Blackening-resistant zinc-aluminum-magnesium coated steel plate and control method

Abstract

The invention discloses a blackening-resistant zinc-aluminum-magnesium coated steel plate, a coating comprises the following components in percentage by mass: 1-3% of Al, 1-2% of Mg, 0.001-0.015% of Fe and the balance of zinc, and the total content E of the added Al and Mg is 2.0-5.0%; the invention further discloses a control method, the working mode of the finishing machine is a rolling force control mode, and the rolling force is 220-420 KN. The technical problem that the occurrence rate of blackening of an existing low-aluminum zinc-aluminum-magnesium steel plate in the long-term storage process is high is effectively solved, and meanwhile the good coating performance of the steel plate is guaranteed. After the method is adopted, the blackening occurrence rate of the steel plate after the storage period is 6 months is reduced to be within 20% from original 80% or above, and the blackening resistance is greatly improved.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a blackening-resistant zinc-aluminum-magnesium coated steel plate and a control method thereof. Background Art

[0002] Hot-dip galvanized aluminum-magnesium steel sheet is made by adding certain elements such as Mg and Al to the existing hot-dip galvanized steel sheet coating. Due to its superior corrosion resistance, self-repairing and good processing performance, it is widely used in the fields of automobiles, home appliances, photovoltaics, and construction. At present, domestic zinc-aluminum-magnesium products can be divided into low-aluminum zinc-aluminum-magnesium (Al content: 1-3%, Mg content: 1-2%, and the rest is Zn) according to the different Al content in the coating; medium-aluminum zinc-aluminum-magnesium (Al content: 3-13%, Mg content: 1-3%, and the rest is Zn) and high-aluminum zinc-aluminum-magnesium (Al content: 47-57%, Mg content: 1-2%, and the rest is Zn). Low-aluminum zinc-aluminum-magnesium has good corrosion resistance and excellent surface quality, and its application in the field of home appliances is becoming more and more extensive.

[0003] During the processing and circulation of parts, zinc-aluminum-magnesium materials inevitably need to be stored in the warehouses of home appliance companies for a period of time, which can be as short as three to four days or as long as several months. After unpacking and during the subsequent unwinding of steel coils, home appliance processing companies often find blackening defects along the edge areas of the steel plate surface. The blackened area accounts for about 1 / 3 of the entire plate width, and the morphology is similar to a "mountain peak". This defect seriously affects the customer's subsequent coating process, especially for appearance parts with high surface quality requirements, resulting in extremely poor appearance defects. In severe cases, they may even fail to pass factory inspection. According to statistics, when the storage period of low-aluminum zinc-aluminum-magnesium materials in the warehouse exceeds 3 months, the proportion of blackening is about 60%, and when the storage period exceeds 6 months, the proportion of blackening is about 80%.

[0004] The blackening is mainly caused by the fact that during the storage of steel plates, substances such as air and water vapor enter the interior of the steel coil through capillary action to form an electrochemical reaction with a difference in oxygen concentration, which in turn causes the blackening of the steel plate surface due to oxidation. Existing technologies usually use methods such as coating a layer of organic film on the surface of the original zinc-aluminum-magnesium material, or increasing the amount of oil applied, to prevent substances such as air and water vapor from penetrating into the interior of the steel coil from the edge area of ​​the steel coil, so as to prevent oxidation of the steel plate surface. Although this post-treatment method can temporarily prevent the blackening of the steel coil, it is not effective for steel coils that have been stored for more than 6 months or even longer. At the same time, the method of coating with organic films has a great impact on the steel plate coating process of home appliance processing companies, and it is easy to cause paint peeling and poor coating problems.

[0005] The Chinese patent document with application publication number CN117344098A published "A method for manufacturing zinc-aluminum-magnesium coated steel coils with excellent blackening resistance and steel coils". This method mainly prevents the defect of "rapid blackening" of offline steel coils by controlling factors such as the chemical composition of the plating solution, zinc liquid temperature, pot entry temperature, quenching tank temperature, pH value, electrical conductivity and flattening elongation during the hot-dip process. Although this method has a certain effect on the blackening problem of offline steel coils in a short period of time, it is not applicable to the blackening of steel coils with a long storage period, and the proportion of blackening of zinc-aluminum-magnesium materials is still high;

[0006] The Chinese patent document with application publication number CN111676434A discloses "A blackening-resistant zinc-aluminum-magnesium coated steel sheet and its preparation method". This method mainly covers the surface of the zinc-aluminum-magnesium coated steel coil with a layer of transparent oxide with a thickness of 50-200nm to isolate the coating from the reaction with water vapor and oxygen to inhibit its blackening. However, since there is a layer of transparent oxide on the surface of zinc-aluminum-magnesium, this method has certain adverse effects on the later coating process of home appliance companies, and is prone to defects such as poor coating.

[0007] The Chinese patent document with application publication number CN117551956A discloses "A zinc-aluminum-magnesium coated steel sheet with excellent blackening resistance and its manufacturing method". This method mainly prevents the blackening of the steel sheet surface by coating a layer of anti-fingerprint film on the surface of the zinc-aluminum-magnesium material and controlling the process parameters such as the thickness of the anti-fingerprint film, baking temperature, cooling temperature and plating solution composition. However, the surface passivation agent required by this method not only increases the manufacturing cost of the production enterprise, but also the anti-fingerprint film on the surface has a certain impact on the later coating and painting effects of home appliance enterprises, and is not suitable for promotion and application in home appliance enterprises. Summary of the invention

[0008] The purpose of the present invention is to solve the above technical problems, provide a blackening-resistant zinc-aluminum-magnesium coated steel plate and a control method, and solve the technical problem that the existing low-aluminum zinc-aluminum-magnesium steel plate has a high blackening rate during long-term storage.

[0009] To achieve the above-mentioned purpose, the present invention provides a blackening-resistant zinc-aluminum-magnesium coated steel plate, comprising a substrate and a coating plated on the substrate, wherein the substrate is a low-aluminum zinc-aluminum-magnesium steel plate, and the coating comprises, by mass percentage, 1-3% Al, 1-2% Mg, 0.001-0.015% Fe, and the remainder is zinc, and the total added content of Al and Mg △E=2.0-5.0%.

[0010] The addition of Al element in the plating solution is mainly to improve the corrosion resistance of the zinc-aluminum-magnesium coating. However, if the Al element is added too much, it is easy to cause the formation of suspended slag in the zinc pot, which can easily cause the surface quality of the steel plate to deteriorate; if the Al element is added too little, it is easy to cause the corrosion resistance of the coating to decrease. The Al content is further preferably 1.5-2.0%.

[0011] The addition of Mg element in the plating solution will increase the sensitivity of the steel plate surface to blackening. If the Mg element in the plating solution is too high, it will easily lead to an excessive amount of ternary eutectic phase in the coating, which is very likely to cause surface oxidation, so the blackening tendency of the steel plate is significantly enhanced; if the Mg element is too low, it will easily cause a decrease in the corrosion resistance of the coating. The preferred Mg content is 1.2-1.5%.

[0012] The total added content △E of Al and Mg elements in the plating solution is controlled to be maintained in the range of 2.0-5.0%. If the total content △E exceeds 5%, the hardness of the coating will be significantly improved, the brittleness will increase, and the surface roughness Rpc value will be difficult to control, which will affect the oil storage capacity of the coating and increase the incidence of blackening; if the total content △E is lower than 2%, the corrosion resistance of the coating will not be guaranteed, and the preferred △E is 2.7-3.5%.

[0013] A control method for the blackening-resistant zinc-aluminum-magnesium coated steel plate as described above is also provided. In the present invention, a rolling force control mode is selected for the working mode of the skin pass mill, rather than a conventional elongation control mode. Selecting the rolling force control mode can effectively control the roughness Rpc value of the zinc-aluminum-magnesium steel plate surface, improve the oil storage effect on the steel plate surface, and reduce the tendency of the steel plate to blacken; and control the rolling force in the range of 220 to 420 KN. If the rolling force is too low, it is easy to cause the roughness Rpc value of the steel plate surface to be too low, reduce the oil storage effect on the steel plate surface, and cause the steel plate to blacken; if the rolling force is too high, it is easy to cause the steel plate to have a poor shape and cannot prevent the intrusion of substances such as air and water vapor. It is further preferred that the rolling force is 320 to 400 KN.

[0014] In the present invention, the roughness Rpc value of the steel plate and the amount of oil applied must satisfy the relationship t=Rpc value×10 -2 / oil coating amount, t = 0.6 ~ 1.0, where the unit of oil coating amount is g / m 2 If the t value is higher than 1.0, the oil film thickness cannot cover the peak points of higher roughness Rpc value (i.e. too many surface bumps), which may easily cause surface blackening; if the t value is lower than 0.6, too much oil will overflow from the edge of the steel plate, causing inevitable pollution and waste.

[0015] Furthermore, a shape meter roller is arranged between the skin-pass mill and the post-tension roller, and a middle-wave shape control is adopted between the skin-pass mill and the post-tension roller to control the straightness of the steel plate to be ≤5UI, instead of the traditional control mode which usually adopts the edge-wave shape control method. The present invention can effectively prevent substances such as air and water vapor from entering the interior of the steel coil under the principle of capillary action, thereby causing the steel coil to turn black, and has a significant effect on steel coils stored for a long period of time.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention adjusts and optimizes the key process parameters of the low-aluminum zinc-aluminum-magnesium steel plate in the hot-dip plating process, such as the plating solution composition, the working mode of the skin-pass mill, the rolling force, the oil coating amount and the roughness Rpc, and adds a plate shape meter roller between the skin-pass mill and the post-tension roller, so as to control the flatness of the steel plate, which not only effectively solves the technical problems such as the high incidence of blackening of the existing low-aluminum zinc-aluminum-magnesium steel plate during long-term storage, but also ensures the good coating performance of the steel plate; after adopting the present invention, the blackening incidence of the steel plate after a storage period of 6 months is reduced from more than 80% to less than 20%, thereby greatly improving the blackening resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic diagram of the layout of the shape meter rollers of the present invention. DETAILED DESCRIPTION

[0018] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0019] A low-carbon aluminum-killed cold-rolled steel sheet with a thickness of 1.2 mm and a width of 1250 mm is used as the substrate, and the coating type is a zinc-aluminum-magnesium coating. The present invention is further described below through some embodiments.

[0020] The control of the hot-dip plating solution composition of the embodiment is shown in Table 1, the control of the key process parameters of the embodiment is shown in Table 2, and the blackening rate of the steel coil after storage of the embodiment is shown in Table 3.

[0021] Table 1 Control of plating solution composition in hot dip plating Example (mass fraction) % Table 1 Control of plating solution composition

[0022]

[0023] Table 2 Key process parameter control of the embodiment

[0024] Table 2 Key process control

[0025]

[0026] like Figure 1 As shown, a shape meter roller is arranged between the skin pass mill and the post-tension roller. The shape meter roller detects the plate shape between the skin pass mill and the post-tension roller and controls the flatness of the steel plate.

[0027] The corresponding galvanized aluminum-magnesium steel plates were obtained by controlling the process parameters of the above-mentioned embodiments 1 to 4 and comparative examples 5 to 6. After being stored in a warehouse for 3 to 6 months, the proportion of the steel plates that turned black was observed and counted, as shown in Table 3.

[0028] Table 3 Blackening rate of steel coils after storage (%)

[0029]

Claims

1. A blackening-resistant zinc-aluminum-magnesium coated steel sheet, comprising a substrate and a coating coated on the substrate, wherein the substrate is a low-aluminum zinc-aluminum-magnesium steel sheet, characterized in that: The composition of the coating includes, by mass percentage, 1-3% Al, 1-2% Mg, 0.001-0.015% Fe, and the balance is zinc, and the total content of Al and Mg added is ΔE=2.0-5.0%.

2. The blackening-resistant zinc-aluminum-magnesium coated steel sheet according to claim 1, characterized in that: The Al content is 1.5-2.0%.

3. The blackening-resistant zinc-aluminum-magnesium coated steel sheet according to claim 1, characterized in that: The Mg content is 1.2-1.5%.

4. The blackening-resistant zinc-aluminum-magnesium coated steel sheet according to claim 1, characterized in that: The ΔE is preferably 2.7 to 3.5%.

5. A control method for the blackening-resistant zinc-aluminum-magnesium coated steel sheet as claimed in claim 1, characterized in that: The working mode of the skin pass mill is the rolling force control mode, and the rolling force is 220~420KN.

6. The control method according to claim 5, characterized in that: The roughness Rpc value of the steel plate and the amount of oil applied must satisfy the relationship t=Rpc value×10 -2 / oil coating amount, t = 0.6 ~ 1.0, where the unit of oil coating amount is g / m 2 .

7. The control method according to claim 5, characterized in that: A shape meter roller is arranged between the skin pass mill and the post-tension roller.

8. The control method according to claim 5, characterized in that: The middle wave plate shape control is adopted between the skin pass machine and the post tension roller.

9. The control method according to claim 8, characterized in that: The straightness of the steel plate is ≤5UI.

10. The control method according to claim 5, characterized in that: The rolling force is 320-400 KN.

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