JP7809674B2 - Method for manufacturing hot-dip Al-Zn-Si-Mg plated steel sheet - Google Patents

Description

The present invention relates to a method for producing hot-dip Al-Zn-Si-Mg-plated steel sheet that can suppress the generation of Mg-containing dross and produce hot-dip Al-Zn-Si-Mg-plated steel sheet with excellent appearance.

Hot-dip Al-Zn coated steel sheets, typified by 55% Al-Zn, are known to exhibit high corrosion resistance among various coated steel sheets because they combine the sacrificial corrosion protection of Zn with the high corrosion resistance of Al, as shown in Patent Document 1, for example. For this reason, due to their excellent corrosion resistance, hot-dip Al-Zn coated steel sheets have been widely used in the building materials field as materials for roofs and walls that are exposed to the outdoors for long periods of time.

Furthermore, in recent years, the building materials market has been demanding further improvements in corrosion resistance in order to extend the life of products and ensure corrosion resistance in severely corrosive environments such as coastal areas, etc. For this reason, as shown in Patent Document 2, for example, many hot-dip Al-Zn-Si-Mg-plated steel sheets have been proposed in which corrosion resistance is improved by adding Mg to the coating, and these hot-dip Al-Zn-Si-Mg-plated steel sheets are expected to be applied not only in the field of building materials but also in the fields of electrical machinery and civil engineering, etc.
In general, hot-dip Al-Zn-Si-Mg-plated steel sheets are manufactured by using a thin steel sheet obtained by hot-rolling or cold-rolling a slab as a base steel sheet, and subjecting the base steel sheet to recrystallization annealing and hot-dip plating treatment in an annealing furnace of continuous hot-dip plating equipment.

However, with the above-mentioned method of hot-dip coating using continuous hot-dip coating equipment, the addition of magnesium, which has a high oxidizing effect, generates lumpy dross containing magnesium (hereinafter referred to as "Mg-containing dross") near the surface of the coating bath, posing a problem in that the amount generated is greater than in the conventional production of hot-dip Al-Zn coated steel sheet. When the amount of this Mg-containing dross increases, it adheres to the steel sheet, causing poor appearance such as uncoated defects and unevenness, resulting in a deterioration of the steel sheet's appearance.

Therefore, as a technique for preventing dross when adding Mg, Patent Document 3 discloses a technique for sealing the space above the plating bath with an inert gas.
Furthermore, Patent Document 4 discloses a technology for reducing the amount of dross generated in the production of hot-dip Al-Zn-Mg coated steel sheets by dissolving Al in a molten Zn bath and then dissolving at least one alloy selected from the group consisting of pure Mg, an Mg-Zn alloy, and an Al-Mg-Zn alloy.

Special Publication No. 46-7161 Japanese Patent Application Laid-Open No. 2001-316791 Special Publication No. 61-33070 Japanese Patent Application Publication No. 11-193452

However, the technology disclosed in Patent Document 3 requires high costs for equipment and maintenance thereof and cannot be easily applied. Furthermore, even when this technology is used in the production of hot-dip Al-Zn-Si-Mg-plated steel sheets, particularly in the production using a bath with a high Mg concentration, it is not possible to sufficiently suppress the generation of Mg-containing dross, and it is not possible to stably obtain an excellent appearance.
Furthermore, the technology of Patent Document 4 does not disclose specific ranges for the compositions of the Mg-Zn alloy or Al-Mg-Zn alloy used, and even if this method is used to produce hot-dip Al-Zn-Si-Mg-plated steel sheets, it does not necessarily have the effect of suppressing the generation of Mg-containing dross, and there is still a need for improvement in terms of stable appearance. Furthermore, when an alloy with a high melting point is used, it takes a long time to melt it, which not only increases the time required to make the bath but also makes it difficult to adjust the composition in a short period of time, making it difficult to apply this method to the production of hot-dip Al-Zn-Si-Mg-plated steel sheets using continuous hot-dip galvanizing equipment in which the bath composition is constantly monitored and fine-tuned.

In light of these circumstances, the present invention aims to provide a method for producing hot-dip Al-Zn-Si-Mg-plated steel sheet that can suppress the generation of Mg-containing dross and produce hot-dip Al-Zn-Si-Mg-plated steel sheet with excellent appearance.

As a result of investigations aimed at solving the above-mentioned problems, the inventors focused on the coating bath preparation process, which is the process most closely related to the generation of dross among the processes for manufacturing hot-dip Al-Zn-Si-Mg-coated steel sheets. They discovered that by appropriately controlling the raw materials added to the coating bath, particularly the Mg raw materials, it is possible to suppress the generation of Mg-containing dross and consistently produce hot-dip Al-Zn-Si-Mg-coated steel sheets with excellent appearance.

The present invention has been made based on the above findings, and its gist is as follows: 1. A method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet using a continuous hot-dip galvanizing facility, comprising:
The method includes a hot-dip galvanizing step of immersing a substrate steel sheet in a coating bath having a composition containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and 1.0 to 10.0 mass% Mg, with the balance being Zn and unavoidable impurities;
A method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet, characterized in that an Al-Mg alloy containing 10 to 90 mass% of Mg is used as a source of Mg in the plating bath.
2. The method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet according to item 1 above, wherein the coating bath further contains 0.01 to 3.0 mass% in total of one or more elements selected from B, Ca, Ti, V, Cr, Mn, Sr, Mo, In, Sn, Sb, Ce, and Bi.
3. The method for producing a hot-dip Al-Zn-Si-Mg coated steel sheet according to 1 or 2 above, wherein the Al-Mg alloy contains 20 to 80 mass % of Mg.
4. The method for producing a hot-dip Al-Zn-Si-Mg plated steel sheet according to 1 or 2 above, wherein the Al-Mg alloy contains 30 to 70 mass % of Mg.

The present invention makes it possible to suppress the generation of Mg-containing dross and produce hot-dip Al-Zn-Si-Mg-plated steel sheets with excellent appearance.

This is a binary calculated phase diagram of an Al-Mg alloy.

The method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention is a method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet using continuous hot-dip plating equipment.
Regarding the hot dip plating process, there are no particular limitations on the conditions other than the composition of the plating bath and the raw material of Mg in the plating bath, which will be described later.

For example, in continuous hot-dip galvanizing equipment, a base steel sheet can be cleaned, heated, and then immersed in a coating bath to form a coating film. In the heating process for the base steel sheet, recrystallization annealing or other treatments are performed to control the structure of the base steel sheet itself. It is also effective to heat the steel sheet in a reducing atmosphere, such as a nitrogen-hydrogen atmosphere, to prevent oxidation and reduce the trace oxide film present on the surface.

The base steel sheet is not particularly limited, and cold-rolled steel sheet, hot-rolled steel sheet, etc. can be used as appropriate depending on the required performance and specifications.
Furthermore, the method for obtaining the base steel sheet is not particularly limited. For example, in the case of the hot-rolled steel sheet, a steel sheet that has been subjected to a hot-rolling process and a pickling process can be used, and in the case of the cold-rolled steel sheet, a cold-rolling process can be added for production. Furthermore, in order to obtain the desired steel sheet properties, a pre-plating process, a recrystallization annealing process, or the like can be performed before the hot-dip galvanizing process.

The method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention includes a hot-dip galvanizing step of immersing a substrate steel sheet in a coating bath having a composition containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and 1.0 to 10.0 mass% Mg, with the balance being Zn and unavoidable impurities.
This allows a hot dip Al-Zn-Si-Mg coating to be formed on the base steel sheet.

The Al content in the coating bath is 45 to 65 mass %, preferably 50 to 60 mass %, in view of the balance between the corrosion resistance of the resulting hot-dip Al-Zn-Si-Mg coated steel sheet and operational aspects.
This is because an Al content of at least 45% by mass causes dendritic solidification of Al, allowing the formation of an α-Al phase dendritic solidification structure in the resulting plating film, which improves corrosion resistance. In addition, the dendritic solidification structure has a structure in which layers are stacked in the thickness direction of the plating film, and the greater the number of layers, the more complex the corrosion progression path from the plating surface becomes, thereby improving corrosion resistance. Therefore, it is preferable that the Al content in the plating bath be 50% by mass or more.
On the other hand, if the Al content in the coating bath exceeds 65% by mass, the resulting coating film changes to a structure in which most of the Zn is solid-solved in α-Al, which increases the dissolution rate during corrosion and deteriorates the corrosion resistance of the hot-dip Al-Zn-Si-Mg-coated steel sheet. Therefore, the Al content in the coating bath must be 65% by mass or less, and preferably 60% by mass or less.

The Si content in the coating bath is 1.0 to 4.0 mass %. ...
In fact, when a steel sheet is immersed in an Al-Zn-Si-Mg coating bath containing Si, an alloying reaction occurs between the Fe on the steel sheet surface and the Al and Si in the bath, resulting in the formation of an Fe-Al and/or Fe-Al-Si intermetallic compound layer at the interface between the base steel sheet and the coating. However, since the growth rate of the Fe-Al-Si alloy is slower than that of the Fe-Al alloy, the higher the proportion of the Fe-Al-Si alloy, the more the growth of the entire interfacial alloy layer is suppressed. Therefore, the Si content in the coating bath must be 1.0 mass% or more.
On the other hand, if the Si content in the plated film exceeds 4.0 mass%, not only will the aforementioned effect of inhibiting the growth of the interfacial alloy layer saturate, but excess Si phase will form in the resulting plated film, reducing workability. Therefore, the Si content is set to 4.0 mass% or less.

Furthermore, Mg in the plating bath is a component contained for the purpose of forming intermetallic compounds such as _Mg2Si and _MgZn2 in the plating film and improving the corrosion resistance of the hot-dip Al-Zn-Si-Mg-plated steel sheet.
If the Mg content in the plating bath is less than 1.0 mass%, sufficient corrosion resistance cannot be ensured because Mg is used to dissolve in the α-Al phase, which is the main phase, rather than to form the intermetallic compounds ( _Mg2Si , _MgZn2 ) in the plating film. On the other hand, if the Mg content in the plating bath is too high, the corrosion resistance improvement effect saturates, the α-Al phase becomes brittle, workability deteriorates, and the amount of dross generated increases. Therefore, the Mg content is set to 10.0 mass% or less.
Therefore, the Mg content in the coating bath needs to be 1.0 to 10.0 mass %, and from the viewpoint of achieving both improved corrosion resistance and suppression of dross, it is preferably 3.0 to 5.0 mass %.

The plating bath contains Zn and unavoidable impurities in addition to the above-mentioned Al, Si, and Mg.
Among these, the inevitable impurities include Fe. This Fe is inevitably contained in the plating film as a result of dissolution of the steel sheet or bath-immersed equipment into the plating bath, and as a result of being supplied by diffusion from the base steel sheet during the formation of the interfacial alloy layer. The Fe content in the plating bath is typically about 0.1 to 0.5 mass%.
Inevitable impurities other than Fe include Ni, Cu, Co, W, etc. These elements may be dissolved into the plating bath from the base steel sheet, stainless steel bath-immersed equipment, or WC-based or Co-Cr-W-based thermal spray coatings applied to bath-immersed equipment, or may be contained as impurities in the metal ingots that are used as raw materials for the plating bath.

The total content of unavoidable impurities in the plating film is not particularly limited, but if contained in excess, it may affect various properties of the hot-dip Al-Zn-Si-Mg-plated steel sheet produced, so it is preferable to control the total content to 5.0 mass% or less.

Next, the raw material of Mg, which is the most important element in the present invention, will be described.
In the method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention, an Al-Mg alloy containing 10 to 90 mass % of Mg is used as the Mg source to be charged into the coating bath, because the use of an Al-Mg alloy as the Mg source makes it possible to suppress the generation of Mg-containing dross in the raw material melting step.
Because magnesium has a strong oxidizing effect, when pure magnesium or an magnesium-zinc alloy is melted, the dissolved magnesium oxidizes violently at the surface of the coating bath, resulting in the formation of a large amount of lumpy dross (magnesium-containing dross) composed primarily of magnesium oxide. However, when an aluminum-magnesium alloy is melted as in the present invention, a thin, dense aluminum oxide film forms on the surface of the coating bath, preventing contact between the coating bath and the atmosphere and suppressing the oxidation of magnesium, thereby preventing the generation of magnesium-containing dross. To achieve this effect of suppressing magnesium-containing dross, the aluminum concentration in the aluminum-magnesium alloy must be at least 10% by mass, i.e., the magnesium concentration must be 90% by mass or less.

The Al-Mg alloy mentioned above is a binary alloy of Al and Mg, and does not include ternary alloys such as Mg-Zn-Al alloys. This is because it more reliably forms a thin, dense Al oxide film on the surface of the plating bath described above. It is sufficient to use an Al-Mg alloy as the Mg raw material, and a small amount of a ternary alloy such as an Mg-Zn-Al alloy can also be added as a material for components other than Mg.

The Mg-containing dross refers to clumped top dross, primarily composed of Mg oxide, that forms near the surface of the plating bath. It is different from the iron-containing oxides (bottom dross) that are unevenly distributed in the bath or at the bottom of the plating bath.

Furthermore, from the perspective of continuous operation, the Al-Mg alloy introduced into the coating bath must have high solubility. Figure 1 shows a binary phase diagram of Al-Mg alloys (source: Binary Alloy Phase Diagrams, vol. 1 ed. by T.B. Massalski, ASM, (1986), 129.). Since Al-Mg alloys of each composition are only liquid at temperatures higher than the liquidus indicated by the arrows in Figure 1, high solubility is achieved when the liquidus temperature is below the bath temperature. Furthermore, even higher solubility can be achieved by using an alloy with a lower liquidus temperature at the same bath temperature. As mentioned above, the bath temperature for the production of the hot-dip Al-Zn-Si-Mg-coated steel sheet of the present invention is not particularly limited, but is generally in the range of 550°C to 650°C.
Therefore, in the production of the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention, in order to obtain high solubility in the Al-Mg alloy when it is melted, the Mg concentration in the Al-Mg alloy needs to be 10 to 90 mass%, preferably 20 to 80 mass%, and more preferably 30 to 70 mass%.

Furthermore, there are no particular restrictions on the raw materials for the bath components other than Mg used when preparing the plating bath. For example, pure Al, Al-Si alloy, Al-Zn alloy, or pure Zn can be used, and the component composition of the plating bath can be adjusted by appropriately mixing and dissolving these together with the Al-Mg alloy.

The component composition of the plating bath can be confirmed by dissolving the alloy solidified from the plating bath in acid and then performing ICP emission spectroscopy, atomic absorption spectroscopy, or the like.
As a specific example, the alloy solidified from the plating bath can be dissolved in hydrochloric acid, the solution filtered, and the filtrate analyzed by ICP emission spectroscopy to quantify components other than insoluble Si. Furthermore, the solids are dried and incinerated in a heating furnace at 650°C, and then melted by adding sodium carbonate and sodium tetraborate. The molten material is dissolved in hydrochloric acid, and the resulting solution is analyzed by ICP emission spectroscopy to quantify insoluble Si. In this case, the Si concentration in the plating film can be calculated by adding the soluble Si concentration obtained by filtrate analysis to the insoluble Si concentration obtained by solid analysis.
However, the ICP emission spectroscopy is merely an example, and any method may be used as long as it can accurately quantify the component composition of the plating film, and there are no particular limitations.

The plating bath may also optionally contain one or more elements selected from B, Ca, Ti, V, Cr, Mn, Sr, Mo, In, Sn, Sb, Ce, and Bi in a total amount of 0.01 to 3.0 mass%. These elements have the effect of improving the stability of corrosion products when the formed plating film corrodes, thereby slowing the progression of corrosion, and of stabilizing the spangle size on the plating surface, thereby improving the surface appearance.

Furthermore, the bath temperature of the coating bath is not particularly limited, but is preferably in the range of (melting point + 20°C) to 650°C. The reason the lower limit of the bath temperature is set at melting point + 20°C is that in order to perform hot-dip coating, the bath temperature must be above the solidification point, and setting it at melting point + 20°C prevents solidification due to a local drop in the bath temperature of the coating bath. On the other hand, the reason the upper limit of the bath temperature is set at 650°C is that if the temperature exceeds 650°C, it becomes difficult to rapidly cool the coating film, and there is a risk that the interfacial alloy layer formed between the coating film and the steel sheet will become thick.

The temperature of the base steel sheet immersed in the coating bath (immersion sheet temperature) is not particularly limited, but it is preferable to control it to within ±20°C of the coating bath temperature in order to ensure the coating characteristics in the continuous hot-dip coating operation and to prevent changes in the bath temperature.

Furthermore, it is preferable that the immersion time of the base steel sheet in the plating bath be 0.5 seconds or more. This is because if it is less than 0.5 seconds, there is a risk that a sufficient plating film will not be formed on the surface of the base steel sheet. There is no particular upper limit to the immersion time, but since a longer immersion time may result in a thicker interfacial alloy layer being formed between the plating film and the steel sheet, it is more preferable to keep it to 8 seconds or less.

In addition, in the manufacturing method of hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention, there are no particular limitations on the processes other than the hot-dip plating process described above, and it is possible to carry out any of the processes used in the manufacture of ordinary hot-dip plated steel sheet as appropriate.

[Samples 1 to 26]
A production test was carried out using a low-carbon cold-rolled steel sheet manufactured by a conventional method, having a thickness of 0.8 mm and a width of 765 mm, as the base steel sheet, using a general continuous hot-dip galvanizing facility, in which (1) the coating bath was made up and (2) the hot-dip galvanizing treatment was carried out for 6 consecutive hours under the conditions shown in Table 1. Details of (1) the coating bath making and (2) the hot-dip galvanizing treatment are as follows.

(1) Preparation of coating bath A coating bath having the composition shown in Table 1 was prepared in a hot dip coating pot of a continuous hot dip coating facility. For the preparation of the bath, the raw materials (ingots) shown in Table 1 were used, and the bath was mixed to have the composition shown in Table 1, and then melted.
In order to make up for the loss of plating bath during the plating process, additional bath is prepared in the hot dip plating pot in the same manner as described above.
The composition of the prepared bath was confirmed by dissolving the alloy pieces solidified from the plating bath in hydrochloric acid, filtering the solution, and then filtering the stripper solution. The filtrate and solids were analyzed separately. Specifically, the filtrate was analyzed by ICP atomic emission spectroscopy to quantify components other than insoluble Si. The solids were dried and incinerated in a heating furnace at 650°C, and then melted by adding sodium carbonate and sodium tetraborate. The molten material was then dissolved in hydrochloric acid, and the solution was analyzed by ICP atomic emission spectroscopy to quantify the insoluble Si. The Si concentration in the plating bath was calculated by adding the soluble Si concentration obtained from the filtrate analysis to the insoluble Si concentration obtained from the solids analysis.

(2) Hot-dip galvanizing treatment After the preparation of the galvanizing bath, a galvanizing treatment was carried out using a general continuous galvanizing equipment. The strip of the base steel sheet was welded, and then continuously degreased, annealed, cooled, and galvanized.
The plating bath temperature was controlled at 600°C for all levels. The plating deposition weight was controlled to 82±0.5 g/ ^m2 per side by gas wiping.

<Evaluation>
Each sample of the hot-dip Al-Zn-Si-Mg-plated steel sheet obtained in the above-mentioned production test was evaluated as follows. The evaluation results are shown in Table 1.
(1) Solubility of Mg Raw Materials The solubility of the Mg raw materials in each production test affects the bath preparation speed, i.e., the plating treatment speed. Therefore, the solubility of each Mg raw material was judged based on the treatment speed (capacity) in the plating treatment using a plating bath prepared using that Mg raw material.
Specifically, the results were compared with the plating treatment speed when a 55% by mass Al-Zn-1.6% by mass plating bath containing no Mg (hereinafter referred to as the reference bath) was used, and the results were evaluated according to the following criteria.
◎: Very high solubility, allowing operation at the same processing speed as when using the standard bath. ○: High solubility, allowing operation at a processing speed of 90% or more compared to plating using the standard bath. △: Low solubility, but allowing operation at a processing speed of 80% or more compared to plating using the standard bath. ×: Very low solubility, making it difficult to operate at a processing speed of 80% or more compared to plating using the standard bath.

(2) Amount of Mg-containing dross generated In each production test, the bath surface during the plating treatment was visually inspected to check the amount of black dross (Mg-containing dross) generated, and evaluated according to the following criteria.
○: Presence of black dross was observed on the surface of the plating bath. ×: Presence of black dross was not observed on the surface of the plating bath.

(3) Appearance of plating film Each sample of the produced hot-dip Al-Zn-Si-Mg-plated steel sheet was checked for the presence or absence of dross defects, and the appearance was evaluated according to the following criteria.
○: No adhesion of granular dross is observed ×: Adhesion of granular dross is observed

The results in Table 1 show that each sample of the invention has high solubility of the Mg raw material, and is excellent in terms of the amount of Mg-containing dross generated and the appearance of the plating film. On the other hand, each sample of the comparative example shows poor results in at least one evaluation item.

The present invention makes it possible to suppress the generation of Mg-containing dross and produce hot-dip Al-Zn-Si-Mg-plated steel sheets with excellent appearance.

Claims (4)

A method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet using a continuous hot-dip plating facility,
The method includes a hot-dip galvanizing step of immersing a substrate steel sheet in a coating bath having a composition containing 45 to 65 mass% Al, 1.0 to 4.0 mass% Si, and 1.0 to 10.0 mass% Mg, with the balance being Zn and unavoidable impurities;
A method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet, characterized in that an Al-Mg alloy containing 10 to 90 mass% of Mg is used as a source of Mg in the plating bath. The method for producing hot-dip Al-Zn-Si-Mg-plated steel sheet according to claim 1, characterized in that the coating bath further contains one or more elements selected from the group consisting of B, Ca, Ti, V, Cr, Mn, Sr, Mo, In, Sn, Sb, Ce, and Bi in a total amount of 0.01 to 3.0 mass%. The method for producing hot-dip Al-Zn-Si-Mg-plated steel sheet according to claim 1 or 2, characterized in that the Al-Mg alloy contains 20 to 80 mass% Mg. 3. The method for producing a hot-dip Al-Zn-Si-Mg plated steel sheet according to claim 1, wherein the Al-Mg alloy contains 30 to 70 mass % of Mg.