Zn-Al-Mg hot-dip galvanized steel sheet

Abstract

This Zn-Al-Mg hot-dip plated steel sheet comprises a steel sheet and a hot-dip plated layer formed on the surface of the steel sheet. The hot-dip plated layer has an average composition including more than 10 mass% and no more than 22 mass% of Al, 1.0-10 mass% of Mg, and a remainder of Zn and impurities. When the thickness of the hot-dip plated layer is denoted by t and a 5-mm square cross-section parallel to the surface is exposed at any of a 3t / 4 position, a t / 2 position, or a t / 4 position from the surface of the hot-dip plated layer, the area fraction of a [Zn phase] of the plating composition in at least one cross section is less than 20%.

Description

[Technical Field]

[0001] The present invention relates to a Zn-Al-Mg hot-dip galvanized steel sheet having a glossy appearance. This application claims priority based on Japanese Patent Application No. 2022-094346, filed in Japan on June 10, 2022, and the contents of that application are incorporated herein by reference. [Background technology]

[0002] Hot-dip galvanized steel sheets are used as steel sheets with good corrosion resistance. Hot-dip galvanized steel sheets, a typical example of hot-dip galvanized steel sheets, are widely used in various manufacturing industries such as automobiles, home appliances, and building materials. Furthermore, with the aim of further improving the corrosion resistance of hot-dip galvanized steel sheets, highly corrosion-resistant hot-dip galvanized steel sheets containing Al or Mg in the hot-dip galvanizing layer have been proposed. For example, Patent Documents 1 to 3 propose Zn-Al-Mg type hot-dip galvanized steel sheets.

[0003] Incidentally, Zn-Al-Mg hot-dip galvanized steel sheets contain mainly four types of phases and structures in the hot-dip galvanized layer: [Al phase], [Zn phase], [MgZn2 phase], and [Al / MgZn2 / Zn ternary eutectic structure]. Furthermore, if Si is also contained in the hot-dip galvanized layer in addition to Zn, Al, and Mg, then mainly five types of phases and structures are included, including the [Mg2Si phase] in addition to the four types of phases and structures mentioned above. Thus, because the hot-dip galvanized layer of Zn-Al-Mg hot-dip galvanized steel sheets contains a mixture of various phases and structures, the surface of the hot-dip galvanized layer exhibits a matte appearance. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-226865 [Patent Document 2] Japanese Patent Application Publication No. 10-306357 [Patent Document 3] Japanese Patent Application Publication No. 2004-68075 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Zn-Al-Mg hot-dip galvanized steel sheets, like hot-dip galvanized steel sheets, are widely used in various manufacturing industries, including automobiles, home appliances, and building materials. In recent years, customer demand for the surface appearance of galvanized steel sheets has increased, and there is a growing need for Zn-Al-Mg hot-dip galvanized steel sheets to have a stronger metallic luster.

[0006] This invention has been made in view of the above circumstances, and aims to provide a Zn-Al-Mg hot-dip galvanized steel sheet that has superior metallic luster on the surface of the hot-dip galvanized layer and also superior corrosion resistance compared to conventional steel sheets. [Means for solving the problem]

[0007] To solve the above problems, the present invention adopts the following configuration. [1] comprising a steel plate and a hot-dip galvanized layer formed on the surface of the steel plate, The aforementioned hot-dip plating layer has an average composition of Al: greater than 10 to 22% by mass, Mg: 1.0 to 10% by mass, with the remainder being Zn and impurities. A Zn-Al-Mg hot-dip galvanized steel sheet characterized in that, with the thickness of the hot-dip galvanized layer being t, when a 5 mm square cross section parallel to the surface is exposed at any of the positions 3t / 4, t / 2, or t / 4 from the surface of the hot-dip galvanized layer, the area fraction of the [Zn phase] in the galvanized structure in at least one of the cross sections is less than 20%. [2] comprising a steel plate and a hot-dip galvanized layer formed on the surface of the steel plate, The aforementioned hot-dip plating layer has an average composition of Al: greater than 10 to 22% by mass, Mg: 1.0 to 10% by mass, with the remainder being Zn and impurities. Furthermore, it contains one or two types selected from the groups A and B described below. A Zn-Al-Mg hot-dip galvanized steel sheet characterized in that, with the thickness of the hot-dip galvanized layer being t, when a 5 mm square cross section parallel to the surface is exposed at any of the positions 3t / 4, t / 2, or t / 4 from the surface of the hot-dip galvanized layer, the area fraction of the [Zn phase] in the galvanized structure in at least one of the cross sections is less than 20%. [Group A]Si:0.0001~2% by mass [Group B] One or more of the following elements: Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, C, in a total of 0.0001 to 2% by mass. [3] The Zn-Al-Mg hot-dip galvanized steel sheet according to [1] or [2], wherein the ratio of the area fraction B of the [Zn phase] to the total area fraction A of the [Zn phase] and [Al / MgZn2 / Zn ternary eutectic structure] of the plating structure in at least one cross section (B / A(%)) is less than 20%. [4] The Zn-Al-Mg hot-dip galvanized steel sheet according to [2], wherein the hot-dip galvanized layer has an average composition containing the A group in mass%. [5] The Zn-Al-Mg hot-dip galvanized steel sheet according to [2], wherein the hot-dip galvanized layer has an average composition containing the B group in mass%. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a Zn-Al-Mg hot-dip galvanized steel sheet that has superior metallic luster on the surface of the hot-dip galvanized layer and also superior corrosion resistance compared to conventional materials. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a schematic cross-sectional view illustrating the exposed surface for measuring the plating structure of the hot-dip galvanized layer in a Zn-Al-Mg hot-dip galvanized steel sheet, which is an embodiment of the present invention. [Figure 2] Figure 2 is a perspective view illustrating an exposed surface for measuring the plating structure of the hot-dip galvanized layer in a Zn-Al-Mg hot-dip galvanized steel sheet, which is an embodiment of the present invention. [Modes for carrying out the invention]

[0010] The inventors of the present invention have investigated in detail the plating layer of a conventional Zn-Al-Mg based hot-dip galvanized steel sheet presenting a pearly appearance. The pearly appearance appears due to the mixture of fine shiny parts showing metallic luster and fine white parts presenting white. Among these, when examining the structure of the plating layer in the shiny parts, it was found that the area fraction of the [Zn phase] on the surface of the plating layer is less than that in the white parts. On the other hand, when examining the structure of the plating layer in the white parts, it was found that the ratio of the [Zn phase] to the [ternary eutectic structure of Al / MgZn2 / Zn] is higher than that in the shiny parts.

[0011] Therefore, in order to obtain a shiny appearance as a whole by increasing the amount of shiny parts and decreasing the amount of white parts in the hot-dip plating layer, the inventors intensively studied and found that by adjusting the chemical composition of the hot-dip plating layer and decreasing the ratio of the [Zn phase], the surface appearance of the plating layer comes to present metallic luster as a whole. Also, in the whole plating layer, it was found that by decreasing the ratio of the [Zn phase] to the total of the [ternary eutectic structure of Al / MgZn2 / Zn] and the [Zn phase], the appearance of the plating layer comes to present more metallic luster.

[0012] Hereinafter, a Zn-Al-Mg based hot-dip galvanized steel sheet which is an embodiment of the present invention will be described.

[0013] The Zn-Al-Mg series hot-dip galvanized steel sheet of this embodiment includes a steel sheet and a hot-dip layer formed on the surface of the steel sheet. The hot-dip layer contains, in terms of average composition, more than 10% to 22% by mass of Al and 1.0% to 10% by mass of Mg, with the balance being Zn and impurities. When a 5 mm square cross-section parallel to the surface of the hot-dip layer is exposed at a position of 3t / 4, t / 2, or t / 4 from the surface of the hot-dip layer, where t is the thickness of the hot-dip layer, the area fraction of the [Zn phase] in the plating structure in at least one cross-section is less than 20%. Here, the 5 mm square cross-section parallel to the surface of the plating layer refers to a square exposed surface that is parallel to the surface and has a size of 5 mm square in plan view.

[0014] The steel material serving as the base for the hot-dip layer is not particularly limited in terms of material. It can be applied to general steel, Al-killed steel, and some high-alloy steels, and there is no particular limitation on the shape. Also, Ni pre-plating may be applied to the steel material. By applying the hot-dip method described later to the steel material, the hot-dip layer according to this embodiment is formed.

[0015] Next, the chemical composition of the hot-dip layer will be described. The hot-dip layer according to this embodiment contains, in terms of average composition, more than 10% to 22% by mass of Al and 1.0% to 10% by mass of Mg, with the balance including Zn and impurities. Furthermore, it may contain one or two selected from the group consisting of the following Group A and Group B. [Group A] Si: 0.0001 to 2% by mass [Group B] Any one or more of Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, C, in a total amount of 0.0001 to 2% by mass

[0016] The Al content is in the range of more than 10% by mass and 22% by mass or less in the average composition. Al is an element necessary to ensure corrosion resistance. If the Al content in the hot-dip plating layer is 10% by mass or less, the effect of improving corrosion resistance becomes insufficient, and if it exceeds 22% by mass, the corrosion resistance decreases for reasons that are unknown. From the viewpoint of corrosion resistance, it is preferably more than 10% by mass and 20% by mass or less. More preferably more than 10% by mass and 18% by mass or less. More preferably 11% by mass or more. And even more preferably 19% by mass or less.

[0017] The Mg content is in the range of 1.0 to 10% by mass in the average composition. Mg is an element necessary to improve the corrosion resistance of the hot-dip plating layer. If the Mg content in the hot-dip plating layer is less than 1.0% by mass, the effect of improving corrosion resistance will be insufficient, and if it exceeds 10% by mass, dross generation in the plating bath will become significant, making it difficult to stably manufacture plated steel materials. From the viewpoint of balancing corrosion resistance and dross generation, it is preferably 1.5% by mass or more. It is also preferably 8% by mass or less. More preferably 2% by mass or more. Furthermore, it is also more preferably 6% by mass or less.

[0018] Furthermore, the hot-dip plating layer may contain Si in the range of 0.0001 to 2 mass%. Si is an effective element for improving the adhesion of the hot-dip plating layer. Since the effect of improving adhesion is exhibited when Si is included at 0.0001 mass% or more, it is preferable to include Si at 0.0001 mass% or more. On the other hand, the effect of improving plating adhesion saturates when the content exceeds 2 mass%, so the Si content should be 2 mass% or less. From the viewpoint of plating adhesion, it may be 0.01 mass% or more, or 1 mass% or less. It may also be 0.03 mass% or more, or 0.8 mass% or more.

[0019] Furthermore, the hot-dip plating layer may contain, in average composition, 0.0001 to 2% by mass of one or more of the following elements: Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, and C. The amount may also be in the range of 0.01 to 2% by mass. Including these elements can further improve corrosion resistance. REM refers to one or more rare earth elements with atomic numbers 57 to 71 in the periodic table.

[0020] The remainder of the chemical composition of the molten plating layer consists of zinc and impurities.

[0021] The composition of the molten plating layer can be measured by the following method. First, the surface coating is removed with a coating remover that does not corrode the plating (for example, Neoriver SP-751 manufactured by Sansai Chemical Co., Ltd.). Then, the molten plating layer is dissolved with hydrochloric acid containing an inhibitor (for example, Hibilon manufactured by Sugimura Chemical Industry Co., Ltd.), and the resulting solution is subjected to inductively coupled plasma (ICP) emission spectroscopy to determine its composition.

[0022] Next, the structure of the hot-dip galvanized layer will be described. The structure of the hot-dip galvanized layer in this embodiment may have, for example, the following structure.

[0023] A hot-dip galvanized layer containing Al, Mg, and Zn comprises an Al phase, a MgZn2 phase, and a Zn phase, as well as an Al / Zn / MgZn2 ternary eutectic structure. Specifically, it has a form in which the Al phase, MgZn2 phase, and Zn phase are embedded within a substrate of the Al / Zn / MgZn2 ternary eutectic structure. Furthermore, if Si is included, a Mg2Si phase may be included within the Al / Zn / MgZn2 ternary eutectic structure substrate.

[0024] [Terneautic structure of Al / Zn / MgZn2] The ternary eutectic structure of Al / Zn / MgZn2 is a ternary eutectic structure consisting of an Al phase, a Zn phase, and an intermetallic compound MgZn2 phase. The Al phase forming this ternary eutectic structure corresponds to, for example, the "Al" phase at high temperatures in the Al-Zn-Mg ternary equilibrium phase diagram (an Al solid solution that dissolves Zn and contains a small amount of Mg). At high temperatures, the Al″ phase usually separates into a fine Al phase and a fine Zn phase at room temperature. The Zn phase in this ternary eutectic structure is a Zn solid solution containing a small amount of Al, and in some cases, an even smaller amount of Mg. The MgZn2 phase in this ternary eutectic structure is an intermetallic compound phase located near the Zn:84 mass% ratio in the Zn-Mg binary equilibrium phase diagram. Judging from the phase diagram, it appears that each phase either does not contain any other additive elements in solid solution, or if it does, only in trace amounts. However, since these amounts cannot be clearly distinguished by conventional analysis, this ternary eutectic structure consisting of these three phases is referred to as the [Al / Zn / MgZn2 ternary eutectic structure] in this specification.

[0025] [Al phase] The [Al phase] is a phase that appears as islands with clear boundaries within the substrate of the [Al / Zn / MgZn2 ternary eutectic structure]. This corresponds to, for example, the "Al″ phase" at high temperatures in the Al-Zn-Mg ternary equilibrium phase diagram (an Al solid solution that dissolves Zn and contains a small amount of Mg). The amount of Zn and Mg dissolved in this high-temperature Al″ phase varies depending on the Al and Mg concentrations in the plating bath. At room temperature, this high-temperature Al″ phase usually separates into fine Al and Zn phases, but the island-like shape observed at room temperature is thought to be due to the shape of the Al″ phase at high temperatures. Judging from the phase diagram, this phase is thought to contain no other additive elements in solid solution, or if so, only in trace amounts. However, since it cannot be clearly distinguished by conventional analysis, this phase, which originates from the Al'' phase at high temperatures and whose shape is similar to that of the Al'' phase, is referred to as the [Al phase] in this specification. The [Al phase] can be clearly distinguished under a microscope from the Al phase that forms a [Terrain eutectic structure of Al / Zn / MgZn2].

[0026] [Zn phase] The [Zn phase] is a phase that appears as islands with clear boundaries within the substrate of the [Al / Zn / MgZn2 ternary eutectic structure], and in reality, it may contain small amounts of Al and Mg in solid solution. Judging from the phase diagram, it is thought that this phase does not contain any other additive elements in solid solution, or if it does, only in trace amounts. The [Zn phase] is a region where the Zn phase has an equivalent circular diameter of 2.5 μm or more, and can be clearly distinguished under a microscope from the Zn phase that forms the [Al / Zn / MgZn2 ternary eutectic structure].

[0027] [MgZn2 phase] The [MgZn2 phase] is a phase that appears as an island with a clear boundary within the [Al / Zn / MgZn2 ternary eutectic structure] substrate, and may actually contain a small amount of dissolved Al. Judging from the phase diagram, it is thought that this phase does not contain any other additive elements in solid solution, or if it does, only in trace amounts. The MgZn2 phase and the MgZn2 phase forming the Al / Zn / MgZn2 ternary eutectic structure can be clearly distinguished under a microscope. In this embodiment, the hot-dip plating layer may not contain the MgZn2 phase depending on the manufacturing conditions, but it is included in the hot-dip plating layer under most manufacturing conditions.

[0028] [Mg2Si phase] The [Mg2Si phase] is a phase that appears as distinct, island-like structures with clear boundaries within the solidification structure of a silicon-added molten plating layer. Judging from the phase diagram, it is thought that Zn, Al, and other additive elements are either not dissolved in the [Mg2Si phase], or are dissolved only in trace amounts. The [Mg2Si phase] can be clearly distinguished from other phases under a microscope in the molten plating layer.

[0029] Next, the content of the [Zn phase] will be explained. In this embodiment, as shown in Figures 1 and 2, the thickness of the hot-dip galvanized layer 2 formed on the steel plate 1 is t, and when a notch is made at one of the positions 3t / 4, t / 2, or t / 4 from the surface 2a of the hot-dip galvanized layer 2 so that a square exposed surface 3, 4, or 5 appears, parallel to the surface 2a and having an area of ​​5 mm on each side in a plan view, the area fraction of the [Zn phase] in the plating structure will be less than 20% in at least one of these exposed surfaces 3 to 5. The area fraction of the [Zn phase] may be 15% or less, less than 15%, 10% or less, or 5% or less. By having an area fraction of 20% or less of the [Zn phase], the proportion of fine glossy areas exhibiting metallic luster on the surface of the hot-dip galvanized layer increases, and the overall appearance of the hot-dip galvanized layer exhibits metallic luster. Furthermore, there is no particular lower limit to the area fraction of the [Zn phase], but it may be greater than 0%, 1% or more, or 2% or more. Note that the schematic cross-sectional view shown in Figure 1 is a cross-sectional view along the AA' plane in Figure 2.

[0030] Furthermore, it is preferable that in at least one of the exposed surfaces 3 to 5, the ratio of the area fraction B of the [Zn phase] to the total area fraction A of the [Zn phase] and [Al / MgZn2 / Zn ternary eutectic structure] of the plating structure (B / A(%)) is less than 20%. The ratio (B / A(%)) may be 15% or less, less than 15%, or 10% or less. There is no particular limit to the lower limit of the ratio (B / A(%)), but it may be 1% or more, 2% or more, or 5% or more.

[0031] Furthermore, the area fraction of the Al phase on the exposed surface where the area fraction of the Zn phase is measured may be, for example, 30 to 80 area percent, or 40 to 65 area percent. Furthermore, the area fraction of the [Al / MgZn2 / Zn ternary eutectic structure] on the exposed surface where the area fraction of the [Zn phase] is measured may be, for example, 10 to 75 area percent, or 20 to 65 area percent. Furthermore, the area fraction of the [MgZn2 phase] on the exposed surface where the area fraction of the [Zn phase] is measured may be, for example, 0 to 60 area percent, or 10 to 40 area percent. Furthermore, the area fraction of the [Mg2Si phase] on the exposed surface where the area fraction of the [Zn phase] is measured may be, for example, 0 to 5 area percent, or 0 to 1 area percent.

[0032] When forming 5 mm square exposed surfaces 3, 4, and 5 parallel to the surface at any of the following positions from the surface 2a of the molten plating layer 2: 3t / 4, t / 2, or t / 4, the molten plating layer is removed by means of grinding or argon sputtering. Furthermore, it is desirable that the exposed surfaces be mirror-finished; for example, it is desirable that the maximum height Rz of the exposed surface be 0.2 μm or less. The exposed surface to be observed may be at any of the following depths from the surface of the molten plating layer: 3t / 4, t / 2, or t / 4. It is preferable to select the exposed surface at the t / 2 position. If the area fraction or B / A ratio of the [Zn phase] satisfies the range of the present invention at the exposed surface at the t / 2 position, there is a high probability that the area fraction or B / A ratio of the [Zn phase] will also satisfy the range of the present invention at the other positions. More preferably, the area fraction or B / A ratio of the [Zn phase] satisfies the range of the present invention at any two of the exposed surfaces at depths from the surface of the molten plating layer: 3t / 4, t / 2, or t / 4. More preferably, the area fraction or B / A ratio of the [Zn phase] satisfies the range of the present invention at all exposed surfaces at depths of 3t / 4, t / 2, or t / 4 from the surface of the molten plating layer.

[0033] For an exposed surface measuring 5 mm x 5 mm, the plating structure is observed using a scanning electron microscope (SEM) to obtain a secondary electron image, and the [Zn phase] and the [Al / MgZn2 / Zn ternary eutectic structure] are identified. When identifying each phase and structure, elemental analysis is performed using an energy-dispersive X-ray elemental analyzer attached to the SEM, confirming the distribution of Zn, Al, and Mg. Specifically, the region where Zn is mainly detected is designated as the Zn phase, the region where Al is mainly detected is designated as the Al phase, and the region where Zn and Mg are mainly detected is designated as the MgZn2 phase. Based on the distribution of each detected phase, the material is classified into [Al phase], [MgZn2 phase], and [Zn phase], and into the [Al / Zn / MgZn2 ternary eutectic structure] according to the method described above. Next, the area fraction of the [Zn phase] on the exposed surface is determined, and further, the ratio of the area fraction B of the [Zn phase] to the total area fraction A of the [Zn phase] and the [Al / MgZn2 / Zn ternary eutectic structure] (B / A(%)) is calculated. The [Zn phase] is measured as the region with an equivalent circular diameter of 2.5 μm or more. This allows for the distinction between the Zn phase in the [Al / MgZn2 / Zn ternary eutectic structure] and the [Zn phase].

[0034] Next, the method for manufacturing the Zn-Al-Mg hot-dip galvanized steel sheet according to this embodiment will be described. When manufacturing the Zn-Al-Mg hot-dip galvanized steel sheet of this embodiment, it is necessary to control the structure of the hot-dip galvanized layer so that the area fraction of the [Zn phase] on the exposed surface parallel to the surface at one of the following positions from the surface of the hot-dip galvanized layer: 3t / 4, t / 2, or t / 4.

[0035] To manufacture Zn-Al-Mg hot-dip galvanized steel sheets by hot-dip galvanizing, the steel sheets are immersed in a molten galvanizing bath with adjusted chemical composition, thereby depositing molten metal onto the surface of the steel sheets. Next, the steel sheets are removed from the galvanizing bath, the amount of deposit is controlled by gas wiping, and then the molten metal is allowed to solidify. During solidification, depending on the composition, an Al phase is formed first, followed by the formation of an Al / Zn / MgZn2 ternary eutectic structure as the temperature of the molten metal decreases. In addition, MgZn2 phases and Zn phases are formed within the Al / Zn / MgZn2 ternary eutectic structure. Furthermore, if Si is contained in the hot-dip galvanized layer, a Mg2Si phase is formed within the Al / Zn / MgZn2 ternary eutectic structure.

[0036] It was found that when a coarse [Zn phase] is formed, the relative proportion of [Al phase] and [MgZn2 phase] in the molten plating layer increases, and these phases are exposed to the plating surface, resulting in a decrease in the quality of the surface appearance. It is presumed that the formation of the [Zn phase] is influenced by the number of Zn nucleation sites. That is, when there are many Zn nucleation sites, the Zn in the liquid phase just before final solidification crystallizes as a fine Zn phase in the [Al / Zn / MgZn2 ternary eutectic structure]. Therefore, the inventors have found that by increasing the number of Zn nucleation sites, the formation of a coarse [Zn phase] can be suppressed and the quality of the surface appearance can be improved. One way to increase the number of Zn nucleation sites is to attach a substance that can act as a Zn nucleation site to the surface of the steel sheet, which is the base material. The manufacturing process is described in detail below.

[0037] Hot-rolled steel sheets are manufactured, and hot-rolled sheets are annealed as needed. After pickling, cold rolling is performed as needed to produce cold-rolled sheets. After degreasing and washing the hot-rolled or cold-rolled sheets, they are annealed, and the annealed hot-rolled or cold-rolled sheets are immersed in a molten plating bath to form a molten plating layer.

[0038] Here, Zn powder is applied to the annealed hot-rolled or cold-rolled sheet before immersion in the molten plating bath. The applied Zn powder does not completely dissolve in the molten plating, but becomes a nucleation site for Zn during the final solidification of the plating. Some of the Zn powder diffuses into the plating bath as a solid. In this embodiment, the manufacturing method is achieved only when the Zn powder is applied after annealing but before immersion in the plating bath. If the Zn powder is applied before annealing, the Zn will alloy with the steel sheet during annealing, inhibiting the formation of the molten plating layer. If the Zn powder is applied after immersion in the molten plating bath, the applied Zn powder may actually cause the surface appearance of the plating to become rough. The Zn powder to be applied can be any Zn powder containing Zn and impurities. The average particle size of the Zn powder can be, for example, in the range of 4 to 6 μm. The amount of Zn powder applied can be, for example, 1 to 5 g / m² per side. 2 The degree is good. If the average particle size and adhesion amount are within this range, the Zn powder can function as a Zn nucleation site. The Zn powder adhesion should be carried out when the plate temperature of the hot-rolled or cold-rolled sheet is in the range of bath temperature + 10°C to 20°C.

[0039] Next, the steel plate is immersed in a molten plating bath. Preferably, the molten plating bath contains Al: more than 10 to 22% by mass, Mg: 1.0 to 10% by mass, with the remainder being Zn and impurities. The molten plating bath may also contain Si: 0.0001 to 2% by mass. Furthermore, the molten plating bath may contain 0.0001 to 2% by mass of one or more of the following elements: Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, and C in total.

[0040] The temperature of the molten plating bath is preferably in the range of 400 to 500°C. This is because a desired molten plating layer can be formed when the temperature of the molten plating bath is within this range. Furthermore, the amount of hot-dip galvanizing layer can be adjusted by means of gas wiping or other methods on the steel sheet after it has been removed from the hot-dip galvanizing bath. The total amount of hot-dip galvanizing layer on both sides of the steel sheet should be 30 to 600 g / m². 2 It is preferable to adjust the amount to be within this range. The amount of adhesion is 30g / m². 2If the amount is less than 600 g / m², the corrosion resistance of the Zn-Al-Mg hot-dip galvanized steel sheet will decrease, which is undesirable. 2 In the case of excessive heat, dripping of molten metal adheres to the steel plate, making it impossible to smooth the surface of the hot-dip plating layer, which is undesirable.

[0041] After adjusting the amount of molten plating layer applied, the steel sheet is cooled. Cooling of the molten metal adhering to the steel sheet begins after the sheet is removed from the molten plating bath. Depending on the composition of the molten plating bath, the Al phase begins to crystallize at around 430°C. Subsequently, MgZn2 begins to crystallize at around 370°C, the Al / Zn / MgZn2 ternary eutectic structure begins to crystallize at around 340°C, and then the Zn phase crystallizes, completing solidification.

[0042] In this process, numerous Zn powder particles adhering to the steel plate act as nucleation sites for Zn, so that the Zn in the liquid phase forms as a fine Zn phase within the [Al / Zn / MgZn2 ternary eutectic structure], while the formation of coarse Zn phases ([Zn phase]) with an equivalent circular diameter of 2.5 μm or more is suppressed.

[0043] When forming a chemical conversion treatment layer on the surface of a hot-dip galvanized layer, the chemical conversion treatment is performed on the hot-dip galvanized steel sheet after the hot-dip galvanized layer has been formed. The type of chemical conversion treatment is not particularly limited, and known chemical conversion treatments can be used. Furthermore, when forming a coating layer on the surface of the hot-dip galvanized layer or the chemical conversion treatment layer, the hot-dip galvanized steel sheet is coated after the hot-dip galvanized layer or the chemical conversion treatment layer has been formed. The type of coating treatment is not particularly limited, and known coating treatments can be used.

[0044] As described above, according to this embodiment, the metallic luster of the hot-dip galvanized layer surface can be improved compared to conventional methods. [Examples]

[0045] Next, embodiments of the present invention will be described. The steel sheet after cold rolling was degreased and washed with water. Then, the steel sheet was cold-rolled and annealed. Zn powder with an average particle size in the range of 4-6 μm was applied to the cold-rolled and annealed steel sheet at a rate of 1-5 g / m² per side. 2 The Zn powder was applied to the sample, immersed in a molten plating bath, and then removed. After that, the amount of Zn powder was adjusted by gas wiping, and then the sample was cooled. In No. 67, Zn powder was applied before annealing and molten plating; in No. 68, Zn powder was applied after molten plating; and in No. 69, molten plating was performed without applying Zn powder. In this way, molten plated steel sheets No. 1 to 69 shown in Tables 1A and 2B were manufactured.

[0046] As shown in Figure 1, 5 mm square exposed surfaces parallel to the surface were formed on the obtained hot-dip galvanized steel sheet at positions t / 4, t / 2, and 3t / 4 from the surface of the hot-dip galvanized layer. The exposed surfaces were formed by grinding away the hot-dip galvanized layer and then performing mirror polishing.

[0047] For each exposed surface measuring 5 mm x 5 mm, the plating structure was observed using a scanning electron microscope (SEM) to obtain secondary electron images, and the [Zn phase] and [Al / MgZn2 / Zn ternary eutectic structure] were identified. In identifying each phase and structure, elemental analysis using an energy-dispersive X-ray elemental analyzer attached to the SEM was also used to confirm the distribution of Zn, Al, and Mg. The area fraction of the [Zn phase] on each exposed surface was then determined, and the ratio of the area fraction B of the [Zn phase] to the total area fraction A of the [Zn phase] and [Al / MgZn2 / Zn ternary eutectic structure] (B / A(%)) was calculated. The [Zn phase] was defined as regions with an equivalent circular diameter of 2.5 μm or more. This allowed for the distinction between the Zn phase in the [Al / MgZn2 / Zn ternary eutectic structure] and the [Zn phase]. The results are shown in Tables 2A and 2B. Also, on the exposed surface where the area fraction of the [Zn phase] of Nos. 1 to 62 was measured, the area fraction of the [Al phase] was in the range of 30 to 80 area%, the area fraction of the [ternary eutectic structure of Al / MgZn2 / Zn] was in the range of 10 to 75 area%, the area fraction of the [MgZn2 phase] was in the range of 0 to 60 area%, and the area fraction of the [Mg2Si phase] was in the range of 0 to 5 area%.

[0048] Also, the surface of the molten plating layer of the obtained molten-plated steel sheet was observed, and visual evaluation was performed based on the following criteria. A and B were regarded as qualified. The results are shown in Tables 2A and 2B.

[0049] A: No appearance spots other than luster are observed even from 0.5 m away. B: Appearance spots other than luster are observed from 0.5 m away, but no appearance spots other than luster are observed from 2 m away. C: Appearance spots other than luster are observed even from 2 m away.

[0050] The corrosion resistance of the molten-plated steel sheet was evaluated by the corrosion weight loss after the CCT test. The plated steel sheet was cut into 150 × 70 mm, and using the CCT conforming to JASO-M609, the corrosion weight loss after 30 cycles of CCT was investigated. The evaluation was that F was for a corrosion weight loss of less than 30 g / m 2 and G was for a corrosion weight loss of 30 g / m 2 or more and less than 50 g / m 2 and P was for a corrosion weight loss of 50 g / m 2 or more, and F and G were regarded as qualified. The results are shown in Tables 2A and 2B.

[0051] For the molten-plated steel sheets of Nos. 1 to No. 62, since the chemical composition of the molten plating layer was within the scope of the present invention and Zn powder was adhered before annealing and before molten plating, as a result of Zn powder functioning as each generation site of Zn during the solidification of the molten plating layer, when an exposed surface of 5 mm square was formed at the 3t / 4 position, t / 2 position or t / 4 position from the surface of the molten plating layer, the area fraction of the [Zn phase] of the plating structure on at least one exposed surface was less than 20%. For this reason, the appearance of the molten plating layer became a shiny appearance. Also, the corrosion resistance was good.

[0052] The hot-dip galvanized steel sheet No. 63 had reduced corrosion resistance due to a low Al content in the hot-dip galvanized layer. The hot-dip galvanized steel sheet No. 64 exhibited reduced corrosion resistance due to an excessive Al content in the hot-dip galvanized layer. The hot-dip galvanized steel sheet No. 65 had reduced corrosion resistance due to a low Mg content in the hot-dip galvanized layer. The hot-dip galvanized steel sheet No. 66 exhibited reduced corrosion resistance due to an excessive Mg content in the hot-dip galvanized layer. Furthermore, the area fraction of the [Zn phase] in the galvanized structure exceeded 20%, resulting in insufficient gloss in the appearance of the hot-dip galvanized layer.

[0053] In the case of hot-dip galvanized steel sheet No. 67, Zn powder was applied before annealing and hot-dip galvanizing. As a result, the Zn powder alloyed with the steel sheet during annealing and did not function as a Zn nucleation site. Consequently, the area fraction of the [Zn phase] exceeded 20% at positions 3t / 4, t / 2, and t / 4 from the surface of the hot-dip galvanized layer, resulting in insufficient glossiness of the hot-dip galvanized layer.

[0054] In the hot-dip galvanized steel sheet No. 68, Zn powder was attached after hot-dip galvanizing, so the Zn powder did not function as a nucleation site for Zn. As a result, the area fraction of the [Zn phase] exceeded 20% at positions 3t / 4, t / 2, and t / 4 from the surface of the hot-dip galvanized layer, resulting in insufficient glossiness of the hot-dip galvanized layer.

[0055] In the case of hot-dip galvanized steel sheet No. 69, no Zn powder was attached, and therefore the Zn powder did not function as a Zn nucleation site. As a result, the area fraction of the [Zn phase] was 20% or more at 3t / 4, t / 2, and t / 4 positions from the surface of the hot-dip galvanized layer, resulting in insufficient glossiness of the hot-dip galvanized layer's appearance.

[0056] [Table 1A]

[0057] [Table 1B]

[0058] [Table 2A]

[0059] [Table 2B] [Industrial applicability]

[0060] The Zn-Al-Mg hot-dip galvanized steel sheet disclosed herein has excellent metallic luster on the surface of the hot-dip galvanized layer and also exhibits excellent corrosion resistance, making it highly suitable for industrial use. [Explanation of Symbols]

[0061] 1...Steel plate, 2...Hot-dip galvanized layer, 2a...Surface of the hot-dip galvanized layer, 3...Cross-section at position t / 4 (exposed surface), 4...Cross-section at position t / 2 (exposed surface), 5...Cross-section at position 3t / 4 (exposed surface).

Claims

1. The invention comprises a steel plate and a hot-dip galvanized layer formed on the surface of the steel plate, The aforementioned hot-dip plating layer has an average composition of Al: more than 10 to 22% by mass, Mg: 1.0 to 10% by mass, with the remainder being Zn and impurities. When a 5 mm square cross section parallel to the surface is exposed at any of the following positions from the surface of the molten plating layer, with the thickness of the molten plating layer being t, the area fraction of the [Zn phase] in the plating structure in at least one of the cross sections is 1% or more and less than 20%. The ratio of the area fraction B of the [Zn phase] to the total area fraction A of the [Zn phase] and [Al / MgZn2 / Zn ternary eutectic structure] in at least one of the cross-sections (B / A (%)) is less than 20%. A Zn-Al-Mg hot-dip galvanized steel sheet characterized by the following features.

2. The invention comprises a steel plate and a hot-dip galvanized layer formed on the surface of the steel plate, The aforementioned hot-dip plating layer has an average composition of Al: more than 10 to 22% by mass, Mg: 1.0 to 10% by mass, with the remainder being Zn and impurities. Furthermore, it contains one or two types selected from the groups A and B described below. When a 5 mm square cross section parallel to the surface is exposed at any of the following positions from the surface of the molten plating layer, with the thickness of the molten plating layer being t, the area fraction of the [Zn phase] in the plating structure in at least one of the cross sections is 1% or more and less than 20%. The ratio of the area fraction B of the [Zn phase] to the total area fraction A of the [Zn phase] and [Al / MgZn2 / Zn ternary eutectic structure] in at least one of the cross-sections (B / A (%)) is less than 20%. A Zn-Al-Mg hot-dip galvanized steel sheet characterized by the following features. [Group A] Si: 0.0001 to 2% by mass [Group B] One or more of the following elements: Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, C, in a total of 0.0001 to 2% by mass.

3. The Zn-Al-Mg hot-dip galvanized steel sheet according to claim 2, wherein the hot-dip galvanized layer has an average composition containing the group A in mass%.

4. The Zn-Al-Mg hot-dip galvanized steel sheet according to claim 2, wherein the hot-dip galvanized layer has an average composition containing the B group in mass%.

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