Highly corrosion-resistant zinc-aluminum-magnesium hot-dip plated steel sheet and method for manufacturing the same

Abstract

The present application provides a kind of high corrosion resistance zinc-aluminum-magnesium hot-dip coating steel plate and its manufacturing method, the steel plate includes steel plate matrix and zinc-aluminum-magnesium hot-dip coating, coating composition is as follows by mass percent: Al 17~25%, Mg 5~10%, Si 0.1~0.5%, Gd 0.02~0.3%, Sm 0.02~0.2%, the rest is Zn and inevitable impurity element;The above manufacturing method includes: preparing the plating solution containing the above components, heating to 460~500 ℃, the steel plate matrix is immersed to carry out hot-dip coating, then cooling at a rate of not less than 8 ℃ / second.The present application produces synergistic effect by simultaneously adding rare earth elements Gd and Sm, not only can refine MgZn₂ phase in coating, but also can improve the forming property of coating, at the same time, form dense mixed oxide film on the surface of plating solution, inhibit the oxidation of high magnesium plating solution, ensure the process stability, and, the steel plate of the present application has high corrosion resistance, excellent workability and good process adaptability.

Description

Technical Field

[0001] This invention relates to the field of hot-dip galvanizing anti-corrosion technology, specifically to a high corrosion-resistant zinc-aluminum-magnesium hot-dip galvanized steel sheet and its manufacturing method. Background Technology

[0002] Hot-dip galvanizing is an important anti-corrosion process that involves immersing metal workpieces in a molten metal bath to form a protective coating on their surface. With the increasing demands on the service life of building materials, traditional hot-dip pure zinc coatings are no longer sufficient to meet the increasingly stringent corrosion resistance requirements.

[0003] To improve the corrosion resistance of coatings, various zinc-based alloy coatings have been developed in existing technologies. For example, Galfan coating and Galvalume coating improve the corrosion resistance of flat plates by increasing the aluminum content, but sacrifice the sacrificial anode protection capability unique to zinc coatings. In order to balance corrosion resistance and sacrificial anode protection performance, Zn-Al-Mg ternary alloy coatings have emerged.

[0004] A search revealed the following: ① Patent US6235410B1 discloses a Zn-(4.0~10.0%)Al-(1.0~4.0%)Mg coating, but its magnesium content is low, limiting the improvement in the corrosion resistance of the flat plate; ② Although patent US6465114B1 increases the Al content to 2~19% and the Mg content to 1~10%, it limits the Al+Mg content to ≤20%, restricting the optimization space for high corrosion-resistant components; ③ Patent WO2018 / 139620Al improves the overall performance by increasing the Mg content to 5~20%, but still has the following technical defects: When the magnesium content in the coating exceeds the threshold of 5.0 wt%, during the solidification process, the magnesium-rich phase undergoes a eutectic reaction with zinc and aluminum elements to generate a large amount of intermetallic compound MgZn2 phase. The MgZn2 phase has a close-packed hexagonal crystal structure and its microhardness is as high as 200~400 HV, which is much higher than that of the Zn matrix and Al matrix. More importantly, the MgZn2 phase exhibits a significant tendency to coarsen grains during the cooling stage after high-temperature hot-dip galvanizing. When the coated steel sheet is subjected to plastic deformation such as bending, stamping, and edge rolling, the coating is prone to cracking.

[0005] On the other hand, from a thermodynamic perspective, the standard Gibbs free energy of magnesium oxide is much lower than that of zinc and aluminum in the typical hot-dip galvanizing temperature range of 440~480℃. Therefore, magnesium has extremely strong chemical activity. When the magnesium concentration in the plating solution exceeds 5%, a loose and porous MgO oxide film will quickly form on the surface of the plating solution. The above oxide film does not have protective properties, but instead becomes a short-circuit channel for oxygen diffusion, leading to a vicious cycle of "oxidation-cracking-re-oxidation".

[0006] To systematically solve the above-mentioned technical problems, the applicant introduced composite rare earth microalloying elements, namely gadolinium (Gd) and samarium (Sm), into the Zn-Al-Mg high alloy coating system. Through the synergistic effect of the two, the applicant achieved dual regulation of coating microstructure refinement and plating solution oxidation resistance. Summary of the Invention

[0007] The purpose of this invention is to address the problems existing in the prior art by providing a high corrosion-resistant zinc-aluminum-magnesium hot-dip coated steel sheet and its manufacturing method, thereby solving the problem of poor corrosion resistance of the coating.

[0008] Therefore, the present invention provides a high corrosion-resistant zinc-aluminum-magnesium hot-dip coated steel sheet, the steel sheet comprising a steel sheet substrate and a zinc-aluminum-magnesium hot-dip coated layer formed on the surface of the steel sheet substrate.

[0009] The steel plate substrate can be a low-carbon cold-rolled substrate or a hot-rolled substrate, including but not limited to DX51, DX52, DX53, Q235, Q355 and SPHC, etc.

[0010] The zinc-aluminum-magnesium hot-dip coating is composed of the following components by mass percentage: Al 17~25%; Mg 5~10%; Si 0.1~0.5%; Gd 0.02~0.3%; Sm 0.02~0.2%; the remainder being Zn and unavoidable impurity elements.

[0011] Of the above elements: Zn is a key element for improving the corrosion resistance of coated flat plates and the ability to protect against sacrificial anodic corrosion.

[0012] Al is an essential element in coatings. By adding Al to the zinc pot, the coating can contain other elements, thus developing a Zn-Al-Mg coating. The aluminum phase formed by Al in the coating can improve the flat plate corrosion resistance of the coating. At the same time, Al also affects the formation of the aluminum-iron alloy layer, which can ensure the adhesion of the coating. When the concentration of Al in the coating is too high, a large number of other elements dissolve in the Al phase. For example, Zn dissolves in the Al phase, reducing the proportion of Zn phase in the coating. This ultimately leads to a decrease in the sacrificial anode protection capability and a reduction in edge protection of the coating.

[0013] Mg is an essential element for improving the sacrificial anode protection performance of coatings. Adding Mg to zinc-aluminum-magnesium coatings can form the MgZn2 phase, which is beneficial to the sacrificial anode protection performance. When the Mg content exceeds 7%, the proportion of the brittle MgZn2 phase increases, and the coating's processability deteriorates. When the Mg content exceeds 10%, the elemental Mg in the zinc pot is prone to spontaneous combustion, making it impossible to complete the hot-dip galvanizing process.

[0014] With its small atomic radius, Si readily dissolves in Fe-Al alloy layers to form interstitial solid solutions. Si can also inhibit the growth of aluminum-iron alloy layers by suppressing the diffusion interfacial reaction kinetics of Al. Furthermore, Si can enhance the corrosion resistance of the coating.

[0015] Gd preferentially segregates on a certain crystal plane of the MgZn2 phase, reducing the surface energy of the aforementioned crystal plane and inhibiting its preferential growth. Sm, on the other hand, is enriched at the intersection of the grain boundaries, increasing the nucleation rate. The synergistic effect of the two can increase the nucleation rate of the MgZn2 phase and reduce the growth rate. In terms of oxidation resistance, Gd and Sm form a mixed oxide film on the surface of the plating solution. The above interface is densely bonded, and the oxygen diffusion coefficient is lower than that of a single MgO film.

[0016] The present invention also provides a method for manufacturing the above-mentioned high corrosion resistance zinc-aluminum-magnesium hot-dip coated steel sheet, comprising the following steps: Prepare a plating solution comprising the following components by mass percentage: Al: 17-25%, Mg: 5-10%, Si: 0.1-0.5%, Gd: 0.02-0.3%, Sm: 0.02-0.2%, with the remainder being Zn and unavoidable impurity elements; The plating solution is heated to 460~500℃ and kept at that temperature; The steel plate substrate is immersed in a heated plating solution for hot-dip plating. The hot-dip galvanized steel sheet is cooled to room temperature at a cooling rate of not less than 8°C / s to obtain the high corrosion-resistant zinc-aluminum-magnesium hot-dip galvanized steel sheet.

[0017] In the above manufacturing method, the temperature of the zinc pot is controlled at 460~500℃; the temperature of the strip before immersion in the zinc pot is 5~20℃ higher than the temperature of the plating solution; and the cooling rate after plating shall not be lower than 8℃ / s.

[0018] The cross-sectional microstructure of the Zn-Al-Mg coating prepared under the above composition and process includes the following microstructures: Al phase, Al-MgZn2 binary eutectic structure, Zn-MgZn2-Al ternary eutectic structure, Zn phase and MgZn2 phase.

[0019] The beneficial effects of this invention are as follows: This invention achieves a synergistic effect by simultaneously adding a specific proportion of rare earth elements Gd and Sm to the Zn-Al-Mg coating system. Specifically: ① Gd and Sm work together to refine the brittle MgZn2 phase, solving the problem of decreased coating processing performance caused by high magnesium content, making the coating less prone to cracking during bending, stamping, and other deformation processes; ② Gd and Sm form a dense mixed oxide film on the surface of the plating solution, solving the problem of easy oxidation of high magnesium plating solutions, ensuring process stability and coating surface quality. Therefore, this invention achieves a balance between high corrosion resistance, excellent processability, and good process stability. Attached Figure Description

[0020] Figure 1 Scanning electron microscope images of the surface morphology of zinc-aluminum-magnesium coatings without the addition of Gd and Sm elements.

[0021] Figure 2 Scanning electron microscope images of the surface morphology of zinc-aluminum-magnesium coatings with added Gd and Sm elements. Detailed Implementation

[0022] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. The scope of protection of the present invention is not limited to the embodiments mentioned herein.

[0023] Example 1 A method for preparing a highly corrosion-resistant zinc-aluminum-magnesium coated steel, comprising: The composition of the plating solution by mass percentage is: Al 20.0%, Mg 7.0%, Gd 0.05%, Sm 0.03%, Si 0.1%, with the balance being Zn and other unavoidable impurity elements.

[0024] The plating solution is heated to 460℃ and kept at that temperature, while N2 protective gas is introduced. After annealing, the 0.8mm thick cold-rolled steel sheet is immersed in the plating solution for 3 seconds, and the coating thickness on both sides is controlled at 180g / m². The steel sheet was cooled to room temperature by air at a rate of 8℃ / s to obtain a hot-dip coated steel sheet.

[0025] The surface quality of the coating meets the FB grade. The cupping test on both sides of the galvanized sheet showed no coating peeling, indicating excellent coating adhesion.

[0026] Example 2 A method for preparing a highly corrosion-resistant zinc-aluminum-magnesium coated steel, comprising: The composition of the plating solution by mass percentage is: Al 22.0%, Mg 6.0%, Gd 0.25%, Sm 0.18%, with the balance being Zn and other unavoidable impurity elements.

[0027] The plating solution is heated to 500℃ and kept at that temperature, while N2 protective gas is introduced. After annealing, the 0.8mm thick cold-rolled steel sheet is immersed in the plating solution for 3 seconds, and the coating thickness on both sides is controlled at 200g / m². The steel sheet is cooled to room temperature by air at a rate of 10℃ / s to obtain a hot-dip coated steel sheet.

[0028] The surface quality of the coating meets the FB grade. The cupping test on both sides of the galvanized sheet showed no coating peeling, indicating excellent coating adhesion.

[0029] Example 3 A method for preparing a highly corrosion-resistant zinc-aluminum-magnesium coated steel, comprising: The plating solution composition by mass percentage is: Al 18.5%, Mg 8.5%, Gd 0.08%, Sm 0.25%, with the balance being Zn and other unavoidable impurity elements.

[0030] The plating solution is heated to 480℃ and kept at that temperature, while N2 protective gas is introduced. After annealing, the 0.8mm thick cold-rolled steel sheet is immersed in the plating solution for 3 seconds, and the coating thickness on both sides is controlled at 180g / m². The steel sheet is cooled to room temperature by air at a rate of 10℃ / s to obtain a hot-dip coated steel sheet.

[0031] The surface quality of the coating meets the FB grade. The cupping test on both sides of the galvanized sheet showed no coating peeling, indicating excellent coating adhesion.

[0032] Comparative Example 1 A method for preparing a highly corrosion-resistant zinc-aluminum-magnesium coated steel, comprising: The plating solution composition, by mass percentage, is: Al 20.0%, Mg 6.0%, with the balance being Zn and other unavoidable impurity elements.

[0033] The plating solution is heated to 493°C and kept at that temperature, while N2 protective gas is introduced. After annealing, the 0.8mm thick cold-rolled steel sheet is immersed in the plating solution for 3 seconds, and the coating thickness on both sides is controlled at 180g / m². The steel sheet is cooled to room temperature by air at a rate of 10℃ / s to obtain a hot-dip coated steel sheet.

[0034] The MgZn2 phase size was measured and a 180° bending test was performed on the coated steel sheets prepared in Examples 1-3 and Comparative Example 1. The results are shown in the table below.

[0035] like Figure 1 As shown, in the zinc-aluminum-magnesium coating surface morphology of Comparative Example 1 without the addition of Gd and Sm elements, the MgZn2 phase is large in size.

[0036] like Figure 2 As shown, in Example 1, the MgZn2 phase size was significantly refined in the surface morphology of the zinc-aluminum-magnesium coating with added Gd and Sm elements.

[0037] The above results show that the present invention effectively refines the MgZn2 phase and improves the processability of the coating by adding Gd and Sm elements.

[0038] Those skilled in the art will understand that the specific component ranges and process parameters in the above embodiments are examples given under specific experimental conditions. In actual industrial production, the immersion time and cooling rate can be adaptively adjusted according to factors such as production line speed, strip steel specifications, zinc pot capacity, and cooling section length. For example, the cooling method is not limited to air cooling; water quenching or mist cooling can also be used, as long as the required cooling rate can be achieved. The protective gas is not limited to nitrogen; a nitrogen-hydrogen mixture can also be used. All the above adjustments do not depart from the technical concept of this invention and are still within the protection scope of this invention.

Claims

1. A high corrosion-resistant zinc-aluminum-magnesium hot-dip galvanized steel sheet, characterized in that, include: Steel plate substrate; And a zinc-aluminum-magnesium hot-dip galvanized layer formed on the surface of the steel plate substrate; The zinc-aluminum-magnesium hot-dip coating is composed of the following components by mass percentage: Al: 17~25%, Mg: 5~10%, Si: 0.1~0.5%, Gd: 0.02~0.3%, Sm: 0.02~0.2%, with the remainder being Zn and unavoidable impurity elements.

2. The high corrosion-resistant zinc-aluminum-magnesium hot-dip coated steel sheet according to claim 1, characterized in that, The mass percentage of Al in the zinc-aluminum-magnesium hot-dip coating is 20-22%.

3. The high corrosion-resistant zinc-aluminum-magnesium hot-dip coated steel sheet according to claim 1, characterized in that, The zinc-aluminum-magnesium hot-dip coating contains 6-8.5% Mg by mass.

4. The high corrosion-resistant zinc-aluminum-magnesium hot-dip coated steel sheet according to claim 1, characterized in that, The mass percentage of Gd in the zinc-aluminum-magnesium hot-dip coating is 0.05~0.25%.

5. The high corrosion-resistant zinc-aluminum-magnesium hot-dip coated steel sheet according to claim 1, characterized in that, The mass percentage of Sm in the zinc-aluminum-magnesium hot-dip coating is 0.03~0.18%.

6. The high corrosion-resistant zinc-aluminum-magnesium hot-dip coated steel sheet according to claim 1, characterized in that, The double-sided adhesion of the zinc-aluminum-magnesium hot-dip coating is 180~200g / m².

7. The high corrosion-resistant zinc-aluminum-magnesium hot-dip coated steel sheet according to claim 1, characterized in that, The microstructure of the zinc-aluminum-magnesium hot-dip coating includes: Al phase, Al-MgZn2 binary eutectic structure, Zn-MgZn2-Al ternary eutectic structure, Zn phase, and MgZn2 phase.

8. A method for manufacturing a high corrosion-resistant zinc-aluminum-magnesium hot-dip coated steel sheet as described in any one of claims 1 to 7, characterized in that, Includes the following steps: Prepare a plating solution comprising the following components by mass percentage: Al: 17-25%, Mg: 5-10%, Si: 0.1-0.5%, Gd: 0.02-0.3%, Sm: 0.02-0.2%, with the remainder being Zn and unavoidable impurity elements; The plating solution is heated to 460~500℃ and kept at that temperature; The steel plate substrate is immersed in a heated plating solution for hot-dip plating. The hot-dip galvanized steel sheet is cooled to room temperature at a cooling rate of not less than 8°C / s to obtain the high corrosion-resistant zinc-aluminum-magnesium hot-dip galvanized steel sheet.

9. The manufacturing method according to claim 8, characterized in that, The temperature of the steel plate substrate before immersion in the plating solution is 5-20°C higher than the temperature of the plating solution.

10. The manufacturing method according to claim 8, characterized in that, The hot-dip plating time is 3 to 10 seconds.

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