An ultra-thick invar alloy plate having a uniform structure and a method of manufacturing the same

Abstract

The application discloses a kind of super-thick invar alloy plate of uniform organization and manufacturing method thereof, 6-9t invar alloy steel ingot is prepared by electric furnace→AOD refining→LF refining→VD refining→IC die casting process;Nb element is added in AOD refining process;After that, invar alloy steel ingot carries out stress relief annealing, and adopts three times upsetting and drawing process, upsetting reduction is according to the principle of increment, gradually refines steel ingot surface and heart grain structure, ensures that steel ingot internal organization is fully broken after recrystallization, then forged to target size after annealing, obtains invar alloy plate of thickness ≥50mm after air cooling;The application can prevent the grain coarsening of the heart of super-thick invar alloy plate with thickness greater than 50mm, and obtain super-thick invar alloy plate with grain size ≥4 levels and uniform organization.

Description

Technical Field

[0001] This invention relates to the field of Invar alloy materials, and more specifically, to an ultra-thick Invar alloy sheet with uniform microstructure and its manufacturing method. Background Technology

[0002] Invar alloys are typical low-expansion alloys, exhibiting a low coefficient of thermal expansion within a temperature range of -180℃ to 100℃. Steel grade 4J36 is a typical low-expansion alloy, compatible with borosilicate glass or ceramic materials, and possesses both excellent mechanical properties and high-temperature expansion performance, making it widely used in aerospace and other fields. Metallurgical theory dictates that to ensure the alloy's linear expansion performance meets the requirements within its service temperature range, the addition of microalloying elements should be avoided. In recent years, with the development of aerospace equipment towards larger and ultra-larger sizes, user requirements for Invar alloy plates have also evolved towards greater width and greater thickness. For example, the slab thickness gradually changes from the original 10-50mm to 100mm or more, but the requirements for the grain size of the alloy slab are not relaxed. The grain size of the entire cross section of the slab is still required to be ≥4, and a very small number of 3-grade grains are acceptable. Mixed grains and grain sizes of 2 grade or below are not allowed. However, in the actual hot working process of thicker slabs, due to the high core temperature, the deformation is insufficient, which causes the core structure of the slab to coarsen at high temperature and grow abnormally. At the same time, due to the mismatch between the deformation temperature and the pressing parameters, it is difficult for the core of the material to recrystallize, and the structure cannot be further refined.

[0003] Chinese patent application CN202411232032.4 discloses an Invar alloy and its rheological forming method. This technology starts with pure metal raw materials, optimizes and improves the production process, simplifies the alloy smelting process, and significantly improves the purity of the melt. It adopts thin strip continuous casting technology, and by adjusting the cooling double roller cooling method and the smaller casting strip thickness, it can achieve rapid solidification of Invar alloy. The casting strip surface is smooth, without vibration marks, inclusions, flash, etc., which can effectively inhibit the segregation of inclusion elements to grain boundaries and reduce the oxidation of the casting strip. The thin strip continuous casting process can shorten or even eliminate the hot rolling process, fundamentally avoiding the problem of hot working cracking. This technology can greatly reduce the Invar alloy smelting production process, improve production efficiency and yield, with a yield of over 80%, shorten the production cycle, reduce production costs, and enable rapid conversion between different products, with a wide range of product applications. However, this technology belongs to the thin strip continuous casting process and does not involve the shaping process of materials such as forging billets, so it is not suitable for the preparation of ultra-thick plates.

[0004] Chinese patent application CN202510036472.0 discloses a production method for Invar alloy 4J36 hot-rolled coils. The method utilizes a reciprocating rolling mill to provide sufficient dynamic recrystallization time for the strip, reducing its anisotropy and significantly minimizing performance differences across the entire sheet. After annealing, the austenite equiaxed grain ratio is high, resulting in high yield strength, tensile strength, and elongation of the hot-rolled black coil, ensuring high strength and toughness in the finished product. The method employs a preheating furnace and heating furnace series production process. Considering the high alloy content and low thermal conductivity of this steel grade, slow heating at low temperatures is used to avoid excessive internal structural and temperature stresses that could cause fracture in the slab. Reasonable control of heating time and temperature prevents excessive oxide scale buildup during rolling. Finishing rolling utilizes furnace temperature control and a reasonable cooling method to ensure forced rapid cooling after high-temperature rolling, further refining the grains and improving the strength of the black coil. However, this technology is only applicable to hot-rolled coils and not to slab materials, especially ultra-thick plates.

[0005] In view of the above, there is an urgent need to develop a manufacturing method for ultra-thick Invar alloy plates with a thickness greater than 50 mm, which can prevent core grain coarsening and ensure uniform grain structure. Summary of the Invention

[0006] To address the deficiencies in existing technologies, the purpose of this invention is to provide an ultra-thick Invar alloy sheet with uniform microstructure and its manufacturing method, which can prevent core grain coarsening in ultra-thick Invar alloy sheets with a thickness greater than 50 mm, and obtain ultra-thick Invar alloy sheets with a grain size ≥ 4 and uniform microstructure.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] The first aspect of the present invention provides a method for manufacturing an ultra-thick Invar alloy plate with uniform microstructure, comprising the following steps:

[0009] S1, smelting, using electric furnace → AOD refining → LF refining → VD refining → IC die casting process to prepare 6-9t Invar alloy steel ingots;

[0010] Nb is added during the AOD refining process to ensure that the Nb content in the Invar alloy steel ingot meets the following requirements: 0.01×Ni+0.1%~0.03×Ni+0.01%, where Ni is the weight percentage content of Ni in the Invar alloy steel ingot.

[0011] S2, Invar alloy steel ingots are sent into an annealing furnace for stress-relief annealing, and the annealing holding time is 20-40 hours;

[0012] S3, after heating the annealed steel ingot, it is shaped and first upset drawing is performed. First, it is shaped until the diameter of the head and tail of the steel ingot is the same. The upsetting temperature of the steel ingot is controlled to be ≥1000℃. The upsetting reduction h meets the requirements of 1 / 4 of the steel ingot body height to 1 / 3 of the steel ingot body height. Then the steel ingot is drawn to the original height of the Invar alloy steel ingot and then put back into the furnace for heat preservation.

[0013] S4. The steel ingot that has been kept warm in step S3 is subjected to a second upsetting and drawing process. The upsetting temperature of the steel ingot is controlled to be ≥950℃. The upsetting reduction h meets the following requirements: 1 / 3 of the steel ingot body height ~ 1 / 2 of the steel ingot height. The steel ingot is then drawn to 4 / 5 to 5 / 6 of the original Invar alloy steel ingot height and then returned to the furnace for heat preservation.

[0014] S5, the steel ingot after heat preservation in step S4 is subjected to a third upsetting and drawing process. The upsetting temperature of the steel ingot is controlled to be ≥930℃, and the upsetting reduction h is satisfied to be 1 / 2 of the steel ingot body height to 3 / 5 of the steel ingot body height. Then the steel ingot is drawn to 2 / 3 to 3 / 4 of the original Invar alloy steel ingot height, and then the thickness direction is pressed to obtain a plate with a thickness ≤3 / 5 of the original Invar alloy steel ingot head diameter. After that, it is returned to the furnace for heat preservation.

[0015] S6. After the plate is heat-insulated in step S5, it is forged to the target size and then annealed. After air cooling, an Invar alloy plate with a thickness of ≥50mm is obtained.

[0016] Preferably, in step S1, the composition of the Invar alloy steel ingot by weight percentage is as follows: 0.01%≤C≤0.3%, 0.01%≤Si≤0.20%, 0.2%≤Mn≤0.3%, 0.01%≤P≤0.15%, 0.01%≤S≤0.25%, 30%≤Ni≤38%, Mg≤0.1%, Al≤0.20%, 0.01×Ni+0.1%≤Nb≤0.03×Ni+0.01%, with the balance being iron and unavoidable impurities;

[0017] The ingot mold used for IC die casting is octagonal in shape.

[0018] Preferably, in step S2: the stress-relief annealing temperature is 800-950℃, and the annealing holding time is 20-40h.

[0019] Preferably, in step S3:

[0020] The heating temperature is 1200-1210℃, and the heating and holding time is 10-20h;

[0021] The reheating temperature is 1150-1160℃, and the holding time is ≥2h.

[0022] Preferably, in step S4, the reheating temperature is 1120-1130℃, and the holding time is ≥2h.

[0023] Preferably, in step S5, the reheating temperature is 1000-1010℃, and the holding time is ≥1h.

[0024] Preferably, in step S6, the annealing temperature is 800-850℃, and the holding time is required to be ≥1h.

[0025] Preferably, in step S6:

[0026] The thickness of the Invar alloy plate is 50-300 mm, and the width is 1000-2000 mm;

[0027] The Invar alloy sheet has a grain size ≥ 4 and its expansion performance at low temperatures of -180 to 0℃ meets the following requirement: 1×10⁻⁶. -6 ≤δ （-180～0℃） ≤2×10 -6 .

[0028] The second aspect of the present invention provides an ultra-thick Invar alloy sheet obtained by the manufacturing method of the ultra-thick Invar alloy sheet with uniform microstructure according to the first aspect of the present invention, wherein the composition of the ultra-thick Invar alloy sheet is as follows by weight percentage: 0.01%≤C≤0.3%, 0.01%≤Si≤0.20%, 0.2%≤Mn≤0.3%, 0.01%≤P≤0.15%, 0.01%≤S≤0.25%, 30%≤Ni≤38%, Mg≤0.1%, Al≤0.20%, 0.01×Ni+0.1%≤Nb≤0.03×Ni+0.01%, with the balance being iron and unavoidable impurities.

[0029] The composition design principle of the above-mentioned ultra-thick Invar alloy plates is as follows:

[0030] C: C is an austenitic strengthening element, and increasing the C content can effectively reduce the nitrogen saturation solubility in molten steel. In terms of mechanical properties, while C increases the strength of the alloy, it reduces the toughness and low-temperature expansion properties of the alloy. Therefore, to ensure the strength and toughness of the alloy, the C content in this invention is controlled at 0.01% ≤ C ≤ 0.3%.

[0031] Si: As a deoxidizer in molten steel, Si can strengthen the matrix and improve the high-temperature oxidation resistance of steel. An appropriate amount of Si can improve the steel's resistance to strong acid corrosion, but too much Si will reduce the steel's hot workability and toughness, and affect its magnetic permeability. Therefore, in this invention, the Si content is controlled at 0.01% ≤ Si ≤ 0.20%.

[0032] Mn: Mn is an austenite-forming element with a very strong ability to stabilize the austenite phase, effectively controlling the low-temperature phase transformation of martensite. Therefore, in this invention, the Mn content is required to be 0.2% ≤ Mn ≤ 0.3%.

[0033] P and S are harmful elements. These two elements tend to segregate near the columnar grain boundaries of steel ingots, which can have a detrimental effect on the welding performance of the material. Therefore, P and S elements are controlled within a low range. Based on the P and S content level of the raw materials, the requirements are 0.01%≤P≤0.15% and 0.01%≤S≤0.25%.

[0034] Ni (Ni): An austenite-forming element that improves the strength and corrosion resistance of steel. However, excessively high Ni content reduces the solubility of nitrogen in the steel, inhibits the precipitation of strengthening phases such as carbides and nitrides, affects strength, and increases cost. The effect of Ni content on the expansion properties of Invar alloys differs between low-temperature and high-temperature service temperature ranges. As the Ni content decreases, the Curie temperature decreases, moving closer to the low-temperature service temperature range. When the Ni content is below 28% or above 35%, its expansion properties cannot meet user standards. Therefore, in this invention, the Ni content is controlled at 30% ≤ Ni ≤ 38%.

[0035] Nb: This alloying element can effectively pin austenite grain boundaries and refine austenite grain size during the hot deformation of the alloy. To avoid excessive Nb addition leading to element segregation during solidification of the steel ingot, and to prevent further deterioration of the material's low-temperature expansion properties, the content of this element should be controlled within the range of 0.01Ni (wt%) + 0.1% ≤ Nb (wt%) ≤ 0.03Ni (wt%) + 0.01%.

[0036] Mg and Al can further remove oxygen from Invar alloys and improve the hot plasticity of steel ingots, but their levels should not be too high. At the same time, adding Mg to Invar alloys can refine austenite grains, pinning them during hot working and inhibiting austenite grain growth; therefore, in this invention, Mg is controlled to be ≤0.1% and Al to be ≤0.20%.

[0037] Preferably, the Invar alloy plate has a thickness of 50-300 mm and a width of 1000-2000 mm;

[0038] The Invar alloy sheet has a grain size ≥ 4 and its expansion performance at low temperatures of -180 to 0℃ meets the following requirement: 1×10⁻⁶. -6 ≤δ （-180～0℃） ≤2×10 -6 .

[0039] This invention is particularly suitable for preparing ultra-thick Invar alloy plates with a thickness greater than 50 mm, where the alloy grain size is required to be uniform (≥4 grade). During the Invar alloy steel ingot smelting process, Nb microalloying elements are added. Based on the Nb precipitation kinetic model, by limiting the amount of Nb added, it is ensured that Nb is completely precipitated, thus inhibiting grain coarsening. Simultaneously, it ensures that Nb exists entirely in the matrix as compounds such as Fe2Nb, avoiding excessive Nb dissolution in the matrix, which could lead to the alloy's expansion properties failing to meet standard requirements. Furthermore, through a three-upsetting and three-drawing process, the initial forging temperature and upsetting reduction for each heat are specified, following the principle of "gradually decreasing initial upsetting temperature and gradually increasing upsetting reduction." This gradually refines the grain structure on the surface and in the core of the steel ingot. After ensuring sufficient fragmentation and recrystallization of the internal structure of the steel ingot, the structure is transformed into a finished product at a low temperature, preventing later coarsening of the core grain structure due to excessively high core temperatures.

[0040] The present invention has the following beneficial effects:

[0041] This invention effectively prevents core grain coarsening in ultra-thick Invar alloy forging blanks with a thickness greater than 50mm, which results in uneven grain structure across the entire cross-section and failure to meet the overall grain size requirement of ≥4. Through microalloying and matching forging deformation parameters, it ensures uniform grain structure across the entire cross-section of the forging blank, meeting the ≥4 requirement. Simultaneously, this Invar alloy exhibits a low-temperature expansion performance of 1×10⁻⁶ at -180℃ to 0℃. -6 ≤δ （-180～0℃） ≤2×10 -6 Standard requirements. Attached Figure Description

[0042] Figure 1 This is a kinetic model diagram of Nb precipitation;

[0043] Figure 2 This is a grain structure morphology diagram of a 100mm thick forging billet prepared using the original process;

[0044] Figure 3 This is a microscopic morphology image of the uniformly structured ultra-thick Invar alloy plate prepared in Example 1 of this invention. Detailed Implementation

[0045] To better understand the above-mentioned technical solutions of the present invention, the technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0046] This invention provides a method for manufacturing an ultra-thick Invar alloy plate with uniform microstructure, comprising the following steps:

[0047] S1, smelting, using electric furnace → AOD refining → LF refining → VD refining → IC die casting process to prepare 6-9t Invar alloy steel ingots.

[0048] This step requires optimizing the composition of the Invar alloy. During the microalloying process in the later stages of the AOD refining process, an Nb-Fe alloy is added to adjust the Nb content in the molten steel. The Nb-Fe alloy contains approximately 90% Nb, with the remainder primarily being Fe. By limiting the amount of Nb added, it is possible to ensure complete Nb precipitation to suppress grain coarsening, while simultaneously ensuring that Nb exists entirely in the matrix as compounds such as Fe2Nb, avoiding excessive Nb dissolution in the matrix that could prevent the alloy's expansion properties from meeting standard requirements. Figure 1 As shown in the Nb precipitation kinetic model diagram, as the Nb content increases, the precipitation temperature of Fe2Nb gradually increases. When the Nb content is 0.4%, Nb begins to precipitate as Fe2Nb from about 1300℃, and the precipitation rate of Fe2Nb is very fast in the high temperature range. When the temperature drops to 1100℃, the precipitation rate decreases significantly, and the precipitation behavior is completed at around 1000℃.

[0049] Based on the above analysis, the Nb content in Invar alloy steel ingots should be ensured to meet the following range: 0.01×Ni+0.1%~0.03×Ni+0.01%, where Ni is the Ni content in the Invar alloy steel ingot. If the amount of Nb added is too small, the microstructure refinement effect will be insignificant; if the amount of Nb added is too large, the low-temperature expansion performance of the material will be severely deteriorated, causing the material properties to fail to meet the standard requirements.

[0050] The ingot mold used for IC die casting is octagonal. This ingot shape is beneficial for subsequent upsetting processes, thereby refining the grain structure. The weight of molten steel that the ingot mold can hold is 6 to 9 tons. For example, the weight of molten steel poured into the ingot mold is 8 tons.

[0051] The composition of the Invar alloy steel ingot obtained above, by weight percentage, is as follows: 0.01%≤C≤0.3%, 0.01%≤Si≤0.20%, 0.2%≤Mn≤0.3%, 0.01%≤P≤0.15%, 0.01%≤S≤0.25%, 30%≤Ni≤38%, Mg≤0.1%, Al≤0.20%, 0.01×Ni+0.1%≤Nb≤0.03×Ni+0.01%, with the balance being iron and unavoidable impurities.

[0052] S2, Invar alloy steel ingots are sent into an annealing furnace for stress-relief annealing, and the annealing holding time is 20-40 hours;

[0053] In this step, after smelting, the Invar alloy steel ingot is subjected to stress-relief annealing. During the annealing process, it is necessary to further control the annealing temperature and annealing holding time. The stress-relief annealing temperature is controlled at 800-950℃, and the annealing holding time is 20-40h. If the annealing temperature is lower than the lower limit, the residual stress in the steel ingot is not effectively removed, and stress cracks are very likely to occur on the surface of the steel ingot during the subsequent forging process, leading to forging cracks. If the annealing temperature is higher than the upper limit, the surface of the steel ingot is severely heated, and the columnar crystal structure on the surface is prone to overheating, resulting in weakened grain boundary bonding and easy forging cracks during forging. In addition, high-temperature heating leads to severe surface oxidation, resulting in significant metal loss and energy consumption in the steel ingot. If the annealing holding time is below the lower limit, the stress in the steel ingot cannot be effectively removed, making it prone to cracking during forging. If the annealing holding time is above the upper limit, the surface of the steel ingot is severely heated, and the columnar crystal structure on the surface is prone to overheating, resulting in weakened grain boundary bonding and making it very easy to generate forging cracks during forging.

[0054] S3 involves heating the annealed steel ingot and then shaping and performing the first upsetting and drawing. First, the ingot is shaped until the diameter of the head and tail is consistent. The upsetting temperature is controlled to be ≥1000℃, and the upsetting reduction h is satisfied to be 1 / 4 of the ingot body height to 1 / 3 of the ingot body height. Then, the ingot is drawn to the original height of the Invar alloy steel ingot and then returned to the furnace for heat preservation.

[0055] In this step, the steel ingot is heated to a temperature of 1200–1210℃ for 10–20 hours. The shaping process is to lightly break the edges of the steel ingot to facilitate subsequent upsetting. This process further reduces the ingot diameter, ensuring that the ingot's head and tail are deformed to maintain a consistent diameter. It also ensures that the fine-grained layer on the surface of the ingot is effectively broken before upsetting, improving the ingot's hot workability and preventing micro-cracks. During upsetting, the initial upsetting temperature is controlled to be ≥1000℃, and the upsetting reduction h should be between 1 / 4 and 1 / 3 of the ingot's height. If the upsetting reduction is greater than 1 / 3 of the ingot's height, the ingot is prone to hot cracking during drawing. If the upsetting reduction is less than 1 / 4 of the ingot's height, the deformation energy stored in the as-cast structure is insufficient, preventing subsequent dynamic recrystallization. After upsetting, the steel ingot is drawn back to its original height.

[0056] The drawn steel ingots are then reheated in the furnace, with the reheating temperature controlled at 1150-1160℃ and the holding time ≥2h.

[0057] S4. The steel ingot that has been kept warm in step S3 is subjected to a second upsetting and drawing process. The surface temperature of the steel ingot is controlled to be ≥950℃ during upsetting. The upsetting reduction h satisfies: 1 / 3 of the steel ingot body height to 1 / 2 of the steel ingot height. The steel ingot is then drawn to 4 / 5 to 5 / 6 of the original Invar alloy steel ingot height, and then returned to the furnace for heat preservation.

[0058] In the second upsetting and drawing process of this step, the initial upsetting temperature of the steel ingot is controlled to be lower than that of the first upsetting and drawing process, while the upsetting reduction is higher than that of the first upsetting and drawing process. The purpose is to gradually refine the grain structure of the steel ingot surface and core, and prevent coarsening of the core structure due to high temperature. In a specific embodiment, the surface temperature of the steel ingot during upsetting is required to be ≥950℃, and the upsetting reduction h is required to be: 1 / 3 of the steel ingot body height to 1 / 2 of the steel ingot height. Then the steel ingot is drawn to 4 / 5 to 5 / 6 of the original Invar alloy steel ingot height, and then it is returned to the furnace for heat preservation. The heat preservation temperature is controlled to be 1120 to 1130℃, and the heat preservation time is ≥2h.

[0059] S5. After the steel ingot has been kept warm in step S4, it is subjected to a third upsetting and drawing process. The upsetting temperature of the steel ingot is controlled to be ≥930℃, and the upsetting reduction h is satisfied to be 1 / 2 of the steel ingot body height to 3 / 5 of the steel ingot body height. Then the steel ingot is drawn to 2 / 3 to 3 / 4 of the original Invar alloy steel ingot height, and then the thickness direction is reduced to obtain a plate with a thickness m ≤ 3 / 5 of the original Invar alloy steel ingot head diameter. After that, it is put back into the furnace for heat preservation.

[0060] In the third upsetting and drawing process of this step, the initial upsetting temperature of the ingot must be ≥930℃, and the upsetting reduction h must meet the following requirements: 1 / 2 to 3 / 5 of the ingot height. The ingot is then drawn to 2 / 3 to 3 / 4 of the original Invar alloy ingot height, followed by thickness reduction, where the thickness m must be ≤3 / 5d, where d is the original ingot head diameter. When the plate is reheated in the furnace, the reheating temperature must be controlled at 1000–1010℃, and the holding time ≥1 hour.

[0061] In the aforementioned third upsetting and drawing process, the ingot's initial upsetting temperature is controlled to be lower than that of the second upsetting and drawing process, while the upsetting reduction is higher than that of the first upsetting and drawing process. Increasing the reduction in both the ingot's initial and thickness directions further refines the grain size. By lowering the reheating temperature and time of the intermediate ingot, coarsening of the ingot's core grain structure due to high temperatures is prevented. If the holding temperature exceeds 1010℃, the grains in the intermediate slab are prone to recrystallization and growth, leading to larger grain sizes that fail to meet the required grain size. If the holding temperature is below 1000℃, the intermediate slab loses heat too quickly during hot deformation, resulting in excessively low surface temperatures and increased deformation resistance, preventing further deformation. Simultaneously, the heating time is reduced to 1 hour. Excessive heating time causes precipitates such as Fe2Nb and NbC formed during the initial hot deformation process to remelt into the matrix, thus losing their function of pinning grain boundaries and preventing growth. Therefore, low-temperature forging is required here to keep the grain size of the forged slab at a level of 4 or higher.

[0062] Through the three upsetting and drawing processes of steps S3 to S5 above, and specifying the upsetting and roughing reduction amount for each steel ingot, following the principle of "gradually decreasing the upsetting temperature and gradually increasing the reduction amount", the surface and core grain structure of the forging billet are gradually refined. After ensuring that the internal structure of the steel ingot is fully broken and recrystallized, the structure is transformed into a material at a low temperature, avoiding the coarsening of the core grain structure due to excessively high core temperature in the later stage.

[0063] S6. After the plate is heat-insulated in step S5, it is forged to the target size and then annealed. After air cooling, an Invar alloy plate with a thickness of ≥50mm is obtained.

[0064] In this step, the sheet material after heat preservation in step S5 is forged to the target dimensions, namely a thickness of 50–300 mm and a width of 1000–2000 mm. Then, during annealing, the annealing temperature is 800–850℃, and the holding time is required to be ≥1 hour. If the annealing temperature is below 800℃, the internal grains of the sheet material cannot undergo static recrystallization, resulting in a large amount of deformed structures within the sheet material and a large number of elongated grains at the edges, failing to meet the requirement of a grain size of grade 4 or higher across the entire cross-section of the slab. If the annealing temperature is above 850℃, after recrystallization and refinement of the internal structure of the slab, the grains will further grow, leading to coarsening and growth of the grain structure, and the grain size will not meet the performance requirements of grade 4 or higher.

[0065] The ultra-thick Invar alloy sheet obtained above has the following composition by weight percentage: 0.01%≤C≤0.3%, 0.01%≤Si≤0.20%, 0.2%≤Mn≤0.3%, 0.01%≤P≤0.15%, 0.01%≤S≤0.25%, 30%≤Ni≤38%, Mg≤0.1%, Al≤0.20%, 0.01×Ni+0.1%≤Nb≤0.03×Ni+0.01%, with the balance being iron and unavoidable impurities.

[0066] The thickness of the ultra-thick Invar alloy sheet is 50-300mm (e.g., 100-220mm, 210-220mm, etc.), and the width is 1000-2000mm.

[0067] The grain size of the ultra-thick Invar alloy sheet is ≥4, and its expansion performance at low temperatures of -180 to 0℃ meets the requirement of 1×10⁻⁶. -6 ≤δ （-180～0℃） ≤2×10 -6 .

[0068] Figure 2The image shows the grain structure of a 100mm thick Invar alloy sheet prepared using the original process. The original process did not follow the principle of "gradually decreasing upsetting temperature and gradually increasing upsetting reduction" during forging deformation. Furthermore, the original process only performed one upsetting and drawing process, and after deformation to the specified dimensions, it did not undergo recrystallization annealing but was directly air-cooled to room temperature. As shown in the image, the sheet's microstructure consists largely of deformed structures, with a grain size less than grade 4.

[0069] Figure 3 The image shows the grain structure of a 100mm thick Invar alloy plate prepared using the alloy composition and method of this invention. As can be seen from the image, the structure is uniform and the grain size is ≥4.

[0070] Example 1

[0071] The alloy composition of the uniformly structured ultra-thick Invar alloy plate in this embodiment is calculated by weight percentage as follows: C 0.01%, Si 0.01%, Mn 0.2%, P 0.01%, S 0.01%, Ni 30%, Nb 0.50%, Mg 0.03%, Al 0.15%, with the balance being Fe and unavoidable impurities.

[0072] The manufacturing method of the uniformly structured ultra-thick Invar alloy plate in this embodiment is as follows: Invar alloy steel ingots are smelted in a 50t electric furnace. The smelting process is: electric furnace → AOD refining → LF refining → VD refining → IC casting. During the AOD refining process, Nb is added to ensure that the Nb content in the Invar alloy steel ingot meets the following requirements: 0.01×Ni+0.1%~0.03×Ni+0.01%, with Nb content controlled at 0.50%. The ingot mold used for IC casting is octagonal. An 8t Invar alloy steel ingot is prepared using the above process. After casting for 8 hours and demolding, it is placed in an annealing furnace for stress-relief annealing at 850℃ for 35 hours. After annealing, the resulting steel ingot (2 meters high, 1 meter head diameter, and 0.8 meter tail diameter) is placed in a heating furnace for heating at 1200℃ for 10 hours. After the heating time is sufficient, the ingot is first shaped to ensure that the diameters of the first and last ends are consistent. Then, the first upsetting and drawing process is performed. During the first upsetting, the ingot's initial upsetting temperature is controlled at 1150℃, and the ingot is upset by 600mm. It is then drawn to its original height. The ingot is then returned to the furnace and heated at 1150℃ for 4 hours. Afterward, the ingot undergoes a second upsetting and drawing process. During the second upsetting, the initial upsetting temperature is 970℃, and the upsetting reduction is 900mm. The ingot is then drawn to 1600mm. The ingot is then returned to the furnace and heated at 1120℃ for 3 hours. During the third upsetting, the initial upsetting temperature is 950℃, and the upsetting reduction is 1100mm. The ingot is then drawn to 1500mm, and finally, the ingot is forged to 600mm in the thickness direction. After the steel ingot was reheated and held at 1000℃ for 2 hours, it was forged into a sheet with a target size of 210×1500×L mm. Then, it was annealed at 850℃ for 1 hour and air-cooled to obtain an ultra-thick Invar alloy sheet. Its properties are shown in Table 3.

[0073] Example 2

[0074] The alloy composition of the uniformly structured ultra-thick Invar alloy plate in this embodiment is calculated by weight percentage as follows: C 0.01%, Si 0.04%, Mn 0.25%, P 0.01%, S 0.1%, Ni 35%, Nb 0.7%, Mg 0.07%, Al 0.12%, with the balance being Fe and unavoidable impurities.

[0075] The manufacturing method of the uniformly structured ultra-thick Invar alloy plate in this embodiment is as follows: Invar alloy steel ingots are smelted in a 50t electric furnace. The smelting process is: electric furnace → AOD refining → LF refining → VD refining → IC casting. Nb is added during the AOD refining process to ensure that the Nb content in the Invar alloy steel ingot meets the following requirements: 0.01×Ni+0.1%~0.03×Ni+0.01%, with Nb content controlled at 0.70%. The ingot mold used for IC casting is octagonal. An 8t Invar alloy steel ingot is prepared using the above process. After casting for 8 hours and demolding, it is placed in an annealing furnace for stress-relief annealing at 850℃ for 35 hours. After annealing, the resulting steel ingot (2 meters high, 1 meter head diameter, and 0.8 meter tail diameter) is placed in a heating furnace for heating at 1200℃ for 10 hours. After heating, the ingot is shaped until the diameter of both ends is consistent. The first upsetting and drawing process is then performed. During the first upsetting, the ingot's initial upsetting temperature is 1140℃, and the upsetting diameter is 650mm. It is then drawn back to its original height. The ingot is then reheated in the furnace at 1150℃ for 3 hours. A second upsetting and drawing process is then performed. During the second upsetting, the initial upsetting temperature is 990℃, and the upsetting reduction is 950mm. The ingot is then drawn back to 1600mm. The ingot is then reheated in the furnace at 1125℃ for 3 hours. During the third upsetting, the initial upsetting temperature is 940℃, and the upsetting reduction is 1200mm. The ingot is then drawn back to 1400mm, and finally forged to a thickness of 550mm. After the steel ingot was reheated and held at 1005℃ for 2 hours, it was forged into a sheet with a target size of 220×1600×L mm. Then, it was annealed at 840℃ for 1 hour and air-cooled to obtain an ultra-thick Invar alloy sheet. Its properties are shown in Table 3.

[0076] Example 3

[0077] The alloy composition of the uniformly structured ultra-thick Invar alloy plate in this embodiment is calculated by weight percentage as follows: C 0.01%, Si 0.12%, Mn 0.28%, P 0.07%, S 0.23%, Ni 30%, Nb 0.75%, Mg 0.04%, Al 0.13%, with the balance being Fe and unavoidable impurities.

[0078] The manufacturing method of the uniformly structured ultra-thick Invar alloy plate in this embodiment is as follows: Invar alloy steel ingots are smelted in a 50t electric furnace. The smelting process is: electric furnace → AOD refining → LF refining → VD refining → IC casting. Nb is added during the AOD refining process to ensure that the Nb content in the Invar alloy steel ingot meets the following requirements: 0.01×Ni+0.1%~0.03×Ni+0.01%, with Nb content controlled at 0.75%. The ingot mold used for IC casting is octagonal. An 8t Invar alloy steel ingot is prepared using the above process. After casting for 8 hours and demolding, it is placed in an annealing furnace for stress-relief annealing at 900℃ for 25 hours. After annealing, the resulting steel ingot (2 meters high, 1 meter head diameter, and 0.8 meter tail diameter) is placed in a heating furnace for heating at 1200℃ for 10 hours. After heating, the ingot is shaped until the diameter of both ends is consistent. Then, the first upsetting and drawing process is performed. During the first upsetting, the ingot's initial upsetting temperature is 1100℃, and it is upset to 630mm before being drawn to its original height. It is then returned to the furnace for heat treatment. The ingot is then heated at 1150℃ for 2.5 hours, followed by a second upsetting and drawing process. During the second upsetting, the initial upsetting temperature is 950℃, and the upsetting reduction is 950mm. The ingot is then drawn to 1600mm. The ingot is then returned to the furnace and heated at 1128℃ for 2 hours. During the third upsetting, the initial upsetting temperature is 930℃, and the upsetting reduction is 1200mm. The ingot is then drawn to 1500mm, and finally, the thickness of the ingot is forged to 500mm. The steel ingot was reheated and held at 1010℃ for 2 hours before being forged into a sheet with dimensions of 215×1700×L mm. It was then annealed at 830℃ for 1.5 hours and air-cooled to obtain an ultra-thick Invar alloy sheet. Its properties are shown in Table 3.

[0079] Example 4

[0080] The alloy composition of the uniformly structured ultra-thick Invar alloy plate in this embodiment is calculated by weight percentage as follows: C 0.20%, Si 0.10%, Mn 0.20%, P 0.08%, S 0.13%, Ni 34%, Nb 0.56%, Mg 0.06%, Al 0.17%.

[0081] The manufacturing method of the uniformly structured ultra-thick Invar alloy plate in this embodiment is as follows: Invar alloy steel ingots are smelted in a 50t electric furnace. The smelting process is: electric furnace → AOD refining → LF refining → VD refining → IC casting. Nb is added during the AOD refining process to ensure that the Nb content in the Invar alloy steel ingot meets the following requirements: 0.01×Ni+0.1%~0.03×Ni+0.01%, with Nb content controlled at 0.80%. The ingot mold used for IC casting is octagonal. A 6t Invar alloy steel ingot is prepared using the above process. After casting for 8 hours and demolding, it is placed in an annealing furnace for stress-relief annealing at 950℃ for 25 hours. After annealing, the resulting steel ingot (2 meters high, 1 meter head diameter, and 1 meter tail diameter) is placed in a heating furnace for heating at 1200℃ for 10 hours. After heating, the ingot is shaped until the diameter of both ends is consistent. Then, the first upsetting and drawing process is performed. During the first upsetting, the ingot's initial upsetting temperature is 1000℃, and after upsetting by 500mm, it is drawn to its original height. It is then returned to the furnace for heat treatment. The ingot is then heated at 1155℃ for 2 hours, followed by a second upsetting and drawing process. During the second upsetting, the initial upsetting temperature is 950℃, and the upsetting reduction is 670mm. The ingot is then drawn to 1600mm. The ingot is then returned to the furnace and heated at 1130℃ for 2.5 hours. During the third upsetting, the initial upsetting temperature is 940℃, and the upsetting reduction is 1000mm. The ingot is then drawn to 1400mm, and finally, the thickness of the ingot is forged to 550mm. The steel ingot was reheated and held at 1000℃ for 1.5 hours before being forged into a sheet with dimensions of 220×1600×L mm. It was then annealed at 820℃ for 1.5 hours and air-cooled to obtain an ultra-thick Invar alloy sheet. Its properties are shown in Table 3.

[0082] Example 5

[0083] The alloy composition of the uniformly structured ultra-thick Invar alloy plate in this embodiment is calculated by weight percentage as follows: C 0.30%, Si 0.20%, Mn 0.30%, P 0.15%, S 0.25%, Ni 38%, Nb 0.80%, Mg 0.1%, Al 0.2%, with the balance being Fe and unavoidable impurities.

[0084] The manufacturing method of the uniformly structured ultra-thick Invar alloy plate in this embodiment is as follows: Invar alloy steel ingots are smelted in a 50t electric furnace, wherein the smelting process is electric furnace → AOD refining → LF refining → VD refining → IC casting. Nb is added during the AOD refining process to ensure that the Nb content in the Invar alloy steel ingot meets the following requirements: 0.01×Ni+0.1%~0.03×Ni+0.01%; the ingot mold used for IC casting is octagonal; 9t Invar alloy steel ingots are prepared through the above process. After casting for 8 hours and demolding, the ingots are placed in an annealing furnace for stress-relief annealing at 900℃ for 30 hours. After annealing, the resulting steel ingots (2 meters high, 1 meter head diameter, and 0.8 meter tail diameter) are placed in a heating furnace for heating at 1205℃ for 10 hours. After heating, the ingot is shaped until the diameter of both ends is consistent. Then, the first upsetting and drawing process is performed. During the first upsetting, the ingot's initial upsetting temperature is 1130℃, and it is upset to 660mm before being drawn to its original height. It is then returned to the furnace for heat treatment. The ingot is then heated at 1160℃ for 2 hours, followed by a second upsetting and drawing process. During the second upsetting, the initial upsetting temperature is 970℃, and the upsetting reduction is 1000mm. The ingot is then drawn to 1600mm. It is then returned to the furnace and heated at 1130℃ for 2.5 hours. During the third upsetting, the initial upsetting temperature is 940℃, and the upsetting reduction is 1100mm. The ingot is then drawn to 1500mm, and finally, the thickness of the ingot is forged to 500mm. The steel ingot was reheated and held at 1010℃ for 2.5 hours before being forged into a sheet with dimensions of 50×1700×L mm. It was then annealed at 810℃ for 2 hours and air-cooled to obtain an ultra-thick Invar alloy sheet. Its properties are shown in Table 3.

[0085] Comparative Example 1

[0086] This comparative example uses the alloy composition and manufacturing method of Example 1, the difference being that the steel ingot mold used in the IC die casting of the smelting process is a flat ingot, and the low-temperature expansion performance of the final ultra-thick Invar alloy plate is δ. （-180～0℃） 1.3×10 -6 / ℃.

[0087] Comparative Example 2

[0088] This comparative example uses the manufacturing method of Example 1, except that the Nb content in the alloy composition is 1.2%.

[0089] Comparative Example 3

[0090] This comparative example uses the manufacturing method of Example 1, except that the Nb content in the alloy composition is 0.25%.

[0091] Comparative Example 4

[0092] This comparative example uses the alloy composition and manufacturing method of Example 2. The difference is that the reheating temperature in the three upsetting and drawing processes is different. Specifically, the reheating temperature after the first upsetting and drawing and the second upsetting and drawing is 1200°C.

[0093] Comparative Example 5

[0094] This comparative example uses the alloy composition and manufacturing method of Example 1. The difference is that the surface temperature of the steel ingot is different in the three upsetting and drawing processes. Specifically, the opening upsetting temperatures of the steel ingot during the three upsetting and drawing processes are 950°C, 940°C, and 900°C, respectively.

[0095] Comparative Example 6

[0096] This comparative example uses the alloy composition and manufacturing method of Example 1. The difference lies in the upsetting reduction amount in the three upsetting and drawing processes, namely, the upsetting reduction amounts in the three upsetting and drawing processes are 480 mm, 550 mm, and 900 mm, respectively.

[0097] Comparative Example 7

[0098] This comparative example uses the alloy composition and manufacturing method of Example 1. The difference is that the upsetting reduction in the three upsetting and drawing processes is not set to gradually increase. That is, the upsetting reduction in the three upsetting and drawing processes are 700 mm, 600 mm and 1300 mm respectively.

[0099] Comparative Example 8

[0100] This comparative example uses the alloy composition and manufacturing method of Example 1, the difference being that the plate after forging was not annealed, and the slab was directly air-cooled to room temperature.

[0101] Table 1: Chemical composition of each example and comparative example

[0102]

[0103] Table 2: Process Parameter Design Table for Example

[0104]

[0105] Table 3: Performance of Products in Each Example and Comparative Example

[0106]

[0107] As shown in Table 3, the ultra-thick Invar alloy plates prepared in Examples 1-5 of this invention have a thickness of 210-220 mm and a width of 1500-1700 mm. The grain structure of the entire cross-section of the plate is uniform, and the grain size at the edge and center of the slab meets ≥4 level. The expansion performance at low temperatures of -180 to 0℃ meets the requirement of 1.2 × 10⁻⁶. -6 / ℃~1.4×10-6 / ℃, the overall performance is better than the comparative example.

[0108] As can be seen from Comparative Example 1 and Example 1, although the ultra-thick Invar alloy plates prepared using flat ingot molds have the same grain size and expansion properties, the surface crack defects of the plates are more serious and cannot meet the requirements of subsequent rolling.

[0109] Based on Comparative Examples 2 and 3 and Example 1, it can be seen that when the Nb content in the alloy composition does not meet the requirement of 0.01Ni (wt%) + 0.1% ≤ Nb (wt%) ≤ 0.03Ni (wt%) + 0.01%, that is, when the Nb content is less than or greater than the above range, the low-temperature expansion performance of the plate is much higher than that of Example 1 and cannot meet the standard requirements.

[0110] Based on Comparative Example 4 and Example 2, it can be seen that when the reheating temperature in the three upsetting and drawing processes is not gradually reduced during reheating, the internal grain structure of the material coarsens during the high-temperature range, resulting in a final product with a grain size of only grade 2, which does not meet the technical standard requirements.

[0111] Based on Comparative Example 5 and Example 1, it can be seen that when the surface temperature of the steel ingot in the three upsetting and drawing processes is not within the range set by the present invention, the columnar crystal structure on the surface of the steel ingot is prone to produce fine microcracks. In the subsequent forging and drawing process, the cracks further expand, causing the slab to crack and making it unusable in the subsequent hot rolling process.

[0112] Based on Comparative Examples 6 and 7 and Example 1, it can be seen that when the upsetting reduction in the three upsetting and drawing processes is carried out in a gradually increasing manner, the grain size of the steel ingot forging meets the requirement of ≥4 level. If the upsetting amount is less than the set value, the grain size of the forging billet does not meet the requirement of 4 level. If the upsetting amount is greater than the set target value, the deformation of the steel ingot is too large, resulting in cracking of the steel ingot forging, making it impossible to continue production and use.

[0113] As can be seen from Comparative Example 8 and Example 1, if annealing is not performed after forging, the grain size of the plate will be only level 1, which cannot meet the technical standard requirements.

[0114] Those skilled in the art should recognize that the above embodiments are merely illustrative of the present invention and are not intended to limit the present invention. Any variations or modifications to the above embodiments that are within the spirit and essence of the present invention will fall within the scope of the claims of the present invention.

Claims

1. A method for manufacturing an ultra-thick Invar alloy plate with uniform microstructure, characterized in that: Includes the following steps: S1, smelting, using electric furnace → AOD refining → LF refining → VD refining → IC die casting process to prepare 6-9t Invar alloy steel ingots; Nb is added during the AOD refining process to ensure that the Nb content in the Invar alloy steel ingot meets the following requirements: 0.01×Ni+0.1%~0.03×Ni+0.01%, where Ni is the Ni content in the Invar alloy steel ingot. The composition of the Invar alloy steel ingot, by weight percentage, is as follows: 0.01%≤C≤0.3%, 0.01%≤Si≤0.20%, 0.2%≤Mn≤0.3%, 0.01%≤P≤0.15%, 0.01%≤S≤0.25%, 30%≤Ni≤38%, Mg≤0.1%, Al≤0.20%, 0.01×Ni+0.1%≤Nb≤0.03×Ni+0.01%, with the balance being iron and unavoidable impurities; S2, Invar alloy steel ingots are fed into an annealing furnace for stress-relief annealing, wherein the stress-relief annealing temperature is 800-950℃ and the annealing holding time is 20-40h; S3, after heating the annealed steel ingot, it is shaped and first upset drawing is performed. First, it is shaped until the diameter of the head and tail of the steel ingot is the same. The upsetting temperature of the steel ingot is controlled to be ≥1000℃. The upsetting reduction h meets the requirements of 1 / 4 of the steel ingot body height to 1 / 3 of the steel ingot body height. Then the steel ingot is drawn to the original Invar alloy steel ingot height and then returned to the furnace for heat preservation. S4. The steel ingot that has been kept warm in step S3 is subjected to a second upsetting and drawing process. The upsetting temperature of the steel ingot is controlled to be ≥950℃. The upsetting reduction h meets the following requirements: 1 / 3 of the steel ingot body height ~ 1 / 2 of the steel ingot height. The steel ingot is then drawn to 4 / 5 to 5 / 6 of the original Invar alloy steel ingot height and then returned to the furnace for heat preservation. S5, the steel ingot after heat preservation in step S4 is subjected to a third upsetting and drawing process. The upsetting temperature of the steel ingot is controlled to be ≥930℃, and the upsetting reduction h is satisfied to be 1 / 2 of the steel ingot body height to 3 / 5 of the steel ingot body height. Then the steel ingot is drawn to 2 / 3 to 3 / 4 of the original Invar alloy steel ingot height, and then the thickness direction is reduced to obtain a plate with a thickness m ≤ 3 / 5 of the original Invar alloy steel ingot head diameter. After that, it is returned to the furnace for heat preservation. Among them, the opening temperature decreases sequentially during the first upsetting and drawing process, the second upsetting and drawing process, and the upsetting reduction h increases sequentially. S6. After the plate is heat-insulated in step S5, it is forged to the target size and then annealed. After air cooling, an Invar alloy plate with a thickness of ≥50mm is obtained. The thickness of the Invar alloy plate is 50-300 mm, and the width is 1000-2000 mm; The Invar alloy sheet exhibits uniform grain structure across its entire cross-section, with grain size at the edges and center of the slab meeting ≥4 grade. Its expansion performance at low temperatures of -180 to 0℃ meets the requirement of 1×10⁻⁶. -6 ≤δ （-180～0℃） ≤2×10 -6 .

2. The method for manufacturing a uniformly structured ultra-thick Invar alloy plate according to claim 1, characterized in that: In step S1, the steel ingot mold used for IC casting is octagonal in shape.

3. The method for manufacturing a uniformly structured ultra-thick Invar alloy plate according to claim 1, characterized in that: In step S3: The heating temperature is 1200-1210℃, and the heating and holding time is 10-20h; The reheating temperature is 1150-1160℃, and the holding time is ≥2h.

4. The method for manufacturing a uniformly structured ultra-thick Invar alloy plate according to claim 1, characterized in that: In step S4, the reheating temperature is 1120-1130℃, and the holding time is ≥2h.

5. The method for manufacturing a uniformly structured ultra-thick Invar alloy plate according to claim 1, characterized in that: In step S5, the reheating temperature is 1000-1010℃, and the holding time is ≥1h.

6. The method for manufacturing a uniformly structured ultra-thick Invar alloy plate according to claim 1, characterized in that, In step S6, the annealing temperature is 800-850℃, and the holding time is required to be ≥1h.

7. An ultra-thick Invar alloy plate obtained by a method for manufacturing an ultra-thick Invar alloy plate with uniform microstructure according to any one of claims 1 to 6, characterized in that: The composition of the ultra-thick Invar alloy plate, by weight percentage, is as follows: 0.01%≤C≤0.3%, 0.01%≤Si≤0.20%, 0.2%≤Mn≤0.3%, 0.01%≤P≤0.15%, 0.01%≤S≤0.25%, 30%≤Ni≤38%, Mg≤0.1%, Al≤0.20%, 0.01×Ni+0.1%≤Nb≤0.03×Ni+0.01%, with the balance being iron and unavoidable impurities; The thickness of the Invar alloy plate is 50-300 mm, and the width is 1000-2000 mm; The Invar alloy sheet exhibits uniform grain structure across its entire cross-section, with grain size at the edges and center of the slab meeting ≥4 grade. Its expansion performance at low temperatures of -180 to 0℃ meets the requirement of 1×10⁻⁶. -6 ≤δ （-180～0℃） ≤2×10 -6 .

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