A method of improving corrosion resistance of 5xxx aluminum alloy sheet and strip

Abstract

The application discloses a method for improving corrosion resistance of 5xxx aluminum alloy plate and strip, and belongs to the field of metal forming, and steps are as follows: 5xxx aluminum alloy is soaked in a heat preservation container containing liquid nitrogen and is sufficiently cooled; the 5xxx aluminum alloy plate which is sufficiently cooled is sent into a rolling mill for rolling, and the 5xxx aluminum alloy plate and the roller are sufficiently cooled before each pass of rolling. The brittleness characteristics of the coarse second phase in a low-temperature environment are utilized to make the coarse second phase in the 5xxx aluminum alloy plate be broken in the process of deep cryogenic rolling. In the process of deep cryogenic treatment, the difference between the volume shrinkage effects of the second phase and the matrix effectively improves the interface bonding strength of the aluminum matrix and the second phase, so that the 5xxx aluminum alloy has better pitting corrosion resistance. Compared with room temperature rolling, the deep cryogenic rolling improves the proportion of small-angle grain boundaries in the 5xxx aluminum alloy, thereby inhibiting the precipitation of beta phase, and realizes the improvement of intergranular corrosion performance. The 5xxx aluminum alloy with better corrosion resistance is of great significance to the development of the industries such as ships, rail transportation, aerospace and military.

Description

Technical Field

[0001] The present invention belongs to the technical field of aluminum alloy material forming and processing, and in particular relates to a method for improving the corrosion resistance of 5xxx aluminum alloy plates and strips. Background Art

[0002] Aluminum alloys are widely used in the manufacturing industry due to their good formability and high specific strength. Among them, 5xxx aluminum alloys cannot be strengthened by heat treatment, but have medium strength after work hardening. Compared with other aluminum alloys, they have higher corrosion resistance and are therefore widely used in fields such as automobile and shipbuilding. In some scenarios, aluminum alloys with both good corrosion resistance and strength may be required to meet actual needs. 5xxx aluminum alloys are mainly strengthened through work hardening, so the accumulation of higher dislocation densities during alloy deformation is more conducive to improving strength. The deep cold environment helps to suppress the dynamic recovery of the alloy during deformation, allowing for high-density dislocation accumulation.

[0003] Corrosion is one of the main forms of component failure. Corrosion expansion can directly lead to component fracture. Even if no fracture occurs, the safety of the component is seriously compromised. When subjected to external loads, stress concentration occurs at the corroded area, which eventually leads to fracture. The corrosion problem seriously affects the service life of aluminum alloy plates and the components made from them. Therefore, properly handling the problem of alloy corrosion resistance will help promote the application of aluminum alloys. The corrosion of 5xxx aluminum alloys is mainly pitting corrosion and intergranular corrosion. Among them, pitting corrosion is mainly caused by coarse second phases in aluminum-magnesium alloys. For example, the Al6 (Mn, Fe) phase is preferentially corroded during the corrosion process because its corrosion potential is lower than that of the aluminum matrix, leaving pitting pits on the corroded surface.

[0004] 5xxx aluminum alloys are prone to intergranular corrosion under certain conditions, especially when they contain a high magnesium content (Mg>3%). High magnesium content will cause the precipitation of magnesium-rich β (Al3Mg2) phase near the grain boundaries, which is prone to anodic dissolution in a humid environment, causing intergranular corrosion. This phenomenon is particularly evident in marine environments and will significantly reduce the durability of the alloy. During the corrosion process, intergranular corrosion occurs along the β phase. The length, width and continuity of the β phase precipitation along the high-angle grain boundaries have a significant effect on the corrosion resistance of the alloy. Deep cold rolling helps to refine the coarse second phase in the alloy, while regulating the location and continuity of the β phase precipitation in the alloy.

[0005] Low-angle grain boundaries (LGBs) typically have angles below 15°. These boundaries are typically formed by clusters of dislocations and have a relatively high dislocation density. They are generally less susceptible to corrosion. Because their internal lattice distortion is minimal and their grain boundary energy is low, their electrochemical activity is relatively low, allowing them to form a continuous passivation film. Consequently, they are less likely to form continuous corrosion pathways, which inhibits pitting and intergranular corrosion. High-angle grain boundaries (LGBs) typically have angles above 15°. These boundaries exhibit greater lattice distortion and high energy, making them preferred targets for corrosion. In 5xxx aluminum alloys, magnesium atoms readily precipitate at LGBs, forming a continuous β phase. This precipitate dissolves preferentially in corrosive environments, making LGBs prime sites for intergranular corrosion. 5xxx aluminum alloys are more susceptible to stress corrosion cracking at LGBs because they are more susceptible to damage under the combined effects of tensile stress and the corrosive environment. This is particularly evident when 5xxx aluminum alloys are used in harsh environments such as marine and acidic environments. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, the purpose of the present invention is to provide a method for improving the corrosion resistance of 5xxx aluminum alloy plates and strips, so as to improve the alloy's resistance to pitting corrosion and intergranular corrosion, and ultimately provide a high-performance aluminum alloy for lightweight manufacturing.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for improving the corrosion resistance of 5xxx aluminum alloy sheet and strip comprises the following steps:

[0009] Step 1, homogenizing annealing the smelted and cast 5xxx aluminum alloy ingot;

[0010] Step 2: milling the annealed 5xxx aluminum alloy ingot, and then hot rolling it at 450° C. to 470° C. to obtain a pre-rolled plate;

[0011] Step 3, cooling the pre-rolled plate to liquid nitrogen temperature and performing deep cold rolling, with a reduction rate of less than 20% per pass;

[0012] Step 4, repeating step 3 until the total deformation of the pre-rolled plate obtained in step 2 reaches 80% to 98% and the final thickness of the plate reaches 0.8 mm to 4 mm;

[0013] Step 5, stretching and straightening the plate obtained in step 4;

[0014] Step 6: Soak the plate obtained in step 5 in a low-temperature liquid nitrogen box for 2 to 4 hours.

[0015] The present invention utilizes the brittle characteristics of the coarse second phase in a low-temperature environment to cause the coarse second phase in the 5xxx aluminum alloy plate to be crushed during the deep cold rolling process. Due to the difference in volume shrinkage effect between the second phase and the matrix during the deep cold treatment process, the interfacial bonding strength of the aluminum matrix and the second phase is effectively improved, thereby having better pitting corrosion resistance. Compared with room temperature rolling, deep cold rolling increases the ratio of small-angle grain boundaries in the 5xxx aluminum alloy, thereby suppressing the precipitation of β phase and achieving improved intergranular corrosion performance.

[0016] In one embodiment, the composition of the 5xxx aluminum alloy ingot is:

[0017] Mg: 3.5%~5.0%, Mn: 0.25%~0.50%, Ti<0.03%, Fe<0.50%, Si<0.50%, Cr: 0.05%~0.25%, and the balance is Al.

[0018] In one embodiment, during the smelting and casting process, online slag removal, degassing and filtering processes are performed, and the thickness of the obtained 5xxx aluminum alloy ingot is greater than 30 mm and less than 50 mm.

[0019] In one embodiment, in step 1, the smelted and cast 5xxx aluminum alloy ingot is placed in a homogenization heat treatment furnace, heated to 460-475° C. and kept warm for 8-12 hours. After the insulation is completed, the ingot is taken out and air-cooled.

[0020] In one embodiment, the step 2 adopts a multi-pass rolling process with a small reduction, and the reduction is reduced as the number of rolling passes increases. The thickness of the plate after hot rolling is 70% to 80% of the ingot thickness.

[0021] In one embodiment, the reduction in reduction as the number of rolling passes increases means that the reduction in each pass is less than 20% of the current plate thickness, and the minimum reduction is 0.2 mm.

[0022] In one embodiment, in step 3, the plate is cooled in liquid nitrogen, and liquid nitrogen spray is used to cool the upper and lower rollers of the rolling mill. The plate cooled to the liquid nitrogen temperature is transferred into the rolling mill for deep cold rolling within 5 seconds. During the rolling process, the liquid nitrogen nozzle continuously sprays liquid nitrogen to cool the plate, ensuring that the plate is in a deep cold environment.

[0023] In one embodiment, in step 6, when cooling the plate in liquid nitrogen, sufficient liquid nitrogen is required, and the plate is completely immersed in the liquid nitrogen to ensure that the plate is in a deep cold environment.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The process of the present invention is simple and easy to produce continuously.

[0026] (2) The method of the present invention has a significant effect on the refinement of coarse second phases in aluminum alloy plates and effectively accumulates high-density dislocations. The 5xxx aluminum alloy plates processed and prepared have an ultimate tensile strength of 400-450 MPa, a yield strength of 350-380 MPa, and an elongation of 8%-12% at room temperature. In the corrosion test conducted in accordance with GB / T7998-2005, the maximum corrosion depth of the 5xxx aluminum alloy plates (taking AA5083 as an example) obtained by the method of the present invention was less than 20 μm, which is much smaller than the room temperature rolled samples of the control group. To accelerate the corrosion test, the alloy was sensitized. The maximum corrosion depth of the 5xxx aluminum alloy plates was 63.8 μm, which is also smaller than the room temperature rolled samples (95.6 μm). The maximum corrosion depth of the samples that were further cryogenically treated after cryogenic rolling was only 52.1 μm. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a process flow chart for the deep cold rolling of 5xxx aluminum alloy (AA5083) plates according to the present invention, wherein (a) is the plate deep cold treatment process; (b) is the plate deep cold rolling process.

[0028] 1-Liquid nitrogen insulation box; 2-Liquid nitrogen; 3-5xxx aluminum alloy pre-rolled plate; 4-Liquid nitrogen nozzle 1; 5-Liquid nitrogen nozzle 2; 6-Liquid nitrogen nozzle 3; 7-Upper roller; 8-Lower roller.

[0029] Figure 2 Schematic diagram of the microstructure of the 5xxx aluminum alloy (AA5083) plate of the present invention, wherein (a) shows the crushed fine second phase in the deep cold rolled 5xxx aluminum alloy, and (b) shows the high density dislocations in the deep cold rolled 5xxx aluminum alloy.

[0030] Figure 3 These are the experimental results of the room temperature tensile test of the 5xxx aluminum alloy (AA5083) plate of the present invention.

[0031] Figure 4 The immersion corrosion test results of the 5xxx aluminum alloy (AA5083) plates of the present invention are shown in Figure 1. (a) is a room temperature rolled plate, (b) is a deep cold rolled plate, (c) is a deep cold rolled + deep cryogenic treatment plate, (d) is a room temperature rolled + 160℃ 100h sensitization plate, (e) is a deep cold rolled + 160℃ 100h sensitization plate, and (f) is a deep cold rolled + deep cryogenic treatment + 160℃ 100h sensitization plate.

[0032] Figure 5 The immersion corrosion test results of the 5xxx aluminum alloy (AA5024) plates of the present invention are shown in Figure 1. (a) is a deep cold rolled plate, (b) is a room temperature rolled plate, (c) is a deep cold rolled plate + 160℃ 100h sensitization plate, and (d) is a room temperature rolled plate + 160℃ 100h sensitization plate. DETAILED DESCRIPTION

[0033] The embodiments of the present invention are described in detail below with reference to the accompanying drawings and examples.

[0034] The present invention is a method for improving the corrosion resistance of 5xxx aluminum alloy sheets and strips. The method has the same or similar effects on alloys containing Mg as the main additive element and a certain amount of Mn, Fe and other elements. The process mainly includes: homogenization annealing, hot rolling, deep cold treatment, deep cold rolling, stretching and straightening, etc. Figure 1 , the method of the present invention is specifically described as follows:

[0035] Step 1, select 5xxx aluminum alloy ingot.

[0036] The present invention adopts 5xxx aluminum alloy ingots obtained by melting and casting as raw materials, and its composition by weight is:

[0037] Mg: 3.5%~5.0%, Mn: 0.25%~0.50%, Ti<0.03%, Fe<0.50%, Si<0.50%, Cr: 0.05%~0.25%, and the balance is Al.

[0038] In order to remove oxides and impurities in the melt, improve the purity of the alloy, reduce the hydrogen content, and avoid the generation of pores, the present invention adds online slag removal, degassing and filtration steps during the smelting process. The thickness of the final 5xxx aluminum alloy ingot is between 30mm and 50mm. The end value of 30mm or 50mm can be determined according to needs, which has no substantial impact on the method of the present invention.

[0039] Step 2: homogenizing annealing the 5xxx aluminum alloy ingot selected in step 1.

[0040] The purpose of this step is to allow these elements to diffuse and eliminate segregation, thereby making the alloy composition more uniform. Specifically, the smelted and cast 5xxx aluminum alloy ingot can be placed in a homogenization heat treatment furnace, heated to 460-475°C and kept at this temperature for 8-12 hours. After the holding period, it can be removed from the furnace and air-cooled.

[0041] Step 3: milling the annealed 5xxx aluminum alloy ingot, then heating it to 450° C. to 470° C., keeping it warm for about 2 hours, and then hot rolling it to obtain a pre-rolled plate.

[0042] The purpose of hot rolling in this step is to change the initial form of the metal, simplify the subsequent cold rolling process, realize the division of labor between rough processing and fine processing, improve processing efficiency and reduce production costs.

[0043] Furthermore, this step utilizes a low-reduction, multi-pass rolling process, with the reduction decreasing with each pass. The thickness of the hot-rolled plate is 70% to 80% of the ingot thickness. This low-reduction, multi-pass rolling process reduces deformation forces, stress and wear on rolling equipment, prevents material hardening or cracking, and improves production yield.

[0044] Furthermore, the reduction in each pass is less than 20% of the current plate thickness, with a minimum reduction of 0.2 mm. After hot rolling in this step, the plate thickness is 5 mm to 6 mm, which is 70% to 80% of the original ingot thickness. The above aluminum alloy plate is selected as the raw material for deep cold rolling.

[0045] Step 4: Cool the obtained pre-rolled plate to liquid nitrogen temperature and perform deep cold rolling, with the relative reduction of each pass being less than 20%.

[0046] The purpose of deep cold rolling in this step is to increase the proportion of small-angle grain boundaries in 5xxx aluminum alloys by controlling process conditions, which can slow down the occurrence of intergranular corrosion to a certain extent and improve the corrosion resistance of the alloy. Appropriate deformation processes (such as deep cold rolling) can adjust the type and distribution of grain boundaries. For example, deep cold rolling will introduce more small-angle grain boundaries and reduce the number of large-angle grain boundaries, which helps to improve the corrosion resistance and microstructural stability of the alloy.

[0047] Specifically, refer to Figure 1 As shown in (a), the 5xxx aluminum alloy pre-rolled plate 3 obtained in step 3 is cooled in a liquid nitrogen insulation box 1 loaded with liquid nitrogen 2. When the liquid nitrogen does not boil, it means that the plate has reached the liquid nitrogen temperature, and then the temperature is kept at this temperature for 3-5 minutes. Figure 1 As shown in (b), a pressure vessel is externally connected to a liquid nitrogen nozzle 1 4, a liquid nitrogen nozzle 2 5 and a liquid nitrogen nozzle 3 6. The liquid nitrogen nozzle 1 4 sprays spray to cool the 5xxx aluminum alloy pre-rolled plate 3, the liquid nitrogen nozzle 2 5 sprays spray to cool the upper roll 7 of the rolling mill, and the liquid nitrogen nozzle 3 6 sprays spray to cool the lower roll 8 of the rolling mill. The roll temperature is below -100°C, the liquid nitrogen flow rate is about 1L / min, and the plate cooled to the liquid nitrogen temperature is transferred to the rolling mill for deep cold rolling within 5 seconds. The roll linear speed is 4-6m / min. During the rolling process, the liquid nitrogen nozzle 1 4 continuously sprays liquid nitrogen to cool the 5xxx aluminum alloy pre-rolled plate 3 to ensure that the 5xxx aluminum alloy pre-rolled plate 3 is in a low temperature environment.

[0048] Step 5: Repeat step 4 until the total deformation of the sheet obtained in step 3 reaches 80% to 98%, and the final sheet thickness reaches 0.8 mm to 4 mm. More preferably, the total deformation reaches 93% to 98%, and the thickness reaches 0.8 mm to 2 mm. Aluminum alloy sheets with this thickness range are widely used in fields such as rail transportation, new energy vehicles, and shipbuilding. This step aims to enhance the effects of deep cold rolling.

[0049] Step 6: stretch and straighten the plate obtained in step 5.

[0050] The purpose of this step is to remove residual stress, eliminate internal stress and bending of the material, improve the machinability of the material, and make subsequent processing smoother.

[0051] Step 7: Soak the plate obtained in step 6 in a low-temperature liquid nitrogen box for 2 to 4 hours. Preferably, there should be sufficient liquid nitrogen during cooling, and the plate should be completely immersed in liquid nitrogen to ensure that the plate is in a low-temperature environment.

[0052] Cryogenic treatment changes the nucleation conditions for precipitates by introducing high-density dislocations and subgrain boundaries. The nucleation of β phase typically requires a certain amount of thermal energy, but in a cryogenic environment, this energy is insufficient to drive the nucleation of precipitates. Even during subsequent elevated-temperature annealing, the suppression of β phase precipitation by low-angle grain boundaries can reduce the formation of precipitation-free zones.

[0053] The 5xxx aluminum alloy sheet obtained by the above method has a high density of dislocations, a significant effect on refining coarse second phases, and performs well in corrosion tests and room temperature tensile tests.

[0054] The main principle of the present invention is to produce more corrosion-resistant 5xxx aluminum alloy sheets through deep cold rolling and deep cryogenic treatment. Specifically, the coarse second phase exhibits brittleness at low temperatures and has poorer deformation and slip capabilities than the aluminum matrix, making it easy to break when subjected to rolling shear strain. The broken second phase is reduced in size and more dispersed. After being corroded in the environment, the depth and width of the corrosion pits left behind by the broken second phase after corrosion or exfoliation are smaller. In addition, deep cold rolling significantly reduces the proportion of high-angle grain boundaries in the alloy, hindering the expansion and precipitation of Mg elements along high-angle grain boundaries. Since β phase mainly precipitates on high-angle grain boundaries during the sensitization process, the deep cold rolling process inhibits the precipitation of β phase on grain boundaries, significantly reducing the intergranular corrosion sensitivity of 5xxx aluminum alloys. Third, deep cold rolling effectively reduces the macroscopic shear bands in the aluminum alloy sheet, significantly improving the deformation uniformity of 5xxx aluminum alloys, thereby avoiding the deterioration of corrosion performance in local areas. Combined with the above three points, deep cold rolling improves the pitting corrosion resistance and intergranular corrosion resistance of 5xxx aluminum alloys. Due to the difference in volume shrinkage effect between the aluminum alloy matrix and the second phase particles under cryogenic conditions, the thermal expansion coefficient of the aluminum alloy is larger, and the volume shrinkage effect is more significant. However, the thermal expansion coefficient of the second phase particles is small, and the volume shrinkage effect is weaker. The larger volume shrinkage effect of the aluminum alloy will cause the aluminum alloy matrix to wrap the second phase more tightly, which plays a role in strengthening the combination effect of the matrix and the second phase. In addition, the non-precipitation zone at the grain boundary is also an important reason for the decline in the corrosion resistance of the alloy. Cryogenic treatment reduces the atomic thermal energy of the alloy, and the diffusion rate of the Mg element decreases, reducing the formation of the non-precipitation zone near the grain boundary and the unfavorable electric coupling effect. In summary, the present invention combines cryogenic rolling with cryogenic treatment to achieve an effective improvement in the corrosion resistance of 5xxx aluminum alloy materials, and achieves a synergistic improvement with mechanical strength, which has broad application prospects in the manufacturing industry.

[0055] Two specific embodiments of the present invention are as follows.

[0056] Example 1

[0057] AA5083 aluminum alloy ingots are heated to 460°C in a muffle furnace, homogenized for 6 hours, and then air-cooled. After milling to a 30mm thick ingot, they are heated to 450°C in a muffle furnace and held for 2 hours before being hot-rolled. The relative reduction per pass is less than 20%. The reduction decreases with each rolling pass, with intermediate annealing steps until the final thickness is 6mm. The hot-rolled 5xxx aluminum alloy sheet exhibits no cracking and excellent surface quality. This serves as the raw material for the next step of deep cold rolling.

[0058] Before the first rolling pass, the AA5083 aluminum alloy sheet was thoroughly cooled in liquid nitrogen for 30 minutes, and the rolls were cooled using a pressurized liquid nitrogen spray nozzle connected to a liquid nitrogen tank. Before each subsequent rolling pass, the sheet was cooled in liquid nitrogen for 5 minutes, and the transfer time from the liquid nitrogen holding tank to the roll nip did not exceed 5 seconds. Each pass achieved a relative reduction of 10%, ultimately reducing the 5083 aluminum alloy sheet to 1.5 mm. After rolling, the sheet was immersed in a liquid nitrogen holding tank for 4 hours.

[0059] The microstructure of deep cold rolled AA5083 aluminum alloy sheet was observed using transmission electron microscopy. Figure 2 In (a), it can be seen that the coarse second phase is broken under stress due to its low temperature brittleness, thereby reducing the size of the second phase. After pitting occurs, the size of the surface pitting pit is smaller. Figure 2 In (b), it can be seen that deep cold rolling produces high-density dislocations, which enables the alloy to achieve better dislocation strengthening.

[0060] The mechanical properties of the alloy were studied by tensile test. Dog bone-shaped tensile specimens were cut along the rolling direction and the tensile rate was set at 1×10 -3 s -1 The tensile test was carried out at room temperature. The ultimate tensile strength of the AA5083 aluminum alloy sheet of Example 1 of the present invention after rolling was measured to be 450 MPa, the yield strength was 380 MPa, and the elongation at break was 10%. Figure 3 shown.

[0061] The corrosion resistance of the alloy was evaluated using an immersion corrosion test. The test was conducted according to the method for determining intergranular corrosion of 5xxx aluminum alloys specified in GB / T 7998-2005, "Determination of Intergranular Corrosion of Aluminum Alloys." Figure 4 The cross section of cryogenically rolled and cryogenically treated AA5083 aluminum alloy sheets after immersion in a NaCl solution containing hydrochloric acid for 24 hours is shown. The control group is an AA5083 aluminum alloy sheet rolled to 1.5 mm at room temperature. Figure 4 As can be seen from (a) to (f), the maximum corrosion depth of the deep cold rolled AA5083 aluminum alloy is 16.5μm, and the maximum corrosion depth of the deep cold rolled + deep cryogenic treated sample is further reduced to 14.2μm. The maximum corrosion depth of intergranular corrosion of the sensitized sample also shows that deep cold rolling and deep cryogenic treatment can significantly improve the corrosion resistance of AA5083 aluminum alloy plate and reduce the intergranular corrosion tendency of the alloy.

[0062] Example 2 uses AA5024 (a 5xxx aluminum alloy containing Sc element) aluminum alloy ingot, which is heated to 460°C in a muffle furnace, homogenized annealed for 6 hours, and then air-cooled. After milling into an ingot with a thickness of 30 mm, it is heated to 450°C in a muffle furnace and kept warm for 2 hours, and then hot rolled. The relative reduction in each pass is less than 20%. As the number of rolling passes increases, the reduction continues to decrease, and the intermediate furnace is annealed once, and finally rolled to 6 mm. Before the first rolling pass, the AA5083 aluminum alloy plate is fully cooled in liquid nitrogen for 30 minutes, and the rollers are cooled using a pressure liquid nitrogen spray nozzle connected to a liquid nitrogen tank. Before each subsequent rolling pass, the plate is cooled in liquid nitrogen for 5 minutes, and the time for transferring the plate from the liquid nitrogen insulation box to the roller bite does not exceed 5 seconds. The relative reduction in each pass is 10%, and the 5083 aluminum alloy plate is finally rolled to 1.5 mm.

[0063] The corrosion resistance of the alloy was evaluated using an immersion corrosion test. The test was conducted according to the method for determining intergranular corrosion of 5xxx aluminum alloys specified in GB / T 7998-2005, "Determination of Intergranular Corrosion of Aluminum Alloys." Figure 5 (a) and (b) are the intergranular corrosion morphologies of AA5024 aluminum alloy after deep cold rolling and room temperature rolling, respectively. The maximum corrosion depths are 9.8 μm and 21.2 μm, respectively. Figure 5 In the samples sensitized at 160°C for 100 hours shown in (c) and (d), the maximum corrosion depth of the deep cold rolled sample (35.2 μm) is also smaller than that of the room temperature rolled sample (53.9 μm).

Claims

1. A method for improving the corrosion resistance of 5xxx aluminum alloy sheet and strip, characterized in that: The steps include: Step 1, homogenizing annealing the smelted and cast 5xxx aluminum alloy ingot; Step 2: milling the annealed 5xxx aluminum alloy ingot, and then hot rolling it at 450° C. to 470° C. to obtain a pre-rolled plate; Step 3, cooling the pre-rolled plate to liquid nitrogen temperature and performing deep cold rolling, with a reduction rate of less than 20% per pass; Step 4, repeating step 3 until the total deformation of the pre-rolled plate obtained in step 2 reaches 80% to 98% and the final thickness of the plate reaches 0.8 mm to 4 mm; Step 5, stretching and straightening the plate obtained in step 4; Step 6: Soak the plate obtained in step 5 in a low-temperature liquid nitrogen box for 2 to 4 hours.

2. The method for improving the corrosion resistance of 5xxx aluminum alloy sheet and strip according to claim 1, characterized in that: The composition of the 5xxx aluminum alloy ingot is by weight: Mg: 3.5%~5.0%, Mn: 0.25%~0.50%, Ti<0.03%, Fe<0.50%, Si<0.50%, Cr: 0.05%~0.25%, and the balance is Al.

3. The method for improving the corrosion resistance of 5xxx aluminum alloy sheet and strip according to claim 1, characterized in that: During the smelting and casting process, online slag removal, degassing and filtering processes are performed, and the thickness of the obtained 5xxx aluminum alloy ingot is greater than 30 mm and less than 50 mm.

4. The method for improving the corrosion resistance of 5xxx aluminum alloy sheet and strip according to claim 1, characterized in that: In the step 1, the smelted and cast 5xxx aluminum alloy ingot is placed in a homogenization heat treatment furnace, heated to 460-475° C. and kept warm for 8-12 hours. After the insulation is completed, the ingot is taken out and air-cooled.

5. The method for improving the corrosion resistance of 5xxx aluminum alloy sheet and strip according to claim 1, characterized in that: In step 2, a multi-pass rolling process with a small reduction is adopted, wherein the reduction is reduced as the number of rolling passes increases, and the thickness of the plate after hot rolling is 70% to 80% of the ingot thickness.

6. The method for improving the corrosion resistance of 5xxx aluminum alloy sheet and strip according to claim 5, characterized in that: The reduction amount as the rolling passes increase means that the reduction amount in each pass is less than 20% of the current plate thickness, and the minimum reduction amount is 0.2 mm.

7. The method for improving the corrosion resistance of 5xxx aluminum alloy sheet and strip according to claim 1, characterized in that: In step 3, the plate is cooled in liquid nitrogen, and the upper and lower rollers of the rolling mill are cooled by liquid nitrogen spray. The plate cooled to the liquid nitrogen temperature is transferred to the rolling mill for deep cold rolling within 5 seconds. During the rolling process, the liquid nitrogen nozzle continuously sprays liquid nitrogen to cool the plate, ensuring that the plate is in a deep cold environment.

8. The method for improving the corrosion resistance of 5xxx aluminum alloy sheet and strip according to claim 1, characterized in that: In step 6, when cooling the plate in liquid nitrogen, sufficient liquid nitrogen is required, and the plate is completely immersed in the liquid nitrogen to ensure that the plate is in a deep cold environment.

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