A 500mpa grade high formability zinc-iron alloy plated automobile outer panel and a method of manufacturing the same

Abstract

The application relates to the field of metallurgy and metal material processing technology, in particular to a 500MPa-grade high-formability zinc-iron alloy (GA) plated steel plate suitable for automobile outer cover parts and an industrialized preparation method thereof. The method is characterized by a'medium-silicon limited-aluminum + chromium micro-alloying' component system, coupling a set of precise process control strategies throughout the whole process of annealing, galvanizing and alloying, especially dew point subsection control and alloying process window optimization, which solves the core problems of poor compatibility between high-strength base plate and GA plating layer, difficult alloying control, and prominent contradiction between surface quality and formability. The reduction annealing process comprises the following steps: heating segment temperature: 720-800 DEG C; soaking segment temperature: 720-800 DEG C; slow cooling segment end temperature: 620-700 DEG C; fast cooling segment end temperature: 450-520 DEG C; heating segment dew point temperature: 0--20 DEG C; soaking segment dew point temperature: -10--30 DEG C; and furnace nose dew point temperature: -10--30 DEG C.

Description

Technical Field

[0001] This invention relates to the field of metallurgy and metal material processing technology, specifically to a 500MPa grade high formability zinc-iron alloy coated (GA) steel sheet suitable for automotive exterior body panels and its industrial preparation method. Background Technology

[0002] With the automotive industry's increasing demands for corrosion resistance, weldability, and paint finish of body materials, zinc-iron alloy coatings (GA) have gradually become one of the preferred coatings for high-end automotive exterior panels due to their excellent coating adhesion, good weldability, and superior paint adhesion. Compared to pure zinc coatings (GI) and zinc-aluminum-magnesium coatings (ZM), GA coatings form stable Fe-Zn alloy phases (such as Γ and δ phases) through an alloying reaction between the coating and the substrate. This not only improves the coating's hardness and wear resistance but also significantly enhances its anti-powdering and weldability, making it particularly suitable for the complex process requirements of stamping, welding, and painting of automotive exterior panels.

[0003] However, applying GA coating to 500MPa high-strength steel outer panels still faces significant technical challenges: 1. Sensitive Alloying Process Control: The alloying process of GA coatings is extremely sensitive to temperature and time, with a narrow process window. Excessive temperature or time can lead to over-alloying, forming too much ζ phase, causing powdering and increased surface roughness. Insufficient temperature or time results in incomplete alloying, leaving residual η phase (pure zinc layer) in the coating, leading to insufficient corrosion resistance, increased welding spatter, and affecting phosphating coating performance.

[0004] 2. Poor compatibility between high-strength substrate and coating: To achieve a strength of 500 MPa, the substrate typically contains high levels of alloying elements such as Si and Mn. These elements readily undergo selective oxidation on the steel surface during continuous annealing, forming dense oxide films such as SiO2 and MnO. This oxide film severely hinders the interdiffusion of iron and zinc during galvanizing, leading to uneven and incomplete alloying reactions, and consequently causing surface defects such as poor coating adhesion, localized missed plating, or alloying spots.

[0005] 3. Conflict between formability and surface quality: GA coatings have high hardness. If the substrate also has high strength, the ability of the coating and substrate to deform together during stamping is required to be even higher. If the coating adhesion is insufficient or the internal stress is not properly controlled, micro-cracks or even peeling of the coating are likely to occur in areas of severe deformation (such as flanges and bumps). This not only affects the surface quality and corrosion resistance of the parts, but may also become a cause of paint film cracking after coating.

[0006] 4. Limitations of Existing Technologies: Currently available technologies primarily focus on the production of GA coatings for low-carbon mild steel, or only on GI / ZM coatings for high-strength steel. A mature, industrially viable method specifically designed for 500MPa-grade high-strength steel, systematically addressing the challenges of interfacial reaction control, surface quality, and formability synergy during the alloying process with GA coatings, remains lacking. For example, simply transplanting the GA process from mild steel to high-strength steel often leads to deteriorated surface quality and uneven alloying; while using the annealing atmosphere control of GI coatings fails to meet the higher cleanliness and activity requirements of GA coatings on the substrate surface.

[0007] Therefore, developing a manufacturing method for 500MPa-grade GA-coated automotive outer panels with reasonable composition design and precise process control, capable of stably producing panels that combine high strength, high formability, excellent coating adhesion, and superior surface quality, is of great practical significance for promoting the wider application of high-strength steel in high-end vehicle body panels and enhancing the overall competitiveness of automotive products. Summary of the Invention

[0008] The primary objective of this invention is to overcome the shortcomings of the prior art and provide a 500MPa grade high formability zinc-iron alloy coating (GA) automotive exterior panel.

[0009] Another objective of this invention is to provide a method for preparing the aforementioned GA-coated automotive exterior panels. This method utilizes a "medium-silicon, aluminum-limited + chromium micro-alloying" composition system coupled with a precise process control strategy that runs through the entire annealing, zinc plating, and alloying process. In particular, the dew point segmentation control and alloying process window optimization collaboratively solve the core problems of poor compatibility between high-strength substrates and GA coatings, difficulty in alloying control, and the prominent contradiction between surface quality and formability. This method can stably produce high-quality GA-coated exterior panels with uniform and dense coatings, strong adhesion, smooth surfaces after forming, and that meet high-end coating requirements.

[0010] To achieve the above objectives, the present invention employs the following technical solution: A 500MPa grade high formability zinc-iron alloy coating (GA) automotive outer panel 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%.

[0011] The microstructure of the outer plate, by volume percentage, is: ferrite ≥ 85%, average grain size 4–8 μm, martensite < 15%, and carbide precipitates < 2%.

[0012] The GA coating on the surface of the outer panel has an Fe content of 8wt% to 12wt%.

[0013] The mechanical properties of the outer plate meet the following requirements: yield strength 300-390MPa, tensile strength ≥490MPa, JIS5# transverse elongation after fracture ≥30%, work hardening index n ≥0.19, plastic strain ratio r ≥0.8, hole expansion rate ≥70%, and minimum relative bending radius R / t=0.

[0014] The surface quality of the outer plate meets the following requirements: waviness Wsa≤0.3μm, Wca≤0.35μm; within a 400mm×400mm area on one side of the coating, there are ≤2 zinc dross defects with a diameter <0.5mm.

[0015] A method for manufacturing the above-mentioned 500MPa grade high formability zinc-iron alloy coating (GA) automotive exterior panel includes steelmaking, hot rolling, cold rolling, reduction annealing, hot-dip galvanizing, and alloying.

[0016] 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.

[0017] 2) Surface treatment: The cold-hardened coil enters the cleaning section, where alkaline cleaning and electrolytic cleaning are used to thoroughly remove residual oil and iron from the surface.

[0018] 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℃; The furnace atmosphere is controlled using a segmented dew point control strategy: heating section dew point temperature: 0~-20℃; soaking section dew point temperature: -10~-30℃; furnace nose dew point temperature: -10~-30℃. This strategy aims to induce internal oxidation of Si and Mn in the heating section, obtain a clean and active surface under the strong reducing atmosphere in the soaking section, and prevent secondary oxidation in the furnace nose section, thus providing an ideal interface for the good wetting and iron-zinc diffusion required for GA coating.

[0019] 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℃.

[0020] 4) The hot-dip galvanizing process includes immersing the annealed substrate in a zinc bath for hot-dip galvanizing, controlling the effective aluminum content in the zinc bath at 0.12–0.18 wt%, and controlling the zinc bath temperature at 465 ± 5℃. A lower aluminum content is beneficial for the initiation and uniformity of the subsequent alloying reaction.

[0021] 5) Coating control: After galvanizing, use an air knife to control the coating thickness. The air knife height is 400-500mm, and the air knife distance is 5-7mm.

[0022] 6) The alloying process is a key step. The galvanized strip steel is heat-treated in an alloying furnace. The alloying temperature is strictly controlled at 520–550℃, and the holding time is 15–25 seconds. Through this process, the Fe and Zn in the coating are fully diffused and reacted to form a uniform alloy layer with δ phase as the main component and a small amount of ζ phase dispersed in the δ phase on the surface, with an Fe content of 8%–12%.

[0023] 7) Post-galvanizing cooling control: After the hot-dip galvanizing and alloying processes, the strip steel is cooled in a controlled manner, with the cooling rate controlled within the range of 30-50℃ / s.

[0024] 8) After the strip cools, it is finished. The finishing elongation is controlled at 0.3% to 0.8% to eliminate the yield plateau, improve the strip shape, and seal any micropores that may exist in the alloy layer, thereby further improving the surface quality.

[0025] Step 3) is a key part of this invention. By precisely controlling the temperature and dew point of each stage, it optimizes the matrix structure (obtaining ≥85% fine-grained ferrite and an appropriate amount of dispersed island martensite) and controls 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 (-10 to -30℃): 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) Alloying process control: The alloying temperature and time window were optimized to suit the characteristics of the high-strength substrate. The relatively high alloying temperature (520-550℃) ensures that the Fe-Zn diffusion reaction has sufficient driving force to proceed smoothly on substrates containing inhibitory elements (such as Si); while the precisely controlled short holding time (15-25s) prevents over-reaction from causing coating embrittlement and surface roughness. The control of the effective Al content also contributes to the uniformity of the alloying process.

[0039] (3) A light finishing elongation of 0.3% to 0.8% is adopted. 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.

[0040] 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 the alloyed plating layer to achieve uniform, dense, and excellent adhesion coverage, achieving the superior outer panel quality required for O5 board grade.

[0041] 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.

[0042] (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.

[0043] 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.

[0044] (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.

[0045] (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.

[0046] (4) Alloying stage: By promoting the enrichment of carbon in the supercooled austenite and releasing the internal stress of the matrix, the final martensite has a higher carbon content, a better lath shape, and reduced internal stress. At the same time, the ferrite undergoes static recovery, and the dislocation structure tends to be stable.

[0047] (5) 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).

[0048] Finally, by applying a finishing elongation rate of 0.3% to 0.8%, the overall performance of the material is effectively improved, the flatness of the sheet is improved, and the stamping stability is enhanced, thereby ensuring the uniform manifestation of mechanical properties in the macroscopic components.

[0049] 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 suitable amount of island-like martensite dispersed on it. This microstructure enables the final product to simultaneously possess high strength (tensile strength greater than 490 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.

[0050] 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.

[0051] 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.

[0052] 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 GA coatings can obtain uniform, dense and strong adhesion coatings. The waviness (Wsa, Wca) after forming is low, which can meet the painting requirements of O5 plates.

[0053] 4. Optimized control of GA coating performance was achieved: An ideal alloy layer structure was obtained through a precise alloying process window (520-550℃, 15-25s). This coating combines excellent corrosion resistance, strong adhesion (anti-powdering, anti-peeling), and good weldability and coating adhesion.

[0054] 5. Stable, versatile, and cost-controllable process: The method of this invention has a clearly defined process window and clear control measures, making it suitable for continuous industrial production. No precious metal Mo is used in the composition; performance targets are achieved through micro-alloying with elements such as Cr and Nb, resulting in a significant cost advantage. Attached Figure Description

[0055] Figure 1 This is a metallographic micrograph of the 500MPa grade hot-dip galvanized iron alloy coated (GA) steel sheet prepared in Example 1.

[0056] Figure 2 This is a micrograph of the coating microstructure of the 500MPa grade hot-dip galvanized iron alloy coated (GA) steel sheet prepared in Example 1. 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 a 500MPa-grade high-formability zinc-iron alloy (GA) coated automotive exterior panel 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 panels 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, as shown in Table 2. The control parameters for the hot 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 reduction annealing 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): The Si content (0.55%) in its chemical composition exceeds the upper limit of this invention (≤0.5%). Although its tensile strength (560 MPa) is relatively high, its elongation (26%) and porosity (58%) are significantly lower than those of the embodiments of this invention, indicating that at the current carbon content level, excessively high Si content significantly impairs the plasticity and flanging performance of the material while ensuring strength. Comparative Example 2 (Inconsistent Si Content and (Mn+Cr)-2×Si Relationship): Its Si content (0.65%) exceeds the upper limit of this invention, and the (Mn+Cr)-2×Si value (0.40) is lower than the lower limit of this invention (≥0.5%). Although some of its mechanical properties are acceptable, due to poor coating adhesion and poor surface oxidation control, the surface quality grade is only FB (defective), and the number of single-sided defects is significantly increased, verifying the serious negative impact of excessively high Si content and imbalanced composition relationship on the surface quality of GA coating.

[0061] In summary, Examples 1-9 of this invention, through specific composition design (especially strict control of Si≤0.5%, (Mn+Cr)-2×Si≥0.5%), combined with precisely matched annealing atmosphere control, zinc plating process, and key alloying steps (520-550℃, 15-25s), successfully prepared 500MPa-grade GA-coated automotive exterior panels that possess high strength (tensile strength ≥490MPa), high formability (elongation ≥30%, hole expansion rate ≥70%, R / t=0), excellent coating performance (Fe content 8-12%), and high surface quality (rippleness meets requirements, surface grade FC). The comparative examples, on the other hand, confirm the necessity of the composition and process control of this invention. The manufacturing method of this invention has been systematically optimized for the characteristics of GA coatings, with a clear process window, enabling the stable production of high-quality GA-coated products that meet the requirements of high-end automotive exterior panels.

Claims

1. A 500MPa grade high-formability zinc-iron alloy coated automotive exterior panel, 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%; A method for manufacturing 500MPa grade high-formability zinc-iron alloy coated automotive exterior panels includes steelmaking, hot rolling, cold rolling, reduction annealing, hot-dip galvanizing, and alloying; 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℃; Dew point temperature of heating section: 0~-20℃; Dew point temperature of soaking section: -10~-30℃; Dew point temperature of furnace nose: -10~-30℃; The alloying process specifically includes: heat-treating the galvanized strip steel in an alloying furnace at an alloying temperature of 520–550°C for 15–25 seconds.

2. The 500MPa grade high formability zinc-iron alloy coated automotive outer panel according to claim 1, characterized in that, The mechanical properties of the high-strength outer steel plate are as follows: yield strength 300~390MPa, tensile strength ≥490MPa, JIS5# transverse elongation after fracture ≥30%, 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.

3. The 500MPa grade high formability zinc-iron alloy coated automotive outer panel according to claim 1, characterized in that, The GA coating on the outer panel has an Fe content of 8wt% to 12wt%, and the surface quality of the outer panel meets the following requirements: waviness Wsa ≤ 0.3μm, Wca ≤ 0.35μm; within a 400mm×400mm area on one side of the coating, there are ≤2 zinc dross defects.

4. The 500MPa grade high formability zinc-iron alloy coated automotive outer panel according to claim 1, characterized in that, The microstructure of the outer plate, by volume percentage, is: ferrite ≥ 85%, average grain size 4–8 μm, martensite < 15%, and carbide precipitates < 2%.

5. A method for manufacturing a 500MPa grade high-formability zinc-iron alloy coated automotive exterior panel as described in any one of claims 1-4, comprising steelmaking, hot rolling, cold rolling, reduction annealing, hot-dip galvanizing, and alloying; 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℃; Dew point temperature of heating section: 0~-20℃; Dew point temperature of soaking section: -10~-30℃; Dew point temperature of furnace nose: -10~-30℃; The alloying process specifically includes: heat-treating the galvanized strip steel in an alloying furnace at an alloying temperature of 520–550°C for 15–25 seconds.

6. The method for manufacturing a 500MPa grade high-formability zinc-iron alloy coated automotive outer panel according to claim 5, 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.

7. The method for manufacturing a 500MPa grade high-formability zinc-iron alloy coated automotive outer panel according to claim 5, characterized in that, The hot-dip galvanizing process includes: immersing the annealed substrate in a zinc bath for hot-dip galvanizing, controlling the effective aluminum content in the zinc bath at 0.12-0.18 wt% and the zinc bath temperature at 465±5℃; after galvanizing, using an air knife to control the coating thickness, with an air knife height of 400-500 mm and an air knife distance of 5-7 mm.

8. The method for manufacturing a 500MPa grade high-formability zinc-iron alloy coated automotive outer panel according to claim 5, characterized in that, After the hot-dip galvanizing and alloying processes, the strip steel is subjected to controlled cooling, with the cooling rate controlled at 30-50℃ / s.

9. The method for manufacturing a 500MPa grade high-formability zinc-iron alloy coated automotive outer panel according to claim 5, characterized in that, After alloying, the product is polished, and the polishing elongation is controlled at 0.3% to 0.8%.

10. The application of a 500MPa grade high-formability zinc-iron alloy coated automotive exterior panel as described in any one of claims 1-4, characterized in that, Used to manufacture automotive exterior body panels, including doors, engine hoods, and trunk covers.

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