A multistage annealing preparation method of high corrosion-resistant magnesium-aluminum-zinc alloy plated steel plate

Abstract

The application discloses a multistage annealing preparation method of high-corrosion-resistance magnesium-aluminum-zinc alloy plated steel plate, and the method comprises the following steps: sequentially performing cover furnace preannealing, intermediate cooling treatment, vertical annealing furnace annealing and surface zinc plating on a preliminarily formed steel plate after continuous casting and rolling. The intermediate cooling treatment adopts a stepped cooling mode, and the cooling rate and the final cooling temperature are controlled to induce grain refinement. The method optimizes the grain structure of the substrate through the multistage annealing process, and significantly improves the comprehensive performance of the steel plate by adjusting the plating layer composition design.

Description

Technical Field

[0001] This invention belongs to the field of steel plate preparation technology, and particularly relates to a multi-stage annealing method for preparing high corrosion-resistant magnesium-aluminum-zinc alloy coated steel plates. Background Technology

[0002] Hot-dip galvanized aluminum-magnesium alloy coated steel sheets are widely used in construction, home appliance, and automobile manufacturing due to their excellent corrosion resistance and processing performance. However, traditional coated steel sheets still have some problems in practical applications: on the one hand, conventional annealing processes (such as single-chamber annealing or vertical annealing) easily lead to uneven matrix grain structure, affecting stamping formability; on the other hand, magnesium in the coating has high reactivity and is prone to filamentary corrosion in corrosive environments, affecting aesthetics and lifespan. In addition, traditional processes add a large amount of alloying elements (such as silicon, manganese, niobium, and titanium) to pursue high strength, which not only increases costs but also easily leads to incomplete coating or reduced adhesion on the coating surface.

[0003] In the prior art, patent CN110669907A proposes to improve the uniformity of substrate microstructure through "two-step annealing" (pre-annealing in a bell-type furnace + annealing in a vertical annealing furnace), but its annealing stage is still relatively simple, and its degree of grain refinement is limited. Meanwhile, the coating composition of this patent is mainly zinc, with relatively low aluminum and magnesium content, leaving room for improvement in corrosion resistance under harsh environments. Furthermore, the magnesium element in the coating easily reacts with moisture in the air, leading to filamentous corrosion under the organic coating, affecting the appearance of the coating.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of insufficient corrosion resistance, poor uniformity of substrate structure, and susceptibility to filamentous corrosion in existing coated steel sheets, and to provide a multi-stage annealing method for preparing high corrosion-resistant magnesium-aluminum-zinc alloy coated steel sheets. This method optimizes the grain structure of the substrate through a multi-stage annealing process.

[0006] Furthermore, by adjusting the coating composition design, the overall performance of the steel plate is significantly improved.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] This invention discloses a multi-stage annealing preparation method for high corrosion-resistant magnesium-aluminum-zinc alloy coated steel sheets. The method involves sequentially pre-annealing the initially formed steel sheet after continuous casting and rolling, performing bell-type furnace pre-annealing, intermediate cooling treatment, vertical annealing furnace annealing, and surface galvanizing. The intermediate cooling treatment adopts a stepped cooling method, controlling the cooling rate and final cooling temperature to induce grain refinement.

[0009] The bell-type furnace pre-annealing: set the heating hood heating rate to 30-50℃ / h, heat to a temperature of 580-620℃, and hold for at least 6 hours; after holding, perform a cooling, control the cooling rate to ≤25℃ / h, and when the temperature drops to 550℃, replace the heating hood with a cooling hood, and cool to a rolling temperature of ≤100℃ before unloading from the furnace.

[0010] The intermediate cooling process includes: rapidly cooling the pre-annealed steel plate to 300–400°C at a rate of ≥15°C / s, holding at that temperature for 1–3 minutes, and then air-cooling to room temperature. This process can promote bainitic phase transformation and refine the grain size.

[0011] The vertical annealing furnace is used for annealing. A vertical radiant tube heating continuous annealing furnace is adopted. The strip speed during the annealing stage is set to 85-95 m / min. The plate temperature in the soaking zone is controlled at 820-880℃, the holding time is 40-60 s, the plate temperature in the slow cooling zone is 600-650℃, and the plate temperature entering the zinc pot is 480-500℃. The oxygen content in the furnace is <800 ppm, the hydrogen mass percentage is 10-15%, and the remaining gas is nitrogen. The higher hydrogen concentration helps to reduce surface oxides and improve the adhesion of the coating.

[0012] Furthermore, before annealing the continuously cast and rolled steel plate, the molten steel undergoes sequential LF and RH refining processes. The RH refining process yields a refined substrate with the following chemical composition: carbon ≤ 0.0040 wt%; silicon ≤ 0.15 wt%; manganese ≤ 0.18 wt%; phosphorus ≤ 0.012 wt%; sulfur ≤ 0.008 wt%; acid-fused aluminum 0.035–0.045 wt%; the sum of titanium and niobium ≤ 0.08 wt%, with the remainder being iron and unavoidable trace elements. This low-carbon, low-silicon design helps reduce surface oxides and improve coating quality.

[0013] Furthermore, after annealing in a vertical annealing furnace, an annealed substrate is obtained. The annealed substrate is then surface-galvanized. The coating material includes: 15-25 wt% aluminum, 4-6 wt% magnesium, 0.05-0.1 wt% nickel, 0.1-0.3 wt% silicon, 0.01-0.05 wt% cerium, with the remainder being zinc and unavoidable impurities. Increasing the aluminum content enhances the passivation effect, magnesium and nickel synergistically improve corrosion resistance, and rare earth cerium refines the coating structure.

[0014] Furthermore, the coating amount of the material forming the coating is 140-150 g / m².

[0015] Furthermore, during zinc pot coating: ensure the temperature of the strip steel entering the zinc pot is 480-500℃, set the zinc pot temperature to 470-490℃, the immersion time to 4-8s, the air knife pressure to 6-12Kpa, and the cooling rate after coating to ≥15℃ / s; use nitrogen purging with a gas flow rate of 500-600ml / min.

[0016] Furthermore, the steel plate has a yield strength ≥150MPa, tensile strength ≥350MPa, elongation after fracture ≥35%, and resistance to red rust under salt spray test ≥1500h.

[0017] Furthermore, the high corrosion-resistant magnesium-aluminum-zinc alloy coated steel sheet is suitable for building roofs, automobile chassis, and photovoltaic brackets.

[0018] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0019] This invention achieves the following advantages through a multi-stage annealing process design (bell-type furnace pre-annealing + intermediate cooling + vertical annealing furnace annealing), combined with optimized composition of high-aluminum-magnesium-nickel-rare-earth alloy coatings:

[0020] The substrate structure is more uniform: multi-stage annealing improves the equiaxedness of the grains, refines the grain size to 8-12μm, and improves the stamping formability (elongation after fracture) by more than 20%.

[0021] Significantly enhanced corrosion resistance: The synergistic effect of aluminum, magnesium, nickel and rare earth cerium in the coating makes the corrosion products denser. The salt spray test resistance to red rust exceeds 1500h, and the cut surface has excellent self-healing ability.

[0022] Suppressing filiform corrosion: The addition of nickel (0.05-0.1 wt%) and rare earth cerium refines the grain boundaries of the coating, delays the dissolution of magnesium-zinc compounds, and reduces the incidence of filiform corrosion by more than 50%.

[0023] Strong coating adhesion: The surface oxides are reduced by a high hydrogen concentration (10-15%) annealing atmosphere, avoiding the problem of incomplete coating. Detailed Implementation

[0024] The present invention will be described in more detail below through specific embodiments. These embodiments are merely descriptions of the best mode of implementation and do not limit the scope of the invention in any way.

[0025] Example 1

[0026] This embodiment provides a multi-stage annealing method for preparing high corrosion-resistant magnesium-aluminum-zinc alloy coated steel sheets, the steps of which are as follows:

[0027] Smelting and Continuous Casting and Rolling: After refining with LF-RH, the molten steel yields a substrate with the following composition: C: 0.0038 wt%, Si: 0.12 wt%, Mn: 0.16 wt%, P: 0.010 wt%, S: 0.007 wt%, Als: 0.040 wt%, Ti+Nb: 0.075 wt%, with the remainder being Fe and impurities. CSP continuous casting and rolling parameters: tundish temperature 1550℃, casting speed 4.5 m / min, finishing rolling temperature 920℃, coiling temperature 630℃.

[0028] Multi-stage annealing:

[0029] Pre-annealing in a bell-type furnace: Heat to 600℃ at 40℃ / h, hold for 6 hours, cool to 550℃ at 20℃ / h, then replace the cooling hood and cool to 90℃ before removing from the furnace.

[0030] Intermediate cooling process: rapidly cool to 350℃ at 20℃ / s, hold for 2 minutes, and then air cool.

[0031] Vertical annealing furnace annealing: strip speed 90m / min, soaking zone temperature 850℃, holding time 50s, hydrogen concentration in furnace 12%, dew point -40℃, zinc pot temperature 490℃.

[0032] Hot-dip galvanizing: Solution composition: Al: 20 wt%, Mg: 5 wt%, Ni: 0.08 wt%, Si: 0.2 wt%, Ce: 0.03 wt%, balance Zn. Zinc pot temperature: 480℃, immersion time: 6s, air knife pressure: 10 kPa, post-galvanizing cooling rate: 20℃ / s.

[0033] Example 2

[0034] This embodiment is similar to Embodiment 1 in its steps, except that:

[0035] The coating composition was adjusted to: Al: 18 wt%, Mg: 4.5 wt%, Ni: 0.06 wt%, Si: 0.15 wt%, Ce: 0.02 wt%.

[0036] Intermediate cooling process: Rapidly cool to 400℃ and then hold for 1 minute.

[0037] Example 3

[0038] This embodiment is similar to Embodiment 1 in its steps, except that:

[0039] Substrate composition: C: 0.0040 wt%, Si: 0.10 wt%, Mn: 0.15 wt%.

[0040] The annealing temperature of the bell-type furnace was adjusted to 580℃ and held for 7 hours.

[0041] Comparative Example 1

[0042] This comparative example uses a conventional two-step annealing process (without intermediate cooling), and the coating has a standard zinc-aluminum-magnesium composition (Al: 11 wt%, Mg: 3 wt%, excluding Ni and Ce). Other steps are the same as in Example 1.

[0043] Performance testing

[0044] The performance of the steel plates obtained in Examples 1-3 and Comparative Example 1 was tested, and the results are shown in the table below:

[0045] Example 1 175 365 38 1650 5 (No filiform corrosion) Example 2 160 350 36 1580 4 (Mild) Example 3 170 360 37 1600 5 (No filiform corrosion) Comparative Example 1 150 330 32 1200 2 (obvious filamentous corrosion)

[0046] *Filament corrosion rating: Level 5 is the best, Level 0 is the worst.

[0047] Test results show that the steel plates of Examples 1-3 of this invention are superior to Comparative Example 1 in terms of strength, plasticity, and corrosion resistance. The multi-stage annealing process combined with the optimization of the coating composition significantly improves the overall performance of the steel plates.

[0048] Conclusion: This invention, through multi-stage annealing and innovative coating composition, solves the problems of uneven matrix structure, insufficient corrosion resistance, and filamentous corrosion in traditional zinc-aluminum-magnesium coated steel sheets. This steel sheet is particularly suitable for building structures and automotive parts in highly corrosive environments.

[0049] 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 multi-stage annealing method for preparing high corrosion-resistant magnesium-aluminum-zinc alloy coated steel sheets, characterized in that, The pre-formed steel plates after continuous casting and rolling are sequentially subjected to bell-type furnace pre-annealing, intermediate cooling treatment, vertical annealing furnace annealing, and surface galvanizing; wherein, the intermediate cooling treatment adopts a stepped cooling method to control the cooling rate and final cooling temperature in order to induce grain refinement. The bell-type furnace pre-annealing: set the heating hood heating rate to 30-50℃ / h, heat to a temperature of 580-620℃, and hold for at least 6 hours; after holding, perform a cooling, control the cooling rate to ≤25℃ / h, and when the temperature drops to 550℃, replace the heating hood with a cooling hood, and cool to a rolling temperature of ≤100℃ before unloading from the furnace. The intermediate cooling process includes: rapidly cooling the pre-annealed steel plate to 300-400°C at a rate of ≥15°C / s, holding it at that temperature for 1-3 minutes, and then air-cooling it to room temperature; this process can promote bainitic phase transformation and refine the grains. The vertical annealing furnace is used for annealing. A vertical radiant tube heating continuous annealing furnace is adopted. The strip speed during the annealing stage is set to 85-95 m / min. The plate temperature in the soaking zone is controlled at 820-880℃, the holding time is 40-60 s, the plate temperature in the slow cooling zone is 600-650℃, and the plate temperature entering the zinc pot is 480-500℃. The oxygen content in the furnace is <800 ppm, the hydrogen mass percentage is 10-15%, and the remaining gas is nitrogen. The higher hydrogen concentration helps to reduce surface oxides and improve the adhesion of the coating.

2. The multi-stage annealing preparation method for high corrosion-resistant magnesium-aluminum-zinc alloy coated steel sheet according to claim 1, characterized in that, Before annealing, the continuously cast and rolled steel plate undergoes sequential LF and RH refining processes. The RH refining process yields a refined substrate with the following chemical composition: carbon ≤ 0.0040 wt%; silicon ≤ 0.15 wt%. Manganese ≤0.18Wt% Phosphorus ≤ 0.012 wt%; Sulfur ≤ 0.008 wt%; Acid-fused aluminum 0.035–0.045 wt%; The sum of titanium and niobium content ≤ 0.08 wt%, with the remainder being iron and unavoidable trace elements.

3. The multi-stage annealing preparation method for high corrosion-resistant magnesium-aluminum-zinc alloy coated steel sheet according to claim 1, characterized in that, After annealing in a vertical annealing furnace, an annealed substrate is obtained. The annealed substrate is then surface-galvanized. The coating material includes: 15-25 wt% aluminum, 4-6 wt% magnesium, 0.05-0.1 wt% nickel, 0.1-0.3 wt% silicon, 0.01-0.05 wt% cerium, with the remainder being zinc and unavoidable impurities. Increasing the aluminum content enhances the passivation effect, magnesium and nickel synergistically improve corrosion resistance, and rare earth cerium refines the coating structure.

4. The multi-stage annealing preparation method for high corrosion-resistant magnesium-aluminum-zinc alloy coated steel sheet according to claim 1, characterized in that, The coating amount of the material forming the coating is 140-150 g / m².

5. The multi-stage annealing method for preparing high corrosion-resistant magnesium-aluminum-zinc alloy coated steel sheet according to claim 1, characterized in that, When coating zinc in a zinc pot: ensure that the temperature of the strip steel entering the zinc pot is 480-500℃, set the zinc pot temperature to 470-490℃, the immersion time to 4-8s, the air knife pressure to 6-12Kpa, and the cooling rate after coating to ≥15℃ / s; use nitrogen purging with a gas flow rate of 500-600ml / min.

6. The multi-stage annealing method for preparing high corrosion-resistant magnesium-aluminum-zinc alloy coated steel sheet according to claim 1, characterized in that, The steel plate has a yield strength ≥150MPa, tensile strength ≥350MPa, elongation after fracture ≥35%, and resistance to red rust under salt spray test ≥1500h.

7. The multi-stage annealing preparation method for high corrosion-resistant magnesium-aluminum-zinc alloy coated steel sheet according to claim 1, characterized in that, The high corrosion-resistant magnesium-aluminum-zinc alloy coated steel sheet is suitable for building roofs, automobile chassis, and photovoltaic brackets.