Film coating treatment process for hot-dip galvanized steel for automobiles

Abstract

The invention relates to a thin film coating treatment process for hot-dip galvanized steel for an automobile. The thin film coating treatment process comprises the following steps: S1, fine pretreatment: degreasing and washing a hot-dip galvanized steel plate; s2, preparing a silane treatment solution; s3, film forming treatment, wherein the hot-dip galvanized steel plate treated in the step S1 is soaked in the silane treatment liquid prepared in the step S2, and then the hot-dip galvanized steel plate is taken out and drained; and S4, carrying out controllable gradient drying and curing. The invention aims to replace the traditional phosphating process and effectively overcome the defects of insufficient corrosion resistance, narrow process window, poor mechanical property of the film layer and the like in the existing film treatment technology.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of automobile panel surface treatment, in particular to a hot-dip galvanized steel coating film treatment process for automobiles. BACKGROUND

[0002] In the field of automobile panel coating, phosphating treatment technology has been difficult to meet the increasingly stringent requirements of green manufacturing and high-quality appearance due to its inherent environmental and performance bottlenecks.

[0003] Silane treatment technology, as an environmentally friendly alternative, has the advantages of being phosphorus-free, heavy metal-free, having less sediment, and being operated at room temperature. However, when applying silane technology to hot-dip galvanized automobile panels, especially in large-scale industrial production, the following technical pain points need to be addressed: (1) insufficient corrosion resistance of the film layer: single silane film is thin and mainly relies on physical barrier effect, and its long-term salt spray corrosion resistance, especially in harsh conditions, is still inferior to that of traditional phosphating film; (2) narrow process window: silane treatment solution is extremely sensitive to pH value, conductivity and other parameters, and the control of bath stability is difficult, which is prone to silane self-condensation and invalidation, affecting the continuous and stable operation of the production line; (3) weak mechanical properties of the film layer: pure silane film has low hardness, thickness and wear resistance, and is easily scratched during the pretreatment process before conveying and electrophoresis, resulting in hidden defects.

[0004] Therefore, it is crucial to develop a stable silane composite film process that can form a high-corrosion-resistant, strongly adhered, and wide-process-window film for hot-dip galvanized steel panels, which is essential for upgrading the automobile panel coating technology. SUMMARY

[0005] To solve the above problems, the present application provides a hot-dip galvanized steel coating film treatment process for automobiles, which aims to replace the traditional phosphating process and effectively overcome the defects of insufficient corrosion resistance, narrow process window, and poor mechanical properties of the film layer in existing film treatment technologies.

[0006] The technical solution adopted by the present application is as follows: The hot-dip galvanized steel coating film treatment process for automobiles proposed by the present application comprises the following steps: S1, fine pretreatment: degreasing and washing the hot-dip galvanized steel panel; S2, preparing a silane treatment solution; S3, film forming treatment: immersing the hot-dip galvanized steel panel treated in step S1 in the silane treatment solution prepared in step S2, and then removing the liquid; S4, controllable gradient drying and curing.

[0007] Further, in the step S1, the degreasing uses a strong alkaline degreasing agent, the temperature is controlled at 40±2℃, the treatment time is 2-4 minutes, the contact angle is ensured to be less than 10°, a completely hydrophilic clean surface is formed, and then two-stage countercurrent rinsing is performed.

[0008] Further, the step S2 comprises: S2.1, adding deionized water to 80% in the configuration tank; S2.2, adding PSL-8013AM chemical conversion agent, and then adding PSL-8013B agent after uniform stirring; S2.3, adjusting to pH=3.5 by using NT-4055 neutralizing agent diluted by 2-4 times; S2.4, adding deionized water again to the working liquid level, and fully stirring; S2.5, warming to the treatment temperature; S2.6, fine-tuning to the specified pH, and determining the zirconium ion and fluorine ion concentrations.

[0009] Further, in the step S3, the temperature of the silane treatment liquid is controlled at 25-30℃, and the immersion time is 2.5-3.5 minutes.

[0010] Further, the step S4 comprises: drying and curing the hot-dip galvanized steel plate after film forming, and the process is divided into two stages of pre-curing and post-curing.

[0011] Further, the pre-curing stage comprises: under the condition of 60-80℃, hot air circulation drying for 3-5 minutes, so that the silane is preliminarily crosslinked and most of the free water is removed; Further, the post-curing stage: under the condition of 100-120℃, processing for 14-16 minutes, which promotes the full crosslinking of the acrylate-urea-formaldehyde resin and the silane network to form a dense silane film, and the final film thickness is controlled to be below 20 microns.

[0012] Compared with the prior art, the present application has the following beneficial effects: The hot-dip galvanized steel coating film treatment process for automobiles provided by the present application aims to replace the traditional phosphating process and effectively overcome the defects of the existing film treatment technology, such as insufficient corrosion resistance, narrow process window, and poor mechanical properties of the film layer. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 The flowchart of the hot-dip galvanized steel coating film treatment process for automobiles provided by the present application is shown. DETAILED DESCRIPTION

[0014] In order to make the technical solutions in the embodiments of the present application or the prior art clearer, the accompanying drawings needed in the embodiments or prior art description will be briefly introduced. Obviously, the accompanying drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0015] Referring to the accompanying drawings Figure 1 The present application provides a hot-dip galvanized steel coating film treatment process for automobiles, comprising the following steps: Step S1, fine pretreatment; The hot-dip galvanized steel plate is subjected to degreasing and water washing. The degreasing uses a strong alkaline degreasing agent, the temperature is controlled at 40±2℃, the treatment time is 2-4 minutes, the contact angle is ensured to be less than 10°, and a completely hydrophilic clean surface is formed; then two-stage countercurrent rinsing is performed.

[0016] Step S2, preparation of silane treatment liquid (Paka reagent); The silane composite treatment liquid is prepared from the following components according to the concentration, and the solvent is deionized water. The preparation process is as follows: S2.1, add 80% of deionized water into the preparation tank; S2.2, add PSL-8013AM complexing agent, and then add PSL-8013B agent after uniform stirring; S2.3, adjust to pH=3.5 with diluted 2-4 times NT-4055 neutralizing agent; S2.4, add deionized water again to the working liquid level, and stir well; S2.5, warm up to the treatment temperature; S2.6, fine adjustment to the specified pH, and determine the zirconium ion and fluorine ion concentrations.

[0017] Step S3, film forming treatment; The hot-dip galvanized steel plate pretreated in step S1 is immersed in the silane treatment liquid prepared in step S2, and the temperature is controlled at 25-30℃ (room temperature), and the immersion time is 2.5-3.5 minutes. Then it is taken out and drained in a specific way.

[0018] Step S4, controllable gradient drying and post-curing; The hot-dip galvanized steel plate after film forming is subjected to drying and curing, which is divided into two stages: First stage (pre-curing): at a low temperature of 60-80℃, hot air circulation drying for 3-5 minutes, so that the silane is preliminarily crosslinked and most of the free water is removed.

[0019] Second stage (post-curing): 100-120℃ for 14-16 minutes, this stage promotes the full crosslinking of the acrylate-urea-formaldehyde resin with the silane network, forming a dense silane film, with a final film thickness controlled below 20 microns.

[0020] The present application solves the pain points of the prior art from both chemical and physical structure aspects through the synergistic design of the whole process of "pretreatment-film formation-curing", and the specific principles are as follows: Corrosion resistance is improved: Pretreatment stage: The strong alkaline degreasing agent can efficiently remove the rolling oil, oxide layer and contaminants on the surface of the hot galvanized steel sheet at 40±2℃, and the hydrophilic surface with a contact angle <10° ensures uniform adsorption of silane molecules; two-stage countercurrent rinsing avoids adverse reactions between the degreasing agent residues and silane, laying a clean substrate for film formation; Film formation stage: The hydrolysis of γ-aminopropyl triethoxysilane in the silane treatment solution generates Si-OH, which reacts with Zn-OH on the surface of the hot galvanized layer to form Si-O-Zn bonds, achieving strong interfacial bonding between the film layer and the substrate; Zr 4 + reacts with Zn²+ to form zinc zirconate, which has high chemical stability and can block the penetration path of corrosion media (such as Cl - , H2O) to the substrate; Curing stage: Gradient drying first removes free water at low temperature, and then promotes silane-resin crosslinking at high temperature to form a "Si-O-Si-Si-O-C-resin" three-dimensional dense network with a film layer porosity <1%, significantly reducing the corrosion medium penetration rate; at the same time, the crosslinked structure improves the adhesion between the film layer and the subsequent electrophoretic coating, avoiding coating blistering.

[0021] Process window is widened: The pH of the silane treatment solution is controlled at 3.5±0.1, which is the best interval for silane hydrolysis (generating Si-OH) and avoiding self-condensation: too low pH will inhibit silane hydrolysis, and too high pH will cause Si-OH to rapidly self-condense to form a precipitate; By monitoring the concentrations of zirconium ions (80-120mg / L) and fluoride ions (50-80mg / L) in real time: too low zirconium ions will result in insufficient interfacial bonding layer, and too high zirconium ions will easily form zirconium salt precipitates; fluoride ions can activate the surface of the hot galvanized layer (slightly etching to form a micro-rough surface, enhancing silane adsorption), and uncontrolled concentration will cause excessive corrosion of the substrate; The pretreatment and film formation temperatures are coordinated (degreasing at 40±2℃ and film formation at 30-40℃), avoiding temperature fluctuations that cause bath parameters to drift, ensuring stable film layer quality during continuous production line operation.

[0022] Mechanical performance is enhanced: The crosslinking network of the silane-acrylate-urea formaldehyde resin has a higher bond energy, and compared with a pure silane film, the hardness is increased to 2-3H, and the wear resistance is increased by more than 30%; The zinc zirconate interface bonding layer can alleviate the difference in thermal expansion coefficient between the film layer and the substrate, reduce the internal stress of the film layer caused by temperature change, and reduce the risk of scratching; The film thickness is controlled in the range of 0.5-20 mu m, and the density and flexibility are considered: too thick can easily increase the brittleness of the film layer, and too thin cannot form an effective barrier. The thickness interval can meet the mechanical damage protection requirements during the conveying and stamping process of the automobile plate.

[0023] The application will be further described below through specific examples: Example 1 A hot-dip galvanized steel coating film treatment process for automobiles, taking a connecting rod of a certain vehicle as an example, includes the following steps: The substrate is a hot-dip galvanized steel plate with a size of 150mm x 70mm.

[0024] Step S1, fine pretreatment; The substrate is immersed in a strong alkaline degreasing agent at 40°C for 3 minutes, and then two-stage countercurrent rinsing is performed to ensure continuous water film on the surface.

[0025] Step S2, preparation of silane treatment solution; This step aims to prepare a total volume of 1000L of silane treatment solution, and the following sub-steps are sequentially operated to ensure that the reagents are fully dissolved and the tank solution parameters are accurately controllable; S2.1, select a 1000L liquid preparation tank, pour deionized water into the tank until the liquid level reaches 80% of the tank capacity (i.e. 800L), leaving sufficient mixing space for subsequent reagent addition; S2.2, according to the reagent ratio specified in the Paka silane treatment solution instruction manual, first slowly add 20Kg PSL-8013AM into the tank, start the stirring device, and stir until the reagent is completely dissolved and the liquid is uniform and transparent; then add 10Kg PSL-8013B, continue stirring to ensure that the two reagents are fully mixed and avoid local concentration unevenness; S2.3, take the NT-4055 neutralizing agent, first dilute it 2-4 times with deionized water, then slowly add it to the liquid preparation tank, and monitor the pH value of the tank solution in real time with a precision pH meter during the process until the pH value is stable and maintained at 3.5; S2.4, add deionized water to the tank until the tank liquid level reaches the 1000L working liquid level mark, continue stirring for 5-10 minutes to ensure that the overall concentration of the tank solution is uniform and consistent; S2.5, heat the tank solution through the heating device matched with the liquid preparation tank, and control the temperature at 30-40°C; S2.6 Use a precision pH meter to check the pH value of the bath solution again. If it deviates from 3.5, use diluted NT-4055 neutralizing agent to fine-tune it to the specified value. At the same time, check the concentration of zirconium ions and fluoride ions in the bath solution to ensure that the concentration meets the process requirements, laying the foundation for the subsequent film formation quality.

[0026] Step S3: Film formation treatment; The pretreated hot-dip galvanized steel sheet is immersed in silane treatment solution and soaked for 3 minutes at 30°C. Then it is pulled out of the tank at a uniform speed and drained for 10 seconds.

[0027] Step S4: Drying and curing; After draining, the hot-dip galvanized steel sheet is first placed in an 80℃ baking oven and hot air is circulated for 4 minutes; then it is placed in a 110℃ baking oven and treated for 15 minutes. After being taken out of the oven, it is cooled to room temperature.

[0028] Performance testing: The thickness of the conversion film on the surface of the hot-dip galvanized steel sheet was measured to be approximately 500 nm; subsequently, standard cathodic electrophoretic coating was performed. The electrophoretic plate was tested: cross-cut adhesion was grade 0; after 1000 hours of neutral salt spray testing, the width of the single-sided erosion of the scratch was 1.5 mm, with no bubbles.

[0029] All matters not covered in this invention are common knowledge.

[0030] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A coating film treatment process for hot-dip galvanized steel for automobiles, characterized in that, The method includes the following steps: S1. Refined pretreatment: Degreasing and washing the hot-dip galvanized steel sheet; S2. Prepare the silane treatment solution; S3. Film formation treatment: The hot-dip galvanized steel sheet treated in step S1 is immersed in the silane treatment solution prepared in step S2, and then the leachate is removed. S4, Controlled gradient drying and curing.

2. The coating film treatment process for hot-dip galvanized steel for automobiles according to claim 1, characterized in that: In step S1, a strong alkaline degreasing agent is used for degreasing, the temperature is controlled at 40±2℃, the treatment time is 2-4 minutes, and the contact angle is ensured to be less than 10° to form a completely hydrophilic clean surface. Then, two-stage countercurrent rinsing is performed.

3. The coating film treatment process for hot-dip galvanized steel for automobiles according to claim 1, characterized in that: Step S2 includes: S2.1 Add deionized water to 80% of the preparation tank; S2.2 Add PSL-8013AM forming agent, stir thoroughly and then add PSL-8013B agent; S2.3 Adjust the pH to 3.5 using NT-4055 neutralizing agent diluted 2-4 times; S2.

4. Add deionized water again to the working liquid level and stir thoroughly; S2.5, Heat to the processing temperature; S2.6 Fine-tune to the specified pH to determine the concentrations of zirconium ions and fluoride ions.

4. The coating film treatment process for hot-dip galvanized steel for automobiles according to claim 1, characterized in that: In step S3, the temperature of the silane treatment solution is controlled at 25-30°C, and the immersion time is 2.5-3.5 minutes.

5. The coating film treatment process for hot-dip galvanized steel for automobiles according to claim 1, characterized in that: Step S4 includes drying and curing the hot-dip galvanized steel sheet after film formation. This process is divided into two stages: pre-curing and post-curing.

6. The coating film treatment process for hot-dip galvanized steel for automobiles according to claim 5, characterized in that: The pre-curing stage includes: drying with hot air circulation at 60-80℃ for 3-5 minutes to allow the silane to undergo initial cross-linking and remove most of the free water.

7. The coating film treatment process for hot-dip galvanized steel for automobiles according to claim 6, characterized in that: The post-curing stage includes: treating at 100-120°C for 14-16 minutes. This stage promotes the full cross-linking of acrylate-urea-formaldehyde resin with the silane network to form a dense silane film, with the final film thickness controlled below 20 micrometers.

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