Preparation method of FCC crystal structure alloy for ultra-low temperature applications with high microstructure homogeneity

Abstract

A method for preparing a crystal structure alloy for ultra-low temperature applications with high microstructure uniformity belongs to the field of metal materials technology. Specifically, it relates to a method for improving the microstructure uniformity of FCC structural alloys for ultra-low temperature applications. By adjusting process parameters, the method increases the nucleation sites of coarse, elongated, non-recrystallized grains in the hot-rolled microstructure during the solid solution process, thereby improving the mixed-crystal microstructure and enhancing the uniformity of the finished product microstructure. The method includes the following steps: (1) preparing FCC structural alloy billets according to the set composition; (2) heating the billets and holding them at the set temperature, followed by multi-pass rough rolling and multi-pass finish rolling; (3) cooling the finish-rolled slab and then performing single-pass rolling to obtain hot-rolled plates; (4) performing solid solution treatment on the hot-rolled plates; (5) water quenching the solution-treated plates to room temperature to obtain medium-thick plates of FCC structural alloys for ultra-low temperature applications with uniform microstructure.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials technology, specifically relating to a method for preparing an FCC crystal structure alloy with high microstructure uniformity for ultra-low temperature applications. Background Technology

[0002] The FCC structural alloy involved in this invention is a fully austenitic metallic material with excellent low-temperature strength, plasticity, and fracture toughness. It exhibits superior mechanical properties at extremely low temperatures and can be used as a primary material for the outer shell of the toroidal field coil in the China Fusion Engineering Experimental Reactor, showing broad development prospects. However, this material has a narrow temperature range for complete dynamic recrystallization during hot deformation (typically above 1100℃ under industrial production conditions), making it difficult to ensure it remains within the complete dynamic recrystallization range during hot rolling. In other words, the last few passes of hot rolling tend to be within the partial dynamic recrystallization range. Furthermore, due to its high strength and large deformation resistance, it is difficult to achieve high-pressure rolling, often resulting in the slab temperature dropping below 950℃ after hot rolling (the static recrystallization temperature of austenitic stainless steel is generally above 950℃). Therefore, even after hot rolling and waiting at a warm temperature, it is difficult to induce sufficient static recrystallization in the hot-rolled plate to obtain a hot-rolled plate with a fully recrystallized structure. This results in a microstructure of hot-rolled sheet composed of coarse, elongated, banded unrecrystallized grains along the rolling direction and fine, equiaxed recrystallized grains—a non-uniform deformation microstructure. In particular, to obtain a heavier hot-rolled slab, a thicker initial billet (≥300mm) or a wider hot-rolled slab is used before hot rolling (increasing width increases rolling torque, reduces rolling pass reduction, increases the number of rolling passes, and slows down the rolling pace). After solution treatment, recrystallized grains form in both deformation microstructure regions, but their size distribution uniformity is significantly different. In hot-rolled microstructures, the large, elongated, non-recrystallized grain regions experience earlier nucleation at grain boundaries than within the grains during solution treatment. Nucleation sites are fewer, especially within the grains, where they are far fewer than at grain boundaries. Furthermore, due to the high deformation energy stored in the non-recrystallized grains, the grain boundary migration rate of newly nucleated grains is high, easily leading to the formation of large, equiaxed grains. Conversely, in hot-rolled microstructures, the fine, dynamically recrystallized grain regions primarily undergo nucleation at grain boundaries during solution treatment, resulting in more nucleation sites and a lower grain boundary migration rate, easily forming fine, equiaxed grains. Ultimately, the solution-treated microstructure contains regions with both large and small grain sizes, forming a mixed-grain structure. This mixed-grain structure leads to uneven deformation in micro-regions and triggers microcrack initiation, deteriorating the material's mechanical and low-temperature properties, reducing structural safety, and causing significant economic losses. Therefore, this is a pressing problem to be solved in the demanding applications of stainless steel plates for nuclear power plants.

[0003] Patent CN114042773A discloses a method for improving the microstructure uniformity of extra-thick austenitic stainless steel plates. By controlling the thickness of the forged slab through die casting, electroslag remelting, and 3D forging, and by setting appropriate rolling ratios, conditions are provided for sufficient recrystallization during deformation. The heating, descaling, and conveying time of the slab are controlled to reduce temperature drop and obtain deformation conditions more conducive to full-thickness recrystallization. During hot rolling, the pass reduction rate is controlled according to the slab thickness, and appropriate dwell time is ensured between passes to further promote full-thickness recrystallization. Ultimately, extra-thick austenitic stainless steel plates (thickness > 60 mm) with uniform microstructure and a full-thickness grain size rating within ±1 grade are obtained. The main purpose of this method is to improve microstructure uniformity by designing the initial hot-rolled slab, heating temperature, and rolling process parameters to ensure sufficient recrystallization throughout the thickness of the steel plate during hot rolling. This method is not suitable for medium-thick plates of FCC structural alloys used in extremely low temperatures, as these materials are difficult to recrystallize fully during hot rolling. Furthermore, this method employs appropriate pauses between hot rolling passes to improve microstructure uniformity, which reduces the rolling pace and affects production efficiency. Additionally, the embodiments of this method primarily target austenitic stainless steel with Cr contents of 17.6% and 18.1% and Mn contents of 1.56% and 1.23%.

[0004] Patent CN110724809A discloses a heat treatment method for controlling the grain size of hot-rolled high-carbon austenitic stainless steel. This method involves controlling the furnace heating rate of the slab to ≤5℃ / min and employing a multi-stage annealing process to gradually release the deformation energy (simultaneous recrystallization nucleation occurs at a lower temperature, followed by solid solution at a higher temperature to achieve the desired grain size). While this method improves the uniformity of the stainless steel's microstructure through multi-stage annealing, controlling the cooling rate to ≤5℃ / min increases the difficulty and makes it challenging to implement in actual production. Furthermore, the multi-stage annealing and holding process is energy-intensive, costly, and has a long heat treatment cycle with low efficiency.

[0005] In addition, scholars both domestically and internationally have developed several controlled rolling and cooling methods to address the low yield strength of austenitic stainless steel. Patent CN101748342A discloses a method for manufacturing high-strength 18Cr-8Ni austenitic stainless steel hot-rolled medium-thick plates. This method involves strictly controlling the proportions of elements such as Cr, Ni, Mo, N, and C in the austenitic stainless steel, employing a high-temperature continuous hot rolling process to ensure that the 18Cr-8Ni austenitic stainless steel is hot-rolled above its recrystallization temperature, thus avoiding dislocation structures and ensuring the elongation of the finished plate. Subsequently, the cooling rate is controlled to regulate the recrystallization grain size, preventing grain growth and ensuring the strength of the finished plate while avoiding corrosion caused by carbide precipitation. Ultimately, offline solution treatment is avoided, reducing costs and significantly improving material strength. This method is only applicable to 18Cr-8Ni austenitic stainless steel (Cr content 17-20%, Mn content ≤2%). Meanwhile, the main purpose of this method is to ensure that the steel plate is finally rolled above the dynamic recrystallization temperature by designing alloy composition, heating temperature, and rolling process parameters, thereby achieving the effect of "avoiding offline solution treatment, reducing costs, and significantly improving material strength". However, this method is not suitable for medium-thick plates made of FCC structural alloys for extremely low temperatures, as these materials are difficult to recrystallize sufficiently during hot rolling.

[0006] Patent JP 60-26619 discloses a method for improving the strength of austenitic stainless steel. The patent primarily describes austenitic stainless steel with the following composition: C ≤ 0.08%, N ≤ 0.25%, Cr 16.00–20.00%, Ni 8.00–16.00%, Mo 0–3.00%, Mn ≤ 2%, and Si ≤ 1%. The method involves rolling at a temperature above Tr = 940 + 30 × (%Mo) to obtain fine recrystallized grains, followed by cooling at a certain rate from a temperature above 800°C to 500°C. In other words, the strength is improved by refining the grains through rolling in the recrystallization zone.

[0007] Japanese Patent Application Publication No. 5-75809 discloses a method for manufacturing high-strength austenitic stainless steel. This patent mainly targets austenitic stainless steel with the following composition: C≤0.08%, N≤0.25%, Cr 16.00~20.00%, Ni 8.00~16.00%, Mn≤2%, and Si≤1%. This method involves rolling an austenitic stainless steel slab at a temperature above Tr = 940 + 30 × (%Mo) (the recrystallization temperature range) to form fine recrystallized grains. Then, a cumulative reduction of 5% to 30% is applied within a temperature range of Tp (600℃ (C≤0.03%) or 5000 × (%C) (C>0.03%)) to (Tr-40)℃ (the non-recrystallization temperature range) to introduce dislocations into the fine recrystallized grains. After rolling, the slab is rapidly cooled to below 500℃ at a certain cooling rate. Finally, the strength of the austenitic stainless steel is improved by refining the grains and introducing dislocations through rolling process control. However, this method does not address the issue of improving the uniformity of the microstructure in the thickness direction of the thicker austenitic stainless steel plate.

[0008] Patent CN101724789A discloses a two-stage hot-rolled austenitic stainless steel medium-thick plate manufacturing method. This patented method primarily aims to obtain high-strength austenitic stainless steel medium-thick plates, mainly targeting austenitic stainless steel containing small amounts of rare elements, with a Cr content of 16-22%, a Mn content of 1-2%, and a microstructure containing 1-10% high-temperature δ-ferrite. The method involves: rolling the designed austenitic stainless steel in the recrystallization range of 1250-1050℃ to obtain fine recrystallized grains; then rapidly cooling it to the non-recrystallization range of 700-950℃ for rolling with a certain deformation amount (cumulative reduction of over 40%). This breaks down the fine recrystallized grains into even finer grains, increasing the dislocation density. In subsequent low-temperature annealing, these fine grains and dislocations serve as nucleation sites, facilitating grain size refinement and thus improving strength. This method mainly refines the microstructure of the finished product by refining the hot-rolled grains and introducing dislocations through alloy composition design, hot rolling process control, and annealing process control, thereby improving the strength of austenitic stainless steel. However, it does not address the issue of improving the microstructure uniformity in the thickness direction of the thick plates of austenitic stainless steel.

[0009] The aforementioned controlled rolling and cooling methods primarily enhance the strength of austenitic stainless steel through one-stage or two-stage rolling. The main method involves inducing full dynamic recrystallization through the first-stage rolling, forming fine recrystallized grains (and introducing dislocations or further fragmenting these fine grains through the second-stage rolling (with a certain amount of small deformation)), ultimately improving the strength of the austenitic stainless steel. It is evident that the first-stage rolling in these methods mainly rolls the initial billet to near the finished product thickness, promoting full recrystallization to obtain fine grains. The second-stage rolling mainly uses a certain amount of small deformation to introduce dislocations or fragment these fine grains, thereby ultimately achieving strength improvement, without considering the improvement of microstructure uniformity. Furthermore, for ultra-low temperature FCC structural alloys, it is difficult to ensure that they remain within the full recrystallization temperature range when hot-rolled to near the finished product thickness. These controlled rolling and cooling methods are not applicable to the ultra-low temperature FCC structural alloys of this invention, which require improved microstructure uniformity. Summary of the Invention

[0010] To address the issue of microstructure uniformity in FCC structural alloys for ultra-low temperatures, this invention aims to provide a method for preparing FCC crystal structure alloys for ultra-low temperatures with high microstructure uniformity. Specifically, it relates to a method for improving the microstructure uniformity of FCC structural alloys for ultra-low temperatures, which involves using FCC structural alloys as raw materials and adjusting process parameters to increase the nucleation sites of coarse, elongated, non-recrystallized grains in the hot-rolled microstructure during the solid solution process, thereby improving the mixed-grain microstructure and enhancing the uniformity of the finished product microstructure.

[0011] This invention provides a method for preparing an FCC crystal structure alloy with high microstructure uniformity for ultra-low temperature applications, comprising the following steps:

[0012] (1) Prepare FCC structural alloy billets according to their composition. The composition, by mass percentage, contains C≤0.03%, N 0.21~0.27%, Cr 11.00~13.00%, Ni 11.00~13.00%, Mo 4.00~6.00%, Mn 9.00~11.00%, P≤0.030%, S≤0.020%, Si≤0.75%, with the balance being Fe and unavoidable impurities;

[0013] (2) Heat the billet to 1200℃±30℃ and hold it for 180~240min, then perform multiple rough rolling and multiple finish rolling.

[0014] (3) The finished slab is cooled and then rolled in a single pass to obtain hot-rolled sheet;

[0015] (4) Solution treatment of hot-rolled sheet metal;

[0016] (5) After solution treatment, the material is quenched in water to room temperature to obtain a medium-thick plate of FCC structural alloy for ultra-low temperature with uniform structure.

[0017] in:

[0018] In step (2), the finishing rolling temperature is 900-950℃.

[0019] In step (3), the slab is cooled at a rate of 50-100°C / s to 590-800°C.

[0020] In step (3), the single-pass reduction rate is 13-20%, and the thickness of the resulting hot-rolled sheet is 10-45 mm.

[0021] In step (3), the cooling equipment used is an ultra-fast cooling device.

[0022] In step (4), the solution treatment temperature is 1050-1100℃ and the holding time is 30-60min.

[0023] In step (5), the average grain size of the thick plate of the obtained FCC structural alloy is 45-90 μm, and the difference in grain size at different positions in the thickness direction is ≤10 μm.

[0024] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0025] The FCC structural alloys used in this invention possess excellent low-temperature mechanical properties. However, the temperature range within which complete dynamic recrystallization occurs during hot deformation is narrow. Simultaneously, they exhibit high strength and significant deformation resistance, making high-pressure rolling difficult. Consequently, the last few passes of hot rolling are prone to being in a partially dynamic / non-recrystallized range, especially for large-weight, wide hot-rolled plates. Ultimately, the microstructure of the hot-rolled plate consists of coarse, elongated deformed grains and fine, equiaxed recrystallized grains, resulting in a non-uniform hot-rolled deformed microstructure. After subsequent solution treatment, a mixed-grain microstructure is formed.

[0026] 1. This invention optimizes the process, resulting in sufficiently refined grains formed in the elongated grain region during solution treatment within a non-uniformly deformed microstructure, while the grains formed in the fine dynamic recrystallized grain region remain largely unchanged. Therefore, the size difference between the new grains formed in the elongated grain region and the fine dynamic recrystallized grain region during solution treatment is reduced, leading to improved microstructure uniformity and elimination of mixed-grain structures.

[0027] 2. The method of the present invention can obtain FCC structural alloy medium-thick plates with uniform microstructure, especially large single-weight and large-width medium-thick plates, reduce welding during use of steel plates, reduce welding risks, and improve the stability and safety of key equipment manufactured based on FCC structural alloys.

[0028] 3. Compared with the technology of using controlled hot rolling process to fully ensure recrystallization of the steel plate throughout the thickness during hot rolling to improve the uniformity of the microstructure, the method of the present invention does not have specification restrictions on the initial hot-rolled billet, does not require excessively high heating temperature, and the hot rolling process can be carried out continuously without the need for appropriate stops between passes. At the same time, ultra-fast cooling equipment is used to quickly cool the steel plate to below 800°C, so the rolling rhythm is fast and the production efficiency is high.

[0029] 4. Compared with the technology of improving the uniformity of microstructure by using multi-stage solution treatment, the solution treatment process of the present invention is simple to operate, highly operable, and easier to realize industrial application; at the same time, the heat treatment cycle is short, the efficiency is high, and the heat treatment energy consumption is low. This will not only greatly reduce production costs, but also achieve energy conservation and emission reduction goals, which is in line with the current "dual carbon" strategy and conducive to the sustainable development of related materials industries. Attached Figure Description

[0030] Figure 1 The images show the IPF (In-Pack Fiber) diagrams of the central layer in the thickness direction of the FCC structural alloy after solution treatment in Examples 1-3 and Comparative Example 2 of this invention; where (a) is Example 1; (b) is Example 2; (c) is Example 3; and (d) is Comparative Example 2.

[0031] Figure 2 The images show the microstructure of the central layer of the FCC structural alloy after hot rolling and solution treatment in Comparative Example 1 of this invention; where (a) is the microstructure of the hot-rolled alloy and (b) is the microstructure of the solution-treated alloy.

[0032] Figure 3 The figures show the grain size differences at different positions along the thickness direction of the FCC structural alloy medium-thick plates in Examples 1-4 and Comparative Examples 1-4 of the present invention; wherein, (a) is Example 1; (b) is Example 2; (c) is Example 3; (d) is Example 4; (e) is Comparative Example 1; (f) is Comparative Example 2; (g) is Comparative Example 3; and (h) is Comparative Example 4. Detailed Implementation

[0033] The main idea of ​​this invention is that the FCC structural alloy involved forms two microstructures during hot rolling deformation: elongated deformed grains that have not undergone dynamic recrystallization and fine dynamically recrystallized grains. The fine dynamically recrystallized grains have a random orientation, while the elongated deformed grains that have not undergone dynamic recrystallization are mainly concentrated near orientations with larger Schmidt factors, thus undergoing preferential deformation during the deformation process. Ultimately, after finishing rolling, the FCC structural alloy involved in this invention is rapidly cooled to below 800°C and subjected to an appropriate reduction in deformation. The elongated deformed grains undergo preferential deformation, bearing more deformation, while the fine dynamically recrystallized grains are less prone to deformation and bear less deformation. This promotes an increase in the number of slip bands in the elongated grain region, the interlacing of slip bands, dislocation accumulation, and entanglement. This deformation characteristic leads to a significant change in the number and manner of nucleation in the elongated grain region during the subsequent solid solution process, while the nucleation manner and number in the fine dynamic recrystallized grain region remain largely unchanged. The nucleation mechanism in the elongated grain region changes from "grain boundary nucleation precedes intragranular nucleation, and there are fewer nucleation sites, especially intragranular nucleation sites are far fewer than grain boundary nucleation" to "there are more nucleation sites, and intragranular nucleation sites are more numerous than grain boundary nucleation, with intragranular nucleation gradually preceding grain boundary nucleation." Ultimately, this results in the grains formed in the elongated grain region during solid solution being sufficiently refined, while the grains formed in the fine dynamic recrystallized grain region remain largely unchanged. Therefore, the size difference between the new grains formed in the elongated grain region and the fine dynamic recrystallized grain region during solid solution is reduced, leading to improved microstructure uniformity and the elimination of mixed-grain structures.

[0034] In this embodiment of the invention, the solution treatment is performed using a box-type resistance furnace, and the solution is then water-cooled and quenched to room temperature after being taken out of the furnace.

[0035] In this embodiment of the invention, hot-rolled plates and solution-treated plates were randomly sampled along their entire thickness, sanded, and electropolished before being installed on a ZEISS ULTRA. TM The Oxford EBSD system on a Type 55 field emission scanning electron microscope was used for scanning, and the Aztec Crystal software was used for grain size determination and IPF plotting.

[0036] The specific embodiments of the present invention will be described in further detail below, but the embodiments of the present invention are not limited thereto.

[0037] Example 1

[0038] A method for preparing an FCC crystal structure alloy with high microstructure uniformity for ultra-low temperature applications, comprising the following steps:

[0039] (1) The composition of the FCC structure alloy, by mass percentage, contains 0.02% C, 0.27% N, 13.00% Cr, 11.00% Ni, 4.00% Mo, 11.00% Mn, 0.030% P, 0.020% S, 0.75% Si, with the balance being Fe and unavoidable impurities;

[0040] (2) Heat the billet to 1200℃±30℃, hold for 180min, and then perform multiple rough rolling and multiple finish rolling. The final rolling temperature of finish rolling is 900℃.

[0041] (3) After finishing rolling, the slab is cooled to 730°C at 70°C / s and rolled in a single pass with a single pass reduction rate of 13% to obtain a hot-rolled plate with a thickness of 10mm.

[0042] (4) The hot-rolled sheet is subjected to solution treatment at a temperature of 1100℃ and a holding time of 60min.

[0043] (5) After solution treatment, the grain size was quenched to room temperature and the average grain size was 90 μm by EBSD. The difference in grain size at different positions in the thickness direction was 10 μm.

[0044] The IPF diagram of the central layer of the FCC structural alloy in the thickness direction after solution treatment is shown below. Figure 1 (a) The grain size difference diagram at different positions along the thickness direction of the thick plate in the FCC structural alloy is shown in Figure 1. Figure 3 (a).

[0045] Example 2

[0046] The method is the same as in Example 1, except that:

[0047] (1) The composition of the FCC structure alloy by mass percentage is C 0.028%, N 0.23%, Cr 12.19%, Ni 11.82%, Mo 4.83%, Mn 10.53%, P 0.030%, S 0.020%, Si 0.75%, with the balance being Fe and unavoidable impurities;

[0048] (2) After finishing rolling, the slab is cooled to 690°C at 80°C / s and rolled in a single pass with a single pass reduction rate of 15% to obtain a hot-rolled plate with a thickness of 25mm.

[0049] (3) The hot-rolled sheet is subjected to solution treatment at a temperature of 1080℃ and a holding time of 45min.

[0050] After solution treatment and water quenching to room temperature, the average grain size was found to be 61.02 μm by EBSD, and the grain size difference at different positions in the thickness direction was 8 μm.

[0051] The IPF diagram of the central layer of the FCC structural alloy in the thickness direction after solution treatment is shown below. Figure 1 (b) The grain size difference diagram at different positions along the thickness direction of the thick plate of FCC structural alloy is shown in the figure. Figure 3 (b)

[0052] Example 3

[0053] The method is the same as in Example 1, except that:

[0054] (1) The composition of the FCC structure alloy by mass percentage is C 0.03%, N 0.21%, Cr 11.00%, Ni 13.00%, Mo 6.00%, Mn 9.00%, P 0.030%, S 0.020%, Si 0.75%, with the balance being Fe and unavoidable impurities;

[0055] (2) After finishing rolling, the slab is cooled to 800°C at 50°C / s and rolled in a single pass with a single pass reduction rate of 20% to obtain a hot-rolled plate with a thickness of 40mm.

[0056] (3) The hot-rolled sheet is subjected to solution treatment at a temperature of 1050℃ and a holding time of 30min.

[0057] After solution treatment and water quenching to room temperature, the average grain size was found to be 45 μm by EBSD, and the grain size difference at different positions in the thickness direction was 6 μm.

[0058] The IPF diagram of the central layer of the FCC structural alloy in the thickness direction after solution treatment is shown below. Figure 1 (c) The grain size difference diagram at different positions along the thickness direction of the thick plate of FCC structural alloy is shown in the figure. Figure 3 (c)

[0059] Example 4

[0060] The raw materials and methods are the same as in Example 1, except that:

[0061] (1) Heat the billet to 1200℃±30℃, hold for 240min, and then perform multiple rough rolling and multiple finish rolling. The final rolling temperature of finish rolling is 950℃.

[0062] (2) After finishing rolling, the slab is cooled to 590°C at a temperature of 100°C / s and rolled in a single pass with a single pass reduction rate of 20% to obtain a hot-rolled plate with a thickness of 45mm.

[0063] (3) The hot-rolled sheet is subjected to solution treatment at a temperature of 1080℃ and a holding time of 30min.

[0064] After solution treatment and water quenching to room temperature, the average grain size was found to be 50.15 μm by EBSD, and the grain size difference at different positions in the thickness direction was 7 μm.

[0065] The grain size difference diagram at different locations along the thickness direction of the FCC structural alloy medium-thick plate is shown below. Figure 3 (d)

[0066] Example 5

[0067] The raw materials and methods are the same as in Example 1, except that:

[0068] (1) Heat the billet to 1200℃±30℃, hold for 230min, and then perform multiple rough rolling and multiple finish rolling. The final rolling temperature of finish rolling is 930℃.

[0069] (2) After finishing rolling, the slab is cooled to 650°C at a temperature of 80°C / s and then rolled in a single pass.

[0070] (3) The hot-rolled sheet is subjected to solution treatment at a temperature of 1100℃ and a holding time of 50min.

[0071] After solution treatment and water quenching to room temperature, the average grain size was found to be 75.63 μm by EBSD, and the grain size difference at different positions in the thickness direction was 9 μm.

[0072] Example 6

[0073] The raw materials and methods are the same as in Example 2, except that:

[0074] (1) Heat the billet to 1200℃±30℃, hold for 180min, and then perform multiple rough rolling and multiple finish rolling. The final rolling temperature of finish rolling is 925℃.

[0075] (2) After finishing rolling, the slab is cooled to 700℃ at a temperature of 90℃ / s and then rolled in a single pass.

[0076] After solution treatment and water quenching to room temperature, the average grain size was found to be 65.76 μm by EBSD, and the grain size difference at different positions in the thickness direction was 7.5 μm.

[0077] Example 7

[0078] The raw materials and methods are the same as in Example 2, except that:

[0079] (1) Heat the billet to 1200℃±30℃, hold for 190min, and then perform multiple rough rolling and multiple finish rolling. The final rolling temperature of finish rolling is 905℃.

[0080] (2) The single-pass reduction rate is 13%, and hot-rolled sheet is obtained;

[0081] (3) The hot-rolled sheet is subjected to solution treatment at a temperature of 1100℃ and a holding time of 45min.

[0082] After solution treatment and water quenching to room temperature, the average grain size was found to be 68.63 μm by EBSD, and the grain size difference at different positions in the thickness direction was 8.5 μm.

[0083] Example 8

[0084] The raw materials and methods are the same as in Example 3, except that:

[0085] (1) Heat the billet to 1200℃±30℃, hold for 200min, and then perform multiple rough rolling and multiple finish rolling. The final rolling temperature of finish rolling is 920℃.

[0086] (2) After finishing rolling, the slab is cooled to 790°C at a temperature of 80°C / s and then rolled in a single pass.

[0087] After solution treatment and water quenching to room temperature, the average grain size was found to be 43.05 μm by EBSD, and the grain size difference at different positions in the thickness direction was 6.5 μm.

[0088] Comparative Example 1

[0089] The raw materials and methods were the same as in Example 1, except that after finishing rolling, no single-pass hot rolling was performed; the material was directly water-cooled to room temperature before solution treatment. After solution treatment, the average grain size was 126.44 μm as determined by EBSD, and the grain size difference at different locations along the thickness direction was 56 μm.

[0090] Hot-rolled and solution-treated microstructures of the product, such as Figure 2 As shown, it can be seen that the hot-rolled plate is a mixed-crystal structure composed of unrecrystallized grains elongated along the rolling direction and fine dynamic recrystallized grains, and the steel plate after solution treatment also has obvious mixed crystal structure.

[0091] Microstructure of the central layer of the hot-rolled FCC structural alloy in the thickness direction is shown in the figure. Figure 2 (a) Microstructure of the central layer of the FCC structural alloy in the thickness direction after solution treatment is shown in Figure 1. Figure 2 (b) The grain size difference diagram at different positions along the thickness direction of the thick plate of FCC structural alloy is shown in the figure. Figure 3 (e).

[0092] Comparative Example 2

[0093] The raw materials and methods are the same as in Example 1, except that the single-pass rolling reduction is 7%; after solution treatment, the average grain size is 100.63 μm and the grain size difference at different positions in the thickness direction is 42 μm.

[0094] The IPF diagram of the central layer of the FCC structural alloy in the thickness direction after solution treatment is shown below. Figure 1 (d) The grain size difference diagram at different positions along the thickness direction of the thick plate of FCC structural alloy is shown in Figure 1. Figure 3 (f).

[0095] Comparative Example 3

[0096] The raw materials and methods are the same as in Example 3, except that the single-pass rolling reduction is 30%; after solution treatment, the average grain size is 39.86 μm and the grain size difference at different positions in the thickness direction is 36 μm.

[0097] The grain size difference diagram at different locations along the thickness direction of the FCC structural alloy medium-thick plate is shown below. Figure 3 (g)

[0098] Comparative Example 4

[0099] The raw materials and methods are the same as in Example 2, except that: the material is cooled to 850°C for single-pass rolling with a reduction of 20%; after solution treatment, the average grain size is 69.79 μm and the grain size difference at different positions in the thickness direction is 34 μm.

[0100] The grain size difference diagram at different locations along the thickness direction of the FCC structural alloy medium-thick plate is shown below. Figure 3 (h).

[0101] The features and effects of the present invention have been described in detail above through examples and comparative examples. However, the invention is not limited to these examples. There are many other equivalent examples that can be made without departing from the concept of the present invention, such as applications to other alloys.

Claims

1. A method for preparing an FCC crystal structure alloy with high microstructure uniformity for ultra-low temperature applications, characterized in that, Includes the following steps: (1) Prepare FCC structural alloy billets according to their composition. The composition, by mass percentage, contains C≤0.03%, N 0.21~0.27%, Cr 11.00~13.00%, Ni 11.00~13.00%, Mo 4.00~6.00%, Mn 9.00~11.00%, P ≤0.030%, S ≤0.020%, Si ≤0.75%, with the balance being Fe and unavoidable impurities; (2) After heating the billet to 1200℃±30℃, hold it for 180~240 min, and then perform multiple rough rolling and multiple finish rolling. The final rolling temperature of the multi-pass roughing and multi-pass finishing rolling is 900~950℃; (3) After the slab is cooled, it is rolled in a single pass to obtain hot-rolled sheet; The slab is cooled at a rate of 50~100℃ / s to 590~800℃, with a single-pass reduction rate of 13~20%, and the resulting hot-rolled sheet has a thickness of 10~45 mm. (4) The hot-rolled sheet is subjected to solution treatment at a temperature of 1050~1100℃ and a holding time of 30~60 min; (5) After solution treatment, the material is quenched in water to room temperature to obtain a medium-thick plate of FCC structural alloy for ultra-low temperature with uniform structure.

2. The method for preparing a high-uniformity, ultra-low temperature FCC crystal structure alloy according to claim 1, characterized in that, In step (3), the cooling equipment used is an ultra-fast cooling device.

3. The method for preparing a high-uniformity, ultra-low temperature FCC crystal structure alloy according to claim 1, characterized in that, In step (5), the average grain size of the thick plate of the obtained FCC structural alloy is 45~90 μm, and the difference in grain size at different positions in the thickness direction is ≤10 μm.

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