A method for preparing a high-strength magnesium alloy plate

Abstract

The application discloses a high-strength magnesium alloy plate preparation method, and chemical components of the magnesium alloy plate are as follows in percentage by mass: 5.5-6.0% of zinc, 2.0-2.5% of ytterbium, 0.6-1.0% of zirconium, and the rest of magnesium. The preparation method involves four steps of alloy smelting, solid solution, inclined rolling and finishing. When the total reduction reaches 80-85% of the initial thickness, the magnesium alloy plate obtained has a room-temperature tensile yield strength of 350-390 MPa and a tensile strength of 370-430 MPa. The magnesium alloy plate prepared by the technology has the characteristics of high strength and less edge cracking, and has the technical advantages of low cost and high efficiency.

Description

Technical Field

[0001] This invention relates to the field of metal material processing, and in particular to a method for preparing high-strength magnesium alloy plates. Background Technology

[0002] Magnesium alloy sheets have broad application prospects in aerospace and automotive manufacturing due to their lightweight characteristics. However, the large-scale application of ordinary magnesium alloy sheets produced by conventional hot rolling processes is severely limited by their insufficient absolute strength and edge cracking problems, and related technical bottlenecks urgently need to be overcome.

[0003] To address the insufficient strength of magnesium alloy rolled sheets, researchers primarily aim to improve yield strength by adding alloying elements and lowering the rolling temperature. This strategy seeks to create a high-density dislocation structure while simultaneously increasing grain boundary density and promoting the precipitation of precipitates to suppress dislocation migration. While this approach is beneficial for strength improvement, it's difficult to achieve the desired strengthening effect with typical alloying elements. More importantly, lower rolling temperatures hinder the activation of non-basal slip and sufficient recrystallization in magnesium alloys. Incomplete recrystallization results in an uneven microstructure with coexisting large and small grains. Combined with the stress concentration effect of coarse second-phase particles, this exacerbates the unevenness of local deformation in the sheet, significantly increasing the tendency for fracture in load-bearing critical areas such as the edges.

[0004] In conventional magnesium alloy flat-roll rolling processes, when the tensile stress on the edge of the sheet reaches its fracture strength, localized uneven deformation will initiate edge cracks. Subsequently, under the interaction of uneven temperature and uneven deformation, the cracks will extend to a certain depth laterally. Therefore, edge crack defects are directly related to the stress distribution in the deformation zone of magnesium alloy rolling. Changing the stress state at the edge, reducing tensile stress, and improving the coordinated deformation capacity of the sheet edge are important ways to suppress the initiation and propagation of edge cracks during the rolling of wide magnesium alloy sheets. Domestic and international research on changing the local load state of magnesium alloy rolled sheets and improving deformation uniformity mainly focuses on modifying rolling tooling and optimizing rolling processes. However, high design and modification costs or lengthy pretreatment procedures are not conducive to large-scale application. Furthermore, improvements in deformation uniformity (machinability) are often achieved at the expense of strength.

[0005] Therefore, this invention aims to explore a method for preparing high-strength magnesium alloy sheets. By optimizing the alloy composition and content and combining it with high-temperature skew roll rolling, high-strength deformed magnesium alloy sheets with minimal edge cracking are obtained. This method has advantages such as a short preparation process, high efficiency, and good performance, and shows promising application prospects in the automotive manufacturing and aerospace fields. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing a high-strength magnesium alloy rolled plate with fewer edge cracks.

[0007] The technical problem this invention aims to solve is to overcome the technical disadvantage of magnesium alloy rolled plates in the prior art, where high strength and edge crack suppression are mutually exclusive. By optimizing the alloying elements and their addition amounts, coupled with a unique high-temperature inclined roll turning process design, a high-strength, high-performance magnesium alloy rolled plate with fewer edge cracks is obtained. The rolled magnesium alloy plate prepared using this method has a room temperature tensile yield strength of 350-390 MPa, a tensile strength of 370-430 MPa, and an elongation of 4-7%. The plate surface is smooth and crack-free, with side crack depth ≤ 1 cm. Subsequently, depending on further application requirements, the alloy can be heat-treated at 150-300℃ for 0.5-50 hours. High-temperature annealing reduces dislocations and homogenizes the microstructure, improving the plate's plasticity, or low-temperature aging precipitates nanoscale phases, further enhancing the plate's strength. After heat treatment, the plate has a tensile yield strength of 200-400 MPa, a tensile strength of 250-450 MPa, and an elongation of 3-15%. Magnesium alloy sheets prepared by this technology are characterized by high strength and few edge cracks, and have the technical advantages of low cost and high efficiency, showing good application prospects in the fields of automobile manufacturing and aerospace.

[0008] The objective of this invention is achieved through the following technical solution:

[0009] First, heavy rare earth element Yb, as well as Zn and Zr, were selected as the main alloying elements. Further optimization of the alloy content was performed: a relatively high addition of Yb was chosen at 2.0~2.5% to fully utilize its excellent strengthening effect on the magnesium matrix (grain refinement, high-density nanophase precipitation, dislocation strengthening, dispersion strengthening, and solid solution strengthening); the addition of Zn was chosen at 5.5~6.0%, a content close to its solid solubility in magnesium, which can fully utilize its age-hardening effect; finally, the addition of Zr was chosen at 0.6~1.0%, a content slightly exceeding its solid solubility in magnesium, which can fully utilize its strong component supercooling and purification effect on the magnesium melt, resulting in sufficiently equiaxed refinement of the solidification structure and improving the quality of the initial ingot from the source.

[0010] After the slab is fully dissolved, it is introduced into a specially designed adjustable tilting roll mill. The high rolling temperature ensures the alloy's forming ability and effectively improves deformation uniformity. At the same time, the temperature of the rolls is stabilized by the heating element embedded in the roll core and thermocouples combined with the electrical control system. This ensures that the slab will not weaken its strength due to the deformation heat effect or cause local cracking due to excessively low temperature caused by severe heat conduction throughout the rolling process. The tilted rolls form a roll gap with a gradient, which can introduce shear stress during the rolling process and change the load state of the strong tensile stress at the edge of the rolled plate. Under the action of the side baffles, the material enters the small gap region of the roll gap in a manner similar to "extrusion", relaxing the strong tensile strain at the edge and suppressing the formation of strong basal texture. The special crystallographic orientation of the grains with the c-axis "tilted" along the rolling direction is conducive to the large-scale activation of basal slip (conventionally the c-axis is perpendicular to the rolling direction), which is beneficial to improving deformation uniformity and suppressing edge cracks.

[0011] After the current rolling pass is completed, the slab is horizontally flipped and cyclically loaded to change the direction of shear stress. This balances the local single load state, reduces opposite-sign dislocations, and partially restores the work hardening capacity of the slab. The increased lateral deformation due to cyclic loading also helps to break down the original coarse deformed grains and residual second phases, inducing the dynamic dispersion of nanoscale precipitates, further improving the strength of the plate. Most importantly, the entire rolling process is continuous, without additional intermediate annealing, effectively improving processing efficiency. The final finishing process ensures the thickness tolerance of the slab.

[0012] The specific technical solution is as follows:

[0013] A method for preparing high-strength magnesium alloy sheet, characterized by the following steps: alloy melting → solution treatment → inclined roll rolling → finishing; the alloy composition by mass percentage is: Zn content 5.5~6.0%, Yb content 2.0~2.5%, Zr content 0.6~1.0%, with the balance being Mg and other unavoidable impurities; the Mg–Zn–Yb–Zr alloy is uniformly preheated to 350~400 ℃ and then continuously reciprocated rolled at a speed of 8~10 m / min in a trapezoidal roll gap with the upper and lower roll axes at 2~5° to 80~85% of the initial thickness; heating elements and thermocouples are pre-embedded in the cores of the upper and lower rolls, and the surface temperature of the rolls is stabilized at 150~200 ℃ during the rolling process, and the hardness is ≥HRC. 50; The rolling process ensures that the reduction of the slab in each pass is 5~8% of the total reduction. After each pass, the slab is horizontally flipped 180° and the rolling is continuous. The slab temperature in the final pass is ≥200 ℃. The finishing step is carried out immediately on the same equipment after the inclined roll rolling is completed by leveling the inclined upper roll. The finishing reduction is ≤3%. The final deformed structure is fully broken and refined, with an average grain size ≤6 μm and high-density dislocations distributed in the grains. The final rolled plate obtained by the above rolling process has a room temperature tensile yield strength of 350~390 MPa, a tensile strength of 370~430 MPa, an elongation of 4~7%, a smooth surface without cracks, and a side crack depth ≤1 cm.

[0014] Furthermore, a method for preparing high-strength magnesium alloy plates is characterized by the following: the as-cast alloy structure after melting is a uniform equiaxed fine grain with an average grain size of 30-40 μm, and irregularly sized Yb and Zr-rich second-phase particles with a length of 0.5-1 μm are dispersed within the grains and at the grain boundaries; the solid solution structure is free from overheating and burning, the equiaxed grains have not grown significantly, and there are still Yb and Zr-rich second-phase particles remaining within the grains and at the grain boundaries, with their size reduced by 50-80% compared to before heat treatment; the rolled structure is fully broken and refined, with a large number of submicron-sized spherical and short rod-shaped Mg-Zn-Yb and Mg-Zn phases dispersed within the grains and at the grain boundaries, as well as micron-sized irregularly shaped Yb and Zr-rich particles, and the typical (0001) pole figure of the alloy exhibits a textured feature of splitting along the rolling direction.

[0015] Furthermore, a method for preparing high-strength magnesium alloy sheets is characterized by the following preparation steps:

[0016] 1) Alloy smelting: Under the protection of SF6 + CO2 gas, pure magnesium ingots, Mg-Zr master alloy, Mg-Yb master alloy and pure zinc ingots are added in sequence at 700~760 ℃. The melting process is stirred thoroughly to assist melting. After adding a covering agent at 710 ℃ and holding at static temperature, the slag is removed and the ingot is cast into a plate shape. Then, it is immediately water quenched. The content of the as-cast alloy is Zn: 5.5~6.0%, Yb: 2.0~2.5%, Zr: 0.6~1.0%, and the balance is Mg and other unavoidable impurities.

[0017] 2) Solution treatment: Place the above ingots in a box-type resistance furnace and keep them at 400~420 ℃ for 36~48 h, then immediately water cool them to room temperature;

[0018] 3) Inclined roll rolling: The solution-treated sheet is held at the rolling temperature for 20-30 minutes to ensure uniform heating. Simultaneously, the upper roll is adjusted so that its axis forms a 2-5° angle with the horizontal lower roll axis. The rolls are heated by heating elements embedded in the rolls to achieve a surface temperature of 150-200°C. The mill is started and the roll speed is stabilized at 8-10 m / min. The billet is then introduced into the relatively rotating roll gap for rolling. The billet enters the roll gap at 1 / 5 of the length of the lower roll closest to the smaller gap side. A baffle is installed between the lower and upper rolls to prevent the billet from shifting horizontally. During the rolling process, the roll surface temperature is adjusted in real time using thermocouples and an electronic control system to maintain a stable temperature of 150-200°C. Inclined roll rolling is performed continuously, and the final rolling amount is 80-85% of the initial thickness.

[0019] 4) Finishing: After the inclined roll is rolled, the upper roll is adjusted to be horizontal to complete the finishing of the rolled plate, ensuring uniform thickness. After completion, it is immediately water cooled to room temperature.

[0020] This technology discloses a method for preparing high-strength magnesium alloy plates. After finishing, the plates can be subjected to targeted heat treatment at 150~300 ℃ for 0.5~50 hours according to the strength and plasticity requirements. The plasticity of the plates is improved by reducing dislocations and homogenizing the structure at high temperature, or by aging at low temperature to precipitate nano-scale precipitates to further improve the strength of the plates. After heat treatment, the tensile yield strength of the plates is 200~400 MPa, the tensile strength is 250~450 MPa, and the elongation is 3~15%.

[0021] Compared with existing magnesium alloy sheets and their preparation methods, the advantages of this technology are as follows:

[0022] 1. The design selects Yb, Zn, and Zr as the main alloying elements, ensuring readily available raw materials, controllable costs, and significant strengthening effects. Unlike other magnesium alloy processing technologies, this preparation process and tooling are simple and universal, offering advantages such as energy saving, short process flow, readily available conventional equipment, and low cost. It possesses advantages not found in complex deformation methods and has good potential for widespread application.

[0023] 2. By using high-temperature rolling to "strike while the iron is hot," the slab is guaranteed to have excellent formability in the initial deformation stage of introducing dislocations, suppressing damage accumulation and uneven strain. Subsequently, the local loading state and uneven deformation are controlled in real time during the rolling strain accumulation stage by using constant-temperature hot rolling rolls and introducing cyclic loading shear stress, effectively suppressing stress concentration and overcoming edge cracks. 3. This technical solution synergistically leverages the beneficial effects and unique advantages of rare earth Yb alloying and the unique high-temperature tilting roll turning rolling process in terms of matrix strengthening and deformation homogenization. With the help of conventional processing technology combinations and rolling tooling design, an unexpected combination of high strength and low edge cracks in magnesium alloy performance is obtained, overcoming the problem of the inversion of high strength and edge crack tendency in existing magnesium alloy plates. Its comprehensive performance is significantly higher than other magnesium alloy plates with similar rare earth additions and preparation costs. Attached Figure Description

[0024] Figure 1 The diagram shows the preparation process mechanism of this process and the traditional flat roll rolling process (a), the skew roll structure diagram of the technology of this application (b), and the crack state of the Mg–6.0 Zn–2.5 Yb–1.0 Zr plate after rolling at 400 ℃ and 85% reduction using the traditional process and this process (c) and (d).

[0025] Figure 2 The microstructure of the Mg–6.0 Zn–2.5 Yb–1.0 Zr alloy in its modified state (a) and orientation distribution diagram and its corresponding (0001) pole figure (b) prepared using the technology of this application, as well as the SEM image of the rolled state and its corresponding elemental surface scan (c).

[0026] Figure 3 The microstructure of the Mg–5.5 Zn–2.0 Yb–0.6 Zr alloy in the modified state prepared using the technology of this application is shown in (a) and orientation distribution diagram and its corresponding (0001) pole figure (b), as well as the SEM image of the rolled state and its corresponding elemental surface scan (c). Figure 4 The EBSD orientation distributions of alloys in Examples 1 and 2 after holding at 250 °C for 1.5 hours are shown in (a) and (b), and the corresponding (0001) pole figures. Detailed Implementation

[0027] The present invention will be specifically described below through embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above description.

[0028] Example 1

[0029] A method for preparing high-strength magnesium alloy sheet, characterized by the following steps: alloy smelting → solution treatment → inclined roll rolling → finishing (see appendix). Figure 1 (As shown in a); the alloy composition by mass percentage is: Zn content 6.0%, Yb content 2.5%, Zr content 1.0%, with the balance being Mg and other unavoidable impurities; the Mg–Zn–Yb–Zr alloy is uniformly preheated to 400 ℃ and then rolled at a speed of 10 m / min in a trapezoidal roll gap with the upper and lower roll axes at 2° (as shown in the attached figure). Figure 1 (As shown in angle θ) The slab is continuously rolled back and forth to 85% of its initial thickness. Heating elements and thermocouples are pre-embedded in the cores of the upper and lower rolls. During the rolling process, the surface temperature of the rolls is stabilized at 200 ℃, and the hardness is HRC 50. The rolling process ensures that the reduction in each pass of the slab is 8% of the total reduction. After each pass, the slab is horizontally flipped 180°, and the rolling is continuous, with the final pass slab temperature reaching 250 ℃. The finishing step is performed immediately after the inclined roll rolling on the same equipment using a leveled inclined upper roll. The finishing reduction is 2%, resulting in a fully broken and refined final structure with an average grain size of 5.15 μm and high-density dislocations distributed within the grains (as shown in the attached figure). Figure 2 (As shown in the b-orientation distribution diagram, the short white lines represent low-angle grain boundaries). The room-temperature tensile yield strength, tensile strength, and elongation of the final rolled plate obtained through the above rolling process along the rolling direction (RD) and rolling transverse direction (TD) are 373 MPa, 415 MPa, and 6.5%, and 352 MPa, 371 MPa, and 5.2%, respectively. The plate surface is smooth and free of cracks, and the side crack depth is <1 cm (see attached diagram). Figure 1 (as shown in d).

[0030] Furthermore, a method for preparing high-strength magnesium alloy plates is characterized by: the as-cast alloy microstructure after melting is a uniform equiaxed fine grain with an average grain size of 30 μm, and irregularly sized Yb- and Zr-rich second-phase particles with a length of 0.8 μm are dispersed within the grains and at the grain boundaries; the solid solution microstructure shows no overheating or burning, the equiaxed grains have not grown significantly, and Yb- and Zr-rich second-phase particles remain within the grains and at the grain boundaries, their size being reduced by 60% compared to before heat treatment; the rolled microstructure is fully broken and refined (as shown in the attached diagram). Figure 2 As shown in Figure a), a large number of submicron-sized spherical and short rod-shaped Mg-Zn-Yb and Mg-Zn phases are dispersed within the crystal and at the grain boundaries, as well as micron-sized irregularly shaped Yb-rich and Zr-rich particles (as shown in the attached figure). Figure 2 As shown in c), the typical (0001) pole figure of the alloy exhibits a textured feature that splits along the rolling direction (as shown in the attached figure). Figure 2 (as shown in b).

[0031] Furthermore, a method for preparing high-strength magnesium alloy sheets is characterized by the following preparation steps:

[0032] 1) Alloy smelting: Under the protection of SF6 + CO2 gas, pure magnesium ingots, Mg-Zr master alloy, Mg-Yb master alloy and pure zinc ingots are added in sequence at 750 ℃. The melting process is stirred thoroughly to assist melting. After adding a covering agent at 710 ℃ and holding at static temperature, the slag is removed and the ingot is cast into a plate shape. Then, it is immediately water quenched. The content of the as-cast alloy is Zn: 6.0%, Yb: 2.5%, Zr: 1.0%, with the balance being Mg and other unavoidable impurities.

[0033] 2) Solution treatment: Place the above ingots in a box-type resistance furnace, keep them at 420 ℃ for 48 h, and then immediately water cool them to room temperature;

[0034] 3) Inclined roll rolling: The solution-treated sheet is held at the rolling temperature for 30 minutes to ensure uniform heating. Simultaneously, the upper roll is adjusted so that its axis forms a 2° angle with the horizontal lower roll axis. The roll is heated to a surface temperature of 200°C by a pre-embedded heating element. The mill is started and the roll speed is stabilized at 10 m / min. The billet is then introduced into the relatively rotating roll gap for rolling. The billet enters the roll gap at 1 / 5 of the length of the lower roll near the smaller gap side. A baffle is installed between the lower and upper rolls to prevent the billet from shifting horizontally. During the rolling process, the roll surface temperature is adjusted in real time and stabilized at 150°C using thermocouples and an electronic control system. Inclined roll rolling is performed continuously, and the final rolling amount is 85% of the initial thickness.

[0035] 4) Finishing: After the inclined roll is rolled, the upper roll is adjusted to be horizontal to complete the finishing of the rolled plate, ensuring uniform thickness. After completion, it is immediately water cooled to room temperature.

[0036] To improve plasticity after finishing, the slab was heat-treated at 250 °C for 1.5 hours (grain orientation and (0001) pole figure as shown). Figure 4 As shown in a), the room temperature tensile yield strength, tensile strength and elongation of the rolled plate along the rolling transverse (TD) are 263 MPa, 311 MPa and 16.3%, respectively.

[0037] Example 2

[0038] A method for preparing high-strength magnesium alloy sheet, characterized by the following steps: alloy smelting → solution treatment → inclined roll rolling → finishing (see appendix). Figure 1 (As shown in a); the alloy composition by mass percentage is: Zn content 5.5%, Yb content 2.0%, Zr content 0.6%, with the balance being Mg and other unavoidable impurities; the Mg–Zn–Yb–Zr alloy is uniformly preheated to 350 ℃ and then rolled at a speed of 8 m / min in a trapezoidal roll gap with the upper and lower roll axes at 5° (as shown in the attached figure). Figure 1(As shown by angle θ in b) The slab is continuously rolled back and forth to 80% of its initial thickness; heating elements and thermocouples are pre-embedded in the cores of the upper and lower rolls, and the surface temperature of the rolls is stabilized at 150 ℃ during the rolling process, with a hardness of HRC 52; the rolling process ensures that the reduction of the slab in each pass is 5% of the total reduction, and the slab is horizontally flipped 180° after each pass, and the rolling is continuous, with the final pass slab temperature >200 ℃; the finishing step is performed immediately after the inclined roll rolling is completed on the same equipment by leveling the inclined upper roll, with a finishing reduction of 3%, resulting in a fully broken and refined final deformed structure with an average grain size of 4.81 μm and high-density dislocations distributed within the grains (as shown in the attached figure). Figure 3 (The short white lines in the b-orientation distribution diagram represent low-angle grain boundaries). The room temperature tensile yield strength, tensile strength, and elongation of the final rolled plate obtained through the above rolling process along the rolling direction (RD) and rolling transverse direction (TD) are 387 MPa, 423 MPa, and 5.6%, and 371 MPa, 389 MPa, and 4.3%, respectively. The plate surface is smooth and free of cracks, and the side crack depth is <1 cm.

[0039] Furthermore, a method for preparing high-strength magnesium alloy plates is characterized by: the as-cast alloy microstructure after melting is a uniform equiaxed fine grain with an average grain size of 40 μm, and irregularly sized Yb- and Zr-rich second-phase particles with a length of 0.8 μm are dispersed within the grains and at the grain boundaries; the solid solution microstructure shows no overheating or burning, the equiaxed grains have not grown significantly, and Yb- and Zr-rich second-phase particles remain within the grains and at the grain boundaries, their size being reduced by 80% compared to before heat treatment; the rolled microstructure is fully broken and refined (as shown in the attached diagram). Figure 3 As shown in Figure a), a large number of submicron-sized spherical and short rod-shaped Mg-Zn-Yb and Mg-Zn phases are dispersed within the crystal and at the grain boundaries, as well as micron-sized irregularly shaped Yb-rich and Zr-rich particles (as shown in the attached figure). Figure 3 As shown in c), the typical (0001) pole figure of the alloy exhibits a textured feature that splits along the rolling direction (as shown in the attached figure). Figure 3 (as shown in b).

[0040] Furthermore, a method for preparing high-strength magnesium alloy sheets is characterized by the following preparation steps:

[0041] 1) Alloy smelting: Under the protection of SF6 + CO2 gas, pure magnesium ingots, Mg-Zr master alloy, Mg-Yb master alloy and pure zinc ingots are added in sequence at 760 ℃. The melting process is stirred thoroughly to assist melting. After adding a covering agent at 710 ℃ and holding at static temperature, the slag is removed and the ingot is cast into a plate shape. Then, it is immediately water quenched. The content of the as-cast alloy is Zn: 5.5%, Yb: 2.0%, Zr: 0.6%, and the balance is Mg and other unavoidable impurities.

[0042] 2) Solution treatment: Place the above ingots in a box-type resistance furnace, keep them at 400 ℃ for 36 h, and then immediately water cool them to room temperature;

[0043] 3) Inclined roll rolling: The solution-treated sheet is held at the rolling temperature for 20 minutes to ensure uniform heating. Simultaneously, the upper roll is adjusted so that its axis forms a 5° angle with the horizontal lower roll axis. The roll is heated to a surface temperature of 150°C by a heating element embedded in the roll. The mill is started and the roll speed is stabilized at 8 m / min. The billet is then introduced into the relatively rotating roll gap for rolling. The billet enters the roll gap at 1 / 5 of the length of the lower roll near the smaller gap side. A baffle is installed between the lower and upper rolls to prevent the billet from shifting horizontally. During the rolling process, the roll surface temperature is adjusted in real time and stabilized at 200°C using thermocouples and an electronic control system. Inclined roll rolling is performed continuously, and the final rolling amount is 80% of the initial thickness.

[0044] 4) Finishing: After the inclined roll is rolled, the upper roll is adjusted to be horizontal to complete the finishing of the rolled plate, ensuring uniform thickness. After completion, it is immediately water cooled to room temperature.

[0045] To improve plasticity after finishing, the slab was heat-treated at 250 °C for 1.5 hours (grain orientation and (0001) pole figure as shown). Figure 4 As shown in b), the room temperature tensile yield strength, tensile strength and elongation of the rolled plate along the rolling transverse (TD) are 289 MPa, 322 MPa and 14.5%, respectively.

[0046] Finally, it should be noted that the above embodiments are only used to more clearly illustrate the working principle and process of the present invention and do not limit the present invention. The present invention can also be applied to hot-rolled magnesium alloys with other contents of Mg–Zn–Yb–Zr as defined in this application; the processing principle and steps are no different from the above examples, so there is no need to repeat the examples. The inventive contribution of the present invention to the prior art lies in obtaining high-strength, high-performance magnesium alloy plates with fewer edge cracks by optimizing the alloying elements and their addition amounts, supplemented by a unique high-temperature inclined roll turning rolling process. This effectively expands the application fields of magnesium alloys, develops the potential of alloy performance, and has the advantages of excellent comprehensive performance, short process, and high efficiency, with very significant beneficial effects.

Claims

1. A method for preparing high-strength magnesium alloy sheet, characterized in that: The preparation process includes four steps: alloy melting → solution treatment → inclined roll rolling → finishing. The alloy composition by mass percentage is: Zn content 5.5~6.0%, Yb content 2.0~2.5%, Zr content 0.6~1.0%, with the balance being Mg and other unavoidable impurities. The as-cast alloy microstructure after melting is uniform equiaxed fine grains with an average grain size of 30~40 μm. Irregularly sized Yb and Zr-rich second-phase particles with a length of 0.5~1 μm are dispersed within the grains and at the grain boundaries. The solution-treated microstructure shows no overheating or burning, and the equiaxed grains have not grown significantly. There are still Yb and Zr-rich second-phase particles remaining within the grains and at the grain boundaries, with their size reduced by 50~80% compared to before heat treatment. The Mg-Zn-Yb-Zr alloy is uniformly preheated to 350~400 °C and then rolled at 8~10 °C. The slab is continuously rolled at a rotation speed of m / min in a trapezoidal roll gap of 2-5° between the upper and lower roll axes to 80-85% of the initial thickness. Heating elements and thermocouples are pre-embedded in the cores of the upper and lower rolls. During the rolling process, the surface temperature of the rolls is stabilized at 150-200°C, and the hardness is ≥HRC 50. The rolling process ensures that the reduction of the slab in each pass is 5-8% of the total reduction. After each pass, the slab is horizontally flipped 180°, and the rolling is continuous. The final pass slab temperature is ≥200°C. The temperature is ℃; the rolled microstructure is fully broken and refined, with a large number of submicron-sized spherical and short rod-shaped Mg-Zn-Yb and Mg-Zn phases dispersed in the grains and grain boundaries, as well as micron-sized irregularly shaped Yb-rich and Zr-rich particles. The typical (0001) pole figure of the alloy shows a textured feature of splitting along the rolling direction; the finishing step is carried out immediately on the same equipment after the inclined roll rolling is completed by leveling the inclined upper roll, the finishing reduction is ≤3%, the average grain size is ≤6 μm, and high-density dislocations are distributed in the grains; the final rolled plate obtained through the rolling process has a room temperature tensile yield strength of 350~390 MPa, a tensile strength of 370~430 MPa, an elongation of 4~7%, a smooth plate surface without cracks, and a side crack depth of ≤1cm.

2. The method for preparing a high-strength magnesium alloy sheet according to claim 1, characterized in that: The preparation steps include the following: 1) Alloy smelting: Under the protection of SF6 + CO2 gas, pure magnesium ingots, Mg-Zr master alloy, Mg-Yb master alloy and pure zinc ingots are added in sequence at 700~760 ℃. The melting process is stirred thoroughly to assist melting. After adding a covering agent at 710 ℃ and holding at static temperature, the slag is removed and the ingot is cast into a plate shape. Then, it is immediately water quenched. The content of the as-cast alloy is Zn: 5.5~6.0%, Yb: 2.0~2.5%, Zr: 0.6~1.0%, and the balance is Mg and other unavoidable impurities. 2) Solution treatment: Place the above-mentioned ingots in a box-type resistance furnace and keep them at 400~420 ℃ for 36~48 h, then immediately water cool them to room temperature; 3) Inclined roll rolling: The solution-treated sheet is held at the rolling temperature for 20-30 minutes to ensure uniform heating. Simultaneously, the upper roll is adjusted so that its axis forms a 2-5° angle with the horizontal lower roll axis. The rolls are heated by heating elements embedded in the rolls to achieve a surface temperature of 150-200°C. The mill is started and the roll speed is stabilized at 8-10 m / min. The billet is then introduced into the relatively rotating roll gap for rolling. The billet enters the roll gap at 1 / 5 of the length of the lower roll closest to the smaller gap side. A baffle is installed between the lower and upper rolls to prevent the billet from shifting horizontally. During the rolling process, the roll surface temperature is adjusted in real time using thermocouples and an electronic control system to maintain a stable temperature of 150-200°C. Inclined roll rolling is performed continuously, and the final rolling amount is 80-85% of the initial thickness. 4) Finishing: After the inclined roll is rolled, the upper roll is adjusted to be horizontal to complete the finishing of the rolled plate, ensuring uniform thickness. After completion, it is immediately water cooled to room temperature.

3. A method for preparing a high-strength magnesium alloy sheet according to any one of claims 1 to 2, characterized in that: After finishing, heat treatment can be carried out at 150~300℃ for 0.5~50 hours according to the strength and plasticity requirements of the sheet. The plasticity of the sheet is improved by reducing dislocations and homogenizing the structure through annealing at high temperature, or by aging at low temperature to precipitate nano-scale precipitates to further improve the strength of the sheet. After heat treatment, the tensile yield strength of the sheet is 200~400 MPa, the tensile strength is 250~450 MPa, and the elongation is 3~15%.

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