Method for improving the strength and toughness and fatigue resistance of austenitic stainless steel at extremely low temperature

Abstract

The application belongs to the technical field of stainless steel, and relates to a method for improving the low-temperature strength, toughness and fatigue resistance of austenitic stainless steel, which comprises the following steps: (1) obtaining molten steel with qualified components through an electric furnace, AOD and LF refining, and obtaining a casting blank after continuous casting; the molten steel comprises C 0.045% to 0.055%, Si 0.55% to 0.6%, Mn 4% to 5%, Cr 16% to 17%, Ni 6% to 6.5%, N 0.15% to 0.18%, Co 0.2% to 0.3%, and the balance of Fe and other inevitable impurity elements; (2) sequentially performing hot rolling, hot coil annealing, cold rolling and cold coil heat treatment processes on the casting blank. The application can improve the low-temperature strength, toughness and fatigue resistance of austenitic stainless steel, and can ensure efficient and stable production of the product, and can meet the use requirements of low-temperature storage tanks.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel technology and relates to a method for improving the ultra-low temperature strength, toughness and fatigue resistance of austenitic stainless steel, and more particularly to a method for improving the ultra-low temperature strength, toughness and fatigue resistance of austenitic stainless steel used in the energy field. Background Technology

[0002] Energy is a key sector crucial to national security and development. The world today is undergoing profound changes unseen in a century, with a new round of technological revolution and industrial transformation converging historically with my country's requirements for high-quality economic development. The goal of "peak carbon and carbon neutrality" brings new opportunities and challenges to the energy revolution and high-quality development.

[0003] Among all types of energy, natural gas and hydrogen are two abundant low-carbon and zero-carbon energy sources, and are gradually becoming one of the important carriers of global energy transformation and development. They are of great significance for building a clean, low-carbon, safe and efficient energy system and achieving high-quality energy development.

[0004] In the hydrogen energy and LNG industry chain, the key to their application lies in safe and efficient hydrogen storage and transportation technologies, which are crucial for the practical application and industrialization of hydrogen energy utilization. Currently, hydrogen storage methods mainly include high-pressure hydrogen storage, liquid hydrogen storage, metal hydride hydrogen storage, and organic liquid hydrogen storage. Hydrogen transportation methods mainly include gaseous hydrogen long-tube trailers, liquid hydrogen tank trucks, and pipeline transportation. High-pressure gaseous storage and transportation technology is relatively mature and is currently the main storage and transportation method; however, only a few experimental pipelines for gaseous hydrogen transportation have been built domestically. Liquid hydrogen is limited by heat exchangers, insulators, and standards, and is mainly used in aerospace and military industries in China, with slow progress in civilian applications. Solid-state hydrogen storage and organic liquid hydrogen storage have high hydrogen density, good safety, and high cost, but have not yet been widely applied.

[0005] Austenitic stainless steel sheets produced using traditional compositions and processes present the following problems in product design, ultra-low temperature service processes, and manufacturing:

[0006] 1) The composition design is unreasonable, which cannot guarantee a good match between strength, toughness and fatigue resistance under extremely low temperature conditions below -163℃, and there is a risk of fracture failure during product use;

[0007] 2) The production process is difficult to control, and the organization and performance of the entire process are out of control. Summary of the Invention

[0008] In order to overcome the above-mentioned shortcomings of existing austenitic stainless steel used in the energy field, this invention provides a method to improve the ultra-low temperature strength, toughness and fatigue resistance of austenitic stainless steel by specially designing the composition of stainless steel and controlling the key process points of casting, hot rolling, cold rolling and heat treatment.

[0009] Specifically, the method for improving the ultra-low temperature strength, toughness, and fatigue resistance of austenitic stainless steel provided by this invention includes:

[0010] (1) Steel with qualified composition is obtained by refining through electric furnace → AOD → LF, and then cast billet is obtained by continuous casting;

[0011] By weight percentage, the molten steel comprises 0.045%–0.055% C, 0.55%–0.6% Si, 4%–5% Mn, 16%–17% Cr, 6%–6.5% Ni, 0.15%–0.18% N, 0.2%–0.3% Co, with the balance being Fe and other unavoidable impurity elements;

[0012] (2) The billet is subjected to hot rolling, hot coil annealing, cold rolling and cold coil heat treatment processes in sequence.

[0013] The above-mentioned method for improving the ultra-low temperature strength, toughness and fatigue resistance of austenitic stainless steel uses aluminum for deoxidation in AOD, with an aluminum addition amount of 1-1.5 kg / ton of molten steel.

[0014] The above-mentioned method for improving the ultra-low temperature strength, toughness and fatigue resistance of austenitic stainless steel involves creating a high-alumina refining slag system at the LF station, which, by weight percentage, includes: 55%–60% CaO, 10%–15% Al2O3, 8%–10% MgO, 10%–15% SiO2, and 3%–5% CaF2.

[0015] The above-mentioned method for improving the ultra-low temperature strength, toughness and fatigue resistance of austenitic stainless steel requires that the [Ca] / [Al] ratio in the molten steel in the LF process be 0.13% to 0.20%, the argon blowing and weak stirring flow rate be 70-80 L / min, and the time be ≥30 min.

[0016] In the above-mentioned method for improving the ultra-low temperature strength, toughness and fatigue resistance of austenitic stainless steel, the billet heating temperature in the hot rolling process is 1250-1260℃, and the holding time is 3-3.5h.

[0017] The above-mentioned method for improving the ultra-low temperature strength, toughness and fatigue resistance of austenitic stainless steel involves the following steps in the hot rolling process: when the billet temperature reaches above 1100℃ in the intermediate specification, rolling is stopped and the dwell time is above 20s. Rolling continues when the temperature drops below 900℃. The deformation per pass is ≥30%. The final rolling temperature is controlled at 800-850℃, and the coiling temperature is controlled at 650-680℃.

[0018] The above-mentioned method for improving the ultra-low temperature strength, toughness, and fatigue resistance of austenitic stainless steel involves a steel plate temperature of 1150-1170℃ and a line speed (TV) of 120-130 mm·m / min during the hot-rolling annealing process.

[0019] The above-mentioned methods for improving the ultra-low temperature strength, toughness and fatigue resistance of austenitic stainless steel require that, in the cold rolling process, the deformation amount of the first pass is ≥25%, the total deformation amount is greater than 70%, and the deformation amount of the last pass is controlled at 5%-7%.

[0020] In the above-mentioned method for improving the ultra-low temperature strength, toughness, and fatigue resistance of austenitic stainless steel, the temperature of the steel plate in the first stage of the cold coil heat treatment process is 900-950℃, and the linear speed is 10-12m / min; the temperature of the steel plate in the second stage is 1100-1120℃, and the linear speed is 50-55m / min.

[0021] On the other hand, the present invention provides an austenitic stainless steel, which adopts the above-mentioned method for improving the ultra-low temperature strength, toughness and fatigue resistance of austenitic stainless steel during the preparation process.

[0022] The technical solution of the present invention has the following beneficial effects:

[0023] (1) This invention can improve the ultra-low temperature strength, toughness and fatigue resistance of austenitic stainless steel, while ensuring efficient and stable production and preparation of the product, which can meet the requirements of cryogenic storage tanks.

[0024] (2) The austenitic stainless steel cold-rolled plate prepared by this invention exhibits significantly improved ultra-low temperature strength and toughness, specifically: 1) Mechanical properties at -163℃: Yield strength Rp0.2 ≥ 700 MPa; Tensile strength Rm ≥ 1300 MPa; Elongation A ≥ 30%; 2) Fracture toughness at -163℃: KIC ≥ 250 MPa. 1 / 2 ;3) Fatigue resistance at -163℃: 1Hz, 500Mpa conditions, cycle count > 30,000 times. Detailed Implementation

[0025] To fully understand the purpose, features, and effects of this invention, the following detailed embodiments are provided. Except as described below, the process methods of this invention employ conventional methods or apparatus in the art. Unless otherwise specified, the terms and expressions used below have the meanings commonly understood by those skilled in the art.

[0026] When a numerical range is disclosed in this invention, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Further, when the range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. Moreover, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed in this invention should be understood to include any and all subranges to which they are incorporated.

[0027] The inventors of this application, through extensive research, have discovered that the main factors affecting ultra-low temperature strength, toughness, and fatigue resistance include alloy composition, steel purity, and the hot-rolling-hot-coiling heat treatment-cold-rolling-cold-coiling solution treatment process. Therefore, the core of this invention in improving ultra-low temperature strength and toughness lies in achieving precise microstructure control during the rolling and heat treatment processes, based on composition design, thereby realizing precise performance control. The overall concept of this invention is as follows:

[0028] 1) Design an excellent alloy composition system to achieve a good match of strength, toughness and fatigue resistance under extremely low temperature conditions while controlling alloy costs, so as to meet the requirements of extremely low temperature use;

[0029] 2) By optimizing the smelting process, the number and size of inclusions are reduced, fatigue fracture sources are blocked, and the fatigue resistance at extremely low temperatures is improved;

[0030] 3) Optimize the matching of the entire process of hot rolling-hot coil annealing-cold rolling-cold coil solution treatment to regulate the microstructure to a state of fine and uniform grains with no obvious precipitates.

[0031] Specifically as follows:

[0032] 1. Ingredient Design

[0033] The main chemical composition (by weight percentage) of the optimized austenitic stainless steel for the energy field, designed according to this invention to improve its ultra-low temperature strength, toughness, and fatigue resistance, is as follows: C 0.045%–0.055%, Si 0.55%–0.6%, Mn 4%–5%, Cr 16%–17%, Ni 6%–6.5%, N 0.15%–0.18%, Co 0.2%–0.3%, with the balance being Fe and other unavoidable impurity elements. The reasons for limiting the content range of the main alloying elements will be explained in detail below.

[0034] C: 0.045%~0.055%

[0035] C solute atoms tend to aggregate in the tensile portion of edge dislocations, making the crystal more stable by reducing lattice distortion. This results in greater resistance to dislocation movement, creating a "Cootherd atmosphere" effect, which macroscopically increases the yield strength. Simultaneously, C is an austenite stabilizing element, and appropriately increasing the C content can improve low-temperature microstructure stability. However, because the M23C6 carbide formed by C and Cr tends to accumulate at grain boundaries and easily becomes a source of fatigue fracture, the C content is limited to 0.045%–0.055%.

[0036] Si: 0.55%–0.6%

[0037] As a deoxidizer added during smelting, silicon (Si) is appropriately increased in this invention to improve the deoxidation effect; however, excessively high Si content will reduce the stability of the microstructure. Therefore, the Si content is limited to 0.55%–0.6%.

[0038] Mn: 4%–5%

[0039] The main function of manganese (Mn) is to stabilize the austenite phase and increase the solubility of nitrogen (N) in steel. Because this invention has a high N content, the Mn content must be above 4% to achieve a fully austenitic microstructure and match the N content. However, excessive addition can easily form MnS inclusions, which can become sources of fatigue fracture. Therefore, the Mn content is limited to 4%–5%.

[0040] Cr: 16%–17%

[0041] Cr is the most important alloying element for forming the passivation film on stainless steel. However, in order to improve the stability of the microstructure, the Cr content is appropriately reduced in this invention; therefore, the Cr content is limited to 16%–17%.

[0042] Ni: 6%–6.5%

[0043] Ni can ensure extremely low-temperature toughness, but adding too much Ni will lead to excessively high costs. Therefore, the Ni content is limited to 6% to 6.5%.

[0044] N: 0.15%–0.18%

[0045] Nitrogen (N) can improve extremely low temperature strength while suppressing the formation of intermetallic phases. In this invention, the N content is further increased to 0.15% or more. However, excessive N will reduce toughness. Therefore, the N content is limited to 0.15% to 0.18%.

[0046] 2. Smelting process

[0047] The smelting process route is electric furnace → AOD → LF refining. In order to effectively remove inclusions and render them harmless, it is mainly achieved through two measures: AOD aluminum-enhanced deoxidation + LF to create high-alumina refining slag, and LF furnace calcium treatment + weak stirring.

[0048] 1) Aluminum-enhanced deoxidation at AOD + high-alumina refining slag production at LF: Aluminum is used for deoxidation at AOD, with an addition rate of 1-1.5 kg / ton of molten steel, which can effectively reduce the oxygen content. However, the Al2O3 generated after aluminum deoxidation does not easily float and tends to form chain-like inclusions in the molten steel. After solidification, these inclusions generally have sharp corners, severely affecting the extremely low temperature strength and toughness. Therefore, a high-alumina refining slag system needs to be produced at the LF station to effectively adsorb and remove Al2O3 inclusions. The typical composition of the slag system is: 55%–60% CaO, 10%–15% Al2O3, 8%–10% MgO, 10%–15% SiO2, and 3%–5% CaF2.

[0049] LF furnace calcium treatment + weak stirring: Alumina inclusions in steel after aluminum deoxidation have a high melting point and are solid at continuous casting temperatures, easily accumulating at the tundish nozzle and causing blockage. Therefore, it is necessary to control the aluminum deoxidation products to be in a liquid state at continuous casting temperatures to allow them to float to the surface in large quantities. Calcium, due to its strong affinity for oxygen and sulfur, can not only be used as a strong deoxidizer and desulfurizer, but also transform easily deformable MnS inclusions in steel into CaS, and can also change the shape of Al2O3 in the molten steel. The addition of calcium allows Al2O3, CaO, and SiO2 to form a low-melting-point CaO·Al2O3·SiO2. In smelting this alloy stainless steel, to ensure that all Al2O3 in the steel is converted into low-melting-point calcium aluminate, the [Ca] / [Al] ratio in the molten steel is optimally controlled at 0.13–0.20%.

[0050] In an LF furnace, bottom-blowing argon removes inclusions larger than 50 μm through air bubbles. Simultaneously, it encourages inclusions smaller than 50 μm to collide, aggregate, and rapidly form larger particles, which are then quickly floated upwards and removed. Blowing argon at a lower flow rate and with a slightly longer blowing time is more effective in removing inclusions, thereby reducing the oxygen content of the molten steel and the average size of inclusions, particularly reducing the number of large inclusions in the steel. The recommended argon blowing flow rate with weak stirring is 70-80 L / min, and the blowing time is ≥30 min.

[0051] 3. Hot rolling, hot coil annealing, cold rolling, and cold coil heat treatment processes

[0052] Because the microstructure of materials is inherited during the process from billet processing to finished steel coil, and the strength, toughness, and fatigue resistance of the product under extremely low temperature conditions are determined by the microstructure, the performance indicators are as follows based on the usage requirements:

[0053] Mechanical properties at -163℃: Yield strength Rp0.2≥700MPa; Tensile strength Rm≥1300MPa; Elongation A≥30%;

[0054] Fracture toughness at -163℃: KIC ≥ 250 MPa.m 1 / 2 ;

[0055] -163℃ fatigue resistance: >30,000 cycles under 1Hz, 500MPa conditions.

[0056] After extensive laboratory research, the performance indicators were transformed into microscopic tissue control targets.

[0057] Grain size grade 8-9, grain size range <2, all austenitic structure, no precipitates.

[0058] Based on the above requirements, the production process parameters are determined as follows:

[0059] (1) Hot rolling:

[0060] The billet is heated to 1250-1260℃ and held for 3-3.5 hours to ensure that all precipitates in the billet are dissolved back.

[0061] For intermediate billet dimensions (20-25mm), the rolling temperature is controlled above 1100℃ to ensure recrystallization is complete. Rolling is then stopped at this point, with a dwell time of at least 20 seconds to allow sufficient time for the microstructure to complete the post-dynamic recrystallization process and completely eliminate the as-cast microstructure. Rolling continues when the temperature drops below 900℃, with a single-pass deformation of ≥30%. Under low temperature and high pressure, further grain breakage is ensured to achieve a uniform, fine-grained state. The final rolling temperature is controlled at 800-850℃, and the coiling temperature is controlled at 650-680℃.

[0062] (2) Hot rolling annealing

[0063] Maintain a steel plate temperature of 1150-1170℃; a line speed (TV value) of 120-130 mm·m / min. Ensure that all stress generated by hot rolling is released, newly generated precipitates are fully dissolved, resulting in a completely recrystallized structure, and that the grains grow appropriately and become more uniform.

[0064] (3) Cold rolling

[0065] The deformation amount in the first pass is ≥25%, and the total deformation amount is greater than 70%, to ensure that the hot-rolled grains are fully broken again. The deformation amount in the last pass is controlled at 5%-7% to prevent the deformation of the fiber structure from being too severe.

[0066] (4) Cold rolling and annealing

[0067] First stage: Steel plate temperature 900-950℃, line speed 10-12m / min, low temperature long-time annealing is adopted, which not only eliminates residual stress, but also slowly consumes the distortion energy generated in the cold rolling process, and reduces the driving force for grain growth in the subsequent high temperature annealing process.

[0068] The second stage involves a steel plate temperature of 1100-1120℃ and a line speed of 50-55m / min. This ensures that recrystallization is completed and the precipitated phase is fully dissolved, while also preventing grain growth due to excessive time.

[0069] On the other hand, the present invention also provides an austenitic stainless steel prepared by the above method, the main chemical composition of which is designed (by weight percentage) as follows: C 0.045-0.055%, Si 0.55-0.6%, Mn 4-5%, Cr 16-17%, Ni 6-6.5%, N 0.15-0.18%, Co 0.2-0.3%, with the balance being Fe and other unavoidable impurity elements.

[0070] This invention significantly improves the yield strength, tensile strength, elongation, fracture toughness, and fatigue resistance of austenitic stainless steel at -163℃ by optimizing the smelting and inclusion removal process and matching it with the entire process of hot rolling-hot coil annealing-cold rolling-cold coil solution treatment.

[0071] Example

[0072] The present invention is further illustrated below by way of examples, but the invention is not limited to the scope of the examples described herein. Experimental methods in the following examples, unless otherwise specified, were performed according to conventional methods and conditions. The chemical composition of the austenitic stainless steel cold-rolled coils prepared in Examples 1-6 and the comparative examples is shown in Table 1.

[0073] Example 1: Finished product specifications: 1.2×1219mm cold-rolled coil

[0074] During smelting, the AOD (aluminum oxide) addition is 1.2 kg / ton of molten steel. A high-alumina refining slag system is created at the LF (leaf ferroalloy) station, with the following composition: 55% CaO, 13% Al2O3, 9% MgO, 12% SiO2, and 4% CaF2. The [Ca] / [Al] ratio in the molten steel at the LF process is 0.15%. Argon blowing with weak stirring is carried out at a flow rate of 75 L / min for 40 min. The billet thickness is 200 mm. Before hot rolling, the billet is heated to 1250℃ and held for 3 hours. The billet is rolled to an intermediate size of 20 mm at a temperature of 1130℃. Rolling is then stopped at this point, with a dwell time of 40 seconds. Rolling continues when the temperature drops to 890℃. The deformation per pass is between 32% and 35%. Final rolling temperature 830℃, coiling temperature 670℃; hot-rolled annealed steel plate temperature 1150℃; line speed (TV value) 125mm·m / min; cold rolling first pass deformation 35%, total deformation 75%, last pass deformation 6%; cold rolling first stage: steel plate temperature 930℃, line speed 11m / min; second stage: steel plate temperature 1110℃, line speed 52m / min.

[0075] Example 2: Finished product specifications: 1.2×1300mm cold-rolled coil

[0076] During smelting, the AOD (aluminum oxide) addition is 1.1 kg / ton of molten steel. A high-alumina refining slag system is created at the LF (leaf ferroalloy) station, with the following composition: 55% CaO, 13% Al2O3, 9% MgO, 12% SiO2, and 4% CaF2. The [Ca] / [Al] ratio in the molten steel at the LF process is 0.16%. Argon blowing with weak stirring is carried out at a flow rate of 73 L / min for 43 min. The billet thickness is 200 mm. Before hot rolling, the billet is heated to 1250℃ and held for 3 hours. The billet is rolled to an intermediate size of 22 mm at a temperature of 1110℃. Rolling is then stopped at this point, with a dwell time of 25 seconds. Rolling continues when the temperature drops to 880℃. The deformation per pass is between 32% and 35%. Final rolling temperature 820℃, coiling temperature 670℃; hot-rolled annealed steel plate temperature 1160℃; line speed (TV value) 122mm·m / min; cold rolling first pass deformation 30%, total deformation 75%, last pass deformation 6%; cold rolling first stage: steel plate temperature 930℃, line speed 11m / min; second stage: steel plate temperature 1100℃, line speed 53m / min.

[0077] Example 3: Finished product specifications: 1.2×1250mm cold-rolled coil

[0078] During smelting, the AOD (aluminum oxide) addition is 1.5 kg / ton of molten steel. A high-alumina refining slag system is created at the LF (leaf ferroalloy) station, with the following composition: 60% CaO, 10% Al2O3, 8% MgO, 14% SiO2, and 4% CaF2. The [Ca] / [Al] ratio in the molten steel at the LF process is 0.18%. Argon blowing with weak stirring is carried out at a flow rate of 78 L / min for 35 min. The billet thickness is 200 mm. Before hot rolling, the billet is heated to 1260℃ and held for 3.5 h. The billet is rolled to an intermediate size of 22 mm at a temperature of 1120℃. Rolling is then stopped at this point, with a dwell time of 23 s. Rolling continues when the temperature drops to 890℃, with a deformation of 32-35% per pass. Final rolling temperature 830℃, coiling temperature 680℃; hot-rolled annealed steel plate temperature 1150℃; line speed (TV value) 128mm·m / min; cold rolling first pass deformation 30%, total deformation 75%, last pass deformation 6%; cold rolling first stage: steel plate temperature 930℃, line speed 11m / min; second stage: steel plate temperature 1100℃, line speed 53m / min.

[0079] Example 4: Finished product specifications are 1.2×1500mm cold-rolled coil

[0080] During smelting, the AOD (aluminum oxide) addition is 1.5 kg / ton of molten steel. A high-alumina refining slag system is created at the LF (leaf ferroalloy) station, with the following composition: 58% CaO, 12% Al2O3, 10% MgO, 10% SiO2, and 4% CaF2. The [Ca] / [Al] ratio in the molten steel at the LF process is 0.19%. Argon blowing with weak stirring is carried out at a flow rate of 70 L / min for 45 min. The billet thickness is 200 mm. Before hot rolling, the billet is heated to 1250℃ and held for 3 hours. The billet is rolled to an intermediate size of 25 mm at a temperature of 1120℃. Rolling is then stopped at this point, with a dwell time of 25 seconds. Rolling continues when the temperature drops to 890℃, with a deformation of 32-35% per pass. Final rolling temperature 840℃, coiling temperature 650℃; hot-rolled annealed steel plate temperature 1170℃; line speed (TV value) 120mm·m / min; cold rolling first pass deformation 30%, total deformation 75%, last pass deformation 6%; cold rolling first stage: steel plate temperature 920℃, line speed 12m / min; second stage: steel plate temperature 1100℃, line speed 50m / min.

[0081] Example 5: Finished product specifications: 1.2×1800mm cold-rolled coil

[0082] During smelting, the AOD (aluminum oxide) addition is 1.2 kg / ton of molten steel. A high-alumina refining slag system is created at the LF (leaf ferroalloy) station, with the following composition: 58% CaO, 12% Al2O3, 10% MgO, 10% SiO2, and 4% CaF2. The [Ca] / [Al] ratio in the molten steel at the LF process is 0.13%. Argon blowing with weak stirring is carried out at a flow rate of 80 L / min for 32 min. The billet thickness is 200 mm. Before hot rolling, the billet is heated to 1250℃ and held for 3 h. The billet is rolled to an intermediate size of 23 mm at a temperature of 1110℃. Rolling is then stopped at this point, with a dwell time of 22 s. Rolling continues when the temperature drops to 880℃. The deformation per pass is between 32% and 35%. Final rolling temperature 840℃, coiling temperature 650℃; hot-rolled annealed steel plate temperature 1170℃; line speed (TV value) 120mm·m / min; cold rolling first pass deformation 30%, total deformation 75%, last pass deformation 6%; cold rolling first stage: steel plate temperature 920℃, line speed 12m / min; second stage: steel plate temperature 1100℃, line speed 50m / min.

[0083] Example 6: Finished product specifications: 1.2×2000mm cold-rolled coil

[0084] During smelting, the AOD (aluminum oxide) addition is 1.0 kg / ton of molten steel. A high-alumina refining slag system is created at the LF (Left-Left) station, with the following composition: 60% CaO, 10% Al2O3, 8% MgO, 15% SiO2, and 4% CaF2. The [Ca] / [Al] ratio in the molten steel at the LF process is 0.15%, and the argon blowing with weak stirring is carried out at a flow rate of 75 L / min for 33 min. The billet thickness is 200 mm. Before hot rolling, the billet is heated to 1260℃ and held for 3 hours. The billet is rolled to an intermediate size of 23 mm at a temperature of 1120℃. Rolling is then stopped at this point, with a dwell time of 24 seconds. Rolling continues when the temperature drops to 890℃, with a deformation of 32-35% per pass. Final rolling temperature 840℃, coiling temperature 650℃; hot-rolled annealed steel plate temperature 1170℃; line speed (TV value) 120mm·m / min; cold rolling first pass deformation 30%, total deformation 75%, last pass deformation 6%; cold rolling first stage: steel plate temperature 920℃, line speed 12m / min; second stage: steel plate temperature 1100℃, line speed 50m / min.

[0085] Comparative Example 304: Finished product specifications are 1.2×1500mm cold-rolled coil

[0086] During smelting, the AOD (aluminum oxide) addition is 0.5 kg / ton of molten steel. A high-alumina refining slag system is created at the LF (leaf-lift) station, with the composition: 65% CaO, 20% Al₂O₃, 5% SiO₂, and 2% CaF₂. The [Ca] / [Al] ratio in the molten steel at the LF process is 0.3%, with argon blowing and weak stirring at a flow rate of 100 L / min for 20 min. The billet thickness is 200 mm. The billet is heated to 1280℃ before hot rolling, held for 2.5 h, and directly rolled to a thickness of 3.5 mm. The deformation per pass is between 20-25%. The final rolling temperature is 920℃, the coiling temperature is 780℃, the hot-rolled annealed steel plate temperature is 1100℃, the line speed (TV value) is 100 mm·m / min, the first cold rolling pass deformation is 20%, the total deformation is 75%, and the last cold rolling pass deformation is 10%. The cold-rolled annealed steel plate temperature is 1080℃, and the line speed is 45 m / min.

[0087] Table 1. Chemical composition (weight percentage) of austenitic stainless steel in Examples 1-6 and Comparative Examples

[0088]

[0089] Performance testing

[0090] The performance of the austenitic stainless steel cold coils prepared in Examples 1-6 and the comparative examples was tested. The evaluation results of the mechanical properties at -163℃ are shown in Table 2, and the evaluation results of the fracture toughness and fatigue resistance at -163℃ are shown in Table 3.

[0091] Table 2 Summary of Evaluation Results of Mechanical Properties of Austenitic Stainless Steel at -163℃

[0092]

[0093]

[0094] Table 3 Summary of Evaluation Results of Fracture Toughness and Fatigue Resistance of Austenitic Stainless Steel at -163℃

[0095]

[0096] The present invention has been disclosed above with reference to preferred embodiments. However, those skilled in the art should understand that these embodiments are merely illustrative of the invention and should not be construed as limiting its scope. It should be noted that any variations and substitutions equivalent to these embodiments should be considered to be covered within the scope of the claims. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A method for improving the ultra-low temperature strength, toughness, and fatigue resistance of austenitic stainless steel, characterized in that, include: (1) Steel with qualified composition is obtained by refining through electric furnace → AOD → LF, and then cast billet is obtained by continuous casting; By weight percentage, the molten steel comprises 0.045%~0.055% C, 0.55%~0.6% Si, 4%~5% Mn, 16%~17% Cr, 6%~6.5% Ni, 0.15%~0.18% N, 0.2%~0.3% Co, with the balance being Fe and other unavoidable impurity elements; (2) The billet is subjected to hot rolling, hot coil annealing, cold rolling, and cold coil heat treatment processes in sequence; In the hot rolling process, rolling is stopped when the billet is rolled to the intermediate specification and the temperature is above 1100℃, and the dwell time is more than 20 seconds. Rolling continues when the temperature drops to below 900℃. The deformation amount per pass is ≥30%, the final rolling temperature is controlled at 800-850℃, and the coiling temperature is controlled at 650-680℃.

2. The method according to claim 1, characterized in that, AOD uses aluminum for deoxidation, with an aluminum addition rate of 1-1.5 kg / ton of molten steel.

3. The method according to claim 1, characterized in that, The high-alumina refining slag system produced at the LF station consists of, by weight percentage: 55%~60% CaO, 10%~15% Al2O3, 8%~10% MgO, 10~15% SiO2, and 3%~5% CaF2.

4. The method according to claim 1, characterized in that, The [Ca] / [Al] ratio in the molten steel during the LF process is 0.13%~0.20%, the argon blowing and weak stirring flow rate is 70-80L / min, and the time is ≥30min.

5. The method according to claim 1, characterized in that, In the hot rolling process, the billet is heated to 1250~1260℃ and held for 3-3.5 hours.

6. The method according to claim 1, characterized in that, In the hot-rolled annealing process, the steel plate temperature is 1150-1170℃; the line speed (TV) is 120-130 mm·m / min.

7. The method according to claim 1, characterized in that, In the cold rolling process, the deformation amount of the first pass is ≥25%, the total deformation amount is greater than 70%, and the deformation amount of the last pass is controlled at 5%-7%.

8. The method according to claim 1, characterized in that, In the cold coil heat treatment process, the steel plate temperature in the first stage is 900-950℃ and the wire speed is 10-12m / min; the steel plate temperature in the second stage is 1100-1120℃ and the wire speed is 50-55m / min.

9. An austenitic stainless steel, characterized in that, The method for improving the ultra-low temperature strength, toughness and fatigue resistance of austenitic stainless steel as described in any one of claims 1 to 8 was used in the preparation process.

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