Surface strengthening process of air knife for hot galvanizing production line

Abstract

The invention discloses a surface strengthening process of an air knife for a hot galvanizing production line, which adopts a physical vapor deposition (PVD) method to deposit a TiN coating on the surface of the air knife, and comprises the following steps: (1) pretreating the air knife; (2) carrying out overall ultrasonic oil removal on the air knife; (3) loading a workpiece and vacuumizing; (4) carrying out ion bombardment cleaning on the air knife; (5) the air knife is subjected to magnetron sputtering deposition of a TiN coating, wherein the deposition comprises deposition of a Ti bonding layer, deposition of a TiN transition layer and deposition of a TiN functional layer; and (6) the air knife is taken out for use after being subjected to vacuum cooling in the PVD furnace. The TiN coating is deposited through the PVD technology to replace electroplating of hard chromium, and the performance requirements of non-zinc adhesion, high abrasion resistance, high corrosion resistance and high bonding strength of the whole air knife, especially the knife edge and the peripheral surface of the air knife are met.

Description

Technical Field

[0001] This invention belongs to the field of surface strengthening technology for hot-dip galvanized sheet production equipment in the metallurgical industry, specifically involving a new surface treatment process using an air knife on a hot-dip galvanizing production line. Background Technology

[0002] In continuous hot-dip galvanizing production lines for strip steel, the air knife is the core equipment for controlling the thickness of the galvanized layer and ensuring the surface quality of the galvanized steel sheet. It removes excess molten zinc-aluminum alloy from the strip surface by blowing flat airflow, resulting in a uniform zinc layer thickness. The surface performance of the air knife directly affects its operational stability and the quality of the galvanized product. Currently, the industry commonly uses electroplating hard chrome to strengthen the surface of the air knife, improving its wear resistance and corrosion resistance.

[0003] However, the electroplating hard chrome process has several significant drawbacks in the application of hot-dip galvanizing air knives: First, molten zinc droplets easily adhere to the surface of the electroplated hard chrome coating. This adhesion alters the gap size between the upper and lower air knives, disrupting the airflow velocity and flow field distribution. Consequently, it leaves zinc particles and zinc scale on the galvanized steel sheet surface, reducing the surface quality of the galvanized sheet and even leading to product downgrading. Second, the electroplated hard chrome coating contains microcracks, allowing molten zinc to easily penetrate into the air knife body through these cracks. This results in insufficient resistance to molten zinc corrosion and limited wear resistance, leading to long-term use... First, the hard chrome plating process is prone to wear and tear, shortening the service life of the air knife. Second, the bonding strength between the hard chrome plating coating and the air knife body is low, and it is easy to peel off under long-term high temperature and airflow impact conditions, affecting the normal operation of the air knife. Third, after the hard chrome plating process, the surface of the air knife needs to be mechanically ground to meet the surface finish requirements, which increases the production process and cost, and the surface finish after grinding is still difficult to meet the requirements for low zinc viscosity. Fourth, the hexavalent chromium produced by the traditional hard chrome plating process is toxic and does not meet environmental protection requirements. It is subject to RoHS, REACH and other standards, and the environmental pressure is increasing.

[0004] The existing hot-dip galvanizing air knife surface electroplating hard chrome process suffers from drawbacks such as zinc adhesion, poor corrosion and wear resistance, low coating adhesion strength, cumbersome subsequent processing, and poor environmental performance. There is an urgent need in the industry to develop a new air knife surface strengthening process that can solve the zinc adhesion problem, improve the corrosion and wear resistance and adhesion strength of the coating, simplify the production process, meet environmental requirements, and thus ensure the stable operation of hot-dip galvanizing production lines and improve the quality of galvanized steel sheets. Summary of the Invention

[0005] The problem this invention aims to solve is to provide a surface strengthening process for air knives used in hot-dip galvanizing production lines. This process uses PVD technology to deposit a TiN coating instead of electroplating hard chrome, achieving the performance requirements of low zinc adhesion, high corrosion resistance, high wear resistance, and high bonding strength on the air knife surface. Simultaneously, this surface strengthening process simplifies the production process, improves the service life of the air knife, and enhances the surface quality of the galvanized steel sheet. Furthermore, this invention is environmentally friendly and pollution-free.

[0006] The present invention relates to a surface strengthening process for air knives used in hot-dip galvanizing production lines, which employs physical vapor deposition (PVD) to deposit a TiN coating on the surface of the air knife.

[0007] The surface strengthening process of the air knife for hot-dip galvanizing production line of the present invention includes the following steps: (1) Pre-treatment of the air knife: the air knife is ground as a whole, and the surface roughness is 0.8μm; the air knife cutting edge and its surrounding surface are mechanically polished to make the local surface roughness 0.4μm; (2) The air knife is ultrasonically degreased as a whole; (3) The workpiece is loaded and vacuumed; the air knife is vertically hoisted into the PVD furnace along the length direction and fixed, and the PVD furnace is vacuumed; (4) The air knife is ion bombarded and cleaned; (5) The air knife is magnetron sputtered to deposit a TiN coating: including deposited Ti bonding layer, deposited TiN transition layer, and deposited TiN functional layer; (6) The air knife is taken out for use after vacuum cooling in the PVD furnace.

[0008] In step (2): Ultrasonic degreasing uses an alkaline degreasing agent, is treated at a temperature of 40-60℃ for 15-30 minutes, rinsed with deionized water and then dried.

[0009] In step (4): Ion bombardment cleaning: gas: pure argon (Ar); gas pressure: 0.1-1.0 Pa; bias voltage: -200V to -1000V; time: 5-30 minutes; workpiece temperature: heated to about 300-450°C by bombardment.

[0010] In step (5): depositing Ti adhesive layer: target material: high purity Ti target (99.9%+); gas: pure Ar; gas pressure: 0.5-2.0 Pa; workpiece bias voltage: -50 V to -150 V; deposition temperature: 300-450 °C; layer thickness: 0.1-0.5 μm.

[0011] In step (5): depositing TiN transition layer: target material: high purity Ti target (99.9%+); gas: N2; gas pressure: 1.0 -3.0 Pa; workpiece bias voltage: -80V to -200V; deposition temperature: 300 - 450°C; layer thickness: 0.2-1.0μm.

[0012] In step (5): depositing TiN functional layer: target material: high purity Ti target (99.9%+); gas: N2; gas pressure: 1.5 - 4.0 Pa; workpiece bias voltage: -100V to -250V; deposition temperature: 400 - 500°C; layer thickness: 3-5μm.

[0013] Steps (4) and (5): are carried out in a vacuum environment; the focus of ion bombardment and PVD deposition coating is on the gas knife edge and its surrounding surface, that is, the direction of sputtering of the gas knife edge and its surrounding surface toward the target material; the bias voltage can be DC or pulsed DC.

[0014] The TiN coating has a microhardness ≥2200HV, a critical load Lc ≥50N for bonding strength with the air knife body, and a surface roughness Ra ≤0.2μm.

[0015] The surface strengthening process of this invention has the following significant beneficial effects: 1. Completely solves the zinc adhesion problem: The PVD TiN coating has a high surface finish (Ra≤0.2μm), which is significantly better than the electroplated hard chrome surface processed by mechanical grinding. This greatly reduces the adhesion of the air knife surface to molten zinc droplets, avoids changes in the air knife gap caused by zinc adhesion, reduces defects such as zinc particles and zinc scale on the galvanized steel sheet surface, improves the surface quality of the galvanized sheet, and reduces the product downgrade rate; 2. Significantly improved corrosion and wear resistance: The microhardness of the TiN coating is ≥2300HV, which is much higher than that of the electroplated hard chrome coating, and the wear resistance is improved by 3-10 times; at the same time, the TiN coating has a dense structure, which can effectively block the contact between the molten zinc liquid and the air knife body. Its resistance to molten zinc liquid corrosion is better than that of electroplated hard chrome, extending the service life of the air knife; 3. High coating bonding strength: Through pretreatment and optimized PVD process parameters, the bonding strength between the TiN coating and the air knife body is much higher than that of the electroplated hard chrome coating. Therefore, the coating is not easy to peel off and can adapt to the harsh working conditions of high temperature and airflow impact in hot-dip galvanizing production lines; 4. 5. Simplified production process and reduced costs: After PVD TiN coating deposition, no machining is required, directly meeting the surface finish requirements for air knife use, reducing production steps and shortening the processing cycle; at the same time, the coating has a long service life, reducing the frequency of air knife replacement and maintenance, further reducing production costs; 6. Environmentally friendly and pollution-free: The PVD process does not emit toxic or harmful substances, and the TiN coating does not contain hexavalent chromium, complying with environmental standards such as RoHS and REACH, avoiding the environmental pollution problems of traditional electroplating hard chrome processes, and responding to the call for green production.

[0016] In summary, this invention achieves the performance requirements of non-stick zinc, high wear resistance, high corrosion resistance, and high bonding strength of the air knife as a whole, especially the air knife cutting edge and its surrounding surface, by replacing electroplated hard chrome with PVD technology to deposit a TiN coating. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the blade structure on the air knife of the present invention.

[0018] Figure 2 This is a schematic diagram of the structure of the lower blade of the air knife of the present invention.

[0019] Figure 3 This is a side view of the upper and lower blades of the air knife of the present invention in operation.

[0020] Figure 4 This is a SEM image of the cross-section of the PVD TiN coating of the air knife of the present invention. Detailed Implementation

[0021] The following describes the air knife (with the upper and lower blades structured as follows) actually used on a hot-dip galvanizing production line for a slender shape (up to 2m in length) made of stainless steel. Figure 1 and Figure 2 As shown in the figure, a TiN coating is deposited using a PVD process. The specific steps are as follows: 1. Air knife pretreatment: First, the entire stainless steel air knife is ground to a surface roughness of 0.8μm; then the cutting edge and its surrounding surfaces are pretreated (see...). Figure 3 Mechanical polishing is performed to reduce the surface roughness from the original 0.8μm to Ra=0.4μm in order to remove surface scratches and impurities.

[0022] 2. Use an alkaline degreasing agent for ultrasonic degreasing at 45℃ for 25 minutes. Rinse with deionized water and blow dry to remove oil stains from the air knife surface.

[0023] 3. Workpiece loading and vacuuming: Since the air knife is long and thin, reaching 2m in length, and the working temperature during PVD deposition of TiN is 300~500℃, in order to prevent the air knife from deforming due to heat during PVD treatment, it is necessary to suspend and fix the air knife in the vacuum chamber inside the PVD furnace (i.e., the air knife is vertically hoisted in along its length, and its upper part is connected to the furnace cover to form a suspension). The PVD furnace is then evacuated to prepare for the following ion bombardment cleaning and deposition processes.

[0024] 4. Ion bombardment: Performed under vacuum: Gas: pure argon (Ar); Pressure: 0.50 Pa; Bias voltage: -600V; Time: 18 minutes; Workpiece temperature: 375°C. Ion bombardment can remove the surface oxide layer, activate the air knife surface, and improve the adhesion of subsequent coatings. Note: Higher voltage results in stronger bombardment; the bombardment time depends on the workpiece material and cleanliness requirements.

[0025] 5. PVD Deposition of Ti Bond Layer: Target: High-purity Ti target (99.9%+); Gas: Pure Ar; Gas Pressure: 1.2 Pa; Workpiece Bias Voltage: -100V; Deposition Temperature: 380°C; Layer Thickness: 0.3 μm. At this stage, the workpiece bias voltage is relatively low to avoid excessive bombardment leading to re-sputtering; the layer thickness is typically very thin.

[0026] 6. PVD deposition of TiN transition layer: Target material: high purity Ti target (99.9%+); Gas: N2; Gas pressure: 2.0 Pa; Workpiece bias voltage: -140V; Deposition temperature: 380°C; Layer thickness: 0.6μm.

[0027] 7. PVD deposition of TiN functional layer: Target material: high purity Ti target (99.9%+); Gas: N2; Gas pressure: 2.8Pa; Workpiece bias voltage: -170V; Deposition temperature: 450°C; Layer thickness: 4μm.

[0028] Steps 4-7: 1. All are performed in a vacuum environment; 2. Although ion bombardment and PVD deposition coating are applied to the entire air knife, the focus is on the air knife cutting edge and its surrounding surface, i.e., the air knife cutting edge and its surrounding surface facing the target sputtering direction. From Figure 3 It can be seen that: "the cutting edge of the air knife and its surrounding surface" includes three parts: ① the cutting edge of the upper (lower) blade, ② the inner surface that forms a straight slit connected to the cutting edge of the upper (lower) blade, and ③ the outer arc surface connected to the cutting edge of the upper (lower) blade; 3. The bias voltage can be DC or pulsed DC.

[0029] 8. After deposition is completed, maintain the vacuum chamber environment and allow the air knife to cool naturally to room temperature. After cooling, open the vacuum chamber and remove the air knife. No additional machining is required; it can be directly put into use on the hot-dip galvanizing production line.

[0030] The performance of the enhanced air knife was tested, and the results are as follows: the microhardness of the TiN coating is 2300 HV, the critical load for bonding strength is 58 N, and the surface roughness Ra near the air knife cutting edge is 0.2 μm (higher than the surface roughness after mechanical polishing). Figure 4 It can be seen that the air knife after being strengthened by the present invention has a dense structure and very high hardness (cleavage plane).

[0031] Therefore, the wear resistance of the air knife of this invention is 3 times higher than that of electroplated hard chrome, and it has excellent resistance to corrosion by molten zinc. After 4 months of use, there are no problems with coating peeling or zinc adhesion, and the surface quality of the galvanized steel plate is stable.

[0032] Among them, the core parameters of the PVD TiN coating deposition in this invention can be flexibly adjusted according to the material of the air knife body and production requirements to ensure stable coating performance, and the thickness of the TiN coating is 4.5-5.5μm.

[0033] In summary, the TiN coating prepared by this invention has a significantly higher surface finish than mechanically ground surfaces, effectively reducing the adhesion of molten zinc droplets to the air knife surface and avoiding surface defects in galvanized steel sheets caused by changes in the air knife gap. Furthermore, the TiN coating outperforms electroplated hard chrome coatings in terms of resistance to molten zinc corrosion, wear resistance, and bonding strength with the air knife body. Moreover, it requires no machining after deposition, simplifying the production process, improving the service life of the air knife and the surface quality of the galvanized steel sheet, and thus possesses significant industrial application value.

Claims

1. A surface strengthening process using an air knife in a hot-dip galvanizing production line, characterized in that: A TiN coating was deposited on the surface of an air knife using physical vapor deposition (PVD).

2. The surface strengthening process according to claim 1, characterized in that: It includes the following steps: (1) Pre-treatment of the air knife: The air knife is ground as a whole, and the surface roughness is 0.8μm; the air knife cutting edge and its surrounding surface are mechanically polished to make the surface roughness of the local area 0.4μm; (2) The air knife is ultrasonically degreased as a whole; (3) The workpiece is loaded and vacuumed; (4) The air knife is ion bombarded and cleaned; (5) The air knife is magnetron sputtered to deposit a TiN coating: including depositing a Ti bonding layer, depositing a TiN transition layer, and depositing a TiN functional layer; (6) The air knife is vacuum cooled in the PVD furnace and then taken out for use.

3. The surface strengthening process according to claim 2, characterized in that: In step (2): Ultrasonic degreasing uses an alkaline degreasing agent, is treated at a temperature of 40-60℃ for 15-30 minutes, rinsed with deionized water and then dried.

4. The surface strengthening process according to claim 2, characterized in that: In step (4): Ion bombardment cleaning: Gas: pure argon (Ar); Air pressure: 0.1-1.0Pa; bias voltage: -200V to -1000V; time: 5-30 minutes; workpiece temperature: heated to approximately 300-450°C by bombardment.

5. The surface strengthening process according to claim 2, characterized in that: In step (5): deposited Ti adhesive layer: target material: high purity Ti target (99.9%+); Gas: Pure Ar; Gas pressure: 0.5-2.0 Pa; Workpiece bias voltage: -50V to -150V; Deposition temperature: 300-450°C; Layer thickness: 0.1-0.5 μm.

6. The surface strengthening process according to claim 2, characterized in that: In step (5): depositing the TiN transition layer: target material: high purity Ti target (99.9%+); gas: N2; gas pressure: 1.0 - 3.0 Pa; workpiece bias voltage: -80V to -200V; deposition temperature: 300 - 450°C; Layer thickness: 0.2-1.0μm.

7. The surface strengthening process according to claim 2, characterized in that: In step (5): Deposition of TiN functional layer: Target material: High purity Ti target (99.9%+); Gas: N2; Gas pressure: 1.5 - 4.0 Pa; Workpiece bias voltage: -100V to -250V; Deposition temperature: 400 - 500°C; Layer thickness: 3-5μm.

8. The surface strengthening process according to claim 2, characterized in that: Steps (4) and (5): are carried out in a vacuum environment; the focus of ion bombardment and PVD deposition coating is on the air knife edge and its surrounding surface, that is, the air knife edge and its surrounding surface in the direction of sputtering towards the target material. The bias voltage can be DC or pulsed DC.

9. The surface strengthening process according to claim 1, characterized in that: The TiN coating has a microhardness ≥2200HV, a critical load Lc ≥50N for bonding strength with the air knife body, and a surface roughness Ra ≤0.2μm.

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