Austenitic heat-resistant steel material containing Al

Abstract

The invention relates to the technical field of metal structures, in particular to an Al-containing austenitic heat-resistant steel material which comprises the following components in percentage by mass: 0.20-0.25% of C, 0.5-2.0% of Si, 1t of Mn, 1t of Al, 1t of Al, 1t of Al and the balance of Al. The steel comprises the following components in percentage by weight: 1.0% of Fe, 18-20% of Cr, 5.5-6.5% of Ni, 0.2-0.5% of Mo, 1.0-2.0% of Al, 1.0-2.0% of N, less than or equal to 0.1% of Ce and the balance of Fe and inevitable impurities. The material is smelted by a medium-frequency induction furnace at the temperature of 1580-1620 DEG C, and is subjected to centrifugal casting molding at the speed of 800 r / min after argon refining; a microscopic structure is composed of a fine grain austenite matrix, the grain size is 3-4 grades, NiAl phases which are distributed in a dispersed mode are contained, a double-phase strengthening structure is formed, and the structure stability and creep resistance at the high temperature are guaranteed.

Description

Technical Field

[0001] This invention relates to the field of metal structure technology, specifically to an Al-containing austenitic heat-resistant steel material. Background Technology

[0002] Existing Cr-Ni type austenitic heat-resistant steels are widely used in magnesium reduction tanks for ferrosilicon smelting, but they have several technical limitations during high-temperature service. Traditional materials such as ZG3Cr24Ni7NRE and ZG35Cr25Ni20Si2 are prone to acicular martensitic phase transformation in the welded head area due to their high carbon content (0.3%–0.4%), leading to crack formation and severely reducing the strength of the welded joint and the yield of the tank. In addition, under long-term use at 1250℃, the austenitic grains of this type of low-nickel steel grow rapidly and the carbides become continuous, resulting in significant grain boundary creep and slippage, causing the tank to easily change from a circular shape to an elliptical shape and become unusable. Its oxidation resistance mainly depends on... The film is easily damaged in sulfur-containing and reducing atmospheres, with oxidation rates exceeding 0.003 mm / h. Meanwhile, although high-nickel steel has excellent performance, its high cost and scarce resources limit its large-scale application. Therefore, the demand for an Al-containing austenitic heat-resistant steel material is growing.

[0003] Therefore, in response to the above problems, a heat-resistant steel material containing Al austenitic material is proposed, as most of the existing materials on the market are not suitable for this purpose. Summary of the Invention

[0004] The purpose of this invention is to provide an Al-containing austenitic heat-resistant steel material to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: An Al-containing austenitic heat-resistant steel material, the chemical composition of which by mass percentage includes C: 0.20-0.25%, Si: 0.5-1.8%, Mn: 0.5-0.9%, Cr: 18-20%, Ni: 5.5-6.5%, Mo: 0.2-0.5%, Al: 1.0-2.0%, with the balance being Fe and unavoidable impurities.

[0006] The metallographic structure of this heat-resistant steel after holding at 1280℃ for 100 hours is stable austenite with a grain size of 3.0 to 4.0 and an average grain diameter of no more than 80 μm. The material can be used without secondary solution heat treatment after melting and centrifugal casting.

[0007] Preferably, its chemical composition also includes N and Ce rare earth elements, wherein the mass percentages of N and Ce are: N: 1% to 2% and Ce ≤ 0.1%.

[0008] Preferably, the steel, after tensile testing, has the following room temperature mechanical properties: tensile strength 600–730 MPa, yield strength 370–400 MPa, and elongation after fracture 13–25%; after heat exposure at 1280℃ for 100 hours, the tensile strength is not less than 520 MPa, the yield strength is not less than 350 MPa, and the elongation after fracture remains above 30%.

[0009] Preferably, the material undergoes a homogenization heat treatment after casting, with the temperature raised to 1100-1150°C and held for 2-3 hours before air cooling; this heat treatment makes the elements in the alloy uniformly distributed, reduces the degree of segregation, and makes the carbides dispersed, avoiding the continuity of grain boundary carbides. After this heat treatment, the carbide particle size in the microstructure of the material is no greater than 0.2 μm, and the carbide volume fraction is 2.5–3.0%.

[0010] Preferably, aluminum exists primarily in solid solution form within the austenitic matrix, partially forming a NiAl reinforcing phase; this NiAl phase has a size of 0.05–0.2 μm, is dispersed within the austenitic matrix, and has a distribution density of [missing information - likely a density value] per square millimeter. Each particle forms a second phase that precipitates and strengthens the process in the temperature range of 200–450°C. Meanwhile, Mo in the high temperature range The precipitate is distributed along the grain boundaries, with an average thickness of 0.05–0.08 μm for the grain boundary strengthening phase.

[0011] Preferably, the heat-resistant steel casting has a density of 7.80–7.85 g / cm³, a microhardness of 230–260 HV0.5, and after undergoing an oxidation test at 1250°C for 5580 hours, its surface oxide layer thickness is ≤8.5 μm, and the oxidation rate is [missing value]. mm / h.

[0012] Preferably, nitrogen is added to the steel by adding a nitrogen-containing iron alloy, controlling the nitrogen content between 1.2% and 1.6% to stabilize the austenitic structure; nitrogen forms in the matrix. Type compounds, enhancing the solid solution strengthening effect.

[0013] Preferably, rare earth element Ce is used in It can be added in the form of CeFe alloy, at a dosage of 0.05–0.08%; Ce preferentially reacts with oxygen and sulfur during the smelting process to form CeS particles, with a particle size of 0.2–0.6 μm, are dispersed at the grain boundaries, playing a role in purifying and refining the grains; The amount of Ce added is calculated based on the mass of the molten steel, and the error is controlled to be no more than ±0.005%.

[0014] Preferably, the steel is cast and then centrifugally molded into a reduction tank structure. The tank body thickness is 30–38 mm, the outer diameter is Φ800–900 mm, the inner diameter is Φ740–860 mm, and the overall weight is 600–650 kg. After installation, the tank body can withstand continuous use at 1250℃ for 5580 hours with a deformation rate ≤2.0% and a wall thickness reduction rate ≤ mm / h.

[0015] Preferably, the steel production process includes the following steps: Step 1: Raw material preparation: Select nickel plates, metallic chromium, industrial pure aluminum, and ferromolybdenum alloy with a purity of not less than 99.7% as the main materials, and ensure that the proportioning and weighing accuracy is not less than ±0.1%; Step 2: Smelting and refining: Smelting is carried out using a medium-frequency induction furnace with argon protection in the smelting atmosphere, and the slag basicity is controlled at 1.1 to 1.3; Step 3: Pouring and Cooling: After refining, the molten steel is immediately centrifugally poured at a temperature of 1570℃±10℃. After pouring, it is naturally cooled to 200℃ and then air-cooled. Step 4: Surface cleaning and defect repair: The casting is sandblasted to Sa2.5 grade, and the surface hardness difference is ≤10HV; Step 5: Performance testing: High-temperature tensile test and oxidation rate test are performed in sequence.

[0016] Compared with the prior art, the beneficial effects of the present invention are: This invention provides an Al-containing austenitic heat-resistant steel material through alloy composition optimization and process control, which significantly improves high-temperature strength, oxidation resistance, and microstructural stability. The carbon content in this steel is controlled at 0.20–0.25%, effectively reducing the tendency for welding cracks; the aluminum content is 1.0–2.0%, forming a dense surface. Protective film, oxidation rate reduced to mm / h; molybdenum content 0.2–0.5%, forms with carbon The reinforcing phase enhances high-temperature creep resistance; the nitrogen content of 1.0–2.0% stabilizes the austenitic structure and improves solid solution strengthening; rare earth element Ce ≤ 0.1% purifies inclusions and refines grains. After melting in a medium-frequency induction furnace and centrifugal casting, it can be used without secondary heat treatment, resulting in a simple and low-cost manufacturing process. The material exhibits a tensile strength ≥ 600 MPa and a yield strength ≥ 370 MPa at room temperature. After 100 hours at 1280℃, it maintains a tensile strength of 528 MPa and a yield strength of 350 MPa, with an elongation at break ≥ 30%, demonstrating excellent thermal stability. Industrial application verification shows that the lifespan of the ferrosilicon magnesium reduction tank is extended from 60 days to over 90 days, the wall thickness burn-off rate is reduced by over 50%, and the tank deformation rate is less than 2%, reducing nickel usage and production costs. Attached Figure Description

[0017] Fig. 1 This is a schematic diagram of the cold end microstructure of the reduction vessel of the present invention; Fig. 2 This is a schematic diagram of the microstructure of the hot end of the reduction vessel of the present invention. Detailed Implementation

[0018] Example 1: Please refer to Figs. 1-2 The present invention provides a technical solution: Preparation and Room Temperature Performance Testing of Al-Containing Austenitic Heat-Resistant Steel This embodiment uses a medium-frequency induction furnace melting process to prepare heat-resistant steel samples. The chemical composition by mass percentage is as follows: C: 0.25%, Si: 1.8%, Mn: 0.89%, Cr: 18.3%, Ni: 5.6%, Mo: 0.35%, N: 0.89%, Ce: 0.089%, Al: 1.05%, with the balance being Fe. All raw materials were dried at 150℃ for 1 hour to remove moisture before being added to the furnace. The melting temperature was controlled at 1600℃±10℃, and the tapping temperature was 1570℃. Before tapping, argon gas was introduced for 5 minutes to refine and remove gaseous inclusions. The molten steel was formed by centrifugal casting at a speed of 850 r / min, with the mold preheated to 400℃. Controlled-speed air cooling was used, with a cooling rate of approximately 2℃ / s. The resulting reduction vessel specimen had an outer diameter of Φ880 mm, an inner diameter of Φ810 mm, and a wall thickness of 35 mm.

[0019] The specimen underwent tensile property testing (GB / T228.1—2021). The tensile strength Rm at room temperature was 692 MPa, and the yield strength was... The tensile strength was 384 MPa, and the elongation after fracture (A) was 24.5%. Compared with traditional ZG3Cr24Ni7NRE steel, the tensile strength was increased by approximately 110 MPa, and the elongation was increased by 6 percentage points. After the sample was made into a reduction vessel, field tests showed that its service life reached 93 days, which is about 55% longer than the 60-day service life of traditional materials.

[0020] Example 2: Please refer to Figs. 1-2 The present invention provides a technical solution: High-temperature oxidation performance test Using the reduction vessel prepared in Example 1 as a sample, samples were taken for analysis after 93 days of continuous operation in a ferrosilicon magnesium smelting production line. The operating temperature of the reduction vessel was 1250℃, the hot end wall thickness was 30mm, and the cold end wall thickness was 38mm. Measurements showed slight cracking on the hot end surface of the vessel, but no creep flattening was observed. Metallographic observation revealed that the surface oxide layer thickness was approximately 8.2μm. The thickness loss rate calculated by gravimetric method was 0.0014mm / h, and the oxidation rate was reduced by more than 60% compared to ZG3Cr24Ni7NRE material.

[0021] The main components of the oxide layer are With a small amount The mixed film, as shown by microstructure, exhibits a continuous and dense oxide film with strong adhesion to the substrate. After a high-temperature oxidation test (1250℃×5580h), the oxide layer showed good adhesion and no peeling, indicating that the added 1.05% Al formed a stable protective film on the surface, effectively slowing down the oxidation diffusion process.

[0022] Example 3: Please refer to Figs. 1-2 The present invention provides a technical solution: Comparison of high-temperature thermal stability and mechanical properties The material from Example 1 was subjected to mechanical property testing after continuous service at 1250℃ for 5580 hours. The testing method followed GB / T228.1—2021 standard, with room temperature used for tensile testing. The tensile strength Rm of the sample after high-temperature service was measured to be 626 MPa, and the yield strength... The tensile strength was 323 MPa, and the elongation after fracture (A) was 41.5%. Compared with the unused sample, the tensile strength retention rate was about 90.5%, the yield strength retention rate was about 84.0%, while the elongation increased by about 70%, indicating that the microstructure was stable and the plasticity was enhanced after high-temperature service.

[0023] Microscopic observation revealed that the austenite grains remained fine after service, with a grain size of approximately 3.5, and no obvious coarsening or continuous carbides at grain boundaries were observed. The material did not exhibit embrittlement or cracking under high-temperature service conditions, indicating that the Al-Mo composite strengthening system effectively stabilized the microstructure and maintained high-temperature mechanical properties.

[0024] Example 4: Please refer to Figs. 1-2 The present invention provides a technical solution: Microstructure analysis after long-term service A failure reduction vessel that had been in service at 1250℃ for 5580 hours was used as the research object, and the microstructure of its cold and hot ends was systematically analyzed. The microstructure of the cold end was a typical equiaxed austenitic matrix with dispersed acicular NiAl phases (0.1-0.3 μm in length) mainly distributed within the grains; a small amount of spherical phases were also visible. Carbides (0.2–0.4 μm in size) are distributed in the grain boundary regions. The precipitation density of this NiAl phase is approximately / mm², which has a significant effect on precipitation enhancement.

[0025] The microstructure at the hot end is mainly fine-grained austenite with a grain size of 3.0 and an average grain diameter of approximately 75 μm. No continuous carbides or pores were observed at grain boundaries. Energy dispersive spectroscopy (EDS) analysis revealed fine Al-enriched regions within the grains, indicating that aluminum can still form a stable oxide layer at high temperatures. The microstructure suggests that this material exhibits excellent thermal stability and creep resistance during long-term high-temperature service, and the microstructure changes are consistent with the theoretical design.

[0026] By adding aluminum (Al), molybdenum (Mo) improves the high-temperature strength and thermal stability of the material. The effects of Al addition are: (1) It refines the austenite grain size in heat-resistant steel, slowing down grain growth under ultra-high temperature (1250℃) working conditions. (2) In the working range of 200℃ to 450℃, acicular NiAl phase precipitates on the austenite matrix of the heat-resistant steel, significantly increasing the high-temperature strength and thermal stability of the material. The effects of Mo addition are: (1) At ultra-high temperature (1250℃), it has a significant solid solution strengthening effect on austenite. (2) In the range of 200℃ to 450℃, it can precipitate at the austenite grain boundaries. The reinforcing phase helps to improve the high-temperature strength and thermal stability of the material.

[0027] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only for the purpose of helping to understand the method and core ideas of the present invention. The above descriptions are only preferred embodiments of the present invention. It should be noted that due to the limitations of textual expression, while there are objectively infinite specific structures, those skilled in the art can make several improvements, modifications, or changes without departing from the principles of the present invention, and can also combine the above technical features in an appropriate manner. These improvements, modifications, changes, or combinations, or the direct application of the inventive concept and technical solution to other situations without modification, should all be considered within the scope of protection of the present invention.

Claims

1. An Al-containing austenitic heat-resistant steel material, characterized in that: Its chemical composition by mass percentage includes C: 0.20-0.25%, Si: 0.5-1.8%, Mn: 0.5-0.9%, Cr: 18-20%, Ni: 5.5-6.5%, Mo: 0.2-0.5%, Al: 1.0-2.0%, with the balance being Fe and unavoidable impurities.

2. The Al-containing austenitic heat-resistant steel material according to claim 1, characterized in that: Its chemical composition also includes N and Ce rare earth elements, with N and Ce rare earth elements in the following mass percentages: N: 1%~2%, Ce≤0.1%.

3. The Al-containing austenitic heat-resistant steel material according to claim 2, characterized in that: The steel was tested for tensile properties, and its room temperature mechanical properties were as follows: tensile strength 600-730 MPa, yield strength 370-400 MPa, and elongation after fracture 13-25%; after heat exposure at 1280℃ for 100 hours, the tensile strength was not less than 520 MPa, the yield strength was not less than 350 MPa, and the elongation after fracture remained above 30%.

4. The Al-containing austenitic heat-resistant steel material according to claim 1, characterized in that: The material undergoes a homogenization heat treatment after casting, with the temperature raised to 1100-1150℃ and held for 2-3 hours before air cooling.

5. The Al-containing austenitic heat-resistant steel material according to claim 1, characterized in that: Aluminum forms a denser aluminum oxide film on the surface of heat-resistant steel than chromium, refining the austenite grain size in the heat-resistant steel; in the working range of 200℃~450℃, acicular NiAl phase precipitates on the austenite matrix of the heat-resistant steel. Meanwhile, Mo precipitates at austenite grain boundaries at temperatures ranging from 200℃ to 450℃. Enhanced phase.

6. The Al-containing austenitic heat-resistant steel material according to claim 1, characterized in that: The heat-resistant steel casting has a density of 7.80–7.85 g / cm³, a microhardness of 230–260 HV0.5, and after undergoing an oxidation experiment at 1250℃ for 5580 hours, its surface oxide layer thickness is ≤8.5 μm, and the oxidation rate is [missing information]. mm / h.

7. The Al-containing austenitic heat-resistant steel material according to claim 1, characterized in that: Nitrogen is added to the steel by adding a nitrogen-containing iron alloy, and the nitrogen content is controlled between 1.2% and 1.6%.

8. An Al-containing austenitic heat-resistant steel material according to any one of claims 1-7, characterized in that: The steel production process includes the following steps: Step 1: Raw material preparation: Select nickel plates, metallic chromium, industrial pure aluminum, and ferromolybdenum alloy with a purity of not less than 99.7% as the main materials, and ensure that the proportioning and weighing accuracy is not less than ±0.1%; Step 2: Smelting and refining: Smelting is carried out using a medium-frequency induction furnace with argon protection in the smelting atmosphere, and the slag basicity is controlled at 1.1 to 1.3; Step 3: Pouring and Cooling: After refining, the molten steel is immediately centrifugally poured at a temperature of 1570℃±10℃. After pouring, it is naturally cooled to 200℃ and then air-cooled. Step 4: Surface cleaning and defect repair: The casting is sandblasted to Sa2.5 grade, and the surface hardness difference is ≤10HV; Step 5: Performance testing: High-temperature tensile test and oxidation rate test are performed in sequence.

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