A high-temperature resistant austenitic-ferritic duplex stainless steel with resistance to combined chloride / fluoride ion corrosion and its preparation method

Abstract

A high-temperature resistant austenitic-ferritic duplex stainless steel resistant to chloride / fluoride ion composite corrosion and its preparation method belong to the field of metallic materials technology. The steel composition includes: 0.20%–0.60% Si; 0.50%–1.50% Mn; 22.0%–26.0% Cr; 12.0%–16.0% Ni; 3.0%–5.0% Mo; 0.5%–1.5% Cu; 1.0%–2.5% W; 0.15%–0.25% N; 0.05%–0.12% RE; 1.8%–2.5% Al; 0.02%–0.08% Ti; 0.15%–0.25% Nb, etc. The preparation method includes smelting, hot working, solution treatment, and stabilization. This invention can form a strong oxide film by preferentially capturing fluoride ions with rare earth elements to form rare earth fluoride oxides, preventing F⁻ from damaging the protective film. It has an austenitic-ferrite dual-phase structure and possesses resistance to high-temperature oxidation, Cl⁻ pitting corrosion, and F⁻ active corrosion.

Description

Technical Field

[0001] This invention relates to corrosion-resistant steel and its preparation method, and particularly to an austenitic-ferritic duplex stainless steel resistant to high-temperature chloride / fluoride ion composite corrosion and its preparation method. Background Technology

[0002] As a key basic material supporting strategic emerging industries such as lithium batteries, semiconductors, and new energy, the supply and efficient recycling of graphite resources are directly related to the security of the national industrial chain. Waste graphite gas thermal purification technology, as a core means of achieving resource recycling, has become an inevitable trend in industry development due to its industrial application.

[0003] Waste graphite gas thermal purification equipment operates under extreme conditions of high temperature and chloride / fluorine composite corrosion, facing severe corrosion problems. Fly ash, impurities, and gases generated during production also cause erosion and friction wear on the metal surface, leading to further rapid corrosion. Chloride ions easily penetrate the passivation film, forming an "activation-repassivation" cycle, causing pitting corrosion; fluoride ions can dissolve the protective layer of metal oxides (such as Cr2O3, converting it into easily volatile CrO2F2 or CrF3), accelerating "active oxidation." Existing austenitic stainless steels (such as 310S and 316L) are widely used in many industrial and engineering fields due to their excellent corrosion resistance and high-temperature performance, but they also suffer from corrosion problems under high temperature and the simultaneous presence of fluoride ions (F... - ), chloride ions (Cl) - Under conditions of high temperature and stress corrosion cracking, austenitic stainless steel is prone to intergranular corrosion and stress corrosion cracking, exhibiting a high corrosion rate and thus not being fully suitable for multi-factor coupled corrosion conditions. Nickel-based alloys (such as Inconel 625 and Hastelloy C-276) offer excellent corrosion resistance but are extremely expensive and prone to failure in high-fluorine environments due to the formation of volatile nickel fluorides. Therefore, developing a steel grade that combines low cost, excellent high-temperature strength, resistance to chloride ion pitting corrosion, and resistance to fluoride ion active oxidation corrosion is of great significance.

[0004] To improve the high-temperature resistance of steel to fluoride ions (F... - ), chloride ions (Cl) -Regarding corrosion resistance, Chinese invention patent CN121852821A discloses a super austenitic stainless steel based on the synergistic effect of Mo and N and its preparation method. The stainless steel, by mass percentage, comprises: Cr 26.0%-26.5%, Mo 3.48%~4.52%, N 0.29%-0.51%, Ni 23.0%-25.0%, C≤0.01%, Mn 0.40%-0.65%, with the balance being Fe and unavoidable impurities. This invention, while maintaining a essentially unchanged PREN (i.e., chromium content plus 3.3 times molybdenum content plus 16 times nitrogen content) value, significantly improves the pitting corrosion resistance of stainless steel in chloride-containing media by synergistically reducing the Mo content and increasing the N content, utilizing the interaction between Mo and N. This synergistic effect is manifested in inhibiting pitting corrosion initiation, slowing pitting corrosion propagation, promoting repassivation, and forming a more protective passivation film structure. This material is particularly suitable for harsh corrosive environments such as marine oil and gas extraction and seawater desalination. Chinese invention patent CN121472730A also discloses an austenitic stainless steel tube for heat exchangers, its preparation method, and its application. The austenitic stainless steel tube for heat exchangers, by mass percentage, comprises the following components: C: ≤0.03%, Si: ≤0.75%, Mn: 3-6%, P: ≤0.04%, S: ≤0.03%, Cr: 16-19%, Ni: 4-7%, Mo: 2-4%, Cu: 1-3%, Hf: 0.3-0.6%, Ti: 0.2-0.5%, Co: 0.1-0.3%, and the balance being Fe and unavoidable impurities. The austenitic stainless steel tube for heat exchangers provided by this invention possesses excellent mechanical properties while being resistant to chloride ions and microbial corrosion. Chinese invention patent CN121451088A also discloses a thin-walled stainless steel tube for fluid transport, its preparation method, and its application. The thin-walled stainless steel pipe for fluid transportation, by mass percentage, comprises the following components: C: 0.01-0.03%, Si: 0.5-0.7%, Mn: 3-6%, P: ≤0.04%, S: ≤0.01%, Cr: 20-25%, Ni: 8-12%, N: 0.2-0.5%, Mo: 2-3%, Nb: 0.05-0.1%, Ce: 0.05-0.15%, Zr: 0.05-0.2%, Te: 0.01-0.02%, and the balance being Fe and unavoidable impurities. This invention, through optimized raw material formulation, yields a thin-walled stainless steel pipe for fluid transportation that exhibits excellent resistance to chloride ions and oxalic acid, and has broad application prospects in residential gas transportation.

[0005] Chinese invention patent CN121250031A discloses an optimized aging treatment method to suppress the precipitation of γ2 phase in solution-treated duplex stainless steel. Addressing the problems of excessive γ2 phase precipitation and poor passivation film stability caused by existing processes (700℃, 8-32 hours), this method reduces the precipitation of low-chromium γ2 phase at the α / γ phase interface by shortening the aging time to 0.5-4 hours. The method includes a solution treatment stage (1100℃, 2 hours, air cooling), an aging treatment stage (700℃, 0.5-4 hours), and furnace cooling. The optimized material exhibits a significantly increased pitting potential, reduced corrosion current density, and improved pitting corrosion resistance by over 30%. It also avoids complex equipment or alloy additions, making it suitable for industrial production. Experiments show that aging within 4 hours can effectively suppress γ2 phase precipitation, enhance passivation film stability and resistance to chloride ion corrosion, reduce pit formation, and significantly extend the service life of the material in nuclear energy, marine, and other environments. Chinese invention patent CN121992289A also discloses a ferritic stainless steel and its applications, heat exchanger components, and a heat exchanger. This ferritic stainless steel comprises the following components by mass percentage: C≤0.025%, Si≤0.8%, Mn≤1.0%, P≤0.04%, S≤0.03%, 17%≤Cr≤20%, 1.5%≤Mo≤2.5%, 0.5%≤Nb≤0.8%, Ti≤0.02%, with the balance being Fe and unavoidable impurities. The ferritic stainless steel provided by this invention possesses high thermal conductivity and low coefficient of thermal expansion, as well as excellent resistance to sensitized intergranular corrosion and chloride ion corrosion. Therefore, it exhibits excellent thermal conductivity and fatigue resistance. Furthermore, the production cost of this ferritic stainless steel is low. Especially when applied to heat exchangers, the heat exchangers demonstrate excellent thermal conductivity and a long service life. This ferritic stainless steel has excellent market application prospects, particularly in the application of heat exchangers, where it possesses significant potential advantages. Chinese invention patent CN120272815A also discloses a stainless steel oil casing for carbon dioxide flooding enhanced oil production and its manufacturing method. The stainless steel oil casing for carbon dioxide flooding enhanced oil production has the following chemical composition by weight percentage: C: ≤0.03%, Si: 0.15%~0.3%, P: ≤0.010%, S: ≤0.005%, Cr: 14.0%~18.0%, Mn: 0.5%~2%, Ni: 2.0%~3%, Mo: 1.0%~3.0%, N: 0.20%~0.30%, Cu: 0.5%~1.0%, Nb: 0.025%~0.1%, V: 0.015%~0.025%, Ca: 0.001%~0.003%, Re: 0.01%~0.03%, with the balance being Fe and unavoidable impurities.The stainless steel oil casing of this invention has a yield strength of 758MPa~931MPa, a tensile strength greater than 862MPa, and an elongation of ≥30%. It has good strength and toughness matching, excellent corrosion resistance and low temperature impact toughness. It is suitable for service temperature range of -50℃~180℃ and can be applied to low temperature environments caused by carbon dioxide capture and injection-production enhancement, as well as corrosive media conditions in which two or three of the following coexist: carbon dioxide, hydrogen sulfide and chloride ions. Chinese invention patent CN117867384A also discloses a martensitic stainless steel with excellent stress corrosion resistance and its manufacturing method. The chemical composition of the steel, by weight percentage, includes: 0.10–0.20% C; 0.2–0.5% Si; 0.4–0.8% Mn; ≤0.030% P; ≤0.005% S; 11.5–13.0% Cr; 0.006%–0.012% Ce; 0.15%–0.35% Al; 0.08–0.15% Ta; 0.005–0.010% Zr; 0.04%–0.08% N; (Ta+Zr+Al) / N: 4.0–7.5, with the balance being Fe and unavoidable impurities. The steel plate production method includes smelting, continuous casting, heating, rolling, and heat treatment. The composite effect of elements promotes the formation of a highly dense passivation film, Ta3N5. Through optimized design of the quenching-partitioning process, the microstructure of the steel is controlled to be tempered martensite + 6–9 vol.% stable retained austenite. The steel of this invention exhibits a yield strength ≥919 MPa, tensile strength ≥1126 MPa, elongation ≥17%, room temperature impact energy ≥94 J, and a stress corrosion cracking time ≥192 h under a 650 MPa load and a chloride ion concentration of 10 mg / L aqueous solution at ≥95°C.

[0006] To address the problems of poor corrosion resistance, high cost, and susceptibility to intergranular corrosion and stress corrosion cracking in existing technologies of steel in high-temperature chloride / fluoride ion composite environments, this invention provides a duplex stainless steel and its preparation method that achieve synergistic improvement in heat resistance and composite corrosion resistance through rare earth microalloying and reasonable phase ratio control. Summary of the Invention

[0007] The purpose of this invention is to provide a heat-resistant and corrosion-resistant steel and its preparation method, ensuring stable operation of waste graphite gas thermal purification equipment under extreme conditions of high temperature and chlorine / fluorine composite corrosion. To solve the above problems, the technical solution adopted by this invention is:

[0008] 1. Chemical composition (mass percentage, %)

[0009] To achieve the above objectives, the chemical composition of the steel of the present invention consists of the following elements (total mass 100%):

[0010] C: 0.03%~0.08%; Si: 0.20%~0.60%; Mn: 0.50%~1.50%; Cr: 22.0%~26.0%; Ni: 12.0%~16.0%; Mo: 3.0%~5.0%; Cu: 0.5%~1.5%; W: 1.0%~2.5%; N: 0.15%~0.25%; Rare earth elements (RE): 0.05%~0.12% (preferably one or more of Ce, La, or Y; furthermore, if two or three are present, the content of each element shall not be less than 0.01%); Ti: 0.02%~0.08%; B: 0.0015%~0.0050%; Nb: 0.15%~0.25%; Al: 1.8%~2.5%; P: ≤0.025%; S: ≤0.025%, with the balance being Fe and unavoidable impurities.

[0011] 2. Alloy Design Principles

[0012] Cr (22-26%): The main corrosion-resistant element, forming a dense Cr2O3 film at high temperatures. High Cr content can resist Cl⁻ damage and reduce the formation rate of volatile fluorides in F-containing environments.

[0013] Al (1.8-2.5%): A core element that preferentially forms a dense, continuous Al2O3 film on the surface at high temperatures, prior to Cr. This film grows extremely slowly and exhibits very high stability against sulfur and chlorine. Content below 1.8% makes it difficult to form a continuous protective film; content above 2.5% leads to severe embrittlement of the steel, making it difficult to hot work.

[0014] Ni (12-16%): Stabilizes the austenitic phase, improving high-temperature plasticity and resistance to stress corrosion. Synergistically with Cr and Mo, it enhances the re-repair capability of the passivation film.

[0015] Mo (3-5%): Significantly improves the stability of the passivation film, especially against Cl. - The addition of Mo can cause pitting corrosion. Mo can form MoO4 under the film. 2- This inhibits Cl⁻ adsorption.

[0016] W (1.0-2.5%): Solid solution strengthening element, improving high-temperature creep strength. Synergistically with Mo, it forms non-volatile tungsten oxide in high-temperature F-containing environments, stabilizing the protective film.

[0017] Cu (0.5-1.5%): Promotes cathode depolarization and enhances passivation in acidic reducing media. It also forms a CuCl precipitate layer in high-temperature, chlorine-containing environments, physically isolating Cl-. - erosion.

[0018] Nb (0.15-0.25%): Forms stable carbonitrides with C and N, refining grain size and improving high-temperature strength and creep resistance. This invention employs a composite alloying technology combining Nb microalloying and Cu alloying to improve the high-temperature corrosion resistance of steel. Nb can form a corrosion-resistant Z-phase around non-metallic inclusions (such as MgAl2O4 and MnS), encapsulating harmful inclusions and significantly improving the interfacial bonding strength between the inclusions and the matrix. This effectively prevents localized corrosion caused by inclusions, thereby reducing the rate of localized corrosion propagation and significantly improving the corrosion resistance of the steel. Furthermore, the formed NbC can refine the grain size, greatly improving the overall mechanical properties of the steel. The introduction of Cu can significantly increase the electrode potential of the matrix and form second-phase particles, further enhancing corrosion resistance and precipitation strengthening.

[0019] C (0.03-0.08%): An essential solid solution strengthening element to ensure high-temperature strength. Too low a concentration will result in insufficient strength, while too high a concentration will lead to the formation of a large amount of carbides, impairing toughness and corrosion resistance.

[0020] N (0.15-0.25%): Interstitial strengthening element, improving yield strength. It also inhibits the precipitation of σ phase (brittle phase) at high temperatures and increases the pitting corrosion equivalent value (PREN, i.e., chromium content plus 3.3 times molybdenum content plus 16 times nitrogen content).

[0021] Rare earth elements (RE, 0.05-0.12%): Core innovation. Rare earth elements (Ce, La, Y) can purify grain boundaries (reacting with S and O) and refine grains; more importantly, rare earth oxides (such as CeO2) can preferentially adsorb fluoride ions to form stable rare earth fluoride oxides (such as CeOF), preventing fluoride from penetrating the Cr2O3 film and significantly alleviating "fluorine-induced active oxidation".

[0022] Ti (0.02-0.08%): Stabilizes carbonitrides and prevents chromium depletion at grain boundaries.

[0023] Boron (0.0015-0.0050%): Adding 0.0015-0.0050% boron to stainless steel can improve its corrosion resistance. Adding trace amounts of boron to austenitic stainless steel can improve its hot plasticity. Boron has extremely low solubility (<0.0082% in the γ phase). Excessive or improper addition will precipitate brittle borides such as (Fe, Cr)₂B, which are distributed in a network along grain boundaries, leading to increased hot brittleness, cold cracking, and a higher tendency for welding hot cracking. Generally, a boron content >0.005% significantly impairs toughness and machinability.

[0024] 3. Micro-organizational control

[0025] This invention controls the room temperature microstructure of steel to be a two-phase structure consisting of 60%-75% austenite (γ) phase and 25%-40% ferrite (δ) phase (the two-phase structure is calculated based on a total volume of 100%). The two-phase structure effectively prevents the propagation of intergranular cracks and combines the toughness of austenite with the stress corrosion resistance of ferrite.

[0026] 4. Preparation method

[0027] This invention also provides a method for preparing the above-mentioned austenitic-ferritic duplex stainless steel, comprising the following steps:

[0028] Smelting: Materials are added according to chemical composition, with nitrogen added in the form of ferrochrome nitride. Induction furnace smelting or electric arc furnace smelting + ladle refining (AOD / VOD) process is employed. The smelting temperature is controlled between 1610℃ and 1630℃. During the refining period, corresponding rare earth elements (in the form of rare earth ferrosilicon master alloy), ferroboron, and ferrotitanium are added to control desulfurization and deoxidation. The chemical composition and mass fraction of the molten steel in the furnace are controlled as follows: 0.03%–0.08% C; 0.20%–0.60% Si; 0.50%–1.50% Mn; 22.0%–26.0% Cr; 12.0%–16.0% Ni; 3.0%–5.0% Mo; 0.5%–1.5% Cu; 1.0%–2.5% W; 0.15%–0.25% N; 0.05%–0.12% RE (preferably one or more of Ce, La, or Y); 0.02%–0.08% Ti; 0.0015%–0.0050% B; 0.15%–0.25% Nb; 1.8%–2.5% Al; ≤0.025% P; ≤0.025% S, with the balance being Fe and unavoidable impurities;

[0029] Casting, forging / rolling: When the molten steel temperature drops to 1520℃-1545℃, it is cast into an ingot in a metal mold; after the ingot solidifies, it is removed from the mold, and after removing the risers and grinding, the ingot is reheated to 1180℃-1220℃ and held for 2-3 hours, then forged or hot-rolled into a steel plate with a thickness of 6mm-20mm; the final forging or final rolling temperature is not lower than 980℃; the deformation is controlled at 50%-80%;

[0030] Solution treatment: for steps The steel plate undergoes solution treatment at a temperature of 1050℃-1080℃ (preferably 1060℃). The holding time is calculated as 20min + 0.5min / mm × steel plate thickness (i.e., based on 20min, an additional 0.5min is added for each mm of steel plate thickness). Subsequently, rapid water cooling is performed (cooling rate ≥ 50℃ / s) to control the precipitated phase.

[0031] Stabilization aging: Stabilization treatment is carried out after solution treatment; the stabilization treatment temperature is 850℃-900℃, the temperature is held for 2-4 hours, and then air-cooled, which is beneficial to improve the resistance to intergranular corrosion.

[0032] Surface pre-passivation film: Before use, austenitic-ferritic duplex stainless steel is treated in an oxygen-containing atmosphere at 500℃-600℃ for 1-2 hours to promote the formation of a Cr-rich and RE-rich oxide film.

[0033] Further in the steps In this process, the rare earth elements are added in the form of rare earth ferrosilicon master alloy 3-5 minutes before tapping, and the slag is ensured to be reducing.

[0034] This steel is used to manufacture thermal purification equipment for waste graphite gas in a temperature range of 500℃-900℃ where the medium contains corrosive components such as chloride and fluoride ions.

[0035] Beneficial effects

[0036] Compared with the prior art, the present invention has the following advantages:

[0037] (1) Dual-phase synergistic corrosion resistance: austenite provides good resistance to pitting corrosion, ferrite provides resistance to stress corrosion cracking, and the dual-phase interface breaks the continuous corrosion channel of Cl⁻.

[0038] (2) Rare earth elements solve the "fluorine corrosion problem": The addition of rare earth elements significantly reduces the rate of damage to the oxide film by F⁻ at high temperatures. In the test of simulated waste graphite gas thermal purification flue gas (containing 200ppm HCl + 50ppm HF, 600℃), the weight loss rate of the steel of this invention is only 1 / 4 of that of traditional 310S stainless steel, and no "peeling" phenomenon is observed on the surface.

[0039] (3) Low cost and high performance: Compared with Hastelloy C-276, the nickel content is significantly reduced (12-16% vs 55% or more), and the raw material cost is reduced by about 40-50%.

[0040] (4) Excellent high-temperature mechanical properties: yield strength ≥280 MPa and tensile strength ≥520 MPa at 600℃, meeting the requirements for steel for gas thermal purification of waste graphite. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to the embodiments, but the present invention is not limited to the following embodiments.

[0042] Example 1:

[0043] The specific process steps of a high-temperature resistant austenitic-ferritic duplex stainless steel with chloride / fluoride ion composite corrosion and its preparation method are as follows:

[0044] Smelting: According to the chemical composition, nitrogen is added in the form of ferrochrome nitride. The process involves induction furnace smelting followed by ladle refining (AOD). The smelting temperature is controlled at 1610℃. During the refining period, rare earth elements (in the form of rare earth ferrosilicon master alloy), ferroboron, and ferrotitanium are added to control desulfurization and deoxidation. The chemical composition and mass fraction of the molten steel in the furnace are controlled as follows: 0.03%; 0.57% Si; 0.66% Mn; 25.83% Cr; 12.46% Ni; 4.85% Mo; 0.52% Cu; 1.19% W; 0.24% N; 0.05% Ce; 0.08% Ti; 0.0015% B; 0.25% Nb; 1.84% Al; 0.021% P; 0.013% S, with the balance being Fe and unavoidable impurities.

[0045] Casting and forging: When the molten steel temperature drops to 1520℃, it is poured into a metal mold to form an ingot; after the ingot solidifies, it is removed from the mold, and after removing the risers and grinding, the ingot is reheated to 1180℃ and held for 3 hours for forging into a 6mm steel plate; the final forging temperature is 992℃; the deformation is controlled at 80%;

[0046] Solution treatment: for steps The steel plate undergoes solution treatment; the heating temperature is 1060℃, and the holding time is 23 min (calculated based on 20 + 0.5 min / mm × steel plate thickness); followed by rapid water cooling (cooling rate ≥ 50℃ / s) to control the precipitated phase;

[0047] Stabilization aging: Stabilization treatment is performed after solution treatment; the stabilization treatment temperature is 850℃, the holding time is 4 hours, and then air cooling is performed, which is beneficial to improving the resistance to intergranular corrosion.

[0048] Surface pre-passivation film: Before use, austenitic-ferritic duplex stainless steel is treated in an oxygen-containing atmosphere at 500℃ for 2 hours to promote the formation of Cr-rich and RE-rich oxide film; the specific mechanical properties of duplex stainless steel are shown in Table 1.

[0049] Example 2:

[0050] The specific process steps of a high-temperature resistant austenitic-ferritic duplex stainless steel with chloride / fluoride ion composite corrosion and its preparation method are as follows:

[0051] Smelting: According to the chemical composition, nitrogen is added in the form of ferrochrome nitride. The process involves electric arc furnace smelting followed by vat-out refining (VOD). The smelting temperature is controlled at 1630℃. During refining, rare earth elements (in the form of rare earth ferrosilicon master alloy), ferroboron, and ferrotitanium are added to control desulfurization and deoxidation. The chemical composition and mass fraction of the molten steel in the furnace are controlled as follows: 0.08% C; 0.27% Si; 1.48% Mn; 22.17% Cr; 15.80% Ni; 3.09% Mo; 1.41% Cu; 2.44% W; 0.17% N; 0.11% Ce+La (including 0.05% Ce and 0.06% La); 0.03% Ti; 0.0047% B; 0.16% Nb; 2.36% Al; 0.024% P; 0.022% S, with the balance being Fe and unavoidable impurities.

[0052] Casting and rolling: When the temperature of molten steel drops to 1545℃, it is cast into an ingot in a metal mold; after the ingot solidifies, it is removed from the mold, and after removing the risers and grinding, the ingot is reheated to 1220℃, held for 2 hours, and then hot-rolled into a 20mm steel plate; the final rolling temperature is 1010℃; the deformation is controlled at 50%;

[0053] Solution treatment: for steps The steel plate undergoes solution treatment; the heating temperature is 1050℃, and the holding time is 30min (calculated based on 20min + 0.5min / mm × steel plate thickness); followed by rapid water cooling (cooling rate ≥ 50℃ / s) to control the precipitated phase;

[0054] Stabilization aging: Stabilization treatment is performed after solution treatment; the stabilization treatment temperature is 900℃, the holding time is 2 hours, and then air cooling is performed, which is beneficial to improving the resistance to intergranular corrosion.

[0055] Surface pre-passivation film: Before use, austenitic-ferritic duplex stainless steel is treated in an oxygen-containing atmosphere at 600℃ for 1 hour to promote the formation of Cr-rich and RE-rich oxide film; the specific mechanical properties of duplex stainless steel are shown in Table 1.

[0056] Example 3:

[0057] The specific process steps of a high-temperature resistant austenitic-ferritic duplex stainless steel with chloride / fluoride ion composite corrosion and its preparation method are as follows:

[0058] Smelting: According to the chemical composition, nitrogen is added in the form of ferrochrome nitride. The process employs electric arc furnace smelting followed by vat-out refining (VOD). The smelting temperature is controlled at 1626℃. During the refining period, rare earth elements (in the form of rare earth ferrosilicon master alloy), ferroboron, and ferrotitanium are added to control desulfurization and deoxidation. The chemical composition and mass fraction of the molten steel in the furnace are controlled as follows: 0.05% C; 0.39% Si; 0.96% Mn; 25.01% Cr; 14.28% Ni; 3.95% Mo; 1.17% Cu; 1.83% W; 0.19% N; 0.08% Y+Ce+La (of which 0.03% Ce, 0.02% La, and 0.03% Y); 0.06% Ti; 0.0036% B; 0.18% Nb; 2.24% Al; 0.020% P; 0.016% S, with the balance being Fe and unavoidable impurities;

[0059] Casting and rolling: When the molten steel temperature drops to 1538℃, it is cast into an ingot in a metal mold; after the ingot solidifies, it is removed from the mold, and after removing the risers and grinding, the ingot is reheated to 1210℃, held for 2.5 hours, and then hot-rolled into a 12mm steel plate; the final rolling temperature is 995℃; the deformation is controlled at 65%;

[0060] Solution treatment: for steps The steel plate undergoes solution treatment; the heating temperature is 1080℃, and the holding time is 26min (calculated based on 20min + 0.5min / mm × steel plate thickness); followed by rapid water cooling (cooling rate ≥ 50℃ / s) to control the precipitated phase;

[0061] Stabilization aging: Stabilization treatment is performed after solution treatment; the stabilization treatment temperature is 880℃, the holding time is 3 hours, and then air cooling is performed, which is beneficial to improving the resistance to intergranular corrosion.

[0062] Surface pre-passivation film: Before use, austenitic-ferritic duplex stainless steel is treated in an oxygen-containing atmosphere at 580℃ for 1.5 hours to promote the formation of Cr-rich and RE-rich oxide film; the specific mechanical properties of duplex stainless steel are shown in Table 1.

[0063] High-temperature simulation experiment: The experiment was conducted in simulated waste graphite gas thermal purification flue gas (containing 200ppm HCl + 50ppm HF, 600℃) for 168 hours.

[0064] Table 1 Mechanical properties of duplex stainless steel

[0065] Polarization curves were measured using a three-electrode system (working electrode: sample; auxiliary electrode: platinum electrode; reference electrode: SCE). The electrolyte was a 3.5% NaCl solution. The potential scan range varied with open-circuit voltage, and the scan rate was set to 1 mV / s. The corrosion potential E was measured. corr The current density I at the point was fitted using the Tafel extrapolation method. corr The experimental results are shown in Table 2.

[0066] Table 2 Electrochemical test results of duplex stainless steel

[0067]

[0068] This invention provides an austenitic-ferritic duplex stainless steel resistant to high-temperature chloride / fluoride ion combined corrosion. The austenite provides excellent resistance to pitting corrosion, while the ferrite provides resistance to stress corrosion cracking. The duplex interface disrupts the continuous corrosion pathway of Cl⁻. The addition of rare earth elements significantly reduces the rate of F⁻ damage to the oxide film at high temperatures. In tests simulating waste graphite gas thermal purification flue gas (containing 200ppm HCl + 50ppm HF, 600℃, experimental time 168h), the weight loss rate of this invention's steel was only 1 / 4 that of traditional 310S stainless steel, and no "peeling" phenomenon was observed on the surface. This invention's steel is low-cost and high-performance: compared to Hastelloy C-276, the nickel content is significantly reduced (12-16% vs. over 55%), reducing raw material costs by approximately 40-50%. The steel of this invention has excellent high-temperature mechanical properties: yield strength ≥280 MPa and tensile strength ≥520 MPa at 600℃, and excellent resistance to high-temperature chloride / fluoride ion composite corrosion, fully meeting the requirements for steel used in the thermal purification of waste graphite gas.

Claims

1. An austenitic-ferritic duplex stainless steel resistant to high-temperature chloride / fluoride ion composite corrosion, characterized in that, The chemical composition consists of the following elements (total mass 100%): C: 0.03%~0.08%; Si: 0.20%~0.60%; Mn: 0.50%~1.50%; Cr: 22.0%~26.0%; Ni: 12.0%~16.0%; Mo: 3.0%~5.0%; Cu: 0.5%~1.5%; W: 1.0%~2.5%; N: 0.15%~0.25%; Rare Earth Elements (RE): 0.05%~0.12%; Ti: 0.02%~0.08%; B: 0.0015%~0.0050%; Nb: 0.15%~0.25%; Al: 1.8%~2.5%; P: ≤0.025%; S: ≤0.025%, with the balance being Fe and unavoidable impurities.

2. The austenitic-ferritic duplex stainless steel resistant to high-temperature chloride / fluoride ion composite corrosion according to claim 1, characterized in that, Rare earth elements (RE) are one or more of Ce, La or Y, and further, if there are two or three, the content of each element is not less than 0.01%.

3. The austenitic-ferritic duplex stainless steel resistant to high-temperature chloride / fluoride ion composite corrosion according to claim 1, characterized in that, The metallographic structure of this steel is a two-phase structure of austenite (γ) and ferrite (δ), with the volume fraction of austenite being 60%-75% and the volume fraction of ferrite being 25%-40%.

4. A method for preparing the high-temperature resistant chloride / fluoride ion composite corrosion steel according to any one of claims 1-3, characterized in that, Includes the following steps: Smelting: According to the chemical composition, nitrogen is added in the form of ferrochrome nitride. Induction furnace smelting or electric arc furnace smelting + ladle refining (AOD / VOD) process is adopted; the smelting temperature is controlled at 1610℃-1630℃; during the refining period, the corresponding rare earth elements, ferroboron and ferrotitanium are added to control desulfurization and deoxidation; and the chemical composition and mass fraction of the molten steel in the furnace are controlled at 0.03%~0.08% C. 0.20%~0.60% Si; 0.50%~1.50% Mn; 22.0%~26.0% Cr; 12.0%~16.0% Ni; 3.0%~5.0% Mo; 0.5%~1.5% Cu; 1.0%~2.5% W; 0.15%~0.25% N; 0.05%~0.12%RE; 0.02%~0.08% Ti; 0.0015%~0.0050% B; 0.15%~0.25% Nb; 1.8%~2.5% Al; ≤0.025% P; ≤0.025% S, balance being Fe and unavoidable impurities; Casting, forging / rolling: When the molten steel temperature drops to 1520℃-1545℃, it is cast into an ingot in a metal mold; after the ingot solidifies, it is removed from the mold, and after removing the risers and grinding, the ingot is reheated to 1180℃-1220℃ and held for 2-3 hours, then forged or hot-rolled into a steel plate with a thickness of 6mm-20mm; the final forging or final rolling temperature is not lower than 980℃; the deformation is controlled at 50%-80%; Solution treatment: for steps The steel plate undergoes solution treatment; the solution temperature is 1050℃-1080℃, and the holding time is calculated as 20min + 0.5 min / mm × steel plate thickness in mm; then it is rapidly water-cooled to control the precipitated phases; Stabilization aging: Stabilization treatment is carried out after solution treatment; the stabilization treatment temperature is 850℃-900℃, the temperature is held for 2-4 hours, and then air-cooled, which is beneficial to improve the resistance to intergranular corrosion. Surface pre-passivation film: Before use, austenitic-ferritic duplex stainless steel is treated in an oxygen-containing atmosphere at 500℃-600℃ for 1-2 hours to promote the formation of a Cr-rich and RE-rich oxide film.

5. The method according to claim 4, characterized in that, In the steps In this process, the rare earth elements are added in the form of rare earth ferrosilicon master alloy 3-5 minutes before tapping, and the slag is ensured to be reducing.

6. The method according to claim 4, characterized in that, step The solution treatment temperature is 1060℃; the rapid water cooling rate is ≥50℃ / s.

7. The application of the high-temperature resistant chloride / fluoride ion composite corrosion steel according to any one of claims 1-3, used to manufacture waste graphite gas thermal purification equipment containing chloride and fluoride ion corrosive components in a temperature range of 500℃-900℃.

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