A prestressed grinding and cold rolling method for suppressing edge cracks in wide magnesium alloy sheets

Abstract

This invention discloses a prestressed grinding cold rolling method for suppressing edge cracks in wide magnesium alloy sheets, relating to the field of magnesium alloy processing and forming. Specifically, for magnesium alloy sheets with a thickness of 5-10 mm and a width of 500-1500 mm, before cold rolling, a certain tensile stress is uniformly applied as a prestress along the rolling direction at both ends of the sheet. While maintaining the prestress, the edges of the sheet are ground and rapidly cooled. After processing, the prestress is removed, resulting in a hardened layer with a certain thickness ratio on the surface of the sheet edge, and residual compressive stress is obtained at the edge. Multiple cold rolling processes are then completed. After prestressed grinding, a hardened layer with a certain thickness ratio is formed on the surface of the magnesium alloy sheet edge. This improves the metal's ability to deform in this area, and the residual compressive stress at the edge largely offsets the tensile stress experienced by the edge during subsequent cold rolling deformation, thereby effectively suppressing the initiation and propagation of edge cracks. This invention has the advantages of short process flow and low execution cost. It can suppress edge cracking defects in the rolling process of magnesium alloy plates, solve the production problems of limited specifications and difficulty in guaranteeing the width of wide magnesium alloy plates, and effectively guarantee the mechanical properties, surface quality and forming accuracy of magnesium alloy plates, and significantly improve the yield of plates.

Description

Technical Field

[0001] This invention belongs to the field of magnesium alloy processing and forming, and specifically relates to a prestressed grinding and cold rolling method for suppressing edge cracks in wide magnesium alloy sheets. Background Technology

[0002] The demand for lightweight manufacturing of equipment components in aerospace, rail transportation, and new energy vehicles is becoming increasingly urgent. Wide-width sheet metal is the most strategically significant application of magnesium alloys and a high-tech, high-value-added bulk product, with broad application prospects in weight reduction of body panels, coverings, and other wall panel components. Rolling can overcome width and length limitations, offering advantages such as high productivity, good economy, and a wide range of applications, making it highly promising for continuous and mass production of wide-width magnesium alloy sheets. Edge cracking is a significant defect in rolled magnesium alloy sheets; its width depth directly determines the amount of edge trimming in subsequent finishing processes, affecting not only the final sheet yield but also being a major reason for the limited width of the finished sheet. Analysis of the stress distribution in the rolling deformation zone reveals that the metal in the middle of the deformation zone experiences triaxial uneven compressive stress, while the metal at the edges experiences biaxial compressive stress and uniaxial tensile stress (along the rolling direction). When the tensile stress reaches the fracture strength of the magnesium alloy, the uneven deformation caused by the tensile stress will initiate edge cracks. Subsequently, under the interaction of uneven temperature and uneven deformation in the width direction, the cracks will extend to a certain depth along the width direction. Therefore, edge crack defects are directly related to the stress distribution in the rolling deformation zone of magnesium alloys. Changing the stress state at the edges, reducing the additional influence of tensile stress, and improving the coordinated deformation capacity of the edge metal are important ways to suppress the initiation and propagation of edge cracks during the rolling of wide magnesium alloy plates.

[0003] Currently, methods to suppress edge cracking during rolling by improving the stress state of magnesium alloy sheets mainly include edge pre-rolling with vertical rolls, edge pre-rolling with horizontal rolls, and edge machining to pre-form convexity. However, these methods or technical solutions are difficult to widely apply due to their complexity, lengthy technical features, the need for additional expensive equipment, or the generation of significant transverse structural inhomogeneity defects. This invention proposes a pre-stressed grinding cold rolling method that can effectively suppress edge cracking in wide magnesium alloy sheets. By pre-stressing the sheet edge, a hardened layer with a certain thickness ratio is formed on its surface, improving the coordinated deformation capacity of the metal in this area and obtaining a certain form of residual compressive stress at the edge. This largely offsets the tensile stress on the edge during rolling deformation, thereby effectively suppressing the initiation and propagation of edge cracks, increasing the width and yield of finished sheets. This method is of great significance for suppressing edge cracking defects during the rolling process of magnesium alloy sheets, solving the production problems of limited specifications and difficulty in guaranteeing the width of wide magnesium alloy sheets, and improving product quality. Summary of the Invention

[0004] To address the problem of severe edge cracking and inability to guarantee the width of wide magnesium alloy sheets with a thickness of 5-10mm and a width of 500-1500mm during rolling deformation due to tensile stress at the edges, this invention provides a prestressed grinding cold rolling method that can effectively suppress edge cracking in wide magnesium alloy sheets. Before cold rolling, a hardened layer with a certain thickness ratio can be formed on the surface of the sheet edge, improving the coordinated deformation ability of the edge metal and obtaining a certain form of residual compressive stress at the edge. During rolling deformation, this largely offsets the tensile stress on the edge, thereby effectively suppressing the initiation and propagation of edge cracks. This solves the production problem of limited specifications and difficulty in guaranteeing the width of wide magnesium alloy sheets, and improves product quality.

[0005] To achieve the above objectives, the thickness is first... 5~10mm, width Prestressed grinding of wide magnesium alloy sheets ranging from 500 to 1500 mm, such as... Figure 1 As shown, before cold rolling, a certain tensile stress is uniformly applied as prestress at both ends of the sheet along the rolling direction. While maintaining the prestress, the edges of the sheet metal are ground and rapidly cooled. After processing, the prestress is removed, resulting in a hardened layer of a certain thickness ratio on the surface of the sheet metal edges, and residual compressive stress is obtained at the edges. Then, multiple cold rolling processes are completed. The process route proposed in this invention is as follows: Figure 2 As shown, the ranges of parameters such as prestressing loading, grinding conditions, grinding parameters, and working roll temperature and reduction for multi-pass cold rolling are clearly given.

[0006] The specific method is as follows:

[0007] (1) Prestressing loading: A certain tensile stress is uniformly applied to the head and tail ends of the plate along the rolling direction using a plate stretching machine as prestress. Considering that excessive prestress can cause microcracks or even breakage on the plate surface, while insufficient prestress results in insignificant residual compressive stress at the edges after unloading, this study... To determine the magnitude of the prestress, in the formula, This represents the yield strength of the magnesium alloy.

[0008] (2) Grinding the edges of the sheet while maintaining prestress loading: Grinding the sheet generates a large amount of grinding heat instantaneously, causing the sheet deformation temperature to rise rapidly. This promotes dynamic recrystallization of the edges, especially the surface metal, before rolling deformation, refining the microstructure and improving the edge metal's ability to coordinate deformation. The edge grinding method uses single-stroke planar grinding, with the grinding direction parallel to the subsequent rolling direction. Due to the hardness of magnesium alloys... Lower, typically around 45HB Within the 75HB range, to ensure timely removal of magnesium alloy grinding chips from the abrasive grains during prestressed grinding and prevent clogging of the abrasive grain gaps that could affect the grinding process, brown corundum abrasive wheels with a hardness grade of J or K are used. Considering both grinding heat control and chip containment requirements, the grinding wheels are bonded with a ceramic binder, with a grit size of 46 and a microstructure of 5. Grinding speed directly affects grinding heat generation, thus determining the instantaneous temperature rise of the machined surface. Maintaining an instantaneous temperature rise of 480~600℃ is crucial for achieving good hardening results after rapid cooling. To stabilize the grinding process and ensure hardening effect and surface finish, the grinding wheel feed rate is... The grinding wheel linear velocity is 1.5~1.8 m / min. Based on the hardness of magnesium alloy To adjust, when 45HB At 60HB, =1180~1320m / min, when 60HB At 75HB, =1020~1180m / min. Considering that the width-direction depth of unilateral edge crack defects in 500~1500mm wide magnesium alloy plates is usually in the range of 30~85mm during actual rolling production, in order to cover the edge crack risk area and achieve a significant suppression effect, the unilateral grinding width is... The width is 38~100mm, which is the same as the initial plate width. Positive correlation, by Determined. The grinding depth directly determines the surface hardening thickness ratio after grinding and directly affects the surface finish. Furthermore, because edge grinding creates a thickness difference along the width of the sheet metal, it leads to uneven deformation distribution along the width during subsequent rolling, resulting in differences in microstructure and properties. Therefore, the grinding depth should not be too large. For this reason, for 5-10mm thick magnesium alloy sheets, the single-sided grinding depth... The thickness is 0.105~0.195mm, from Sure.

[0009] (3) Rapid cooling during prestressed grinding: High-speed grinding generates a large amount of grinding heat. After rapid cooling, a metal hardened layer with a certain thickness ratio will form on the surface. The hardened layer of magnesium alloy is characterized by refined structure and sufficient recrystallization, and has good coordinated deformation ability. In order to obtain the edge hardened layer and prevent the surface metal from oxidizing under continuous high temperature, a large amount of oil-based coolant is used during grinding, and the cooling rate can reach 60℃ / s. The obtained hardened layer thickness ratio is 11.6%~12.8%. Magnesium alloy material itself has good thermal conductivity. The grinding heat generated at the edge will be conducted to the metal from the width to the inside, causing the adjacent parts of the edge to heat up. Therefore, the wetting width of the coolant is... Need to cover the grinding width on one side , and by Confirmed, when When the diameter is 38~100mm, The value range is 48~125mm.

[0010] (4) Removal of prestress after prestress grinding: At the edge, due to the influence of the surface hardened layer, uneven springback occurs in the thickness direction after the prestress is removed. This is manifested in the fact that the springback of the central layer is greater than that of the surface layer, thus forming residual compressive stress. Its magnitude depends on the prestressing load. Considering that the tensile stress on the edge during rolling deformation is related to the reduction, in order to largely offset the tensile stress, the residual compressive stress on the edge... The pressure ranges from 23 to 52 MPa, adjusted based on the initial reduction in subsequent passes. A larger reduction requires more pressure. The larger the width, the more effectively it can suppress edge cracking during rolling and increase the width of the finished sheet.

[0011] (5) Multi-pass cold rolling: Magnesium alloys have a close-packed hexagonal crystal structure and poor room temperature deformation ability, which can be significantly improved under heating conditions. However, residual compressive stress is obtained at the edge of the plate after prestressed grinding. Heating the sheet metal eliminates this type of internal stress. Therefore, subsequent multi-pass rolling processes utilize cold-roll and hot-roll conditions, maintaining the work roll temperature at 250-300℃ to achieve temperature compensation and ensure the deformation performance of the magnesium alloy. Since the pre-stressed grinding of the sheet metal before rolling improves the coordinated deformation ability of the edge metal, a larger rolling reduction of 35%-45% can be applied in the first pass to fully refine the deformed microstructure and improve the uniformity of the thickness microstructure. To further ensure the accuracy of the finished sheet metal's width and thickness, subsequent passes use smaller reductions to achieve the target thickness, with reductions of 10%-15%.

[0012] Advantages and positive effects of the present invention:

[0013] The prestressed grinding cold rolling method for suppressing edge cracks in wide magnesium alloy sheets of the present invention involves prestressing the edges of the magnesium alloy sheet before rolling. On the one hand, a hardened layer with a certain thickness ratio is formed on the surface of the edge, which has a refined structure and sufficient recrystallization, thus improving the coordinated deformation ability of the edge metal during rolling deformation. On the other hand, a certain form of residual compressive stress can be obtained at the edge, which largely offsets the tensile stress on the edge during rolling deformation, changes the stress state of the edge, and reduces the additional influence of tensile stress, thereby effectively suppressing the initiation and propagation of edge cracks, and increasing the width of the finished sheet by 6%.

[0014] The prestressed grinding and cold rolling method for suppressing edge cracks in wide magnesium alloy sheets of the present invention not only eliminates the sheet preheating step, resulting in a shorter process flow and lower execution cost, but also enhances the metal coordination deformation capability of the sheet edge before rolling and largely eliminates the additional influence of edge tensile stress during deformation. Subsequent rolling processes can all adopt room temperature deformation, effectively ensuring the mechanical properties, surface quality, and forming accuracy of the magnesium alloy sheet, and significantly improving the sheet yield. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the prestressed grinding process, in which... This is the initial plate thickness. The initial plate width, For prestressing, The linear velocity of the grinding wheel. This refers to the grinding wheel feed rate. For single-sided grinding depth, For single-sided grinding width, RD represents the coolant wetting width, ND represents the plate rolling direction, ND represents the plate thickness direction, and TD represents the plate width direction.

[0016] Figure 2 A schematic diagram of the process flow for a prestressed grinding and cold rolling method to suppress edge cracking in wide magnesium alloy sheets.

[0017] Figure 3 This is a schematic diagram of the residual stress measurement range in the embodiment. Detailed Implementation

[0018] This invention provides a prestressed grinding and cold rolling method for suppressing edge cracking in wide magnesium alloy sheets.

[0019] Two identical rolled AZ31 magnesium alloy plates with dimensions of 1000mm*500mm*5mm (RD*TD*ND) were used as test samples for the example. Their chemical composition and microstructure are shown in Table 1. The hardness of the test samples is as follows. All are 55HB.

[0020] .

[0021] For specimen No. 1, a 500-ton plate tensile tester was used to uniformly apply tensile stress along the rolling direction (RD direction) at both ends of the specimen plate. =48MPa as the prestress, while maintaining the prestress, the edges of the plate are ground and rapidly cooled. A single-stroke upward grinding surface is performed using a brown corundum abrasive ceramic bond grinding wheel with a hardness grade of K, grit size of 46, and microstructure number of 5. During the grinding process, the wheel linear velocity... =1300m / min, grinding wheel feed speed =1.5m / min, single-sided grinding depth = 0.10mm, single-sided grinding width =38mm, during grinding, an oil-based coolant with a cooling rate of up to 60℃ / s is used to cool the grinding area, wetting width It is 48mm.

[0022] The thickness, ratio, hardness, recrystallization volume fraction, average grain size, and residual stress distribution of the fine grain layer on the surface of sample No. 1 after prestressed grinding and sample No. 2 in the initial rolled state were measured. The measurement results are shown in Table 2.

[0023] .

[0024] Samples No. 1 and No. 2 were cold-rolled. The rolls were heated to 280°C. The reduction in the first pass was 38%, and the reduction in subsequent passes was 12%. The samples were rolled to the target thickness of 1.64 mm in multiple passes. After rolling, the width-direction depth of the edge cracks and the distribution of residual stress were measured.

[0025] Residual stress measurement range as follows Figure 3 As shown, residual compressive stress was measured at 7 locations at equal intervals within a 400mm range in the middle section of the rolling direction. To reduce errors, 6 measurement points were uniformly selected along the width direction at each location within a 38mm range from the edge in the width direction, for a total of 42 measurement points. The average value was taken as the measurement result, and the measurement results are shown in Table 3 (- indicates compressive stress).

[0026] Table 3. Crack depth and residual stress measurement results

[0027]

[0028] The No. 1 plate sample, after undergoing prestressed grinding, exhibited a hardened fine-grained layer on its edge surface. The thickness of this fine-grained layer increased by up to 210%, the fine-grained layer thickness ratio increased by 8.4%, and the hardness increased by 20.98%. The hardening effect was significant, the microstructure refined, the recrystallization degree improved by 18.61%, the average grain size decreased by 37.53%, and a residual compressive stress of 45 MPa was achieved. After cold rolling, the width-to-depth crack of the edge cracks in the No. 1 sample decreased by 40.63%. The residual compressive stress largely offset the tensile stress experienced at the edge during rolling, significantly suppressing edge crack defects. Therefore, the method of this invention can effectively suppress edge cracking problems during magnesium alloy rolling, thereby expanding the width specifications of finished magnesium alloy plates and improving the yield and product quality.

Claims

1. A prestressed grinding and cold rolling method for suppressing edge cracking in wide magnesium alloy sheets, characterized in that, This method addresses the problem of edge cracking during the rolling process of magnesium alloy sheets with a thickness H of 5–10 mm and a width B of 500–1500 mm, which compromises the required width. A prestressed grinding cold rolling method is proposed to effectively suppress edge cracking. Specifically, before cold rolling, a certain tensile stress σ is uniformly applied along the rolling direction at both ends of the sheet as a prestress. While maintaining the prestress σ, the edges of the sheet are ground and rapidly cooled. After grinding, the prestress is removed, resulting in a hardened layer with a certain thickness ratio on the surface of the sheet edges, and residual compressive stress σ is obtained at the edges. c Then, multiple cold rolling processes are completed; During grinding, an oil-based coolant with a cooling rate of up to 60℃ / s is used to rapidly cool the sheet metal. The wetting width L covers the grinding width b on one side, determined by L = 1.25b, and ranges from 48 to 125 mm. This prevents high-temperature oxidation of the magnesium alloy sheet edge surface while obtaining a hardened layer with a thickness ratio of 11.6% to 12.8%, and forms a residual compressive stress σ of 23 to 52 MPa in this area. c On the one hand, the hardened metal layer has good coordination deformation ability; on the other hand, the residual compressive stress can largely offset the tensile stress on the edge during subsequent rolling, thereby effectively suppressing rolling edge cracks and increasing the width of the finished plate. After the edge of the prestressed hardened plate is ground, the working roll temperature is maintained at 250-300℃ in the subsequent multiple cold rolling processes. The first pass uses a larger reduction to refine the microstructure and improve the uniformity of the microstructure in the thickness direction, with a reduction of 35%-45%. Subsequent passes use smaller reductions to roll to the target thickness, with a reduction of 10%-15%.

2. The prestressed grinding and cold rolling method for suppressing edge cracks in wide magnesium alloy sheets as described in claim 1, characterized in that, During the prestressed grinding process, the prestress σ is applied uniformly using a sheet metal stretching machine, and the magnitude of the prestress is determined by σ = (0.2~0.3)σ. s Determine, in the formula, σ s This represents the yield strength of the magnesium alloy.

3. The prestressed grinding and cold rolling method for suppressing edge cracks in wide magnesium alloy sheets as described in claim 1, characterized in that, During the prestressed grinding process, the edge grinding method is a single-stroke planar grinding, with the grinding direction parallel to the subsequent rolling direction. A brown corundum abrasive ceramic-bonded grinding wheel with a hardness grade of J or K, a grit size of 46, and a microstructure of 5 is used. The grinding wheel linear velocity v is 1020–1320 m / min, and the grinding wheel feed rate v0 is [missing information]. f The grinding speed is 1.5–1.8 m / min, the single-sided grinding depth a is 0.105–0.195 mm, and the single-sided grinding width b is 38–100 mm. The grinding wheel linear velocity is related to the magnesium alloy hardness H. a Related, when 45HB≤H a When ≤60HB, v=1180~1320m / min, when 60HB≤H a When ≤75HB, v=1020~1180m / min, the single-sided grinding depth a and single-sided grinding width b are determined by… In the formula, H is the initial plate thickness and B is the initial plate width.

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