A zinc-based high-strength outer panel having a gi plating layer or a zm plating layer and excellent formability and a method of manufacturing the same

Abstract

A zinc-based high-strength outer sheet with excellent formability and equipped with a GI or ZM coating, and its manufacturing method, are disclosed. The chemical composition of the steel, by mass percentage, is as follows: C: 0.04%–0.1%, Si: 0.2%–0.5%, Mn: 1.2%–1.8%, P≤0.02%, S≤0.01%, Al: 0.02%–0.05%, Cr≤0.5%, Nb≤0.02%, N≤0.006%, with the balance being Fe and unavoidable impurities. The content of Si, Mn, and Cr satisfies the condition: (Mn+Cr)−2×Si≥0.5%. This method, through optimized composition design and precise control of the annealing-galvanizing process, synergistically solves the core contradiction of achieving high strength, high formability, and high surface quality in high-strength outer sheets. In particular, it effectively inhibits surface oxidation of the substrate, ensuring the uniformity, density, and adhesion of the coating, and can stably produce high-quality outer sheet products suitable for pure zinc (GI) and zinc-aluminum-magnesium (ZM) coatings.

Description

Technical Field

[0001] This invention relates to the field of metallurgical-metal material processing, and in particular to a zinc-based high-strength outer plate with excellent formability having a GI coating or a ZM coating, and a method for manufacturing the same. Background Technology

[0002] With the global automotive industry's increasing demand for energy conservation, emission reduction, safety, environmental protection, and lightweighting, the application of high-strength steel in automobile bodies is constantly increasing. Among them, high-strength steel with a tensile strength of 500MPa has become a highly promising material for manufacturing exterior body panels such as car doors, hoods, and trunk lids due to its good formability and significant lightweighting potential. These exterior panels not only require materials with sufficient strength to withstand assembly stresses and operating loads to achieve thinner profiles and cost reductions, but also have extremely stringent requirements for surface quality and corrosion resistance to ensure the aesthetics and long-term durability of the vehicle's appearance.

[0003] Currently, most automotive exterior panels on the market are made of low-carbon mild steel with relatively low tensile strength (currently, the mainstream is 300MPa level), and are coated with pure zinc (GI) or zinc-aluminum-magnesium (ZM) through hot-dip galvanizing to meet basic corrosion protection and coating requirements. The production process of these traditional low-strength exterior panel products is mature, and surface quality (such as coating uniformity, smoothness, and defect-free properties) is relatively easy to control, but their strength limit restricts further lightweighting options.

[0004] To overcome this limitation, the industry began experimenting with using high-strength steel of 500MPa and above to manufacture outer panels. However, applying high-strength steel to outer panel manufacturing faces numerous severe technical challenges, making it difficult to achieve the surface quality standards required for high-end automotive exterior parts.

[0005] Surface quality control challenges: First, high-strength steel typically contains high levels of alloying elements (such as Mn, Si, Cr, Mo, etc.) for strengthening. These elements are prone to selective oxidation on the steel plate surface during annealing, forming external oxides. These oxides severely deteriorate the wettability between the coating and the substrate, leading to defects such as poor coating adhesion, incomplete coating, and uneven coating. Subsequent coating processes can result in surface streaks, unevenness, and other problems, significantly affecting the appearance. Second, the higher C and alloy content of high-strength steel makes it more prone to forming more zinc dross (including suspended dross and floating dross) in the zinc bath compared to mild steel. This leads to various zinc dross adhesion defects on the coating surface, which also affects the appearance quality after coating.

[0006] Coating performance and adhesion issues: For high-performance coatings such as zinc-aluminum-magnesium (ZM), their excellent corrosion resistance depends on the specific microstructure of the coating. The interaction between the high-strength substrate and the plating solution is more complex. Traditional annealing and coating process parameters are difficult to make the coating form an ideal, dense and well-adhesive coating structure on the high-strength substrate, which easily leads to phenomena such as coating cracking and powdering.

[0007] The contradiction between material formability and surface consistency: High-strength steel has high yield strength and is extremely sensitive to process windows such as annealing temperature, slow cooling, and over-aging during continuous annealing. Slight deviations in process control can not only affect the mechanical properties of the material (such as elongation, n-value, and r-value) but also amplify the aforementioned surface problems. Traditional annealing-coating processes for low-strength steel used in outer panels cannot be directly applied. A completely new precision manufacturing method must be developed that can synergistically control the matrix microstructure (to ensure strength and formability) and surface condition (to ensure coating quality).

[0008] In summary, existing technologies lack a mature and reliable industrial production method specifically designed for 500MPa-grade high-strength steel outer panels, capable of stably producing zinc-based coatings (including GI and ZM) with excellent surface quality, superior formability, good coating adhesion, and outstanding paintability. Developing such a method would have significant practical and economic value in promoting the large-scale application of high-strength steel in high-end automotive outer panels and achieving the automotive industry's goals of lightweighting and cost reduction.

[0009] In the prior art, patent document CN113604637B discloses a method for producing hot-dip galvanized steel sheet with a tensile strength of 500MPa. The chemical composition of the cast billet is: C: 0.05-0.1%, Mn: 1.2-1.6%, Si≤0.05%, P≤0.02%, S≤0.01%, Al: 0.02%-0.06%, Cr: 0.15-0.4%, Mo: 0.15%-0.3%, N≤0.004%, with the balance being Fe. Although the hot-dip galvanized steel sheet produced by this method can achieve a strength of over 500MPa and has excellent coating performance, it does not mention the control parameters affecting surface quality or the evaluation of the number of defects on the plate surface after galvanizing. In terms of formability, as an outer plate, it has high requirements for edge banding and flanging, but the evaluation of bending performance and flanging performance is not mentioned. In addition, the addition of the precious metal Mo to the substrate will significantly increase the material cost, thus failing to provide the advantage of lightweight and cost-reducing outer plate production.

[0010] Patent document CN116837285A discloses a novel 490MPa grade duplex steel outer plate and its production method. Although this outer plate product can be used to replace traditional mild steel such as IF steel and BH steel for manufacturing car door outer plates, engine hood outer plates, and fender outer plates, especially for cars, it has good dent resistance, flanging ability, formability, and high surface quality that meets the requirements of O5 plates, and has excellent comprehensive performance, it does not mention how to achieve the high surface quality control requirements of O5 plates.

[0011] Patent document CN119913425A discloses a high-strength outer plate with excellent flanging forming performance and its preparation method. The chemical composition of the high-strength outer plate, by mass fraction, includes: C: 0.05-0.14%, Si: 0.1%-0.4%, Mn: 0.8-1.4%, P≤0.01%, S≤0.005%, Al: 0.5%-1.0%, Ti: 0.04%-0.04%, and the matrix element Fe. However, during the steelmaking process, aluminum, as a strong deoxidizer, generates a large amount of Al2O3 inclusions, which can easily cause blockage of the continuous casting nozzle and deteriorate the purity of the molten steel. During hot rolling, the high deformation resistance brought about by aluminum solid solution strengthening and the precipitation of aluminum nitride (AlN) at the grain boundaries will drastically reduce the high-temperature plasticity of the steel plate, leading to a surge in the risk of edge cracking during hot rolling or cold rolling. At the same time, aluminum easily forms a dense oxide film on the surface of the steel plate, which poses a severe challenge to the control of coating wettability and adhesion in the subsequent hot-dip galvanizing process, directly affecting the surface quality of the outer plate. Summary of the Invention

[0012] The main objective of this invention is to overcome the shortcomings of existing technologies and provide a zinc-based high-strength outer sheet with excellent formability and a method for manufacturing the same, featuring a GI or ZM coating. This method, through optimized composition design and precise control of the annealing-galvanizing process, synergistically resolves the core contradiction of achieving high strength, high formability, and high surface quality simultaneously in high-strength outer sheets. In particular, it effectively inhibits oxidation of the substrate surface, ensuring the uniformity, density, and adhesion of the coating, and enabling the stable production of high-quality outer sheet products suitable for pure zinc (GI) and zinc-aluminum-magnesium (ZM) coatings.

[0013] To achieve the above objectives, the present invention employs the following technical solution: A zinc-based high-strength outer plate with excellent formability has the following chemical composition by mass percentage: C: 0.04%–0.1%, Si: 0.2%–0.5%, Mn: 1.2%–1.8%, P≤0.02%, S≤0.01%, Al: 0.02%–0.05%, Cr≤0.5%, Nb≤0.02%, N≤0.006%, with the balance being Fe and unavoidable impurities; wherein the content of Si, Mn, and Cr satisfies: (Mn+Cr)-2×Si≥0.5%.

[0014] The microstructure of the steel plate, by volume percentage, is as follows: ferrite ≥ 85%, average grain size 4–8 μm, martensite < 15%, and carbide precipitates < 2%. Waviness: GI coating Wsa ≤ 0.25 μm, Wca ≤ 0.3 μm; ZM coating Wsa ≤ 0.25 μm, Wca ≤ 0.3 μm. Surface defects: Within a 400 mm × 400 mm area on one side of the coating, there are ≤ 2 zinc dross defects with a defect diameter < 0.5 mm.

[0015] The mechanical properties of the high-strength outer steel plate are as follows: yield strength 290~380MPa, tensile strength 490~600MPa, longitudinal elongation after fracture ≥30% for A80, work hardening index n value ≥0.19, plastic strain ratio r value ≥0.8, hole expansion rate ≥70%, and minimum relative bending radius R / t=0.

[0016] A method for manufacturing a zinc-based high-strength outer sheet with excellent formability suitable for GI or ZM coatings includes steelmaking, hot rolling, cold rolling, reduction annealing, and hot-dip galvanizing.

[0017] 1) The hot rolling process includes: smelting and continuous casting according to the above chemical composition to obtain a billet. The billet is heated to 1200-1250°C and subjected to hot continuous rolling at a final rolling temperature of 860-920°C and a coiling temperature of 580-640°C to obtain a hot-rolled coil. Subsequently, pickling and cold rolling are performed to obtain a cold-rolled substrate of the required thickness.

[0018] 2) Surface treatment: The cold-hardened coil enters the cleaning section and is cleaned with alkaline solution and electrolytic cleaning to thoroughly remove residual oil and iron from the surface.

[0019] 3) The reduction annealing specifically includes: sending the cleaned substrate into a continuous annealing furnace for annealing treatment, the annealing conditions being: Heating section temperature: 720~800℃; Soaking section temperature: 720~800℃; Slow cooling section end temperature: 620~700℃; Rapid cooling section end temperature: 450~520℃; Furnace atmosphere control: Heating section dew point temperature: 0~-20℃; Soaking section dew point temperature: -10~-30℃; Furnace nose dew point temperature: 0~-20℃.

[0020] Furthermore, the preferred reduction annealing temperatures are: heating zone temperature: 740~780℃; soaking zone temperature: 740~780℃; slow cooling zone end temperature: 640~660℃; rapid cooling zone end temperature: 470~490℃.

[0021] 4) The hot-dip galvanizing process includes immersing the annealed substrate in a zinc bath for hot-dip galvanizing. The zinc bath composition and subsequent processes are adjusted according to the target coating type. Production of GI coating: The effective Al content in the zinc bath is controlled at 0.20wt%~0.25wt% (preferably 0.21wt%~0.24wt%), and the zinc bath temperature is 460±5℃.

[0022] Production of ZM coating: The zinc bath composition is a Zn-Al-Mg alloy, wherein the Al content is 1.0wt%~3.0wt% (preferably 1.0wt%~2.0wt%), the Mg content is 1.0wt%~3.0wt% (preferably 1.0wt%~2.0wt%), and the zinc bath temperature is 440±10℃.

[0023] 5) Coating Control and Cooling: After galvanizing, an air knife is used to control the coating thickness. For GI coating: air knife height 300-400mm, air knife distance 6-8mm; for ZM coating: air knife height 250-350mm, air knife distance 5-7mm. The strip is then cooled under controlled conditions, with the cooling rate maintained within the range of 30-50℃ / s.

[0024] 6) After hot-dip galvanizing, the strip undergoes finishing and tension leveling. The cooled strip enters the finishing mill and is subjected to finishing rolling with an elongation of 0.2% to 1.0% to improve shape and straightness. Optionally, tension leveling can be performed to further eliminate residual stress.

[0025] The reduction annealing process in step 3 is one of the key processes of this invention. By precisely controlling the temperature and dew point of each stage, it achieves the optimization of the matrix structure (obtaining ≥85% fine-grained ferrite and a small amount of dispersed martensite) and the control of the surface state (effectively reducing surface oxides), laying the foundation for obtaining excellent forming performance and coating quality in the future.

[0026] The design principles of each alloying element in this invention are as follows: C (0.04%~0.1%): This is the core element that ensures the formation of the martensitic phase and achieves a strength of 500MPa. If the content is too low, there will be insufficient martensite and the strength will not meet the standard; if the content is too high, it will worsen the weldability, plasticity and coating control.

[0027] Si (0.2%–0.5%): Provides solid solution strengthening and strongly inhibits cementite precipitation, while promoting carbon enrichment into austenite. Strictly controlling its upper limit at 0.5% is to maximize its metallurgical benefits while minimizing the negative impact of selective surface oxidation on coating adhesion.

[0028] Mn (1.2%–1.8%) and Cr (≤0.5%), with (Mn+Cr)⁻²×Si ≥ 0.5%: Mn is the core austenite stabilizing element. The introduction of Cr assists Mn in stabilizing austenite and, through its "internal oxidation" properties, forms oxide particles beneath the matrix surface, unlike Si and Mn which form a continuous oxide film on the surface. This provides a cleaner matrix surface for zinc molten metal wetting. The range of (Mn+Cr)⁻²×Si integrates the interactions of Si, Mn, and Cr. When (Mn+Cr)⁻²×Si ≥ 0.5%, it indicates that the Si content is sufficiently low or the Mn / Cr content is sufficiently high to offset the negative effects of Si, thus simultaneously ensuring plate quality and formability. This is crucial for surface quality control in this design.

[0029] Nb (≤0.02%): Through the precipitation of Nb(C,N), fine grain strengthening and precipitation strengthening are produced, allowing for an appropriate reduction in C and Mn content, which is beneficial to improving weldability, plasticity and galvanizing performance.

[0030] Al (0.02%~0.05%): Primarily used as a deoxidizer; residual acid-soluble aluminum can refine grains. Its content must be strictly controlled to avoid problems such as continuous casting nozzle blockage, hot rolling edge cracking, and surface alumina film caused by high aluminum content.

[0031] The design principle of this invention to achieve excellent outer panel quality: To achieve excellent surface quality in zinc-based high-strength outer panels, the manufacturing method of this invention establishes a systematic control scheme. The core of this scheme lies in the precise dew point control throughout the continuous annealing process, in conjunction with a specific composition system and other process parameters, fundamentally solving the industry challenges of poor plating suitability and surface oxide control in high-strength steel substrates.

[0032] During reduction annealing, precise control of the dew point temperature in each functional section of the furnace is crucial for ensuring excellent adhesion between the coating and the substrate, as well as a low-waviness surface. This control strategy is specifically manifested in the synergy of three stages: (1) Dew point control in the heating section (0 to -20℃): In this stage, a moderate oxidizing atmosphere is created by controlling the dew point within this specific range. This environment can promote the selective internal oxidation of alloying elements such as silicon and manganese on the surface of the steel plate, so that fine, discontinuous oxide particles are formed below the substrate surface, thereby effectively avoiding the formation of a continuous and dense outer oxide film on the outermost layer of the steel.

[0033] (2) Dew point control in the soaking zone (-10 to -30℃): After entering the soaking zone, a lower dew point is used, switching to a strongly reducing atmosphere. The core function of this stage is: firstly, to completely reduce the trace surface oxides that may form in the heating zone; secondly, to create conditions for "internal oxidation" of chromium introduced in the composition design. The internal oxidation products of chromium can further "capture" oxygen atoms and inhibit the diffusion of silicon and manganese to the surface, thereby providing a clean and active ferrite surface on the substrate surface, laying the foundation for good wetting of the subsequent zinc liquid and coating adhesion.

[0034] (3) Dew point control at the furnace nose section (0 to -20℃): This area is the last barrier before the strip enters the zinc pot. Maintaining the dew point at this point within a slightly oxidizing range can isolate the interaction between the furnace cavity and the molten zinc, preventing zinc ash from continuously adhering to the strip surface, and effectively preventing instantaneous secondary oxidation of the strip before it enters the zinc pot. Stable control of this step is crucial for consolidating the aforementioned surface treatment effects and avoiding defects such as incomplete plating.

[0035] The chemical composition system of this invention is highly synergistic with the dew point control strategy described above, creating inherent conditions for achieving clean surfaces: (1) Limitation of silicon content (Si: 0.2% to 0.5%): By strictly limiting the silicon content to below 0.5%, the total amount of strong external oxidizing elements is reduced from the source, significantly reducing the surface oxidation trend, so that the dew point control can be implemented more effectively and stably.

[0036] (2) Selective introduction and content ratio of chromium (Cr≤0.5%, and (Mn+Cr)-2×Si≥0.5%): The selective addition of chromium is one of the key designs of this invention. Under the aforementioned dew point atmosphere, it preferentially undergoes internal oxidation, and the resulting internal oxide particles can effectively pin the grain boundaries, hindering the diffusion channels of elements such as silicon and manganese to the surface, thus playing a physical barrier role in "blocking" the formation of harmful external oxides. The content relationship of Si, Mn, and Cr is limited to ensure the optimal balance between austenite stabilization and surface quality control.

[0037] Other process parameters and core control methods work closely together to ensure the achievement of the final product performance: (1) The specific temperature windows of the heating section, the slow cooling section and the fast cooling section ensure that the matrix structure forms an ideal microstructure dominated by fine-grained ferrite and a small amount of martensite dispersed in the matrix. This uniform and fine structure itself provides favorable conditions for obtaining a smooth matrix surface and uniform coating growth.

[0038] (2) A light finishing elongation of 0.2% to 1.0% is adopted, and a tension elongation of 0.1% to 0.3% can be selected. This design can effectively eliminate the yield plateau and improve the flatness of the plate while avoiding damage to the integrity and density of the coating due to excessive plastic deformation, thereby ensuring extremely low waviness after stamping.

[0039] In summary, the control method provided by this invention constructs a complete technical system centered on precise dew point control throughout the entire process, based on a composition system that limits silicon content and utilizes chromium internal oxidation, and synergistically integrates optimized heat treatment regimes and gentle post-treatment techniques. This system ensures the stable acquisition of a clean and active pre-plating surface on high-strength substrates, ultimately enabling uniform, dense, and highly adhesive coverage of both GI and ZM plating layers with different properties, achieving the superior outer panel quality required for O5 board grade.

[0040] The design principle of this invention to achieve excellent forming performance: The chemical composition system of the outer panel of this invention is the intrinsic basis for its excellent mechanical properties. The elements are not simply superimposed, but work together through the following synergistic mechanism: (1) Carbon element (C: 0.04%~0.1%): As a core strengthening element, its content range ensures that austenite can be transformed into the necessary amount of martensite during subsequent annealing and cooling processes, thereby providing a guarantee for achieving tensile strength of 500MPa or higher.

[0041] (2) Composite design of silicon with manganese and chromium (Si: 0.2%~0.5%; Mn: 1.2%~1.8%; Cr≤0.5%, and (Mn+Cr)-2×Si≥0.5%): The core role of silicon is solid solution strengthening, and it strongly inhibits the precipitation of cementite in ferrite, promoting the enrichment of carbon atoms in the untransformed austenite region, thereby "purifying" ferrite. This lays the metallurgical foundation for obtaining high elongation (A80≥30%) and high work hardening index (n value); Manganese and chromium, as austenite stabilizing elements, work together to improve the hardenability of steel, ensuring that during continuous cooling, carbon-rich austenite can be transformed into dispersed martensite, rather than pearlite or bainite. The synergistic control of the contents of the two is the key to achieving the balance of ferrite-martensite dual-phase structure; (3) Microalloying element niobium (Nb≤0.02%): By precipitating Nb(C,N) particles, significant precipitation strengthening and grain refinement effects are produced. This allows for an appropriate reduction in the content of main alloying elements such as carbon and manganese while ensuring strength, which is beneficial for further improving plasticity, weldability and galvanizing performance.

[0042] The continuous annealing process of this invention is the core step in transforming the potential inherent in compositional design into actual mechanical properties. It achieves ideal microstructure matching through precise temperature control in the following stages: (1) Heating and soaking stage (720-800℃): The temperature is maintained in this range to allow the cold-rolled matrix to recrystallize sufficiently and form a suitable ratio of austenite and ferrite two-phase structure. This temperature window is the first key node for controlling the ratio of the two phases in the final structure. Too high a temperature will result in too much austenite and too high martensite content, thus sacrificing plasticity; too low a temperature will result in insufficient austenitization, leading to insufficient strength.

[0043] (2) Slow cooling stage (ending at 620-700℃): This stage is the key period for the precipitation of new ferrite. By controlling the slow cooling rate and the ending temperature, some austenite is transformed into pure, soft polygonal ferrite, while carbon is further driven into the remaining austenite, making it more stable. This directly determines the grain size, purity, and volume fraction (>85%) of the final ferrite, and is the main mechanism for achieving high elongation and high r-value.

[0044] (3) Rapid cooling stage (ends at 450-520℃): Rapidly cool to a temperature range above the martensitic transformation point and below the bainitic transformation zone, in order to suppress the formation of other medium-temperature transformation products and to supercool the carbon-rich, highly stable austenite to the galvanizing temperature range, in order to prepare for subsequent phase transformation.

[0045] (4) Post-plating cooling control: By controlling the post-plating cooling rate within the aforementioned range (30-50℃ / s), the remaining supercooled austenite is transformed into a small amount of uniformly dispersed martensite (<15%) during subsequent cooling. This "island-film" structure, in which hard martensite islands are dispersed on a soft and tough ferrite matrix, is an ideal microstructure for achieving high strength, high initial work hardening rate (high n value), excellent porosity (≥70%), and zero-radius bending capability (R / t=0).

[0046] Finally, by applying a finishing elongation of 0.2% to 1.0% and optionally performing tension straightening of 0.1% to 0.3%, the overall performance of the material is effectively improved, the flatness of the sheet is improved, and the stability of stamping is enhanced, thereby ensuring the uniform manifestation of mechanical properties in the macroscopic components.

[0047] In summary, through the precise synergy of the aforementioned component design and annealing process, this invention successfully constructs an ideal dual-phase microstructure with fine-grained ferrite as the matrix and a small amount of martensite dispersed on it. This microstructure enables the final product to simultaneously possess high strength (tensile strength 490–600 MPa), high plasticity (elongation after fracture ≥30%), high work hardening capacity (n value ≥0.19), and excellent local formability (pore expansion rate ≥70%, R / t=0), fully meeting the stringent requirements for comprehensive mechanical properties of high-strength automotive outer panels.

[0048] Compared with existing technologies, the beneficial effects of this invention are: 1. Innovative and economical composition design: Through a composite design of "medium manganese + medium silicon + low chromium + micro niobium" and strict control of Al content, while ensuring 500MPa strength and excellent formability, the plating properties of high-strength steel substrate are fundamentally improved, avoiding the use of precious metal Mo, resulting in a significant cost advantage.

[0049] 2. Balanced and excellent performance: The finished steel plate not only meets the strength standard, but also has extremely high elongation (≥30%), high n value and r value, high bake hardening value (≥50MPa), as well as excellent hole expansion rate (≥70%) and zero radius bending capability (R / t=0), while meeting the stringent requirements of the outer plate for overall drawing and forming as well as local flanging and edge wrapping.

[0050] 3. Excellent surface quality: Through optimized annealing dew point control and utilization of the internal oxidation characteristics of Cr, the formation of harmful surface oxides is effectively suppressed, so that both GI and ZM coatings can obtain uniform, dense and strong adhesion coatings. The waviness (Wsa, Wca) after forming is extremely low, which can meet the painting requirements of O5 plates.

[0051] 4. High process versatility: Based on the same set of composition and annealing process, both GI and ZM coating products can be stably produced by adjusting the zinc pot composition and parameters, which is highly flexible in production and easy to promote industrially. Attached Figure Description

[0052] Figure 1 This is a metallographic micrograph of 500MPa grade hot-dip pure zinc (GI) obtained by the present invention (Example 1).

[0053] Figure 2 This is a photograph of the expanded hole sample of 500MPa grade hot-dip pure zinc (GI) prepared according to the present invention (Example 1).

[0054] Figure 3 This is a photograph of a bending sample of 500MPa grade hot-dip pure zinc (GI) prepared according to the present invention (Example 1).

[0055] Figure 4The image shows a photograph of a car door outer panel made from the high-strength outer panel (GI) obtained by the invention, which is actually stamped. (Example 1)

[0056] Figure 5 The image shows a photograph of a car door outer panel made from the high-strength outer sheet (ZM) obtained by the invention, which is actually stamped. (Example 3) Detailed Implementation

[0057] The present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto.

[0058] This invention demonstrates the manufacturing process and performance of zinc-based high-strength outer plates through specific embodiments. All embodiments were carried out within the chemical composition and key process parameters required by this invention. By fine-tuning the composition system (such as the addition or absence of Cr and Nb elements) and adapting the coating process, the effectiveness, flexibility, and versatility of this invention were verified. Comparative Examples 1 and 2 are used for comparison to highlight the advantages of the technical solution of this invention. The standards for testing the mechanical properties and surface quality of the hot-dip galvanized finished plates of this invention are as follows: The basic mechanical property testing methods shall be in accordance with GB / T 228.1-2021; The test method for hole expansion test shall be in accordance with GB / T 15825.4-2008 (Cylindrical punch hole expansion test); The bending test method shall be in accordance with GB / T 15825.5-2008 (180° bending test). The waviness test method shall comply with GB / T 2523-2022; The surface quality assessment method shall comply with GB / T 2518-2019.

[0059] The specific implementation details are summarized as follows: The chemical composition of the billet is shown in Table 1. After hot rolling and cold rolling, the billet undergoes continuous annealing and hot-dip galvanizing. The continuous annealing process is shown in Table 2. The control parameters of the heating rolling process and hot-dip galvanizing process are shown in Table 3. The mechanical properties and surface quality of the hot-dip galvanized finished steel plate are shown in Table 4. The microstructure of the finished steel plate is shown in Table 5.

[0060] Table 1 Chemical composition of steel (wt%) Table 2 Control parameters for annealing hot-dip galvanizing process Table 3 Control parameters for hot rolling and hot-dip galvanizing processes Table 4 Mechanical properties and surface quality of finished steel plates Table 5 Microstructure of finished steel plates Comparative Example 1 (excessive Si content): Chemical composition: Si content (0.55%) exceeds the upper limit of this invention. Although the strength (573 MPa) is relatively high, the elongation after fracture (27%) and the porosity (56%) are significantly lower than those of the embodiments of this invention, indicating that the excessive alloy content deteriorates the plasticity and flanging performance of the material. Comparative Example 2 (excessive Si content and (Mn+Cr)-2×Si): Chemical composition: Si content (0.65%) exceeds the upper limit of this invention, and (Mn+Cr)-2×Si exceeds the lower limit of this invention. Although the mechanical properties are acceptable, due to the presence of plating defects, its surface quality grade is FB (defective), proving that the high Si content has a significant negative impact on the surface quality of the coating and cannot meet the stringent surface requirements of the outer panel.

[0061] In summary, Examples 1-9 of this invention, through specific composition design (especially controlling Si ≤ 0.5%, (Mn+Cr)⁻²×Si ≥ 0.5%, and Al at 0.02-0.05%) and precisely matched annealing and galvanizing processes, successfully produced zinc-based high-strength outer sheets possessing high strength (tensile strength 490-600 MPa), high formability (elongation ≥ 30%, hole expansion rate ≥ 70%, R / t = 0), and high surface quality (low waviness Wsa / Wca, surface grade FC). The comparative examples, on the other hand, demonstrate the importance of composition control in this invention. The manufacturing method of this invention is highly versatile and can stably produce both GI and ZM coated products, meeting the application requirements of automotive exterior panels.

Claims

1. A zinc-based high-strength outer sheet with excellent formability, having a GI coating or a ZM coating, characterized in that, The chemical composition of the steel, by mass percentage, is as follows: C: 0.04%–0.1%, Si: 0.2%–0.5%, Mn: 1.2%–1.8%, P≤0.02%, S≤0.01%, Al: 0.02%–0.05%, Cr≤0.5%, Nb≤0.02%, N≤0.006%, with the balance being Fe and unavoidable impurities; wherein, the contents of Si, Mn, and Cr satisfy: (Mn+Cr)-2×Si≥0.5%; The Al content in the zinc bath used to prepare the GI coating is 0.20wt%~0.25wt%, and the zinc bath temperature is 460±5℃; The zinc bath used to prepare the ZM coating is a Zn-Al-Mg alloy, with an Al content of 1.0wt% to 3.0wt% and a Mg content of 1.0wt% to 3.0wt%, and the zinc bath temperature is 440±10℃.

2. A zinc-based high-strength outer plate with excellent formability having a GI coating or a ZM coating as described in claim 1, characterized in that, The microstructure of the steel plate by volume percentage is as follows: ferrite ≥ 85%, average grain size 4-8 μm, martensite < 15%, carbide precipitates < 2%; waviness: GI coating Wsa ≤ 0.25 μm, Wca ≤ 0.3 μm; ZM coating Wsa ≤ 0.25 μm, Wca ≤ 0.3 μm; Surface defects: Within a 400mm×400mm area on one side of the coating, there are ≤2 zinc dross defects with a diameter of <0.5mm.

3. A zinc-based high-strength outer plate with excellent formability having a GI coating or a ZM coating as described in claim 1, characterized in that, The mechanical properties of the high-strength outer steel plate are as follows: yield strength 290~380MPa, tensile strength 490~600MPa, longitudinal elongation after fracture ≥30% for A80, work hardening index n value ≥0.19, plastic strain ratio r value ≥0.8, hole expansion rate ≥70%, and minimum relative bending radius R / t=0.

4. A method for manufacturing a zinc-based high-strength outer plate with excellent formability having a GI coating or a ZM coating as described in any one of claims 1-3, comprising steelmaking, hot rolling, cold rolling, reduction annealing, and hot-dip galvanizing; characterized in that, The reduction annealing specifically includes: Heating section temperature: 720~800℃; Soaking section temperature: 720~800℃; Slow cooling section end temperature: 620~700℃; Rapid cooling section end temperature: 450~520℃; Furnace atmosphere control: Heating section dew point temperature: 0~-20℃; Soaking section dew point temperature: -10~-30℃; Furnace nose dew point temperature: 0~-20℃; The hot-dip galvanizing process includes: producing a GI coating: the effective Al content in the zinc bath is controlled at 0.20wt%~0.25wt%, the zinc bath temperature is 460±5℃; the air knife height is 300-400mm, and the air knife distance is 6-8mm; Production of ZM coating: The zinc bath composition is a Zn-Al-Mg alloy, with Al content of 1.0wt%~3.0wt% and Mg content of 1.0wt%~3.0wt%; the zinc bath temperature is 440±10℃; the air knife height is 250~350mm and the air knife distance is 5~7mm.

5. A method for manufacturing a zinc-based high-strength outer plate with excellent formability having a GI coating or a ZM coating according to claim 4, characterized in that, The hot rolling specifically includes: heating the billet to 1200-1250°C, performing hot continuous rolling, with a final rolling temperature of 860-920°C and a coiling temperature of 580-640°C to obtain a hot-rolled coil.

6. A method for manufacturing a zinc-based high-strength outer plate with excellent formability having a GI coating or a ZM coating according to claim 4, characterized in that, After hot-dip galvanizing, the strip steel is cooled in a controlled manner, with the cooling rate controlled at 30-50℃ / s.

7. A method for manufacturing a zinc-based high-strength outer plate with excellent formability having a GI coating or a ZM coating according to claim 4, characterized in that, After hot-dip galvanizing, the strip undergoes finishing and tension leveling. After cooling, the strip enters the finishing mill and is subjected to finishing rolling with an elongation of 0.2% to 1.0%.

8. An application of a zinc-based high-strength outer sheet with excellent formability having a GI coating or a ZM coating as described in any one of claims 1-3, characterized in that, Used to manufacture automotive exterior body panels, including doors, engine hoods, and trunk covers.

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