A cold rolled invar alloy strip having excellent weldability and a method for producing the same

Abstract

The application discloses a cold-rolled invar alloy strip with excellent welding performance and a production method thereof. The chemical components of the cold-rolled invar alloy strip are as follows in terms of percentage by weight: C: 0.02-0.06%, Si: 0.40%, Mn: 0.20-0.60%, P: 0.008%, S: 0.003%, Ni: 34.8-37.5%, Nb: 0.01-0.08%, Al: 0.005-0.02%, Ca: 0.0001-0.003%, and the balance of Fe and inevitable impurities, wherein the total amount of the inevitable impurities is less than 0.05%. The linear expansion coefficient of the cold-rolled invar alloy strip at a temperature of-180-0 DEG C is 1.0-2.0*10 ‑6 / DEG C. The application can improve the thermal crack resistance of the invar alloy, so that the post-weld metal is not prone to thermal cracks, and the cold-rolled invar alloy strip has excellent welding performance.

Description

Technical Field

[0001] This invention relates to the field of precision alloy materials, and more specifically, to a cold-rolled strip of Invar alloy with excellent weldability and its production method. Background Technology

[0002] Invar alloy is a low-expansion coefficient iron-nickel alloy with a nickel content of approximately 36%. Its core value lies in maintaining excellent dimensional stability at extreme temperatures. This characteristic gives it irreplaceable application advantages in fields sensitive to temperature deformation, such as energy transportation, aerospace, and precision instruments. Especially in the energy transportation field, Invar alloy is a key material for the containment system of liquefied natural gas (LNG) ship cargo tanks. It needs to withstand extremely low temperature environments, which places higher demands on the material's welding performance. For example, the weld morphology must be good and crack-free, the maximum load-bearing capacity of the weld must be ≥4025N, and the welding performance must be evaluated through lap welding tests before application.

[0003] A search of existing technologies revealed the following technologies related to Invar alloys:

[0004] Chinese Patent Publication No. CN116904841A discloses an Invar alloy precision strip and its preparation method. The method mainly involves smelting molten steel in a vacuum induction furnace to obtain a square ingot; after surface grinding, the ingot is smelted and cast into a flat billet in a vacuum electron beam cold hearth furnace; after surface grinding, the flat billet is heated, rough rolled, and fine rolled, and then coiled into a hot coil; the hot coil is annealed, pickled, cold rolled, and bright annealed. The purpose of this technology is to improve product purity and yield, improve strip shape, and reduce surface roughness. However, the technology does not evaluate welding performance or mention welding-related properties, so the welding performance of the material cannot be determined.

[0005] Chinese Patent Publication No. CN107119234A discloses a method for fine-grain strengthening of Invar alloy strip, whose main chemical composition is: C 0.01-0.1%, Si 0.01-0.04%, Mn 0.01-0.05%, Ni 36%, S < 0.01%, P < 0.01%, with the balance being Fe. The alloy preparation process includes smelting, casting, hot rolling, solution treatment, cryogenic rolling, and low-temperature recrystallization annealing. This technology solves the technical problems of coarse grain size and poor mechanical properties in Invar alloy strips produced by existing processes. It is low-cost, pollution-free, and conducive to the development of high-strength Invar alloy strips. However, this technology also does not evaluate weldability or mention weldability, making it impossible to know the weldability of the material.

[0006] In view of the above, it is necessary to study an Invar alloy with excellent welding performance that can be used in extremely low temperature environments. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide an Invar alloy cold-rolled strip with excellent weldability and its production method. Based on the Fe-Ni36 alloy, trace amounts of Nb, Al, and Ca elements are added to improve the mechanical properties of the heat-affected zone metal of the weld without significantly increasing the alloy's coefficient of thermal expansion. This enhances the Invar alloy's resistance to hot cracking, making the welded metal less prone to hot cracking and ensuring that the Invar alloy cold-rolled strip has excellent weldability.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] The first aspect of the present invention provides an Invar alloy cold-rolled strip with excellent weldability, the chemical composition of which, by weight percentage, is as follows: C: 0.02-0.06%, Si≤0.40%, Mn: 0.20-0.60%, P≤0.008%, S≤0.003%, Ni: 34.8-37.5%, Nb: 0.01-0.08%, Al: 0.005-0.02%, Ca: 0.0001-0.003%, with the balance being Fe and unavoidable impurities, the total amount of said unavoidable impurities being less than 0.05%;

[0010] The flatness of the Invar alloy cold-rolled strip is ≤0.5mm / m, and the coefficient of linear expansion at temperatures ranging from -180℃ to 0℃ is 1.0~2.0×10⁻⁶. -6 / ℃;

[0011] The maximum load-bearing capacity of the weld metal after lap welding of the Invar alloy cold-rolled strip is ≥4025N.

[0012] The aforementioned Invar alloy cold-rolled strip is based on the Fe-Ni36 alloy with the addition of trace amounts of Nb, Al, and Ca elements. This allows for the synergistic effect of Nb and Al without significantly increasing the alloy's coefficient of thermal expansion, inhibiting grain growth during subsequent welding and thus improving the alloy's weldability. Furthermore, the addition of Ca promotes MnS spheroidization in the alloy, altering the morphology of sulfides, thereby enhancing the impact toughness of the weld heat-affected zone and reducing the alloy's tendency to hot crack. The specific composition design principle is as follows:

[0013] Carbon (C): C reacts with niobium (Nb) to form carbides, thereby refining the grain structure of the weld metal, increasing the strength of the weld zone, improving metal fluidity during welding, and reducing the tendency for hot cracking. However, when the C content is higher than 0.06%, the coefficient of thermal expansion in the alloy increases significantly; when the C content is lower than 0.02%, the alloy strength is weakened, and the risk of welding deformation and cracking increases. Therefore, this invention controls the C content to be between 0.02% and 0.06%.

[0014] Silicon (Si): Si mainly plays a deoxidizing role in alloys, but excessive Si content will reduce the impact resistance of weld metal and lead to a significant increase in the coefficient of thermal expansion of the material. Therefore, this invention controls the Si content to ≤0.40%, preferably 0.05-0.40%.

[0015] Manganese (Mn): Adding Mn to alloys can significantly improve desulfurization efficiency and effectively inhibit the formation of hot cracks. However, if the Mn content is too high (above 0.60%), it will lead to an increase in the coefficient of thermal expansion; for every 0.1% increase in manganese content, the coefficient of thermal expansion increases by approximately 0.1 × 10⁻⁶. -6 / ℃; if the Mn content is too low (below 0.20%), the desulfurization and deoxidation effects will be significantly weakened. Therefore, this invention controls the Mn content to be between 0.20% and 0.60%.

[0016] Sulfur (S): The presence of S in the alloy readily leads to the formation of a low-melting-point Fe-Ni-S eutectic (melting point approximately 650°C) during solidification; to effectively prevent welding solidification cracks, the lower the S content in the alloy, the better. Therefore, this invention controls the S content to ≤0.003%.

[0017] Phosphorus (P): P in alloys can segregate at grain boundaries, leading to material embrittlement, especially under thermal stress, which promotes microcrack formation and increases the cold brittleness of the weld metal. Therefore, the P content in the alloy should be controlled as low as possible. In this invention, P is controlled to be ≤0.008%.

[0018] Nickel (Ni): The Ni content in the alloy is the most critical factor affecting the coefficient of thermal expansion. Both excessively high and low Ni content will lead to an increase in the coefficient of thermal expansion of the alloy. Therefore, the Ni content in the alloy should be precisely controlled. In this invention, the Ni content is set between 34.8% and 37.5%.

[0019] Niobium (Nb): Adding Nb to alloys can form NbN and NbC, which can pin grain boundaries and refine weld grains, thereby improving weldability. However, when the Nb content is greater than 0.08%, coarse NbN and NbC will form, deteriorating the performance. Therefore, this invention controls the Nb content to be between 0.01% and 0.08%.

[0020] Aluminum (Al): Al is primarily added to the alloy to deoxidize, and it also forms AlN, which, together with NbN and NbC, synergistically controls grain growth, thereby effectively improving the properties of the weld metal. When the Al content exceeds 0.02%, the impact toughness of the weld decreases. Therefore, this invention controls the Al content to be between 0.005% and 0.02%.

[0021] Calcium (Ca): Adding an appropriate amount of Ca to the alloy can promote the spheroidization of sulfide morphology (such as MnS), thereby improving the impact toughness of the weld heat-affected zone; however, excessive addition will lead to the formation of inclusions and deteriorate the alloy properties. Therefore, the present invention controls the Ca content to be between 0.0001 and 0.003%.

[0022] A second aspect of the present invention provides a method for producing Invar alloy cold-rolled strip with excellent weldability according to the first aspect of the present invention, comprising the following steps:

[0023] S1, annealing the Invar alloy hot-rolled coil with a thickness of 5±0.3mm at an annealing temperature of 1050±10℃;

[0024] S2, the annealed Invar alloy hot-rolled coil is pickled. The pickling solution is a mixed acid solution of sulfuric acid and hydrofluoric acid, with the concentration of sulfuric acid being 350-400 g / L and the concentration of hydrofluoric acid being 50-60 g / L.

[0025] S3, the pickled Invar alloy hot-rolled coil is subjected to a single cold rolling process, which is carried out in 5 passes to obtain a single cold-rolled coil; the deformation amount of the single cold rolling pass is set in a manner that first increases and then decreases, and the deformation amount of the pass is controlled between 0.1 and 0.5 mm.

[0026] S4, the cold-rolled coil is subjected to a bright annealing in a bell-type annealing furnace, and the annealing temperature is controlled at 650±5℃.

[0027] S5, the first cold-rolled coil after bright annealing is subjected to a second cold rolling, with deformation controlled in 6 or 8 passes, to obtain a second cold-rolled coil; the deformation amount per pass of the second cold rolling is controlled to be 0.1 to 0.5 mm;

[0028] S6, the second cold-rolled coil is subjected to second bright annealing in a continuous bright annealing furnace, and the annealing temperature is controlled at 980±5℃.

[0029] S7 involves leveling or straightening the secondary cold-rolled coil after secondary bright annealing, with tension controlled between 3 and 15 tons and elongation controlled at ≤1.5%.

[0030] Preferably, in step S1:

[0031] The chemical composition of the Invar alloy hot-rolled coil, by weight percentage, is as follows: C: 0.02-0.06%, Si≤0.40%, Mn: 0.20-0.60%, P≤0.008%, S≤0.003%, Ni: 34.8-37.5%, Nb: 0.01-0.08%, Al: 0.005-0.02%, Ca: 0.0001-0.003%, with the balance being Fe and unavoidable impurities, the total amount of which is less than 0.05%.

[0032] The preparation method of the Invar alloy hot-rolled coil is as follows: molten steel is produced by one of the following smelting processes: electric furnace + ladle refining / electroslag remelting or vacuum induction + electroslag remelting. The molten steel is then cast into steel ingots, which are then hot-rolled to obtain Invar alloy hot-rolled coils.

[0033] The annealing is performed using a continuous annealing furnace or a bell-type annealing furnace. When using a continuous annealing furnace, the running speed of the Invar alloy hot-rolled coil strip is controlled at 12-16 mm / min. When using a bell-type annealing furnace, the annealing holding time is controlled at 4-6 h.

[0034] The Invar alloy hot-rolled coil used in step S1 can be cast into steel ingots using conventional smelting methods, such as electric furnace + ladle refining, electric furnace + electroslag remelting, or vacuum induction + electroslag remelting, and then hot-rolled coils with a thickness of about 5±0.3mm can be obtained by conventional hot rolling process.

[0035] In the annealing process of step S1, a continuous annealing furnace can be used. At this time, the feed speed of the Invar alloy hot-rolled strip needs to be controlled at 12–16 mm / min. Too fast a feed speed will result in incomplete annealing, failing to completely eliminate residual stress generated during rolling, which is detrimental to subsequent cold rolling. Too slow a feed speed will increase energy consumption. Alternatively, a bell-type annealing furnace can be used, with the annealing holding time controlled at 4–6 hours. Through the above annealing process, the stress generated during hot rolling can be eliminated, which is beneficial for subsequent cold rolling. Furthermore, controlling the annealing temperature at 1050±10℃ aims to eliminate residual stress in a shorter time.

[0036] Preferably, in step S2, the pickling temperature is 80±5℃.

[0037] In step S2, the pickling process uses a mixed acid solution consisting of sulfuric acid with a concentration of 350–400 g / L and hydrofluoric acid with a concentration of 50–60 g / L, and the pickling temperature is 80 ± 5 °C. The purpose is to thoroughly remove the oxide scale from the surface of the hot-rolled coil and ensure that the surface quality of the Invar alloy hot-rolled coil and cold-rolled strip meets the requirements. If the concentration of sulfuric acid is lower than 350 g / L or the concentration of hydrofluoric acid is lower than 50 g / L, it will result in under-pickling and incomplete removal of oxide scale. If the concentration of sulfuric acid is higher than 400 g / L or the concentration of hydrofluoric acid is higher than 60 g / L, it will result in over-pickling and the formation of corrosion defects on the surface of the strip. The pickling temperature must be controlled within the range of 80 ± 5 °C to avoid incomplete removal of oxide scale due to under-pickling or surface defects due to over-pickling.

[0038] Preferably, in step S3, the deformation amounts of the five rolling passes are 0.3 mm, 0.4 mm, 0.5 mm, 0.2 mm, and 0.1 mm, respectively.

[0039] Step S3 involves a single cold rolling process with five passes. The deformation amount for each pass needs to be reasonably allocated to ensure uniform deformation and obtain a good sheet shape, which is beneficial for subsequent rolling processes. In this invention, the deformation amount for each pass is set in an order of increasing first and then decreasing. If this order is not used for Invar alloy hot-rolled coils with a thickness of 5±0.3mm, it will result in poor sheet shape control. In a preferred embodiment, the deformation amounts for the five rolling passes can be 0.3mm, 0.4mm, 0.5mm, 0.2mm, and 0.1mm, respectively, to obtain a single cold-rolled coil with a thickness of 3.5mm.

[0040] Preferably, in step S4, the annealing holding time is 0.5 hours longer than the holding time of the Invar alloy hot-rolled coil.

[0041] The primary bright annealing process in step S4 is carried out in a bell-type annealing furnace, with the annealing temperature controlled at 650±5℃ and the annealing holding time at 4-6 hours. This process can completely eliminate work hardening and residual stress in the strip steel. If the annealing temperature is higher than 655℃, it will lead to energy waste; if it is lower than 645℃, the work hardening will not be completely removed.

[0042] Preferably, in step S5:

[0043] The secondary cold rolling process employs a 6-pass deformation rolling technique, with pass deformation amounts of 0.4 mm, 0.4 mm, 0.5 mm, 0.3 mm, 0.3 mm, and 0.1 mm, respectively. The thickness of the resulting secondary cold-rolled coil is 1.5 ± 0.05 mm; or

[0044] When the secondary cold rolling is carried out in 8 passes, the deformation amounts per pass are 0.4mm, 0.4mm, 0.5mm, 0.5mm, 0.4mm, 0.3mm, 0.2mm, and 0.1mm, respectively, and the thickness of the secondary cold-rolled coil after cold rolling is 0.7±0.03mm.

[0045] In the secondary cold rolling process of step S5, the 3.5mm thick secondary cold-rolled coil can be further cold-rolled, with deformation controlled in 6 or 8 passes. The deformation amount in each pass is reasonably allocated to ensure uniform deformation and obtain a good sheet shape. In a specific embodiment, the deformation amounts in the 6-pass deformation rolling are 0.4mm, 0.4mm, 0.5mm, 0.3mm, 0.3mm, and 0.1mm, respectively. In another specific embodiment, the deformation amounts in the 8-pass deformation rolling are 0.4mm, 0.4mm, 0.5mm, 0.5mm, 0.4mm, 0.3mm, 0.2mm, and 0.1mm, respectively.

[0046] Preferably, in step S6:

[0047] The strip with a thickness of 1.5±0.05mm is subjected to a strip travel speed of 7-9m / min during secondary bright annealing; or

[0048] The secondary cold-rolled coil with a thickness of 0.7±0.03mm is subjected to a strip speed of 10.5~12.5m / min during secondary bright annealing.

[0049] The secondary bright annealing process in step S6 is carried out in a continuous bright annealing furnace, with the annealing temperature set at 980±5℃. The purpose is to eliminate work hardening and residual stress in the secondary cold-rolled coil / strip. If the annealing temperature is below 975℃, the work hardening and residual stress will not be completely removed; if the annealing temperature is above 985℃, the mechanical properties of the strip will decrease. In a specific embodiment, the travel speed of the secondary cold-rolled coil with a thickness of 1.5±0.05mm during the secondary bright annealing process is controlled at 7–9 m / min. The purpose is to remove residual stress and obtain the required mechanical properties. The travel speed of the secondary cold-rolled coil with a thickness of 0.7±0.03mm during the secondary bright annealing process is controlled at 10.5–12.5 m / min. The purpose is to remove residual stress and obtain the required mechanical properties.

[0050] Preferably, in step S7:

[0051] The 1.5±0.05mm thick secondary cold-rolled coil undergoes a leveling process after secondary bright annealing, with tension controlled at 5–15 tons and elongation controlled at 0.8%–1.5%; or

[0052] The secondary cold-rolled coil with a thickness of 0.7±0.03mm is subjected to a tension leveling process after secondary bright annealing, with tension controlled at 3 to 5 tons and elongation controlled at 0.4% to 0.6%.

[0053] The leveling or tension straightening process in step S7 ensures that the secondary cold-rolled coil has a high degree of flatness. In a specific embodiment, the 1.5±0.05mm secondary cold-rolled coil undergoes a leveling process with tension controlled at 5–15 tons and elongation controlled at ≤1.5%. The 0.7±0.03mm secondary cold-rolled coil undergoes a tension straightening process with tension controlled at 3–5 tons and elongation controlled at 0.4–0.6%. Good sheet shape ensures the implementation of subsequent lap welding. The flatness of the Invar alloy cold-rolled strip obtained after the above leveling or tension straightening process is controlled within the range of ≤0.5mm / m.

[0054] This invention relates to a cold-rolled Invar alloy strip with excellent weldability and its production method. Based on the Fe-Ni36 alloy, trace amounts of Nb, Al, and Ca elements are added to significantly improve weldability and effectively reduce crack resistance. A combined process is employed: annealing → pickling → primary cold rolling → primary bright annealing → secondary cold rolling → secondary bright annealing → leveling / tension straightening. The annealing process eliminates internal stress in the hot-rolled Invar alloy, while pickling thoroughly removes oxide scale from the hot-rolled coil surface, laying a good foundation for subsequent cold rolling. A special cold rolling process (primary cold rolling → primary bright annealing → secondary cold rolling → secondary bright annealing → leveling or tension straightening) is then used, controlling the deformation amount in each cold rolling pass within a reasonable range to ensure strip deformation uniformity and shape quality. Bright annealing after each cold rolling effectively eliminates work hardening and residual stress, and the final leveling or tension straightening process further improves the strip's flatness. This invention achieves stable assurance of the excellent welding performance of Invar alloy cold-rolled strip through precise design of alloy composition and coordinated control of preparation process.

[0055] The present invention has the following beneficial effects:

[0056] This invention, based on the Fe-Ni36 alloy, simultaneously adds Nb and Al to leverage their synergistic effect, ensuring that grain growth is suppressed during subsequent welding and improving the weldability of Invar alloy cold-rolled strip. Simultaneously, the addition of a certain amount of Ca alters the sulfide morphology (spheroidized MnS), thereby enhancing the mechanical properties of the weld heat-affected zone metal, making the weld metal less prone to hot cracking, and improving the hot cracking resistance of the Invar alloy cold-rolled strip, resulting in excellent weldability. The Invar alloy cold-rolled strip designed by this invention exhibits excellent weldability and high weld metal strength, making it highly suitable for low-temperature environments, such as the construction of membrane storage tanks for LNG carriers and onshore storage facilities. Attached Figure Description

[0057] Figure 1 This is a flowchart of the production method of the Invar alloy cold-rolled strip with excellent weldability according to the present invention;

[0058] Figure 2 These are weld morphology diagrams of the Invar alloy cold-rolled strip prepared according to the embodiments of the present invention. (a) is a weld morphology diagram of the Invar alloy cold-rolled strip of Example 1, (b) is a weld morphology diagram of the Invar alloy cold-rolled strip of Example 2, and (c) is a weld morphology diagram of the Invar alloy cold-rolled strip of Example 3.

[0059] Figure 3 This is a diagram showing the weld morphology of a cold-rolled strip of Invar alloy prepared in proportion. Detailed Implementation

[0060] 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.

[0061] Based on the chemical composition and production method designed according to this invention, three batches of Invar alloy cold-rolled strip were produced. The composition is shown in Table 1. Subsequent production of Invar alloy cold-rolled strip was performed using non-consumable electrode gas shielded welding (GEG). The weld morphology was then observed, and the weld metal properties were tested. The results are shown in Table 2. Specifically, GB / T708-2019 "Dimensions, Shape, Weight and Permissible Deviations of Cold-Rolled Steel Sheets and Strips" was used to test the thickness and straightness of the Invar alloy cold-rolled strip; GB / T 4339-2008 "Determination of Characteristic Parameters of Thermal Expansion of Metallic Materials" was used to test the coefficient of linear expansion of the Invar alloy cold-rolled strip; and GB / T 228.1-2021 "Tensive Testing of Metallic Materials—Part 1: Test Method at Room Temperature" was used to determine the maximum load-bearing capacity of the weld metal. Weld metal samples with a length of 140 mm and a width of 40 mm were taken and subjected to tensile testing on a WANCE-TSE105D microcomputer-controlled electronic universal testing machine, with the tensile speed controlled at 2 mm / min.

[0062] Example 1

[0063] Combination Figure 1 As shown, the production method of Invar alloy cold-rolled strip in this embodiment is as follows:

[0064] Molten steel with the chemical composition shown in Table 1 was produced by electric furnace + ladle refining smelting method, cast into steel ingots, and then hot-rolled into Invar alloy hot-rolled coils of approximately 5 mm thickness. First, the 5 mm Invar alloy hot-rolled coils were annealed in a continuous annealing furnace at a temperature controlled at 1050℃ and a strip speed of 14 m / min. The annealed Invar alloy hot-rolled coils were then pickled in a pickling solution with a sulfuric acid concentration of 350 g / L and a hydrofluoric acid concentration of 60 g / L, at a temperature controlled at approximately 80℃. The pickled 5 mm Invar alloy hot-rolled coils were then cold-rolled once to obtain a 3.5 mm thick cold-rolled coil, rolled in 5 passes with deformation amounts of 0.3 mm, 0.4 mm, 0.5 mm, 0.2 mm, and 0.1 mm respectively per pass. A 3.5mm thick primary cold-rolled coil was subjected to a first bright annealing in a bell-type annealing furnace at a temperature of 650℃ for 5 hours. The 3.5mm primary cold-rolled coil after the first bright annealing was then subjected to a second cold rolling process to 1.5mm and 0.7mm thicknesses, with deformation controlled in 6 and 8 passes respectively. The deformation amounts for the 6 passes were controlled as follows: 0.4mm, 0.4mm, 0.5mm, 0.3mm, 0.3mm, 0.1mm, resulting in a 1.5mm thick secondary cold-rolled coil. The deformation amounts for the 8 passes were controlled as follows: 0.4mm, 0.4mm, 0.5mm, 0.5mm, 0.4mm, 0.3mm, 0.2mm, 0.1mm, resulting in a 0.7mm thick secondary cold-rolled coil. The finished products undergo bright annealing at 980℃. The running speed for 1.5mm thick secondary cold-rolled strip is 8m / min, and for 0.7mm thick secondary cold-rolled strip, it is 11.5m / min. The annealed secondary cold-rolled strip is then leveled or tension-straightened: the 1.5mm thick strip undergoes leveling with a tension controlled at 10 tons and an elongation controlled at 1.0%; the 0.7mm thick strip undergoes tension straightening with a tension controlled at 5 tons and an elongation controlled at 0.5%.

[0065] Welding performance evaluation: Lap welding tests were conducted on the two thicknesses (1.5 mm and 0.7 mm) of Invar alloy cold-rolled strip produced above using non-consumable electrode gas shielded welding. The weld morphology was good, with no welding hot cracks. (See...) Figure 2 As shown in (a) above, the performance is shown in Table 2.

[0066] Example 2

[0067] Combination Figure 1 As shown, the production method of Invar alloy cold-rolled strip in this embodiment is as follows:

[0068] Molten steel with the chemical composition shown in Table 1 was produced by electric furnace + ladle refining smelting method, cast into steel ingots, and then hot-rolled into Invar alloy hot-rolled coils of approximately 5 mm thickness. First, the 5 mm Invar alloy hot-rolled coils were annealed in a continuous annealing furnace at a temperature controlled at 1040℃ and a strip speed of 12 m / min. The annealed Invar alloy hot-rolled coils were then pickled in a pickling solution with a sulfuric acid concentration of 400 g / L and a hydrofluoric acid concentration of 50 g / L, at a temperature controlled at approximately 75℃. The pickled 5 mm Invar alloy hot-rolled coils were then cold-rolled once to obtain a 3.5 mm thick cold-rolled coil, rolled in 5 passes with deformation amounts of 0.3 mm, 0.4 mm, 0.5 mm, 0.2 mm, and 0.1 mm respectively per pass. A 3.5mm thick primary cold-rolled coil was subjected to a bright annealing process in a bell-type annealing furnace at a temperature of 645℃ for 4 hours. The 3.5mm primary cold-rolled coil after the bright annealing was then subjected to a second cold rolling process to achieve thicknesses of 1.5mm and 0.7mm, respectively, with deformation controlled in 6 and 8 passes. The deformation amounts for the 6 passes were controlled as follows: 0.4mm, 0.4mm, 0.5mm, 0.3mm, 0.3mm, 0.1mm, resulting in a 1.5mm thick secondary cold-rolled coil. The deformation amounts for the 8 passes were controlled as follows: 0.4mm, 0.4mm, 0.5mm, 0.5mm, 0.4mm, 0.3mm, 0.2mm, 0.1mm, resulting in a 0.7mm thick secondary cold-rolled coil. The finished products undergo bright annealing at 985℃. The running speed for 1.5mm thick secondary cold-rolled strip is 9m / min, and for 0.7mm thick secondary cold-rolled strip, it is 12.5m / min. The annealed secondary cold-rolled strip is then leveled or tension-straightened: the 1.5mm thick strip undergoes leveling with tension controlled at 5 tons and elongation controlled at 1.5%; the 0.7mm thick strip undergoes tension straightening with tension controlled at 4 tons and elongation controlled at 0.4%.

[0069] Welding performance evaluation: Lap welding tests were conducted on the two thicknesses (1.5 mm and 0.7 mm) of Invar alloy cold-rolled strip produced above using non-consumable electrode gas shielded welding. The weld morphology was good, with no welding hot cracks. (See...) Figure 2 As shown in (b) above, the performance is shown in Table 2.

[0070] Example 3

[0071] Combination Figure 1 As shown, the production method of Invar alloy cold-rolled strip in this embodiment is as follows:

[0072] Molten steel with the chemical composition shown in Table 1 was produced by electric furnace + ladle refining smelting method, cast into steel ingots, and then hot-rolled into Invar alloy hot-rolled coils of approximately 5 mm thickness. First, the 5 mm Invar alloy hot-rolled coils were annealed in a continuous annealing furnace at a temperature controlled at 1060℃ and a strip speed of 16 m / min. The annealed Invar alloy hot-rolled coils were then pickled in a pickling solution with a sulfuric acid concentration of 380 g / L and a hydrofluoric acid concentration of 56 g / L, at a temperature controlled at approximately 85℃. The pickled 5 mm Invar alloy hot-rolled coils were then cold-rolled once to obtain a 3.5 mm thick single-pass cold-rolled coil, rolled in 5 passes with deformation amounts of 0.3 mm, 0.4 mm, 0.5 mm, 0.2 mm, and 0.1 mm respectively. A 3.5mm thick primary cold-rolled coil was subjected to a bright annealing process in a bell-type annealing furnace at a temperature of 655℃ for 6 hours. The 3.5mm primary cold-rolled coil was then further cold-rolled to 1.5mm and 0.7mm thicknesses, respectively, with deformation controlled in 6 and 8 passes. The deformation amounts for the 6 passes were controlled as follows: 0.4mm, 0.4mm, 0.5mm, 0.3mm, 0.3mm, 0.1mm, yielding a 1.5mm thick secondary cold-rolled coil. The deformation amounts for the 8 passes were controlled as follows: 0.4mm, 0.4mm, 0.5mm, 0.5mm, 0.4mm, 0.3mm, 0.2mm, 0.1mm, yielding a 0.7mm thick secondary cold-rolled coil. The finished products undergo bright annealing at 975℃. The running speed for 1.5mm thick secondary cold-rolled strip is 7m / min, and for 0.7mm thick secondary cold-rolled strip, it is 10.5m / min. The annealed secondary cold-rolled strip is then leveled or tension-straightened: the 1.5mm thick strip undergoes leveling with tension controlled at 15 tons and elongation controlled at 0.8%; the 0.7mm thick strip undergoes tension straightening with tension controlled at 3 tons and elongation controlled at 0.6%.

[0073] Welding performance evaluation: Lap welding tests were conducted on the two thicknesses (1.5 mm and 0.7 mm) of Invar alloy cold-rolled strip produced above using non-consumable electrode gas shielded welding. The weld morphology was good, with no welding hot cracks. (See...) Figure 2 As shown in (c) in the figure, the performance is shown in Table 2.

[0074] Comparative Example 1

[0075] The production method of the Invar alloy cold-rolled strip in this comparative example is as follows:

[0076] Molten steel with the chemical composition shown in Table 1 was produced by electric furnace + ladle refining. This was cast into ingots and then hot-rolled into approximately 5mm thick hot-rolled coils. First, the 5mm hot-rolled coils were annealed in a continuous annealing furnace at a temperature controlled at 1060℃ and a strip speed of 16m / min. The annealed hot-rolled coils were then pickled in a pickling solution with a sulfuric acid concentration of 380g / L and a hydrofluoric acid concentration of 56g / L, at a temperature controlled at approximately 85℃. The pickled 5mm hot-rolled coils were then cold-rolled once to obtain a 3.5mm thick cold-rolled coil. This was done in five passes with deformation amounts of 0.3mm, 0.4mm, 0.5mm, 0.2mm, and 0.1mm respectively. Finally, the 3.5mm cold-rolled coils underwent a bright annealing process in a bell-type annealing furnace at a temperature controlled at 655℃ for 5 hours. The 3.5mm primary cold-rolled coil, after a single bright annealing, is further cold-rolled to 1.5mm and 0.7mm thicknesses, respectively, with deformation controlled in 6 and 8 passes. The deformation amounts for the 6 passes are: 0.4mm, 0.4mm, 0.5mm, 0.3mm, 0.3mm, 0.1mm, resulting in a 1.5mm thick secondary cold-rolled coil. The deformation amounts for the 8 passes are: 0.4mm, 0.4mm, 0.5mm, 0.5mm, 0.4mm, 0.3mm, 0.2mm, 0.1mm, resulting in a 0.7mm thick secondary cold-rolled coil. The finished products are then bright annealed at 980℃. The running speed for the 1.5mm thick secondary cold-rolled coil is 11m / min, and the running speed for the 0.7mm thick secondary cold-rolled coil is 14m / min. The annealed secondary cold-rolled coils and strips are leveled or tension-straightened: For secondary cold-rolled coils with a thickness of 1.5mm, a leveling process is used, with tension controlled at 3 tons and elongation controlled at 0.8%. For secondary cold-rolled coils with a thickness of 0.7mm, a tension-straightening process is used, with tension controlled at 2 tons and elongation controlled at 0.3%.

[0077] Welding performance evaluation: Lap welding tests were conducted on the two thicknesses (1.5 mm and 0.7 mm) of cold-rolled strip produced above using non-consumable electrode gas shielded welding. Although no welding hot cracks were observed, the weld morphology was poor. (See attached image) Figure 3 As shown, the maximum load-bearing capacity of the weld is low and does not meet the standard requirements. The performance is shown in Table 2.

[0078] Comparative Example 2

[0079] This comparative example uses the production method of Example 1, the difference being that the chemical composition of the alloy strip is: C: 0.038%, Si: 0.31%, Mn: 0.52%, P: 0.006%, S: 0.002%, Ni: 36.1%, Nb: 0.006%, Al: 0.002%, Ca: 0.00008%, with the balance being Fe and unavoidable impurities, the total amount of which is less than 0.05 wt%.

[0080] The welding performance of the cold-rolled strip produced in this comparative example is shown in Table 2.

[0081] Comparative Example 3

[0082] This comparative example uses the production method of Example 1, the difference being that the chemical composition of the alloy strip is: C: 0.04%, Si: 0.25%, Mn: 0.46%, P: 0.009%, S: 0.0015%, Ni: 35.8%, Nb: 0.10%, Al: 0.03%, Ca: 0.0035%, with the balance being Fe and unavoidable impurities, the total amount of which is less than 0.05 wt%.

[0083] The welding performance of the cold-rolled strip produced in this comparative example is shown in Table 2.

[0084] Comparative Example 4

[0085] This comparative example uses the production method of Example 1, the difference being that the primary cold rolling is performed in 4 passes with deformation amounts of 0.5mm, 0.6mm, 0.6mm, and 0.3mm respectively, to obtain a primary cold-rolled coil with a thickness of 3mm; the primary cold-rolled coil with a thickness of 3mm is then rolled in 6 and 8 passes respectively in the secondary cold rolling to obtain secondary cold-rolled coils with thicknesses of 1.1mm and 0.2mm. The secondary cold-rolled coil with a thickness of 1.1mm is processed using a leveling process, and the secondary cold-rolled coil with a thickness of 0.2mm is processed using a tension leveling process.

[0086] The welding performance of cold-rolled strips with thicknesses of 1.1 mm and 0.2 mm produced in this comparative example is shown in Table 2.

[0087] Comparative Example 5

[0088] This comparative example uses the production method of Example 1, the difference being that the secondary cold rolling is performed in 7 passes, with the deformation amounts per pass being 0.4mm, 0.4mm, 0.5mm, 0.4mm, 0.4mm, 0.4mm, and 0.3mm, respectively, to obtain a secondary cold-rolled coil with a thickness of 0.7mm, which is then subjected to a tension leveling process.

[0089] The thickness of the cold-rolled strip produced in this comparative example is 0.7 mm, and the welding performance is shown in Table 2.

[0090] Table 1 Chemical composition of the alloy (wt%)

[0091]

[0092] Table 2

[0093]

[0094] As shown in Table 1, the linear expansion coefficient and maximum load-bearing capacity of the Invar alloy cold-rolled strip prepared in Examples 1-3 are superior to those of the cold-rolled strip prepared in Comparative Example 1. The flatness of the Invar alloy cold-rolled strip in Examples 1-3 is 0.2–0.4 mm / m, and the linear expansion coefficient at -180℃ to 0℃ is 1.43–1.67 × 10⁻⁶. -6 At ℃, the maximum load-bearing capacity of the weld metal after lap welding is 4179~4201N, and the weld morphology is good with no welding hot cracks. It can be seen that the Invar alloy cold-rolled strip of the present invention has excellent expansion performance and welding performance.

[0095] As can be seen from Example 1 and Comparative Examples 2-3, the Nb, Al, and Ca contents in the alloy composition are lower than the chemical composition range of the alloy of the present invention, resulting in poor weldability of the cold-rolled strip. The maximum load-bearing capacity of the weld metal after lap welding of the strip is only 3595N, which is far lower than that of the Invar alloy cold-rolled strip prepared in Example 1. The Nb, Al, and Ca contents are higher than the chemical composition range of the alloy of the present invention, resulting in increased expansion properties of the alloy. The linear expansion coefficient of the strip at -180℃ to 0℃ is significantly higher than that of the Invar alloy cold-rolled strip prepared in the examples. Therefore, if the chemical composition of the alloy of the present invention is not used, the expansion and weldability of the prepared cold-rolled strip will not meet the requirements for use.

[0096] As can be seen from Example 1 and Comparative Example 4, when the process parameters of the present invention are not used in a single cold rolling process, the alloy deforms unevenly during cold rolling, and the local residual stress is large, resulting in the flatness failing to meet the requirements. At the same time, the maximum load-bearing capacity of the weld metal after the strip lap welding is only 1525N, which is far below the standard requirement.

[0097] As can be seen from Example 1 and Comparative Example 5, if the process parameters of the present invention are not used in the secondary cold rolling, the alloy will deform unevenly during cold rolling, resulting in large local residual stress and flatness that cannot meet the requirements.

[0098] 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 producing Invar alloy cold-rolled strip, characterized in that: Includes the following steps: S1, annealing the Invar alloy hot-rolled coil with a thickness of 5±0.3mm at an annealing temperature of 1050±10℃; S2, the annealed Invar alloy hot-rolled coil is pickled. The pickling solution is a mixed acid solution of sulfuric acid and hydrofluoric acid, with the concentration of sulfuric acid being 350-400 g / L and the concentration of hydrofluoric acid being 50-60 g / L. S3, the pickled Invar alloy hot-rolled coil is subjected to a single cold rolling process, which is carried out in 5 passes to obtain a single cold-rolled coil; the deformation amount of the single cold rolling pass is set in a manner that first increases and then decreases, and the deformation amount of the pass is controlled between 0.1 and 0.5 mm. S4, the cold-rolled coil is subjected to a bright annealing in a bell-type annealing furnace, and the annealing temperature is controlled at 650±5℃. S5, the first cold-rolled coil after bright annealing is subjected to a second cold rolling, with deformation controlled in 6 or 8 passes, to obtain a second cold-rolled coil; the deformation amount per pass of the second cold rolling is controlled to be 0.1 to 0.5 mm; S6, the second cold-rolled coil is subjected to second bright annealing in a continuous bright annealing furnace, and the annealing temperature is controlled at 980±5℃. S7 involves leveling or tensioning the second cold-rolled coil after secondary bright annealing, with tension controlled between 3 and 15 tons and elongation controlled at ≤1.5%. The chemical composition of the Invar alloy cold-rolled strip, by weight percentage, is as follows: C: 0.02-0.06%, Si≤0.40%, Mn: 0.20-0.60%, P≤0.008%, S≤0.003%, Ni: 34.8-37.5%, Nb: 0.01-0.08%, Al: 0.005-0.02%, Ca: 0.0001-0.003%, with the balance being Fe and unavoidable impurities, the total amount of which is less than 0.05%. The flatness of the Invar alloy cold-rolled strip is ≤0.5mm / m, and the coefficient of linear expansion at temperatures ranging from -180℃ to 0℃ is 1.0~2.0×10⁻⁶. -6 / ℃; The maximum load-bearing capacity of the weld metal after lap welding of the Invar alloy cold-rolled strip is ≥4025N.

2. The method for producing Invar alloy cold-rolled strip according to claim 1, characterized in that: In step S1: The chemical composition of the Invar alloy hot-rolled coil, by weight percentage, is as follows: C: 0.02-0.06%, Si≤0.40%, Mn: 0.20-0.60%, P≤0.008%, S≤0.003%, Ni: 34.8-37.5%, Nb: 0.01-0.08%, Al: 0.005-0.02%, Ca: 0.0001-0.003%, with the balance being Fe and unavoidable impurities, the total amount of which is less than 0.05%. The preparation method of the Invar alloy hot-rolled coil is as follows: molten steel is produced by one of the following smelting processes: electric furnace + ladle refining / electroslag remelting or vacuum induction + electroslag remelting. The molten steel is then cast into steel ingots, which are then hot-rolled to obtain Invar alloy hot-rolled coils. The annealing is performed using a continuous annealing furnace or a bell-type annealing furnace. When using a continuous annealing furnace, the running speed of the Invar alloy hot-rolled coil strip is controlled at 12-16 mm / min. When using a bell-type annealing furnace, the annealing holding time is controlled at 4-6 h.

3. The method for producing Invar alloy cold-rolled strip according to claim 1, characterized in that: In step S2, the pickling temperature is 80±5℃.

4. The method for producing Invar alloy cold-rolled strip according to claim 1, characterized in that: In step S3, the deformation amounts of the five rolling passes are 0.3 mm, 0.4 mm, 0.5 mm, 0.2 mm, and 0.1 mm, respectively.

5. The method for producing Invar alloy cold-rolled strip according to claim 1, characterized in that: In step S4, the annealing holding time for the first bright annealing is 4 to 6 hours.

6. The method for producing Invar alloy cold-rolled strip according to claim 1, characterized in that: In step S5: The secondary cold rolling process employs a 6-pass deformation rolling technique, with pass deformation amounts of 0.4 mm, 0.4 mm, 0.5 mm, 0.3 mm, 0.3 mm, and 0.1 mm, respectively. The thickness of the resulting secondary cold-rolled coil is 1.5 ± 0.05 mm; or When the secondary cold rolling is carried out in 8 passes, the deformation amounts per pass are 0.4mm, 0.4mm, 0.5mm, 0.5mm, 0.4mm, 0.3mm, 0.2mm, and 0.1mm, respectively, and the thickness of the secondary cold-rolled coil after cold rolling is 0.7±0.03mm.

7. The method for producing Invar alloy cold-rolled strip according to claim 6, characterized in that: In step S6: The strip speed of the 1.5±0.05mm thick secondary cold-rolled coil during secondary bright annealing is 7-9m / min; or The secondary cold-rolled coil with a thickness of 0.7±0.03mm is subjected to a strip speed of 10.5~12.5m / min during secondary bright annealing.

8. The method for producing Invar alloy cold-rolled strip according to claim 6, characterized in that: In step S7: The 1.5±0.05mm thick secondary cold-rolled coil undergoes a leveling process after secondary bright annealing, with tension controlled at 5–15 tons and elongation controlled at 0.8%–1.5%; or The secondary cold-rolled coil with a thickness of 0.7±0.03mm is subjected to a tension leveling process after secondary bright annealing, with tension controlled at 3 to 5 tons and elongation controlled at 0.4% to 0.6%.

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