Hot-dip galvannealed steel sheet having excellent adhesion and method for manufacturing the same

Abstract

This invention belongs to the technical field of hot-dip galvanized aluminum-magnesium steel, and discloses a hot-dip galvanized aluminum-magnesium steel with excellent adhesion and its manufacturing method. The chemical composition of the substrate, by mass percentage, includes: C: 0.05-0.07%, Si: 0.05-0.20%, Mn: 0.3-0.5%, Als: 0.025-0.055%, P: 0.025-0.035%, S≤0.010%, Nb: 0.01-0.02%, with the balance being Fe and other unavoidable impurities. The chemical composition of the coating on the substrate, by mass percentage, includes: Al: 12.0-25.0%, Mg: 4.0-10.0%, Si: 0.06-0.5%, Ca: 0.01-0.2%, and the Al / Mg mass ratio ≥2.5, the Al / Si mass ratio: 50-160, with the balance being Zn and other unavoidable impurities. The substrate of this invention is designed with low Si and low Mn composition to ensure coating adhesion, and P and Nb are used to enhance mechanical properties. The plating solution is designed with high magnesium and aluminum composition to ensure high corrosion resistance. The coating microstructure and the composition and morphology of the interface reaction layer between the coating and the substrate are controlled by the Al / Mg mass ratio and Al / Si mass ratio to improve the adhesion of the coating, thereby taking into account both corrosion resistance and adhesion.

Description

A hot-dip galvanized aluminum-magnesium steel with excellent adhesion and its manufacturing method Technical Field

[0001] This invention belongs to the technical field of hot-dip galvanized aluminum-magnesium steel, specifically relating to a hot-dip galvanized aluminum-magnesium steel with excellent adhesion and its manufacturing method. Background Technology

[0002] Zinc-aluminum-magnesium (ZAM) coatings have been a major development trend in high-corrosion-resistant coatings for the past 20 years. "Chunglung" ZAM coatings (Al content 3-11%), by adding appropriate amounts of Al and Mg to traditional zinc-based coatings, exhibit corrosion resistance 3-5 times higher than pure zinc (GI) coatings. In industries such as agricultural facilities, industrial buildings, road infrastructure, and photovoltaic brackets, "Chunglung" ZAM coatings are rapidly replacing traditional zinc-based coatings. Currently, the Mg content in widely used "Chunglung" ZAM coatings does not exceed 3%. For applications in harsh environments, such as photovoltaic projects in coastal or high-humidity and high-temperature areas, fishery and livestock facilities, urban underground utility tunnels, and industrial building facilities, the corrosion resistance of coating products needs to be further improved compared to traditional ZAM coatings.

[0003] Further increasing the Al and Mg content can enhance the corrosion resistance of the coating. However, increasing the Al and Mg content leads to changes in the microstructure of the coating and the reaction layer. Compared to zinc-aluminum-magnesium (ZAMg) coatings with low Mg content (<3%), the content of the hard and brittle compound phase MgZn2 increases, and MgZn2 mainly forms and grows before the final multi-element eutectic reaction, forming larger blocky MgZn2 phases. Increasing the Al content leads to a rise in the plating bath temperature, and due to the vigorous reaction between Al and Fe, controlling the reaction layer between the coating and the steel substrate becomes more complex. Furthermore, increasing the Al and Mg content leads to an increase in zinc ash and zinc dross. These factors negatively impact the adhesion of the coating, thus affecting the processing performance of the coated product. For example, processing with small deformation may be acceptable, but for processing with large deformation, the risk of coating peeling is higher, limiting the application range of such high corrosion-resistant products. Therefore, achieving excellent adhesion while improving the corrosion resistance of the coating is crucial for expanding the product's usability. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to address the shortcomings of the prior art by providing a hot-dip galvanized aluminum-magnesium steel with excellent adhesion and its manufacturing method. The substrate is designed with a low Si and low Mn composition to ensure coating adhesion, and P and Nb are used to enhance mechanical properties. The plating solution is designed with a high magnesium and aluminum composition to ensure high corrosion resistance. The adhesion of the coating is improved by controlling the microstructure of the coating and the composition and morphology of the interface reaction layer between the coating and the substrate through the Al / Mg mass ratio and the Al / Si mass ratio, thereby taking into account both corrosion resistance and adhesion.

[0005] To address the technical problems raised in this invention, this invention provides a hot-dip galvanized aluminum-magnesium steel material with excellent adhesion, comprising a substrate and a coating adhered to the surface of the substrate.

[0006] In the above scheme, the chemical composition of the substrate, by mass percentage, includes: C: 0.05-0.07%, Si: 0.05-0.20%, Mn: 0.3-0.5%, Als: 0.025-0.055%, P: 0.025-0.035%, S≤0.010%, Nb: 0.01-0.02%, with the balance being Fe and other unavoidable impurities.

[0007] In the above scheme, the thickness of the substrate is 0.4 to 2.5 mm.

[0008] In the above scheme, the chemical composition of the coating, by mass percentage, includes: Al: 12.0-25.0%, Mg: 4.0-10.0%, Si: 0.06-0.5%, Ca: 0.01-0.2%, and the Al / Mg mass ratio is ≥2.5, the Al / Si mass ratio is 50-160, and the balance is Zn and other unavoidable impurities.

[0009] In the above scheme, the single-sided adhesion amount of the coating is 40-300 g / m². 2 .

[0010] In the above scheme, the structure of the coating includes primary (Al) phase, bulk MgZn2 phase encapsulated (Al) phase, bulk (Zn) phase, Mg2Si phase, and multi-element eutectic Al / Zn / MgZn2 structure.

[0011] In the above scheme, there is an interface reaction layer between the coating and the substrate, which is a dense, approximately equiaxed granular Al-Fe compound coated with a Ca-containing compound.

[0012] In the above scheme, the hot-dip galvanized aluminum-magnesium steel has a yield strength ≥380MPa, a tensile strength ≥450MPa, and an elongation ≥25%.

[0013] This invention also provides a method for manufacturing hot-dip galvanized aluminum-magnesium steel with excellent adhesion, comprising the following steps:

[0014] 1) After smelting the molten iron, it is refined and continuously cast in thin slabs to obtain a cast billet;

[0015] 2) After the billet is homogenized, it is hot rolled, cooled, coiled and leveled to obtain hot-rolled raw material coils;

[0016] 3) Hot-rolled raw material coils are pickled and cold-rolled to obtain a substrate;

[0017] 4) After the substrate is annealed in the annealing furnace, it is hot-dip galvanized aluminum-magnesium steel in a zinc pot, cooled after being removed from the pot, and hot-dip galvanized aluminum-magnesium steel with excellent adhesion is obtained.

[0018] In the above scheme, the continuous casting speed is 4.5 to 5.0 m / min, and the billet thickness is 60 to 85 mm.

[0019] In the above scheme, the billet is pre-descaled before homogenization, and the descaling pressure is 140-180 bar.

[0020] In the above scheme, the furnace entry temperature for homogenizing the billet is 850-950℃, the heating time is 25-40 min, and the furnace exit temperature is 1130-1180℃.

[0021] In the above scheme, the billet is descaled by high-pressure water before hot rolling, and the descaling pressure is 280-380 bar.

[0022] In the above scheme, the controlled rolling speed of the hot rolling is 7-13 m / s, and the final rolling temperature is 780-820℃.

[0023] In the above scheme, the coiling temperature is 500-580℃, and the steel coil is leveled after the temperature drops below 50℃, resulting in a hot-rolled raw material coil thickness of 2.0-6.0mm.

[0024] In the above scheme, the temperature of the pickling acid solution is controlled at 80-90℃.

[0025] In the above scheme, the total reduction rate of cold rolling is 58-80%.

[0026] In the above scheme, the substrate is first heated to 760-800°C at a rate of 10-50 K / s in the annealing furnace, held at that temperature for 2-200s, and then cooled to 10-15°C above the temperature of the zinc plating solution at a rate greater than 10 K / s.

[0027] In the above scheme, the hydrogen concentration in the annealing furnace is ≥10%, the dew point is ≤-35℃, and the dew point of the furnace nose is -15~-5℃.

[0028] In the above scheme, the temperature of the plating solution in the zinc pot is 40-50°C higher than the melting point of the coating.

[0029] In the above scheme, during the cooling process after the coating is removed from the pot, the average cooling rate from the start of solidification to the end of solidification is greater than 30 K / s.

[0030] The design concept for the substrate composition in this invention is as follows:

[0031] To improve coating adhesion, a low-Si, low-Mn composition was selected for the substrate alloying elements. However, low Si and Mn content cannot guarantee that the target strength will be achieved. Lowering the annealing temperature to introduce incomplete recrystallization to increase strength would result in a significant decrease in elongation, failing to reach the target value of 25%. Furthermore, a lower annealing temperature is detrimental to the reduction of residual iron oxide on the strip surface. Residual iron oxide can affect the wetting of the plating solution on the steel substrate, leading to defects such as incomplete plating or zinc stripping during processing. Therefore, to effectively reduce residual iron oxide on the strip surface and ensure the target strength is achieved at this temperature, small amounts of P and Nb are added to the composition. The main alloying element contents are controlled as follows:

[0032] C: C is the main strengthening element in steel. C dissolves in ferrite and forms carbides, increasing the strength of steel. The higher the C content, the higher the strength of the steel, but the lower the plasticity and toughness. Since the peritectic region must be avoided in the CSP process of thin slab continuous casting and rolling, this invention controls the C content to 0.05-0.07%.

[0033] Si: Si is a commonly used deoxidizer in steelmaking. Si does not form carbides and is dissolved in ferrite, which can improve the strength of steel. However, excessively high Si content will significantly reduce the plasticity, toughness, and weldability of the steel. Simultaneously, Si tends to accumulate on the surface of steel plates, which can affect the wetting of the plating solution on the steel substrate, resulting in poor adhesion. To further improve the adhesion of the coating, this invention controls the Si content to 0.05–0.20%.

[0034] Mn: Mn is an excellent deoxidizer and desulfurizer in steelmaking. Mn can reduce the hot brittleness of steel caused by sulfur, thereby improving the hot working properties of steel. Mn is dissolved in ferrite, increasing the strength of steel. However, Mn tends to accumulate on the surface of steel plates, which can affect the wetting of the plating solution on the steel substrate, resulting in poor adhesion. To further improve the adhesion of the coating, this invention controls the Mn content to 0.3–0.5%.

[0035] P: To control surface oxidation of the substrate during annealing, Si and Mn elements are kept at low levels in this invention. Appropriate amounts of P are added to ensure the strength of the steel. P can significantly strengthen ferrite through solid solution strengthening, but excessive P can cause a significant decrease in plasticity and impact toughness, especially at low temperatures, making the steel significantly brittle. In this invention, the P content is controlled within the range of 0.025% to 0.035%.

[0036] Nb: The addition of a small amount of Nb to steel is mainly to utilize its precipitation strengthening and grain refinement strengthening effects to improve yield strength and tensile strength. Grain refinement strengthening does not lead to a decrease in elongation. Too low a Nb content results in an insignificant strengthening effect, while too much Nb increases alloy costs. Considering the tensile performance objectives of this invention, the Nb content is controlled at 0.01–0.02%.

[0037] The coating composition is designed as follows in this invention:

[0038] Al: Increasing the Al content can improve the corrosion resistance of the coating. When the Al content increases, the overall melting point of the coating increases, requiring a corresponding increase in the temperature of the zinc bath during production. Compared to zinc baths with low Al content, the amount of zinc dross and ash will increase significantly. When the Al content is greater than 25%, the increase in planar corrosion resistance of the coating slows down with further increases in Al content, while the corrosion resistance at cut edges decreases. Therefore, the Al content in the coating of this invention is controlled at 12-25%.

[0039] Mg: Increasing the Mg content in the coating promotes the rapid formation of dense corrosion products containing Zn, Al, and Mg on the coating surface, which is beneficial for protecting the interior of the coating and the substrate. Furthermore, these corrosion products are fluid and can protect the cut edges. However, increasing the Mg content leads to an increase in the compound phase MgZn2, and the amount of MgZn2 phase formed before the final multi-element eutectic reaction increases significantly. These blocky, hard, and brittle MgZn2 phases are prone to cracking during processing, leading to dezincification problems. Therefore, the Mg content in the coating of this invention is controlled at 4.0–10.0%.

[0040] Al / Mg mass ratio: To improve coating adhesion while ensuring high corrosion resistance, the growth of the MgZn2 phase needs to be controlled. Firstly, this means avoiding the MgZn2 phase being the primary crystalline phase; secondly, ensuring that the blocky MgZn2 phase in the coating structure encapsulates the tough (Al) phase, thereby inhibiting the propagation of cracks formed in the MgZn2 phase. This requires controlling the ratio of Al to Mg content. Therefore, in this invention, the Al to Mg content ratio in the coating is controlled to be no less than 2.5.

[0041] Si: The main role of trace element Si in the plating bath is to control the growth of the Fe-Al reaction layer at the interface between the substrate and the coating, which is crucial for the adhesion of the coating. Si not only effectively inhibits the erosion of the substrate by Al in the high-temperature plating bath, but also inhibits the excessive growth of the interface reaction layer. Too little Si cannot effectively control the interface reaction layer, while too much Si cannot completely dissolve in the plating bath, resulting in the formation of more Si-containing compounds that adhere to the substrate or the coating, thus affecting the adhesion of the coating. Therefore, the Si content in the coating of this invention is controlled at 0.06% to 0.5%.

[0042] The Al / Si mass ratio is crucial for controlling the morphology of the Al-Fe reaction layer at the interface and the compounds present thereon. An excessively high Al / Si ratio hinders the formation of dense, equiaxed granular Fe-Al compounds, while a ratio that is too low leads to excess Si reacting with Al and Mg at the interface between the coating and the substrate to form numerous Si-containing compounds, negatively impacting the bonding between the coating and the substrate. Therefore, the Al / Si content ratio is controlled between 50 and 160.

[0043] Ca: Due to the high Al and Mg content in the plating bath, a large amount of Al and Mg oxides will form on the surface of the bath. These oxides are loose and porous, and cannot prevent further oxidation of the plating bath. When the strip enters the plating bath, the Al and Mg oxides will adhere to the substrate or the coating, thus affecting the adhesion of the coating. Ca can react with oxygen to form dense oxides, thereby preventing further oxidation of the plating bath and effectively reducing the Al and Mg oxides on the surface of the plating bath. Excessive Ca cannot completely dissolve in the plating bath and will form various compounds with elements such as Al, Mg, and Si, which will adhere to the substrate or the coating, thereby reducing the adhesion of the coating. Therefore, this invention controls the Ca content to be between 0.01% and 0.2%.

[0044] The design concept of the manufacturing process in this invention is as follows:

[0045] To improve coating adhesion, thorough cleaning of the substrate surface before plating and control of the hot-dip galvanizing process are crucial. Manufacturing processes primarily consider controlling iron oxide scale during billet heating and hot rolling, cleaning iron oxide scale during pickling, reducing residual iron oxide scale on the strip surface during hot-dip galvanizing, controlling zinc slag and zinc ash in the zinc pot, and controlling the plating bath temperature and post-plating cooling rate. Compared to conventionally hot-rolled billets, CSP billets have a thinner surface iron oxide scale, making it difficult to form appropriately thick scale blocks for removal during descaling. Therefore, process control is essential during billet heating to form appropriately thick iron oxide scale blocks on the billet surface, which is beneficial for subsequent high-pressure water descaling. Furthermore, compared to conventionally hot-rolled billets, CSP slabs are thinner. For the same finished thickness, the compression ratio is smaller, resulting in a higher surface roughness due to proportional compression compared to conventional hot-rolled slabs. Therefore, to improve substrate surface quality, a larger reduction rate of 58-80% is used during cold rolling after pickling, further enhancing surface quality. In terms of plating bath composition design, due to the high Al and Mg content, there will be more zinc dross and zinc ash during production, which will affect the coating quality. Nitrogen humidification at the furnace nose is used to control zinc ash, and the plating bath temperature is controlled to control zinc dross. The main process controls are as follows:

[0046] Pre-descaling of the billet before it enters the furnace: Before entering the furnace, the billet is pre-descaled to remove the protective slag and large iron oxide scale from its surface. In this invention, the pre-descaling pressure is set to 140–180 bar.

[0047] Billet heating time and tapping temperature: During the heating process, iron oxide scale will form on the surface of the billet. Excessively thick or thin iron oxide scale is detrimental to high-pressure water descaling. This invention sets the billet heating time to 25–40 minutes and the tapping temperature to 1130–1180°C. At this heating time and temperature, iron oxide scale of moderate thickness forms on the billet surface, which is beneficial for subsequent high-pressure water descaling.

[0048] High-pressure water descaling before hot rolling: High water pressure is beneficial for removing iron oxide scale formed on the surface of the billet during the heating process. In this invention, the high-pressure water descaling pressure is set to 280-380 bar.

[0049] Hot rolling final rolling temperature: A lower final rolling temperature can reduce the formation of secondary iron oxide scale on the surface of the steel coil during the post-rolling cooling process. This invention sets the final rolling temperature to 780-820℃. Too high a temperature will lead to the formation of more secondary iron oxide scale during cooling, while too low a temperature will lead to the formation of excessive ferrite during rolling, thereby resulting in a decrease in the strength of the final product.

[0050] Coiling temperature: A lower coiling temperature helps suppress the formation of tertiary iron oxide scale during the air cooling process after coiling. This invention sets the coiling temperature to 500-580℃. Too high a temperature will lead to the formation of more tertiary iron oxide scale during cooling, while too low a temperature will exceed the cooling capacity of the cooling unit and increase the risk of coil tip curling during coiling, thus affecting production.

[0051] Pickling temperature: The pickling temperature of this invention is set to 80-90℃. Within this temperature range, the iron oxide scale can be fully removed from the hot-rolled substrate without over-pickling. If the temperature is too high, the substrate surface will be corroded while removing the iron oxide scale; if the temperature is too low, the iron oxide scale will not be removed sufficiently.

[0052] Annealing temperature: The annealing temperature not only determines the mechanical properties but is also crucial for the cleanliness of the strip surface. At higher annealing temperatures, the residual iron oxide scale on the strip surface after pickling can be effectively reduced. The reduced surface iron has higher activity and reacts more strongly with the plating solution, which is beneficial to the adhesion of the coating. Since a small amount of P and Nb are added to the substrate composition of this invention, even though the contents of Si and Mn are low, there is no need to increase strength by lowering the annealing temperature. Therefore, this invention sets the annealing temperature to 760–800°C. An excessively high annealing temperature will reduce the strength of the steel, while an excessively low annealing temperature will prevent the effective reduction of iron oxide scale on the strip surface.

[0053] Hydrogen concentration in the annealing furnace: To enhance the reduction of residual iron oxide on the strip surface and improve the heat transfer efficiency of the gas in the furnace, the hydrogen concentration needs to be controlled at no less than 10%. Too low a hydrogen concentration will not be able to effectively reduce the residual iron oxide on the strip surface, resulting in surface defects such as exposed iron and zinc dross on the coating surface during the hot-dip galvanizing process, which is not conducive to the adhesion of the coating.

[0054] Dew point in annealing furnace: To suppress oxidation of the strip steel in the annealing furnace, the dew point inside the furnace needs to be controlled at -35℃ or below. Excessively high dew points will cause localized oxidation of the strip steel surface, resulting in surface defects such as exposed iron and zinc dross on the coating surface during hot-dip galvanizing, which is detrimental to the adhesion of the coating.

[0055] Furnace nose dew point: To reduce the impact of zinc vapor evaporating from the liquid surface inside the furnace nose on the substrate, nitrogen humidification is used at the furnace nose to control the dew point at the furnace nose to -15 to -5°C. Too low a dew point cannot effectively suppress the formation of zinc ash on the zinc liquid surface.

[0056] Plating bath temperature: The melting point of the plating system is related to its composition. Excessively high plating bath temperatures will increase defects such as zinc ash and zinc dross on the plating surface, which is detrimental to the adhesion of the plating. Therefore, the plating bath temperature in this invention is set to be 40–50°C higher than the melting point of the plating.

[0057] Post-plating cooling rate: Due to the increase in Al and Mg content, the solidification range of the coating will increase significantly, resulting in a longer solidification time. Excessive cooling time will lead to an excessively large size of the MgZn2 phase, which is detrimental to the adhesion of the coating. Therefore, the post-plating cooling rate of this invention is set to be greater than 30 K / s.

[0058] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0059] (1) The present invention provides a high corrosion-resistant zinc-aluminum-magnesium coated steel sheet with excellent adhesion, with a yield strength ≥380MPa, tensile strength ≥450MPa, elongation ≥25%, and thickness specifications as low as 0.4mm. The product is suitable for coastal or high humidity and high temperature areas, as well as facilities in factory areas.

[0060] (2) Based on the design and process control of substrate and coating composition, while increasing the Al and Mg content of the coating to improve corrosion resistance, the coating adhesion is ensured to be excellent and can be used for processing with large deformation.

[0061] (3) The manufacturing process adopts CSP hot rolling and cold rolling hot-dip galvanized aluminum-magnesium. The CSP process can not only provide thin-gauge raw material coils, which helps the cold rolling production line to produce high-strength thin-gauge products, but also effectively shorten the process flow and reduce energy consumption. Attached Figure Description

[0062] Figure 1 is a picture of the coating structure of Example 6.

[0063] Figure 2 is an image of the interface reaction layer in Example 6.

[0064] Figure 3 shows an image and energy dispersive spectroscopy (EDS) analysis of the Ca-containing compound on the interfacial reaction layer of Example 6.

[0065] Figure 4 shows the coating structure of Comparative Example 6.

[0066] Figure 5 shows an image of the interface reaction layer in Comparative Example 7.

[0067] Figure 6 shows an image of the interface reaction layer in Comparative Example 8. Detailed Implementation

[0068] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0069] In the following examples and comparative examples, the chemical composition of the substrates used includes five types, as detailed in Table 1.

[0070] Table 1 Chemical composition of the substrate

[0071] Substrate Number C (%) Si (%) Mn (%) Nb (%) Als (%) P (%) S (%) 1 0.06 0.05 0.42 0.012 0.025 0.025 0.0082 0.05 0.12 0.30 0.01 0.031 0.035 0.0073 0.07 0.2 0.50 0.020 0.055 0.031 0.0084 0.07 0.13 0.42 0.006 0.032 0.029 0.0095 0.06 0.15 0.36 0.015 0.037 0.007 0.007 surface

[0072] In the following examples and comparative examples, the chemical composition of the coatings used includes 10 types, as shown in Table 2.

[0073] Table 2 Chemical composition and melting point of the coating

[0074]

[0075] The following embodiments illustrate a method for manufacturing hot-dip galvanized aluminum-magnesium steel with excellent adhesion, comprising the following steps:

[0076] 1) After smelting the molten iron, it is refined and continuously cast in thin slabs at a casting speed of 4.5 to 5.0 m / min to obtain a slab with a thickness of 60 to 85 mm.

[0077] 2) The billet is pre-descaled at a pressure of 140–180 bar; then it is homogenized in a furnace at a temperature of 850–950°C for 25–40 minutes, and exited at a temperature of 1130–1180°C; the billet is then descaled with high-pressure water at a pressure of 280–380 bar; then it is hot-rolled at a controlled rolling speed of 7–13 m / s and a final rolling temperature of 780–820°C; after hot rolling, it is cooled to 500–580°C and coiled; then the temperature of the coil is reduced to below 50°C and leveled to obtain a hot-rolled raw material coil with a thickness of 2.0–6.0 mm.

[0078] 3) The hot-rolled raw material coil is pickled, and the acid temperature is controlled at 80-90℃; then it is cold-rolled, and the total reduction rate of cold rolling is 58-80% to obtain a substrate with a thickness of 0.4-2.5mm.

[0079] 4) The substrate enters the annealing furnace and is first heated to 760-800℃ at a rate of 10-50K / s, held for 2-200s, and then cooled to 10-15℃ higher than the temperature of the plating solution in the zinc pot at a rate greater than 10K / s. The hydrogen concentration in the annealing furnace is ≥10%, the dew point is ≤-35℃, and the dew point of the furnace nose is -15 to -5℃. Then, it is hot-dip galvanized aluminum-magnesium steel in the zinc pot, where the temperature of the plating solution is 40-50℃ higher than the melting point of the coating. During the air cooling process after removal from the pot, the average cooling rate of the coating from the start of solidification to the end of solidification is greater than 30K / s, resulting in hot-dip galvanized aluminum-magnesium steel with excellent adhesion.

[0080] The specific process conditions used in the following examples and comparative examples include 17 types, as shown in Tables 3 to 5.

[0081] Table 3. Relevant parameters for continuous casting, billet homogenization, and hot rolling.

[0082]

[0083] Table 4. Relevant parameters for hot rolling, pickling, and cold rolling.

[0084]

[0085]

[0086] Table 5. Relevant parameters for annealing in an annealing furnace and hot-dip galvanizing in a zinc pot with aluminum and magnesium alloys.

[0087]

[0088] Table 6 shows the substrate composition, coating composition, and process conditions used in each embodiment and comparative example. To better compare the effects, the coating thickness of the test samples was 120 g / m² on one side. 2 The effects of each embodiment and comparative example were tested. The corrosion resistance was reflected by the red rust resistance cycle of the CCT surface, and the test was conducted according to standard JASO M609-91. The specific test conditions were: one cycle of 8 hours; including 2 hours of salt spray using 5% NaCl solution at 35°C; followed by 4 hours of drying at 60°C with a relative humidity of 20-30%; followed by 2 hours of wetting at 50°C with a relative humidity of 95%; the above steps were repeated until red rust appeared; the cycle from the start of the experiment to the appearance of red rust was defined as the CCT surface red rust resistance cycle. The adhesion of the coating was reflected by testing for cracking or peeling during 0T and 1T bending. No cracking or peeling was marked as √ (qualified), while cracking or peeling was marked as × (unqualified).

[0089] Table 6. Specific conditions and results for each embodiment and comparative example.

[0090]

[0091]

[0092] As shown in Table 6, the hot-dip galvanized magnesium-aluminum steels obtained in each embodiment of the present invention exhibit good mechanical properties and corrosion resistance, and the coating has excellent adhesion. Figure 1 is a microstructure diagram of the coating in Example 6. As can be seen from the figure, the microstructure of the coating includes primary (Al) phase, bulk MgZn2 phase encapsulating (Al) phase, bulk (Zn) phase, Mg2Si phase, and multi-element eutectic Al / Zn / MgZn2 microstructure. Among them, the bulk MgZn2 phase encapsulating (Al) phase and the tough (Al) phase help to suppress the propagation of MgZn2 cracks. Figure 2 is the interface reaction layer of Example 6. As can be seen from the figure, it is a dense, approximately equiaxed granular Al-Fe compound. Figure 3 shows the image and energy dispersive spectroscopy (EDS) analysis of the Ca-containing compound on the interface reaction layer of Example 6. The white plus sign in the left figure indicates the Ca-containing compound, and the EDS analysis also confirms the presence of the Ca-containing compound.

[0093] In Comparative Examples 1 and 2, the yield strength of the resulting steel was significantly reduced due to the low Nb or P content in the substrate composition. Figure 4 shows the coating microstructure of Comparative Example 3. It can be seen from the figure that when the Al / Mg mass ratio is too low, some MgZn2 phases are formed independently, resulting in decreased adhesion and dezincification during 0T bending. Figure 5 shows the interface reaction layer of Comparative Example 4. It can be seen from the figure that when the Al / Si mass ratio is too high, the Al and Fe reaction layer consists of columnar particles, resulting in decreased adhesion and dezincification during 0T bending. Figure 6 shows the interface reaction layer of Comparative Example 5. It can be seen from the figure that when the Al / Si mass ratio is too low, excess Si forms a large number of Si-containing particles on the interface reaction layer between the coating and the substrate, resulting in decreased adhesion and dezincification during 0T bending. Zinc descaling occurred during bending. In Comparative Example 6, due to the low Ca content, the surface of the plating solution could not form enough dense oxides containing Ca to inhibit further oxidation of the plating solution. As a result, more Al and Mg oxides were generated on the surface of the plating solution. These oxides would adhere to the substrate or the plating layer, thereby reducing the adhesion of the plating layer. Zinc descaling occurred during 0T bending. In Comparative Example 7, due to the high Ca content, too much Ca could not be completely dissolved in the plating solution. It formed various compounds with elements such as Al, Mg, and Si, which adhered to the substrate or the plating layer, thereby reducing the adhesion of the plating layer. As a result, both the 0T and 1T bending tests failed.

[0094] Comparative Example 8 shows that when the pre-descaling pressure before entering the furnace is too low, the protective slag or large pieces of iron oxide scale on the surface of the billet cannot be effectively removed. The protective slag or iron oxide scale remaining on the substrate surface is not conducive to hot-dip galvanizing and affects the adhesion of the coating after hot-dip galvanizing. Comparative Example 9 shows that when the heating time is too short, iron oxide scale of appropriate thickness cannot be formed on the surface of the billet, which is not conducive to subsequent high-pressure water descaling. The iron oxide scale remaining on the substrate surface is not conducive to hot-dip galvanizing and affects the adhesion of the coating after hot-dip galvanizing. Comparative Example 10 shows that when the billet exit temperature is too high, the primary iron oxide scale is thick, and subsequent high-pressure water descaling cannot completely remove it, resulting in a thick layer of iron oxide scale remaining on the substrate surface, which cannot be completely removed in subsequent processes, affecting the adhesion of the coating after hot-dip galvanizing. Comparative Examples 11 and 12 show that when the final rolling temperature or coiling temperature is too high, excessive secondary or tertiary iron oxide scale will be generated, which cannot be completely removed in subsequent processes, reducing the adhesion of the coating after hot-dip galvanizing. Comparative Example 13 shows that when the pickling temperature is too low, the acid solution cannot effectively dissolve the iron oxide scale on the substrate surface, and the residual iron oxide scale will reduce the adhesion between the coating and the substrate. Comparative Example 14 shows that when the annealing temperature is too low, the residual iron oxide scale on the strip surface cannot be effectively reduced to iron, which will affect the wetting of the plating solution on the substrate during the hot-dip galvanizing process, thereby reducing the adhesion of the coating. Comparative Example 15 shows that when the hydrogen concentration is too low, the iron oxide scale on the substrate surface cannot be effectively reduced, and the residual iron oxide scale will reduce the adhesion between the coating and the substrate. Comparative Example 16 shows that when the dew point in the annealing furnace is too high, the strip surface will be locally oxidized, affecting the wetting of the plating solution on the substrate during the hot-dip galvanizing process, thereby reducing the adhesion of the coating. Comparative Example 17 shows that when the dew point at the furnace nose is too low, the atmosphere at that location cannot effectively suppress the zinc ash formed on the zinc liquid surface, and the zinc ash adhering to the substrate will reduce the adhesion between the coating and the substrate. Comparative Example 18 shows that when the plating bath temperature is too high, the zinc ash and zinc dross increase significantly, and the zinc dross remaining in the coating will reduce the adhesion of the strip steel. Comparative Example 19 shows that when the cooling rate after plating is too low, the size of the generated blocky MgZn2 phase will increase, making it prone to cracking during bending deformation and reducing the adhesion of the coating.

[0095] The above embodiments are merely examples for clear illustration and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations, and any obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for manufacturing hot-dip galvanized aluminum-magnesium steel with excellent adhesion, characterized in that, Includes the following steps: 1) After smelting the molten iron, it is refined and continuously cast into thin slabs to obtain a billet; 2) The billet is pre-descaled, then homogenized in a furnace at a temperature of 850~950℃ for 25~40 minutes, and exited at a temperature of 1130~1180℃; the billet is descaled by high-pressure water, then hot-rolled at a final rolling temperature of 780~820℃; after hot rolling, it is cooled to 500~580℃ and coiled, then the temperature of the coil is reduced to below 50℃ and leveled to obtain a hot-rolled raw material coil; 3) The hot-rolled raw material coil is pickled at an acid temperature of 80~90℃; then cold-rolled with a total reduction of 58~80% to obtain a base plate; The chemical composition of the substrate, by mass percentage, includes: C: 0.05~0.07%, Si: 0.05~0.20%, Mn: 0.3~0.5%, Als: 0.025~0.055%, P: 0.031~0.035%, S≤0.010%, Nb: 0.01~0.02%, with the balance being Fe and other unavoidable impurities; 4) The substrate enters the annealing furnace and is first heated to 760~ The solution is held at 800℃ and then cooled to 10-15℃ higher than the temperature of the zinc bath. The hydrogen concentration in the annealing furnace is ≥10%, the dew point is ≤-35℃, and the dew point at the furnace nose is -15~-5℃. Then, it is hot-dip galvanized in a zinc bath with the solution temperature 40-50℃ higher than the melting point of the coating. After cooling, the average cooling rate from the start of solidification to the end of solidification is greater than 30K / s, resulting in hot-dip galvanized aluminum-magnesium steel with excellent adhesion. The steel has a yield strength ≥380MPa, tensile strength ≥450MPa, and elongation ≥25%; the hot-dip galvanized aluminum-magnesium steel includes a substrate and a coating attached to the surface of the substrate; the chemical composition of the coating, by mass percentage, includes: Al: 12.0~25.0%, Mg: 4.0~10.0%, Si: 0.06~0.5%, Ca: 0.01~0.2%, and the Al / Mg mass ratio ≥2.5, the Al / Si mass ratio: 50~160, and the balance is Zn and other unavoidable impurities; the microstructure of the coating includes primary (Al) phase, blocky MgZn2 phase encapsulated (Al) phase, blocky (Zn) phase, Mg2Si phase, and multi-element eutectic Al / Zn / MgZn2 microstructure; there is an interfacial reaction layer between the coating and the substrate, which is a dense, approximately equiaxed granular Al-Fe compound coated with a Ca-containing compound.

2. The method for manufacturing hot-dip galvanized aluminum-magnesium steel with excellent adhesion according to claim 1, characterized in that, The thickness of the substrate is 0.4~2.5mm; the single-sided adhesion weight of the coating is 40~300g / m². 2 .

3. The method for manufacturing hot-dip galvanized aluminum-magnesium steel with excellent adhesion according to claim 1, characterized in that, The descaling pressure for the pre-descaling is 140~180 bar; the descaling pressure for the high-pressure water descaling is 280~380 bar.

4. The method for manufacturing hot-dip galvanized aluminum-magnesium steel with excellent adhesion according to claim 1, characterized in that, The heating rate of the substrate in the annealing furnace is 10~50K / s, the holding time after heating is 2~200s, and the cooling rate after holding is greater than 10K / s.

5. The method for manufacturing hot-dip galvanized aluminum-magnesium steel with excellent adhesion according to claim 1, characterized in that, The continuous casting speed is 4.5~5.0 m / min, and the resulting billet thickness is 60~85 mm; the hot rolling controlled rolling speed is 7~13 m / s, and the resulting hot-rolled raw material coil thickness is 2.0~6.0 mm.

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