Ultra-low permeability engineering structural steel under low temperature environment and manufacturing method thereof

Abstract

The application relates to the technical field of stainless steel and discloses an ultra-low magnetic permeability engineering structure steel under a low-temperature environment and a manufacturing method thereof, the chemical components of the steel are as follows: C 0.01-0.03%, Si 0.25-0.45%, Mn 5.00-8.00%, Cr 20.0-25.0%, Ni 10-15% and N 0.3-0.5%. The manufacturing method is that a slab is heated and then subjected to high-strain-rate dynamic plastic deformation in a quick forging machine, and then is annealed after quick forging. The thickness of the steel plate is 20-80 mm, under a 4.2K ultra-low-temperature environment, the yield strength of the steel plate is above 1.4GPa, the fracture elongation is above 23%, the fracture toughness is above 180 MPa*m1 / 2, and the magnetic permeability is below 1.01. The application solves the problem that the coarse-grained low-magnetic stainless steel has low strength under an ultra-low-temperature environment and is difficult to meet engineering requirements, and the tensile plasticity can be obviously increased while the high strength is ensured, and the comprehensive performance is excellent.

Description

Technical Field

[0001] The present invention discloses non-magnetic austenitic stainless steel for ultra-low temperature structures, and more particularly, relates to an engineering structural steel with ultra-low magnetic permeability in low temperature environments and a method for manufacturing the same. Background Technology

[0002] With the transformation and upgrading of key industries and the implementation of major national projects, the demand for ultra-low magnetic permeability structural materials is constantly increasing in fields such as aerospace, power engineering, and nuclear fusion engineering. In initial industrial applications, ordinary austenitic stainless steel was typically used as low-magnetic stainless steel (LMSS). Although ordinary austenitic stainless steel exhibits a single-phase face-centered cubic (FCC) structure at room temperature and has a low relative magnetic permeability, its austenitic stability is relatively poor. It is prone to martensitic phase transformation and paramagnetic damage after ultra-low temperature plastic deformation, failing to meet stringent service requirements. Therefore, the need to develop an LMSS with high austenitic stability has arisen. Developing a high-yield-strength LMSS while ensuring good plasticity, corrosion resistance, and paramagnetism is a formidable technical challenge, and improving the yield strength of LMSS sheets under these conditions is even more challenging. Therefore, achieving this technological breakthrough has significant scientific and engineering value.

[0003] For LMSS (Metal-Liquid Metal-Steel Slabs), highly stable single-phase austenite is essential for its paramagnetic stability. Martensitic transformation cannot occur during processing or cryogenic service; therefore, inverse transformation strengthening is unsuitable for LMSS. Strengthening using dispersed precipitation of carbide, nitride, or intermetallic compound particles can lead to complex second-phase particles that affect magnetic properties and deteriorate plasticity and toughness. Therefore, strengthening methods for LMSS should focus more on solid solution strengthening and grain refinement. While methods such as high-pressure torsion, equal-channel angular extrusion, ultrasonic impact treatment, surface mechanical rolling, and cryogenic rolling can effectively reduce grain size and improve alloy strength, they are difficult to apply to large-size LMSS plates and lack advantages for large-scale industrial production. Using medium-thick plate mills for recrystallization rolling or non-recrystallization rolling results in large grain sizes, failing to meet the high strength and toughness requirements of cryogenic environments. Therefore, it is necessary to explore grain refinement methods for large-size LMSS plates to meet the needs of cryogenic structural engineering. Summary of the Invention

[0004] The problem this invention aims to solve is that coarse-grained LMSS steel exhibits low strength at 4.2 K ultra-low temperature, making it difficult to meet engineering requirements. To address this issue, this invention discloses a compositional design, deformation process, and heat treatment method for LMSS steel with excellent strength at 4.2 K ultra-low temperature. It utilizes dynamic plastic deformation at high strain rates to prepare a nanocrystalline structure with embedded nanotwins in a bundle-like form. After annealing, more stable nanotwin bundles than nanocrystalline grains are formed within the recrystallized grains. These nanotwin bundles with austenitic structures can be considered "hard grains" to strengthen the coarse-grained austenitic structure. The steel plate produced by this method can be applied in the field of ultra-low temperature structural engineering.

[0005] The present invention employs a chemical composition design with high paramagnetic stability (by weight): C: 0.01~0.03%, Si: 0.25~0.45%, Mn: 5.00~8.00%, Cr: 20.0~25.0%, Ni: 10~15%, N: 0.3~0.5%, with the remainder being Fe and unavoidable inclusions.

[0006] The carbon content is controlled at 0.01~0.03% to ensure the stability of austenite and prevent the induction of martensitic phase transformation during ultra-low temperature plastic deformation. The carbon atoms at the above content are dissolved in austenite, and the low-temperature strength of the steel plate can be significantly improved through solid solution strengthening.

[0007] The Si content is controlled between 0.25% and 0.45% to balance the dissolved oxygen content in the steel and ensure its cleanliness. The silicon content at this level can effectively improve the low-temperature fatigue performance of the steel plate.

[0008] The Mn content is controlled at 5.00~8.00%. Its function is to reduce stacking fault energy, promote deformation twin formation, alleviate local stress concentration caused by dislocation entanglement, and change the phase of the crystal through twinning, thereby stimulating further slip, promoting the continuation of plastic deformation, and improving the strength and toughness at ultra-low temperatures.

[0009] The Cr content is controlled at 20.0-25.0%, which effectively reduces the activity coefficient of nitrogen, matching the nitrogen content of 0.3-0.5%, and improving the solubility of nitrogen in steel.

[0010] By controlling the Ni content to 10-15%, high-temperature δ-ferrite can be generated in the nickel-poor chromium-rich region due to phase transformation caused by fluctuations in the nickel composition.

[0011] The nitrogen content is controlled at 0.3-0.5%. Its function is to stabilize austenite, inhibit the precipitation of intermetallic phases, expand the hot working temperature range, and design this nitrogen content range. After annealing, the nitrogen is completely dissolved, which induces lattice distortion and improves room temperature and ultra-low temperature strength by pinning dislocations, while ensuring toughness.

[0012] The present invention also provides a method for manufacturing steel for engineering structures with ultra-low magnetic permeability in a low-temperature environment, including the following:

[0013] The slab is first heated, with the heating temperature controlled between 1220 and 1280 ℃. Considering the high deformation resistance of stainless steel, a higher heating temperature can soften the slab, providing a sufficiently high initial temperature to ensure that the slab deformation process is completed within the full austenitization temperature range. Next, high-strain-rate dynamic plastic deformation is performed in a high-speed forging mill. After high-speed forging, annealing is carried out to induce static recrystallization in the nanocrystalline region between the original shear bands of the nanotwin bundles. 730 ℃ is the critical point for static recrystallization. As the annealing temperature increases, the proportion of static recrystallization increases, reaching a maximum at 800 ℃. Above 800 ℃, the statically recrystallized grains grow.

[0014] Preferably, a single-fire forging process is adopted: the initial forging temperature of the first stage does not exceed 1100 ℃, the final forging temperature is not lower than 950 ℃, the cumulative deformation rate is 55~75%, and the thickness of the intermediate billet is controlled at 40~100mm; the initial forging temperature of the second stage does not exceed 850 ℃, the final forging temperature is not lower than 750 ℃, the cumulative deformation rate is 20~55%, and air cooling is used between the two stages to allow the temperature to warm up.

[0015] Preferably, rapid forging deformation adopts a unidirectional drawing method.

[0016] Preferably, the total strain ε in rapid forging is controlled between 1.2 and 1.6. The total strain is defined as ε = ln(L / L). 0 / L f ), where L0 and L f These refer to the slab thickness and the final finished steel plate thickness, respectively, with the slab thickness controlled between 90 and 350 mm.

[0017] Preferably, the strain rate of the rapid forging process is 0.08~1s. -1 .

[0018] Preferably, the steel plate is annealed at 730 ~ 800 °C for 20 ~ 80 min after plastic deformation.

[0019] By using the above-mentioned rapid forging process parameters, a nanocrystalline structure with nanotwins embedded in a bundle form can be obtained, and adiabatic heating can be induced during high strain rate deformation to suppress strain-induced martensitic phase transformation.

[0020] Preferably, the thickness of the finished steel plate is controlled between 20 and 80 mm.

[0021] According to the composition design of this invention, LMSS steel with high paramagnetic stability can be obtained. Solid solution strengthening of LMSS is achieved through nitrogen alloying, and dynamic plastic deformation at high strain rates is performed using a high-speed forging mill to prepare a bulk nanostructured ultra-low temperature austenitic stainless steel composed of nano-sized grains and nanoscale deformation twins. Through annealing, a single-phase austenitic structure composed of statically recrystallized micron-sized grains and nanotwins is obtained. After annealing, statically recrystallized grains preferentially form in the shear band region between the nanotwin bundles, thus forming a tough network. While ensuring high strength, tensile plasticity is also significantly increased. At an ultra-low temperature of 4.2 K, the steel's yield strength reaches over 1.4 GPa, tensile strength is 1800–1880 MPa, elongation at break exceeds 23%, impact energy is 250–280 J, and fracture toughness exceeds 180 MPa·m. 1 / 2 With a permeability of less than 1.01, it exhibits excellent overall performance. Attached Figure Description

[0022] Figure 1 This is a typical cross-sectional bright-field TEM image.

[0023] Nanotwins are embedded in the nanocrystal matrix in the form of bundles. The thickness of the twin / matrix layer is 20–50 nm. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of an exemplary embodiment of the experimental apparatus and method is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] The following embodiments are merely some preferred embodiments of the present invention and do not limit the scope and technical means of the invention in any way. Table 1 lists the components involved in each embodiment, Table 2 lists the heating and forging results of each embodiment, Table 3 lists the heat treatment results of each embodiment, and Table 4 lists the mechanical properties of each embodiment.

[0026]

[0027] According to the composition design of this invention, LMSS steel with high paramagnetic stability can be obtained. As shown in the table, at an ultra-low temperature of 4.2K, the steel's yield strength exceeds 1.4 GPa, tensile strength is between 1800 and 1880 MPa, elongation at break exceeds 23%, impact energy is between 250 and 280 J, and fracture toughness exceeds 180 MPa·m. 1 / 2 With a permeability of less than 1.01, it exhibits excellent overall performance.

Claims

1. A type of engineering structural steel with ultra-low magnetic permeability for low-temperature environments, characterized in that, The chemical composition of the steel plate is as follows: C: 0.01%–0.03%, Si: 0.25%–0.45%, Mn: 5.00%–8.00%, Cr: 20.0%–23.45%, Ni: 10%–15%, N: 0.35%–0.5%, with the remainder being Fe and unavoidable inclusions. The steel is manufactured using deformation and heat treatment processes. The slab is first heated to a temperature controlled at 1220–1280℃. Next, it undergoes high-strain-rate dynamic plastic deformation on a high-speed forging mill, employing a unidirectional elongation method. The total forging strain ε is controlled at 1.2–1.6, and the forging process strain rate is 0.08–1 s. -1 After rapid forging, the steel plate is annealed at 730–800℃ for 20–80 min. The steel plate microstructure contains nanotwin bundles embedded in a nanocrystalline matrix, and the thickness of the twin / matrix lamellar is 20–50 nm.

2. The engineering structural steel with ultra-low magnetic permeability under low-temperature conditions according to claim 1, characterized in that, The thickness of the finished steel plate is controlled between 20 and 80 mm.

3. The engineering structural steel with ultra-low magnetic permeability under low-temperature conditions according to claim 1, characterized in that, At an ultra-low temperature of 4.2K, the steel exhibits a yield strength exceeding 1.4 GPa, a fracture elongation exceeding 23%, and a fracture toughness exceeding 180 MPa·m. 1 / 2 The permeability is less than 1.

01.

4. A method for manufacturing ultra-low magnetic permeability engineering structural steel under low-temperature conditions as described in any one of claims 1 to 3, comprising a deformation process and a heat treatment method, characterized in that, The slab is first heated to a temperature controlled between 1220 and 1280°C. Then, it undergoes high-strain-rate dynamic plastic deformation on a high-speed forging mill, using a unidirectional elongation method. The total forging strain ε is controlled between 1.2 and 1.6, and the forging process strain rate is between 0.08 and 1 s. -1 After rapid forging, anneal at 730-800℃ for 20-80 minutes.

5. The method for manufacturing steel for engineering structures with ultra-low magnetic permeability under low-temperature environments according to claim 4, characterized in that, When forging slabs quickly, one-fire forging is used: the initial forging temperature of the first stage should not exceed 1100℃, the final forging temperature should not be lower than 950℃, the cumulative deformation rate should be 55-75%, and the thickness of the intermediate slab should be controlled between 40-100mm; the initial forging temperature of the second stage should not exceed 850℃, the final forging temperature should not be lower than 750℃, the cumulative deformation rate should be 20-55%, and air cooling should be used between the two stages to allow the slabs to warm up.

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