METHOD FOR PRODUCING HOT-DIP Al-Zn-Si-Mg-PLATED STEEL SHEET

Abstract

To provide a method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet, wherein the occurrence of Mg-containing dross can be suppressed, and a hot-dip Al-Zn-Si-Mg-plated steel sheet with a superior external appearance can be obtained.SOLUTION: The present invention provides a method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet using continuous hot-dip plating equipment, characterized by comprising a hot-dip plating step in which a substrate steel sheet is immersed in a plating bath that has a composition including 45-65 mass% Al, 1.0-4.0 mass% Si, and 1.0-10.0 mass% Mg, the remainder consisting of Zn and unavoidable impurities, wherein an Al-Mg alloy including 10-90 mass% Mg is used as a source material for the Mg in the plating bath.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet, which can suppress the generation of Mg-containing dross and provide a hot-dip Al-Zn-Si-Mg-plated steel sheet having excellent appearance. [Background technology]

[0002] Hot-dip Al-Zn-plated steel sheets, typified by 55% Al-Zn, are known to exhibit high corrosion resistance among various plated steel sheets because they combine the sacrificial corrosion protection of Zn with the high corrosion resistance of Al, as shown in, for example, Patent Document 1. Because of their excellent corrosion resistance, hot-dip Al-Zn-plated steel sheets have therefore been widely used in the field of building materials as materials for roofs and walls that are exposed to the outdoors for long periods of time.

[0003] Moreover, in recent years, the building materials market has been demanding further improvement in corrosion resistance in order to extend the life of products and ensure corrosion resistance in severe corrosive environments such as coastal regions, 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 Mg is added to the plating to improve corrosion resistance, 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 sheet is 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.

[0004] However, in the above-mentioned method of performing hot-dip galvanizing treatment using continuous hot-dip galvanizing 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 galvanizing bath, and the amount of generated dross is greater than that in the conventional production of hot-dip Al-Zn-coated steel sheets. If the amount of this Mg-containing dross increases, it adheres to the steel sheet, causing appearance defects such as uncoated defects and uneven defects, and deteriorating the appearance.

[0005] Therefore, as a technique for preventing dross when adding Mg, Patent Document 3 discloses a technique for sealing the upper part of the plating bath with an inert gas. Furthermore, Patent Document 4 discloses a technique for reducing the amount of dross generated in the production of hot-dip Al-Zn-Mg coated steel sheet by dissolving Al in a molten Zn bath and then dissolving at least one selected from the group consisting of pure Mg, an Mg-Zn alloy, and an Al-Mg-Zn alloy. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 46-7161 [Patent Document 2] JP 2001-316791 A [Patent Document 3] Special Publication No. 61-33070 [Patent Document 4] Japanese Patent Application Publication No. 11-193452 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the technology disclosed in Patent Document 3 requires high equipment and maintenance costs and cannot be easily applied. Furthermore, even if 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. In addition, the technology of Patent Document 4 does not disclose specific ranges for the compositions of the Mg-Zn alloy and Al-Mg-Zn alloy used, and even if this method is used in the production of 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 to improve the stable appearance. Furthermore, when an alloy with a high melting point is used, it takes time to melt, which not only increases the time required for bath construction but also makes it difficult to adjust the components in a short time, making it difficult to apply this to the production of hot-dip Al-Zn-Si-Mg-plated steel sheets using continuous hot-dip plating equipment in which the bath composition is constantly monitored and finely adjusted.

[0008] In view of the above circumstances, the present invention has an object to provide a method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet, which can suppress the generation of Mg-containing dross and provide a hot-dip Al-Zn-Si-Mg-plated steel sheet excellent in appearance. [Means for solving the problem]

[0009] The present inventors have conducted studies to solve the above-mentioned problems. As a result, they have focused on the coating bath preparation process, which is the process most related to the generation of dross among the processes for producing hot-dip Al-Zn-Si-Mg-plated steel sheets, and have found that by appropriately controlling the raw materials charged into the coating bath, particularly the Mg raw materials, it is possible to suppress the generation of Mg-containing dross and stably produce hot-dip Al-Zn-Si-Mg-plated steel sheets having excellent appearance.

[0010] The present invention has been made based on the above findings, and the gist of the present invention is as follows: 1. A method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet using a continuous hot-dip plating facility, comprising the steps of: The method includes a hot-dip galvanizing process in which a base steel sheet is immersed 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; 2. A method for producing a hot-dip Al-Zn-Si-Mg plated steel sheet, comprising using an Al-Mg alloy containing 10 to 90 mass % of Mg 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, characterized in that the plating 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 plated steel sheet according to 1 or 2 above, characterized in that 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, characterized in that the Al-Mg alloy contains 30 to 70 mass % of Mg. Effect of the Invention

[0011] According to the present invention, the generation of Mg-containing dross can be suppressed, and it is possible to produce a hot-dip Al-Zn-Si-Mg-plated steel sheet having excellent appearance. [Brief description of the drawings]

[0012] [Figure 1] This is a binary calculated phase diagram of an Al-Mg alloy. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] 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 a continuous hot-dip plating facility. Regarding the hot dip plating process, there are no particular limitations on the conditions other than the composition of the plating bath and the source of Mg in the plating bath, which will be described later.

[0014] For example, in a continuous hot-dip galvanizing facility, a base steel sheet is washed, heated, and then immersed in a galvanizing bath to form a coating film. In the heating process of the base steel sheet, recrystallization annealing or the like is performed to control the structure of the base steel sheet itself, and heating in a reducing atmosphere such as a nitrogen-hydrogen atmosphere is effective in preventing oxidation and reducing a small amount of oxide film present on the surface.

[0015] The base steel sheet is not particularly limited, and a cold-rolled steel sheet, a hot-rolled steel sheet, or the like 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, one 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, it can be manufactured by further adding a cold rolling process. Furthermore, in order to obtain the properties of the steel sheet, it is also possible to go through a pre-plating process, a recrystallization annealing process, etc. before the hot-dip plating process.

[0016] The method for producing a hot-dip Al-Zn-Si-Mg plated steel sheet of the present invention includes a hot-dip galvanizing treatment step of immersing a base steel sheet in a plating 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 makes it possible to form a hot-dip Al-Zn-Si-Mg coating on the base steel sheet.

[0017] The Al content in the coating bath is 45 to 65 mass %, and 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, if the Al content is at least 45 mass%, dendritic solidification of Al occurs, and the resulting plating film can form a dendritic solidification structure of the α-Al phase that 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 more the number of layers increases, the more complex the corrosion progression path from the plating surface becomes, improving corrosion resistance, so it is preferable that the Al content in the plating bath is 50 mass% or more. On the other hand, if the Al content in the coating bath exceeds 65 mass%, the coating film formed changes to a structure in which most of the Zn is dissolved 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 mass% or less, and is preferably 60 mass% or less.

[0018] The Si content in the coating bath is 1.0 to 4.0 mass% and is contained in order to suppress the growth of an Fe-Al and / or Fe-Al-Si interfacial alloy layer formed at the interface with the base steel sheet and to prevent the deterioration of the adhesion between the resulting coating and the steel sheet. In fact, when a steel sheet is immersed in an Al-Zn-Si-Mg-based coating bath containing Si, the Fe on the steel sheet surface reacts with the Al and Si in the bath to form an Fe-Al and / or Fe-Al-Si intermetallic compound layer at the interface between the base steel sheet and the coating film, but since the growth rate of the Fe-Al-Si alloy is slower than that of the Fe-Al alloy, the higher the ratio 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 plating film exceeds 4.0 mass%, not only will the aforementioned effect of inhibiting the growth of the interfacial alloy layer be saturated, but also an excess Si phase will be formed in the obtained plating film, resulting in reduced workability. Therefore, the Si content is set to 4.0 mass% or less.

[0019] Furthermore, Mg in the plating bath is a component that is 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. When the Mg content in the plating bath is less than 1.0% by mass, 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, and sufficient corrosion resistance cannot be ensured. On the other hand, when the Mg content in the plating bath is high, the effect of improving corrosion resistance is saturated, and the α-Al phase becomes brittle, resulting in a decrease in workability and an increase in the amount of dross generated. Therefore, the Mg content is set to 10.0% by mass or less. Therefore, the Mg content in the coating bath must 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 %.

[0020] The plating bath further 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 interface alloy layer. The Fe content in the plating bath is usually about 0.1 to 0.5 mass%. Inevitable impurities other than Fe include Ni, Cu, Co, W, etc. These elements may be dissolved in the plating bath from the WC-based or Co-Cr-W-based thermal spray coating applied to the base steel sheet, stainless steel bath-immersed equipment, or bath-immersed equipment, or may be contained as impurities in the metal blocks that are the raw material for the plating bath.

[0021] Although there are no particular limitations on the total content of unavoidable impurities in the plating film, if contained in excess, there is a possibility that various properties of the hot-dip Al-Zn-Si-Mg-plated steel sheet to be produced may be adversely affected, and therefore it is preferable to control the total content to 5.0 mass% or less.

[0022] 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 raw material to be charged into the coating bath, because by using an Al-Mg alloy as the Mg raw material, it is possible to suppress the generation of Mg-containing dross in the raw material melting step. Since Mg has a high oxidizing effect, when pure Mg or an Mg-Zn alloy is melted, the melted Mg is oxidized violently on the bath surface of the plating bath, and a large amount of lump-shaped dross (Mg-containing dross) mainly composed of Mg oxide is formed. However, when an Al-Mg alloy is melted as in the present invention, a thin and dense Al oxide film is formed on the bath surface of the plating bath, which prevents the plating bath from coming into contact with the atmosphere and suppresses the oxidation of Mg, thereby making it possible to suppress the generation of Mg-containing dross. In order to obtain this effect of suppressing Mg-containing dross, it is necessary to make the Al concentration in the Al-Mg alloy at least 10 mass% or more, that is, the Mg concentration is 90 mass% or less.

[0023] Here, the Al-Mg alloy is a binary alloy of Al and Mg, and does not include ternary alloys such as Mg-Zn-Al alloys. This is from the viewpoint of more reliably forming a thin and dense Al oxide film on the bath surface of the above-mentioned plating bath. Note that it is sufficient to use an Al-Mg alloy as the raw material for the Mg, and a small amount of a ternary alloy such as an Mg-Zn-Al alloy can be added as a material for a component other than Mg.

[0024] The Mg-containing dross refers to clumped top dross, mainly composed of Mg oxides, which occurs near the surface of the coating bath, and is different from the oxides containing iron (bottom dross) which are unevenly distributed in the bath or at the bottom of the coating bath.

[0025] Furthermore, from the viewpoint of continuous operation, the Al-Mg alloy to be added to the plating bath must have high solubility. FIG. 1 shows a binary system calculation phase diagram of Al-Mg alloy (source: Binary Alloy Phase Diagrams, vol. 1 ed. by TB Massalski, ASM, (1986), 129.). Since the Al-Mg alloy of each composition is only liquid at a higher temperature side than the liquidus line indicated by the arrow in FIG. 1, high solubility is obtained when the liquidus temperature is lower than the bath temperature. In addition, in the case of the same bath temperature, even higher solubility can be obtained by using an alloy with a lower liquidus temperature. As described above, in the production of the hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention, the bath temperature 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 melting of the Al-Mg alloy, 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%.

[0026] In addition, the raw materials of the bath components other than Mg when preparing the plating bath are not particularly limited. For example, pure Al, Al-Si alloy, Al-Zn alloy, and 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.

[0027] The component composition of the plating bath can be confirmed by dissolving the alloy that has solidified from the plating bath in acid and performing ICP emission spectrometry, atomic absorption spectrometry, or the like. As a specific example, the alloy solidified from the plating bath is dissolved in hydrochloric acid, the solution is filtered, and the filtrate is analyzed by ICP emission spectroscopy, allowing the components other than insoluble Si to be quantified. Furthermore, the solid content is dried and incinerated in a heating furnace at 650°C, and then melted by adding sodium carbonate and sodium tetraborate, and the molten material is dissolved in hydrochloric acid and the solution is analyzed by ICP emission spectroscopy, allowing the amount of insoluble Si to be quantified. 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 content analysis. However, the ICP emission spectrometry is merely one example, and any method that can accurately quantify the component composition of the plating film may be used, and there is no particular limitation.

[0028] The plating bath may further contain, as an optional component, one or more 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 can impart the effect of improving the stability of the corrosion product when the formed plating film corrodes, thereby delaying the progress of corrosion, and the effect of stabilizing the spangle size on the plating surface, thereby improving the surface appearance.

[0029] 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 why the lower limit of the bath temperature is set to melting point + 20°C is that in order to perform hot-dip coating, the bath temperature needs to be equal to or higher than the solidification point, and by setting the temperature to melting point + 20°C, solidification due to a local drop in the bath temperature of the coating bath is prevented. On the other hand, the reason why the upper limit of the bath temperature is set to 650°C is that if the bath 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.

[0030] 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.

[0031] Furthermore, the immersion time of the base steel sheet in the plating bath is preferably 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 may 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 cause a thicker interface alloy layer to form between the plating film and the steel sheet, it is more preferable to set it to 8 seconds or less.

[0032] In the method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet of the present invention, there are no particular limitations on the steps other than the above-mentioned hot-dip galvanizing treatment step, and it is possible to appropriately carry out steps used in the production of a normal hot-dip galvanized steel sheet. EXAMPLES

[0033] [Sample 1-26] A production test was carried out using a low carbon cold rolled steel sheet with a thickness of 0.8 mm and a width of 765 mm, manufactured by a conventional method, as the base steel sheet, and using a general continuous hot-dip galvanizing equipment, in which (1) the coating bath was made up and (2) the hot-dip galvanizing treatment was carried out continuously for 6 hours under the conditions shown in Table 1. The details of (1) the coating bath making and (2) the hot-dip galvanizing treatment are as follows.

[0034] (1) Preparation of plating bath A plating bath having the composition shown in Table 1 was prepared in a hot-dip plating pot of a continuous hot-dip plating facility. To prepare the bath, the raw materials (ingots) shown in Table 1 were used, mixed to obtain the composition shown in Table 1, and melted. In order to replenish the plating bath that is lost during the plating process, additional bath is made 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 stripping solution, and analyzing the filtrate and solid content. Specifically, the filtrate was analyzed by ICP atomic emission spectroscopy to quantify components other than insoluble Si. The solid content was dried and incinerated in a heating furnace at 650°C, and then melted by adding sodium carbonate and sodium tetraborate. The molten material was 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 by the filtrate analysis to the insoluble Si concentration obtained by the solid content analysis.

[0035] (2) Hot-dip plating After the plating bath was prepared, a typical continuous hot-dip plating facility was used for plating. The strips of the base steel sheet were welded, and were continuously degreased, annealed, cooled, and plated. The plating bath temperature was controlled at 600°C for all levels. The plating weight was controlled to 82±0.5g / m per side by gas wiping. 2 It was controlled so that

[0036] <Evaluation> Each sample of the hot-dip Al-Zn-Si-Mg-plated steel sheet obtained in the above-mentioned manufacturing test was evaluated as follows. The evaluation results are shown in Table 1. (1) Solubility of Mg raw material The solubility of the Mg raw material in each production test affects the bath preparation speed, i.e., the plating treatment speed, so 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 speed when a Mg-free 55 mass % Al-Zn-1.6 mass % plating bath (hereinafter referred to as the reference bath) was used, and judged according to the following criteria. ◎: Extremely high solubility, allowing operation at the same processing speed as when using the standard bath ○: High solubility, allowing operation at 90% or more of the processing speed compared to plating using a standard bath. △: Low solubility, but operation is possible at 80% or more of the processing speed compared to plating using the standard bath. ×: The solubility is very low, and it is difficult to operate at a processing speed of 80% or more compared to plating using the standard bath.

[0037] (2) Amount of magnesium-containing dross generated In each production test, the bath surface during plating treatment was visually inspected, and the amount of black dross (Mg-containing dross) generated was confirmed and evaluated according to the following criteria. ○: Black dross is observed on the plating bath surface. ×: No black dross was observed on the plating bath surface.

[0038] (3) Appearance of plating film For each sample of the produced hot-dip Al-Zn-Si-Mg plated steel sheet, the presence or absence of dross defects was confirmed, and the appearance was evaluated according to the following criteria. ○: No adhesion of granular dross is observed ×: Granular dross adhesion is observed

[0039] [Table 1]

[0040] From the results in Table 1, it can be seen that each sample of the invention has high solubility of Mg raw material, and is also excellent in 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 a poor result in at least one evaluation item. [Industrial Applicability]

[0041] According to the present invention, the generation of Mg-containing dross can be suppressed, and it is possible to produce a hot-dip Al-Zn-Si-Mg-plated steel sheet having excellent appearance.

Claims

1. A method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet using a continuous hot-dip plating facility, comprising: The method includes a hot-dip galvanizing step of immersing a base 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. 2. The method for producing a hot-dip Al-Zn-Si-Mg-plated steel sheet according to claim 1, characterized in that 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. 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 20 to 80 mass % of Mg.

4. 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.

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