Martensitic stainless steel with high strength and hydrogen sulfide stress corrosion resistance and manufacturing method thereof

Abstract

The martensitic stainless steel comprises the following components in percentage by weight: 0.01 to 0.04 percent of C, 0.1 to 0.4 percent of Si, 0.20 to 1.0 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.005 percent of S, less than or equal to 0.004 percent of O, 0.01 to 0.1 percent of Al, 0.0001 to 0.004 percent of Ca, 12.0 to 14.0 percent of Cr, 4.0 to 6.0 percent of Ni, 1.0 to 3.0 percent of Mo, less than or equal to 0.020 percent of N, 0.01 to 0.15 percent of Nb, 0.03 to 0.15 percent of V, 0.0001 to 0.3 percent of rare earth elements and the balance of Fe and inevitable impurities. The yield strength of the martensitic stainless steel reaches 125ksi grade (the yield strength is larger than or equal to 862MPa), and meanwhile, the martensitic stainless steel meets the application capacity in an H2S partial pressure environment containing 0.1 Bar and a high-concentration CO2 environment with the temperature as high as 177 DEG C; products such as bars, plates and steel pipes can be manufactured, and the device can be used for petroleum or natural gas production equipment and CO2 sealing equipment.

Description

Technical Field

[0001] This invention relates to martensitic stainless steel, and more specifically to a high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion and its manufacturing method. Background Technology

[0002] With the increasing number of deep and ultra-deep wells being developed, products such as corrosion-resistant alloy casing and tubing must evolve towards higher strength. Martensitic stainless steel, due to its relatively low alloy cost and excellent CO2 corrosion resistance, has a wide range of applications in high-CO2 oil and gas resource development and CCUS (Complex Corrosion Under Scrap) applications. However, as the lowest-cost corrosion-resistant alloy, it is widely used in these fields. But when H2S is present in the application environment, the resistance to sulfide stress corrosion cracking of traditional martensitic stainless steel, and even super-martensitic stainless steel, becomes a key factor limiting its application. The ISO 15156-3 standard limits the upper strength of super-martensitic stainless steel resistant to sulfide stress corrosion cracking to 724 MPa. This forces the use of expensive super duplex stainless steel or even nickel-based alloys in oil and gas resource development with high CO2 content and small amounts of H2S. Therefore, improving the resistance to sulfide stress corrosion cracking of martensitic stainless steel is crucial to expanding its applications.

[0003] Chinese Patent Publication No. CN115768914 discloses a martensitic stainless steel with a yield strength of over 125 ksi (862 MPa), excellent low-temperature toughness and excellent corrosion resistance in ultra-low temperature environments, and its manufacturing method. The martensitic stainless steel of this patent has the following composition: C: less than 0.030%, Si: less than 1.00%, Mn: 0.05-2.00%, Cr: 11.50-14.00%, Ni: 5.00-7.50%, Mo: 1.10-3.50%, Cu: 0.50-3.50%, Co: 0.01-0.30%, Al: 0.001-0.100%, N: 0.001-0.100%, and the balance being Fe and impurities. Its microstructure consists of 0-15 vol% retained austenite, 0-10 vol% ferrite, and the balance being martensite. The yield strength is above 862 MPa, and the number density of Cu precipitates is 3.0 × 10⁻⁶. 21 ~50.0×10 21 pcs / m 3 .

[0004] Chinese Patent Publication No. CN109563581 discloses a high-strength stainless steel seamless pipe for oil wells, possessing excellent low-temperature toughness, resistance to carbon dioxide corrosion, resistance to sulfide stress corrosion cracking, and resistance to sulfide stress cracking. The seamless steel pipe contains, by mass%, less than 0.05% C, less than 0.5% Si, 0.15-1.0% Mn, less than 0.030% P, less than 0.005% S, 14.5-17.5% Cr, 3.0-6.0% Ni, 2.7-5.0% Mo, 0.3-4.0% Cu, 0.1-2.5% W, 0.02-0.20% V, and less than 0.10% Al. N: less than 0.15%, C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy a specific formula, Cu, Mo, W, Cr, and Ni satisfy other specific formulas, the balance consists of Fe and unavoidable impurities, with martensite phase: greater than 45%, ferrite phase: 10-45%, retained austenite phase: less than 30%, the total amount of precipitated Cr, precipitated Mo, and precipitated W is less than 0.75% by mass, and the yield strength is greater than 862 MPa.

[0005] Chinese Patent Publication No. CN104884658 discloses a method, by mass percent, containing the following components to satisfy -5.9×(7.82+27C-0.91Si+0.21Mn-0.9Cr+Ni-1.1Mo+0.2Cu+11N)≥13.0: C: less than 0.05%, Si: less than 0.5%, Mn: 0.15~1.0%, P: less than 0.030%, S: less than 0.005%, Cr: 15.5~17.5%, Ni: 3.0~6.0%, Mo: 1.5~5.0%, Cu: less than 4.0%, W: 0.1~2.5%, and N: less than 0.15%. Therefore, it is possible to manufacture high-strength stainless steel seamless pipes with excellent corrosion resistance, exhibiting excellent resistance to carbon dioxide corrosion in high-temperature environments (up to 200°C) containing CO2 and Cl-, and excellent resistance to sulfide stress cracking and sulfide stress corrosion cracking in corrosive environments containing H2S. Additionally, it may contain V, and / or Al, and / or one or more of Nb, Ti, Zr, and B, and / or one or two of REM, Ca, and Sn. "High strength" as used herein refers to a yield strength of 110 kSi, i.e., a yield strength of 758 MPa or higher.

[0006] Chinese Patent Publication No. CN106414785 discloses a high-strength stainless steel seamless pipe for oil wells with excellent hot workability, resistance to sulfide stress corrosion cracking, and corrosion resistance, and its manufacturing method. The pipe has a composition containing Cr and Ni in a manner satisfying Cr / Ni ≤ 5.3. Furthermore, it has a microstructure dominated by tempered martensite, with a surface microstructure in which a white phase, etched using a Villeroy & Boch etchant, has a thickness of 10 μm to 100 μm in the wall thickness direction from the outer surface of the pipe. The white phase, etched using a Villeroy & Boch etchant, is dispersed at a concentration of 50% or more on the outer surface of the pipe. The composition, by mass percent, includes C: 0.005–0.05%, Si: 0.05–1.50%, Mn: 0.2–1.8%, P: less than 0.02%, S: less than 0.005%, Cr: 11–18%, Ni: 0.10–8.0%, Mo: 0.6–3.5%, with the balance being Fe and unavoidable impurities. This steel pipe possesses high strength, with a yield strength exceeding 654 MPa.

[0007] In recent years, the development of deep and ultra-deep wells has been increasing, leading to a growing demand for corrosion-resistant alloy materials with high yield strength (≥862MPa) above 125ksi and resistance to high concentrations of CO2 and small amounts of H2S. Currently, corrosion-resistant alloys capable of handling H2S content down to 0.1 bar typically require iron-nickel based alloys or nickel-based alloys, but these are expensive.

[0008] The four patents mentioned above propose different martensitic stainless steels with resistance to sulfide stress corrosion. Martensitic stainless steel, as the most economical corrosion-resistant alloy, has a significant cost advantage. However, Chinese patent publication number CN115768914 proposes the addition of precious metals such as Cu and Co. The addition of Cu and Co will reduce the hot working performance of the steel, thereby affecting the surface quality and yield.

[0009] Chinese patent publications CN109563581 and CN1048846583 propose solutions with a Cr content greater than 14.5%, i.e., 17Cr. While CN109563581 proposes a material with a yield strength of over 862 MPa, it incorporates a large amount of precious metals such as Mo and W. CN104884658 also proposes a high-strength 110 steel grade, i.e., over 758 MPa, and incorporates precious metals such as Cu and W. The 17Cr steels involved in CN109563581 and CN1048846583 have a high ferrite content, posing a significant risk of cracking during hot working. Furthermore, the high austenite content can easily lead to excessively high austenite content during implementation, making it difficult to achieve the required high strength.

[0010] Chinese Patent Publication No. CN106414785 proposes a relatively traditional martensitic stainless steel, which has resistance to sulfide stress corrosion, but its strength is only above 655 MPa. Summary of the Invention

[0011] The purpose of this invention is to provide a high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion and its manufacturing method. The martensitic stainless steel has a high strength of yield strength of 125 ksi (yield strength ≥ 862 MPa) or higher, and can be used in environments with partial pressure of H2S containing 0.1 Bar and high concentration of CO2 at temperatures up to 177°C. It can be used to manufacture products such as bars, plates and steel pipes for use in oil or natural gas production equipment and CO2 storage equipment.

[0012] To achieve the above objectives, the technical solution of the present invention is as follows:

[0013] In order to reduce alloy costs and improve hot working performance, this invention controls the amount of alloying elements such as Cr and Mo added to reduce the ferrite content, and does not add Cu, which is prone to copper embrittlement during hot working.

[0014] Specifically, the high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion described in this invention has the following composition by weight percentage: C: 0.01-0.04%, Si: 0.1-0.4%, Mn: 0.20-1.0%, P≤0.02%, S≤0.005%, O≤0.004%, Al: 0.01-0.1%, Ca: 0.0001-0.004%, Cr: 12.0-14.0%, Ni: 4.0-6.0%, Mo: 1.0-3.0%, N≤0.020%, Nb: 0.01-0.15%, V: 0.03-0.15%, rare earth elements: 0.0001-0.3%, and the balance includes Fe and other unavoidable impurity elements; wherein, rare earth elements include, but are not limited to, Nd, Ce, Er, Pr, Pm, Dy or La.

[0015] Furthermore, the remainder consists of Fe and other unavoidable impurity elements.

[0016] Preferably, C: 0.012%–0.035%, and / or, Si: 0.1%–0.3%, and / or, Mn: 0.2%–0.5%, and / or, P ≤ 0.015%, and / or, O ≤ 0.003%, and / or, Al: 0.015%–0.08%, and / or, Ca: 0.0005%–0.0035%, Cr: 12.2%–13.5%, and / or, Ni: 4.5%–5.5%, and / or, Mo: 1.5%–2.5%, and / or, N ≤ 0.015%, and / or, Nb: 0.015%–0.1%, and / or, V: 0.05%–0.10%, and / or, rare earth elements: 0.001%–0.2%.

[0017] The microstructure of the martensitic stainless steel described in this invention comprises: 2-10% austenite by volume, 0-2% ferrite, and the remainder being tempered martensite; wherein the austenite is the sum of retained austenite and inverted austenite.

[0018] The martensitic stainless steel described in this invention has a yield strength ≥862MPa, an SSC resistance of 80% SMYS threshold value under H2S partial pressure of 0.1Bar, and a corrosion rate ≤0.1mm / a in a high-concentration CO2 environment at a temperature of 177℃.

[0019] In the martensitic stainless steel described in this invention, the B and D type inclusions, mainly composed of Ca and Al oxides, are rated below level 1.0.

[0020] In the compositional design of the martensitic stainless steel described in this invention:

[0021] In martensitic stainless steel, carbon (C) acts as an austenite-forming element. Increasing the C content can increase the percentage of austenitization at high temperatures, thereby obtaining martensite at room temperature and improving strength. Furthermore, C in this invention also plays a role in regulating the ratio of Nb and V precipitates; to achieve these effects, the C content must be above 0.01%. However, excessive C content will decrease the corrosion resistance and toughness of the stainless steel. Therefore, in this invention, to obtain better performance, the C content is limited to 0.01%–0.04%, preferably 0.012%–0.035%.

[0022] Si is an important deoxidizer in steelmaking, but in stainless steel with high Cr content, Si risks promoting the formation of σ phase and ferrite phase. σ phase and ferrite have adverse effects on the toughness and corrosion resistance of stainless steel. Therefore, this invention limits the Si content to 0.1–0.4%, preferably 0.1–0.3%.

[0023] Mn can improve the strength of stainless steel. In this invention, to ensure the required strength for use as an oil casing, Mn is added at 0.2% or more. However, if Mn exceeds 1.0%, the toughness decreases. Therefore, this invention limits the Mn content to 0.2% to 1.0%, preferably 0.2% to 0.5%.

[0024] Phosphorus (P) is a harmful element that reduces CO2 corrosion resistance at high temperatures and also adversely affects hot working properties. If the P content exceeds 0.02%, the corrosion resistance cannot meet the requirements of high-temperature environments. Therefore, this invention limits the P content to ≤0.02%, preferably P≤0.015%.

[0025] Sulfur (S) is a harmful element that reduces hot working properties and adversely affects impact toughness. If the S content exceeds 0.005%, steel pipes cannot be manufactured properly. Therefore, this invention limits the S content to ≤0.005%.

[0026] Oxygen (O) is a harmful element in steel, causing poor toughness. Furthermore, when O forms complex inclusions with elements like Al and Ca, it incompletely affects the passivation film on the stainless steel surface, leading to localized corrosion and electrochemical corrosion in corrosive media, thus reducing resistance to general corrosion and stress corrosion. To ensure corrosion resistance, the O content should be controlled to ≤0.004%, preferably ≤0.003%.

[0027] Al is used as a deoxidizer in the smelting process. When the Al content is too low, it cannot achieve the desired deoxidation effect. However, when the Al content is too high, it cannot exert a greater deoxidation effect either. Furthermore, excessively high Al content can cause the molten steel to become viscous, making it difficult for the slag to float and resulting in an increase in the number and size of inclusions. Therefore, this invention controls the Al content to be 0.01–0.1%, preferably 0.015–0.08%.

[0028] Ca treatment can modify inclusions and has a positive effect on improving the internal quality and anisotropy of steel. However, when the Ca content in steel is too high, it can have some adverse effects on the steel's properties. Excessive calcium may lead to segregation in the steel, that is, uneven distribution of calcium in the steel. This uneven distribution may affect the mechanical properties and processing properties of the steel, and may also form large inclusions. These inclusions may have poor distribution and stability in the steel, thus adversely affecting the toughness of the steel. Therefore, in this invention, the Ca content is controlled at 0.0001-0.004%, preferably 0.0005-0.0035%.

[0029] Cr is an important element for improving the corrosion resistance of stainless steel. The addition of Cr allows the stainless steel surface to quickly form a corrosion-resistant passivation film even in air, improving the resistance of oil casing to CO2 corrosion at high temperatures. To obtain CO2 corrosion resistance at 177°C, the Cr content in the stainless steel system of this invention must reach 12.0% or more. On the other hand, in the alloy system of this invention, the addition of Cr exceeding 14% increases the risk of ferrite precipitation, which adversely affects the hot working properties and corrosion resistance of the product. Therefore, this invention limits the Cr content to 12.0% to 14.0%, preferably 12.2% to 13.5%.

[0030] Ni expands the austenite region, thereby improving the corrosion resistance and toughness of stainless steel, especially its resistance to stress corrosion cracking under high-temperature conditions. To achieve this effect, the Ni content should be greater than or equal to 4.0%. However, Ni is also a relatively expensive alloying element, and its content in the stainless steel system of this invention does not exceed 6%. Therefore, this invention limits the Ni content to 4.0–6.0%, preferably 4.5–5.5%.

[0031] Mo is an element that enhances the resistance of stainless steel to Cl ion pitting corrosion. To achieve corrosion resistance in high-temperature environments above 150°C, at least 1.0% Mo needs to be added in this invention. However, Mo is a precious metal element, and if the Mo content exceeds 3.0% in this invention, a large amount of ferrite will be formed, which will adversely affect the hot working properties and corrosion resistance of the product. Therefore, this invention limits the Mo content to 1.0–3.0%, preferably 1.5–2.5%.

[0032] Nitrogen (N) can improve the pitting resistance of stainless steel. As an austenite-forming element, N can increase the martensite ratio of stainless steel and thus improve its strength. However, in this invention, N is a residual element. Since N causes distortion in the crystal lattice, it will reduce the impact toughness of stainless steel. In order to improve the low-temperature impact toughness of steel, this invention controls the N content to be ≤0.020%, preferably N≤0.015%.

[0033] Nitrogen (Nb) is an element that inhibits the precipitation of inverse austenite during the tempering process. The amount of austenite directly affects the strength of martensitic stainless steel. By adding Nb, the precipitation of austenite during tempering is effectively suppressed, thus preserving the high strength characteristics of martensite. To obtain the above-mentioned beneficial effects, the amount of Nb added should be ≥0.01%. If the Nb content is greater than 0.15%, its beneficial effects cannot be further exerted, and the cost of the alloy is significantly increased. Therefore, this invention limits the Nb content to 0.01–0.15%, preferably 0.015–0.1%.

[0034] Vanadium (V) is a precipitation strengthening element. In this invention, V is added to form V carbides during annealing before cold rolling. The vanadium carbide formed during annealing becomes nucleation sites during cold rolling, changing the direction of dislocation movement and thus refining the grain size in the subsequent quenching process. To achieve this effect, the amount of V added should be above 0.03%. However, a V content exceeding 0.15% will increase strength but decrease toughness. Therefore, this invention limits the V content to 0.03–0.15%, preferably 0.05–0.10%.

[0035] The rare earth elements mentioned in this invention include, but are not limited to, Nd, Ce, Er, Pr, Pm, Dy, and La. Adding rare earth elements to molten steel in the later stages of smelting can effectively purify the steel. The oxides formed by the combination of rare earth elements and O in the molten steel have good dispersibility and are not prone to agglomeration, thus ensuring that the B and D type inclusions related to Ca and Al oxides are controlled below level 1.0.

[0036] It is particularly noteworthy that the combined addition of Nb and rare earth elements in this invention modifies the size of the inclusions. Furthermore, the presence of rare earth and Nb elements in the Ca and Al oxide inclusions reduces the potential difference between the inclusions and the matrix, minimizing the risk of localized electrochemical corrosion and improving corrosion resistance. To achieve these effects, the amount of rare earth elements added should be above 0.0001%. However, excessively high amounts of rare earth elements can hinder the flotation of steel slag and easily lead to nodule formation during steel casting, affecting the casting process. To obtain beneficial effects, the rare earth element content is limited to the range of 0.0001% to 0.3%, preferably within the range of 0.001% to 0.2%.

[0037] The microstructure of the martensitic stainless steel of the present invention after final heat treatment comprises 2-10% austenite, 0-2% ferrite, and the remainder being tempered martensite. The austenite is the sum of retained austenite and inverted austenite. In this invention, "the remainder being tempered martensite" means that the volume fraction of other microstructures besides austenite, ferrite, and tempered martensite is negligible. That is, the total amount of other precipitates and inclusions besides austenite, ferrite, and tempered martensite is negligible.

[0038] The microstructure of the steel of this invention contains 2-10% austenite, where austenite refers to the sum of retained austenite and inverted austenite. Retained austenite refers to the austenite content of the steel in the quenched state, and inverted austenite refers to the austenite content precipitated in the steel after tempering. The austenite content is measured by X-ray diffraction on the tempered steel. The addition of Nb in this invention can control the austenite volume fraction. If the austenite volume fraction is too high, the strength controllability is poor. To ensure the steel meets the requirements of 125ksi steel grade (yield strength ≥ 862MPa), the upper limit of the austenite content is 10%, and the lower limit is 2%.

[0039] The microstructure of the steel of this invention contains 0-2% ferrite, which is a Cr-rich phase formed by the segregation of elemental components. The ferrite content is measured by metallographic methods according to ASTM E562 standard. The presence of a small amount of ferrite acts as a heterogeneous structure, hindering crack propagation and thus achieving better fracture toughness and resistance to stress corrosion cracking. However, ferrite content >2% reduces the impact properties and corrosion resistance of the steel.

[0040] The inclusion rating of the steel of this invention is based on the GB / T 10561 standard, which rates Class B and Class D inclusions that affect resistance to H2S stress corrosion cracking. To obtain better resistance to H2S stress corrosion cracking, both the coarse and fine series of Class B and Class D inclusions are ≤1.0.

[0041] In summary, through extensive research, the inventors discovered that Nb plays a unique role in inhibiting the precipitation of inverted austenite. By adding Nb, under the same quenching and tempering heat treatment conditions, the precipitation of inverted austenite can be effectively reduced. Furthermore, a major mechanism of stress corrosion cracking in martensitic stainless steel in H2S environments is the localized micro-area corrosion occurring on the surface of an incomplete passivation film. By controlling the phase ratio of austenite, ferrite, and tempered martensite, and through the combined addition of rare earth elements and Nb, the inclusions are modified, resulting in Ca, Al, and other oxide inclusions below grade 1.0, and reducing the potential difference between the inclusions and the matrix, significantly improving its resistance to H2S stress corrosion.

[0042] The ability to perform applications in environments containing H2S and high concentrations of CO2 was verified using the following methods.

[0043] 1) The test piece was immersed in a 5% NaCl aqueous solution (liquid temperature: 24±3℃, H2S atmosphere at 0.1 atm, CO2 atmosphere at 0.9 atm) with an aqueous solution containing acetic acid and sodium acetate added to adjust the pH to 4.2. The immersion time was set to 720 hours, and 80% of the nominal yield strength was applied as the stress. The test piece did not crack after the test.

[0044] 2) The test piece was immersed in a 25% NaCl aqueous solution (liquid temperature: 177±3℃, H2S atmosphere at 0.1 atm, CO2 atmosphere at 30 atm) with an aqueous solution containing acetic acid and sodium acetate added to adjust the pH to 4.2. The immersion time was set to 720 hours. The corrosion weight loss rate of the test piece after the test was less than 0.1 mm / a.

[0045] The shape of the high-strength, hydrogen sulfide-resistant, stress-corrosion-resistant martensitic stainless steel described in this invention is not particularly limited; it can be a bar, plate, or tube.

[0046] The method for manufacturing high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion according to the present invention includes the following steps:

[0047] 1) Smelt and cast steel billets according to the above composition;

[0048] 2) The steel billet is heated to 1150-1200℃ and then hot-formed;

[0049] 3) Quenching treatment, quenching temperature is AC3+30℃, holding time t1=(3~10)×h, t1 is in min; h is plate thickness in mm; then cool to below 150℃ at a cooling rate greater than 0.5℃ / s.

[0050] 4) Tempering treatment, tempering temperature is 500~650℃, holding time t2=(5~15)×h, t2 is in min; h is the plate thickness in mm; then air cool to room temperature.

[0051] Preferably, step 1) involves smelting in a converter, electric furnace, or vacuum induction furnace, followed by continuous casting, ingot casting, primary rolling, or forging to produce steel billets.

[0052] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0053] Existing martensitic stainless steels with a yield strength of 125 ksi (yield strength ≥ 862 MPa) and resistance to hydrogen sulfide stress corrosion and CO2 corrosion are produced by adding alloying elements such as Ni, Mo, and Cu, which is costly and results in poor hot working performance due to the addition of Cu.

[0054] This invention uses Cr, Ni, and Mo as the main alloying components. By controlling the amount of alloying elements such as Cr and Mo added, the content of ferrite is reduced, and Cu, which is prone to copper embrittlement during hot working, is not added. Furthermore, it does not add expensive alloying elements such as Mo and Cu, thus having the advantages of low cost and good hot working performance.

[0055] Existing martensitic stainless steels with yield strengths reaching 125 ksi (yield strength ≥ 862 MPa) and resistance to hydrogen sulfide stress corrosion and CO2 corrosion all have a high ferrite content, resulting in a high risk of cracking during hot working. Furthermore, the high austenite content can easily lead to excessively high austenite levels during implementation, making it difficult to achieve the required high strength.

[0056] The martensitic stainless steel of this invention contains 2-10% austenite, 0-2% ferrite, and the remainder is tempered martensite in its microstructure; the austenite is the sum of retained austenite and inverted austenite. The martensitic stainless steel of this invention exhibits high strength, reaching a yield strength of 125 ksi (yield strength ≥ 862 MPa), and its resistance to susceptibility to susceptibility to corrosion cracking (SSC) at a H₂S partial pressure of 0.1 Bar can reach 80% of the SMYS threshold value. Furthermore, its corrosion rate in a high-concentration CO₂ environment at 177°C is ≤ 0.1 mm / a. It can be used to manufacture bars, plates, and pipes for use in oil or natural gas production equipment and CO₂ storage equipment.

[0057] The martensitic stainless steel of the present invention has a grade of less than 1.0 for Class B and Class D inclusions, which are mainly composed of Ca and Al oxides.

[0058] The mechanism by which rare earth element REM improves sulfide stress corrosion cracking is not clear in existing technologies. This invention proposes that the combined addition of rare earth element and Nb element can improve the size of inclusions and reduce the potential difference between inclusions and the matrix, thereby improving the resistance of high-strength martensitic stainless steel to hydrogen sulfide stress corrosion cracking. Detailed Implementation

[0059] The present invention will be further described below with reference to the embodiments.

[0060] The components of the embodiments of the present invention are shown in Table 1, with the remainder being Fe and unavoidable impurities; Table 2 shows the manufacturing process parameters of the embodiments of the present invention; Table 3 shows the performance parameters of the embodiments of the present invention.

[0061] The following tests were conducted on the steel after the above heat treatment.

[0062] Yield strength test: The steel was processed into threaded tensile test specimens according to ASTM A370 standard, and the average value was obtained after testing according to ASTM E8 standard, as shown in Table 3.

[0063] Charpy V-type impact absorption energy (i.e. impact toughness) test: Take a V-type impact specimen with a transverse volume of 10*10*55 (mm) on the steel, test it according to GB / T 229 standard, take the average value, and convert it to the full size of 10*10*55 (mm) according to API 5CT standard and list it in Table 3. The test temperature is -10℃.

[0064] Austenite content testing: Full-thickness samples were taken from the steel, and the volume fraction of austenite on a cross-section perpendicular to the rolling direction was determined using a D8 DISCOVER X-ray diffractometer and TOPAS4 analysis software. The austenite content of the examples and comparative examples is listed in Table 3.

[0065] Determination of ferrite volume fraction: Following ASTM E562-2019 standard, 30 metallographic photographs covering the entire thickness of the steel section along the rolling direction were taken longitudinally using the equidistant method. The ferrite volume fraction was then determined using the point counting method. The ferrite content of the examples and comparative examples is listed in Table 3.

[0066] Inclusion rating: GB / T 10561 standard was adopted, and the steel was sampled longitudinally along the full thickness section along the rolling direction for rating. The rating results of Class B and Class D coarse and fine inclusions in the examples and comparative examples are listed in Table 3.

[0067] H2S stress corrosion test: Method A in NACE TM0177 standard was adopted. The solution was 5% NaCl (liquid temperature: 24±3℃, H2S atmosphere at 0.1 atm, CO2 atmosphere at 0.9 atm) with added aqueous solution containing acetic acid and sodium acetate to adjust the pH to 4.2. The immersion time was set to 720 hours. 80% of the nominal yield strength was applied as the stress. After the test, the samples were observed for macroscopic and microscopic cracks by visual inspection and 10X stereomicroscope. The results are listed in Table 3.

[0068] Corrosion test at high temperature H2S, CO2, Cl - Coexistence corrosion test: The test piece was immersed in a 25% NaCl aqueous solution (liquid temperature: 177±3℃, H2S atmosphere at 0.1 atm, CO2 atmosphere at 30 atm) with an aqueous solution containing acetic acid and sodium acetate added to adjust the pH to 4.2. The immersion time was set to 720 hours. The corrosion weight loss rate of the test piece after the test is listed in Table 3.

[0069] As shown in Table 3, the stainless steel and the steel obtained by the manufacturing method of the present invention have high strength with a yield strength YS of over 862 MPa, and simultaneously meet the requirements of resistance to stress corrosion cracking and high-temperature corrosion in environments containing H2S and high concentrations of CO2. Compared with the prior art, it achieves the significant advantages of high strength and resistance to H2S corrosion without significantly adding high contents of precious metal elements such as Cr, Mo, and Cu.

[0070] It should be noted that the above embodiments are merely examples of high-strength, hydrogen sulfide-resistant, stress-corrosion-resistant martensitic stainless steel used to implement this application. This application is not limited to the above embodiments, and modifications within the scope of this application can achieve the technical effects pointed out in this application.

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Claims

1. A high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion, comprising the following composition by weight percentage: C: 0.01–0.04%, Si: 0.1–0.4%, Mn: 0.20–1.0%, P≤0.02%, S≤0.005%, O≤0.004%, Al: 0.01–0.1%, Ca: 0.0001–0.004%, Cr: 12.0–14.0%, Ni: 4.0–6.0%, Mo: 1.0–3.0%, N≤0.020%, Nb: 0.01–0.15%, V: 0.03–0.15%, rare earth elements: 0.0001–0.3%, with the balance including Fe and other unavoidable impurity elements; wherein, Rare earth elements include, but are not limited to, Nd, Ce, Er, Pr, Pm, Dy, or La.

2. The high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion as described in claim 1, characterized in that, The remainder consists of Fe and other unavoidable impurity elements.

3. The high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion as described in claim 1 or 2, characterized in that, C: 0.012%–0.035%, and / or, Si: 0.1%–0.3%, and / or, Mn: 0.2%–0.5%, and / or, P ≤ 0.015%, and / or, O ≤ 0.003%, and / or, Al: 0.015%–0.08%, and / or, Ca: 0.0005%–0.0035%, Cr: 12.2%–13.5%, and / or, Ni: 4.5%–5.5%, and / or, Mo: 1.5%–2.5%, and / or, N ≤ 0.015%, and / or, Nb: 0.015–0.1%, and / or, V: 0.05–0.10%, and / or, rare earth elements: 0.001–0.2%.

4. The high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion as described in claim 1, 2, or 3, characterized in that, The microstructure of the martensitic stainless steel includes: 2-10% austenite by volume, 0-2% ferrite, and the remainder is tempered martensite; wherein the austenite is the sum of retained austenite and inverted austenite.

5. The high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion as described in claim 1, 2, 3, or 4, characterized in that, The martensitic stainless steel has a yield strength ≥862MPa, an SSC resistance of 0.1Bar H2S partial pressure reaching 80% SMYS threshold value, and a corrosion rate ≤0.1mm / a in a high-concentration CO2 environment at a temperature of 177℃.

6. The high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion as described in claim 1, 2, 3, 4, or 5, characterized in that, In the martensitic stainless steel, the B and D type inclusions, mainly composed of Ca and Al oxides, are rated below level 1.

0.

7. The method for manufacturing high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion as described in any one of claims 1 to 6, characterized in that, Includes the following steps: 1) Smelting and casting into slabs according to the composition described in claim 1, 2 or 3; 2) The slab is heated to 1150-1200℃ and then hot-formed; 3) Quenching treatment, quenching temperature is AC3+30℃, holding time t1=(3~10)×h, t1 is in min; h is plate thickness in mm; then cool to below 150℃ at a cooling rate greater than 0.5℃ / s. 4) Tempering treatment, tempering temperature is 500~650℃, holding time t2=(5~15)×h, t2 is in min; h is the plate thickness in mm; then air cool to room temperature.

8. The method for manufacturing high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion as described in claim 7, characterized in that, Step 1) Smelting is carried out using converter, electric furnace or vacuum induction furnace, and then slabs are manufactured by continuous casting and ingot rolling.

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