Hot-dip Zn-Al-Mg coated steel sheet, automotive parts, and manufacturing method thereof

Abstract

To provide a fused Zn-Al-Mg-based plated steel sheet capable of improving initial corrosion resistance.SOLUTION: There is employed a fused Zn-Al-Mg-based plated steel sheet that comprises a steel sheet and a plating layer formed on a surface of the steel sheet, wherein the plating layer contains 1.0 mass% or more and less than 3.0 mass% of Al and 1.0 mass% or more and 2.0 mass% or less of Mg, in terms of average composition, and the remainder is composed of Zn and impurities, the plating layer includes a η-Zn phase, [a ternary lamellar structure of Zn / Mg2Zn11 / MgZn2], and [a quaternary lamellar structure of Al / Zn / Mg2Zn11 / MgZn2], and Mg2Zn11 is detected in an X-ray diffraction pattern of a surface of the plating layer measured by using Cu-Kα rays and under the condition that an X-ray output has 50 kV and 200 mA.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a hot-dip Zn-Al-Mg coated steel sheet, an automotive part, and a method for producing the same. [Background technology]

[0002] Conventionally, plated steel sheets (zinc-based plated steel sheets), in which a zinc-based plating layer (zinc-based plating layer) is formed on the surface of the steel sheet, have been used in a wide range of applications, such as automobiles, building materials, and home appliances. The formation of a zinc-based plating layer gives the steel sheet excellent corrosion resistance.

[0003] In the automotive industry, galvannealed steel sheets, a type of zinc-based coated steel sheet, have traditionally been used. Recently, in order to further extend the life of automobiles, the use of hot-dip Zn-Al-Mg coated steel sheets has been considered in place of galvannealed steel sheets. Hot-dip Zn-Al-Mg coated steel layers can contain primary η-Zn crystals, which are the first to crystallize from the molten state, as well as a binary eutectic structure [MgZn2 / Zn] and a ternary eutectic structure [Al / MgZn2 / Zn].

[0004] For example, Patent Document 1 describes a highly corrosion-resistant hot-dip zinc alloy-plated steel sheet containing 1-3% Al, 1.5-4.0% Mg, and the remainder being Zn and unavoidable impurities, with Al+Mg: 2.5-7.0% and Al / (Al+Mg): 0.38-0.48, and the coating structure of the hot-dip zinc alloy coating layer is a coating structure in which a Zn-Al-MgZn2 ternary eutectic structure is used as a base structure and a Zn-MgZn2 binary eutectic structure is dispersed, with the Zn single-phase structure being contained in an amount of 10% or less and the balance being an MgZn2 structure. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5764672 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the plated steel sheet described in Patent Document 1 has a problem in that, in an atmospheric corrosive environment, MgZn2 in the plated structure corrodes preferentially and becomes a corrosion base, resulting in inferior initial corrosion resistance compared to conventional galvannealed steel sheets.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-dip Zn-Al-Mg coated steel sheet, an automotive part, and methods for manufacturing the same, which are capable of improving initial corrosion resistance. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention employs the following configuration. [1] A steel plate and a plating layer formed on the surface of the steel plate are provided. The plating layer has an average composition of Al: 1.3 % or more by mass but less than 3.0% by mass, Mg: 1.3 % by weight or more and 0.02% by weight or less, with the remainder consisting of Zn and impurities, The plating layer contains η-Zn phase and [Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 / MgZn2 quaternary lamellar structure], In the X-ray diffraction pattern of the surface of the plating layer measured using Cu-Kα rays under conditions of an X-ray output of 50 kV and 200 mA, Mg2Zn 11 Hot-dip Zn-Al-Mg coated steel sheet. [2] The hot-dip Zn-Al-Mg coated steel sheet according to [1], wherein the coating layer further contains 0.0001 to 0.1 mass % of Si in its average composition. [3] The hot-dip Zn-Al-Mg coated steel sheet according to [1] or [2], wherein the coating layer further contains, in terms of average composition, 0.0001 to 2 mass% in total of one or more of Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, and Hf. [4] In the cross section of the coating layer, the area ratio of the η-Zn phase is 30 to 70 area %, and [Zn / Mg 2 Zn 11 / MgZn 2 The area ratio of the ternary lamellar structure is 5 to 40% by area, and the area ratio of the ternary lamellar structure is 5 to 40% by area. 2 Zn 11 / MgZn 2 The hot-dip Zn-Al-Mg coated steel sheet according to any one of [1] to [3], wherein the area ratio of the four-element lamellar structure is 10 to 60 area %. [ 5 The present invention provides a steel plate having a plating layer formed on a surface of the steel plate, The plating layer has an average composition of Al: 1.3 % or more by mass but less than 3.0% by mass, Mg: 1.3 % by mass or more and 2.0% by mass or less, with the remainder consisting of Zn and impurities, The plating layer contains η-Zn phase and [Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 / MgZn2 quaternary lamellar structure], In the X-ray diffraction pattern of the surface of the plating layer measured using Cu-Kα rays under conditions of an X-ray output of 50 kV and 200 mA, Mg2Zn 11 are detected, automotive parts. [ 6 The plating layer further contains 0.0001 to 0.1 mass% of Si in its average composition. 5 ] Automotive parts described in the above. [ 7 The plating layer further contains, in an average composition, 0.0001 to 2 mass% in total of one or more of Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, and Hf. 5 ]or[ 6 ] Automotive parts described in the above. [8] In the cross section of the coating layer, the area ratio of the η-Zn phase is 30 to 70 area %, and [Zn / Mg 2 Zn 11 / MgZn 2 The area ratio of the ternary lamellar structure is 5 to 40% by area, and the area ratio of the ternary lamellar structure is 5 to 40% by area. 2 Zn 11 / MgZn 2 The automobile part according to any one of [5] to [7], wherein the area ratio of the four-component lamellar structure is 10 to 60 area %. [ 9 a plating step of forming a plating layer on the surface of a steel sheet by a hot-dip galvanizing method, the plating layer having the average composition according to any one of [1] to [3] and including an η-Zn phase, a Zn / MgZn2 binary eutectic structure, and an Al / Zn / MgZn2 ternary eutectic structure; a heat treatment step of performing heat treatment after the plating layer step, In the heat treatment step, when the soaking temperature is T (°C) and the soaking time is t (minutes), the soaking temperature T (°C) is calculated by the following formula (A): max (℃) or more and 310℃ or less, T max If the temperature (℃) is less than 90℃, the soaking temperature T (℃) is 90℃ or higher, [1] to [ 4 10. A method for producing a hot-dip Zn-Al-Mg coated steel sheet according to claim 9. T max =(1300 / lnt)-273 …(A) In the above formula (A), t is the soaking time (minutes), which is set to be longer than 0 minutes and equal to or shorter than 45 minutes. [ 10 ] Hot-dip plating is used to coat the surface of the steel sheet with [ 5 ]~[ 7 a plating step of forming a plating layer having an average composition according to any one of the above items (1) to (4), and including an η-Zn phase, a binary eutectic structure of Zn / MgZn2, and a ternary eutectic structure of Al / Zn / MgZn2; a forming step of forming the steel sheet on which the plating layer has been formed after the plating step; a heat treatment step of performing heat treatment after the molding step, In the heat treatment step, when the soaking temperature is T (°C) and the soaking time is t (minutes), the soaking temperature T (°C) is calculated by the following formula (B): max (℃) or more and 310℃ or less, T max If (℃) is less than 90℃, the soaking temperature T (℃) shall be 90℃ or higher. 5 ]~[ 8] A method for manufacturing an automobile part according to any one of the above. T max =(1300 / lnt)-273 …(B) In the above formula (B), t is the soaking time (minutes), where t is longer than 0 minutes and is equal to or shorter than 45 minutes. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a hot-dip Zn-Al-Mg plated steel sheet, an automobile part, and a method for producing the same, which are capable of improving initial corrosion resistance. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing an X-ray diffraction pattern of a coating layer of a hot-dip Zn-Al-Mg coated steel sheet according to an embodiment of the present invention. [Figure 2] FIG. 2 shows backscattered electron images of the cross sections of the hot-dip Zn-Al-Mg coated steel sheets No. 5 and No. 27 and the results of element distribution measurement by EPMA. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present inventors have conducted extensive research into means for improving the initial corrosion resistance of hot-dip Zn-Al-Mg-coated steel sheets. It was speculated that the decrease in the initial corrosion resistance of hot-dip Zn-Al-Mg-coated steel sheets is due to preferential corrosion of MgZn2, an electrochemically less noble metal compound. Therefore, the present inventors have investigated ways to suppress the corrosion of MgZn2 by arranging a structure or intermetallic compound around the MgZn2 that prevents the progression of corrosion of MgZn2.

[0012] When a coating layer containing 1.0 to less than 3.0% by mass of Al, 1.0 to 2.0% by mass of Mg, and the remainder being Zn and impurities is formed by hot-dip coating, in addition to the η-Zn phase, an Al / Zn / MgZn2 ternary eutectic structure and a Zn / MgZn2 binary eutectic structure are formed in the coating layer. These ternary eutectic structures and binary eutectic structures contain Zn and MgZn2.

[0013] Here, Mg2Zn is used as a metal compound with an intermediate composition between Zn and MgZn2. 11 It is known that Mg2Zn 11 is electrochemically more noble than MgZn2, so it may be possible to inhibit the corrosion of MgZn2. 11 Therefore, the inventors have conducted extensive research and found that by heat treating a coating layer containing an [Al / Zn / MgZn2 ternary eutectic structure] and a [Zn / MgZn2 binary eutectic structure] under specific conditions, Mg2Zn can be formed near the boundary between Zn and MgZn2 or around MgZn2. 11 It was found that this can improve the initial corrosion resistance of the plating layer.

[0014] Hereinafter, a hot-dip Zn-Al-Mg coated steel sheet, an automotive part, and a method for producing the same according to embodiments of the present invention will be described.

[0015] The hot-dip Zn-Al-Mg coated steel sheet of the present embodiment includes a steel sheet and a coating layer formed on the surface of the steel sheet. The coating layer contains, on an average composition, Al: 1.0 mass % or more but less than 3.0 mass %, Mg: 1.0 mass % or more and 2.0 mass % or less, with the balance being Zn and impurities. The coating layer contains a η-Zn phase and a [Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 The X-ray diffraction pattern of the coating layer surface measured using Cu-Kα radiation at an X-ray output of 50 kV and 200 mA showed that the coating layer contained a quaternary lamellar structure of Mg2Zn 11This is a hot-dip Zn-Al-Mg coated steel sheet in which

[0016] The automotive part of the present embodiment includes a steel material and a plating layer formed on the surface of the steel material. The plating layer contains, on an average composition, Al: 1.0 mass % or more and less than 3.0 mass %, Mg: 1.0 mass % or more and 2.0 mass % or less, with the balance being Zn and impurities. The plating layer contains a η-Zn phase and a [Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 The X-ray diffraction pattern of the coating surface measured using Cu-Kα radiation at an X-ray output of 50 kV and 200 mA showed that the coating layer contained a quaternary lamellar structure of Mg2Zn 11 It is an automotive part that is detected.

[0017] There are no particular limitations on the material of the steel sheet that constitutes the hot-dip Zn-Al-Mg-plated steel sheet. Various types of steel sheet can be used, such as general steel, Ni-preplated steel, Al-killed steel, ultra-low carbon steel, high carbon steel, various high-tensile steels, and some high-alloy steels (steels containing elements that strengthen corrosion resistance, such as Ni and Cr). Furthermore, there are no particular limitations on the conditions for the steel sheet manufacturing method (hot rolling, pickling, cold rolling, etc.). Furthermore, pre-plated steel sheets, the surface of which have been pre-plated with a plating layer of less than 1 μm, such as Zn, Ni, Sn, or an alloy thereof, may also be used.

[0018] Furthermore, the steel materials constituting automotive parts are formed into desired shapes by forming the above-mentioned steel plates. There are no particular limitations on the shape of the steel materials, and the steel materials may be used for automobile exterior panels, automobile parts (such as suspension parts), steel pipes, civil engineering and construction materials (such as fences, corrugated pipes, drainage ditch covers, sand-blocking plates, bolts, wire mesh, guardrails, and water-stop walls), prefabricated parts, residential walls, roofing materials, and home appliance parts (such as the housings of air conditioner outdoor units), or may be steel structural members formed by welding.

[0019] Next, the plating layer will be described. The plating layer according to this embodiment includes a Zn-Al-Mg alloy layer. The addition of alloying elements such as Al and Mg to Zn improves corrosion resistance, so a thin film, for example, about half the thickness of a normal Zn plating layer, can provide equivalent corrosion resistance. Similarly, the present invention also ensures corrosion resistance equivalent to or greater than that of a Zn plating layer with a thin film. The plating layer may also include an Al-Fe alloy layer.

[0020] The Zn-Al-Mg alloy layer is made of a Zn-Al-Mg alloy, which means a ternary alloy containing Zn, Al, and Mg. The Al-Fe alloy layer is an interfacial alloy layer between the steel plate or steel material and the Zn-Al-Mg alloy layer.

[0021] That is, the plating layer may have a single-layer structure of a Zn-Al-Mg alloy layer, or a laminate structure including a Zn-Al-Mg alloy layer and an Al-Fe alloy layer. In the case of a laminate structure, the Zn-Al-Mg alloy layer is preferably a layer that forms the surface of the plating layer.

[0022] When an Al-Fe alloy layer is present in the coating layer, it bonds the steel material to the Zn-Al-Mg alloy layer. The thickness of the Al-Fe alloy layer as an interfacial alloy layer can be controlled in any way by adjusting the coating bath temperature and immersion time during the production of the coated steel sheet. In hot-dip coated steel sheet manufacturing methods centered on the Sendzimir process, the Zn-Al-Mg alloy layer forms the majority of the coating layer, and the thickness of the Al-Fe alloy layer is sufficiently small that it has little effect on the corrosion resistance of the coating layer. Furthermore, because it is formed near the interface, it has almost no effect on the corrosion resistance in the early stages of corrosion or on the appearance of the coating layer.

[0023] The Al-Fe alloy layer is formed at the interface between the coating and the steel sheet or the interface between the coating and the steel material (specifically, between the steel sheet or steel material and the Zn-Al-Mg alloy layer), and is a layer with the Al5Fe2 phase as its main phase in terms of structure. The Al-Fe alloy layer is formed by atomic diffusion between the base steel (steel sheet or steel material) and the coating bath. When hot-dip coating is used as the manufacturing method, the Al-Fe alloy layer is likely to form in coating layers that contain Al. This is because the coating bath contains Al at a certain concentration or higher. The Al5Fe2 phase is the most commonly formed phase. However, atomic diffusion takes time, and there are also areas where the Fe concentration is higher near the base steel. Therefore, the Al-Fe alloy layer may partially contain small amounts of AlFe phase, Al2Fe phase, Al3Fe phase, etc. Depending on the conditions, a disk-shaped Fe4Al 13 It has also been reported that, since the plating bath contains a certain concentration of Zn, the Al-Fe alloy layer also contains a small amount of Zn.

[0024] The plating layer according to this embodiment may contain Si. Si is particularly likely to be incorporated into an Al-Fe-based alloy layer, and may form an Al-Fe-Si intermetallic compound phase. The identified intermetallic compound phase is the AlFeSi phase, and isomers include α-, β-, q1-, and q2-AlFeSi phases. Therefore, these AlFeSi phases may be detected in the Al-Fe-based alloy layer. An Al-Fe-based alloy layer containing these AlFeSi phases is also referred to as an Al-Fe-Si-based alloy layer.

[0025] The thickness of the entire plating layer (adhesion weight) is affected by the plating conditions, so there are no particular upper or lower limits for the thickness of the entire plating layer. In particular, changes in appearance over time when the plating is used in an outdoor atmospheric environment are only affected by the condition of the plating layer in the surface layer of a few micrometers, and are not affected by its thickness, because corrosion of the plating layer is very slow. Furthermore, for example, the thickness of the entire plating layer is related to the viscosity and specific gravity of the plating bath in conventional hot-dip plating methods. Furthermore, the amount of plating weight is adjusted by the drawing speed of the steel (base steel) and the strength of wiping.

[0026] Next, the average chemical composition of the plating layer will be described. When the plating layer has a single-layer structure of a Zn-Al-Mg alloy layer, the average chemical composition of the entire plating layer is the average chemical composition of the Zn-Al-Mg alloy layer. When the plating layer has a laminated structure of an Al-Fe alloy layer and a Zn-Al-Mg alloy layer, the average chemical composition is the combined average chemical composition of the Al-Fe alloy layer and the Zn-Al-Mg alloy layer.

[0027] Typically, in hot-dip galvanizing, the chemical composition of the Zn-Al-Mg alloy layer is nearly identical to that of the plating bath, since the reaction that forms the plating layer is almost always completed within the plating bath. Furthermore, in hot-dip galvanizing, the Al-Fe alloy layer forms and grows instantly immediately after immersion in the plating bath. Furthermore, the reaction that forms the Al-Fe alloy layer is completed within the plating bath, and its thickness is often significantly smaller than that of the Zn-Al-Mg alloy layer. Therefore, unless special heat treatment, such as a thermal alloying treatment, is performed after plating, the average chemical composition of the entire plating layer is substantially identical to that of the Zn-Al-Mg alloy layer, and components such as the Al-Fe alloy layer can be ignored.

[0028] The elements contained in the plating layer will be described below.

[0029] Al: 1.0 mass% or more, less than 3.0 mass% Al is contained to ensure corrosion resistance. If the Al content in the coating layer is 1.0 mass% or more, the effect of improving corrosion resistance is further enhanced. However, if the Al content is 3.0 mass% or more, the effect of improving corrosion resistance saturates. Therefore, the Al content is set to 1.0 mass% or more and less than 3.0 mass%. From the viewpoint of corrosion resistance, the Al content is preferably 1.2 to 2.5 mass%, and more preferably 1.3 to 2 mass%.

[0030] Mg: 1.0 mass% or more, 2.0 mass% or less The Mg content is in the range of 1.0 to 2.0 mass% in terms of average composition. Mg is preferably added to improve corrosion resistance. If the Mg content in the coating layer is 1.0 mass% or more, the effect of improving corrosion resistance is further enhanced. However, if the Mg content exceeds 2.0 mass%, significant dross generation in the coating bath occurs, making it difficult to stably produce hot-dip galvanized steel sheets. Therefore, the Mg content is set to 2.0 mass% or less. From the viewpoint of the balance between corrosion resistance and dross generation, the Mg content is preferably set to 1.2 to 1.8 mass%, more preferably 1.3 to 1.7 mass%.

[0031] Si:0.0001~0.1% by mass The plating layer may further contain 0.0001 to 0.1 mass% of Si in its average composition. Si inhibits the growth of the Al-Fe alloy layer and improves the corrosion resistance of the plating layer. When a trace amount of Si is contained, it forms Al-Si compounds and also forms an interstitial solid solution in the Al-Fe alloy layer. The formation of an Al-Fe-Si intermetallic compound phase in the Al-Fe alloy layer has already been explained above. If Si is incorporated into these compounds, it does not cause any change in the performance of the plating layer. Therefore, if the Si content is less than 0.0001 mass%, most of the Si is trapped in these compounds, which does not change the appearance of the plating layer, or affect its sacrificial corrosion protection or other performance characteristics such as corrosion resistance. Therefore, the lower limit of Si content is set to 0.0001 mass% or more.

[0032] On the other hand, if there is an excess of Si, the excess Si crystallizes in the coating layer, the eutectic structure is not formed sufficiently, and the corrosion resistance and workability of the coating layer are reduced. Therefore, the Si content is set to 0.1 mass% or less.

[0033] The hot-dip coating layer may further contain, in terms of average composition, one or more of Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, and Hf in a total amount of 0.0001 to 2 mass%. By containing these elements, the corrosion resistance of the coating layer can be further improved. REM is one or more of the rare earth elements having atomic numbers 57 to 71 in the periodic table.

[0034] The remainder of the chemical composition of the plating layer is zinc and impurities, some of which are unavoidably contained in zinc and other base metals, and some of which are introduced as a result of the steel dissolving in the plating bath.

[0035] The average composition of the 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 (e.g., Neo River SP-751 manufactured by Sansai Kako Co., Ltd.), then the plating layer is dissolved with hydrochloric acid containing an inhibitor (e.g., Hibilon manufactured by Sugimura Chemical Industry Co., Ltd.), and the resulting solution is subjected to inductively coupled plasma (ICP) atomic emission spectrometry. Furthermore, if there is no surface coating, the process of removing the surface coating can be omitted.

[0036] Next, the structure of the plating layer will be described. The coating layer containing Al, Mg and Zn is composed of η-Zn phase and [Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2ZN 11 The dendritic η-Zn phase contains a quaternary lamellar structure of Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 The morphology has a quaternary lamellar structure of MgZn2.

[0037] In addition, when the plating layer of this embodiment contains Si, [Al / Zn / Mg2Zn 11 The matrix of [Al / Zn / Mg2Zn] contains the intermetallic compound Mg2Si. 11The matrix of the quaternary lamellar structure of [MgZn2 / MgZn2 phase] may contain [MgZn2 phase] and [Al phase].

[0038] [Al / Zn / Mg2Zn 11 / MgZn2 quaternary lamellar structure] is a structure of Al phase, Zn phase, intermetallic compound MgZn2 phase and intermetallic compound Mg2Zn 11 It is a quaternary lamellar structure of [Al / Zn / Mg2Zn 11 The quaternary lamellar structure of η-Zn / MgZn2 exists, filling the spaces between the dendritic η-Zn.

[0039] The Al phase forming this quaternary lamellar structure corresponds to the "Al" phase (an Al solid solution with Zn and containing a small amount of Mg) at high temperatures in the Al-Zn-Mg ternary equilibrium phase diagram. At room temperature, this Al" phase at high temperatures usually appears separated into a fine Al phase and a fine Zn phase. Also, the [Al / Zn / Mg2Zn 11 The Zn phase in the [Al / Zn / Mg2Zn quaternary lamellar structure] is a Zn solid solution containing a small amount of Al as a solid solution, and in some cases a small amount of Mg as a solid solution. 11 The MgZn2 phase in the [Al / Zn / MgZn2 quaternary lamellar structure] is an intermetallic compound phase that exists near Zn: approximately 84 mass% in the Zn-Mg binary equilibrium phase diagram. As far as the phase diagram is concerned, it is thought that other added elements are not dissolved in each phase, or even if they are dissolved, they are in extremely small amounts, but the amounts cannot be clearly distinguished by normal analysis. Also, in the [Al / Zn / Mg2Zn 11 / MgZn2 quaternary lamellar structure 11 The quaternary lamellar structure consisting of these four phases is referred to as [Al / Zn / Mg2Zn 11 / MgZn2 quaternary lamellar structure].

[0040] The η-Zn phase is the same as the Al / Zn / Mg2Zn 11It is a phase that appears as islands with clear boundaries in the matrix of the [Zn / MgZn2 quaternary lamellar structure]. The η-Zn phase corresponds to the η-Zn phase in the Al-Zn-Mg ternary equilibrium phase diagram. The η-Zn phase has a dendritic shape. It is thought that other added elements are not solid-dissolved in the η-Zn phase, or even if other elements are solid-dissolved, the amount is extremely small. The η-Zn phase is similar to the η-Zn phase in the matrix of the [Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 The Zn phase contained in the quaternary lamellar structure of MgZn2 can be clearly distinguished under a microscope.

[0041] [Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] is a structure of the Zn phase, the intermetallic compound MgZn2 phase, and the Mg2Zn formed at the interface between the Zn phase and the MgZn2 phase. 11 It is a ternary lamellar structure consisting of phases.

[0042] The Zn phase forming this ternary lamellar structure is a Zn solid solution containing a small amount of Al as a solid solution and, in some cases, a small amount of Mg as a solid solution. 11 The MgZn2 phase in the ternary lamellar structure of [Zn / MgZn2] is an intermetallic compound phase that exists in the vicinity of Zn: approximately 84 mass% in the Zn-Mg binary equilibrium phase diagram. 11 / MgZn2 ternary lamellar structure 11 The phase is an intermetallic compound phase that exists near Zn: approximately 93 mass% in the Zn-Mg binary equilibrium phase diagram. As can be seen from the phase diagram, it is thought that other added elements are not dissolved in each phase, or even if they are dissolved, the amount is extremely small. However, since the amount cannot be clearly distinguished by normal analysis, the ternary lamellar structure consisting of these three phases is referred to in this specification as [Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure].

[0043] [Zn / Mg2Zn 11 The ternary lamellar structure of Zn / MgZn2 can be clearly distinguished from the η-Zn phase under microscopic observation.11 In the backscattered electron image of a scanning electron microscope, the ternary lamellar structure [Al / Zn / Mg2Zn] is a lamellar structure that does not contain a black phase (Al). Therefore, this ternary lamellar structure is observed as a lamellar structure that contains a black phase (Al). 11 / MgZn2 quaternary lamellar structure] in backscattered electron images.

[0044] [MgZn2 phase] is the same as the [Al / Zn / Mg2Zn 11 This phase appears as an island with clear boundaries in the matrix of the [Al / Zn / Mg2Zn quaternary lamellar structure], and may actually contain a small amount of solid solution of Al. As far as the phase diagram is concerned, it is thought that other added elements are not solid-dissolved in this phase, or even if other added elements are solid-dissolved, the amount is extremely small. This [MgZn2 phase] is 11 / MgZn2 quaternary lamellar structure] and [Zn / Mg2Zn 11 The MgZn2 phase can be clearly distinguished from the MgZn2 phase that forms the [ternary lamellar structure of MgZn2 / MgZn2] under microscope observation. The plating layer of this embodiment contains the [MgZn2 phase].

[0045] When the coating layer contains Si, Mg2Si is a phase that appears as islands with clear boundaries in the solidification structure of the coating layer. Mg2Si can be clearly distinguished in the coating phase under a microscope.

[0046] The [Al phase] is the above-mentioned [Al / Zn / Mg2Zn 11This is a phase that appears as islands with clear boundaries in the matrix of [Al / Zn / MgZn2 quaternary lamellar structure]. [Al phase] corresponds, for example, to the "Al" phase (an Al solid solution with Zn, containing a small amount of Mg) at high temperatures in the Al-Zn-Mg ternary equilibrium phase diagram. The amount of Zn and Mg dissolved in this Al" phase at high temperatures varies depending on the Al and Mg concentrations in the plating bath. This Al" phase at high temperatures normally separates into a fine Al phase and a fine Zn phase at room temperature, but the island-like shape seen at room temperature can be seen as a remnant of the Al" phase at high temperatures. Judging from the phase diagram, this phase is thought to contain no other added elements, or if they do, only trace amounts. However, since this cannot be clearly distinguished by ordinary analysis, this phase, which is derived from the Al" phase at high temperatures and retains the morphology of the Al" phase, is referred to as the [Al phase] in this specification. This [Al phase] is the same as the aforementioned [Al / Zn / Mg2Zn 11 In microscopic observation, this phase can be clearly distinguished from the Al phase that forms the [Al phase / MgZn2 quaternary lamellar structure]. The plating layer of this embodiment contains the [Al phase].

[0047] The content of the η-Zn phase in the coating layer is preferably, for example, 30 to 70 area %. If the content of the η-Zn phase is 30 area % or more, the relatively ductile η phase will be abundant, and the coating layer will have excellent workability, which is preferable. If the content of the η-Zn phase is 70 area % or less, the MgZn2 will not decrease, and the long-term corrosion resistance of the coating layer will be improved, which is preferable. A more preferable range for the content of the η-Zn phase is 35 to 55 area %.

[0048] The plating layer [Zn / Mg2Zn 11 The content of the [Zn / Mg2Zn ternary lamellar structure] is preferably, for example, 5 to 40 area %. 11 If the content of the [Zn / Mg2Zn ternary lamellar structure] is 5% by area or more, the MgZn2 content will be high, and the long-term corrosion resistance of the coating layer will be excellent, which is preferable. 11If the content of the η-Zn phase is 40% by area or less, the content of the η-Zn phase does not decrease and the workability of the plating layer does not deteriorate, which is preferable.

[0049] [Al / Zn / Mg2Zn 11 The content of the quaternary lamellar structure of [Zn / Mg2Zn] is preferably 10 to 60 area %. The reason for this limitation is that 11 The majority of the Al in the coating layer first solidifies as an [Al / Zn / MgZn2 ternary eutectic lamellar structure], so the [Al / Zn / Mg2Zn 11 The content of the Al / MgZn2 quaternary lamellar structure corresponds to the amount of Al as the average composition of the plating layer.

[0050] The balance is the sum of the [MgZn2 phase], Mg2Si, [Al phase] and other intermetallic compounds.

[0051] The plating layer according to this embodiment is made of [Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 / MgZn2 quaternary lamellar structure]. 11 The presence of Mg2Zn can be detected in the X-ray diffraction pattern of the plating layer surface measured using Cu-Kα radiation at an X-ray output of 50 kV and 200 mA. 11 is electrochemically more noble than MgZn2. 11 The plating layer containing MgZn2 can suppress corrosion of MgZn2.

[0052] Mg2Zn 11 The composition of the plating layer according to this embodiment is intermediate between Zn and MgZn2. 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 These eutectic structures contain Zn and MgZn2. Therefore, Mg2Zn 11exists at the boundary between Zn and MgZn2 in these eutectic structures. 11 may also be present on the surface of MgZn2. 11 The presence of in the vicinity of MgZn2 makes it possible to suppress the corrosion of MgZn2.

[0053] Mg2Zn in the plating layer 11 There is no particular restriction on the content of Mg2Zn. 11 The presence or absence of Mg2Zn can be easily confirmed by X-ray diffraction. In other words, if two or more diffraction peaks are detected in the X-ray diffraction pattern at the diffraction angle 2θ of 14.64° ((110) plane), 23.24° ((210) plane), 37.90° ((320) plane), 39.39° ((123) plane), 43.60° ((322) plane), or 67.45° using a Cu tube (CuKα radiation) as shown in JCPDS card No. 01-071-9624, then the coating layer contains Mg2Zn. 11 Diffraction peaks other than these may overlap with those of Zn, Al, MgZn2, etc. in the Zn-Al-Mg based plating layer, or may have low intensity, and are therefore not suitable for identification.

[0054] The X-ray diffraction measurement results shown in Fig. 1(a) to Fig. 1(e) show that Mg2Zn 11 The figure shows an example of the detection of MgZn, and shows the measurement results for Sample 1, which was not heat-treated, Sample 2, which was soaked at 170°C for 20 minutes, and Sample 3, which was soaked at 205°C for 30 minutes, for a coating layer containing 1.5% Al, 1.5% Mg, and the balance Zn. Samples 2 and 3 showed diffraction peaks at 14.64°, 23.24°, and 39.39°, indicating the presence of MgZn 11 On the other hand, no diffraction peaks were observed in sample 1, and Mg2Zn 11 has not been detected.

[0055] As an X-ray source, X-rays (CuKα rays) targeting Cu are the most convenient because they can obtain average information about the constituent phases in the plating layer. Other measurement conditions include an X-ray output voltage of 50 kV and a current of 200 mA. There are no particular limitations on the X-ray diffraction device, but for example, a horizontal sample-mounted high-power X-ray diffraction device such as the RINT-TTR III manufactured by Rigaku Corporation can be used.

[0056] The measurement conditions other than the X-ray source are as follows: a goniometer TTR (horizontal goniometer) is used, a Kβ filter is not used, the length limiting slit is 5 mm, the receiving slit is 0.15 mm, the receiving slit 2 is open, the scan speed is 0.60 deg. / min, the step width is 0.01 deg, and the scan axis 2θ is 5 to 90°.

[0057] For the measurement, X-rays are irradiated onto the surface of the plating layer. No sample preparation is required, and the measurement is performed on the plating layer as is. If the plating is painted, the surface paint film is removed with a paint stripper that does not corrode the plating (for example, Neo River SP-751 manufactured by Sansai Kako Co., Ltd.), and then the X-ray diffraction is measured.

[0058] The coating weight of the coating layer is 30 to 600 g / m2 on one side of the steel sheet. 2 The coating weight is preferably 30 g / m 2 If the coating weight is less than 600 g / m, the corrosion resistance of the hot-dip Zn-Al-Mg coated steel sheet or automotive parts will decrease, which is not preferable. 2 If the thickness is more than 1000 nm, the molten metal adhering to the steel sheet will drip, making it impossible to smooth the surface of the coating layer, which is undesirable.

[0059] Next, a method for producing the hot-dip Zn-Al-Mg coated steel sheet and the automotive part of this embodiment will be described.

[0060] The method for producing a hot-dip Zn-Al-Mg-plated steel sheet according to this embodiment includes a plating step of forming a plating layer on the surface of a steel sheet by hot-dip plating, and a heat treatment step of performing heat treatment after the plating step.

[0061] The method for manufacturing an automotive part of this embodiment includes a plating step of forming a plating layer on the surface of a steel sheet by hot-dip galvanizing, a forming step of forming the steel sheet with the plating layer formed thereon after the plating step, and a heat treatment step of performing heat treatment after the forming step.

[0062] In the following explanation, the plating process and heat treatment process required for manufacturing hot-dip Zn-Al-Mg plated steel sheet and automotive parts will be described, and then the forming process required for manufacturing automotive parts will be described.

[0063] (Plating process) In the plating process, hot-dip plating is performed on a steel sheet manufactured through steelmaking, casting, and hot-rolling. After the hot-rolling, pickling, hot-rolled sheet annealing, cold rolling, and cold-rolled sheet annealing may be performed, followed by hot-dip plating. The hot-dip plating is a continuous hot-dip plating method in which the steel sheet is continuously passed through a hot-dip plating bath.

[0064] The hot-dip galvanizing bath preferably contains 1.0 mass% or more but less than 3.0 mass% Al, 1.0 mass% or more but less than 2.0 mass% Mg, with the balance being Zn and impurities. Alternatively, the hot-dip galvanizing bath may contain 1.0 mass% or more but less than 3.0 mass% Al, 1.0 mass% or more but less than 2.0 mass% Mg, with the balance being Zn and impurities. Furthermore, the hot-dip galvanizing bath may contain 0.0001 to 0.1 mass% Si. Furthermore, the hot-dip galvanizing bath may contain 0.0001 to 2 mass% in total of one or more of Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, and Hf. The average composition of the plating layer in this embodiment is approximately the same as the composition of the hot-dip plating bath.

[0065] The temperature of the hot-dip galvanizing bath varies depending on the composition, but is preferably in the range of, for example, 400 to 500° C. This is because if the temperature of the hot-dip galvanizing bath is in this range, a desired hot-dip plated layer can be formed.

[0066] The coating weight of the hot-dip coating layer can be adjusted by gas wiping or other means on the steel sheet pulled out of the hot-dip coating bath. The coating weight of the hot-dip coating layer on one side of the steel sheet is 30 to 600 g / m 2 It is preferable to adjust the range to be within the range.

[0067] In this embodiment, the cooling rate for the steel sheet immediately after being pulled up from the hot-dip galvanizing bath is not particularly limited, but may be, for example, 5 to 30°C / s. The temperature range may be from the sheet temperature at the time of pulling up to a temperature slightly below 340°C, the solidification completion point.

[0068] As a result of the above, a coating layer containing the η-Zn phase, a binary eutectic structure of Zn / MgZn2, and a ternary eutectic structure of Al / Zn / MgZn2 is formed on the surface of the steel sheet.

[0069] (Heat treatment process) Next, the plated steel sheet on which the plating layer is formed is subjected to a heat treatment process. By the heat treatment, the Mg2Zn 11 Mg2Zn is formed. 11 As a result of the formation of the ternary eutectic structure, the binary eutectic structure becomes a ternary lamellar structure, and the ternary eutectic structure becomes a quaternary lamellar structure. In the heat treatment process, when the soaking temperature is T (°C) and the soaking time is t (minutes), the soaking temperature T (°C) is calculated by the following formula (A): max Heat treatment is carried out in a range of 310°C or more and 310°C or less.

[0070] T max =(1300 / ln(t))-273 …(A) In the above formula (A), t is the soaking time (minutes), which is set to be longer than 0 minutes and equal to or shorter than 45 minutes.

[0071] The soaking temperature is T max If the thickness is less than 1000 nm, the plating layer contains Mg2Zn. 11Furthermore, if the soaking temperature exceeds 310°C, the temperature approaches the ternary eutectic point of the plating layer, which is undesirable as it may cause the plating layer to melt and its crystalline structure to change. However, T max If the value is less than 90°C, the soaking temperature should be between 90°C and 310°C. There is an empirical rule that for atomic diffusion to occur in a metal structure, a temperature of more than half the melting point in absolute temperature is required, and the melting point of Zn is 419°C (692K), and half of that is 73°C (346K). Therefore, 90°C is set as the lower limit of the temperature at which atomic diffusion will definitely occur.

[0072] T max is preferably defined by the following formula (A1), may be defined by the following formula (A2), or may be defined by the following formula (A3).

[0073] T max =(1400 / ln(t))-273 …(A1) T max =(1500 / ln(t))-273 …(A2) T max =(1600 / ln(t))-273 …(A3) In the above formulas (A1) to (A3), t is the soaking time (minutes), which is set to be longer than 0 minutes and equal to or shorter than 45 minutes.

[0074] The heat treatment is preferably carried out in an inert gas atmosphere or a vacuum atmosphere to prevent oxidation of the plating layer. Examples of inert gas atmospheres include argon and nitrogen. As described above, when the heat treatment is carried out after plating, the plating layer is not oxidized even in the air, so the heat treatment may be carried out in the air.

[0075] The hot-dip Zn-Al-Mg plated steel sheet of this embodiment is manufactured through the plating step and heat treatment step as described above.

[0076] (molding process) Next, the forming step will be described. The forming step may be performed between the plating step and the heat treatment step. In the forming step, the plated steel sheet after the plating step is formed into a desired part shape. The forming method used in the forming step may be ordinary press forming, or so-called hot press forming in which the plated steel sheet is heated and then press-formed. If the plating layer structure after hot pressing is the same as that after solidification of the hot-dip plating, the effects of the present invention can be obtained.

[0077] The automotive component of this embodiment is manufactured by carrying out the plating step, molding step, and heat treatment step as described above. The heat treatment step after the molding step may be carried out before or after the painting and baking step on the surface of the plating layer.

[0078] According to the hot-dip Zn-Al-Mg coated steel sheet and automotive part of this embodiment, the coating layer contains Mg2Zn 11 Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 / MgZn2 quaternary lamellar structure], which can improve initial corrosion resistance. Furthermore, according to the method for producing a hot-dip Zn-Al-Mg coated steel sheet and the method for producing an automobile part of the present embodiment, the coating layer contains Mg2Zn 11 Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 Since the alloy can contain a quaternary lamellar structure of Zn-Al-Mg / MgZn2, the initial corrosion resistance of hot-dip Zn-Al-Mg coated steel sheets and automotive parts can be improved. [Example]

[0079] After alkaline degreasing of a 0.7 mm thick SPCC (JIS G3141) plate, it was subjected to a heating reduction treatment at 800°C for 60 seconds in an N2-H2 atmosphere using a hot dip galvanizing simulator manufactured by Rhesca Corporation. After cooling to the plating bath temperature, it was immersed for 3 seconds in a plating bath with the same composition as the average composition of the plating layer shown in Table 1. Thereafter, the plating coating weight per side was adjusted by N2 wiping, and the plate was cooled under the conditions shown in Table 1.

[0080] Next, the steel sheets were heat-treated in a heating furnace under an argon atmosphere at the soaking temperatures and times shown in Table 1. In this way, plated steel sheets Nos. 1 to 34 were produced.

[0081] The average composition of the plating layer was measured by peeling off and dissolving the plating layer, and then analyzing the content of elements contained in the plating layer by inductively coupled plasma atomic emission spectrometry.

[0082] The η-Zn phase in the coating layer, Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure], [Al / Zn / Mg2Zn 11 The area ratio of the η-Zn phase, [Zn / Mg2Zn quaternary lamellar structure] was determined by taking backscattered electron images of five locations on the cross section of the coating layer at 1000x magnification using a scanning electron microscope. The photographs were taken so that the entire thickness of the coating layer was visible. The photographing positions were selected randomly. Furthermore, an energy dispersive X-ray elemental analyzer attached to the scanning electron microscope was used to obtain elemental mapping data corresponding to the photographs, and the area ratio of the η-Zn phase, [Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 The total cross-sectional area of ​​these phases and structures that appeared in all cross-sectional photographs was measured, and this was divided by the cross-sectional area of ​​the plating layer that appeared in all cross-sectional photographs to determine the area ratio of these phases and structures.

[0083] In Table 2, [Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] is described as "ternary lamellar structure", and [Al / Zn / Mg2Zn 11 / MgZn2 quaternary lamellar structure] is described as "quaternary lamellar structure."

[0084] In addition, X-ray diffraction measurements were performed on the plating layer to determine the Mg2Zn 11 The presence or absence of ions was confirmed. A horizontal sample-mounted high-power X-ray diffractometer RINT-TTR III manufactured by Rigaku Corporation was used, with CuKα radiation as the X-ray source. The X-ray output conditions were a voltage of 50 kV and a current of 200 mA. Measurement conditions other than the X-ray source included a goniometer TTR (horizontal goniometer), no Kβ filter, a length limiting slit of 5 mm, a receiving slit of 0.15 mm, receiving slit 2 open, a scan speed of 0.60 deg. / min, a step width of 0.01 deg., and a scan axis 2θ of 5 to 90°.

[0085] Mg2Zn in the plating layer 11 The presence of Mg2Zn in the plating layer can be confirmed by detecting two or more diffraction peaks at the following angles 2θ using CuKα radiation in the X-ray diffraction pattern, as shown in JCPDS card No. 01-071-9624: 14.64° ((110) plane), 23.24° ((210) plane), 37.90° ((320) plane), 39.39° ((123) plane), 43.60° ((322) plane), and 67.45°. 11 It is said that this includes:

[0086] (Initial corrosion resistance) The resulting plated steel sheets were cut into 150mm x 70mm pieces, degreased, pretreated with a liquid chemical pretreatment, and chemically treated to form dense zinc phosphate crystals. They were then subjected to cathodic electrodeposition coating to form a 20μm thick coating. The edges and backsides were sealed with tape, and a cross-cut scratch was made in the center of the front side with a utility knife, reaching down to the base steel sheet, and then a corrosion test was conducted. The test was conducted in accordance with JASO M609, with a corrosion cycle consisting of a 2-hour salt spray process (5% salt water spray at 35°C), a 4-hour dry process (60°C and 20-30% relative humidity), and a 2-hour wet process (50°C and 95% relative humidity). The specimens were first removed after the 30th cycle, and the coating blister caused by corrosion originating from the cross-cut was observed. The average width of the top four corrosion blister locations was calculated, and the specimens were then subjected to the corrosion cycle test again. The specimens were also removed after the 60th and 90th cumulative cycles and evaluated in the same manner. The initial corrosion resistance was evaluated based on the width of corrosion blisters after 30 cycles. The evaluation criteria were as follows, with ⊚ and ◯ representing pass.

[0087] Note that this evaluation is a relative evaluation of the invention example and the comparative example, and does not match the current standards for the acceptance of automotive steel sheets. For example, the GI plating of comparative example No. 24 may also be applicable to automotive materials depending on the applied parts and coating.

[0088] ◎: Corrosion width is 0.1 mm or less after 30 cycles. ○: Corrosion width after 30 cycles is over 0.1 mm and less than 0.2 mm. △: 30-cycle corrosion width is over 0.2 mm and 0.3 mm or less. ×: Corrosion width exceeds 0.3 mm after 30 cycles.

[0089] In Nos. 1 to 25, the average chemical composition of the coating layer was within the range of the present invention, and the manufacturing method was appropriate. Therefore, the coating layer structure contained η-Zn phase, [Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 The X-ray diffraction pattern also showed that the Mg2Zn 11 This indicates that the initial corrosion resistance is good.

[0090] When the corrosion test was continued for No. 1 to No. 25 up to 60 and 90 cycles, the difference between them and the comparative examples (No. 27 and No. 30) that were not subjected to heat treatment narrowed. This is thought to be because the improvement effect of heat treatment diminished as corrosion progressed.

[0091] No. 26 had a zinc-plated layer and an average chemical composition outside the range of the present invention. Therefore, although it exhibited initial corrosion resistance equivalent to that of conventional zinc-plated steel sheets, its initial corrosion resistance was inferior to that of the inventive examples.

[0092] No. 27 and No. 30 were not subjected to the heat treatment process, so the Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 / MgZn2 quaternary lamellar structure] is not formed, and in the X-ray diffraction pattern, 11 This resulted in poor initial corrosion resistance.

[0093] No. 28 and No. 31 are heat treatment processes with a soaking temperature of T max Since the value was lower than that of Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 / MgZn2 quaternary lamellar structure] is not formed, and in the X-ray diffraction pattern, 11 This resulted in poor initial corrosion resistance.

[0094] No. 29 and No. 32 were heated to a temperature exceeding 310°C during the heat treatment process, resulting in the formation of a Zn / Mg2Zn 11 / MgZn2 ternary lamellar structure] and [Al / Zn / Mg2Zn 11 / MgZn2 quaternary lamellar structure] is not formed, and in the X-ray diffraction pattern, 11 This resulted in poor initial corrosion resistance.

[0095] Nos. 33 and 34 had a 55% Al-Zn-based coating layer with an average chemical composition outside the range of the present invention, and were not subjected to heat treatment. Therefore, they exhibited initial corrosion resistance equivalent to that of conventional 55% Al-Zn-based coated steel sheets, but were inferior in initial corrosion resistance to the examples of the present invention.

[0096] FIG. 2 shows the results of elemental mapping measurement by EPMA of the plated cross sections of Comparative Example No. 27 (Zn-1.5%Al-1.5%Mg plating, no heat treatment) and Example No. 5 (Zn-1.5%Al-1.5%Mg plating, heat treatment at 205°C for 30 minutes).

[0097] X-ray diffraction results revealed that the constituent phases of No. 27 were Zn, MgZn2, and Al. Comparing the elemental mapping and backscattered electron image, the white areas in the backscattered electron image are thought to be Zn, the gray areas MgZn2, and the black areas Al.

[0098] On the other hand, the constituent phases of No. 5 were Zn, MgZn2, Al and Mg2Zn according to the X-ray diffraction results. 11 Comparing the elemental mapping of No. 5 and No. 27, a steep compositional difference was observed at the boundary between Zn and Mg2Zn in No. 27 before heat treatment, but the compositional difference at the boundary between Zn and Mg2Zn in No. 5 after heat treatment was gentler than in No. 27. Mg2Zn 11 is an intermediate composition between Zn and MgZn2, and 2MgZn2 + 7Zn → Mg2Zn 11 In the reaction, Mg2Zn is formed at the interface between Zn and MgZn2. 11 is thought to have been generated.

[0099] [Table 1]

[0100] [Table 2]

Claims

1. The steel plate has a steel sheet and a plating layer formed on the surface of the steel sheet. the plating layer contains, in an average composition, Al: 1.3 mass% or more and less than 3.0 mass% and Mg: 1.3 mass% or more and 2.0 mass% or less, with the balance being Zn and impurities; The plating layer contains η-Zn phase and [Zn / Mg ２ Zn １１ / MgZn ２ ternary lamellar structure] and [Al / Zn / Mg ２ Zn １１ / MgZn ２ and a four-component lamellar structure, In the X-ray diffraction pattern of the surface of the plating layer measured using Cu-Kα rays under conditions of an X-ray output of 50 kV and 200 mA, Mg ２ Zn １１ The hot-dip Zn-Al-Mg coated steel sheet is found to have the following characteristics:

2. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1, wherein the plated layer further contains 0.0001 to 0.1 mass % of Si in an average composition.

3. The hot-dip Zn-Al-Mg plated steel sheet according to claim 1 or 2, characterized in that the plating layer further contains, in terms of average composition, 0.0001 to 2 mass% in total of one or more of Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, and Hf.

4. A hot-dip Zn-Al-Mg plated steel sheet according to any one of claims 1 to 3, wherein in a cross section of the plated layer, the area ratio of the η-Zn phase is 30 to 70 area %, the area ratio of a ternary lamellar structure of Zn / Mg 2 Zn 11 / MgZn 2 is 5 to 40 area %, and the area ratio of a quaternary lamellar structure of Al / Zn / Mg 2 Zn 11 / MgZn 2 is 10 to 60 area %.

5. The steel plate includes a steel material and a plating layer formed on a surface of the steel material, the plating layer contains, in an average composition, Al: 1.3 mass% or more and less than 3.0 mass% and Mg: 1.3 mass% or more and 2.0 mass% or less, with the balance being Zn and impurities; The plating layer contains η-Zn phase and [Zn / Mg ２ Zn １１ / MgZn ２ ternary lamellar structure] and [Al / Zn / Mg ２ Zn １１ / MgZn ２ and a four-component lamellar structure, In the X-ray diffraction pattern of the surface of the plating layer measured using Cu-Kα rays under conditions of an X-ray output of 50 kV and 200 mA, Mg ２ Zn １１ are detected, automotive parts.

6. 6. The automotive part according to claim 5, wherein the plating layer further contains 0.0001 to 0.1 mass % of Si in an average composition.

7. The automotive part according to claim 5 or 6, characterized in that the plating layer further contains, in an average composition, 0.0001 to 2 mass% in total of one or more of Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, and Hf.

8. An automotive part according to any one of claims 5 to 7, wherein in a cross section of the plating layer, the area ratio of the η-Zn phase is 30 to 70 area %, the area ratio of the ternary lamellar structure of Zn / Mg 2 Zn 11 / MgZn 2 is 5 to 40 area %, and the area ratio of the quaternary lamellar structure of Al / Zn / Mg 2 Zn 11 / MgZn 2 is 10 to 60 area %.

9. The surface of a steel sheet is formed by a hot-dip plating method, and the steel sheet has the average composition according to any one of claims 1 to 3 and contains an η-Zn phase and a [Zn / MgZn ２ Binary eutectic structure] and [Al / Zn / MgZn ２ a plating step of forming a plating layer containing a ternary eutectic structure of a heat treatment step of performing heat treatment after the plating layer step, In the heat treatment step, when the soaking temperature is T (°C) and the soaking time is t (minutes), the soaking temperature T (°C) is calculated by the following formula (A): ｍａｘ (°C) or more and 310°C or less, and T ｍａｘ The method for producing a hot-dip Zn-Al-Mg plated steel sheet according to any one of claims 1 to 4, wherein the soaking temperature T (°C) is set to 90°C or higher when the soaking temperature T (°C) is lower than 90°C. T ｍａｘ =(1300 / lnt)-273 …(A) In the above formula (A), t is the soaking time (minutes), which is set to be longer than 0 minutes and equal to or shorter than 45 minutes.

10. The surface of a steel sheet is formed by a hot-dip galvanizing method, and the steel sheet has the average composition according to any one of claims 5 to 7, and contains an η-Zn phase and a [Zn / MgZn ２ Binary eutectic structure] and [Al / Zn / MgZn ２ a plating step of forming a plating layer containing a ternary eutectic structure of a forming step of forming the steel sheet on which the plating layer has been formed after the plating step; a heat treatment step of performing heat treatment after the molding step, In the heat treatment step, when the soaking temperature is T (°C) and the soaking time is t (minutes), the soaking temperature T (°C) is calculated by the following formula (B): ｍａｘ (°C) or more and 310°C or less, and T ｍａｘ 9. The method for manufacturing an automobile part according to claim 5, wherein when the soaking temperature T (°C) is less than 90°C, the soaking temperature T (°C) is set to 90°C or higher. T ｍａｘ =(1300 / lnt)-273 …(B) In the above formula (B), t is the soaking time (minutes), where t is greater than 0 minutes and is equal to or less than 45 minutes.

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