours followed by water cooling.

[0041] Specifically, in the above step 4, since the austenite in the structure of the present invention is very likely to undergo martensitic transformation during the cold deformation process, the cold deformation amount needs to be controlled within a smaller range, for example, the cold deformation amount is controlled to be 5% to 25% (for example, 10% to 20%).

[0042] Specifically, the composition of the high-strength, high-corrosion-resistant, economical oil well pipe steel includes, by mass percentage, C: ≤0.03%, Cr: 17.4% to 18.4%, Mo: 0.5% to 2.5%, Ni: 1.0% to 2.8%, Mn: 1.0% to 3.5%, Si: 0.1% to 0.5%, N: 0.1% to 0.25%, P: ≤0.03%, S: ≤0.01%, and the balance being Fe and unavoidable impurities.

[0043] Specifically, the composition of the aforementioned high-strength, high-corrosion-resistant, economical oil well tubular steel is Cr + 3.3Mo + 16N (i.e., PREN) ≥ 26, and 50Ni + 17Mn - 12Mo - 800N ≤ 0. Cr, Mo, N, Ni, and Mn refer to the mass percentage of these elements multiplied by 100. For example, if the mass percentage of Cr is 18.4%, then Cr here is 18.4.

[0044] The following is a detailed description of the effects and dosage of the components in the present invention:

[0045] C: Although the addition of carbon can significantly improve the hardness and strength of steel, due to the high diffusion capacity of carbon, it is easy to combine with Cr to form M 23 C6 carbides precipitate rapidly (≤0.5 hours) primarily within the temperature range of 700-900°C, or when held at 550-700°C for extended periods. The precipitation of carbides causes localized Cr depletion, increasing the material's susceptibility to intergranular corrosion and deteriorating its corrosion resistance. Therefore, in the present invention, the combined effects of strength and corrosion resistance are considered, and its content is controlled to ≤0.03%.

[0046] Cr: The main element that ensures the corrosion resistance of stainless steel. It is also a ferrite-forming element. Improper control of Cr content will increase the ferrite content in the organization and reduce the strength. Considering comprehensive considerations, its content is controlled at 17.4% to 18.4%.

[0047] Ni: It is an element that strongly forms and stabilizes austenite and expands the austenite phase region. Ni is an element that improves the toughness and corrosion resistance of stainless steel in reducing media. Ni is also an important element for improving the austenite phase's resistance to transgranular stress corrosion in various media. At the same time, compared with Mn, Ni can reduce the cold work hardening tendency of the austenite phase. Considering cost control, the present invention controls its content within a range of 1.0% to 2.8%.

[0048] Mn: It can increase the solubility of nitrogen in steel and inhibit the precipitation of harmful phase chromium nitride, but Mn reduces the toughness of stainless steel. Compared with Ni, Mn is an economical austenite-forming element. Mn has different hardening effects on the austenite phase of stainless steel and the overall material. Therefore, the present invention controls its content within 1.0% to 3.5%.

[0049] Mo: In addition to improving the corrosion resistance of stainless steel in oxidizing media, it also has a good effect on improving the corrosion resistance of stainless steel in reducing media. At the same time, in order to ensure pitting resistance, alloying elements such as Cr and Mo that increase the PREN value should not be too low. At the same time, alloy cost control and phase balance must also be taken into account. Comprehensive considerations should be taken to control its content to 0.5% to 2.5%.

[0050] Si: It is added as a deoxidizer during the smelting process to achieve a deoxidizing effect. Excessive addition will deteriorate the material's intergranular corrosion resistance. The present invention controls its content to 0.1% to 0.5%.

[0051] Nitrogen: Like nickel, it is an element that strongly forms and stabilizes austenite and expands the austenite phase. Nitrogen also significantly improves the pitting corrosion resistance of the austenite phase in stainless steel, thereby improving the overall pitting corrosion resistance of the stainless steel. In some media, nitrogen has a positive effect on the stress resistance of stainless steel, and there is an optimal value. Further increasing the nitrogen content decreases its stress corrosion resistance. Nitrogen is also a relatively economical alloying element, but it reduces the hot workability of stainless steel. Therefore, the present invention controls its content to 0.1% to 0.25%.

[0052] S: It significantly reduces the hot working properties of steel and easily reacts with Mn to form MnS inclusions, which reduces the material's resistance to pitting and crevice corrosion. Therefore, its content is controlled to ≤ 0.01%.

[0053] P: It is a harmful impurity element that will not only deteriorate the plasticity of the material, but also reduce the corrosion resistance of the material. Therefore, its content is controlled to be ≤0.03%.

[0054] Specifically, this application comprehensively considers three aspects: ensuring corrosion resistance, raw material costs caused by alloy element matching, harmful phase precipitation caused by alloy element matching, and cost changes caused by hot working yield. In addition to controlling the content of individual elements of the main alloying elements Cr, Ni, Mn, and N in steel, in order to ensure corrosion resistance, Cr+3.3Mo+16N≥26 is controlled. In order to make the maximum precipitation temperature of harmful phases lower than 750°C and ensure that the material has good hot working properties, and considering that the three-phase ratio can be controlled during the hot working process between 900 and 1180°C, it is necessary to control the content of Ni, Mn, Mo, and N elements to meet the following requirements: 50Ni+17Mn-12Mo-800N≤0.

[0055] To further improve the comprehensive performance of the high-strength, high-corrosion-resistant, economical oil-well tubular steel, the composition of the high-strength, high-corrosion-resistant, economical oil-well tubular steel, in mass percentage, may be: C: 0.01%-0.03%, Cr: 17.4%-18.3%, Mo: 1.1%-2.2%, Ni: 1.0%-2.4%, Mn: 1.2%-3.2%, Si: 0.1%-0.5%, N: 0.1%-0.2%, P: ≤0.03%, S: ≤0.01%, with the balance being Fe and unavoidable impurities.

[0056] Specifically, 50Ni+17Mn-12Mo-800N≤-3.

[0057] Specifically, the microstructure of the high-strength, high-corrosion-resistant, economical oil-well tubular steel after solution treatment is a mixed structure of ferrite, austenite, and martensite, with the volume percentage of austenite being 10% to 30%. The austenite in this structure is in a metastable state and can further transform into martensite during plastic deformation. In particular, the quenched martensite already present in the structure of the present invention promotes the phase transformation during deformation (providing nucleation sites for the phase transformation and increasing the stress state of the austenite). By utilizing the characteristic that austenite in the structure after solution treatment readily undergoes martensitic transformation, a portion of the austenite phase undergoes deformation-induced martensitic transformation through quantitative cold rolling. With a relatively small deformation amount (5% to 25%), a high-strength, high-corrosion-resistant, economical oil-well tubular steel with a mixed structure of ultrafine martensite (25% to 50% by volume), ferrite (30% to 50% by volume), and austenite (10% to 30% by volume) can be obtained.

[0058] Specifically, in the microstructure of the high-strength, high-corrosion-resistant, economical oil well tubular steel after cold rolling, martensite is dispersed in the austenite in the form of ultrafine flakes, alternately stacked and divided with the austenite, resulting in a smaller austenite grain size (e.g., an austenite grain size of less than 3 μm), and the thickness of the martensite flakes is less than 100 nm.

[0059] Compared to existing technologies, the high-strength, highly corrosion-resistant, economical oil well pipe steel of the present invention features a three-phase structure primarily composed of ferrite and martensite with a limited amount of austenite. The steel's low Cr and alloying element content yields higher strength, while simultaneously offering high strength, resistance to intergranular corrosion and stress corrosion, high toughness, and excellent weldability, resulting in excellent overall performance. Furthermore, the high martensite content in the steel's structure provides superior mechanical strength properties to conventional ~22Cr duplex stainless steel, reaching grades 110 and 125. This multiphase composite structure also offers superior corrosion resistance to H2S stress corrosion and intergranular corrosion compared to single-phase martensitic stainless steel.

[0060] Compared to existing technologies, the high-strength, highly corrosion-resistant, economical oil well pipe steel of the present invention precisely controls the mass percentages of Cr, Ni, Mn, Mo, and N in the steel. Combining a hot rolling, solutionizing, and cold rolling process, the steel achieves a mixed structure of ferrite, austenite, and martensite after solutionizing. Furthermore, through quantitative cold deformation, the austenite portion of the structure that has not undergone the quenching martensitic transformation undergoes a deformation-induced martensitic transformation, resulting in a mixed structure composed primarily of ferrite and martensite, with a small amount of austenite. Because the steel of the present invention contains nearly 50% martensite, its tensile strength far exceeds that of duplex stainless steel, reaching grades 110 and 125, achieving high strength comparable to martensitic stainless steel in a multiphase structure. Compared to existing martensitic stainless steel, the multiphase structure of the present steel exhibits superior intergranular corrosion resistance and H2S stress corrosion resistance. Furthermore, the steel of the present invention, with its moderate austenite content, exhibits superior plasticity while maintaining strength comparable to martensitic stainless steel.

[0061] The steel of the present invention not only ensures high strength, good ductility and excellent corrosion resistance, but also ensures a yield strength greater than 800 MPa (e.g., 812-1020 MPa), a tensile strength greater than 1000 MPa (e.g., 1095-1300 MPa), and an elongation greater than 10% (e.g., 11%-20%). The CO2 corrosion rate at 180°C is 0.035 g / m 2 h or less (e.g. 0.0289 to 0.0324 g / m 2 h); (200℃) CO2 corrosion rate 0.12g / m 2 h or less (e.g. 0.104 to 0.112 g / m 2 h); pitting corrosion rate 2.6g / m 2 h or less (e.g. 2.138 to 2.521 g / m 2 h); crevice corrosion rate 6.1g / m 2 h or less (e.g. 5.421 to 6.023 g / m 2 h); H2S stress corrosion test passed.

[0062] The steel of this invention offers excellent strength, toughness, and corrosion resistance, combining the high strength of super martensitic stainless steel oil well pipe with the stress corrosion, intergranular corrosion, and low-level H2S stress corrosion resistance of duplex stainless steel. Due to its low Cr and Ni content, the raw material cost of this steel is comparable to or slightly lower than that of existing super 13Cr martensitic stainless steel. Furthermore, the steel exhibits excellent hot workability, resulting in a low-cost, economical, and excellent performance.

[0063] Examples 1-4

[0064] The advantages of precise control of the composition and process parameters of the steel of the present invention are demonstrated below with specific examples and comparative examples. 13Cr is selected as a comparison object for corrosion performance.

[0065] Examples 1-4 of the present invention provide a highly corrosion-resistant and economical oil well pipe steel and a preparation method thereof. The chemical compositions of the steels of Examples 1-4 are shown in Table 1.

[0066] The preparation method of Example 1 comprises:

[0067] Step 1: Melt the metal raw materials into molten steel; use the die casting method to smelt the molten steel into steel ingots;

[0068] Step 2: After heating the steel ingot to 1200℃ and keeping it at that temperature for 2 hours, it is forged and opened to form a forging billet, which is kept at 1200℃ for 2 hours and hot-rolled into a hot-rolled plate in multiple passes;

[0069] Step 3: The hot-rolled sheet is held at 1100°C for 1 hour and then water-cooled. Cold rolling is then performed at a controlled cold rolling deformation of 10%, yielding an economical oil well tubular steel with both high strength and corrosion resistance. In this step, the cold rolling deformation is controlled at 10% to adjust the volume fraction of austenite transforming to martensite. The final microstructure consists of 40% ferrite, 45% martensite, and 15% austenite.

[0070] The preparation method of Example 2 comprises:

[0071] Step 1: Melt the metal raw materials into molten steel; use the die casting method to smelt the molten steel into steel ingots;

[0072] Step 2: After heating the steel ingot to 1180℃ and keeping it at that temperature for 2 hours, it is forged and opened to form a forging billet, which is kept at 1210℃ for 2 hours and hot-rolled into a hot-rolled plate in multiple passes;

[0073] Step 3: The hot-rolled plate is held at 1050°C for 1.5 hours and then water-cooled. Cold rolling is then performed at a controlled cold rolling deformation of 20%, yielding an economical oil-well tubular steel with both high strength and corrosion resistance. In this step, the cold rolling deformation is controlled at 20% to adjust the volume fraction of austenite transforming to martensite. The final microstructure consists of 35% ferrite, 49% martensite, and 16% austenite.

[0074] The preparation method of Example 3 comprises:

[0075] Step 1: Melt the metal raw materials into molten steel; use the die casting method to smelt the molten steel into steel ingots;

[0076] Step 2: After heating the steel ingot to 1210℃ and keeping it at that temperature for 1 hour, it is forged and opened to form a forging billet. The forging billet is kept at 1190℃ for 2 hours and hot-rolled into a hot-rolled plate in multiple passes.

[0077] Step 3: The hot-rolled plate is held at 1140°C for 1 hour and then water-cooled. Cold rolling is then performed with a controlled cold rolling deformation of 10%, yielding an economical oil-well tubular steel with both high strength and corrosion resistance. In this step, the cold rolling deformation is controlled to 10% to adjust the volume fraction of austenite transforming to martensite. The final microstructure consists of 42% ferrite, 42% martensite, and 16% austenite.

[0078] The preparation method of Example 4 comprises:

[0079] Step 1: Melt the metal raw materials into molten steel; use the die casting method to smelt the molten steel into ingots;

[0080] Step 2: After heating the ingot to 1200℃ and keeping it at that temperature for 1 hour, it is forged and opened to form a forging billet, which is kept at 1200℃ for 2 hours and hot-rolled into a hot-rolled plate in multiple passes;

[0081] Step 3: The hot-rolled plate is held at 1150°C for 1 hour and then water-cooled. Cold rolling is then performed at a controlled cold rolling deformation of 15%, yielding an economical oil-well tubular steel with both high strength and corrosion resistance. In this step, the cold rolling deformation is controlled at 15% to adjust the volume fraction of austenite transforming to martensite. The final microstructure consists of 42% ferrite, 47% martensite, and 11% austenite.

[0082] The controlled cold rolling parameters of Examples 1-4 are shown in Table 1, and the mechanical properties of Examples 1-4 are shown in Table 2. Figure 1 This is a microstructure diagram of the steel of Example 1 of the present invention. The white arrows point to ferrite, and the gray phase at the black arrows is a mixed structure of martensite and austenite. Figure 2 This is an EBSD two-phase distribution diagram of the final structure of the steel of Example 1 of the present invention. The blue phase is austenite that has not undergone martensitic transformation and has a content of 15%.

[0083] Figure 3-Figure 4 The microstructures of the steels of Examples 2-4 of the present invention are shown below; the microstructures of Examples 1-4 are shown in Table 3 below. The microstructure of the economical oil well tubular steel of the present invention is a mixed structure of ferrite, austenite, and martensite, wherein the volume percentage of austenite is 10% to 30% (e.g., 11% to 16%), the volume percentage of martensite is 25% to 50% (e.g., 42% to 49%), and the volume percentage of ferrite is 30% to 50% (e.g., 35% to 45%).

[0084] In the cold-rolled microstructure of the high-strength, high-corrosion-resistant, economical oil-well tubular steels of Examples 1-4, ultrafine martensite is dispersed in the austenite in the form of flakes, alternately stacked and divided with the austenite, resulting in a smaller austenite grain size (e.g., an austenite grain size of <3 μm) and a martensite flake thickness of less than 100 nm.

[0085] Table 1 Chemical composition of Examples 1-4 (wt%)

[0086]

[0087] Table 2 Mechanical properties of Examples 1-4

[0088]

[0089] Table 3 Microstructure of the steel of Examples 1-4

[0090]

[0091] To demonstrate the corrosion resistance of the steel of the present invention, the corrosion resistance of the steels of Examples 1-4 was measured under five different conditions and compared with 13Cr under the same test conditions. This experiment evaluated the corrosion resistance of the steel of the present invention in different corrosive environments. The results of the corrosion test are shown in Table 4, and the corresponding test conditions are as follows:

[0092] (180℃ or 200℃) The corrosion conditions of CO2 corrosion rate are as follows:

[0093] 1. Medium (formation water) mg / L: CO3 2- / 0,HCO3 - / 189,OH - / 0,Cl - / 128000,SO4 2- / 430,Ca 2+ / 8310,Mg 2+ / 561,K + / 6620,Na + / 76500);

[0094] 2. CO2 partial pressure: 4.48MPa, total test pressure: 10MPa;

[0095] 3. Test temperature: 180℃ or 200℃;

[0096] 4. Test time: 720 hours.

[0097] The corrosion conditions for pitting corrosion rate are as follows:

[0098] 1. Test temperature: 22±2℃;

[0099] 2. Medium: 100g reagent grade ferric chloride FeCl3·6H2O dissolved in 900mL IV reagent grade water (mass ratio about 6%);

[0100] 3. Test time: 72 hours;

[0101] 4. Sample size: 50×25×3mm, polished to 2000# sandpaper.

[0102] The corrosion conditions for crevice corrosion rate are as follows:

[0103] 1. The test temperature is 22±2℃;

[0104] 2. Dissolve 100g of reagent-grade ferric chloride (FeCl3·6H2O) in 900mL of Type IV reagent-grade water (approximately 6% by mass).

[0105] 3. Test time: 72 hours;

[0106] 4. Sample size: 50×25×3mm, polished to 2000# sandpaper.

[0107] The corrosion conditions of the H2S stress corrosion test are as follows:

[0108] 1. The test temperature is 22±2℃;

[0109] 2. Dissolve 50 g NaCl, 25 g [23.8 mL] CH3COOH, and 4.1 g CH3COONa in 921 mL IV reagent grade water (approximately 5 wt% sodium chloride, 2.5 wt% glacial acetic acid, and 0.41 wt% sodium acetate by weight);

[0110] 3. Test time: 720 hours;

[0111] 4. Sample size: 50×5×2mm, polished to 2000# sandpaper;

[0112] 5. Four-point bending test, loading force is 80% yield stress.

[0113] As shown in Table 4, the (180°C) CO2 corrosion rate of the highly corrosion-resistant economical oil well pipe steel of Examples 1-4 is 0.035 g / m 2 h or less (e.g. 0.0289 to 0.0324 g / m 2 h); (200℃) CO2 corrosion rate 0.12g / m 2 h or less (e.g. 0.104 to 0.112 g / m 2 h); pitting corrosion rate 2.6g / m 2 h or less (e.g. 2.138 to 2.521 g / m 2 h); crevice corrosion rate 6.1g / m 2 h or less (e.g. 5.421 to 6.023 g / m 2h); H2S stress corrosion test was passed. The corrosion resistance of Examples 1-4 of the present invention is better than that of 13Cr, especially the CO2 corrosion resistance at 200°C. The present invention is far better than super 13Cr and can be used in more harsh environments, that is, in environments with greater well depths. Figure 5 Shown is the H2S stress corrosion test result of Example 1.

[0114] Table 4 Corrosion test data

[0115]

[0116] The inventors have conducted extensive research. To further highlight the advantages of the present invention in terms of performance and composition, super 13Cr martensitic stainless steel is now listed as a comparative example as follows:

[0117] Comparative Example 1

[0118] This comparative example provides a corrosion-resistant oil-well tubular steel and a preparation method thereof. The corrosion-resistant oil-well tubular steel in this comparative example is 13Cr and is prepared using a currently mature 13Cr process. The corrosion-resistant oil-well tubular steel in this comparative example has a composition of 0.03% C, 13% Cr, 5% Ni, 2% Mo, 0.5% Mn, and 0.5% Si. The preparation method involves heating and forging at 1150°C, water cooling, reheating to 860°C, holding for 2 hours, quenching, and tempering at 620°C for 1 hour.

[0119] The microstructure of the steel of this comparative example is martensite (93%) + austenite (7%). The mechanical properties are shown in Table 3 above, and the corrosion resistance results are shown in Table 4 above.

[0120] Comparison of the Examples and Comparative Examples shows that the high-strength, high-corrosion-resistant, economical oil-well tubular goods steel of the present invention achieves martensitic transformation of the austenite portion by precisely controlling the mass percentages of Cr, Ni, Mn, Mo, and N in the steel, combined with a specific hot rolling, solutionizing, and cold rolling process, thereby obtaining a mixed structure composed primarily of ferrite and martensite with a small amount of austenite. This significantly improves the corrosion resistance and toughness of the oil-well tubular goods steel, achieving strength comparable to that of the prior art super 13Cr martensitic stainless steel while lowering the raw material cost. Furthermore, the corrosion resistance of the steel of the present invention is superior to that of the super 13Cr martensitic stainless steel. Therefore, the steel of the present invention is low in overall cost, economical, and practical.

[0121] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing high-strength, high-corrosion-resistant, economical oil well pipe steel, characterized in that: The components of the high-strength, high-corrosion-resistant economical oil well pipe steel include, by mass percentage, C: ≤0.03%, Cr: 17.4%-18.4%, Mo: 1.1%-2.5%, Ni: 1.0%-2.8%, Mn: 1.0%-3.5%, Si: 0.1%-0.5%, N: 0.1%-0.14%, P: ≤0.03%, S: ≤0.01%, and the balance is Fe and unavoidable impurities; 50Ni+17Mn-12Mo-800N≤0; Mo, N, Ni, Mn refer to the mass percentage of these elements × 100; The preparation method comprises: Step 1: Melt the metal raw materials into molten steel; Step 2: smelting the molten steel into continuous casting billets or ingots; Step 3: Forging the continuous casting billet or ingot into a forging billet; hot rolling the forging billet into a hot rolled plate; the forging billet temperature is 1150~1250℃; Step 4: The hot-rolled plate is solution treated, and then the solution-treated hot-rolled plate is quantitatively cold rolled with a deformation amount of 5% to 25%, and finally an oil well pipe steel with a three-phase mixed structure of ferrite + austenite + martensite is produced; the content of the three-phase mixed structure is: 30% to 50% by volume of ferrite, 25% to 50% by volume of martensite, and 10% to 30% by volume of austenite; the martensite is dispersed in the austenite in the form of ultrafine flakes, alternately stacked with the austenite, and divided from each other, so that the austenite has a smaller grain size, the austenite grain size is less than 3 μm, and the thickness of the martensite flake is less than 100 nm.

2. The preparation method according to claim 1, characterized in that In step 3, the forging temperature is 1180-1250°C.

3. The preparation method according to claim 1, characterized in that In the step 3, the hot rolling is followed by water cooling to room temperature.

4. The preparation method according to any one of claims 1 to 3, characterized in that In step 4, the solution treatment process includes: keeping the temperature at 1000-1150° C. for 1-3 hours and then cooling with water.

5. A high-strength, high-corrosion-resistant, economical oil well pipe steel, characterized by: The preparation method is described in any one of claims 1 to 4.

6. The high-strength, high-corrosion-resistant, economical oil well pipe steel according to claim 5, characterized in that: In the composition of the high-strength, high-corrosion-resistant economical oil well pipe steel, Cr+3.3Mo+16N≥26; wherein Cr, Mo, and N refer to the mass percentage of these elements × 100.

7. The high-strength, high-corrosion-resistant, economical oil well pipe steel according to claim 5, characterized in that: The components of the high-strength, high-corrosion-resistant economical oil well pipe steel include, by mass percentage, C: 0.01%-0.03%, Cr: 17.4%-18.3%, Mo: 1.1%-2.2%, Ni: 1.0%-2.4%, Mn: 1.2%-3.2%, Si: 0.1%-0.5%, N: 0.1%-0.14%, P: ≤0.03%, S: ≤0.01%, and the balance is Fe and unavoidable impurities.

8. The high-strength, high-corrosion-resistant, economical oil well pipe steel according to any one of claims 5 to 7, characterized in that: The composition of the high-strength, high-corrosion-resistant economical oil well pipe steel is 50Ni+17Mn-12Mo-800N≤-3; wherein Mo, N, Ni, and Mn refer to the mass percentage of these elements × 100.

Images