Method for producing high-quality surface of galvanized automobile sheet in high humidity and heat environment

Abstract

The application discloses a production method of a high-quality surface of a galvanized automobile plate in a high-humid and hot environment, and when the relative humidity of the environment is greater than or equal to 70%, the strip steel is subjected to segmented and zoned gradient drying before being wound after being plated, a front drying section, a gradient main drying section and a uniform temperature drying section are sequentially arranged along the running direction of the strip steel, the gradient main drying section is divided into a middle zone and double-side edge zones according to the width of the strip steel, the temperature of each drying section is closed-loop linked with the running speed of the strip steel, the edge condensation and uneven spreading of the oil-water mixed film are effectively inhibited, the edge wave oil spot defects are basically eliminated under the high-humid and hot working condition, and the cutting edge is not needed to be judged, the yield is significantly improved while the performance of the galvanized layer and the oil film is ensured, and the production method is suitable for application of various hot-dip galvanized automobile plate production lines.

Description

Technical Field

[0001] This invention belongs to the technical field of galvanized automotive sheet production process, specifically relating to a method for producing a high-quality surface of galvanized automotive sheet under high humidity and heat conditions. Background Technology

[0002] Hot-dip galvanized automotive steel sheets are core raw materials for automobile bodies and structural components. They require extremely high surface quality, color uniformity, and appearance precision. The absence of any oil spots, color differences, or texture defects is a core indicator for qualified automotive steel sheets to leave the factory. Currently, all domestic automotive steel sheet manufacturers face a common technical challenge: every summer under high humidity and heat conditions, finished galvanized automotive steel sheets develop wavy oil spot defects on the edges of the strips in batches.

[0003] This defect is concentrated on both sides of the strip, exhibiting a regular wavy pattern. The surface color of the defective area is significantly darker than that of the normal surface, and its appearance is highly similar to oil stains. Despite long-term process exploration, equipment debugging, and raw material optimization, this seasonal defect has remained unresolved for many years, becoming a common and persistent problem in the industry. There is no effective prevention or repair process for this defect; currently, the only recourse is to subsequently cut off the defective edges in strips and downgrade the product. This not only significantly reduces the yield of galvanized automotive steel sheets and increases the cost of slitting processes but also results in a large waste of high-quality steel. Statistics show that a single automotive steel sheet production line incurs millions of yuan in re-grading and scrap losses annually due to this summer edge stain defect, severely hindering companies' production cost control and the reputation of high-end automotive steel sheet quality. A targeted process method is urgently needed to completely solve this seasonal production defect problem.

[0004] Extensive experimental analysis revealed that the oil stains were caused by high moisture content in the air. After galvanizing and oiling, the surface temperature of the steel strip differed from the ambient temperature, making it easy for dew to adhere to the steel surface, especially at the edges where heat dissipation is faster and condensation is more likely. This formed a small amount of condensate film, which mixed with the subsequent anti-rust oil to create an uneven oil-water mixture film, ultimately leading to edge corrosion. Since galvanized automotive steel sheets do not require drying after galvanizing, an attempt was made to improve the process by introducing a constant-temperature drying process for galvanized appliance sheets. However, this approach has significant technical limitations: appliance sheets use a single-stage drying mode with uniform temperature and fixed linear speed throughout, failing to consider the product characteristics of galvanized automotive steel sheets. Firstly, automotive steel sheets have thin galvanized layers and require stringent surface precision; full-area high-temperature drying easily leads to oxidation of the galvanized layer and aging of the anti-rust oil, damaging the sheet's appearance and corrosion resistance. Secondly, the heat dissipation rate at the edges of wide-width galvanized automotive steel sheets is much higher than in the center; single-area drying cannot solve the problem of preferential condensation at the edges. Thirdly, the operating speed of automotive steel sheet production lines fluctuates frequently; the mismatch between fixed heating parameters and dynamic linear speed easily leads to incomplete dehydration or localized overheating. Meanwhile, traditional coiling tension is divided into fixed parameter ranges based solely on steel grade, without dynamically adapting to ambient humidity, residual moisture on the strip surface, and production line operating speed. Even with a simple addition of a drying process, oil stain defects caused by uneven coupling of moisture and tension cannot be completely eliminated.

[0005] In summary, traditional galvanized automotive steel sheet production lacks post-galvanizing drying, directly transplanting the drying process from appliance steel sheets and employing static, fixed tension parameters. This approach fails to consider the specific working conditions and defect formation mechanisms of galvanized automotive steel sheets, representing a flawed technological path. This invention breaks with existing industry biases, abandoning the simple application of mature processes from other steel sheets. By combining the strip temperature field distribution, moisture diffusion patterns, and coiling mechanical properties, it develops a proprietary, multi-process collaborative production process, fundamentally solving the problem of wavy oil stains on the edges of galvanized automotive steel sheets under high humidity and heat conditions. Summary of the Invention

[0006] The present invention aims to solve the technical problem of wavy oil spot defects on the edges of galvanized automotive steel sheets under high humidity and heat conditions in the prior art, and to provide a production method for high-quality surface of galvanized automotive steel sheets under high humidity and heat conditions that eliminates defects and reduces costs.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for producing high-quality surfaces of galvanized automotive steel sheets under high humidity and heat conditions involves selecting a drying mode based on the relative humidity of the environment after the galvanizing process and before coiling. When the relative humidity is ≥70% (typically from May to September), the strip steel is dried in sections and zones using a gradient drying process. Along the strip steel's running direction, a pre-drying section, a gradient main drying section, and a uniform temperature drying section are sequentially set up. The pre-drying section has a heating rate of 2~5℃ / s, slowly increasing the overall surface temperature of the strip steel to reduce the temperature difference between the plate surface and the ambient dew point, suppressing premature condensation. The temperature of the pre-drying section is controlled at 60±5℃. The gradient main drying section is divided into a central zone and two side zones according to the strip steel width. The temperature control range for the side zones extends 80±20mm inward from the edge. Considering the rapid heat dissipation and severe condensation at the edges, the temperature in the central zone is controlled at 70±5℃, and the temperature on both sides is controlled at 75±5℃. The uniform temperature drying section is used to eliminate the lateral temperature difference remaining after the section drying process, ensuring that the strip steel surface temperature remains stable within a safe range below the oxidation threshold of the galvanized layer and above the ambient dew point after leaving the drying area. Therefore, the temperature of the uniform temperature drying section is controlled at 65±5℃. Meanwhile, the temperature of each drying section is linked to the strip running speed in a closed loop. When the strip running speed is ≤60m / min, the temperature of each section is reduced by 3~5℃; when the strip running speed is >120m / min, the temperature of each section is increased by 3~5℃; when 60m / min < strip running speed ≤120m / min, the temperature of each section is not adjusted and is maintained at the standard parameters.

[0008] To address the shortcomings of traditional fixed tension methods that cannot adapt to high humidity, uneven temperature, and speed fluctuations, this invention establishes a dynamic tension model based on steel strength and considering four dimensions: ambient temperature and humidity, strip running speed, and strip thickness. This model corrects the winding tension in real time and includes a localized edge tension compensation mechanism to ensure consistent winding tightness between the center and edges of the strip, eliminating minute gaps during winding. Details are as follows: During the coiling stage, the coiling tension is adjusted in real time based on the relative humidity of the environment, the running speed of the strip, the strength of the steel grade, and the thickness of the plate. A local reinforcing tension of 1~2MPa is applied to both sides of the strip using an edge micro-tension compensation mechanism to solve the inherent problem of the middle of the wide strip being tight and the edges being loose, and to prevent the oil-water mixture from accumulating and spreading unevenly in the gaps. The standard coiling tension for mild steel and deep-drawing automotive panels (such as IF steel, DX53D, DC56D, etc.) is 20~28MPa. The standard coiling tension for high-strength galvanized automotive structural steel (such as HC340 / 420LA, DP590, DP780) is... The standard coiling tension for high-strength steel grades is 28~40MPa. When the relative humidity is ≥70%, the standard coiling tension is increased by 5%~8%, and when the relative humidity is <70%, the standard coiling tension is used. When the strip running speed is >120m / min, the standard coiling tension is increased by 3%~5%, when the strip running speed is 60m / min <120m / min, the standard coiling tension is used, and when the strip running speed is ≤60m / min, the standard coiling tension is decreased by 2%~3%. When the plate thickness is <0.8mm, the standard coiling tension is increased by 4%~6%, when the plate thickness is 0.8mm ≤1.2mm, the standard coiling tension is used, and when the plate thickness is ≥1.2mm, the standard coiling tension is decreased by 2%~4%.

[0009] When the relative humidity of the environment is less than 70% (usually from October to April of the following year), the strip steel is not dried in sections or zones. Only during the coiling stage, the unit coiling tension is adjusted in real time according to the relative humidity of the environment, the running speed of the strip steel, the strength of the steel grade and the thickness of the plate. A local reinforcing tension of 1~2MPa is applied to both sides of the strip steel by an edge micro-tension compensation mechanism.

[0010] Furthermore, the temperature in the edge zone of the gradient main drying section is always 3-5°C higher than that in the middle zone.

[0011] This invention employs segmented and zoned gradient drying combined with a closed-loop linear velocity linkage to suppress condensation. By offsetting the heat dissipation differences at the edges of wide strip steel through a relative high-temperature gradient in the edge area, the temperature uniformity of the strip steel across its entire width is controlled within ±5℃. This prevents the formation of a non-uniform oil-water mixture film between the rust-preventive oil and condensate from the source. Furthermore, the drying temperature range does not damage the galvanized layer structure, the performance of the rust-preventive oil, or the mechanical properties of the strip steel. The combined effect of dynamic winding tension and edge micro-tension compensation ensures that the winding tightness of the strip steel at the edges and center is consistent, eliminating edge slack gaps and suppressing local accumulation and uneven spreading of the oil-water mixture between layers.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention abandons the conventional technical route commonly used in the industry, which is to dry automotive steel sheets after plating without drying and to dry appliance steel sheets in a single stage with constant temperature and static tension after plating. In response to the unique characteristics of galvanized automotive steel sheets, such as wide structure, easy condensation at the edges under high humidity and heat conditions, and sensitive coating, this invention adopts a segmented and zoned gradient drying + linear velocity closed-loop linkage process, while dynamically adjusting the tension. This solves the drawbacks of traditional processes, such as rust after condensation at the edges and poor adaptability to working conditions.

[0013] 2. This invention employs a synergistic process of segmented and zoned gradient drying under high humidity and heat conditions, linear speed-linked temperature control, and dynamic adjustment of winding tension to accurately address the core causes of defects. The incidence of edge wavy oil spots in high humidity and heat environments is reduced to zero. The product does not require subsequent slitting, edge trimming, or downgrading and re-judgment, and its surface quality fully meets the supply standards for high-end new energy vehicle panels.

[0014] 3. This invention does not require modification of the core equipment of the production line. It only adds a zone heating module, a speed measurement and temperature control linkage system, and a tension fine-tuning module. The modification cost is low, the manual operation is simple, and the process is highly controllable. It can be quickly and scalably applied to all hot-dip galvanized automotive sheet production lines. A single production line can reduce waste, rework, and product downgrade losses by millions of yuan per year, and significantly improve the yield and economic benefits of enterprises.

[0015] 4. All temperature and tension ranges of this invention have been verified through batch tests of multiple steel types and specifications, and are strictly controlled within the safe threshold range. This will not damage the structure of the galvanized layer or the performance of the anti-rust oil film, nor will it change the mechanical properties and dimensional accuracy of the strip steel, resulting in superior overall product quality. Attached Figure Description

[0016] Figure 1 This is a diagram showing the experimental results of Comparative Example 1. Detailed Implementation

[0017] The present invention will be further described below with reference to specific embodiments.

[0018] Example 1: Production of Deep-Drawing Galvanized Automotive Panels in High Humidity and Heat During Summer Steel grade produced: DC56D deep-drawing galvanized automotive outer sheet Product specifications: 0.7mm × 1500mm Production period: June (summer high humidity period) Environmental parameters: Relative humidity in the workshop: 82% Production line operating conditions: Average strip speed 90m / min (normal speed range) Process execution: Drying process: The three-section zone gradient heating + speed linkage mode is activated. The heating rate of the pre-drying section is 2℃ / s, the temperature of the pre-drying section is 60℃, the temperature of the middle zone of the gradient main drying section is 70℃, the temperature of the edge zone is 75℃, and the temperature of the uniform temperature drying section is 68℃. The operating speed of 90m / min is within the normal range, and no additional temperature correction is made.

[0019] Tension process: The coiling tension benchmark value of 24MPa is adopted for mild steel, and a comprehensive correction is made by adding 6% for high humidity conditions and 5% for thin sheet metal. The actual coiling tension value is 26.712MPa. A local reinforcement tension of 1.5MPa is added to the edge of the strip, and the comprehensive edge tension is 28.212MPa.

[0020] Production results: The strip steel was free of condensation film, the steel coil was tightly wound without gaps, the finished product had no defects such as edge wavy oil spots, color difference, dark spots, etc., the plate surface had uniform color, all products were qualified, and no re-judgment or waste was generated.

[0021] Example 2: High-speed production of high-strength galvanized automotive structural steel in summer Steel grade produced: DP780 deep-drawing galvanized automotive outer sheet Product specifications: 1.2mm × 1500mm Production period: July (summer high humidity period) Environmental parameters: Relative humidity in the workshop: 78% Production line operating conditions: Average strip speed 135m / min (high-speed operation) Process execution: Drying process: The three-section zone gradient heating + speed linkage mode is activated. Due to the high-speed operation (running speed 135m / min), the temperature of each module is increased by 4℃. The temperature of the pre-drying section is 64℃, the temperature of the middle zone of the gradient main drying section is 74℃, the temperature of the edge zone is 79℃, and the temperature of the uniform temperature drying section is 72℃.

[0022] Tension process: The coiling tension benchmark value of 35MPa is adopted for high-strength steel. Combined with the 5% increase for high humidity conditions and the 3% decrease for thin-gauge plates, the actual coiling tension value is 37.0734MPa. A local reinforcement tension of 1.2MPa is added to the edge of the strip, and the comprehensive edge tension is 38.2734MPa.

[0023] Production results: The temperature of the strip is uniform in the middle and at the edges, the residual moisture on the plate surface is completely removed, the steel coil is tightly wound as a whole, and there are no defects such as wavy oil spots; the galvanized layer, the anti-rust oil film, and the mechanical properties of the strip all meet the standards and fully meet the requirements for use in high-end automotive structural components.

[0024] Comparative Example 1: Traditional Process Summer Production The same steel type, specifications, raw materials, and production line as in Example 1 were selected, but drying was not performed, and the fixed winding tension was used.

[0025] Production result: Finished batches showed defects such as wavy oil spots on the edges (e.g.) Figure 1 As shown in the figure, with a defect rate of over 80%, all products need to be slit and trimmed or downgraded for re-judgment, resulting in a significant increase in material loss and processing costs.

[0026] Comparative Example 2: Copying the appliance panel drying process Using the same raw materials and production line as in Example 1, the process employs a single-section whole-area constant temperature drying method (uniform 70℃ throughout) for general-purpose household appliance plate plating, coupled with a fixed benchmark winding tension, without zoned temperature control, wireless speed linkage, or dynamic tension correction.

[0027] Production results: The dehydration effect in the middle of the strip is good, but there is still a small amount of condensation at the edge of the wide plate due to the faster heat dissipation. The local oil spot defect rate is about 25%. During high-speed operation, the plate surface experienced local overheating, and the anti-rust oil aged slightly, resulting in appearance defects. This proves that simply transplanting the process of home appliance plates cannot completely solve the seasonal defect problem of galvanized automotive plates.

[0028] The above description is merely 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 method for producing a high-quality surface finish on galvanized automotive steel sheets under high humidity and heat conditions, characterized in that, Includes the following steps: After the galvanizing process and before coiling, select the drying mode based on the relative humidity of the environment: When the relative humidity of the environment is ≥70%, the strip steel is dried in sections and zones in a gradient manner. A pre-drying section, a gradient main drying section, and a uniform temperature drying section are set up in sequence along the running direction of the strip steel. The gradient main drying section is divided into a central area and two side areas according to the width of the strip steel. The temperature of the pre-drying section is 60±5℃, the temperature of the central area of ​​the gradient main drying section is 70±5℃, the temperature of the side area is 75±5℃, and the temperature of the uniform temperature drying section is 65±5℃. When the strip running speed is ≤60m / min, the temperature of each section is reduced by 3~5℃; when the strip running speed is >120m / min, the temperature of each section is increased by 3~5℃; when 60m / min < strip running speed ≤120m / min, the temperature of each section is not adjusted.

2. The method for producing a high-quality surface of galvanized automotive steel sheet under high humidity and heat conditions according to claim 1, characterized in that: Also includes: During the coiling stage, the coiling tension is adjusted in real time based on the relative humidity of the environment, the running speed of the strip, the strength of the steel grade, and the thickness of the plate. Specifically: the standard coiling tension for mild steel and deep-drawn automotive panels is 20~28MPa, and the standard coiling tension for high-strength galvanized automotive structural steel is 28~40MPa; when the relative humidity is ≥70%, the standard coiling tension is increased by 5%~8%, and when the relative humidity is <70%, the standard coiling tension is used; when the strip running speed is >120m / min, the standard coiling tension is increased by 3%~5%, when the strip running speed is 60m / min <120m / min, the standard coiling tension is used, and when the strip running speed is ≤60m / min, the standard coiling tension is decreased by 2%~3%; when the plate thickness is <0.8mm, the standard coiling tension is increased by 4%~6%, when the plate thickness is 0.8mm ≤1.2mm, the standard coiling tension is used, and when the plate thickness is ≥1.2mm, the standard coiling tension is decreased by 2%~4%.

3. The method for producing a high-quality surface of galvanized automotive steel sheet under high humidity and heat conditions according to claim 1, characterized in that: The heating rate of the pre-drying section is 2~5℃ / s.

4. The method for producing a high-quality surface of galvanized automotive steel sheet under high humidity and heat conditions according to claim 1, characterized in that: The temperature control range of the edge area extends inward from the edge of the plate by 80±20mm.

5. The method for producing a high-quality surface of galvanized automotive steel sheet under high humidity and heat conditions according to claim 1, characterized in that: The temperature in the edge zone of the gradient main drying section is always 3-5°C higher than that in the middle zone.

6. The method for producing a high-quality surface of galvanized automotive steel sheet under high humidity and heat conditions according to claim 1, characterized in that: During the winding stage, a local reinforcing tension of 1-2 MPa is applied to both sides of the strip.

7. The method for producing a high-quality surface of galvanized automotive steel sheet under high humidity and heat conditions according to claim 1, characterized in that: When the relative humidity of the environment is less than 70%, the strip steel is not subjected to segmented and zoned gradient drying.

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