Method for improving corrosion resistance of 5xxx aluminum alloy plate strip

Abstract

The invention discloses a method for improving the corrosion resistance of a 5xxx aluminum alloy plate strip, which belongs to the field of metal forming and comprises the following steps: soaking 5xxx aluminum alloy in a heat preservation container filled with liquid nitrogen and fully cooling; and the fully cooled 5xxx aluminum alloy plate is fed into a rolling mill to be rolled, and the 5xxx aluminum alloy plate and a roller are fully cooled before each pass of rolling. The brittleness characteristic of the coarse second phase in the low-temperature environment is utilized, so that the coarse second phase in the 5xxx aluminum alloy plate is broken in the deep cooling rolling process. In the subzero treatment process, due to the difference of the volume shrinkage effect between the second phase and the base body, the interface bonding strength of the aluminum base body and the second phase is effectively improved, and therefore better pitting corrosion resistance is achieved. Compared with room-temperature rolling, deep cooling rolling improves the proportion of small-angle grain boundaries in the 5xxx aluminum alloy, so that separation of a beta phase is inhibited, and the intergranular corrosion performance is improved. The more corrosion-resistant 5xxx aluminum alloy has great development significance in industries such as ships, rail transit, spaceflight and military affairs.

Description

Technical Field

[0001] The 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, have medium strength after work hardening, and have higher corrosion resistance than other aluminum alloys. Therefore, they are 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 by work hardening, so the accumulation of higher density dislocations when the alloy is deformed is more conducive to improving strength. The deep cold environment helps to suppress the dynamic recovery of the alloy during deformation and achieve 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 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 large-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] The grain boundary angle of low-angle grain boundaries is generally less than 15°. This type of grain boundary is usually formed by dislocation aggregation, and the dislocation density is relatively high. Low-angle grain boundaries are usually not very sensitive to corrosion. Because the internal lattice distortion is small and the grain boundary energy is low, the electrochemical activity is relatively low, and it is easy to form a continuous passivation film. Therefore, it is not easy to form a coherent corrosion channel at the low-angle grain boundary, which has a certain inhibitory effect on pitting and intergranular corrosion. The grain boundary angle of high-angle grain boundaries is generally above 15°. This type of grain boundary has a large lattice distortion and high energy, and is easy to become a priority attack area for corrosion. In 5xxx aluminum alloys, magnesium atoms are easy to precipitate at high-angle grain boundaries to form a continuous β phase. This precipitated phase is easy to dissolve preferentially in a corrosive environment, making the high-angle grain boundary the main site of intergranular corrosion. 5xxx aluminum alloys are more prone to stress corrosion cracking at high-angle grain boundaries because the high-angle grain boundaries are more easily destroyed under the combined action of tensile stress and 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 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, soaking 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, so that the coarse second phase in the 5xxx aluminum alloy plate is broken 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 interface bonding strength between 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 proportion of small angle grain boundaries in the 5xxx aluminum alloy, thereby suppressing the precipitation of the 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 steps 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, the step 1 is to place the smelted and cast 5xxx aluminum alloy ingot in a homogenization heat treatment furnace, heat it to 460-475° C. and keep it warm for 8-12 hours. After the insulation is completed, take it out and air cool it.

[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 thickness of the ingot.

[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 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 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 the coarse second phase in the aluminum alloy plate, and effectively accumulates high-density dislocations. The ultimate tensile strength of the processed 5xxx aluminum alloy plate at room temperature is 400-450MPa, the yield strength is 350-380MPa, and the elongation is 8%-12%. In the corrosion experiment implemented according to GB / T7998-2005, the maximum corrosion depth of the 5xxx aluminum alloy plate (taking AA5083 as an example) obtained by the method of the present invention is less than 20μm, which is much smaller than the room temperature rolled sample of the control group. In order to accelerate the corrosion experiment, the alloy is sensitized. The maximum corrosion depth of the 5xxx aluminum alloy plate is 63.8μm, which is also smaller than the room temperature rolled sample (95.6μm). The maximum corrosion depth of the sample further cryogenically treated after deep cold rolling is only 52.1μm. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The deep cold rolling process flow chart of the 5xxx aluminum alloy (AA5083) plate of the present invention is shown in FIG. (a) is the deep cold treatment process of the plate; and (b) is the deep cold rolling process of the plate.

[0028] 1-liquid nitrogen insulation box; 2-liquid nitrogen; 3-5xxx aluminum alloy pre-rolled plate; 4-liquid nitrogen nozzle one; 5-liquid nitrogen nozzle two; 6-liquid nitrogen nozzle three; 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) is the broken fine second phase in the deep cold rolled 5xxx aluminum alloy, and (b) is the high density dislocation in the deep cold rolled 5xxx aluminum alloy.

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

[0031] Figure 4 The results of the immersion corrosion test of the 5xxx aluminum alloy (AA5083) plate of the present invention are shown in Figure 1. (a) is a plate rolled at room temperature, (b) is a plate rolled at low temperature, (c) is a plate rolled at low temperature + treated with low temperature, (d) is a plate rolled at room temperature + sensitized at 160℃ for 100h, (e) is a plate rolled at low temperature + sensitized at 160℃ for 100h, and (f) is a plate rolled at low temperature + treated with low temperature + sensitized at 160℃ for 100h.

[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 sensitized plate, and (d) is a room temperature rolled plate + 160℃ 100h sensitized 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 sheet and strip, which has the same or similar effect on alloys with Mg as the main additive element and a certain amount of Mn, Fe and other elements in the alloy. 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 produced 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 filtering 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 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 warm for 8-12 hours, and then taken out of the furnace and air-cooled after the insulation is completed.

[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 shape 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 adopts a small reduction multi-pass rolling process, and the reduction decreases as the number of rolling passes increases, and the thickness of the plate after hot rolling is 70% to 80% of the ingot thickness. The small reduction multi-pass rolling process can reduce deformation force, reduce pressure and loss on rolling equipment, avoid material hardening or cracking, and improve production yield.

[0044] Furthermore, the reduction in each pass is less than 20% of the current plate thickness, and the minimum reduction is 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, cooling the obtained pre-rolled plate to liquid nitrogen temperature, and performing deep cold rolling, with the relative reduction amount 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 will help 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 plate is kept warm 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 out spray to cool the 5xxx aluminum alloy pre-rolled plate 3, the liquid nitrogen nozzle 2 5 sprays out spray to cool the upper roller 7 of the rolling mill, and the liquid nitrogen nozzle 3 6 sprays out spray to cool the lower roller 8 of the rolling mill. The roller 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 5s. The roller 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 plate obtained in step 3 reaches 80% to 98%, and the final thickness of the plate reaches 0.8 mm to 4 mm, and more preferably the total deformation reaches 93% to 98%, and the thickness reaches 0.8 mm to 2 mm. Aluminum alloy plates with this range of thickness are widely used in rail transportation, new energy vehicles, shipbuilding and other fields. The purpose of this step is to enhance the effect of deep cold rolling.

[0049] Step 6, stretching and straightening 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, soaking the plate obtained in step 6 in a low-temperature liquid nitrogen box for 2 to 4 hours. Preferably, sufficient liquid nitrogen is required during cooling, and the plate is completely immersed in liquid nitrogen to ensure that the plate is in a low-temperature environment.

[0052] Cryogenic treatment changes the nucleation conditions of the precipitate phase by introducing high-density dislocations and subgrain boundary structures. The nucleation of the β phase usually requires a certain amount of thermal energy, but in a cryogenic environment, the thermal energy is not enough to drive the nucleation of the precipitate phase. Even in the subsequent temperature-raising annealing process, the small-angle grain boundaries inhibit the precipitation of the β phase and reduce the formation of the precipitation-free zone.

[0053] The 5xxx aluminum alloy sheet obtained by the method has high density dislocations and has a significant refinement effect on the coarse second phase, and has good performance in corrosion tests and room temperature tensile tests.

[0054] The main principle of the present invention is to prepare a more corrosion-resistant 5xxx aluminum alloy sheet by deep cold rolling and deep cold treatment. Specifically, the coarse second phase shows brittleness at low temperature, and has a worse deformation and sliding ability than the aluminum matrix, so it is easy to break when subjected to rolling shear strain. The size of the broken second phase is reduced and more dispersed. After being corroded in the environment, the depth and width of the corrosion pit left after the broken second phase is corroded or peeled off are smaller. In addition, deep cold rolling significantly reduces the proportion of high-angle grain boundaries in the alloy, hinders the expansion and precipitation of Mg elements along the high-angle grain boundaries, and the β phase is mainly precipitated on the high-angle grain boundaries during the sensitization process. Therefore, the deep cold rolling process inhibits the precipitation of the β phase on the grain boundaries, significantly reducing the intergranular corrosion sensitivity of the 5xxx aluminum alloy. Third, the macroscopic shear band in the aluminum alloy sheet is effectively reduced by deep cold rolling, so that the deformation uniformity of the 5xxx aluminum alloy is greatly improved, thereby avoiding the deterioration of corrosion performance in local areas. Combined with the above three points, deep cold rolling improves the pitting resistance and intergranular corrosion resistance of the 5xxx aluminum alloy. Due to the difference in the volume shrinkage effect of the aluminum alloy matrix and the second phase particles under the cryogenic environment, 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 weak. 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 of the grain boundary is also an important reason for the decrease in the corrosion resistance of the alloy. The cryogenic treatment reduces the atomic heat 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 electrical coupling effect. In summary, the present invention combines deep cold rolling with deep 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 used, heated to 460℃ in a muffle furnace, homogenized and annealed for 6 hours, and then air-cooled. After milling into an ingot with a thickness of 30mm, it is heated to 450℃ in a muffle furnace and kept warm for 2 hours, followed by hot rolling. The relative reduction of 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 6mm. The hot-rolled 5xxx aluminum alloy plate has no cracks and good surface quality. This is used as the raw material for the next step of deep cold rolling.

[0058] Before the first rolling, the AA5083 aluminum alloy sheet was fully cooled in liquid nitrogen for 30 minutes, and the rollers were cooled using a pressure liquid nitrogen spray nozzle connected to a liquid nitrogen tank. Before each subsequent rolling, the sheet was cooled in liquid nitrogen for 5 minutes, and the time for the sheet to be transferred from the liquid nitrogen insulation box to the roller bite did not exceed 5 seconds. The relative reduction for each pass was 10%, and the 5083 aluminum alloy sheet was finally rolled to 1.5 mm. The rolled sheet was soaked in an insulation box filled with liquid nitrogen 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 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 It can be seen in (b) that deep cold rolling produces high-density dislocations, which enables the alloy to achieve better dislocation strengthening.

[0060] The tensile test was used to study the mechanical properties of the alloy. 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 reached 450MPa, the yield strength reached 380MPa, 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 experiment was conducted in accordance with the method for determining intergranular corrosion of 5xxx aluminum alloys in GB / T 7998-2005 "Determination of intergranular corrosion of aluminum alloys". Figure 4 The cross-section of the cryogenically rolled and cryogenically treated AA5083 aluminum alloy sheet 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 It can be seen from (a) to (f) that the maximum corrosion depth of deep cold rolled AA5083 aluminum alloy is 16.5μm, and the maximum corrosion depth of deep cold rolled + deep cryogenic treatment samples is further reduced to 14.2μm. The maximum corrosion depth of intergranular corrosion of sensitized samples also shows that deep cold rolling and deep cryogenic treatment can significantly improve the corrosion resistance of AA5083 aluminum alloy plates 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 and annealed for 6 hours, and then air-cooled. After milling the ingot into a thickness of 30mm, it is heated to 450°C in a muffle furnace and kept warm for 2 hours, followed by hot rolling. The relative reduction of 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 6mm. Before the first rolling, the AA5083 aluminum alloy sheet is fully cooled in liquid nitrogen for 30 minutes, and the roller is cooled using a pressure liquid nitrogen spray nozzle connected to a liquid nitrogen tank. Before each subsequent rolling, the sheet is cooled in liquid nitrogen for 5 minutes, and the time for the sheet to be transferred from the liquid nitrogen insulation box to the roller bite does not exceed 5s. The relative reduction of each pass is 10%, and the 5083 aluminum alloy sheet is finally rolled to 1.5mm.

[0063] The corrosion resistance of the alloy was evaluated using an immersion corrosion test. The experiment was conducted in accordance with the method for determining intergranular corrosion of 5xxx aluminum alloys in GB / T 7998-2005 "Determination of intergranular corrosion of aluminum alloys". Figure 5 (a) and (b) show 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 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, soaking 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 steps 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, and after the insulation is completed, 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 the step 2, a multi-pass rolling process with a small reduction is adopted, 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 thickness of the ingot.

6. The method for improving the corrosion resistance of 5xxx aluminum alloy sheet and strip according to claim 5, characterized in that: 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.

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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