DP500 grade dual-phase steel automobile outer plate and hot-dip galvanizing production method thereof

Abstract

This invention relates to the field of metal production technology, specifically to a DP500 grade duplex steel automotive outer panel and its hot-dip galvanizing production method. It is composed of the following chemical composition by weight percentage: C: 0.055%~0.095%, Si: 0.45%~0.65%, Mn: 1.50%~1.90%, Cr: 0.40%~0.80%, P≤0.020%, S≤0.015%, Als: 0.015%~0.060%, with the remainder being Fe and unavoidable impurities. The substrate microstructure of the automotive outer panel is: ferrite content >90%, average grain size 4~8μm, martensite content <10%, and carbide precipitates <2%. The hot-dip galvanizing production method involves gradient dew point control of the atmosphere within the continuous annealing furnace: dew point of -5℃ to -15℃ in the heating section, -15℃ to -25℃ in the soaking section, and -8℃ to -15℃ in the furnace nose section, which is 4-6℃ lower than the conventional O5 process. An intermittent production mode of "2 rolls of transition material + 2 rolls of DP500-O5 material" is employed. The resulting product meets the DP500 standard in mechanical properties and achieves O5 grade surface quality, solving the technical challenge of consistently obtaining high-surface-quality outer panels from high-silicon duplex steel.

Description

Technical Field

[0001] This invention relates to the field of metal production technology, specifically to a DP500 grade duplex steel automotive outer panel and its hot-dip galvanizing production method. Background Technology

[0002] With the increasing demands for lightweighting and safety in automobiles, the application of advanced high-strength steel in automotive body-in-white is steadily rising. Dual-phase steel, due to its excellent strength-plasticity match, low yield strength ratio, and high work hardening capacity, is increasingly in demand for exterior body panels such as doors and fenders. Simultaneously, to meet automotive corrosion resistance requirements, hot-dip galvanized duplex steel is also being widely used in automotive exterior panels. Compared to cold-rolled duplex steel, hot-dip galvanized duplex steel, while maintaining mechanical properties, also requires good plating compatibility, surface quality, and coating adhesion, placing higher demands on composition design and manufacturing processes.

[0003] However, the high content of alloying elements such as silicon (Si), manganese (Mn), and chromium (Cr) added to DP500 duplex steel to ensure mechanical properties makes it highly susceptible to selective surface oxidation during continuous hot-dip galvanizing and annealing, forming oxides such as MnO and SiO2. These oxides severely degrade the wettability of the molten zinc to the steel substrate, leading to surface defects such as incomplete plating and bright spots in the plated product. As a result, the surface quality of the product usually only meets the requirements of automotive inner panels (such as grade 4 or 7), and it is difficult to reach the highest grade O5 surface (no visible defects) standard for outer panels.

[0004] To address the above problems, existing technologies mainly employ two technical approaches: One approach is to reduce surface oxide formation at the source by limiting the content of alloying elements in the steel. For example, Chinese patent applications CN107267727A and CN105401071A limit the silicon content in steel to extremely low levels (Si≤0.005% or ≤0.01%) to improve plating availability. The drawback of this method is that it severely restricts the freedom of alloy design. Silicon, as an important solid solution strengthening element, significantly contributes to the strength and work hardening ability of duplex steel. While excessively suppressing silicon content is beneficial for plating availability, it can impair the mechanical properties of the material or force the addition of more expensive alloying elements (such as Mo) to compensate, leading to increased costs and making it unsuitable for commercially available DP500 grades with higher silicon content.

[0005] Another technical approach is to improve surface quality by controlling the atmosphere within the annealing furnace. Chinese patent application CN110230000A employs a slightly oxidizing atmosphere (dew point 0 to +10°C) in the heating section, while Chinese patent applications CN107267727A and CN117758144A emphasize a strong reducing atmosphere throughout the process (dew point ≤ -40°C). These methods either employ relatively simple process control strategies or fail to provide differentiated and coordinated fine-grained control over the oxidation / reduction kinetics of high-Si, Mn-content duplex steels at each stage of continuous annealing (heating, soaking, furnace nose). For substrates with high silicon content, simple oxidation or excessively strong reduction is insufficient to effectively control the surface state throughout the entire heat treatment process. This easily leads to the formation of a harmful oxide-rich layer before the strip enters the zinc pot, resulting in frequent defects such as bright spots and incomplete plating during the production of O5 outer plates. The surface quality is unstable, the yield is low, and stable mass production is difficult to achieve.

[0006] In summary, existing technologies either sacrifice material performance and economy through stringent composition restrictions or adopt general or single annealing atmosphere control strategies, neither of which can provide a complete solution that can be compatible with reasonable alloy compositions and stably produce O5-grade surface DP500 outer plates through refined collaborative processes. Summary of the Invention

[0007] To overcome the shortcomings of the prior art, the present invention provides a DP500 grade duplex steel automotive outer panel and a hot-dip galvanizing production method thereof, thereby solving the problem that high-silicon duplex steel is difficult to stably obtain O5 grade surface quality.

[0008] To achieve the above objectives, the present invention employs the following technical solution: A DP500 grade duplex steel automotive exterior panel, wherein the base material of the automotive exterior panel is LC490 steel, and the chemical composition comprises the following weight percentages: C: 0.055%~0.095%, Si: 0.45%~0.65%, Mn: 1.50%~1.90%, Cr: 0.40%~0.80%, P≤0.020%, S≤0.015%, Als: 0.015%~0.060%, with the remainder being Fe and unavoidable impurities; the substrate microstructure of the automotive outer panel is as follows: ferrite content >90%, average grain size 4~8μm, martensite content <10%, and carbide precipitates <2%.

[0009] The effect of selecting the above alloying elements and their contents: 1. Carbon: Carbon is the most basic strengthening element in duplex steel, mainly improving the strength of the steel through solid solution strengthening and the formation of martensite. If the carbon content is too low, the volume fraction of martensite is insufficient, making it difficult to guarantee the tensile strength of DP500 level; if the carbon content is too high, it will lead to excessively high martensite hardness and morphological deterioration, reducing the elongation and low-temperature impact toughness of the steel plate, while also increasing the difficulty of spot welding and reducing corrosion resistance. Therefore, this invention precisely controls the carbon content to 0.055%~0.095%.

[0010] 2. Silicon (Si): Silicon is an important solid solution strengthening element that can significantly improve the strength, hardenability, and work hardening ability of steel. Silicon can dissolve in ferrite and austenite to increase the strength of materials; the higher the silicon content, the higher the strength, thus improving the yield strength ratio of duplex steel. However, excessive silicon content can easily generate iron oxide scale that is difficult to pickle, affecting the surface quality of the steel plate. During hot-dip galvanizing annealing, it is prone to selective oxidation on the steel plate surface to form SiO2, severely deteriorating the wettability of zinc liquid to the steel substrate, leading to surface defects such as incomplete plating and bright spots. Therefore, while ensuring mechanical properties, the Si content must be controlled within a reasonable range.

[0011] When the Si content is <0.45%, its contribution to DP500-grade strength is insufficient, requiring a significant increase in the content of other expensive alloying elements, which is uneconomical. Conversely, when the Si content is >0.65%, even with a strong reducing atmosphere, the risk of surface oxidation increases dramatically, making stable control difficult. Therefore, this invention precisely controls the Si content between 0.45% and 0.65%, ensuring the DP500-grade strength requirements while also providing the possibility for subsequent process control of surface oxidation.

[0012] 3. This invention employs a synergistic formulation of "medium silicon (0.45%~0.65%) + medium chromium (0.40%~0.80%)". Si ensures the required solid solution strengthening strength and work hardening capacity; while the introduced Cr, with its strengthening and hardenability-enhancing functions similar to Mn and Si, but based on thermodynamic principles (higher Gibbs free energy), preferentially maintains stability in the annealing atmosphere compared to Si and Mn, thus significantly reducing the tendency for the formation of wetting harmful oxides such as SiO2 and MnO on the steel plate surface. This makes the medium silicon design possible, laying the foundation for obtaining high surface quality from the material's inherent nature.

[0013] 4. Mn: Manganese is an austenite stabilizing element that can significantly improve the hardenability of steel, delay the transformation of pearlite and bainite, and promote the formation of martensite during cooling. Manganese can also improve the strength of steel through solid solution strengthening and refining ferrite grains. When the manganese content is too low, the hardenability is insufficient, making it difficult to obtain the required martensite ratio; when the manganese content is too high, on the one hand, it will aggravate the selective oxidation of manganese on the surface of the steel plate, affecting platingability, and on the other hand, it is easy to generate segregation in the center of the slab, reducing the low-temperature toughness of the steel plate. Therefore, this invention precisely controls the Mn content at 1.50%~1.90%.

[0014] 5. Cr: Chromium is an effective element for improving hardenability, significantly delaying the transformation of pearlite and bainite, thereby transforming austenite into martensite and ensuring the strength of duplex steel. Simultaneously, chromium forms a dense passivation film in steel, improving the corrosion resistance of the steel plate. Compared to silicon and manganese, chromium oxide has a higher Gibbs free energy for formation, making it less prone to selective surface oxidation during annealing and having less impact on plating applicability. Therefore, adding an appropriate amount of chromium can partially alleviate the plating applicability problems caused by high silicon and high manganese content while ensuring strength. Furthermore, chromium has a significant cost advantage over molybdenum and niobium, significantly reducing costs. Therefore, this invention precisely controls the Cr content to 0.40%~0.80%.

[0015] 5. P and S: Phosphorus and sulfur are harmful impurity elements in steel. Phosphorus tends to segregate at grain boundaries, increasing the brittleness of steel and reducing its stamping and welding properties; sulfur forms MnS inclusions, impairing the plasticity and anisotropy of steel. Therefore, their content should be reduced as much as possible. This invention controls P ≤ 0.020% and S ≤ 0.015%.

[0016] 6. Als (acid-soluble aluminum): Aluminum is a major deoxidizing element in steel, effectively reducing the oxygen content in molten steel and improving its purity. Aluminum can also combine with nitrogen to form AlN, refining grains and improving the toughness and strength of steel. If the aluminum content is too low, the deoxidation effect is insufficient; if the aluminum content is too high, large alumina inclusions are easily formed, affecting the surface quality and low-temperature impact toughness of the steel plate. Therefore, this invention precisely controls the Al content to 0.015%~0.060%.

[0017] Substrate microstructure: Based on the above chemical composition design, an ideal substrate microstructure can be obtained through subsequent hot rolling, cold rolling, and continuous annealing processes: ferrite content >90%, average grain size 4~8μm, martensite content <10%, and carbide precipitates <2%. The fine ferrite grains ensure good formability and plasticity of the steel sheet, the dispersed martensite provides the required strength, and the small amount of carbide precipitates contributes to further strengthening. This microstructure characteristic lays the foundation for the DP500 automotive outer panel of this invention, which ultimately possesses both excellent mechanical properties and O5-grade surface quality.

[0018] The surface quality grade of the automotive outer panel is O5, and its mechanical properties meet the following requirements: tensile strength Rm≥490MPa, yield strength Rp0.2 is 290~380MPa, elongation A80≥26%, work hardening index n≥0.19, and plastic strain ratio r≥0.75.

[0019] Furthermore, its coating is a pure zinc coating with a surface waviness (Wsa) ≤ 0.35 μm, meeting the requirements for automotive steel without intermediate coating. The product exhibits excellent dent resistance and high formability, with a hole expansion rate ≥ 70%.

[0020] A hot-dip galvanizing production method for DP500 grade duplex steel automotive exterior panels, the method comprising the step of gradient dew point control of the atmosphere in a continuous annealing furnace: the dew point in the heating section is controlled at -5 to -15℃; the dew point in the soaking section is controlled at -15 to -25℃. The dew point of the furnace nose section is controlled at -8 to -15℃.

[0021] Furthermore, the dew point of the furnace nose section is controlled by adjusting the flow rate of humidifying nitrogen to the furnace nose to the minimum or turning it off.

[0022] Furthermore, the method employs an intermittent production mode, where at least two rolls of transitional material that does not meet the O5 grade requirement are produced continuously before producing DP500 grade duplex steel automotive exterior panels with a surface quality grade of O5.

[0023] Furthermore, a production model is adopted that alternates between 2 rolls of transition material and 2 rolls of DP500 grade duplex steel automotive outer sheet material with a surface quality grade of O5.

[0024] Furthermore, the preparation method of the LC490 steel substrate includes: hot rolling and cold rolling sequentially on the slab after smelting and continuous casting according to the chemical composition of claim 1 to obtain a cold-rolled coil; wherein the final rolling temperature of the hot rolling process is controlled at 850~930℃, the coiling temperature is controlled at 580~650℃, and the total reduction rate of the cold rolling process is controlled at 62%~80%.

[0025] Furthermore, the method also includes the following steps: cleaning the cold-rolled steel coil; hot-dip galvanizing the cleaned steel strip, controlling the production line speed to be 90~120m / min, the zinc pot temperature to be 457~463℃, and the aluminum mass percentage content in the zinc pot to be 0.20%~0.25%; and finishing the galvanized steel strip with a finishing elongation of 0.4%~0.7%.

[0026] Compared with the prior art, the beneficial effects of the present invention are: 1. The synergistic effect of composition design and process control has broken through the technical prejudice that "low silicon must be strictly controlled in order to ensure plating properties".

[0027] To circumvent plating issues, existing technologies typically limit Si content to extremely low levels of ≤0.01%, which severely undermines silicon's contribution as a solid solution strengthening element to strength, work hardening ability, and low yield strength ratio. The composition system designed in this invention (C: 0.055%~0.095%, Si: 0.45%~0.65%, Mn: 1.50%~1.90%, Cr: 0.40%~0.80%) has a clear synergistic effect: Si and Mn ensure the strength and hardenability required for DP500 grade; in particular, the introduction of 0.40%~0.80% Cr is crucial. Cr's role in improving hardenability and promoting martensite formation is comparable to that of Mn and Si, but the Gibbs free energy of its oxide formation is significantly higher than that of Si and Mn oxides, making Cr less prone to surface selective oxidation in the annealing environment. Therefore, by replacing part of the Mn / Si strengthening effect with Cr in this composition system, the tendency of harmful oxides (such as MnO and SiO2) to form on the steel plate surface is reduced from the source without sacrificing mechanical properties, laying a feasible material foundation for obtaining high surface quality through subsequent processing methods.

[0028] 2. The targeted design of the gradient dew point control process has enabled stable control of the surface quality of high-Si steel.

[0029] Existing atmosphere control strategies are mostly based on single oxidizing or strong reducing throughout the process, which cannot be adapted to the oxidation / reduction kinetics of high-Si and Mn steels at various annealing stages.

[0030] This invention proposes gradient dew point control: The heating section (dew point -5~-15℃) uses a relatively high dew point (micro-oxidation) to promote the "internal oxidation" of elements such as Si and Mn below the surface of the steel plate, rather than enriching them to the outermost surface to form a continuous oxide layer. This reduces obstacles to subsequent reactions with the zinc liquid.

[0031] The soaking zone (dew point -15~-25℃) lowers the dew point and creates a strong reducing atmosphere, reducing the extremely thin layer of iron oxide that may form in the heating zone and exposing a clean, active iron matrix surface.

[0032] Furnace nose section (dew point -8~-15℃, and 4~6℃ lower than conventional O5 process): Using ultra-low dew point (achieved by turning off or minimizing humidifying nitrogen gas) ensures that the surface of the strip steel is in an absolutely clean and highly reduced state in the final stage before entering the zinc pot, completely eliminating the possibility of missed plating or bright spots caused by the enrichment of trace oxides.

[0033] This gradient strategy of "first inducing internal oxidation, then strong reduction, and finally maintaining ultra-cleanliness" is tailored for the specific components (medium Si and medium Mn) of this invention. Its synergistic effect with the component design enables LC490 substrates with high Si content (0.45%~0.65%) to stably obtain O5-level surface quality.

[0034] 3. The “2+2” production model ensures extreme stability of the process window and achieves reliability in mass production.

[0035] On a continuous annealing-galvanizing line, stable furnace atmosphere and zinc pot conditions are prerequisites for obtaining uniform surface quality. This invention employs a production mode of "first continuously producing at least two rolls of non-O5 grade transition material, then alternating between two rolls of transition material and two rolls of DP500-O5 material." Utilizing transition material production allows the entire system (furnace atmosphere, zinc bath activity, etc.) sufficient time to reach and stabilize within the specific process window optimized for DP500 composition. This eliminates the impact of product changes and process fluctuations on high-surface-requirement products (O5 material), achieving stable batch production of high-surface-quality products.

[0036] 4. The synergistic effect of composition and process yields an ideal microstructure, achieving a unity of mechanical properties and forming properties.

[0037] Through the synergistic effect of the above-mentioned composition design and hot-dip galvanizing process, this invention achieves an ideal substrate microstructure: ferrite content >90% (ensuring high plasticity and formability), average grain size refined to 4~8μm (fine grain strengthening, improving strength and toughness), martensite content <10% (providing a reinforcing phase), and carbide precipitates <2% (avoiding harmful brittleness). These microstructure characteristics directly lead to excellent overall performance. High strength and good plasticity are combined: tensile strength Rm≥490MPa, yield strength Rp0.2 is 290~380MPa, elongation A80≥26%, fully meeting and exceeding the DP500 grade standard.

[0038] Excellent formability: A work hardening index (n) ≥ 0.19 and a plastic strain ratio (r) ≥ 0.75 ensure uniform deformation and resistance to thinning during stamping; a hole expansion rate ≥ 70% indicates excellent flanging performance and resistance to localized cracking.

[0039] 5. The coating has excellent surface quality, meeting the requirements for high-end automotive exterior panel applications.

[0040] The DP500 grade hot-dip galvanized automotive exterior panels obtained by this invention achieve an O5 grade surface quality (no bright spots, no uncoated defects), with a surface waviness Wsa ≤ 0.35μm. This meets the stringent requirements for substrate surface flatness in the "no intermediate coat" painting process for automotive steel panels, directly reducing downstream manufacturing costs. The product exhibits excellent dent resistance and can be widely used in automotive exterior panels such as doors, fenders, and hoods. It achieves lightweighting of the vehicle body without sacrificing appearance quality and manufacturability, demonstrating promising market application prospects. Attached Figure Description

[0041] Figure 1 This is a metallographic microstructure image of Embodiment 1 of the present invention.

[0042] Figure 2 This is a comparison image of the surface of LC490-based DP500-O5 steel plate produced using the process of this invention (without bright spots) and the surface of the same material steel plate produced using conventional processes (with bright spot defects).

[0043] Figure 3 This is a schematic diagram of the "2+2" production mode of the present invention (showing the effect of alternating production of transition material and O5 material on the stability of the atmosphere inside the furnace).

[0044] Figure 4 These are macroscopic photographs of the surface of a DP500 grade duplex steel automotive exterior panel prepared using this invention. The surface is uniform and free of bright spots or defects.

[0045] Figure 5 This is a part drawing of a car door outer panel actually stamped from DP500 grade duplex steel automotive outer panel prepared using this invention. Detailed Implementation

[0046] This invention discloses a DP500 grade duplex steel automotive outer panel and its hot-dip galvanizing production method. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments, and those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0047] This invention demonstrates the manufacturing process and performance of the DP500 grade duplex steel automotive outer panel through specific embodiments. The implementation steps are as follows: 1. Steelmaking and continuous casting: After being smelted in a converter and refined in an LF furnace according to the set composition, slabs are continuously cast to obtain continuously cast slabs with chemical composition that meet the requirements.

[0048] 2. Hot rolling: The continuously cast billet is heated to 1200~1250℃, and after rough rolling and finish rolling, the final rolling temperature is controlled at 850~930℃. Then, it is rapidly cooled to the coiling temperature of 580~650℃ at a cooling rate of ≥15℃ / s to obtain hot-rolled coil.

[0049] 3. Cold rolling: After pickling the hot-rolled coil, it is cold-rolled with a total reduction rate controlled at 62%~80% to obtain a cold-rolled coil.

[0050] 4. Cleaning: The cold-hardened coil enters the cleaning section, where alkaline cleaning and electrolytic cleaning are used to remove residual oil and iron from the surface.

[0051] 5. Continuous Annealing and Hot-Dip Galvanizing: The cleaned steel strip is fed into a continuous annealing furnace for annealing, and the atmosphere inside the annealing furnace is controlled with gradient dew point: the dew point in the heating section is controlled at -5℃ to -15℃, the dew point in the soaking section is controlled at -15℃ to -25℃, and the dew point in the furnace nose section is controlled at -8℃ to -15℃, which is 4~6℃ lower than the dew point setting for the furnace nose section when producing conventional low-carbon steel O5 automotive outer panels. The dew point in the furnace nose section is achieved by adjusting the flow rate of humidifying nitrogen gas to the furnace nose to the minimum or shutting it off. The production line speed is 90~120m / min.

[0052] The intermittent production mode is adopted. Before producing DP500-O5 automotive outer panels with LC490 steel as the base material, two rolls of transitional material that does not meet the O5 grade requirements are produced continuously, and then the production of two rolls of DP500-O5 material is switched.

[0053] The annealed steel strip is hot-dip galvanized at a temperature of 457-463℃, with an aluminum content of 0.20%-0.25%.

[0054] 6. Finishing: The galvanized steel strip enters the finishing mill and is subjected to finishing rolling with an elongation of 0.4% to 0.7%.

[0055] The chemical composition of the specific embodiments of the present invention is shown in Table 1, the hot-dip galvanizing process control parameters are shown in Table 2, and the mechanical properties and surface quality of the finished steel plates are shown in Table 3.

[0056] Table 1 Chemical composition (wt%) and rolling process parameters of the steel in the embodiments of the present invention Table 2. Hot-dip galvanizing process control parameters in embodiments of the present invention. Table 3 Mechanical properties and surface quality of finished steel plates according to embodiments of the present invention Comparative Example 1: To verify the technical effect of the present invention, Comparative Example 1 was set up for comparison. Comparative Example 1 used a chemical composition similar to that of Example 1 (C: 0.070%, Si: 0.55%, Mn: 1.70%, Cr: 0.58%, P: 0.009%, S: 0.004%, Als: 0.038%), but the hot-dip galvanizing process used a conventional strong reducing atmosphere throughout (annealing furnace dew point ≤ -40℃), and gradient dew point control and the "2+2" production mode were not adopted.

[0057] The mechanical properties of the product in Comparative Example 1 are: yield strength 340 MPa, tensile strength 532 MPa, and elongation 26.5%, which meet the DP500 standard. However, surface quality inspection shows obvious bright spots and localized plating defects, and the surface quality grade is only inner panel grade (07 grade), which cannot meet the O5 outer panel requirements.

[0058] Comparative Example 2: To verify the technical effects of the present invention, a comparative example was set up for comparison. Comparative Example 2 used a different chemical composition than Example 1 (C: 0.070%, Si: 0.06%, Mn: 1.36%, Cr: 0.72%, P: 0.019%, S: 0.0035%, Als: 0.006%). The Si content of Comparative Example 2 was much lower than that of Example 1, the Mn% was slightly lower than that of Example 1, while the Cr% was higher than that of Example 1. The hot-dip galvanizing process used a conventional strong reducing atmosphere throughout (annealing furnace dew point ≤ -40℃), and gradient dew point control and the "2+2" production mode were not used.

[0059] The mechanical properties of Comparative Example 2 are: yield strength 298 MPa, tensile strength 541 MPa, and elongation 29.5%. Although the mechanical properties meet the DP500 standard, the yield strength is close to the lower limit (range 290~380 MPa). However, surface quality inspection shows obvious bright spots, many unplated areas, and zinc dross defects. The surface quality grade is only inner plate grade (07 grade), which cannot meet the O5 outer plate requirements.

[0060] Results Analysis As shown in Table 3, the finished steel plates using the technical solution of this invention (Examples 1-30) all achieved ideal substrate microstructure: ferrite content 91%-95%, average grain size 5.2-7.0 μm, martensite content 4%-8%, and carbide precipitates ≤1%. The mechanical properties fully meet the DP500 standard: yield strength 318-356 MPa, tensile strength 518-553 MPa, and elongation 26.0%-28.0%. Surface quality inspection (QM) showed no bright spots, no unplated areas, and no zinc slag; the surface quality met the O5 grade requirements for automotive outer panels.

[0061] Compared with the comparative example, the present invention successfully solved the surface oxidation problem in the hot-dip galvanizing process of high silicon duplex steel, stably obtained O5 grade surface quality, and maintained excellent mechanical properties.

[0062] This invention successfully solves the global challenge of producing O5 surface outer plates from high-Si, Mn-content duplex steel by combining a chemical composition system tailored to LC490 substrates with a dedicated hot-dip galvanizing process. It achieves a perfect balance between product performance and surface quality, demonstrating significant innovation and industrial application value.

[0063] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A DP500 grade duplex steel automotive exterior panel, characterized in that, The base material of the automotive outer panel is LC490 steel, which is composed of the following chemical composition by weight percentage: C: 0.055%~0.095%, Si: 0.45%~0.65%, Mn: 1.50%~1.90%, Cr: 0.40%~0.80%, P≤0.020%, S≤0.015%, Als: 0.015%~0.060%, with the remainder being Fe and unavoidable impurities; the substrate microstructure of the automotive outer panel is as follows: ferrite content >90%, average grain size 4~8μm, martensite content <10%, and carbide precipitates <2%.

2. The DP500 grade duplex steel automotive exterior panel according to claim 1, characterized in that, The surface quality grade of the automotive outer panel is O5, and its mechanical properties meet the following requirements: tensile strength Rm≥490MPa, yield strength Rp0.2 is 290~380MPa, and elongation A80≥26%. The work hardening index n value is ≥0.19, and the plastic strain ratio r value is ≥0.

75.

3. The DP500 grade duplex steel automotive outer panel according to claim 1, characterized in that, Furthermore, its coating is a pure zinc coating, with a surface waviness Wsa≤0.35μm and a porosity ≥70%.

4. A method for hot-dip galvanizing DP500 grade duplex steel automotive exterior panels as described in any one of claims 1-3, characterized in that, The method includes the step of gradient dew point control of the atmosphere inside a continuous annealing furnace: The dew point of the heating section is controlled at -5 to -15℃; The dew point of the heat spreader is controlled at -15~-25℃; The dew point of the furnace nose section is controlled at -8 to -15℃.

5. The hot-dip galvanizing production method of DP500 grade duplex steel automotive outer panels according to claim 4, characterized in that, The dew point of the furnace nose section is controlled by adjusting the flow rate of humidifying nitrogen to the furnace nose to the minimum or turning it off.

6. The hot-dip galvanizing production method of DP500 grade duplex steel automotive outer panels according to claim 4, characterized in that, The method employs an intermittent production mode, where at least two rolls of transitional material that does not meet the O5 grade requirement are produced continuously before producing DP500 grade duplex steel automotive exterior panels with a surface quality grade of O5.

7. The hot-dip galvanizing production method of DP500 grade duplex steel automotive outer panels according to claim 6, characterized in that, The production process employs an alternating production model of 2 rolls of transition material and 2 rolls of DP500 grade duplex steel automotive outer panel material with a surface quality grade of O5.

8. The hot-dip galvanizing production method of DP500 grade duplex steel automotive outer panels according to claim 4, characterized in that, The method for preparing the LC490 steel substrate includes: hot rolling and cold rolling sequentially on a slab that has been smelted and continuously cast according to the chemical composition of claim 1 to obtain a cold-rolled coil; The final rolling temperature of the hot rolling process is controlled at 850~930℃, and the coiling temperature is controlled at 580~650℃; the total reduction rate of the cold rolling process is controlled at 62%~80%.

9. The hot-dip galvanizing production method of DP500 grade duplex steel automotive outer panels according to claim 4, characterized in that, The method further includes the following steps: Clean the cold-hardened roll; The cleaned steel strip is then hot-dip galvanized, with the production line speed controlled at 90~120m / min, the zinc pot temperature at 457~463℃, and the aluminum content in the zinc pot at 0.20%~0.25% by mass. The galvanized steel strip is then finished with a finishing elongation of 0.4% to 0.7%.

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